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

• Published in: Journal of the American Chemical Society Vol. 118; no. 3; pp. 612 - 620
• Main Authors: Clement, Omoshile, Roszak, Aleksander W, Buncel, Erwin
• Format: Journal Article
• Language: English
• Published: American Chemical Society 24.01.1996
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja951615c

A kinetic study of hydrogen−deuterium (H/D) exchange in Pt(II)−1-methylimidazole complexes has been performed in D2O/NaOD solution, at 60 °C, by means of 1H 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]Cl2 (3), trans-[Pt(NH3)2(MeIm)2]Cl2 (4), [Pt(en)(MeIm)2]Cl2 (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. 102 to 103, relative to the neutral substrate. However, it is ca. 104−105 times less effective compared to H+ and CH3 +. 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)−CH3 position. An X-ray crystal structure determination of 6 was also performed. The complex crystallizes in the triclinic space group P1̄, 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.