First N-Heterocyclic Carbenes Relying on the Triazolone Structural Motif: Syntheses, Modifications and Reactivity

Article abstract of DOI:10.1002/chem.201502685

4-Phenylsemicarbazide and 1,5-diphenylcarbazide are suitable starting materials for the syntheses of N-heterocyclic carbene (NHC) compounds with new backbone structures. In the first case, cyclisation and subsequent methylation leads to a cationic precurs

Full text of DOI:10.1002/chem.201502685

DOI: 10.1002/chem.201502685
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Organometallic Chemistry |Hot Paper|
First N-Heterocyclic Carbenes Relying on the Triazolone Structural
Motif: Syntheses, Modifications and Reactivity
Markus Jonek, Janina Diekmann, and Christian Ganter*[a]
Abstract: 4-Phenylsemicarbazide and 1,5-diphenylcarbazide
are suitable starting materials for the syntheses of N-hetero-
cyclic carbene (NHC) compounds with new backbone struc-
tures. In the first case, cyclisation and subsequent methyla-
tion leads to a cationic precursor whose deprotonation af-
fords the triazolon-ylidene 2, which was converted to the
corresponding sulfur and selenium adducts and a range of
metal complexes. In contrast, cyclisation of diphenylcarba-
zide affords a neutral betain-type NHC-precursor 7, which is
not in equilibrium with its carbene tautomer 7a. Precursor 7
can either be deprotonated to give the anionic NHC 8 or
methylated at the N or O atom of the backbone resulting in
two isomeric cationic species 16 and 20. Deprotonation of
the latter two provides neutral NHC compounds with a car-
boxamide or carboximidate backbone, respectively. The
ligand properties of the new NHC compounds were evaluat-
ed by IR and 77Se NMR spectroscopy. Tolman electronic pa-
rameter (TEP) values range from 2050 to 2063 cmÀ1 with the
anionic NHC 8 being the best overall donor.
Introduction
a commercially available triazole-based zwitterionic compound
bearing an amido group at C3, provides access to carbene C
by a tautomeric proton shift. Likewise, the carbon analogue of
Nitron, D’, reported by CØsar, Lavigne et al.,[10] also represents
a “crypto-NHC”, as it features the reactivity of its carbene tau-
tomer. Triazolylidenes are particularly successful as organocata-
lysts,[11] while when coordinated as ligands to metal fragments
they are less donating than structurally related imidazolyliden-
es.[2e]
Commencing with the first isolated derivative reported by Ar-
duengo and co-workers in 1991,[1] most of the stable N-hetero-
cyclic carbenes (NHCs) are still based on the unsaturated imi-
dazole-2-ylidene.[2] However, in the meantime a variety of relat-
ed NHCs have been prepared, ranging from three-membered
up to seven-membered heterocycles. Triazole-based NHCs
have also attracted considerable attention (Figure 1). The first
In continuation of our previous work directed towards the
syntheses and reactivity studies of amido[12] or mixed amino-
amido[13] NHCs based on the imidazoline or triazine framework,
we became interested in new backbone structures. We won-
dered if commercially available 4-phenylsemicarbazide and 1,5-
diphenylcarbazide could be suitable starting materials for the
preparation of new amido-functionalized triazole-based NHCs.
Our first results concerning these investigations are reported
herein.
Figure 1. Selected examples of carbenes or crypto-carbenes relevant to the
present study.
triazolylidene A was synthesized by Enders et al. in 1995[3] and
a number of differently modified 1,2,4-triazolylidenes have sub- Results and Discussion
sequently been reported.[4] Biscarbenes relying on the 1,2,4-tri-
Semicarbazide-based NHCs
azole core have been reported by the groups of Bertrand[5]
and Peris.[6] 4-Amino-substituted triazolylidenes B were investi-
gated by Schottenberger and co-workers[7] and Lassaletta,[8]
while Siemeling and co-workers[9] demonstrated that Nitron,
For the synthesis of triazolonium salt 1·BF4, 4-phenylsemicarba-
zide was cyclized with formamidinium acetate in DMF accord-
ing to a procedure recently reported by Zhang and Yao.[14] The
resulting neutral triazole was alkylated in two steps; first it was
treated with iodomethane in the presence of potassium car-
bonate to give the methylated amide backbone intermedi-
ate,[15] which was then further methylated using trimethyloxo-
nium tetrafluoroborate to afford the amidinium cation (1·BF4)
in 74% yield (Scheme 1). 1·BF4 was fully characterized by mass
[a] M. Jonek, J. Diekmann, Prof. Dr. C. Ganter
Institut für Anorganische und Strukturchemie
Heinrich-Heine Universität Düsseldorf
Universitätsstrasse 1, 40225 Düsseldorf (Germany)
E-mail: christian.ganter@uni-duesseldorf.de
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201502685.
1
spectrometry, elemental analysis, H and 13C{1H} NMR spectros-
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A variety of metal complexes of NHC 2 could be obtained
starting from the triazolonium precursor 1. For example, reflux-
ing a THF solution of 1·BF4 with nickelocene in the presence of
sodium iodide afforded the nickel half-sandwich complex 6 in
51% yield. In this reaction, one of the cyclopentadienyl ligands
of the nickelocene acts as a base for precursor deprotonation
and sodium iodide is added to saturate the coordination
sphere of the nickel atom. The molecular structure of complex
6 is depicted in Figure 3. The geometrical parameters are
within the range typically observed for CpNi half-sandwich
complexes with other NHC ligands.[17]
Scheme 1. Preparation of triazolonium salt 1·BF4.
4
copy. Notably, a JHH coupling between the amidinium proton
and the methyl group was observed as evidenced by the ap-
pearance of a quartet and a doublet, respectively, in the
4
1H NMR spectrum with JHH =0.9 Hz.
Deprotonation of precursor 1·BF4 with bases such as sodium
bis(trimethylsilyl)amide (NaHMDS) or potassium tert-butoxide
generated in situ the carbene 2. While all attempts to isolate
the free carbene failed, its intermediate existence was proven
by trapping reactions with sulfur and selenium leading to thio-
urea 3a[16] and selenide 3b in 70 and 46% yield, respectively
(Scheme 2). Slow diffusion of n-hexane into a dichloromethane
solution of 3a led to crystals suitable for X-ray diffraction stud-
Figure 3. Molecular structure of complex 6 in the solid state. Thermal ellip-
soids are drawn at the 35% probability level. Selected interatomic distances
[Š] and bond angles [8]: Ni1ÀI1 2.5072(6), Ni1ÀC2 1.8539(17), N1ÀC2
1.325(2), N3ÀC2 1.371(2); N1-C2-N3 104.6(2).
Furthermore, precursor 1 was also deprotonated with potas-
sium tert-butoxide in the presence of [{M(cod)Cl}2] to yield the
corresponding rhodium and iridium complexes [M(cod)(2)Cl]
(M=Rh, Ir) 4a,b. A convenient measure for the overall ligand
properties of NHCs is the Tolman electronic parameter (TEP),
which can be easily obtained by IR spectroscopy of carbonyl
complexes containing the respective ligands. Therefore, di-
chloromethane solutions of the cod complexes were stirred
while a slow stream of carbon monoxide was bubbled through
the solutions for several minutes resulting in a complete cod
! CO exchange. The IR spectra of both dicarbonyl derivatives
(5a,b) feature the typical symmetrical and anti-symmetrical CO
stretching vibrations, which were converted using well estab-
lished relations[2c,18] to a TEP value of 2060 cmÀ1 for ligand 2,
which is slightly higher than for other triazole-based
NHCs.[2e,6,9,19] A specific measure of the p-acceptor character of
an NHC is provided by the chemical shift of its phosphinidene
or selenium adduct in the 31P or 77Se NMR spectrum, respec-
tively.[20,21] The 77Se NMR spectrum of the corresponding sele-
Scheme 2. Deprotonation of the triazolonium salt 1·BF4.
ies. The molecular structure is depicted in Figure 2. As expect-
ed, the carbon–oxygen bond (121.4(2) pm) is much shorter
than the carbon–sulfur bond (165.8(2) pm). Both distances fall
in the range usually observed for related compounds. The het-
erocycle is essentially flat and forms an interplanar angle of
66.6(1)8 with the phenyl ring plane.
nide 3b was recorded and revealed
a resonance at
d=137 ppm, suggesting that the p-acidity of 2 is comparable
to that of the saturated five-membered ring NHC 1,3-di-(mesi-
tyl)imidiazolidin-2-ylidene (SIMes, d=116 ppm). However, tria-
zolylidene 2 is significantly less p-acidic than the five-mem-
bered ring monoamido carbene 1,3-di(mesityl)imidiazolidin-4-
one-2-ylidene (E) with d(77Se)=295 ppm.[22] In this compound,
the carbonyl group withdraws electron density only from the
N atom adjacent to the carbene-C atom, while in the triazole
Figure 2. Molecular structure of 3a in the solid state. Thermal ellipsoids are
drawn at the 35% probability level. Selected interatomic distances [Š] and
bond angles [8]: S1ÀC2 1.6578(18), O1ÀC1 1.214(2), N2ÀC1 1.361(2), N1ÀC2
1.326(2), N2ÀN1 1.392(2), N3ÀC2 1.382(2), N3ÀC1 1.382(2); N1-C2-N3
104.97(14), N2-C1-N3 104.18(14).
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derivative 2 the electron-withdrawing effect of the carbonyl
group is attenuated by the presence of the additional N atom
remote to the carbene-C atom. In addition to being less p-
acidic than E, the slightly higher TEP value of 2 (2060 cmÀ1) in
comparison to that of E (2058 cmÀ1) suggests that 2 should
also be a weaker s-donor.
Replacement of the cod ligand in 4b by two CO molecules
leads to the typical geometrical features of a dicarbonyl com-
plex in 5b. While the cod ligand renders the metal fragment in
4b electron rich and capable of significant back bonding to
the NHC, the strongly p-acidic CO group diminishes the p-
component in the NHC–Ir bond in complex 5b. Thus, a longer
C2ÀIr1 bond (206.6(1) pm) is observed in 5b, compared with
that in the cod derivative 4b (202.8(3) pm). Again, chloride is
the better donor than the NHC, leading to increased back-
bonding to the trans-CO, and as a result the Ir1ÀC11 bond is
shorter (183.4(2) pm) than the Ir1ÀC12 bond trans to the NHC
(189.0(1) pm). The analogous Rh compound 5a is isostructural
to complex 5b with only marginally different geometrical pa-
rameters.
The molecular structures of the cod–iridium complex 4b
and the dicarbonyl complexes 5a and 5b were determined by
X-ray diffraction and their perspective drawings are depicted in
Figure 4.
Carbazide-based NHCs
In the presence of acetic acid, 1,5-diphenylcarbazide under-
goes a cyclisation reaction in refluxing triethyl orthoformate,
leading to the formation of betaine 7 in 71% isolated yield
under concomitant release of three equivalents of ethanol
(Scheme 3). The five-membered ring is formed exclusively, in
which one amide-nitrogen atom and one aniline-nitrogen
Scheme 3. Formation of betaine 7.
atom are connected to the CH fragment delivered by the or-
thoformate. Betaine 7 is an air-stable white solid, which was
characterized by X-ray diffraction, and various spectroscopic
and analytical methods; its molecular structure is depicted in
Figure 5. Betaine 7 features a delocalized negative charge in
Figure 4. Molecular structures of Ir complexes 4b (top) and 5b (bottom) in
the solid state. Thermal ellipsoids are drawn at the 35% probability level. H
atoms have been omitted for clarity. Selected interatomic distances [Š] and
bond angles [8]: 4b: Ir1ÀC2 2.028(3), Ir1ÀC11 2.114(3), Ir1ÀC12 2.116(3), Ir1À
C16 2.178(3), Ir1ÀC15 2.200(3), Ir1ÀCl1 2.3696(8), C1ÀO1 1.227(4), N1ÀC2
1.322(4), C2ÀN3 1.374(4), C1ÀN2 1.359(4); N1-C2-N3 105.5(3); 5b: Ir1ÀC11
1.8336(16), Ir1ÀC12 1.8896(13), Ir1ÀC2 2.0657(11), Ir1ÀCl1 2.3591(5), O1ÀC1
1.2181(15), N1ÀC2 1.3271(16), C2ÀN3 1.3682(15), O2ÀC11 1.145(2), O3ÀC12
1.1364(17); N1-C2-N3 105.28(10).
All structures feature slightly distorted square–planar metal
environments. The NHC ligand is oriented almost perpendicu-
larly to the coordination plane with interplanar angles in the
range of 80–868. As is usually observed, the geometric param-
eters regarding the bonding of the two olefin moieties in the
cod complex 4b are quite different: the IrÀC distances to the
C15ÀC16 double bond trans to the NHC are significantly
longer (av. 219 pm) than those trans to the chloride ligand
(C11ÀC12, av. 212 pm), indicating that the latter enforces stron-
ger backbonding to the trans-olefin than the NHC. Accordingly,
the C11ÀC12 double bond appears longer (142.3(4) pm) than
C15ÀC16 (138.8(4) pm). Compound 4b crystallizes with two in-
dependent molecules in the asymmetric unit that do not differ
significantly regarding their geometrical data. The structural
parameters of complex 4b agree well with those reported for
closely related complexes.[9,19,23]
Figure 5. Molecular structure of betaine 7 in the solid state. Thermal ellip-
soids are drawn at the 30%. probability level. The co-crystallized DMSO mol-
ecule has been omitted for clarity. Selected interatomic distances [Š] and
bond angles [8]: N1ÀC2 1.3169(15), N1ÀN2 1.3893(13), C1ÀO1 1.2443(13),
C1ÀN2 1.3450(14), C1ÀN3 1.4195(15), C2ÀN3 1.3386(14), N4ÀN3 1.3947(13),
N1-C2-N3 106.09(10).
the amide backbone and a delocalized positive charge in the
formamidinium fragment (Scheme 3).
In the 1H NMR spectrum of 7, the signals for C2ÀH and
N4ÀH were detected at d=10.2 ppm and 9.3 ppm, respective-
ly. We found no indication that betaine 7 is in a tautomeric
equilibrium with its neutral carbene tautomer 7a, as no reac-
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tion occurred with 7 in the presence of sulfur, selenium or
carbon disulfide. This is in marked contrast to Siemeling’s
nitron[9] and the mesoionic 4-amido-imidazolium cation report-
ed by CØsar and Lavigne,[10] which were both found to exhibit
the reactivity of the corresponding free carbenes. The meso-
ionic structures were shown to be thermodynamically more
stable, but still served as sources for the carbenes in low con-
centration. Probably, in the case of 7, due to the delocalization
of the negative charge the basicity is not sufficient to allow for
the existence of the neutral carbene tautomer even in trace
amounts. However, deprotonation at C2 is easily accomplished
by addition of an external base such as KOtBu or NaHMDS
yielding the negatively charged NHC 8 with potassium or
sodium as a counter ion depending on the base used
(Scheme 4). In the presence of carbon disulfide or selenium,
with the dithiocarboxylate of C (d=219.9 ppm).[9] In contrast
to other dithiocarboxylates, compound 9 shows only a limited
stability,[24] which is already apparent from the ESI mass spec-
trum; besides a peak for 9 centered at m/z 327, a peak for 8 at
m/z 251 is observed as well. The weak carbene–CS2 bond is
also reflected by the reactivity of 9, which can serve as
a source of free NHC. Thus, refluxing of 9 in the presence of
sulfur provided the corresponding thiourea 10 in 76% yield.
Unfortunately, apart from the reaction with sulfur, all other at-
tempts to use 9 as a source for 8 met with failure.
Next, we turned our attention to the coordination chemistry
of 8. Deprotonation of 7 with potassium tert-butoxide in the
presence of [{Rh(cod)Cl}2] afforded the neutral rhodium com-
plex 13 in 64% yield under elimination of potassium chloride.
Initially, we assumed that the anionic ligand would coordinate
to the Rh atom through the carbene-C atom, analogous to the
behaviour of another anionic NHC reported by CØsar and Lav-
igne.[25] However, NMR investigations revealed that the coordi-
nation had instead occurred in a N,O-chelating manner, where-
as the formamidinium fragment was still present, featuring
1
a resonance at d=8.68 ppm in the H NMR spectrum. Accord-
ingly, the corresponding formamidinium carbon resonance ap-
peared as a doublet at d=124.1 ppm in the proton-coupled
1
13C NMR spectrum with a JCH coupling constant of 218 Hz. The
coordination of the hard N- and O-atoms in a chelating
manner leading to a square–planar (cod)Rh complex is pre-
ferred compared to the monodentate coordination through
the soft C atom of the carbene tautomer. This observation
leads to the question whether betaine 7 might initially be de-
protonated at the secondary NH function before it eventually
undergoes a tautomeric proton shift leading to 8. Such a be-
haviour was observed for the mesoionic 4-amido-imidazolium
cation reported by CØsar and Lavigne.[10] Further investigations
concerning this question are currently in progress.
The above-mentioned tautomerism was also observed when
one equivalent of PPh3 was added to the N,O-chelate complex
13, which resulted in the formation of the neutral rhodium car-
bene complex 14 under concomitant proton shift. The coordi-
nation of the phosphine was confirmed by 31P{1H} NMR spec-
troscopy, where a doublet was detected at d=24 ppm with
Scheme 4. Reactions of the anionic NHC 8.
the in situ generated carbene 8 reacted cleanly to give the di-
thiocarboxylate 9 or the selenide 11. A high-field signal at d=
91 ppm was recorded in the 77Se NMR spectrum of compound
11, which indicated that compared to other NHCs,[21] carbene 8
has only a weak acceptor character, in accord with its negative-
ly charged backbone (compare selenide 3b at d=137 ppm,
vide supra). Interestingly, treatment of 11 with methyl iodide
resulted in the selective alkylation at the selenium atom to
afford the neutral mesoionic selenoether 12. The selenium sat-
a
1JPRh coupling constant of 157 Hz. Furthermore, the
13C{1H} NMR spectrum delivered a doublet of doublets for the
1
carbene atom due to a JCRh coupling constant of 47 Hz in
2
combination with a JCP coupling constant of 12 Hz. Likewise,
addition of NBu4Cl to the cod–complex 13 was sufficient to
induce the tautomeric rearrangement of the ligand, leading to
the anionic complex [(8)Rh(cod)Cl]À. While this species could
not be isolated in a pure form, its 13C{1H} NMR spectrum fea-
tured a doublet for the Rh-bound carbene-C atom with a typi-
1
1
ellites in the H NMR spectrum of 12 (2JHSe =12 Hz) prove that
cal JCRh coupling constant of 47 Hz.
the alkylation occurred at selenium and not at the nitrogen or
oxygen sites in the backbone. The selenium methylation result-
ed in a downfield shift of 90 ppm and a resonance at d=
181 ppm was detected in the 77Se NMR spectrum of selenoeth-
er 12. The identity of the CS2-adduct 9 was deduced by mass
The corresponding dicarbonyl complex [(8)RhCl(CO)2]À (15)
was generated in situ from the cod complex 13 by treatment
with CO in CH2Cl2 solution and in the presence of NBu4Cl as
a chloride source. The chloride ion coordinates to the rhodium
and initiates the internal amide-to-carbene rearrangement re-
action. Unfortunately, complex 15 could not be isolated in
a pure form but was characterized by two strong CO stretch-
1
spectrometry, H and 13C{1H} NMR spectroscopy. The signal for
the N2-C-CS2 atom is located at d=217.2 ppm, comparable
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ing bands in the IR spectrum, leading to a TEP value of
2050 cmÀ1.
Both cationic species could be successfully converted to the
respective neutral NHCs by deprotonation with sterically de-
manding strong bases (Scheme 5). Thus, treatment of the O-
methylated derivative 16 with NaHMDS in the presence of se-
lenium afforded the neutral selenide 17 which was purified by
column chromatography with diethyl ether as eluent. The
compound has a very good solubility in solvents of different
polarity from methanol to n-hexane. Its resonance at d=
119 ppm in the 77Se NMR spectrum is intermediate between
those of the neutral 3b (d=137 ppm) and the anionic deriva-
tive 11 (d=91 ppm). Thus, carbene 8 is the weakest
p-acceptor due to the anionic nature of the back-
bone. On the other hand, O-methylation of the
anionic backbone to afford the neutral carboximidate
unit in 17 enhances the acceptor character, while it is
even more pronounced in derivative 3b with the car-
boxamide functional group. Unfortunately, the corre-
sponding Se-adduct of the N-methylated precursor
20 could not be prepared.
Due to the delocalized negative charge on the backbone of
7, the nitrogen and the oxygen atoms are potential sites for
attack by electrophiles. Both positions could be addressed de-
pending on the reaction conditions leading to positively
charged NHC precursors, which were subsequently converted
to the respective NHCs (Scheme 5). Under relatively mild con-
ditions (oil bath, reflux) and with the weak alkylating agent
methyl iodide, the nitrogen atom was alkylated to afford the
In order to gauge the overall ligand properties of
the NHCs obtained from the isomeric precursors 16
and 20, the Rh(CO)2 complexes 19 and 22 were pre-
pared by routine procedures, involving deprotona-
tion of the cations in the presence of [{Rh(cod)Cl}2] at
low temperature. The (cod)Rh complexes 18 and 21
thus obtained were then exposed to CO in dichloro-
methane solution, and IR spectra were recorded for
the resulting carbonyl derivatives 19 and 22. TEP
values of 2058 and 2063 cmÀ1 were calculated for the
NHCs 16ÀH+ and 20ÀH+, respectively. Thus, the rel-
ative order of p-acceptor strength suggested above
Scheme 5. Selective alkylation of the anionic backbone in betaine 7.
triazolonium salt 20·I (20·BF4 after anion exchange) with an
amide backbone. On the other hand, the oxygen atom was se-
lectively methylated to give the methoxytriazolium salt 16·OTf
with a carboximidate backbone under more forcing conditions
(MW, 1008C) and with the more powerful alkylating agent
methyl trifluoromethansulfonate. The reaction with other elec-
trophiles like trimethylsilyl chloride was unsuccessful. Both tria-
zolium salts were characterized by spectroscopic and analytical
methods and the molecular structures were elucidated for
both compounds by X-ray diffraction analysis (Figure 6 and
Figure 7). The most significant structural differences are found
in the backbones and are in accord with the chemical intuition
(Table 1): a long N2ÀC1 and a short C1ÀO1 bond length is
found for the urea fragment in the NÀMe compound 20, while
the opposite is true for the OÀMe derivative 16 with a C1ÀN2
double and a C1ÀO1 single bond, respectively. Intermediate
values for 7 nicely reflect the delocalized nature of the betaine
compound 7. Analogously, the C2ÀN distances within the ami-
dinium moieties in compounds 7, 16 and 20 are found within
a narrow range of 131 and 134 pm, intermediate between the
typical values for single and double bonds. In all three struc-
tures the C2ÀN1 bond is consistently shorter than the C2ÀN3
bond. Analogous structural trends have been observed by Lav-
igne and CØsar for a pair of compounds related to 7 and 16
based on a 4-amino/amido-substituted imidazolium cation.[10]
Figure 6. Molecular structure of triazolium salt 16·OTf in the solid state.
Thermal ellipsoids are drawn at the 35% probability level. The anion has
been omitted for clarity. Selected geometrical parameters are compiled in
Table 1.
Figure 7. Molecular structure of triazolonium salt 20·I in the solid state. Ther-
mal ellipsoids are drawn at the 35% probability level. The anion has been
omitted for clarity. Selected geometrical parameters are compiled in Table 1.
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Conclusion
Table 1. Selected interatomic distances and bond angles of 7, 16 and 20.
A series of new NHCs with O-functionalized triazole
backbones could be prepared from cheap starting
materials. 4-Phenylsemicarbazide was cleanly con-
verted to the neutral triazolon-ylidene 2, which could
be incorporated into metal complexes of Ni, Rh and
Ir. Its ligand properties as determined by the TEP
value of 2060 cmÀ1 are in accord with the presence
of the electron-withdrawing amido group in the
backbone. On the other hand, 1,5-diphenylcarbazide
turned out to be an even more versatile starting ma-
terial. Its cyclisation afforded betaine 7, in which a cat-
ionic amidinium fragment is connected to an anionic
imidate moiety. The betaine is however not in equilibrium with
its carbene tautomer 7a. Deprotonation of betaine 7 and sub-
sequent treatment with electrophilic trapping reagents leads
to derivatives of the corresponding anionic NHC 8 or to the ex-
ceptional formation of the anionic N,O-coordinated chelate
complex 13. Furthermore, betaine 7 can be selectively methy-
lated at the N- or O-atom of the backbone depending on the
reaction conditions. The result-
N2ÀC1 [Š]
C1ÀO1 [Š]
N1ÀC2 [Š]
N3ÀC2 [Š]
N3ÀC1 [Š]
N3-C2-N1 [8]
1.362(3)
1.204(3)
1.307(3)
1.331(3)
1.415(3)
108.3(2)
1.3450(14)
1.2443(13)
1.3169(15)
1.3386(14)
1.4195(15)
106.1(1)
1.294(4)
1.316(3)
1.311(3)
1.344(3)
1.380(3)
107.4(2)
on the basis of the 77Se NMR data is well reflected by the over-
all ligand properties as indicated by the TEP values. An analo-
gous sequence of values was established by CØsar and Lavigne
for a related series of compounds (see Figure 8 for a compila-
tion of relevant data).[22]
Interestingly, while a CD2Cl2 solution of 16·OTf showed no
changes in the 1H NMR spectrum within a couple of days,
ing cationic species 16 and 20
allow access to the isomeric neu-
tral NHCs with an iminoether or
carboxamide function, respec-
tively. The range of TEP values
obtained from the (CO)2ClRh–
NHC complexes indicates that
the anionic NHC 8 is the best
overall donor in this series of li-
gands (2050 cmÀ1), while the do-
nating ability is significantly di-
minished for the OÀMe deriva-
tive (2058 cmÀ1) and even more
Figure 8. Comparison of TEP values of the compounds synthesized with those reported in the literature.
attenuated for the NÀMe func-
tionalized NHC (2063 cmÀ1).
a new signal set started to appear overnight when the com-
pound was dissolved in [D6]DMSO. The spectra indicated that
the newly formed species was the betaine 7 formed by formal
loss of a methyl cation from 16, which was subsequently trans-
ferred to the nucleophilic DMSO molecule resulting in the for-
mation of the well-known cation Me(CD3)2SO+. A related slow
demethylation was also observed by Hudnall and co-workers
for their amino–acrylamido carbene precursor.[26] Thus, the tria-
zolium salt 16 is obviously a good electrophilic methylating
agent. To confirm this, 16 was treated with a series of N-nucle-
ophiles such as triethylamine, methylimidazole, pyridine and
DABCO, which were all cleanly converted to the corresponding
N-methylated cationic derivatives (dicationic in the case of
DABCO) while 16 was converted back to 7. However, we never
observed a methyl transfer from O to the N-atom resulting in
the formation of cation 20. Furthermore, it should be noted
that this N-methyl derivative 20 did not show any alkylating
reactivity at all.
Thus, by simple chemical modifications of the basic structural
motif the electronic properties of the corresponding NHCs can
be varied considerably, leading to a spread of TEP values of 13
wavenumbers.
Experimental Section
General information: All reactions were performed in an oxygen-
free, dry nitrogen atmosphere using standard Schlenk techniques.
The microwave-assisted reactions were carried out in a CEM-Dis-
cover-System microwave. Diethyl ether and THF were dried and
distilled over sodium/benzophenone, dichloromethane over CaH2
and n-hexane over sodium. The NMR spectra were recorded on
a Bruker Avance DRX 200, a Bruker Avance DRX 500, a Bruker
1
Avance III 300 or a Bruker Avance III 600. All H and 13C{1H} NMR
spectra are referenced to the chemical shifts of residual undeuter-
ated solvent signals. 77Se NMR spectra were referenced to external
KSeCN in D2O with specific concentration and a chemical shift of
d=À316.5 ppm (4.0 molLÀ1) or À329.0 ppm (0.25 molLÀ1). Mass
spectra were recorded on a UHR-QTOF maxis 4G (HR-ESI), a GC/
MS-System Finnigan Trace DSQ, a MALDI-TOF Ultraflex or a Triple-
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Quadrupol-mass spectrometer TSQ 7000 (EI-MS). Elemental analy-
ses were recorded on an Elementar vario Micro cube. X-ray crystal
structure data were collected on a Bruker Apex Duo. IR spectra
were obtained with a Shimadzu IR Affinity-1 spectrometer. Re-
agents such as potassium tert-butoxide and NaHMDS (2m in THF)
were purchased from Acros Organics or Sigma Aldrich and used as
received. [{Rh(cod)Cl}2] and [{Ir(cod)Cl}2] were synthesized accord-
ing to a literature procedure.[27]
methane as eluent. In the case of the iridium complex 4b, the
crude product was dissolved in dichloromethane and filtered over
Celite. The clear solution was reduced in vacuo and precipitated by
addition of n-hexane. The resulting yellow solid was washed a few
times with n-hexane and dried in vacuo.
Compound 4a: Deprotonation of 1·BF4 (114 mg, 0.41 mmol) with
potassium tert-butoxide (53 mg, 0.47 mmol) in the presence of
[{Rh(cod)Cl}2] (100 mg, 0.20 mmol) led to 4a (50%, 85 mg,
0.20 mmol) as a yellow solid. 1H NMR (200 MHz, CDCl3): d=8.10–
8.03 (m, 2H; PhÀH), 7.61–7.41 (m, 3H; PhÀH), 5.20–5.07 (m, 1H;
codolef.), 5.06–4.93 (m, 1H; codolef.), 4.21 (s, 3H; NÀMe), 3.48 (s, 3H;
NÀMe), 3.42–3.31 (m, 1H; codolef.), 2.71–2.58 (m, 1H; codolef.), 2.45–
2.18 (m, 2H; codaliph.), 2.11–1.72 (m, 4H; codaliph.), 1.59–1.40 ppm (m,
Synthesis of triazolonium salt 1·BF4: A 100 mL Schlenk flask was
charged
with
1-methyl-4-phenyl-1,2,4-triazol-5-one
(3.0 g,
17.1 mmol) and trimethyloxonium tetrafluoroborate (2.6 g,
17.9 mmol). After addition of acetonitrile (30 mL) the solution was
stirred for 12 h at room temperature. The solvent was reduced in
vacuo and the product was precipitated by addition of diethyl
ether (80 mL). After filtration the pale yellow solid was dried in
vacuo to afford compound 1 in 87% yield (4.2 g, 15.0 mmol).
1H NMR (200 MHz, CD3CN): d=9.07 (q, 4J(H,H)=0.94 Hz, 1H;
1
2H; codaliph.); 13C{1H} NMR (126 MHz, CDCl3): d=189.4 (d, J(Rh,C)=
51 Hz; NCN), 151.5 (s; C=O), 135.5 (s; PhÀC), 129.0 (s; PhÀC), 128.7
(s; PhÀC), 126.7 (s; PhÀC), 100.4 (d, 1J(Rh,C)=7 Hz; codolef.), 100.2
1
1
(d, J(Rh,C)=7 Hz; codolef.), 69.6 (d, J(Rh,C)=14 Hz; codolef.), 69.4 (d,
1J(Rh,C)=14 Hz; codolef.), 37.3 (s; NÀMe), 33.6 (s; codaliph.), 31.7 (s;
codaliph.), 30.4 (s; NÀMe), 29.0 (s; codaliph.), 28.5 ppm (s; codaliph.); MS
(MALDI): m/z 399.9 [MÀCl]+; elemental analysis calcd (%) for
C18H23N3ORhCl: C 49.61, H 5.32, N 9.64; found: C 49.75, H 5.43, N
9.67.
4
NCHN), 7.69–7.50 (m, 5H; PhÀH), 3.94 (d, J(H,H)=0.94 Hz, 3H; NÀ
Me), 3.59 ppm (s, 3H; NÀMe); 13C{1H} NMR (126 MHz, CD3CN): d=
149.2 (s; C=O), 139.3 (s; NCHN), 132.4 (s; PhÀC), 131.7 (s; PhÀC),
131.4 (s; PhÀC), 125.4 (s; PhÀC), 37.5 (s; NÀMe), 31.0 ppm (s; NÀ
Me); MS (MALDI): m/z 189.6 [M]+; elemental analysis calcd (%) for
C10H12N3OBF4: C 43.36, H 4.37, N 15.17; found: C 43.11, H 4.24, N
15.05.
Compound 4b: Deprotonation of 1·BF4 (164 mg, 0.59 mmol) with
potassium tert-butoxide (68 mg, 0.61 mmol) in the presence of
[Ir(cod)Cl]2 (199 mg, 0.3 mmol) led to 4b (57%, 177 mg,
0.34 mmol) as a yellow solid. 1H NMR (200 MHz, CDCl3): d=7.93–
7.86 (m, 2H; PhÀH), 7.54–7.41 (m, 3H; PhÀH), 4.82–4.70 (m, 1H; co-
dolef.), 4.68–4.56 (m, 1H; codolef.), 4.09 (s, 3H; NÀMe), 3.52 (s, 3H; NÀ
Me), 2.99–2.89 (m, 1H; codolef.), 2.39–2.28 (m, 1H; codolef.), 2.23–2.05
(m, 2H; codaliph.), 1.87–1.49 (m, 4H; codaliph.), 1.36–1.23 ppm (m, 2H;
codaliph.); 13C{1H} NMR (126 MHz, CDCl3): d=185.2 (s; NCN), 151.8 (s;
C=O), 135.1 (s; PhÀC), 128.8 (s; PhÀC), 128.7 (s; PhÀC), 127.1 (s;
PhÀC), 88.0 (s; codolef.), 87.5 (s; codolef.), 53.3 (s; codolef.), 53.2 (s;
codolef.), 36.8 (s; NÀMe), 34.0 (s; codaliph.), 32.6 (s; codaliph.), 30.5 (s;
NÀMe), 29.4 (s; codaliph.), 29.2 ppm (s; codaliph.); MS (MALDI): m/z
525.1 [M]+; elemental analysis calcd (%) for C18H23N3OIrCl: C 41.17,
H 4.42, N 8.00; found: C 40.95, H 4.25, N 7.84.
Synthesis of thiourea 3a and selenide 3b: General procedure: A
mixture of precursor 1·BF4, S8 (or Se) and potassium tert-butoxide
was cooled to À808C. After 10 min THF (30 mL) was added and
the resulting suspension was stirred for 12 h. The mixture was al-
lowed to warm up to room temperature. The solvent was removed
in vacuo and the crude product was dissolved in dichloromethane.
After filtration over Celite, the clear solution was reduced in vacuo
and precipitated by addition of n-hexane to give compound 3a as
a pale yellow and compound 3b as a white solid. Diffusion of n-
hexane into a THF solution of 3a delivered suitable crystals for X-
ray diffraction studies.
Compound 3a: The deprotonation of 1·BF4 (200 mg, 0.72 mmol)
with potassium tert-butoxide (82 mg, 0.73 mmol) in the presence
of S8 (26 mg, 0.81 mmol) led to 3a (70%, 112 mg, 0.51 mmol).
1H NMR (200 MHz, CDCl3): d=7.56–7.37 (m, 5H; PhÀH), 3.72 (s, 3H;
NÀMe), 3.44 ppm (s, 3H; NÀMe); 13C{1H} NMR (126 MHz, CDCl3): d=
170.1 (s; C=O), 152.4 (s; NCN), 133.0 (s; PhÀC), 129.5 (s; PhÀC),
129.4 (s; PhÀC), 128.0 (s; PhÀC), 33.9 (s; NÀMe), 32.1 ppm (s; NÀ
Me); MS (GC-MS): m/z 221 [M]+; elemental analysis calcd (%) for
C10H11N3OS: C 54.28, H 5.01, N 18.99, S 14.49; found: C 54.22, H
5.11, N 19.06, S 14.30.
Synthesis of dicarbonyl complexes 5a and 5b: General proce-
dure: Carbon monoxide was bubbled through a solution of 4a
(4b) in dichloromethane for 10 min. The solvent was evaporated in
vacuo.
Compound 5a: 1H NMR (300 MHz, CDCl3): d=7.69–7.62 (m, 2H;
PhÀH), 7.55–7.46 (m, 3H; PhÀH), 4.04 (s, 3H; NÀMe), 3.56 ppm (s,
3H; NÀMe); 13C{1H} NMR (75 MHz, CDCl3): d=184.7 (d, 1J(Rh,C)=
56 Hz; RhÀCO), 181.3 (d, 1J(Rh,C)=73 Hz; RhÀCO), 179.9 (d,
1J(Rh,C)=45 Hz; NCN), 150.8 (s; C=O), 134.6 (s; PhÀC), 129.6 (s;
PhÀC), 129.5 (s; PhÀC), 127.0 (s; PhÀC), 38.0 (s; NÀMe), 30.4 ppm
(s; NÀMe); MS (MALDI): m/z 348.0 [MÀCl]+; IR (CH2Cl2): u˜ =2089
(RhÀCO), 2011 (RhÀCO), 1746 cmÀ1 (amideÀCO); elemental analysis
calcd (%) for C12H11N3O3RhCl: C 37.57, H 2.89, N 10.95; found: C
37.28, H 2.89, N 10.68.
Compound 5b: 1H NMR (500 MHz, CDCl3): d=7.57–7.49 (m, 2H;
PhÀH), 7.47–7.42 (m, 3H; PhÀH), 3.97 (s, 3H; NÀMe), 3.54 ppm (s,
3H; NÀMe); 13C{1H} NMR (126 MHz, CDCl3): d=179.72 (s; IrÀCO),
177.5 (s; IrÀCO), 166.8 (s; NCN), 150.9 (s; C=O), 134.2 (s; PhÀC),
129.8 (s; PhÀC), 129.5 (s; PhÀC), 127.5 (s; PhÀC), 37.7 (s; NÀMe),
30.4 ppm (s; NÀMe); MS (MALDI): m/z (%): 438.2 [MÀCl]+; IR
(CH2Cl2): u˜ =2076 (IrÀCO), 1994 (IrÀCO), 1749 cmÀ1 (amideÀCO); el-
emental analysis calcd (%) for C12H11N3O3IrCl: C 30.48, H 2.34, N
8.89; found: C 30.26, H 2.24, N 8.79.
Compound 3b: The deprotonation of 1·BF4 (100 mg, 0.36 mmol)
with potassium tert-butoxide (45 mg, 0.40 mmol) in the presence
of selenium (32 mg, 0.41 mmol) led to 3b (46%, 45 mg,
1
0.17 mmol). H NMR (200 MHz, CDCl3): d=7.58–7.38 (m, 5H; PhÀH),
3.88 (s, 3H; NÀMe), 3.51 ppm (s, 3H, NÀMe); 13C{1H} NMR (75 MHz,
CDCl3): d=166.5 (s; C=O), 151.9 (s; NCN), 133.8 (s; PhÀC), 129.8 (s;
PhÀC), 129.5 (s; PhÀC), 128.3 (s; PhÀC), 35.2 (s; NÀMe), 31.9 ppm
(s; NÀMe); 77Se NMR (115 MHz, [D6]Acetone): d=137 ppm (s;
C=Se); MS (EI): m/z 269 [M]+; elemental analysis calcd (%) for
C10H11N3OSe: C 44.79, H 4.13, N 15.67; found: C 44.92, H 4.09, N
15.67.
Synthesis of rhodium complex 4a and iridium complex 4b: Gen-
eral procedure: Precursor 1·BF4, potassium tert-butoxide and
[{M(cod)Cl}2] (M=Rh or Ir) were charged into a 100 mL Schlenk
flask and cooled to À808C. After a few minutes THF (30 mL) was
added and the solution was stirred overnight. Afterwards the sol-
vent was evaporated in vacuo. The rhodium complex 4a was puri-
fied by flash chromatography on aluminium oxide with dichloro-
Synthesis of nickel complex 6: A mixture of precursor 1·BF4
(202 mg, 0.73 mmol), sodium iodide (253 mg, 1.69 mmol) and nick-
Chem. Eur. J. 2015, 21, 15759 – 15768
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elocene (138 mg, 0.73 mmol) was refluxed in THF (35 mL) for 4 h.
All volatiles of the resulting red solution were removed in vacuo.
The residue was dissolved in dichloromethane and filtered over
Celite. The solvent was reduced in vacuo and the product was pre-
cipitated by addition of n-hexane (35 mL) as a red solid (162 mg,
(0.5 mL, 1 mmol, 2m in THF) was diluted in THF (5 mL), and added
drop wise to the reaction mixture and stirred for 17 h. The solvent
was removed and the residue dissolved in dichloromethane. After
filtration over Celite, the solvent was removed and the product
1
was obtained in 68% yield (189 mg, 0.53 mmol). H NMR (200 MHz,
1
0.37 mmol, 51%). H NMR (200 MHz, CDCl3): d=7.92–7.83 (m, 2H;
[D6]DMSO): d=8.32 (s, 1H; NHÀPh), 8.27–8.19 (m, 2H; PhÀH),
7.44–7.34 (m, 2H; PhÀH), 7.28–7.19 (m, 1H; PhÀH), 7.18–7.08 (m,
2H; PhÀH), 6.78–6.68 (m, 1H; PhÀH), 6.65–6.55 ppm (m, 2H; PhÀ
H); 13C{1H} NMR (50 MHz, [D6]DMSO): d=158.5 (s; CÀOÀ), 154.2 (s;
NCN), 147.8 (s; PhÀC), 140.6 (s; PhÀC), 128.4 (s; PhÀC), 127.8 (s;
PhÀC), 125.5 (s; PhÀC), 123.4 (s; PhÀC), 118.8 (s; PhÀC), 113.0 ppm
(s; PhÀC); 77Se NMR (115 MHz, [D6]acetone): d=91 ppm (s; C=Se);
MS (MALDI): m/z 330.8 [MÀNa]À; elemental analysis calcd (%) for
C14H11N4OSeNa: C 47.61, H 3.14, N 16.86; found: C 47.21, H 3.54, N
16.46.
PhÀH), 7.65–7.51 (m, 3H; PhÀH), 4.92 (s, 5H; NiÀCp), 4.32 (s, 3H;
NÀMe), 3.53 ppm (s, 3H; NÀMe); 13C{1H} NMR (75 MHz, CDCl3): d=
179.5 (s; NCN), 151.7 (s; C=O), 135.9 (s; PhÀC), 129.4 (s; PhÀC),
129.3 (s; PhÀC), 127.9 (s; PhÀC), 92.8 (s; NiÀCp), 39.3 (s; NÀMe),
30.8 ppm (s; NÀMe); MS (MALDI): m/z 439.0 [M]+; HRMS (ESI): m/z
calcd for C15H16N3NiO: 312.0641; found: 312.0640.
Synthesis of betaine 7: A 250 mL Schlenk flask was charged with
1,5-diphenylcarbazide (5.0 g, 20.6 mmol) and triethylorthoformate
(50 mL). After addition of acetic acid (0.1 mL), the suspension was
refluxed for 3 h while a white solid precipitated from the solution.
The product was filtered and washed with diethyl ether (3x20 mL).
After drying in vacuo the betaine 7 was obtained analytical pure in
71% yield (3.7 g, 14.7 mmol). 1H NMR (200 MHz, [D6]DMSO): d=
10.17 (s, 1H; NCHN), 9.31 (s, 1H; NHÀPh), 7.91–7.79 (m, 2H; PhÀ
Hortho), 7.63–7.53 (m, 2H; PhÀHmeta), 7.50–7.40 (m, 1H; PhÀHpara),
7.30–7.18 (m, 2H; NHPhÀHmeta), 6.94–6.84 (m, 1H; NHPhÀHpara),
6.80–6.70 ppm (m, 2H; NHPhÀHortho); 13C{1H} NMR (126 MHz,
[D6]DMSO): d=157.4 (s; CÀOÀ), 146.7 (s; NHPhÀCquart.), 136.5 (s;
NCHN), 135.9 (s; PhÀCquart.), 129.6 (s; PhÀCmeta), 129.1 (s; NHPhÀ
Cmeta), 128.4 (s; PhÀCpara), 120.8 (s; NHPhÀCpara), 118.9 (s; PhÀCortho),
113.2 ppm (s; NHPhÀCortho); MS (MALDI): m/z 252.7 [M]+; elemental
analysis calcd (%) for C14H12N4O: C 66.65, H 4.79, N 22.21; found: C
66.43, H 4.54, N 22.29.
Synthesis of selenoether: 12: Selenide 11 (113 mg, 0.33 mmol)
was dissolved in a 100 mL Schlenk flask with dichloromethane
(20 mL). Methyl iodide (80 mL, 1.29 mmol) was added to the clear
solution. After a few minutes a white solid precipitated from the
solution. After 5 h the suspension was filtered over Celite, the sol-
vent reduced in vacuo and the product precipitated with n-
hexane. The selenoether 12 was obtained in 56% yield (61 mg,
0.18 mmol). 1H NMR (200 MHz, CDCl3): d=9.25 (s, 1H; NHÀPh),
7.75–7.68 (m, 2H; PhÀH), 7.63–7.54 (m, 3H; PhÀH), 7.28–7.18 (m,
2H; PhÀH), 6.92–6.83 (m, 1H; PhÀH), 6.80–6.72 (m, 2H; PhÀH),
2.26 ppm (s, 3H; SeÀMe); 13C{1H} NMR (75 MHz, [D6]DMSO): d=
158.1 (s; CÀOÀ), 146.4 (s; NHPhÀCquart), 137.6 (s; NCN), 137.3 (s; PhÀ
Cquart), 129.6 (s; PhÀCpara), 129.1 (s; PhÀCmeta), 129.0 (s; NHPhÀCmeta),
125.7 (s; PhÀCortho), 120.4 (s; NHPhÀCpara), 113.0 (s; NHPhÀCortho),
9.7 ppm (s; SeÀMe); 77Se NMR (114 MHz, [D6]acetone): d=181 ppm
(s; CÀSeÀMe); HRMS (ESI): m/z calcd for C15H15N4OSe+: 347.0406;
found: 347.0410.
Synthesis of dithiocarboxylate 9: The betaine
7 (102 mg,
0.40 mmol) and potassium tert-butoxide (46 mg, 0.41 mmol) were
charged in a 100 mL Schlenk flask and cooled to À808C. After
a few minutes THF (20 mL) was added and the suspension was
stirred while the colour changed to bright green. After 10 min
carbon disulfide (50 mL, 0.83 mmol) was added and the reaction
mixture was stirred for a further six hours while the mixture was al-
lowed to warm up to room temperature. The precipitated off-
white solid was filtered and washed first with THF and then n-
hexane. The product was obtained in 68% yield (100 mg,
0.27 mmol). 1H NMR (200 MHz, [D6]DMSO): d=10.41 (s, 1H; NHÀ
Ph), 7.83–7.75 (m, 2H; PhÀH), 7.66–7.52 (m, 4H; PhÀH), 7.46–7.39
(m, 1H; PhÀH), 7.37–7.27 (m, 2H; PhÀH), 7.26–7.17 ppm (m, 1H;
PhÀH); 13C{1H} NMR (125 MHz, [D6]DMSO): d=217.2 (s; NCN), 156.6
(s; CS2À), 145.5 (s; COÀ), 136.4 (s; PhÀC), 136.2 (s; PhÀC), 129 (s;
PhÀC), 129.6 (s; PhÀC), 128.7 (s; PhÀC), 128.1 (s; PhÀC), 126.6 (s;
PhÀC), 118.4 ppm (s; PhÀC); MS (ESI): m/z 327 [M]À; HRMS (ESI):
m/z calcd. for C15H11N4OS2À: m/z 327.0380; found: 327.0376.
Synthesis of rhodium complex 13: A mixture of betaine 7
(201 mg, 0.8 mmol), potassium tert-butoxide (91 mg, 0.81 mmol)
and [{Rh(cod)Cl}2] (200 mg, 0.4 mmol) was charged in a 100 mL
Schlenk flask and precooled to À808C. After addition of THF
(25 mL) the reaction mixture was allowed to warm up to room
temperature, while stirring was continued for 15 h. All volatiles
were removed in vacuo, the residue dissolved in dichloromethane
and filtered over Celite. The clear yellow solution was reduced in
vacuo and the product precipitated after addition of n-hexane. The
product was filtered off and washed with n-hexane (3”20 mL). The
solid was dried in vacuo to yield the rhodium complex 13 (64%,
237 mg, 0.51 mmol). 1H NMR (300 MHz, CD2Cl2): d=8.68 (s, 1H;
NCHN), 7.53–7.38 (m, 4H; PhÀH), 7.36–7.21 (m, 3H; PhÀH), 6.98–
6.84 (m, 3H; PhÀH), 4.26–4.08 (m, 2H; codolef.), 3.62–3.45 (m, 2H;
codolef.), 2.48–2.29 (m, 4H; codaliph.), 1.83–1.65 ppm (m, 4H; codaliph.);
13C{1H} NMR (75 MHz, CD2Cl2): d=165.1 (s; CÀOÀ), 152.7 (d,
2J(C,Rh)=2 Hz; NÀNÀÀPhquart.), 137.6 (s; NÀPhquart.), 130.2 (s; PhÀC),
130.1 (s; PhÀC), 128.2 (s; PhÀC), 124.1 (s; NCHN), 122.3 (s; PhÀC),
120.7 (s; PhÀC), 118.8 (s; PhÀC), 77.0 (d, 1J(C,Rh)=13 Hz; codolef.),
74.5 (d, 1J(C,Rh)=15 Hz; codolef.), 31.6 (s; codaliph.), 30.2 ppm (s;
codaliph.); MS (EI): m/z 462 [M]+; elemental analysis calcd (%) for
C22H23N4ORh: C 57.15, H 5.01, N 12.12; found: C 57.10, H 5.05, N
11.93.
Synthesis of thiourea 10: A 100 mL Schlenk flask was charged
with the dithiocarboxylate 9 (200 mg, 0.55 mmol) and S8 (32 mg,
1.00 mmol). After addition of THF (25 mL), the reaction mixture
was refluxed for 3 h. The solution was cooled to room tempera-
ture, filtrated over Celite and then reduced in vacuo. After addition
of n-hexane (40 mL), the product precipitated in 76% yield
(134 mg, 0.42 mmol). 1H NMR (200 MHz, [D6]DMSO): d=8.30 (m,
2H; PhÀH), 8.26 (s, 1H; NHÀPh), 7.37 (m, 2H; PhÀH), 7.14 (m, 3H;
PhÀH), 6.71 (m, 1H; PhÀH), 6.59 ppm (m, 2H; PhÀH); 13C{1H} NMR
(76 MHz, [D6]DMSO): d=161.2 (s; CÀOÀ), 157.2 (s; NCN), 148.0 (s;
PhÀC), 140.3 (s; PhÀC), 128.4 (s; PhÀC), 127.7 (s; PhÀC), 124.5 (s;
PhÀC), 122.1 (s; PhÀC), 118.5 (s; PhÀC), 112.6 ppm (s; PhÀC); HRMS
[ESI]: m/z calcd for C14H11N4OSÀ: m/z 383.0659; found: 383.0657.
Synthesis of rhodium complex 14: Rhodium complex 13 (150 mg,
0.32 mmol) and triphenyl phosphine (91 mg, 0.35 mmol) were
charged in a 100 mL Schlenk flask. After addition of dichlorome-
thane (20 mL) the solution was stirred for 5 h. The solvent was re-
duced in vacuo and the target compound was precipitated by ad-
dition of n-hexane (40 mL). After filtration the yellow solid was
washed with n-hexane and 14 was obtained (63%, 146 mg,
Synthesis of selenide 11: A suspension of 7 (199 mg, 0.79 mmol)
and selenium (128 mg, 1.62 mmol) in THF (35 mL) was cooled to
1
À808C. After
a
few minutes, sodium bis(trimethylsilyl)amide
0.20 mmol). H NMR (200 MHz, CDCl3): d=8.70–8.59 (m, 2H; PhÀH),
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7.50–7.29 (m, 17H; PhÀH), 7.26–7.15 (m, 3H; PhÀH), 6.94–6.85 (m,
1H; PhÀH), 6.80–6.70 (m, 2H; PhÀH), 6.26 (bs, 1H; NHÀPh), 5.27–
5.04 (m, 1H; codolef.), 4.74–4.51 (m, 1H; codolef.), 4.13–3.88 (m, 2H;
codolef.), 2.42–1.86 ppm (m, 8H; codaliph.); 13C{1H} NMR (75 MHz,
4.18 (s, 3H; OÀMe), 2.87–2.76 (m, 1H; codolef.), 2.68–2.57 (m, 1H;
codolef.), 1.81–1.06 ppm (m, 8H; codaliph.); 13C{1H} NMR (126 MHz,
1
CDCl3): d=187.6 (d, J(Rh,C)=51 Hz; NCN), 159.6 (s; CÀOMe), 147.1
(s; PhÀC), 140.0 (s; PhÀC), 129.6 (s; PhÀC), 128.8 (s; PhÀC), 128.6 (s;
PhÀC), 123.6 (s; PhÀC), 122.9 (s; PhÀC), 116.0 (s; PhÀC), 100.6 (d,
1J(Rh,C)=7 Hz; codolef.), 98.6 (d, 1J(Rh,C)=7 Hz; codolef.), 71.6 (d,
1J(Rh,C)=14 Hz; codolef.), 70.6 (d, 1J(Rh,C)=14 Hz; codolef.), 59.3 (s;
OÀMe), 32.2 (s; codaliph.), 31.9 (s; codaliph.), 28.9 (s; codaliph.), 28.3 ppm
(s; codaliph.); HRMS (ESI): calcd for C23H26N4ORh+: m/z 477.1156;
found: 477.1153.
1
2
CD2Cl2): d=176.9 (dd, J(C,Rh)=48 Hz, J(C,P)=12 Hz; NCN), 160.9
2
(s; CÀOÀ), 147.6 (s; PhÀC), 141.2 (s; PhÀC), 134.2 (d, J(C,P)=12 Hz;
1
4
PPh3ÀCortho), 132.2 (d, J(C,P)=40 Hz; PPh3ÀCquart), 130.9 (d, J(C,P)=
3
2 Hz; PPh3ÀCpara), 129.5 (s; PhÀC), 129.0 (d, J(C,P)=9.7 Hz; PPh3À
Cmeta), 128.7 (s; PhÀC), 126.3 (s; PhÀC), 121.5 (s; PhÀC), 120.3 (s,
PhÀC), 114.2 (s, PhÀC), 98.1 (m, codolef), 93.4 (m; codolef), 92.1 (m;
codolef), 90.6 (m; codolef), 31.3 (s; codaliph), 30.8 (s, codaliph), 30.7 (s,
codaliph), 30.6 ppm (s, codaliph); 31P NMR (121 MHz, CD2Cl2): d=
23.98 ppm (d, 1J(P,Rh)=159 Hz; PPh3); MS (MALDI): m/z 724.5 [M]+.
Synthesis of triazolonium salt 20·I: Betaine 7 (1.47 g, 5.8 mmol)
and methyl iodide (1.5 mL, 24.1 mmol) were stirred together at
708C in DMF (20 mL). After 23 h the solvent was removed in vacuo
and the residue was dissolved in dichloromethane (5 mL). The
product was precipitated completely by addition of n-hexane
(70 mL). The white solid was filtered off and washed first with di-
chloromethane (10 mL) and then with diethyl ether (50 mL). The
product was dried in vacuo to give the triazolonium salt in 78%
yield (1.8 g, 4.6 mmol). Suitable crystals for X-ray diffraction studies
were obtained by slow diffusion of diethyl ether in an acetonitrile
solution of 20·I. 1H NMR (200 MHz, [D6]DMSO): d=10.70 (s, 1H;
NCHN), 9.74 (s, 1H; NHÀPh), 7.98–7.88 (m, 2H; PhÀH), 7.85–7.74
(m, 3H; PhÀH), 7.38–7.28 (m, 2H; PhÀH), 7.14–7.06 (m, 2H; PhÀH),
7.05–6.96 (m, 1H; PhÀH), 3.39 ppm (s, 3H; NÀMe); 13C{1H} NMR
(75 MHz, [D6]DMSO): d 147.9 (s; C=O), 144.8 (s; NHPhÀCquart), 143.6
(s; NCN), 132.8 (s; PhÀCquart), 130.3 (s; PhÀCmeta), 129.8 (s; PhÀCpara),
129.1 (s; NHPhÀCmeta), 127.3 (s; PhÀCortho), 121.9 (s; NHPhÀCpara),
113.8 (s; NHPhÀCortho), 31.6 ppm (s; NÀMe); MS (MALDI): m/z 267.1
[M]+; elemental analysis calcd (%) for C15H15IN4O: C 45.70, H 3.84, N
14.21; found: C 45.58, H 3.81, N 14.06.
Synthesis of triazolium salt 16·OTf: A microwave tube was
charged with betaine 7 (304 mg, 1.21 mmol) and methyl trifluoro-
methansulfonate (2 mL, 18.2 mmol). The reaction mixture was
heated at 1008C for one minute under microwave irradiation
(200 W). From the resulting brown clear solution the product was
precipitated by addition of diethyl ether (30 mL) and the suspen-
sion was stirred for 1 h. The grey precipitate was filtered off and
washed with diethyl ether (20 mL) to give 16·OTf in 67% yield
(339 mg, 0.81 mmol). 1H NMR (200 MHz, [D6]DMSO): d=11.10 (s,
1H; NCHN), 10.02 (s, 1H; NHÀPh), 8.01–7.91 (m, 2H; PhÀH), 7.78–
7.61 (m, 3H; PhÀH), 7.37–7.26 (m, 2H; PhÀH), 7.08–6.96 (m, 3H;
PhÀH), 4.27 ppm (s, 3H; OÀMe); 13C{1H} NMR (75 MHz, [D6]DMSO):
d=157.2 (s; CÀOMe), 144.5 (s; NCHN), 142.6 (s; PhÀC), 135.1 (s;
PhÀC), 130.4 (s; PhÀC), 130.0 (s; PhÀC), 129.4 (s; PhÀC), 122.4 (s;
PhÀC), 120.2 (s; PhÀC), 113.7 (s; PhÀC), 60.9 ppm (s; OÀMe); MS
(MALDI): m/z 267.0 [M]+; elemental analysis calcd (%) for
C16H15F3N4O4S: C 46.15, H 3.63, N 13.46, S 7.70; found: C 46.17, H
3.93, N 13.45, S 8.00.
Synthesis of triazolonium salt 20·BF4: The triazolonium salt 20·I
(200 mg, 0.51 mmol) and silver tetrafluoroborate (100 mg,
0.51 mmol) were stirred at room temperature in acetonitrile
(20 mL). After 3 h the precipitated solid was filtered off and the re-
sulting clear colourless solution was reduced in vacuo. The addi-
tion of diethyl ether led to precipitation of the target compound in
89% yield (161 mg, 0.46 mmol). 1H NMR (200 MHz, [D6]DMSO):
d=10.66 (s, 1H; NCHN), 9.75 (s, 1H; NHÀPh), 7.94–7.85 (m, 2H;
PhÀH), 7.83–7.75 (m, 3H; PhÀH), 7.39–7.27 (m, 2H; PhÀH), 7.11–
6.95 (m, 3H; PhÀH), 3.38 ppm (s, 3H; NÀMe); 13C{1H} NMR (75 MHz,
[D6]DMSO): d=147.9 (s; C=O), 144.8 (s; PhÀC), 143.6 (s; NCN),
132.8 (s; PhÀC), 130.3 (s; PhÀC), 129.9 (s; PhÀC), 129.2 (s; PhÀC),
127.2 (s; PhÀC), 122.0 (s; PhÀC), 113.8 (s; PhÀC), 31.5 ppm (s; NÀ
Me); MS (MALDI): m/z 266.9 [M]+; elemental analysis calcd (%) for
C15H15N4OBF4: C 50.88, H 4.27, N 15.82; found: C 50.72, H 4.50, N
16.09.
Synthesis of selenide 17: The precursor 16·OTf (239 mg,
0.57 mmol) and selenium (79 mg, 1.00 mmol) were added in THF
(25 mL) and cooled to À808C. Sodium bis(trimethylsilyl)amide
(0.3 mL, 0.60 mmol, 2 m in THF) was diluted in THF (5 mL) and
added drop wise to the cooled reaction mixture. The reaction mix-
ture was stirred for 16 h and then allowed to warm up to room
temperature. After evaporation to dryness the crude product was
purified by flash chromatography on alumina with diethyl ether.
The product was obtained as a brown solid in 62% yield (122 mg,
1
0.35 mmol). H NMR (200 MHz, [D6]DMSO): d=9.24 (s, 1H; NHÀPh),
8.10–8.02 (m, 2H; PhÀH), 7.59–7.46 (m, 3H; PhÀH), 7.28–7.19 (m,
2H; PhÀH), 6.91–6.83 (m, 1H; PhÀH), 6.72–6.65 (m, 2H; PhÀH),
4.11 ppm (s, 3H; OÀMe); 13C{1H} NMR (75 MHz, [D6]DMSO): d=
161.5 (s; CÀOMe), 157.1 (s; NCN), 145.3 (s; PhÀC), 138.6 (s; PhÀC),
129.0 (s; PhÀC), 128.7 (s; PhÀC), 128.2 (s; PhÀC), 124.4 (s; PhÀC),
120.5 (s; PhÀC), 112.8 (s; PhÀC), 58.7 ppm (s; OÀMe); 77Se NMR
(115 MHz, [D6]Acetone): d=119 ppm (s; C=Se); MS (MALDI): m/z
347.1 [M+H]+; elemental analysis calcd (%) for C15H14N4SeO: C
52.18, H 4.09, N 16.23; found: C 52.27, H 4.32, N 16.11.
Synthesis of rhodium complex 21: The triazolonium salt 20·BF4
(142 mg, 0.40 mmol), [{Rh(cod)Cl}2] (100 mg, 0.20 mmol) and potas-
sium tert-butoxide (45 mg, 0.40 mmol) were cooled to À808C. THF
(20 mL) was added and the solution was stirred for 18 h. After
evaporation of the solvent in vacuo, the residue was dissolved in
dichloromethane and filtered over Celite. The solvent was reduced
in vacuo and after addition of n-hexane (40 mL) the product pre-
cipitated as a yellow solid. After filtration the rhodium complex
Synthesis of rhodium complex 18: A 100 mL Schlenk flask was
charged with the triazolium salt 16·OTf (144 mg, 0.35 mmol),
[{Rh(cod)Cl}2] (85 mg, 0.17 mmol) and potassium tert-butoxide
(43 mg, 0.38 mmol). The mixture was cooled to À808C. After
10 min THF (20 mL) was added and the solution was stirred for
20 h. The solvent was evaporated in vacuo and the residue dis-
solved in dichloromethane. After filtration over Celite, the solvent
was evaporated and the resulting yellow solid was washed with n-
hexane (2”20 mL). The rhodium complex 18 was obtained in 47%
yield (82 mg, 0.16 mmol). 1H NMR (200 MHz, CDCl3): d=8.49–8.39
(m, 2H; PhÀH), 8.34 (bs, 1H; NHÀPh), 7.57–7.49 (m, 2H; PhÀH),
7.48–7.42 (m, 1H; PhÀH), 7.41–7.32 (m, 2H; PhÀH), 7.14–7.05 (m,
1H; PhÀH), 7.04–6.96 (m, 2H; PhÀH), 5.09–4.97 (m, 2H; codolef.),
1
was obtained in 40% yield (83 mg, 0.16 mmol). H NMR (200 MHz,
CDCl3): d=7.96–7.86 (m, 2H; PhÀH), 7.83 (s, 1H; NHÀPh), 7.69–7.56
(m, 3H; PhÀH), 7.40–7.29 (m, 2H; PhÀH), 7.10–7.00 (m, 3H; PhÀH),
5.09–4.90 (m, 2H; codolef.), 3.32 (s, 3H; NÀMe), 2.89–2.77 (m, 1H;
codolef.), 2.48–2.36 (m, 1H; codolef.), 2.04–1.84 (m, 2H; codaliph.), 1.72–
1.55 (m, 2H; codaliph.), 1.50–1.14 ppm (m, 4H; codaliph.); 13C{1H} NMR
(126 MHz, CDCl3): d=194.6 (d, 1J(Rh,C)=51 Hz; NCN), 153.2 (s;
C=O), 147.3 (s; PhÀC), 135.0 (s; PhÀC), 130.7 (s; PhÀC), 129.8 (s;
PhÀC), 129.5 (s; PhÀC), 127.5 (s; PhÀC), 122.7 (s; PhÀC), 116.1 (s;
Chem. Eur. J. 2015, 21, 15759 – 15768
www.chemeurj.org
15767
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Full Paper
PhÀC), 101.6 (d, 1J(Rh,C)=7 Hz; codolef.), 100.1 (d, 1J(Rh,C)=7 Hz;
codolef.), 71.4 (d, 1J(Rh,C)=14 Hz; codolef.), 70.8 (d, 1J(Rh,C)=14 Hz;
codolef.), 32.8 (s; NÀMe), 32.3 (s; codaliph.), 31.9 (s; codaliph.), 28.5 (s;
codaliph.), 28.2 ppm (s; codaliph.); MS (MALDI): m/z 477.1 [MÀCl]+; ele-
mental analysis calcd (%) for C23H26ClN4ORh: C 53.87, H 5.11, N
10.92; found: C 53.68, H 4.97, N 10.83.
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Dicarbonyl complex 19: 1H NMR (200 MHz, CDCl3): d=8.11 (m,
3H; PhÀH), 7.54–7.47 (m, 2H; PhÀH), 7.52 (bs, 1H; NHÀPh), 7.33
(m, 2H; PhÀH), 7.07 (m, 1H; PhÀH), 6.80 (m, 2H; PhÀH), 4.24 ppm
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1
1J(Rh,C)=56 Hz; NCN or RhÀCO), 179.5 (d, J(Rh,C)=44 Hz; NCN or
RhÀCO), 159.5 (s; COÀMe), 145.6 (s; PhÀC), 139.5 (s; PhÀC), 129.6
(s; PhÀC), 129.5 (s; PhÀC), 129.2 (s; PhÀC), 128.8 (s; PhÀC), 123.5 (s;
PhÀC), 115.9 (s; PhÀC), 66.0 (s; OÀCH3); IR (CH2Cl2): u˜ =2089 cmÀ1
(strong; RhÀCO), 2014 cmÀ1 (strong; RhÀCO).
Dicarbonyl complex 22: 1H NMR (200 MHz, CDCl3): d=7.65 (m,
5H; PhÀH), 7.55 (bs, 1H; NHÀPh), 7.36–7.29 (m, 2H; PhÀH), 7.09–
7.01 (m, 1H; PhÀH), 6.94–6.89 (m, 2H; PhÀH), 3.37 ppm (s, 3H; NÀ
CH3); 13C{1H} NMR (126 MHz, CDCl3): d=151.5 (s; C=O), 145.6 (s;
PhÀC), 134.3 (s; PhÀC), 131.8 (s; PhÀC), 130.4 (s; PhÀC), 129.5 (s;
PhÀC), 127.9 (s; PhÀC), 123.3 (s; PhÀC), 116.1 (s; PhÀC), 32.4 ppm
(s; NÀCH3). MS (MALDI): m/z 396.9 [M-CO-Cl]+; IR (CH2Cl2): u˜ =2092
(strong; RhÀCO), 2015 (strong; RhÀCO), 1761 cmÀ1 (strong; amideÀ
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Keywords: carbenes · heterocyclic compounds · iridium ·
rhodium · selenium
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Received: July 9, 2015
Published online on September 23, 2015
Chem. Eur. J. 2015, 21, 15759 – 15768
www.chemeurj.org
15768
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim