Dynamic coordinative exchange in rhodium(I) complexes of chiral diphosphines bearing pendant pyridyl donor groups

meso-{Ph2PCH(Ph)CH(Ph)PPh2}, meso-{Ph2PCH(pyr)CH(pyr)PPh2}, erythro-{Ph2PCH(Ph)CH(pyr)PPh2}, rac-{Ph2PCH(pyr)CH(pyr)PPh2}, threo-{Ph2PCH(Ph)CH(pyr)PPh2}, and threo-{Ph2PCH(Ph)CH(pym)PPh2}, [pyr = 2-pyridyl, pym = 2-pyrimidyl] reacted with [Rh(COD)Cl](2) (COD = 1,5-cyclooctadiene) to give cationic rhodium(I) complexes [Rh(COD){L}](+) (L = diphosphine ligand), which were isolated as their PF6- salts, 1-6 respectively. In 2 and in 3 the phosphine ligand adopts a P,P',N-coordination mode whereas 1, and 4-6 exhibit simple P,P'-coordination for the parent ligands and no evidence for N-coordination is observed. In solution 2 undergoes a fluxional process involving interchange of the coordinated and non- coordinated pyridyl environments. Variable temperature NMR studies revealed an enthalpy of activation (Delta H double dagger) of 64.3 kJ mol(-1) and an entropy of activation (Delta S double dagger) of 0.005 kJ K-1 mol(-1) for this process in ortho-dichlorobenzene solution. Complex 3 exhibits no similar uxional behaviour. A single-crystal X-ray analysis of 2 revealed a nitrogen-rhodium distance of 2.369(3) Angstrom for the coordinated pyridyl group, which is slightly longer than each of the phosphorus-rhodium distances [2.2868(7) Angstrom and 2.3649(8) Angstrom]. This suggests a relatively weak nitrogen- rhodium bonding interaction.