Kinetic and equilibrium studies of reactions of N-heterocycles with dimeric and monomeric oxorhenium(V) complexes

Equilibrium constants have been evaluated for the reaction {MeReO(edt)}(2) + 2 L --> 2 MeReO(edt)L, where edt is 1,2-ethanedithiolate and L is any of 13 N-donor heterocyclic ligands. The values of K range from 1.37(27)x10(-2) for pyrimidine to 1.95(6)x10(6) for imidazole at 25 degreesC in chloroform. A successful correlation of log K with log (K-a) of HL+ was realized except in the case of the 2-substituted ligands 2-picoline and quinoline, where steric effects make K smaller than expected from the proton basicity of L. The kinetics of the same reactions were studied; the rate law for the reaction in the forward direction is given by -d[{MeReO(edt)}(2)]/dt = {k(a) + k(b)[L]}[L] x [{MeReO(edt)}(2)]. Except for 2-picoline and quinoline, the major pathway is provided by the term that shows the quadratic dependence on [L]. Values of log (k(b)) also correlate with log K, and therefore necessarily with log (K-a).