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(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 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(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.
Institute of Inorganic and Analytical Chemistry, University of Münster, Corrensstrasse 30, 48149 Münster, Germany.
Eva-Maria
EM
Schnöckelborg
Marat M
MM
Khusniyarov
Bas
B
de Bruin
František
F
Hartl
Thorsten
T
Langer
Matthias
M
Eul
Stephen
S
Schulz
Rainer
R
Pöttgen
Robert
R
Wolf
D000873:Anthracenes / Q000737:chemistry*; D018360:Crystallography, X-Ray; D004578:Electron Spin Resonance Spectroscopy; D004583:Electrons; D005290:Ferric Compounds / Q000138:chemical synthesis / Q000737:chemistry*; D005296:Ferrous Compounds / Q000138:chemical synthesis / Q000737:chemistry*; D009682:Magnetic Resonance Spectroscopy; D008958:Models, Molecular; D015394:Molecular Structure; D009281:Naphthalenes / Q000737:chemistry*; D011789:Quantum Theory; D013056:Spectrophotometry, Ultraviolet; D015204:Spectroscopy, Mossbauer
D000873:Anthracenes; D005290:Ferric Compounds; D005296:Ferrous Compounds; D009281:Naphthalenes; C031721:naphthalene; C034020:anthracene