Preferential Geometry and Reactivity of Neutral Iridium(III) and Rhodium(III) Complexes Bearing a Flexible Heterochelate PN Ligand (PN = o-Ph₂PC₆H₄CH₂OCH₂C₅H₄N-2)

Reactions of [IrCl(coe)₂]₂ (coe = cyclooctene) with 1 equiv of the PN ligand (PN = o-Ph₂PC₆H₄CH₂OCH₂C₅H₄N-2) in the presence of several phosphines or pyridine (1 equiv) at room temperature afforded a neutral iridium(III) hydride complex, (PCN-κ³P,C,N)Ir(H)(Cl)(L) (PCN = Ph₂PC₆H₄CHOCH₂C₅H₄N) [L = PPh₃ (1a), PCy₃ (1b), PBu₃ (1c), PMePh₂ (1d), Py (1e)] in good isolated yield. The PCN ligand is coordinated in a meridional manner, which was confirmed by spectral analyses and X-ray analysis of 1a. The related rhodium(III) hydride complex (PCN-κ³P,C,N)Rh(H)(Cl)(L) (PCN = Ph₂PC₆H₄CHOCH₂C₅H₄N) [L = PPh₃ (4a), PCy₃ (4b)] was also prepared from the reactions of [RhCl(coe)₂]₂ with 1 equiv of the PN ligand in the presence of 1 equiv of PPh₃ or PCy₃. The structure of 4a and 4b was determined by spectral analyses and X-ray analysis of 4a. In contrast to the iridium complexes, the PCN ligand in the rhodium(III) complexes coordinated to the metal center in a facial manner. A ligand exchange reaction of the PPh₃ iridium(III) complex 1a with PCy₃ did not proceed well (20% conversion), while a similar reaction of the PPh₃ rhodium(III) complex 4a with PCy₃ afforded the PCy₃ complex 4b quantitatively. We examined the ligand exchange reactions of the iridium(III) and rhodium(III) complexes with various phosphines to explain the different reactivities. Using Tolman's parameter, we demonstrated that the ligand exchange reaction in the iridium(III) complexes bearing the meridional-coordinated PCN ligand is controlled by the steric factor of the phosphines and the ligand exchange reaction in the rhodium(III) complexes bearing the facial-coordinated PCN ligand is controlled by their electronic factors. The meridional coordination of the PCN ligand makes the environment around the iridium center sensitive to the steric nature due to the steric repulsion between the diphenyl group in the PCN ligand and the phosphines. The facial coordination of the PCN ligand decreases their repulsion, and reactivity of the metal center depends on the basicity of the phosphines.