Modulation of Mn³⁺ Spin State by Guest Molecule Inclusion

Spin state preferences for a cationic Mn³⁺ chelate complex in four different crystal lattices are investigated by crystallography and SQUID magnetometry. The [MnL₁]⁺ complex cation was prepared by complexation of Mn³⁺ to the Schiff base chelate formed from condensation of 4-methoxysalicylaldehyde and 1,2-bis(3-aminopropylamino)ethane. The cation was crystallized separately with three polyatomic counterions and in one case was found to cocrystallize with a percentage of unreacted 4-methoxysalicylaldehyde starting material. The spin state preferences of the four resultant complexes [MnL₁]CF₃SO₃·xH₂O, (1), [MnL₁]PF₆·xH₂O, (2), [MnL₁]PF₆·xsal·xH₂O, (2b), and [MnL₁]BPh₄, (3), were dependent on their ability to form strong intermolecular interactions. Complexes (1) and (2), which formed hydrogen bonds between [MnL₁]⁺, lattice water and in one case also with counterion, showed an incomplete thermal spin crossover over the temperature range 5-300 K. In contrast, complex (3) with the BPh₄^-, counterion and no lattice water, was locked into the high spin state over the same temperature range, as was complex (2b), where inclusion of the 4-methoxysalicylaldehyde guest blocked the H-bonding interaction.