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

Abstract

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 P1 h, with a ) 8.219 (1) Å, b ) 9.424(1) Å, c ) 9.139(3) Å; R ) 107.68(2)°, ) 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.

Characterization of nonderivatized plant cell walls using high‐resolution solution‐state NMR spectroscopy

“…Imidazoles are much less prone to isotope exchange than their corresponding imidazolium salts. [35,36] …”

A Monoclinic Form of Tetrakis(1-methylimidazole-N3)platinum(II) Diperchlorate

“…We have begun a study of the synthesis and characterization of a number of ptU-amine complexes (Clement, 1995;Buncel & Clement, 1995;Roszak, Clement & Buncel, 1996;Clement, Roszak & Buncel, 1996a,b) in continuation of our study of metal ion-biomolecule interactions (Buncel, Joly & Jones, 1986;Buncel, Joly & Yee, 1989;Buncel, Clement, Fan, Joly, Jones & Onyido, 1991;Buncel, Clement & Onyido, 1994;Buncel, Fan, Moir & Onyido, 1995). In this regard, we have recently described the triclinic form of the title compound, tetrakis(1-methylimidazole)platinum(II) diperchlorate, [Pt(Melm)4](C104)2, (I), where Melm is the 1-methylimidazole ligand (Clement, Roszak & Buncel, 1996a). The complex was isolated as small colorless prisms from a methanolic solution of cis-[Pt(DMSO)2C12] and excess 1-methylimidazole, following the modified procedure of Reedijk and co-workers (Graves, Hodgson, van Kralingen & Reedijk, 1978).…”

“…It is therefore uncertain if the orientation of ligands is responsible for the observed differences in the monoclinic form. In the triclinic form, the [Pt(MeIm)4] 2÷ cations are all packed in a parallel fashion and the dominant intermolecular interactions are the 7r-stacking interactions between the imidazole rings of adjacent molecules located at the four comers of the ac face of the unit cell (Clement, Roszak & Buncel, 1996a). In the monoclinic form described here (Fig.…”

“…The title complex was obtained from a methanolic solution of cis-[Pt(DMSO)2CI2] and excess l-methylimidazole in methanol (Graves, Hodgson, van Kralingen & Reedijk, 1978;Clement, Roszak & Buncel, 1996a), and was recrystallized from water.…”

“…It is of note, however, that the geometries of the two crystallographically independent ligands in the triclinic form, which are both far from being coplanar with the Pt coordination plane, were found to be equal within two standard deviations and similar to the geometry of uncoordinated imidazole. The N13---C14 and N23--C23 bond lengths in the triclinic form were found to be 1.367(11) and 1.383(11),~,, respectively (Clement, Roszak & Buncel, 1996a). It is therefore uncertain if the orientation of ligands is responsible for the observed differences in the monoclinic form.…”