Simplified and versatile access to low valent Ni complexes by metal-free reduction of NiII precursors

Article abstract of DOI:10.1039/c9dt00668k

The reduction of [Ni(DME)Cl₂] with 2 equiv. of bis(trimethylsilyl)-1,4-tetramethyldihydropyrazine in the presence of a ligand L and an excess of olefin cleanly leads to [Ni(L)(alkene)₂] complexes. When reduction is performed in the presence of 1,5-cyclooctadiene (COD), [Ni(COD)₂] is obtained. Such an approach also allows access to the Ni^I dimer [Ni(bis(dicyclohexylphosphino)propane)Cl]₂.

Full text of DOI:10.1039/c9dt00668k

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Simplified and Versatile access to low valent Ni
complexes by Metal-free reduction of NiII
precursors.†
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
E. Moser[a], E. Jeanneau[b], N. Mézailles[c]*, H. Olivier-Bourbigou[a], Pierre-Alain R. Breuil[a]*
www.rsc.org/
The reduction of [Ni(DME)Cl2] with 2 equiv. of bis(trimethylsilyl)- one pot. We also report that with strongly donating bis-phosphine
1,4-tetramethyldihydropyrazine in presence of a ligand L and an ligands, the reaction leads to Ni(I) complexes instead.
excess of olefin cleanly leads to [Ni(L)(alkene)2] complexes. When
Our work started with the synthesis of the emblematic and
sensitive [Ni(COD)2] complex. To date it is synthesized by reduction
of Ni(acac)2 with alkylaluminum compounds such as
triethylaluminium or DIBAL-H in presence of COD and butadiene at
reduction is done in presence of 1,5-cyclooctadiene (COD),
[Ni(COD)2] is obtained. Such approach also allows access to the NiI
dimer [Ni(bis(dicyclohexylphosphino)propane)Cl]2.
Low valent, ligand stabilized (phosphines, N-heterocyclic carbenes = low temperature, followed by low temperature crystallization and
NHC) Ni(0) complexes are important species, being catalytically filtration to eliminate aluminium side products.13 The room
relevant in many transformations, such as cross coupling temperature reduction of [Ni(DME)Cl2] with two equivalents of 1
processes.1 Among the known Ni(0) complexes, stable was carried out in a first stage in THF with the presence of a large
[Ni(L)(alkene)2] or [Ni(L)(bis-alkene)] species (L=phosphines, NHC) excess (20 equiv. of COD). Formation of black precipitate (likely Ni
are rare because of a lack of efficient, versatile synthetic route or metal) pointed to a fast reduction compared to coordination by
availability
of
the
appropriate
precursors.2
Indeed, COD. The low temperature reduction (-78°C) was attempted to
[Ni(phosphine)(C2H4)2] have been reported by displacement of the prevent Ni(0) precipitate formation, but the reaction does not
poly-ene ligand from the extremely sensitive [Ni(CDT)] precursor proceed at this temperature within a day. The optimized reduction
(CDT = 1,5,9-cyclododecatriene), whereas other [Ni(L)(bis-alkene)] temperature was found to be at -20°C. In this case, we were able to
species have been prepared from air, light and heat sensitive synthesize [Ni(COD)2] as crystalline material in 54% isolated yield,
1
[Ni(COD)2].3,8 In turn, both Ni(0) complexes [Ni(CDT)] and [Ni(COD)2] fully characterized by H and 13C NMR (Scheme 1). The moderate
require the use of pyrophoric alkylaluminium derivatives to be yield can be explained by the remaining formation of nickel black
synthesized. It is thus desirable to devise a rational, experimentally particles during the reduction, which can nevertheless be readily
simpler, versatile method to access families of Ni(0) species.
eliminated by filtration prior to crystallization of [Ni(COD)2] from
cold toluene.
Recently, bis(trimethylsilyl)-1,4-tetramethyl-dihydropyrazine 1 has
been shown to behave as a one or two-electron reducing agent for
numerous transition metal complexes.9,10,11 It was also used to
reduce several late transition metal halide precursors to the
corresponding metallic nanoparticles in the absence of stabilizing
ligand.12 In the case of Ni, nanoparticles active in carbon-carbon
cross coupling process were obtained. We have hypothesized that if
the in situ generated reduced Ni center could be efficiently trapped
by ligands the aforementioned desirable Ni(0) complexes would be
readily accessible. In the present work, we show that from the
simple, commercially available, [Ni(DME)Cl2] precursor (DME =
dimethoxy-ethane), several Ni(0) complexes could be synthesized in
1 (2 eq.)
Ni
[Ni(DME)Cl2] + 20 eq.
THF, -20°C to -10°C, 3h
Si
N
Y = 54%
1 =
N
Si
Scheme 1 Synthesis of [Ni(COD)2] by organic reduction of Ni(II)
halide
a. IFP Energies nouvelles, Rond-point de l’échangeur de Solaize, BP 3, 69360 Solaize,
France
Beyond the synthesis of [Ni(COD)2] itself, the reaction proved our
hypothesis right, clearly indicating that monometallic Ni(0) can be
b. Centre de Diffractométrie Henri Longchambon, Site CLEA - Bât. ISA, 3ème étage,
5 rue de La Doua; 69100 Villeurbanne, France
c. Laboratoire Hétérochimie Fondamentale et Appliquée UMR 5069 CNRS,
Université Paul Sabatier 118 Route de Narbonne ; 31062 Toulouse, France
†Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
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DOI: 10.1039/x0xx00000x
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trapped. The next level of complexity concerns the selective With this very encouraging result, we targeted more challenging
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DOI: 10.1039/C9DT00668K
stabilization by two different types of ligands. Indeed, in order to be [Ni(L)(C2H4)2] complexes. Indeed, examples of these are very scarce.
able to generate efficient catalytic species, it is crucial for the metal The synthesis of [Ni(PCy3)(C2H4)2] was only reported via the
to bear strongly bound ligands as well as weakly bound ligands that substitution of CDT from the highly sensitive complex [Ni(CDT)] in
will be displaced during the transformation. For example, Lappert 1971.5 Following our strategy, the same complex [Ni(PCy3)(C2H4)2],
et al. studied the coordination of 1,3-divinyltetramethyldisiloxane 3, was formed exclusiely (100% 31P NMR) at room temperature
(dvtms) to Ni(0) centers in order to synthesize hydrosilylation after 45 minutes (Scheme 3).
catalysts.6,8 For that purpose, they synthesized [Ni(PR3)(dvtms)]
(R=Ph, Cy and p-Tol) complexes from [Ni2(dvtms)3] or [Ni(COD)2].
1 (2 eq.)
[Ni(DME)Cl2] + C2H4 (1atm) + PCy3
Cy3P
Ni
We therefore turned our attention to the synthesis of Ni(0)
complexes stabilized by phosphine and alkene ligands. Pleasingly,
the reduction of [Ni(DME)Cl2] in THF with two equiv. of 1, in
presence of two equiv. of dvtms and one equiv. of phosphine ligand
appeared remarkably selective. Indeed, in each case, a single
complex is formed as shown by 31P NMR spectroscopy. The isolated
yields (not optimized) are between 24 and 77%, because of their
high solubility (Scheme 2).
THF, 45min, RT
3
Scheme 3 Synthesis of a Ni(0)-phosphine bis-ethylene complex
Complex 3 is thermally sensitive and decomposes almost instantly
at room temperature. Crystallization of the bulk solution at -34°C
yielded crystals that were analyzed by XRD.3 The measurement of
the P-Ni-Centroid 116.3° and 116.7° (with Centroid being the
centers of the two ethylene ligands) confirms the slightly distorted
trigonal planar structure. The CC bond lengths of the ethylene
ligands measured at 1.39 Å (average) attest here also the back
donation from the nickel center into the * orbitals of ethylene.
Si
Si
Si
Si
1 (2 eq.)
O
O
R3P
Ni
[Ni(DME)Cl2] + 1 eq. PR3 + 2eq.
THF, 1h, RT
PR3 = PCy3 (Y = 77%) (2a)
PPh3 (Y = 43%) (2b)
PiPr3 (Y = 43%) (2c)
PtBuiPr2 (Y = 24%) (2d)
Scheme 2 Synthesis of [Ni(PR3)(dvtms)] complexes (2a-2d)
Complexes 2a-d were characterized by 1H, 13C and 31P NMR in
accordance with literature.8 The structure of complex 2d was not
known and is presented here (Figure 1). The local geometry at the
three-coordinate nickel atom is planar and the angles P-Ni-L1 and P-
Ni-L2 (with L1 and L2 the centers of the double bounds coordinated)
of 116.9° and 113.7° confirm the slightly distorted trigonal
structure. The six-membered ring (Ni-C13-Si-O-Si-C18) adopts a
chair structure.6 The CC bond lengths of the olefins ligands
measured at 1.40 Å (average) attest here the back donation from
the nickel center into the π* orbitals of the olefins coordinated.
Figure 2 ORTEP drawings of 3, hydrogen atoms were omitted for
clarity. Bond distances [Å] and angles [°] : Ni-P 2.1873(10), Ni-C22
2.000(4), Ni-C21 1.987(4), Ni-C23 1.985(4), Ni-C24 1.988(4), C22-
C21 1.392(6), C23-C24 1.389(6), P-Ni-C22 95.89(12), P-Ni-C21
136.69(13), P-Ni-C23 137.14(15), P-Ni-C24 96.26(13), C21-Ni-C23
86.01(19).
Having shown on several examples the efficiency of the reduction
method for the synthesis of phosphine stabilized Ni(0) complexes,
we then turned our attention to the related NHC complexes. We
chose the known complex 4 as a representative example. Complex
4 has been obtained by COD substitution in [Ni(COD)2] by NHCIPr
(1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) in the presence
of styrene with a 90% yield.7 Our one pot synthesis from
[Ni(DME)Cl2] (Scheme 4) allowed the isolation of complex 4 in 44%
yield.
Figure 1 ORTEP drawings of 2d, Hydrogen atoms were omitted for
clarity. Bond distances [Å] and angles [°] : Ni-P 2.2284(10), Ni-C14
2.010(3), Ni-C13 2.018(3) Ni-C19 2.008(3), Ni-C18 2.030(3), C13-C14
1.402(4), C18-C19 1.404(4), P-Ni-C13 133.98(9) , P-Ni-C14 93.36(9),
P-Ni-C19 96.585(10), P-Ni-C18 137.27(9).
1 (2 eq.)
[Ni(DME)Cl2] +
+ 1.2 eq. NHCIPr
NHCIPr
Ni
THF, -20°C, 1h
(excess)
4 (Y = 44%)
2 | J. Name., 2012, 00, 1-3
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Scheme 4 Synthesis of a Ni(0) NHC-stabilized complex
have moreover proposed that coordination of the reducing agent is
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needed for the electron transfer and TMSClDeOlimI: i1n0a.1t0io3n9/.9C9DT00668K
These examples show the versatility of the reduction method,
employing monodentate phosphines or NHC in conjunction with
alkenes or dienes. In all cases, the reaction was highly selective in
the formation of the desired [Ni(L)(alkene)2] or [Ni(L)(diene)]
complexes.
It was therefore of interest to investigate the mechanism of the
reduction of
a Ni(II) complex, using both NMR and EPR
spectroscopies. We chose to study the reduction of [Ni(DME)Cl2] in
presence of one equiv. of PCy3 and two equiv. of dvtms into
complex [Ni(PCy3)(dvtms)] 2a. Most interestingly, with one equiv. of
species 1, complex 2a is formed with a ca 50% yield (31P NMR)
within 5 minutes at room temperature, along with ca 50% of free
PCy3. Interestingly, at this point already, the 1H NMR spectrum is
very poorly resolved, which prevented quantification of reagent 1
consumption, and points to the formation of paramagnetic species.
After 30 min the intensity of the signal had drastically decreased to
a very poorly defined 31P NMR spectrum (ESI). A well-defined EPR
spectrum was observed, consistent with the formation of Ni(I)
species. Indeed, the room temperature X-band EPR solution is
triplet with hyperfine coupling to two equivalent phosphorus nuclei.
Simulation of this spectrum gives a g value of 2.143 and a coupling
constant with P of 73 G. This data is fully consistent with literature
precedence for [(bidentate phosphine)NiCl] species.15 Formation of
[Ni(PCy3)2Cl] either as a monomer or dimer is therefore evidenced
during the process, following a comproportionation mechanism
between generated Ni(0) and Ni(II) precursor.16 est ce que ce
mélange est alors stable ou évolue t il vers le produit très
lentement? Nonetheless, when a second equiv. of 1 was added, the
quantitative formation of complex 2a was observed by 31P NMR
spectroscopy together with the loss of the EPR signal. These
experimental observations are only consistent with the mechanism
presented in Scheme 6. It involves a fast reduction of half of the
[Ni(DME)Cl2] into the [Ni(PCy3)(dvtms)] complex 2a via a two
electron transfer and coordination of the ligands followed by
comproportionation to form a Ni(I) species. This intermediate can
be reduced to Ni(0) efficiently by the addition of a second equiv. of
1. Overall, our experiments confirm that two equivalents of the
reducing agent are essential for an efficient reduction, despite the
fact that only one equivalent is consumed.
Mechanistically, the reaction sequence can be proposed to involve
either reduction of the Ni(II) salt to unsaturated Ni(0) species to
which the ligands bind with very fast kinetics, or coordination of the
ligand(s) to the Ni(II) precursor prior to reduction followed again by
coordination of other ligands to complete the coordination sphere.
In addition, the intermediacy of Ni(I) species remains an open
question. In order to provide preliminary information on the
mechanism, we investigated the influence of the Ni precursor on
the nature of the reduced Ni complexes. When the complex
[Ni(PCy3)2Cl2] was reduced with 2 equiv. of 1 in presence of
ethylene, the known complex [Ni(PCy3)(C2H4)2] 3 was observed
together with liberation of one equiv. of PCy3. This experiment
showed that the redox potential of species 1 is low enough to
reduce electron-rich Ni(II) into Ni(0). Interestingly in this case,
despite the presence of two PCy3 in the medium, no trace of the
[Ni(PCy3)2(C2H4)] complex was detected. In order to favor the
formation of such type of complex, we turned to more strongly
bound bidentate diphosphine ligands. Unexpectedly, the reduction
of complex [Ni(dcpp)Cl2] with 2 equiv. of 1 in presence of ethylene
did not lead to the expected Ni(0) complex [Ni(dcpp)(C2H4)] as no
signal was observed in the 31P NMR spectrum of the crude mixture.
An EPR spectrum revealed an intense signal for a Ni(I) species. From
this mixture, the known paramagnetic Ni(I) dimer 5 [Ni(dcpp)Cl]2
was isolated with a 70% yield (Scheme 5).14
Cy2
P
Cy2
P
C2H4 (1 atm)
THF, 4h
Cl
Cl
+ 1 (2eq.)
Ni
Ni
P
P
Cy2
Cy2
THF, 5h
Si
Si
[Ni(DME)Cl2] + PCy3 + 1
THF, RT
5 min
O
Ni(0)
Cy3P
Ni
Si
Cy2
Cy2
O
P
P
+ 2
Cl
Cl
2a, Y = ca 50% (NMR)
Si
Ni
Ni
P
P
Cy2
Cy2
5
+ [Ni(DME)Cl2]
(70%)
[Ni(PCy3)2Cl]
Scheme 5 Isolation of Ni(I)dccp dimer
Si
Si
+ 1
Ni(0)
O
or [Ni(PCy3)2Cl]2
Cy3P
Ni
Ni(I)
2a, Y = ca 100% (NMR)
This observation raised further mechanistic questions. Mashima
and coworkers have investigated the reduction process using
species 1 with several metal centers. They have shown that despite
the fact that 1 is a potential 2-electron reducing agent, an excess (2
equiv.) of 1 was needed to reduce efficiently Ni(II) into Ni(0)
nanoparticles and only half of reductant 1 was converted.12 They
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Scheme 6 Proposed reduction mechanism
8
9
P. B. Hitchcock, M. F. Lappert and H. MVaiecwieAjertwiclsekOi,nlinJe.
Organomet. Chem., 2000, 605, 221–225.
DOI: 10.1039/C9DT00668K
T. Saito, H. Nishiyama, H. Tanahashi, K. Kawakita, H. Tsurugi
and K. Mashima, J. Am. Chem. Soc., 2014, 136, 5161–5170.
A major difference can be seen between the complexes featuring
mono and bidentate ligands. Indeed, starting from [Ni(dcpp)Cl2], a
stable Ni(I) dimer is formed. The four-coordinate Ni center adopts a
square planar geometry and it is likely that the bulkiness of the
ligand prevents coordination of the reducing agent necessary to
allow further electron transfer.
10 a) R. Arteaga-Muller, H. Tsurugi, T. Saito, M. Yanagawa, S. Oda
and K. Mashima, J. Am. Chem. Soc., 2009, 131, 5370–5371; b)
H. Tsurugi, H. Tanahashi, H. Nishiyama, W. Fegler, T. Saito, A.
Sauer, J. Okuda and K. Mashima, J. Am. Chem. Soc., 2013, 135,
5986–5989.
11 H. Tsurugi and K. Mashima, Chemistry A Eur. J., 2018, DOI:
10.1002/chem.201803181.
12 T. Yurino, Y. Ueda, Y. Shimizu, S. Tanaka, H. Nishiyama, H.
Tsurugi, K. Sato and K. Mashima, Angew. Chem. Int. Ed., 2015,
54, 14437–14441.
In summary we have demonstrated the potential of organic
reducing agent 1 to synthesize stabilized Ni(0) complexes from Ni(II)
halides. We have shown the versatility of this method toward
diversified ligands such as alkenes, tertiary phosphines and NHCs.
Mechanistically, a two electron reduction of the Ni(II) precursor to 13 a) e-Eros Encyclopedia of reagents for organic Synthesis, 2015,
1; b) G. Wilke et al., Angew. Chem. Int. Ed., 1966, 5, 151–266; c)
Phillips Petroleum Company, US5130458, 1992;
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S. Sigman and A. M. Arif, Inorg. Chem., 2005, 44, 3774–3776;
Ni(0) followed by comproportionation to Ni(I) species is involved.
When a monodentate trialkyl phosphine ligand is used, further
reduction to Ni(0) is possible, while with the strongly donating
bidentate ligand dcpp, the reaction stops at the Ni(I) stage.
Preliminary experiments showed promising results in the use of this
method to access active species in various catalytic
transformations.
We thank Lionel Rechignat for EPR Measurements and IFP Energies
nouvelles for financial support.
Notes and references
1
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Various well defined low valent Ni complexes were obtained by salt-free reduction of a commercially
available Ni(II) precursor.
Ni(II) = [Ni(DME)Cl2
]
+ PCy3
+ Ethylene
Cy3
P
Ni
Ni(II) = [Ni(dcpp)Cl2
]
+ PR3
Si
+ dvtms
O
R3
P
Ni
Si
Si
N
Cy2
P
Cy2
P
Cl
Ni(II) + 2eq.
Ni
Cl
Ni
N
P
P
Cy2
Cy2
Si
+ COD
Ni
1
+ NHCIPr
+ Styrene
NHCIPr
Ni