1,2-Addition versus σ-Bond Metathesis Reactions in Transient Bis(cyclopentadienyl)zirconium Imides: Evidence for a d0 Dihydrogen Complex

Exposure of a series of zirconocene amido and hydrazido hydride complexes, (η5-C5Me4H)2Zr(NHR)H (R = tBu, NMe2, Me, H), to 4 atm of D2 gas at 56 °C produced isotopic exchange in both the N−H and Zr−H positions. In general, the relative rates of 1,2-elimination can be rationalized on the basis of ground-state effects, whereby amido compounds with the strongest N−H bonds, as judged by the corresponding free amine, undergo the slowest isotopic exchange. For the compound with the strongest N−H bond in the series, (η5-C5Me4H)2Zr(NH2)H, the barrier for 1,2-elimination is sufficiently high such that σ-bond metathesis becomes the dominant intermolecular exchange pathway. For the other amido zirconocene hydrides, the rate constants for deuterium exchange into the N−H position are faster than for the Zr−H position. This behavior is a result of a faster intramolecular isomerization process driven by an equilibrium isotope effect favoring N−D over Zr−D bond formation. Computational studies on a related model compound, (η5-C5H5)2Zr(NHtBu)H, successfully reproduce these observations and support a pathway involving the formation of rare d0 dihydrogen complexes.