Nitron: A stable N-heterocyclic carbene that has been commercially available for more than a century

Article abstract of DOI:10.1039/c1cc16460k

The analytical reagent Nitron is a mesoionic compound whose structure consists of a cationic triazolium unit with an anionic PhN substituent. We provide clear evidence for the existence of a singlet diaminocarbene tautomer whose structure is composed of a

Full text of DOI:10.1039/c1cc16460k

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Nitron: a stable N-heterocyclic carbene that has been commercially
available for more than a centurywz
¨
Christian Farber, Michael Leibold, Clemens Bruhn, Martin Maurer and Ulrich Siemeling*
Received 17th October 2011, Accepted 26th October 2011
DOI: 10.1039/c1cc16460k
The analytical reagent Nitron is a mesoionic compound whose
structure consists of a cationic triazolium unit with an anionic
PhN substituent. We provide clear evidence for the existence of
a singlet diaminocarbene tautomer whose structure is composed
of a 1,2,4-triazol-5-ylidene unit bearing an NHPh substituent.
The report of the first N-heterocyclic carbene (NHC) by Arduengo
and co-workers in 19911 triggered the tremendous development of
such singlet carbenes from laboratory curiosities to powerful work-
horses in synthesis and catalysis.2 They are most commonly
generated by deprotonation of the corresponding azolium cations
with a strong base.2b,3 Conveniently, a limited number of NHCs
have become commercially available, with the 1,2,4-triazol-5-ylidene
derivative 1 (Fig. 1) introduced by Enders and co-workers4
being the earliest such example.2f A major drawback, however,
is that prices exceed several hundred US$ per gram. We found
that the low-cost analytical reagent Nitron,5 a relative of 1 that
has been commercially available for more than a century,5c
Scheme 1 Reactions of Nitron with typical carbene trapping reagents.
exhibits a hitherto unrecognised reactivity which is not compatible
with its conventional Lewis structure 2, but rather is expected
for its NHC-type tautomer 20 (Fig. 1) and closely resembles that
of 1 (Scheme 1).
Reagents and conditions: (i) S8, THF, room temp.; (ii) CS2, THF,
reflux; (iii) [{Rh(m-Cl)(CO)2}2], dichloromethane, room temp.;
(iv) [{Rh(m-Cl)(COD)}2], THF, room temp.
Elemental sulfur reacts smoothly and swiftly with Nitron
dissolved in THF, cleanly affording the triazolinethione derivative
20QS, whose CQS unit gives rise to a diagnostic 13C NMR
signal at d = 164.5 ppm, close to the value of 168.5 ppm
reported for 1QS.6 This reaction is typical of nucleophilic
carbenesꢀ.7 In the same vein, the betainic dithiocarboxylate
20+–CS2 was obtained from the nucleophilic addition reaction
of Nitron with CS2. The 13C NMR signal due to the CS2ꢀ unit
ꢀ
is located at d = 219.9 ppm and the characteristic nas(CS2
)
vibrational band is observed at 1065 cmꢀ1 in the IR spectrum
(CH2Cl2 solution). Essentially identical values have been
ꢀ 6 ꢀ
.
20+–CS2 was structurally charac-
reported for 1+–CS2
terised by a single-crystal X-ray diffraction study (see ESIz). Bond
Fig. 1 The 1,2,4-triazol-5-ylidene 1 (with numbering scheme for the
heterocyclic ring), the conventional structure of Nitron (2) and the
structure of its NHC-type tautomer 20.
parameters are unexceptional and compare well with those of
ꢀ 8
closely related compounds of the type NHC+–CS2
.
We have synthesised the rhodium carbonyl complex
cis-[RhCl(20)(CO)2] in order to elucidate the donor properties
of 20. This compound was obtained in essentially quantitative
yield from [{Rh(m-Cl)(CO)2}2] and Nitron in THF. Recrystalli-
sation from a mixture of dichloromethane and diethyl ether
afforded single-crystals of the solvate [RhCl(20)(CO)2]ꢁOEt2
suitable for an X-ray diffraction study (Fig. 2). The exocyclic
amino substituent of the carbene ligand 20 is associated with the
solvent molecule through an N–HꢁꢁꢁO hydrogen bond of
Institute of Chemistry, University of Kassel, Heinrich-Plett-Str. 40,
D-34132 Kassel, Germany. E-mail: siemeling@uni-kassel.de
w Presented in part at the 19th EuCheMS International Conference on
Organometallic Chemistry, Toulouse, July 2011.
z Electronic supplementary information (ESI) available: Experimental
details including analytical and spectroscopic data, crystallographic
and computational details, Fig. S1–S7, Tables S1–S3. CCDC
848861–848863. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c1cc16460k
c
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Chem. Commun., 2012, 48, 227–229 227

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Fig. 2 Molecular structure of cis-[RhCl(20)(CO)2]ꢁOEt2 in the crystal
(ellipsoids drawn at the 30% probability level).
moderate strength according to commonly accepted criteria
[NꢁꢁꢁO 2.886(6) A, HꢁꢁꢁO 1.921(6) A, N–H–O 156(4)1].9 The
Rh–Ccarbene bond length is 2.077(5) A. The significantly different
Rh–CO distances of 1.844(7) and 1.909(6) A for the carbonyl
ligand cis and trans to 20, respectively, reflect the strong trans-
influence of NHC ligands.10 These values agree well with those
reported for closely related compounds.11 The 13C NMR spectrum
of cis-[RhCl(20)(CO)2] reveals three low-field doublets, which can
Fig. 3 Molecular structure of [RhCl(20)(COD)] in the crystal (ellipsoids
drawn at the 30% probability level).
perpendicular to this empty orbital which contains the lone
pair of electrons at the divalent carbon atom.7 Since the
chemical behaviour of 20 is very similar to that of the
1,2,4-triazol-5-ylidene 1 (including the essentially identical TEP
values), the energies of their corresponding relevant molecular
orbitals are expected to be very similar. This is indeed the case
according to the results of our DFT calculations (BP86/def2-
SVP). The energy of the molecular orbital which corresponds to
the vacant p-type orbital is ꢀ1.73 eV for 1 and ꢀ1.93 eV for 20,
while that of the molecular orbital which corresponds to the
Ccarbene lone pair is ꢀ5.15 eV for 1 and ꢀ5.18 eV for 20 (see ESIz).
In summary, the hitherto unrecognised reactivity of the well
known analytical reagent Nitron towards carbene trapping
reagents reflects the presence of the NHC-type tautomer 20 in
solution. This tautomer, which is closely related to Enders’
1,2,4-triazol-5-ylidene 1, has never been considered before.14
This instant carbene has been commercially available for more
than a century, and is by far the cheapest NHC, available at a
fraction of the price of 1.
be assigned to the carbonyl (d = 184.9 and 181.7 ppm, 1JCRh
=
55.8 and 73.8 Hz, respectively) and carbene C atom (d =
175.9 ppm, 1JCRh = 43.8 Hz) resonances. The Tolman electronic
parameter (TEP) was calculated to be 2057.4 cmꢀ1 from the two
n(CO) bands at 2086 and 2007 cmꢀ1 observed in the IR spectrum
(CH2Cl2 solution), using a well-established linear regression.2b
This TEP value is essentially identical to that reported for 1
(2057.3 cm )
ꢀ1 12 and indicates an NHC of moderate donor strength.
In an analogous reaction, [RhCl(20)(COD)] (COD = cyclo-
octa-1,5-diene) was obtained from [{Rh(m-Cl)(COD)}2] and Nitron
in THF. The complex was structurally characterised by a single-
crystal X-ray diffraction study (Fig. 3). The Rh–Ccarbene distance is
2.030(4) A. Owing to the strong trans-influence of 20 (vide supra),
the Rh–CCOD bond lengths are significantly longer for the carbon
atoms trans to the NHC ligand than for those trans to the chlorido
ligand (average value ca. 2.23 vs. 2.12 A). These structural features
compare well with those reported for other complexes of the type
[RhCl(NHC)(COD)].13
We thank the Deutsche Forschungsgemeinschaft for generous
financial support and the Center for Scientific Computing of
the Goethe University Frankfurt for computer time.
The reactions summarised in Scheme 1 clearly indicate that the
NHC-type tautomer 20, although present at concentrations below
the detection limit of NMR spectroscopy (see ESIz), plays an
important role for the reactivity of Nitron in solution. This is
supported by results of DFT calculations (BP86/def2-SVP) which
reveal that in the gas phase 20 is only slightly less stable than 2
(DG0 = 5.7 kcal molꢀ1 for the process 2 - 20).
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The reactivity of singlet carbenes is governed by the
presence of a vacant p-type orbital and an occupied orbital
c
228 Chem. Commun., 2012, 48, 227–229
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¨
c
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Chem. Commun., 2012, 48, 227–229 229