Tetranuclear complexes of [Fe(CO)₂(C₅H₅)]+ with TCNX ligands (TCNX = TCNE, TCNQ, TCNB): intramolecular electron transfer alternatives in compounds (μ₄-TCNX)[ML_n]₄

Abstract

The complexes {(μ ₄-TCNX)[Fe(CO)₂(C₅H₅)]₄}(BF₄)₄ were prepared as light-sensitive materials from [Fe(CO)₂(C₅H₅) (THF)](BF₄) and the corresponding TCNX ligands (TCNE = tetracyanoethene, TCNQ = 7,7,8,8-tetracyano-p-quinodimethane, TCNB = 1,2,4,5-tetracyanobenzene). Whereas the TCNE and TCNQ complexes are extremely easily reduced species with reduction potentials >+0.3 V vs ferrocenium/ferrocene, the tetranuclear complex of TCNB exhibits a significantly more negative reduction potential at about -1.0 V. Even for the complexes with strongly p-accepting TCNE and TCNQ, the very positive reduction potentials, the unusually high nitrile stretching frequencies >2235 cm^-1, and the high-energy charge-transfer transitions indicate negligible metal-to-ligand electron transfer in the ground state, corresponding to a largely unperturbed (TCNX°)(Fe^II)₄ formulation of oxidation states as caused by orthogonality between the metal-centered HOMO and the π LUMO of TCNX. Mö ssbauer spectroscopy confirms the low-spin iron(II) state, and DFT calculations suggest coplanar TCNE and TCNQ bridging ligands in the complex tetracations. One-electron reduction to the 3+ forms of the TCNE and TCNQ complexes produces EPR spectra which confirm the predominant ligand character of the then singly occupied MO through isotropic g values slightly below 2, in addition to a negligible g anisotropy of frozen solutions at frequencies up to 285 GHz and also through an unusually well-resolved solution X band EPR spectrum of {(μ₄-TCNE)[Fe(CO)₂(C₅H₅)]₄}³⁺ which shows the presence of four equivalent [Fe(CO)₂(C₅H₅)]⁺ moieties through ⁵⁷Fe and ¹³C(CO) hyperfine coupling in nonenriched material. DFT calculations reproduce the experimental EPR data. A survey of discrete TCNE and TCNQ complexes [(μ₄-TCNX)(ML_n)₄] exhibits a dichotomy between the systems {(μ₄-TCNX)[Fe(CO)₂(C₅H₅)]₄}⁴⁺ and {(μ₄-TCNQ)[Re(CO)₃(bpy)]₄}⁴⁺ with their negligible metal-to-ligand electron transfer and several other compounds of TCNE or TCNQ with Mn, Ru, Os, or Cu complex fragments which display evidence for a strong such interaction, i.e., an appreciable value d in the formulation {(μ₄-TCNX^{δ -})[Mx+&^{#948; /4}L_n]4}. Irreversibility of the first reduction of {(μ4-TCNB)[Fe(CO)₂(C₅H₅)]₄}(BF₄)₄ precluded spectroelectrochemical studies; however, the high-energy CN stretching frequencies and charge transfer absorptions of that TCNB analogue also confirm the exceptional position of the complexes {(μ₄-TCNX)[Fe(CO)₂(C₅H₅)]₄}(BF₄)₄.