Evidence for the Involvement of 5f Orbitals in the Bonding and Reactivity of Organometallic Actinide Compounds: Thorium(IV) and Uranium(IV) Bis(hydrazonato) Complexes
Migratory insertion of diphenyldiazomethane into both metal−carbon bonds of the bis(alkyl) and bis(aryl) complexes (C5Me5)2AnR2 yields the first f-element bis(hydrazonato) complexes (C5Me5)2An[η2-(N,N′)-R−N−N═CPh2]2 [An = Th, R = CH3 (18), PhCH2 (15), Ph (16); An = U, R = CH3 (17), PhCH2 (14)], which have been characterized by a combination of spectroscopy, electrochemistry, and X-ray crystallography. The two hydrazonato ligands adopt an η2-coordination mode leading to 20-electron (for Th) and 22-electron (for U) complexes that have no transition-metal analogues. In fact, reaction of (C5H5)2Zr(CH3)2 or (C5Me5)2Hf(CH3)2 with diphenyldiazomethane is limited to the formation of the corresponding mono(hydrazonato) complex (C5R5)2M[η2-(N,N′)-CH3−N−N═CPh2](CH3) (M = Zr, R = H or M = Hf, R = CH3). The difference in the reactivities of the group 4 metal complexes and the actinides was used as a unique platform for investigating in depth the role of 5f orbitals on the reactivity and bonding in actinide organometallic complexes. The electronic structure of the (C5H5)2M[η2-(N,N′)-CH3−N−N═CH2]2 (M = Zr, Th, U) model complexes was studied using density functional theory (DFT) calculations and compared to experimental structural, electrochemical, and spectroscopic results. Whereas transition-metal bis(cyclopentadienyl) complexes are known to stabilize three ligands in the metallocene girdle to form saturated (C5H5)2ML3 species, in a bis(hydrazonato) system, a fourth ligand is coordinated to the metal center to give (C5H5)2ML4. DFT calculations have shown that 5f orbitals in the actinide complexes play a crucial role in stabilizing this fourth ligand by stabilizing both the σ and π electrons of the two η2-coordinated hydrazonato ligands. In contrast, the stabilization of the hydrazonato ligands was found to be significantly less effective for the putative bis(hydrazonato) zirconium(IV) complex, yielding a higher energy structure. However, the difference in the reactivities of the group 4 metal and actinide complexes does not arise on thermodynamic grounds but is primarily of kinetic origin. Unfavorable steric factors have been ruled out as the sole influence to explain these different behaviors, and electronic factors were shown to govern the reactivity. For the actinides, both the C5H5 and more realistic C5Me5 ligands have been taken into account in computing the energy surface. The reaction profile for the C5Me5 system differs from that with the C5H5 ligand by a uniform shift of ∼5 kcal/mol in the relative energies of the transition state and products. The insertion of a second diazoalkane molecule into the sole metal−carbon bond in the mono(hydrazonato) complexes involves a high energy barrier (∼20 kcal/mol) for the zirconium(IV) system, whereas the actinides can facilitate the approach of the diazoalkane by coordination (formation of an adduct) and its insertion into the An−C bond with a very low barrier on the potential energy surface.