Tetranuclear Complexes of [Fe(CO)2(C5H5)]~+with TCNX Ligands (TCNX=TCNE,TCNQ,TCNB):Intramolecular Electron Transfer Alternatives in Compounds (mu 4-TCNX)[ML_n]_4

摘要

The complexes {(mu_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 jr-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~(deg))(Fe~(II))_4 formulation of oxidation states as caused by orthogonality between the metal-centered HOMO and the n* LUMO of TCNX.Mossbauer spectroscopy confirms the low-spin iron(ll) 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 {(mu_4-TCNE)[Fe(CO)2(C5H5)]4}~(3+) which shows the presence of four equivalent [Fe(CO)2(C5H5)]~+moieties through ~(57)Fe and ~(13)C(CO) hyperfine coupling in nonenriched material.DFT calculations reproduce the experimental EPR data.A survey of discrete TCNE and TCNQ complexes [(mu_4-TCNX)(ML_n)4] exhibits a dichotomy between the systems {(mu_4-TCNX)[Fe(CO)2(C5H5)]4}~(4+) and {(mu_4-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 {(mu_4-TCNX~(delta-))[M~(x+delta/4)L_n]_4}.Irreversibility of the first reduction of {(mu 4-TCNB)[Fe(CO)_2(C5H5)]_4}(BF_4)_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 {(mu_4-TCNX)[Fe(CO)2(C5H5)]4}(BF4)_4.