A palladacyclic N-heterocyclic carbene system used to probe the donating abilities of monoanionic chelators

Article abstract of DOI:10.1039/c8dt01618f

A series of NHC-containing [C^N]- or [C^C′]-type palladacyclic complexes of the general formula [PdBr(ⁱPr₂-bimy)(L^X)] (5-8, 11, 12, ⁱPr₂-bimy = 1,3-diisopropylbenzimidazolin-2-ylidene) have been synthesized and fully characterized. Using these complexes, the donating abilities of monoanionic chelators were probed for the first time. The [C^N]-type palladacycles 5-8 were prepared from acetato-bridged dipalladium complexes [Pd(μ-CH₃COO)(C^N)]₂ (1-4) and ⁱPr₂-bimy·H⁺Br^-as precursors. In the case of the [C^C′]-type NHC palladacycles (11, 12), the hetero-bis(NHC) complexes trans-[PdBr₂(ⁱPr₂-bimy)(trz)] (8, 9, trz = 1,2,3-triazolin-5-ylidene) containing the ⁱPr₂-bimy probe were first prepared followed by acetate-assisted cyclopalladations. The ¹³C_carbene NMR signals of the ⁱPr₂-bimy ligands in all complexes (i.e. HEP and HEP2 values) are found to rationally reflect the donating abilities of the incorporated trz or [L^X]-type chelators with the exception of the Bzpy ligand (Bzpy = 2-(2-pyridinylmethyl)phenyl-C,N). This has been attributed to its larger bite angle, the resulting varied coordination geometry and the lack of electronic delocalization between the two donor units. The donicities of [L^X]-type chelators studied in this work were found to surpass those of all other bidentate ligands evaluated by HEP2 thus far.

Full text of DOI:10.1039/c8dt01618f

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A palladacyclic N-heterocyclic carbene
system used to probe the donating abilities
Cite this: DOI: 10.1039/c8dt01618f
of monoanionic chelators†
a
a
a
a
a
Xuechao Yan, Rui Feng, Chunhui Yan, Peng Lei, Shuai Guo * and
b
Han Vinh Huynh
*
A series of NHC-containing [C^N]- or [C^C’]-type palladacyclic complexes of the general formula
i
i
[
2 2
PdBr( Pr -bimy)(L^X)] (5–8, 11, 12, Pr -bimy = 1,3-diisopropylbenzimidazolin-2-ylidene) have been syn-
thesized and fully characterized. Using these complexes, the donating abilities of monoanionic chelators
were probed for the first time. The [C^N]-type palladacycles 5–8 were prepared from acetato-bridged
i
+
−
dipalladium complexes [Pd(μ-CH
3
COO)(C^N)]
2
(1–4) and Pr
2
-bimy·H Br as precursors. In the case of
i
the [C^C’]-type NHC palladacycles (11, 12), the hetero-bis(NHC) complexes trans-[PdBr
2
( Pr
2
-bimy)(trz)]
i
(
8, 9, trz = 1,2,3-triazolin-5-ylidene) containing the Pr
2
-bimy probe were first prepared followed by
13
i
acetate-assisted cyclopalladations. The
C
carbene NMR signals of the Pr
2
-bimy ligands in all complexes
(
i.e. HEP and HEP2 values) are found to rationally reflect the donating abilities of the incorporated trz or
Received 23rd April 2018,
Accepted 16th May 2018
[L^X]-type chelators with the exception of the Bzpy ligand (Bzpy = 2-(2-pyridinylmethyl)phenyl-C,N). This
has been attributed to its larger bite angle, the resulting varied coordination geometry and the lack of
electronic delocalization between the two donor units. The donicities of [L^X]-type chelators studied in
this work were found to surpass those of all other bidentate ligands evaluated by HEP2 thus far.
DOI: 10.1039/c8dt01618f
rsc.li/dalton
stronger donating ligand
L
in complexes of the type
Introduction
i
i
trans-[PdBr ( Pr -bimy)(L)]
( Pr -bimy 1,3-diisopropyl-
=
2
2
2
13
The properties of transition metal complexes are highly depen- benzimidazolin-2-ylidene) leads to a more downfield
C
carbene
i
5
dent on the electronic nature of their ligands. During the past NMR signal of the Pr -bimy ligand (Chart 1, system I). In con-
2
few decades, various methods to measure the electron-donat- trast to carbonyl-based methods, the HEP utilizes a more Lewis-
1
ing abilities of ligands have been developed. In this regard, acidic palladium(II) center, which is known to be a weaker
the best known metric is undoubtedly the Tolman electronic π-donor. Moreover, planar ligands, e.g. NHCs or pyridines, bind
2
parameter (TEP), in which the A1 carbonyl bands of perpendicularly to the coordination plane in the HEP system,
3 3
[Ni(CO) (PR )] complexes serve as an indicator for the net- thus making the delocalization of d-electrons into their π-cloud
donor strength of the phosphine of interest. Along with the less efficient. As such, the donor–acceptor ability of a ligand can
3
rapid development of NHC chemistry (NHC = N-heterocyclic be better separated, and it is primarily the σ-donating ability
carbene), the TEP and related methods based on iridium and that is being evaluated. By using the HEP, the electron-donating
1
a–c,4
rhodium have also been used to evaluate NHCs.
However, abilities of various monodentate Werner-type and organo-
1d,6
it is noteworthy that in certain cases, competing backdonation metallic donors have been gauged and compared.
to the ligand of interest becomes non-negligible, complicating
More recently, the HEP has been extended to the so-called
HEP2, which probes the donor strengths of charge-neutral and
1
b
the interpretation of data.
1
3
In 2009, Huynh and co-workers reported a C NMR-based symmetrical L^L bidentate ligands using cationic complexes of
i
7
2 6
electronic parameter (HEP), which makes use of the fact that a the formula [PdBr( Pr -bimy)(L^L)]PF (Chart 1, System II).
Crowley et al. have also used HEP2 to study a few neutral
unsymmetrical L ^L chelators with two different N-donors
1
2
8
a
Department of Chemistry, Capital Normal University, Beijing, People’s Republic of
(HEP2, Chart 1, System III). A point to note is that the HEP
for monodentate ligands and HEP2 for bidentate ligands are
structurally and electronically different systems, but rooted on
the same concept. For example, cyclic ligands in the HEP
probes usually coordinate in a perpendicular fashion with
respect to the coordination plane. In HEP2 probes, however, a
China. E-mail: guoshuai@cnu.edu.cn
b
Department of Chemistry, National University of Singapore, 3 Science Drive 3,
Singapore 117543
†
Electronic supplementary information (ESI) available: Selected crystallographic
data and NMR spectra. CCDC 1824536–1824539. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c8dt01618f
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i
Thus, a series of [PdBr(C^N)( Pr
2
-bimy)] complexes bearing
these types of chelators were initially targeted.
Acetato-bridged dinuclear complexes 1–4 can be conveniently
accessed via reported procedures starting from their corres-
11
ponding pyridine derivatives with Pd(OAc) (Scheme 1). These
2
acetato complexes can serve as potential basic precursors for
further metalation reactions. Hence, complexes 1–4 were treated
i
+
−
with two equivalents of Pr -bimy·H Br (a) in CH Cl , affording
2
2
2
the desired NHC-incorporated palladacycles 5–8 in decent yields.
Moreover, it was found that improved yields can be achieved in
the presence of K CO as an additional and external base, which
2
3
was therefore included in the optimized procedures.
1
In the H NMR spectra of all complexes 5–8, the signals
assigned to the NCHN protons of salt a are absent suggesting the
successful carbene coordination. The isopropyl C–H protons give
signals at 6.11 ppm (for 5 and 6) and 6.19 ppm (for 7 and 8)
Chart 1 Previously reported HEP-based systems used to determine the respectively, owing to which these resonances are not diagnostic
donor strengths of monodentate ligands (I), neutral symmetric (II) and for the distinction of the incorporated [C^N]-type chelators. In
asymmetric bidentate ligands (III), and the target system (IV) in this work.
13
13
the C NMR spectra of complexes 5–8, the
Ccarbene signals are
observed at 184.6 ppm (5), 184.7 ppm (6), 183.7 ppm (7) and
13
183.6 ppm (8). These
Ccarbene signals are found to correlate to
coplanar coordination mode is enforced by chelation. The
different orientations will result in different electronic contri-
butions to the reporter ligand, and thus the comparison of
ligands can only be made within a given system. Moreover,
coplanar attachment of a π-system via chelation potentially
the expected donating abilities of the corresponding bidentate
ligands with the exception of complex 8 (vide infra).
The solid-state structures of complexes 5–7 were elucidated
by X-ray crystallographic studies of single crystals obtained
from slow evaporation of their solutions in a mixture of
9
allows the metal to mesomerically act as a donor. For such
2 2
CH Cl and n-hexane. As a representative, the molecular struc-
ligands, HEP2 therefore detects the net donor strengths.
Although monoanionic [L^X]-type bidentate ligands,
especially those containing aryl-derived carbanions, have been
extensively employed in organometallic chemistry and tran-
sition metal mediated catalysis, the donating abilities of
such chelators have not been systematically probed due to the
lack of suitable and straightforward methodologies.
ture of 5 is depicted in Fig. 1. The molecular structures of 6–7,
selected crystallographic data and bond parameters are given
in the ESI.† In all three cases, the aryl moieties of [C^N]-type
chelators are cis to the carbene ligand. Moreover, chelation
enforces a coplanar arrangement between the chelator and the
square-planar coordination plane. A similar coordination be-
1
0
1
2
havior was also observed in the known analogues.
Herein, we report the syntheses and structural elucidations
of various neutral palladacycles of type IV incorporating
Pr -bimy as the reporter ligand and different aryl-based,
2
monoanionic [L^X]-type (L = N, C; X = C′) chelators. Additionally,
Syntheses of [C^C′]-type palladacycles bearing carbene-based
asymmetric chelators
i
Various NHCs with N-aryl wingtips can undergo cyclopallada-
it should be noted that the synthesis of HEP2-based complex
1
3
i
i
tions leading to the formation of [C^C′]-type chelators. To
2 1 2 6 2
probes of the type [PdBr( Pr -bimy)(L ^L )]PF (III) or [PdBr( Pr -
bimy)(L^X)] (IV) bearing asymmetric bidentate ligands is less
straightforward compared to that of type II due to the potential
formation of two geometric isomers (Chart 1). The latter are
expected to give two different HEP2 values, and thus the knowl-
edge of the exact binding modes is crucial for the donor strength
evaluations using these systems. Using complexes of system IV,
the donating abilities of asymmetric and monoanionic bidentate
ligands are evaluated and compared for the first time.
Results and discussion
Syntheses of [C^N]-type palladacycles bearing pyridine-based
asymmetric chelators
Pyridine-derived [C^N]-type chelators are commonly used as
1
0
monoanionic bidentate ligands in coordination chemistry.
Scheme 1 Syntheses of NHC-containing [C^N]-type palladacycles 5–8.
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1
In the H NMR spectrum of 9, two characteristic multiplets
at 6.22 and 5.96 ppm for the isopropyl C–H protons are
observed suggesting the loss of 2-fold rotational symmetry of
i
the Pr
2
-bimy ligand. Similar signals (6.20 and 5.98 ppm) are
1
3
also found in the case of 10. In the C NMR spectra of 9 and
i
1
0, the Pr
and 180.8 ppm, and the two
59.1 ppm are assigned to 1,2,3-triazole-derived mesoionic
2
-bimy ligands show the carbene resonances at 181.1
1
3
Ccarbene signals at 158.5 and
1
carbene ligands.
Single crystals of complex 10 suitable for X-ray diffraction
analysis were obtained by slow evaporation of a saturated solu-
tion of 10 in a mixture of CH Cl and n-hexane. The molecular
2 2
Fig. 1 Molecular structures of complexes 5 showing 50% probability
ellipsoids. Hydrogen atoms and solvent molecules are omitted for clarity.
structure is depicted in Fig. SI-2,† and the selected crystallo-
graphic data and bond parameters are summarized in Tables
SI-1 and SI-2 (see the ESI†). The triazolin-5-ylidene ligand is
found to be trans to the reporter carbene and perpendicular to
the coordination plane, which is in line with earlier
assess the electron-donating abilities of such bidentate
ligands, two new monocationic 1,2,3-triazole-derived salts c
and d were synthesized as the precursors via a two-step pro-
cedure (Scheme SI-1, see the ESI†). In the H NMR spectra of c
and d, two downfield singlets at 8.64 ppm (c) and 10.23 ppm
6
f
1
observations.
Previously, complexes of the general type trans-[PdBr
i
2
2
( Pr -
bimy)(trz)] (trz = 1,2,3-triazolin-5-ylidene) have been employed
to determine the donating abilities of 1,2,3-triazole-derived
(
d) characteristic of the triazolium C–H protons are observed.
With the two precursor salts c and d in hand, and in analogy
6
f
mesoionic carbenes (vide supra). The syntheses of 9 and 10
to the syntheses of 1–4, the preparation of acetato-bridged
dinuclear [C^C′]-type palladacycles was initially attempted. Such
complexes could be potentially applied as basic precursors in
further metallations to synthesize NHC-containing [C^C′]-type
palladacycles. Notably, some analogues have been previously
further broaden the scope of the HEP for these ligands, and
i
13
the Pr
2
-bimy
C
carbene NMR signals of a series of trans-
i
[
PdBr
pared on a Pd
2 2
( Pr -bimy)(trz)] complexes including 9 and 10 are com-
II 13
C NMR spectroscopic scale (Fig. 2). Based on
1
4
the comparison of the HEP values, a decreasing donor
strength of trz ligands in the order of i > v > ii/iii/vi > iv can be
inferred. This trend clearly underlines the presence of substi-
tuent effects in this series of trz ligands. For example, the fact
that ligand vi is stronger donating than iv can be rationalized
by the increased +I effect of the n-butyl versus phenyl group.
reported by Sankararaman et al. However, in our hands,
several attempts either using the silver-NHC transfer method or
by direct deprotonation with Pd(OAc)
tures of inseparable products were always obtained.
Thus, different synthetic strategy was employed
Scheme 2). First, salts c and d were treated with Ag O to yield
silver–carbene species, and subsequent transmetalation to the
2
were to no avail, and mix-
a
(
2
1
In the H NMR spectra of 11 and 12, the isopropyl C–H
i
15
protons show only one multiplet at 5.98 ppm indicating
greater symmetry upon cyclometalation. Characteristic doub-
lets at 6.40 ppm (11) and 6.42 ppm (12) were observed, which
known dimeric complex [PdBr ( Pr -bimy)]
2
led to the for-
2
2
mation of hetero-bis(carbene) complexes 9 and 10 in moderate
yields. Subsequently, C–H activations assisted by NaOAc as an
external base in DMSO successfully afforded the cyclopalla-
dated complexes 11 and 12 as off-white solids, although in low
yields. Notably, similar NHC-containing palladacycles were
typically obtained through cyclopalladations followed by
b
are assigned to the Ar–H protons (Fig. SI-3, see the ESI†)
1
2a–f
carbene ligations.
To the best of our knowledge, the
“cyclopalladation at the final step” synthetic strategy presented
herein is employed for the first time to access such complexes.
13
Scheme 2 Syntheses of bis(carbene) complexes 9/10 and NHC-palla-
Fig. 2 Relative donor abilities of trz ligands on a HEP scale. C NMR
dacycles 11/12. Conditions for step (i): (a) Ag
2
O, TBAB (only for c); (b)
spectra were recorded in CDCl
signal at 77.7 ppm.
3
and internally referenced to the solvent
i
2 2 2
[
PdBr ( Pr -bimy)] . Conditions for step (ii): NaOAc, DMSO.
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ortho to the aryl carbon donors. Similar signals were also Table 1 Summary of the Pr2-bimy
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i
13
C
carbene signals (HEP2) in com-
plexes bearing pyridine-containing chelators
found for the [C^N]-type analogues 5–8.
In principle, HEP2 complex probes of the general formula
a
i
Complex
L
HEP2
[
(
PdBr( Pr
2
-bimy)(L^X)] can have two geometric isomers
i
vide supra). The fact that only one set of signals is observed in [PdBr( Pr
the NMR spectra of 11/12 clearly implicates the presence of a
single isomer in both cases. Notably, the precursor complexes
/10 bear two mutually trans-standing carbenes, which expect- [PdBr( Pr
edly retain such coordination fashions after C–H activations of
aryl wingtips. These ligand orientations of 11/12 are un-
ambiguously determined by 1D NOESY NMR spectroscopic [PdBr( Pr
2
2
2
2
2
2
2
2
2
2
2
-bimy)(Ppy)] (5)
-bimy)(Tpy)] (6)
-bimy)(Bzq)] (7)
-bimy)(Bzpy)] (8)
Ppy
Tpy
Bzq
Bzpy
Bipy
Phen
Phen-dione
reg-Pytri-Ph
reg-Pytri-Bn
inv-Pytri-Ph
inv-Pytri-Bn
184.6
184.7
183.7
i
[
[
[
PdBr( Pr
i
PdBr( Pr
i
PdBr( Pr
183.6b
162.7b
161.4
i
9
-bimy)(Bipy)]PF
-bimy)(Phen)]PF
-bimy)(Phen-dione)]PF
-bimy)(reg-Pytri-Ph)]PF
6
i
[
[
[
PdBr( Pr
6
i
b
PdBr( Pr
6
160.0c
i
PdBr( Pr
6
156.7
i
c
-bimy)(reg-Pytri-Bn)]PF
-bimy)(inv-Pytri-Ph)]PF
-bimy)(inv-Pytri-Bn)]PF
6
6
156.7c
i
[
[
PdBr( Pr
158.3
158.2
analyses (Fig. SI-3, SI-4, see the ESI†). Several attempts to grow
single crystals of 11/12 were to no avail. However, an in-plane
coordination enforced by chelation can be expected for the
C^C′ bidentate ligands.
i
c
PdBr( Pr
6
a
3
Measured in CDCl and internally referenced to the solvent signal at
b
7
7.7 ppm relative to tetramethylsilane (TMS). Data from ref. 7. Bipy = 2,2′-
bipyridine, Phen = 1,10-phenanthroline, Phen-dione = 1,10-phenanthroline-
1
3
13
c
Two
C
carbene signals were detected in the C NMR spectra 5,6-dione. Data from ref. 8 were referenced to 77.16 instead. The HEP2
values shown here were obtained by adding 0.54 ppm to the reported data
of 11 and 12. The resonances at 162.0 ppm (11) and 162.7 ppm
for proper comparison. reg-Pytri-Ph = 1-(2-pyridyl)-4-phenyl-1,2,3-triazole,
reg-Pytri-Bn = 1-(2-pyridyl)-4-benzyl-1,2,3-triazole, inv-Pytri-Ph = 1-phenyl-4-
(
12) assigned to 1,2,3-triazole-derived carbenes are found to be
comparable to those in earlier reported palladium triazolin-5- (2-pyridyl)-1,2,3-triazole, inv-Pytri-Bn = 1-benzyl-4-(2-pyridyl)-1,2,3-triazole.
1
3
ylidene complexes. The much more downfield signals at
93.0 ppm (11) and 192.9 ppm (12) are assigned to the Ccarbene
1
i
2
atoms of Pr -bimy ligands, which allows direct comparison of
the donor strengths for the [C^C′]-type chelators incorporated
into 11 and 12 versus the [C^N]-type bidentate ligands of 5–8
1
3
on a unified C NMR spectroscopic scale (vide infra).
Donor strength comparisons of pyridine-based [L^X]-type
chelators
1
3
i
The Ccarbene NMR resonances of the Pr -bimy reporter
2
ligand (i.e. HEP2 values) in complexes 5–8, 11 and 12 allow for Fig. 3 Donor strength comparison of selected pyridine-based chelators
on a HEP2 scale.
the donor strength determinations and comparisons of [L^X]-
type monoanionic bidentate ligands. Complexes 5–8 all bear
pyridine-based bidentate ligands, which are first internally
ranked and then further compared with previously reported
symmetric or asymmetric pyridine-based chelators.
A
summary of all these structurally related complexes is given in
1
3
Table 1, and their HEP2 values are shown on a C NMR spec-
troscopic scale (Fig. 3 and 4).
In the [C^N]-type series synthesized here, a stepwise upfield
shift going from 184.7 ppm (Tpy) to 184.6 ppm (Ppy) and
further to 183.7 ppm (Bzq) was observed (Fig. 3). This is
rational since (i) the attachment of a methyl group to Ppy sup-
posedly enhances the donating ability through a +I effect (cf.
Tpy and Ppy); and (ii) a phenyl ring fused to Ppy is expected to
reduce the donor strength (cf. Bzq and Ppy).
Interestingly, the bidentate ligand Bzpy leads to a slightly
more upfield HEP2 compared to Ppy, although the former is
assumed to be a stronger bidentate donor on grounds of the +I
Fig. 4 Donor strength comparison of selected charge-neutral and
monoanionic pyridine-based chelators on a HEP2 scale.
effect of the additional CH
communication between the two donor-containing rings is
less feasible, which diminishes mesomeric contributions. series of complexes fac-[ReBr(CO)
Moreover, it is reasonable to assume that the structural diNHC) with variations in alkylene bridges between the two
2
spacer. Nevertheless, electronic
3
(L^L)] (L^L = chelating
1
6
changes induced by an increased bite-angle of Bzpy and the carbenes. They did not find any clear influence of the alky-
resulting 6-membered palladacycle lead to sterically induced lene bridges on the carbonyl IR vibrations and NMR reso-
dilution of the electronic effects. A similar observation was pre- nances and attributed this to the variations in ligand bite
viously made by Herrmann and Kühn et al., who reported a angles and the resulting change in molecular geometries.
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iPr2-bimy 13
Such steric interferences on HEP-related methods remain to Table 2 Summary of
be addressed in the near future with greater detail.
C
carbene signals (HEP2) in complexes
bearing carbene-containing chelators
The HEP2 value of Ppy in 5 was further compared to
selected counterparts bearing pyridine-based symmetric or
a
Complex
L
HEP2
193.0
i
asymmetric [N^N]-type chelators (Fig. 4). Note that in all cases [PdBr( Pr
2
2
2
2
2
2
2
2
2
-bimy)(Et-pty)] (11)
-bimy)(Bn-pty)] (12)
Et-pty
Bn-pty
A
B
i
i
[PdBr( Pr
192.9b
the pyridine moieties are exclusively trans to the Pr
2
-bimy
i
[
[
PdBr( Pr
-bimy)(A)]PF
-bimy)(B)]PF
-bimy)(C)]PF
-bimy)(D)]PF
-bimy)(E)]PF
6
177.1
i
b
probe leading to the formation of 5-membered metallacycles.
By comparison of the HEP2 values, a decreasing donating [PdBr( Pr
ability in the order of Ppy > Bipy > inv-Pytri-Ph > reg-Pytri-Ph
can be deduced. The fact that Ppy is by far the strongest chela-
PdBr( Pr
6
178.7b
i
6
C
179.9
i
b
b
[
[
[
PdBr( Pr
6
D (R = Bn, n = 1)
E (R = Pr, n = 1)
F (R = Bn, n = 2)
G (R = Bn, n = 3)
179.51
i
i
b
PdBr( Pr
6
180.0
i
PdBr( Pr
-bimy)(F)]PF
-bimy)(G)]PF
6
179.54
i
b
tor is reasonable, since the anionic carbon donor in Ppy is [PdBr( Pr
6
180.3
more electron releasing compared to neutral N-donors in all
the other cases. Additionally, it should be noted that steric 77.7 ppm relative to tetramethylsilane (TMS). Data from ref. 7.
interferences on the HEP2 values of Ppy and Bipy can be
a
3
Measured in CDCl and internally referenced to the solvent signal at
b
essentially excluded, because they are structurally very similar.
The same statement can be made for the case of Bzq versus
Phen (Fig. 4).
donor units may additionally contribute to a weaker donicity
compared to that of the monoanionic [C^C′] chelators. Again,
more detailed future studies are required to address this
specific issue.
Donor strength comparisons of carbene-based [L^X]-type
chelators
A comparison among the [C^C′] chelators in complexes 11
Previously, Huynh et al. determined the HEP2 values of sym- versus 12 reveals that a change of the N-benzyl substituent to a
metric and classical dicarbene ligands with simple alkyl more electron-releasing ethyl group brings a small, but yet
linkers (system II, vide supra), which allow donor strength com- detectable shift of the HEP2 value, although this variation is
i
18
parison with the carbene-containing [C^C′]-type chelators in considerably far away from that of the Pr -bimy probe. This
2
the newly synthesized complexes 11/12. The HEP2 values of observation also highlights that HEP2 is sufficiently sensitive
1
3
these ligands are shown on a C NMR spectroscopic scale to detect subtle electronic differences of monoanionic
(
Fig. 5), and a more detailed summary is given in Table 2.
The HEP2 values of Et-pty and Bn-pty in 11 and 12 are
chelators.
Finally, the HEP2 results obtained also demonstrate that
1
93.0 ppm and 192.9 ppm, respectively. They are significantly monoanionic [C^C′]-type chelators are significantly more
larger than those of all previously studied dicarbene ligands donating than all [C^N]-type chelators studied herein. Again,
177.1–180.3 ppm), which implies that their donicities surpass this observation is in line with the fact that mesoionic 1,2,3-
those of all dicarbenes evaluated by HEP2 thus far. This result triazolin-5-ylidenes are much stronger donors compared to
(
5
,6f
is well in line with intuition, since (i) mesoionic 1,2,3-triazole- pyridines.
derived carbenes are known to be stronger donors compared
The reasonability of all results discussed above collectively
6
f,17
to classical Arduengo-type NHCs;
and (ii) the aryl carba- and clearly indicates that palladacyclic NHC complexes of the
i
nion is expected to be a stronger donor than neutral carbon general formula [PdBr( Pr -bimy)(L^X)] accessed in this work
2
donors. In addition to these reasonable arguments, one must can serve as a suitable system to probe the donating abilities
also note that electronic communication between the two of monoanionic chelators.
donors in the dicarbene ligands is interrupted by the alkyl
bridge. The lack of mesomeric interactions between the two
Conclusion
We have reported the syntheses of a series of new palladacycles
bearing various monoanionic and asymmetric [L^X]-type che-
lators (L = C, N; X = C) and 1,3-diisopropylbenzimidazolin-2-
ylidene as the constant reporter ligand. The specific coordi-
nation mode and arrangement of the incorporated non-sym-
metric bidentate ligands were verified by either X-ray diffrac-
tion analyses or 1D NOESY NMR spectroscopy.
These complexes have allowed for the systematic determi-
nation and comparison of the donating abilities of [L^X]-type
chelators for the first time using the HEP2 method. The
rational results obtained provide useful knowledge for the
design of the cyclometallated complexes. More importantly,
they can facilitate the understanding of structure–activity
Fig. 5 Donor strength comparison of selected carbene-based chelators
on a HEP2 scale.
relationships observed in applications of related metallacycles
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in the areas such as homogeneous catalysis or photo- 137.9, 134.4, 130.1, 124.9, 124.5, 123.1, 122.9, 118.8, 113.5 (Ar–
luminescence. The fact that Bzpy (Bzpy = 2-(2-pyridinylmethyl) C), 55.3 (CH(CH ) ), 21.5, 21.4 (CH(CH ) ). ESI-MS (positive)
3
2
3 2
+
phenyl-C,N) does not follow the expected order on grounds of m/z 462 [M − Br] . Anal. Calcd for C24
H
3
26BrN Pd: C, 53.10; H,
inductive effects has been tentatively attributed to a larger bite 4.83; N, 7.74%; found: C, 53.16; H, 4.51; N, 7.68%.
angle, the severely varied coordination geometry and the lack Complex 6 was obtained in a yield of 65% following the
of electronic delocalization brought about by a different struc- general procedure with complex 2 as the metal precursor. H
ture of this ligand. Research addressing this issue and other NMR (CDCl , 600 MHz): δ 9.52–9.51 (m, 1H, Ar–H), 7.79–7.78
1
3
limitations to the method rooted in steric interferences as well (m, 1H, Ar–H), 7.68–7.66 (m, 3H, Ar–H), 7.48–7.47 (m, 1H,
as further broadening the scope of bidentate ligands is Ar–H), 7.29–7.28 (m, 2H, Ar–H), 7.19–7.17 (m, 1H, Ar–H),
3
currently underway in our laboratories.
6.90–6.88 (m, 1H, Ar–H), 6.11 (septet, 2H, CH(CH
3
)
2
, JH,H
=
7
6
7
1
1
2
Hz), 6.09–6.08 (m, 1H, Ar–H), 2.07 (s, 3H, Tpy-CH ), 1.77 (d,
3
3
3
H, CH(CH
3
)
2
,
J
H,H = 7 Hz), 1.61 (d, 6H, CH(CH
Hz). C NMR (CDCl , 150 MHz): 184.7 (NCN), 165.0, 156.0,
51.8, 144.4, 140.1, 139.0, 138.6, 134.4, 125.7, 124.3, 122.8,
22.6, 118.5, 113.5 (Ar–C), 55.3 (CH(CH ), 22.3 (Tpy-CH ),
1.6, 21.4 (CH(CH ). ESI-MS (positive) m/z 476 [M − Br] .
3 2 H,H
) , J =
1
3
Experimental section
General considerations
3
3
)
2
3
+
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. H, C and NOESY
NMR spectra were recorded on a Varian 600 MHz spectrometer
and the chemical shifts (δ) were internally referenced to the
3 2
)
Anal. Calcd for C H BrN Pd: C, 53.92; H, 5.07; N, 7.55%;
found: C, 54.16; H, 5.45; N, 7.36%.
2
5
28
3
1
13
Complex 7 was obtained in a yield of 71% following the
1
general procedure with complex 3 as the metal precursor. H
NMR (CDCl , 600 MHz): δ 9.73–9.72 (m, 1H, Pyr–H), 8.31–8.29
3
(m, 1H, Ar–H), 7.77–7.75 (m, 1H, Ar–H), 7.70–7.69 (m, 2H,
Ar–H), 7.64–7.62 (m, 1H, Ar–H), 7.59–7.56 (m, 2H, Ar–H),
1
13
residual solvent signals relative to (CH ) Si ( H, C). ESI mass
3
4
spectra were recorded using an Agilent 6540 Q-TOF mass
spectrometer. X-ray single crystal diffractions of complexes 5
and 7 were carried out using an Agilent Xcalibur Eos Gemini
diffractometer at the Beijing University of Chemical
Technology. X-ray single crystal diffractions of complexes 6 and
7.31–7.30 (m, 2H, Ar–H), 7.23–7.21 (m, 1H, Ar–H), 6.47–6.46
3
(
3 2
m, 1H, Ar–H), 6.19 (heptet, 2H, CH(CH ) , JH,H = 7 Hz), 1.79
3
3
(
d, 6H, CH(CH ) , J
= 7 Hz), 1.62 (d, 6H, CH(CH ) , J
=
3
2
H,H
3 2
H,H
1
3
7
1
1
3
Hz). C NMR (CDCl , 150 MHz): 183.7 (NCN), 155.0, 154.1,
1
0 were carried out using a SuperNova, Dual, Cu at zero,
50.7, 137.7, 134.6, 134.5, 134.4, 129.6, 129.3, 127.3, 124.2,
23.4, 122.9, 122.4, 113.5 (Ar–C), 55.4 (CH(CH ) ), 21.6, 21.4
AtlasS2 diffractometer (for complex 6) at Beijing Normal
University and a Rigaku XtaLAB P200 MM003 diffractometer
3
+
2
(
CH(CH
3
)
2
). ESI-MS (positive) m/z 486 [M − Br] . Anal. Calcd
(
for complex 10) at Capital Normal University. Elemental ana-
for C26
H
26BrN Pd: C, 55.09; H, 4.62; N, 7.41%; found: C, 55.06;
3
lyses were carried out on a vario EL cube elemental analyzer at
the Beijing University of Chemical Technology.
H, 4.79; N, 7.27%.
Complex 8 was obtained in a yield of 62% following the
general procedure with complex 4 as the metal precursor. H
1
General synthetic procedure for complexes 5–8
NMR (CDCl , 600 MHz): δ 9.38–9.37 (m, 1H, Ar–H), 7.67 (t, 1H,
3
3
The acetato-bridged dipalladium complex [Pd(μ-CH
C^N)]
solved in CH Cl (15 mL). To the resulting solution, K CO
3
COO) Ar–H, JH,H = 7 Hz), 7.60–7.59 (m, 2H, Ar–H), 7.40 (d, 1H, Ar–
2
(0.1 mmol) and salt a (56.6 mg, 0.2 mmol) were dis- H, JH,H = 7 Hz), 7.23–7.22 (m, 2H, Ar–H), 7.18 (t, 1H, Ar–H,
3
(
3
3
J
= 7 Hz), 7.12 (d, 1H, Ar–H, J
= 7 Hz), 6.87 (t, 1H, Ar–
2
2
2
3
H,H
H,H
3
(
28 mg, 0.2 mmol) was added. The reaction mixture was H,
J
H,H = 7 Hz), 6.68–6.63 (m, 2H, Ar–H), 6.19 (br s, 2 H,
), 4.41 (br s, 2 H, Bzpy-CH ), 1.80 (d, 6H, CH(CH
JH,H = 6 Hz), 1.47 (br s, 6H, CH(CH3)2). C NMR (CDCl3,
10 mL) was added. The organic phase was collected and dried 150 MHz): 183.6 (NCN), 159.2, 154.5, 149.3, 139.2, 138.7,
over Na SO . All the volatiles were removed under vacuum. An 134.1, 127.6, 124.6, 122.7, 111.5, 113.3 (Ar–C), 55.3 (CH(CH ),
analytically pure compound was obtained by crystallization 50.3 (CH ), 21.7 (CH(CH ) ). ESI-MS (positive) m/z 476
stirred and heated under reflux for 24 h. The resulting suspen- CH(CH
sion was cooled to ambient temperature, and deionized water
(
3
)
2
2
3 2
) ,
3
13
2
4
3 2
)
2
3 2
+
from the solution of the crude product in a mixture of CH
2
Cl
2
3
[M − Br] . Anal. Calcd for C25H28BrN Pd: C, 53.92; H, 5.07; N,
and n-hexane.
7.55%; found: C, 53.80; H, 5.35; N, 7.19%.
Complex 5 was obtained in a yield of 76% following the
general procedure with complex 1 as the metal precursor. H
1
Synthesis of compound c
NMR (CDCl
m, 1H, Ar–H), 7.75–7.74 (m, 1H, Ar–H), 7.68–7.65 (m, 2H, oxonium tetrafluoroborate (380.0 mg, 2 mmol) were mixed in a
Ar–H), 7.60–7.58 (m, 1H, Ar–H), 7.29–7.27 (m, 2H, Ar–H), 25 mL Schlenk tube. To this mixture, dry 1,2-dichloroethane
.23–7.22 (m, 1H, Ar–H), 7.08–7.06 (m, 1H, Ar–H), 6.87–6.84 (1 mL) was added. The tube was sealed and the reaction
m, 1H, Ar–H), 6.24–6.23 (m, 1H, Ar–H), 6.11 (septet, 2H, mixture was stirred at 90 °C for 24 h. All the volatiles were
3
, 600 MHz): δ 9.55–9.54 (m, 1H, Ar–H), 7.84–7.81 4-Butyl-1-phenyl-1,2,3-triazole (201.2 mg, 1 mmol) and triethyl-
(
7
(
3
3
CH(CH
1
1
3
)
2
, JH,H = 7 Hz), 1.77 (d, 6H, CH(CH
3
)
2
, JH,H = 7 Hz), removed in vacuo. To the residue, 2 mL of CH
H,H = 7 Hz). C NMR (CDCl and the resulting suspension was stirred at ambient tempera-
50 MHz): 184.6 (NCN), 164.9, 156.1, 152.0, 147.1, 139.1, ture for 0.5 h exposed to air to remove excess oxonium salt. All
3
OH was added
3
13
.61 (d, 6H, CH(CH
3
)
2
,
J
3
,
Dalton Trans.
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Paper
3
the volatiles were removed in vacuo, and then the residue was CH
2
CH
2
CH
̲
2
CH
3
), 1.11 (t, 3H, CH
2
CH
2
CH
2
CH
̲
3
, JH,H = 6 Hz).
1
3
i
washed with ethyl acetate (3 × 10 mL) and dried under vacuum
C NMR (CDCl , 150 MHz): 181.1 (C
– Pr -bimy), 158.5
2
3
carbene
affording the product as an off-white powder (132.5 mg, (Ccarbene–trz), 145.4, 141.0, 134.3, 134.2, 130.1, 129.2, 126.4,
1
0
.42 mmol, 42%). H NMR (CDCl
3
, 600 MHz,): δ 8.64 (s, 1H, 122.3, 113.11, 113.06 (Ar–C), 54.3, 54.2 (NCH), 45.4
NCH), 7.78–7.75 (m, 2H, Ar–H), 7.46–7.43 (m, 3H, Ar–H), 4.51 (NC
̲
̲
̲
2
3
2
2
2
3
2
2
2
3
3
(
q, 2H, NCH
̲
2
CH
3
, JH,H = 12 Hz), 2.79 (t, 2H, CH
CH CH CH
= 12 Hz), 1.37–1.28 (m, 2H, CH CH CH
̲
2
CH
2
CH
2
CH
3
,
23.8 (CH
), 21.7 (CH(C
CH CH ). ESI-MS (positive) m/z: 618
2CH3), [M − Br] . Anal. Calcd for C H Br N Pd: C, 46.47; H, 5.34; N,
3
J
H,H = 12 Hz), 1.70–1.63 (m, 2H, CH
2
̲
2
2
3
), 1.56 (t, 3H, (NCH
3
NCH CH
0
1
(
2
(
̲
, J
̲
2
3
H,H
2
2
3
13
.81 (t, 3H, CH
2
CH
2
CH
2
CH
̲
3
, JH,H = 12 Hz). C NMR (CDCl
3
,
10.04%; found: C, 46.58; H, 5.65; N, 9.86%.
50 MHz): 145.8, 135.2, 132.0, 130.5, 126.2, 121.6 (Ar–C), 47.3
Synthesis of complex 10
NC
2.4
̲
H CH ), 29.1 (C
̲
H CH CH CH ), 23.0 (CH C
̲
2
3
2
2
2
3
2
2
2
3
(NCH
2
C
̲H
3
),
14.0
(CH
2
CH
2
C̲
2
CH
13.7 A mixture of salt d (132 mg, 0.35 mmol), Ag
0.21 mmol) and dimeric complex [PdBr
166.6 mg, 0.17 mmol) was stirred in CH Cl at ambient temp-
CH
2
CH
2
CH
2
C
̲
H
3
). ESI-MS (positive) m/z 230 [M − BF
(
Synthesis of compound d
erature for 12 h. Then the suspension was filtered over Celite
A mixture of 4-butyl-1-phenyl-1,2,3-triazole (201.2 mg, 1 mmol) and the solvent of the filtrate was removed in vacuo to give a
and benzyl bromide (1 mL) was stirred in a 25 mL Schlenk crude product. Then the crude product was purified using
tube and heated at 120 °C for 72 h. After the mixture was column chromatography (SiO , CH Cl /petroleum ether (v : v) =
2 2 2
cooled to ambient temperature, diethyl ether (10 mL) was 2 : 1). The solvent system contains 3‰ triethyl amine. The
added and the resulting suspension was stirred overnight. product was afforded as a yellow solid (124 mg, 0.16 mmol,
1
Then the precipitate was washed with a copious amount of 48%). H NMR (CDCl
3
, 600 MHz): δ 8.38–8.36 (m, 2H, Ar–H),
diethyl ether and dried under vacuum to give the desired com- 7.61–7.58 (m, 3H, Ar–H), 7.52–7.49 (m, 2H, Ar–H), 7.40–7.37
1
pound as an off-white powder (294.2 mg, 0.79 mmol, 79%). H (m, 3H, Ar–H), 7.27–7.26 (m, 2H, Ar–H), 7.15–7.14 (m, 2H,
5
3
NMR (CDCl
m, 2H, Ar–H), 7.60–7.58 (m, 3H, Ar–H), 7.42–7.41 (m, 3H, Ar– NCH,
H), 7.38–7.36 (m, 2H, Ar–H), 5.99 (s, 2H, NCH ), 3.01 (t, 2H, CH
3
, 600 MHz): δ 10.23 (s, 1H, C triazole–H), 8.21–8.20 Ar–H), 6.20 (heptet, 1H, NCH, JH,H = 7 Hz), 5.98 (heptet, 1H,
3
(
J
H,H = 7 Hz), 5.28 (s, 2H, C
6
H
5
CH
2
̲ ), 3.07 (t, 2H,
3
̲
CH CH CH ,
JH,H
=
6
Hz), 2.08–2.05 (m, 2H,
2
2
2
2
3
3
3
CH
CH
̲
2
CH
2
CH
2
CH
3
,
J
H,H
=
6
Hz), 1.71 (br s, 2H, CH
), 1.37–1.31 (m, 2H, CH CH CH CH
Hz).
, 150 MHz): 146.7 (C triazole), 135.4, 132.5, 131.7, 131.1,
30.5, 130.2, 129.1, 128.9, 122.0 (Ar–C), 56.6 (NCH ), (Ccarbene–trz), 145.9, 140.7, 134.0, 133.9, 133.1, 130.0, 129.7,
0.0 (CH CH CH CH ), 24.6 (CH2CH CH CH ), 22.9 129.4, 129.0, 128.1, 126.2, 122.2, 112.92, 112.86 (Ar–C), 54.04,
2
CH
̲
2
CH
2
CH
3
), 1.78 (d, 6H, CH(CH
̲
3
)
2
, JH,H = 6 Hz), 1.63 (d,
3
2
CH
̲
2
CH
2
CH
3
2
2
̲
2
3
), 0.83 6H, CH(CH
3
)
2
,
J
H,H
=
6
Hz), 1.53–1.49 (m, 2H,
3
13
3
(t, 3H, CH CH CH CH
̲
3,
JH,H
=
6
C
NMR CH CH CH CH ), 1.01 (t, 3H, CH CH CH CH
̲
̲
3
i
, J
= 12 Hz).
2
2
2
2
2
2
3
2
2
2
H,H
5
13
(CDCl
3
3 2
C NMR (CDCl , 150 MHz): 180.8 (Ccarbene– Pr -bimy), 159.1
1
3
2
̲
̲
2
2
2
3
2
2
3
(CH
2
CH
2
C
̲
H
2
CH
3
), 14.1 (CH
2
CH
2
CH
2
C̲
3
). ESI-MS (positive) 54.00 (C
̲
3
)
2
), 53.8 (C
6
H
5
C
̲
2
) 31.7 (C
CH CH
H ). ESI-MS (positive) m/z:
̲
2
CH
2
CH
2
CH
3
), 26.1
+
m/z 292 [M − Br] .
(CH
2
C
̲
2
2
3
), 23.5 (CH
2
2
C
̲
2
3 2
̲H )
),
2
6
̲
3
2
+
2
2
2
3
Synthesis of complex 9
2 5
80 [M − Br] . Anal. Calcd for C32H39Br N Pd: C, 50.58; H,
i
A mixture of dimeric complex [PdBr
2
( Pr
.2 mmol) and tetra-butyl ammonium bromide (128.9 mg,
.4 mmol) was stirred in CH Cl (10 mL) at ambient tempera-
2 2
-bimy)] (187.4 mg, 5.17; N, 9.22%; found: C, 50.44; H, 5.22; N, 8.86%.
0
0
Synthesis of complex 11
2
2
ture for 2 h. Then salt c (126.8 mg, 0.4 mmol) and Ag
2
O
A mixture of 9 (54.9 mg, 0.08 mmol) and NaOAc (32.3 mg,
(
55.6 mg, 0.24 mmol) were added, and the reaction mixture 0.4 mmol) was stirred in DMSO (5 mL) at 120 °C for 2 h. Then
was stirred at ambient temperature for 18 h in the dark. Then the mixture was suspended in CH Cl (20 mL) and washed
the suspension was filtered over Celite. Then the filtrate was with deionized H O (3 × 10 mL). Drying of the organic phase
washed with deionized H O (3 × 10 mL). Drying of the organic over Na SO followed by removal of the solvent in vacuo
2
2
2
2
2
4
phase over Na
in vacuo afforded the crude product as an yellow powder. Then product was purified using column chromatography (SiO
the crude product was purified using column chromatography CH Cl /petroleum ether (v : v) = 2 : 1). The solvent system con-
2 4
SO followed by the removal of the solvent afforded the crude product as a yellow powder. Then the crude
2
,
2
2
(
SiO
2 2 2
, CH Cl /petroleum ether (v : v) = 3 : 1). The eluent con- tains 3‰ triethyl amine. The product was afforded as a yellow
1
tains 3‰ triethyl amine. The product was afforded as a yellow solid (10 mg, 0.016 mmol, 20%). H NMR (CDCl
solid (128 mg, 0.18 mmol, 46%). H NMR (CDCl , 600 MHz): δ 7.64–7.63 (m, 2H, Ar–H), 7.53 (dd, 1H, Ar–H, J
δ 8.33 (d, 2H, Ar–H, JH,H = 6 Hz), 7.61–7.49 (m, 5H, Ar–H),
7
3
, 600 MHz):
1
3
= 12 Hz,
3
H,H
3
4
3
4
J
J
H,H = 1.2 Hz), 7.26–7.24 (m, 2H, Ar–H), 7.04 (td, 1H, Ar–H,
3
4
3
.16–7.15 (m, 2H, Ar–H), 6.22 (heptet, 1H, NCH,
J
H,H = 7.2
H,H = 6 Hz, JH,H = 1.2 Hz), 6.81 (td, 1H, Ar–H, JH,H = 6 Hz,
3
3
4
Hz), 5.96 (heptet, 1H, NCH, J
= 7 Hz), 4.37 (q, 2H, NCH ,
JH,H = 1.2 Hz), 6.40 (dd, 1H, Ar–H, JH,H = 12 Hz, JH,H =
H,H
2
3
3
3
J
H,H = 6 Hz), 3.12 (t, 2H, CH
m, 2H, CH CH CH CH ), 1.79 (d, 6H, CH(CH
.66–1.61 (m, 11H, CH(CH3)2 and NCH CH
2
CH
2
CH
2
CH
3
, JH,H = 6 Hz), 2.23 1.2 Hz), 5.98 (heptet, 2H, NCH, JH,H = 7.2 Hz), 4.37 (q, 2H,
3
3
3
(
1
2
̲
2
2
3
̲
3
)
2
, JH,H = 6 Hz), NCH
̲
2
CH
3
, JH,H = 7.2 Hz), 3.33 (t, 2H, CH
̲
2
CH
2
CH
2
CH
3
, JH,H
=
̲
̲
and 6 Hz), 1.79–1.72 (m, 8H, NCH(CH
̲ )
3 2
and CH CH
̲
CH CH ),
2
3
2
2
2
3
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Dalton Trans.

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Dalton Transactions
3
1
.61 (t, 3H, NCH
2
CH
̲
3
, JH,H = 7.2 Hz), 1.60 (d, 6H, NCH(CH
3
)
2
,
data is given in Table SI-1.† CCDC 1824536–1824539† contain
3
JH,H = 7.2 Hz), 1.56–1.50 (m, 2H, CH CH CH
̲
CH ), 0.99 the supplementary crystallographic data for this paper.
3
JH,H = 6 Hz). C NMR (CDCl ,
3
2
2
2
3
13
(t, 3H, CH
2
CH
2
CH
2
CH
̲
3
,
i
1
1
1
3
50 MHz): 193.0 (Ccarbene– Pr -bimy), 162.0 (Ccarbene–trz),
2
48.6, 148.0, 144.0, 139.7, 134.4, 129.2, 128.3, 126.5, 124.6, Conflicts of interest
22.5, 114.9, 113.4 (Ar–C), 54.5 (C
3.1 (C CH CH CH ), 24.9 (CH
H ) ), 16.0 (NCH C
̲
3
)
C
2
), 44.7 (NC
CH CH ), 23.3
H ),
2 3
̲H CH ),
There are no conflicts to declare.
̲
H
̲
CH
2
2
2
3
̲
2
2
3
(CH CH C
H CH ), 21.7, 21.6 (CH(C
̲
2
2
2
3
2
3
+
1
4.7 (CH
2
2
2 3
CH C ̲H ). ESI-MS (positive) m/z: 536 [M − Br] .
Acknowledgements
Anal. Calcd for C27
H
36BrN
5
Pd: C, 52.56; H, 5.88; N, 11.35%;
found: C, 53.01; H, 5.55; N, 11.16%.
The authors thank the Beijing Natural Science Foundation
(
Grant No. 2164057) and the National Natural Science
Synthesis of complex 12
Foundation of China (Grant No. 21502122) for financial
support.
A mixture of 10 (68.4 mg, 0.09 mmol) and NaOAc (36.9 mg,
0
.45 mmol) was stirred in DMSO (5 mL) at 120 °C for 2 h.
Then the mixture was suspended in CH Cl (20 mL) and
washed with deionized H O (3 × 10 mL). Drying of the organic
2
2
2
Notes and references
2 4
phase over Na SO followed by removal of the solvent in vacuo
afforded the crude product as a yellow powder. Then the crude
product was purified using column chromatography (SiO2,
CH Cl /petroleum ether (v : v) = 5 : 1). The solvent system con-
2 2
tains 3‰ triethyl amine. The product was afforded as a yellow
1 For selected reviews on the donor strength determinations,
see: (a) T. Dröge and F. Glorius, Angew. Chem., Int. Ed.,
2010, 49, 6940; (b) D. J. Nelson and S. P. Nolan, Chem. Soc.
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Coord. Chem. Rev., 2007, 251, 874; (d) Q. Teng and
H. V. Huynh, Dalton Trans., 2017, 46, 614; (e) D. G. Gusev,
Organometallics, 2009, 28, 763; (f) H. V. Huynh, Chem. Rev.,
DOI: 10.1021/acs.chemrev.8b00067, ASAP.
1
solid (13.4 mg, 0.02 mmol, 22%). H NMR (CDCl , 600 MHz):
3
3
δ 7.65–7.62 (m, 2H, Ar–H), 7.59 (dd, 1H, Ar–H, JH,H = 6 Hz,
H,H = 1.2 Hz), 7.41–7.36 (m, 3H, Ar–H), 7.30–7.29 (m, 2H,
Ar–H), 7.26–7.24 (m, 2H, Ar–H), 7.06 (td, 1H, Ar–H, JH,H
Hz, JH,H = 1.2 Hz), 6.83 (td, 1H, Ar–H, JH,H = 6 Hz, JH,H
4
J
3
=
=
4
3
4
6
1
2 C. A. Tolman, J. Am. Chem. Soc., 1970, 92, 2953.
3
4
.2 Hz), 6.42 (dd, 1H, Ar–H, JH,H = 6 Hz, JH,H = 1.2 Hz), 5.98
3 For selected reviews on N-heterocyclic carbenes, see:
(a) F. E. Hahn and M. C. Jahnke, Angew. Chem., Int. Ed.,
2008, 47, 3122; (b) S. Gu, C. Chen and W. Chen, Curr. Org.
Chem., 2011, 15, 3291; (c) W. Zhao, V. Ferro and
M. V. Baker, Coord. Chem. Rev., 2017, 339, 1; (d) J. C. Y. Lin,
R. T. W. Huang, C. S. Lee, A. Bhattacharyya, W. S. Hwang
and I. J. B. Lin, Chem. Rev., 2009, 109, 3561;
(e) K. F. Donnelly, A. Petronilho and M. Albrecht, Chem.
Commun., 2013, 49, 1145; (f) L. A. Schaper, S. J. Hock,
W. A. Herrmann and F. E. Kühn, Angew. Chem., Int. Ed.,
2013, 52, 270; (g) S. Díez-González, N. Marion and
S. P. Nolan, Chem. Rev., 2009, 109, 3612; (h) A. T. Normand
and K. J. Cavell, Eur. J. Inorg. Chem., 2008, 2781; (i) E. Peris,
Chem. Rev., 2017, DOI: 10.1021/acs.chemrev.6b00695.
4 (a) A. R. Chianese, X. Li, M. C. Janzen, J. W. Faller and
R. H. Crabtree, Organometallics, 2003, 22, 1663;
(b) R. A. Kelly III, H. Clavier, S. Giudice, N. M. Scott,
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1487.
3
(
(
heptet, 2H, NCH, J
t, 2H, CH
= 7.2 Hz), 5.53 (s, 2H, C H CH
̲
2), 3.27
, JH,H = 7.8 Hz), 1.74 (d, 6H, CH(CH
Hz), 1.63–1.59 (m, 8H, CH(CH and
CH ), 0.91 (t,
H,H
6
5
3
̲
2
CH
6
2
CH
2
CH
3
3 2
̲ ) ,
3
J
H,H
=
3 2
̲ )
CH CH
̲
CH CH ), 1.49–1.45 (m, 2H, CH CH CH
̲
2
2
2
3
2
2
2
3
3
13
3
1
1
1
3
H, CH
2
CH
2
CH
2
CH
̲
3
,
J
H,H
=
6 Hz).
3
C NMR (CDCl ,
i
50 MHz): 192.9 (Ccarbene– Pr
49.2, 139.7, 134.4, 134.0, 129.8, 129.6, 128.4, 128.3, 124.6,
22.5, 115.1, 113.4 (Ar–C), 54.5 (NCH), 53.4 (C ),
2.7 (C CH CH CH ), 25.0 (CH CH CH ), 23.3
H ) ), 21.6 (CH(CH ) ), 14.6
2
-bimy), 162.7 (Ccarbene–trz),
6
5 2
H C̲H
̲
H
̲
2
2
2
3
2
C
̲
2
2
3
(
CH CH C
H CH ), 21.7 (CH(C
̲
̲
2
2
2
3
3 2
3 2
+
(CH
2
CH
2
CH
2
C
̲
H
3
). ESI-MS (positive) m/z: 598 [M − Br] . Anal.
Pd: C, 56.60; H, 5.64; N, 10.31; found: C,
7.31; H, 5.25; N, 9.56%. Although the elemental analysis
Calcd for C32H38BrN
5
5
results are slightly outside the range viewed as establishing
analytical purity, they are provided to illustrate the best values
obtained to date.
X-ray diffraction studies
X-ray data for complexes 5 and 7 were collected with an Agilent
Xcalibur Eos Gemini diffractometer using Mo-K radiation. For
α
complex 6, the data were collected on a SuperNova, Dual, Cu at
zero, AtlasS2 diffractometer. For complex 10, the data were col-
lected on a Rigaku XtaLAB P200 MM003 diffractometer.
5 H. V. Huynh, Y. Han, R. Jothibasu and J. A. Yang,
Organometallics, 2009, 28, 5395.
6 (a) M. Meier, T. T. Y. Tan, F. E. Hahn and H. V. Huynh,
Organometallics, 2017, 36, 275; (b) J. R. Wright, P. C. Young,
N. T. Lucas, A. L. Lee and J. D. Crowley, Organometallics,
2013, 32, 7065; (c) S. Guo, H. Sivaram, D. Yuan and
H. V. Huynh, Organometallics, 2013, 32, 3685;
(d) J. C. Bernhammer and H. V. Huynh, Dalton Trans., 2012,
1
9
Structural solution was carried out with the Olex2 program.
The structure was solved using direct methods. All non-hydro-
gen atoms were generally given anisotropic displacement para-
meters in the final model. All H-atoms were put at calculated
positions. A summary of the most important crystallographic
Dalton Trans.
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4
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1
i
13
(b) J. Broggi, H. Clavier and S. P. Nolan, Organometallics,
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This journal is © The Royal Society of Chemistry 2018
Dalton Trans.