Chemistry of polynuclear metal complexes with bridging carbene or carbyne ligands. Part 89. Tetra- and penta-nuclear tungsten–rhodium complexes: crystal structures of [W3Rh2(µ-CO)2(µ-CMe){µ-C(Me)C(O)}(µ-PPh2)2(µ3-CMe)(CO)2(η-C5H5)3] and [W3Rh2(µ-CO)3(µ-CMe){µ-C(Me)PPh2}(µ3-CMe)-(CO)2(ηC5H5)3]

Abstract

Treatment of [Rh₂(µ-PPh₂)₂(cod)₂](cod = cyclo-octa-1,5-diene) in thf (tetrahydrofuran) at room temperature with 2 equivalents of [W(CMe)(CO)₂(η-C₅H₅)] rapidly affords the tetranuclear metal complex [W₂Rh₂(µ-CMe)₂(µ-PPh₂)₂(CO)₄(η-C₅H₅)₂](4). If the same reaction is carried out using an excess of the ethylidynetungsten complex two metal cluster compounds are obtained: [W₂Rh₂(µ-CO){µ-C(Me)C(O)}(µ-PPh₂)2(µ₃-CMe)(CO)₂(η-C₅H₅)₂](5)(formed as a separable mixture of two diastereoisomers) and [W₃Rh₂(µ-CO)₂(µ-CMe){µ-C(Me) C(O)}(µ-PPh₂)₂(µ₃-CMe)(CO)₂(η-C₅H₅)₃](6). The structure of the latter was established by X-ray diffraction. The molecule has an essentially planar W(1), Rh(1), W(2), Rh(2), W(3) framework. Carbonyl groups bridge the Rh(1)–W(1) and Rh(2)–W(2) bonds, and PPh₂ ligands span the Rh(1)–W(2) and Rh(2)–W(3) linkages. Ethylidyne groups edge-bridge the Rh(1)–W(1) bond and triply bridge the atoms Rh(1)W(2)Rh(2), but the Rh(1)⋯ Rh(2) separation [3.115(4)Å] is non-bonding. The Rh(2)–W(3) bond is bridged by a C(Me)C(O) ketenyl group, and the terminal W atoms in the chain each carry a CO ligand and a C₅H₅ ring. A third C₅H₅ group ligates W(2). The metal–metal distances fall into two groups [Rh(1)–W(2) 2.791(6), Rh(2)– W(3) 2.813(4); and Rh(1)–W(1) 2.696(5), Rh(2)–W(2) 2.664(5)Å], with the shorter separations corresponding to double bonds and the longer to single bonds. Compound (6) is produced by addition of a molecule of [W(CMe)(CO)₂(η-C₅H₅)] to (5), while the latter is formed by isomerisation of (4), a process necessitating migration of µ-CMe and µ-PPh₂ between the metal centres. After several hours solutions of (6) in thf afford quantitatively the complex [W₃Rh₂(µ-CO)₃(µ-C(Me){µ-C(Me)PPh₂}(µ-PPh₂)(µ-CMe)(CO)₂(η-C₅H₅)₃](7) an isomer of (6). The structure of (7) was established by X-ray diffraction. An essentially planar metal atom chain W(1), Rh(1), W(2), Rh(2), W(3) forms the spine of the molecule. The various ligands ligate the rhodium and tungsten atoms in a similar manner to those in (6), with the exception of the groups which span the Rh(2)–W(3) bond. This bond is now bridged by a λ⁵- phospha-alkyne C(Me) PPh₂ fragment such that the phosphorus atom ligates W(3) and the carbon atom CMe is bonded to Rh(2) and W(3). The Rh(2)–W(3) bond is also bridged by a CO ligand. Thus the conversion of (6) into (7) involves a novel interchange of CO and PPh₂ fragments such that in the former a µ-C(Me)C(O) group is present and in the latter a µ- C(Me)PPh₂ moiety. The n.m.r. data (¹H, ¹³C-{¹H), and ³¹P-{¹H}) for the new compounds are reported and discussed.