Modulation of the spin crossover behaviour in four manganese(iii) complexes through cation–anion interactions

Abstract

Here we report the synthesis, structural and magnetic characterization of a series of manganese(III) monomers, formulated as [Mn(L)]X with X = PF₆⁻ (1), BF₄⁻ (2), I⁻ (3) and Br⁻/NO₃⁻ (4). These four compounds have been prepared with a Schiff base ligand (H₂L) formed by the 2 : 1 condensation of an eugenol scaffold aldehyde and N,N′-bis(3-aminopropyl)ethylene-diamine. The structures of compounds 1–4 show in all cases the presence of the same [Mn^III(L)]⁺ cation surrounded by different X⁻ anions. Interestingly, the anions play a key role in determining not only the symmetry and cation–anion packing, which are quite different in compound 1 compared to those of compounds 2–4, but also the magnetic properties of the [Mn^III(L)]⁺ cation. Variable temperature magnetic measurements show that in compound 1 the Mn^III centre is in the high spin S = 2 state at any temperature, whereas compounds 2–4 show a smooth and incomplete spin crossover (SCO) from the low spin (S = 1) to the high spin (S = 2) ground state in the temperature range of 200–400 K. At 400 K the high spin fractions are around 31%, 27% and 42% for compounds 2–4, respectively. Isothermal magnetization measurements at low temperatures confirm the spin ground states in all cases. The magnetic data can be very well reproduced with a simple monomeric S = 2 (in 1) or S = 1 (in 2–4, for the low spin state) model with a zero-field splitting (ZFS) contribution (and a very weak intermolecular antiferromagnetic interaction in 2 and 4). Additionally, a detailed analysis of the structural parameters in compounds 2–4 shows that the Mn–N bond distances, the continuous shape parameters, the trigonal distortion angles (Φ) and the octahedral distortion parameters (Σ) show a significant increase when the temperature is increased from 100 to 398 K, confirming the presence of an incomplete SCO between 100 and 398 K in compounds 2–4.