Kinetic and mechanistic study with optically active, four-coordinate nickel(II) complexes: Stereoselectivity in ligand substitution

Conventional and rapid scan stopped-flow spectrophotometry as well as polar imetry was used to study the kinetics of ligand substitution in six chiral bis N-alkylsalicylardiminato nickel(II) complexes NiA(2) by different chira l salen-type ligands H2B, according to NiA(2) + H2B - NiB + 2HA, in acetone at 298 K and, partly, at variable temperature. In most cases ligand substi tution was found to follow monophasic second-order kinetics, rate = k x [Ni A(2)] x [H2B]. Second-order rate constant k, lying in the range 10(-2) - 40 0 M(-1)s(-1) at 298 K,was determined for the various combinations of enanti omers in a given system NiA(2)/H2B, namely, R-NiA(2)/R-H2B, S-NiA(2)/R-H2B, R-NiA(2)/S-H2B, and S-NiA(2)/S-H2B. It was found that ligand substitution is subject to chiral discrimination. The ratio of second-order rate constan ts, k(fast)/k(slow), with k(fast) being rate constant k for the faster reac ting pair of enantiomers and vice versa, lies in the range 1.0-3.0, dependi ng on the nature of the N-alkyl groups in NiA2 and organic groups attached to the ethylene bridge in the salen ligands H2B. The rate discrimination fa ctor of 3.0, as obtained for NiA(2) = bis [N-dehydroabietylsalicylaldiminat o]nickel(II) reacting with the R- and with the S-enantiomer of H2B = N,N'-d isalicylidene-1,2-diamino-4-methylpentane, appears to be the highest stereo selectivity reported so far for ligand substitution in nickel(II) complexes .. With NiA2 = R- and S-bis[N-(1-phenylethyl)-5-nitrosalicylaldiminato]nick el(II) and H2B = R- and S-N,N'-disalicylidene-1,2-diamino-4-methylpentane, the kinetics of ligand substitution are biphasic, describing initial adduct formation between NiA(2) and H2B (equilibrium constant K) and stepwise los s of the two bidentate ligands HA (first-order rate constants k(1) and k(2) ) The data for K, k(1), and k(2) for one of the combinations of enantiomers were determined at variable temperature, and the corresponding activation parameters are presented.