Dual reactivity of N-heterocyclic carbenes towards copper(ii) salts

Article abstract of DOI:10.1039/c0dt01630f

Complexes of N-heterocyclic carbenes (NHCs) with copper(ii) halogenides are unstable. Upon formation, these complexes decompose to give haloamidinium salts. Contrastingly, O-substituted copper(ii) NHC complexes are fairly stable. A series of new five-, si

Full text of DOI:10.1039/c0dt01630f

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Dual reactivity of N-heterocyclic carbenes towards copper(II) salts†‡
Eugene L. Kolychev,a Viacheslav V. Shuntikov,b Victor N. Khrustalev,b Alexander A. Busha,c and
Mikhail S. Nechaev*a,c
Received 24th November 2010, Accepted 21st January 2011
DOI: 10.1039/c0dt01630f
Complexes of N-heterocyclic carbenes (NHCs) with copper(II) halogenides are unstable. Upon
formation, these complexes decompose to give haloamidinium salts. Contrastingly, O-substituted
copper(II) NHC complexes are fairly stable. A series of new five-, six- and seven-membered ring NHC
complexes of Cu(OAc)2 have been synthesised and characterised in the solid state.
Copper(I) complexes of N-heterocyclic carbenes (NHCs) have re-
cently received much attention. Various catalytic,1photophysical,1a
as well as medical applications2 have been reported. On the other
hand, copper(II) NHC complexes are less well studied. Although
postulated as catalysts in a number of reactions, there are only a
few Cu(II) complexes characterised in the solid state (1,3 2,4 3 and
4,5 and 5;6 Scheme 1). All of these complexes bear CŸO chelating
carbene or acetate ligands. Syntheses of (NHC)CuBr2 as well as
(NHC)2CuBr2 via direct carbene interaction with CuBr2 have been
reported.7 However, no reliable structure characterisation of the
corresponding complexes have been provided. To fill in this gap,
we performed a thorough investigation of the interaction of stable
diaminocarbenes with copper(II) salts.
The most common way of synthesising copper(I) complexes
with five-membered ring carbenes is ligand exchange between
(NHC)AgX and CuX.8 Recently, we utilised this approach for
the synthesis of copper(I) complexes bearing six- and seven-
membered ring diaminocarbenes.9 In this work we tried this
approach for the synthesis of copper(II) complexes as well. It was
found that Cu(OTf)2, CuF2·2H2O and Cu(OAc)2 do not react with
(NHC)AgBr (see the NHCs in Scheme 2).
Scheme 1 Copper(II) NHC complexes.
aA.V. Topchiev Institute of Petrochemical Synthesis, RAS, Leninsky
Prosp., 29, Moscow, 119991, Russian Federation. E-mail: nechaev@
nmr.chem.msu.ru; Fax: +7(495)9392677
Scheme 2 The N-heterocyclic carbenes under study.
bA.N. Nesmeyanov Institute of Organoelement Compounds, RAS, Vavilov
Str., 28, Moscow, 119991, Russian Federation
In contrast to previous reports,1 the interaction of (NHC)AgBr
with CuCl2 or CuBr2 in DCM at room temperature (rt) in our
hands led to the formation of haloamidinium salts in quantitative
yields (Scheme 3).
For the reaction of (6-Dipp)AgBr with CuBr2, crystallisation
of the product from a concentrated acetonitrile solution gave two
types of crystals; dark-green and colourless. Single crystal X-ray
diffraction analysis (see ESI Figs S1 and S2‡) has shown that
cDepartment of Chemistry, M.V. Lomonosov Moscow State University,
Leninskie Gory 1/3, Moscow, 119991, Russian Federation
† Dedicated to Professor Salambek N. Khadzhiev on the occasion of his
70th birthday.
‡ Electronic supplementary information (ESI) available: Experimental
procedures and crystallographic details. CCDC reference numbers 794664
and 794668. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c0dt01630f
3074 | Dalton Trans., 2011, 40, 3074–3076
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Scheme 5 Oxidation of a copper(I) NHC complex by bromine.
Scheme 3 The interaction of Ag(I) carbene complexes with copper(II)
salts.
both compounds are bromoamidinium salts bearing Cu2Br26- and
CuBr-2 anions, respectively. Other reaction mixtures were analysed
using 13C and 1H NMR.
To obtain a benchmark for 13C NMR chemical shifts, we
synthesised a series of bromoamidinium salts bearing a bromide
anion. Oxidation of (NHC)AgBr with molecular bromine in DCM
at rt leads to the corresponding salts in excellent yields (Scheme 4).
The structure of the six-membered ring bromoamidinium bromide
[(6-Dipp)Br][Br] has been unambiguously established by X-ray
crystallography (see ESI Fig. S3‡).
Scheme 6 The reaction of free carbenes with copper(II) salts.
has been recently reported that in situ generated IMes reacts with
Cu(II) salts to give unexpected complexes bearing [(NHC)2Cu]+
cations.15
The proposed mechanism (Scheme 6) of the reaction proceeds
from the initial formation of a dimer [(NHC)CuX2]2 followed by
the migration of one of the halogen atoms to the ylidene carbon,
with the formation of a complex CuX-2 anion and a copper(I)
NHC complex. This mechanism is supported by the observation
of rapid dissolution of CuX2 sediment in Et2O upon addition of
a free carbene. A dark solution is initially formed, which becomes
colourless within several hours.
Scheme 4 The preparation of bromoamidinium bromides.
According to 13C NMR, the only organic products from the
interaction of (NHC)AgBr with CuX2 (Scheme 3) are haloami-
dinium salts. C2 chemical shifts at 126–128 ppm (for unsaturated
salts)10 and 148–155 ppm (for saturated salts) are indicative
of haloamidinium salts. The corresponding H-substituted ami-
dinium salts and free carbenes exhibit 13C chemical shifts at 130–
132 (for unsaturated salts), ~160 ppm (for saturated salts) and 220–
260 ppm regions, respectively. In addition, for chloroamidinium
salts, which exhibit 13C chemical shifts similar to the corresponding
amidinium salts,10–11 we checked the absence of amidinium proton
signals in the 1H NMR spectra.
In contrast to the reactions of free carbenes with CuX2, the
interaction with Cu(OAc)2 in Et2O at room temperature gives
(NHC)Cu(OAc)2 in moderate to good yields (Scheme 7).
It has been previously reported that the interaction of a chiral
imidazolium carbene silver(I) complex with CuCl2 leads to the
formation of a chloroamidinium salt.12 In this report we have
shown that this type of reaction is general for five-, six-, and seven-
membered ring diaminocarbenes. The exceptions are bidentate
CŸO chelating carbenes, for which the interaction of silver(I)
complexes with CuCl2 leads to the formation of the corresponding
(NHC)Cu(II) complexes 13 and 5.6
Scheme 7 The reaction of free carbenes with copper(II) salts.
Two complexes, (IMes)Cu(OAc)2 and (6-Mes)Cu(OAc)2, were
characterised by X-ray crystallography. Their molecular struc-
tures, along with the atomic numbering schemes and selected
geometrical parameters, are shown in Figs 1 and 2.
It has been previously reported that AuBr3 carbene complexes
can be readily obtained via oxidation of (NHC)AuBr with molecu-
lar bromine.13 We attempted the same approach for the synthesis of
(NHC)CuBr2. However, only the formation of bromoamidinium
salt has been observed (Scheme 5). Analogous results have been
recently reported for the oxidation of (NHC)CuX with various
oxidants.14
Finally, we attempted to synthesize (NHC)CuX2 complexes via
direct interaction of free carbenes with CuX2 (Scheme 6). In
all cases the formation of (NHC)CuX and haloamidinium salts
[(NHC)X][CuX2] in a 1 : 1 ratio was observed. Additionally, it
Both of the copper(II)–NHC diacetate complexes adopt sim-
ilar geometries. The mesityl ligands, as well as the CuOAc
fragments, are arranged almost perpendicular to the carbene
plane (99.1, 98.8, 109.4 and 98.4◦ for (IMes)Cu(OAc)2 and 98.0,
94.7, 100.6 and 93.9◦ for (6-Mes)Cu(OAc)2). Also, the CuOAc
fragments are practically perpendicular to each other (84.9◦ for
(IMes)Cu(OAc)2 and 89.6◦ for (6-Mes)Cu(OAc)2). The Cu–Ccarb
and the two covalent Cu–O distances in (IMes)Cu(OAc)2 (1.960(3)
˚
and 1.989(3), 1.963(3) A) and in (6-Mes)Cu(OAc)2 (1.981(3)
˚
and 1.957(3), 1.962(3) A) are slightly longer than those in the
4
˚
analogous (IDipp)Cu(OAc)2 (1.942(4) and 1.941(3), 1.941(3) A).
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Scheme 8 The reaction of (5-Dipp)Cu(OAc)2 with water.
In conclusion, we discussed previously reported data and
performed a thorough investigation of the reactivity of free NHCs
and their Ag(I) complexes towards Cu(II) salts. It was found
that (NHC)CuX2 complexes are unstable. They decompose to
give haloamidinium salts. On the other hand, oxygen-containing
ligands, such as OAc or CŸO chelating NHCs make Cu(II)
complexes fairly stable. The nature of this stabilizing effect is yet
to be determined.
Fig. 1 The molecular structure of (IMes)Cu(OAc)2. Anisotropic displace-
ment ellipsoids are draw◦n at the 40% probability level. Selected bond
˚
distances (A) and angles ( ): Cu1–C1 1.960(4), Cu1–O1 1.989(3), Cu1–O2
2.077(3), Cu1–O3 1.963(3), Cu1–O4 2.356(3), N1–C1 1.337(4), N2–C1
1.332(5), O1–Cu1–O3 164.2(1), O1–Cu1–C1 99.0(1), O3–Cu1–C1 96.8(1),
N1–C1–N2 105.5(3).
Mixtures of in situ generated carbenes with CuX2 were previ-
ously reported as efficient pre-catalysts in a number of catalytic
transformations.1 In this report we have clearly shown that
(NHC)CuX2 complexes are not operative. We provided solid
evidence that Cu(I) should preferably be considered as the true
catalytic species. As a result, we believe that the data presented here
will give rise to knowledge-based development of new, efficient,
copper NHC catalytic systems.
Notes and references
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Fig. 2 The molecular structure of (6-Mes)Cu(OAc)2. Anisotropic dis-
placement ellipsoids are drawn ◦at the 40% probability level. Selected
˚
bond distances (A) and angles ( ): Cu1–C1 1.981(3), Cu1–O1 1.957(3),
Cu1–O2 2.254(3), Cu1–O3 1.962(3), Cu1–O4 2.252(3), N1–C1 1.339(5),
N2–C1 1.333(4), O1–Cu1–O3 160.1(2), O1–Cu1–C1 96.1(2), O3–Cu1–C1
103.8(2), N1–C1–N2 118.3(2).
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the lengths of the dative Cu–O bonds for the two diacetate
4
˚
ligands in (IDipp)Cu(OAc)2 (2.259(3) and 2.259(3) A) and (6-
˚
Mes)Cu(OAc)2 (2.254(3) and 2.252(3) A) are equal, whereas
8 H. M. J. Wang and I. J. B. Lin, Organometallics, 1998, 17, 972–
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˚
those in (IMes)Cu(OAc)2 (2.077(3) and 2.356(3) A) are signif-
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icantly different. This fact, apparently, determines the different
symmetries of the crystal packing of the three copper(II)–NHC
diacetate complexes. So, complex (IDipp)Cu(OAc)2 crystallises in
the monoclinic C2/c space group, with the molecule occupying a
special position on the two-fold axis.4 Complex (6-Mes)Cu(OAc)2
crystallises in the monoclinic Cc space group, with the molecule
lying on the pseudo-crystallographic two-fold axis. The complex
¯
(IMes)Cu(OAc)2 crystallizes in the triclinic Pspace group.
Some reactivity studies of (5-Dipp)Cu(OAc)2 have been per-
formed. We attempted to exchange the OAc groups with bromide
ions. Unfortunately, (5-Dipp)Cu(OAc)2 does not react with LiBr
in acetone, nor with [Bu4N]Br in chloroform.
It was found that (5-Dipp)Cu(OAc)2 is sensitive to water
(Scheme 8). Decomposition in wet chloroform proceeds within
a few hours to give the corresponding open ring formamide 8.
3076 | Dalton Trans., 2011, 40, 3074–3076
This journal is
The Royal Society of Chemistry 2011
©