Dual Modulation of Spin-Crossover Behavior in [Fe(tpma)(xbim)]²⁺ Complexes by Chemical and Mechanical Pressures

We present a comprehensive investigation into effects of both chemical and mechanical compression on the spin-crossover behavior in a family of Fe(II) complexes, [Fe(tpma)(xbim)]X₂ (tpma = tris(2-pyridylmethyl)amine; xbim = 1,1'-(α,α'-o-xylyl)-2,2'-biimidazole; X^- = BPh₄^- (1), I^- (2), BF₄^- (3), and ClO₄^- (4)). By systematically varying the counteranion (X), we probe the influence of crystal packing on spin-state energetics. Single-crystal X-ray diffraction, Monte Carlo-based packing efficiency analysis, and magnetic susceptibility measurements reveal that complexes with tighter crystal packing (corresponding to a higher "chemical pressure") exhibit higher spin transition temperatures (T_1/2) due to stabilization of the low-spin (LS) state. In contrast, loosely packed structures tend to stabilize the high-spin state, resulting in lower T_1/2 values. Under hydrostatic mechanical pressure, 4 exhibits a continuous increase in T_1/2 from 199 K at ambient pressure to 315 K at 0.46 GPa. The pressure-induced widening of thermal hysteresis and higher T_1/2 values reflect enhanced elastic interactions and lattice compression, which favors the smaller-volume LS state. All compounds also exhibit light-induced excited spin-state trapping effects. Together, these findings provide a unified view of chemical and physical compression effects on the spin transition behavior, offering guidance for designing novel pressure- and temperature-responsive switches and sensors.