Electron and Hydrogen-Atom Self-Exchange Reactions of Iron and Cobalt Coordination Complexes

Reported here are self-exchange reactions between iron 2,2‘-bi(tetrahydro)pyrimidine (H2bip)
complexes and between cobalt 2,2‘-biimidazoline (H2bim) complexes. The 1H NMR resonances of [FeII(H2bip)3]2+ are broadened upon addition of [FeIII(H2bip)3]3+, indicating that electron self-exchange occurs with
kFe,e- = (1.1 ± 0.2) × 105 M-1 s-1 at 298 K in CD3CN. Similar studies of [FeII(H2bip)3]2+ plus [FeIII(Hbip)(H2bip)2]2+ indicate that hydrogen-atom self-exchange (proton-coupled electron transfer) occurs with kFe,H• =
(1.1 ± 0.2) × 104 M-1 s-1 under the same conditions. Both self-exchange reactions are faster at lower
temperatures, showing small negative enthalpies of activation:  ΔH⧧(e-) = −2.1 ± 0.5 kcal mol-1 (288−320 K) and ΔH⧧(H•) = −1.5 ± 0.5 kcal mol-1 (260−300 K). This behavior is concluded to be due to the
faster reaction of the low-spin states of the iron complexes, which are depopulated as the temperature is
raised. Below about 290 K, rate constants for electron self-exchange show the more normal decrease with
temperature. There is a modest kinetic isotope effect on H-atom self-exchange of 1.6 ± 0.5 at 298 K that
is close to that seen previously for the fully high-spin iron biimidazoline complexes. The difference in the
measured activation parameters, EaD − EaH, is −1.2 ± 0.8 kcal mol-1, appears to be inconsistent with a
semiclassical view of the isotope effect, and suggests extensive tunneling. Reactions of [Co(H2bim)3]2+-d24
with [Co(H2bim)3]3+ or [Co(Hbim)(H2bim)2]2+ occur with scrambling of ligands indicating inner-sphere
processes. The self-exchange rate constant for outer-sphere electron transfer between [Co(H2bim)3]2+ and
[Co(H2bim)3]3+ is estimated to be 10-6 M-1 s-1 by application of the Marcus cross relation. Similar application
of the cross relation to H-atom transfer reactions indicates that self-exchange between [Co(H2bim)3]2+ and
[Co(Hbim)(H2bim)2]2+ is also slow, ≤10-3 M-1 s-1. The slow self-exchange rates for the cobalt complexes
are apparently due to their interconverting high-spin [CoII(H2bim)3]2+ with low-spin Co(III) derivatives.