Spin-Crossover Effects in Reversible O2 Binding on a Dinuclear Cobalt(II) Complex

Reversible and selective binding of O2 has been demonstrated previously in a series of experiments with di-nuclear and tetra-nuclear cobalt­(II) complexes, including materials containing the redox-active di-cobalt cluster [Co­(II)2(bpbp)­CH3COO]2+ (bpbp– = 2,6-bis­(N,N-bis­(2-pyridylmethyl)­aminomethyl)-4-tert-butylphenolato). In this paper, the electronic mechanism of O2 binding by this di-nuclear cobalt­(II) cluster was studied using density functional theory calculations with a hybrid functional and dispersion correction. O2 physically adsorbs in a high-spin nonet state (S = 4) and chemisorbs in a low-spin singlet state (S = 0). The details of the spin-forbidden reaction mechanism connecting these states reveal a double spin-crossing process where a superoxido cobalt­(III) is an intermediate prior to the generation of the corresponding peroxido dimer. The O2 binding process was shown to be barrierless, while the desorption process had a barrier of 56 kJ/mol. The binding affinity of O2 and N2 with the complex was explored by tuning the electronegativity of the functional groups that are close to the cobalt atoms.