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).