Tetranuclear Complexes of [Fe(CO)2(C5H5)]+ with TCNX Ligands (TCNX = TCNE, TCNQ, TCNB): Intramolecular Electron Transfer Alternatives in Compounds (![]()
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- 作者：Amarendra N. Maity ; Brigitte Schwederski ; Biprajit Sarkar ; Stanislav Zá ; li
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- 刊名：Inorganic Chemistry
- 出版年：2007
- 出版时间：September 3, 2007
- 年：2007
- 卷：46
- 期：18
- 页码：Gütlich, and Wol
- 全文大小：249K
- 年卷期：v.46,no.18(September 3, 2007)
- ISSN：1520-510X
文摘
The complexes {(
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 extremelyeasily reduced species with reduction potentials >+0.3 V vs ferrocenium/ferrocene, the tetranuclear complex ofTCNB exhibits a significantly more negative reduction potential at about -1.0 V. Even for the complexes withstrongly 
-accepting TCNE and TCNQ, the very positive reduction potentials, the unusually high nitrile stretchingfrequencies >2235 cm-1, and the high-energy charge-transfer transitions indicate negligible metal-to-ligand electrontransfer in the ground state, corresponding to a largely unperturbed (TCNX
)(FeII)4 formulation of oxidation statesas caused by orthogonality between the metal-centered HOMO and the * LUMO of TCNX. Mössbauer spectroscopyconfirms the low-spin iron(II) state, and DFT calculations suggest coplanar TCNE and TCNQ bridging ligands inthe complex tetracations. One-electron reduction to the 3+ forms of the TCNE and TCNQ complexes producesEPR spectra which confirm the predominant ligand character of the then singly occupied MO through isotropic gvalues slightly below 2, in addition to a negligible g anisotropy of frozen solutions at frequencies up to 285 GHzand also through an unusually well-resolved solution X band EPR spectrum of {(4-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 couplingin nonenriched material. DFT calculations reproduce the experimental EPR data. A survey of discrete TCNE andTCNQ complexes [(4-TCNX)(MLn)4] exhibits a dichotomy between the systems {(4-TCNX)[Fe(CO)2(C5H5)]4}4+and {(4-TCNQ)[Re(CO)3(bpy)]4}4+ with their negligible metal-to-ligand electron transfer and several other compoundsof TCNE or TCNQ with Mn, Ru, Os, or Cu complex fragments which display evidence for a strong such interaction,i.e., an appreciable value 
in the formulation {(4-TCNX-)[Mx+/4Ln]4}. Irreversibility of the first reduction of{(4-TCNB)[Fe(CO)2(C5H5)]4}(BF4)4 precluded spectroelectrochemical studies; however, the high-energy CN stretchingfrequencies and charge transfer absorptions of that TCNB analogue also confirm the exceptional position of thecomplexes {(4-TCNX)[Fe(CO)2(C5H5)]4}(BF4)4.
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 extremelyeasily reduced species with reduction potentials >+0.3 V vs ferrocenium/ferrocene, the tetranuclear complex ofTCNB exhibits a significantly more negative reduction potential at about -1.0 V. Even for the complexes withstrongly -accepting TCNE and TCNQ, the very positive reduction potentials, the unusually high nitrile stretchingfrequencies >2235 cm-1, and the high-energy charge-transfer transitions indicate negligible metal-to-ligand electrontransfer in the ground state, corresponding to a largely unperturbed (TCNX)(FeII)4 formulation of oxidation statesas caused by orthogonality between the metal-centered HOMO and the * LUMO of TCNX. Mössbauer spectroscopyconfirms the low-spin iron(II) state, and DFT calculations suggest coplanar TCNE and TCNQ bridging ligands inthe complex tetracations. One-electron reduction to the 3+ forms of the TCNE and TCNQ complexes producesEPR spectra which confirm the predominant ligand character of the then singly occupied MO through isotropic gvalues slightly below 2, in addition to a negligible g anisotropy of frozen solutions at frequencies up to 285 GHzand also through an unusually well-resolved solution X band EPR spectrum of {(4-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 couplingin nonenriched material. DFT calculations reproduce the experimental EPR data. A survey of discrete TCNE andTCNQ complexes [(4-TCNX)(MLn)4] exhibits a dichotomy between the systems {(4-TCNX)[Fe(CO)2(C5H5)]4}4+and {(4-TCNQ)[Re(CO)3(bpy)]4}4+ with their negligible metal-to-ligand electron transfer and several other compoundsof TCNE or TCNQ with Mn, Ru, Os, or Cu complex fragments which display evidence for a strong such interaction,i.e., an appreciable value in the formulation {(4-TCNX-)[Mx+/4Ln]4}. Irreversibility of the first reduction of{(4-TCNB)[Fe(CO)2(C5H5)]4}(BF4)4 precluded spectroelectrochemical studies; however, the high-energy CN stretchingfrequencies and charge transfer absorptions of that TCNB analogue also confirm the exceptional position of thecomplexes {(4-TCNX)[Fe(CO)2(C5H5)]4}(BF4)4.