Abstract
The reaction of [Pt(N-N)(2)](2+) [N-N = 2,2'-bipyridine (bpy) or 4,4'-dimethyl-2,2'-bipyridine (4,4'-Me(2)bpy)] with phosphine ligands [PPh3 or PPh(PhSO3)(2)(2-)] in aqueous or methanolic solutions was studied by multinuclear (H-1, C-13, P-31, and Pt-195) 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 N: 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 P-31-Pt-195 coupling in the P-31 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)}]center dot 5.5H(2)O 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.