Chemistry of polynuclear metal complexes with bridging carbene or carbyne ligands. Part 66. Carbaboranetungsten–platinum complexes. Polyhedral rearrangements of a 12-vertex cage system; crystal structures of [PtW(CO)2(PEt3)2{µ6-C2B9H8(CH2C6H4Me-4)Me2}]·CH2Cl2, [PtW(µ-H){µ-σ:η5- C2B9H7(CH2C6H4-Me-4)Me2}(CO)2(PMe3)(PEt3)2], and related compounds

Abstract

In acetone, the salts [N(PPh₃)₂][W(CC₆H₄Me-4)(CO)₂(η⁵-C₂B₉H₉Me₂)] and [PtH(Me₂CO)(PEt₃)₂][BF₄] yield the dimetal compound [PtW(CO)₂(PEt₃)₂{η⁶-C₂B₉H₈(CH₂C₆H₄Me-4)Me₂}](2a), the structure of which has been established by X-ray diffraction. The very short Pt–W bond [2.602(1)Å] is semi-bridged by the two carbonyl ligands [W–C–O 165(1) and 168(1)°], while a novel η⁶-C₂B₉H₈(CH₂C₆H₄Me-4)Me₂ group ligates the tungsten atom. Six atoms [graphic omitted] in the face of the ligand are within bonding distance of the metal, but the C ⋯ C separation (2.88 Å) within the ring is non-bonding. The CH₂C₆H₄Me-4 substituent is attached to the central boron of the B₃ unit, and is derived from the Pt–H, B–H, and CC₆H₄Me-4 groups present in the reactants. Treatment of [N(PPh₃)₂][W(CMe)(CO)₂(η⁵-C₂B₉H₉Me₂)] with [ PtH (Me₂CO)(PEt₃)₂][BF₄] affords an inseparable 1 : 2 mixture of the two complexes [PtW(CO)₂(PEt₃)₂{η⁶-C₂B₉H₈(R)Me₂}][(2b), R = H; (2c), R = Et]. Reactions between (2a) and the electron pair donor molecules L = PMe₃, CO, CNBu^t, or PHPh₂ yield the complexes [PtW(µ-H){µ-σ:η⁵-C₂B₉H₇(CH₂C₆H₄Me-4)Me₂}(CO)₂(L)(PEt₃)₂](3). The structure of (3a)(L = PMe₃) was established by X-ray diffraction, confirming that the Pt–W bond [2.843(2)Å] is bridged by a hydrido ligand, the presence of which was revealed in the ¹H n.m.r. spectrum. The tungsten atom is ligated by two terminal CO groups, the PMe₃ group, and an η⁵-C₂B₉H₇(CH₂C₆H₄Me-4)Me₂ group. In the latter the central boron of the nido-C₂B₃ face carries the CH₂C₆H₄Me-4 substituent, while another boron atom in the face forms an exo- polyhedral bond to the platinum [2.123(5)Å]. The synthesis of (3a) from (2a) thus represents an unprecedented iso-closo(or hyper-closo) to closo transformation in a C₂B₉W 12-vertex framework. The mixture of compounds (2b) and (2c)(ca. 1 : 2) reacts with CO to give three complexes [PtW(µ-H){µ-σ:η⁵-C₂B₉H₇(R)Me₂}(CO)₃(PEt₃)₂][(3e), R = H; (3f), R = Et] and [PtW(CO)₃(PEt₃)₂(η⁵-C₂B₉H₉Me₂)](4), formed in a ratio (3e) : (3f) : (4) of ca. 1 : 4 : 1. Moreover, i.r. and n.m.r. studies showed that there was an equilibrium in solution between (3e) and (4). Column chromatography on alumina failed to separate the three complexes completely, and an X-ray study on a crystal from the mixture revealed that co-crystallisation had occurred. The asymmetric unit contains two crystallographically independent and chemically inequivalent molecules. One molecule proved to be (4), having a structure in which the Pt–W bond [2.818(1)Å] is bridged by a B–H ⇀ Pt three-centre two-electron linkage. The latter involves the central boron atom in the nido face of the tungsten-ligating η⁵-C₂B₉H₉Me₂ group. The other unit in the crystal exhibited disorder. This arises from its correspondence to the two products (3e) and (3f), which differ only in the occupancy [R = H (20%) or Et (80%)] of the exo-polyhedral site at the central boron in the C₂B₃nido face of the carbaborane ligand. The structures of (3e) and (3f), with their exo-polyhedral B–Pt σ bonds and tungsten-ligated η⁵-C₂B₉H₇(R)Me₂ groups are thus closely related to that of (3a). Protonation (HBF₄·Et₂O) of the complexes (3)(L = PMe₃, CO, or CNBu^t) affords the salts [PtW(µ-H)(CO)₂(L)(PEt₃)₂{η⁵-C₂B₉H₈(CH₂C₆H₄Me-4)Me₂}][BF₄]. The n.m.r. data (¹H, ¹³C-{¹H}, ¹¹ B-{¹H}, and ³¹P-{¹H}) for the new compounds are eported, and are discussed in relation to their structures.