Half-Sandwich Nickel(II) NHC-Picolyl Complexes as Catalysts for the Hydrosilylation of Carbonyl Compounds: Evidence for NHC-Nickel Nanoparticles under Harsh Reaction Conditions

Article abstract of DOI:10.1002/ejic.202100371

The cationic [NiCp(Mes-NHC-CH₂py]Br complex 2 a was prepared directly by the reaction of nickelocene with 1-(2-picolyl)-3-mesityl-imidazolium bromide (1), and its PF₆^? derivative 2 b, by subsequent salt metathesis. X-ray diffraction studies and Variable Temperature ¹H NMR experiments run with 2 a and 2 b strongly suggest the bidentate coordination of the picolyl-functionalized carbene to the nickel both in the solid state and in solution in both cases. These data suggest the absence of hemilabile behavior of the latter, even in the presence of a coordinating anion. Both complexes show similar activity in aldehyde hydrosilylation, further implying the absence of hemilability of the picolyl-functionalized carbene, and effectively reduce a broad scope of aldehydes in the absence of additive under mild conditions. In the case of ketones, effective hydrosilylation is only observed in the presence of a catalytic amount of potassium t-butoxide at 100 °C. Dynamic light scattering, scanning transmission electron microscopy and X-ray photoelectron spectroscopy show evidence for the involvement of NHC-picolyl-Ni nanoparticles under these conditions.

Full text of DOI:10.1002/ejic.202100371

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Half-Sandwich Nickel(II) NHC-Picolyl Complexes as Catalysts
for the Hydrosilylation of Carbonyl Compounds: Evidence
for NHC-Nickel Nanoparticles under Harsh Reaction
Conditions
Franck Ulm,[a] Saurabh Shahane,[a] Lai Truong-Phuoc,[b] Thierry Romero,[b]
Vasiliki Papaefthimiou,[b] Matthieu Chessé,[a] Michael J. Chetcuti,*[a] Cuong Pham-Huu,[b, c]
Christophe Michon,*[a, c] and Vincent Ritleng*[a]
The cationic [NiCp(Mes-NHC-CH2py]Br complex 2a was pre-
aldehyde hydrosilylation, further implying the absence of
hemilability of the picolyl-functionalized carbene, and effec-
tively reduce a broad scope of aldehydes in the absence of
additive under mild conditions. In the case of ketones, effective
hydrosilylation is only observed in the presence of a catalytic
pared directly by the reaction of nickelocene with 1-(2-picolyl)-
À
3-mesityl-imidazolium bromide (1), and its PF6 derivative 2b,
by subsequent salt metathesis. X-ray diffraction studies and
1
Variable Temperature H NMR experiments run with 2a and 2b
°
strongly suggest the bidentate coordination of the picolyl-
functionalized carbene to the nickel both in the solid state and
in solution in both cases. These data suggest the absence of
hemilabile behavior of the latter, even in the presence of a
coordinating anion. Both complexes show similar activity in
amount of potassium t-butoxide at 100 C. Dynamic light
scattering, scanning transmission electron microscopy and X-
ray photoelectron spectroscopy show evidence for the involve-
ment of NHC-picolyl-Ni nanoparticles under these conditions.
Introduction
reducible functionalities.[2] Beside catalysts based on noble
metals, attention has recently turned to catalysts based on
earth-abundant 3d metals, due to sustainability concerns.[3]
Among these developed catalysts,[4] half-sandwich [Ni(η5-C5R5)
L(NHC)](+) complexes have shown interesting activities for the
hydrosilylation of carbonyl derivatives. In 2012, some of us
reported that [NiCpCl(IMes)] A (Cp=η5-C5H5, IMes=1,3-dimesi-
tylimidazol-2-ylidene) effectively catalyzed the hydrosilylation of
Hydrosilylation is a reaction of interest for the selective
reduction of carbonyl compounds under mild conditions using
transition-metal-based catalysts.[1,2] Indeed, the use of hydro-
silanes as reductants, which proceeds without the need of high-
pressures or elevated temperatures, is an advantageous
surrogate to hydrogenation procedures. As the reactivity of
hydrosilanes is modular and depends on their substitution
patterns, the hydrosilylation may behave as a highly chemo-
and regioselective reduction process that tolerates other
°
a broad range of aldehydes and ketones at 25 C in the
presence of NaHBEt3 as a co-catalyst with a turnover frequency
(TOF) that reached values up to 390 hÀ 1 for the hydrosilylation
of benzaldehyde (Scheme 1).[4e] At the same time, Royo et al.
described the synthesis of a related half-sandwich nickel(II)
complex B bearing a bidentate tetramethylcyclopentadienyl-
[a] Dr. F. Ulm, Dr. S. Shahane, M. Chessé, Prof. M. J. Chetcuti, Dr. C. Michon,
Prof. V. Ritleng
Université de Strasbourg,
Ecole Européenne de Chimie, Polymères et Matériaux,
CNRS, LIMA, UMR 7042,
25 rue Becquerel, 67087 Strasbourg, France
E-mail: michael.chetcuti@unistra.fr
cmichon@unistra.fr
vritleng@unistra.fr
http://lima.unistra.fr/
[b] Dr. L. Truong-Phuoc, T. Romero, Dr. V. Papaefthimiou, Dr. C. Pham-Huu
Université de Strasbourg,
Institute of Chemistry and Processes for Energy,
Environment and Health (ICPEES), UMR 7515 CNRS,
25 rue Becquerel, 67087 Strasbourg, France
https://icpees.unistra.fr/
[c] Dr. C. Pham-Huu, Dr. C. Michon
University of Strasbourg Institute for Advanced Study (USIAS),
5 allée du Général Rouvillois, 67083 Strasbourg, France
http://www.usias.fr/
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/ejic.202100371
Scheme 1. Previously reported [Ni(η5-C5R5)L(NHC)](+) precatalysts for the
hydrosilylation of carbonyl compounds.
Part of the joint “ECPM Strasbourg Institute Feature” Collection with EurJOC.
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functionalized carbene ligand, [Ni(Cp*-NHCÀ Me)(OtBu)], that
catalyzed the reduction of carbonyl derivatives at temperatures
°
°
ranging from 25 C (aldehydes) to 100 C (ketones) with a TOF
up to 2 300 hÀ 1 for the hydrosilylation of 4-trifluorometh-
ylbenzaldehyde (Scheme 1).[4f] In 2016, Albrecht et al. reported a
potentially hemilabile pyridyl-triazolylidene NiCp precatalyst C
for the hydrosilylation of aldehydes with a TOF as high as 13
350 hÀ 1 observed with 4-methoxybenzaldehyde at 60 C
°
(Scheme 1).[4w]
The high TOF observed with the latter complex was
tentatively rationalized as the result a well-balanced hemilabile
ligand, with a C,N-chelate that can both stabilize the catalyst by
a strong bidentate chelation in the resting state and enable an
effective reactivity at the nickel center by ring opening on the
substrate’s approach.[4w] The hemilabile balance of chelates may
indeed be of utmost importance to develop efficient catalysts.
This is especially true for [Ni(η5-C5R5)(NHCÀ Y)](+) complexes,
which do not have potentially available coordination site aside
from those resulting from decoordination of the Lewis basic Y
atom and/or Cp ring slippage from η5- to η3- or even η1- or
decoordination.[5,6] Following a recent work in our group on
nickel complexes such as D (Scheme 1) bearing hemilabile k2-
C,S-thioether-functionalized NHCs,[7] we wondered whether a
NHC-picolyl ligand could afford a suitable balance in such type
of complexes for nickel-catalyzed hydrosilylation reactions.
Herein we describe the synthesis of the cationic [NiCp(Mes-
NHC-CH2py]Br complex bearing a k2-C,N-1-(2-picolyl)-3-mesityl-
imidazol-2-ylidene ligand directly from nickelocene and the
corresponding imidazolium bromide. X-ray diffraction and
Scheme 2. Synthesis of the cationic NHC-picolyl-Ni(II) complexes 2a and 2b.
uncoordinated, and a cationic species bearing a k2-C,N-picolyl-
NHC ligand was obtained. The subsequent metathesis of Br to
PF6 anion led to complex 2b with a 93% yield. It is noteworthy
À
that the synthesis of the BF4 analogue of 2a and 2b was
recently reported by Valerga et al. through a two-step proce-
dure involving reaction of 1 with Ag2O and subsequent trans-
metalation to nickel by reaction with the air-sensitive [NiCp
(COD)](BF4).[5b]
°
Single crystals of 2a and 2b were obtained at À 28 C by
diffusion of n-pentane in a THF solution of 2a and by slow
solvent evaporation from a solution of 2b in chloroform. X-ray
diffraction studies of 2a and 2b established the molecular
2
structures and k -C,N-coordination pattern of the 1-(2-picolyl)-
3-mesityl-imidazol-2-ylidene ligand for both complexes in the
solid state (Figure 1, Figures S1-S4). Key bond distances and
angles are listed in Table 1 and selected crystallographic data
and data collection parameters can be found in Table S1.
1
Variable Temperature (VT) H NMR studies strongly suggest the
preferential coordination of the pyridine side arm over the
bromide anion, both in solution and in the solid state, therefore
suggesting no hemilabile behavior of the picolyl-functionalized
Complex 2a crystallizes in the orthorhombic space group
P212121. The coordination of the Ni atom is characterized by a
Ni–C(1) bond distance of 1.864 Å, a Ni–N(3) distance of 1.922 Å
and a Ni-Cpcent distance of 1.751 Å (Table 1) [C(1)=the carbene
carbon atom]. The corresponding Ni···Br anion distance is
7.528 Å. Whereas C(1)À NiÀ N(3) bite angle is orthogonal with a
À
carbene ligand. The latter complex and its PF6 derivative
obtained by salt metathesis show similar activity in aldehyde
hydrosilylation under mild conditions, which further suggests
the absence of hemilabile behavior. In the case of ketones, the
addition KOtBu to the catalytic medium and a temperature of
°
°
100 C are requested to observe effective reductions. Diffusion
value of 91.5 , C(1)À NiÀ Cpcent and N(3)À NiÀ Cpcent angles are
°
°
Light Scattering (DLS), Scanning Electron Microscopy in the
Transmission mode (SEMÀ T) and X-ray Photoelectron Spectro-
scopy (XPS) show evidence for the involvement of NHCÀ Ni
nanoparticles under these conditions.
larger at 134.6 and 133.9 , respectively. This is characteristic of
[Ni(η5-C5R5)(NHC)L](+) type complexes.[5b,7,9] One can note how-
ever the relatively smaller C(1)À NiÀ N(3) bite angle of 2a
compared to the corresponding C(1)À NiÀ S and C(1)À NiÀ C
2
[7]
°
angles of the k -C,S-NHC-thioether complex D (95.94 ) and of
Results and Discussion
°
Table 1. Selected distances (Å) and angles ( ) in 2a, 2b1 and 2b2 with esd’s
in parentheses.
Our investigations started with the synthesis of complex 2a by
the reaction of the imidazolium bromide 1[8] with nickelocene in
THF under reflux (Scheme 2). The use of a conventional oil bath
heater resulted in the obtention of 2a with a 50% yield after
2a
2b1
2b2
NiÀ C(1)
NiÀ N(3)
NiÀ Cpcent
1.864(3)
1.922(3)
1.751
2.124
91.5(1)
134.6
1.875(5)
1.930(4)
1.747
2.119
92.9(2)
133.9
1.875(5)
1.916(4)
1.745
2.121
92.2(2)
133.8
[a]
°
4 days. Heating through microwave (2.45 GHz) at 110 C allowed
NiÀ CCp av[b]
a substantial reduction in the reaction time, affording 2a in
41% yield after just 0.5 h. In contrast to what was observed for
the reactions of nickelocene with thioether-functionalized
imidazolium bromides that yielded neutral complexes bearing
monodentate carbenes,[7] the bromide anion was found
C(1)À NiÀ N(3)
C(1)À NiÀ Cpcent
N(3)À NiÀ Cpcent
133.9
133.2
133.8
[a] Cpcent =centroid of the Cp group. [b] Average NiÀ C distance to the Cp
ring. [c] Ni···P distance.
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chelate, the H NMR spectrum of 2a at room temperature (RT)
exhibits a broad singlet at 6.32 ppm that integrates for two
protons for the methylene protons. Similarly, the 1H NMR
spectrum of 2b at RT shows these protons as a singlet
integrating for two protons, however as a sharper and slightly
more shielded (5.68 ppm) peak in this case. These surprising
observations suggested the existence of a dynamic process in
1
solution at RT in both complexes. VT H NMR experiments were
then performed on CD2Cl2 solutions of 2a and 2b between 298
and 203 K to check if these dynamic processes could be frozen
on the NMR time scale at low temperature. In the case of 2a,
the singlet of the NCH2py methylene group that is observed at
5.56 ppm at 298 K in CD2Cl2 broadens as the temperature
decreases and decoalesces between 213 and 208 K to give two
very broad signals integrating for one proton each at 5.47 and
5.43 ppm at 203 K. The free energy of activation (ΔG�) for this
process, based on this (de)coalescence temperature of the
NCH2py group (ca. 210 K) is of ca. 16 kcalmolÀ 1.[11] In contrast,
no decoalescence could be observed for 2b, indicating a fast
dynamic process involving the NCH2py arm on the NMR time
À
scale. Considering the weak coordinating ability of the PF6
anion, it is unlikely that the latter can displace the pyridine
group from the nickel center. Thus, it is reasonable to postulate
that the absence of diastereotopy observed for 2b, even at low
temperature, is likely due to small angle oscillations about the
N3À Ni and N1À C1 bonds, and/or to conformational isomer-
ization of the “chair–boat” type as proposed by Valerga et al. for
closely related k2-C,N-[Ni(η5-C5R5)(Ar-NHC-CH2-Py)](BF4) com-
plexes. In the case of 2a, a similar process most probably occurs
as the decoalescence of the methylene signal observed at low
temperature strongly suggest the coordination of the pyridyl
group to the nickel center in solution as well.[12] This assumption
is further corroborated by the green color of the solution,
characteristic of cationic [Ni(η5-C5R5)L(NHC)]+XÀ complexes,[7,12]
as well as by the chemical shift of the pyridine nitrogen atom at
217 ppm in the 15N NMR spectrum of 2a at 298 K (Figure S5),
characteristic of pyridine nitrogen atom coordination to a metal
center.[13]
Figure 1. (A) Molecular structure of complex 2a showing all non-H atoms of
the cation and one BrÀ anion. (B) Molecular structure of complex 2b
+
showing all non-H atoms of cation 2b1 and one PF6À anion. Disorder on the
latter, one molecule of CHCl3 and another complex were deleted for clarity.
Ellipsoids are shown at the 50% probability level, and key atoms are
labelled.[10]
a
related k2-C,C-6- membered half-sandwich nickelacycle
[9d]
°
°
(94.0 ). Similar values (89.5–91.7 ) were observed in the NiCp
and NiCp* (Cp*=η5- C5Me5) complexes described by Valerga
et al., bearing closely related k2-C,N-picolylimidazol-2-ylidene
ligands.[5b]
By comparison, complex 2b crystallizes in the triclinic space
Complexes 2a,b were subsequently applied as catalysts for
the reduction of benzaldehyde 3a to benzylic alcohol 4a
through hydrosilylation using solely phenyl silane, and subse-
quent basic hydrolysis (Scheme 3). At a loading of 2 mol%
+
group PÀ 1, with two independent cations, 2b1 and 2b2+, two
À
PF6 anions and one molecule of chloroform in the crystal unit
+
+
cell (Figure 1B, Figures S3-S4). The cations 2b1 and 2b2 are
not identical having slightly different structural parameters
(Table 1) and the fluorine atoms of one PF6 anion exhibit some
disorder over two positions. Thus, the coordination of the Ni
°
under mild reaction conditions (e.g. 4 h at 40 C), complexes 2a
and 2b both allowed the effective reduction of 3a into 4a with
a similar yield of 92%, thus showing no difference of reactivity
despite the different coordinating ability of their respective
counter-ions.
+
+
atoms in 2b1 and 2b2 is characterized by the following
distances: NiÀ C(1)=1.875/1.875 Å, Ni–N(3)=1.930/1.916 Å and
Ni-Cpcent =1.747/1.745 Å (Table 1). Stronger similarities are note-
The substrate scope of the reaction was then explored
investigating the hydrosilylation of substituted benzaldehydes
3b–g to the corresponding alcohols 4b–g (Scheme 3). By using
2a as catalyst and similar conditions, these reactions also
proceeded in high yields independently of the electron-donor
or -acceptor character of the substituents. It is worth noting
that substrates 3c and 3d bearing bromo or methoxy groups
°
worthy for the C(1)–Ni–N(3) bite angles with 92.9 and 92.2 , the
°
C(1)À NiÀ Cpcent angles with 133.9 and 133.8 and the N-
°
(3)À NiÀ Cpcent angles with 133.2 and 133.8 , respectively.
The main spectroscopic features of 2a,b are in agreement
with their X-ray structures. The carbene carbon is observed at
ca. 162 ppm in the 13C NMR (CDCl3) spectra while the Cp
1
°
carbons appear at ca. 92.5 ppm. In the H NMR spectra, the Cp
required heating to 60 C in order to proceed smoothly.
protons of 2a and 2b resonate as a singlet at 4.99 ppm in both
cases. However, in contrast to what is expected in the case of a
Interestingly, the hydrosilylation of aldehydes 3f and 3g
proceeded chemoselectively, the nitrile and ester functions
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Table 2. Reaction conditions optimization for the hydrosilylation of
acetophenone 5a.[a]
Ent.
Cat.
Solvent
Additive
[mol%]
T
Time
[h]
Yield
[%][b]
°
[ C]
1
2
3
4
5
6
7
8
2a
2b
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
–
–
–
40
40
60
40
40
40
40
40
40
40
60
40
60
60
100
100
4
4
19
4
4
4
4
4
4
4
19
4
19
19
19
24
16
16
25
6
6
1
NaSbF6 (2)
NaBArF24 (2)
KBArF24 (2)
NBu4OTf (2)
KNTf2 (2)
Kacac (4)
KOAc (4)
KOAc (4)
KOtBu (2)
KOtBu (2)
KOtBu (2)
KOtBu (2)
KOtBu (2)
1
7
9
10
21
38
15
47
49
74[c]
96
10
11
12
13
14
15
16
THF
Toluene
Toluene
Toluene
[a] Experiments run on a 0.25 mmol acetophenone scale. [b] Isolated
yields after hydrolysis. [c] 22% yield without KOtBu additive.
Scheme 3. Hydrosilylation of aldehydes 3a–l to alcohols 4a–l using catalyst
2a. NMR yields (%). [a] Same result using catalyst 2b.
19 h and an almost quantitative 96% yield in 24 h (entries 14–
16).
being not reduced. In addition, 1-naphtaldehyde 3h and furan-
2- carbaldehyde 3i were effectively converted to the corre-
sponding alcohols in 67% and 90% yields, respectively
With these conditions in hand, the substrate scope of the
reaction was then explored by investigating the hydrosilylation
of various aryl and alkyl ketones 5a–l into the corresponding
alcohols 6a–l (Scheme 4). Though the hydrosilylation of
°
(Scheme 3). Finally, the hydrosilylation of 3l proceeded at 40 C
with a 64% yield and without any reduction of the conjugated
CÀ C double bond. By comparison, the reaction of alkyl
°
aldehydes 3k and 3l required a 60 C heating in order to afford
similar yields.
We next focused our attention on the catalytic activity of
complexes 2a,b for the hydrosilylation of acetophenone 5a
(Table 2). By using similar reaction conditions as for the
°
reduction of aldehydes (e.g. 40 C, 4 h), the catalysts 2a,b again
showed a similar activity, leading to the corresponding alcohol
6a in a similar 16% yield (entries 1–2). The latter was slightly
°
increased to 25% by heating at 60 C for 19 h (entry 3). In order
to check the effect of the counter-anion and various additives
on the catalysis,[14] an anion screening was carried out
(entries 4–10). Apart from acetate that led to a 21% yield for 6a
À
°
after 4 h reaction at 40 C (entry 10), none of the BArF24
(tetrakis[(3,5-trifluoromethyl)phenyl]borate), SbF6 , OTfÀ , NTf2
À
À
or acacÀ anions afforded a better yield than 2a,b without
additives (entries 4–9). An increase of the reaction temperature
°
to 60 C and time to 19 h resulted in a 38% yield of 6a with
KOAc (entry 11). A similar trend was observed with KOtBu as
additive (entries 12 and 13),[4f,15,16] but a higher yield of 47%
yield was obtained (entry 13), though this additive effect was
°
not significant at 40 C (entry 12). A change of solvent to
toluene allowed a further increase of the reaction temperature
°
to 100 C and afforded benzyl alcohol 6a with a 74% yield in
Scheme 4. Hydrosilylation of ketones 5a–l into alcohols 6a–l using catalyst
2a. GC yields (%).
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acetophenone 5a and benzophenone 5b proceeded in high
conditions, but without addition of KOtBu. DLS indicated the
presence of nanoparticles of average 90 nm sizes in this case
(Figure S16). However, these nanoparticles proved to be
relatively ineffective for the hydrosilylation of acetophenone 5a
compared to the particles generated in the presence of KOtBu
yields, the reduction of propiophenone 5c proved to be harder,
a reaction time of 61 h being required in order to reach 74%
yield. Moreover, the hydrosilylation of more sterically hindered
ketones was limited.[17] Indeed, the reductions of 2-methyl-1-
phenylpropan-1-one 5d, dicyclohexylmethanone 5e and 2-
acetylmesitylene 5f led to much lower yields. Furthermore, it is
worth noting that the hydrosilylation of the conjugated ketone,
4-phenylbut-3-en-2-one 5g, proceeded with 75% yield in 24 h
and implied the reduction of both the carbonyl and the
conjugated CÀ C double bond, thus leading to the saturated
alcohol 6h. By comparison, the reduction of 4-phenylbutan-2-
one 5h led to 6h in a similar yield. The hydrosilylation of
cyclohexanone 5i afforded the corresponding cyclohexanol 6i
in 57% yield after 24 h reaction, but an extended reaction time
to 61 h resulted in 92% yield. The reduction of other cyclic
ketones 5j–l comprising an additional aromatic proceeded in
moderate to good yields.
°
(22% yield after 19 h at 100 C vs. 74% yield in the presence of
KOtBu, see Table 2, entry 15).
Afterwards, we evaporated a DLS sample of 2a (2 mol%),
°
KOtBu (2 mol%) and PhSiH3 prepared in toluene at 100 C on a
glass support in an argon filled glovebox; the remaining oily
solid residue was subsequently analyzed by X-ray diffraction
(XRD) in the reflexion mode (Figure S17 and Figure S18). Most
of the observed diffractions were fitted with the diffraction
patterns of KBr. Such a result can be attributed to the poor
crystallinity of the sample. This rendered the observation of NiO
(most probably formed after air exposure) difficult, and there
was no evidence for the presence of Ni(111), e.g. Ni(0) species.
However, XRD is mainly a bulk technique, which is hardly
accurate for the analysis of such small and dispersed samples.
Two similar samples, one fresh and one aged, were then
analyzed by XPS in order to determine the chemical composi-
tion at the topmost surface (i.e.: 9�2 nm) of the catalysts. The
survey scan and high-resolution spectra of the freshly prepared
sample revealed the core levels of C1s, O1s, N1s, Ni2p, Br3d and
Si2p elements (Figure S19 and Figure S20). More specifically,
the peaks values for Ni2p3/2 and 2p3/2 satellite of the Ni2p region
are found at 853.6 and ca. 861.7 eV, respectively (Figure 2, red).
These values are consistent with the presence of NiO, which
could result from the oxidation of the Ni(0) species during the
transport of the sample in air, as observed by XRD. However, it
is worthy to note that, due to the weak signal-to-noise ratio, the
assignment of the Ni(0) vs. Ni(2+) is not straightforward, and
the presence of some residual Ni(0) cannot be completely ruled
out in this sample (Figure S20 and Figure S21).[22] Raman
spectroscopy, carried out on a sample whose exposure to air
was avoided, indeed showed no evidence of Ni oxides (150–
430 cmÀ 1) and Ni hydroxides (3100–3650 cmÀ 1) (Figure S22).
Regarding the activation effect of KOtBu[15,16,18] and the
reaction mechanism at play in these ketones’ hydrosilylations, it
is of interest that the reductions of 5a–l were characterized by
a striking color change from clear brown to deep black within a
few minutes (Figure S6 and Figure S7). To gain some insight
into the nature of the active species, we prepared a toluene-d8
solution comprising complex 2a (2 mol%), KOtBu (2 mol%) and
phenylsilane, but without the ketone reagent, and heated up to
°
100 C for 1 h. The same color change was observed. After
cooling the reaction medium to room temperature, a sample
1
was placed in a sealed tube and analyzed by H and 29Si NMR
spectroscopy. The resulting 1H NMR spectrum did not show any
hydride intermediate[4e,18,19] nor any [Ni(I)Cp(NHC)][18,20] or other
well-defined Ni species, and further interpretations proved to
be difficult. The 29Si NMR spectrum did not allow us to observe
the pentacoordinate siliconate species, [PhSiH3(OtBu)]À (at
δ29Si=À 96.2 ppm), that was reported by Thomas et al. to act as
an in situ generated universal reducing species of a relatively
wide range of pre-catalysts of Fe(II), Co(II) and Mn(II) for
hydrosilylation reactions when NaOtBu was used as an activator
in conjunction with PhSiH3.[15]
Despite the change of color to deep black, the course of the
catalytic reduction of acetophenone under the conditions of
Scheme 4 remained unchanged when performing a hot filtra-
tion test after 19 h reaction, and similarly afforded 6a in 96%
yield after 24 h reaction. Furthermore analyses of both a sample
of 2a (2 mol%), KOtBu (2 mol%) and PhSiH3 and a sample of 2a
°
(2 mol%) and PhSiH3, prepared in toluene at 100 C, by GC-MS
and IR spectroscopy showed no evidence of NHC ligand
degradation to the corresponding azolone (Figures S8–S14),
therefore allowing us to rule out an Ananikov type activation/
deactivation mechanism.[16] Nevertheless, when we performed a
measurement by DLS[21] on a similar sample of 2a (2 mol%),
°
KOtBu (2 mol%) and PhSiH3 prepared in toluene at 100 C, we
observed a fine distribution of particles of ca. 310 nm wide
(Figure S15). To establish whether these particles were gener-
°
ated by the combined action of PhSiH3 and KOtBu at 100 C or
the sole action of PhSiH3 at this temperature, we next prepared
a sample from complex 2a and PhSiH3 in toluene under similar
Figure 2. XPS spectra of Ni 2p3/2. Red: fresh sample; orange: aged sample.
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Interestingly, no Ni was detected by XPS in the aged sample
(Figure 2, orange), suggesting the progressive agglomeration of
the nanoparticles (filtration onto 200 nm PTFE filters was carried
out before analysis), and thus their instability. Furthermore, the
% surface ratio of all the elements confirmed the presence of C,
O, N, Si, Br and Ni elements in the fresh sample and the Ni/N
ratio of 1:4 suggested the presence of NHC-picolyl-coordinated
Ni particles. As expected, the old sample contained only C, O, N
and Si elements (Table S3).
Evaporation in an argon filled glovebox of a similar sample
of 2a (2 mol%), KOtBu (2 mol%) and PhSiH3 on a membrane
support allowed analyses by SEM and SEMÀ T microscopies.
Some faceted particles with a dimension of ca. 200 to 600 nm
were observed which could be attributed to the Ni-based
material (Figure 3 and Figure S22). SEM-EDX chemical analysis
range of aldehydes without additive and of ketones in the
presence of KOtBu at 100 C. Single-crystal and VT 1H NMR
°
studies tend to show the absence of a hemilability of the
picolyl-functionalized carbene ligand. Studies by DLS, trans-
mission mode SEM, XPS, ICP-AES and CHN elemental analyses
showed evidence for the involvement of NHC-picolyl Ni
particles in the catalytic reactions when KOtBu is used as an
additive. Though KOtBu clearly acts has an activator leading to
the formation of catalytically active particles, together with
PhSiH3, their detailed implication in this process remains
unclear. Further studies on such NHC-stabilized-Ni nanocatalysts
are currently in progress in our laboratories and will be reported
in the future.
confirmed that the observed particles were composed of C, N, Experimental Section
O, Ni, Br, and Si elements, at least on their surface (Figure S23,
Table S3). To get further insight into the bulk composition of
these 310 nm particles and get a reliable estimation of the
number of NHC-picolyl ligands that remain bound to the nickel,
two samples were then analyzed by ICP-AES and CHN elemental
analyses. Mass percentages of 6.3�0.2 (Ni) and 3.16�0.4 (N)
were measured, which corresponds to a Ni:N molar ratio of
1:2, thus to a Ni:NHC ratio of 1.5:1. These results confirm the
coordination of NHC-picolyl ligands to the Ni nanoparticles and
highlight, to the best of our knowledge, only the third example
of NHC stabilized nanoparticles.[23,24]
Synthesis of [NiCp{Bn-NHC-(CH2)-Py}](Br) (2a)
A 10 mL vial containing a stirring bar was charged with nickelocene
(106 mg, 0.561 mmol), 1-mesityl-3-(2-picolyl)imidazolium bromide 1
(200 mg, 0.558 mmol), and THF (5 mL). After sealing the vial, the
°
green suspension was heated at 110 C for 0.5 h in a Discover CEM
S-class microwave operating at 2.45 GHz. The resulting suspension
was filtered through a Celite pad that was washed with THF (3×
10 mL). The solvent was then removed under vacuum, and the
resulting residue triturated in pentane (5 mL) to afford 2a as a
green powder after solvent removal with a syringe and vacuum
drying (109 mg, 0.223 mmol, 41% yield). Anal. calcd for
C23H24BrN3Ni: C, 57.43; H, 5.03; N, 8.74; found: C, 57.12; H, 4.96; N,
8.81. ESI-MS (TOF): m/z 400.1318 calcd. for C23H24N3Ni [MÀ Br]+,
1
400.1318, found 400.1298. H NMR (CDCl3, 300 MHz): δ 8.61 (s, 1 H,
Conclusion
3
3
NCH), 8.51 (br. d, J=7.5, 1H, 3- or 6-HPy), 8.30 (d, J=5.4, 1H, 3- or
3
3
6-HPy), 7.84 (t, J=7.5, 1H, 4- or 5-HPy), 7.08 (t, J=5.1, 1H, 4- or 5-
HPy), 7.05 (s, 2H, m-HMes), 6.75 (s, 1H, NCH), 6.32 (s, 2H, CH2), 4.99 (s,
5H, C5H5), 2.39 (s, 3H, p-CH3), 2.03 (s, 6H, o-CH3). 13C{1H} NMR (CDCl3,
75 MHz): δ 161.0 (NCN), 158.3 (3-Cpy), 157.1 (1-Cpy), 140.1 (ipso-CMes),
139.4 (5-Cpy), 135.6 (p-CMes), 135.2 (o-CMes), 129.5 (m-CMes), 128.1 (6-
Cpy), 126.1 (4-Cpy), 124.2 (NCH), 122.9 (NCH), 92.4 (C5H5), 53.8 (CH2),
21.3 (p-CH3), 18.2 (o-CH3).
In this work, we have shown that an accessible cationic half-
sandwich Ni(II)-NHC-picolyl complex 2a bearing a bromide
counter-ion effectively catalyzes the hydrosilylation of a broad
Synthesis of [NiCp{Bn-NHC-(CH2)-Py}](PF6) (2b)
An oven dried Schlenk tube containing a stirring bar was loaded
with 2a (300 mg, 0.624 mmol), KPF6 (0.115 mg, 0.625 mmol) and
THF (4 mL). The resulting green suspension was stirred at room
°
temperature overnight and at 40 C for 40 min. The reaction
medium was then cooled to room temperature and its contents
filtered over a pad of Celite that was washed with THF until the
solvent went colorless. The filtrate was evaporated to dryness to
give 2b as a green powder (338 mg, 0.619 mmol, 93% yield). Anal.
calcd for C23H24F6N3NiP+1= C4H8O: C, 51.57; H, 4.85; N, 7.22; found:
2
C, 51.48; H, 5.01; N, 6.95. ESI-MS (TOF): m/z 400.1318 calcd. for
C23H24N3Ni [M-PF6]+, found 400.1310. 1H NMR (CDCl3, 300 MHz): δ
8.36 (d, 3J=5.7, 1H, 3-HPy), 7.89 (m, 1H, 4- or 5-HPy), 7.85 (dd, 3J=7.5,
3
3
4J=1.5, 1H, 6-HPy), 7.82 (d, 3J=1.8, 1H, NCH), 7.12 (ddd, J=7.2, J=
5.7, 4J=1.5, 1H, 4- or 5-HPy), 7.06 (s, 2H, m-HMes), 6.80 (d, 3J=1.8, 1H,
NCH), 5.68 (s, 2H, CH2), 4.99 (s, 5H, C5H5), 2.39 (s, 3H, p-CH3), 2.05 (s,
6H, o-CH3). 13C{1H} NMR (CDCl3, 75 MHz): δ 162.5 (NCN), 158.8 (3-
Cpy), 156.2 (1-Cpy), 140.2 (ipso-CMes), 139.5 (5-Cpy), 135.4 (p-CMes), 135.2
(o-CMes), 129.6 (m-CMes), 126.9 (6-Cpy), 125.0 (4-Cpy), 124.6 (NCH),
123.4 (NCH), 92.6 (C5H5), 54.4 (CH2), 21.3 (p-CH3), 18.1 (o-CH3).
Figure 3. MEB-STEM image of Ni NHC-picolyl nanoparticles.
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General procedure for the hydrosilylation of aldehydes 3into
alcohols 4
Keywords: Carbene ligands · Carbonyls · Hydrosilylation ·
Nanoparticles · Nickel
An oven dried Schlenk tube containing a stirring bar was loaded
with the nickel pre-catalyst 2a (9.6 mg, 0.02 mmol) and dry THF
(4 mL). To the resulting green solution was added the aldehyde
(1.00 mmol) and PhSiH3 (148 μL, 1.20 mmol), in this order, and the
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1
The conversion was determined by H NMR spectroscopy, and the
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Acknowledgements
The CNRS, the University of Strasbourg, and the Agence Nationale
de la Recherche (ANR 2010 JCJC 716 1) are acknowledged for
supporting and funding partially this work (post-doc of S.
Shahane). The Institut Universitaire de France is acknowledged for
its support (V.R.), and the Région Grand Est for the doctoral
fellowship of F. Ulm. This work has also benefitted from support
provided by the University of Strasbourg Institute for Advanced
Study (USIAS) for a Fellowship, within the French national
program “Investment for the future” (IdEx-Unistra) (C.M. and C.P.-
H.) Mrs. Corinne Bailly and Dr. Lydia Karmazin are acknowledged
for the X-ray structural determinations. Mrs. Alexandra Sutter
(ICPEES) is gratefully thanked for her assistance with DLS measure-
ments and discussion. Dr. Emeric Wasielewski (LIMA) is thanked
for his assistance with NMR experiments.
Conflict of Interest
The authors declare no conflict of interest.
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Manuscript received: May 6, 2021
Revised manuscript received: July 7, 2021
Accepted manuscript online: July 8, 2021
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FULL PAPERS
Dr. F. Ulm, Dr. S. Shahane, Dr. L.
Truong-Phuoc, T. Romero, Dr. V. Pa-
paefthimiou, M. Chessé, Prof. M. J.
Chetcuti*, Dr. C. Pham-Huu, Dr. C.
Michon*, Prof. V. Ritleng*
1 – 9
Half-Sandwich Nickel(II) NHC-
Picolyl Complexes as Catalysts for
the Hydrosilylation of Carbonyl
Compounds: Evidence for NHC-
Nickel Nanoparticles under Harsh
Reaction Conditions
A half-sandwich nickel NHC-picolyl
complex effectively catalyzed the hy-
drosilylation of aldehydes and
ketones. Studies by DLS, STEM, XPS,
ICP-AES and elemental analyses
showed evidence for the involvement
of NHC-stabilized nickel nanoparticles
when potassium t-butoxide is used as
an activator.