Impact of ligand fields on Kubas interaction of open copper sites in MOFs with hydrogen molecules: an electronic...

We investigate hydrogen sorption on open copper sites in various ligand coordinations of metal-organic frameworks (MOFs), including the triangular T(CuL₃) in MFU-4l, the linear L(CuL₂) in NU2100, and the paddlewheel P(CuL₄)₂ in HKUST-1 from an electronic structure perspective using DFT calculations. The ligand-field-induced splitting of d states and spd hybridizations in copper are thoroughly examined. The hybridization between Cu s, p, and d orbitals occurs in various forms to optimize the Coulomb repulsion of different ligand fields. Despite the Cu⁺ oxidation state, which is typically conducive to strong Kubas interactions with hydrogen molecules, the vacant spd hybrid orbitals of the open copper site in the L(CuL₂) coordination are unsuitable for facilitating electron forward donation, thereby preventing effective hydrogen adsorption. In contrast, the vacant spd hybrid orbitals in the T(CuL₃) and P(CuL₄)₂ coordinations can engage in electron forward donations, forming bonding states between the Cu spd and H₂ σ bonding orbitals. The forward donation in the T(CuL₃) configuration is significantly stronger than in the P(CuL₄)₂ configuration due to both the lower energy of the vacant orbitals and the larger contributions of p and d characters to the hybrid orbital. Additionally, the occupied Cu pd and pd hybrid orbitals in the T(CuL₃) configuration promote electron back donation to the H₂ σ* antibonding orbital, leading to the formation of π bonding states. In the P(CuL₄)₂ coordination, the repulsion from the electron density distributed over the surrounding ligands prevents the H₂ molecule from approaching the copper center closely enough for the back donation to occur. The complete Kubas interaction, involving both forward and back electron donations, results in a large dihydrogen-copper binding energy of 37.6 kJ mol^-1 in the T(CuL₃) coordination. In contrast, the binding energy of 10.6 kJ mol^-1 in the P(CuL₄)₂ coordination is primarily driven by electrostatic interactions with a minor contribution of the Kubas-like forward donation interaction. This analysis highlights the pivotal role of coordination environments in determining the hydrogen sorption properties of MOFs.