Multinuclear (1H, 31P and 195Pt) NMR study and dynamical analysis of binuclear mu-hydrido mu-carbonyl Pt(I) cations with chelating diphosphines

The dynamic behaviour of the binuclear mu-hydrido mu-carbonyl cations with chelating diphosphines, [Pt2(P-P)2(mu-H)(mu-CO)]+ [P-P = dppe, 1, dppp, 2, and dppb, 3] have been investigated by multinuclear (1H, 31P and 195Pt) variable temperature NMR spectroscopy. The 195Pt and 1H results are consistent with intramolecular mutual exchange of the P atoms with respect to the bridging ligands in all of the complexes 1-3. A detailed dynamical analysis carried out on complexes 2 and 3 shows that the dynamical process exchanges the P atoms within a single diphosphino ligand, and excludes the simultaneous P atom exchange in both ligands. The bite of the diphosphino ligands affects the rate of this process in the order 3 > 2 > 1. The process follows an activation law with deltaH++ = 67 and 60 kJ mol(-1) for 2 and 3, respectively, so that P-Pt bond breaking should not be involved. The positive activation entropy (17-19 J K(-1) mol(-1)) hints at a mechanism where the intermediate(s) have a less ordered structure than that of the stable complex. In accordance with the NMR results, two reactivity experiments provided further evidence of the intramolecular nature of the observed dynamics and exclude any equilibration path via Pt-P and/or Pt-Pt bond breaking. On these grounds, a mechanism involving rotation about a Pt-Pt bond could be proposed.