State of Palladium Complexes in the PdBr2–LiBr–CH3CN–H2O Catalytic System, Used to Obtain Succinic Anhydride

Article abstract of DOI:10.1134/S0036024419010230

Abstract: UV–Vis spectroscopy is used to study the PdBr₂–LiBr–CH₃CN system at different concentrations of palladium and lithium bromides. The resulting data are mathematically processed using hypotheses that include the formation of monomeric and dimeric palladium complexes. Equilibrium constants of the stages of monomer and dimer complexation are calculated, along with the extinction coefficients of palladium complexes. The spectra of individual monomeric and dimeric palladium complexes are reproduced within the model, allowing for the formation of four monomeric and three dimeric complexes. UV–Vis and IR spectroscopy are used to analyze the state of palladium complexes in the PdBr₂–LiBr–CH₃CN system after contact with СО, and directly during the carbonylation of acetylene into succinic anhydride.

Full text of DOI:10.1134/S0036024419010230

ISSN 0036-0244, Russian Journal of Physical Chemistry A, 2019, Vol. 93, No. 2, pp. 222–230. © Pleiades Publishing, Ltd., 2019.
Russian Text © A.Yu. Putin, E.A. Katsman, L.G. Bruk, 2019, published in Zhurnal Fizicheskoi Khimii, 2019, Vol. 93, No. 2, pp. 199–207.
CHEMICAL KINETICS
AND CATALYSIS
State of Palladium Complexes in the PdBr –LiBr–CH CN–H O
2
3
2
Catalytic System, Used to Obtain Succinic Anhydride
a,
a
a
A. Yu. Putin *, E. A. Katsman , and L. G. Bruk
a
Moscow Technological University, Moscow, 119454 Russia
*
e-mail: putinalekse@yandex.ru
Received February 20, 2018
Abstract—UV–Vis spectroscopy is used to study the PdBr –LiBr–CH CN system at different concentra-
2
3
tions of palladium and lithium bromides. The resulting data are mathematically processed using hypotheses
that include the formation of monomeric and dimeric palladium complexes. Equilibrium constants of the
stages of monomer and dimer complexation are calculated, along with the extinction coefficients of palla-
dium complexes. The spectra of individual monomeric and dimeric palladium complexes are reproduced
within the model, allowing for the formation of four monomeric and three dimeric complexes. UV–Vis and
IR spectroscopy are used to analyze the state of palladium complexes in the PdBr –LiBr–CH CN system
2
3
after contact with СО, and directly during the carbonylation of acetylene into succinic anhydride.
Keywords: succinic anhydride, maleic anhydride, acetylene carbonylation, palladium bromide complexes,
dimeric palladium complexes, palladium carbonyl complexes
DOI: 10.1134/S0036024419010230
INTRODUCTION
system where Hal is Br or I, M is Li or Na dissolved in
an organic solvent (acetone, acetonitrile, benzonitrile,
acetylacetone, methyl butyl ketone, methyl ethyl
ketone) was developed in [2].
Succinic anhydride is a product of organic synthesis
that is used in industry and medicine. It is used to obtain
various pharmaceutical preparations (including vita-
mins А, В , and Е); as intermediates in the production of
6
Kinetics was studied in [3, 4], and a mechanism
was proposed for acetylene carbonylation in the
PdBr –LiBr–CH CN–H O catalytic system. The
insecticides, agrochemicals, dyes, esters of succinic acid,
and surfactants; as a crosslinking agent in the fabrication
of ion-active membranes, epoxy curing agents; and as a
starch modifier [1].
2
3
2
most likely mechanism of the process is illustrated in
Scheme 1 [3]. Maleic anhydride forms by a mecha-
A technique for obtaining succinic anhydride by nism that includes the insertion of acetylene and car-
acetylene carbonylation in a PdHal –MHal catalytic bon monoxide into the Pd–Pd bond of the Pd(I) car-
2
k1
Pd Br (CO) + C H
Pd Br (CO)(C H ) + CO
2 2 2 2
2
2
2
2
2
K1
k2
Pd Br (CO)(C H ) + CO
Pd Br (CO)(C H )
2 2 2 2
2
2
2
2
Pd Br (CO)(C H ) + H O
(MA)Pd Br (H)
2 2 2
2
2
2
2
2
k3
(
MA)Pd Br (H) + C H
MA + Pd Br (H) (C H )
2 2 2 2 2
2
2
2
2
2
k4
(
MA)Pd Br (H) + CO
(CO)Pd Br (H)(CH CH )
2 2
2
2
2
2
K4
OC CO
O
(
CO)Pd Br (H)(CH CH )
2 2
2
k5
OC CO + CO
O
Pd Br (CO) + AA
2 2 2
Scheme 1. Mechanism of acetylene carbonylation.
222

STATE OF PALLADIUM COMPLEXES IN THE PdBr –LiBr–CH CN–H O
223
2
3
2
bonyl cluster. Succinic anhydride is obtained through palladium complexes were reproduced using the cal-
the formation of maleic anhydride coordinated by pal- culated extinction coefficients.
ladium hydride complex [3].
Mainly qualitative data on the state of palladium
However, the distribution of palladium complexes complexes in organic solvents have been obtained. The
in the initial solution and changes in this distribution
during the process remain unknown. Answers to these
questions are essential for a better understanding of
the mechanism of this conjugated process.
−
3
2−
authors of [24, 25] believed that PdBr and PdBr
4
complexes were present in the PdBr –LiBr–n-
2
С H OH system. The possibility of dimeric palladium
4
9
halide complexes forming was assumed in [26–30].
In this work, we systematically studied the state of
The PdCl –LiCl–CH CN system was investigated
palladium complexes in PdBr –LiBr–CH CN,
2
3
2
3
in [30]. It was found that palladium chloride exists in
the solution as a binuclear complex with bridging
chlorine atoms. This was confirmed by the results
from measuring a Raman spectrum in the range of
PdBr –LiBr–CH CN + СО model systems and the
2
3
PdBr –LiBr–CH CN–H O + СО, С Н reaction
2
3
2
2
2
system via UV–Vis and IR spectroscopy. Information
on the equilibrium of palladium complexation will
allow us to refine the mechanism of this process:
−1
4
00–200 cm . Bands with frequencies of 380–
−1
3
50 cm corresponding to the stretching vibrations of
Pd(II) complexation with alkali metal halides in
aqueous solutions has been studied in detail, and the
the terminal Pd–Cl group, were detected in the spec-
trum of an acetonitrile solution of palladium chloride
2
n
−n
formation of PdX (n = 1–4) complexes in aqueous ([PdCl ] = 0.01 M), along with bands at frequencies of
2
−1
solutions of Pd(II) salts in the presence of halide ions 310–300 and 270–250 cm corresponding to the
has been established. General and stepwise constants stretching vibrations of the bridging Pd–Cl groups
of the stability of these complexes were determined in [30]. In the presence of chlorine ions, the destruction
[
5–21].
In aqueous PdBr 24 − solutions, water substitutes for
of the binuclear complex is preceded by complexation
with the chloride ion [30]. It was thus shown that this
system can contain dimeric palladium complexes:
–
Br ions, while a set of aqua complexes of the compo-
2
4
+
2−
6
Pd Cl (CH CN) , Pd Cl (CH CN) , and Pd Cl
2
– n
2
2
3
2
4
3
2
2
sition [PdBr (H O) ]
4 – n
(n = 1–4) is in equilib-
n
2
[
30].
rium [22, 23]. In [14, 21], data on the equilibrium of
the formation of these complexes were quantitatively
In [31], we quantitatively studied the equilibrium of
–LiBr–THF system via
processed (the stability constants and extinction coef- the complexation in the PdBr
2
ficients were calculated), and the spectra of individual UV–Vis spectroscopy. Different models were used for
mathematical treatment of the resulting data (Table 1).
The best description of the equilibria was obtained
using a model that considered the formation of four
Table 1. Considered models of complexation
−
2−
+
Stages of complexation
Models of complexation
monomeric (PdBr , PdBr , PdBr , PdBr ) and three
2
3 4
2
+
2−
1) Pd2+ + Br– ⇆ PdBr+
dimeric (Pd Br , Pd Br , Pd Br ) palladium com-
2 6
2 2 4 2
(
“0, 1, 2, 3, 4”*—stages 1, 2,
plexes (Table 1, steps 2, 3, 4, 5, and 6). According to
3, 4
+
−3
the calculation results, PdBr (∼1.94 × 10 mol/L)
+
–
−3
(
(
(
(
2) PdBr + Br ⇆ PdBr
“1, 2, 3, 4”—stages 2, 3, 4
“2, 3, 4”—stages 3, 4
“3, 4”—stages 4
and PdBr (∼1.40 × 10 mol/L) complexes predomi-
2
2
−
nate when [PdBr ] = 0.005 M and [LiBr] ~ 0.048 M)
2 Σ
–
3) PdBr + Br ⇆ PdBr
2
3
[
31]. The results showed that in the initial PdBr –
2
−
3
–
2−
4
4) PdBr + Br ⇆ PdBr
CuBr –THF catalytic system used in the conjugated
2
process of alkene hydroxycarboxylation [32, 33], the
−
2−
6
2−
6
5) 2PdBr ⇆ Pd Br
Model with Pd Br —stages
+
3
2
2
predominant palladium complexes were PdBr and
2, 3, 4, 5
PdBr .
2
(
6) PdBr ⇆ Pd Br
Model with Pd Br —stages
Below, we present the results from a quantitative
analysis of complexation in the initial catalytic system
2
2
4
2
4
2, 3, 4, 6
of acetylene carbonylation PdBr –LiBr–CH CN,
+
2+
2
2
3
(
7) 2PdBr ⇆ Pd Br
Model with Pd Br ,
2
2
4
along with information on the state of palladium com-
plexes in this system after contact with СО and during
carbonylation.
2
6
−
Pd Br —stages 2, 3, 4, 5, 6
2
2
+
Modelь с Pd Br , Pd Br ,
2
2
2
4
2
−
Pd Br —stages 2, 3, 4, 5, 6, 7
2
6
EXPERIMENTAL
*
Palladium-containing compounds are designated by the num-
2+
+
−
bers in quotation marks: 0, Pd ; 1, PdBr ; 2, PdBr ; 3, PdBr ; 4,
PdBr2− .
2
3
In this work, we used palladium dibromide PdBr
(pure), lithium bromid LiBr (chemically pure), and
2
4
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 93 No. 2 2019

2
24
PUTIN et al.
1
.5
.2
.9
.6
.3
0
respectively; N is the total number of measurements of
absorbance at all wavelengths under all experimental
conditions. The error of measuring the experimental
1
2
3
4
5
6
7
8
9
1
0
0
0
absorbance value becomes constant when A ≤ 0.2;
iexp
in formula (1), the difference (A − Aicalc) is divided
iexp
by 0.2.
Experiments (with model and reaction systems) in
which the investigated solutions were purged with
gases were conducted in a gas-flow thermostatic reac-
tor with intense mixing of the gaseous and liquid
phases. Portions of the compounds were loaded into
the reactor and the required amount of the solvent was
added. The resulting mixture was stirred for 15–
2
0 min, and the reactor was purged with gases without
200
250
300
350
400
450
500
, nm
mixing for around 20 min. The moment each experi-
ment began was considered to be the start of mixing
when the reactor with the solution was purged with
gases. During spectral experiments, several samples of
the liquid phase were taken to measure the electronic
and IR spectra.
ꢀ
Fig. 1. Electronic absorption spectra of PdBr , LiBr solu-
tions in acetonitrile ([PdBr ] = 5 × 10 M). Thickness of
2
−3
2
Σ
the cuvette, 0.01 cm; [LiBr] = (1) 0.01, (2) 0.04, (3) 0.15,
4) 0.3, (5) 0.54, (6) 0.76, (7) 1.04, (8) 1.61, and (9) 2.6 М.
(
RESULTS AND DISCUSSION
acetonitrile (HPLC 99.9%) (Lab-Scan Analytical Sci-
ences, Poland).
Equilibrium of Complexation
in the PdBr –LiBr–CH CN System
2
3
The investigated solutions were prepared as fol-
lows: The required portions of palladium bromide and
pre-dried lithium bromide were placed in flasks and
the necessary amounts of acetonitrile was added. The
contents of each closed flask were then mixed thor-
oughly for around 30 min until the compounds dis-
solved completely. Two series of solutions with palla-
The PdBr –LiBr–CH CN system ([PdBr ] = 5 ×
0 М) was studied with variation of the LiBr concen-
2
3
2
−3
1
tration (from 0.01 to 2.6 М). Upon an increase in the
bromide ion concentration, the intensity of absorption
bands with maxima at 253 and 340 nm grew in the
electronic spectrum (Fig. 1). As [LiBr] rose, the
absorption bands shifted in the regions of 340 nm and
−3
dium bromide concentrations of 5 × 10 and 5 ×
4
00–410 nm (from 327 to 342 and 410 to 400 nm,
respectively).
‒
4
1
0 mol/L were prepared. The lithium bromide con-
centration was varied. The water content in the solu-
tions did not exceed ∼0.2 wt %.
For a wider change in the composition of the com-
plexes, we measured the absorption spectra of three
solutions of the PdBr –LiBr–CH CN system at
The solutions were analyzed via UV−Vis spectros-
copy on a Specord M-40 spectrophotometer. The
spectra were measured relative to the solvent (acetoni-
trile) at a temperature of 23 ± 2°С in quartz cuvettes
2
3
−4
[
PdBr ] = 5 × 10 M with lithium bromide concen-
2 Σ
trations of 0.014, 0.108, and 0.93 М (Figs. 2a, 2b). The
absorption spectra were measured with two quartz
cuvettes (0.01 and 0.5 cm thick). The solutions were
orange–brown and became more orange as the con-
centration of lithium bromide grew in the solution.
0
.01 and 0.5 cm thick. The error in measuring absor-
bance was ±5%.
Mathematical treatment of the spectral data (cal-
culating the equilibrium constants of the complex-
ation steps for monomers and dimerization, and the
extinction coefficients of the respective complexes)
was done using the Kinetika software [31, 34].
Different models of complexation in the PdBr –
2
LiBr–CH CN system were considered (Table 1). The
3
equilibrium constants of the stages and extinction coeffi-
cients of the complexes were calculated using the experi-
To quantitatively estimate and compare the quality
of models for each of these, the relative mean square
deviations in describing the experimental results were
calculated using the Kinetika software [31, 34]:
−
3
mental data (solutions with [PdBr ] = 5 × 10 M had
2 Σ
nine experimental points, Fig. 1) and with [PdBr ] =
2 Σ
–4
5 × 10 M (three experimental points, Figs. 2a, 2b).
The absorbance of an acetonitrile solution with a cer-
tain composition, measured at five selected wave-
(1) lengths, we label one experimental point.
2
N



Aiexp − Aicalc
Aiexp



1
σ =
×100%,

N i=1
Using the Kinetika software [31, 34] and experi-
where σ is the relative mean square error in the mental data (Figs. 1 and 2a, 2b), we calculated the
description of the experimental results; A , A
are equilibrium constants of the respective stages and the
icalc
iexp
the experimental and calculated absorbance values, extinction coefficients of separate complexes at five
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A
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STATE OF PALLADIUM COMPLEXES IN THE PdBr –LiBr–CH CN–H O
225
2
3
2
А
(a)
А
(b)
0
0
0
0
.12
1.6
.09
.06
.03
0
1.2
0.8
1
2
3
0.4
0
1
2
3
2
30
250
270
290
300
400
500
600
ꢀ, nm
ꢀ
, nm
−4
Fig. 2. Electronic absorption spectra of the PdBr –LiBr–CH CN system at [PdBr ] = 5 × 10 M. ꢀidth of the cuvette:
2
3
2 Σ
(a) 0.01, (b) 0.5 cm; [LiBr] = (1) 0.014, (2) 0.108, and (3) 0.930 М.
wavelengths (253, 272, 340, 385, and 410 nm). The log K , log K , log K ) are, respectively, 0.266 ± 0.397,
5
6
7
upper limit of the extinction coefficients was taken to 2.194 ± 0.147, 0.662 ± 0.088, 1.846 ± 0.692, 5.759, and
5
−1
−1
be 1.5 × 10 L mol cm , since (according to quan- 2.465 ± 0.500. ꢀe were unable to estimate the accu-
5
−1
−1
tum theory) ε must not exceed 1.5 × 10 L mol cm
racy of determining the constant of Pd Br dimer for-
2 4
[35].
mation (Table 1, stage 6), since the calculated PdBr
2
concentration was negligible (close to 0) over the range
Table 2 lists the mean square errors in the descrip-
tion of the experimental results for models that allow
the formation of monomeric and dimeric complexes.
of [LiBr].
Figure 3 and Table 3 give the dependences of the
All values lie within the experimental error (<5%). All experimental and calculated values of absorbance of
of the considered models thus describe the experimen- the solutions on the lithium bromide concentrations at
tal data well.
five wavelengths and concentrations [PdBr ] = 5 ×
2
Σ
−3
−4
Based on the literature data for the PdCl –LiCl– 10 M, [PdBr ] = 5 × 10 M.
2
2 Σ
CH CN system [30] and the mean square errors of the
3
Table 4 lists the decimal logarithms of the extinc-
description of the experimental results (Table 2), the
model that allows the formation of three dimeric com-
−1
−1
tion coefficients logε (ε, L mol cm ) of individual
complexes.
2
2
+
2−
6
plexes (Pd Br , Pd Br , Pd Br ) is better for describ-
Figure 4 presents the concentration distribution
2
2
4
2
ing the equilibrium of complexation in the PdBr – curves of the respective complexes upon varying
2
−
3
LiBr–CH CN system.
[LiBr] and [PdBr ] = 5 × 10 M. Table 5 summarizes
3
2 Σ
the concentrations of complexes at [PdBr ] = 5 ×
10 M and three concentrations of lithium bromide.
According to this model, the system contains com-
2 Σ
−4
2
2
+
2−
6
+
plexes Pd Br , Pd Br , Pd Br , PdBr , PdBr ,
2
2
4
2
2
2
2
+
+
−
2−
The concentrations of the Pd Br , PdBr com-
PdBr , and PdBr . Complexation can be described
by stages 2, 3, 4, 5, 6, 7 (Table 2).
The decimal logarithms of the equilibrium con- curves of the Pd
stants of stages 2, 3, 4, 5, 6, 7 (log K , log K , log K , plexes have maxima. The concentration of PdBr and
2
3
4
plexes diminish as [LiBr] grows. The distribution
2
6
−
−
3
Br , Pd
4
Br
, PdBr , PdBr
2
com-
2
2
2
3
4
2
Table 2. Mean square errors in describing the experimental results for models that allow for the formation of monomeric
and dimeric complexes
Model
Error
“3, 4”
4.78%
“2, 3, 4”
“1, 2, 3, 4”
“0, 1, 2, 3, 4”
3.39%
4.51%
Pd Br
3.71%
2
−
2−
6
2+
2−
6
Model
Pd Br
Pd Br , Pd Br
Pd Br , Pd Br , Pd Br
2
6
2
4
2
4
2
2
2
2
4
2
Error, %
3.25%
3.57%
2.99%
2.76%
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 93 No. 2 2019

2
26
PUTIN et al.
(
а)
Concentration of the complex, mol/L
.005
А
0
1
.2
.8
.4
1
1
2
2
3
3
,1
,2
,1
,2
,1
,2
1
2
3
4
5
6
7
0.004
0.003
0.002
0.001
0
0
0
1
2
3
с(LiBr), mol/L
А
0.20
0.16
0.12
0.08
0.04
0
1
2
(b)
с(LiBr), mol/L
Fig. 4. Concentration distribution curves of complexes in
2+
the model that allows for the formation of Pd Br
,
2
2
4
,1
Pd Br , and Pd Br ([PdBr2]Σ = 5 × 10−3 M). Com-
2
−
2
4
2
6
4,2
plexes: (1) Pd Br2 , (2) Pd Br , (3) Pd Br , (4) PdBr+,
+
2−
5,1
2 2 4 2
2 6
5,2
−
3
2−
(
5) PdBr , (6) PdBr , (7) PdBr
.
2
4
0
1
2
3
2
+
с(LiBr), mol/L
The electronic spectrum of the Pd Br2 complex
2
contains absorption bands with maxima at 239 nm
Fig. 3. Dependences of the experimental absorbances and
1
−1
−1
−1
(
12720 М− cm ), 276 nm (48070 М cm ), and
ones calculated using the model that allows for the forma-
−1
−1
tion of Pd Br2 , Pd Br , and Pd Br on the concentra-
+
2−
413 nm (5794 М cm ), plus а shoulder near 327 nm
2
2
2
4
2
6
−1
−1
tion of lithium bromide at wavelengths of (a) 253, 272,
3
(5437 М cm ).
−3
40 nm and (b) 385, 410 nm ([PdBr ] = 5 × 10 M):
2 Σ
2
2
3
53 nm (exp.), 1,1; 253 nm (calc.), 1,2; 272 nm (exp.), 2,1;
72 nm (calc.), 2,2; 340 nm (exp.), 3,1; 340 nm (calc.),
,2; 385 nm (exp.), 4,1; 385 nm (calc.), 4,2; 410 nm (exp.),
Table 3. Experimental absorbances and ones calculated
2+
using the model that allows for the formation of Pd Br ,
2
2
5,1; 410 nm (calc.), 5,2.
2
−
Pd Br , and Pd Br , depending on the concentration of
2
4
2
6
–4
lithium bromide in the solutions at [PdBr ] = 5 × 10
M
2
Σ
2
−
Pd Br complexes is negligible over the range of (where l is the length of the cuvette)
2
6
[
LiBr], which explains the great extinction coefficients
С(LiBr), mol/L
2
−
obtained for these complexes (Table 4). The PdBr4
concentration grows along with [LiBr].
l, cm
λ, nm
0.014
0.108
0.93
absorbance, А
The electronic spectra of individual compounds
were constructed for this model, which assumes the
presence of three dimeric complexes. Using the Kine-
tika software [34] and our equilibrium constants of the
complexation stages, we calculated the extinction
coefficients of the complexes at wavelengths in the
range of 230 to 470 nm. The resulting electronic spec-
tra of the complexes are shown in Fig. 5.
0
.01
.01
253 (exp.)
0.0486
0.0492
0.1187
0.1199
0.6234
0.6233
0.3080
0.3080
0.5188
0.5190
0.0603
0.0602
0.0910
0.0913
0.9307
0.9311
0.4213
0.4215
0.4173
0.4173
0.1069
0.1032
0.0797
0.0799
1.5706
1.5470
0.6631
0.6500
0.3907
0.3880
253 (calc.)
0
272 (exp.)
272 (calc.)
340 (exp.)
0.5
0.5
0.5
3
40 (calc.)
385 (exp.)
85 (calc.)
+
The electronic spectrum of PdBr has absorption
−1
−1
bands with a maximum at 273 nm (27290 М cm ), a
low-intensity shoulder near 336 nm (1858 М−1 cm ),
and a weak absorption band with a maximum at
3
−1
410 (exp.)
−1
−1
410 (calc.)
4
10 nm (1754 М cm ).
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A
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STATE OF PALLADIUM COMPLEXES IN THE PdBr –LiBr–CH CN–H O
227
2
3
2
Table 4. Decimal logarithms of extinction coefficients (log ε) of individual complexes in the model that allows for the for-
2
2
+
2−
6
mation of Pd Br , Pd Br , and Pd Br
2
2
4
2
PdBr+
−
PdBr 24 −
2+
2−
6
λ, nm
PdBr2
Pd Br
2 4
PdBr3
Pd Br2
Pd Br
2
2
2
2
3
3
53 3.975 ± 0.141 4.972 ± 0.323
—
4.401 ± 0.012
—
4.377 ± 0.052 5.176 ± 0.559
72 4.432 ± 0.083
—
4.256 ± 0.080 4.186 ± 0.014 4.666 ± 0.080 4.523 ± 0.043 5.120 ± 0.552
40 3.212 ± 0.218 4.682 ± 0.186
85 2.898 ± 0.233 4.311 ± 0.204
—
—
3.877 ± 0.013 3.074 ± 0.359 3.784 ± 0.043 4.557 ± 0.540
3.500 ± 0.014 3.170 ± 0.246 3.499 ± 0.055 4.268 ± 0.556
410 3.245 ± 0.114 4.196 ± 0.310 2.682 ± 0.397 3.244 ± 0.034 3.748 ± 0.086 3.543 ± 0.057 4.291 ± 0.595
The electronic spectrum of the Pd Br complex
PdBr –LiBr–CH CN System
2 3
2
4
contains absorption bands with a shoulder near
before and after Purging with СО
−
1
−1
2
47 nm (22130 М cm ) and maxima at 272 nm
IR spectroscopy (Fig. 6). ꢀhen the reactor with the
solution was purged with carbon monoxide, the solu-
tion became light yellow. After 15 min of purging the
PdBr –LiBr–CH CN system with carbon monoxide,
−1
–1
−1
(
33360 М−1 cm ), 340 nm (6076 М cm ), and
00 nm (3525 М cm ).
−1
−1
4
−
2
3
The electronic spectrum of PdBr has an absorption
3
an absorption band corresponding to the terminal car-
−1
−1
band with a maximum at 273 nm (18220 М cm ).
−1
bonyl group at 2120 cm was observed in the IR spec-
trum. After 1 h of contact with СО, the absorption
band corresponding to the bridging carbonyl group
The electronic spectrum of the PdBr 24 complex
has a broad, intense absorption band with a maximum
at 253 nm (22050 М−1 cm ) and an absorption band
−
−1
appeared at ∼1908 сm . The PdBr –LiBr–CH CN
2
3
−1
system thus contained palladium(II) and palladium(I)
carbonyl complexes. After contact with СО, a band
likely corresponding to the bending vibrations of water
−1
−1
with a maximum at 342 nm (7630 М cm ).
e were unable to reproduce the electronic spectra
ꢀ
–1
was measured at 1632 cm in the IR spectra. ꢀater
2
6
−
of the PdBr and Pd Br complexes, due to the very vapor can penetrate into this system along with СО
2
2
low concentrations of these compounds that were supplied from a gas meter whose sealing fluid was an
obtained through calculations.
aqueous salt solution.
T, %
−
1
−1
Extinction coefficient, М сm
100
50000
8
0
1
2
3
4
5
30000
1
2
3
4
60
1
0000
2
4
0
2
200
2000
1800
1600
ꢁ, сm
ꢂ1
00
250
300
350
400
450
500
λ, nm
Fig. 6. IR spectra of the PdBr –LiBr–CH CN system
2
3
before and after purging with СО ([PdBr ] = 0.005 M,
+
−
3
2
Fig. 5. Electronic spectra of (1) PdBr , (2) PdBr , (3)
[
LiBr] = 0.01 M); (1) PdBr –LiBr–CH CN; (2–4) СО
2 3
PdBr2 , (4) Pd Br2 , (5) Pd Br complexes.
−
2+
4
2
2
4
for 15 min, 1 h, and 2 h, respectively.
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 93 No. 2 2019

2
28
PUTIN et al.
+
Table 5. Concentration distribution of complexes in the 0.005 mol/L and [LiBr] ∼ 0.01 mol/L: PdBr ,
2
+
2
+
+
model that allows for the formation of Pd Br , Pd Br , and
2
2
2
4
Pd Br , and Pd Br (the PdBr concentration was
2
2 2 4
2
−
−4
highest). All of these complexes then contributed to
the absorbance of the above bands (Table 4).
Pd Br ([PdBr ] = 5 × 10 M)
2
6
2 Σ
с(LiBr), mol/L
The absorption bands corresponding to palla-
dium(II) bromide complexes virtually disappeared
after 15 min of contact with СО under vigorous stir-
ring; new absorption bands that can be attributed to
the palladium(II) carbonyl complex (by analogy with
Complex
0.014
0.108
0.93
concentrations of the complexes, mol/L
.07 × 10–5
the PdBr –LiBr–THF+СО system [36]) appeared
2
2
+
−
3
0
0
2
Pd Br
2
with maxima at 240 and 390 nm. A band also appeared
at 225 nm after contact with СО, and the intensity of
the absorption band at 280 nm grew as well. These
bands can be attributed to the palladium(I) carbonyl
complex.
Pd Br4
4.25 × 10–5
0
7.20 × 10–5
0
0
2
2.49 × 10–6
2
6
Pd Br
2
PdBr+
PdBr2
3
.24 × 10–4
5.64 × 10–5
0
8.59 × 10–6
1.93 × 10–5
1.12 × 10–5
State of Palladium Complexes in the PdBr –LiBr–
2
0
CH CN–H O Catalytic System during Acetylene
3
2
1.88 × 10–4
9.43 × 10–5
Carbonylation
−
PdBr3
The initial solution containing acetonitrile, 0.05 M
2
trations of palladium and lithium bromides were cho-
sen because of the high rate of the process with them.
1.28 × 10–6
9.33 × 10–5
4.03 × 10–4
2
4
−
PdBr
PdBr , and 0.1 M LiBr was dark brown. The concen-
UV–Vis spectroscopy (Fig. 7). The electronic spec-
trum of the initial mixture contained the following
absorption bands: a shoulder near 230 nm, a band with
According to our calculations, the concentrations
of palladium complexes in the initial system of acety-
lene
carbonylation
PdBr –LiBr–CH CN
at
2
3
a maximum at 272 nm (the most intense band in the [PdBr ] = 0.05 M, [LiBr] = 0.1 M were, respectively,
2
Σ
Σ
spectrum), a band near 340 nm, and a band with a
maximum at 410 nm. According to the results from
studying the equilibrium of palladium complexation in
+
−3
−4
[
PdBr ] = 1.10 × 10 M, [PdBr ] = 1.93 × 10 M,
2
2−
4
−
3
−3
−3
[
PdBr ] = 2.87 × 10 M, [PdBr ] = 1.26 × 10 M,
2
2
2
6
+
−
4
−2
the PdBr –LiBr–CH CN system, the following com- [Pd
Br
] = 3.52 × 10 M, [Pd
Br
] = 2.14 × 10 M,
2
3
2
2
4
−
plexes predominated in this system at [PdBr ] =
−4
2
∑
[Pd Br ] = 5.79 × 10 M. The calculations were per-
2
formed using the Kinetika software with the calculated
equilibrium constants and extinction coefficients of
these complexes in a model that allowed for the forma-
tion of three dimeric complexes. It should be noted
that experimental point [PdBr ] = 0.05 M, [LiBr] =
A
2
.4
.6
.8
0
1
2
3
4
2
Σ
Σ
0
.1 M was not used in combination with the spectral
−3
−4
data at [PdBr ] = 5 × 10 and 5 × 10 M to calculate
2
Σ
1
0
the equilibrium constants and extinction coefficients
of the complexes, since a high concentration of the
solution can result in large deviations from the Buger–
Lambert–Beer law [37]. ꢀe assume that when
[
PdBr ] is raised to 0.05 M, the qualitative composi-
2
Σ
tion of palladium complexes does not change in the
investigated system.
The Pd Br dimeric complex thus predominates in
2
4
2
00
300
400
500
, nm
the PdBr –LiBr–CH CN system at [PdBr ]
=
2
3
2 Σ
ꢀ
0
.05 M, [LiBr] = 0.1 M.
Σ
After the onset of acetylene carbonylation (t =
Fig. 7. Electronic spectra of the PdBr –LiBr–CH CN
2
3
system before and after purging with СО ([PdBr ] =
40°С, PCO : P
∼ 2.5), the solution became light
2
2 2
C H
0
.005 M, [LiBr] = 0.01 M); (1) PdBr –LiBr–CH CN;
orange. The concentration of water was held constant
2
3
(2–4) СО for 15 min, 1 h, and 2 h, respectively.
during the process ([H O] ∼ 0.1 М).
2
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Vol. 93
No. 2
2019

STATE OF PALLADIUM COMPLEXES IN THE PdBr –LiBr–CH CN–H O
229
2
3
2
T, %
20
A
1
1
1
0
0
.4
1
2
3
4
.0
.6
.2
8
0
0
4
1
2
3
4
0
2
200
2000
1800
1600
200
300
400
500
ꢀ, nm
, сmꢂ1
ꢁ
Fig. 8. Evolution of IR spectra during acetylene carbonyla-
tion ([PdBr ] = 0.05 M, [LiBr] = 0.1 M); (1) PdBr –
Fig. 9. Evolution of electronic spectra during acetylene
carbonylation ([PdBr ] = 0.05 M, [LiBr] = 0.1 M);
2
2
2
LiBr–CH CN; (2–4) С Н + СО for 25 min, 1 h, and 2 h,
(1) PdBr –LiBr–CH CN; (2–4) С Н + СО for 25 min,
3
2
2
2 3 2 2
respectively.
1 h, and 2 h, respectively.
IR spectroscopy (Fig. 8). An absorption band that band with a maximum at 272 nm (most intense band
corresponded to the bridging carbonyl group was in the spectrum), a band near 340 nm, and a band with
−1
detected at 1924 cm in the IR spectrum of the system a maximum at 410 nm. According to our results (see
during acetylene carbonylation. This band was above), the main contribution to the absorbance of
attributed to the palladium(I) carbonyl complex. The these bands came from the Pd Br complex.
2
4
coordination sphere of this complex likely contains
acetylene, since the detected band lay at a lower region
The intensity of the absorption bands at 272 and
4
10 nm diminished during acetylene carbonylation,
−1
of wavelengths, relative to the 1908 cm band present
while the intensity of bands near 220–230 nm and
in the IR spectrum of the model PdBr –LiBr–
2
3
40 nm grew. This was most likely due to the forma-
CH CN + СО system. No absorption bands corre-
3
tion of the palladium complex with maleic anhydride.
sponding to the terminal carbonyl group were mea-
sured during the process. The broad band at
Palladium was thus present during acetylene carbo-
nylation in the form of palladium(I) carbonyl com-
plex, the coordination sphere of which apparently
contained acetylene and Pd(I) complex with maleic
anhydride.
−
1
∼
1632 cm corresponds to the bending vibrations of
water, and probably dissolved acetylene as well. The
formation of acetylene carbonylation products was
established: succinic anhydride (the bands at 1864,
−
1
The simplest way of forming palladium(I) carbonyl
complex is naturally through the reduction of dimeric
palladium(II) complexes, since their palladium atoms
are close enough to one another to form the Pd–Pd
bonds in palladium(I) carbonyl. In light of the
acquired data, we may assume that during the process,
1
1
780 cm ), maleic anhydride (the bands at 1852,
−
1
780 cm ), carboxylic acids (succinic, maleic) (the
−1
band at 1736 cm ). The formation of these products
was also confirmed via gas chromatography, the pro-
cedure for which was given in [4].
UV–Vis spectroscopy (Fig. 9). In this system, the active palladium(I) carbonyl is mainly formed from
electronic spectra were measured in a quartz cuvette the Pd Br dimeric complex. Scheme 2 of the forma-
2
4
0
.001 cm thick. The electronic spectrum of the initial tion of carbonyl from Pd Br is given below.
2 4
mixture contained an absorption band near 240 nm, a
CONCLUSIONS
CO
Br
Br
Br
AN
Br
Br
AN
Br
Based on the our data, we managed to refine the
mechanism of acetylene carbonylation (Scheme 1) to
show the most likely way of forming an active palla-
dium(I) carbonyl complex is associated with the trans-
formation of the Pd Br dimeric complex (Scheme 2)
CO, H
2
O
Pd
Pd
Pd
Pd
−
2HBr, CO
2
AN
AN
CO
2
4
Scheme 2. Formation of carbonyl from Pd Br (AN is ace-
present in the initial solution. Based on our spectral
data, we were able to confirm the presence of palla-
2
4
tonitrile).
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 93 No. 2 2019

2
30
PUTIN et al.
dium(I) carbonyl complex in the catalytic solution 19. A. A. Biryukov and V. I. Shlenskaya, Vestn. Mosk.
Univ., Ser. Khim. 19 (5), 81 (1964).
assumption it was coordinated with acetylene 20. V. I. Shlenskaya and A. A. Biryukov, Zh. Neorg. Khim.
1, 54 (1966).
1. C. J. Le Roux and R. J. Kriek, Hydrometallurgy 169,
47 (2017).
2. A. A. Grinberg and N. V. Kiseleva, Zh. Neorg. Khim.
, 1804 (1958).
under conditions of the process and validate the
1
(
Scheme 1, Pd Br (CO) (C H ) complex), along with
2
2
2
2
2
2
2
2
Pd(I) complex and maleic anhydride (Scheme 1,
4
(
МА)Pd Br (H) complex).
2
2
2
3
ACKNOꢀLEDGMENTS
This work was supported by the Russian Founda-
3. Yu. A. Kushnikov, V. F. Vozdvizhenskii, L. N. Zine-
vich, and A. B. Fasman, Zh. Neorg. Khim. 11, 2193
(
1966).
tion for Basic Research, project code 16-33-00371.
2
4. A. N. Nyrkova, Cand. Sci. (Chem.) Dissertation
(
Lomonosov Moscow State Univ. of Fine Chem. Tech-
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Translated by L. Chernikova
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A
Vol. 93
No. 2
2019