Synthesis, crystal structures and dynamic behaviour of the pentanuclear mixed-metal cluster compounds [Au2Ru3(µ-H)(µ3-COMe){µ-Ph2P(CH2)nPPh2}(CO)9] (n =1 or 5)‡

Abstract

The cluster compound [Ru₃(µ-H)₃(µ₃-COMe)(CO)₉] reacted with the complex [Au₂{µ-Ph₂P(CH₂)_nPPh₂}Me₂] (n = 1 or 5) in diethyl ether solution to afford the new mixed-metal cluster compounds [Au₂Ru₃(µ-H)(µ₃-COMe){µ-Ph₂P(CH₂)_nPPh₂}(CO)₉] (n = 1 1 or 5 2) in ca. 30–40% yield. Compounds 1 and 2 have been characterized by IR and NMR spectroscopy and by single-crystal X-ray diffraction. The X-ray diffraction studies show that the Au₂Ru₃ metal cores of 1 and 2 both adopt similar distorted square-based pyramidal structures, with the basal plane defined by the two Au atoms and two of the Ru atoms and the third Ru atom forming the apex of the pyramid [Au–Au 2.878(1), Au–Ru 2.688(1)–2.863(1), Ru–Ru 2.887(1)–2.891(1) Å for 1 and Au–Au 3.109(1), Au–Ru 2.716(3)–2.787(3), Ru–Ru 2.859(2)–2.895(3) Å for 2]. The Ph₂P(CH)_nPPh₂ (n = 1 or 5) ligand bridges the two Au atoms, the methoxycarbyne caps the Ru₃ face and the hydride bridges the basal Ru–Ru edge of the metal framework. Each ruthenium atom is bonded to three terminal carbonyl groups. Variable-temperature NMR spectroscopic studies suggested that the metal skeletons of 1 and 2 are stereochemically non-rigid at high temperature in solution. The novel fluxional process is thought to involve a migration of the diphosphinedigold group around the three possible edge-bridging sites on the trigonal-planar triruthenium unit, together with a concomitant movement of the edge-bridging hydride ligand. Band-shape analysis of variable-temperature ¹H and ¹³C-{¹H} NMR spectra afforded values of ΔG ^‡ for these proposed metal-core rearrangements of 67.1 ± 0.2 (¹H) and 67.0 ± 0.2 kJ mol^–1 (¹³C-{¹H}) for 1 and 58.9 ± 0.1 kJ mol^–1 (¹³C-{¹H}) for 2. These free energies of activation are surprisingly high compared with those previously reported for skeletal rearrangements in gold–ruthenium heteronuclear cluster compounds.