Synthesis of bis N-heterocyclic carbenes, derivatives and metal complexes

Article abstract of DOI:10.1016/j.crci.2013.01.016

A method for the synthesis and isolation of 1,1′-methylene-bis-(3- aryl-imidazol-2-ylidene) ligands, aryl = 2,6-diisopropyl-phenyl (DiPP), L ^DiPP, mesityl (mes), L^mes, is reported, which provides synthetically useful quantities of hi

Full text of DOI:10.1016/j.crci.2013.01.016

G Model
CRAS2C-3701; No. of Pages 9
C. R. Chimie xxx (2013) xxx–xxx
Contents lists available at SciVerse ScienceDirect
Comptes Rendus Chimie
www.sciencedirect.com
´
Full paper/Memoire
Synthesis of bis N-heterocyclic carbenes, derivatives and metal
complexes
*
Arlanda Huffer, Ben Jeffery, Benjamin J. Waller, Andreas A. Danopoulos
Laboratoire de chimie de coordination, Institut de chimie (UMR 7177 CNRS), universite´ de Strasbourg, 4, rue Blaise-Pascal, 67081 Strasbourg, France
A R T I C L E I N F O
A B S T R A C T
A method for the synthesis and isolation of 1,10-methylene-bis-(3-aryl-imidazol-2-
Article history:
ylidene) ligands, aryl = 2,6-diisopropyl-phenyl (DiPP), LDiPP mesityl (mes), Lmes
, , is
Received 19 October 2012
Accepted after revision 24 January 2013
Available online xxx
reported, which provides synthetically useful quantities of high purity. Derivatisation
of LDiPP with chalcogenides gave the adducts LDiPPE2, E = S, Se, Te. Reaction of LDiPP with
[Pd(tmeda)Me2], [Pt(m-SMe2)Me2]2, [Ir(1,5-COD)(m-Cl)]2/KPF6 and [NiBr2(dme)] gave
[Pd(LDiPP)Me2] (1), [Pt(LDiPP)Me2] (2), [Ir(LDiPP)(1,5-COD)](PF6) (3) and [Ni(LDiPP)Br2] (4),
respectively. The latter was reduced in the presence of CO to [Ni(LDiPP)(CO)2] (5). The
structures of Lmes, LDiPPTe2, and 1–5 are also reported.
Keywords:
N-heterocyclic carbenes
Tellurium
Palladium
ß 2013 Published by Elsevier Masson SAS on behalf of Acade´mie des sciences.
Platinum
Nickel
´
´
R E S U M E
Iridium
Crystal structures
0
´
´
`
`
´
`
Nous decrivons une methode de synthese des ligands bis-carbenes 1,1 -methylene-bis-(3-
DiPP
mes
`
´
´
aryl-imidazol-2-ylidene) (aryl = 2,6-diisopropyl-phenyl (DiPP), L
; mesityl (mes), L
)
qui permet de les isoler en grandes quantite´s et haute purite´. La re´action de LDiPP avec les
chalcoge´nures a conduit aux de´rive´s LDiPPE2, E = S, Se, Te. Les re´actions de LDiPP avec
´
[Pd(tmeda)Me2], [Pt(m-SMe2)Me2]2, [Ir(1,5-COD)(m-Cl)]2/KPF6 et [NiBr2(dme)] ont donne
respectivement [Pd(LDiPP)Me2] (1), [Pt(LDiPP)Me2] (2), [Ir(LDiPP)(1,5-COD)](PF6) (3) et
[Ni(LDiPP)Br2] (4). Ce dernier fut re´duit en pre´sence de CO en [Ni(LDiPP)(CO)2] (5). Les
structures de Lmes, LDiPPTe2, and 1-5 sont egalement decrites.
´
´
´
´
ß 2013 Publie par Elsevier Masson SAS pour l’Academie des sciences.
1. Introduction
Since then, chelating dicarbene ligands (Scheme 1) have
been studied mainly on Pd and Pt centres due to the
emerging potential of the corresponding complexes in
homogeneous catalysis (cross coupling, copolymerisation,
carbonylation, C–H activation, CO2 activation), [3] stabi-
lisation of high oxidation states [4] and, more recently,
photophysical applications [5]. Other metals have not been
studied as extensively. Early work on Ni demonstrated the
potential of Ni-NHC complexes in catalysis [6], and on Cr
the stabilisation of organometallics of relevance to alkene
polymerisation and oligomerisation [7]. In most cases, the
commonly used ligand with a methylene spacer between
the heterocyclic donors (Scheme 1 A–C) is not being
isolated, but introduced to the metal centre by the reaction
with metal precursors coordinated to an internal base
Chelating bis-N-heterocyclic carbene (NHC) complexes
followed as early developments after the introduction of
the NHC functionality in the coordination chemistry,
which gained momentum with the isolation of ‘bottleable’
NHCs by Arduengo [1]. Bis-imidazolium salts linked by
alkylene spacers served as precursors to the corresponding
NHCs and proved the reagents of choice as in situ sources
of the NHC ligands leading to the synthesis of Pd
complexes [2].
* Corresponding author.
E-mail address: danopoulos@unistra.fr (A.A. Danopoulos).
1631-0748/$ – see front matter ß 2013 Published by Elsevier Masson SAS on behalf of Acade´mie des sciences.
http://dx.doi.org/10.1016/j.crci.2013.01.016
Please cite this article in press as: Huffer A, et al. Synthesis of bis N-heterocyclic carbenes, derivatives and metal
complexes. C. R. Chimie (2013), http://dx.doi.org/10.1016/j.crci.2013.01.016

G Model
CRAS2C-3701; No. of Pages 9
2
A. Huffer et al. / C. R. Chimie xxx (2013) xxx–xxx
an improved synthesis of ligands Lmes and LDiPP in
synthetically useful quantities and high purity. We also
provide some examples of novel transition metal organo-
metalllics that demonstrate some advantages of using the
free ligands in synthesis by simple substitution reactions. A
summary of the chemical transformations is given in
Scheme 2.
2. Results and discussion
Scheme 1.
2.1. Synthesis of ligands and derivatives
(e.g. acetate, acac, etc.), or by deprotonating the di-
imidazolium precursors in situ with an external base
The bis-imidazolium salts (LmesH2)Br2 and (LDiPPH2)Br2
were prepared by adaptation of established literature
procedures [3d,7,8]. Analytically pure, colourless solids
were obtained after washing the precipitated products that
were formed by the quaternisation of the imidazoles with
CH2Br2 in xylenes, with small amounts of acetone. For
structural characterisation of the (LDiPPH2)2+, (LDiPPH2)Br2
was converted to the triflate salt and crystallised from
CH2Cl2/petrol. The structure of the dication in (LDiP-
PH2)(OTf)2 determined crystallographically is depicted in
Fig. 1. It is worth commenting on some differences
observed in the structure of the (LDiPPH2)(OTf)2 and the
known (LDiPPH2)Br2ꢀH2O [7]. The vectors of the two C–H
imidazolium bonds in the former are pointing to the same
direction; this is certainly due to the formation of H-
bonding between these C–H hydrogen atoms with one
oxygen atom of the triflate anion. The interaction may
also influence the angle between the two heterocycles
(NaOAc, Et3N) in the presence of
precursor.
a suitable metal
Attempts to isolate the free di-NHC ligand have been
reported for type A ligands (R = Bu,t [6a] benzyl [6b]). In
addition, the ligand of type C (R = DiPP) has been prepared
by deprotonation of the corresponding imidazolium salt
with KN(SiMe3)2 in THF at room temperature [7]. This
method, albeit attractive, proved in our hands problematic
leading to intensely colored and spectroscopically impure
materials, which we were unable to purify by common
methods. On the other hand, the advantage of the product
stoichiometry control provided by the bulky ligands of
type C (i.e. formation of complexes with ligand-to-metal
stoichiometry 1:1 rather 2:1, the latter being common
with NHC ligands of type A), prompted us to reinvestigate
and expand the synthetic methodology to access the
aforementioned ligands of type C. In this paper we describe
Scheme 2. Synthetic transformations leading to the ligands, derivatives and complexes described in this paper. Reagents: (i) S8, Se or Te; (ii)
[Pd(tmeda)Me2] (M = Pd); [Pt( -SMe2)Me2]2 (M = Pt); (iii) [Ir(1,5-COD)( -Cl)]2/KPF6; (iv) [NiBr2(dme)]; (v) Na(Hg)/CO.
m
m
Please cite this article in press as: Huffer A, et al. Synthesis of bis N-heterocyclic carbenes, derivatives and metal
complexes. C. R. Chimie (2013), http://dx.doi.org/10.1016/j.crci.2013.01.016

G Model
CRAS2C-3701; No. of Pages 9
A. Huffer et al. / C. R. Chimie xxx (2013) xxx–xxx
3
Fig. 1. Representation of the dication in the structure of (LDiPPH2)(OTf)2;
ellipsoids are at 30% probability level. Only one disordered position of one
i
˚
Pr groups of one DiPP ring is shown for clarity. Selected bond lengths (A)
and angles (deg): C13–N1 1.332(6), C13–N2 1.329(7), N2–C13–N1
108.4(4), N2–C16–N20 110.1(6), N2–C13–N1 108.4(4).
(75.208) and the N2-C16-N20 angles. The vectors of the two
C–H imidazolium bonds in the (LDiPPH2)Br2ꢀH2O are
pointing to opposite directions, being involved in H-
bonding with different Brꢁ in the lattice. Lastly, the
situation in (LmesH2)Br2 [9] is analogous with that in the
triflate salt mentioned above (i.e. imidazolium C–H vectors
are pointing to the same direction and H-bonding with the
same Brꢁ) even though the N-C-N angle at the bridging C
atom is ca. 1128 (cf. ca. 1108 in (LDiPPH2)(OTf)2). Therefore,
it appears that the relative orientations of the C–H bonds,
the interplanar angle between the imidazole heterocycles
and the angle at the bridging C atom are dependent on
crystal packing and H-bonding interactions between the
imidazolium hydrogens and the anion(s).
Fig. 2. Representation of the structure of Lmes; ellipsoids are at 30%
˚
probability level. Selected bond lengths (A) and angles (deg): N1–C10
1.3702(12), N2–C10 1.3617(12), N3–C14 1.3674(12), N4–C14 1.3646(13),
N2–C10–N1 101.56(8), N3–C13–N2 112.02(8), N4–C14–N3 101.50(8).
Prior to the deprotonation reactions described below,
the imidazolium salts were dried azeotropically with
toluene.
The molecule adopts a conformation where the lone
pairs at CNHC are pointing towards opposite directions,
with an interplanar angle between the heterocycles of
77.858. In addition, there is a reduction of the size of the
angles at the CNHC compared to the corresponding angles in
the imidazolium salts.
In order to access the free Lmes and LDiPP, the conditions
of deprotonation described previously [7] have been
modified to suppress product decomposition which is
noticeable above –20 8C by the intense brown coloration of
the reaction mixture. The decomposition slowed down if
the deprotonation was carried out in diethyl ether and the
product when dissolved or suspended was maintained
below –20 8C. However, after isolation of the product as a
solid, it was stable for longer periods (at least 0.5 h) at
room temperature under inert atmosphere without
appreciable visual or spectroscopic (1H-NMR) change.
We were unable to establish the nature of the colored
decomposition product. The off-white to yellow LDiPP and
Lmes were characterised by NMR spectroscopic methods
(1H and 13C{1H}). The spectra support a symmetrical
structure in solution, while the CNHC in the 13C{1H}
Derivatisation of the NHCs by the reaction with
chalcogens has been used as a chemical diagnostic method
for their transient presence and characterisation of free
NHCs and their indirect structural identification. The most
commonly employed chalcogen for this purpose is S8
leading to air stable thioureas. Less stable selenoureas and
telluroureas are expected from the reaction with Se and Te,
respectively. Taking advantage of this methodology in
order to confirm the successful preparation of the free NHC
ligands as detailed above, we reacted the isolated LDiPP
with S8, Se and Te and obtained the corresponding ureas
that were characterised spectroscopically and analytically.
In addition, the structure of the telluro-urea LDiPPTe2 was
determined crystallographically and is shown in Fig. 3. It
constitutes a rare example of structurally characterised
compound of this type [10].
spectrum for both ligands appear at ca.
d 218. Interest-
ingly, X-ray quality crystals of Lmes were obtained and
used for structural determination by crystallography.
Due to the poor crystal quality as evidenced from the
observation of low diffraction angles, the data set was
finally collected at the Daresbury synchrotron facility,
and was suitable to establish the unit cell contents and
atom connectivity and extract metrical data with
reasonable accuracy. A diagram of the molecule is shown
in Fig. 2.
The heterocyclic rings arrange at a dihedral angle of
45.128, while the C5Te vectors have an antiorientation. The
CTe–N bond lengths are virtually identical with the
corresponding CNHC–N in the free NHC, however the
values of the endocyclic angle at the CTe are significantly
larger than the corresponding at the CNHC, which may
Please cite this article in press as: Huffer A, et al. Synthesis of bis N-heterocyclic carbenes, derivatives and metal
complexes. C. R. Chimie (2013), http://dx.doi.org/10.1016/j.crci.2013.01.016

G Model
CRAS2C-3701; No. of Pages 9
4
A. Huffer et al. / C. R. Chimie xxx (2013) xxx–xxx
indicate a varying degree of
the adjacent N atoms.
p-interaction of the CTe and
2.2. Transition metal complexes
The availability of the free chelating bulky Lmes and
LDiPP ligands prompted us to carry out some preliminary
explorations of their scope by means of the synthesis of
representative examples of transition metal complexes. It
is known that the coordination of the free chelating
dicarbene ligands reported previously [6,7] with group 10
metal centres (Ni2+, Pd2+) leads to homoleptic complexes of
ligand-to-metal stoichiometry 2:1 due to the kinetic
control of the initial complexation reaction and the fact
that strong metal-NHC bonds in combination with the
chelate effect slow down ligand dissociation and redistri-
bution. Therefore, the chemoselectivity of the LDiPP and
Lmes towards formation of complexes of 1:1 stoichiometry
is a potentially attractive feature to consider.
Reactions of LDiPP with one equiv. of [Pd(tmeda)Me2]
(tmeda is N, N, N0, N-tetramethylethylenediamine) or 0.5
equiv. of [Pt(m-SMe2)Me2]2 resulted to the clean formation
Fig. 3. Representation of the structure of LDiPPTe2; ellipsoids are at 30%
of the complexes [Pd(LDiPP)Me2] and [Pt(LDiPP)Me2],
respectively. The complexes were characterised analyti-
cally, and by NMR spectroscopy. The 1H-NMR spectrum
indicates highly symmetric structures in solution as
evidenced by the presence of two doublets and one septet
for the isopropyl groups of the DiPP substituents. However,
the solid-state structures determined crystallographically
(see Figs. 4 and 5 for complexes 1 and 2, respectively)
indicate lower symmetry, originating from the puckered
six membered chelating ring.
probability level. One molecule of crystallisation solvent C6D6 in the
˚
asymmetric unit is omitted for clarity. Selected bond lengths (A) and
angles (deg): C13–N1 1.370(6), C13–N2 1.371(6), C13–Te1 2.065(5), C17–
N4 1.353(6), C17–N3 1.373(6), C17–Te2 2.073(5), N2–C13–N1 108.4(4),
N2–C16–N3 116.4(3), N4–C17–N3 105.3(4).
The bond lengths and angles are similar to those
previously observed for analogous species [3a,11]. How-
ever, it is interesting to comment on a trend of M-CH3 and
M-CNHC bond lengths in these very similar species: in the
Pd complex 1, the lengths of the two types of bonds are
almost equal (within the measured e.s.d.s) but in the Pt
complex 2 the CNHC bonds are significantly shorter than the
Pt-CH3 bonds. This may imply a different balance of
s- and
p
-bond components for the two types of bonds but
detailed understanding is not yet possible. The only other
Pt dimethyl complex with a ligand of type A (R = But)
(Scheme 1) was recently reported [3a] in conjunction with
its reactivity toward CO2; it was prepared by transmetalla-
tion from Ag to Pt. Bis-alkynyl complexes with a ligand of
type A (R = C6H11) were studied in relation to their
luminescence properties; they were prepared by direct
metallation of the imidazolium salts with PtCl2 in the
presence of NaOAc, followed by reaction of the halides
with alkynes [5b]. Pd dimethyl complexes with Lmes have
been previously prepared by in situ deprotonation of the
salt (LmesH2)Br2 [3c] but not structurally characterised.
Reactions of LDiPP with 0.5 equiv. of [Ir(1,5-COD)(
m-
Cl)]2 followed by anion exchange with KPF6 gave complex
3, which was characterised analytically, spectroscopically
and crystallographically. In contrast to 1 and 2, the 1H NMR
spectrum of 3 showed a non-symmetrical structure in
solution as supported by the presence of four doublets and
two septets for the isopropyl groups of the DiPP substitu-
ent. The non-symmetrical structure is also seen in the solid
Fig. 4. Representation of the structure of 1; ellipsoids are at 30%
probability level. One disordered THF molecule in the asymmetric unit is
˚
omitted for clarity. Selected bond lengths (A) and angles (deg): C1–N1
1.361(3), C1–N4 1.361(3), C1–Pd1 2.071(2), C3–Pd1 2.065(2), C3–N3
1.375(3), C3–N2 1.372(3), C32–Pd1 2.069(2), C33–Pd1 2.074(2), C3–Pd1–
C1 87.95(8), C32–Pd1–C1 94.04(9), C32–Pd1–C33 85.33(9), C3–Pd1–C33
92.75(9).
Please cite this article in press as: Huffer A, et al. Synthesis of bis N-heterocyclic carbenes, derivatives and metal
complexes. C. R. Chimie (2013), http://dx.doi.org/10.1016/j.crci.2013.01.016

G Model
CRAS2C-3701; No. of Pages 9
A. Huffer et al. / C. R. Chimie xxx (2013) xxx–xxx
5
Fig. 6. Representation of the structure of the cation in 3; ellipsoids are at
30% probability level. The anion (PF6)ꢁ and one THF molecule in the
˚
asymmetric unit are omitted for clarity. Selected bond lengths (A) and
angles (deg): C1–N2 1.345(7), C1–N1 1.379(7), C1–Ir1 2.076(5), C5–N3
1.339(7), C5–Ir1 2.058(6), C32–Ir1 2.191(6), C35–Ir1 2.215(6), C5–N4
1.378(7), C39–Ir1 2.209(6), N2–C1–N1 103.1(5), N3–C5–N4 103.5(5), C5–
Ir1–C1 87.4(2).
Fig. 5. Representation of the structure of 2; ellipsoids are at 30%
probability level. One THF molecule in the asymmetric unit is omitted for
˚
clarity. Selected bond lengths (A) and angles (deg): C1–Pt1 2.088(5), C2–
Pt1 2.088(5), C3–Pt1 2.037(5), C4–Pt1 2.020(5), C3–N3 1.367(6), N4–C3
1.372(6), C4–N1 1.390(6), N2–C4 1.380(5), N3–C3–N4 102.3(4), C2–Pt1–
C1 85.35(19), N2–C4–N1 100.4(4), C3–Pt1–C1 93.43(18), C4–Pt1–C3
88.15(17), C4–Pt1–C2 93.14(18).
Fig. 7. Representation of the structure of 4; ellipsoids are at 30%
˚
probability level. Selected bond lengths (A) and angles (deg): Ni1–Br1
2.3501(10), Ni1–Br2 2.4009(10), C13–Ni1 1.976(7), C17–Ni1 = 1.975(6),
N2–C13–N1 102.8(5), Br1–Ni1–Br2 123.00(4), C13–Ni1–Br2 102.75(17),
C17–Ni1–Br2 96.92(16), C13–Ni1–Br1 115.20(18), C17–Ni1–C13 91.5(2).
state as depicted in the diagram of the cation (Fig. 6)
obtained from a crystal structure determination of 3.
The Ir centre adopts a distorted square planar geometry
with virtually equal Ir–CNHC bonds within the measured
e.s.d.s. The Ir–CCOD bond lengths fall into the range 2.165–
obtained were invariably of small size, which in combina-
tion with the low solubility of 4 in inert solvents (a fact that
eliminates controlled recrystallisations, see Experimental
Section) hampered an accurate crystal structure determi-
nation. Eventually a data set obtained at the synchrotron
X-ray facility, Daresbury, UK, was adequate for solution
and satisfactory refinement of the model, which estab-
lished unequivocally the atom connectivity in the complex
and its essential geometrical and metrical features (Fig. 7).
The Ni centre is in a distorted tetrahedral geometry
with wide Br–Ni–Br (123.00(4)8) and relatively small
˚
2.216 A and there is elongation of the coordinated olefinic
bonds due to increased back-bonding from the electron
rich metal due to the strong
s-donor ability of the
coordinated NHCs. Complex 3 is the first example of IrI
centre with ligands of the type shown in Scheme 1. Two
examples of IrIII complexes with ligands of type A (R = Pri)
have been obtained by the deprotonation of the corre-
sponding bis-imidazolium salts with NaOAc or Et3N in the
CNHC–Ni–CNHC (91.5(2)8) angles. The Ni–CNHC bonds fall in
˚
the long end of the observed range (average 1.90 A, range
˚
presence of [Ir(1,5-COD)(
m
-Cl)]2 and [Cp*IrCl(
m
-Cl)]2, in
1.85–2.08 A).
processes that may also involve C–H activation of the C2-H
by the Ir center [12]. The catalytic activity of 3 is currently
under investigation.
Finally, LDiPP was reacted with [NiBr2(dme)] in THF to
afford the pink, paramagnetic tetrahedral complex 4,
which was characterised by analytical and X-ray diffrac-
tion methods. The analytical data support a ligand-to-
metal stoichiometry of 1:1. Unfortunately, the crystals
In view of the thermal and air sensitivity of LDiPP, which
may limit its use for the synthesis of the interesting
complex 4, we developed a methodology based on the in
situ deprotonation of the imidazolium salt (LDiPPH)2Br2
with KN(SiMe3)2. However, the low solubility of 4 in the
reaction medium resulted in its contamination with KBr,
the salt metathesis product from the deprotonation.
Unexpectedly, purification of the mixture of 4 with KBr
Please cite this article in press as: Huffer A, et al. Synthesis of bis N-heterocyclic carbenes, derivatives and metal
complexes. C. R. Chimie (2013), http://dx.doi.org/10.1016/j.crci.2013.01.016

G Model
CRAS2C-3701; No. of Pages 9
6
A. Huffer et al. / C. R. Chimie xxx (2013) xxx–xxx
were either not previously accessible or via multistep
synthetic schemes. We expect that this development will
open the way for the more widespread use of 1,10-
methylene-bis-(3-aryl-imidazol-2-ylidene) ligands across
the Periodic Table.
4. Experimental
4.1. General
All manipulations were performed under nitrogen in a
MBraun glovebox or using standard Schlenk techniques,
unless stated otherwise. Solvents were dried using
standard methods and distilled under nitrogen prior use.
The light petrol used throughout had a b.p. of 40 to 60 8C.
The starting materials were obtained from commercial
sources or prepared by modification of the literature
procedures as follows: crude, usually colored bis-imida-
zolium (LmesH2)Br2 [8] and (LDiPPH2)Br2 [7], were washed
with minimum amount of acetone to afford colorless
powders that were dried under vacuum and finally
azeotropically with toluene. The salt (LDiPPH2)(OTf)2 that
was prepared to facilitate crystallographic characterisation
of the bis-imidazolium cation, was obtained by anion
exchange from (LDiPPH2)Br2, with NaO3SCF3 in CH2Cl2 and
crystallised from CH2Cl2/petrol; [Pd(tmeda)Me2] [14] and
Fig. 8. Representation of the structure of 5; ellipsoids are at 30%
˚
probability level. Selected bond lengths (A) and angles (deg): C17–Ni1
1.955(2), C13–Ni1 1.957(2), C32–Ni1 1.759(3), C33–O2 1.158(3), C32–O1
1.159(3), C33–Ni1 1.764(2), C32–Ni1–C33 111.16(11), C32–Ni1–C17
112.93(10), C33–Ni1–C17 113.18(10), C32–Ni1–C13 113.68(10), C33–
Ni1–C13 112.65(10), C17–Ni1–C13 92.06(9).
[PtMe2(m-SMe2)]2[15] were prepared according to litera-
ture methods.
Elemental analyses were carried out by the London
Metropolitan University microanalytical laboratory. NMR
data were recorded on Bruker AV-300 and DPX-400
spectrometers, operating at 300 and 400 MHz (1H),
respectively. The spectra were referenced internally using
the signal from the residual protio-solvent (1H) or the
signals of the solvent (13C).
could be carried out by quick washing with degassed water
on
a glass sinter followed by quick drying giving
analytically pure 4. The observed selectivity for the
formation of the complex of 1:1 metal-to-ligand stoichi-
ometry as observed with 4 should be contrasted with the
other NiII complexes with ligands of type A in Scheme 1,
where the preferred products are square planar species of
1:2 metal-to-ligand stoichiometry and may be ascribed to
4.1.1. 1,10-methylene-bis-(3-diisopropylphenyl-imidazol-2-
ylidene) LDiPP
(LDiPPH2)Br2 (2.50 g, 4.00 mmol) was weighed out into a
Schlenk and suspended in diethyl ether (30 ml). In a
separate Schlenk, KN(SiMe3)2 (1.74 g, 8.70 mmol) was
dissolved in diethyl ether (20 ml). The solution of
KN(SiMe3)2 was cooled to -78 8C and added portionwise
with efficient stirring to the suspension of the salt at the
same temperature, the suspension turning cream upon full
addition. The mixture was stirred at -78 8C for 1 h and
allowed to warm to –30 8C slowly over 2 h and stirred at
this temperature for 30 min. At this point a light yellow
suspension is formed. The reaction mixture was filtered
through Celite into a cold (–30 8C) receiver flask. The
precipitate was washed with cooled (–20 8C) diethyl ether
(4 ꢂ 50 ml). The solvent was removed under reduced
the steric requirements of LDiPP
.
Preliminary reactivity studies showed that 4 could be
reduced in the presence of CO to the [Ni(LDiPP)(CO)2], a Ni0
complex, which was structurally characterised (Fig. 8).
Here too, the metal is in a tetrahedral environment with
C
NHC–Ni–CNHC angle of 92.06(9)8 very similar to the one
observed in 4. This value may relate to the LDiPP ligand
natural bite angle [13]. The Ni–CNHC bond lengths are
marginally shorter than in 4 while the Ni–CCO are shorter
than the Ni–CNHC bonds indicating a good degree of
back-bonding.
p-
3. Conclusions
pressure to give
spectroscopically pure. Yield: 1.46 g, 78%. NMR (C6D6):
1H,
a lemon yellow solid, which was
A method for the synthesis of free 1,10-methylene-bis-
(3-aryl-imidazol-2-ylidene) ligands of high purity and in
synthetically useful quantities will provide alternative or
complementary routes for the synthesis of organometallic
complexes that are sensitive to strong bases or the
oxidising conditions of transmetallation from silver. This
idea has been exemplified in this work by the synthesis of
some novel organometallics of group 10 and Ir metals that
d
H 1.10, 1.30 (2 ꢂ 6H, d, CH(CH3)2, J = 7.32 Hz), 2.80 (2H,
septet, CH(CH3)2, J = 7.32 Hz), 6.30 (2H, s, CH2 bridge), 6.50
(2H, s, 5-imidazol-2-ylidene), 7.20 (2H, s, backbone
imidazol-2-ylidene), 7.20 (4H, d, aromatic DiPP, J = 7.32),
7.30 (2H, t, aromatic DiPP, J = 7.32). 13C{1H}:
dC, 23.7, 24.5
(iPr CH3), 28.5 (iPr C(CH3)2), 65.6 (-CH2-), 118.1, 122.8 (4,5-
imidazol-2-ylidene), 128.3, 128.9, 132.0, 146.2 (aromatic),
Please cite this article in press as: Huffer A, et al. Synthesis of bis N-heterocyclic carbenes, derivatives and metal
complexes. C. R. Chimie (2013), http://dx.doi.org/10.1016/j.crci.2013.01.016

G Model
CRAS2C-3701; No. of Pages 9
A. Huffer et al. / C. R. Chimie xxx (2013) xxx–xxx
7
218.9 (imidazol-2-ylidene). The solid can be stored at
–35 8C under nitrogen without decomposition.
(C6D6): 1H, dH 0.90 and 1.20 (2 d, 24H, (CH3)2CH), 2.40
(sept, 4H, (CH3)2CH), 5.90 [(s, 2H, imidazol-2-ylidene), 6.30
(s, 2H, CH2 bridge), 7.00 (d, 4H, aromatic), 7.10 (t, 2H,
4.1.2. 1,10-methylene-bis-(3-mesityl-imidazol-2-ylidene)
Lmes
aromatic)], 8.00 (s, 2H, imidazol-2-ylidene). 13C{1H}: dC
,
22.3 and 23.0 ((CH3)2CH), 27.7 ((CH3)2CH), 57.9 (CH2
bridge), 118.8 and 119.4 (backbone imidazol-2-ylidene),
123.0, 129.0, 133.5 and 145.0 (aromatic), 161.3 (C = Se).
Anal. Calcd for C31H40N4Se2 (%): C, 59.42; H 6.43; N 8.94;
found (%): C, 58.22; H, 7.13; N, 8.72.
(LmesH2)Br2 (1.50 g, 2.75 mmol) was weighed out into a
Schlenk and suspended in ether (30 ml). In a separate
Schlenk, KN(SiMe3)2 (1.20 g, 6.05 mmol) was dissolved in
diethyl ether (20 ml). The solution of KN(SiMe3)2 was
cooled to –78 8C and added portionwise with efficient
stirring to the suspension of the salt at the same
temperature. After completion of the addition, the mixture
was stirred at –78 8C for 1 h and allowed to warm to –20 8C
slowly over 2 h and stirred at this temperature for 30 min.
The solution was filtered through a pad of Celite (2 cm) into
a Schlenk that was kept at –78 8C. The pad was washed
with pre-cooled diethyl ether until the washings were not
colored (2 ꢂ 30 ml). The combined washings were concen-
trated under reduced pressure to ca. 1/3 of the original
volume and the solution was cooled at –35 8C affording a
yellow solid that was isolated by decanting the mother
liquor with a cannula and washed with petrol (30 ml) and
dried under vacuum. Yield: 0.62 g, 60.5%. NMR (C6D6): 1H,
4.1.5. Preparation of LDiPPTe2
Tellurium (0.08 g, 0.64 mmol) in THF (20 ml) was added
to a stirred solution of LDiPP (0.15 g, 0.32 mmol) in THF
(30 ml) at RT. After 72 h, a brown solution was obtained.
The THF was removed under vacuum and the solid residue
was dissolved in diethyl ether (30 ml). Some black
precipitate formed on addition of ether was removed by
filtration. The filtrate was layered with petrol to give
yellow brown crystals. NMR (C6D6): 1H,
dH, 0.80 and 1.30
(two d, 24H, (CH3)2CH), 2.30 (sept, 4H, (CH3)2CH), 6.10 (s,
2H, imidazol-2-ylidene), 6.40 (s, 2H, methylene bridge
protons), 7.00 (d, 4H, aromatic), 7.10 (t, 2H, aromatic), 8.6
(s, 2H, imidazol-2-ylidene). 13C{1H}:
dC, 22.1 and 23.3
d
H 2.10 (12H, s, o-CH3), 2.20 (6H, s, p-CH3), 6.30 (2H, s, CH2),
((CH3)2CH), 27.9 ((CH3)2CH), 64.0 (CH2 bridge), 121.3 and
121.4 (imidazol-2-ylidene), 123.2, 129.3, 133.5 and 144.7
(aromatic). C = Te was not observed. Anal. Calcd for
C31H40N4Te2 (%): C, 55.43; H 5.57; N 7.74; found (%): C,
55.39; H, 5.66; N, 7.85. X-ray quality crystals were
obtained by layering a C6D6 solution with petrol in an
NMR tube.
6.40 (2H, s, backbone imidazol-2-ylidene), 6.80 (4H, s,
mesityl aromatic-H), 7.20 (2H, s, backbone imidazol-2-
ylidene). 13C{1H}:
dC, 17.9, 20.9 (mesityl-CH3), 65.7 (-CH2-
), 118.1, 121.7 (4,5-imidazol-2-ylidene), 129.0, 135.3,
137.3, 138.9 (aromatic), 218.0 (imidazol-2-ylidene). The
solid can be stored at –35 8C under nitrogen without
decomposition. X-ray quality crystals were obtained by
prolonged cooling of ether solutions at –35 8C.
4.1.6. Synthesis of [PdLDiPPMe2] (1)
To a solution of [Pd(tmeda)Me2] (0.10 g, 0.39 mmol) in
diethyl ether at –78 8C, was added by cannula a solution of
LDiPP (0.18 g, 0.39 mmol) in the same solvent. Upon
completion of the addition, the colour had changed from
light to intense yellow. The solution was allowed to warm
to room temperature overnight becoming brown-red.
Evaporation of the solvent under reduced pressure left a
dark green-purple solid, which was dissolved in THF and
layered with petrol to give colorless crystals. NMR
(acetone-d6): 1H, dH –1.00 (s, 6H, Pd-(CH3)2), 0.90 (d,
12H, (CH3)2CH), 1.10 (d, 12H, (CH3)2CH), 1.60 (m, 4H, THF),
2.60 (sept, 4H, (CH3)2CH), 6.15 (s, 2H, CH2 bridge), 6.85 (s,
2H, NHC backbone), 7.10 (d, 4H, DiPP), 7.20 (d, 2H, DiPP),
7.40 (s, 2H, NHC backbone). 13C{1H}: dC –3.8 [Pd-(CH3)2],
23.8 ((CH3)2CH on Dipp), 25.6 ((CH3)2CH), 55.0 ((CH3)2CH),
70.0 (CH2 bridge), 120.0 (imidazol-2-ylidene), 123.8
(imidazol-2-ylidene backbone), 125.0 (C = C backbone),
126.3 (aromatic of DiPP), 130.0 (aromatic of DiPP), 146.3
(aromatic of DiPP), 212.0 (CNHC). Anal. Calcd. for
4.1.3. Preparation of LDiPPS2
Oven dried sulphur (0.02 g, 0.64 mmol) in THF (20 ml)
was added to LDiPP (0.15 g, 0.32 mmol) in THF (30 ml) at RT.
The mixture was stirred for 72 h to give a pale orange
solution. The THF was evaporated under reduced pressure,
the solid residue was dissolved in ether (30 ml) and layered
with petrol (30 ml). The pale orange solid precipitate was
collected and dried under vacuum. NMR (C6D6): 1H,
dH 0.90
and 1.20 (d, 2 ꢂ 12H, (CH3)2CH), 2.50 (sept, 4H, (CH3)2CH),
5.80 (s, 2H, backbone imidazol-2-ylidene), 6.00 (s, 2H,
methylene bridge), 7.00 (doublet, 4H, aromatic DiPP), 7.10
(triplet, 2H, arom. DiPP), 7.50 (s, 2H, backbone imidazol-2-
ylidene). 13C{1H}: dC 22.3 and 22.9 ((CH3)2CH), 27.5
((CH3)2CH), 54.6 (CH2 bridge), 116.6 and 117.4 (imida-
zol-2-ylidene backbone), 123.0, 128.9, 132.9 and 145.4
(aromatic), 180.3 (C = S). Anal. Calcd for C31H40N4S2 (%): C,
69.88; H 7.57; N 10.52; found (%): C, 69.78; H, 7.64; N,
10.43.
C
33H48N4Pd.THF (%): C, 65.60; H 8.30; found (%): C,
4.1.4. Preparation of LDiPPSe2
65.62; H, 8.35.
Dried selenium (0.05 g, 0.64 mmol) in THF (20 ml) was
added to a stirred solution of LDiPP (0.15 g, 0.32 mmol) in
THF (30 ml) at RT. The mixture was stirred at RT for 72 h to
giving a pale green/brown solution. The volatiles were
removed under reduced pressure and the solid residue
dissolved in ether (30 ml). Small amounts of black
precipitate were removed by filtration and the filtrate
evaporated to dryness giving a dark orange solid. NMR
4.1.7. Synthesis of [PtLDiPPMe2] (2)
To a solution of [(Pt(m-SMe2)2Me2]2 (0.04 g, 0.07 mmol)
in diethyl ether at –78 8C, was added by cannula a solution
of LDiPP (0.07 g, 0.14 mmol) in the same solvent. After
stirring for 72 h the volatiles were removed under reduced
pressure to give a black solid residue which was washed
with pentane and ether and dissolved in THF giving a
Please cite this article in press as: Huffer A, et al. Synthesis of bis N-heterocyclic carbenes, derivatives and metal
complexes. C. R. Chimie (2013), http://dx.doi.org/10.1016/j.crci.2013.01.016

G Model
CRAS2C-3701; No. of Pages 9
8
A. Huffer et al. / C. R. Chimie xxx (2013) xxx–xxx
purple solution which on concentration and standing at
room temperature overnight gave colorless crystals. More
crystals could be obtained from the supernatant by
layering it with petrol. NMR (acetone-d6): 1H, dH -0.10
(t, 6H, (CH3)2-Pt satellites), 1.25 (d, 12H, (CH3)2CH), 1.50
(doublet, 12H, (CH3)2CH), 2.00 (m, 4H, THF), 3.05 (sept, 4H,
(CH3)2CH), 3.85 (4H, m, THF), 6.30 (s, 2H, CH2 bridge), 7.20
(s, 2H, NHC backbone), 7.40 (d, 4H, aromatic DiPP), 7.55 (d,
2H, aromatic DiPP), 7.70 (s, 2H, NHC backbone). Anal.
Calcd. for C33H48N4PtꢀTHF (%): C, 57.90; H 7.30; N, 7.29;
found (%): C, 57.93; H, 7.00; 7.19.
4.1.8. Synthesis of [IrLDiPP(1,5-COD)]PF6 (3)
A solution of LDiPP (0.11 g, 0.24 mmol) in THF (20 ml)
was added at –78 8C to a solution of [Ir(1,5-COD)(m-Cl)]2
(0.08 g, 0.12 mmol) in the same solvent (10 ml). After the
addition the cooling bath was removed and the orange
reaction mixture was allowed to reach room temperature,
when it became brown. A suspension of KPF6 (0.02 g,
0.12 mmol) in THF (20 ml) was then added to the reaction
mixture and the whole was stirred at room temperature
overnight. After evaporation of the volatiles under reduced
pressure, the solid residue was washed with petrol
(2 ꢂ 15 ml) and toluene (15 ml) and crystallised from
THF/petrol affording dark red crystals. NMR (acetone-d6):
1H,
dH 7.95 (d, 2H, NCHCHN), 7.66 (d, 2H, NCHCHN), 7.50–
7.60 (m, 2H, DiPP aromatic), 7.35–7.50 (m, 4H aromatic
DiPP), 6.68 (d, 1H, CH2 bridge), 6.52 (d, 1H, CH2 bridge),
4.29 (br s, 4H, COD vinyl), 3.58 (m, THF), 3.20 (br s, 8H, COD
allyl), 2.84 (sept, CH(CH3)2), 2.24 (sept, CH(CH3)2), 1.48 (d,
3H, CH(CH3)2), 1.39 (d, 3H, CH(CH3)2), 1.11 (d, 3H,
CH(CH3)2), 0.98 ppm (d, 3H, CH(CH3)2); 13C{1H}: dC
172.7 (NCN), 136.4, 131.5, 127.3, 125.2, 121.5 (aromatic
DiPP), 73.2 (COD vinyl), 68.1 (CH2 bridge), 63.9 (iPr CH),
31.8, 26.2 (iPr CH3) ppm; 19F NMR: –72.4 (d). Anal. Calcd.
for C39H43F6IrN4P (%): C, 51.76; H, 4.79; N, 6.19; found: C,
51.66; H, 4.77; N, 6.16.
4.1.9. Preparation of [NiLDiPPBr2] (4)
LDiPP (0.15 g, 0.32 mmol) in THF (30 ml) was added at –
78 8C with stirring to [NiBr2(dme)] (0.10 g, 0.32 mmol) in
THF (30 ml). Stirring at –78 8C was continued for 10 min
and then the mixture was allowed to warm to room
temperature when it became dark orange. The volatiles
were removed under reduced pressure to give a pink solid,
which was insoluble in inert solvents but sparingly soluble
in THF. X-ray quality crystals were obtained by dissolving a
small amount in THF (30 ml) and layering with diethyl
ether.
4.1.10. Preparation of [NiLDiPPBr2] (4) by generating the LDiPP
in situ
KN(SiMe3)2 (2.40 g, 12.00 mmol) in diethyl ether
(100 ml) was added at –78 8C portionwise with rapid
stirring to (LDiPPH2)Br2 (3.00 g, 4.80 mmol) in diethyl
ether (100 ml). This was followed by addition at the same
temperature of [NiBr2(dme)] (0.11 g, 3.70 mmol, equi-
molar amount based on 75% carbene yield) in THF
(100 ml). Cold THF (200 ml at –78 8C) was used to dilute
the reaction mixture. The mixture was allowed to warm
to room temperature and then stirred overnight. A brown
Please cite this article in press as: Huffer A, et al. Synthesis of bis N-heterocyclic carbenes, derivatives and metal
complexes. C. R. Chimie (2013), http://dx.doi.org/10.1016/j.crci.2013.01.016

G Model
CRAS2C-3701; No. of Pages 9
A. Huffer et al. / C. R. Chimie xxx (2013) xxx–xxx
9
suspension with some pink precipitate was observed. The
mixture was filtered on a glass sinter and washed with
degassed H2O (30 ml) followed by EtOH (30 ml) and then
Et2O (50 ml). The dark pink solid was dried under
vacuum. Anal. Calcd. for C31H40N4Br2Ni (%): C, 54.18;
H, 5.87; N, 8.15; found: C, 54.01, H, 5.74; N, 8.03. The
product is paramagnetic giving featureless NMR spectra
in THF-d8.
discussions and Dr Mark E. Light, Dr Simon J. Coles
(University of Southampton) and Dr Pierre de Fremont
(Laboratoire de chimie de coordination, UMR 7177) for the
data collection of Lmes and 4 and assistance in structure
solution and refinement, respectively.
Appendix. Supplementary material
4.1.11. Formation of [NiLDiPP(CO)2] (5)
Supplementary material associated with this article can
be found at http://www.sciencedirect.com, at http://dx.doi.
org/10.1016/j.crci.2013.01.016.
In a Fisher-Porter bottle, to a solution of [NiLDiPPBr2]
(0.04 g, 0.06 mmol) in THF (10 ml) was added Na/Hg
(1.00 g of 0.4% w/w, excess), the mixture was pressurised
with CO (100 psi), and stirred at room temperature for
12 h. After releasing of the pressure the solution was
decanted from the Hg pool and the THF removed under
reduced pressure. The residue was dissolved in ether,
filtered, the filtrate concentrated and cooled (4 8C) to give
small quantity of yellow crystals that were characterised
crystallographically.
References
[1] A.J. Arduengo, R.L. Harlow, M. Kline, J. Am. Chem. Soc. 113 (1991) 361.
[2] W.A. Herrmann, M. Elison, J. Fischer, C. Ko¨cher, G.R.J. Artus, Angew.
Chem. Int. Ed. Engl. 34 (1995) 2371.
[3] (a) S. Jamali, D. Milic, R. Kia, Z. Mazloomi, H. Abdolahi, Dalton Trans. 40
(2011) 9362 ;
(b) A.D. Yeung,P.S. Ng, H.V. Huynh, J. Organomet. Chem. 696(2011) 112 ;
(c) S.S. Subramanium, L.M. Slaughter, Dalton Trans. (2009) 6930 ;
(d) T. Scherg, S.K. Schneider, G.D.J. Frey, R. Schwarz, E. Herdtweck, W.A.
Herrmann, Synlett (2006) 2894 ;
(e) S. Budagumpi,R.A.Haque, A.W.Salman,Coord. Chem.Rev. 256(2012)
1787 ;
(f) R. Corbera´n, E. Mas-Marza´, E. Peris, Eur. J. Inorg. Chem. (2009) 1700 ;
(g) M. Muehlhofer,T.Strassner, W.A.Herrmann, Angew. Chem.Int.Ed. 41
(2002) 1745.
4.1.12. X-ray crystallography
The data sets for (LDiPPH2)(OTf)2 and complexes 2, 3 and
5 were collected on a Enraf-Nonius Kappa CCD area
detector diffractometer with an FR591 rotating anode (Mo
Ka
radiation) and an Oxford Cryosystems low-tempera-
[4] (a) A.S. McCall, H. Wang, J.M. Desper, S. Kraft, J. Am. Chem. Soc. 133
(2011) 1832 ;
(b) D. Meyer, S. Ahrens, T. Strassner, Organometallics 29 (2010) 3392.
[5] (a) Y. Unger, A. Zeller, M.A. Taige, T. Strassner, Dalton Trans. 38 (2009)
4786 ;
ture device, operating in scanning mode with and
v
c
v
scans to fill the Ewald sphere. The crystals were mounted
on a glass fiber with silicon or fluorocarbon grease, from
Fomblin vacuum oil. The programs used for control and
integration were Collect, Scalepack, and Denzo [16]. All
solutions and refinements were performed using the
WinGX [17] package and all software packages within.
All non-hydrogen atoms were refined using anisotropic
thermal parameters, and hydrogens were added using a
riding model. Complexes 1, 2, 3 and 5 contain one THF
solvent molecule of crystallization in the asymmetric unit.
In the structures of (LDiPPH2)(OTf)2 and 3 one of the
isopropylphenyl groups was disordered over two positions
that were refined anisotropically without any restraints.
The structure of 1 contains one THF solvent molecule of
crystallization which was disordered over two positions
and refined anisotropically. The crystals of Lmes, LDiPPTe2
and of the complexes 1 and 4 were very small, and the data
sets were collected at the synchrotron facility at Darebury,
UK [18]. Crystal data for all structurally characterised
compounds are given in Table 1.
(b) Y. Zhang, J.A. Garg, C. Michelin, T. Fox, O. Blacque, K. Venkatesan,
Inorg. Chem. 50 (2011) 1220 ;
(c) K. Li, X. Guan, C.W. Ma, W. Lu, Y.C.M. Chen Che, Chem. Commun. 47
(2011) 9075.
[6] (a) R.E. Douthwaite, D. Ha¨ussinger, M.L.H. Green, P.J. Silcock, P.T.
Gomes, A.M. Martins, A.A. Danopoulos, Organometallics 18 (1999)
4584 ;
(b) T.A.P. Paulose, S.C. Wu, J.A. Olson, T. Chau, N. Theaker, M. Hassler,
J.W. Quail, S.R. Foley, Dalton Trans. 41 (2012) 251.
[7] K.A. Kreisel, G.P.A. Yap, K.H. Theopold, Organometallics 25 (2006)
4670.
[8] M.G. Gardiner, W.A. Herrmann, C.P. Reisinger, J.R. Schwarz, M. Spiegler,
J. Organomet. Chem. 572 (1999) 239.
[9] S. Ahrens, T. Strassner, Inorg. Chim. Acta. 359 (2006) 4789.
[10] A.J. Arduengo, F. Davidson, H.V.R. Dias, J.R. Goerlich, D. Khasnis, W.J.
Marshall, T.K. Prakasha, J. Am. Chem. Soc. 119 (1997) 12742.
[11] A.A.D. Tulloch, S. Winston, A.A. Danopoulos, G. Eastham, M.B. Hurst-
house, Dalton Trans. (2003) 699.
[12] (a) M. Albrecht, J.R. Miecznikowski, A. Samuel, J.W. Faller, R.H. Crab-
tree, Organometallics 21 (2002) 3596 ;
(b) M. Vogt, V. Pons, D.M. Heinekey, Organometallics 24 (2005) 1832.
[13] P. Dierkes, P.W.N.M. van Leeuwen, J. Chem. Soc. Dalton Trans. 28 (1999)
1513.
[14] W. De Graaf, J. Boersma, W.J.J. Smeets, A.L. Spek, G. Van Koten, Orga-
nometallics 8 (1989) 2907.
[15] G.S. Hill, M.J. Irwin, C.J. Levy, L.M. Rendina, R.J. Puddephatt, R.A.
Andersen, L. McLean, Inorganic Syntheses, John Wiley & Sons Inc.,
1998 , pp. 149.
Acknowledgements
We thank CNRS for a fellowship (to AAD). AAD is also
[16] Z. Otwinoski, W. Minor, Methods Enzymol 276 (1997) 307.
[17] L.J. Farrugia, J. Appl. Crystallogr. 32 (1999) 83.
[18] R.J. Cernik, W. Clegg, C.R.A. Catlow, G. Bushnell-Wye, J.V. Flaherty, G.N.
Greaves, M. Hamichi, I. Burrows, D.J. Taylor, S.J. Teat, J. Synchrotron
Radiat. 4 (1997) 279.
´
´
grateful to the Region Alsace, the Departement du Bas-Rhin
´
and the Communaute urbaine de Strasbourg for the award
of a Gutenberg Excellence Chair (2010–2011). We also
thank Dr Pierre Braunstein for support and fruitful
Please cite this article in press as: Huffer A, et al. Synthesis of bis N-heterocyclic carbenes, derivatives and metal
complexes. C. R. Chimie (2013), http://dx.doi.org/10.1016/j.crci.2013.01.016