Variable-temperature nuclear magnetic resonance spectroscopic studies of the dynamic behaviour of the mixed-metal cluster compounds [MM′Ru4H2(µ-dppf)(CO)12][M = M′= Cu, Ag or Au; M = Cu, M′= Au; dppf = Fe(η5-C5H4PPh2)2] and the crystal structures of [MM′Ru4H2(µ-dppf)(CO)12](M = Cu or Au, M′= Au)

Abstract

The free energies of activation (ΔG^‡) for the fluxional processes in the clusters [M₂Ru₄H₂(µ-dppf)(CO)₁₂][M = Cu 1, Ag 2 or Au 3; dppf = 1,1′-bis(diphenylphosphino)ferrocene] have been determined by band-shape analysis of variable-temperature ¹H or ³¹P-{¹H} NMR spectra. In solution, the dppf ligand attached to the coinage metals in each of 1–3 undergoes dynamic behaviour involving inversion at the phosphorus atoms, together with twisting of the cyclopentadienyl rings, and values of 47.0 ± 0.2, 51.5 ± 0.1 and 48.8 ± 0.2 kJ mol^–1 were obtained for ΔG^‡ of this fluxional process in 1–3, respectively. The metal cores of these three clusters are also stereochemically non-rigid in solution and values of ΔG^‡ of 47.7 ± 0.2 and 39.9 ± 0.4 kJ mol^–1 have been calculated for a coinage-metal site-exchange process in 1 and 2, respectively. The new trimetallic cluster [AuCuRu₄(µ₃-H)₂(µ-dppf)(CO)₁₂]4, which is closely related to the bimetallic clusters 1–3, has been synthesized in ca. 40% yield by treating [Cu₂Ru₄(µ₃-H)₂(µ-dppf)(CO)₁₂] with the complex [AuCl(SC₄H₈)]. Variable-temperature ¹H NMR studies on 4 demonstrate that the dppf fluxion still occurs in solution (ΔG^‡ 49.2 ± 0.2 kJ mol^–1), but the metal skeleton of the cluster is stereochemically rigid, which is in marked contrast to those of the bimetallic analogues 1–3. The structures of [MM′Ru₄H₂(µ-dppf)(CO)₁₂](M = Cu or Au, M′= Au) have been established by single-crystal X-ray diffraction studies. The metal framework of 4 consists of a tetrahedron of ruthenium atoms with one face capped by the copper atom and one CuRu₂ face of the CuRu₃ tetrahedron so formed is capped by the gold atom to give an overall capped trigonal-bipyramidal metal-core geometry [Cu–Au 2.641(1), Cu–Ru 2.755(1)–2.909(2), Au–Ru 2.780(1) and 2.815(1), Ru–Ru 2.783(1)–2.998(1)Å]. The other two CuRu₂ faces are capped by triply bridging hydrido ligands, the dppf ligand bridges the two coinage metals and each ruthenium atom is bonded to three terminal CO groups. The skeletal geometry of 3 is reasonably similar to that of 4, with a second gold atom replacing the copper atom. However, one of the Au–Ru distances [3.558(2)Å] is too long for any significant bonding interaction between the two metal atoms, so the metal framework of 3 is somewhat distorted towards a capped square-pyramid [Au–Au 2.901(1), Au–Ru 2.771(1)–2.846(1), Ru–Ru 2.775(2)–3.016(1)Å]. As observed for 4, the bidentate diphosphine ligand bridges the two Group 11 metals in 3 and each ruthenium atom is bonded to three terminal CO groups, but one of the hydrido ligands bridges a Ru–Ru vector rather than capping an AuRu₂ face.