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-Ph2PCH
CHPPh2; 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-Ph2PCH
CHPPh2 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-Ph2PCH
CHPPh2 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-Ph2PCH
CHPPh2 ligand in (21) undergoes a novel fluxional process involving intramolecular site exchange of the two phosphorus atoms between the two silver atoms.