Structural insights into the counterion effects on the manganese(iii) spin crossover system with hexadentate Schiff-base ligands

Abstract

A series of new salts [Mn(5-MeO-sal-N-1,5,8,12)]Y (Y = ClO₄ for 1, Y = BF₄ for 2, Y = NO₃ for 3 and Y = CF₃SO₃ for 4) based on the six-coordinated mononuclear manganese(III) Schiff-base complex cation [Mn(5-MeO-sal-N-1,5,8,12)]⁺, has been investigated to determine the impact of counter anion effects, intramolecular ligand distortion and intermolecular supramolecular structures on the spin crossover (SCO) behavior. The SCO in salt 1 has resulted in a crystallographic observation of the coexistence of high-spin (HS, S = 2) and low-spin (LS, S = 1) manganese(III) complex cations in equal proportions around 100 K. At room temperature, the two crystallographically distinct manganese centers are both close to the complete HS state. Only one of the two slightly different units undergoes SCO in the temperature range 300–180 K, whereas the other remains in the HS state down to 20 K. For salts 2 and 3, crystal structural analysis indicates change in the anion from ClO₄⁻ to BF₄⁻ and NO₃⁻ was led to the close arrangement of the cations and the stacking between phenyl groups from the ligands. With CF₃SO₃⁻ as the counterion, although the cations and the anions separate clearly in one direction, the close arrangement of cations in other directions precludes the spin transformation of the Mn(III) cations. Magnetic measurements on 2–4 indicate that the manganese(III) complex cations remain in the HS state in the temperature range 2–300 K.