Five-Coordinate [PtII(bipyridine)2(phosphine)]n+ Complexes: Long-Lived Intermediates in Ligand Substitution Reactions of [Pt(bipyridine)2]2+ with Phosphine Ligands

The reaction of [Pt­(N–N)2]2+ [N–N = 2,2′-bipyridine (bpy) or 4,4′-dimethyl-2,2′-bipyridine (4,4′-Me2bpy)] with phosphine ligands [PPh3 or PPh­(PhSO3)22–] in aqueous or methanolic solutions was studied by multinuclear (1H, 13C, 31P, and 195Pt) NMR spectroscopy, X-ray crystallography, UV–visible spectroscopy, and high-resolution mass spectrometry. NMR spectra of solutions containing equimolar amounts of [Pt­(N–N)2]2+ and phosphine ligand give evidence for rapid formation of long-lived, 5-coordinate [PtII(N–N)2(phosphine)]n+ complexes. In the presence of excess phosphine ligand, these intermediates undergo much slower entry of a second phosphine ligand and loss of a bpy ligand to give [PtII(N–N)­(phosphine)2]n+ as the final product. The coordination of a phosphine ligand to the Pt­(II) ion in the intermediate [Pt­(N–N)2(phosphine)]n+ complexes is supported by the observation of 31P–195Pt coupling in the 31P NMR spectra. The 5-coordinate nature of [Pt­(bpy)2­{PPh­(PhSO3)2}] is confirmed by X-ray crystallography. X-ray crystal structural analysis shows that the Pt­(II) ion in [Pt­(bpy)2{PPh­(PhSO3)2}]·5.5H2O displays a distorted square pyramidal geometry, with one bpy ligand bound asymmetrically. These results provide strong support for the widely accepted associative ligand substitution mechanism for square planar Pt­(II) complexes. X-ray structural characterization of the distorted square planar complex [Pt­(bpy)­(PPh3)2]­(ClO4)2 confirms this as the final product of the reaction of [Pt­(bpy)2]2+ with PPh3 in CD3OD. The results of density functional calculations on [Pt­(bpy)2]2+, [Pt­(bpy)2­(phosphine)]n+, and [Pt­(bpy)­(phosphine)2]n+ indicate that the bonding energy follows the trend of [Pt­(bpy)­(phosphine)2]n+ > [Pt­(bpy)2­(phosphine)]n+ > [Pt­(bpy)2]2+ for stability and that the formation reactions of [Pt­(bpy)2­(phosphine)]n+ from [Pt­(bpy)2]2+ and [Pt­(bpy)­(phosphine)2]n+ from [Pt­(bpy)2­(phosphine)]n+ are energetically favorable. These calculations suggest that the driving force for the formation of [Pt­(bpy)­(phosphine)2]n+ from [Pt­(bpy)2]2+ is the formation of a more energetically favorable product.