Synthesis and coordination properties of nitrogen(I)-based ligands

Article abstract of DOI:10.1002/anie.200906168

Chemical Equation Reprentation Cive me four: In contrast to typical imines, the central nitrogen atom in cyclopropenyl-derived carbeneimines (see example; C gray, N blue) can donate up to four electrons to form coordination complexes.

Full text of DOI:10.1002/anie.200906168

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
under http://dx.doi.org/10.1002/anie.200906168.
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Figure 1. Structure of 11 in the solid state (hydrogen atoms removed
for clarity; ellipsoids set at 50% probability).[17]
after coordination to rhodium, the cyclopropenyl system
regains aromaticity, accompanied by an elongation of the
ꢀ
C1 N1 bond (Figure 2).
Scheme 3. Reagents and conditions (yields): a) MesNH2, MeCN,
1608C (microwave) 1 h (48%); b) KH, THF, 458C, overnight (56%);
c) [{RhCl(CO)2}2], THF, 20 min (88%); d) MesNH2, nBuLi, THF, reflux,
12 h (78%); e) KHMDS, toluene, ꢀ208C!RT (96%);
f) [{RhCl(CO)2}2], THF, 20 min (61%); g) MesNH2, THF, reflux, 24 h
(93%); h) KH, THF, 608C, overnight (91%); i) [{RhCl(CO)2}2], THF,
20 min (72%). Mes=2,4,6-trimethylphenyl, KHMDS=potassium
hexamethyldisilazide.
Figure 2. Molecular structure of 12 in the solid state (hydrogen atoms
removed for clarity; ellipsoids set at 50% probability).[17]
The cyclopropenyl moiety in these molecules can be
thought of as an electron reservoir. In the free ligand, it
accepts p electron density from the imino nitrogen, thereby
losing some of its aromatic character. Upon coordination it
compensates the electron density transferred from the nitro-
gen atom to the metal, recovering aromaticity. This effect also
explains why the total negative charge calculated on the
nitrogen atom remains effectively constant, or surprisingly
even increases after formation of some gold complexes (see
the Supporting Information). The same behavior of the
flanking ligands has been previously observed in carbon(0)
species.[2,4a]
ꢀ
the solid state (Figure 1). The three C C bond lengths in the
cyclopropenyl ring of 10 are quite similar (1.379–1.398 ꢂ),
suggesting that the positive charge largely resides on the
aromatic cyclopropenium ring, represented by the mesomeric
extreme 10’. After deprotonation, the structure of 11 indi-
cates, however, that mesomeric form 11’ loses importance.
ꢀ
The C1 N3 bond in 11 (1.295 ꢂ) is substantially shorter than
in 10 (1.334 ꢂ) and has a typical value for carbon–nitrogen
double bonds. Furthermore, the three-membered ring is now
less aromatic, with two longer (1.420 and 1.411 ꢂ) and one
ꢀ
shorter (1.371 ꢂ) C C bonds (Scheme 3). Even though these
data suggest that the mesomeric structure 11 gains impor-
tance in the free ligands, the outstanding donor quality of
these compounds can not be explained by considering only its
imine character. In fact, the X-ray structure of 12 reveals that,
Once the bis(diisopropylamino)cyclopropenylylidene was
identified as the most appropriate carbene to produce
extremely electron-rich structures C’, the influence of the
R group was investigated. Therefore, compounds 17–20 were
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prepared and their electron properties evaluated as previ-
ously (Scheme 4). In general, R groups have less influence on
the donor ability than the carbene L, and only in the case of
the 1-pyrrolidine substituent could a clear albeit small
increase of the donor properties be observed.
Scheme 5. Reagents and conditions (yields): a) [AuCl(Me2S)], THF, RT
(R=Mes, 76%; R=1-pyrrolidine, 84%); b) B(C6F5)3, toluene, RT
(72%); c) CS2 (3 equiv), THF (quantitative).
Scheme 4. Evaluation of the influence of the R groups on ligands of
type C. Reagents and conditions: a) KH, THF, 17 (92%), 18 (97%), 19
(80%), 20 (93%); b) [{RhCl(CO)2}2], THF; 21 (98%), 22 (72%), 23
(66%); 24 (79%).
Carbodicarbene A2 reacts with electrophiles such as S8,
CO2 or CS2 at the central carbon atom to form zwitterionic
adducts.[19] No reaction was observed between 11 and S8 or
CO2 (1 atm), however, upon addition of CS2 to a solution of
=
=
In parallel, the bonding situation in compounds 3, 7, 11,
and 17–20 was evaluated using density functional theory
computations at the BP86(RI)/TZVP level. Inspection of the
frontier orbitals reveals that in all the cases studied, the
HOMO and HOMOꢀ1 correspond to p- and s-type lone-pair
orbitals with maximum coefficients at the central nitrogen
atom (Figure 3).
11, metathesis of the C S and C N bonds takes place yielding
a 1:1 mixture of thioisocyanate 30 and thioketone 31 instead
of any adduct. This reactivity resembles that of guanidines.[20]
Evidence of whether the central nitrogen atom in these
species is a nitrogen(I) is given by the isolation of compounds
in which they formally donate two electron pairs. This aim has
been achieved herein in two different ways. When 18 was
submitted to methylation conditions using an excess of
Me3OBF4, the dialkylated salt 33 was obtained. Even more
interesting, owing to its possible implications in catalysis, is
the fact that dimeric structure 34 is able to react with two
equivalents of [PdCl2(CH3CN)2]. The resulting complex 35
has an unprecedented coordination mode for neutral imine-
type ligands in which the central nitrogen atoms donate four
electrons each (Scheme 6). The X-ray structure of 35
unequivocally confirms the nitrogen(I) nature of this ligand
as predicted by our calculations (Figure 4). Remarkably,
despite extensive studies on the coordination chemistry of
closely related bis(imidazolin-2-imine) ligands,[21] dimetala-
tion of these compounds has not been achieved.
Figure 3. Highest occupied molecular orbitals of 14: HOMOꢀ1 (left)
and HOMO(right).
This finding, together with the calculated first and second
proton affinity, support the idea that compounds 11 and 17–20
should be considered as nitrogen(I)-containing species. Inter-
estingly, in some cases the energies of the calculated lone-pair
orbitals are even higher than those in carbodiphosphoranes or
carbodicarbenes, indicating their propensity towards coordi-
nation (see the Supporting Information).[2a]
Encouraged by this analysis, the reactivity and coordina-
tion properties of these new compounds were studied. Both
11 and 17 react smoothly with soft Lewis acids, such as
[AuCl(Me2S)], affording the desired metal complexes 27 and
28 in good yields (Scheme 5). A hard Lewis acid, such as
B(C6F5)3, also reacts cleanly with 11 to form the correspond-
ing adduct 29 as colorless crystals (see the Supporting
Scheme 6. Reagents and conditions (yields): a) Me3OBF4, CH2Cl2, RT
(84%); b) Me3OBF4, CH2Cl2, reflux (54%); c) [PdCl2(MeCN)2]
(2 equiv), THF (21%).
[18]
ꢀ
Information). The short B1 N3 bond (1.587 ꢂ)
points to
ꢀ
an enhanced B N interaction.
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Figure 4. Molecular structure of 35 in the solid state (hydrogen atoms
and solvent molecules removed for clarity; ellipsoids set at 50%
probability).[17]
In conclusion, the synthesis, bonding situation, and
coordination behavior of several highly electron-rich imines,
and in particular cyclopropenylylidenimines, has been stud-
ied. The central nitrogen atom in these compounds exhibits all
the attributes of a nitrogen(I) atom, namely the availability of
two lone pairs essentially localized on that atom, and that
occupy the HOMO and HOMOꢀ1 orbitals, and the ability to
use these electrons in reactions with Lewis acids. The
application of these compounds as ligands in homogeneous
catalysis is currently under investigation in our laboratory.
[12] A. Krebs, A. Gꢁntner, S. Versteylen, S. Schulz, Tetrahedron Lett.
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[13] P. Imhoff, C. J. Elsevier, C. H. Stam, Inorg. Chim. Acta 1990, 175,
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[14] A 4-iminopyridine recently synthesized by Doster and Johnson,
which might be considered a compound of type C with L =
1-methylpyridine-4-ylidene, has a similar donor strength: M. E.
Doster, S. A. Johnson, Angew. Chem. 2009, 121, 2219 – 2221;
Angew. Chem. Int. Ed. 2009, 48, 2185 – 2187.
Received: November 2, 2009
Revised: January 25, 2010
Published online: April 13, 2010
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[17] CCDC 752823 (2), 752824 (6), 752819 (10), 752825 (11), 752821
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uk/data_request/cif.
Keywords: coordination chemistry · cyclopropenylylidenes ·
imines · nitrenes · nitrogen ligands
.
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