Electrochemical and Spectroscopic Characterization of the Novel Charge-Transfer Ground State in Diimine Complexes of Ytterbocene
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- 作者：Ryan E. Da Re ; Christopher J. Kuehl ; Mac G. Brown ; Reginaldo C. Rocha ; Eric D. Bauer ; Kevin D. John ; David E. Morris ; Andrew P. Shreve ; and John L. Sarrao
- 刊名：Inorganic Chemistry
- 出版年：2003
- 出版时间：September 8, 2003
- 年：2003
- 卷：42
- 期：18
- 页码：5551 - 5559
- 全文大小：122K
- 年卷期：v.42,no.18(September 8, 2003)
- ISSN：1520-510X
文摘
The novel charge-transfer ground state found in 
,'-diimine adducts of ytterbocene (C5Me5)2Yb(L) [L = 2,2'-bipyridine (bpy) and 1,10-phenanthroline (phen)] in which an electron is spontaneously transferred from the f14metal center into the lowest unoccupied (
*) molecular orbital (LUMO) of the diimine ligand to give an f13-L
-ground-state electronic configuration has been characterized by cyclic voltammetry, UV-vis-near-IR electronicabsorption, and resonance Raman spectroscopies. The voltammetric data demonstrate that the diimine ligandLUMO is stabilized and the metal f orbital is destabilized by ~1.0 V each upon complexation for both bpy andphen adducts. The separation between the ligand-based oxidation wave (L0/-) and the metal-based reductionwave (Yb3+/2+) in the ytterbocene adducts is 0.79 V for both bpy and phen complexes. The UV-vis-near-IR absorptionspectroscopic data for both the neutral adducts and the one-electron-oxidized complexes are consistent with thosereported recently, but previously unreported bands in the near-IR have been recorded and assigned to ligand(*)-to-metal (f orbital) charge-transfer (LMCT) transitions. These optical electronic excited states are the converseof the ground-state charge-transfer process (e.g., f13-L- 
f14-L0). These new bands occur at ~5000 cm-1 inboth adducts, consistent with predictions from electrochemical data, and the spacings of the resolved vibronicbands in these transitions are consistent with the removal of an electron from the ligand * orbital. The unusuallylarge intensity observed in the f 
f intraconfiguration transitions for the neutral phenanthroline adduct is discussedin terms of an intensity-borrowing mechanism involving the low-energy LMCT states. Raman vibrational data clearlyreveal resonance enhancement for excitation into the low-lying * * ligand-localized excited states, andcomparison of the vibrational energies with those reported for alkali-metal-reduced diimine ligands confirms thatthe ligands in the adducts are reduced radical anions. Differences in the resonance enhancement pattern for themodes in the bipyridine adduct with excitation into different * * levels illustrate the different nodal structuresthat exist in the various low-lying * orbitals.
,'-diimine adducts of ytterbocene (C5Me5)2Yb(L) [L = 2,2'-bipyridine (bpy) and 1,10-phenanthroline (phen)] in which an electron is spontaneously transferred from the f14metal center into the lowest unoccupied (*) molecular orbital (LUMO) of the diimine ligand to give an f13-L-ground-state electronic configuration has been characterized by cyclic voltammetry, UV-vis-near-IR electronicabsorption, and resonance Raman spectroscopies. The voltammetric data demonstrate that the diimine ligandLUMO is stabilized and the metal f orbital is destabilized by ~1.0 V each upon complexation for both bpy andphen adducts. The separation between the ligand-based oxidation wave (L0/-) and the metal-based reductionwave (Yb3+/2+) in the ytterbocene adducts is 0.79 V for both bpy and phen complexes. The UV-vis-near-IR absorptionspectroscopic data for both the neutral adducts and the one-electron-oxidized complexes are consistent with thosereported recently, but previously unreported bands in the near-IR have been recorded and assigned to ligand(*)-to-metal (f orbital) charge-transfer (LMCT) transitions. These optical electronic excited states are the converseof the ground-state charge-transfer process (e.g., f13-L- f14-L0). These new bands occur at ~5000 cm-1 inboth adducts, consistent with predictions from electrochemical data, and the spacings of the resolved vibronicbands in these transitions are consistent with the removal of an electron from the ligand * orbital. The unusuallylarge intensity observed in the f f intraconfiguration transitions for the neutral phenanthroline adduct is discussedin terms of an intensity-borrowing mechanism involving the low-energy LMCT states. Raman vibrational data clearlyreveal resonance enhancement for excitation into the low-lying * * ligand-localized excited states, andcomparison of the vibrational energies with those reported for alkali-metal-reduced diimine ligands confirms thatthe ligands in the adducts are reduced radical anions. Differences in the resonance enhancement pattern for themodes in the bipyridine adduct with excitation into different * * levels illustrate the different nodal structuresthat exist in the various low-lying * orbitals.