3 Boron Michael A. Beckett Department of Chemistry, University of Wales, Bangor, Gwynedd, UK LL57 2UW 1 Introduction This report takes a similar format to that used last year1 and reviews the chemistry of boron compounds reported during 1998. The literature has been surveyed by use of Chemical Abstracts, volumes 128 and 129, in conjunction with independent searches of BIDS and the principal chemical journals. 2 Reviews The reader is directed specifically to two chapters in Specialist Periodical Report Organometallic Chemistry (volume 28) 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 Specific review articles have appeared on the following topics: ‘Transition-metal complexes of boron’,2c ‘Five- and sixcoordinate nitrogen in azaborane clusters’,2d ‘Porphyrins like boron after all’,2e and ‘Chemistry of neutron capture therapy’.2f A special issue of J.Organomet. Chem. was published at the beginning of 1998 to celebrate the 65th anniversary of the birthday of Prof.K. Wade; the numerous articles on boron chemistry therein are included in the text of this report.2g The book ‘The borane, carborane, carbocation continuum’ titled after the symposium held in December 1995 to honour the 70th birthday of Dr R. E. Williams, has now been published and it contains 18 very relevant chapters organised into four sections: ‘Patterns of structures in boranes and carboranes’, ‘The carboranecarbocation continuum’, ‘Untangling molecular structures’, and ‘New species of boranes and carboranes’.2h 3 Polyhedral species Boranes Spectroscopic and theoretical studies have been reported on unusual hydrogen bonds involving hydride atoms of boron hydrides as proton acceptors; the interaction of Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 23–43 23di§erent proton donors with ionic [NBu 4 ][BH 4 ] and neutral BH 3 NEt 3 , BH 3 P(OEt) 3 species were described.3a The results of IGLO/NMR and GIAO/NMR calculations on [closo-R-XB 5 H 5 ]2~ (X\B: R\H, Me, CN; X\C: R\H, Me) and their monoprotonated counterparts have been correlated with experimental data.3b The isomeric composition of B 4 H 8 (CO) in the gas phase and in solution and its reactions with ethene or propene which a§orded R 4 B 4 H 4 (R\Et, Pr) with all four wing-tip H atoms of the tetraboranecarbonyl replaced by alkyl groups were described.3c Computational studies on eight-vertex clusters with nido electron counts were carried out using ab initio methods and the results indicated that the nido ‘six-membered open face geometry’ was usually the preferred configuration over the nido ‘five-membered open face’ geometry.3d The conventional preparation of 4-(L)-arachno-B 9 H 13 derivatives by ligand exchange on 4-(SMe 2 )-arachno-B 9 H 13 has been shown by NMR spectroscopy to also generate the previously unreported isomeric compounds, 5-(L)-arachno-B 9 H 13 ; the molecular structure of 5-(4@-PhC 5 H 4 N)-arachno-B 9 H 13 was determined.3e Relatively high potential barriers for the intramolecular rearrangement of [B 9 H 9 ]2~ were computed by ab initio methods via single (28.4 kcal mol~1) and double (21.3 kcal mol~1) diamond –square–diamond mechanisms.3f Two high-yield routes to nido-6-alkyldecaborane( 14) derivatives via one-pot syntheses were reported starting from either [nido-B 10 H 13 ]~ or arachno-6,9-(SMe 2 ) 2 B 10 H 12 with X-ray di§raction results described for the following intermediates/products: nido-8-(SMe 2 )B 10 H 12 , nido-6- (CMe 2 CHMe 2 )-8-(SMe 2 )B 10 H 11 , and nido-6-(CMe 2 CHMe 2 )B 10 H 13 .3g The partial insertion of the 9-BBN unit into the nido-B 10 H 14 framework was observed in the formation of (9-BBN)B 10 H 13 from Na[B 10 H 13 ] and 9-Br-BBN; the product was deprotonated with ‘proton sponge’ MPS, 1,8-bis(dimethylamino)naphthaleneNto a§ord [PSH][(9-BBN-B 10 H 12 ].3h Mechanistic experiments on the formation of arachno-6,9- (Me 2 S) 2 B 10 H 12 from nido-B 10 H 14 and its subsequent reaction with 2,3-dimethylbut- 2-ene to form nido-5-(Me 2 S)-9-(CMe 2 CHMe 2 )B 10 H 11 indicated that the movement of Me 2 S from B(6) to B(5) did not involve a concerted rearrangement of the boron cage but rather a migration of H and Me 2 S on an otherwise static borane cluster.3i An ab initio/IGLO/NMR investigation was undertaken on the structures of the following arachno- and hypho-MB 10N clusters and their possible Lewis base adducts: [B 10 H 12 ]2~, [B 10 H 12 L]2~, [B 10 H 12 L 2 ]2~, [B 10 H 13 ]~, [B 10 H 13 L]~, B 10 H 12 L 2 ; the recently proposed 6,6-(py) 2 B 10 H 12 structure was not supported computationally.3j The singlecrystal X-ray structures of the neutral substituted decaboranes closo-1,6- (PPh 3 ) 2 B 10 H 8 and arachno-2,4-Cl 2 -endo-6-exo-9-(PMe 2 Ph) 2 B 10 H 12 were reported. 3k,l The molecular structures of closo-1,12-B 12 H 10 (CO) 2 and its dihydrate closo-1,12-B 12 H 10MC(OH) 2N2 ·4H 2 O were reported; the latter was the first structurally characterised species with two carbene diol groups.3m The palladium-catalysed coupling (trans-[Pd(PPh 3 ) 2 Cl 2 ] and CuI) of [closo-B 12 H 12 ]2~ with Grignard reagents (RMgBr) in thf or dioxane a§orded in good yields the corresponding arylated or alkylated borane anions, [closo-B 12 H 11 R]2~.3n A new class of inorganic self-assembled monolayers have been prepared by the spontaneous adsorption of [closo- B 12 H 11 S]3~ onto gold; the monolayers were characterised by surface-enhanced Raman spectroscopy using 735nm excitation.3o The structurally characterised [Ml- MeC(NH) 2NB 20 H 16 ]~ anion was obtained from the benzoquinone oxidation of a2- [Ml-MeC(NH) 2NB 20 H 16 ]3~ in acidic aqueous solution at room temperature.3p A new Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 23–43 24Fig. 1 ORTEP representation of the dianion cis-[B 20 H 18 ]2~ (reproduced by permission from Angew. Chem., Int. Ed., 1998, 37, 1869). isomer of [B 20 H 18 ]2~ was isolated and characterised: the oxidation of ae-[B 20 H 18 ]4~ resulted in the unprecedented polyhedral borane anion (Fig. 1) designated cis- [B 20 H 18 ]2~; the anion consisted of two intact [B 10 H 9 ]~ cages linked by two 3c–2e bonds.3q The thermodynamic stabilities of larger unknown closo-boranes [BnHn]2~ (n\13–17) have been calculated; the ‘three dimensional aromaticity’ of all of these larger closo species was demonstrated and the clusters with n\16 or 17 were calculated to be thermodynamically more stable than [B 12 H 12 ]2~.3r Quantum chemical calculations of proposed multicage boron fullerenes have also been reported.3s Metallaboranes There were a few reports on metal-rich polyhedral metallaboranes during 1998.4a–c An improved synthesis of [HRu 6 (CO) 17 B] was described.4a The formation of the octahedral hexaruthena derivatives [N(PPh 3 ) 2 ][Ru 6 (CO) 16 (PPh 3 )B], [HRu 6 (CO) 16 - (PPh 3 )B], and [Ru 6 (CO) 16 (PPh 3 )B(AuPPh 3 )], and some P(OMe) 3 -substituted species, including the crystal structure of [Ru 6 (CO) 16MP(OMe) 3NB(AuPPh 3 )] were reported. 4b Similarly, a selection of octahedral dirhoda-tetraruthena clusters, including the single-crystal structures of [Rh 2 Ru 4 (CO) 16 B(AuPPh 3 )] and [Rh 2 Ru 4 (CO) 14 - (PPh 3 ) 2 B(AuPPh 3 )] were reported.4c There were a number of papers during 1998 concerned with boron-rich metalla- Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 23–43 25boranes.5a–y The reduction of TaCl 5 with Li[BH 4 ] in the presence of the bidentate ligand dmpm produced the novel [Ta 2 (l-BH 3 )(l-dmpm)(g2-BH 4 ) 2 ] metallaborane with an interesting bridging MBH 3N unit.5a The gas-phase molecular structure of H 2 GaB 3 H 8 was redetermined from electron di§raction data using the SARACEN method of analysis and structural trends in the family of compounds H 2 MB 3 H 8 (M\B, Al, Ga, or In) were investigated by ab initio molecular orbital calculations.5b Similarly, ab initio calculations for Me 2 MB 3 H 8 (M\B, Al, Ga, or In) and gas-phase electron di§raction studies of Me 2 MB 3 H 8 (M\Al, Ga) were reported.5c The dichromatetraborane, [MCr(CO) 4N2 (BH 2 BH 2MdmpmN)], was formed from BH 3 ·dmpm·BH 3 and [Cr(CO) 6 ] in toluene under vacuum using a medium pressure Hg lamp at 0 °C; the structure was comparable to that of arachno-B 4 H 10 with the MCr(CO) 4N fragments in the wing-tip positions.5d The first example of a alkylidyne metallatetraborane, [NBu 4 ][WBr(CO) 2 (CR)(B 3 H 8 )] (R\C 6 H 3 Me 2 -2,6), and its facile conversion via a metal templated triboronate condensation reaction to [NBu 4 ] 2 [B 12 H 12 ] was reported.5e The reactions of nido-2,4-(CoCp*) 2 B 3 H 7 with [Fe 2 (CO) 9 ] and [Co 2 (CO) 8 ] generated, by metal fragment substitution or metal fragment degradation, the seven skeletal electron pair nido or arachno species nido-1- MCoCp*N-2-MFe(CO) 3NB 3 H 7 and arachno-MCoCp*(CO)NB 3 H 7 , respectively.5f The reaction between [Cp*TaCl 4 ] and BH 3 ·thf at 40 °C yielded the pale-red electronically unsaturated air-stable cluster nido-2-MCp*TaCl 2NB 4 H 8 with an electron count two fewer than that required for its nido five vertex structure.5g The ‘unsaturated’ chromaborane MCp*CrN2 B 4 H 8 readily underwent reduction by Na/Hg to Na[MCp*CrN2 B 4 H 8 ] and changes in the paramagnetic 11B NMR spectra of mixtures of the two species indicated that their structures were closely related.5h It has been demonstrated by Fenske–HallMOcalculations that the dinuclear MCp 2 Cr 2N fragment in MCp*CrN2 B 4 H 8 is able to provide an additional low energy (filled) or high energy (unfilled) orbital to the cluster bonding with only a small distortion of the MCr 2 B 4N cluster core geometry.5i A new coordination mode for [closo-B 6 H 6 ]2~ has been described in the compound [NBu 4 ] 2 [Cd(B 6 H 6 ) 2 ] where the Cd(II) centre is symmetrically coordinated by two hexaborate clusters each g3-bound via one MB 3N face.5j A capped nido geometry, based upon a square-based pyramid, was obtained for MRuCp*N3 B 3 H 8 where a BH fragment capped the M1,2,3-Ru 3N triangular face; the cluster was formed in the reaction between [Cp*RuCl 2 ]n with Li[BH 4 ].5k The targeted high-yield synthesis of a number of new clusters was achieved by the controlled addition of MBHN, MFe(CO) 3N or MCo(CO) 3N fragments to dinuclear Group 6 metallaboranes; the synthesis and characterisation of (Cp*Cr) 2 B 5 H 9 , (Cp*Cr) 2 B 4 H 8MFe(CO) 3N, (Cp*Cr) 2 B 4 H 7MCo(CO) 3N and (Cp*Mo) 2 B 5 H 9MFe(CO) 3N have been described.5l The metallaboranes closo-(Cp*Re) 2 B 7 H 7 and closo- (Cp*W) 2 B 7 H 9 were found to display unusual and identical core structures and to have skeletal electron-pair counts of n[3; their structures were considered as molecular metallaborane counterparts of hypoelectronic main-group cluster Zintl phases.5m The first bimetallanonaborane, arachno-6,8-M(dppe)PtN2 B 7 H 11 , was characterised as the final product from the reaction of [PtCl 2 (dppe)] with nido-B 5 H 9 ; its structure was based on the uncommon n-B 9 H 15 cluster framework.5n The monometalla and bimetalla clusters 1-(g6-Pr*C 6 H 4 Me)-isocloso-1-RuB 9 H 9 and 1,5-(g6-Pr*C 6 H 4 Me) 2 -isocloso- 1,5-Ru 2 B 8 H 8 were reported as the first ‘clean’ examples of such cage geometries. 5o The bimetallaundecaboranes nido-[7,7-(PMe 2 Ph) 2 -9-(g6-Pr*C 6 H 4 Me)-7,9- Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 23–43 26Fig. 2 Drawing of [(PMe 2 Ph) 2 PtB 16 H 17 PtB 10 H 11 (PMe 2 Ph)] with P-organyl groups omitted for clarity (reproduced by permission from J. Chem. Soc., Dalton Trans., 1998, 2777).PtRuB 9 H 11 ] and closo-[1-(g6-Pr*C 6 H 4 Me)-4,4-(PMe 2 Ph) 2 -1,4-RuPtB 9 H 9 ], each formally with two skeletal electrons fewer than expected for the observed geometries, were obtained from the reaction between nido-[6-(g6-Pr*C 6 H 4 Me)-6-RuB 9 H 13 ], KH, and [PtCl 2 (PMe 2 Ph) 2 ].5p The 11-atom boron rich cluster MCp*WN3 B 8 H 9 was obtained as a co-product with MCp*WN2 B 5 H 9 from the pyrolysis reaction of Cp*H 3 WB 4 H 8 ; the larger cluster’s structure was interpreted as close-packed.5q The synthesis of some new stanna- and germa-undecaboranes was described and the reaction of MX 2 (SnBr 2 or Gel 2 ) with a thf solution of Na 2 [B 10 H 12 ] a§orded the airand moisture-sensitive salts Na[7-X-7-MB 10 H 12 ] from which metathesis with [Ph 3 PMe]Br gave [Ph 3 PMe][7-X-7-MB 10 H 12 ]; treatment of [7-Br-7-SnB 10 H 12 ]~ with MeLi at [78 °C in thf gave the linked anionic species [7,7@-(SnB 10 H 12 ) 2 ]2~ which had the clusters joined by an Sn–Sn bond.5r The synthesis and molecular structures of 11-vertex metallaboranes with exocyclic thiobenzoate rings was described with reports of closo-(PPh 3 )(PhCOS) 2 RuB 10 H 8 ·CH 2 Cl 2 5s and nido- [(PPh 3 )(PhCOS)PtB 10 H 11 ·0.5CH 2 Cl 2 .5t The molecular structure of closo- MCpNiN2 B 10 H 10 has been reported; the Ni–Ni distance of 2.4233Å, although similar to that found in other dinickel metallaborane clusters, was the longest yet reported.5u Treatment of [Me 3 NH][nido-B 11 H 14 ] with 4 equivalents of lithium alkyl in thf at 5 °C followed by addition of [PdBr 2 (PMe 2 Ph) 2 ] gave in low yield the 12-vertex 1,4-Br 2 - Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 23–43 271,2,5-(PMe 2 Ph) 3 -closo-1-PdB 11 H 8 cluster as the first metallaborane complex to contain a doubly charge-compensated ‘ollide’ ligand.5v Multiple cluster fusion between [arachno-(PMe 2 Ph) 2 PtB 8 H 12 ] and molten nido-B 10 H 14 resulted in the novel macropolyhedral 26-boron species [(PMe 2 Ph) 2 PtB 16 H 17 PtB 10 H 11 (PMe 2 Ph)] (Fig. 2) in low yield.5w The syntheses of [NEt 4 ] 2 [Pt(anti-B 18 H 20 ) 2 ] and [PSH] 2 [Pt(syn- B 18 H 20 ) 2 ] from [Pt(cod)Cl 2 ] and anti-B 18 H 22 and syn-B 18 H 22 respectively have been reported with their crystal structures revealing intramolecular steric crowding and unusual intimately packed anion layers.5x The co-thermolysis reaction between [arachno-(PMe 2 Ph) 2 PtB 8 H 12 ] and anti-B 18 H 22 in benzene solution resulted in cluster fusion and generation of the contiguous triple-cluster species [(PMe 2 Ph)PtB 26 H 26 (PMe 2 Ph)] which consisted of a nido 11-vertex MPtB 10N cage, fused with a MPtB 2N triangular face in common, to a second nido-MPtB 10N cage, which in turn was fused to a nido 10-vertex MB 10N cage via a MB 2N edge.5y Heteroboranes The reader is directed to a Royal Society of Chemistry publication, Specialist Periodical Report Organometallic Chemistry for a comprehensive review of the 1998 literature concerning carbaboranes.2a Monocarbaboranes are reported first.6a–d A series of halogen derivatives of the monocarbaboranes, nido-7-Me 3 N-7-CB 10 H 12 , closo-2-Me 3 N-2-CB 10 H 10 and closo- 1-Me 3 N-1-CB 11 H 11 have been reported by electrophilic iodination or bromination (AlCl 3 –X 2 ) or electrophilically induced substitution (HI–AlCl 3 ) of nido-7-Me 3 NCB 10 H 11 with subsequent oxidation, or insertion by reaction with PhBCl 2 , of the readily deprotonated nido cage halogenated products.6a Treatment of the unsolvated [CpA2 Ln] (Ln\Sm, Eu) or [MCpA2 LnIN2 ] with, respectively, 1 or 2 equivalents of [Ag(CB 11 Br 6 H 6 )] in pure toluene at room temperature gave the ‘Lewis-base free’ cationic lanthanide metallocene complexes [CpA2 Ln][CB 11 Br 6 H 6 ] in good yield.6b Trialkylation of 1-amino-1-carba-closo-dodecaborane with 1-bromo-3,3-bis(2-bromoethyl) octane a§orded in 29% yield the closo quinuclidin-1-yl derivative 1-(4-penthylquinuclidin- 1-yl)-1-carba-closo-dodecaborane; this was iodinated to the corresponding closo 12-iodo species in 40% yield and was characterised by X-ray crystallography.6c Comprehensive ab initio calculations at the RMP2(fc)/6-31G* level on the closo-monocarborane anions [CBn~1 Hn]~ and the neutral closo-dicarbaboranes, C 2 Bn~2 Hn, (n\5–12) showed that the relative energies of all the positional isomers agreed with the qualitative connectivity considerations of Williams, and with the topological charge stabilization rule of Gimarc.6d The highly fluorinated, chlorinated, brominated and iodinated icosahedral carbaborane anions [1-H-CB 11 X 11 ]~, [1-MeCB 11 X 11 ]~ (X\F, Cl, Br, and I) and [1-BrCB 11 Br 11 ]~ have been described; 6e,f the fluorinated derivatives were weakly coordinating and the structure of [NBu/ 4 ][ClCuCB 11 F 11 ] was determined by X-ray di§raction methods.6f Dicarbaboranes are considered next.7,8a–d The molecular structure of nido-1,2- C 2 B 3 H 7 was determined in the gas phase by electron di§raction; this dicarbaborane was the principal volatile product from the quenched gas-phase reaction of B 4 H 10 with ethyne at 70 °C whilst other carbaboranes identified in this complicated reaction included 2,3-C 2 B 4 H 8 , 2-Me-2,3,4-C 3 B 3 H 6 , 4-Me-2-CB 5 H 8 , 2,4-Me 2 -2,3-C 2 B 4 H 6 , 5- Et-2,3-C 2 B 4 H 7 , 2,5-Me 2 -2,3-C 2 B 4 H 6 , 4-Et-2,3-C 2 B 4 H 7 , 1-Me-2,3,4-C 3 B 3 H 6 , 2-Me-2- Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 23–43 28CB 5 H 8 , 3-Me-2-CB 5 H 8 , and 2,3,4,5-C 4 B 2 H 6 .7a The experimentally observed C 2 B 4 Et 6 H 2 , claimed to be the first nido-2-carbapentaborane(8) derivative, has been shown by the ab initio/GIAO/NMR method to be a substituted nido-2,4-dicarbahexaborane( 8) cluster.7b A computational study of the reaction of B 4 H 10 with ethene leading to the basket compound (CH 2 CH 2 )B 4 H 8 has been undertaken and the preferred mechanism found to involve H 2 dissociation from B 4 H 10 as the rate determining step.7c A convenient route to carbon-substituted derivatives of nido-5,6-C 2 B 8 H 12 , based upon the reaction of 4-(Me 2 S)-arachno-B 9 H 13 with alkynes in toluene under reflux, was reported.7d A topological analysis of the electron density distribution in the crystal of 8,9,10,12-F 4 -1,2-C 2 B 10 H 8 was performed with the aid of high resolution low-temperature (120 K) single-crystal X-ray di§raction data; it was concluded that the fluorine atoms cause considerable redistribution of electron density within the molecule with a shift from the more electron rich C–C bond to B–C bonds.7e The analysis of 13C NMR data and UV–VIS spectra for a series of substituted aryl-pcarborane (1,12-dicarba-closo-dodecaborane) derivatives established that the p-carborane moiety was able to transmit electronic e§ects.7f Deboronation of o-carborane to [nido-C 2 B 9 H 12 ]~ was achieved in high yield by the use of KF supported on alumina in dry acetonitrile; addition of [PPh 3 Me]Br to the reaction solution allowed isolation of the product as [PPh 3 Me][nido-C 2 B 9 H 12 ].7g In the presence of aqueous [NBu 4 ]F the addition of o-carborane to various aldehydes proceeded smoothly at room temperature and gave in high yields the corresponding carbinols.7h The Barton reaction has been adapted to carborane chemistry in a report where the reaction of deca-B-methyl-1-hydroxymethyl-1,12-dicarbadodecaborane(12) with NOCl followed by UV light to yield nona-B-methyl-1-hydroxymethyl-2-hydroxyimino-1,12-dicarbadodecaborane( 12) was described.7i There were a number of reports of polyhedral ligands and their complexes where either a substituted [nido-7,8-C 2 B 9 H 12 ]~ cage7j–m,x–z,8a or a substituted closo-1,2- C 2 B 10 H 12 cage7n–w supported substituents with donor atoms of P, N, S, or Si; examples included [7,8-(PPh 2 ) 2 -7,8-C 2 B 9 H 11 -7-(AuPPh 3 )-8-(AuC 6 F 5 )]·CH 2 Cl 2 ,7j [IrCp*Cl(7,8-l-(SCH 2 CH 2 S)-7,8-C 2 B 9 H 10 )],7k [RhCp*Cl(7,8-l- (SMCH 2 CH 2 (OCH 2 CH 2 ) 3NS)-7,8-C 2 B 9 H 10 )],7k [Ru(p-cymene)ClM7,8-(SPh) 2 -7,8- C 2 B 9 H 11N],7l [Pd 2 (l-Cl) 2M7,8-(PPr* 2 ) 2 -7,8-C 2 B 9 H 10N2 ],7m 2,6-bisM[(1-methyl-1,2- dicarba-closo-dodecaboranyl)thio]methylNpyridine,7n [AgM1,2-(C 5 H 4 NS) 2 -1,2- C 2 B 10 H 10N(PPh 3 )]OTf,7o [Au 2 (1-S-1,2-C 2 B 10 H 11 ) 2 (l-dppe)],7p rac- and meso-1,2- (PPhH) 2 -1,2-C 2 B 10 H 10 ,7q 1-(PPhH)-1,2-C 2 B 10 H 12 ,7q l-S-1,2-(PMSNPh) 2 -1,2- C 2 B 10 H 10 ,7q rac-[MMo(CO) 4N-1,2-(PPhH) 2 -1,2-C 2 B 10 H 10 ],7r rac- and meso- [MCuCl(PPh 3 )N-1,2-(PPhH) 2 -1,2-C 2 B 10 H 10 ],7r [1-MAuClN-1-PPh 2 -2-Ph-1,2- C 2 B 10 H 10 ],7s [MNi(PEt 3 ) 2N-1,2-(SiMe 2 ) 2 -1,2-C 2 B 10 H 10 ],7t 4@-(ortho-carboranyl)- 2,2@: 6@,2A-terpyridine,7u [4-MeC 5 H 4 NMe] 2 [Pd(S 2 C 2 B 10 H 10 )I 2 ],7v and (R,R/S,S)- [PdCl 2 (1,2-PPhCl) 2 C 2 B 10 H 10 ].7w The synthesis and applications in organolanthanide chemistry of the new versatile ligand Me 2 Si(Cp)C 2 B 10 H 11 , obtained from the reaction of Me 2 SiCpCl with Li 2 [C 2 B 10 H 10 ] followed by hydrolysis, together with the analogous monoanionic, dianionic and trianionic species was described.8b Treatment of Li[RC 2 B 10 H 10 ] (R\Me, Ph) with [M(CO) 6 ] (M\Cr, W) followed by quenching with [Me 3 O][BF 4 ], a§orded in moderate yields the first examples of a new class of ‘Fischer-type’ carbene complexes, [(CO) 5 MMC(OMe)(RC 2 B 10 H 10 )N].8c The dicarbaborane derivative [rac-Zr(g5;g1-CpCMe 2 CB 10 H 10 C) 2 ], prepared from the reaction Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 23–43 29of [ZrCl 4 ] with Li 2 [CpCMe 2MCB 10 H 10 CN], catalysed the formation of syndiotactic poly(methyl methacrylate) in thf in the absence of any alkylating reagent or cationic centre generator.8d A few papers appeared during 1998 concerned with carbaboranes containing more than two carbon atoms.Hydroboration of diethyl(1-propynyl)borane with tetraethyldiborane( 6), in the presence of catalytic amounts of tributyl- or trimethyltin chloride gave organosubstituted carbaboranes with either 2,3,4- or 2,3,5-tricarba-nido-hexaborane( 7) skeletons along with polymeric material and the known 1-carba-arachnopentaborane( 10) derivative and pentaethyl-1,5-dicarba-closo-pentaborane(5).8eA new hexacarborane system based on the hexacarba-arachno-dodecaborane(12) structure, was also described.8f A few reports appeared during 1998 concerned with heteroboranes containing heteroatoms other than, or in addition to, carbon.The transformation of 3,4- bis(isopropylidene)-2,5-dichloro-1,2,5-dithiaborolane, obtained from 3,4-bis(dichloroboryl)- 2,5-dimethylhexa-2,4-diene and (Me 3 Si) 2 S, by reaction with Li[RBH 3 ] (R\H, Ph, C 6 H 4 Me) to the corresponding derivatives of nido-4,5-diisopropyl-2,4,5- thiadicarbahexaborane in low yield was reported; the formation of the nine-vertex cluster closo-5,6-Pr* 2 -4,5,6-SC 2 B 6 H 6 was also detected by GC-MS and identified by the ab initio/IGLO/NMR method.9a The single-crystal molecular structure of the mono-ligand adduct 9-(PPh 3 )-arachno- 6-SB 9 H 11 has been reported.9b The primary amines RNH 2 (R\Bu/, Pr*, Bu5) were found to react with 4-arachno-B 9 H 13 (SMe 2 ) to yield the hypho-type azaboranes, (RH 2 N)B 8 H 11 NHR, in an analogous manner to that reported for R\Et; the crystal structures of (Pr*H 2 N)B 8 H 11 NHPr* and (Pr*H 2 N)B 8 H 11 NHBu5 were reported.9c Treatment of an acidified solution of [nido-7,8-C 2 B 9 H 12 ]~ with Na[NO 2 ] at 0°C resulted in the formation of two 11-vertex dicarbaazaundecaboranes, nido-10,7,8- NC 2 B 8 H 11 and arachno-1,8,11-NC 2 B 8 H 13 in yields of 15 and 35% respectively; the arachno cage was converted in 68% yield to the nido species by treatment with PS and acetone.9d A range of nido 11-vertex thia- and phospha-boranes, thia- and phosphadicarbaboranes, and the first thiaphosphaborane, nido-10-Ph-7,10-SPB 9 H 9 , have been produced by synthetic sequences involving the reaction of an organophosphorus dihalide or sulfur dihalide with monoanionic boron clusters followed by in situ dehydrohalogenation initiated by PS.9e The reaction of closo-1,2-(MeSi) 2 B 10 H 10 with [Zr(NEt 2 ) 4 ] in thf gave a surprising closo 12-vertex cluster anion adduct as observed in the salt [Zr(NEt 2 ) 3 (thf) 2 ][(Et 2 N)(MeSi) 2 B 10 H 10 ] (Fig. 3); the Si–Si cluster edge was bridged by the diethylamido group.9f A number of sulfur containing macropolyhedral double-cluster species have been reported: thus, treatment of [S 2 B 17 H 18 ]~ with oxidising acids quantitatively yielded [S 2 B 17 H 17 ] characterised as its anion [S 2 B 17 H 16 ]~,9g and the double cluster anion [S 2 B 18 H 19 ]~ was prepared from the interaction of elemental sulfur with [syn-B 18 H 21 ]~.9h 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 Non-carbon containing metallaheteroboranes are described first.Reaction between Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 23–43 30Fig. 3 PLATON representation of the cluster anion [(Et 2 N)(MeSi) 2 B 10 H 10 ]~ (reproduced by permission from Angew.Chem., Int. Ed., 1998, 37, 1412). arachno-4,6-S 2 B 7 H 9 and the metal organophosphine complexes [Pt(PPh 3 ) 3 ], [PtCl 2MP(OMe) 3N2 ] and [RhCl(PPh 3 ) 3 ] yielded the ten-vertex MMS 2 B 7N cluster compounds [9,9-(PPh 3 ) 2 -9,6,8-PtS 2 B 7 H 7 ], [9,9-MP(OMe) 3N2 -9,6,8-PtS 2 B 7 H 7 ] and [9,9- (PPh 3 ) 2 -9,6,8-RhS 2 B 7 H 8 ]; heating a solution of the rhodadithiadecaborane in benzene isomerised it to [5,5-(PPh 3 ) 2 -5,6,10-RhS 2 B 7 H 8 ].10a An 11-vertex MM 2 S 2 B 7N cluster [(PPh 3 ) 2 HRh(PPh 3 )ClS 2 B 7 H 7 ] was also reported.10a The ten-vertex oxametallaborane [9,9-(PMe 2 Ph) 2 -arachno-9,6-PtOB 8 H 10 ] was found by X-ray crystallography to have the classical arachno ten-vertex geometry, with the MON and MPt(PMe 2 Ph) 2N vertices in the prow 6- and 9-positions, and with the oxygen bound contiguously to three boron atoms.10b The orange–yellow [6-Cp*-nido-6-RhB 9 H 12 -l- 8,9-(NEt 2 )] and yellow [5-Cp*-nido-5-RhB 9 H 12 -l-9,10-(NEt 2 )] cluster compounds, obtained by reaction of 4-(NHEt 2 )-arachno-B 9 H 13 with [MRhCl 2 Cp*N2 ] and NaH, have been characterised by single-crystal X-ray di§raction studies and NMRspectroscopy. 10c The structures of [8,8-(PPh 3 ) 2 -8,7-nido-RhSB 9 H 10 ] and [9,9-(PPh 3 ) 2 -9,7,8- RhC 2 B 8 H 11 ] were analysed by RMS-misfit calculations which in apparent contravention of Wade’s rules confirmed their 11-vertex cluster geometries as nido.10d The unique cluster compound [2-I-2-(Bu5NC)-3-(Bu5NHCH)-closo-2,1-PdTeB 10 H 9 ] contained the secondary carbene (Bu5NHCH) bonded to a cage boron atom.10e The monometallic closo cluster anion in [NEt 4 ][1-MRu(CO) 3N-2-(MeSi)B 10 H 10 ] was obtained in almost quantitative yield from the interaction of 3 equivalents of [NEt 4 ]- [MeSiB 10 H 12 ] with 1 equivalent of [Ru 3 (CO) 12 ]; equimolar amounts of the two reagents a§orded [NEt 4 ][Ru 3 (CO) 8 (g5-MeSiB 10 H 10 )].10f The platinathiaborane species [(PMe 2 Ph) 2 PtSB 8 H 12 ], [(PMe 2 Ph) 2 PtSB 10 H 10 ] and [(PMe 2 Ph) 2 PtS 2 B 15 H 14 (NHCOMe)] were obtained by reaction of [PtMe 2 (PMe 2 Ph) 2 ] with arachno-4-SB 8 H 12 , nido-7-SB 10 H 12 , and (anti)-9,9@- Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 23–43 31Fig. 4 Molecular structure of the macropolyhedral anion [Cp*IrSB 18 H 19 ]~ (reproduced by permission from Inorg.Chem. Commun., 1998, 1, 97). S 2 B 16 H 16 , respectively.10g Reaction of syn-[Cp*IrB 18 H 20 ], obtained from [MIrCl 2 Cp*N2 ] with syn-B 18 H 22 and base, with elemental sulfur a§orded by direct heteroatom insertion the 20-vertex cluster anion [Cp*IrSB 18 H 19 ] (Fig. 4).10h Small metallacarbaborane species are considered next. The disodium or dilithium salts of [nido-2,4-(SiMe 3 ) 2 -2,4-C 2 B 4 H 4 ]2~ reacted with gaseous HCl in 1: 1 stoichiometry in thf and a§orded the monoprotonated species [1-M-(thf) 2 -2,4- (SiMe 3 ) 2 -2,4-C 2 B 4 H 5 ] (M\Na, Li) in excellent yields.10i Pale-yellow crystals of [1,1-(Bu5OH) 2 -1-(Bu5O)-2,3-(SiMe 3 ) 2 -l-4,5-MLi(thf)ClN-closo-g5-1-Sm-2,3-C 2 B 4 H 4 ]· thf were obtained from reaction of anhydrous SmCl 3 with the dilithiocarbaborane [closo-exo-l-4,5-Li(thf) 2 -1-Li(thf) 2 -2,3-(SiMe 3 ) 2 C 2 B 4 H 4 ] in 1: 1 ratio in dry benzene at 0 °C.10j A synthetic spectroscopic and structural investigation of C-trimethylsilylsubstituted half- and full-sandwich magnesacarbaboranes of 2,3- and 2,4-MC 2 B 4N carbaborane ligands was reported.10k The dichlorotantalum cluster species [CpCl 2 Ta(Et 2 C 2 B 4 H 4 )] gave the dimeric system [MCpTa(H)(Et 2 C 2 B 4 H 4 )N2 -l-Cl] upon treatment with Li[AlH 4 ] in thf; this hydrido species underwent alkyne insertion with p-tolyl acetylene and generated exclusively [trans-CpCl(p- MeC 6 H 4 CH––CH)Ta(Et 2 C 2 B 4 H 4 )].10l The thermally stable tantalum carbaborane complex [CpTaMe 2 (Et 2 C 2 B 4 H 4 )] was cleanly converted to the vinyl tantalum species Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 23–43 32Fig. 5 Molecular structure of [CpCrMl-g6:g6-(l-1,2-C 3 H 6 -1,2-C 2 B 4 H 4 )NCtrCp] (reproduced by permission from Inorg. Chem., 1998, 37, 608). [CpTaMe(CR––CRMe)(Et 2 C 2 B 4 H 4 )] by photochemical insertion of alkynes; the analogous diphenyl complex [CpTaPh 2 (Et 2 C 2 B 4 H 4 )] was thermally active, and reacted by elimination of benzene followed by trapping reactions involving alkynes and the derived benzyne intermediate.10m The enyne insertion of HC–– – CCMe–– CH 2 with arachno-[(CO)(PMe 3 ) 2 (H)Ir(B 8 H 12 )] in xylene at 138 °C a§orded two iridadicarbaboranes [7-MCMeCH 2N-9,9,9-(CO)(PMe 3 ) 2 -nido-9,7,8-IrC 2 B 8 H 10 ] and [5,5,5- (CO)(PMe 3 ) 2 -l-6,7-MCHCMeCH 2N-nido-5,6-IrCB 8 H 11 ] in low yields.10n The synthesis and crystal structure of the EPR-silent polymeric caesium carbaborane system [Mexo-Cs(tmen)-1-Cs-2,4,7,9-(SiMe 3 ) 4 -2,4,7,9-C 4 B 8 H 8Nn] has been reported.10o The synthesis of molybdenum and tungsten carbonyl multidecker sandwiches e.g.[Cp*Co(Et 2 C 2 B 3 H 3 )M(CO) 4 ] (M\Mo, W), [MCp*Co(Et 2 C 2 B 3 - H 3 )N2 Mo(CO) 2 ] were described with the compounds being characterised by X-ray di§raction studies.10p The triple-decker complex [(CpCr) 2 -l-g6:g6-(l-1,2-C 3 H 6 -1,2- C 2 B 4 H 4 )N] (Fig. 5) with an unusual 24-electron count contained a metal-stabilised planar tetraborabenzene bridging ligand.10q There were a number of reports of metalladicarbaborane clusters based upon Annu. Rep. Prog. Chem., Sect.A, 1999, 95, 23–43 33Fig. 6 A view of the ‘head-set’ and ‘ear-mu§’ molecular structure of [M(nido- C 2 B 9 H 11 )Zn(NMe 3 )N2 ] (reproduced by permission from Chem. Commun., 1998, 1713). derivatives of the icosahedral 12-vertex closo MMC 2 B 9 H 11N11a–e or MMC 2 B 9 H 10 LN11f–j cage structures, examples include: [3,3-(PEt 3 ) 2 -1-Ph-3,1,2- PtC 2 B 9 H 10 ]·0.5CH 2 Cl 2 ,11a [Cp*(g5:g1-4-CHMeO-C 2 B 9 H 10 )Ti],11b [Ru(CO) 2 (MeCCPh)(g5-7,8-C 2 B 9 H 11 )],11c [1-M(g5-C 5 H 4 )FeCpN-3-(p-cym)-3,1,2- RuC 2 B 9 H 10 ],11d [1-C 4 H 3 S-3-(cod)-3,1,2-MC 2 B 9 H 10 ] (M\Pd, Pt),11e [3-(PPh 3 )- 3,3-(I) 2 -4-SMe 2 -3,1,2-RhC 2 B 9 H 10 ],11f [3-Cp*-4-SMe 2 -3,1,2-RuC 2 B 9 H 10 ],11g [3-(g3- C 3 H 5 )-3,3-(CO) 2 -4-SMe 2 -3,1,2-MoC 2 B 9 H 10 ],11h [FeMC 2 B 9 H 10 (SMe 2 )N2 ],11i and [1,8-Ph 2 -2-(g3-C 3 H 5 )-2,2-(CO) 2 -6-SMe 2 -2,1,8-MoC 2 B 9 H 8 ].11j The alkane elimination reaction between ZnMe 2 and [NMe 3 H][C 2 B 9 H 12 ] gave the macropolyhedral dimer [M(nido-C 2 B 9 H 11 )Zn(NMe 3 )N2 ], which contained an unprecedented planar diamond- shaped MZn 2 B 2N ring at its core (Fig. 6).11k The synthesis and structural characterisation of the thioether bridged commometallabis( dicarbollide) species, [3,3@-Co-(l-1,1@-MCH 2 SCH 2N-1,2-C 2 B 9 H 10 ) 2 ]~, was described and developed as a model for ‘venus-flytrap’ radiotransition metal carriers. 11l The cobaltabis(dicarbollide) [3,3@-Co-(1-Me-2-R-1,2-C 2 B 9 H 9 ) 2 ]~ (R\MCH 2N6 OMCH 2N3 Me) showed a higher e¶ciency in transport of 152Eu at low acidity than the well known calixarene derivatives.11m The sandwich complexes [3,3@- Co(1-Ph-1,2-C 2 B 9 H 10 ) 2 ]~ and [3,3@-Co(1,7-Ph 2 -1,2-C 2 B 9 H 9 ) 2 ]~ were tested for liquid–liquid extraction and transport for 137Cs, 90Sr, and 152Eu;11n [3,3@-Co-(1-Me-2- M(CH 2 ) 3 OEtN-1,2-C 2 B 9 H 9 ) 2 ]~ was also tested for its 137Cs- and 90Sr-extracting capacity. 11o Electrochemical studies on [Li(thf) 4 ] 2 [(g5-C 2 B 9 H 11 ) 2 UBr 2 ] were also reported. 11p Metallamonocarbaboranes are reported next. The synthesis and crystal structures of the platinacarborane complexes [PtCl(PMe 2 Ph) 2 (g5-7-CB 10 H 11 )], [Pt 2Ml- Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 23–43 34r,g5: r,g5{-8,9@-I(H)-(7-CB 10 H 10 ) 2 (PMe 2 Ph) 4 ] and [Pt 2 (PEt 3 ) 4Mg5:g5{-9,9@-I(H)-(7- CB 10 H 10 ) 2 ] were described following protonation reactions of [Na][Pt(PR 3 ) 2 (g5-7- CB 10 H 11 )] salts;12a reactions of the PEt 3 derivative with PhSeCl, PhSeSePh, and PhTeI yielded products with chalcogen substituents on platinum, or platinum and boron.12b The salts [X] 2 [Re(CO) 3 (g5-CB 10 H 11 )] [X\N(PPh 3 ) 2 , NEt 3 (CH 2 Ph)] were synthesised from the reaction of Na 3 [nido-7-CB 10 H 11 ] with [ReBr(thf) 2 (CO) 3 ] followed by addition of [X]Cl.12c Neutral bimetallic species [RePt(CO) 3 L 2 (g5- CB 10 H 11 )]12c (L\PPh 3 , PEt 3 , 1/2dppe) and [ReM(CO) 3 Cp*(g5-CB 10 H 11 )]12d (M\Rh, Ir) were prepared by treatment of [N(PPh 3 ) 2 ] 2 [Re(CO) 3 (g5-CB 10 H 11 )] with [PtCl 2 L 2 ] in the presence of Tl[PF 6 ], or [M(NCMe) 3 Cp*][BF 4 ] 2 , respectively; CO/PR 3 exchange reactions of the Group 9 bimetallic clusters were also reported.Treatment of Na[MeSi(C 2 B 10 H 11 )(C 5 H 4 )] with SmI 2 (thf)x gave [MeSi(C 2 B 10 H 11 )(C 5 H 4 )Sm(thf) 2 ] as the first structurally characterised example of a mixed lanthanacarborane incorporating g5-cyclopentadienyl and g6-carboranyl ligands.12e The three new metallacarboranes [closo-1-CoCp-2-(NC)-2,3-C 2 B 10 H 11 ], [nido-2-FeCp-8-(CH 3 OCO)-6,7,8,9-C 4 B 7 H 11 ] and [arachno-CoCp-8-(CH 3 OCO)- 7,8,9,10-C 4 B 7 H 12 ], with cage geometries derived from supraicosahedral frameworks, were produced by metal insertion reactions into the [arachno-8-(NC)-7,8-C 2 B 10 H 14 ]~ and [arachno-8-(MeOCO)-7,8,9,10-C 4 B 8 H 12 ]~ anions.12f 4 Organometallic boron species General The synthesis and structure of the first terminal-borylene complexes were reported with a ‘nearly linear’ arrangement observed for the W–B–N unit of [(CO) 5 WBN(SiMe 3 ) 2 ].13a Treatment of Cp*BCl 2 with K 2 [Fe(CO) 4 ] gave [(CO) 4 FeMBCp*N]; a single-crystal X-ray di§raction study confirmed that the complex displayed a Fe–B r bond and that the Cp* was g5-bonded to boron.13b The reaction of aminodichloroboranes, R 2 NBCl 2 , with Na[C 5 R 5 Fe(CO) 2 ] yielded, depending upon the nature of the amino group bound to boron, either boryl or bridging borylene complexes of iron.13c The reaction of the organodiborane M(l-H)(BC 5 H 10 )N2 with Lewis bases NMe 3 , PMe 3 , NH 3 , orH~ produced cyclic adducts L·HBC 5 H 10 through symmetric cleavage of the hydrogen-bridge bonds.13d The synthesis of a series of cyclic hydroborate complexes of metallocenes have been described and the structures of zirconoceneboracyclohexane derivatives [Cp 2 Zr(X)M(l-H) 2 BC 5 H 10N] (X\H, Me, H 2 BC 5 H 10 )] reported.13e Pentafluorophenylborane derivatives The neutral compound H 2 O·B(C 6 F 5 ) 3 ·2H 2 O was formed in the interaction of H 2 O with B(C 6 F 5 ) 3 whilst the reaction of KOH/H 2 O in the presence of dibenzo-18- crown-6 gave [K(dibenzo-18-crown-6)][(HO)B(C 6 F 5 ) 3 ] co-crystallised with H 2 O·B(C 6 F 5 ) 3 ; the new binuclear borate anion [M(C 6 F 5 ) 3 BN2 (l-OH)]~ was also reported. 13f The phosphorus ylide Ph 3 P––CH 2 when reacted with B(C 6 F 5 ) 3 a§orded the zwitterionic species Ph 3 PCH 2 B(C 6 F 5 ) 3 characterised by an X-ray crystal structure Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 23–43 35analysis.13g The interaction of B(C 6 F 5 ) 3 with oxo- and peroxo-molybdenum, and nitrido-rhenium and -osmium complexes has been reported and the products, typified by cis-[MoOMOB(C 6 F 5 ) 3N(g2-ONEt 2 ) 2 ]13h and [ReMNB(C 6 F 5 ) 3N(PMe 2 Ph)(S 2 CNMe 2 ) 2 ] were described.13i The reaction of [MCp@YMeN2 ] (Cp@\Cp, C 5 H 4 SiMe 3 ) with B(C 6 F 5 ) 3 a§orded the complexes [Cp@YMMeB(C 6 F 5 ) 3N] in which the ‘anion’ was co-ordinated to the metal via one ortho-fluorine atom and agostic interactions from two of the methyl hydrogens; these compounds acted as initiators for carbocationic polymerisation of isobutene.13j The zwitterionic compound [Cp 2 Zr(PPh 2 Me)CH 2 CH 2 B(C 6 F 5 ) 3 ], formed from the reaction of B(C 6 F 5 ) 3 with [ZrCp 2 (g2-CH 2 CH 2 )(PPh 2 Me)], was an ethene polymerisation catalyst either with or without additional B(C 6 F 5 ) 3 .13k Addition of B(C 6 F 5 ) 3 to [(ArO) 2 TiMe 2 ] gave unstable species which were also able to polymerise ethene and propene.13l The cationic benzyl derivative of [(C 10 H 6 CH 2 ) 2 NMe]Zr(CH 2 Ph) 2 with [B(C 6 F 5 ) 4 ]~ as counter ion catalytically polymerised olefins, whereas with the g6-[PhCH 2 B(C 6 F 5 ) 3 ]~ counter ion the system was inactive.13m The triamide complex, [NEt 4 ]- [MC 5 H 4 B(C 6 F 5 ) 3NZr(NMe 2 ) 3 ], formed from [NEt 4 ][C 5 H 5 B(C 6 F 5 ) 3 ] and [Zr(NMe 2 ) 4 ] in good yield, was readily converted to the crystallographically characterised dimeric trichloride [NEt 4 ] 2 [(MC 5 H 4 B(C 6 F 5 ) 3NZr(l-Cl)Cl 2 ) 2 ].13n The bifunctional boranes RCH–– CMB(C 6 F 5 ) 2N2 (R\Bu5, Ph, C 6 F 5 ) were formed from the regioselective hydroboration of corresponding 1-boraalkynes using HB(C 6 F 5 ) 2 .13o The synthesis and characterisation of the amino(pentafluorophenyl)boranes (Me 3 Si)HNB(C 6 F 5 ) 2 , HNMB(C 6 F 5 ) 2N2 , and (Me 3 Si) 2 NB(C 6 F 5 ) 2 together with the structure of the latter were reported.13p Unsaturated ring systems containing boron and related derivatives The first heterometallic borole complexes of Fe and Au were prepared by the reaction of [HFeMg5-C 4 H 4 BPhN(CO) 2 ]~ with [Au(PPh 3 )Cl] inCH 2 Cl 2 which yielded [Fe(g5- C 4 H 4 BPh)(CO) 2MAu(PPh 3 )N2 ]; the cationic MFeAu 3N cluster species with a tetrahedral metal core was obtained by further reaction with [Au(PPh 3 )Cl] and Tl[PF 6 ].14a The polymeric salt catena-[MRh(l,g5:g6-C 4 H 4 BPh)N(BF 4 )]x was obtained by loss of acetonitrile under vacuum from [Rh(NCMe) 3 (C 4 H 4 BPh)][BF 4 ].14b X-Ray structure determinations of [Cp*Mg5-C 4 H 4 BNH(CHMe 2 ) 2NHfCl(C–– – CSiMe 3 )], [Cp*Mg5- C 4 H 4 BN(CHMe 2 ) 2NHfCl(PMe 3 )] and [Cp*Mg5-C 4 H 4 B(C–– – CSiMe 3 )NHfCl(PMe 3 )] were reported.14c The reaction of TaCl 5 with 2 equivalents of AlCl 3 and Li[C 4 H 4 BN(CHMe 2 ) 2 ]·thf gave in 47% yield [MC 4 H 4 BN(CHMe 2 ) 2NTaCl 3 ], and from which a number of tantalum borollide complexes were prepared.14d The synthesis of the bridged boratabenzene zirconium complexes [ZrCl 2Mg6:g6- (C 5 H 4 BNPr* 2 ) 2 -l-CH 2 CH 2N], [ZrCl 2Mg6:g6-(C 5 H 4 BNPr* 2 ) 2 -l-SiMe 2N] and [ZrCl 2Mg5:g6-(C 5 H 4 )(C 5 H 4 BNPr* 2 )-l-CMe 2N] were reported and the bridging SiMe 2 and CMe 2 derivatives characterised by X-ray crystallogoraphy.14e The lithium salt of the new benzothiaborolide heteroaromatic anion [C 6 H 4 SC(H)B(NPr* 2 )]~ has been prepared by a multistep synthesis starting from thioanisole; the anion was converted to a MCp*RuN-g5 adduct and characterised crystallographically.14f The synthesis of the cycloheptatrienyl(dipropyl)borane, C 7 H 7 BPr 2 , was accomplished by an exchange reaction of C 7 H 7 SnMe 3 with Pr 2 BCl; it was found to equilibrate with its valence tautomer 7-exo-(dipropylboryl)norcaradiene.14g The bisboryl cobaltocenes Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 23–43 36[Co(g5-C 5 H 4 BR 2 ) 2 ] (R\NMe 2 , NEt 2 ) were prepared from [CoBr 2 (dme)] and M[C 5 H 4 BR 2 ] (M\Li, Na) and were oxidised to ionic cobaltocenium chlorides by C 2 Cl 6 .14h The synthesis of borylindenides has been achieved by borylation of the lithium indenide and the structure of [Li(N,N@,NA-Me 3 -1,3,5-C 3 H 6 N 3 )]- [1-C 9 H 6MB(NMe 2 ) 2N] was reported.14i The formation of 1,5-bis(dimethylamino) dibenzo[b,f ]-1,5-bidorocane was accomplished via sodium reduction of 1-(Me 2 NCH 2 )-2-BCl 2 C 6 H 4 .14j 5 Catecholate derivatives and boration reactions The synthesis and characterisation of a series of bis-catecholate, bis-dithiocatecholate, and tetraalkoxydiborane(4) compounds have been described together with the bis(adducts) [B 2 (cat) 2 (NHMe 2 ) 2 ] and [B 2 (1,2-O 2 -3,5-Bu5 2 C 6 H 2 ) 2 (NHMe 2 ) 2 ];15a XRay structures were reported for [B 2 (cat) 2 ], [B 2 (1,2-O 2 -4-Bu5C 6 H 3 ) 2 ], [B 2 (1,2-O 2 - 3,5-Bu5 2 C 6 H 2 ) 2 ], [B 2 (1,2-S 2 C 6 H 4 ) 2 ], [B 2 (1,2-S 2 -4-MeC 6 H 3 ) 2 ], and [B 2 (OCH 2 CMe 2 CH 2 O) 2 ] which revealed planar MB 2 O 4N or MB 2 S 4N units.15b Oxidative-addition reactions of B–B, B–Cl, and B–Br bonds to Pt(0) complexes to yield mono(boryl) Me.g.trans-[PtX(PPh 3 ) 2 (Bcat)] (X\Cl, Br)N or bis(boryl) Me.g. cis-[Pt(PPh 3 ) 2 (Bcat) 2 ]N derivatives were reported.15c Oxidative addition of [B 2 (cat) 2 ] with either [RhCl(PPh 3 ) 3 ] or [MRh(l-Cl)(PPh 3 ) 2N2 ] a§orded the colourless Rh(III) bis(boryl) species [RhCl(PPh 3 ) 2 (Bcat) 2 ]; the Rh coordination sphere was squarebased pyramidal with a boryl group apical and the two PPh 3 ligands mutually trans in the basel plane.15d A number of neutral and cationic six-coordinate osmium boryl complexes derived from [Os(Bcat)Cl(CO)(PPh 3 ) 2 ] have been prepared and characterised crystallographically.15e The reactions of [L 2 Rh(acac)] (L\alkene, triorganophosphine) with B 2 (cat) 3 cleanly yielded the zwitterionic complexes [L 2 Rh(g6-cat- Bcat)] with [(acac)Bcat]; [(dppm)Rh(g6-catBcat)] was found to be an excellent catalyst for the diboration of vinylarenes and the unstrained internal alkenes cis- and trans-stilbene, and trans-b-methylstyrene.15f The clean and quantitative platinumcatalysed diborations of a range of prochiral 1,3-dienes with the compounds [B 2MR,R-O 2 CH(CO 2 Me)CH(CO 2 Me)N], [B 2 (S-O 2 CH 2 CHPh) 2 ] and [B 2 (R,RO 2 CHPhCHPh) 2 ] were studied but the observed de values were low or non-existent. 15g Di§erences between the Pd(0) and Pt(0) catalysts and between thioboration and diboration reactions have been studied by hybrid density (B3LYP) calculations.15h 6 Boron–pnictogen species The elimination of HF from mes*B(F)CH 2 CH 2 B(F)mes* (mes*\2,4,6-Bu5 3 C 6 H 2 ) resulted in mes*B–– – NCH 2 CH 2 N–– – Bmes* as the first example of a compound containing B–– – N triple bonds; this compound was thermally stable but readily hydrated to a§ord mes*B(OH)NHCH 2 CH 2 NHB(OH)mes*.16a The preparation of a series of differently substituted 2-chloro-, 2-fluoro-, and 2-iodo-2,3-dihydro-1H-1,3,2-diazaboroles were reported.16b Eleven new 2-aminoethyl- and 3-aminopropyl-borinate derivatives with a coordinate N]B bond have been synthesised by condensation reactions between piperidine- and piperazine-alcohols and diphenylborinic acid.16c Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 23–43 37Fig. 7 A CAMERON drawing of MFe(g5-C 5 H 4 Me)(CO) 2N2 ClB 3 N 3 H 3 (reproduced by permission from Eur. J. Inorg.Chem., 1998, 291). The first tricyclic N-pyrrollylborane with an exceptionally stable B–N bond was obtained from hydroboration of 2,5-diallylpyrrole using a mixture of Et 2 BH and Et 3 B; this borane was readily converted into adducts or borates.16d Theoretical and experimental evidence for a S N 2-type mechanism for the dissociation of B–N coordination bonds has been obtained for 2,6-bis(dimethylaminomethyl)phenylborane derivatives. 16e The electronic structure of [Cr(g6-B 3 N 3 H 6 )(CO) 3 ] was investigated in detail using high quality density functional calculations; puckering of the borazine ring was associated with repulsive interactions between high lying boron dominated ring r-bonding orbitals and the metal’s ‘lone-pairs’.16f A theoretical study of the structures, energetics and magnetic properties of B 3 E 3 H 6 (E\N, P, As) and B 3 E@3 H 3 (E@\O, S, Se) has been reported and the discussion centered on the relative aromaticity of these ring compounds.16g The synthesis and structures of the first g1-borazine complexes were reported from the reaction of Na[(g5-C 5 R 5 )Fe(CO) 2 ] with Cl 3 B 3 N 3 H 3 ; Fe–B p-interactions were ruled out from spectroscopic and structural results (Fig. 7).16h New silylborazines have been prepared by the interaction between Li[Si(SiMe 3 ) 3 ] with ClMe 2 B 3 N 3 Me 3 , Cl2 MeB 3 N 3 Me 3 , Cl3 B 3 N 3 Me 3 , and Cl 3 B 3 N 3 H 3 ; pyrolysis of an oligomer obtained from M(Me 3 Si) 3 SiNCl 2 B 3 N 3 H 3 and (Me 3 Si) 2 NH in hexane resulted in composites containing BN and SixNyCz.16i Bicyclic cage compounds containing B, P and transition metals (Ni, Pd, Pt) were isolated and characterised from 1: 1 reactions of (Pr* 2 N)BP(H)B(NPr* 2 )PLi·dme and Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 23–43 38Fig. 8 Molecular structure of [B 2 O 2 (BCl 3 ) 2 (tcp)] (reproduced by permission from Angew. Chem., Int. Ed., 1998, 37, 1112). (tmp)BP(H)B(tmp)PLi·dme with [(R 3 P) 2 MCl 2 ].16j Skeletal bonding in closo-1,5- X 2 B 3 Y 3 (X\N, P; Y\NH 2 , CH 3 , H) cages was shown by bonding, energetic, and magnetic analyses to be dependent upon the substituents at boron.16k 7 Boron–chalcogen species Solid state NMR spectroscopy has provided evidence for the selective association of [Na]` with tetrahedral MBO 4N units and [H]`with trigonal MBO 3N units in the dehydrated zeolite B-ZSM-5.17a A new 1-D inorganic chain polymer [H 2 en] 4 - [Hen] 2 [V 6 B 22 O 53 H 8 ]·5H 2 O consisting of [V 6 B 20 O 50 H 6 ] cluster sub-units linked together through diborate bridges has been synthesised by a molten boric acid ‘flux’ method in which H 3 BO 3 , V 2 O 5 , and en were heated together at 180 °C for 3 days.17b Boronic acids were used for selective fluorescence detection of fluoride17c and for optical sensing properties of sugars.17d The crystal structure of the adduct (Ph 3 SiO) 3 B·Ph 3 SiOH consisted of molecules of (Ph 3 SiO) 3 B and Ph 3 SiOH linked by a weak B · · ·O (silanol) donor–acceptor bond and additionally stabilised by a OH(silanol) · · · O(siloxy) hydrogen bond; the MBO 3N fragment remained ‘trigonal’ with the interaction to the silanol axial.17e New divinyl- and diallyl-borasiloxanes have been synthesised in good yield from corresponding boronic acids and their behaviour Annu.Rep. Prog. Chem., Sect. 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