Power of Three: Incremental Increase in the Ligand Field Strength of N‑Alkylated 2,2′-Biimidazoles Leads to Spin Crossover in Homoleptic Tris-Chelated Fe(II) Complexes

Homoleptic complexes [Fe­(L_n)]­X₂ (L₁ = 1,1′-(α,α′-o-xylyl)-2,2′-biimidazole, L₂ = 1,1′-(α,α′-3,4-dibromo-o-xylyl)-2,2′-biimidazole, L₃ = 1,1′-(α,α′-2,5-dimethoxy-o-xylyl)-2,2′-biimidazole; X = BF₄^– or ClO₄^–) were synthesized by direct reactions of the Fe­(II) precursor salts and bidentate ligands L₁, L₂, or L₃. All mononuclear complexes undergo gradual temperature-driven spin-crossover (SCO) between the high-spin (HS, S = 2) and low-spin (LS, S = 0) states. Complexes with ligands L₁ and L₂ synthesized in methanol exhibit complete SCO with the midpoint of the LS↔HS conversion varying from 233 to 313 K, while complexes with ligand L₃, crystallized from an ethanol/dichloromethane mixture, exhibit incomplete SCO with the residual HS/LS ratio of ∼1:4 for [Fe­(L₃)₃]­(BF₄)₂ and ∼1:1 for [Fe­(L₃)₃]­(ClO₄)₂. Complexes with L₁ can also be recrystallized from ethanol/dichloromethane, in which case they exhibit very gradual and incomplete SCO, similar to those of the complexes with L₃. The differences in magnetic behavior have been traced back to peculiarities of molecular packing observed in the corresponding crystal structures. Density-functional theoretical calculations provide justification to the SCO behavior of these complexes, as compared to the HS-only behavior observed for the parent [Fe­(bim)₃]²⁺ complex with nonalkylated 2,2′-biimidazole (bim).