Ionic composite of palladium(II)/iron bis(dicarbollide) for catalytic oxidative carbonylation in the formation of diphenyl carbonate

Article abstract of DOI:10.1016/j.jorganchem.2017.05.040

The carborane-based sandwich iron complex, [n-Bu₄N]{Fe(3,3′)-[1,2-(PPh₂)₂-1,2-C₂B₉H₉]₂}, was synthesized in 53.1% yield. A catalyst composite of PdAc₂/[n-Bu₄N]{Fe(3,3′)-[1,2-(PPh₂)₂-1,2-C₂B₉H₉]₂} was found to be highly active for the oxidative carbonylation of phenol, with the formation of diphenyl carbonate (DPC). A DPC yield of 46% and a turnover number (TON) of 511 were achieved in 4 h using the composite at 110 °C. For comparison, the reaction was also investigated using catalyst composites of PdAc₂/Mn(acac)₃, PdAc₂/Fe(acac)₃, PdAc₂/Co(acac)₃ and PdAc₂/Ce(acac)₃ (acac = acetylacetone) under the same conditions of temperature and pressure. The DPC yield was determined by gas chromatography with flame ionization detector (GC-FID). All new products were characterized by elemental analysis, and by ¹H, ¹³C, ¹¹B and ³¹P NMR and FT-IR spectroscopy.

Full text of DOI:10.1016/j.jorganchem.2017.05.040

Journal of Organometallic Chemistry xxx (2017) 1e6
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Ionic composite of palladium(II)/iron bis(dicarbollide) for catalytic
oxidative carbonylation in the formation of diphenyl carbonate
,
*
Algin Oh Biying a, Karen Tang Yuanting a, Narayan S. Hosmane b, Yinghuai Zhu a
a Institute of Chemical and Engineering Sciences (ICES), 1 Pesek Road, Jurong Island, 627833, Singapore
b Department of Chemistry and Chemical Biology, Northern Illinois University, DeKalb, IL 60115-2862, USA
a r t i c l e i n f o
a b s t r a c t
The carborane-based sandwich iron complex, [n-Bu4N]{Fe(3,30)-[1,2-(PPh2)2-1,2-C2B9H9]2}, was synthe-
sized in 53.1% yield. A catalyst composite of PdAc2/[n-Bu4N]{Fe(3,30)-[1,2-(PPh2)2-1,2-C2B9H9]2} was
found to be highly active for the oxidative carbonylation of phenol, with the formation of diphenyl
carbonate (DPC). A DPC yield of 46% and a turnover number (TON) of 511 were achieved in 4 h using the
composite at 110 ꢀC. For comparison, the reaction was also investigated using catalyst composites of
PdAc2/Mn(acac)3, PdAc2/Fe(acac)3, PdAc2/Co(acac)3 and PdAc2/Ce(acac)3 (acac ¼ acetylacetone) under
the same conditions of temperature and pressure. The DPC yield was determined by gas chromatography
with flame ionization detector (GC-FID). All new products were characterized by elemental analysis, and
by 1H, 13C, 11B and 31P NMR and FT-IR spectroscopy.
Article history:
Received 22 January 2017
Received in revised form
17 May 2017
Accepted 18 May 2017
Available online xxx
Keywords:
Carborane
Organometallics
Oxidative carbonylation
Diphenyl carbonate
Iron complexes
© 2017 Published by Elsevier B.V.
1. Introduction
processes, transesterification reaction between dimethylcarbonate
(DMC) and phenol (Eq. (3)) [5e11], and oxidative carbonylation
Diphenylcarbonate (DPC) is an important intermediate for the
preparation of polycarbonates (PC), which is one of the major en-
gineering thermoplastics with excellent mechanical, electrical and
heat resistance properties [1e3]. PC materials are widely used in
our daily life, such as in DVDs and blue-ray discs. Melting DPC with
bisphenol (4,4'-(propane-2,2-diyl)diphenol) produces poly-
carbonates as shown in Eq (1) [1e3]. Conventionally, DPC is mainly
produced by reaction of phenol with phosgene (Eq. (2)) [4]. In this
process, the highly toxic and corrosive phosgene is used as raw
material. Therefore, the process cannot be considered as green. To
avoid using toxic phosgene and to develop environmentally benign
reaction of phenol to form DPC have been investigated (Eq. (4))
[12e23]. However, it is crucial for transesterification reaction to
completely convert DMC to DPC under mild conditions because the
reaction is kinetically and thermodynamically unfavorable. At
higher temperature, an anisole by-product is formed by the
methylation of phenol [5e11]. Whereas polycarbonates produced
from DPC, obtained by oxidative carbonylation processes, generally
require fewer reaction steps from raw material in the total syn-
thesis procedure, when compared with the current commercialized
or phosgene-free processes.
(1)
* Corresponding author.
E-mail address: yinghuaizhu2016@gmail.com (Y. Zhu).
http://dx.doi.org/10.1016/j.jorganchem.2017.05.040
0022-328X/© 2017 Published by Elsevier B.V.
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A.O. Biying et al. / Journal of Organometallic Chemistry xxx (2017) 1e6
(2)
(3)
(4)
The production of DPC by the oxidative carbonylation of phenol
iron bis(dicarbollide) and its utility as a co-catalyst in the oxidative
carbonylation reaction of phenol to form DPC.
is well documented [12e23]. In general, a noble metal catalyst such
as a palladium complex is used in the process with carbon mon-
oxide (CO), oxygen (O2), a co-catalyst such as manganese (III) ace-
tylacetonate (Mn(acac)3) and additives such as sodium hydroxide/
tetrabutyl ammonium bromide. The reactions are commonly car-
ried out in acetonitrile (MeCN). A DPC yield of 13.8% has been re-
ported by homogeneous catalyst PdAc2 in the presence of
Mn(acac)3 [12]. The reaction was carried out in a Batch model
reactor in acetonitrile at 100 ꢀC in the presence of CO and O2 with a
ratio of VCO/VO2 ¼ 11. The oxidative carbonylation of phenol was
also conducted neat at 80 ꢀC [13]. In this process, Mn(acac)3 was
also used as co-catalyst and the molten phenol played the role of
solvent. The highest yield reported is 10.4% with a selectivity of
100% [13]. It has been well recognized that co-catalyst Mn(acac)3 is
crucial to enable the catalytic cycle for the palladium based ho-
mogeneous catalysts to operate, as shown in Fig.1 [12e23]. It is also
recognized that other by-products may be formed during DPC
synthesis, as shown in Scheme 1. Nevertheless, the existing catalyst
systems suffer from low catalytic activity. A high-performance
catalyst composite is required for the oxidative carbonylation
process.
2. Results and discussion
Closo-1,2-Bis-(diphenylphosphino)-1,2-carborane (2) was pre-
pared from closo-1,2-carborane (1) following literature procedures,
as shown in Scheme 2 [27]. The functional closo-carborane was
decapitated by reacting with potassium hydroxide in ethanol to
form an intermediate carboranyl mono anion, nido-7,8-bis-
(diphenylphosphino)-7,8-C2B9H10 (3) in 64.1% yield. Further
deprotonation of compound 3 was accomplished by refluxing with
sodium hydride (NaH) in THF, that generated in situ the dianionic
intermediate,
[nido-7,8-bis-(diphenylphosphino)-7,8-C2B9H9]2.
Ferrous chloride was then reacted with the dianion in THF followed
by treating with an aqueous solution of tetrabutylammonium
bromide, which led to the formation of the tetrabubutylammonium
salt of iron bis(dicarbollide) (4) in 53.1% yield. Compound 3 showed
the typical absorptions of phenyl and methyl functional groups, as
well as the B-H bonds in nido-carborane cage in the 1H-NMR
spectra (DMSO-d6). The 13C-NMR spectra exhibited the peaks for
phenyl and methyl groups and for carbons of the cage. In 11B-NMR
spectra, 3 showed a splitting pattern of 2:3:2:1:1. Compound 4 was
analyzed by elemental analysis and FT-IR as described in the
experimental section. In FT-IR spectra, strong absorption at
2518 cmꢁ1 was observed for both of 3 and 4, which are attributed to
B-H stretching mode of vibrations (nBH). Polarographic measure-
ments in acetone-water (v/v ¼ 1/:1) were conducted with a 0.1 N
LiClO4 supporting electrolyte with use of a dropping mercury
electrode. A single-wave reduction polarogram was obtained at E1/
On the other hand, the iron metal center shows the variable
chemical valence of FeII/FeIII in its bis(dicarbollide) complexes
[24,25], therefore it could possibly be used as an in situ regenerable
co-oxidant, replacing Mn(acac)3. It has been reported that bis(di-
carbollide) complexes are efficient co-catalyst for selective oxida-
tion reactions of alcohol to produce the corresponding carbonyl
compounds in the presence of palladium-based catalysts [26].
Herein, we report the synthesis of the tetrabutylammonium salt of
Fig. 1. Proposed mechanism of DPC synthesis.
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A.O. Biying et al. / Journal of Organometallic Chemistry xxx (2017) 1e6
3
Scheme 1. By-reactions of oxidative carbonylation.
Scheme 2. Synthesis of the tetrabubutylammonium salt of iron bis(dicarbollide) (4).
¼ ꢁ0.49 V and the reduced species being the formal iron (II)
reaction after the catalyst [12e23]. Therefore, commonly used co-
catalysts were investigated and the results are listed in Table 1.
The phenol conversion, along with DPC yield and DPC selectivity,
were also calculated as following. A typical GC-FID spectrum of the
product mixture was shown in Fig. 2. Peaks arising from by-
products were observed near the DPC peak and they are labeled.
To investigate the by-products of the oxidative carbonylation of
PhOH, the product mixture was analyzed by GC-MS (gas
2
derivatives as indicated. The result is consistent with literature
observations [25].
Oxidative carboranylation of phenol to produce DPC was carried
out in a Parr reactor using acetonitrile as solvent at 110 ꢀC with a
volume rate of CO to O2 of 25/1 in which the CO concentration is
below the CO explosion limit [28] in the reaction processes. As per
previous results, co-catalyst plays the second important role in the
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A.O. Biying et al. / Journal of Organometallic Chemistry xxx (2017) 1e6
Table 1
Catalyst performance in the semi-continuous model.a,b
Entry
Catalyst
Co-catalyst
Phenol Conv.(%)
DPC Yield (%)
DPC Sel. (%)
TON
1
2
3
4
5
6
7
8
9
PdAc2
PdAc2
PdAc2
PdAc2
e
Mn(acac)3
Fe(acac)3
Co(acac)3
Ce(acac)3
24
29
20
30
3
51
47
44
45
4
2
3
2
18
6
12
6
175
98
157
71
Mn(acac)3
0
0
0
PdAc2
PdCl2
PdCl2(MeCN)2
Pd/AC
4
4
4
4
46
23
26
9
89
49
61
19
511
242
357
195
a
Reaction conditions: catalyst loading amounts: [Pd]/[co-catalyst]/[PhOH] ¼ 1:5:1610; PhOH 10.0 g, weight ratio of PhOH/MeCN ¼ 1/4; 4 Å MS: 2.0 g, NaOH: 20 mg,
nBu4NBr: 170 mg; PCOþO2 ¼ 60 bar (at room temperature), VCO/VO2 ¼ 25:1; reaction temperature 110 ꢀC; reaction time ¼ 4 h.
b
Yield (%) is determined by GC-FID.
Fig. 2. GC-FID of the product mixture from the oxidative carbonylation reaction.
chromatography-mass spectrometry) and GPC (gel permeation
chromatography). The GC-MS and GPC spectra of the reaction
mixture are presented in Figs. S-1 and S-2. Based on GC-MS results,
compounds formed by O-C and C-C coupling reaction between
PhOH are the main by-products. In addition, high molecular weight
products (around 966) are observed. They could be generated by
multiple cross-coupling reactions within the PhOH substrate.
bis(dicarbollide) is found to be significantly effective for the
oxidative carbonylation of phenol to form DPC in comparison with
other compounds examined. A fairly high DPC yield of 46% was
reached with 89% selectivity. The results further demonstrate that
co-catalysts are essential for the reaction under same conditions of
temperature and pressure. It is also demonstrated that a PdAc2
catalyst is more active than PdCl2, PdCl2(MeCN)2 and heteroge-
neous catalyst Pd/C. The real mechanism is not known at this stage.
However, considering the homogeneous catalyst, PdAc2 has been
well explored in the oxidative carbonylation reaction of phenol, a
similar proposed mechanism in Fig. 1 could be applicable for the
catalyst composite of PdAc2/[n-Bu4N]{Fe(3,30)-[1,2-(PPh2)2-1,2-
C2B9H9]2}. A study of its mechanism is currently underway in our
laboratory.
Phenol conversion ¼ (total amount of phenol consumed) x 100% /
(total amount of phenol added)
DPC selectivity ¼ (amount of phenol reacted to produce
DPC) x 100% / (total amount of phenol consumed)
DPC yield ¼ phenol conversion x DPC selectivity
TON ¼ moles of DPC / moles of palladium
3. Conclusions
The co-catalyst, tetrabubutylammonium salt of iron
The tetrabubutylammonium salt of iron bis(dicarbollide) has
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A.O. Biying et al. / Journal of Organometallic Chemistry xxx (2017) 1e6
5
been synthesized and shown to be an efficient co-catalyst for the
oxidative carbonylation of phenol to produce diphenyl carbonate. A
good yield of 46% was reached in the presence of a palladium ac-
etate catalyst. Considering the high performance of the co-catalyst,
we expect our prototype co-catalyst to find broader applications
both in academia and in the fine chemical industry, especially for
oxidative reactions.
(m, s), 2959 (w, s), 2518 (vs, s, nBH), 2340 (m, s), 1975 (w, s), 1922 (w,
s), 1824 (w, s), 1590 (m, s), 1482 (s, s), 1438 (vs, s), 1308 (w, s), 1182
(vs, s), 1127 (vs, s), 1069 (w, s), 1024 (w, s), 948 (m, s), 919 (w, s), 744
(m, s), 720 (m, s), 695 (s, s), 553 (m, s), 523 (w, s), 486 (m, s).
4.2. Synthesis of complex [n-Bu4N]{Fe(3,30)-[1,2-(PPh2)2-1,2-
C2B9H9]2} (4)
4. Experimental
Compound (4) was synthesized by a modified literature pro-
cedure [24e26]. [Me3NH] [nido-7,8-(PPh2)2-7,8-C2B9H10] (1.69 g,
3 mmol) was added slowly added to a solution of NaH (0.17 g,
7 mmol) in 30 mL of anhydrous THF. The mixture was heated to
reflux for 4 h with continuous stirring. The resulting trimethyl-
amine was removed by passing a stream of argon. After reaction,
the unreacted NaH was filtered off and the filtrate was added to a
stirring suspension of ferrous chloride (0.76 g, 6 mmol) in 16 mL
anhydrous THF. The resulting mixture was stirred at room tem-
perature for 1.5 h in argon followed by 1.5 h in air. The solvent was
removed in vacuum, and the residue was dissolved in 20 mL of de-
ionized water and treated with aqueous tetrabutylammonium
chloride (1.45 g, 4.5 mmol). The precipitating red solid was
collected and purified by recrystallizing from a mixed solvent of
acetone and water (v/v ¼ 2.5:1) to produce 2.07 g (4) in 53.1% yield.
Elemental analysis for C68H94B18NP4Fe: Calculated (%) C 62.83, H
7.29, B 14.97, N 1.08, Fe 4.30; Found: C 62.54, H 7.06, B 14.55, N 1.14,
Fe 4.10 (B and Fe were analyzed by ICP analysis). IR (KBr pellet,
cmꢁ1), 3415 (m, br), 2963 (s, s), 2924 (vs, s), 2854 (s, s), 2518 (s, s,
nBH), 1734 (m, s), 1654 (w, s), 1459 (m, s), 1438 (m, s), 1378 (m, s),
1261 (w, s), 1129 (m, s), 1024 (m, s), 726 (m, s), 695 (m, s), 459 (w, s).
All synthetic procedures and operations were carried out in an
argon atmosphere using standard Schlenk techniques or a glove
box. Organic solvents such as tetrahydrofuran (THF) and hexane
were dried using standard procedures and distilled under argon
before use [29]. The 1,2-Bis-(diphenylphosphino)-carborane and
the trimethylammonium salt of [7,8-bis-(diphenylphosphino)-7,8-
dicarba-nido-C2B9H10
cedures [24e27]. Closo-1,2-carborane, trimethylammonium chlo-
ride, n-butyllithium solution (1.6 in hexane),
diphenylphosphrous chloride, tetrabutylammonium bromide and
other chemicals were purchased from Sigma-Aldrich Pte Ltd. 1H, 13
-
]
were prepared as per the literature pro-
M
C
and 11B NMR were recorded on a Bruker Fourier-Transform multi-
nuclear NMR spectrometer at 400, 100.6 and 128.4 MHz, relative to
external Me4Si (TMS) and BF3$OEt2 standards. Infrared (IR) spectra
were measured using a BIO-RAD spectrophotometer with KBr
pellets technique and presented in the sequence of signal strength
as strong (s), medium (m) and weak (w), and peak pattern as single
(s), multiple (m) and broad (br). Elemental analyses were carried
out on a CHNSO Elemental Analyzer. ICP analysis was determined
using a VISTA-MPX, CCD Simultaneous ICP-OES analyzer. Product
mixtures were analyzed in an Agilent GC (GC-FID).
4.3. Procedures of catalytic oxidative carbonylation of phenol
4.1. Synthesis of complex [Me3NH][nido-7,8-(PPh2)2-7,8-C2B9H10
(3)
]
Before conducting a reaction, the reactor was checked for
leakage using helium gas. Palladium catalyst (~1mol%), co-catalyst
(Mn(acac)3 or Co(acac)3 or Ce(acac)3 or 4), and molecular sieves
(2 g) were charged to an autoclave of internal volume 160 mL.
Under a He atmosphere, a solution of phenol (5.0 g in 40 g aceto-
nitrile) was charged by syringe. The vessel was then fed with
~57.7 bar of carbon monoxide followed by ~2.3 bar of oxygen. The
vessel was heated to 110 ꢀC with constant stirring followed by the
work up the reaction after the desired number of hours. The
resulting mixtures were analyzed by GC-FID chromatography. The
experimental results are summarized in Table 1.
The literature method was used to prepare the trimethy-
lammonium salt of dicarbadodecahydroundecaborate anion
[24e27]. A solution of 0.70 g (12 mmol) of KOH in 25 mL of anhy-
drous ethanol was added in a 100-mL two-necked round bottom
flask equipped with a magnetic stirring bar and a reflux condenser.
After the KOH dissolving completely, closo-1,2-bis-(diphenylphos-
phino)-carborane (2.56 g, 5 mmol) was added and resulting
mixture was heated to reflux for 10 h until hydrogen evolution
ceased. The solution was cooled to room temperature and CO2 gas
was bubbled to the solution with continually stirring to precipitate
the excess KOH. After filtration to remove the resulting solid and
washed with 10 mL of ethanol, the combined filtrate was evapo-
rated under reduced pressure. The crude potassium salt was dis-
solved in 20 mL of de-ionized water, and treated with an aqueous
solution of 0.76 g (8 mmol) Me3HNCl in 10 mL of de-ionized water.
The precipitated trimethylammonium salt was isolated by filtra-
tion, washed with 5 mL cold water and dried in vacuo. The crude
product was further purified by re-crystallization from water to
obtain pure product [Me3NH] [7,8-(PPh2)2-7,8-C2B9H10] 1.80 g in
64.1% yield. Elemental analysis for C29H40B9NP2: Calculated (%) C
61.99, H 7.18, B 17.32, N 2.49; Found (%) C 62.23, H 7.00, B 17.04, N
2.15 (B was analyzed by ICP analysis). 1H NMR (DMSO-d6, ppm),
Acknowledgements
We thank the Institute of Chemical and Engineering Sciences,
Agency for Science, Technology and Research, Singapore, and
Singapore-MIT Alliance for Research and Technology Innovation
Centre (NG120510ENG(IGN)) for financial support. NSH thanks the
support from the National Science Foundation (CHE-0906179).
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.jorganchem.2017.05.040.
d
¼ 8.91 (s, 1H, NH), 7.75e7.32(m, br, 20H, 4C6H5), 2.72 (s, 9H, 3
References
CH3), 1.79e1.19 (m, br, 9H, 9BH), ꢁ1.92- -2.31 (br, 1H, bridge B-H).
13C NMR (DMSO-d6, ppm),
d
¼ 133.01,132.98,130.76,130.60,129.55,
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d
¼
ꢁ13.68 (2B, JBH
¼
119 Hz), ꢁ16.94 (3B,
1JBH
¼
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