Canopied trans-chelating bis(N-heterocyclic carbene) ligand: Synthesis, structure and catalysis

Article abstract of DOI:10.1039/b815739a

The terphspan scaffold was employed to support a bis(N-heterocyclic carbene) ligand (1) that provides a m-terphenyl canopy over one side of its metal complexes. Single-crystal X-ray diffraction studies on a silver complex of {[Ag(1)]AgBr₂}₂ revealed an unusual tetranuclear silver core with a Ag-Ag bond distance of 3.0241(8) with 1 as a trans-chelating ligand (C-Ag-C = 171°). A preliminary X-ray structure of pseudo-square planar [PdCl₂(1)] showed a similar binding mode of 1 (C-Pd-C = 177°). High yields were obtained in Suzuki-Miyaura coupling reactions utilizing [PdCl₂(1)] as the procatalyst and the results were compared with analogous complexes of trans-spanning diphosphine (2) and diphosphinite (3) complexes. The diphosphinite complex, [PdCl₂(3)], decomposes to [μ-ClPd(PPh₂OH)(PPh₂O)]₂ at room temperature. The Royal Society of Chemistry 2009.

Full text of DOI:10.1039/b815739a

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PAPER
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Canopied trans-chelating bis(N-heterocyclic carbene) ligand: synthesis,
structure and catalysis†
Brad P. Morgan, Gabriela A. Galdamez, Robert J. Gilliard, Jr. and Rhett C. Smith*
Received 10th September 2008, Accepted 11th November 2008
First published as an Advance Article on the web 30th January 2009
DOI: 10.1039/b815739a
The terphspan scaffold was employed to support a bis(N-heterocyclic carbene) ligand (1) that provides
a m-terphenyl canopy over one side of its metal complexes. Single-crystal X-ray diffraction studies on a
silver complex of {[Ag(1)]AgBr2}2 revealed an unusual tetranuclear silver core with a Ag–Ag bond
◦
˚
distance of 3.0241(8) A with 1 as a trans-chelating ligand (C–Ag–C = 171 ). A preliminary X-ray
structure of pseudo-square planar [PdCl2(1)] showed a similar binding mode of 1 (C–Pd–C = 177◦).
High yields were obtained in Suzuki–Miyaura coupling reactions utilizing [PdCl2(1)] as the procatalyst
and the results were compared with analogous complexes of trans-spanning diphosphine (2) and
diphosphinite (3) complexes. The diphosphinite complex, [PdCl2(3)], decomposes to
[m-ClPd(PPh2OH)(PPh2O)]2 at room temperature.
catalysis remain in their early stages of development. This point
Introduction
is highlighted by the small number of bidentate bis(NHC)s whose
transition metal complexes exhibit crystallographically character-
ized trans-chelating binding modes, representative examples of
which are shown in Chart 1 (CSD version 5.29 Nov. 2007).29–33
The development of N-heterocyclic carbene (NHC) complexes
in the 1960’s1,2 was followed by eventual isolation and crystallo-
graphic characterization of a stable, metal-free NHC by Arduengo
et al. in 1991.3 The unexpected stability and advantageous elec-
tronic properties of NHCs lead to an extensive investigation into
their utility in synthesis, as ligands for transition metal complexes
and in homogeneous catalysis.4–8 Sterically encumbered NHCs
have been particularly beneficial as ligands in next-generation
catalysts with improved performance and thermal stability for
olefin metathesis.9–13 The ability of certain NHCs to dimerize,
sometimes reversibly, has been exploited for the construction of
new classes of p-conjugated materials for optoelectronic applica-
tions and self-healing polymers.14–19 More recently, Robinson et al.
has pioneered the application of sterically encumbered carbenes
20
=
to stabilize reactive functionalities such as an Si Si bond.
Like phosphines, NHCs are good s-donor ligands and they
have consequently been explored as supporting ligands in catalytic
transformations in which phosphines have long proven their value.
One of the recent thrusts in phosphine-supported catalysis has
been to affect selectivity by controlling the phosphine bite angle
(P–M–P angle, b).21–24 Specifically, there is an ongoing interest
in wide bite angle (b greater than ~110◦) or trans-spanning
diphosphine ligands, which can provide superior regioselectivity
in industrially important catalytic processes, notably rhodium
catalyzed hydroformylation.25–27 Because chelating bis(NHC)s
have only been studied for the past two decades and the first
chiral chelating NHC complex was not reported until 2000,28 cor-
responding studies on bite angle dependence of NHC-supported
Chart
1
Examples of crystallographically characterized bidentate
bis(NHC) complexes with C–M–C angles of ≥170◦.
Previously, wide bite angle diphosphines built on the flexible
terphspan scaffold have shown good activity for use in Pd-
catalyzed Suzuki–Miyaura and Mizoroki–Heck couplings34 and
Rh-catalyzed 1,4-addition reactions of arylboronic acids to a,b-
unsaturated ketones.35 Protasiewicz et al. has further utilized
C–X bond activation of terphspan diphosphines, diphosphinites
and bis(imines) to yield cyclometallated C2-symmetric terpincer
ligands that are good candidates for asymmetric catalysis.36–38
These promising initial data points suggest that the terphspan
scaffold could be a worthy target for preparation of bis(NHC)
complexes.
Department of Chemistry and Center for Optical Materials Science and
Engineering Technologies (COMSET), Clemson University, Clemson, SC
29634. E-mail: rhett@clemson.edu; Fax: +1 864 656 6613; Tel: +1 864 656
6112
Herein, we describe a trans-chelating bis(NHC) built on the
terphspan scaffold and compare its utility as a supporting ligand
with that of terphspan diphosphine and diphosphinite ligands in
Pd-catalyzed C–C bond formation via Suzuki–Miyaura coupling.
† Electronic supplementary information (ESI) available: 1H, 13C and
other NMR spectra. CCDC reference numbers 701814–701816. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/b815739a
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Results and discussion
Synthesis and reactivity
The preparation of the diphosphine 2 and diphosphinite 3 used
in this study was carried out by modification of the literature
reports.39,40 The NHC complexes were prepared from the easily
accessible bis(imidazolium) salt [1H2](Br)2 via the sequence shown
in Scheme 1. The precursor [1H2](Br)2 is a viscous, hygroscopic
oil melting near room temperature, and is difficult to obtain in a
pure (>95%) form. The sample used for subsequent preparations
was obtained by precipitation of the material as a white powder
upon addition of a saturated dichloromethane solution into ethyl
ether at -40 ◦C under a dry atmosphere followed by cold filtration
Chart 2 Structures of canopied ligands that chelate metals via carbene
(1), phosphine (2) and diphosphinite (3) moieties.
The [1H2](Br)2 salt was employed to prepare the Ag(I) complex
[Ag(1)]AgBr2, (isolated as the dimer {[Ag(1)]AgBr2}2 shown in
Fig. 1 in the solid state, vide infra) with the aim of using it as
a convenient precursor to other complexes via transmetallation.
[Ag(1)]AgBr2 was readily prepared from [1H2](Br)2 and Ag2O
following a modified literature procedure for conversion of imida-
zolium salts to silver carbene complexes.43 Analytically pure, but
hygroscopic and light-sensitive, crystals of [Ag(1)]AgBr2 (Fig. 1)
were obtained either by slow diffusion of pentane into a saturated
methylene chloride solution, or by slow evaporation of a saturated
and found to be pure (≥95%) by NMR spectroscopy (1H and 13
C
NMR spectra are provided in the ESI†).
1
solution in acetone. Complete assignment of H and 13C NMR
resonances for [Ag(1)]AgBr2 were determined through a variety
1
of NMR spectrometric techniques including H–1H correlation
spectroscopy (COSY), heteronuclear multiple quantum coherence
(HMQC), distortionless enhancement by polarization transfer
(DEPT) and differential nuclear Overhauser effect (NOE). The
original spectra from all of these techniques and the structural
assignments derived from them are provided in the ESI.†
Scheme 1 Preparation of [1H2](Br)2: (i) (1) nBuLi, -78 ◦C, THF, (2)
2-tolymagnesium bromide, -78 ◦C, THF, 6h, (3) D, 12 h (4) HCl,
0
◦C; (ii) N-bromosuccinimide/benzoylperoxide, CHCl3, D, 48 h;
(iii) N-(n-butyl)imidazole, CH3CN, D,12 h.
It was of interest to determine whether the free carbene ligand
1 could be generated or if the two carbene fragments would
dimerize to form a tetraazafulvalene, as can sometimes occur
for NHCs, a tendency that may be enhanced by the scaffold
structure in bis(NHC)s.41,42 The reaction of [1H2](Br)2 with 2 equiv.
of NaH in anhydrous DMSO-d6 at room temperature under a
1
nitrogen atmosphere was thus undertaken and monitored by H
and 13C NMR spectroscopy. The 1H NMR resonance at 9.07 ppm,
attributed to the protons in the imidazolium 2 position, rapidly
disappeared from the 1H NMR spectrum and a new resonance at
210.9 ppm was observed in the 13C NMR spectrum. The latter peak
is comparable to that observed in the initial report by Arduengo
et al. on an isolable NHC (211.4 ppm in C6D6),3 and was attributed
to the free carbene unit in 1. Unfortunately, X-ray quality crystals
of free 1 have not been obtainable thus far. Although the depiction
in Chart 2 suggests that bis(NHC) 1 may be predisposed to
intramolecular dimerization, molecular modeling (semi-empirical
PM3 level) revealed that dimerization would place significant
strain on the scaffold and would require distortion of the central
aryl ring of the m-terphenyl unit. The other possibility would be
intermolecular dimerization to produce a polymer. However, we
did not observe any increase in viscosity of reaction solutions even
at very high concentrations (as high as 100 mg mL-1) or evidence
from NMR spectroscopy for polymerization, i.e., broadening of
resonances and disruption of molecular symmetry.
Fig. 1 ORTEP drawing (50% probability ellipsoids) of the molecular
structure of {[Ag(1)]AgBr2}2. Hydrogen atoms are omitted for clarity.
The complex [Ag(1)]AgBr2 was useful for the preparation of
catalytically interesting late transition metal complexes by trans-
metallation. For example, [Ag(1)]AgBr2 was cleanly converted
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◦
˚
to [PdCl2(1)] by transmetallation with [PdCl2(NCPh)2] in
dichloromethane at room temperature, and crystalline
[PdCl2(1)]·PhCH3 was obtained by diffusion of toluene into
a saturated solution in chlorobenzene. Whereas the phosphine
analogue [PdCl2(2)] shows broad and solvent dependent 1H
NMR resonances resulting from structural fluxionality, [PdCl2(1)]
shows sharp, well-resolved resonances. Even in 1H and 13C NMR
spectra of the crystalline material, small side peaks are visible on
some of the resonances attributable to atoms nearest the metal
coordination site. This was initially thought to be indicative of
two isomers that could possibly interconvert through different
conformations of the 14-membered ring that includes the Pd
atom, analogous to what was reported for [PdCl2(2)].40 The
nature of these two isomers was elucidated by single-crystal
X-ray diffraction studies, which identified the minor isomer as
arising from variable occupancy of Br for some Cl on Pd (vide
infra). Unfortunately, the two isomers could not be separated by
preparative TLC or crystallization methods, and the material was
therefore utilized as a mixture of isomers without separation.
The 13C NMR signal attributed to the carbene carbon (in
CDCl3), observed at 168.9 ppm, is comparable to the chemical
shift observed in other trans-[PdCl2(bis(NHC))] complexes
(165.6–171.7 ppm).44–46
Table 1 Selected bond lengths (A) and angles ( ) for {[Ag(1)]AgBr2}2
˚
Bond lengths/A
Ag(1)–Ag(2)
Ag(2)–Br(1)
Ag(2)–Br(2)
Ag(2)–Br(2)Aa
Ag(1)–C(14)
Ag(1)–C(27)
3.0241 (8)
2.5486 (8)
2.6965 (11)
2.7231 (8)
2.100 (5)
2.104 (5)
Bond angles/◦
C(27)–Ag(1)–C(14)
C(27)–Ag(1)–Ag(2)
C(14)–Ag(1)–Ag(2)
Br(1)–Ag(2)–Br(2)
Br(1)–Ag(2)–Br(2)Aa
Br(2)–Ag(2)–Br(2)Aa
Br(1)–Ag(2)–Ag(1)
Br(2)–Ag(2)–Ag(1)
Br(2)Aa–Ag(2)–Ag(1)
Ag(2)–Br(2)–Ag(2)Aa
171.16 (18)
62.88 (12)
125.84 (13)
129.40 (3)
122.82 (3)
99.45 (2)
80.86 (3)
134.14 (2)
84.37 (2)
80.55 (2)
a Symmetry transformations used to generate equivalent atoms: -x + 1, -y
+ 1, -z.
m-Br tetrametallic core of {[Ag(1)]AgBr2}2 (type 2 in Chart 3)
has been observed only infrequently in Ag(I) NHC complexes
previously,48–54 and {[Ag(1)]AgBr2}2 appears to be the first example
of a crystallographically characterized chelating NHC complex
adopting this mode in the solid state.
Structure
A single-crystal X-ray diffraction structure of {[Ag(1)]AgBr2}2
provided a structure (Fig. 1) in which a type 2 (Chart 3)
binding mode is observed with a AgAg-m-Br2AgAg core being
observed. Selected bond angles and distances listed in Table 1, and
refinement details are provided in Table 2. The salient features are
a Ag(1)–Ag(2) distance of 3.024 A indicative of Ag–Ag bonding,
and a cross-core Ag–Ag distance of 3.504 A, which is not within
the sum of the van der Waals radii (3.44 A). The unusual bis-
In one case of a complex exhibiting a type 2 binding mode
(Chart 3), Lin, et al. aptly noted that the binding geometry can
depend on the crystallization conditions, suggesting that different
coordination modes may be operative in solution vs. the solid
state.48 Specifically, recrystallization of the type 2 crystals in the
presence of a potentially coordinating solvent such as acetone
allowed isolation of a type 1 binding mode (Chart 3). Following
their lead, we collected structures of crystals grown either by
diffusion of pentane into a CH2Cl2 solution of the silver complex
or by slow evaporation of acetone. However, we observed the same
binding mode from both types of crystals (the data from crystals
grown in acetone refined better and yielded the data in Fig. 1 and
Tables 1 and 2), indicating that this unusual binding motif is likely
enforced by the scaffold.
˚
˚
˚
47
On the basis of previous X-ray diffraction studies on Ag(I)-NHC
complexes, we considered the possibility that one of the alternative
binding modes shown in Chart 3 may be present in solution. Of
these possibilities, type 1 and 3 binding modes require too far a
separation between carbene fragments to be accommodated by
a single 1 unit (as gauged by simple modelling using molecular
mechanics), and are inconsistent with NMR data. The other
binding modes may all be accommodated by the flexible terphenyl
scaffold. Type 4 binding is ruled out by NMR spectrometry,
which suggests a symmetry plane through the central aryl ring
of the m-terphenyl unit. Type 5 binding, in which one carbene
fragment from each unit of 1 binds to each of the two Ag(I) ions,
cannot be ruled out on the basis of NMR data. This mode also
seems plausible in light of a related structure that is formed for
a dipalladium complex of Terphspan diphosphine 2.40 However,
in diphosphine complexes utilizing the same scaffold, including
[RhCl(CO)(2)] and [MCl2(2)] (M = Ni, Pd, or Pt), as well as in
Chart 3 Some binding modes of silver(I) complexes featuring mono- (top)
and bis- (bottom) N-heterocyclic carbene ligands that have been observed
in the solid-state by X-ray diffraction.
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Table 2 Crystal data and structure refinement details
{[Ag(1)]AgBr2}2
[m-ClPd(PPh2OH)(PPh2O)]2
[PdX2(1)]·PhCH3
Molecular formula
Formula weight/g mol-1
Temperature/K
Wavelength/A
Crystal system
C68H76Ag4Br4N8
1756.49
153 (2)
C48H42Cl2O4P4Pd2
1090.40
153 (2)
C34H38Cl1.43Br0.57N4Pd ◊ ◊ ◊ C7H8
797.46
153(2)
˚
0.71073
0.71073
0.71073
Triclinic
Monoclinic
P21/n (#14)
15.262(3)
10.348(2)
21.561(4)
90.00
103.32(3)
90.00
3316.2(12)
2
Triclinic
¯
¯
Space group
P1 (#2)
P1 (#2)
˚
a/A
11.855(2)
13.167(3)
17.352(4)
68.76(3)
78.84(3)
63.27(3)
2253.2(8)
2
1.607
1.103
1096
10.392(2)
13.675(3)
15.092(3)
109.66(3)
91.31(3)
111.21(3)
1857.2(6)
2
˚
b/A
˚
c/A
a/◦
b/◦
g /◦
3
˚
V/A
Z
Dc/Mg m-3
1.759
3.620
1736
1.426
1.250
940
m/mm-1
F(000)
Crystal size/mm
Crystal colour and shape
q range for data collection/◦
Limiting indices
0.46 ¥ 0.22 ¥ 0.12
Colourless chip
2.74–25.10
-18 < h < 18
-12 < k < 12
-25 < l < 25
21 729
5901
25.10 (99.2%)
0.6504
0.53 ¥ 0.48 ¥ 0.36
Yellow prism
3.15–25.10
-14 < h < 14
-15 < k < 14
-20 < l < 18
16 023
7784
25.10 (97.0%)
0.6923
0.22 ¥ 0.17 ¥ 0.10
Colourless chip
2.43–25.10
-10 < h < 12
-16 < k < 14
-18 < l < 18
13 822
6539
25.10 (98.8%)
0.8852
Reflections collected
Independent reflections
Completeness to q
Max. transmission
Min. transmission
0.2781
0.5926
0.7705
Refinement method
Data/restraints/parameters
Goodness of fit on F2
Full-matrix least-squares on F2
5901/0/381
1.013
Full-matrix least-squares on F2
7784/0/567
1.004
Full-matrix least-squares on F2
6539/6/473
1.024
Final R indices (I > 2s(I))
R1
wR2
0.0389
0.0732
0.0375
0.0715
0.0583
0.1073
R indices (all data)
R1
wR2
0.0465
0.0779
0.0436
0.0759
0.0922
0.1196
[PdCl2(1)] (vide infra), the ligand adopts a trans-spanning mode
such as that observed in the crystal structure for {[Ag(1)]AgBr2}2.
Furthermore, due to the chelate effect it is entropically unlikely
that the ligand-bridged type 5 core would form in the presence
of coordinating bromide anions unless argentophilic interactions
are strong enough to provide the necessary stabilization. The
type 6 core is the most likely species to be present in solution
on the basis of NMR and X-ray diffraction data. This species
maintains the ligand-preferred trans-spanning mode and it can
be easily envisioned that the type 2 core would be formed upon
concentration of the solution without gross reorganization of the
core.
similar mixed halogen occupation in a [PdCl2(NHC)] complex
prepared by the same method we used to prepare [PdCl2(1)].46
The partial occupancy in the current structure can be most easily
denoted from the bond distances and angles listed in Table 3.
˚
˚
Specifically, Pd–X(1) (2.377 A) and Pd–X(2) (2.347 A) interatomic
˚
distances are longer than the standard Pd–Cl bond (2.31 A), which
can be attributed to the larger van der Waals radius of bromide.
The amount of bromide in X1 is more prominent as seen in the
larger of the two bond distances (location of the X1 is in the less
sterically hindered side). The C(14)–Pd(1)–C(25) angle of 176.8◦
is quite similar to the P–Pd–P angle observed in the terphspan
phosphine complex [PdCl2(2)] 172.97◦.40
The diffusion of toluene into a saturated solution of putative
[PdCl2(1)] in chlorobenzene gave a preliminary structure of a bis-
NHC palladium carbene with a trans chelating structure (Fig. 2).
X-Ray diffraction analysis of one of these colourless crystals
revealed that the halogen sites are both partially occupied by Br in
addition to the expected Cl (Fig 2: X(1) = 61.63% Cl, 38.37% Br
and X(2) = 81.18% Cl, 18.82% Br). Erker has reported observing
NMR spectroscopic data also revealed important differences in
the structural aspects of the terphspan carbene (1) and phosphine
(2) complexes. Whereas the phosphine analogue [PdCl2(2)] shows
broad and solvent-dependent 1H NMR resonances resulting from
structural fluxionality,40 [PdCl2(1)] shows sharp, well-resolved
resonances in both 1H and 13C NMR spectra. Another difference
between the spectra of the phosphine complex [PdCl2(2)] and
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Suzuki coupling: comparison of terphspan bis(NHC), diphosphine
and diphosphinite
Table 3 Selected bond lengths (A) and angles ( ) for [PdX2(1)]
˚
Bond lengths/A
The canopied terphspan diphosphine 2 produces moderately ac-
tive catalysts for Pd-catalyzed Suzuki–Miyaura coupling.34 How-
ever, chelating NHC Pd complexes have produced a wide range
of yields, from poor to excellent for this coupling reaction44,55–68
and thus Suzuki–Miyaura coupling was the first reaction exam-
ined with terphspan carbene 1. Because the coupling partners
reported for reactions supported by 2 were rather limited in
scope (PhB(OH)2 was the only boronic acid employed), we
expanded the set to include electron rich and electron deficient
boronic acids as well as a heteroatom-containing aryl halide
(2-iodothiophene) to gauge the compatibility of these species
with reaction conditions (Table 3). That [PdCl2(1)] supports
high yields employing 2-iodothiophene illustrates the use of
heteroatom-containing aryl halides as successful reagents for
coupling reactions. It is also noteworthy that 1 exhibits high
yields in all cases (≥81%) when phenyl boronic acid is a coupling
partner, while both electron donating (4-methoxyphenylboronic
acid) and withdrawing (4-acetylphenylboronic acid) group sub-
stituted boronic acids sacrifice some efficiency, though modest
yields are still achievable. The 4-acetylphenylboronic acid has
substantially lower yields than reactions employing phenylboronic
acid or 4-methoxyphenylboronic acid. Good to excellent yields
are achievable utilizing a variety of aryl halides including aryl
chlorides, bromides and iodides. This represents an improvement
over coupling reactions employing diphosphine 2 under identical
conditions, in which the highest yields achievable for coupling aryl
chlorides and phenylboronic acid was 5%.34 Modest to good yields
(28–81%) were also observed utilizing [PdCl2(1)] for sterically
encumbered aryl halides, such as 2,4,6-trimethylbromobenzene.
Having demonstrated that complexes of 1 can participate in
efficient catalytic reactions, it was of interest to compare the
relative efficiency of canopied carbene (1), diphosphine (2) and
diphosphinite (3) ligands under identical reaction conditions. We
therefore selected a subset of the reactions listed in Table 4 for
testing with each of the three ligands (Table 5). As anticipated, cat-
alytic reactions employing the diphosphinite ligand were generally
much less efficient than analogous bis(NHC)- and diphosphine-
supported reactions. Depressed yields of reactions employing
the diphosphinite may be due to Pd-mediated decomposition
of the ligand to species, such as [ClPd(m-Cl)(PPh2OH)]2 or [m-
ClPd(PPh2OH)(PPh2O)]2, though such species have also been em-
ployed in Stille–Miyaura coupling,69–73 and related diphosphinite
hydrolysis products have proven active in a variety of catalytic
processes.74–76 To determine whether ligand decomposition could
be occurring in the current case, a solution of [PdCl2(NCPh)2]
and 3 in a 1 : 2 ratio was allowed to stand at room temperature
under nitrogen in the absence of other reagents (Scheme 2).
Pd(1)–C(16)
Pd(1)–C(25)
Pd(1)–X(1)
Pd(1) X(2)
N(1)–C(16)
N(2)–C(16)
N(3)–C(25)
N(4)–C(25)
2.015 (6)
2.031 (5)
2.3768 (15)
2.3470 (16)
1.343 (8)
1.360 (8)
1.353 (7)
1.343 (7)
Bond angles/◦
C(16)–Pd(1)–C(25)
X(1)–Pd(1)–X(2)
C(16)–Pd(1)–X(1)
C(25)–Pd(1)–X(1)
C(16)–Pd(1)–X(2)
C(25)–Pd(1)–X(2)
N(1)–C(16)–N(2)
N(3)–C(25)–N(4)
176.8 (3)
175.75(6)
90.69(18)
90.26(17)
88.08 (18)
91.19 (17)
105.3(5)
105.2(4)
Fig. 2 ORTEP drawing (50% probability ellipsoids) of the molecular
structure of [X2Pd(1)]. Hydrogen atoms and the co-crystallized toluene
solvent are omitted for clarity. C18, C19 and C20 show only one of the
representative examples of the butyl partial occupancy observed. X1 =
61.63% Cl, 38.37% Br and X2 = 81.18% Cl, 18.82% Br.
the carbene complex [PdCl2(1)] are the positions of resonances
attributable to H atoms on the central m-terphenyl ring. In
[PdCl2(2)], these resonances are distributed over a broad range
from 5.96–8.02 ppm due to interaction between the Pd–Cl and
the aryl ring. However, in [PdCl2(1)], the larger Pd-containing ring
size (14-atom vs. 12-atom in [PdCl2(2)]) apparently positions the
metal center too far from the central ring to engage in this type
of interaction, with closest solid state Pd–Cl ◊ ◊ ◊ Haryl distances of
˚
3.69 and 3.48 A for [PdCl2(1)] and [PdCl2(2)], respectively, in the
Scheme 2 Decomposition of [Cl2Pd(3)] to form [m-ClPd(PPh2OH)-
solid state.
(PPh2O)]2.
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Table 6 Selected bond lengths (A) and angles (◦) for [m-
Table 4 Yields (GC-MS vs. internal calibration) of Suzuki–Miyaura
coupling reactions in the presence of [Cl2Pd(1)]
˚
ClPd(PPh2OH)(PPh2O)]2
˚
Bond lengths/A
Pd(1)–P(1)
Pd(1)–P(2)
Pd(1)–Cl(1)
Pd(1)–Cl(1)Aa
P(1)–O(1)
2.2578 (10)
2.2510 (12)
2.4198 (10)
2.4284 (12)
1.537 (3)
96
81
83
52
99b
28
P(2)–O(2)
1.539 (3)
Pd(2)–P(3)
Pd(2)–P(4)
Pd(2)–Cl(2)
Pd(2)–Cl(2)Ab
P(3)–O(3)
2.2410 (13)
2.2449 (11)
2.4251 (13)
2.4297 (10)
1.542 (3)
P(4)–O(4)
1.536 (3)
97
33
0
Bond angles/◦
P(1)–Pd(1)–P(2)
92.56 (4)
171.87 (3)
89.77 (4)
93.85 (4)
177.06 (3)
83.99 (4)
96.01 (4)
91.69 (5)
168.14 (4)
91.09 (4)
93.84 (4)
175.05 (3)
84.19 (4)
95.81 (4)
82
90a,b
89a,b
96a
60
8
P(1)–Pd(1)–Cl(1)
P(1)–Pd(1)–Cl(1)Aa
P(2)–Pd(1)–Cl(1)
P(2)–Pd(1)–Cl(1)Aa
Cl(1)–Pd(1)–Cl(1)Aa
Pd(1)–Cl(1)–Pd(1)Aa
P(3)–Pd(2)–P(4)
P(3)–Pd(2)–Cl(2)
P(3)–Pd(2)–Cl(2)Ab
P(4)–Pd(2)–Cl(2)
P(4)–Pd(2)–Cl(2)Ab
Cl(2)–Pd(2)–Cl(2)Ab
Pd(2)–Cl(2)–Pd(2)Ab
90a
90a
43
~100
~100
~100
65a
57a
Symmetry transformations used to generate equivalent atoms:a -x, -y +
1, -z + 1. b -x, -y + 2, -z + 2.
~100
~100
95a,b
39a
a Aryl halide homocoupling was observed. b A small amount of boronic
acid self coupling was also observed.
Table 5 Yields of Suzuki–Miyaura coupling reactions in the presence of
[Cl2Pd(L)], where L = 1, 2, or 3
ArB(OH)2: Ar =
ArX
Ligand
Yield(%)
4-methoxyphenyl
4-methoxyphenyl
4-methoxyphenyl
4-acetylphenyl
4-acetylphenyl
4-acetylphenyl
Ph
PhCl
PhCl
PhCl
PhCl
PhCl
PhCl
2-bromotoluene
2-bromotoluene
2-bromotoluene
1
2
3
1
2
3
1
2
3
83
100
82
99
2
2
90
100
100
Ph
Ph
Crystals of [m-ClPd(PPh2OH)(PPh2O)]2 were indeed isolated from
this mixture after 18 h (Fig. 3 and Table 6), and the initial
yellow colour attributable to the putative [PdCl2(3)] complex
slowly fades to colourless. As seen in Table 4, the data referring
to 4-acetylphenylboronic acid has considerable differences in
yields depending on ligand substrate. The NHC ligand 1 emerges
Fig. 3 ORTEP drawing (50% probability ellipsoids) of the molecular
structure of [m-ClPd(PPh2OH)(PPh2O)]2. Hydrogen atoms are omitted
for clarity however, H49 and H49A are shown to demonstrate hydrogen
bonding.
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as an obvious choice for coupling due to the advantageous
yields achieved. It is also noteworthy that while diphosphine 2
provides poor (<5%) yields for coupling of aryl chlorides with
either phenylboronic acid or 4-acetylphenylboronic acid, near
quantitative coupling is achieved when 4-methoxyphenylboronic
acid is employed. The generally good yields observed for all ligands
used in this study for the coupling of phenylboronic acid with aryl
bromides and iodides are also noteworthy.
Preparation of [Ag(1)] AgBr2
To a solution of 0.556 g (0.837 mmol) [1H2](Br)2 in 25 mL
dichloromethane 0.239 g (1.03 mmol) silver(I) oxide was added.
The suspension was refluxed for 24 h in the absence of light. The
resulting solution was filtered through Celite and the filtrate was
concentrated under reduced pressure then vacuum dried for 3 h to
afford the product as a white powder. A minimal amount of DCM
was used to dissolve the material and pentane was diffused into the
vial at 0 ◦C to afford the pure complex as clear colourless crystals
(0.632 g, 86.0%). High quality crystals for single-crystal X-ray
diffraction were also attainable by slow evaporation of acetone.
1H NMR (300 MHz, DMSO-d6) d/ppm: 7.75 (dd, 6.0 Hz, 2H),
7.48 (t, 7.5 Hz, 2H), 7.47 (d, 3.0 Hz, 2H), 7.43 (t, 7.5 Hz, 2H),
7.38 (t, 9 Hz, 1H), 7.38 (d, 3.0 Hz, 2H), 7.34 (bs, 1H), 7.21 (dd,
6.0 Hz, 2H), 6.95 (dd, 6.0 Hz, 2H), 5.43 (s, 4H), 3.96 (t, 7.5 Hz,
4H), 1.66 (quintet, 6.0 Hz, 4H), 1.20 (sextet, 9.0 Hz, 4H), 0.87 (t,
7.5 Hz, 6H). 13C{H} NMR (75.4 MHz, DMSO-d6) d/ppm: 179.8,
142.2, 140.8, 134.1, 131.2, 131.2, 129.6, 129.1, 129.1, 128.6, 128.6,
122.8, 121.5, 53.1, 51.0, 33.6, 19.6, 13.9. Elemental analysis calcd
for C68H76Ag4Br2N8·C4H8O: (fw = 1662.13) C 47.29, H 4.63,
N 6.13. Found: C 47.77, H 4.28, N 6.31.
Conclusions
We have prepared and characterized a trans-chelating bis(NHC)
silver complex {[Ag(1)]AgBr2}2 that exhibits a unique tetranu-
clear core in the solid state. The synthesis of [PdCl2(1)] was
acheived via transmetallation and was shown to exhibit similar
binding as [PdCl2(2)]. The catalytic activity of canopied trans-
chelating bis(NHC) ligands along with phosphine and diphosphi-
nite analouges were explored for catalytic activity of C–C bond
formation reactions via Suzuki–Miyaura coupling. Moderate to
high yields and distinct differences in catalytic activity highlight
the influence of trans-chelating complexes in catalytic processes.
Additional studies on catalytic applications of wide bite angle
bis(NHC) ligands are important ongoing efforts.
Preparation of [PdCl2(1)]
To a vigorously stirring solution of 0.0646 g (0.168 mmol)
bis(benzonitrile)
palladium(II)
chloride
(10
mm
in
Experimental
dichloromethane) a 10 mm solution of 0.1493 g (0.157 mmol)
[Ag(1)]AgBr2 in dichloromethane was added dropwise. This
solution was allowed to stir overnight then filtered and the filtrate
was concentrated in vacuo then washed with hexane to afford a
light yellow powder (0.0863 g, 80.9%) that was used immediately
for coupling reactions. 1H NMR (300 MHz, CDCl3) d/ppm:
7.63–7.30 (m, 10H) (dd, 6.0 Hz, 2H), 7.24 (dd, 7.7 Hz, 2H), 6.86
(d, 1.8 Hz, 2H), 6.76 (d, 2.1 Hz, 2H), 6.11 (s, 4H), 4.42 (t, 7.5 Hz,
4H), 2.03 (quintet, 7.5 Hz, 4H), 1.45 (sextet, 7.8 Hz, 4H), 1.00 (t,
7.2 Hz, 6H). 13C{H} NMR (75.4 MHz, CDCl3) d/ppm: 168.9,
143.5, 140.1, 132.6, 132.4, 131.6 (131.7), 131.0, 130.8, 130.2,
128.5, 128.4, 120.8 (120.9), 119.1 (119.2), 52.4 (52.8), 50.4 (50.6),
33.4 (33.2), 20.1, 13.8. (peaks attributable to minor isomers are
provided in parentheses; see ESI for original NMR spectra).†
ESI-MS: m/z 689, (100%) [BrPd(1)]+; m/z 645 (73.3%) [ClPd(1)]+;
m/z 608 (49.5%) [Pd(1)]+.
General considerations
All air sensitive procedures were carried out in an inert atmosphere
of nitrogen using an MBraun dry box or with standard Schlenk
techniques. Compounds 240 3,39 477 and 5,78 were prepared as
reported previously. Solvents were purified by passage through
alumina columns under a N2 atmosphere employing an MBraun
solvent purification system. All other reagents were used as
received from Aldrich Chemical Company, Acros, TCI and Alfa
Aesar. NMR spectra were collected using a Bruker Avance 300
instrument operating at 300 MHz for proton and 75.4 MHz
for 13C. Gas chromatographs and mass spectra were collected
using a Shimadzu GCMS-QP2010 and analyzed using the GCMS
solution software.
Preparation of [1H2](Br)2
General conditions for Suzuki coupling reactions
A solution of 1,3-bis[2-(bromomethyl)phenyl]benzene (1.00 g,
2.40 mmol) and N-(n-butyl)imidazole (0.656 g, 5.28 mmol) in
acetonitrile (200 mL) was refluxed for 15 h. Once cooled to room
temperature, the solvent was removed under reduced pressure
to give an oily residue. Further purification was affected by re-
precipitation from a saturated DCM solution into ethyl ether at
-40 ◦C followed by cold filtration. The resulting solid was then
dried in vacuo to yield a white powder that was used without
further purification (1.46 g, 91.6%). 1H NMR (300 MHz, DMSO-
d6) d/ppm: 9.07 (s, 2H), 7.81 (s, 2H), 7.63–7.44 (m, 7H), 7.44–7.33
(m, 4H), 7.33–7.24 (m, 2H), 7.21 (s, 1H), 5.50 (s, 4H), 4.12 (t,
9.0 Hz, 4H), 1.69 (quintet, 6.0 Hz, 4H), 1.19 (sextet, 6.0 Hz, 4H),
0.85 (t, 7.5 Hz, 6H). 13C{H} NMR (CD3CN, 75.4 MHz) d/ppm:
12.6, 18.9, 31.5, 49.4, 51.3, 122.4, 122.5, 128.1, 128.7, 128.8, 129.1,
129.6, 130.0, 130.8, 130.9, 135.3, 140.1, 141.7.
Conditions were identical to those reported for Suzuki coupling
reactions utilizing ligand 2.34 A 5 mL vial equipped with a spin
vain was charged with the requisite aryl halide (0.30 mmol),
phenylboronic acid (0.45 mmol), Cs2CO3 (0.90 mmol), and 1 mol%
of [PdCl2(1)]. The components were then taken up in 2 mL 1,4-
dioxane and heated at 100 ◦C under nitrogen for 25 h. Following
the reaction, the crude reaction mixtures were diluted with diethyl
ether for direct GC-MS analysis.
X-Ray crystallography
Intensity data were collected using a Rigaku Mercury CCD
detector and an AFC8S diffractometer. Data collection and cell
refinements were achieved using CrystalClear.79 Data reduction
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and absorbance corrections were attained using REQAB.80 The
structures were solved by direct methods and subsequent Fourier
difference techniques, and refined anisotropically, by full-matrix
least squares, on F2 using SHELXTL 6.1081 All molecular graphics
were generated using SHELXTL 6.10. Hydrogen atom positions
were calculated from ideal geometry with coordinates riding on the
parent atom however H49 and H50 in [m-ClPd(PPh2OH)(PPh2O)]2
were located in the Fourier difference map, then refined. Partial
occupancy was used to account for disorder observed in C47 and
C48 of a phenyl ring in this structure. Disorder was observed in
[PdX2(1)]·C7H8 on one of the nbutyl functionalities and accounted
for using partial occupancies of C18, C19 and C20 with their
counterparts being C18A, C19A and C20A, respectively. The
disorder in the co-crystallized toluene molecule in [PdX2(1)]·C7H8
was accounted for using partial occupancy of C41 with its
counterpart being C41A.
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