Enabling Valence Delocalization in Iron(III) Macrocyclic Complexes through Ring Unsaturation

Abstract

The complexes [FeIII(HMC)­(C2DMA)2]­CF3SO3 ([2]­OTf) and [FeIII(HMTI)­(C2Y)2]­CF3SO3 ([3a–c]­OTf) have been prepared and thoroughly characterized (HMC = 5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane; HMTI = 5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradeca-1,3,8,10-tetraene; Y = Fc (ferrocenyl, [3a]­OTf), 4-(N,N-dimethyl)­anilino (DMA, [3b]­OTf), or 4-(N,N-bis­(4-methoxyphenyl)­anilino (TPA, [3c]­OTf); OTf– = CF3SO3 –)). Vibrational and electronic absorption spectroelectrochemical analyses following one-electron oxidation of the ethynyl substituent Y revealed evidence of strong coupling in the resultant mixed valent species for all HMTI-based complexes. However, the analogous mixed valent ion based on [2]­OTf appeared to be more localized. Thus, the tetra-imino macrocycle HMTI has enabled significant valence delocalization along the −C2–FeIII–C2– bridge. Electron paramagnetic resonance and Mössbauer spectroscopic studies of [3b]­OTf reveal that the π-acidity of HMTI lowers the energy of the FeIII dπ orbitals compared to the purely σ-donating HMC. This observation provides a basis for the interpretation of the macrocycle-dependent valence (de)­localization.

Fe(III) Bis-Alkynyls Supported by the TIM Macrocycle: Molecular and Electronic Structures and Altering the Nature of Charge Transfer Transitions through Reduction

“…Electronic absorption spectra and voltammetric data indicate that TIM is a more electron-rich macrocycle than HMTI, which results in higher energy dπ orbitals in Fe­(TIM) than those in Fe­(HMTI). Similar to the previously reported [Fe­(HMTI)­(C 2 Y) 2 ] + , both [Fe­(TIM)­Cl 2 ] + and [Fe­(TIM)­(C 2 R) 2 ] + are low spin d 5 species based on their EPR characteristics. Among many interesting features revealed by SEC analysis is the emergence of the low-energy MLCT band(s) with the concurrent disappearance of the LMCT band(s) upon the first one-electron reduction of both [ 1 ] + and [ 2 ] + .…”

“…Although the yields are not impressive, the synthesis of [Fe­(TIM)­(C 2 R) 2 ] + is based on a one-step reaction of [ 1 ]­PF 6 with the appropriate LiC 2 R, a significant improvement over that of Fe­(HMTI) alkynyl species. − For the latter, the oxidative dehydrogenation of the saturated macrocycle in the Fe­(HMC) precursor complex is required and is achieved in situ by the addition of excess n -butyllithium/HC 2 R followed by the subsequent exposure of the reaction mixture to air. However, the tetra-imine nature of the TIM ligand in [ 1 ]­PF 6 eliminates the need for both the oxidative dehydrogenation step and the ensuing purification to remove the byproducts.…”

“…Access to the spectrum of [Fe­(HMTI)­(CN) 2 ] −1 helped to ascertain the MLCT absorption in the transient spectra of Fe­(HMTI)­(CN) 2 . The extraction of the [Fe­(HMTI)­(C 2 Y) 2 ] +2 spectrum allowed the establishment of an IVCT band associated with the [Y–Y] + pair that is consistent with a highly delocalized class II mixed valency …”

“…EPR simulation of [ 1 ] + –[ 4 ] + shows that all complexes are axial low-spin S = 1/2 Fe III species (Figure ), with the exception of complex [ 1 ] + displaying a slightly more rhombic signal (Figure S9). According to the Griffith theory, first developed for low-spin ferric porphyrin complexes, a large g max ( g ≈ 3) and a Σ g 2 = 16 are typical for a (d xy ) 2 (d xz ,d yz ) 3 ground state, whereas a (d xz ,d yz ) 4 (d xy ) 1 ground state usually has an axial EPR spectrum with a g max < 2.6 and Σ g 2 < 16. − The g values for the alkynyl complexes [ 2 ] + –[ 4 ] + were found to be in the same range as seen for the previously reported Fe­(HMTI) complexes, with a slightly higher Σ g 2 values ranging from 14.40 to 14.46 (Table ). This suggests that the alkynyl complexes [ 2 ] + –[ 4 ] + have a (d xz ,d yz ) 4 (d xy ) 1 ground state.…”

“…Our laboratory has studied a number of 3d metal alkynyl complexes supported by both cyclam (1,4,8,11-tetraazacyclotetradecane) and its C-substituted derivatives during the past decade. , While interesting structural features were observed, the conjugation along the metal–alkynyl backbone was weak at best . The investigation of the [Fe III (HMTI)­(C 2 R) 2 ] + -type complexes revealed that the dπ­(Fe) → π*­(α-diimine) interaction is substantial due to the enhanced π-acidity of the unsaturated HMTI. , Subsequent study of the [Y–Y] + mixed valency in the [Fe III (HMTI)­(C 2 Y) 2 ] + -type complexes (Y = DMAP and Fc) revealed facile hole delocalization across the –C 2 –Fe III –C 2 – backbone, further proving the significance of the π-acidity of HMTI in enabling metal–alkynyl conjugation . Nonetheless, further investigation of electronic and optical properties of Fe­(HMTI) species is hindered by their low-yielding syntheses, which are generally based on aerobic dehydrogenation of the corresponding Fe­(HMC) complexes (HMC = 5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane) .…”