Synthesis, properties, and reactivity with carbon dioxide of (allyl) 2Ni(L) complexes

Article abstract of DOI:10.1021/om2002238

A family of nickel allyl complexes of the type (allyl)₂Ni(L) (L = PMe₃ (1), PEt₃ (2), PPh₃ (3), NHC (4); NHC = 1,3-bis(2,6-diisopropylphenyl)-4,5-dihydro-2H-imidazol-2-ylidene) and (2-methylallyl)2Ni(L) (L = PMe₃ (5), PEt₃ (6), PPh ₃ (7), NHC (8)) have been prepared using literature methods. Compounds 5, 7, and 8 were characterized by X-ray crystallography. Whereas compounds 5 and 7 are 18-electron species with two n³-allyl ligands, the NHC-supported complexes are 16-electron species with one n¹- and one n³-allyl ligand. Using DFT we have identified the key factors in predicting whether complexes of the type (allyl)2M(L) (M = Ni, Pd) are 16- or 18-electron species. Compounds 1, 2, and 5-8 readily decompose to give a mixture of products, while compound 4 decomposes to give the unusual Ni0 species (n⁴-1,5-hexadiene)Ni(NHC) (9), which was characterized by X-ray crystallography. The reactions of 1-8 with CO₂ were investigated. Compounds 5-8 react rapidly with CO₂ at low temperature to form well-defined unidentate nickel carboxylates of the type (n³-2- methylallyl)Ni(OC(O)C₄H₇)(L) (L = PMe₃ (10), PEt₃ (11), PPh₃ (12), NHC (13)). The structure of 13 was elucidated using X-ray crystallography. In contrast, compounds 1-4 do not react with CO₂.We believe that the difference in reactivity between the 2-methylallyl-supported complexes and the allyl-supported complexes is related to the mechanism of CO₂ insertion.

Full text of DOI:10.1021/om2002238

ARTICLE
pubs.acs.org/Organometallics
Synthesis, Properties, and Reactivity with Carbon Dioxide of
(allyl)2Ni(L) Complexes
Jianguo Wu, Nilay Hazari,* and Christopher D. Incarvito
The Department of Chemistry, Yale University, P.O. Box 208107, New Haven, Connecticut 06520, United States
S Supporting Information
b
ABSTRACT: A family of nickel allyl complexes of the type
(allyl)2Ni(L) (L = PMe3 (1), PEt3 (2), PPh3 (3), NHC (4);
NHC = 1,3-bis(2,6-diisopropylphenyl)-4,5-dihydro-2H-imida-
zol-2-ylidene) and (2-methylallyl)2Ni(L) (L = PMe3 (5), PEt3
(6), PPh3 (7), NHC (8)) have been prepared using literature
methods. Compounds 5, 7, and 8 were characterized by X-ray
crystallography. Whereas compounds 5 and 7 are 18-electron species with two η3-allyl ligands, the NHC-supported complexes are
16-electron species with one η1- and one η3-allyl ligand. Using DFT we have identified the key factors in predicting whether
complexes of the type (allyl)2M(L) (M = Ni, Pd) are 16- or 18-electron species. Compounds 1, 2, and 5ꢀ8 readily decompose to
give a mixture of products, while compound 4 decomposes to give the unusual Ni0 species (η4-1,5-hexadiene)Ni(NHC) (9), which
was characterized by X-ray crystallography. The reactions of 1ꢀ8 with CO2 were investigated. Compounds 5ꢀ8 react rapidly with
CO2 at low temperature to form well-defined unidentate nickel carboxylates of the type (η3-2-methylallyl)Ni(OC(O)C4H7)(L) (L
= PMe3 (10), PEt3 (11), PPh3 (12), NHC (13)). The structure of 13 was elucidated using X-ray crystallography. In contrast,
compounds 1ꢀ4 do not react with CO2. We believe that the difference in reactivity between the 2-methylallyl-supported complexes
and the allyl-supported complexes is related to the mechanism of CO2 insertion.
’ INTRODUCTION
3-bis(2,6-diisopropylphenyl)-4,5-dihydro-2H-imidazol-2-ylidene),
which contain one η1- and one η3-2-methylallyl ligand. All of
these species rapidly insert CO2 at low temperature to form the
unidentate carboxylate species VꢀVIII. Using a combination of
computational and experimental techniques, we performed me-
chanistic studies on the carboxylation reaction. We proposed that
the reaction of η1-Pd allyls with CO2 does not proceed via direct
insertion of CO2 into the PdꢀC bond but through nucleophilic
attack of the terminal olefin on electrophilic CO2, followed by
ligand substitution to give the O-bound product (Scheme 1). As a
result only nucleophilic η1-allyls will react with CO2 and electro-
philic η3-allyls are inert. Wendt and co-workers reached a similar
conclusion by performing mechanistic studies on a system in
which a PCP pincer ligand supported an η1-Pd allyl (Scheme 1).26
We subsequently demonstrated that catalysts based on VIII were
someof the most active for thecarboxylationof allylstannanes with
CO2 and were also able to carboxylate allylboranes.22
Although Ni is significantly cheaper than Pd (the cost of pure
Ni is approximately 1000 times cheaper than the cost for pure Pd
on May 1, 2011, on the London Metal Exchange) and there is
significant precedent for the reaction of Ni(0) with CO2,3 there
has been little work investigating the reactions of Ni allyls with
CO2. In fact, studies of the insertion of CO2 into NiꢀR σ-bonds
(R = H, alkyl) remain rare.20,28,29 Early work by Jolly and co-
workers in the late 1970s established that CO2 could react with
(2-methylallyl)2Ni(L) (L = PMe3, PCy3) but the products of the
The potential environmental consequences of continuing to
utilize fossil fuels as an energy source and the decline in our
petroleum reserves has led to considerable research into alter-
native carbon sources.1 CO2 is a particularly attractive feedstock
for the synthesis of both commodity chemicals and complex
organic molecules, due to its low toxicity and cost, high abun-
dance, and relative ease of transport.2ꢀ9 However, the catalytic
conversion of CO2 into value-added products is complicated by
its high kinetic and thermodynamic stability. One approach for
the functionalization of CO2 is to utilize transition metals to
weaken the strong CdO double bond, and several transition-
metal complexes have been developed as catalysts for carboxyla-
tion reactions.10ꢀ22 The key step in many of these cycles is
proposed to involve the insertion of CO2 into a metalꢀcarbon
bond, and one of the most well-studied and successful reactions
involves the insertion of CO2 into Pdꢀallyl bonds. A number of
catalytic cycles are proposed to involve the formation of Pd
carboxylates from the reactions of Pd allyl species with CO2.
These include systems for the coupling of CO2 with butadiene to
form lactones,23ꢀ25 the carboxylation of allylstannanes10,13
and -allenes,15 and the carboxylative coupling of allylstannanes
with allyl halides.11
In order to further understand the carboxylation of allylstannanes,
both Wendt's and our group have recently reported studies
describing the mechanism of CO2 insertion into monomeric Pd
η1-allyls.13,26,27 Our group initially prepared a family of thermally
sensitive Pd allyl complexes of the type (2-methylallyl)2Pd(L)
(L = PMe3 (I), PEt3 (II), PPh3 (III), NHC (IV); NHC = 1,
Received: March 13, 2011
Published: May 16, 2011
r
2011 American Chemical Society
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Scheme 1
reaction were not well characterized.30 Subsequently, Tsuda and
co-workers demonstrated that (allyl)2Ni could be reacted with
CO2 to form five-membered lactones in polar solvents.31 Re-
cently, we demonstrated that CO2 could insert into a well-
defined Ni PCP η1-allyl to form an η1-carboxylate and presented
evidence that this reaction proceeds through a mechanism
similar to that described for η1-Pd allyls (Scheme 1).32 Unfortu-
nately, these Ni species were not active catalysts for the carbox-
ylation of allylstannanes and allylboranes. In this work, we
synthesize a series of complexes of the type (2-methylallyl)2Ni-
(L), including those supported by NHC ligands, and explore
their structures and decomposition pathways. We build on Jolly’s
preliminary results and demonstrate that these species react
cleanly with CO2 to form η1-carboxylates. Furthermore, we
use our new results to make comparisons about CO2 insertion
into related Pd and Ni systems and draw conclusions about the
design of future Ni catalysts for CO2 reduction.
species with two η3-allyl ligands.33 They also performed low-
1
temperature H NMR spectroscopy on 1 at ꢀ110 °C, which
suggested that the solution-state structure was consistent with
that observed in the solid state.33 In contrast, even at ꢀ80 °C
only two broad peaks were observed for the allyl or 2-methylallyl
groups in the 1H NMR spectra of 2ꢀ8 and the binding mode of
the allyl or 2-methylallyl ligands was not clear. We recently
prepared the Pd species (2-methylallyl)2Pd(L) (L = PMe3 (I),
PEt3 (I-I), PPh3 (III), NHC (IV)), and in all cases the 1H NMR
spectra indicated that at low temperature (ꢀ80 °C) the η1:η3-
isomer of the complex was present.27 These results were
supported by the solid-state structure of III, which unequivocally
showed one η1- and one η3-2-methylallyl ligand.27
’ RESULTS AND DISCUSSION
Synthesis and Structures of (allyl)2Ni(L) Complexes. Using
the route first described by Jolly and co-workers,33 a family of
complexes of the type (allyl)2Ni(L) (L = PMe3 (1), PEt3 (2)-
PPh3 (3), NHC (4); NHC = 1,3-bis(2,6-diisopropylphenyl)-4,5-
dihydro-2H-imidazol-2-ylidene) and (2-methylallyl)2Ni(L) (L =
PMe3 (5), PEt3 (6), PPh3 (7), NHC (8)) were prepared through
the reaction of (allyl)2Ni or (2-methylallyl)2Ni with the appro-
priate free ligand at low temperature (eq 1). Although the
phosphine-supported complexes had previously been prepared
by Jolly et al.,33 to the best of our knowledge this is the first
example of the preparation of NHC-supported complexes of this
type. Complexes 1ꢀ7 were all unstable in solution at room
temperature and displayed highly fluxional 1H NMR spectra. The
1H NMR spectrum of compound 8 was also highly fluxional, but
8 could be left at room temperature for days with minimal
decomposition. Jolly et al. had previously crystallized
(allyl)2Ni(PMe3) (1) and demonstrated that it is an 18-electron
In order to properly compare the Pd and Ni systems, we were
interested in gaining greater clarity about the structures of 1ꢀ8.
We were able to crystallize compounds 5, 7, and 8 from saturated
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Figure 1. (a) X-ray structure of 5 (hydrogen atoms omitted for clarity). Selected bond lengths (Å) and angles (deg): Ni(1)ꢀP(1) 2.2517(7),
Ni(1)ꢀC(1) = 2.063(4), Ni(1)ꢀC(2) = 2.015(3), Ni(1)ꢀC(3) = 2.060(3), Ni(1)ꢀC(5) = 2.057(3), Ni(1)ꢀC(6) = 2.018(2), Ni(1)ꢀC(7) =
2.057(3), C(1)ꢀC(2) = 1.403(4), C(2)ꢀC(3) = 1.407(5), C(2)ꢀC(4) = 1.506(4), C(5)ꢀC(6) = 1.405(4), C(6)ꢀC(7) = 1.402(5), C(6)ꢀC(8) =
1.507(4); Ni(1)ꢀC(1)ꢀC(2) = 68.08(19), Ni(1)ꢀC(2)ꢀC(3) = 71.52(17), Ni(1)ꢀC(5)ꢀC(6) = 68.36(16), Ni(1)ꢀC(6)ꢀC(7) = 72.08(17). (b)
X-ray structure of 7 (hydrogen atoms omitted for clarity). Selected bond lengths (Å) and angles (deg): Ni(1)ꢀP(1) = 2.2925(9), Ni(1)ꢀC(1) =
2.083(3), Ni(1)ꢀC(2) = 2.024(3), Ni(1)ꢀC(3) = 2.041(3), Ni(1)ꢀC(5) = 2.091(3), Ni(1)ꢀC(6) = 2.025(2), Ni(1)ꢀC(7) = 2.043(3), C(1)ꢀC(2) =
1.397(4), C(2)ꢀC(3) = 1.399(5), C(2)ꢀC(4) = 1.499(5), C(5)ꢀC(6) = 1.389(4), C(6)ꢀC(7) = 1.399(4), C(6)ꢀC(8) = 1.515(5); Ni(1)ꢀ
C(1)ꢀC(2) = 67.83(19), Ni(1)ꢀC(2)ꢀC(3) = 70.5(2), Ni(1)ꢀC(5)ꢀC(6) = 67.73(14), Ni(1)ꢀC(6)ꢀC(7) = 70.56(15). (c) X-ray structure of 8
(hydrogen atoms omitted for clarity). Selected bond lengths (Å) and angles (deg): Ni(1)ꢀC(1) = 1.889(3), Ni(1)ꢀC(28) = 1.966(3), Ni(1)ꢀC(32) =
2.058(4), Ni(1)ꢀC(33) = 2.003(3), Ni(1)ꢀC(34) = 2.001(4), C(28)ꢀC(29) = 1.467(5), C(29)ꢀC(30) = 1.354(8), C(29)ꢀC(31) = 1.388(7),
C(32)ꢀC(33) = 1.394(6), C(33)ꢀC(34) = 1.400(5), C(33)ꢀC(35) = 1.496(5); Ni(1)ꢀC(28)ꢀC(29) = 121.7(2), Ni(1)ꢀC(32)ꢀC(33) =
67.81(19), Ni(1)ꢀC(33)ꢀC(34) = 69.45(17).
solutions of pentane at low temperature (Figure 1). Whereas
compounds 5 and 7 are both square-pyramidal 18-electron
species with two η3-2-methylallyl ligands, compound 8 is a
square-planar 16-electron species with one η1- and one η3-
2-methylallyl ligand. The structures of 5 and 7 are similar and
feature a syn arrangement of the 2-methylallyl ligands. The
bonds of the Ni to the two 2-methylallyl ligands are virtually
identical, and each 2-methylallyl ligand is symmetrically bound
to the metal center. For example, in the structure of 5, the
Ni(1)ꢀC(1) distance of 2.063(4) Å is the same (within
experimental error) as the Ni(1)ꢀC(3) distance of 2.060(3)
Å. Unsurprisingly, the Niꢀphosphorus distance is longer in 7
(Ni(1)ꢀP(1) = 2.2925(9) Å), which contains the less donating
and more sterically bulky PPh3 ligand, in comparison with the
Niꢀphosphorus distance in 5 (Ni(1)ꢀP(1) = 2.2517(7) Å),
which contains the PMe3 ligand. The structures of 5 and 7
are analogous to that reported for 1 by Jolly,33 but the 18-
electron square-pyramidal structure for 7 stands in contrast to
the 16-electron square-planar structure observed for its direct
Pd analogue III.27
The structure of the NHC-supported complex 8 unambigu-
ously shows that the molecule contains one η1- and one η3-
2-methylallyl ligand. In fact, 8 is a rare example of a crystal-
lographically characterized Ni η1-allyl.32,34 The Niꢀη1-allyl
bond length is 1.966(3) Å, which is significantly shorter than
the Niꢀη1-allyl bond length observed in a pincer-supported
complex (in (PCP)Ni(η1-allyl), which contains two indepen-
dent molecules in the unit cell, the Niꢀallyl bond lengths are
2.094(5) and 2.071(6) Å) presumably because of the strong trans
influence of the anionic carbon in the pincer ligand.32 However,
the bond length is comparable with the only other example of a
Ni complex which features two different coordination modes of
an allyl ligand (the Niꢀη1-allyl bond lengths in [Li(tmeda)]þ-
[(η3-C3H5)Ni(η1-C3H5)2]ꢀ are 1.953(4) and 1.979(3) Å).34
The Niꢀη3-allyl bond lengths are comparable to those observed
in 5 and 7, although the 2-methylallyl ligand is no longer
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symmetrically bound. The Ni(1)ꢀC(32) bond length, 2.058(4)
Å, is longer than the Ni(1)ꢀC(34) bond length, 2.001(4) Å,
seemingly because C(32) is trans to the η1 2-methylallyl ligand,
which exerts a stronger trans influence than the NHC ligand,
which is trans to C(34). Overall, the structure of 8 is similar
to that described for (η1-2-methylallyl)(η3-2-methylallyl)Pd-
(PPh3) (III).27
Our experimental results clearly indicate that, depending on
the metal, ancillary ligand, and substitution of the allyl ligand,
complexes of the type (allyl)2M(L) (M = Ni, Pd) can adopt
either a 16-electron square-planar or 18-electron square-pyrami-
dal geometry. In order to further probe the factors that cause
these species to have either two η3-allyl ligands or one η1- and
one η3-allyl ligand, a series of DFT calculations were performed.
The structures of the molecules that were calculated are shown in
Figure 2, and the relative energies are given in Table 1. In the case
of the bis η3:η3-allyl isomers, both anti and syn isomers were
calculated. It should be noted that we have used a simplified
NHC ligand to model 4 and 8; however, we do not believe that
steric factors cause large changes to our results.
Figure 2. Model compounds used for computational calculations.
Table 1. Relative SCF Energies of Different Isomers of
(allyl)2M(L) and (2-methylallyl)2M(L) (M = Ni, Pd)a
compd
ligand
PH3
metal
allyl
η3-syn
η3-anti
η1
Ni
Pd
Ni
Pd
Ni
Pd
Ni
Pd
Ni
Pd
C3H5
C3H5
C3H5
C3H5
C3H5
C3H5
C3H5
C3H5
C3H5
C3H5
0
10.0
b
7.63
0
27.6
0
In general, the calculated energies are in excellent agreement
with our experimental results and good agreement was obtained
between calculated and experimental geometries. The only case
where the calculated energy suggests that an isomer which was
not experimentally observed is the most stable isomer, is for
(2-methylallyl)2Ni(PMe3) (5). For 5, calculations suggest that
the 16-electron complex is more stable than the syn 18-electron
complex by 9.4 kJ molꢀ1, even though the X-ray structure
showed two syn η3-2-methylallyl ligands. In this case the
calculated difference in energy is small and can probably be
attributed to either error associated with the calculation or
crystal-packing effects. All complexes show a preference for the
syn 18-electron configuration over the anti 18-electron config-
uration, which is also consistent with experimental results.
On the basis of both our experimental and computational
trends we believe that the following conclusions can be made
about the preferred geometry and properties of complexes of
type (allyl)2M(L) (M = Ni, Pd). (i) Electron-donating ancillary
ligands increase the electron richness of the metal center and
promote the formation of 16-electron η1:η3-allyl complexes,
rather than 18-electron η3:η3-allyl complexes. Thus, 16-electron
complexes are favored for NHC-supported species compared
with phosphine-supported species. (ii) The 2-methylallyl ligand
inductively increases the donating power of the allyl ligand, and
hence 16-electron compounds are more favorable for 2-methy-
lallyl ligands than for allyl ligands. On this basis we suggest that if
an even more strongly electron donating group were present at
the 2-position (i.e. OMe), a 16-electron compound would
become even more favorable. (iii) A 16-electron isomer is more
favored for Pd than for Ni. We believe this is related to the fact
that Pd forms stronger metalꢀligand bonds and therefore the
olefin of the second allyl group is less likely to coordinate. (iv) In
the systems that we studied, the energy difference between the
different isomers is generally significantly larger for Pd compared
with Ni. This could explain why it is possible to freeze out the
lowest energy conformation using 1H NMR spectroscopy for Pd
complexes,27 but for Ni complexes even at low temperatures
fluxional NMR spectra are observed.
PMe3
PEt3
14.2
56.5
15.5
57.5
10.2
b
2.10
0
43.5
3.20
40.1
0
0
0
PPh3
NHC0
1.31
0
36.5
29.1
67.2
49.6
84.2
0
0
PH3
Ni
Pd
Ni
Pd
Ni
Pd
Ni
Pd
Ni
Pd
C4H7
C4H7
C4H7
C4H7
C4H7
C4H7
C4H7
C4H7
C4H7
C4H7
1.63
33.7
9.43
45.0
11.6
42.4
0
5.10
b
0
0
PMe3
PEt3
20.3
b
0
0
22.9
b
0
0
PPh3
NHC0
6.22
b
0.70
0
35.7
37.8
72.9
48.3
0
85.7
0
a Relative energies are given in kJ molꢀ1, with the lowest energy
structures assigned as being at 0 kJ molꢀ1
optimized structure was not achieved.
.
b Convergence on an
contrast, Jolly showed that 3 decomposes to give a Ni0 species
containing a bidentate 1,5-hexadiene ligand. We observed that at
room temperature complexes 1, 2, and 5ꢀ7 all decomposed to
give a mixture of products. Either 1,5-hexadiene or 2,6-dimethyl-
1,5-hexadiene was detected by 1H NMR spectroscopy in all cases,
but the metal-containing products could not be identified. The
NHC-supported complex 8 is significantly more stable than
compounds 5ꢀ7 and decomposed very slowly at room tempera-
ture to give a complex mixture. On the other hand, the
unsubstituted NHC-supported compound 4 decomposed
cleanly to give a single product (eq 3). On the basis of 1H and
13C NMR spectroscopy we propose that this complex is the Ni0
hexadiene species 9. Unlike compound 4, the room-temperature
1H NMR spectrum of 9 was not fluxional and displayed three
separate signals for the six protons corresponding to the co-
Decomposition of (allyl)2Ni(L) Complexes. It is relatively
well-known that compounds of the type (allyl)2Pd(L) and
(2-methylallyl)2Pd(L) (L = PR3, NHC) decompose readily in
solution to give PdI bridging allyl dimers (eq 2).33,35,36 In
1
ordinated olefins. The H NMR spectrum showed that the
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resonance corresponding to the central allyl proton in 4 was
shifted from δ 5.05 to 4.10 ppm when it became part of the
coordinated olefin in 9. The carbene resonance in the 13C
spectrum was observed at δ 206.1 ppm, which is considerably
further downfield than the shift in 4 (δ 198.2 ppm), consistent
with the decrease in oxidation state from 4 to 9.
Figure 3. X-ray structure of 9 (hydrogen atoms omitted for clarity).
Selected bond lengths (Å) and angles (deg): Ni(1)ꢀC(1) = 1.916(3),
Ni(1)ꢀC(28) = 2.045(4), Ni(1)ꢀC(29) = 2.0022(5), Ni(1)ꢀC(32) =
1.981(6), Ni(1)ꢀC(33) = 2.022(5), C(28)ꢀC(29) = 1.362(6), C-
(29)ꢀC(30) = 1.543(8), C(30)ꢀC(31) = 1.553(11), C(31)ꢀC(32) =
1.477(10), C(32)ꢀC(33)
=
1.416(7); C(1)ꢀNi(1)ꢀC(28)
=
98.70(13), C(1)ꢀNi(1)ꢀC(29) = 137.83(16), C(1)ꢀNi(1)ꢀC(32) =
143.12(18), C(1)ꢀNi(1)ꢀC(33) = 101.82(16), C(28)ꢀNi(1)ꢀ
C(32) = 117.5(2), C(28)ꢀNi(1)ꢀC(33) = 157.48(20).
Further confirmation of the structure of 9 was obtained
through X-ray crystallography (Figure 3). In the solid state the
geometry around the pseudo-three-coordinate Ni is approxi-
mately trigonal planar. The hexadiene ligand is clearly bound to
Ni in an η4 fashion, and the CꢀC bond lengths in the 1,
5-hexadiene unit show that the olefins are in terminal positions
with a saturated linker. The CꢀC bond lengths of the coordi-
nated olefins (C(28)ꢀC(29) = 1.362(6) Å and C(32)ꢀC(33) =
1.416(7) Å) are consistent with moderate back-donation from
the Ni to the olefins and indicate that the structure is
unsymmetric.37 Surprisingly, the NiꢀNHC bond length in 9
(Ni(1)ꢀC(1) = 1.916(3) Å) is similar to that observed in 8,
despite the change in formal oxidation state at the metal center.
In general transition-metal complexes containing coordinated
1,5-hexadiene units are rare.38ꢀ42 Two previous examples of
structurally characterized Ni0 species containing a coordinated
η4-1,5-hexadiene are known, but these complexes are both 18-
electron species featuring two additional L type donors alongside
the hexadiene.43,44 However, a 16-electron Pt complex featuring
one η4-1,5-hexadiene ligand and a coordinated maleic anhydride
group is known.45 The structural parameters observed in that
complex are similar to those observed in 9.
At this stage it is unclear why the Ni species 4 decomposes to
give the monomeric Ni0 species 9, whereas the analogous Pd
complex (allyl)2Pd(NHC) decomposes to give a dimeric PdI
species with bridging allyl ligands.36 It should be noted that
extended heating of the PdI bridging allyl dimers does eventually
lead to the formation of Pd0 but there is no evidence to suggest
that a dimeric species is an intermediate in the formation of 9.46
Reactions of (allyl)2Ni(L) Complexes with CO2. Compounds
5ꢀ8 all reacted with CO2 at low temperature (eq 4) to cleanly
form the unidentate carboxylate species 10ꢀ13, which were fully
characterized. These reactions are similar to those described for
analogous Pd systems (IꢀIV), and the rates of reaction are
comparable.27 IR spectroscopy indicated that in all cases the
difference between the symmetric and asymmetric CO2 stretch
was greater than 200 cmꢀ1, which is consistent with the presence
of a unidentate carboxylate.47 The structure of 13 was elucidated
by X-ray crystallography, which unequivocally confirmed the
presence of both an η3-allyl and a unidentate carboxylate
(Figure 4). The coordination geometry around Ni is square
planar, and the bond lengths and angles are unsurprising. The η3-
allyl ligand is again bound to Ni in an asymmetric fashion due to
the differing trans influences of the NHC and carboxylate ligands,
with the NHC being the stronger donor.
The resonances associated with the terminal hydrogens of the
1
2-methylallyl ligands in the H NMR spectra of 10ꢀ13 are all
fluxional. At low temperature (ꢀ70 °C) four distinct resonances
are observed for the terminal protons in compounds 10, 11, and
13, while for the PPh3-supported complex 12 no resonances are
visible. At room temperature one broad peak integrating to four
protons is observed for 10, two broad peaks each integrating to
two protons each are observed for 11 and 12, and none of the
terminal protons of the 2-methylallyl ligand can be detected for
13, as they have presumably broadened into the baseline.
Unfortunately, the spectra were too complicated for line shape
analysis to be used to accurately determine the rates of the
fluxional processes and investigate the mechanism. However, it
appears that two processes are occurring: an initial process which
causes the four inequivalent protons to coalesce into two sets of
two protons, followed by another process requiring more energy
which results in all four protons becoming equivalent. Further
support for this hypothesis is obtained by heating samples of 11
and 12 to 80 °C, which causes coalescence of the two sets of
protons, so that only one broad peak integrating to four protons
is observed. Presumably if the PPh3-supported complex could be
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Organometallics
ARTICLE
than with Pd and will require the use of transmetalating agents
that form extremely strong metalꢀoxygen bonds.
’ CONCLUSIONS
Figure 4. X-ray structure of 13 (hydrogen atoms and methyl groups of
NHC ligand omitted for clarity). Selected bond lengths (Å) and angles
(deg): Ni(1)ꢀC(1) = 1.921(2), Ni(1)ꢀC(28) = 1.970(4), Ni(1)ꢀC-
(29) = 1.985(3), Ni(1)ꢀC(30) = 2.072(3), Ni(1)ꢀO(1) = 1.9136(20),
C(28)ꢀC(29) = 1.406(4), C(29)ꢀC(30) = 1.393(4), C(29)ꢀC(31) =
1.504(4), O(1)ꢀC(32) = 1.290(3), C(32)ꢀO(38) = 1.227(3), C-
(32)ꢀC(33) = 1.529(4), C(33)ꢀC(34) = 1.504(4), C(34)ꢀC(35) =
We have prepared a series of complexes of the type
(allyl)2Ni(L) and (2-methylallyl)2Ni(L), including the first
species supported by NHC ligands. NMR spectroscopy shows
that these complexes are fluxional, while X-ray crystallography
demonstrates that they can either be 18-electron species with two
η3-allyl ligands or 16-electron species with one η1- and one
η3-allyl ligand. In general the 16-electron configuration is more
likely to be preferred for complexes with strongly donating
ancillary ligands, for complexes with methyl groups on the
2-position of the allyl ligand compared with an unsubstituted
allyl ligand, and for Pd relative to Ni. In fact, to the best of our
knowledge all known Pd species adopt the 16-electron confor-
mation, while a mixture of 16- and 18-electron species is ob-
served for Ni. Another difference between the properties of
(allyl)2Ni(L) and (2-methylallyl)2Ni(L) complexes in compar-
ison with those of their Pd analogues is that Pd species decom-
pose relatively cleanly to give PdI dimers, while Ni species
decompose to give either monomeric Ni0 hexadiene species or
complex mixtures. This suggests that, at least in the cases where
clean reactions occur, the Pd decomposition pathway is bimole-
cular, whereas the Ni pathway is unimolecular. At this stage
further studies are required to fully understand the mechanistic
pathway by which decomposition occurs.
The reactivity of complexes of the type (2-methylallyl)2Ni(L)
with CO2 is similar to that previously reported for related Pd
species.27 The trend in the rate of reaction as the ancillary ligand
is varied is identical, and we propose that the reaction follows the
same mechanistic pathway. Unfortunately the unidentate car-
boxylate complexes formed from the insertion of CO2 into
(2-methylallyl)2Ni(L) do not undergo transmetalation to regen-
erate (2-methylallyl)2Ni(L) species. This prevents (2-methyl-
allyl)2Ni(L) species from being used as catalysts for the carbox-
ylation of allylstanannes and allylboranes using CO2. If Ni
catalysts for these reactions are to be developed, it appears that
alternative methods for the liberation of the carboxylate fragment
will need to be explored.
1.489(4), C(34)ꢀC(36)
=
1.317(5); Ni(1)ꢀC(28)ꢀC(29)
=
69.75(19), Ni(1)ꢀC(29)ꢀC(30) = 73.32(18), Ni(1)ꢀO(1)ꢀC(32) =
127.71(16), C(1)ꢀNi(1)ꢀC(28) = 94.30(11), C(30)ꢀNi(1)ꢀO(1) =
97.79(11), O(1)ꢀNi(1)ꢀC(1) = 96.07(8).
cooled past ꢀ70 °C, four separate resonances would eventually
be observed.
The effect on rate of varying the ancillary ligand for CO2
insertion into 5ꢀ8 is similar to that observed for Pd.27 Thus, the
rate of reaction increases as the ancillary ligand becomes more
electron rich; therefore, PPh3 < PEt3 < PMe3 < NHC. Our
previous studies on CO2 insertion into Pd allyl complexes IꢀIV
and a pincer-supported Ni η1-allyl species suggest that CO2
insertion occurs via nucleophilic attack of the terminal olefin of
the η1-allyl on electrophilic CO2 (Scheme 1).27,32 Our results
with complexes 5ꢀ8 are consistent with this mechanism, and in
all cases we believe that reaction occurs from the nucleophilic
η1:η3-2-methylallyl isomer. Further support for this mechanism
is provided by the observation that the unsubstituted allyl
complexes 1ꢀ4 do not react with CO2. At temperatures
aroundꢀ10°C decomposition of1ꢀ4 occurs beforeany insertion
of CO2. In our proposed mechanism the barrier for CO2 insertion
into an unsubstituted allyl is significantly higher than for the
2-methylallyl ligands because a secondary carbocation is formed in
the transition state rather than a tertiary carbocation.27
In the case of the Pd carboxylate complexes VꢀVIII, treat-
ment with allylstannanes or allylboranes results in transmetala-
tion and the regeneration of the starting bis(allyl) complexes
IꢀIV and a tin or boron carboxylate.22 This reaction is a key
component of our catalytic cycle for the carboxylation of
allylstannanes and allylboranes with CO2. In contrast, no reac-
tion is observed between the Ni carboxylates 10ꢀ13 and either
tri-n-butyl(2-methylallyl)stannane or (pincaol)(2-methylallyl)-
borane, even at elevated temperatures. Calculations using
trimethyl(2-methylallyl)stannane and the Pd complex V and
the Ni complex 10 show that, whereas transmetalation is favor-
able for Pd, it is unfavorable for Ni (eq 5). This suggests that
achieving catalytic reactions with Ni will be significantly harder
’ EXPERIMENTAL DETAILS
General Methods. Experiments were performed under a dinitro-
gen atmosphere in an M. Braun drybox or using standard Schlenk
techniques. (Under standard glovebox conditions purging was not
performed between uses of petroleum ether, diethyl ether, benzene,
toluene, and tetrahydrofuran; thus, when any of these solvents were used,
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Organometallics
ARTICLE
traces of all these solvents were in the atmosphere and could be found
intermixed in the solvent bottles.) Moisture- and air-sensitive liquids were
transferred by stainless steel cannula on a Schlenk line or in a drybox. The
solvents for air- and moisture-sensitive reactions were dried by passage
through a column of activated alumina followed by storage under dinitro-
gen. All commercial chemicals were used as received, except where noted.
Anhydrous nickel chloride was purchased from Alfa Aesar, while PMe3,
PEt3, andPPh3 were purchased from Aldrich or Strem. Anhydrous CO2 was
obtained from Airgas, Inc. and was not dried prior to use. C6D6 and toluene-
d8 were dried over sodium metal. NMR spectra were recorded on Bruker
AMX-400 and -500 spectrometers at ambient probe temperatures unless
noted. Chemical shifts are reported with respect to residual internal protio
solvent for 1H and 13C{1H} NMR spectra and to an external standard for
31P spectra (85% H3PO4 at 0.0 ppm). Atom numbering for the peak
assignments is given below. All assignments are based on two-dimensional
1H,13C-HMQC and HMBC experiments. IR spectra were measured using a
Diamond Smart Orbit ATR on a Nicolet 6700 FT-IR instrument.
Robertson Microlit Laboratories, Inc. performed the elemental analyses
(inert atmosphere). Literature procedures were followed to prepare the
following compounds: 1,3-bis(2,6-diisopropylphenyl)-4,5-dihydro-2H-
imidazol-2-ylidene,48 (allyl)2Ni,49 (2-methylallyl)2Ni,49 (allyl)2Ni-
(PMe3) (1),33 (allyl)2Ni(PEt3) (2),33 (allyl)2Ni(PPh3) (3),33 (2-
methylallyl)2Ni(PMe3) (5),33 (2-methylallyl)2Ni(PEt3) (6),33 and (2-
methylallyl)2(PPh3)33. Compounds 5 and 7 were characterized by X-ray
crystallography, and the single crystals were grown from a saturated
pentane solution at ꢀ35 °C.
stirred for 2 h. The resulting precipitate was collected by filtration and
dried at ꢀ40 °C to give 4 as an orange solid. Yield: 38 mg (81%). This
compound is thermally unstable and was stored at ꢀ30 °C (no
elemental analysis was obtained due to the thermal instability). The
1H NMR spectrum of 4 is provided in the Supporting Information.
1H NMR (500 MHz, toluene-d8, ꢀ70 °C): δ 7.14 (d, 4H, H13 and
H15), 6.96 (m, 2H, H14, overlapping with solvent peak), 6.13 (s, 2H,
H10), 5.05 (s, 2H, allyl-central H), 3.16 (br s, 4H, H17 and H21), 2.70
(br s, 8H, allyl-end H), 1.36 (br s, 12H, H18, H19, H20 or H22), 1.05
(br s, 12H, H18, H19, H20, or H22). 13C{1H} NMR (125.8 MHz,
toluene-d8, ꢀ70 °C): δ 198.2 (s, C23), 145.9 (s, C11), 137.0 (s, C12 and
C16), 130.2 (s, C13 and C15), 125.3 (s, allyl-central C, overlapping with
solvent peak), 124.4 (s, C14), 123.9 (s, C10), 57.4 (br s, allyl-end C),
29.1 (s, C17 and C21), 26.6 (s, C18, C19, C20, or C22), 23.0 (s, C18,
C19, C20, or C22).
(η1-2-methylallyl)(η3-2-methylallyl)Ni(NHC) (8). A solution of NHC
(64.5 mg, 0.17 mmol) in toluene (0.5 mL) was added to a stirred
solution of (2-methylallyl)2Ni (28 mg, 0.17 mmol) in pentane (4 mL)
at ꢀ40 °C. The mixture was stirred for 2 h. The resulting precipitate was
collected by filtration and dried at ꢀ40 °C to give 8 as an orange solid.
Orange single crystals for X-ray analysis were grown from pentane
at ꢀ35 °C. Yield: 78 mg (85%). Anal. Calcd (found) for C35H50N2Ni:
C, 75.41 (75.18); H, 9.04 (8.91); N, 5.02 (5.12).
1H NMR (500 MHz, toluene-d8, ꢀ70 °C): δ 7.14 (d, 4H, H13 and
H15), 6.96 (m, 2H, H14, overlapping with solvent peak), 6.13 (s, 2H,
H10), 5.05 (s, 2H, allyl-central H), 3.16 (br s, 4H, H17 and H21), 2.70
(br s, 8H, allyl-end H), 1.36 (br s, 12H, H18, H19, H20, or H22), 1.05
(br s, 12 H, H18, H19, H20, or H22). 13C{1H} NMR (125.8 MHz,
toluene-d8, ꢀ70 °C): δ 199.1 (s, C23), 146.3 (s, C12), 145.9 (s, C16),
139.0 (s, C11), 137.6 (s, C12), 130.2 (s, C13 and C15), 124.2 (s, C14),
124.0 (s, C10), 121.6 (s, allyl-central C), 36.1 (s, allyl-end C), 29.1
(s, C17), 29.0 (s, C21), 26.6 (s, C18, C19, C20, or C22), 25.6 (s, C18,
C19, C20, or C22), 25.3 (s, Me from allyl), 23.6 (s, C18, C19, C20, or
C22), 23.0 (s, C18, C19, C20, or C22).
(1,5-hexadiene)Ni(NHC) (9). NHC (26 mg, 0.07 mmol) was added to
a stirred solution of (allyl)2Ni (9.3 mg, 0.07 mmol) in toluene (2 mL)
at ꢀ40 °C. The mixture was warmed to room temperature and heated to
40 °C for 3 h. The mixture was evaporated to dryness to give 9 as an
orange powder. Orange single crystals for X-ray analysis were grown
from pentane at ꢀ35 °C. Yield: 32 mg (91%). Anal. Calcd (found) for
C33H46N2Ni: C, 74.87 (72.65); H, 8.76 (8.84); N, 5.29 (5.28). The
1H NMR spectrum of 9 is provided in the Supporting Information.
1H NMR (500 MHz, C6D6): δ 7.18 (t, 8 Hz, 2H, C14), 7.62 (d, J =
8 Hz, 4H, C13 and C15), 6.64 (s, 2H, H10), 4.10 (m, 2H, H2), 3.23
(m, 4H, H17 and H21), 2.61 (m, 2H, CH2), 2.58 (d, J = 9 Hz, 2H, H1),
2.20 (d, J = 13 Hz, 2H, H10), 1.52 (m, 2H, CH2), 1.32 (d, J = 7 Hz, 12H,
12H, H18, H19, H20, or H22), 1.08 (d, J = 7 Hz, 12H, H18, H19, H20,
or H22). 13C{1H} NMR (125.8 MHz): δ 206.1 (s, C23), 146.6 (s, C11),
138.4 (s, C16), 129.7 (s, C14), 128.7 (s, C10), 124.1 (s, C13 and C15),
123.2 (s, C12 and C16), 76.7 (s, C2), 54.4 (s, C1), 36.5 (s, CH2), 29.0
(s, C17 and C21), 26.2 (s, C18, C19, C20, or C22), 23.1 (s, C18, C19,
C20 or C22).
Computational Methods. Density functional calculations were
carried out using Gaussian 09 Revision A.02.50 Calculations were
performed using the B3LYP functional. The LANL2DZ basis set and
pseudopotential were used for Ni, Pd, and Sn, and the 6-31Gþþ(d,p)
basis set was used for all other atoms. Initial geometries were obtained
using the coordinates from X-ray structures, and all optimized structures
were verified using frequency calculations to check that they did not
contain any imaginary frequencies. All energies given in the Results and
Discussion are SCF energies. The coordinates for the optimized
structures are given in the Supporting Information.
X-ray Crystallography. The diffraction experiments were carried
out on a Rigaku Mercury275R CCD (SCX mini) diffractometer with a
sealed tube at 23 °C using graphite-monochromated Mo KR radiation
(λ = 0.710 73 Å). The software used was SMART for collecting frames of
data, indexing reflections, and determination of lattice parameters,
SAINT for integration of intensity of reflections and scaling, SADABS
for empirical absorption correction, and SHELXTL for space group
determination, structure solution, and least-squares refinements on |F|2.
The crystals were mounted at the end of glass fibers and used for the
diffraction experiments. Anisotropic thermal parameters were refined
for the rest of the non-hydrogen atoms. Hydrogen atoms were placed in
their ideal positions. Details of the crystal and refinement data for
complexes 5, 7ꢀ9, and 13 are given in the Supporting Information.
Synthesis and Characterization of New Compounds. The
following numbering scheme is used for the purposes of assigning NMR data
(η3-2-methylallyl)(η1-CO2C4H7)Ni(PMe3) (10). To a solution of
(2-methylallyl)2Ni (235 mg, 1.4 mmol) in toluene (2 mL) at ꢀ78 °C
was added PMe3 (0.15 mL, 1.40 mmol). The mixture was then degassed
using three freezeꢀpumpꢀthaw cycles and warmed to ꢀ40 °C. Excess
1 atm CO2 was then added via a dual-manifold Schlenk line at ꢀ40 °C.
The mixture was slowly warmed to room temperature. After 3 h the
mixture was evaporated to dryness to give 10 as a brown powder. Yield:
390 mg (97%). Anal. Calcd (found) for C12H23O2PNi: C, 49.88
(49.21); H, 8.02 (7.98).
(η1-allyl)(η3-allyl)Ni(NHC) (4). A solution of NHC (32 mg, 0.08
mmol) in toluene (0.5 mL) was added to a stirred solution of (allyl)2Ni
(12.5 mg, 0.08 mmol) in pentane (4 mL) at ꢀ40 °C. The mixture was
IR (cmꢀ1): 1581 (νasym,CO ), 1282 (νsym,CO). 1H NMR (500 MHz,
2
C6D6): δ 5.06 (m, 1H, H8), 4.96 (m, 1H, H80), 3.39 (s, 2H, H6), 2.62 (br
s, 4H, H1 and H3), 2.15 (s, 3H, H9), 1.64 (s, 3H, H4), 0.99 (d, J = 5 Hz,
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9H, PMe3). 13C{1H} NMR (101 MHz, toluene-d8, ꢀ15 °C): δ 176.4
(s, CO2), 143.9 (s, C7), 124.0 (s, C2), 112.7 (s, C8), 69.4 (s, C3), 47.8
(s, C6), 42.6 (s, C1), 24.1 (s, C9), 23.8 (s, C4), 14.0 (d, JCꢀP = 24 Hz,
PCH3). 31P{1H} NMR (121 MHz, C6D6): δ ꢀ11.4 (s).
(allyl)2Ni(PPh3) (3), or (allyl)2Ni(NHC) (4) was dissolved in 0.4 mL
of toluene-d8 in a J. Young NMR tube at ꢀ78 °C. The mixture was then
degassed using three freezeꢀpumpꢀthaw cycles, and excess 1 atm CO2
was added via a dual-manifold Schlenk line at ꢀ78 °C. No reaction was
observed with CO2 before decomposition of the starting material
occurred at approximately ꢀ10 °C.
(η3-2-methylallyl)(η1-CO2C4H7)Ni(PEt3) (11). To a solution of
(2-methylallyl)2Ni (225 mg, 1.33 mmol) in toluene (2 mL)
at ꢀ78 °C was added PEt3 (0.20 mL, 1.33 mmol). The mixture was
then degassed using three freezeꢀpumpꢀthaw cycles and warmed
to ꢀ40 °C. Excess 1 atm CO2 was added via a dual-manifold Schlenk
line at ꢀ40 °C. The mixture was slowly warmed to room temperature.
After 3 h the mixture was evaporated to dryness to give 11 as a brown-
orangeoil. Yield: 430mg(97.5%). Anal. Calcd(found) for C15H29O2PNi:
’ ASSOCIATED CONTENT
S
Supporting Information. Figures giving NMR spectra of
b
selected compounds, tables and CIF files giving details of the
X-ray structures, and tables giving xyz coordinates for optimized
structures. This material is available free of charge via the Internet
at http://pubs.acs.org.
1
C, 54.42 (51.19); H, 8.83 (8.64). The H NMR spectrum of 11 is
provided in the Supporting Information.
IR (cmꢀ1): 1580 (νasym,CO ), 1284 (νsym,CO ). 1H NMR (500 MHz,
2
2
toluene-d8): δ 4.93 (s, 1H, H8), 4.87 (s, 1H, H80), 3.20 (s, 2H, H6), 2.70
(br s, 2H, H1 and H3), 2.15 (br s, 2H, H10 and H30, overlapping with
solvent), 2.02 (s, 3H, H9), 1.98 (s, 3H, H4), 1.20 (m, 6H, PCH2CH3),
0.89 (m, 9H, PCH2CH3). 13C{1H} NMR (101 MHz, toluene-d8):
δ 175.5 (s, CO2), 143.6 (s, C7), 123.3 (s, C2), 111.8 (s, C8), 47.7
(s, C6), 23.3 (s, C4), 23.1 (s, C9), 15.9 (d, PCH2, JCꢀP = 17 Hz), 8.14
(s, PCH2CH3). 31P{1H} NMR (121 MHz, C6D6): δ ꢀ19.1 (s).
(η3-2-methylallyl)(η1-CO2C4H7)Ni(PPh3) (12). To a solution of
(2-methylallyl)2Ni (12.1 mg, 0.086 mmol) in toluene (4 mL) at ꢀ78 °C
was added PPh3 (23 mg, 0.086 mmol). The mixture was then degassed
using three freezeꢀpumpꢀthaw cycles and warmed to ꢀ40 °C. Excess
1 atm CO2 was added via a dual-manifold Schlenk line at ꢀ40 °C. The
mixture was slowly warmed to room temperature. After 10 h the mixture
was evaporated to dryness. The resulting residue was extracted with
pentane, and the extract was dried to give 12 as an orange powder. Yield:
10.1 mg (30.1%). Anal. Calcd (found) for C27H29O2PNi: C, 68.25
(68.04); H, 6.15 (5.93).
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: nilay.hazari@yale.edu.
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(2-methylallyl)2Ni (168 mg, 1.0 mmol) in toluene (2 mL) at ꢀ78 °C
was added NHC (388 mg, 1.0 mmol), and the mixture was stirred for 2 h.
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and warmed to ꢀ40 °C. Excess 1 atm CO2 was added via a dual-manifold
Schlenk line at ꢀ40 °C. The mixture was slowly warmed to room
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dryness to give 13 as a yellow powder. Colorless single crystals for X-ray
analysis were grown from toluene/pentane at ꢀ35 °C. Yield: 445 mg
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(28) Darensbourg, D. J.; Darensbourg, M. Y.; Goh, L. Y.; Ludvig, M.;
Wiegreffe, P. J. Am. Chem. Soc. 1987, 109, 7539.
(29) Laird, M. F.; Pink, M.; Tsvetkov, N. P.; Fan, H.; Caulton, K. G.
Dalton Trans. 2009, 1283.
IR (cmꢀ1): 1602 (νasym,CO ), 1327 (νsym,CO ). 1H NMR (500 MHz,
2
toluene-d8, ꢀ35 °C): δ 7.23 (2t, 2H, H14), 7.14 (m, 4H, H13 and H15,
overlapping with solvent peak), 6.48 (s, 2H, H10), 5.03 (s, 2H, H8), 3.16
(m, 4H, H17 and H21), 2.91 (br s, 2H, H6), 3.69 (s, 1H), 2.53 (s, 1H),
1.61 (s, 1H) and 0.85 (s, 1H), 2.62 (H1 and H3), 2.05 (s, 3H, H9), 1.73
(s, 3H, H4), 1.45 (br s, 12H, H18, H19, H20, or H22), 1.03 (m, 12 H,
H18, H19, H20, or H22). 13C{1H} NMR (125.8 MHz, C6D6): δ 188.7
(s, C23), 176.2 (s, C5), 146.4 (s, C11), 144.6 (s, C7), 137.1 (s, C16),
130.4 (s, C14), 128.7 (s, C2), 124.9 (s, C10), 124.6 (s, C12 and C16),
111.5 (s, C8), 47.8 (s, C6), 29.1 (s, C21), 26.4 (s, C20 and C22), 23.8
(s, C9), 23.2 (s, C18 and C19), 22.3 (s, C4).
Reactions between CO2 and (allyl)2Ni(L) Complexes. In an
typical reaction 8 mg of (allyl)2Ni(PMe3) (1), (allyl)2Ni(PEt3) (2),
3149
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