Hydrogen-deuterium exchange studies in platinum(II) complexes of 1-methylimidazole

A kinetic study of hydrogen-deuterium (H/D) exchange in Pt(II)-1-methylimidazole complexes has been performed in D2O/NaOD solution, at 60 degrees C, by means of H-1 NMR spectroscopy. Isotopic exchange has been observed at C(2)-H, C(4)-H, and C(5)-H of the imidazole moiety. Kinetic data analysis of H/D exchange in the complexes cis-[Pt(NH3)(2)(MeIm)(2)]Cl-2 (3), trans-[Pt(NH3)(2)(MeIm)(2)]Cl-2 (4), [Pt(en)CMeIm)(2)]Cl-2 (5), and [Pt(MeIm)(4)](ClO4)(2) (6) revealed that Pt(II) enhances C(2)-H exchange in the coordinated 1-methylimidazole (MeIm) by ca. 10(2) to 10(3), relative to the neutral substrate. However, it is ca. 10(4)-10(5) times less effective compared to H+ and CH3+. In complex 5, C(5)-H exchange is ca. 5 times faster than C(4)-H exchange, in contrast with expectations based on an inductive/field effect by Pt(II). The observation of the relative reactivity order, C(5)-H exchange > C(4)-H exchange in 5, is discussed by consideration of contributing resonance structures of the intermediates formed upon abstraction of C(4)-H and C(5)-H, which would place the positive charge at the more favorable N(1)-CH3 position. An X-ray crystal structure determination of 6 was also performed. The complex crystallizes in the triclinic space group P (1) over bar, with a = 8.219(1) Angstrom, b = 9.424(1) Angstrom, c = 9.139(3) Angstrom; alpha = 107.68(2)degrees, beta = 83.72(2)degrees, gamma = 114.87(1)degrees; and Z = 1. The complex has a center of symmetry with the 1-methylimidazole ligands coordinated in a square-planar arrangement around the Pt(II) atom.