Dipalladium complexes with triazolidin-diylidene bridges and their catalytic activities

Article abstract of DOI:10.1021/om3003625

The 1,2,4-trimethyltriazolidin-3,5-diylidene (ditz) bridged dipalladium heterotetracarbene complex [PdBr₂(ⁱPr₂-bimy)] ₂(μ-ditz) (3) (ⁱPr₂-bimy = 1,3-diisopropylbenzimidazolin-2-ylidene) was prepared by Ag-carbene transfer involving the 1,2,4-trimethyltriazolium dication (C), Ag₂O, and the precursor complex (ⁱPr₂-bimyH)[PdBr₃( ⁱPr₂-bimy)] (2). Bromido substitution of 3 with AgO ₂CCH₃ and AgO₂CCF₃afforded the carboxylato complexes [Pd(O₂CCH₃)₂( ⁱPr₂-bimy)]₂(μ-ditz) (4) and [Pd(O ₂CCF₃)₂(ⁱPr₂-bimy)] ₂(μ-ditz) (5). Multinuclei NMR spectroscopies and X-ray diffraction analyses showed that the all-trans isomers of 3, 4, and 5 are the predominant products in all three cases. In addition, the decomposition product of complex 4, trans-[Pd(O₂CCH₃)₂( ⁱPr₂-bimy)₂] (trans-6) was structurally determined by X-ray crystallography. A comparative catalytic study revealed the superiority of complexes 3, 4, and 5 over the previously reported mononuclear bis(benzimidazolin-2-ylidine) complexes without ditz bridges in the Mizoroki-Heck coupling reaction. Overall, complex 4 bearing acetato coligands showed the best catalytic performance.

Full text of DOI:10.1021/om3003625

Article
pubs.acs.org/Organometallics
Dipalladium Complexes with Triazolidin-Diylidene Bridges and Their
Catalytic Activities
Shuai Guo and Han Vinh Huynh*
Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543, Republic of Singapore
S
* Supporting Information
ABSTRACT: The 1,2,4-trimethyltriazolidin-3,5-diylidene
(ditz) bridged dipalladium heterotetracarbene complex
[PdBr2(iPr2-bimy)]2(μ-ditz) (3) (iPr2-bimy = 1,3-diisopropyl-
benzimidazolin-2-ylidene) was prepared by Ag−carbene trans-
fer involving the 1,2,4-trimethyltriazolium dication (C), Ag2O,
and the precursor complex (iPr2-bimyH)[PdBr3(iPr2-bimy)] (2).
Bromido substitution of 3 with AgO2CCH3 and AgO2CCF3
afforded the carboxylato complexes [Pd(O2CCH3)2(iPr2-
bimy)]2(μ-ditz) (4) and [Pd(O2CCF3)2(iPr2-bimy)]2(μ-ditz) (5). Multinuclei NMR spectroscopies and X-ray diffraction
analyses showed that the all-trans isomers of 3, 4, and 5 are the predominant products in all three cases. In addition, the
decomposition product of complex 4, trans-[Pd(O2CCH3)2(iPr2-bimy)2] (trans-6) was structurally determined by X-ray
crystallography. A comparative catalytic study revealed the superiority of complexes 3, 4, and 5 over the previously reported
mononuclear bis(benzimidazolin-2-ylidine) complexes without ditz bridges in the Mizoroki−Heck coupling reaction. Overall,
complex 4 bearing acetato coligands showed the best catalytic performance.
mild Ag−carbene transfer method. In relation to our previous
study on the catalytic activities of mononuclear bis(benzimidazolin-
2-ylidene) Pd(II) complexes,8 a comparative catalytic study on the
Mizoroki−Heck coupling reaction was also conducted, which shows
the superiority of all the new dinuclear complexes with triazolidin-
diylidene bridges.
INTRODUCTION
■
Transition-metal complexes bearing N-heterocyclic carbenes
(NHCs) are now ubiquitous in the field of organometallic
chemistry and homogeneous catalysis.1 In addition to various
types of monodentate NHCs, di-NHCs incorporating two
linked carbene ligands have been investigated in great detail as
well. Generally, the two carbene moieties in a di-NHC can be
joined by a variety of linker groups, such as a flexible chain,2 a
rigid aromatic ring,3 and, less common, a fused aromatic ring.4
The nature of the chosen bridge determines the function of the
resulting di-NHC as a chelating or bridging ligand. Triazolidin-
diylidene5 (ditz, Figure 1), as an unique di-NHC, bears two
carbene donors within a single five-membered ring, which
prevents chelation and brings the two bound metal centers
fairly close (∼6 Å) to each other. This unique feature could also
facilitate electronic communication between the two metal centers.
In 1997, Bertrand et al. reported a polymeric Ag−triazolidin-
diylidene complex (I, Figure 1),6 which could potentially serve as a
dicarbene transfer agent. Thereafter, Peris et al. reported a range of
dinuclear triazolidin-diylidene complexes of different metals, most
of which were synthesized using the free dicarbene or in situ
deprotonation of the dicationic ligand precursor by a basic metal
source.7 Notably, less attention has been paid to dipalladium
triazolidin-diylidene complexes, and to the best of our knowledge,
only two examples (II and III, Figure 1) have been reported.7d,e
Overall, previous work has suggested that the catalytic perform-
ance could be enhanced due to the synergistic effect of two
catalytically active metal centers linked by a triazolidin-diylidene
ligand.7a,b
RESULTS AND DISCUSSION
■
Synthesis of Triazolidin-Diylidene Precursor Salt (C).
The 1,2,4-trimethyltriazolium tetrafluoroborate salt C, which
serves as the triazolidin-diylidene precursor, was synthesized by
a modified procedure9 in two steps (Scheme 1). To avoid the
use of toxic iodomethane in the first step, N-methylation of
1,2,4-triazole was accomplished with more benign dimethyl
carbonate (DMC), yielding a mixture of 1-methyltriazole (A)
and 4-methyltriazole (B) as a colorless liquid in a total yield of
69%. This mixture was directly used for the second and third
alkylation using the stronger electrophilic Meerwein’s salt,
affording the desired dicationic salt C in 60% yield.
Synthesis and Characterization of Pd(II) Heterocar-
bene Complexes. Although the first ditz complex reported
bears Ag(I) centers (vide infra), the potentially useful dicarbene
transfer from such species to other metal centers is still rare.7b
To explore such a mild methodology for the preparation of
ditz-bridged heterocarbene complexes of Pd(II), the reaction of
precursor salt C with 2 equiv of AgOAc in THF under reflux
was carried out. This reaction supposedly afforded the Ag−carbene
species {[Ag(ditz)]BF4}n (1), in which the aforementioned
Herein, we describe the synthesis of three dinuclear Pd(II)−
triazolidin-diylidene heterotetracarbene complexes using the
Received: May 1, 2012
Published: May 31, 2012
© 2012 American Chemical Society
4565
dx.doi.org/10.1021/om3003625 | Organometallics 2012, 31, 4565−4573

Organometallics
Article
Figure 1. Triazolidin-3,5-diylidene (ditz) and reported Ag(I) and Pd(II) complexes.
Scheme 1. Synthesis of 1,2,4-Trimethyltriazolium Salt C
Prolonging the reaction time did not change the observed
isomeric ratio significantly.
Complex trans-3 is well soluble in halogenated solvents,
CH3CN, DMSO, and DMF, but insoluble in nonpolar solvents,
such as diethyl ether, hexane, and toluene. Even in solution, it is
stable, and no palladium black is observed upon standing for
several weeks in CHCl3.
For the predominant product trans-3, the three N-methyl
groups of the triazolidin-diylidene ligand give two signals at
4.62 and 4.36 ppm with a ratio of 1:2 in the 1H NMR spectrum,
which indicates a 2-fold symmetry of the molecule.
Interestingly, two mutiplets of equal intensity are found for
the isopropyl C−H protons at 6.16 and 6.02 ppm, suggesting
two inequivalent N substituents of the terminal benzimidazole-
analogue I has been demonstrated to be polymeric in the solid
state.6 Without further isolation, complex 1 was treated with
our previously reported complex (iPr2-bimyH)[PdBr3(iPr2-bimy)]
(2, iPr2-bimy = 1,3-diisopropylbenzimidazolin-2-ylidene),10 afford-
ing the desired dipalladium complex trans-[PdBr2(iPr2-bimy)]2-
(μ-ditz) (trans-3) as the major product along with minor amounts
of another inseparable isomer in a ratio of ∼9:1, as indicated by
1H NMR spectroscopy (Scheme 2).
i
derived carbene Pr2-bimy. The significant downfield shifts
(ΔδH = 0.81, 0.95 ppm) of these protons, compared with that
10
i
in the ligand precursor salt Pr2-bimyH+Br− (cf. 5.21 ppm),
Scheme 2. Synthesis of Pd(II) Complexes 3
indicates the presence of anagostic (or preagostic) interactions
between the isopropyl C−H protons and the Pd(II) metal
centers in trans-3, which are commonly observed for Pd(II)−
iPr2-bimy complexes.8,10 Despite the inequivalence of the iso-
1
propyl groups in trans-3, as indicated by H NMR spectros-
copy, the 13C NMR spectrum shows only two resonances at
54.8 and 21.7 ppm for isopropyl C−H and CH3 groups, res-
pectively, which is most likely due to accidental overlap. Finally,
two signals at 179.6 ppm (NCNditz) and 175.2 ppm (NCNbimy
)
are found in the downfield region of the 13C NMR spectrum,
i
which have been unambiguously assigned to the ditz and Pr2-
bimy carbene donors using 2D-HMBC NMR experiments (see
the Supporting Information).
Previously, we reported the determination of ligand donor
strengths by 13C NMR spectroscopy on complexes of the type
i
trans-[PdBr2(iPr2-bimy)(L)], in which the Pr2-bimy ligand is
used as a spectroscopic probe to evaluate the σ-donating ability
of L.11 In the same way, we can evaluate the donating ability of
the ditz dicarbene ligand in complex trans-3 using the carbenoid
i
resonance of the Pr2-bimy ligand at 175.2 ppm. This chemical
shift is the most highfield among all the trans-[PdBr2(iPr2-
bimy)(NHC)] complexes on the “Pd scale” and even more
highfield than that of trans-[PdBr2(iPr2-bimy)(PCy3)], which
indicates that 1,2,4-triazolidin-3,5-diylidene is a relatively weak
electron donor to two metal centers comparable to that of
phosphines rather than NHCs. This result is in line with the
reported donor strength determined using a dinuclear Ir(I)−
carbonyl complex.7a
Theoretically, there are four possible isomers for [PdX2L]2-
(μ-ditz) type complexes adopting (all-)trans, (all-)cis-syn, (all-)
cis-anti, and trans−cis configurations, respectively (Chart 1).
Chart 1. Possible Isomers of [PdX2L]2(μ-ditz) Complexes
1
Structural characterization of the minor isomer by H NMR
spectroscopy was hampered due to the low intensity of its
signals, which also overlap with those of trans-3. However, it
is noteworthy that three equally intense singlets (4.64, 4.39,
4.35 ppm) are observed for the N-methyl groups of the bridg-
ing ditz dicarbene ligand. The absence of 2-fold symmetry in
this minor isomer may point to a complex with a mixed cis−
trans configuration.
The formation of trans-3 is also supported by positive mode
ESI mass spectrometry, which shows a peak at m/z = 1072 for
4566
dx.doi.org/10.1021/om3003625 | Organometallics 2012, 31, 4565−4573

Organometallics
Article
Figure 2. Ball-and-stick representation of trans-3·3CHCl3. Hydrogen atoms and solvent molecules are omitted for clarity. Selected bond lengths [Å]
and angles [deg]: Pd1−C1 2.011(7), Pd1−C14 2.034(7), Pd1−Br1 2.4191(10), Pd1−Br2 2.4282(10), Pd2−C19 2.004(7), Pd2−C15 2.026(7),
Pd2−Br4 2.4373(11), Pd2−Br3 2.4433(11); C1−Pd1−Br1 87.3(2), C14−Pd1−Br1 90.9(2), C1−Pd1−Br2 89.2(2), C14−Pd1−Br2 92.7(2), C1−
Pd1−C14 177.1(3), Br1−Pd1−Br2 176.32(4), C19−Pd2−Br3 89.5(2), C15−Pd2−Br3 92.9(2), C19−Pd2−Br4 87.0(2), C15−Pd2−Br4 90.6(2),
Br4−Pd2−Br3 176.29(3), C19−Pd2−C15 177.4(3).
the [M + Na]+ species. However, its identity was finally con-
Scheme 3. Synthesis of Tetracarboxylato-Heterocarbene
firmed by X-ray diffraction analyses on single crystals obtained
by evaporation of a chloroform/hexane solution. The molecular
structure is shown in Figure 2, and crystallographic data are
listed in Table 3. The dinuclear complex contains two essen-
tially square-planar palladium centers, each of which is coordinated
by one terminal benzimidazolin-2-ylidene, two bromido ligands in
a trans fashion and linked by one triazolidin-3,5-diylidene. As
expected, the benzimidazolin-2-ylidene and triazolidin-3,5-
diylidene ring planes are oriented almost perpendicularly to
the PdC2Br2 coordination planes with average dihedral angles
of 83.67° and 67.30°, respectively. The distance between the
two Pd centers is ∼6.165 Å, and the average Pd−Cditz bond
length is 2.030 Å, which is significantly longer than those in the
reported complexes [trans-PdCl2(CH3CN)]2(μ-ditz) (1.949 Å)7d
and [trans-PdCl2(py)]2(μ-ditz) (1.960 Å).7e This is mainly due to
the increasing steric repulsion and the lower Lewis acidity of the
Pd centers induced by the coordination of the strongly donating
iPr2-bimy ligands. The Pd−Cbimy bond lengths, on the other hand
(2.011(7), 2.004(7) Å), are similar to those in the mononuclear
trans-[PdBr2(iPr2-bimy)]2 complex,8 and generally markedly
shorter than the Pd−Cditz bond lengths. The trans standing two
bromido ligands at each Pd center deviate from linearity (∼176°)
and are bent toward the bulky iPr2-bimy ligands. This bending has
been attributed to a “back-donation” of the bromido lone pair into
the formally vacant p orbital at the carbene carbon.10 Such an
interaction may also explain the limited rotation and the
Pd(II) Complexes 4 and 5
spectrum shows two sets of signals due to the presence of trans-4
and supposedly cis-anti-4 in a ratio of ∼2.6:1. The major product
trans-4 gives one singlet at 1.70 ppm for the four equivalent CH3
groups of the acetato ligands, and correspondingly one singlet at
22.8 ppm is observed in the 13C NMR spectrum. Surprisingly and
in contrast to trans-3, only one multiplet at 6.33 ppm and one
doublet at 1.76 ppm are detected for the N-isopropyl protons,
1
i
inequivalence of the isopropyl C−H protons observed by H
NMR spectroscopy (vide supra).
which may indicate a lower rotation barrier for the Pr2-bimy
ligand despite the presence of bulkier acetato ligands (vide infra).
The two sets of N-CH3 groups of the dicarbene ligand resonate at
4.63 and 4.42 ppm in an intensity ratio of 1:2, respectively.
For the minor cis isomer, two singlets at 1.71 and 1.66 ppm
of equal intensity are observed for the inequivalent acetato CH3
groups, and consequently, two signals at 23.0 and 22.8 ppm
are found in the 13C NMR spectrum. Interestingly, the C−H
isopropyl protons of cis-4 give rise to only one multiplet at 6.44
ppm, which is more downfield compared to that in trans-4. The
exact conformation of cis-4 as the syn or anti form (Chart 1)
cannot be further determined, although a cis-anti structure is more
likely due to sterical reasons. The 13C NMR signals due to the
carbene donors and the carbonyl groups of trans- and cis-4 fall in a
narrow range (175.4−180.2 ppm) and have not been assigned
further.
It has been reported that substitution of the halido ligands
with more labile coligands leads to better catalyst precursors,
which would undergo faster reduction to catalytically active
Pd(0) species.12 Thus, complex 3 was treated with 4 equiv of
AgO2CCH3 and AgO2CCF3 in CH3CN, affording the carboxylato
complexes [Pd(O2CCH3)2(iPr2-bimy)]2(μ-ditz) (4) and [Pd-
(O2CCF3)2(iPr2-bimy)]2(μ-ditz) (5) (Scheme 3). In both cases
and similar to complex 3, the product mixture contained
inseparable geometrical isomers, as indicated by NMR spectros-
copy (vide infra).
The formation of complex 4 bearing acetato ligands is
supported by ESI mass spectrometry with dominant peaks at
m/z = 906, 937, and 988 for [M − OAc]+, [M − OAc +
1
MeOH]+, and [M + Na]+ species, respectively. Its H NMR
4567
dx.doi.org/10.1021/om3003625 | Organometallics 2012, 31, 4565−4573

Organometallics
Article
Figure 3. Ball-and-stick representation of trans-4. Hydrogen atoms and disordered atoms are omitted for clarity. Selected bond lengths [Å] and
angles [deg]: Pd1−C18 2.013(12), Pd1−C1 2.029(11), Pd1−O1 2.026(11), Pd1−O3 2.036(11); C18−Pd1−O1 89.3(5), O1−Pd1−C1 89.6(4),
C18−Pd1−O3 90.0(5), C1−Pd1−O3 91.1(4), C18−Pd1−C1 178.5(5), O1−Pd1−O3 179.1(4). Bond lengths and angles related to Pd2 are not
given due to the disorder in the crystal.
Figure 4. Ball-and-stick representation of trans-5·CH3CN. Hydrogen atoms, disordered parts, and solvent molecules are omitted for clarity. Selected
bond lengths [Å] and angles [deg]: Pd1−C18 2.023(10), Pd1−C1 2.035(10), Pd1−O1 2.026(6), Pd1−O3 2.036(6), Pd2−C19 2.045(9), Pd2−C23
2.060(9), Pd2−O5 2.017(9), Pd2−O7 2.024(6); C18−Pd1−O1 87.7(3), O1−Pd1−C1 92.6(3), C18−Pd1−O3 91.3(3), C1−Pd1−O3 88.3(3),
C18−Pd1−C1 177.0(4), O1−Pd1−O3 178.0(3), O5−Pd2−C19 90.3(3), O7−Pd2−C19 91.2(3), O5−Pd2−C23 89.7(3), O7−Pd2−C23 88.9(3),
C19−Pd2−C23 179.8(5), O5−Pd2−O7 165.3(3).
Despite the presence of isomers in solution, several attempts
to grow single crystals always afforded crystals of solely trans-4.
A sample suitable for X-ray diffraction studies was obtained by
diffusion of pentane into a concentrated CH3CN solution, and
the molecular structure is shown in Figure 3. Since the atoms
around Pd2 are disordered, only the parameters of the complex
fragment containing Pd1 are discussed here. The acetato
ligands are monodentate and arranged in a trans fashion around
the square-planar Pd center. Furthermore, the dihedral angle
In contrast to the preparation of 4, complex trans-5 was
obtained almost exclusively as the major product, and the
amount of other isomers was negligible. Its ESI mass spectrum
is comparable to that of the acetato analogue, showing intense
isotopic patterns at m/z = 1068 and 1099 originating from
[M − CF3CO2]+ and [M − CF3CO2 + MeOH]+ fragments, res-
1
pectively. The H NMR spectrum of trans-5 in CDCl3 also
displays essentially the same characteristics as that of trans-4, apart
from an additional signal arising from the acetato ligands observed
for the latter.
i
between the Pr2-bimy heterocyclic ring and the Pd1C2O2
The 13C NMR spectrum, on the other hand, shows distinct
differences due to the presence of the trifluoroacetato coligands.
As a result of 13C−19F heteronuclear couplings, the signals for the
coordination plane has decreased to 67.31° compared with
that in trans-3. This is not surprising because this twist can
reduce the steric repulsion between the acetyl groups and the
iPr2-bimy ligand. The Pd−Ccarbene bond lengths remain essen-
tially unchanged compared to those in trans-3, and also the
average Pd−O bond length of 2.031 Å is comparable to those
in mononuclear trans-configured analogues.13 Notably and
different from trans-3, the two anionic coligands trans to each
other do not deviate from linearity (∼179°). The absence of
any intramolecular interactions of the O donors with the
2
CO (162.8 ppm, q, J(C,F) = 37.3 Hz) and CF3 groups (114.7
1
ppm, q, J(C,F) = 289.9 Hz) of these ligands resonate as two
distinct quartets, and their equivalency is in line with the proposed
all-trans configuration in solution. Finally, the two carbene carbon
resonances of trans-5 (176.3, 169.5 ppm) are found upfield
compared to those in trans-3 (179.6, 175.2 ppm), indicating a
lower Lewis acidity of the Pd(II) centers and a shielding effect of
the CF3CO2− ligands.14
i
carbene carbon may account for the free rotation of the Pr2-
bimy ligand and the equivalence of the isopropyl groups in
solution (vide supra). Attempts to obtain single crystals of
better quality and less disorder were to no avail.
Single crystals of trans-5 suitable for X-ray diffraction studies
were grown by diffusion of pentane into a concentrated CH3CN
solution. The molecular structure depicted in Figure 4 shows an
4568
dx.doi.org/10.1021/om3003625 | Organometallics 2012, 31, 4565−4573

Organometallics
Article
all-trans arrangement around the square-planar Pd(II) centers,
which was also observed in solution (vide supra). The inter-
palladium distance amounting to 6.146 Å is similar to that in
trans-3. The benzimidazolin-2-ylidene and triazolidin-diylidene
ring planes are oriented almost perpendicularly to the PdC2O2
coordination planes with average dihedral angles of 75.19° and
78.70°, respectively. Other structural parameters are nonexcep-
tional and do not require further comments.
Isomerization and Decomposition. In our previous study
on mononuclear dicarboxylato-bis(carbene)Pd(II) complexes, we
have noted an interesting trans−cis isomerization.8,12b Thus, the
isomerization behavior of 4 and 5 was studied by time-dependent
1H NMR spectroscopy. Figure 5 shows the H NMR spectra of
1
Figure 6. Ball-and-stick representation of trans-6. Hydrogen atoms are
omitted for clarity. Selected bond lengths [Å] and angles [deg]: Pd1−
O1 2.013(2), Pd1−O1A 2.013(2), Pd1−C1 2.032(3), Pd1−C1A
2.032(3); O1−Pd1−C1A 88.23(10), O1A−Pd1−C1A 91.77(10),
O1−Pd1−C1 91.77(10), O1A−Pd1−C1, 88.23(10), C1A−Pd1−C1
180.0, O1−Pd1−O1A 180.00(12).
1H NMR spectrum of the major product trans-6 shows only
one multiplet at 6.48 ppm and one doublet at 1.81 ppm for the
N-isopropyl group, indicating the free rotation of Pd−Ccarbene
bonds. Its 13C NMR spectrum reveals the carbene carbon
resonance at 180.1 ppm, which is almost the same with that
(180.0 ppm) of its precursor complex trans-[PdBr2(iPr2-bimy)2].8
On the other hand, two doublets at 1.85 and 1.84 ppm are
detected for the inequivalent isopropyl CH3 groups of the minor
isomer cis-6. The two acetato ligands in cis-6 are also different,
giving rise to two singlets at 1.72 and 1.62 ppm. Furthermore, the
mass spectrum shows a base peak for the [M − O2CCH3 + L]+
species, further supporting the formation of complex 6.
Catalysis. All triazolidin-diylidene complexes 3, 4 ,and 5
were tested as catalyst precursors in the Mizoroki−Heck
reaction. The coupling of aryl halides with tert-butyl acrylate in
DMF with a 0.5 mol % catalyst loading and a reaction time of
24 h was selected as a standard test reaction condition. The
results obtained are summarized in Table 1. Entries 1−3 reveal
that all three complexes can couple activated 4-bromobenzal-
dehyde in near-quantitative yields without any additives. Using
the same conditions, the yields for the coupling of deactivated
4-bromoanisole decreased generally (entries 4, 6, and 8, Table 1).
It is surprising that the drop in yield is especially severe with
precatalysts 4 and 5, whereas complex 3 still afforded 78%. The
difference in yields is also reflected in the rate of Pd black
formation during these reactions (∼4 h for entry 4, ∼1.5 h for
entries 6 and 8). As expected, addition of NBu4Br led to a dra-
matic improvement, giving essentially quantitative product yields
for the coupling of 4-bromoanisole (entries 5, 7, and 9, Table 1)
and 4-chlorobenzaldehyde (entries 10−12, Table 1). All the
reactions gave selectively the corresponding E-cinnamate, and a
further increase of the reaction temperature to 140 °C lowers the
selectivity to a slight extent and gave Z isomers as minor products.
To further study any potential cooperative effects introduced
by the ditz bridges, the activity of dimetallic trans-3 was
compared with that of the previously reported monopalladium
complex trans-[PdBr2(iPr2-bimy)2].8 The coupling reactions
were conducted under the same condition using the same
catalyst loading based on Pd (Table 1, entries 13 and 14), and
the cooperativity index (a), proposed by Jones and James,15
was calculated using eqs 1 and 2.
1
Figure 5. Time-dependent H NMR spectra showing the trans−cis
isomerization of complex 5 in CDCl3.
complex trans-5 in the range of ∼6.10−6.60 ppm recorded over
a time span of 110 h in CDCl3. Notably, the intensity of the
multiplet at 6.42 ppm due to the isopropyl C−H protons of the
cis isomer increased gradually until eventually a trans/cis
isomeric ratio of ∼1:1 is reached. On the other hand, the
isomerization of trans-4 is sluggish with an insignificant change
of the isomeric ratio from an initial value of ∼2.6:1 to finally
∼2.2:1. This finding is not surprising, since a cis configuration is
expected to be electronically more favorable for bis(carbene)
−
complexes bearing weaker donating CF3CO2 coligands. The
fact that the trans isomers do not fully convert to the cis form,
as observed for mononuclear species, may be attributed to
competing contribution of the enhanced sterical crowding in
the resulting cis-configured dinuclear complex.
In contrast to the tetrabromido complex 3, the tetracarbox-
ylato compounds 4 and 5 are less stable, particularly in their
solutions. Formation of Pd black was observed in CH3CN
solutions upon standing for a couple of days. From the crude
decomposition mixture of complex 4, we were able to obtain a
few single crystals, which were found to be the mononuclear
complex trans-[Pd(O2CCH3)2(iPr2-bimy)]2 (trans-6). The
molecular structure determined by X-ray crystallography and
shown in Figure 6 consists of one Pd(II) center coordinated by
two acetato ligands and two benzimidazolin-2-ylidene ligands in
a trans fashion. Although the decomposition pathway of
complex 4 is currently not known, the formation of Pd black
and trans-6 points to a process involving reductive elimination
of the bridging ditz ligand.
trans-6 can be selectively synthesized by treating trans-
[PdBr2(iPr2-bimy)2]8 with 2 equiv of AgO2CCH3 in CH3CN
(Scheme 4). Similar with complex 4, the product was obtained
as a mixture of trans- and cis-6 with a final trans/cis isomeric
AP
n
A =
̅
1
(1)
ratio of ∼3.5:1, as indicated by H NMR spectroscopy. The
4569
dx.doi.org/10.1021/om3003625 | Organometallics 2012, 31, 4565−4573

Organometallics
Article
Scheme 4. Synthesis of Pd(II) Acetato Complex 6
a
Table 1. Mizoroki−Heck Coupling Reactions Catalyzed by Complexes 3, 4, and 5
b
entry
catalyst
mol %
aryl halide
temp [°C]
yield [%]
1
2
3
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
1
4-bromobenzaldehyde
4-bromobenzaldehyde
4-bromobenzaldehyde
4-bromoanisole
120
120
120
120
120
120
120
120
120
120
120
120
140
140
>99
>99
90
4
3
5
4
3
78
c
5
3
4-bromoanisole
94
6
4
4-bromoanisole
10
>99
8
c
7
4
4-bromoanisole
8
5
4-bromoanisole
c
9
5
4-bromoanisole
93
c
10
11
12
13
14
3
4-chlorobenzaldehyde
4-chlorobenzaldehyde
4-chlorobenzaldehyde
4-bromoanisole
97
c
4
98
c
5
96
c
3
97
c,d
trans-[PdBr2(iPr2-bimy)2]
4-bromoanisole
72
a
Reaction conditions generally not optimized. Reaction conditions in entries 1−12: 1 mmol of aryl halide; 1.5 mmol of tert-butyl acrylate; 3 mL of
DMF; 1.5 equiv of NaHCO3; 0.5 mol % of catalyst; 120 °C; 24 h. Reaction conditions in entries 13 and 14 are the same with those in entries 1−12
b
except for the use of NaOAc as the base and 140 °C as the reaction temperature. Yields were determined by 1H NMR spectroscopy for an average
of two runs. With addition of 1.5 equiv of [N(n-C4H9)4]Br. Data are obtained from a previous study.8
c
d
AO − AP
CONCLUSION
■
a =
(2)
A
̅
In summary, we have reported the synthesis of new dinuclear
heterocarbene Pd(II) complexes 3−5 bearing triazolidin-
diylidene bridges and terminal benzimidazolin-2-ylidenes.
NMR spectroscopy and X-ray diffraction analysis revealed
that the all-trans isomers are generally more favorable. A pre-
liminary catalytic study on Mizoroki−Heck reactions reveals
the superiority of 3 compared to the previously reported mono-
nuclear Pd(II) bis(carbene) complexes with a cooperativity
index of a = 0.69, which warrants further study into the co-
operativity behavior of complexes bearing ditz bridges. Overall,
complex 4 bearing an acetato ligand appears to be the best
precatalyst. Further studies are underway to study the detailed
isomerization behavior of such dinuclear ditz bridged com-
plexes and to extend their scope to metallo-supramolecular
carbene chemistry.
In our case, AO and AP were taken as the yields obtained after
24 h in the bimetallic and monometallic systems, respectively,
which gave the cooperativity index a = 0.69. This positive value
indicates cooperativity in the dinuclear system of trans-3, the
nature of which remains to be explored in the future. Overall,
the three dinuclear complexes 3−5 showed superiority over the
mononuclear bis(carbene) complexes trans-[PdBr2(iPr2-bimy)2]
and cis-[Pd(O2CCF3)2(iPr2-bimy)2] at the same catalyst loading
based on Pd, which were reported by us previously.8
Table 1 also revealed that the acetato complex 4 shows a
slightly better performance than the other two complexes,
which should result from the ability of the acetato ligand to
stabilize active Pd0 species.12b Encouraged by the good
performance of precatalyst 4, we extended its scope to a few
deactivated substrates, dihalides, and heteroarenes. The results
summarized in Table 2 show that excellent yields are
obtained with aryl- and heteroaryl bromides. Substrates with
N- or S-donor atoms can undergo the coupling reactions in
high yields without deactivation of the catalysts via com-
peting coordination (entries 3−5). However, for the more
challenging 2-chloro-pyridine substrate, only negligible
conversion was observed.
EXPERIMENTAL SECTION
■
General Considerations. Unless otherwise stated, all the
manipulations were carried out without taking precautions to exclude
air and moisture. All chemicals and solvents were used as received
without further purification if not mentioned otherwise. 1,2-Dichloro-
ethane was dried with CaH2 and distilled under N2 using standard
Schlenk techniques. Complex 210 and trans-[PdBr2(iPr2-bimy)2]8 were
1
synthesized as previously reported. H, 13C, and 19F NMR spectra
4570
dx.doi.org/10.1021/om3003625 | Organometallics 2012, 31, 4565−4573

Organometallics
Article
a
b c
,
Table 2. Mizoroki−Heck Coupling Reactions Catalyzed by 4
a
Reaction conditions: 1 mmol of aryl halide; 1.5 mmol of tert-butyl acrylate; 3 mL of DMF; 1.5 equiv of NaHCO3; 0.5 mol % of complex 4; 120 °C;
b
c
1
24 h. Yields were determined by H NMR spectroscopy for an average of two runs. With addition of 1.5 equiv of [N(n-C4H9)4]Br.
were recorded on Bruker ACF 300 and Bruker AMX 500 spectro-
meters, and the chemical shifts (δ) were internally referenced to the
residual solvent signals relative to (CH3)4Si (1H, 13C) or externally to
CF3CO2H (19F). ESI mass spectra were measured using a Finnigan
MAT LCQ spectrometer. Elemental analyses were done on an Elementar
Vario Micro Cube elemental analyzer at the Department of Chemistry,
National University of Singapore.
1-Methyl-1,2,4-triazole (A) and 4-Methyl-1,2,4-triazole (B).
Dimethyl carbonate (30 mL, 356 mmol) was added to a mixture of
1,2,4-triazole (3.45 g, 50 mmol) and K2CO3 (3.45 g, 25 mmol). The
resulting suspension was heated under reflux overnight. All the
volatiles were removed under vacuum, and then CHCl3 (20 mL) was
added to the residue. After filtration, the solvent was removed to yield
a colorless liquid (2.85 g, 34.3 mmol, 69%). Analytical data of 1a and
1b were identical with literature values.16
solvent of the filtrate was removed under vacuum. The yellow residue
was redissolved in CHCl3 (30 mL) and washed with water (20 mL × 3).
The organic phase was collected and dried over Na2SO4. Filtration and
removal of the solvent under vacuum afforded the product as a yellow
1
solid (1042 mg, 0.979 mmol, 90%). trans-3: H NMR (500 MHz,
CDCl3): δ 7.58−7.57 (m, br, 4H, Ar-H), 7.23−7.21 (m, 4H, Ar-H),
6.16 (m, 2H, CH(CH3)2, 3J(H,H) = 7.0 Hz), 6.02 (m, 2H, CH(CH3)2,
3J(H,H) = 7.0 Hz), 4.62 (s, 3H, NCH3), 4.36 (s, 6H, NCH3), 1.82,
1.80 (d, 24H, 3J(H,H) = 7.0 Hz). 13C{1H} NMR (125.76 MHz,
i
CDCl3): 179.6 (s, NCHN, ditz), 175.2 (s, NCHN, Pr2-bimy), 134.2,
134.1, 122.9, 113.4 (s, Ar-C), 54.8 (s, CH(CH3)2), 41.3 (s, NCH3),
37.7 (s, NCH3), 21.7 (s, CH(CH3)2). Signals of other minor isomers
have not been assigned due to their very low intensity. MS (ESI):
m/z = 1072 [M + Na]+. Anal. Calcd for C32H49Br4N7Pd2: C, 36.11; H,
4.64; N, 9.21%. Found: C, 35.58; H, 4.25; N, 9.22%.
1,2,4-Trimethyl-1,2,4-triazolium tetrafluoroborate (C). In a
modification of a literature procedure,9 the isomeric mixture of A
and B (130 mg, 1.56 mmol) was dissolved in dry 1,2-dichloroethane
(0.5 mL), and the resulting solution was transferred to a suspension of
trimethyloxonium tetrafluoroborate (692 mg, 4.68 mmol) in dry 1,2-
dichloroethane (1.5 mL) via cannula. The reaction mixture was heated
under reflux under N2 for 30 min and then cooled to ambient tem-
perature. All the volatiles were removed under vacuum. The residue
was washed with CH2Cl2 (3 × 3 mL) and then dried under vacuum to
Pd(II) Acetato Complex 4. CH3CN (10 mL) was added to a
mixture of AgO2CCH3 (70 mg, 0.42 mmol) and complex 3 (106 mg,
0.1 mmol). The resulting green-yellowish suspension was stirred
overnight at ambient temperature shielded from light and filtered
through Celite. The solvent of the filtrate was removed under reduced
pressure, yielding a yellow solid (84 mg, 0.087 mmol, 87%). Overlapp-
ing and signals that cannot be unambiguously assigned are indicated
1
with an asterisk (*). trans-4: H NMR (300 MHz, CDCl3): δ 7.57−
7.54 (m, 4H, Ar-H*), 7.19−7.14 (m, 4H, Ar-H*), 6.33 (m, 4H,
1
yield a white solid (262 mg, 0.91 mmol, 59%). H NMR (500 MHz,
3
CH(CH3)2, J(H,H) = 7.2 Hz), 4.63 (s, 3H, NCH3), 4.42 (s, 6H,
CD3CN): δ 9.85 (s, 2H, NCHN), 4.32 (s, 6H, NCH3), 4.15 (s, 3H,
NCH3). 13C{1H} NMR (125.76 MHz, CD3CN): 146.8 (s, NCHN),
39.1 (s, NCH3), 38.8 (s, NCH3). 19F{1H} NMR (282.37 MHz,
CD3CN): −75.52 (s, 10BF4), −75.57 (s, 11BF4). MS (ESI): m/z = 130
[M − 2BF4 + OH]+.
Pd(II) Bromido Complex 3. THF (25 mL) was added to a
mixture of 1,2,4-trimethyl-1,2,4-triazolium tetrafluoroborate (C) (300
mg, 1.10 mmol) and AgOAc (365 mg, 2.19 mmol). The resulting
suspension was heated under reflux for 2 h shielded from light. After
cooling the reaction mixture to ambient temperature, all volatiles were
removed under vacuum, and CH3CN (25 mL) was added to the off-
white residue. The solution formed was transferred to a solution of
complex 2 (1632 mg, 2.17 mmol) in CH2Cl2 (30 mL). A yellow pre-
cipitate formed immediately, and the suspension was stirred overnight
at ambient temperature. The mixture was filtered over Celite, and the
NCH3*), 1.76 (d, 24 H, 3J(H,H) = 7.2 Hz), 1.70 (s, 12 H, CH3COO).
13C{1H} NMR (75.47 MHz, CDCl3): 180.2, 178.4, 177.9, 177.1,
176.4, 175.4 (s, NCHN* and CH3COO*), 133.5, 122.7, 113.5 (s, Ar-C),
54.2 (s, CH(CH3)2), 39.5 (s, NCH3), 37.1 (s, NCH3), 22.8 (s,
CH3COO), 21.8 (s, CH(CH3)2). cis-4: 1H NMR (300 MHz, CDCl3):
δ 7.57−7.54 (m, 4H, Ar-H*), 7.19−7.14 (m, 4H, Ar-H*), 6.44 (m,
4H, CH(CH3)2, 3J(H,H) = 6.9 Hz), 4.58 (s, 3H, NCH3), 4.42 (s, 6H,
3
NCH3*), 1.77 (d, 24 H, J(H,H) = 6.9 Hz), 1.71 (s, 6H, CH3COO),
1.66 (s, 6H, CH3COO). 13C{1H} NMR (75.47 MHz, CDCl3): 180.2,
178.4, 177.9, 177.1, 176.4, 175.4 (s, NCHN* and CH3COO*), 133.7,
122.4, 113.3 (s, Ar-C), 53.9 (s, CH(CH3)2), 39.4, 37.3, 37.2 (s,
NCH3), 23.0, 22.8 (s, CH3COO), 21.9 (s, CH(CH3)2). MS (ESI):
m/z = 906 [M − CH3COO]+, 937 [M + MeOH − CH3COO]+, 988
[M + Na]+, 1871 [2M − CH3COO]+.
4571
dx.doi.org/10.1021/om3003625 | Organometallics 2012, 31, 4565−4573

Organometallics
Article
Table 3. Selected X-ray Crystallographic Data for Complexes trans-3·3CHCl3, trans-4, trans-5·CH3CN, and trans-6
trans-3·3CHCl3
trans-4
trans-5·CH3CN
trans-6
formula
fw
C31H45Br4N7Pd2·3CHCl3
1406.28
C39H57N7O8Pd2
964.72
C39H45F12N7O8Pd2·CH3CN
1221.67
C30H42N4O4Pd
629.08
color, habit
cryst size (mm)
temp (K)
cryst syst
space group
a (Å)
yellow, block
0.30 × 0.24 × 0.08
100(2)
colorless, thin-plate
0.50 × 0.20 × 0.12
223(2)
colorless, block
0.40 × 0.26 × 0.18
100(2)
colorless, block
0.36 × 0.20 × 0.12
223(2)
monoclinic
P2(1)/c
orthorhombic
P2(1)2(1)2(1)
9.1677(6)
15.6062(9)
31.6509(17)
90
monoclinic
P2(1)/c
orthorhombic
Pbca
14.2884(11)
11.9763(11)
31.944(3)
90
14.4073(17)
26.565(3)
13.7018(17)
90
16.9882(9)
9.9514(5)
18.5623(10)
90
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
V (Å3)
101.585(2)
90
90
104.890(3)
90
90
90
90
5355.0(8)
4
4528.4(5)
4
5068.0(11)
4
3138.1(3)
4
Z
Dc (g cm−3
)
1.744
1.415
1.601
1.332
radiation used
μ (mm−1
Mo Kα
Mo Kα
Mo Kα
Mo Kα
)
4.137
0.848
0.809
0.630
θ range (deg)
1.82−27.50
12 275
1.83−25.00
7971
1.53−25.00
8931
2.19−27.50
3595
no. of unique data
max, min transmission
final R indices (I > 2σ(I))
0.7332, 0.3700
R1 = 0.0635
wR2 = 0.1493
R1 = 0.1176,
wR2 = 0.1706
1.031
0.9051, 0.6765
R1 = 0.0909
wR2 = 0.2202
R1 = 0.1135
wR2 = 0.2352
1.108
0.8680, 0.7378
R1 = 0.0882
wR2 = 0.2222
R1 = 0.1131
wR2 = 0.2418
1.039
0.9283, 0.8051
R1 = 0.0465
wR2 = 0.0949
R1 = 0.0784
wR2 = 0.1062
1.047
R indices (all data)
goodness of fit on F2
peak/hole (e Å−3
)
1.751/−1.315
1.555/−0.411
2.027/−1.624
0.630/−0.364
Pd(II) Trifluoroacetato Complex 5. Complex 5 was synthesized
in analogy to complex 4 using AgO2CCF3 (93 mg, 0.42 mmol) and
complex 3 (106 mg, 0.1 mmol) and isolated as a yellow solid (108 mg,
22.06, 21.9, 21.8 (s, CH3COO* and CH(CH3)2*). MS (ESI): m/z =
771 [M − CH3COO + L]+. Anal. Calcd for C30H42N4O4Pd: C,
57.28; H, 6.73; N, 8.91%. Found: C, 57.26; H, 6.37; N, 8.53%.
General Procedure for Mizoroki−Heck Catalysis. In a typical
run, a 25 mL Schlenk tube was charged with a mixture of aryl halide
(1.0 mmol for monohalides, 0.5 mmol for dihalides), sodium bicarbonate
(1.5 mmol), tert-butyl acrylate (1.5 mmol), catalyst (0.005 mmol), and
DMF (3 mL). The tube was sealed, and the reaction mixture was stirred
and heated at 120 °C for 24 h. After the mixture was cooled to ambient
temperature, dichloromethane (10 mL) was added. The organic layer was
then washed with water (5 × 10 mL) and dried over sodium sulfate. The
solvent was removed by evaporation to give a crude product, which was
1
0.091 mmol, 91%). trans-5: H NMR (300 MHz, CDCl3): δ 7.65−
7.62 (m, 4H, Ar-H), 7.30−7.28 (m, 4H, Ar-H), 6.28 (m, 4H,
3
CH(CH3)2, J(H,H) = 6.9 Hz), 4.73 (s, 3H, NCH3), 4.48 (s, 6H,
NCH3), 1.83 (d, 24H, 3J(H,H) = 6.9 Hz). 13C{1H} NMR (75.47
i
MHz, CDCl3): 176.3 (s, NCHN, ditz), 169.5 (s, NCHN, Pr2-bimy),
162.8 (q, 2J(C,F) = 37.3 Hz, CF3COO), 133.6, 123.7, 113.8 (s, Ar-C),
114.7 (q, 1J(C,F) = 289.9 Hz, CF3COO), 54.8 (s, CH(CH3)2), 39.5 (s,
NCH3), 37.4 (s, NCH3), 22.1 (s, CH(CH3)2). 19F NMR (282.38 MHz,
CDCl3): 2.19 (s, 12 F, CF3). Signals of other minor isomers have not
been assigned due to their very low intensity. MS (ESI): m/z = 1068
[M − CF3COO]+, 1099 [M − CF3COO + CH3OH]+. Anal. Calcd for
C39H45F12N7O8Pd2: C, 39.67; H, 3.84; N, 8.30%. Found: C, 39.74; H,
3.74; N, 8.15%.
1
analyzed by H NMR spectroscopy.
X-ray Diffraction Studies. X-ray data for trans-3·3CHCl3, trans-4,
trans-5·CH3CN, and trans-6 were collected with a Bruker AXS
SMART APEX diffractometer, using Mo Kα radiation at 223(2) K
(for trans-4 and trans-6) or at 100(2) K (for trans-3·3CHCl3, trans-
5·CH3CN) with the SMART suite of Programs.17 Data were pro-
cessed and corrected for Lorentz and polarization effects with SAINT,18
and for absorption effects with SADABS.19 Structural solution and
refinement were carried out with the SHELXTL suite of programs.20
The structure was solved by direct methods to locate the heavy atoms,
followed by difference maps for the light, non-hydrogen atoms. All
non-hydrogen atoms were generally given anisotropic displacement
parameters in the final model. All H atoms were put at calculated
positions. A summary of the most important crystallographic data is
given in Table 3.
Pd(II) Acetato Complex 6. CH3CN (10 mL) was added to a
mixture of AgO2CCH3 (37 mg, 0.22 mmol) and trans-[PdBr2(iPr2-
bimy)2] (67 mg, 0.1 mmol). The reaction mixture was stirred for 3 h at
ambient temperature shielded from light. The resulting green-
yellowish suspension was then filtered through Celite, and the solvent
of the filtrate was removed under reduced pressure, yielding an off-
white solid (51 mg, 0.081 mmol, 81%). Overlapping and signals that
cannot be unambiguously assigned are indicated with an asterisk (*).
1
trans-6: H NMR (300 MHz, CDCl3): δ 7.60−7.58 (m, 4H, Ar-H),
3
7.20−7.18 (m, 4H, Ar-H), 6.48 (m, 4H, CH(CH3)2, J(H,H) = 7.0
3
Hz), 1.81 (d, 24H, J(H,H) = 7.0 Hz), 1.70 (s, 12H, CH3COO).
13C{1H} NMR (75.47 MHz, CDCl3): 180.1 (s, NCHN), 176.5 (s,
CH3COO), 133.8, 122.5, 113.4 (s, Ar-C), 54.0 (s, CH(CH3)2), 23.1 (s,
CH3COO), 21.9 (s, CH(CH3)2). cis-6: 1H NMR (300 MHz, CDCl3):
δ 7.63−7.58 (m, 4H, Ar-H*), 7.24−7.22 (m, 4H, Ar-H), 6.55−6.45
(m, 4H, CH(CH3)2*, 3J(H,H) = 6.5 Hz), 1.85 (d, 12H, 3J(H,H) = 6.5 Hz),
ASSOCIATED CONTENT
■
S
* Supporting Information
13C NMR and HMBC spectra of complex 3 and crystallo-
graphic data for trans-3·3CHCl3, trans-4, trans-5·CH3CN, and
trans-6 as CIF files. This material is available free of charge via
the Internet at http://pubs.acs.org.
3
1.84 (d, 12H, J(H,H) = 6.5 Hz), 1.72 (s, 6H, CH3COO), 1.62 (s, 6H,
CH3COO). 13C{1H} NMR (75.47 MHz, CDCl3): The Ccarbene signal could
not be detected due to its low concentration. 176.0 (CH3COO*),
134.0, 133.0, 122.5, 113.6, 113.3 (s, Ar-C), 54.6 (s, CH(CH3)2), 22.10,
4572
dx.doi.org/10.1021/om3003625 | Organometallics 2012, 31, 4565−4573

Organometallics
Article
(14) Huynh, H. V.; LeVan, D.; Hahn, F. E.; Hor, T. S. A. J.
Organomet. Chem. 2004, 689, 1766.
(15) Jones, N. D.; James, B. R. Adv. Synth. Catal. 2002, 344, 1126.
(16) (a) Belletire, J. L.; Bills, R. A.; Shackelford, S. A. Synth. Commun.
2008, 38, 738. (b) Elguero, J.; Marzin, C.; Roberts, J. D. J. Org. Chem.
1974, 39, 357.
(17) SMART, version 5.628; Bruker AXS Inc.: Madison, WI, 2001.
(18) SAINT+, version 6.22a; Bruker AXS Inc.: Madison, WI, 2001.
(19) Sheldrick, G. W. SADABS, version 2.10; University of
AUTHOR INFORMATION
Corresponding Author
*E-mail: chmhhv@nus.edu.sg.
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the National University of Singapore and the
Singapore Ministry of Education for financial support (WBS
R-143-000-410-112). Technical assistance from the staff at the
CMMAC of our department is appreciated. In particular, we
thank Ms. Geok Kheng Tan, Ms. Hong Yimian, and Prof. Lip
Lin Koh for determining the X-ray molecular structures.
Gottingen: Gottingen, Germany, 2001.
̈
̈
(20) SHELXTL, version 6.14; Bruker AXS Inc.: Madison, WI, 2000.
REFERENCES
■
(1) (a) Hahn, F. E. Angew. Chem., Int. Ed. 2006, 45, 1348.
(b) Kantchev, E. A. B.; O’Brien, C. J.; Organ, M. G. Angew. Chem., Int.
Ed. 2007, 46, 2768. (c) Herrmann, W. A. Angew. Chem., Int. Ed. 2002,
́
41, 1290. (d) Cesar, V.; Bellemin-Laponnaz, S.; Gade, L. H. Chem. Soc.
Rev. 2004, 33, 619. (e) Peris, E.; Crabtree, R. H. Coord. Chem. Rev.
2004, 248, 2239. (f) Bourissou, D.; Guerret, O.; Gabbaï, F. P.;
Bertrand, G. Chem. Rev. 2000, 100, 39.
(2) Selected examples: (a) Wells, K. D.; Ferguson, M. J.; McDonald,
R.; Cowie, M. Organometallics 2008, 27, 691. (b) Huynh, H. V.;
Jothibasu, R. Eur. J. Inorg. Chem. 2009, 1926. (c) Huynh, H. V.; Seow,
H. X. Aust. J. Chem. 2009, 62, 983. (d) Han, Y.; Lee, L. J.; Huynh, H.
V. Chem.Eur. J. 2010, 16, 771. (e) Yeung, A. D.; Ng, P. S.; Huynh,
H. V. J. Organomet. Chem. 2011, 696, 112. (f) Huynh, H. V.; Jothibasu,
R. J. Organomet. Chem. 2011, 696, 3369.
(3) (a) Rit, A.; Pape, T.; Hahn, F. E. Organometallics 2011, 30, 6393.
(b) Rit, A.; Pape, T.; Hepp, A.; Hahn, F. E. Organometallics 2011, 30,
334. (c) Rit, A.; Pape, T.; Hahn, F. E. J. Am. Chem. Soc. 2010, 132,
4572. (d) Mercs, L.; Neels, A.; Stoeckli-Evans, H.; Albrecht, M. Dalton
Trans. 2009, 35, 7168.
(4) (a) Boydston, A. J.; Williams, K. A.; Bielawski, C. W. J. Am. Chem.
Soc. 2005, 127, 12496. (b) Khramov, D. M.; Boydston, A. J.; Bielawski,
C. W. Angew. Chem., Int. Ed. 2006, 45, 6186.
(5) In previous reports, the ditz ligand has been referred to as
triazolediylidenes (e.g., 1,2,4-triazole-3,5-diylidene) or triazolyldiyli-
dene. However, the correct carbene Lewis structure shown in Figure 1
for ditz contains a fully reduced heterocycle (e.g., no double bonds).
According to the nomenclature for heterocyclic systems, ditz should
thus be termed triazolidin-diylidene (e.g., 1,2,4-trimethyltriazolidin-
3,5-diylidene).
(6) Guerret, O.; Sole, S.; Gornitzka, H.; Teichert, M.; Trinquier, G.;
Bertrand, G. J. Am. Chem. Soc. 1997, 119, 6668.
(7) (a) Mas-Marza, E.; Mata, J. A.; Peris, E. Angew. Chem., Int. Ed.
2007, 46, 3729. (b) Viciano, M.; Sanau, M.; Peris, E. Organometallics
2007, 26, 6050. (c) Zanardi, A.; Corberan, R.; Mata, J. A.; Peris, E.
Organometallics 2008, 27, 3570. (d) Zanardi, A.; Mata, J. A.; Peris, E.
Organometallics 2009, 28, 1480. (e) Zanardi, A.; Mata, J. A.; Peris, E.
Organometallics 2009, 28, 4335. (f) Zanardi, A.; Mata, J. A.; Peris, E.
Chem.Eur. J. 2010, 16, 13109.
(8) Han, Y.; Huynh, H. V.; Koh, L. L. J. Organomet. Chem. 2007, 692,
3606.
(9) Curphey, T. J.; Prasad, K. S. J. Org. Chem. 1972, 37, 2259.
(10) Huynh, H. V.; Han, Y.; Ho, J. H. H.; Tan, G. K. Organometallics
2006, 25, 3267.
(11) (a) Huynh, H. V.; Han, Y.; Jothibasu, R.; Yang, J. A.
Organometallics 2009, 28, 5395. (b) Yuan, D.; Huynh, H. V.
Organometallics 2012, 31, 405.
(12) (a) Huynh, H. V.; Ho, J. H. H.; Neo, T. C.; Koh, L. L. J.
Organomet. Chem. 2005, 690, 3854. (b) Huynh, H. V.; Neo, T. C.;
Tan, G. K. Organometallics 2006, 25, 1298.
(13) (a) Kuhn, N.; Maichle-Moβmer, C.; Niquet, E.; Walker, I. Z.
̈
Naturforsch. 2002, 57b, 47. (b) Konnick, M. M.; Guzei, I. A.; Stahl, S.
S. J. Am. Chem. Soc. 2004, 126, 10212.
4573
dx.doi.org/10.1021/om3003625 | Organometallics 2012, 31, 4565−4573