Communications
DOI: 10.1002/anie.200906168
Ligand Development
Synthesis and Coordination Properties of Nitrogen(I)-Based Ligands**
Hans Bruns, Mahendra Patil, Javier Carreras, Arcadio Vꢀzquez, Walter Thiel, Richard Goddard,
and Manuel Alcarazo*
In most organic compounds, nitrogen uses three of its five
valence electrons to form bonds with surrounding atoms
whilst keeping the other two as a lone pair. Similarly, with the
exception of carbon monoxide, carbenes, and isonitriles,
carbon is normally tetravalent in organic molecules. However,
following seminal work from Ramꢀrez[1] and inspiring theo-
retical studies from Frenking,[2] Bertrand[3] and Fꢁrstner[4]
have synthesized and evaluated the coordination behavior
of carbodicarbenes A1 and A2 and carbophosphinocarbenes
A3 (Scheme 1). These compounds may be considered as
consisting of two carbene ligands, or a carbene and a
phosphine, coordinated to a central zerovalent carbon atom.[5]
species.[7] A further structural change, directed to retain
charge neutrality, is therefore essential.
Replacement of one of the carbene moieties in B2 by
anionic Rꢀ or Arꢀ groups will eliminate the global positive
charge yet keep the system isoelectronic with carbodicar-
benes. To favor the particular electronic configuration in
which the nitrogen atom is able to donate four electrons over
that typical of imines, an adequate selection of the ligand L is
also crucial. It must be a strong s donor and at the same time a
very poor p acceptor to minimize back-donation from the
central nitrogen atom to the carbene moiety (Scheme 2).[8]
Scheme 2. Conceivable resonance extremes of C-type imines.
This extreme situation, represented by resonance struc-
ture C’, may be interpreted as a coordination complex in
which a carbene donates two electrons into the empty orbital
of a nitrene. The aim herein is to study the synthesis, bonding
situation, and coordination properties of compounds of this
general structure either to confirm or disprove this view.
The preparation of compounds 3, 7, and 11 (Scheme 3)
begins with the condensation of readily available chloroimi-
nium species 1 or 5 or chlorocyclopropenium salts[9] 9 with
mesitylamine, followed by deprotonation with KH or
KHMDS. This route, which is complementary to the one
developed by Tamm et al. for the synthesis of imidazoline-2-
imines,[10,11] avoids the use of free carbenes; furthermore, it
allows the synthesis of compounds of type Cc with more
structural diversity than previously reported.[12]
Scheme 1. Representative structures of carbodicarbenes A1 and A2,
carbophosphinocarbenes A3, bis(phosphine)iminium cations B1, and
bis(carbene)iminium cations B2.
It can be envisaged that this bonding situation can in
principle be extrapolated to compounds B1 and B2, in which
the central carbon atom of carbodiphosphoranes or carbodi-
carbenes has been formally replaced by an isoelectronic N+
fragment.[6] However, the positive charge introduced in the
systems by such substitution precludes interaction with metal
centers. In fact, B1 ions have been extensively used as non-
interfering cations for the structural characterization of -ate
The donor ability of these ligands was evaluated by
analysis of the CO stretching frequencies for complexes of
type [RhCl(CO)2C]; the data suggest that 3 has a donor
capability similar to iminophosphoranes.[13,14] Compound 7
surpasses the strongest electron-releasing N-heterocyclic
carbenes, whilst 11 clearly rivals (amino)(ylide) carbene[15]
and carbodicarbenes.[3,4b] These exceptional donor properties
are highlighted by comparison with other nitrogen-based
ligands, such as pyridines or imines.[16]
[*] H. Bruns, Dr. M. Patil, J. Carreras, A. Vꢀzquez, Prof. Dr. W. Thiel,
Dr. R. Goddard, Dr. M. Alcarazo
Max-Planck-Institut fꢁr Kohlenforschung
45470 Mꢁlheim an der Ruhr (Germany)
Fax: (+49)208-306-2994
E-mail: alcarazo@mpi-muelheim.mpg.de
[**] We thank Prof. A. Fꢁrstner for generous support and constant
encouragement. Financial support from the Max Planck-Institut fꢁr
Kohlenforschung is also gratefully acknowledged.
Especially informative is a comparison between the
structures of 10 (see the Supporting Information) and 11 in
Supporting information for this article is available on the WWW
3680
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2010, 49, 3680 –3683