Lattice solvent- and substituent-dependent spin-crossover in isomeric iron(II) complexes

Spin-state switching in iron(II) complexes composed of ligands featuring moderate ligand-field strength-for example, 2,6-bi(1H-pyrazol-1-yl)pyridine (BPP)-is dependent on many factors. Herein, we show that spin-state switching in isomeric iron(II) complexes composed of BPP-based ligands-ethyl 2,6-bis(1H-pyrazol-1-yl)isonicotinate (BPP-COOEt, L1) and (2,6-di(1H-pyrazol-1-yl)pyridin-4-yl)methylacetate (BPP-CH₂OCOMe, L2)-is dependent on the nature of the substituent at the BPP skeleton. Bi-stable spin-state switching-with a thermal hysteresis width (ΔT_1/2) of 44 K and switching temperature (T_1/2) = 298 K in the first cycle-is observed for complex 1·CH₃CN composed of L1 and BF₄^- counter anions. Conversely, the solvent-free isomeric counterpart of 1·CH₃CN-complex 2a, composed of L2 and BF₄^- counter anions-was trapped in the high-spin (HS) state. For one of the polymorphs of complex 2b·CH₃CN-2b·CH₃CN-Y, Y denotes yellow colour of the crystals-composed of L2 and ClO₄^- counter anions, a gradual and non-hysteretic SCO is observed with T_1/2 = 234 K. Complexes 1·CH₃CN and 2b·CH₃CN-Y also underwent light-induced spin-state switching at 5 K due to the light-induced excited spin-state trapping (LIESST) effect. Structures of the low-spin (LS) and HS forms of complex 1·CH₃CN revealed that spin-state switching goes hand-in-hand with pronounced distortion of the trans-N{pyridyl}-Fe-N{pyridyl} angle (ϕ), whereas such distortion is not observed for 2b·CH₃CN-Y. This observation points that distortion is one of the factors making the spin-state switching of 1·CH₃CN hysteretic in the solid state. The observation of bi-stable spin-state switching with T_1/2 centred at room temperature for 1·CH₃CN indicates that technologically relevant spin-state switching profiles based on mononuclear iron(II) complexes can be obtained.