Synthesis and reactivity of a mononuclear parent amido nickel complex. Structures of Ni[C6H3-2,6-(CH2P iPr2)2] (NH2) and Ni[C 6H3-2,6-(CH2PiPr2) 2](OMe)

Article abstract of DOI:10.1021/om049312r

The first example of a nickel parent amido complex, Ni(PCP)(NH ₂), has been synthesized and structurally characterized. This compound reacts quantitatively with water and methanol to give the mononuclear hydroxide and methoxide derivatives Ni(PCP)(OH) and Ni(PCP)(OMe). Ni(PCP)(NH₂) behaves as a strong nucleophile and adds rapidly to benzaldehyde, affording an equimolecular mixture of the aldimido derivative Ni-(PCP)(N=CHPh), the hydroxide Ni(PCP)(OH), and ammonia.

Full text of DOI:10.1021/om049312r

© Copyright 2004
Volume 23, Number 24, November 22, 2004
American Chemical Society
Communications
Synthesis and Reactivity of a Mononuclear Parent
Amido Nickel Complex. Structures of
Ni[C6H3-2,6-(CH2PiPr2)2](NH2) and
Ni[C6H3-2,6-(CH2PiPr2)2](OMe)
Juan Ca´mpora,* Pilar Palma,* Diego del R´ıo, M. Mar Conejo, and
Eleuterio AÄ lvarez
Instituto de Investigaciones Quı´micas, Consejo Superior de Investigaciones
Cientı´ficas-Universidad de Sevilla, Avda. Ame´rico Vespucio 49, 41092 Sevilla, Spain
Received September 4, 2004
Summary: The first example of a nickel parent amido
amides, usually interpreted as a result of the combina-
tion of a “soft” late transition metal with a “hard” donor
ligand,6 poses to their synthesis and isolation, especially
in the case of the parent species. Furthermore, amido
and alkoxo complexes display a pronounced tendency
to associate, forming binuclear or polynuclear species.7
It has not been until recently that some parent amido
complexes of middle and late transition metals, such as
iridium4 and iron and ruthenium,5 have been reported.
Similarly, alkoxo complexes of the late transition ele-
ments remain uncommon. In this communication we
report the synthesis, structural characterization, and
complex, Ni(PCP)(NH2), has been synthesized and struc-
turally characterized. This compound reacts quantita-
tively with water and methanol to give the mononuclear
hydroxide and methoxide derivatives Ni(PCP)(OH) and
Ni(PCP)(OMe). Ni(PCP)(NH2) behaves as a strong nu-
cleophile and adds rapidly to benzaldehyde, affording
an equimolecular mixture of the aldimido derivative Ni-
(PCP)(NdCHPh), the hydroxide Ni(PCP)(OH), and am-
monia.
The synthesis and reactivity of late-transition-metal
complexes possessing amido (NR2-) and alkoxide (OR-)
ligands have received considerable attention in recent
years.1 These complexes have been proposed as inter-
mediates in a number of catalytic processes.2 Despite
these efforts, examples of the parent amides, M-NH2,
continue to be scarce.3-5 This is possibly due to the
difficulty that the high reactivity of late-transition-metal
(3) (a) Dewey, M. A.; Knight, D. A.; Gladysz, J. A. Chem. Ber, 1992,
125, 815. (b) Joslin, F. L.; Jonson, M. P.; Mague, J. T.; Roundhill, D.
M. Organometallics 1991, 10, 41. (c) Tahmassebi, S. K.; McNeil, W.
S.; Mayer, J. M. Organometallics 1997, 16, 5342. (d) Jayaprakash, K.
N.; Conner, D.; Gunnoe, T. B. Organometallics 2001, 20, 5254.
(4) Kanzelberger, M.; Zhang, X.; Emge, T. J.; Goldman, A. S.; Zhao,
J.; Incarvito, C.; Hartwig, J. F. J. Am. Chem. Soc. 2003, 125, 13644.
(5) (a) Fulton, J. R.; Bouwkamp, M. W.; Bergman, R. G. J. Am.
Chem. Soc. 2000, 122, 8799. (b) Fulton, J. R.; Sklenak, S.; Boukamp,
M.; Bergman, R. G. J. Am. Chem. Soc. 2002, 124, 4722. (c) Holland,
A. W.; Bergman, R. G. J. Am. Chem. Soc. 2002, 124, 14684. (d) Kaplan,
A. W.; Ritter, J. C. M.; Bergman, R. G. J. Am. Chem. Soc. 1998, 120,
6828. (e) Fox, D. J.; Bergman, R. G. J. Am. Chem. Soc. 2003, 125, 8984.
(f) Fox, D. J.; Bergman, R. G. Organometallics 2002, 23, 1656.
(6) For a recent discussion see for instance: Holland, P. L.; Ander-
sen, R. A.; Bergman, R. G. Comments Inorg. Chem. 1999, 21, 115.
(7) A search in the CCDC database (version 5.27, Nov 2003) afforded
67 structures of metal complexes containing µ-NH2 ligands.
* To whom correspondence should be addressed. E-mail: campora@
iiq.csic.es (J.C.); ppalma@iiq.csic.es (P.P.).
(1) See for instance: Fulton, J. R.; Holland, A. W.; Fox, D. J.;
Bergman, R. G. Acc. Chem. Res. 2002, 35, 44 and references herein.
(2) (a) Cornils, B.; Herrmann, W. A. Applied Homogeneous Catalysis
with Organometallic Compounds; VCH: New York, 1996; Vol. 1. (b)
Cotton, F. A.; Wilkinson, G.; Murillo, C. A.; Bochmann, M. Advanced
Inorganic Chemistry, 6th ed.; Wiley: New York, 1999. (c) Bryndza, H.
E.; Tam, W. Chem. Rev. 1988, 88, 1163. (d) Roundhill, D. M. Chem.
Rev. 1992, 92, 11.
10.1021/om049312r CCC: $27.50 © 2004 American Chemical Society
Publication on Web 10/27/2004

5654 Organometallics, Vol. 23, No. 24, 2004
Communications
Scheme 1
some reactivity studies of a mononuclear Ni complex
that features a terminally bonded amido unit, Ni-NH2.
In addition, we report the crystal structure of a mono-
nuclear methoxide derivative. Dimerization of these
complexes has been prevented by the use of the triden-
tate pincer ligand 2,6-bis((diisopropylphosphino)meth-
yl)phenyl (PCP). The two complexes reported herein
constitute very unusual examples in the chemistry of
group 10 metals. To our knowledge, no structural data
for mononuclear group 10 M-NH2 complexes have been
hitherto reported.
Figure 1. ORTEP diagram of Ni(PCP)(NH2) (1). Selected
bond distances (Å) (one of two independent molecules):
Ni1-N1 ) 1.872(2), Ni1-C1 ) 1.931(2), Ni1-P1 ) 2.1585-
(6), Ni1-P2 ) 2.1587(6).
We have recently shown that sonication of a suspen-
sion of the precursors M(PCP)X (M ) Ni, X ) Br; M )
Pd, X ) NO3) and NaOH in THF leads to the mono-
nuclear nickel and palladium hydroxides M(PCP)(OH).8
Use of NaNH2 in place of the hydroxide reagent allows
the preparation of the Ni amido complex Ni(PCP)(NH2)
(1)9 as an orange crystalline solid in 70% isolated yield
(Scheme 1). For M ) Pd, a new complex is formed
(detected as a singlet resonance at δ 59.0 ppm by 31P-
{1H} NMR spectroscopy), but its high reactivity has
hitherto prevented its isolation.
Complex 1 is very soluble in common organic solvents.
Its 31P{1H} NMR spectrum displays a singlet at δ 60.8
ppm, whereas in the 1H NMR spectrum, a slightly broad
triplet at δ -1.00 ppm (3JHP ) 8.3 Hz) can be assigned
to the NH2 protons. IR absorptions associated with νas
and νs NH2 stretching modes are observed at 3355 and
3295 cm-1, respectively.
The structural assignment of the amido complex 1 has
been confirmed by X-ray crystallography.10 Crystals
suitable for an X-ray diffraction study were obtained
from a concentrated hexamethyldisiloxane solution at
room temperature. Two crystallographically indepen-
dent molecules displaying similar bond distances and
angles form the asymmetric unit of complex 1. At
variance with the analogous hydroxide Ni(PCP)(OH)
(2),8 no intermolecular contacts can be found in the solid
state. Figure 1 shows an ORTEP perspective of one such
molecule. The Ni-N bond in complex 1 (average 1.872-
(2) Å), while slightly shorter than in Ni(2,4,6-Me3C6H2)-
(NHPh)(PMe3)211 (1.931(3) Å), is appreciably longer than
Figure 2. ORTEP diagram of Ni(PCP)(OMe) (3). Selected
bond distances (Å) and angles (deg) (one of two independent
molecules): Ni1-O1 ) 1.855(2), Ni1-C1 ) 1.922(3), Ni1-
P1 ) 2.1914(9), Ni1-P2 ) 2.1543(9), O1-C21 ) 1.289(5);
Ni1-O1-C21 ) 122.4(2).
the formally double NidN bond length of 1.702(2) Å
reported by Hillhouse et al. for the three-coordinate
nickel-imido complex Ni(dNC6H3-2,5-iPr2)(dtbpe).12
Even though the amido hydrogen atoms were located
in positions that suggest a planar configuration of the
NH2 group, this is probably the result of the averaging
of two equally populated orientations of the pyramidal
NH2 fragment.5e DFT calculations on models of Ni-
(PCP)(NH2) support this assumption.13
Complex 1 is exceedingly sensitive to moisture, as it
readily reacts with traces of water, forming 2 (Scheme
1). The reaction with MeOH proceeds in a similar
fashion, to afford the methoxide complex Ni(PCP)(OMe)
(3) (Scheme 1).14 This compound is characterized by a
1
31P{1H} signal at δ 52.6 ppm and by H and 13C{1H}
resonances at δ 3.90 and 59.9 ppm, respectively, due to
the Ni-OCH3 linkage.
Very few monomeric nickel methoxides has been
reported before,15 and therefore, the structure of com-
(8) Ca´mpora, J.; Palma, P.; del R´ıo, D.; AÄ lvarez, E. Organometallics
2004, 23, 1652.
(9) A solution of 476 mg (1 mmol) of Ni(PCP)(Br) in 30 mL of THF
was added to 390 mg (10 mmol) of NaNH2. The reaction mixture was
sonicated for 3 h and centrifuged and the solvent removed under
reduced pressure. The solid residue was extracted with hexane, the
resulting suspension centrifuged, and the solvent removed under
vacuum. The solid residue was extracted with hexamethyldisiloxane.
The complex Ni(PCP)(NH2) (1) was obtained as an orange crystalline
solid after cooling to -30 °C. Yield: 70%.
(12) Mindiola, D. J.; Hillhouse, G. L.; J. Am. Chem. Soc. 2001, 123,
4623.
(13) The geometry of Ni[C6H3-2,6-(CH2Me2)2](NH2) was optimized
using the B3LYP functional and the 6-311+G** basis set for all
atoms: Ni-N ) 1.883 Å, Ni-N-H(1) ) 118.4°, Ni-N-H(2) ) 119.2°,
H(1)-N-H(2) ) 105.5°.
(14) A solution of 476 mg (1 mmol) of Ni(PCP)(Br) in 30 mL of THF
was added to 390 mg (10 mmol) of NaNH2. The reaction mixture was
sonicated for 3 h and centrifuged and the solvent removed under
reduced pressure. The solid residue was extracted with hexane and
the resulting suspension centrifuged again. The orange solution of
complex 1 was treated with 50 µL (1.2 mmol) of MeOH, the solvent
removed under vacuum, and the solid residue dissolved in hexane.
Complex 3 was isolated as a dark orange crystalline solid after adding
some hexamethyldisiloxane and cooling to -30 °C. Yield: 60%.
(10) Crystal data for 1 at 100(2) K:
C20H37NNiP2, fw 412.16,
monoclinic, space group P21/n, a ) 10.8790(6) Å, b ) 13.7440(8) Å, c
) 28.7731(17) Å, R ) 90°, â ) 95.7910(10)°, γ ) 90°, V ) 4280.2(4) Å3,
Z ) 8. The final r factor was 0.0422 for 10 027 independent reflections
with I > 2σ(I) (wR2 ) 0.0922). GOF ) 1.040.
(11) Van der Lende, D. D.; Abboud, K. A.; Boncella, J. M. Inorg.
Chem. 1995, 34, 5319.

Communications
Organometallics, Vol. 23, No. 24, 2004 5655
Scheme 2
2, reaction of the water generated with the excess of 1
being responsible for the formation of 2 and NH3.
Interestingly, the addition of further amounts of ben-
zaldehyde to this reaction mixture results in the slow
consumption of 2, along with additional amounts of 4.
Since PhCHO does not react separately with the pure
hydroxide or with NH3 under the reaction conditions,19
it seems likely that, in the presence of NH3, the
hydroxide is in equilibrium with small amounts of the
amido complex, the latter reacting with benzaldehyde
to produce 4.
In summary, the first example of a parent nickel
amido complex, namely Ni(PCP)(NH2) (1), has been
synthesized and structurally characterized. This com-
pound reacts quantitatively with water and methanol
to give the mononuclear hydroxide and methoxide
derivatives Ni(PCP)(OH) (2) and Ni(PCP)(OMe) (3). In
addition, it behaves as a strong nucleophile and adds
rapidly to benzaldehyde, affording an equimolecular
mixture of the aldimido derivative 4, the hydroxide 2,
and ammonia. The slow formation of 4 upon addition
of PhCHO to a mixture of 2 and NH3 suggests that small
amounts of the amido complex exist in equilibrium with
the hydroxide and NH3. Further studies on the reactiv-
ity of the hydroxo, amido, and alkoxo complexes of Ni
and Pd are under way.
plex 3 has been determined by an X-ray diffraction
study.16 Figure 2 gives an ORTEP representation of one
of the two independent molecules found in the asym-
metric unit. The average Ni-O bond distance, 1.856(2)
Å, is only slightly shorter than that in the related
hydroxide complex 2, 1.865(2) Å.8 The C-O bond length
in complex 3, 1.307(4) Å (average), is significantly
shorter than the standard C-O bond (ca. 1.43 Å), or
the distance found in early-transition-metal alkoxides.
Late-transition-metal alkoxides often display anoma-
lously short C-O bonds.2c It has been suggested that
the shortening of the C-O distance in alkoxides bearing
very polar M-O bonds is due to an attractive electro-
static interaction between the O and C atoms.17 This
effect is accompanied by an increase of the frequency
of the IR C-O stretching band of 3 (1088 cm-1) of 50
cm-1 above the standard value observed in MeOH and
other methyl ethers (ca. 1030 cm-1), which suggests
some strengthening of the C-O bond.
Alkylamido or arylamido complexes often react with
CO, giving formal insertion products.5f The reactivity
of the parent amido complexes trans-(dmpe)2Fe(H)-
(NH2), is somewhat different, as CO inserts into the
N-H bond of the amido moiety.5d-f In contrast, treat-
ment of a solution of complex 1 with dry CO (excess) or
CN(2,6-Me2C6H3) (1 equiv) in C6D6 solution results in
the formation of complex mixtures of products that could
not be identified. The formation of ammonia was
Acknowledgment. Financial support from the DGI
(Project PPQ2003-000975), the Ministerio de Educacio´n,
Cultura y Deporte (research studentship, D.d.R.), and
the Junta de Andaluc´ıa is gratefully acknowledged. We
thank Prof. E. Carmona for helpful discussions.
1
detected in both cases by H NMR spectroscopy (broad
1
1:1:1 triplet, δ 0.17 ppm, JHN ) 42.0 Hz).
The nucleophilic character of compound 1 is evidenced
by its reaction with benzaldehyde. Thus, its treatment
with 0.5 equiv of PhCHO (Scheme 2), yields the new
aldimido complex Ni(PCP)(NdC(H)Ph) (4), together
with equivalent amounts of 2 and NH3. Complex 4 has
been isolated from the reaction mixture, and it has been
characterized by analytical and spectroscopic methods.18
Its formation can be rationalized as shown in Scheme
Supporting Information Available: Text and tables
giving experimental procedures and characterization data for
all new complexes and computational details and CIF files
giving X-ray crystallographic data. This material is available
free of charge via the Internet at http://pubs.acs.org.
OM049312R
(15) (a) Holland, P. L.; Andersen, R. A.; Bergman, R. G.; Huang, J.;
Nolan, S. P. J. Am. Chem. Soc. 1997, 119, 12815. (b) Jian, P.; Wang,
S.; Suo, J.; Wang, L.; Wu, Q. Acta Crystallogr., Sect. C: Cryst. Struct.
Commun. 1995, 51, 1071.
(18) To a solution of 150 mg (0.36 mmol) of complex 1 in 8 mL of
benzene was added 74 µL (0.73 mmol) of PhCHO. The mixture was
stirred overnight at room temperature and then taken to dryness and
the residue extracted with hexane (10 mL). After filtration, concentra-
tion, and cooling to -30 °C several crops of orange crystals of complex
4 were isolated, some of them with a small amount of the hydroxide
derivative 2.
(16) Crystal data for 3 at 100(2) K:
C21H38NiOP2, fw 427.16,
monoclinic, space group P21/n, a ) 11.0090(7) Å, b ) 13.8960(9) Å, c
) 29.403(2) Å, R ) 90°, â ) 97.7530(10)°, γ ) 90°, V ) 4457.0(5) Å3, Z
) 8. The final r factor was 0.0520 for 10 386 independent reflections
with I > 2σ(I) (wR2 ) 0.0898). GOF ) 1.026.
(19) Verified in independent experiments. Addition of NH3 to a
mixture of PhCHO and 2 causes the slow conversion of the latter in 4.
(17) Wiberg, K. B. J. Am. Chem. Soc. 1990, 112, 3379.