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

Article abstract of DOI:10.1021/ja952882l

The first cyclopentadienylnickel amido complexes have been isolated and characterized as [(η-C₅Me₄R')Ni(μ-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(μ-NH(p-tol))]₂ (cis-1'), trans-[Cp(Et)Ni(μ-NH(2,6-xylyl))]₂ (trans-3'), and cis-[Cp(Et)Ni(μ-NH(t)Bu)]₂ (4') (Cp(Et) = 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 (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 PMe₃ to give the unstable monomeric amido complex Cp*Ni(PMe₃)(NH(p-tol)) (5).

Full text of DOI:10.1021/ja952882l

1092
J. Am. Chem. Soc. 1996, 118, 1092-1104
Synthesis, Characterization, Isomerization, and Reactivity of
Dimeric Cyclopentadienylnickel Amido Complexes
Patrick L. Holland, Richard A. Andersen,* and Robert G. Bergman*
Contribution from the Department of Chemistry, UniVersity of California,
Berkeley, California 94720
ReceiVed August 21, 1995X
Abstract: The first cyclopentadienylnickel amido complexes have been isolated and characterized as [(η-C5Me4R′)Ni(µ-
t
NHR)]2 (R ) Ph, p-tol, 2,6-xyl, 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-[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
1
) η-C5Me4Et). Resonances in the H 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).
Introduction
(Cp*) would provide a stabilizing steric influence to allow the
isolation of stable amidonickel(II) complexes. This expectation
has now been realized: this paper describes nickel amides of
the type [Cp*Ni(µ-NHR)]2 along with their solid state stuctures,
solution properties, and reactivity.
There is ample reason to believe that amido, imido, alkoxo,
hydroxo, and oxo complexes of the “late” (groups 8-10)
transition metals will be reactive.1 On the basis of HSAB
(hard-soft acid-base theory) arguments, a mismatch exists
between the “hard” anionic base and the “soft” low-valent late-
transition-metal center, resulting in a relatively weak M-N
bond. Alternatively, using orbital arguments, nonbonding
interactions between filled orbitals on the metal and on the
ligand lead to destabilization.2 The weakness predicted for this
bond is expected to lead to high reactivity in both stoichiometric
and catalytic reactions.
For this reason, our groups have synthesized amido and imido
complexes of the d6 metals Ru(II),3 Os(II),4 and Ir(III)5 and
probed their exchange, elimination, and insertion reactions. In
this contribution, these metal-amido studies are extended to a
d8 metal center. Simple amido complexes of Ni(II), Pd(II), and
Pt(II) have usually been synthesized by metathesis of a
platinum-metal halide or pseudo-halide with an alkali metal
amide6 or by deprotonation of coordinated amines,7 although
imide insertion,8 â-attack on coordinated imines,9 and fragmen-
tation of a tetrazenide10 have also been used. While these
syntheses provide many examples of palladium and platinum
amides, nickel amides are much less common. At the start of
this work, no cyclopentadienylnickel amides were known. We
believed, however, that the pentamethylcyclopentadienyl ligand
Results and Discussion
Synthesis of Amido Complexes. A synthetic precursor to
Cp* complexes of nickel, Cp*Ni(acac) (acac ) O,O-η2-acetyl-
acetonate) has been described by Manriquez, but has found sur-
prisingly little use as a route to Cp*Ni(L)(X) complexes (L )
neutral ligand, X ) anionic ligand).11,12 The red Cp*Ni(acac)
can be synthesized easily in one step from MgCp*2 and commer-
cially available [Ni(acac)2]n, using a modified preparation.12
Addition of THF to a mixture of Cp*Ni(acac) and the
appropriate lithium amide at -40 or -78 °C gave dimeric amido
(6) (a) Fryzuk, M. D.; MacNeil, P. A.; Rettig, S. J.; Secco, A. S.; Trotter,
J. Organometallics 1982, 1, 918. (b) Bartlett, R. A.; Chen, H.; Power, P. P.
Angew. Chem. Int. Ed. Engl. 1989, 28, 316. (c) Bradley, D. C.; Hursthouse,
M. B.; Smallwood, R. J.; Welch, A. J. Chem. Commun. 1972, 872. (d)
Power, P. P. Comments Inorg. Chem. 1989, 8, 177 and references therein.
(e) Okeya, S.; Yoshimatsu, H.; Nakamura, Y.; Kawaguchi, S. Bull. Chem.
Soc. Jpn. 1982, 55, 483. (f) Tenten, A.; Jacobs, H. Z. Anorg. Allg. Chem.
1991, 604, 113. (g) Villanueva, L. A.; Abboud, K. A.; Boncella, J. M.
Organometallics 1994, 13, 3921. (h) Cowan, R. L.; Trogler, W. C. J. Am.
Chem. Soc. 1989, 111, 4750. (i) Seligson, A. L.; Cowan, R. L.; Trogler,
W. C. Inorg. Chem. 1991, 30, 3371. (j) Driver, M. S.; Hartwig, J. F. J. Am.
Chem. Soc. 1995, 117, 4708. (k) Hope, H.; Olmstead, M. M.; Murray, B.
D.; Power, P. P. J. Am. Chem. Soc. 1985, 107, 712. (l) Cetinkaya, B.;
Hitchcock, P. B.; Lappert, M. F.; Misra, M. C.; Thorne, A. J. J. Chem.
Soc., Chem. Commun. 1984, 148. (m) Rahim, M.; Bushweller, C. H.;
Ahmed, K. J. Organometallics 1994, 13, 4952.
(7) See ref 1b and: (a) Kita, M.; Nonoyama, M. Polyhedron 1993, 12,
1027. (b) Alcock, N. W.; Bergamini, P.; Kemp, T. J.; Pringle, P. G. J.
Chem. Soc., Chem. Commun. 1987, 235. (c) Park, S.; Rheingold, A. L.;
Roundhill, D. M. Organometallics 1991, 10, 615.
(8) (a) Matsunaga, P. T.; Hess, C. R.; Hillhouse, G. L. J. Am. Chem.
Soc. 1994, 116, 3665. (b) Pizzotti, M.; Cenini, S.; Porta, F.; Beck, W.;
Erbe, J. J. Chem. Soc., Dalton Trans. 1978, 1155. (c) Beck, W.; Bauder,
M. Chem. Ber. 1970, 103, 583.
X Abstract published in AdVance ACS Abstracts, January 1, 1996.
(1) For reviews on the subject, see: (a) Bryndza, H. E.; Tam, W. Chem.
ReV. 1988, 88, 1163. (b) Fryzuk, M. D.; Montgomery, C. D. Coord. Chem.
ReV. 1989, 95, 1.
(2) Mayer, J. M. Comments Inorg. Chem. 1988, 8, 125.
(3) (a) Hartwig, J. F.; Andersen, R. A.; Bergman, R. G. Organometallics
1991, 10, 1875. (b) Michelman, R. I. Ph.D. Thesis, University of California,
Berkeley, 1993.
(4) (a) Michelman, R. I.; Andersen, R. A.; Bergman, R. G. Organome-
tallics 1993, 12, 2741. (b) Michelman, R. I.; Ball, G. E.; Bergman, R. G.;
Andersen, R. A. Organometallics 1994, 13, 869.
(5) (a) Glueck, D. S.; Bergman, R. G. Organometallics 1991, 10, 1479.
(b) Glueck, D. S.; Winslow, L. J. N.; Bergman, R. G. Organometallics
1991, 10, 1462. (c) Glueck, D. S.; Wu, J.; Hollander, F. W.; Bergman, R.
G. J. Am. Chem. Soc. 1991, 113, 2041. (d) Dobbs, D. A.; Bergman, R. G.
J. Am. Chem. Soc. 1993, 115, 3836.
(9) Villanueva, L. A.; Abboud, K.; Boncella, J. M. Organometallics 1991,
10, 2969.
(10) Lee, S. W.; Trogler, W. C. Inorg. Chem. 1990, 29, 1099.
(11) Bunel, E. E.; Valle, L.; Manriquez, J. M. Organometallics 1985, 4,
1680.
(12) Smith, M. E.; Andersen, R. A. Submitted for publication.
0002-7863/96/1518-1092$12.00/0 © 1996 American Chemical Society

Study of Cyclopentadienylnickel Amido Complexes
J. Am. Chem. Soc., Vol. 118, No. 5, 1996 1093
Scheme 1
Figure 1. Geometric isomers of [Cp*Ni(µ-NHR)]2.
Figure 2. Isomers of [CpEtNi(µ-NHR)]2, viewed down the Ni-Ni axis.
complexes [Cp*Ni(µ-NHR)]2 (R ) p-tolyl (1), phenyl (2), 2,6-
xylyl (3), tert-butyl (4)) (Scheme 1). In the cases of 1 and 2,
the initial green-blue solution observed at low temperature turned
brown as the mixture was warmed or the solvent was removed;
workup gave 1 or 2 as purple solids in 60-70% yield. When
synthesizing 3 or 4, allowing the reaction mixtures to warm to
room temperature resulted in extensive decomposition. How-
ever, removal of solvent at low temperature gave residues from
which purple (3) or black (4) crystals could be obtained in
reasonable yields. Dimer 3 was less soluble than 1 or 2, and
could be crystallized from a mixture of toluene and pentane.
Compound 4, on the other hand, was extremely soluble in
nonpolar solvents. Crystallization from acetonitrile (in which
the crude product is poorly soluble) proved to be a high-yield
purification technique for 4. Each of these nickel amides was
highly air and water sensitive in the solid state or in solution.
Since alkylamides of group 10 metals are not very stable,1,6g,13
it is notable that 4 was isolable. However, the lower stability
of alkylamides relative to arylamides was reflected in the thermal
stabilities of 1-4 in solution: the arylamides 1-3 were stable in
solution to 80-90 °C (with rigorous exclusion of water and
air, as demonstrated below), while 4 decomposed within hours
in solution at 45 °C or within 1 day at room temperature.
Analogous procedures led to representative ethyltetrameth-
ylcyclopentadienyl (CpEt) analogues of 1-4. The products 1′,
3′, and 4′ (as well as the precursors MgCpEt2 and CpEtNi(acac))
were more soluble than their Cp* analogues in hydrocarbon
solvents, and invariably gave higher-quality crystals. The
isolated yields of 1′, 3′, and 4′ were generally lower than for
the Cp* analogues, due to their high solubilities in common
solvents. Spectroscopically, however, they behaved very simi-
larly to 1, 3, and 4, and we shall assume that our crystallographic
and spectroscopic findings on the CpEt compounds are also
applicable to their Cp* analogues.14
(p-tol) by low-temperature NMR spectroscopy, the reaction was
performed in an NMR tube using THF-d8. However, NMR
analysis at -50 °C was complicated by the paramagnetic starting
material and the poorly-soluble lithium salts, and the only
identifiable amidonickel complex was 1.
Nevertheless, it was possible to obtain evidence supporting
the intermediacy of the monomer with a low-temperature
trapping experiment. Addition of trimethylphosphine to the
green-blue solution at -78 °C immediately gave a blue solution
of Cp*Ni(PMe3)(NH(p-tol)) (5). Reaction of the dimer 1 with
PMe3 gives 5 only after several hours at room temperature (Vide
infra), so this low-temperature reaction cannot be preceded by
dimerization. Hence, it appears that PMe3 is trapping mono-
meric Cp*Ni(NH(p-tol)) that persists under the reaction condi-
tions at low temperature. Attempted trapping by pyridine did
not hinder the formation of dimer, although the presence of
pyridine made the reaction proceed cleanly at a higher temper-
ature. The syntheses of 1 and 2 described in the Experimental
Section demonstrate both methods for making these arylamido
dimers.
Solution Structural Characterization. Mass spectrometry
indicated that these nickel amides were dimers in the gas phase;
we also desired confirmation that the dimeric structures persist
in solution and in the solid state. Both solution NMR techniques
and X-ray crystal structure analysis proved useful in this effort.
An M2(µ-NHR)2 skeleton allows the presence of two geo-
metric isomers, cis and trans, that have idealized C2V and C2h
symmetries, respectively (Figure 1). In the case of 4, only one
isomer was observed, but for 1, 2, and 3, mixtures of isomers
were generally obtained. It was not possible to separate these
isomers by crystallization, but cis and trans isomers could be
1
distinguished in solution by their different H NMR spectra.
The cis and trans isomers of 1-4 are expected to have the same
number of resonances in their NMR spectra, so there was no
definitive way to determine which isomer corresponded to each
set of resonances. However, the lower symmetry of the CpEt
ligand allowed us to distinguish cis (Cs symmetry) from trans
(Ci symmetry) isomers in 1′, 3′, and 4′ (Figure 2). In the cis
(Cs) isomers, methyl groups a and b are rendered chemically
equivalent by the molecular mirror plane, as are methyl groups
c and d, so two singlets are expected in the proton and carbon
NMR spectra, corresponding to CpEt methyl groups. However,
in the trans (Ci) isomers, there are no mirror planes, so all four
methyl groups are chemically inequivalent and four different
resonances are expected for configurationally stable dimers. The
resonances attributable to the amido ligands in 1′, 3′, and 4′
were observed at virtually the same chemical shifts as in the
Cp* analogues, so it is possible to assign the resonances in the
1H NMR spectra of 1-4 to the appropriate isomers. This
The following three sections of this paper will be devoted to
showing that complexes 1-4 and 1′-4′ are dimers, based on
evidence from NMR spectroscopy, variable-temperature phe-
nomena, and X-ray crystallography. It is reasonable to believe
that the formation of these dimers proceeds through initial
formation of an unsaturated monomeric amido complex
Cp*NiNHR; this is consistent with the observation of a transient
green-blue intermediate in the reaction between Cp*Ni(acac)
and LiNH(p-tol). In an attempt to observe monomeric Cp*NiNH-
(13) For an extremely unstable nickel alkoxide complex, see: Baranwal,
B. P.; Mehrotra, R. C. Aust. J. Chem. 1980, 33, 37.
(14) No N-H stretching bands were observed in the IR spectra of 1 and
3, and only weak resonances appeared in the IR spectra of the other dimeric
amides. These bands are often weak in metal amides, see: Chisholm, M.
H.; Rothwell, I. P. In ComprehensiVe Coordination Chemistry; Wilkinson,
G., Ed.; Pergamon: Oxford, 1987; Vol. 2, p 161.
(15) Johnson, C. K., Report ORNL-3794, Oak Ridge National Laboratory,
Oak Ridge, TN, 1965.

1094 J. Am. Chem. Soc., Vol. 118, No. 5, 1996
Holland et al.
analysis shows that the predominant isomers formed in the
syntheses of 1-3 were the trans isomers, but the cis isomer
was the exclusive product in the preparation of 4. The same
preference for the cis isomer has been observed in a (µ-tert-
butylamido)iridium dimer.6n (The observation of two distinct
isomers for 1-3 at temperatures up to 90 °C also shows that
the amido ligands of 1-3 do not rotate on the NMR time scale.)
The crystallographic studies described below provide a basis
for rationalizing the observed isomeric distributions.
Another piece of evidence supporting a dimeric structure for
1
1-3 in solution is the observation of variable-temperature H
NMR data consistent with restricted rotation about the N-C
bonds. Dimers 1 and 2 had broad aryl resonances in their proton
NMR spectra at room temperature; these coalesced upon
warming to 90 °C to two doublets (for 1) or a doublet and two
multiplets (for 2) for each isomer, and decoalesced to compli-
cated patterns upon cooling to -75 °C. However, heating a
sample of 3 to 100 °C in toluene-d8 did not result in coalescence
of any resonances. This variable-temperature behavior is
consistent with an aryl group “sandwiched” between Cp* ligands
that make it difficult for the aryl groups to rotate out of the
plane parallel to the Cp* rings. Since 3 has large o-methyl
groups, it cannot rotate within the confines of the Cp* rings.
Figure 3. ORTEP diagram of cis-1′, using 50% probability ellipsoids
(except the N-bound H atom, which has arbitrary size). Carbon-bound
hydrogen atoms have been omitted for clarity.
Since several of the above phenomena (observation of
multiple isomers and hindered rotation about the N-C bonds)
applied only to the arylamides, we desired confirmation that
the alkylamide 4 also exists as a dimer in solution. This was
accomplished by performing a reaction analogous to the one
used to synthesize 4, but using a 1:1 mixture of LiNHtBu and
LiNHtAmyl (tAmyl ) CMe2Et). After workup, the product had
a 1H NMR spectrum consistent with a mixture of three different
nickel amides. The resonances for two of these corresponded
to the independently synthesized compounds [Cp*Ni(µ-NHt-
Bu)]2 (4) and [Cp*Ni(µ-NHtAmyl)]2: the third had resonances
in similar positions, consistent with the mixed dimer Cp*Ni-
(µ-NHtBu)(µ-NHtAmyl)NiCp*. These three complexes were
produced in the statistically expected ratio of 1:1:2. Incorpora-
tion of both amide ligands into one molecule indicates that 4 is
a dimer in solution. Thus, 1-4 are all dimers, even in the
solution phase.
Figure 4. ORTEP diagram of trans-3′, using 50% probability
ellipsoids. Carbon-bound hydrogen atoms have been omitted for clarity.
X-ray Crystal Structures of cis-1′, trans-3′, and 4′. To date,
it has not been possible to grow reasonably-sized single crystals
of 1-4. However, by using the CpEt ligand, it was possible to
grow crystals of 1′, 3′, and 4′ that were suitable for X-ray
structural characterization. In each case, data collection (at low
temperature) and refinement proceeded without problems; the
refinements converged with residuals of 4.2%, 5.4%, and 2.9%,
respectively (Rw ) 5.2, 6.3, 4.3%). Data collection for 3′ was
hampered somewhat by the thinness of the crystals (0.08 mm),
but we were able to obtain a satisfactory structure determination
using a CCD area detector system through judicious choice of
crystal, a higher power setting, and a large semiempirical
absorption correction (Tmin/Tmax ) 0.45).
The structures of 1′, 3′, and 4′ are shown as ORTEP15
diagrams in Figures 3-5. Selected intramolecular distances and
angles are in Tables 1-3. In the cases of both 1′ and 4′, the
single crystals were exclusively the cis isomers, and they
crystallized in the monoclinic space group C2/c. In each of
these structures, the asymmetric unit consists of only one
CpEtNiNHR unit; the other half of the molecule is related by a
crystallographic 2-fold rotation axis normal to the Ni2N2 plane.
In the case of 3′, the single crystal contained the trans isomer,
and the dimer was found to lie on a crystallographic inversion
center in space group P1h. Again, each half of the molecule is
crystallographically equivalent, and in this case the inversion
Figure 5. ORTEP diagram of 4′, using 50% probability ellipsoids.
Carbon-bound hydrogen atoms have been omitted for clarity.
center enforces planarity of the Ni2N2 ring. No unusually close
intermolecular contacts were observed in any of the structures.
The Ni-N bonds in cis-1′ (1.951(3), 1.950(3) Å), trans-3′
(1.957(5), 1.962(6) Å), and 4′ (1.944(2), 1.942(2) Å) are
symmetric and longer than those in the only other structurally
characterized complex with a simple Ni2(amide)2 core, the
homoleptic dimer [(Ph2N)Ni(µ-NPh2)]2, where Ni-N(bridge)
distances ranged from 1.90 to 1.92 Å.6k This contrast is
indicative of a weaker Ni-N interaction in our system,
presumably because of the electron-donating power of the
cyclopentadienyl ligand relative to the terminal amide ligand.
The most striking difference between the structures of cis-
1′, trans-3′, and 4′ is that the Ni2N2 core is planar in cis-1′ and
trans-3′ (cis-1′, mean distance from the least-squares plane )
0.014 Å; trans-3′, crystallographically required to be planar),
but it is significantly puckered (mean distance from the least-

Study of Cyclopentadienylnickel Amido Complexes
J. Am. Chem. Soc., Vol. 118, No. 5, 1996 1095
Table 1. Selected Intramolecular Distances (Å) and Angles (deg)
in cis-1′ a
Ni-N
1.951(3)
1.950(3)
2.085(4)
2.165(4)
2.170(4)
2.088(4)
2.161(4)
1.76
N-Ni-N′
Ni-N-Ni′
Cp-Ni-N
Cp-Ni-N′
87.7(1)
92.3(1)
135.6
Ni-N′
Ni-C(1)
Ni-C(2)
Ni-C(3)
Ni-C(4)
Ni-C(5)
Ni-Cp
136.6
Ni-N-C(12)
Ni′-N-C(12)
120.4(3)
119.9(3)
Figure 6. The “ene-allyl” distortion in cis-1′. Distances are in
angstroms (Å).
Ni-Ni′
2.813(1)
C(1)-C(2)-C(3)
C(2)-C(3)-C(4)
C(3)-C(4)-C(5)
C(4)-C(5)-C(1)
C(5)-C(1)-C(2)
107.6(4)
107.1(4)
109.4(4)
106.2(4)
109.0(4)
are observed because only the cis isomers can accommodate
the large, nonplanar ligands.
N-C(12)
1.410(5)
C(1)-C(2)
C(2)-C(3)
C(3)-C(4)
C(4)-C(5)
C(1)-C(5)
1.461(7)
1.393(6)
1.450(7)
1.422(6)
1.408(6)
In the crystal structure of cis-1′, there are clear signs of an
“ene-allyl” distortion of the cyclopentadienyl rings; this is
depicted in Figure 6. The C(4)-C(5) and C(5)-C(1) bonds
are nearly equivalent and short, as is the C(2)-C(3) bond; these
correspond to the “allyl” and “ene” portions of the ring,
respectively. This geometry is similar to those observed in the
crystal structures of [CpNi]2(biallyl)16 and Cp*Ni(acac),12 where
this type of distortion was explained by a significant energy
difference between the “in-plane” and “out-of-plane” CpM
fragment orbitals in the CpML2 system. However, an “ene-
allyl” distortion is not observed in the crystal structures of trans-
3′ and 4′. Although there are significant distortions from ideal
5-fold symmetry in the cyclopentadienyl rings of both trans-3′
and 4′, they are distorted in a “diene-yl” fashion, with two short
and three long C-C bonds. It is possible that the steric strain
from the large amido ligands keeps 3′ and 4′ from “slipping”
into a more electronically favorable “ene-allyl” conformation.
The hydrogen atoms in the structures of trans-3′ and 4′ were
located, and this allowed us to examine the geometry of the
nitrogen-bound hydrogen atoms in 4′ whose resonances lie
a
“Cp” refers to the centroid of the five cyclopentadienyl carbon
atoms.
Table 2. Selected Intramolecular Distances (Å) and Angles (deg)
in trans-3′ a
Ni-N
Ni-N′
1.957(5)
1.962(6)
2.146(6)
2.115(6)
2.197(6)
2.208(7)
2.093(7)
1.78
N-Ni-N′
Ni-N-Ni′
Cp-Ni-N
Cp-Ni-N′
85.4(2)
94.6(2)
137.1
Ni-C(1)
Ni-C(2)
Ni-C(3)
Ni-C(4)
Ni-C(5)
Ni-Cp
137.4
Ni-N-C(12)
Ni′-N-C(12)
119.9(4)
121.0(4)
Ni-Ni′
2.880(1)
C(1)-C(2)-C(3)
C(2)-C(3)-C(4)
C(3)-C(4)-C(5)
C(1)-C(5)-C(4)
C(2)-C(1)-C(5)
108.4(5)
107.9(4)
107.6(4)
108.5(4)
107.0(4)
N-C(12)
N-H(N)
1.426(7)
0.99(5)
1
unusually far upfield in its H NMR spectrum (Vide infra). In
C(1)-C(2)
C(2)-C(3)
C(3)-C(4)
C(4)-C(5)
C(1)-C(5)
1.395(7)
1.478(7)
1.384(8)
1.453(7)
1.437(7)
both trans-3′ and 4′, the NH hydrogen atoms lie in the plane
defined by the nitrogen atoms and the nitrogen-bound carbons,
and there is no sign that the hydrogen atoms “lean” toward either
nickel atom. There is a relatively close contact between the
NH hydrogen and the nickel atom (2.3 Å) in 4′, but this appears
to be induced by steric forces, not by any direct nickel-
hydrogen interaction.
a “Cp” refers to the centroid of the five cyclopentadienyl carbon
atoms.
Table 3. Selected Intramolecular Distances (Å) and Angles (deg)
in 4′ a
The Ni-Ni distances in cis-1′, trans-3′, and 4′ (2.813(1),
2.880(1), and 2.7029(5) Å) do not suggest any Ni-Ni bonding,
as opposed to Power’s amide dimer, where a Ni-Ni separation
of 2.327(2) Å indicated the presence of a metal-metal bond.6k
Rather, these distances reflect the size of the organic ligand that
is trapped between the CpEt ligands.
Spectroscopy. The most striking spectroscopic characteristic
of complexes 1-4 is the chemical shifts of the NH protons,
which vary from δ 2.59 ppm in the cis isomer of 3 to δ -5.72
ppm in 4 (Table 4). An agostic interaction was immediately
suspected as a possible reason for the upfield chemical shifts,
but this explanation was discounted by the crystal structure
Ni-N
Ni-N′
1.944(2)
1.942(2)
2.155(2)
2.173(2)
2.198(2)
2.104(2)
2.192(3)
1.79
N-Ni-N′
Ni-N-Ni′
Cp-Ni-N
Cp-Ni-N′
85.50(9)
88.16(8)
135.9
Ni-C(1)
Ni-C(2)
Ni-C(3)
Ni-C(4)
Ni-C(5)
Ni-Cp
135.2
Ni-N-C(12)
Ni′-N-C(12)
124.7(1)
126.7(1)
Ni- Ni′
2.7029(5)
C(1)-C(2)-C(3)
C(2)-C(3)-C(4)
C(3)-C(4)-C(5)
C(4)-C(5)-C(1)
C(5)-C(1)-C(2)
108.6(2)
106.5(2)
109.6(2)
106.6(2)
108.3(2)
N-C(12)
N-H(N)
1.500(3)
0.80(3)
determinations of trans-3′ and 4′ and by the “normal” values
C(1)-C(2)
C(2)-C(3)
C(3)-C(4)
C(4)-C(5)
C(1)-C(5)
1.457(3)
1.392(3)
1.438(3)
1.426(4)
1.401(3)
1
15
1
of J N- H in 1, 3, and 4 in solution (Table 4), since an agostic
Ni‚‚‚H-N interaction would be expected to weaken the N-H
bond and reduce this coupling constant. Four other potential
explanations are as follows: (a) an inductive effect from the
basic nitrogen atom, which has been used to explain similar
upfield chemical shifts for bridging ligands;17 (b) ring-current
anisotropy from the aromatic cyclopentadienyl rings; (c) an
equilibrium between a diamagnetic ground state and a thermally
a “Cp” refers to the centroid of the five cyclopentadienyl carbon
atoms.
squares plane ) 0.229 Å) in 4′. The puckered conformation
of 4′ is enforced by the sterically bulky tert-butyl groups because
they cannot fit as easily between parallel CpEt ligands as the
flat aryl groups in 1′ and 3′ can. To relieve strain, they “push”
the CpEt ligands toward each other and gain more room on one
side of the Ni2N2 ring. Thus, only the cis isomers of 4 and 4′
(16) Smith, A. E. Inorg. Chem. 1972, 11, 165.
(17) (a) Lo´pez, G.; Garcia, G.; Ruiz, J.; Sa´nchez, G.; Garc´ıa, J.; Vicente,
C. J. Chem. Soc., Chem. Commun. 1989, 1045. (b) Lo´pez, G.; Ruiz, J.;
Garc´ıa, G.; Vicente, C.; Marti, J. M.; Hermoso, J. A.; Vegas, A.; Mart´ınez-
Ripoll, M. J. Chem. Soc., Dalton Trans. 1992, 53.

1096 J. Am. Chem. Soc., Vol. 118, No. 5, 1996
Holland et al.
Table 4. 1H NMR Chemical Shifts and 15N-1H Coupling
moment measurements19 showed that 1′ was diamagnetic within
experimental error (µeff < 0.2), and the chemical shift of the
upfield NH resonance in 4 was temperature independent between
-80 °C and room temperature. For these reasons, it is unlikely
that a spin equilibrium of the type observed in Cp*Ni(acac) is
responsible for the unusual NMR spectra.
Constantsa
1
15
1
δ
NH, ppm
J
N- H, Hz
cis-[Cp*Ni(µ-NHtol)]2 (cis-1)
trans-[Cp*Ni(µ-NHtol)]2 (trans-1)
cis-[Cp*Ni(µ-NHxyl)]2 (cis-3)
trans-[Cp*Ni(µ-NHxyl)]2 (trans-3)
cis-[Cp*Ni(µ-NHtBu)]2 (4)
-1.79
-2.08
2.59
-0.11
-5.72
-1.05
65
66
64
62
68
Thus, it seems most likely that ring-current anisotropy is the
source of the unusual chemical shifts observed in 1-4. The
unusually high-field NH resonance in 4 can be rationalized
through a simple steric argument. Repulsion between the tert-
butyl groups causes the Ni2N2 ring to pucker, as shown in the
solid-state structure of 4′ (Figure 5). This causes the NHtBu
groups to rotate about the Ni-Ni axis, bringing the N-bound
protons closer to the center of the aromatic cyclopentadienyl
ring. Since the effect of the aromatic rings is to shield protons
directly above the ring, these protons resonate far upfield. The
same effect is undoubtedly present in 1, 2, and 3, but it is
apparently complicated by the ring-current anisotropy of the
aromatic substituents on the nitrogen atom, and the trends are
less simply explained. Ring-current anisotropy can also explain
the very different chemical shifts of the xylyl methyl protons
in 3, because the crystal structure of trans-3′ clearly shows that
these methyl groups lie at different distances from the cylinder
described by the CpEt rings (Figure 7c). Thus, a combination
of inductive effects (which affects the NH protons as in other
bridging hydroxides and amides)17,20 and ring-current anisotropy
(which affects the xylyl methyl protons differently) can account
for the unusual proton NMR shifts in Table 4.
Cp*Ni(PMe3)(NHtol) (5)
a Coupling constants were measured using natural-abundance samples,
using a pulse sequence (details in the Experimental Section) with a
double-quantum filter to “remove” resonances from protons attached
to 14N. Thus, one observes only resonances from protons attached to
15N (I ) 1/2), and the result is a doublet, from which the coupling
constant can be easily measured.
Isomerization. After purification by crystallization, 1 and
2 were typically obtained as a mixture of trans and cis isomers
in a ratio of greater than 10:1, and 3 was obtained with an
isomeric ratio of approximately 1:1. (Repeated crystallization
gives 3 as exclusively the trans isomer.) However, heating
solutions of 1 or 2 converted them to equilibrium mixtures with
trans/cis ratios of about 2:1. When 3 was heated, it also
1
isomerized, with the product being exclusively (>98% by H
NMR spectroscopy) the trans isomer.
In initial attempts to determine the mechanism of these
isomerizations by H NMR kinetics, we found that heating
Figure 7. (a, b) Perspective representations of the cis and trans isomers
of 3; (c) view down the Ni-Ni axis in the crystal structure of trans-3′
(cyclopentadienyl substituents have been removed for clarity).
1
solutions of 1-3 caused significant decomposition. In order
to inhibit decomposition, a small amount of Cp2ZrMe2 was
added to the NMR tube reactions; this has been used success-
fully in our group as an oxygen and water scavenger.21 In the
presence of the zirconium complex, less than 2% decomposition
of the nickel complexes (by integration Vs an internal standard)
was observed over a typical kinetic run at 77 °C for 10 h. In
these experiments, a small amount of Cp2Zr(Me)(µ-O)Zr(Me)-
Cp2 (1H NMR: δ 5.9 ppm), the product of the reaction between
Cp2ZrMe2 and water,22 was observed. This shows that the
zirconium complex destroyed traces of water on the NMR tube
surface and/or in the solvent that had not been removed by
conventional drying techniques.
populated electronically excited triplet state, as in Cp*Ni(acac);12
and (d) an unusually large contact shift from the nickel atom.18
An unusual observation in the proton NMR spectrum of 3 is
that the xylyl methyl protons resonate at δ 1.75 and 5.39 ppm
in the trans isomer and at δ 2.32 and 4.05 ppm in the cis isomer
(Figure 7a,b). In attempting to explain both the unusual NH
shifts and the unusual xylyl methyl shifts using a single reason,
the first and last explanations can be eliminated. Since inductive
effects normally travel through bonds, the basicity of the
nitrogen atom (explanation (a) in the previous paragraph) would
not be able to affect the distant methyl groups. (In addition,
the very basic LiNHR compounds do not have NH resonances
this far upfield: for example, the NH proton in LiNHtBu
resonates at δ -1.50 ppm in THF-d8.) Likewise, local
“paramagnetic” contributions to the chemical shift due to the
metal (explanation (d)) would not be able to travel through bonds
to the xylyl methyl groups.18
Thus, mixtures of 1, 2, or 3, dimethylzirconocene and
ferrocene (as an internal standard) were heated to 76.9 °C in
THF-d8 and monitored by one-pulse proton NMR spectroscopy,
(19) (a) Bax, A.; Griffey, R. H.; Hawkins, B. L. J. Magn. Reson. 1988,
55, 301. (b) Evans, D. F. J. Chem. Soc. 1959, 2003. (c) Baker, M. V.;
Field, L. D.; Hambley, T. W. Inorg. Chem. 1988, 27, 2872. (d) Jolly, W.
L. The Synthesis and Characterization of Inorganic Compounds; Wave-
land: Prospect Heights, IL, 1970.
(20) (a) Park, S.; Rheingold, A. L.; Roundhill, D. M. Organometallics
1991, 10, 615. (b) Blake, R. E.; Heyn, R. H.; Tilley, T. D. Polyhedron
1992, 11, 709.
(21) (a) Proulx, G.; Bergman, R. G. J. Am. Chem. Soc. 1995, 117, 6382.
(b) Hanna, T. A.; Baranger, A. M.; Walsh, P. J.; Bergman, R. G. J. Am.
Chem. Soc. 1995, 117, 3292.
(22) Marsella, J. A.; Folting, K.; Huffman, J. C.; Caulton, K. G. J. Am.
Chem. Soc. 1981, 103, 5596.
There are several pieces of evidence that argue against a spin
equilibrium (explanation (c)) in 1-4. The carbon NMR
resonances did not occur in unusual positions, and all resonances
in the proton and carbon NMR spectra were sharp (except for
broadness attributable to the 14N quadrupole). Evans magnetic
(18) For a discussion of paramagnetic contributions to the chemical shift,
see: Drago, R. S. Physical Methods for Chemists; Saunders College
Publishing: Orlando, 1992; pp 232-243.

Study of Cyclopentadienylnickel Amido Complexes
J. Am. Chem. Soc., Vol. 118, No. 5, 1996 1097
strong base (nBuLi).26 Two series of palladium amide dimers
with M2(NHR)2 cores have been isolated; in one case, no
exchange between isomers occurred at high temperature,6e and
in the other, isomerization took place at room temperature, but
the process involved several isomers that could not be un-
equivocally identified.6g
The data allow us to distinguish the mechanisms shown in
Scheme 2. Pathway (a), which is dissociative in nature, is ruled
out because only a small amount (<5%) of crossover (Vide infra)
occurred between 1 and 2 when they were heated together under
the conditions used for the kinetic runs. Associative mecha-
nisms like (d) are not satisfactory because plots of log |[A] -
[A]eq| vs time were linear, and the observed first-order rate
constant did not change upon doubling the concentration of
nickel complex. Moreover, when the isomerization of 1 was
performed in toluene-d8 instead of THF-d8, kobs was 3.7 ( 0.4
× 10-4 s-1. Since the coordination ability of the solvent did
not affect the rate constant, it is unlikely that the rate-determining
step involves any significant interaction with solvent as in (c).
Thus, the mechanism most consistent with the data is (b), in
which the isomerization proceeds by one dative bond breaking
followed by the amido ligand rotating; rapid inversion at the
three-coordinate nitrogen then allows the amido ligand to bind
with the opposite orientation. We have not been able to identify
a trapping agent that is capable of diverting the presumed ring-
opened intermediate into a stable product.
The values of k1 for the isomerizations of 1-3 (Table 5)
parallel the coalescence temperatures for the aryl protons in their
1H NMR spectra (Vide supra): the tolyl and phenyl groups in
1 and 2 can rotate with a relatively low barrier, while the 2,6-
xylyl group in 3 does not rotate. This suggests that the rate-
determining step in the isomerization process is the rotation of
the amido ligand, because the sterically hindered 2,6-xylylamido
complex 3 isomerizes an order of magnitude more slowly than
1 and 2. The observation of a slower rate with larger ligands
further refutes dissociative mechanisms (including mechanism
(a)) for the isomerization.
Figure 8. Plot of [cis-1] (+) and [trans-1] (b) Vs time at 76.9 °C.
Table 5. Rate Constants Calculated from Kinetic Runs in THF-d8
at 76.9 °C
kobs
/
k1/
k-1/
10-4 s-1
Keq
10-4 s-1 10-4 s-1
[Cp*Ni(µ-NHtol)]2 (1) 3.5 ( 0.3
[Cp*Ni(µ-NHPh)]2 (2) 5.5 ( 0.5
[Cp*Ni(µ-NHxyl)]2 (3) 0.34 ( 0.04
2.3 ( 0.3
2.4 ( 0.3
>50
2.4
3.9
0.34
1.1
1.6
with data points taken every 6 min. Some of the peaks for cis
and trans isomers overlapped, but when peaks which did not
overlap were monitored, good first-order time dependence
curves were obtained. There were no signs of reaction of Cp2-
ZrMe2 with the nickel complexes or with the ferrocene.
Repeating the isomerization of trans-2 without Cp2ZrMe2
present gave an identical isomerization rate, but as stated earlier,
this was accompanied by decomposition of the nickel com-
plexes.
A typical plot of [cis-1] and [trans-1] Vs time is shown in
Figure 8. The isomerizations of 1-3 were found to be first
order to approximately 5 half-lives.23 The first-order rate
constants (kobs) for these processes are displayed in Table 5. In
1
the cases of 1 and 2, values for Keq estimated from H NMR
Minimal crossover occurred in thermolyses (77 °C for 10 h)
of 1-3 in the presence of dimethylzirconocene, in C6D6 or THF-
d8 solutions, as mentioned earlier. However, a mixture of 1
and 2 in C6D6 solution was converted to the mixed amido
complex Cp*Ni(µ-NH(p-tol))(µ-NHPh)NiCp* over several weeks
at 45 °C (eq 2).
data allow us to derive k1 and k-1 from kobs as shown in eq 1. In
the case of 3, since the equilibrium lies completely on the side
of the trans isomer, one can make the approximation k1 ≈ kobs
.
cis yk1z trans Keq ) k1/k-1; kobs ) k1 + k-1
(1)
k-1
[Cp*Ni(µ-NH(p-tol))]2 (1) + [Cp*Ni(µ-NHPh)]2 (2) h
Cp*Ni(µ-NH(p-tol))(µ-NHPh)NiCp* (2)
Few studies have been done on the isomerization of dimers
like 1-3. Several groups have looked at dimers [R2Al(µ-
NHR)]2 and their isomerization and conversion to trimers and
imides.24 Mechanistic studies of these reactions indicated that
this isomerization proceeds through a ring-opened Lewis base
adduct.24e One group has proposed a π-cycloreversion/cycload-
dition mechanism for this process, but their data do not support
that conclusion.25 Mays and co-workers have studied the
isomerization of [(CO)4Mn(µ-PHPh)]2; their study indicates that
this proceeds through a mechanism where one Mn-P bond is
broken, but the isomerization can also take place through a
deprotonation/reprotonation mechanism in the presence of a
This process was slowed, but not totally inhibited, by the
addition of Cp2ZrMe2, implying that it occurs concurrently Via
water-catalyzed and -uncatalyzed mechanisms. The proton
NMR resonances for the mixed amido complex lie very close
to the resonances for 1 and 2, and the characteristic singlets for
the Cp* protons in both isomers of all three compounds can be
distinguished in C6D6 solution. The appearance of a new
envelope at m/z 584 in the mass spectrum of the product (Figure
9) is consistent with our formulation of this product.
(25) Beachley observed an equilibrium ratio of ca. 5:1 for cis- and trans-
[Me2AlN(Me)(Ph)]2, and rationalized the predominance of the cis isomer
on the basis of a transition-state argument involving cycloadditions.
However, we were somewhat puzzled by this discussion, because near the
end of ref 25b the authors make clear that the observed cis/trans ratios are
thermodynamically controlled. See: (a) Beachley, O. T.; Tessier-Youngs,
C. Inorg. Chem. 1979, 18, 3188. (b) Beachley, O. T.; Bueno, C.; Churchill,
M. R.; Hallock, R. B.; Simmons, R. G. Inorg. Chem. 1981, 20, 2423.
(26) Brown, M. P.; Buckett, J.; Harding, M. M.; Lynden-Bell, R. M.;
Mays, M. J.; Woulfe, K. W. J. Chem. Soc., Dalton Trans. 1991, 3097.
They saw faster isomerization rates in polar solvents; this implicates a
mechanism with solvent coordination, as in (c).
(23) The isomerization of 3 was too slow at 76.9 °C to follow for more
than 1 half-life, so this isomerization was repeated at 97.6 °C; we observed
first-order behavior to 5 half-lives, with a rate constant of kobs ) 5.5 ( 0.4
× 10-4 s-1
.
(24) (a) Bowen, R. E.; Gosling, K. J. Chem. Soc., Dalton Trans. 1974,
964. (b) Wakatsuki, K.; Tanaka, T. Bull. Chem. Soc. Jpn. 1975, 48, 1475.
(c) Al-Wassil, A.-A. I.; Hitchcock, P. B.; Sarisaban, S.; Smith, J. D.; Wilson,
C. L. J. Chem. Soc., Dalton Trans. 1985, 1929. (d) Sauls, F. C.; Czekaj, C.
L.; Interrante, L. V. Inorg. Chem. 1990, 29, 4688. (e) Choquette, D. M.;
Trimm, M. J.; Hobbs, J. L.; Rahim, M. M.; Ahmed, K. J.; Planalp, R. P.
Organometallics 1992, 11, 529.

1098 J. Am. Chem. Soc., Vol. 118, No. 5, 1996
Holland et al.
Scheme 2
Scheme 3
Figure 9. A portion of the electron-impact mass spectrum of the
product obtained by heating a mixture of 1 (M+, m/z 598) and 2 (M+,
m/z 570) at 45 °C for 2 weeks. The envelopes on the left are assigned
as the fragments [Cp*Ni]2(µ-NHPh)+ and [Cp*Ni]2(µ-NHtol)+, respec-
tively.
although amine was generally observed as a byproduct (by 1H
NMR spectroscopy). When carbon monoxide (1 atm) was
added to 4, the solution turned red over the course of several
days. As this is the same time scale as that associated with
decomposition of 4, the reaction was repeated at higher pressure.
Under 15 atm of CO, the solution rapidly turned red, but the
These mixed amides can also be formed by reaction of
Cp*Ni(acac) with a mixture of lithium amides using procedures
analogous to those in Scheme 1; using this methodology, we
have also prepared Cp*Ni(µ-NHtBu)(µ-NHtAmyl)NiCp* (tAmyl
) CMe2Et) (Vide supra) and Cp*Ni(µ-NH(p-tol))(µ-NHxyl)-
NiCp*. However, the materials isolated from these reactions
were inseparable mixtures of the symmetrically substituted
dimers and the mixed products.27 The tert-butylamido/tert-
amylamido and p-tolylamido/2,6-xylylamido mixed dimers are
notable in that they cannot be formed by “crossover” reactions
analogous to eq 2; heating a mixture of 1 and 3, for example,
gives only decomposition. The higher reactivity of 1 (with
respect to the bulkier complexes 3 and 4) in crossover reactions
parallels its reactivity toward small molecules, which will be
presented in the following section.
Reactions of 1 with Small Molecules. We hoped to replace
the dative Ni-N bond in these dimers with a strongly
coordinating neutral Lewis base such as carbon monoxide,
isonitrile, or phosphine, to give monomeric complexes Cp*Ni-
(L)(NHR). However, this strategy was not successful for
complexes 3 or 4; when treated with 2 equiv of PMe3, each
slowly gave Ni(PMe3)4 as the only identifiable nickel-containing
product. Similarly, reaction of 3 or 4 with tert-butyl isonitrile
slowly gave Ni(CNtBu)4. We were not able to determine the
fate of the amido ligand or the Cp* ligand in these reactions,
28
starting material and [Cp*Ni(µ-CO)]2 were again the only
identified nickel-containing products. We also identified the
urea, (tBuNH)2CdO, by NMR spectroscopy and by analogy to
the product, ((p-tol)NH)2CdO, identified in an analogous
reaction described below. Other Lewis bases such as pyridine
and amines did not react with 3 or 4 prior to decomposition.
Thus, it has not been possible to cleave these dimers while
maintaining the nickel-nitrogen bond; this is consistent with
the inaccessibility of the Ni2N2 core that is evident in the crystal
structures of trans-3′ and 4′.
Dimer 1, on the other hand, was easily cleaved by CO,
tBuNC, or PMe3 to give monomeric products (Scheme 3).
When a solution of 1 in toluene was stirred under an atmosphere
of carbon monoxide for 12 h, a red solution resulted which
contained both the known red compound, [Cp*Ni(µ-CO)]2,28
and the new nickel carboxamide complex Cp*Ni(CO)(C(O)-
NH(p-tol)) (6). Dilute reaction conditions and lowered tem-
perature maximized the amount of 6 in this mixture. At ca.
0.5 mM concentration of 1 and 0 °C, we obtained [Cp*Ni(µ-
CO)]2 and 6 in a 3:2 ratio and isolated 6 in 39% yield as an
orange powder after removal of volatile materials and washing
with hexamethyldisiloxane to remove [Cp*Ni(µ-CO)]2.
(27) Cp*Ni(µ-NHtBu)(µ-NHtAmyl)NiCp* was obtained in a roughly
statistical ratio of products, as mentioned above. Cp*Ni(µ-NH(p-tol))(µ-
NHxyl)NiCp* could be separated from 1 by crystallization, but we were
unable to separate the two isomers of the mixed product from the two
isomers of 3.
(28) Mise, T.; Yamazaki, H. J. Organomet. Chem. 1979, 164, 391.

Study of Cyclopentadienylnickel Amido Complexes
J. Am. Chem. Soc., Vol. 118, No. 5, 1996 1099
Samples of 6 were stable in solution at room temperature
for short periods of time, but decomposed within hours in the
solid state or in solution at room temperature. This thermal
instability precluded mass spectrometric characterization, but
contain only (>95%) the desired monomer Cp*Ni(PMe3)(NH-
(p-tol)) (5) after 10 h at room temperature. Unfortunately, it
proved impossible to isolate this complex, because it decom-
posed to mixtures of 1 and Ni(PMe3)4 upon standing in solution
or exposure to vacuum. Addition of excess PMe3 resulted in
formation of Ni(PMe3)4. In the proton NMR spectrum of 5, an
1
elemental analysis and H NMR spectroscopy were consistent
with our formulation of this product. The infrared spectrum of
6 displays absorptions at 2015 and 1647 cm-1, indicating the
presence of CtO and CdO bonds, respectively. When the
reaction was repeated using 13CO, two resonances were observed
in the 13C{1H} NMR spectrum, a doublet (2JCC ) 9 Hz) at δ
192.4 ppm and a broad resonance at δ 185.2 ppm. These are
consistent with the nickel-bound carbons of a terminal carbonyl
ligand and of a carboxamide ligand, coupled to each other (the
resonance of the latter is broadened by the 14N quadrupole).
When a C6D6 solution of 6 was allowed to stand at ambient
temperature for 2 days, it decomposed to a red solution and a
white powder: the supernatant solution consisted of [Cp*Ni-
upfield resonance (δ -1.05 ppm) was observed; its assignment
1
15
1
as the NH proton was confirmed by observation of J
)
N- H
68 Hz in a 15N-filtered proton NMR spectrum (Table 4).
Coordination of the phosphine ligand was evident from phos-
phorus coupling to the Cp* ligand (J ) 1.6 Hz) and a new
resonance in the 31P{1H} NMR at δ -5.9 ppm.
The reaction of 1 with PMe3 occurred much more quickly
than the cis/trans isomerization described earlier. It is possible
that the reaction with PMe3 proceeds through the ring-opened
intermediate that eventually leads to cis/trans isomerization.
However, the data do not rule out the intermediacy of an
associative intermediate such as Cp*Ni(PMe3)(µ-NH(p-tol))2-
NiCp*. Twenty-electron Cp*Ni complexes have been isolated
and crystallographically characterized as such,12 and they are
not unreasonable intermediates in this process if steric hindrance
is not excessive. Ring slippage is also a possibility in transition-
metal cyclopentadienyl complexes. The instability of 5 in
solution and its intolerance to added phosphine have prevented
us from performing mechanistic studies to resolve this question.
We have also found that arylamines exchange with the amide
ligands in 1 and 2. Thus, reaction of 1 with a large excess of
aniline at 45 °C slowly gave mixtures of 1, 2, and Cp*Ni(µ-
NH(p-tol))(µ-NHPh)NiCp*, until eventually the solution con-
tained almost exclusively 2. Other Brønsted acids such as water
and p-cresol reacted with 1, but did so slowly and/or not cleanly.
We have not been able to characterize the products of these
reactions, but they were clearly paramagnetic, giving 1H NMR
spectra with broad resonances at unusual chemical shifts (for
example, the product of 1 and H2O gives one broad resonance
at δ 196 ppm in C6D6!). It is likely that these products are
polynuclear clusters.
1
(µ-CO)]2 (greater than 90% purity by H NMR spectroscopy),
and the urea ((p-tol)NH)2CdO was the major component of
the powder.
The reaction of 1 with tert-butyl isonitrile gave the analogous
amidinylnickel complex Cp*Ni(CNtBu)(C(NtBu)NH(p-tol)) (7),
which was isolated as a brown solid in 63% yield. This product
was much more stable than its carbonyl analogue. The proton
NMR spectrum of 7 contains tert-butyl resonances at δ 0.92
and 1.57 ppm, and the IR spectrum displays absorptions for
CtN and CdN at 2120 and 1557 cm-1, respectively. It was
possible to characterize 7 by mass spectrometry and elemental
analysis, and each of these confirmed our description of this
complex as an insertion product analogous to 6. Alkenes and
alkynes did not insert into the nickel-nitrogen bond of 1.
The new ligands in 6 and 7 are interesting in that they are
the hypothetical products from the protonation of a coordinated
isocyanate or carbodiimide, respectively. Such a route has been
employed in synthesizing vanadium and cobalt complexes
containing amidinyl ligands.29 Even more interesting is viewing
the amidinyl ligand in 7 as a deprotonated diaminocarbene
complex. A platinum amidinyl complex has recently been
synthesized using this method.30 The insertion of an isocyanide
into a metal-nitrogen bond as described here is a complemen-
tary entry into this ligand system.
Finally, we have attempted several routes for removing the
hydrogen atom from the nitrogen in the amide ligand to give
an imido complex. None of the reagents we have tried (MeLi,
KOtBu, Ph3CBF4, tBuONO, (p-tBuC6H4)3C• 32 ) reacted cleanly
with 1 or with 4.
Insertion into the M-N bond in amido complexes is
uncommon but not unprecedented for late-transition-metal
amides.1b In some recent studies, it is difficult to determine
whether CO inserts into an M-N bond or an M-C bond
because the insertion is immediately followed by reductive
elimination.8a,31 It is often assumed that the carbonyl group
inserts into the M-C bond, despite examples of complexes in
which M-N insertion is preferred over M-C insertion.1a In
the reactions described here, though, it is clear that CO and
tBuNC insertion into the Ni-N bond of 1 is facile. The most
reasonable mechanism involves cleavage of the dimer to give
Conclusions
Although the bulky Cp* ligand might be expected to give
monomeric amide products from the reaction conditions in
Scheme 1, only dimeric [Cp*Ni(µ-NHR)]2 complexes have been
isolated. This indicates that, despite the entropic and steric costs
of dimerization, the nitrogen-nickel dative bond provides more
stabilization than the hypothetical nickel-nitrogen π bond of a
Cp*NiNHR monomer. These Cp*Ni complexes seemingly go
to great lengths (including the toleration of severe steric strain
in 4′, as evidenced by its crystal structure) to dimerize in order
to avoid coordinative unsaturation at the metal center. However,
the strong steric forces produce effects such as unusual chemical
shifts in the proton NMR and hindered rotation about the C-N
single bonds. We have also presented a striking example of
the effects of ring-current anisotropy, which is the best way to
explain the abnormally high field at which the N-H protons in
these complexes (especially 4) resonate.
t
Cp*Ni(L)(NH(p-tol)) (L ) CO, BuNC) as an intermediate,
followed by migratory insertion and coordination of another
molecule of CO or tBuNC. While it is interesting that cleavage
of the M2N2 core is possible, we desired a method of cleavage
that would retain the nickel-nitrogen bond.
In the presence of 2 equiv of PMe3, 1 was converted to a
blue solution that was judged by proton NMR spectroscopy to
(29) (a) Pasquali, M.; Gambarotta, S.; Floriani, C.; Villa, A. C.; Guastini,
C. Inorg. Chem. 1981, 20, 165. (b) Horlin, G.; Mahr, N.; Werner, H.
Organometallics 1993, 12, 1993.
(30) Crociani, B.; DiBianca, F.; Fontana, A.; Forsellini, E.; Bombieri,
G. J. Chem. Soc., Dalton Trans. 1994, 407.
We have also completed a mechanistic study on the cis/trans
isomerization of the Ni2N2 core of 1-3. The data indicate that
this process is initiated by cleavage of a Ni-N dative bond,
(31) Yamamoto, T.; Sano, K.; Osakada, K.; Komiya, S.; Yamamoto, A.;
Kushi, Y.; Tada, T. Organometallics 1990, 9, 2396.
(32) Eisenberg, D. C.; Lawrie, C. J. C.; Moody, A. E.; Norton, J. R. J.
Am. Chem. Soc. 1991, 113, 4888.

1100 J. Am. Chem. Soc., Vol. 118, No. 5, 1996
Holland et al.
as C6D6 solutions. In the 1H NMR spectra of 1, 2, 1′, and 2′, the
broadness of some aryl peaks at room temperature (Vide supra)
followed by rotation of the amido ligand and recoordination.
This study showed that the aforementioned steric effects
constrain the rotation of the NHR unit, making this the slow
step for the process. The mechanism is similar to that observed
for aluminum amido dimers;24e however, the bulkiness of this
nickel system prevents catalysis by Lewis bases.
Finally, we have shown that insertion into the metal-nitrogen
bond in 1 by carbon monoxide and tert-butyl isonitrile is a facile
process, and that phosphines can also effect the cleavage of the
dimer. These reactions are inefficient with large amido ligands
or with large phosphines, which is almost certainly a result of
the steric constraints on these bulky dimeric nickel complexes.
However, the fact that 1 will react with Lewis bases stands in
contrast to similar complexes of iridium and platinum, in which
the M2N2 core is unreactive even to fairly stringent condi-
tions.6e,7b,33 This indicates that the first-row transition metal
indeed has higher reactivity, and we expect that this reactivity
will eventually be utilized for catalytic applications.
1
prevented accurate integration of these peaks. 15N-filtered H NMR
spectra were obtained using a pulse sequence similar to that described
by Griffey,19a modified to give only the one-dimensional 1H NMR
spectrum. Effective magnetic moment (µeff) measurements were made
using Evans’ method19b using C6D6 solutions, with the standard
formula19c and values for diamagnetic corrections.19d IR spectra were
recorded on a Mattson Galaxy Series FTIR 3000 spectrometer, as Nujol
mulls on NaCl plates, as pressed KBr pellets, or as solutions between
NaCl plates in a solution cell, and the frequencies are reported in cm-1
.
Melting points of samples in sealed capillary tubes under N2 were
recorded on a Thomas Hoover melting point apparatus, and are
uncorrected. Elemental analyses were performed at the UCB Mi-
croanalysis Facility. Mass spectral analyses were performed by the
UCB Mass Spectrometry Laboratory on AEI-MS12, Kratos MS-50, or
VG ProSpec instruments. Mass spectral results are reported in m/z
using the most intense peak in each envelope; relative abundances are
listed in parentheses. Unless otherwise indicated, the isotopic distribu-
tion of the parent ion envelope matched the simulated spectrum.
[Cp*Ni(µ-NH(p-tol))]2 (1). A flask was charged with Cp*Ni(acac)
(619 mg, 2.11 mmol), LiNH(p-tol) (250 mg, 2.21 mmol), and a stir
bar and capped with a vacuum adapter. The flask was cooled to -196
°C, THF (60 mL) was condensed into the flask, and the mixture was
warmed to -40 °C with stirring. After 30 min, the blue-green solution
was allowed to warm to room temperature. After an additional hour
of stirring, the volatile materials were removed in Vacuo, leaving a
purple-brown residue. In the glovebox, the residue was extracted into
pentane (130 mL); the resulting solution was filtered through a Celite
pad (2 × 1 cm) and concentrated to 70 mL. Slow cooling to -80 °C
gave a purple powder. Concentration and cooling of the mother liquor
gave a second crop for a total yield of 414 mg (65.3%) of 1. 1H
NMR: δ 7.62 (br m, 2, aryl), 6.86 (m, 2, aryl), 2.16 (s, 3, C6H4Me,
trans), 2.11 (s, 3, C6H4Me, cis), 1.12 (s, 15, C5Me5, cis), 1.11 (s, 15,
C5Me5, trans), -1.79 (s, 1, NH, cis), -2.08 (s, 1, NH, trans). 13C{1H}
NMR (trans isomer,37 THF-d8): δ 157.0 (aryl), 128.7 (aryl), 127.4
(aryl), 124.1 (aryl), 98.1 (C5Me5), 20.7 (C6H4Me), 8.9 (C5Me5). Mp:
199 - 201 °C dec. IR (C6D6): 1608 (w), 1502 (s), 1246 (m). EI-MS:
299 (70), 493 (100), 598 (79, M+). Anal. Calcd for C34H46Ni2N2: C,
68.04; H, 7.73; N, 4.67. Found: C, 67.86; H, 7.73; N, 4.75.
Experimental Section
General. All manipulations were carried out in oven-dried glassware
either using standard Schlenk techniques or in a Vacuum Atmospheres
circulating inert atmosphere glovebox. Pentane, diethyl ether, hexanes,
hexamethyldisiloxane, toluene, toluene-d8, C6H6, C6D6, THF, THF-d8,
and PMe3 were dried over Na/benzophenone. Methylene chloride,
acetonitrile, propionitrile, chloroform-d, aniline, propionitrile, tert-
amylamine and tert-butylamine were dried over calcium hydride.
p-Toluidine, ferrocene, and Cp2ZrMe2 were sublimed prior to use. tert-
Butyl isonitrile was dried over 4 Å molecular sieves. Silica gel (Merck,
60 Å, 230-400 mesh, grade 9385) and Celite were dried overnight at
200 °C under dynamic vacuum. Lithium amides were prepared by
adding a slight deficiency of n-butyllithium or methyllithium (in hexanes
or diethyl ether, respectively) to a hexanes or toluene solution of the
appropriate amine, washing with hexanes or toluene, and removing
volatile materials in Vacuo. MgCp*234 and MgCpEt2 were prepared by
heating Cp*H or CpEtH and Bu2Mg (0.85 M in heptane solution, 0.5
equiv) to reflux overnight, crystallized by cooling the heptane solution
to -80 °C, and recrystallized from hexanes. Successful synthesis of
the new compound MgCpEt2 was verified by the 1H NMR spectrum of
the product in C6D6 (δ 2.38 (q, J ) 7.5 Hz, 2, Et), 1.94 (s, 6, Me),
1.93 (s, 6, Me), 1.01 (t, 3, Et)). Ni(acac)2 and Cp*Ni(acac) were
synthesized according to published procedures.12,35 Cp*Ni(acac) was
generally sublimed before use. Carbon monoxide was obtained from
Matheson and used as received. All other starting materials were of
reagent grade and were purified according to standard procedures, as
necessary.36 Proton NMR spectra were recorded on a Bruker AMX-
300 spectrometer (300 MHz) and 13C{1H} and 31P{1H} NMR spectra
were recorded on a Bruker AMX-400 spectrometer (100.6 MHz for
carbon, 162.1 MHz for phosphorus). Chemical shifts are reported in
parts per million (δ), coupling constants are reported in hertz (Hz) and
integrations are reported in number of protons. Unless otherwise
indicated, 13C{1H} NMR resonances are singlets. Chemical shifts are
referenced internally to the C6D6 solvent peaks at δ 7.15 (1H) and 128.0
(13C) ppm, the THF-d8 solvent peaks at δ 3.58 (1H) and 67.4 (13C)
ppm, or the CDCl3 solvent peak at δ 7.26 ppm. In some cases, DEPT
was used to assign the 13C{1H} NMR resonances as CH3, CH2, CH, or
quaternary. Unless otherwise indicated, NMR spectra were recorded
[Cp*Ni(µ-NHPh)]2 (2). To a three-necked flask charged with
Cp*Ni(acac) (277 mg, 0.945 mmol) and fitted with a septum, a stopper,
and a gas inlet, pyridine (15 mL) was added Via cannula. The orange
solution was cooled to -40 °C, and a solution of LiNHPh (101 mg,
1.02 mmol) in THF (10 mL), also cooled to -40 °C, was slowly added
Via cannula, and the solution became very dark. The solution was
stirred for 1 h at -40 °C, then warmed to room temperature, and volatile
materials were removed in Vacuo. The purple residue was extracted
with pentane (50 mL), filtered through a Celite pad (2 × 1 cm), and
concentrated to 30 mL. Slow cooling to -80 °C gave analytically
pure purple crystals (176 mg, 65.3%) that contained only the trans
1
isomer of 2 (as judged by H NMR spectroscopy). 1H NMR: δ 7.04
(br m, 2, aryl), 6.68 (tt, J ) 7 Hz, 1 Hz, 1, para), 1.06 (s, 15, Cp*),
-2.03 (s, 1, NH). 13C{1H} NMR (THF-d8): δ 160.1 (aryl), 128.1 (aryl),
124.3 (aryl), 118.6 (aryl), 98.3 (C5Me5), 8.8 (C5Me5). Mp: 197-200
°C dec. IR (Nujol): 3279 (m), 3080 (m), 2728 (w), 2057 (w), 1921
(w), 1833 (w), 1601 (s), 1237 (s), 1209 (m), 1157 (s), 1070 (s), 1025
(s), 996 (m), 944 (w), 883 (m), 816 (s), 758 (m), 690 (s). Anal. Calcd
for C32H42Ni2N2: C, 67.18; H, 7.40; N, 4.90. Found: C, 67.04; H,
7.42; N, 4.89.
(33) (a) Kolel-Veetil, M. K.; Ahmed, K. J. Inorg. Chem. 1994, 33, 4945.
(b) Kolel-Veetil, M. K.; Curley, J. F.; Yadav, P. R.; Ahmed, K. J.
Polyhedron 1994, 13, 919. (c) Kolel-Veetil, M. K.; Rheingold, A. L.;
Ahmed, K. J. Organometallics 1993, 12, 3439. (d) Casalnuovo, A. L.;
Calabrese, J. C.; Milstein, D. Inorg. Chem. 1987, 26, 971.
(34) (a) Andersen, R. A.; Blom, R.; Boncella, J. M.; Burns, C. J.; Volden,
H. C. Acta Chem. Scand. 1987, A41, 24. (b) Robbins, J. L.; Edelstein, N.;
Spencer, B.; Smart, J. C. J. Am. Chem. Soc. 1982, 104, 1882.
(35) Charles, R. G.; Pawlikowski, M. A. J. Phys. Chem. 1958, 62, 440.
(36) Perrin, D. D.; Armarego, W. L. F.; Perrin, D. R. Purification of
Laboratory Materials; Pergamon: Oxford, 1980.
[Cp*Ni(µ-NH(2,6-xyl))]2 (3). A flask was charged with Cp*Ni-
(acac) (370 mg, 1.26 mmol), LiNH(2,6-xyl) (177 mg, 1.39 mmol), and
a stir bar and capped with a vacuum adapter. The flask was cooled to
-196 °C, THF (30 mL) was condensed into the flask, and the mixture
was warmed to -40 °C with stirring. After 30 min, the green solution
was allowed to warm to room temperature, and volatile materials were
removed in Vacuo, leaving a purple-brown residue. In the glovebox,
the residue was washed with hexamethyldisiloxane (10 mL). The
remaining solid was extracted into toluene (75 mL); the resulting
solution was filtered through a Celite pad (2 × 1 cm) and the volatile
materials were removed to give a purple solid (284 mg, 71.6%), that
was judged to be pure 3 (as a mixture of isomers) by 1H NMR
(37) This product is not very soluble in any common NMR solvent, and
only the resonances for the most abundant isomer were observed in the
13C{1H} NMR spectrum.

Study of Cyclopentadienylnickel Amido Complexes
J. Am. Chem. Soc., Vol. 118, No. 5, 1996 1101
spectroscopy. Analytically pure material could be obtained by crystal-
lization from a mixture of toluene, pentane, and hexamethyldisiloxane.
1H NMR: δ 7.27 (m, 1, aryl), 6.92 (m, 1, aryl), 6.71 (m, 2, aryl), 6.59
(m, 2, aryl), 5.39 (s, 3, C6H3(Me)(Me), trans), 4.05 (s, 3, C6H3(Me)-
(Me), cis), 2.59 (br s, 1, NH, cis), 2.32 (s, 3, C6H3(Me)(Me), cis), 1.75
(s, 3, C6H3(Me)(Me), trans), 1.20 (s, 15, C5Me5, cis), 0.99 (s, 15, C5-
Me5, trans), -0.11 (br s, 1, NH, trans). 13C{1H} NMR (trans isomer,37
THF-d8): δ 131.0 (aryl), 129.3 (aryl), 129.1 (aryl), 127.6 (aryl), 118.5
(aryl), 99.2 (C5Me5), 22.9 (C6H3(Me)(Me)), 19.2 (C6H3(Me)(Me)), 8.5
(C5Me5). Mp: 247-250 °C dec. IR (C6D6): 1589 (m), 1466 (m),
1417 (m), 1250 (m), 1221 (s), 1098 (s), 1020 (m), 758 (s). EI-MS:
313 (100), 506 (46), 626 (30, M+). Anal. Calcd for C36H50Ni2N2: C,
68.83; H, 8.02; N, 4.46. Found: C, 68.44; H, 8.30; N, 4.36.
[Cp*Ni(µ-NHtBu)]2 (4). A Schlenk tube was charged with Cp*Ni-
(acac) (195 mg, 0.665 mmol), LiNHtBu (109 mg, 1.38 mmol, 2.07
equiv), and a stir bar. The Schlenk tube was brought out of the box
and chilled to -78 °C and THF (10 mL, cooled to -78 °C) was added
Via cannula, with stirring. The orange-brown mixture was warmed to
0 °C and stirred for 20 min, during which time it became brown and
homogeneous. The volatile materials were removed at room temper-
ature in Vacuo. The tube was returned to the glovebox and pentane
(25 mL) was added to the green-brown residue. This mixture was
filtered through a Celite pad (2 × 1 cm) and the volatile materials
were removed from the filtrate in Vacuo. This crude product was
extracted into 120 mL of acetonitrile filtered and the dark green filtrate
was cooled slowly to -40 °C. Removal of the supernatant solution
and drying in Vacuo yielded 118 mg (66.7%) of black crystals of 4. 1H
NMR: δ 1.70 (s, 15, C5Me5), 1.44 (s, 9, NHtBu), -5.72 (s, 1, NHtBu).
13C{1H} NMR: δ 97.1 (C5Me5), 50.1 (CMe3), 37.2 (CMe3), 10.1-
(C5Me5). Mp: 98-100 °C. IR (Nujol): 3263 (w), 2723 (w), 1454
(m), 1352 (m), 1261 (w), 1184 (m), 1157 (w), 1097 (w), 1020 (m),
922 (w), 904 (w), 800 (w), 756 (w), 606 (w), 598 (w), 542 (w). EI-
MS: 58 (100), 119 (59), 134 (66), 192 (54), 266 (29), 268 (71), 281
(26), 457 (27), 530 (4, M+). FAB MS (sulfolane): 250 (47), 265 (69),
457 (100), 530 (67, M+). Anal. Calcd for C28H50Ni2N2: C, 63.20; H,
9.47; N, 5.26. Found: C, 63.33; H, 9.32; N, 5.33.
[Cp*Ni(µ-NHtAmyl)]2. The procedure used to prepare 4 was
followed, using 141 mg (0.480 mmol) of Cp*Ni(acac), 100 mg (1.07
mmol) of LiNHtAmyl, and 8 mL of THF. The solution was stirred for
10 min, extracted with 20 mL of pentane, and crystallized from 80 mL
of acetonitrile, giving 63.6 mg (47.5%) of black plates of [Cp*Ni(µ-
NHtAmyl)]2. Although this material appeared to be pure by 1H NMR,
several attempts to obtain satisfactory elemental analyses were unsuc-
cessful. 1H NMR: δ 1.74 (s, 15, C5Me5), 1.47 (s, CMe2CH2CH3), 1.47
(q, J ) 7.5 Hz, CMe2CH2CH3), 0.84 (t, 9, J ) 7.5 Hz, CMe2CH2CH3),
-5.67 (s, 1, NHtBu). 13C{1H} NMR: δ 98.2 (C5Me5), 53.2 (CMe2-
CH2CH3), 42.2 (CMe2CH2CH3), 33.2 (CMe2CH2CH3), 11.2 (CMe2-
CH2CH3), 10.3 (C5Me5). Mp: >250 °C. IR (Nujol): 3257 (w), 2719
(m), 1574 (w), 1516 (w), 1352 (m), 1315 (w), 1286 (m), 1261 (m),
1173 (m), 1147 (s), 1059 (m), 1020 (s), 935 (m), 874 (m), 800 (m),
727 (m). Anal. Calcd for C30H54Ni2N2: C, 64.32; H, 9.72; N, 5.00.
Found: C, 63.79; H, 9.80; N, 4.88.
Preparation of a Mixture of [Cp*Ni(µ-NHtBu)]2 (4), [Cp*Ni(µ-
NHtAmyl)]2, and Cp*Ni(µ-NHtBu)(µ-NHtAmyl)NiCp*. The pro-
cedure used to prepare 5 was followed, using 166 mg (0.567 mmol) of
Cp*Ni(acac), 50 mg (0.63 mmol, 1.1 equiv) of LiNHtBu, and 62 mg
(0.67 mmol, 1.2 equiv) of LiNHtAmyl for the reaction and a mixture
of 60 mL of acetonitrile and 25 mL of propionitrile for the crystal-
lization. This gave a mixture (1:1:2) of the amides as black crystals.
1H NMR (all integrations referenced to 4): δ 1.74 (s, 15, C5Me5 of
[Cp*Ni(µ-NHtAmyl)]2), 1.72 (s, 30, C5Me5 of mixed product), 1.70
(s, 15, C5Me5 of 4), 1.47 (s, 9, methyl), 1.45 (m, 2 × CMe2CH2CH3),
1.44 (s, 9, methyl), 1.43 (s, 9, methyl), 0.84 (dt, 6, J ) 7.5 Hz, 2 ×
CMe2CH2CH3), -5.61 (s, 1, NH of mixed product), -5.67 (s, 1, NH
of [Cp*Ni(µ-NHtAmyl)]2), -5.71 (s, 1, NH of 4), -5.77 (s, 1, NH of
mixed product).
a dark solid. 1H NMR (all integrations referenced to trans-3): δ 7.41
(m, 0.4, aryl), 7.27 (m, 1.5, aryl), 6.91 (m, 1, aryl), 6.71 (m, 2, aryl),
6.60 (m, 2, aryl), 5.39 (s, 3, trans-3), 5.17 (s, 0.6, mixed product),
4.95 (s, 0.6, mixed product), 4.05 (s, 2.3, cis-3), 2.59 (s, 0.4, cis-3),
2.32 (s, 2.3, cis-3), 2.16 (s, 0.7, mixed product), 2.11 (s, 0.7, mixed
product), 2.06 (s, 0.6, mixed product), 1.87 (s, 0.7, mixed product),
1.75 (s, 3, trans-3), 1.41 (s, 0.2, mixed product), 1.20 (s, 11, cis-3),
1.06 (s, 6, mixed product), 1.03 (s, 6, mixed product), 0.99 (s, 15, trans-
3), 0.36 (s, 0.2, mixed product), -0.11 (s, 1, trans-3), -0.70 (s, 0.2,
mixed product), -2.41 (s, 0.2, mixed product). Heating a solution of
this mixture in THF-d8 to 105 °C for several hours did not result in
any isomerization, although 1H NMR indicated that significant decom-
position had taken place.
CpEtNi(acac). A solution of CpEt2Mg (520 mg, 1.61 mmol) in THF
(15 mL) was added to a slurry of Ni(acac)2 in THF (10 mL) (CpEt
)
C5Me4Et). This caused an immediate color change from green to deep
red. This solution was stirred for 90 min, and volatile materials were
removed in Vacuo. The resulting dark red residue was extracted with
diethyl ether (25 mL), filtered through a Celite pad (2 × 1 cm), and
concentrated to 7 mL. Slow cooling to -80 °C gave dark red crystals
(576 mg, 58.3%) that were suitable for synthesis. Sublimation at 70
°C onto a 5 °C cold finger gave analytically pure material. 1H NMR
(C6D6, 298 K): δ 64.4 (br s, ν1/2 ) 76 Hz, 6, CpEt methyls), 60.9 (br
s, ν1/2 ) 69 Hz, 6, CpEt methyls), 33.3 (br s, ν1/2 ) 31 Hz, 2, CpEt
ethyl), 5.0 (br s, ν1/2 ) 15 Hz, 3, CpEt ethyl), -2.3 (s, ν1/2 ) 6 Hz, 6,
acac methyls), -10.1 (s, ν1/2 ) 11 Hz, 1, acac methine). Mp: 74.5-
75.5 °C. IR (Nujol): 3077 (sh), 2741 (w), 1599 (s), 1518 (s), 1261
(m), 1198 (m), 1155 (m), 1080 (w), 1055 (w), 1020 (m), 928 (m), 796
(m), 766 (w), 660 (m), 623 (m). µeff (298 K): 1.2 µB. Anal. Calcd
for C16H24NiO2: C, 62.58; H, 7.88. Found: C, 62.37; H, 8.05.
[CpEtNi(µ-NH(p-tol))]2 (1′). A flask was charged with CpEtNi(acac)
(275 mg, 0.895 mmol), LiNH(p-tol) (105 mg, 0.928 mmol), and a stir
bar and capped with a vacuum adapter. The flask was cooled to -196
°C, THF (30 mL) was condensed into the flask, and the mixture was
warmed to -78 °C with stirring. The blue solution was allowed to
warm to room temperature over 1 h with stirring. The volatile materials
were removed in Vacuo, leaving a brown residue. In the glovebox,
the residue was extracted into pentane (65 mL); the resulting solution
was filtered through a Celite pad (2 × 1 cm) and concentrated to 15
mL. Slow cooling to -80 °C gave purple crystals (153 mg, 54.4%)
of 1′ that contained a roughly 1:1 mixture of isomers (as judged by 1H
NMR spectroscopy). Analytically pure material was obtained by
recrystallization from pentane. 1H NMR: δ 7.61 (m, aryls), 6.88 (m,
aryls), 2.15 (s, 3, C6H4Me, trans), 2.12 (s, 3, C6H4Me, cis), 1.97 (q, J
) 7.6 Hz, 2, CpEt ethyl, cis), 1.88 (m, 2, CpEt ethyl, trans), 1.36 (s, 3,
CpEt methyl, trans), 1.17 (s, ca. 6, CpEt methyls, cis), 1.17 (t, J ) 7.5
Hz, ca. 3, CpEt ethyl), 1.04 (t, J ) 7.5 Hz, 3, CpEt ethyl), 1.02 (s, 3,
CpEt methyl, trans), 0.99 (s, 3, CpEt methyl, trans), 0.90 (s, 6, CpEt
methyls, cis), 0.84 (s, 3, CpEt methyl, trans), -1.91 (s, 1, NH, cis),
-2.01 (s, 1, NH, trans). 13C{1H} NMR: δ 156.1 (aryl), 155.4 (aryl),
128.6 (aryl), 127.2 (aryl), 127.1 (aryl), 123.5 (aryl), 123.3 (aryl), 106.1
(C5Me4Et), 105.4 (C5Me4Et), 100.8 (C5Me4Et), 98.6 (C5Me4Et), 97.2
(C5Me4Et), 96.4 (C5Me4Et), 94.1 (C5Me4Et), 20.7 (C6H4Me), 20.7
(C6H4Me), 17.6 (C5Me4CH2CH3), 17.6 (C5Me4CH2CH3), 15.5 (CpEt
methyl), 15.0 (CpEt methyl), 9.0 (CpEt methyl), 8.8 (CpEt methyl), 8.8
(CpEt methyl), 8.6 (CpEt methyl), 8.6 (CpEt methyl), 8.6 (CpEt methyl).
Mp: 151-153 °C. IR (Nujol): 3278 (w), 2734 (w), 1869 (m), 1749
(w), 1611 (s), 1270 (s), 1102 (m), 1022 (s), 966 (w), 939 (w), 817 (m),
748 (w), 710 (w). Anal. Calcd for C36H50Ni2N2: C, 68.83; H, 8.02;
N, 4.46. Found: C, 69.05; H, 8.08; N, 4.39.
[CpEtNi(µ-NH(2,6-xylyl))]2 (3′). A flask was charged with CpEt-
Ni(acac) (191 mg, 0.623 mmol), LiNH(2,6-xylyl) (90 mg, 0.71 mmol),
and a stir bar and capped with a vacuum adapter. The flask was cooled
to -196 °C, THF (30 mL) was condensed into the flask, and the mixture
was warmed to -40 °C with stirring. After 30 min, the blue-green
solution was allowed to warm to 0 °C, and volatile materials were
removed in Vacuo, leaving a purple-brown residue. In the glovebox,
the residue was extracted into pentane (60 mL); the resulting solution
was filtered through a Celite pad (2 × 1 cm) and concentrated to 15
mL. Slow cooling to -80 °C gave purple crystals (113 mg, 55.4%)
of 3′ as a roughly 1:1 mixture of isomers (as judged by 1H NMR
spectroscopy). Although recrystallization gave material that appeared
Preparation of a Mixture of 1, 3, and Cp*Ni(µ-NH(p-tol))(µ-
NH(2,6-xylyl))NiCp*. The procedure used to prepare 1 was followed,
using 107 mg (0.365 mmol) of Cp*Ni(acac), 22.7 mg (0.201 mmol,
0.55 equiv) of LiNH(p-tol), 25.4 mg (0.200 mmol, 0.55 equiv) of LiNH-
(2,6-xylyl), and 20 mL of THF for the reaction and 25 mL pentane for
the crystallization. This gave a mixture of the mixed amide and 3 as

1102 J. Am. Chem. Soc., Vol. 118, No. 5, 1996
Holland et al.
pure by NMR spectroscopy, repeated attempts at elemental analysis
gave unsatisfactory results (see below). 1H NMR: δ 7.28 (d, J ) 7
Hz, 1, aryl), 6.9 (m, 2, aryl), 6.72 (t, J ) 7 Hz, 1, aryl), 6.62 (d, J )
7 Hz, 1, aryl), 5.41 (s, 3, C6H3(Me)(Me), trans), 4.09 (s, 3, C6H3(Me)(Me),
cis), 2.64 (s, 1, NH, cis), 2.34 (s, 3, C6H3(Me)(Me), cis), 1.80 (m, C5-
Me4CH2CH3, cis), 1.77 (s, 3, C6H3(Me)(Me), trans), 1.67 (q, J ) 7 Hz,
2, C5Me4CH2CH3, trans), 1.27 (s, 6, C5Me4Et, cis), 1.14 (s, 6, C5Me4-
Et, cis), 1.06 (s, 3, C5Me4Et, trans), 0.99 (s, 3, C5Me4Et, trans), 0.97
(s, 3, C5Me4Et, trans), 0.93 (s, 3, C5Me4Et, trans), 0.79 (t, J ) 7 Hz, 3,
C5Me4CH2CH3, cis), 0.74 (t, J ) 8 Hz, 3, C5Me4CH2CH3, trans), -0.06
(s, 1, NH, trans). 13C{1H} NMR (THF-d8): δ 152.4 (aryl), 150.4 (aryl),
130.9 (aryl), 129.9 (aryl), 129.2 (aryl), 129.0 (aryl), 128.4 (aryl), 127.7
(aryl), 127.5 (aryl), 118.9 (aryl), 118.5 (aryl), 105.1 (C5Me4Et), 101.8
(C5Me4Et), 100.7 (C5Me4Et), 99.7 (C5Me4Et), 98.7 (C5Me4Et), 97.7 (C5-
Me4Et), 97.2 (C5Me4Et), 22.9 (C6H3(Me)(Me), trans), 22.5 (C6H3(Me)-
(Me), cis), 19.5 (C6H3(Me)(Me), cis), 19.2 (C6H3(Me)(Me), trans), 17.6
(C5Me4CH2CH3, trans), 17.1 (C5Me4CH2CH3, cis), 14.2 (CpEt methyl),
8.5 (CpEt methyl), 8.3 (CpEt methyl), 8.2 (CpEt methyl), 8.0 (CpEt
methyl), 7.7 (CpEt methyl). Mp: 219-220 °C dec. IR (Nujol): 3305
(w), 3284 (w), 2734 (w), 1896 (w), 1588 (s), 1411 (m), 1330 (m), 1251
(s), 1218 (s), 1155 (m), 1096 (s), 1020 (s), 984 (m), 837 (m), 755 (m).
Anal. Calcd for C30H54Ni2N2: C, 69.55; H, 8.29; N, 4.27. Found: C,
69.17; H, 8.41; N, 4.72. EI-MS: 106 (46), 121 (100), 327 (30), 534
(64), 654 (45, M+). High-resolution EI-MS: Calcd (58Ni2/58Ni60Ni):
654.2994/656.2948. Found: 654.2986/656.2956.
[CpEtNi(µ-NHtBu)]2 (4′). A flask was charged with CpEtNi(acac)
(193 mg, 0.628 mmol), LiNHtBu (56 mg, 0.71 mmol), and a stir bar
and capped with a vacuum adapter. The flask was cooled to -196
°C, THF (30 mL) was condensed into the flask, and the mixture was
warmed to -78 °C with stirring. After 30 min, the blue-green solution
was allowed to warm to -40 °C, and the volatile materials were
removed in Vacuo, leaving a purple-red residue. In the glovebox, the
residue was extracted into pentane (40 mL); the resulting solution was
filtered through a Celite pad (2 × 1 cm) and concentrated to 4 mL.
Slow cooling to -30 °C gave large black crystals of 4′ in a yield of
65.5 mg (37.2%). Attempts to collect further crops gave only brown
solids. 1H NMR: δ 2.22 (q, J ) 7.4 Hz, 2, C5Me4CH2CH3), 1.74 (s,
6, C5Me4CH2CH3), 1.72 (s, 6, C5Me4CH2CH3), 1.45 (s, 9, CMe3), 1.04
(t, J ) 7.4 Hz, 3, C5Me4CH2CH3), -5.71 (s, 1, NH). 13C{1H} NMR:
δ 103.4 (C5Me4Et), 98.9 (C5Me4Et), 97.2 (C5Me4Et), 51.0 (CMe3), 37.5
(CMe3), 19.1 (C5Me4CH2CH3), 15.2 (CpEt methyl), 10.4 (CpEt methyl),
10.2 (CpEt methyl). Mp: 102-103.5 °C dec. IR (Nujol): 3260 (w),
2719 (w), 1455 (sh), 1354 (sh), 1304 (m), 1261 (w), 1181 (s), 1155
(s), 1047 (m), 1018 (s), 968 (m), 955 (m), 919 (m), 906 (s), 788 (m),
754 (m). Anal. Calcd for C30H54Ni2N2: C, 64.32; H, 9.72; N, 5.00.
Found: C, 64.34; H, 9.86; N, 4.97.
red solution that was found to contain almost exclusively [Cp*Ni(µ-
CO)]2, as judged by 1H NMR spectroscopy. The light orange residue
was dissolved in diethyl ether (10 mL), filtered, and dried in Vacuo to
give analytically pure 6 as a light orange powder (12 mg, 39%). 1H
NMR: δ 7.35 (d, J ) 8 Hz, 2, aryl), 6.90 (d, J ) 8 Hz, 2, aryl), 6.67
(br s, 1, NH), 2.05 (s, 3, C6H4Me), 1.65 (s, 15, C5Me5). IR (C6D6):
3147 (w), 2015 (vs), 1647 (s), 1511 (m), 1472 (m), 1390 (w), 1286
(m), 1224 (m), 1081 (s), 865 (w). Anal. Calcd for C19H23NiNO2: C,
64.08; H, 6.51; N, 3.93. Found: C, 64.01; H, 6.68; N, 4.08. When
this powder was left at room temperature for several days, it turned
1
red, and H NMR spectroscopy confirmed production of [Cp*Ni(µ-
CO)]2.28
Cp*Ni(13CO)(13C(O)NH(p-tol)). This was synthesized in a fashion
analogous to that used for the natural-abundance sample above, except
that the reaction was performed in an NMR tube with 5 mg of 1, 0.6
mL of C6D6, and 375 Torr of 13CO, in order to monitor the reaction at
room temperature. Similar workup yielded Cp*Ni(13CO)(13CONH(p-
2
tol)) as an orange solid. 13C{1H} NMR: δ 192.4 (d, J
) 9 Hz,
CC
Ni-CO), 185.2 (br s, ν1/2 ) 22 Hz, C(O)NH(p-tol)).
Decomposition of 6 in Solution. In the glovebox, 6 (4 mg) was
placed in a vial and C6D6 (0.6 mL) was added. The orange solution
was transferred to an NMR tube, which was placed on a Cajon adapter,
brought out of the glovebox, frozen at -196 °C, degassed, and flame
sealed. The tube was warmed to room temperature and monitored by
1H NMR spectroscopy. After 2 days at room temperature, the red
solution was almost exclusively [Cp*Ni(µ-CO)]2 (Vide supra), and a
white powder had appeared. Repetition of the reaction on a larger scale
(ca. 20 mg of 6) gave enough white powder to allow 1H NMR and IR
analysis. 1H NMR (CDCl3): δ 7.22 (d, J ) 8.4 Hz, 2, aryl), 7.14 (d,
J ) 8.3 Hz, 2, aryl), 6.44 (br s, 1, NH(p-tol)), 2.33 (s, 3, C6H4Me). IR
(KBr): 3306 (m), 1637 (vs), 1593 (s), 1563 (s). For a sample of ((p-
tol)NH)2CdO prepared independently by mixing pentane/toluene (6:
1) solutions of p-toluidine (684 mg, 6.38 mmol) and p-tolyl isocyanate
(854 mg, 6.41 mmol), filtering through a glass frit, and washing the
white solid with pentane, all 1H NMR and IR resonances were identical
1
except the NH resonance in the H NMR spectrum, which resonated
at δ 6.50 ppm in the authentic sample.
Reaction of 4 with tBuNC. In the glovebox, 4 (5.8 mg, 11 µmol)
was placed in a vial and C6D6 (0.6 mL) was added. tert-Butyl isonitrile
(12.5 µL, 110 µmol) was added to the solution Via syringe. The
greenish-black solution was transferred to an NMR tube, which was
placed on a Cajon adapter, brought out of the glovebox, frozen at -196
°C, degassed, flame sealed, warmed to room temperature, and monitored
by 1H NMR spectroscopy. After 2 days at room temperature, the
product was almost exclusively Ni(CNtBu)4 (Vide infra); no other
products or intermediates were detected.
Reaction of 4 with CO. In a glovebox, 4 (4.3 mg, 8.1 µmol) was
placed in a vial and C6D6 (0.6 mL) was added. The green-black solution
was transferred to a J. Young NMR tube, which was closed, brought
out of the glovebox, and degassed with two freeze/pump/thaw cycles.
The tube was repressurized to 1 atm with CO and the tube was shaken
Reaction of 3 with tBuNC. In the glovebox, 3 (3.4 mg, 5.4 µmol)
was placed in a vial and C6D6 (0.6 mL) was added. The purple solution
was transferred to an NMR tube, which was placed on a Cajon adapter,
brought out of the glovebox, frozen at -196 °C, and degassed. tert-
Butyl isonitrile (23.58 mL at 21.3 Torr, 16.8 µmol) was condensed
into the tube, which was then flame sealed, warmed to room
temperature, and monitored by 1H NMR spectroscopy. After 1 day at
room temperature, the product was almost exclusively Ni(CNtBu)4 (Vide
infra), and no other products or intermediates were detected.
Cp*Ni(CNtBu)(C(NtBu)NH(p-tol)) (7). In the glovebox, a solution
of 1 (32.6 mg, 54.3 µmol) and tBuNC (18.0 mg, 217 µmol) in toluene
(7 mL) was stirred for 4 h, and then the volatile materials were removed
in Vacuo. The brown residue was dissolved in pentane (4 mL), filtered,
concentrated to 1 mL, and cooled slowly to -30 °C. The cold solution
gave a small quantity of orange crystals of Ni(CNtBu)4 (1H NMR
(C6D6): δ 1.09 (s). Lit. (toluene-d8, -20 °C):38a δ 1.03 (s). IR
(C6D6): 2023 cm-1. Lit. (C6H6):38b 2026 cm-1) and a brown solution.
The brown supernatant was evaporated in Vacuo to give a brown residue
(32 mg, 63%) of 7 in >90% purity. Silica chromatography using
1
and monitored by H NMR spectroscopy. Over several days at room
temperature, the solution changed to a red color and new peaks
increased in intensity. After 3 weeks at room temperature, the 1H NMR
spectrum showed a large resonance at δ 1.75 ppm due to [Cp*Ni(µ-
CO)]2 (lit. δ 1.75 ppm),28 and other resonances at δ 5.33 (s, 2), 3.18
(br s, 1), 1.68 (t, 12), 1.28 (s, 9), 1.09 (br s, ca. 15), and 0.59 (s, 1)
ppm (integrations referenced to [Cp*Ni(µ-CO)]2 ) 15). After removal
of volatile materials and dissolution in C6D6, 1H NMR resonances
remained for [Cp*Ni(µ-CO)]2 and another nonvolatile product (reso-
nances at δ 3.18, 1.28), in addition to some unreacted starting material.
Dissolution in diethyl ether and filtration through a silica plug removed
starting material, but left the other nonvolatile products unchanged, as
determined by 1H NMR spectroscopy. IR (C6H12 solution): 1857 (s),
1815 (vs). (Lit. for [Cp*Ni(µ-CO)]2 in C6H12: 1857, 1815.)28
Cp*Ni(CO)(C(O)NH(p-tol)) (6). In a glovebox, 1 (26 mg, 43 µmol)
was dissolved in toluene (75 mL) and placed in a flask with a stir bar.
This purple solution was brought out of the box, frozen, and degassed
and CO (250 Torr) was expanded into the headspace (ca. 30 mL). The
solution was warmed to 0 °C and stirred for 10 h, over which time it
turned clear red. The volatile materials were then removed in Vacuo.
The residue was washed with hexamethyldisiloxane (10 mL) to give a
(38) (a) Otsuka, S.; Nakamura, A.; Tatsuno, Y. J. Am. Chem. Soc. 1969,
91, 6994. (b) Nast, R.; Schulz, H.; Moerler, H.-D. Chem. Ber. 1970, 103,
777.
(39) Tolman, C. A. J. Am. Chem. Soc. 1970, 92, 2956.
(40) Amman, C.; Meier, P.; Merbach, A. E. J. Magn. Reson. 1982, 46,
319.
(41) Kaleidagraph v.2.1.3, Abelbeck Software, 1991.

Study of Cyclopentadienylnickel Amido Complexes
J. Am. Chem. Soc., Vol. 118, No. 5, 1996 1103
pentane/diethyl ether as eluent gave analytically pure 7. 1H NMR: δ
7.41(d, J ) 8 Hz, 2, aryl), 7.20 (d, J ) 8 Hz, 2, aryl), 4.42 (br s, 1,
NH), 2.34 (s, 3, C6H4Me), 1.71 (s, 15, C5Me5), 1.57 (s, 9, C(NtBu)-
NH(p-tol)), 0.92 (s, 9, Ni-CNtBu). 13C{1H} NMR: δ 172.5 (Ni-CNt-
Bu), 151.9 (Ni-CNtBu), 139.5 (aryl), 128.7 (aryl), 128.5 (aryl), 122.7
(aryl), 100.0 (C5Me5), 52.6 (CMe3), 30.5 (CMe3), 29.7 (CMe3), 21.1
(C6H4Me), 10.2 (C5Me5). Mp: 100-103 °C. IR (C6D6): 3441 (vw),
3233 (vw), 2120 (vs), 1557 (vs), 1502 (s), 1465 (m), 1432 (s), 1382
(m), 1356 (m), 1232 (m), 1212 (m), 1132 (s), 882 (m). EI-MS: 309
(39), 382 (100), 465 (45, M+). Anal. Calcd for C27H41NiN3: C, 69.54;
H, 8.86; N, 9.01. Found: C, 69.37; H, 9.10; N, 8.84.
Table 6. Selected Crystal and Data Parameters for cis-1′, trans-3′,
and 4′
cis-1′
trans-3′
4′
formula
a (Å)
b (Å)
C36H50Ni2N2
17.574(5)
16.360(3)
11.102(2)
90
91.18(2)
90
3191.3(20)
C2/c (C62h, #15)
C38H54Ni2N2 C30H54Ni2N2
8.4268(5)
10.6600(6)
10.9110(7)
115.251(1)
102.957(1)
98.529(1)
830.16(8)
19.7934(1)
9.1003(1)
17.9028(2)
90
113.616(1)
90
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
2954.69(5)
P1h (C1i , #2) C2/c (C26h, #15)
space group
Cp*Ni(PMe3)(NH(p-tol)) (5). In the glovebox, 1 (5.2 mg, 8.7 µmol)
was placed in a vial and C6D6 (0.6 mL) was added. The purple solution
was transferred to an NMR tube, which was placed on a Cajon adapter,
brought out of the glovebox, frozen at -196 °C, and degassed.
Trimethylphosphine (6.6 mL at 54 Torr, 19 µmol) was condensed into
the tube, which was then flame sealed, warmed to room temperature,
Z
4
1
4
diffractometer
CAD4
Siemens
SMART
Siemens
SMART
detector
crystal scintillation
counter, with PHA
-90 ( 1
CCD area
detector
-140 ( 2
2318
CCD area
detector
-138 ( 1
4490a
temp (°C)
no. of unique
reflctns
1
and monitored by H and 31P{1H} NMR spectroscopy. After 10 h at
2812
room temperature, the product was almost exclusively Cp*Ni(PMe3)-
(NH(p-tol)) (5). Attempts to isolate this complex from solution
invariably resulted in partial conversion to a mixture of 1 and Ni(PMe3)4
(1H NMR (C6D6): δ 1.15 (s). 31P NMR (C6D6): δ -21.4. Lit.
Tmin/Tmax
no. of variables
R; Rw
0.95
181
0.042; 0.052
0.45
271
0.93
235
0.054, 0.063 0.029; 0.043
R (including zeros) 0.065
0.065
2.09
0.031
1.95
1
(toluene-d8):39 δ 1.17 (s), δ -22.2). For 5, H NMR: δ 6.99 (d, J )
goodness of fit
1.64
8 Hz, 2, aryl), 6.87 (d, J ) 8 Hz, 2, aryl), 2.35 (s, 3, C6H4Me), 1.52 (d,
JPH ) 1.6 Hz, 15, C5Me5), 0.73 (d, JPH ) 9 Hz, 9, PMe3), -1.05 (br s,
1, NH). 13C{1H} NMR: δ 158.6 (aryl), 129.6 (aryl), 123.7 (aryl), 117.7
(aryl), 100.5 (C5Me5), 15.1 (C6H4Me), 10.2 (C5Me5). 31P{1H} NMR:
δ -5.9.
indicator
a Includes systematic absences.
have been described.42 The final cell parameters and specific data
collection parameters for this data set are given in Table 6 and the
supporting information. The 2919 raw intensity data were converted
to structure factor amplitudes as described earlier.42 Inspection of the
azimuthal scan data43 showed a variation Imin/Imax ) 0.95 for the average
curve. An empirical correction based on the observed variation was
applied to the data. The choice of space group C2/c was confirmed
by inspection of the systematic absences and the successful solution
and refinement of the structure. The structure was solved by Patterson
methods and refined via standard least-squares and Fourier techniques.
Hydrogen atoms were assigned idealized locations and values of Biso
approximately 1.25 times the Bequiv of the atoms to which they were
attached. They were included in structure factor calculations, but not
refined. The final residuals for the 2051 data for which F2 > 3σ(F2)
were R ) 4.20%, wR ) 5.15%, and GOF ) 1.64. The R value for all
2812 data was 6.49%. The maximum and minimum peaks on the final
difference Fourier map corresponded to 0.38 and -0.37 e-/Å3,
respectively. Data reduction and refinement formulae, anisotropic
thermal parameters, and the positional parameters of all atoms are
available as supporting information.
X-ray Crystal Structure of 3′. An irregularly-shaped purple plate
of 3′ (approximate dimensions 0.25 × 0.20 × 0.08 mm) was obtained
by cooling a pentane solution to -30 °C. The crystal was mounted
on a glass fiber using Paratone N hydrocarbon oil and centered on the
beam. It was cooled to -140 °C by a nitrogen flow low-temperature
apparatus that had been previously calibrated by a thermocouple placed
at the sample position. All measurements were made on a Siemens
SMART diffractometer with a CCD area detector45a using graphite
monochromated Mo KR radiation.
Reaction of 1 with Aniline. In the glovebox, 1 (5.3 mg, 8.8 µmol)
was placed in a vial and C6D6 (0.6 mL) was added. Aniline (1 drop,
excess) was added to the solution Via pipet. The purple solution was
transferred to an NMR tube, which was placed on a Cajon adapter,
brought out of the glovebox, frozen at -196 °C, degassed, flame sealed,
warmed to room temperature, and monitored by 1H NMR spectroscopy.
After 15 h at room temperature, the solution contained mostly 2 and
ca. 0.5 equiv of Cp*Ni(µ-NH(p-tol))(µ-NHPh)NiCp*. 1H NMR (mixed
product, as cis and trans isomers): δ 2.06 (s, C6H4Me), 2.01 (s,
C6H4Me), 1.08 (s, Cp*), 1.07 (s, Cp*), -1.73 (s, NH), -2.02 (s, NH).
Detection of aryl resonances and accurate integrations were impossible
due to significant overlap of the mixed product peaks with those from
other reactants and products.
Crossover of 1 and 2. In the glovebox, 2 (8.4 mg, 14.7 µmol) and
1 (8.5 mg, 14.2 µmol) were placed in a vial and C6D6 (0.5 mL) was
added. The purple solution was transferred to an NMR tube, which
was placed on a Cajon adapter, brought out of the glovebox, frozen at
-196 °C, flame sealed, warmed to room temperature, and monitored
by 1H NMR spectroscopy. After 1 week at 45 °C, the solution was a
mixture (ca. 1:1:1) of 1, 2, and Cp*Ni(µ-NH(p-tol))(µ-NHPh)NiCp*
(Vide supra). Further heating resulted in a ca. 1:1:2 ratio of compounds.
After several weeks at 45 °C, the solvent was evaporated to give a
purple powder that was used for EI-MS characterization. EI-MS: 93
(75), 106 (100), 192 (24), 285 (18), 299 (33), 477 (34), 491 (36), 570
(14, M+, 2), 584 (26, M+, mixed product), 598 (15, M+, 1).
Rates of Isomerization. All kinetic runs were performed using
variants of the following procedure. In the glovebox, trans-2 (5.0 mg),
ferrocene (1.7 mg), and Cp2ZrMe2 (0.7 mg) were dissolved in THF-d8
(0.4 mL) and transferred to a new medium-wall NMR tube. The tube
was placed on a Cajon adapter, brought out of the glovebox, frozen at
-196 °C (but not degassed), and flame sealed. The NMR probe was
warmed to 76.9 °C, as determined from a peak separation in an ethylene
glycol standard sample of 1.143 ppm.40 The T1’s of the protons were
checked, and they were all invariably shorter than 15 s. The sample
was inserted into the probe and scans with a 90° pulse width were
recorded every 6 min. Integrations were determined vs the ferrocene
resonance at δ 4.0 ppm. Rates and errors were determined by
exponential curve-fitting on KaleidaGraph.41 Plots of log |[A] - [A]eq|
vs time were always linear to 3 half-lives, after which time the noise
often made line-fitting difficult; in several cases, however, the lines
were linear to >5 half-lives.
Good crystal quality was confirmed by well-shaped, intense spots
appearing to 2θ g 45°. Collection of 60 10-s frames, followed by
spot integration and least-squares refinement, gave a preliminary
orientation matrix and cell constants. A full hemisphere of reciprocal
space was measured, using 20-s frames of width 0.3° in ω. The raw
data were integrated (XY spot spread ) 1.60°; Z spot spread ) 0.60°)
using SAINT.45b The unit cell parameters were refined, giving the final
values shown in Table 6 and the supporting information. Data analysis
was performed using Siemens XPREP.45c The unit cell parameters
indicated a primitive triclinic cell; the choice of P1h was confirmed by
(42) Dobbs, D. A.; Bergman, R. G. Inorg. Chem. 1994, 33, 5329.
(43) Reflections used for azimuthal scans were located near ø ) 90°
and the intensities were measured at 10° increments of rotation of the crystal
about the diffraction vector.
(44) Cromer, D. T.; Waber, J. T. International Tables for X-ray
Crystallography; The Kynoch Press: Birmingham, England, 1974; Vol.
IV.
X-ray Crystal Structure of 1′. A mixture of clear purple blocky
and flakelike crystals of 1′ were obtained by slow cooling of a
concentrated pentane solution. The data collection methods used here

1104 J. Am. Chem. Soc., Vol. 118, No. 5, 1996
Holland et al.
successful solution and refinement of the structure. The data were
corrected for Lorentz and polarization effects. No correction for
decomposition was necessary, but a semiempirical ellipsoidal absorption
correction (Tmax ) 0.842, Tmin ) 0.381) was applied.
parameters in Table 6, except for the following changes. Inspection
of the systematic absences revealed a monoclinic C-centered cell, with
possible space groups Cc and C2/c; the choice of C2/c was confirmed
by successful solution and refinement of the structure. Least-squares
refinement converged with unweighted and weighted agreement factors
of R ) 2.9%, Rw ) 4.3%, and GOF ) 1.95. Using all unique data
(including systematic absences), R ) 3.1%, Rw ) 4.4%. The maximum
and minimum peaks on the final difference Fourier map corresponded
to 0.28 and -0.32 e-/Å3, respectively. Data reduction and refinement
formulae, anisotropic thermal parameters, and positional parameters
of all atoms are available as supporting information.
Of the 3353 reflections that were measured, 2318 were unique (Rint
) 10.5%); equivalent reflections were merged. The structure was
solved and refined with the teXsan software package45d using direct
methods45e and expanded using Fourier techniques.45f The non-
hydrogen atoms were refined anisotropically. The hydrogen atoms were
located in a Fourier synthesis; their coordinates were refined but their
isotropic B’s were held fixed at 2.5. The final cycle of full-matrix
least-squares refinement was based on 1967 observed reflections (I >
3σ(I)) and 271 variable parameters and converged with unweighted
and weighted agreement factors of R ) 5.4%, Rw ) 6.3% and GOF )
2.09. Using all 2318 unique data (including systematic absences), R
) 6.5% and Rw ) 6.6%. The weighting scheme was based on counting
statistics and included a factor (p ) 0.030) to downweight the intense
reflections. Neutral atom scattering factors and mass attenuation
Acknowledgment. The authors would like to thank Dr.
Frederick J. Hollander, staff crystallographer at the CHEXRAY
facility at the University of California, Berkeley, for performing
the crystal structure determination of 1′ and for assisting with
the other crystal structure determinations. The authors also
1
thank Dr. Graham Ball for assistance with the 15N-filtered H
coefficients were given standard values.44,45g Anomalous dispersion
45g,h
effects were included in Fcalc
.
The maximum and minimum peaks
spectra and the National Institutes of Health (Grant No. R37
GM25459) for financial support.
on the final difference Fourier map corresponded to 0.63 and -1.06
e-/Å3, respectively. Data reduction and refinement formulae, aniso-
tropic thermal parameters, and positional parameters for all atoms are
available as supporting information.
X-ray Crystal Structure of 4′. A large black block of 4′ was
obtained by cooling a pentane solution to -30 °C. A shard (0.20 ×
0.21 × 0.26 mm) was cleaved from the large crystal and mounted.
The data collection, reduction, and refinement were carried out as
described for 3′, using the data collection, reduction, and refinement
Note Added in Proof: While this work was in press,
Boncella reported dimeric square-planar nickel amido dimers
which react similarly with PMe3 and have similar upfield NH
resonances: VanderLande, D. D.; Abboud, K. A.; Boncella, J.
M. Inorg. Chem. 1995, 34, 5319.
Supporting Information Available: Additional information
on the collection, processing, and refinement of the X-ray crystal
structures of cis-1′, trans-3′, and 4′; anisotropic thermal
parameters and positional parameters for all atoms in the X-ray
crystal structures of cis-1′, trans-3′, and 4′; and full tables of
intramolecular distances and angles (13 pages). This material
is contained in many libraries on microfiche, immediately
follows this article in the microfilm version of the journal, can
be ordered from the ACS, and can be downloaded from the
Internet; see any current masthead page for ordering information
and Internet access instructions.
(45) (a) SMART Area-Detector Software Package; Siemens Industrial
Automation, Inc.: Madison, WI, 1993. (b) SAINT: SAX Area-Detector
Integration Program; v. 4.024; Siemens Industrial Automation, Inc.:
Madison, WI, 1995. (c) XPREP (v.5.03): Part of the SHELXTL Crystal-
Structure Determination Package; Siemens Industrial Automation: Madison,
WI, 1994. (d) TEXSAN: Crystal Structure Analysis Package; Molecular
Structure Corporation, 1992. (e) Hai-Fu, F. SAPI91: Structure Analysis
Programs with Intelligent Control; Rigaku Corp.: Tokyo, 1991. (f)
Beurskens, P. T.; Admiraal, G.; Beurskens, G.; Bosman, W. P.; Garcia-
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