Amidine- and amidinate-functionalised N-heterocyclic carbene complexes of silver and chromium

Article abstract of DOI:10.1039/c2dt31619f

A new family of N,N′-bis-(2,6-diisopropylphenyl)-(2,6- diisopropylphenyl-imidazolium)-acetamidines have been developed as NHC proligands and ligands that are functionalised with neutral amidine and anionic amidinato moieties. On coordination they adopt diverse binding modes, depending on the nature of the metal and the reaction conditions. In the Ag, K and Cr complexes reported in this paper, monodentate κ¹(NHC), bimetallic bridging-(κ¹-NHC-κ¹-amidinato) and bidentate (κ¹-NHC-κ¹-amidinato) binding modes were observed, respectively. Two of the novel Cr complexes, in the presence of activators, were tested as catalysts for the polymerisation of ethylene showing low activity.

Full text of DOI:10.1039/c2dt31619f

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PAPER
Amidine- and amidinate-functionalised N-heterocyclic carbene complexes of
silver and chromium†‡
Susana Conde-Guadano,a Martin Hanton,a Robert P. Tooze,a Andreas A. Danopoulos*b and
Pierre Braunsteinb
Received 20th July 2012, Accepted 22nd August 2012
DOI: 10.1039/c2dt31619f
A new family of N,N′-bis-(2,6-diisopropylphenyl)-(2,6-diisopropylphenyl-imidazolium)-acetamidines
have been developed as NHC proligands and ligands that are functionalised with neutral amidine and
anionic amidinato moieties. On coordination they adopt diverse binding modes, depending on the nature
of the metal and the reaction conditions. In the Ag, K and Cr complexes reported in this paper,
monodentate κ1(NHC), bimetallic bridging-(κ1-NHC–κ1-amidinato) and bidentate (κ1-NHC–κ1-
amidinato) binding modes were observed, respectively. Two of the novel Cr complexes, in the presence of
activators, were tested as catalysts for the polymerisation of ethylene showing low activity.
The versatility of the amidine ligands has further been expanded
thanks to their tuning scope by relatively straightforward and
Introduction
The use of functionalised N-heterocyclic carbene (NHC) ligands
for the tailoring of the metal coordination sphere appeared very
early in the history of development of NHC ligands.1,2 The demon-
stration of the versatility, scope and potential of this ligand
design concept is now well established and attracting interest
from researchers from diverse areas of organometallic, bioinor-
ganic chemistry and catalysis.3 NHC ligands functionalised with
neutral N-donors are particularly attractive, especially with
amine,4 pyridine,5 imine6 and other heterocycles. In comparison,
fewer examples of NHCs functionalised with anionic N-donors,
i.e. carboxylic acid amides7 and dialkylamides,8–10 have also
been studied, the latter mainly on electropositive metals.
adaptable synthetic methods and the additional functionalisation
of the amidine core by the attachment of pendant arms bearing
classical donor groups at the amine terminus (A in Scheme 1)
such as ω-dialkylamino-alkyl-, β-hydroxyalkyl-, β-(2-pyridyl)-
ethyl-, 2-lutidinyl-, ferrocenyl-.17–22
Less frequently and relatively recently pendant groups have
been attached to the amidine and amidinato functionality at the
C atom of the amidine using a flexible linker (this excludes gua-
nidines and phosphaguanidines) as shown in B, Scheme 1.
Known examples of L2 donors include 2-pyridyl-,23 2-furyl-,24
carboranyl-,25 and phenoxy-26 groups, giving rise to tridentate or
‘heteroscorpionate’ ligand architectures,27 with potential
dynamic behaviour, hemilability and denticity changes. These
new ligands are intensely studied in catalytic applications invol-
ving electropositive metals.
The usefulness of amidine and amidinato donors has been
amply demonstrated in coordination chemistry, in particular with
electropositive metals, where the amidinato as an anionic ligand
is usually coordinating in a κ2 chelating bidentate mode acting
as a 6e− donor11 and in catalysis applications.12,13 Diverse and
rich coordination chemistry of the amidinato ligand has been
observed that is dependent on the steric and electronic properties
of the metal coligands and the amidine moiety substituents.14 In
contrast, neutral amidines preferentially coordinate from the
Amidine and amidinato functionalised NHC ligands and com-
plexes have not been described, albeit they are expected to
exhibit high potential for the tuning of the metal coordination
sphere. In principle, functionalisation via the N-terminus or the
C atom of the amidine is conceivable (Scheme 2) leading to a
range of coordination modes depending on the length of the
more basic and sterically accessible imine
N
atom.15,16
aSasol Technology UK Ltd, Purdie Building, North Haugh, St Andrews,
Fife, UK, KY16 9ST
bLaboratoire de Chimie de Coordination, Institut de Chimie (UMR 7177
CNRS) Université de Strasbourg, 4 rue Blaise Pascal, F-67081
Strasbourg, France
†This manuscript is dedicated to the outstanding chemist, colleague and
supportive friend Professor David Cole-Hamilton on the occasion of his
retirement and for his outstanding contribution to transition metal
catalysis.
‡CCDC 892658–892663. For crystallographic data in CIF or other elec-
tronic format see DOI: 10.1039/c2dt31619f
Scheme 1 Schematic representation of the functionalisation of the
amidine moiety by pendant arms attached to the N-terminal atom or the
amidine C atom; R1–R3 are alkyl or aryl groups, L1 is amine, pyridine
etc., L2 is bis-(pyrazolyl)methyl, phenoxide etc. Only one of the poss-
ible isomers is shown in each case.
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Scheme 2 Plausible coordination modes of (i) an anionic ligand with the amidinato moiety and NHC donors tethered from the amidine C atom
(C and D) or N atom (H, I); (ii) a neutral ligand with one amidine moiety and an NHC donor tethered from the amidine C atom (E) or N atom (J);
(iii) a neutral ligand with NHC coordination and the dangling amidine moiety (F and K); and (iv) an anionic ligand with amidinato and NHC donors
bridging two metals (G and L). The complexes described here display modes D, E, F and G. Different geometrical and conformational isomers are not
depicted.
linker, the bulk of the substituents R1–R3, the nature and oxi-
dation state of the metal and the metallation reaction conditions
(kinetic control).
arises from the presence of the Z and E and syn and anti isomers
in solution (Scheme 4).29
The existence of isomers and the corresponding tautomers is
common in amidine chemistry and their rate of interconversion
(by inversion at the N, rotation around the CvN bond and/or
tautomerisation) is dependent on the nature of the amidine sub-
stituents and the reaction medium.28,29 We were not able to carry
out full assignment of the 1H-NMR spectra. However, support of
the presence of two isomers in CDCl3 was provided by the two
peaks in the imidazolium proton region as well as for the
methylene protons of the linker. Finally, the isopropyl substitu-
ents of the diisopropylphenyl rings were non-equivalent and
gave three septets and six doublets for each isomer. The
1H-NMR and 13C{1H}-NMR spectra in d6-DMSO were much
simpler, with the peaks corresponding to the Z and E isomers
converging to one average signal. Therefore the higher polarity
of DMSO may favor the interconversion between Z and E
isomers in solution possibly by increasing the rate of tautomeri-
sation. The presence of H-bonding interactions in solution is also
possible.
Herein we report the first examples of pro-ligands and ligands
in which the imidazolium or the corresponding NHC groups are
tethered to the C atom of the amidine via a methylene linker.
We also describe their K, Ag and Cr complexes which illustrate
the scope, diversity and potential of the ligand design.
Complexes with other early and late transition metals as well as
N-tethered NHC ligand designs will be the subject of forthcom-
ing papers. Preliminary catalytic testing of two Cr complexes in
the oligomerisation/polymerisation of ethylene is also reported.
Results and discussion
The synthesis of the amidine proligands
The synthetic approach for the proligand (H2L)+Cl− (4) and
ligands (HL) (5) and (K+L−) (6) used in this work is depicted in
Scheme 3.
The chloroacetamidines were obtained from the chloroacet-
amide 1 with PCl5 in two steps by a modification of the Wolmer-
shäuser methodology,28 consisting of isolation of the imidoyl
chloride 2 by vacuum distillation and subsequent reaction with
Additional insight into the structure of the imidazolium
chloride 4 in the solid state was gained by a single crystal X-ray
diffraction study of crystals obtained from chloroform; a
representation of the cation in 4 is given in Fig. 1.
The cation that crystallised was the E-anti isomer. All bond
lengths in the structure of 4 were unremarkable. There are short
contacts between the imidazolium proton as well as the amidine
proton and the chloride anion (2.56 and 2.45 Å, respectively).
There are also close contacts of the chloride anion with one of
the backbone hydrogens of the imidazolium ring (2.77 Å) and of
the chloroform (2.37 Å).
aniline to the amidine 3. Quaternisation of the (2,6-Prii C6H3)-
2
imidazole by the chloroacetamidine 3 took place only under
forcing conditions. Interestingly, self-quaternisation of 3 was not
a competing reaction, possibly due to steric reasons.
1
The appearance of the H-NMR spectra of 4 in CDCl3 was
complex, showing clearly the presence of two main isomers in
an approximately 2/3 ratio. This isomeric mixture probably
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Scheme 3 Synthetic transformations for the proligands and ligands. Reagents and conditions: (i) 1 equiv. PCl5, C6H6; (ii) 1 equiv. 2,6-Pri2C6H3NH2,
RT; (iii) (2,6-Pri2C6H3)-imidazol; (iv) 1 equiv. KN(SiMe3)2, C6H6; (v) KN(SiMe3)2, C6H6.
Scheme 4 Possible geometrical and conformational isomers of the
imidazolium salt 4.
The synthesis of the neutral NHC amidine and the anionic NHC
amidinato potassium ligands
Fig. 1 Ball-and-stick representation of the structure of the cation in 4.
Solvent molecules (CHCl3) in the asymmetric unit are omitted for
clarity. Selected bond lengths (Å) and angles (°): C17–N4 = 1.273(5),
C13–N1 = 1.322(5), C13–N2 = 1.334(5), N4–C17–C16 = 125.1(3),
The reaction of the imidazolium chloride 4 with 1 equiv. of
KN(SiMe3)2 in benzene gave the neutral NHC ligand 5 which
was isolated as a light yellow air and moisture sensitive powder
1
and was characterised by spectroscopic techniques. The H- and
13C{1H}-NMR spectra in d6-benzene gave unequivocal evidence
that the first deprotonation occurred specifically at the C–H acidic
site rather than the amidine. This was inferred by the disappear-
ance of the imidazolium proton signals at ca. δ 10.95, the per-
sistence of the signals assignable to the amidine protons at
δ 8.05 and the appearance of a signal at δ 217.0 in the 13C{1H}
spectrum assignable to the NHC carbon. Generally, the 1H-NMR
spectrum of 5 showed rather broad signals and was considerably
simpler compared to the spectrum of 4 in CDCl3 described
earlier. This is certainly due to a faster interconversion between
the Z- and E-isomers in solution although the cause and the
mechanism of this process have not been further explored.
N4–C17–N3 = 121.4(3), N3–C17–C16 = 113.4(3), N1–C13–N2 =
108.6(3).
Deprotonation of 5 with a second equivalent of KN(SiMe3)2
in benzene gave the potassium salt 6 in good yield as an extre-
mely air and moisture sensitive white solid. It was characterised
by analytical and spectroscopic methods. The 1H-NMR spectrum
of 6 in d5-pyridine showed relatively broad features. However,
the signals corresponding to the isopropyl substituents of the
NHC bound aromatic rings could be easily distinguished from
the signals of the corresponding substituents of the amidinate
bound aromatic groups. The CNHC in the 13C{1H}-NMR
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distances [in the range between 2.862 and 2.953 Å, see figures]
are comparable to those found in the literature (2.810–3.127 Å,
average 2.929 Å).34 All K–Namidinate bonds (2.946–3.035 Å) fall
at the long end of the range of anionic potassium–amide bonds
found in the CCDC or are even longer (2.578–2.973 Å, average
2.766 Å). All amidinato N atoms are almost planar (angle sums
at N atoms ca. 355.5–359.5°), while the Camidinato–Namidinato
(bound to K) and Camidinato–Namidinato (unbound) are virtually
equal.
Fig. 2 (a) Ball-and-stick representation of the one-dimensional struc-
ture of 6 showing the different repeat units. Only the ipso C of the
2,6-diisopropylphenyl groups at the NHC ring is shown; Pri substituents
of the 2,6-diisopropylphenyl groups at the amidine N atoms as well as
THF solvents of crystallisation are omitted for clarity. (b) Ball-and-stick
representation of the coordination environment of K1 and K3. Selected
bond lengths (Å) and angles (°): K1–N5 = 2.803(2), K1–C53 =
2.862(3), K3–N6 = 2.946(2), K3–C79 = 3.302(2), K3–C83 = 3.276(3),
K3–C84 = 3.146(3). (c) Ball-and-stick representation of the coordination
environment of K2. Selected bond lengths (Å) and angles (°): K2–N2 =
3.035(2), K2–C34 = 3.248(2), K2–C33 = 3.256(2), K2–C35 = 3.260(2),
K2–C32 = 3.279(2), K2–C30 = 3.307(2).
The synthesis of the silver complex 7
Initial insight into the coordination behavior of the ligand HL
was sought by the reaction of the imidazolium salt 4 and Ag2O,
which is known to provide access to silver complexes with syn-
thetic potential in NHC transmetallation reactions, and could
therefore expand the scope and applications of the new ligand
design. The metallation took place in refluxing dichloroethane
giving complex 7 in good yields (Scheme 5).
Complex 7 was characterised by analytical and spectroscopic
methods. An X-ray diffraction study was also performed,
however due to the poor crystal and data quality the model
serves only to unequivocally establish the atom connectivity in
the molecule (Fig. 3) and not to extract accurate metrical data.
The ligand is exclusively metallated at the CNHC with dangling
amidine functionality. In the crystal the amidine isomer present
is Z-syn (in contrast to the imidazolium 4 above) possibly in
order to reduce steric interactions. The Ag centre adopts an
almost linear geometry which is common with analogous com-
spectrum was observed at δ 214.6. In larger scale preparations of
6, the neutral NHC ligand 5 was not isolated but subjected to the
second deprotonation by KN(SiMe3)2 after separation of the KCl
from the reaction mixture by filtration. The salt 6 was crystallised
from THF–petrol and its structure was determined crystallo-
graphically. It features one-dimensional polymeric ‘zig-zag’
chains comprising potassium cations, bridged by L as shown in
Fig. 2a–c. There are a limited number of structurally charac-
terised potassium amidinates.30–33 In most reported structures two
amidinate ligands adopt a symmetrical κ1, κ1 coordination bridg-
ing two K centers. In the structure of 6 arene ring–potassium inter-
actions, NHC–potassium and Namidinato–potassium interactions
are involved in the formation of the chain. In this respect, the
structure of 6 has similarities with the structure of the potassium
bis(2,6-diisopropylphenyl)-formamidinate·nTHF reported by
Junk and Cole.32
pounds previously described.35 The H-NMR spectrum of 7 is
1
simpler than that observed for 4 in CDCl3 where the presence of
isomers was evident. The methyls of the Pri substituents of all
the aromatic rings are diastereotopic giving rise to six doublets
(CH(CH3)2) and three septets (CH(CH3)2). This is in accordance
with the presence of one isomer and presumably the persistence
of the solid state structure in solution.
The synthesis of the chromium complexes
More specifically, 6 exhibits two types of alternating repeat
units in the polymeric chain distinguishable by the different
coordination spheres of the potassium centers (Fig. 2b and 2c).
In Fig. 2b are shown in detail the coordination environments of
K(1) and K(3). K(1) is bound only to one Namidinate and two
CNHC atoms [C(3) and C(53)] originating from two NHC func-
tionalities of different ligands L. K(3) is ‘sandwiched’ by two
aromatic rings, and two nitrogen atoms originating from the ami-
dinate end of different ligands that are arranged in a ‘tail-to-tail’
fashion. Each aromatic ring in proximity to K(3) is nonsymme-
trically disposed as evidenced by the K–Caromatic distances:
[K(3)–C(83) = 3.276(3) Å, K(3)–C(84) = 3.146(2) Å, K(3)–
C(79) = 3.302(2) Å] supporting two η3-K(3)–arene interactions
with two different aromatic rings.
The coordination sphere of K(2) is shown in Fig. 2c. K(2) is
also ‘sandwiched’ between two aromatic rings arranged in an
eclipsed face-to-face stacking. In addition, K(2) is coordinated to
two Namidinate donor atoms. The variation of K(2)–Carene dis-
tances is small [3.248–3.330 Å, K(2)–arene centroid = 2.967 Å]
and possibly dictated by steric factors. All K–CNHC bond
The development of new chromium complexes with catalytic
potential in the oligomerisation and polymerisation of ethylene
and other 1-alkenes is a current topical area in organometallic
chemistry. In addition current research is aimed at the elucidation
of the operating mechanisms of the above reactions, which are
not understood.36 Having this motive, we decided to prepare Cr
complexes with the new ligand with the aim to explore their cat-
alytic potential and reactivity with ethylene. The chromium com-
plexes described below are summarised in Scheme 5.
The reaction of the precursor [CrCl2Me(THF)3] with 6 in THF
at low temperature gave the purple paramagnetic complex 8 in
good yields, which was characterised by X-ray crystallography.
A diagram of the molecule is shown in Fig. 4. The metal centre
adopts a distorted square-pyramidal geometry with the methyl
group [C(51)] at the apical position. The ligand L acts as an
anionic bidentate donor (κ1-NHC, κ1-amidinato cf. structure D
in Scheme 2) implying that the coordination of the NHC
is favored over the imine N atom of the amidinato moiety. A
tridentate ‘scorpionate’ type coordination is also disfavored
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Scheme 5 Transformations leading to the complexes described in this paper. Reagents and conditions: (i) Ag2O, 1,2-dichloroethane; (ii) 1 equiv.
[CrMeCl2(THF)3] in THF; (iii) [CrCl2(THF)2] in THF; (iv) [CrCl3(THF)3] in THF. Ar is 2,6-diisopropylphenyl.
Fig. 4 Representation of the structure of 8; ellipsoids are at 30% level.
Pri substituents of the 2,6-diisopropylphenyl groups are omitted for
clarity; only one conformation of the coordinated disordered THF is
shown. Selected bond lengths (Å) and angles (°): Cr1–N3 = 2.030(2),
Fig. 3 Ball-and-stick representation of complex 7; see text for details.
Cr1–C51 = 2.055(3), Cr1–C1 = 2.107(3), Cr1–Cl2 = 2.3064(9), N3–
C30 = 1.370(4), N4–C30 = 1.290(4), N3–Cr1–C51 = 102.01(12), N3–
presumably due to steric reasons. The C–Namidine bond lengths
(C30–N3 and C30–N4) give evidence for a localised electronic
structure. This coordination mode of the amidinato group is rare
with transition metals, in particular electronically unsaturated.37
The symmetrical chelating bidentate mode has been observed
with amidinato ligands in CrIII, 4,38 CrII 39 or with the related
2-amido-pyridines.40 The Cr–Namidinato at 2.030 Å lies at the
long end of the Cr–Namido distance range (1.760–2.080 Å,
average 1.845 Å for Cr complexes and 1.803–1.909 Å, average
1.856 Å for CrIII complexes) and is comparable with the bond
length in sterically congested terminal CrII–NPh2. The
Cr1–C1 = 88.64(10), C51–Cr1–C1 = 89.25(12).
coordinated Namidinato atom is planar (angle sum 359.4°). The
bite angle subtended by the bidentate chelate ligand is 88.6°.
The Cr–CNHC bond length [2.107(3) Å] is in the range
reported for other Cr–NHC complexes (2.041–2.245 Å, average
2.134 Å).41 The difference between the exocyclic [N(1)–C(1)–
Cr(1)] and endocyclic [N(2)–C(1)–Cr(1)] angles at the CNHC is
18.8° indicating a trend to release chelate ring strain on coordi-
nation and reduced directionality of the Cr–CNHC bond.
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Fig. 5 Representation of the structure of 9; ellipsoids are at 30% level.
Pri substituents of the 2,6-diisopropylphenyl groups are omitted for
clarity. Selected bond lengths (Å) and angles (°): Cl1–Cr1 = 2.3884(8),
N3–Cr1 = 2.0675(19), C17–N3 = 1.358(3), C17–N4 = 1.296(3), C1–
Cr1 = 2.163(2), N3–Cr1–Cl1 = 94.46(6), C1–Cr1–Cl1 = 93.04(6), N3–
Cr1–Cl1 = 176.37(6), N3–Cr1-C1 = 90.39(8).
Fig. 6 Representation of the structure of 10; ellipsoids are at 30%
level. Pri substituents of the 2,6-diisopropylphenyl groups and solvent
molecules (CH2Cl2 and ether) present in the asymmetric unit are omitted
for clarity. Selected bond lengths (Å) and angles (°): N7–C68 = 1.402(5),
N7–C67 = 1.292(5), N3–C17 = 1.342(5), N4–C17 = 1.308(5), Cr2–Cl1
= 2.6311(17), Cr2–Cl2 = 2.4241(16), Cr2–Cl3 = 2.3824(15), Cr2–C51
= 2.120(4), Cr2–N8 = 2.050(3), Cr1–Cl3 = 2.4126(15), Cr1–Cl2 =
2.3996(16), Cr1–Cl1 = 2.3500(17), Cr1–Cl4 = 2.2645(15), Cr1–N4 =
2.124(3), Cr1–C1 = 2.093(4), N8–Cr2–Cl1 = 106.45(10), C51–Cr2–Cl2
= 178.21(10), C51–Cr2–Cl3 = 96.33(12), N8–Cr2–Cl3 = 169.29(10),
N8–Cr2–C51 = 89.76(14), N4–Cr1–Cl1 = 172.72(9), C1–Cr1–Cl1 =
92.06(11), N4–Cr1–Cl4 = 93.24(9), C1–Cr1–Cl4 = 94.73(11), C1–Cr1–
N4 84.41(13).
The reaction of the precursor [CrCl2(THF)2] with 6 in THF at
low temperature gave the blue-purple paramagnetic very air sen-
sitive complex 9 in moderate yields. Analytically pure indigo
blue powder was isolated by extraction of the reaction residue in
diethyl ether, filtration through Celite and removal of the vola-
tiles under reduced pressure. X-ray quality crystals were obtained
by slow diffusion of pentane into THF solutions of the complex.
The structure of the molecule determined crystallographically is
shown in Fig. 5.
Complex 9 is a centrosymmetric dimer with bridging chlor-
ides; here too the ligand L acts as an anionic bidentate donor
(κ1-NHC, κ1-amidinato cf. structure D in Scheme 2) with puck-
ered chelate rings. Each metal center adopts a distorted square
planar geometry. The C–N distance involving the exocyclic
N atoms is shorter (1.296 Å) than the endocyclic (1.358 Å).
Both values are comparable to those observed for the corre-
sponding bond lengths in the salt 4 and the complexes 7 and 8
indicating that the ligand π-system is not significantly altered on
coordination. The planarity of the coordinated nitrogen (358.5°)
supports anionic amide binding, however, the Cr–Namidinato bond
length (2.068 Å) is slightly longer than other previously reported
terminal CrII–N amides (1.904–2.062 Å, average 1.984 Å).
There is no evidence for Cr–Cr interaction (3.619 Å).
Finally, the reaction of 6 with [CrCl3(THF)3] gave the para-
magnetic air sensitive complex 10 in good yields. Crystals of 10
were obtained from CH2Cl2–ether and studied by single crystal
X-ray diffraction. A diagram of the molecule is shown in Fig. 6.
The binuclear complex comprises two different metal centers:
Cr1 is coordinated by six donors in a distorted octahedral geo-
metry and Cr2 by five donors in a distorted trigonal bipyramidal
geometry. The amidinato ligand acts again as bidentate via the
NHC and one nitrogen atom of the amidine functionality.
Careful perusal of the metrical data shows additional subtle
differences. The bidentate ligand at Cr2 features exocyclic and
endocyclic C–N and Cr–N bond lengths (1.292 Å, 1.340 Å, and
2.050 Å, respectively) comparable to those in complexes 8
and 9. Consequently, the coordination mode is analogous and
described by structure D in Scheme 2. In contrast, the bidentate
ligand at Cr1 features exocyclic and endocyclic C–N and Cr–N
bond lengths at 1.308 Å, 1.340 Å and 2.125 Å, respectively,
which support a coordination mode best described by the struc-
ture E in Scheme 2, and the involvement of a neutral amidine
ligand (HL). Therefore, the two metal centers Cr1 and Cr2 are in
oxidation states III and II, respectively. This is further supported
by the Cr–CNHC bond lengths (Cr1–C 2.093 Å and Cr2–C
2.120 Å) which are significantly different and in line with the
trend observed for 8 (2.107 Å) and 9 (2.164 Å), respectively.
Interestingly, in 10 the intermetallic distance is much shorter
(3.109 Å). The mechanism of formation of 10 has not been elu-
cidated. It is plausible to postulate that the initial formation of a
(CrCl2L)n, n = 1,2, by a salt metathetical reaction is followed by
protonation of an anionic amidinato group. The source of the
proton may be THF or solvents of crystallisation.
Preliminary catalytic testing using complexes 8 and 9 for the
oligomerisation/polymerisation of ethylene was carried out in the
presence of EtAlCl2 as an activator. The results show that 8 is a
low activity polymerisation catalyst, while 9 was inactive.
However, the latter may be decomposing before the catalytic
reaction as indicated by the color change observed after dissol-
ving it in chlorobenzene, the solvent of choice for the catalytic
experiments. Further catalytic studies are underway.
In conclusion, we have introduced a novel NHC functionalisa-
tion strategy involving the versatile amidine functionality. The
easy availability of well defined ligand transfer reagents (neutral
ligand HL, silver complex Ag(HL) and the KL) provides options
for the synthesis of complexes with metals of different nature in
various environments and oxidation states. The new ligand
systems exhibit a range of coordination modes (monodentate,
bidentate and bridging) which have been observed crystallo-
graphically with the metals used in the present study. We are
now focussing our efforts on the exploration of the scope of the
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mixed donor amidine NHCs across the periodic table, the
development and refinement of analogous systems with pendant
arms at the N-termini of the amidine, guanidine functionalisation
and selected catalytic applications of complexes with the new
ligand designs.
31.15 g, 72%. 1H-NMR (CDCl3, 300 MHz): (mixture of E/Z
isomers) 7.28–6.88 (6H, m, H-aromatic); 6.11 (0.6H, s, NH),
5.35 (0.4H, s, NH); 3.96 (1.2H, s, CH2Cl); 3.89 (0.8, sCH2Cl);
3.25 (1.2H, sept, J = 7.5 Hz, CH(CH3)2); 3.09 (2H, sept, J = 7.5
Hz, CH(CH3)2); 2.81 (0.8H, sept, J = 7.5 Hz, CH(CH3)2);
1.20–0.91 (24H, m, CH(CH3)2). 13C{1H}-NMR (CDCl3,
75 MHz): 151.7; 150.7; 147.2; 147.0; 143.8; 139.2; 138.9;
132.8; 132. 2; 128.9; 128.2; 123.9; 123.8; 123.3; 123.1; 123.0;
40.8 (–CH2–); 39.3 (–CH2–); 28.7; 28.3; 28.2; 27.7; 25.3; 24.3;
23.8; 23.7; 22.8; 22.4. M.S. (ES+): 415.6 (M + H)+; 413.5
(M + H)+.
Experimental section
Elemental analyses were carried out by the London Metropolitan
University microanalytical laboratory. All manipulations invol-
ving metals were performed under nitrogen in a Braun glove box
or using standard Schlenk techniques, unless stated otherwise.
Solvents were dried by passing through alumina columns
(pentane, toluene) or by distillation from sodium benzophenone
ketyl (ether, THF) and kept under Ar over carefully activated
sieves until use. NMR solvents were dried and distilled from KH
and stored in the glove box over activated molecular sieves. The
3-(2,6-Diisopropylphenyl)-1-[N,N′-bis(2,6-diisopropylphenyl)-
acetamidyl]imidazolium chloride 4
2-Chloro-N,N′-bis(2,6-diisopropylphenyl)acetimidamide 3 (20 g,
50 mmol) and 2,6-diisopropylphenylimidazol (12.56 g,
55 mmol) were heated in a sealed Youngs’ ampoule under
partial vacuum at 150 °C for 4 days. The resulting material was
dissolved in the minimum amount of CH2Cl2 and triturated with
diethyl ether to give a white solid which was dried azeotropically
with toluene. It was finally washed with dry diethyl ether, dried
under vacuum and stored in the glove box. Yield: 19.69 g, 65%.
X-ray quality crystals were obtained by layering with diethyl
ether a concentrated CH2Cl2 solution. 1H-NMR (CDCl3,
300 MHz): (mixture of E/Z isomers) 10.95 (0.4H, s, imidazo-
lium-H); 10.91 (0.6H, s, imidazolium-H); 9.69 (0.4H, s, NH);
7.50 (0.4H, t, J = 6 Hz, aromatic-H); 7.40 (0.6 H, t, J = 6 Hz,
aromatic-H); 7.30–6.90 (8 H, m, aromatic-H and 0.8 H NHC-
backbone); 6.90 (0.6 H, broad s, NHC-backbone); 6.05 (0.6 H,
broad s, NHC-backbone); 5.53 (0.6 H, s, NH); 5.49 (0.8, s,
CH2); 5.49 (1.2, s, CH2); 3.30–3.11 (2H, m, CH(CH3)2); 2.95
(1.2H, two overlapped sept, J = 6.5 Hz, CH(CH3)2); 2.81 (0.8H,
sept, J = 6.5 Hz, CH(CH3)2); 2.35 (1.2H, sept, J = 6.5 Hz, CH-
(CH3)2); 2.15 (0.8H, sept, J = 6.5 Hz, CH(CH3)2); 1.40, 1.35,
1.30, 1.15, 1.10, 1.00 and 0.90 (36H, d (some are overlapping),
J = 6.5 Hz, CH(CH3)2). 13C{1H}-NMR (CDCl3, 300 MHz):
148.7; 147.2; 146.6; 145.6; 145.1; 142.3; 142.0; 139.5; 139.0;
138.7; 133.4; 132.1; 131.7; 130.8; 130.4; 130.1; 129.5; 127.6;
124.8; 124.5; 124.3; 124.1; 123.9; 123.2; 123.2; 123.0; 122.9;
122.7; 65.8; 49.6 (–CH2–); 46.1 (–CH2–); 29.0; 28.7; 28.6; 28.5;
28.3; 25.8; 25.0; 24.5; 24.4; 24.3; 24.1; 24.0; 23.3; 22.3; 22.2;
starting
materials
N,N′-bis(2,6-diisopropylphenyl)-chloro-
acetamide42 and [MeCrCl2(THF)3]43 were prepared according to
the literature procedures. [CrCl2(THF)2] was prepared by con-
tinuous Soxhlet extraction of commercial anhydrous [CrCl2]
(Aldrich) with THF for 48 h.
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).
2-Chloro-N,N′-bis(2,6-diisopropylphenyl)acetamidine 3
A modification of the method published by Wolmershäuser is
followed.28 The imidoyl chloride was isolated and reacted in a
separate step with 2,6-diisopropylaniline to form the
chloromethylamidines.
To a suspension of 2-chloro-N-(2,6-diisopropylphenyl)acet-
amide (31.72 g, 0.120 mol) in dry benzene (ca. 100 cm3) was
added portionwise at room temperature solid PCl5 (28.53 g,
0.137 mol). Gas evolution (HCl) took place following the
addition of each portion. After the completion of the addition the
mixture was slowly brought to reflux (oil bath temperature
75–80 °C) and kept at this until no further HCl evolution
through the bubbler is noticeable (usually 20–30 min). The
reflux condenser was replaced by a distillation head equipped
with a short (10 cm) fractionation column. The benzene was
removed at atmospheric pressure, followed by the POCl3 at
reduced pressure and finally the imidoyl chloride 2 as an off
white liquid (82–84 °C, 0.36 mbar). Yield: 27.75 g, 85%. The
product 2 can be stored in Youngs’ ampoules under N2. 1H
NMR (CDCl3): δ 1.28 (d, 12H, CH3)2CH), 2.83 (septet, 2H,
(CH3)2CH), 4.61 (s, 2H, CH2Cl), 7.2–7.4 (m, 4H, aromatics).
2,6-Diisopropylaniline (19 g, 0.105 mol) in benzene (75 cm3)
was added to a solution of the imidoyl chloride 2 (27.2 g,
0.10 mol) in benzene (75 cm3) and the reaction mixture was
refluxed for 20 h. After cooling a white precipitate appeared
which was washed with petrol. To liberate the free amidine base
the solid was dissolved in ethanol (250 cm3) and treated with
aqueous ammonia (25%, 6 × 60 cm3). The solid product was
collected by filtration, recrystallised from ethanol and dried
under reduced pressure to give small white crystals of 3. Yield:
1
21.8; 15.3. H-NMR (d6-DMSO, 300 MHz): 9.62 (1H, s, imid-
azolium-H); 8.11 (1H, s broad, NHC-backbone); 7.77 (1H, s
broad, NHC-backbone); 7.70–6.95 (9H, m, aromatic-H); 4.70
(2H, s, CH2); 3.25 (2H, sept, J = 7.5 Hz, CH(CH3)2); 2.95 (2H,
sept, J = 7.5 Hz, CH(CH3)2); 2.30 (2H, sept, J = 7.5 Hz, CH-
(CH3)2); 1.38 (6H, d, J = 7 Hz, CH(CH3)2); 1.33 (6H, d, J =
7 Hz, CH(CH3)2); 1.20–0.90 (24H, m, CH(CH3)2). 13C{1H}-
NMR (d6-DMSO, 75 MHz): 147.4; 146.9; 145.1; 142.9; 139.9;
137.9; 132.5; 131.4; 130.5; 128.5; 124.3; 123.7; 122.7; 122.6;
49.6 (CH2); 27.8; 27.7; 24.4; 23.9; 23.5; 22.9; 22.4. M.S. (ES+):
607.7 (M − Cl)+; 606.7 (M − Cl)+; 605.7 (M − Cl)+.
Deprotonation procedures
3-(2,6-Diisopropylphenyl)-1-[N,N′-bis(2,6-diisopropylphenyl)-
acetamidyl]imidazol-2-ylidene 5. The imidazolium chloride 4
12564 | Dalton Trans., 2012, 41, 12558–12567
This journal is © The Royal Society of Chemistry 2012

View Article Online
(1.88 g, 3 mmol) was combined with 1 equiv. of KN(SiMe3)2
(0.60 g, 3 mmol) in benzene (20 cm3) and the yellow suspension
was stirred overnight. The precipitated KCl was removed by filt-
ration through Celite and the solution was evaporated to dryness.
The yellow air sensitive solid was characterised by NMR
spectroscopy. 1H-NMR (C6D6): 8.05 (1H, broad s, NH);
7.20–6.85 (9H, m, H-aromatic); 6.30 and 6.15 (2H, d, J =
6.5 Hz, NHC-backbone); 4.45 (2H, s, CH2); 3.50 (2H, sept, J =
7.5 Hz, CH(CH3)2); 3.25 (2H, sept, J = 7.5 Hz, CH(CH3)2);
2.65 (2H, sept, J = 7.5 Hz, CH(CH3)2); 1.35–0.8 (36H, m, CH-
(CH3)2). 13C{1H}-NMR C6D6: 217.0 (NHC-C); 150.9; 145.7;
145.1; 144.3; 137.7; 136.8; 133.1; 127.8; 122.5; 122.0; 121.8;
120.9; 119.0; 48.4 (–CH2–); 27.7; 26.9; 26.8; 23.0; 22.9; 22.8;
22.4; 21.6.
d, J = 6 Hz, CH(CH3)2); 0.96 (6H, d, J = 6 Hz, CH(CH3)2). 13
C
{1H}-NMR (CDCl3, 75 MHz): 148.8; 147.1; 145.8; 142.9;
138.6; 134.7; 131.2; 130.4; 129.4; 124.3; 124.2; 123.1; 122.2;
122.1; 52.0; 35.1; 28.8; 28.6; 28.1; 24.8; 24.5; 24.3; 24.2; 23.6;
21.7.
Complex 8
A precooled (−78 °C) solution of 6 (0.30 g, 0.47 mmol) in THF
(30 cm3) was added to a solution of [CrCl2Me(THF)3] in THF
(20 cm3) at −78 °C. The light green reaction mixture was
allowed to reach room temperature and stirred for 1 h when the
colour turned to purple. The volatiles were removed under
reduced pressure, the residue extracted with toluene and filtered
through Celite. Evaporation of the solvent under reduced
pressure gave the analytically pure 8 as purple powder. Yield:
0.19 g, 58%. X-ray quality crystals were obtained by slow diffu-
sion of petrol in a concentrated diethyl ether solution. Elemental
analysis: Calculated (%) for (M − THF) C42H58ClCrN4: C,
71.31; H, 8.41; N, 7.92. Found (%): C, 71.33; H, 8.17; N, 7.83.
The 1H-NMR spectrum of 8 is broad and featureless.
3-(2,6-Diisopropylphenyl)-1-[2-N,N′bis(2,6-diisopropylphenyl-
amidinate)ethyl]imidazol-2-ylidene potassium 6. The imidazo-
lium chloride 4 (1.88 g, 3 mmol) was combined with 1 equiv. of
KN(SiMe3)2 (0.60 g, 3 mmol) in benzene (20 cm3) and the
yellow suspension was stirred overnight. The precipitated KCl
was removed by filtration through Celite and the solution of the
ligand 5 was subjected to a second deprotonation with an
additional equivalent of KN(SiMe3)2 (0.60 g, 3 mmol) in
benzene (20 cm3). The orange suspension was stirred overnight.
The precipitated extremely air sensitive, beige product 6 was iso-
lated on a frit, washed with benzene (2 cm3) and pentane
(10 cm3) and dried under vacuum. Yield: 1.15 g, 61%. X-ray
quality crystals were obtained by layering a concentrated THF
solution of 6 with pentane. Elemental analysis: Calculated for
C43H59N4K1·0.5 THF (%): C, 75.99; H, 8.69; N 8.25. Found
(%): C, 76.09; H, 8.67; N, 8.95. 1H-NMR (d5-pyridine,
300 MHz): 7.45–7.00 (11H, m, aromatic-H and NHC carbene
backbone); 4.95 (2H, s, CH2); 3.76 (4H, m, CH(CH3)2); 2.91
(2H, m, CH(CH3)2); 1.35–0.89 (36H, m, CH(CH3)2). 13C{1H}-
NMR (d5-pyridine, 75 MHz): 214.6 (NHC); 165.1; 156.8;
156.3; 147.3; 142.4; 140.1; 138.4; 129.2; 122.7; 121.2; 119.98;
53.71; 51.37; 28.7; 28.5; 25.4; 25.0; 24.8; 24.4; 24.1.
Complex 9
A pre-cooled (−78 °C) solution of 6 (0.30 g, 0.47 mmol) in
THF (30 cm3) was added to a solution of [CrCl2(THF)2] in the
same solvent (20 cm3) at −78 °C. The light grey-green reaction
mixture was allowed to reach room temperature and stirred for
1 h when it became indigo. The solution was concentrated to ca.
10 cm3 and filtered through Celite. Slow diffusion of pentane
into the THF gave dark blue-purple prisms. Yield: 0.12 g, 36%.
Elemental analysis: Calculated for C82H110N8Cr2Cl2 (%): C,
71.12; H, 8.15; N, 8.09. Found (%): C, 71.15; H, 8.02; N, 8.10.
Complex 10
A precooled (−78 °C) solution of 6 (1.00 g, 1.56 mmol) in THF
(20 cm3) was added to a solution of [CrCl3(THF)3] in the same
solvent (20 cm3) and temperature. The mixture was allowed to
reach room temperature and stirred overnight. During this time it
turned from purple to brown. THF was removed under reduced
pressure, the residue extracted with toluene and filtered through
Celite. Removal of the volatiles under reduced pressure gave the
analytically pure product in almost quantitative yield. Yield:
1.10 g, 97%. The product can be crystallized by slow diffusion
of ether in a concentrated CH2Cl2 solution. Elemental analysis:
Calculated for C82H111N8Cr2Cl4 (%): C, 67.76; H, 7.57; N, 7.71.
Found (%): C, 67.78; H, 7.64; N, 7.81.
Silver complex 7
Ag2O (0.54 g, 2.3 mmol) was added to a solution of 4 (1.00 g,
1.5 mmol) in 1,2-dichloroethane (50 cm3). The mixture was
refluxed for 20 h and after cooling to room temperature, the
black suspension was filtered through Celite. Removal of the
volatiles under reduced pressure and dissolving the residue in the
minimal amount of CH2Cl2 (5 cm3), precipitation with diethyl
ether (20 cm3) and filtration of the resulting colorless solid gave
7 which was washed with diethyl ether (3 × 15 cm3) and dried
under vacuum. Yield: 0.40 g, 53%. Elemental analysis: Calcu-
lated for C41H56N4AgCl (%): C, 65.81, H, 7.54, N, 7.49. Found
1
(%): C, 65.89; H, 7.76; N, 8.49. H-NMR (CDCl3, 300 MHz):
X-ray crystallography
7.35 (1H, t, J = 9 Hz, C-H aromatic); 7.20 (1H, t, J = 9 Hz, C-H
aromatic); 7.19–7.09 (7H, m, C-H aromatic); 7.00 (1H, dd, J =
6 Hz, NHC-backbone); 6.88 (1H, dd, J = 3 Hz, NHC-backbone);
5.46 (1H, s, NH); 4.69 (2H, s, CH2-bridge); 3.20 (2H, sept, J =
6 Hz, CH(CH3)2); 2.98 (2H, sept, J = 6 Hz, CH(CH3)2); 2.37
(2H, sept, J = 6 Hz, CH(CH3)2); 1.39 (6H, d, J = 6 Hz, CH
(CH3)2); 1.29 (6H, d, J = 6 Hz, CH(CH3)2); 1.08 (6H, d, J =
6 Hz, CH(CH3)2); 1.03 (6H, d, J = 6 Hz, CH(CH3)2); 1.02 (6H,
A summary of the crystal data, data collection and refinement for
complexes 4, 6, 7, 8, 9 and 10 is given in Table 1. The crystals
were mounted on a glass fiber with grease, from Fomblin
vacuum oil.
Data sets for 4, 6, 7 and 8 were collected on an Enraf-Nonius
Kappa CCD area detector diffractometer with an FR591 rotating
anode (Mo-Kα radiation) and an Oxford Cryosystems low
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 12558–12567 | 12565

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Table 1 Crystal data for complexes 4, 6, 7, 8, 9 and 10
4
6
7
8
9
10
Chemical formula
C41H57ClN4·1.5CHCl3 C43H59KN4O0.5 C41H56AgClN4 C46H66ClCrN4O C82H110Cl2Cr2N8 C82H112Cl4Cr2 N8·
C4H10O·2(CH2Cl2)
CCDC
679.04
Triclinic
CCDC
748.22
CCDC
778.48
Orthorhombic
14.4973(4)
16.0212(4)
18.7006(4)
90.00
90.00
90.00
4343.48(19)
120(2)
P212121
4
CCDC
Formula mass
Crystal system
a/Å
b/Å
c/Å
α/°
β/°
γ/°
820.41
Monoclinic
22.5237(3)
12.7743(2)
16.6305(2)
90.00
102.6610(10)
90.00
5591
1382.68
Monoclinic
12.2847(9)
15.9373(11)
22.3137(13)
90.00
119.069(3)
90.00
3818.4(4)
120(2)
1699.57
Monoclinic
14.433(10)
21.648(15)
29.447(19)
90.00
97.642(7)
90.00
1699.57
120(2)
Triclinic
10.2894(7)
12.4817(13)
16.3801(17)
80.103(4)
86.108(5)
74.035(5)
748.22
13.80520(10)
14.34640(10)
20.6098(2)
90.5980(10)
94.8700(10)
95.8140(10)
4045.47(6)
120(2)
Unit cell volume/Å3
Temperature/K
Space group
Formula units/cell, Z
Absorption
coefficient, μ/mm−1
No. of reflections
measured
120(2)
Cc
4
120(2)
ˉ
P1
4
0.166
ˉ
P1
P21/c
2
0.402
P21/c
4
0.520
2
0.371
0.605
0.363
50 959
5357
18 443
13 353
8364
3367
9596
7847
11 240
5891
15 908
9457
No. of independent
reflections
Rint
0.0537
0.0495
0.0565
0.0722
0.2321
0.0702
0.0625
0.0520
0.0698
0.0568
0.0908
0.0670
Final R1 values
(I > 2σ(I))
Final wR(F2) values
(I > 2σ(I))
0.1387
0.0615
0.1439
1.045
0.1516
0.1081
0.1739
1.081
0.1191
0.2433
0.1637
0.937
0.0962
0.0752
0.1069
1.075
0.1301
0.1384
0.1653
0.993
0.1778
0.1224
0.2097
1.026
Final R1 values
(all data)
Final wR(F2) values
(all data)
Goodness of fit on
F2
temperature device operating in ω scanning mode with ψ and ω
scans to fill the Ewald sphere. The programs used for control and
integration were Collect,44 Scalepack, and Denzo.45 All non-
hydrogen atoms were refined using anisotropic thermal para-
meters, and hydrogens were added using a riding model.
(2010–2011). We also thank Dr Mark E. Light, Simon J. Coles
(University of Southampton) and Dr Lydia Berlot (Universitè de
Strasbourg) for assistance with crystallographic data collection
and structure determination.
The data set for 9 was collected on a Bruker APEX II DUO
Kappa-CCD diffractometer equipped with an Oxford Cryo-
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