Selective, Tunable O_2 Binding in Cobalt(Ⅱ)-Triazolate/Pyrazolate Metal-Organic Frameworks

摘要

The air-free reaction of CoCl_2 with 1,3,5-tri(1H-1,2,3-triazol-5-yl)benzene (H_3BTTri) in N,N-dimethylforma-mide (DMF) and methanol leads to the formation of Co-BTTri (Co_3[(Co_4Cl)_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(Ⅰ) 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) A. Replacement of one of the triazolate linkers with a more electron-donating pyrazolate group leads to the isostructural framework Co-BDTriP (Co_3[(Co_4Cl)_3(BDTriP)_8]_2•DMF; H_3BDTriP = 5,5'-(5-(1H-pyrazol-4-yl)-l,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.