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(η4-C10H8)}] (K1), [K­(18-crown-6)­{Cp*Fe­(η4-C14H10)}] (K2), [Cp*Fe­(η4-C10H8)] (1), and [Cp*Fe­(η4-C14H10)] (2) were synthesized
and characterized by NMR, UV–vis, and 57Fe Mössbauer
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(η4-C10H8)]− (1–) and [Cp*Fe­(η4-C14H10)]− (2–) and reversibly oxidized to the cations [Cp*Fe­(η6-C10H8)]+ (1+) and [Cp*Fe­(η6-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 FeII 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 FeI ion coordinated by the neutral
naphthalene or anthracene ligand L, and, on the other hand, a low-spin
FeII 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.