ChemComm
Communication
Table 1 Selectivity reaction/catalytic reduction of unsaturated organic
compounds with Cu8-H
Cu8-H
equiv.
Yield
Entry Substrate
Product
(TON)c %
1
2
0.17a
0.17
90
—
No reaction
Fig. 3 1H NMR (CD2Cl2) spectrum of (a) 1H and (b) 1D.
3
4
5
0.17
70
opposite Cu3 triangles to form two tetrahedra in Cu8-H. Thus,
the two m3-HÀ in Stryker’s reagent turn into m4-HÀ in Cu8-H.
The 1H NMR peak of Cu8-H shows a singlet resonance at
2.46 ppm in CD2Cl2 which is not observed in Cu8-D (Fig. 3). The
integration ratio of H in dppy ligands to hydrides is calculated
to be 14.4 : 1 (14 : 1 in theory, Fig. S8, ESI†), which evidences
that the number of hydrides is 6. The 2H NMR peak of Cu8-D in
CH2Cl2 exhibits a broad resonance at 2.54 ppm (Fig. S9, ESI†),
also confirming the presence of hydrides in the cluster. The
chemical shift for the hydrides in Cu8-H has a similar value to
that of different hexameric clusters: the hydride signal of
0.3%b
0.3%
98 (326)
65 (215)
6
0.3%
No reaction
—
Cu6H6(PPh3)6 appears near 3.50 ppm13 and
a singlet
a
Reaction conditions: 28 mg Cu8-H (8 Â 10À3 mmol), PhCRCH 5.5 mL
b
(2.81 ppm) is detected when P(NMe2)3 is used as the ligand.21
The presence of two types of hydrides in Cu8-H is confirmed by
the 1H NMR resonance peaks at 2.81 and 2.19 ppm (2 : 1) in
CD2Cl2 at À80 1C (Fig. S10, ESI†). In addition, the 31P NMR
peak of Cu8-H in CD2Cl2 shows one broad singlet at 0.47 ppm
(full width at half-height B90 Hz) due to the fluxional behavior
of the cluster, which makes it impossible to determine the JP–H
(Fig. S11, ESI†).
(0.05 mmol), H2O 2 mL, DCM 3 mL, stirred for 4 h. Reduction
conditions: 1 mg Cu8-H (3 Â 10À4 mmol), trans-Benzalacetone 14 mL
(0.1 mmol), Ph2SiH2 25 mL (0.2 mmol), EtOH 2 mL, stirred for 12 h at
c
30 1C. The turnover number is based on Cu8-H. The conversion and
selectivity were determined by NMR analysis.
yield, and no 1,2-reduction of the carbonyl moiety was
observed. The comparable activity and selectivity of Cu8-H to
Stryker’s reagent make it a useful hydrogenation reagent.8,10
Due to the different nature between Stryker’s reagent and
Cu8-H, such as charges and solubility, we believe that Cu8-H
may play a complementary role in different solvents.
Stryker’s reagent is air-sensitive. Interestingly, Cu8-H was
found to be stable under ambient conditions. In the solid state,
Cu8-H is not sensitive to air and moisture at least for a couple of
weeks, and can stay intact in CH2Cl2 at room temperature for
more than 6 hours as evidenced by the 31P NMR spectra
(Fig. S12, ESI†). It is much more stable in an anaerobic solvent
or at low temperature (5 1C). The good stability of Cu8-H makes
it a promising copper hydride complex for various applications.
We examined the reaction of Cu8-H with organic com-
pounds containing unsaturated groups, such as phenylacety-
lene and cinnamaldehyde. As shown in Table 1 (entry 1), 90%
conversion and 100% selectivity for styrene were obtained in
CH2Cl2 (Fig. S13, ESI†). It should be noted that water is
necessary to supply protons.10 Because a small amount of H2
was produced as detected by 1H NMR, it is suggested that more
than 1/6 equivalent of Cu8-H is required to completely consume
the starting materials. In contrast, no 1,2-diphenylethyne was
reduced under the same conditions, indicating that the inter-
nal alkyne is difficult to adsorb on the catalyst. Although a 91%
yield was obtained at elevated temperature (80 1C), only trace
conversion was observed in benzene at room temperature after
24 h with Stryker’s reagent,10 which is very similar to our case
with Cu8-H in CH2Cl2. In addition, the conjugated reduction of
a,b-unsaturated carbonyl compounds has also been realized.
Cinnamaldehyde was reduced to 3-phenylpropanal at 70%
Moreover, Cu8-H was found to show excellent performance
toward site-specific catalytic reduction of a,b-unsaturated
organic compounds in the presence of Ph2SiH2. Two substrates,
trans-Benzalacetone (entry 4) and b-Nitrosytrene (entry 5) were
examined, and only CQC hydrogenation products were
detected with the turnover number up to 326 and 215 based
on Cu8-H, respectively. These facts indicate that Cu8-H provides
active catalytic centers that can be regenerated with silyl
hydrides, and the hydrogenation takes place through the 1,
4-reduction mechanism as previously described with Cu6H6
(PPh3)6.13 Therefore, Cu8-H is a very useful alternative to
Stryker’s reagent. When 3-nitrostyrene was tested under the
same conditions, neither CQC nor nitro group was found to be
reduced. This means that Cu8-H is completely inert toward
unactivated alkenes.
In summary, a novel copper hydride reagent, [Cu8H6
(dppy)6](OTf)2, was prepared with the stabilization of dppy
ligands. The precise structure was determined using X-ray
single crystal diffraction. The facile preparation and the stabi-
lity under ambient conditions make Cu8-H a promising hydro-
genation catalyst, which offers a very useful alternative to
Stryker’s reagent.
This journal is © The Royal Society of Chemistry 2021
Chem. Commun., 2021, 57, 4315–4318 | 4317