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

The complexes [Fe III (HMC)(C 2 DMA) 2 ]CF 3 SO 3 ([ 2 ]OTf) and [Fe III (HMTI)(C 2 Y) 2 ]CF 3 SO 3 ([ 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 - = CF 3 SO 3 - )). 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 -C 2 -Fe III -C 2 - bridge. Electron paramagnetic resonance and Mössbauer spectroscopic studies of [ 3b ]OTf reveal that the π-acidity of HMTI lowers the energy of the Fe III dπ orbitals compared to the purely σ-donating HMC. This observation provides a basis for the interpretation of the macrocycle-dependent valence (de)localization.