Activation of Si-H bonds across the nickel carbene bond in electron rich nickel PCcarbeneP pincer complexes

Article abstract of DOI:10.1039/c5cc09349j

Silicon-hydrogen bonds are shown to add to a nickel carbon double bond to yield nickel α-silylalkyl hydrido complexes. Kinetic and isotope labeling studies suggest that a concerted 4-centred addition across the NiC bond is operative rather than a mechanism involving Si-H oxidative addition. This constitutes an example of Si-H bond activation via ligand cooperativity.

Full text of DOI:10.1039/c5cc09349j

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Activation of Si-H bonds across the nickel carbene bond in
electron rich nickel PCcarbeneP pincer complexes†
Etienne A. LaPierre, Warren E. Piers, Denis M. Spasyuk and David W. Bi
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Silicon-hydrogen bonds are shown to add to a nickel carbon key step leading to product. We were interested in how these
double bond to yield nickel α-silylalkyl hydrido complexes. Kinetic compounds would interact with E-H bonds in which the
and isotope labeling studies suggest that a concerted 4-centred polarity is reversed and the hydrogen is more hydridic in
addition across the Ni=C bond is operative rather than
a
character. Accordingly, we have explored the reactivity of this
mechanims involving Si-H oxidative addition. This constitutes an system with various tertiary silanes and describe evidence for
example of Si-H bond activation via ligand cooperativity.
direct, concerted addition of Si-H across the Ni=C bond such
that the silicon is delivered to the nucleophilic carbene carbon.
The tert-butyl nitrile stabilized PCcarbeneP complex 19 reacts
with triphenylsilane in benzene or THF to yield a yellow
solution of a nickel hydrido product with a characteristic
upfield triplet resonance at -13.56 ppm (2JHP = 62 Hz). The
disappearance of the carbene carbon signal at 110.2 ppm in
the 13C NMR
The homogeneously catalysed hydrosilation of unsaturated
functions is a commercially important process that relies on
the facile activation of Si-H bonds. While mechanisms that rely
on sigma bond metathesis or heterolytic hydride abstraction
pathways are operative in early transition metal1 and main
group element catalysts,2 respectively, for late metals this
activation typically takes place via oxidative addition.3
Activation mechanisms that involve
a ligand engaging
cooperatively in the Si-H bond cleavage step are less
common.4-8
Recently, we described a family of nickel PCcarbeneP pincer
complexes which rapidly added protic E-H bonds (E = RCC-,
R2N- and RO-) across the anchoring C=Ni linkage such that the
nucleophilic E group ends up bonded to nickel (Scheme 1).9, 10
In this system, the Ni carbene adopts a nucleophilic, “Schrock-
like” character11, 12 and likely acts to deprotonate E-H in the
Scheme 1. Addition of protic E-H bonds to Ni=C.
Scheme 2. Addition of tertiary silanes to Ni=C; synthesis of compounds 3.
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, and detection of a 29Si signal at -4.2 ppm via a appear to be an agostic interaction present between the C-H
1H-29Si HMBC experiment suggested the Ph3Si group is bond on C(39) and the Ni(1) center based on the long Ni(1)-
attached to anchoring carbon of the PCalkylP ligand as depicted C(39) distance of 3.051Å. Furthermore, the open C(1)-Si(1)-
for 3Ph in Scheme 2. Purification of 3Ph from these reactions C(38) angle of 116.5(2)˚ is suggestive of a repulsive rather than
was complicated by the nitrile byproduct, so a second protocol an attractive interaction.
for the preparation of 3Ph was developed, wherein the PCalkyl
nickel (II) bromide
9 was dehydrohalogenated using KHMDS in protocol can be used to prepare the related derivatives 3p-Me-
the presence of triphenylsilane. Here, the presumed THF 3p-OMe-Ph and 3PhMe2 in moderate to good yields. Each was
solvated PCcarbeneP intermediate shown in Scheme 2 is rapidly fully characterized by multinuclear NMR spectroscopy and X-
trapped by the silane rather than the nitrile ligand as in the ray crystallography; details can be found in the ESI (Figures S1-
synthesis of . We propose as the intermediate in this 3). The phenyldimethylsilyl derivative PhMe2 was also prepared
process because the known compound (PCalkylP)NiN(SiMe3)2 by treating 3Ph with a moderate excess of PhMe2SiH in THF
does not form when
is treated with KHMDS in THF solvent;9 (Scheme 2). Notably, this reaction does not take place in
furthermore, separately prepared amido complex does not benzene solution, suggesting that incorporates ligated THF.
react with silane under the conditions of this experiment. For these two silanes, the equilibrium lies strongly towards the
Since the preformed carbene complex also engages in this less hindered 3PhMe2 derivative, but with addition of an excess
reactivity, it seems likely that is the key species in the of Ph3SiH to PhMe2, the equilibrium can be driven partially back
formation of 3Ph Thus, as is generated through towards Ph. These observations are significant because they
dehydrohalogenation of in THF, it reacts rapidly with the illustrate that silane addition to the nickel carbene linkage of
silane present to yield the observed product. By the putative is reversible and thus provide a means by which the
dehydrohalogenation method, 3Ph is easily isolated from the mechanism of the activation of Si-H by can be probed.
non-basic HN(SiMe3)2 byproduct in 80% yield as analytically Given the system of chemistry depicted in Scheme 2, there
pure yellow crystals; its structure was confirmed by X-ray are two avenues possible for examination of this mechanism:
P
As depicted in Scheme 2, this dehydrohalogenative
2
,
Ph
I
1
I
3
2
I
1
I
3
.
I
3
2
I
I
crystallography.
study of the reactions of
1
with various silanes, or investigation
Si via the trapping of by
The molecular structure of
3
Ph is shown in Figure 1, along of the loss of silane from compounds
3
I
with selected metrical data. The Ni(1)-C(1) distance of another reagent. We chose the latter, since in that instance
2.043(4)Å is elongated by 0.04-0.06Å in comparison to related the reactivity central to Si-H bond transformation is likely rate
PCalkylP compounds9,
10
due to the strong trans influencing limiting; in the reactions of 1, loss of the nitrile ligand is rate
hydrido donor and the steric presence of the Ph3Si group, limiting and so the key Si-H bond activation step is not probed
which also causes the phenyl group linking C(1) and P(1) to tilt directly. By the principle of microscopic reversibility,13 the
44.2(6)˚ out of the plane defined by the aryl group connecting mechanism of Si-H bond activation by
I can be inferred from
P(2) and C(1). Although the plane of one of the phenyl rings on details of Si-H bond formation from compounds
3.
the silicon atom lies closely along the C(1)-Ni(1) vector, there
does not
Accordingly, we examined the kinetic behaviour of the
reactions of compounds 3Si with an excess of phenol, which
rapidly—and irreversibly9—traps the carbene intermediate
I
to
form the (PCalkylP)NiOPh
4 (Scheme 3). Phenolate 4
was
prepared separately and fully characterized, including via X-ray
crystallography (see Figure S2 in the ESI for details). It is
characterized by a resonance in the 31P NMR spectrum at 38.4
ppm, distinct from those of compounds 3Si which all resonate
at around 56 ppm. Therefore, the conversions of 3Si to
monitored conveniently by 31P NMR spectroscopy.
4 were
Scheme 3. Kinetics of silane elimination from compounds 3Si
.
Figure 1. Molecular structure of 3Ph. Most hydrogen atoms are omitted for
clarity. Selected bond and non-bonded distances (Å): Ni(1)-C(1), 2.043(4); Ni(1)-
P(1), 2.1344(14); Ni(1)-P(2), 2.1079(14); Ni(1)-H(1), 1.32(5); C(1)-Si(1), 1.916(4);
C(39)-Ni(1), 3.051. Selected bond angles (˚): P(1)-Ni(1)-P(2), 163.42(5); C(1)-Ni(1)-
P(1), 89.99(12); C(1)-Ni(1)-P(2), 89.94(12); Si(1)-C(1)-Ni(1), 109.8(2); C(1)-Si(1)- to
C(38), 116.5(2).
These experiments show that conversion of compound 3Ph
4
is first order in [3Ph] (Figure S3) with a rate constant of
5.7(2) x 10-4 s-1. The rate of the reaction is independent of the
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amount of added phenol (2 – 10 equivalents), indicating that dissociates silane to give
the reaction of phenol with the product of silane loss from 3Ph gives
is rapid. Furthermore, the rate is not affected by the inclusion these scenarios, silicon-hydrogen bond cleavage involves
of additional excess triphenylsilane (0 – equivalents), nucleophilic attack by the carbene carbon on the silicon in the
indicating that k-1 is also small relative to k2. The first order concerted addition of Si-H across the Ni=C bond.
rate constants, however, did vary slightly with the nature of
Although based on the more commonly observed3 Si-H
the substitution on the silicon atom‡ and the rate constants oxidative addition, path
is unlikely for a few reasons. First, it
are shown in Scheme 3. Two labelling experiments were is not entirely consistent with the kinetic data obtained (see
conducted. In the first, the rate of conversion of Ph-d1 discussion below). Second, while oxidative addition of Si-H to
(labelled in the nickel hydride position) to
was followed. The Ni(0) is known,14-17 addition to Ni(II) to give Ni(IV) compounds
I; alternatively, direct loss of silane
I
without the intermediacy of III (path C). In either of
6
A
3
4
label was exclusively localized in the Ph3SiD product, and an is less precedented.18 Third, it is not easy to understand why a
inverse KIE (kH/kD) of 0.90(3) was recorded (Figure S4-5). In the species such as II would selectively lead to 3Ph in the reverse
second labelling experiment, the reaction was performed using reaction, since migration of the hydride ligand to the carbene
C6D5OD as the trapping agent and here the label ended up carbon to form the isomeric complex 5Ph should be
exclusively in the benzylic position of the PCalkylP ligand in
4
competitive with Ph3Si migration (Scheme 5).8 In support of
(Figure S6) and proteo Ph3SiH was selectively produced. this latter hypothesis, as shown in Scheme 5, the nickel silyl
Finally, an Eyring analysis of rate constants obtained for the complex 5Ph can be separately prepared via
reaction of 3Ph with PhOH (10 equiv) at four temperatures
between 50 and 80˚C gave activation parameters of
33(2) kcal mol-1 and S‡ = 24(2) cal mol-1 K-1 (Figure S7).
ꢀ =
H‡
ꢀ
Potential mechanisms for the elimination of silane from
compounds 3Si (and therefore the reverse, activation of silane
by the PCcarbenePNi fragment) are shown in Scheme 4. Path
A
invokes a migration of the R3Si group from carbon to nickel via
a 3-centred transition state to give the formally Ni(IV) silyl
hydride II, which reductively eliminates silane to generate
(perhaps via silane complex III), which is trapped by phenol to
give product . This would imply that silane addition to would
occur by oxidative addition of the Si-H bond to the Ni(II)
center. The other two plausible paths ( and in Scheme 4,
I
4
I
B
C
Scheme 5. Separate synthesis of nickel silyl isomer 5Ph
.
depending on whether or not silane complex III is on the path
for silane elimination) proceed through direct Si-H bond
formation via the 4-centred transition state depicted. Here,
the geometry of the nickel centre likely distorts towards
tetrahedral in order to achieve the spatial arrangement
necessary for Si-H bond formation. Such a transition state
treatment of
experiments and computations indicate that it is
thermodynamically preferred over 3Ph
2 with in situ generated KSiPh3 and both
.
17, 19, 20
Nickel silyl14,
isomer 5Ph is characterized by a 31P
NMR resonance at 52.8 ppm that exhibits 29Si satellites (2JP-Si
=
could lead to III, (
B
), which either reacts with phenol directly or
Ph
iPr2
Ph
PiPr2
Ph
P
Si
Ni
PiPr2
SiPh3
H
Ni
H
A
PiPr2
II
3 center TS
PiPr2
PiPr2
ꢀH‡ = 33(2) kcal molꢀ1
H
ꢀS‡ = 24(2) e.u.
Ph3Si
PhOH
SiPh3
H
Ni
Ni
4
PiPr2
PiPr2
B
III
3Ph
PhOH
PiPr2
Ph3Si
H
+THF
ꢀTHF
C
PiPr2
Ni
+ HSiPh3
Ni THF
PiPr2
P
iPr2
C
I
4 center TS
Figure 2. Molecular structure of 5Ph. Most hydrogen atoms are omitted for
clarity. Selected bond and non-bonded distances (Å): Ni(1)-C(13), 2.030(8); Ni(1)-
P(1), 2.194(2); Ni(1)-P(2), 2.168(2); Ni(1)-Si(1), 2.338(2). Selected bond angles (˚):
P(1)-Ni(1)-P(2), 153.26(9); C(13)-Ni(1)-Si(1), 161.7(2); C(13)-Ni(1)-P(1), 84.1(2);
C(13)-Ni(1)-P(2), 83.6(2); Si(1)-Ni(1)-P(1), 105.06(9); Si(1)-Ni(1)-P(2), 93,75(7).
Scheme 4. Potential mechanisms for Si-H bond formation/cleavage at
(PCcarbeneP)Ni fragments.
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45 Hz) and a triplet in the 29Si NMR spectrum at -0.2 ppm ((2JP-Si Funding for this work was provided by NSERC of Canada in the
= 44 Hz). Its molecular structure, determined by X-ray form of a Discovery Grant and an Accelerator Supplement to
crystallography, is shown in Figure 2 and features a Ni(1)-Si(1) W.E.P. W.E.P. also thanks the Canada Research Chair
bond length of 2.338(2)Å. When 3Ph is heated at reflux in THF secretariat for a Tier I CRC (2013–2020) and the Alexander von
for 24 hours, it converts to 5Ph, albeit with about 60% Humboldt Stiflung for a Research Award. The authors would
decomposition (Figure S8); these other products likely arise like to thank Lauren Doyle for performing the ground state
from decomposition of THF ligated carbene I, which clearly energy computations on compounds 3Ph and 5Ph.
forms reversibly under these conditions. In contrast, the 31P
NMR spectrum of 5Ph is unchanged after heating under the
same conditions for several days (Figure S9). Ground state
energy computations by DFT (B3LYP using the 6-31G** basis
set) on isomers 3Ph and 5Ph indicate that the silyl complex is 7.3
kcal mol-1 more stable than the hydride isomer 3Ph (Figure
S10). This indicates that 5Ph is the thermodynamic isomer,
further suggesting that it should be observed in the reactions
Notes and references
‡The variation in rate constant with silane substituent does not
correlate with the Hammett parameters. Rather, the rate
variations appear to be related to slight changes in Si-H bond
strength with changing substitution.
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(Scheme 4). We do not at present know much about the
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energy process than that which leads to elimination of R3SiH
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discussed above make it clear that silyl isomer 5Ph is not
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6.
4
7.
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8.
transition (
B
/
C
, Scheme 4) state are more consistent with our
. The substantially positive entropy of
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data, particularly path
C
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
23.
24.
activation is indicative of a transition state in which the silane
is well dissociated. The observed inverse KIE is also consistent
with these mechanisms; in a rapid equilibrium between 3Ph
and I + HSiPh3, the right hand side should be slightly favoured
when deuterium is incorporated since deuterium will be
preferred in the Si-H position (νSiH = 2119 cm-1) than the Ni-H
position
(νNiH
=
1781 cm-1). This would increase the
in the deuterated isotopologue leading to
concentration of
I
overall increased rates of production of
4 in the deuterated
system. The involvement of a silane complex akin to III is an
open question, but the lack of dependence on the rate of
added silane and the positive activation entropy favour direct
dissociation, i.e., path
In conclusion, we propose that the reaction of silanes with
the (PCcarbeneP)Ni(THF) intermediate occurs via concerted Si-H
C.
I
addition across the Ni=C linkage to produce 3Ph as the
kinetically preferred product. This mode of silane activation is
rare, having been proposed on the basis of kinetic and
labelling data only for highly nucleophilic Schrock type
carbenes21 where oxidative addition paths were not
available.22, 23 Here, the high energy expected for an oxidative
addition pathway involving Ni(IV) directs the reactivity to the
concerted path in a late metal carbene system. This reactivity
attests to the high nucleophicity associated with the Ni=C
linkage in these complexes. The reversibility of the silane
activation disclosed here is potentially exploitable in catalytic
cycles featuring ligand cooperativity.24
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