Substituent-dependent spin-state switching in isomeric iron(II) complexes: from bi-stable spin-state switching with T1/2 centred at room temperature to trapped high-spin state

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—examples include lattice solvent, counter anion, and substituent. Herein, we show that spin-state switching in isomeric iron(II) complexes (1·CH3CN and 2) composed of tridentate all nitrogen coordinating 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 controlled by the nature of substituent at the fourth position of the pyridine ring of the BPP skeleton. Complex 1·CH3CN, crystallized with acetonitrile solvent, undergoes abrupt and hysteretic spin-state switching, hence bistable switching, with a thermal hysteresis width (ΔT1/2) of 44 K and switching temperature (T1/2) = 298 K in the first cycle. Conversely, the isomeric counterpart of 1·CH3CN—complex 2—crystallized with no lattice solvent; the complex was trapped in the high-spin (HS) state upon cooling from 300 K. Molecular structures of the LS and HS forms of complex 1·CH3CN revealed that spin-state switching induces a pronounced angular distortion, creating an energy barrier separating the LS and HS states. Traversing the barrier requires substantial molecular rearrangement in the presence of constraints imposed by the crystal lattice, rendering the spin-state switching of 1·CH3CN hysteretic in the solid-state. The observation of bistable spin-state switching with T1/2 centred at room temperature for 1·CH3CN as well as the attribution of pronounced angular distortion and conformational variation of the COOEt substituent as causes behind the observed hysteretic spin-state switching indicates that technologically relevant spin-state switching profiles based on mononuclear iron(II) complexes can be obtained.