Selective reduction of CO2 with silanes over platinum nanoparticles immobilised on a polymeric monolithic support under ambient conditions

Article abstract of DOI:10.1002/chem.201304845

Here, we demonstrate the use of Pt⁰ nanoparticles immobilised on a polymeric monolithic support as a ligand-free heterogeneous catalytic system for the reduction of ¹³CO₂ at room temperature and atmospheric pressure. The described system effectively reduces ¹³CO₂ with dihydrosilanes as the hydrogen source to yield a mixture of silylformates, silylacetals and methoxysilanes, which upon further hydrolysis with D₂O, produces their respective C1-type products, that is H¹³COOD, ¹³CH₂(OD)₂ and ¹³CH₃OD. If a monohydrosilane was used as the hydrogen source, a selective reduction of ¹³CO₂ to a product mixture of only silylformates was observed. Addition of diethylamine to this reaction mixture results in the formation of H¹³COOH and Et ₂N¹³CHO. This robust catalytic system is not only maintenance-free and simple to handle, as compared with organometallic and organocatalyst systems, but also shows 3- to 11-fold better catalytic activity and exhibits higher turnover numbers (TONs) up to 21 900 (activity=6.22 kg CO 2 g_Pt^-1 bar^-1).

Full text of DOI:10.1002/chem.201304845

DOI: 10.1002/chem.201304845
Communication
&
Catalysis
Selective Reduction of CO2 with Silanes over Platinum
Nanoparticles Immobilised on a Polymeric Monolithic Support
under Ambient Conditions
Vijay P. Taori, Rajendar Bandari, and Michael R. Buchmeiser*[a]
er, these catalysts are usually sensitive to moisture, air and/or
Abstract: Here, we demonstrate the use of Pt0 nanoparti-
to solvents, which can significantly restrict their applicability.
cles immobilised on a polymeric monolithic support as
a ligand-free heterogeneous catalytic system for the re-
duction of 13CO2 at room temperature and atmospheric
pressure. The described system effectively reduces 13CO2
with dihydrosilanes as the hydrogen source to yield a mix-
ture of silylformates, silylacetals and methoxysilanes,
which upon further hydrolysis with D2O, produces their re-
spective C1-type products, that is H13COOD, 13CH2(OD)2
and 13CH3OD. If a monohydrosilane was used as the hy-
drogen source, a selective reduction of 13CO2 to a product
mixture of only silylformates was observed. Addition of di-
ethylamine to this reaction mixture results in the forma-
tion of H13COOH and Et2N13CHO. This robust catalytic
system is not only maintenance-free and simple to handle,
as compared with organometallic and organocatalyst sys-
tems, but also shows 3- to 11-fold better catalytic activity
and exhibits higher turnover numbers (TONs) up to 21900
Also, the sensitivity of these catalysts does not allow for de-
tailed characterization of the reaction mixture and only in few
instances[1g,6b,c] have the C1-type intermediates and products
been characterised and reported. Also, for the purpose of recy-
cling CO2, Cantat and co-workers[2a] emphasise the importance
of a “diagonal approach”. This involves formation of other
functional-group compounds, such as formamides, esters or
tertiary amines from the intermediates of the reduced-CO2.
This approach, however, definitely requires that the catalysts
used are not very sensitive towards substrates (e.g., amines)
used for converting intermediates, such as formates, acetals or
methyl ethers, to “derivative compounds” such as formamides
or tertiary amines. In addition, the amount of catalyst required
can be high,[2a] and collectively these reasons have resulted in
very low turnover numbers (TONs) of <8300,[1g,6a–c] with little
or no possibility for the synthesis of derivative compounds
based on a diagonal approach. Most of these systems also lack
the ability to selectively reduce CO2 to different C1-type inter-
mediates upon a change in hydrogen source and show a very
limited possibility for application in a continuous process.
Previously, our group has immobilised platinum and palladi-
um on polymeric monolithic supports;[10] the resulting hetero-
geneous catalysts performed well in both the continuous hy-
drosilylation of olefins[10a] and in CÀC coupling[10b] reactions,
showing high TONs. Here, we demonstrate the use of a Pt-
loaded, polymeric monolithic support as a ligand-free catalytic
system for the reduction of 13CO2. As shown in Scheme 1, the
use of the system with dihydrosilanes (methylphenylsilane,
MPS) as a hydrogen source results in the formation of a 13C-la-
belled C1-type product mixture consisting of methoxysilanes,
silylacetals and silylformates.
(activity=6.22 kgCO gPtÀ1 barÀ1).
2
Despite the high thermodynamic stability of carbon dioxide
(CO2),[1] its use as a C1 building block[1e,f,2] is of ultimate inter-
est, especially since the industrial use of C1-type chemicals
occurs at very high annual volumes.[3] CO2 is usually used as
precursor for C1-type products at high temperatures and high
pressures.[1c,3,4] Straightforward reduction of CO2 to products
with oxidation states +2 or below at low cost is therefore
highly desirable and would have enormous economic and eco-
logical impact.[1e,f,2,5]
To this end, a limited number of studies have been carried
out to achieve the reduction of CO2 at room temperature and
atmospheric pressure using either hydrosilanes[1d,6] or hydro-
boranes[1g,7] as the source of hydrogen. These studies exclu-
sively use organocatalysts,[2a] N-heterocyclic carbenes[6b,8]
(NHCs) or organometallic catalysts[1g,6a,c,d,9] in solution. Howev-
[a] Dr. V. P. Taori, Dr. R. Bandari, Prof. M. R. Buchmeiser
Institute fꢀr Polymerchemie
Universitꢁt Stuttgart
Pfaffenwaldring 55, 70550 Stuttgart (Germany)
Fax: (+49)711-685-64050
E-mail: michael.buchmeiser@ipoc.uni-stuttgart.de
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201304845.
Scheme 1. Overall reaction depicting target C1 and derivative compounds.
Chem. Eur. J. 2014, 20, 3292 – 3296
3292
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
Notably, when this catalytic system was used with a monohy-
drosilane (dimethylphenylsilane, DMPS) as the hydrogen
source, the selective reduction of 13CO2 to silylformate was ach-
ieved. As outlined for numerous other catalytic systems immo-
bilised on monolithic supports,[11] the polymeric-monolith-
based heterogenic catalytic system shown here also allows in
principle for a continuous setup.
termined by inductively coupled plasma-optical emission spec-
trometry (ICP-OES). Transmission electron microscopy (TEM,
Figure 1c) revealed Pt nanoparticle diameters in the range of 7
to 10 nm. In addition, ICP-OES measurements found 46 ppm
inert Ru (RuO2) contamination from the deactivated catalyst.
Please refer to the Supporting Information for details.
Reduction of 13CO2 was studied by adding 10.1 mg of
crushed monolith (28.4 nmol of Pt) to an NMR tube equipped
with a J. Young valve fitting. Due to its good solubility parame-
ters for CO2,[13] [D6]DMSO (700 mL) was added to the NMR tube
both as a reaction and NMR solvent and (CH3)PhSiH2 (MPS)
was chosen as a hydrogen source and was added in batches
to determine the activity of the catalyst. In batch I, MPS
(21.5 mg, 0.176 mmol; 0.352 mmol SiÀH) was added to an
NMR tube (Table 1) that was then evacuated by a freeze-
The cross-linked polymeric monolith was prepared by the
ring opening metathesis polymerization (ROMP) of a cis-cyclo-
octene-based monomer and cross-linker using a 3rd-genera-
tion Grubb’s catalyst. The detailed synthesis of the precursors
and monolith is described in the literature[10a,12] and in the Sup-
porting Information. Further chemical modifications outlined in
Scheme 2 were carried out to produce vic-diol sites. PtCl4 was
Table 1. Activity of the catalyst.
Reaction time
[h]
Moles of SiÀH
[mmol]
TON[a,b] TOF[a,b] Integral TON[a,c]
[hÀ1
]
loaded reacted
24[d]
48[d]
24[e]
0.352[d] 0.217
0.352[d] 0.352
0.400[e] 0.097
0.400[e] 0.138
0.400[e] 0.222
0.400[e] 0.270
7645
12394
3409
4862
7824
318
258
142
101
47
–
–
–
–
48[e]
168[e]
336[e]
–
9501
28
21900
[a] Based on 28.4 nmol of Pt. [b] Based on the number of SiÀH bonds re-
acted from either batch I or batch II. [c] Based on the total number of SiÀ
H bonds reacted (0.622 mmol) from batch I and II. [d] Batch I. [e] Batch II.
TOF=turnover frequency.
Scheme 2. Synthesis of Pt-loaded monolith and 13CO2 reduction with meth-
ylphenylsilane (MPS): i) [RuCl2(pyridine)2(IMesH2)(CHPh)], 2-propanol, toluene,
À308C (0.5 h), RT (16 h); ii) ethyl vinyl ether (EVE):DMSO:THF (20:40:40); iii)
0.1 N H2SO4, 658C, 24 h; iv) PtCl4, THF; v) 10 wt% NaBH4.
then introduced into the monolithic structure and reduced
using NaBH4. The Pt0 that formed is stabilised by both the vic-
diol sites and double bonds present in the polymeric support
(Figure 1a). The morphology of the monolith was characterised
using scanning electron microscopy (SEM, Figure 1b), which
shows the globular morphology of the monolith with globule
diameters in the range of 1–2 micrometers. The specific surface
area of the monolith was found to be 8.5 m2 gÀ1 and an
amount of 550 mg of Pt per 1 g of monolith (550 ppm) was de-
pump-thaw cycle and pressurised with 13CO2 (0.8 bar) at room
temperature (RT). In the 13C NMR spectrum, 13CO2 was ob-
served at d=124.1 ppm (Figures S2 and S3 in the Supporting
Information).
As shown in Scheme 3, Pt activates 13CO2,[1e,f,2b,14] which is
then subjected to reduction in the presence of MPS. It is also
evident from these spectra and Figure 2 (1) that new signals in
Figure 1. a) Representation of the monolithic structure and proposed coor-
dination of Pt. b) SEM image of the monolith shows the globular morpholo-
gy with diameters ranging from 1 to 2 mm. The scale bar represents 200 nm.
c) TEM image of monolith prior to 13CO2 reduction. The size of the Pt nano-
particles is around 7–10 nm. Scale bar represents 50 nm. d) TEM image of
the monolith after 16 days of 13CO2 reduction in [D6]DMSO. The size of the
Pt nanoparticles increased to 50 nm. Scale bar represents 200 nm.
Scheme 3. Possible silylformate intermediates and formation of formic acid,
methanediol and methanol from CO2 and methylphenylsilane (MePhSiH2,
MPS).
Chem. Eur. J. 2014, 20, 3292 – 3296
www.chemeurj.org
3293
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
(Figure S7; Figure 2, 2 for 48 h). Clearly, this depletion of the
hydrogen source concomitantly results in the formation of in-
termediates bearing the SiÀO (either HÀSiÀO or OÀSiÀO)
moiety. The OÀSiÀH moiety (Figure S9) showed up as quartets
3
in the region d=5.10–5.20 ppm with coupling constants JHÀH
in the range 2.75–2.97 Hz. Further acquisition as shown in
Figure 2 (2) and Figure S8 (2) indicate that within 48 h, all SiÀH
species (HÀSiÀH and HÀSiÀO) were consumed. A negative con-
trol reaction in the absence of the monolith showed only ex-
tremely slow reduction (!0.5% within 24 h, Figure S10), clear-
ly underlining the role of Pt. After this time, only one very
minor peak was observed by 13C NMR spectroscopy in the silyl-
formate region.
Next, another 25 mg (0.400 mmol SiÀH) of MPS (batch II)
were added at RT to the earlier-discussed reaction mixture,
which was then again pressurised with 13CO2 (0.8 bar). More
prominent peaks for SiO13CHO, SiO13CH2OSi and SiO13CH3 inter-
mediates were visible at the end of this reaction (Figure S11 in
the Supporting Information).
Figure 2. 13C NMR spectra of the reaction mixture. 1) Spectrum acquired
13
&
*
10 min after the addition of CO2 ( ) and 0.176 mmol of MPS (batch I; ). 2)
Spectrum acquired 48 h after the addition of 0.176 mmol of MPS. 3) Spec-
&
trum acquired 1 h after the addition of 0.200 mmol of MPS (batch II; ). 4)
Spectrum acquired 264 h after the addition of 0.200 mmol of MPS (batch II).
All spectra are normalised and referenced using the solvent peak. [D6]DMSO
(13C: d=39.51 ppm), 13CO2: d=124.15 ppm.
Once little further reduction was observed, the intermediates
were hydrolyzed by addition of 70 mL (excess) of D2O and NMR
characterization of the hydrolysis mixture was carried out after
15 h. 1H NMR spectroscopy shows a doublet centred at d=
1
the region around dꢀ163 ppm form within the first 10 min (+
30 min acquisition time) after addition of 13CO2 to the NMR
tube. Within the same time interval, a new resonance in the
13C NMR spectrum evolved at d=À1.0 ppm, which represents
the methyl carbon attached to the Si atom of the Si-O-species
(either HÀSiÀO or OÀSiÀO) at the expense of the resonance for
8.12 ppm, JCÀH =215.5 Hz (Figure 3) for formic acid (H13COOD).
1
Its formation was also confirmed by 13C-coupled H NMR spec-
1
the methyl carbon in MPS at d=À8.0 ppm. The H NMR spec-
trum taken after 3 h (Figure S4 in the Supporting Information)
reveals the evolution of two new peaks in the formyl region.
These two peaks represent a doublet centred at d=8.29 ppm
1
1
with a coupling constant JCÀH of 229.6 Hz resulting from H-
13C coupling and thus confirm the formation of a 13C-labelled
silylformate as a result of the reduction of 13CO2. The men-
1
tioned JCÀH coupling constant was also verified from 13C-cou-
1
pled H NMR spectroscopy (Figure S5) for a doublet centred at
d=162.0 ppm with a coupling constant 1JCÀH =229.6 Hz. In
principle, the formation of many different silylformate inter-
mediates is possible (Scheme 3a–e) due to the presence of
two SiÀH bonds in MPS. And in fact, numerous products were
1
observed in the H NMR spectrum (Figure S6) as the silylform-
Figure 3. 1H NMR spectrum acquired 15 h after the addition of excess of D2O
to the reaction mixture of MPS, 13CO2 and Pt-loaded monolith. The spectrum
is referenced using the solvent peak. [D6]DMSO (1H: d=2.50 ppm).
ate peaks show significant broadening and the 13C NMR spec-
trum shows multiple peaks evolving around dꢀ162 ppm
(Figure 2, Figure S7). In the 13C NMR spectrum new peaks also
evolve at dꢀ85 ppm within three hours (Figure S7, 1) after the
addition of 13CO2 while after another three hours new peaks
evolve at dꢀ50 ppm (Figure S7, 2) suggesting the formation
of disilyloxymethane- (RPhCH3SiO)213CH2 (R=H, 13CHO,
troscopy (Figure 4), which shows a doublet at d=163.3 ppm,
1JCÀH =215.5 Hz. Methanol (13CH3OD) showed up as a doublet
centred at d=3.16 ppm, 1JCÀH =139.2 Hz (Figure 3) and was
1
Scheme 3c,
d
and g) and methoxysilyl-intermediates
further confirmed by a quartet centred at d=48.8 ppm, JCÀH
=
(RPhCH3SiO13CH3, R=H, 13CHO Scheme 3e, f).
139.2 Hz (Figure 4). Again, 1ꢁxꢁ3 in 13CHx stated here was va-
lidated by 13C-coupled 1H NMR spectroscopy (Figure 3). In
Figure 3, hydrolysis products of silylacetal-type intermediates
are visible as doublets centred around d=4.60 ppm (J
ꢀ158.9 Hz) which can also be seen in Figure 4 as triplets in
the range of 65<d<85 ppm. These triplets confirm the pres-
1
Following the reaction by H NMR spectroscopy (Figure S6 in
the Supporting Information) revealed that the HÀSiÀH species
3
of MPS at d=4.28 ppm (q, JHÀH =4.21 Hz, 2H; SiH2) was con-
stantly consumed over 24 h. This was further confirmed by the
disappearance of the methyl shift at d=À8.0 ppm within 24 h
Chem. Eur. J. 2014, 20, 3292 – 3296
www.chemeurj.org
3294
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
Figure 5. 13C NMR spectra acquired after the addition of excess of diethyla-
1
Figure 4. 13C-coupled H NMR spectrum acquired after the addition of excess
mine to the reaction mixture containing DMPS 13CO2 and Pt-loaded mono-
of D2O to the reaction mixture of MPS, 13CO2 and Pt-loaded monolith. The
spectrum is referenced using the solvent peak. [D6]DMSO (13C: d=
39.51 ppm).
1
lith. 1) 13C-coupled H NMR spectrum, 2) 13C NMR spectrum. Spectra are nor-
malised and referenced using the solvent peak. [D7]DMF (13C: d =30.1 ppm).
13
ence of different À CH2À motifs. Although the exact analysis
was confirmed by comparison with commercially available
compounds (Figure S18–S24).
of these compounds is beyond the scope of this study, we sus-
pect the formation of methandiol as one of the major prod-
ucts.
In summary, the system reported here shows high activity
for the reduction of 13CO2 and allows for the synthesis of C1-
type products such as 13CH3OD, H13COOD and formamides and
other derivative compounds. With a monohydridosilane, selec-
tive formation of silylformates is achieved. Notably, the ab-
sence of any ligand in this heterogeneous-type catalyst offers
access to a simple and continuous setup reducing the woes
for separation and strengthening the concept of “diagonal ap-
proach”[2a] for CO2 recycling. Studies to immobilise bimetallic Pt
alloys for the heterolytic splitting of H2 as a source of hydrogen
for CO2 reduction are underway.
As shown in Table 1, the heterogeneous catalyst shows high
activity with TONs of about 12400 within 48 h (TOF=258 hÀ1,
batch I, based on the consumption of the SiÀH bond). Upon
addition of further MPS and 13CO2, the TOF decreased to 28 hÀ1
within 16 days. This is attributed to the growth of the Pt nano-
particles due to Ostwald ripening, a common phenomenon
observed with metallic nanoscale systems.[10a,15] In fact, TEM
images of the monolith acquired after its use in the reduction
reaction (Figure 1, d) revealed an increase in size of the Pt
nanoparticles from about 7 nm to about 50 nm. Notably, the
overall TON obtained in this reaction was around about 21900
Acknowledgements
(activity=6.22 kgCO gPtÀ1 barÀ1), which is 3- to 11-fold higher
2
than reported TONs,[1g,6a–c] even though the entire reaction was
carried out at room temperature and a very low pressure of
0.8 bar.
The authors wish to thank PD Dr. Michael Schweikert, Biolo-
gisches Institut - Abteilung Zoologie, Universitꢁt Stuttgart, for
TEM measurements.
When CO2-reduction was carried out with a monohydrosi-
lane, that is, with dimethylphenylsilane (DMPS), dimethylphe-
nylsilyl formate (Figure S13 in the Supporting Information)
formed selectively. The reaction was also carried in [D7]DMF
and was found to proceed faster than in DMSO. Upon termina-
tion of this reaction with excess of diethylamine, 13C NMR spec-
troscopy provided clear evidence for the formation of diethyl-
formamide (d=162.5 ppm) and formic acid (d=167.7 ppm)
(Figure 5 and Figure S17). GC-MS data confirm the formation
of a siloxane compound (Figure S14) and of 13C-labelled DEF
(Figure S15). Thus, a commercially available sample of DEF
showed exactly the same retention time and identical frag-
mentation (Figure S16). A similar reaction pathway as with
DMPS was observed when excess morpholine was added to
the reaction mixture containing 13CO2, MPS and the Pt-loaded
monolith in [D6]DMSO. Formation of 13C-labelled 4-formylmor-
pholine, bis(4-dimorpholinyl)methane and N-methylmorpholine
Keywords: carbon dioxide · monolith · platinum · reduction ·
silanes
[1] a) F. Goettmann, A. Thomas, M. Antonietti, Angew. Chem. 2007, 119,
2773–2776; Angew. Chem. Int. Ed. 2007, 46, 2717–2720; b) S. Wessel-
baum, U. Hintermair, W. Leitner, Angew. Chem. 2012, 124, 8713–8716;
Angew. Chem. Int. Ed. 2012, 51, 8585–8588; c) T. J. Schmeier, G. E. Do-
bereiner, R. H. Crabtree, N. Hazari, J. Am. Chem. Soc. 2011, 133, 9274–
9277; d) Y. Jiang, O. Blacque, T. Fox, H. Berke, J. Am. Chem. Soc. 2013,
135, 7751–7760; e) M. Aresta, A. Dibenedetto, Dalton Trans. 2007,
2975–2992; f) M. Cokoja, C. Bruckmeier, B. Rieger, W. A. Herrmann, F. E.
Kꢂhn, Angew. Chem. 2011, 123, 8662–8690; Angew. Chem. Int. Ed. 2011,
50, 8510–8537; g) S. Chakraborty, J. Zhang, J. A. Krause, H. Guan, J. Am.
Chem. Soc. 2010, 132, 8872–8873.
[2] a) C. D. N. Gomes, O. Jacquet, C. Villiers, P. Thuꢃry, M. Ephritikhine, T.
Cantat, Angew. Chem. 2012, 124, 191–194; Angew. Chem. Int. Ed. 2012,
51, 187–190; b) K. Huang, C.-L. Sun, Z.-J. Shi, Chem. Soc. Rev. 2011, 40,
2435–2452.
Chem. Eur. J. 2014, 20, 3292 – 3296
www.chemeurj.org
3295
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
[3] T. Schaub, R. A. Paciello, Angew. Chem. 2011, 123, 7416–7420; Angew.
Chem. Int. Ed. 2011, 50, 7278–7282.
[4] K. Tominaga, Y. Sasaki, M. Kawai, T. Watanabe, M. Saito, J. Chem. Soc.
Chem. Commun. 1993, 629–631.
[5] W. Leitner, Angew. Chem. 1995, 107, 2391–2405; Angew. Chem. Int. Ed.
Engl. 1995, 34, 2207–2221.
[6] a) S. Park, D. Bezier, M. Brookhart, J. Am. Chem. Soc. 2012, 134, 11404–
11407; b) S. N. Riduan, Y. Zhang, J. Y. Ying, Angew. Chem. 2009, 121,
3372–3375; Angew. Chem. Int. Ed. 2009, 48, 3322–3325; c) T. Matsuo, H.
Kawaguchi, J. Am. Chem. Soc. 2006, 128, 12362–12363; d) A. Jansen, S.
Pitter, J. Mol. Catal. A 2004, 217, 41–45.
[10] a) R. Bandari, M. R. Buchmeiser, Catal. Sci. Technol. 2012, 2, 220–226;
b) R. Bandari, T. Hçche, A. Prager, K. Dirnberger, M. R. Buchmeiser, Chem.
Eur. J. 2010, 16, 4650–4658.
[11] a) M. R. Buchmeiser, Chem. Rev. 2009, 109, 303–321; b) E. B. Anderson,
M. R. Buchmeiser, ChemCatChem 2012, 4, 30–44.
[12] R. Bandari, A. Prager-Duschke, C. Kꢂhnel, U. Decker, B. Schlemmer, M. R.
Buchmeiser, Macromolecules 2006, 39, 5222–5229.
[13] S. T. Perisanu, J. Solution Chem. 2001, 30, 183–192.
[14] a) D. H. Gibson, Chem. Rev. 1996, 96, 2063–2095; b) D. A. Palmer, R. Va-
neldik, Chem. Rev. 1983, 83, 651–731.
[15] a) M. S. Wilson, F. H. Garzon, K. E. Sickafus, S. Gottesfeld, J. Electrochem.
Soc. 1993, 140, 2872–2877; b) C. G. Granqvist, R. A. Buhrman, J. Catal.
1976, 42, 477–479; c) G. A. Gruver, R. F. Pascoe, H. R. Kunz, J. Electro-
chem. Soc. 1980, 127, 1219–1224; d) A. Honji, T. Mori, K. Tamura, Y.
Hishinuma, J. Electrochem. Soc. 1988, 135, 355–359.
[7] M.-A. Courtemanche, M.-A. Lꢃgarꢃ, L. Maron, F.-G. Fontaine, J. Am.
Chem. Soc. 2013, 135, 9326–9329.
[8] S. N. Riduan, J. Y. Ying, Y. G. Zhang, ChemCatChem 2013, 5, 1490–1496.
[9] a) T. C. Eisenschmid, R. Eisenberg, Organometallics 1989, 8, 1822–1824;
b) G. Menard, D. W. Stephan, J. Am. Chem. Soc. 2010, 132, 1796–1797;
c) R. Shintani, K. Nozaki, Organometallics 2013, 32, 2459–2462; d) M.
Khandelwal, R. J. Wehmschulte, Angew. Chem. 2012, 124, 7435–7439;
Angew. Chem. Int. Ed. 2012, 51, 7323–7326.
Received: December 11, 2013
Published online on February 19, 2014
Chem. Eur. J. 2014, 20, 3292 – 3296
www.chemeurj.org
3296
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim