Tetranuclear Complexes of Fe(CO)2(C5H5)+ with TCNX Ligands (TCNX = TCNE, TCNQ, TCNB): Intramolecular Electron Transfer Alternatives in Compounds (m4-TCNX)MLn4
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Inorganic Chemistry 46 (18): 7312--7320 (2007)

(μ4-TCNX)Fe(CO)2(C5H5)4(BF4)4 were prepd. 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 redn. potentials \textgreater+0.3 V vs. ferrocenium/ferrocene, the tetranuclear complex of TCNB exhibits a significantly more neg. redn. potential at $\sim$-1.0 V. Even for the complexes with strongly p-accepting TCNE and TCNQ, the very pos. redn. potentials, the unusually high nitrile stretching frequencies \textgreater2235 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°)(FeII)4 formulation of oxidn. states as caused by orthogonality between the metal-centered HOMO and the p* LUMO of TCNX. Mossbauer spectroscopy confirms the low-spin Fe(II) state, and DFT calcns. suggest coplanar TCNE and TCNQ bridging ligands in the complex tetracations. One-electron redn. 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 \textless2, in addn. to a negligible g anisotropy of frozen solns. at frequencies up to 285 GHz and also through an unusually well-resolved soln. X band EPR spectrum of (μ4-TCNE)Fe(CO)2(C5H5)43+ which shows four equiv. Fe(CO)2(C5H5)+ moieties through 57Fe and 13C(CO) hyperfine coupling in nonenriched material. DFT calcns. reproduce the exptl. EPR data. A survey of discrete TCNE and TCNQ complexes (m4-TCNX)(MLn)4 exhibits a dichotomy between the systems (μ4-TCNX)Fe(CO)2(C5H5)44+ and (μ4-TCNQ)Re(CO)3(bpy)44+ with their negligible metal-to-ligand electron transfer and several other compds. 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-TCNXδ-)Mx+δ/4Ln4. Irreversibility of the 1st redn. of (μ4-TCNB)Fe(CO)2(C5H5)4(BF4)4 precluded spectroelectrochem. studies; however, the high-energy CN stretching frequencies and charge transfer absorptions of that TCNB analog also confirm the exceptional position of (μ4-TCNX)Fe(CO)2(C5H5)4(BF4)4. on SciFinder(R)
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