A hexacarbene complex derived from 1,1′-methylenebis(4-butyl-1H-1,2,4-triazolium) diiodide. Synthesis, characterization and catalytic activity

Article abstract of DOI:10.1016/S0022-328X(02)01730-8

The synthesis and characterization of a hexacarbene complex (1) derived from 1,1′-methylenebis(4-butyl-1H-1,2,4-triazolium) diiodide are reported. The organic backbone was constructed prior to the introduction of metal atoms by reaction with palladium(II) acetate. The catalytic activity of 1 in a Heck reaction has been tested.

Full text of DOI:10.1016/S0022-328X(02)01730-8

Journal of Organometallic Chemistry 660 (2002) 50ꢁ54
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www.elsevier.com/locate/jorganchem
A hexacarbene complex derived from 1,1?-methylenebis(4-butyl-1H-
1,2,4-triazolium) diiodide. Synthesis, characterization and catalytic
activity
Enrique D´ıez-Barra *, Javier Guerra, Rene´ I. Rodr´ıguez-Curiel, Sonia Merino,
Juan Tejeda *
Facultad de Qu´ımica, Universidad de Castilla-La Mancha, 13071 Ciudad Real, Spain
Received 22 April 2002; accepted 17 July 2002
Abstract
The synthesis and characterization of a hexacarbene complex (1) derived from 1,1?-methylenebis(4-butyl-1H-1,2,4-triazolium)
diiodide are reported. The organic backbone was constructed prior to the introduction of metal atoms by reaction with
palladium(II) acetate. The catalytic activity of 1 in a Heck reaction has been tested. # 2002 Elsevier Science B.V. All rights
reserved.
Keywords: 1,2,4-Triazole-5-ylidene; Carbene; Palladium; Catalysis
1. Introduction
ylidene) [7] and 1,1?-methylenebis(4-methyl-4,5-dihydro-
1H-1,2,4-triazole-5-ylidene)-1,1?-methylenebis(3-meth-
yl-4,5-dihydro-1H-imidazole-5-ylidene) palladium(II)
diiodide [8]. A biscarbene derived from 1H-1,2,4-tria-
zole has been used in the preparation of a polymeric
silver complex [9]. In 1999 we reported on some mono-
nuclear and dinuclear palladium(II) carbene complexes
derived from 1,1?-methylenebis(4-alkyl-1,2,4-triazolium)
diiodides [10].
Two important reviews focused on the dendritic
catalyst have recently been published by van Leeuwen
and Astruc [11]. In these are described the inherent
advantages associated with this type of catalysts. The
active catalytic center in a metallodendrimer can be
situated in three different areas: (i) metal atom as the
dendrimer core, (ii) metal atoms in the dendrimer
branches, and (iii) metal atoms in the periphery. The
synthesis of the third type of metallodendrimer may be
undertaken by two strategies. Firstly, it is possible to
build the dendrimer and then incorporate the metal
atoms in the final stage or, secondly, the metal atoms
can be incorporated within the molecular fragments
used to build the dendrimer.
In the past decade, significant effort has been directed
towards the theoretical understanding [1], synthesis,
metal coordination and catalytic applications of N-
heterocyclic carbenes [2]. A special issue covering all
previous studies and approaches has been published [3].
A large number of papers related to free carbenes and
carbene complexes based on imidazole and imidazoli-
dine rings have also been published [4]. However,
carbenes based on the 1,2,4-triazole ring, where an
additional nitrogen atom is present in the ring, have
received little attention. Despite the fact that several
stable free imidazole-2-ylidenes are known, 1,3,4-triphe-
nyl-4,5-dihydro-1H-1,2,4-triazole-5-ylidene is the only
1,2,4-triazole derivative to have been isolated as the free
carbene [5]. Several rhodium, ruthenium, and cis- and
trans-palladium complexes derived from 4,5-dihydro-
1H-1,2,4-triazole-5-ylidene have been prepared [6] as
well as one tungsten compound with a similar structure
to the chelate carbene complexes derived from 1,1?-
methylenebis(4-methyl-4,5-dihydro-1H-1,2,4-triazole-5-
We report here an example involving the use of the
first method to build a dendritic catalyst bearing
palladium(II)-stabilized peripheral carbene atoms.
* Corresponding author. Tel.: ꢀ
5318
E-mail address: enrique.diez@uclm.es (E. D´ıez-Barra).
/
34-9-2629-5300; fax: ꢀ34-9-2629-
/
0022-328X/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 0 2 2 - 3 2 8 X ( 0 2 ) 0 1 7 3 0 - 8

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2. Results and discussion
dopalladium(II), afforded the hexa-carbene 1 in 82%
yield (Scheme 1).
All intermediate compounds (2, 3, 4, and 5) were fully
The synthesis of compound 1 was accomplished
through the following sequence of reactions: (i) pre-
paration of bis(1,2,4-triazol-1-yl)methane derivative 2;
(ii) coupling with 1,3,5-trischlorocarbonylbenzene; (iii)
alkylation with butyl tosylate; (iv) exchange of tosylate
by iodide anions; and (v) treatment of the hexa-salt 5
with palladium(II) acetate.
1
characterized by H- and 13C-NMR spectroscopy, MS
and microanalysis.
Compound 1 is a yellow solid and was fully char-
1
acterized by H- and 13C-NMR spectroscopy and MS.
1
The H- and 13C-NMR spectra are consistent with the
1
proposed structure. The H-NMR spectrum shows the
signals (d, ppm) for the hydrogen atoms of the core
(9.03), the para-system (7.52, 7.65), the hydrogen of the
bridge between the 1,2,4-triazole rings (8.41), H-3 of the
1,2,4-triazole ring (9.06) and the hydrogen atoms of the
aliphatic chains (0.89, 1.23, 1.84, 4.20 and 4.89). 13C
resonances (d, ppm) of the significant carbon atoms are
78.3 for the carbon of the bridge, 145.1 for C-3 in the
heterocyclic moiety and 166.5 for the carbene atom.
In our previous work [10], compounds 6a and 6b (Fig.
1) were fully characterized, including an X-ray structure
of 6a. Comparison of the significant 1H and 13C
resonances of compound 1 with those of 6a and 6b
can be made from the data in Table 1. Examination of
the chemical shifts for the two compounds allows us to
confirm the structure of 1.
Compound 2, 4-[bis(1,2,4-triazol-1-yl)methyl]phenol,
was prepared by heating a mixture of 4-hydroxybenzal-
dehyde dimethyl acetal [12] and 1,2,4-triazole at 120 8C
without solvent for 4.5 h according to literature
procedures [13]. Condensation of 2 with 1,3,5-trischlor-
ocarbonylbenzene [14] afforded compound 3, which can
be regarded as a first generation dendrimer. Reaction of
the hydroxyl group of compound 2 with appropriate
AB2-type monomers can be employed to build higher
dendrimers [15].
Unexpectedly [16], the alkylation of compound 3 with
alkyl iodides failed, although the use of butyl tosylate
did afford the hexa-1,2,4-triazolium tosylate 4 in 70%
yield.
Anion exchange was performed by stirring hexa-
tosylate 4 with lithium iodide at room temperature for
1 h in THF. Filtration gave hexa-iodide 5 in 80% yield.
Reaction of hexa-iodide 5 with palladium(II) acetate
in THF at 0 8C, following the procedure previously
reported by us [10] for the synthesis of 1,1?-methylene-
bis(4-alkyl-4,5-dihydro-1H-1,2,4-triazole-5-ylidene)diio-
Fig. 1. Reported structure for 6a and 6b.
Scheme 1. Synthesis of the hexacarbene 1. i) DMAP, THF, r.t., 2 h. ii) n-BuOTs, 100 8C, 24 h. iii) LiI, THF, r.t., 30 min, twice. iv) Pd(OAc)2, THF,
0 8C, 24 h.

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EtOAc. The organic solvent was evaporated to give a
yellow oil, which was purified by chromatography
Table 1
Chemical shifts (d, ppm) for representative atoms of compounds 1, 6a
and 6b
(SiO2, hexaneꢁ
duct from EtOAcꢁ
yielded colorless crystals. Yield: 2.31 g (9.54 mmol),
/EtOAc, 1:3). Crystallization of the pro-
/
carbon tetrachlorideꢁEt2Oꢁhexane
/
/
Group
1
6a
9.12
6b
1
H-3
9.06
8.92
48%. m.p.: 158ꢁ
6.83 (A of ABq, Jꢂ
Jꢂ8.7 Hz, 2H) (OC6H4), 8.11 (s, 2H) and 8.65 (s, 2H)
/
160 8C. H-NMR (Me2SO-d6, d, ppm):
N-CH2
C-3
4.20 and 4.89
ꢁ
/
4.18 and 4.80
/8.7 Hz, 2H) and 7.25 (B of ABq,
145.1
166.5
49.5
142.3
166.7
144.7
166.3
48.9
/
Carbene C
N-CH2
[H3(Tz) and H5(Tz)], 8.23 (s, 1H, CHTz2), 9.86 (broad
s, 1H, OH). 13C-NMR (Me2SO-d6, d, ppm): 72.1, 115.6,
123.9, 129.2, 144.5, 152.3, 158.6. EIMS, m/z: 242 [M]ꢀ,
174, 120, 93, 65. Anal. Calc. for C11H10N6O (242.24): C,
54.24; H, 4.16; N, 34.69. Found: C, 54.26; H, 3.83; N,
35.09%.
ꢁ/
The catalytic activity of hexacarbene 1 was tested by
reacting p-bromobenzaldehyde and n-butyl acrylate
under Heck conditions following a procedure similar
to that reported by Herrmann [17]. The use of 0.16%
mol of compound 1 as a catalyst gave a conversion of
74% after 5 h. Under the same reaction conditions 0.5%
mol of 6b, a palladium(II) biscarbene whose structure is
comparable to the palladacycle moieties of hexacarbene
1, gave rise to a conversion of only 50% in the same
time. The two reactions attain a conversion of 100%
after 8 and 50 h, respectively. The hexacarbene, which
has a structure related to dendrimers, showed better
catalytic activity than the monomeric catalyst (Scheme
2).
3.2. Tris{4-[bis(1,2,4-1H-triazol-1-
yl)methyl]phenyl}benzene-1,3,5-tricarboxylate (3)
A mixture of 400 mg (1.65 mmol) of compound 2 and
220 mg (1.8 mmol) of 4-dimethylaminopyridine was
dissolved in 10 ml of anhyd. THF under Ar in a 100 ml
Schlenk tube. After 5 min, a solution of 133 mg (0.5
mmol) of 1,3,5-trischlorocarbonylbenzene in 5 ml of
anhyd. THF was added and the reaction mixture was
stirred for 2 h at r.t. The amine hydrochloride was
filtered off and the solvent from the filtrate was
evaporated. The solid residue was dissolved in CH2Cl2
and the solution was washed with water. The organic
layer was dried with anhyd. Na2SO4. The pure product
was obtained as colorless crystals after filtration and
solvent evaporation. Yield: 353 mg (0.40 mmol), 80%.
m.p.: 135 8C (dec.). 1H-NMR (Me2SO-d6, d, ppm): 7.50
(s, 12H, OC6H4), 8.18 (s, 6H) and 8.78 (s, 6H) [H3(Tz)
and H5(Tz)], 8.50 (s, 3H, CHTz2), 9.02 (s, 3H, C6H3).
13C-NMR (Me2SO-d6, d, ppm): 71.5, 122.4, 129.1,
130.7, 132.1, 135.2, 144.8, 151.1, 152.5, 162.7. MS
3. Experimental
Solvents were purified by distillation from appropri-
ate drying agents before use. Melting points were
determined in capillary tubes on a Gallenkamp appara-
tus and are uncorrected. Elemental analyses were
performed on a PerkinꢁElmer 2400 CHN microanaly-
/
zer. NMR spectra were recorded on a Varian Unity
spectrometer operating at 299.980 MHz for 1H and
75.423 MHz for 13C. Chemical shifts are expressed in
parts per million (d) relative to Me4Si as internal
standard. The resonances of compounds were assigned
by difference NOE and Hetcor experiments.
(FAB, m-nitrobenzylalcohol), m/z: 883 [Mꢀ
1]ꢀ, 814,
/
545, 244. Anal. Calc. for C42H30N18O6 (882.82): C,
57.14; H, 3.42; N, 28.56. Found: C, 57.41; H, 3.38; N,
28.78%.
3.1. 4-[Bis(1,2,4-triazol-1-yl)methyl]phenol (2)
3.3. 1,3,5-Benzenetriyltris[1,1?-(p-
carbonyloxybenzylidene)bis(4-butyl-1H-1,2,4-
triazolium)] hexatosylate (4)
A mixture of 2.76 g (40 mmol) of 1H-1,2,4-triazole,
3.36 g (20 mmol) of p-hydroxybenzaldehyde dimethyla-
cetal [14] and 34 mg (0.2 mmol) of monohydrated p-
toluenesulfonic acid was heated to 120 8C. Further
quantities of monohydrated p-toluenesulfonic acid (34
mg, 0.2 mmol) were added every 15 min until a reaction
time of 4.5 h had been reached. The mixture was cooled
to room temperature (r.t.) and was extracted with
A mixture of 441 mg (0.5 mmol) of compound 3 and
10 ml of butyl tosylate was stirred and heated at 100 8C
for 24 h. The mixture was cooled to r.t. and poured onto
cooled EtOAc (25 ml), at which point a colorless solid
precipitated. The solid was filtered off, washed with
cooled EtOAc and crystallized from a mixture of
Scheme 2. Heck olefination of p-bromobenzaldehyde with n-butyl acrylate catalyzed by 1 and 6b.

E. D´ıez-Barra et al. / Journal of Organometallic Chemistry 660 (2002) 50ꢁ
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54
53
EtOHꢁ
220ꢁ
222 8C. 1H-NMR (Me2SO-d6, d, ppm): 0.90 (t, Jꢂ
7.4 Hz, 18H, NCH2CH2CH2CH3); 1.32 (sxt, Jꢂ7.4 Hz,
12H, NCH2CH2CH2CH3); 1.84 (qui, Jꢂ7.6 Hz, 12H,
NCH2CH2CH2CH3); 2.28 (s, 18H, p-CH3C6H4SO3);
4.31 (t, Jꢂ7.4 Hz, 12H, NCH2CH2CH2CH3); 7.11,
7.46 (AA?BB? system, Jꢂ7.9 Hz, 24H, p-CH3C6-
H4SO3); 7.60, 7.80 (AA?BB? system, Jꢂ8.6 Hz, 12H,
/
EtOAc. Yield: 785 mg (0.35 mmol), 70%. m.p.
3.5. 1,3,5-Benzenetriyltris{[1,1?-(p-
carbonyloxybenzylidene)bis(4-butyl-4,5-dihydro-1H-
1,2,4-triazole-5-ylidene)]diiodopalladium(II)} (1)
/
/
/
/
A suspension of 199 mg (0.1 mmol) of compound 5
and 74 mg (0.33 mmol) of palladium acetate in 200 ml of
THF in a 1 l Schlenk tube was stirred at 0 8C for 24 h
under an Ar atmosphere. The solvent was then evapo-
rated under vacuum. The residue was dissolved in 50 ml
of MeCN, the solution was stirred for 15 min at r.t. and
the solvent was evaporated under vacuum. The residue
was washed with Et2O and the product was obtained as
a brown solid. Yield: 189 mg (0.08 mmol) 82%. Crystal-
/
/
/
OC6H4); 8.96 [s, 3H, CH(1,2,4-triazole)2]; 9.04 (s, 3H,
C6H3); 9.52 (s, 6H) and 10.54 (s, 6H) (H3 and H5). 13C-
NMR (Me2SO-d6, d, ppm): 13.4 (NCH2CH2CH2CH3);
18.9 (NCH2CH2CH2CH3); 20.8 (p-CH3C6H4SO3); 30.6
(NCH2CH2CH2CH3); 48.0 (NCH2CH2CH2CH3); 74.1
[CH(1,2,4-triazole)2]; 122.7, 127.7, 130.7, 152.2
(OC6H4); 125.5, 128.2, 137.9, 145.5 (p-CH3C6H4SO3);
130.7, 135.4 (C6H3); 145.3, 145.9 (C3 and C5); 162.8
(COO). MS (FABꢀ, m-NBA), m/z (%): 57 (50), 126
lization of the product was not possible due to decom-
1
position. H-NMR (Me2SO-d6, d, ppm): 0.89 (t, Jꢂ
/
7.2
7.4 Hz,
7.0 Hz, 12H,
6.8 Hz, 6H,
NCH2CH2CH2CH3); 4.89 (bs, 6H, NCH2CH2-
CH2CH3); 7.52, 7.65 (A2B2 system, Jꢂ8.6 Hz, 12H,
Hz, 18H, NCH2CH2CH2CH3); 1.23 (sxt, Jꢂ
12H, NCH2CH2CH2CH3); 1.84 (m, Jꢂ
NCH2CH2CH2CH3); 4.20 (qui, Jꢂ
/
/
/
(100), 230 (61), 354 (12), 2081 (3) [Mꢃ
/
TsO]ꢀ.
C108H126N18O24S6 (2252.68): Anal. Calc. for C, 57.58;
H, 5.64; N, 11.19. Found: C, 57.48; H, 5.42; N, 11.52%.
/
OC6H4); 8.41 [s, 3H, CH(1,2,4-triazole)2]; 9.03 (s, 3H,
C6H3); 9.06 (s, 6H, H3). 13C-NMR (Me2SO-d6, d, ppm):
13.4 (NCH2CH2CH2CH3); 18.9 (NCH2CH2CH2CH3);
31.8 (NCH2CH2CH2CH3); 49.5 (NCH2CH2CH2CH3);
78.3 [CH(1,2,4-triazole)2]; 122.4, 127.9, 133.2, 151.2
(OC6H4); 130.7, 135.2 (C6H3); 145.1 (C3); 162.8
(COO); 166.5 (C5). MS: (MALDI), m/z (%): 693
(100), 1433 (25), 1580 (50), 2045 (20), 2172 (50), 2298
(5) [M]ꢀ.
3.4. 1,3,5-Benzenetriyltris[1,1?-(p-
carbonyloxybenzylidene)bis(4-butyl-1H-1,2,4-
triazolium)] hexaiodide (5)
To a mixture of 226 mg (0.1 mmol) of compound 4
and 20 ml of THF was slowly added a solution of 96 mg
(0.72 mmol) of lithium iodide in 10 ml of THF. The
resulting suspension was stirred for 30 min at r.t. The
solid was filtered off and a further quantity of 96 mg
(0.72 mmol) of lithium iodide in 20 ml of THF was
added to the filtrate. The resulting suspension was
stirred for 30 min at r.t. The solid was again filtered
Acknowledgements
Financial support from the Spanish DGES (PB97-
0425 and 1FD97-0167) is acknowledged. We are in-
debted to the Junta de Comunidades de Castilla-La
Mancha for a grant (J.G.).
off and washed with THF (3ꢄ
was obtained as a yellow solid after crystallization from
a mixture of MeOHꢁEtOAc. Yield: 159 mg (0.08
mmol), 80%. m.p. 182ꢁ
184 8C. 1H-NMR (Me2SO-d6,
d, ppm): 0.93 (t, Jꢂ7.3 Hz, 18H, NCH2CH2CH2CH3);
1.35 (sxt, Jꢂ7.4 Hz, 12H, NCH2CH2CH2CH3); 1.87
(qui, Jꢂ7.6 Hz, 12H, NCH2CH2CH2CH3); 4.34 (t, Jꢂ
7.5 Hz, 12H, NCH2CH2CH2CH3); 7.64, 7.81 (AA?BB?
system, Jꢂ8.8 Hz, 12H, OC6H4); 8.95 [s, 3H, CH(1,2,4-
/
20 ml). The pure product
/
/
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[17] W.A. Herrmann, M. Elison, J. Fischer, C. Kocher, G.R.J. Artus,
¨
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