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 Jeremy J. Hrudka Hoa Phan Jeff Lengyel Andrey Yu. Rogachev Michael Shatruk 10.1021/acs.inorgchem.8b00223.s001 https://acs.figshare.com/articles/journal_contribution/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/6157484 Homoleptic complexes [Fe­(L<sub><i>n</i></sub>)]­X<sub>2</sub> (<b>L</b><sub><b>1</b></sub> = 1,1′-(α,α′-<i>o</i>-xylyl)-2,2′-biimidazole, <b>L</b><sub><b>2</b></sub> = 1,1′-(α,α′-3,4-dibromo-<i>o</i>-xylyl)-2,2′-biimidazole, <b>L</b><sub><b>3</b></sub> = 1,1′-(α,α′-2,5-dimethoxy-<i>o</i>-xylyl)-2,2′-biimidazole; X = BF<sub>4</sub><sup>–</sup> or ClO<sub>4</sub><sup>–</sup>) were synthesized by direct reactions of the Fe­(II) precursor salts and bidentate ligands <b>L</b><sub><b>1</b></sub>, <b>L</b><sub><b>2</b></sub>, or <b>L</b><sub><b>3</b></sub>. 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><sub><b>1</b></sub> and <b>L</b><sub><b>2</b></sub> 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><sub><b>3</b></sub>, crystallized from an ethanol/dichloromethane mixture, exhibit incomplete SCO with the residual HS/LS ratio of ∼1:4 for [Fe­(<b>L</b><sub><b>3</b></sub>)<sub>3</sub>]­(BF<sub>4</sub>)<sub>2</sub> and ∼1:1 for [Fe­(<b>L</b><sub><b>3</b></sub>)<sub>3</sub>]­(ClO<sub>4</sub>)<sub>2</sub>. Complexes with <b>L</b><sub><b>1</b></sub> 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><sub><b>3</b></sub>. 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)<sub>3</sub>]<sup>2+</sup> complex with nonalkylated 2,2′-biimidazole (bim). 2018-04-18 19:49:59 BF LS L 3 L 2 L 1 HS ligand L 3 SCO Complexe Ligand Field Strength Fe xylyl bidentate ligands L 1 ligands L 1 exhibit complex