Parahydrogen derived illumination of pyridine based coordination products obtained from reactions involving rhodium phosphine complexes

Abstract

The reactions of RhCl(PBz₃)₃ with H₂ and pyridine or 4-methylpyridine yield RhCl(H)₂(PBz₃)₂(py) and RhCl(H)₂(PBz₃)₂(4-Me-py), respectively. These species undergo hydride site exchange via the loss of the pyridyl donor and formation of RhCl(H)₂(PBz₃)₂ which contains equivalent hydride ligands; for the py system the activation free energy, ΔG^‡₃₀₀, is 57.4 ± 0.1 kJ mol⁻¹ while for 4-Me-py the value is 59.6 ± 0.3 kJ mol⁻¹. These products only showed parahydrogen enhancement in the corresponding hydride resonances when a sacrificial substrate was added to promote hydrogen cycling. When RhCl(PPh₃)₃ was used as the precursor similar observations were made, while when RhCl(PCy₃)₂(C₂H₄) was examined, H₂ addition led to the formation of the binuclear complex (H)₂Rh(PCy₃)₂(μ-Cl)₂Rh(H)₂(PCy₃)₂ which was differentiated from RhCl(H)₂(PCy₃)₂ on the basis of the similarity in diffusion coefficient (5.5 × 10⁻⁹ m² s⁻¹) to that of (H)₂Rh(PPh₃)₂(μ-Cl)₂Rh(PPh₃)₂ (5.3 × 10⁻⁹ m² s⁻¹). The detection of RhCl(H)₂(PCy₃)₂(py) was facilitated when pyridine was added to a solution of RhCl(PCy₃)₂(C₂H₄) before the introduction of H₂. During these reactions trace amounts of the double substitution products, RhCl(H)₂(phosphine)(py)₂, were also detected.