Preparation of a novel homogeneous bimetallic Rhodium/Palladium ionic catalyst and its application for the catalytic hydrogenation of nitrile butadiene rubber

Article abstract of DOI:10.1016/j.jorganchem.2016.09.009

A new hybrid second-generation poly (propylene imine) dendrimer terminated by nitrogen-containing triolefinic macrocycle on the periphery (G2-M) was synthesized. The bimetallic Rhodium (III)/Palladium (II) (Rh³⁺/Pd²⁺) dendrimer-stabilized catalysts (G2-M(Rh³⁺_xPd²⁺_10-x)) were prepared by a co-complexation route within G2-M and analyzed by¹H NMR, XRD and XPS. The catalytic activity, selectivity and separation capability for the hydrogenation of nitrile-butadiene rubber (NBR) catalyzed by the G2-M(Rh³⁺_xPd²⁺_10?x) have been researched. As a novel catalyst system, the bimetallic ions of Rh³⁺and Pd²⁺have influenced on the catalytic activity for the hydrogenation of NBR, which can be related to electron-deficiency effect of Rh³⁺resulting from the interaction of Rh³⁺and Pd²⁺within G2-M.

Full text of DOI:10.1016/j.jorganchem.2016.09.009

Journal of Organometallic Chemistry 823 (2016) 76e82
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Preparation of a novel homogeneous bimetallic Rhodium/Palladium
ionic catalyst and its application for the catalytic hydrogenation of
nitrile butadiene rubber
Wei Zhou, Xiaohong Peng*
Department of Polymer Science and Engineering, South China University of Technology, Guangzhou, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A new hybrid second-generation poly (propylene imine) dendrimer terminated by nitrogen-containing
triolefinic macrocycle on the periphery (G2-M) was synthesized. The bimetallic Rhodium (III)/Palla-
dium (II) (Rh3þ/Pd2þ) dendrimer-stabilized catalysts (G2-M(Rh3þxPd2þ10-x)) were prepared by a co-
complexation route within G2-M and analyzed by 1H NMR, XRD and XPS. The catalytic activity, selec-
tivity and separation capability for the hydrogenation of nitrile-butadiene rubber (NBR) catalyzed by the
G2-M(Rh3þxPd2þ10ꢀx) have been researched. As a novel catalyst system, the bimetallic ions of Rh3þ and
Pd2þ have influenced on the catalytic activity for the hydrogenation of NBR, which can be related to
electron-deficiency effect of Rh3þ resulting from the interaction of Rh3þ and Pd2þ within G2-M.
© 2016 Elsevier B.V. All rights reserved.
Received 29 July 2016
Received in revised form
6 September 2016
Accepted 12 September 2016
Available online 15 September 2016
Keywords:
Poly(propylene imine) dendrimer
Catalytic hydrogenation
Bimetallic ions
Nitrile-butadiene rubber
1. Introduction
obstacles. The levels of the metal residues reported are about
160e800 ppm for HNBR before post treatment, which will result in
The copolymer of nitrile-butadiene rubber (NBR) has been
widely applied in various fields. However, the residual unsaturated
carbon-carbon bonds make NBR susceptible to degradation when
exposed to oxygen, ozone, ultraviolet light, high temperature and
some other extreme conditions [1,2]. Normally, this deficiency can
be alleviated by selectively hydrogenating NBR to yield hydroge-
nated nitrile-butadiene rubber (HNBR) [3,4].
At present, the solution hydrogenation of NBR has been the
main approach of preparing HNBR by using a homogeneous or
heterogeneous catalyst. To contrast the difference between using
the two catalysts, the former displays higher catalytic activity.
Generally, the Rhodium (Rh) and Palladium (Pd) catalysts are the
frequently adopted ones for the homogeneous catalytic hydroge-
nation of NBR [5e10]. Particularly, the Rh catalysts are the most
popular homogeneous catalysts because they can provide excellent
hydrogenation degree without reduction of nitrile group [11].
However, the high cost results from the high-price of Rh compared
with other metals for hydrogenation, such as Pd, and the poor
separation of the catalysts from substrates have been the major
high producing cost of HNBR. Moreover, the large amount of Rh
residue can result in crosslinking after hydrogenation [12].
Although the ion extraction, organic extraction and chemical pre-
cipitation methods have been applied to reduce the Rh residue, the
removal of Rh residue is very time-consuming and costly. There-
fore, the preparation of an active bimetallic catalyst, which is
consisted of Rh and Pd and can be facilely separated, will be very
promising.
Dendritic polymer has a stereoregular, highly branched and
well-defined structure. Therefore, the dendrimer-stabilized
metallic particles can be obtained by transferring the catalytic
active centers to the periphery of the functional dendrimer [13,14].
The peripherally modified dendrimer can stabilize the metallic
particles, enhance the catalytic activity, and be easy to remove or
recycle. The double bonds in the nitrogen-containing 15-
membered triolefinic macrocyclic ligands are excellent electron
donators for the transition metallic atoms, and the macrocyclic li-
gands have high antioxide stabilization and tend to coordinate with
metallic atoms to form complexes, which are beneficial to stabilize
and recover metals [15,16]. One can see that it might be applicable
for the catalytic reaction of polymer substrate. However, few re-
ports have been made on the macrocyclic complexes in a process
for the catalytic hydrogenation of NBR.
* Corresponding author.
E-mail address: renewable_2008@126.com (X. Peng).
http://dx.doi.org/10.1016/j.jorganchem.2016.09.009
0022-328X/© 2016 Elsevier B.V. All rights reserved.

W. Zhou, X. Peng / Journal of Organometallic Chemistry 823 (2016) 76e82
77
In this paper, the hybrid poly (propylene imine) dendrimer (G2-
M) has been obtained by modifying the surface of the second-
generation PPI dendrimer with 15-membered triolefinic macro-
cycles. Pd2þ was employed to partly replace Rh3þ in the preparation
of the novel Rh3þ/Pd2þ catalysts (G2-M(Rh3þxPd2þ10ꢀx)). Further-
Trifluoroacetic anhydride (TFAA), CH2Cl2, room temperature; Yield:
89.3%; VI. K2CO3, refluxing CH3CN, trans-1, 4-dibromo-2-butene (1
equiv); Yield: 76.5%.
Characterization data of the arenesulfonamide 3: m. p.: 102 ꢁC;
elemental analyses: found C, 39.02; H, 4.27; N, 4.96; S, 11.76. Calc.
for C9H12BrNSO2: C, 38.85; H, 4.32; N, 5.04; S, 11.51. MALDI-TOF MS
(matrix alpha-Cyano-4-hydroxycinnamic acid): m/z Calc. for
C9H12BrNSO2, 278 [Mþ]; found, 302.4 [Mþ Naþ]. IR (KBr) 3358,
3273, 2930, 2851, 1595, 1550, 1329, 1168, 902 cmꢀ1.1H NMR (D-
more, the structure and catalytic activity of the bimetallic Rh3þ
Pd2þ catalysts were also investigated.
/
2. Experimental
DMSO, 600 MHz, ppm):
d
¼ 1.28 (m, 2H),
d
¼ 1.95 (t, 2H),
d
¼ 2.68
(m, 2H),
J ¼ 8.8 Hz, 2H).
d
¼ 6.48 (s, 2H),
d
¼ 6.59 (d, J ¼ 8.8 Hz, 2H),
d
¼ 6.92 (d,
2.1. Materials
Characterization data of the azamacrocycle 6: m. p.: 139 ꢁC;
elemental analyses: found C, 61.13; H, 7.38; N, 4.17; S, 9.27. Calc. for
The second-generation poly (propylene imine) dendrimer (G2-
PPI) with a diaminobutane core were purchased from SyMO-
Chem B. V. in Netherland. The nitrogen-containing triolefinic
macrocycle (MAC) poly (propylene imine) and the dendrimer
modified by MAC (G2-M) were synthesized by the pathway
shown in Scheme 1 and Scheme 2, respectively. RhCl3.3H2O (99%)
and Pd(OAc)2 (99%) and triphenyl phosphate (TPP, 99%) were
purchased from Macklin company (China). NBR (N31, ACN:
33.5 wt%) was obtained from Shanghai Nessen International
Trading Company.
C
51H74BrN3S3O6: C, 61.20; H, 7.40; N, 4.20; S, 9.60. MALDI-TOF MS
(matrix alpha-Cyano-4-hydroxycinnamic acid): m/z Calc. for
51H74BrN3S3O6, 1000 [Mþ]; found, 1024.4 [Mþ Naþ]. IR (KBr) 2955,
2858, 1595, 1313, 1155, 972 cmꢀ1. 1HNMR (CDCl3, 600 MHz, ppm):
C
d
d
d
¼ 1.24 (m, 36H),
d
¼ 2.18 (t, 2H),
d
¼ 2.87 (m, 4H),
d
¼ 3.39 (t, 2H),
¼ 3.78 (m, 12H),
d
¼ 4.08 (m, 4H),
d
¼ 5.77 (m, 6H),
d
¼ 7.15 (d, 4H),
¼ 7.36 (d, J ¼ 8.8 Hz, 2H),
d
¼ 7.74 (d, J ¼ 8.8 Hz, 2H).
The triolefinic 15-membered macrocycle 6 (MAC) has been ob-
tained in excellent yields. For the preparation of 5 the NH2 groups
must be protected in 1 and 2 to avoid N-dialkylation in step III, and
to obtain a successful reaction between 2 and the arenesulfona-
mide 2 (step IV).
2.2. Preparation of G2-M
MAC (0.48 g, 4.8 ꢂ 10ꢀ1 mmol) was reacted with G2-PPI
(0.052 g, 6.7 ꢂ 10ꢀ2 mmol) using K2CO3 (0.33 g) as an acid-trap
in CH3CN (30 ml) at 82 ꢁC for 24 h under N2. The crude product
was obtained by filtration and rotary evaporation. Finally, the
product was washed with ethyl acetate-pentane (1:6) twice to
remove the unreacted MAC and dried to obtain pure G2-M as light
yellow powdery solid. The preparation route of G2-M is shown in
Scheme 2.
Scheme 1. Conditions: I. (1) HSO3Cl, 0 ꢁC (2) NH3$H2O, 13 ꢁC;
Yield: 33.4%; II. (tert-BuOCO)2O, Et3N, dimethylaminopyridine
(DMAP, 0.1 equiv), CH2Cl2; Yield: 92%; III. K2CO3, CH3CN, trans-1,4-
dibromo-2-butene (4 equiv), reflux; Yield: 82.1%; IV. K2CO3, CH3CN,
arenesulfonamide (0.5 equiv), reflux; Yield: 67.5%; V.
Scheme 1. The synthetic route of MAC.

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W. Zhou, X. Peng / Journal of Organometallic Chemistry 823 (2016) 76e82
Scheme 2. The synthetic route of G2-M.
2.3. Preparation of G2-M(Rh3þxPd2þ
)
maintained for 7 h. The system was cooled down after a given re-
action time. HNBR was obtained by unloading the crude product,
10ꢀx
A equivalent mixture of G2-M, and metal complexes of the
RhCl3$3H2O and Pd(OAc)2 (Rh: Pd ¼ 10:0, 7:3, 5:5, 0:10 (mol:mol))
in tetrahydrofuran (20 ml) was stirring for 24 h at room tempera-
ture under nitrogen. Then the solvent was removed under reduced
pressure to afford solid G2-M(Rh3þxPd2þ10-x)(see Scheme 3).
adding ethanol to flocculate and drying the filtrated rubber. The
organic solution was evaporated, and the G2-M(Rh3þxPd2þ
was recovered.
)
10ꢀx
2.5. Characterization
2.4. Hydrogenation of NBR
The Fourier Transform Infrared Spectra (FT-IR) and Attenuated
Total Reflection Infrared Spectra (ATR-IR) were collected by Bruker
VERTEX70 Infrared Spectroscopy. The G2-M and G2-
M(Rh3þxPd2þ10ꢀx) were determined by Proton Nuclear Magnetic
Resonance (1H- NMR, Bruker Advance III-HD, 600 MHz, CDCl3). The
elemental analysis was carried out with Vario EL III organic element
analyzer. The Matrix Assisted Laser Desorption Ionization Time of
Flight Mass Spectrometry (MALDI-TOF MS) tests were carried out
with Autoflex III smartbean mass spectrometer. The X-ray photo-
electron spectroscopy (XPS) was measured with an X-ray
The hydrogenation of NBR was carried out in an autoclave
reactor. A certain amount of NBR (0.9 g) dissolved in the mono-
chlorobenzene (35 mL) was first added to the reactor, and then G2-
M(Rh3þxPd2þ10ꢀx) (catalyst/NBR ¼ 0.020e0.022 wt%) was added to
the NBR solution. Thereafter H2 was introduced to the reactor by a
hydrogen adapter and maintained at 5.5 MPa after the NBR mixture
was degassed with H2 for three times. The hydrogenation tem-
perature at 100 ꢁC and the stirring speed of 600 rpm were
Scheme 3. The preparation of G2-M(Rh3þxPd2þ10-x).

W. Zhou, X. Peng / Journal of Organometallic Chemistry 823 (2016) 76e82
79
photoelectron spectrometer (Kratos Axis Ulra DLD). The XRD
spectra were collected by an X-ray diffractometer (D8 ADVANCE).
The Rh and Pd residues were measured by an atomic absorption
spectrometer (Z-2000). The hydrogenation degree X is obtained:
Â
ꢀ
Ãꢀ
ꢀ
Ã
X ¼ 1 ꢀ A0970þ917 A20 236 ½A970þ918 A2236
where the absorption peaks at 2236 cmꢀ1, 970 cmꢀ1 and 917 cmꢀ1
723 cmꢀ1correspond to the eC^N, trans-CH]CH and eCH]CH2
groups in NBR, respectively, A0 is the area sum of the corresponding
the peaks of HNBR in FT-IR spectra, and A is the area sum of the
peak of NBR in FT-IR spectra.
3. Results and discussion
3.1. Characterization of G2-M
Fig. 1 shows the FT-IR absorption spectra of G2-PPI, MAC and
G2-M. As shown in Fig. 1, the absorption bands at 3380 cmꢀ1
,
1604 cmꢀ1 and 1329 cmꢀ1 respectively ascribed to eNHe, -C]C-
and O]S]O groups appear in G2-M with the disappearance of the
double peaks at 3285-3350 cmꢀ1 ascribed to the eNH2 groups in
G2-PPI, which indicates the successful preparation of G2-M. The 1H
NMR spectrum of G2-M is illustrated in Fig. 2. As shown in Fig. 2,
the peaks of G2-M at 2.4 ppm(n), 1.22e1.26 ppm(j) and 3.75 ppm (f,
i) are respectively ascribed to the protons of -N-CH2-CH2-, -C-CH3
and NeCH2eC]C [14]. And the characteristic absorption peaks at
5.70e5.80 ppm (g, h) ppm and 7.15e7.73 ppm (l, d, e) attributed to
the protons of -C-CH]CHeC- and aromatic groups are detected in
G2-M, respectively [15]. Moreover, the result of elemental analysis
for G2-M is shown in Table 1. The test values of C, H, N and S in G2-
M are very similar with the calculated ones. And the m/z of 8145
[Mþ Naþ] for G2-M was found by MALDI-TOF MS (matrix of dihy-
droxybenzoic acid) analysis, which is identical with the m/z
calculated for G2-M (C448H680N38S24O48, 8121 [Mþ]). Combined
with all of the characterization and analyses, it can be confirmed
that G2-PPI reacted with MAC the completely.
Fig. 2. 1H NMR spectrum of G2-M (600 MHz, CDCl3)
¼ 1.54e1.62 (d, 44H, b), ¼ 2.40e2.45 (s, 36H, n),
(m, 32H, c, m), ¼ 4.10 (m, 32H, k),
¼ 3.75 (m, 96H, f, i),
¼ 7.15 (d, 32H, l), ¼ 7.33 (d, J ¼ 8.8 Hz, 16H, d),
d
¼ 1.22e1.26 (m, 288H, j),
d
d
d
¼ 2.63e2.72 (m, 32H, a), ¼ 2.87
d
d
d
d
¼ 5.70e5.80 (m, 48H, g, h),
d
d
d
¼ 7.70 (d, J ¼ 8.8 Hz, 16H, e).
Table 1
Elemental analysis and calculated values of G2-M.
Element
C
H
N
S
Cal./% Test/% Cal./% Test/% Cal./% Tes/% Cal./% Test/%
66.17 65.79 8.37 8.22 6.55 6.61 9.45 9.31
G2-M
3.2. Characterization of G2-M(Rh3þxPd2þ
)
10-x
The presence of Rh3þ and Pd2þ coordination with the endocyclic
olefins of the macrocycles can be deduced from the 1H NMR spectra
in Fig. 3. The G2-M shows signals for the endocyclic olefinic protons
Fig. 3. 1H NMR spectra of G2-M and G2-M(Rh3þxPd2þ10ꢀx).
at 5.70e5.80 ppm, whereas the signal intensity at 5.70e5.80 ppm
decreases significantly in G2-M(Rh3þxPd2þ10ꢀx), and a new up-field
shifts at 2.22e2.51 ppm for G2-M(Rh3þxPd2þ10ꢀx) is observed,
which indicates that macrocycles coordinated with Rh3þ and Pd2þ
successfully.
The XRD patterns of RhCl3$3H2O, Pd(OAc)2, G2-M and G2-
M(Rh3þxPd2þ10ꢀx) are illustrated in Fig. 4. The characteristic
peaks at 9.49ꢁ, 36.75ꢁ, 52.75ꢁ and 65.94ꢁ are assignable to
Fig. 1. FT-IR spectra of (a) G2-PPI, (b) MAC, (c) G2-M.

80
W. Zhou, X. Peng / Journal of Organometallic Chemistry 823 (2016) 76e82
Fig. 4. XRD spectra of RhCl3$3H2O and Pd(OAc)2(A); G2-M and G2-M(Rh3þxPd2þ10ꢀx) (B).
RhCl3$3H2O (Fig. 4A). The characteristic peaks of Pd(OAc)2 are
located at 10.95ꢁ and 12.24ꢁ (Fig. 4A). The G2-M exhibits the
characteristic peaks at 12.57ꢁ, 20.37ꢁ, 26.53ꢁ and 38.54ꢁ (Fig. 4B).
However, after the coordination between macrocycles and the
complexes of RhCl3$3H2O and Pd(OAc)2, the peaks at 26.53ꢁ and
38.54ꢁ of the macrocycles disappear, and the peaks of G2-
M(Rh3þxPd2þ at 12.57ꢁ and 20.37ꢁ of the macrocycles
become weaker, indicating the coordination destroyed the crys-
tallinity of G2-M. Moreover, the characteristic peaks of RhCl3$3H2O
and Pd(OAc)2 were not detected in G2-M(Rh3þxPd2þ10ꢀx) because
)
10ꢀx
Fig. 5. XPS scans for the G2-M(Rh3þ10) (A), G2-M(Rh3þ7Pd2þ3) (B, C) and G2-M(Pd2þ10) (D).

W. Zhou, X. Peng / Journal of Organometallic Chemistry 823 (2016) 76e82
81
Fig. 6. The FT-IR spectra of HNBR catalyzed by G2-M(Rh3þxPd2þ10-x) (A); the hydrogenation degrees for HNBR catalyzed by G2-M(Rh3þxPd2þ10-x), G2-M(Rh3þx) and the physical
mixture of G2-M(Rh3þx)/G2-M(Pd2þ10ꢀx) (B). Reaction condition: catalyst concentration ¼ 0.020 wt%, 100 ꢁC, 600 rpm, 5.5 MPa, 7 h.
of the strong interaction of Rh3þ and Pd2þ with the cores of the
triene heterocycles destroying the crystallinity of RhCl3$3H2O and
Pd(OAc)2.
RhCl(PPh3)3 (the most popular catalyst for the hydrogenation of
NBR, concentration ¼ 0.022 wt%) was only 95.8%, and the Rh res-
idue in HNBR was as high as 265 ppm. The high catalytic capability
of G2-M(Rh3þ7Pd2þ3) compared with the RhCl(PPh3)3 can be
ascribed to the peripheral multi-active centers which can enhance
the catalytic activity of the catalyst and the electron-deficiency
effect resulting from the interaction of the bimetallic ions of Rh3þ
and Pd2þ. It can also be seen that the strong coordination between
G2-M and the bimetallic ions of Rh3þ and Pd2þ impeded the metals
leaching in the hydrogenation process and the favourable solubility
of G2-M(Rh3þ7Pd2þ3) in the co-solvents of ethanol and mono-
chlorobenzene reduced the catalyst encapsulation in HNBR during
the separation process.
Fig. 5 presents the XPS scans of the Rh3þ and Pd2þ binding en-
ergy regions for the G2-M(Rh3þ10), G2-M(Rh3þ7Pd2þ3) and G2-
M(Pd2þ10). The XPS peaks of Rh3þ 3d for G2-M(Rh3þ Pd2þ
)
7
3
(Fig. 5C) are present at 309.4 eV (3d5/2) and 313.9 eV (3d3/2), which
are higher than those for Rh3þ 3d of G2-M(Rh3þ10) (Fig. 5A): 308.7
(3d5/2) and 313.5 eV (3d3/2), respectively. The XPS peaks of Pd2þ 3d
for G2-M(Rh3þ7 Pd2þ3) (Fig. 5B) are present at 342.0 eV (3d5/2) and
336.9 eV (3d3/2), which are lower than those for Pd2þ 3d of G2-
M(Pd2þ10) (Fig. 5D): 342.8 eV (3d5/2) and 337.6 eV (3d3/2), respec-
tively. The binding energy shifts of Rh3þ 3d and Pd2þ 3d in G2-
M(Rh3þ Pd2þ3) may be attributed to the shift of electron density
7
from the Rh3þ to the Pd2þ within the macrocycles of G2-M.
4. Conclusions
The homogeneous G2-M(Rh3þxPd2þ10ꢀx) catalysts were pre-
pared by the coordination between G2-M and the complex of
3.3. Catalytic hydrogenation
RhCl3$3H2O and Pd(OAc)2. G2-M(Rh3þxPd2þ
)
displayed
10ꢀx
Fig. 6A shows the FT-IR spectra of HNBR catalyzed by G2-
M(Rh3þxPd2þ10ꢀx). The absorption peaks at 2236 cmꢀ1, 970 cmꢀ1
and 917 cmꢀ1 correspond to the eCN, trans-CH]CH- and eCH]
CH2 groups of NBR. As shown in Fig. 6A, the absorption eNH2 peak
at 3300e3400 cmꢀ1 does not appear after the hydrogenation,
indicating the excellent selectivity of G2-M(Rh3þxPd2þ10ꢀx). The
relative intensity (A970þ918/A2236) of the double bond decreases
with the Rh3þ ratio in G2-M(Rh3þxPd2þ10ꢀx) increasing, as the Rh3þ
is a more active catalytic metallic ion than Pd2þ [17]. As shown in
Fig. 6B, the hydrogenation degrees for HNBR catalyzed by G2-
M(Rh3þxPd2þ10-x) (x ¼ 7, 5) are higher than the ones catalyzed by
G2-M(Rh3þx) (x ¼ 7, 5) and the physical mixture of G2-M(Rh3þx)/
G2-M(Pd2þ10ꢀx) (x ¼ 7, 5). This can be ascribed to the more sig-
nificant electron deficiency of Rh3þ resulting from the interaction
between Rh3þ and Pd2þ, which can make the Rh3þ more active
because the NBR with double bond favors the electron-deficient
surface. Although the hydrogenation degree of HNBR catalyzed by
G2-M(Rh3þ7Pd2þ3) (95.5%) was lower than the HNBR catalyzed by
G2-M(Rh3þ10) (99.8%) when the catalyst concentrations were both
0.020 wt%, the hydrogenation degree of the HNBR catalyzed by G2-
remarkable catalytic activity, high selectivity and facile separation
capability for the hydrogenation of NBR. The hydrogenation degree
for the HNBR catalyzed by G2-M(Rh3þ7Pd2þ3) can reach 99.9%.The
composition of bimetallic ions influenced on the catalytic activity of
G2-M(Rh3þ7Pd2þ3) because of the electron deficiency effect of Rh3þ
resulting from the interaction between Rh3þ and Pd2þ. The metal
(Rh and Pd) residue for the HNBR catalyzed by G2-M(Rh3þ7Pd2þ
)
3
decreased 90.6% compared with the traditional catalyst of
RhCl(PPh3)3. The recovering rate of G2-M(Rh3þ7Pd2þ3) can reach as
high as 94.1% after the first hydrogenation of NBR.
Acknowledgment
This work was supported financially by the National Natural
Science Foundation of China (Project No. 51273071).
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)
3
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82
W. Zhou, X. Peng / Journal of Organometallic Chemistry 823 (2016) 76e82
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