[{Cu(IPr)}2(μ-OH)][BF4]: Synthesis and halide-free CuAAC catalysis

Article abstract of DOI:10.1039/c4cc03346a

The preparation under protic conditions of the first μ-hydroxo dicopper(i)-NHC complex is reported. Its application as a CuAAC catalyst was investigated, evidencing a remarkable enhancement of catalytic efficiency in the presence of 4,7-dichloro-1,10-phen

Full text of DOI:10.1039/c4cc03346a

ChemComm
View Article Online
View Journal | View Issue
COMMUNICATION
[{Cu(IPr)}2(l-OH)][BF4]: synthesis and halide-free
CuAAC catalysis†
Cite this: Chem. Commun., 2014,
50, 7154
Houssein Ibrahim,ab Regis Guillot,c Federico Cisnetti*ab and Arnaud Gautier*ab
´
Received 5th May 2014,
Accepted 12th May 2014
DOI: 10.1039/c4cc03346a
www.rsc.org/chemcomm
The preparation under protic conditions of the first l-hydroxo
dicopper(I)–NHC complex is reported. Its application as a CuAAC
catalyst was investigated, evidencing a remarkable enhancement of
catalytic efficiency in the presence of 4,7-dichloro-1,10-phenanthroline
and highlighting the beneficial effect of the absence of coordinating
halides.
Classically, the direct metalation of azolium salts involves the use of
appropriate metal salts and strong bases under anhydrous anoxic
conditions.1a This is particularly true for copper(I), which is prone to
oxidation.1b Alternatively, CuI–NHC complexes may be obtained by
transmetalation of silver precursors but this generates large amounts
Scheme 1 Outcome of the metalation of 2.
of inorganic waste.1c Recently, there has been a drive for the discovery and [Cu(NHC)(OH)], leading to [Cu(NHC)H2O]+ in water. We report in
of novel, alternative and simplified syntheses of Group 11 metal–NHC this communication the facile preparation of [{Cu(IPr)}2(m-OH)][BF4]
complexes,2 as these offer unique catalytic opportunities. Mild bases (1) by a two-step one-pot procedure (Scheme 1) and its catalytic
and aqueous conditions have been highlighted as suitable media for potential for the copper-catalysed azide–alkyne cycloaddition (CuAAC)
the synthesis of CuI–NHC complexes. We have reported the prepara- alone or in the presence of 4,7-dichloro-1,10-phenanthroline (phen*)
tion of hetero- and homoleptic CuI–NHCs in aqueous or ethanolic as an additive. Indeed, it has been shown that the addition of
ammonia as basic and coordinating medium.2d
Phenanthroline Enhances the Copper (‘‘PECu’’) performances
Previous reports have highlighted the potential of [M((S)IPr)(OH)] toward CuAAC reactions.5
(M = Cu, Au) and [{Au(IPr)}2(m-OH)]+ complexes in catalysis
The synthesis of (1) was discovered serendipitously while
and as useful synthons.3 To the best of our knowledge, no studying an ammonia-based metalation in ethanol.2d Indeed, in
[{Cu(NHC)}2(m-OH)]+ compound was reported. Such a complex contrast to (S)IMesÁHBF4, for which reaction with Cu2O (0.25 eq.)/
would be of catalytic interest because of (i) the absence of halide NH3 in ethanol yielded the homoleptic complexes as filterable solids,
ions – cationic halide-free complexes displayed improved activity IPrÁHBF4 (2) afforded a colorless solution. 1H NMR of a sample of the
in CuAAC and hydrosilylation reactions, which was rationalized reaction mixture revealed that it contained an equimolar amount of 2
by a more efficient pathway for the activation of the precatalyst4 – and an unknown species (3) as well as a small amount (5–10%) of
and (ii) its possible equivalence with a combination of [Cu(NHC)]+ [Cu(IPr)2]BF4, 4. Increasing the amount of Cu2O to 0.5 eq. afforded 3
1
as the major component. The H NMR signature of 3 was different
from that of 4 and [Cu(IPr)OH] (5).† It displayed a broad signal
a
´
´
Clermont-Universite, Universite Blaise Pascal, ICCF, 24 Av. des Landais,
(2.26 ppm) roughly integrating 3 protons. Therefore, 3 was tentatively
identified to be [Cu(IPr)(NH3)]+. While treating the reaction mixture
with water, a white solid separated. The 1H-NMR spectrum revealed
the presence of [Cu(IPr)2]BF4 (4) and 2 (5% and 10%, respectively) as
well as a major new compleÀx, 1. Infrared analysis showed that 1 is a
cationic complex with a BF4 counter-ion (1051 cmÀ1).
`
63177 Aubiere, France. E-mail: federico.cisnetti@univ-bpclermont.fr,
arnaud.gautier@univ-bpclermont.fr; Tel: +33 473407110
b CNRS, UMR 6296, ICCF, 63171 Aubiere, France
c
´
´
ˆ
Institut de Chimie Moleculaire et des Materiaux d’Orsay, Bat. 420,
´
Universite Paris-Sud, UMR CNRS 8182, 91405 ORSAY Cedex, France
† Electronic supplementary information (ESI) available: Preparation and crystal-
lographic data for 1; the mechanism of formation of 1, NMR spectra of 3 and the
phenanthroline adduct of 1. CCDC 989122. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/c4cc03346a
Preparative recrystallization affords prisms (1, majority) and
needles (4, minority), both being stable for months on the benchtop.
7154 | Chem. Commun., 2014, 50, 7154--7156
This journal is ©The Royal Society of Chemistry 2014

View Article Online
Communication
ChemComm
Table 1 CuAAC performed using Cu(IPr)-containing species
Time (h) Yielda (%)
Entry Catalyst
x (mol%) Cu Alkyne
1
2
3
4
5
6
7
8
1
7a
7b
1
1
1
2
4
4
1
0.5
1
4
2
4
8a (–H)
8a (–H)
8a (–H)
8a (–H)
8a (–H)
8a (–H)
8a (–H)
8a (–H)
8a (–H)
8a (–H)
8a (–H)
8a (–H)
8a (–H)
8a (–H)
8a (–H)
8a (–H)
19
19
19
19
19
3
19
19
19
19
19
19
3
90
19
53
83
46
10
39
20
80
68
89
76
55
17
18
0
7a + 10 (4 mol%)
7a + 10 (2 mol%)
7b + 10 (4 mol%)
7b + 10 (2 mol%)
1 + 10 (2 mol%)
1 + 10 (1 mol%) 0.5
1 + 10 (2 mol%)
1 + 10 (1 mol%)
9
10
11
12
13
14
15
16
17
18
19
2
1
Fig. 1 EllÀipsoid plot of [{Cu(IPr)}2(m-OH)](BF4) (1) (50% probability
level, BF4 and most hydrogens omitted). Cu–C: 1.87 Å, Cu–O: 1.84 Å,
C–Cu–O: 1171 (average values), Cu–O–Cu: 127.87(8).
1
1
3
19
19
1 + 10 (0.5 mol%) 0.25
1 + ClÀ b
1
1
1
1
1
1
Identity and purity of 1 were established by X-ray crystallography
(Fig. 1) and elemental analysis.† The structure is quite similar to
[{Au(IPr)}2(m-OH)]+.3b Optimization of the reaction conditions
1
1
1
1
1
8b (–OMe) 19
8c (–NH2) 19
8d (–F)
8e
76
70
83
70
47
72
72
72
allowed this species to be obtained 95% pure in the crude solid. 20
21
8f
The formation of 1 could be explained by the replacement of the
ammonia ligand of 3 by water furnishing [Cu(IPr)(OH2)] (6) which
a
b
Pure compound isolated by filtration. NaCl or NH4Cl (3.0 eq.).
is the conjugate acid of [Cu(IPr)OH], (5). The metal-bound water of
6 is expected to be displaced by 5 in an olation step furnishing the
m-OH complex 1 which precipitates from the reaction medium in
the presence of a BF4À counterion, thus providing a driving force
for the reaction (Scheme S1, ESI†).
of halides.4b,9 As blank experiment we added 1 to NaCl solution
and recovered [Cu(IPr)Cl] nearly instantaneously – the same
observations applied for BrÀ or IÀ anions. The catalyst was also
efficient with electron-rich (entries 17 and 18) as well as electron-
poor alkynes, albeit at a lower rate (entries 19 and 20). The
reaction hardly functions with bulky substituents (entry 21)
compared to the triazolium-derived halide free homoleptic
abnormal CuI–NHCs reported by Sarkar.9a
Having established 1 as a CuAAC catalyst, we decided to
investigate the kinetic profile of the CuAAC reaction (Fig. 2)
under homogenous conditions and at higher concentration.
Using 1, the reaction starts after an induction period of
B20 minutes and reaches completion in approximately 120 min
probably because 1 behaves as a precatalyst. Phen* exhibits a
remarkable acceleration effect. At the same catalyst concentration,
the inductive period is shortened to 4 min and completion reached
in less than 7 min. A decrease of the catalyst loading lengthens
A first evaluation of the CuAAC potential of 1 under benchmark
conditions such that the products are isolated by simple filtration
was performed. It revealed that the catalyst allowed the CuAAC
reaction to be performed under true click reaction conditions: open
flask and pure products by filtration (Scheme 2, Table 1, entry 1).6
For the sake of comparison, we tested [Cu(IPr)Cl]7 (7a) and
[Cu(IPr)(OtBu)]8 (7b) under the same conditions (especially, the
same metal loading). Entry 2 shows a low efficiency of 7a whereas
entry 3 displays a higher but still limited yield for the chloride free
catalyst 7b (19 vs. 53%). Catalyst 1 (entries 4 and 5) was more active
furnishing good isolated yields of 11a at loadings of 1 and
0.5 mol%. A shorter reaction time (entry 6) resulted in a lower
isolated yield. The PECu effect was evaluated in entries 7–11.
Addition of phen* to 7a showed a better but still limited perfor-
mance (entries 2 vs. 7), exhibiting a strong positive action on 7b
(entries 9, 10) and an even more marked effect on 1 (entries 11
and 12). Moderate yields were obtained only after 3 h using two
equivalents of phen* per 1 (entry 13) while a lower yield
was observed with 1 equivalent (entry 14). The lower limit of
catalyst concentration was reached using 0.25 mol% (entry 15).
Interestingly, chloride free catalysts 1 and 7b perform better
than [Cu(IPr)Cl] activated by phen* (90 and 53% vs. 39%; entries
1, 3 vs. 7). In the presence of chloride, the catalysis was totally
inhibited (entries 4 vs. 16) highlighting the detrimental role
Fig. 2 1H NMR kinetic survey at 298 K (log scale) of the CuAAC reaction
of phenylacetylene (8a) and benzyl azide (9) catalysed by 1. [8a] = 0.5 M;
t
[9] = 0.5 M, 1,4-dimethoxybenzene as internal standard, BuOH/H2O v/v
3 : 1.; ’: 1 (2 mol%) + phen* (4 mol%); m: 1 (1 mol%) + phen* (2 mol%);
E: 1 (0.5 mol%) + phen* (1 mol%); Â: 1 (2 mol%).
Scheme 2 Catalytic CuAAC experiments.
This journal is ©The Royal Society of Chemistry 2014
Chem. Commun., 2014, 50, 7154--7156 | 7155

View Article Online
ChemComm
Communication
Applications pertaining to water-soluble functionalized CuI–NHCs
may be of special interest.13
´
Financial support from Region Auvergne (PNC) is acknowledged.
´
Kevin Fauche and Maxime de Sousa Lopes Moreira participated to
this study as part of their undergraduate training.
Notes and references
1 (a) N-Heterocyclic Carbenes. From Laboratory Curiosities to Efficient
´
´
Synthetic Tools, ed. S. Dıez-Gonzalez, RSC catalysis series, Cambridge,
´
´
2010; (b) S. Dıez-Gonzalez, E. C. Escudero-Adan, J. Benet-Buchholz,
E. D. Stevens, A. M. Z. Slawin and S. P. Nolan, Dalton Trans., 2010,
39, 7595; (c) J. C. Y. Lin, R. T. W. Huang, C. S. Lee, A. Bhattacharyya,
W. S. Hwang and I. J. B. Lin, Chem. Rev., 2009, 109, 3561.
`
2 Using carbonate: (a) M. Fevre, J. Pinaud, A. Leteneur, Y. Gnanou,
J. Vignolle and D. Taton, J. Am. Chem. Soc., 2012, 134, 6776;
(b) S. Zhu, R. Liang and H. Jiang, Tetrahedron, 2012, 68, 7949;
(c) O. Santoro, A. Collado, A. M. Z. Slawin, S. P. Nolan and
C. S. J. Cazin, Chem. Commun., 2013, 49, 10483. Using NH3:
(d) C. Gibard, H. Ibrahim, A. Gautier and F. Cisnetti, Organometallics,
2013, 32, 4279. Electrosynthesis: (e) B. R. M. Lake, E. K. Bullough,
T. J. Williams, A. C. Whitwood, M. A. Little and C. E. Willans, Chem.
Commun., 2012, 48, 4887. Continuous flow: ( f ) S. M. Opalka, J. K. Park,
A. R. Longstreet and T. D. McQuade, Org. Lett., 2013, 15, 996. Micro-
waves: (g) B. Lander and O. Navarro, Eur. J. Inorg. Chem., 2012, 2980.
3 (a) S. Gaillard, C. S. J. Cazin and S. P. Nolan, Acc. Chem. Res., 2012,
Scheme 3 Proposed mechanistic path.
the inductive period and completion time by 10 min and 30 min
(1 and 0.5 mol%), respectively.
´
45, 778; (b) S. Gaillard, J. Bosson, R. S. Ramon, P. Nun,
A. M. Z. Slawin and S. P. Nolan, Chem. – Eur. J., 2010, 16, 13729.
A probable mechanism of catalysis by 1 is displayed in
Scheme 3.10 The reaction is expected to proceed through the
formation of a s, p dicuprated complex 12. Reversible coordination
of azide 9 followed by annulation through intermediate 14 and
protonation of the resulting triazolide copper–NHC (15)11 delivers
the target 1,2,3-triazole 11 as well as the copper–NHC species 6
which could enter into the catalytic cycle through the coordination
of a new alkyne delivering 12 or reforming 1.
´
´
4 (a) S. Dıez-Gonzalez, E. D. Stevens, N. M. Scott, J. L. Petersen and
´
´
S. P. Nolan, Chem. – Eur. J., 2008, 14, 158; (b) S. Dıez-Gonzalez and
S. P. Nolan, Angew. Chem., Int. Ed., 2008, 47, 8881.
¨
5 (a) M.-L. Teyssot, A. Chevry, M. Traıkia, M. El-Ghozzi, D. Avignant
and A. Gautier, Chem. – Eur. J., 2009, 15, 6322; (b) M.-L. Teyssot,
L. Nauton, J.-L. Canet, F. Cisnetti, A. Chevry and A. Gautier, Eur.
J. Org. Chem., 2010, 3507; (c) S. Hohloch, B. Sarkar, L. Nauton,
F. Cisnetti and A. Gautier, Tetrahedron Lett., 2013, 14, 1808. For
selected applications: (d) Z. Chamas, X. Guo, J.-L. Canet, A. Gautier,
D. Boyer and R. Mahiou, Dalton Trans., 2010, 39, 7091; (e) C. Gaulier,
A. Hospital, B. Legeret, A. F. Delmas, V. Aucagne, F. Cisnetti and
A. Gautier, Chem. Commun., 2012, 48, 4005.
6 H. C. Kolb, M. G. Finn and B. K. Sharpless, Angew. Chem., Int. Ed.,
2001, 40, 2004.
7 V. Jurkauskas, J. P. Sadighi and S. L. Buchwald, Org. Lett., 2003,
5, 2417.
8 N. P. Mankad, D. S. Laitar and J. P. Sadighi, Organometallics, 2004,
23, 3369.
9 (a) S. Hohloch, D. Scheiffele and B. Sarkar, Eur. J. Inorg. Chem., 2013,
3956; chloride inhibition has been noticed for other ligands:
(b) J. E. Hein and V. V. Fokin, Chem. Soc. Rev., 2010, 39, 1302. For
triazolium-derived CuI–NHCs see also: (c) T. Nakamura,
T. Terashima, K. Ogata and S.-i. Fukuzawa, Org. Lett., 2011, 13, 620.
Addition of 2 eq. of phen* disrupts 1 into [Cu(IPr)(phen*)]+
(16)12 – liberating simultaneously one equivalent of base which
could facilitate alkyne deprotonation to access 12. Phen* could
also block the reformation of inactive 1 at the end of the catalytic
cycle. Moreover, phen* may tune the electronic properties at
several intermediates and transition states – especially, by analogy
to previous reports, for the dicuprated complex 12 and the
transition state between 13 and 144a,b,9b
In summary, we have reported the synthesis of [{Cu(IPr)}2-
(m-OH)](BF4), (1), the first m-hydroxo dicopper(I)–NHC. This
species is easily synthesized in hydro alcoholic media using
ammonia. Thanks to the absence of coordinating halides, the
catalytic activity of (1) in CuAAC is greatly increased compared
to [Cu(IPr)Cl], (7). Moreover, the addition of phen* to (1) leads to a
marked enhancement of the catalytic efficiency. Compared to
10 Mechanistic investigation using
a dinuclear copper complex:
(a) J. Straub, E. Schreiner, S. Mader, F. Rominger and B. F. Straub,
Adv. Synth. Catal., 2012, 354, 3445. For a deep mechanistic study:
(b) B. T. Worrell, J. A. Malik and V. V. Fokin, Science, 2013, 340, 457.
See also ref. 5a.
11 (a) C. Nolte, P. Mayer and B. F. Straub, Angew. Chem., Int. Ed., 2007,
46, 2101; (b) B. F. Straub, Chem. Commun., 2007, 3868.
previously reported systems containing halides, such a PECu effect 12 Addition of phen to 1 (see ESI†) results in previously reported
[Cu(IPr)(phen)]+ species: V. A. Krylova, P. I. Djurovich, M. T. White
and M. E. Thompson, Chem. Commun., 2010, 46, 6696.
13 (a) Ref. 5e; (b) W. Wang, J. Wu, C. Xia and F. Li, Green Chem., 2011,
is optimally efficient. The synthesis of other [{Cu(NHC)}2(m-OH)]+
complexes is currently under investigation as well as their use
as catalysts for other reactions under halide-free conditions.
13, 3440.
7156 | Chem. Commun., 2014, 50, 7154--7156
This journal is ©The Royal Society of Chemistry 2014