Five-Coordinate [Pt<sup>II</sup>(bipyridine)<sub>2</sub>(phosphine)]<sup><i>n</i>+</sup> Complexes: Long-Lived Intermediates in Ligand Substitution Reactions of [Pt(bipyridine)<sub>2</sub>]<sup>2+</sup> with Phosphine Ligands
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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.
