Tuning Affinity And Reversibility For O-2 Binding In Dinuclear Co(Ii) Complexes

The O-2 binding affinity of a series of dicobalt(II) complexes can be tuned between p(O-2)(50%) = 2.3 x 10(-3) and 700 x 10(-3) atm at 40 degrees C by varying the number of H and Cl atoms in the bridging acetato ligands of [CO2(bpbp)(CH(3-n)ClnCO2)(CH3CN)(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 [CO2(bpbp)(O-2)(CH3CO2)](2+), [Co-2(bpbp)(O-2)(CH2ClCO2)](2+), [Co-2(bpbp)(O-2)(CHCl2CO2)](2+) and [Co-2(bpbp)(O-2)(CCl3CO2)](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(center dot-) 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) angstrom) 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)(C10H7O2)](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.