摘要:

DALTON PERSPECTIVE J. Chem. Soc., Dalton Trans., 1999, 97–105 97 Rhenium sulfide cluster chemistry † Taro Saito Department of Chemistry, The University of Tokyo, Hongo, Tokyo 113-0033, Japan Received 24th August 1998, Accepted 6th October 1998 Rhenium sulfides are still very rare except for octahedral cluster compounds. The chemistry of trinuclear, tetranuclear and octahedral cluster compounds is reviewed in comparison with that of molybdenum sulfide clusters and in the light of the relationships between molecular and solid state cluster compounds. A promising outlook is expected. 1 Introduction Rhenium is one of the rarest elements on the earth and its concentration in the earth’s crust is only 0.0004 ppm.1 No ore deposit containing rhenium as the main metal component is known and rhenium is produced as a by-product in the metallurgy of other metals, especially of molybdenum. A pure rhenium mineral with a composition between ReS2 and Re2S3 was discovered recently from a volcano in the Kuril islands.2 This is claimed to be the first natural mineral with Re as the only cation and the first example of macroscopic Re mineralization.There are two known rhenium sulfides. One is the disulfide ReS2 and the correct structure was determined recently by single crystal X-ray analysis.3,4 The other is the heptasulfide Re2S7 which has been obtained only in amorphous forms and the structure is still unknown.5 Both solid state and molecular sulfide cluster compounds are less abundant than those of molybdenum which otherwise is in many ways similar to rhenium.6,7 Because of the paucity of good starting com- † Dedicated to Professor Warren Roper on the occasion of his 60th birthday.Taro Saito was born in 1938 and received a Doctor Degree of Engineering from the University of Tokyo in 1966. He spent two years (1967–1969) as a Ramsay fellow at the Inorganic Chemistry Laboratory, University of Oxford, UK while he was a research associate of the University of Tokyo.He moved to the Department of Chemistry in 1970. He became a professor at the Department of Synthetic Chemistry, Osaka University in 1982 and came back to the University of Tokyo as a professor at the Department of Chemistry in 1989. He is now a professor emeritus at the department. His current research interests are rational synthesis of metal cluster compounds and the relationships between the molecular and solid state clusters. Taro Saito pounds, the chemistry of the sulfide cluster compounds of rhenium is still in a very early stage of development.There is, however, good reason to expect that the chemistry of the rhenium sulfide clusters and their congeners of selenium and tellurium will be as rich as that of molybdenum. The present article focuses on the characteristics of this chemistry compared with those of the early or late transition metals, molybdenum in particular,8 and on the relationship between the molecular and solid state clusters.9 2 Position of rhenium in the periodic table Rhenium being a Group 7 element located between the early and late transition metals, forms compounds having properties intermediate between those expected for these extremes.The ground state electronic configuration is [Xe]4f145d56s2 which allows for the formation of rhenium compounds in oxidation states from 2III to 1VII (d10 to d0).1 The number of valence electrons in the same oxidation states is larger by one than that of molybdenum which is located in a diagonal position to the upper left.If the isoelectronic analogy holds, we can expect compounds of rhenium in an oxidation state one electron higher than those of molybdenum. In cluster compounds with a metal nuclearity of n and with the same number of anionic ligands, the isoelectronic relationship can be realized by changing n monovalent anions to n divalent anions, or by introducing counter anions Xn2.The replacement of halogens by chalcogens has proved very fruitful in the preparation of new rhenium compounds with similar cluster frameworks to those of molybdenum. This has been the case especially for the octahedral cluster compounds. The number and variety of the rhenium sulfide clusters are still limited as seen in Table 1 which lists the reported trinuclear and tetranuclear cluster compounds. 3 Triangular clusters The first halogen–sulfur mixed anion clusters were reported by Table 1 Trinuclear and tetranuclear rhenium sulfide cluster compounds Compound [Re3S7Cl6]Cl [Re3S7Cl6]AlCl4 [Re3S7Br6]Br [PEt3H][Re3S4Cl6(PEt3)3] [Re3S4Cl4(PEt3)3] [Re3S4Br3(PEt3)4] [Et4N][Re3S3(SO2)Cl6(PEt3)3] [NH4]4[Re4S22] [Ph4P]4[Re4S4(CN)12] [Cs2K2][Re4S4(CN)12] [Re4S4Cl8(TeCl2)4] R4S4Te4 K8[Re4S2(SO2)4(CN)10] ReS2 MCE 66689 10 8 12 12 12 12 12 14 12 Ref. 10 11 11 14 20 20 22 26 28 30 31 34 24 3, 4098 J. Chem. Soc., Dalton Trans., 1999, 97–105 Timoshchenko et al.10,11 Re3S7Cl7 1 (Fig. 1) was prepared by the reaction of ReOCl4 with sulfur in a solution in S2Cl2 at 200 8C. The product is a black crystalline compound and relatively stable in air. It is insoluble in S2Cl2, CCl4, CS2, hydrocarbons, and acetonitrile, but soluble in pyridine, DMSO, and DMF. The yield was 80% based on ReOCl4 and the reaction equation is considered to be 6 ReOCl4 1 27 S 2 Re3S7Cl7 1 3 SO2 1 5 S2Cl2 Similar compounds Re3S7Br7 and Re3S7Cl6AlCl4 were prepared by the reaction of Re2O7 with S2Br2, and with a solution of AlCl3 and S2Cl2, respectively.The structures of these compounds comprise a triangular rhenium framework, a capping sulfur and three edge-bridging S2, and six terminal halogen ligands. Thus the structures are formulated as [Re3(m3-S)(m-S2)3X6]X. The counter anion has weak “secondary” bonding with the S2 ligands. The whole structures are very like those of the molybdenum analogue [Et4N]2[Mo3S7Cl6],12 which is isoelectronic with the rhenium cluster.The oxidation state of rhenium is 1V (d2), while that of molybdenum 1IV (d2). The Re–Re distances (2.698 Å for 1) are somewhat shorter than the Mo–Mo distances (2.758 Å) reflecting the higher oxidation state. The molybdenum cluster has been prepared from a chain cluster compound Mo3S7Cl2Cl4/2 13 by the scission of the bridging chlorine with concomitant introduction of two terminal chlorine ligands. In order to keep the isoelectronic relationship, the pentavalent rhenium atoms require three extra halogens and the result is the formation of the ionic cluster compounds instead of the bridged chain compound.Trinuclear rhenium sulfide cluster complexes with triethylphosphine ligands have been prepared by the reaction of Re3S7Cl7 with triethylphosphine in benzene. One of the sulfur atoms in the S2 bridging ligands is removed as triethylphosphine sulfide and the vacant coordination sites are occupied by triethylphosphine ligands.Two cluster complexes have been isolated from these reactions. The first one, crystallized from a benzene solution, is [PPh3H][Re3(m3-S)(m-S)3Cl6(PEt3)3] 2 which has a triangular rhenium core capped and bridged by sulfur ligands.14 Each rhenium is coordinated by a triethylphosphine and two chlorine ligands. All the triethylphosphine ligands are directed below the Re3 plane capped by the m3-S atom and the chlorine ligands are above the plane (Fig. 2). The Re–Re distances are 2.715 Å, 2.716 Å, and 2.725 Å and the Re–Re–Re angles range from 59.878 to 60.238.Therefore the cluster core distorts slightly from a regular triangle. The cluster part is a monovalent anion with 8 MCE (metal cluster electron) and the rhenium atoms are in the mixed valence oxidation state of 2 × Re(IV) and Re(V) Fig. 1 Structure of [Re3(m3-S)(m-S2)3Cl6]1 in 1 (Re black, S yellow, Cl light green). (= 113/3). The Re–Re distances are slightly longer than those in the starting 6-electron compound (2.701 Å),10 reflecting the lower oxidation states of rhenium atoms.The Re–Re distances are shorter than those of [Mo3(m3-S)(m-S)3Cl4(PEt3)3(MeOH)2] (Mo–Mo 2.738–2.780 Å)15 and may be explained by the smaller Re metallic radius (1.29 Å vs. 1.31 Å for Mo).16 The two cluster complexes diVer in the number of MCE, and are not isoelectronic. The reason why an 8-electron cluster complex forms preferentially from the 6-electron cluster compound Re3S7Cl7 is not clear.Probably the reducing conditions with excess triethylphosphine are responsible for the formation of the more reduced cluster. The cluster core is very similar to the molybdenum analogues 8 which have proved useful for the construction of larger cluster compounds.17 For example [Mo3Ni(m3-S)4- (H2O)10]41 has been prepared by the condensation of [Mo3S4- (H2O)9]41 with metallic nickel.18 The reaction of the rhenium cluster 2 with Ni(cod)2 formed a mixed metal tetrahedral cluster complex [Re3Ni(m3-S)4Cl6(PEt3)4] 3 (Fig. 3).19 The other trinuclear cluster complex formed under similar reaction conditions, but crystallized from acetone, is [Re3- (m3-S)2(m-S)2(m-Cl)Cl3(PEt3)3] 4.20 (Fig. 4). The cluster core consists of three rhenium atoms with two capping sulfur, two bridging sulfur and a bridging chlorine atom. A chlorine atom is located in the bridging position by comparison of the structure with that of an analogous 10-electron cluster complex [Re3(m3-S)2(m-S)2(m-Br)Br2(PEt3)4].20 The Re–Re distances are 2.628–2.677 Å (mean 2.653 Å) and the Re–Re bond with the bridging chlorine is the longest one.The distances are considerably shorter than the mono-capped triangular rhenium cluster. Each rhenium is coordinated by a terminal chlorine and a triethylphosphine ligand. This complex has 9 MCE with 3 Re(IV) Fig. 2 Structure of [Re3(m3-S)(m-S)3Cl6(PEt3)3]2 in 2 (Re black, S yellow, Cl light green, P red, C blue). Fig. 3 Structure of [Re3Ni(m3-S)4Cl6(PEt3)4] 3 (Re black, Ni light blue, S yellow, Cl light green, P red).J.Chem. Soc., Dalton Trans., 1999, 97–105 99 centers in contrast with the mono-capped cluster 2 and is paramagnetic. The bi-capped triangular cluster of rhenium resembles the molybdenum cluster [Mo3(m3-S)2(m-S)3(PMe3)6] with 8 MCE.21 The Mo–Mo distance is 2.714 Å and is longer than the Re–Re distances. In the bi-capped molybdenum and rhenium clusters, the number of MCE is larger than necessary for 3 M–M 2c–2e bonds (6 electrons) and the excess electrons are considered to enter the a10 orbital for the molybdenum cluster and e9 orbital for the rhenium cluster.The molybdenum cluster has a regular triangular core and the rhenium cluster has an isosceles triangle one. The reaction of Re3S7Cl7 with PEt3 in benzene at room temperature under a nitrogen atmosphere for 1 week and with addition of Et4NCl to the reaction solution formed large black crystals of a compound with the formula [Et4N][Re3(m3-S)- (m-SO2)(m-S)2Cl6(PEt3)3]?2Me2CO 5.22 The Re atoms form a monocapped triangle and are bridged by two S and one SO2 ligands (Fig. 5). The Re–Re distances are significantly longer (mean 2.81 Å) than in any of previously observed triangular Re–S clusters, the shortest one is bridged by SO2 (2.79 Å). Examples of SO2 bridged Re–Re bonds are also found in binuclear [Re2(SO2)2(CN)8]62 (2.636 Å) 23 and rhombic [Re4- (m3-S)2(m-SO2)4(CN)10]82 (2.837 Å).24 Another feature is the very short distances between the Re and the m-S atoms.At 2.24–2.26 Å, this is considerably shorter than those in other compounds having Re–S–Re bridges (average value is about 2.30 Å).14 Thus the incomplete cubane framework Re3S4 has a high degree of distortion in 5. The S]] O distance (1.47 Å ) is slightly shorter than that found in both the above mentioned rhenium clusters with bridging Fig. 4 Structure of [Re3(m3-S)2(m-S)2(m-Cl)Cl3(PEt3)3] 4 (Re black, S yellow, Cl light green, P red, C blue). Fig. 5 Structure of [Re3(m3-S)(m-SO2)(m-S)2Cl6(PEt3)3]2 in 5 (Re black, S yellow, Cl light green, P red, C blue). SO2 (1.49 Å). Only two of the three PEt3 ligands are found in positions trans to the capping sulfur, the third phosphine ligand being cis to m3-S and trans to m-SO2. It is assumed that the SO2 ligand is acting as a 4-electron donor, SO2 22. It is likely that the cluster [Re3(m3-S)(m-S)3Cl6(PEt3)3]2 forms first and then is oxidized by oxygen to give 5.In the electron-rich 8-electron [Re3- (m3-S)(m-S)3Cl6(PEt3)3]2, the two extra electrons in the Re3S4 51 core are delocalized over the Re3(m-S)3 ring, given the closeness of energy of Re 5d and S 3p AO as shown in the MO calculations for analogously built incomplete Mo3S4 41 cubes.25 This delocalization enhances negative partial charge on bridging sulfur atoms and facilitates attack by an electrophilic agent (O2) to give SO2. 4 Tetrahedral clusters The first tetrahedral cluster of rhenium [NH4]4[Re4(m3-S)4- (m-S3)6] 6 was synthesized by Müller et al.upon heating a solution of [NH4][ReO4] with an aqueous ammonium polysulfide solution.26,27 The cluster core consists of a Re4 tetrahedron capped by a sulfur atom on each face and bridged by a S3 ligand on each of the six edges (Fig. 6). The Re–Re distances are 2.763 Å and the Re–m3-S distances are 2.319 Å. The S3 ligands bridge the rhenium atoms in envelope shapes with S–S distances of 2.140 Å.The compound is diamagnetic with 4 Re(IV) and 12 MCE to be assigned to the 6 Re–Re bonds. GriYth et al. obtained [Re4Q4(CN)12]42 7 in attempts to repeat the preparation of the salts of [Re(CN)6]2 from K2[ReCl6], KSCN and excess KCN.28 The structures of the tetraphenylphosphonium salts turned out to be tetrahedral clusters [Ph4P]4[Re4Q4(CN)12]?3H2O (Q = S, Se) (Fig. 7). The sulfide anion contains a tetrahedron of bonded rhenium atoms Fig. 6 Structure of [Re4(m3-S)4(m-S3)6]42 in 6 (Re black, S yellow).Fig. 7 Structure of [Re4(m3-S)4(CN)12]42 in 7 (Re black, S yellow, C blue, N orange).100 J. Chem. Soc., Dalton Trans., 1999, 97–105 (mean Re–Re 2.755 Å) with one sulfur atom per face of the tetrahedron, equally bonded to each rhenium atom (mean Re–S 2.34 Å). Three CN groups are attached to each Re atom making the coordination number of the rhenium atom 9 including the Re–Re bonds. The cluster compound is diamagnetic and the number of MCE is 12 for the tetravalent rhenium atoms (d3) consistent with 6 metal–metal single bonds.In the isoelectronic and isostructural molybdenum cluster K8[Mo4S4(CN)12]? 4H2O,29 the Mo–S distances (2.38 Å) are almost the same but the metal–metal distances are much longer (2.854 Å) reflecting not only a larger ionic radius of Mo(III) than that of Re(IV) but also probably a weaker metal–metal bonding interaction. Recently, a high-yield synthesis of the [Re4S4(CN)12]42 anion has been attained by the reaction of an aqueous solution of Re3S7Br7 11 and KCN at room temperature for 60 min.30 The addition of CsCl formed Cs2K2[Re4S4(CN)12]?2H2O in 78% yield.The reaction is believed to proceed via trinuclear [ReV 3S4]71 and binuclear [ReVI 2S4]41 yielding the [ReIV 4S4]81 compound. A new series of tetrahedral rhenium chalcogenide cluster compounds with novel TeCl2 ligands has recently been reported.31 The sulfide (8) among [Re4(m3-Q)4Cl8(TeCl2)4] (Q = S, Se, Te) is prepared by the reaction of ReCl5, elemental sulfur, and elemental tellurium at 400 8C for 48 h in 93% yield.The Re–Re (2.706–2.742 Å) and Re–S (2.332–2.349 Å) distances are near those of the other Re4 clusters mentioned above and the most prominent feature is the presence of the coordinated TeCl2 ligand (Fig. 8) (Re–Te 2.725 Å, Te–Cl 2.328 Å, and Cl–Te–Cl 96.58). The TeCl2 is stabilized upon bonding to the rhenium cluster core. The cluster compound is insoluble in organic solvents and water but reacts slowly with DMF to form [Re4(m3-S)4Cl8(DMF)4] and with KCN in water to form K4[Re4- (m3-S)4(CN)12].A rhenium telluride [Re4(m3-Te)4Br8(TeBr2)4] having a similar tetrahedral Re4 cluster core and TeBr2 ligands has also been reported.32 The tetrahedral rhenium cluster Re4S4Te4 9 was synthesized by Fedorov et al.33 and characterized by a single crystal study as well as by powder X-ray diVraction.34 The compound was obtained from the reaction of rhenium metal, elemental sulfur and tellurium in the ratio of 1:1:6 at 900 8C for 3 weeks.The Re4 cluster core is a tetrahedron capped by a sulfur atom on each face forming a distorted cube of Re4S4. The Re–Re distance is 2.785 Å and the Re–S distance is 2.337 Å. Each rhenium atom is further coordinated by three bridging tellurium atoms at a distance of 2.790Å (Fig. 9). The short Re–Re distance results from the tetravalent rhenium atoms which leave 12 MCE for the 6 metal–metal single bonds.The structure is of the type found in those of the chalcogeno halides of niobium and molybdenum M4Q4X4 (M = Nb, Mo; Q = S, Se; X = Cl, Br, I) 35,36 which have been described as an NaCl-type arrangement Fig. 8 Structure of [Re4(m3-S)4Cl8(TeCl2)4] 8 (Re black, S yellow, Cl light green, Te green). of M4Q4 clusters and tetrahedral X4 fragments. The rhenium analogue can also be regarded as a NaCl-type or zinc-blendetype arrangement of Re4S4 clusters and Te4 tetrahedra, or a distorted spinel in which the octahedral cavities formed by sulfur and tellurium atoms are occupied by rhenium atoms and the tetrahedral cavities are vacant.The NaCl type (Te–Te 3.60 Å) and zinc-blende type (Te–Te 3.49 Å) are diVerent in the assumed tetrahedra of tellurium atoms. X-Ray emission and X-ray photoelectron studies of Re4S4Te4 have been reported.37 5 Rhomboidal clusters The reaction of amorphous Re2S7 with aqueous CN2 solution at 85 8C formed a crystalline cluster compound [Re4(m3-S)2- (m-SO2)4(CN)10]82 10 together with [Re4(m3-S)4(CN)12]42.24 The cluster anion has a rhomboidal Re4 framework capped by two m3-S atoms.The four edges are bridged by SO2 ligands which are considered to be formed by the oxidation of the m-S ligands in the intermediate product ‘[Re4(m3-S)2(m-S)4(CN)10]82’. The oxidation states of the rhenium atoms are III and IV and the number of MCE is 14. As 5 Re–Re bonds require only 10 electrons, Müller claims that the localization of negative charge on the central Re(III) atoms leads to the formation of considerable double-bond character in the transannular Re–Re bond.24 Rhomboidal molybdenum sulfide clusters [Mo4(m3-S)2(m-S)4- X2(PMe3)6] (X = SH, Cl, Br, I, SCN) have been reported.38,39 The molybdenum atoms are in mixed valence states represented by Mo(IV)2Mo(III)2.The number of MCE is 10 corresponding to an electron-precise cluster core with 5 Mo–Mo bonds (2.817–2.828 Å) for the Cl derivative. The transannular Mo–Mo distance (2.817 Å) is considerably longer than the transannular Re–Re distance (2.740 Å) reflecting the single bond nature. The DV-Xa MO calculations on the model compound [Mo4S6Cl2(PH3)6] have shown that the Mo–Mo bonding orbitals are strongly mixed with Mo–S orbitals and the HOMO–LUMO gap is 1.55 eV.The gap is larger than that in the [Re4S2(SO2)4]21 fragment (1.3 eV) obtained by EH-SCCC MO calculations.24 According to Müller, the greater number of valence electrons for the rhenium compound results in stronger M–M bonding, which is further strengthened because of the larger 5d–5d overlap than the 4d–4d overlap.Also it is suggested from the comparison of the rhenium and molybdenum cluster compounds that the rhomboidal rhenium cluster framework is stable in the presence of 4 excess MCE. However, whether these electrons enter the “transannular p* orbital” forming a Re–Re doublebond does not seem very clear, since the transannular metal–metal distances in [Mo4S6Cl2(PMe3)6] (10-electron, electron-precise), ReS2 (12-electron, electron-precise) and [Re4S2(SO2)4(CN)10]82 (14-electron, excess electron) are always the shortest ones.Therefore the shortness may be due to a steric Fig. 9 Connectivity in Re4S4Te4 9 (Re black, S yellow, Te green).J. Chem. Soc., Dalton Trans., 1999, 97–105 101 constraint in the cluster framework composed of amalgamation of two M3(m3-S) triangular units.Rhenium disulfide had been considered to be isostructural with ReSe2 before the single-crystal X-ray diVraction study of Murray et al.3 Layers of nearly hcp arrays of sulfur atoms stack along the a axis and are nearly parallel to the bc plane of the unit cell. The Re atoms occupy the octahedral sites between every other pair of the hcp layers of sulfur atoms. Each rhenium atom is coordinated by six sulfur atoms which in turn have trigonal pyramidal coordination to three rhenium atoms.Consequently, rhenium layers are sandwiched between two sul- fide layers forming a distorted CdCl2-type structure. The distances between the rhenium atoms are close enough to invoke Re–Re bonds and rhenium atoms form Re4 parallelograms with Re–Re distances 2.790 Å and 2.824 Å and the shorter distance between the apexes is 2.695 Å (Fig. 10). The study has indicated that the crystal is not isostructural with the selenide in the sense that the fractional coordinates of corresponding atoms in different structures are similar.However, more recent comparative studies of ReSe2 and ReS2 have suggested that Murray et al. overlooked the doubling of the c axis in their structure description. 40 The new crystallographic study has indicated that the corresponding Re–Re distances in the Re4 cluster are 2.805 Å, 2.800 Å, and 2.693 Å. A scanning probe microscopy study of ReS2 has been reported.4 The rhomboidal rhenium cluster framework is the characteristic feature of the tetravalent d3 rhenium disulfide and diVers from molybdenum disulfide in which each molybdenum atom is in a trigonal prismatic coordination site and no apparent Mo– Mo bonds exist.41 It is considered that metal–metal bonding is more favourable for rhenium than for molybdenum because of the larger 5d–5d overlap than the 4d–4d overlap.24 The number of MCE for the Re4S8 unit is 12 resulting in 5 intracluster Re– Re bonds and 1 intercluster Re–Re bond.The molybdenum is in the tetravalent d2 state and has one less electron for each metal.Therefore if a triangular molybdenum cluster forms Mo3S6 (=MoS2) can use 6 MCE with 3 Mo–Mo bonds. This form of molybdenum disulfide may have Mo3(m3-S)(m-S)3- (m3-S)6/3.42 Re2S7 has been obtained only in amorphous phases. The Fig. 10 Connectivity in ReS2 (Re black, S yellow). structure of the compound has been studied by a radial distribution analysis using experimental intensities from the powder patterns.5 The study shows that it contains a Re4 cluster framework similar to that of ReS2.The Re–Re distances range from 2.6 to 2.9 Å and the shortest Re–S distance is 2.38 Å. The formal oxidation state of rhenium is heptavalent but Müller and his co-workers consider that the compound contains tetrahedral and rhomboidal Re4 cluster units linked irregularly through Sx 22 ligands.24 Thus the actual oxidation state of rhenium is lower than tetravalent and the compound can be considered a “quasi-solid” solution of clusters in Sx 22 solvent.The intramolecular oxidation of x S22 into Sx 22 by 2 x electrons with the concomitant reduction of ReVII into lower oxidation states is the reason for the presence of reduced metal cluster moieties. Also the presence of diVerent kinds of metal clusters and Sx 22 anions with diVerent chain lengths may be responsible for the failure of crystallization of Re2S7. The reaction of the compound with a CN2 solution forms two kinds of discrete cluster compounds, [ReIV 4S4(CN)12]42 and [Re4S2(SO2)4(CN)10]82.24 6 Octahedral clusters Octahedral rhenium cluster compounds with 3-D to 0-D dimensionality have been prepared by solid state synthesis.Neutral ligands are not contained in these cluster compounds. Halogen bridges in the 1-D or 2-D compounds can sometimes be split by either MX or neutral ligands to form 0-D clusters (Fig. 11). No solution methods to prepare discrete octahedral cluster compounds by condensation have been reported.Condensation of octahedral molecular clusters is a possible route to the solid state clusters of higher dimensionality. Table 2 lists the known and possible types of 0-D octahedral sulfide clusters with the combination of cations, monoanions, and neutral ligands. The MCE is fixed to 24-electron, because other oxidation states are rarely encountered. There is also the possibility of geometrical isomers such as cis, trans, mer, fac or isomerism due to the positions of the m3 ligands.It is easily seen that by elaborate combination of cations, divalent chalcogens, monovalent halogens, or neutral ligands, the total MCE remains 24-electron. As is evident from the small number of known examples, these combinations of ligands are not always easy to realize. The correspondence of the Werner type octahedral complexes and octahedral cluster complexes has been noted 43 and some geometrical isomers of the octahedral molybdenum,44 tungsten,45 and rhenium,46,47 cluster complexes have been isolated. The syntheses of several geometrical isomers of the octahedral rhenium cluster compounds coordinated by triethylphosphine ligands are remarkable.47 Some of the geometrical isomers have been intended for use as precursors in Fig. 11 Structure of [Re6(m3-S)8Cl6]42 (Re black, S yellow, Cl light green).102 J.Chem. Soc., Dalton Trans., 1999, 97–105 bridged assemblies such as [Re12Se16(PEt3)10][SbF6]2 or [(PEt3)5- Re6Se8(4,49-bipy)Re6Se8(PEt3)5].46–48 Similar dodecanuclear cluster complexes of cobalt,49 chromium,50 and molybdenum51 have been reported.Another type of bridged twin cluster [Bu4N]4[(Re6S5OCl7)2O] has been obtained by the reaction of rhenium metal, ReCl5, KCl and sulfur in the presence of a controlled amount of water at 850 8C for 4 days.52 In the structure of this compound, two identical [Re6S5OCl7]22 cores are linked by an apical oxygen ligand.Ligand substitutions are very general reactions used to prepare various kinds of complexes in Werner type or organometallic transition metal chemistry. These reactions have as yet been little explored for the early transition metal clusters with p-donor ligands.53 The conversion of terminal chlorine ligands in the octahedral chloride clusters of molybdenum and tungsten into alkyls oVers one of the rare examples.44,45,54 In this respect, the preparation of [Re6Q5(NR)Cl8]22 (R = methyl, benzyl) (Fig. 12) and [Re6Q5(NSiMe3)Cl8]22 (Q = S, Se) and the nucleophilic conversion of the NSiMe3 group into NH by Bu4F are important as new synthetic methods to prepare inorganic– organic hybrid compounds starting from cluster compounds prepared by high-temperature solid state synthesis.55 One of the face-capping chlorines in [Bu4N][Re6Q5Cl9] can be transformed also into divalent anions to form [Bu4N]2[Re6Q5ECl8] (E = O, S, Se, Te) by means of (Me3Si)2E.55 In solid state chemistry, a number of octahedral rhenium chalcogenide clusters have been reported by Fedorov,56–62 Table 2 Types of zero-dimensional octahedral rhenium sulfide clusters and reported examples Type M4[(Re6S8)X6] M3[(Re6S8)LX5] M2[(Re6S8)L2X4] M[(Re6S8)L3X3] [(Re6S8)L4X2] [(Re6S8)L5X]X [(Re6S8)L6]X2 M3[(Re6S7X)X6] M2[(Re6S7X)LX5] M[(Re6S7X)L2X4] [(Re6S7X)L3X3] [(Re6S7X)L4X2]X [(Re6S7X)L5X]X2 [(Re6S7X)L6]X3 M2[(Re6S6X2)X6] [(Re6S6X2)L2X4] [(Re6S6X2)L3X3]X [(Re6S6X2)L4X2]X2 [(Re6S6X2)L5X]X3 [(Re6S6X2)L6]X4 M[(Re6S5X3)X6] [(Re6S5X3)LX5] [(Re6S5X3)L2X4]X [(Re6S5X3)L3X3]X2 [(Re6S5X3)L4X2]X3 [(Re6S5X3)L5X]X4 [(Re6S4X4)X6] [(Re6S4X4)LX5]X [(Re6S4X4)L2X4]X2 [(Re6S4X4)L3X3]X3 [(Re6S4X4)L4X2]X4 [(Re6S3X5)X6]X [(Re6S3X5)LX5]X2 [(Re6S3X5)L2X4]X3 [(Re6S3X5)L3X3]X4 [(Re6S2X6)X6]X2 [(Re6S2X6)LX5]X3 [(Re6S2X6)L2X4]X4 [(Re6SX7)X6]X3 [(Re6SX7)LX5]X4 Example KCs3[(Re6S8)(CN)6] [Bu4N]2[cis-{(Re6S8)(PEt3)2Br4}] [Bu4N][mer-{(Re6S8)(PEt3)3Br3}] cis-[(Re6S8)(PEt3)4Br2] [(Re6S8)(PEt3)5Br]Br [(Re6S8)(PEt3)6]Br2 [Bu4N]3[(Re6S7Cl)Cl6] K2[(Re6S6Br2)Br6] K[(Re6S5Br3)Br6] [(Re6S4Cl4)Cl6] Ref. 68 47 47 47 47 47 71 70 67 71 M = Monocation or M2 = dication; X = monoanion; L = neutral ligands. Perrin,63–70 Batail,71,72 Bronger,73–82 Holm,46,83 Ibers 84,85 and others.86,87 They are prepared by high-temperature solid state synthesis in evacuated sealed tubes from the combinations of (a) metal 1 halogen 1 chalcogen, (b) metal chalcogenide 1 halogen, (c) metal halide 1 chalcogen, (d) metal halide 1 metal 1 chalcogen, (e) metal chalcogenide 1 metal halide, (f) metal 1 chalcogen halide, (g) metal 1 hydrogen sulfide, (h) metal 1 chalcogen 1 hydrogen, and the chemical compositions depend on the ratio of the reactants and heating conditions. 82,88,89 Recently the octahedral clusters Re6Q4Br10 and Re6Q8Br2 (Q = S, Se, Te) have been prepared by condensation from triangular rhenium bromide Re3Br9 and PbQ or CdQ.62 It is considered that the choice of lead and cadmium chalcogenides is very important to promote the reaction by thermodynamic control of the formation of lead or cadmium bromides.This kind of formal condensation of trinuclear rhenium clusters was once suggested by Perrin and Sergent.6 Some of the A4Re6Q12 type compounds have been prepared by an ion exchange method from Tl4Re6Q12.86,87 Topotactic oxidation of Na4Re6S12 at ambient temperature forms Re6S12.90 The octahedral rhenium chalcogenide clusters invariably have 6 Re(III) (d4) atoms (24 MCE) with 8 capping and 6 terminal ligands [Re6(m3-L)8L96 type].Depending on the ratio of chalcogen to halogen ligands, the connectivity of the Re6 cluster units changes to become molecular or 1-, 2- and 3-D cluster compounds. Replacement of two halogen atoms by a chalcogen atom or introduction of counter ions can make the oxidation state of the rhenium atoms the same (trivalent). The connectivity of the Re6 cluster frameworks is determined by the sum of the halogen and chalcogen atoms.Smaller numbers lead to higher connectivity. Table 3 lists the representative higher Fig. 12 Structure of [Re6(m3-S)5(m3-NMe)(m3-Cl)2Cl6]22. The m3-NMe group is disordered onto two opposite faces of the Re6 octahedron (Re black, S yellow, N orange, C blue, Cl light green). Table 3 Connectivity and dimensionality of octahedral rhenium chalcogenide clusters Compound Rb4Re6S13 Rb4Re6S12 Ba2Re6S11 Re6S8Br2 Re6S7Br4 Cs6Re6Se15 Re6Se8Te7 Re6S12 Re6Se6Cl6 Re6Se8Cl2 TlRe6Se8Cl3 CsRe6Se8I3 Re6S5Cl8 Cs2Re6Se8Br4 Connectivity [Re6(m3-S)8S2/2(S2)4/2]42 [Re6(m3-S)8S4/2(S2)2/2]42 [Re6(m3-S)8S6/2]42 Re6(m3-S6)(m4-S)2/2Br4/2S2/2 Re6(m3-S)7(m3-Br)Br6/2 [Re6(m3-Se)8(Se2)6/2]42 Re6(m3-Se)8(Te7)6/6 Re6(m3-S)8S4/2S2 Re6(m3-Se)6(m3-Cl)2Cl2Cl4/2 Re6(m3-Se)4(m4-Se)4/2Cl2Se4/2 [Re6(m3-Se)5(m4-Se)3/2Cl3Se3/2]2 [Re6(m3-Se)6(m4-Se)2/2I2I2/2Se2/2]2 Re6(m3-S)5(m3-Cl)3Cl4Cl2/2 [Re6(m3-Se)6(m4-Se)2/2Br4Se2/2]22 Dimensionality 3-D 3-D 3-D 3-D 3-D 3-D 3-D 2-D 2-D 2-D 2-D 2-D 1-D 1-D Ref. 89 81 75 97 66 82 106 90 66 63 92 83 71 83J. Chem. Soc., Dalton Trans., 1999, 97–105 103 dimensional clusters. As sulfides do not cover all the known types of compounds, selenides are included. In cases where the sulfides and selenides have equivalent compositions, they usually have equivalent structures. Therefore it is very likely that pairs of compounds should exist even if one of the pair has not yet been synthesized. The dimensionality and connectivity of the octahedral rhenium chalcogenide clusters have been described thoroughly before,6,7,46,83,91 and the possible and realized structures are given in these references.83 Consequently, it may be suYcient to summarize the connectivity in Table 3 and present figures (Fig. 13–15) of representative types of compounds. One of the most important features of octahedral cluster chemistry is that it oVers the general concept of the relationships between the solid state and molecular inorganic compounds.9,91 One aspect is the so-called dimensionality reduction 83,92 and the other one is the condensation of molecular cluster compounds to form larger clusters.46,48,50,51 We can envisage the relation better in cluster compounds than in ionic compounds without apparent metal–metal bonding.The constraint imposed by the cluster frameworks and the limitation of the bridging modes may simplify the possible packing schemes compared with more flexible packings in “mononuclear” ionic compounds.However, the relationship between the preparative conditions and the structure of the product is not always clear. Replacement of chalcogens [e.g. Re6S8Cl2 (3-D) and Re6Se8Cl2 (2-D)] or halogens [e.g. Re6Se8Cl2 (2-D) and Re6Se6Br2 (3-D)] changes the connectivity. Although the varieties of oxidation states in [Mo6Q8]n2 (Q = S, Se, Te; n = 0 to ª4) have been realized, 93 only 24-electron clusters have been well characterized in rhenium chemistry.The cluster Re6S12 (20-electron) prepared by Fig. 13 Three-dimensional connectivity in Ba2[Re6|S8|S6/2] (Re black, S yellow). Fig. 14 Two-dimensional connectivity in Re6|Se6Cl2|Cl2Cl4/2 (Re black, Cl light green, Se omitted). topotactic oxidation of Na4Re6S12 appears to be the sole exception. 90 Chevrel phase type 3-D compounds have not yet been reported. The mixed metal cluster Mo4Re2Te8 (22-electron) has a similar intercluster linkage to those of the Chevrel phases and is a superconducting material.94 7 Physical properties and catalysis Despite the structural diversity of the octahedral rhenium cluster chalcogenides, the electronic structure remains the same (24-electron) and most of them are either insulators or semiconducting.The temperature dependence of the resistivity of A4Re6Q12 (A = Tl, Na, K, Rb, Cs; Q = Se, S) phases has been measured.86,87 The 2-D clusters Re6Q8Cl2 are n-type semiconductors with band gaps of 1.42 eV (Q = S) 95 and 0.83 eV (Q = Se).96 The p-type behaviour of Re6Q8Br2 (Q = S, Se) was characterized photoelectrochemically and the photocurrent action spectra indicated band gaps of 1.65 eV and 0.84 eV for the sulfide and selenide, respectively.97 Another 3-D cluster Re6Se7Br4 also behaves as a p-type semiconductor and the band gap is about 1.78 eV.98 Photoelectric properties of ReS2 and ReSe2 single crystals have also been reported.99 The attempts to prepare conductive complexes using electron donors have met with considerable success and Batail et al.have synthesized a metallic complex by the combination of electron donors and 0-D soluble selenide clusters, [BEDT-TTF]4[Re6- Se5Cl9][guest] [BEDT-TTF = 3,4,39,49-bis(ethylenedithio)-2,29, 5,59-tetrathiafulvalene; guest = DMF, THF, dioxane].100 Conducting organic cation radical slabs are sandwiched by inorganic cluster monoanion layers in these cluster complexes. They are metallic at room temperature and their electrical conductivity at low temperatures depends on the size, shape, and symmetry of the neutral guest molecules.Especially in the case of dioxane, the metallic regime is sustained down to 4.5 K. Stiefel has reviewed catalysis by transition metal sulfides in industrial reactions.101 Major reactions are hydrodesulfurization (HDS), hydrodenitrogenation, hydrodeoxygenation and hydrodemetallation in the petrochemical industries. It was reported that Re2S7 and ReS2 are more active than MoS2 or CoSx as liquid-phase hydrogenation catalysts.102 The activity and selectivity of Re2S7 in the hydrogenation of nitric oxide to nitrous oxide or dinitrogen and sulfur dioxide to hydrogen sulfide was reported.103 Comparative studies on the hydrode- Fig. 15 One-dimensional connectivity in Cs2[Re6|Se6Se2/2|Br4Se2/2] (Re black, Se yellow, Br pink).104 J. Chem. Soc., Dalton Trans., 1999, 97–105 sulfurization of dibenzothiophene by various transition metal sulfides indicated that the activity of ReS2 is higher than those of the early transition metal sulfides but lower than Ru, Rh, Os, or Ir sulfides.104 Although the rhenium chalcogenide cluster compounds described above may be active for HDS reactions, few reports are available at present.The influence of d-state density on catalytic electrochemical dihydrogen evolution and dioxygen reduction in acid medium was studied using a mixed metal octahedral cluster Mo2Re4Se8.105 8 Perspectives of rhenium sulfide clusters The solid state and discrete chalcogenide cluster compounds of rhenium so far synthesized have triangular, tetrahedral, rhomboidal and octahedral cluster frameworks.The frameworks are essentially determined by the number of MCE, and reported triangular clusters have 6–10, tetrahedral 12, rhomboidal 14 and octahedral 24 electrons. The tetrahedral and octahedral clusters are electron-precise having 6 and 12 Re–Re single bonds, respectively. The connectivity of the cluster units depends on the number of anionic ligands.Also, the presence of Sx 22 polysulfide units modifies the situation. The coordination of neutral ligands is favourable for the formation of molecular clusters. It seems that there are a number of possible cluster compounds of various sizes by the appropriate combination of anionic and neutral ligands as well as cations. These potential clusters will be prepared by the discovery of appropriate reaction conditions in the case of the solid state compounds, and mainly by ligand substitution and condensation reactions for the molecular clusters.Little of the chemical and physical properties of the known rhenium cluster compounds have been studied so far and they will provide us with good opportunities for interesting studies of cluster compounds. 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ISSN:1477-9226    

DOI:10.1039/a806651e

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

DALTON COMMUNICATION J. Chem. Soc., Dalton Trans., 1999, 107–108 107 Synthesis and structure of [{As2(NCy)4}2Li4], containing an imido As(III) dianion Michael A. Beswick, Eilis A. Harron, Alexander D. Hopkins, Paul R. Raithby and Dominic S. Wright * Chemistry Department, University of Cambridge, Lensfield Road, Cambridge, UK CB2 1EW. E-mail: dsw1000@cus.cam.ac.uk Received 22nd October 1998, Accepted 27th November 1998 Reaction of [As(NMe2)3] with CyNH2 (1 :1 equivalents), followed by the addition of [CyNHLi] (1 equivalent) gives the heterobimetallic cage complex [{As2(NCy)4}2Li4], the first example of a complex containing an imido As(III) anion; the missing link in a series of Group 15 anions of the type [{E2(NCy)4}]22 (E 5 Bi, Sb and P).In previous work we showed that a series of imido Bi and Sb anions could be prepared utilising dimethyl amido derivatives.1 The complexes [{Bi2(NtBu)4}Li2?2thf] 2 and [{Sb2(NCy)4}2Li4] 3 are obtained from the in situ reactions of [E(NMe2)3] (E = Sb, Bi) with primary amines [RNH2; R = tBu, Cy (cyclohexyl)], followed by the reaction of the resulting [(Me2N)E(m-NR)]2 dimers with [RNHLi].Whereas [{Sb2(NCy)4}2Li4] has an elaborate cage structure consisting of two interlocked broken [Sb2N4Li2] cubanes in the solid state,3 such aggregation is precluded in the case of [{Bi2(NtBu)4}Li2?2thf ] by the solvation of the Li1 cations by thf (the complex remaining as a discrete cubane).2 Recently it has also been shown that a P analogue of these systems [{P2(NtBu)4}Li2?2thf ], isostructural with the previous Bi complex, can be obtained by deprotonation of [(tBuNH)P(m-NtBu)]2 with nBuLi in thf.4 Transmetallation of the Sb complex with metal salts provides a versatile strategy to heterobimetallic complexes containing [E2(NR)4]22 ligands.5 In view of the current interest in these species as ligand systems and in the light of the recent application of related alkali metal/Sb(III) phosphinidene cages as sources of photoemissive materials,6 we have initiated studies of the corresponding As systems as potential precursors to GaAs.We present here the synthesis and structure of [{As2(NCy)4}2- Li4] 1, containing an [As2(NR)4]22 anion; the missing link in the series of Group 15 containing [E2(NR)4]22 ligands and the first such polyimido anion of As(III) to be reported. Complex 1 is prepared by the reaction of [As(NMe2)3] with [CyNH2] (1:1 equivalent) followed by the addition of [CyNHLi] (1 equivalent) in toluene [eqn.(1)].† 4[As(NMe2)3] (i) 14CyNH2 (ii) 14CyNHLi [{As2(NCy)4}2Li4] 1 12Me2NH (1) The low-temperature X-ray structure determination of 1 ‡ shows that it possesses a cage structure constructed from the association of two interlocked ‘broken’ [As2(NCy)4Li2] cubanes (Fig. 1). The roughly tetrahedral arrangement of the Li1 cations at the centre of the cage and the pattern of the coordination of the Li1 cations by the m-N [Li–N range 2.08(1)– 2.132(9) Å] and terminal CyN groups [Li–N range 1.941(9)– 2.043(9) Å] of the [As2(NCy)4]22 anions are very similar to that occurring in the Sb analogue [m-N–Li range 2.07(2)–2.14(2), terminal N–Li range 1.96(3)–2.03(2) Å].3 This similarity with the Li4N4 substructure of 1 occurs despite the presence of significantly shorter As–N bonds in the [As2(NCy)4]22 anion (m-N–As average 1.92 Å, terminal As–N average 1.79 Å7), which at first sight may be anticipated to result in a markedly smaller ligand bite.However, the overall compression in the [As(m-NCy)]2 ring units of 1 compared to the [Sb(m-NCy)]2 units of the Sb counterpart is largely oVset by the greater exocyclic N–As–N angles in 1 [average m-N–As–N (terminal) 100.28; cf. average 90.88 in the Sb complex3], so that the bite of the terminal CyN groups is almost identical in 1 and its Sb Fig. 1 (a) Structure of 1. H atoms have been omitted for clarity. Key bond lengths (Å) and angles (8): As(1)–N(5) 1.916(4), As(1)–N(7) 1.790(3), As(1)–N(8) 1.929(4), As(2)–N(5) 1.932(4), As(2)–N(6) 1.789(4), As(2)–N(8) 1.915(4), As(3)–N(1) 1.917(4), As(3)–N(3) 1.792(4), As(3)–N(4) 1.943(4), As(4)–N(1) 1.922(4), As(4)–N(2) 1.797(4), As(4)–N(4) 1.914(4), N(1)–Li(3) 2.10(1), N(2)–Li(1) 1.941(9), N(2)–Li(3) 2.039(9), N(3)–Li(2) 1.966(9), N(3)–Li(4) 2.043(9), N(4)– Li(4) 2.130(9), N(5)–Li(1) 2.08(1), N(6)–Li(1) 2.008(9), N(6)–Li(4) 2.012(9), N(7)–Li(3) 1.979(9), N(7)–Li(2) 2.037(9), N(8)–Li(2) 2.132(9), C(21) ? ? ? Li(1) 2.614(9), C(31) ? ? ? Li(2) 2.79(1), C(36) ? ? ? Li(2) 2.78(1), C(72) ? ? ? Li(3) 2.774(9); As–(m-N)–As mean 96.2, (m-N)–As–(m-N) mean 82.5, exo-(m-N)–As–N mean 100.2, (m-N)–Li–N mean within SbN2Li rings 85.1, sum of N–Li–N angles about Li 348.7; (b) core of 1.108 J.Chem. Soc., Dalton Trans., 1999, 107–108 analogue [N(2,6) ? ? ? N(3,7) average 4.20 Å in 1; cf. average 4.27 Å in the Sb complex]. The only noticeable concession to the presence of a more compact dianion ligand in 1 is the more acute N–Li–N angles made with the chelating m-N and terminal-N centres (average 85.18; cf. 90.48 in the Sb analogue 3). There is also some eVect on the pattern of peripheral agostic C(–H) ? ? ? Li interactions with the Cy groups. In the Sb analogue the a-C–H of each of the pendant CyN groups are orientated towards and involved with adjacent Li1 cations (eVectively reinforcing the association of the cubane units).3 However, a far less regular pattern of C(–H) ? ? ? Li interactions is present in 1, involving both the a and b carbons of Cy groups.Despite the diVerences in the steric demands of the tBu and Cy groups present in the structurally characterised complexes [{E2(NtBu)4}Li2?2thf] (E = P,4 Bi2) and [{E2(NCy)4}2Li4] (E = As, Sb 3), and the presence of diVerent Group 15 elements and Lewis base solvation, it is now possible to obtain some general structural trends from this series.In particular the N–E–N (range 79.6–82.88) and E–N–E (range 96.2–98.68) angles in the [E(m-NR)]2 ring units of the [E2(NR)4]22 dianions in all of these species are surprisingly similar. One of the most significant diVerences in the geometry of the dianions occurs in the exocyclic N–E–N angles which exhibit an overall reduction going from P (average 99.48) to Bi (average 87.98), consistent with the idea of increased s-character in the lone pair and increased p-character in the E–N bonds as Group 15 is descended.This eVect oVsets the increase in E–N bond lengths so that coordination of the Li1 cations can be achieved without major structural modification of the [E2(NR)4Li2] units. Dimerisation of the cubane substituents of 1 and the Sb analogue is made possible by puckering of the E2N2 ring units (the N centres being an average of 18.48 out of the plane in 1 and an average of 21.28 in the Sb complex). This expands the ligand bite and allows inter-cubane Li–N bonding to be established.The use of [As(NMe2)3] as a precursor should allow other imido anions of As(III) to be prepared {e.g., [As(NR)3]32} and the coordination chemistry of these species to be explored. Of potential technological relevance is the synthesis of As(III)/ Group 13 (Ga, In) heterometallics. Notes and references † Synthesis of 1: [As(NMe2)3] (6.0 mmol, 2.4 ml, 2.5 mol dm23 solution in toluene) was added to a solution of CyNH2 (6.0 mmol, 0.70 ml) in toluene (20 ml) at 25 8C.The mixture was brought to reflux briefly and a pale yellow solution was formed. This was added to a suspension of [CyNHLi] (6.0 mmol, made by the in situ reaction of CyNH2 with nBuLi) in hexanes. The solid dissolved immediately and a bright yellow solution was produced after heating to reflux. The solvent was reduced to ca. 6 ml and a colourless solid precipitated. This was warmed back into solution and storage at 5 8C for 24 h gave crystals of 1; yield 0.37 g (22%).Decomp. ca. 75 8C to red semi-solid, darkens and becomes black at ca. 200 8C. IR (Nujol), nmax/cm21: 1225.4s, 1143.3m, 1056.2vs (br), 973.3s, 921.5m, 890.4s, 845.7s, 766.6s. 1H NMR (125 8C, 400 MHz, d6-benzene): 3.46 (2H, a-C–H Cy), 3.27 (2H, a-C–H Cy), 2.7–1.0 (40H, overlapping multiplets, CH2 Cy) (ca. 0.33 molecules of toluene were also present per molecule of 1, CH3 at 2.13). 7Li NMR (100.6 MHz, d8-toluene, relative to LiCl–D2O, 50 mg per 0.5 mol dm23): d 1.25 (s, line width 23 Hz, 125 8C) [Found: C, 54.8; H, 8.2; N, 10.1.Calc.: C, 52.2; H, 8.0; N, 10.1% (the high %C is a result of minor amounts of toluene, up to ca. 0.33 per molecule of 1 as confirmed by 1H NMR]. ‡ Crystal data for 1: C48H88As4Li4N8, M = 1104.70, triclinic, space group P1� , a = 10.415(5), b = 11.809(8), c = 23.502(13) Å, a = 97.75(4), b = 100.35(4), g = 103.30(4)8, U = 2720(3) Å3, Z = 2, Dc = 1.349 Mg m23, l = 0.71073 Å, T = 180(2) K, m(Mo–Ka) = 2.475 mm21.Data were collected on a Siemens-Stoe AED diVractometer. Of a total of 11222 data collected (3.50 £ q £ 24.018) 8458 were independent (Rint = 0.0988). The structure was solved by direct methods and refined by full-matrix least-squares on F2 to final values of R1[F > 4s(F)] = 0.040 and wR2 = 0.122 (all data); largest peak and hole in the final diVerence map 0.713 and 20.820 e Å23. CCDC reference number 186/1261. 1 M. A. Beswick, M. E. G. Mosquera and D. S. Wright, J. Chem. Soc., Dalton Trans., 1998, 2437. 2 D. Barr, M. A. Beswick, A. J. Edwards, J. R. Galsworthy, M. A. Paver, M.-A. Rennie, C. A. Russell, P. R. Raithby, K. L. Verhorevoort and D. S. Wright, Inorg. Chim. Acta, 1996, 248, 9. 3 R. A. Alton, D. Barr, A. J. Edwards, M. A. Paver, P. R. Raithby, M.-A. Rennie, C. A. Russell and D. S. Wright, J. Chem. Soc., Chem. Commun., 1994, 1481. 4 I. Schranz, L. Stahl and R. J. Staples, Inorg. Chem., 1998, 37, 1493. 5 M. A. Beswick, C. N. Harmer, M. A. Paver, P. R. Raithby, A. Steiner and D. S. Wright, Inorg. Chem., 1997, 36, 1740. 6 M. A. Beswick, N. Choi, C. N. Harmer, A. D. Hopkins, M. Mc- Partlin and D. S. Wright, Science, 1998, 1500. 7 Although very diVerent, the terminal and bridge As–N bond lengths are within the range occurring in other As–N compounds (in which only minimal pp–dp bonding occurs), see; A. L. Atwood, A. H. Cowley, W. E. Hunter and S. K. Mehritra, Inorg. Chem., 1982, 21, 1354; R. Bohra, H. W. Roesky, M. Noltemeyer and G. M. Sheldrick, Acta Crystallogr., Sect. C, 1984, 40, 1150; J. Weiss and W. Einenhuth, Z. Anorg. Allg. Chem., 1967, 350, 9; M. G. Begley, D. B. Sowerby and R. J. Tillott, J. Chem. Soc., Dalton Trans., 1974, 2527. Communication 8/0821

ISSN:1477-9226    

DOI:10.1039/a808214f

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

DALTON COMMUNICATION J. Chem. Soc., Dalton Trans., 1999, 109–110 109 Introduction of ·-hydroxymethylserine residues in a peptide sequence results in the strongest peptidic, albumin-like, copper(II) chelator known to date Piotr M�ynarz,a Wojciech Bal,a Teresa Kowalik-Jankowska,a Marcin Stasiak,b Miros�aw T. Leplawyb and Henryk Koz�owski a* a Faculty of Chemistry, University of Wroc�aw, F. Joliot-Curie st. 14, 50-383 Wroc�aw, Poland. E-mail: henrykoz@wchuwr.chem.uni.wroc.pl b Institute of Organic Chemistry, Technical University, Zÿ eromski st. 116, 90-924 £ódz�, Poland Received 26th October 1998, Accepted 24th November 1998 A tripeptide amide HmS–HmS–His–NH2 is the strongest peptidic CuII chelator known to date, due to the steric shielding of the chelate plane as well as electronic effects.a-Hydroxymethylserine (HmS) is a non-proteinaceous amino acid, found as the N-terminal residue in antibiotic peptides, antrimicin1 and cirratiomycin.2 It diVers from serine by having another –CH2OH function at the a carbon.This substitution generates specific constraints on the conformational freedom of a peptide containing HmS, which is the likely reason for its existence. From the co-ordination point of view, the presence of two alcoholic functions in HmS enhances its binding abilities. The direct involvement of alcoholic functions in co-ordination was found for oxovanadium(IV) and copper(II) complexes of the HmS amino acid.3 Indirect, conformational phenomena also contributed to the stabilisation of particular complex species in di- and tri-peptides containing HmS residues.4,5 Peptides containing the N-terminal sequence Xaa–Yaa–His exhibit a particular aYnity towards CuII and NiII, resulting from the formation of three fused chelate rings and a flat fournitrogen (4N) co-ordination sphere around the metal ion.6,7 We have previously shown that the stability of such complexes can be influenced by conformational and electronic eVects resulting from substitutions of amino acids Xaa and Yaa.Substitutions of non-bonding Gly with bulkier Val and Ile provided a stability gain of two orders of magnitude, and the introduction of a positive Arg residue in position 1 was even more eVective.8,9 The study presented in this communication was aimed at finding out whether the presence of HmS residues can augment the binding capabilities of Xaa–Yaa–His peptides. The peptides, H–HmS–HmS–His–OH (1) and H–HmS– HmS–His–NH2 (2), were prepared using optimised methodology for incorporation of HmS into the peptide chain.10,11 Their co-ordination to CuII was studied by potentiometry and spectroscopy (UV/VIS, CD, EPR) in conditions analogous to those applied previously.5 Table 1 contains the stability constants (log b values) and spectroscopic parameters of complexes formed.Fig. 1 presents species distribution diagrams for these complexes, derived from potentiometric and spectroscopic measurements.Table 2 Table 1 Stability constants and spectroscopic characterisation of complexes formed by 1 and 2 UV/VISb CDb EPR Species log b a l (e) l (De) A|| c g|| HmS–HmS–His 1 HL H2L H3L CuL CuH21L CuH22L CuH23L 7.140(1) 13.176(1) 15.848(2) 7.66(1) 4.223(3) 0.064(2) 210.41(2) 510 510 (108) d (114) d 564 487 307 563 487 308 (20.31) d (10.73) d (10.80) e (20.32) d (10.74) d (10.79) e 210 210 2.17 2.17 HmS–HmS–His–NH2 2 HL H2L CuH21L CuH22L CuH23L 6.636(3) 12.322(3) 4.09(4) 1.271(7) 210.15(3) 510 510 (99) d (110) d 559 484 319 287 559 483 323 288 (20.26) d (10.69) d (10.06) f (20.31) g (20.29) d (10.74) d (10.06) f (10.39) g 207 210 2.18 2.18 a b(CuHiL) = [CuHiL]/{[Cu21][H1]i[L]}. Standard errors on the last digits are included in parentheses.Three titrations were performed for each system. b UV/VIS and CD units: l/nm, e/dm3 mol21 cm21, De/dm3 mol21 cm21. c EPR unit: A||/G. d d–d transition. e Nim aA CuII and N2 aA CuII CT transitions.f Nim aA CuII CT transition. g N2 aA CuII CT transition.110 J. Chem. Soc., Dalton Trans., 1999, 109–110 provides protonation-corrected stability constants for a range of complexes of Xaa–Yaa–His peptides that allow one to compare their metal binding capabilities directly. Spectroscopic parameters indicate that the binding mode in the 4N complex CuH22L, which predominates at pH 4–10 for both ligands, is identical to that of complexes of Gly–Gly–His and other Xaa– Yaa–His peptides.It involves the N-terminal amine (HmS-1), the amide nitrogens of HmS-2 and His-3 and the N-3 nitrogen of His-3 imidazole. There is no evidence for the direct involvement of alcoholic groups of HmS in the binding. The potential C-terminal donors, carboxyl in 1, or amide in 2, do not participate in CuII co-ordination as well. The dramatic stability gain of 3.3 log units vs. Gly–Gly–His, seen for the CuH22L species (Table 2), is likely to originate from the partial shielding of the CuII binding site from the bulk of solution both from above and below the co-ordination plane.Fig. 1 Superimposed species distributions for CuII and 1 (? ? ?) or 2 (——), calculated for CuII concentrations of 1 × 1023 mol dm23 and ligand concentrations of 1.2 × 1023 mol dm23. Fig. 2 CD spectra of CuH22L complexes of 1 (? ? ?) or 2 (——), recorded at pH 7.0. Concentrations used were 1.88 × 1023 mol dm23 (CuII) and 2.28 × 1023 mol dm23 (1), and 1.75 × 1023 mol dm23 (CuII) and 2.1 × 1023 mol dm23 (2).Table 2 Comparison of log *K values for the 4N complexes of X–X–His peptides with CuII Peptide Gly–Gly–His b Gly–Gly–His c Gly–Gly–His–OMec Gly–Gly–His–Gly–Gly c Gly–Gly–hist d Arg–Thr–His–Gly–Asn–NH2 e Arg–Thr–His–Gly–Asn–(15) f HmS–HmS–Hisg HmS–HmS–His–NH2 g log *Ka 216.43 216.14 214.97 214.59 217.14 214.24 213.13 213.12 211.04 a log *K = log b(CuH22L) 2 log b(H2L). b Ref. 6. c Ref. 12, 37 8C. d Ref. 14, hist stands for histamine. e Ref. 9. f Ref. 9, pentadecapeptide. g This paper. In this way, the access of water molecules to metal ion-bound amide nitrogens is limited, and the dissociation reaction is slowed. Complexes of Xaa–Yaa–His peptides composed of L-amino acids can provide such shielding only at one side. The NMR-derived solution structure of the NiII complex of Val– Ile–His–Asn, exhibiting this phenomenon, correlates with the stability gain of 2 log units.7 Quite surprisingly though, the amidation of the carboxylic function in 2 results in a further hundredfold increase of complex stability (to 5.4 log units vs.Gly–Gly–His), making it the strongest peptidic CuII chelator known to date. There is no reference data for appropriate amides (e.g. Gly–Gly–His–NH2) in the literature, but the stability constants for Gly–Gly–His–OMe and Gly–Gly–His–Gly– Gly indicate that the carboxylate charge neutralisation by means of esterification or peptide chain extension may increase the complex stability by ca.one log unit.12 The much bigger eVect seen in our complexes presumably results from more specific interactions. The only major spectroscopic diVerence between the complexes of 1 and 2 is the sign of the amide nitrogen to CuII CT, positive with 1, and negative with 2 (Fig. 2). Complexes of other Xaa–Yaa–His peptides studied previously 7–9 and of human and bovine serum albumins, sharing a similar metal binding site,13 exhibited a positive CT band. HmS residues are not chiral, so the sign of the CT band is governed by the conformation of the 6-membered chelate ring between the His amide and the imidazole nitrogens.However, any major conformational change in this ring ought to be reflected in the d–d bands, while this region of the spectra is practically identical for both complexes, indicating the unchanged metal ion environment. At this point we conclude that there is a specific interaction in the CuH22L complex of 2, which is probably related to its extremely high stability, but the reasons for this eVect remain to be elucidated.Further studies of the complexes presented above and of their nickel counterparts are currently in progress in our laboratory and will be reported soon. Acknowledgements This work was financially supported by the Polish State Committee for Scientific Research (KBN 3T09A-10514 and 3T09A- 11108), within the framework of the COST D8/0018/98 programme. References 1 N.Shimada, K. Morimoto, H. Naganawa, T. Takita, M. Hamada, K. Maeda, T. Takeuchi and H. Umezawa, J. Antibiot., 1981, 34, 1613. 2 T. Shiroza, N. Ebisawa, K. Furihata, T. Endo, H. Seto and N. Otake, Agric. Biol. Chem., 1982, 46, 865. 3 T. Kowalik-Jankowska, H. Koz�owski, K. Kocio�ek, M. T. Leplawy and G. Micera, Transition Met. Chem., 1995, 20, 23. 4 T. Kowalik-Jankowska, H. Koz�owski, M. Stasiak and M. T. Leplawy, J. Coord. Chem., 1996, 40, 113. 5 T. Kowalik-Jankowska, M. Stasiak, M. T. Leplawy and H. Koz�owski, J. Inorg. Biochem., 1997, 66, 193. 6 R. W. Hay, M. M. Hassan and C. You-Quan, J. Inorg. Biochem., 1993, 52, 17. 7 W. Bal, M. I. Djuran, D. E. Margerum, E. T. Gray, Jr., M. A. Mazid, R. T. Tom, E. Nieboer and P. J. Sadler, J. Chem. Soc., Chem. Commun., 1994, 1889. 8 W. Bal, G. N. Chmurny, B. D. Hilton, P. J. Sadler and A. Tucker, J. Am. Chem. Soc., 1996, 118, 4727. 9 W. Bal, M. Jez· owska-Bojczuk and K. S. Kasprzak, Chem. Res. Toxicol., 1997, 10, 906. 10 M. Stasiak, W. M. Wolf and M. T. Leplawy, J. Pept. Sci., 1998, 4, 46. 11 M. Stasiak and M. T. Leplawy, Lett. Pept. Sci., 1998, 5, 449. 12 R. Agarwal and D. Perrin, J. Chem. Soc., Dalton Trans., 1977, 53. 13 W. Bal, J. Christodoulou, P. J. Sadler and A. Tucker, J. Inorg. Biochem., 1998, 70, 33. 14 T. Gajda, B. Henry, A. Aubry and J.-J. Delpuech, Inorg. Chem., 1996, 35, 586. Communication

ISSN:1477-9226    

DOI:10.1039/a808269c

出版商:RSC    

年代:1999

数据来源: RSC

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DALTON COMMUNICATION J. Chem. Soc., Dalton Trans., 1999, 111–113 111 Self-assembly of triatomic gold units as supporting frames for a large gold diphenylphosphinite cage molecule† Christian Hollatz, Annette Schier, Jürgen Riede and Hubert Schmidbaur * Anorganisch-chemisches Institut der Technischen Universität München, Lichtenbergstrasse 4, D-85747 Garching, Germany Received 4th November 1998, Accepted 26th November 1998 A novel hexanuclear cage-type double-decker cation [FB- (OPPh2Au)3Cl3(AuPPh2O)3BF]1 is obtained in high yield as the tetrafluoroborate salt from a dinuclear diphenylphosphinous acid complex [Ph2P(OH)AuCl]2 upon treatment with BF3?OEt2.Intra- and inter-molecular metal–metal contacts between the closed-shell Au(I) centres of two-coordinate gold complexes are now recognized to contribute significantly to the stoichiometry, structure and conformation of all compounds of this type.1–3 The energy associated with these interactions is similar to the energetics of hydrogen bonds,4–9 and therefore this phenomenon has a great influence on the molecular and supramolecular chemistry of gold.10 Small complex molecules are found to associate into pairs, rings, chains, or multidimensional frameworks the structural pattern of which is often solely determined by “aurophilic” Au ? ? ?Au attractions.11 There are also systems where hydrogen bonding and aurophilic bonding are cooperative forces.12,13 We now report another striking case where the build-up of gold–gold contacts induces the formation of large cage-type molecules in which two Au3-triples represent supporting framework units.The reaction of diphenylphosphinous acid with chloro- (dimethyl sulfide)gold(I) in dichloromethane at room temperature gives the 1 : 1 complex [Ph2P(OH)AuCl]2 1, with liberation of dimethyl sulfide [eqn. (1)]. The colourless product (93% 2 (Me2S)AuCl 1 2 Ph2P(O)H CH2Cl2 22 Me2S [Ph2(OH)AuCl]2 (1) 1 yield, mp 128 8C) has been fully characterized by standard analytical and spectroscopic data.‡ In the crystal (triclinic, space group P1� , Z = 4),§ the compound is a dimer the monomeric units of which are tied together by a central Au ? ? ? Au bond [3.1112(7) Å] and two peripheral O–H ? ? ? Cl hydrogen bonds (Fig. 1). It is obvious that the two Cl–Au–P units are bent to allow a close contact of the metal atoms. The structure approaches quite closely non-crystallographic twofold symmetry as shown in Fig. 2. Related structures have recently been found for compounds of the type [R2P(OH)–Au–P(O)R2]2.13 3 [Ph2P(OH)AuCl]2 BF3?OEt2 (excess) 23 HCl/23 HF 1 [FB(OPPh2Au)3Cl3(AuPPh2O)3BF]1BF4 2 (2) 2 Treatment of compound 1 with an excess of BF3?OEt2 in dichloromethane at 20 8C leads to the liberation of HCl and HF, the latter being trapped by the excess BF3 to give HBF4 and BF4 2 counter ions. The net reaction is represented by eqn. (2). The only gold-containing product in this reaction, 2, is isolated almost quantitatively (96% yield) as a colourless, crystalline solid (mp 152 8C with decomposition), soluble in dichloromethane. The solutions are stable only at lower temperatures and the NMR spectra show a singlet resonance for 31P and two singlet 11B resonances (intensity ratio 2 : 1).There is only one set of phenyl 13C and 1H resonances with the expected 1H- and 13C-31P splittings, respectively.‡ These data suggest a very high symmetry for the components of the product in solution.Crystals of 2?3CH2Cl2 (from CH2Cl2–Et2O, hexagonal, space group P63/m, Z = 2) § contain cage-like hexanuclear cations with crystallographically imposed point group C3h symmetry (Fig. 3). At the opposite ends of the cation two BF bridgehead units are each connected to three diphenylphosphinite units via the oxygen atoms. The tentacles of the resulting tripodal donor anions [FB(OPPh2)3]2 are attached via their phosphorus atoms to three V-shaped digoldchloronium groups [Au2Cl]1 to close three 16-membered rings which have only the two BF bridgeheads in common.In the lattice the BF4 2 counter ions are disordered and associated with the CH2Cl2 molecules via weak F ? ? ? H–C hydrogen bonds (virtual C3h symmetry). The structure of the cation is remarkable mainly for two Fig. 1 Molecular structure of compound 1 (ORTEP20 drawing with 50% probability ellipsoids, C–H atoms omitted for clarity). Selected bond lengths (Å) and angles (8): Au(1)–P(1) 2.218(2), Au(1)–Cl(1) 2.306(2), P(1)–O(1) 1.597(5), Au(1) ? ? ?Au(2) 3.1112(7), Au(2)–P(2) 2.224(2), Au(2)–Cl(2) 2.309(2), P(2)–O(2) 1.582(6); P(1)–Au(1)–Cl(1) 169.18(7), P(2)–Au(2)–Cl(2) 170.85(7); hydrogen bridges: O(1)–- H(1) ? ? ? Cl(2): O(1)–H(1) 0.986, H(1) ? ? ? Cl(2) 2.029, O(1) ? ? ? Cl(2) 2.994; O(1)–H(1) ? ? ? Cl(2) 165.6; O(2)–H(2) ? ? ? Cl(1): O(2)–H(2) 0.921, H(2) ? ? ? Cl(1) 2.105, O(2) ? ? ? Cl(1) 3.004; O(2)–H(2) ? ? ? Cl(1) 168.1.Fig. 2 Projection of the molecular structure of compound 1 along the Au(1) ? ? ?Au(2) axis.112 J.Chem. Soc., Dalton Trans., 1999, 111–113 reasons. (1) The gold atoms are arranged in two triangular groups with short Au ? ? ?Au contacts [3.1725(5) Å]. These two units clearly stabilize the framework of the cage like two rings of a barrel. The same phenomenon, but with only one Au3- triple, has recently been observed in the structure of the trinuclear cation [FB(OPPh2AuPPh2O)3BF]1.14 (2) The two triangles of gold atoms, which together form a trigonal prism, are linked through three chloride anions which are thus converted into di(gold)chloronium centres already known in salts of the type [Cl(AuPR3)2]1.15 The Au–Cl–Au angles in 2 [106.51(10)8] are not as small as in open-chain reference compounds [82.7(2)8 for R = Ph],16 but probably still small enough to allow some weak Au ? ? ?Au bonding.The overall double decker arrangement may thus be taken as a hexanuclear gold cluster with three chlorine atoms bridging the three vertical edges of the trigonal prism (Fig. 3 and 4). Triangular Au3 units have previously been encountered with various other tripodal ligands.9,14,17–19 The mechanism of the formation of 2 probably involves stepwise substitution of fluoride in the BF3?OEt2 agent by phosphinite nucleophiles [ClAuPPh2O]2. The second and third steps are increasingly promoted by the opportunity to form pairs and triples of gold atoms. The reaction is terminated by closure of the cluster via only three chloride anions.The prismatic unit Au3Cl3Au3 is remarkably robust and withstands attack by HCl and HBF4, the by-products of the reaction. All P–Au–Cl units are close to linear, but nevertheless bent in the direction required for intimate Au ? ? ?Au interactions. Acknowledgements This work was supported by Deutsche Forschungsgemeinschaft, by Fonds der Chemischen Industrie, and by Degussa AG and Heraeus GmbH. Fig. 3 Molecular structure of the cation of compound 2 (ORTEP drawing with 50% probability ellipsoids, H atoms omitted for clarity). Selected bond lengths (Å) and angles (8): Au(1)–P(1) 2.238(2), Au(1)–Cl(1) 2.357(2), Au(1) ? ? ?Au(1a) 3.1725(5), P(1)–O(1) 1.576(5), O(1)–B(1) 1.464(7), B(1)–F(1) 1.38(2); P(1)–Au(1)–Cl(1) 171.38(8), Au(1)–Cl(1)–Au(1c) 106.51(10), Au(1a) ? ? ?Au(1) ? ? ?Au(1b) 60.0. Fig. 4 Projection of the molecular structure of the cation of compound 2 along the threefold axis.Notes and references † Dedicated to Professor E. Niecke on the occasion of his 60th birthday. ‡ Preparations. 1: (Me2S)AuCl (177 mg, 0.60 mmol) and Ph2P(O)H (121 mg, 0.60 mmol) were dissolved in CH2Cl2 (15 mL) and the resulting mixture was stirred for 2 h at 20 8C. The solvent was evaporated under vacuum to leave a volume of 3 mL, and pentane (30 mL) was added to precipitate a white solid, which was recrystallized from CH2Cl2–pentane to give colourless crystals. Yield 243 mg, 93%; mp 128 8C, stable to air and moisture, soluble in tetrahydrofuran, di- and tri-chloromethane, and insoble in diethyl ether and pentane. 1H NMR (CDCl3, 20 8C): d 8.70 (br s, OH); 7.21–7.91 (m, C6H5). 13C-{1H} NMR (CDCl3, 20 8C): d 134.7 (d, 1JPC = 74.4, i-C6H5), 132.1 (d, 4JPC = 2.3, p-C6H5), 131.4 (d, 2JPC = 16.1, o-C6H5), 128.8 (d, 3JPC = 13.0 Hz, m- C6H5). 31P-{1H} NMR (CDCl3, 20 8C): d 90.4 (s). MS (FAB): m/z 1000 [{Ph2P(OH)}3Au2]1, 833 [2M 2 Cl]1, 601 [{Ph2P(OH)}2Au]1, 399 [M 2 Cl]1, 202 [M 2 AuCl]1 (Found: C, 34.11; H, 2.79.Calc. for C12H11AuClOP?0.125C5H12: C, 34.18; H, 2.84%). 2: a solution of compound 1 (140 mg, 0.32 mmol) in CH2Cl2 (10 mL) was treated with 1 mL of BF3?OEt2 for 2 h at 20 8C. The solvent was evaporated to leave a volume of 2 mL, and Et2O was added to precipitate the product 2, which was recrystallized from CH2Cl2–Et2O at 4 8C to give colourless crystals. Yield 135 mg, 96%; mp 152 8C (decomp.), stable to air and moisture, soluble in tetrahydrofuran and methanol, and insoluble in diethyl ether and pentane.Product 2 decomposes slowly in dichloromethane and rapidly in chloroform, at 20 8C. 1H NMR (CD2Cl2, 20 8C): d 7.25–8.00 (m, C6H5). 13C-{1H} NMR (CD2Cl2, 20 8C): d 132.6 (s, p-C6H5), 131.2 (d, 2JPC = 16.9, o-C6H5), 128.9 (d, 3JPC = 13.8 Hz, m-C6H5), i-C6H5 not detected. 31P-{1H} NMR (CD2Cl2, 20 8C): d 82.3 (s). 11B-{1H} NMR (CD2Cl2, 20 8C): d 20.75 [s, (PO)3BF], 21.05 (s, BF4 2) (Found: C, 32.45; H, 2.44.Calc. for C72H60Au6B3Cl3- F6O6P6: C, 32.74; H, 2.29%). § Crystal structure determinations. Crystal data for C12H11AuClOP 1. Mr = 434.59, colorless crystals (0.45 × 0.35 × 0.30 mm), triclinic, a = 10.357(2), b = 10.806(2), c = 11.689(2) Å, a = 101.18(1), b = 98.49(2), g = 98.00(2)8, space group P1� , Z = 4, V = 1250.2(4) Å3, rcalc = 2.309 g cm23, F(000) = 808; T = 278 8C. Data were corrected for Lorentz, polarization, and absorption eVects [m(Mo-Ka) = 120.83 cm21]. 5436 measured [(sin q/l)max = 0.64 Å21], 5435 unique reflections (Rint = 0.0058); 289 refined parameters, wR2 = 0.0918, R = 0.0361 for 5166 reflections with Fo > 4s(Fo) used for refinement. Crystal data for C75H66Au6B3Cl9F6O6P6 (2?3CH2Cl2), Mr = 2896.38, colorless crystals (0.40 × 0.35 × 0.35 mm), hexagonal, a, b = 15.709(1), c = 23.705(1) Å, space group P63/m, Z = 2, V = 5066.0(5) Å3, rcalc = 1.899 g cm23, F(000) = 2700; T = 277 8C. Data were corrected for Lorentz, polarization, and absorption eVects [m(Mo-Ka) = 90.40 cm21]. 7974 measured [(sin q/l)max = 0.64 Å21], 3755 unique reflections (Rint = 0.0502); 175 refined parameters, wR2 = 0.0954, R = 0.0363 for 3265 reflections with Fo > 4s(Fo) used for refinement. CCDC reference number 186/1260. See http://www.rsc.org/suppdata/dt/1999/111/ for crystallographic files in .cif format. 1 H. Schmidbaur, Chem. Soc. Rev., 1995, 24, 391. 2 A. Grohmann, J. Riede and H. Schmidbaur, Nature (London), 1990, 345, 140. 3 P. G. Jones, Gold Bull., 1993, 16, 114. 4 H. Schmidbaur, W. Graf and G. Müller, Angew. Chem., Int. Ed. Engl., 1988, 27, 417. 5 D. E. Harwell, M. D. Mortimer, C. B. Knobler, F. A. L. Anet and M. F. Hawthorne, J. Am. Chem. Soc., 1996, 118, 2679. 6 K. Dziwok, J. Lachmann, D. L. Wilkinson, G. Müller and H. Schmidbaur, Chem. Ber., 1990, 123, 423; H. Schmidbaur, K. Dziwok, A. Grohmann and G. Müller, Chem. Ber., 1989, 122, 893. 7 R. Narayanaswany, M. A. Young, E. Parkhust, M. Ouelette, M.E. Kerr, D. M. Ho, R. C. Elder, A. E. Bruce and M. R. M. Bruce, Inorg. Chem., 1993, 32, 2506. 8 D. Braga, F. Grepioni and G. R. Desiraju, Chem. Rev., 1998, 98, 1375. 9 J. Zank, A. Schier and H. Schmidbaur, J. Chem. Soc., Dalton Trans., 1998, 323. 10 H. Schmidbaur, Gold Bull., 1990, 23, 11. 11 F. Scherbaum, A. Grohmann, B. Huber, C. Krüger and H. Schmidbaur, Angew. Chem., Int. Ed. Engl., 1988, 27, 1544. 12 W. Schneider, A. Bauer and H. Schmidbaur, Organometallics, 1996, 15, 5445; J.-C. Shi, B.-S. Kang and T. C. W. Mak, J. Chem. Soc., Dalton Trans., 1997, 2171; D. M. P. Mingos, J. Yau, S. Menzer and D. J. Williams, J. Chem. Soc., Dalton Trans., 1995, 319; J. Vicente, M. T. Chicote, M. D. Abrisqueta, R. Guerro and P. G. Jones, Angew. Chem., Int. Ed. Engl., 1997, 36, 1203. 13 C. Hollatz, A. Schier and H. Schmidbaur, J. Am. Chem. Soc., 1997, 119, 8115.J. Chem. Soc., Dalton Trans., 1999, 111–113 113 14 C. Hollatz, A. Schier and H. Schmidbaur, Inorg. Chem. Commun., 1998, 1, 115. 15 R. Usón, A. Laguna and M. V. Castrillo, Synth. React. Inorg. Met.- Org. Chem., 1979, 9, 317. 16 P. G. Jones and G. M. Sheldrick, Acta Crystallogr., Sect. B, 1980, 36, 1486; A. Bayler, A. Bauer and H. Schmidbaur, Chem. Ber., 1997, 130, 115. 17 A. L. Balch and E. Y. Fung, Inorg. Chem., 1990, 29, 4764. 18 A. Stützer, P. Bissinger and H. Schmidbaur, Chem. Ber., 1992, 125, 367. 19 C. M. Che, H. K. Yip, V. W. W. Yam, P. Y. Cheung, T. F. Lai, S. J. Shieh and S. M. Peng, J. Chem. Soc., Dalton Trans., 1992, 427. 20 C. K. Johnson, ORTEP, Report ORNL-5138, Oak Ridge National Laboratories, Oak Ridge, TN, 1997. Communication 8/0857

ISSN:1477-9226    

DOI:10.1039/a808570f

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

DALTON COMMUNICATION J. Chem. Soc., Dalton Trans., 1999, 115–117 115 When the ligands go marching in: a step-scan Fourier transform infrared spectroscopic study of ligand attack at the transient species W(CO)5(CyH) Richard H. Schultz*a and S. Krav-Amib a Department of Chemistry, Bar-Ilan University, Ramat-Gan 52900, Israel. E-mail: schultr@mail.biu.ac.il b Scientific Equipment Corporation, Holon, Israel. E-mail: ska–sec@mail.inter.net.il Received 2nd October 1998, Accepted 1st December 1998 Time-resolved step-scan Fourier transform infrared spectroscopy (S2FTIR) is used to probe the reactions of the transient species W(CO)5(CyH), produced by photolysis of W(CO)6 in cyclohexane solution, with a series of incoming ligands L.This study marks the first time that S2FTIR has been used to obtain mechanistic information for an irreversible chemical process. Although photolytic ligand substitution reactions of Group 6 carbonyls have long been of interest,1 some of the most basic questions about these systems remain unresolved.One of these is the mechanism by which the transient intermediate “M(CO)5- (solv)” (solv = solvent molecule) reacts with an incoming ligand L to form the stable complex M(CO)5L [reaction (1)]. M(CO)5(solv) 1 L æÆ M(CO)5L (1) In alkane solution, reaction (1) appears to proceed through an associative or associative interchange (A or Ia) mechanism in which there is more bond forming than bond breaking in the transition state,2–5 although in some cases the data are also consistent with a more dissociative mechanism.1,2 Furthermore, because of the direct participation of the solvent in the reaction and the large rate constants typically encountered (ca. 106–107 M21 s21), purely kinetic experiments (i.e., experiments in which the only experimental parameter measured is the reaction rate) rarely lead to unambiguous mechanistic conclusions in these systems. DiVerentiation among the various mechanistic possibilities must therefore be made by chemical studies that determine the relationship between the reaction kinetics and some other aspect of the the reaction system.2b,2d,3–6 To this end, we have undertaken a time-resolved S2FTIR spectroscopic study † of the reactions of the transient “W(CO)5(CyH)” complex with a variety of incoming ligands in order to determine the relationship (if any) between the properties of the attacking ligand and the kinetics of reaction (1). We monitored the reaction of W(CO)5(CyH) with a series of ligands L of the form cyclo- C4HnE, where E = O (n = 4, 6, 8), NH (n = 4, 8), or CH2 (n = 6),‡ as well as with L = 2-MeTHF and 2,5-Me2THF.While S2FTIR has been used extensively in investigation of reversible processes, 7,8 its use in irreversible processes has been limited to detection and identification of reaction intermediates.9 This study marks the first use, to our knowledge, of S2FTIR to obtain mechanistic information about a bimolecular chemical reaction, as well as the first systematic study of the influence of L on reaction (1) for any M(CO)5(alkane) complex.The experiments reported here were performed with a Bruker Equinox 55 S2FTIR system. A continuously flowing solution of W(CO)6 (5–6 × 1024 mol L21) in CyH containing a large excess of L (for hex-1-ene and cyclopentene, [L] = 0.149 mol L21; for the other ligands, [L] = 0.015 ± 0.001 mol L21) was photolyzed in an 0.5 mm CaF2 cell at room temperature (20 8C) by the pulsed output of an excimer laser (XeCl, 308 nm, 5–6 Hz).At each FTIR mirror position, the time-dependent IR signal was measured by a fast detector (<55 ns risetime), digitized, averaged over 8–15 photolysis shots, and converted to time-resolved interferograms and spectra. A detailed description of the instrument is given elsewhere.3 HPLC grade cyclohexane (either freshly opened or stored over molecular sieves and used within 2 days) and other reagents were used without further purifi- cation; ligand purities of at least 97% were confirmed by NMR.Typical time-resolved S2FTIR diVerence spectra are shown in Fig. 1. At the laser flash, a bleach, due to loss of W(CO)6, appears at 1981 cm21. Simultaneously, positive absorbances appear at 1954 cm21 and 1928 cm21, indicating formation of W(CO)5(CyH).10 These peaks decrease in intensity with time while two new peaks, assigned to E and A1 symmetry C–O stretches of W(CO)5L,11 grow in with the same time dependence.The IR peaks observed in this study for W(CO)5L (Table 1) are consistent with literature values for those complexes for which IR spectra have been reported.11 No spectroscopic or kinetic evidence for significant amounts of side reaction [e.g., reaction with trace amounts of H2O3,10b or with unphotolyzed W(CO)6] was seen. Sample kinetic traces (i.e. the time dependence of the absorption intensity) for the reaction of W(CO)5- (CyH) with various ligands are shown in Fig. 2. Pseudo-first order reaction rates were determined from single-exponential fits to such kinetic traces and converted to second-order rate constants (Table 1). The kinetic behavior of the ligands studied here falls into three categories: (a) for all L except for 2-MeTHF, 2,5-Me2- THF, and the alkenes, there is an inverse correlation between Fig. 1 Time-resolved S2FTIR spectra (4 cm21 resolution) showing changes in sample absorbance in the 1910–1990 cm21 carbonyl stretching region following photolysis of W(CO)6 in the presence of 0.015 mol L21 furan.Shown are spectra for t = 0 and for t = 10 ms, 20 ms, and 45 ms after photolysis. The W(CO)6 bleach appears as a negative peak (1981 cm21). Positive peaks that decrease in size with time (1954 cm21 and 1928 cm21) are attributed to the W(CO)5(CyH) intermediate, and the peaks that increase in intensity with time (1949 cm21 and 1936 cm21) to the W(CO)5(furan) product.11b116 J.Chem. Soc., Dalton Trans., 1999, 115–117 the rate of reaction (1) and nCO of the product W(CO)5L (Table 1);§ (b) 2-MeTHF and 2,5-Me2THF react at about half the rate of THF despite having nCO similar to those of THF; (c) the alkenes react much more slowly than any of the other ligands. These results are easily rationalized in terms of the CDD “backbonding” model of the metal–ligand interaction.12 According to this model, the C–O stretching frequencies in any carbonyl complex will be inversely related to the electron density at the metal center.In the product W(CO)5L complexes, the relative amount of electron density at the metal (and thus the values of nCO) should depend primarily on the relative electron donating ability of the ligand L. The data in Table 1, shown graphically in Fig. 3, reveal that for the C4HnE ligands from pyrrolidine to furan, the rate of reaction (1) correlates directly with the electron-donating ability of the ligand in the W(CO)5L product.The correlation of the reaction rate with properties of the reaction product implies that for these ligands, the transition state for reaction (1) is nearer to the products than to the reactants; that is, the spectroscopic data for reaction (1) implies an associative (A or Ia) mechanism in which the transition state occurs while the solvent CyH molecule is still in the coordination sphere of the W atom. The more electron-donating the incoming ligand is, the better able it is to stabilize the transition state by continuing to maintain electron density at the metal center as the CyH molecule leaves.The trend in the rates of reaction Fig. 2 Time dependence of the W(CO)5(CyH) IR absorbance at 1954 cm21 for three incoming ligands (reaction 1), normalized to the same DA at t = 0. Shown are results for reaction of W(CO)5(CyH) with 0.15 mol L21 cyclopentene (j), 0.015 mol L21 THF (m), and 0.015 mol L21 pyrrolidine (d). The lines are single-exponential fits to the data (cf.Table 1). Table 1 Observed W(CO)5L nCO frequencies for complexes W(CO)5L and room-temperature second-order rate constants a (kobs) for reaction (1) in cyclohexane solution Ligand (L) Hex-1-ene Cyclopentene Furan Cyclohexane Pyrrole 2,3-DHF 2,5-DHF THF Pyrrolidine 2-MeTHF 2,5-Me2THF nCO b/cm21 1963, 1948 1960, 1943 1949, 1936 1954, 1928 1939, 1919 1936, 1915 1934, 1913 1933, 1911 1926, 1917 1933, 1910 1930, 1909 1026 kobs/L mol21 s21 0.68 ± 0.04 c 0.64 ± 0.03 c 2.58 ± 0.15 d — 6.14 ± 0.18 6.41 ± 0.40 12.4 ± 0.7 13.4 ± 0.7d 20.9 ± 1.0 6.59 ± 0.32 6.48 ± 0.18 a Second-order reaction rate constants (20 8C) calculated from observed pseudo-first order reaction rates measured at [L] = 0.015 ± 0.001 mol L21 are given with 1 standard deviation relative uncertainties.b Peak positions for E and A1 C–O stretches; 4 cm21 resolution. c Reaction rates for these two ligands were measured at [L] = 0.149 mol L21.d Reported values for these two ligands include results from an IR laser flash kinetic study performed in our laboratory.3 (1) observed here is consistent with the relative values of DH‡ for those cases in which it is known (L = THF, DH‡ = 14 ± 3 kJ mol21; 3 L = furan, DH‡ = 23 ± 3 kJ mol21; 3 L = hex-1-ene, DH‡ = 30 ± 2 kJ mol21 13) and with the negative values of DS‡ observed for reaction (1) in these cases. 2-MeTHF, 2,5-Me2THF, and the alkenes react more slowly than one would predict from their W(CO)5L C–O stretching frequencies. In the cases of 2-MeTHF and 2,5-Me2THF, the IR spectra of the product complexes show that these two ligands are strongly electron-donating.For these two ligands, steric hindrance appears to inhibit access of the incoming ligand to the associative, leading to slower reaction. Indeed, although the average nCO is lower for 2,5-Me2THF than for MeTHF, it does not react any more quickly. This observation can be explained by the additional inductive eVect of the second methyl group (shown by the lower values of nCO) being oVset by its additional steric repulsion.On the other hand, for the strongly electronwithdrawing alkene ligands [which have higher values of nCO than the W(CO)5(CyH) intermediate does], any transition-state stabilization due to electron donation will necessarily be much less significant, so according to the model developed here, the rate of reaction (1) should be much slower in these systems.Indeed, a dissociative (D or Id) mechanism has been proposed for reaction (1) for L = hex-1-ene,2c and we observe here that reaction (1) proceeds at the same rate for hex-1-ene and cyclopentene despite the significant diVerences in nCO of the two product compounds. Thus, for ligands of the type C4HnX (X = O or NH), the kinetics and spectroscopy observed here are entirely consistent with an associative (Ia) type of mechanism for ligand substitution at W(CO)5(CyH).The reaction rate correlates to the electron-donating ability of the incoming ligand as measured by the C–O stretching frequencies of the product W(CO)5L, implying a late transition state. This study marks the first time that such a correlation has been observed, as well as being the first time that S2FTIR has been used to draw mechanistic conclusions about an irreversible chemical process. Additional studies are underway to further probe the steric and electronic influences on the reaction kinetics and mechanism and to determine the activation parameters of the reactions studied here.Acknowledgements This research was supported in part by the Israel Science Foundation, founded by the Israel Academy of Sciences and Humanities, and by funds provided by the Bar-Ilan University Research Authority. The authors would also like to thank Dr Heinz Frei (Lawrence Berkeley Laboratory) for helpful suggestions about the experimental setup. Fig. 3 Inverse dependence of the reaction rate constant on the CO stretching frequency. Shown is 107 kobs 21 (Table 1) as a function of the average of nCO(A1) and nCO(E) of W(CO)5L.The circles represent results for (in order of increasing nCO) L = pyrrolidine; THF; 2,5-DHF; 2,3-DHF; pyrrole; furan. The dashed line is a linear fit through these data points. The triangles represent results for L = 2,5-Me2THF and 2-MeTHF.J. Chem. Soc., Dalton Trans., 1999, 115–117 117 Notes and references † Abbreviations: S2FTIR = Step-scan Fourier transform infrared; CyH = cyclohexane; CDD = Chatt–Dewar–Duncanson; THF = tetrahydrofuran; DHF = dihydrofuran; MeTHF = methyltetrahydrofuran; Me2THF = dimethyltetrahydrofuran (mixture of cis and trans).‡ Since these ligands have essentially the same geometry, but diVering basicities, diVerences in reactivity among them will presumably be due primarily to electronic eVects. § DA for the additional, very weak, A1 symmetry C–O stretch for a W(CO)5L species (usually found around 2075 cm21) was below the detectibility limit (ca. 0.002 absorbance units) of our instrument. The frequency of this peak tends to be less sensitive to changes in the ligand L in W(CO)5L10,11 than the other two IR-active C–O stretching frequencies, however. Furthermore, its value invariably correlates (at least qualitatively) with the average of the other two. Thus, the correlation shown in Fig. 3 is unlikely to be aVected by including or not including the third peak in the calculation of the average nCO. 1 J. A. S. Howell and P. M. Burkinshaw, Chem. Rev., 1983, 83, 557; C. Hall and R. N. Perutz, Chem. Rev., 1996, 96, 3125. 2 (a) S. Zhang and G. R. Dobson, Inorg. Chim. Acta, 1991, 181, 103; (b) G. R. Dobson and M. D. Spradling, Inorg. Chem., 1990, 29, 880; (c) G. R. Dobson, K. J. Asali, C. D. Cate and C. W. Cate, Inorg. Chem., 1991, 30, 447; (d ) S. Zhang, G. R. Dobson, V. Zang, H. C. Bajaj and R. van Eldik, Inorg. Chem., 1990, 29, 3477. 3 R. Paur-Afshari, J. Lin, A. Lugovskoy, S. Lugovskoy and R. H. Schultz, unpublished work. 4 C. J. Breheny, J. M. Kelly, C. Long, S. O’KeeVe, M. T. Pryce, G. Russell and M. M Walsh, Organometallics, 1998, 17, 3690. 5 G. K. Yang, V. Vaida and K. S. Peters, Polyhedron, 1988, 7, 1619. 6 A. Drjlaca, C. D. Hubbard, R. van Eldik, T. Asano, M. V. Basilevsky and W. J. le Noble, Chem. Rev., 1998, 98, 2167. 7 W. Uhmann, A. Becker, C. Taran and F. Siebert, Appl. Spectrosc., 1991, 45, 390; J.-R.Burie, W. Leibl, E. Nabedryk and J. Breton, Appl. Spectrosc., 1993, 47, 140; S. E. Plunkett, J. L. Chao, T. J. Tague and R. A. Palmer, Appl. Spectrosc., 1995, 49, 702; X. Hu, H. Frei and T. G. Spiro, Biochemistry, 1996, 35, 13001; W. Hage, M. Kim, H. Frei and R. A. Mathies, J. Phys. Chem., 1996, 100, 16026; A. K. Dioumaev and M. S. Braiman, J. Phys. Chem. B, 1997, 101, 1655; R. Rammelsberg, B. Hessling, H. Chorongiewski and K. Gerwert, Appl. Spectrosc., 1997, 51, 558; R.A. Palmer, S. E. Plunkett, P. Chen, J. L. Chao and T. J. Tague, Mikrochim. Acta, Suppl., 1997, 14, 603. 8 J. R. Schoonover, G. F. Strouse, B. D. Dyer, W. D. Bates, P. C. Chen and T. J. Meyer, Inorg. Chem., 1996, 35, 273; J. R. Schoonover, G. F. Strouse, K. M. Omberg and R. B. Dyer, Comments Inorg. Chem., 1996, 18, 165; H. Sun and H. Frei, J. Phys. Chem. B, 1997, 101, 205; P. Chen, K. M. Omberg, D. A. Kavaliunas, J. A. Treadway, R. A. Palmer and T. J. Meyer, Inorg. Chem., 1997, 36, 954; R. A. Palmer, P. Chen, S. E. Plunkett and J. L. Chao, Mikrochim. Acta, Suppl., 1997, 14, 595. 9 S. E. Bromberg, H. Yang, M. C. Asplund, T. Lian, B. K. McNamara, K. T. Kotz, J. S. Yeston, M. Wilkens, H. Frei, R. G. Bergman and C. B. Harris, Science, 1997, 278, 260; J. S. Bridgewater, B. Lee, S. Berhard, J. R. Schoonover and P. C. Ford, Organometallics, 1997, 16, 5592. 10 (a) D. R. Tyler and D. P. Petrylak, J. Organomet. Chem., 1981, 212, 389; (b) H. Hermann, F.-W. Grevels, A. Henne and K. SchaVner, J. Phys. Chem., 1982, 86, 5151. 11 (a) G. W. A. Fowles and D. K. Jenkins, Inorg. Chem., 1964, 3, 257; (b) I. Stolz, H. Haas and R. K. Sheline, J. Am. Chem. Soc., 1965, 87, 716; (c) R. J. Dennenberg and D. J. Darensbourg, Inorg. Chem., 1972, 11, 72. 12 D. M. P. Mingos, in Comprehensive Organometallic Chemistry, eds. G. Wilkinson, F. G. A. Stone and E. W. Abel, Pergamon, New York, 1982, vol. 3, p. 1. 13 Calculated from the rate data reported in the Supplementary Material to ref. 2(c). The authors of that study interpreted their results in terms of a dissociative mechanism and derived DH‡ = 34 ± 2 kJ mol21, DS‡ = 215 ± 0.5 kJ mol21 K21 for the W–CyH bond breaking step. Communication 8/07672C

ISSN:1477-9226    

DOI:10.1039/a807672c

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

DALTON FULL PAPER J. Chem. Soc., Dalton Trans., 1999, 119–125 119 Combined versus individual labilising eVects of H1, Na1 and nucleophile on catalysed substitution reactions: studies on [Fe4S4X4]22 (X 5 Cl or PhS)† Richard A. Henderson John Innes Centre, Nitrogen Fixation Laboratory, Norwich Research Park, Colney, Norwich, UK NR4 7UH. E-mail: richard.henderson@bbsrc.ac.uk Received 6th October 1998, Accepted 27th November 1998 The reactions between [Fe4S4X4]22 (X = PhS or Cl) and Et2NCS2 2 to form [Fe4S4X2(S2CNEt2)2]22 have been studied in MeCN.The kinetics are consistent with a dissociative mechanism under all conditions. The addition of Na1 led to an increase in rate for [Fe4S4(SPh)4]22 and analysis of the kinetics indicates that a single Na1 binds and labilises the cluster. Comparison is drawn with the established eVect of H1 on the lability of this cluster. The presence of a thiolate ligand is necessary to bind Na1 since the reaction between [Fe4S4Cl4]22 and Et2NCS2 2 is unaVected by the addition of Na1.The addition of acid to [{Fe4S4(SPh)4}Na]2 further accelerates the rate of substitution. Quantitative analysis shows that the combined labilising eVect of Na1 and H1 is no more than that expected from the individual labilisation aVorded by each cation. Similar analyses show the same is true for H1 and nucleophile in acid-catalysed associative substitution mechanisms, and two H1 in the acid-catalysed dissociative mechanisms of Fe–S-based clusters.The generality of these observations is discussed. Introduction There are many substitution reactions which are catalysed or inhibited in the presence of other species; for example, substitution reactions 1 in the presence of H1. Elaborations of this chemistry could involve catalysis by several components [A, B, C, etc. . . ., as illustrated in equation (1)]. For example, A and B could both be H1. M–X 1 Y A, B, C, etc. . . . M–Y 1 X (1) An important mechanistic question in such systems is, “Is the combined labilising eVect of all these species diVerent to that expected from each contributor or is there a cooperative eVect when all the components A, B, C, etc.are present?” Specifically, for the acid-catalysed reactions, “Is the eVect of two H1 different from that expected from compounding the eVect from one H1 with another H1?” Although these are fundamental mechanistic questions which relate to the reactions of many compounds, we are unaware of any study which addresses this problem.This is because such a study requires that the elementary rate constants for the dissociation of M–X to be determined in the presence of A, in the presence of B, as well as in the presence of A and B together. In most systems it is not possible to ‘dissect’ kinetically the dissociation rate constants from the binding constants of A and B. However, our studies on the acid-catalysed substitution reactions of synthetic Fe–S-based clusters 2–7 have shown that the binding of H1 or nucleophile are rapid equilibrium reactions which are followed by the slow dissociation of the leaving group.Analysis of the kinetic data invariably allows us to determine the dissociation rate constants. Herein, we report kinetic studies on the substitution reaction shown in equation (2) (X = Cl or PhS) and: (i) compare the eVects that Na1 and H1 have on the lability † Supplementary data available: kinetic data.For direct electronic access see http://www.rsc.org/suppdata/dt/1999/119/, otherwise available from BLDSC (No. SUP 57466, 5 pp.) or the RSC Library. See Instructions for Authors, 1999, Issue 1 (http://www.rsc.org/dalton). [Fe4S4X4]22 1 2Et2NCS2 2 H1, Na1, etc. [Fe4S4(S2CNEt2)2X2]22 1 2X2 (2) of the clusters; (ii) quantify the combined eVect of binding Na1 and H1 on the lability of the cluster in terms of the individual labilising eVects of these two cations, and (iii) a quantitative analysis of the relative labilising eVects of binding H1 and PhSH in the associative substitution mechanisms of [Fe4S4Cl4]22. In order to investigate the eVect of Na1 it has been necessary to use Et2NCS2 2 as the nucleophile.Previously similar studies used RS2 or ArS2 as the nucleophile.2–7 However, both NaSR and NaSAr are very poorly soluble in MeCN, and precipitation of these compounds precludes studying the reactions. NaS2CNEt2 is suYciently soluble in MeCN to avoid this complication.Clearly, in the reactions of [Fe4S4(SPh)4]22 in the presence of Na1, some NaSPh will be produced. However, the low amounts of NaSPh formed ([NaSPh] £ 0.2 mmol dm23) are suYciently soluble in MeCN for the reaction to remain homogeneous. Results and discussion EVects of H1 on the reactivity of [Fe4S4(SPh)4]22 In a series of kinetic studies we have been studying the acidcatalysed substitution reactions of a variety of synthetic Fe–Sbased clusters: reactions essential in understanding the multiproton, multi-electron, substrate transformation chemistry of these compounds.8 The generalised picture which has emerged from these studies is exemplified by that of [Fe4S4(SPh)4]22 shown in Fig. 1. Initial protonation occurs at the thiolato-S and subsequently at two m3-S atoms. It is protonation of these m3-S which labilises the cluster towards substitution. Protonation of the thiolate ligand is, apparently, not appreciably labilising. The reasons for this have been discussed in detail earlier 5,8 but, briefly, are a consequence of protonation at this site decreasing the s-donor but increasing the p-acceptor abilities of the ligand.The nett120 J. Chem. Soc., Dalton Trans., 1999, 119–125 Fig. 1 EVect of successive addition of H1 to [Fe4S4(SPh)4]22 on the lability of the cluster in the dissociative substitution reactions with EtS2. For simplicity only one PhS ligand is shown; d = Fe, s = S. eVect is that the Fe–thiolate and Fe–thiol bond strengths are very similar, and consequently the lability is unchanged.This proposal is consistent with structural studies on mononuclear thiolate complexes.9 These reactions of [Fe4S4(SPh)4]22 operate by an acidcatalysed dissociative substitution mechanism and analysis of the kinetics gives the values of k0 = 1.0 ± 0.2 × 1022 s21, kH = 8.0 ± 0.2 × 1022 s21 and kHH = 0.39 ± 0.02 s21. The ratio, kH/ k0 = 8.4 ± 1.6, describes the labilising eVect a single protonation has on the dissociation of the leaving group.Similarly kHH/ k0 = 41 ± 8 describes the eVect of diprotonation. It is evident, that to a reasonable approximation, kHH/k0 = (kH/k0)2. That is, the labilising eVect of each successive H1 is compounded. Clearly, we are not looking for an exact relationship here. Merely a guide as to whether there are orders of magnitude diVerence between the combined and the individual labilising eVects of each contributor to the activated complex.In the remainder of this paper we will see that similar equations describe the labilisation of the leaving group in Fe–S clusters by the combined eVects of: (i) H1 and Na1 and (ii) H1 and nucleophile. The clusters studied in this work10,11 and the products of the reactions [equation (2)] have already been structurally well characterised. Earlier synthetic studies showed that the addition of at least two mole equivalents of Et2NCS2 2 to [Fe4S4X4]22 (X = Cl or PhS) results in the formation of [Fe4S4- (S2CNEt2)2X2]22 and X-ray crystallography has established that the Et2NCS2-ligands are bound in a bidentate fashion to the Fe atoms.12 EVect of Na1 on the reactivity of [Fe4S4(SPh)4]22 and [Fe4S4Cl4]22 When studied on a stopped-flow apparatus, the reaction between [Fe4S4(SPh)4]22 and an excess of [NBun 4]S2CNEt2 is associated with a biphasic absorbance–time curve, provided [Et2NCS2 2] < 20 mmol dm23.The initial absorbance is that of [Fe4S4(SPh)4]22 and the final absorbance corresponds to [Fe4S4(SPh)2(S2CNEt2)2]22.At higher concentrations of Et2- NCS2 2 the absorbance–time curve becomes more complicated, with an increasing absorbance over protracted times (>20 s). For simplicity we have: (i) studied the kinetics only when [Et2NCS2 2] < 20 mmol dm23 and (ii) restricted the discussion to the first substitution reaction, corresponding to the initial phase of the absorbance–time curve. In order to get accurate rate constants for this first phase a method has been adopted, used in earlier studies, involving fitting the entire curve to two exponentials, and from this analysis obtaining the rate constant for the faster phase.The rate of the initial substitution reaction exhibits a first order dependence on the concentration of [Fe4S4(SPh)4]22 but is independent of the concentration of Et2NCS2 2 (kobs = 2.0 ± 0.5 × 1022 s21). This value is in good agreement with the rate constant measured using EtS2 or ButS2 in earlier studies.The first-order dependence on the concentration of cluster is indicated by the exponential shape of the absorbance–time curve, and is confirmed by experiments in which the concentration of the cluster was varied ([Fe4S4(SPh)4 22] = 0.02–0.2 mmol dm23) whilst keeping the concentration of Et2NCS2 2 constant (5.0 mmol dm23). Under these conditions the value of kobs did not vary. These kinetics are consistent with the uncatalysed dissociative mechanism shown in the centre of Fig. 2, in which dissociation of the Fe–SPh bond has to occur before Et2NCS2 2 binds to the cluster. The addition of Na[BPh4] results in an increase in the rate of the reaction as shown in Fig. 3. The dependence on the concentration of Na1 is complicated. At low concentrations of Na1 the rate exhibits a first order dependence on the concentration of Na1, but at high concentrations the rate becomes independent of the concentration of Na1.Experiments in which the concentration of Et2NCS2 2 was varied (maintaining [Na1] = 10.0 mmol dm23), showed that the rate of the reac-J. Chem. Soc., Dalton Trans., 1999, 119–125 121 Fig. 2 Dissociative substitution pathways in the reactions of [Fe4S4(SPh)4]22 with Et2NCS2 2 in MeCN at 25.0 8C. Shown (from left to right) are: (i) acid-catalysed pathway; (ii) uncatalysed pathway; (iii) Na1-catalysed pathway and (iv) Na1- with acid-catalysed pathway. Only one PhS ligand is shown for simplicity; d = Fe, s = S.tion is independent of thiolate. Analysis of these data by the usual “double reciprocal” graph13 gives the rate law shown in equation (3). 2d[Fe4S4(SPh)4 22] dt = {(2.0 ± 0.5) × 1022 1 (1.5 ± 0.1) × 102[Na1]} 1 1 (5.1 ± 0.3) × 102[Na1] [Fe4S4(SPh)4 22] (3) Fig. 3 Kinetics for the reaction between [Fe4S4(SPh)4]22 (0.1 mmol dm23) and Et2NCS2 2 in MeCN at 25.0 8C. The bottom curve illustrates the eVect of Na1 on the rate of the reaction (d); the curve is that defined by equation (3).The top curve shows the combined eVect of Na1 and H1; data points correspond to: [NHEt3 1] = 10 mmol dm23 (s), [NHEt3 1] = 20 mmol dm23 (g); [Et2NCS2 2] = 2.0 mmol dm23, [NEt3] = 1–20 mmol dm23. The curve is that defined by equation (6). This behaviour is consistent with the pathway shown in Fig. 2 in which Na1 rapidly binds to the cluster and this labilises the Fe–SPh bond to dissociation. This behaviour is directly analogous to that observed with H1, and the eVects of Na1 and H1 will be compared below. First, the way Na1 binds to [Fe4S4- (SPh)4]22 will be considered.The binding of Na1 to other Fe–S clusters has been observed crystallographically. Thus, in [Na2{Fe6S9(SMe)2}2]62, each Na1 is bound to three m3-S;14 in [a-Na2Fe18S30]82 and [b-Na2Fe18- S30]82, each Na1 is bound to four m-S 15,16 and in [Na9Fe20Se38]92 each Na1 is bound to four m-Se.16 Finally, there is evidence that Na1 interacts with the “double-cubane” [{MoFe3S4(SEt)2- (Cl4cat)}2(m-SEt)2]42 (Cl4cat = C6Cl4O2 22).17 Molecular modelling studies indicate that a Na1 could bind to two m3-S and two m-SEt residues.With these precedents in mind, it seems likely that Na1 binds to [Fe4S4(SPh)4]22 using one SPh and two m3-S as shown in Fig. 2. The rate law for the reactions in the presence of Na1 is readily derived by assuming that binding Na1 is a rapid equilibrium reaction (complete within the dead-time of the stopped-flow apparatus, 2 ms), and that subsequent dissociation of the Fe–SPh bond is rate-limiting.The result is shown in equation (4), and comparison of equations 2d[Fe4S4(SPh)4 22] dt = {k0 1 kNaKNa[Na1]} 1 1 KNa[Na1] [Fe4S4(SPh)4 22] (4) (3) and (4) gives k0 = 1.5 ± 0.5 × 1022 s21, kNa = 0.30 ± 0.04 s21 and KNa = 5.1 ± 0.3 × 102 dm3 mol21. A quantitative measure of the labilising eVect of Na1 is given by kNa/k0 = 31 ± 6. Equation (4) is directly comparable to the rate law describing the eVect of [NHEt3]1 on the substitution reactions 2 of [Fe4- S4(SPh)4]22.In this case the total ‘proton’ concentration is expressed as [NHEt3 1]/[NEt3], and the rate law is that shown in equation (5), with k0 = 1.0 ± 0.2 × 1022 s21, kH = 8.0 ± 0.1 × 1022 s21 and KH = 1.2 ± 0.1.122 J. Chem. Soc., Dalton Trans., 1999, 119–125 2d[Fe4S4(SPh)4 22] dt = {k0 1 kHKH[NHEt3 1]/[NEt3]} 1 1 KH[NHEt3 1]/[NEt3] [Fe4S4(SPh)4 22] (5) By comparing kNa and kH derived from equations (4) and (5) respectively, a quantitative measure of the relative labilising eVects of H1 and Na1 is obtained, kNa/kH = 4.1 ± 0.3.Although this is not a large diVerence it is, at first sight a surprising result. Intuitively, it might be expected that H1 would be more labilising than Na1 since H1 is a more polarising cation. In addition, in the reactions with acid, H1 is labilising a thiol ligand (Fig. 1) whereas Na1 is labilising a thiolate ligand (Fig. 2), making the greater labilising power of Na1 even more unexpected.The reasons for this are not entirely clear but we suggest that (at least) part of the reason is because a single Na1 is suYciently large to interact with both the thiolate ligand and two m3-S simultaneously. Consequently, the labilising interactions of Na1 with the leaving group and m3-S are always in concert. In contrast, with the smaller H1, such a concerted interaction is not possible and multiple protonations must occur to attain the same eVect.The importance of the thiolate ligand in facilitating the binding of Na1 to [Fe4S4(SPh)4]22 is emphasised in studies with [Fe4S4Cl4]22. The kinetics of the reactions of [Fe4S4Cl4]22 with [NBun 4]S2CNEt2 are independent of the concentration of Et2NCS2 2, with k0 C = 3.0 ± 0.5 s21. This rate constant is in good agreement with that observed earlier for the dissociative substitution pathway using PhS2 as the nucleophile (k0 C = 2.0 ± 0.3 s21).4 The rate of the reaction of [Fe4S4Cl4]22 with Et2NCS2 2 is unaVected by the presence of Na1.Since the geometries of the cluster cores of [Fe4S4(SPh)4]22 and [Fe4S4Cl4]22 are essentially identical, this indicates that Cl is a poorer ligand than PhS for Na1. Assuming that the rate law shown in equation (4) operates in the reactions of [Fe4S4Cl4]22, a limit for the value of KNa C (the binding constant of Na1 to [Fe4S4Cl4]22) can be calculated. Since, there is no evidence for the binding of Na1 even when [Na1] = 20.0 mmol dm23, KNa C < 5 dm3 mol21 (i.e.Na1 is bound to [Fe4S4Cl4]22 at least 100 times more weakly than it is to [Fe4S4(SPh)4]22). Combined eVect of Na1 and H1 on the lability of [Fe4S4(SPh)4]22 When [Na1] � 10 mmol dm23, all of the [Fe4S4(SPh)4]22 in solution has a Na1 bound to it, i.e. [{Fe4S4(SPh)4}Na]2. Under these conditions, the addition of [NHEt3]1 results in a further increase in the rate as shown in Fig. 3. Analysis of the kinetics shows that the reaction exhibits a non-linear dependence on, [NHEt3 1]/[NEt3], such that at low values of this ratio the rate exhibits a first order dependence on [NHEt3 1]/[NEt3], but is independent of the ratio at high values of [NHEt3 1]/ [NEt3].In additional experiments, [NHEt3 1]/[NEt3] was kept constant and the concentration of PhSH varied. Under these conditions the rate of the reaction does not depend on the concentration of PhSH. The rate law which fits these data is shown in equation (6). 2d[Fe4S4(SPh)4 22] dt = {0.30 ± 0.03 1 (0.38 ± 0.04)[NHEt3 1]/[NEt3]} 1 1 (0. ± 0.02)[NHEt3 1]/[NEt3] × [Fe4S4(SPh)4 22] (6) This rate law is identical to that observed with essentially every Fe–S-based cluster we have studied to date,2–8 the only diVerence is that in this case Na1 is additionally bound to the cluster. The mechanism is shown in Fig. 2. Rapid protonation of [{Fe4S4(SPh)4}Na]2 labilises the cluster towards dissociation of the Fe–SPh bond. The kinetics clearly demonstrate that H1 does not displace the bound Na1, otherwise kobs at high [NHEt3 1]/[NEt3] would correspond to the value observed in the presence of [NHEt3]1 and defined by equation (5).By considering all the pathways shown in Fig. 2, the general rate law shown in equation (7) can be derived, assuming that 2d[Fe4S4(SPh)4 22] dt = {k0 1 kNaKNa[Na1] 1 kNaHKNaKNaH[Na1][NHEt3 1]/[NEt3]} 1 1 KNa[Na1] 1 KNaKNaH[Na1][NHEt3 1]/[NEt3] × [Fe4S4(SPh)4 22] (7) binding of Na1 and H1 are rapidly established equilibria complete within the dead-time of the stopped-flow apparatus, and dissociation of the Fe–SPh bonds are the rate-limiting steps.Under conditions where [Na1] > 10 mmol dm23, KNa[Na1] @ 1 and equation (7) simplifies to equation (8). 2d[Fe4S4(SPh)4 22] dt = {k0 1 kNa 1 kNaHKNaH[NHEt3 1]/[NEt3]} 1 1 KNaH[NHEt3 1]/[NEt3] × [Fe4S4(SPh)4 22] (8) Comparison of equations (6) and (8) gives (k0 1 kNa) = 0.30 ± 0.03 s21, and using k0 = 1.5 ± 0.5 × 1022 s21 (the mean value of k0 derived from this and earlier studies), kNa = 0.28 ± 0.03 s21.This value is in good agreement with that derived from studies in the presence of only Na1 [equation (4)]; in addition, kNaH = 1.5 ± 0.2 s21 and KNaH = 0.25 ± 0.02. The question which must now be addressed is, “Where does this proton bind?” Using the value of KNaH = 0.25 ± 0.02 derived from these studies together with the pKa of [NHEt3]1 in MeCN (18.46),18 the pKa = 17.9 of [{Fe4S4(SPh)4}Na]2 can be calculated.This value is slightly smaller than for the parent [Fe4S4(SPh)4]22 (pKa = 18.6),5 consistent with the presence of the electron-withdrawing Na1 bound to the cluster. Most important the pKa associated with [{Fe4S4(SPh)4}Na]2 falls in the range 17.9 £ pKa £ 18.9, observed for all Fe–S-based clusters in MeCN.5 This is consistent with protonation occuring at the cluster core; most probably a m3-S. Since, the above analyses have yielded the values of kNaH and kNa, and earlier work2 gave kH we are in a position to discuss quantitatively the relative labilising eVects of Na1, H1 and the combined eVect of both Na1 and H1 on the cluster. From the studies with [NHEt3]1 alone [equation (5)], kH/ k0 = 8.4 ± 1.6, and in studies where only Na1 is added [equation (4)], kNa/k0 = 31 ± 6.The addition of both Na1 and H1 results in an increase in the rate [equation (8)], kNaH/k0 = 170 ± 20. This is close to the value which can be calculated using the simple equation, kNaH/k0 = (kNa/k0)(kH/k0) = 260 ± 50.That is, the labilising eVect of Na1 and H1 together is not appreciably diVerent from the product of the individual labilising components. We will see in the next section that similar behaviour is observed in the eVects of H1 and nucleophile on the lability of the cluster in an associative mechanism. EVect of H1 on the dissociative lability of [Fe4S4Cl4]22 Previous studies showed that the substitution reaction between [Fe4S4Cl4]22 and PhS2 occurs predominantly by an associativeJ.Chem. Soc., Dalton Trans., 1999, 119–125 123 mechanism, and protonation (by [NHEt3]1) accelerates the rate.4 Previously, the relative contributions to the labilisation of the cluster from binding H1 and PhSH could not be assessed. However, because Et2NCS2 2 is a poor nucleophile the substitution reaction with [Fe4S4Cl4]22 occurs exclusively by a dis- Fig. 4 Kinetics for the reaction between [Fe4S4Cl4]22 (0.1 mmol dm23) and Et2NCS2 2 in the presence of [NHEt3]1 in MeCN at 25.0 8C.Data shown: [NHEt3 1] = 10.0 mmol dm23 (d), [NHEt3 1] = 20.0 mmol dm23 (g); [Et2NCS2 2] = 2.0 mmol dm23, [NEt3] = 0.7–20 mmol dm23. Curve drawn is that defined by equation (9). sociative pathway. This permits a quantification of the eVect H1 alone has on the rate of dissociation of the chloro-group. Comparison with the earlier studies allows us to estimate the individual eVects that binding H1 and PhSH have on the labilisation of the chloro-group in the associative pathway.The addition of [NHEt3]1 to the reaction between [Fe4S4- Cl4]22 and Et2NCS2 2 leads to an increase in the rate of reaction as shown in Fig. 4. The rate of reaction exhibits a first order dependence on the concentration of [Fe4S4Cl4]22 (as indicated by the exponential shape of the absorbance–time curve) and the usual non-linear dependence on the ratio, [NHEt3 1]/[NEt3]. The rate law consistent with these data is shown in equation (9). 2d[Fe4S4Cl4 22] dt = {3.0 ± 0.5 1 (30.2 ± 0.2)[NHEt3 1]/[NEt3]} 1 1 (2.0 ± 0.2)[NHEt3 1]/[NEt3] × [Fe4S4Cl4 22] (9) This is consistent with the dissociative mechanism shown in Fig. 5. The rate law associated with this mechanism is shown in equation (10), assuming that protonation is a rapidly estab- 2d[Fe4S4Cl4 22] dt = {k0 C 1 kH CKH C[NHEt3 1]/[NEt3]} 1 1 KH C[NHEt3 1]/[NEt3] [Fe4S4Cl4 22] (10) lished equilibrium and dissociation of the chloro-group is ratelimiting. Comparison of equations (9) and (10) gives: k0 C = 3.0 ± 0.5 s21; kH C = 15.0 ± 1.0 s21 and KH C = 2.0 ± 0.2.The value of Fig. 5 Summary of the uncatalysed dissociative, and the acid-catalysed dissociative and associative substitution pathways for the reaction between [Fe4S4Cl4]22 and RSH (R = Et2NCS or Ph). Only one Cl ligand is shown for simplicity; d = Fe, s = S.124 J. Chem. Soc., Dalton Trans., 1999, 119–125 Table 1 Comparison of the individual and combined eVects of H1, Na1 and PhSH on the dissociation of the leaving group in the substitution reactions of [Fe4S4X4]22 (X = Cl or PhS) kAB/k0 Cluster [Fe4S4(SPh)4]22 A HH kA/k0 a 8.4 ± 1.6 8.4 ± 1.6 B H Na kB/k0 8.4 ± 1.6 31 ± 6 Obs. 41 ± 8 170 ± 20 Calc. 71 ± 13 260 ± 50 kAB/k0 C kAk0 C b kBk0 C Obs. Calc. [Fe4S4Cl4]22 H 5.0 ± 0.3 PhSH 83 ± 4 �250 415 ± 20 a k0 = (1.0 ± 0.2) × 1022 s21, for studies with [Fe4S4(SPh)4]22. b k0 C = 3.0 ± 0.5 s21, for studies with [Fe4S4Cl4]22. KH C is in good agreement with that determined in the earlier studies with PhSH (KH C = 2.2 ± 0.1).4 Earlier studies on the reaction between [Fe4S4Cl4]22 and PhSH in the presence of [NHEt3]1 showed that the mechanism involved rapid protonation of the cluster, followed by the binding of PhSH (KT), then rate-limiting cleavage of Fe–Cl (kTH)4 (Fig. 5). Analysis of the kinetics gave KTkTH = 1.5 × 104 dm3 mol21 s21. A limit to the value of KT can be estimated since, even at the highest concentration of PhSH used ([PhSH] = 5.0 mmol dm23), there is no kinetic evidence for the accumulation of appreciable amounts of the cluster with PhSH bound; hence (5.0 × 1023)KT £ 0.1, and KT £ 20 mol dm23; consequently kTH � 7.5 × 102 s21.The labilisation aVorded by binding H1 and PhSH is kTH/k0 C � 250. We are now in a position to estimate the individual labilising eVects of H1 (kH C/k0 C) and nucleophile (kT/k0 C). The studies with Et2NCS2 2, reported herein, show that protonation of the cluster core labilises the chloro-group to dissociation, kH C/ k0 C = 5.0 ± 0.3.Earlier studies 4 showed that the dissociation of the chloro-group after binding of PhS2 was associated with a rate constant, kT = 2.5 ± 0.1 × 102 s21. Consequently, labilisation aVorded by binding of PhS2 is kT/k0 C = 83 ± 4. Although, strictly, this is the labilisation aVorded by PhS2 rather than PhSH, currently it is the best we can do, and does at least give an estimate of the eVect of PhSH.Using these values we can calculate (kT/k0 C)(kH/k0 C) = 415 ± 20, consistent with the simple relationship, kTH/k0 C = (kT/k0 C)(kH/k0 C). Previously, in a study 5 on id-catalysed substitution reactions of the linear trinuclear cluster, [Cl2FeS2VS2FeCl2]32 we came to the conclusion that H1 alone was not particularly labilising but for maximum labilisation both protonation and binding a thiol was necessary. Herein, a detailed quantitative analysis confirms our earlier proposal.Labilisation by multiple components Throughout this paper we have emphasised that the labilising eVect of adding more than one reactant (H1, Na1 or nucleophile) to a cluster is not appreciably more labilising than that expected from the individual eVects of each contributor. This is born out by the summary of the results shown in Table 1, where we return to the generalised designations (A, B, C, etc. . . .) introduced in equation (1). Thus, two species A and B will aVect the lability of the leaving group by an amount described by the simple relationship shown in equation (11), where k0 is the rate constant associated with the uncatalysed reaction.kAB/k0 = (kA/k0)(kB/k0) (11) Close inspection of Table 1 reveals that our (perhaps oversimplistic) equations consistently over-emphasise the labilising power of the combination of several components. This is probably not too surprising considering the electronic origins of the eVects we are discussing.The strength, and hence lability, of Fe–Cl, Fe–SPh or Fe–SHPh bonds are defined by the s- and p-orbital overlap between ligand and Fe. The electron density distribution within these s- and p-orbital components is perturbed by the presence of H1, Na1 or nucleophile. It seems likely that in the presence of several components the electron distribution is distorted predominantly by one component such that the others do not have the eVect (when acting in concert) that they do when acting individually.However, it is clear that this is a rather minor eVect and that (at least in these systems) there is no cooperative labilising eVect from having several components present. Experimental All manipulations were routinely performed under an atmosphere of dinitrogen using Schlenk or syringe techniques as appropriate. [NBun 4]2[Fe4S4(SPh)4] 19 and [NBun 4]2[Fe4S4Cl4] 11 were prepared by the literature methods and characterised as described earlier.MeCN was dried by distillation from CaH2 under an atmosphere of dinitrogen. Na[BPh4] was purchased from Aldrich and used as received. NaS2CNEt2?3H2O (Aldrich) was recrystallised from methanol– diethyl ether and dried in vacuo. [NHEt3]BPh4 was prepared by the literature method.20 Preparation of [NBun 4]S2CNEt2 [NBun 4]Br (2.9 g, 8.9 mmol) was added to a solution of NaS2CNEt2?3H2O (2.0 g, 8.9 mmol) in methanol (ca. 50 mL), and the solution stirred for 30 min.The solvent was then removed in vacuo to leave a pale yellow solid. MeCN (ca. 20 mL) was added to the solid and after stirring for 30 min the mixture was filtered through Celite to remove NaBr. Diethyl ether (ca. 60 mL) was added to the clear filtrate which went cloudy (a further small amount of NaBr). The mixture was again filtered through Celite, then addition of a large excess of diethyl ether (ca. 200 mL) to the clear solution produced no further cloudiness. The solution was cooled to 220 8C overnight to produce fine, pale yellow needles of the product, which was removed by filtration, washed with diethyl ether and then dried in vacuo.Kinetic studies The reactions were studied on a Hi-Tech Stopped-Flow apparatus modified to handle air-sensitive solutions.21 The temperature was maintained at 25.0 8C using a Grant LE8 thermostat tank. The spectrophotometer is interfaced to a Viglen computer via an analogue-to-digital convertor. All solutions were prepared immediately prior to study and used within 1 h.J.Chem. Soc., Dalton Trans., 1999, 119–125 125 Under all conditions the reactions exhibited exponential absorbance–time curves which were fitted using a computer program. The dependence on the concentration of other reagents was established using conventional graphical methods as presented in Results and discussion. Acknowledgements The BBSRC is acknowledged for supporting this work. References 1 R. G. Wilkins, Kinetics and Mechanism of Reactions of Transition Metal Complexes, VCH, Weinheim, 2nd edn., 1991, ch. 1 and 4. 2 R. A. Henderson and K. E. Oglieve, J. Chem. Soc., Dalton Trans., 1993, 1467. 3 R. A. Henderson and K. E. Oglieve, J. Chem. Soc., Dalton Trans., 1993, 1473. 4 R. A. Henderson and K. E. Oglieve, J. Chem. Soc., Chem. Commun., 1994, 377. 5 K. L. C. Grönberg and R. A. Henderson, J. Chem. Soc., Dalton Trans., 1996, 3667. 6 R. A. Henderson and K. E. Oglieve, J. Chem. Soc., Dalton Trans., 1998, 1731. 7 V. R. Almeida, C. A. Gormal, K. L. C. Grönberg, R. A. Henderson, K. E. Oglieve and B. E. Smith, Inorg. Chim. Acta, submitted. 8 K. L. C. Grönberg, R. A. Henderson and K. E. Oglieve, J. Chem. Soc., Dalton Trans., 1998, 3093. 9 D. Sellmann and J. Sutter, Acc. Chem. Res., 1997, 30, 460, and refs. therein. 10 B. A. Averill, T. Herskovitz, R. H. Holm and J. A. Ibers, J. Am. Chem. Soc., 1973, 95, 3523. 11 G. B. Wong, M. A. Bobrik and R. H. Holm, Inorg. Chem., 1978, 17, 578. 12 M. G. Kanatzidis, D. Coucouvanis, A. Simopoulos, A. Kostikas and V. Papaefthymiou, J. Am. Chem. Soc., 1985, 107, 4925. 13 Ref. 1, p. 24. 14 H. Strasdeit, B. Krebs and G. Henkel, Inorg. Chem., 1984, 23, 1816. 15 J.-F. You, B. S. Snyder, G. C. Papefthymiou and R. H. Holm, J. Am. Chem. Soc., 1990, 112, 1067. 16 J.-F. You, G. C. Papefthymiou and R. H. Holm, J. Am. Chem. Soc., 1992, 114, 2697. 17 J. Huang, C. Goh and R. H. Holm, Inorg. Chem., 1997, 36, 356. 18 K. Izutsu, Acid–Base Dissociation Constants in Dipolar Aprotic Solvents, Blackwell Scientific, Oxford, 1990, p. 17. 19 B. V. Pamphilis, B. A. Averill, T. Herskovitz, L. Que, jun. and R. H. Holm, J. Am. Chem. Soc., 1974, 96, 4159. 20 J. R. Dilworth, R. A. Henderson, P. Dahlstrom, T. Nicholson and J. A. Zubieta, J. Chem. Soc., Dalton Trans., 1987, 529. 21 R. A. Henderson, J. Chem. Soc., Dalton Trans., 1982, 917. Paper 8/07769J

ISSN:1477-9226    

DOI:10.1039/a807769j

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

DALTON FULL PAPER J. Chem. Soc., Dalton Trans., 1999, 127–133 127 Selective electrochemical recognition of sulfate over phosphate and phosphate over sulfate using polyaza ferrocene macrocyclic receptors in aqueous solution Paul D. Beer,*a James Cadman,a José M. Lloris,b Ramón Martínez-Máñez,*b Miguel E. Padilla,b Teresa Pardo,b David K. Smith a and Juan Soto b a Inorganic Chemistry Laboratory, University of Oxford, South Parks Road, Oxford, UK OX1 3QR. E-mail: paul.beer@inorganic-chemistry.oxford.ac.uk b Departamento de Química, Universidad Politécnica de Valencia, Camino de Vera s/n, 46071 Valencia, Spain.E-mail: rmaez@qim.upv.es Received 7th September 1998, Accepted 30th October 1998 Potentiometric and electrochemical studies have been carried out with a family of ferrocene redox-functionalised polyamines (L1–L5) and have been directed towards the discrimination, using electrochemical techniques, between the two oxoanions phosphate and sulfate and the electrochemical sensing of ATP.Potentiometric titrations were carried out in THF–water (70 : 30 v/v, 0.1 mol dm23 tetrabutylammonium perchlorate, 25 8C) for L1, L2, L3, L5 and in water (0.1 mol dm23 potassium nitrate, 25 8C) for L4. Potentiometric data indicate that all receptors studied form stable complexes with sulfate, phosphate and ATP. Distribution for the ternary diagram system sulfate–phosphate–L2 shows pH dependent selectivity patterns; [L2HjSO4]j 2 2 species exist at greater than 90% in the pH range 3–4, whereas the corresponding phosphate complexes are the main species in the neutral and basic pH range.The electrochemical studies are in agreement with the speciation results. Sulfate produces in all cyclic receptors maximum cathodic shifts of the redox potential of the ferrocenyl groups around pH 3–4, whereas maximum cathodic shifts for phosphate were found between pH 7 and 8. This behaviour is not observed for the open-chain tetraamine L5.Selective quantitative electrochemical recognition of sulfate and phosphate in the presence of competing anions in aqueous solution has been achieved using the redox-active polyaza ferrocene macrocyclic L2, L3 and L4 receptors. Additionally ATP is able to cathodically shift the oxidation potential of the ferrocenyl groups of L2 and L3 receptors by up to 100 mV. The electrochemical response of L3 against ADP and AMP is also reported. Introduction Taking into account the importance of oxoanions in environmental and biological processes, the development of new oxoanion-sensing receptors is of considerable interest in fields such as environmental chemistry.In fact most of the sensors which have been developed for phosphate, sulfate, etc. do not fulfil requirements such as suYcient selectivity. With the aim of developing new chemical sensor technology, considerable interest is currently being shown in the synthesis of new receptors containing redox-active groups and binding sites for the electrochemical recognition of cationic, anionic and neutral substrates.1 This class of receptors has proved eVective in transforming host–guest interactions into measurable perturbations of the redox potential of the ligand.Examples of water soluble redox responsive receptors designed to electrochemically sense concentrations of guests in the aqueous environment are rare.2,3 This is specially so in anion-sensing where most of the studies have been carried out in non-aqueous solvents and very little is known about the potential use of ferrocene functionalised receptors as anion-sensing molecules in water.Polyamines are well known to bind anions in aqueous solution at certain pH values via favourable protonated ammonium–anion electrostatic and hydrogen bonding interactions.4 By means of incorporating the redox-active ferrocene moiety into polyamine ligand frameworks we report the study of the potential sensing behaviour against sulfate, phosphate and ATP anions of a family of ferrocene-functionalised polyamines (L1–L5) in water and THF–water mixtures.Experimental The synthesis of receptors L1, L2, L3, L4 and L5 have been published elsewhere.5–7 Physical measurements Electrochemical data were obtained with a programmable function generator Tacussel IMT-1, connected to a Tacussel PJT 120-1 potentiostat. The working electrode was graphite with a saturated calomel reference electrode separated from the test solution by a salt bridge containing the solvent/ supporting electrolyte.The auxiliary electrode was platinum wire. Potentiometric titrations were carried out in THF–water (70 : 30 v/v, 0.1 mol dm23 tetrabutylammonium perchlorate, 25 8C) for L1, L2, L3, L5 and in water (0.1 mol dm23 potassium nitrate, 25 8C) for L4, using a reaction vessel waterthermostatted at 25.0 ± 0.1 8C under nitrogen. The titrant was added by a Crison microburette 2031. The potentiometric measurements were made using a Crison 2002 pH-meter and a combined glass electrode. The titration system was automatically controlled by a PC.The electrode was calibrated by titration of well-known amounts of HCl with CO2-free KOH solution and determining the equivalence point by Gran’s method8 which gives the standard potential E98 and the ionic product of water (K9w = [H1][OH2]). The computer program SUPERQUAD9 was used to calculate the protonation and stability constants. The titration curves for each system (ca. 250 experimental points corresponding to at least three titration128 J. Chem. Soc., Dalton Trans., 1999, 127–133 N N N N NH HN NH HN NH HN HN NH HN HN NH NH NH NH NH NH Fe Fe Fe Fe Fe Fe Fe Fe Fe Fe Fe L1 L2 L3 L4 L5 curves, pH = 2log[H], range investigated 2.5–10, concentration of the ligand and anions was ca. 1.2 × 1023 mol dm23) were treated either as a single set or as separate entities without significant variations in the values of the stability constants.Results and discussion Potentiometric anion binding studies Phosphate and sulfate complexation. Speciation studies have been carried out in THF–water (70 : 30 v/v, 0.1 mol dm23 tetrabutylammonium perchlorate, 25 8C) for L1, L2, L3 and L5 and in water (0.1 mol dm23 potassium nitrate, 25 8C) for L4. Tables 1 and 2 report the stability constants found for the L–H1–A systems (L = L1, L2, L3, L4, A = sulfate, phosphate). It is well known that macrocyclic polyamines in solution form protonated species which can interact with anions via electrostatic forces and hydrogen bonds.10 With receptors L1–L4 an additional favourable electrostatic interaction with the anionic guest will result from the oxidised ferrocenium moieties in electrochemical experiments (see below).Table 1 gives the stoichiometry of the species formed and the stability constants with phosphate. There is interaction between the receptors and the phosphate anion in a wide pH range (ca. 1–10).Despite the use of diVerent solvents (THF–water and water) the stoichiometries found in solution for the phosphate complexes formed are quite similar. In all cases 1 : 1 complexes were found. Fig. 1 shows the distribution diagram of the species for the L2–H1–phosphate system. Taking into account the complexity of the studied system the evaluation of the existing species in solution throughout the pH range studied is rather diYcult.11 Table 1 Logarithms of the stability constants for the interaction of L1, L2, L3, L4 or L5 with phosphate in THF–water (70 : 30 v/v, 25 8C, 0.1 mol dm23 tetrabutylammonium perchlorate) for L1, L2, L3, L5 and water (25 8C, 0.1 mol dm23 potassium nitrate) for L4 a Reaction L 1 2H 1 PO4 H2LPO4 b L 1 3H 1 PO4 H3LPO4 L 1 4H 1 PO4 H4LPO4 L 1 5H 1 PO4 H5LPO4 L 1 6H 1 PO4 H6LPO4 H2L 1 PO4 H2LPO4 H3L 1 PO4 H3LPO4 H4L 1 PO4 H4LPO4 H4L 1 HPO4 H5LPO4 H4L 1 H2PO4 H6LPO4 L1 41.63(2) 48.23(2) 50.28(3) 15.55 10.11 4.65 L2 31.03(5) 37.72(4) 43.96(6) 49.49(3) 9.21 11.01 5.21 2.68 L3 40.51(3) 48.58(1) 53.12(2) 12.65 8.85 5.08 L4 25.66(1) c 36.27(1) 45.13(1) 52.60(1) 59.51(1) 5.54 8.14 10.36 5.8 5.68 L5 24.88(2) 33.49(1) 41.18(1) 46.94(1) 50.07(3) 8.74 11.82 16.23 10.14 4.90 a Basicity constants for L1 ref. 13, L2 ref. 6. L3 in THF–water (70 : 30 v/v, 25 8C, 0.1 mol dm23 tetrabutylammonium perchlorate): logb1 = 9.00(1), logb2 = 16.89(1), logb3 = 24.00(1), logb4 = 27.86(1). L5 in THF–water (70 : 30 v/v, 25 8C, 0.1 mol dm23 tetrabutylammonium perchlorate): logb1 = 8.83(1), logb2 = 16.14(1), logb3 = 21.67(1), logb4 = 24.95(1), phosphate in THF–water (70 : 30 v/v, 25 8C, 0.1 mol dm23 tetrabutylammonium perchlorate): logb1 = 11.85(1), logb2 = 20.22(1), logb3 = 24.41(1).L4 in water (25 8C, 0.1 mol dm23 potassium nitrate): logb1 = 10.67(1), logb2 = 20.12(1), logb3 = 28.13(2), logb4 = 34.77(3). b Charges have been omitted for clarity. c Values in parentheses are the standard deviations in the last significant digit.Table 2 Logarithms of the stability constants for the interaction of L1, L2, L4 or L5 with sulfate in THF–water (70 : 30 v/v, 25 8C, 0.1 mol dm23 tetrabutylammonium perchlorate) for L1, L2, L5 and water (25 8C, 0.1 mol dm23 potassium nitrate) for L4 a Reaction L 1 H 1 SO4 HLSO4 b L 1 2H 1 SO4 H2LSO4 L 1 3H 1 SO4 H3LSO4 L 1 4H 1 SO4 H4LSO4 L 1 5H 1 SO4 H5LSO4 HL 1 SO4 HLSO4 H2L 1 SO4 H2LSO4 H3L 1 SO4 H3LSO4 H4L 1 SO4 H4LSO4 H4L 1 HSO4 H5LSO4 L1 30.98(5) 35.02(5) 4.9 5.77 L2 29.80(2) 35.16(4) 3.09 5.28 L4 37.01(1) 40.83(1) 2.24 3.52 L5 12.05(3) c 20.05(2) 26.64(1) 31.89(1) 35.09(2) 3.22 3.91 4.97 6.94 6.86 a Basicity constants for sulfate in THF–water (70 : 30 v/v, 25 8C, 0.1 mol dm23 tetrabutylammonium perchlorate): logb1 = 3.28(1).b Charges have been omitted for clarity. c Values in parentheses are the standard deviations in the last significant digit.J. Chem. Soc., Dalton Trans., 1999, 127–133 129 Nevertheless bearing in mind the protonation constants of the receptors and the phosphate we have tentatively assigned the complexes H4LPO4 to the interaction of H2L21 1 H2PO4 2 and H5LPO4 to H3L31 1 H2PO4 2, taking into account that the H2PO4 2 is in greatest abundance in the pH ranges 4.06–8.08 (in THF–water) and 4.31–8.31 (in water).Assuming these interactions between species, the logarithms of the stability constants for the equilibria H2L21 1 H2PO4 2 H4LPO4 and H3L31 1 H2PO4 2 H5LPO4 (L = L1 to L4) are in the range 1.59–6.28 and 2.04–5.88, respectively. The complex H6LPO4 exists at maximum concentration at pH 4–5 and probably involves H4L41 and H2PO4 2.The nature of the remaining complexes is less clear. The stability constants corresponding to the equilibrium of L1, L2 and L4 with sulfate have also been determined by pH-metric titrations. Stability constants are reported in Table 2. For receptors L1, L2 and L4 receptor–sulfate interactions have only been found at pH lower than 7.Tentatively H4LSO4 and H5LSO4 species are attributed to H4L41 1 SO4 22 and H4L41 1 HSO4 2, respectively. Fig. 2 shows the distribution diagram for the L2–H1–sulfate system. One of our main goals in this study was the development of selective electrochemical sensing receptors able to discriminate between the oxoanions sulfate and phosphate. In order to detect selectivity and determine which are the prevailing species in solution in a mixture of sulfate and phosphate with the receptors L1, L2 and L4, we have calculated the distribution diagram Fig. 1 Distribution diagram of the species for the system L2–H1– phosphate. Fig. 2 Distribution diagram of the species for the system L2–H1– sulfate. of the ternary sulfate–phosphate–L systems by plotting the overall percentages of the free receptors and the sulfate–L and phosphate–L complexes as a function of the pH.11 These diagrams show the competition between sulfate and phosphate (equimolecular amounts) to interact with a target receptor.Fig. 3 shows the ternary diagram for the L2–sulfate–phosphate system. The figure clearly displays the pH dependent selectivity patterns. [L2HjSO4]j 2 2 species exist at greater than 90% in the pH range 3–4, whereas the corresponding phosphate complexes are the main species in the neutral and basic pH range. Similar ternary diagrams are obtained for L1–sulfate–phosphate systems, with predominant sulfate complexes at acid pH and predominant phosphate complexes at neutral and basic pH.This trend is also observed for L4 but phosphate predominates in the presence of sulfate in the pH range studied. This data strongly suggests that some receptors are able to selectively complex sulfate or phosphate by pH modulation. For the sake of comparison the protonation and formation of sulfate and phosphate complexes with the open-chain tetraamine L5 have also been determined in THF–water 70 : 30 v/v. Tables 1 and 2 list the stability constants found.Despite the diVerent geometric architecture of L5 (open-chain against cyclic) the stoichiometries and stability constants of the complexes are in general similar to those found for the cyclic receptors L1, L2, L3 and L4. Additionally the ternary diagram for L5–sulfate–phosphate also displays sulfate species as predominant at acid pH and phosphate complexes as the main species at neutral pH. Fig. 3 Distribution diagram for the ternary system sulfate– phosphate–L2.The sum of percentages of complexed species are plotted vs. pH. [L2] = [sulfate] = [phosphate] = 8 × 1023 mol dm23. Fig. 4 Distribution diagram of the species for the system L1–H1–ATP.130 J. Chem. Soc., Dalton Trans., 1999, 127–133 Table 3 Logarithms of the stability constants for the interaction of L1, L2, L3, L4 or L5 with ATP in THF–water (70 : 30 v/v, 25 8C, 0.1 mol dm23 tetrabutylammonium perchlorate) for L1, L2, L3, L5 and water (25 8C, 0.1 mol dm23 potassium nitrate) for L4 a Reaction L 1 H 1 ATP HLATPb L 1 2H 1 ATP H2LATP L 1 3H 1 ATP H3LATP L 1 4H 1 ATP H4LATP L 1 5H 1 ATP H5LATP L 1 6H 1 ATP H6LATP L 1 7H 1 ATP H7LATP HL 1 ATP HLATP H2L 1 ATP H2LATP H3L 1 ATP H3LATP H4L 1 ATP H4LATP H4L 1 HATP H5LATP H4L 1 H2ATP H6LATP H4L 1 H3ATP H7LATP L1 28.99(4) 36.45(5) 41.72(3) 44.84(3) 6.15 10.37 7.78 4.82 L2 33.69(5) 38.27(4) 43.67(4) 6.98 3.70 3.02 L3 12.67(7) c 22.29(5) 30.43(6) 38.59(5) 45.47(5) 50.46(7) 3.67 5.40 6.43 10.73 10.10 11.33 L4 23.32(9) 31.96(9) 39.58(8) 45.99(11) 50.40(14) 53.44(16) 3.20 3.83 4.81 4.44 4.84 5.86 L5 21.39(1) 29.34(1) 35.75(1) 39.54(1) 41.07(9) 5.25 7.67 10.80 7.08 4.60 a Basicity constants for ATP in THF–water (70 : 30 v/v, 25 8C, 0.1 mol dm23 tetrabutylammonium perchlorate): logb1 = 7.51(1), logb2 = 11.52(1), logb3 = 14.07(3).Basicity constants for ATP in H2O (25 8C, 0.1 mol dm23 potassium nitrate): logb1 = 6.78(1), logb2 = 10.79(2), logb3 = 12.81(5).b Charges have been omitted for clarity. c Values in parentheses are the standard deviations in the last significant digit. ATP complexation. In Table 3 the stability constants of the cyclic L1, L2, L3 and L4 and the open-chain L5 polyamines with ATP are reported. Stability constants found due to the interaction of the protonated forms of the receptors with ATP are generally higher in THF–water 70 : 30 v/v than those found for L4 in water.Fig. 4 shows the distribution diagram for the L1– H1–ATP system. Receptor L4 is fully protonated (H4L4)41 at pH lower than 6.6. On the other hand the first protonation of free ATP in water is ca. 6.7. Therefore the complexes expected to exist in solution involve the interaction of HjLj1 species and ATP42 (H2L, H3L and H4L for species H2LATP, H3LATP and H4LATP species in Table 3). Further protonated species H5LATP and H6LATP are probably related to the interaction of H4L41 with HATP32 and H2ATP22, respectively. The value found for the open-chain tetraamine spermine [H2N(CH2)3- NH(CH2)4NH(CH2)3NH2] in water for its tetraprotonated form with ATP has been reported to be 3.97 which is a value close to that found for (H4L4)41 and ATP42.12 In THF–water with receptors L1, L2 and L3 the situation is more complex. The first protonation constant of ATP in THF–water is ca. 7.51. On the other hand the last protonation constants for L1, L2, L3 and L5 are ca. 3.2–4.8.The diVerence between the first protonation constant of ATP and fully protonated species L1, L2, L3 and L5 is now larger than for L4 in water and therefore several species can coexist in solution and it is more diYcult to determine the nature of the complexes taking into account only stability constant values. Electrochemical anion recognition investigations One of the most interesting features in receptors L1 to L5 is the presence near co-ordination sites of redox-active groups.These can be aVected by the presence of closely bound anionic guest species and transform the receptor–substrate interaction into a macroscopic electrochemical response. The shift of the redox potential of the ferrocenyl groups as a function of the pH in the presence and absence of sulfate, phosphate, ATP and nitrate anions was monitored in THF–water (70 : 30 v/v, 0.1 mol dm23 tetrabutylammonium perchlorate, 25 8C) for L1, L2, L3, L5 and in water (0.1 mol dm23 potassium nitrate, 25 8C) for L4.A unique oxidation potential wave was observed for all the receptors throughout the pH range, except for L4 at neutral pH in which two unresolved waves were observed. Plots of E1/2 vs. pH show for all receptors that a steady anodic shift of the redox potential occurs when the solution is acidified. The diVerence found between the oxidation potential at basic pH (pH = 12) and acidic pH (pH = 0) (obtained by extrapolation of the curves E1/2 vs. pH because of the instability of ferrocenyl groups at pH lower than 2) was 100, 260, 250, 326 and 110 mV, for L1–L5 respectively.As a general rule the fewer the number of ferrocenyl centres and the closer the N-donor atoms are to the redox-centres, the larger is DE1/2.13 Electrochemical response towards sulfate and phosphate. The electrochemical response of sulfate, phosphate and nitrate anions was monitored as a function of pH range. Plots of E1/2 vs. pH for the systems L–H1–A, (L = L1 to L5; A = sulfate, phosphate, nitrate) with a ligand-to-anion molar ratio of 1 : 1 have been determined.Fig. 5 graphically displays the electrochemical anion response found for receptors L1, L2, L3, L4 and L5 as a function of the pH [DE1/2 defined as E1/2 (receptor) 2 E1/2 (anion–receptor)]. Nitrate does not produce any significant redox potential shift at any pH value. Sulfate produces in all receptors maximum cathodic shifts of the redox potential of the ferrocenyl groups around pH 3–5, whereas maximum cathodic shifts for phosphate were found between pH 6 and 8.Maximum selective redox potential shifts (DE1/2) of 54 and 50 mV were observed for sulfate and phosphate, using receptors L2 and L4 at pH 4 and 7, respectively (see Fig. 5). If we compare the potentiometric data and the electrochemical response as a function of the pH the results appear to suggest that the contributions to DE1/2 of the diVerent species found in solution are not the same.For example from Fig. 1 and Fig. 5 it can be observed that although phosphate interacts with L2 in the range pH 2 to 9, the maximum electrochemical response was found in the pH range 5–7 suggesting that only the [H5L2PO4]21 and [H4L2PO4]1 species are able to significantly perturb the oxidation potential of the ferrocenyl moiety, whereas the [H6L2PO4]31 and [H3L2PO4] complexes are not capable of doing so. This is also observed for the remaining receptors L1, L3 and L4, for which the maximum phosphate–receptor interaction always coincides with the pH range of existence of the [H5LPO4]21 and [H4LPO4]1 species.Assuming that [H5LPO4]21 and [H4LPO4]1 species are associated with the interaction of the H2PO4 2 anion with H2L21 and H3L31 species it can be concluded that receptors L1 to L4 are able to selectively detect the presence of the H2PO4 2 anion. From our point of view this is of importance because the data suggests for the first time, to the best our knowledge, that there is a selective electrochemical speciation in the sense that not all the HjPO4 j 2 3 species produce the same oxidation potential shift of the ferrocene groups.For all the L1, L2, and L4 receptors sulfate produces oxidation potential shifts at pH values lower than 7, where the species [H4L2SO4]21 and [H5L2SO4]31 exist. In order to demonstrate the potential use of redoxfunctionalised receptors as practical sensors we have carried out studies on the selective quantitative determination of sulfateJ. Chem.Soc., Dalton Trans., 1999, 127–133 131 Fig. 5 Redox potential shift (DE1/2) for L1, L2, L3, L4 and L5 in the presence of phosphate and sulfate as a function of the pH. and phosphate using receptors L2, L3 and L4. Although the following studies, from a practical point of view, can probably not be applied to a real analytical problem they point out the selective nature of the interaction and reinforce the arguments stated above.For example Fig. 6 shows DE1/2 at pH = 4.0 versus sulfate-to- L2 ratios in the presence and absence of phosphate ([L2] = 50 × 1025 mol dm23; [phosphate] = 52 × 1025 mol dm23). Apart from the selectivity exhibited for sulfate in the presence of phosphate, Fig. 6 indicates that 1 : 1 complexes are formed. This is in agreement with the ternary diagram in Fig. 3 which indicates that in a mixture of sulfate and phosphate at pH 4 the L2 receptor selectively forms complexes with sulfate.We have also determined DE1/2 vs. phosphate-to-L ratios for receptor L3 and L4 at pH 8 and 7, respectively. The linear range of the curve in Fig. 6 (sulfate anion-to-receptor ratios < 0.9 : 1) can be used132 J. Chem. Soc., Dalton Trans., 1999, 127–133 Table 4 Determination of the concentration of sulfate in the presence of phosphate, nitrate, chloride or acetate with receptor L2 in THF–water (70 : 30 v/v) at pH 4.0 by using electrochemical methods a [sulfate] × 105 14.3(8) a [15.2] b 29(1) [29] 39(2) [42] [sulfate] × 105 22.0(7) c [15.0] b 31(1) [29] 41(1) [42] [sulfate] × 105 12.8(6) d [13.4] b 25(1) [26] 33(2) [37] [sulfate] × 105 11.1(3) e [11.1] b 23(1) [21] 29(1) [31] [sulfate] × 105 12.2(8) f [11.3] b 23(1) [22] 31(2) [32] a Concentration (mol dm23) determined by electrochemical methods.Values in parentheses are the standard deviations in the last significant digit. b Sulfate concentration (mol dm23). c [sulfate] determined in the presence of phosphate, [phosphate] = 52 × 1025 mol dm23. d [sulfate] determined in the presence of nitrate, [nitrate] = 46 × 1025 mol dm23.e [sulfate] determined in the presence of chloride, [chloride] = 38 × 1025 mol dm23. f [sulfate] determined in the presence of acetate, [acetate] = 38 × 1025 mol dm23. as a calibration curve for the quantitative determination of sulfate, whereas linear ranges in DE1/2 vs. phosphate-to-L ratio curves for receptor L3 and L4 have been used for the quantitative determination of phosphate. Table 4 shows the selective determination of sulfate in the presence of phosphate, nitrate, chloride, or acetate.The presence of chloride or acetate, which are able to interact with protonated polyamines, does not appear to significantly aVect the sulfate determination indicating that sulfate can be selectively determined in the presence of these competing anions. Table 5 gives the results found in the selective quantitative determination of phosphate using receptor L3 employing electrochemical methods, whereas Table 6 reports the selective determination of phosphate using L4 in water in the presence of sulfate. Sulfate is not able to perturb the electrochemical response against phosphate at pH 7 in agreement with the tertiary diagram of the L4–sulfate– phosphate system which shows predominant L4–phosphate versus L4–sulfate species.Of particular note is the selective quantitative phosphate determination in water even in the presence of other anions such as sulfate and nitrate (often present in water), at the environmentally typical neutral pH.The importance of the molecular architecture is noteworthy when the comparison is drawn between the electrochemical response found for the cyclic receptors L1, L2, L3 and L4 and the open-chain tetraamine L5. The half-wave potential of the open-chain tetraamine L5 is also pH dependent, but neither the presence of nitrate nor phosphate produce any significant change in the oxidation potential of the ferrocenyl groups in clear contrast with that found for the corresponding cyclic tetraamines L1 to L4.On the contrary at acid pH L5 is able to electrochemically recognise sulfate. Bearing in mind that both cyclic and acyclic tetraamines form stable complexes with Fig. 6 Redox potential shift (DE1/2) of L2 vs. sulfate-to-L2 ratios in the absence (s) and presence of phosphate (r). sulfate and phosphate (see above), the diVerent electrochemical response can only be attributed to a diVerent molecular architecture (cyclic versus acyclic).In considering the electrochemically observed behaviour one should be aware of the nature of the interaction process between the ferrocene/ferrocenium groups and the anion. In a first step for a determined pH the anion interacts with the poly-amine/-ammonium cavity via electrostatic forces and/or hydrogen bonds. In a second step when the ferrocene groups are oxidised to ferrocenium an additional cation(ferrocenium)– anion interaction would occur.This ferrocenium–anion interaction would probably be the factor having the largest contribution to the oxidation potential shift found using electrochemical techniques. This interaction would be favoured if the ferrocene groups are fixed and are in close proximity to the anion bound within the cavity. By considering the molecular architecture of cyclic and acyclic receptors it seems clear that most of these factors can be better accommodated by receptors L1, L2, L3 and L4 than by the open-chain molecule L5 and in general one would expect to obtain a greater degree of selectivity and larger DE1/2 shifts in the presence of anions in cyclic rather than in acyclic receptors.Electrochemical response towards ATP. The electrochemical response of receptors L1, L2, L3, L4 and L5 towards ATP in THF–water (70 : 30 v/v) has also been monitored as a function of the pH.Fig. 7 shows DE1/2 [DE1/2 defined as E1/2 (receptor) 2 E1/2 (anion–receptor)] for the L–H1–ATP systems. Although all the receptors L1, L2, L3, L4 and L5 have been found to form stable complexes with ATP their electrochemical response is quite diVerent. First it is interesting to point out that Table 5 Determination of the concentration of phosphate in the presence of sulfate and nitrate with receptor L3 in THF–water (70 : 30 v/v) at pH 8.0 by using electrochemical methods a [PO4 23] × 105 14.8(8) a [11.8] b 21(1) [23] 33(2) [33] [PO4 23] × 105 11(2) c [12] b 21(1) [23] 33(2) [33] [PO4 23] × 105 14(1) d [15] b 28(3) [28] 39(2) [41] a Concentration (mol dm23) determined by electrochemical methods.Values in parentheses are the standard deviations in the last significant digit. b Sulfate concentration (mol dm23). c [PO4 23] determined in the presence of sulfate, [SO4 22] = 42 × 1025 mol dm23. d [PO4 23] determined in the presence of nitrate, [NO3 2] = 50 × 1025 mol dm23.Table 6 Determination of the concentration of phosphate in the presence of sulfate with receptor L4 in water at pH 7.0 by using electrochemical methods a [phosphate] × 105 14(2) a [14] b 27(2) [27] 39.1(9) [39.0] [phosphate] × 105 15(1) c [15] b 27(2) [29] 41(2) [42] a Concentration (mol dm23) determined by electrochemical methods. Values in parentheses are the standard deviations in the last significant digit. b Phosphate concentration (mol dm23).c [phosphate] determined in the presence of sulfate, [sulfate] = 52 × 1025 mol dm23.J. Chem. Soc., Dalton Trans., 1999, 127–133 133 ATP is able to cathodically shift the oxidation potential of the ferrocenyl groups of receptors L2 and L3 by up to 100 mV. Thus DE1/2 found in aqueous solutions for ATP is quite large and is even larger than some of the DE1/2 values found for the interaction of polyazaalkanes with metal ions. In general for the same receptor transition metal ions form more stable complexes than anions, however the large DE1/2 found for L2 and L3 with ATP suggest that there is no direct relation between stability constants and oxidation potential shift.The L5 receptor displays the lowest oxidation potential shift (DE1/2 lower than 20 mV) in the presence of ATP. This appears to reinforce the fact that macrocyclic receptors compared to acyclic structures generally exhibit an enhanced electrochemical recognition eVect.There is also a contrast between the electrochemical response of receptors L2, L3, L4 and L1 which could be explained by taking into account the smaller cyclic cavity in L1 when compared with L2, L3 and L4. Additionally we have also carried out preliminary studies on the electrochemical recognition of ADP and AMP. ATP, ADP and AMP are a series of anions where the charge and the size is steadily reduced from ATP to AMP. Fig. 8 shows DE1/2 [DE1/2 defined as E1/2 (receptor) 2 E1/2 (anion–receptor)] for the L3–H1–A (A = ATP, ADP, AMP) systems as a function of the pH.Maximum oxidation potential shift was found about pH 6–7, where the anions are in their deprotonated form ATP41, ADP31 and AMP21. Fig. 7 Redox potential shift (DE1/2) for L1, L2, L3, L4 and L5 in the presence of ATP. Fig. 8 Redox potential shift (DE1/2) for L3 in the presence of ATP, ADP and AMP anions. Conclusions In summary we have shown that redox-active ferrocene polyazamacrocyclic receptors L1–L4 can, through an electrochemical response, selectively detect at certain pH values sulfate and phosphate in the presence of competing anions in the aqueous environment.A diVerent electrochemical response has been found for open-chain receptor L5 pointing out the importance of the molecular architecture in the electrochemical recognition process. Maximum selective redox potential shifts (DE1/2) of 54 and 50 mV were observed for sulfate and phosphate, using receptors L2 and L4 at pH 4 and 7, respectively.Larger cathodic DE1/2 shifts of up to 100 mV have been found for ATP and L2 and L3. Both the selectivity and the large redox potential shift found for some anions strongly suggest the potential use of these receptors as transducers in amperometric sensor devices in the near future. Of particular note is the selective quantitative phosphate determination in water in the presence of competing anions at the environmentally common neutral pH.Acknowledgements We should like to thank the DGICYT (proyecto PB95-1121- C02-02) for support. We also thank the EPSRC and British Petroleum for studentships (J. C., D. K. S.) and the EPSRC for use of the mass spectrometry service at University College Swansea. References 1 See for example, P. D. Beer, M. G. B. Drew and R. Jagessar, J. Chem. Soc., Dalton Trans., 1997, 881; P. D. Beer, A. R. Graydon, A. O. M. Johnson and D. K. Smith, Inorg. Chem., 1997, 36, 2112; P.D. Beer, Chem. Commun., 1996, 689; P. D. Beer, Chem. Soc. Rev., 1989, 18, 409; P. D. Beer, M. G. B. Drew, D. Hesek, J. Kingston, D. K. Smith and S. E. Stokes, Organometallics, 1995, 14, 3288; P. D. Beer, Z. Chen, M. G. B. Drew and P. A. Gale, J. Chem. Soc., Chem. Commun., 1995, 1851. 2 M. E. Padilla-Tosta, R. Martínez-Máñez, T. Pardo, J. Soto and M. J. L. Tendero, Chem. Commun., 1997, 887; J. M. Lloris, R. Martínez-Máñez, T. Pardo, J. Soto and M. E. Padilla-Tosta, Chem. Commun., 1998, 837. 3 P. D. Beer, Z. Chen, M. G. B. Drew, J. Kingston, M. Ogden and P. Spencer, J. Chem. Soc., Chem. Commun., 1993, 1046. 4 A. Bianchi, K. Bowman-James and E. García-España (Editors), Supramolecular Chemistry of Anions, Wiley-VCH, New York, 1997. 5 P. D. Beer, J. E. Nation, S. L. W. McWhinnie, M. E. Harman, M. B. Hursthouse, M. I. Ogden and A. H. White, J. Chem. Soc., Dalton Trans., 1991, 2485. 6 M. J. L. Tendero, A. Benito, R. Martínez-Máñez, J. Soto, J. Paya, A. J. Edwards and P. R. Raithby, J. Chem. Soc., Dalton Trans., 1996, 343; J. M. Lloris, R. Martínez-Máñez, M. E. Padilla-Tosta, T. Pardo and J. Soto, J. Chem. Soc., Dalton Trans., 1998, 3657. 7 P. D. Beer, Z. Chen, M. G. B. Drew, A. O. M. Johnson, D. K. Smith and P. Spencer, Inorg. Chim. Acta, 1996, 246,143. 8 G. Gran, Analyst (London), 1952, 77, 661. F. J. Rossotti and H. J. Rossotti, J. Chem. Educ., 1965, 42, 375. 9 P. Gans, A. Sabatini and A. Vacca, J. Chem. Soc., Dalton Trans., 1985, 1195. 10 B. Dietrich, M. W. Hosseini and J. M. Lehn, J. Am. Chem. Soc., 1981, 103, 1282; A. Bencini, A. Bianchi, C. Giorgi, P. Paoletti and B. Valtancoli, Inorg. Chem., 1996, 35, 1114. 11 B. Dietrich, J. Guilhem, J. M. Lehn, C. Pascard and E. Sonveaux, Inorg. Chim. Acta, 1984, 67, 91; A. Andres, J. Aragó, A. Bencini, A. Bianchi, A. Domenech, V. Fusi, E. Garcia-España, P. Paoletti and J. A. Ramírez, Inorg. Chem., 1993, 32, 3418; A. Andres, C. Bazzicalupi, A. Bencini, A. Bianchi, V. Fusi, E. Garcia-España, N. Nardi, P. Paoletti and J. A. Ramírez, J. Chem. Soc., Perkin Trans. 2, 1994, 2367. 12 B. Dietrich, L. Fyles, T. M. Fyles and J. M. Lehn, Helv. Chim. Acta, 1979, 2763; A. Bencini, A. Bianchi, M. I. Burguete, A. Domenech, E. García-España, S. V. Luis, M. A. Niño and J. A. Ramírez, J. Chem. Soc., Perkin Trans. 2, 1991, 1445. 13 A. Benito, R. Martínez-Máñez, J. Soto and M. J. L. Tendero, J. Chem. Soc., Faraday Trans., 1997, 93, 2175. Paper 8/06944A

ISSN:1477-9226    

DOI:10.1039/a806944a

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

DALTON FULL PAPER J. Chem. Soc., Dalton Trans., 1999, 135–140 135 Chemistry of ruthenium(II) complexes of the tridentate NNS donor methyl 2-pyridyl ketone 4-(4-tolyl)thiosemicarbazone. Isolation and structural characterisation of a novel ruthenium(II) complex containing a co-ordinated imine of an ·-N heterocyclic ketone Milan Maji,a Madhumita Chatterjee,a Saktiprosad Ghosh,*a Shyamal Kumar Chattopadhyay,b Bo-Mu Wu c and Thomas C. W. Makc a Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, Calcutta 700 032, India b Department of Chemistry, Bengal Engineering College (Deemed University), Howrah 711103, India c Department of Chemistry, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong Received 11th August 1998, Accepted 30th October 1998 A series of ruthenium(II) complexes of the NNS donor ligand methyl 2-pyridyl ketone 4-(4-tolyl)thiosemicarbazone (HL) has been synthesized using RuCl3?xH2O and [Ru(PPh3)3Cl2]: [Ru(HL)2][ClO4]2 1, [Ru(L)(PPh3)2Cl] 2, [Ru(HL)- (PPh3)2Cl]Cl 3, [Ru(HL)(PPh3)2Cl]PF6 4, [Ru(L)(PPh3)(bpy)]PF6 5, [Ru(L)(PPh3)(dppe)]PF6 6, [Ru(HL)(PPh3)- (pic)]PF6 7 and [Ru(HL)(PPh3)(mpi)]Cl2 8 [where bpy = 2,29-bipyridine, dppe = 1,2-bis(diphenylphosphino)ethane, Hpic = pyridine-2-carboxylic acid, mpi = methyl(2-pyridyl)methyleneimine].Chemical and electrochemical studies have been carried out. Structures of the compounds 3?CH2Cl2 3 and 8?CH2Cl2?3H2O have been determined by single crystal X-ray diVraction.The thione form of the ligand (HL) is chelated to the ruthenium centre through the pyridine nitrogen, imine nitrogen and the thione sulfur atom. The existence of a new unstable ligand methyl(2-pyridyl)- methyleneimine (mpi) co-ordinated to RuII through the pyridine and imine nitrogen atoms was confirmed from the crystal structure of compound 8. The chemistry of ruthenium bound to nitrogen–sulfur donor ligands has evinced considerable interest in recent years primarily due to their ability to form complexes with unusual stereochemistry,1 uncommon co-ordination number,2,3 interesting electronic structure and bonding situations and with intricate electron-transfer characteristics.4,5 Thiosemicarbazides and thiosemicarbazones constitute an important class of nitrogen– sulfur donor ligands, because of their highly interesting chemical 5–9 and biological properties.10 As a part of our programme to investigate ruthenium complexes of thiosemicarbazides and thiosemicarbazones in general, we undertook the study of ruthenium complexes of methyl 2-pyridyl ketone 4-(4-tolyl)thiosemicarbazone (HL).During the course of our investigations we came across two very interesting phenomena. (1) Under certain reaction conditions RuII-catalysed reductive cleavage of the hydrazinic N–N bond of the thiosemicarbazone moiety occurred. Such reductive cleavage is rather common in the molybdenum complexes of hydrazine,11 but never observed previously in the metal complexes of thiosemicarbazides and thiosemicarbazones.(2) We isolated and structurally characterised the first metal complex of the imine of a 2(N)-heterocyclic ketone, i.e. a ruthenium(II) complex of methyl(2-pyridyl)- methyleneimine formed during the process mentioned earlier. Most ketone imines (HN]] CR1R2) are unstable at room temperature 12 and are found to react with some metal ions as a monodentate ligand through the imine nitrogen or as an exobidentate ligand bridging two metal ion centres through its deprotonated iminate nitrogen.However, no report on a metal complex containing a co-ordinated imine of a 2(N)- heteroaromatic ketone has appeared previously. This paper reports the results of our studies on several ruthenium complexes involving HL as well as the complex [Ru(HL)(PPh3)- (mpi)]Cl2 containing methyl(2-pyridyl)methyleneimine (mpi). Structures of two complexes, [Ru(HL)(PPh3)2Cl]Cl?CH2Cl2 and [Ru(HL)(PPh3)(mpi)]Cl2?CH2Cl2?3H2O, are described and discussed.Experimental Materials and instrumentation Elemental analysis were performed with a Perkin-Elmer 240 CHN analyser. Those of complexes 3 and 8 were before crystallisation. The IR and electronic spectra were recorded on a Perkin-Elmer 783 spectrophotometer (as KBr disks) and on a Shimadzu UV-VIS recording spectrophotometer respectively. Solution conductances were measured on a Systronics direct reading conductivity meter (model 304) and magnetic susceptibility (at room temperature) was determined with a PAR vibrating sample magnetometer using Hg[Co(SCN)4] as the calibrant.The NMR spectra were recorded on a Bruker 300 MHz spectrometer using SiMe4 as an internal standard. Electrochemical data were collected with a BAS CV-27 and a BAS model X-Y recorded at 298 K. Cyclic voltammetry experiments were carried out with platinum working and auxiliary electrodes and a Ag–AgCl reference electrode. The compound RuCl3?xH2O was obtained from Arora Matthey (Calcutta, India) and 2-acetylpyridine from Aldrich. 4-(4-Tolyl)thiosemicarbazide 10 and [Ru(PPh3)3Cl2] 13 were prepared according to published procedures. Acetonitrile (pure) obtained from E. Merck (India) was freshly distilled over calcium hydride for electrochemical experiments. Other reagents were used without further purification. Preparations Methyl 2-pyridyl ketone 4-(4-tolyl)thiosemicarbazone (HL). 4-(4-Tolyl)thiosemicarbazide (2.42 g, 0.02 mol) was dissolved in ethanol (100 cm3) by heating and 2-acetylpyridine (3.62 g,136 J.Chem. Soc., Dalton Trans., 1999, 135–140 0.02 mol) was added. The mixture was stirred for 45 min. Then acetic acid (2 cm3) was added and stirred again for 3 h. The product was filtered oV, washed with water and diethyl ether and dried. The yield was 90%. 1H NMR (CDCl3, room temperature): d 9.32 (s, 1 H, NH), 8.88 (s, 1 H, NH), 8.76 (d), 8.61 (d), 8.01 (d), 7.87 (t), 7.74 (t), 7.56 (t), 7.37 (t), 7.31 (t), 7.19 (t), 2.49 (s, 3 H, CH3) and 2.36 (s, 3 H, CH3).[Ru(HL)2][ClO4]2 1. CAUTION! perchlorate salts of metal complexes with organic ligands are potentially explosive. Only a small amount of compound should be prepared, and handled with caution. The ligand (HL) (568 g, 0.2 mmol) was suspended in methanol (30 cm3) and RuCl3?xH2O (261 mg, 0.2 mmol) dissolved in methanol (25 cm3) was added drop by drop. The mixture was stirred for 3 h.It was filtered and the filtrate concentrated to about 10 cm3 using a rotary evaporator. An aqueous solution of lithium perchlorate was added to the concentrated solution and the desired compound precipitated. It was washed with water followed by diethyl ether and dried over fused calcium chloride (Found: C, 41.03; H, 3.60; N, 12.90. Calc. for C30H32Cl2N8- O8RuS2: C, 41.47; H, 3.69; N, 12.90%). Conductance in CH3CN (LM): 230 W21 cm2 mol21. Electronic spectrum in CH3CN [lmax/ nm(103emax/M21 cm21)]: 650 (2.536), 382 (86.032), 255 (100.9) and 210 (151.58).[Ru(L)(PPh3)2Cl] 2, [Ru(HL)(PPh3)2Cl]Cl 3 and [Ru(HL)- (PPh3)2Cl]PF6 4. The ligand (HL) (71 mg, 0.25 mmol) was dissolved in ethanol (20 cm3) and [Ru(PPh3)3Cl2] (240 mg, 0.25 mmol) added. The mixture was refluxed for 4 h under dry nitrogen, then cooled. The solid product (2) was filtered oV, washed with ether and dried in a calcium chloride desiccator. The filtrate was concentrated in a rotary evaporator to about 10 mL.Compound 4 was isolated by adding saturated aqueous ammonium hexafluorophosphate to the concentrated solution. It was filtered oV, washed thoroughly with water and then with ether and finally dried over fused calcium chloride. Alternatively, the chloride compound 3 was obtained by concentrating the filtrate to about 10 mL and adding ether. The solid separated was filtered oV, washed thoroughly with ether and recrystallised from dichloromethane (Found: C, 64.63; H, 4.78; N, 6.01.Calc. for C51H45ClN4P2RuS 2: C, 64.86; H, 4.77; N, 5.93%). Conductance in CH3CN (LM): 15.4 W21 cm2 mol21. Electronic spectrum in CH3CN [lmax/nm(103emax/M21 cm21)]: 580 (0.9007), 481 (2.256), 398 (9.760), 376 (11.192), 274 (11.870) and 215 (40.85) (Found: C, 62.67; H, 4.83; N, 5.59. Calc. for C51H46Cl2N4P2RuS 3: C, 62.45; H, 4.69; N, 5.71%). Conductance in CH3CN 123.09 W21 cm2 mol21. Electronic spectrum in CH3CN [lmax/nm(103emax/M21 cm21)]: 478 (0.3218), 445 (9.381), 402 (10.67), 378 (12.58) and 210 (73.15). 1H NMR (CDCl3, room temperature): d 12.16 (s, 1 H, NH), 11.12 (s, 1 H, NH), 8.76 (d), 7.41 (m), 7.36 (t), 7.24 (m), 7.06 (t), 6.79 (t), 2.31 (s, 3 H, CH3) and 2.16 (s, 3 H, CH3) (Found: C, 56.27; H, 4.34; N, 5.19.Calc. for C51H46ClF6N4P3RuS 4: C, 56.17; H, 4.22; N, 5.14%). Conductance in CH3CN 153.24 W21 cm2 mol21. Electronic spectrum in CH3CN [lmax/nm(103emax/M21 cm21)]: 478 (0.3712), 442 (11.37), 403 (16.15), 362 (12.37), 265 (35.41) and 212 (52.27).[Ru(L)(PPh3)(bpy)]PF6 5. The complex [Ru(L)(PPh3)2Cl] (119 mg, 0.12 mmol) was dissolved in dichloromethane (20 cm3). 2,29-Bipyridine (219.5 mg, 0.12 mmol) followed by methanol (25 cm3) was added. The mixture was refluxed for 8 h. After cooling the solution was concentrated in a rotary evaporator to about 10 cm3. Compound 5 was isolated by adding saturated aqueous ammonium hexafluorophosphate to the concentrated solution.The precipitated compound was filtered oV, washed thoroughly with distilled water and dried over fused calcium chloride. It was then washed with ether and dried (Found: C, 54.93; H, 4.29; N, 9.1. Calc. for C43H38F6N6P2RuS: C, 54.49; H, 4.01; N, 8.87%). Conductance in CH3CN 125.21 W21 cm2 mol21. Electronic spectrum in CH3CN [lmax/nm (103emax/M21 cm21)]: 456 (8.01), 376 (15.59), 296 (28.57) and 209 (81.19). [Ru(L)(PPh3)(dppe)]PF6 6. The complex [Ru(L)(PPh3)2Cl] (120 mg, 0.12 mmol) was dissolved in dichloromethane (20 cm3). 1,2-Bis(diphenylphosphino)ethane (49 mg, 0.12 mmol) was added followed by methanol (25 cm3). The mixture was refluxed for 8 h then concentrated in a rotary evaporator. The compound was isolated by adding an aqueous solution of ammonium hexafluorophosphate. It was filtered oV, washed with water and dried over fused calcium chloride. The dry compound was finally washed with ether and dried (Found: C, 60.13; H, 4.67; N, 4.59. Calc. for C59H54F6N4P4RuS: C, 59.54; H, 4.54; N, 4.71.Conductance in CH3CN 127.10 W21 cm2 mol21. Electronic spectrum in CH3CN [lmax/nm(103emax/M21 cm21)]: 481 (0.820), 424 (5.94), 373 (11.91), 261 (21.75) and 216 (45.96). [Ru(HL)(PPh3)(pic)]PF6 7. The complex [Ru(L)(PPh3)2Cl] (120 mg, 0.12 mmol) was dissolved in dichloromethane (20 cm3). Pyridine-2-carboxylic acid (16 mg, 10.12 mmol) was added, followed by methanol (25 cm3). The mixture was refluxed for 8 h (Hpic) then concentrated in a rotary evaporator. Compound 7 was isolated by adding an aqueous solution of ammonium hexafluorophosphate. It was filtered oV, washed thoroughly with water and dried over calcium chloride.The dry compound was washed again with ether and dried (Found: C, 50.8; H, 4.03; N, 4.75. Calc. for C39H34F6N5O2P2RuS: C, 51.26; H, 3.72; N, 7.67%). Conductance in CH3CN 139.30 W21 cm2 mol21. Electronic spectrum in CH3CN [lmax/nm(103emax/M21 cm21)]: 577 (0.811), 481 (4.82), 376 (21.93), 256 (23.09) and 220 (54.13).[Ru(HL)(PPh3)(mpi)]Cl2 8. The complex [Ru(L)(PPh3)2Cl] (120 mg, 0.12 mmol) was dissolved in ethanol (40 cm3) and a sixfold excess of ligand (HL) (0.72 mmol) added. The mixture was refluxed for 24 h then cooled and filtered. The filtrate was evaporated to dryness. The solid residue was stirred with n-hexane to wash out excess of ligand and triphenylphosphine, filtered oV, dried and recrystallised from a mixture of dichloromethane and n-hexane. The components in the filtrate were separated by column chromatography using neutral silica gel.The first component was triphenylphosphine eluted with light petroleum (bp 60–80 8C), the middle fraction was N-(p-tolyl)thiourea and the last fraction with the ligand (HL) eluted with 10% ethyl acetate in light petroleum (Found: C, 57.21; H, 4.62; N, 10.38. Calc. for C40H39Cl2N6PRuS 8: C, 57.28; H, 4.65; N, 10.02%). Conductance in CH3CN 210.00 W21 cm2 mol21. Electronic spectrum in CH3CN [lmax/nm(103emax/ M21 cm21)]: 464 (3.469), 378 (10.94), 317 (15.65), 260 (20.94) and 216 (45.96). 1H NMR (CDCl3, room temperature): d 12.25 (s, 1 H, NH), 8.14 (d), 8.04 (t), 7.87 (q), 7.6 (m), 7.3 (s), 7.05 (d), 6.66 (s), 2.31 (s, 3 H, CH3) and 2.35 (s, 3 H, CH3) (Found: C, 57.68; H, 5.93; N, 17.03. Calc. for N-(p-tolyl)thiourea (C8H10- N2S): C, 57.83; H, 6.02; N, 16.87%). IR in CHCl3: n(NH) 3400, 3380, n(SH) 2420, n(C–S) 840 cm21. 1H NMR (CDCl3, room temperature): d 8.62 (d, 1 H, Ph), 8.24 (d, 1 H, Ph), 7.77 (t, 1 H, Ph), 7.31 (t, 1 H, Ph) and 3.37 (s, 3 H, CH3).X-Ray crystallography Brown prismatic crystals were grown by the slow diVusion of n-hexane into dichloromethane solution of complexes 3 and 8 at room temperature. Single crystals 0.15 × 0.19 × 0.20 and 0.10 × 0.20 × 0.40 mm were chosen for diVraction study respectively. Crystal data are in Table 1. Intensity data were collected at 294 K on a MSC/Rigaku-IIC imaging plate diVractometer using graphite-monochromatized Mo-Ka (l = 0.71073J.Chem. Soc., Dalton Trans., 1999, 135–140 137 Å) radiation from a rotating anode generator. For 3 a total of 15190 reflections were collected, with 5221 independent reflections (Rint = 6.44%).14 For complex 8, 8178 (Rint = 0.00) unique data were collected. The intensities were corrected for Lorentzpolarisation eVects and absorption using the ABSCOR program. 15 The structure of 3 and 8 were solved by Patterson methods and direct methods respectively.All non-hydrogen atoms were refined anisotropically by full matrix least squares, with a riding model for hydrogen atoms, using the SHELXTL PLUS (PC Version) package.16 For compound 3 with 2966 [F > 6.0s(F)] observed reflections, refinement converged with Rf = 0.042 and R9 = 0.049. Largest diVerence peak and hole are 0.83 and 20.93 e Å23 respectively. For compound 8 with 6127 observed reflection (|Fo| � 6s|Fo|), refinement converged with Rf = 0.072 and R9 = 0.078.Largest diVerence peak and hole are 0.95 and 20.93 e Å23 respectively. Selected bond lengths and bond angles are given in Table 2. CCDC reference number 186/1231. Results and discussion Reaction of ruthenium chloride with HL aVords the bis chelate complex [Ru(HL)2][ClO4]2 1. The compound is diamagnetic and behaves as a 1 : 2 electrolyte in acetonitrile solution. Previous works 17–19 with thiosemicarbazones of 2-acetylpyridine have established that the ligand behaves as a planar NNS donor, co-ordinating through the pyridine nitrogen, the imine nitrogen and the thione sulfur atom.The IR spectrum of compound 1 indicates a similar co-ordination behaviour of the ligand. Thus 1 may be considered as a ruthenium(II) bis chelate, where each tridentate NNS ligand occupies a meridional plane. Reaction of (HL) with [Ru(PPh3)3Cl2] in refluxing ethanol leads to the isolation of the monochelates [Ru(L)(PPh3)2Cl] 2 and [Ru(HL)- (PPh3)2Cl]X [X = Cl 3 or PF6 4].The neutral complex 2 separated out from the reaction mixture, whereas the cationic complexes [3 and 4] were isolated from the mother-liquor by addition of the appropriate anion. Compounds 2 and 3/4 can easily be converted into each other by the addition of acid and base respectively. Crystal structure analysis of 3 established that in the distorted octahedral complex the two triphenylphosphine moieties are trans to each other, while the three NNS donor points of the ligand and the co-ordinated chloride constitute the equatorial square plane.The electronic spectrum of the bulk compound 3 in acetonitrile is identical to that of the crystals dissolved in the same solvent, indicating that the bulk compound is the trans isomer. The ready interconversion of 2 and 3 and very similar IR and electronic spectra suggest that the trans structure also prevails in 2. The steric repulsion between the two bulky triphenylphosphine moieties, as well as the p-tolyl moiety of the ligand leads to the formation of only the trans compounds.When compound 2 is dissolved in acetonitrile the coordinated chloride is solvolysed and the resulting solution behaves as a 1 : 1 electrolyte. However, compounds 3 and 4 did not suVer such a change. It is well known that RuII, a low spin d6 system, undergoes substitution by a dissociative mechanism. 20omplex 2 being neutral, can dissociate the chloride ion much more easily than 3 and 4 which are monocationic. Such an eVect of the overall charge of the complex unit on the dissociation of chloride ion is well documented.21 Compound 2 reacts with bidentate donors like bipyridine (bpy) and dppe to give [Ru(L)(PPh3)(bpy)]PF6 5 and [Ru(L)(PPh3)(dppe)]PF6 6 (Scheme 1).However, reaction with Hpic produces [Ru(HL)- (PPh3)(pic)]PF6 7 in which the ligand is present in its protonated form, picolinic acid displaying its usual behaviour by acting in the monoanionic bidentate fashion.The proton dissociated from picolinic acid appears to transform the deprotonated ligand into its protonated form. A very interesting reaction took place when compound 2 was refluxed with an excess of ligand HL. From the reaction medium it was possible to isolate the complex [Ru(HL)(PPh3)(mpi)]Cl2 8, in which the ligand HL is in its protonated form while the imine (mpi) retains its neutral (non-deprotonated) form. Though a number of diphenylmethyleneimine complexes are reported in the literature involving a variety of co-ordination modes, no [a(N)- heterocyclic]methyleneimine complexes have been reported to date.To our knowledge this is the first report of an [a(N)- heterocyclic]iminato complex which has been fully characterised by X-ray crystallography. The formation of the imine complex from the thiosemicarbazone may be visualised to proceed via a two-electron reductive cleavage of the hydrazinic N–N bond of the thiosemicarbazone by the ruthenium(II) acceptor centre in 2.The resulting ruthenium(IV) complex could be reduced subsequently by the triphenylphosphine or by the excess of ligand present in the system. If the ligand plays the role of reductant, it should be converted into the N-(4- tolyl)thiourea. The latter is actually isolated from the reaction medium and identified by its characteristic NMR and IR spectra and elemental analysis. The two-electron reductive cleavage of the N–N bond is one of the elementary reaction steps in the reduction of nitrogen to ammonia.Examples of such reductive cleavage are abundant in molybdenum complexes of hydrazine. 10 It is also known that diphenylmethyleneimine complexes may be generated by the reaction of an appropriate precursor metal complex and azines like Ph2C]] N–N]] CPh2. However, this is the first report of the generation of an imine complex by such cleavage of the N–N bond of a thiosemicarbazone coordinated to a metal centre.Structures of complexes 3 and 8 In both complexes 3 and 8 the ligand occupies a meridional plane co-ordinating through pyridine nitrogen [N(1)], imine nitrogen [N(2)] and the thiolate sulfur [S(1)] atom. Along with these three donor atoms a chlorine [Cl(1)] atom in 3 (Fig. 1) and an imine nitrogen [N(6)] in 8 (Fig. 2) complexes a square plane around the metal ion. The Ru–Cl(1) distance (2.459 Å) in 3 is somewhat long {cf. Ru–Cl distance of 2.387 Å in [Ru(PPh3)- Cl2] 13}. Two trans triphenylphosphine groups in 3 and one triphenylphosphine and one pyridine nitrogen [N(5)] of methyl- (2-pyridyl)methyleneimine ion 8 complete the octahedron.It is worthwhile to make a comparison of structures of 3 and 8 with that of [Ru(L9)(PPh3)2]ClO4 9; [L9 = monoanion of 2,6-diacetylpyridine 4-(4-tolyl)thiosemicarbazone].6 The trans triphenyl- Scheme 1138 J. Chem. Soc., Dalton Trans., 1999, 135–140 phosphine groups in 3 have identical Ru–P bond lengths (2.399 Å).These bonds are longer than reported (2.370, 2.373 Å) for trans-[Ru(L9)(PPh3)2]ClO4 but similar to that observed in [Ru(CO)(C2HN2S3)2(PPh3)2] 22 (2.397 Å, 2.399 Å). However, the Ru–P bond lengths in both 3 and 9 are longer than that observed in 8 (2.334 Å), the latter being on the shorter side of the range normally observed for Ru–P bonds.23 Again, in the 2-acetylpyridine SchiV base complexes 3 and 8, the Ru–N (py) distances are larger than the Ru–N (imine) distances, but in 2,6- Fig. 1 Perspective view of the [Ru(HL)(PPh3)2Cl]1 cation of [Ru- (HL)(PPh3)Cl]Cl?CH2Cl2 with atom labelling. Fig. 2 Perspective view of the [Ru(HL)(PPh3)(mpi)]21 cation of [Ru- (HL)(PPh3)(mpi)]Cl2?CH2Cl2?3H2O with atom labelling. Table 1 Crystal data for [Ru(HL)(PPh3)2Cl]Cl?CH2Cl2 3 and [Ru- (HL)(PPh3)(mpi)]Cl2?CH2Cl2?3H2O 8 Formula M Space group a/Å b/Å c/Å a/8 b/8 g/8 V/Å3 ZF (000) Dc/g cm23 RR 9 C52H48Cl4N4P2RuS 1065 Pnma 19.279(4) 15.947(3) 15.752(3) 4843(2) 4 2184 1.462 0.0418 0.0493 C41H47Cl4N6O3PRuS 977.7 P1� 11.286(1) 13.629(1) 16.109(2) 98.36 97.19 109.02(1) 2277.99(11) 2 1000 1.424 0.072 0.078 diacetylpyridine monothiosemicarbazone complex 9 the opposite trend is observed.6 In 9 the Ru–N (py) distance is appreciably shorter than the normally observed value and the Ru–N (py) distances increase in the order 9 < 3 < 8.The Ru–N(2) (imine) distances are slightly shorter than the Ru–N(1) (py) distances, but they follow the same order, e.g. 9 < 3 < 8. The Ru–S(1) distances are normal, but they follow an order opposite to that of the Ru–N(1) distances, e.g. 9 > 3 > 8. The C(8)–S(1) distances in all the three compounds are similar (1.69–1.71 Å) and close to the C]] S distances observed in the free thiosemicarbazides and thiosemicarbazones.24,25 Again, though the imine C(6)– N(2) distances are close to their expected values, both the C(8)– N(3) bond distances in the thiosemicarbazone moiety are appreciably shorter than the C–N single bond distance.The C(8)–N(4) distance in 8 is shorter than the C(8)–N(3) distance, but in complex 9 the opposite is true. Similarly the N(2)–N(3) distances in all the complexes are appreciably shorter than that reported for free thiosemicarbazide or for hydrazine.23,24 It has been suggested that in thiosemicarbazides and thiosemicarbazones there is an extensive p delocalisation over the entire chain, so that none of the bonds can be considered a true single or double bond.Rheingold and co-workers 9 proposed that, even in deprotonated thiosemicarbazones, the iminothiolate sulfur S(1) undergoes rehybridisation to sp2 and the lone pair on the p orbital can participate in conjugation with the imine moiety. Such extensive p delocalisation within the ligand moiety coupled with the p backbonding from the metal is responsible for the apparent anomalies in bond distances mentioned above. Electrochemistry The electrochemical data for the complexes are given in Table 3.The electrochemistry of the complexes is dominated by a reversible RuII–RuIII oxidation. Peak potential separations between anodic and cathodic peaks, Epa 2 Epc, vary between 60 and 90 mV and are virtually independent of scan rate. These peak separations, though larger than the ideal Nernstian value of 59 mV, are commonly observed for this type of com- Table 2 Selected bond distances (Å) and angles (8) for [Ru(HL)- (PPh3)2Cl]Cl?CH2Cl2 3 and [Ru(HL)(PPh3)(mpi)]Cl2?CH2Cl2?3H2O 8 Ru(1)–N(1) Ru(1)–N(2) Ru(1)–S(1) Ru(1)–P(1) Ru(1)–P(1A) Ru(1)–Cl(1) C(6)–N(2) N(2)–N(3) N(3)–C(8) C(8)–S(1) C(8)–N(4) C(5)–C(6) N(4)–C(9) N(1)–Ru(1)–N(2) N(1)–Ru(1)–S(1) N(1)–Ru(1)–Cl(1) N(2)–Ru(1)–Cl(1) N(2)–Ru(1)–S(1) Cl(1)–Ru(1)–S(1) P(1)–Ru(1)–N(1) P(1)–Ru(1)–N(2) P(1)–Ru(1)–S(1) P(1)–Ru(1)–Cl(1) P(1)–Ru(1)–P(1A) P(1A)–Ru(1)–N(1) P(1A)–Ru(1)–N(2) P(1A)–Ru(1)–S(1) P(1A)–Ru–Cl(1) C(5)–C(6)–N(2) 2.085(5) 1.984(5) 2.386(2) 2.399(1) 2.399(1) 2.459(2) 1.304(7) 1.383(7) 1.359(7) 1.707(6) 1.334(8) 1.462(9) 1.416(8) 77.4(2) 160.8(1) 99.0(1) 176.4(1) 83.3(1) 100.2(1) 91.9(1) 91.9(1) 88.7(1) 88.2(1) 175.2(1) 91.9(1) 91.9(1) 88.7(1) 88.2(1) 112.5(5) Ru(1)–N(1) Ru(1)–N(2) Ru(1)–S(1) Ru(1)–P(1) Ru(1)–N(5) Ru(1)–N(6) C(6)–N(2) N(2)–N(3) N(3)–C(8) C(8)–S(1) C(8)&ash;N(4) C(5)–C(6) N(4)–C(9) N(1)–Ru(1)–N(2) N(1)–Ru(1)–S(1) N(1)–Ru(1)–N(6) N(2)–Ru(1)–N(6) N(2)–Ru(1)–S(1) N(6)–Ru(1)–S(1) P(1)–Ru(1)–N(1) P(1)–Ru(1)–N(2) P(1)–Ru(1)–S(1) P(1)–Ru(1)–N(6) P(1)–Ru(1)–N(5) N(5)–Ru(1)–N(1) N(5)–Ru(1)–N(2) N(5)–Ru(1)–S(1) N(1)–Ru(1)–N(6) C(5)–C(6)–N(2) C(38)–C(39)–N(6) 2.092(6) 1.991(5) 2.358(2) 2.334(2) 2.110(6) 2.075(7) 1.336(8) 1.371(9) 1.347(8) 1.698(7) 1.344(11) 1.478(12) 1.418(9) 78.0(2) 161.1(2) 98.7(3) 169.5(2) 83.4(2) 98,9(2) 97.0(2) 92.0(2) 87.3(1) 98.3(2) 171.6(1) 90.4(2) 93.5(2) 87.0(2) 76.5(2) 110.8(6) 115.3(7)J.Chem. Soc., Dalton Trans., 1999, 135–140 139 plexes.6,26,27 In most of the cases no well defined peaks are observed at the cathodic side of the cyclic voltammograms. This is probably due to the reduction of the ligand followed by decomposition of the resultant complex.For complexes 5 and 8 a reductive couple observed around 21.5 V may be ascribed to a ligand (bipyridine/mpi) centered reduction.28 It may be noted that in this study we have extensively varied the co-ordination environment around the ruthenium(II) acceptor centre employing a variety of nitrogen, sulfur, phosphorus, oxygen and chloride donors.So, it is worthwhile to follow the trend in the variation of RuIII–RuII potential with the change of donor environment around the metal ion, particularly because such studies are rather scanty.29 It is well established that such potentials are aVected by both the nature of the donor sets as well as the overall charge of the complex, the latter being the dominating factor. So, a meaningful correlation is possible only when one compares complexes having identical charges.Thus, we may compare the ERu(III)/Ru(II) of Ru(bpy)3 21 (1.38 V, N6 donors) with that of [Ru(HL)2]21 (20.005 V, N4S2 donors) and conclude that replacement of two pyridine nitrogens by two thiocarbonyl sulfurs has stabilised the RuIII by 1.43 V. This may be rationalised by referring to the higher polarisability and poorer p-accepting capability of the thiocarbonyl sulfur compared to pyridyl nitrogen, and both the factors tend to stabilise the ruthenium(III) state.Again, we can compare the RuIII–RuII potential of the complex [Ru(L)(PPh3)2Cl] (0.36 V, N2P2SCl donors) with that of [Ru(bpy)2Cl2] (0.34 V, N4Cl2 donors); in this case the replacement of two pyridine nitrogens and a chloride by two phosphorus and a thiolato donor set keeps the potential almost unaltered. Though thiolato ligands are known to be eYcient in stabilising higher (III and IV) oxidation states of ruthenium, in the present case that eVect is compensated by the introduction of two phosphine donors, which are even more eYcient in stabilising ruthenium(II) than the pyridine nitrogens.Similarly one can compare the series of five monocationionic complexes [Ru(HL)(PPh3)2Cl]- Cl (0.65 V, N2P2SCl donors), [Ru(HL)(PPh3)2Cl]PF6 (0.70 V, N2P2SCl donors), [Ru(L)(PPh3)(bpy)]PF6 (0.65 V, N4PS9 donors), [Ru(L)(PPh3)(dppe)]PF6 (0.77 V, N2P3S9 donors) and [Ru(HL)(PPh3)(pic)]PF6 (0.47 V, N3PSO donors) and conclude that the presence of bipyridine nitrogen or imine nitrogen as well as phosphine donors tends to stabilise the ruthenium(II) state, whereas thiolato and carboxylato donors stabilise the ruthenium(III) state.One may also compare the ERu(III)/Ru(II) values of [Ru(bpy)2(SPh)2] (20.28 V, N4S2 2 donors) 29 and [Ru(bpy)2(pybt)]1 [0.32 V, N5S2 donors; pybt = 2-(2-pyridyl)benzenethiolate] 28 with those of thiolato complexes reported in this paper and conclude that the benzenethiolato group is more eYcient in stabilising RuIII than the iminethiolates described in this paper, a fact which correlates Table 3 Cyclic voltammetric data a of the complexes in acetonitrile at 298 K E2� 1 /V (DEp/mV) Compound 1 [Ru(HL)2][ClO4]2 2 [Ru(L)(PPh3)2Cl] 3 [Ru(HL)(PPh3)2Cl]Cl 4 [Ru(HL)(PPh3)2Cl]PF6 5 [Ru(L)(PPh3)(bpy)]PF6 6 [Ru(L)(PPh3)(dppe)]PF6 7 [Ru(HL)(PPh3)(pic)]PF6 8 [Ru(HL)(PPh3)(mpi)]Cl2 Oxidation 0.005(90) 0.36(75) 0.65(60) 0.70(60) 0.65(60) 0.77(80) 0.47(60) 0.67(60) Reduction 21.42(100) 21.56(80) Donor sites b N4S2 N2P2S9Cl N2P2SCl N2P2SCl N4PS9 N2P3S9 N3PSO N4PS a Conditions: supporting electrolyte, NEt4ClO4 (0.1 M); working electrode, platinum; reference electrode, Ag–AgCl; solute concentration, 1023 M.E2� 1 is calculated as the average of anodic (Epa) and cathodic (Epc) peak potentials; DEp = Epa 2 Epc, Ipc /Ipa = 1, and scan rate = 50 mV s21. b S refers to thiocarbonyl sulfur and S9 to thiolato sulfur. well with the lower basicity of the latter as described by Rheingold and co-workers.9 Electronic spectra The electronic spectra of low-spin d6 complexes are generally dominated by metal to ligand charge transfer in the visible region.30–32 As most of the complexes discussed in this work are of Cs or lower symmetry all the d orbitals are non-degenerate.So, a number of MLCT transitions are expected. However, due to the small energy separation between some of these d orbitals, as well as poor overlap between them and the excited state orbitals, some of the expected MLCT transitions may not be resolved.In general, all the complexes exhibit two well resolved MLCT transitions around 420–480 (band I) and 373–378 nm (band II). When the energy of band I is plotted against E8Ru(III)/Ru(II) a nice linear correlation EMLCT = 1.18 E8Ru(III)/Ru(II) 1 2.42 is obtained (Fig. 3). Besides, for some of the complexes, there is an additional low energy band at 480–580 nm (band III). While bands I and III are substituent dependent, II is unaVected by substituents.The two highest energy bands at 260–290 and 210–220 nm are probably due to intraligand transitions. 1H NMR spectra The 1H NMR spectrum of the ligand (HL) exhibits signals at d 2.49 (3 H) and 2.35 (3 H) which are assigned to the CH3 group of the p-tolyl moiety and that attached to the imine moiety of the ligand. The signals at d 9.32 (1 H) and 8.9 (1 H) are due to NH protons and all aromatic protons exhibit signals in the region d 7.19–8.61.33 For complex 3 the signal of the CH3 group attached to the imine moiety was shifted upfield to d 2.16, whereas the CH3 proton signal of the p-tolyl moiety remains unaVected at d 2.31.The NH proton signals are at d 12.16 and 11.12. The aromatic protons are observed between d 6.8 and 8.76. Compound 8 exhibits three CH3 proton signals at d 2.31 (3 H), 2.35 (3 H) and 2.46 (3 H). The signal at d 2.35 is due to the CH3 group of the p-tolyl residue.One NH signal is observed at d 12.25. The broken organic fragment isolated from the reaction mixture [N-(p-tolyl)thiourea] exhibits a signal at d 3.37 due to the CH3 group33 of the tolyl part. The phenyl protons are observed between d 7.3 and 8.62. The NH proton signals are not observed and similar observations were reported 34 previously in the case of N-(p-nitrophenyl)thiourea. Conclusion This paper describes the ruthenium(II) complexes of methyl 2-pyridyl ketone 4-(4-tolyl)thiosemicarbazone, in which the Fig. 3 Plot of EMLCT versus RuIII–RuII potential (Eobs, on NHE scale).140 J. Chem. Soc., Dalton Trans., 1999, 135–140 ligand behaves either as a monoanionic tridentate NNS donor (thioenol form) or as a neutral tridentate NNS donor (thione form). The pH-dependent interconversion of the compounds [Ru(L)(PPh3)2Cl] and [Ru(HL)(PPh3)2Cl]Cl is a manifestation of thione–thioenol tautomerisation of the co-ordinated ligand. Formation of the compound [Ru(HL)(PPh3)(mpi)]Cl2 from the complex [Ru(L)(PPh3)2Cl] is an extremely interesting manifestation of the unusual reactivity of the co-ordinated thiosemicarbazone moiety.The complex [Ru(HL)(PPh3)(mpi)]Cl2, produced through reductive cleavage of the hydrazinic N–N bond of the thiosemicarbazone ligand, is the first structurally characterised metal complex of an imine of a heterocyclic ketone. The crystal structure of the compound [Ru(HL)- (PPh3)2Cl]Cl?CH2Cl2 has been of great help in understanding the rather unusual chemiceaction leading to the formation of [Ru(HL)(PPh3)(mpi)]Cl2.Acknowledgements M. M. gratefully acknowledges the Council of Scientific and Industrial Research (CSIR), New Delhi for the award of a fellowship. Financial assistance from the Department of Science and Technology (DST), Government of India, New Delhi is also gratefully acknowledged. T. C. W. M. acknowledges support from the Hong Kong Research Grants Council. References 1 M.Maji, S. Ghosh and S. K. Chattopadhyay, Transition Met. Chem., 1998, 23, 81. 2 D. Sellmann, U. Reineke, G. Huttner and L. Zsolnai, J. Organomet. Chem., 1986, 310, 83; D. Sellmann and O. Kappler, Angew. Chem., Int. Ed. Engl., 1988, 27, 689; D. Sellmann, R. Ruf, F. Knoch and M. Mol, Inorg. Chem., 1995, 34, 4745. 3 M. Hossain, S. K. Chattopadhyay and S. Ghosh, Polyhedron, 1997, 16, 143; Transition Met. Chem., 1997, 22, 497. 4 M. Hossain, S. K. Chattopadhyay and S. Ghosh, Polyhedron, 1997, 16, 4313. 5 F. Basuli, S. M. Peng and S. Bhattacharya, Inorg. Chem., 1997, 36, 5645. 6 M. Maji, S. Ghosh, S. K. Chattopadhyay and T. C. W. Mak, Inorg. Chem., 1997, 36, 2938. 7 S. Purohit, A. P. Koley and S. Ghosh, Polyhedron, 1990, 9, 881. 8 C. E. Forbes, A. Gold and R. H. Holm, Inorg. Chem., 1971, 10, 2479. 9 Z. Lu, C. White, A. L. Rheingold and R. H. Crabtree, Inorg. Chem., 1993, 32, 3391. 10 F. Bregant, S. Pacor, S. Ghosh, S. K. Chattopadhyay and G. Sava, Anticancer Res., 1993, 13, 1007; S.K. Chattopadhyay and S. Ghosh, Inorg. Chim. Acta, 1987, 131, 15; 1989, 163, 245. 11 M. Y. Mohammed and C. J. Pickett, J. Chem. Soc., Chem. Commun., 1988, 1119; S. N. Anderson, M. E. Fakley, R. L. Richards and J. Chatt, J. Chem. Soc., Dalton Trans., 1981, 1973. 12 P. L. Andreu, J. A. Cabeja, I. Rio, V. Riera and C. Boiss, Organometallics, 1996, 15, 3004. 13 P. S. Hallman, T. A. Stephenson and G. Wilkinson, Inorg. Synth., 1970, 12, 237. 14 M. Sato, M.Yamamoto, K. Imada, N. Tanaka and T. Higashi, J. Appl. Crystallogr., 1992, 25, 146; K. L. Kraus and G. N. Philips, Jr., J. Appl. Crystallogr., 1992, 25, 146. 15 ABSCOR, An empirical absorption correction based on Fourier coeYcient fitting, T. Higashi Rigaku Corporation, Tokyo, 1995. 16 G. M. Sheldrick, Computational Crystallography, ed. D. Sayre, Oxford University Press, New York, 1982, pp. 506–514. 17 D. X. West and D. S. Galloway, Transition Met. Chem., 1988, 13, 410. 18 D. X. West and D. S. Galloway, Transition Met. Chem., 1988, 13, 415. 19 D. W. West, R. D. Profilit and J. L. Hines, Transition Met. Chem., 1988, 13, 467. 20 F. Basolo and R. G. Pearson, Mechanism of inorganic reaction. A study of metal complexes in solution, Wiley, New York, 1958, pp. 108, 163. 21 J. E. Huheey, E. A. Keiter and R. L. Keiter, Inorganic Chemistry. Principles of Structure and Reactivity, 4th edn., Harper Collins College Publishers, 1993, p. 551. 22 P. Mura, B. G. Olby and S. D. Robinson, Inorg. Chem., 1985, 108, 45. 23 S. R. Fletcher and A. C. Skapski, J. Chem. Soc., Dalton Trans., 1972, 635. 24 A. K. Nandi, S. Chaudhuri, S. K. Chaudhuri and S. Ghosh, Acta Crystallogr., Sect. C, 1984, 40, 1993. 25 A. K. Nandi, S. Chaudhuri, S. K. Chaudhuri and S. Ghosh, J. Chem. Soc., Perkin Trans. 2, 1984, 1729. 26 B. P. Sullivan, D. J. Solmon and T. J. Mayer, Inorg. Chem., 1978, 17, 3334. 27 R. W. Callahan, F. R. Keene, T. J. Mayer and D. J. Solmon, J. Am. Chem. Soc., 1977, 99, 1064. 28 M. J. Root, B. P. Sullivan, T. J. Mayer and E. Deutch, Inorg. Chem., 1985, 24, 2731. 29 A. M. W. Cargill Thompson, D. A. Bardwell, J. C. JeVery, L. H. Rees and M. D. Ward, J. Chem. Soc., Dalton Trans., 1997, 726. 30 M. Haga, E. S. Dodsworth and A. B. P. Lever, Inorg. Chem., 1986, 25, 447. 31 E. S. Dodsworth and A. B. P. Lever, Chem. Phys. Lett., 1986, 124, 152. 32 M. A. Greaney, C. L. Coyle, M. A. Harmer, A. Jordan and E. I. Stifel, Inorg. Chem., 1989, 28, 912. 33 W. Kemp, Organics Spectroscopy, 3rd edn., ELBS with Mcmillan, 1994, p. 126. 34 The Aldrich Library of NMR Spectra, 11th edn., C. J. Poucher, Library of Congress Catalog Card No. 83-70633, 1983, vol. 2, p. 387. Paper 8/06341I

ISSN:1477-9226    

DOI:10.1039/a806341i

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

DALTON COMMUNICATION J. Chem. Soc., Dalton Trans., 1997, Pages 137–138 137 Grafting of versatile lanthanide silylamide precursors onto mesoporous MCM-41† Reiner Anwander* and Rainer Roesky Institut für Technische Chemie I, Universität Stuttgart, D-70550 Stuttgart, Germany Chemical anchoring of organometallic lanthanide silylamides onto the internal walls of dehydrated MCM-41 in n-hexane at ambient temperature was monitored by FTIR spectroscopy and nitrogen adsorption/desorption. In the past decade surface organometallic chemistry 1 has been challenged by the discovery of the intriguing class of mesoporous support materials composed of, for example, aluminosilicates and metal oxides.2,3 Like the commonly used silica and alumina supports 4 the new systems are capable of surface reactions via terminal silanol groups.However, the prevailing structural patterns such as hexagonally arranged, uniform mesopores ensure a more detailed characterization by means of nitrogen adsorption/desorption, X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM).5 Furthermore, the interplay of the proposed readily tunable pore radius (20–100 Å) and the accommodation of both bulky metal complexes and reactants might even allow substrate transformations in a regime of stereo- and shape-selectivity.6 Given the current high interest in the field of ‘stereoselective catalysis by organolanthanide complexes’ 7 we anticipated the grafting of such reactive moieties onto mesoporous material MCM-41 via formation of a thermodynamically stable lanthanide– siloxide s bond.8 MCM-41-derived hybrids containing maingroup- 9 and d-transition-organometallics 10 have recently been described.Herein, we report our initial results on a heterogeneously performed silylamide route employing MCM-41 1 and amides of type [Nd{N(SiMe3)2}3] 2 and [Nd{N(SiHMe2)2}3(thf)2] (thf = tetrahydrofuran) 3. Such highly soluble, monomeric silylamides are known to act as key precursors in homogeneous, protolytic exchange reactions under mild conditions.11 Dehydrated MCM-41 was synthesized according to the literature employing [N(C14H29)Me3]Br as a templating agent.2 After calcination (N2: 500 8C, 5 h, heating rate 1.5 8C min21; air: 500 8C, 5 h) and dehydration (1025 Torr: 280 8C, 4 h, heating rate 1 8C min21),‡ 1 was characterized by XRD, nitrogen adsorption and desorption isotherms (BET surface area: 1005 m2 g21; pore diameter: 26 Å (desorption); pore volume: 0.78 cm3 g21), elemental analysis (ICP, Si :Al ª 18), and IR spectroscopy [n(OH): 3695 cm21].12 The grafting procedure involves the addition of silylamide desolved in n-hexane to a suspension of MCM-41 in n-hexane within a period of 5 min (Scheme 1).Upon stirring for 2 h, followed by several n-hexane washings, the resulting bluish hybridic materials were dried under vacuum for at least 5 h.§ This way, approximately 1 mmol of silylamide per g of 1 could be loaded.However, quantitative gas chromatography allowed the detection of only 0.3–0.4 mmol of released silylamine ligand probably due to retention in the mesopores by metal complexation or silylation reaction.¶ † Non-SI unit employed: Torr ª 133.322 Pa. ‡ Dehydration at temperatures >350 8C led to partial collapse of the mesopores as indicated by nitrogen adsorption and desorption isotherms (recorded on a micromeritics ASAP 2000). The consumption of all terminal silanol groups and the disappearance of the SiO mode at 980 cm21 (strained Si–O]Si bonds) were unequivocally proven by Fourier-transform IR spectroscopy (Fig. 1). Type-3 silylamide was employed not only for reasons of changed reactivity during immobilization and following ligand exchange reactions, but also for its unique behaviour as an IR probe. Two n(Si]H) stretches appear in the spectra of hybrid material 5. The lower energy one at 2058 cm21 can be assigned to metal bonded amide moieties.11 Independently performed reactions of the silylamines HN(SiMe3)2 and HN(SiHMe2)2 with 1 in n-hexane revealed formation of silylated 6a and 6b under these mild conditions and allowed the assignment of the Si]H stretching frequency at 2144 cm21 to 6b-analogous ‘OSiHMe2’ moieties.The competing silylation reaction is in agreement with the findings from quantitative gas chromatography and points out a self-supported spacing of the lanthanide centres by steric restrictions or diffusion-controlled reactions within the mesopores.For comparison, silylation reactions along the homogeneously performed silylamide route are observed only when an excess of more acidic alcohols such as HOCH(CF3)2 14 or HOSiBut 3 15 are employed. Calculations from the elemental analysis of 6a reveal that approximately 13% of the MCM-41 silicon atoms are carrying reactive sites available for silylation.|| Also, elemental analyses of the hybrid materials 4 and 5 favour the formation of ‘MCM-41] O2Ln[N(SiMe3)2]’ over bis(amide) moieties. Scheme 1 Possible surface species of the immobilization of neodymium silylamides on MCM-41: (i) hexane, room temperature, 20 h; thf is not shown for 5 § All manipulations were performed in a dinitrogen-filled glove box (MB Braun MB150B-G-II).The initial grafting runs were carried out with coloured silylamide solutions of neodymium to visualize completeness of the reactions (titration). Prolonged reaction times (>24 h) gave no further immobilization.Also, after prolonged Soxhlet extractions with thf only traces of what we assume to be a non-chemically anchored complex were isolated. X-Ray diffraction spectra of airexposed hybrid materials showed the characteristic pattern of 1, however, with decreased intensity. For 1 (stirred in n-hexane, evacuated for 5 h, N2-filled) (Found: C, 0.28; H, 0.27; N, 0.41%. Synthesis of 4 from 2: 2 (0.300 g, 0.48 mmol) in n-hexane (10 cm3) was added to 1 (0.368 g) in n-hexane (10 cm3) over 5 min.The mixture was stirred for 20 h, 4 was separated by centrifugation, washed with nhexane (20 cm3) and dried in vacuo for at least 5 h (Found: C, 9.45; H, 2.45; N, 1.95; Nd, 11.1%). IR (Nujol): 831s, 771m, 661m, 604m cm21 [N(SiMe3)2]. Synthesis of 5 from 3: as above using 3 (0.330 g, 0.48 mmol) and 1 (0.248 g) (Found: C, 10.0; H, 2.35; N, 2.00; Nd, 8.8%). IR (Nujol): 2144m, 2058m, 902s, 836s, 681m, 626m cm21 [N(SiHMe2)2].¶ Hexamethyldisilazane was thoroughly studied as a trimethylsilylating agent for silica (gels).13138 J. Chem. Soc., Dalton Trans., 1997, Pages 137–138 Nitrogen adsorption and desorption isotherms of 4 and 5 clearly demonstrate the filling of the mesopores (Fig. 2). The host-characteristic type-IV isotherm (1) is replaced by type-I isotherms (4, 5) indicating the presence of microporous materials. The original pore volume of 0.78 cm3 g21 is reduced to approximately 0.30 cm3 g21 (after activation at 100 8C under high vacuum). 16 The silylated materials still display type-IV isotherms and the pore characteristics reveal effective pore-size engineering [BET surface area: ª840 m2 g21 (6a, 6b); pore diameter (desorption): 21 Å (6a, 6b); pore volume: 0.53 cm3 g21 (6a), 0.54 cm3 g21 (6b)]. Further evidence for a competing silylation reaction stems from the reaction of monomeric [Nd(NPri 2)3(thf)] 17 7 with 1.* Evaluation of the free isopropylamine results in complete desorption at T < 150 8C under high vacuum.As a result, approximately 2 mmol of 7 can be immobilized to yield hybrid material 8 [17.3% Nd (m/m) detected by ICP analysis]. The pore volume is decreased to 0.10 cm3 g21 (Fig. 2). All anchored amide complexes can be ‘degrafted’ as alkoxide complexes by treatment with HOC(CF3)3 (pKa 5.7) 18–thf solutions. The support residues display nitrogen adsorption and desorption isotherms which are similar to the silylated materials (4, 5) and to original 1 (8).A direct conclusion of these findings is that by varying the ratio of silylamide to silylamine different metal loadings will be available by simultaneous consumption of all terminal silanol groups, and hence the spacing of the metal centres will be effected. Preliminary ligand exchange reactions with less acidic alcohols such as ethanol and binaphthol (stoichiometric) reveal that we can direct future investigations towards the generation of both unsaturated lanthanide centres (small ligand chemistry) Fig. 1 FTIR spectra (Nujol). MCM-41 (1), [Nd{N(SiHMe2)2}3(thf)2] (3), MCM-41 + [Nd{N(SiHMe2)2}3(thf)2] (5), MCM-41 + HN(SiHMe2) 2 (6b, 1023 Torr, 2 h, 250 8C) * Synthesis of 6a from 1: to a suspension of 1 (0.490 g) in n-hexane was added HN(SiMe3)2 (excess, 0.5 cm3). After stirring for 20 h the unreacted silylamine and n-hexane were removed in vacuo. Material 6a was dried in vacuo for at least 5 h at 20 8C, then heated at 250 8C under high vacuum for 2 h (Found: C, 6.50; H, 1.60%).IR (Nujol): 864s, 846s, 757m, 563m cm21 [N(SiMe3)2]. Synthesis of 6b from 1: as above using 1 (0.247 g) and HN(SiHMe2)2 (excess, 0.3 cm3) (Found: C, 5.15; H, 1.50%). IR (Nujol): 2145m, 904s, 837s, 773m, 629m cm21 [N(SiHMe2)2]. Synthesis of 8 from 7: 7 (0.185 g, 0.36 mmol) was added to 1 (0.342 g) in n-hexane (10 cm3). The mixture was stirred for 2 h and additional 7 (0.182 g, 0.35 mmol) added.More 7 (0.104 g, 0.20 mmol) was added and the solution stirred for 20 h, 8 was separated by centrifugation, the residue washed with n-hexane (20 cm3) and dried in vacuo for at least 5 h (Found: C, 14.50; H, 2.65; N, 2.20; Nd, 17.3%). IR (Nujol): 918m, 823m, 799m cm21 (NPri 2). Only small amounts of silylamide complexes could be grafted on pre-silylated material, probably reflecting some nonsilylated bridging OH-moieties. and ‘immobilized chirality’, two main features of organolanthanides and stereoselective catalysis.Acknowledgements We thank the Deutsche Forschungsgemeinschaft (DFG) and Hoechst AG for financial support of this research project and the DFG for the award of a fellowship to R. A. Additionally, generous support from Professor J. Weitkamp is gratefully acknowledged. We are also grateful to Dr. Thomas Röser for valuable technical assistance. References 1 J. M. Basset, B. C. Gates, J. P. Candy, A. Choplin, H. Leconte, F. Quignard and C.Santini, (Editors), Surface Organometallic Chemistry, Molecular Approaches to Surface Catalysis, Kluwer, Dordrecht, 1988. 2 C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli and J. S. Beck, Nature (London), 1992, 359, 710; J. S. Beck, J. C. Vartuli, W. J. Roth, M. E. Leonowicz, C. T. Kresge, K. D. Schmitt, C. T.-W. Chu, D. H. Olson, E. W. Sheppard, S. B. McCullen, J. B. Higgins and J. L. Schlenker, J. Am. Chem. Soc., 1992, 114, 10834. 3 D. M. Antonelli, A. Nakahira and J.Ying, Inorg. Chem., 1996, 35, 3126. 4 W. C. Finch, R. D. Gillespie, D. Hedden and T. J. Marks, J. Am. Chem. Soc., 1990, 112, 6221. 5 F. Schüth, Ber. Bunsen-Ges. Phys. Chem., 1995, 99, 1306. 6 See, for example, A. Corma, M. Iglesias and F. Sánchez, J. Chem. Soc., Chem. Commun., 1995, 1635. 7 M. A. Giardello, V. P. Conticelli, L. Brard, M. R. Gagne and T. J. Marks, J. Am. Chem. Soc., 1994, 116, 10241; H. Tsukube, H. Shiba and J.-I. Uenishi, J. Chem. Soc., Dalton Trans., 1995, 181; H.Sasai, T. Arai, Y. Satow, K. N. Houk and M. Shibassaki, J. Am. Chem. Soc., 1995, 117, 6194. 8 J. L. Sessler, B. L. Iverson, V. Kral, R. E. Thomas, D. A. Smith and D. Magda, PCT Int. Appl., WO 95 29,702 (Cl. A61K47/48), 1995; N. E. Drysdale and N. Herron (du Pont de Nemours, E. I. and Co.), PCT Int. Appl., WO 95 02,625 (Cl. CO8G65/10), 1995. 9 C. Huber, K. Moller and T. Bein, J. Chem. Soc., Chem. Commun., 1994, 2619. 10 T. Maschmeyer, F. Rey, G. Sankar and J.M. Thomas, Nature (London), 1995, 378, 159. 11 W. A. Herrmann, R. Anwander, F. C. Munck, W. Scherer, V. Dufaud, N. W. Huber and G. R. J. Artus, Z. Naturforsch., Teil B, 1994, 49, 1789; W. A. Herrmann, R. Anwander, V. Dufaud and W. Scherer, Angew. Chem., 1994, 106, 1338; Angew. Chem., Int. Ed. Engl., 1994, 33, 1285; J. P. Kinney and R. H. Staley, J. Phys. Chem., 1983, 87, 3735. 12 J. Chen. Q. Li, R. Xu and F. Xiao, Angew. Chem., 1995, 107, 2898; Angew. Chem., Int. Ed. Engl., 1995, 34, 2898. 13 W. Hertl and M. L. Hair, J. Phys. Chem., 1971, 75, 2181; D. W. Sindorf and G. E. Maciel, J. Phys. Chem., 1982, 86, 5208. 14 D. C. Bradley, H. Chudzynska, M. E. Hammond, M. B. Hursthouse, M. Motevalli and W. Ruowen, Polyhedron, 1992, 11, 375. 15 K. J. Covert, D. R. Neithammer, M. C. Zonnevylle, R. E. LaPointe, C. P. Schaller and P. T. Wolczanski, Inorg. Chem., 1991, 30, 2494. 16 A. C. Greenwald, W. S. Rees jun. and U. W. Lay, in Rare Earth Doped Semiconductors, eds. G. S. Pomrenke, P. B. Klein and D. W. Langer, MRS, Pittsburgh, PA, 1993. 17 H. C. Aspinell and M. R. Tillotson, Polyhedron, 1994, 13, 3229. 18 C. J. Willis, Coord. Chem. Rev., 1988, 88, 133. Received 30th October 1996; Communication 6/07391C Fig. 2 Nitrogen adsorption and desorption isotherms (77.4 K): 1, 4, 5, 8 (1023 Torr, >5 h, room temperature); 6a, 6b (1023 Torr, >5 h, room temperature and 2 h, 250 8C)

ISSN:1477-9226    

DOI:10.1039/a607391c

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

DALTON COMMUNICATION J. Chem. Soc., Dalton Trans., 1997, Pages 139–140 139 Co-ordinative activation of phosphaalkynes: methyl neopentylidene phosphorane complexes of ruthenium(II); crystal structure of [Ru(MeP] CHBut)Cl(I)(CO)(PPh3)2] Robin B. Bedford,a Anthony F. Hill,*,†,a Cameron Jones,*,‡,b Andrew J. P. Whitec and James D. E. T. Wilton-Ely a a Department of Chemistry, Imperial College of Science, Technology and Medicine, South Kensington, London SW7 2AY, UK b Department of Chemistry, University of Wales, Swansea, Singleton Park, Swansea SA2 8PP, UK c Chemical Crystallography Laboratory, Department of Chemistry, Imperial College of Science, Technology and Medicine, South Kensington, London SW7 2AY, UK The reactions of [Ru(P]] CHBut)Cl(CA)(PPh3)2] (A = O or S) with iodomethane provided the phosphaalkene complexes [Ru(PMe]] CHBut)Cl(I)(CA)(PPh3)2] (I trans to phosphaalkene), with one example (A = O) having been crystallographically characterised.Phosphaalkenes (A) 1 (Scheme 1) are generally difficult to isolate unless they bear kinetically stabilising substituents capable of protecting the reactive unsaturated P]] C linkage.One class of phosphorus substituent which has been shown to confer remarkable stability, both thermodynamic and kinetic, on such compounds are transition metals (B). Recently we have provided access to such compounds via a route of unprecedented simplicity, viz the hydrometallation of phosphaalkynes.2 Thus the complex [Ru(P]] CHBut)Cl(CO)(PPh3)2] 1a 2 and its thiocarbonyl analogue [Ru(P]] CHBut)Cl(CS)(PPh3)2] 1b3 result in high yield from the reaction of P]] ] CBut with the hydride complexes [RuH(Cl)(CA)(PPh3)3] (A = O or S).The nucleophilicity of the phosphorus atom of the phosphaalkenyl ligand has been demonstrated in reactions with Brønsted acids 4 and in this report we wish to discuss the reactions of these complexes with carbon-based electrophiles which lead to complexes of the otherwise unstable methyl neopentylidene phosphorane (C).Treating a solution of [Ru(P]] CHBut)Cl(CO)(PPh3)2] 1a in dichloromethane with an excess of methyl iodide leads to slow decolourisation and formation of a pale yellow complex which is formulated as [Ru(PMe]] CHBut)Cl(I)(CO)(PPh3)2] 2a (Scheme 2) on the basis of spectroscopic data §. Most conspicuous and informative amongst the spectroscopic data is the clearly resolved AX2 spin system apparent in the 31P-{1H} NMR spectrum of 2a.Alkylation of 1a is accompanied by a dramatic shift in the resonance due to the phosphaalkenyl ligand from d 450.4 in the precursor to d 225.1 in 2a. This latter datum may be compared with that observed at d 187.9 for the ‘parent’ phosphaalkene complex [Ru(HP]] CHBut)Cl2(CO)- (PPh3)2] 3 obtained by addition of HCl to 1a.2 The formulation was confirmed by single-crystal X-ray diffraction analysis,¶ the results of which are summarised in Fig. 1. The geometry at ruthenium is distorted octahedral with cis interligand angles in the range 80.7(2)–97.2(1)8, the angle between the cis co-ordinated chloride and iodide being noticeably enlarged.The bond lengths between ruthenium and the atoms I, Cl, P(28), P(9) and C(7) are unremarkable for divalent † E-Mail: a.hill@ic.ac.uk ‡ E-Mail: c.a.jones@swansea.ac.uk Scheme 1 Scheme 2 L = PPh3, R = But, A = O or S § Data for 2a. Yield 79% (0.20 mmol scale) IR: (Nujol) 1978 [n(CO)], 1717, 1259, 899, 853 cm21; (CH2Cl2) 1976 [n(CO)] cm21.NMR (CD2Cl2, 25 8C): 1H, d 0.90 (s, 9 H, CMe3), 2.95 [br d, 3 H, PMe, J(PH) = 12.9], 6.35 [d, 1 H, P]] CH, J(PH) = 7.6 Hz], 7.28–8.03 (m, 30 H, PC6H5), 13C- {H}, d 197.4 (m, CO), 165.3 [d, P]] C, J(PC) = 55.4], 135.3–126.8 (PC6H5), 118.8 [d, PMe, J(PC) = 89.3], 40.4 [d, CCH3, J(PC) = 16.1], 31.0 [d, CCH3, J(PC) = 12.5 Hz]; 31P-{1H}, d 225.1 [t, J(PP) = 39.0], 10.4 [d, J(PP) = 40.7 Hz]. FAB-MS: m/z 897 [M 2 Cl] +, 820 [M + H2O 2 I] +, 805 [M 2 I] +, 780 [RuI(CO)(PPh3)2] +, 689 [RuCl- (CO)(PPh3)2] +, 654 [RuCl(PPh3)2] +, 625 [Ru(PPh3)2] +, 363 [RuPPh3] +.Data for 2b. Yield 74% (20 mmol scale) IR: (Nujol) 1290 [n(CS)], 894, 853 cm21. NMR (CDCl3, 25 8C): 1H, d 0.89 (s, 9 H, CMe3), 3.18 [br d, 3 H, PMe, J(PH) = 13.2 Hz], 6.63 [br d, 1 H, P]] CH, J(PH) = not resolved], 7.16–8.04 (m, 30 H, PC6H5), 13C-{H}, d 295.5 [dt, CS, J(P2P) ª J(PP) ª 12.5], 161.8 [d, P]] C, J(PC) = 57.1], 119.0 [d, PMe, J(PC) = 91.1], 39.6 [d, CCH3, J(PC) = 17.8], 31.1 (d, CCH3, J(PC) = 12.5 Hz]; 31P-{1H}, d 219.5 [t, J(PP) = 37], 11.6 [d, J(PP) = 41 Hz].FAB-MS: m/z 913 [M 2 Cl] +, 821 [M 2 I] +, 780 [RuI- (CS)[PPh3)2] +, 705 [RuCl(CO)(PPh3)2] +, 669 [RuCl(PPh3]2] +, 651 [M 2 Cl 2 PPh3] +, 625 [Ru(PPh3)2] +, 363 [RuPPh3] +.140 J. Chem. Soc., Dalton Trans., 1997, Pages 139–140 ruthenium. The ligand of primary interest is the phosphaalkene which has trigonal geometry at phosphorus [intersubstituent angles in the range 114.8(4)–123.4(3)8], the planarity of which extends to include C(2) and the remaining ligands in the equatorial ruthenium co-ordination plane [maximum deviation from planarity of 0.09 Å by C(6)].The P]C(1) bond length of 1.657(8) Å is clearly multiple in nature, and significantly shorter than the single bond of 1.803(8) Å to C(6), and that of the pbound phosphaalkene ligand in, e.g. [Rh(h2-CH2PPh)(CO)(h- C5Me5)] [1.740(4) Å].5 Indeed this value lies marginally below the range 1.68–1.72 Å associated with free phosphaalkenes.6 The Ru]P separation is substantially shorter [2.280(2) Å] than those to the phosphines [P(28), 2.417(2); P(9), 2.412(2) Å].This may be interpreted as indicating a pronounced p-acceptor role for the phosphaalkene ligand, a feature presumably enhanced by the p-dative capacity of the iodide, and reflected in the n(CO) value (1978 cm21) which is comparatively high for neutral divalent ruthenium. The trans arrangement of the iodide and MeP]] CHBut ligands suggests that the mechanism is in fact a two-step Fig. 1 Molecular geometry for complex 2a. Hydrogen atoms and phenyl groups omitted ¶ Crystal data for 2a. C43H43ClIOP3Ru?0.75CH2Cl2?0.5Et2O, M = 1032.9, monoclinic, space group P21/c, a = 12.168(2), b = 16.728(2), c = 22.585(3) Å, b = 101.00(1) 8, U = 4513(1) Å3, Z = 4, Dc = 1.520 g cm23, m(Mo-Ka) = 13.2 cm21, l = 0.710 73 Å, F(000) = 2082. A yellow cube with dimensions 0.44 × 0.33 × 0.27 mm was used.Data were measured on a Siemens P4/PC diffractometer with graphite-monochromated Mo-Ka radiation (w scans). 7947 Independent reflections were measured (2q < 508) of which 5424 had |Fo| > 4s(|Fo|) and were considered to be observed. The structure was solved by the heavy-atom method and the major occupancy nonhydrogen atoms were refined anisotropically by full-matrix least squares based on F2 using absorption-corrected data to give R1 = 0.055, wR2 = 0.120 for the observed data and 474 parameters.Atomic coordinates, thermal parameters and bond lengths and angles, have been deposited at the Cambridge Crystallographic Data Centre (CCDC). See Instructions for Authors, J. Chem. Soc., Dalton Trans., 1997, Issue No. 1. Any request to the CCDC for this material should quote the full literature citation and the reference number 156/351. process, presumably involving initial nucleophilic displacement of iodide from MeI by 1a to provide the 16-electron complex [Ru(MeP]] CHBut)Cl(CO)(PPh3)2] +.Notably, s/p co-ordination (reminiscent of three-electron vinyl co-ordination) cannot be excluded as a means of temporarily stabilising the co-ordinative unsaturation in such an intermediate. The nucleophilicity of the phosphaalkenyl phosphorus in 1a is itself noteworthy, in that phosphaalkenyl ligands bound to 15-electron metal centres typically show electrophilic behaviour at phosphorus as a result of the linear M]] P]] CR2 linkage.In the case of 1a, as with formally isoelectronic nitrosyls of the late transition metals, e.g. [OsCl(NO)(CO)(PPh3)2], such a linear arrangement does not appear to be required, despite effective atomic number considerations. Thus despite formal co-ordinative unsaturation at the ruthenium centre of 1a, the phosphaalkenyl ligand retains nucleophilic character. Perhaps the most surprising feature of this approach is the apparent lack of generality. Whilst the thiocarbonyl complex 1b reacts with methyl iodide in a similar manner to provide [Ru(PMe]] CHBut)Cl(I)(CS)(PPh3)2] 2b,§ attempts to broaden the range of carbon-based electrophiles have all met with failure.Thus 1a fails to react cleanly with the carbon electrophiles EtI, [Et3O]BF4, N]] N]] CHCO2Et, PhCH2Cl and Me2NC(]] S)Cl. Under more forcing conditions or with prolonged reaction times, the latter two reagents provide only traces of 3, presumably due to hydrolysis of the organic halide by adventitious water.In a similar manner, 3 is the only product of the reactions of 1a with Me3SnCl or Ph3SiCl. Furthermore, treating 1a with such electrophiles in the presence of carbon monoxide does not appear to induce reaction, even though (reversible) co-ordination of CO to 1a results in [Ru(P]] CHBut)Cl- (CO)2(PPh3)2] which must have a bent (and accordingly nucleophilic) Ru]P]] CHBut linkage. Although the range of carbon electrophiles to which 1a and 1b are succeptible appears to be very narrow, preliminary results indicate that metal-based electrophiles offer a much broader array of reagents for electrophilic attack, a subject on which we will report subsequently.3 Acknowledgements We gratefully acknowledge the generous loan of ruthenium salts by Johnson Matthey Chemicals and the Engineering and Physical Sciences Research Council (UK), the Royal Society and the Nuffield Foundation for financial support.References 1 L. Weber, Angew. Chem., Int. Ed. Engl., 1996, 35, 271 and refs. therein. 2 R. B. Bedford, A. F. Hill and C. Jones, Angew. Chem., Int. Ed. Engl., 1996, 35, 547. 3 R. B. Bedford, A. F. Hill, C. Jones, J. D. E. T. Wilton-Ely, A. J. P. White and D. J. Williams, Chem. Commun., in the press. 4 R. B. Bedford, D. E. Hibbs, A. F. Hill, M. B. Hursthouse, K. M. A. Malik and C. Jones, Chem. Commun., 1996, 1985. 5 H. Werner, W. Paul, J. Wolf, M. Steinmetz, R. Zolk, G. Müller, O. Steigelmann and J. Riede, Chem. Ber., 1989, 122, 1061. 6 R. Appel, F. Knoll and I. Ruppert, Angew. Chem., Int. Ed. Engl., 1981, 20, 731; A. H. Cowley, R. A. Jones, J. G. Lasch, N. C. Norman, C. A. Stewart, A. L. Stuart, J. L. Atwood, W. E. Hunter and H.-M. Zhang, J. Am. Chem. Soc., 1984, 106, 7015. Received 14th October 1996; Communication 6/07016G

ISSN:1477-9226    

DOI:10.1039/a607016g

出版商:RSC    

年代:1997

数据来源: RSC