Hydrogen−Deuterium Exchange Studies in Platinum(II) Complexes of 1-Methylimidazole1

A kinetic study of hydrogen−deuterium (H/D) exchange in Pt(II)−1-methylimidazole complexes has been performed in D 2 O/NaOD solution, at 60 °C, by means of 1 H 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(NH 3 ) 2 (MeIm) 2 ]Cl 2 ( 3 ), trans -[Pt(NH 3 ) 2 (MeIm) 2 ]Cl 2 ( 4 ), [Pt(en)(MeIm) 2 ]Cl 2 ( 5 ), and [Pt(MeIm) 4 ](ClO 4 ) 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 CH 3 + . In complex 5, C(5)−H exchange is ca. 6 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)−CH 3 position. An X-ray crystal structure determination of 6 was also performed. The complex crystallizes in the triclinic space group P 1̄, with a = 8.219(1) Å, b = 9.424(1) Å, c = 9.139(3) Å; α = 107.68(2)°, β = 83.72(2)°, γ = 114.87(1)°; 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.