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

Abstract

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.

Iron Complexes Bearing Diphosphine Ligands with Positioned Pendant Amines as Electrocatalysts for the Oxidation of H2

“…Information about the EIE for [4-FeD(NH)] + ⇄ [4-FeH(ND)] + can be obtained from NMR spectroscopic studies; it requires knowledge of the "frozen out" chemical shifts of the This Fe system has a sufficiently low exchange barrier that this information cannot be directly obtained experimentally, as we were unable to observe decoalescence for 63 Chirik and co-workers reported an intramolecular EIE of K eq = 0.41 for the equilibrium between (C 5 Me 4 H) 2 Zr-(H)(ND t Bu) and (C 5 Me 4 H) 2 Zr(D)(NH t Bu) at 56°C; as in the reactions reported here, the MH/ND isotopomer is favored over MD/NH. 71 In CD 2 Cl 2 , the averaged resonance for [4-FeH(NH)] + appears at −5.36 ppm (slightly shifted from its value of −7.06 ppm in PhCl-d 5 ) and remains almost unchanged upon cooling to −80°C. Upon cooling a solution of the HD adduct, the 2 H NMR resonance gradually shifted downf ield from 1.38 to 4.61 ppm as the temperature was lowered from 0 to −80°C ( Figure 6).…”

1,2-Addition and cycloaddition reactions of niobium bis(imido) and oxo imido complexes

“…While there are numerous examples of nitrene transfer chemistry involving p-loaded metal imido complexes, catalytic reactions without nitrene transfer are rare. High-valent transition metal complexes of groups 3-5 [39][40][41][42][43][44] and actinide 45,46 imido complexes have been shown to activate dihydrogen, silanes and/or boranes across imido bonds, but in all of these cases, only stoichiometric reactivity could be observed. A cationic V bis(imido) complex was shown to catalytically hydrogenate alkynes to alkenes via a 1,2-addition and 1,2-elimination pathway.…”

H/D Exchange on Silica-Grafted Tantalum(V) Imido Amido [(≡SiO)2Ta(V)(NH)(NH2)] Synthesized from Either Ammonia or Dinitrogen: IR and DFT Evidence for Heterolytic Splitting of D2

“…In the specific field of N1-chemistry, D 2 labelling and other H/D exchange reactions are finding similarly useful application for probing metal-amide reactivity [36] and shedding light on hydrogenation reaction mechanisms of N-based substrates [37][38][39][40], both of which are fields of relevance to catalysis [28,[41][42][43].…”