Investigation of the Defect Cu2+ Centers in the Paddle Wheel MOF: Defective Structures, EPR Spectra, and H2O Adsorption

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Due to the different Cu 2+ centers, the ∞ 3 [Cu 2 I Cu 2 II (H 2 O) 2 L 2 Cl 2 ] MOF has been widely studied using electron paramagnetic resonance (EPR) spectra, particularly regarding its capacity for gas molecule adsorption. In the present work, based on the occupation sites of Cu 2+ in ∞ 3 [Cu 2 I Cu 2 II (H 2 O) 2 L 2 Cl 2 ], three corresponding fragments, dimer [CuO 5 ]–[CuO 5 ] (C 0 ), defective [CuO 5 ] (C 1 ), and [CuN 4 ] (C 2 ), are established to investigate the two different EPR signals (S a and S b ) and the local environment of Cu 2+ . By applying the order perturbation formulas of the spin-Hamiltonian parameters, C 0 and C 2 can be excluded because of the EPR silence of the antiferromagnetically coupled Cu 2+ pair at low (7 K) temperatures and the silent signal of Cu 1+ . Thus, two mononuclear Cu 2+ ion defect species (C 1 a and C 1 b ) in ∞ 3 [Cu 2 I Cu 2 II (H 2 O) 2 L 2 Cl 2 ] can account for signals S a and S b, indicating two distinct surrounding environments of Cu 2+ . When the results from the density functional theory calculations are combined, the defective copper paddle wheel units are superior to the pristine ones in the adsorption of H 2 O, which is beneficial to the electrochemical hydrogen evolution reaction (HER) and oxygen evolution reactions (OER). Therefore, the reliability of the perturbation method to investigate the EPR spectra and the adsorption behavior of MOFs is powerfully demonstrated, which can reduce the time consumed by current first-principles calculations.

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