Unraveling the Electronic Structures of Low-Valent Naphthalene and Anthracene Iron Complexes: X-ray, Spectroscopic, and Density Functional Theory Studies

Naphthalene and anthracene transition metalates are potent reagents, but their electronic structures have remained poorly explored. A study of four Cp*-substituted iron complexes (Cp* = pentamethylcyclopentadienyl) now gives rare insight into the bonding features of such species. The highly oxygen- and water-sensitive compounds [K(18-crown-6){Cp*Fe(eta(4)-C10H8)}] (K1), [K(18-crown-6)[Cp*Fe(eta(4)-C14H10)}] (K2), [Cp*Fe(eta(4)-C10H8)] (1), and [Cp*Fe(eta(4)-C14H10)] (2) were synthesized and characterized by NMR, UV-vis, and Fe-57 Mossbauer spectroscopy. The paramagnetic complexes 1 and 2 were additionally characterized by electron paramagnetic resonance (EPR) spectroscopy and magnetic susceptibility measurements. The molecular structures of complexes K1, K2, and 2 were determined by single-crystal X-ray crystallography. Cyclic voltammetry of 1 and 2 and spectroelectrochemical experiments revealed the redox properties of these complexes, which are reversibly reduced to the monoanions [Cp*Fe(eta(4)-C10H8)](-) (1(-)) and [Cp*Fe(eta(4)-C14H10)](-) (2(-)) and reversibly oxidized to the cations [Cp*Fe(eta(4)-C10H8)](+) (1(+)) and [Cp*Fe(eta(4)-C14H10](+) (2(+)). Reduced orbital charges and spin densities of the naphthalene complexes 1(-/0/+) and the anthracene derivatives 2(-/0/+) were obtained by density functional theory (DFT) methods. Analysis of these data suggests that the electronic structures of the anions 1(-) and 2(-) are best represented by low-spin Fe-II ions coordinated by anionic Cp* and dianionic naphthalene and anthracene ligands. The electronic structures of the neutral complexes 1 and 2 may be described by a superposition of two resonance configurations which, on the one hand, involve a low-spin Fe-I ion coordinated by the neutral naphthalene or anthracene ligand L, and, on the other hand, a low-spin Fe-II ion coordinated to a ligand radical L. Our study thus reveals the redox noninnocent character of the naphthalene and anthracene ligands, which effectively stabilize the iron atoms in a low formal, but significantly higher spectroscopic oxidation state.