Cyclotriveratrylene-tethered trinuclear palladium(ii)-NHC complexes; Reversal of site selectivity in Suzuki-Miyaura reactions

Article abstract of DOI:10.1039/c9dt03400e

The trinuclear complexes [{PdI₂(pyCl)}₃(L1)] C1 and [{PdI₂(pyCl)}₃(L2)] C2, where pyCl = 3-chloropyridine, L1 = methyl(cyclotriguaiacylenyl)methylbenzimidazol-2-ylidene and L2 = benzyl(cyclotriguaiacylenyl)methy

Full text of DOI:10.1039/c9dt03400e

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Cyclotriveratrylene-tethered trinuclear
palladium(II)–NHC complexes; reversal of
site selectivity in Suzuki–Miyaura reactions†‡
Cite this: Dalton Trans., 2019, 48,
14687
Jonathan M. Fowler,
Edward Britton, Christopher M. Pask,
Charlotte E. Willans * and Michaele J. Hardie
*
2 3 2 3
The trinuclear complexes [{PdI (pyCl)} (L1)] C1 and [{PdI (pyCl)} (L2)] C2, where pyCl = 3-chloropyridine,
L1 = methyl(cyclotriguaiacylenyl)methylbenzimidazol-2-ylidene and L2 = benzyl(cyclotriguaiacylenyl)
methylbenzimidazol-2-ylidene, each feature three palladium N-heterocyclic carbene (NHC) centres teth-
ered onto a host-type cyclotriguaiacylene scaffold. Crystal structures of different solvates of complex C1
reveal different host–guest motifs including intra-cavity binding of dioxane guests concomitant with
intramolecular halogen bonding interactions of C1. Mononuclear NHC analogues of C1 and C2, namely
2 2
[PdI (pyCl)(L3)] C3 and [PdI (pyCl)(L4)] C4, where L3 = (3-chloropyridyl)-1-(2-methoxyphenyoxy)methyl-
3-methylbenzimidazol-2-ylidene and L4 = (3-chloropyridyl)-1-(2-methoxyphenyoxy)methyl-3-benzyl-
Received 20th August 2019,
Accepted 14th September 2019
benzimidazol-2-ylidene, were also synthesised and their crystal structures determined. Complexes C1–
C4 are competent catalysts for Suzuki Miyaura cross-coupling, and interestingly exhibit a switch in the
normal regioselectivity observed for reactions of 2,4-dibromopyridine with aryl boronic acids, usually C2-
selective, yielding C4-arylated product preferentially over C2-arylated product.
DOI: 10.1039/c9dt03400e
rsc.li/dalton
ated with this class of molecular hosts have seldom been studied.
An exception is the class of organometallic CTV-type complexes
Introduction
18,19
Cyclotriveratrylene (CTV) is a bowl-shaped molecule, which where the arene faces of the CTV/CTG scaffold
or extended
9
20,21
6
can behave as a molecular host for a variety of guest species ligand, or of cryptophanes
have been employed as η -donors
1,2
including fullerene-C . The CTV family of cavitands have a in polymetallic Fe(II) and Ru(II) sandwich complexes.
60
shallow cavity built around a tribenzo[a,d,g]cyclononene core in a
Since their first isolation in 1991 by Arduengo and co-
bowl conformation. Cyclotriguaiacylene (CTG) is the tris-hydroxyl workers, N-heterocyclic carbenes (NHCs) have become an
22
analogue of CTV, and upon reaction about the phenolic moieties important ligand class in organometallic chemistry. They are
with alkyl halides or acid chlorides, give rise to tripodal hosts regularly used as persistent ligands on Pd(II) precatalysts for
2
3–27
with extended arms at the upper-bowl rim. Cyclotriguaiacylene cross-coupling reactions.
The Pd(II)–NHC library is diverse
and its derivatives possess helical chirality about the upper rim. and ever-expanding. One commonly used class of Pd(II)–NHCs
Attaching ligand functionality to the CTG scaffold in this, or are the PEPPSI (pyridine enhanced precatalyst preparation
similar, fashions gives tripodal ligands capable of forming trime- stabilisation and initiation) complexes pioneered by Organ
3
–9
tallic complexes.
Such ligands can also form coordination and co-workers, which catalyse
a
plethora of coupling
PEPPSI complexes are readily synthesised in
complex interlocked architectures. Ligand groups that have 3-chloropyridine solvent, often under aerobic conditions.
been appended to the CTG-scaffold have generally been N-donor There have been relatively few examples of CTV catalysts.
or O-donor moieties and, as such, organometallic systems associ- Zheng et al. reported a C -symmetric imidazolium salt which,
10–13
14–17
28–32
polymer materials,
coordination cages,
and topologically reactions.
14
1
upon in situ deprotonation of the imidazolium NCHN proton,
behaved as an organocatalyst in the Michael addition of alco-
hols to unsaturated α,β-unsaturated ketones, but coordination
School of Chemistry, University of Leeds, Woodhouse Lane, Leeds LS2 9JT, UK.
E-mail: c.e.willans@leeds.ac.uk, m.j.hardie@leeds.ac.uk
3
3
of this carbene to metal centres was not reported. Kück and
†
‡
Data Accessibility: Data are available at https://doi.org/10.5518/716.
Peris reported a Rh(I)–NHC complex fused onto the closely
Electronic supplementary information (ESI) available: NMR, IR, mass spectra
34
related tribenzotriquinacene (TBTQ) scaffold. Dutasta et al.
and details of crystallography and additional catalysis studies. CCDC
905408–1905413. For ESI and crystallographic data in CIF or other electronic
have developed a range of CTV-based hemicryptophanes with
1
3
5
format see DOI: 10.1039/c9dt03400e
endohedral metallation that function as catalysts. Herein, we
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report the synthesis and characterisation of the first NHC benzyl(methylbenzimidazol-1-yl)cyclotriguaiacylenyl bromide
complex on a CTV scaffold. Catalytic activity in the Suzuki– (H L2·3Br) as a white solid. The benzyl group was selected as
3
Miyaura cross-coupling of 4-methyoxyphenylboronic acid with both a bulkier moiety, which also imparts some flexibility to
2
,4-dibromopyridine has been studied.
an otherwise rigid system. The CTG axial and equatorial reso-
nances appear at 4.74 and 3.54 ppm respectively. The benzimi-
dazolium NCHN proton resonance in the H NMR spectrum
1
shifted downfield to 10.18 ppm. The main peak in the HRMS
was the anticipated +3 tris-imidazolium cation at m/z =
Results and discussion
Ligand synthesis
3
57.1624, again with fragmentation evident.
Salt metathesis of H L1·3I with ammonium hexafluoro-
phosphate led to the isolation of H L1·3(PF ), confirmed by
The tripodal benzimidazole precursor, P1, was prepared
according to previous methodology.
nitrogen atoms were methylated using MeI to give the C -sym-
metric benzimidazolium salt tris-methyl(methylbenzimidazol-
3
1
6
The benzimidazole
3
6
1
9
31
3
F and P NMR spectroscopy, and the infrared spectrum con-
tained a sharp peak at 834 cm , typical of a P–F stretch.
Crystals of H L1·3(PF ) were grown from the diffusion of
−
1
1
3
-yl)cyclotriguaiacylenyl iodide (H L1·3I), according to
3
6
Scheme 1. Compounds of the CTV family contain two charac-
teristic axial and equatorial resonances in the H NMR spec-
trum corresponding to the methylene bridge protons in the
crown conformation, and occur at δ = 3.55 (H ) and 4.73 (H )
diethyl ether into an acetonitrile solution of the ligand, and
their crystal structure determined using synchrotron radiation.
1
3
+
The [H L1] cation adopts the expected bowl-shape and does
3
eq
ax
not have crystallographic C -symmetry as all three benzimida-
3
ppm here, with the Heq-resonance overlapping with the
methoxy peak (confirmed by relative integrals, COSY and
HMQC evidence). The characteristic downfield shift of NCHN
proton to 9.86 ppm after functionalisation of a benzimidazole
is also apparent. The high-resolution mass spectrum (HRMS)
contains peaks corresponding to tricationic benzimidazolium
salt minus one to three iodide ligands. Cleavage of one benz-
imidazolium arm is also observed in the mass spectrum (m/z =
zolium arms adopt different conformations with respect to the
CTG-scaffold, and one exhibits positional disorder (Fig. 1a).
There is extensive π–π stacking throughout the crystal lattice.
3
+
3
Enantiomeric pairs of [H L1] cations form a “hand-shake”
3
49.1559), but this is a fragmentation on ionisation, noting
1
3
retention of C -symmetry of the molecule evident by H NMR
spectroscopy. Addition of benzyl bromide, rather than methyl
iodide, under milder reaction temperatures to P1 yields tris-
Fig. 1 From the crystal structure of H
3
L1·3(PF
6
). (a) Asymmetric unit
−
with PF
6
anions in ball-and-stick representation; (b) hand-shake motif
Scheme 1 Synthesis of imidazolium salts of CTG-type ligands, with
formed with π–π stacking interactions, only one position of disorder
monomeric analogues shown in inset.
shown for clarity.
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host–guest motif with the disordered benzimidazolium arm of
3
+
one [H L1] occupying the CTG-bowl of the other and vice
3
versa through face-to-face benzimidazolium-phenyl π–π stack-
ing interactions at ring centroid separation of 3.72 Å (Fig. 1b).
Overall, the cations stack into columns and the PF6− anions
occupy spaces between them (Fig. S66, ESI‡).
3 4
Imidazolium salts HL ·I and HL ·Br were also prepared
in good yields using analogous procedures from the prev-
iously reported 1-(2-methoxyphenoxy)methylbenzimidazole
3
6
(
Scheme 1). These are analogous to H
but monomeric in nature and without the potential for supra-
molecular host–guest interactions. Single crystals of HL ·I were
3 3
L1·3I and H L2·3Br
3
grown from the diffusion of diethyl ether vapours into an
acetonitrile solution of the salt, and their structure determined
using data collected with synchrotron radiation. The asym-
3
metric unit comprises two molecules of HL ·I. One imidazo-
lium salt contains a weak intramolecular hydrogen bond
between methylene bridge proton and methoxy group at a
C⋯O distance of 2.84 Å and an N–C–O–C torsion angle of
−63.3°. The second imidazolium salt has an N–C–O–C torsion
angle of 73.5°, and is therefore aligned in the opposite direc-
tion, with no such hydrogen bond present (Fig. 2).
Palladium(II) complexes
Palladium complex [{PdI
-chloropyridine, was prepared by reaction of ligand H
with PdI2 under basic conditions (Scheme 2). The NCHN
2
(pyCl)}
3
(L1)] C1, where pyCl is
3
3
L1·3I
1
3
proton seen in the H NMR spectrum of H L1·3I is absent for
1
C1 which is indicative of carbene formation. The H NMR
spectrum of C1 is typical of a C -symmetric species with dia-
3
stereotopic methylene bridges of the tribenzo[a,d,g]cyclono-
Scheme 2 Synthesis of trimetallic complexes with monometallic ana-
nene resonating at 4.44 and 3.41 ppm. The N–CH
2
O protons
logues in inset.
are also diastereotopic, appearing as two doublets at 6.61 and
1
3
1
6
.40 ppm. C{ H} NMR spectroscopy confirmed the presence
of a carbenic species with a resonance at 161.69 ppm. The [{PdI
(pyCl)} (L2)] C2 from H L2·3Br utilised the same meth-
3 3
2
HRMS spectrum contains a single peak at m/z 1794.5960 odology, with the addition of KI to prevent halide mixing.
corresponding to the desired complex less all 3-chloropyridyl HRMS contained a single monocationic peak at m/z 2022.6935,
ligands and one iodide ion. The preparation of matching the desired complex less one iodide ion and three
1
3
-chloropyridine ligands. The H NMR spectrum was also con-
clusive with two sets of diastereotopic methylenic bridges, com-
prising four doublets resonating at 6.84 and 6.67, and 6.19 and
5
2 2
.94 ppm, and corresponding to N–CH –O and benzyl CH
13
1
respectively. The C{ H} NMR spectrum bore the diagnostic car-
benic shift at 162.69 ppm, 1 ppm further downfield than in C1.
Monomeric guaiacol-tethered complexes [PdI
2
(pyCl)(L3)]
C3 and [PdI (pyCl)(L4)] C4 were prepared using similar meth-
2
odologies (Scheme 2). The 2-methoxyphenoxymethyl was
selected due to it being the same residue as one arene face of
the CTG scaffold. These complexes have similar NMR features
to their trimetallic counterparts, with methoxy, N-methyl (C1
2
and C3), and benzylic CH (C2 and C4) all showing similar
chemical shifts. The aromatic regions for complexes C3 and C4
were less straightforward, primarily due to the tribenzo[a,d,g]
cyclononene core only contributing two singlets to the aromatic
1
region of H NMR spectra for C1 and C2, as opposed to four
Fig. 2 Asymmetric unit of the crystal structure of HL
3
·I.
peaks of more convoluted multiplicity in C3 and C4.
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Two different solvate forms of complex C1 were obtained. prises of both ligands twisted with respect to one another.
Crystals of [C1]·2(C H )·1/2(CH Cl )·3/2(H O) C1_p were Pd–C bond lengths of 1.931(13) to 1.943(13) Å, and Pd–N bond
5
14
2
2
2
grown by diffusion of pentane vapours into a dichloromethane lengths of 2.06(2) to 2.110(12) Å are all similar to previously
3
7–39
solution, while crystals of [C1]
2
·11/2(C
4
H
8
O
2
) C1_d were grown reported PEPPSI-type complexes.
Adjacent C1 complexes
from diffusion of diethyl ether vapours into a 1,4-dioxane solu- in the crystal lattice form an enantiomeric hand-shake motif
tion. The crystal structure of C1_p has one C1 complex in the with the 3-chloropyridyl ligand of one complex behaving as a
asymmetric unit alongside pentane, dichloromethane and guest for a neighbouring bowl and vice versa, with a separation
water solvent positions, some refined at low occupancy or with between cyclononene centroid and 3-chloropyridine 5-C of
disorder. All three benzimidazol-2-ylidene moieties of the tri- 3.94 Å. All three benzo-backbones of NHC ligands π–π stack
nuclear C1 complex have similar rotations, with (Nim–C–O–CAr
)
with neighbouring complexes at ring centroid separations of
torsion angles between −66.9° and −77.2°. This places the 3.68 to 3.70 Å. Imidazol-2-ylidene groups are of opposite orien-
metal complex moieties largely below the upper rim of the tations in the stacking arrangements. These intermolecular
cavitand (Fig. 3). The three Pd(II) centres have the expected interactions echo those seen in the structure of parent com-
near square planar geometry and trans arrangement of I− pound H L1·3(PF ).
3
6
ligands with Pd–I distances 2.5836(12) to 2.5957(15) Å. The
The dioxane solvate C1_d forms a remarkably different
rotations of 3-chloropyridyl ligands with respect to the NHC crystal structure to that of C1_p with the presence of halogen
are different at each metal site on the complex. For two of the bonding interactions and intra-cavity binding of dioxane mole-
Pd(II) centres, the 3-chloropyridyl and NHC groups are near- cules within the hydrophobic cavity of the L1 ligands. The
coplanar, of which one environment contains a 3-chloropyridyl crystal structure was solved in space group P ˉ1 and there are
ligand disordered over two positions, whilst the other site com- two distinct C1 complexes in the asymmetric unit, one of
which shows a disorder of one pyCl ligand similar to that seen
in C1_p, alongside numerous dioxane molecules (Fig. S71‡).
Metal–carbene bond lengths were similar to the pentane
solvate, with Pd–C bond lengths measured at 1.935(16)–
1.974(12) Å. Both C1 complexes have similar orientations of
their metal complex groups with (Nim–C–O–C ) torsion angles
Ar
all geometrically similar, ranging between ca. 174 and 180°, so
that the metal-binding moieties effectively crown the cavity.
Orientation of the PdI (pyCl) groups is such that there is a
2
ring of intramolecular Pd–I⋯Cl–py contacts between adjacent
metal-containing moieties at distances ranging from 3.586 to
3.872 Å (only contacts between ordered positions considered).
These contacts are near linear at the organic halogen with
C–Cl⋯I angles between 169.97 to 176.10°, and bent at iodo
ligand with Pd–I⋯Cl angles between 94.1 and 111.6°. These
contacts are consistent with formation of M–X⋯X′–R halogen
4
0
bonds
around the upper rim of the cavitand complex
(
Fig. 4a). Crystal structures which show Pd–I⋯Cl–arene contacts
<
3.7 Å are rare, and have been previously observed as inter-
3
9,41
2
molecular contacts for two PdI (NHC)(pyCl) complexes.
Each C1 complex contains a molecule of dioxane as an intra-
cavity guest with a hydrophobic (CH fragment directed into
2 2
)
the hydrophobic cavity of the L1 ligand. For each complex the
guest dioxane forms a second sphere interaction with one Pd(II)
centre at Pd⋯O distances 4.48 and 4.52 Å respectively. In the
crystal lattice, distinctive capsule-like arrangements of two
head-to-head (C1∩dioxane) host–guest assemblies form which
sandwich a third molecule of dioxane between them (Fig. 4b).
Each capsule-like arrangement is homochiral and contains one
of each crystallographically distinct C1 complex. There are π–π
stacking interactions between 3-chloropyridyl groups of one
capsule and the NHC benzo-backbone of two capsules adjacent
to it in the crystal lattice at centroid–centroid distances of separ-
ation of ca. 3.67 Å.
5 2 2 2
Fig. 3 From the crystal structure of [C1]·2(C H14)·1/2(CH Cl )·3/2(H O)
C1_p. (a) One C1 complex; (b) two enantiomeric C1 complexes forming
a hand-shake host–guest motif, minor disorder positions not shown in
Single crystals of both mononuclear complexes C3 and C4
were grown from the diffusion of pentane vapours into a di-
(
b) for the sake of clarity.
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Fig. 5 Monometallic complexes from the X-ray crystal structures of (a)
C3; and (b) C4 (see Fig. S76‡ for other conformation).
nificant difference between both molecules is the Carene–O–C–
N
NHC torsion angle, with one at a near antiperiplanar 172.8°,
and the other a more strained −72.0°.
Catalytic studies
To assess the catalytic viability of complexes C1 and C2, a
cross-coupling reaction between 2,4-dibromopyridine and
4-methoxybenzeneboronic acid was examined. The crystal
structure of C1 displays second-sphere coordination of a
dioxane molecule included within the CTV bowl, indicating
that the complex may be capable of molecular recognition,
which may lead to regioselective catalysis. The bromo substitu-
ents in 2,4-dibromopyridine are electronically similar, thus a
statistical mixture of products would be expected in a non-
regioselective reaction. Few examples of regioselectivity in this
reaction have been reported, with most precedent favouring
4
2–44
the formation of the 2-substitued product a.
Rao and co-
workers reported a Suzuki–Miyaura reaction where 4-substi-
tution of 2,4-dibromopyridine (i.e. product b) was the major
4
5
product when using tri-phenylbismuth reagents. Monomeric
complexes C3 and C4 were also assessed in the same reaction,
in addition to 1 : 2 Pd(OAc)
2 3
: PPh as a benchmark, Table 1.
Using 1 : 2 Pd(OAc) /PPh as the catalyst results in no regio-
2
3
selectivity under the conditions tested. All four NHC com-
plexes displayed a low affinity towards monoarylation in the
2
-position, with a detected as a minor product and products b
2 4 8 2
Fig. 4 From the crystal structure of [C1] ·11/2(C H O ) C1_d. (a) One
and c being observed in a roughly 1 : 1 ratio. Although coopera-
tivity-driven catalysis does not appear to be in operation due to
all four complexes producing similar results, such selectivity
for the 4-substituted product b over the 2-substituted product
C1 complex with Pd–I⋯Cl–py distances indicative of halogen bonding
shown; (b) capsule-like arrangement of C1 and guest dioxane (shown in
space filling mode).
4
6
a is rare. Similarly, reaction of 2,5-dibromopyridine with
chloromethane or acetonitrile solution respectively, and their 4-methoxybenzeneboronic acid using complexes C1–C4 as cat-
structures determined by X-ray diffraction (Fig. 5). Complex C3 alysts showed a significant increase in the proportion of 5-sub-
has one molecule in the asymmetric unit. The Pd–C bond stituted product obtained in comparison to the same reaction
length of 1.9568(15) Å was similar to that of complex C1, as with 1 : 2 Pd(OAc) /PPh as the catalyst (see ESI, Table S2‡).
2
3
were the C–Pd–N and I–Pd–I bond angles at 173.37(6)° and Complexes C1 to C4 gave relatively low conversions for the
1
72.219(6)°. The (NNHC-C–O-CAr) torsion angle is −169.8°. simpler Suzuki–Miyaura cross-coupling reaction between
There are two molecules in the asymmetric unit of C4, both 4-bromotoluene and benzeneboronic acid (ESI, Table S3‡).
with similar bond lengths and angles about the Pd centres, Work is in progress to further examine such Pd complexes in
namely Pd-C bond lengths of 1.957(7) and 1.969(7) Å, and regioselective cross-coupling reactions, and determine the
C-Pd-N bond angles of 179.6(3)° and 176.7(3)°. The most sig- possible role of the ligand substituents and Pd speciation in
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Table 1 Conversions and product ratios for reaction of 2,4-dibromo- solid. Yield 117 mg, 0.09 mmol, 74%. HRMS (ESI+) 485.1450
pyridine and 4-methoxybenzeneboronic acid
2+
+
{
{
C H IN O }
1097.1956 {C H I N O } , 1242.1345
51 51 2 6 6
5
1
51
6
6
+
C
51
H
55
1
I
3
N
7
O
6
} calcd 485.1452, 1097.1954, 1242.1342 respect-
ively. H NMR (300 MHz, DMSO-d
6
) δ (ppm) 9.86 (s, 3H,
CHimine), 8.04 (d, 3H, J = 8.3 Hz, Ar), 7.84 (d, 3H, J = 8.3 Hz,
Ar), 7.69 (t, 3H, J = 7.4 Hz, Ar), 7.58 (t, 3H, J = 7.5 Hz, Ar), 7.33
(
2
s, 3H, ArCTV), 7.08 (s, 3H, ArCTV), 6.37 (s, 6H, O–CH –N), 4.73
(
d, 3H, J = 13.5 Hz, ax-CH ), 4.10 (s, 9H, N–CH ), 3.56 (s, 9H,
2
3
1
1
3
O–CH
3
2
), 3.55 (d, 3H, J = 13.5 Hz, eq-CH ). C{ H} (75 MHz,
6
DMSO-d ) δ 148.95, 143.24, 142.90, 136.43, 132.19, 131.78,
1
5
30.43, 126.96, 126.81, 120.59, 114.49, 113.79, 113.64, 75.92,
6.10, 34.79, 33.56 (full NMR assignments for this and all
−
1
0
.25 mol% Pd
0.5 mol% Pd
other compounds are given in ESI‡). FTIR ν (cm ) 2026, 1968,
1509, 1269, 756 (all very weak signals). M.p. 192–194 °C (dec.).
a
a
Conv.
(%)
Ratio
a : b
c
(%)
Conv.
(%)
Ratio
a : b
c
(%)
51 3 6 6 2
Elemental analysis for C51H I N O ·2(H O) calcd C 48.59;
H 4.40; N 6.67. Found C 48.11; H 4.05; N 7.07.
Catalyst
: 2
1
—
—
79
25 : 33
21
Pd(OAc)
C1
C2
C3
2 3
, PPh
Synthesis of tris-methyl(methylbenzimidazol-1-yl)
cyclotriguaiacylenyl hexafluorophosphate, H L1·3(PF )
44
41
39
47
2 : 19
2 : 17
2 : 16
3 : 20
23
22
21
24
68
60
58
54
3 : 32
4 : 27
3 : 27
4 : 26
33
29
28
24
3
6
H L1·3I (50 mg, 0.04 mmol) and ammonium hexafluoro-
3
C4
phosphate (20 mg, 0.12 mmol) were dissolved in MeCN (1 mL)
and MeOH (5 mL). The pale yellow solution was stirred for 1 h,
during which a white solid precipitated, which was collected
via vacuum filtration, washed with methanol (5 mL) and
diethyl ether (3 × 5 mL), affording the title compound as a
white solid. Yield 43 mg, 0.033 mmol, 83%. HRMS (ESI+)
a
Conversions are an average of two experiments.
affecting the overall regioselectivity switches observed in this
study.
3
+
2+
2
81.1282 {C51
H
51
N
6
O
6
} , 349.1499 {C42
H
F
42
N
4
O
6
} , 494.2698
2
+
+
{
C
51
H
51
F
6
N
6
O
6
P} , 1133.3018 {C51
H
51
12
N
6
O
6
P
2
}
calcd
1
2
81.1285, 349.1547, 494.1751, 1133.3149 respectively. H NMR
) δ (ppm) 9.86 (s, 3H, CH), 8.04 (d, 3H, J =
.3 Hz, Ar), 7.84 (d, 3H, J = 8.3 Hz, Ar), 7.69 (t, 3H, J = 7.4 Hz,
Conclusions
(
300 MHz, DMSO-d
6
Four new ligands and Pd(II)–NHC complexes have been pre-
pared and characterised, including the first two metal–NHC
complexes affixed onto a cyclotriguaiacylene scaffold. The tri-
metallic complex C1 displayed solvent-dependent host–guest
interactions in clathrate materials, forming either self-comp-
lementary hand-shake associations or intra-cavity binding of
8
Ar), 7.58 (t, 3H, J = 7.5 Hz, Ar), 7.33 (s, 3H, ArCTV), 7.08 (s, 3H,
ArCTV), 6.37 (s, 6H, O–CH –N), 4.73 (d, 3H, J = 13.5 Hz, ax-
CH ), 4.10 (s, 9H, NMe), 3.56 (s, 9H, OMe), 3.55 (d, 3H, J = 13.5
Hz, eq-CH2). C{ H} (101 MHz, DMSO-d ) δ (ppm) 149.00,
2
2
1
3
1
6
1
1
{
43.28, 142.97, 136.43, 132.18, 131.80, 130.45, 126.96, 126.82,
1
9
1
,4-dioxane with second-sphere interactions to Pd. Catalytic
20.62, 114.45, 113.76, 113.62, 75.91, 55.96, 34.86, 33.47.
F
1
activities of the complexes in the Suzuki–Miyaura of 2,4-dibro-
mopyridine and 4-methoxybenzeneboronic acid were assessed.
Tenfold selectivities towards para- and bis-coupling were dis-
played, with C1–C4 all giving similar activities and selectivites,
suggesting that the CTV bowl behaves as a bulky ligand for
three catalytically independent sites, rather than facilitating
cooperativity.
H} NMR (376 MHz, DMSO-d ) δ (ppm) −70.15 (d, J = 711.8
6
F–P
3
1
1
Hz). P{ H} NMR (162 MHz, DMSO-d
6
) δ (ppm) −144.19
1
(
hept, JP–F = 711.5 Hz). FTIR ν (cm− ) 834 (P–F), 557; elemental
analysis for C H F N O P calcd C 47.90; H 4.02; N 6.57.
5
1
51 18 6 6 3
Found: C 47.77; H 3.93; N 6.64.
Synthesis of tris-benzyl(methylbenzimidazol-1-yl)
cyclotriguaiacylenyl iodide, H L2·3I
3
(
0
±)-Tris-(1-methylbenzimidazolyl)cyclotriguaiacylene (100 mg,
.12 mmol) was dissolved in acetonitrile (5 mL) and benzyl
Experimental
Synthesis of tris-methyl(methylbenzimidazol-1-yl)
bromide (0.06 mL, 0.54 mmol) was added. The colourless solu-
tion was heated to 60 °C for 18 h, during which time a white
solid precipitated from solution. The solid was filtered,
3
cyclotriguaiacylenyl iodide, H L1·3I
(±)-Tris-(1-methylbenzimidazolyl)cyclotriguaiacylene (102 mg, washed with diethyl ether (20 mL) and dried in air, affording
0
.13 mmol) was dissolved in MeCN (5 mL) in a J. Youngs L2·3I as a white solid. Yield 129 mg, 0.22 mmol, 82%. HRMS
3+
2+
ampoule and methyl iodide (0.1 mL, 1.7 mmol) added. The (ESI+) 357.1624 {C H N O } , 490.2127 {C H N O }
6
9
63
6
6
62 56 6 6
1
ampoule was sealed and heated to 80 °C for 18 h and the solu- calcd 357.1598, 490.2125 respectively. H NMR (300 MHz,
tion turned orange. Diethyl ether (40 mL) was added dropwise DMSO-d ) δ (ppm) 10.18 (s, 3H, CH), 8.00 (d, 3H, J = 8.1 Hz,
and the title compound precipitated from solution as a cream Arim), 7.91 (d, 3H, J = 8.1 Hz, Arim), 7.61 (m, 6H, Arim), 7.40 (bs,
6
14692 | Dalton Trans., 2019, 48, 14687–14695
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Paper
1
3
1
6
H, Arbz, ArCTV), 7.09 (s, 3H, ArCTV), 6.43 (m, 6H, O–CH
2 2 2
–N), CH ), 5.80 (s, 2H, O–CH –N), 3.54 (s, 3H, OMe). C{ H} NMR
5
.81 (s, 6H, N–CH –C), 4.74 (d, 3H, J = 13.6 Hz, ax-CH ), 3.54 (101 MHz, DMSO-d ) δ (ppm) 150.84, 143.67, 143.08, 133.66,
2
2
6
1
3
1
(d, 3H, J = 13.4 Hz, eq-CH
2
), 3.45 (s, 9H, OMe). C{ H} NMR 130.77, 129.02, 128.80, 128.17, 127.25, 127.06, 125.47, 120.89,
(
1
1
75 MHz, DMSO-d ) 149.17, 142.98, 142.51, 136.71, 133.68, 120.02, 114.14, 114.04, 113.06, 76.06, 55.42, 49.95. FTIR
6
32.25, 130.76, 130.72, 129.03, 128.83, 128.17, 127.09, 127.02, ν (cm− ) 2900, 1505, 1252, 1206, 745. Elemental analysis for
1
21.18, 114.57, 114.11, 114.00, 76.24, 56.02, 49.92, 34.80. FTIR 21BrN calcd C 62.13; H, 4.98; N, 6.59. Found C 61.96;
C
22
H
2 2
O
−
1
ν (cm ) 3432, 2936, 1511, 1269, 766. Elemental analysis for H 4.88; N 6.67.
C H Br N O (HBr) calcd C 59.89; H 4.05; N 6.07. Found: C
6
9
63
3 6 6
Synthesis of (±)-tris-(3-chloropyridyl)(methyl
cyclotriguaiacylenyl)methylbenzimidazol-2-ylidene)
5
9.50; H 4.38; N 6.25.
(
Synthesis of 1-(2-methoxyphenyoxy)methyl-3-
methylbenzimidazolium iodide, HL3·I
palladium(II) iodide, C1
Palladium(II) iodide (100 mg, 0.28 mmol), potassium carbon-
Sodium hydride (60% dispersion, 800 mg, 19.48 mmol) was ate (192 mg, 1.4 mmol) and H
added to anhydrous, degassed N,N′-dimethylformamide dissolved in 3-chloropyridine (3 mL) and the reaction mixture
5 mL) under an N2 atmosphere. Guaiacol (400 mg, 0.36 mL, degassed. The orange suspension was stirred at 80 °C under
.20 mmol) was added dropwise over a period of 20 min, and an N atmosphere for 24 h. Water (10 mL) and CH Cl (10 mL)
solution stirred at for 1 h. 1-(Chloromethyl)benzimidazole were added. The organic layer was separated, and washed with
hydrochloride (786 mg, 3.2 mmol) was added and the solution water (3 × 5 mL) before being dried over MgSO . Solvents were
3
L1·3I (113 mg, 0.09 mmol) were
(
3
2
2
2
4
stirred for 16 h. The reaction was quenched with water and the removed in vacuo, and the residue triturated in pentane
product extracted into dichloromethane (3 × 200 mL). The (5 mL). The resulting solid was filtered, affording C1 as an
organic layers were combined, washed with brine, and dried orange solid. Yield 171 mg, 0.08 mmol, 84%. HRMS (ESI+)
+
4
48 5 6 6 3
over MgSO . Solvents were removed in vacuo, and the residue 1794.5960 calcd for {M − I − 3(3-Clpy): C51H I N O Pd }
1
triturated in petroleum ether (60–80), giving 2-(methoxyphe- 1794.5962. H NMR (300 MHz, CDCl ) δ (ppm) 9.05 (d, 3H, J =
noxymethyl)benzimidazole as a white solid (441 mg, 58%). 2.2 Hz, Arpy), 8.95 (dd, 3H, J = 5.4, 1.1 Hz, Arpy), 7.77–7.72 (m,
-(Methoxyphenoxymethyl)benzimidazole (50 mg, 0.19 mmol) 3H, Arpy), 7.34–7.27 (m, 6H, Arpy, Arim), 7.20 (s, 3H, ArCTV), 7.10
3
4
7
2
and acetonitrile (2 mL) were added to a J. Youngs ampoule. (t, 3H, J = 7.7 Hz, Arim), 6.98 (d, 3H, J = 8.2 Hz, Arim), 6.79 (s,
Methyl iodide (0.12 mL, 1.9 mmol) was added, and the 3H, ArCTV), 6.73 (t, 3H, J = 7.8 Hz, Arim), 6.61 (d, 3H, J =
2 2
ampoule sealed and the pale yellow solution heated to 80 °C 10.2 Hz, O–CH –N), 6.40 (d, 3H, J = 10.2 Hz, O–CH –N), 4.44
for 16 h. Diethyl ether (20 mL) was added and a white solid (d, 3H, J = 13.8 Hz, ax-CH ), 4.23 (s, 9H, NMe), 3.83 (s, 9H,
2
1
3
1
precipitated from solution. The powder was filtered and OMe), 3.41 (d, 3H, J = 13.8 Hz, eq-CH
washed with diethyl ether (5 mL) to give L3·I as a white powder. (75 MHz, CDCl ) δ (ppm) 161.69, 152.88, 151.92, 149.97,
Yield 60 mg, 0.15 mmol, 84%. HRMS (ESI+) 269.1246 143.99, 138.10, 136.55, 134.96, 134.68, 132.54, 132.07,
2
). C{ H} NMR
3
+
1
{
C H N O
16 17 2 2
} calcd 269.1285. H NMR (300 MHz, DMSO-d
δ (ppm) 9.87 (s, 1H, CH), 8.11–8.01 (m, 1H, Arim), 8.00–7.92 (m, 57.07, 36.36, 35.76. FTIR ν (cm− ) 1506, 1263, 1203, 795,
H, Arim), 7.77–7.67 (m, 2H, Arim), 7.16–7.00 (m, 3H, Ar), 6.88 743. M.p. 165–167 °C (dec.). Elemental analysis for
ddd, 1H, J = 8.3, 7.2, 1.7 Hz, Ar), 6.45 (s, 2H, CH ), 4.12 (s, 3H, Pd calcd C 35.04; H 2.67; N 5.57. Found C
NMe), 3.65 (s, 3H, OMe). C{ H} NMR (75 MHz, DMSO-d 35.15; H 2.80; N 5.30.
δ (ppm) 150.60, 144.13, 143.36, 131.82, 130.48, 127.10, 126.83,
6
)
125.04, 123.54, 123.41, 123.36, 114.19, 111.15, 109.78, 79.20,
1
1
(
2
C
66
H
60Cl
3
I
6
N
9
O
6
3
13
1
6
)
Synthesis of (±)-tris-(3-chloropyridyl)(benzyl
(cyclotriguaiacylenyl)methylbenzimidazol-2-ylidene)
palladium(II) iodide, C2
1
3
25.17, 120.84, 119.36, 113.82, 113.72, 112.98, 75.66, 55.53,
3.57. FTIR ν (cm−1) 1491, 1253, 1124, 1054, 1015, 745.
M.p. 145–147 °C. Elemental analysis for C H IN O calcd C
16
17
2 2
4
8.50; H 4.30; N 7.07. Found C 48.59; H 4.26; N 7.14.
Palladium(II) iodide (41 mg, 0.11 mmol), potassium carbonate
(
79 mg, 0.57 mmol), potassium iodide (95 mg, 0.57 mmol)
Synthesis of 1-(2-methoxyphenyoxy)benzyl-3-
methylbenzimidazolium bromide, HL4·Br
and H L2·3I (50 mg, 0.04 mmol) were dissolved in 3-chloro-
pyridine (3 mL) and the reaction mixture degassed. The orange
3
2
-(Methoxyphenoxymethyl)benzimidazole (200 mg, 0.78 mmol) suspension was stirred at 80 °C under an N
2
atmosphere for
was dissolved in acetonitrile (10 mL) and benzyl bromide 24 h. Water (10 mL) and CH Cl (10 mL) were added. The
2
2
(
0.92 mL, 7.8 mmol) added. The colourless solution was organic layer was separated, and washed with water (3 × 5 mL)
heated to 60 °C overnight, during which a white solid precipi- before being dried over MgSO . Solvents were removed
tated. The precipitate was filtered, washed with diethyl ether in vacuo, and the residue triturated in pentane (5 mL). The
20 mL) and dried, affording L4·Br as a white solid. Yield resulting solid was filtered, affording C2 as a yellow-orange
93 mg, 0.68 mmol, 88%. HRMS (ESI+) 345.1611 solid. Yield 70 mg, 0.028 mmol, (70%). HRMS (ESI+)
4
(
2
{
(
2
7
+
1
+
1
C H N O } calcd 345.1598. H NMR (300 MHz, DMSO-d ) δ 2022.6935 {C H I N O Pd } calcd 2022.6951.
H
NMR
) δ (ppm) 9.01, (d, 3H, J = 2.3 Hz, Arpy), 8.89
H, Arim), 7.39 (s, 5H, Arbz), 7.12 (t, 1H, J = 7.0 Hz, Ar), (dd, 3H, J = 5.5, 1.4 Hz, Arpy), 7.70 (tdd, 3H, J = 8.5, 2.4, 1.3 Hz,
2
2
21
2
2
6
69 60 5
6
6
3
ppm) 10.04 (s, 1H, CH), 8.06–7.95 (m, 2H, Arim), 7.77–7.60 (m, (300 MHz, CDCl
3
.06–6.94 (m, 2H, Ar), 6.86 (t, 1H, J = 7.0 Hz, Ar), 6.47 (s, 2H, Ar ), 7.63–7.57 (m, 6H, Ar ), 7.42–7.28 (m, 18H, Ar), 7.21 (d,
py
bz
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Dalton Transactions
6
H, J = 8.2 Hz, Arim), 6.94 (d, 3H, J = 6.3 Hz, Arim), 6.91 (s, 3H, 6.87 (td, 1H, J = 7.7, 1.6 Hz, Ar), 6.72 (s, 2H, O–CH
2
–N), 6.03 (s,
1
3
1
ArCTV), 6.84 (ddd, 3H, J = 8.3, 6.1, 2.3 Hz, Arim), 6.67 (d, 3H, J = 2H, CH2bz), 3.96 (s, 3H, OMe). C{ H} NMR (75 MHz, CDCl )
3
1
3
3
1
1
1
1
2 2
0.0 Hz, O–CH –N), 6.52 (d, 3H, J = 9.9 Hz, O–CH –N), 6.19 (d, δ (ppm) 162.57, 152.95, 151.96, 151.54, 145.39, 137.98, 135.26,
H, J = 15.7 Hz, CH2bz), 5.94 (d, 3H, J = 15.8 Hz, CH2bz) 4.53 (d, 134.86, 134.42, 132.51, 128.98, 128.42, 128.39, 125.07, 124.90,
H J = 13.8 Hz, ax-CH ), 3.84 (s, 9H, OMe), 3.49 (d, 3H, J = 123.69, 123.60, 121.55, 121.46, 112.92, 112.05, 111.72, 79.99,
2
1
3
1
−1
3.5 Hz, eq-CH
2
). C{ H} NMR (75 MHz, CDCl
62.69, 152.90, 151.94, 150.17, 144.07, 138.06, 136.60, 135.24, Elemental analysis for C27
3
) δ (ppm) 56.13, 55.01; FTIR ν (cm ) 1504, 1252, 1212, 1026, 745.
24ClI Pd calcd C 39.64; H
H
2 3 2
N O
34.47, 134.41, 132.52, 132.15, 129.00, 128.49, 125.01, 124.47, 2.96; N 5.14. Found C 39.55; H 2.89; N 5.07.
23.51, 114.45, 111.50, 111.46, 79.64, 57.12, 54.90, 35.97. FTIR
−
1
ν (cm ) 1414, 1356, 1266, 1028, 742. Elemental analysis for General protocol for catalytic procedure
C H Cl I N O Pd calcd C 40.51; H 2.91; N 5.06. Found C
8
4
72 3 6
9
6
3
1,4-Dioxane (1.25 mL) and water (1.25 mL) were added to a
Schlenk flask and thoroughly degassed. 2,4-Dibromopyridine
4
0.55; H 2.86; N 5.04.
(
1
236.9 mg, 1 mmol), 4-methoxybenzeneboronic acid (182 mg,
.2 mmol), caesium carbonate (650 mg, 2 mmol), 1,4-
Synthesis of trans-(3-chloropyridyl)-1-(2-methoxyphenyoxy)
methyl-3-methylbenzimidazol-2-ylidene palladium(II) iodide, C3
dimethoxybenzene (69 mg, 0.5 mmol) and catalyst were added,
and the reaction mixture heated to 80 °C for 2 h. A 0.1 mL
aliquot of reaction mixture was added to 3 mL of CH Cl and
passed through a Celite plug, dried over MgSO , the solvent
4
removed in vacuo and the residue was added to an NMR tube
HL3·I (40 mg, 0.12 mmol), palladium(II) iodide (44.2 mg,
0
.12 mmol) and potassium carbonate (84.7 mg, 0.6 mmol)
were dissolved in anhydrous, degassed 3-chloropyridine
2 mL). The reaction mixture was heated to 80 °C under an N
atmosphere for 24 h and turned dark brown. Water (20 mL)
was added and the product was extracted into CH Cl (3 ×
0 mL). The organic layer was washed with water (3 × 20 mL)
2
2
(
2
containing 0.4 mL CDCl . The conversions were calculated by
3
1
2
2
H NMR spectroscopy by comparing methyl integrals of 1,4-
2
dimethoxybenzene (3.77 ppm) and that of 4-methylbiphenyl
(2.41 ppm), while 6-pyridyl proton resonances were examined
for products a (8.43 ppm), b (8.35 ppm) and/or c (8.67 ppm)
due to overlap of their methoxy proton resonances at
3.85 ppm.
and dried over MgSO , and then removed in vacuo. The orange
4
residue was triturated in hexane, giving C3 as an orange solid.
Yield 44.3 mg, 0.05 mmol, 50%. HRMS (ESI+) 500.9284
+
+
{
5
C H IN O Pd} , 1130.7631 {C H I N O Pd }
calcd
1
6
16
2
2
32 32 3
4
4
2
1
00.9286, 1130.7626 respectively. H NMR (300 MHz, CDCl
3
)
(
ppm) 9.07 (d, 1H, J = 2.3 Hz, Arpy), 8.97 (dd, 1H, J = 5.4,
1
1
.4 Hz, Ar ), 7.76 (dt, 1H, J = 8.2, 1.8 Hz, Ar ), 7.64–7.54 (m,
py py
H, Arim), 7.41–7.19 (m, 5H, Ar), 7.07 (td, 1H, J = 7.8, 1.7 Hz,
Conflicts of interest
Ar), 6.97 (dd, 1H, J = 8.2, 1.6 Hz, Ar), 6.82 (td, 1H, J = 7.6, 1.6
There are no conflicts to declare.
Hz, Ar), 6.65 (s, 2H, O–CH –N), 4.20 (s, 3H, NMe), 3.96 (s, 3H,
2
1
3
1
OMe). C{ H} NMR (126 MHz, CDCl
3
) δ (ppm) 161.36, 152.72,
1
1
7
1
51.72, 151.29, 145.11, 137.76, 135.28, 134.52, 132.32, 124.83,
Acknowledgements
24.77, 124.69, 123.43, 121.40, 121.18, 112.69, 111.70, 109.75,
9.38, 55.88, 36.21. FTIR ν (cm−1) 2160, 1500, 1461, 1387, This work was supported by Diamond Light Source
201, 747. M.p. 145 °C (dec.). Elemental analysis for (MT-15059) and EPSRC (DTG studentships for JMF and EB),
C H ClI N O Pd calcd. C 33.99; H 2.72; N 5.66. Found C equipment grant EP/K039202/1.
2
1
20
2 3 2
3
3.87; H 2.64; N 5.62.
Synthesis of trans-(3-chloropyridyl)-1-(2-methoxyphenyoxy)
methyl-3-benzyllbenzimidazol-2-ylidene palladium(II) iodide, C4
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1
7
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14694 | Dalton Trans., 2019, 48, 14687–14695
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Dalton Transactions
Paper
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