LABILIZATION BY STERIC CROWDING - HYDROLYSIS AND RACEMIZATION KINETICS OF CO(TMEN)33+, TMEN = 2,3-DIAMINO-2,3-DIMETHYLBUTANE

Hydrolysis and racemization of Co(tmen)33+ (tmen=2,3-diamino-2,3-dimethylbutane) follow the rate law, rate = k[Co(tmen)33+][OH-] in the pH range 4.8 to 13.8. Kinetic parameters are k(hydr) = 2.5 ± 0.8 M-1 s-1 (298 K),ΔH*(hydr) = 1324 ± 3 kJ mol--1, ΔS*(hydr) = 144 ± 23 J K-1 mol-1, ΔV*(hydr) = 57.64±0.9 cm3 mol-1; k(rac) = 273 ± 14 M-1 s-1 (298 K), ΔH*(rac) = 117 ± 3 kJ mol-1, ΔS*(rac) = 87 ± 20 J K-1 mol-1. A conjugate base mechanism D(CB) is assigned to the hydrolysis reaction which proceeds from the singly deprotonated complex via a five-coordinate intermediate followed by a rapid dissociation of the monodentate trans-ligand and cis-trans isomerization to the final trans-Co(tmen)2(H2O)23+. The operation of a D(CB) mechanism is supported by a large positive activation entropy and a remarkably large activation volume of +60 cm3 mol-1. For the faster racemization the deprotonation is followed by the inversion process without CoN bond cleavage corresponding to a Bailar twist. As a consequence of the steric crowding of the twelve peripheral methyl groups, both processes are faster by at least nine orders of magnitude than for the parent Co(en)33t complex. © 1990.