Modeling molecular crystals formed by spin-active metal complexes by atom–atom potentials

Abstract

We apply the atom-atom potentials to molecular crystals of iron (II) complexes with bulky organic ligands. The crystals under study are formed by low-spin or high-spin molecules of Fe(phen)2(NCS)2 (phen = 1,10-phenanthroline), Fe(btz)2(NCS)2 (btz = 5,5 ′ ,6,6 ′ -tetrahydro-4H,4 ′ H -2,2 ′ -bi-1,3-thiazine), and Fe(bpz)2(bipy) (bpz = dihydrobis(1-pyrazolil)borate, and bipy = 2,2 ′ -bipyridine). All molecular geometries are taken from the X-ray experimental data and assumed to be frozen. The unit cell dimensions and angles, positions of the centers of masses of molecules, and the orientations of molecules corresponding to the minimum energy at 1 atm and 1 GPa are calculated. The optimized crystal structures are in a good agreement with the experimental data. Sources of the residual discrepancies between the calculated and experimental structures are discussed. The intermolecular contributions to the enthalpy of the spin transitions are found to be comparable with its total experimental values. It demonstrates that the method of atom-atom potentials is very useful for modeling organometalic crystals undergoing the spin transitions.

Phenomenological model of spin crossover in molecular crystals as derived from atom–atom potentials

“…Molecular crystals of coordination compounds of d-elements, capable of spin crossover transitions (SCT), are at present under active investigation. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] The average crystal field splitting parameter 10Dq in these crystals is comparable with the average pairing energy of the d-electrons which makes it possible that the high-spin (HS) and low spin (LS) forms have close energies and may coexist in a crystal. The external thermodynamical parameters, such as temperature or pressure, affect the equilibrium fractions of the spin forms.…”

“…This model treats the solid undergoing a spin transition as a regular solution (mixture) of molecules in the LS and HS states. The Gibbs energy of the system is: G(x,T) = G(0,T) + x(DH À TDS) + Gx(1 À x) À TS mix , (1) where x is the fraction of the HS molecules; DH and DS are the variations of the enthalpy and the entropy in the course of the SCT (they may be independently obtained from calorimetric measurements, like in ref. 22, or phenomenologically fitted to describe the experimental x(T) curve, like in ref.…”

“…), which prevent construction of a complete theory, especially due to importance of the short intermolecular contacts (p-p interactions, SÁ Á ÁH-C interactions, hydrogen bonds, etc. 1,3 ) tentatively responsible for the cooperativity effects (see, however, section IV).…”

Elektroneninduzierter Spin‐Crossover von Einzelmolekülen in einer Doppellage auf Gold

“…Moleküle in der zweiten Lage zeigen eine entgegengesetzte Orientierung (Abbildung 1 b). Diese Geometrie ermöglicht eine günstige π‐π Wechselwirkung der Phenanthrolingruppen, wie es von Kristallen ähnlicher Moleküle bekannt ist 29…”

Rapid growing cystic variant of choroid plexus papilloma in a fetal cerebral hemisphere

“…Second‐layer molecules exhibit an opposite orientation (Figure 1 b). This geometry enables a favorable π–π interaction of the phenanthroline groups as known from bulk crystals of similar molecules 29…”