Synthesis, structure, and reactivity of the heteronuclear clusters [Os5H(CO)15{M(PR3)}] and [Os5(CO)15{M(PR3)}2](M = Cu, Ag, or Au; R = Ph or Me); the X-ray crystal structure of [Os5H(CO)15{Au(PPh3)}]

Abstract

Reaction of the salt [N(PPh₃)₂][Os₅H(CO)₁₅] with a slight excess of [MCl(PR₃)](M = Cu, Ag, or Au; R = Ph or Me), in the presence of TIPF₆, proceeds instantaneously to afford a neutral brown cluster [Os₅H(CO)₁₅{M(PR₃)}][M = Cu, R = Ph (1); M = Ag, R = Ph (2); M = Au, R = Ph (3); M = Au, R = Me(4)]. The copper and silver complexes, (1) and (2), are less stable than the gold complexes, (3) and (4), and decompose back to the starting anion on standing in solution. Complexes (3) and (4) have been fully characterised on the basis of their i.r., ¹H n.m.r., and mass spectra, and the structure of (3) has been confirmed by a single-crystal X-ray analysis. The five Os atoms define a trigonal bipyramid, one face of which is asymmetrically capped by the gold triphenylphosphine group, so that the whole metal framework may be described as a bicapped tetrahedron. In co-ordinating solvents, such as MeCN, (3) and (4) dissociate with the loss of Au(PR₃) to regenerate the anion [Os₅H(CO)₁₅]^–. With bases, such as NEt₃ and dbu (1,8-diazabicyclo[5.4.0]undec-7-ene)(3) and (4) deprotonate to give [Os₅(CO)₁₅{Au(PR₃)}]^–[R = Ph (5) or Me (6)]. Careful protonation with HBF₄ regenerates (3) and (4). Clusters (5) and (6) can also be synthesised by the interaction of [N(PPh₃)₂]₂[Os₅(CO)₁₅] with 1 equivalent of [AuCl(PR₃)]. Similarly, the reaction of the dianion [Os₅(CO)₁₅]^2– with 1 equivalent of [MCl(PPh₃)](M = Cu or Ag) affords the anionic cluster [Os₅(CO)₁₅{M(PPh₃)}]^–[M = Cu (7) or Ag (8)]. The clusters [Os₅(CO)₁₅{M(PR₃)}₂][M = Cu, R = Ph (9); M = Ag, R = Ph (10), M = Au, R = Ph (11); M = Au, R = Me (12)] may be synthesised in one step by the treatment of the salt [N(PPh₃)₂]₂[Os₅(CO)₁₅], in the presence of TIPF₆, with a slight excess of [MCl(PR₃)](M = Cu, Ag, or Au; R = Ph or Me). The complexes (9)–(12) have been characterised on the basis of i.r., ¹H and ³¹P n.m.r., and mass spectrometry. These products exhibit identical i.r. spectra and are believed to possess similar metal frameworks. The ³¹P n.m.r. data show only one signal, and variable-temperature studies indicate that the Au(PPh₃) groups occupy equivalent positions and are not fluxional. The clusters (9)–(12) are stable in non-co-ordinating solvents, but in co-ordinating solvents such as MeCN undergo fragmentation with the loss of a M(PR₃) unit (M = Cu, Ag, or Au) to give the corresponding anionic clusters (5)–(8) in quantitative yield. With bases such as NEt3 and dbu, complexes (9)–(12) react with the elimination of a M(PR₃) unit to produce the anions (5)–(8).