A stable anionic N-heterocyclic carbene and its zwitterionic complexes

Article abstract of DOI:10.1021/ja804296t

Pyrimidinium betaines (1), readily accessible via a straightforward modular synthesis from a formamidine and a monosubstituted malonic acid, are readily deprotonated by nBuLi (or KHMDS) to give the stable carbene species [2]Li⁺ (abbreviated as maloNHC). The latter represents the archetype of a subgroup of N-heterocyclic carbenes incorporating a malonate as remote anionic functional group within their heterocyclic backbone. While playing the dual role of monodentate 2 e^- L type donor and noncoordinating charge carrier X, such ligands are seen to provide a rational route to zwitterionic complexes, as illustrated here by three examples (Rh, Fe, Ag). In particular, the reaction of [2]Li⁺ with [RhCl(1,5-COD)]₂ produces the neutral 14 e^- complex Rh(maloNHC)(COD) (3) in which coordinative unsaturation at the metal is relieved in the solid state by an uncommon labile bonding interaction between the C_ipso of one of the mesityl arms and the Rh center. Copyright

Full text of DOI:10.1021/ja804296t

Published on Web 08/05/2008
A Stable Anionic N-Heterocyclic Carbene and Its Zwitterionic Complexes
Vincent Ce´sar,* Noe¨l Lugan, and Guy Lavigne*
Laboratoire de Chimie de Coordination du CNRS, 205 route de Narbonne, 31077 Toulouse Cedex 04, France
Received June 13, 2008; E-mail: Vincent.Cesar@lcc-toulouse.fr; Guy.Lavigne@lcc-toulouse.fr
With very few exceptions, N-heterocyclic carbenes (NHCs)1 are
traditionally neutral “L” type ligands according to the Green
formalism. Though their functionalization by anionic noncoordi-
nating substituents has been achieved,2 the present account focuses
on the design of stable monodentate ligands whose main charac-
teristic will be the presence of a remote anionic functional group
within the heterocyclic backbone.
The anticipated benefit of such ligands in coordination chemistry
is that the anionic moiety pointing toward the outer coordination
sphere will not interfere with the metal’s cavity shape, controlled
Figure 1. Molecular structure of the lithium salt of the carbene 2a.Li+(thf)2
exclusively by the size of the ring and the nature of the nitrogen
revealing a trimeric annular association (ellipsoids drawn at 30% probability
level).
substituents. Roesler and co-workers recently reported the unique
archetype of such a family, consisting of a six-π-electron B2N2C2
framework, but the coordination chemistry of such a derivative
remains unexplored.3
Scheme 2
Direct reaction of monodentate anionic NHCs with transition
metal complexes is prone to produce neutral zwitterionic species
exhibiting valuable advantages relative to classical cationic com-
plexes.4 This concept was previously applied to N, P, and S ligands
containing tetraorganoborate, indenyl, or substituted boratabenzene
anions.5,6
While thinking of malonate as an alternate anionic function to
be installed within the heterocyclic framework, we reasoned that
pyrimidinium betaïnes (1) might constitute convenient precursors
for the intended anionic carbene since they can be prepared in high
yield with various substituents by a classical double peptide-type
coupling between a formamidine and a monosubstituted malonic
acid (Scheme 1, first equation).7
carbenic heterocycle is almost planar (dihedral angles: C11-N21-
C41-C31, 2.5°; C41-C31-C21-C11, 0.6°) and the averaged
angle NCN in the carbene 2a.Li+ (112.7°) is more acute than in
the corresponding betaïne 1a (122.1°) as always observed when
comparing geometrical parameters of NHCs and their NHC.H+
precursors.
In a first attempt to obtain a Rh(I) complex prototype, the carbene
2 was generated in situ and reacted with 0.5 equiv of [RhCl(1,5-
COD)]2 (Scheme 2). This readily produced the neutral 14e-
complex Rh(maloNHC)(COD) (3), which was isolated (3a, 73%
yield; 3b, 77% yield) and fully characterized both in solution and
in the solid state. Clearly, the anionic carbene 2 behaves as a halide
scavenger, and the negative charge of the resulting zwitterionic
Rh(I) species 3 is delocalized on the remote malonate-type ligand
backbone,10 in the outer-coordination sphere of the complex.
Interestingly, coordinative unsaturation at the metal is relieved
in the solid state by an uncommon labile bonding interaction
between the Cipso of one of the mesityl arms and the Rh center,
albeit with no observable structural change of metrical values within
the mesityl group. This must be a dynamic process in solution,
given that 1H NMR data revealed the occurrence of a symmetrical
structure, even at -80 °C. A closely related structural pattern was
previously identified in the case of the cationic NHC Rh(I) complex
Scheme 1
The new anionic, six-membered carbene 2 (abbreviated as
maloNHC) was generated cleanly and quantitatively by treatment
of 1 with nBuLi (or KHMDS) (Scheme 1, second equation). The
appearance of a highly deshielded 13C NMR (THF-d8) signal at δ
243.7 ppm for 2a.Li+ was consistent with its formulation as a
NHC.8,9 An X-ray structure analysis of 2a.Li+(thf)2 (Figure 1)
revealed the occurrence of a trimeric annular association where three
cationic Li(thf)2+ units serve as linkers between neighboring anionic
carbenic units through the formation of O-Li-O bridges. The
-
[Rh(NHC)(COD)]+PF6 involving the neutral NHC ligand 1,3-
dimesityl-3,4,5,6-tetrahydropyrimidin-2-ylidene.11
9
11286 J. AM. CHEM. SOC. 2008, 130, 11286–11287
10.1021/ja804296t CCC: $40.75
2008 American Chemical Society

C O M M U N I C A T I O N S
Scheme 3
(maloNHC)(CO)2 (5b) (Scheme 3, left reaction) which was fully
characterized (Figure 3). The observation of ν(CO) bands at 2038
and 1993 cm-1 for 5b tends to indicate that the anionic maloNHC
2b is a better donor than IMes or SIMes.
In view of the general synthetic approach disclosed here, it can
be reasonably anticipated that a much broader range of zwitterionic
complexes will now become accessible from our new ligand and
its congeners. Taking advantage of the modularity of the present
synthesis, we will now concentrate our efforts on the evaluation of
their scope in homogeneous catalysis.
In line with these observations, we were logically prompted to
validate the generality of the present approach by reacting 2 with
a series of simple basic transition-metal complexes.
Acknowledgment. Financial support by the CNRS is gratefully
acknowledged.
Beginning with silver, we found that 2b.K+ reacts cleanly with
the phosphine-stabilized precursor Ph3PAgOTf (Scheme 3) to
produce the zwitterionic complex (maloNHC)Ag(PPh3) 4b (84%
yield), characterized by an X-ray structure analysis (see Supporting
Information).
Supporting Information Available: Experimental details for the
preparation of the reported compounds and crystallographic data (CIF
files) for 1a, 2a.Li(THF)2+, 3a, 4b, and 5b. This material is available
free of charge via the Internet at http://pubs.acs.org.
In further work aimed at estimating the donor properties of 2 as
compared with those of a “normal” NHC by using CO stretching
frequencies as a probe,12 we were led to prepare a simple carbonyl
maloNHC complex having a known cationic NHC-complex equiva-
lent. As a standard reference, we chose the known cationic species
[CpFe(NHC)(CO)2]+I- previously prepared both with NHC ) IMes
(ν(CO): 2050, 2006 cm-1) and SIMes (ν(CO): 2049, 2005
cm-1).13,14 Our parallel reaction of CpFe(CO)2I with 2b.K+
generated in situ produced the zwitterionic equivalent CpFe-
References
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D. F.; Dautel, O.; Duong, H.; Helgeson, R.; Wudl, F. Angew. Chem., Int.
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Figure 2. Molecular structure of the zwitterionic complex 3a (ellipsoids
drawn at 50% probability level) revealing an uncommon Rh-C(ipso)
interaction. Selected bond distances (Å) and angles (deg): Rh1-C1,
2.036(2); Rh1-C21, 2.346(2); Rh1-C6, 2.091(2); Rh1-C13, 2.130(2);
Rh1-C9,2.248(2);Rh1-C10,2.230(2);C1-Rh1-C21,62.78(6);N1-C1-N2,
115.5 (1).
(9) The countercation brought by the base has a significant role since the lithium
salt of 2 is soluble in THF whereas the potassium salt precipitates
immediately.
(10) Structural evidence provided as Supporting Information.
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690, 4433. (b) Zhang, Y.; Wang, D.; Wurst, K.; Buchmeiser, M. R. J.
Organomet. Chem. 2005, 690, 5728.
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a neutral complex of type RhCl(NHC)(CO)2 is inoperative in our case.
For more details, see: (a) Fu¨rstner, A.; Alcarazo, M.; Krause, H.; Lehmann,
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(14) IMes and SIMes correspond respectively to 1,3-dimesitylimidazol-2-ylidene
and 1,3-dimesitylimidazolin-2-ylidene.
Figure 3. Molecular structure of complex 5b (ellipsoids drawn at 50%
probability level). Selected bond distances (Å) and angles (deg): Fe1-C1,
1.786 (1); Fe1-C2, 1.779(1); Fe1-C3, 2.041(1); N1-C3-N2, 113.42(8);
Fe1-C1-O1, 169.89(10); Fe1-C2-O2, 170.13(10).
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