1 Introduction 2 Reviews The reader is directed specifically to two chapters in Specialist Periodical Reports Organometallic Chemistry (Volume 27) for two reviews complementary to this report. The first review is a comprehensive account of the chemistry of carbaboranes and metallacarbaboranes,2a and the second review is a general account of the organometallic chemistry of Group 13 elements.2b The book Advances in Boron Chemistry summarises the results and advances discussed at the ninth international meeting on boron chemistry (IMEBORONIX) held in Heidelberg, Germany in July 1996; it is divided into 11 chapters and contains 74 articles.2cAspecial edition of Collect.Czech. Chem. Commun. appeared during 1997 to celebrate the 70th birthday of J. Ples¢ek: it contains a number of articles relating to carbaborane, metallaborane and metallacarbaborane chemistry.2d–m Specific review articles2n–u have also appeared on the following topics: ‘Pentafluorophenylboranes: from obscurity to applications’, ‘Boryl metal complexes, boron complexes, and catalytic (hydro)boration’, ‘Recognition of electrondonating guests by carborane supported multidentate macrocyclic Lewis acid hosts: mercuracarborand chemistry’, ‘Semi-sandwich platinum metals metallacarboranes derived fromnido-[C2B9H12]~: chemistry and structural studies’, ‘Main-group-based rings and polymers’, ‘Recent advances in the chemistry of heterocarborane complexes incorporating s- and p-block elements’, ‘Synthesis, structure, and reactions of hydride, borohydride, and aluminohydride compounds of the f-elements’, and ‘Chalcogenoboron hydrides’.3 Boron By M.A. BECKETT Department of Chemistry, University of Wales, Bangor, Gwynedd LL57 2UW, UK This report takes a similar format to that used last year1 and reviews the chemistry of boron compounds reported during 1997. The literature has been surveyed by use of Chemical Abstracts, volumes 126 and 127, in conjunction with independent searches of BIDS and the principal chemical journals.19M.A.Beckett 20 2 2F) was found to generate in yields of 25–37% novel 3.3b Details of the reaction of 3H6(PMe3)2]`, have now been reported; the reaction 4 2 3 Polyhedral species 2]` and twoH2 molecules.3a The reaction between BH3·thf or B2H6 and 2 4H10, B5H9 and B5H11.3f The reaction of Me 2H6, 2Se2 with nido-B10H14 yielded arachno-6,7-k-(MeSe)B10H13 10H13 was formed by the reaction of nido-B10H14 2S3.3g The reaction of B10H14 with PMe2Ph at 200Kresulted in the separable whereas under identical conditions Me2S2 failed to react; however, the analogous thiomethyl derivative 6,7-k-(MeS)B with Me bis(ligand) adducts exo,exo- and exo,endo-6,9-(PMe Ph)2-arachno-B10H12 which were characterised by single-crystal X-ray di§raction studies; the halogenated nido boranes 2-BrB10H13 and 2,4-Cl2B10H12 exclusively gave the exo,endo isomers, and in contrast to the chlorinated product, the brominated products isomerised to the exo,exo isomer upon heating.3h A mixture of products was obtained from the interaction of [NBu ][B10H10] with 2-aminopyridine and the structure of one such product, [PPh4 ][2,9-MN,N@-[2- NH(C5H4N)]NB10H8], was determined by X-ray analysis.3i The reaction of Boranes 2H4(PMe3)2 with [CPh3][BF4], which had previously been shown to yield the The cationic species [BH6]` was readily produced at room temperature in the gas phase under the high-pressure conditions of the flowing afterglow-selected ion flow tube (FA-SIFT) apparatus; the structure of the cation was described as a complex between [BH RCN (R\Me, Et, Bu5 or CH bicyclic carboraza systems related to dihydronaphthalene; these products were in addition to the expected borazines, (RCH NBH) Barachno triborane cation [B proceeded via cluster expansion of [B2H3(PMe3)2]` by reaction with B2H4(PMe3)2.3c The reaction of Bu5BBr with Na–K alloy in toluene yielded a closo derivative, Bu54B4H2, with two bridging H-atoms; the same product was obtained in 74% yield by reduction of Bu54B4 in thf by Na–K alloy followed by protonation by HCl.3d The molecular structure of B4H8(PF3), as determined in the gas phase by electron-di§raction, was shown to be the endo isomer with C symmetry.3e The non-linear pair population analysis was extended to the ab initio SCF closed shell level of theory and this new approach was successfully applied to bonding in the simple boranes B B 3 4 [B10H10]2~ with (SCN)2 in CH2Cl2 gave the thiocyanato derivatives [1- (SCN)B10H9]2~ and [(SCN)2B10H8]2~ which were characterised crystallographically as their bis(tetraphenyl-phosphonium) and -arsonium salts, respectively.3j The crystal structure of [PPh4]2[1,10-(O2N)2B10H8], formed from reaction of [NBu4]2- [B10H10] with [NO2]~ in aqueous MeCN, was also reported.3k The structure of [PPh H][B11H14] had the expected 11-vertex icosahedral-fragment geometry with crystallographically imposed mirror symmetry.3l The synthesis and crystal structures of the ammonia adducts of the closed 10- and 12-vertex species [PPh ][1- 4 (NH3)B10H9] and Cs[(NH3)B12H11] were reported.3m The reactions of closo- [(NH3)B12H11]~ with alkyl halides have been studied in detail; the degree of alkylation of the nitrogen was found to be dependent upon the steric demands of the alkyl groups with four representative compounds characterised by single-crystal X-ray di§raction studies.3n Ring opening of the tetrahydropyran ring of [B12H11O(CH2)5]~Boron 21 Fig. 1 Reduction and oxidation reactions of closo 10- and 20-vertex borane anions (Reproduced by permission from Inorg. Chem., 1997, 36, 5419) 20H18]4~, [a2-B20H18]4~ and [B10H10]2~ to [B20H18]2~ by to produce [B12H11O(CH2)5X]2~ was observed in thf solution for X\F~ orOH~.3o The oxidation of [e2-B organic oxidants was evaluated for general applicability for reactions of polyhedral boranes in non-aqueous systems (Fig. 1).3p 10] 3 2 4 Metallaboranes [(k-H) ter analogue [(k-H) stituted products were obtained from the photolysis of [RuH(CO) presence of PHPh and the structure of [Ru H(CO) (PHPh crystal X-ray di§raction study.4b Metal-rich metallaboranes are reported first.4a,b Hydroboration of [(k-H )Os (CO) by BH3·SMe2 at room temperature yielded the new borylidine cluster analogue 2Os3(CO)9(k-H)2BH] whilst at 65 °C the previously reported ketenylidene clus- 3Os3(CO)9(k3-BCO)] was obtained.4a Mono-, di- and tri-sub- 12BH2] in the 2; spectroscopic data allowed the substitution sites to be assigned 2)2BH2]·CH3CN was confirmed by a single- Boron-rich metallaboranes are considered next.4c–l The preparation and character- 2 2 10N 10 isation of a series of 2-arachno-argenta- and 2-arachno-cupra-tetraboranes were described: solid-state 11B MAS NMR spectroscopy indicated that at room temperature intramolecular boron exchange occurred.4c The directed synthesis of Cr and Mo metallaborane clusters was achieved in the preparations of [(Cp*Cr)2B5H9], [(Cp*Mo)2B5H9] and [(Cp*MoCl)2B4H8] from monoboron fragment sources.4d The product [(PPh3)2(CO)OsB5H9(PPh3)], obtained from the reaction of [(PPh3)2(CO)OsB5H9] with PPh3, had a structure characterised as a nido-osmapentaborane with a pendant BH2·PPh3 group; the degradation of metallahexaboranes to clusters containing pendant boron atoms had previously been suggested from ab initio calculations.4e The dizirconaborane [(Cp Zr)2B5H8][B11H14], exhibiting a novel cluster type, was prepared from the interaction of [ZrCp2Cl2] with two equivalents of Li[B5H8] at low temperature (\[35 °C), followed by exposure to air in CH2Cl2 solution; the reaction proceeded via the [k-2,3-(Cp ClZr)B5H8] stable isolable intermediate.4f The closo hexaborate [B6H6]2~ was observed as a bridging ligand in the co-ordination compounds [M2(k-bis-g3-B6H6)(PPh3)2] (M\Au or Cu) and was characterised by single-crystal X-ray di§raction studies.4g The nido 11-vertex MNiB cages in [(PhCOS) NiB10H8(PPh3)] and [(PPh3)(PhCOS)2NiB10H10]·0.5C6H14 2M.A.Beckett 22 Fig. 2 Molecular structure of [(PMe3)2IrB26H24Ir(CO)(PMe3)2] (Reproduced by permission from Chem. Commun., 1997, 2405) have been synthesised and characterised by X-ray di§raction analyses; both structures displayed ortho-cycloboronation of the thiobenzoates with the formation of fivemembered Ni–S–C–O–B rings.4h,i 2 Macropolyhedral metallaborane chemistry made significant advances during 1997.The 18-vertex species 7,7-(PMe3)2-syn-7-IrB17H20 was obtained in low yield from the thermolysis of (PMe3)2(CO)HIrB8H12 with molten B10H14.4j The 28-vertex cluster with a polyboron core, [(PMe3)2IrB26H24Ir(CO)(PMe3)2] (Fig. 2) was also obtained from this co-themolysis reaction; it was described as a triple-cluster species that consisted of a closo 12-vertex MIrB11N cage and a closo 10-vertex MIrB9N cage fused by a common MIrB2N triangular face with an additional nido 9-vertex MIrB8N subcluster.4k The 21-vertex metallaborane (PMe Ph) HReB20H15Ph(PHMe2) made up of a closo 12-vertex MB12N unit and a nido 11-vertex MB11N unit fused with a common triangular face, and a MReH(PMe Ph) 3 3N fragment capping with three Re–H–B linkages exo to 2Boron 23 2 5 one face of the nido cage, was obtained by reaction of [ReH (PMe Ph)3] with B20H16.4l 2 2 Heteroboranes The reader is directed to a Royal Society of Chemistry publication, Specialist Periodical Reports Organometallic Chemistry for a comprehensive review of the 1997 literature concerning carbaboranes.2a Monocarbaboranes are reported first.5a–d The quenched gas phase reaction between 2 4H10 and allene (CH2CCH2) yielded a fluxional monocarbapentaborane system 4H8 in addition to arachno-1-Me- 4H7.5a Deprotonation occurred at the exo nitrogen atom of nido-7- 2Bu5-7-CB10H12 upon reaction with LiBu/, and addition of [NEt3CH2Ph]Cl gave 3CH2Ph][nido-7-NHBu5-7-CB10H12] which was characterised by X-ray 3)-closo-1-CB9H9 was ob- 3N·BH3 and 6-(NH3)-nido-6-CB9H11 4]; related derivatives of formula 1-L-closo-CB9H9 2S, NH2~ or Me3N) were also reported.5c The computed properties of the nHn`1 (n\4, 9 or 11) radicals and anions were reported and the stability of the 11Me12 radical was seen as remarkable in view of the large predicted ionisation 11H12]~ anion.5d 3 Binvolving arachno-1,3- and arachno-1,2-Me -1-CB 2,5-k-CH -1-CB NH the salt [NEt analysis.5b The monocarbaborane ligand derivative 1-(NH tained in 75% yield from the reaction between Et in thf in the presence of Na[BH (L\Me CB CB potential (4.32 eV) of the [CB Dicarbaboranes are considered next.6a–l Two nido intermediates have been separated, isolated, and characterised by NMR spectroscopy, from the conversion-reaction of arachno-1-carbapentaborane(10) derivatives to closo-pentaethyl-1,5-dicarbapentaborane(5).6a The two-electron reduction of dicarbapentaborane(5) derivatives led to the 4n anti-aromatic five-membered ring compounds, 1,2-diborata-4-boracyclopentadienes.6b The crystal structure of the zwitterionic salt [7,8-(2-SNHC5H5)2-7,8- C2B9H10][CF3SO3] was determined.6c The kinetics of ortho-C2B10H12 formation from acetylenes (propargyl bromide, but-2-yne-1,4-diactate or non-1-yne) and B10H12L2 [L\Me2S, Ph2S, Bu52S, (C6F5)2S, MePhS, Me(C6F5)S or Me(Bu4)S] were investigated in detail: the rate constants decreased with both an increase in the electronegativity and/or an increase in size of substituent on S, and yields also increased as the size and or basicity of the Lewis base increased.6d An improved synthesis and full characterisation of all three 12-vertex C-hydroxycarba-closododecaboranes was reported.6e Two nido anions [7-(4-FC6H4)-7,8-C2B9H11]~ and [7-(4-FC6H4)-7,9-C2B9H11]~ were obtained from closo-1-(4-FC6H4)-1,2-C2B10H11 and closo-1-(4-FC6H4)-1,7-C2B10H11 by interaction with two equivalents of [Bu4N]F in thf or CH CN; the NMR spectra of the initial anionic monoboron product was consistent with the new fluoroborate [HOBHF2]~.6f The compound S––CS2C2B10H10, which displayed a stacked arrangement of the S2C––S moieties in the solid state, was obtained from the reaction of 1,2-(HS) -1,2-C2B10H10 with thiophosgene.6g Parameters have been developed which allow conformational calculations to be carried out on 12-vertex boranes, carbaboranes and their derivatives.6h The functionalisation of o-C2B10H12 with propargylphthalimide or (bromoalkyl)phthalimides and their subsequent conversion into isocyanate-substituted derivatives was reported; the reaction of these isocyanate derivatives towards amino- and alcohol-containing molecules was also described.6i Superacid-promoted polycondensation between bis(4- phenoxyphenyl) derivatives of o- or m-carbaborane and organic dicarboxylic acidsM.A.Beckett 24 2B10H12 Fig. 3 Host–guest interaction of CTV with o-C (Reproduced by permission from Angew.Chem., Int. Ed. Engl., 1997, 36, 504) 3 resulted in linear polyether ketones containing rings and icosahedral carbaborane cages; ‘semi-inorganic’ polymers of molecular weight[150,000 were readily obtained.6j Novel host–guest/inclusion complexes of o-carbaborane with aza- and 7,16- diaza-18-crown-6 macrocycles have been reported and their structures determined by single-crystal X-ray di§raction.6k The inclusion chemistry of o-carbaborane was extended by a report on the complexation reactions with the rigid ‘bowl shaped’ cyclotriveratrylene (CTV) and the symmetrically tris(allyl)-substituted analogue: CTV formed a 2:1 adduct in the solid state but only one of the ligands was found to be bound to the cluster (Fig. 3).6l A few papers appeared during 1997 concerning carbaboranes with more than two carbon atoms.7 Deamination of 7-(Me N)-nido-7,8,9-C3B8H10 led principally to either the parent tricarbollide anion nido-[7,8,9-C3B8H11]~ or to the neutral tricarbaborane, nido-7,8,9-C3B8H12.7a Heating 7-(Me3N)-nido-7,8,9-C3B8H10 or nido- [7,8,9-C3B8H11]~ at 350 °C resulted in rearrangement of the carbon atoms in the cluster open face and produced high yields of the isomeric tricarbollides 10-(Me N)- 3 nido-7,8,10-C3B8H11 and [10-(NMe3)-nido-7,8,10-C3B8H11]~, respectively.7a The 6- trimethylstannyl and 6-triphenylstannyl derivatives of pentaalkyl-2,3,4,5-tetracarbahexaborane(6) were prepared by reaction of the triorganylstannyllithiums with the 6-bromotetracarbaborane derivatives; reactions of the 6-triphenylstannyl derivative towards various electrophiles was studied.7bBoron 25 A few papers appeared during 1997 concerning heteroboranes with heteroatoms other than carbon.8 An improved synthesis of [OB11H12]~ by oxidation of [B11H14]~ with aqueous NaOH was described; the nido-oxaborane anion was obtained in 24% yield and isolated as its tetraethylammonium salt.8a The results of a theoretical investigation of the [OB11H12]~ anion, based on the DFT/GIAO/NMR method, confirmed the nido nature of the cluster with a three-co-ordinate cage oxygen atom.8b Ab initio theoretical studies on all possible isomers of X2B10H10 (X\CH, SiH, N, P or Sb) have been undertaken but a low-energy conventional 1,2 isomer forN was not found; the general trends of stabilities followed the order 1,12[1,7[1,2.8c The structure and bonding characteristics of the dehydro derivative 1,2-Si was compared, with calculations at the HF/6-31-G* and B3LYP/6-31-G* levels of theory, with those of 1,2-dehydro-o-carbaborane and 1,2-dehydrobenzyne; in view of these calculations 1,2-Si 2B10H10 2B10H10 was described as an interesting experimental target.8d 3 3 3 2 Metallaheteroboranes Continuing the tradition of previous years, a survey of the more important developments in metallacarbaborane chemistry is included in this section; a comprehensive review of this area is available elsewhere.2a Metallamonocarbaborane derivatives are described first.9a–f Two novel 10-vertex monocarbon ruthenacarbaboranes 2-Cl-2,5-(PPh3)2-2-H-3,9-(MeO)2-2,1-RuCB8H6 and 2,2-(PPh3)2-2-H-3,9-(MeO)2-2,1-RuCB8H7, and a small amount of the 8-vertex (PPh3)2HRuCB6H4(OMe)3, were obtained from polyhedral contraction by reaction of Cs[nido-B10H12CH] with [RuCl2(PPh3)3] in refluxing methanol; the 10-vertex clusters were regarded as hypercloso species with n rather than n]1 skeletal electron pairs.9a The trinuclear osmium complex/cluster [Os (CO)8(g5-7-NMe3-7-CB10H10)] was synthesised by heating [Os (CO)12] with nido-7-NMe3-7-CB10H12 in bromobenzene; the structure, as established by X-ray analysis, showed the MOsCB10N icosahedral cage, retained the Os triangle, and had two agostic B–H· · ·Os interactions (Fig. 4).9b The closo species 2,2-(PPh tion of [RhCl(PPh synthesis of the salt [Na][Pt(PEt drido complex [PtH(PEt platinamonocarbaborane anion reagent with [AuCl(PPh )], [MCuCl(PPh [HgClPh] yielded dimetallic species.9d Reaction of 5-Me S-6-[(Me Si) 3 3)2-2-H-1-(NMe3)-2,1-RuCB10H10 was synthesised by reac- 3)3] with nido-B10H12CNMe3 under alkaline conditions.9c The 3)2(g5-7-CB10H11)] and its protonation to the hy- 3)2(CB10H11)] was described; metallation of the 3)N4] and 2C––CH]- B10H11 with [MNiCp(CO)N2] a§orded two isomeric 12-vertex cluster derivatives, 10H10 and 1-Cp-2-(Me3Si)2CH-1,2-NiCB10H10; the 1,7 2 3 3 1-Cp-7-(Me Si) CH-1,7-NiCB isomer was characterised by single-crystal X-ray di§raction methods.9e The crystal structures of all three Ag(CB11H6X6) compounds (X\Cl, Br or I) were reported as one-dimensional co-ordination polymers with the hexahalogenocarbaborane anions acting as bridging ligands.9f The tantalum carbaborane complexes [(Et2C2B4H4)CpTaR2] (R\Me or Ph) underwent insertion reactions with nitriles and isocyanides; the thermal and photochemical g2-iminoacyl isomerisation reactions of the isomers [(Et2C2B4H4)CpTaCH3(g2-C,N-CM––NBu5NCH3)] were described.10a The bent-sandwich complex [Li(thf)3][g5-C2B4H4(SiMe3)2][Cp*ZrCl2] was synthesised by reaction of [Cp*ZrCl3] with [C2B4H4(SiMe3)2]2~.10b The synthesis, structures, EPR spectra and reactivities of the bent-sandwich and the half-sandwich titanacar-M.A.Beckett 26 3 Fig. 4 Molecular structure of [Os (CO)8(g5-7-NMe3-7-CB10H10)] (Reproduced by permission from J. Organomet. Chem., 1997, 536/537, 537) 2B4N carbaborane cage systems have been reported. The single-crystal baboranes,10c and the half-sandwich and full-sandwich gallacarbaboranes,10d of the 2,3- and 2,4-MC X-ray di§raction structure of the isonido-metalladicarbaborane 1-H-1,1-(PMe3)2-6- Cl-1,2,4-IrC2B8H9 has been determined and showed a four-membered Ir1-C2-C4-B7 open face.10e The syntheses and structures of hydridoruthenacarbaborane double-decker and triple-decker sandwich complexes [Cp*RuH(Et2C2B4H4)] and [Cp*Co(Et2C2B3- H )RuHCp*] were reported; the monometallic species was characterised crystallog- 3Boron 27 2 Fig. 5 Molecular structure of [CpCoMk5-[(CEt) (BEt) CMe]NCo(S2B6H8)] (Reproduced by permission from Chem. Ber./Receuil, 1997, 130, 329) 2 2 2 raphically.10f The crystal structure of [CpCoMk5-[(CEt2)2(BEt)2CMe]N-7-Co-6,8- (S2B6H8)] (Fig. 5) confirmed the triple-decker arrangement for this complex with the bridging 1,3-diborolyl ring and the terminal thiaborane ligand; the compound was prepared by the three-component reaction of [CpCoMg5-[(CEt) (BEt) CMe]N]~ with CoCl and arachno-[6,8-S2B7H8]~.10g A series of novel bis(cobaltacarbaboranyl), (CoC2B4)2X, dicluster complexes whose apical B7 atoms were linked by organic moieties were prepared by an extension of the ‘recapitation’ method by treatment of 6-vertex nido-MCoC2B3N dianions with monoboron reagents; the characterisation of 7,7@-[Cp*Co(2,3-Et2C2B4H3)]2 and [Cp*Co(2,3-Et2C2B4H3-7)]2X (X\MeCH, CH––CH or C– 22 – –C) were reported.10h Metalladicarbaboranes with the MC -3,1,2-closo-RhC 2B9N cage are reported next.11a–x A series of papers concerning the synthesis, structural characterisation, and reactions of the B8–B8@ bridged derivatives of the bis(1,2-dicarbollido)-3-cobalta(1[) anion,11a–c or the B6–B6@ bridged derivatives of the bis(1,7-dicarbollido)-3-cobalta(1[) anion11c–e were reported.The synthesis and reactivity of the sterically encumbered chargecompensated carbaboranes 7-Ph-11-SMe2-nido-C2B9H10 and 1-Ph-3,3-(CO)2-7- SMe 2B9H8 was described.11f The unique semipseudocloso carbaruthenaborane 1-(PhCC)-2-Ph-3-(cym)-3,1,2-RuC2B9H9, characterised by multinuclear NMR spectroscopy and by single-crystal X-ray di§raction, had a cage geometry between that expected for closo and pseudocloso structures and closer to theM.A. Beckett 28 Fig. 6 Perspective view of the non-icosahedral anion [(CO) C3H5)MoPh2C2B9H9]~ (Reproduced by permission from Angew. Chem., Int. Ed. Engl., 1997, 36, 645) 3 2 3 2B9H10]~ and its reaction with 2] to yield 1,1-(PMe2Ph)2-2,4-Ph2-1,2,4-closo-PtC2B9H9 have been 2 2(g3- latter.11g A crystallographic study of [Me N][(CO)2(g3-C3H5)MoPh2C2B9H9] revealed a non-icosahedral closed cage geometry with two four-cage-connected vertices (both occupied by C) and two six-cage-connected vertices (one occupied by Mo) (Fig.6); the cluster contained only one Mo–C connectivity and the two C atoms were substantially separated.11h The related C-monophenyl derivative [BnMe N]- [MMo(CO)2(g3-C3H5)NPhC2B9H10] had the expected icosahedral arrangement con- firmed by a single-crystal X-ray di§raction study.11h Synthetic routes towards the C,C@-diphenyl nido carbaborane [7,9-Ph -7,9-nido-C [PtCl (PMe Ph) described; this compound, with the 1,2,4-MMC2B9N cage geometry, indicated that a hextuple-concerted diamond–square–diamond rearrangement mechanism cannot be operative in the spontaneous isomerisation of 1,2-Ph -3,3-(PMe Ph) -3,1,2-closo- 2 2 2 2 22 2 2B9H11 3)2-3-H-3,1,2-closo-RhC2B9H9-9,12-Br2.11k The first thorium com- 4 2 PtC2B9H9 to 1,1-(PMe2Ph)2-2,8-Ph2-1,2,8-closo-PtC2B9H9.11i The complexes 3-[g1- -3,1,2-closo-RhC SC(H)NPh]-3,3-(PMe Ph) and 2-[g1-SC(H)NPh]-2,2- (PMe Ph) -2,1-closo-RhTeB10H10 have been structurally characterised by X-ray crystallography and described as the first thioformamidate complexes to be isolated.11j A closed icosahedral cage geometry with adjacent Rh and C atoms was found for the cluster 3,3-(PPh plexes incorporating carbollide ligands [Li(thf)4]2[Th(g5-C2B9H11)2X2] (X\Cl, Br or I) and [Li(thf) ][Th(g5-C2B9H11Br3(thf)] were prepared and characterised by a combination of spectroscopic and analytical techniques.11l The molecular structure has been determined of the key catalyst for the hydrogenation of methacycline to doxycycline and epidoxycycline viz.closo-3,3-(g2:g3-C7H7CH2)-3,1,2-RhC2B9H11.11mBoron 29 The palladacarbaborances 1-C4H2RS-3,3-(PMe2Ph)2-3,1,2-PdC2B9H10 and 1- 4H2RS-3,3-(PMe2Ph)2-8-PMe2Ph-3,1,2-PdC2B9H9 (R\Me or H) were obtained 2 2(PMe Ph)2] [7-(C4H2RS)-7,8-nido-C2B9H10]11n The novel bimetallic carbaboranyl haf- 2B9H11)Hf(k,g5: g1-C2B9H11)HfCp*(H)] was ob- 2(g5-C2B9H11)2Hf2Me2] with H2.11o The dinuclear 2(g5-C2B9H11)2Hf2Me2] was found to Cas the major and minor products respectively, from the reaction of [PdCl with Tl nium hydride complex [Cp*(g5-C tained by the reaction of [Cp* hafnium dicarbollide dimethyl complex [Cp* regioselectively dimerise terminal alkynes RC– stituted but-1-en-3-ynes CH C(M\Mo or W), yielded the products [MRu(k-CC CCbaboranes [M(C2B9H11)Mo(k-SPh2)2N2]n~ (n\1 or 2) were prepared by successive oxidation of [(C2B9H11)Mo(CO)2(SPh)2]2~ with Ph2S2 and then PhIO.11r An improved synthesis of [3,3@-Co-(1-R-2-R@-1,2-C2B9H9)2]~ derivatives was reported together with the synthesis of cobaltabis(dicarbollyl) complexes incorporating exo cluster SR substituents.11s The first example of B,C@-bridging in a commo bis(carbollide) complex was observed for [1,8@-k-SEt-3,3@-Co(1-Ph-2-SEt-1,2-C2B9H9)(9@-Ph-1@,9@- C2B9H8)].11t Synthetic routes, structures, and the mechanistic implications involved in the synthesis of mixed cobaltacarbaboranes incorporating g5-pyrrolyl and dicarbollide ligands were reported.11u The synthesis and characterisation of [CoM7- C4H4N(CH2)3-8-Me-7,8-C2B9H9N(g5-NC4H4)], the first structurally characterised molecule with [g5-NC4H4]~ and an organic C–NC4H4 group, was presented.11v Reduction of Cs[Co(1,2-C2B9H11)2] by one equivalent of Na–Hg amalgam yielded the cobalt(II) anion [Co(1,2-C2B9H11)2]2~ which was converted back into the cobalt(III) species by oxidation with 0.5 equivalent of I2; lithiation of the cobalt(III) species followed by reaction with an alkyl halide furnished a new synthetic route to the C-substituted derivatives [Co(RC2B9H10)(C2B9H11)]~ and [Co(RC2B9H10)2]~ (R\Me or C6H13).11w Condensation polymerisation of a cobalt dicarbollide dicarboxylic acid derivative with hexamethylenediamine led to the formation of the first oligomeric amide compounds with cobalt dicarbollide in the main chain.11x There were a number of publications in 1997 relating to metal complexes of substituted dicarbaborane ligands in which the metal remained exo to the cage.12a–g The synthesis of gold(I) and gold(III) complexes with 1-Me-2-SH-1,2-C2B10H10 of the type [Au(SCB10H10CMe)L] (L\PPh3, PPh2Me or AsPh3) or [NBu4]- [Au(C6F5)3(SCB10H10CMe) have been prepared by reaction of the dicarbaborane with suitable gold(I) or gold(III) precursors.12a Crystal structures of the gold(I) complexes related to 1,2-(SH) -1,2-C 2 2 – –CH (R\Me, Pr/ or Bu*) to 2,4-disub- 2––C(R)C–– –CR.11p The complex [Ru(thf)(CO)2(g5-7,8- 2B9H11)], when reacted with the alkylidyne reagents [M(–– –CC6H4Me-4)(CO)2Cp] 6H4Me-4)(CO)4(g5-7,8- 2B9H11)Cp] which readily isomerised to [MRu(CO)4Mp,g5-9-CH(C6H4Me-4)-7,8- 2B9H10NCp].11q Unprecedented tetrathiolate-bridged dinuclear ionic molybdacarhave been reported: [Au2(k- S2C2B10H10)(PPh3)2], [Au2(k-S2C2B10H10)(k-M(PPh2)2(CH––CH)N], [Au2(k- S2B10H10)(k-M(PPh2)2(C6H4)N].12b The synthesis and degradation reactions of the gold(III) complex [Au(S [Au(S X-ray di§raction studies.12c The synthesis and structural characterisation of the novel compound [Au (PPh reported: the molecular structure consisted of a tetrahedral MAu 2B10H10 2B10H10)2]~ was described and the complexes [NBu4]- 2B10H10)2] and [NBu4][Au(S2C2B9H10)(S2C2B10H10)] were characterised by 4M(PPh2)2C2B9H10N2(AsPh3)2], obtained by treatment of 2)2C2B10H10 with [AuCl(AsPh3)] in the molar ratio 1: 2 in refluxing ethanol, was 4N core in which two ofM.A.Beckett 30 Fig. 7 Molecular structure of a small gold cluster with carbaborane ligands: [Au4M(PPh2)2C2B9H10N2(AsPh3)2] (Reproduced by permission from Angew. Chem., Int. Ed. Engl., 1997, 36, 993) 2 2B9H10]2~ 2 2 the gold atoms were chelated by the anionic diphosphine ligand and the other two gold atoms were bonded to one arsine ligand each (Fig. 7).12d The gold(III) complex [AuCl2M(PPh2)2C2B9H10N]·CHCl3 contained the expected cis square-planar gold(III) units and the dicarbaundecaborate anion acted as a P,P@-bidentate ligand.12e The S,S@-thioether–thioester chelating ligand [7,8-k-SCH C(O)-7,8-C2B9H10]~ maintained its original cyclic nature upon reaction with [RhCl(PPh)3] in ethanol, but yielded a complex of the transesterified ligand [7-S-8-CH C(O)OEt-7,8-C upon reaction with [PdCl (PPh3)2].12f Structural characterisation of the late transition-metal complexes of bis(o-carbaborane) e.g.[M4~nM(C NiII or CuII), and [CuM(C2B10H10)2N2]2~ have been described and these studies confirmed the structural assignments previously reported.12g 2B10H10)2N2]n~ (M\CoII, Metallaheteroboranes containing more than two cage carbons are reported 3B7H9 [R\H, NCCH2 or MeOC(O)CH2], were prepared from the reacnext.13a–c A series of closo 11-vertex ferratricarbaboranyl complexes, closo-1-CpFe-3- R-2,3,4-C tions of [CpFe(CO)2I] with arachno-[6-R-5,6,7-C3B7H11]~ anions; another isolated product included the first metallapentacarbaborane nido-2-CpFe-7-Me-7,8,9,10,12-Boron 31 10H10)N2]2~ Fig. 8 A PLATON representation of [MHFe(MeSiB (Reproduced by permission from Angew. Chem., Int. Ed. Engl., 1997, 36, 888) C5B6H10.13a When a mixture of cyclopentadienyl anions and arachno-[6-(NCCH2)- 5,6,7-C3B7H11]~ were reacted with cobalt chloride, cobaltatricarbaboranes of the general formula closo-1-CpCo-(NCCH2)C3B7H9 along with the unique multimetal cluster commo-Co-[closo-1-Co-8,9-(CpCo) -2,3,5-C3B7H10][closo-2@-Co-3@-(NCCH2- )-1@,10@-C CH 2B7H8] were obtained.13a The synthesis of paramagnetic closo-[1-CpFe- 2-Me-2,3,4-C3B7H9][X] (X\AsF6 or SbF6) salts from nido-[6-Me-5,6,9-C3B7H9]~ and [CpFe(CO)2I] followed by oxidation by AgI, and their antineoplastic bioactivity has been investigated.13bA new route to the tetracarba-nido-octaborane(8) species and the molecular structure of nido-6,9-[Fe(CO) 2 2 10H12]~ with K[BHEt3] and FeBr2 10H10)N2]2~, isolated in 66% yield as its 2 3]2-5,7,8,10-C4H4B4Et4 was described.13c There were a few reports during 1997 of non-carbon containing metallaheteroboranes, although not as many as in previous years.14a–d The closed 10-vertex 1-Et-6,7-Cp* -1,6,7-NRh2B7H7 was isolated in low yield from the reaction between [MRhCp*ClN2], NaH and EtH2NB8H11NHEt in thf.14a The two new isoelectronic nido species k-9,10-(SMe)-8,8-(PPh3)2-8,7-IrSB9H9 and k-9,10-(SMe)-8-(g4-Cp*H)- 8,7-IrSB9H9 have been characterised by single-crystal X-ray di§raction studies.14b The first transition-metal complexes of silaboranes were reported during 1997: thus, the reaction of the sila-nido-undecaborate [MeSiB gave the cluster dianion [MHFe(MeSiB bis(tetrabutylammonium) salt14c (Fig. 8). The sandwich anions [Cp*M(MeSiB10H10)]~ (M\Co, Rh or Ir) were obtained from reaction of the monodeprotonated silaborane with [MCp*MCl2N2].14dM.A. Beckett 32 3 4 Organometallic boron species Pentafluorophenylborane derivatives A review2n relating to the history and recent developments/applications of penta- fluorophenylboranes was alluded to in Section 2, and a number of important publications in this area appeared during 1997. The new compounds [TiCp*Me2E] (E\C6F5 or OC6F5) and [TiCp*Me(OC6F5)2] were found to react with the borane B(C6F5)3 to form the highly electrophilic, thermally unstable, olefin polymerisation initiators [TiCp*Me(E)(k-Me)B(C6F5)3] and [TiCp*(OC6F5)2][BMe(C6F5)3];15a their solution structures and the exchange phenomena of these complexes were also reported.15b The yellow-orange insoluble solid, [MRN(CH2)3NRNTiMe2MB(C6F5)3N], prepared by the reaction of the titanium diamide complex [MRN(CH2)3NRNTiMe2] (R\C6H3Pr*2-2,6) with B(C6F5)3, acted as a catalyst for the living-polymerisation of hex-1-ene.15c Zr(k-MeC Me)B(C Bis(pentafluoro)boron fluoride, which was readily obtained from BF3·OEt2 and two equivalents of C6F5MgBr, was found to react with fluorenyllithium to a§ord (flu)B(C6F5)2, whilst reaction with indenyllithium led to the regioisomers 1- and 2-(ind)B(C6F5)2.15d The cyclopentadienylborane C5H4(SiMe3)B(C6F5)2, underwent smooth dehalogenosilation with [TiCl4] and a§orded [C5H4B(C6F5)2TiCl3] which in the presence of low concentrations of AlEt was active as an ethene polymerisation catalyst.15d The reactions of the so called ‘tuck-in’ permethylzirconocene compounds [Cp*(g5: g1-C5Me4CH2)ZrX] (X\Cl, Ph or Me) with the electrophilic boranes HB(C6F5)2 and B(C6F5)3 produced zwitterionic products which were also olefin catalysts;15e polymerisation [Cp*Mg5- the zwitterionic product, C5Me4CH2B(C6F5)2NZrPh], reacted readily with acetone or acetophenone to yield the stable adducts [Cp*Mg5-C5Me4CH2B(C6F5)2NZrPh(OCMeR)] (R\Me or Ph).15f Bis(propynyl)zirconocene reacted with B(C6F5)3 and gave the C–C coupled [Cp 6F5)3] betaine product.15g 4 2 Heterocycles containing boron 4H4BR)N4] and bis(borole)iodorhodium complexes [[RhI(g5-C4H4BR)2].16b The borirene molecules, (CH)2BH, was identified by FT-IR spectroscopy of a matrixisolation product and density functional theory (DFT) calculations.16a The triple-decker complexes containing borole moieties, [(k-C4H4BR)MRh(g5- C4H4BR)N2] (R\Me or Ph) were readily oxidised by elemental I2 in MePh or CH2Cl2 solution at ambient temperature and produced the heterocubanes [MRh(k3- I)(k5-C The reversible carbonylation and Lewis-base degradations of [MRh(k -I)(g5- C4H4BPh)N4] were reported together with the synthesis and structure of the dinuclear complex [Cp*Rh(k-I) Rh(g5-C 3 was reported and described as the first hetero-p-terphenyl analogue to be structurally characterised.16d Crystallographic evidence for the simultaneous p- and n-donation by a carbonyl group to a divalent boron Lewis acid was obtained by analysis of the structure of Mg6-borabenzene[3-(dimethylamino)acrolein]Nchromium tricarbonyl (Fig. 9); this compound was prepared by B ligand exchange of (g6-borabenzene)(tetrahydrofuran)chromium tricarbonyl.16e The reaction of lithium M[1-(3- 3 4H4BPh)].16c The synthesis and crystal structure of the borabenzene-4-phenylpyridine complexBoron 33 Fig. 9 Crystal structure of Mg6-borabenzene [3-(dimethylamino)acrolein]Nchromium tricarbonyl (Reproduced by permission from Angew. Chem., Int. Ed. Engl., 1997, 36, 267) 3 3 dimethylamino)propyl]borabenzeneN with [Mn(CO) (CH CN) ][PF6] a§orded [M1- (3-dimethylamino)propylNborabenzene]manganese(I) tricarbonyl which has an intramolecular N]B interaction in the solid state but which exists in solution in an equilibrium mixture with its ring-opened isomer; 11B NMR spectroscopy was used to measure the equilibrium constants over the temperature range [35 to ]48 °C and allowed an evaluation of *HL– (ca. 25 kJ mol~1).16f 3 The reaction of Li[C5H5BMe] with electrophiles Me3ECl (E\Si, Ge, Sn or Pb) produced the 1,2-dihydroborines 2-(Me3E)C5H5BMe; the crystal structure of the trimethylstannyl derivative showed a long Sn–C bond (228.7 pm) and in solution all derivatives were fluxional with [1,3] sigmatropic migrations of the MMe3EN groups from C2 to C6.16g The borabenzene derivative InMe(C5H5BMe)2 was synthesised from 2-(Me Sn)C5H5BMe and InMe3.16h The complexes [TiCl3(g6-C5H5BMe)], [MMCl3(g6-C5H5BMe)Nx] and [MCl2(g6-C5H5BMe)] (M\Zr or Hf) were obtained 4] (M\Ti, Zr or Hf) and Li[C5H5BMe], 2-(Me3Sn)C5H5BMe, or 2- 3 3 3 from [MCl (Me Si)C5H5BMe in excellent yields; borabenzene complexes were also prepared from the cyclopentadienyl precursors [MCl Cp] (M\Ti or Zr) and [MCl Cp*] (M\Zr or Hf).16i The reaction of two equivalents of Li[C5H5BPh] with [ZrCl4] in Et2) a§orded [ZrCl2(g6-C5H5BPh)2]16j whilst the ethoxyborabenzene complex [ZrCl2(g6- C5H5BOEt)2] was obtained similarly in 71% yield;16k these bis(borabenzene)zirconium(IV) complexes were found to be useful catalysts for a-olefin production.The borabenzene complexes of zirconium(II), [Zr(g6-C5H5BR)2(PMe3)2] (R\NPr*2 or Ph), were prepared and the PMe ligands were found to be easily displaced under very mild conditions.16l The reaction of 1-substituted borepins C 33 3 6H6BX (X\Me, Ph or Cl) with [(py) Mo(CO)] and BF3·OEt2 a§orded the corresponding borepinmolybedum tricarbonyl complexes; reaction of [(g6-C6H6BCl)Mo(CO)3] with appropriate nucleophiles gave the boron-substituted complexes [(g6-C6H6BX)Mo(CO)3] [X\H, OMe, OH, O0.5, NPr*2, NMe2, N(C6H12)2 or NBnMe].16m The relative aromaticity of annelated borepins and related systems has been calculated at the B3LYP/6-311]G** level.16nM.A.Beckett 34 3 2 Fig. 10 Crystal structure of [TiCp (PMe )(HBcatF-3)] (Reproduced by permission from Angew. Chem., Int. Ed. Engl., 1997, 36, 1510) 2 2 5 Boration reactions and metal–boryl derivatives Ti(g2-CH 2 3 2 A theoretical study has been undertaken on the mechanism of Pt0-catalysed alkyneand alkene-diboration reactions using the B3LYP density functional method; the observed di§erences between these reactions originated from the energetics of the insertion of the alkyne/alkene into the Pt–B bond.17a In the presence of a catalytic amount of [Pt(dba)2] at 50 °C, bis(pinacolato)diboron added selectively to terminal and cyclic alkenes and provided a route to bis(boryl)alkanes in 76–86% yields.17b Diboron(4) compounds reacted, in the presence of 5 mol% [Pt(C2H4)(PPh3)2] at 80 °C, with a,b-unsaturated ketones and produced the 1,4-addition products.17c Solutions of [Cp* 2––CH2)] catalysed the reaction between ethylene and HBOp (benzo-1,3,2-diazoborolane) and CH2––C(H)BOp was isolated in reasonable (58%) yield at low catalyst loading.17d The synthesis, structure, and reactivity of [TiCp (PMe )(HBcatF-3)] have been reported, and the structure was described as a co-ordination complex involving the B–H bonding pair (Fig. 10).17e The olefin complexes [Cp* M(CH2––CHR)H] (R\H or Me; M\Nb or Ta) reacted cleanly with catecholborane (HBcat) and HBO2C6H3Bu5-4 (HBcat@) to yield [Cp*2M(H2Bcat)] and [Cp* M(H Bcat@)] and the anti-Markovnikov hydroboration products 2 catBCH2CH2R and cat@BCH2CH2R; the solid state structures of [Cp*2Nb(g2- H2BO2C6H3Bu5-3)] and [Cp*2Nb(BH4)] were determined by X-ray di§raction.17f The reaction between the rhodium(III) bis(boryl) complex [RhCl(PPh3)2(Bcat)2] and diborane(4) compounds B2(O2R)2 (O2R\1,2-O2C6H3Me-4, 1,2-O2C6H2Bu52-3,5, and dimethyl-L-tartrate) a§orded the unsymmetrical [RhCl(PPh3)2(Bcat)(BO2R)] and (cat)B–B(O2R) with the possible mechanism involving p-bond metathesis.17g A numBoron 35 2 2 ber of five-co-ordinate ruthenium(II) and osmium(II) boryl complexes Me.g.[M(Bcat)Cl(CO)(PPh3)2] (M\Ru or Os)N resulted from the reactions of [RuHCl(CO)(PPh3)2] or [OsPhCl(CO)(PPh3)2] with the appropriate borane; the new complexes were characterised by IR and NMR spectroscopy.17h Ethyne was found to insert readily into the Ru–B bond of [Ru(Bcat)Cl(CO)(PPh3)2] to form the six-coordinate borylalkenyl complex [RuMCH––CH(BOC6H4O)NCl(CO)(PPh3)2], which was characterised by spectroscopy and by X-ray di§raction methods; the relevance of the observed ethyne insertion for metal catalysed hydroboration reactions was discussed.17i The first low-valent metal complexes containing dialkylboryl ligands have been synthesised: thus, the reaction of the Fischer carbyne complexes [Tp@(CO) M(CR)] (M\Mo, R\p-tolyl; M\W, R\Me or p-tolyl) at room temperature (R\Me) or 60 °C (R\p-tolyl) with the hydroborating agent ‘Et BH’ gave the products [(Tp@)(CO)2MMB(Et)CH2RN] with agostic stabilisation from one H atom of the CH2R group to the metal.17j New borylene complexes of the type [k-BXM(g5- C5H4Me)Mn(CO)2N2] (Mn–Mn) (X\NMe2, Cl, NHBu5, NHPh, OMe, OEt or OH) were prepared by substitution reactions of the metal co-ordinated borylene moiety.17k 2 6 Boron–pnicogen species 2 2 2 6 The molecular structure of (Me2N)2B(NMe)B(NMe2)2, as determined by X-ray diffraction, displayed two-fold symmetry and a planar boron–nitrogen framework.18a (Dimethylamino)bis(trifluoromethyl)borane, (F3C)2B(NMe2), underwent a number of [2]2] cycloaddition reactions withN-sulfinylsulfonamides, aminoiminophosphines, carbodiimides, and a ketenimine and yielded a variety of four-membered heterocyclic ring systems.18b A series of benzo-1,3,2-diphosphaborolanes, C6H4(PR)2BR@ (R\H, Pr* or SiMe3; R@\R2N, R) featuring pyramidal phosphorus atoms with substituents in antiperiplanar positions, were synthesised and characterised by spectroscopic and X-ray di§raction methods; the phosphorus atoms acted as co-ordination sites for the MCr(CO)5N fragment.18c The synthesis of benzo-1,4,2,3-diphosphadiborinane derivatives and benzo-1,5,2,3,4-diphosphatriborepanes were also reported.18c The 1,3,2,4- diphosphadiboretanes were found to be useful starting materials for the e¶cient synthesis of five- and six-membered cage species of the general type [(R NB)2P2E] (E\R NB, R2Si, R2Ge18d or R2Sn18e) and [(R2NB)2P2(E2)] (E2\Me2SiSiMe2).The same methods were extended to the formation of new spirocyclic 9-vertex cages e.g.[(R NB)2P2SiP2(BNR2)2] (R2 N\Pr*2N or tmpip) by reaction of [(Pr* N)BP(H)B(NPr*2)P·Li(DME)] with SiCl4.18f A triple-cage compound [P (Pr* NB)6Si2] (Fig. 11), with 14 atoms in the cage-core, was produced in the reaction of [(Pr* N)BP(H)B(NPr* 2 2 2)P·Li(DME)] with Si2Cl6.18g Addition of one equivalent of BH3·thf to the u-alkenyldiphenylphosphanes H2C––CH(CH2)nPPh2 (n\0–2) gave the corresponding phosphane–borane adducts H2C––CH(CH2)nPPh2·BH3 without further reactions, whilst treatment of the u-alkenyldiphenylphosphanes with 9-borabicyclo[3.3.1]nonane gave the cyclic hydroborated phosphane–borane adducts, (C8H14)B(CH2)nPPh2 (n\3 or 4).18h The phosphane–borane (C8H14)B(CH2)4PPh2, characterised in the crystalline state by X-ray di§raction as a cyclic adduct,18h showed temperature dependent ring-opening and -closure in solution with *HL– of [30.5 kJ mol~1.18i The reactions ofM.A.Beckett 36 6 Fig. 11 Schematic drawing of the triple-cage species [P (Pr* NB)6Si2] (Reproduced by permission from Inorg.Chem., 1997, 36, 1534) 2 2 2 (Me S)BH2Br and (Me2S)BHBr2 with equimolar quantities of 1,2-bis(diphenylphosphino)ethylene (dppee) or 1,2-bis(diphenylphosphino)benzene (dppbn) led to cyclic cationic bis(phosphane)boranes: [LBH ]Br or [LBHBr]Br (L\dppee or dppbn); the bicyclic [L@BH]Br was obtained from the triphosphane [bis(2-diphenylphosphino)phenyl]phenylphosphane (L@).18j 2 2 3 2 1,2-Bis(dimethylamino)-1,2-dibora-[2]ferrocenophane was obtained from the reaction of 1,1-dilithioferrocene with Cl(Me N)BB(NMe )Cl; variable-temperature NMR spectroscopy revealed that a dynamic process in addition to the hindered rotation (*Gt[80 kJ mol~1) about the B–Nbond occurred in solution.18k The highly strained [1]boraferrocenophane [1,1@-Fe(C5H4)2-(k-BN(SiMe3)2] was synthesised from dilithioferrocene·tmen with (Me Si) NBCl2.18l Reaction of the readily available diboryl ferrocenes [1,1@-Fe(C5H4)2(BBrR)2] (R\Br, Me or OEt) with LiPPh2 afforded the ferrocenophanes [1,1@-Fe(C5H4)2-MB(k-PPh2)RN2] (R\Br, Me or PPh2) and [1,1@-Fe(C 2 5H4)2-MB(k-PPh2)OEtNMB(k-PPh2)PPh2N].18m 7 Boron–chalcogen species A series of biborate derivatives of the ‘salen-type’ ligand systems with general formula salenMB(OR)2N2 (R\Me or Et) have been prepared; analogous N,N@-1,n-alkylenebis(3,5-di-tert-butylsalicylideneimine) derivatives were also reported.19a NewBoron 37 of complex metallaboroxide the structure 12 Molecular 2 2 Fig.[CuMO3B2(mes)2N2MLi(CH3CN)(py)N2] (Reproduced by permission from Polyhedron, 1997, 16, 2637) macrocyclic oligoboronate esters have been reported: the reaction of 2,6-pyridinedimethanol with PhB(OH) gave a tetrameric macrocyclic boronate whilst 2-(salicylideneamino)-1-hydroxyethane with PhB(OH) provided a dimeric cycloboronate; both compounds were characterised by single-crystal di§raction methods.19b The controlled hydrolysis of (diorganoamino)dihalogenoboranes substituted with bulky dialkyl or alkyl/aryl groups, resulted in the formation of bis(diorganoamino)dihalogeno-1,3-diboroxanes which were used as ‘building blocks’ for boron heterocyles with the BOB linkage.19c Treatment of CuBr with Li[OB(mes)2] in thf a§orded, after work-up and addition of excess pyridine, the monomeric copper(II) boroxide [CuMO3B2(mes)2N2MLi(CH3CN)(py)N2] (Fig. 12) which contained the new ligand, (mes)B(O)OB(O)(mes), as a result of loss of mesitylene and formation of a B–O–B link.19d A number of 1: 1 adducts of the triarylboroxine (2-MeC6H4)B3O3 with N-donor ligands (4-methylpyridine, cyclohexylamine, pyridine, 3-methylpyridine, piperidine, isoquinoline, and benzylamine) were prepared by reaction of equimolar quantities of amine and (2-MeC temperature; the molecular structures of the related compounds (4-MeC and 4-Mepy·(4-MeC toborane (Tbt)B(SH) followed by treatment with electrophiles such as [Cp TiCl 2 6H4)3B3O3 in Et2O at room 6H4)3B3O3 6H4)3B3O3 were also reported.19e Dilithiation of the dimercap- 2], 2 2M.A.Beckett 38 12 Fig. 13 Molecular structure of the anion [B (BSe (Reproduced by permission from Angew. Chem., Int.Ed. Engl., 1997, 36, 2189) 2 n 3)6]8~ (mes) GeBr2, Ph2SnCl2, (Tbt)SbBr2 resulted in the isolation of novel four-membered 1,3-dithia-2-boretane rings SB(Tbt)SER [ERn\TiCp2, Ge(mes)2, SnPh2 or Sb(Tbt)]; the structures of these ring systems were confirmed in all cases by single-crystal X-ray di§raction studies.19f The novel chalcogenoborate Cs8[B12(BSe3)6] (Fig. 13) was obtained from the reaction of caesium selenide, boron, and selenium by means of a high-temperature solid state synthesis;19g the retention the iscosahedral boron network during such a reaction was highlighted as unusual.19h 3 2 8 Boron–halide species The formation of 1: 1 van der Waals’ complexes between BF and ethene or propene were observed in liquid argon and liquid nitrogen; their IR spectra (4000–400cm~1) were recorded at di§erent temperatures and the complexation enthalpies, *HL–, where calculated to be ca.[8 kJ mol~1.20a Hydrogen bonds and anomeric e§ects were both found to be important in determining the preferred conformations of RR@CO·H BF; MO calculations indicated that these e§ects were ca.[6 and [10 kJ mol~1, respectively, and details should be useful in designing systems which take advantage of such interactions.20b The addition of BX3 (X\Cl, Br or I) to tricarbonylchromium(0) complexes of benzylic alcohols a§orded the analogous benzylic halide complexes in excellent yield.20c The synthesis and characterisation of volatile allylic dihalogenoboranes (R@BX2) were reported; the stability of such compounds in CDCl3 solution was found to be dependent on the halide and ranged from a few minutes (X\Br) to several days (X\F).20dBoron 39 References 1 M.A.Beckett, Annu. 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