The heteronuclear cluster chemistry of the Group 1B metals. Part 15. Effect of the nature of the Group 1B metals and the cone angles of the attached phosphine ligands on the metal framework structures of heteronuclear cluster compounds. Synthesis, structures, and dynamic behaviour of the bimetallic hexanuclear cluster compounds [M2Ru4H2(CO)12(PR3)2](M = Cu, R = CHMe2 or C6H11; M = Ag or Au, R = CHMe2, C6H11, or CMe3)
Abstract
Treatment of the salt [N(PPh3)2]2[Ru4(µ-H)2(CO)12] with 2 equivalents of the complex [M(NCMe)4]PF6 at –30 °C, followed by the addition of 2 equivalents of PR3, affords the hexanuclear cluster compounds [M2Ru4(µ3-H)2(CO)12(PR3)2][M = Cu, R = CHMe2, or C6H11(C6H11= cyclohexyl); M = Ag, R = CHMe2, C6H11 or CMe3] in ca. 50–65% yield. The analogous gold-containing species [Au2Ru4H2(CO)12(PR3)2](R = CHMe2, C6H11, or CMe3) were prepared in ca. 30–50% yield from the reaction of [N(PPh3)2]2[Ru4(µ-H)2(CO)12] with 2 equivalents of the compound [AuCl(PR3)], in the presence of TIPF6. Despite the relatively large size of the P(C6H11)3 ligand, the clusters [M2Ru4H2(CO)12{P(C6H11)3}2](M = Ag or Au) still adopt the capped trigonal-bipyramidal skeletal geometry, with the Group 1 B metals in close contact, which previous work has shown is preferred by clusters of general formula [M2Ru4H2(CO)12L2](M = Cu, Ag, or Au) when L is a smaller monodentate phosphine or phosphite ligand. However, the smaller size of the copper atom relative to silver and gold means that the P(C6H11)3 ligand is too bulky to allow two adjacent Cu{P(C6H11)3} fragments to be accommodated in the metal framework of [Cu2Ru4(µ3-H)2-(CO)12{P(C6H11)3}2]. Thus, the cluster is forced to adopt a sterically less-demanding skeletal geometry, which consists of a Ru4 tetrahedron with one edge bridged by a Cu{P(C6H11)3} unit and a non-adjacent face capped by the second such group. When the phosphine ligand P(CMe3)3, which is larger than P(C6H11)3, is attached to the Group 1 B metals in the clusters [M2Ru4(µ3-H)2-(CO)12{P(CMe3)3}2](M = Ag or Au), the silver- and gold-containing species are also forced to adopt a similar sterically less-demanding edge-bridged trigonal-bipyramidal metal core structure. In addition, the P(CMe3)3 ligand seems to be too bulky to allow a hexanuclear cluster of formula [Cu2Ru4H2(CO)12{P(CMe3)3}2] even to adopt an edge-bridged trigonal-bipyramidal metal framework structure and an attempt to prepare this species afforded the pentanuclear cluster [CuRu4(µ3-H)3-(CO)12{P(CMe3)3}] instead. The phosphine ligand P(CHMe2)3, which is smaller than P(C6H11)3, is not sufficiently bulky to cause the metal cores of [M2Ru4(µ3-H)2(CO)12{P(CHMe2)3}2](M = Cu, Ag, or Au) to change from the preferred capped trigonal-bipyramidal skeletal geometry in the solid state, but a second isomer of the copper-containing cluster, which probably has two face-capping Cu{P(CHMe2)3} units with no bonding interaction between them, is also present in solution at low temperatures. Variable-temperature 31P-{1H} and 1H n.m.r. spectroscopic studies demonstrate that the new Group 1 B metal heteronuclear cluster compounds undergo a variety of interesting dynamic processes in solution.
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