RATES OF DIMETHYL-SULFOXIDE EXCHANGE IN MONOALKYL CATIONIC PLATINUM(II) COMPLEXES CONTAINING NITROGEN BIDENTATE LIGANDS - A PROTON NMR-STUDY

A series of monoalkyl square-planar complexes of the type [Pt(N-N)(CH3 )(Me(2)SO)]PF6 (1-14), where N-N represents chelating diamines or diim ines of widely different steric and electronic characteristics, was sy nthesized? and the complexes were fully characterized as solids and in solution. The substrates were tailored to offer only one site of exch ange to a neutral molecule, i.e. Me(2)SO, in a noncoordinating solvent . No evidence for fluxionality of the N-N ligands was found, except fo r the case of complex 11 formed by 2,9-dimethyl-1,10-phenanthroline. I n solution this complex is fluxional with the phenanthroline oscillati ng between nonequivalent bidentate modes by a mechanism which involves rupture of the metal-nitrogen bond and rapid interconversion of two c oordinatively unsaturated T-shaped 14-electron three-coordinate molecu lar fragments. Rates of this fluxion were measured by NMR spectroscopy from the exchange effects on the H-1 signals of the methyl and aromat ic hydrogens. The Delta G double dagger value for the fluxion is 49.6 +/- 4 kJ mol(-1). Dimethyl sulfoxide exchange with all the complexes h as been studied as a function of ligand concentration by H-1 NMR line- broadening, isotopic labeling, and magnetization transfer experiments with deuterated acetone as the solvent. Second-order rate constants we re obtained from linear plots of k(obs) vs [Me(2)SO] and activation pa rameters were obtained from exchange experiments carried out at differ ent temperatures. Second-order kinetics and negative entropies of acti vation indicate an associative mechanism. The lability of dimethyl sul foxide 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-diaminoe thane, k(2)(298) = (1.15 +/- 0.1) x 10(-6) mol(-1) s(-1)) and the last (N-N = 2,9-dimethyl-1,10-phenanthroline, k(2)(298) = (3.81 +/- 0.005) x 10(4) mol(-1) s(-1)) members of the series being greater than 10 or ders of magnitude, as a result of a well-known phenomenon of steric re tardation (for the first complex) and an unprecedented case of steric acceleration (for the last complex). Other factors of primary importan ce in controlling the reactivity are (i) the presence of an extensive pi system on the ligand N-N, (ii) the ease with which this pi system i nteracts with nonbonding d electrons of the metal, and (iii) the flexi bility and ease of elongation of the chelate bite distance. The basici ty plays a somewhat minor role, except in the restricted range of the same class of compounds such as substituted phenanthrolines.