Isolation and Structures of 1,2,3-Triazole-Derived Mesoionic Biscarbenes with Bulky Aromatic Groups

Article abstract of DOI:10.1002/ejoc.201501626

1,2,3-Triazole-derived mesoionic biscarbenes bridged by a pyridylene group were synthesized and their structures were determined. Bulky substituents on the carbene rings stabilized the carbenes, which enabled their crystal structures to be determined. As implied by DFT calculations and the experimental results, the carbenes are partly reduced to anionic radical species in the presence of strong bases. This suggests that the pyridylene and triazolylidene moieties endow molecules with both electron-acceptor and -donor properties. A rare example of an iron complex with a pincer carbene was synthesized. 1,2,3-Triazole-derived mesoionic biscarbenes bridged by a pyridylene group are synthesized and characterized. The carbenes are thought to be partly reduced to anionic radical species in the presence of strong bases, which suggests that these moieties endow molecules with both electron-acceptor and -donor properties. A rare example of an iron complex with a pincer carbene is synthesized.

Full text of DOI:10.1002/ejoc.201501626

DOI: 10.1002/ejoc.201501626
Communication
Mesoionic Carbenes
Isolation and Structures of 1,2,3-Triazole-Derived Mesoionic
Biscarbenes with Bulky Aromatic Groups
Haruka Iwasaki,[a] Yuji Yamada,[a] Ryuta Ishikawa,[a] Yuji Koga,[a] and Kouki Matsubara*[a]
Abstract: 1,2,3-Triazole-derived mesoionic biscarbenes bridged tal results, the carbenes are partly reduced to anionic radical
by a pyridylene group were synthesized and their structures species in the presence of strong bases. This suggests that the
were determined. Bulky substituents on the carbene rings stabi- pyridylene and triazolylidene moieties endow molecules with
lized the carbenes, which enabled their crystal structures to be both electron-acceptor and -donor properties. A rare example
determined. As implied by DFT calculations and the experimen- of an iron complex with a pincer carbene was synthesized.
Introduction
pounds containing various triazolylidene ligands have been re-
ported.[7]
The chemistry of stable carbenes has been widely studied in
both organic and inorganic chemistry.[1] Such compounds are
used as organic catalysts on the basis of their strongly basic
properties for organic synthesis[2] and as ligands for the con-
struction of metal complexes for catalysts or other functional
materials.[3] The singlet electron pair can be stabilized by
heteroatoms adjacent to the carbene carbon atom and by the
introduction of large substituents around it.[1] Various stable
carbene core structures are known, including the familiar N-
heterocyclic carbenes (NHCs) and mesoionic carbenes (MICs), in
which the heteroatom positions are different from those in
NHCs (Figure 1).[4]
Several synthetic pathways to MIC complexes have been re-
ported.[7] However, tridentate bisMICs have not been isolated,
although a silver-stabilized carbene was reported as an inter-
mediate in the synthesis of a series of luminescent ruthenium
complexes by Schubert et al.[8] Bertrand et al. reported the
structure of a potassium-stabilized tridentate MIC.[9] In this pa-
per, we report the first isolation of free tridentate carbenes and
an iron complex. Schubert's sequential method was used to
synthesize the carbenes, as shown in Scheme 1. Furthermore,
we were interested in the effects of a pyridine moiety placed
between two triazolylidene moieties. In conjunction with the
pyridine ring directly bound to the triazolylidene moieties, this
type of molecule could have specific electronic properties de-
rived from extended π conjugation, which cannot be observed
in monodentate carbene molecules. The experimental and the-
oretical results and a discussion concerning the expanded π
conjugation in such free carbene molecules have not been re-
ported to the best of our knowledge.
Figure 1. Representative examples of NHCs and MICs.
Results and Discussion
Various MICs have been developed and used as new alterna-
tives to NHCs as strong σ-donor ligands in metal complexes.[4]
Bertrand et al. first reported a 1,2,3-triazole-derived MIC (tri-
azolylidene) ligand.[5] MICs have been attracting increasing at-
tention, because a click reaction, that is, the copper-catalyzed
[2+3] Huisgen cycloaddition to form 1,2,3-triazoles,[6] makes this
ligand easier to synthesize than other carbene ligands. The σ
donation by a 1,2,3-triazole-derived carbene to metals is
stronger than that by common NHCs.[5] Many metal com-
A [2+3] Huisgen cyclization by using 2,6-bisethynylpyridine and
an aryl azide afforded 1,2,3-triazoles 1a (Ar = 2,6-diisopropyl-
phenyl) and 1b (Ar = 2,4,6-trimethylphenyl) in high yields
(Scheme 1). Triazolium salts 2a and 2b were obtained by reac-
tions of 1a and 1b with trimethyloxonium tetrafluoroborate.
The obtained triazolium salts contained tetrafluoroborate an-
ions. Desired carbenes 3a and 3b were synthesized from 2a
and 2b with sodium tert-butoxide at room temperature; the
products were obtained after filtration through Celite. The sol-
vents used for carbene preparation were critical in terms of
efficient formation and isolation of the products. Ether was the
best solvent for the reduction. The carbene molecules were sta-
ble in toluene, and therefore, the reaction mixtures were ex-
tracted with toluene and filtered through Celite after removal
of ether from the reaction mixture. These results were encour-
[a] Department of Chemistry, Fukuoka University,
8-19-1 Nanakuma, Jonan-ku, Fukuoka 814-0180, Japan
E-mail: kmatsuba@fukuoka-u.ac.jp
http://www.sc.fukuoka-u.ac.jp/~psc/main_polymer.htm
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/ejoc.201501626.
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aging, because it has been reported that potassium tert-butox- these are smaller than those of the NHC analogue, which are
ide and bis(trimethylsilyl)amide generally reduce triazolium 101.1(3).[10] The smaller carbene carbon atom angles in 3a sug-
salts with up to moderate conversions.[4]
gest that the σ-donor ability of the triazolylidene is stronger
than that of an NHC, as expected.[7a,7b]
Scheme 1. Synthesis of MICs 3a and 3b.
Compound 2a was identified by using common spectro-
scopic methods (see the Supporting Information); it is similar
1
to 2b, which was previously reported.[8] In the H NMR spectra
of 3a and 3b, the signals from the methyl protons on the tri-
azole nitrogen atom appear at δ = 3.77 and 3.75 ppm, respec-
tively. The doublet signals from the methyl protons of the iso-
propyl groups in 3a are split into two signals (δ = 1.30 and
1.22 ppm), which indicates that free rotation of the aromatic
ring on the triazolylidene moiety is restricted because of steric
repulsion between the triazole ring and the isopropyl groups
at the 2,6-positions (see the Supporting Information). The 13C
resonances from the carbene carbon atoms in 3a and 3b are
observed at δ = 206.78 and 201.54 ppm, respectively; these are
higher than the resonance of an NHC analogue, which appears
at δ = 219 ppm.[10] These values are comparable to those of
other triazolylidenes, which confirmed triazolylidene forma-
tion.[11]
A single crystal of 3a suitable for X-ray crystallography was
obtained from a toluene/n-hexane (5:1) solution at –30 °C.[12]
Figure 2 shows the structure of 3a. This is the first example of
a crystal structure of a free tridentate mesoionic carbene. The
three directly connected central rings are mainly located in the
same plane, although one of the triazole rings is slightly twisted
[the torsion angle N1–C5–C8–N7 is 16.8(4)°] because of steric
repulsion between the methyl groups on the triazole nitrogen
atom. This is in contrast to the two NHC and pyridine rings of
the bisNHC analogue, which has no methyl groups; the crystal
structure shows that the rings are in the same plane.[10] The C–
C bond lengths between the three central rings C1–C6 and C5–
C8 are 1.475(4) and 1.476(4) Å, respectively. These bonds are
about 0.06 Å shorter than a C–C single bond, which supports
the existence of extended π conjugation. The 2,6-diisopropyl-
phenyl groups on the triazole rings are twisted perpendicularly
from the three central rings, as suggested by the 1H NMR spec-
trum. Similar to the reported triazolylidene, the five-membered
ring has a conjugated structure:[5,11] the five bond lengths of
one of the triazolylidene rings, N2–N3, N3–N4, N4–C6, C6–C7,
and C7–N2 are 1.350(4), 1.329(4), 1.370(4), 1.411(4), and
1.368(4) Å, respectively. The carbene carbon atom angles, C6–
C7–N2 and C8–C9–N5, are 99.6(3) and 99.7(3), respectively;
Figure 2. ORTEP diagrams of 3a (thermal ellipsoid probability 50 %): (a) side
view and (b) bottom view. Two molecules of cocrystallized toluene (used as
the solvent) and hydrogen atoms are omitted for clarity.
A density functional theory (DFT) calculation with use of the
B3LYP functional and the 6-31G(d) basis set suggests easy one-
electron reduction of 3a. Reduced anionic analogue 3a′ is
–10.39 kcal mol–1 more stable than its neutral counterpart 3a
on the basis of DFT calculations. A tBuO– anion and solvent
are proposed as possibilities of reductants to generate 3a′ as
follows:
3a + tBuO– + toluene → 3a′ + tBuOH + benzyl radical
Here, charge transfer from the tBuO– anion to 3a gives rise
to 3a′ and the tBuO· radical, which promptly abstracts a
hydrogen atom from the toluene solvent to give a benzyl radi-
cal. This reaction energy was calculated to be +3.67 kcal mol–1,
and consequently, this reaction equilibrium sufficiently provides
3a′ at room temperature.
The singly occupied molecular orbital (SOMO) of radical an-
ion 3a′ calculated by DFT calculations also showed predomi-
nant single-electron distribution in the delocalized π* orbitals
of pyridylene and both triazolylidenes (Figure 3). Because of its
stability, 3a could easily accept a single-electron transfer from
Figure 3. SOMO of 3a′ obtained by DFT calculations at the B3LYP/6-31G(d)
level.
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Scheme 2. Formation of a possible radical anion of MIC 3a.
the reductant. Although the formation of the radical anion of
pyridine derivatives has been reported,[13] the radical anion of
pyridylene between two N-heterocyclic carbenes is not known
to the best of our knowledge.
Supporting these calculation results, we found experimental
evidence of an anionic radical species of 3a. Products 3a and
3b were dark purple after filtration through Celite, whereas
their crystals were colorless. No color change was noted in the
preparative process of the reported NHC analogue.[10] The
1H NMR spectrum of product 3a shows that prominent signals
from byproducts cannot be detected, and only signals of 3a
and those of an excess amount of reductant are observed (see
the Supporting Information). We suspect that in the presence
Figure 4. (a) EPR spectrum of a mixture of 3a and 3a′ in the solid state at
room temperature and (b) the UV/Vis spectrum of the same mixture in THF.
of an excess amount of a strong base such as potassium or change reagent to stabilize the compound (36 % conversion
sodium tert-butoxide and bis(trimethylsilyl)amide, the molecule from carbene). Unfortunately, the purification procedure, alu-
is reduced to form radical anion 3a′ in part (Scheme 2). The mina column chromatography and subsequent recrystallization,
radical nature of 3a′ was confirmed by using solid-state elec- diminished the yield of the product (2 % yield). MS (ESI) in
tron paramagnetic resonance (EPR) spectroscopy at room tem- acetonitrile solution showed the generation of dicationic iron
perature. A single band was observed at g = 2.005 (Figure 4, complex 4 bearing two triazolylidene ligands (Scheme 3). The
a)[13] by using the solid of the dark purple product after filtra- 1H NMR spectrum of 4-PF6 indicated that the compound is dia-
tion. The electronic spectrum of a THF solution of the product magnetic. Single-crystal X-ray crystallography showed that
showed broad absorption bands at around λ = 400 and 750 nm complex 4 has octahedral geometry around the iron(II) cen-
(shoulders), as shown in Figure 4 (b). The UV absorption band ter.[12,16]
at around λ = 400 nm and a near-IR band at λ = 750 nm is
consistent with the simulated electronic transition spectrum of
an anionic isomer of 3a (see the Supporting Information). An
alkali-metal-coordinated anion radical complex[9] is also possi-
ble, because the stabilization energy by coordination with
metal cations was comparable to that of the free anion radical,
according to the calculations. Other possibilities such as the
generation of triazolylidene-to-pyridine charge transfer induced
by the positive metal center can be ruled out on the basis of
the calculations. The resulting radical species generated from
the reductant cannot be detected, probably because of facile
radical trap by the solvent molecules. These findings suggest
that the heteroaromatic rings in 3a are potential electron ac-
ceptors, although both MICs are strong donors as ligands.
Scheme 3. Reaction of 3a with iron(II) chloride.
The pyridylene-bridged biscarbene can stabilize an iron com-
plex.[14] The above results prompted us to use a tridentate pin-
cer-type ligand to form an iron complex, because there has only
Conclusions
been one reported example of a triazolylidene ligand bound In summary, 1,2,3-triazole-derived biscarbenes were synthe-
to iron.[15] The room-temperature reaction of in situ generated sized. The two carbene groups were bound to the central pyri-
triazolylidene 3b with iron(II) chloride in THF/ether afforded a dylene and had bulky 2,6-diisopropylphenyl or 2,4,6-trimethyl-
dark purple solution at room temperature. Ammonium hexa- phenyl groups. The biscarbenes were characterized by NMR
fluorophosphate was added to the solution as an anion-ex- spectroscopy and their structures were confirmed by X-ray crys-
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Experimental Section
Preparation of 3a: Typically, a Schlenk tube was charged with 2a
(100 mg, 0.136 mmol), NaOtBu (29.7 mg, 0.306 mmol), and diethyl
ether (0.80 mL) in a glovebox. The mixture was stirred for 1 h, and
then, the volatiles were removed under reduced pressure. After ex-
traction with toluene, the solution was filtered through Celite. The
solvent was removed under reduced pressure to give 3a as a pur-
1
ple-white solid (71.0 mg, 80 %). H NMR (400 MHz, C6D6): δ = 8.83
(d, J = 8.0 Hz, 2 H, Py-H), 7.33 (t, J = 8.0 Hz, 2 H, Pro-Ph), 7.27 (t, J =
7.6 Hz, 1 H, Py-H), 7.21 (d, J = 8.0 Hz, 4 H, Pro-Ph), 3.77 (s, 6 H, Trz-
H), 2.92 (sept, J = 7.2 Hz, 4 H, Pro-Ph), 1.30 (d, J = 6.8 Hz, 12 H, Pro-
Ph), 1.22 (d, J = 6.8 Hz, 12 H, Pro-Ph) ppm. 13C NMR (100 MHz,
C6D6): δ = 206.78 (carbene), 151.37, 147.11, 145.59, 139.54, 137.60,
129.95, 124.26, 123.84, 37.79, 28.96, 24.26, 24.10 ppm. MS (ESI-TOF):
m/z = 562.37 [M + H]+ (C35H43N7·H+ requires 562.45).
Acknowledgments
This work was supported by the Japan Society for the Promo-
tion of Science (Grant-in-Aid for Scientific Research, 22550104
and 25410123) and a fund from the Central Research Institute
of Fukuoka University (grant number 117106).
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Keywords: Carbene ligands · Pincer-type ligands · Iron ·
Nitrogen heterocycles · Radical ions
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Received: December 31, 2015
Published Online: March 2, 2016
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