Spin Crossover in Fe(II) Complexes with N₄S₂ Coordination

Reactions of Fe­(II) precursors with the tetradentate ligand S,S′-bis­(2-pyridylmethyl)-1,2-thioethane (bpte) and monodentate NCE^– coligands afforded mononuclear complexes [Fe­(bpte)­(NCE)₂] (1, E = S; 2, E = Se; 3, E = BH₃) that exhibit temperature-induced spin crossover (SCO). As the ligand field strength increases from NCS^– to NCSe^– to NCBH₃^–, the SCO shifts to higher temperatures. Complex 1 exhibits only a partial (15%) conversion from the high-spin (HS) to the low-spin (LS) state, with an onset around 100 K. Complex 3 exhibits a complete SCO with T_1/2 = 243 K. While the γ-2 polymorph also shows the complete SCO with T_1/2 = 192 K, the α-2 polymorph exhibits a two-step SCO with the first step leading to a 50% HS → LS conversion with T_1/2 = 120 K and the second step proceeding incompletely in the 80–50 K range. The amount of residual HS fraction of α-2 that remains below 60 K depends on the cooling rate. Fast flash-cooling allows trapping of as much as 45% of the HS fraction, while slow cooling leads to a 14% residual HS fraction. The slowly cooled sample of α-2 was subjected to irradiation in the magnetometer cavity resulting in a light-induced excited spin state trapping (LIESST) effect. As demonstrated by Mössbauer spectroscopy, an HS fraction of up to 85% could be achieved by irradiation at 4.2 K.