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

A green complex [Fe(L 3 )] ( 1 ), supported by the deprotonated form of a hexadentate noninnocent redox-active thioether-appended 2-aminophenolate ligand (H 4 L 3 = 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 CH 2 Cl 2 , 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 (PF 6 )·CH 2 Cl 2 ( 2 ) and a gray solid [Co( η 5 -C 5 H 5 ) 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 [Fe III {(L AP O , N ) 2- }{(L ISQ O , N ) •- }{(L S , S ) 0 }] ( 1 ) (two tridentate halves are electronically asymmetric-ligand mixed-valency), [Fe III {(L ISQ O , N ) •- }{(L ISQ O , N ) •- }{(L S , S ) 0 }] + ( 1 + ), and [Fe III {(L AP O , N ) 2- }{(L AP O , N ) 2- }{(L S , S ) 0 }] - ( 1 - ) [dianionic 2-amidophenolate(2-) (L AP O , N ) 2- and monoanionic 2-iminobenzosemiquinonate(1-) π-radical ( S rad = 1/2) (L ISQ ) •- 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) [Fe III {(L AP O , N ) 2- }{(L ISQ O , N ) •- }{(L S , S ) 0 }] ⇌ (symmetric) [Fe II {(L ISQ O , N ) •- }{(L ISQ 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 1 H 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 (L ISQ ) •- redox level [ls-Fe(III) ( S Fe = 1/2) is strongly antiferromagnetically coupled to one of the (L ISQ ) •- radicals ( S rad = 1/2)], and 1 - / 3 is a hs-Fe(III) complex, supported by (L 3 ) 4- with two-halves in the (L AP ) 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 (CH 2 Cl 2 ; 298 K). Variable-temperature (218-298 K; CH 2 Cl 2 ) absorption spectral (400-1000 nm) studies on 1 justify the presence of VT equilibrium in the solution-state.