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-C₅Me₄)₂Zr(NHR)H (R = ¹Bu, 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 groundstate 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(NH ^tBu)H, successfully reproduce these observations and support a pathway involving the formation of rare d⁰ dihydrogen complexes.