H-Bonding Assisted Self-Assembly of Anionic and Neutral Ligand on Metal: A Comprehensive Strategy to Mimic Ditopic Ligands in Olefin Polymerization

Article abstract of DOI:10.1021/acs.inorgchem.7b01923

Self-assembly of two neutral ligands on a metal to mimic bidentate ligand coordination has been frequently encountered in the recent past, but self-assembly of an anionic ligand on a metal template alongside a neutral ligand remains an elusive target. Suc

Full text of DOI:10.1021/acs.inorgchem.7b01923

Article
pubs.acs.org/IC
H‑Bonding Assisted Self-Assembly of Anionic and Neutral Ligand on
Metal: A Comprehensive Strategy To Mimic Ditopic Ligands in Olefin
Polymerization
Nilesh R. Mote,† Ketan Patel,† Dinesh R. Shinde,‡ Shahaji R. Gaikwad,† Vijay S. Koshti,†
Rajesh G. Gonnade,§ and Samir H. Chikkali*,†,#
†Polyolefin Lab, Polymer Science and Engineering Division, CSIR-National Chemical Laboratory, Dr. Homi Bhabha Road,
Pune-411008, India
‡Central NMR facility, CSIR-National Chemical Laboratory, Dr. Homi Bhabha Road, Pune-411008, India
§Center for Materials Characterization, CSIR-National Chemical Laboratory, Dr. Homi Bhabha Road, Pune-411008, India
#Academy of Scientific and Innovative Research (AcSIR), Anusandhan Bhawan, 2 Rafi Marg, New Delhi-110001, India
S
* Supporting Information
ABSTRACT: Self-assembly of two neutral ligands on a metal to
mimic bidentate ligand coordination has been frequently
encountered in the recent past, but self-assembly of an anionic
ligand on a metal template alongside a neutral ligand remains an
elusive target. Such a self-assembly is hampered by additional
complexity, wherein a highly negatively charged anion can form
intermolecular hydrogen bonding with the supramolecular motif,
leaving no scope for self-assembly with neutral ligand. Presented
here is the self-association of anionic ligand 3-ureidobenzoic acid
(2a) and neutral ligand 1-(3-(diphenylphosphanyl)phenyl)urea
(1a) on a metal template to yield metal complex [{COOC6H4-
NH(CO)NH2}{Ph2PC6H4NH(CO)NH2}PdMeDMSO] (4a).
The identity of 4a was established by NMR and mass spectroscopy. Along the same lines, 3-(3-phenylureido)benzoic acid
(2b) and 1-(3-(diphenylphosphanyl)phenyl)-3-phenylurea (1b) self-assemble on a metal template to produce palladium complex
[{COOC6H4NH(CO)NHPh}{Ph2PC6H4NH(CO)NHPh}PdMePy] (5c). The existence of 5c was confirmed by Job plot, 1−
2D NMR spectroscopy, deuterium labeling, IR spectroscopy, UV−vis spectroscopy, model complex synthesis, and DFT
calculations. These solution and gas phase investigations authenticated the presence of intramolecular hydrogen bonding
between hydrogen’s of 1b and carbonyl oxygen of 2b. The generality of the supramolecular approach has been validated by
preparing six complexes from four monodentate ligands, and their synthetic utility was demonstrated in ethylene polymerization.
Complex 4a was found to be the most active, leading to the production of highly branched polyethylene with a molecular weight
of 55700 g/mol and melting temperature of 112 °C.
generate a large catalyst library using a small number of
monodentate ligands was introduced by Reek,17−22 Breit,23−25
and others.26−28 It has been demonstrated that two supra-
molecular phosphine ligands with phosphorus as neutral-donor
can self-assemble on a metal to produce a metal complex
(Figure 1, left).29−31 The self-assembled metal complexes were
found to be active in asymmetric hydrogenation,32−35
asymmetric hydroformylation,36,37 allylic amination,38 nitro-
aldol reaction,39 and decarboxylative hydroformylation−hydro-
genation,40 and on many occasion these self-assembled
complexes outperform their parent covalent metal cata-
lysts.17−22 However, the success of supramolecular ligands has
been so far limited to the self-assembly of two neutral-donor
INTRODUCTION
■
Bidentate ligands play a prominent role in homogeneous
catalysis1−6 and perform better than their monodentate
counterparts, with few exceptions.7−9 The enhanced regio-
and stereoselection can be attributed to the chelating ability of
bidentate ligands and enforcement of a confined environment
around the metal center.10 These attributes provide a better
discrimination between the two faces of incoming substrate and
lead to enhanced selectivities.11,12 However, synthesis of
bidentate ligands is largely a tedious, multistep, and time-
consuming process. This is particularly inconvenient if a large
ligand library has to be screened to meet the desired
selectivity.13 Although combinatorial approaches have been
designed to exactly address this bottleneck, they usually make
use of covalently synthesized ligands and suffer from the lack of
ligand libraries.14−16 A supramolecular approach that can
Received: July 28, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.7b01923
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−6.05 and −5.96 ppm and disappearance of diphenylphosphine
resonance clearly indicated formation of 1a and 1b. Along the
same line, reaction of 3-aminobenzoic acid with potassium-
cyanate produced 2a. Ligand 2b [3-(3-phenylureido)benzoic
acid] was prepared by treating 3-aminobenzoic acid with
phenylisocyanate in THF.53,54 Thus, the four ligands with an
appended supramolecular motif were prepared in a single step
in good to excellent yields.
With a set of four monodentate supramolecular ligands in
hand, we set out to investigate the self-assembly between an
anionic and a neutral ligand. A stoichiometric reaction between
1a, the sodium salt of 2a, and [(COD)PdMeCl] in DMSO lead
to a turbid mixture. The resultant reaction mixture was passed
through Celite, and the volatiles were stripped off to obtain a
gray solid. The 31P NMR of this solid revealed a broad
resonance at 37.21 ppm, suggesting coordination of phosphine
1a. The corresponding proton NMR displayed a characteristic
broad singlet at 0.35 ppm, which can be assigned to metal
bound methyl protons (Pd−Me). The observed chemical shift
of Pd−Me protons is similar to those reported previously.55−60
In addition, proton NMR of the complex displayed a downfield
shift of NH protons to 8.99 and 9.23 ppm as compared to free
ligands (1a: 8.56, 2a: 8.84). The phosphorus and proton NMR
findings were further supported by 13C NMR, which revealed a
3 ppm downfield shift of the carboxylate carbon, suggesting that
the carboxylate oxygen is covalently bound to the palladium.
Additional evidence that supports the existence of complex 4a
(formed per Scheme 2) was obtained from ESI-MS analysis. A
Figure 1. Classical supramolecular approach in homogeneous catalysis
(left) and self-assembly of x-type and n-type ligands on a metal for
olefin polymerization (right).
ligands only.41−43 To the best of our knowledge, self-assembly
of an anionic and a neutral ligand to mimic ditopic ligand
behavior generating a heterocomplex has never been
reported.44,45 Although highly desirable, supramolecular cata-
lyst designing strategies via the self-assembly of anionic and a
neutral donor to mimic the ditopic ligand coordination have
never been scouted and their application in olefin polymer-
ization remains unexplored.46 This may be due to the additional
complexity of stabilization of the anion (from X-type ligand) by
the supramolecular motif, which will hamper the self-assembly.
Therefore, the choice of the anionic ligand and the supra-
molecular motif would be very crucial. A highly negatively
charged anion may be stabilized by intermolecular interactions
with the supramolecular motif, leaving no scope for self-
assembly with neutral donor. In this context, we pondered if a
monodentate supramolecular carboxylate and supramolecular
phosphine unit can be self-assembled via secondary interaction
on a metal template, which can then mimic the classical SHOP
type catalyst47−49 (Figure 1, right) and might catalyze olefin
oligomerization/polymerization. Apart from metal catalyzed
polymerization, metal free organocatalytic approaches have
been reported and an organo-catalytic ring opening polymer-
ization of lactide has been reported in the recent past.50,51
Scheme 2. Formation of Complexes 4a−c and 5a−c via Self-
Association of an Anionic Donor and a Neutral-Donor
Ligand on a Metal Template
RESULT AND DISCUSSION
■
In our pursuit to realize the supramolecular strategy, we
attempted the synthesis of monodentate supramolecular ligands
equipped with a hydrogen bonding motif (Scheme 1). The
supramolecular phosphines 1a and 1b were prepared by
treating iodourea with diphenylphosphine in almost quantita-
tive yields (90%).52 The appearance of a 31P NMR resonance at
Scheme 1. Supramolecular Phosphine and Carboxylate
Ligands
positive mode ESI-MS revealed a molecular ion peak at m/z =
621.08 Da [M − DMSO + H]+ and the corresponding sodium
salt at m/z = 643.07 Da [M − DMSO + Na]+, authenticating
the identity of 4a. The observed and simulated isotopic patterns
match exactly, confirming formation of 4a (SI Figure S21).
Although DMSO complex 4a allowed characterization, it was
found to be unstable in solution state and decomposes after a
few hours. To prevail over this, we choose pyridine as a donor
solvent with relatively small dielectric constant (≈12), and an
alternate synthetic protocol was established. The three
components of the self-assembly {1a:2a:[(TMEDA)PdMe2]}
were simultaneously mixed in pyridine at room temperature.61
Volatiles were stripped off after 2 h, and remaining solid was
B
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treated with pyridine and washed with hexane to yield a
powder. A single 31P resonance at 39.48 ppm, and the
appearance of a methyl proton at 0.35 ppm indicated formation
of self-assembled complex 5a. Complex 5a was found to be
stable enough for full scale characterization, and identity of 5a
was established using a combination of 1−2D NMR and mass
spectroscopy.
In a decisive through-bond direct C−H correlation spec-
troscopy (HSQC), metal bound methyl protons at 0.35 ppm
revealed a cross peak to a carbon at 0.43 ppm (SI Figure S26).
Similarly, pyridine protons at 8.58 ppm displayed a cross peak
to a carbon at 150.1 ppm. The NOESY spectrum (SI Figure
S27) unveiled a through space correlation between the methyl
proton on palladium (Pd−Me) and the pyridine protons, which
indicates that the methyl group and pyridine are placed cis to
each other.62,63 As a consequence of this arrangement,
phosphine (1a) and carboxylate ligand (2a) are situated cis to
each other (Figure S27). A comparative proton NMR spectrum
of the two ligands and the self-assembled complex 5a (Figure
S29) revealed significant downfield shift of the internal NH
protons, suggesting intramolecular hydrogen bonding in 5a.
The deshielding of the urea protons after self-assembly is in line
with the previous reports.64 The solution structure of complex
5a was corroborated by positive mode electrospray ionization
mass spectrometry (ESI-MS). A methanol solution of 5a
revealed pseudomolecular ion peaks at m/z = 621.08 [M − Py
+ H]+; 643.06 [M − Py + Na]+ (SI S28). To increase the
solubility, complexes 4b−c and 5b−c were prepared by
replacing one of the urea protons with a phenyl group. The
thus prepared complexes 4b, 4c, 5b, and 5c were characterized
using a combination of spectroscopic and analytical methods.61
The thus obtained data confirmed formation of 4b, 4c, 5b, and
5c. In our attempts to fully elucidate the self-assembly and map
the molecular structure, we choose complex 5c as it was found
to be soluble in nondonor solvents, such as dichloromethane
and chloroform. The phosphorus resonates at 41.17 ppm as the
only peak (SI Figure S44), and the urea protons were found to
be deshielded to appear at 9.93 and 10.59 ppm (SI Figure S45).
13C NMR revealed a downfield shifted carbonyl carbon at 173.3
ppm, indicating covalent bonding with palladium.65 The NOE
experiment established a through space correlation between the
methyl (Pd-Me) protons and pyridine protons, indicating that
the two groups are cis to each other. This was further
corroborated by a long-range P−H correlation (HMBC)
experiment, which revealed a cross peak between a phosphorus
and Pd-Me protons (Figure 2, SI Figure S50). The other two
cross peaks can be attributed to minor equilibrium (pyridine
coordination−decoordination) products that appear while
recording 2D NMR. The NMR findings were supported by
ESI-MS, which revealed a pseudomolecular ion peak at m/z =
773.15 [M − Py + H]+ (Figure S52).
Figure 2. Expanded view of a long-range P−H correlation (HMBC)
spectrum of 5c.
cm−1 further corroborated the NMR data (Figure S63). A weak
d−d transition band at λmax = 354 nm indicates the existence of
a covalent bond between the carboxylate anion and the metal
center.68 Multiple attempts to crystallize 5c lead to glassy
crystals (SI, Figure S54). To obtain detailed insights, we sought
assistance of computational methods (DFT), and the energy
optimized structures for complex cis-5c and trans-5c are
depicted in Figure 3. The energy optimized structures revealed
that complex cis-5c is lower in energy (by 7.63 kcal/mol) than
complex trans-5c.61
Figure 3. Energy (in kcal/mol) optimized structure of cis-5c and trans-
5c (H atoms on phenyl rings have been omitted for clarity).
A careful evaluation of the structural features of self-
assembled complexes 4a−c and 5a−c suggested that these
complexes meet all the requirements to be an active catalyst for
olefin polymerization. Just to demonstrate the synthetic utility,
performance of self-assembled complexes 4a−c was evaluated
in ethylene polymerization and Table 1 presents the most
important results. Complex 4a was found to be the most active
and complex 4c revealed the lowest activity in the ethylene
polymerization. The reduced activity in 4b and 4c can be
ascribed to the decreasing strength of hydrogen bonding in 4b
and 4c. It is well documented in the literature that substituting
NH protons of urea with an aryl substituent leads to reduced
intermolecular hydrogen bonding due to (i) increased steric
crowding around the remaining NH protons, (ii) and the
remaining NH protons can form hydrogen bonding with ortho-
In addition, the existence of self-assembled complex 5c was
demonstrated by deuterium labeling experiments, by NMR
titration (Job plot), by IR and UV−vis spectroscopy, by
synthesizing a model complex 6, and by computational
methods (SI section 4 and 5). A NMR titration experiment
revealed that the chemical shift of the NH proton changes with
increasing concentration of 1b, indicating intermolecular
hydrogen-bonding in 1b.66 On the contrary, the NH resonance
in complex 5c does not shift with increasing concentration of
5c, a characteristic feature of a self-assembled complex with
intramolecular hydrogen-bonding (SI Figure S62).67 The
change in CO stretching frequencies from 1645 to 1698
C
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a
DFT calculations. The synthetic utility of these complexes was
demonstrated by employing 4a−c in ethylene polymerization.
Complex 4a was found to be the most active and led to the
production of highly branched polyethylene, whereas 5a−c
could not catalyze the polymerization, indicating the limitation
of the present method. The ideal polymerization condition was
found to be 80 °C at 10 bar ethylene pressure, although the
best activity under these conditions is an order of magnitude
lower than that of the corresponding phosphine−sulfonate
complex.57
Table 1. Polymerization of Ethylene Catalyzed by 4a−c
Temp.
(°C)
Yield
(g)
TOF (mol of PE/mol of
Pd/h)
Tm
(°C)
Entry Cat.
1
2
3
4
5
6
7
8
4a
4a
4a
4a
4b
4b
4c
4c
60
70
80
90
70
80
70
80
0.014
0.103
0.183
0.053
0.040
0.043
0.025
0.030
14.2
105.1
186.7
54.04
44.64
47.99
30.68
36.94
ND
109
112
99.8
ND
ND
ND
ND
EXPERIMENTAL SECTION
■
Materials and methods. Unless noted otherwise, all manipu-
lations were carried out under an inert atmosphere of argon using
standard Schlenk line techniques or an M-Braun glovebox. Diethyl
ether and tetrahydrofuran were distilled from sodium/benzophenone
under argon atmosphere. Acetonitrile, dimethyl sulfoxide, and
methylene chloride were distilled on calcium-hydride. Potassium
cyanate and hydrochloric acid were purchased from Loba Chemie. 3-
amino benzoic acid and 3-iodo aniline were obtained from Alfa Aesar.
Phenylisocyante was purchased from Spectrochem Pvt. Ltd. Ligands
1a and 1b were synthesized by modifying a literature procedure.26
Ligands 2a and 2b were synthesized by improved literature
protocols.53,54 Other chemicals, such as [Pd(COD)MeCl]78 and
[Pd(TMEDA)Me2], were synthesized by following known procedures.
Ethylene polymerization was carried out in a 250 mL stainless steel
high pressure Buchi (glasuster cyclone 075) reactor equipped with a
mechanical stirrer and a heating/cooling jacket. NMR was recorded on
Bruker 400 and 500 MHz instruments. Chemical shifts are referenced
to external reference TMS (1H and 13C). Coupling constants are given
as absolute values. Multiplicities are given as follows: s: singlet, d:
doublet, t: triplet, m: multiplet. High temperature NMR was recorded
on a Bruker Avance 500 MHz instrument at 130 °C in a C6D6 + TCB
(10:90) mixture. Mass spectra were recorded on a Thermo scientific
Q-Exactive mass spectrometer, the column specification is Hypersil
gold C18 column 150 × 4.6 mm diameter 8 μm particle size, and the
mobile phase used is 90% methanol + 10% water + 0.1% formic acid.
Chromatography: separations were carried out on Spectrochem silica
gel (0.120−0.250 mm, 60−120 mesh). IR spectra were recorded on
Bruker ALPHA spectrometer. UV−vis spectra were recorded using a
Shimadzu UV-1800 spectrometer. The pictures of glassy crystal were
captured using a Leica MC 170HD microscope. MALDI-ToF-MS was
performed on AB SCIEX TOF/TOFTM 5800 and matrix used is
dithranol. High-temperature gel permeation chromatography (HT-
GPC) of the polymers was recorded in 1,2,4-trichlorobenzene at 160
°C on a Viscotek GPC (HT-GPC module 350A) instrument equipped
with triple detector system. The columns were calibrated with linear
polystyrene standards and the reported molecular weights are with
respect to polystyrene standards. C, H, and N analyses were carried
out using PerkinElmer 2400 instrument.
a
Reaction conditions: Catalyst = 35 μmol (DMSO − 0.5 mL + DCM-
2 mL), toluene = 100 mL; time = 1 h, ethylene pressure = 10 bar, ND
= not determined, NO = not observed.
CH protons of the aryl ring.69−71 Complex 4a catalyzes the
polymerization of ethylene at 60 °C (Table 1, entry 1), and the
activity increases with increasing polymerization temperature.
The best polymerization activity was observed at 80 °C, and
increasing the temperature to 90 °C lead to lower activity
(Table 1, entry 4).57 It is most likely that 4a becomes less active
at 90 °C due to weak or no intramolecular hydrogen bonding.
The same constraints are presumably responsible for the
vanished activity of 5a−c (Table S3). This hypothesis was
supported by variable temperature proton NMR of 5a, which
revealed that the NH protons of the two ligands merge with
each other at 90 °C, indicating collapse of intramolecular
hydrogen bonding and ditopic behavior of 5a (SI Figure S75).
This finding is in line with the common observation
encountered in supramolecular catalysis for hydrogenation
and hydroformylation.29−31,72 However, there could be other
reasons for the reduced activity at high temperature, which
cannot be ruled out at this stage.
The resultant polyethylene revealed a molecular weight of
55700 g/mol with a polydispersity of 2.2 (SI Figure S76). The
HT-GPC data was further corroborated by MALDI-ToF-MS
data which displayed a repeat unit mass of 28 Da (SI Figure
77). High temperature NMR of the polymer displayed methyl
and methylene resonances at 0.79 and 0.87 and 1.31 ppm,
confirming the existence of highly branched polyethylene.73
This apart, the proton NMR revealed resonance at 1.71, 1.88,
2.13, and 2.23 ppm, which can be ascribed to internal double
bonds in the polyethylene backbone and CH protons at
branching points. Thus, comparison of the NMR data indicates
that the polyethylene is highly branched with internal double
bonds (SI, Figure S78−S79).74−76 In accordance with the
branched structure, the melting temperature of polyethylene
was found to be 112 °C (Figure S74).77
Computational details.79−82 All the calculations have been
conducted using density functional theory (DFT), employing the
Turbomole 7.0 suite of programs. Geometry optimizations were
performed using the Perdew, Burke, and Ernzerhof (PBE) functional.
The triple-ζ basis set augmented by a polarization function
(Turbomole basis set TZVP) was used for all the atoms. The
resolution of identity (RI) along with the multipole accelerated
resolution of identity (marij) approximations were employed for an
accurate and efficient treatment of the electronic Coulomb term.
1-(3-(Diphenylphosphanyl)phenyl)urea (1a). 3-Iodoaniline was
dissolved in 2 M hydrochloric acid and then diluted with water.
Potassium cyanate (KOCN) was dissolved in a minimum amount of
water and was dropwise added to the above solution. The reaction
mixture was stirred at room temperature for 12 h, filtration and
washing with toluene lead to intermittent formation of 1-(3-
iodophenyl)urea. The resultant 1-(3-iodophenyl)urea was dissolved
in a THF/DMF (3:1) mixture, and diphenylphosphine and triethyl-
amine were successively added to it. 0.5 mol % of [Pd(OAc)2] was
CONCLUSION
■
In conclusion, self-assembly of an anionic and a neutral
supramolecular ligand on a metal template has been
demonstrated for the first time. A supramolecular carboxylate
(X-type) ligand, supramolecular phosphine (L-type) ligand, and
metal precursor were found to self-assemble through
deliberately installed urea motifs to generate a small library of
palladium complexes 4a−c and 5a−c that mimic the bidentate
coordination in SHOP-type catalysts. The existence of self-
assembled complex 5c was demonstrated using 1−2D NMR
spectroscopy, deuterium labeling, NMR titration, IR spectros-
copy, UV−vis spectroscopy, model complex synthesis, and
D
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added as a catalyst and the resultant mixture was refluxed overnight at
70 °C. Next, volatiles were evaporated, 10 mL of degassed water was
added to the residue, and the organic compound was extracted with
ethyl acetate. Ethyl acetate was stripped off; the resultant crude solid
was dissolved in dichloromethane and filtered over a plug of silica. The
silica plug was washed with dichloromethane to remove impurities,
and then the product was pushed through with ethyl acetate. A faint
yellow solid was obtained in 90% isolated yield.
[C14H13O3N2]+ m/z = 257.09 [M + H]+; for [C14H12N2O3Na]+ m/
z = 279.07 [M + Na]+.
Palladium complex [PdMe(2a)(1a)DMSO] (4a). Ligand 2a (0.027
g; 0.150 mmol) was treated with sodium hydride (0.0043 g; 0.181
mmol) in THF for 24 h at room temperature. After evaporation of
THF, the sodium salt of ligand 2a was dissolved in DMSO which was
followed by addition of [Pd(COD)MeCl] (0.040 g; 0.150 mmol) and
ligand 1a (0.048 g; 0.150 mmol). The reaction mixture was stirred at
room temperature for 16 h. The resulting yellow brown solution was
passed through the bed of Celite. After the evaporation of volatiles, a
gray solid was obtained in 63% (0.094 mmol) isolated yield, which was
identified as complex 4a. DMSO in 4a could not be accounted due to
the overlapping resonance of DMSO-d6.
31P NMR (500 MHz in DMSO-d6): δ = −6.05. 1H NMR (500 MHz
in DMSO-d6): δ = 8.56 (s, 1H, NH), 7.51 (s, 1H, Ar−H), 7.39 (s, 6H,
Ar−H), 7.25 (s, 6H, Ar−H), 6.74 (s, 1H, Ar−H), 5.82 (s, 2H, NH2).
13C NMR (125 MHz in CDCl3): δ = 156.5 (CO), 138.5, 136.7,
133.8, 133.6, 129.4, 128.8, 128.5, 125.4, 121.3. IR (cm−1) = 3301
(NH), 1668 (CO).
1
31P NMR (500 MHz in DMSO-d6): δ = 37.21 (broad). H NMR
(400 MHz in DMSO-d6): δ = 9.23 (s, 1H, NH), 8.99 (s, 1H, NH),
7.59 (s, 9H, Ar−H), 7.47 (s, 9H, Ar−H), 6.05 (s, 2H, NH2), 5.99 (s,
2H, NH2), 0.35 (s, 3H, Pd-Me). 13C NMR (100 MHz in DMSO-d6): δ
= 170.7, 156.4, 156.1, 140.9, 140.7, 139.8, 134.3, 130.4, 128.5, 128.2,
127.2, 126.6, 123.1, 122.0, 119.6, 118.9, 118.5, 0.9 (Pd-Me). ESI-MS
(+ve): for [C28H28O4N4PPd]+ m/z = 621.08 [M − DMSO + H]+; for
[C28H27O4N4NaPPd]+ m/z = 643.07 [M − DMSO + Na]+.
Palladium complex [PdMe(2a)(1a)Py] (5a). An equimolar mixture
of 1a (0.177 g; 0.555 mmol), 2a (0.100 g; 0.555 mmol), and
[(TMEDA)PdMe2] (0.140 g; 0.555 mmol) was add to pyridine (8
mL) at room temperature and stirred for 2 h. The volatiles were
evaporated, and pyridine (8 mL) was added to the solid. After 15 min
volatiles were stripped off and the off white residue was washed with
hexane (15 mL × 3) to obtain 5a in 90% (0.351 g; 0.501 mmol)
isolated yield.
1-(3-(Diphenylphosphanyl)phenyl)-3-phenylurea (1b). Phenyl-
isocyanate was dissolved in DCM to which 3-iodoaniline was dropwise
added at 0 °C and the mixture was stirred at room temperature for 70
h. The reaction progress was monitored by TLC. Volatiles were
stripped off to obtain off white solid, which was dried under vacuum
for 6−7 h. The intermittent 1-(3-iodophenyl)-3-phenylurea was
dissolved in THF/DMF (3:1), diphenylphosphine and triethylamine
were successively added under argon. A catalytic amount of
[Pd(OAc)2] (0.5 mol %) was added as a catalyst, and the mixture
was refluxed overnight at 70 °C. Solvent was evaporated, 10 mL of
degassed water was added, and the organic compound was extracted
with ethyl acetate. The thus obtained ethyl acetate was stripped off,
and crude solid was dissolved in dichloromethane and filtered over a
plug of silica. The silica plug was washed with dichloromethane to
remove impurities, and then the product was pushed through a silica
plug with ethyl acetate. A faint yellow solid was obtained in 90%
isolated yield.
31P NMR (400 MHz in DMSO-d6): δ = 39.48 (s, broad). 1H NMR
(400 MHz, DMSO-d6): δ = 9.23 (s, 1H, NH), 8.75 (s, 1H, NH), 8.58
(s, 3H, Ar−H), 7.78−7.74 (m, 2H, Ar−H), 7.67 (m, 2H, Ar−H), 7.58
(m, broad, 4H, Ar−H), 7.52 (m, 2H, Ar−H), 7.43 (m, 7H, Ar−H),
7.36 (m, 3H, Ar−H), 7.29 (m, 2H, Ar−H), 7.08 (s, 2H, Ar−H), 6.08
(s, 2H, NH2), 5.91 (s, 2H, NH2), 0.35 (s, 3H, Pd-Me). 13C NMR (100
MHz, DMSO-d6): δ = 170.3 (COO), 156.1 (CO), 155.9 (CO), 150.1
(C-Py), 140.8, 139.7, 138.7, 137.0, 133.8, 130.5, 128.7, 128.4, 127.3,
124.4, 122.9, 122.0, 119.8, 118.8, 0.43 (Pd-Me). ESI-MS: for
[C28H28O4N4PPd]+ m/z = 621.08 [M − Py + H]+; for
[C28H27O4N4NaPPd]+ m/z = 643.06 [M − Py + Na]+. Elemental
analysis (%) calculated for C33H32N5O4PPd: C 56.62, H 4.61, N 10.00;
found: C 55.02, H 4.11, N 10.57.
Palladium complex [PdMe(2b)(1a)DMSO] (4b). Ligand 2b (0.100
g; 0.390 mmol) was treated with sodium hydride (0.010 g; 0.429
mmol) in THF (8 mL) for 24 h at room temperature. After
evaporation of THF, the sodium salt of ligand 2b was dissolved in
DMSO (8 mL) which was followed by addition of [Pd(COD)MeCl]
(0.103 g; 0.390 mmol) and ligand 1a (0.125 g; 0.390 mmol). The
above reaction mixture was stirred at room temperature for 16 h. The
resulting brown solution was passed through a bed of Celite under
argon. After the evaporation of volatiles, a gray solid was obtained in
71% (0.215 g; 0.277 mmol) yield, which was identified as complex 4b.
DMSO in 4b could not be accounted due to the broad overlapping
resonance of DMSO-d6.
31P NMR (500 MHz in DMSO-d6): δ = −5.96. 1H NMR (500 MHz
in DMSO-d6): δ = 8.69 (s, 1H, NH), 8.57 (s, 1H, NH), 7.53 (m, 1H,
Ar−H), 7.37 (m, 8H, Ar−H), 7.24 (m, 8H, Ar−H), 6.92 (t, 1H, Ar−
H), 6.79 (t, 1H, Ar−H). IR (cm−1) = 3301 (NH), 1644 (CO). ESI-
MS (+ve): for [C25H22ON2P]+ m/z = 397.14 [M + H]+; 419.12 [M +
Na]+.
3-Ureidobenzoic acid (2a). 3-Amino benzoic acid (7.2 mmol) was
dissolved in 2 M hydrochloric acid (20 mL) and was diluted with
water (100 mL). KOCN (29 mmol) was dissolved in a minimum
amount of water and was dropwise added to the above solution with
constant stirring at room temperature. After addition, the reaction
mixture was stirred for 40 h. The formed precipitate was separated by
filtration and the resultant solid was dried to obtain white powder in
51% isolated yield.
1H NMR (500 MHz in DMSO-d6): δ = 12.85 (s, 1H, COOH), 8.84
(s, 1H, NH), 8.05 (s, 1H), 7.59 (d, 1H, Ar−H), 7.47 (d, 1H, Ar−H),
7.32 (m, 1H, Ar−H), 5.94 (s, 2H, NH2). 13C NMR (100 MHz in
DMSO-d6): δ = 167.4, 155.9, 140.8, 131.2, 128.8, 121.9, 118.4. IR
(cm−1) = 3310 (NH), 1673 (CO). ESI-MS (+ve): for [C8H9O3N2]+
m/z = 181.06 [M + H]+; for [C8H8N2NaO3]+ m/z = 203.04 [M +
Na]+.
3-(3-Phenylureido)benzoic acid (2b). 3-Amino benzoic acid (7.2
mmol) was dissolved in THF (100 mL), and phenylisocyanate (7.2
mmol) was dropwise added to the above solution with constant
stirring. The reaction mixture was stirred at room temperature for 60
h. After the stipulated time, the formed precipitate was separated by
filtration. The resultant white solid was dried and washed with diethyl
ether (20 mL × 2) and dichloromethane (20 mL × 2) to obtain white
powder in 92% yield, which was identified as 3-(3-phenylureido)-
benzoic acid (2b).
31P NMR (500 MHz in DMSO-d6): δ = 37.37. 1H NMR (500 MHz
in DMSO-d6): δ = 11.18 (s, broad, 1H, NH), 10.70 (s, broad, 1H,
NH), 9.00 (s broad, 1H, NH), 7.89 (s, broad, 2H, Ar−H), 7.64 (s,
broad, 3H, Ar−H), 7.55 (s, broad, 6H, Ar−H), 7.45 (s, broad, 6H,
Ar−H), 7.21 (s, broad, 4H, Ar−H), 7.04 (s, 1H, Ar−H), 6.87 (s, 1H,
Ar−H), 5.95 (s, broad, 2H, NH2), 0.40 (s, 3H, Pd-Me). 13C NMR
(125 MHz in DMSO-d6): δ = 170.8, 155.9, 153.6, 141.1, 140.0, 134.2,
130.6, 128.5, 127.3, 123.0, 122.4, 120.8, 119.9, 119.3, 118.0, 0.91 (Pd-
Me). ESI-MS (+ve): for [C34H32O4N4PPd]+ m/z = 697.12 [M + H]+,
for [C34H31O4N4PNaPd]+ m/z = 719.10 [M + Na]+.
Palladium complex [PdMe(2b)(1a)Py] (5b). A stoichiometric
amount of 1a (0.125 g; 0.390 mmol), 2b (0.100 g; 0.390 mmol),
and [(TMEDA)PdMe2] (0.098 g; 0.390 mmol) was mixed in pyridine
(20 mL) at room temperature and stirred for 2 h. Solvents were
evacuated, and pyridine (12 mL) was added to the solid. After stirring
for 15 min at room temperature, volatiles were stripped off to obtain
1H NMR (500 MHz in DMSO-d6): δ = 12.91 (s, 1H, COOH), 8.88
(s, 1H, NH), 8.69 (s, 1H, NH), 8.14 (d, 1H, Ar−H), 7.64 (d, 1H, Ar−
H), 7.56 (d, 1H, Ar−H), 7.47 (d, 2H, Ar−H), 7.40 (t, 1H, Ar−H),
7.29 (t, 2H, Ar−H), 6.98 (t, 1H, Ar−H). 13C NMR (125 MHz in
DMSO-d6): δ = 167.3 (CO), 152.5 (CO), 140.0, 139.5, 131.4
(C−Ar), 128.9 (C−Ar), 128.8 (C−Ar), 122.7 (C−Ar), 122.4 (C−Ar),
122.4 (C−Ar), 121.9 (C−Ar), 118.9 (C−Ar), 118.4 (C−Ar). IR
(cm−1) = 3312 (NH), 1688 (CO). ESI-MS (+ve): for
E
DOI: 10.1021/acs.inorgchem.7b01923
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
an off white solid which was washed with hexane (15 mL × 3). The
resultant residue was dried under vacuum to obtain a gray solid in 90%
(0.271 g; 0.349 mmol) isolated yield, which was identified as
compound 5b (with an extra pyridine molecule) after analysis.
31P NMR (500 MHz in DMSO-d6): δ = 42.61 (s, broad). 1H NMR
(500 MHz in DMSO-d6): δ = 9.53 (s, 2H, NH), 8.93 (s, 1H, NH),
8.61 (s broad, 6H, Py-H), 7.78 (s, 2H, Ar−H), 7.60 (s, 10H, Ar−H),
7.54 (m, 10H, Ar−H), 7.41 (m, 3H, Ar−H), 6.93 (s, 2H, Ar−H), 5.99
(s, 2H, NH2), 0.32 (s, 3H, Pd-Me). 13C NMR (125 MHz in DMSO-
d6): δ = 170.6, 155.9, 153.1, 149.9, 140.50, 139.5, 137.1, 133.8, 130.6,
128.6, 127.5, 124.5, 122.5, 121.2, 119.9, 119.3, 118.1, 0.76 (Pd-Me).
ESI-MS (+ve): for [C34H32O4N4PPd]+ m/z = 697.11 [M − Py + H]+.
Elemental analysis (%) calculated for C39H36N5O4PPd: C 60.35, H
4.68, N 9.02; found: C 60.42, H 4.68, N 10.29.
Palladium complex [PdMe(2b)(1b)DMSO] (4c). Ligand 2b (0.072
g; 0.281 mmol) was treated with sodium hydride (0.074 g; 0.309
mmol) in THF (8 mL) for 24 h at room temperature. THF was
evaporated and the residue (sodium salt of ligand 2b) was dissolved in
DMSO (1 mL), which was followed by the addition of [Pd(COD)-
MeCl] (0.074 g; 0.281 mmol) and ligand 1b (0.111 g; 0.281 mmol).
The reaction mixture was stirred at room temperature for 16 h. The
resulting brown solution was passed through the bed of Celite under
argon. Volatiles were stripped off to obtain a gray solid in 68% (0.163
g; 0.191 mmol) isolated yield. A combined spectroscopic and
analytical characterization established that the above gray solid is
compound 4c.
NH-protons after self-assembly. Then these 1b NH protons in 5c can
be tracked via NMR titration to establish the fate of inter- or
intramolecular hydrogen bonding.
Thus, the deuterium labeled ligand 1b′ was self-assembled with 2b
to obtain a self-assembled complex 5c′. As evident from SI section
4.1.2 and proton NMR (Figure S59) of 5c′, only 0.58 protons for the
NH group of 1b′ could be detected at 10.32 ppm. Similarly, instead of
2H only 1.36H were observed at 9.25 ppm, indicating that the
remaining is ND belonging to 1b′. This observation indicates that the
NH protons from 1b appear at 10.32 and 9.25 ppm after self-assembly.
Having established the chemical shift of 1b NH protons after self-
assembly, we performed NMR titration experiment as presented in SI
section 4.2. The change in chemical shift as a function of concentration
of 1b and concentration of 5c is presented in Table S1, and the same is
plotted in Figure S62. As it is evident from this figure; the 1b NH
proton shifts downfield with increasing concentration of 1b, suggesting
intermolecular hydrogen bonding. Whereas, there was hardly any
change in the chemical shift of 5c NH protons with increasing
concentration of 5c, suggesting intramolecular hydrogen bonding.
Deuterium labeled 1-(3-(diphenylphosphanyl)phenyl)-3-phenyl-
urea (1b′). Ligand 1b (0.120 g, 0.303 mmol) was dissolved in 2 mL of
dichloromethane, and 1 mL of methanol-d4 was added. After stirring at
room temperature for 2 h, solvents were stripped off. The resultant
solid was dried and was analyzed by NMR spectroscopy.
31P NMR (500 MHz in CDCl3): δ = −5.11. 1H NMR (500 MHz in
CDCl3): δ = 7.55 (d, 1H, Ar−H), 7.31 (s, 9H, Ar−H), 7.26 (m, 5H,
Ar−H), 7.09 (m, 2H, Ar−H), 6.97 (m, 2H, Ar−H).
31P NMR (500 MHz in DMSO-d6): δ = 37.21. 1H NMR (400 MHz
in DMSO-d6): δ = 10.74 (s, broad, 1H, NH), 10.45 (m, broad, 2H,
NH), 10.16 (s, broad, 1H, NH), 7.92 (s, broad, 1H, Ar−H), 7.86 (s,
broad, 2H, Ar−H), 7.60 (m, 8H, Ar−H), 7.46 (m, 7H, Ar−H), 7.22
(m, 7H, Ar−H), 6.88 (m, 3H, Ar−H), 0.41 (s, 3H, Pd-Me). 13C NMR
(100 MHz in DMSO-d6): δ = 170.9, 153.5, 153.0, 140.9, 139.9, 134.2,
130.9, 130.5, 128.6, 128.4, 127.4, 123.4, 122.5, 121.4, 120.9, 120.3,
119.3, 118.1, 116.8, 0.82 (Pd-Me). ESI-MS (+ve): for
[C40H36O4N4PPd]+ m/z = 773.15 [M − DMSO + H]+; for
[C40H35O4N4PNaPd]+ m/z = 795.13 [M − DMSO + Na]+.
Palladium complex [PdMe(2b)(1b)Py] (5c). An equimolar mixture
of 1b (0.156 g; 0.396 mmol), 2b (0.101 g; 0.396 mmol), and
[(TMEDA)PdMe2] (0.100 g; 0.396 mmol) was added to pyridine (8
mL) at room temperature and stirred for 2 h. Volatiles were
evaporated, and a second batch of pyridine (6 mL) was added to the
residue, and the mixture was stirred at room temperature for 15 min.
Finally, volatiles were stripped off, and the remaining residue was
washed with hexane (15 mL × 3) to obtain a yellow solid in 86%
(0.290 g; 0.340 mmol) isolated yield. Solid was crystallized from the
mixture of chloroform:hexane at 0 °C. An extra molecule of pyridine
was found to be trapped in the complex. No homocomplex could be
observed by NMR.
Deuterium labeled palladium complex [PdMe(2b)(1b)Py] (5c′).
Ligand 1b′ (0.100 g, 0.252 mmol), ligand 2b (0.064 g, 0.252 mmol),
and [Pd(TMEDA)Me2] (0.063 g, 0.252 mmol) were mixed in
pyridine (8 mL) at room temperature and stirred for 3 h. Volatiles
were stripped off, and pyridine (8 mL) was added to the residue. After
stirring for 15 min, volatiles were evaporated and the resultant yellow
solid was washed with diethyl ether (10 mL × 2). The thus obtained
solid was dried under vacuum to yield complex 5c′ in 88.7% yield
(0.190 g, 0.222 mmol).
31P NMR (500 MHz in DMSO-d6): δ = 39.88. 1H NMR (400 MHz
in CDCl3): δ = 10.32 (0.58H, NH/ND), 9.60 (1H, NH), 9.26 (1.36H,
NH/ND), 8.56 (4H, Py), 7.93 (5H, Ar−H), 7.55 (7H, Ar−H), 7.33
(5H, Ar−H), 7.09 (8H, Ar−H), 6.73 (4H, Ar−H), 0.51 (s, 3H, Pd-
Me).
NMR titration to establish self-assembly 5c. NMR titration for 1-
(3-(diphenylphosphanyl)phenyl)-3-phenylurea (1b). 0.0039 g (20
mM) of 1b was taken in a NMR tube, and 0.5 mL CDCl3 was added to
the NMR tube. Proton NMR at 23 °C was recorded, and the chemical
shift of NH protons was noted. Subsequently, the remaining quantity
of ligand 1b was added to the NMR tube to make a 40 mM and 60
mM solution of 1b and proton NMR was recorded.
31P NMR (500 MHz in CDCl3): δ = 41.17. 1H NMR (400 MHz in
CDCl3): δ = 10.59 (s, 1H, NH), 9.93 (s, 1H, NH), 9.75 (s, br., 1H,
NH), 9.58 (s, br., 1H, NH), 8.57 (s br., 4H, Py-H), 8.18 (m, 2H, Py-
H), 7.96 (s, 3H, Py-H), 7.60 (s, 6H, Ar−H), 7.40 (s, 4H, Ar−H), 7.23
(m, 9H, Ar−H), 7.10 (s, 4H, Ar−H), 7.01 (m, 2H, Ar−H), 6.78 (m,
4H, Ar−H), 0.48 (s, 3H, Pd-Me). 13C NMR (100 MHz in CDCl3): δ
= 173.3, 154.2, 150.1, 139.9, 137.1, 134.1, 131.0, 128.8, 125.4, 124.6,
123.2, 122.2, 121.5, 121.2; 120.4, 119.5, 119.2, 2.68 (Pd-Me). IR
(cm−1) = 3332 (NH), 1698 (CO). ESI-MS (+ve): calculated for
[C40H36O4N4PPd]+ m/z = 773.15 [M − Py + H]+; for
[C40H35O4N4PNaPd]+ m/z = 795.13 [M − Py + Na]+. Elemental
analysis (%) calculated for C45H40N5O4PPd: C 63.43, H 4.73, N 8.22;
found: C 63.21, H 4.88, N 9.18.
Demonstration of self-assembled complex 5c. Deuterium
labeling experiment. The purpose of deuterium labeling is to
establish the identity of NH protons originating from ligand 1b when
it self-assembles. If the NH protons belonging to 1b are labeled with
deuterium (D), then these would be absent (or partially present if
there is H/D exchange) after self-assembly with 2b. Those protons
that remain absent (or partially present if there is H/D exchange) in a
deuterium labeled self-assembled complex 5c′ can therefore only
belong to 1b. This exercise would establish the chemical shift of 1b
NMR titration for palladium complex (5c). 0.0085 g (20 mM) of
5c was taken in a NMR tube, 0.5 mL CDCl3 was added to the tube,
and a proton NMR at 23 °C was recorded. Subsequently, the
remaining quantity of 5c was added to the above NMR tube to bring
the concentration to 40 mM and 60 mM and the change in chemical
shift was noted.
IR studies. The existence of hydrogen bonding was attested by the
change in CO stretching frequencies from 1645 (free 1b) to 1698
cm−1(Figure S63). Similar frequencies have been accounted for
hydrogen bonding interactions.32
UV−vis studies. A weak d−d transition band at λmax = 354 nm
indicates formation of a covalent bond between the carboxylate group
and the metal center. The UV−vis analysis is supported by literature
data which reports a similar λmax for a palladium complex, as well as a
model complex 6 that we prepared (see SI section 4.5 for the synthesis
and characterization of model complex 6).
Synthesis of model complex 6. In our attempts to establish the
covalent behavior of carboxylate ligand, we synthesized model complex
6. The molecular structure of complex 6 (Figure S67) unambiguously
established the covalent bond between the carboxylate oxygen and
palladium (Pd−O1 = 2.105 Å).83 Triphenylphosphine (0.206 g, 0.790
mmol), benzoic acid (0.048 g, 0.395 mmol), and [Pd(TMEDA)Me2]
F
DOI: 10.1021/acs.inorgchem.7b01923
Inorg. Chem. XXXX, XXX, XXX−XXX

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Article
(0.100 g, 0.395 mmol) were taken in a precooled (at −30 °C) Schlenk
tube. A suitable quantity of DCM (6 mL) was added into the Schlenk
tube, and the mixture was stirred overnight at 0 °C. Volatiles were
evaporated at 0 °C, and crude solid was washed with hexane (5 mL ×
3). The resultant residue after washing was dried under vacuum to
yield complex 6 in 84.7% yields (0.255 g, 0.332 mmol).
Recrystallization from dichloromethane:heptane at 0 °C afforded
crystals suitable for single crystal X-ray measurement.
REFERENCES
■
(1) Fliedel, C.; Ghisolfi, A.; Braunstein, P. Functional short-bite
ligands: synthesis, coordination chemistry, and applications of N-
functionalized bis(diaryl/dialkylphosphino)amine-type ligands. Chem.
Rev. 2016, 116, 9237−9304.
(2) Chikkali, S. H.; van der Vlugt, J. I.; Reek, J. N. H. Hybrid
diphosphorus ligands in rhodium catalysed asymmetric hydro-
formylation. Coord. Chem. Rev. 2014, 262, 1−15.
1
31P NMR (500 MHz, CDCl3): δ = 29.23. H NMR (500 MHz,
(3) Clarke, M. L. Recent developments in the homogeneous
hydrogenation of carboxylic acid esters. Catal. Sci. Technol. 2012, 2,
2418−2423.
CDCl3): δ = 7.75 (s, 12H, Ar−H), 7.33 (s, 18H, Ar−H), 7.21 (s, 2H,
Ar−H), 7.14 (t, 1H, Ar−H), 7.02 (t, 2H, Ar−H), −0.05 (s, 3H, Pd-
Me).
(4) Gillespie, J. A.; Dodds, D. L.; Kamer, P. C. J. Rational design of
diphosphorus ligands − a route to superior catalysts. Dalton Trans.
2010, 39, 2751−2764.
Ethylene polymerization. Ethylene polymerization was carried out
in a 250 mL stainless steel high pressure Buchi (glasuster cyclone 075)
reactor equipped with a mechanical stirrer and a heating/cooling
jacket. Prior to the experiment, the reactor was heated in vacuum to 90
°C for 30 min, cooled to room temperature, and filled with argon.
Next, the reactor was flushed with ethylene (3 times, 12 bar) and was
charged with an appropriate quantity of toluene under a positive
ethylene stream. The reactor was then pressurized to 10 bar and
saturated with ethylene for 30 min at the desired reaction temperature.
After cooling at room temperature, catalyst solution (25 mg, 0.5 mL
DMSO + 2 mL DCM) was introduced into the reactor at room
temperature. The reactor was finally pressurized to 10 bar with stirring.
The polymerization was generally carried out for 1 h, the excess
ethylene was slowly vented off, and the reactor was allowed to cool
down to room temperature. Solid was obtained after evaporating the
volatiles which were further dried under reduced pressure at 60 °C for
10 h. Important polymerization experiments are summarized in Table
S3.
(5) Freixa, Z.; van Leeuwen, P. W. N. M. Bite angle effects in
diphosphine metal catalysts: steric or electronic? Dalton Trans. 2003,
2003, 1890−1901.
(6) Tang, W.; Zhang, X. New chiral phosphorus ligands for
enantioselective hydrogenation. Chem. Rev. 2003, 103, 3029−3070.
(7) Fu, W.; Tang, W. Chiral monophosphorus ligands for asymmetric
catalytic reactions. ACS Catal. 2016, 6, 4814−4858.
(8) Ayora, I.; Ceder, R. M.; Espinel, M.; Muller, G.; Rocamora, M.;
Serrano, M. Modular approach to new chiral monodentate
diamidophosphite ligands. Application in palladium-catalyzed asym-
metric hydrovinylation of styrene. Organometallics 2011, 30, 115−128.
(9) van den Berg, M.; Minnaard, A. J.; Schudde, E. P.; van Esch, J.; de
Vries, A. H. M.; de Vries, J. G.; Feringa, B. L. Highly enantioselective
rhodium-catalyzed hydrogenation with monodentate ligands. J. Am.
Chem. Soc. 2000, 122, 11539−11540.
(10) Wang, Q.; Gonell, S.; Leenders, S. H. A. M.; Duerr, M.;
Ivanovic-Burmazovic, I.; Reek, J. N. H. Self-assembled nanospheres
with multiple endohedral binding sites pre-organize catalysts and
substrates for highly efficient reactions. Nat. Chem. 2016, 8, 225−230.
(11) van Leeuwen, P. W. N. M.; Kamer, P. C. J.; Claver, C.; Pamies,
O.; Dieguez, M. Phosphite-containing ligands for asymmetric catalysis.
Chem. Rev. 2011, 111, 2077−2118.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.7b01923.
■
S
Synthetic procedure, spectroscopic and analytical data,
and polymer analysis. (PDF)
(12) Koblenz, T. S.; Wassenaar, J.; Reek, J. N. H. Reactivity within a
confined self-assembled nanospace. Chem. Soc. Rev. 2008, 37, 247−
262.
Accession Codes
(13) Breit, B. Supramolecular approaches to generate libraries of
chelating bidentate ligands for homogeneous catalysis. Angew. Chem.,
Int. Ed. 2005, 44, 6816−6825.
(14) Raynal, M.; Ballester, P.; Vidal-Ferran, A.; van Leeuwen, P. W.
N. M. Supramolecular catalysis. Part 1: non-covalent interactions as a
tool for building and modifying homogeneous catalysts. Chem. Soc.
Rev. 2014, 43, 1660−1733.
(15) Wieland, J.; Breit, B. A combinatorial approach to the
identification of self-assembled ligands for rhodium-catalysed asym-
metric hydrogenation. Nat. Chem. 2010, 2, 832−837.
(16) Meeuwissen, J.; Reek, J. N. H. Supramolecular catalysis beyond
enzyme mimics. Nat. Chem. 2010, 2, 615−621.
(17) Jans, A. C. H.; Gomez-Suarez, A.; Nolan, S. P.; Reek, J. N. H. A
switchable gold catalyst by encapsulation in a self-assembled cage.
Chem. - Eur. J. 2016, 22, 14836−14839.
(18) Garcia-Simon, C.; Gramage-Doria, R.; Raoufmoghaddam, S.;
Parella, T.; Costas, M.; Ribas, X.; Reek, J. N. H. Enantioselective
hydroformylation by a Rh-catalyst entrapped in a supramolecular
metallocage. J. Am. Chem. Soc. 2015, 137, 2680−2687.
(19) Leenders, S. H. A. M.; Gramage-Doria, R.; de Bruin, B.; Reek, J.
N. H. Transition metal catalysis in confined spaces. Chem. Soc. Rev.
2015, 44, 433−448.
(20) Dydio, P.; Reek, J. N. H. Supramolecular control of selectivity in
transition-metal catalysis through substrate preorganization. Chem. Sci.
2014, 5, 2135−2145.
CCDC 1550942 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_
request@ccdc.cam.ac.uk, or by contacting The Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2
1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
*E-mail: s.chikkali@ncl.res.in.
ORCID
Rajesh G. Gonnade: 0000-0002-2841-0197
Samir H. Chikkali: 0000-0002-8442-1480
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We gratefully acknowledge the financial support from SERB-
DST (SR/S2/RJN-11/2012 and EMR/2016/005120) India,
AvH foundation, Bonn, and CSIR-NCL, India. We thank Dr.
Dipa Mandal for HT-GPC analysis.
(21) Bellini, R.; Chikkali, S. H.; Berthon-Gelloz, G.; Reek, J. N. H.
Supramolecular control of ligand coordination and implications in
hydroformylation reactions. Angew. Chem., Int. Ed. 2011, 50, 7342−
7345.
DEDICATION
■
Dedicated to Prof. Dietrich Gudat on the occasion of his 60th
birthday.
G
DOI: 10.1021/acs.inorgchem.7b01923
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Inorganic Chemistry
Article
(22) Sandee, A. J.; Reek, J. N. H. Bidentate ligands by supramolecular
chemistrythe future for catalysis? Dalton Trans. 2006, 3385−3391.
(23) Koepfer, A.; Breit, B. Rhodium-catalyzed hydroformylation of
1,1-disubstituted allenes employing the self-assembling 6-DPPon
system. Angew. Chem., Int. Ed. 2015, 54, 6913−6917.
(24) Gellrich, U.; Seiche, W.; Keller, M.; Breit, B. Mechanistic
insights into a supramolecular self-asembling catalyst system: evidence
for hydrogen bonding during rhodium-catalyzed hydroformylation.
Angew. Chem., Int. Ed. 2012, 51, 11033−11038.
(25) Laungani, A. C.; Slattery, J. M.; Krossing, I.; Breit, B.
Supramolecular bidentate ligands by metal-directed in situ formation
of antiparallel β-Sheet structures and application in asymmetric
catalysis. Chem. - Eur. J. 2008, 14, 4488−4502.
(26) Koshti, V. S.; Mote, N. R.; Gonnade, R. G.; Chikkali, S. H.
Highly enantioselective Pd-catalyzed synthesis of P-stereogenic
supramolecular phosphines, self-assembly, and implication. Organo-
metallics 2015, 34, 4802−4805.
to valuable β- and γ-aldehyde intermediates using supramolecular
catalysis. J. Am. Chem. Soc. 2014, 136, 8418−8429.
(42) Straub, A. T.; Otto, M.; Usui, I.; Breit, B. Room temperature
ambient pressure (RTAP)-hydroformylation in water using a self-
assembling ligand. Adv. Synth. Catal. 2013, 355, 2071−2075.
(43) Kluwer, A. M.; Kapre, R.; Hartl, F.; Lutz, M.; Spek, A. L.;
Brouwer, A. M.; van Leeuwen, P. W. N. M.; Reek, J. N. H. Self-
assembled biomimetic [2Fe2S]-hydrogenase-based photocatalyst for
molecular hydrogen evolution. Proc. Natl. Acad. Sci. U. S. A. 2009, 106,
10460−10465.
(44) Self-assembly of two neutral phosphorus ligands, with a dangling
squaramide motif, on platinum, has been reported to deliver a C−C
coupling reaction, see: McGuirk, C. M.; Mendez-Arroyo, J.; Lifschitz,
A. M.; Mirkin, C. A. Allosteric regulation of supramolecular
oligomerization and catalytic activity via coordination-based control
of competitive hydrogen-bonding events. J. Am. Chem. Soc. 2014, 136,
16594−16601.
(27) Pignataro, L.; Bovio, C.; Civera, M.; Piarulli, U.; Gennari, C. A
Library approach to the development of BenzaPhos: highly efficient
chiral supramolecular ligands for asymmetric hydrogenation. Chem. -
Eur. J. 2012, 18, 10368−10381.
(28) Kleij, A. W. Zinc-centred salen complexes: versatile and
accessible supramolecular building motifs. Dalton Trans. 2009,
4635−4639.
(45) In a two pronged approach, a bidentate (neutral donor) ligand
with a dangling sulfonate group self-assembles on platinum to yield a
supramolecular complex. The sulfonate moiety was found to catalyze
the ring opening polymerization of cyclic esters, see: McGuirk, C. M.;
Mendez-Arroyo, J.; d’Aquino, A. I.; Stern, C. L.; Liu, Y.; Mirkin, C. A.
A concerted two-prong approach to the in situ allosteric regulation of
bifunctional catalysis. Chem. Sci. 2016, 7, 6674−6683.
(29) Zhang, D.; Martinez, A.; Dutasta, J.-P. Emergence of
hemicryptophanes: from synthesis to applications for recognition,
molecular machines, and supramolecular catalysis. Chem. Rev. 2017,
117, 4900−4942.
(46) Self-assembly of two neutral donor ligands on nickel and their
implication in ethylene dimerization has been reported, see: Boulens,
P.; Pellier, E.; Jeanneau, E.; Reek, J. N. H.; Olivier-Bourbigou, H.;
Breuil, P. A. R. Self-assembled organometallic nickel complexes as
catalysts for selective dimerization of ethylene into 1-butene.
Organometallics 2015, 34, 1139−1142.
(47) The SHOP catalyst family was extended to rigid congeners by
Keim, Bazan, and others, see: Bonnet, M. C.; Dahan, F.; Ecke, A.;
Keim, W.; Schulz, R. P.; Tkatchenko, I. Synthesis of cationic and
neutral methallyl nickel complexes and applications in ethene
oligomerisation. J. Chem. Soc., Chem. Commun. 1994, 615−616.
(48) Komon, Z. J. A.; Bu, X.; Bazan, G. C. Synthesis of butene−
ethylene and hexene−butene−ethylene copolymers from ethylene via
tandem action of well-defined homogeneous catalysts. J. Am. Chem.
Soc. 2000, 122, 1830−1831.
(49) Cai, Z.; Shen, Z.; Zhou, X.; Jordan, R. F. Enhancement of chain
growth and chain transfer rates in ethylene polymerization by
(phosphine-sulfonate)PdMe catalysts by binding of B(C6F5)3 to the
sulfonate group. ACS Catal. 2012, 2, 1187−1195.
(50) Schreiner, P. R. Metal-free organocatalysis through explicit
hydrogen bonding interactions. Chem. Soc. Rev. 2003, 32, 289−296.
(51) Zhang, X.; Jones, G. O.; Hedrick, J. L.; Waymouth, R. M. Fast
and selective ring-opening polymerizations by alkoxides and thioureas.
Nat. Chem. 2016, 8, 1047−1053.
(52) Synthesis of 1a and 1b is reported in refs 26 and 32.
(53) Synthesis of 2a was achieved by modifying the literature
reported procedure: Scott, J. R.; Cohen, J. B. CCCLXXVII.The
condensation of aromatic amino-sulphonic acids with isocyanic acid,
phenylcarbimide, and cyanamide. J. Chem. Soc., Trans. 1923, 123,
3177−3191.
(54) Ligand 2b was prepared as reported previously: Kadam, S. A.;
Martin, K.; Haav, K.; Toom, L.; Mayeux, C.; Pung, A.; Gale, P. A.;
Hiscock, J. R.; Brooks, S. J.; Kirby, I. L.; Busschaert, N.; Leito, I.
Towards the discrimination of carboxylates by hydrogen-bond donor
anion receptors. Chem. - Eur. J. 2015, 21, 5145−5160.
(30) Davis, H. J.; Phipps, R. J. Harnessing non-covalent interactions
to exert control over regioselectivity and site-selectivity in catalytic
reactions. Chem. Sci. 2017, 8, 864−877.
(31) Brown, C. J.; Toste, F. D.; Bergman, R. G.; Raymond, K. N.
Supramolecular catalysis in metal−ligand cluster hosts. Chem. Rev.
2015, 115, 3012−3035.
(32) Breuil, P. A. R.; Patureau, F. W.; Reek, J. N. H. Singly hydrogen
bonded supramolecular ligands for highly selective rhodium-catalyzed
hydrogenation reactions. Angew. Chem., Int. Ed. 2009, 48, 2162−2165.
(33) Weis, M.; Waloch, C.; Seiche, W.; Breit, B. Self-assembly of
bidentate ligands for combinatorial homogeneous catalysis: asym-
metric rhodium-catalyzed hydrogenation. J. Am. Chem. Soc. 2006, 128,
4188−4189.
(34) Laungani, A. C.; Breit, B. Supramolecular phanePhos-analogous
ligands through hydrogen-bonding for asymmetric hydrogenation.
Chem. Commun. 2008, 844−846.
(35) Birkholz, M. N.; Dubrovina, N. V.; Jiao, H.; Michalik, D.; Holz,
J.; Paciello, R.; Breit, B.; Boerner, A. Enantioselective hydrogenation
with self-assembling rhodium phosphane catalysts: influence of ligand
structure and solvent. Chem. - Eur. J. 2007, 13, 5896−5907.
(36) Breit, B.; Seiche, W. Hydrogen bonding as a construction
element for bidentate donor ligands in homogeneous catalysis:
regioselective hydroformylation of terminal alkenes. J. Am. Chem.
Soc. 2003, 125, 6608−6609.
(37) Goudriaan, P. E.; Kuil, M.; Jiang, X. B.; van Leeuwen, P. W. N.
M.; Reek, J. N. H. SUPRAPhos ligands for the regioselective rhodium
catalyzed hydroformylation of styrene forming the linear aldehyde.
Dalton Trans. 2009, 1801−1805.
(38) Takacs, J. M.; Reddy, D. S.; Moteki, S. A.; Wu, D.; Palencia, H.
Asymmetric catalysis using self-assembled chiral bidentate P,P-ligands.
J. Am. Chem. Soc. 2004, 126, 4494−4495.
(39) Park, J.; Lang, K.; Abboud, K. A.; Hong, S. Self-assembled
dinuclear cobalt(II)-salen catalyst through hydrogen-bonding and its
application to enantioselective nitro-aldol (henry) reaction. J. Am.
Chem. Soc. 2008, 130, 16484−16485.
(40) Diab, L.; Gellrich, U.; Breit, B. Tandem decarboxylative
hydroformylation−hydrogenation reaction of α,β-unsaturated carbox-
ylic acids toward aliphatic alcohols under mild conditions employing a
supramolecular catalyst system. Chem. Commun. 2013, 49, 9737−9739.
(41) Dydio, P.; Detz, R. J.; de Bruin, B.; Reek, J. N. H. Beyond
classical reactivity patterns: hydroformylation of vinyl and allyl arenes
(55) There are ample reports on the chemical shift of methyl protons
in [Pd−Me] complexes with the phosphinobenzenesulfonate ligand
system, see: (a) Drent, E.; van Dijk, R.; van Ginkel, R.; van Oort, B.;
Pugh, R. I. Palladium catalysed copolymerisation of ethene with
alkylacrylates: polar comonomer built into the linear polymer chain.
Chem. Commun. 2002, 744−745.
(56) Kochi, T.; Noda, S.; Yoshimura, K.; Nozaki, K. Formation of
linear copolymers of ethylene and acrylonitrile catalyzed by phosphine
sulfonate palladium complexes. J. Am. Chem. Soc. 2007, 129, 8948−
8949.
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Article
(57) There is no neutral Pd-catalyst derived from 2-(diphenylphos-
phanyl) benzoic acid (a covalent analogue of the current ligand
system), and hence no direct comparison is possible. The closest
possible neutral palladium complex is based on the phosphine
sulfonate system. Although it is not a fair comparison, the Pd-
phosphinesulfonate system is an order of magnitude more active than
4a, see: Guironnet, D.; Roesle, P.; Ruenzi, T.; Goettker-Schnetmann,
I.; Mecking, S. Insertion polymerization of acrylate. J. Am. Chem. Soc.
2009, 131, 422−423.
(74) For branched polyethylene, see: (a) Zhang, Y.; Huang, C.;
Wang, X.; Mahmood, Q.; Hao, X.; Hu, X.; Guo, C.-U.; Solan, G. A.;
Sun, W.-H. Highly branched unsaturated polyethylenes achievable
using strained imino-cyclopenta[b]pyridyl-nickel precatalysts. Polym.
Chem. 2017, 8, 995−1005.
(75) Jung, M.; Lee, Y.; Kwak, S.; Park, H.; Kim, B.; Kim, S.; Lee, K.
H.; Cho, H. S.; Hwang, K. Y. Analysis of chain branch of polyolefins by
a new proton NMR approach. Anal. Chem. 2016, 88, 1516−1520.
(76) Jiang, H.; Wu, Q.; Zhu, F.; Wang, H. Preparation of highly
branched polyethylene using (α-diimine) nickel complex covalently
supported on modified SiO2. J. Appl. Polym. Sci. 2007, 103, 1483−
1489.
(58) Shen, Z.; Jordan, R. F. Self-sssembled tetranuclear palladium
catalysts that produce high molecular weight linear polyethylene. J.
Am. Chem. Soc. 2010, 132, 52−53.
(77) A characteristic feature of branched PE is low Tm; for the
influence of branching on Tm, see: Zhu, F.; Xu, W.; Liu, X.; Lin, S.
Synthesis of branched polyethylene by ethylene homopolymerization
with a novel (α-diimine)nickel complex in the presence of
methylaluminoxane. J. Appl. Polym. Sci. 2002, 84, 1123−1132.
(78) Salo, E. V.; Guan, Z. Late-transition-metal complexes with bis-
azaferrocene ligands for ethylene oligomerization. Organometallics
2003, 22, 5033−5046.
(59) Gaikwad, S. R.; Deshmukh, S. S.; Gonnade, R. G.; Rajamohanan,
P. R.; Chikkali, S. H. Insertion copolymerization of difunctional polar
vinyl monomers with ethylene. ACS Macro Lett. 2015, 4, 933−937.
(60) Gaikwad, S. R.; Deshmukh, S. S.; Koshti, V. S.; Poddar, S.;
Gonnade, R. G.; Rajamohanan, P. R.; Chikkali, S. H. Reactivity of
difunctional polar monomers and ethylene copolymerization: a
comprehensive account. Macromolecules 2017, 50, 5748−5758.
(61) See the Supporting Information for experimental/computational
details.
(62) For inter-/intramolecular NOE experiments, see: (a) Anglister,
J.; Srivastava, G.; Naider, F. Detection of intermolecular NOE
interactions in large protein complexes. Prog. Nucl. Magn. Reson.
Spectrosc. 2016, 97, 40−56.
(63) Pregosin, P. S.; Anil Kumar, P. G.; Fernandez, I. Pulsed gradient
spin−echo (PGSE) diffusion and 1H,19F heteronuclear overhauser
spectroscopy (HOESY) NMR methods in inorganic and organo-
metallic chemistry: something old and something new. Chem. Rev.
2005, 105, 2977−2998.
(64) Meeuwissen, J.; Kuil, M.; van der Burg, A. M.; Sandee, A. J.;
Reek, J. N. H. Application of a supramolecular-ligand library for the
automated search for catalysts for the asymmetric hydrogenation of
industrially relevant substrates. Chem. - Eur. J. 2009, 15, 10272−10279.
(65) A similar 31P NMR chemical shift is reported for Ni-complexes
derived from phosphine-carboxylate ligand; see compound 17 in the
following reference: Keim, W.; Schulz, R. P. Chelate control in the
nickel-complex catalysed homogeneous oligomerization of ethylene. J.
Mol. Catal. 1994, 92, 21−33.
(79) For computational methods, see: Ahlrichs, R.; Bar, M.; Haser,
M.; Horn, H.; Kolmel, C. Electronic structure calculations on
workstation computers: the program system turbomole. Chem. Phys.
Lett. 1989, 162, 165−169.
(80) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized gradient
approximation made simple. Phys. Rev. Lett. 1996, 77, 3865−3868.
̈
(81) Eichkorn, K.; Treutler, O.; Ohm, H.; Haeser, M.; Ahlrichs, R.
Auxiliary basis sets to approximate Coulomb potentials. Chem. Phys.
Lett. 1995, 240, 283−290.
(82) Sierka, M.; Hogekamp, A.; Ahlrichs, R. Fast evaluation of the
Coulomb potential for electron densities using multipole accelerated
resolution of identity approximation. J. Chem. Phys. 2003, 118, 9136−
9148.
(83) For a representative Pd−O covalent bond distance, see:
Thirupathi, N.; Amoroso, D.; Bell, A.; Protasiewicz, J. D. Reactivity
studies of cationic palladium(II) phosphine carboxylate complexes
with lewis bases: substitution versus cyclometalation. Organometallics
2007, 26, 3157−3166.
(66) Sandee, A. J.; van der Burg, A. M.; Reek, J. N. H. UREAphos:
supramolecular bidentate ligands for asymmetric hydrogenation. Chem.
Commun. 2007, 864−866.
(67) Knight, L. K.; Freixa, Z.; van Leeuwen, P. W. N. M.; Reek, J. N.
H. Supramolecular trans-Coordinating Phosphine Ligands. Organo-
metallics 2006, 25, 954−960.
(68) Azmi, S. N. H.; Iqbal, B.; Al-Humaimi, N. S. H.; Al-Salmani, I. R.
S.; Al-Ghafri, N. A. S.; Rahman, N. Quantitative analysis of cefixime via
complexation with palladium(II) in pharmaceutical formulations by
spectrophotometry. J. Pharm. Anal. 2013, 3, 248−256.
(69) The effect of various substituents on the hydrogen bonding
ability of urea and urea-derivatives has been investigated, see: Koshti,
V. S.; Thorat, S. H.; Gote, R. P.; Chikkali, S. H.; Gonnade, R. G. The
impact of modular substitution on crystal packing: The tale of two
ureas. CrystEngComm 2016, 18, 7078−7094.
(70) Reddy, L. S.; Basavoju, S.; Vangala, V. R.; Nangia, A. Hydrogen
bonding in crystal structures of N,N¢-Bis(3-pyridyl)urea. Why is the
N-H···O tape synthon absent in diaryl ureas with electron-withdrawing
groups? Cryst. Growth Des. 2006, 6, 161−173.
(71) Zheng, W. R.; Xu, J. L.; Huang, T.; Yang, Q.; Chen, Z. C.
Hydrogen bonding interaction between ureas or thoureas and nitro-
compounds. Res. Chem. Intermed. 2011, 37, 31−45.
(72) Gadzikwa, T.; Bellini, R.; Dekker, H. L.; Reek, J. N. H. Self-
assembly of a confined rhodium catalyst for asymmetric hydro-
formylation of unfunctionalized internal alkenes. J. Am. Chem. Soc.
2012, 134, 2860−2863.
(73) For details on chain-walking and branching, see: Guan, Z.;
Cotts, P. M.; McCord, E. F.; McLain, S. J. Chain walking: a new
strategy to control polymer topology. Science 1999, 283, 2059−2062
and the reference therein.
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