Thermochemistry of Paddle Wheel MOFs: Cu-HKUST-1 and Zn-HKUST-1

Metal organic framework (MOF) porosity relies upon robust metal-organic bonds to retain structural rigidity upon solvent removal. Both the as-synthesized and activated Cu and Zn polymorphs of HKUST-1 were studied by room temperature acid solution calorimetry. Their enthalpies of formation from dense assemblages (metal oxide (ZnO or CuO), trimesic acid (TMA), and N,N-dimethylformamide (DMF)) were calculated from the calorimetric data. The enthalpy of formation (Delta H-f) of the as-synthesized Cu-HKUST-H2O ([Cu(3)TMA(2)center dot 3H(2)O]center dot 5DMF) is -52.70 +/- 0.34 kJ per mole of Cu. The Delta H-f for Zn-HKUST-DMF ([Zn(3)TMA(2)center dot 3DMF]center dot 2DMF) is -54.22 +/- 0.57 kJ per mole of Zn. The desolvated Cu-HKUST-dg [Cu(3)TMA(2)] has a Delta H-f of 16.66 +/- 0.51 kJ/mol per mole Cu. The Delta H-f for Zn-HKUST-amorph [Zn(3)TMA(2)center dot 2DMF] is -3.57 +/- 0.21 kJ per mole of Zn. Solvent stabilizes the Cu-HKUST-H2O by -69.4 kJ per mole of Cu and Zn-HKUST-DMF by at least -50.7 kJ per mole of Zn. Such strong chemisorption of solvent is similar in magnitude to the strongly exothermic binding at low coverage for chemisorbed H2O on transition metal oxide nanoparticle surfaces. The strongly exothermic solvent-framework interaction suggests that solvent can play a critical role in obtaining a specific secondary building unit (SBU) topology.