Long-range electronic communication in free-base meso-poly(ferrocenyl)-containing porphyrins

Abstract

H_2FcPh_3P [FcPh_3P = 5-ferrocenyl-10,15,20-triphenyl porphyrin(2-)], cis-H_2Fc_2Ph_2P [cis-Fc_2Ph_2P = 5,10-bisferrocenyl-15,20-diphenyl porphyrin(2-)], trans-H_2Fc_2Ph_2P [trans-Fc_2Ph_2P = 5,15-bisferrocenyl-10,20-diphenyl porphyrin(2-)], and H_2Fc_3PhP [Fc_3PhP = 5,10,15-trisferrocenyl-20-phenyl porphyrin(2-)] along with H_2TPP [TPP = 5,10,15,20-tetraphenylporphyrin] and H_2TFcP [TFcP = 5,10,15,20-tetraferrocenyl porphyrin(2-)] were isolated from the direct cross-condensation reaction between pyrrole, benzaldehyde, and ferrocene carboxaldehyde or from the reaction between ferrocenyl-2,2'-dipyrromethane and benzaldehyde, suggesting a scrambling reaction mechanism for the last approach. All compounds were characterized by UV-vis, MCD, and NMR spectroscopy; APCI MS and MS/MS methods; as well as high-resolution ESI MS spectrometry. The conformational flexibility of ferrocene substituents in all compounds was confirmed using variable-temperature NMR and computational methods. DFT calculations were employed to understand the degree of nonplanarity of the porphyrin core as well as the electronic structure of ferrocene-containing porphyrins. In all cases, a set of occupied, predominantly ferrocene-based molecular orbitals was found between the highest occupied and the lowest unoccupied, predominantly porphyrin-based molecular pi orbitals. The redox properties of all ferrocene-containing porphyrins were investigated in a CH_2Cl_2/TFAB [TFAB = tetrabutylammonium tetrakis(perfluorophenyl)borate] system using cyclic voltammetry, differential pulse voltammetry, and square wave voltammetry methods. In all cases, oxidations of individual ferrocene substituent(s) along with porphyrin core oxidation(s) and reductions have been observed. Mixed-valence [cis-H_2Fc_2Ph_2P]^+, [trans-H_2Fc_2Ph_2P]^+, [H_2Fc_3PhP]^+, and [H_2Fc_3PhP]^(2+) complexes were formed in situ under spectroelectrochemical and chemical oxidation conditions and were characterized using UV-vis and MCD approaches. Analysis of intervalence charge-transfer bands observed in the NIR region for all mixed-valence complexes suggests electron localization and thus class II behavior in the Robin-Day classification.