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 polarimetry was used to study the kinetics of ligand substitution in six chiral bis N-alkylsalicylaldiminato nickel(II) complexes NiA2 by different chiral salen-type ligands H2B, according to NiA2 + H2B --> NiB + 2HA, in acetone at 298 K and, partly, at variable temperature. In most cases ligand substitution was found to follow monophasic second-order kinetics, rate = k x NiA2] x H2B]. Second-order rate constant k, lying in the range 10(-2)-400 M(-1) s(-1) at 298 K, was determined for the various combinations of enantiomers in a given system NiA2/H2B, namely, R-NiA2/R-H2B, S-NiA2/R-H2B, R-NiA2/S-H2B, and S-NiA2/S-H2B. It was found that ligand substitution is subject to chiral discrimination. The ratio of second-order rate constants, kfast/kslow, with kfast being rate constant k for the faster reacting pair of enantiomers and vice versa, lies in the range 1.0-3.0, depending 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 factor of 3.0, as obtained for NiA2 = bisN-dehydroabietylsalicylaldiminato]nickel(II) reacting with the R- and with the S-enantiomer of H2B = N,N'-disalicylidene-1,2-diamino-4-methylpentane, appears to be the highest stereoselectivity reported so far for ligand substitution in nickel(II) complexes. With NiA2 = R- and S-bisN-(1-phenylethyl)-5-nitrosalicylaldiminato]nickel(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 NiA2 and H2B (equilibrium constant K) and stepwise loss of the two bidentate ligands HA (first-order rate constants k1 and k2). The data for K, k1, and k2 for one of the combinations of enantiomers were determined at variable temperature, and the corresponding activation parameters are presented.