Oxygenation of Co(II) Complexes with Tripodal Ligands

The kinetics of O 2 ‐uptake of five‐coordinated Co 2+ /tren complexes (tren = 2,2′, 2″‐tris(2‐aminoethyl)amine) have been studied extensively. The kinetics of formation of (tren)Co(O 2 , OH)Co(tren) 3+ exhibits two steps. The rate law of O 2 ‐addition, the first step, was of the form: rate = ( k   1 ′ [H + ] + k   1 ″K a )/([H + ] + K a ) [Co(tren) 2+ ][O 2 ]. Second‐order rate constants k   1 ′= 220 ± 19 M −1 s −1 and k   1 ″= 1.8 ± .035 · 10 3 M −1 s −1 agreed well from O 2 ‐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 (p K a = 10.03) agreed well with that derived from kinetic data (p K a = 9.93); k   1 ′and k   1 ″are about a factor 100 smaller than those for the pseudooctahedral Co(trien) (H 2 O)   2 2+ . 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 Co 2+ /tren complexes involves fast preequilibria between Co(tren) (H 2 O) 2+ and Co(tren) (H 2 O)   2 2+and only the latter is assumed to be reactive. The enhanced rate at high pH is explained by rate determining H 2 O‐exchange in the O 2 ‐addition step and the ability of coordinated OH − to labilize the neighbouring H 2 O. This mechanism is furthermore supported by the formation of one kinetically preferred isomer of the peroxo‐bridged dicobalt(III) complex (O 2 cis to the tertiary N‐atom) and the large negative activation entropy (−30 eu). The second step is the intramolecular bridging reaction:is independent of [Co(tren) 2+ ] and [O 2 ] but exhibits a pH‐dependence of the form k 3 = k ′ 3 [H + ]/( K a + [H + ]); k −3 ( = 5 · 10 −5 s −1 ) was determined independently and from the two rate constants the equilibrium constant was calculated as ≈ 10 5 . 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.