Electrochemical and Spectroscopic Characterization of the Novel Charge-Transfer Ground State in Diimine Complexes of Ytterbocene

Abstract

The novel charge-transfer ground state found in alpha,alpha'-diimine adducts of ytterbocene (C(5)Me(5))(2)Yb(L) [L = 2,2'-bipyridine (bpy) and 1,10-phenanthroline (phen)] in which an electron is spontaneously transferred from the f(14) metal center into the lowest unoccupied (pi*) molecular orbital (LUMO) of the diimine ligand to give an f(13)-L(*)(-) ground-state electronic configuration has been characterized by cyclic voltammetry, UV-vis-near-IR electronic absorption, and resonance Raman spectroscopies. The voltammetric data demonstrate that the diimine ligand LUMO is stabilized and the metal f orbital is destabilized by approximately 1.0 V each upon complexation for both bpy and phen adducts. The separation between the ligand-based oxidation wave (L(0/-)) and the metal-based reduction wave (Yb(3+/2+)) in the ytterbocene adducts is 0.79 V for both bpy and phen complexes. The UV-vis-near-IR absorption spectroscopic data for both the neutral adducts and the one-electron-oxidized complexes are consistent with those reported recently, but previously unreported bands in the near-IR have been recorded and assigned to ligand (pi*)-to-metal (f orbital) charge-transfer (LMCT) transitions. These optical electronic excited states are the converse of the ground-state charge-transfer process (e.g., f(13)-L(*-) <--> f(14)-L(0)). These new bands occur at approximately 5000 cm(-1) in both adducts, consistent with predictions from electrochemical data, and the spacings of the resolved vibronic bands in these transitions are consistent with the removal of an electron from the ligand pi* orbital. The unusually large intensity observed in the f --> f intraconfiguration transitions for the neutral phenanthroline adduct is discussed in terms of an intensity-borrowing mechanism involving the low-energy LMCT states. Raman vibrational data clearly reveal resonance enhancement for excitation into the low-lying pi* --> pi* ligand-localized excited states, and comparison of the vibrational energies with those reported for alkali-metal-reduced diimine ligands confirms that the ligands in the adducts are reduced radical anions. Differences in the resonance enhancement pattern for the modes in the bipyridine adduct with excitation into different pi* --> pi* levels illustrate the different nodal structures that exist in the various low-lying pi* orbitals.

Thermal Dihydrogen Elimination from Cp*2Yb(4,5-diazafluorene)

“…Thus, the most deshielded resonance, due to 2H's in 1 in toluene-d 8 , is a singlet at 71.1 ppm that moves upfield to 45.6 or 35.8 ppm in either thf-d 8 or pyridine-d 5, respectively. The other resonances due to the diazafluorene ligand shift slightly (± 3 to 4 ppm) and those due to the Cp * resonances are shielded by about 1 ppm in either thf-d 8 or pyridine-d 5 ; the values are given in the Experimental Section. Although the resonance at = 71.1 ppm in toluene-d 8 is not assigned with certainty, it is likely due to the CH that is to N, since these resonances are the most strongly deshielded in the bipy adducts due to their proximity to the paramagnetic center giving rise to a large dipolar contribution to the total chemical shift tensor.…”

“…1. Synthetic support for this postulate is obtained by heating 1 in pyridine-d 5 for several days at 60 ºC in an NMR tube. The resulting material has NMR resonances due to Cp * 2 Yb(py) 2 and free diazafluorene.…”

“…Toluene and pentane were dried over sodium and distilled while CH 2 Cl 2 was purified by passage through a column of activated alumina. Toluene-d 8 , pyridine-d 5 and C 6 D 6 were dried over sodium. All the solvents were degassed prior to use.…”

“…The spectrum of 1 and 3 are very similar and present a very broad and intense band in the Vis-NIR region (700-1100 nm), which is attributed to * * transitions since the spectral profile is very similar to that of both Cp * 2 Yb(bipy) and Na(bipy). 5,14 This band has a maximum at 909 nm. On the other hand, the spectrum from 2 displays only a set of three low intensity, yet relatively sharp, bands in the same range that are characteristic of f 13 f f transitions.…”

Application of the Hubbard Model to Cp*2Yb(bipy), a Model System for Strong Exchange Coupling in Lanthanide Systems

“…16 The absorption spectra of 3 and related complexes have been extensively studied, and a low-lying transition at 4750(250) cm -1 has been assigned as a ligand-to-metal charge transfer band (E LMCT ). 38 Using these values of c 1 2 and E LMCT , t and U are 0.13(1) eV and 0.42(4) eV, respectively, since c 1 2 = c 2 in eqn 3, and E LMCT = U + 4J = U+4(t/U) 2 (see Figure 4). The resulting value of 2J is -0.09(1) eV or -700(90) cm -1 , which is in good agreement with the value of -0.11(2) eV determined from the susceptibility of 3.…”