π-Conjugated polymeric phthalocyanine for the oxidative coupling of amines

Article abstract of DOI:10.1039/c9cc09422a

A cross-linked π-conjugated polymeric cobalt phthalocyanine material was developed by a facile, economical, scalable, and solid-phase synthesis method. In addition to being highly recyclable, this material showed greatly enhanced activity for the aerobic

Full text of DOI:10.1039/c9cc09422a

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p-Conjugated polymeric phthalocyanine for the
oxidative coupling of amines†
Cite this:DOI: 10.1039/c9cc09422a
b
Haihong Wu,‡ab Yanwei Cao,‡b Gangli Zhu, *b Dehong Zeng,b Xiang Zhu,
*
b
Jimin Dua and Lin He
*
Received 4th December 2019,
Accepted 6th February 2020
DOI: 10.1039/c9cc09422a
rsc.li/chemcomm
A cross-linked p-conjugated polymeric cobalt phthalocyanine material especially when the oxidizing reagents and alkaline additives are
was developed by a facile, economical, scalable, and solid-phase involved. Nevertheless, most of them require extra additives such as
synthesis method. In addition to being highly recyclable, this material ligands and bases in homogenous conditions, which may restrict
showed greatly enhanced activity for the aerobic oxidative coupling of their potential large-scale applications due to the difficulties in
amines compared with a molecular catalyst before heterogenization, their separation and regeneration.14
indicating good prospects for the material with a p-conjugated
To solve the problem of separation, the heterogenization
of molecular catalysts may be naturally conceived, e.g., by
immobilizing, supporting or anchoring the homogeneous catalysts
electronic character.
The integration of homogeneous and heterogeneous catalysts over solids.15–22 However, as Vries and Farina addressed,23 it is
has been one of the key issues in catalysis since the symposium general that the heterogenized homogeneous catalysts may lead
organized at Brussels in 1974. Homogeneous transition-metal- to some negative influences: catalyst deactivation, low turnover
based molecular complexes with well-defined structures and numbers, metal leaching and increased complexity. Another
atomic-level structural tunability offer attributes that make concern is that complicated preparation and high extra cost
them potentially compelling choices because the mechanisms may be incurred in immobilizing the catalysts.
in the homogeneous systems are well-clarified and controlled.
Herein, aiming at the oxidative coupling of amines, we
They hold great promise of achieving high activity and selectivity developed a cross-linked p-conjugated polymeric cobalt phthalo-
after delicate adjustment. Take the synthesis of imines as an cyanine material, which showed good performance in electro-
example, whose products can be widely applied in pharmaceutical chemistry.24–27 The preparation method is facile, scalable and
syntheses and organic transformations such as reduction, addition, cost-effective. The heterogenized p-conjugated polymer exhibited
cyclization and aziridination reactions;1–3 thus, the oxidative remarkable catalytic activity and high selectivity without furnishing
coupling of amines provides an attractive alternative to the by products such as unstable aldehydes. In addition to good
traditional synthetic protocol, where the materials are prepared recyclability, the catalytic activity of the p-conjugated material was
by the condensation of carbonyl compounds with amines in the greatly enhanced compared with that of the molecular catalyst
presence of Lewis acid catalysts.4,5 Considerable efforts have before heterogenization, proving the concept of heterogenizing
been devoted to this reaction during the past decade;6 however, a homogeneous catalytic systems in redox reactions. The solid
great challenge remains: homogeneous reaction systems such as p-conjugated polymeric cobalt phthalocyanine material having
CoTPP(CF3)4,7 MnTPPCl,8 FePc,9 and MnTE4Py10 may make the black colour (denoted as PCoPc-a) was prepared by a facile solid-
separation and purification of unstable imines very difficult,11–13 phase synthesis method (the details are given in Scheme S1, ESI†).
It is easy to scale this material up to 50 g with inexpensive raw
materials due to the simplicity of this process.
a Henan Province Key Laboratory of New Opto-Electronic Functional Materials,
Scanning electron microscopy (SEM) and transmission
College of Chemistry and Chemical Engineering, Anyang Normal University,
electron microscopy (TEM) (Fig. S2a–c, ESI†) were employed to
observe the morphology of the polymer, and some layer-stacked 2D
structures were found. High-angle annular dark-field (HAADF)
imaging with energy dispersive spectroscopy (EDS) elemental
mapping via scanning transmission electron microscopy (STEM)
confirmed the existence of C, N, and Co with good dispersion
(Fig. S2d and e, ESI†). The powder X-ray diffraction (PXRD)
Anyang, Henan, 455000, China
b State Key Laboratory for Oxo Synthesis and Selective Oxidation (OSSO), Suzhou
Research Institute of LICP, Lanzhou Institute of Chemical Physics (LICP), Chinese
Academy of Sciences (CAS), Lanzhou 730000, China. E-mail: zhugl@licp.cas.cn,
xiang@licp.cas.cn, helin@licp.cas.cn
† Electronic supplementary information (ESI) available: Experimental details,
analytical data and GC-MS spectra. See DOI: 10.1039/c9cc09422a
‡ These authors contributed equally to this work.
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Fig. 1 (a) XRD patterns of PCoPc-a. The calculated crystal structure of (b) and
(c) a and b forms of PCoPc-a. (d) FTIR spectra of prepared CoPc, PCoPc-a and
commercial CoPc. (e) Solid-state 13C NMR spectrum of PCoPc-a.
patterns (Fig. 1a) show the existence of crystalline domains
within the polymers, and they may be deconvoluted into two
p–p stacked crystal forms: the evenly aligned a form of CoPPc-a
and the staggered b form of CoPPc-a (Fig. 1b and c).28,29
The FTIR spectrum shows peaks at ca. 1095, 1332, 1520 and
1606 cmÀ1, representing the characteristic signals of the phthalo-
cyanine skeleton.30–32 The peaks at 804 (w) and 913 cmÀ1 can be
Fig. 2 Comparison of the (a) Co 2p, (b) C 1s, (c) N 1s XPS spectra of
PCoPc-a and CoPc. (d) Co K-edge XANES spectra and (e) EXAFS fitting
result for PCoPc-a using Co foil as a reference. (f) Electronic absorption
spectra of CoPc and PCoPc-a before and after benzylamine addition.
assigned to the vibration of the metal–ligand bond (Co–N).28,33,34 The and the band at 285.4 eV originates from the carbon atom linked to
peaks at 1709 and 1771 cmÀ1 indicate that there are carboxyl the two nitrogen atoms in the isoindole structure (Cg). The
functional group residuals in the polymer. The peaks for the stoichiometric ratio of Ca, Cb and Cg of the carbon species in CoPc
phthalocyanine skeleton and Co–N structure can also be observed is 2 : 1 : 1, which is in excellent agreement with that reported in
in the FTIR spectrum of PCoPc-a, providing definitive evidence literature.40 The satellite peaks at 286.0 and 287.5 eV can be
toward the formation of the polymeric skeleton. In particular, the attributed to p–p* excitations.38 Notably, these p–p* excitation-
decrease in the intensity of the aromatic C–H vibrational band at related satellite peaks observed for PCoPc-a were significantly
1120 cmÀ1 indicates that the phthalocyanine monomer is integrated stronger than that for CoPc. In the N 1s spectra (Fig. 2c), the peaks
into the conjugated polymer in the polycondensation reaction.28
at the BE of 398.3 and 398.8 eV can be attributed to the nitrogen
The 13C cross-polarization magic-angle spinning (13C CP-MAS) atoms adjacent to the central Co atom (Na) and the nitrogen atom
spectrum (Fig. 1e) exhibits broad signals at 117 and 138 ppm for the linking the isoindole rings (Nb), respectively.38,41 The Co K-edge
aromatic carbons in the benzene ring.35 The weak-intensity peaks at EXAFS and XANES spectra of PCoPc-a (Fig. 2d and e) show the
193 and 215 ppm can be attributed to the carboxyl functional existence of Co–Na and Co–Nb coordination and no Co–Co bond is
groups. More importantly, the formation of the phthalocyanine ring found, indicating that no Co nanoparticles are formed in the
within the architecture can be confirmed by the presence of a peak catalyst.
for an sp2 carbon at 170 ppm, confirming the conjugated structure
of the phthalocyanine-based polymer.24,36
The electronic absorption spectra are shown in Fig. 2f. Typically,
the N band at ca. 280 nm, B band (Soret band) at ca. 300 nm and
X-ray photoelectron spectroscopy (XPS) was employed to study Q band at 700 nm, which can be attributed to the allowed p–p*
the surface chemistry. The Co 2p spectrum of PCoPc-a (Fig. 2a) transitions, can be found in the spectra.32,42 The Q absorbance
shows two prominent peaks at 780.0 and 795.4 eV, corresponding band can be assigned to the a1u(p)–eg(p*) transition from the
to Co 2p3/2 and Co 2p1/2 with an area ratio of ca. 2 : 1 along with two highest occupied molecular orbital (HOMO) to the lowest unoccu-
satellite peaks at 789.0 and 803.7 eV. The peak at 782.5 eV can be pied molecular orbital (LUMO). The N band and B band originate
assigned to the spin-state-dependent satellite.37,38 These typical from the transition from a lower occupied orbital to an eg(p*)
characteristic peaks were also observed for the Co(II)Pc sample. orbital.42 Besides, in the PCoPc-a spectrum, an absorption band can
However, PCoPc-a showed a significantly broader full width at half- be found at 623 nm, which can be attributed to the intramolecular
maximum (FWHM) of Co 2p3/2 than Co(II)Pc, indicating that there coupling between the phthalocyanine units of the multi-nuclear
was a degree of Co(II) disproportionation.39 The C 1s spectrum structure.39,43 Compared with the observation for molecular CoPc,
(Fig. 2b) can be deconvoluted into five bands. The band at 284.0 eV the B band of PCoPc-a blue-shifts from 325 to 317 nm, while the
can be attributed to the phenyl carbon atoms (Ca), the band at Q band red-shifts from 657 to 691 nm, which is consistent with
284.5 eV can be assigned to the carbon atoms at the b position (Cb) the observations in the synthesis of polymeric phthalocyanine.26
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Table 1 Catalyst performance in oxidative couplinga
This can be essentially explained by the fact that the two ground-
state energy levels (a1u(p) and a2u(p)) move toward opposite direc-
tions due to polymerization.26,31 This means that the transition of
electrons from a1u(p) to the LUMO orbital in PCoPc-a would be much
easier than in CoPc.26
Entry
Catalyst
Con.b (%)
Sel.b (%)
It is reasonable that while the electron transitions are enhanced
in the p-conjugated polymeric skeleton, the electron distribution
change at the central metal ion may also be observed. It was found
that when benzylamine was added, the electronic absorption
spectrum changed significantly. The Q band shifted to 698 nm
and a band at 370 nm together with the very weak band at 768 nm
appeared, which is similar to the case of tetrabutylammonium
hydroxide, indicating the disproportionation of Co valence.43 As
explained, the alkaline hydroxyl/benzylamine groups can bind
strongly to Co(III), thereby leading to disproportionation.43,44 The
p-conjugated structure not only enhanced the delocalization and
electronic character of the central metal ion but also increased the
electric conductivity of the polymer. The electric conductivity of
PCoPc-a was measured to be 2.14 Â 10À2 S mÀ1, which was five
orders of magnitude greater than that of CoPc (3.31 Â 10À7 S mÀ1).
The change in the electronic characters and high conductivity of
PCoPc-a might facilitate fast electron transfer and oxidative coupling.
To study the catalytic performance of PCoPc-a in the oxidative
coupling reaction, first, benzylamine was selected as the model
substrate. A blank experiment was carried out in the absence of the
1
2
3
4
5
6
7
None
Commercial CoPc
CoPc
—
NA
97
97
499
98
98
99
97
99
99
98
499
99
499
99
98
12.5
13.1
91.3
35.3
46.7
62.7
11.6
63.0
75.1
3.8
87.4
84.2
87.1
89.7
4.5
PCoPc-a
PFePc-a
PMnPc-a
PCuPc-a
PPc
PCoPc-a
PCoPc-a
PCoPc-a
PCoPc-a
PCoPc-a
PCoPc-a
PCoPc-a
CoCl2
8
9c
10d
11e
12f
13g
14h
15i
16
17j
CoCl2 + PPc
78.1
99
a
Conditions: substrate (0.3 mmol), catalyst (5 mg), solvent (octane,
b
2.5 mL), O2 balloon, 15 h, 120 1C, in the absence of light. Determined
by GC-MS and GC, selectivity relating to by-product of benzonitrile. Air
balloon. 100 1C. N2 balloon. 10 mol% BHT added. 50 mg PCoPc-a
and 5 mmol benzylamine without solvent. 5 equiv. H2O added. Under
c
d
e
f
g
h
i
j
the natural light in lab. Mixture of CoCl2 and PPc was used.
catalyst with 0.3 mmol of benzylamine at 120 1C for 15 h under the The Co species in the filtrate were not detected by ICP. In addition,
atmospheric pressure of oxygen. As shown in Table 1, interestingly, when PCoPc-a was filtrated from the reaction mixture, the benzyl-
molecular CoPc (commercial or self-prepared) shows a low benzyl- amine conversion ceased (Fig. S5, ESI†), indicating that the observed
amine conversion of 13% (entry 2 and 3), while PCoPc-a gives a catalysis was truly heterogeneous.
conversion of 91.3% with selectivity 499% (entry 4). Compared
Considering that the reactions were carried out in the
with the activity of molecular CoPc, the activity was much higher absence of light (Fig. S1, ESI†), photocatalysis for the oxidation
although the surface of PCoPc-a had not been fully employed of amines in this work is excluded and not discussed (entry 15)
due to its low specific area (Fig. S3, ESI†), indicating that the although phthalocyanine has the intrinsic properties of the
p-conjugated system, which facilitated the electron transfer, played strong absorption of light.45 The reactions were also conducted
an important role in the oxidative coupling reaction. Furthermore, under natural light keeping the other conditions constant; the
the activity of the metal phthalocyanine polymers appeared to be results showed that the conversion did not change. When the
dependent on the nature of their central ions. PCoPc-a was more reaction was performed in an N2 atmosphere, the conversion
effective than the phthalocyanine polymers with iron, copper or decreased drastically to 3.8% (entry 11), confirming that oxygen
manganese (entry 5–7), suggesting that cobalt was suitable for this as an oxidant was indispensable for the reaction. Furthermore,
reaction. Furthermore, only 4.5% conversion could be obtained a series of substrates were tested under the optimized condi-
when catalyzed by an equal amount of CoCl2 (entry 16). These tions and exhibited high conversion (refer to Table S2, ESI†). In
results indicated that there was a synergistic interaction between addition, the catalyst was tested to verify its stability. As shown
the phthalocyanine skeleton and central Co ions. To further prove in Fig. S6 (ESI†), there is no significant decrease in the catalytic
this, metal-free polymeric phthalocyanine (PPc) was prepared performance after four cycles. The high performance and
(for the method, please see section 1.2 in ESI†). A conversion of stability of PCoPc-a may originate from the robust polymeric
11.6% was achieved under the same reaction conditions, which structure and strong binding between Co species and abundant
was significantly inferior to that of PCoPc-a (entry 8). Interest- N nests in phthalocyanine.
ingly, the conversion increased to 78.1% when PPc was mixed
The results from controlled tests have demonstrated that the
with CoCl2 (entry 17). This is because PPc can absorb Co ions cobalt phthalocyanine polymer with a p-conjugated structure is
due to its strong binding ability to form cobalt-containing essential for the oxidative coupling of amines and the reaction
polymeric phthalocyanine.
starts smoothly without an induction period and has high
To make the reaction more environmentally friendly, we further selectivity for imines even in the initial period (Fig. S5, ESI†).
tested the reaction under solvent-free conditions. The results Meanwhile, it was found that the reaction was not inhibited
revealed that the conversion of benzylamine could still be main- when a free-radical scavenger (BHT) was added (entry 12),
tained to about 84.2% (entry 13). After the oxidative coupling which is quite different from MnTPPCl.8 Thus, the possibility
reaction, the catalyst was easily recovered by filtration (Fig. S4, ESI†). of a radical chain pathway in the dark could be excluded.
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We gratefully thank the Youth Innovation Promotion Asso-
ciation CAS (2018453) for their support, the National Natural
Science Foundation of China (91645118, 21773270, 21703267,
21972152), the Foundation research project of Jiangsu Province
(BK20170423) and Beam line BL14W1 at the Shanghai Synchro-
tron Radiation Facility (SSRF).
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There are no conflicts to declare.
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