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} (1), {[Zn₂(cca)₂(2-Pn)]·DMF} (2), {[Zn₂(thdc)₂(2-Pn)]·3DMF} (3), and {[Zn₂(1,4-ndc)₂(2-Pn)]·1.5DMF} (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 (Q) gave commonly decreased Q 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.