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

Ferric bis-alkynyl complexes bearingredox-active substituentsare thoroughly characterized, and the mixed valent properties of thesingly oxidized species are investigated through visible-nearIR and IR absorption spectroelectrochemistry. The degree of valence(de)localization through the metal center depends strongly on thenature of its supporting macrocyclic ligand. Valence delocalizationis enhanced for compounds containing a tetraimine macrocycle, whicheffect appears related to the altered ground state of the unoxidizedcomplexes. The complexes [Fe-III(HMC)(C(2)DMA)(2)]CF3SO3 ([2]OTf) and [Fe-III(HMTI)(C2Y)(2)]CF3SO3 ([3a-c]OTf) have been preparedand thoroughlycharacterized (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 spectroelectrochemicalanalyses following one-electron oxidation of the ethynyl substituentY revealed evidence of strong coupling in the resultant mixed valentspecies for all HMTI-based complexes. However, the analogous mixedvalent ion based on [2]OTf appeared to be more localized.Thus, the tetra-imino macrocycle HMTI has enabled significant valencedelocalization along the -C-2-Fe-III-C-2- bridge. Electron paramagnetic resonanceand Mo''ssbauer spectroscopic studies of [3b]OTfreveal that the & pi;-acidity of HMTI lowers the energy of the Fe-III d & pi; orbitals compared to the purely & sigma;-donatingHMC. This observation provides a basis for the interpretation of themacrocycle-dependent valence (de)localization.

Automated summary

Ferric bis-alkynyl complexes bearing redox-active substituents were characterized and the mixed valent properties of the singly oxidized species were investigated. The nature of the supporting macrocyclic ligand strongly influenced the degree of valence delocalization through the metal center. A tetraimine macrocycle enhanced valence delocalization, which was related to the altered ground state of the unoxidized complexes. The study provides insights into macrocyclic ligand-dependent valence delocalization and the role of π-acidity in the electronic structure of the complexes.