Theoretical approach to the one-step versus two-step spin transitions in Hofmann-like FeII SCO metal-organic frameworks

Guest molecules in the 3D Hofmann-type FeII spin-crossover (SCO) metal-organic frameworks modulate the magnetic properties of the host, modifying the spin state, transition temperature, width of the hysteresis loop and are responsible for the occurrence of a single-step or multistep spin transition. In this work we explore the guest-dependent SCO properties of the Hofmann-like FeII SCO clathrates with formula {Fe(bpb)[Pt(CN)4]}⋅2G. We use a computational strategy based on DFT periodic calculations on the whole system, CASSCF/NEVPT2 calculations on single metal fragments and plots of the reduced density gradients to identify and quantify the dominant effects governing the occurrence of one-step or two-step spin transitions in these systems. Our results inform about the strength of the ligand field around Fe sites, the relative stability of the different spin states, the amplitude and nature of the host-guest and guest-guest interactions, and the role of the vibronic effects on the SCO properties of Hofmann-type FeII clathrates.