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

Homoleptic complexes [Fe­(L_<i>n</i>)]­X₂ (<b>L</b>_<b>1</b> = 1,1′-(α,α′-<i>o</i>-xylyl)-2,2′-biimidazole, <b>L</b>_<b>2</b> = 1,1′-(α,α′-3,4-dibromo-<i>o</i>-xylyl)-2,2′-biimidazole, <b>L</b>_<b>3</b> = 1,1′-(α,α′-2,5-dimethoxy-<i>o</i>-xylyl)-2,2′-biimidazole; X = BF₄^– or ClO₄^–) were synthesized by direct reactions of the Fe­(II) precursor salts and bidentate ligands <b>L</b>_<b>1</b>, <b>L</b>_<b>2</b>, or <b>L</b>_<b>3</b>. All mononuclear complexes undergo gradual temperature-driven spin-crossover (SCO) between the high-spin (HS, <i>S</i> = 2) and low-spin (LS, <i>S</i> = 0) states. Complexes with ligands <b>L</b>_<b>1</b> and <b>L</b>_<b>2</b> synthesized in methanol exhibit complete SCO with the midpoint of the LS↔HS conversion varying from 233 to 313 K, while complexes with ligand <b>L</b>_<b>3</b>, crystallized from an ethanol/dichloromethane mixture, exhibit incomplete SCO with the residual HS/LS ratio of ∼1:4 for [Fe­(<b>L</b>_<b>3</b>)₃]­(BF₄)₂ and ∼1:1 for [Fe­(<b>L</b>_<b>3</b>)₃]­(ClO₄)₂. Complexes with <b>L</b>_<b>1</b> can also be recrystallized from ethanol/dichloromethane, in which case they exhibit very gradual and incomplete SCO, similar to those of the complexes with <b>L</b>_<b>3</b>. 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).