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).