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

Abstract

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 (ΔH^‡) of 64.3 kJ mol⁻¹ and an entropy of activation (ΔS^‡) of 0.005 kJ K⁻¹ mol⁻¹ for this process in ortho-dichlorobenzene solution. Complex 3 exhibits no similar fluxional behaviour. A single-crystal X-ray analysis of 2 revealed a nitrogen–rhodium distance of 2.369(3) Å for the coordinated pyridyl group, which is slightly longer than each of the phosphorus–rhodium distances [2.2868(7) Å and 2.3649(8) Å]. This suggests a relatively weak nitrogen–rhodium bonding interaction.