Spin Crossover and Valence Tautomerism in Neutral Homoleptic Iron Complexes of Bis(pyridylimino)isoindolines

Abstract

Homoleptic iron complexes of six bis(pyridylimino)isoindoline (bpi) ligands with different substituents (H, Me, Et, tBu, OMe, NMe2) at the 4-positions of the pyridine moieties have been prepared and studied with regard to temperature-dependent spin and redox states by a combination of (57)Fe Mössbauer spectroscopy, SQUID magnetometry, single-crystal X-ray diffraction analysis, X-band EPR, and (1)H NMR spectroscopy. While the H-, methyl-, and ethyl-substituted complexes remain in a pure high-spin state irrespective of the temperature, the 4-tert-butyl-substituted derivative shows spin-crossover behavior. The methoxy- and dimethylamino-substituted compounds were found to easily undergo oxidation. In the crystalline state, valence tautomeric behavior was observed for the methoxy derivative as a thermally activated charge-transfer transition, accompanied by a spin crossover above 200 K. The valence tautomerism leads to a chelate with one of the bpi ligands as a dianion radical L(2-·) and with an effective spin of S=2.

Solution and Solid‐State Study of the Spin‐Crossover [FeII(R‐bik)3](BF4)2 Complexes (R = Me, Et, Vinyl)

“…The continuous‐shape‐measures analysis (CShM), allows a more precise quantification of the deviation of the coordination spheres from an ideal geometry. The distortion of the coordination sphere reflects the spin state of the Fe II ion: in the present case, very small values of the shape factor, are observed in 1 – 3 (Table ), indicating an almost perfect octahedral surrounding, whereas a higher one is obtained in 4 , as expected for a HS Fe II complex.…”

“…82.8°). [12b] The sum of the deviation from orthogonality of the twelve cis ‐N–Fe–N angles (denoted as Σ ) is relatively small in 1 – 3 [respectively: 13.5(4)°, 8.9(3)° and 21.2(5)°] and indicates a moderate distortion of the octahedral coordination sphere. In contrast, this value reaches Σ = 47.6° within the related LS [Fe(phen) 3 ] 2+ complex.…”

Thermally-Induced Spin Crossover and LIESST Effect in the Neutral [FeII(Mebik)2(NCX)2] Complexes: Variable-Temperature Structural, Magnetic, and Optical Studies (X = S, Se; Mebik = bis(1-methylimidazol-2-yl)ketone)

“…This might be correlated to the decrease of the Fe(1)-N(1)-C(1) angle ( ca . 6° in both 1 and 2 ) between 200 and 400 K. This angle variation might allow in part the absorption of the strain caused by the increase of Fe-N bond lengths accompanied by the spin-transition (Pap et al, 2011a,b,c; Scheja et al, 2015). The bending also indicates that the Fe-N = C = X mesomeric form (X = S or Se) coexists with the Fe-N=C-X − , which is expected to be linear.…”

An Iron(III) Complex with Pincer Ligand—Catalytic Water Oxidation through Controllable Ligand Exchange

“…According to the reported single crystal structure of [Fe III Cl 2 (tia-BAI)] the complex is a five-coordinate, distorted trigonal bipyramidal with the tridentate, anionic tia-BAI − ligand occupying the two apical and one equatorial positions in meridional topology [51]. The central pyrrolic nitrogen atom ( Figure 1) resides closer to the iron center than the two thiazolic nitrogen atoms (the Fe-N bond distances are 2.019(2) and 2.095(2) Å in avg., respectively) due to the greater Lewis basicity of the former, and the Fe-Cl distances are equally~2.23 Å, altogether in agreement with a high-spin ferric center, in contrast with the homoleptic [Fe III (tia-BAI) 2 ] + exhibiting shorter bond distances of~1.95 and 2.00 Å, or similar bis-BAI − complexes with a low-spin ferric center [54]. Importantly, in the earlier report two electrochemically responsive species could be detected, when [Fe III Cl 2 (tia-BAI)] was dissolved in N,N-dimethylformamide solvent that has been attributed to the exchange of chloride to solvent upon reduction due to the more labile ferrous species [51].…”