Synthesis, characterization, isomerization, and reactivity of dimeric cyclopentadienylnickel amido complexes

The first cyclopentadienylnickel amido complexes have been isolated and characterized as [(eta-C(5)Me(4)R')Ni(mu-NHR)](2) (R = Ph, p-tol, 2,6-xyl, (t)Bu; 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-[Cp(Et)Ni(mu-NH(p-tol))](2) (cis-1'), trans-[Cp(Et)Ni(mu-NH(2,6-xylyl))](2) (trans-3'), and cis-[Cp(Et)Ni(mu-NH(t)Bu)](2) (4') (Cp(Et) = eta-C(5)Me(4)Et). Resonances in the H-1 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(mu-NH(p-tol))](2) (1) reacts with CO and with (t)BuNC to give the insertion products Cp*Ni(CO)(C(O)NH(p-tol)) (6) and Cp*Ni(CN(t)Bu)(C(N(t)Bu)NH(p-tol)) (7), respectively, and with PMe(3) to give the unstable monomeric amido complex Cp*Ni(PMe(3))(NH(p-tol)) (5).