Syntheses of sterically shielded stable carbenes of the 1,2,4-triazole series and their corresponding palladium complexes: Efficient catalysts for chloroarene hydrodechlorination

Article abstract of DOI:10.1039/c4dt01353k

The new sterically shielded 1,3,4-trisubstituted 1,2,4-triazol-5-ylidenes 8b-d were synthesized by a three step method starting from 2-phenyl-1,3,4-oxadiazole. The syntheses of palladium complexes 9a-d and 10a-d (including the sterically shielded derivatives 9c,d and 10a-d) were carried out via the reactions of the stable carbenes 8a-d with palladium halogenide salts in THF or toluene solution. Complexes 9c,d and 10a-d were found to be excellent catalysts for the reductive dechlorination (hydrodechlorination) of p-dichlorobenzene. The structures of 8c, 9a,b, and 10a were determined by single-crystal X-ray diffraction.

Full text of DOI:10.1039/c4dt01353k

Dalton
Transactions
View Article Online
View Journal
PAPER
Syntheses of sterically shielded stable carbenes of
the 1,2,4-triazole series and their corresponding
palladium complexes: efficient catalysts for
chloroarene hydrodechlorination†
Cite this: DOI: 10.1039/c4dt01353k
Nataliya V. Glinyanaya,a Vagiz Sh. Saberov,a Nikolai I. Korotkikh,*a Alan H. Cowley,b
Rachel R. Butorac,b Daniel A. Evans,b Tatyana M. Pekhtereva,a Anatolii F. Popova and
Oles P. Shvaikaa
The new sterically shielded 1,3,4-trisubstituted 1,2,4-triazol-5-ylidenes 8b–d were synthesized by a three
step method starting from 2-phenyl-1,3,4-oxadiazole. The syntheses of palladium complexes 9a–d and
10a–d (including the sterically shielded derivatives 9c,d and 10a–d) were carried out via the reactions of
the stable carbenes 8a–d with palladium halogenide salts in THF or toluene solution. Complexes 9c,d
and 10a–d were found to be excellent catalysts for the reductive dechlorination (hydrodechlorination) of
p-dichlorobenzene. The structures of 8c, 9a,b, and 10a were determined by single-crystal X-ray
diffraction.
Received 7th May 2014,
Accepted 24th June 2014
DOI: 10.1039/c4dt01353k
www.rsc.org/dalton
for example, ref. 13–15). Furthermore, 1,3,4-triphenyl-1,2,4-
triazol-5-ylidene has proven to be not only an effective catalyst
Introduction
Recently, stable carbenes have proven to be versatile reagents for organic reactions (e.g. benzoin and Stetter condensations),2
and catalysts for many important organic reactions.1–3 Hetero- but also a ligand for metal catalyzed reactions.3 To the best of
aromatic imidazol-2-ylidenes and 1,2,4-triazol-5-ylidenes are our knowledge, sterically shielded carbenes of this general
particularly well known examples of stable carbenes.4 The type have not been synthesized thus far. However, such com-
former type has been studied extensively; however, as individ- pounds could prove to be useful for catalytic applications and
ual compounds, 1,2,4-triazol-5-ylidenes are relatively unex- for the development of new methods for haloarene
plored. The first stable carbene of this type, namely a 1,3,4- hydrodehalogenation.
triphenyl substituted derivative, was prepared and investigated
Halogenaromatic compounds (haloarenes) are widely used
by Enders and co-workers.5,6 Subsequently, new types of 1-ada- as e.g. pesticides, pharmaceuticals, and high-temperature
mantyl- and 1-tert-butyl substituted 1,2,4-triazol-5-ylidenes fluids. Unfortunately, most of the aforementioned materials
have been described7,8 along with their transformations upon are highly toxic. Moreover, these toxic haloarenes have accu-
heating9 and reaction with a malonic ester.10 Biscarbenes with mulated in large amounts throughout the world. Several of
enhanced thermal and chemical stabilities featuring conjugated these haloarene compounds require particular attention,
3,3′- and 4,4′-bis-1,2,4-triazol-5-ylidenes have been synthesized specifically DDT, hexachlorobenzene, dioxins, polychlorodi-
and used as ligand supports for copper(I) complexes.11,12 In a benzofurans (furans), and polychlorobiphenyls.16 All of the
series of studies 1,2,4-triazol-5-ylidene complexes were pre- foregoing compounds were included in the Stockholm Conven-
pared in situ from the corresponding salts (for the latest see, tion of May 22nd 2001 in a list of the 12 most hazardous “per-
sistent organic pollutants” (POPs). The safe management of
these troublesome toxic POPs represents a persistent environ-
mental concern.
aThe L.M. Litvinenko Institute of Physical Organic and Coal Chemistry, Ukrainian
Academy of Sciences, 70, R. Luxemburg, Donetsk, 83114, Ukraine.
The most common pathways for haloarene detoxification
E-mail: nkorotkikh@ua.fm; Fax: +(380 62)311-68-30; Tel: +(380 62)311-68-35
bDepartment of Chemistry, The University of Texas at Austin, 1 University Station
are as follows: (1) oxidative methods (combustion at high
temperatures or oxidation in critical water, etc.), (2) reductive
methods such as catalytic reduction with hydrogen, reduction
with metals and nanometals, and (3) biodegradation
methods.17 The first group of methods for haloarene detoxifi-
cation has some disadvantages such as the use of high
A5300, Austin, Texas 78712-0165, U.S.A.. E-mail: acowley@cm.utexas.edu;
Tel: +1(512)471-74-84
†Electronic supplementary information (ESI) available: X-ray crystallographic
data for 8c, 9a,b, 10c; NMR spectra; CIF files for 8c, 9a,b, 10c. CCDC 1000720
(8c), 1000718 (9a), 1000719 (9b) and 1000721 (10a). For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c4dt01353k
This journal is © The Royal Society of Chemistry 2014
Dalton Trans.

View Article Online
Paper
Dalton Transactions
temperature and/or high pressure conditions, the difficulty in efficiencies for the reactions of monochlorobenzenes at room
recovering the vaporized polychloroarenes, and the high risk temperature27 (TON 100, TOF 200 h−1) and complex 2 has
of producing dioxins. The second group of detoxification been shown to be active for the hydrodehalogenation of
methods also presents some unfavorable aspects, particularly p-dichlorobenzene at 60 °C (TON 194, TOF 111 h−1).28 Further-
those that concern the formation of corrosive hydrohalogen- more, the use of microwave irradiation at 120 °C resulted in
ides and the high pressures that are necessary for hydrogen- the highest rate for this reaction. In this case, a virtually quan-
ation and acidic reductions,18 the use of dangerous or titative yield of the product was obtained in only 2 min (TON
expensive reducing reagents such as alkali metals and metalor- 4000, TOF 120 000 h−1 for p-chlorotoluene; TON 3800, TOF
ganic compounds,19 microwave techniques,20 all of which 114 000 h−1 for p-dichlorobenzene).28
result in low decomposition rates or efficiencies. For example,
The catalytic efficiency of the nickel complex 3 that was pre-
the promising use of metallic calcium in ethanol for hydrode- pared via the reaction of Ni(PPh3)2Br2 with the corresponding
halogenation requires large quantities of the metal (10–40 fold carbene29 is not high for the hydrodebromination transform-
excess per 1 Cl) and involves the formation of several by-pro- ation of p-bromofluorobenzene or for the hydrodechlorination
ducts of the ring reduction.21–23 Noble metal nanoparticles of p-chlorofluorobenzene (activated haloarenes) (TON 15–22
allow recycling of the catalyst. However, they require approxi- and TOF 4–44 h−1 at 25 °C).
mately 1 mol% Pd to achieve the necessary activity for detoxifi-
The nickel(I) complex 4 was used for the hydrodefluorina-
cation.24,25 Furthermore, biodegradation methods with aerobic tion of fluorobenzenes.30 It was found that in the presence of
microbes are only suitable for the reduction of low haloge- Et2CHONa at 100 °C catalysts of the type L2Ni are less efficient
nated arenes and hence they are not useful as a general detoxi- for hydrodehalogenation (TON 12 and TOF 4 h−1 for L = IMes)
fication method (such organisms are contained in small than LNi (TON 18–31 and TOF 6–10 h−1, respectively). Taken
amounts in the first few mm of soil).16,17 Moreover, anaerobic collectively, complexes 1–4 exhibit insufficient catalytic activi-
microorganisms are suitable for reductive dechlorination ties. However, the highest efficiencies for polyhalogenated
(hydrodechlorination) which results in the conversion of arenes were obtained with the dimeric (IPrPdCl2)2 catalyst 5
more chlorinated hydrocarbons. However, such species only (R = Dipp)31 (for the hydrodehalogenation of 1,2,4,5-tetrachloro-
react with m- and p-substituted compounds and do not activate benzene in isopropanol at 80 °C TON 19 650 and TOF 819 h−1
o-substituted haloarenes.
and for the reaction of 2,2′,3,3′-tetrachlorobiphenyl with a TON
Thus, among the known methods, catalytic reductive deha- of 10 000 and with a TOF 417 h−1, respectively).
logenation (hydrodehalogenation) promises the greatest poten-
Given the foregoing information, sterically shielded 1,2,4-
tial for success. In the interest of achieving high catalytic triazol-5-ylidenes were investigated as ligands for haloarene
activities and cost effectiveness, catalytic reductive dehalogena- hydrodehalogenation catalysis. In the present contribution, we
tion could represent the best solution for the neutralization of describe (i) the syntheses of new sterically shielded carbenes
toxic POPs. Moreover, palladium carbene complexes are well 1-alkyl-3,4-diaryl-1,2,4-triazol-5-ylidenes; (ii) the syntheses
known to be the suitable catalysts for the foregoing reaction. of mono- and biscarbene palladium halogenide complexes of
For example, it is known that an in situ generated catalyst the 1,2,4-triazol-5-ylidenes; (iii) the X-ray crystal structures of
based on Pd(dba)2 and 1,3-disubstituted imidazolin-2-ylidenes the sterically shielded carbene 1-tert-butyl-3-phenyl-4-(2,6-di-
affords high yields of hydrodehalogenation products when isopropylphenyl)-1,2,4-triazol-5-ylidene; three PdI2 and PdCl2
carried out in dioxane at 100 °C (TON up to 28–50, TOF up to carbene
complexes
bis-[1-tert-butyl-3-phenyl-4-(4-bromo-
28–50 h−1).26 Compound 1 (Fig. 1) provides moderate catalytic phenyl)-1,2,4-triazol-5-yliden]palladium chloride, bis-[1-tert-
butyl-3-phenyl-4-(4-bromophenyl)-1,2,4-triazol-5-yliden]palla-
dium iodide and the sterically shielded [1-tert-butyl-3-phenyl-4-
(2,6-diisopropylphenyl)-1,2,4-triazol-5-yliden]palladium chlor-
ide; (iv) catalytic efficiencies of the foregoing complexes with
respect to the hydrodechlorination of p-dichlorobenzene.
Results and discussion
Synthesis of carbenes and their palladium complexes
In order to obtain the desired carbene complexes, along with
the known carbene 8a,9 a series of new stable carbenes 8b–d
was synthesized in the following three stages: (1) syntheses of
sterically hindered 4-substituted 3-phenyl-4-aryl-1,2,4-triazoles
6b,c via the ring transformation of 2-phenyl-1,3,4-oxadiazole
with the appropriate anilines, (2) quaternization of 1,2,4-tri-
Fig. 1 Palladium and nickel catalysts for the hydrodehalogenation reac-
azoles 6b,c using sterically hindered alkylhalogenides (tert-
tions (1, 2, 5 R = Dipp (2,6-diisopropylphenyl); 3, 4 R = Mes (2,4,6-
trimethylphenyl).
butyl iodide and 1-bromoadamantane), (3) deprotonation of
Dalton Trans.
This journal is © The Royal Society of Chemistry 2014

View Article Online
Dalton Transactions
Paper
other form was evident from the presence of only one signal
for the methyl groups (δ 1.19 ppm) and a downfield signal for
the methine CH-groups (δ 3.02 ppm).
The meso-proton CHN resonances for these forms were
found at 8.10 and 8.30 ppm, respectively. Presumably, the
second form relates to a symmetrical structure with a close to
90° angle for the N(4)-aryl group. On the other hand, the alter-
nate form could be a rotational form with a smaller bond
angle. In DMSO-d6 solution the second form converted to the
first one (the pertinent signals were located at δ 0.88, 1.09, and
2.24 ppm, respectively). Presumably for the same reason, in
DMSO-d6 solution, salt 7c shows only the first form of the
isomers (δ 0.88, 1.09, and 2.24 ppm). However, the NMR
spectrum of salt 7d in CDCl3 corresponds to only the first
form (δ 0.91, 1.21, and 2.24 ppm, respectively). Interestingly,
compounds 6b,c were also obtained recently33 by a similar
method to the one described above. Following purification,
only one form of this compound was isolated.
Scheme 1 Syntheses of carbenes 8a–d. Reaction conditions: (a) ArNH2,
TFA, o-DCB, 190 °C, 12 h; (b) (1) RX, AcOH, 120 °C, 10–18 h; (2) NaClO4;
(c) t-BuOK, PhMe, i-PrOH, r.t. Compounds: 6a Ar = 4-BrC6H4; 6b Ar =
Mes; 6c Ar = Dipp; 7a, 8a R = t-Bu; Ar = 4-BrC6H4; 7b, 8b R = 1-Ad; Ar =
Mes; 7c, 8c R = t-Bu; Ar = Dipp; 7d, 8d R = 1-Ad; Ar = Dipp; Dipp =
2,6-i-Pr2C6H3.
The most useful information, for carbenes 8a–d, are the
13C NMR signals for the carbene carbon atom C(5)
(δ 211.1–212.7 ppm). It was found that the NMR signals
for complexes 9a–d and 10a–d are shifted upfield
(δ 153.3–171.9 ppm). Interestingly, two closely spaced reso-
nances were detected for compound 9b in CDCl3 solution.
These resonances can be assigned to the trans–trans and cis–
Fig. 2 Mono and biscarbene palladium complexes (9a X = Cl, R = t-Bu,
Ar = p-BrC6H4; 9b X = I, R = t-Bu, Ar = p-BrC6H4; 9c X = I, R = Ad, Ar =
Mes; 9d X = I, R = t-Bu, Ar = Dipp; 10a X = Cl, R = t-Bu, Ar = Dipp; 10b X
= I, R = t-Bu, Ar = Dipp; 10c X = Cl, R = Ad, Ar = Dipp; 10d X = I, R = Ad,
Ar = Dipp).
the aforementioned salts 7b–d using potassium tert-butoxide trans isomers of 9b (the first designation relates to the disposi-
in a mixture of toluene and isopropanol akin to previously
published work from our laboratory7 (Scheme 1), most of
tion of the rotating triazole rings with respect to each other
and the second one relates to the disposition of the halogen
which (8b–d) include sterically hindered groups (t-Bu or 1-Ad atoms at the metal center). The first isomer was isolated in a
in position 1 with Mes or Dipp in position 4).
pure state and subsequently characterized on the basis of a
single crystal X-ray diffraction study. The 1H and 13C NMR
The syntheses of the new biscarbene complexes 9a–d
(Fig. 2) were carried out via the direct reaction of the free car- spectra for complex 10a in CDCl3 solution exhibited two
benes 8a–d with palladium halogenides (chloride and iodide)
forms, presumably due to rotation of the N(4)-aromatic ring.
in a molar ratio of 2 : 1 in THF solution according to the perti-
The presence of two different signals of the tert-butyl protons
nent literature method.32 An analogous method was used for confirmed this interpretation (δ 2.18 and 2.29 ppm). However,
only one methyl signal was evident in the 13C NMR spectrum
the syntheses of the monocarbene complexes 10a–d using the
appropriate molar ratio of 1 : 1. For purposes of comparison,
the known complex 5 was prepared via the reaction of the (δ 64.3 ppm). Furthermore, the presence of two groups of
(δ 31.4 ppm), along with one signal for the ipso-C atom
1
appropriate 1,3-diarylimidazol-2-ylidene with Pd(PhCN)2Cl2
according to the literature procedure.31
methyl and CH-resonances is evident in the H NMR spectra
(δ 0.54–0.70 ppm and 1.16–1.57 ppm, and 2.50, 2.61 ppm,
Compounds 6a–c, 7a–d, 8a–d, 9a–e, 10a–d were character- respectively). These resonances indicate the existence of non-
ized by 1H and 13C NMR spectroscopy, and their elemental
equivalent isopropyl and methyl groups in both isomers.
compositions were established on the basis of elemental ana-
However, only one isomer was detected in the single crystal
lysis. The 13C NMR spectra of the sparingly soluble complexes X-ray structure (see below).
The solid state 13C NMR spectra of complexes 9c,d and 10c,
d exhibited only one isomeric form for each compound (single
specific signals for the mesityl or dipp groups, the resonances resonances are apparent for the i-Pr CH, Ad ipso-C, C(3) and
9c,d and 10b,d were recorded in the solid state. In the case of
1
the H NMR spectra of triazoles 6b,c with the exception of the
of the meso-protons CHN were evident (δ 8.73, 8.88 ppm in
DMSO-d6). In the case of the spectra of the triazolium salts
C(5)Pd atoms at δ 47.4–49.1, 64.1–65.6, 148.2–156.4, and
154.9–171.9 ppm, respectively). The C(5)Pd signals for the
7a–d the NMR signals were shifted significantly downfield chloride complexes (10a,c) were shifted upfield (δ 153.3–
(δ 10.61–10.91 ppm). It is noteworthy that two forms of the
triazole 6c were detected in CDCl3 solution. The NMR spec-
154.9 ppm) relative to those for similar resonances (δ 167.0–
171.9 ppm) that were observed for the iodide complexes (9c,d
trum of one of these forms was characterized on the basis of and 10b,d). Four resonances were evident for the carbenoid
different signals for the methyl groups (δ 0.90, 1.10 ppm) and
the presence of an upfield signal for the methine CH-groups (δ
carbon of complex 10b and can be explained by the presence
of four fixed rotational forms in the solid state. The H NMR
1
2.37 ppm) of the corresponding isopropyl fragments. The spectrum for 10b in DMSO-d6 solution indicates that only
This journal is © The Royal Society of Chemistry 2014
Dalton Trans.

View Article Online
Paper
Dalton Transactions
two forms of the complex were present in solution (for the
i-Pr group CH3C δ 0.62, 1.36; CHC 2.94 ppm and CH3C
δ 0.83, 1.25; CHC 4.11 ppm and one signal of the t-Bu CH3C
group).
X-ray diffraction studies
The single crystals of compound 8c were grown from toluene
solution and studied subsequently by X-ray diffraction. The
structure of 8c is presented in Fig. 3 and the bond lengths (Å),
bond orders, bond angles and dihedral angles (degrees) for
structure 8c are presented in Table 1. As can be seen from
these data, the planar structure of 8c is reminiscent of those
for the 1,2,4-triazol-5-ylidenes.9,10,34 The bond angle at the
carbene carbon atom is approximately 100.5° and the high
bond order of 1.718 for the C(1)–N(3) bond implies a substan-
tial ylide contribution to the carbene structure. Moreover, the
C(2)-aromatic ring is twisted relative to the plane of the triazole
ring by 36.3° (the bond order is 1.284), and furthermore the
N(1)-aromatic ring is twisted almost orthogonally with respect
to the triazole ring (87.4°) (the bond order is 1.139).
Fig. 4 POV-Ray diagrams for bis-(1-tert-butyl-3-phenyl-4-bromo-
phenyl-1,2,4-triazol-5-yliden)palladium chloride 9a with thermal ellip-
soids displayed at 50% probability. All hydrogen atoms have been
removed for clarity.
The single crystals of the palladium complexes 9a,b and
10a that were used for the X-ray diffraction studies were grown
from acetonitrile. The crystal structures of 9a,b and 10a are
displayed in Fig. 4–6 and the bond lengths, bond orders, bond
angles and dihedral angles are presented in Tables 2–4.
Complexes 9a,b adopt predominant trans–trans isomeric
orientations. The bond orders for N(2)–C(1) in these molecules
(p 1.770–1.782) closely resemble that of the initial carbene
Fig. 5 POV-Ray diagrams for bis-(1-tert-butyl-3-phenyl-4-bromo-
phenyl-1,2,4-triazol-5-yliden)palladium iodide 9b with thermal ellipsoids
displayed at 50% probability. All hydrogen atoms have been removed for
clarity.
Fig. 3 POV-Ray diagrams for 1-tert-butyl-3-phenyl-4-(2,6-diisopropyl-
phenyl)-1,2,4-triazol-5-ylidene 8c with thermal ellipsoids displayed at
50% probability. All hydrogen atoms have been removed for clarity.
Table 1 Selected bond lengths and angles for carbene 8c
Bond
Bond
Length (Å) order Angle
°
C(1)–N(3)
C(1)–N(1)
C(2)–N(1)
C(2)–N(3)
C(2)–C(15)
C(3)–N(1)
N(2)–N(3)
1.3491(14) 1.718 N(3)–C(1)–N(1)
1.3843(15) 1.516 N(1)–C(2)–N(2)–N(3)
1.3860(14) 1.506 N(1)–C(1)–N(3)–C(21) 177.70(10)
1.3134(14) 1.923 N(1)–C(2)–C(9)–C(10)
1.4781(15) 1.284 C(1)–N(1)–C(3)–C(4)
100.53(9)
−0.48(11)
36.3(6)
87.40(13)
—
—
—
Fig. 6 POV-Ray diagrams for 1-tert-butyl-3-phenyl-4-(2,6-diisopropyl-
phenyl)-1,2,4-triazol-5-yliden)palladium chloride 10a with thermal ellip-
soids displayed at 50% probability. All hydrogen atoms and an
acetonitrile molecule have been removed for clarity.
1.4498(13) 1.139
1.3834(13) 1.336
—
—
—
N(3)–C(21) 1.4917(14) 0.898
Dalton Trans.
This journal is © The Royal Society of Chemistry 2014

View Article Online
Dalton Transactions
Paper
The bond angles at the carbenoid carbon atom (103°) are
significantly larger than those for the carbene (100.3°) and are
noticeably smaller than that observed for the salt complex
(110°).7,34 Furthermore, the Pd–C bond distances (2.025,
2.034 Å) are notably larger than the sum of the covalent radii
for these atoms (1.95 Å) and significantly larger than the sum
of covalent radius of the carbon atom and the ionic radius of
palladium(II) (1.75 Å) that is indicative of a C–Pd coordination
bond in these complexes.
The Pd–Cl bond distances in complex 9a (2.330 Å) are inter-
mediate in length between those of covalent and ionic bonds
(the sum of the covalent radii is 2.19 Å, while the sum of the
ionic radii is 2.67 Å). Overall, however it is closer to a covalent
bond (the extent of Pd(1)–Cl(1) ionization is approximately
29%). In the case of complex 9b, the Pd–I bond (2.60 Å) is also
intermediate in character with respect to covalency vs. ionicity
(the sum of the covalent radii is 2.53 Å and the sum of the ionic
radii is 3.06 Å). Overall, however the Pd–I bond more closely
resembles a covalent bond (the extent of Pd(1)–I(1) ionization is
13%). Thus, complexes 9a,b possess typical structures with C–Pd
coordination bonds and shortened Pd–halogen bonds.
Compound 10a is a monocarbene–acetonitrile palladium
complex. The bond order of the N(1)–C(1) bond in this mole-
cule (p 1.833) is larger than that of the initial carbene (1.718).
The Pd–C bond distance (1.963 Å) is close to the sum of the
covalent radii of the constituent atoms (1.95 Å) and signifi-
cantly larger than the sum of the covalent radius of the carbon
atom and the ionic radius of palladium(II) (1.75 Å). The latter
observation, along with the bond orders, indicates that there is
a stronger interaction between the carbene carbon and the
palladium atoms than in the case of the biscarbene complexes
9a,b. Moreover, the Pd–Cl bond in the structure of 10a
(2.308 Å) is intermediate in length between covalent and ionic
bonds (the sum of the covalent radii is 2.19 Å and the sum of
the ionic radii is 2.67 Å), but closer to that of a covalent bond
(the extent of Pd(1)–Cl(1) ionization is approximately 25%).
The bond angle of the carbenoid carbon atom (103.4°) is
significantly larger than that of the initial carbene (100.5°)
however the salt carbenoid angle is larger by comparison
(110°).7,34 Furthermore, the C(2) nucleus is twisted relative to
that of the triazole ring by 151.0° as opposed to 29.9° for the
initial carbene. Furthermore, the N(3)-nucleus is twisted with
respect to the triazole ring by 102.1° versus 51.1° for the same
Table 2 Selected bond lengths and angles for complex 9a
Bond
Bond
Length (Å) order Angle
°
C(1)–N(2)
C(1)–N(1)
C(2)–N(3)
C(2)–N(1)
C(2)–C(9)
C(3)–N(1)
N(2)–N(3)
N(2)–C(15)
C(6)–Br(1)
C(1)–Pd(1)
1.338(4)
1.368(5)
1.308(5)
1.377(4)
1.469(5)
1.443(4)
1.386(4)
1.500(5)
1.895(4)
2.025(4)
1.782 N(1)–C(1)–N(2)
1.609 C(1)–Pd(1)–Cl(1)
103.7(3)
89.70(10)
180.00(5)
−0.2(4)
−1.7(4)
36.3(6)
1.954 Cl(1)–Pd(1)–Cl(1a)
1.557 N(3)–C(2)–N(1)–C(1)
1.330 N(1)–C(1)–N(2)–N(3)
1.178 N(1)–C(2)–C(9)–C(10)
1.323 C(4)–C(3)–N(1)–C(1)
0.851 Pd(1)–C(1)–N(1)–C(2)
69.3(5)
−174.1(2)
—
—
—
Pd(1)–C(1)–N(2)–C(15) −4.4(6)
N(1)–C(1)–Pd(1)–Cl(1)
N(2)–C(1)–Pd(1)–Cl(1)
86.0(3)
92.4(4)
Pd(1)–Cl(1) 2.3301(10)
Table 3 Selected bond lengths and angles for complex 9b
Bond
Bond
Length (Å) order Angle
°
C(1)–N(2)
C(1)–N(1)
C(2)–N(3)
C(2)–N(1)
C(2)–C(9)
C(3)–N(1)
N(2)–N(3)
1.340(7)
1.368(7)
1.308(7)
1.386(7)
1.474(7)
1.435(7)
1.376(6)
1.770 N(1)–C(1)–N(2)
1.609 C(1)–Pd(1)–I(1)
1.954 I(2)–Pd(1)–I(1)
1.506 N(3)–C(2)–N(1)–C(1)
1.305 N(1)–C(1)–N(2)–N(3)
1.224 N(1)–C(2)–C(9)–C(10)
1.374 C(4)–C(3)–N(1)–C(1)
0.902 Pd(1)–C(1)–N(1)–C(2)
103.4(4)
90.02(15)
176.59(2)
−0.2(6)
0.8(6)
12.3(10)
69.3(5)
N(2)–C(15) 1.491(7)
−179.7(4)
C(6)–Br(1)
C(1)–Pd(1)
Pd(1)–I(1)
1.907(5)
2.034(5)
2.6014(7)
—
—
—
Pd(1)–C(1)–N(2)–C(15) −6.3(9)
N(1)–C(1)–Pd(1)–I(1)
N(2)–C(1)–Pd(1)–I(1)
85.8(5)
−97.6(5)
Table 4 Selected bond lengths and angles for complex 10a
Bond
Bond
Length (Å) order Angle
°
C(1)–N(2)
C(1)–N(1)
C(2)–N(3)
C(2)–N(1)
N(1)–N(2)
C(2)–C(7)
N(3)–C(14) 1.457(4)
C(3)–N(1)
C(1)–Pd(1)
Pd(1)–Cl(1) 2.3078(8)
Pd(1)–N(4) 2.075(3)
1.329(4)
1.366(4)
1.384(4)
1.309(4)
1.375(3)
1.482(4)
1.833 N(1)–C(1)–N(3)
104.6(2)
92.36(8)
173.88(3)
0.4(3)
1.621 C(1)–Pd(1)–Cl(1)
1.517 Cl(1)–Pd(1)–Cl(2)
2.142 C(1)–N(1)–N(2)–C(2)
1.379 N(3)–C(1)–N(1)–N(2)
1.264 C(19)–C(14)–N(3)–C(1) 102.1(3)
1.098 N(3)–C(2)–C(7)–C(8)
0.770 Pd(1)–C(1)–N(3)–C(2)
0.1(3)
151.0(3)
175.50(19)
1.514(4)
1.963(3)
—
—
—
Pd(1)–C(1)–N(2)–C(15) −6.3(9)
N(1)–C(1)–Pd(1)–Cl(1)
N(1)–C(1)–Pd(1)–Cl(2)
−84.3(3)
89.6(3)
(1.799).10 For comparison, similar features were observed for carbene. The bond orders for C(2)–C(7) and N(3)–C(14) are
the linear bistriazolylidene complexes of copper(I) chloride9,16 1.264 and 1.098, respectively. Thus, complex 10a adopts a
and the related bistriazolylidene chelate complex of copper(I) typical structure with a strong C–Pd coordination bond and a
iodide.11
The dihedral angles of the aromatic rings in molecules 9a,b
are also close to that of the initial carbene. The C(2)-nucleus is
shortened bond Pd–Cl.
Catalytic tests
twisted relative to the triazole ring by 36.3° (9a) and 12.3° (9b), Test reactions for the hydrodechlorination catalysis were
respectively versus 29.9° for the initial carbene.
carried out by heating a reaction mixture of the haloarene with
Furthermore, the N(1) nucleus is twisted with respect to the potassium or sodium isopropoxides, which had been gener-
same plane by 69.3° (9a,b) versus 51.1° for the initial carbene. ated from potassium or sodium tert-butoxide or hydroxide, in
The bond orders for C(2)–C(9) and N1–C(3) are 1.31–1.33 and the presence of a catalyst in isopropanol at 80 °C. Control of
1.18–1.22, respectively, since the C(2) nucleus is naturally more the conversion (on 1 chlorine atom) was calculated according
conjugated with the triazole ring than with the N(1)-nucleus.
to the masses of the sodium or potassium chlorides, both of
This journal is © The Royal Society of Chemistry 2014
Dalton Trans.

View Article Online
Paper
Dalton Transactions
complex 10c is relatively low (TON 1140 cycles after 8 h),
however, in the presence of 1 mol% lithium tert-butoxide the
TON is enhanced up to 8000 cycles after 24 h.
Table 5 Catalytic efficiencies of carbene complexes 9a–d, 10a–d, 5 for
the reaction of p-dichlorobenzene with potassium tert-butoxide in iso-
propanol at 80 °C
When sodium tert-butoxide was used for these hydrode-
halogenation reactions (Table 6), a significant increase in the
efficiency indices by almost an order of magnitude was
observed for the triazolylidene complexes 9d, 10b,c (TON
7200–15 400, TOF 300–2230 h−1). In fact, these values exceeded
those for the etalon complex 5 (TON 6400, TOF 266 h−1).
The hydrodechlorination reaction was examined in the
presence of sodium hydroxide and isopropanol using com-
plexes 5 and 10c,d. The use of sodium hydroxide is particularly
appropriate in view of the possible development of a techno-
logy for haloarene (production wastes) neutralization (detoxifi-
cation) since it is an inexpensive reagent. It was found
(Table 7) that the triazolylidene complexes 10c,d exhibited
high efficiencies for the reaction of p-dichlorobenzene with
sodium hydroxide (TON 2000–6800, TOF 176–284 h−1).
However, for 10d this represents threefold decrease with
respect to the use of potassium tert-butoxide.
Interestingly, the known complex 5 appeared to be some-
what more effective for the reaction with sodium hydroxide
(TON 19 200, TOF 800 h−1), than those with potassium tert-but-
oxide (TON 17 600, TOF 734 h−1) and sodium tert-butoxide
(TON 6400, TOF 266 h−1).
On the basis of gas chromatographic data, noticeable
quantities of haloarenes were not observed in the experi-
ments that resulted in quantitative conversion of the initial
haloarene.
Catalyst amount
(mol%)
Reaction
time (h)
Conv.
(%)
TON per
1 Cl
TOF
(h−1
Catalyst
)
9a
9b
9c
9d
10a
10b
10c
10ca
10d
5
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.01
0.01
0.01
0.01
8
8
8
8
8
8
8
24
24
8
25
11
70
38
66
58
72
40
100
63
88
500
220
1400
760
1320
1160
1140
8000
20 000
12 600
17 600
62
28
175
95
165
145
180
334
840
1560
734
5
24
a The reagent is potassium tert-butoxide to which 1 mol% lithium tert-
butoxide has been added.
Table 6 Catalytic efficiencies of carbene complexes 9d, 10b,c, 5 for
the reaction of p-dichlorobenzene with sodium tert-butoxide in iso-
propanol at 80 °C
Catalyst amount
(mol%)
Reaction
time (h)
Conv.
(%)
TON per
1 Cl
TOF
Catalyst
(h−1
)
9d
10b
10c
5
0.01
0.01
0.01
0.01
24
24
6
53
36
77
32
10 600
7200
15 400
6400
440
300
2230
266
24
In conclusion, the highest efficiency is apparent for the cat-
alysts that feature the sterically bulky 1-(1-adamantyl)-3-phenyl-
4-(2,6-diisopropylphenyl)-1,2,4-triazol-5-ylidene ligand.
Table 7 Catalytic efficiencies of carbene complexes 9d, 10a–d for the
reaction of p-dichlorobenzene with sodium hydroxide in isopropanol at
80 °C
The high TON of 20 000 cycles (10d) with potassium tert-
butoxide and TON 15 400 with sodium tert-butoxide exceed the
efficiency level of complex 5 (TON 17 600 and 6400, respect-
ively). Another factor that contributes to the catalytic efficiency
is the presence of iodide as the second ligand which raises the
efficiency of the catalyst (TON 20 000 cycles for 10d, TON 8000
cycles for 10c).
Catalyst amount
(mol%)
Reaction
time (h)
Conv.
(%)
TON per
1 Cl
TOF
Catalyst
(h−1
)
10c
10c
10d
5
0.1
8
24
24
24
100
34
21
2000
6800
4200
250
284
176
800
0.01
0.01
0.01
96
19 200
As far as we know, the mechanism of haloarene (i.e.
p-dichlorobenzene) hydrodehalogenation2,3 involves the for-
mation of palladium(0) complex (D) as the first step via the
interaction of the palladium(II) complex with metal iso-
propoxide, followed by reaction with the haloarene E thereby
affording the palladium(II) species (F) (Scheme 2).
which are insoluble in isopropanol. The most efficient catalysis
trials were examined on the basis of gas chromatography. The
catalytic efficiencies were estimated using the values of TON
and TOF.
A summary of the results pertaining to the studies of the
catalytic efficiencies for the carbene complexes 5, 9a–d, and
10a–d with respect to the model compound p-dichlorobenzene
and a variety of bases is presented in Tables 5–7.
Examination of Table 5 reveals that the hydrodehalogena-
tion of p-dichlorobenzene with potassium tert-butoxide pro-
ceeds efficiently with the majority of the carbene catalyst
complexes that were tested. The most rapid reaction was
observed with the triazolylidene monocarbene complex 10d
(after 24 h of the reaction the maximum TON and TOF values
reached 20 000 and 840 h−1 respectively). The efficiency of
The activated complex F reacts subsequently with the metal
isopropoxide via the nucleophilic substitution of the halogen,
thus forming the adduct of the H-arene with palladium
species H due to decomposition of the previously formed iso-
propoxide complex G. Finally, decomposition of the latter pro-
duces H-arene I and the palladium(0) complex D. The enthalpy
changes that take place during chlorobenzene conversion cata-
lyzed by complex 10a were evaluated on the basis of DFT,
B3LYP5, 3-21G calculations (Scheme 2, in kcal mol−1). These
calculations indicate that a strong exothermic reaction takes
place between palladium(0) compound D and chlorobenzene
Dalton Trans.
This journal is © The Royal Society of Chemistry 2014

View Article Online
Dalton Transactions
Paper
using
a
Rigaku AFC12 Saturn 724+ CCD diffractometer
equipped with a Rigaku XStream low temperature device that
is operated at 100 K, a Rigaku SCX-Mini Diffractometer
equipped with a Rigaku XStream low temperature device that
is operated at 100 K, or a Nonius-Kappa CCD diffractometer
with an Oxford cryostream 600 that is operated at 153 K. All
instruments used a graphite-monochromated Mo Kα radiation
source (λ = 0.71075 Å). Corrections were applied for Lorentz
and polarization effects. The structures were solved by direct
methods and refined by full-matrix least-squares cycles on F2
using the Siemens SHELXTL PLUS 5.0 (PC) software package35
and PLATON.36 All non-hydrogen atoms were refined aniso-
tropically. The hydrogen atoms were placed in fixed, calculated
positions using a riding model.
Thin-layer chromatography was performed on silica gel
with chloroform or a 10 : 1 mixture of chloroform and metha-
nol as the eluent, followed by development with iodine.
Elemental analyses were carried out at the Analytical Labora-
tory of the Litvinenko Institute of Physical Organic and Coal
Chemistry. The quantum chemical calculations were per-
formed with the help of Firefly 8.0 program.
Scheme 2 Mechanism of haloarene hydrodehalogenation.
(ΔH −48.0 kcal mol−1) and that the conversion of compound
G to H is endothermic (ΔH +30.7 kcal mol−1). An important
stage in the foregoing process may be the interaction of the
initial precatalyst A with the metal isopropoxide (ΔH −16.0 kcal
mol−1), which is the initial stage in the formation of these mole-
cular complexes. As might be expected, the stabilities of these
complexes depend on steric protection of the palladium complex.
A possible reason for the increase in catalytic efficiency
for sterically shielded carbene complexes, particularly 10c,d,
is attributed to the steric protection which results in an
increased stability of the intermediate Pd(0) species in the
reaction medium.
3,4-Diaryl-1,2,4-triazoles (6b,c) (general procedure)
3,4-Diaryl-1,2,4-triazoles were obtained according to the modi-
fied procedure of the work37 by the ring transformation of
2-phenyl-1,3,4-oxadiazole with the respective anilines in the
presence of trifluoroacetic acid at 190 °C. The mixture of aro-
matic amine (20 mmol) and trifluoroacetic acid (20 mmol) in
o-dichlorobenzene (1–2 mL) was added to 2-phenyl-1,3,4-oxa-
diazole (20 mmol) and heated at 190 °C for 12 h. The resulting
oil or crystalline product was washed 5–6 times with petroleum
ether (50 ml). Then 10% aqueous solution of potassium
hydroxide was added and the mixture was stirred for
15–20 min. The resulting crystalline precipitate was filtered
off, washed with water, then with a mixture of petroleum
ether–isopropanol (3 : 1) and petroleum ether and dried. The
obtained products were recrystallized from benzene.
Conclusions
Along with the syntheses of the new biscarbene palladium
complexes 9a,b from the known carbene 8a, a series of steri-
cally shielded 1,3,4-trisubstituted 1,2,4-triazol-5-ylidenes 8b–d
and their palladium complexes 9c,d, 10a–d have been syn-
thesized in the present work. It was found that sterically
shielded compounds (9c,d, 10a–d) are effective catalysts for
p-dichlorobenzene hydrodechlorination. The efficiencies for
the monocarbene complexes 10c,d were shown to be somewhat
higher than those for the biscarbene complexes 9c,d. However,
the latter results are strongly dependent on the nature of the
reagent.
3-Phenyl-4-(2,4,6-trimethylphenyl)-1,2,4-triazole (6b)
Yield 2.63 g, 50%, mp 125–126 °C (benzene). δH (400 MHz;
CDCl3; Me4Si): 1.86 (s, 6H, o-CH3C), 2.31 (s, 3H, p-CH3C), 7.09s
(2H, Ar), 7.39s (5H, Ar), 8.73s (1H, C5HN). Compound 6b was
identical to the product of the same reaction in ionic liquids
isolated by column chromatography33 (mp 125–126 °C).
3-Phenyl-4-(2,6-diisopropylphenyl)-1,2,4-triazole (6c)
Yield 3.05 g, 50%, mp 98–100 °C (benzene). Found: C, 78.7; H,
7.4; N, 13.9. Calc. for C20H23N3: C, 78.7; H, 7.6; N, 13.8%. δH
(400 MHz; DMSO-d6; Me4Si): 0.88 (d, 6H, CH3C, J 6.8 Hz), 1.09
Experimental section
General
1H NMR and 13C NMR spectra were recorded using a Bruker (d, 6H, CH3C, J 6.8 Hz), 2.24 (qr, 2H, CHC, J 6.8 Hz), 7.25–7.42
Avance II 400 spectrometer (400 MHz for H NMR spectra and (m, 7H, Ar), 7.58 (m, 1H, Ar), 8.88 (s, 1H, C5HN). δH (400 MHz;
1
100 MHz for 13C NMR spectra) in DMSO-d6, C6D6 or CDCl3 CDCl3; Me4Si): 1 form: 0.90 (d, 6H, CH3C, J 6.8 Hz), 1.10 (d,
solution or in the solid state. The 1H NMR and 13C NMR 6H, CH3C, J 6.8 Hz), 2.37 (qr, 2H, CHC, J 6.8 Hz) (i-Pr), 8.10 (s,
chemical shifts are reported relative to tetramethylsilane (TMS) 1H, C5HN); 2 form: 1.19 (d, 12H, CH3C), 3.02 (qr, 2H, CHC,
(solution) and sodium 2,2-dimethyl-2-silapentan-5-sulfonate J 6.8 Hz) (i-Pr); 8.30 (s, 1H, C5HN); general for both forms:
(DSS) (solid state). The X-ray diffraction data were collected 7.20–7.37 (m, 12H, Ar), 7.45 (d, 3H, Ar, J 7.6 Hz), 7.51 (dd, 1H,
This journal is © The Royal Society of Chemistry 2014
Dalton Trans.

View Article Online
Paper
Dalton Transactions
Ar, J 7.6 Hz). The first form of compound 6c was identical to isopropanol and toluene (1 : 3) or in THF and stirred for 1.5 h.
the product of the same reaction in ionic liquids isolated by The solvent was evaporated and the oil was obtained. This oil
column chromatography33 (mp 114–115 °C).
was then stirred with petroleum ether and the subsequent pre-
cipitate that was formed was filtered off and dried. Then, an-
hydrous toluene (10–20 mL) was added to the precipitate and
stirred for 1 h. The resulting inorganic salt was filtered off.
1-Alkyl-3,4-diaryl-1,2,4-triazolium perchlorates (7b–d) (general
procedure)
The mixture of sodium iodide (50–60 mmol) and 1.5-fold The mother liquor was evaporated and the resulting product
access of tert-butyl chloride in acetic acid (10 mL) was boiled was crystallized by stirring with petroleum ether. The precipi-
for 1.5 h. One equivalent (to iodide amount) of the respective tate was filtered off, washed several times with petroleum
3,4-diaryl-1,2,4-triazole 6a,c was added and boiled for 21 h. ether, dried, and recrystallized from toluene.
Then water (20 mL) and sodium sulfite were added to the
1-(1-Adamantyl)-3-phenyl-4-(2,4,6-trimethylphenyl)-1,2,4-
triazol-5-ylidene (8b)
obtained mixture to discolor the solution. The mixture was
diluted by water (0.5 L) and activated carbon (1–3 g) was
added. The subsequent mixture was heated to boiling and fil- Yield 50%, mp 168–169 °C (toluene). Found: C, 81.7; H, 7.8;
tered hot. Sodium perchlorate was added in an amount exceed- N, 10.5%. Calc. for C27H31N3: C, 81.6; H, 7.9; N, 10.6%. δH
ing equivalence by 10–20%. The precipitate that formed was (400 MHz; C6D6; Me4Si): 1.64 (m, 3H, Ad), 1.73 (m, 3H, Ad),
filtered off, washed with isopropanol and ether, recrystallized 1.97 (s, 6H, o-CH3C), 2.06 (s, 3H, p-CH3C), 2.13 (m, 3H, Ad),
from the respective solvent, and subsequently dried. The salts 2.66 (m, 6H, Ad), 6.69 (s, 2H, Ar), 6.94 (s, 3H, Ar), 7.61 (d, 2H,
7b,d were obtained similarly using 1-bromoadamantane Ar, J 3.6 Hz). δC (100 MHz, C6D6, Me4Si): 18.3 (o-CH3C), 21.0
instead of tert-butyl iodide. The heating time was 10–18 h.
(p-CH3C), 30.2 (CHC, Ad), 36.7 (CH2C, Ad), 44.2 (CH2C, Ad),
59.4 (ipso-C, t-Bu), 127.7, 128.4, 128.6, 129.5, 130.2 (Ar), 135.5,
136.8, 137.9 (ipso-C, Ar), 151.1 (C3), 211.5 (C5).
1-(1-Adamantyl)-3-phenyl-4-(2,4,6-trimethylphenyl)-1,2,4-
triazolium perchlorate (7b)
1-tert-Butyl-3-phenyl-4-(2,6-diisopropylphenyl)-1,2,4-triazol-
5-ylidene (8c)
The heating time was 10 h. Yield 85%, mp > 300 °C (acetic
acid). Found: C, 65.0; H, 6.5; Cl, 7.1; N, 8.5. Calc. for
C27H32ClN3O4: C, 65.1; H, 6.5; Cl, 7.1; N, 8.4%. δH (400 MHz; Yield 70%, mp 112–114 °C (toluene). Found: C, 79.8; H, 8.5; N,
CDCl3; Me4Si): 1.79 (s, 6H, Ad), 2.02 (s, 6H, CH3C), 2.30 (s, 3H, 11.7. Calc. for C24H31N3: C, 79.7; H, 8.6; N, 11.6%. δH
Ad), 2.35 (s, 3H, p-CH3C), 2.36 (s, 6H, o-CH3C), 7.19 (s, 2H, Ar), (400 MHz; C6D6; Me4Si): 0.85 (d, 6H, J 7.2 Hz), 1.18 (d, 6H,
7.44 (dd, 2H, Ar, J 7.6 Hz), 7.51 (dd, 2H, Ar, J 8.0 Hz), 7.62 (dd, J 7.2 Hz) (CH3C, i-Pr), 1.83 (s, 9H, CH3C, t-Bu), 2.74 (qr, 2H, J
1H, Ar, J 7.2 Hz), 10.59 (s, 1H, C5HN).
7.2 Hz, CHC, i-Pr), 6.90 (m, 3H), 7.10 (d, 2H, J 7.6 Hz), 7.24
(dd, 1H, J 7.6 Hz), 7.61 (m, 2H) (Ar). δC (100 MHz; C6D6;
Me4Si): 22.5, 24.5 (CH3C, i-Pr), 29.0 (CH, i-Pr), 30.9 (CH3C, t-
Bu), 59.3 (ipso-C, t-Bu), 128.2, 128.5, 129.5, 129.6 (Ar), 124.2,
1-tert-Butyl-3-phenyl-4-(2,6-diisopropylphenyl)-1,2,4-triazolium
perchlorate (7c)
Yield 64%, mp 232–234 °C (acetic acid–water, 1 : 1). Found: 136.6, 146.1 (ipso-C, Ar), 151.8 (C3), 213.0 (C5).
C, 62.5; H, 6.9; Cl, 7.6; N, 9.2. Calc. for C24H32ClN3O4: C, 62.4;
H, 7.0; Cl, 7.7; N, 9.1%. δH (400 MHz; DMSO-d6; Me4Si): 0.91
(d, 6H, J 6.8 Hz), 1.16 (d, 6H, J 6.8 Hz) (CH3C, i-Pr), 1.81 (s, 9H,
1-(1-Adamantyl)-3-phenyl-4-(2,6-diisopropylphenyl)-1,2,4-
triazol-5-ylidene (8d)
CH3C, t-Bu), 2.40 (qr, 2H, CHC, i-Pr, J 6.8 Hz), 7.44 (d, 2H, J Yield 68%, mp 151–153 °C (toluene). Found: C, 82.1; H, 8.4; N,
7.6 Hz), 7.51 (d, 2H, J 7.6 Hz), 7.54 (d, 2H, J 7.6 Hz), 7.63 (dd, 9.6. Calc. for C30H37N3: C, 82.0; H, 8.5; N, 9.6%. δH (400 MHz;
1H, J 7.6 Hz), 7.73 (dd, 1H, J 7.6 Hz) (Ar), 10.91 (s, 1H, C5HN).
C6D6; Me4Si): 0.87 (d, 6H, J 6.8 Hz), 1.20 (d, 6H, J 6.8 Hz)
(CH3C, i-Pr), 1.64 (m, 3H), 1.71 (m, 3H), 2.12 (m, 3H), 2.63 (m,
6H) (Ad), 2.76 (qr, 2H, CHC, i-Pr, J 6.8 Hz), 6.94 (m, 3H), 7.12
(d 2H, J 7.6 Hz), 7.26 (dd, 1H, J 7.6 Hz), 7.62 (m, 2H) (Ar). δC
1-(1-Adamantyl)-3-phenyl-4-(2,6-diisopropylphenyl)-1,2,4-
triazolium perchlorate (7d)
The heating time was 18 h. Yield 59%, mp 210–212 °C (acetic (100 MHz, C6D6, Me4Si): 22.5, 24.5 (CH3C, i-Pr), 29.4 (CH, i-Pr),
acid–water, 1 : 1). Found: C, 66.8; H, 7.0; Cl, 6.6; N, 7.7. Calc. 30.5 (CHC), 36.6, 44.6 (CH2C) (Ad), 59.5 (ipso-C, Ad), 124.2,
for C30H38ClN3O4: C, 66.7; H, 7.1; Cl, 6.7; N, 7.8%. δH 128.1, 128.5, 129.4, 129.5 (Ar), 127.6, 136.8, 146.3 (ipso-C, Ar),
(400 MHz; CDCl3; Me4Si): 0.91 (d, 6H, J 4.4 Hz), 1.28 (d, 6H, 151.5 (C3), 211.8 (C5N).
J 4.4 Hz) (CH3C, i-Pr), 1.87 (m, 6H, Ad), 2.23 (qr, 2H, CHC, i-Pr,
trans–trans-Bis(1-tert-butyl-3-phenyl-4-p-bromophenyl-1,2,4-
J 4.4 Hz), 2.42 s (3H, Ad), 2.48 (s, 6H, Ad), 7.37 (m, 4H), 7.42
triazol-5-ylidene)palladium chloride (9a)
A powder of carbene 8a9 (2 g, 0.56 mmol) and anhydrous
dimethylsulfoxide (0.03 mL) were added to the suspension of
palladium chloride (0.05 g, 0.28 mmol) in THF (5 mL) and
(m, 2H), 7.51 (m, 1H), 7.63 (m, 1H) (Ar), 10.28 (s, 1H, C5HN).
1-Alkyl-3,4-diaryl-1,2,4-triazol-5-ylidenes (8b–d) (general
procedure)
A
suspension of 1-tert-butyl-3,4-diaryl-1,2,4-triazolium per- stirred for 0.5 h. The suspension of complex 9a was evaporated
chlorate 7a–d and potassium tert-butoxide (95% from the cal- to dryness and stirred with petroleum ether. The precipitate
culated amount) was added to the mixture of anhydrous was filtered off and dried. Yield 0.25 g (100%), mp 160–163 °C
Dalton Trans.
This journal is © The Royal Society of Chemistry 2014

View Article Online
Dalton Transactions
Paper
(subl.) (acetonitrile). Found: C, 48.7; H, 4.0; Br, 18.3; Cl, 8.1; N, [1-tert-Butyl-3-phenyl-4-(2,6-diisopropylphenyl)-1,2,4-triazol-5-
9.4. Calc. for C36H36Br2Cl2N6Pd: C, 48.6; H, 4.1; Br, 18.0; Cl, yliden]palladium chloride (10a)
8.0; N, 9.4%. δH (400 MHz; CDCl3; Me4Si): 1.65 (s, 18H, CH3C),
7.26 (m, 10H), 7.53 (m, 4H) 7.69 (m, 4H) (Ar). δC (100 MHz,
A mixture of carbene 8c (0.25 g, 0.68 mmol) and palladium
chloride (0.12 g, 0.68 mmol) in anhydrous dimethylsulfoxide
(2.5 mL) was stirred for 3 days. The precipitate that formed
was filtered off, washed by petroleum ether, and dried.
CDCl3, Me4Si): 30.6 (CH3C), 61.9 (CH3C), 128.5, 128.6, 130.3,
131.6, 132.9 (Ar), 124.0, 125.1, 136.3 (ipso-C), 151.8 (C3), 171.3
(C5Pd).
Dichloromethane (5 mL) and water (5 mL) were added to the
mother liquor and stirred again. The phases that formed were
then subsequently separated. The dichloromethane phase was
then dried by anhydrous sodium sulfate. The solvent was evap-
Bis(1-tert-butyl-3-phenyl-4-p-bromophenyl-1,2,4-triazol-5-
ylidene)palladium iodide (9b)
The same procedure that was used for complex 9a was applied
orated; the oil that formed was stirred with petroleum ether.
to the synthesis of 9b. Yield 75%, mp 119–120 °C (acetonitrile).
The precipitate that formed was filtered off and dried. Yield
Found: C, 40.4; H, 3.4; Br, 14.8; I, 23.6; N, 7.9. Calc. for
C36H36Br2I2N6Pd: C, 40.3; H, 3.4; Br, 14.9; I, 23.7; N, 7.8%. The
0.15 g (41%), mp 196–198 °C (acetonitrile). Found: C, 53.4; H,
5.9; Cl, 13.2; N, 7.7%. Calc. for C24H31Cl2N3Pd: C, 53.5; H, 5.8;
product was a mixture of two isomers trans–trans- and presum-
Cl, 13.2; N, 7.8%. δH (400 MHz; CDCl3; Me4Si): 1 form: 0.54 d,
ably cis–trans-forms:
0.63 d, 1.15 d, 1.30 d (12H, CH3C, J 4.8 Hz, i-Pr), 2.29 (9H,
For the trans–trans-isomer: δH (400 MHz, CDCl3, Me4Si):
t-Bu), 2.50 (m, 2H, CHC, i-Pr); 2 form: 0.57 d, 0.71 d, 1.38 d,
1.58 (s, 18H, CH3C), 7.24 (m, 10H), 7.61 (m, 8H, Ar). δC
1.53 d (12H, CH3C, J 4.8 Hz, i-Pr), 2.18 (9H, t-Bu), 2.61 (m, 2H,
(100 MHz; CDCl3; Me4Si): 30.9 (CH3C), 62.2 (CH3C), 128.6,
CHC, i-Pr); a general part of two forms: 7.17 (m, 4H), 7.26–7.48
128.7, 130.5, 132.4, 132.8 (Ar), 124.0, 125.3, 136.3 (ipso-C),
(m, 3H), 7.26 (m, 1H) (Ar). δC (100 MHz, CDCl3, Me4Si): 25.1,
152.5 (C3), 168.7 (C5Pd).
25.4, 28.4, 28.6, 28.7 (CH3C, i-Pr), 31.0 (CH3C, t-Bu), 24.3, 24.5
For the cis–trans-isomer: δH (400 MHz; CDCl3; Me4Si): 2.11
(CH, i-Pr), 64.3 (CH3C, t-Bu), 125.3, 125.7, 127.9, 128.4, 130.9
(s, 18H, CH3C), 7.01 (m, 4H), 7.33 (m, 14H); δC (100 MHz;
(Ar), 131.3, 132.1, 146.8, 147.2 (ipso-C, Ar), 149.1 (C3), 153.3
CDCl3; Me4Si): 31.9 (CH3C), 63.0 (CH3C), 128.5, 128.6, 130.3,
(C5Pd).
131.4, 132.0 (Ar), 123.4, 125.5, 136.1 (ipso-C), 152.8 (C3), 167.7
(C5Pd).
[1-tert-Butyl-3-phenyl-4-(2,6-diisopropylphenyl)-1,2,4-triazol-5-
yliden]palladium iodide (10b)
The trans–trans-isomer was isolated from the mixture by
additional recrystallization from acetonitrile and identified by
A solution of carbene 8c (0.25 g, 0.68 mmol) and anhydrous
X-ray diffraction study.
dimethylsulfoxide (0.03 mL) in anhydrous toluene (1 mL) was
Bis-[1-(1-adamantyl)-3-phenyl-4-(2,4,6-trimethylphenyl)-1,2,4-
triazol-5-ylidene]palladium iodide (9c)
added to a mixture of palladium iodide (0.246 g, 0.68 mmol)
in toluene (5 mL) and stirred for 8 h. Then the mixture was
boiled for 2.5 h. The precipitate that formed was filtered off,
washed with petroleum ether, and dried. Yield 0.40 g (81%),
mp 254–255 °C (acetonitrile). Found: C, 40.0; H, 4.2; I, 35.1; N,
5.9%. Calc. for C24H31I2N3Pd: C, 39.9; H, 4.3; I, 35.2; N, 5.8%.
δH (400 MHz; CDCl3; Me4Si): 1 form: 0.62 d, 1.36 d (12H,
CH3C, J 4.8 Hz, i-Pr), 2.94 (m, 2H, CHC, i-Pr); 2 form: 0.83 d,
1.25 s (12H, CH3C, J 4.8 Hz, i-Pr), 4.11 (s, 2H, CHC, i-Pr); a
general part of two forms: 2.09 (s, 9H, t-Bu), 7.19 (m, 2H), 7.30
(m, 2H), 7.39 (m, 3H), 7.60 (m, 1H) (Ar). δC (100 MHz; solid
state; DSS): 23.7, 25.8, 26.5, 27.2, 29.6 (CH3C, i-Pr), 31.7, 32.3
(CH3C, t-Bu), 47.3 (CHC, i-Pr), 64.5 (CH3C, t-Bu), 126.0, 127.7,
The powder carbene 8b (0.4 g, 1 mmol) was added to a solu-
tion of palladium iodide (0.18 g, 0.5 mmol) in THF (4 mL), the
mixture was stirred for 30 h and boiled for 2 h. The precipitate
formed was filtered off and dried. Yield 0.42 g (72%), mp >
300 °C (dimethylformamide). Found: C, 56.3; H, 5.3; I, 21.9; N,
7.4%. Calc. for C54H62I2N6Pd: C, 56.1; H, 5.4; I, 22.0; N, 7.3%.
δC (100 MHz; solid state; DSS): 21.3, 23.7 (o-CH3C), 24.6
(p-CH3C), 30.9 (CHC, Ad), 37.8, 45.2 (CH2C, Ad), 64.1 (ipso-C,
Ad), 127.7, 130.0, 131.2, 133.3, 135.6, 139.8, 140.7 (Ar), 154.8
(C3), 168.2 (C5Pd).
Bis[1-tert-butyl-3-phenyl-4-(2,6-diisopropylphenyl)-1,2,4-triazol- 128.6, 131.0, 131.5, 132.7, 134.2, 136.3, 137.3 (Ar), 146.6, 147.2,
5-ylidene]palladium iodide (9d)
149.2, 154.2 (ipso-C), 156.1, 157.6 (C3), 167.0, 169.2, 170.0,
171.3 (C5Pd).
The powder palladium iodide (0.12 g, 0.34 mmol) and
dimethylsulfoxide (0.04 mL) were added to a solution of
carbene 8c (0.25 g, 0.68 mmol) in toluene (5 mL), stirred at
room temperature for 8 h and boiled for 2 h. The precipitate
[1-(1-Adamantyl)-3-phenyl-4-(2,6-diisopropylphenyl)-1,2,4-
triazol-5-ylidene]palladium chloride (10c)
that formed was filtered off, washed with petroleum ether, and A solution of carbene 8d (0.2 g, 0.455 mmol) in toluene (1 mL)
dried. Yield 0.29 g (78%), mp 260–261 °C (acetonitrile). Found: and anhydrous dimethylsulfoxide (0.03 mL) was added to a
C, 53.3; H, 5.8; I, 23.3; N, 7.7%. Calc. for C48H62I2N6Pd: C, suspension of palladium chloride (0.08 g, 0.455 mmol) in
53.2; H, 5.8; I, 23.4; N, 7.8%. δC (100 MHz; solid state; DSS): anhydrous toluene (4 mL) for 10 min, and the mixture was
25.5, 27.1, 27.8 (CH3C, i-Pr), 31.6 (CH3C, t-Bu), 47.4, 48.1 stirred for 8 h. The precipitate that formed was filtered off,
(CHC, i-Pr), 64.3 (CH3C, t-Bu), 126.4, 127.2, 128.9, 130.1, 132.1, washed by petroleum ether, and dried. Yield 0.19 g (68%), mp
132.8 (Ar), 134.0, 146.7, 147.4 (ipso-C), 154.9 (C3), 171.9 (C5Pd). 146 °C (dec.) (acetonitrile). Found: C, 58.3; H, 6.1; Cl, 11.5; N,
This journal is © The Royal Society of Chemistry 2014
Dalton Trans.

View Article Online
Paper
Dalton Transactions
6.9. Calc. for C30H37Cl2N3Pd: C, 58.4; H, 6.0; Cl, 11.5; N, 6.8%.
δH (400 MHz, CDCl3, Me4Si): 0.65 s, 13.8 s (12H, CH3C, i-Pr),
2.25 (m, 9H, Ad), 2.62 (m, 6H, Ad; 2H, CHC, i-Pr), 7.19–7.60
(m, 8H, Ar). δC (100 MHz; CDCl3; Me4Si): 24.0 (CH3C, i-Pr),
24.4 (CH, i-Pr), 28.5 (CH, Ad), 31.3, 41.0 (CH2, Ad), 64.2 (ipso-C,
Ad), 125.4, 125.6, 127.9, 128.5, 130.9 (Ar), 131.4, 132.1, 147.0
(ipso-C), 149.1 (C3), 153.3 (C5Pd). δC (100 MHz; solid state;
DSS): 26.2, 27.1, 29.6, 31.0 (CH3C, i-Pr), 31.0, 36.6, 44.5 (CH2,
Ad), 41.8, 48.6 (CH, i-Pr), 65.6 (ipso-C, Ad), 126.0, 127.2, 130.5,
131.9, 133.2 (ipso-C, Ar), 148.2 (C3), 154.9 (C5Pd).
M. Soleilhavoup and G. Bertrand, Organometallics, 2011,
30, 5304; (d) W. Kirmse, Angew. Chem., Int. Ed., 2010, 49,
8798.
5 D. Enders, K. Breuer, G. Raabe, J. Runsink, J. H. Teles,
J. P. Melder, K. Ebel and S. Brode, Angew. Chem., Int. Ed.
Engl., 1995, 34(9), 1021.
6 D. Enders, K. Breuer, J. Runsink and J. H. Teles, Liebigs
Ann. Chem., 1996, 2019.
7 N. I. Korotkikh, G. F. Rayenko, O. P. Shvaika,
T. M. Pekhtereva, A. H. Cowley, J. N. Jones and
C. L. B. Macdonald, J. Org. Chem., 2003, 68(14),
5762.
8 N. I. Korotkikh, O. P. Shvaika, G. F. Rayenko, A. V. Kiselyov,
A. V. Knishevitsky, A. H. Cowley, J. N. Jones and
C. L. B. Macdonald, ARKIVOC, 2005, 8, 10.
9 N. I. Korotkikh, N. V. Glinyanaya, A. H. Cowley, J. A. Moore,
A. V. Knishevitsky, T. M. Pekhtereva and O. P. Shvaika,
ARKIVOC, 2007, 16, 156.
10 N. I. Korotkikh, A. H. Cowley, J. A. Moore, N. V. Glinyanaya,
I. S. Panov, G. F. Rayenko, T. M. Pekhtereva and
O. P. Shvaika, Org. Biomol. Chem., 2008, 1, 195.
11 A. V. Knishevitsky, N. I. Korotkikh, A. H. Cowley,
J. A. Moore, T. M. Pekhtereva, O. P. Shvaika and G. Reeske,
J. Organomet. Chem., 2008, 693, 1405.
12 A. V. Kiselyov, N. I. Korotkikh, A. H. Cowley, J. A. Moore,
T. M. Pekhtereva and O. P. Shvaika, ARKIVOC, 2008, 15,
329.
[1-(1-Adamantyl)-3-phenyl-4-(2,6-diisopropylphenyl)-1,2,4-
triazol-5-yliden]palladium iodide (10d)
A mixture of carbene 8d (0.25 g, 0.55 mmol), palladium iodide
(0.19 g, 0.55 mmol) and anhydrous dimethylsulfoxide (0.3 mL)
in anhydrous toluene (5 mL) was stirred for 4 h, and then the
mixture was boiled for 3 h. The precipitate that formed was fil-
tered off, washed by petroleum ether, and dried. Yield 0.37 g
(84%), mp > 310 °C (dimethylformamide). Found: C, 45.0; H,
4.8; I, 31.6; N, 5.2%. Calc. for C30H37I2N3Pd: C, 45.1; H, 4.7; I,
31.7; N, 5.3%. δC (100 MHz; solid state; DSS): 26.9, 27.6, 29.0,
29.9 (CH3C, i-Pr), 31.6, 37.5, 43.3 (CH2, Ad), 47.5, 49.1 (CH,
i-Pr), 66.2 (ipso-C, Ad), 125.7, 128.0, 129.0, 130.5, 130.9, 132.0,
133.2 (Ar), 136.3, 146.9, 148.6 (ipso-C, Ar), 156.4 (C3), 169.5
(C5Pd).
13 C. Dash, M. M. Shaikh, R. J. Butcher and P. Ghosh, Dalton
Trans., 2010, 39, 2515.
Acknowledgements
14 V. Hornillos, J. Guerra, A. de Cózar, P. Prieto, S. Merino,
M. A. Maestro, E. Dıez-Barra and J. Tejeda, Dalton Trans.,
2011, 40, 4095.
15 J. Turek, I. Panov, M. Horáček, Z. Černošek, Z. Padělková
and A. Růžička, Organometallics, 2013, 32, 7234.
16 T. I. Gorbunov, V. I. Saloutin and O. N. Chupakhin, Russ.
Chem. Rev., 2010, 79(6), 565.
17 C. L. Geiger, K. Carvalho-Knighton, S. Novaes-Card,
P. Maloney and R. DeVor, in Environmental Applications of
Nanoscale and Microscale Reactive Metal Particles, ACS Sym-
posium Series, American Chemical Society, Washington,
DC, 2010, p. 1–20.
We thank the Ukrainian Academy of Sciences for financial
support (Grant no. 118, 26.02.2014), and the generous finan-
cial support provided by the Robert A. Welch Foundation
(Grant F-0003) (A.H.C.).
Notes and references
1 Y. Cheng and O. Meth-Cohn, Chem. Rev., 2004, 104, 2507.
2 (a) D. Enders, O. Niemeier and A. Henseler, Chem. Rev.,
2007, 107, 5606; (b) N-Heterocyclic Carbenes in Synthesis, ed.
S. P. Nolan, Wiley VCH Verlag GmbH
& Co KgaA,
Weinheim, 2006, p. 68; (c) J. Izquierdo, G. E. Hutson, 18 V. Dichiarante, M. Fagnoni and A. Albini, Green Chem.,
D. T. Cohen and K. A. Scheidt, Angew. Chem., Int. Ed., 2012, 2009, 11, 942.
51, 11686; (d) M. J. Fuchter, Chem. – Eur. J., 2010, 16, 19 K. A. Cannon, M. E. Geuther, C. K. Kelly, S. Lin and
12286; (e) P.-C. Chiang and J. Y. W. Bode, in RSC Catalysis A. H. R. MacArthur, Organometallics, 2011, 30, 4067.
Series No. 6: N-Heterocyclic Carbenes: From Laboratory Curio- 20 W. M. Czaplik, S. Grupe, M. Mayer and A. J. von Wangelin,
sities to Efficient Synthetic Tools, ed. S. Diez-Gonzalez, Royal
Society of Chemistry, 2013, ch. 14, p. 399.
3 N. I. Korotkikh and O. P. Shvaika, Carbene and carbene
Chem. Commun., 2010, 46, 6350.
21 Y. Mitoma, T. Uda and N. Egashira, Environ. Sci. Technol.,
2004, 38, 1216.
complex catalysis of organic reactions, DonNU, Donetsk, 22 Y. Mitoma, M. Kakeda, A. M. Simion, N. Egashira and
Ukraine, 2013, p. 372. C. Simion, Environ. Sci. Technol., 2009, 43, 5952.
4 (a) D. Bourissou, O. Guerret, F. P. Gabbaï and G. Bertrand, 23 A. M. Simion, H. Miyata, M. Kakeda, N. Egashira,
Chem. Rev., 2000, 100, 39; (b) M. C. Jahnke and F. E. Hahn,
in RSC Catalysis Series No. 6: N-Heterocyclic Carbenes: From
Y. Mitoma and C. Simion, Sep. Purif. Technol., 2013, 103,
267.
Laboratory Curiosities to Efficient Synthetic Tools, ed. S. Dıez- 24 Y. Huang, S. Liu, Z. Lin, W. Li, X. Li and R. Cao, J. Catal.,
Gonzalez, RSC, 2013, ch. 1, p. 1; (c) D. Martin, M. Melaimi, 2012, 292, 111.
Dalton Trans.
This journal is © The Royal Society of Chemistry 2014

View Article Online
Dalton Transactions
Paper
25 P. Selvam, S. U. Sonavane, S. K. Mohapatra and
G. V. Dudarenko, N. A. Bumagin and O. P. Shvaika, Chem.
Hetercycl. Compd., 2012, 47(12), 1551.
R. V. Jayaram, Tetrahedron Lett., 2004, 45, 3071.
26 M. S. Viciu, G. A. Grasa and S. P. Nolan, Organometallics, 33 X. Chen, R. Liu, Y. Xu and G. Zou, Tetrahedron, 2012, 68,
2001, 20(16), 3607. 4813.
27 O. Navarro, H. Kaur, P. Mahjoor and S. P. Nolan, J. Org. 34 N. I. Korotkikh, G. F. Raenko, T. M. Pekhtereva,
Chem., 2004, 69(9), 3173.
28 O. Navarro, N. Marion, Y. Oonishi, R. A. Kelly and
S. P. Nolan, J. Org. Chem., 2006, 71(2), 685.
29 C. J. E. Davies, M. J. Page, C. E. Ellul, M. F. Mahon and
M. K. Whittlesey, Chem. Commun., 2010, 46, 5151.
30 S. Kuhl, R. Schneider and Y. Fort, Adv. Synth. Catal., 2003,
345(3), 341.
31 S. Akzinnay, F. Bisaro and C. S. J. Cazin, Chem. Commun.,
2009, 5752.
O. P. Shvaika, A. H. Cowley and J. N. Jones, Russ. J. Org.
Chem., 2006, 42(12), 1822.
35 (a) G. M. Sheldrick, SHELXTL-PC, version 5.03, Siemens
Analytical X-ray Instruments, Inc., Madison, WI, 1994. See
also; (b) G. M. Sheldrick, Acta Crystallogr., Sect. A:Fundam.
Crystallogr., 2008, 64, 112.
36 A. L. Spek, PLATON, A Multipurpose Crystallographic Tool,
Utretcht University, The Netherlands, 1998.
37 N. I. Korotkikh, A. V. Kiselyov, A. V. Knishevitsky,
G. F. Rayenko, T. M. Pekhtereva and O. P. Shvaika, Chem.
Heterocycl. Compd., 2005, 7, 1026.
32 N. I. Korotkikh, V. Sh. Saberov, A. V. Kiselev,
N. V. Glinyanaya, K. A. Marichev, T. M. Pekhtereva,
This journal is © The Royal Society of Chemistry 2014
Dalton Trans.