Angewandte
Chemie
alkynoic acids. The Rh complex 3 was also tested in the latter
reaction. Complex 2 turns out to be an excellent catalyst in the
reduction of ketones and imines. The reactions were carried
out in iPrOH, using KOH as base. Conversions of aliphatic
ketones were complete in 30 min, while N-benzylideneaniline
and aromatic ketones were completely reduced in 1h under
the reaction conditions used (see the Supporting Informa-
tion). This result confirms that 2 is a very active species in this
type of reaction. Its catalyst efficiency is close to that recently
reported by Crabtree and co-workers for monodentate
triazole-derived NHC complexes of Ir[11] and better than the
imidazole NHC Ir compounds reported by Nolan and co-
workers.[12]
Probably more relavant is the catalytic activity in the
intramolecular cyclization of acetylenic carboxylic acids.
Compounds 2 and 3 show full converion of 4-pentynoic acid
in 3 and 8 h, respectively, under the reaction conditions used
(see the Supporting Information). The more inert 5-hexynoic
acid is fully converted into the corresponding cyclic com-
pound in 7 or 8 days (see the Supporting Information), when 2
or 3 was used, respectively. These results are clearly better
than our previously reported results on this type of reac-
tion,[13,14] where trinuclear complexes of Rh and Ir were also
used.[13] Remarkably, 2 is the first Ir complex to be highly
active in this type of reaction, since other Ir catalysts are
clearly less efficient than their Rh analogues.[15]
In summary, we have reported the coordination of a
triazolediylidene ligand to Rh and Ir and have fully charac-
terized the resulting complexes. The electron-donating ability
of the ligand has been determined in terms of the TEP
parameter, from which it was found to be as strongly electron-
donating as alkylphosphines. The connection of two catalyti-
cally active metal fragments through an aromatic linker may
also have interesting implications for their catalytic cooper-
ativity. The preparation of these new molecules allows novel
strategies for the design of new homo- and hetero-binuclear
species with potentially improved catalytic properties. In fact,
the effective preparation of the Ir–Rh species 5 opens the
possibility for an extremely wide range of bimetallic combi-
nations, especially if we take into account the high coordina-
tion versatility of NHCs. This property makes the triazole-
diylidene ligand a unique building block for the preparation
of tandem catalysts, an effort that is currently underway.
Keywords: carbene ligands · heterocycles ·
homogeneous catalysis · iridium · rhodium
.
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[16] CCDC-635919 (2) and CCDC-635920 (4) contain the supple-
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obtained free of charge from The Cambridge Crystallographic
Received: February 6, 2007
Published online: April 5, 2007
Angew. Chem. Int. Ed. 2007, 46, 3729 –3731
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