Selective, Tunable O2 Binding in Cobalt(II)–Triazolate/Pyrazolate Metal–Organic Frameworks

• Published in: Journal of the American Chemical Society 2016-06, Vol.138 (22), p.7161-7170
• Main Authors: Xiao, Dianne J, Gonzalez, Miguel I, Darago, Lucy E, Vogiatzis, Konstantinos D, Haldoupis, Emmanuel, Gagliardi, Laura, Long, Jeffrey R
• Format: Article
• Language: English
• Subjects: Binding; Cobalt; Electronic structure; Electronics; Heat of adsorption; Materials selection; Metal-organic frameworks; Methyl alcohol
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/jacs.6b03680

The air-free reaction of CoCl2 with 1,3,5-tri­(1H-1,2,3-triazol-5-yl)­benzene (H3BTTri) in N,N-dimethylformamide (DMF) and methanol leads to the formation of Co-BTTri (Co3[(Co4Cl)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 O2 over N2, 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 O2 binds end-on to each framework cobalt center in a 1:1 ratio with a Co–O2 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 (Co3[(Co4Cl)3(BDTriP)8]2·DMF; H3BDTriP = 5,5′-(5-(1H-pyrazol-4-yl)-1,3-phenylene)­bis­(1H-1,2,3-triazole)), which demonstrates markedly higher yet still fully reversible O2 affinities (Q st = −47(1) kJ/mol at low loadings). Electronic structure calculations suggest that the O2 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 O2 adsorption capacity of these materials render them promising new adsorbents for air separation processes.