Diamine bis(phenolate) and pendant amine bis(phenolate) ligands: Catalytic activity for the room temperature palladium-catalyzed Suzuki-Miyaura coupling reaction

Article abstract of DOI:10.1002/aoc.3396

A series of amine bis(phenolate) ligands bearing aryl substituents of varying steric bulk are reported and characterized using single-crystal X-ray diffraction, NMR spectroscopy and high-resolution mass spectrometry experiments. Palladium complexes derived in situ from these ligands are evaluated as catalysts for the Suzuki-Miyaura coupling of phenylboronic acid and aryl bromides. High conversions are observed for these reactions in methanol solvent at low catalyst loadings (0.01 mol%), short reaction times (30 min) and mild temperatures (30°C). Conversion is observed for a range of substrates, and is found to depend on the nature of the external base and solvent employed. These findings demonstrate the utility of catalysts derived from late transition metal complexes of amine bis(phenolate) ligands, particularly those bearing bulky cumyl substituents.

Full text of DOI:10.1002/aoc.3396

Full paper
Received: 22 July 2015
Revised: 2 September 2015
Accepted: 28 September 2015
Published online in Wiley Online Library: 6 November 2015
(wileyonlinelibrary.com) DOI 10.1002/aoc.3396
Diamine bis(phenolate) and pendant amine bis
(phenolate) ligands: catalytic activity for the
room temperature palladium-catalyzed Suzuki–
Miyaura coupling reaction
Andrew K. Bowsera, Amelia M. Anderson-Wilea, Dean H. Johnstonb
and Bradley M. Wilea*
A series of amine bis(phenolate) ligands bearing aryl substituents of varying steric bulk are reported and characterized using
single-crystal X-ray diffraction, NMR spectroscopy and high-resolution mass spectrometry experiments. Palladium complexes de-
rived in situ from these ligands are evaluated as catalysts for the Suzuki–Miyaura coupling of phenylboronic acid and aryl bro-
mides. High conversions are observed for these reactions in methanol solvent at low catalyst loadings (0.01 mol%), short
reaction times (30min) and mild temperatures (30°C). Conversion is observed for a range of substrates, and is found to depend
on the nature of the external base and solvent employed. These findings demonstrate the utility of catalysts derived from late
transition metal complexes of amine bis(phenolate) ligands, particularly those bearing bulky cumyl substituents. Copyright ©
2015 John Wiley & Sons, Ltd.
Additional supporting information may be found in the online version of this article at the publisher’s web site.
Keywords: palladium; cross-coupling reactions; Mannich; Suzuki; amine bis(phenolate)
ligands have been reported,[31–50] with most employed as support
for group 4 metal complexes in the polymerization of 1-hexene or
Introduction
The Suzuki coupling of aryl halides and arylboronic acids/esters has
become an increasingly important catalytic transformation,
allowing the facile preparation of species containing biaryl or
substituted aromatic systems.[1–7] Palladium complexes have long
been employed as the pre-catalyst of choice, in part due to their in-
creased oxygen tolerance and robust reactivity when compared to
other late transition metals.[5–7] Many of the palladium complexes
known to serve as efficient catalysts to date have employed
mono- or bidentate phosphine ligands, sometimes employing a
mixed-donor set. In general, increasing the steric bulk about the
palladium center has led to improved catalyst performance for
these phosphine complexes.[8–12] However, these complexes may
be oxidized under aerobic conditions to the corresponding phos-
phine oxides, leading to decomposition and hindering catalyst
performance. In addition, the cost of these phosphine ligands
may be significant, especially for chiral variants.
cyclic ethers. Many of these reports have focused on phenolate sub-
stituents that are sterically unencumbered and electron withdraw-
ing, as these features have been shown to promote exceptionally
active or long-lived 1-hexene polymerization catalysts.[35,50–52]
Fewer studies have explored the utility of late transition metal
complexes bearing bulky amine bis(phenolate) ligands. Kol and
co-workers have employed bipyrrolidine bis(phenolate) complexes
as a scaffold for helically chiral zinc complexes,[53] while Periana and
co-workers have reported that rhodium complexes of pendant
amine bis(phenolate) ligands are active for the C–H activation of
benzene.[54] Bimetallic complexes of cobalt[38,55,56] and nickel[57] li-
gated by both diamine bis(phenolate) and pendant amine bis(phe-
nolate) ligands have exhibited interesting magnetic properties. Iron
complexes of both diamine bis(phenolate) and pendant amine bis
(phenolate) ligands serve as interesting model compounds for a va-
riety of iron-oxo species implicated in non-heme metalloenzymes
such as catechol dioxygenases, as well as exhibiting intriguing reac-
tivity with molecular oxygen.[46,58–62] Recent reports from Kozak
In an effort to circumvent these problems, the development of
new ligand architectures bearing more oxygen-tolerant functional-
ity is desirable. The inclusion of a mixed-donor set is also targeted,
as it may afford longer-lived complexes, resulting in more produc-
tive catalysis or unexpected reactivity.[13–23] A variety of nitrogen-
and oxygen-based ligands have shown exceptional activity for the
Suzuki coupling reaction, including amines,[3,24] imines,[25–28]
*
Correspondence to: Bradley M. Wile, Department of Chemistry and Biochemistry,
Ohio Northern University, 525 South Main Street, Ada, OH 45810, USA. E-mail:
b-wile@onu.edu
substituted bipyridines[29] and amino alcohols.[30] Group
4
a Department of Chemistry and Biochemistry, Ohio Northern University, 525 South
Main Street, Ada, OH, 45810, USA
isopropoxide complexes of diamine bis(phenolate) or pendant
amine bis(phenolate) ligands (Fig. 1) have exhibited excellent air
and moisture sensitivity, and the ligand framework incorporates
both amine and phenolate donors. A number of variants of these
b Department of Chemistry, Otterbein University, 1 South Grove Street, Westerville,
OH, 43081, USA
Appl. Organometal. Chem. 2016, 30, 32–39
Copyright © 2015 John Wiley & Sons, Ltd.

Amine bis(phenolate) Pd complexes as catalysts for Suzuki coupling
Figure 1. (a) Diamine bis(phenolate) and (b) pendant amine bis(phenolate)
ligand architectures reported in this study (R = CH3, C(CH3)3, C(CH3)2Ph;
n = 1, 2).
and co-workers have described the Kumada coupling of alkyl ha-
lides with aryl grignards mediated by iron complexes bearing a va-
riety of amine bis(phenolate) ligands.[36,37,39–43,63]
We have only identified five reports of palladium complexes of
amine bis(phenolate) ligands to date, all of which employ diamine
bis(phenolates) (Fig. 1(a)) rather than pendant amines (Fig. 1(b)).
Balakrishna and co-workers reported the preparation of a pyrazine
bis(phenolate) ligand (Fig. 2(a)) and corresponding activity of in situ
generated palladium complexes for the Suzuki–Miyaura and
Mizoroki–Heck cross-coupling reactions.[44,64] Related work by
Zhou and co-workers describes the catalytic efficiency of a system
derived from cyclic diamine bis(phenolates) (Fig. 2(b)) with compa-
rable findings.[65] Lü and co-workers have reported the formation of
a distorted square-planar palladium complex featuring a ligand de-
rived from o-phenylenediamine and 2-methoxyphenol (Fig. 2(c)),
and accompanying activity as a styrene polymerization catalyst.[66]
Chaudhary and Bedekar have reported a series of bidentate ligands
derived from a 1-(α-aminobenzyl)-2-naphthol framework, as well as
the related piperazine bis(naphtholate) ligand (Fig. 2(d)).[67] Palla-
dium complexes of these ligands have been employed as catalysts
for a multicomponent Wittig–Suzuki reaction to generate styryl bi-
phenyl compounds. All five of these reports focus on ligands with a
diamine core, rather than a pendant amine; this may be due to the
structural similarities between diamine bis(phenolate) ligands and
the ubiquitous salen ligands.
Scheme 1. Synthesis of diamine bis(phenolate) (1–3) and pendant amine
bis(phenolate) (4–9) ligands used in this study. 1: R = CH3; 2: R = C(CH3)3; 3:
R = C(CH3)2Ph; 4: R = CH3, n = 1; 5: R = C(CH3)3, n = 1; 6: R = C(CH3)2Ph, n = 1;
7: R = CH3, n = 2; 8: R = C(CH3)3, n = 2; 9: R = C(CH3)2Ph, n = 2.
of different metal centers. The first reports utilized the ligands to
produce model molybdenum,[71] copper[72,73] and iron[74] com-
plexes for the enzymes molybdenum hydroxylase, galactose oxi-
dase and catechol 1,2-dioxygenase, respectively. Early work on
this ligand class also included reports of copper,[75] tungsten,[47]
molybdenum[47] and titanium[31] complexes. Particular success
was seen with the group 4 metal complexes used in the polymeri-
zation of olefins and cyclic esters. We have recently reported that
these amine bis(phenols) and their corresponding titanium com-
plexes may be easily prepared in high yield and purity by under-
graduate students in the teaching laboratory.[76]
As part of our interest in expanding the range of available
phosphine-free, low-cost and robust ligand frameworks for the
palladium-catalyzed Suzuki–Miyaura coupling reaction, we sought
to prepare new variants of these ligands. We report herein the syn-
thesis and characterization of several new amine bis(phenolate) li-
gands bearing bulky cumyl substituents obtained via a Mannich-
type coupling reaction in poly(ethylene glycol) (PEG) solvent. The
in situ activity of the palladium complexes of these ligands was
assessed using the coupling of 4′-bromoacetophenone and
phenylboronic acid as a model reaction.
The ligands used in this study are obtained from a double
Mannich reaction (Scheme 1) utilizing two equivalents of a
substituted phenol, two equivalents of formaldehyde and a single
equivalent of a primary amine or a secondary diamine. The synthe-
sis of amine bis(phenols) was originally reported by Weatherbee
and co-workers in 1952,[68–70] although it was not until much later
that amine bis(phenolates) were employed as ligands for a variety
Experimental
General considerations
Palladium precursors were purchased from Strem Chemicals Inc.,
2,4-di-tert-butylphenol and 2,4-dimethylphenol were purchased
from Alfa Aesar, 1-bromo-4-nitrobenzene and potassium carbonate
were purchased from Acros Organics, and all other reagents were
purchased from Sigma-Aldrich Ltd. All reagents were of high purity,
and were used without further purification unless otherwise noted.
Ligands were prepared using reported literature procedures.[76,77]
1
All products were characterized using H NMR and 13C{1H} NMR,
1
with peak assignments assisted by the use of DEPT-135, H–13C
HSQC and 1H–13C HMBC experiments. NMR spectra were recorded
with a Bruker Avance 400 spectrometer using a BBO probe operat-
ing at 400.1325 MHz (1H) or 100.6208 MHz (13C). Chemical shifts are
reported in ppm downfield of SiMe4, and referenced using the
Figure 2. Previously reported amine bis(phenolate) ligands to support
palladium.
Appl. Organometal. Chem. 2016, 30, 32–39
Copyright © 2015 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/aoc

A. K. Bowser et al.
solvent residual peak. Fourier transform infrared (FT-IR) spectra
were recorded with a Thermo Nicolet 6700 FT-IR spectrometer as
KBr pellets. GC-MS was conducted using a Varian 3900 GC/Saturn
2100 T MS with a fused silica capillary (Phenomenex Zebron ZB-
5MSi, 30 m × 0.25 mm× 0.25μm). Accurate mass determination
was conducted by the Ohio State University Mass Spectrometry
and Proteomics Facility in Columbus, OH using a Bruker Maxis
electrospray ionization quadrupole TOF mass spectrometer.
Elemental analyses were performed by Midwest Microlab LLC,
Indianapolis, IN.
effective distance of 0.84(0.02) Å to best fit the data at 200 K. ORTEP
diagrams were generated using CrystalMaker®, a crystal and molec-
ular structures modeling program for Mac and Windows
(CrystalMaker Software Ltd, Oxford, UK, www.crystalmaker.com).
The structural reports described in this work have been deposited
at the Cambridge Crystallographic Data Centre (numbers CCDC
1408668–1408670) and may be obtained at http://www.ccdc.cam.
ac.uk/.
Results and discussion
Ligand syntheses
Characterization of ligands
Amine (6mmol), phenol (12 mmol), 37 wt% aqueous formaldehyde
(12 mmol) and 2.0g of PEG-400 were combined in a 20 ml scintilla-
tion vial with a Teflon cap and stirred at 75°C for 24 h in a heated
block reactor. The mixture was cooled to room temperature, 2 ml
of CH2Cl2 was added to the vial to loosen the solid, the mixture
was filtered and washed with methanol. The solid was ground with
a mortar and pestle, dissolved in a minimal amount of hot CH2Cl2,
precipitated in 200 ml of rapidly stirring methanol and filtered. Li-
gands requiring further purification were recrystallized from ethyl
acetate. Full experimental details for the preparation and character-
ization of 3, 6, 7 and 9 are provided in the supporting information.
Ligands 1–9 were prepared in moderate to good yield (Scheme 1)
using a procedure modified from that reported by Kerton and co-
workers.[77] The use of PEG-400 as a solvent allowed for slightly
higher reaction temperatures and reduced solvent volume when
compared to literature procedures employing alcohols or water as
the solvent. As an additional benefit, a large number of ligand var-
iants can be prepared simultaneously using an aluminium reactor
block without the need for a typical reflux condenser setup. All li-
gands were purified by precipitation from a dichloromethane–
methanol mixture, and recrystallized from ethyl acetate to yield a
colorless crystalline solid.
Ligands 3, 6, 7 and 9 are reported here for the first time, and ex-
hibit spectral and structural features similar to those expected
based on comparisons to related species. Single crystals of 3, 6
and 7 suitable for X-ray diffraction studies were obtained by slow
evaporation of a saturated solution in ethyl acetate at room tem-
perature (3 and 6) or 0°C (7). ORTEP[81] diagrams of 3 (Fig. 3), 6
(Fig. 4) and 7 (Fig. 5) are shown with selected bond lengths and an-
gles collected in the corresponding figure caption, and relevant ex-
perimental parameters for all three compounds are presented in
Table 1. All three ligands exhibit intramolecular hydrogen bonding
between phenol and amine functional groups, as has been ob-
served for related amine bis(phenols).[36,37,40–42,44,66,82] Heavy atom
(O…N) interatomic distances are consistent with the presence of
hydrogen bonds, and are within the range of those reported for
similar amine bis(phenolate) ligands.
General conditions for Suzuki coupling reactions
To a 20ml scintillation vial equipped with a magnetic stir bar, 4-
bromoacetophenone (0.102g, 0.50 mmol, 1 eq.), phenylboronic
acid (0.091 g, 0.75mmol, 1.5eq.), potassium carbonate (0.142g,
1.00 mmol, 2 eq.) and methanol (5ml) were added. Amounts of
0.10 ml of a 0.00050 M palladium precursor stock solution in CH2Cl2
and 0.10 ml of a 0.00050 M ligand stock solution in CH2Cl2 were
added via a syringe. The vial was placed in an aluminium block
heater regulated at 30°C, and stirring was initiated. After 30 min,
the vial was removed from the heat and partitioned between
diethyl ether (8ml) and water (8ml). The aqueous layer was ex-
tracted with diethyl ether (2 × 6 ml), and the combined organic
layers were dried over magnesium sulfate and filtered. An aliquot
of the sample was diluted with CH2Cl2 and analyzed using GC-MS
(50°C, hold 2 min; 20°CminÀ1 to 250°C, hold 10 min; 2.0mlminÀ1
flow rate, 50:1 split ratio) to determine percent conversion. Identity
of products was determined using GC-MS and confirmed using 1H
NMR spectroscopy.
The structure of 3 exhibits a crystallographically imposed inver-
sion center, such that half of the molecule lies within the
Crystallographic solution and refinement details
Crystallographic data were obtained at 200 ( 2) K with a Bruker
SMART X2S benchtop diffractometer using graphite-
monochromated Mo Kα (λ = 0.71073 Å) radiation, employing sam-
ples that were mounted in inert oil and transferred to a cold gas
stream on the diffractometer. Using Olex2,[78] the structure was
solved with the ShelXT[79] structure solution program using direct
methods and refined with the ShelXL[80] refinement package using
least squares minimization (on F2) with R based on Fo2 ≥ 2σ(Fo2) and
wR2 based on Fo2 ≥ À3σ(F2o). Anisotropic displacement parameters
were employed throughout for the non-hydrogen atoms, and hy-
drogen atoms attached to carbon atoms were added at calculated
positions and refined by use of a riding model employing isotropic
displacement parameters based on the isotropic displacement pa-
rameter of the attached atom. Phenol hydrogen atom positions
were located in the difference map, but were restrained to an
Figure 3. ORTEP diagram for 3 shown with 50% displacement ellipsoids
and the numbering scheme for non-hydrogen atoms shown. Hydrogen
atoms are draw arbitrarily small for clarity. Selected bond distances (Å) and
angles (°) for 3: N1–C7 1.471(2); N1–C8 1.464(2); N1–C9 1.465(2); O1–C1
1.3664(18); O1–H1 0.902(16); O1…N1 2.601; C7–N1–C8 109.93(12); C7–N1–
C9 111.21(12); C8–N1–C9 111.48(13); O1–C1–C2 120.57(14); O1–C1–C6
118.64(13).
wileyonlinelibrary.com/journal/aoc
Copyright © 2015 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2016, 30, 32–39

Amine bis(phenolate) Pd complexes as catalysts for Suzuki coupling
and 327.10°, respectively. Weak hydrogen bonding is observed be-
tween the phenol OH and neighboring nitrogen atoms, with O…N
interatomic distances of 2.764 and 2.824 Å. The steric bulk of 6 is
similar to that observed for 5, as evidenced by the sum of the an-
gles around the quaternary carbon of the cumyl (325.29° and
325.57°) or t-butyl (324.11° and 324.92°) group in the 2-position.[82]
In the molecular depiction of 7, the nitrogen atoms adopt a
distorted pyramidal geometry, with the sum of angles about N1
and N2 equal to 330.10° and 327.27°, respectively. Weak hydrogen
bonding is observed between the phenol OH and neighboring ni-
trogen atoms, with O…N interatomic distances of 2.706 and
2.730Å. Though the lengths of the methylene chain in 6 and 7 dif-
fer, the solid-state structure of these two ligands are quite similar to
one another, as well as to related amine bis(phenolate) ligands
bearing pendant donors.[37,40,42]
Figure 4. ORTEP diagram for 6 shown with 50% displacement ellipsoids
and the numbering scheme for selected non-hydrogen atoms shown.
Disordered phenyl ring left intact. Hydrogen atoms are draw arbitrarily
small for clarity. Selected bond distances (Å) and angles (°) for 6: N1–C1
1.465(2); N1–C5 1.471(2); N1–C30 1.479(2); N2–C2 1.457(2); N2–C3 1.462(2);
N2–C4 1.469(2); O1–C11 1.3666(19); O2–C36 1.360(2); O1–H1 0.877; O1…
N1 2.824; O2–H2 0.878; O2…N2 2.764; C1–N1–C5 111.36(13); C1–N1–C30
111.87(12); C5–N1–C30 110.49(12); C2–N2–C3 110.94(15); C2–N2–C4
110.19(15); C3–N2–C4 108.54(14); O1–C11–C10 118.48(14); O1–C11–C16
120.01(14); O2–C36–C31 122.60(15); O2–C36–C35 117.13(15).
1
In the H NMR spectra (CDCl3) for these ligands, 3 exhibits a
broad singlet at 10.25 ppm attributed to the phenol OH, while 6,
7 and 9 exhibit similar broad signals between 9.40 and 9.44 ppm.
The difference in chemical shift may be attributed to the structural
differences between the ligands, namely the diamine bis(pheno-
late) (3) and pendant amine bis(phenolate) (6, 7 and 9). These
chemical shifts lie well within the reported range for similar
ligands.[37,40,43,44,63] Other spectral features in the 1H NMR and
13C NMR spectra support the proposed structural formulation
depicted in Scheme 1, and are consistent with the solid-state struc-
tures of 3, 6 and 7 (see above). The equivalence of the proton sig-
nals corresponding to the methylene adjacent to the phenol
residue suggests that the hydrogen bonding observed in the solid
state is not present in CDCl3 solution. Were this not the case, hy-
drogen bonding would lead to restricted rotation of the phenol
ring, and cause these methylene signals to be split into two
diasterotopic resonances. While the 1H NMR spectra for ligands 3,
6 and 9 feature a large number of overlapping aromatic signals,
these correlate with well-separated peaks in the 13C{1H} spectrum,
and complete assignments may be made with the assistance of
data from HSQC and HMBC experiments. Bands between 3414
and 3453 cmÀ1 in the FT-IR spectra are observed for ligands 3, 6,
7 and 9, and are assigned as the phenolic OH stretch. These values
are consistent with those reported in the literature for related
ligands.[36,37,43,44,63,66]
Figure 5. ORTEP diagram for 7 shown with 50% displacement ellipsoids
and the numbering scheme for non-hydrogen atoms shown. Disordered
methyl groups C8 and C17 left intact. Hydrogen atoms are draw arbitrarily
small for clarity. Selected bond distances (Å) and angles (°) for 7: N1–C1
1.474(2); N1–C19 1.4727(19); N1–C10 1.4759(19); N2–C21 1.475(2); N2–C22
1.461(2); N2–C23 1.468(2); O1–C7 1.3732(19); O2–C16 1.3640(18); O1…N1
2.730; O2…N2 2.706; C1–N1–C10 109.98(12); C1–N1–C19 108.99(13); C10–
N1–C19 111.15(12); C21–N2–C22 111.25(13); C21–N2–C23 107.71(13); C22–
N2–C23 108.30(14); O2–C16–C15 115.43(14); O2–C16–C11 123.97(13); O1–
C7–C6 117.58(14); O1–C7–C2 121.09(14).
Catalytic studies
In order to assess the properties of these ligands relative to other
reported amine bis(phenols), catalytic studies were undertaken
using palladium complexes generated in situ from a 1:1 mixture
of the chosen ligand (1–9) and palladium(II) acetate. For ease of
characterization, the initial survey of catalyst activity (Table 2) was
conducted at 30°C in methanol, using 4′-bromoacetophenone
and phenylboronic acid as the coupling partners, and potassium
carbonate as an external base. Using ligand 1 in conjunction with
palladium(II) acetate at a catalyst loading of 1 mol% (Table 2, entry
1) or 0.1 mol% (entry 2), no unreacted 4′-bromoacetophenone is
asymmetric unit. In the molecular depiction of 3, the nitrogen atom
adopts a distorted pyramidal geometry, with the sum of angles
about N1 equal to 332.62°. Hydrogen bonding is observed between
the phenol OH and neighboring nitrogen atom, with an O…N inter-
atomic distance of 2.601Å. This distance is notably shorter than
those observed for 6 and 7, and is similar to the value of 2.633 Å ob-
1
observed in the H NMR spectrum after 30min, indicating >99%
conversion to the coupled product; these findings were confirmed
using GC-MS. Further reducing the catalyst loading to 0.01 mol%
(entry 3) generates a reaction mixture with 56.3% of the available
4′-bromoacetophenone converted to the coupled product, as de-
termined by comparing the integrated area for each species in
the GC-MS chromatogram.
served for a pyrazine-bridged diamine bis(phenolate) ligand.[44]
A
cursory examination of the structure for 3 reveals less steric conges-
tion than is observed for pendant amine bis(phenolates) (i.e. 4–9),
facilitating a stronger hydrogen bonding interaction. In the molec-
ular depiction of 6, the nitrogen atoms adopt a distorted pyramidal
geometry, with the sum of angles about N1 and N2 equal to 330.12°
With these conditions established, the remaining ligands (2–9)
were evaluated using 0.01 mol% Pd(OAc)2 (Table 2, entries 4–11).
Appl. Organometal. Chem. 2016, 30, 32–39
Copyright © 2015 John Wiley & Sons, Ltd.
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A. K. Bowser et al.
Table 1. Crystallographic data for ligands 3, 6 and 7
Compound
3
6
7
Empirical formula
Formula weight
C
54H64N2O2
C54H64N2O2
773.07
C23H34N2O2
370.52
773.07
Crystal size
Crystal system
Space group
a (Å)
0.62 × 0.31 × 0.16
Triclinic
0.48 × 0.4 × 0.32
Triclinic
0.55 × 0.48 × 0.24
Monoclinic
P21/c
P-1
P-1
8.7770(14)
9.5027(18)
14.581(3)
107.320(6)
102.688(6)
99.219(6)
1098.9(3)
1
8.2644(9)
10.9137(14)
25.659(3)
94.108(4)
97.725(4)
98.263(4)
2259.8(5)
2
17.951(2)
8.4276(10)
14.5978(18)
90
b (Å)
c (Å)
α (°)
β (°)
107.074(4)
90
γ (°)
V (Å3)
2111.1(4)
4
Z
ρ
cacld (g cmÀ3
)
1.168
1.136
1.166
μ (mmÀ1
F(000)
)
0.070
0.068
0.074
418.0
836.0
808.0
Radiation
Mo Kα (λ = 0.71073)
4.578 to 50.164
À10 ≤ h ≤ 10
À11 ≤ k ≤ 11
À17 ≤ l ≤ 17
10 789
Mo Kα (λ = 0.71073)
4.25 to 50.13
À9 ≤ h ≤ 9
À12 ≤ k ≤ 12
À30 ≤ l ≤ 30
22 177
Mo Kα (λ = 0.71073)
4.748 to 50.124
À21 < h < 21
À10 < k < 10
À17 < l < 17
19 889
2θ limit (°)
Total data collected
No. of independent reflections
No. of observed reflections
Rint
3861
7901
3734
2836
5453
2880
0.0385
0.0350
0.0400
Abs correction
Multiscan (SADABS)
0.6286–0.7452
3861/1/270
0.0461
Multiscan (SADABS)
0.6982–0.7452
7901/84/572
0.0477
Multiscan (SADABS)
0.6936–0.7452
3734/2/258
0.0420
Range of transmission
Data/restraints/parameters
R1 [F2o ≥ 2σ(F2o)]
wR2 [F2o ≥ À3σ(F2o)]
GOF
0.1149
0.1140
0.1014
1.035
1.042
1.049
Largest peak/hole (e ÅÀ3
)
0.19/À0.23
0.16/À0.23
0.17/À0.20
In general, percent conversion increases as the steric bulk and
electron-donating ability of the ligand increase, within a set of sim-
ilar ligands. For example, the smaller ligand 4 bearing methyl sub-
stituents generates a conversion of 51.5% (entry 6), while the more
electron-donating ligand bearing t-butyl groups (5) results in a no-
table increase to 65.9% (entry 7). Having similar sterics and elec-
tronics to 5, the cumyl ligand (6) demonstrates a comparable
conversion of 65.0% (entry 8). A parallel trend has been observed
for catalyst systems employing phosphine ligands, where in-
creased steric bulk and electron-rich ligands lead to improved cat-
alyst performance.[8–12] It is notable that previous reports of
palladium systems featuring similar ligand sets have focused on
the diamine bis(phenolates), rather than pendant amine bis
(phenolates).[44,64–67] In our hands, the corresponding pendant
amine bis(phenolate) ligands exhibit slightly enhanced catalyst
performance when compared with the corresponding diamine
bis(phenolates). As an example, when paired with Pd(OAc)2, the
cumyl substituted diamine ligand 3 (entry 5) exhibits 47.3% con-
version, while the corresponding cumyl-substituted pendant
amine ligands 6 and 9 (entries 8 and 11, respectively) exhibit
higher conversions of 65.0% and 63.8% under the same condi-
tions. Given the relatively high percent conversion for systems con-
taining the newly reported cumyl-substituted ligands (3, 6 and 9),
further studies were conducted using ligand 9. The electronic and
steric properties of these cumyl-substituted ligands appear at first
glance to be similar to those of related t-butyl-substituted ligands,
allowing for a clear assessment of ligand performance.
Employing bis(benzonitrile) palladium(II) chloride as the metal
precursor in conjunction with 9 results in a decrease in catalytic per-
formance (Table 2, entry 12; 38.7%) when compared to palladium(II)
acetate. No cross-coupling activity is observed when palladium(II)
chloride is employed as a precursor (entry 13), though this may
be attributed in part to the reduced solubility of the salt. Palla-
dium(0) precursors (entries 14 and 15) are also found to be effective
at forming catalytically active species, albeit with reduced efficiency
compared with palladium(II) acetate.
Using the conditions established by this survey, the effects of sol-
vent and external base on catalyst performance were evaluated
(Table 3). The reaction is found to proceed in ethanol (entry 2,
33.3%), 2-propanol (entry 3, 26.4%) and PEG (entry 4, 17.4%) sol-
vents. It is notable that increasing alkyl or ether chain length corre-
sponds to a decrease in catalyst performance. Negligible or no
catalytic activity is observed for the polar aprotic solvents diethyl
ether (entry 5) and acetonitrile (entry 6), as well as for toluene (entry
7), a relatively non-polar solvent. The identity of the external base is
found to play a significant role in catalyst performance. Catalyst per-
formance comparable to that observed with potassium carbonate
is noted when potassium hydroxide (entry 8) or sodium phosphate
(entry 11) is employed. The relatively weak bases sodium bicarbon-
ate (entry 9) and sodium acetate (entry 10) support moderate
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Copyright © 2015 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2016, 30, 32–39

Amine bis(phenolate) Pd complexes as catalysts for Suzuki coupling
Table 2. Survey of catalytic activity
conversions under these conditions. In general, percent conversion
increases as a function of basicity, as assessed by a comparison of
pKa values for the conjugate acids.[83]
To further probe the utility of this catalyst system, a series of
cross-coupling reactions were conducted in which the identity
of the aryl halide was varied (Table 4). While moderate conversion
to the coupled product is noted for bromobenzene (entry 1,
32.4%), 4-bromotoluene (entry 2, 33.4%) and 4-bromoanisole
(entry 4, 40.1%), catalytic activity is notably decreased when the
bulky methyl substituent is moved closer to the aryl bromide
(entries 4 and 5). Increased conversion is observed when both
the reaction time (2h) and temperature (60°C) are increased
(entries 3 and 6). Using substrates featuring electron-donating
groups para to the aryl–bromine bond such as 4-bromoaniline
(entry 8) results in a marked decrease in coupling efficiency
(4.7%), while the strongly electron-withdrawing groups in 1-
bromo-4-nitrobenzene (entry 10, 94.1%) and 4-bromobenzonitrile
(entry 11, >99%) facilitate near quantitative conversion to the
coupled products. Moderate conversion (entry 9, 73.6%) is noted
when 4-bromoaniline is coupled at elevated temperatures for 2 h.
Bromoaryl ketones such as 4-bromoacetophenone (entry 12,
84.5%) are well tolerated, as is the aldehyde functional group in
4-bromobenzaldehyde (entry 13, >99%). Moving these substitu-
ents to the meta position results in a marked decrease in catalytic
Entry Ligand
Pd
source
Catalyst
Time Conversion
(h)
loading (mol%)a
(%)b
1
1
1
1
2
3
4
5
6
7
8
9
9
9
9
9
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(PhCN)2Cl2
PdCl2
1
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
>99
>99
56.3
60.7
47.3
51.5
65.9
65.0
59.7
58.0
63.8
38.7
ndc
2
0.1
3
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
4
5
6
7
8
9
10
11
12
13
14
15
d
Pd2(dba)3
17.8
3.2
Pd(PPh3)4
aCatalyst loading relative to 4-bromoacetophenone. Reaction
conditions: 4-bromoacetophenone (0.50 mmol, 1 eq.), phenylboronic
acid (0.75 mmol, 1.5 eq.), K2CO3 (1.0 mmol, 2 eq.), ligand (0.10 ml of a
0.0005 M stock solution in CH2Cl2), palladium(II) acetate (0.10 ml of a
0.0005 M stock solution in CH2Cl2), methanol (5.0 ml).
Table 4. Survey of substrate scopea
bConversion of 4-bromoacetophenone to coupled product by
comparison of integrated area in the GC-MS chromatogram.
cNo coupled product detected.
ddba = dibenzylideneacetone, C17H14O.
Entry
R
Time (h)
Conversion (%)b
1
H
0.5
0.5
2d
32.4
33.4
86.0
40.1
3.4
2
p-Me
p-Me
Table 3. Dependence of observed catalytic activity on external base
and solventa
3
4
p-OMe
0.5
0.5
2d
5
o-Me-p-Me
o-Me-p-Me
m-Me-p-Me
p-NH2
6
36.7
6.9
7
0.5
0.5
2d
8
4.7
Entry
Solvent
Base
Conversion (%)b
9
p-NH2
73.6
94.1
>99
84.5
>99
23.2
ndc
10
11
12
13
14
15
16
17
18
19
20
p-NO2
0.5
0.5
0.5
0.5
24
96d
96d
0.5
2d
1
MeOH
EtOH
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
KOH
62.8
33.3
26.4
17.4
2.8
p-CN
2
p-C(O)Ph
p-C(O)H
m-C(O)H
o-C(O)Me
p-C(O)Me-PhCl
o-F
3
i-PrOH
PEG-400
Et2O
4
5
6
MeCN
Toluene
MeOH
MeOH
MeOH
MeOH
ndc
3.9
7
0.9
12.2
73.6
36.4
99.1
8
57.8
27.7
32.6
65.6
o-F
9
NaHCO3
NaOAc
Na3PO4
o-F-PhI
0.5
2d
10
11
o-F-PhI
aReaction conditions: aryl halide (0.50 mmol, 1 eq.), phenylboronic acid
(0.75 mmol, 1.5 eq.), base (1.0 mmol, 2 eq.), ligand 9 (0.10 ml of a
0.0005 M stock solution in CH2Cl2), palladium(II) acetate (0.10 ml of a
0.0005 M stock solution in CH2Cl2), solvent (5.0 ml).
aReaction conditions: 4-bromoacetophenone (0.50 mmol, 1 eq.),
phenylboronic acid (0.75 mmol, 1.5 eq.), base (1.0 mmol, 2 eq.), ligand
9 (0.10 ml of a 0.0005 M stock solution in CH2Cl2), palladium(II) acetate
(0.10 ml of a 0.0005 M stock solution in CH2Cl2), solvent (5.0 ml).
bConversion of 4-bromoacetophenone to coupled product by
comparison of integrated area in the GC-MS chromatogram.
cNo coupled product detected.
bConversion of 4-bromoacetophenone to coupled product by
comparison of integrated area in the GC-MS chromatogram.
cNo coupled product detected.
dReaction conducted at 60°C.
Appl. Organometal. Chem. 2016, 30, 32–39
Copyright © 2015 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/aoc

A. K. Bowser et al.
activity (entry 14, 23.2%), even after extended reaction times.
Similarly, no coupled product is observed when 2′-
bromoacetophenone is employed (entry 15), despite extended re-
action times and elevated temperatures. This is likely a result of
increased steric bulk near the aryl–bromine bond rendering the
substrate too large to interact with the metal complexes featuring
the bulky cumyl substituents. Although the inductive withdrawal
of electron density and donation of the lone pair of a fluoride
substituent are often closely balanced, the additional steric bulk
in the ortho position results in somewhat reduced conversion
(entry 17, 12.2%); increasing the reaction time and temperature
results in increased coupling activity (entry 18, 73.6%). Using the
more facile aryl iodide, enhanced conversion at both 30°C (entry
19, 36.4%) and 60°C (entry 20, 99%) is observed. Aryl chlorides
are significantly less reactive, showing only trace conversion to
the coupled product (entry 16, 3.9%) after four days at 60°C.
Our data suggest that the system reported in this work is compa-
rable, if not superior, to similar catalyst systems based on amine bis
(phenolate) scaffolds reported in the literature to date. Using a ste-
rically less demanding pyrazine-bridged bis(naphtholate) ligand
(Fig. 2(d)) and Pd(OAc)2 at 0.1mol% catalyst loading in dioxane–
water solvent, Bedekar and co-workers obtained biphenyl in 85%
yield (turnover number of 854) after 4 h at 95°C.[67] This coupling re-
action relied on iodobenzene as the aryl halide, and required longer
reaction times at higher catalyst loadings than are required for the
system reported in the present work. Balakrishna and co-workers
have also reported using palladium complexes featuring pyrazine-
bridged bis(phenolate) ligands (Fig. 2(a)) for cross-coupling
applications.[44,64] At catalyst loadings of 0.5mol%, quantitative
conversion of several aryl bromides to the corresponding biphenyl
species was observed after 10 min at room temperature in metha-
nol solution. The authors also report high yields (95%) at
0.0125 mol% catalyst loading, similar to those observed for the cat-
alysts derived from 1–9 as reported here. Zhou and co-workers ob-
served yields of 40% under conditions comparable to those
described in this work, with higher yields (85–99%) observed upon
refluxing the reaction mixture in acetone–water solvent for 1–
3 h.[65] Collectively, these findings support the claim that palladium
complexes derived from amine bis(phenolate) ligands are highly ef-
fective catalysts for the Suzuki–Miyaura coupling of aryl bromides
and phenylboronic acid. Our results also suggest that pendant
amine bis(phenolate) ligands represent an underexploited and
highly active system for palladium-mediated cross-coupling reac-
tions under mild conditions.
identity on catalyst performance are underway in our laboratory,
and will be reported in due course.
Acknowledgements
Funding for this research was provided by the Getty College of Arts
and Sciences and the Office of the Provost at Ohio Northern Univer-
sity, including a summer research fellowship for AKB and a faculty
fellowship for BMW. Crystallographic facilities at Otterbein Univer-
sity were acquired under a grant from the National Science Founda-
tion (DUE 0942850). The Ohio State University Mass Spectrometry
and Proteomics Facility is supported under grants from the NIH
(P30 CA016058) and NSF (1040302).
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Supporting information
Additional supporting information may be found in the online
version of this article at the publisher’s web site.
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