The heteronuclear cluster chemistry of the Group 1 B metals. Part 12. Variable temperature nuclear magnetic resonance spectroscopic studies of the dynamic behaviour of the mixed-metal cluster compounds [M2Ru4(µ3-H)2(µ-L)(CO)12][M = Cu or Ag, L = Ph2P(CH2)nPPh2(n= 1–6) or cis-Ph2PCHCHPPh2; M = Cu, L = Ph2 As (CH2)nPPh2(n= 1 or 2)] in solution

Abstract

The free energies of activation (ΔG^‡) for the intramolecular metal core rearrangements which the clusters [M₂Ru₄(µ₃-H)₂(µ-L)(CO)₁₂][M = Cu or Ag, L = Ph₂P(CH₂)_nPPh₂(n= 1–6); M = Cu, L = Ph₂ As (CH₂)_nPPh₂(n= 1 or 2)] undergo in solution have been calculated by bandshape analysis of variable temperature ³¹P-{¹H} or ¹H n.m.r. spectra or estimated from the coalescence temperature in ³¹P-{¹H} n.m.r. spectra. Changes in the nature of the bidentate ligands attached to the coinage metals cause relatively large alterations of up to ca. 13 and ca. 1O kJ mol^–1 in the values of ΔG^‡ for the copper- and silver-containing species, respectively. The formal replacement of two PPh₃ groups in [Ag₂Ru₄(µ₃-H)₂(CO)₁₂(PPh₃)₂] by a Ph₂PCH₂PPh₂ or Ph₂P(CH₂)₂PPh₂ ligand reduces the value of ΔG^‡ by ca. 8 and ca. 4 kJ mol^–1, respectively. These decreases, together with the previously reported distortions in metal framework geometry from capped trigonal bipyramidal to or towards capped square-based pyramidal, which occur for the same formal change of ligands in the analogous gold-containing species, provide some indirect evidence to support a restricted Berry pseudo-rotation mechanism for the Group 1 B metal site-exchange process in the clusters studied. The new heteronuclear cluster compounds [M₂Ru₄H₂(µ-cis- Ph₂PCHCHPPh₂)(CO)₁₂][M = Cu (2O), Ag (21), or Au (22)] have been synthesized and the effect of the cis-Ph₂PCHCHPPh₂ ligand on the value of ΔG^‡ for the metal core rearrangement of (21) and on the skeletal geometry adopted by (22) is also consistent with the restricted Berry pseudorotation mechanism. The magnitude of ΔG^‡ for the silver-containing species (21) is ca. 7 kJ mol^–1 lower than that for the analogous PPh₃-ligated cluster and the metal framework geometry of the gold-containing compound (22) is altered from capped trigonal bipyramidal to capped squarebased pyramidal when two PPh₃ groups are formally replaced by the cis-Ph₂PCHCHPPh₂ ligand. In addition, variable temperature ¹H and ³¹P-{¹H} n.m.r. spectroscopic studies show that the hydrido ligands in the copper and silver clusters containing Ph₂E(CH₂)_nPPh₂(E = P, n= 1–6; E = As, n= 1 or 2) are rendered equivalent by dynamic behaviour which involves conformational changes in the methylene backbones of the bidentate ligands and that the cis-Ph₂PCHCHPPh₂ ligand in (21) undergoes a novel fluxional process involving intramolecular site exchange of the two phosphorus atoms between the two silver atoms.