Unraveling the electronic structures of low-valent naphthalene and anthracene iron complexes: X-ray, spectroscopic, and density functional theory studies

摘要

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(η ~4-C _(10)H _8)}] (K1), [K(18-crown-6){Cp*Fe(η ~4-C _(14)H _(10))}] (K2), [Cp*Fe(η ~4-C _(10)H _8)] (1), and [Cp*Fe(η ~4-C _(14)H _(10))] (2) were synthesized and characterized by NMR, UV-vis, and ~(57)Fe M?ssbauer 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(η ~4-C _(10)H _8)] ~- (1 ~-) and [Cp*Fe(η ~4-C _(14)H _(10))] ~- (2 ~-) and reversibly oxidized to the cations [Cp*Fe(η ~6-C _(10)H _8)] ~+ (1 ~+) and [Cp*Fe(η ~6-C _(14)H _(10))] ~+ (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.