Iron(III) Complexes of a Hexadentate Thioether-Appended2-Aminophenol Ligand: Redox-Driven Spin State Switchover br

A green complex [Fe(L3)] (1), supported by the deprotonated form of a hexadentatenoninnocent 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 structurallycharacterized at 100(2) K and 298(2) K. In CH2Cl2,1displays two oxidative and a reductive one-electron redox processes atE1/2values of-0.52 and 0.20 V, and-0.85 V versus the Fc+/Fc redoxcouple, respectively. The one-electron oxidized1+and one-electron reduced1-forms, isolated as ablackish-blue solid1(PF6)middotCH2Cl2(2) and a gray solid [Co(eta 5-C5H5)2]1middotDMF (3), have beenstructurally characterized at 100(2) K. Structural parameters at 100 K of the ligand backbone and metrical oxidation state valuesunambiguously establish the electronic states as [FeIII{(LAPO,N)2-}{(LISQO,N)center dot-}{(LS,S)0}] (1) (two tridentate halves areelectronically asymmetric-ligand mixed-valency), [FeIII{(LISQO,N)center dot-}{(LISQO,N)center dot-}{(LS,S)0}]+(1+), and [FeIII{(LAPO,N)2-}-{(LAPO,N)2-}{(LS,S)0}]-(1-) [dianionic 2-amidophenolate(2-)(LAPO,N)2-and monoanionic 2-iminobenzosemiquinonate(1-)pi-radical (Srad= 1/2) (LISQ)center dot-redox level]. Mo''ssbauer spectral data of1at 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, in1, theoccurrence of a thermally driven valence-tautomeric (VT) equilibrium (asymmetric) [FeIII{(LAPO,N)2-}{(LISQO,N)center dot-}{(LS,S)0}](symmetric) [FeII{(LISQO,N)center dot-}{(LISQO,N)center dot-}{(LS,S)0}] (80-295 K) is implicated. Mo''ssbauer spectral parameters unequivocallyestablish that1+is a ls-Fe(III) complex. In contrast, the monoanion1-contains a high-spin (hs)-Fe(III) center (SFe= 5/2), as isdeduced from its Mo''ssbauer and EPR spectra. Complexes1-3possess total spin ground statesSt= 0, 1/2, and 5/2, respectively,based on1H NMR and EPR spectra, the variable-temperature (2-300 K) magnetic behavior of2, and the mu effvalue of3at 300 K.Broken-symmetry density functional theory (DFT) calculations at the B3LYP-level of theory reveal that the unpaired electron in1+/2is due to the (LISQ)center dot-redox level [ls-Fe(III) (SFe= 1/2) is strongly antiferromagnetically coupled to one of the (LISQ)center dot-radicals(Srad= 1/2)], and1-/3is a hs-Fe(III) complex, supported by (L3)4-with two-halves in the (LAP)2-redox level. Complex1can haveeither a symmetric or asymmetric electronic state. As per DFT calculation, the former state is stabilized by-3.9 kcal/mol over thelatter (DFT usually stabilizes electronically symmetric structure). Time-dependent (TD)-DFT calculations shed light on the originof observed UV-vis-NIR spectral absorptions for1-3and corroborate the results of spectroelectrochemical experiments (300-1100 nm) on1(CH2Cl2; 298 K). Variable-temperature (218-298 K; CH2Cl2) absorption spectral (400-1000 nm) studies on1justify the presence of VT equilibrium in the solution-state