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 (ΔHf) of the as-synthesized Cu-HKUST-H2O ([Cu3TMA2·3H2O]·5DMF) is −52.70 ±
0.34 kJ per mole of Cu. The ΔHf for
Zn-HKUST-DMF ([Zn3TMA2·3DMF]·2DMF)
is −54.22 ± 0.57 kJ per mole of Zn. The desolvated Cu-HKUST-dg
[Cu3TMA2] has a ΔHf of 16.66 ± 0.51 kJ/mol per mole Cu. The ΔHf for Zn-HKUST-amorph [Zn3TMA2·2DMF] 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.