J. Chang et al.
Inorganica Chimica Acta 515 (2021) 120088
be associated with the co-catalytic role of some in situ generated thiolato
species [45].
energies were evaluated at SMD (toluene)/B3LYP-D3/6–311++G(d,p)/
SDD(Ni) level [56].
Encouraged by the excellent catalytic activity of the above nickel
thiolate complexes in catalyzing the hydroboration of carbon dioxide,
3. Results and discussion
we decided to apply these nickel pincer complexes to the catalytic
t
hydrosilylation of aldehydes. We found that complexes [2,6-( Bu
2-
3.1. Catalytic activity of complexes 1–5 in catalyzing the hydrosilylation
of aldehydes
t
PO)
2
6
C H
3
]NiSH (1) and [2,6-( Bu
PO)
2 2
6
C H
3
]NiSBn (2) are active in
catalyzing hydrosilylation of aldehydes with phenylsilane. The corre-
sponding alcohols were isolated in good to excellent yields following
basic hydrolysis of the resultant hydrosilylation products. Complexes 1
The nickel pincer complexes used in the present study are shown in
Chart 1. It was reported that toluene is the best solvent for the hydro-
silylation of aldehydes and ketones catalyzed by the corresponding
POCOP pincer ligated nickel and platinum hydride complexes [37,49].
Therefore, our investigation began with the reactions of benzaldehyde
with different silanes in the presence of 0.2 mol % of the pincer nickel
and 2 serve as catalyst precursors which generate the corresponding
t
nickel hydride complex [2,6-( Bu
2
PO) C
2 6
3
H ]NiH in situ, and the nickel
hydride complex is the active species that catalyzes this hydrosilylation
process. The catalytic efficiency with complex 1 is comparable to that
with the corresponding POCOP pincer nickel hydride complexes.
◦
complexes (1–5) at 70 C in toluene. The reactions were monitored by
GC–MS and the final reaction mixtures were treated with 10 wt% of
aqueous sodium hydroxide solution followed by flash column chroma-
tography to isolate the resultant benzyl alcohol. The results of the initial
investigation are summarized in Table 1.
2
. Experimental section
2
.1. General information
It can be seen from Table 1 that both complexes 1 and 2 are active in
catalyzing the hydrosilylation of benzaldehyde and complex 1 is more
active than complex 2. Phenylsilane is the best silyl reagent for the
present hydrosilylation process. The efficiency of the five silyl reagents
Standard Schlenk techniques were used for the present study. All of
the solvents were distilled from standard drying agents [48] and
degassed before use. NMR spectra were recorded on a Bruker Advance
6
00 or 400 MHz spectrometer. The residual solvent resonances were
investigated follow the series of PhSiH
3
≫ Ph
2 2 3 3
SiH > (EtO) SiH, Ph SiH,
1
13
used to reference the chemical shift values internally for H and
C
3
Et SiH, which is similar to the hydrosilylation of aldehydes catalysed by
NMR spectra and H PO (85%) was used as a standard to calibrate the
3
4
POCOP pincer nickel hydride complexes [37]. When complex 3 or 4 or 5
was used as the catalyst or when the reaction was carried out in the
absence of the nickel complexes (the last entry), the hydrosilylation
3
1
chemical shifts externally for P NMR spectra. GC–MS analyses were
carried out by using a SHIMADZU-(GCMS-QP2020) instrument. Com-
plexes 1–5 were synthesized according to the published procedures
3
reaction did not occur even the best silyl reagent (PhSiH ) was used. It
[
41–44].
must be noted that no hydrosilylation was detected if the reaction was
◦
carried out at temperatures below 70 C in toluene even 5 mol% of
2
.2. General procedure for catalytic reactions
complex 1 or 2 was used.
The scope of the aldehyde substrate for the present hydrosilylation
was then explored using 0.2 mol % of complex 1 as the catalyst and a
Aldehyde (10 mmol) was mixed with silane (12 mmol) in a 10 mL
flame-dried Schlenk flask. Solvent (6 mL) containing the nickel complex
slight excess of PhSiH
3
(1.2 equiv.) as the silyl reagent in three different
◦
◦
(
0.02 mmol) was then added. The reaction mixture was stirred at 70 C
solvents (toluene, THF and DMF) at 70 C. The results are summarized in
Table 2.
and the reaction was monitored by using GC–MS. The reaction was
stopped after a period of time or after aldehyde was completely
consumed. The solvent was removed under vacuum and the resultant
residue was treated with a 10 wt% of aqueous sodium hydroxide solu-
tion (10 mL). The aqueous solution was extracted with diethyl ether for
No significant side reactions occurred no matter what solvent was
used for the reaction as indicated by GC–MS analysis. As shown in
Table 2, toluene is not the best solvent for the present hydrosilylation
process. Instead, polar solvents (DMF and THF) are better. In most cases,
the efficiency of the three solvents follow the series of DMF > THF >
toluene. This is different from the hydrosilylation of aldehyde catalyzed
by the corresponding POCOP pincer nickel or platinum hydride com-
plexes [37,49]. The reactions are tolerant of many functional groups
2 4
three times, dried over anhydrous Na SO , and concentrated under
vacuum. The desired alcohol was further purified by flash column
chromatography on silica gel using petroleum ether/EtOAc as the
eluent. Conversions were calculated based on the GC analysis. The
characterization data of the isolated alcohol products are provided in
Appendix A. The NMR spectra were in good agreement with those re-
ported in the literature [37,49–54].
–
–
including methoxyl group, halogens, nitro group and C C. In addition
to substituted benzaldehydes, other aromatic aldehydes can also be
efficiently reduced. 2-pyridinecarboxaldehyde was reduced very quickly
ꢀ 1
◦
with a turnover frequency (TOF) of 2500 h in DMF at 70 C (entry 13),
which is higher than most of the reported TOF values for nickel cata-
lyzed hydrosilylation of carbonyl compounds [28–36]. However, it
would be difficult to rationalize the relationship between the structure
and property of the substrate and the relative hydrosilylation rate from
Table 2 without detailed kinetic studies for each substrate.
2
.3. Experimental procedure for the NMR tube reactions
The pincer nickel complex (0.01 mmol) was dissolved in toluene‑d
8
(
3
0.5 mL) in a NMR tube. Benzaldehyde (0.20 mmol) or PhSiH (0.20
3
1
1
mmol) was then added. The reaction was monitored by P{ H} NMR
spectroscopy at room temperature for 48 h. Then the NMR tube was
Under the same conditions, hydrosilylation of acetophenone was
explored in toluene, THF and DMF using 5 mol% of complex 1 as the
◦
heated to 70 C and the reaction was further monitored at this tem-
perature for an additional 24 h.
2
.4. Density Functional Theory (DFT) calculation
X
tBu
Ni
P Bu
t
P
2
X = SH, 1
SCH2Ph, 2
2
The DFT calculation was carried out by using Gaussian 09 program
[
55]. The B3LYP functional was used with a standard 6–31 + G(d) basis
O
O
SPh,
3
set (SDD basis set for Ni) to optimize the geometries of the structures.
The frequency calculations were performed for all stationary points to
confirm them as local minima or transition state (TS) structures and to
NCS, 4
N3, 5
ꢀ 1
derive Gibbs free energies (ΔG, kcal mol at 298 K). The single point
Chart 1. The nickel pincer complexes used in the present study.
2
Chang, Jiarui
