John Staves

The chemistry of isomers of icosaborane(26), B20H26: synthesis and nuclear magnetic resonance study of various isomers of platinahenicosaboranes and diplatinadocosaboranes, and the X-ray crystal and molecular structures of 7,7-bis(dimethylphenylphosphine)-nido-7-platinaundecaborane and 4-(2′-nido-decaboranyl)-7,7-bis(dimethylphenylphosphine)-nido-7-platinaundecaborane

An improved synthesis of the known compound [(PMe2Ph)2(PtB10H12)] has been developed by deprotonation of B10H14 with NNN′N′-tetramethylnaphthalene-1,8-diamine followed by treatment with cis-[PtCl2(PMe2Ph)2]. This reaction has been applied to the 2,2′, 2,6′, and 1,5′ isomers of (B10H13)2 to prepare various isomeric platinahenicosaborane clusters [(PMe2Ph)2(PtB10H11–B10H13)] which differ either in the position of the conjuncto-linkage or the site of the platinum atom in the cluster. Appropriate modification of the reaction stoicheiometry in the case of 2,2′-(B10H13)2 led to the isolation of cisoid and transoid diplatinadocosaboranes [{(PMe2Ph)2(PtB10H11)}2]. The X-ray crystal structure of [(PMe2Ph)2(PtB10H12)] showed it to contain a platinaundecaborane cluster in which the tetrahapto B10H12 group is twisted by ca. 20° with respect to the PtP2 plane. Similarly, the molecular structure of the isomer of [(PMe2Ph)2Pt(η4-B10H11–B10H13)] obtained from 2,2′-(B10H13)2 is distorted by a twist of ca. 8°. A detailed n.m.r. study of a number of these clusters has been made, using the resonances of 1H, 11B, 31P, and 195Pt. In addition to permitting structural assignments, the data reveal a novel mutual pseudo-rotation of the η4-B10H11X group (X = H or B10H13) and the (PMe2Ph)2 grouping about the central Pt atom. For [(PMe2Ph)2(PtB10H12)] the two sets of 1H-{31P} methyl resonances at 100 MHz coalesce at 71.5 °C with an implied activation energy ΔG‡ of 79 ± 5 kJ mol–1 for the fluxional process. Similar activation energies were deduced for the various isomers of [(PMe2Ph)2(PtB20H24)].

Dicarbonyl(η-cyclopentadienyl)iron(II) derivatives of pentaborane(9)

The [B5H8]– anion reacts with [Fe(η-C5H5)(CO)2I] to give [Fe(2-B5H8)(η-C5H5)(CO)2](1). This can be deprotonated with KH to give the anion [Fe(2-B5H7)(η-C5H5)(CO)2]– which can react with another equivalent of [Fe(η-C5H5)(CO)2I] to give [{Fe(η-C5H5)(CO)2}2(2,4-B5H7)](3); this is the first example of a compound with two transition-metal atoms bound to the pentaborane skeleton. Boron-11 and 1H n.m.r. and mass spectroscopic data confirm the structures assigned to (1) and (3); these are consistent with the hypothesis that nido-penta-borate anions act as 2–3-η ligands in 16-electron transition metal compounds and as 2-σ ligands in 18-electron species. The electron-impact fragmentation of the compounds is briefly discussed.

The chemistry of isomeric icosaboranes, B20H26. Molecular structures and physical characterization of 2,2′- bi(nido-decaboranyl) and 2,6′-bi-(nido-decaboranyl)

The structures of the large neutral boranes 2,2′-bi(nido-decaboranyl), 2,2′-(B10H13)2, m.p. 178 °C, and 2,6′- bi(nido-decaboranyl), 2,6′-(B10H13)2, m.p. 154 °C, have been established by single-crystal X-ray diffraction. The 2,2′ isomer is tetragonal, space group l41cd, with a= 11.901(3), c= 23.135(4)A, molecular symmetry C2; the 2,6′ isomer is orthorhombic, space group Pbca, with a= 14.673(3), b= 19.765(4), and c= 11.580(3)A. The intercluster B–B bond lengths are respectively 1.692(3) and 1.679(3)A, and there is a lengthening of 0.015 A of the adjacent cluster B–B distances when compared with the corresponding distances in decaborane(14) itself. The 1H-{11B} and 11B n.m.r. behaviour of the two isomeric conjuncto-boranes has been investigated and the results for the 2,2′ isomer are discussed in some detail. Infrared and u.v. spectroscopic data are presented. The unambiguous determination of the structures permits a discussion of possible synthetic mechanisms.

NMR Studies on Halogeno-cis-bis(phosphine)-nido-pentaboranyl Derivatives of Nickel, Palladium, and Platinum

Abstract An examination of the 1H-{11B}, 11B, 11B-{1H}, and 31P-{1H} NMR behaviour of the compounds [M(B5H8)(X)(dppe)] has been made [M = Ni, Pd and Pt; X = halogen; dppe = bis(diphenylphosphino)ethane]. Some relaxation phenomena are rationalised, and it is concluded that all the compounds examined have a static metallo -nido-penta -borane structure in which the metal atom occupies a bridging position between two basal boron atoms. Some temperature and solvent effects are also presented and briefly discussed.

Isomers of B20H26: structural characterisation by X-ray diffraction of 2,2′-Bi(nido-decaboranyl)

One of the isomers of B20H26 obtained by the photolysis of B10H14 has been shown by single-crystal X-ray structure analysis to be 2,2′-bi(nido-decaboranyl).

Reactions of decaborane derivatives with bis(diphosphine) complexes of iridium, and the crystal structure of cis-bis[1,2-bis(diphenylphosphino)ethane]dihydroiridium(III) tetradecahydrononaborate(1–), [Ir-(dppe)2H2][B9H14]

The four- and five-co-ordinate iridium(I) cations [Ir(dppe)2]+ and [Ir(CO)(dppe)2]+ react with B10H14, B10H13(Cl-6), and [B10H13]– to give a number of ionic compounds. The reaction between [Ir(dppe)2]Cl and B10H13X (X = H or 6-Cl) yields [IrIIICl(dppe)2H][B10H12X]. The complex [Ir(CO)(dppe)2]Cl reacts with [B10H13]–, in methanol solution, to give [IrI(CO)(dppe)2][B10H13] and, as a product of methanolic degradation, [IrIII(dppe)2H2][B9H14]. The latter crystallizes as pale yellow needles in the orthorhombic space group Ccca with a= 15.456(4), b= 23.936(4), c= 28.529(7)A, and Z= 8. The [B9H14]– anion is grossly disordered, but it is possible to determine the structure of the cation.

Tetraorganophosphonium and tetraphenylarsonium salts of B5H8− and B6H9−

Abstract Eight new quaternary phosphonium or arsonium salts of the polyhedral borane anions B 5 H 8 − and B 6 H 9 − have been prepared in order to study the influence of cation size on stability. The salts are, indeed, considerably more stable than those previously prepared, the most stable being the tetraphenylphosphonium and tetraphenylarsonium derivatives. IR and NMR data are reported.

Complexes of (diphosphine)halogenonickel(II) with the anions of pentaborane(9) and 1-bromopentaborane(9)

Diphosphine complexes of nickel(II) halides react readily at low temperature with K[B5H8] and K[1-BrB5H7] to give the corresponding complexes [Ni(µ-B5H8)X(dppe)] and [Ni(µ-1-BrB5H7)X(dppe)][X = Cl, Br, or I, dppe = 1,2-bis(diphenylphosphino)ethane]. The complex [Ni(µ-B5H8)Br(dppp)] has also been prepared [dppp = 1,3-bis(diphenylphosphino)propane]. The complexes are monomeric and non-conducting in solution and their n.m.r. spectra (1H, 11B, and 31P) are best interpreted in terms of a square-planar NiX(dppe) group inserted via a three-centre two-electron B–Ni–B bond into a bridging position between two basal boron atoms of the pentaborane cluster. The four basal boron atoms and three bridging hydrogen atoms are equivalent on the n.m.r. time scale, implying fluxional behaviour. In this, the nickel complexes differ from ‘isoelectronic’(16-electron) copper(I) and cadmium(II) complexes of the type [Cu(µ-B5H8)(PPh3)2] and [Cd(µ-B5H8)Cl(PPh3)] which are non-fluxional.

Cage expansion in metallopentaborane chemistry: the preparation and structure of 2-{carbonylbis(triphenylphosphine)irida}-nido-hexaborane, [(IrB5H8)(CO)(PPh3)2]

The reaction of K[B5H8] with [Ir(CO)Cl(PPh3)2] at low temperature yields [(IrB5H8)(CO)(PPh3)2] which crystallises as yellow blocks in the monoclinic space group P21/c with a= 10.097(2), b= 20.708(5), c= 16.493(3)A, β= 92.85(2)°, and Z= 4. Single-crystal X-ray diffraction analysis shows that the compound has a novel structure in which the transition-metal atom has inserted into the polyhedral pentaborane cluster; the iridium and boron atoms form a pentagonal pyramid in which the metal occupies a basal position, and the structure is therefore that of a metallahexaborane. The co-ordination number of the iridium atom is seven and the geometry is a distorted, monocapped, trigonal prism which probably results from formally octahedral hybridisation about the iridium(III) atom, one of the bonds being a two-electron three-centre bond involving the apical and a basal boron atom in the metallaborane cluster. This is consistent with formal skeletal electron-counting rules.

Copper(I), silver(I), and gold(I) complexes with nido-pentaborane anions

The 16-electron copper(I) complexes [Cu(µ-1-BrB5H7)(PPh3)2] and [Cu(µ-B5H8)(dppe)](dppe = Ph2PCH2- CH2PPh2) have been prepared and shown to be similar to [Cu(µ-B5H8)(PPh3)2] whose structure has recently been established by single-crystal X-ray analysis. The corresponding silver(I) complex [Ag(µ-B5H8)(PPh3)2] has been prepared as a colourless photosensitive solid which decomposes fairly rapidly in solution even in the absence of light; however, 11B n.m.r. spectroscopy shows that the complex is non-fluxional in dichloromethane. A gold complex, possibly [Au(B5H8)(PPh3)], is formed at –78 °C but is too unstable to isolate.