The heteronuclear cluster chemistry of the Group 1B metals. Part 2. Synthesis, X-ray crystal structures, and dynamic behaviour of the bimetallic hexanuclear Group 1B metal cluster compounds [M2Ru4H2(CO)12(PPh3)2](M = Cu, Ag, or Au)
Treatment of dichloromethane solutions of the salt [N(PPh3)2]2[Ru4(µ-H)2(CO)12] with two equivalents of the complex [M(NCMe)4]PF6(M = Cu or Ag) at –30°C, followed by the addition of two equivalents of PPh3, affords the mixed-metal cluster compounds [M2Ru4(µ3-H)2(CO)12(PPh3)2][M = Cu (1) or Ag(2)] in ca. 75% yield. The analogous gold species [Au2Ru4(µ3-H)(µ-H)(CO)12(PPh3)2](5)(ca. 60% yield) and reduced yields (ca. 50%) of (1) and (2) can be obtained by treating acetone solutions of the salt [N(PPh3)2]2[Ru4(µ-H)2(CO)12] with a dichloromethane solution containing two equivalents of the appropriate complex [MX(PPh3)](M = Cu or Au, X = Cl; M = Ag, X = I), in the presence of TlPF6. Single-crystal X-ray diffraction studies on each member of this series of Group 1B metal congeners show that all three clusters adopt the same metal core structure. Each metal skeleton consists of a tetrahedron of ruthenium atoms capped by a M(PPh3)(M = Cu, Ag, or Au) moiety, with one of the MRu2 faces of the MRu3 tetrahedron so formed further capped by a second M(PPh3) fragment to give a capped trigonal bipyramidal metal core geometry. For (1) and (2), the other two faces of the MRu3 tetrahedron are each capped by triply bridging hydrido ligands but, in the case of (5), one of these hydrido ligands bridges a ruthenium–ruthenium edge only. In all three clusters, each ruthenium atom is ligated by three terminal carbonyl groups. Variable-temperature n.m.r. studies show that, at ambient temperature in solution, the coinage metals in all three clusters are exchanging between the two distinct sites in the capped trigonal bipyramidal metal cores, the CO ligands all exhibit dynamic behaviour involving complete intramolecular site exchange and, in addition, the PPh3 groups of (2) are undergoing intermolecular exchange between clusters.