First step in preparation of multifunctional spin crossover material based on Fe(II) complex of cyclam-based ligand. Magnetism and DFT studies

Abstract

With the aim to make the first step in preparation of magnetic multifunctional material combining spin crossover (SCO) together with single-molecule magnet (SMM) properties in one molecule (polynuclear complex), a specially designed ligand L-NH2 (1,8-bis(4-aminobenzyl)-4,11-bis(pyridin-2-ylmethyl)-1,4,8,11-tetraazacyclotetradecane) was synthesized. It contains (i) macrocyclic cyclam core substituted with two 2-pyridylmethyl pendant arms suitable for complexation of Fe(II), because such system was previously proven to provide SCO, and (ii) two p-aminobenzyl pendant arms introducing NH2 groups suitable for consequent attachment of SMM molecule (e.g. transformation to functional groups for selective complexation of other metals). Furthermore, Fe(II) complex [Fe(L-NH2)]Cl2·2.5H2O was prepared and characterized. Unfortunately, according to the magnetic measurements, SCO behavior was not observed and the complex remained in high-spin state with relatively high magnetic anisotropy (D = –17.7 cm−1, E/D = 0.31). This magnetic behavior was rationalized by the DFT computational study, which identified high-spin state as preferential for prepared complex in contrast to other structurally similar SCO systems with low-spin state preference. The most probable reason behind this difference appeared to be the electron-withdrawing effect of benzyl groups providing weaker ligand field of macrocyclic nitrogen donor atoms.