An N-heterocyclic carbene ligand with an oxalamide backbone

Article abstract of DOI:10.1021/om100728n

The preparation of the novel N-heterocyclic carbene ligand 3 based on an oxalamide backbone is described. Carbene 3 is not stable as a monomeric species at ambient temperature but dimerizes to the olefin 4 in the absence of trapping reagents. Carbene 3 reacts with elemental sulfur, tert-butylisocyanide, or [M(COD)Cl]₂ (M = Rh, Ir) to give thione 5, ketenimine 6, or complexes [(3)M(COD)Cl] (7), respectively. The dicarbonyl complexes [(3)M(CO) ₂Cl] (8) (M = Rh, Ir) show high-energy CO vibrations. The carbene ligand 3 behaves as a remarkably poor net electron donor compared to other NHC ligands.

Full text of DOI:10.1021/om100728n

4418 Organometallics 2010, 29, 4418–4420
DOI: 10.1021/om100728n
An N-Heterocyclic Carbene Ligand with an Oxalamide Backbone
Markus Braun, Walter Frank,† Guido J. Reiss,† and Christian Ganter*
€
Institut fu€r Anorganische Chemie und Strukturchemie, Heinrich-Heine-Universitat Dusseldorf,
€
Universita€tsstrasse 1, D-40225 Du€sseldorf, Germany. †X-ray structure determinations
Received July 26, 2010
Summary: The preparation of the novel N-heterocyclic carbene
Scheme 1. Synthesis and Reactivity of Carbene Precursor 1
ligand 3 based on an oxalamide backbone is described. Carbene 3
is not stable as a monomeric species at ambient temperature but
dimerizes to the olefin 4 in the absence of trapping reagents.
Carbene 3 reacts with elemental sulfur, tert-butylisocyanide, or
[M(COD)Cl]2 (M = Rh, Ir) to give thione 5, ketenimine 6, or
complexes [(3)M(COD)Cl] (7), respectively. The dicarbonyl
complexes [(3)M(CO)2Cl] (8) (M = Rh, Ir) show high-energy
CO vibrations. The carbene ligand 3 behaves as a remarkably
poor net electron donor compared to other NHC ligands.
of carbonyl groups into the backbone, thus to proceed from
diamino to diamidocarbenes. The latter approach has been
5
ꢀ
reported independently by the groups of Cesar and
Bielawski,6 respectively, based on easily available six-mem-
bered cyclic malonamide derivatives. In addition to the
typical NHC behavior as nucleophilic ligands toward transi-
tion metal fragments, some electrophilic character was also
observed for these diamido carbenes. In this communication
we wish to report our own preliminary results concerning the
synthesis and properties of a cyclic five-membered diamido
carbene based on an oxalamide backbone. In a very recent
study Roesler reported attempts to prepare related oxala-
mide-based NHCs with various N-aryl substituents, which
proved elusive.7 Furthermore, Cesar and Lavigne as well as
Glorius reported the preparation and properties of five-ring
NHCs with one carbonyl function in the backbone.8
Introduction
Understanding and modifying the electronic structure of
N-heterocyclic carbenes (NHCs) is an active field of current
research.1 While NHCs were soon recognized as superb
donor ligands, their capability to exert some π-acceptor
character as well has been appreciated only recently. The
net donor properties of NHCs have been modified, for
example, by introduction of electron-releasing or -withdraw-
ing groups to either the N-bonded aryl groups or the C4 and
C5 atoms of imidazol-2-ylidene rings.2 While it is difficult
to separate donor and acceptor contributions to the metal-
NHC bond from experimental data, various theoretical
studies indicate clearly that the acceptor contribution is not
negligible.3 Alternative approaches to modify the electronic
properties of NHCs included the evaluation of cationic
carbenes based on triazol structures4 and the introduction
Results and Discussion
Reaction of N,N0-dimesitylformamidine with oxalyl chloride
in chloroform according to a protocol reported by Richter9
afforded the neutral five-membered oxalamide 1 in 98% yield as
an off-white solid (Scheme 1). The formation of a neutral
product with a chlorine atom attached to the C2 atom of
*To whom correspondence should be addressed. E-mail: christian.
ganter@uni-duesseldorf.de.
(1) For reviews see: (a) Bourissou, D.; Guerret, O.; Gabbaı, F. P.;
ꢀ
¨
Bertrand, G. Chem. Rev. 2000, 100, 39. (b) Díez-Gonzalez, S.; Nolan, S. P.
ꢀ
the ring is in accord with Cesar’s observation in the case of the
six-membered malonamide.5 In contrast, Bielawski reported
the formation of a cationic malonamide when chloride was
exchanged for the less nucleophilc triflate anion.6 The C-Cl
bond in 1 is highly labile, and upon addition of methanol the
2-methoxy derivative 2 was obtained in nearly quantitative yield
and fully characterized by spectroscopic methods (Scheme 1).
Coord. Chem. Rev. 2007, 251, 874. (c) Jahnke, M. C.; Hahn, F. E. Angew.
Chem., Int. Ed. 2008, 47, 3122. (d) Wu€rtz, S.; Glorius, F. Acc. Chem. Res.
2008, 41, 1523. (e) Hirai, K.; Itoh, T.; Tomioka, H. Chem. Rev. 2009, 109,
3275. (f) Droge, T.; Glorius, F. Angew. Chem. 2010, DOI: 10.1002/
ange.201001865.
(2) For selected examples of this approach see: (a) Sanderson, M. D.;
€
Kamplain, J. W.; Bielawski, C. W. J. Am. Chem. Soc. 2006, 128, 16514.
€
(b) Leuthausser, S.; Schwarz, D.; Plenio, H. Chem.-Eur. J. 2007, 13, 7195.
(c) Khramov, D. M.; Lynch, V. M.; Bielawski, C. W. Organometallics 2007,
26, 6042. (d) Khramov, D. M.; Rosen, E. L.; Er, J. A. V.; Vu, P. D.; Lynch,
V. M.; Bielawski, C. W. Tetrahedron 2008, 64, 6853. (e) Bittermann, A.;
ꢀ
(5) (a) Cesar, V.; Lugan, N.; Lavigne, G. J. Am. Chem. Soc. 2008, 130,
€
ꢀ
Harter, P.; Herdtweck, E.; Hoffmann, S. D.; Hermann, W. A. J. Organomet.
11286. (b) Cesar, V.; Lugan, N.; Lavigne, G. Eur. J. Inorg. Chem. 2010, 361.
Chem. 2008, 693, 2079. (f) Ogle, J. W.; Miller, S. A. Chem. Commun. 2009,
5728.
(3) (a) Srebro, M.; Michalak, A. Inorg. Chem. 2009, 48, 5361. (b)
Jacobsen, H.; Correa, A.; Poater, A.; Costabile, C.; Cavallo, L. Coord. Chem.
(6) (a) Hudnall, T. W.; Bielawski, C. W. J. Am. Chem. Soc. 2009, 131,
16039. (b) Hudnall, T. W.; Moerdyk, J. P.; Bielawski, C. W. Chem. Commun.
2010, 4288. (c) Hudnall, T. W.; Moorhead, E. J.; Gusev, D . G.; Bielawski,
C. W. J. Org. Chem. 2010, 75, 2763.
ꢀ
Rev. 2009, 253, 687. (c) Antonova, N.; Carbo, J. J.; Poblet, J. M. Organo-
(7) Hobbs, M. G.; Forster, T. D.; Borau-Garcia, J.; Knapp, C. J.;
metallics 2009, 28, 4283. (d) Kausamo, A.; Tuonen, H. M.; Krahulic, K.;
Roesler, R. Inorg. Chem. 2008, 47, 1145.
Tuononen, H. M.; Roesler, R. New J. Chem. 2010, 34, 1295.
ꢀ
(8) (a) Benhamou, L.; Cesar, V.; Gornitzka, H.; Lugan, N.; Lavigne,
ꢀ
ꢀ
(4) (a) Zanardi, A.; Corberan, R.; Mata, J. A.; Peris, E. Organome-
ꢀ
G. Chem. Commun. 2009, 4720. (b) Benhamou, L.; Vujkovic, N.; Cesar, V.;
tallics 2008, 27, 3570. (b) De Bo, G.; Berthon-Gelloz, G.; Tinant, B.; Marko,
Gornitzka, H.; Lugan, N.; Lavigne, G. Organometallics 2010, 29, 2616.
€
I. E. Organometallics 2006, 25, 1881. (c) Buron, C.; Stelzig, L.; Guerret, O.;
Gornitzka, H.; Romanenko, V.; Bertrand, G. J. Organomet. Chem. 2002,
664, 70.
(c) Biju, A. T.; Hirano, K.; Frohlich, R.; Glorius, F. Chem.-Asian J. 2009, 4,
1786.
(9) Barsa, E. A.; Richter, R. J. Org. Chem. 1986, 51, 4483.
r
pubs.acs.org/Organometallics
Published on Web 09/21/2010
2010 American Chemical Society

Communication
Scheme 2. Synthesis and Reactivity of Carbene 3
Organometallics, Vol. 29, No. 20, 2010 4419
Figure 1. Molecular structure of olefin 4 (ellipsoids drawn at
30% probability; H atoms and the solvating CH2Cl2 molecule
omitted for clarity). Selected distances (pm) and angles (deg):
C1-C4 135.7(4), C1-N1 142.9(3), C1-N2 143.1(3), C2-N1
138.6(3), C3-N2 138.0(4), C2-O1 120.8(3), C3-O2 120.6(3),
N1-C1-N2 104.7(2).
Treatment of precursor 1 with NaHMDS (NaHMDS =
NaN(SiMe3)2) in THF at -80 °C leads to deprotonation
and chloride elimination with subsequent formation of the
neutral diamidocarbene 3, which proved too unstable to
be characterized. Instead, after workup the olefin 4 was
obtained as a result of carbene dimerization in 70% yield
(Scheme 2). Olefin 4 is water-stable and can be handled in air
for a short period of time. For the related six-membered
malonamide-based carbenes, no dimerization was observed,
and some derivatives could be characterized by NMR spec-
troscopy in solution at low temperature5,6 and in one case by
X-ray diffraction.6b Interestingly, Roesler reported that the
treatment of molecular precursors analogous to 1 bearing
other aryl substituents at nitrogen with bases resulted mainly
in decomposition. However, he succeeded in isolating a side
product in low yield, which was identified as a dimer con-
nected by a C2-C2 single bond with additional H atoms
located at the bridging C atoms.7
Single crystals suitable for X-ray diffraction were obtained
by slow diffusion of n-hexane into a solution of 4 in dichlor-
omethane at room temperature. The structure is shown in
Figure 1 together with some representative geometric para-
meters. Olefin 4 is the first structurally characterized tetra-
amido ethylene. Both rings deviate only slightly from
planarity, with a maximum atomic distance from the mean
planes of 3.6(2) pm. All oxalamide ring atoms feature sp2-
hybridization with most significant deviations for N1 (sum
of angles 355.2°) and N3 (352.7°). In di(imidazolidin-2-
ylidenes) the N atoms are usually significantly pyramidalized
with angle sums of 330-345°.10 The length of the interan-
nular C-C bond is 135.7(4) pm. The torsion angle between
the heterocyclic rings is 19.2(2)o, a significant twist compared
to di(imidazolidin-2-ylidenes), which usually feature torsion
angles between 0° and 15°.10,11 The geometrical parameters
for both rings are very similar, with mean values for CdO of
120.7 pm and OC-CO of 148.2 pm, longer C-N bonds to
the olefinic C (142.8 pm), and shorter C-N bonds to CdO
carbons (138.4 pm). The N-bonded mesityl groups are in
a pairwise parallel orientation, with interplanar angles
between respective planes of 10.0(2)o (rings containing
C11 and C31) and 4.4(2)o (rings containing C21 and C41).
The formation of the carbene 3 as an intermediate species
was corroborated by further trapping reactions. Addition of
elemental sulfur to a solution of carbene 3 prepared in situ
resulted in the formation of the expected thione 5, which was
fully characterized by spectroscopic methods. Furthermore,
carbene 3 also reacted cleanly with tert-butylisocyanide to
afford the respective ketenimine 6 in good yield. Usually, the
formation of ketenimines from isonitriles is observed only
for highly electrophilic carbenes, available for example from
diazoalkanes, and not for diamino NHCs characterized by a
large HOMO-LUMO gap due to the stabilization exerted
by the N atoms. Bielawski reported recently the formation of
ketenimines from a malonamide-type carbene and attributed
this reactivity to an energetically low-lying LUMO centered
on the carbene carbon atom, providing this diamidocarbene
with significant electrophilic character.6c In addition, the
malonamide NHC studied by Bielawski reversibly formed a
ketene upon reaction with CO and inserted into a secondary
isopropyl C-H bond of a diisopropylphenyl group bound to
the N atom.6a These latter two reactions were again taken as
evidence for the electrophilicity of the malonamide NHC. In
contrast, the oxalamide NHC 3 did not produce the respec-
tive ketene when exposed to CO. Instead, the olefin 4 was
formed as the only product under these reaction conditions.
The propensity of the new NHC 3 to act as a ligand toward
transition metal fragments was also evaluated. When a carbene
solution was treated with a solution of [M(COD)Cl]2 (M =
Rh, Ir) in THF at -80 °C, the complexes [(3)M(COD)Cl] 7a
(M = Rh) and 7b (M = Ir) were isolated in 65% and 59%
yield after chromatographic workup (Scheme 3). In contrast,
treatment of the olefin 4 with [Rh(COD)Cl]2 did not result in
the formation of complex 7a.
(10) (a) Hitchcock, P. B. J. Chem. Soc., Dalton Trans. 1979, 1314. (b)
C-etinkaya, E.; Hitchcock, P. B.; Jasmin, H. A.; Lappert, M. F.; Spyropoulos,
K. J. Chem. Soc., Perkin Trans. 1 1992, 561. (c) C-etinkaya, E.; Hitchcock,
P. B.; K€uc-€ukbay, H.; Lappert, M. F.; Al-Juaid, S. J. Organomet. Chem. 1994,
481, 89. (d) Denk, M. K.; Thadani, A.; Hatano, K.; Lough, A. J. Angew.
Chem., Int. Ed. Engl. 1997, 36, 2607. (e) Hahn, F. E.; Wittenbecher, L.;
The most prominent spectroscopic feature of the Rh
complex 7a is the low-field carbene resonance in the 13C
NMR spectrum at 248 ppm, which appears as a doublet
(54.6 Hz) due to the coupling to the Rh nucleus. For the
analogous Rh complex with the six-membered malonamide
NHC ligand, the resonance for the carbene-C was found at
€
Boese, R.; Blaser, D. Chem.-Eur. J. 1999, 5, 1931. (f) Hahn, F. E.;
€
Wittenbecher, L.; Le Van, D.; Frohlich, R. Angew. Chem., Int. Ed. 2000,
39, 541. (g) Luan, X.; Mariz, R.; Gatti, M.; Costabile, C.; Poater, A.; Cavallo,
L.; Linden, A.; Dorta, R. J. Am. Chem. Soc. 2008, 130, 6848.
(11) For olefins arising from benzimidazolin-2-ylidenes with excep-
tionally high torsion angles of 21° and 27° see ref 10c and: Kamplain,
J. W.; Lynch, V. M.; Bielawski, C. W. Org. Lett. 2007, 9, 5401.

4420 Organometallics, Vol. 29, No. 20, 2010
Braun et al.
Scheme 3. Formation of Carbene Metal Complexes
245 ppm.5b The 1H NMR spectrum of complex 7a shows two
signals for the olefinic protons of the COD ligand at 3.62 and
5.12 ppm. The spectroscopic properties of the Ir complex 7b
are quite similar to those of the Rh analogue 7a. In addition,
crystals of 7b suitable for X-ray diffraction were obtained
by slow evaporation of a dichloromethane solution. The
molecular structure of complex 7b is depicted in Figure 2
together with some geometrical parameters. The Ir atom is
in a square-planar environment with a remarkably short
Ir-C(NHC) distance of 193.1(9) pm. In (COD)IrCl com-
plexes bearing NHCs derived from imidazole or imidazoli-
dine the Ir-C(NHC) distance is usually found in the range
201-208 pm.12 The bonds between Ir and the COD C atoms
trans to the NHC are fairly long (228.0(9) and 225.6(8) pm)
compared to other (NHC)Ir(COD)Cl complexes, where they
are usually found in the range 217-219 pm.12c The Ir-
C(COD) bonds trans to the chloride are significantly shorter
(215.1(9) and 218.2(9) pm), as is usually observed. The CdC
bond lengths of the COD ligand are also strongly affected by
the groups bound in trans position to Ir: the CdC bond trans
to NHC is short (136.3(15) pm), while the one trans to Cl is
considerably longer (141.1(13) pm).
Figure 2. Molecular structure of Ir complex 7b (ellipsoids drawn
at 30% probability; H atoms omitted for clarity). Selected dis-
tances (pm) and angles (deg): Ir1-Cl1 235.9(2), Ir1-C1 193.1(9),
Ir1-C22 215.1(9), Ir1-C23 218.2(9), Ir1-C26 225.6(8), Ir1-C27
228.0(9), N1-C1-N2 105.1(7), C1-Ir1-Cl1 95.8(2).
2030-2045 cm-1 13,8b
.
ꢀ
In a very interesting study, Cesar
reported a series of complexes bearing five-membered NHCs,
derived from a mixed amino-amido heterocycle with a
CO-CH2 backbone. This species can act either as a neutral
NHC ligand or, after deprotonation, as an anionic enolate, or
as a neutral enolether after O-functionalization of the enolate
with appropriate electrophiles. The Rh(CO)2Cl complexes of
all three ligand types were examined, and νav(CO) values were
found to be 2048.5, 2029.5, and 2037 cm-1, respectively.8b All
experimental data indicate that the oxalamide NHC 3 acts as
one of the poorest net electron donors among the N-hetero-
cyclic carbenes. Moreover, theoretical work carried out
recently by Roesler indicates that oxalamide-based NHCs
feature a quite small HOMO-LUMO gap and, in particular,
a low-lying LUMO, which endows these ligands with con-
siderable π-acceptor character,7 much more pronounced than
in the majority of imidazole- and imidazolidine-based NHCs.3
The exceptionally short Ir-C(NHC) bond (vide supra) may
thus be considered as arising from this pronounced π-acceptor
contribution. The fact that carbene 3 is not stable as a
monomeric species but dimerizes to the olefin 4 can be
regarded as additional evidence for the small HOMO-LUMO
gap as well.14
In conclusion, we have prepared a novel type of N-hetero-
cyclic carbene ligand, 3, based on an oxalamide backbone.
While the carbene is not stable as a monomeric species at
ambient temperature, it gives rise to the formation of the
olefin 4 as a result of dimerization in the absence of trapping
reagents. In the presence of either elemental sulfur or tert-
butylisocyanide the carbene thione 5 and the ketenimine 6
are obtained, respectively. NHC 3 also reacts cleanly with the
dimeric species [M(COD)Cl]2 (M = Rh, Ir) to afford the
complexes [(3)M(COD)Cl] (7). The dicarbonyl complexes
[(3)M(CO)2Cl] (8) show high-energy CO vibrations, which
proves the poor net electron donor property of the carbene
ligand 3, while theoretical work indicates that this character
is at least partly due to a considerable π-acceptor contribu-
tion. Further work to evaluate the reactivity of the new
ligand type is currently in progress.
In order to use IR spectroscopy to evaluate the electronic
properties of the NHC ligand 3, the carbonyl complexes 8a,b
were prepared from the COD complexes by bubbling a slow
stream of COthrough a solution of complexes 7a,b (Scheme 3).
The carbonyl derivatives 8 are much more sensitive than the
COD complexes, and only the Rh derivative 8a could be
obtained in analytically pure form by chromatography. In
the 13C NMR spectrum of the Rh complex 8a the signals for
the carbene-C and amide-C atoms are observed at 242 and 152
ppm, respectively, while the signals for the carbonyl C atoms
are found at 181 and 184 ppm. For the Ir complex 8b the signal
for the NHC carbon appeared at 229.8 ppm and the signals for
the CO ligands were detected at 167.4 and 179.5 ppm. The IR
spectra of 8a,b show two strong CO stretching vibrations at
2017 and 2103 cm-1 (8a, νav = 2060 cm-1) and 2003 and 2089
cm-1 (8b, νav = 2046 cm-1), respectively. Compared to other
M(CO)2Cl (M=Rh, Ir) complexes bearing NHC ligands,
the stretching vibrations of complexes 8a,b are located at very
high wavenumbers. Specifically, in the case of Rh complexes
Bielawski reported CO bands located at 2017 and 2099 cm-1
for the complex with the 4,5-dicyanoimidazol-2-ylidene ligand,
which was shown to exert a reasonable degree of π-acceptor
character.2c For the related complex with a malonamide NHC
ligand, νav(CO) was found to be 2045 cm-1 5b
Usually, Rh-
.
(CO)2Cl complexes with diaminocarbenes based on imidazol-
or imidazolidin-2-ylidene have their νav(CO) in the range
Supporting Information Available: Experimental details for
the preparation of the reported compounds and crystallo-
graphic data (CIF files) for compounds 4 and 7b. This material
is available free of charge via the Internet at http://pubs.acs.org.
(12) (a) Kownacki, I.; Kubicki, M.; Szubert, K.; Marciniec, B. J.
Organomet. Chem. 2008, 693, 321. (b) Reference 2b. (c) Kelly, R. A., III;
Clavier, H.; Giudice, S.; Scott, N. M.; Stevens, E. D.; Bordner, J.; Samardjiev,
I.; Hoff, C. D.; Cavallo, L.; Nolan, S. P. Organometallics 2008, 27, 202.
(d) Chianese, A. R.; Mo, A.; Datta, D. Organometallics 2009, 28, 465.
(13) Wolf, S.; Plenio, H. J. Organomet. Chem. 2009, 694, 1487.
(14) Poater, A.; Ragone, F.; Giudice, S.; Costabile, C.; Dorta, R.;
Nolan, S. P.; Cavallo, L. Organometallics 2008, 27, 2679.