Chemistry of polynuclear metal complexes with bridging carbene or carbyne ligands. Part 84. Carbaborane tungsten–platinum complexes having a µ-CC6H3Me2-2,6 ligand; crystal structures of [WPt(µ-CC6H3Me2-2,6)(CO)n(PEt3)(µ-σ:η5-C2B9H8Me2)](n= 2 or 3)

Abstract

The reaction between the salts [PtH(Me₂CO)(PEt₃)₂][BF₄] and [NEt₄][W(CC₆H₃Me₂-2,6)(CO)₂(η⁵-C₂B₉H₉Me₂)] in acetone at low temperature (ca. –30 °C) affords the dimetal compound [WPtH(µ-CC₆H₃Me₂-2,6)(CO)₂(PEt₃)(η⁵-C₂B₉H₉Me₂)], in which the carbaborane ligand forms a B–H⇀Pt three-centre bond by employing a BH group in the pentagonal face of the cage. This product readily affords, via loss of hydrogen, the complex [WPt(µ-CC₆H₃Me₂-2,6)(CO)₂(PEt₃)(µ-σ:η⁵-C₂B₉H₈Me₂)] as a mixture of two isomers. The structure of the major isomer (ca. 80%) has been established by X-ray diffraction. The W–Pt bond [2.728(1)Å] is asymmetrically bridged by the CC₆H₃Me₂-2,6 group [µ-C–W 1.88(2), µ-C–Pt2.15(2)Å]. Indeed with a W–µ-C–C¹(C₆H₃) angle of 175(1)° these three atoms are nearly linear. The carbaborane ligand has undergone ‘slippage’ along the W–Pt vector to accommodate formation of a direct exopolyhedral B–Pt σ bond [2.01(2)Å]. This linkage involves the central boron in the [graphic omitted] face of the icosahedral fragment η⁵ bonded to tungsten. The PEt₃ ligand attached to the platinum atom [Pt–P, 2.251(7)Å] is transoid to the µ-C and W atoms [µ-C–Pt–P, 160.6(4)°, W–Pt–P 155.9(1)°]. The n.m.r. data (¹H, ¹³C-{¹H}, ¹¹B-{¹H}, and ³¹P-{¹H}) are in accord with the structure established by the X-ray diffraction study. Similar data for the minor isomer reveal that this species had a structure in which the B–Pt σ bond involves a boron atom [graphic omitted] in the face of the cage, adjacent to a CMe group. The two isomers may be separated by column chromatography, but solutions equilibrate on standing. Formation of the major isomer from [WPtH (µ-CC₆H₃Me₂-2,6)(CO)₂(PEt₃)(η⁵-C₂B₉H₉Me₂)] can be partially reversed by treating solutions of the former with hydrogen. Mechanisms are proposed for these various transformations based in part on deuteriation studies employing [PtD(Me₂CO)(PEt₃)₂][BF₄] as a precursor to the W–Pt species. The compound [WPt(µ-CC₆H₃Me₂-2,6)(CO)₂(PEt₃)(µ-σ:η⁵-C₂B₉H₈Me₂)](major isomer) is relatively inert to the addition of ligands at the formally 14-electron platinum centre, presumably due to the presence of the bulky xylyl group. However, reactions very readily occur with rod-like ligands (L = CO or CNBu^t) to give the compounds [WPt(µ-CC₆H₃Me₂-2,6)(CO)₂(PEt₃)(L)(µ-σ:η⁵-C₂B₉H₈Me₂)]. Indeed the tricarbonyl complex is a major constituent of the mixture of products obtained by treating the complex [NEt₄][W(CC₆H₃Me₂-2,6)(CO)₂(η⁵-C₂B₉H₉Me₂)] with [PtH(Me₂CO)(PEt₃)₂][BF₄]. An X-ray study of the carbon monoxide adduct revealed a structure similar to the precursor with parameters W–Pt, 2.728(1),µ-C–W 1.89(1), µ-C–Pt 2.29(1), B–Pt 2.16(1)Å, W–µ-C–C¹(C₆H₃) 160.0(8), and P–Pt–W 138.5(1)°. The significantly lower W–µ-C–C¹(C₆H₃) angle (ca. 160°) compared with that in the precursor is presumably a consequence of the presence of the additional CO ligand terminally bound to the platinum and cisoid to the alkylidyne group [µ-C–Pt–CO 83.5(4)°].