Homoleptic crown N-heterocyclic carbene complexes

Article abstract of DOI:10.1002/anie.200702138

(Figure Presented) Two ways about it: The first metal N-heterocyclic carbene complexes derived from a cyclic tetraimidazolium salt show a remarkable versatility of ligand conformation and coordination geometry. With Pd ^II, a mononuclear square-planar complex is obtained, but with Cu ^I and Ag^I, an unprecedented dinuclear motif with a short metal-metal interaction is observed (see structure; N blue, C white ellipsoids, H white circles).

Full text of DOI:10.1002/anie.200702138

Angewandte
Chemie
DOI: 10.1002/anie.200702138
Macrocyclic Carbene Ligands
Homoleptic Crown N-Heterocyclic Carbene Complexes**
Ross McKie, John A. Murphy,* Stuart R. Park, Mark D. Spicer,* and Sheng-ze Zhou
Since the first crystal structure, reported by Arduengo et al. in
1991,[1] N-heterocyclic carbenes (NHCs)and their metal
complexes have become the subject of intensive study.[2] Their
relative ease of synthesis from readily accessible precursors,
together with their favorable donor properties, have made
them the ligand of choice in many applications. Mono- and
bidentate NHC ligands are common, and hybrid NHC ligands
with pyridine, oxazoline, phosphine, and other pendant
donors are also known.[3]
Our interest is in generating a convenient and easily
adapted route to metal complexes in which the metals are
complexed exclusively by macrocyclic NHC ligands; such
complexes are likely to have unique reactivity profiles that
could be attractive in synthesis and in coordination chemistry
and are likely to have interesting electrochemical properties
compared with known types of complexes.
the ligands coordinate to cis coordination sites on the metal
center and act more like classical bidentate ligands rather
than encapsulating the metal in the manner typically observed
with macrocyclic ligands. Only one example (3)of a
tetradentate macrocyclic ligand with two imidazolium-
derived NHC donors containing a metal ion within the
macrocyclic cavity has been reported.[6] Very recently, the first
example of a homoleptic platinum(II)complex of a tetrakis
NHC macrocycle has been obtained (4). This species was
synthesized not from an imidazolium precursor but rather by
an elegant, multistep metal-templated cyclization of 2-azido-
phenylisocyanide.[7] It is, however, unlikely that this proce-
dure will be widely adopted by coordination chemists in its
present form, since the use of platinum salts on a multigram
scale will be prohibitively expensive. In addition, removal of
platinum from the macrocycle to yield free carbene ligand is
likely to be problematic.
Surprisingly, to date only a small handful of metal
complexes with cyclic poly-NHC ligands are known. The
majority of such complexes contain cyclic dicarbenes, gen-
erated either from activated alkenes such as 1[4] or from
bisimidazolium cyclophanes 2.[5] However, in these species
Our approach is different, involving the synthesis of a
preformed macrocycle bearing multiple imidazolium salts as
precursors of NHCs. Appropriate choice of macrocycle
should allow adaptive binding to different metals.
The most easily accessible imidazolium macrocycle is the
bis-imidazolium salt (5H2)2+(IÀ)2, easily formed from imida-
zole and 1,3-diiodopropane;[8a] however, we have adapted the
synthesis to afford its tetrameric homologue (6H4)4+(IÀ)4 in
multigram quantities (Scheme 1).[14] An alternative and more
lengthy route to (6H4)4+(PF6 )4 has been developed inde-
À
pendently by Beer and co-workers,[8b] but for different
purposes. The 1H and 13C NMR spectra of the tetramer
(6H4)4+ are very simple, thus attesting to the high degree of
symmetry present; the formulation is supported by mass
spectrometry. The parent ion is observed, together with ions
corresponding to the association of one, two, and three iodide
anions. We have confirmed the formulation by X-ray crys-
tallography,[15] which indicates a tetracationic species with
four iodide counterions, one of which interacts strongly with
À
the cation through both imidazolium and alkyl C H···I
contacts (Figure 1). The potential of this salt as a precursor
to a range of previously inaccessible macrocyclic NHC ligand
complexes was immediately recognized, and we have pro-
ceeded to investigate this aspect of their chemistry. The
widespread use of silver carbene complexes as ligand transfer
agents[9] prompted us to attempt the synthesis of a silver
complex from our cyclic proligand. Thus, reaction of
(6H4)4+(IÀ)4 with Ag2O in DMSO in the presence of sodium
acetate results in the formation of a white solid that is
sparingly soluble in polar solvents.[14] Mass spectrometry
suggests the presence of two silver cations per ligand, while
once more the NMR spectra are very simple, thus indicating
the equivalence of the four imidazole groups. X-ray crystal-
lography (Figure 2)[15] confirms that two silver(I)cations are
complexed by a single ligand, each having linear two-
[*] R. McKie, Prof. Dr. J. A. Murphy, S. R. Park, Dr. M. D. Spicer,
Dr. S.-Z. Zhou
Department of Pure & Applied Chemistry
University of Strathclyde
295 Cathedral Street, Glasgow G1 1XL (UK)
Fax: (+44)141-552-0876
E-mail: john.murphy@strath.ac.uk
m.d.spicer@strath.ac.uk
[**] We thank the EPSRC for funding, the EPSRC Mass Spectrometry
Service, Swansea, for mass spectrometric data, the EPSRC crystal-
lographic service at Southampton University for X-ray data collec-
tion and acknowledge the use of the EPSRC’s Chemical Data Service
at Daresbury.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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structure in the crystal is also of interest, since the
cations assemble into infinite, one-dimensional
chains via intermolecular contacts between silver
ions and the carbene carbon atoms in the neigh-
boring molecules. It is tempting to describe these
assemblies as insulated molecular wires with
chains of silver ions sheathed by the macrocyclic
ligands. In this case, the association is relatively
weak, but the generation of a more robust linkage
between the units could be envisaged, thus leading
to 1D coordination polymers.
Finally, the adventitious [Ag4I8]4À anion is a
fourth, previously unreported, structural isomer of
this anion. The three crystallographically charac-
terized structures in the literature include a linear
diiodo-bridged
species
[I{Ag(m2-I)2}3AgI]4À,
(7),[12a] a {Ag4I4} cubane with an additional termi-
nal iodide on each silver center (8),[12b] and a
structure based on a silver rhombus with terminal,
Scheme 1. Synthesis of cyclic imidazolium salts and crown carbene complexes.
Figure 1. X-raÀy crystal structure of the macrocyclic tetraimidazolium
salt (6H4)4+(I )4 showing C H···I distances of less than 3.4 Š. Thermal
À
ellipsoids are drawn at the 50% level.
coordinate geometry. The silver centers are coordinated to
alternate NHC donors in the macrocycle, which results in a
mutually orthogonal orientation of the two L-M-L units (C-
Ag-Ag-C torsion angles, e.g. C(1)-Ag(1)-Ag(2)-C(7), lie in
the range 88.8–91.58). The flexibility of the ligand allows the
incorporation of two metal ions into the macrocycle cavity
and apparently also constrains them in close proximity; the
Figure 2. The X-ray crystal structure of the [Ag2(6)]2+ cation and its
counteranion [Ag4I8]4À (top), and the long-range association of the
[Ag2(6)]2+ cations in [Ag2(6)]2[Ag4I8] (bottom). Thermal ellipsoids are
drawn at the 50% probability level.
À
À
ligand geometry dictates that a Ag Ag interaction (d(Ag
Ag) = 2.8349(6)Š) is present. This distance is somewhat
À
shorter than the average Ag Ag separation (3.056 Š)
extracted from the Cambridge Crystallographic Database.[10]
While linear {AgI(NHC)2} units are reasonably common,[11] in
the cases where these form Ag Ag interactions, they
di- and tri-bridged iodide ions (9).[12c] In our complex, a planar
À
À
invariably bond to AgX2 units (X = halide, CN). Our silver
complex is the first compound to exhibit strong argentophilic
interactions between two {Ag(NHC)2} units. The extended
{Ag4I4} raft is doubly bridged along the external Ag Ag
bonds by iodide ions (10). It is clear that these structures lie on
a shallow potential energy surface, with the synthesis and
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Chemie
crystallization conditions strongly influencing the structural
outcome.
The copper complex [Cu2(6)]2+ is prepared similarly to its
silver analogue by reaction of Cu2O with the cyclic imidazo-
lium salt in DMSO in the presence of acetate ions;[14] it
crystallizes as the simple diiodide salt. The cation is isostruc-
tural with the silver analogue (Figure 3),[15]and once more the
Figure 4. The X-ray crystal structure of [Pd(6)]I2 with thermal ellipsoids
drawn at the 50% probability level.
The contrast between the structures of the copper and
silver complexes and that of the palladium complex is
marked. Two very different ligand conformations are
observed. In the Group 11 complexes, the {M(NHC)2} units
form mutually parallel planes, which are linked by the
propylene spacers, thus resulting in a lanternlike structure
that is remarkably compact and can readily stack end-to-end
in the solid state. By contrast, in the palladium complex the
ligand adopts a conformation topologically equivalent to the
seams on a tennis ball. The consequence of this arrangement
is that the vacant coordination sites at the palladium center
are located in protected pockets formed by the propylene
linkers between the heterocycles. It will be of interest to
explore the extent to which small ligands (substrates)can
infiltrate these pockets.
The conjunction of NHC and macrocyclic ligand types
generates an important new class of ligands. This first foray
into their chemistry with transition metals indicates a
remarkable flexibility of coordination mode, with the ability
to form both mono- and dinuclear complexes, presaging a rich
and varied coordination chemistry from this and related
systems.
Figure 3. The X-ray crystal structure of [Cu2(6)]I2 with thermal ellip-
soids drawn at the 50% probability level. H atoms are omitted for
clarity.
Received: May 15, 2007
Published online: July 23, 2007
metal–metal distance (2.553(2)Š) is substantially shorter
than the mean value (2.751(4)Š) determined from the
Cambridge Crystallographic Database.[10] Short, unbridged
copper–copper interactions in NHC complexes are unprece-
dented. In the copper complex, there is no association of
cations to form coordination polymers.
Keywords: carbene ligands · copper · macrocyclic ligands ·
.
palladium · silver
Treatment of (6H4)4+(IÀ)4 with PdI2 and NaOAc in DMSO
solution[14] results in the formation of a stable white solid,
which once more has spectroscopic properties consistent with
high symmetry. X-ray crystallography[15] indicates that this
complex has the formulation [Pd(6)]I2, and that the metal ion
is tetracoordinated by the macrocyclic ligand in a square-
planar geometry (Figure 4). The complex broadly resembles a
palladium complex bearing four monodentate NHCs
reported by Fehlhammer et al.[13]
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