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

Abstract

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)2(2-)] in aqueous or methanolic solutions was studied by multinuclear ((1)H, (13)C, (31)P, and (195)Pt) 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 [Pt(II)(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 [Pt(II)(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 (31)P-(195)Pt coupling in the (31)P 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.