Carbon−Hydrogen Bond Activation with a Cyclometalated Zirconocene Hydride: Mechanistic Differences between Arene and Alkane Reductive Elimination

Thermolysis of the cyclometalated zirconocene hydride (η5-C5H3-1,3-(SiMe3)2)(η5-C5H3-3-SiMe3-1-η1-SiMe2CH2)ZrH, at 45 °C in benzene-d6 or toluene-d8, resulted in H/D exchange arising from reversible
C−H activation of sp2-hybridized bonds. More careful inspection of the isotopic exchange reactions by
NMR spectroscopy revealed formation of zirconocene aryl hydride complexes in equilibrium with the
cyclometalated hydride compound. In contrast, activation of (dimethylamino)pyridine (DMAP) was
complete at 23 °C and afforded the ortho-metalated (dimethylamino)pyridyl hydride. Performing the
thermolysis of the cyclometalated zirconocene hydride in alkane solvents resulted in activation of a second
[SiMe3] group on the opposite cyclopentadienyl ring, forming the C2-symmetric double-cyclometalated
complex (η5-C5H3-3-SiMe3-1-η1-SiMe2CH2)2Zr. Isotopic labeling studies demonstrated that neopentane
reductive elimination to yield the cyclometalated hydride proceeds through a cyclometalated assisted
pathway, while the analogous extrusion of benzene occurs by direct C−H bond reductive coupling, possibly
a consequence of the lower orbital reorganization energy associated with the sp2-hybridized carbon.
Activation of DMAP follows a different course, where the nucleophilicity of the heterocycle induces
reductive elimination and subsequent ortho metalation. In contrast, conversion of the cyclometalated
zirconocene hydride to the double-cyclometalated derivative most likely occurs by σ-bond metathesis,
highlighting the various pathways available for carbon−hydrogen bond activation.