Tricarbonyl(carbaborane) complexes of the Group 6 metals as reagents for preparing heteronuclear trimetal compounds involving rhodium

Abstract

The reaction between the salt [Rh(NCMe)₃(η-C₅Me₅)][BF₄]₂ and the reagents Na₂[M(CO)₃(η⁵-7,8-C₂B₉H₉R′₂)](M = Mo or W, R′= H or Me) in tetrahydrofuran affords [Rh(CO)₂(η-C₅Me₅)] and the trimetal complexes [MRh₂(µ-H)(µ-CO)(µ-σ:η⁵-7,8-C₂B₉H₈R′₂)(CO)₂(η-C₅Me₅)₂]. An X-ray diffraction study of the compound with M = W and R′= Me revealed a structure based on a triangulated metal core [Rh(1)–Rh(2) 2.859(1), W–Rh(1) 2.736(1), W–Rh(2) 2.822(1)Å] with the Rh–Rh bond bridged by a hydrido ligand. Two of the carbonyl groups are terminally attached to the tungsten atom, while the third [W–C–O 154(1)°] semi-bridges a W–Rh bond. The open pentagonal C₂B₃ face of the carbaborane framework is η⁵ co-ordinated to the tungsten, but the cage also bridges the metal triangle through exopolyhedral B–Rh and B–H⇀Rh bonds, using boron atoms in the C₂B₃ ring attached to the tungsten. The B–Rh bond bridges the W–Rh vector which is asymmetrically bridged by the CO group, and involves the boron atom α to the carbons in the CCBBB ring. The B–H⇀Rh three-centre two-electron bond utilises the boron atom which is β to the carbons in the CCBBB ring. Examination of the NMR spectra (¹H, ¹³C-{¹H}, ¹¹B-{¹H}, ¹¹B) of the trimetal complexes revealed that the structures of these molecules at –60 °C were similar to that established in the solid state for M = W and R′= Me. However, at ambient temperatures three of the complexes (M = Mo, R′= H or Me; M = W, R′= Me) isomerised to a structure [MRh₂(µ-H)₂(µ-CO)(µ-σ:σ′:η⁵-7,8-C₂B₉H₇R′₂)(CO)₂(η-C₅Me₅)₂] in which the carbaborane cage bridged the metal triangle via two B–Rh bonds.