Selective, Tunable O(2) Binding in Cobalt(II)–Triazolate/Pyrazolate Metal–Organic Frameworks

[Image: see text] The air-free reaction of CoCl(2) with 1,3,5-tri(1H-1,2,3-triazol-5-yl)benzene (H(3)BTTri) in N,N-dimethylformamide (DMF) and methanol leads to the formation of Co-BTTri (Co(3)[(Co(4)Cl)(3)(BTTri)(8)](2)·DMF), a sodalite-type metal–organic framework. Desolvation of this material generates coordinatively unsaturated low-spin cobalt(II) centers that exhibit a strong preference for binding O(2) over N(2), with isosteric heats of adsorption (Q(st)) of −34(1) and −12(1) kJ/mol, respectively. The low-spin (S = 1/2) electronic configuration of the metal centers in the desolvated framework is supported by structural, magnetic susceptibility, and computational studies. A single-crystal X-ray structure determination reveals that O(2) binds end-on to each framework cobalt center in a 1:1 ratio with a Co–O(2) bond distance of 1.973(6) Å. Replacement of one of the triazolate linkers with a more electron-donating pyrazolate group leads to the isostructural framework Co-BDTriP (Co(3)[(Co(4)Cl)(3)(BDTriP)(8)](2)·DMF; H(3)BDTriP = 5,5′-(5-(1H-pyrazol-4-yl)-1,3-phenylene)bis(1H-1,2,3-triazole)), which demonstrates markedly higher yet still fully reversible O(2) affinities (Q(st) = −47(1) kJ/mol at low loadings). Electronic structure calculations suggest that the O(2) adducts in Co-BTTri are best described as cobalt(II)–dioxygen species with partial electron transfer, while the stronger binding sites in Co-BDTriP form cobalt(III)–superoxo moieties. The stability, selectivity, and high O(2) adsorption capacity of these materials render them promising new adsorbents for air separation processes.