Spin crossover in metal-organic frameworks: A crystal embedded multi-reference study

Spin crossover (SCO) in transition metal (TM)-containing solid state materials remains a challenge for the electronic structure calculations as some of the electronic states may have a significant multi-reference character. The periodic effective Hamiltonian of crystal field (pEHCF) method accurately describes strong correlations in TM-containing crystalline systems. In this work, pEHCF has been applied to study the electronic structure of the high spin and low spin states in the Fe(pyridine)2Ni(CN)4 metal-organic framework (MOF). The relative energy of the spin states involved in SCO has been calculated, and the degeneracy line exhibiting a strong dependence on the distance between an Fe ion and the CN groups has been identified. The degeneracy line also displays a step-like dependence on the position of the pyridine ligands in the narrow interval of 2.08-2.10 Å, while outside this interval, the dependence is weak. Low-temperature paramagnetism of the Fe(pyridine)2Ni(CN)4 SCO-MOF has been explained by the triplet ground state of Ni in the square-planar coordination with the CN groups. The electronic structure of a recently synthesized Fe2(H0.67bdt)3 SCO-MOF has been also investigated. This MOF contains two types of Fe ions and exhibits unusual spin crossover behavior above room temperature. Our calculations confirm that in the temperature range of 300-423 K, Fe2 ions undergo a spin transition from quintet (S = 2) to singlet (S = 0), while Fe1 ions exist in the low-spin configuration in both initial (300 K) and final structures (423 K).