Synthesis and Reactions of [Cp*2Yb]2(μ-Me) and [Cp*2Yb]2(μ-Me)(Me) and Related Yb2(II, III) and Yb2(III, III) Compounds

A new type of synthesis, referred to as oxidative methylation, is developed for [Cp*2Yb]2(μ-X) and [Cp*2Yb]2(μ-X)­(X), where X = Me, using MeCu or Cp*2VMe as the methyl transfer reagent and Cp*2Yb. The synthetic methodology is extended to other X derivatives such as the halides and BH4. Reaction of [Cp*2Yb]2(μ-Me)­(Me) and H2 yields the mixed-valent hydride [Cp*2Yb]2(μ-H), which eliminates H2 on gentle heating, forming Cp*2Yb. When Cp*2VX is replaced by Cp*2TiX, 1:1 adducts based upon Ti­(III,d1) are isolated. The X-ray crystal structure of [Cp*2Yb]­(μ-Me)­[TiCp*2] shows that the methyl group bridges the two different decamethylmetallocene fragments in a near-linear fashion, a geometry that is likely to resemble the transition state of the single-electron-transfer precursor complex. A CASSCF computational study on the mixed-valent hydride [Cp*2Yb]2(μ-H) shows that the ground state is a spin doublet in which the hydride forms a symmetric bridge to both Yb atoms. The three spins forming the ground-state doublet are aligned as Yb­(f13(α),(dz2)0)···H···Yb­(f13(α),(dz2)1(β)), and the unpaired d electron is delocalized between the dz2 orbitals of the two Yb centers via the hydride bridge using the σ* orbital of the Yb­(dz2)–H bond. The first excited state lies 0.09 eV (725 cm–1) higher in energy and is a spin quartet in which the three spins are aligned as Yb­(f13(α),(dz2)0)···H···Yb­(f13(α),(dz2)1(α)), also giving rise to delocalization of the d electron between the dz2 orbitals of the two Yb centers. The second spin doublet resembles the Lewis structure with an asymmetric μ-H bridge in which the Yb­(II) metallocene has a closed-shell electronic configuration and is approximately 0.15 eV (1210 cm–1) higher in energy than the ground-state delocalized open-shell doublet. The electronic structure of the mixed-valent methyl is closely related to that of the hydride, but the methyl group is localized on the Cp*2YbIII fragment. Electronic energies (ΔE) computed at the DFT (B3PW91) level of theory provide insights into the thermochemistry of the formation and decomposition of [Cp*2Yb]2(μ-H). The BDE for Yb–H is ca. 15 kcal/mol stronger than that for the corresponding Yb–Me in the monomeric metallocenes. In contrast, formation of [Cp*2Yb]2(μ-CH3) is ca. 60 kcal/mol more exothermic than the formation of [Cp*2Yb]2(μ-H). This difference is ascribed to enhanced intramolecular steric repulsion between the Cp*2Yb moieties in the linear Yb–H–Yb unit.