Quantification of the thermodynamic effects of the low-spin - high-spin interaction in molecular crystals of a mononuclear iron(ii) spin crossover complex.

A method is proposed to estimate the energetic and entropic effects of spins of neighbouring molecules on the spin transition of a mononuclear spin crossover (SCO) complex in a molecular crystal. Density functional theory (DFT) methods have been used to model the SCO material [FeII(Lnpdtz)2(NCS)2] (Lnpdtz = 2-naphthyl-5-pyridyl-1,2,4-thiadiazole) exhibiting numerous pi-pi interactions using a 2D arrangement of 15 molecules. The modelling considers only the effects in the crystallographical ac plane with a particularly pronounced stacking but paves the way for future work with 3D arrangements which are computational much more costly. It involves the optimisation and normal mode calculation of the molecules in a rigid matrix of both low-spin (LS) and high-spin (HS) neighbours. This procedure has been used to calculate the previously defined cooperativity parameter Hcoop (S. Rackwitz, W. Klopper, V. Sch & uuml;nemann and J. A. Wolny, Phys. Chem. Chem. Phys., 2013, 15, 15450). For [FeII(Lnpdtz)2(NCS)] we obtain Hcoop = 11 kJ mol-1, a value which is comparable to those found for 3D polynuclear spin crossover materials. A normal mode analysis of the optimised centrally located molecule indicates that the vibrational entropy of the spin transition is somewhat higher (5 J K-1 mol-1) for the LS to HS transition in the LS matrix than in the HS one. The calculations show that the interactions with the neighbours influence the low-frequency modes with wave numbers <65-70 cm-1. These cause the main difference in the vibrational entropy of the spin transition for the vicinity of high- and low-spin molecules. Furthermore, a deformation of the coordination sphere of the central molecule is observed when the spins of the surrounding centres are switched. This deformation is accompanied by a change in the equatorial Fe-N bond lengths.