Synthesis and reactivity of nickel-hydride amino-bis-phosphinimine complexes

Article abstract of DOI:10.1039/c3dt33021d

Cationic hydride complexes [R-N(1,2-CH₂CH₂NPPh ₃)₂NiH][PF₆] (R = H, Me) are prepared and shown to react with ethylene to produce NiEt complexes and with LiHBEt₃ in bromobenzene to produce N

Full text of DOI:10.1039/c3dt33021d

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Synthesis and reactivity of nickel-hydride amino-bis-
phosphinimine complexes†
Cite this: Dalton Trans., 2013, 42, 4237
Received 17th December 2012,
Accepted 18th January 2013
Renan Cariou, Todd W. Graham and Douglas W. Stephan*
DOI: 10.1039/c3dt33021d
www.rsc.org/dalton
Cationic hydride complexes [R–N(1,2-CH2CH2NvPPh3)2NiH][PF6] afforded the species [R–N(1,2-CH2CH2NvPPh3)2NiH][PF6]
(R = H, Me) are prepared and shown to react with ethylene to (R = H 3, Me 4) in yields of 52 and 84%, respectively
produce NiEt complexes and with LiHBEt3 in bromobenzene to (Scheme 1). Compounds 3 and 4 display the expected diamag-
produce NiPh complexes. These species are also thermolysed at netic NMR spectra for square planar complexes, with hydride
80 °C to orthometallate one of the phenyl groups of the NvPPh3 resonances at −27.33 and −28.65 ppm, respectively. Whereas 3
substituents.
shows a singlet at 3.58 ppm attributable to the amine proton,
4 displays a singlet at 2.80 ppm corresponding to the NMe
fragment. The phosphinimine moieties in 3 and 4 give rise to
31P singlet resonances at 31.9 and 32.3 ppm, respectively. In
addition, diagnostic multiplets around −144.6 ppm for the PF6
anions are observed for both species. The synthesis of the
corresponding deuteride complex 3-d1 from the reaction with
LiDBEt3 confirmed the source of the deuteride, showing
a singlet at −27.10 ppm in the 2H NMR spectrum. The
Metal hydride complexes have attracted interest for many
decades as they are intermediates to a variety of catalytic pro-
cesses. In particular, nickel hydride complexes have been
identified as intermediates in various nickel-based catalysis,
such as olefin isomerization and polymerization,1–4 hydrosilyl-
ation,5,6 desulfurization,7 hydrodesulfurization,8,9 C–C, C–H
and H2 activation,10 as well as reduction of CO2.11 Recently,
Guan and coworkers successfully employed a pincer complex,
POCOPNi-H, in the catalytic reduction of CO2 in combination
with boranes.12,13 In a similar way, Labinger and Bercaw
described the CO2 reduction to formate assisted by trialkyl-
borane and late transition metal hydrides including the cationic
nickel hydride [(dmpe)2NiH]+.14 In recent years, a number of
nickel hydride complexes, stabilized by “pincer” type ligands
have been isolated and characterized.15–22 In an effort to
develop new ligands for applications in catalysis, we reported
recently the synthesis of complexes of Ni and Pd bearing tri-
dentate phosphinimine ligands.23 This class of tridentate
ligand was shown to provide steric protection for the binding
site trans to the central donor, although the sterically demand-
ing substituents are somewhat removed from the metal in
comparison to more conventional PNP and PCP pincer
ligands. Herein, we exploit this feature to access stable nickel-
hydride complexes and to explore the resulting reactivity.
The cationic nickel complexes [R–N(1,2-CH2CH2Nv
PPh3)2NiCl][PF6] (R = H 1, Me 2) were prepared by methods
recently described.24,25 Subsequent reactions with LiHBEt3
Department of Chemistry, 80 St. George St.University of Toronto, Toronto, Ontario,
Canada, M5S 3H6. E-mail: dstephan@chem.utoronto.ca
†Electronic supplementary information (ESI) available: Synthetic and spectro-
scopic data are deposited. CCDC 915323 and 915324. For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/c3dt33021d
Scheme 1 Synthesis of 3–10.
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reported Ni-pincer hydride complexes.16 While stable in the
solid state, compound 3 is prone to H2 loss and subsequent
decomposition in C6D5Br or CD3CN solutions over the course
of 12 h at room temperature. On the other hand, storage of 3
under an atmosphere of H2 prolonged stability in solution for
several days. In contrast, complex 4, where the N–H proton is
replaced by N–Me, showed increased stability as it remains
stable in solution for several days.
Compounds 3 and 4 react with an atmosphere of ethylene
to form the new species 5 and 6, respectively. In each case, the
1H NMR spectra show diagnostic triplet and quartet reson-
ances at −0.49, −1.08 ppm and −0.44, −0.98 ppm for 5 and 6,
respectively. The corresponding 31P{1H} NMR spectra displayed
a small shift of the singlet resonances to 33.4 and 34.2 ppm.
These NMR data were consistent with the formulations of 5
and 6 as RN(1,2-CH2CH2NvPPh3)2NiEt][PF6] (R = H 5, Me 6)
(Scheme 1). While 5 and 6 were fully characterized spectro-
scopically, attempts to isolate 5 resulted in the recovery of the
nickel hydride 3 while efforts to isolate 6 gave a mixture of 4
and 6. This difficulty is consistent with the expected facile
β-hydride elimination of ethylene affording regeneration of the
Ni-hydride species.
Upon heating 3 to 80 °C for 3 hours, the Ni-hydride reso-
nance was lost and a new product 7 was observed in low yield in
addition to the generation of nickel-black and an unidentified
paramagnetic species. Although 7 could not be isolated, the
31P{1H} NMR spectrum showed two phosphorus environments
with chemical shifts at 44.7 and 36.7 ppm. In contrast, the
analogous thermolysis of 4, afforded the isolation of the
species 8. This species exhibited 31P{1H} NMR signals at 45.7
and 35.9 ppm. In addition, disappearance of the hydride
signal and the presence of four new upfield aromatic protons
(6.24–6.58 ppm) is observed in the 1H NMR spectra. These
data were consistent with the formulation of 8 as the product
arising from the orthometallation of one of the phosphorus-
bound arene rings and thus [MeN(CH2CH2NvPPh3)
(CH2CH2NvPPh2(C6H4))Ni][PF6]. The observation of a mixture
of products from 3 suggests thermolytic loss of H2 from 3 and
subsequent degradation of the transient amide species com-
petes with orthometallation. The analogous loss of methane
from 4 is disfavoured and thus cleanly affords the orthometal-
lation product 8.
Treatment of the cationic nickel hydride complex 3 with an
additional equivalent of LiHBEt3 in C6H5Br resulted in the
evolution of H2 and 1H and 11B{1H} NMR spectroscopy that
were consistent with the generation of BEt3, the loss of the
hydride resonance and the appearance of new aryl peaks. The
new product 9 gives rise to 31P{1H} NMR resonances at
35.6 ppm. This, together with the 1H NMR data, confirmed the
formulation of 9 as [HN(1,2-CH2CH2NvPPh3)2Ni(C6H5)][PF6].
An analogous bromide salt was previously prepared from the
reaction from the equimolar reaction of Ni(COD)2 and the
ligand HN(1,2-CH2CH2NvPPh3)2 in bromobenzene.25
Fig. 1 POV-ray depictions of the cations of (a) 3 and (b) 4. Hydrogen atoms
except for the Ni–H and N–H were omitted for clarity. C: black, P: orange, N:
aqua, Ni: teal.
corresponding signal for 4d1 was observed at −28.65 ppm. The
structure of these hydride complexes was further confirmed by
X-ray diffraction (Fig. 1). The molecular structures of 3 and 4
reveal pseudo-square planar geometries at the Ni cations as
well as the PF6 anions and each co-crystallized with one mole-
cule of acetonitrile. In the cation of 3, the Ni–H bond length
refines to 1.45(4) Å, in agreement with previously reported
Ni–H bond lengths.5,17,19,26 The Ni–N bond lengths for the
phosphinimine fragments were found to be 1.891(3) Å and
1.902(3) Å in 3 with an N–Ni–N angle of 168.8(1)°. The central
amine, gives rise to a Ni–N distance of 1.959(3) Å resulting in
N–Ni–N′ angles within the adjacent five-membered rings of
85.36(13)° and 86.31(13)° and a N–Hi–H angle of 173.3(14)°. In
the case of 4, crystallographic symmetry results in a twofold
symmetric cation.‡ Nonetheless, the corresponding geometry
of the cation of 4 is similar to that of 3, with the phosphin-
imine N atoms giving rise to Ni–N bond lengths of 1.895(3) Å
and an N–Ni–N angle of 170.3(2)°. The central amine N atom
gives a Ni–N bond length of 1.965(5) Å, and a N–Ni–N′ angle of
85.1(1)°. Disorder in the methylene groups adjacent N and the
twofold symmetry generates uncertainty in the location of the
Ni-bound hydride and thus no comment can be made in this
case.
These hydride complexes 3 and 4 proved to be highly sensi-
tive to halogenated solvents. Indeed, in presence of CH2Cl2
these species rapidly revert to the chloro-cation precursors 1
and 2. This sensitivity is similar to that observed for previously
In the present reaction, the avenue to effect the observed
hydride for aryl exchange was considered. The addition of an
equivalent of LiHBEt3 to 3 is thought to generate a transient
4238 | Dalton Trans., 2013, 42, 4237–4239
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Notes and references
‡Crystallographic data: In both structures the hydride hydrogen atoms were
located and refined. CCDC: 915323 and 915324.
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Scheme 2 Proposed mechanism of arylation via transient Ni dihydride.
neutral dihydride complex. Reductive loss of H2 would afford a
Ni(0) species that then undergoes oxidative addition of bromo-
benzene affording the Ni-aryl derivative 9 (Scheme 2). In
support of this mechanism, the deuteride analogue 3-d1 was
treated with LiHBEt3. Monitoring the initial stages of the reac-
tion of 3-d1 with LiHBEt3 revealed the appearance of the
hydride peak at −27 ppm attributable to 3. The formation of
HD is observed by the 1H NMR a 1 : 1 : 1 triplet at 4.57 ppm
1
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2
new peak at 7.3 ppm was observed in the H NMR spectrum
indicating deuterium incorporation in a phenyl ring of the
NvPPh3 fragment. This infers that the transient neutral
hydride–deuteride complex can also participate in Ni-D/C-H
exchange via a sigma bond metathesis (Scheme 2).
In summary, cationic nickel hydride complexes stabilized
by phosphinimine based pincer ligands were synthesized by
reaction with LiHBEt3. As postulated, the phosphinimine
ligand stabilizes the hydride moiety by shielding the coordi-
nation site. Nonetheless, these species undergo reversible
ethylene insertions, thermolytic orthometallation of a P-bound
aryl ring to generate a transient nickel dihydride which pro-
vides access to a reactive Ni(0) species. This observed reactivity
demonstrates that residing in a protected pocket, these Ni-
hydride species are quite reactive. Utilizing bis-phosphinimine
ligands to control the reaction pathway of the resulting metal
complexes continues to be of interest in our laboratories.
Financial support of NSERC of Canada is gratefully
acknowledged. DWS is grateful for the award of a Canada
Research Chair.
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Dalton Trans., 2013, 42, 4237–4239 | 4239