N-Heterocyclic carbene-initiated hydrosilylation of styryl alcohols with dihydrosilanes: A mechanistic investigation

Article abstract of DOI:10.1039/c3dt32983f

Investigation of the mechanism of the NHC-initiated hydrosilylation of styryl alcohols in the presence of a dihydrosilane suggests a general base catalysis mechanism and not the activation of the dihydrosilane by the NHC.

Full text of DOI:10.1039/c3dt32983f

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N-Heterocyclic carbene-initiated hydrosilylation of
styryl alcohols with dihydrosilanes: a mechanistic
investigation†
Cite this: Dalton Trans., 2013, 42, 7458
David Gatineau,a Qiwu Zhao,a Dennis P. Curran,b Max Malacria,a Emmanuel Lacôte,c
Louis Fensterbank*a and Jean-Philippe Goddard*a
Received 12th December 2012,
Accepted 8th February 2013
Investigation of the mechanism of the NHC-initiated hydrosilylation of styryl alcohols in the presence of a
dihydrosilane suggests a general base catalysis mechanism and not the activation of the dihydrosilane by
the NHC.
DOI: 10.1039/c3dt32983f
www.rsc.org/dalton
The catalytic hydrogenation of multiple bonds is an essential
transformation of synthetic organic chemistry.1 Organocataly-
tic reductions are attractive because they avoid the use of toxic
and expensive transition metals as catalysts or of stoichio-
metric amounts of metal hydrides.2 Thus, the development of
new methodologies to reduce olefins, particularly non-acti-
vated ones, is an important goal.
N-Heterocyclic carbenes (NHCs) have been widely used as
ligands or as catalysts in organic and organometallic syn-
thesis.3 They also activate tetravalent silicon reagents.4 Car-
benes add to silicon-based pro-nucleophiles, thus initiating
polymerizations5 and catalyzing 1,2-addition reactions on
carbonyls6 or aziridines.7
Scheme 1 The NHC-mediated reduction of styryl and propargylic alcohols
(IPr = 1,3-bis(2,6-di-iso-propylphenyl)imidazol-2-ylidene).
Moreover, the Lewis base activation of silanes to form penta-
coordinate hydrosilicates is a well-established method to
activate the silicon–hydrogen bond during the hydrosilylation
of carbonyls and alkynes.8 Using this concept, Lee and co-
workers accomplished the hydrosilylation of alkynes promoted
by tert-butoxide and they suggested that silanes can transfer a
hydride via a hypervalent intermediate.9
We reported that carbenes are effective catalysts for the
hydrosilylation of a variety of styryl and propargylic alcohols
with dihydrosilanes (Scheme 1),10 and discussed some prelimi-
nary mechanistic investigations. When the reduction of 1 was
Scheme 2 (a) Hydrosilylation of
(b) Reduction of trisubstituted alkene 3.
1
with deuterated diphenylsilane.
carried out with D2SiPh2, the only product obtained was 2a,
deuterated at the homobenzylic position and isolated as a
single diastereomer (Scheme 2a). Moreover, the reduction of
trisubstituted allyl alcohol 3 gave the anti diastereoisomer 4
with good diastereoselectivity (Scheme 2b). The syn relative
configuration of 2a has been attributed by analogy with 4
(Scheme 2). This led us to propose that the reductions pro-
ceeded through an intramolecular hydrosilylation of the
alkenes triggered by NHC complexation of the Si atom.
aInstitut Parisien de Chimie Moléculaire (UMR CNRS 7201)-FR 2769 UPMC Univ
Paris 06, 4 place Jussieu, C. 229, 75005 Paris, France.
E-mail: louis.fensterbank@upmc.fr, jean-philippe.goddard@upmc.fr;
Fax: (+33) 1-44-27-73-60
bDepartment of Chemistry, University of Pittsburgh, 4200 Fifth Avenue, Pittsburgh,
Pennsylvania 15260, USA
cUniversité de Lyon, Institut de chimie de Lyon UMR 5265 CNRS-Université Lyon
I-ESCPE Lyon, 43 Bd du 11 novembre 1918, 69616 Villeurbanne, France
†Electronic supplementary information (ESI) available: Experimental procedures
and spectroscopic data for all new reagents and products. See DOI:
10.1039/c3dt32983f
However, we still had some uncertainty about the actual
role of the catalytic carbene. Two main mechanisms can be
7458 | Dalton Trans., 2013, 42, 7458–7462
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Scheme 3 Hydrosilylation of deuterated alcohol 1-D.
Fig. 1 Formation of the catalytic species in (a) Lewis acid–base interaction;
(b) Brønsted based initiation.
proposed for the generation of the hypervalent species pre-
sumably involved in the hydrosilylation step. First, a Lewis
acid–base NHC–Si interaction as the catalytically relevant
species can be involved as proposed for the tert-butoxide-
mediated hydrosilylation (Fig. 1a).9 Another plausible mechan-
ism relies on the Brønsted basicity of NHCs in a general base-
catalyzed mechanism in which an alcoholate is formed, which
adds to the silicon-containing reagent (Fig. 1b).9b,11
To obtain more mechanistic insight, we further investigated
the one-pot NHC-mediated reduction of styryl alcohol in more
depth.
Isotopic labelling experiments under the reaction con-
ditions described in our previous paper were carried out. A solu-
tion of Ph2SiH2 (1.1 equiv.) and the substrate (1 equiv.) in dry
DMF was added to the free carbene solution in DMF. A solu-
tion of tetra-n-butylammonium fluoride (TBAF) in THF was
added after complete conversion to quench all traces of the
silicon derivative (see condition A below).‡ The reduction of 1
in DMF-d7 led to alcohol 2b with no incorporation of deuter-
ium. The same result was observed upon quenching with D2O.
In contrast, deuterated alcohol 1-D gave reduced product 2c
in which a deuterium was incorporated at the benzylic posi-
tion (D content: 90%, Scheme 3). Importantly, the deuteration
at this position was not diastereoselective.
Scheme 4 Reaction of silyl ether 5 and cyclic siloxane 6 to alcohols 2.
To confirm the intramolecular pathway, we prepared the
putative intermediates of the reduction, namely silyl ether 5
and cyclic siloxane 6.12 When silyl ether 5 was placed in the
presence of a catalytic amount of triazolylidene NHC A (see
Scheme 1), 2b was obtained in 53% yield after treatment with
TBAF (Scheme 4a). Deutero-silane 5-D was converted into 2a as
a single diastereoisomer in 50% yield. The homobenzylic posi-
tion was deuterated with 83% D-incorporation (Scheme 4a).
Starting from siloxane 6, a complex mixture of silylated com-
pounds was obtained and incomplete conversion was observed
before the addition of TBAF. However, 2b was isolated in 75%
yield after TBAF treatment (Scheme 4b).
The benzylic position was deuterated at 70% when the
cyclic siloxane 6 was treated with 10 mol% of A and 10 equiva-
lents of deuterated methanol (Scheme 4c). No reaction
occurred without carbene, and addition of TBAF resulted in
40% of deuterium incorporation at the benzylic position,
again without stereoselectivity.
‡Experimental part: All reactions were performed under an inert atmosphere.
Free carbene IPr was bought in order to avoid potential traces of NaH in the reac-
tion media and triazolylidene A was prepared in situ by addition of triazolium
chloride salt (0.1 mmol, 0.1 equiv.) to a suspension of NaH (0.1 mmol, 4 mg,
60% in mineral oil, 0.1 equiv.) in dry DMF (1 mL) at room temperature.
Conditions A: A solution of Ph2SiH2 (1.1 mmol, 203 mg, 1.1 equiv.) and sub-
strate 1, 3, 5, 6 or 7 (1 mmol, 1 equiv.) in dry DMF (1 mL) were added to the free
carbene (or base) solution in DMF (1 mL).
Conditions B: The substrate 1, 3, 5, 6 or 7 (1 mmol, 1 equiv.) was added to a
solution of the free carbene (or base) (0.1 mmol, 0.1 equiv.) in DMF (2 mL). The
reaction mixture was stirred at room temperature for 30 min and diphenylsilane
(1.1 mmol, 203 mg, 1.1 equiv.) was then added.
Thus, silyl ether 5 and siloxane 6 could be converted into
the expected product under our reaction conditions, though
with less efficiency, and a proton (or deuterium) was trans-
ferred from an external alcohol present in the medium. In our
previous communication, we had shown that a directing
Conditions C: Diphenylsilane (1.1 mmol, 203 mg, 1.1 equiv.) was added to a
solution of the free carbene (or base) (0.1 mmol, 0.1 equiv.) in DMF (2 mL). The
reaction mixture was stirred at room temperature for 30 min and substrate 1, 3,
5, 6 or 7 (1 mmol, 1 equiv.) was then added.
General work-up for conditions A, B and C: The reaction was monitored by TLC hydroxyl group is essential for the reduction. So, the non-
until no substrate was left. TBAF (1 mmol, 1 mL, 1.0 M in THF, 1 equiv.) was
stereoselective incorporation of deuterium at the benzylic pos-
ition from the alcohol 1-D or MeOD-d4 in the presence of both
NHC and TBAF seems to suggest a base-mediated mechanism,
then added to the resulting mixture, which was stirred for a further 30 min and
quenched with H2O (10 mL). The mixture was extracted with EtOAc (3 × 10 mL)
and the combined organic layers were washed with brine (10 mL), dried with
while the high diastereoselectivity at the homobenzylic pos-
ition tends to indicate a rigid transition state.
anhydrous Na2SO4, filtered, and the solution was concentrated under vacuum.
The crude product was purified by flash column chromatography over silica gel.
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completely shut down when the sodium hydride was left with
the silane before introduction of the alcohol (Table 1, entry 7,
condition C). We believe that the silane reacts with NaH in
that case, since an important gas evolution was observed when
the silane was introduced into the reaction mixture.
Table 1 Influence of the experimental conditions in the reduction of 1
Entry Conditionsa Baseb
Time (h) Conversiond (%) Yielde (%)
1
2
3
4
5
6
7
A
A
A
B
C
B
C
EtONa
NaH
IPrc
1
1
6
3
3
1
1
100
100
100
83
83
100
0
73
92
82
79
76
90
—
In THF the conversion was almost the same as in DMF
(94%) but the temperature increased sharply.
IPrc
IPrc
NaH
NaH
The addition of 1 equivalent or 0.1 equivalent of TBAF in
solution in THF to a solution of alcohol 1 and diphenylsilane
in DMF gave 2b with complete conversion after 0.5 hour at
room temperature. The yields of the reaction were respectively
78% and 72% for a stoichiometric and sub-stoichiometric
quantity of TBAF, which shows that the latter can also promote
the hydrosilylation.
Then, we reduced alcohol 7 with deuterated diphenylsilane
to observe the diastereoselectivity of the reaction in a more
strained environment. The relative configuration of the
hydroxyl group and the deuterium on the cyclohexyl ring can
be attributed by NMR using a selective 1D version of the 2D
NOESY experiment.14 IPr was able to perform the reaction but
the two diastereomers 8 and 9 were obtained in only 37%
yield. The yield increased to 89% when the reaction was
accomplished with NaH (Scheme 6).
a See Scheme 5. b Reaction performed with 10 mol% of base.
1
c Commercially obtained and stored in a glove box. d Base on H NMR.
e Isolated yield.
This prompted us to test the reactivity of other bases as
initiators of the reduction of 1 with diphenylsilane (Table 1).
Sodium ethoxide led to complete conversion in 1 h, and 2b
was isolated in 73% yield (Table 1, entry 1). Similarly, sodium
hydride yielded 92% of 2b after only 1 h (Table 1, entry 2).
The fact that the reaction could go on without NHC par-
tially contradicted our previous report, and we wanted to get to
the bottom of this puzzling result. Three different conditions
were set up to examine the influence of addition order of the
reagents.
To finish with this mechanistic study, when the reaction
was performed with a sub-stoichiometric quantity of sodium
hydride, high deuteration at the homobenzylic position (D
content 94%) was observed with Ph2SiD2 (giving 2a). In con-
trast, the deuterium transfer from 1-D to the benzylic position
with 10 mol% or one full equivalent of NaH led to 2c with an
incorporation of only 63 and 21% deuterium respectively.
The 1H NMR spectrum of alcohol 1 was recorded in C6D6 in
the presence or absence of IPr. The hydroxyl proton of
complex 10 displays a significant downfield shift in the 1H
NMR spectrum (Δδ = 3.32 ppm) but the formation of imidazo-
lium 11 was not observed in 1H or 13C NMR (Scheme 7).15
Thus, even if NHCs are basic16 it is unlikely that they fully
In a first series of experiments, we added a solution of
alcohol 1 and diphenylsilane in DMF to a solution of base in
DMF, as described in our previous publication (Scheme 5, con-
ditions A).10
In a second series of experiments, alcohol 1 was added to a
solution of base in DMF and after 0.5 hour of mixing at room
temperature, the silane was added (Scheme 5, condition B). In
the last series of experiments, the silane was first added to a
solution of base in DMF followed by alcohol 1 (Scheme 5, con-
dition C). In all reactions the mixture was stirred for 3 hours at
room temperature and the reaction was quenched with TBAF.
Also, all reactions were carried out at the same concentration
and on the same scale.‡
The same reactivity was observed following conditions B
independently of the base used as the catalyst (Table 1, entries
4 and 6). When the hydrosilylation was catalyzed with IPr,13
the conversion was not affected by the order of addition of the
reagents (Table 1, entries 3–5). However, the reaction was
Scheme 6 Diastereoselective reduction of cyclic alcohol 7.
Scheme 5 Experimental conditions in the reduction of 1.
7460 | Dalton Trans., 2013, 42, 7458–7462
Scheme 7 Alcohol–carbene complex 10 in C6D6.
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anion, which ensures turnover by deprotonation of one further
molecule of starting material. The NHC is a catalytic base that
in that case initiates the reaction.
This work shows the problems one faces with the NHC–Si
interaction. Compared to boranes,17 complexation of silanes
with NHCs is much less favorable, and other pathways become
competitive. Nonetheless, this should facilitate the develop-
ment of new reaction methodologies based on the activation
of silicon reagents. Further work will focus on finding reac-
tions and structures which involve real NHC–Si bond
formation.
Scheme 8 Proposed mechanism for the hydrosilylation of 1 with deuterated
diphenylsilane in the presence of 10 mol% of IPr.
Acknowledgements
This work was supported by grants from UPMC, CNRS, IUF
(L.F. and M.M.) and ANR: Borane 08-CEXC-011–01 and NHCX
(ANR-11-BS07-008). Technical assistance was generously
offered through FR2769.
deprotonate the alcohol. However, a hydrogen-bonded NHC–
alcohol complex15b,c such as 10 is possible under the reaction
conditions, which can be in equilibrium with the alkoxide 12
in DMF.
Notes and references
On the basis of these observations, the Lewis acid–base
NHC–Si interaction leading to a hypervalent species may be
ruled out under our conditions and we propose a rationali-
zation involving Brønsted base catalysis (Scheme 8).
The base (carbene or NaH) deprotonates alcohol 1 to gener-
ate traces of alkoxide 12. It forms the hypervalent complex I
with diphenylsilane. The hydrosilylation could then lead to
cyclic siloxane II. The ring opening of II leads to benzylic
anion III which undergoes protonolysis to release silyl ether IV
and alkoxide 9. It seems reasonable that the released alkoxide
anion could instead go on to catalyze the hydrosilylation reac-
tion without further intervention of the NHC.
The stereochemical outcome of the hydrosilylation of 1 and
7 could be rationalized by a five-membered transition state
and the addition of the deuterium is directed by the alcohol.
In the case of acyclic alcohols, an eclipsed conformation could
be adopted in order to reduce allylic strain (Fig. 2a). Due to
steric demand, alcohol 7 would lead to another transition state
with the insertion of deuterium directed by the OH group
(Fig. 2b).
In conclusion, our mechanistic investigation of the one-pot
reduction of a styryl alcohol with diphenylsilane suggests that
this reaction proceeds via Brønsted base activation of the
alcohol and subsequent ate-complex I formation followed by a
diastereoselective intramolecular hydrosilylation. The reaction
then likely proceeds via opening of the sila-cycle to a benzylic
1 The Handbook of Homogeneous Hydrogenation, ed. J. G. de
Vries and C. J. Elsevier, Wiley-VCH, Weinheim, 2007.
2 (a) B. J. Marsh, E. L. Heath and D. R. Carbery, Chem.
Commun., 2011, 47, 280; (b) Y. Imada, H. Iida and T. Naota,
J. Am. Chem. Soc., 2005, 127, 14544; (c) S. Hoffmann,
A. M. Seayad and B. List, Angew. Chem., Int. Ed., 2005, 44,
7424; (d) J. B. Tuttle, S. G. Ouellet and D. W. C. MacMillan,
J. Am. Chem. Soc., 2006, 128, 12662; (e) M. Rueping,
A. P. Antonchick and T. Theissmann, Angew. Chem., Int.
Ed., 2006, 45, 3683; for a metal-free hydrosilylation, see:
(f) R. Shchepin, C. Xu and P. Dussault, Org. Lett., 2010, 12,
4772; (g) D. J. Parks, J. M. Blackwell and W. E. Piers, J. Org.
Chem., 2000, 65, 3090; (h) L. Ilies, H. Tsuji and
E. Nakamura, Org. Lett., 2009, 11, 3966.
3 (a) Topics in Organometallic Chemistry 21: N-Heterocyclic
Carbenes in Transition Metal Catalysis, ed. F. Glorius,
Springer, Berlin, 2007(b) S. Díez-González, N. Marion and
S. P. Nolan, Chem. Rev., 2009, 109, 3612; (c) M. Poyatos,
J. A. Mata and E. Peris, Chem. Rev., 2009, 109, 3677;
(d) F. E. Hahn and M. C. Jahnke, Angew. Chem., Int. Ed.,
2008, 47, 3122; (e) N. E. Kamber, W. Jeong,
R. M. Waymouth, R. C. Pratt, B. G. G. Lohmeijer and
J. L. Hedrick, Chem. Rev., 2007, 107, 5813; (f) D. Enders,
O. Niemeier and A. Henseler, Chem. Rev., 2007, 107, 5606;
(g) E. A. B. Kantchev, C. J. O’Brien and M. G. Organ, Angew.
Chem., Int. Ed., 2007, 46, 2768.
4 M. J. Fuchter, Chem.–Eur. J., 2010, 16, 12286.
5 (a) J. Raynaud, A. Ciolino, A. Baceiredo, M. Destarac,
F. Bonette, T. Kato, Y. Gnanou and D. Taton, Angew. Chem.,
Int. Ed., 2008, 47, 5390; (b) M. D. Scholten, J. L. Hendrick
and R. M. Waymouth, Macromolecules, 2008, 41, 7399;
(c) J. Raynaud, Y. Gnanou and D. Taton, Macromolecules,
2009, 42, 5996; (d) M. Rodriguez, S. Marrot, T. Kato,
Fig. 2 Proposed five-membered transition state in acyclic alcohol (a) and cyclic
alcohol (b).
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S. Stérin, E. Fleury and A. Baceiredo, J. Organomet. Chem.,
2007, 692, 705; (e) S. Marrot, F. Bonnette, T. Kato, L. Saint-
A. Weickgenannt and M. Oestreich, Chem.–Asian J., 2009, 4,
406.
Jalmes, E. Fleury and A. Baceiredo, J. Organomet. Chem., 10 Q. Zhao, D. P. Curran, M. Malacria, L. Fensterbank,
2008, 693, 1729. J.-P. Goddard and E. Lacôte, Chem.–Eur. J., 2011, 17, 9911.
6 (a) Y. Fukuda, Y. Maeda, S. Ishii, K. Kondo and T. Aoyama, 11 (a) R. W. Alder, P. R. Allen and S. J. Williams, J. Chem. Soc.,
Synthesis, 2006, 589; (b) J. J. Song, F. Gallou, J. T. Reeves,
Z. Tan, N. K. Yee and C. H. Senanayake, J. Org. Chem., 2006,
71, 1273; (c) Y. Suzuki, A. Bakar, K. Muramatsu and
M. Sato, Tetrahedron, 2006, 62, 4227; (d) T. Kano, K. Sasaki,
Chem. Commun., 1995, 1267; (b) Y.-J. Kim and
A. Streitwieser, J. Am. Chem. Soc., 2002, 124, 5757;
(c) A. M. Magill, K. J. Cavell and B. F. Yates, J. Am. Chem.
Soc., 2004, 126, 8717.
T. Konishi, H. Mii and K. Maruoka, Tetrahedron Lett., 2006, 12 (a) Silyl ether 5 has been synthesized using the procedure
47, 4615; (e) Y. Fukuda, K. Kondo and T. Aoyama, Synthesis,
2006, 2649; (f) J. J. Song, Z. Tan, J. T. Reeves, F. Gallou,
N. K. Yee and C. H. Senanayake, Org. Lett., 2005, 7, 2193;
(g) J. J. Song, Z. Tan, J. T. Reeves, N. K. Yee and
C. H. Senanayake, Org. Lett., 2007, 9, 1013; (h) Q. Zhao,
of Bosnich: S. H. Bergens, P. Noheda, J. Whelan and
B. Bosnich, J. Am. Chem. Soc., 1992, 114, 2123; (b) Cyclic
siloxane 6 has been synthesized using the procedure of
Smith III; A. B. Smith III, R. Tong, W.-S. Kim and
W. A. Maio, Angew. Chem., Int. Ed., 2011, 50, 8904.
D. P. Curran, M. Malacria, L. Fensterbank, J.-P. Goddard 13 It was necessary to make sure that the catalytic activity
and E. Lacôte, Synlett, 2012, 23, 433.M. Tan, Y. Zhang and
J. Y. Ying, Adv. Synth. Catal., 2009, 351, 1390; R. Blanc,
P. Nava, M. Rajzman, L. Commeiras and J.-L. Parrain, Adv.
Synth. Catal., 2012, 354, 2038.
7 J. Wu, X. Sun, S. Ye and W. Sun, Tetrahedron Lett., 2006, 47,
4813.
derived only from carbene activation. For this purpose,
base-free samples of IPr were used from commercial
sources (Strem Chemicals Inc.) or following the synthesis
described by Rovis et al.: J. Read de Alaniz and T. Rovis,
J. Am. Chem. Soc., 2005, 127, 6284.
14 The pulse sequence used for a selective 1D NOESY experi-
ment on Bruker equipment was selnogp with a mixing
time of 0.8 ms.
8 For additional reviews, see: (a) Silicon in Organic, Organo-
metallic, and Polymer Chemistry, ed. M. A. Brook, Wiley-Inter-
science, New York, 1999; for reviews of pentacoordinate 15 (a) While the C2 resonances of imidazolium salts are typi-
silicates, see: ; (b) R. R. Holmes, Chem. Rev., 1996, 96, 927;
(c) C. Chuit, R. J. P. Corriu, C. Reye and J. C. Young, Chem.
Rev., 1993, 93, 1371; (d) H. Nishikori, R. Yoshihara and
A. Hosomi, Synlett, 2003, 561; (e) F. J. LaRonde and
M. A. Brook, Tetrahedron Lett., 1999, 40, 3507;
(f) F. Iwasaki, O. Onomura, K. Mishima, T. Maki and
Y. Matsumura, Tetrahedron Lett., 1999, 40, 2065;
(g) R. Schiffers and B. H. Kagan, Synthesis, 1997, 1175–1178
cally >60 ppm upfield relative to the corresponding
carbene C2 resonance, the reversible formation of an
undetectable concentration of imidazolium alkoxide
cannot be ruled out. For complete characterization of
alcohol–carbene complexes see: (b) M. Movassaghi and
M. A. Schmidt, Org. Lett., 2005, 7, 2453; (c) M. A. Schmidt,
P. Müller and M. Movassaghi, Tetrahedron Lett., 2008, 49,
4316.
and references therein; (h) R. J. P. Corriu, C. Guerin, 16 T. Dröge and F. Glorius, Angew. Chem., Int. Ed., 2010, 49,
B. Henner and Q. Wang, Organometallics, 1991, 10, 2297
and references therein.
9 (a) S. V. Maifeld and D. S. Lee, Org. Lett., 2005, 7, 4995;
(b) For another activation of silane with tert-butoxide see:
6940.
17 D. P. Curran, A. Solovyev, M. Makhlouf Brahmi,
L. Fensterbank, M. Malacria and E. Lacôte, Angew. Chem.,
Int. Ed., 2011, 50, 10294.
7462 | Dalton Trans., 2013, 42, 7458–7462
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