Tuning affinity and reversibility for O_2 binding in dinuclear Co(ii) complexes

摘要

The O_2 binding affinity of a series of dicobalt(ii) complexes can be tuned between p(O_2)_(50%) = 2.3 × 10 ~(-3) and 700 × 10~(-3) atm at 40 °C by varying the number of H and Cl atoms in the bridging acetato ligands of [Co _2(bpbp)(CH_((3-n))Cl_nCO_2)(CH _3CN)_2]~(2+), where bpbp- = 2,6-bis(N,N-bis(2-pyridylmethyl)aminomethyl)-4-tert-butylphenolate and n = {0, 1, 2, 3}. O_2 binds most strongly to the deoxy complex containing the acetato bridge and the O_2 affinity decreases linearly as the number of Cl atoms is increased from 0 to 3 in [Co_2(bpbp)(O _2)(CH_3CO_2)]~(2+), [Co _2(bpbp)(O_2)(CH_2ClCO_2)] ~(2+), [Co_2(bpbp)(O_2)(CHCl_2CO _2)]~(2+) and [Co_2(bpbp)(O_2)(CCl _3CO_2)]~(2+). The O_2 affinities can be qualitatively correlated with both the pK_a value of the parent acetic or chloroacetic acid and the redox potential of the O_2 ~(2-)/O_2 - couple measured for the peroxide-bridged complexes. The redox potential varies between 510 mV (vs. Fc~(0/+)) for the acetato-bridged complex to 696 mV for the trichloroacetato-bridged system. Despite the clear difference in reactivity in solution, there are no clear trends which can be correlated to O_2 affinity in the O-O bond lengths in the X-ray crystal structures at 180 K (1.415(4)-1.424(2) ?) or in the frequencies of the peroxido O-O stretch in the solid-state resonance Raman spectra at 298 K (830-836 cm~(-1)). Using density functional theory calculations, we conclude that the Co(ii) atoms of the deoxy complexes coordinate solvent molecules as auxiliary ligands and that a conformation change of the ligand is involved in the reversible O_2 binding process. The alternative of five coordination in the deoxy Co(ii) complexes is therefore seen as less likely. The crystal structure and p(O_2)_(50%) are also reported for the 1-naphthoato-bridged oxy complex [Co_2(bpbp) (O_2)(C_(10)H_7O_2)]~(2+), and the O_2 binding affinity in that case is also qualitatively consistent with the expectation from the pK_a of the parent 1-naphthoic acid. This journal is