Molecular spectroscopy of uranium(IV) bis(ketimido) complexes. Rare observation of resonance-enhanced Raman scattering from organoactinide complexes and evidence for broken-symmetry excited states

Electronic absorption and resonance-enhanced Raman spectra for ketimido (azavinylidene) complexes of tetravalent uranium, (C5Me5)(2)U[-N=C(Ph)(R)](2) (R = Ph, Me, and CH2Ph), have been recorded. The absorption spectra exhibit four broad bands between 13 000 and 24 000 cm(-1). The highest-energy band is assigned to the ketimido-localized p(perpendicular to)(N)-->pi*(N=C) transition based on comparison to the spectra of (C5H5)(2)Zr[-N=CPh2](2) and (C5Me5)(2)Th[-N=CPh2](2). Upon excitation into any of these four absorption bands, the (C5Me5)2U[-N=C(Ph)(R)](2) complexes exhibit resonance enhancement for several Raman bands attributable to vibrations of the ketimido ligands. Raman bands for both the symmetric and nominally asymmetric N=C stretching bands are resonantly enhanced upon excitation into the p(perpendicular to)(N)-->pi*-(N=C) absorption bands, indicating that the excited state is localized on a single ketimido ligand. Raman excitation profiles for (C5Me5)2U[-N=CPh2]2 confirm that at least one of the lower-energy electronic absorption bands (E-max similar to 16300 cm(-1)) is a charge-transfer transition between the U(IV) center and the ketimido ligand(s). The observations of both charge-transfer transitions and resonance enhancement of Raman vibrational bands are exceedingly rare for tetravalent actinide complexes and reflect the strong bonding interactions between the uranium 5f/6d orbitals and those on the ketimido ligands.