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.
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