Paddlewheel SBU based Zn MOFs: Syntheses, Structural Diversity, and CO(2) Adsorption Properties

Four Zn metal–organic frameworks (MOFs), {[Zn(2)(2,6-ndc)(2)(2-Pn)]·DMF}(n) (1), {[Zn(2)(cca)(2)(2-Pn)]·DMF}(n) (2), {[Zn(2)(thdc)(2)(2-Pn)]·3DMF}(n) (3), and {[Zn(2)(1,4-ndc)(2)(2-Pn)]·1.5DMF}(n) (4), were synthesized from zinc nitrate and N,N′-bis(pyridin-2-yl)benzene-1,4-diamine (2-Pn) with naphthalene-2,6-dicarboxylic acid (2,6-H(2)ndc), 4-carboxycinnamic acid (H(2)cca), 2,5-thiophenedicarboxylic acid (H(2)thdc), and naphthalene-1,4-dicarboxylic acid (1,4-H(2)ndc), respectively. MOFs 1–4 were all constructed from similar dinuclear paddlewheel {Zn(2)(COO)(4)} clusters and resulted in the formation of three kinds of uninodal 6-connected non-interpenetrated frameworks. MOFs 1 and 2 suit a topologic 4(8)·6(7)-net with 17.6% and 16.8% extra-framework voids, respectively, 3 adopts a pillared-layer open framework of 4(8)·6(6)·8-topology with sufficient free voids of 39.9%, and 4 features a pcu-type pillared-layer framework of 4(12)·6(3)-topology with sufficient free voids of 30.9%. CO(2) sorption studies exhibited typical reversible type I isotherms with CO(2) uptakes of 55.1, 84.6, and 64.3 cm(3) g(−1) at 195 K and P/P(0) =1 for the activated materials 1′, 2′, and 4′, respectively. The coverage-dependent isosteric heat of CO(2) adsorption (Q(st)) gave commonly decreased Q(st) traces with increasing CO(2) uptake for all the three materials and showed an adsorption enthalpy of 32.5 kJ mol(−1) for 1′, 38.3 kJ mol(−1) for 2′, and 23.5 kJ mol(−1) for 4′ at zero coverage.