The heteronuclear cluster chemistry of the group IB metals—18. Synthesis, structural characterization and dynamic behaviour of the bimetallic hexanuclear group IB metal cluster compounds [M 2Ru 4H 2(μ-dppf)(CO) 12] [M = Cu, Ag or Au; dppf = Fe(η 5-C 5H 5PPh 2) 2]. X-ray crystal structure of [Cu 2Ru 4(μ 3-H) 2(μ-dppf)(CO) 12

Abstract

Treatment of a dichloromethane solution of the salt [N(PPh 3] 2] 2[Ru 4(μ-H) 2(CO) 12] with two equivalents of the complex [M(NCMe) 4]PF 6 (M = Cu or Ag) at −30°C, followed by the addition of one equivalent of 1,1′-bis(diphenylphosphino)ferrocene (dppf) affords the mixed-metal clusters [M 2Ru 49μ-H) 2(μ-dppf)(CO) 12] [M = Cu ( 1) or Ag ( 2)] in ca 45–60% yield. The analogous gold-containing species [Au 12Ru 4H 2(μ-dppf)(CO) 12] ( 3) was obtained in ca 70% yield by treating an acetone solution of [N(PPh 3) 2] 2[Ru 4(μ-H) 2(CO) 12] with a dichloromethane solution of the complex [Au 2(μ-dppf)Cl 2], in the presence of T1PF 6. The novel cluster compounds 1–3 have been characterized by IR and NMR spectroscopy and the structure of 1 has been determined by a single-crystal X-ray diffraction study. The metal core structure of 1 consists of a tetrahedron of ruthenium atoms capped by a copper atom, with one of the CuRu 2 faces of the CuRu 3 tetrahedron so formed further capped by a second copper atom to give a capped trigonal bipyramidal skeletal geometry. The other two CuRu 2 faces of the CuRu 3 tetrahedron are each capped by a triply bridging hydrido ligand, the bidentate diphosphine ligand bridges the two coinage metals and each ruthenium atom is bonded to three terminal CO groups. The spectroscopic data of the silver- and gold-containing species 2 and 3 are closely similar to those of 1, which suggests that 2 and 3 adopt similar metal core structures to that established for 1. However, the possibility that the capped trigonal bipyramidal metal framework of 3 is distorted by the dppf ligand towards a capped square-based pyramidal skeletal geometry cannot be excluded with the evidence available. Variable-temperature 1H and 31P{ 1H} NMR studies show that, at ambient temperatures in solution, the metal frameworks of all of the clusters undergo dynamic behaviour involving coinage metal site exchange, even though the two Group IB metals are linked together by the bidentate diphosphine ligand dppf. Free energies of activation ( ΔG ‡ ) at the coalescence temperature of 47±1, 40±1 and ca 33 kJ mol −1 have been calculated for the skeletal rearrangements from the coalescence temperatures observed in the variable-temperature 31P{ 1H} NMR spectra of clusters 1, 2 and 3, respectively. These values of ΔG ‡ are compared with those of structurally related analogous clusters. In addition, the dppf ligand attached to the coinage metals in each of 1–3 is also stereochemically non-rigid in solution at room temperature and it undergoes a process involving inversion of configuration at the phosphorus atoms, together with twisting of the cyclopentadienyl rings. The skeletal rearrangement process and the fluxional behaviour of the dppf ligand are definitely independent for clusters 2 and 3.