Counter-Anion-Directed Supramolecular Organization Modulates Spin Crossover in Mononuclear Mn(III) Schiff−Base Complexes

Abstract

The influence of counter-anion-directed supramolecular organization on the spin-crossover (SCO) behavior of Mn(III) complexes has been systematically investigated using a series of seven mononuclear complexes containing the same SCO-active [Mn(5-Cl-3-Me-sal2-323)]+ cation paired with ClO4− (1), BF4− (2), BPh4− (3), PF6− (4), CF3SO3− (5), NO3− (6), and I− (7) counter anions. Variable-temperature magnetic susceptibility measurements reveal markedly different magnetic behaviors, ranging from a persistent high-spin (HS) state to nearly complete thermally induced spin crossover, despite the complexes possessing nearly identical MnN4O2 coordination environments. Variable-temperature single-crystal X-ray diffraction demonstrates that spin-state changes are accompanied by the characteristic anisotropic structural expansion of the equatorial MnN4 plane, while the axial Mn−O bonds remain essentially unchanged. Detailed analyses of the supramolecular structures reveal that the counter anions direct distinct hydrogen-bonding topologies and crystal packing arrangements. The absence of conventional hydrogen-bonding interactions is associated with the persistent HS state in complex 3, one-dimensional hydrogen-bonded chains are associated with broad and gradual spin crossover in complexes 1, 2, 4, and 5, whereas additional supramolecular cross-linking in complex 6 is associated with incomplete spin-state conversion and increased lattice rigidity. In contrast, an extended two-dimensional hydrogen-bonding network in complex 7 is associated with the largest extent of spin-state conversion in the series despite the comparatively weak individual N−H···I interactions. These results demonstrate that spin-crossover behavior is strongly influenced by the interplay between hydrogen-bond topology, crystal packing, and lattice flexibility, providing mechanistic insights for the rational design of Mn(III)-based spin-crossover materials.