Alkylidyne(carbaborane) complexes of the Group 6 metals. Part 6. exo-nido-Tungstacarbaborane compounds

Abstract

Protonation of mixtures of [NEt₄][Mo(CC₆H₄Me-4)(CO){P(OMe)₃}(η⁵-C₂B₉H₉Me₂)] and [W(CR)(CO)₂(η-C₅R′₅)](R = C₆H₄Me-4 or Me, R′= H; R = C₆H₄Me-4, R′= Me) with HBF₄·Et₂O in CH₂Cl₂ at –78 °C affords the mononuclear tungsten complexes [exo-nido-9,10-{W(CO)₂(η-C₅R′₅)}-9,10-(µ-H)₂-7,8-C₂B₉H₈-7,8-Me₂]. An X-ray diffraction study (R′= Me) reveals a structure which may be regarded formally as an ion pair, with a [nido-7,8-C₂B₉H₁₀Me₂]^– anion co-ordinated to a [W(CO)₂(η-C₅Me₅)]⁺ cation by two exopolyhedral three-centre two-electron B–H⇀W bonds. The boron atoms forming these linkages lie in the open pentagonal [graphic omitted] face of the cage and are adjacent. The remaining B–B connectivity in this face is bridged by a hydrogen atom. Data from ¹¹B–¹H and ¹¹B–¹¹B correlation NMR spectroscopy of the compound are reported and are fully in agreement with the structure. Treatment of an equimolar mixture of [NEt₄][W(CC₆H₄Me-4)(CO)₂(η⁵-C₂B₉H₉Me₂)] and [W(CC₆H₄Me-4)(CO)₂(η-C₅H₅)] in CH₂Cl₂ at –78 °C with HBF₄·Et₂O affords the dimetal complex [W₂(µ-CC₆H₄Me-4)(CO)₃(η⁵-C₂B₉H₉Me₂)(η-C₅H₅)]. In contrast, if the salt [PPh₄][W(CC₆H₄Me-4)(CO)₂(η⁵-C₂B₉H₁₁)] is employed in a similar reaction, carried out at room temperature, the product obtained, as shown by NMR spectroscopy, exists in solution as an equilibrium mixture (ca. 10 : 1) of two isomers, the major species being highly fluxional. Crystals of the latter were obtained from solution, and an X-ray diffraction analysis revealed the complex to be [exo-nido-9,11-(CH₂C₆H₄Me-4)₂-5,10-{W(CO)₂(η-C₅H₅)}-5,10-(µ-H)₂-7,8-C₂B₉H₈]. In this unusual structure the two boron atoms adjacent to the carbons in the open [graphic omitted] face of the nido-C₂B₉ anion carry CH₂C₆H₄Me-4 substituents. The remaining boron atom forms a B–H⇀W bridge to the exopolyhedral [W(CO)₂(η-C₅H₅)]⁺ fragment, which is also attached to the cage through a second B–H⇀W linkage, involving a boron atom in the pentagonal boron layer below the face. The NMR data suggest that the minor isomer existing in solution is a closo-icosahedral species [WH(CO)₂{η³-7,8-C₂B₉H₉-9,11-(CH₂C₆H₄Me-4)₂}(η-C₅H₅)].