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

Abstract

Homoleptic complexes [Fe(L )]X (L = 1,1'-(α,α'- o-xylyl)-2,2'-biimidazole, L = 1,1'-(α,α'-3,4-dibromo- o-xylyl)-2,2'-biimidazole, L = 1,1'-(α,α'-2,5-dimethoxy- o-xylyl)-2,2'-biimidazole; X = BF or ClO) were synthesized by direct reactions of the Fe(II) precursor salts and bidentate ligands L, L, or L. 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 L and L synthesized in methanol exhibit complete SCO with the midpoint of the LS↔HS conversion varying from 233 to 313 K, while complexes with ligand L, crystallized from an ethanol/dichloromethane mixture, exhibit incomplete SCO with the residual HS/LS ratio of ∼1:4 for [Fe(L)](BF) and ∼1:1 for [Fe(L)](ClO). Complexes with L can also be recrystallized from ethanol/dichloromethane, in which case they exhibit very gradual and incomplete SCO, similar to those of the complexes with L. 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)] complex with nonalkylated 2,2'-biimidazole (bim).

A Strain-Deformation Nexus within Pincer Ligands: Application to the Spin States of Iron(II) Complexes

“…We then consider ligand strain within bis­(pincer)­iron­(II) complexes that exhibit SCO activity. Understanding and predicting SCO in metal complexes is still an important challenge, with recent reports focusing on the electronic effects of the ligands and avoiding steric considerations, by systematically modifying ligands that are structurally similar. , It has long been known (and recently exploited by Shatruk and co-workers) , that the magnetic properties of iron­(II) complexes can be tuned through steric effects (i.e., strain). ,− Others have considered the strain within the first coordination sphere of iron­(II) complexes with tridentate pincer ligands and the magnetic properties which result. Halcrow, in particular, has proposed four geometric parameters from the L–Fe–L bond angles within the complexes to characterize the distortion, and therefore strain, of the coordination geometry around each iron­(II) center, and these have now been widely accepted. − Our work is related but tells the other half of the story: what happens to the ligand as it binds these centers.…”

Abrupt Spin Transition in a Heteroleptic Fe(II) Complex with Pendant Naphthalene Functionality

“…On the other hand, the T 1/2 values for the two samples differ by ∼200 K, indicating a relatively higher stability of the LS state in 1 ·py. Such an effect can emerge from the larger distortion of the coordination environment of Fe II sites in structure 1 due to intermolecular packing forces. , This assumption could be probed by the analysis of the angular distortion parameter, Σ 90 , which equals to 69.67(6) and 70.6(1)° for the LS structures of 1 ·py and 1 , respectively (Table ). The Fe II coordination in 1 appears to be slightly more distorted, but the difference of ∼1° is rather small, taking into account that Σ 90 is a result of summation over 12 different cis -N–Fe–N angles.…”

Assessing the Performance of Approximate Density Functional Theory on 95 Experimentally Characterized Fe(II) Spin Crossover Complexes

“…Steep SCO behavior observed in these complexes is attributed to the presence of intermolecular interactions 193 . Thus, overall, disagreement between DFA-predicted T1/2 values and experimentally reported T1/2 values could be attributed at least in part due to the lack of explicit solvent 53,194 or crystal packing 63,158,195 as well as the absence of noncovalent interactions 196 between the complex and its counterions 63,193,197,198 in the DFT calculations that are present in the crystal structures. Another source of discrepancy would be hysteresis during SCO 196,[199][200][201][202] or two-step SCO behavior 49,53,155,184,199,203,204 , both of which are not accounted for by the modelling protocol based…”