Iron(III) Complexes of a Hexadentate Thioether-Appended 2-Aminophenol Ligand: Redox-Driven Spin State Switchover

Abstract

A green complex [Fe­(L3)] (1), supported by the deprotonated form of a hexadentate noninnocent redox-active thioether-appended 2-aminophenolate ligand (H4L3 = N,N′-bis­(2-hydroxy-3,5-di-tert-butylphenyl)-2,2′-diamino­(diphenyldithio)­ethane), has been synthesized and structurally characterized at 100(2) K and 298(2) K. In CH2Cl2, 1 displays two oxidative and a reductive one-electron redox processes at E 1/2 values of −0.52 and 0.20 V, and −0.85 V versus the Fc+/Fc redox couple, respectively. The one-electron oxidized 1 + and one-electron reduced 1 – forms, isolated as a blackish-blue solid 1(PF6)·CH2Cl2 (2) and a gray solid [Co­(η 5–C5H5)2]1·DMF (3), have been structurally characterized at 100(2) K. Structural parameters at 100 K of the ligand backbone and metrical oxidation state values unambiguously establish the electronic states as [FeIII{(LAP O,N )2–}­{(LISQ O,N )•–}­{(L S,S )0}] (1) (two tridentate halves are electronically asymmetric–ligand mixed-valency), [FeIII{(LISQ O,N )•–}­{(LISQ O,N )•–}­{(L S,S )0}]+ (1 +), and [FeIII{(LAP O,N )2–}­{(LAP O,N )2–}­{(L S,S )0}]− (1 −) [dianionic 2-amidophenolate(2−) (LAP O,N )2– and monoanionic 2-iminobenzosemiquinonate(1−) π-radical (S rad = 1/2) (LISQ)•– redox level]. Mössbauer spectral data of 1 at 295, 200, and 80 K reveal that it has a major low-spin (ls)-Fe­(III) and a minor ls-Fe­(II) component (redox isomers), and at 7 K, the major component exists exclusively. Thus, in 1, the occurrence of a thermally driven valence-tautomeric (VT) equilibrium (asymmetric) [FeIII{(LAP O,N )2–}­{(LISQ O,N )•–}­{(L S,S )0}] ⇌ (symmetric) [FeII{(LISQ O,N )•–}­{(LISQ O,N )•–}­{(L S,S )0}] (80–295 K) is implicated. Mössbauer spectral parameters unequivocally establish that 1 + is a ls-Fe­(III) complex. In contrast, the monoanion 1 – contains a high-spin (hs)-Fe­(III) center (S Fe = 5/2), as is deduced from its Mössbauer and EPR spectra. Complexes 1–3 possess total spin ground states S t = 0, 1/2, and 5/2, respectively, based on 1H NMR and EPR spectra, the variable-temperature (2–300 K) magnetic behavior of 2, and the μ eff value of 3 at 300 K. Broken-symmetry density functional theory (DFT) calculations at the B3LYP-level of theory reveal that the unpaired electron in 1 +/2 is due to the (LISQ)•– redox level [ls-Fe­(III) (S Fe = 1/2) is strongly antiferromagnetically coupled to one of the (LISQ)•– radicals (S rad = 1/2)], and 1 –/3 is a hs-Fe­(III) complex, supported by (L3)4– with two-halves in the (LAP)2– redox level. Complex 1 can have either a symmetric or asymmetric electronic state. As per DFT calculation, the former state is stabilized by −3.9 kcal/mol over the latter (DFT usually stabilizes electronically symmetric structure). Time-dependent (TD)-DFT calculations shed light on the origin of observed UV–vis–NIR spectral absorptions for 1–3 and corroborate the results of spectroelectrochemical experiments (300–1100 nm) on 1 (CH2Cl2; 298 K). Variable-temperature (218–298 K; CH2Cl2) absorption spectral (400–1000 nm) studies on 1 justify the presence of VT equilib...

Electronic Transition and Magnetic Coupling Regulation in Trimetallic Complexes Featuring a New Bridging Ligand Obtained by Oxidative Addition

“…However, there are “noninnocent” ligands named by Jørgensen, which could also participate in redox process. − It has been well determined that the interaction of metals with noninnocent ligands is important to promote electron transfer processes in biological or chemical reactions. − In addition, the noninnocent ligands deeply offer more features in establishing “correct” electronic distribution . Hence, a profusion of noninnocent ligands with the N, O, and S atoms is utilized more frequently in the coordination, organometallic chemistry, and biochemistry, such as dithiolenes, , semiquinone (SQ) − or the hybrid o -aminophenol, − glyoxal-bis­(2-mercaptoanil) (H 2 gma), − etc.…”