.
Angewandte
Communications
DOI: 10.1002/anie.201302686
Homogeneous Catalysis
Triphenylene-Based Tris(N-Heterocyclic Carbene) Ligand:
Unexpected Catalytic Benefits**
Sergio Gonell, Macarena Poyatos, and Eduardo Peris*
Sophisticated ligands that do far more than fulfill their
traditional spectator roles are attracting close attention
because they can provide additional functions which can be
improve catalytic effects, especially in some asymmetric
inductions. We believe that a similar effect may have some
[
7]
consequences in other homogeneously catalyzed reactions
[1]
[7a]
useful in catalysis. In the search for new precursors to
multifunctional materials, polytopic N-heterocyclic carbene
involving aromatic substrates,
reactions in which the
catalyst–substrate interactions may be facilitated by p stack-
[2]
ligands (NHCs) have emerged as very promising tools.
ing. Given the high affinity of NHCs for a broad range of
[
2,8]
Among them, those featuring rigid geometrically isolated
carbene moieties, which prevent chelation have recently
found great utility as building blocks for accessing improved
metal fragments,
this new D -symmetry system should
3h
provide us with an excellent building block for the prepara-
tion of a wide variety of trimetallic complexes whose proper-
ties may be easily compared to those of their monometallic
analogues.
[
3]
multimetallic catalysts, and as novel macromolecules with
structurally dynamic characteristics and useful electronic
[4]
properties. While most of these carbenes refer to Janus-
The synthesis of the triphenylene-based trisazolium salt 2
starts from the multifold amination of hexabromotripheny-
[
3a,d–f,5]
type (facially opposed) bis(NHC)s,
the only known
[9]
rigid tris(NHC)s with geometrically isolated carbenes is
restricted to the example reported by Williams and Bielawski
lene and subsequent trisannulation with triethylorthofor-
mate in the presence of HBF (Scheme 1). The deprotonation
4
[6]
in 2010 (1; Figure 1). This tris(NHC) holds potential in
reticular chemistry and may serve for accessing an assortment
of functional organic and metal–organic frameworks, but
lacks an extended p-conjugated system (the p delocalization
is disrupted by the presence of the triptycene core), which
limits its utility in the formation of structurally dynamic
of the trisazolium salt with potassium hexamethyldisilazane-
(KHMDS) in tetrahydrofuran and subsequent addition of
[{PdCl(allyl)} ] and [AuCl(SMe )] yields the corresponding
2
2
tris(NHC) complexes of Pd (3) and Au (4), respectively. All
[
4a,b]
materials.
edents in mind, we sought to
obtain star-shaped tris-
NHC) in which the three
With these prec-
a
(
carbenes are connected by
a p-delocalized polyaromatic
system derived from a triphe-
nylene core. Polyaromatic-
based ligands are also inter-
esting tools in the design of
efficient homogeneous cata-
Figure 1. Triptycene-based tris-
(
(
NHC) described by Bielawski
R=nBu and nHex). The com-
lysts
because
aromatic
p stacking between ligands
pound with R=H has been pre-
pared for this work.
and substrates is known to
[*] S. Gonell, Dr. M. Poyatos, Prof. E. Peris
Dpto. de Quꢀmica Inorgꢁnica y Orgꢁnica
Universitat Jaume I
Avda. Sos Baynat, 12071 Castellꢂn (Spain)
E-mail: eperis@uji.es
[**] We gratefully acknowlegde financial support from the MEC of Spain
(
CTQ2011-24055/BQU) and Bancaixa (P1.1B2010-02 and
P1.1B2011-22). We would also like to thank the Ramꢂn y Cajal
program (M.P.). S.G. thanks the Ministerio de Ciencia e Innovaciꢂn
for a fellowship. The authors are grateful to the Serveis Centrals
d’Instrumentaciꢂ Cientꢀfica (SCIC) of the Universitat Jaume I for
providing us with spectroscopic and X-Ray facilities. We also thank
Bob Crabtree for thoughtful discussions.
Scheme 1. Synthesis of the D -symmetric trisazolium salt 2 and its
3
h
coordination to Pd and Au. IPrH·HCl=1,3-bis(2,6-diisopropylphenyl)-
imidazolium chloride, THF=tetrahydrofuran.
2
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2013, 52, 1 – 6
These are not the final page numbers!