Synthesis, Characterization, and Reactivity of the Group 11 Hydrido Clusters [Ag6H4(dppm)4(OAc)2] and [Cu3H(dppm)3(OAc)2]

Article abstract of DOI:10.1021/acs.inorgchem.6b02385

The group 11 hydride clusters [Ag₆H₄(dppm)₄(OAc)₂] (1) and [Cu₃H(dppm)₃(OAc)₂] (2) (dppm = 1,1-bis(diphenylphosphino)methane) were synthesized in moderate yields from the reactio

Full text of DOI:10.1021/acs.inorgchem.6b02385

Article
pubs.acs.org/IC
Synthesis, Characterization, and Reactivity of the Group 11 Hydrido
Clusters [Ag6H4(dppm)4(OAc)2] and [Cu3H(dppm)3(OAc)2]
Andrew W. Cook, Thuy-Ai D. Nguyen, William R. Buratto, Guang Wu, and Trevor W. Hayton*
†Department of Chemistry and Biochemistry, University of California, Santa Barbara, California 93106, United States
S
* Supporting Information
ABSTRACT: The group 11 hydride clusters [Ag6H4(dppm)4(OAc)2] (1) and [Cu3H-
(dppm)3(OAc)2] (2) (dppm = 1,1-bis(diphenylphosphino)methane) were synthesized in
moderate yields from the reaction of M(OAc) (M = Ag, Cu) with Ph2SiH2, in the presence
of dppm. Complex 1 is the first structurally characterized homometallic polyhydrido silver
cluster to be isolated. Both 1 and 2 catalyze the hydrosilylation of (α,β-unsaturated)
ketones. Notably, this represents the first example of hydrosilylation with an authentic silver
hydride complex.
[Ag6H4(dppm)4(OAc)2] (1), as a colorless, crystalline solid
in 47% yield (Scheme 1).
opper hydrides have proven to be potent catalysts for the
Chydrogenation and hydrosilylation of α,β-unsaturated
ketones, carbon−carbon multiple bonds, and CO2.1−14 In
contrast, only a handful of examples of silver-catalyzed
hydrogenation and hydrosilylation are known.15−20 For
example, AgOTf has been shown to catalyze the hydrosilylation
of aryl aldehydes in the presence of Me2PhSiH and PEt3.16
Similarly, the semihydrogenation of alkynes by a hetero-
bimetallic Ag/Ru catalyst has been reported.19 In both cases,
catalysis is thought to proceed via a Ag−H intermediate, but
this hypothesis has yet to be confirmed, though recently a silver
hydride dimer, [{(SIDipp)Ag}2(μ-H)]X (SIDipp = 1,3-bis(2,6-
Scheme 1. Synthesis of Complexes 1 and 2
diiso-propylphenyl)imidazolin-2-ylidene; X− = OTf− or BF4 ),
−
was shown to stoichiometrically reduce CO2 to formate.21 This
limited understanding of Ag−H reactivity is due, in part, to the
difficulty in generating stable silver hydride complexes, which is
a consequence of this metal’s relatively high M(I)/M(0) half-
cell potential (0.80 V).22 Because of this property, AgI−H
complexes more easily decompose to yield Ag0 and H2 versus
their Cu counterparts. Consequently, AgI−H complexes have
proven to be relatively rare.21,23−38 Moreover, the examples
isolated thus far are either homometallic monohydrides, for
example, [Ag7H{Se2P(OiPr)2}6],31 heterometallic polyhydrides,
for example, [Ag(μ-H)4{Re2(μ-H)(CO)8}2]−,33 or polyhy-
drides observed via electrospray ionization (ESI) mass
spectrometry (e.g., [Ag10H8(dppm)6]2+)34 (dppm = 1,1-bis-
(diphenylphosphino)methane) or NMR spectroscopy (e.g.,
[Ag18H16(Ph3P)10]2+).39 Herein, we report the synthesis and
characterization of the first structurally characterized homo-
metallic silver polyhydrido cluster, [Ag6H4(dppm)4(OAc)2]. In
addition, we examined its ability to catalyze the 1,4- and 1,2-
hydrosilylations of (α,β-unsaturated) ketones.
Complex 1 crystallizes in the triclinic space group P1 as the
̅
benzene solvate 1·2.5C6H6 (Figure 1). In the solid state, the Ag
atoms of the [Ag6]6+ core are arranged in an octahedron. The
four dppm ligands coordinate to an equatorial belt formed by
four Ag atoms. Each dppm ligand is deflected out of the [Ag4]4+
plane, with two arranged above the plane and two below the
plane in an alternating fashion. While the four hydride ligands
were not located in the difference Fourier map, the arrange-
ment of the dppm ligands likely requires that the hydride
ligands occupy the four trigonal faces that are opposite a dppm
moiety with a μ3-binding mode. Lastly, the two acetate
counterions are bound to Ag atoms at axial positions of the
[Ag6]6+ core, via κ1 and κ2 binding modes.
The average Ag−Ag bond length within the [Ag4]4+ belt is
3.08 Å, which is longer than the average Agcap−Agbelt bond
Addition of 1 equiv of dppm and 0.5 equiv of diphenylsilane
(Ph2SiH2) to a slurry of Ag(OAc) in benzene results in a rapid
color change from white to dark red-brown. Workup of the
solution after stirring at room temperature for 15 h results in
the isolation of the silver polyhydrido cluster,
Received: September 29, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.6b02385
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plausible mechanism for cluster assembly, as dimerization of
[Ag3(dppm)3H(OAc)]+, concomitant with dppm dissociation,
would generate the Ag6 core.
We also briefly explored the chemical properties of 1.
Complex 1 is modestly soluble in C6H6 and tetrahydrofuran
(THF), insoluble in MeCN, Et2O, and nonpolar solvents, and
reasonably soluble in 1,2-dichlorobenzene. Complex 1 exhibits
only modest thermal stability. On standing overnight in C6D6,
it partially decomposes to H2 and silver metal, among other
products (Figure S23). An ESI mass spectrum of this sample
suggests that a mixture of larger silver nanoclusters is generated
(Figure S41); however, their identities remain unknown.
Complex 1 also reacts slowly with excess H2O (10 equiv) in
C6D6, forming Ag0, as well as other products.
To better understand the formation of 1 we recorded 1H and
31P NMR spectra of the crude reaction mixture. These spectra
reveal the presence of free dppm, complex 1, and an
unidentified silver hydride cluster, as indicated by broad
hydride ligand resonances at 2.40 and 3.60 ppm (Figure S21)
1
in the H NMR spectrum. Accordingly, we speculate that the
Figure 1. Ball-and-stick diagram of 1. All hydrogen atoms and solvent
molecules are omitted for clarity. Color legend: Ag = purple; P =
yellow-orange; O = red; C = gray wireframe.
modest yield of 1 is due to the competing formation of these
byproducts, as well as its gradual decomposition over the
course of the 15 h reaction time. The formation of these other
Ag clusters also likely accounts for the dark brown color of the
reaction mixture. Interestingly, if the reaction is performed
using a Ag-to-dppm ratio that corresponds to the molecular
formula (6:4), the yield of 1 drops precipitously. We suggest
that the Ag-to-dppm ratio shown in Scheme 1 (i.e., 1:1) results
in higher concentrations of dppm, which helps to stabilize
transient “AgI−H” monomers and reduces the probability of
decomposition.
length of 2.88 Å. The Agcap−Agbelt interaction is within the
covalent radius for Ag,40 and the Agbelt−Agbelt interaction lies
well within twice the van der Waals radius for Ag,41 suggestive
of considerable argentophilic interactions within the [Ag6]6+
core. These values are also similar to those reported for other
AgI−H clusters21,28−32,35,37,38 and Ag6 octahedra.42−48 Finally,
the average Ag−P distance (2.51 Å) is typical of Ag−P
bonds.32,35,38,49−54
In agreement with the high symmetry of complex 1 in the
solid state, its 31P{1H} NMR spectrum features a single 31P
resonance at 8.06 ppm in C6D6. This resonance features a
complicated J-coupling pattern, due to one- and two-bond
coupling to the 107Ag and 109Ag nuclei in the [Ag6]6+ core. The
1H NMR spectrum of complex 1 in C6D6 shows a single acetate
resonance at 2.92 ppm, while the diastereotopic methylene
protons of the dppm ligand appear at 3.02 and 5.44 ppm. Most
notably, a complicated multiplet, centered at 5.82 ppm and
integrating for 4H, is assignable to the four equivalent hydride
For further comparison, we endeavored to synthesize the Cu
congener of 1. Previous attempts to make a Cu−H dppm
cluster ended with decomposition because of the high basicity
of the [Cu(OtBu)]4 starting material.56 However, the combined
use of Cu(OAc) and Ph2SiH2 to generate the CuI−H moiety
obviates the use of an alkoxide precursor. Thus, addition of 1
equiv of dppm and 0.5 equiv of Ph2SiH2 to a slurry of Cu(OAc)
in benzene results in the gradual color change from pale green
to yellow. Workup of the solution after 20 h affords the copper
hydride cluster [Cu3H(dppm)3(OAc)2] (2), as colorless
crystals in 83% yield (Scheme 1). Complex 2 is closely related
to the known CuI−H clusters, [Cu3H(dcpm)3]2+ (dcpm = 1,1-
bis(dicyclohexylphosphino)methane) and [Cu3(BH4)H-
(dppa)3]+ (dppa = bis(diphenylphosphino)amine).57,58
Complex 2 crystallizes as the benzene solvate 2·2C6H6
(Figure 2) in the monoclinic space group P21/n. In the solid
state, complex 2 contains a triangular [Cu3]3+ core with an
average Cu−Cu distance of 2.91 Å. This distance is comparable
to the average Cu−Cu distance of 2.882(1) Å reported for
[Cu3H(dcpm)3]2+.57 The average Cu−P distance in 2 (2.28 Å)
is consistent with Cu−P bonds in other phosphine-supported
CuI−H clusters.13,56,57,59−64 The hydride ligand was located in
the difference Fourier map, and was found to display a μ3
binding mode. Lastly, the two acetate counterions both bind to
the Cu3 core, via κ1 and κ2 binding modes.
2
moieties. The H NMR spectrum of the isotopically labeled
analogue, 1-d4, features a resonance at 5.76 ppm in 1,2-
C6H4Cl2/C6D6 (99:1, v/v), confirming our assignment of this
feature as a hydride resonance. The 109Ag NMR spectrum of 1
in 1,2-C6D4Cl2, recorded using an INEPT pulse sequence,55
features a single resonance at 1095 ppm. This chemical shift is
in good agreement with the 109Ag signals observed for the AgI−
H dichalcogenatophosphate clusters reported by Liu and co-
workers.28−31 However, only one resonance was observed in
the spectrum, despite the presence of two unique Ag
environments in this complex, which may be a consequence
of poor signal-to-noise due to its low solubility. Finally, the ESI
mass spectrum of 1 features a signal at 2246.942 m/z
corresponding to [M−OAc]+ (calculated m/z 2246.954). For
comparison, the ESI mass spectrum of 1-d4 gave rise to a signal
at 2250.978 m/z (calculated m/z 2250.979), a shift of 4 m/z
versus 1, as anticipated for a tetrahydrido complex. Interest-
ingly, the ESI mass spectrum of 1 contains another major
feature at 1537.098 m/z, which is assignable to the
[Ag3(dppm)3H(OAc)]+ fragmentation product, consistent
with O’Hair’s observation that the Ag3(μ3-H) core is especially
stable.32,34−36,38 Its facile formation may also indicate a
While complex 2 appears to have Cs symmetry in the solid
state, there is only one resonance observed in its 31P{1H} NMR
spectrum, at −6.66 ppm, in MeCN-d3. Likewise, complex 2
1
exhibits a sharp singlet at 1.91 ppm in its H NMR spectrum,
assignable to the methyl group of a single acetate environment.
Overall, these data are consistent with fast exchange of the
acetate moieties at room temperature, which results in a higher
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Table 1. 1,4-Hydrosilylation of 2-cyclohexen-1-one
Catalyzed by 1, 2, and [CuH(PPh3)]6
a
b
catalyst
reaction time (h)
conversion (%)
4a/4b/4c
1
2
24
79
76:0:3
24
96
57:29:10
[CuH(PPh3)]6
<0.25
>99
79:21:0
a
b
For experimental details see Supporting Information. Total
1
conversion to 4 determined by H NMR integration with long pulse
delay (d1 = 60 s) against an internal standard of hexamethyldisiloxane.
hydrosilylation reaction. Surprisingly, in the absence of silane,
neither 1 nor 2 can perform the stoichiometric reduction of 3.
A similar observation has been reported for other copper
hydrides68,69 and can be rationalized by assuming that
formation of the copper enolate intermediate is reversible,
but its equilibrium concentration is very low. As a result, the
presence of silane is required to trap the enolate and drive the
reaction to completion.68,70 For comparison, we also monitored
the ability of [CuH(PPh3)]6 to catalyze 1,4-hydrosilylation of 3
under similar conditions. Of the three complexes tested, this
reagent proved the most effective, reaching >99% total
conversion in only 15 min.
Figure 2. Ball-and-stick diagram of 2. All hydrogen atoms (except the
hydride ligand) and solvent molecules are omitted for clarity. Color
legend: Cu = green; H = magenta; P = yellow-orange; O = red; C =
gray wireframe.
We also screened the ability of 1, 2, and [CuH(PPh3)]6 to
catalyze the 1,2-hydrosilylation of cyclohexanone (5). Thus,
addition of Ph2SiH2 (1.3 equiv) to 5, in the presence of 1 (0.10
H− equiv), in C6D6 resulted in 42% conversion to the silyl ether
6 after 24 h (Table 2). Complex 2 is an even better precatalyst
effective symmetry in solution than observed in the solid-state.
1
A septet centered at 2.10 ppm in the H NMR spectrum (2JPH
= 14.4 Hz) is assignable to the lone hydride moiety. This signal
collapses to a singlet upon 31P decoupling. To further confirm
the presence of a single hydride ligand, we synthesized the
isotopically labeled complex, 2-d1. Gratifyingly, complex 2-d1
2
Table 2. 1,2-Hydrosilyation of Cyclohexanone Catalyzed by
1, 2, and [CuH(PPh3)]6
features a singlet at 1.74 ppm in the H NMR spectrum in
C6H6/C6D6 (9:1, v/v), assignable to the deuteride ligand.
Finally, complex 2 produced a signal at 1379.120 m/z in the
ESI mass spectrum, corresponding to the [Cu3H(dppm)3Cl]+
ion (calculated m/z 1379.125). For comparison, complex 2-d1
features a signal at 1380.123 m/z in its ESI mass spectrum,
corresponding to the [Cu3D(dppm)3Cl]+ ion (calculated m/z
1380.131), a shift of 1 m/z. A parent peak for 2 was not found
in the mass spectrum, likely due to facile exchange of acetate for
Cl− during the ESI mass spectrometry ionization process.
We also explored the chemical properties of 2. Complex 2 is
soluble in MeCN, partially soluble in C6H6 and THF, and
insoluble in Et2O and nonpolar solvents. It is stable in MeCN
for at least 3 d, showing no signs of decomposition over this
time. In contrast to complex 1, complex 2 does not appear to
react with excess H2O, even over the course of 24 h.
a
b
catalyst
reaction time (h)
conversion (%)
1
2
24
2
42
>99
[CuH(PPh3)]6
24
26
a
b
For experimental details see Supporting Information. Total
1
conversion to 6 determined by H NMR integration with long pulse
delay (d1 = 60 s) against an internal standard of hexamethyldisiloxane.
Group 11 hydrides are known to catalyze both 1,4- and 1,2-
hydrosilylations of (α,β-unsaturated) ketones, in the presence
of a silane.6,7,16,65,66 Given this precedent, we screened the
ability of complex 1 to catalyze the 1,4-hydrosilylation of 2-
cyclohexen-1-one (3). Thus, addition of Ph2SiH2 (1.5 equiv) to
3, in the presence of 1 (0.05 H− equiv), in C6D6 results in 76%
conversion to the corresponding silyl enol 4a after 24 h (Table
1). Also formed in small amounts is the 1,2-hydrosilylation
product, 4c.67 To our knowledge, this transformation
represents the first example of hydrosilylation with an authentic
silver hydride complex.68 Complex 2 is also an effective
precatalyst for the 1,4-hydrosilylation of 3, achieving 96% total
conversion after 24 h using a similar catalyst loading. However,
this precatalyst features somewhat lower selectivity for the 1,4-
for the 1,2-hydrosilylation of 5, achieving >99% conversion
after just 2 h, using a similar catalyst loading. Surprisingly,
[CuH(PPh3)]6 was the least effective precatalyst for this
reaction, achieving only 26% conversion after 24 h. It is not
readily apparent why complex 2 is a more effective precatalyst
relative to 1 or [CuH(PPh3)]6; however, the changes in
efficacy, relative to the results observed for the 1,4-hydro-
silylation (Table 1), could indicate a change in the reaction
mechanism.71
In summary, we have isolated and characterized the
diphosphine ligated clusters [Ag6H4(dppm)4(OAc)2] (1) and
[Cu3H(dppm)3(OAc)2] (2). Complex 1 is the first homo-
metallic silver polyhydrido cluster to be structurally charac-
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AUTHOR INFORMATION
Corresponding Author
*E-mail: hayton@chem.ucsb.edu.
■
ORCID
Trevor W. Hayton: 0000-0003-4370-1424
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the Center for Sustainable Use of
Renewable Feedstocks (CenSURF), a National Science
Foundation (NSF) Center for Chemical Innovation (CCI).
We thank C. Ravelo for preliminary experiments in this area.
We also thank Dr. J. Hu of the Materials Research Laboratory
(MRL) at UCSB for assistance with the Ag NMR spectroscopy.
NMR spectral acquisition was supported by the NIH Shared
Instrument Grant No. 1S10OD012077-01A1. Mass spectrom-
etry was performed at the MRL Shared Experimental Facilities
supported by the MRSEC Program of the NSF under Award
No. DMR 1121053; a member of the NSF-funded Materials
Research Facilities Network.
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