摘要:

Russian Chemical Reviews 71 (11) 867 ± 868 (2002) Institute of Physical and Organic Chemistry at the Rostov State University This issue of the journal is composed of reviews prepared by scientists working at the Research Institute of Physical and Organic Chemistry at the Rostov State University (IPOC). The Institute was founded in 1971 as a subdivision of the North- Caucasian Scientific Centre of Higher School established a year before. The Institute includes six departments and four laborato- ries. Currently, the staff of the Institute comprises 200 associates, among them there are 14 Doctors of Science and 79 Ph.Ds. The Nuclear Magnetic Resonance Laboratory equipped with Varian spectrometers has the status of a Shared Access Centre of the Russian Foundation for Basic Research (RFBR).For RFBR grant holders from the south of Russia, similar functions are performed by the Laboratory for Quantum Chemistry, which implements the RFBR and Ministry for Industry, Science and Technology projects on computer network development and molecular simulation for higher education institutes in the south of Russia. Since 1998, the Institute has been under the guidance of the Division of General and Technical Chemistry of the Russian Academy of Sciences (since 2001, the Division of Chemistry and Materials Sciences). This fact, together with the creation of joint laboratories with the Institute of the Problems of Chemical Physics and the Institute of Physical Chemistry of the RAS reflects the tendency towards the closer integration of Russian science and education, which has taken shape in the last decade, as a vital necessity dictated by the development of society.Related to this, an obvious advantage of research institutes affiliated to a univer- sity is the absence of departmental or other problems hampering the selection and recruitment of the most gifted students to work on scientific projects. The IPOC actively uses this opportunity in cooperation with the Department of Chemistry of the Rostov State University. The post-graduate course and the facilities for working towards doctorate degree, and the Academic Council for defending Ph.D. and Doctoral theses functioning at the Institute, provide the conditions necessary for raising the scientific level of young scientists.The main fields of research developed at the Institute are related to fine organic synthesis, photochemistry, the synthesis #2002 Russian Academy of Sciences and Turpion Ltd and study of the properties of metal coordination compounds, structural theory, and the mechanisms of organic reactions. Of the interesting results of investigations performed at the Institute in recent years, noteworthy is the theoretical develop- ment of the principle of structural design of compounds with dynamic molecular behaviour able to undergo reversible low- barrier intramolecular rearrangements. This principle has under- lain the targeted synthesis of bi-stable compounds existing as two or several stable forms whose interconversions can be induced by electromagnetic or electric fields.Studies into photocontrolled bi-stable structures have resulted in the development of new types of photochromic and fluorescent systems functioning as abiotic converters of light energy, highly sensitive photochemical chemo- sensors, effective molecular switches and 3D data recording media (molecular memory). A traditional field of investigation at the Institute is the synthesis of polycyclic nitrogen- and oxygen-containing hetero- cycles and monosaccharides. The scientists working at the Insti- tute have received more than 100 patents, including foreign ones, for physiologically active compounds exhibiting more than 50 types of biological activity including antidiabetic, antiaggre- gant, antiarrhythmic, antiulcer, local anaesthetic, antioxidant and others.Some of these products have already been authorised for clinical trials and the antiarrhythmic drug Ritmidazol has passed these trials and is allowed to be used in clinical practice by the Russian Federation Pharmacological Committee. The organic ligands synthesised at the Institute serve for the preparation of coordination compounds of transition and non- transition elements with variable polyhedral types of chelate units. Efficient polymerisation catalysts, compounds with valuable magnetic characteristics, UV light converting luminophores, laser dyes, additives to lubricating oils, and so on, have been found among the new metal complexes. The applied studies carried out at the Institute concentrate on electrochemistry (study of metal corrosion mechanisms, develop- ment of effective corrosion inhibitors for drilling tools operating in highly acidic and hydrogen sulfide environments), geochem- istry (regional analytic surveys, landscape pollution mapping, analysis of man-caused pollution), development of new compo-868 sites, coatings, lubricants and antifriction materials for aerospace and the automotive industry (some materials elaborated at the Institute were used in the spaceshuttle Buran).The process of manufacture of the Drotaverin drug (No-spa analogue) from Russian raw materials developed at the IPOC is currently under industrial development at the Volgograd chemical plant.During the last 5 years, the scientists working at the Institute have published sixteen scientific monographs (six of them were published by companies in the USA, Germany and the Nether- lands) with more than 500 scientific papers published in Russian and other international journals. More than 80 patents were granted, two inventions being patented abroad (in 23 countries). There are three State Prize laureates and a number of A M Butlerov and Alexander von Humboldt Prize winners, and winners of other prizes. As in other scientific institutions of Russia, the Institute has faced severe financial problems and a shortage of personnel during the last decade. Suffice it to say that today's budgetary financing of the Institute does not exceed 5% of the pre-reform finances (on a comparable price schedule).The scientific potential and the `critical mass' of the Institute and the intake of young specialists needed for future prospects are maintained almost exclusively by RFBR grants (the Institute has annually 10 to 12 initiative grants, and also leading scientific school grants and grants of the shared access laboratory `Molecular spectroscopy' and of the library and database electronic access network), grants and programmes of the Ministry of Education and the Ministry for Industry, Science and Technology of the Russian Federation. Of special significance are international programmes and grants from the European Union (INTAS), Civilian Research and Development Foundation (CRDF, USA), Alexander von Hum- boldt and Volkswagen Stifftung Foundations (Germany), Inter- national Science and Technical Centre (ISTC), Council of National Research and Science (CNRS, France), SPECS and BioSPECS companies (Netherlands), the Servier pharmaceutical company (France), etc.Other forms of international collabora- tion and exchange are also used. Research dealing with photo- chromic and fluorescent chemosensors carried out at the Institute is a part of a Project executed by the Basic Research and Higher Education Centre at the Rostov, Kuban' and Taganrog Radio Engineering Universities financed by the CRDF, the Ministry of Education of the Russian Federation and the administrations of the Rostov Region and Krasnodar Territory. Within the frame- work of this project, more than 30 students and post-graduates were recruited on a competitive basis to work at the Institute where they received an additional scholarship.The reviews presented in this issue illustrate the main fields of investigations carried out at the Institute and also some results of the collaboration with Universities in the USA and France. The paper by A D Dubonosov, V A Bren and V A Chernoivanov entitled `Norbornadiene ± quadricyclane as an abiotic system for the storage of solar energy' and the paper by V Lokshin, A Samat (Marseilles University) and A V Metelitsa dealing with the syn- thesis and photochromic properties of spirooxazines represent the rapidly progressing research into photochemically active systems � molecular switches and optical memory elements, abiotic converters of light energy and fluorescent chemosensors.The review by Yu A Zhdanov and Yu E Alekseev entitled `Basic achievements in the coordination chemistry of modified monosaccharides' characterises the new trends in the development of carbohydrate chemistry, in particular, the possibility of prepar- ing effective enantioselective catalysts by chirality transfer from the carbohydrate ligand to the metal centre. The review by A D Garnovskii and I S Vasil'chenko `Rational design of metal coordination compounds with azo- methine ligands' gives a concentrated survey of the many-year research into targeted synthesis of mono- and polynuclear clelates and molecular complexes with coordination units of the type MNxZy (where Z = NR, O, S, Se, Te), typical of the most important metal enzymes and metal proteins.An original field of investigation carried out at the Institute is the chemistry of organotellurium compounds. Organotellurium compounds are not only precursors of some efficient semiconduc- tor materials but, owing to specific features of the valence and hypervalent bonds formed by tellurium, they are also useful synthons for the synthesis of new heterocycles; this allows devel- opment of a peculiar synthetic strategy. I D Sadekov's review `b-Telluroacroleins and b-tellurovinyl ketones: synthesis, reac- tions and structure' is devoted to one of the classes of organo- tellurium compounds studied at the IPOC. Finally, the paper by V I Minkin, R M Minyaev, R Hoff- mann `Non-classical structures of organic compounds: unusual stereochemistry and hypercoordination' summarises the main results of investigations in this intriguing field of theoretical chemistry, including the results obtained in cooperation with the group of theorists headed by Nobel Prize winner, a foreign member of the RAS, Roald Hoffmann (Cornell University, USA). Of course, the size of the issue does not permit all the fields of research carried out at the Institute of Physical and Organic Chemistry to be covered. Additional information concerning the Institute and its personnel can be found at the Institute websites (http://www.ipoc.rsu.ru and http://rec.ipoc.rsu.ru). The increase in students' interest in scientific research and in the intake of post-graduates and young scientists, which has become evident in recent years, allows one to hope that the studies published in this issue will be continued in future years. Academician V I Minkin, Director of the Institute of Physical and Organic Chemist

ISSN:0036-021X    

出版商:RSC    

年代:2002

数据来源: RSC

摘要:

Russian Chemical Reviews 71 (11) 869 ± 892 (2002) Non-classical structures of organic compounds: unusual stereochemistry and hypercoordination V I Minkin, RMMinyaev, R Hoffmann Contents I. Introduction II. Tetracoordinate carbon atom with a planar configuration of bonds in the molecules and ions of organic and organoelement compounds: stereoelectronic strategies of stabilisation III. The mechanism of intramolecular inversion of the tetrahedral configuration of bonds at the carbon atom IV. Structures with a pyramidal configuration of bonds at a tetracoordinate carbon atom V. Tetracoordinate carbon atom with a bisphenoidal configuration of bonds and inverted tetracoordinate carbon atom VI. Carbonium ions with penta-, hexa- and heptacoordinate carbon atoms VII.Planar hexacoordinate carbon atom inside a cyclic borocarbon cage VIII. Molecules and ions containing planar penta-, hepta- and octacoordinate carbon atoms or atoms of other non-transition elements IX. Conclusion Abstract. are compounds organic of structures Non-classical Non-classical structures of organic compounds are defined non-tetrahedral containing molecules as defined as molecules containing non-tetrahedral tetracoordinate tetracoordinate and/or the of evolution The atoms. carbon hypercoordinate and/or hypercoordinate carbon atoms. The evolution of the views views on theoretical accumulated the and considered is subject this on this subject is considered and the accumulated theoretical and and experimental dynamic and structures the on data experimental data on the structures and dynamic transformations transformations of is It systematised.are compounds organic non-classical of non-classical organic compounds are systematised. It is shown shown that the and methods the using analysis computational that computational analysis using the methods and the software software potential acquired now has chemistry quantum modern of potential of modern quantum chemistry has now acquired high high predictive on data of source important most the is and capacity predictive capacity and is the most important source of data on the the structures bibliography The compounds. non-classical of structures of non-classical compounds. The bibliography includes includes 227 227 references. references.I. Introduction Molecular structure is a key concept of theoretical organic chemistry. The description of molecular structures of organic compounds is underlain by the fundamental concept of two-centre two-electron covalent bonds, the tetravalence of carbon, and the tetrahedral geometry of the four single bonds it forms. Using only these concepts, which can be easily extended to other Main Group elements of the Periodic Table, it is possible to describe and predict all the key types of organic structures and kinds of isomerism. By means of a molecular meccano, these views can assume a material form as simple stereochemical models; these models appear to be an invention of Jacobus Henricus van't Hoff.{ The fundamental nature of the principles that underlie these models can be compared only with the simplicity of their practical implementa- tion, which can be understood by looking at the cardboard models V I Minkin, RMMinyaev Institute of Physical and Organic Chemistry, Rostov State University, prosp.Stachki 194/2, 344090 Rostov-on-Don, Russian Federation. Fax (7-863) 243 46 67. Tel. (7-863) 243 47 00. E-mail: minkin@ipoc.rsu.ru (V I Minkin). Tel. (7-863) 243 40 88. E-mail: minyaev@ipoc.rsu.ru (RM Minyaev). R Hoffmann Department of Chemistry and Chemical Biology, Cornell University, 14853-1301, Ithaca, NY, USA. Fax (1-607) 255 57 07. Tel. (1-607) 255 34 19. E-mail: rh34@cornell.edu Received 15 May 2002 Uspekhi Khimii 71 (11) 989 ± 1014 (2002); translated by Z P Bobkova #2002 Russian Academy of Sciences and Turpion Ltd DOI 10/1070.RC2002v071n11ABEH000729 869 870 875 876 879 881 887 889 890 Figure 1.Three-dimensional sterochemical models hand-made person- ally by van't Hoff (courtesy Leyden Museum of Natural History). made personally by van't Hoff (Fig. 1). By the same procedure, one can easily assemble molecular models of millions of organic and organoelement compounds. Actually, the prediction of the double-helix DNA structure (which is, perhaps, the most impor- tant discovery of the last century) required only one further structural concept�the notion of the hydrogen bond. The development and extensive use of new methods for the investigation of molecular structure and dynamics as well as the development of organometallic chemistry, which linked organic chemistry to images and theoretical views of coordination chem- istry, have extended the scope of the classical structural theory.Thus, it has become impossible to describe the whole diversity of the new types of structures and their dynamic transformations using only the ideas and the language of the classical structural theory. This triggered the development of two interrelated con- cepts, namely, the concepts of stereochemically non-rigid (flux- ional) compounds and multicentre bonds. The development of the { The Russian version of van't Hoff's article `Sur les formules de structure dans l'espace' 1 and his original drawings illustrating the construction of spatial structures of molecules can be found in the monograph by Bykov.2870 multicentre bond concept, in turn, has entailed the notions of hypervalence and hypercoordination.Thus, the description of molecular structures in terms of the notion of multicentre bonds can be likened to a complication in the standard molecular meccano by adding a set of new units, i.e., multicentre bonds (mc ± ne, where n=1, 2 and m>n), for example, three-centre two-electron bonds (3c ±2e). Examples of consistent application of this approach to the analysis of chemical structures of com- pounds containing hypercoordinate carbon atoms and derivatives of polyhedral boranes can be found in monograph.3 The orbital approach to the analysis of molecular structures and their transformations, developed in the second half of the XX century and based on semiquantitative orbital interaction theory, is more general and has a much more extensive predictive capacity.4 ±6 This theory focuses on the closure of the electron shells of valence molecular orbitals and on the presence of a rather large energy gap between the frontier MO as the key factors that influence the thermodynamic stability of a molecular structure.Kinetic stability of a molecular structure with respect to rear- rangements or fragmentation may be due to the possibility that these processes are thermally forbidden by the orbital symmetry rules. An important advantage of the orbital approach as a qualitative theory of modern theoretical chemistry is that its views and notions can be directly transferred to and used in organometallic chemistry, in the coordination chemistry of tran- sition metals and even in solid-state chemistry.7, 8 Detailed anal- ysis of the data accumulated by the early 1990s on the structures and properties of non-classical organic compounds carried out using the orbital interaction theory has been reported in our monographs 9, 10 and reviews.11, 12 The reliability of any qualitative views needs to be validated by strict theoretical calculations.By the early 1990s, the capacity for performing such calculations for compounds with more than 6 ± 7 non-hydrogen atoms was quite limited; therefore, calculations for molecular systems were mainly carried out in the valence approx- imation by semiempirical quantum chemistry methods. The semi- empirical methods have played and still play an important role in the theoretical simulation of structural chemistry problems.However, these methods are hardly applicable to non-classical structures, as they are parametric techniques in which parameters are selected by adjusting the calculated data to well-known experimental results obtained for classical molecular structures (references). The situation has markedly improved during the last decade. Enormous progress in the manufacture of a new generation of high-performance computers and inhe development of concom- itant software made possible (and almost routine now) calcula- tions of rather complicated molecules by means of high-level ab initio quantum chemistry techniques.13 The accuracy of calcula- tions of this type is quite comparable with that attained in the experiment.This opens up new routes both for additional analysis of the previously developed qualitative structural theory of non- classical organic compounds made at a higher quantitative level and for the computational design of new molecular systems and new structural patterns with unusual geometry and unusual coordination types. The purpose of this review is to consider the main results of the above-mentioned evolution of the views on non-classical organic structures. Whereas in the early stages of development, consid- erations of non-classical structures were focused on discussion of the structures of carbonium cations with a hypercoordinate carbon atom,3, 14 in recent years, they have been markedly extended due to the appearance of new data on unusual stereo- chemistry of organic compounds.Currently, non-classical organic structures constitute a rather diverse field of theoretical and structural chemistry, which cannot be covered within the frame- work of a single review. Therefore, we restricted ourselves mainly to the analysis of recent data concerning non-classical types of carbon coordination. V I Minkin, RMMinyaev, R Hoffmann Defining of the problem is as important for each investigation as the results obtained during its solution. Therefore, to preserve the general outlook, we have composed the first sections of this review in such a way as to allow the evolution of each problem to be followed starting from its background up to the most recent results. This strategy is employed to consider the structures of compounds containing a non-tetrahedral tetracoordinate carbon atom { (so-called anti-van't Hoff ± LeBel chemistry) and struc- tures of compounds with penta- and hexacoordinate carbon atoms.The final sections of the review are devoted to the theoretical design of compounds in which hypercoordinate (with coordina- tion numbers from five to eight) carbon atoms, and atoms and ions isoelectronic with them, are encapsulated into planar organic and organoelement cages. II. Tetracoordinate carbon atom with a planar configuration of bonds in the molecules and ions of organic and organoelement compounds: stereoelectronic strategies of stabilisation The idea of the existence of a tetracoordinate carbon atom with a planar bond configuration } in organic molecules was rejected long ago by van't Hoff and LeBel 2 because it fails to explain the numbers of isomers of methane derivatives.Subsequently, this idea did not attract the attention of researchers for almost a century until Hoffmann, Alder, and Wilcox,15, 16 relying on the orbital interaction method, defined the problem of stabilisation of such a centre and proposed ways in which its structure could exist. Seemingly paradoxical at first glance, this problem immediately became (and still remains) one of the most intriguing challenges to the imagination and the capabilities of theoreticians and experi- mentalists. The initial and later stages of development of this problem have been the subject of detailed reviews.9, 17 ± 23 Our goal is to consider the most recent results and to distinguish the key strategies directed at solving the problem of stabilising a planar tetracoordinate carbon atom.In this context, the data of earlier studies are resorted to for discussion. The reasons for the instability of the planar bond configura- tion at a tetracoordinate carbon atom become clear on examining the Walsh diagram for the methane molecule (Fig. 2). It can be seen that one of the triply degenerate bonding t1u MO of the tetrahedral (Td) structure is transformed in the planarD4h form (1) into a nonbonding a2u MO, which is the pz AO of the carbon atom and is occupied by two electrons.H H sp2 sp2 sp2 H H 1 Thus, only six electrons are used in the formation of four C7Hbonds in the planar structure. According to the most precise calculations, the planar D4h form of the methane molecule is unstable with respect to dissociation giving a hydrogen atom and a methyl radical. This structure is 138.4 [QCISD(T)(fc)/6-311+ G(3d f,2p)//CISD(fc)/6-311G**] 24 136.2 kcal mol71 or [CCD(full)/6-311++G**] 25 energetically less favourable than the tetrahedral Td form.} Moreover, the D4h configuration does { Here and below, we mean the topological tetrahedral type rather than the exact tetrahedral geometry with angles of 109 828'.} Below, carbon atoms with this bond configuration are referred to as planar tetracoordinate carbon atoms. } Expansion of the abbreviations and description of calculation tech- niques and the orbital basis sets can be found in the book by J B Foresman, A Frish Exploring Chemistry with Electronic Structure Methods (Pittsburg: Gaussian Inc., 1996).Non-classical structures of organic compounds: unusual stereochemistry and hypercoordination E (a.u.) b1g 0 70.2 1a2u 70.4 70.6 1t1u 1e1u 70.8 1a1g 1a1 71.0 Td D4h Figure 2. Correlation diagram of the molecular orbitals for tetrahedral and square-planar configurations of the methane molecule. not even correspond to any local minimum in the potential energy surface (PES) of methane.As predicted by both earlier (MINDO/3) and later (RHF/4-31G) calculations,9, 26 the vibra- tional spectrum of the planar structure contains four imaginary frequencies, which imply the occurrence of four types of deforma- tions resulting in barrier-free rearrangements of the molecule. A different planar form ofmethane with C2u symmetry, structure 2, is preferred from the energy standpoint. H H 1.172 A H H C C 0.879A 131.68 H 1.079A H H H 2 This form is not matched by any local minimum in the PES either, but it presents interest as illustrating the adaptation of a planar structure to the electron deficiency experienced by itsC7H bonds. This structure and the energetically more favourable structure 3g (Cs symmetry), which is shown in Fig.3, can be considered as complexes formed by singlet methylene (1A1) and a hydrogen molecule. Both structures (2 and 3g) are stable with respect to dissociation into CH2 and H2 . Figure 3 shows the geometric structures of methane corresponding to all the sta- tionary points in the PES found in calculations.25 The electronic and steric structures of the model methane molecule with D4h symmetry provide a key to the quest for stabilisation routes for compounds with a planar tetracoordinate carbon atom. Now we consider these routes. 1. p-Acceptor and s-donor substituents The replacement of hydrogen atoms in planar methane by p-acceptor groups results in delocalisation of the lone electron pair in the 1a2u orbital (see Fig.2), while s-donor substituents partially make up for the electron deficiency of the s-bonds in the D4h and Cs structures of methane. The concept of electronic stabilisation first put forward by Hoffmann et al.15, 16 and soon supported by extensive ab initio calculations 27 is the key and the most successful strategy used in computational and experimental quests for compounds with a planar tetracoordinate carbon atom. A recent review 21 contains a rather comprehensive collection of data on the structures of a broad range of so-called polar organo- metallic compounds�derivatives of methane, ethene, and cyclo- propane in which the hydrogen atoms have been replaced by lithium or sodium atoms or BeH, MgH, BH2 or AlH2 groups. This review considers both the results of early theoretical studies and the recent results obtained by Schleyer's group 28 using the density functional theory with an extended orbital basis set (B3LYP/6- 871 1.109 1.092 1.101 1.069 1.089 65.2 3c (C2u) l=3, DE=132.2 3b (D4h):1 l=4, DE=136.2 3a (Td) l=0, DE=0 1.079 1.144 0.879 131.6 109.8 1.172 1.231 1.147 1.117 78.9 3d (C2u):2 l=2, DE=127.6 1.114 l=3, DE=110.0 3e (C4u) 98.6 0.889 80.2 1.154 64.0 3g (Cs) l=1, DE=110.01 3f (Cs) l=2, DE=110.9 Figure 3.Geometric parameters of the structures corresponding to sta- tionary points in the PES of the methane mecule found 25 by CCD(full)/ 6-311++G** calculations. Here and below, l is the number of negative eigenvalues of the Hess matrix in the given stationary points: for a minimum, l=0; for a transition state (first-order saddle point), l=1; for a second-order saddle point (the top of a two-dimensional hill), l=2.The relative energies (DE /kcal mol71) of configurations are corrected for the zero-point energy (ZPE) of harmonic vibrations. Here and below, the bond lengths are given in A Ê ngstroÈ ms and the bond angles are in degrees. 311++G**). The results fully confirm the expected effect, namely, a substantial decrease in the energy gap between the planar and tetrahedral structures (here and below, the tetrahedral topology is meant). For compounds such as 1,1-dilithiocyclopro- pane or 1,2- and 1,1-dilithioethenes, structures with planar tetracoordinate carbon centres (structures 4 ± 6, respectively) are energetically the most favourable.Li Li Li H H H2C C C C C C H H2C Li H Li6 (Cs) 4 (C2u) Li 5 (C2h) Divanadium complex 7 was, apparently, the first compound with a planar tetracoordinate carbon atom to be studied exper- imentally. Its structure was established by X-ray diffraction analysis.29 In this compound and in the structurally related dizirconocene complex 8, the planar configuration exists due to the multicentre bond formed by the carbon sp2 orbital of the phenyl anion, similar to binding in the C2u form of methane (structure 2). Complex 9 belongs to the extensive class of bimet- allic complexes in which planar tetracoordinate carbon atoms are linked to transition (or nontransition) metal atoms.30 A virtually planar geometry of the carbon centre was found in compound 10, which is related to substituted 2-lithiocyclopropene,31 and in a number of carbides, for example, in Ca4Ni3C5, which was studied in detail both experimentally 32 and theoretically.33 Numerous examples of bimetallic complexes with structures similar to 7 ± 10 and a description of the methods for their synthesis can be found in reviews.22, 23 All these data can serve as a good illustration for872 the effectiveness of R Hoffmann's concept of electronic stabilisa- tion of a planar tetracoordinate carbon centre.CH3 OMe MeO 2 ZrCp2 Cp2Zr C V V OMe MeO C OMe MeO 8 2 7 tmeda Me But Me Li But Me Me O Me C Me Me Li Li B Me C Me O Me Me CpCo C Me But CoCp Li But C tmeda BSiMe3 SiMe3 10 9 Cp is cyclopentadienyl, tmeda is tetramethylethylenediamine. 2.Carbon atom in the centre of the annulene ring Yet another idea of stabilising a planar configuration of carbon, also proposed by R Hoffmann and coworkers,16 implies incorpo- ration of this carbon atom into an annulene ring having an aromatic (4n+2) electron shell. This gives rise to structures such as, for example, compounds 11 and 12. Indeed, extended HuÈ ckel calculations showed that these structures actually have a closed electron shell characterised by a rather wide energy gap between the highest occupied and lowest unoccupied molecular orbitals. Meanwhile, structures 13 ± 15 with antiaromatic annulene rings proved to be unstable.15 14 13 12 11 The MINDO/3 34 and MNDO35 semiempirical calculations with geometry optimisation confirmed the instability of planar structures 13 ± 15. However, according to these calculation tech- niques, the planar configuration is not realised for compounds 11 or 12 either. Figure 4 shows the stable conformations of fenes- trane molecules 11, 12 and 15 which we determined by B3LYP/6- 311+G** calculations. It can be seen that the central carbon atom in these structures retains the tetrahedral configuration of bonds, although the angular distortions may reach 30 8. It is of interest that in compounds 11 and 12, the charges on the central carbon atom are +3.6 and +3.7, respectively,{ i.e., all the valence electrons of carbon are displaced to the periphery of the molecule. The aromaticity of the 14-membered rings (*18 p electrons) is retained and the effective radius of the central atom decreases, which results in a lower steric strain.However, in compound 15, the charge on the central carbon atom is 2e lower (+1.9) and, as a consequence, the conjugated 12-membered ring contains 14 p electrons, which complies with the aromaticity condition. { From here on, Mulliken charges are considered. 129.2 103.1 98.111 137.8 98.3 96.7 15 3. Carbon atom in a small ring One conclusion drawn by Schleyer and coworkers 27 on the basis of extensive calculations states that stabilisation of a planar tetracoordinate carbon atom could be attained by incorporating it into a small ring.This was explained by the fact that the angular strain in three- and four-membered rings (the HCH angle in planar structure 1 is 90 8) would be lower than in the case of the tetrahedral configuration (the HCH angle is 109.5 8). This con- clusion is also supported by the fact 26 that the HCH angle in molecule 2 (C2u symmetry) is close to the bond angles in small rings. The role of this factor shows itself, for example, in comparison of the relative stabilities of the acyclic dilithiomethane and 1,1-dilithiocyclopropane. In the latter case, structure 4 with a planar carbon atom is more stable, whereas for the former case, the tetrahedral configuration is 2.5 kcal mol71 energy preferred (B3LYP/6-311++G** calculation).21 4.Planar tetracoordinate carbon atoms in organoboron cages. The stabilising role of ligand ± ligand interactions This approach to stabilisation of structures with a planar tetra- coordinate carbon atom is focused on the combined realisation of the electronic and steric effects considered above. Even early ab initio calculations by the Hartree ± Fock procedure 35 showed that incorporation of a tetracoordinate carbon atom into a three- membered 1,2-diboracyclopropane ring creates favourable con- ditions for ring flattening: the boron atoms have vacant pz orbitals involved in the delocalisation of the lone electron pair of carbon and, hence, they exhibit s-donor properties. However, allowance for electron correlation in terms of the MP2/6-31G* (see Refs 36, 37) or B3LYP/6-311+G* (see Ref.21) approximations showed that neither structure 16 H B H C H B 16 H nor planar 2,3-diboraspiropentane 17c, incorporating a dibora- cyclopropane ring, is a true minimum on the PES. According to the most precise MP2(full)/6-311++G** calculations,38 com- pound 17c is a transition state of an almost barrier-free (the activation energy DE=0.23 kcal mol71) enantiotopomerisation of the tetrahedral conformation. V I Minkin, RMMinyaev, R Hoffmann 128.2 106.3 97.3 12 Figure 4. Stable structures and geometric characteristics of fen- estranes 11, 12 and 15 determined by B3LYP/6-311+G** calcula- tions.Non-classical structures of organic compounds: unusual stereochemistry and hypercoordination B C (1.575) (1.490) 1.567 1.485 C 56.9 (57.0) 51.7 (51.6) 1.295 (1.298) C B 1.073 (1.077) 18a (C2u) l=0(0), DE=48.5(47.8) (157.5) 157.7 (1.405) 1.072 (1.076) C 1.400 C Figure 5.Geometric characteristics, relative energies (DE, kcal mol71), and l values for boron-containing heterocyclic compounds 18a ± c, 19a,b and 20 determined by MP2(full)/6-31G** and MP2(full)/6-311++G** calculations (the values in parentheses).38 H H1 (1.503) 1.494 (1.179) 1.176 C (1.613) 1.603 1.412 (1.417) 91.1 (91.5) B B 1.172 (1.176) 19a (C2u) l=0(0), DE=3.9(4.4) H1 H B C C B C H H2 H2 H 17a H H2 H B C C B C H H1 17b H On passing to 2,3-diboraspirocyclopentene, the energy levels of the tetrahedral (18b) and planar (18a) structures (Fig.5) are reversed. In the planar structure 18a, each three-membered ring contains two p electrons, i.e., possesses a certain degree of aromaticity; this provides additional stabilisation to this struc- ture. It can be seen from Fig. 6 that two bonding p MOs of structure 18a fully correspond to the bonding p MO of the cyclopropenium ion. As a consequence, structure 18a is indeed a true minimum on the PES. According to MP2(full)/ 6-311++G**+ZPE calculations, the structure with the planar carbon atom is 58.2 kcal mol71 energetically more favourable than structure 18b with the tetrahedral spiro carbon atom. The latter corresponds to the top of a hill in the PES (l=3).Figure 5 presents the data on the geometry of structures 18a and 18b and the type of PES stationary points corresponding to these structures. The same Figure also shows data for the isomeric stable species 19a and 19b, containing a planar tetracoordinate carbon atom. The structure 19a is only 3.9 kcal mol71 energeti- cally less favourable than the singlet carbene structure 20, corre- sponding to the global minimum in the PES of C3B2H4 . The stabilisation of the planar carbon centre in compounds 18a and 19a,b is due to a combination of all factors including the s-donor and p-acceptor effects of the neighbouring boron atoms, the incorporation of this carbon in the small ring, and the presence (1.817) B 1.799 C 1.463 (1.471) B18b (C2u) l=3(3), DE=111.7(107.5) 1.172 (1.173) (108.5) 108.1 B (1.691) 1.683 51.4 (51.3) (1.378) 1.373 C 1.445 (1.452) (1.074) 1.070 19b (Cs) l=0(0), DE=18.2(18.4) = H H B C C B C H H 17c (1.083) 1.078 C (1.404) 1.404 1.355 1.079 (1.085) 57.7 (58.0) (1.361) B C 1.674 (1.685) 18c (Cs) l=0(0), DE=103.5(102.3) C (1.432) 1.428 C 56.9 (56.8) (1.417) 1.411 1.531 B B (1.539) (1.176) 1.169 20 (C2u) l=0(0), DE=0(0) E /eV 6 HB C 1a2 B 2 H 72 H 76 2b1 710 HH H HB 714 B 1b1 H H H B H C B H H 18a Figure 6.Shapes of the p-molecular orbitals of planar structures 18a and 19a.38 of the stable p-conjugated system.The last-mentioned factor is nothing but creation of the conditions for stabilising ligand ± ligand interactions, which are weaker or totally missing in the structures with tetrahedral carbon atoms. The enhancement of 873 C (1.521) (1.180) 1.513 1.176 C B C 1.477 (1.484) C (1.452) 1.444 C (1.849) 1.842 C (1.178) B 1.175 1.562 (1.570) 2b1 H H C 1a2 H H H 1b1 H H H H H C B B H H 19a874 this interaction following an extension of the p-system of the ligand environment provides additional stabilisation to structures with a planar carbon atom, for example, in compounds 21 ± 23.38 H H2.800 Ga Si Si Si Ga Ga 24b l=0, DE=0.0 24a l=0, DE=2.01 2.819 Al Al Ge C 1.945 2.038 Ge Ge Al 25b l=0, DE =2.87 25a l=0, DE=0.0 2.728 Al Si Si C 1.827 2.026 Si Al Al 26b l=1, DE =1.24 26a l=1, DE=0.01 Figure 7.Geometric parameters, relative energies (DE /kcal mol71) and l values for the CGa2Si2 (24a ± c), CAl2Ge2 (25a ± c) and CSi2Al2 (26a ± c) clusters calculated by the MP2(fc)/6-31G** (for CGa2Si2 and CAl2Ge2) and MP2(full)/6-311++G** (for CSi2Al2) techniques.40 3.218 3.013 3.042 C B B 21 H H 5. The Jahn ± Teller instability of tetrahedral structures 4 2.615 2.415 2.403 4 4 The ligand ± ligand bonding interactions play an especially impor- tant role in the pentaatomic structures CX2Y2, CX3 Yn7 and CXn¡ (X=Al, Ga; Y=Si, Ge; n=0, 1) which incorporate a planar tetracoordinate carbon atom.39 ± 42 For example, the pla- nar Al¡ cage in CAl¡ behaves as an aromatic 2p-electron system.43 According to calculations,43 the stability of the planar structures for these compounds is directly related to the formation of bonding four-centre s- and p-type orbitals in the ligand.In addition, the higher stability of the planar structures with respect to the tetrahedral isomers can be attributed to the fact that the 17- or 18-electron valence shells in the tetrahedral compounds CX4 (1a121t262a122t261e1 and 1a121t262a122t261e2) correspond to degenerate electronic states and, hence, they are subject to Jahn ± Teller deformations.40 Meanwhile, 16-electron compounds with the closed 1a121t262a122t26 electron shell, for example, CAl4 , retain the tetrahedral structure.It can be seen from the data shown in Fig. 7 that the structures of the CSi2Ga2 (24c) and CGe2Al2 (25c) 18-electron compounds with tetrahedral carbon atoms are energetically less favourable than the planar cis- (24a and 25a) and trans-isomers (24b and 25b). They do not correspond to local minima on the PES, being transition states. It is of interest that in the case of CSi2Al2 (26a), the four-membered ring formed by the C 1.835 2.122 X H H B B C B B B B C H H 23 22 H H X= O, NH. 2.834 Ga Si Ga 2.212 C 1.772 2.362 C 2.083 Si 1.870 Ga Si 24c l=1, DE=28.65 2.820 Ge Al Ge 2.009 2.726 2.529 1.891 C 2.092 3.015 C 1.995 Al Ge 3.140 Al 25c l=1, DE=25.83 2.739 Al Si Al 2.086 C 1.824 3.031 C 1.775 2.416 2.024 Si Al Si 26c l=0, DE=0.0 V I Minkin, RMMinyaev, R Hoffmann ligands is not sufficiently large for the carbon atom to occupy its centre.As a consequence, the carbon atom is expelled from the ring plane to form a somewhat more stable pyramidal form 26c. However, the energy gap and the vibration frequency with a negative force constant are so small that the fluctuating structure CSi2Al2 should be considered as having C2u effective symmetry. 4 , CAl3Si7 and The photoelectron spectra of the CAl¡ CAl3Ge7 anions generated by laser vaporisation of the corre- sponding carbide clusters and detected using time-of-flight photo- electron spectroscopy 40 ± 42 are consistent with a planar structure of these anions.Theoretical and experimental facts supporting the planar structure for the 17-electron SiAl¡4 and GeAl¡4 anions were obtained in a similar way. Planar and nearly planar Cs forms were found for the 16-electron species, SiAl4 and GeAl4 .44 The simple planar clusters such as 24 ± 26 represent, in principle, new types of structural patterns that can exist in solid-state compounds with properties useful for high-technology materials.43 ± 45 6. Flattening of the tetrahedral configuration of the carbon atom in sterically strained saturated systems The strategy for steric stabilisation of the planar angular defor- mation of bonds at a tetrahedral carbon atom by placing the atom at the centre of a saturated polycyclic system is similar to that considered above in Section I.2.Numerous theoretical and exper- imental studies have been devoted to the development of this approach (see the reviews 17 ± 20, 46 and references therein). Partic- ular attention was drawn to [m.n.p.q]fenestranes 27 and, subse- quently, to bowlanes 28. The most important result of these studies, as in the case of compounds of the type 11 and 12, was the conclusion that complete flattening of the tetrahedral struc- ture cannot be attained. (CH2)n73 m73(H2C) (CH2)p73 q73(H2C) 27 28a Both semiempirical (MINDO/3 12, 47 and MNDO48) and ab initio 49, 50 calculations showed that the [4.4.4.4]fenestrane mole- cule (29), which is the best candidate to form a planar structure, exists preferentially as a flattened tetrahedral D2d form (29a and 29b).Contrary to suggestions,51 the isomeric pyramidal C4u conformation proved to be energetically less favourable [by 28.5 4 4 1 3 2 2 C C4u C0 2 4 C 3 1 C2u 3 2 2 C 1 4 1 C2u 29b, D2d (R) C C 28b4 1 1 C C 3 3 2 29a, D2d (S) 2u 1 C C 3 4 2 C02u 29, D4h C 3 C 2 3 4 4 1 C04uNon-classical structures of organic compounds: unusual stereochemistry and hypercoordination or 48.3 kcal mol71 according to MINDO/3 47 and MP2(fc)/ 4-31G 50 calculations, respectively]. Interconversion of the D2d and C4u forms proceeds via a transition state with C2u symmetry, which is 17 kcal mol71 higher in energy than the C4u form.The planarD4h form 29 is unstable; it corresponds to the top of a hill on the PES. The pattern of conformational transitions given below reflects the general topology of the PES of [4.4.4.4]fenestrane.47 Analogous structural transformations take place in the case of bowlane 28. According to RHF/6-31G* calculations,52 the energy minimum corresponds to the flattened tetrahedral structure 28a, while the structure 28b with a pyramidalised quaternary carbon atom is a transition state in the interconversion of the mirror topomers 28a. 7. Planar tetracoordinate carbon atom inside a rigid three- dimensional cage of bonds A successful (in theory) route of development of the steric (mechanic) strategy for planarisation of bonds around a tetra- coordinate carbon atom has been provided by computational design of alkaplanes and spiroalkaplanes.These polycyclic struc- tures can be constructed from planar neopentane and spiropen- tane units by incorporating peripheral carbon atoms into rigidly connected cycloalkane fragments.53 ± 56 Complete flattening of bonds around the central carbon atom is not attained in alka- planes; according to RHF/6-31G* calculations, the C7C7C angles in the hexaplane 30 and octaplane 31 molecules are 168.6 8 and 168.8 8.53, 54 In the case of spirooctaplane 32, the deviation from the ideally planar configuration is only 3.1 8. The complete planarisation of bonds around the central carbon is found in dimethanospiro[2.2]octaplane 33; note that this result has been obtained by a rather high-level calculation (MP2/6-311+G**).55 The higher occupied MO of each of the compounds 30 ± 33 is a pz orbital localised on the planar carbon atom.This accounts for the exceptionally low ionisation poten- tials of these hydrocarbons (4.5 ± 5.0 eV), wich are comparable with the ionisation potentials of alkali metals. C C C 32 (D2) 31 (S4) 30 (D2d ) B C B C B B 34 (D4h) 33 (D2h) An interesting route of development of the alkaplane design can be found in a study by Wang and Schleyer.57 In this case, flattening of a tetracoordinate carbon atom was attained using not only steric (mechanical) factors but also electronic factors which are favourable for this type of deformation. The calculations (B3LYP/6-311+G**) carried out by the researchers cited showed that boraplane 34, formed from octaplane 31 via replacement of four carbon atoms adjacent to the central atom by boron atoms, has D4h symmetry and, hence, contains a planar carbon atom.A peculiar feature of structure 34 is the perpendicular orientation of two B7C bonds with respect to the bonds of the central carbon atom. Unlike octaplane 31, in the case of boraplane 34, the highest occupiedMOis not localised at the central atom but is distributed over the perimeter of the borocarbon skeleton. This involvement 875 of the lone electron pair in multicentre bonding is responsible for the additional stabilisation of the planar form, realised in neither octaplane 31 nor the C(BH2)4 fragment simulating the structure of the boraplane mirror plane. III.The mechanism of intramolecular inversion of the tetrahedral configuration of bonds at the carbon atom The results of theoretical and experimental studies of molecules and ions in the ground electronic state considered in the previous Section attest to the possibility of stabilisation of a planar tetracoordinate carbon atom. This brings up the question � whether it is possible to stabilise the structures of transition states of these compounds in such a way as to decrease substantially the energy barrier to the inversion of tetrahedral forms and to identify the classes of organic and organoelement compounds stereo- chemically non-rigid with respect to to this inversion.{ Even early semiempirical and ab initio (RHF/DZ) calcula- tions 58,59 of the PES for enantiotopomerisation of the methane molecule carried out assuming that the four C7H bonds remain equivalent along the whole reaction path showed that the square- planar structure 1 does not correspond to a first-order saddle point and does not represent a transition state of stereoisomerisa- tion.When this constraint was eliminated,9, 60 MINDO/3 calcu- lations showed that inversion of the tetrahedral configuration of the methane molecule proceeds as an asymmetrical digonal twist deformation, the transition state being aCs structure the geometry of which is quite similar to the geometry of the transition Cs structure 3g and to other structures of this type determined using rigorous ab initio calculation techniques.24, 61, 62 The data pre- sented in Table 1 show that the relative energy of the Cs transition state is approximately 25 ± 30 kcal mol71 lower than the energy of the planar methane structure with D4h symmetry.However, this structure is still unstable with respect to the CH4?CH3+H. decomposition, which requires 104 kcal mol71. This means that the non-dissociation route of methane inversion is impracticable under usual conditions. Table 1. Inversion barriers of the methane molecule obtained in the most precise ab initio calculations. Ref. Method DEa /kcal mol71 61 61 24 24 24 125.6 117.9 109.2 117.9 110.2 24 105.1 MCSCF/TZV++G(d, p) SOCI/TZV++G(d, p) MP2(full)/6-311+G** CISD(fc)/6-311G** QCISD(T)(fc)/6-311+G(3df,2p)// CISD(fc)/6-311G** QCISD(T)(fc)/6-311+G(3df,2p)// CISD(fc)/6-311G** +ZPE 62 25 25 109.4 115.4 110.1 B3LYP/6-311G**+ZPE CCD/6-311++G** CCD/6-311++G**+ZPE a Energy difference between structures 3g (transition state with Cs sym- metry) and 3a (ground state with Td symmetry).The inversion of the tetrahedral structure is one of the most important structural types of polytopal rearrangements. The reaction trajectories corresponding to digonal twist { Stereochemically non-rigid molecules have low barriers to an intra- molecular rearrangement (too low to be detected on theNMRtime scale); therefore, they undergo fast (on theNMRtime scale) reversible rearrange- ments.876 (Td ±D2 ± T¡d) or tetrahedral compression (Td ±D2d ± T¡d) deforma- tions are symmetry allowed.15, 63, 64 The latter mechanism is also referred to as edge inversion 65, 66 (as opposed to vertex inversion, characteristic of the stereoisomerisation of tricoordinated pyra- midal structures).Detailed investigations 61, 62 of the reaction pathway of methane stereoisomerisation using the method of intrinsic reaction coordinate (i.e., the trajectory obtained by moving down from the point on the PES corresponding to the transition state along the transition vector 67) showed that these trajectories are actually more intricate (Fig. 8). As follows from Fig.8, the change in the geometry can be described by the digonal twist pattern only at early stages of inversion, while after the initial rotation of the plane of oneCH2 fragment relative to the other, the central atom undergoes pyramidalisation, which is not taken into account in the above schemes. DE /kcal mol71 120 2 1 2 80 1 3 2 4 1 3 40 2 4 1 3 0 4 21 3 Td Figure 8. Geometry evolution of the methane molecule along the energy profile of the tetrahedral configuration Td ±Cs ± T¡d according to ab initio calculations.25, 62 It would be of interest to find out how the decrease in symmetry caused by partial replacement of the hydrogen atoms in the methane molecule by other atoms or groups would influence the type of reaction trajectory in the inversion process.Such calculations based on the intrinsic reaction coordinate technique have so far been performed only for the difluoromethane molecule (the Hartree ± Fock method in a small STO-3G basis set).68 Two transition states (planar cis- and trans-structures) were identified; they correspond to the reaction pathways typical of the digonal twist and tetrahedral compression mechanisms, respectively. The question of what structural factors could stabilise the Cs form of the transition state of the inversion of type 3g tetrahedral configuration has not been discussed in the literature. In principle, it is clear that a solution of this problem is the same as the solution of the problem of stabilisation of a planar tetracoordinate carbon atom.Thus in complex 35, due to the presence of two electro- positive metal atoms at the carbon center, the barrier to inversion decreases to a level that can be determined by dynamic 1H NMR from coalescence of the signals for the methylene group protons. According to EHMO calculations, stereoisomerisation of com- plex 35 proceeds via a non-dissociative mechanism and the transition state contains a planar tetracoordinate carbon atom.69 The energy gap between the tetrahedral and planar structures formed by Main Group elements is known to narrow with a decrease in the electronegativity of the central atom.9, 70 On passing from carbon atom to isoelectronic Be27, B7, Al7, Si, 4 2 3 3 2 1 4 4 1 3 2 4 1 TS (Cs) Reaction coordinate 34 3 2 1 4 4 2 1 33 21 4 T¡d V I Minkin, RMMinyaev, R Hoffmann O O Ge Th N N HH HH C Zr H H 36 35 R1 R1 R2 N N + R3 7 X XM N R2 O B R2 38 R1 37 R1, R2, R3=Alk, Ar, Hal, NO2 .M=Be, Zn, Cd, Hg; X=O, S, Se; R1, R2=Alk, Ar, Hal, NO2 . and Ge ams and to d10 atoms (Zn, Cd, Hg), the ordering in energy of the frontier orbitals is reversed; as a consequence, stabilisation of the planar configuration can be attained by introducing electronegative substituents containing lone electron pairs. There are quite a few examples of stereochemically non- rigid compounds of this type, in particular, one can mention bis(ethanolamino)germanium derivative 36 (see Ref.71) and the extensive group of bis-chelate complexes 37 (see Refs 72, 73). It should be noted, however, that in the general case, the non- dissociative intramolecular inversion of configuration is not the only mechanism responsible for the exchange processes observed by NMR.72 ± 74 Among compounds 35 ± 37, quite reliable proof for the non-dissociative mechanism of inversion has been obtained only for complexes 37 with d10 metals,72, 75 whereas for complexes 35 and 36, a mechanism involving cleavage ± recombi- nation of one bond at the central atom also cannot be ruled out. This mechanism is realised for beryllium complexes 37 and 1,3,2- oxazaboroles 38.72, 76, 77 IV. Structures with a pyramidal configuration of bonds at a tetracoordinate carbon atom Analysis of the stereochemical configurations and chirality of compounds containing tetracoordinate atoms, in particular, tet- racoordinate carbon atoms, is based on consideration of topo- logical characteristics of three possible simplexes (configurations whose symmetry ensures equivalence of all four bonds).78 Apart from the tetrahedral Td and planarD4h structures, this condition is satisfied only by the square-pyramidal C4u form.Early EHMO and ab initio (RHF/DZ) 58, 59 calculations predicted that the C4u structure 3e of methane (see Fig 3) is energetically more favour- able than the D4h form; it is this structure that is formed as the transition state in the inversion of the tetrahedral configuration. This sets the task of stabilising a pyramidal tetracoordinate carbon atom in the molecules of organic compounds.High-level ab initio calculations carried out in recent years 24, 25 fully con- firmed the first conclusion: the total energy of the C4u configu- ration of the methane molecule (calculated with allowance for the zero-point energy) is lower than the total energy of theCs structure 3g. However, it was found that the stationary point corresponding to the methane molecule with C4u symmetry on the PES of methane is a third-order saddle-point, and, hence, this structure is not a transition state. The search for routes for stabilisation of compounds contain- ing tetracoordinate carbon atoms with a pyramidal configuration of bonds (below, pyramidal carbon) is mainly carried out in two ways: (1) electronic stabilisation and (2) steric stabilisation.The most important results obtained along the former route include the theoretical prediction of stability of the parent compound of877 Non-classical structures of organic compounds: unusual stereochemistry and hypercoordination b a C this class, namely, tetracyclo[2.1.0.0.1,302,5]pentane (39), which was called pyramidane, interpretation of the nature of the stability of structure 39 and a description of the possible methods for pyramidane synthesis.59, 79, 80 C 40 3 C 2 C 4 1 C 390 40 (D2d ) 39a 39 (C4u) C 3 1 4 2 39b0 The fact that structure 39 is a rather deep minimum on the PES of C5H4 was first established in a theoretical (EHMO, MINDO/3, HF/4-31G) study 12, 59 of isomerisation routes of spiropentadiene 40, the simplest spiroalkene, which was successfully synthesised only relatively recently.81, 82 C C 4 2 4 1 3 3 1 2 39b 39 Figure 9.Topography of the PES of C5H4 for structures 39 and 40 during deformation of the latter along the angular coordinates a and b.59 It can be seen from Fig. 9 that deformation of structure 40 with variation of the angle coordinates a and b gives rise to four topomers (degenerate isomers) of pyramidane 39, having lower energies. However, structures 39 and 40 are separated from each other by a high energy barrier, which prevents the exothermic rearrangement 40?39. This is the reason for the kinetic stability of spiropentadiene and the possibility of its synthesis.The pyramidane structure does not correspond to the global energy minimum in the PES of C5H4; however, as in the case of spiropentadiene, it is characterised by high kinetic stability with respect to the possible decomposition and rearrangement reac- tions. Pyramidane 39 is computed to be the simplest organic system with a pyramidal carbon atom. As [3.3.3.3]fenestrane, this com- pound is the first member of the [m.n.p.q]fenestrane 27 family. Therefore, it comes as no surprise that study of the structure and properties of this compound was the object of a large number of studies,83 ± 88 in which ab initio calculations were carried out at different approximation levels.The results of calculations of the total and relative energies of the pyramidane 39 molecule and its most stable isomers 40 ± 43 and some other parameters of these molecules are summarised in Table 2. Table 2. Total (7Etot /a.u.) and relative (DE /kcal mol71) energies of the pyramidane molecule 39 and its isomers 40 ± 43 in the ground singlet electronic state, charges on the quaternary carbon atoms and dipole moments calculated by ab initio quantum-chemistry methods. Calculation method Ref. Calculation method Ref. 7Etot DE qC 7Etot DE qC m /D m /D Isomer 41a a (C2u) Pyramidane 39 (C4u) 3.2 70.43 RHF/3G RHF/6-31G* MP2(fc)/6-31G* RHF/3G RHF/6-31G* MP2(fc)/6-31G* 84 7 7 86 7 7 86 7 7 86 88 88 88 189.06525 191.41790 192.03046 OCISD(T)//MP2/6-31G* 192.0954 (192.72932) 192.22987 192.26066 0000000 189.08749 191.45834 192.09980 OCISD(T)//MP2/6-31G* 192.15317 (192.78107) 192.30990 192.31312 70.102 1.43 84 1.79 86 7 7 86 7 7 86 1.83 88 1.77 88 1.84 88 3.65 3.67 3.41 B3LYP/6-311+G** MP2(full)/6-311+G** CCSD/6-311+G** B3LYP/6-311+G** MP2(full)/6-311+G** CCSD/6-311+G** 13.7 70.24 24.4 42.8 35.4 32.5 70.34 50.2 70.29 32.9 70.22 70.46 70.42 70.46 Isomer 42 (Cs) Isomer 40 (D2d ) 191.44637 191.44109 192.06529 6.2 +0.25 192.07803 12.1 7 6.8 7 5.0 +0.40 RHF/3G RHF/6-31G* MP2(fc)/6-31G* OCISD(T)//MP2/6-31G* 191.13154 (192.76380) 192.26111 192.28738 84 86 86 86 88 88 88 RHF/3G RHF/6-31G* MP2(fc)/6-31G* OCISD(T)//MP2/6-31G* 192.13976 (192.71311) 192.26986 25.1 +0.59 192.29504 11.3 +0.48 189.07558 27.3 70.123 0000000 B3LYP/6-311+G** MP2(full)/6-311+G** CCSD/6-311+G** B3LYP/6-311+G** MP2(full)/6-311+G** CCSD/6-311+G** 7 7 7 7 84 86 7 7 86 7 7 86 88 88 88 3.97 3.86 3.90 10.3 70.84 7 21.3 13.2 10.8 70.40 30.6 b 70.36 16.1 b 70.55 Isomer 43 (Cs) Isomer 41 (Cs) 7 189.11556 717.8 1.1 191.48997 719.9 70.34 7 191.43272 15.8 70.77 7 7 7 7 7 84 86 192.06434 22.3 7 7 86 (192.81089) 718.7 +0.15 4.4 +0.28 192.30280 RHF/3G RHF/6-31G* MP2(fc)/6-31G* OCISD(T)//MP2/6-31G* 192.17408 713.7 B3LYP/6-311+G** MP2(full)/6-311+G** CCSD/6-311+G** RHF/3G RHF/6-31G* MP2(fc)/6-31G* OCISD(T)//MP2/6-31G* 192.12006 20.8 7 7 86 4.26 88 4.53 88 4.31 88 B3LYP/6-311+G** MP2(full)/6-311+G** CCSD/6-311+G** 1.02 0.68 192.32741 79.0 70.155 1.46 (192.74965) 19.7 70.46 192.26209 30.0 70.24 192.28218 19.4 70.36 84 86 192.10866 76.2 c 7 7 86 7 7 86 88 88 88 a The C2u structure 41a is a transition state for the conformational isomerisation of 41 proceeding according to the pattern of wagging vibrations of the carbene centre with respect to the plane of the other four carbon atoms.85, 87, 88 b At this level of approximation, the Cs structure 42 has one imaginary frequency in the vibrational spectrum.c At this approximation level, the Cs structure 43 is a flattened region in the C5H4 PES (a first-order saddle point). The adjacent minimum corresponds to a C2 cumulene form whose total energy is 0.6 kcal mol71 lower than the energy of the Cs structure.878 C C C C 43 (Cs) 42 (Cs) 41 (Cs) 41a (C2u) It follows from the above data that the structure of pyrami- dane 39 is only 9 kcal mol71 energetically less favourable than the structure of its carbene isomer 43 which is most stable in the singlet electronic state.The singlet ± triplet splitting calculated for struc- ture 39 is rather large (46.8 kcal mol71) and the strain energy per C7C bond (19 kcal mol71) is lower than the strain energy in the tetrahedrane molecule (*25 kcal mol71).9, 84 However, it can be expected that pyramidane 39 has a relatively high kinetic stability.The lowest vibration frequencies calculated for the C4u structure of pyramidane 85, 88 are rather high: 453.6 and 478.9 cm71 (CCD/ 6-311+G**). The PES of C5H4 contains no reaction valley to connect the two minima corresponding to isomers 39 and 43.12, 85 The pyramidane structure is energetically more favourable than the structures of other classical carbenes, 41 and 42. As shown by semiempirical 79, 83 and ab initio 85, 87, 88 calculations, both car- benes can serve as precursors of pyramidane and appropriate precursors can be found for each of these carbenes.89, 90 It has been reported 87 that the potential barrier to thermal isomerisation of bicyclo[2.1.0]pent-2-en-5-ylidene (42) into pyramidane 39 is equal to 16.3 kcal mol71, while a similar transformation of tri- cyclo[2.1.0.02,5]pent-3-ylidene (41) requires overcoming a poten- tial barrier of only 3.5 kcal mol71.This implies that the generation of these precursors would be accompanied by imme- diate isomerisation in pyramidane. Simultaneously, structure 39 is separated from more stable isomers, for example 3-ethynylcyclo- propene, by relatively high barriers corresponding to a half-life of more than 4 ± 5 h at room temperature. Thus, synthesis of pyramidane should be possible. The nature of the stability of the pyramidane molecule and features of its geometric structure (Fig. 10) are adequately inter- preted using the scheme of orbital interactions (Fig. 11) between the fragments forming this molecule � the basal cyclobutadiene fragment and the apical carbon atom (structure 39a).Stabilisation is mainly due to the formation of the bonding 1e MO of the pyramidane molecule upon overlap of the px,py AO of the apical carbon atom with the degenerate eg MO of the cyclobutane fragment. The spz AO pair of the carbon atom and the cyclo- butadiene a2u MO form a bonding (1a1), a nonbonding (2a1), and a high-lying antibonding MO of molecule 39; only the first two of these orbitals are occupied with electrons. This type of electronic structure is typical of all the pyramidal molecules formed by a p-conjugated cyclic fragment and an apical atom or group. Pyramidal structures of this type have only four bonding (or three bonding and one nonbonding) MOs, which can be populated by p-electrons of the basal fragment and by all valence electrons of the apical atom (or group).The eight-electron 1.653(DFT) 1.653(MP2) 1.642(CCD) 7.88(DFT) 8.08(MP2) 8.58(CCD) 1.450(DFT) 1.453(MP2) 1.451(CCD) 39 (C4u) Figure 10. Geometric characteristics of the pyramidane molecule (39) calculated by the B3LYP/6-311+G**(DFT), MP2(full)/6-311+G** (MP2) and CCD(full)/6-311+G** (CCD) techniques.88 E /eV b2u 710 eg a2u 715 H Figure 11. Diagram of the orbital interactions of the fragments of pyramidane molecule 39$39a according to Refs 9 and 12. The p-MO energy levels of the cyclobutadiene molecule with D4h symme- try were calculated using the EHMO method, the energy levels of the hybrid orbitals of carbon correspond to those given in Ref.91. rule determining the stability of pyramidal structures follows from the foregoing.9, 11, 12 This rule, attributed to three-dimensional aromaticity conditions,92, 93 holds for both heteroatomic systems and transition metal p-complexes (provided that isolobality relations are taken into account).7 Numerous examples of pyr- amidal structures can be found in reviews 9, 12, 94 ± 97 and recent works.25, 98 The stability of non-classical pyramidal structure 39 is com- pletely dictated by electronic factors. The efficiency of steric (mechanical) stabilisation of organic compounds with a tetra- coordinate pyramidal carbon atom was demonstrated by Ras- mussen and Radom.86 As in alkaplanes and spiroalkaplanes (see Section II.7), in compounds with a pyramidal carbon atom, the rigid cage of bonds is based on neopentane and spironeopentane structures with an additional cycloalkane fragment attached.The B3LYP/6-31G* and MP2(fc)/6-31G* calculations showed that the stable hemialkaplane (44, 45) and hemispiroalkaplane (46 ± 48) hydrocarbon structures obtained in this way contain a pyramidal quaternary carbon atom. The degree of pyramidalisa- tion of bonds at the carbon atom in hemialkaplanes is low but in spirohemialkaplanes, this value approaches that in pyramidane 39 and the calculated lengths of the C7C bonds involving the apical carbon atom (1.632 ± 1.650 A) are similar to the corresponding bond lengths in pyramidane. C 44 (C2) V I Minkin, RMMinyaev, R Hoffmann b2 C 2a1 px,py 1e spz,spz 1a1 C C H H H H C H H H C 45 (C2)Non-classical structures of organic compounds: unusual stereochemistry and hypercoordination C C C 48 (C2u) 47 (C2u) 46 (C2u) The strain energies of structures 44 �} 48 are comparable with the strain energies of well-studied strained structures such as cubane and prismane. Thus, there are reasons for assuming that these compounds could be synthesised. Relying on an efficient method for the preparation of bridged spiropentanes,99 �} 101 Rasmussen and Radom 86 proposed the following method for the synthesis of hemispirobioctaplane 46 from the available hydrocarbon 49.Br Br CBr2 MeLi 46 49 It follows from the data of Table 2 that the apical carbon atom in structure 39 bears a considerable negative charge, while the highest occupiedMOis localised almost entirely on this atom and can actually be regarded to be a lone electron pair orbital (see Fig.10). Similar results have also been obtained for hydrocarbons 44 �} 48. This fact is responsible for fairly low (4.5 �} 5.0 eV) ionisa- tion energies calculated for pyramidane, hemialkaplanes, and hemispiroalkaplanes, commensurate in magnitude with the ion- isation potentials of light alkali metals, as well as the exceptionally high basicity of these compounds. The proton affinities of com- pounds with a pyramidal tetracoordinate carbon atom calculated using various methods (they are given in Table 3) are record- breaking values for organic compounds including superbases such as proton sponges.Formerly, it has been considered that [m.n.p.q]paddlanes 50, like [m.n.p.q]fenestranes 27, can also contain pyramidal carbon atoms in the molecule.51, 104 C (H2C)q (CH2)n (CH2)m (H2C)pC50 However, all paddlanes known to date contain at least one large ring (m>8), which leads to flattening of the tetrahedral configuration of the quaternary carbon atoms. The structures of lower [1.1.1.1]- and [2.2.2.2]paddlanes (structure 51) do not Table 3. Proton affinity PA (kcal mol71) in the gas phase (298 K) for structures with a tetracoordinate pyramidal carbon atom. PA Ref. Calculation method Compound 39 CCD/6-311+G** 236.8 88 HF/3-21G 253.3 82 MP2/6-311+G**//MP2/6-31G* 230.6 86 MP2/6-311+G**//MP2/6-31G* 269.3 86 MP2/6-311+G**//MP2/6-31G* 280.1 86 MP2/6-311+G**//MP2/6-31G* 280.8 86 MP2/6-311+G**//MP2/6-31G* 281.8 86 MP2/6-311+G**//MP2/6-31G* 285.1 86 28b 46 47 48 Tetramethyl-sub- stituted hemi- spirooctaplane 47 MP2/6-311+G**//HF/6-31G* 245.5 102 245.7 103 1,8-Bis(dimethyl- amino)naphthalene Experimental data (proton sponge) 879 correspond to minima in the PES;48, 52, 105 according to the results of MINDO/3 andMNDO semiempirical calculations,48 structure 51 relaxes towards unusual structure 51a in which the distance between two planar carbon atoms is only 1.56 A.51a 51 Essentially pyramidalised tetracoordinate carbon atoms are found in yet another group of sterically strained compounds, tricyclo[n.1.0.01,3]alkanes.99 �} 101, 106 �} 109 The first member of this series of bridged spiropentanes, compound 52, has a sufficient lifetime along the pyramidane-to-spiropentane rearrangement pathway to be detected at 755 8C.107, 110 Tricyclo[2.1.0.01,3]al- kane (53) is still unknown but the next members of this series D compounds 54, 55 and some of their derivatives D have been prepared and isolated.99 �} 101 H H H C C C C H H H 52 H55 H54 53 Table 4 summarises the data characterising the spatial struc- tures and strain energies �} 55.As the ring size increases, i.e., on passing from compound 52 to compound 55, the CH27C7CH2 angle rapidly decreases, together with the strain energy of the molecule.For compound 55, the strain energy almost does not differ from that of spiropentane (63 kcal mol71). Table 4. Angular parameters characterising the configuration of the quaternary carbon atom in molecules 52 �} 55 and strain energies of these structures found by MP2/6-31G** ab initio calculations.101 n Strain energy /kcal mol71 Twisting angle b /deg Folding angle a /deg Com- pound 137.2 115.5 79.2 66.0 71.5 47.8 26.1 15.0 52 53 54 55 0123 21.3 6.1 6.3 1.5 a The folding angle is 18087€ ACE; b the twisting angle is 90 87g (g is the dihedral angle between the ABC and CDE planes). B A C (CH2)n D E V. Tetracoordinate carbon atom with a bisphenoidal configuration of bonds and inverted tetracoordinate carbon atom The size of theCH2�}C7CH2 angle in tricyclo[2.1.0.01,3]alkane 53 approaches 180 8, and the bond configuration at a quaternary carbon atom is fairly close to that found in bisphenoid 56 (`butterfly' conformation).It has been proposed to call tetracoor- dinate carbon centres of this type `half planar'.64 In terms of the topological definition,9 according to which structures with tetra- coordinate carbon atoms are divided into classical and non- classical (anti-van't Hoff �} LeBel) ones, depending on the arrange- ment of the four bonds around carbonDeither in both or in only one hemisphere, respectively, D structures of type 56 should be considered non-classical. For compounds containing tetracoordi-880 nate carbon atoms, the only other non-classical topological form is represented by structure 57 with an inverted configuration of bonds at the carbon atom.C C 57 56 The half planar tetracoordinate carbon atom occurs in the structures of carbide clusters 58 111 ± 113 and in zirconium complex 59.114 C (CO)3Fe Fe(CO)3 (CO)3Fe Fe(CO)3 X58 X=H+, CO. The electronic factors promoting stabilisation of the half planar carbon atom in these structures are similar to those effective in structures with planar or pyramidal centres (Fig. 12). In this case, as in the case of planar (D4h) or pyramidal (C4u) structures, the highest occupied MO of the methane molecule having C2u symmetry is formed by the 2a1 AO of the lone electron pair of carbon (this AO is delocalised to a very low extent over equatorial C7H bonds).The electrons of this orbital are not involved in bonding, which results in weakening of the s bonds formed by a half planar carbon atom. Therefore, s-donor (for example, BHAr¡2 groups in compound 59) and p-acceptor groups are expected to stabilise the tetracoordinate half planar carbon atom. These views have been confirmed by calculations 64 for model compounds 60. The results of these calculations are given in Table 5. Xa HbC H X 60 Energy /eV 714 t2u 716110 110 2a1 H H 1b2 H H Figure 12. Walsh diagram illustrating the transformation of the t2u orbitals of tetrahedral methane (see Fig. 2) during its deformation into the structure with a half planar carbon atom.64 F5C6 C6F5 B H Zr C HH B H C6F5 F5C6 59 b a 2a1 1b1 1b2 180 a /deg 120 b /deg H H 1b1 H H V I Minkin, RMMinyaev, R Hoffmann Table 5.Deformation energies (eV) of tetrahedral structures (the a and b angles correspond to structures with the minimum energy) and charges on the carbon atom in compounds 60 determined by EHMO calculations.64 X qC DEb /eV DE0 c /eV b a /deg a a /deg 0.98 1.15 2.65 0.93 0.54 109.5 105 117 107 102 109.5 117 112 114 120 HMe OH CN BH¡ 70.44 70.39 +0.82 70.01 70.96 3 3.06 3.76 6.46 2.70 2.06 a The a and b values are given for the tetrahedral configuration corres- ponding to the global minimum in the PES of disubstituted methane 60.b The deformation energy of the tetrahedral structure into a structure with a half planar carbon atom (a=180 8, b=140 8). c The deformation energy of the tetrahedral structure into a structure with a carbon atom geometry identical to that in complex 59 (a=180 8, b=140 8). In derivative 60 with s-donor substituents (X=BH¡3 ) in the axial positions, the energy gap between the tetrahedral and bisphenoidal conformations decreases by 1.0 eV, mainly, due to substantial charge transfer to the central carbon atom (cf. the charges qC on half planar carbon atoms). It should be noted that the conformation 56 is destabilised by the strong effect of the p-donor and s-acceptor hydroxy group, pulling the electron density off the half planar carbon.As a result, the carbon atom acquires a positive charge. Stabilisation of configuration 56 in complex 59 and the rare earth metal complexes (Me3SiCH2)Y[(m-CH2)2SiMe2][(m-OBut)Li(THF)2]2 (see Ref. 115) and (Me3SiCH2)Sm[(m-OPh)(m-CH2SiMe3)Li(THF)][(m-OPh)2. .Li(THF)] (see Ref. 116) is also promoted by the additional coordination of a half planar carbon atom to a metallic centre formed due to electron donation from the carbon 2a1 AO to the metal AO. An inverted (umbrella) configuration of bonds at the carbon atom (structure 57) can be found in the molecules of some bicyclobutane derivatives, for example, in structures 61117 and 62,118 and undoubtedly, in the molecule of highly strained [1.1.1]propellane (63), which was synthesised by Wiberg and Walker 119 in 1982.SiMe3 Me3Si C6H2But3-2,4,6 P C C P Me Me C 2,4,6-But3H2C6 Si Si SiMe3 Me Me 62 61 C C CH2 CH2 CH2 CH2 H2C H2C C C63 63a The structures, methods of synthesis and the chemistry of [1.1.1]propellane and its derivatives and higher homologues have been considered in numerous reviews 120 ± 122 and theoretical studies.85, 123 ± 126 As noted by Wiberg,122 [1.1.1]propellane appears to be the first multiatomic molecule whose stability, structure, vibrational and phoelectron spectra, and the enthalpy of formation were predicted before its synthesis and all the theoretical predictions were successfully confirmed by subsequent experimental research.Theoretical investigations of compound 63 were mainly focused on elucidating the nature of the central C7C bond and the possibility of its dissociation to give the isomeric biradical 63a. The results of ab initio calculations 122, 125, 126 con- vincingly confirm the conclusion, drawn on the basis of orbital interaction analysis,123 stating that the non-dissociated form 63 is preferred. According to HF/6-31G* calculations, the energy benefit is equal to 65 kcal mol71. The energy gap between structures 63 and 63a, interrelated by `bond-stretch isomer-Non-classical structures of organic compounds: unusual stereochemistry and hypercoordination ism',127, 128 is rather narrow (the difference between the heats of formation of these compounds is only *5 kcal mol71).The situation is reversed on passing to [2.2.2]propellatriene (64). Although the structure 64 with an inverted carbon atom is an energy minimum, the singlet biradical 64a is energetically more favourable (by 5.1 kcal mol71), as shown by CASSCF/6-31G* calculations.129 CC64 64a The inverted configuration of carbon atoms has been pre- dicted by MP2/6-31G** and B3LYP/6-31G** calculations (see Ref. 130) for a series of polycyclic structures with a bicyclooctane skeleton. VI. Carbonium ions with penta-, hexa- and heptacoordinate carbon atoms The preceding Sections of the review were devoted to unusual (i.e., other than tetrahedral) types of carbon coordination. In this and subsequent Sections, we consider compounds of hypercoordinate carbon in which a carbon atom is linked by covalent bonds to five and more atoms.Since a carbon atom has only four valence electrons, it can form only four two-electron (classical) bonds; hence hypercoordination of carbon implies delocalisation of s-electrons.131 This effect does not fit into the scope of classical octet theory.3, 132 It is noteworthy that the notion of non-classical structure was first introduced to describe the structures of hyper- coordinate carbon.131, 133 1. Methonium ion 5 The methonium ion is the parent compound for pentacoordinate carbon derivatives. Protonated methane (65) was first detected in the gas phase by mass spectroscopy by Talrose and Lubi- mova 134, 135 and in superacid liia by Olah and cow- orkers.136 ± 138 The methonium ion is important for astrophysics, as its microwave spectrum serves as an indicator of methane content in molecular galactic clouds.The methonium ion is the prototype of intermediate structures formed upon ion ± molecular interactions 139 and, what is more important, in the reactions of saturated hydrocarbons with electrophiles;3, 140 ± 143 therefore, it comes as no surprise that the study of its structure and properties has received particular attention from theoreticians. The results of calculations (up to the MP4SDQ/6-311G**//MP2/6-31G* level) carried out before 1984 are summarised in a monograph.144 The possible structures of the CHá5 cation are described by conformations 65a ± e.The global minimum in the PES of CHá corresponds to structure 65a with Cs symmetry, which can be regarded as a complex of the CHá3 (C3u) cation with an H2 molecule. The stability of this complex is ensured by the formation of the 3c± 2e bond (according to theoretical and experimental data, the energy of dissociation into fragments is 42 ± 44 kcal mol71). This conclusion was confirmed by recent calculations at the most advanced (of those attainable to date) theoretical level.145 ± 149 + + + + H1 H1 H1 H2 H3 H H C C C 0.95 H H4 H5 H C H3 H5 H5 H4 H2 H H3 H2 H465b (Cs) 65a (Cs) 65a0(Cs) + + + H5 H1 H1 H5 H1 H2 H4 C C C H2 H2 H4 H3 H5 H3 H3 65c (C2u) H465a (Cs) 65a00 (Cs) 881 + + H H H H C C H H H H 65e (D3h) HH65d (C4u) Structure 65b (Cs) corresponds to a first-order saddle point in the PES and serves as the transition state for rotation of the H2 fragment; the potential barrier to this process found using various calculation methods is exceptionally low (less than 0.1 kcal mol71).Structure 65c (C2u) is also a transition state but of another process, namely, the exchange by hydrogen atoms between the CH3 and H2 fragments. A number of experimental studies have been devoted to isotope exchange reactions of this type proceeding at high rates.3, 137, 140 The energy of structure 65c is only 0.9 kcal mol71 higher than the energy level of structure 65a; with allowance for the zero-point vibrations, structures 65a ± c are nearly equivalent in energy.This means that proton `exchange' in the methonium ion is almost barrier-free even at 0K. Thus it has been hypothesised that the CHá5 cation does not have any definite structure at all. It was proposed that the methonium cation should be considered as an intramolecular liquid and its structure described by statistical functions rather than atomic coordinates (see the publication 150 concerning the structure of the methonium cation entitled `Cheshire cat smiles'). Powerful exper- imental proof pointing to an extremely high degree of stereo- chemical non-rigidity (fluxionality) of methonium cation was obtained when a high-resolution IR spectrum of this cation was finally recorded (after many years of unfruitful efforts) in a matrix of a small number (n=1 ± 6) of hydrogen molecules whose presence is needed for retarding the exchange processes.This spectrum proved to be much more complicated than would be expected for a structure having any definite type of symmetry. Indeed, in the 2770 ± 3150 cm71 range alone, *900 spectral lines were found. The question of whether the methonium cation with its unusual fluxional behaviour can be described by a particular molecular structure is still debatable.148, 150 ± 152 However, such a question does not arise for its derivatives. There exist a fairly broad spectrum of compounds in which structures like 65a,c 3, 138, 139, 149 or 65b are realised. The latter is found, in particular, in the structures of s-complexes formed by metal ions (from Sc+ to Cu+) with the methane molecule.The structure and stereodynamics of these complexes have been studied theoretically (using density functional theory techniques) in relation to simu- lation of the problem of decreasing the barrier to the stereo- mutation of the methane molecule.62 It was shown that, when this transformation is catalysed by metal ions, the potential barrier to enantiotopomerisation decreases to 43 ± 50 kcal mol71. In addi- tion to the stationary points corresponding to structures 65a ± c, the PES of CHá5 was found to contain higher-order stationary points corresponding to highly symmetrical forms 65d,e. Stabilisation of such bond configurations at a pentacoordi- nate carbon atom can be achieved only in more complicated structures.2. The pyramidal cation (CH)á5The (CH)á5 cation (66) is a classical example of a structure with a square-pyramidal configuration of bonds about a pentacoordi- nate carbon atom.153 ± 155 + H H+ C C H H C C H H C C C C H H C C H 66 66a H882 The possibility of the existence of a stable cation of this type was pointed out by Williams.156 This prediction soon received theoretical substantiation based on the analysis of orbital inter- actions and EMHO calculations.127 Almost simultaneously, Masamune et al.157 synthesised the first derivative of the pyramidal (CH)á5 cation, compound 67, its bishomo-derivatives and other analogues (68 ± 73).158 ± 160 OH H Me OH Me C+ Me Me H FSO3H FSO3H Me H OH H Me Me H 67 Me H H H C+ C+ C+ 68 69 70 H H H C+ C+ C+ 73 71 72 Just as the CHá5 cation (65a) resembles the structure of the stable 153 ± 155 isoelectronic and isolobal pentahydridoborane 74 with a hypercoordinate boron atom, the (CH)á5 cation (66) has the structure of the isolobal nido-pentaborane 75.+ H H H H H H H H B H74 C H65a + H H C B H H C B H H H B C H C B H H C B H H H H 75 66 The geometry of the pyramidal cation 66 is shown in Fig. 13. In the (CH)á5 cation, unlike pyramidane 39, which is its conjugate base, the C7C bonds involving the apical carbon atom are only slightly elongated compared with usual single C7C bonds. The lengths of the C7C bonds in the four-membered ring are intermediate between the lengths of standard single and double bonds. The C7H bonds in this ring are deflected toward the apical centre, as in pyramidane 39, which is due to favourable overlap of the px and py orbitals of the apical carbon with the s*-orbitals of the C7H bonds.The pyramidal cation 66 has an unusual electronic structure: its electron density is concentrated on the apical carbon atom. Thus according to CCD/6-311+G** calculations, the negative charge on this carbon equals70.18 and the whole positive charge V I Minkin, RMMinyaev, R Hoffmann 1.653(DFT) 1.653(MP2) 1.642(CCD) 7.88(DFT) 8.08(MP2) 8.58(CCD) 1.450(DFT) 1.453(MP2) 1.451(CCD) 39 (C4u) + 1.573(DFT) 1.572(MP2) 1.570(CCD) 7.28(DFT) 8.38(MP2) 7.88(CCD) 1.466(DFT) 1.469(MP2) 1.468(CCD) 66 (C4u) Figure 13.Geometric characteristics of the pyramidane molecule 39 and the (CH)á5 (66) cation determined by the B3LYP/6-311+G**(DFT), MP2(full)/6-311+G** (MP2) and CCD(full)/6-311+G** (CCD) calcu- lations.88 is dispersed in the basal plane.88 This type of charge distribution accounts for the typical pattern of the 1Hand 13CNMRspectra of pyramidal cations 66, 68 ± 73, namely, the carbon signals of the apical groups are shifted to exceptionally high fields [the d13C chemical shifts are from +2.4 (for structure 69) to 733.6 (for structure 72)] and the signals for the basal carbon atoms are shifted downfield. This provides the possibility of easy detection of the formation of non-classical pyramidal cations in the course of reactions.The synthesis of the 1,3,5-trimethyl derivative 76 from 1,5- dimethyl-3-methylenetricyclo[2.1.0.02,5]pentane (77) serves as a good illustration for the mechanism of transformation of the tricyclo[2.1.0.02,5]pentane skeleton in the pyramidal structure precursors 78.161 It was found that the CD2H group formed upon protonation of the deuterated compound 77 occupies only the basal position in cation 76, i.e., during the transformation of compound 77 into cation 76, the deuterium label is not transferred from the basal carbon to the apical one. Thus, the formation of cation 76 does not proceed along the C2u-symmetry pathway but follows the Cs pathway, allowed by the principle of orbital symmetry conservation.4 The chemical shifts (d 13C) of the apical quaternary carbon atom and the methyl-group carbon atom attached to it in compound 76 are about 720.89 and 73.20 ppm, respectively.CD2H + C H C2u Me = CD2H CD2 Me 76a H + H H H H FSO3H Me 75 8C, SO2 + C Me Me Me Me H 78 77 CD2H Cs Me 76 H The eight-electron rule, like more general electron count rules for polyhedral structures,92, 162, 163 explains adequately the kinetic stability of the (CH)á5 cation and its derivatives. The only differ- ence in the orbital interaction diagram of cation 66 from the orbital interaction diagram of the pyramidane molecule presented in Fig. 11 is the fact that the nonbonding 2a1 MO localised on theNon-classical structures of organic compounds: unusual stereochemistry and hypercoordination apical carbon atom in pyramidane 39 is transformed into the orbital of the C7H s-bond in cation 66.9 The square-pyramidal structure 66 is not the most stable form of (CH)á5 .According to the data of ab initio calculations 164 ± 166 (Table 6), the global minimum in the PES of (CH)á5 is the vinyl- cyclopropenyl cation (79), which is only 2 ± 3 kcal mol71 more favourable than the D5h structure of the cyclopentadienyl cation (80) in the triplet electronic state. + + +79 (Cs) 80 (D5h) 78 (C2u) However, the rearrangements 66?79 and 66?80 do not take place, the former � due to the absence of a reaction valley connecting the cations 66 and 79 in the PES of the singlet electronic state of (CH)á5 and the latter rearrangement is spin forbidden.Therefore, it is important to know to what extent the pyramidal structure 66 is energetically less favorable than the other possible structures of the (CH)á5 cation which are stationary points in the PES of the singlet electronic state. Early MINDO/3 semiempirical calculations 167 ± 169 provided the conclusion that the nonplanar Cs structure of the cyclo- pentadienyl cation 81 is preferred from the energy standpoint. However, more recent ab initio calculations 88, 164, 165 showed that the Cs structure (81) does not correspond to a minimum on the PES and relaxes to the planar C2u form (82), which is only 3 ± 4 kcal mol71 energetically more favourable than the pyrami- dal structure 66 and is separated from it by a rather high potential barrier.In the case of the cyclopentadienyl cation, one more C2u Table 6. Total and relative energies of the pyramidal (CH)á5 cation 66 and isomeric structures 78, 82 and 83 in the ground singlet electronic state calculated by ab initio quantum chemistry methods. Calculation method 7Etot /a.u. Isomer 66 (C4u) 191.86520 192.5263 192.58611 HF/6-31G* MP2/6-31G**//6-31G* MP4SDTQ/6-31G**// MP2/6-31G* B3LYP/6-311+G** MP2(full)/6-311+G** CCSD/6-311+G** (193.1679) 192.68932 192.70330 b Isomer 78 (C2u) (193.0098) 192.64397 192.66056 B3LYP/6-311+G** MP2(full)/6-311+G** CCSD/6-311+G** Isomer 82 (C2u) 191.90215 191.9105 192.52005 HF/6-31G* MP2/6-31G**//6-31G* MP4SDTQ/6-31G**// MP2/6-31G* Isomer 83 (C2u) 191.90235 192.51986 HF/6-31G* MP4SDTQ/6-31G**// MP2/6-31G* a The values in parentheses are the energies of the relatively most stable cyclic D5h structure 80 of the cyclopentadienyl cation in the triplet state.b Calculation by the G2 method [approximately equivalent to QCISD(T)/ 6-311+G(3df,2dp) in the approximation level with allowance for the zero- point vibration energies] gives the values 7Etot=192.68503 a.u. and DE=13.1 kcal mol71 for structure 66 (relative to the energy of 80) (see Ref. 166). c First-order saddle point (transition state). + 81 (Cs) Ref. l DE /kcal mol71 165 164 165 0 (42.7)a 0 (12.9)a 7 000 777 0 88 0 88 0 8888 88 88 777 1 c 1 c 1 c 165 164 165 723.2 73.7 73.5 01 c 0 165 165 723.3 73.4 01 c form (83) is possible; it is almost equivalent in energy to the C2u structure 82.According to the most advanced calculations,88, 165 structure 82 is a local minimum on the PES, while structure 83 is a first-order saddle point. Structure 83 corresponds to the transition state between the degenerate isomers (topomers) of structure 82. The potential barrier to interconversion of the topomers of the singlet cyclopentadienyl cation 82 is only 0.1 ± 0.2 kcal mol71, i.e., the reaction proceeds almost without a barrier (cf. the topomerisation of CHá5 ).88, 165 The most important results of calculations 88, 92, 164 ± 166 are summarised in Table 6.+ + 83 (C2u) 82 (C2u) + 82b 83a The C4u configuration of bonds at the pentacoordinate carbon atom can be realised in electrically neutral structures formed upon replacement of theCH+group by the isolobalBHgroup. This can give rise to a series of stable square-pyramidal nido-carboranes whose electronic structure would satisfy the eight-electron rule. Carboranes 84b,c, which are isomers of borole 84a, shown in Fig. 14 are examples of such structures. As shown by calcula- tions,98 structure 84c containing a pentacoordinate pyramidal B 1.583(DFT) 1.582(MP2) C CC C 1.343(DFT) 1.353(MP2) 1.518(DFT) 1.513(MP2) 84a (C2u) 1.573(DFT) 1.572(MP2) C 1.507(DFT) 1.523(MP2) C 1.723(DFT) 1.698(MP2) B C C 1.559(DFT) 1.491(DFT) 1.566(MP2) 1.489(MP2) 84c (C2u) B 1.735(MP2) 1.733(DFT) 1.7520.005(exp.) B C 1.716(MP2) 1.706(DFT) 1.7210.015(exp.) C B B 85b (C2u) 1.624(MP2) 1.629(DFT) 1.6050.005(exp.) Figure 14.Geometric characteristics and relative energies of the struc- tures of borole (84a), its non-classical pyramidal isomers 84b and 84c, and 1,6- (85a) and 1,2-closo-carboranes C2B4H6 (85b) calculated using the MP2(full)/6-311+G** and B3LYP/6-311+G** methods.98, 172 (Experi- mental data for closo-carboranes C2B4H6 were taken from Ref. 172.) 883 + 82a ... + B 1.649(DFT) 1.647(MP2) C C C C 1.464(DFT) 1.468(MP2) 84b (C2u) C 1.627(MP2) 1.624(DFT) 1.6350.004 (exp.) 1.6330.004 (exp.) B B B B C 85a (D2h) 1.716(MP2) 1.712(DFT) 1.7250.012 (exp .) 1.7200.004 (exp.) 1.540(MP2) 1.543(DFT) 1.5400.005(exp.)884 6 carbon atom, being thermodynamically less stable than structures 84a,b, is likely, nevertheless, to have rather high kinetic stability.A similar configuration is found for the carbon atoms in square- bipyramidal closo-carboranes whose structures have been studied in detail both experimentally 3 and theoretically.170 ± 172 The stability of the bipyramidal systems produced by adding apical groups to a p-conjugated ring is controlled by the ten- electron rule.9 Irrespective of the size of the central ring, these structures have only five bonding MO, and the total number of electrons (the number of ring p-electrons +the number of valence electrons in the apical group) in a stable bipyramidal molecule or ion should not exceed 10.The simplest hydrocarbon system which meets this condition is the octahedral C6H4á ion. However, this compound is unstable for electrostatic reasons. The replacement of fourCH+groups byBHgroups results in two stable electrically neutral bipyramidal 1,6- (85a) and 1,2-carboranes (85b), which contain pyramidal pentacoordinate carbon centres (see Fig. 14). 3. Trigonal-bipyramidal configuration of bonds at a pentacoordinate carbon atom A trigonal-bipyramidal configuration of bonds is not realised in the methonium ion CHá5 , but can be stabilised by replacing the hydrogen atoms by electropositive atoms or groups.According to HF/4-31G calculations,173 the D3h conformation is preferred for disubstituted cations 86. As shown by X-ray diffraction analysis, this configuration occurs in penta-aurated methonium ion 87 174, 175 (the ligand AuL is isolobal to single-electron H and CH3 groups). + + M AuPPh3 AuPPh3 H BF¡ C H C Ph3PAu 4 H AuPPh3 AuPPh3 M 87 86 M=Li, BeH. The electronic structure of compounds with the D3h config- uration of bonds at the central atom, which is a Main Group element, can be represented by a combination of three equatorial 2c± 2e bonds formed by the sp2 orbitals of the central atom and one hypervalent 3c± 2e bond of the central atom with two apical ligands.The greater the difference between the electronegativities of the atoms forming this bond, e lower its electron deficiency and the greater the contribution of the electrostatic component which stabilises the structure (for discussion of factors determin- ing the structure of hypervalent compounds, see Refs 5, 176 ± 182). Higher stability of such polarised structures is attained when ligands with like charges are separated in space, i.e., placed in the apical positions of the D3h structure. By following this scheme, one can also propose another way of stabilising the trigonal-bipyramidal configuration of bonds at pentacoordinate carbon atoms in carbonium ions. In this case, a considerable difference between the electronegativities of the central atom and the apical ligands is attained by using atoms or groups with high electronegativity as the ligands.This type of stabilisation of trigonal-bipyramidal structures of transition states is peculiar to SN2 reactions at tetrahedral carbon atoms involving compounds with highly nucleofugal (including positively charged) groups. Examples are 183 two model reactions, namely, degenerate gas-phase hydrolysis of protonated methanol [reaction (1)] and intramolecular rearrangement of hypothetical cation 88 [reac- tion (2)]. H + + O O C (1) CH3OH+H2O H H H H H 89 (C2) V I Minkin, RMMinyaev, R Hoffmann H H H H H H + + + C C F F F F F F C (2) B B B 88b 90 88a The results of MP2(full)/6-31G** and MP2(full)/ 6-311++G** calculations showed that putative intermediates 89 and 90 containing pentacoordinate carbon atoms with the trigonal-bipyramidal configuration of bonds, which are formed presumably in these reactions, are considerably stabilised: the differences between the energies of the intermediates and the initial structures amount to 12.8 and 3.8 kcal mol71.However, neither these nor other similar structures correspond to minima in the PES and, hence, they are not intermediates but are transition states. A similar result has been obtained in an attempt to `freeze' (this term was proposed by Martin 184) the pentacoordinate structure of transition state 91 formed during the rearrangement of 1,8-bis(arylthio)anthracene-9-carbonium ion.185 In this cation, the heteropentalene fragment is incorporated in a rigid cage, which fixes both the attacking and leaving groups in the orienta- tion required by the SN2 reaction conditions.10, 186 ± 188 The structure 91 corresponds to a transition state (rather than an intermediate) of a reaction with a low potential barrier whose height is 10 ± 20 kcal mol71 (depending on the solvent and the substituents).R1 R2 R1 R2 R3 R3 R3 R3 + C+ S S S C S 91 91a R1 R2 + R3 R3 S C S 91b Stabilisation of a structure with a trigonal-bipyramidal con- figuration of bonds at the pentacoordinate carbon atom was achieved by Akiba et al.189 They modified the dynamic system 91 by replacing the sulfur atoms by more electronegative oxygen atoms and thus ensured better conditions for collinearity of the axial bonds. They isolated and characterised the salt 92, the cation of which contains a pentacoordinate carbon atom with a trigonal- bipyramidal configuration of bonds. The complete structural analogy between compound 92 and compounds with a pentacoor- dinate boron atom was confirmed by X-ray diffraction analysis of neutral pentacoordinate compounds of boron 93 (10-B-5) pre- pared recently by the same researchers.190 R + Me Me MeO OMe 2.44 Me Me O O C X X B O O B2F¡7 92 93 X=O, S; R=H, OMe.Thus, by varying the structure, one can embody all the theoretically possible types of coordination of a pentacoordinate carbon atom. Incidentally we would like to mention interesting results of theoretical 155, 191, 192 and experimental 175, 192, 193 studies of the structures of dications containing pentacoordinate nitrogen atoms, which are isoelectronic to carbonium cations.In these studies, relying on high-level (QCISD/6-311G** and CCSD(T)/Non-classical structures of organic compounds: unusual stereochemistry and hypercoordination 5 aug-cc-pVTZ) calculations, a structure with C4u symmetry was attributed to the NH2á dication (according to X-ray diffraction analysis,193 its penta-aurated analogue 94 has a perfect D3h configuration). 2+ Ph3PAu AuPPh3 N 2BF¡ Ph3PAu 4 AuPPh3 Ph3PAu 94 4. Hexa- and heptacoordinate carbonium ions 6 . 6 6 6 In the most symmetrical octahedral Oh configuration, structures of type AH6 , where A is a Group IV element, have four bonding molecular orbitals, which can be occupied by eight electrons (1a1g2t1u6eg2a1g).5, 6, 9 One might expect that this type of octahe- dral structure would exist for the diprotonated methane CH2á Nonempirical calculations 194 ± 197 confirm the expected stability of the CH2á dication.Besides, they indicate that the C2u form in which the eight valence electrons are distributed over two 2c ±2e and two 3c± 2e C7H bonds, rather than the Oh form, is preferred for dication 95 for energy reasons. As for the methonium ion 65, dication 95 is a stereochemically non-rigid structure. According to MP3/6-31G**//6-31G** calcu- lations,195 redistribution of hydrogen atoms between the C7H bonds in the structure 95 in the gas phase requires overcoming a small potential barrier (only 4.3 kcal mol71).The octahedral bond configuration, which is unfavourable for the diprotonated methane, can be stabilised for its derivatives. Examples of this type are the CLi2á (96)198, 199 and C(AuPPh3)2á (97) dications.174 The latter has been isolated preparatively as the tetrafluoroborate salt.174 2+(0) 2+ 2+ Li H AuPPh3 AuPPh3 Li Li Ph3PAu C C C Li Li Ph3PAu H Li HHHH AuPPh3 AuPPh3 97 96 (Oh) 95 (C2u) The pyramidal dication 98 is the prototype of polyhedral organic structures containing a hexacoordinate carbon atom. 2+ 2+ H H C C H H H H H 98a 98 The hexamethyl derivative 99 of this dication has been prepared by Hogeveen and Kwant 200, 201 in superacid media using various precursors (Scheme 1).5 6 The 1H and 13C NMRsignals of the apical centres in 99 occur at a high field (d 13C72.0 ppm).200, 201 As in the case of the (CH)á (66) cation, the pyramidal structure of (CH)2á (98) does not correspond to the deepest minimum in the PES. The transition of this structure to the energetically favourable isomer (according to HF/3-21G calculations), the fulvene dication, is associated with overcoming a high potential barrier.202 The kinetic stability of the dication 98 is interpreted by the eight-electron rule.9 This rule also predicts the existence of another pentagonal-pyramidal non- classical structure 100, which is the conjugate base of the dication 98 (in the same way as pyramidane 39 is the conjugate base for the pyramidal cation 66).Nonempirical calculations 88 carried out at different approximation levels indicate that structure 100 actually corresponds to a minimum on the C6Há5 PES. As can be seen from the data presented in Fig. 15, the bonds connecting the apical and the basal carbon atoms in the dication 98 are substantially Me Me Me OH OH Me Me Me FSO3H Me C Me Me Me 99 FSO3H+SbF5 Me Me Me Me Me elongated with respect to similar bonds in the square-pyramidal ion 66. In the cationic carbene 100, the lengths of these bonds approach the greatest values known for C7C bonds.203, 204 The longest single C7C bond known to date (1.80 A) has been found in a [1.1.1]propellane derivative.46 + C H H H H 100 H A hexacoordinate carbon atom can exist not only in ions but also in electrically neutral species.The octahedral CLi6 (96) molecule can serve as an example.144, 198, 205 This molecule con- tains two valence electrons more than could be accommodated in the four bonding MO of the octahedral structure. These electrons 15.08(MP2) 16.18(DFT) 98 (C5u) 12.68(MP2) 13.88(DFT) 100 (C5u) Figure 15. Geometric characteristics of the C6Há5 cation (100) and C6H26á (98) dication calculated by the B3LYP/6-311+G** and MP2(full)/6- 311+G** methods.88 Me Me + Me Me Me Me FSO3H 2+ Me Me FSO3H Me Me Me + C 100a 2+ 1.682(MP2) 1.637(DFT) 1.439(MP2) 1.440(DFT) + 1.804(MP2) 1.754(DFT) 1.420(MP2) 1.422(DFT) 885 Scheme 1 OH Me886 are localised in the C7Li bonds.An additional bonding a1g MO having a spherical symmetry is formed by delocalisation of orbitals of the lithium atoms surrounding the central carbon atom. In relation to the C7Li bonds, this MO is antibonding; nevertheless, it makes a substantial contribution to the molecular stabilisation due to ligand ± ligand interactions [cf. stabilisation of the tetracoordinate planar carbon atom in structures 24 ± 26 (see Fig. 7)]. Multicentre bonds also determine the stability of a series of carbide clusters 101 and carboranes 102 in which the formal coordination number of carbon is six. In nickel and cobalt clusters 103 with an antiprism structure, the coordination number is as high as eight.94, 206 HC C C 101 103 16 , C 102 H C2B10H12 CRu6(CO)13 , CFe6(CO)2¡ CNi8(CO)2¡ CCo8(CO)2¡ 6 18 The simplest stable structure containing a heptacoordinate carbon atom is triprotonated methane 104.In this compound, eight valence electrons are distributed over three 3c± 2e C7H2 bonds and one 2c± 2eC7Hbond.197 The results of MP2/6-31G** and QCISD(T)/6-311G** calculations indicate that the whole positive charge in structure 104 is concentrated on the hydrogen atoms (each carries a charge of*+0.6), whereas the carbon atom is negatively charged (qC=71.27). Thus, the stability of this three-charged ion is mainly due to electrostatic forces. 3+ H 2+ 3+ C H C H C H HHHH 105 (C6u) 106 (C6u) 104 (C3u) Electrostatic interactions are also important for stabilisation of pyramidal cations with a six-membered basal ring.According to MP2/6-311+G** and B3LYP/6-311+G** calculations,88 the di- and trication structures 105 and 106 in which the apical carbon atoms are coordinated to six and seven centres, respectively, exist, as predicted by the octet rule, as fairly deep minima in the PES. The geometry calculated for structures 105 and 106 is shown in Fig. 16. In both cases, the Cap±Cbas distances exceed the critical values for the single C7C bond lengths and the electron popula- tions of the Cap7Cbas bonds are rather low. 8 8 8 8 The presence of one 2c± 2e C7H bond in the structure of triprotonated methane 104 provides an opportunity to replace this bond by a three-centre 3c± 2e C7H2 bond and thus to approach theCH4á 8 tetracation.However with a charge of +4, the Coulomb repulsion of the hydrogen atoms, which carry the entire positive charge, becomes the predominant destabilising factor. The results of MP2/6-31G** calculations showed that the CH4á ion is unstable. The corresponding PES contains no minima for any of the possible CH4á structures.143, 197 Meanwhile, the isolobal trinuclear boronium ion BH3¡ is kinetically stable, although the calculation indicates that various pathways of its dissociation are highly exothermic. The structure of BH3á (107) found by MP2/6-31G** and QCISD(T)/6-311G** calculations has four tetrahedrally oriented 3c± 2e B7H2 bonds.207 3+ H H H H B H H H H 107 (Td ) V I Minkin, RMMinyaev, R Hoffmann 2.009(MP2) 1.993(DFT) 2+ 9.38(MP2) 10.38(DFT) 1.417(MP2) 1.419(DFT) 105 (C6u) 3+ 1.833(MP2) 1.823(DFT) 14.68(MP2) 15.58(DFT) 1.436(MP2) 1.438(DFT) 106 (C6u) 6 trication (106) calculated by the B3LYP/6-311+G** and 7 Figure 16.Geometric characteristics of the C7H2á dication (105) and C7H3á MP2(full)/6-311+G** methods.88 VII. Planar hexacoordinate carbon atom inside a cyclic borocarbon cage A new line in the study of compounds with hypercoordinate carbon atoms is the quest for structures with a planar hexacoordi- nate carbon atom. The problem of stabilisation of these structures has been attacked simultaneously by two research groups, which have arrived at similar solutions.One approach 25, 208, 209 is based on expansion of the rigid cage of bonds formed by a planar tetracoordinate carbon atom in cyclic structures like 21 ± 23. This expansion results in a higher coordi- nation number of the carbon atom. For example, `doubling' of molecule 23 (by reflecting it in a mirror accommodating the central carbon atom and two boron atoms in the plane of the mirror) furnishes structure 108 containing a planar hexacoordi- nate carbon atom. H H B B B B B B X X X X C C B B B B B B H H 108a,b (D2h) 23 BCB 23 X=O (a), NH (b). According to MP2(full)/6-311+G** calculations, the result- ing structure 108 has D2h symmetry and corresponds to a mini- mum on the PES.The calculated geometry of molecule 108a,b is shown in Fig. 17. The C7B bond lengths are only *0.1 A longer than the length of the typical single C7B bond.210 The replacement of the central carbon atom in structure 108 by the B7isolobal centre, as expected, does not weaken the stability of the structure. The anion 109 and molecules 110a,b (Fig. 18) are minima on the corresponding PES.209 Li Li B B B B B B B HN HN B B 7 B B HN NH B B B NH NH B B B B B 109 B 110a B 110bNon-classical structures of organic compounds: unusual stereochemistry and hypercoordination 1.002 (1.006) N (1.414) 1.412 86.0 (86.7) B B 1.925 (1.942) (1.541) 1.533 1.677 (1.688) (54.9)55.0 B C B 63.6 1.643 (1.654) (63.7) 70.0 (70.2) B B N 108a (D2h) Figure 17.Geometric characteristics of molecules 108a,b (X=NH, O) containing hexacoordinate carbon atoms calculated by the MP2(full)/6- 31G** and MP2(full)/6-311++G** methods (values in parentheses).25 The lengths of the B7B bonds formed by the central boron atom (*1.7 A) are within the limits of values typical of these bonds.211 Analysis of the electronic structure of compounds 108 ± 110 shows that they are 6p-electron aromatic systems. The two electrons lent by the central carbon atom or by the B7 anion are delocalised in the p-system of the ligands. As a consequence, the carbon atom in the structure 108 bears a positive charge (+0.8 to +0.9), which decreases its effective radius and reduces the steric strain in the planar system. The aromatic nature of molecule 110 is emphasised by the presence of shortened peripheral B7B bonds, which are 0.06 ± 0.08 A shorter than the double B=B bond (*1.63 A).212 The second approach to solving the problem of stabilisation of a hexacoordinate carbon centre 213 is also underlain by the idea of surrounding this centre by a rigid cage composed of boron atoms.At the first stage of the investigation,213 carried out by B3LYP/6- 311+G** calculations using the density functional theory, a model planar structure 111 was considered with various atoms or ions X being placed at the midpoint of the benzene ring. For none of the structures designed in this way (even for the structure with X=He or C4+), were the researchers able to locate an energy minimum on the PES.Since the geometry of structure 111 (X=C4+) was quite realistic (the Ccentre7C bond lengths were normal, 1.516 A) and the instability of the structure was mainly due to excess charge, subsequent modification of the structure 111 with X=C4+ was carried out in the usual way. H H H X H H H 111 The total charge of the molecule was neutralised by successive replacement of the carbon centres in the ring by boron atoms. Along this route, a number of stable structures 112 ± 116, contain- ing a planar hexacoordinate carbon atom, have been identified. Two factors contribute to the stability of these structures: first, expansion of the inner cavity containing the carbon centre caused by the fact that C7B bonds are longer than C7C bonds; and second, aromaticity of these structures.It was noted 213 that, although structures 112 ± 116 are not the most stable isomers, O (1.391)1.394 83.1 (83.9) B B 1.848 (1.860) (1.548) 1.541 1.639 (1.648) B B C 61.5 1.660 (1.669) 68.6 (61.5) (68.7) (55.7)55.6 B B O 108b (D2h) 887 64.1(DFT) 63.4(MP2) 63.9(RHF) 1.708(DFT) 1.699(MP2) 1.704(RHF) 1.549(DFT) 1.557(MP2) 1.548(RHF) 1.435(DFT) 1.975(DFT) 1.947(MP2) 1.973(RHF) 1.435(MP2) 1.432(RHF) N N 70.6(DFT) 69.9(MP2) 70.7(RHF) 54.7(DFT) 55.0(MP2) 54.6(RHF) 1.662(DFT) 1.670(MP2) 1.667(RHF) 109 (D2h) 2.155(DFT) 2.172(MP2) 2.216(RHF) 2.537(DFT) 2.568(MP2) 2.606(RHF) Li 1.721(DFT) 1.712(MP2) 1.719(RHF) 2.882(DFT) 2.912(MP2) 2.971(RHF) N 2.334(DFT) 2.274(MP2) 2.332(RHF) 1.672(DFT) 1.683(MP2) 1.674(RHF) N 1.424(DFT) 1.423(MP2) 1.418(RHF) 1.436(DFT) 1.538(DFT) 1.440(MP2) 1.556(DFT) 1.548(MP2) 1.435(RHF) 1.569(MP2) 1.536(RHF) 1.566(RHF) 1.732(DFT) 1.726(MP2) 1.727(RHF) 110a (Cs) Li 2.506(DFT) 2.521(MP2) 2.551(DFT) 2.540(MP2) 2.569(RHF) 2.645(RHF) 2.136(DFT) 2.136(MP2) 1.189(RHF) 1.727(DFT) 1.720(MP2) 1.722(RHF) N N 1.429(DFT) 1.547(DFT) 1.429(MP2) 1.559(MP2) 1.424(RHF) 1.553(RHF) 1.674(DFT) 1.686(MP2) 1.673(RHF) 110b (C1), l=1 Figure 18.Geometric characteristics of anion 109 and molecules 110a,b containing hexacoordinate boron atoms calculated by the RHF/6-31G** (RHF), MP2(full)/6-31G** (MP2) and B3LYP/6-311++G** (DFT) methods.208, 209 The molecular configuration of 110b is the transition state structure for rotation of the lithium atom above the ring (topomerisation) with a potential barrier of 0.20 (RHF), 0.63 (MP2) or 0.03 kcal mol71 (DFT).they are separated from the latter by rather high potential barriers and, therefore, the synthesis of these species may be practical. H 27 1.526 C B B B C B B B B1.594 1.66 1.555 C C C B B B B B B B B B 1.490 H113 (Cs) 112 (D6h) 114 (C2u) 1.497 1.443 C C B B B B C C 1.55 1.604 B B C B C B 116 (D2h) 115 (C2u)888 VIII.Molecules and ions containing planar penta-, hepta- and octacoordinate carbon atoms or atoms of other non-transition elements The possibility of the existence of stable compounds incorporating a planar heptacoordinate carbon atom is pointed out by the results of B3LYP/6-311+G** calculations for the cyclic CB¡ anion.214 In this approximation, the D7h- symmetric structure 117a is a local minimum in the PES, which is separated from another local minimum corresponding to a more stable (by 9 kcal mol71) isomer 117b by a fairly high potential barrier. The ion pairs formed by anions 117a and 117b with the Li+ counter- ions have the same energy. Like the cyclic structures with a hexacoordinate carbon atom considered above, the seven-mem- bered analogue 117a is also aromatic.Figure 19 shows the shapes of the four p MO of anion 117a, three occupied orbitals and the lowest unoccupied one. The B7B (1.523 A) and C7B (1.389 A) bond lengths in the seven- and eight-membered rings in anions 117a and 117b are close to the C7B bond length in the H2C=BH molecule (1.376 AÜ and B7B bond length in the HB=BH molecule (1.523 A), calculated by the same technique;213 this indicates that these are conjugated double bonds. 7 1.523 B B B C 1.755 B B B B117a (D7h) The ring in compound 117a is larger than the B6 and C2B4 rings in compounds 108 ± 116; this entails an increase in the Ccentre7B bond lengths in anion 117a by approximately 0.2 A. Due to this elongation, the bond becomes markedly weaker than the usual bonds in carboranes.215 Therefore, even minor symme- try distortions can lead to substantial changes in the type of structure.The symmetry of the structure of ion pair 118 is reduced LiB B B B C B B B 118a (Cs) l=0 E /eV 4 p3 20 p2 72 74 p1 76 Figure 19. Shapes and energy levels of the p-orbitals of anion 117a calculated by the B3LYP/6-311G** method (see Ref. 214). 7 1.389 C B B B 7 B B B B 117b (C2u) Li Li B B B B B B B B ... C C B B B B B B 118b (Cs) l=0 119 (TS) (Cs) l=1 7 B B B B B 117a (D7h) B B 1.567 B 1.477 1.588 B B B 121a (C2u) Figure 20. Geometric characteristics of anion 117a and molecules 120 and 121a,b calculated by the B3LYP/6-311+G(2df ) methods.214 The symmetrical D8h structure 122 resulting from insertion of a carbon atom inside an eight-membered ring composed of boron atoms is unstable, according to the results of B3LYP/6- 311G**calculations.218 On the PES of the CB8 molecule, this structure is matched by a flattened top of a hill (two imaginary vibration frequencies). Although the B7B bonds in the rings are double, the size of the eight-membered ring is so large that the B7C distances in structure 122 exceed the longest known value for B7C bond length.If the carbon atom in structure 122 is replaced by an isoelectronic atom or cation (Si or P+) with a greater atomic radius, structures 123 and 124 obtained in this way will be stable (matched by potential energy minima).The calcu- lated B7Si (2.038 A) and B7P (2.041 A) bond lengths exceed only slightly the corresponding sums of the covalent radii (1.98 and 1.91 A). Structures 123 and 124, like structures 112 ± 117, contain aromatic sextets of p-electrons. 1.509 B B B B B B C 1.625 51.4 1.755 V I Minkin, RMMinyaev, R Hoffmann to Cs . These structures should be fluxional, following a bond switching isomerisation pattern (see Refs 216, 217), the calculated potential barrier being not higher than 0.1 kcal mol71. A similar structure and the same type of structural rigidity are typical of neutral C2B6 (120) and NB7 (121a,b) rings. The cyclic structure 120 is formed from cyclic structure 117a through replacement of the B7 anion by an isoelectronic carbon atom, while 121 is produced when the central carbon is replaced by nitrogen.Calculations of the geometry of these molecules (Fig. 20) demonstrate that the central carbon atom in structure 120 forms only five (of the seven possible) rather short (within the range of usual lengths of C7B bonds) C7B bonds, and the nitrogen atoms in structures 121a and 121b form four and five short N7B bonds. However, the calculated potential barriers to migration of the central atoms inside the seven-membered rings do not exceed 0.8 kcal mol71, which permits these atoms to be regarded as effectively heptacoordinate planar atoms.214 1.523 B B C B 2.161 1.546 1.565 N 2.402 B 1.972 B B B B 123 (D8h) l=0 122 (D8h) l=2 For the cyclic CB8 molecule, the stable form is structure 125 (C2u symmetry) in which the C7B bond lengths are 1.627 and B B 1.551 1.528 1.656 1.520 C 1.618 1.845 B B 1.687 1.856 1.530 1.747 1.669 B B 1.410 1.532 1.411 C 120 (Cs) B B 1.542 1.554 B 1.597 2.290 1.465 B N B B B 121b (C2u), l=11.562 B B B B + B P B Si B2.041 1.559 1.566 B 2.038 B B B B B 124 (D8h) l=0Non-classical structures of organic compounds: unusual stereochemistry and hypercoordination 1.753 A, and the other C_B distances (non-bonding contacts) occur within the limits of 2.5 ¡À 2.9 A.This structure, like analo- gous seven-membered structures 118, 120 and 121, tend to undergo low-barrier rearrangements [the potential barrier calcu- lated in the CCD(fc)/6-311G**//B3LYP/6-311G** approxima- tion is 2.05 kcal mol71] resulting in fast exchange of ligand positions in the environment of the central atom.Thus, the carbon atom in structure 125 is effectively octacoordinated. = B B B B B B B B B B B B C C C B B B B B B B B B B B B 125b (C2u) 126a (C2u) 125a (C2u) = B B B B B B B B C C ... B B B B B B B B 125c (C2u) 126b (C2u) * * * As this review was being prepared for publication, new important studies devoted to derivatives of planar tetracoordinate and hypercoordinate carbon were published. Wang and Schleyer 219 further developed the strategy described in their preliminary communication 57 involving deloc- alisation of the non-bonding higher occupied MO of alkaplanes over the molecular cage by replacing the carbon centers in various positions of the cage by boron atoms and carried out B3LYP/ 6-31G* calculations for a series of new boroalkaplanes with a planar tetracoordinate carbon atom.The same researchers also proposed a general approach to the design of stable organoboron structures incorporating planar pentacoordinate carbon centres.220 It was found that the replacement of two-electron p-donor groups (O, NH, HC=CH) in structures 21 ¡À 23 contain- ing a planar tetracoordinate carbon atom by single-electron p-donor B or C atoms provides the possibility of increasing the coordination number of the planar carbon center.The insertion of non-classical substituents 127a ¡À c thus obtained into the skeletons of various aromatic hydrocarbons in place of the three-carbon (CH)3 chains gives rise to structures with pentacoordinate planar carbon atoms (for example, structures 128 ¡À 130), whose stability was confirmed by B3LYP/6-311+G** calculations. B B B B B B B B B C C C C C C B B B 127c 127b 127a B B B B B B B B B C C C C C B B C B H C C H C C C C C C H H C C H H H H H H 128 130 129 This result is also confirmed by our MP2/6-311+G** ab initio calculations for a number of other structures (for example, isomers 131) with a pentacoordinate carbon atom.221 We showed that by observing the above conditions of aromatic stabilisation of non-classical structures (see Sections II.3 and VII), stable struc- tures with a pentacoordinate carbon atom can also be produced by other permutations of atoms in units 127 and in structures of type 128 ¡À 130.889 C B B C C C B C B C H H H H C C C C B B B B C B C B B B H H 131a 131c 131b Compound DE/ kcal mol71 MP2/6-311+G** B3LYP/6-311+G** 131a 131b 131c 0 07.1 21.9 11.6 32.4 The requirement of aromatic stabilisation and the selection of appropriate fragments of type 21 ¡À 23, 127a ¡À c and others allow one to predict rather reliably the existence of stable molecules and ions with hypercoordinate planar centres of any other element. The B3LYP/6-311+G** calculations for stable structure 132 with a planar pentacoordinate nitrogen atom serves as an example.221 B B B 1.553 1.558 B B 1.611 1.495 C 1.411 C H 132 HIn relation to the discussion of the pyramidal (CH)�¢5 cation 5 and its isomers (see Section VI.2), of interest is the debate initiated by the recent publication 222 reporting the synthesis of the unex- pectedly stable pentamethyl derivative of the singlet cyclopenta- dienyl cation (CMe)�¢ (133). According to X-ray diffraction analysis data, the cation structure corresponds to structural type 83.However, the subsequent high-level quantum-chemical calcu- lations 223 suggested that in reality, the researchers were dealing with the dihydro derivative 134 rather than with the cation 133; this was recognised as proven after additional analysis.224, 225 Me Me Me + B(C6F5)¡¦4Me Me H Me H 134 + Me Me Ph3C B(C6F5)¡¦4Me H Me 1.439 N Me Me Me + B(C6F5)¡¦4 Me Me 133 The outcome of this debate is an illustrative example of the increasing role of precise quantum¡Àmechanical calculations in the study of structures with non-standard geometric and electronic characteristics.IX. Conclusion The results of the theoretical and experimental studies considered demonstrate the diversity of structural types of molecules con- taining atoms with a non-standard spatial orientation of bonds and coordination numbers other than those dictated by the valence rules. Not that these compounds are necessarily thermo- dynamically or kinetically stableDthey often are metastable local minima.But one can think of generating them, and studying them. The advances in theoretical simulation methods and the stupen- dous progress in the development of the computing power of modern quantum chemistry enabled quite substantiated state-890 ment of the problem of looking for new structures which might have been rejected only a few years ago by researchers with a classical way of thinking on the basis that `that can't be so because it can never be so' (see, for example, the caustic remark 226 concerning the studies searching for compounds with a planar tetracoordinate carbon atom). Meanwhile, these studies initiated by theoretical analysis have received some experimental valida- tion. Many compounds of this type have already been synthesised.Currently, theoretical investigations of new non-classical organic compounds are directed at elucidating the factors influ- encing stability and investigating the unusual dynamics and properties of such molecules and ions. Elucidation of unusual structural patterns presents substantial interest in the design of new high-tech materials with specific properties, while technolog- ical developments, especially ultralow-temperature laser vapor- isation,227 allow one to expect that more theoretical predictions will be confirmed by experimental proof. The situation in this field seems to resemble the history of the development of fullerene chemistry. As has already been noted, high-level ab initio calculations with obligatory allowance for the electron correlation energy are the most accurate and reliable methods for theoretical identifica- tion and evaluation of the possible existence of non-classical structures.The question thus arises of how one might choose particular structures on which calculations are to be performed. It is clear that exhaustive screening of atomic compositions cannot be a suitable strategy. There is good reason to believe that analysis of orbital interactions of molecular fragments based on the perturbation theory still remains conceptually the most compre- hensive and the most fruitful way of designing stable non-classical structures. This conclusion is based on the fact that orbital interactions make, as a rule, the major contribution to stabilisa- tion or, conversely, destabilisation of molecular structures.Yet another, more utilitarian line is to simulate a non-classical organic structure that would resemble one or another inorganic or organometallic complex or cluster. The stability of molecules and ions of such compounds is ensured by the formation of multi- centre bonds, of a type which is dictated by the structure of the complex or cluster and can be reproduced in the organic analogue on the basis of the isolobal analogy. It is evident that the principles of stabilisation of non-classical organic structures, i.e., carbon compounds, can be directly extrapolated to the compounds of other non-transition elements. Some examples of unusual geometric configurations and types of coordination of boron, nitrogen, silicon, and phosphorus atoms have been included in this review.The notion of `non-classical compounds' appeared 50 years ago when the first examples of these structures became known, the term itself being introduced mainly to designate exceptions to customary rules. Nowadays, the exceptions appear to become indistinguishable from the rules. 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出版商:RSC    

年代:2002

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Russian Chemical Reviews 71 (11) 917 ± 927 (2002) Norbornadiene ± quadricyclane as an abiotic system for the storage of solar energy A D Dubonosov, V A Bren, V A Chernoivanov Contents I. Introduction II. Photosensitisation of the valence isomerisation of norbornadiene into quadricyclane III. `Donor ± acceptor' norbornadienes IV. 2,3-Disubstituted norbornadienes V. Polymer-bound norbornadienes and sensitisers VI. Conclusion Abstract. and norbornadiene of isomerisation valence the on Data Data on the valence isomerisation of norbornadiene and its quadricyclanes corresponding the into derivatives its derivatives into the corresponding quadricyclanes published published between described and considered are 2001 and 1990 between 1990 and 2001 are considered and described systemati- systemati- cally.The applicability of this reaction for the storage of solar cally. The applicability of this reaction for the storage of solar energy is discussed. The bibliography includes 112 references energy is discussed. The bibliography includes 112 references. I. Introduction About ten years have passed since the publication of our review 1 in which the norbornadiene ± quadricyclane system was consid- ered as a promising accumulator and converter for solar energy. However, the interest in the problem of solar energy accumulation has not slackened. This results from the fact that irreplaceable natural energy sources, i.e., coal, oil and natural gas, are still being consumed almost without control;2 ±4 furthermore, the problems related to climate changes on the Earth due to the emission of enormous amounts of carbon dioxide, as well as products of incomplete combustion of natural fuel, into the atmosphere are becoming ever more critical.5 The possible ways to utilise solar energy are extremely diverse.6, 7 They include the use of solar energy to perform various photocatalytic processes,8±10 photodecomposition of water and photolytic production of hydrogen,11, 12 as well as various photo- biological processes, including artificial photosynthesis.13 ± 15 An alternative abiotic approach to the transformation of solar energy is to accumulate it as the strain energy in metastable photoinduced isomers of organic compounds.The norbornadiene ± quadricy- clane system is most promising for this purpose.16, 17 Its advantage is that the valence isomerisation of bicyclo[2.2.1]hepta-2,5-diene (norbornadiene, NBD) (1) into tetracyclo[3.2.0.02, 7.04, 6]heptane (quadricyclane, Q) (2) better fits the requirements for organic photochromes intended as solar energy accumulators.1 hn + DH.D or Cat 1 2 A D Dubonosov, V A Bren, V A Chernoivanov Institute of Physical and Organic Chemistry, Rostov State University, prosp. Stachki 194/2, 344090 Rostov-on-Don, Russian Federation. Fax (7-863) 243 46 67. Tel. (7-863) 243 47 77. E-mail: dubon@ipoc.rsu.ru (A D Dubonosov), bren@ipoc.rsu.ru (V A Bren) Received 24 June 2002 Uspekhi Khimii 71 (11) 1040 ± 1050 (2002); translated by S S Veselyi #2002 Russian Academy of Sciences and Turpion Ltd DOI 10.1070/RC2002v071n11ABEH000745 917 917 919 920 923 925 The photoisomerisation of NBD results in a metastable quadricyclane structure containing highly strained cyclobutane and two cyclopropane fragments.As a result, the reverse thermal or catalytic reaction 2?1 is characterised by a considerable thermal effect (DH=789 kJ mol71) (see Ref. 18), which is greater than the DH value for other photochromic systems. There is a wide range of homo- and heterogeneous catalysts that enable this reaction to be performed and hence thermal energy to be evolved,1, 16 but the high activation barrier to the thermal transformation 2?1 (even at 140 8C, the half-life period of quadricyclane 2 is 14 h) 19 makes it possible to store the accumu- lated solar energy for a long time.However, the system 1?2 has a number of considerable drawbacks. First, the longwave absorption edge of non-substi- tuted NBD 1 (lae , e=1 litre mol71 cm71) is no higher than 300 nm.20 Second, the quantum yield (F) of the valence isomer- isation 1?2 in the absence of sensitising agents is *0.05 (see Refs 20, 21). In order that this system could be used on industrial scale as a solar energy accumulator, one has to chemically modify NBD for shifting the values of lae towards the maximum of the light energy of the Sun (400 ± 600 nm) and at the same time for increasing the photoisomerisation quantum yield. This review presents a generalisation of the studies that dealt with this problem and were carried out between 1990 and 2001.The following general directions can be distinguished: (1) the use of sensitisers for the valence isomerisation 1?2, including those chemically bound to the NBD molecule; (2) modification of the structure of the starting NBD to increase lae and F; (3) the use of various polymeric systems containing NBD (or its derivatives) in the lateral or main chain of the macromolecule, as well as polymer- bound sensitisers. II. Photosensitisation of the valence isomerisation of norbornadiene into quadricyclane As a rule, the isomerisation 1?2 involves the triplet state of NBD.22 ± 25 Therefore, at the initial stage of the studies under discussion, the photosensitisation was performed mostly by various carbonyl compounds (acetophenone, propiophenone, benzophenone, acridinone, Michler's ketone and some others) known as triplet sensitisers (S).24 ± 27 Since the energy of the triplet state (ET) of NBD is very high (*257 kJ mol71),22, 28 only a small number of sensitisers with high ET fit the classical triplet ± triplet transfer scheme:NBD hn 3S S 3+NBD S+3Q S+Q.1S S918 Irradiation is normally carried out in the absorption band of the sensitiser, which is first converted to a singlet excited state 1S and then, through intersystem crossing, to the triplet state 3S. The triplet ± triplet energy transfer to NBD is followed by its adiabatic isomerisation into 3Q and then relaxation to the ground state (Q). The highest quantum yields F1?2 were obtained using acetophe- none (0.91, lae=366 nm, ET=301 kJ mol71) and benzophe- none (0.45, lae=380 nm, ET=290 kJ mol71); in the latter case, photoacoustic calorimetry revealed the formation of 3NBD with a life-time of *7 ns.25 However, the use of sensitisers of this type for solar energy storage by means of the photoreaction 1?2 was quite limited in recent years.This is due to both difficulties associated with the selection of new systems with ET values not exceeding 257 kJ mol71 and the formation of side addition products of sensitisers to norbornadiene or quadricyclane.29 ± 33 Therefore, the main attention of researchers was paid to the use of metal complexes 6 or derivatives of carbonyl compounds 31 with ET<257 kJ mol71.In this case, the sensitisation of the photo- reaction 1?2 occurs through a stage which involves the forma- tion of sensitiser ± norbornadiene complexes in electron-excited states:23, 25, 34 NBD hn S+Q. 3S [3S NBD] 1S SMost commonly, the products are exciplexes or charge-trans- fer complexes; however, more complicated structures can be formed, e.g., triplexes.31 Cooperative (combined) sensitisation is carried out using the fluoroborate complex 3 (BFC) in combina- tion with aromatic hydrocarbons (A), namely, toluene, ethyl- benzene, biphenyl or durene.35 F F B O O 3+A, hn 3, hn Ph Ph 1 2 3 The use of the sensitiser 3 alone is not efficient, as the reaction of the singlet-excited 1BFC with norbornadiene or quadricyclane results in radical cations NBD+.and Q+.. The latter is readily rearranged toNBD+., as its energy level is 38 ± 46 kJ mol71 lower than the level ofQ+. (see Refs 36, 37). If a co-sensitiserAis added, the process changes basically: the radical ion pair is stabilised and a triplet-excited state is realised; the latter then decomposes to 3NBD, which undergoes recombination into quadricyclane. BFC hn 1BFC 1[BFC BFC] 2 BFC NBD 1[BFC NBD] NBD+.] 1[BFC7 BFC+NBD NBD A 1 1 [BFC A] [BFC7. (A NBD)+. ] Q 3[BFC7. (A NBD)+.] BFC+A+3NBD Efficient sensitisation of the 1?2 transformation due to electron transfer without formation of triplet-excited 3NBD is observed in the presence of 3,30,4,40-benzophenonetetracarboxylic dianhydride (BTA):25 NBD hn 3BTA 1BTA BTA [BTA7.NBD+.] BTA+3Q Q. The quantum yield of the reaction (in CDCl3) is *0.9. N-(a-Naphthyl)carbazole (NCA) increases the efficiency of the 1?2 isomerisation owing to intramolecular charge transfer from the carbazole moiety of the sensitiser molecule in the excited state to the naphthyl radical. In this case, these fragments turn so as to become mutually perpendicular [the so-called Twisted Intramo- lecular Charge Transfer (TICT) state].38 A D Dubonosov, V A Bren, V A Chernoivanov hn 3 1 NCA [NCA] [NCA] 3 1[N7.7CA+.] TICT NCA+Q. [NCA NBD] 3 Metal complex sensitisers of the Rh(phen)3á type (phen is 1,10-phenanthroline),34 which can form exciplexes with NBD, shift the lae values to the4400 nm region, but the quantum yield of the photoreaction is low (F&0.01).Metal complexes which can form complexes with NBD in the ground state are more efficient: hn S+Q. [S NBD] S+NBD Such complexes are characterised by an intense charge trans- fer band in the region l>300 nm where the starting sensitiser and NBD do not have considerable absorption.39 ± 41 Excitation of NBD complexes with [CuCl(PR12 R2)]n (R1 , R2=Et, 2-MeC6H4, 4-MeC6H4, C6F5CH2) in THF with light with a wavelength (lirr) of 366 nm gives quadricyclanes with quantum yields F=0.03 ± 0.49. The use of [Cu2L2(m-NBD)] complexes [where L is 2-methyl- 8-quinolinolato, 5,7-dichloro-2-methyl-8-quinolinolato, 4-acri- dinolato, 2-(benzotriazol-2-yl)-4,5-di-tert-butylphenolato] allows irradiation to be carried out in a longer-wave spectral region (lirr=405 nm).39 Although the quantum yield of the formation of quadricyclane is rather low (F=0.029), the number of Q molecules formed per S molecule is *5000. This fact makes the above system very promising for the practical storage of solar energy.Although sensitisation of the 1?2 photoreaction with univalent copper compounds is well known,1 the mechanism of this process could not be explained theoretically until recently.42 An ab initio method (B3LYP) was used to study the geometry of the ground state of the complex 4, i.e., NBD bound to CuL as the sensitiser (L is 8-quinolinolato). 4 3 N 5 Cu 6 2 O 1 4 The calculated geometrical parameters of the system 4 agree well with the values obtained by X-ray diffraction analysis.39 It is of note that the C(2)=C(3) bond in the complex 4 is elongated to 1.409 A in comparison with the similar bond in non-substituted NBD (1.339 A).Analysis of the electronic structure of the adduct 4 makes it possible to conclude that the interaction of NBD with the sensitiser is in fact donation of the p-electrons of the copper atom to the p*-antibonding orbital of NBD, which decreases the p ± p*-splitting in the norbornadiene fragment from 5.91 to 3.36 eV. Owing to this, complexes of the type 4 can absorb light in the visible spectral region, which makes them applicable for the storage of the sunlight. The covalent binding of norbornadiene to a sensitiser makes it possible to avoid side addition photoreactions of the sensitiser to NBD or polymerisation of NBD.Photoinduced intramolecular electron transfer from the carbazole moiety of molecule 5 to carbon ± carbon double bonds of the norbornadiene fragment (lirr5350 nm) favours the efficient formation of quadricyclane 6.43 N N CO2(CH2)n CO2(CH2)n hn CO2Me CO2Me 6 5 n=2, 6.Norbornadiene ± quadricyclane as an abiotic system for the storage of solar energy The intramolecular energy transfer from the second triplet (via the T2 state) of the anthracene fragment to the norbornadiene moiety of compound 7 causes its isomerisation to quadricyclane 8.44 This mechanism was proven using two-photon flash-photo- lysis (l1exc=260 ± 290 nm, l2exc>410 nm); the quantum yield of the photoreaction in benzene is 0.022.CO2CH2 CO2CH2 hn 8 7 In the past decade, intense studies were carried out on the photophysical and photochemical features of intramolecular processes of energy or electron transfer through a rigid steroid bridge (SB), which links benzophenone (BP) or benzidine (BZ) with a norbornadiene molecule.45 ± 48 For example, the dichromo- phoric 3b-{[2-(methoxycarbonyl)bicyclo-[2.2.1]hepha-2,5-dien-3- yl]carbonyloxy}androst-5-en-17b-ylbenzophenone-4-carboxylate (9a, NBD ± SB ± BP) undergoes isomerisation upon steady-state irradiation (lirr>350 nm) to give a photo-product 10a (Q ± SB ± BP) with a quantum yield F=0.038. R CO2 hn 9a,b CO2Me R CO2 CO2Me 10a,b Me Me .; b: R=MeN CO NMe2 a: R=OCO Me Me It should be noted that only the benzophenone chromophore can absorb irradiation under these conditions. Singlet-excited benzophenone forms the triplet state 3BP by fast and efficient intersystem crossing. The main pathway of 3BP deactivation involves triplet ± triplet energy transfer to the norbornadiene fragment through the system of s-bonds of the steroid bridge. 3NBD± SB ± BP NBD± SB ± 3BP NBD± SB ± 1BP Q±SB±BP 10a hn NBD± SB ± BP 9a The quantum yield of isomerisation of 3b-{[2-methoxy- carbonyl)bicyclo[2.2.1]hepta-2,5-dien-3-yl]carbonyloxy}androst- 5-en-17b-yl-2,2 0,6,6 0,N,N0,N0-heptamethylbenzidine (9b, NBD± SB ± BZ) in cyclohexane is 0.07 (lirr>300 nm), with quantitative formation of the photo-product 10b and with no side reactions. In this case, the isomerisation scheme suggested by Tung et al.45 involves not only the triplet ± triplet energy transfer but also the formation of a triplet radical-ion state, which then undergoes recombination to the triplet-excited state of NBD.919 k1 1 3 [NBD7.±SB±BZ+.] [NBD7.±SB±BZ+.] NBD± SB ± BZ (360 kJ mol71) (320 ± 330 kJ mol71) k2 k3 3NBD±SB ± BZ hn NBD± SB ± 3BZ (264 kJ mol71) (222 kJ mol71) NBD± SB ± BZ 9b Q±SB±BZ 10b The rate constants for the corresponding steps are as follows: k1=1.16107 s71, k2=5.26107 s71, k3=5.26105 s71 (the values in parentheses correspond to the energies of the states with respect to the ground state of compound 9b). The latter examples of valence isomerisation open a new approach to sensitisation, in which a coloured chromophore sensitiser that is remote from the norbornadiene fragment can undergo photoexcitation.III. `Donor ± acceptor' norbornadienes Non-substituted NBD is a formally non-conjugated, non-planar, strained diene. The steric strain in its molecule allows through- space interaction of the C=C bond p-orbitals.49 ± 51 As a result, non-substituted NBD absorbs light in a longer-wave spectrum region than two isolatedC=Cbonds. The systems 11 in which the interaction of electron-donating substituents at one of the car- bon ± carbon double bonds in the molecule with electron-with- drawing substituents at another bond results in a low-intensity charge transfer band (e<102 litre mol71 .cm71) were called `donor ± acceptor' NBD.52 The long-wave absorption edge of these compounds (lae) can be as high as 500 ± 560 nm. R6 R6 R5 R1 R5 R1 hn X X D or Cat R4 R2 R4 R2 R3 R3 12 11 X=CH2, CMe2; R1, R2=H, Me, Ph, 4-MeOC6H4; R4, R5=CN, CO2Me, CO2Et, CONHPh; R3, R6=H, Me. The geometrical structure and electronic absorption spectrum of norbornadiene were studied 53 ± 55 with consideration of indirect conjugation of double bonds and p,s-interaction. This interaction was observed experimentally when studying the electronic absorp- tion spectra of bicyclo[2.2.n]alka-2,5-dienes 13.52 (CH2)n Me CN Me CN y13 When n changes from 3 to 1, the angle y decreases gradually. This is accompanied by a bathochromic shift of the long-wave absorption band (lmax) from 305 to 338 nm with simultaneous increase in the molar extinction coefficient (emax ) from 178 to 200 litre mol71 cm71.The value of lae in the spectra of norbor- nadienes 11 can be as high as 560 nm, while the quantum yields of the photoreaction 11?12 are high (F=0.26 ± 0.75). However, despite high F values, the applicability of compounds 11 for the storage of solar energy is limited due to the following factors: (1) the low extinction coeficient (emax&1000 litre mol71 s71) makes the efficiency of the 11?12 process low and requires prolonged exposure to light; (2) many quadricyclanes 12 are unstable at room temperature and undergo spontaneous rear-920 rangement to the original NBD 11.Although some attempts to increase the thermal stabilities of quadricyclanes by introduction of perfluoroalkyl substituents have been undertaken a long time ago,56 it is only in recent years that reliable methods for the synthesis of the `donor ± acceptor' NBD 14 have been developed; the latter are not only thermally stable at room temperature but also absorb solar energy quite efficiently (Table 1).57, 58 R1 R1 CF3 CF3 hn Me R2 Me R2 Me CF3 Me CF3 15 14 R1, R2=4-MeOC6H4, 4-Me2NC6H4, 2-benzofuryl, 2-thienyl. Table 1. Spectral characteristics of norbornadienes 14 and thermal stabil- ities of quadricyclanes 15.57, 58 R2 t1/2 a lae lmax /nm /nm /h (1074 emax Com- R1 pound 14 /litre mol71 cm71) 346 (0.51) 390 (0.85) 374 (0.85) 385 (0.78) 414 (2.00) 407 (1.29) 440 see b 510 9600 500 see b 520 500 480 72 580 31 4-MeOC6H4 4-Me2NC6H4 4-Me2NC6H4 4-Me2NC6H4 2-benzofuryl 4-Me2NC6H4 abcdef 4-MeOC6H4 4-Me2NC6H4 4-MeOC6H4 2-thienyl 2-benzofuryl 2-benzofuryl a Half-life of quadricyclane.b Quadricyclane is stable at 20 8C. Unlike compounds 11, norbornadienes 14 have high values of emax ranging from 5000 to 20 000 litre mol71 cm71. Their absorption edge increases as the electron-donating properties of the substituents R1 and R2 increase; in fact, lae=580 nm for norbornadiene 14a. The thermal stabilities of quadricyclanes 15 are sufficiently high for the practical storage of solar energy; the number of cycles 14.15 reaches 103 without considerable destruction of the starting compound.IV. 2,3-Disubstituted norbornadienes The synthesis of `donor ± acceptor' norbornadienes faces consid- erable difficulties: polysubstituted cyclopentadiene derivatives and disubstituted acetylene derivatives are first synthesised and then brought into the Diels ± Alder reaction. 2,3-Disubstituted norbornadienes can also be synthesised from non-substituted cyclopentadiene and 1,2-disubstituted acetylene. The studies carried out over the past ten years have shown that in many cases, the presence of an electron-donating substituent and an electron-withdrawing substituent at one of the two double bonds of the norbornadiene molecule is sufficient for their spectral and photochemical characteristics not to be inferior, and sometimes even to be superior, to the corresponding parameters of the `donor ± acceptor' NBD.Two substituents of the same nature shift the lae value only slightly towards the solar radiation maximum.59, 60 For example, the absorption of 2,3-di(tert-butyl- sulfonyl)norbornadiene (16) is characterised by a slight batho- chromic shift with respect to the maximum absorption of non- substituted NBD 1, although the 16?17 isomerisation occurs quantitatively in ether under irradiation with light with lirr>280 nm. SO2But SO2But hn SO2But SO2But 17 16 A D Dubonosov, V A Bren, V A Chernoivanov The valence isomerisation of the 2,3-dicyanonorbornadie- ne ± pentaamminoruthenium(II) complex 18 is an exception.61 The long-wave absorption maximum of norbornadiene 18 in methanol is observed around 490 nm (e&14 000 litre mol71 s71); the lae is 700 nm, but the quantum yield of the formation of the quadricyclane 19 upon irradiation with light with lirr=500 nm is as low as 8.361074.4+ 4+ CN Ru(NH3)5 CN Ru(NH3)5 hn CN Ru(NH3)5 CN Ru(NH3)5 18 19 Combination of an electron-withdrawing and an electron- donating substituents at one of the carbon ± carbon double bonds in NBD favours an increase in the isomerisation quantum yield. The electronic absorption spectra and the quantum yields of the phototransformation of 3-phenylnorbornadiene-2-carboxylic acid (20a) and its esters 20b ± d are listed in Table 2.62, 63 Although these systems absorb in the spectral region l<360 nm, the fast and quantitative photoisomerisation 20?21, which takes a few seconds in DMSO, acetonitrile, ethanol and THF (c=561075 mol litre71), allows us to consider the norborna- dienes 20 to be an efficient system for the accumulation of solar energy.Ph Ph hn CO2R CO2R 21a ± d 20a ± d R = H (a), CH2Ph (b), CH2C6H4OMe-4 (c), (CH2)6OC6H4OMe-4 (d). Table 2. Spectral characteristics of norbornadienes 20 and quantum yields of the photoreactions 20?21 in acetonitrile.62, 63 lae lmax /nm /nm (1074 emax Com- R pound 20 F(lirr= 284 nm) /litre mol71 cm71) 292 (0.56) 0.32 0.36 0.27 0.25 291 (1.12), 222 (0.66) 7350 297 (0.74), 225 (2.45) 355 291 (0.71), 224 (1.62) 340 abcd HPhCH2 4-MeOC6H4CH2 4-MeOC6H4O(CH2)6 2-Trifluoroacetyl-3-phenylnorbornadiene (22) absorbs light at lae4460 nm; its photoisomerisation into the quadricyclane 23 in benzene occurs quantitatively with a quantum yield F=0.62 ± 0.86 (depending on the starting concentration of nor- bornadiene).64, 65 Ph Ph hn H+ COR COR 23, 25 22, 24 R=CF3 (22, 23), NHAr (24, 25).Involvement of the carbon ± carbon double bond of norbor- nadiene in the conjugation chain Ph7C=C7CONHR affects the lae and F values even more significantly. We have studied the spectral and photochemical features of 3-phenylnorbornadiene-2-carboxamides 24.66, 67 Compounds 24 are characterised by absorption in the region 280 ± 305 nm (Table 3) and lae=375 ± 435 nm.66, 67 Irradiation with light with a wavelength corresponding to the long-wave absorption max- imum (lirr=313 nm) results in their isomerisation to give quad- ricyclanes 25 with high quantum yields.Electron-withdrawing substituents in the N-phenyl ring increase the F value in compar- ison with non-substituted 3-phenyl-2-(N-phenylcarbamoyl)nor- bornadiene 24a, whereas electron-donor substituents decrease thisNorbornadiene ± quadricyclane as an abiotic system for the storage of solar energy Table 3. Spectral characteristics of 3-phenylnorbornadiene-2-carboxamides 24 and quadricyclanes 25 and quantum yields of the photoreaction 24?25 in propan-2-ol.66, 67 Compound 24 R lmax /nm (1074 emax /litre mol71 cm71) 233 (1.98), 280 (0.9) 232 (1.78), 285 (0.78) 229 (1.74), 280 (0.76) 235 (1.33), 290 (0.76) 221 (1.30), 295 (1.28) 278 (0.93), 310 (0.74) 232 (2.28), 305 (0.87) 223 (1.47), 295 (0.28) 234 (3.78), 300 (1.00) Ph 4-MeC6H4 2-MeC6H4 4-MeOC6H4 4-MeCOC6H4 4-EtO2CC6H4 3-NO2C6H4 a-naphthyl b-naphthyl abcdefghivalue. However, it should be noted that as electron-withdrawing groups are introduced, the lae values of norbornadienes 24 become closer to those of quadricyclanes 25, hence the efficiency of solar energy accumulation decreases.The quadricyclanes obtained upon valence isomerisation 24?25 are stable at room temperature and in the presence of acid catalysts. In the presence ofMoO3 as a heterogeneous catalyst, they undergo rearrangement to the original NBD with a rate constant of k=561072 ± 1073 s71 at 293 K.Since the triplet mechanism of isomerisation of norbornadiene into quadricyclane can occur with high quantum yields in the presence of aromatic ketones, we studied norbornadienes 26 with one of the C=C bonds involved in the conjugation chain of the Ph7C=C7C=X fragment (Table 4).67 ± 71 R2 R2 hn X X R1 27 R1 26 R1=H, Me, OMe, CH=CHPh; R2=Br, Ar; X=O, CHCOAr. Irradiation of NBD 26 in propan-2-ol at a wavelength corresponding to the maximum absorption in the long-wave region (lirr=313 or 365 nm) results in quadricyclane structures 27. Absorption at the highest wavelengths is observed in the case of 2-[2-(3-nitrobenzoyl)vinyl]-3-phenylnorbornadiene (26 l): its absorption edge approaches 500 nm while the quantum yield of isomerisation is rather high (F=0.25).The highest value F=0.60 is observed for norbornadiene 26f. Quadricyclanes 27 are stable at 293 K; however, they readily undergo isomerisation into the original compounds under conditions of homogeneous catalysis if a small amount of trifluoroacetic acid Table 4. Spectral characteristics of carbonyl-containing norbornadienes 26 and quadricyclanes 27 and quantum yields of the photoreaction 26?27 in propan-2-ol.67 ± 71 1 X R R2 Compound 26 a O H Ph 222 (1.44), 318 (0.96) b O Me Ph 250 (0.89), 297 (0.92) c O Ph Br 259 (1.02), 286 (0.39) d O CH=CHPh Ph 300 (2.48), 345 (1.10) e CHCOPh H Ph 231 (1.70), 377 (1.69) 4-NO2C6H4 f 263 (2.03), 387 (2.26) CHCOPh H g O OMe 4-NO2C6H4 322 (1.53) h O OMe 4-NH2C6H4 237 (1.70), 353 (2.07) i CHCOC6H4Me-4 H Ph 375 (1.63) CHCOC6H4Cl-4 H Ph j 258 (4.09), 380 (1.72) k CHCOC6H4Br-4 H Ph 257 (3.76), 381 (1.68) l CHCOC6H4NO2-3 H Ph 250 (3.15), 389 (1.55) m CHCOC6H4Ph-4 H Ph 275 (3.12), 390 (1.89) a For compounds 26a ± c,g, lirr=313 nm; for compounds 26d,h, 365 nm; for compounds 26e,f,i ±m, 436 nm.F (lirr=313 nm) lae(24) /nm 0.53 0.46 0.49 0.15 70.71 0.66 0.0 0.22 375 380 380 400 380 380 390 380 380 (1072± 1073 mol litre71) is added to their solutions in heptane (561075 mol litre71). Since heterogeneous catalysis is more suitable for practical solar energy accumulation,1 it seemed reasonable to study the effect of oxides, viz., V2O5, WO3 and MoO3, on quadricyclanes 27.The best catalytic activity was observed for molybdenum(VI) oxide, which is characterised by the highest metal electronegativity in this series.72 Stirring of solutions of quadricyclanes 27 in heptane (561075 mol litre71) with MoO3 (0.2 ± 0.5 mg per 2 ml of a solution) results in the original norbornadienes 26 with a rate constant k=1072± 561073 s71 at 291 K. Conditions for the repeated phototransformation 26.27 have been determined. The calcul- ated turnover number with the loss of the working substance not exceeding 3% is 86102 ±103. Elongation of a conjugation chain involving one of the double bonds in norbornadiene may be a way to increase the lae and F values.In view of this, we studied the spectral and photochemical properties of 3-arylnorbornadiene-2-carbaldehyde derivatives 28, namely, oximes, imines, hydrazones, etc. (Table 5).67, 69, 71, 72 By varying the electron-donating and electron-withdrawing substitu- ents at the C=C double bond in norbornadienes 28, it became possible to shift the values of lmax to 500 nm and lae to 620 nm. Heterogeneous catalysis with Mo(VI) oxide described above proved to be efficient for the reverse reaction of the majority of quadricyclanes 29. C6H4R-p hn CH X 28 29 R=H, NO2, NH2; X=NOH, NOMe, NAr, NNPh, C(CN)2, C(CN)CO2Et, C(CO2Et)2. lae(26) /nm lmax /nm (1074 emax /litre mol71 cm71) 400 400 360 435 450 470 415 445 470 475 475 500 500 921 lae(25) /nm 295 295 295 310 370 310 300 7320 C6H4R-p CH X Fa lae(27) /nm 330 350 330 420 415 400 380 330 410 420 420 420 440 0.40 0.34 0.33 70.10 0.60 0.01 0.20 0.12 0.15 0.16 0.25 0.08922 Table 5.Spectral characteristics of norbornadienes 28 and quadricyclanes 29 and quantum yields of the photoreaction 28?29 in propan-2-ol.67, 69, 71, 72 R X Compound 28 lmax /nm (1074 emax /litre mol71 cm71) HHH NOH NOMe NPh H NC6 H4OMe-4 H NC6 H4NMe2-4 223 (1.68), 310 (1.36) 227 (1.70), 313 (1.48) 235 (2.16), 343 (1.66) 238 (2.86), 365 (2.37) 252 (1.70), 320 (1.51), 405 (2.00) 257 (1.67), 373 (3.06) 251 (1.39), 303 (0.60), 382 (2.15) 302 (1.79), 386 (2.98) 235 (1.58), 282 (2.44), 505 (3.62) 249 (1.62), 298 (0.62), 376 (2.18) 233 (1.75), 350 (0.76) HHNO2 NH2 HH abcdefghijk NNHPh C(CN)2 C(CN)2 C(CN)2 C(CN)CO2Et C(CO2Et)2 a For compounds 28a ± c,k , lirr=365 nm.More complete data on the effect of substituents on the state of C=C double bonds in norbornadiene were obtained from an X-ray diffraction study of compounds 28c and 28g (see Ref. 73) and comparison with literature data on the structure of non- substituted NBD53, 74 and 2-(2,2-diphenylvinyl)norbornadiene-3- carboxylic acid 30.75 4 R1 3 R1 5 hn 6 2 R2 R2 1 2, 29c,g, 31 1, 28c,g, 30 R2 R1 Compounds 1, 2 28c, 29c 28g, 29g 30, 31 CH=CPh2 H H Ph CH=NPh Ph CH=C(CN)2 CO2H The lengths of single bonds in the molecule 28c have ordinary values (1.57 A on average), but the lengths of the C(2)=C(3) (1.342 A) and C(5)=C(6) double bonds (1.310 A) differ markedly from the lengths of similar bonds in non-substituted NBD 1 (1.333 A).X-Ray diffraction data suggest that the C(2)=C(3) bond is conjugated with the azomethine C=N bond and with the p-bonds in the phenyl substituents; this results in its elongation with simultaneous shortening of the C(5)=C(6) bond. In the molecule 28g, which contains a stronger electron-withdrawing substituent, the C(2)=C(3) bond is elongated considerably [to 1.38(1) A], whereas the C(5)=C(6) bond is shortened [to 1.29(2) A]. The quantum yield of this photoreaction increases in the order: 0.05 (1)<0.15 (28c)<0.74 (28g).A similar pattern of changes in the lengths of double bonds was found for norborna- diene 30;75 the C(2)=C(3) bond in this molecule is elongated to 1.359 A due to the conjugation with the p-systems of the sub- stituents, while the C(5)=C(6) bond is shortened to 1.271(4) A. The presence of two electron-withdrawing substituents at the C=C double bond in norbornadienes 32a,b affects the distribu- tion of the bond lengths less significantly.76, 77 For example, the elongation of the C(2)=C(3) bond in the molecule of 2,3-di- cyanonorbornadiene (32a) is insignificant [to 1.339(2) A], whereas the C(5)=C(6) bond is shortened to 1.315 A [in the molecule 32b, 4 3 R 56 2 R 1 32a,b R=CN (a), CONH2 (b).A D Dubonosov, V A Bren, V A Chernoivanov lae(29) /nm Fa (lirr=436 nm) lae(28) /nm 310 330 350 360 400 370 330 340 380 360 330 0.19 0.16 0.15 0.12 0.10 0.05 0.74 0.75 0.10 0.69 0.63 360 370 425 465 525 455 480 480 620 490 430 the lengths of these bonds are 1.341(3) and 1.285(4) A, respec- tively]. X-Ray diffraction data suggest convincingly that if one of the double bonds in norbornadiene is involved in a conjugation chain with an electron-donating and an electron-withdrawing substitu- ents, its length increases in parallel with the lae and F values. In view of this, it seemed attractive to use organic cations as the electron-withdrawing component, although it has been shown previously 78, 79 that the presence of pyridinium or pyrylium cations gives high values of lae and F but results in irreversible opening of the cyclopropane fragment in the quadricyclanes formed.We have studied the photochemical properties of 2-ben- zooxazolyl and 2-benzoimidazolyl derivatives of 3-phenylnor- bornadiene 33a,b.67, 69, 80 The benzooxazole derivative 33a is characterised by lae=550 nm, but the 33a?34a rearrangement occurs irreversibly. The lae value for the benzoimidazolyl deriva- tive 33b is 340 ± 365 nm, depending on the type of the counter ion (I7, ClO¡4 , BF¡4 , SbCl¡6 ), while the quantum yield of the photo- reaction is 0.41 ± 0.45. None of the known catalysts of the reverse reaction 1 affects quadricyclanes 34b, whereas thermal initiation (343 K, solution in acetonitrile) results in the original NBD with a rate constant k=661074 s71, with complete restoration of the original spectrum.The cycle 33b.34b can be repeated many times. Ph Ph hn R R 33a,b 34a,b Me N (a), R= (b). ONH + N + Me In order to increase the amount of accumulated solar energy per mol of the original compound, we have studied the dinorbor- nadiene systems 35a ± e.81 Dinorbornadienes 35a ± e absorb light in the region of 345 ± 450 nm; the lae values for compounds 35c ± e are as high as 530 ± 560 nm. Irradiation of these compounds with light with a wavelength of 365 nm (a, b) or 436 nm (c ± e) results in valence isomerisation into the corresponding monoquadricy- clanes 36 and then into diquadricyclanes 37.The two-step pattern of the photoreaction postulated previously for dinorbornadienyl derivatives of diamides 82 was confirmed for 3-phenylnorborna- diene-2-carbaldazine (35b). The primary photoproduct 36b is characterised by a new absorption band at 380 nm (lirr=436 nm, F=0.023); its subsequent irradiation with a wavelength within this band (lirr=365 nm, F=0.028) gives the diquadricyclane 37b. The reverse transformation into the original dinorborna- dienes can be carried out under conditions of homogeneous orNorbornadiene ± quadricyclane as an abiotic system for the storage of solar energy heterogeneous catalysis (addition of CF3CO2H or MoO3, respec- tively).Ph Ph hn1 hn2 X X 36a ± e Ph Ph 35a ± e Ph X Ph 37a ± e OMe NHCO (a), X = CONH ( ) CH N (b), 2 MeO O O O HC HC CH (d), CH (c), HC CH (e). V. Polymer-bound norbornadienes and sensitisers One of the leading approaches to the accumulation of solar energy developed in recent years involves the use of polymeric systems containing non-substituted NBD or its derivatives in a lateral or main chain of the macromolecule as well as polymer-bound sensitisers of the valence isomerisation NBD?Q. This is primar- ily due to the fact that norbornadiene fragments incorporated in a polymer are more stable than free NBD molecules with the same substituents. In many cases, this makes it possible to avoid side reactions and to increase the quantum yield of NBD?Q photo- isomerisation and the turnover number.The first studies along this direction were aimed at the synthesis of polymer-bound benzophenone or acetophenone, but the photo-stability of such sensitisers proved to be rather low.83, 84 Polymeric photosensitisers of a new type (38) containing ammo- nium or phosphonium groups (for binding the substrate) together with sensitising nitroaryloxy groups were used to carry out the 39?40 photoisomerisation.85 CH CH CH2 CH2 CH2 CH2 38 O X NO2 [YR3]+Cl7 X , = ;Y=P, N; R = Prn, Bun, n-C6H13. CO2K CO2K hn Ph Ph 39 40 The quantum yield of this reaction was 0.19 at lirr=366 nm in the absence of a sensitiser and 0.42 in the presence of the polymer 38, suggesting that intermolecular energy transfer occurred from the sensitiser (38) to the norbornadiene (39).Irradiation of the polymers 41 in the visible spectral region (lirr>350 nm) results in their valence isomerisation into the quadricyclane derivatives 42.86 A mechanism of this process was suggested, which involved the intramolecular electron transfer from the carbazole (CA) sensitiser to the norbornadiene fragment: 923 * hn 1CA7NBD 1[CA+7NBD7] CA7NBD 41 3[CA+7NBD7] CA73NBD CA7NBD. 42 The reverse reaction occurs in the presence of catalytic amounts of (5,10,15,20-tetraphenyl-21H,23H-porphyrinato)- cobalt(II) (Co-TPP). P P*O2C *O2C hn Co-TPP O2C N 42 41 Me P = ( ) CH2 C n * CO2(CH2)3 The valence isomerisation of polyesters 43 containing benzo- phenone and norbornadiene fragments occurs both in polymeric films and in solutions in dichloromethane (F=0.62 ± 0.84) and involves a stage of efficient energy transfer from the sensitiser to NBD.87 Compounds 44 are rather stable; the reverse reaction 44?43 releases *90 kJ mol71 of the accumulated thermal energy.P*O2C P*O2C hn D R *(CH2)3 O2C R 44 43 O O O O P =( )O( ) CH2CH O O; CH2CH 10 90 CH2 * * CH2 * Y , , N(Me) CO CO Y=O CO N(Me) NMe2;R=CON , CONPrn2 , CO2Me. Polyamides 45 containing non-substituted norbornadiene fragments in the main chain are characterised by values lae4440 nm.88 ± 90 Irradiation of these compounds in this spec- tral region results in the selective isomerisation of NBD into quadricyclanes without side reactions.Incorporation of a sensi- tiser, e.g., Michler's ketone or 4-(N,N-dimethylamino)benzophe- none, in the polymer increases the efficiency of the 45?46 process. The reverse transformation occurs upon addition of Co- TPP to a solution of the polyamide 46 in dichloromethane or upon irradiation of polymeric films of 46 with light with lirr=235 ± 260 nm. hn X(CH2)3X X(CH2)3X O O O O 46 45 CH2NH X , HNH2C = , , N N N Y NH(CH2)6NH , NH N ; Y =CH2, O. H924 Polyesters 47 with similar composition also readily undergo photoisomerisation on exposure to sunlight in polymeric films or as solutions in THF to give quadricyclane derivatives 48.91 hn O O Y NH HN X hn 0 O O O O 47 O Y O NH X HN O O O O 48 X= ; R1 Me Me ; , O , R1 = CH2 ; Y = CH2CH(OH)CH2OR2OCH2CH(OH)CH2 Me Me R2 = , CMe2 , .(CH2)2 Me Me The reverse transformation of solid polymers 48 occurs efficiently under irradiation with light with lirr=272 nm and is accompanied by liberation of heat (*84 kJ mol71). `Donor ± acceptor' NBD can also be incorporated in the main chain of a polymeric molecule.92, 93 Owing to the existence of a charge transfer band, polyesters 49 have lae4500 nm. Irradia- tion of polymeric films of compound 49 with visible light yields isomeric compounds 50; the reverse reaction occurs slowly at room temperature. R2 R1 R2 R1 X X hn R3 R3 R3 R3 D O O Y O Y O O O O 50 ; R1=4-MeOC6H4; O 49 X = CMe2 R2=Me, 4-MeOC6H4; R3=H, Me; Y = CH2CH(OH)CH2OR4OCH2CH(OH)CH2 ;Me Me = R4 , , CMe2 Me Me , .(CH2)4 The majority of studies in this field deal with polymers in which the NBD residues are `attached' to the main chain of the macromolecule.5, 94 ± 103 For example, polyisoprene 51 is isomer- ised in a benzene solution in the presence of (PPh3)2CuBr as the sensitiser.94 The reaction 52?51 is thermally reversible on heat- ing of the polymer to 180 8C. hnD 51 52 A D Dubonosov, V A Bren, V A Chernoivanov Irradiation of polymers 53, which contain the residue of 3-phenylnorbornadiene-2-carboxylic acid, with light with lirr=313 nm results in their isomerisation into quadricyclanes 54.95, 96 The reverse transformation is performed upon irradiation at lirr=248 nm, heating to 100 8C, or adding Co-TPP to a solution of the polymer 54 in dichloromethane.CH CH CH2 CH2 X X hn OOC OOC hn 0, D or Cat Ph Ph 53 54 . , X = O (CH2)2 CH2 Polymer-bound amides 55a ± e were obtained in order to shift lae to higher wavelengths; they are characterised by absorption at lmax=323 ± 351 nm and an absorption edge at lae= 425 ± 440 nm.97 Under irradiation of compounds 55a ± e with light of a mercury lamp or with sunlight, the valence isomerisation occurs readily, both in solid phase and in solutions of the polymers in THF. Its efficiency increases in the series: 55e<55d455a<55b<55c. Addition of Co-TPP to solutions of the quadricyclanes 56a ± e at 298 K results in their isomer- isation into norbornadienes 55a ± e with a rate constant k=(0.5 ± 0.7)6103 s71.The amount of the accumulated solar energy is evaluated to be 60 kJ mol71, although the presence of an N,N-disubstituted amide group in the norbornadiene fragment increases it to 80 ± 86 kJ mol71 (see Ref. 98). CH CH2 CH CH2 hn CH2OCO CH2OCO Co-TPP p-RC6H4NHCO p-RC6H4NHCO 56a ± e 55a ± e R = H (a), Me (b), OMe (c), Cl (d), COMe (e). Binding of NBD moieties with a polyester macromolecule gave photoactive systems 57.5 Valence isomerisation of the latter into quadricyclanes 58 occurs readily under irradiation of the polymer at lirr4400 nm, both in solid phase and in solutions in THF (F=0.33 ± 0.41). The reverse transformation 58?57 is carried out using low-temperature initiation (62 ± 75 8C).The amount of solar energy accumulated in this photoreaction is very high, viz., about 90 kJ mol71. X X O CH O O CH O CH2 CH2 O O O O hn CH2 CH2 D OCO OCO R R 58 57 , , ; , (CH2)2 X = C6H4 R=Ph, CONPr2, CO2Me. Polymers 59 with the residues of `donor ± acceptor' NBD were obtained in order to shift the lae values farther towards the solar radiation maximum.99 However, they were found to have lae4350 nm. Nevertheless, these compounds are characterisedNorbornadiene ± quadricyclane as an abiotic system for the storage of solar energy by high turnover rate of the 59.60 transformation and can store 55 ± 74 kJ mol71 of thermal energy. CH CH2 CH2 X R4 hn R3 CH2OCO Y Ph R2 R1 59 ; X = OCH2 C6H4 , Y = CH2 R1=R2=R3=R4=H, Me.Polymers 61 containing trifluoromethyl-substituted NBD present an improved version of systems of this type. Their efficient phototransformation into the isomers 62 can be carried out under irradiation in the visible spectral region (up to lae4505 nm).100 P2 P1 P2 P1 hn X X F3C CF3 F3C CF3 61 62 P1 = R1 P2 = ; X=CMe2; S * * R1=CO: R2=CO(CH2)6O7, HN C CH2 * * Me R1=H: R2=CO2CH2CH(OH)CH2OCO, CH The main drawback of the majority of the polymeric systems described above is their insufficient photo- and thermal stability, which limits the NBD.Q turnover number. When norborna- dienes 63 bound to a rigid polymeric system undergo photo- isomerisation, they maintain the most important spectral and photochemical features of theNBD?Qtransformation, but they have an enhanced stability in the cyclic reactions 63.64.101, 102 PCH2OCO P* * CH2OCO hn MeO , N ** * CH2 .CO2CH2 D R 63 P = CO CON CO Ar CO * * Me R=Ph, CONHAr. Norbornadiene-containing polystyrenes insoluble in organic solvents are even more stable.103 Irradiation of their suspensions in dichloromethane results in isomeric quadricyclanes; in the presence of Co-TPP, the reverse reaction occurs with liberation of accumulated energy (44 ± 77 kJ mol71).CH XCH2OCO Ph 60, CHMe R2; S NH; CH CH2 CO2(CH2)2O, 64 CCF3 * * * R CF3 925 VI. Conclusion R4 R3 Y R2 R1 ; Analysis of recently published studies on the valence isomer- isation of norbornadiene and its derivatives into the correspond- ing quadricyclanes revealed two major trends in relation to the accumulation of solar energy: (1) the total number of publications on this subject has decreased somewhat; (2) at the same time, we observe a significant increase in the number of studies that not only pose a certain problem but also suggest specific ways to solve it practically.In our opinion, this indicates that this problem is nearly solved rather than that the interest in the accumulation of solar energy with the use of NBD is being lost. As mentioned previously,1 the thermal energy obtained by the method in question is more expensive than that generated by conventional methods, but cost issues will inevitably become less important as natural fuel resources are exhausted.We distinguished four main competing approaches that are used for enhancing the efficiency of isomerisation of norbornadienes into quadricyclanes. It can be concluded that considerable success has been reached along each of these directions. Efficient ways for the photosensitisation of this reaction have been developed using metal complexes or sensitisers chemically bound with the NBD molecule; in certain cases, the number of working cycles can be as high as 5000. Owing to the enhanced thermal stabilities of `donor ± acceptor' systems in which NBD molecules contain perfluoroalkyl substituents, such derivatives are promising for practical application.2,3-Disubsti- tuted NBD are serious competitors to these systems. This is primarily due to the facts that they are easier to synthesise and that they can absorb sunlight in the spectral region l>400 nm. Polymer-bound NBD can provide acceptable cyclicity and allow up to 94 kJ mol71 of solar energy to be accumulated. It is also obvious that, beside the accumulation of solar energy, the norbornadiene ± quadricyclane system finds diverse new fields of application: creating molecular switches 61, 104, 105 and optical waveguides on their basis,106, 107 developing new types of photo- chromic chemosensors for metal cations 108, 109 and photo-switch- able organic magnetic materials.110 ± 112 Furthermore, NBD- containing polymers can be used as energy-boosting additives for rocket fuel.94 This review was financially supported by the Russian Foun- dation for Basic Research (Project Nos 02-03-32527 and 00-15- 97320) and the Civilian Research and Development Foundation (CRDF) ± at the Russian Federation Ministry for Education (Grant No.REC-004). References 1. 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年代:2002

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Russian Chemical Reviews 71 (11) 929 ± 941 (2002) b-Telluroacroleins and tellurovinyl ketones: synthesis, reactions and structures I D Sadekov Contents I. Introduction II. Synthesis and reactions of b-tellurovinylcarbonyl compounds III. Structures and spectral characteristics of b-tellurovinylcarbonyl compounds IV. Conclusion Abstract. and reactions synthesis, the on data published The The published data on the synthesis, reactions and structures organotellurium novel of structures of novel organotellurium synthones, synthones, viz., b-telluro- vinylcarbonyl compounds, are systematised and generalised. vinylcarbonyl compounds, are systematised and generalised. Special in compounds these of use the to given is attention Special attention is given to the use of these compounds in the the synthesis The heterocycles.tellurium-containing of synthesis of tellurium-containing heterocycles. The bibliography bibliography includes references 57 includes 57 references. I. Introduction b-Tellurovinylcarbonyl compounds (aldehydes and ketones) are of particular interest as starting compounds for the synthesis of various tellurium-containing heterocycles and metal chelates with the coordination sites MO2Te2 and MN2Te2 (M is metal). How- ever, the chemistry of these compounds has not been adequately investigated. The first b-telluroacroleins and b-aryltellurovinyl ketones were prepared by nucleophilic addition of arenetelluro- late anions ArTe7 to a-acetylenic aldehydes and ketones (yields 65%± 75%).1 Bis(b-acylvinyl) tellurides were synthesised in 3%± 11% yields 2 using a similar approach, i.e., by nucleophilic addition of the telluride anion Te27 to a-acetylenic ketones (the reasons for the low yields of ketone will be discussed below).Two other types of b-tellurovinylcarbonyl compounds, viz., aryl b-chlorotellurovinyl ketones and aryl b-trihalogenotellurovinyl ketones were prepared in the 1980's.3 The former were synthesised in four steps; the final step included the rearrangement of 3-(aryltelluro)propenoyl chlorides (yields 50%± 95%).3 Aryl (b-trihalogenotellurovinyl) ketones were obtained in good yields by oxidation of tellurenyl halides with halogens.4 Earlier, b-tellurovinylcarbonyl compounds were used exclu- sively for the preparation of tellurophenes containing strong electron-withdrawing groups in position 2 (NO2, CHO, COMe, CO2Et) 5 and 1,6-dioxa-6a-tellurapentalenes.6 Significant progress in the chemistry of b-telluroacroleins and b-tellurovinyl ketones has been achieved over the last 8 ± 10 years owing to the development of methods for their synthesis.(b-For- mylvinyl)(methyl)tellurium dihalides, a novel class of b-tellurovi- nylcarbonyl compounds, have proved to be convenient starting compounds in the synthesis of (b-halogenotellurovinyl)carbonyl I D Sadekov Institute of Physical and Organic Chemistry of Rostov State University, prosp. Stachki 194/2, 344090 Rostov-on-Don, Russian Federation. Fax (7-863) 243 46 67. Tel. (7-863) 228 38 94. E-mail: sadek@ipoc.rnd.runnet.ru Received 24 June 2002 Uspekhi Khimii 71 (11) 1051 ± 1063 (2002); translated by R L Birnova #2002 Russian Academy of Sciences and Turpion Ltd DOI 10.1070/RC2002v071n11ABEH000746 929 929 938 940 derivatives and the previously unknown b-halogenotellurovinyl aldimines.Preparative procedures for the synthesis of various tellurium-containing heterocycles based on b-tellurovinylcar- bonyl compounds have been developed. The distinguishing feature of organotellurium compounds is the formation of intramolecular hypervalent coordinative bonds between the oxygen (or nitrogen) atom and the tellurium atom [O(N)?Te]; their lengths vary over a wide range, sometimes being close to the lengths of ordinary covalent bonds. Similar bond types were studied mostly for aromatic derivatives in which tellurium-centred groups [TeHal, TeOR, TeOCOR, TeSCN, TeR, Te(R)Hal2, TeHal3, etc.] were in the ortho-position relative to the substituents containing sp2- or sp3-hybridised oxygen or nitrogen atoms (CHO, COR, CO2R, NO2, CH=N, N=N, CH2NR2).7 ±9 The intramolecular coordination O(N)?Te ensures stabilisation of various organotellurium compounds, primarily of dicoordi- nated tellurium derivatives, which are thermodynamically and kinetically unstable in the absence of these bonds, and signifi- cantly affects their reactivities.The preparation of pure arenetel- lurenyl halides,7 ±10 azides,8, 10 acylates,7, 8, 10 and alkoxides 8, 10 is possible provided they contain the coordinative bond O(N)?Te.Tellurium-centred cyclopentadienes are stable if they contain tellurium-containing substituents with o-formyltelluranylbenzene or their respective azomethine fragments.11 However, data on X-ray diffraction analysis of b-tellurovinylcarbonyl compounds and (b-halogenotellurovinyl) aldimines containing intramolecular coordinative bonds O(N)?Te are very scarce. II. Synthesis and reactions of b-tellurovinylcarbonyl compounds In this review, the synthesis and reactions of b-telluroacroleins and b-tellurovinyl ketones will be considered separately for each individual group of b-tellurovinylcarbonyl compounds. Reac- tions typical of carbonyl groups and tellurium-containing sub- stituents (reactions involving double bonds are still unknown) and conversions resulting in the formation of tellurium-containing heterocycles from b-tellurovinylcarbonyl compounds and their derivatives will be discussed.1. Synthesis of b-alkyl(aryl)telluroacroleins and b-telluro- vinyl ketones Two main approaches to the synthesis of b-alkyl(aryl)telluroacro- leins and b-substituted tellurovinyl ketones of the general formula RTeC(R1)=C(R2)COR3 are known, viz., nucleophilic substitu- tion of the tellurolate anions RTe7 for chlorine atoms in b-chlorovinylcarbonyl compounds and nucleophilic addition of RTe7 to the triple bonds of a-acetylenic aldehydes and ketones.930 The former approach is more general and allows the preparation of compounds containing a broad range of substituents in the carbon triad C=C7C(O).As far as the nucleophilic addition is concerned, this yields only b-tellurovinylcarbonyl derivatives devoid of a-substituents. b-Aryltelluroacroleins (1) and b-aryltellurovinyl ketones (2) were prepared by reaction of b-chlorovinylcarbonyl compounds 3 and 4 with arenetellurolate anions 12 ± 15 generated in the reaction of aryllithium with tellurium powder in THF or by reduction of diaryl ditellurides with lithium in THF in the presence of catalytic amounts of naphthalene (Tables 1 and 2). Table 1. Synthesis of b-aryltelluroacroleins 1 from b-chloroacroleins 3. CHO R2 CHO R2 THF +ArTe7 710 to715 8C; D TeAr R1 Cl R1 1 3 Ref. Ar R2 R1 Z: E ratio Yield (%) Com- pound 1 Ph Ph Ph Ph Ph Ph 12 ± 15 14, 15 14, 15 14, 15 14, 15 14, 15 13 ± 15 14 ± 16 12 ± 16 1 : 0 1 : 0 1 : 2 1 : 9 1 : 19 1 : 10 1 : 0 1 : 0 1 : 0 Ph 4-EtOC6H4 Ph 4-EtOC6H4 Ph 4-MeOC6H4 Ph 4-MeC6H4 4-EtOC6H4 HHMe Me Et Et (CH2)4 (CH2)4 (CH2)4 abcdefghi 37 45 60 52 65 58 57 52 37 1Note.The ratios of the Z- and E-isomers were determined from the H NMR spectra. Table 2. Synthesis of b-aryltellurovinyl ketones 2 from b-chlorovinyl ketones 4. COR COR H H THF +ArTe7 710 to715 8C; D TeAr Cl H H 2 4 Ref. Yield (%) Ar R Com- pound 2 12 ± 15 12, 14 ± 16 13 ± 15 14, 15 14 ± 16 12, 14, 15 12, 14 ± 16 12, 14, 15 14, 15 47 45 52 39 53 45 39 55 47 Ph 4-MeOC6H4 4-EtOC6H4 4-MeC6H4 4-EtOC6H4 4-MeC6H4 4-EtOC6H4 4-MeC6H4 4-EtOC6H4 Ph Ph Ph 4-MeC6H4 4-MeC6H4 4-MeOC6H4 4-MeOC6H4 4-BrC6H4 4-BrC6H4 abcdefghi The stereochemistry of reactions of nucleophilic substitution of arenetellurolate anions for the chlorine atom in b-chlorovinyl- carbonyl compounds was studied in the example of b-chlorovinyl ketones 4 (R=Ph, 4-MeC6H4, 4-BrC6H4, 4-MeOC6H4),12, 14, 15 since the configurations of substituents at the double bond of the starting compounds and the derived tellurovinyl ketones 2 could easily be established from the values of coupling constants of olefinic protons.Ketones 4 synthesised by treatment of sodium salts of b-oxoaldehydes 5 with SOCl2 represent a mixture of Z- and E-isomers where the latter predominate.I D Sadekov C(O)R H C(O)R H ArTe7 1) NaOH 2) SOCl2 Cl H O H 4 5 R TeAr H H TeAr H C(O)R O 7Cl7 Te C(O)R H Cl H H Cl A Ar a ± i TeAr H TeAr R(O)C H H R(O)C Cl H Cl As described earlier,14, 15 the ratios of Z- and E-isomers in chlorovinyl ketones 4 depend on the nature of the substituent in position 4 of the phenyl ring and are equal to 0 : 1 (R=4-BrC6H4), 1 : 12 (R=Ph), 1 : 5 (R=4-MeC6H4), 1 : 1,5 (R=4-MeOC6H4). However, aryl b-aryltellurovinyl ketones 2a ± i prepared from them represent exclusively Z-isomers.12, 14, 15 Thus, substitution of ArTe7 for the chlorine atom results in complete reversal of the configuration of the E-isomers of aryl b-chlorovinyl ketones 4 and complete retention in the case of Z-isomers.This was rationalised in terms of a two-step vinylic nucleophilic substitution mechanism, viz., vinylic substitution through addition ± elimination.17 The first step of this reaction includes addition of highly nucleophilic arenetellurolate anions 18 to the double bond of chlorovinyl ketones, the electrophilicity of the double bonds being enhanced owing to the presence of the oxo group. The conformers A stabilised by the coordinative bond O?Te certainly possess the highest stabilities compared with the other conformers existing due to the rotation of the carbanions formed around the simple bond. Subsequent elimination of the chloride ion from conformers A yields the Z-isomers of aryl b-aryltellurovinyl ketones 2a ± i, which are additionally stabilised by the coordinative bond O?Te.14, 15 b-Chloro-b-phenylpropenal obtained as a mixture of Z- and E-isomers in a 9 : 1 ratio 19 reacts with arenetellurolate anions to yield exclusively the Z-isomers of compounds 1a,b.14, 15 The reaction of arenetellurolate anions with 2-chlorocyclohex-1-ene- carbaldehyde has the same stoichiometric result, i.e., exclusive formation of Z-isomers of compounds 1g ± i (see Table 1).14, 15 In contrast to chloroacroleins and chlorovinyl ketones non- substituted in the a-position, compounds 3 (R1=Ph, R2=Me, Et) react with arenetellurolate anions to afford mixtures of E- and Z-isomers of compounds 1c ± f; their isomeric ratios vary widely (see Table 1).In this case, the O?Te interaction, which stabilises conformers A, competes with the steric repulsion of bulky phenyl and alkyl groups, as a result of which the stereospecificity of the reaction is lost.14, 15 Among b-alkyltellurovinylcarbonyl compounds, b-methyltel- luroacroleins were the first to be synthesised by nucleophilic substitution of the chlorine atom in b-chlorovinylcarbonyl deriv- atives.14, 15, 20 Thus 2-methyltellurocyclohex-1-enecarbaldehyde 6 was prepared by reaction of b-chlorocyclohex-1-enecarbaldehyde with lithium methanetellurolate obtained from methyllithium and tellurium in tetrahydrofuran. This reaction proceeds smoothly only at770 8C. CHO CHO THF,770 8C +MeTe7 Cl TeMe 6 (54%)931 b-Telluroacroleins and tellurovinyl ketones: synthesis, reactions and structuresTable 3.Synthesis of b-aryltelluroacroleins and b-aryltellurovinyl ketones from b-acylvinyltriethylammonium chlorides 11. COR3 R2 COR3 R2 MeOH, D The approach to the synthesis of b-methyltelluroacrolein 7 is based on the reaction of equimolar amounts of the corresponding b-chloroacrolein with lithium telluride with subsequent alkylation of intermediate lithium tellurolate 8 with methyl iodide.21 +ArTe7 + R1 TeAr R1 NEt3Cl7 CHO H CHO H MeI THF,710 8C 11 1h ± j, 2e,g,i,j +Li2Te TeLi 4-BrC6H4 Cl 4-BrC6H4 Ar R2 R3 R1 8 Yield Ref. (%) Com- pound CHO H TeMe 4-BrC6H4 7 (22%) 4-MeC6H4 4-EtOC6H4 4-ClC6H4 4-EtOC6H4 14 ± 16 12 ± 16 14 ± 16 14 ± 16 12, 14 ± 16 14, 15 12, 14 ± 16 71 70 79 79 83 85 86 HHH H 4-MeC6H4 H 4-MeOC6H4 4-EtOC6H4 4-EtOC6H4 4-MeC6H4 1h 1i 1j 2e 2g 2i 2j H Ph (CH2)4 (CH2)4 (CH2)4 HHH H 4-BrC6H4 H b-Butyltellurovinyl ketones 9a ± e were synthesised by nucleo- philic substitution of the chlorine atom in the corresponding b-chlorovinyl ketones 10 using the reaction with the butanetellur- olate anion generated by reduction of dibutyl ditelluride with diisobutylaluminium hydride.22 Compounds 9a ± e were obtained as mixtures of E- and Z-isomers,22 however, their ratios were not indicated.than their chlorine-containing analogues. Moreover, the prepara- tion of arenetellurolate anions is preparatively more simple, and the yields of the reaction products are usually higher.COR3 R2 COR3 R2 +BuTe7 R1 TeBu R1 9a ± e Enol phosphates 12 were also used as starting compounds in the synthesis of b-substituted tellurovinylcarbonyl compounds.24 Their reactions with lithium butanetellurolate prepared from butyllithium and tellurium powder in THF result in b-butyl- tellurovinyl methyl ketones 9a,f,g. COR3 R2 COR3 R2 Cl 10a ± e R1=R3=Me, R2=H(a); R1±R3=(CH2)2, R2 = H (b); R1±R3=(CH2)3, R2=H(c); R1=Me, R2±R3=(CH2)3 (d); R1=Me, R2±R3=(CH2)4 (e). +BuTeLi 7P(O)(OEt)2OLi R1 R1 OP(O)(OEt)2 12 TeBu 9a,f,g R1=R3=Me, R2=H(a); R1=Ph, R2=H, R3=Me (f); R1±R2=(CH2)3, R3=Me (g) (b-Acylvinyl)triethylammonium chlorides 11 obtainable in high yields upon treatment of chlorovinylcarbonyl derivatives with triethylamine 23 can be used in the synthesis of b-aryltelluro- vinyl aldehydes 1 and ketones 2 instead of b-chlorovinylcarbonyl compounds.14 ± 16 The reactions of compounds 11 with sodium tellurophenolates prepared by reduction of diaryl ditellurides with sodium borohydride are carried out in methanol with subsequent short-term boiling of the reaction mixture (Table 3).A large variety of telluroacroleins and tellurovinyl ketones non-substituted in the a-position were prepared by nucleophilic addition of alkane(arene)tellurolate anions generated by reduc- tion of dialkyl(aryl) ditellurides with sodium borohydride in methanol or ethanol to the triple bonds of acetylenic aldehydes or ketones 13 (Table 4).1, 14, 15, 25, 26 The reaction proceeds under This approach to the synthesis of b-aryltellurovinylcarbonyl compounds is more convenient than that from chlorovinylcar- bonyl compounds, since ammonium salts are more storage-stable Table 4.Synthesis of b-alkyl(aryl)telluroacroleins and b-alkyl(aryl)tellurovinyl ketones by addition of tellurolate anions to acetylenic carbonyl compounds. COR2 H R1C CCOR2 + R3Te7 13 TeR3 R1 Ref. Z: E ratio Yield (%) R3 R2 Compound R1 Ph Ph HPh Ph HHHBu Bu 1a 1b 1k 1l 1m 3b 3c 3j 3k 3l 3m 14, 15, 25 14, 15 26 25 25 14, 15 14, 15 11, 25 125 90 61 80 76 68 86 89 71 68 71 84 Ph 4-EtOC6H4 Ph 4-MeC6H4 4-MeOC6H4 4-MeC6H4 4-EtOC6H4 4-MeOC6H4 4-MeOC6H4 4-MeOC6H4 Ph 1 : 0 a 1 : 0 a 1 : 0 b 771 : 0 b 1 : 0 b 7777 HHHHHPh Ph Ph Ph Me Ph OH 25 126 86 74 84 Ph Ph Bu Ph 4-MeOC6H4 H Ph Ph H 7789 : 11 3n 3o 9h a The assignment was made on the basis of the identity of the compounds synthesised to those prepared from b-chloroacroleins. b The assignment was made on the basis of coupling constants of olefinic protons in the 1H NMR spectra.932 mild conditions and results in b-alkyl(aryl)telluroacroleins and b-alkyl(aryl)tellurovinyl ketones (yields 60%± 90%). The stereochemistry of the nucleophilic addition of arenetel- lurolate anions to the triple bond was studied for the acetylenic derivatives 13, where R1=H, R2=H, Ph.14, 15, 26 It was found that all these reactions yield Z-isomers in high yields.In the case of b-aryltellurovinyl phenyl ketones 2b,c, the coupling constants for the alkene protons 3JHH are equal to 8.79 Hz, which corresponds to the cis-configuration. The Z-configuration of the substituents in b-phenyltelluroacrolein (1k) was also determined from the 1H NMR spectra. However, reaction of the butanetellurolate anion with pro- piolaldehyde yields a mixture of Z- and E-isomers of the aldehyde 9h in an 89 : 11 ratio.26 This seems to be due to the lower electro- negativity of the butyl substituent as compared with the phenyl one. Indeed, according to quantum-chemical calculations,27 ± 30 the lengths and, hence, the stabilities of the intramolecular coordinative bond O(N)?Te(II) depend on the electronegativ- ities of substituents at the tellurium atom: the higher this value the shorter the coordinative bond.Weakening of the coordinative bond on going from R=Ph to R=Bu results in partial cleavage of this bond in the butyl derivative and the appearance of the E-isomer of compound 9h in the solution. Arenetellurolate anions, which are the strongest nucleophiles in the series of structurally related anions ArM7 (M=O, S, Se, Te),18 react with symmetrical diacetylenic ketones to give only the bisadducts 14 (yields 84% ± 88%),25 whereas alcohols and phenols add to one or both triple bonds depending on the reaction conditions.EtOH, 20 8C PhC CCOC CPh+ArTe7 ArTe(Ph)C CHCOCH C(Ph)TeAr 14 Ar=Ph, 4-MeC6H4. Due to the relatively low electrophilicity of the triple bond in b-aminovinyl ethynyl ketones 15, they do not react with nucleo- philes, such as amines, alcohols and phenols. However, arenetel- lurolate anions readily add to compounds 15. Thus ketone 16 was prepared in 68% yield.25 Acid hydrolysis of this ketone results in the elimination of diethylamine and the formation of diketone 17.25 H+ PhC CHCCH CPh PhC CCCH CPh+ArTe7 TeAr O NEt2 O NEt2 16 15 PhC CHCCH2CPh O O TeAr17 (75%) Ar=4-MeC6H4. Aspecific synthetic procedure was used to prepare b-trifluoro- acetylvinyl tellurides 18a ± g.31 Like other heterosubstituted alkenes (vinyl sulfides, vinyl ethers, N-vinylamides, carbazoles, etc.) (see Ref.31 and references cited therein), vinyl tellurides RTeCH=CH2 react with trifluoroacetic anhydride in THF to give ketones 18a ± g (yields 50% ± 84%). However, in contrast to the previously mentioned heterosubstituted alkenes, the use of catalysts is necessary, among them N,N,N0,N0-tetramethyl- ethylenediamine (TMEDA) is the most efficient. The yields of alkylvinyl tellurides 18a ± d are higher than in the case of com- pounds 18e ± g. I D Sadekov H COCF3 TMEDA, THF +(CF3CO)2O RTe 50 8C or D H TeR 18a ± g R=Bun (a), Bui (b), EtO2C(CH2)2 (c), NC(CH2)2 (d), Ph (e), O NCH2CH CH2 (f), PhCH CH (g). It is of note that the 1,1-disubstituted vinyltelluride, viz., 1-butyltelluro-1-phenylethylene, does not enter into this reac- tion.31 2.Reactions of b-alkyl(aryl)telluroacroleins and b-alkyl(aryl)tellurovinyl ketones Reactions of b-alkyl(aryl)tellurovinylcarbonyl compounds have been studied relatively little. Like other aldehydes and ketones, they are reduced with sodium borohydride to 3-aryltelluroprop-2- en-1-ols 19,13 which can be used as the starting compounds in the synthesis of 5H-1,2-oxatelluroles. Compounds 19 (R1=R2=H) were prepared by nucleophilic addition of arenetellurolate anions to the triple bond of propargyl alcohol.13, 32, 33 COR3 R2 R3 NaBH4 R2 OH TeAr R1 R1 TeAr ArTe7+HC CCH2OH 19 R1=R2=H,R3=Ar=Ph; R1=Ph, R2=R3=H: Ar=Ph, 4-EtOC6H4; R1±R2=(CH2)4, R3=H: Ar=Ph, 4-EtOC6H4; R1=R2=R3=H: Ar=Ph, 4-MeOC6H4, 4-EtOC6H4.Azomethine 20 was prepared by condensation of b-aryl- telluroacrolein 1b with p-toluidine.15 CHO H EtOH, D +4-MeC6H4NH2 Ph TeC6H4OEt-4 1b H CH NC6H4Me-4 Ph TeC6H4OEt-4 20 b-Alkyl(aryl)telluroacroleins, particularly compounds 1k and 9h, enter into the Wittig reaction with in situ generated methyl- enetriphenylphosphorane to give dienyl tellurides 21 and 22.26 The configurations of the starting alkenes are retained. H H CHO Ph3PMeI, ButOK, Et2O 20 8C, 30 min H TePh 1k H TePh Z-21 (89%) H CHO H Ph3PMeI, ButOK, Et2O 20 8C, 30 min TeBu H 9h H TeBu (Z,E)-22 (92%) Z-b-Alkyl(aryl)tellurovinyl trifluoromethyl ketones 18a,b,e react with zinc cuprates 23 at low temperatures (from 778 8C to 730 8C) to give E-isomers of a,b-unsaturated trifluoromethylb-Telluroacroleins and tellurovinyl ketones: synthesis, reactions and structures ketones 24.34 However, compounds 24 are obtained in high yields only with cuprates 23 where R2=Ar and CH=CHPh.In the case of alkylzinc cuprates, the yields of ketones 24 do not exceed 70%; dialkylated ketones 25 are formed as side products (yields 11%± 19%).34 H C(O)CF3 THF,778 to730 8C +R22 CuX 23 TeR1 H18a,b,e H C(O)CF3 +R22 CHCH2C(O)CF3 H R2 25 24 R1=Bun (a), Bui (b), Ph (e); R2=Ph, 4-MeC6H4, 4-MeOC6H4, 4-EtO2CC6H4, PhCH=CH, C8H17, C10H21, C12H25; X=(CN)(ZnCl)2, ZnCl. It is noteworthy that the reaction of organocopper derivatives Ph2CuLi, Ph2CuMgBr, Ph2Cu(CN)Li2 and Ph2Cu(CN)(MgBr)2 with ketones 18a,b,e is accompanied by the predominant forma- tion of the diphenyl derivatives 25, whereas ketones 24 are the side products.34 Only one reaction of b-alkyl(aryl)telluroacroleins involving the two-coordinate tellurium atom is presently known.Like other vinyl tellurides,35 b-methyltelluroacroleins 6 and 7 are oxidised to the four-coordinate tellurium derivatives 26a ± d by halogens or thionyl chloride under mild conditions (PhH, 0 ± 5 8C).15, 20, 21 CHO R2 CHO R2 X2 (SOCl2) TeMe R1 TeMe R1 X 6, 7 X 26a ± d (87% ± 97%) R1±R2=(CH2)4, X = Cl (a), Br (b), I (c); R1=4-BrC6H4, R2=H,X=Br (d). 3. Synthesis of tellurium-containing heterocycles a. Tellurophenes A series of substituted tellurophenes 27 containing strong elec- tron-withdrawing groups in position 2 were synthesised by reac- tion of b-chloroacroleins with sodium telluride and the a-halogenomethyl derivatives 28.5, 36, 37 This reaction proceeds under mild conditions and results in the heterocycles 27 in 25%± 30% yields.Presumably, b-substi- tuted telluroacroleins 29 are the intermediate products in this reaction. CHO R2 DMF, 0 ± 20 8C +Na2Te+XCH2R3 28 Cl R1 R2 CHO R2 7H2O R3 R1 R1 TeCH2R3 Te 27 29 R1=But, R2=H:R3=CHO, COMe, CO2Et, NO2; R1=3-MeOC6H4, 4-MeOC6H4; R2=H,R3=CO2Et; R1±R2=(CH2)4: R3=COMe, CO2Et, NO2; R1±R2=(CH2)5, R3=CO2Et; X=Cl, Br. 2,4-Disubstituted tellurophenes 30 were prepared from a-aryl- b-chloroacroleins.38 933 Ar CHO Ar +Na2Te+BrCH2CO2Et CO2Et Te H Cl 30 The above-described methods for the synthesis of telluro- phenes are similar to the reactions used in the synthesis of substituted thiophenes and selenophenes.b. 2-Aryl-2-chloro-5H-1,2-oxatelluroles 3-Aryltelluroprop-2-en-1-ols 19 13, 32, 33 were used as starting com- pounds in the synthesis of four-coordinate derivatives of 5H-1,2- oxatellurole.13, 33 Thus 2-aryl-2-chloro-5H-1,2-oxatelluroles 31a,b were synthesised in 44%± 82% yields by oxidation of alcohols 19 with ButOCl. This reaction seems to proceed through the inter- mediate formation of s-telluranes 32 which are further converted into the heterocycles 31a,b with elimination of ButOH.13 R3 R3 R2 R2 ButOCl OH OH 7ButOH OBut Te R1 TeAr R1 19 Ar Cl 32 H R3 R3 H R2 R2 + O O Te Te R1 R1 Ar Ar Cl Cl 31a 31b R1=R2=R3=H: Ar=Ph, 4-MeOC6H4, 4-EtOC6H4; R1=R2=H, R3=Ar=Ph; R1=Ph, R2=R3=H: Ar=Ph, 4-EtOC6H4.Oxatelluroles 31 containing substituents in position 5 of the heterocycle are formed as a mixture of two isomers as can be evidenced from the 1H NMR spectra of these compounds. Thus the signals of the methine protons of 2-chloro-2,5-diphenyl-5H- 1,2-oxatelluroles are observed as two singlets at d=6.02 and 6.27. 4. Bis(b-acylvinyl) tellurides Of the methods for the synthesis of bis(b-acylvinyl) tellurides that have been studied, the data about the reactions, structures and applications of these compounds in the synthesis of tellurium- containing heterocycles are virtually absent in the literature.The syntheses of bis(b-acylvinyl) tellurides 33a ± e, like those of b-alkyl(aryl)telluroacroleins and b-alkyl(aryl)tellurovinyl ketones, were carried out using the nucleophilic addition of potassium telluride to the triple bond of a-acetylenic ketones.2 To this end, acetylenic ketones were made to react with potassium telluride and hydroxylamine-O-sulfonic acid in an aqueous solution of sodium acetate. COR2 H Te R1C CCOR2+Te27 R1 2 33a ± e R1=H:R2=Me (a, 11%), Et (b, 6%), Pr (c, 3%); R1=R2=Me (d, traces); R1=Ph, R2=Me (e, 3%). The low yields of ketones 33a ± e are due to the competing heterocyclisation of the intermediate oximes 34 into isotellur- azoles 35.2, 39 Under these conditions, 3-mono- (35a ± c) and 3,5- disubstituted isotellurazoles (35d ± f) are obtained in nearly the same yields (3% ± 10%) as ketones 33.934 R1C CCOR2+H2NOSO2OH R2 1) AcONa 2) K2Te R1C CC NOSO2OH R1 NOSO2O7 R2 R2 Te7 34a ± f N R1 Te 35a ± f 35: R1=H: R2=Me (a, 6%), Et (b, 5%), Ph (c, 4%); R1=R2=Me (d, 10%); R1=Ph, R2=Me (e, 7%); R1=CH2=CH, R2=Me (f, 5%).This method is inapplicable to the synthesis of isotellurazoles non-substituted in position 3, since bis(b-cyanovinyl) tellurides (NCCH=CR)2Te (R=Hand Ph), are the only reaction products (yields 20% and 14%) when a-acetylenic aldehydes are used instead of ketones. Nucleophilic substitution of the telluride anion for the chlor- ine atom in b-chlorovinylcarbonyl compounds is used in the preparative synthesis of ketones and aldehydes 33.14, 15 Lithium telluride, prepared from lithium and elementary tellurium in THF in the presence of catalytic amounts of naphthalene, reacts with b-chloroacroleins and b-chlorovinyl ketones to give compounds 33f ± h in 39%± 60% yields.14, 15 COR3 R2 COR3 R2 THF,715 8C; D (30 min) Te +Te27 R1 Cl R1 2 33f ± h R1=R2=H:R3=Ph (f), 4-MeC6H4 (g); R1±R2=(CH2)4, R3=H(h).The synthesis of bis(b-benzoylvinyl) tellurides 33f,i was car- ried out using b-chlorotellurovinyl phenyl ketones 36 and b-tri- chlorotellurovinyl phenyl ketones 37.4 Reduction of the former with lithium triethylborohydride in THF affords ketones 33f,i in 53%± 92% yields; the yields in the latter case were not indicated.COPh H COPh H THF Te +LiBEt3H R R TeCln 2 36, 37 33f,i R = H (33f), Ph (33i); n = 1 (36), 3 (37). This method obviously holds little promise for the synthesis of ketones 33f,i, since chlorotellurium derivatives 36 are prepared in four steps from a-acetylenecarboxylates,3 and trichlorotelluro- vinyl ketones are produced upon their oxidation. Special mention should be made of the unusual course of the reduction of compounds 36 and 37. Tellurenyl halides 40 and tellurium trihalides 41 devoid of the coordinative bond O(N)?Te are reduced to the corresponding ditellurides. In the case of the chlorotellurium derivatives 36, ditellurides 38 are formed in trace amounts, apparently due to the presence of strong intramolecular coordinative bonds O?Te in the starting tellur- enyl chlorides.COPh H Te2 R 2 38 An analysis of the 1H NMR spectrum of ketone 33f 14, 15 [a ten-proton multiplet of aromatic protons at d=7.24 ± 8.10 I D Sadekov and anAX system (two two-proton doublets at d=8.36 and 9.14, 3JHH=9.67 Hz)] prompts a conclusion that the benzoylvinyl fragments of this compound exist in conformations with the cis- arrangement of the hydrogen atoms. 5. b-Halogenotellurocarbonyl compounds and their analogues; b-halogenotellurovinyl aldimines a. Synthesis Aryl b-chlorotellurovinyl ketones 36 are among the most well- studied b-tellurovinylcarbonyl compounds XTeC(R1)=C(R2). .COR3 where X is an electronegative substituent. They were prepared in four steps starting from a-acetylenecarboxylates 39.3, 4, 42 ± 44 The first step includes nucleophilic addition of arene tellurolate anions generated by reduction of the corresponding diaryl ditellurides with sodium borohydride in EtOH ±THF to esters 39.This leads to methyl(ethyl) 3-aryltelluropropenoates 40 (predominantly, Z-isomers) (yields 58%± 95%).3 However, ester 40 (R=H, Ar=Ph) is formed as a mixture of Z- and E-isomers (13 : 1).3 Hydrolysis of esters 40 with KOH and subsequent acid- ification of the reaction mixture result in b-(aryltelluro)acrylic (41, R=H) or b-(aryltelluro)cinnamic acids (41, R=Ph) (yields 68%± 95%). Treatment of acids 41 with oxalyl chloride gives 3-(aryltelluro)propenoyl chlorides 42. The latter are rearranged into ketones 36 (yields 31%± 96%) on boiling in deuteriochloro- form or by treatment with AlCl3.3, 4, 42 ± 44 Telluraflavones are formed in some cases in addition to ketones 36 (see below).42 EtOH ±THF R1C CCO2R2+ArTe7 39 H CO2H H CO2R2 1) KOH, EtOH ±H2O 2) H+ (COCl)2 TeAr R1 R1 TeAr 41 40 COCl H COAr H AlCl3 or D TeAr R1 TeCl R1 42 36 R2=Me, Et.Yield (%) Ar R1 Ref. 3, 43 43 43 4, 43 3, 4 342 33, 4 42 H Ph 82 3 76 7731 81 75 92 92 96 784 42 50 82 770 7 Ph 4-FC6H4 4-MeOC6H4 4-Me2NC6H4 Ph 2-MeC6H4 3-MeC6H4 4-MeC6H4 4-MeOC6H4 3-FC6H4 3-MeOC6H4 4-MeCOC6H4 1-C10H7 Ph Ph Ph Me Me Me Me Ph Ph Ph Ph Ph Ph Ph Ph Ph 4-MeOC6H4 2,5-(MeO)2C6H3 4-ButC6H4 33444 4 The rates of both thermal and catalytic rearrangements of compounds 42 are governed by the electronic nature of p-sub- stituents in the aryl nuclei.3 Thus in the case of b-(phenyltelluro)- cinnamoyl chlorides, compound 42 (R=Ph, Ar=4-MeOC6H4) could not be isolated at all, since it undergoes rearrangement into ketone 36 (R=Ph, Ar=4-MeOC6H4) in nearly quantitative yield even at 20 8C.The half-transformation time of the acid chloride 42 (R=Ph, Ar=4-MeC6H4) in boiling CDCl3 is 1 h, whereas that for compound 42 (R=Ar=Ph) is 30 h underb-Telluroacroleins and tellurovinyl ketones: synthesis, reactions and structures identical conditions. The presence of strong electron-withdrawing substituents (e.g.,Ar=4-MeCOC6H4) inhibits almost completely the thermal rearrangement, the yield of ketone 36 (R=Ph, Ar=4-MeCOC6H4) being<1%on boiling in CDCl3 for 72 h.The putative mechanism of the catalytic rearrangements of acid chlorides 42 into compounds 36 involves an attack of the acylium cation on the aromatic carbon atom bound to tellurium (ipso-acylation) to yield the spirocyclic s-complex 43. Subsequent nucleophilic attack of the chloride ion on the tellurium atom yields ketones 36. O H H AlCl3 Cl R Te R O H + Te R Cl7 43 It should be noted that such rearrangements were first observed for o-(aryltelluro)benzoyl chlorides o-ClCOC6H4TeAr (Ar=Ph, 4-MeC6H4, 2-C4H3S). Heating of these compounds with ZnCl2 gave (o-chlorotelluro)benzophenones o-ClTeC6H4..COAr in high yields.45 ± 47 The structures of the rearrangement products of aroyl chlor- ides 42 depend on the position and the nature of substituents in the ArTe fragments. In the case of p-substituted b-(aryltelluro)prope- noyl chlorides 42, the only reaction products are ketones 36.3 However, the introduction of donor substituents into the m-posi- tion of aryl substituents results in the activation of the o-positions towards electrophilic attack and prevents ipso-acylation, which results in either a mixture of ketones 36 and telluraflavones 44 or only the latter compounds.42, 44 O H R3 Cl AlCl3 Te R1 R2 42 R3 O H R2 R1 TeCl36 R3 R2 R1 HHFHOMe HHHOMe OMe OMe Me FFOMe OMe OMe OMe OMe OMe OMe OMe Ph Ph Ph Ph Ph 4-MeOC6H4 3,4-(MeO)2C6H3 2,5-(MeO)2C6H3 4-MeOC6H4 3,4-(MeO)2C6H3 2,5-(MeO)2C6H3 The above-described method for the synthesis of ketones 36 has certain limitations: it is a multistep procedure and affords only those aroyl derivatives that are not substituted in the a-position.O+ Te H COPh R TeCl 36 R3 O + Te R2 R1 44 Ref. Yield of Yield of 44 (%) 36 (%) 92 78 740555000 33, 42 42 3344 44 44 44 44 44 0518 78 90 44 68 752 57 72 935 A method for the preparation of b-halogenotellurenylvinyl- carbonyl compounds based on the use of b-methyldihalogenotel- lurovinylcarbonyl compounds 26 is devoid of these drawbacks.Until now, such compounds were mostly employed for the preparation of 2-halogenotellurocyclohex-1-enecarbaldehydes 45,15, 20 they can undoubtedly be used in the synthesis of vinyl- tellurenyl halides containing a broad range of substituents in the carbon triad. Like 2-formylphenyl(alkyl)tellurium dibromides,48 com- pounds 26 (their syntheses were discussed in Section II.2) undergo reductive elimination of methyl halide upon boiling in acetic acid in the presence of catalytic amounts of the corresponding hydro- halic acids.15, 20 CHO CHO AcOH, HX, D 7MeX TeX Te Me 45 (63% ± 67%) 26b,c X X X=Br, I. It is noteworthy that reductive elimination of methyl halides is extremely sensitive to the admixtures present in the reagents used and is therefore poorly reproducible.The mesylate derivative 46 is an example of tellurenyl deriv- atives containing electronegative substituents other than halogen atoms, which has been prepared by direct synthesis.44 This compound is an unstable oil; it was synthesised by heating a mixture of b-(phenyltelluro)-2,5-dimethoxycinnamic acid (47c), phosphoric anhydride and methanesulfonic acid. It is of note that sulfur (47a) and selenium analogues (47b) undergo cyclisation into benzocyclopentenones 48a,b under identical conditions.44 COPh OMe M=Te TeOSO2Me OMe CH CHCO2H OMe 46 P2O5, MeSO3H, D OMe O MPh M=S, Se OMe 47a ± c MPh OMe 48a,b M = S (a), Se (b), Te (c). b. Reactions Reactions of (b-halogenotellurovinyl)carbonyl compounds char- acteristic of the constituent two-coordinate tellurium atom have been studied largely for aryl b-halogenotellurovinyl ketones 36.Ketones 36 enter into reduction, oxidation and exchange reactions of chlorine atoms by other anionoid substituents. Aryl b-chlorotellurovinyl ketones 36 and their bromo analogues 49 undergo oxidative addition of chlorine and bromine under mild conditions (CH2Cl2, 0 8C) to give aryl b-trichlorotellurovinyl ketones 37a ± k and aryl b-tribromotellurovinyl ketones 50a ± c, respectively.4 COAr H COAr H CH2Cl2, 0 8C +X2 TeX R TeX3 R37a ± k, 50a ± c 36, 49 37: X=Cl; R=H, Ar=Ph (a, 70%); R=Me: Ar=Ph (b, 74%), 4-FC6H4 (c, 92%), 4-Me2NC6H4 (d, 39%); R=Ph: Ar=Ph (e, 66%), 3-FC6H4 (f, 88%), 4-FC6H4 (g, 88%), 2-MeC6H4 (h, 82%), 4-MeOC6H4 (i, 55%); R =4-MeOC6H4, Ar=Ph (j, 85%); R=4-ButC6H4, Ar=Ph (k, 79%).50: X=Br; Ar=Ph: R=H (a, 52%), Me (b, 70%), Ph (c, 89%).936 Iodine, possessing the weakest (in comparison with other halogens) oxidating properties, does not oxidise tellurenyl iodides. Thus phenyl b-iodotelluro-b-phenylvinyl ketone ITeC(Ph)=CH. .COPh is not converted into the corresponding tellurium triiodide in a reaction with iodine. The attempts to prepare mixed tellurium trihalides were unsuccessful. Thus the reaction of tellurenyl bromide 49 (R=Me, Ar=Ph) with chlorine gave a mixture of tellurium trichloride 37b and tellurium tribromide 50b in a 2 : 1 ratio, whereas treatment of tellurenyl chloride 36 (R=Me, Ar=Ph) with bromine yields a mixture of analogous tellurium trihalides, but in a 1 : 2 ratio.4 COPh H Cl2 TeBr Me COPh H COPh H CH2Cl2, 0 8C 49 + Me Me TeCl3 TeBr3 COPh H Br2 37b 50b Me TeCl 36 At the same time, the reaction of tellurenyl iodide 51 with bromine results in tellurium tribromide 50c.4 COPh H COPh H +Br2 Ph Ph TeI TeBr3 50c 51 The chlorine atoms in tellurenyl chlorides 36 are highly mobile and readily exchangeable for other anions (Br7, I7, CF3COO7), which affords the tellurenyl derivatives 49, 51, 52 in high yields.3 H COPh LiBr, Me2CO TeBr Me 49 COAr COAr H H NaI, Me2CO TeI TeCl R R 36 51 R=Ar=Ph (89%); R=H, Ar=Ph (75%); R=Me, Ar=Ph (93%) COAr H AgOCOCF3, C6H6 R TeOCOCF3 52 R=Ar=Ph (86%); R=Ph, Ar=4-MeOC6H4 (50%); R=Ph, Ar=4-MeC6H4 (63%) Quite unexpected results were obtained in the reaction of aryl b-chlorotellurovinyl ketone 36 (R=Ar=Ph) with AgBF4 (see Ref.3). b-Fluorotellurovinyl phenyl ketone 53 was isolated instead of the expected tetrafluoroborate. Obviously, this reaction results in the decomposition of the complex ion BF¡4 , but the reasons for this decomposition have not been established. COPh H COPh H AgBF4, MeCN TeF Ph Ph TeCl 53 36 I D Sadekov The condensation of aldehydes 45 with aromatic amines (boiling of equimolar amounts of reactants), which affords azomethines 54a,b, is the only well-studied reaction of (b-halo- genotellurovinyl)carbonyl compounds 45 involving the carbonyl group.15, 20 CH CHO NC6H4Me-4 MeOH, D +4-MeC6H4NH2 TeX TeX 45 54a,b (*90%) X=Br (a), I (b).Earlier,10 it was shown that the reaction of o-butyldibromo- tellurobenzaldehyde (55) with aromatic amines is accompanied by reductive elimination of butyl bromide to give azomethines 56. NAr CH CHO ArNH2 TeBr Te Bun Br Br 55 56 Ar=Ph, 4-MeC6H4, 4-MeOC6H4. 2-Methyldihalogenotellurocyclohex-1-enecarbaldehydes 26a ± c and aldehyde 56d react with aromatic amines in a similar manner. The yields of azomethines 24a ± d, 57 amount to 86%± 91%.15, 20, 21 NAr CH CHO MeOH, D, 30 min +ArNH2 7MeX,7H2O TeX Te Me 54a ± d (86% ± 91%) X X 26a ± c Ar=4-MeC6H4: X =Cl (54a), Br (54b), I (54c); Ar=4-MeOC6H4: X=Br (54d). NAr CH H CHO H ArNH2 7MeBr TeBr 4-BrC6H4 TeBr2Me 4-BrC6H4 57 26d Ar=4-MeC6H4, 4-MeOC6H4.This procedure for azomethine synthesis has certain advan- tages over those mentioned above, since it excludes the conversion of aldehydes 26 into the tellurenyl derivatives 45. o-Aminophenols 58 react with aldehyde 26b in a similar way: the yields of azomethines 59 vary from 78% to 85%.49 H2N R2 CHO MeOH, D, 15min + 7MeBr, 7H2O HO R1 Te Me 58 Br Br 26b N R2 CH R1 Te HO 59 Br R1=R2=H;R1=H, R2=Me; R1=NO2, R2=H. Reactions of 2-methyldibromotellurocyclohex-1-enecarbal- dehyde 26b and o-butyldibromotellurobenzaldehyde (55) with aromatic acid hydrazides proceeds in the same way as with aromatic amines. Short-term (10 min) boiling of equimolar amounts of the reactants results in N-aroylhydrazones of 2-bromotellurocyclohex-1-enecarbaldehyde or o-bromotelluro- benzaldehyde 60.50b-Telluroacroleins and tellurovinyl ketones: synthesis, reactions and structures CHO R2 MeOH, D +H2NNHCOAr 7R3Br,7H2O Te R3 R1 Br Br 26b, 55 NNHCOAr CH R2 R1 TeBr 60 (78% ± 90%) R1±R2=(CH2)4, R3=Me: Ar=4-BrC6H4, 4-MeOC6H4; R1±R2=(CH=CH)2, R3=Bu, Ar=4-MeOC6H4.6. Synthesis of tellurium-containing heterocycles b-Halogenotellurovinylcarbonyl compounds and their aza ana- logues were used as starting compounds in the synthesis of tellurium-containing heterocycles comprising one or two hetero- atoms in addition to the tellurium atom. a. Isotellurazoles The synthesis of isotellurazoles 35a ± f by the reaction of a-ace- tylenic ketones with hydroxylamine-O-sulfonic acid and potas- sium telluride (Section II.4) has significant disadvantages in comparison with other methods due to very low yields of isotellurazoles and the impossibility of obtaining compounds non-substituted in position 3.The procedure for the synthesis of isotellurazole from compounds of the type 45 is devoid of these drawbacks.20 Thus 4,5-tetramethyleneisotellurazole 35g was pre- pared in 70% yield by passage of ammonia through a solution of 2-bromotellurocyclohex-1-enecarbaldehyde 45 in benzene. The putative mechanism of this reaction involves the intermediate formation of imine 61; dehydrobromination of the latter under the action of ammonia yields isotellurazole 35g.21 CHO CH NH NH3 +NH3 7H2O TeBr TeBr 45 61 N Te 35g The synthesis of isotellurazoles from aldehydes 26 as starting compounds is preparatively more convenient, since it excludes the conversion of aldehydes into the tellurenyl derivatives of the type 45. Passage of ammonia through benzene solutions of tellurium dibromides 26b,d gave isotellurazoles 35g,h in *70% yields.21 Allowing for the fact that the reaction of tellurium dibromides 26b,d with aromatic amines proceeds through elimination of methyl bromide, a mechanism for the formation of isotellurazoles has been proposed.21 Its initial step includes the formation of imines 62, which are further converted into imines 61 by elimi- nation of methyl bromide; their cyclisation into isotellurazoles is described above.NH CH R2 CHO R2 H2O +NH3 7MeBr Te Me R1 Te Me R1 Br Br Br Br 26b,d 62 937 R2 NH CH R2 NH3 N Te R1 R1 35g,h TeBr 61 R1±R2=(CH2)4 (26b, 35g); R1=4-BrC6H4, R2=H(26d, 35h). This method for the synthesis of isotellurazoles is similar to that proposed by Renson for the synthesis of isoselenazoles.51 However, due to the high thermal stability of tellurodibromides 26 (s-telluranes of the type R1R2TeX2), the respective reactions are carried out at room temperature, while those of less stable selenium analogues should be carried out only at778 8C. b. N-Arylisotellurazolium salts N-Arylisotellurazolium perchlorates 63 were prepared in 78%± 86% yields by treatment of solutions of aldimines 54 in acetone orDMFwith the silver perchlorate ± acetonitrile complex in acetone.21 R2 R2 + AgClO4 NAr Ar ClO¡4 TeBr R1 R1 54 N Te63 R1±R2=(CH2)4: Ar=4-MeC6H4, 4-MeOC6H4; R1=4-BrC6H4, R2=H: Ar =4-MeC6H4, 4-MeOC6H4. c. 1,6-Dioxa-6a-tellurapentalenes and 1-oxa-6-aza-6a- tellurapentalenes Aryl b-chlorotelluro-b-methylvinyl ketones of the type 36 were used as starting compounds in the synthesis of 1,6-dioxa-6a- tellurapentalenes 64.6, 43 Condensation of these compounds with carboxylic acid chlorides in CH2Cl2 or MeCN in the presence of excess of a base (triethylamine, pyridine, 2,6-lutidine) gives heterocycles 64 (yields 11%± 85%). This reaction occurs with various (saturated, unsaturated, aromatic, heterocyclic, etc.) acid chlorides.O ClTe O Te O B, 20 8C or D +RCOCl Ar R Ar Me 64 36 Ar=Ph: R =Me, Ph, 4-O2NC6H4, 4-MeOC6H4; Ar=4-FC6H4: R =Me, Ph, 4-FC6H4, 4-O2NC6H4, 4-NCC6H4, 4-MeOC6H4, 2,4-(NO2)2C6H3, 3,5-(NO2)2C6H3, CH CH, O CH CH, 4-MeOC6H4CH=CH; S Ar=4-MeOC6H4: R=Me, 4-O2NC6H4, 4-MeOC6H4; Ar=4-Me2NC6H4: R=4-O2NC6H4, 4-NCC6H4. The use of imidoyl chloride 65 instead of acid chlorides has led to 1-oxa-6-aza-6a-tellurapentalene 66.43 O ClTe PhCCl MeCN, 2,6-lutidine, D, 3 h + NMe Me Ph 36 65 NMe Te O Ph Ph 66 (11%)938 d. 2-Aroyl-1,2,3-telluradiazines Dehydrobromination of aldimines is the key step in the synthesis of isotellurazoles. A similar approach was used in the synthesis of 2-aroyl-1,2,3-telluradiazines 67.50 These heterocycles were syn- thesised in 76%± 82% yields by treatment of benzene solutions of N-aroylhydrazones 60 with triethylamine.R2 R2 NNHCOAr N Et3N, PhH, D, 10min TeBr R1 R1 60 Te N COAr 67 R1±R2=(CH2)4: Ar=4-BrC6H4, 4-MeOC6H4; R1±R2=(CH=CH)2: Ar=4-MeOC6H4. e. 1,2,6-Oxatellurazocine benzo-derivatives Benzo-1,2,6-oxatellurazocine derivatives 68 were prepared by dehydrobromination of the corresponding b-bromotellurovinyl- aldimines 59 in the presence of an equimolar amount of triethyl- amine with subsequent short-term boiling of the reaction mixture.49 R2 N Et3N HO Te R1 Br 59 N R2 O Te R1 68 (71% ± 87%) R1=R2=H;R1=H, R2 =Me; R1=NO2, R2=H. 7. b-Trihalogenotellurovinylcarbonyl compounds The only available data in the literature concerns aryl b-trihalo- genotellurovinyl ketones X3TeC(R)=CHCOAr (X=Cl, Br).a. Synthesis The only known method for the synthesis of aryl b-trichlorotel- lurovinyl ketones 37 and aryl b-tribromotellurovinyl ketones 50, viz., the oxidative addition of chlorine and bromine to aryl b-chloro- (36) and aryl b-bromotellurovinyl ketones (49), respec- tively, is described in Section II.5.b. b. Reactions Reactions of aryl b-trihalogenotellurovinyl ketones were studied primarily for the chloro derivatives 37. The latter are reduced to bis(b-aroylvinyl) tellurides with lithium triethylborohydride and react with various nucleophiles. It should be noted that under the action of hypophosphorous acid as reducing agent tellurotrichlor- ides 37b,e are reduced to tellurenyl chlorides 36b,f.4 Of special interest is the striking difference between the structures of the reaction products of aryl b-trichlorotellurovinyl ketones 37 and tellurotrichloride derivatives containing no intramolecular coor- dinative bondsO?Te with nucleophiles.41 Reactions of the latter with nucleophiles result in the exchange of anionoid substituents for chlorine atoms or reduction of tellurium trichlorides to ditellurides, no detellurination occurs.The Michaelis addition of nucleophiles to the enone fragment of the ketones 37 proceeds with the elimination of the tellurium atom; sometimes, the trichlorides are reduced to tellurenyl chlorides. Reaction of tellurium trichlorides 37b,e with aqueous ammo- nia yields diketones 69, tellurenyl chlorides 36 and metallic Te.4 I D Sadekov COPh H MeCN, H2O, 20 8C +NH4OH 7Te TeCl3 R37b,e COPh H PhC(O)CH2C(O)R + TeCl R 69 36 Yield of 69 (%) Yield of 36 (%) R Compound 37 be 13 36 56 63 Me Ph Ketones 69 and methyl ethers of enols 70 (a mixture of Z- and E-isomers) are formed in the reaction of tellurium trichlorides 37b,e with triethylamine.4 COPh H Et3N, 20 8C MeOH±CH2Cl2 (1 : 1) TeCl3 R37b,e OMe O R PhC(O)CH2C(O)R + Ph 69 70 Yield of 69 (%) Yield of 70 (%) R Compound 37 56 80 38 15 Me Ph be The reaction of tellurium trichloride 37e with sodium benze- nethiolate which proceeds with elimination of the tellurium atoms yields the Z-isomer of b-phenyl-b-phenylthiovinyl phenyl ketone 71.The reaction of tellurium trichloride 37e with hydrazine gives a mixture of pyrazole 72 and tellurenyl chloride 36 (R=Ph).4 COPh H PhSNa, CH2Cl2, 20 8C 7Te COPh H Ph SPh 71 (74%) Ph TeCl3 COPh H N2H4, CH2Cl2 NH N 37e + Ph Ph TeCl Ph 72 (44%) 36 (22%) 8. b-Methyldihalogenotelluroacroleins The synthesis of b-methyldihalogenotelluroacroleins was consid- ered in Section II.2. Reductive elimination of methyl halides from these compounds results in b-halogenotelluroacroleins. Reaction with aromatic amines and acid hydrazides yields b-halogenotel- lurovinylaldimines (Section II.5.b). The use of b-methyldihaloge- notelluroacroleins in the synthesis of isotellurazoles was considered in Section II.6.a.III. Structures and spectral characteristics of b-tellurovinylcarbonyl compounds b-Tellurovinylcarbonyl compounds are convenient objects for the study of intramolecular coordinative bonds betweenOorNatoms and tellurium-containing substituents and the effects of these bonds on the structural and chemical properties of compounds under study.b-Telluroacroleins and tellurovinyl ketones: synthesis, reactions and structures The molecular and crystal structures of five b-tellurovinylcar- bonyl compounds of the general formula A have been studied 3, 4, 15, 31 using X-ray diffraction analysis (Tables 5 and 6). Y Te X R2 R1 A Compounds containing two-coordinate tellurium atoms (e.g., carbonyl derivatives 1b, 18c, 36 and azomethine 20) possess Z-configuration of the C=C bond, which ensures the formation of the intramolecular coordinative bonds O?Te and N?Te.The configuration of the substituents at the Te atom in the above- mentioned compounds can be described as a slightly distorted T-shaped structure, since the value of the angle X7Te7Y is close to 180 8 (164.4 8 ± 170.9 8). According to the N-X-L nomencla- ture,52 these compounds represent 10-Te-3-telluranes. The lengths of theC7Te bonds in the two-coordinate tellurium derivatives 1b, 18c, 36 and 20 (2.06 ± 2.119 A) and the values of the bond angles X7Te7C (93.6 8 ± 98.5 8) are similar to those of other two- coordinate tellurium derivatives.8, 9, 53 The five-membered tellurium-containing rings in compounds 1b and 20 are virtually planar; the phenyl rings in these com- pounds deviate from the planes of the heterocycles by 57.3 8 and 60.0 8, respectively.In addition to the presence of the intramolecular coordinative bond C=O?Te, the distinguishing feature of the trifluoro- methyl derivative 18c 31 is the interaction of the carbonyl atom of the ethoxycarbonyl group with theTe atom. However, the length of this bond (3.92 A) exceeds the sum of the van der Waals radii of O and Te (3.70 A54) and can therefore be neglected in the analysis of the polyhedron of the tellurium atom in this compound. The coordination polyhedron of the Te atom in tellurium tribromide 50c, a four-coordinate tellurium derivative, represents a trigonal bipyramid; the intramolecular coordinative bond O?Te is not taken into account.4 Here, the tellurium atom, the vinyl fragment and one bromine atom lie in the equatorial plane, while the other two bromine atoms are in the axial positions.The Te7Brequv. bond (2.513 A) is shorter than the Te7Brax. bond (the average length is 2.660 A). Taking account of the intra- molecular coordinative bond O?Te (2.362 A), the geometry of the bonds at the tellurium atom can be regarded as an octahedron. Table 5. The bond lengths in compounds of the type A (in AÜ: Compound R1 R2 X Y C7Te Te7X C(R1)=CH CH7C(R2) C(R2)=Y Y?Te w Ph C6H4OEt-4 3 O (CH2)2CO2Et OO 2.06 2.08 1b 18c 36 20 50c H H CF Ph Ph Ph C6H4OMe-4 Cl HPh C6H4OEt-4 Br3 O 2.101 2.124 2.14 2.47 NC6H4Me-4 2.119 2.138 2.175 2.513 a 2.660 b Note.w is the covalence factor (see text). a TheTe7Brequiv. bond length. b Average length of the Te7Brax. bond. Table 6. The values of the bond angles in compounds of the type A (in degrees). R2 X Y X7Te7 Te7C(R1)= C(R1)=CH7 CH7C(R2)= C(R2)= Com- R1 pound 7C(R1) =CH C6H4OEt -4 O (CH2)2CO2Et OO 119.4 126 113.7 120.5 114.0 98.5 94.1 93.6 NC6H4Me-4 95.6 O Ph H H CF3 Ph C6H4OMe-4 Cl Ph H Ph Ph C6H4OEt-4 Br3 95.0 a 1b 18c 36 20 50c a The Brequiv.7Te7C angle. 939 In addition to intramolecular interactions, the crystals of com- pound 50c, like those of aryltellurium tribromides,55 contain the intermolecular bond Te_Br (3.757 A), which enables binding of individual molecules into polymeric chains.Apart from bond lengths, the intramolecular coordinative bond O(N)?Te is characterised by the covalence factor (w), which is calculated by the formula 56 w=ÖRA á RBÜ ¡ ÖrA á rBÜ , ÖRA á RBÜ ¡ dAB where R and r are the van der Waals and covalent radii of the non- bonded elements A and B and dAB is the distance between the atoms A and B. Obviously, the higher the value of w the shorter the bond. The Te7Y bond lengths in b-tellurovinylcarbonyl compounds (see Table 5, compound 18c will be mentioned separately) depend on the nature of the substituent X and increase in the following order: Cl<Br3<C6H4OEt-4 (for Y=O)% C6H4OEt-4 (for Y=NC6H4Me-4). Depending on the nature of X, similar changes in the bond lengths corroborated by the results of quantum-chemical calculations 27 ± 30 were observed in the case of o-tellurinated phenylcarbonyl derivatives and (b-halogenotel- lurovinyl)aldimines.7 ±9 2-Trifluoroacetylvinyl 2-ethoxycarbonylethyl telluride (18c) presents special interest.31 The intramolecular coordinative bond in this compound (3.22 A, w=0.24) is the longest in the series of organotellurium compounds (e.g., the length of the O?Te bond in aldehyde 1b is 2.725 A). The reason for the significant elonga- tion of the O?Te bond in the ethoxycarbonyl derivative 18c seems to be due to the decrease in the electron density of the oxygen atom of the trifluoroacetyl group which, in turn, is determined by specific electron-withdrawing properties of the trifluoromethyl substituent. b-Halogenotellurenylvinyl ketones of the general formula RC(TeX)=CHCOAr were studied by 125Te NMR spectroscopy (Table 7).3, 57 Two main conclusions can be drawn from the data presented in Table 7.First, the chemical shifts of the Te atoms in these halogenotellurium derivatives are primarily affected by the elec- tronegativity of the halogen atom: the higher the electronegativity the more low-field the 125Te signal. The nature of substituents at Ref. 0.59 15 0.24 31 0.95 3 0.58 15 0.83 4 2.725 3.22 2.19 2.771 2.362 1.219 1.24 1.27 1.278 1.26 1.437 1.42 1.425 1.442 1.47 1.368 1.34 1.354 1.336 1.33 Y?Te7 Ref. 7C(R2) =Y =Y?Te 7X 168.3 103.2 15 7 7 31 315 4 123.7 124 116.8 120.7 118.4 122.4 112 118.6 125.1 121.1 170.9 164.4 7 114.2 104.5 111.9940 Table 7.The 125Te NMR spectra of compounds RC(TeX)=CHCOAr in CD2Cl2. R ArPh Ph Ph Ph Ph Ph Ph Ph Me Me Me Me Me Ph Ph Ph Ph Ph Ph 2,5-(MeO)2C6H3 a Bis(diethylcarbamato)tellurium served as the standard; the d 125Te values are given relative to Me2Te, these are calculated according to the formula d(Me2Te)=d[Te(S2CNEt2]2+833. the carbon atoms only weakly affects the d 125Te value (the maximum d value does not exceed*50 ppm). IV. Conclusion The data presented in this review suggest that the chemistry of b-tellurovinylcarbonyl compounds has progressed considerably in recent years.This can be exemplified by the development of procedures for the preparative synthesis of these compounds and azomethines prepared from them as well as by studies of the reactivity of b-tellurovinylcarbonyl compounds and the effect of the intramolecular coordinative bond O(N)?Te on the reaction features. The use of b-tellurovinylcarbonyl compounds and related aldimines has made it possible to develop preparative procedures for the synthesis of isotellurazoles, N-arylisotellur- azolium salts and novel tellurium-containing heterocycles, such as 5H-1,2-oxatelluroles, 1,6-dioxa-6a-tellurapentalenes, 1,2,3-tel- luradiazines and 1,2,6-oxatellurazocines. Further investigations in this field will presumably be carried out along the following lines: 1.Synthesis of novel b-tellurovinylcarbonyl compounds (ditellurides, telluronium ylides, tellurimides, telluronium salts). 2. In-depth studies of reactions of b-tellurovinylcarbonyl compounds which have been studied insufficiently even for known compounds. 3. Further studies of possible applications of compounds considered and newly prepared in the synthesis of miscellaneous tellurium-containing heterocycles. 4. Synthesis of complex compounds including metal chelates containing the coordinative site MN2Te2 from b-tellurovinyl aldimines. 5. Systemic studies of the effect of the nature of tellurium- containing substituents and donor sites (COR, CH=NR) on the lengths and stabilities of the intramolecular coordinative bonds O?Te and N?Te.This review has been written with the financial support of the Russian Foundation for Basic Research (Project No. 00-15- 97320) and INTAS (Grant 884-01). Ref. 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J. Gen. Chem. (Engl. Transl.) b�Dokl. Chem. (Engl. Transl.) c�Chem. Heterocycl. Compd. (Engl. Transl.) d�Russ. J. Org. Chem. (Engl. Transl.) e�Mendeleev Chem. J. (Engl

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年代:2002

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Russian Chemical Reviews 71 (11) 943 ± 968 (2002) Rational design of metal coordination compounds with azomethine ligands A D Garnovskii, I S Vasil'chenko Contents I. Introduction II. Azomethine metal chelates III. Molecular complexes with azomethine ligands IV. Bi- and polynuclear azomethine complexes V. Conclusion Abstract. coordination the in art of state the surveys review This This review surveys the state of art in the coordination chemistry amino(hydroxy)- azomethine chelating of chemistry of chelating azomethine systems, systems, viz., ., amino(hydroxy)- azomethines, and ketones, b-aminovinyl -aminovinyl ketones, b-aminovinylimines -aminovinylimines and their Variations analogues. selenium-containing and sulfur- their sulfur- and selenium-containing analogues.Variations in in the fine structure of azomethine ligands allow one to perform the the fine structure of azomethine ligands allow one to perform the targeted bi- mono-, molecular, and chelate of synthesis targeted synthesis of chelate and molecular, mono-, bi- and and polynuclear, homo- and heterometallic structures. The bibliogra- polynuclear, homo- and heterometallic structures. The bibliogra- phy references 425 includes phy includes 425 references. I. Introduction Advances in coordination chemistry are to a large extent associ- ated with the construction of new types of ligand systems. However, there are some well-known rather simple ligands, which can also allow progress in the chemistry of complex compounds relying on the principles of rational design (variation in the fine structure of ligand systems). Among these ligands are, for example, Schiff bases (azomethines) and their structural analogues, which are the `oldest' 1, 2 and most studied 3± 24 ligands in coordination chemistry.The complexes with these ligands have attracted interest for more than 150 years primarily because the nature of substituents in azomethine ligand systems 1 can be varied over a wide range and it is possible to prepare mononuclear metal chelates 2 exhibiting various physicochemical properties and containing coordination units with different geometry. R3 R3 R3 X X X H H R2 R2 R2 M/n N N N R4 R1 R4 R1 R4 R1 2 1a 1b R1, R2, R3, R4=H, Alk, Ar, HetAr; R2±R3=CH=CH±CH=CH, ;X=NR5, O, S, Se;R5=H, Alk, Ar, Ts; Ts=SO2C6H4Me-4.HC CH A D Garnovskii, I S Vasil'chenko Institute of Physical and Organic Chemistry, Rostov State University, prosp. Stachki 194/2, 344090 Rostov-on-Don, Russian Federation. Fax (7-863) 243 46 67. Tel. (7-863) 243 47 76. E-mail: garn@ipoc.rsu.ru (A D Garnovskii), vas@ipoc.rsu.ru (I S Vasil'chenko) Received 30 September 2002 Uspekhi Khimii 71 (11) 1064 ± 1089 (2002); translated by T N Safonova #2002 Russian Academy of Sciences and Turpion Ltd DOI 10.1070/RC2002v071n11ABEH000759 943 944 956 958 961 In earlier reviews 2 ±23 and numerous more recent papers, most attention has been concentrated on azomethine-based chelate compounds. However, not only chelates 2 but also other struc- tural types of metal complexes of chelate-forming azomethines have been much studied.20, 22, 24 ± 33 Taking into account the classification proposed by one of the authors of the present review,24 ± 26 complex compounds of Schiff bases and their analogues can be divided into the following three types: chelates 2; molecular complexes 3; bi- and polynuclear compounds (4 and 5) and polymeric structures (6).Solv MAn R4 X Y N X M M H R3 Y N N X Solv R1 R2 3 4 R1±R4=Alk, Ar; X, Y=NTs, O, S; M=Ni, Cu, Zn, A=Hal, NO3; M=B, Al, Sn, Ti, Zr, Hf, Th, Zn, Cd, Hg; n=2±4. M2An Me R4 X R3 M1/n N N R1 R2 O 5 M O N Me n 6 M=Co, Ni, Cu. All attempts to synthesise the fourth type of complexes, viz., p-complexes,24 ± 26 with azomethine ligands failed because, appa- rently, of a shortage of complex-forming agents and the lack of procedures for the synthesis, which could be used for the prepa- ration of these (predominantly organometallic 24, 34 ± 36) com- pounds. However, it should be noted that the reactions of N,N0- bis(salicylidene)ethylenediamine with typical p-complex-forming944 agents, viz., metal hexacarbonyls, afforded chelate compounds 7.34 O O (CO)4M M(CO)4 N N 7 M=Cr, W.Metal chelates involve deprotonated (anionic) di- or polyden- tately coordinated azomethine ligand systems. Neutral chelate- forming azomethine compounds 3, most of which are monoden- tately coordinated to the metal atom, serve as ligands in molecular complexes.Chelates 4 as well as compounds 5, which are adducts of chelate compounds 2 and Lewis acids (M2An),24, 30 and metal- containing azomethine polymers 631 ± 33 can be assigned to bi- and polynuclear structures. The present review surveys the data (published predominantly in the last decade) on the synthesis and structures of different types of azomethine metal complexes. Particular attention is given to the choice and modifications of ligand systems (rational design) and procedures for the synthesis of complexes of Schiff bases and their analogues, which are of importance in the preparation of compounds with the desired mode of arrangement of the coordi- nation bond and geometry of the metallocycles as well as in the construction of complexes of different nuclearity and with differ- ent degrees of oligomerisation.II. Azomethine metal chelates This section is devoted primarily to the employment of azome- thine compounds, which have an intramolecular hydrogen bond giving rise to a five- or six-membered coordination unit and contain varying aldehyde and amine fragments, as chelating ligands. Chelate compounds based on these ligands can be prepared by reactions of ligand systems with metal salts as well as with the use of template and electrochemical synthesis meth- ods.24 1. Chelate compounds with five-membered metallocycles a. Complexes with N,N-donor atoms Aromatic azomethine chelates containing the five-membered MN4 coordination unit became available in 1977.37 These compounds are prepared by the reactions of Schiff bases of monotosylated o-phenylenediamine with metal acetates (method a) 17, 37 ± 39 or metals under the conditions of electrosyn- thesis.The latter method involves the reactions of zero-valent metals (anode) with ligands in an electrolytic cell equipped with a platinum cathode (method b).39 ± 43 Compounds 8 contain aromatic 37, 38 and heterocyclic 38 ± 43 fragments as the substituent R. a Ts N Ts N H M/2 b N N R R 9 8 R=Ar, HetAr; Mare 3d metals. (a) M(OAc)2, MeOH, Me2CO; (b) M0,72e7, MeCN. In compounds 9, ligands containing the pyrrole substituents, which are capable of forming additional coordination bonds, can serve as both bidentate ligand systems 10 40, 41 and tridentate ligand systems 11.41 A D Garnovskii, I S Vasil'chenko Ts N Ts N L M M/2 N N N HN 10: M=Zn, Cd.11: M=Co, Ni; L=H2O. The quinone-imine groups can be involved in chelates 12 containing a five-membered coordination unit.44 ± 48 Ts N M/2 N NN N NR NR But But M/2 M/2 O 12 14 13 Complexes with Schiff bases of 2-formylpyrrole 13 5, 12, 20, 21, 49 ± 55 and 2-formylbenzimidazole 14 43, 56 ± 60 studied earlier belong to azomethine complexes containing the five- membered MN4 coordination unit. It should be emphasised that chelates of type 13 were generated under the conditions of chemical 5, 12, 20, 21 and electrochemical 49 ± 55 syntheses, whereas chelates 14 were prepared only by the electrochemical method 56 ± 60 because of the rather low acidity of the N7H bond in benzimidazole. This approach was also employed in the syn- thesis of chelate compounds 15 and 16 with the use of pyrrole- based aldimine ligands possessing nitrogen- 55 or sulfur-contain- ing 61 fragments.Tropone imine complexes also contain the MN4 coordination unit with the azomethine bond.20 N NMe2 S S N N Cd N N Zn N N N Me2N 15 16 b. Chelates with N,O-donor atoms Azomethines 17, which were derived from o-aminophenol and contain an intramolecular hydrogen bond, serve as bidentate ligands and form bicyclic chelate compounds 18 with the five- membered MN2O2 coordination unit.20, 22, 25, 62, 63 O M/2 N O M(OAc)2 R H 18 MeOH N O R M/2+ArCHO 17 NH2 19 R=Ar, HetAr; M=Cu, Mn, Co, Ni, Zn, Cd etc.These reactions, particularly those used for the preparation of copper(II) chelates 62 (18, M=Cu), are accompanied by partial hydrolysis of the exocyclic azomethine bond resulting in contam- ination of chelate compounds 18 with o-aminophenol complex 19. Because of this, copper chelate 18 (R=Ph, M=Cu) was pre- pared in the presence of an excess of benzaldehyde.62Rational design of metal coordination compounds with azomethine ligands The template synthesis of manganese complexes 20 64 is of interest. O Mn/2 R R N OH+XC6H4CHO+Mn(OAc)2 NH2 XC6H4 20 R=H, 4-NO2; X=H, 2-NO2, 3-NO2, 2-MeO, 4-MeO, 4-Me2N. The reactions of compounds 17 with acetates of other metals (Co, Ni, Zn, Cd) in methanol afforded virtually pure complexes with composition ML2.20, 22, 24, 65, 66 The exceptions are the palla- dium chelates PdL2, which were prepared 66 from azomethines 17 [R=C6H4R1; R1=H, 4-Cl, 4-NMe2, 2,4-(OMe)2] in an Me2CO±MeOH mixture.Of complexes 18 with varying substituents R, noteworthy are chelate compounds containing heterocyclic fragments, for exam- ple 21 67, 68 and 22 69 prepared by chemical and electrochemical methods. O M/2 O N M/2 R N N MeN HN 21: M=3d metals. 22: R=H, 5,6-C4H4-cyclo; M=Co, Mn, Ni, Zn, Cd. It was proposed (although this assumption was not supported by X-ray diffraction data) that complexes analogous to com- pounds 23, which are generated from o-aminophenol and hetero- aromatic aldehydes, can contain tridentate ligands.70 A O M/n N R N MeN 23 R=H, C4H4-cyclo; n=1, 2; A is absent or is NO3, OCOMe; M=Ni, Pb, Th, UO2.Compounds 17 whose aldehyde fragments possess substitu- ents capable of being involved in additional coordination can serve as tridentate ligands as well.71 Thus, the P,N,O-tridentate mode of coordination of the ligand was found in chelate 25, which was prepared from compound 24 and the [RuCl2 . 4DMSO] complex by the ligand-exchange reaction 24 in THF and structur- ally characterised. O O DMSO DMSO Cl Ru H [RuCl2 . 4 DMSO] N N PPh2 PPh2 24 25 In most cases, bidentate ligands 17 are involved in tetracoor- dinated complexes with composition ML2. However, monocyclic chelates 26 of the L1L2MCl type, which are composed of one monoanion of Schiff bases of o-aminophenol, were also synthes- ised 72 by the reactions of the corresponding ligands with the 945 [(R3P)(Cl)M(m-Cl)]2 (M=Pd, Pt), [(Z5-Cp)(Cl)Ir(m-Cl)]2 and [(4-PriC6H4Me-1)(Cl)Ru(m-Cl)]2 complexes in methanol.Cl O M L N R1 26 Me OMe (R2=H, OMe); C6H4NMe2-4; R1= Me R2 Me Fc; C5H4FeC5H5(Fc); C6H4NMe2-4; Me M=Pd, Pt; L=PR33 ; R3=Et, Bun, Ph; M=Ir; L=C5H4(Cp);M =Ru, L=4-PriC6H4Me-1. Recently,73 complexes 27 and 28 containing three metallo- cycles each were prepared by the template synthesis. O O O O M M N N N N H O R R R 28 27 c. Chelate compounds with N,S(Se)-donor atoms The synthesis of chelates containing the five-membered MN2S2 coordination unit involves the well-studied reactions of Schiff bases of o-mercaptoaniline with metal salts.20, 22, 24, 39, 74, 75 S S S H MA2 H M/2 ROH R N N NH R R 29 2 31 30 R=Ar, HetAr, Fc;M=Co, Ni, Pt, Pd, Zn; A is an anion.The reactions of Schiff bases of o-mercaptoaniline, which occur predominantly in the closed form of 2,3-dihydrobenzothia- zole 29,76, 77 with metal salts in alcohols (methanol, ethanol) afforded chelate compounds 30 containing the open deprotonated form of o-mercaptophenylazomethine 31 as the ligand system. In these chelates, aromatic 78 ± 87 or heterocyclic 39, 69, 75, 88 ± 90 as well as organometallic systems (for example, ferrocene 74) are most often involved as the substituent R. Chelate compounds 30 exhibit a specific type of valence tautomerism.83, 84 For example, heating of complexes 32 in toluene over a short period of time gave rise to cobalt and nickel 2,20-bis(1,2-diphenylethylenediimine)benzothiolates 33 through the formation of the C7C bond.S S S S 7 7 M M D, PhMe N N N N H H 33 32 M=Co, Ni. It should be noted that this transformation was not observed for chelates 30 containing the 3,5-di-tert-butyl-4-hydroxyphenyl fragments as the substituents R.86 In some cases, the reactions afforded tri- (34) 85 and tetrameric (35) 81, 82 structures along with complexes 30.946 N Fe S Pt Pt S S Pt N34 N S M N M S S M N M S N35 Most of chelates of tridentate azomethine ligands character- ised by the N,S-ligand environment are azomethine complexes prepared from heterocyclic aldehydes.39, 69, 75, 89 ± 90 Compounds 36 (X=NMe,39 S 69) serve as examples of this type of chelate compounds. OCOMe M S N N X 36 M=Pd, Zn, Cd; X=NMe, S.2-(2-Pyridyl)-2,3-dihydrobenzothiazole 89 and 2-methyl-2-(2- pyridyl)-2,3-dihydrobenzothiazole 75 act as donor ligands of the N2S type to give complexes 37 and 38, respectively. 2-(2-Pyridyl)- 2,3-dihydrobenzothiazole also serves as the tridentate ligand in tetracyclic chelates 39.90 Cl S PPh3 Ru N N Me 38 The complex formation involving bis-2,3-dihydrobenzothia- zoles 40 gives rise to tricyclic chelates 41 with composition ML, in which mercaptoazomethines act as tetradentate ligands.91 A D Garnovskii, I S Vasil'chenko S S R S S M Fe N N N R H NH 40 R R 41 N R=H, Me, Ph;M=Cd, Zn, Hg.The use of 2,6-dicarbonyl derivatives of pyridine 42a,b (LH2 ligands) as the aldehyde (ketone) fragment made it possible to prepare chelates with pentadentate ligands.88, 92, 93 The structure of complex 43 was established by X-ray diffraction analysis.92 Fe S HN R=Me Zn(OAc)2 N DMF S R R HN 42S S Zn N N N Me Me 43 If the nature of the ligand system in compounds 42 (R=H) and the reaction conditions are changed,94 the complex formation is accompanied by a redox process with the resulting changes in the type and denticity of ligand system 42 giving rise to chelate compound 44. AcO S Zn Zn(OAc)2 N N 42 (R=H) DMF:Me2CO (9 : 1) H N S 44 O Cl Cl Re S N N Apparently, chelate compounds prepared on the basis of 2,5- bis(2,3-dihydrobenzothiazol-2-yl)thiophene derivatives 45 95 can also be assigned to five-membered azomethine chelates containing N,S-donor centres.37 Se S S M/2 S N R R HN NH R 45 46 N . M=Zn, Cd; R=Ph, S N N S There are only a few complexes 46, which belong to azo- methine chelates containing five-membered metallocycles and the MN2Se2 coordination unit. These complexes were prepared by template synthesis from metal o-aminobenzeneselenolates and the corresponding aldehydes.96 Co S N N Similar reactions involving the o-aminobenzenetellurol che- late compound 97 afforded metal-free ditelluride rather than a tellurium-containing chelate analogous to complex 46.98 39 2.Chelate compounds with six-membered coordination units a. Complexes with N,N-donor atoms Not only numerous classical bis-b-aminovinyliminates 47 4, 12, 17, 20, 22, 24, 99 ± 104 but also monoanionic complexes 48 101, 105 ± 116 and binuclear structures 49 108, 109, 112, 114, 117 are examples of six-membered metal chelates prepared on the basis of bidentate azomethine N,N-donors 1 (X=NR5).Rational design of metal coordination compounds with azomethine ligands R1 R3 R3 R1 R3 R1 R1 R3 A R2 R2 R2 R4 R2 n A N MN NM N NM/n N NM N R1 R3 R1 R3 R1 R1 R3 48 47 R3 49 A=Hal, OAc. Most of chelate compounds 47 (n=2) typical of b-amino- vinylimines were obtained by the reactions of ligands 1 (X=NR5) with metal salts in alcohols.In this connection, of particular interest is the synthesis of the first structurally characterised tris- diiminate complex 47 (R1=R3=Me, R2=H, M=Al, n=3), which was prepared by the ligand-exchange reaction of 2-methyl- amino-4-methyliminopent-2-ene with Me3N. AlH3 in toluene.103 First publications on complexes 48 appeared in the mid- 1990's. These studies were devoted primarily to the directed synthesis of chelates, in which the metal centre is shielded by bulky substituents, and were aimed at constructing new catalysts of polymerisation and other processes. titanium,101, 105, 131 cop- b-Aminovinylimines 1 bearing bulky substituents (predomi- nantly, C6H3-2,6-Pri2) at the nitrogen atom were used for the preparation of this type of chelate compounds with lithium,113 barium,118 magnesium,119 ± 121 boron,107 aluminium,110, 122 ± 124 gallium and indium,110 scandium,125, 126 samarium and gadoli- nium,127, 128 ytterbium,129 tin,130 zirconium,101, 132, 133 vanadium,105 chromium,105, 134 per,106, 109, 111, 114, 115 ± 117, 135 zinc,108, 112, 114 platinum 116 and rho- dium.136 Chelates 48 were synthesised either from lithium complexes 50 (predominantly) or by the reactions of b-aminovinylimines with different complex-forming agents.Thus, a series of boron-containing complexes of b-diketimines 51 ± 54 were prepared.107 Me Tol Tol Me N N BF3 . OEt2 F B Li F N N Me Tol Me Tol 51 ( 16%) 50 Me Tol N MeLi Me B 7LiF F N Me Tol 52 + Me Tol Me Tol N N Me B(C6F5)3 2MeLi B B Me [MeB(C6H5)2]7 Me 72LiF N N Tol Me Tol 54 Me53 (70%) Tol=4-MeC6H4.Chelate 51, which was used as the starting compound in the synthesis of complexes 52 ± 54, was prepared by the reaction of BF3 . OEt2 with lithium derivative 50 and isolated from a solution in toluene. b-Aminovinylimines were used for the synthesis of complexes of aluminium, gallium and indium 48a,110 titanium and zirconium 48b 101 and chromium 48c.134 R3 R1 48a: R1=H, Alk, Ar; R2=H; R3=2,6-Pri2C6H3; R2 R4n NM N R1 R4=Me, Cl, I;M=Al, Ga, In; n=2 48b: R1=Me; R2=H; R3=4-MeC6H4; R4=Cl, NMe2;M=Ti, Zn; n=3 48c: R1=Me; R2=H, R3=2,6-Pri2C6H3; R3 48a ± c R4=Cl(O2CMe) .THF;M =Cr; n=1 947 The metal-exchange method was also used for the preparation of copper complexes with b-aminovinylimines.109, 111, 115 Note- worthy are the syntheses using lithium salts 55a,b and the CuCl2 . 0.8THF complex,111 i.e., procedures combining the metal- and ligand-exchange reactions.24 These reactions afforded mononuclear chelate compounds 56a,b containing the labile chlorine atom, which can be replaced by the OAr or CNAr groups.111 Me Me C6H3Pri2-2,6 C6H3Pri2-2,6 N N CuCl2 . 0.8THF R R CuCl Li THF N N Me Me C6H3Pri2-2,6 C6H3Pri2-2,6 56a,b 55a,b R=H(a), Me (b). Monovalent copper chelates 57 and 58 were synthesised analogously.109, 115 Me Me C6H3Me2-2,6 N RCH=CH2 , CuBr .SMe2 Cu N Me C6H3Me2-2,6 NTl NC6H3Me2-2,6 R C6H3Me2-2,6 Me 57 R=H, Ph.R1 R1 C6H3Pri2-2,6 C6H3Pri2-2,6 N N [Cu(MeCN)4]CF3SO3 Cu(MeCN) Li R2 R2 N N R1 R1 C6H3Pri2-2,6 C6H3Pri2-2,6 58 R1=H, Me, But; R2=H, Ph. Copper chelate 59 114 and zinc chelate 60 112 were synthesised from the corresponding ligands and Cu(OAc)2 or ZnMe2, respec- tively. The structures of virtually all chelate compounds described above were established by X-ray diffraction analysis. Me C6H3Pri2-2,6 N Cu(OAc)2 Cu(OAc) N Me C6H3Pri2-2,6 Me C6H3Pri2-2,6 N 59 Me NHC6H3Pri2-2,6 C6H3Pri2-2,6 Me N ZnMe2 ZnMe N Me C6H3Pri2-2,6 60 Binuclear complexes 49 were prepared primarily according to analogous methods, viz., by the metal- or ligand-exchange reac- tions or with the use of a combined procedure.Thus, the reaction of aminovinylimine 61 with copper(II) chloride in alcohol gave rise to copper complex 62, which was isolated and structurally characterised.117 C6H3Pri2-2,6 N NHC6H3Pri2-2,6 CuCl2 Ph Ph CuCl/2 EtOH N N 62 61 C6H3Pri2-2,6 C6H3Pri2-2,6 Lithium complex 55a was used for the preparation of bridged binuclear zinc chelates 49 (M=Zn; A=m-Cl, m-Br, m-OAc).108948 Binuclear complex 63, which served as the starting compound for the preparation of chromium-containing chelates 64 ± 67, was prepared by the ligand-exchange reaction of lithium salt 55a with the CrCl3 . (THF)3 complex in THF.134 Ar Me Me Ar Ar Cl Me Cl N N N Cr Li Cr N N N Cl Me Cl Me Ar Me Ar Ar 55a 63Ar Me Ar Me Me Cl N N Cr Cr N N Cl Me Me Me Ar Ar 64 Ar Me Ar Me Cl N N Cr Cr N N Cl Me Me Ar Ar 65R Ar Me O O N Cr THF N Me Cl Ar 66 Me Ar R O N Cr O N Me R Cl Ar 67 Ar=C6H3Pr2i-2,6. In some cases, binuclear complexes 49 were synthesised with the employment of specific techniques.For example, structures 49 (R1=Me, R2=H, R3=Ar, M=Zn; A=H, F, OAc, OH, OMe) were prepared from mononuclear zinc complexes 48 [R1=Me, R2=H, R3=Ar, M=Zn; R4=Me, Et, N(SiMe3)2] and reagents, which were used for the formation of bridging fragments.108, 112, 137, 138 Recently,139 it has been demonstrated that the reactions of b-aminovinylimines with CuCl2 in alcohols can give rise to dimeric complexes 68 in which the aminoimine ligand is monodentately coordinated to the metal centre.R HPh Cl N H R Cu O N N O Cu R Cl H N PhH R 68 Chelates 69,17, 20, 22, 24, 39, 99, 140 ± 145 70,146 71 45, 147 and 72 148 take an important place among chelate compounds containing the A D Garnovskii, I S Vasil'chenko six-membered MN4 coordination unit, which were prepared on the basis of N,N-donor bidentate azomethine ligand systems. R4 R3 R3 N N R2 M/2 N M/2 N N N R1 R2 70 69 R1 Ts N M/2 N N TiCl2 N But R 2 71 O But 72 These complexes were synthesised predominantly by the direct reactions of the corresponding ligands with salts (predominantly, acetates) of 3d metals.In some cases, chelates 69 were prepared by template 140 ± 142 or electrochemical 39, 143, 144 synthesis methods. It should be noted that azomethines based on N-unsubstituted 2-aminobenzaldehyde and aliphatic (aromatic) monoamines are unstable. Because of this, most of the syntheses were carried out with the use of 2-monotosylaminobenzaldehyde imines as precur- sors.140 ± 145 Complexes 72 (R=Ph, C6H3F2-2,6, C6H2F3-2,4,6) were prepared by the reactions of the lithium salt of 7-(N- aryliminomethyl)indole with titanium tetrachloride.148 The Schiff base derived from 2-aminobenzaldehyde and ethylenediamine is quite stable. Hence, this Schiff base and sources of complex- forming metal were used for the preparation of chelate com- pounds 73a149 with tetradentate ligands.R R Ph N NPh N N N HN N HN M Ni Ph Ph N N N (CH2)2 74 N (CH2)2 73a ± c R=H, Me; M=Ni (a), Cu (b), FeCl(Br) (c). Tricyclic chelates 73a 149 were prepared by the reactions of the corresponding ligands with nickel acetate in methanol. Complexes 73b were synthesised by the reaction of a suspension of Cu(OMe)2 in methanol with the ligand. Chelates 73c were prepared by the reactions of FeCl3 .6H2O or FeBr3 with the corresponding ligands in acetone.150 The synthesis of nickel complex 74 was described in the study.151 b. Chelate compounds with N,O-donor atoms Metal chelates of bidentate hydroxyazomethines 1 (X=O) belong to the best-studied class of chelate compounds containing the six-membered MN2O2 coordination unit.2, 4, 7, 9 ± 16, 18 ± 25 These compounds still attract interest today.Considerable atten- tion is given to the synthesis and properties of chelate compounds with poorly studied metals, viz., aluminium,152 scandium and yttrium,153 titanium,154 ± 156 rhenium 157 and molybdenum.158 Aluminium(III) chelate compounds were synthesised from azo- methine 75 and Me3Al. These reactions can afford not only neutral chelates 76 but also cationic complexes 77. The structure of coordination compound 77 was established by X-ray diffrac- tion analysis.152Rational design of metal coordination compounds with azomethine ligands But AlMe3 PhMe But NR OH 75 But B(C6F5)3 But THF NR O Al 76 Me Me R=C6H3Me2-2,6, C6H3Pri2-2,6, C6H3(CF3)2-3,5 etc.The reactions of Schiff bases 78 with organometallic deriva- tives 79a,b gave rise to scandium and yttrium chelate compounds 80a,b.153 OH N +M(CH2SiMe2Ph)3(THF)2 78 O N M CH2SiMe2Ph N O 80a,b M=Sc (a), Y (b). Titanium chelate complexes 81 were synthesised by the metal- exchange method from lithium salts of the ligands and titanium tetrachloride.154 ± 156 But 2 OLi R1 N R2 R5 R3 R4 R1±R5=H, F. R1 R2 OH Zn[N(SiMe3)2] PhMe,778 8C Pri N Pri Chelates of Re(V) 82 157 and Mo(VI) 83a,b 158 were prepared with the employment of unusual sources of complex-forming metals. R2 O R2 [NBu4][ReOCl4] H + But NR1 R1=Ph, cyclo-C6H11; R2=H, Cl.[MeB(C6H5)3]7 But NR O OH+K2Na[Mo(CN)4O2] .6 H2O+H2NR Al O THF 2+ Me 77 O CN O Mo CN N CN R 83a,b R=Me (a), (CH2)nNH2; n=2, 3 (b). C6H14 0 8C, 24 h 79a,b An important line of investigation in the chemistry of com- plexes with hydroxyazomethine ligands deals with the synthesis and studies of chelate compounds 2 (X=O) containing various substituents R1±R3, including fragments annelated to the metal- locycle. As an example we refer to the synthesis of zinc complexes 84 ± 86 (Scheme 1).159 The reactions of azomethines, which were prepared from salicylaldehyde and chiral a-phenylethylamines, with zinc acetate afforded optically active zinc complexes 87.160 O H N +Zn(OAc)2 H Me But R2 R1 Cl O Ti/2 TiCl4 N R1 Et2O R2 R5 R3 R4 2 Metal chelates in which heterocyclic fragments are annelated to metallocycles were examined in the studies.20, 22 For example, benzofuran derivatives of chelate compounds 88 were pre- pared.161, 162 Chelates 89 and 90 were synthesised on the basis of b-aminovinyl ketones 163 ± 168 or o-hydroxybenzaldimines 169, 170 and the nitroxyl imidazoline radicals and their magnetic pro- perties were studied.Complexes 89 and 90 were prepared 81 R1=H, R2=NO2 O O2N Zn/2Pri N Pri 84 But But OH N(SiMe3)2 But But O But R1=R2=But Zn/2Pri N PhH, 20 8C Pri 85 949 R2 O R2 ReOCl NR1 2 82 (PPh4)27 O Zn/2 N NaHCO3 H EtOH, D Me R2 R1 87 Scheme 1 But But O O Zn But Pri N Pri 86950 predominantly from the corresponding ligands and metal acetates in aqueous ethanol.R1 O M/2 R2 X N Me Me Me Y M/2 N N Me Me Me O R 88: X,Y=O; 90: M=Co, Ni R=Alk, Ar; M=Co, Ni, Cu 89: R1=H, Me; R2=Alk, CF3, Ph, CO2Et, CONH2; M=Co, Ni, Cu, Zn, Hg, Cd In some cases, o-hydroxyazomethine chelates were synthes- ised by the electrochemical method.24, 43, 60, 171, 172 An increase in denticity of the hydroxyazomethine ligands can be achieved by the introduction of the substituents R containing centres which can be involved in additional coordination, in particular, the alkylamino, hydroxy, alkoxy or alkylthioalkyl fragments as well as their aryl analogues. Such ligand systems are involved in complexes 91 ± 94.12, 24 Y(CH2)n X N M N X 91 (CH2)nY Y (CH2)n X N M X N X H 92 (CH2)nY N (CH2)nY Solv X M N Y 93 (CH2)n Y (CH2)n X N M 94 N X (CH2)n Y X=O; Y=NR2, OR, SR; R=H, Alk, Ar; n=2±4.Salicylaldehyde-based azomethine complexes have been studied for many years.12, 173 In these complexes, additional coordination can occur through the dimethylaminoalkyl substitu- ents. For example, a ligand of this type is present in the structurally characterised manganese chelate 95.174 SC O O N Mn(OAc)2, NaNCS O H KOH, MeOH Mn O N NMe2 N NMe2 Me Me 95 This type of metal binding was observed in complex 96.175 A D Garnovskii, I S Vasil'chenko But But Me But O O But Me Al H AlMe3 PhMe, 20 8C, 12 h O N N NMe2 NMe2 M/2 96 N Me By contrast, X-ray diffraction analysis demonstrated that the N Me O nitrogen atom of the aminoalkyl substituent is not involved in additional coordination in molybdenum complexes 83b synthes- ised by the template method.158 Among chelate compounds 91 ± 94, complexes of salicylide- neamino alcohols have received the most study.Early investiga- tions devoted to their synthesis, structures and properties have been surveyed in reviews.176 ± 178 Recently, chelate compounds titanium(IV),179 of vanadium(V),180, 181 dioxomolybde- num(VI),180, 182, 183 cobalt(III),184 nickel(II) 185 ± 187 and cop- per(II) 186 were prepared and characterised. Titanium complexes of type 93 containing the deprotonated OH groups were synthes- ised from the ligands and Ti(OPri)4.179 Complexes 93 with vanadium 180, 181 and molybdenyl 182, 183 were prepared by the ligand-exchange reactions with the use of acetylacetonates of these metals. As an example we refer to compound 97.O Ti/2 O N97 The structures in which theOHgroups are retained are typical of nickel (98) 186 and copper (99) 187 complexes synthesised by the template method. H2N(CH2)2OH O O S EtOH, D, 2 h Ni/2 S Ni/2 N O 2 98 (CH2)2OH 2 O Cl Cu CuCl2 .H2O OH+H2N OH N MeOH, 20 8C CH2OH Me CH2OH O Me HO 99 In the fluorine-containing barium b-ketoiminate complexes Ba[CF3COCH=C(NR)CF3]2 [R=CH2(CH2CH2O)2, (CH2)2. .(CH2CH2O)2 or CH2CH(CH2CH2O)3], which were prepared from the corresponding ligands and BaH2 in a mixture of heptane and THF,188 the metal atom is coordinated by the oxygen atoms of the alkoxy groups.Most of the complexes prepared on the basis of mercapto- alkylimines are bi- 189, 190 or trinuclear.190 However, Zelentsov and Suvorova 191 expected that the mononuclear nickel complex could be isolated. Iminothiol complexes 100 were prepared by the electrochem- ical cleavage of the disulfide bond.192 O O MeCN Sn/2 H +Sn0 7e7 N S S N 2 2 100Rational design of metal coordination compounds with azomethine ligands Salicylideneanilines 101 containing various proton- and elec- tron-donating groups YR in the ortho position of the N-aryl fragment are very widespread azomethine tridentate ligands. OH N YRn Y=NTs, P, O, S; R=H, Alk, Ar.101 Although most of the complexes prepared on the basis of ligands 101 have binuclear structures (see Section IV), mono- nuclear chelates (predominantly of type 102) were prepared with the employment of some complex-forming agents. Thus, mono- nuclear complexes of boron,193 aluminium,175 tin,194, 195 rhenium,196 ± 198 molybdenum,199 tungsten,199 cobalt,200 ± 204 nickel,201 ± 208 copper 201, 203, 207, 208 and cadmium 202, 209 are known. A series of chelate compounds under consideration were synthesised by the direct reactions of compounds 101 with differ- ent sources of metals, viz., AlMe3 (in PhMe), tin, nickel, zinc, copper and cadmium salts (in alcohols). Analogously, boron complexes 103 were synthesised by the reactions with B(OR)3, where R2=Me, Et, Pr or Bun, in a mixture of toluene and THF.193 OR2 Ln O O R1 B M O N Y N 102 103 NH2 102: Y =NTs, P, O, S; L=Py, bipy, etc..; n=1, 2; NH2 103: R1=H, Cl, Br; R2=Me, Et, Prn, Bun.Oxorhenium chelates 102 (M=ReO) were prepared by the ligand-exchange reactions of [ReOCl3(PPh3)2], compounds 101 and ligands L. In particular, 2-(2-hydroxyphenyl)benzo- oxazole,197, 198 2,3-dihydro-2-(2-hydroxyphenyl)benzothiazole 197 and triphenylphosphines containing the proton-donating group (OH or CO2H) in the ortho position of one of the benzene rings 198 were used as the ligands L. Dioxomolybdenum and dioxotungsten complexes were prepared 199 from Schiff bases 104 and 105 and acetylacetonates of MoO2 and WO2, respectively (in MeOH).R R O H O H OH N CH2OH N 105 104 R=H, 5,6-C4H4-cyclo. The electrochemical method is widely employed for the preparation of chelate compounds 102.200 ± 204, 207 This method proved to be particularly useful in the case of chalcogen-contain- ing tridentate azomethines. Thus, chelates 106 were synthesised from the corresponding disulfides and diselenides.194, 210 O OH 74 e7 Sn/2Y N +Sn0 Y N MeCN, Me4NClO4 2 106 2 Y=S, Se. 951 An important advantage of this method is that it allows one to use various N-donor ligands,200 ± 208 including ambidentate 1-alkyl-2-aminobenzimidazoles.201 ± 204 Chelates 102, like other complexes, can be prepared by the template electrochemical method.202 O NHTs M0,72 e7 MeCN, Et4NClO4, 20 8C HO + NH2 L O M NTs N 102 M=Ni, Zn;L=H2O, MeCN.It should be noted that the donor centres Y in azomethines 101, like those in other compounds containing N-donor fragments at the nitrogen atom,24 are not necessarily involved in coordina- tion to the metal atom. Thus, X-ray diffraction analysis demon- strated that the nitrogen atom of the pyridyl fragment in chelates 107, which were obtained from the potentially tridentate pyridine- substituted azomethine by the reactions with metal salts or under the conditions of the electrochemical synthesis, is not bound to the metal atom.211, 212 O O H M/2 M0,72e7 or M(OAc)2 N N N N 107 M=Cu, Ni.By contrast, X-ray diffraction analysis revealed the Zn ±Npy interaction (the distance is*2.8 A) in the analogous zinc chelate 108.213 It was suggested that additional coordination also occurs in complex 109.214 O M/2 O N Zn/2 N N N S N Ar 109 (M=Cu, Ni) 108 The difference in the behaviour of the coordinatively active pyridyl substituent in complexes 107 and 108 was attributed to the instability of four-membered metallocycles.215 Actually, stable bicyclic chelate compounds with tridentate ligands were obtained under conditions favourable for the formation of five-membered coordination units due to the insertion of the methylene group (complex 110) 175 or the quinoline substituent (complexes 111 216 and 112 217). But Ts N OAc Me Cu But O Me Al N N N N 111 110 Bissalicylidene derivatives of aliphatic, aromatic and hetero- cyclic diamines 113 2, 9, 12, 20 ± 22, 24, 218 serve as tetradentate N,O- donor azomethine ligands, which gave rise predominantly to tricyclic chelates 114.952 R2 Ar R2 O O O Ni/2 H H N N N N R R1 R1 Z 113 112 R2 R2 An O O M N N R1 R1 Z 114 R1=H, Alk; R2=H, Alk, Ar, Hal, NO2, OAlk; A=Alk, Hal; Z=(CR3R4)m, Ar, HetAr; R3, R4=H, Alk, Ar; n=0, 1, 2.In recent years, chelate compounds 114 with alumi- nium,218 ± 221 gallium,218, 222 indium,218, 222 ± 225 tin,226 titanium,227 manganese,228 ± 237 vanadium,238, 239 chromium,230, 240 molybde- num,241 cobalt,236, 241 iron,231, 236, 241 nickel,231, 236, 241, 242 ruthe- nium,243 ± 245 palladium,246 osmium,247 copper 231, 236, 246, 248 ± 250 and zinc 251, 252 have been prepared.Aluminium, gallium and indium complexes were synthesised by the reactions of ligands 113 with alkyl and other derivatives of these metals.218 Chelate compounds of Group III metals were prepared with the use of a mixture of acetonitrile and THF221 or cyclohexane 222 as the solvent. Tin complexes of the general formula R2SnL (R=Me, Bun, Ph) were synthesised in benzene with the addition of triethylamine under an atmosphere of nitrogen starting from the corresponding tin(IV) dichloride and tetradentate Schiff bases derived from salicylaldehyde, 3-methoxysalicylaldehyde, 4-ben- zoyl-3-methyl-1-phenylpyrazol-5-one, o-phenylenediamine and 1,3-propylenediamine.226 Complexes 114 (M=Mn: A is either absent 236 or is ClO4,228, 233 Cl,229, 231, 237 H2O;229 M=Cr: A=Cl 240) were pre- pared from ligands 113 and the corresponding manganese or chromium salts in alcohol. Adducts of bis(salicylidene)ethylenedi- amine chelates of manganese and chromium with atomic nitrogen (115) were synthesised from azido complexes.230, 253 N3 N hn O O O O PhH M M N N N N 115 M=Mn, Cr.Osmium nitride complexes 114 [M=Os: R1=H, R2=H, Cl, Me, OMe; A=N, MeOH; Z=(CH2)2] were prepared by the reactions of the corresponding ligand 113 with [NBu4][Os(N)Cl4] in MeCN or MeOH.247 The direct reactions of the reagents were used in the synthesis of chelates 114 containing nickel (M=Ni; A is absent),242 ruthenium (M=Ru; A=Cl),244 copper [M=Cu; A is absent; R2=5-(N=NPh),249 OMe250] or zinc [M=Zn; A=THF, Py; R2=3,5-But, 3-But-5-(4-Py); L=Py or absent].251, 252 Ruthe- nium chelate compounds were also prepared by ligand exchange in the RuCl2(PPh3)3 complex.245 The same synthetic approach R2 R2 O O H H N N R1 R1 Z 116 R1=Me, CF3; R2=H, Me, CMe3; Z=(CH2)2, (CH2)3, MeCHCH2, CH(CH2)4CH.was applied in the preparation of chelates of Ru(II) 254 and Ru(III) 255 from bis-b-aminovinyl ketones 116. Ruthenium chelate compounds 117 were synthesised by combining the ligand- and metal-exchange reactions 254 with the use of the triphenylphosphine complex RuCl2(PPh3)3 and its arsine analogues RuY2(AsPh3)3 (Y=Cl, Br) as the source of metal.255 X3C ONa NaON N Me X=H, F.Copper and nickel chelate compounds with ligand 116 [R1=H; R2=C6H4OMe-4; Z=(CH2)3, 1,2-C6H10-cyclo] were synthesised in methanol with the use of acetates of the corre- sponding metals.256 A series of studies were devoted to modifications of ligand systems of type 113 by varying the Z bridge (for example, complexes 118,257 119,258 120,259 121 260 and 122 260) and aldehyde fragment (complexes 123 226 and 124 261). N O M N O 118:M =Ni, Cu. R But N O M Fe O N But R 121: R =H, But. Ph N R1 R1 N O O Sn Me N N Ph R2 123: R1=Bun, Ph; R2=(CH2)3, C6H4-o. It was demonstrated 262 ± 264 that potentially penta- or hexa- dentate N,O-donor azomethine systems containing the bis(salicy- lidene)ethylenediamine fragment, like the above-considered compounds 118 ± 123, form chelates with tetracoordinated ligands, such as complexes 125 262 and 126.264 Chelates 117 and 120 ± 125 were synthesised from the corresponding ligands and various metal sources.Complex 118 was prepared by the template method. A D Garnovskii, I S Vasil'chenko CX3 CX3 X3C PPh3 O O Ru THF, 25 8C, RuCl2(PPh3)3 PPh3, NaCl N N Me Me Me 117 PPh3 N O N O N N O M M N N O N O N 120:M =Ni, Cu, Co, Zn, Cd. 119:M =Ni, Cu, Co, Zn, Cd. R But N O Ph Zr Fe Ph O N But R 122: R =H, But. Ph O O N N H2O O La H2O Me OH2O OH2 O O Ph N N 124953 Rational design of metal coordination compounds with azomethine ligands Cl The use of chelates 128 enables one to construct coordination Me OMe compounds 127 containing various substituents at the nitrogen atom, viz., aryl,267, 268 pyridyl,269 methoxy 270 and phenylamino groups,271 as well as alkylthiovinyl-bridged complexes 129.272 The method a is more universal than the method b because it H OMe O O N N O O U ONO2 Mg HOMe OH2 allows one to prepare not only chelate compounds 127 containing the MN2S2 coordination unit but also complexes of Schiff bases with the MN2Se2 chelate unit.273 O N O O N O O Chelates 127 can also be prepared starting from bisazome- thine disulfide 130 and iron pentacarbonyl.274 S S S +Fe(CO)5 Cl 126 125 Cl Fe/2 N N N 127 R R R 130 R=C6H4OMe-4.According to the X-ray diffraction data,261 the ligand in complex 124 is coordinated through the crown fragment rather than through the azomethine group.It should be emphasised that ligands in complexes 124 ± 126 can be considered as ambidentate ligand systems.27 Taking into account that the lanthanum cation is the hardest acid of all the cations involved in these complexes,24 the structure assigned to complex 124 seems to be justified. Complex compounds 131 synthesised from benzotellurazole 132 can be considered as tellurium-containing analogues of chelates 127.275 OC Fe(CO)3 N N Fe Fe3(CO)12 2 Te Te 75 CO N Te 132 Fe(CO)3 131 c. Chelate compounds with N,S(Se)-donor atoms In 1968, the template synthesis (method a) of metal chelates 127 of sulfur-containing bidentate Schiff bases generated from the sodium derivative of o-mercaptobenzaldehyde was carried out by two independent research groups.265, 266 These compounds were the first representatives of azomethine chelates containing the MN2S2 coordination unit.Method a Tridentate sulfur-containing azomethine complexes 133 were first prepared according to an original procedure employing the reactions of nickel iodide with Schiff base 134.20 R2OH XNa X +R1NH2+M(OAc)2 I NiI2, D M/2 SMe S O N Ni 127 R1 N N NEt2 NEt2 134 133 X=S, Se; R1=Alk, Ar, AlkO etc.; R2=Me, Bun; M=Co, Ni, Cu, Zn etc. Tetracoordinated nickel complexes 135 were synthesised analogously.276 Complexes 127 are also often prepared by the reactions of stable mercaptosalicylaldehyde chelates with amines and their derivatives (method b).20, 24 HN N N Method b NiCl2, D RNH2 S EtOH or CHCl3 M/2 ButS SBut N Cl R R=Ar, Py S NH2 Ni H2NOMe NH N S MeOH 135 M/2 S N M/2 O OMe Complexes 136,277 137 274 and 138 278 were constructed by the template method.128 H2NNHPh + S SH O CHCl3, EtOH S M/2 H2N(CH2)2NH2 Fe/2 Cl7 N FeCl3 N NH2 NHPh NH2 136 2 H2NN=CSAlk . HI Fe/2 S S EtOH, H2O, NaOH H2N(CH2)2NH2 O S M S S Fe MeO(CH2)2OH N N N N 137 SAlk 129 M=Co, Ni, Cu, Zn.A D Garnovskii, I S Vasil'chenko 954 Py X Ni/2 Ni XH S O S N Py, D + Investigations of the synthesis, structures and properties of chelate compounds of heteroaromatic Schiff bases containing N,S(Se)-donor centres 146 ± 150 were started in the late 1960s ± early 1970s.8, 161, 287 ± 290 A NH2 X X X X 138 A A M/2 M/2 M A X=NTs, O, S.N N N N R 148 147 146 Z R A=O, S; X=S, Se; M=Co, Ni, Cu, Zn. The template synthesis 279 was used for the preparation of chelates of tetradentate N,S-donor azomethine ligands 139,274 140 280 and 141.281, 282 These complexes have still attracted attention in subsequent CO years.20, 22, 24, 267, 269, 291 ± 307 S S CO Fe 137 Py N N Py 139 Azomethines derived from thio-substituted heterocyclic alde- hydes (for example, compound 149a), which exist in the thione- amine form 149b,77 are more stable than their aromatic analogues. Because of this, chelate compounds 150 are synthesised primarily by direct reactions of the corresponding ligands with metal salts.24 O O R3 R3 2+ N R2 N R2 NR3 R2 HN NH2 MAn H M/2 2 Cl7 CH2Cl2 + S S Ni/2 R4OH N N N S S SH NR1 NR1 NR1 2 NH2 150 149b 149a + HN2 S Cl7 Ni N NH 140 O (CH2)nN N NMe2 Ni S S S H2N(CH2)nNH2, Ni(ClO4)2 .6 H2O DMF, 100 8C O O O O In some cases, chelates of type 88 (X=S, Y=O) were prepared by template synthesis.161 Of the chelate compounds under consideration, complexes 150 containing the pyrazole fragment annelated to the metallo- cycle deserve most attention.20, 288 ± 307 In these complexes, d metals (Co, Ni, Pd, Cu, Zn, Cd, Hg) act as complex-forming agents.20 The synthesis of copper complexes of this type has presented substantial difficulties for many years.The first repre- sentative of these complexes (150, M=Cu; R1=Pri; R2=Me, R3=2-Py) was prepared as late as 1992.269 Later on, these chelates became readily available.299 ± 301, 303 ± 307 141 n=2, 3. Some chelate compounds with tridentate ligands 149 [R3=(CH2)2OH,185, 308 2-pyridyl,269, 294, 309 8-quinolyl 298 ± 300] were synthesised. The structure of compound 151 was unambig- uously established by X-ray diffraction analysis.298 + The synthesis and structures of the structural analogues of these chelate compounds, viz., chelates 142 and 143, were described in the studies cited in review 20 and monograph.24 (CH2)n R2 R2 (CH2)2 N N Me Me N N N N A7 OH N M M S S N Ni/2 FeIII/2 N S S S X MeS SMe R1 R1 143 142 RN NPh 2 151 152 n=2 ± 4;M=Ni, Cu. M=Co, Ni, Zn; R1=Alk, Ar; R2=H, Alk.N R1 N N M/2 S RN2 153: R1, R2=Alk, Ar;M=Ni, Cu. Let us emphasise that the rational design of chelate com- pounds of azomethine N,S(Se)-donors is of fundamental impor- tance in the synthesis of complexes with selenium-containing ligands (for example, 144 283) or complexes 145 20 containing alterdentate ligands, which are difficult to prepare.284 ± 286 The latter complexes play an important role in studies of the stereo- dynamics of tetracoordinated azomethine structures.286 Pri N The coordination of the oxygen atom of the hydroxy group to the metal atom is observed in complex 152.185 By contrast, the potentially tridentate pyridine-containing ligand in compound 153 is in fact (according to the X-ray diffraction data) biden- S Se tate.269, 292, 294 M/2 Zn/2 N N Pri Pri 144 145:M=Zn, Cd.Nickel complexes of 154 296 and 155 291 and iron complex 156 298 can be cited as examples of chelates with tetradentate N,S- donor ligands. These chelates were synthesised by the reactions of the corresponding ligands with metal salts [Ni(OAc)2, MeOH; FeCl3, EtOH].Rational design of metal coordination compounds with azomethine ligands R1 But Me N N N NR2 N NPh O S S S N N N R Ni Ni Cl Fe N N N S S S O NR2 N NPh N N N R1 Me But 155: n=1±5. 156 154: R=Alk; R1, R2=Me, Ph. Examples of heteroaromatic azomethine chelates containing the six-membered MN2Se2 coordination unit are known, among which are complexes of nickel 157 309 and copper 158.304 A C6H11-cyclo Me n Me N N Me N Cu Ni/2 N N Se Se N Se NPh NPh NPri 157 158: A=(CH2)2.In conclusion, it should be noted that in searching for azomethine ligands on the basis of heterocyclic aldehydes, researchers were oriented primarily to five-membered hetero- cycles, viz., furan, thiophene, their benzo analogues and azole systems. However, chelate compounds, such as complexes 159 310 and 160,311 can also be synthesised. (CH2)n N N M 2 ClO¡ N Pri + + S S 4 N N S N M/2 Ph Ph 2 159: n=2, 3. 160:M =Zn, Cd. These complexes contain Schiff bases as chelating ligands, which were prepared from six-membered azine heteroaromatic alde- hydes.Chelates 160 are of importance in the investigation of the problem of the stereochemical non-rigidity of azomethine metal complexes.285, 286 3. Structures of azomethine metal chelates The structures of most of the above-considered chelates were unambiguously established by X-ray diffraction analysis. The detailed discussion of X-ray diffraction data is beyond the scope of the present review. Let us only briefly consider the structural aspects of the directed construction of azomethine metal chelates of a desired stereochemistry by varying the fine structure of ligand systems and electronic configuration of complex-forming metals. Although data from X-ray diffraction analysis are available only for a limited number of five-membered metal chelates formed by bidentate Schiff bases, they provide evidence that chelate compounds containing the MN4 or MN2O2 coordination units are tetrahedral.20, 22, 24, 65 Metal chelates containing the MN2S2 coordination units (M=Ni, Pd) have planar structures with a cis arrangement of the ligands.79 ± 86, 312 The structures of azomethine chelate complexes containing six-membered metallocycles and the same set of donor centres were studied in more detail.2, 4, 12, 14, 20 ± 22, 313 ± 319 It was demon- strated that tetrahedral or distorted tetrahedral polyhedra are typical of chelate compounds containing the MN4 coordination unit (M=Co, Ni, Cu).17, 20, 24 955 Most of complexes with the MN2O2 coordination unit (M=Ni, Cu) have planar structures with a trans arrangement of the ligands.12, 20, 21, 24, 313 ± 315 Palladium complexes are character- ised exclusively by the square-planar coordination.21 The tetrahe- dral coordination is observed in all beryllium complexes 317 and in most cobalt,316 zinc, cadmium and mercury complexes.285, 286 Chelate compounds of d 8 elements containing the MN2S2 coor- dination unit have cis-planar structures.20, 22, 24, 25 These struc- tural features of chelate azomethine complexes have been discussed in the literature many times, including from the view- points of the mutual effect of metals and ligands 21 and stereo- chemistry of metal chelates of the ML2 type.320 Most alkylene- or arylene-bridged chelate compounds with tetradentate bisazomethine ligands have planar tricyclic struc- tures.In addition to the above-discussed bis(salicylidene)ethyle- nediamine chelates characterised by X-ray diffraction analysis, complexes 161,150 162 318 and 163 319 also have analogous struc- tures. Br N O N O N NH Os M (CH2)3 Ni N O N N O NH 163 162 Br 161:M=Ni, Cu. The coordination units in chelates of the MN4, MN2O2 and MN2S2 types adopt a tetrahedral configuration as a result of annelation of heteroaromatic fragments to metallocycles.20 ± 22, 24 For example, these structures are typical of benzofuran- and pyrazole-based azomethine chelates (164a and 150, respec- tively).20 S Ni/2 N R X 164a,b: X = O (a), S (b); R=Bun, Ph. A substantial flattening of the structure is observed on going from azomethines of the benzofuran series 164a to azomethines of the benzothiophene series 164b.20 When examining the dependence of the stereochemistry of azomethine chelate compounds of type 2 on the nature of the substituent R4, the effects associated with the volume of this substituent or with the presence of additional donor centres are generally considered.The former effect causes primarily steric distortions of the polyhedra.12, 21 The latter effect leads to an increase in the coordination number of complex-forming metal from 4 to 5 or 6.22, 24, 25 Relying on these notions, the directed synthesis of complexes with the desired geometry of the coordi- nation unit can be carried out. Since the steric effects of the substituents at the nitrogen atom (R4) have been discussed in sufficient detail,12, 20, 21, 24 we will consider only some examples of these effects on the structures of azomethine chelates 2.In the crystalline state, nickel chelates 2 (M=Ni; R4=H, Alk, Ar) have square-planar or distorted tetrahedral configura- tions depending on the volume of theR4 substituent,21, 313 whereas the complex configurational equilibria including planar, tetrahe- dral and octahedral (intermolecularly associated) structures, were observed in solutions of these chelate compounds.21 Distortions of the tetrahedral structures in copper chelates of type 2, which are caused by substituents at the nitrogen atom exhibiting different steric effects, were unambiguously established by X-ray diffrac- tion analysis. However, there is no consensus of opinion on the956 structures of these complexes in solution (cf.Ref. 21). It is believed that square-pyramidal or distorted tetrahedral config- urations as well as square ± tetrahedron equilibria can occur in solution. The influence of the nature of the substituents at the nitrogen atom, which are able to form additional coordination bonds, on the structures of complexes 2 has been the subject of investigation over the past 40 years. The presence of these substituents leads primarily to an increase in the coordination number of the central atom in chelates 2, i.e., the coordination is changed from square- planar or tetrahedral to penta- or hexacoordinated.This fact allows one to perform the directed synthesis of chelate compounds of the azomethine series with pyramidal or octahedral structures. In recent years,101, 105 ± 115 considerable attention has been given to the investigation of the structures of b-aminovinylimine chelates 48 containing the bulky diisopropylphenyl substituents R3 at the nitrogen atom. In chelates with composition MLR3, the access to complex-forming metals is sterically hindered. Ligands containing the same substituents can form chelates of different stereochemical types.22, 24, 215 In complexes of pyridine- containing azomethines, the nitrogen atom of the heterocycle can either form a bond with the metal atom (compound 108 213) or not (compound 107 212).The quinoline nitrogen atom is generally coordinated to the metal atom (compounds 111 215, 216 and 112 217). The dependence of the stereochemistry of chelate compounds on the nature of the metal atoms was partially considered at the beginning of this section. Noteworthy also is structure 165 characterised by the trans arrangement of the donor atoms and the `umbrella' conformation of the coordination unit untypical of azomethine chelates of d 8 metals with the N,S-ligand environ- ment.292, 293 The formation of this structure is associated with several factors. In palladium complexes, the ligands generally exhibit the square-planar coordination mode. However, the pyrazole fragment and bulky cyclohexyl substituents at the nitro- gen atoms induce structural distortions.Pd S N Pri S N N N N Pri N Me 165 Planar geometry with a cis arrangement of ligands S Ni/2 NX Ni/2 N X=S, Se S Ni/2 N S Figure 1. Coordination modes in nickel complexes. The structural analysis of nickel chelates with Schiff bases of aromatic and heterocyclic o-hydroxy(thio)aldehydes clearly showed the influence of the fine structure of ligand systems on the structures of azomethine metal chelates. Three coordination modes, viz., planar with a cis or trans arrangement of the ligands and tetrahedral, were found in the known complexes. The coordination mode is changed if the nature of the heteroatom in the chelate ring is changed, bulky substituents are inserted at the azomethine nitrogen atom, or on going from derivatives of aliphatic or aromatic aldehydes to heterocyclic analogues (Fig.1). III. Molecular complexes with azomethine ligands o-Hydroxyazomethines serve as classical chelating ligands (Sec- tion II.2.b). However, as was first demonstrated in 1964,321 these compounds can also be used for the preparation of molecular complexes 166.20, 22, 24, 28, 29, 322 ± 325 R1 R=Alk, Ar;M=Cu, Zn, B, Al, Sn, Ti, Th; A=Hal, NO3, NCS; m=1, 2; n=2±4. The reaction path depends on the nature of the solvent.323 Thus, metal chelates (Section II.2.b) are formed predominantly in media with high dielectric permeability (alcohols; path a), whereas molecular complexes 166 are generated primarily in aprotic non-polar media with low basicity (hydrocarbons, their halogen derivatives, ethers or mixtures of these solvents; path b).Me Some reactions proceed in mixtures of benzene and alcohols. Thus, the direct reaction of the ligands with ZnCl2 afforded complex 167.326 Planar geometry with a trans arrangement of ligands O O Ni/2 N N O Ni/2 N S Pri N S N Ni/2 Me N Me Me A D Garnovskii, I S Vasil'chenko O MAn H NR2 R1 Path a O MAn NR2 MAn R1 R1 Path b MAn O O H H N N R2 R2 m 166 Tetrahedral geometry N Ni/2 O Ni/2 O N Ni/2 N N O N S Ni/2 X N Ni/2 N N O X =S, SeRational design of metal coordination compounds with azomethine ligands PhH, MeOH 2 +ZnCl2 N N O O Ph Ph ZnCl2 H H 2 167 complexes Bis(salicylidene)alkylenediamine molecular 168a 327 and 168b 328 were synthesised analogously. R R MAn O O H H N N 168a,b (CH2)m 168a: R=H,M=Ca, A=NO3, m=3, n=2; 168b: R=H, Me, OMe;M=Ti, Zr; A=F, Cl; m=2, n=4.The template synthesis methods are employed more rarely. However, this approach was used for the preparation of adducts 169.329 O EtOH Q NH2+SnCl4 H+H2N O O O . SnCl4 H H N N Q 169 . , Q=(CH2)2, , S Molecular complex 170 was synthesised by ligand exchange in tungsten complex 171.330 H H R R O O N N CH2Cl2 . WO2Cl2 7Ph3PO WO2Cl2 . Ph3PO+ 171 170 It was demonstrated that molecular complexes 172 could be prepared from chelates 173 under the action of hydrogen hal- ides.331 RN O O Pd Me2CO .PdX2 H O +HX N N Ph 2 172 Ph 173 X=Cl, Br, I. Recently,332 metal chelates 174 were synthesised starting from a molecular complex analogous to complex 167. O Cl NaOH Zn/2 O O Zn N N H H N Cl Prn Prn Prn 174 957 Taking into account the data on the synthesis of molecular complex 172,331 the possibility of the interconversion of chelates 2 and molecular complexes 3 may be considered as proved. Among recent studies, let us mention the investigations of adducts of metal halides with b-aminovinyl ketones 175 101, 333 and their analogues 176 334 and 177.335 Ar Me N H O TiCl4 Me 2 175 Ph H N Cl N R1 R2 .MoO2Cl2 HN Cu S Cl H O176 But 177 R1=H, Br; R2=C7H15, Bz, C6H4Me-4, C6H4OMe-4, C6H4Br-4.Molecular complexes 178 and 179 of metal halides with hydroxyazomethines containing five- or six-membered chelating fragments 336 ± 338 were prepared by the reactions of the corre- sponding ligands with metal chlorides in anhydrous benzene (M=Sn, Ti) or methanol (M=Cu, Zn, Cd). Palladium com- plexes 178 and 179 were synthesised by the ligand-exchange reactions of Schiff bases with PdCl2(PhCN)2 . O O H N H . . N MAn MAn X X N N m m 179 178 X=NR, O, S; M=Sn, Ti, Pd, Cu, Zn, Cd; m=1, 2; n=2, 4. Several structural types were proposed for the molecular complexes under consideration.20, 27, 29, 322 ± 324 X-Ray diffraction studies demonstrated that hydroxyazomethines and b-aminovinyl ketones in molecular complexes 180 and 181a,b act as monoden- tate ligands containing the O-donor centre.For the first time, this coordination mode was unambiguously established by X-ray diffraction analysis of complexes with bis(salicylidene)ethylenedi- amine 180 339 and tris(acetylacetonato)(4-aminopent-3-en-2- one)ytterbium 181a.15, 340 Me2SnCl2 R Me2SnCl2 Yb O O O H H H2N N N Me 180 181: R =Me (a), Ph (b). It is believed 339 that complexes 180, like compound 168a,327 contain the ligand in the hydroxyimine form. However, X-ray diffraction analysis of complex 167, which made it possible to locate the positions of protons in the structure, showed that the ligand occurs as the quinone-imine tautomer.326 According to the results of X-ray diffraction analysis, the ligands in complexes 182 341, 342 and 183 343 also occur in the form of b-aminovinyl ketones.958 Cl Cl Hal H H H H Mo R Zn R O N O O NPri O N PriN O OMe Me Me R Me Hal R 183: R=Me, Pri . 182: R=Ph, C7H15 .It should be emphasised that IR spectroscopy cannot be employed for revealing the mode of location of the coordination bond and establishing the tautomeric form of the hydroxyazome- thine or b-aminovinyl ketone ligands because the absorption bands corresponding to both types of coordination (at the nitro- gen atom of theC=Nbond or at the oxygen atom of the carbonyl group) have similar frequencies. The 1H NMRspectra also do not provide conclusive information for the solution of these prob- lems.344 The data surveyed in this section provide the basis for the directed stereochemical 24 and regioselective 24, 345 syntheses of azomethine metal chelates.IV. Bi- and polynuclear azomethine complexes Bi- and polynuclear complexes containing azomethine ligand systems are formed by the reactions of bi- and polydentate Schiff bases or their complexes as well as of polymeric azomethines with metal sources or upon polymerisation of monomeric metal com- plexes. 1. Bi- and polynuclear complexes of polydentate Schiff bases and their analogues A series of chelating azomethines form not only mononuclear complexes but also bi- and polynuclear structures. Such structures were constructed and characterised for chelates prepared on the basis of bidentate Schiff bases of o-aminophenol 17 64, 346 and its thio analogue 31,347 tridentate imino alcohols 83 (Y=OR) and iminothio alcohols 83 (Y=SR).24, 177 Reactions of substituted benzaldehydes 184 with aminoetha- nol and nickel acetate afforded binuclear chelates 185.348 Na2S, EtOH XCN+H2N(CH2)2OH +Ni(OAc)2 D, 30 min O 184 (CH2)2N X O Ni Ni X O N (CH2)2 185 X=S, Se.The lead complex with composition [Pb2L]Cl was prepared by the reaction of the tripodal N(CH2CH2N=CH7C6H3OMe-3- OH-2)3(H3L) ligand with PbCl2 in methanol in the presence of a base.349 + N N N N Pb Cl7 O O O OMe OMe OMe Pb A D Garnovskii, I S Vasil'chenko Binuclear complexes 186 with tridentate azomethine ligands 2, 24, 280, 350 ± 353 were synthesised primarily from the ligands and the corresponding metal acetates.350 ± 353 (MeOH)m N Y X M M X Y N (HOMe)m 186 M=Co, Ni; X, Y=NTs, O, S; m=0, 1, 2.Binuclear nickel complex 186 (X=Y=S, M=Ni, m=0) was prepared by the template method from disulfide 187 and complex 188.280 S S 186 Ni (2-OHCC6H4S)2+ 187 NH2 HN2 188 Tetradentate azomethine ligand systems of the bis(salicyl- idene)ethylenediamine series 34, 218 form not only mono- but also binuclear chelates.225, 251, 354 ± 363 Such chelates were prepared for boron,218, 357 aluminium,218, 354, 359 gallium,218 indium,218 tita- nium,358 europium and yttrium,225, 357 cobalt,355, 363 chromium and molybdenum,34 iron,356 nickel and copper,361 and zinc.251, 360 Complexes containing the bridging bis(salicylidene)alkylidene- amino ligands 189a,b, such as 190 357 and 191,354 are the most widespread binuclear chelate compounds.R1 R1 PhMe, D O O R1 R1+B(OR2)3 H H N N (CH2)n 189a,b R1 R1 R1 O O R1 (R2O)2B B(OR2)2 N N (CH2)n 190 But But AlMeCl2 But O 189b O But Me Me THF Al Al Cl Cl N N 191 R1 = H (a), But (b); R2=Me, Et; n=2±6. Conceivably, these binuclear structures are generated through the formation of intermolecular dimers due to coordination of the metal atoms by the oxygen atoms of the adjacent complex molecules.34 Bisbidentate Schiff bases 192,364 193,365 194 365 and 195 366 show promise for the construction of bi- and polynuclear azome- thine complexes.Rational design of metal coordination compounds with azomethine ligands Binuclear structures were prepared on the basis of compounds 192 ± 194.Tetranuclear complexes were derived from ligand 195. Tripodal hexadentate ligands 196 gave rise to bi- and trinuclear nickel complexes.367 R=H, OMe. Various bridging atoms, such as oxygen (for example, com- pound 197 175) or chlorine (compound 198 368), play an important role in the formation of binuclear structures. But But But In some cases, atomic oxygen serves as an intermolecular bridge to form, for example, oxotitanium dimeric,369 ± 372 trimeric O H N OH NN N HO H O 193 192N O H SO2 H OH O N R R 195 Me N N Al But But MeO OMe But Al N N Me 197 THFN N Y But O O But But Cl Cl But But O O But Y N N 198 THF 959 O and tetrameric 370, 373 complexes of tetradentate Schiff bases.Structure 199 provides an example of dimeric complexes.372 H N N N Ti O O SO2 O O O O Ti N N N H 199 O 194 There are data on oxygen-bridged complexes derived from bidentate Schiff bases. For example, complexes of di-m-oxodi- manganese(IV) with N-alkyl-substituted salicylideneamines were synthesised.374 Binuclear chelates 200 375 formed by the complex polydentate ligand containing two b-ketoiminate fragments also belong to this type of complexes. Me R3 R1 O O N N N N M R2 R2 M N N O NHO OH O R1 R3 R 200 196 R1=H, Me; R2=H, COMe; R3=Me, Ph, C6H4NO2-4;M=Ni, Cu.Trinuclear oxygen-bridged complexes 201 376, 377 were pre- pared based on dendrimers with salicylideneaminate groups. CHC6H4OH-2 N (CH2)3 Co(OAc)2 N (CH2)2 N N (CH2)3 CHC6H4OH-2 3 N (CH2)3 O Co N (CH2)2 N O N (CH2)3 3 201 2. Bi- and polynuclear complexes with metal-containing azomethine ligands In the 1960s, it was demonstrated 378 ± 381 that the reactions of chelate compounds containing the azomethine ligandsM1Lm with metal salts M2An can give rise to bi- (202 380, 381) and trinuclear (203 378 and 204 379) structures. M2An M2An R1N O O O M1 M1 O N N N R1 202 203 M2An960 M2An R1 R2 R2 N O M1 O N R2 R2 R1 M2An 204 R1 =Alk, Ar;M1=Ni, Cu;M2=Co, Ni, Cu, B, Al, Ti, Sn; A=Hal, NO3. The direct reactions of ligands with sources of complex- forming metals were used for the preparation of complexes 205,358 206 382 and 207.383 Me Me Al X O O Ti N N X AlMe2Y 205: X =Cl, Br; Y =Me, Cl.But THF THF O O But Al + N N 207 It should be noted that alkali salts in the heteronuclear structures under consideration can act as Lewis acids (M2An). In particular, this is evidenced 382 by the formation of adducts 206 in the reactions of salicylaldimine complexes of divalent metals (Co, Ni, Cu, Zn) with sodium, potassium and cesium salts. b-Amino- vinyliminate-based complexes 206 were described in the study.384 Unlike the above-considered binuclear halide complexes 202, which were prepared by the reactions of the corresponding ligands with metal halides, coordination compounds of metal carbonyls were synthesised by ligand-exchange reactions.385 These reactions afforded two types of complex compounds with theM17M2 and M17M27M1 bonds, viz., M1LM2(CO)5 (208) and (M1L)2M2(CO)4 (209), respectively.O N M1 M2(CO)5 O N 208 M1=Ge, Sn, Pb;M2=Cr, W. Binuclear complexes can be prepared by the reactions of metal chelates, for example, of bis(salicylidene)ethyleneaminates or bis(tris)-b-diketonates, in chloroform.386 ± 391 In azomethine che- late compounds, Ni(II) 387 ± 391 and Cu(II),386, 389, 391 serve as the ON Me Me H O O O MeO O M2 O O M1 N N 206: M1=Co, Ni, Cu, Zn; M2=Na, K, Cs. But But [Co(CO)4]7 O N M1 M2(CO)4 O N 2 209 M1 metal and hexafluoroacetylacetonates of Ba and Mg,390 Y(III),386 ± 389, 391 La(III) 389, 391 and Gd(III) 386, 389, 391 act as metal salts (M2An).The structures of these complexes were established by X-ray diffraction analysis. Like in complexes 202, the coor- dinatively unsaturated oxygen atoms of the bis(salicylidene)ethy- lenediaminate anion are involved in coordination to lanthanides. Bi- and trinuclear complexes were synthesised starting from coordination compounds of crown ethers containing the chelating hydroxyazomethine fragments.392 ± 394 As an example of these compounds, we refer to heterotrinuclear complex 210 generated from heterobinuclear complex 211.392 N N HO OH O O M1X2 O OO O HO OH N N211 M1=Ba(CF3SO3)2; M2=Cu, Ni.It should also be noted that Schiff bases containing organo- metallic fragments can be used as azomethine ligands. Ferrocene derivatives 212 395, 396 and 213 397 as well as derivatives of benzene chromium tricarbonyls 214 398 and 215 399 are examples of these Schiff bases. Fe Fe N H O 212 O H N 214 Cr(CO)3 Ligands 212, 213 and 214 give rise only to metal chelates, whereas azomethine 215 produces molecular complexes. Ruthenium complex 216 shows promise as a ligand for the preparation of tri- and oligonuclear coordination compounds.400 The former complexes can be synthesised by the reactions of ligand 216 with divalent metal acetates and the latter complexes can be prepared on the basis of metal salts in oxidation states 3 and higher.A D Garnovskii, I S Vasil'chenko N N M2 O O O O M2(OAc)2 M1X2 O O MeOH O O O O M2 N N 210 N N H H O O Fe O O H H N N 213 Cr(CO)3 Me NH N S NPh 215Rational design of metal coordination compounds with azomethine ligands O H Ru NC6H4R-4 216 3. Azomethine metal polymers Main methods for the preparation of polymers containing che- lates of Schiff bases and their analogues involve the reactions with chelating macroligands and polymerisation of metal mono- mers.31 ± 33, 401 ± 405 Chelating polymeric azomethine ligands are used in synthetic reactions similar to those involving monomeric Schiff bases and their analogues (see Sections II and III). Thus, the reactions of the ligands with metal salts afforded polymers containing chelates (for example, 217,406 218 407) and molecular complexes (219 405). The metal exchange gave rise to metal polymers 220.32 O O (CH2)7 O N O Cu O N O (CH2)7 217 H15C7O C6H4 N O Cu O N OR1xR2y218 R1=(CH2)12O; R2=(CH2CH2O)3; x=0 ± 100; y=1007x.ONO2 Ph H N U O ONPh O OH ONO2 219 O Ni/2 N Me n 220 Polymer 222 was prepared from ligand 221 with the use of the template method.32 This approach is particularly efficient when metal polymers cannot be prepared by the reactions of ligands with metal salts or through metal exchange. 961 Me O NH2 H2N CH2 Ni/2 O n Me 221 Me Me N N Ni CH2 CH2 O O Me Me n 222 The oxidative polymerisation of monomeric chelates was used for the preparation of polymers with bissalicylidene derivatives of aromatic 408, 409 or heterocyclic 410, 411 amines.O O O O M M N N N N n S S In some cases, polymeric azomethine complexes can be prepared in the course of usual syntheses. In particular, the reaction of ligand 223 with La(III) nitrate afforded polymeric structure 224 containing fragments of molecular complexes with the bis(salicylidene)ethylenediaminate ligand coordinated through the oxygen atom.412 O O O O n H H MeOH, CH2Cl2 +La(NO3)3 N N 20 8C 223 HOMe NO3 O3N O OC7H15 O La N N n O3NMeOH n 224 Polymeric complexes of Schiff bases can also involve bridging fragments, for example, oxygen 413 or metal chelates.414 V.Conclusion n The available data on metal complex compounds with azomethine ligands make it clear that the employment of the techniques of rational design in the search for ligand systems and synthesis procedures can be a help in performing the directed synthesis of complexes in which the mode of coordination and geometry of metallocycles are prespecified. In addition to synthetic and structural problems arising in research on metal coordination compounds with azomethine ligands, theoretical and applied aspects are also of great impor- tance, including competitive coordination,24, 27 standard and non- standard binding of metals to chelating ligands,28, 29 stabilisation through the complex formation of tautomeric forms untypical of free ligand systems 77 and isomerism of coordination units within chelates.24, 27 Complexes of Schiff bases and their structural analogues are of practical importance because they allow one to perform the directed design of new types of azomethine coordination com-962 pounds exhibiting catalytic, magnetically anomalous and liquid- crystalline properties.In this connection, it should be emphasised that metal chelates of Schiff bases in which the complex-forming metal atom is shielded by bulky substituents (compounds of type 48: R1=C6H3Pri2), belong to new-generation catalysts for poly- merisation, copolymerisation, enantioselective synthesis and oxi- dation of alkenes (predominantly, epoxidation).In addition to the studies cited in the present review 101, 105 ± 136 these problems have also been considered in the studies devoted to the same sub- ject.415 ± 417 Bi- and trinuclear complexes of type 204 (A=Hal; see also Ref. 418) serve as efficient catalysts for oligo- and polymerisation as well as for hydrogenation. 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ISSN:0036-021X    

出版商:RSC    

年代:2002

数据来源: RSC

摘要:

Russian Chemical Reviews 71 (11) 969 ± 980 (2002) Basic achievements in the coordination chemistry of modified monosaccharides Yu A Zhdanov, Yu E Alekseev Contents I. Introduction II. Metal complexes of monosaccharide O-derivatives III. Metal complexes of nitrogen-containing monosaccharide derivatives IV. Metal complexes of phosphorus-, sulfur- and selenium-containing monosaccharide derivatives V. Conclusion Abstract. chemically of complexation the on data published The The published data on the complexation of chemically modified acetylated and (acetalated carbohydrates modified carbohydrates (acetalated and acetylated monosacchar- monosacchar- ides, as well as coronands and podands carbohydrate ides, carbohydrate podands and coronands as well as nitrogen-, nitrogen-, phosphorus-, sulfur- and selenium-containing monosaccharide phosphorus-, sulfur- and selenium-containing monosaccharide derivatives) are generalised and systematised.The structural derivatives) are generalised and systematised. The structural diversity and demonstrated is formed complexes metal the of diversity of the metal complexes formed is demonstrated and the the effect the of chiralities the on chirality ligand the of effect of the ligand chirality on the chiralities of the corresponding corresponding coordination includes bibliography The discussed. is points coordination points is discussed. The bibliography includes 155 155 references. I. Introduction Various aspects of the coordination chemistry of natural carbo- hydrates have been detailed in a number of reviews.1 ±8 The coordination chemistry of chemically modified carbohydrates has been studied in far less detail; the data available 8 ±11 discuss only some aspects of this field of research. The interest in this topic is connected with further improvement of methods of synthetic chemistry of carbohydrates.Chemical modification of monosaccharides aimed at the preparation of ligands consists predominantly in the coupling of natural monosaccharides, which are continuously reproduced upon photosynthesis, to a priori complex-forming (in the extreme case, chelatogenic) fragments. This approach allows a consider- able increase in the structural diversity of carbohydrate complex- ones, which are chiral by nature.Such complexones and their metal complexes are necessary for the preparation of chirally pure pharmaceuticals. This review is an attempt to get a better insight into this branch of chemistry. The material presented herein is arranged according to the principle used in our previous review.6 The ligands are classified into oxygen, nitrogen-, phosphorus-, sulfur- and sele- nium-containing monosaccharide derivatives according to the type of donor heteroatoms. Metal cations for each ligand class are cited in the order of complexity of their electronic configu- rations and in conformity with their positions in the Periodic Table, i.e., s-, p-, d- and f-metals. Yu A Zhdanov, Yu E Alekseev Institute of Physical and Organic Chem- istry, Rostov State University, prosp.Stachki 194/2, 344090 Rostov-on- Don, Russian Federation. Fax (7-863) 243 46 67. Tel. (7-863) 265 04 77. E-mail: bell@ipoc.rsu.ru (Yu A Zhdanov) Tel. (7-863) 243 46 00 (Yu E Alekseev) Received 2 August 2002 Uspekhi Khimii 71 (11) 1090 ± 1102 (2002); translated by R L Birnova #2002 Russian Academy of Sciences and Turpion Ltd DOI 10.1070/RC2002v071n11ABEH000758 969 969 972 976 978 II. Metal complexes of monosaccharide O-derivatives 1. Complexes with s-metal cations Free water-soluble monosaccharides represent 2, 3, 6 weak com- plexones of s-metal cations owing to strong competition of water molecules for the coordination of these cations. O-Alkylation (acylation) or acetalation of monosaccharides increases their hydrophobicities as a result of which the corresponding deriva- tives become soluble in organic solvents where they play the role of moderate s-metal complexones due to the lack of water.This can be exemplified by the acetalated monosaccharide 1 (see Ref. 12) and the di-O-methyl derivative 2.13 Thus substitution of the O(CH2)2OH and O(CH2)2O(CH2)2OEt fragments for the methoxy groups in compounds 1 and 2 yields 12, 13 carbohydrate podands (acyclic analogues of crown esters);2 their affinities for s-metal cations markedly exceed those of the starting monosac- charides. This is especially typical of podands 3 (R is a per-O- acetylated monosaccharide residue) 14 and 4.15 MeO O OH O O Me MeO OMe O O O O OR O RO Me 3 2 Me 1 Glyc1 Glyc1 O O n O4 Ch OO O (Ch is cyclohexylidene); n=1 ± 13.Glyc1= OOCh As can be judged from qualitative reactions, regioisomeric mixtures of podands and coronands (e.g., compounds 5 and 6) 15 ± 19 readily bind alkali metal cations. O O O O O O Glyc2 Glyc2 Glyc2 Glyc2 O O HO O OH 6 5 OMe O Glyc2= . OO Ch970 The coordination of coronands 7,20 821 and 9 (see Ref. 22) involves carbohydrate fragments. Ethers 10 (see Ref. 23) and a coronand 11 coordinate s-metals in a similar way, i.e., with the involvement of carboxy groups.24 This imparts additional stabil- ity to complexes of the above-mentioned ligands. O O O O O O7 X=CH2COCH2, CH2; Glyc is an acetalated or acetylated monosaccharide residue.OMe O O O O O O O O O 9 Ph O O O O O O OMe MeO 11 2. Complexes with p-metal cations This type of complex, like complexes with s-metal cations, are relatively scarce. They include a series of borates of acetalated sugars (e.g., compound 12 and its derivatives).25 The O-carboxy- methyl derivative 13 binds Al(III) and some d-metal cations.26 In a homoleptic complex 14, the aluminium atom and three carbohy- drate ligands form a trigonal pyramid.27 Glyc37OBEt2 12 OO Me Me O Glyc3= OO Me Me The Sn(IV) complexes of the general formula 15 are the most extensively documented in the literature.28 ± 30 Organotin com- pounds with organometal fragments directly bound to the carbo- hydrate core 31, 32 are characterised by intramolecular coordination of the type 16a.31 GlycO (CH2)nSnR3 15 R=Me, Bu, Ph; Glyc is an acetalated monosaccharide residue; n=0±2.The stannylene carbohydrate acetals 17 form dimers 18.33, 34 Some analogous derivatives form trimers and even pentamers.34 O O X OR OGlyc O O O O O8 R=H, Ac. R R O R O O HO2C HO2C R O O O 10 R=H, CO2H. Py OCH2CO2H OH Al Glyc3O HO HO OGlyc3 OGlyc3 14 OH Py is pyridine. OH 13 O Ph OHO O OMe MPh3 16a ± c M=Sn (a), Si (b), Pd (c). Yu A Zhdanov, Yu E Alekseev O OMe O Ph O O O O SnBu2 OMe O Bu2Sn O O Bu2Sn O 17 MeO O18 For other p-metals, only complexes of the type 16b,c and 19a,b have been described.30MPh3 O O O O Me O Me O Me M=Si (a), Pb (b).Me 19a,b 3. Complexes with d-metal cations d-Metal complexes are especially fully and diversely represented in the coordination chemistry as a whole and in the coordination chemistry of modified monosaccharides, in particular, which to some extent is characteristic of complexes of monosaccharide O-derivatives. Thus the above-mentioned O-carboxymethyl derivative 13 is coordinated 26 not only with the p-cation of Al3+, but also with d-cations Cu2+, Fe3+, Co2+, Ni2+ and Mn2+. The formation of copper bischelates 20 in the selective O-alkylation (acylation) of the acetalated glycoside 21 in organic solvents in the presence of copper(II) has been described.35 O O O Ph O Cu OH OMe O O O 20 21 OH We failed to retrieve any literature data on complexes of monosaccharide O-derivatives with d-metal cations of subgroups IB and IIIB.d-Metal complexes of subgroup IVB were studied in some papers devoted to the use of monosaccharide Ti(IV) com- plexes in modern organic synthesis.36 ± 41 Ti(IV) complexes con- taining D- and L-tartaric acid derivatives, which are employed in the highly efficient enantioselective Sharpless epoxidation of allylic alcohols (optical yields 80%± 100%) have been studied in most detail.36 ± 38 (Sharpless was awarded a Nobel prize in 2001.42) The Sharpless catalysts form very diverse structures in solu- tion 43, 44 with cyclic (22) 36 ± 38, 45 and bridge-type (23) dimers predominating.38, 45, 46 O OPri OPri O O PriO OPri O O PriO O O OPri PriO OPri O Ti Ti Ti OPri PriO Ti O O O O O OPri OPri PriO O PriO O OPri O PriO 23 22 Titanium complexes 24 serve as nucleophilic reagents in some enantioselective transformations.39 ± 41 The chlorine atom in com- plex 25,47 which is a structural analogue of the complex 14, is readily exchanged for alkyl and other groups, e.g., the allyl group.48 Like organometallic compounds, the substitution prod- uct adds enantioselectively to the carbonyl group.48Basic achievements in the coordination chemistry of modified monosaccharides Ph Ph OGlyc3 Glyc3O O OPri O R1 Ti Ti R2 O Me O Ph Ph 24 25 Cl R1=H, Me; R2=H, Ph, But.The chiral ligands in the homoleptic complexes 26 based on diacetone glucose 27 and the oxygen-rich periphery form cavities which favour further coordination.X-Ray diffraction analysis data suggest that two pyridine and two carbohydrate ligands in the complex 27 are localised in the equatorial plane of the pseudo- octahedral structure.27 OGlyc3 Py Py Zr M(OGlyc3)4 Glyc3O 26 OGlyc3 OGlyc3 27 M=Ti, Zr, Hf. In the case of d-metals of subgroup VB, the vanadiumdi-O- isopropylidene-D-glucose complex 28 is the only complex described so far.49 According to X-ray diffraction analysis data, the ligands in this complex form a propeller-shaped structure having a L-configuration. Reaction of the complex 28 with lithium and sodium alcoholates of di-O-isopropylidene-D-glucose yields complexes [V(OGlyc3)6M3] (M=Li, Na) where the carbo- hydrate ligands form an environment for M+ which resembles that of the solvate complex (structure 29).50 Thus, metal com- plexes of modified sugars can in principle be used as ligands for group IA metal ions: +M OGlyc3 Py OGlyc3 Glyc3O OGlyc3 V V Glyc3O M+ OGlyc3 Glyc3O OGlyc3 28 Py OGlyc3 +M 29 M=Li, Na. Carbohydrate-containing d-metal complexes of subgroup VIB are represented in the literature by carbene complexes of two types.The first of them includes complexes 30 51 where the acetalated carbohydrate moiety is in the b-position relative to the carbene carbon atom. In the second type of complex (31),52 the acetylated carbohydrate fragment is linked directly to the carbene carbon atom.OEt AcO OAc OR Ph (OC)5M M(CO)5 AcO OAc 31 30 OGlyc3 R=Me, Et; M=Cr, Mo, W. M=Cr, W. Carbene complexes of the second type (e.g., 32) have been known for d-metals of subgroup VIIB.53 R (OC)3M R=Ph, tolyl; M=Mn, Re; Glyc is a protected monosaccharide residue. OGlyc 32 Complexes 33 were prepared 54 from acetylated glycosyl bromides by exchange reaction with NaMn(CO)5 . Interactions of 1,6-anhydrohexopyranoses 34 with Mn2+, Mn3+ and Mn4+ follow the Angyal rule;3, 6 the ligand complexes with the axial- equatorial-axial orientation of the three vicinal hydroxy groups are the most stable.55 971 O RO O O HO RO X OH HO OR RO 34 33: R=Ac; X=Mn(CO)5; 35: R=Me; X=Fe(CO)2Cp (Cp is cyclopentadienyl).The structurally unique iron carbonyl complexes 35,56 36 52 and 37 57 and the p-complex 38 58 are of interest among carbo- hydrate compounds of group VIII metals. m3-Carbine clusters, e.g., compound 39, have been prepared.59 Fe(CO)2Cp AcO OAc OR OMe O Fe(CO)4 OAc AcO OAc36 BzO 37 OAc R=Me, Et. RO OAc O O O O CCo3(CO)9 AcOAcO AcO OEt Fe(CO)4 39 38 R=H, Ts. In strongly alkaline aqueous media, vicinal diol groups of partially protected monosaccharides undergo deprotonation; the deprotonated forms of these compounds coordinate the metal complex trans-[(en)2CoCl2]Cl (en is ethylenediamine) to yield complexes 40 with a predominant L- or D-configuration depend- ing on the stereochemistry of the monosaccharide.60 NH2 O 3+Co H2N O H2N 40 NH2 Complex [Pd(OH)2(en)] reacts with 1,6-anhydro-b-D-glucose (levoglucosan) in strongly alkaline aqueous solutions7 to afford complex 41 (by analogy with the complex 40).The complex [PtMe3(Me2CO)3]+BF¡4 is coordinated with acetalated aldoses (e.g., di-O-isopropylidene-D-glucose Glyc3OH) to give the com- plex 42 with elimination of the terminal isopropylidene protective group. Per-O-acetylated aldoses form complexes similar to 43.7, 61 + HO O OH OH Me Pt O Me OH Me BF¡4 O O O Pd O O NH2 H2N Me Me 42 41 + Me Pt Me AcO AcO Me BF¡4O OAc AcO OAc 43972 III. Metal complexes of nitrogen-containing monosaccharide derivatives The predominance of nitrogen-containing ligands in coordination chemistry on the whole 62 is also characteristic of the coordination chemistry of carbohydrates. Nitrogen-containing monosacchar- ide derivatives coordinate virtually all known metals.1. Complexes with s-metal cations The majority of s-metal complexones in the series of nitrogen- containing monosaccharides, e.g., the amino derivatives 44, 45,63 46 ± 48,64 49 and 65, 66 50 67, 68 and the amido derivatives 51 ± 59, have simple structures.69 Typical s-metal complexones, such as the azapodands 60 (see Ref. 70) and 61 have also been prepared.69 The ability of these compounds to form complexes with s-metals was estimated by qualitative reactions.64 Glyc4 NH(CH2)2OH 44 O Ch Ch Glyc5 CH2NH(CH2)2OH 45 OO O OHCh O Ch Glyc5= Glyc4= O O O O Glyc5 COX 51 ± 54 Glyc2 CH(OH)CH2X 46 ± 50 Glyc4 OCH2COX 57 ± 59 Glyc1 OCH2COX 55, 56 X Compound 46 NH2 47, 51, 55, 57 NH(CH2)2OH N(CH2CH2OH)2 NHC(CH2OH)3 48, 52, 56, 58 49 50 NHCHRCO¡2 Na+ (R=H, Me, CH2Me, CH2OH, CH2C6H4OH-4) NHCH2CO¡2 Na+ 53, 59 54 NHCHMeCO¡2 Na+ O Me O O O HN HO Glyc5 Glyc5 O NH NH 2 Clyc2 60 61 The podand activity of N-glycosylaminocarboxylates is prob- ably due to their ability 70, 71 to form the dimers 62 in non-polar media.The formation of the tetramer 64 from the acetalated N-glycoside, viz., guanosine 63, which also occurs in non-polar media, is yet another example of aggregation of cavity-free compounds into those with cavities.72 ± 74 The tetramer 64 forms stable L2M complexes (L=64) with K+ and Cs+ where the coordination apparently involves the carbonyl oxygen atoms.An Glyc2 O R NH2 Na+ 7 N Me NH O O N NH SiO But O N O H H Me O O HNO O 7 Na+ O R Me Me Glyc2 63 62 R=H, Et. Yu A Zhdanov, Yu E Alekseev H N N N N H N N H N N N N H O H H H O O N OHN H N N H N N H N N N N H 64 important role is played by the structure of the carbohydrate fragment; thus substitution of O-acetyl groups for the isopropy- lidene protective group in compound 63 prevents aggregation.74 A series of carbohydrate azacrown ethers 65 form stable complexes with Na+ and K+ ions [logKst=4 ± 6 (in CDCl3)].75 The azacoronands 66 and 67 (see Ref. 64) catalyse the enantiose- lective reduction of a prochiral ketone acetophenone under phase- transfer conditions with optical yields of 48% and 20%, respec- tively.76 OMe N O O O n O O n=3, 4.65 Ph O O O O N N O N HO HO HO O O O Glyc2 Glyc2 Glyc2 67 66 2. Complexes with d-metal cations In contrast to the poorly investigated coordination of nitrogen- containing monosaccharide derivatives with p-metal cations, there is a considerable body of data on analogous complexes with d-metal cations. a. Complexes of N-glycosides and related compounds N-Glycosides containing chelate-forming fragments as aglycons are unstable; therefore, their complexes with metals are prepared using different approaches, e.g., reactions of aldoses with ethyl- enediamine with subsequent addition of copper(II) halides to the reaction mixtures.77 At first, aldose 68a as the aldehyde (form 68b) reacts with ethylenediamine to produce the Schiff base 69, which is immediately cyclised into N-glycoside 70.In turn, the free amino group in compound 70 reacts with the aldose resulting in bis-N- glycoside, which yields complex 71 upon addition of copper(II) salts. 77 In the case of N,N0- and N,N-dimethylethylenediamine, only one amino group is involved in the formation of N-glyco- sides; the resulting complexes have the structures 72a and 72b, respectively. R R NH2CH2CH2NH2 OH O O HO OH HO OH HO OH HO 68b 68aBasic achievements in the coordination chemistry of modified monosaccharides R OH HO N NH2 OH HO 69R O 1 HO NH2 2 HN OH HO 70 R=H, CH2OH.R1 R2 N NH HN N 1 1 1 R3 2 Cu Cu 2 2 X X OH OH OH 72a,b 71 X=Cl, Br. 72a: R1=R2=Me, R3=H; 72b: R1=H, R2=R3=Me. The reactions of aldoses with [Ni(en)3]2+ (73) afford the biscarbohydrate complexes 74.9, 78 Apparently, the aldehyde form 68b first reacts with the amino groups N(1) and N(2) of the complex 73 as is shown in the previous scheme, 68b?69?70, the reacting ligands being localised in the coordination sphere of the complex. Further, the hydroxy groups at C(2) of the carbohydrate residues incorporated into the metal complex displace the ligand with the amino groups at N(3) and N(4) from the coordination sphere (as in the case of formation of the complex 40) and are coordinated with the central metal atom (the formation of metal complexes by ligand exchange 62).The reactions with ketoses yield monocarbohydrate complexes 75;9, 78, 79 noteworthy, the coordi- nation involves the primary hydroxy group at C(1) in addition to the secondary hydroxy group at C(1). H2N H2N H2N 1 NH2 H2N HN HN NH2 NH2 O 2 2+ 2 1 2+ Ni 2+ NH2 NH Ni Ni 2 3 NH2 4 3 NH2 H2N HO HO HO 1 2 OH 1 75 74 73 Reaction of D-mannose with the Ni(II) ±N,N0-dimethylethy- lenediamine complex yields compound 76 having unusual bridg- ing structure;9, 80 one of the carbohydrate ligands in this complex assumes a furanose form seldom occurring in carbohydrate metal complexes. The [Ni(tn)3]2+ complex (tn is trimethylenediamine) reacts with aldoses like [Ni(en)3]2+ to yield products analogous to the complex 74.9, 81 A similar result was obtained in the case of the Ni(II) b-alanine complex.9, 82 HN 2 2 HO OH 1 1 3+ NH2 H2N 2+ 2+ Co NMe MeN NMe MeN Ni Ni NH O 1 HO O O Me 1 OH O 3 HO 2 2 H 4 OH OH 5 HO 77 76 OH Substitution of Co3+ for Ni2+ significantly changes the mode of coordination of the carbohydrate ligand: the conformation of the six-membered ring of the single carbohydrate ligand in 973 complex 77 is considerably distorted, which facilitates involve- ment of the hydroxy group at C(3) in the coordination.9, 83, 84 The main principles of the geometry of the metal complexes under discussion are retained in the case of a structurally more complex amino component, viz., N,N-bis(2-aminoethyl)- ethylenediamine (tren) 78.Thus the reaction with nickel(II) yielded mono- (79) (in the case of D-glucose) and biscarbohydrate (80) complexes (in the case of D-mannose).85 2 1 OH H2N HN 2+ NH2 NH2 NH2 Ni N OH2 N 78 H2N 79 1 2 2 1 HO HN Ni 2+ HO NH NH2 N 80 Triscarbohydrate complexes 81 with Ni2+ (see Refs 9, 85 and 86) and Co3+ (see Refs 9, 85 and 87) using N-glycosides 82 as ligands could also be synthesised. 1 2 N OH HO Glyc Glyc Glyc 2 Mn+ HN 1 NH NH N N N OH 1 2 82 81 M=Ni2+, Co3+. All the metal complexes synthesised are characterised by a specific anomeric configuration (a or b) of the glycosidic imino group, l- or d-conformation of the five-membered metallocycles and helical L- or D-configuration 88 of the chelate point owing to the transfer of chirality from the carbohydrate ligands to the metal centre.Reversible chiral inversion of the L-diastereomer into the D-diastereomer in complex 81 (M=Co3+) after substitution of the sulfate ion as the counterion for the halide ion has been described for the first time.87 The synthesis of a bridging double- charged complex 83 based on D-glucose is another step forward.89 1 2 1 2 NH OH HO RHN 3+ NH Co Co3+ O OH2N NHR N N NH2 83 R is D-glucose residue. In a similar scheme (viz., by the reaction of a carbohydrate ligand with a metal-containing derivative), starting from N-glyco- side 84 and alkynylcarbene complexes 85, a mixture of complexes 86 and 87 (3 : 2) was obtained.90 Ferrocenyl derivatives 88 and 89 were synthesised analogously.91 OEt (OC)5M C C CHPh GlycNH2 84 85974 OEt NHGlyc NHGlyc (OC)5M CHC C C C (OC)5M Glyc7CHCH2Fc Ph 88, 89 86 87 R Ph PivO O 84, 86, 87: Glyc=PivO (Piv=ButCCO); OPiv PivO 85 ± 87:M=Cr, W; 88, 89: Glyc is a protected or free monosaccharide; Fc is ferrocenyl; R=H (88), CH2Fc (89).The N,O-coordination of amino acidN-glycosides 90 (as salts) is a characteristic of complexes 91 with salts M12 [M2Cl4] (M1=Na, K; M2=Pt, Pd).7, 92 N,N-Coordination (structure 92) is observed only in the case of histidine.7, 92 Glyc O R O NH Glyc NHCHCO¡2M HN O 90 R R O 91 Glyc R=H, Me, CH2OH, CH(OH)Me, CH2CONH2, (CH2)2CO2Na.HNGlyc 7O2C HO N NH 2+M O OH ;M=Pt, Pd. Glyc=HO N HN OH Glyc CO¡292 NH b. Complexes of O- and C-substituted monosaccharides with nitrogen-containing fragments The central metal atom in the copper complex 93 of O-(b-D- xylopyranosyl)-L-serine is surrounded 93 by four carbohydrate ligands with a distorted octahedral configuration. HO HO O O NH2 O O O HO C C O O O O H2N Cu 2+OO O 93 The D-glucopyranosyl ester of glycine 94a and its partially (94b) and fully substituted (94c) derivatives coordinate Fe(III) exclusively through the oxygen atoms of the carboxy groups to yield complexes 95a ± c,94 their structures are simpler the larger the number of protective groups.L O RO L OH2 H2O OO OO O O Fe3+ Fe3+ X OR O RO O O O O OR L L 94a ± c (O7L7O) L L O O O O Fe3+ 95a OH2 a:R=H, X = NH3 .CF3CO2H; b: R=Ac, X=NH3 .CF3CO2H; c: R=Ac, X=NHCO2But. Yu A Zhdanov, Yu E Alekseev L O O O OH2 O O O H2O Fe3+ Fe3+ L L Fe3+ L H2O O O O O OH2 O O L 95c 95b O-Glycoside analogues 96 of the ferrocenyl derivatives 88 and 89 have been prepared.95 OR1 R2 O R1O N O (CH2)n R1O Fc OR1 96 R1=H, CH2Ph; R2=Me, CH2Fc; n=2, 3, 5. The type of coordination in 2-[poly(hydroxyalkyl)]thiazol- idine-4-carboxylic acids 97 depends on the nature of the metal ion.96 ± 98 The Cr2+ cation in complexes ML2 (L=97), as in a-amino acid metal complexes, is coordinated through the imino group and the carboxy group of the heterocyclic fragment.96 Similar structures are formed with Zn2+, the latter being coordi- nated at the hydroxy group through the C(2) atom of the carbohydrate residue.97 The Pd2+ cation is coordinated through the nitrogen and sulfur atoms of the heterocyclic fragment 98 (the so-called competitive coordination 62).It is interesting to note that the stability constants of zinc complexes depend on the chirality of the C(1) atom of the carbohydrate fragment.97 S 1 HOCH2(CHOH)nCHOH CO2H 97 NH n=2, 3 Metal complexes 98 ± 102 were prepared in high yields using the electrochemical method.99, 100 The copper chelates 98, 99a and 101 possess a distorted planar configuration of the chelate point, and other complexes have octahedral structures.99, 100 O Ch O Cu/2 O O Ch O O N N S O M/2 99a ± c 98 M=Cu (a), Ni (b), Co (c).Me O Cu/2 N CuOAc N N O Me CuOAc OH 102 101 100 c. Complexes of amino sugars and their derivatives D-Glucosamine (103a), D-mannosamine (103b) and D-galactos- amine (103c) react with [Ni(en)3]2+ to yield complexes 104 9, 101 similar to complexes 74 containing an amino group at C(2) instead of a hydroxy group. Reactions of D-glucosamine 103a with [Co(NH3)3]3+, [Co(en)3]3+ and [Co(phen)2]3+ (phen is phenan- throline) yield complexes 105 by a ligand exchange mecha- nism.9, 102 The use of an alternative workup of the D-glucosamine (103a) ± [Co(en)3]3+ mixture (see Ref. 103) gave eight monocarbohydrate metal complexes.Basic achievements in the coordination chemistry of modified monosaccharides H2N OH HN R3 1 2+ NH2 O OH R1 OH 2 NH R4 Ni NH2 H2N R2 1 103a ± c 2 104 a: R1=R3=H,R2=NH2, R4=OH; b: R1=NH2, R2=R3=H,R4=OH; c: R1=R4=H, R2=NH2, R3=OH. Cl NH2 H2N NH2 H2N Co2+ HO NH2 H2N Co3+ NH2 1 N H2N NH2 2 2 1 H 106 105 H2N H2N NH2 NH2 H2N H2N Co2+ Co2+ HN NH2 NH2 HN 1 HO 2 O 3 HO HO OH OH OH HO OH 108 107 Among the complexes isolated,103 only compound 106 with a pyranose form of the carbohydrate ligand is a normal product.In the other complexes,D-glucosamine 103a is transformed into both D-mannosamine 103b by the Lobry de Bruyn ± van Ekenstein rearrangement,104, 105 and D-fructosylamine by the Amadori rearrangement 105 (complex 107).Complex 108 where D-glucos- amine exists in an unusual open aminal form is of most interest. The complexes formed are diastereomerically pure and have a D- or L-configuration at the chelate point. The study 103 is undoubt- edly one of the key studies in the coordination chemistry of carbohydrates, since it demonstrates extreme flexibility of unpro- tected monosaccharides in complexation with metals. The oxidation products of the glycosidic hydroxy group of 2-amino sugars, i.e., the carbohydrate a-amino acids 109, coor- dinate d-metal ions only through the carboxy and amino groups.106 ± 108 Complexes 110 provide yet another illustrative example of the non-involvement of the carbohydrate fragment in the coordination.109 HO CO2H O CO O NH2 N M OH O (CH2OH)n CO O HO OH CO R O 110 109 n=3, 4.R =H, OH;M=Re, Tc. Monoamino sugars of the type 103a ± c form complexes 111 with platinum metals.9, 110 The protected amino sugar 112 yields complexes of analogous structures.9, 110 In the case of synthetic vicinal diamino sugars, coordination (e.g., with the formation of the complex 113) follows the same scheme.9 cis-[MCl2(H2NGlyc)2] 111 M=Pd, Pt; Glyc is a protected or free monosaccharide. 975 OMe O HO NH2 H2N Glyc6 H2N Pt 112 113 Cl Cl O O Me O Glyc6= . O Me O Me Me The reaction of protected N-glycosylaminocarboxylates 114 with platinum metal complexes gives complexes 115 ± 117.9, 111 Me Me R N O HN CO¡ O 2 Pd Glyc6 HN 115 114 Glyc6 R=H, CH2CHMe2, CH2Ph, (CH2)2SMe.Cp Cp M M O Cl Cl O N O O Glyc6 N Glyc6 H Me 116 Me 117 SMe M=Rh, Ir. Complexes of N-glycosylaminocarboxylate 50 with Cu2+ (118) have a square-planar structure, whereas with Co2+ and Ni2+ ions, octahedron 119 is formed involving the hydroxy groups at the C(5) atom of the carbohydrate fragment.19, 112 ± 115 H H Glyc2 Glyc2 5 OH OH H H H H 2+ N N N N H R H M2+ Cu R H R O O O O O O 5 HO HO H Glyc2 H Glyc2 119 118 M=Ni(II), Co(II). The structures of complexes of a carbohydrate amino deriva- tive 49 with d-metal cations depend on the basicity of the medium. In alkaline media, reactions with the corresponding metal acetates yield monocarbohydrate complexes 120 and 121, while in neutral media, biscarbohydrate complexes 122 are formed.66 HO HO AcO H2O Co Cu O O HN NH HO HO HO HO 121 120 Glyc2 Glyc2O OH M HO HN HN OH HO HO O HO 122 Glyc2 Glyc2 M=Cu(II), Ni(II).Some papers describe metal complexes of the Schiff bases prepared from natural and synthetic amino sugars. Thus the976 Schiff bases 123a,b of both unprotected and protected D-glucos- amine form ordinary bischelates 124,116 ± 118 while the carboxylic derivative 123c reacts with Cu(II) to give the binuclear bridging complex 125.117 In the case of Pt(II), the coordination involves the nitrogen atoms to yield complex 126 7 due to the absence of a donor substituent in the ortho-position of the aromatic ring (all the hydroxy groups in the carbohydrate fragment are acetylated).R1O OR2 O OR1 R1O N HOX 123a ± c a: R1=Ac, R2=Me, X=H; b: R1=R2=X=H; c: R1=Ac, R2=Me, X=CO2H. O N O Cu O O Cu O N O 125 Mixed zero-valent palladium complexes 127 in which the double Schiff base is used as a carbohydrate ligand and dimethyl fumarate, fumarodinitrile and maleic anhydride are used as alkene components, have been synthesised.119. N N Glyc7 Glyc7 Pd C C OAc O Glyc7= . OAc AcO 127 The coordination capabilities of amino sugars can be signifi- cantly enlarged owing to the introduction of coordinating frag- ments into the amino group as can be exemplified by the b-oxoenamino derivatives 128, which form binuclear complexes 129 with copper.120 Me O O OH Me HN O OH HO O O Me Me 128 The ability of isocyanides, e.g., compound 130 obtained from D-glucosamine, to enter into ligand exchange reactions was successfully employed 121 in the preparation of complexes 131 ± 136 with different structures.Complexes 132 and 134 can add other isocyanides as well as amines and alcohols, e.g., with the formation of complexes 137.122 N O M N O 124 M=Cu(II), Zn(II), Co(II) VO2+. OMe Cl N Pt N Cl 126 OMe OAc O Me Me O OH NH Cu + O O O O + Cu HN O Me Me 129 O AcO Cl M CNR1 O OAc OAc AcO + N C7 130 (CNR1) M=Pd, Pt. PPh3 + Cl Au CNR1 Cl Pt CNR1 BF¡4PPh3 133 Cp Cl Au C M CNR1 Cl Cl 136 M=Rh, Ir.R2=But, Ph. Special mention should be made of a yet another interesting peculiarity of the complexation of the compounds under discus- sion. The feasibility of a transfer of chirality from a carbohydrate ligand to a metal centre in metal complexes of chemically modified carbohydrates was demonstrated above. A most illustrative example of this phenomenon is the reaction of the diol derivatives 138 (obtained from tartaric acid) with silver triflate,122 which yields either the polymeric complex 139 in the form of a right- or left-handed helix (supramolecular polymeric complexes, heli- cates 123), or, in the case of the meso-diol, the cyclic dimer 140, depending on the chirality of the starting diol.O NN O 138 O O N N Ag+ Ag+N N O O140 IV. Metal complexes of phosphorus-, sulfur- and selenium-containing monosaccharide derivatives The discovery by Wilkinson of the ability of chlorotris(triphenyl- phosphine)rhodium(I) to efficiently catalyse homogeneous hydro- genation of alkenes at room temperature 124 gave a strong impetus to the appearance of a vast number of publications devoted to the molecular design and applications of this type of catalyst includ- ing enantioselective ones. This, in turn, generated the need for the synthesis of some organophosphorus carbohydrate derivatives, such as phosphines, phosphinites, phosphites and hydrogen phosphonates and phosphates. Chiral phosphines DIOP (141) are the most popular phos- phine ligands used in catalysis; the ruthenium complex DIOP ±Ru2Cl4[(7)-DIOP]3 has a hypothetical structure 142.125, 126 Yu A Zhdanov, Yu E Alekseev Cl Cl Cl Pt CNR1 CNR1 PPh3 132 131 (OC)5M CNR1 135 134 M=Cr, W. NR1H NR2H 137 O O N Ag+ N NAg+ N 139Basic achievements in the coordination chemistry of modified monosaccharides PPh2 Cl Cl O Me P P P Ru Ru P Me P P O PPh2 Cl Cl 142 141 (P P) The monophosphine ligand 143 reacts with Pd(II) to give complex 144.127 O Cl Ph Ph Ph O HO Pd P Ph Glyc P Cl Glyc Ph OMe OPPh2 144 143 (GlycPPh2) A series of [Rh(cod)(PPh2Glyc)2]ClO¡4 complexes (cod is cyclooctadiene) have been synthesised from acetalated monosac- charide monophosphine derivatives, e.g., compound 145.7 PPh2 O O Ph O O O Me Me O O O O OMe Ph2P O + Me (cod)Rh PPh2 Me 146 145 (GlycPPh2) Synthetically more accessible phosphinites react with Rh(III) to yield complexes 146 3, 128 ± 132 similar to monophosphine com- plexes of the acetalated ligands 145.Many studies by Russian investigators have been devoted to the complexation of carbohydrate phosphites (see, e.g., a review 133). Thus hydrogen phosphonates derived from acetalated monosaccharides 147 yield palladium complexes 148.134 ± 137 OR OR Glyc O P(O)HOR Cl 147 Pd H Pd Cl O O P O OGlyc H OR POGlyc O P OGlyc ROPOGlyc R=Et, Pr, Pri; Glyc is an acetalated monosaccharide residue.148 Treatment of the readily available phosphite (Glyc3O)3P with Cu(CO)Cl gives the tetramer {CuCl[P(OGlyc3)3]}4 having a cubic structure.138 The bicyclophosphite complexes LCuX (L=149; X=Br, I),139 ± 142 with a cubic structure (150) in the solid state, have been prepared.143 LCu O X X O CuL O O X P CuL O LCu X O 150 149 Ch Phosphorodiamidites 151 form complexes (Z3-C3H5)Pd(L)Cl and L2Rh(CO)Cl 144 as well as trans-[PtCl2L2] (L=151).145 Their structures and the coordination sites of metal ions have not been established.144, 145 Glyc is an acetalated monosaccharide residue. Glyc OP(NEt2)2 151 Carbohydrate phosphates have been investigated in less detail. The preparation of a cobalt complex with the phosphonate 152 146 having a hypothetical dimeric structure 153 has been reported. 977 OMe O Glyc2 O MeO P Co P O O Co P OMe O O O Glyc2 OMe OH Glyc2 152 153 Some papers are devoted to sulfur- and selenium-containing monosaccharides, particularly thiolates and selenolates.These compounds are, in principle, soft bases, therefore the complexes of the corresponding carbohydrate derivatives with soft cations, such as Au+, and platinum metal cations (the principle of hard and soft acids and bases 147), have been studied first. Thus dithiol 154a forms a mixed complex 154b with gold(I).148 The HS7SH group in the dithiol ± rhodium complex 155a forms a part of the chelate centre (structure 156).7 The iridium cation in complex 157 prepared from the thioethers 155b (R=Me, Pri, Ph) is coordi- nated in a similar way.7 SR SH SX O O HO O O HO SH O SR O Me Me Me Me 155b (RS SR) 155a (HS SH) SX 154a,b a: X =H b: X=AuPEt3.SR + S S (cod)Ir BF¡4 SR Rh(CO)2 (OC)2Rh 157 156 The readily available carbohydrate dithianes react smoothly with K2[PtCl]4 ; thus compound 158 yields the complex 159.7 S S H Glyc OH HO S Cl OH Pt Cl S OH 159 OH S Glyc 158 S A fairly large number of papers are devoted to complexes of acetylated b-D-thioglucose 160. Gold complexes 161 are used in medicine 149, 150 along with analogous gold complexes containing a deacetylated carbohydrate fragment. Iron, ruthenium, tungsten and platinum complexes 162 ± 165 with acetylated thioglucose have been described.7 Cp Fe SGlyc7 OC Glyc77SH 160 CO Glyc77SAuR3 161 R=Me, Et, Pri, Bu.162 Cp Cp SGlyc7 OC W Pt SGlyc7 Ru Ph3P SGlyc7 SGlyc7 Ph3P OC SGlyc7 CO CO 165 164 163 Some complexes of sulfur- and nitrogen-containing carbo- hydrate ligands have been investigated. Thiocarbamates 166 react978 with bridge binuclear platinum complexes to yield complexes 167 ± 172.7 Cp Me Me S Cl M NPd Glyc7 NH Cl OR Cl OR S 166 S RO NHGlyc7 NHGlyc7 R=Me, Et. 168 167 M=Rh, Ir. + Cl Bu3P Ph3P M Pt + OR OR BF¡4Cl Ph3P S S NHGlyc7 NHGlyc7 170 169 M=Pd, Pt. OR H RO N Glyc7 Glyc7 NH S Cl Cl Cl S Pd Pt Pt Cl Cl S S Cl OR Glyc7 Glyc7 N OR H NH 172 171 The metal atom in complexes with the thiourea ligand 173 (L) [M(Z5-Cp)Cl2L] (M=Rh, Ir), [Pd(o-C6H4CH2NMe2)ClL] and [MCl2L2] (M=Pd, Pt) is coordinated through the nitrogen atoms.7 The ligand 174 containing an N,N-dithiocarbamoyl frag- ment forms the complex 175 with bidentate S,S-coordination.7 Cl S S S Pt Cl S Glyc6 Glyc7 NH NEt2 S Et2N NH2 Glyc6 174 173 175 The dihydrooxazole thioglucose derivatives 176 form com- plexes 177 and 178 with palladium.7 By virtue of its allylic structure, the latter complex is represented by two diastereomers differing in the position of the central allylic proton relative to the dihydrooxazole substituent R.N S S N + O Pd PF¡6Pd Glyc7 N S Cl Me Ph Ph R 178 177 176 7S N R=Pri, Ph. p-Allylic complexes are also typical of ligands 179 and 180 containing both sulfur and phosphorus. Thus the aryl ligand 179 forms complexes 181,7, 151 whereas the ferrocenyl ligand 180 yields the complex 182.7, 152 Zero-valent palladium complexes 183 have also been prepared.An analogous derivative of the unsaturated thio sugar 184 coordinates the Rh(II) cation to yield the structure 185.7 Me S Glyc7 S PR2 Glyc7 H 179 FeCp Ph2P 180 R=Ph, Ch. Yu A Zhdanov, Yu E Alekseev + Me Pd Glyc7 Glyc7 S S R2P S Ph2P + Pd Ph Glyc7 PPh2 Pd Ph 182 181 O O X OAc P 183 + AcO O (cod)Rh BF¡ S PPh2 4 S 185 184 S P Selenium-containing monosaccharides described in the liter- ature are represented only by the selenoglucose derivative 186, which was used as a source of complexes 187 ± 191 having different structures.7, 153 The latter complex represents a mixture of three diastereomers 191a ± c.153 Se Glyc7 Ph3P Pt R3P Au Se Glyc7 SeH Glyc7186 187 Se Glyc7 Ph3P 188 R=Me, Et, Ph.CO Cp Cp Se Glyc7 Glyc7 Glyc7 Se Se Fe Ti M(CO)3 (OC)3M Se Cp Se Glyc7 Cp Glyc7 191a 190 189 Glyc7 Glyc7 Glyc7 Se Se Glyc7 Se Se M(CO)3 (OC)3M M(CO)3 (OC)3M 191b 191c M=Fe, Ru. V. Conclusion The progress in the coordination chemistry of carbohydrates is favoured by three main factors, viz., inexhaustibility of this class of compound due to continuous photosynthetic reproduction, the diastereomeric purity of carbohydrates and the chirality of metal complexes formed from them, which is associated with chirality transfer from the ligands to the metal centres.Chemical modification of monosaccharides enables one to enhance considerably their coordination capabilities, particularly owing to the diversity of their oxygen donor centres. Moreover, hydrophobisation of starting monosaccharides makes possible the preparation and further utilisation of their metal complexes in non-aqueous media. Localisation of coordination points of monosaccharide metal complexes localised in close proximity to the ligands often changes their conformation, anomeric configuration and even the nature as a result of molecular rearrangements (see, e.g., Ref. 6 and this review). 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年代:2002

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