An ambidentate Janus-type ligand system based on fused carbene and imidato functionalities

Article abstract of DOI:10.1002/chem.201102767

The new anionic heterocycle imidNHC (4,6-dioxo-1,3,5-triazin-2-ylidene-5- ide), combining a carbene and an anionic imidate, was readily prepared through a coupling reaction between 1,3-dimesityl formamidine and N-phenoxy carbonyl isocyanate followed by de

Full text of DOI:10.1002/chem.201102767

COMMUNICATION
DOI: 10.1002/chem.201102767
An Ambidentate Janus-Type Ligand System Based on Fused Carbene and
Imidato Functionalities
Nadia Vujkovic, Vincent Cꢀsar,* Noꢁl Lugan, and Guy Lavigne*[a]
Thanks to the extensive development of rational synthetic
routes to N-heterocyclic carbenes and related species,[1]
a
growing diversity of exciting new archetypes of persistent
carbenes are now becoming accessible. Of particular interest
are “functional” carbenes with their backbone being deco-
rated by chemically or electrochemically responsive entities
enabling a suitable control of the electronic donor proper-
ties of the carbene center, with observable consequences on
catalytic performances and/or product selectivity.[2,3] Where-
as most of these carbenes are designed to bind only one
metal center, several original ideas to devise ditopic NHC-
based ligands for organometallic polymer synthesis, or for
the design of multifunctional tandem catalysts, have recently
appeared (Scheme 1). These are illustrated, for example, by
heterobimetallic complexes of 1,2,4-triazolyl-3,5-diylidenes
(A),[4] benzobis(imidazolylidene)s (B),[5] 4-phosphinoimida-
zol-2-ylidenes (C)[6] and 1,3-imidazol-2,4-ylidenes (D)[7] as
well by the gold(I) complex E supported by a Fischer car-
bene-containing NHC (Scheme 1).[8]
Scheme 1. Representative examples of ambidentate bimetallic complexes
incorporating a ditopic NHC-based ligand.
In a logical continuation of our original work on the con-
struction of modular anionic and neutral NHCs incorporat-
ing a malonate backbone,[3f,9] which are receiving growing
attention,[10] we reasoned that, based on the isolobal analogy
Scheme 2. Relationship between maloNHC and imidNHC.
À
between “C R’ꢀ group and ”N’ꢀ atom, it would be of interest
to synthesize the N-heterocyclic carbene imidNHC
(Scheme 2) incorporating an imidato backbone (a biologi-
cally relevant functionality)[11] susceptible to act as a labile
donor group toward various transition metals.[12]
of Me3SiCl and N-phenoxycarbonyl isocyanate afforded
compound 2·H in 86% yield (Scheme 3).
The mesoionic precursor 1,3-dimesityl-s-triazine-4,6-dione
2·H was prepared in high yield through decisive modifica-
tions of a procedure which had been previously found to
give only poor yields of analytically impure samples of the
compound.[13] In our hands, treatment of 1,3-dimesitylforma-
midine with n-butyl lithium, followed by stepwise addition
The crucial role of Me3SiCl is to scavenge the released
phenoxide, which otherwise would add to the N-phenoxycar-
bonyl isocyanate substrate, thereby reducing the yield in
final product. The compound was fully characterized by var-
ious spectroscopic techniques,[14a] and its molecular structure
was elucidated by X-ray crystallography (Figure 1).[14b] Com-
pound 2·H can be described as a four p-electron formamidi-
[a] Dr. N. Vujkovic, Dr. V. Cꢁsar, Dr. N. Lugan, Dr. G. Lavigne
CNRS, LCC (laboratoire de chimie de coordination)
205 route de Narbonne, 31077 Toulouse Cedex 4 (France) and
Universitꢁ de Toulouse, UPS, INPT
31077 Toulouse (France)
E-mail: vincent.cesar@lcc-toulouse.fr
guy.lavigne@lcc-toulouse.fr
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201102767: Experimental proce-
dures and CIF files for the X-ray structure analyses of 2·H, 3H, 5 and
8.
Scheme 3. Synthesis of the precursor 2·H and of anionic NHC 2·K: i) 1)
nBuLi, THF, 258C, 20 min; 2) Me3SiCl, 08C, 2 min; 3) N-phenoxycarbon-
yl isocyanate, 0 to 258C, overnight. ii) KHMDS, THF, 08C, 20 min. Mes=
2,4,6-trimethylphenyl.
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Indeed, in situ generation of 2À in THF followed by addi-
tion of 0.5 equiv of [RhCl(1,5-cod)]2 led to abundant precipi-
AHCTUNGTRENNUNG
tation of a red species, which was readily and cleanly pro-
tonated by simple treatment with silicagel in dichlorome-
thane. Further purification by flash chromatography afford-
ACTHNUTRGNEUNG
ed the bright orange complex [RhCl(2H)(1,5-cod)] (3H),
which was fully characterized (Figure 2).[14b]
Very characteristically, its spectral and geometrical char-
acteristics appeared to be comparable with those previously
ascribed to a diamidocarbene complex, due to i) the pres-
ence of a highly deshielded 13C NMR resonance of the car-
bene (d=240.6 ppm),[9b,16] ii) the occurrence of relatively
Figure 1. Molecular structure of 2·H (ellipsoids drawn at 30% probability
level). Hydrogen atoms on the mesityl groups were omitted for clarity.
À
À
À
À
short N1 C2 and N2 C3 bonds (N1 C2: 1.405(3) ꢃ, N2
C3: 1.389(3) ꢃ), reflecting electron delocalization along the
À
À
À
Selected bond lengths [ꢃ]: C1 N1 1.3142(13), C1 N2 1.3210(14), N1 C3
À
À
À
1.4651(14), N2 C2 1.4538(14), C2 N3 1.3367(13), C3 N3 1.3420(15),
À
lateral amido groups NCO, and, iii) a very short Rh Ccarbene
À
À
C2 01 1.2178(15), C3 O2 1.2139(13).
nium unit linked to a six p-elec-
tron imidate unit with only neg-
ligible interactions between the
two systems, as indicated by the
magnitude of the bond lengths
À
À
N1 C3 (1.4651(14) ꢃ) and N2
C2 (1.4538(14) ꢃ) falling within
À
the range of single N Csp2
bonds.[15]
Treatment of 2·H with potas-
sium
bis(trimethylsilyl)amide
(KHMDS) in THF cleanly af-
forded the potassium salt of the
anionic NHC 2À. The 13C NMR
resonance of the carbene nu-
cleus was observed at
d
258.9 ppm, a value appearing
downfield relative to that found
in the corresponding maloNHC
carbene (d 243.7 ppm), thus re-
flecting a reduced donicity of
the carbene, with the imidate
nitrogen atom being more elec-
Figure 2. Molecular structure of 3H (ellipsoids drawn at 30% probability level). Hydrogen atoms on mesityl
À
À
and cyclooctadiene have been omitted for clarity. Selected bond lengths[ꢃ]: Rh1 C1 1.998(3), C1 N1
À
À
À
À
À
À
1.365(4), C1 N2 1.382(3), N1 C2 1.405(3), N2 C3 1.389(3), C2 N3 1.380(3), C3 N3 1.366(3), C2 01 1.198(3),
À
C3 O2 1.215(3). Selected bond angles [8]: C1-Rh1-Cl1 83.86(7), N1-C1-Rh1-Cl1 85.32.
bond length 1.998(3) ꢃ (the shortest reported so far within
tronegative than the malonic carbon atom of the maloNHC
derivative.[9a] Such differences were also substantiated in
terms of chemical reactivity (Scheme 4).
such a ligand family) typical of those found in diamidocar-
bene-Rhodium complexes.[9b,16] The electronic nature of 2H
was inferred from the IR spectra of the carbonyl-derivative
[RhCl(2H)(CO)2] (4H), obtained by simple carbonylation of
av
3H under ambient conditions. Its average nCO frequency of
2046 cmÀ1 is comparable to the value recorded for the ma-
av
loNHC-type
diamidocarbene
analogue
(nCO
=
2045 cmÀ1).[9b] Complex 4H could be readily deprotonated
with triethylamine to yield (4À)(HNEt3)+. Further addition
AHCTUNGTRENNUNG
of HCl led to clean regeneration of 4H, demonstrating that
the present metal/ligand system is switchable.[17] Infrared
data collected from 4À (nCOav =2036 cmÀ1) indicate that the
relevant anionic ligand 2À is much more electron-donating
than 2H, but less than the anionic maloNHC analogues
(nCOav =2028 cmÀ1).[3f]
Scheme 4. Generation of Rh complexes 3H, 4H and 4À. i) 1) KHMDS,
THF, 08C, 20 min; 2) [RhClACHTNUGTRNENUG(cod)]2, 258C, 30 min; 3) SiO2, CH2Cl2. ii)
CO, CH2Cl2, 15 min. iii) NEt3, CH2Cl2, 258C. iv) HCl in Et2O, CH2Cl2,
258C.
The ambidentate nature of 2À was further established
through the directed formation of polymetallic silver and
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Chem. Eur. J. 2011, 17, 13151 – 13155

An Ambidentate Janus-Type Ligand System
COMMUNICATION
gold complexes. Due to the availability of two potentially
coordinating sites in the NHC, two strategies could be envi-
sioned, starting either from 2À or 2·H, and thus determining
the order in which the donor groups were to be connected
to a metal center. Various tries led us to adopt the more
convenient procedure, where the ligand 2À is first selectively
bound to a first transition metal through the carbene center,
thus giving a starting complex that is subsequently used as
an “imidato” ligand for a second metal center. As shown in
Scheme 5, treatment of 2·H with one equivalent of KHMDS
the four p-electron diaminocarbene unit of the ligand 2À.
À
This is reflected in the bond lengths N1 C2 (1.460(4) ꢃ)
À
and N2 C3 (1.466(5) ꢃ) falling within the range of single
À
N C bonds. By contrast, as soon as ligand 2 is bridging two
metal centers such as in the trimetallic silver(I) complex 8,
À
À
the same bonds slightly shorten (N1 C3: 1.430(3) ꢃ, N2
À
À
C2: 1.424(3) ꢃ, N4 C5: 1.430(3) ꢃ, N5 C6: 1.430(3) ꢃ), re-
flecting an electronic situation intermediate between that of
an anionic diaminocarbene and a diamidocarbene.[3f,9b,10]
Such a behavior is also clearly illustrated by the low-field
drift of the 13C NMR resonance of the carbenic carbon in
the gold complexes series (d=210.3 ppm in 5, d=215.6 ppm
in 6 and d=218.2 ppm in 5H).
followed by addition of AuCl
selectively gave the zwitterionic heteroleptic complex
[Au(2)
(PPh3)] (5) in excellent yields (96% yield).[18]
ACHTUNGTNER(NUNG tht) and triphenylphosphine
ACHTUNGTRENNUNG
Scheme 5. Formation of polymetallic complexes 6–8. i) 1) KHMDS, THF,
08C, 20 min; 2) AuCl(tht), PPh3, RT, 6 h; ii) HBF4·OEt2, CH2Cl2, 08C,
10 min; iii) for 6: [(PPh3)Au(CH3CN)](BF4), for 7: AgBF4, PPh3, CH2Cl2,
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
RT, 1 h. iv) 1) KHMDS, THF, 08C, 15 min; 2) AgOTf (0.5 equiv), RT,
1.5 h; 3) PPh3 (2 equiv), AgOTf (1 equiv), RT, 3 h. tht=tetrahydrothio-
furane.
Its subsequent treatment with a second cationic metallic
fragment gave in good yields the stacked bimetallic com-
CHTUNGTRENNUNG CAHTUNTGREN(NUGN PPh3); 7: MLn =Ag-
+A(BF4)À (6: MLn =Au
Figure 3. Molecular structure of the zwitterionic complex 5 (ellipsoids
drawn at 30% probability level). Hydrogen atoms have been omitted for
plexes [LnM(5)]
(PPh3)2, which were both fully characterized. The playful
ACHTUNGTRENNUNG
À
À
clarity. Selected bond lengths [ꢃ]: Au1 C1 2.049(3), Au1 P1 2.2811(7),
possibility to construct even more elaborated polymetallic
complexes in a one-pot reaction is nicely illustrated here by
À
À
À
À
À
C1 N1 1.332(4), C1 N2 1.324(4), N1 C2 1.460(4), N2 C3 1.466(5), C2
À
À
À
N3 1.343(5), C3 N3 1.348(6), C2 01 1.215(4), C3 O2 1.208(5). Selected
bond angle [8]: C1-Au1-P1 173.29(8).
the preparation of the trimetallic silver complex
(Scheme 5, reaction iv).
8
Here, the central unit [Ag(2)2]À where the cationic silver
atom is encapsulated between two molecules of the anionic
carbene ligand 2À both coordinated through their carbene
center, was selectively obtained by simply adjusting the re-
quired metal/ligand stoichiometry, whereas the remote silver
centers were further introduced to obtain a closed-shell spe-
cies. The syntheses of these complexes nicely illustrate some
of the multiple possibilities of this new ligand in coordina-
tion chemistry.
Interestingly, a close examination of the molecular struc-
tures of complexes 5 (Figure 3)[14b] and 8 (Figure 4)[14b] re-
veals that the electronic distribution within the carbenic het-
erocycle 2 is subject to a redistribution apparently governed
by the substitution patterns. Very characteristically, in the
zwitterionic gold(I) complex 5, there is no apparent commu-
nication between the anionic six p-electron imidate unit and
In conclusion, we have disclosed here a new type of ambi-
dentate Janus-type ligand combining two different donor
functionalities—a carbene and an anionic imidate—within
the same heterocyclic framework. This anionic carbene, rep-
resenting a synthetic challenge, is yet available in high yield
through a simple and highly efficient original method. It ap-
pears to be suitable for the directed construction of a variety
of homo-and/or hetero-polymetallic complexes in a stepwise
approach. The present work also reveals that, as a polyfunc-
tional carbene is engaged in bonding with several metal
atoms, its electronic structure is no longer a purely intrinsic
ligand property, but is becoming subject to the influence of
the metal atoms, these tending to shepherd mobile electrons
between the two parts of the heterocycle. Further synthetic
possibilities and applications of this fascinating new carbenic
Chem. Eur. J. 2011, 17, 13151 – 13155
ꢂ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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13153

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À
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À
À
À
À
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À
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Keywords: ambident compounds
organometallic compounds · polymetallic compounds
· carbenes · gold ·
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An Ambidentate Janus-Type Ligand System
COMMUNICATION
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Received: September 5, 2011
Published online: October 13, 2011
Chem. Eur. J. 2011, 17, 13151 – 13155
ꢂ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
13155