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

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 (<b>H</b>_<b>2</b><b>L</b>) formed by the 2 : 1 condensation of an eugenol scaffold aldehyde and <i>N</i>,<i>N</i>'-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 <i>S</i> = 2 state at any temperature, whereas compounds 2-4 show a smooth and incomplete spin crossover (SCO) from the low spin (<i>S</i> = 1) to the high spin (<i>S</i> = 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 <i>S</i> = 2 (in 1) or <i>S</i> = 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 (<i>Φ</i>) and the octahedral distortion parameters (<i>Σ</i>) 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.