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

Abstract

Due to the different Cu²⁺ centers, the _∞³[Cu₂^ICu₂^II(H₂O)₂L₂Cl₂] 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 Cu²⁺ in _∞³[Cu₂^ICu₂^II(H₂O)₂L₂Cl₂], three corresponding fragments, dimer [CuO₅]-[CuO₅] (C₀), defective [CuO₅] (C₁), and [CuN₄] (C₂), are established to investigate the two different EPR signals (S_a and S_b) and the local environment of Cu²⁺. By applying the order perturbation formulas of the spin-Hamiltonian parameters, C₀ and C₂ can be excluded because of the EPR silence of the antiferromagnetically coupled Cu²⁺ pair at low (7 K) temperatures and the silent signal of Cu¹⁺. Thus, two mononuclear Cu²⁺ ion defect species (C₁^a and C₁^b) in _∞³[Cu₂^ICu₂^II(H₂O)₂L₂Cl₂] can account for signals S_a and S_b, indicating two distinct surrounding environments of Cu²⁺. 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 H₂O, 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.