Oxygenation of Co(II) Complexes with Tripodal Ligands

Abstract

AbstractThe kinetics of O2‐uptake of five‐coordinated Co2+/tren complexes (tren = 2,2′, 2″‐tris(2‐aminoethyl)amine) have been studied extensively. The kinetics of formation of (tren)Co(O2, OH)Co(tren)3+ exhibits two steps. The rate law of O2‐addition, the first step, was of the form: rate = (k[H+] + kKa)/([H+] + Ka) [Co(tren)2+][O2]. Second‐order rate constants k = 220 ± 19 M−1s−1 and k = 1.8 ± .035 · 103M−1s−1 agreed well from O2‐uptake and (stopped‐flow) spectrophotometric measurements. The protonation constant of the hydroxo complex obtained by equlibrium measurements (spectrophotometric and by pH‐titration) in anaerobic conditions (pKa = 10.03) agreed well with that derived from kinetic data (p Ka = 9.93); k and k are about a factor 100 smaller than those for the pseudooctahedral Co(trien) (H2O). This and the fact that several other Co(II) complexes with five‐coordinated geometry do not exhibit oxygen affinity led to the proposal that the oxygenation mechanism for Co2+/tren complexes involves fast preequilibria between Co(tren) (H2O)2+ and Co(tren) (H2O) and only the latter is assumed to be reactive. The enhanced rate at high pH is explained by rate determining H2O‐exchange in the O2‐addition step and the ability of coordinated OH− to labilize the neighbouring H2O. This mechanism is furthermore supported by the formation of one kinetically preferred isomer of the peroxo‐bridged dicobalt(III) complex (O2 cis to the tertiary N‐atom) and the large negative activation entropy (−30 eu). The second step is the intramolecular bridging reaction: equation image is independent of [Co(tren)2+] and [O2] but exhibits a pH‐dependence of the form k3 = k′3[H + ]/(Ka + [H+]); k−3 ( = 5 · 10−5 s−1) was determined independently and from the two rate constants the equilibrium constant was calculated as ≈ 105. The ligand combination as in Co(tren)2+ was shown to provide an excellent balance to form a reversible oxygen carrier; possible reasons for this are discussed.