Zinc complexes with 1,2-disubstituted benzimidazole ligands: Experimental and theoretical studies in the catalytic cycloaddition of CO2 with epoxides

Article abstract of DOI:10.1016/j.poly.2019.114134

The synthesis and characterization of four zinc(II) complexes with monodentate 1,2-disubstituted benzimidazole ligands and their catalytic activity in the cycloaddition of CO₂ and epoxides are reported. The complexes were characterized by

Full text of DOI:10.1016/j.poly.2019.114134

Polyhedron 173 (2019) 114134
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Polyhedron
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Zinc complexes with 1,2-disubstituted benzimidazole ligands:
Experimental and theoretical studies in the catalytic cycloaddition of
CO with epoxides
2
a,1
a
a
Jorge Luiz Sônego Milani , Werberson de Almeida Bezerra , Ana Karoline Silva Mendanha Valdo ,
a
b,2
b
Felipe Terra Martins , Lilian Tatiane Ferreira de Melo Camargo , Valter Henrique Carvalho-Silva ,
c
a
a,
⇑
Sailer Santos dos Santos , Danielle Cangussu , Rafael Pavão das Chagas
a
Instituto de Química, Universidade Federal de Goiás, 74690-900 Goiânia, GO, Brazil
Grupo de Química Teórica e Estrutural de Anápolis, Campus de Ciências Exatas de Anápolis, Universidade Estadual de Goiás, 75132-400 Anápolis, GO, Brazil
Departamento de Química, Universidade Federal de Santa Maria, 97105-900 Santa Maria, RS, Brazil
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis and characterization of four zinc(II) complexes with monodentate 1,2-disubstituted benz-
Received 30 April 2019
Accepted 26 August 2019
Available online 31 August 2019
2
imidazole ligands and their catalytic activity in the cycloaddition of CO and epoxides are reported. The
1
13
complexes were characterized by H and C NMR, thermal analysis, infrared absorption and FT-Raman
spectroscopy and ESI-HRMS. Complex Zn3 had its structure determined by single-crystal X-ray diffrac-
tion and a description of intra- and intermolecular interactions has been done. All complexes were able
to catalyze the production of cyclic carbonates selectively, and complex Zn1 was used in the transforma-
tion of various epoxides to carbonates, using tetrabutylammonium bromide as cocatalyst. Quantum
chemistry approaches were used to comprehend the initial step of catalytic cycle: the epoxide coordina-
tion to the metallic center. The calculations show that one benzimidazole ligand should be dislocated
from the metal center before the catalyst interacts with the epoxide to activate it.
Keywords:
Zinc complex
Carbon dioxide
Cyclic carbonate
Catalysis
DFT
Ó 2019 Elsevier Ltd. All rights reserved.
1
. Introduction
neous and homogeneous catalytic systems have been described
2
to perform the coupling of CO and epoxides. Among homogeneous
The future exhaustion of oil resources and the high levels of car-
bon dioxide in the atmosphere due to anthropogenic emissions
have impelled the scientific community to seek ways to utilize
molecular catalysts, a number of zinc complexes have been
reported in the last years [14–27]. Complexes with monodentate
ligands have not been much studied and reports are sparse
[28,29]. The utilization of zinc is advantageous due to its low price,
low toxicity and high stability, associated with good catalytic
activities.
Considering the complexity in the mechanism involved in the
cycloaddition reaction of carbon dioxide and epoxides to produce
cyclic carbonates, application of molecular electronic structure
methods to give some insight into the reaction mechanism
becomes extremely important. There are several theoretical works
involving the process of cycloaddition reaction of carbon dioxide
and epoxides using different catalysts. Wang et al. [30], using
B3PW91 functional with the 6–311++G(d,p) basis set studied the
system of 1,2-benzenediol/tetrabutylammonium bromide (TBAB)
2 2
CO as a feedstock. Moreover, CO is an inexhaustible, cheap, non-
toxic and non-flammable green source of carbon. The cycloaddition
reaction of carbon dioxide and epoxides to produce cyclic carbon-
ates is one of the most studied and attractive transformations
1–10]. Cyclic carbonates provide a wide range of applications as
polar aprotic solvents, electrolytes in Li-ion batteries, synthetic
intermediates, and monomers for the production of polycarbonates
[
[
11,12].
Since CO
satisfactory results in its transformation [13]. Diverse heteroge-
2
is inert by nature, a catalyst is necessary to achieve
⇑
Corresponding author.
E-mail address: rpchagas@ufg.br (R.P. das Chagas).
Present address: Instituto de Ciências Exatas, Departamento de Química, Univer-
2
and heterogeneous poly-ionic liquids, for the fixation of CO to cyc-
lic carbonates. They observed that the hydrogen bond can signifi-
cantly decrease the barrier height of the reaction, since it
activates the epoxide and stabilizes the stationary points of the
1
sidade Federal de Juiz de Fora, 36036-900 Juiz de Fora, MG, Brazil.
2
Present address: Instituto Federal de Educação, Ciência e Tecnologia de Goiás,
Campus Anápolis, 75131-457 Anápolis, GO, Brazil.
https://doi.org/10.1016/j.poly.2019.114134
0
277-5387/Ó 2019 Elsevier Ltd. All rights reserved.

2
J.L.S. Milani et al. / Polyhedron 173 (2019) 114134
Scheme 1. Synthesis of Zn1–Zn4.
reaction more effectively. Using BP86 DFT functional, Castro-
Gómez et al. provided an useful information on the limitations of
catalysts Zn(salphen)/TBAB and revealed conditions to better
design the catalysis of these reactions [31]. Several other DFT cal-
culations have been used to evaluate the role of catalysts in the
cycloaddition reactions of CO and epoxides, presenting excellent
2
agreement with experimental results, which suggest opening ring
as a limiting-rate step [32–34].
2.2. Synthesis of 1,2-disubstituted benzimidazoles (L1–L4)
Aldehyde (6.0 mmol) and 1,2-phenylenediamine (3.0 mmol)
were added to a solution of HCl (5.0 equiv., 1.5 mL of concentrated
hydrochloric acid) in H
The progress of the reaction was monitored by TLC. After comple-
tion of the reaction, saturated solution of NaHCO was added to the
2
O (15 mL), and the mixture stirred at 50 °C.
3
reaction mixture until neutral pH, and the resulting precipitate fil-
tered off. The filtrate was purified by column chromatography in
silica-gel as stationary phase and n-hexane:ethyl acetate (9:1) as
eluent to give analytically pure product.
Recently, we have described a zinc complex with 2-(2-thienyl)-
1
-(2-thienylmethyl)-1H-benzimidazole as the first example of the
utilization of 1,2-disubstituted benzimidazole complex in the
chemical fixation of CO [35]. The facility of synthesizing and alter-
2
ing steric and electronic characteristics by changing the sub-
stituents of 1,2-disubstituted benzimidazole suggests that they
may be a very versatile type of ligands. Now we turn to describe
the synthesis and characterization of four zinc complexes with
2.2.1. 1-Benzyl-2-phenyl-1H-benzoimidazole (L1)
1
White powder, 90% yield. H NMR (298 K, CDCl ): d (ppm)
3
= 5.46 (s, 2H), 7.11 (d, J = 6.95, 2H), 7.22 (t, J = 8.51, 2H), 7.31 (m,
4H), 7.46 (m, 3H), 7.69 (dd, J = 1.51 and 7.77, 2H) and 7.87 (d,
1
3
1
,2-disubstituted benzimidazole ligands (Scheme 1) and the cat-
J = 8.04, 1H). C NMR (298 K, CDCl ): d (ppm) = 48.40, 110.50,
3
alytic studies in the cycloaddition of CO with epoxides. We also
2
120.03, 122.66, 123.02, 125.99, 127.77, 128.74, 129.05, 129.29,
provide theoretical calculations to give some insight into the reac-
tion mechanism, specifically in the epoxide coordination step. The
129.89, 130.15, 136.09, 136.43, 143.24, 154.19. IR (KBr disc,
ꢀ1
cm ): 574 (w), 588 (w), 634 (w), 701 (w), 731 (s), 742 (s), 763
(m), 778 (m), 824 (w), 850 (w), 891 (w), 908 (w), 928 (w), 969
(w), 989 (w), 1003 (w), 1030 (w), 1078 (w), 1108 (w), 1163 (w),
1180 (w), 1251 (w), 1278 (sh), 1285 (w), 1312 (w), 1333 (m),
1360 (m), 1394 (m), 1441 (s), 1448 (s), 1472 (m), 1482 (w), 1492
(w), 1602 (w), 1615 (w), 2957 (w), 3029 (m), 3060 (m), 3082
2 2
four zinc(II) complexes of formula [ZnCl (L) ] were characterized
1
13
by H and C NMR, thermal analysis infrared absorption and FT-
Raman spectroscopy and ESI-HRMS. Complex Zn3 had its structure
determined by single-crystal X-ray diffraction and a description of
intra- and intermolecular interactions has been done.
ꢀ
1
(
w), 3437 (br). FT-Raman (cm ): 71 (m), 90 (m), 191 (w), 207
(w), 773 (w), 1003 (m), 1031 (w), 1071 (w), 1084 (w), 1113 (w),
150 (w), 1181 (w), 1252 (m), 1289 (w), 1316 (w), 1366 (w),
1392 (w), 1441 (s), 1473 (s), 1492 (w), 1516 (s), 1549 (w), 1603
s), 2950 (w), 3059 (w). DSC peaks (°C): 134 (endo), 138 (endo),
2
. Experimental
1
2.1. Materials and methods
(
Zinc chloride, 2-methoxybenzaldehyde, 4-chlorobenzaldehyde
243 (endo), 364 (exo).
and sodium carbonate were purchased from Sigma-Aldrich and
used as received. o-Phenylenediamine was purchased from TCI
Chemicals and it was recrystallized in hot toluene before use. Ben-
zaldehyde and furfural were purchased form Sigma–Aldrich and
2.2.2. 2-(Furan-2-yl)-1-(furan-2-ylmethyl)-1H-benzoimidazole (L2)
1
Brownish powder, 88% yield. H NMR (298 K, CD
3
OD): d (ppm)
= 5.57 (s, 2H), 6.35 (m, 2H), 6.72 (dd, J = 1.79 and 3.47, 1H), 7.32 (m,
1
3
they were distillated before use. For the catalytic tests, CO
2
gas
3
4H), 7.66 (d, J = 7.98, 2H) and 7.86 (s, 1H). C NMR (298 K, CD OD):
(
99.99%) was purchased from Air Liquide; TBAB, TBAI, TBAC, PPNCl
d (ppm) = 41.06, 108.20, 110.10, 110.46, 111.72, 113.15, 118.24,
and epoxides were purchased from TCI Chemicals and used with-
out further purification.
122.96, 123.96, 135.15, 141.86, 142.74, 143.84, 144.45, 144.76
ꢀ1
and 159.63. IR (KBr disc, cm ): 564 (w), 594 (w), 738 (s), 752
(s), 761 (s), 800 (w), 823 (w), 848 (w), 885 (w), 907 (w), 928 (w),
1008 (m), 1019 (m), 1073 (w), 1142 (w), 1148 (w), 1168 (w),
1237 (w), 1258 (w), 1287 (w), 1327 (w), 1342 (m), 1367 (w),
1358 (w), 1403 (w), 1428 (m), 1447 (m), 1461 (m), 1498 (w),
1516 (m), 1607 (w), 1710 (w), 2849 (w), 2925 (w), 3064 (w),
FT-IR spectra were recorded on a PerkinElmer Spectrum 400 FT-
ꢀ1
IR instrument (KBr discs) in the 4000–400 cm region. FT-Raman
spectra were recorded on a Bruker MultiRAM Raman spectrometer
with a laser line of 1064 nm at the power of 50 mW, with 64 scans
ꢀ1
1
13
taken in the 3500–50 cm range. Solution H and C NMR spectra
were measured with a Bruker Avance III 11.75 Tesla spectrometer
at ambient temperature using TMS as internal reference. TG anal-
ysis was performed with a DTG 60/60H Shimadzu instrument in
dry nitrogen at a heat ingrate of 10 °C/min. DSC analysis was per-
formed with a Shimadzu DSC – 60 plus instrument in dry nitrogen.
Mass spectra were recorded in a Bruker micrOTOF-Q III mass spec-
trometer in positive mode using methanol as solvent.
ꢀ1
3090 (m), 3132 (w), 3149 (w), 3430 (br). FT-Raman (cm ): 86
(w), 109 (w), 224 (w), 772 (w), 907 (w), 1013 (w), 1078 (w),
1149 (w), 1228 (w), 1254 (w), 1273 (w), 1289 (w), 1369 (w),
1409 (w), 1428 (w), 1460 (w), 1515 (s), 1585 (w), 1606 (s), 1621
(s), 1784 (w), 2060 (w), 2076 (w), 2092 (w), 2126 (w), 2157 (w),
2183 (w), 2206 (w), 2219 (w), 2242 (w), 2265 (w), 2282 (w),
2293 (w), 2386 (w), 2976 (w), 3063 (w), 3093 (w), 3137 (w),

J.L.S. Milani et al. / Polyhedron 173 (2019) 114134
3
3
154 (w). DSC peaks (°C): 96 (endo), 210 (endo), 221 (endo), 409
(w), 3061 (w), 3436 (br). FT-Raman (cmꢀ1): 74 (m), 94 (m), 133
(w), 164 (w), 192 (w), 213 (w), 287 (w), 789 (w), 1003 (m), 1016
(
exo).
(
w), 1032 (w), 1088 (w), 1161 (w), 1176 (w), 1239 (w), 1258 (w),
2
.2.3. 1-(4-Chlorobenzyl)-2-(4-chlorophenyl)-1H-benzoimidazole (L3)
1296 (w), 1368 (w), 1455 (m), 1470 (m), 1519 (m), 1605 (s),
White powder, 84% yield. 1H NMR (298 K, CDCl
2945 (w), 2969 (w), 3059 (w). ESI-HRMS: [MꢀCl ] Calc.
+
3
): d (ppm)
+
=
4
5.40 (s, 2H), 7.03 (t, J = 9.64, 2H), 7.20 (d, J = 7.98, 1H), 7.31 (m,
H), 7.43 (dd, J = 3.92 and 10.60, 2H), 7.59 (m, 2H) and 7.86 (t,
667.1607; observed 667.1421; [M+H] calcd. 703.1374; observed
703.1157. DSC peaks (°C): 248 (endo), 318 (endo), 395 (endo).
1
3
J = 10.17, 1H). C NMR (298 K, CDCl
3
): d (ppm) = 47.81, 110.30,
1
20.19, 123.08, 123.50, 127.26, 129.16, 129.39, 130.46, 134.63
2
2.3.2. ZnCl -2-(furan-2-yl)-1-(furan-2-ylmethyl)-1H-benzoimidazole
ꢀ1
and 152.87. IR (KBr disc, cm ): 485 (w), 505 (w), 584 (w), 726
(Zn2)
1
6
(
(
m), 742 (s), 763 (m), 801 (m), 826 (m), 842 (m), 925 (w), 934
w), 988 (w), 1009 (m), 1092 (s), 1113 (w), 1163 (w), 1179 (w),
Yield, 89%. H NMR (298 K, DMSO-d ): d (ppm) = 5.77 (s, 2H),
6.38 (dd, J = 1.84, 3.22, 1H), 6.47 (d, J = 3.23, 1H), 6.76 (dd,
J = 1.77, 3.46, 1 h), 7.27 (m, 3H), 7.54 (dd, J = 0.79, 1.77, 1H), 7.65
(d, J = 7.54, 1H), 7.73 (d, J = 7.53, 1H), 8.01 (dd, J = 0,71, 1.71, 1H).
1
1
187 (w), 1250 (w), 1275 (w), 1287 (w), 1296 (w), 1325 (w),
354 (w), 1383 (w), 1404 (s), 1441 (m), 1454 (s), 1470 (s), 1495
1
3
6
(
m), 1595 (w), 1612 (w), 2849 (w), 2928 (w), 3033 (w), 3059 (w),
C NMR (298 K, DMSO-d ): d (ppm) = 40.93, 108.66, 110.56,
110.87, 112.11, 112.86, 119.00, 122.49, 122.84, 135.29, 142.47,
ꢀ1
3076 (w), 3432 (br). FT-Raman (cm ): 78 (m), 90 (m), 110 (sh),
154 (w), 166 (w), 240 (w), 729 (w), 771 (w), 1010 (w), 1079 (w),
1181 (w), 1251 (m), 1292 (w), 1369 (w), 1410 (w), 1443 (w),
1458 (m), 1474 (w), 1522 (m), 1603 (s), 2928 (w), 2954 (w),
3048 (w), 3060 (w). DSC peaks (°C): 143 (endo), 282 (endo), 402
ꢀ1
143.14, 143.40, 144.73, 145.02, 149.75. IR (KBr disc, cm ): 564
(w), 593 (w), 601 (w), 750 (s), 778 (m), 800 (w), 830 (w), 887
(w), 914 (w), 930 (w), 971 (w), 1016 (m), 1074 (w), 1094 (w),
1146 (w), 1175 (w), 1221 (w), 1234 (w), 1249 (w), 1295 (w),
1347 (m), 1409 (m), 1455 (m), 1467 (s), 1501 (w), 1512 (w),
(
exo).
1
592 (w), 1611 (w), 2946 (w), 3031 (w), 3114 (m), 3130 (sh, w),
ꢀ1
2
.2.4. 1-(2-Methoxybenzyl)-2-(2-methoxyphenyl)-1H-benzoimidazole
L4)
White powder, 75% yield. 1H NMR (298 K, CDCl
3.61 (s, 3H), 3.80 (s, 3H), 5.26 (s, 2H), 6.72 (m, 1H), 6.79 (td,
J = 0.74 and 7.50, 1H), 6.84 (t, J = 11.71, 1H), 6.98 (d, J = 8.25, 1H),
3145 (w), 3462 (br., w). FT-Raman (cm ): 75 (m), 161 (w), 331
(w), 780 (w), 1017 (w), 1075 (w), 1094 (w), 1124 (w), 1153 (w),
1221 (w), 1249 (w), 1276 (w), 1296 (w), 1334 (w), 1347 (w),
1366 (w), 1393 (m), 1426 (w), 1467 (w), 1512 (s), 1612 (s), 2945
(
3
): d (ppm)
=
+
(w), 3074 (w). ESI-HRMS: [MꢀCl ] Calc. 627.0778; observed
7
.07 (td, J = 0.86 and 7.49, 1H), 7.23 (m, 3H), 7.29 (ddd, J = 1.81,
.73 and 6.50, 2H), 7.47 (ddd, J = 1.75, 7.59 and 8.39, 1H), 7.56
627.0459. DSC peaks (°C): 297 (endo), 302 (exo).
5
(
dd, J = 1.72 and 7.50, 1H). 13C NMR (298 K, CDCl
3
): d (ppm)
2
2.3.3. ZnCl -1-(4-chlorobenzyl)-2-(4-chlorophenyl)-1H-
=
1
1
5
43.51, 55.14, 55.21, 109.95, 110.74, 110.85, 119.81, 120.40,
20.79, 121.92, 122.44, 124.60, 127.78, 128.37, 131.38, 132.40,
35.55, 143.36, 152.45, 156.53 and 157.63. IR (KBr disc, cm ):
37 (w), 559 (w), 591 (w), 623 (w), 752 (s), 766 (s), 784 (w), 859
benzoimidazole (Zn3)
Yield, 71%. H NMR (298 K, DMSO-d ): d (ppm) = 5.59 (s, 2H),
7.01 (d, J = 8.50, 2H), 7.27 (m, 2H), 7.35, (d, J = 8.49, 2H), 7.49 (m,
1
6
ꢀ1
1
3
6
1H), 7.60 (m, 2H) and 7.74 (m, 3H). C NMR (298 K, DMSO-d ): d
(ppm) = 46.82, 111.07, 119.36, 122.42, 122.98, 128.00, 128.77,
128.90, 130.77, 132.07, 134.75, 135.81, 142.55 and 152.04. IR
(
w), 891 (w), 952 (w), 973 (w), 1009 (sh, m), 1020 (m), 1041 (w),
1
1
1
109 (w), 1127 (w), 1166 (m), 1180 (w), 1191 (w), 1255 (s),
263 (sh, F), 1273 (m), 1291 (m), 1330 (w), 1348 (w), 1370 (w),
388 (w), 1427 (w), 1438 (w), 1459 (s), 1491 (m), 1498 (s), 1556
ꢀ1
(KBr disc, cm ): 750 (s), 786 (w), 1026 (m), 1046 (w), 1077 (m),
1111 (sh), 1117 (w), 1161 (w), 1178 (w), 1232 (sh), 1254 (s),
1291 (m), 1332 (sh), 1345 (w), 1414 (s), 1448 (sh), 1461 (s),
1477 (m), 1494 (m), 1523 (w), 1587 (w), 1607 (w), 2836 (w),
2938 (m), 2958 (m), 3008 (w), 3068 (w), 3458 (m, br). FT-Raman
(
w), 1584 (m), 1607 (s), 2639 (w), 2722 (w), 2839 (w), 2938 (w),
ꢀ1
3
021 (w), 3189 (sh), 3277 (sh), 3386 (br). FT-Raman (cm ): 76
(
s), 169 (w), 772 (w), 789 (w), 1006 (w), 1045 (w), 1054 (w),
ꢀ1
1
1
1
077 (w), 1162 (w), 1239 (w), 1252 (m), 1285 (w), 1376 (w),
393 (w), 1437 (w), 1455 (m), 1474 (w), 1520 (m), 1587 (w),
605 (s), 2842 (w), 2931 (w), 2961 (w), 3012 (w), 3066 (m). DSC
(cm ): 75 (s), 105 (m), 167 (w), 241 (w), 308 (w), 318 (w), 731
(w), 763 (w), 1016 (w), 1079 (w), 1095 (w), 1176 (w), 1247 (w),
1292 (w), 1429 (w), 1459 (w), 1519 (m), 1527 (m), 1604 (s),
peaks (°C): 155 (endo), 262 (endo), 380 (exo).
2952 (w), 2982 (w), 3028 (w), 3051 (w), 3067 (m). ESI-HRMS: [M
+
+H] Calc. 838.9815; observed 838.9716. DSC peaks (°C): 259
2.3. Synthesis of complexes Zn1–Zn4
(endo), 394 (exo)
An ethanolic solution of the pro-ligand L1–L4 (2 mmol) was
2
2.3.4. ZnCl -1-(2-methoxybenzyl)-2-(2-methoxyphenyl)-1H-
added in an ethanolic solution of zinc chloride (1 mmol), a precip-
itated was formed with time. After refluxing for 16 h a white
benzoimidazole (Zn4)
1
6
Yield, 86%. H NMR (298 K, DMSO-d ): d (ppm) = 3.67 (d, 6H),
5.21 (s, 2H), 6.59 (d, J = 7.48, 1H), 6.76 (t, J = 7.45, 1H), 6.93 (d,
J = 8.23, 1H), 7.08 (t, J = 7.46, 1H), 7.20 (m, 4H), 7.38 (d, J = 7.14,
1H), 7.42 (d, J = 7.45, 1H), 7.53 (t, J = 7.90, 1H) and 7.70 (m, 1H).
(
brownish for Zn2) solid was filtered off and it was washed with
cold ethanol (3 ꢁ 10 mL) and ethyl ether (2 ꢁ 10 mL) to furnish
the pure product after drying in vacuum.
1
3
6
C
NMR (298 K, DMSO-d ): d (ppm) = 42.77, 55.24, 55.27,
2
.3.1. ZnCl
2
-1-benzyl-2-phenyl-1H-benzoimidazole (Zn1)
111.80, 111.98, 111.48, 118.98, 120.08, 120.47, 121.71, 122.24,
124.08, 127.48, 128.80, 131.58, 131.88, 135.10, 151.82, 156.35
1
6
Yield, 83%. H NMR (298 K, DMSO-d ): d (ppm) = 5.59 (s, 2H),
.00 (d, J = 7.51, 2H), 7.26 (m, 5H), 7.47 (dd, J = 1.90, 6.22), 7.54
ꢀ1
7
and 157.07. IR (KBr disc, cm ): 751 (s), 783 (w), 1020 (m), 1045
(w), 1076 (w), 1110 (w), 1119 (w), 1162 (w), 1181 (w), 1232 (w),
1253 (s), 1290 (m), 1333 (w), 1345 (w), 1413 (s), 1451 (s, sh),
1462 (s), 1478 (m), 1493 (m), 1523 (s), 1585 (s), 1609 (m), 2835
(s), 2936 (s), 2959 (s), 3007 (s), 3068 (s), 3450 (m, br). FT-Raman
1
3
(
(
dd, J = 1.77, 4.95, 3H), 7.73 (dd, J = 2.87, 60.70, 3H). C NMR
298 K, DMSO-d ):
6
d
(ppm) = 47.43, 111.07, 119.24, 122.18,
1
1
5
22.66, 126.07, 127.44, 128.84, 129.01, 129.79, 130.12, 135.85,
36.89, 142.64 and 153.24. IR (KBr disc, cm ): 458 (w), 487 (w),
54 (w), 598 (w), 697 (s), 718 (m), 736 (s), 754 (s), 764 (m), 824
ꢀ1
ꢀ1
(cm ): 75 (s), 154 (w), 250 (w), 306 (w), 674 (w), 720 (w), 755
(w), 787 (w), 1015 (w), 1045 (w), 1076 (w), 1130 (w), 1166 (w),
1248 (w), 1293 (m), 1335 (w), 1377 (w), 1415 (w), 1460 (m),
1523 (m), 1609 (s), 2842 (w), 2947 (m), 3010 (w), 3072 (s).
(
w), 922 (w), 937 (w), 984 (w), 999 (w), 1028 (w), 1076 (w),
1
1
175 (w), 1237 (w), 1255 (w), 1292 (w), 1335 (w), 1354 (w),
411 (s), 1456 (s), 1467 (s), 1495 (m), 1607 (w), 2928 (w), 3031

4
J.L.S. Milani et al. / Polyhedron 173 (2019) 114134
+
ESI-HRMS: [MꢀCl] calcd. 787.2030; observed 787.1882. DSC
peaks (°C): 216 (endo), 270 (endo), 406 (exo).
package program was used to study the above indexes [46]. Recent
theoretical studies in complexation processes support the quan-
tum chemistry apparatus used here [47–49].
2.4. X-ray crystallography
3
. Results and discussion
Single-crystal with appropriate dimensions for X-ray diffraction
3
.1. Synthesis and characterization
data collection of Zn3 was selected out from the crystallization
batch in acetonitrile and mounted on a Bruker-AXS Kappa Duo
diffractometer to measure the intensity data. The diffraction
Production of complexes Zn1–Zn4 was evidenced by the pre-
cipitation of a white or brownish powder in the reaction medium.
The solids were isolated by simple filtration and subsequently
characterized. Infrared absorption spectra indicated coordination
through the imidazole nitrogen. By comparing the spectra of the
ligands with the respective zinc complexes, most of the peaks are
frames were recorded by / and
x scans using APEX2 [36], under
CuK radiation at room temperature. The raw dataset treatment
a
was performed using the program SAINT and SADABS [36]. Multi-scan
absorption correction has been employed to all dataset [37]. The
structure was solved by direct methods with SHELXS-2014/7 [38],
wherein C, N, Cl and Zn were readily assigned from the Fourier
map. Hydrogen atoms were stereochemically positioned and
refined with fixed individual isotropic displacement parameters
shifted to lower wavenumber. The characteristic imidazole
m
(
C@N) and (C@C) vibrations are very close to each other and occur
m
ꢀ1
between 1411 and 1609 cm [50]. It was observed, in the region
of antisymmetric and symmetric C@NAC@C stretching vibrations,
other peaks that can be, most probably, attributed as overtones or
combination bands [51,52]. Benzene and imidazole ring vibrations,
2
3
[Uiso(H) = 1.2Ueq(Csp) or 1.5Ueq(Csp)] using a riding model with
fixed CAH bond lengths of either 0.93 or 0.97 Å, respectively. All
full-matrix refinements were performed on F using SHELXL-2014/7
2
CAH in plane (benzene ring) and CH
zole ring), were observed above 1300 cm . The CH stretching of
2
wagging and twisting (imida-
[
38]. The MERCURY [39], ORTEP3 [40] programs were used within the
ꢀ
1
WINGX [41] software package to prepare artwork representations.
Crystallographic details are available in the Supporting Informa-
tion in CIF format, the CCDC number is 1881938.
ꢀ1
2
CH
900–3100 cm
FT-Raman spectra of the ligands L1-L4 showed the same profile,
with peaks distributed in four characteristic ranges: the aromatic
2
was observed around 2800 cm , and aromatic Csp AH in
ꢀ1
2
.
ꢀ1
CAH stretching area (2840–3154 cm ), the ring skeletal area
2.5. Catalysis experiments
ꢀ
1
(
7
2
1000–1620 cm ), aromatic CAH out-of-plane bending (729–
ꢀ
1
86 cm ) and the lattice and fundamental peaks area (50–
40 cm ). The FT-Raman spectra of complexes Zn1–Zn4 pre-
Catalytic tests were conducted in a Parr reactor system (model
560 with controller model 4848) equipped with a 300 mL stain-
ꢀ1
4
sented the same features of the ligands plus the rise of peaks in
less steel vessel. In a general procedure, the vessel was charged
with one of the catalysts Zn1–Zn4, a cocatalyst, and the epoxide.
Carbon dioxide was pressurized into the mixture and the reaction
was performed under predetermined conditions. When the reac-
tion was complete, the vessel was cooled to 0 °C and the pressure
was slowly released. The conversion was calculated based on the
ꢀ
1
the range of 287–331 cm that can be attributed to the ZnAN
stretching. For both L2 and Zn2, the spectra are broadened owing
thermal interference caused by the heating of the sample [52,53].
The NMR analyses were performed for the four zinc tetrahedral
complexes using DMSO-d
6 2
as solvent. The CH link was observed,
1
1
for all complexes, between 5.21 and 5.77 ppm in H NMR. The
H NMR spectrum of the crude reaction mixture.
1
3
2
chemical shifts in the C NMR spectrum for the CH were observed
in 47.43 (Zn1), 40.93 (Zn2), 46.82 (Zn3) and 42.77 ppm (Zn4). For
2.6. Computational details
Zn4, the methoxyl groups CH O were observed as two peaks (55.24
3
and 55.27 ppm), which proves the different chemical environment
of these two carbon atoms in the complex.
The molecular systems were optimized with Density functional
theory (DFT) at B3LYP functional combined with the Pople split-
valence basis set 6-311G(d) for the representative atoms C, H, N,
O, Cl and S; and LANL2DZ pseudo-potential for the heavy metal
Zn(II) (a mixed basis set 6-311G(d)/LANL2DZ). The stationary
points were characterized by analytic harmonic frequency calcula-
tions. The absence of imaginary frequencies ensures that the opti-
mized structures are local minimum energy. The global charge in
the complexes were considered zero. DFT energies were corrected
employing a Petersson–Frisch empirical dispersion term. The inter-
action energies have been corrected by including basis set super-
position error (BSSE) through counterpoise method proposed by
Boys and Bernardi [42]. All the quantum chemistry calculations
were carried out with GAUSSIAN 09 [43].
Thermal analyzes showed the expected increase in the decom-
position temperature of the complexes compared to the free
ligands (see ESI). A weight loss of 1–2% was observed in all ana-
lyzes in the initial curves, which is associated with absorbed or lat-
tice water [54–56]. All TG curves presented a decomposition at
213 °C (Zn1), 213 °C (Zn2), 254 °C (Zn3) and 215 °C (Zn4), indicat-
ing the loss of one ligand moiety. This reveals that the Zn3 is more
thermally stable than the other complexes, probably because chlo-
rine atoms at para position of the ligand substituent give a more
ionic characteristic to ZnANbenzimidazole bond in Zn3. For all com-
plexes, after the first ligand decomposition, the residual [ZnCl L]
2
species loses mass without any clear stage. Increasing the temper-
ature, the three-coordinate [ZnCl L] species decomposes very
2
The complexation energies,
D
E, were estimated considering the
rapidly, which demonstrates its thermal instability.
interaction between the complexes (Zn1 and Zn3) with styrene
oxide (SO) and 1,2-epoxybutane (EB) as follow (where Ep desig-
nates the epoxide).
3.2. Description of structure of Zn3
Well-shaped single crystals of Zn3 were obtained from the
recrystallization of the complex in acetonitrile. Crystallographic
data and refinement parameters of the structure are summarized
D
E ¼ EZn ꢀ EEp
ð1Þ
Bond order parameters [44,45] were performed in terms of
Mayer, Fuzzy and Wiberg indexes for metal and specific atoms
involved in the interaction at WB97XD/6-311++G(d,p)/Def2-
TZVP//B3LYP/6-311G(d)/LANL2DZ level of calculations. A MULTIWFN
in Table 1. The crystal structure contains one unit of Zn3 ([ZnL
Cl ]) in its asymmetric unit (Fig. 1) and was solved in the Pbca
space group (see Table 1 for crystallographic details). The Zn is
2
-
2
2+

J.L.S. Milani et al. / Polyhedron 173 (2019) 114134
5
Table 1
Crystal data and refinement statistics of crystal structure of Zn3.
Chemical formula
Fw (g/mol)
Crystal system
Space group
Z
C
40
H
28Cl
6
N
4
Zn
b range for data collection (°)
2.088–66.614
ꢀ22 to 9
ꢀ11 to 11
ꢀ49 to 50
34 514
842.75
orthorhombic
Pbca
Index ranges
h
k
l
8
Observed reflections [ I > 2r(I)]
T (K)
296(2)
18.7914(7)
9.5522(3)
42.3087(17)
90
90
90
7594.4(5)
1.474
5.060
Unique reflections
Symmetry factor (Rint
Completeness to hmax (%)
F(0 0 0)
Refined parameters
6562
0.0795
97.8
3424
460
1.012
0.0467
0.1232
Unit cell dimensions
a (Å)
b (Å)
c (Å)
)
a
(°)
b (°)
(°)
2
a
Goodness-of-fit on F (S)
b
c
Final R
1
factor [I > 2
r
(I)]
V (Å3)
c
2
wR factor (all data)
calc. (Mg/m3)
Largest difference peak/hole (e Å
CCDC deposit no.
ꢀ3
)
0.325/ꢀ0.298
D
Absorption coefficient
l
(mmꢀ1)
1881938
"
P
#
"P
#
2
1=2
Pjj
2
2
2
1=2
2
2
ð
F
ðhklÞobsꢀFðhklÞcalc
Þ
F
ꢀ F
ðhklÞobs
ð
F
ðhklÞobs ꢀFðhklÞcalc
Þ
whkl
ðhklÞobsj j ðhklÞcalc jj ; wR
whkl
aS ¼
; bR ¼
Pj
2
¼
P
2
:
j
N
r
ꢀN
p
j
F
j
F
whkl
ðhklÞobs
benzimidazole ring atoms from both ligands is almost perpendicu-
lar, with an angle of 86.11(7)°.
This type of metal complex with benzimidazole ligands substi-
tuted with p-Cl-phenyl is being reported here for the first time. On
the other hand, complexes containing benzimidazole ligands sub-
stituted only with phenyl rings are well known, as those coordi-
2
+
2+
2+
nated to Zn
complexes with p-methoxyphenyl rings (without crystal structure)
60]. The main difference between the previous ones and the Zn3 is
the loss of intramolecular interactions in the latter due to
or Cd
[58,59], as well as analogous Zn
[
pꢂ ꢂ ꢂp
halogen substitution enabling other interactions, such as Clꢂ ꢂ ꢂ
p
Fig. 1. Atom labeling scheme of Zn3 for C8A and C8B (described in the text) and all
non-carbon atoms, which constitutes the asymmetric unit of the crystal structure.
Thermal ellipsoids are drawn at the 50% probability level, hydrogen atoms were
omitted for clarity.
contacts.
3.3. Catalytic studies
All complexes were able to catalyze the cycloaddition of CO
2
coordinated to two nitrogen atoms from distinct benzimidazole
ligands (A and B) and two chloride ligands, assuming a tetrahedral
geometry. The coordination bond lengths ranged from 2.040(3) Å
to 2.2555(11) Å and angles from 105.14(9)° to 113.7(9)°. Coordina-
tion bond lengths and angles are summarized in Table 2. The devi-
ation parameter value from the idealized T geometry for Zn was
d
calculated with continuous shape measurement [57] and resulted
in a low value of 0.268.
with propylene oxide (PO) producing the respective cyclic carbon-
ate with good conversions and high selectivity. Table 3 summa-
rizes the initial screening reactions conducted under 30 bar of
2
CO , at 100 °C and using tetrabutylammonium bromide (TBAB) as
2
+
cocatalyst. The conversion values varied from 44% to 65%. Although
complex Zn3 possesses an electron-withdrawing substituent in
phenyl ring, it presented similar conversion values to Zn4 and
lower conversions than Zn1 and Zn2. It could be expected that
electron-withdrawing groups in the ligands would increase the
Lewis acidity of zinc metal center facilitating the activation of
the epoxide, and the electron donating groups would lead to the
contrary. However, this correlation was not observed. It is possible
to assume that the steric factors and/or lability of the ligands have
a significant influence on the system.
In the spirit of the molecular configuration accessed by X-ray
structure of Zn3 and as it will be demonstrated below, theoretical
calculations suggest that the reaction intermediate is a four-coor-
dinate complex produced from the exchange of a benzimidazole
ligand by the epoxide. Thus, the initiation of the reaction depends
on the ligand exchange by the epoxide. A ligand containing an elec-
tron withdrawing group, as in Zn3, would weaken the metal–li-
gand bond and the ligand exchange between L3 and the epoxide
should be faster than the displacement of L4 in Zn4. On the other
The coordination compound assumed a so-called locked-geom-
etry due to its non-classical hydrogen bonding intramolecular
(
CAHꢂ ꢂ ꢂCl) and intermolecular (CAHꢂ ꢂ ꢂ
p
) interactions, as seen in
Fig. 2. In the same figure is verified the intermolecular Clꢂ ꢂ ꢂ
p
con-
tacts. In addition, this conformation assumed by the ligand can be
expressed as the angle between the least-square (l.s.) planes fitted
through the benzimidazole ring atoms and those four l.s. planes fit-
ted through each p-chlorobenzyl atoms for ligands A and B. The p-
chlorophenyl groups bonded to N1A (or N1B) and C8A (or C8B)
form angles with benzimidazole of 62.01(10)° and 84.63(9)° in
ligand A, and 69.77(12)° and 89.86(9)° in ligand B. Whilst, in the
whole coordination compound, the l.s. plane fitted through
Table 2
Coordination bond lengths (Å) and angles (°) in Zn3.
Length (Å)
Angle (°)
hand, the last step (ring closure, after epoxide opening and CO
2
insertion; for mechanistic details see references [61–63]) should
be more difficult because the alkoxide-zinc bond strength of the
intermediate is enhanced by the electron withdrawing effect in
the benzimidazole ligand. In contrast, Zn4 should suffer slower
ligand exchange, since the methoxyl group attached to the ligand
can contribute to increase the donating ability of the ligand and
Zn1AN1A
Zn1AN1B
Zn1ACl1
Zn1ACl2
2.040(3)
2.054(3)
2.2555(11)
2.2283(11)
N1AAZn1AN1B
N1AAZn1ACl1
N1AAZn1ACl2
N1BAZn1ACl1
N1BAZn1ACl2
Cl1AZn1ACl2
112.59(12)
105.14(9)
110.87(9)
113.77(9)
105.28(9)
109.25(5)

6
J.L.S. Milani et al. / Polyhedron 173 (2019) 114134
Fig. 2. (a) Intra and (b and c) intermolecular interactions found in crystal structure. Dashed cyan lines denote CAHꢂ ꢂ ꢂCl interactions and dashed black lines draw CAHꢂ ꢂ ꢂ
p and
Clꢂ ꢂ ꢂ contacts. The centroids Cg1 and Cg2 were calculated through the atoms of benzene ring and p-chlorophenyl (bonded to C8A), respectively, of ligand A.
p
Table 3
The temperature has a considerable impact on the system. As
observed in several reports with distinct metallic ions [67–70],
lower temperatures results in much lower activities and in our
case at 70 °C only 6% of conversion was obtained (Table 4, entry
3). Under screening conditions (100 °C, Table 4, entry 1) 65% of
propylene cyclic carbonate was produced and raising the temper-
ature to 120 °C and 150 °C led to an increase in conversion to
2
Cycloaddition of propylene oxide and CO catalyzed by Zn1–Zn4.
O
Zn1-4
TBAB
O
O
O
CO (30 bar)
2
1
00 ºC, 6h
7
1% and 82%, respectively (Table 4, entries 4 and 5). However, at
1
50 °C (Table 4, entry 5), the production of side products was found
Entry
Catalyst
Conv. (%)a
TONb
TOFc
1
in H NMR spectra; except for this condition, all other experiments
1
2
3
4
5
6
Zn1
Zn2
Zn3
Zn4
65
60
44
45
60
19
1623
1506
1106
1136
1488
–
271
251
184
189
248
–
furnished the product with high selectivity (>99%).
Decreasing pressure to 5 bar, much less CO
reaction medium and kinetics of the reaction is modified [71],
thereby the CO insertion is slow under this condition (conversion
2
is soluble in the
d
Ref. [35]
2
e
of 23%, Table 4, entry 6). In contrast, the decrease in conversion of
the experiment under 20 bar compared to 30 bar is slighter
Reactions conditions: 0.01 mmol Zn catalyst (0.04 mol%), 0.01 mmol TBAB
0.04 mol%), PO (25.00 mmol), T = 100 °C, P[CO2] = 30 bar, t = 6 h.
Conversions determined on basis of H NMR analysis. The selectivity was >99%
in all experiments.
(
Table 4, entries 1 and 8), indicating that in high pressure the
CO insertions are fast.
Binary catalytic systems for the coupling of epoxides and CO
(
a
1
2
2
b
Turnover number (mol of carbonate produced/mol catalyst).
Turnover frequency (TON.h ).
Previously reported zinc complex with 2-(2-thienyl)-1-(2-thienylmethyl)-1H-
involve the Lewis acid catalyst to activate the epoxide and a cocat-
alyst as a nucleophile source to open the epoxide ring. Different
cocatalysts were tested in this work and the studies revealed an
interesting result; regardless of the nature of the cation, both
cocatalysts providing chloride as nucleophile, tetrabutylammo-
nium chloride (TBAC) and bis(triphenylphosphine)iminium chlo-
ride (PPNC), showed nearly the same conversions (Table 4,
entries 9 and 11). This is a strong evidence that, in our system,
the cation of the nucleophile source cocatalyst does not influence
the reaction outcome, as already observed by our group in systems
c
ꢀ1
d
benzimidazole.
e
Blank experiment with TBAB only.
consequently ZnANL4 bond is stronger than ZnANL3 bond. In the
ring closure step, the nucleophilic alkoxide attack is faster for
Zn4 compared to Zn3. Thus, the similar conversions for Zn3 and
Zn4 may be explained by the fact that Zn3 had more effective
epoxide activation while Zn4 promotes more effective nucleophilic
attack to ring closure, being both rate determining steps [64,65].
Complex Zn2 with the ligand containing a furan substituent pre-
sented a catalytic behavior similar to the already reported analo-
gous complex [35] with the more aromatic thiophene substituent
in the ligand [66]. The best activity, under screening conditions,
was obtained with complex Zn1 (Table 3, entry 1) and it was
selected to perform the optimization.
III
III
involving Mn –porphyrin/TBAC and Mn –porphyrin/PPNC [72].
When tetrabutylammonium iodide (TBAI) was employed as cocat-
alyst, it resulted in 53% of conversion (Table 4, entry 10), higher
than in the experiment with TBAC (23%, Table 4, entry 9), showing
that the stronger CACl bond hinders the ring closure. Chloride
anion is much more nucleophilic than iodide in polar aprotic
media, so, the ring opening by TBAC should be faster compared
to TBAI. However, the reaction outcome depends on the epoxide

J.L.S. Milani et al. / Polyhedron 173 (2019) 114134
7
Table 4
Effect of reaction parameters on the cycloaddition of propylene oxide and CO
2
catalyzed by Zn1.
Entry
T
P
[CO2]
Conv. (%)a
TONb
TOFc
(°C)
(bar)
1
2
3
4
5
6
7
8
100
40
70
30
30
30
30
30
5
10
20
30
30
30
65
2
6
1623
62
156
1786
2049
563
1404
1295
579
271
10
26
298
342
94
234
216
96
120
150*
100
100
100
100
100
100
71
82
23
56
52
23
51
22
d
9
1
1
e
0
1
1269
540
212
90
f
Reactions conditions: 0.01 mmol Zn catalyst (0.04 mol%), 0.01 mmol TBAB (0.04 mol%), PO (25.00 mmol), t = 6 h.
a
b
c
d
e
f
1
Conversions determined on basis of H NMR analysis. The selectivity was >99% in all experiments.
Turnover number (mol of carbonate produced/mol catalyst).
ꢀ1
Turnover frequency (TON.h ).
TBAC.
TBAI.
PPNC.
Side products.
*
opening and ring closure, which is affected by both the nucle-
ophilicity and leaving ability of the anion [73]. The balance of both
properties leads to the observed conversion order for the cocata-
lysts: TBAB > TBAI > TBAC ꢃ PPNC.
The versatility of the system was evaluated in the cycloaddition
reaction of CO with a series of different epoxides, using Zn1 as
catalyst. The results are shown in Table 5. Except for cyclohexene
oxide (entry 8), all epoxides presented high conversion values to
2
Table 5
Zn1/TBAB-catalyzed cycloaddition reactions with various epoxides.
Entry
1
Epoxide
Conversion (%)a
90
TONb
2252
TOFc
750
2
3
96
99
2403
2475
801
825
4
5
6
70
1748
1879
2500
583
626
833
75
>99
7
8
97
27
2427
677
809
225
9
96
2408
802
1
1
0
1
>99
>99
2500
2500
833
833
(
continued on next page)

8
J.L.S. Milani et al. / Polyhedron 173 (2019) 114134
Table 5 (continued)
Entry
Epoxide
Conversion (%)a
93
TONb
2336
TOFc
778
1
2
1
3
83
2083
694
Reaction conditions: Catalyst Zn1 (0.01 mmol, 0.04 mol%), cocatalyst TBAB (0.1 mmol, 0.4 mol%), epoxide (25 mmol), 30 bar CO
2
, 100 °C, 3 h.
a
1
Conversions was determined on basis of H NMR analysis. The selectivity was >99% for the cyclic carbonate in all experiments, with the exception of glycidol, for which it
was 77% (entry 6).
b
Turnover number (TON = mole of carbonate produced per mole of catalyst).
Turnover frequency (TOF = TON.h ).
c
ꢀ1
their respective cyclic carbonates. Cyclohexene oxide is sterically
hindered and in general results in much lower conversion. In all
cases, the reaction was selective to produce the cyclic carbonate
3.4. Theoretical studies
Theoretical calculations were performed in order to understand
some aspects of the mechanism for the activation of the epoxide in
our system. As mentioned above in Table 3, the electronic effects of
ligands with opposed electronic characteristics did not lead to sig-
nificant modifications in the catalytic activities when comparing
complexes Zn3 and Zn4 (p-chlorophenyl and p-methoxyphenyl
substituents, respectively). An important role of steric effects in
(
>99%), with the exception of glycidol, for which the selectivity
was 77% (entry 6).
Other zinc metal complexes have been described to catalyze the
same transformation, in similar conditions [15,16,19,21,23]. In the
present article, we could obtain high conversions values using very
lower catalyst/cocatalyst loads compared to other works.
Fig. 3. Calculated structures of possible catalytic active species obtained at B3LYP/6-311G(d) level of calculation with dispersion and BSSE correction. Geometric coordinates
of the optimized complexes can be found in ESI. PO and SO stand for propylene oxide and styrene oxide, respectively.

J.L.S. Milani et al. / Polyhedron 173 (2019) 114134
9
the coordination of epoxide at the metal center was then expected.
For the reaction to proceed, an interaction between the epoxide
and the catalyst is required. Zinc complexes have a tetrahedral
geometry and the coordination of an epoxide molecule at the metal
center would lead to breakdown of the geometry with a significant
energetic cost, as can be seen in Fig. 3, where Zn1 and Zn3 are
coordinated to PO (propylene oxide) and SO (styrene oxide),
respectively. From these calculated molecular structures, it can
be suggested that the energetic profile for a penta-coordinate trig-
onal bipyramidal geometry is much higher than for the four-coor-
dinate tetrahedral species, and from these data, it may be supposed
that to achieve an active catalytic species, firstly, a ligand exchange
between L (L1, L2, L3 or L4) and the epoxide should occur to initi-
ate the catalytic cycle. In the case of Zn1/PO, the optimized molec-
ular structure for bipyramidal intermediate results in
analogues, suggesting that in the first step the substitution of the
benzimidazole ligand by the epoxide in the coordination sphere
of the metal center should occur to initiate the catalytic cycle.
The results reported here shed light on some aspects of the mech-
anism of the cycloaddition of CO
2
and epoxides catalyzed by homo-
geneous metal complexes, and reinforce that zinc based catalysts
are promising candidates to promote the chemical fixation of CO .
2
Acknowledgments
This work was financially supported by Fundação de Amparo à
Pesquisa do Estado de Goiás (No 201610267001033) and by Con-
selho Nacional de Desenvolvimento Científico e Tecnológico
(CNPq). J. L. S. Milani, W. A. Bezerra and L. T. F. M. Camargo thank
the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior
(CAPES), CNPq and FAPEG for fellowships. Valter H. Carvalho-Silva
thanks Brazilian Agency CNPq for the research funding program
[Universal 01/2016 – Faixa A – 406063/2016-8] and the support
given by the High-Performance Computing Center at the Universi-
dade Estadual de Goiás. S. S. dos Santos thanks FINEP CT INFRA –
PROINFRA 02/2010.
ꢀ1
D
D
E = ꢀ10.32 kcal mol , whereas, for the tetrahedral analogue,
ꢀ1
E = ꢀ22.21 kcal mol . These values support that the active spe-
cies is tetrahedral derived from the exchange of one benzimidazole
ligand by the epoxide. The thermal analyzes support this mecha-
nistic proposition, since all the complexes have lost a benzimida-
zole ligand (L1–L4) instead of a chloride ligand. The steric effects
plays a key role in the displacement of the ligands, besides that,
the tetrahedral [ZnCl
the hypothetical [ZnClL
2
L(Ep)] should be much more stable than
Appendix A. Supplementary data
+
2
(Ep)] (where Ep is the epoxide).
The same behavior was observed for Zn3/SO system, as can be
CCDC 1881938 contains the supplementary crystallographic
data for Zn3. These data can be obtained free of charge via
http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the
Cambridge Crystallographic Data Centre, 12 Union Road, Cam-
bridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail:
deposit@ccdc.cam.ac.uk. Supplementary data (experimental
details) to this article can be found online at https://doi.org/10.
seen by comparing the values obtained for optimized molecular
ꢀ
1
structures:
E = ꢀ26.91 kcal mol for tetrahedral. In the system Zn3/SO the
difference between the bipyramidal and tetrahedral analogues
D
E = ꢀ6.79 kcal mol
for
bipyramidal
and
ꢀ1
D
ꢀ1
(
DDE = 20.12 kcal mol ) are much higher than in the system
ꢀ
1
Zn1/PO (DDE = 11.89 kcal mol ) and it is connected to the steric
effects, i.e., the higher steric hindrance of SO compared to PO.
The electronic effect of the ligand was evaluated by NBO
indexes, the parameters of the complexes presented in Fig. 3 are
listed in Table S1. Assuming the complexation process as an impor-
tant step in the catalytic cycle, negligible NBO indexes modifica-
tions in three-atoms ring of the epoxide were observed (ꢃ4%):
corroborating that the change in electron-withdrawing character
of the ligands did not lead to significant modifications in covalent
bonds of the three-atoms ring of the epoxide and consequently on
the epoxide opening, which it is considered the limiting-rate step
in catalytic cycle.
1
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