The Electronic Structure of Iron Corroles: A Combined Experimental and Quantum Chemical Study

• Published in: Chemistry : a European journal Vol. 14; no. 34; pp. 10839 - 10851
• Main Authors: Ye, Shengfa, Tuttle, Tell, Bill, Eckhard, Simkhovich, Liliya, Gross, Zeev, Thiel, Walter, Neese, Frank
• Format: Journal Article
• Language: English
• Published: Weinheim WILEY-VCH Verlag 26.11.2008; WILEY‐VCH Verlag
• Subjects: Computer Simulation; corroles; density functional calculations; Electrons; Ferric Compounds - chemistry; iron; Ligands; Metalloporphyrins - chemistry; Models, Chemical; Molecular Conformation; N ligands; non-innocent ligands; Quantum Theory; Spectroscopy, Mossbauer; Stereoisomerism; Temperature
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.200801265

There is a longstanding debate in the literature on the electronic structure of chloroiron corroles, especially for those containing the highly electron‐withdrawing meso‐tris(pentafluorophenyl)corrole (TPFC) ligand. Two alternative electronic structures were proposed for this and the related [FeCl(tdcc)] (TDCC=meso‐tris(2,6‐dichlorophenyl)corrole) complex, namely a high‐valent ferryl species chelated by a trianionic corrolato ligand ([FeIV(Cor)3−]+) or an intermediate‐spin (IS) ferric ion that is antiferromagnetically coupled to a dianionic π‐radical corrole ([FeIII(Cor).2−]+) yielding an overall triplet ground state. Two series of corrole‐based iron complexes ([Fe(L)(Cor)], in which L=F, Cl, Br, I, and Cor=TPFC, TDCC) have been investigated by a combined experimental (Mössbauer spectroscopy) and computational (DFT) approach in order to differentiate between the two possible electronic‐structure descriptions. The experimentally calibrated conclusions were reached by a detailed analysis of the Kohn–Sham solutions, which successfully reproduce the experimental structures and spectroscopic parameters: the electronic structures of [Fe(L)(Cor)] (L=F, Cl, Br, I, Cor=TPFC, TDCC) are best formulated as ([IS‐FeIII(Cor).2−]+), similar to chloroiron corrole complexes containing electron‐rich corrole ligands. The antiferromagnetic pathway is composed of singly occupied Fe d z 2 and corrole a2u‐like π orbitals, with coupling constants that exceed those of analogous porphyrin systems by a factor of 2–3. In the corroles, the combination of lower symmetry, extra negative charge, and smaller cavity size (relative to the porphyrins) leads to exceptionally strong iron–corrole σ bonds. Hence, the Fe d x 2−y 2‐based molecular orbital is unavailable in the corrole complexes (contrary to the porphyrin case), and the local spin states are SFe=3/2 in the corroles versus SFe=5/2 in the porphyrins. The consequences of this qualitative difference are discussed for spin distributions and magnetic properties. Intermediate spins: Two series of halogenoiron corrole complexes [Fe(L)(Cor)] (L=F, Cl, Br, I, Cor=TPFC, TDCC) were investigated by a combined experimental and theoretical approach. The electronic structures of [Fe(L)(Cor)] are best formulated as an intermediate‐spin (IS) ferric ion that is antiferromagnetically coupled to a dianionic π‐radical corrole ([FeIII(Cor).2−]+) yielding an overall triplet ground state ([IS‐FeIII(Cor).2−]+) (see scheme). (TPFC=tris(pentafluorophenyl)corrole, TDCC=meso‐tris(2,6‐dichlorophenyl)corrole.)