Synthesis, Characterization, Isomerization, and Reactivity of Dimeric Cyclopentadienylnickel Amido Complexes

The first cyclopentadienylnickel amido complexes have been isolated and characterized as [(η-C5Me4R‘)Ni(μ-NHR)]2 (R = Ph, p-tol, 2,6-xyl, tBu; R‘ = Me, Et). These complexes are dimers in solution and in the solid state, as shown by the synthesis of mixed amido complexes, by NMR spectroscopy, and by crystallographic studies on cis-[CpEtNi(μ-NH(p-tol))]2 (cis-1‘), trans-[CpEtNi(μ-NH(2,6-xylyl))]2 (trans-3‘), and cis-[CpEtNi(μ-NHtBu)]2 (4‘) (CpEt = η-C5Me4Et). Resonances in the 1H NMR spectra of these diamagnetic dimers display unusual chemical shifts that are explained on the basis of ring-current anisotropy and inductive effects. The dimers undergo reversible cis/trans isomerization at elevated temperatures; mechanistic studies indicate that this process proceeds through cleavage of one dative nitrogen−nickel bond, rate-limiting rotation of the amido group, and recoordination to regenerate the bridge. Dimethylzirconocene was essential as a scavenger for trace water in these studies. The dimer [Cp*Ni(μ-NH(p-tol))]2 (1) reacts with CO and with tBuNC to give the insertion products Cp*Ni(CO)(C(O)NH(p-tol)) (6) and Cp*Ni(CNtBu)(C(NtBu)NH(p-tol)) (7), respectively, and with PMe3 to give the unstable monomeric amido complex Cp*Ni(PMe3)(NH(p-tol)) (5).