Optimization of crystal packing in semiconducting spin-crossover materials with fractionally charged TCNQ(δ−) anions (0 < δ < 1)

Co-crystallization of the prominent Fe(ii) spin-crossover (SCO) cation, [Fe(3-bpp)(2)](2+) (3-bpp = 2,6-bis(pyrazol-3-yl)pyridine), with a fractionally charged TCNQ(δ−) radical anion has afforded a hybrid complex [Fe(3-bpp)(2)](TCNQ)(3)·5MeCN (1·5MeCN, where δ = −0.67). The partially desolvated material shows semiconducting behavior, with the room temperature conductivity σ(RT) = 3.1 × 10(−3) S cm(−1), and weak modulation of conducting properties in the region of the spin transition. The complete desolvation, however, results in the loss of hysteretic behavior and a very gradual SCO that spans the temperature range of 200 K. A related complex with integer-charged TCNQ(−) anions, [Fe(3-bpp)(2)](TCNQ)(2)·3MeCN (2·3MeCN), readily loses the interstitial solvent to afford desolvated complex 2 that undergoes an abrupt and hysteretic spin transition centered at 106 K, with an 11 K thermal hysteresis. Complex 2 also exhibits a temperature-induced excited spin-state trapping (TIESST) effect, upon which a metastable high-spin state is trapped by flash-cooling from room temperature to 10 K. Heating above 85 K restores the ground-state low-spin configuration. An approach to improve the structural stability of such complexes is demonstrated by using a related ligand 2,6-bis(benzimidazol-2′-yl)pyridine (bzimpy) to obtain [Fe(bzimpy)(2)](TCNQ)(6)·2Me(2)CO (4) and [Fe(bzimpy)(2)](TCNQ)(5)·5MeCN (5), both of which exist as LS complexes up to 400 K and exhibit semiconducting behavior, with σ(RT) = 9.1 × 10(−2) S cm(−1) and 1.8 × 10(−3) S cm(−1), respectively.