Pyridinium-DerivedN-Heterocyclic Carbene Complexes of Platinum: Synthesis, Structure and Ligand Substitution Kinetics

Abstract

A series of [(R-iso-BIPY)Pt(CH₃)L ]⁺X^- complexes [R-iso-BIPY = N-(2-pyridyl)-R-pyridine-2-ylidene; (R = 4-H, 1; 4-tert-butyl, 2; 4-dimethylamino, 3; 5-dimethylamino, 4); L = SMe₂, b; dimethyl sulfoxide (DMSO), c; carbon monoxide (CO), d; X = OTf^- = trifluoromethanesulfonate and/or [BPh₄]^-] were synthesized by cyclometalation of the [R-iso-BIPY−H]⁺[OTF]^- salts 1a − 4a ([R-iso-BIPY−H]⁺ = N-(2-pyridyl)-R-pyridinium) with dimethylplatinum-μ-dimethyl sulfide dimer. X-ray crystal structures for 1b, 2c − 4c as well as complexes having bipyridyl and cyclometalated phenylpyridine ligands, [(bipy)Pt(CH₃)(DMSO)]⁺ (5c) and (C₁₁H₈N)Pt(CH₃)(DMSO) (6c), have been determined. The pyridinium-derived N-heterocyclic carbene complexes display localized C−C and C−N bonds within the pyridinium ligand that are indicative of carbene π-acidity. The significantly shortened platinum−carbon distance, for “parent” complex 1b, together with NMR parameters and the ν(CO) values for carbonyl cations 1d − 4d support a degree of Pt−C10 multiple bonding, increasing in the order 3 < 4 < 2 < 1. Degenerate DMSO exchange kinetics have been determined to establish the nature and magnitude of the trans-labilizing ability of these new N-heterocyclic carbene ligands. Exceptionally large second-order rate constants (k₂ = 6.5 ± 0.4 M^-1·s^-1 (3c) to 2300 ± 500 M^-1·s^-1 (1c)) were measured at 25 °C using ¹H NMR magnetization transfer kinetics and variable temperature line shape analysis. These rate constants are as much as 4 orders of magnitude greater than those of a series of structurally similar cationic bis(nitrogen)-donor complexes [(N−N)Pt(CH₃)(DMSO)]⁺ reported earlier, and a factor of 32 to 1800 faster than an analogous charge neutral complex derived from cyclometalated 2-phenylpyridine, (C₁₁H₈N)Pt(CH₃)(DMSO) (k₂ = 0.21 ± 0.02 M^-1·s^-1 (6c)). The differences in rate constant are discussed in terms of ground state versus transition state energies. Comparison of the platinum−sulfur distances with second order rate constants suggests that differences in the transition-state energy are largely responsible for the range of rate constants measured. The π-accepting ability and trans-influence of the carbene donor are proposed as the origin of the large acceleration in associative ligand substitution rate.