Paddlewheel SBU based Zn MOFs: Syntheses, Structural Diversity, and CO₂ Adsorption Properties

Four Zn metal⁻organic frameworks (MOFs), {[Zn2(2,6-ndc)2(2-Pn)]·DMF}n (1), {[Zn2(cca)2(2-Pn)]·DMF}n (2), {[Zn2(thdc)2(2-Pn)]·3DMF}n (3), and {[Zn2(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-H2ndc), 4-carboxycinnamic acid (H2cca), 2,5-thiophenedicarboxylic acid (H2thdc), and naphthalene-1,4-dicarboxylic acid (1,4-H2ndc), respectively. MOFs 1⁻4 were all constructed from similar dinuclear paddlewheel {Zn2(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 48·67-net with 17.6% and 16.8% extra-framework voids, respectively, 3 adopts a pillared-layer open framework of 48·66·8-topology with sufficient free voids of 39.9%, and 4 features a pcu-type pillared-layer framework of 412·63-topology with sufficient free voids of 30.9%. CO2 sorption studies exhibited typical reversible type I isotherms with CO2 uptakes of 55.1, 84.6, and 64.3 cm3 g−1 at 195 K and P/P0 =1 for the activated materials 1′, 2′, and 4′, respectively. The coverage-dependent isosteric heat of CO2 adsorption (Qst) gave commonly decreased Qst traces with increasing CO2 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