Rates of Dimethyl Sulfoxide Exchange in Monoalkyl Cationic Platinum(II) Complexes Containing Nitrogen Bidentate Ligands. A Proton NMR Study

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Abstract
A series of monoalkyl square-planar complexes of the type [Pt(N-N)(CH3)(Me2SO)]PF6 (1−14), where N-N represents chelating diamines or diimines of widely different steric and electronic characteristics, was synthesized, and the complexes were fully characterized as solids and in solution. The substrates were tailored to offer only one site of exchange to a neutral molecule, i.e. Me2SO, in a noncoordinating solvent. No evidence for fluxionality of the N-N ligands was found, except for the case of complex 11 formed by 2,9-dimethyl-1,10-phenanthroline. In solution this complex is fluxional with the phenanthroline oscillating between nonequivalent bidentate modes by a mechanism which involves rupture of the metal−nitrogen bond and rapid interconversion of two coordinatively unsaturated T-shaped 14-electron three-coordinate molecular fragments. Rates of this fluxion were measured by NMR spectroscopy from the exchange effects on the 1H signals of the methyl and aromatic hydrogens. The ΔG⧧ value for the fluxion is 49.6 ± 4 kJ mol-1. Dimethyl sulfoxide exchange with all the complexes has been studied as a function of ligand concentration by 1H NMR line-broadening, isotopic labeling, and magnetization transfer experiments with deuterated acetone as the solvent. Second-order rate constants were obtained from linear plots of kobsvs [Me2SO] and activation parameters were obtained from exchange experiments carried out at different temperatures. Second-order kinetics and negative entropies of activation indicate an associative mechanism. The lability of dimethyl sulfoxide in the complexes depends in a rather unexpected and spectacular way upon the nature of the coordinate N-N ligands, the difference in reactivity between the first (N-N = N,N,N‘,N‘-tetramethyl-1,2-diaminoethane, k2298 = (1.15 ± 0.1) × 10-6 mol-1 s-1) and the last (N-N = 2,9-dimethyl-1,10-phenanthroline, k2298 = (3.81 ± 0.005) ×104 mol-1 s-1) members of the series being greater than 10 orders of magnitude, as a result of a well-known phenomenon of steric retardation (for the first complex) and an unprecedented case of steric acceleration (for the last complex). Other factors of primary importance in controlling the reactivity are (i) the presence of an extensive π system on the ligand N-N, (ii) the ease with which this π system interacts with nonbonding d electrons of the metal, and (iii) the flexibility and ease of elongation of the chelate bite distance. The basicity plays a somewhat minor role, except in the restricted range of the same class of compounds such as substituted phenanthrolines.

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