C-C coupling formation using nitron complexes

Article abstract of DOI:10.1039/d0dt02937h

A series of RuII (1), RhIII (2), IrIII (3, 4), IrI (5) and PdII (6-9) complexes of the 'instant carbene' nitron were prepared and characterized by 1H- and 13C-NMR, FT-IR and elemental analysis. The molecular structures of complexes 1-4 and 6 were determined by X-ray diffraction studies. The catalytic activity of the complexes (1-9) was evaluated in alpha(α)-alkylation reactions of ketones with alcohol via the borrowing hydrogen strategy under mild conditions. These complexes were able to perform this catalytic transformation in a short time with low catalyst and base amounts under an air atmosphere. Also, the PdII-nitron complexes (6-9) were applied in the Suzuki-Miyaura C-C coupling reaction and these complexes successfully initiated this reaction in a short time (30 minutes) using the H2O/2-propanol (1.5?:?0.5) solvent system. The DFT calculations revealed that the Pd0/II/0 pathway was more preferable for the mechanism

Full text of DOI:10.1039/d0dt02937h

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C–C coupling formation using nitron complexes†
Mehmet Sevim,a Serdar Batıkan Kavukcu, a Armağan Kınal,a Onur Şahinb and
Cite this: DOI: 10.1039/d0dt02937h
a
Hayati Türkmen
*
A series of RuII (1), RhIII (2), IrIII (3, 4), IrI (5) and PdII (6–9) complexes of the ‘instant carbene’ nitron were
1
prepared and characterized by H- and 13C-NMR, FT-IR and elemental analysis. The molecular structures
of complexes 1–4 and 6 were determined by X-ray diffraction studies. The catalytic activity of the com-
plexes (1–9) was evaluated in alpha(α)-alkylation reactions of ketones with alcohol via the borrowing
hydrogen strategy under mild conditions. These complexes were able to perform this catalytic transform-
ation in a short time with low catalyst and base amounts under an air atmosphere. Also, the PdII–nitron
complexes (6–9) were applied in the Suzuki–Miyaura C–C coupling reaction and these complexes suc-
cessfully initiated this reaction in a short time (30 minutes) using the H2O/2-propanol (1.5 : 0.5) solvent
system. The DFT calculations revealed that the Pd0/II/0 pathway was more preferable for the mechanism.
Received 21st August 2020,
Accepted 28th October 2020
DOI: 10.1039/d0dt02937h
rsc.li/dalton
metric anion analysis in 1905.7 Later, Kriven’ko and Morozova
proposed a different synthesis method for nitron and obtained
Introduction
N-Heterocyclic carbenes (NHCs) were first introduced by Öfele good yields.8 Baker et al. suggested that nitron (B′) was a
half a century ago.1 The first stable NHC was isolated and mesoionic carbene.9 Due to the tautomerization of its struc-
characterized by Arduengo et al.2 Many of the structural modi- ture, nitron can easily bond with the metal from the unpaired
fications undertaken focused on the σ-donation and pairs of electrons on carbon in solution and does not require
π-accepting properties of NHCs,3 which were achieved through any base for this process. Therefore, it is also known as
the modification of either the groups attached to the nitrogen “instant carbene”. In 2012, Siemeling et al. synthesized RhI
atoms or the backbone structure of the imidazole ring. The complexes bearing nitron and demonstrated that they structu-
latter approach includes changing the ring size and the back- rally and electronically resembled the typical Enders carbene
bone topology by attaching functional groups or heteroatoms according to the results of the Tolman electronic parameter
to the backbone. NHCs derived from five-membered hetero- (TEP).10 The authors also prepared RuCl(nitron)(PPh3)2 com-
cycles with different numbers of nitrogen atoms ranging from plexes containing nitron under anaerobic conditions.11
two to four lead formally either to normal N-heterocyclic or Recently, in another study, coinage metal complexes of nitron
mesoionic carbenes with, in some cases, the same skeletal were developed, such as Cu, Ag, and Au. Different copper-
structure. 1,2,4-Triazol-5-ylidine (A) from the NHC family is the halides and copper-dimer complexes were obtained from
result of a replacement of a nitrogen atom with CH at posi- nitron.12 This material is commercially available and can also
tions 4 or 5 of the imidazole ring (Fig. 1). They were found by be used as a ditopic ligand due to its structure containing an
Enders et al. in 1995 and called the ‘Enders carbene’.4,5 They anilite group providing electrons.13 Transition metal com-
are rather unexplored although they might lead to interesting plexes of 1,2,4-triazol-5-ylidine have recently been used for
systems because of their weak σ-donating and strong determining catalytic activity in many organic reactions.14
π-accepting properties as compared to the most studied classic However, there are no catalytic applications of nitron metal
NHC moiety.6
complexes in the literature.
Nitron (B) of the 1,2,4-triazol-5-ylidine derivative was first
presented as Busch’s analytical reagent and utilized in gravi-
aDepartment of Chemistry, Ege University, 35100 BornovaIzmir, Turkey.
E-mail: hayatiturkmen@hotmail.com
bScientific and Technological Research Application and Research Center, University
of Sinop, 57000 Sinop, Turkey
†Electronic supplementary information (ESI) available. CCDC 1883005–1883008
and 1841200. For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/D0DT02937H
Fig. 1 Enders carbene and resonance of the nitron compound.
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C–C bond forming reactions are highly demanded in
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chemical synthesis.15g Cross-coupling is an important way for
synthetic chemists to prepare specific organic compounds
such as natural products, industrial materials and drugs.15a–i
The Suzuki–Miyaura and α-alkylation of ketones are two of the
most important C–C bond forming reactions in organic syn-
thesis. Herrmann pioneered the use of PdII–NHC complexes in
cross-coupling reactions15j,k and this interest has increased
over the past decade.15l,m The α-alkylation of ketones with
alcohol is very useful in the preparation of biologically and
synthetically organic compounds through the borrowing
hydrogen strategy. However, conventional methods have major
disadvantages; e.g., the use of toxic and contaminant alkyl
halides, the formation of salt as waste, low atom economy or
efficiency, and the need to use an excessive amount of a strong
base. The α-alkylation reaction of ketones with alcohol elimin-
ates these undesirable effects, offering benefits, such as green
chemistry, high atom economy, and water formation as a by-
product.15h Transition metal complexes such as Ru, Rh, Ir and
other metals have been used as catalysts for this reaction.16
In this study, to develop an effective strategy for forming C–
C bonds, we prepared a series of ruthenium (1), rhodium (2),
iridium (3–5), and palladium (6–9) complexes of nitron. We
investigated the catalytic properties of these complexes in the
α-alkylation of ketones with alcohol and Suzuki–Miyaura C–C
cross-coupling reactions. We present a highly selective and
effective α-alkylation of ketones with alcohol and Suzuki–
Miyaura C–C cross-coupling reaction catalyzed by nitron
complexes.
Scheme 1 Synthetic pathway for the related compounds (1–5).
nance of the complexes was observed at δ 188.3 (1), 182.0 (2),
199.1 (3), and 181.3 (5) ppm.
The molecular structures of the complexes (1–4) were deter-
mined by single-crystal X-ray diffraction. An intermolecular
Results and discussion
Synthesis and characterization of complexes
As seen in Scheme 1, the nitron complexes (1–5) were prepared π⋯π contact occurred between the triazole and phenyl rings of
using the corresponding starting materials [RuCl2(p- neighbouring molecules. The distance between the ring cen-
cymene)]2, [RhCl2(Cp*)]2, [IrCl2(Cp*)]2 or [IrCl(cod)]2 with troids was 3.403 Å. The combination of these interactions pro-
nitron in CH2Cl2 at room temperature. The piano-stool com- duced a 2D network. The crystallographic analyses revealed
plexes (1–3) were air- and moisture-stable orange solids con- that complexes 2 and 3 were very similar. The asymmetric
taining the chelating NHC ligand formed by nitron through units of complexes 2 and 3 contained three crystallographically
the ortho-metalation of the 1-phenyl substituent. The related independent molecules, while the asymmetric unit of com-
ortho-metallated NHC complexes with M–Caryl are well known plexes 1 and 4 contained one molecule. In complex 2, the
in the literature.17 Surprisingly, we observed the formation of 4 bond distances of Rh–C were 1.993(10) and 2.052(12) Å in Rh1,
as a by-product in a very low amount in the solution during 2.052(12) and 2.046(10) Å in Rh2, and 2.015(10) and 2.054(11)
the synthesis of 3. These complexes 1–5 have good solubility in Å in Rh3, while those of Ir–C in complex 3 were 1.989(12) and
DCM, alcohols, and DMSO. The complexes 1–5 were character- 2.063(12) Å in Ir1, 2.036(11) and 2.047(14) Å in Ir2, and 2.052
1
ized by H-, 13C-NMR spectroscopy and elemental analysis. In (13) and 2.005(13) Å in Ir3. The Ru–Cl bond length was 2.396
the 1H-NMR spectra of complexes 1, 2, 3, and 5, the Ph–NH (3) Å in Rh1, 2.408(3) Å in Rh2, and 2.411(3) Å in Rh3, while
proton in the backbone of nitron has been observed at δ 6.14, the bond length Ir–Cl was 2.403(4) Å in Ir1, 2.411(3) Å in Ir2,
6.41, 6.20, and 6.04 ppm, respectively. This indicated a slight and 2.407(3) Å in Ir3. In complex 1, the bond distances of Ru–
shift to a lower location compared to the signal in nitron (δ C were 2.003(6) and 2.085(6) Å, while the bond distance of Ru–
10.22 ppm). The Ph–NH proton was located at δ 11.85 ppm in Cl was 2.4277(17) Å. In complex 4, the bond distances of Ir–Cl
the 1H-NMR of complex 4. Complex 1 was detected to have were 2.400(2), 2.413(2), and 2.433(2) Å (Fig. 2 and 3).
four doublets for the aromatic protons of p-cymene, and the
The synthesis pathways of the PdII complexes (6–9) are
Cp* methyl protons for complexes 2 and 3 were detected as a shown in Scheme 2. Complex 6 was prepared through the reac-
singlet. In the 13C-NMR spectra, the characteristic CNHC reso- tion of nitron with [Pd(Py)2Br2]. The reaction of complex 6 with
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Fig. 2 The molecular structure of complex 1.
triphenylphosphine resulted in complex 7. The reaction of
nitron with PdBr2(MeCN)2 produced complex 8 with a yield of
24%. Complex 9 was prepared by the reaction of PdBr2 in the
presence of NaBr in DMSO. These complexes were obtained as
air- and moisture-stable yellow solids and were fully character-
ized by 1H- and 13C-NMR. The solubility of complexes (6–9)
was good in apolar and polar solvents. In the 1H-NMR spectra,
the chemical shifts in the NH protons also supported the for-
mation of PdII–nitron complexes. The NH protons were
observed at 9.01, 9.05, 9.16, and 8.95 ppm for complexes 6–9,
respectively. In the 1H-NMR spectrum of complexes 8 and 9,
the 8/9 balance was observed. The chemical shifts of the
Ccarbene peak of the PdII complexes (6–9) were observed at
154.7, 139.7, 159.2, and 151.6 ppm, respectively. In complex 6,
the PdII ion was coordinated by two bromine [Pd1–Br1 =
2.4159(17) Å and Pd1–Br2 = 2.4576(16) Å], one nitrogen [Pd1–
N1 = 2.095(9) Å] and one carbon [Pd1–C6 = 1.959(10) Å] atoms,
thus showing distorted square planar coordination geometry.
The molecules of complex 6 were connected by C–H⋯π and
π⋯π interactions. Atom C10 in the molecule at (x, y, z) acted as
a hydrogen-bond donor to the C14–C19 phenyl ring in the
molecule at (x + 1, y, z), thereby forming a C(9) chain running
parallel to [100] (Fig. 3).
Catalytic studies
Alpha-alkylation of ketones with alcohols
To investigate the potential of the nitron complexes (1–9) for
α-alkylation reactions, acetophenone (20a) and benzyl alcohol
(21a) were selected as benchmark substrates (Table 1). The
reaction was carried out in toluene (1 mL) at 130 °C under an
air atmosphere for 2 h in the presence of complexes 1–9 as the
catalyst and KOH (0.1 mmol). According to the literature, for
this reaction, KOH is one of the best bases and toluene is the
best solvent.18 All the complexes, except 5, transformed the
reaction with 100% yield. The selectivity of the products
(ketone : alcohol ratio) of complexes 1–5 was 76 : 24, 72 : 28,
94 : 6, 82 : 18 and 87 : 6%, respectively. The IrIII complexes of
nitron (3 and 4) exhibited better selectivity than RuII and RhIII
complexes towards the formation of ketone. The effect of
Fig. 3 The molecular structure of complexes 2, 3, 4 and 6 (from top to
bottom).
different metal catalysts and oxidation states like IrI/IrIII on the phenone to form 1-phenyl ethanol, while low valent IrI
catalytic activity was examined. In this reaction, high valent complex 5 showed lower activity, especially when the IrIII
IrIII complex 3 performed better in the α-alkylation of aceto- center was stabilized by the strongly chelating 1-Ph substituent
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Table 1 The effects of catalyst, base and temperature on the alkylation
of 20a with 21aa
Selectivity
(%)
Entry Cat.
Base
Conversion (%) 22aa 23aa
1
2
3
4
5
6
7
8
1
2
3
4
5
6
7
8
KOH
KOH
KOH
KOH
KOH
KOH
KOH
KOH
KOH
KOH
100
100
100
100
78
100
100
100
100
90
76
72
94
82
87
60
46
53
75
51
53
48
81
29
88
95
75
24
28
6
18
6
40
54
47
25
49
47
52
19
37
12
5
9
9
10b
11
12
13
14
15c
16d
17e
[Cp*IrCl2]2
[Cp*IrCl2]2/nitron KOH
100
100
100
3
3
3
3
3
3
NaOH
KOtBu
NaOtBu 67
KOH
KOH
KOH
100
100
85
25
a Reaction conditions: 20a (1.00 mmol), 21a (1.00 mmol), base
(0.10 mmol), catalyst (0.10 mol%), toluene (1.00 mL), time (2 h), temp-
erature (130 °C), under air. b Cat. (0.05 mol%). c T = 120 °C. d T =
140 °C. e Catalyst (0.01 mol%). Yields were determined using 1H-NMR
spectroscopy using diethyleneglycol-di-n-butylether as an internal
standard.
Scheme 2 Synthetic pathway for complexes 6–9.
NaOtBu could not even complete the conversion (Table 1,
entries 12–14). The ideal temperature was set at 130 °C
of nitron. Complex 3 was selected as the main catalyst for (Table 1, entries 15 and 16). Therefore, the conversion was con-
further studies although all catalysts (except 5) completed the ducted to explore the effect of base and catalyst towards selec-
reaction with 100% yield, since the ketone product is the tive alpha(α)-alkylation production. Finally, reaction para-
desired product in this reaction. In the catalytic reaction with meters were optimized with iridium catalyst (3) for 2 hours in
5, 1-phenylethanol was observed as a by-product in the solu- 1 mL of toluene at 130 °C. When the catalyst amount was
tion after 2 hours and the conversion was not completed decreased to 0.05 mol%, it was observed that the conversion
(Table 1, entry 5). According to the results of the in situ study, was not completed (Table 1, entry 17). With the optimal reac-
the selectivity of the α-alkylation of ketone was reduced tion conditions determined, the α-alkylation of benzyl alcohol
(Table 1, entry 11). Also, [Cp*IrCl2]2 had the same selectivity as with different ketone derivatives was investigated, and the
the in situ study. Inspired by this progress, we also focused on results are outlined in Table 2. The reactions of benzyl alcohol
the α-alkylation of acetophenone with benzyl alcohol using with ketone derivatives bearing halogen atoms, namely 4′-
PdII–nitron complexes 6–9 (Table 1, entries 6–9). To the best of chlorine, 2′-chlorine and 4′-bromine produced yields of
our knowledge, the catalytic properties of PdII–NHC complexes 82–92% (Table 2, entries 2–4). For the electron-donating sub-
on the alpha-alkylation of ketones with alcohol are yet to be stituents, such as methoxy, methyl, and dimethyl, α-alkylated
explored although the α-alkylation reaction of ketones with products were obtained with high yields of 98, 95 and 91%,
alcohols is also a kind of C–C coupling reaction. Among PdII– respectively (Table 2, entries 5–7). Higher catalytic yields were
nitron complexes, the highest selectivity to 22aa was obtained achieved using 1-(naphthalene-2-yl)ethanone and 1-tetralone,
with dimeric PdII–nitron complex 9. It was observed that the resulting in products 1-(naphthalen-2-yl)-3-phenylpropan-1-
catalytic selectivity of the palladium complexes increased with one (95%) and 2-benzyl-3,4-dihydronaphthalen-1(2H)-one
the amount of metal. With our preliminary studies, Pd–NHC (96%), respectively (Table 2, entries 8 and 9). The α-alkylation
catalysts can also perform this reaction and we think that it of acetophenone with alcohol derivatives was also investigated
should be evaluated in detail in future studies. Replacing KOH to further explore the scope of the reaction (Table 2).
with NaOH or KOtBu did not improve the selectivity and Transformation of acetophenone with various primary alco-
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Table 2 Scope of α-alkylation ketones of alcoholsa
Table 2 (Contd.)
Ketone (20) or
Alcohol
Ketone (20) or
Alcohol
Entry
18
Product (22)
Yield 22 : 23%
Entry
1
Product (22)
Yield 22 : 23%
86 : 14
92 : 8 (90 : 10)b
19
20
78 : 22 (72 : 28)b
64 : 36
2
3
4
5
90 : 10 (87 : 13)b
92 : 8
a Reaction conditions: ketone (20) (1.00 mmol), alcohol (21)
(1.00 mmol), KOH (0.10 mmol), 3 (0.10 mol%), toluene (1.00 mL),
time (2 h), temperature (130 °C), under air, analysed by NMR.
b Isolated yields. Benzyl alcohol was used as alcohol in the entries
1–10. Acetophenone was used as ketone in the entries 11–20.
82 : 18 (81 : 19)b
98 : 2 (98 : 2)b
hols, including electron-donating, electron-withdrawing, het-
eroaromatic, and aliphatic alcohols, produced yields ranging
from 64 to 97% (Table 2, entries 11–20). We also reported iso-
lated product yields for a select few examples (Table 2).
α-Alkylated products and their derivatives have biological uses
in pharmacology and medicine, such as Phloretin.26 Under
similar reaction conditions, we achieved a TOF value of
230 h−1 with [chloro(N-phenyl-N-octylethylenediamine)(η6-p-
cymene)ruthenium(II)] hexaflorophophate.15h The IrIII complex
(3) in our study reached a TOF value of 470 h−1. Chen and co-
workers prepared bidentate RuII complexes with a pyridonate
fragment and reported a TOF value reaching 3680 h−1 in the
α-alkylation of ketones with alcohols using KOtBu as a base.15c
The results due obtained from this study are considered to be
promising for the easy synthesis of the catalyst and the com-
mercial availability of the NHC ligand at a low cost. The
α-alkylation of 20a with 21a was monitored by NMR spec-
troscopy in d8-toluene to gain further insight into the reaction
mechanism. Products and Ir–H formation were observed as
soon as the reaction started. The Ir–H species persisted
throughout the reaction. A hydride peak appeared in the hydri-
6
7
95 : 5 (91 : 9)b
91 : 9
8
9
95 : 5 (87 : 13)b
96 : 4
10
11
79 : 21
97 : 3 (97 : 3)b
12
92 : 8
13
14
88 : 12 (85 : 15)b
89 : 11
1
dic region of the H-NMR spectrum (δ = −17.4 ppm) with the
formation of 1,3-diphenylpropan-1-one and 1,3-diphenylpro-
pan-1-ol, which suggested that the catalytically active species
were related to Ir–H formation (Fig. 4). The peaks of the sub-
strates (20a and 21a) disappeared in the 1H-NMR spectrum
within 15 minutes after the reaction began. For a practical
availability, gram-scale reaction was carried out using aceto-
phenone with benzyl alcohol, and the ketone 1,3-diphenylpro-
pan-1-one was obtained in an 89% product yield (Scheme 3).
We monitored the time profile of the α-alkylation of ketone
15
16
17
91 : 9 (88 : 12)b
93 : 7
1
reaction with H-NMR spectroscopy (Fig. 5). According to the
observations, the conversion of substrates to 22aa and 23aa
completed within 15 min. The possible mechanism of the
α-alkylation of ketones with primary alcohols is given in
Scheme 4. The mechanism began with the dehydrogenation of
84 : 16 (82 : 18)b
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Scheme 4 Plausible reaction mechanism for α-alkylation of ketones
with primary alcohols.
Fig. 4 1H-NMR monitoring of the hydride signal of Ir-hydride (located
at −17.40 ppm and zoomed in): 20a (1.0 mmol), 21a (1.0 mmol), KOH
(0.1 mmol), cat. 3 (5 mmol %), toluene-d8 (1 ml), temperature (130 °C).
base. The α, β-unsaturated ketone (24) intermediate was
reduced with iridium hydride, which resulted in ketone (22)
and alcohol (23) products. Finally, the oxidation of alcohol (23)
increased the ketone product (22) over time. The most impor-
tant points of this mechanism are that the hydrogen released
by dehydrogenation of the alcohol is borrowed by the catalyst
for the formation of iridium hydride and this hydrogen is
further used in the reduction of enone.16d If benzaldehyde is
used directly instead of alcohol in the first step, the cross-aldol
Scheme 3 Gram-scale synthesis of 22aa.a aReaction conditions: 20a
(1.0 g, 8.33 mmol), 21a (0.9 g, 8.33 mmol), 3 (0.1 mol%), KOH (5.6 mg, condensation reaction occurs.19
0.10 mmol), toluene (10.0 mL), 130 °C, under an air atmosphere.
Isolated yield.
Suzuki–Miyaura cross-coupling reactions
In the last decade, palladium-catalyzed Suzuki–Miyaura reac-
tions have become the most popular cross-coupling method in
classical chemical synthesis.14 In the current study, catalytic
reactions were carried out in H2O/2-propanol (1.5 : 0.5 mL) at
room temperature under an air atmosphere in the presence of
a catalyst and KOH (0.5 mmol). To compare the effect of the
catalytic activity of PdII–nitron complexes 6–9, the reaction of
bromobenzene with phenylboronic acid was selected as a refer-
ence reaction. The progress of the reaction was monitored by
gas chromatography. The catalytic yields of complexes 6–9
were 88, 74, 82, and 76%, respectively (Table 3). The catalytic
activity of complex 6 was 88%, which was relatively higher
compared to complexes 7, 8, and 9 (Table 3, entries 1–4). The
ligands on the metal atom had a substantial effect on the cata-
lytic activity. The solvent system was a mixture of H2O/2-propa-
nol (1.5/0.5 mL), which was more effective than H2O and 2-pro-
panol (Table 3, entries 5 and 6). H2O helps to activate phenyl-
boronic acid, while 2-propanol enhances the activity of aryl
halides. In this study, decreasing the ratios of catalyst loading
from 0.1% to 0.025% resulted in 1,1′-biphenyl with 78% yield
(Table 3, entry 9). Besides, only 18% yield was obtained using
[PdBr2Py2], which indicated the importance of the nitron
ligand for the catalytic reaction (Table 3, entry 10). A control
experiment was applied without any precatalyst, and no
Fig. 5 Time profile of the α-alkylation of acetophenone (20a) and
benzyl alcohol (21a). Reaction conditions: 20a (1.00 mmol), 21a
(1.00 mmol), KOH (0.10 mmol), 3 (0.10 mol%), toluene (1.00 mL), time
(2 h), temperature (130 °C), under air. Conversions and ratios were deter-
mined by 1H NMR analyses.
the primary alcohol (21) to the carbonyl compound (21′) with product formation was detected (Table 3, entry 11). Different
the aid of catalyst 3. During aldehyde (21′) formation, the bases, such as NaOH, NaHCO3, Cs2CO3, K2CO3, and KOtBu,
system releases hydrogen and this hydrogen may help to form which are soluble in water, were also scanned (Table 3, entries
iridium hydride. The two carbonyl compounds (20 and 21′) 12–16). KOH and KOtBu were identified as the ideal bases for
produced α, β-unsaturated ketone (24) in the presence of a this reaction with 88 and 87% yields, respectively (Table 3,
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Table 3 The effects of solvent, base and catalysts on the Suzuki–
Miyaura reaction of 24a with 25aa
Table 4 Suzuki–Miyaura coupling reactions of aryl bromides and
phenylboronic acids under the optimized conditionsa
Aryl bromide
(25)
Arylboronic acid
(26)
Product, yield
(27%)
Cat.
(mol%)
Yield
(%)
Entry
1
Entry Complex
Solvent
Base
1
2
3
4
5
6
7
8
6
7
8
9
6
6
6
6
6
0.1
0.1
0.1
0.05
0.1
H2O/2-propanol KOH
H2O/2-propanol KOH
H2O/2-propanol KOH
H2O/2-propanol KOH
2-Propanol
H2O
H2O/2-propanol KOH
H2O/2-propanol KOH
H2O/2-propanol KOH
H2O/2-propanol KOH
H2O/2-propanol KOH
H2O/2-propanol NaOH
88
74
82
76
65
73
81
80
78
18
—
76
2
3
4
KOH
KOH
0.1
0.075
0.050
0.025
9
10
11
12
13
14
15
16
17b
18b
19b
20b
PdBr2Py2 0.1
6
6
6
6
6
6
6
6
6
6
—
0.1
0.1
0.1
0.1
0.1
H2O/2-propanol NaHCO3 41
H2O/2-propanol Cs2CO3
H2O/2-propanol K2CO3
H2O/2-propanol KOtBu
62
65
87
72
68
67
52
5
6
7
2nd recycle H2O/2-propanol KOH
3rd recycle
4th recycle
5th recycle
H2O/2-propanol KOH
H2O/2-propanol KOH
H2O/2-propanol KOH
a Reaction conditions: 4-bromobenzene (1.0 mmol), phenylboronic
acid (1.0 mmol), base (0.5 mmol), 0.5 mL 2-propanol + 1.5 mL H2O or
solvent (2.0 mL), RT, 0.5 h, under air. Yields were determined by gas
chromatography. b Reuse experiments.
8
entries 1 and 16). The possibility of reusing catalysts 6 for the
Suzuki–Miyaura coupling reaction was examined. After each
reaction, the reaction mixture was filtered, and the black solid
residue was used for the next reaction. The yields for the
second, third, fourth, and fifth cycles were 72, 68, 67, and
52%, respectively. The results showed that the catalyst was still
active at the end of the fifth cycle (Table 3, entries 17–20).
The electron-donating properties of N-heterocyclic carbenes
change with the effect of heteroatoms on the ring, especially at
the backbone of the structure.20 The literature similarly con-
tains reports on these types of Pd-(1,2,4-triazol-5-ylidine) com-
plexes.21 Chernyshev et al. synthesized alkyl- and benzyl-substi-
tuted PdII-(1,2,4-triazol-5-ylidine) complexes and observed that
these complexes could be used as efficient catalysts for the
Suzuki–Miyaura reaction.22 Padelkova et al. also investigated
the effects of 1,2,4-triazol-5-ylidine substituents on the cata-
lytic activity.24
9
10
11
12
13
14
Under the optimized conditions, we extended the reaction
scope using different aryl bromides and arylboronic acid
derivatives (Table 4). Firstly, phenylboronic acids containing
electron-donating groups at the para position, namely OCH3
and CH3 were tested, and good yields were obtained with
4-methoxy-1,1′-biphenyl and 4-methyl-1,1′-biphenyl (Table 4,
entries 2 and 3). Phenylboronic acids containing electron-with-
drawing groups such as F, Cl, and CHO produced 4-fluoro-1,1′-
biphenyl, 4-chloro-1,1′-biphenyl, and [1,1′-biphenyl]-4-carbal-
15
16
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Table 4 (Contd.)
2-methyl-1,1′-biphenyl and 58% for 2-fluoro-1,1′-biphenyl
(Table 4, entries 7 and 8). Furthermore, different aryl bromides
were also examined (Table 4, entries 9–17). Aryl bromides con-
sisting of electron-donating substituents, OCH3, CH3, and
C(CH3)3, gave the corresponding products 4-methoxy-1,1′-
biphenyl, 4-methyl-1,1′-biphenyl and 4-isopropyl-1,1′-biphenyl
with good yields of 74, 84 and 78%, respectively (Table 4,
entries 9,10 and 13). Aryl bromides containing electron-with-
drawing groups, namely CF3 and CN also resulted in good
yields (82% and 71%) in coupling reactions with the products
of 4-(trifluoromethyl)-1,1′-biphenyl and [1,1′-biphenyl]-4-carboni-
trile (Table 4, entries 11 and 12). However, when sterically hin-
dered aryl bromides were used, the product yields were decreased.
Fluorinated aromatic compounds were used in medically active
compounds to inhibit the oxidation of the aromatic ring and
increase lipophilicity, which inhibits drug delivery.23 The reaction
of 4-bromobenzotrifluoride with 4-flouro-phenylboronic acid and
4-methoxyphenylboronic acid produced 79% and 85% yields for
the coupling products. Our results demonstrated that the PdII
complexes containing nitron can be used in Suzuki–Miyaura
cross-coupling reactions.
Aryl bromide
(25)
Arylboronic acid
(26)
Product, yield
(27%)
Entry
17
18
19
20
a Reaction conditions: aryl bromide (1.0 mmol), arylboronic acid
(1.0 mmol), KOH (0.5 mmol), 6 (0.1 mol%), 0.5 mL 2-propanol +
1.5 mL H2O, RT, 30 min, under air. b Isolated yield. Yields were deter-
mined by gas chromatography.
Liu et al. proposed two possible reaction pathways involving
a Pd0/II/0 and a PdII/IV/II catalytic cycle for the Suzuki–Miyaura
dehyde with excellent yields of 87%, 83%, and 68%, respect- coupling reactions.25 We performed DFT calculations to inves-
ively (Table 4, entries 4–6). When the F and CH3 groups at the tigate which of these mechanisms were preferred by our com-
para position of phenylboronic acids were displaced by the plexes during the Suzuki–Miyaura coupling reaction (Fig. 6).
ortho position, the catalytic activity was reduced to 64% for To determine which pathway the Suzuki–Miyaura Coupling
Fig. 6 The Pd0/II/0 pathway and its free energy profile calculated with the DFT-B3LYP method in water. Energies are given in kcal mol−1
.
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reaction would prefer, both reaction pathways were computa-
tionally investigated using the DFT-B3LYP method. The Pd0/II/0
Conclusions
pathway was calculated with the DFT-B3LYP method (Fig. 6). In conclusion, a series of RuII, RhIII, IrI,III and PdII complexes
Unfortunately, we were unable to locate any transition state bearing the mesoionic NHC ligand were successfully syn-
structure for the oxidation step of the other pathway PdII/IV/II
.
thesized and fully characterized. The X-ray diffraction spectra
Our scan jobs indicate that the approach of PhBr to the of complexes 1, 2, 3, 5, and 6 were obtained by the diffusion of
PdII center continuously increases the electronic energy diethyl ether into the concentrated solutions of the complexes
instead of passing from a maximum. This may infer the PhBr in dichloromethane. Complexes 1–9 exhibited high catalytic
ligand not having enough power to oxidize PdII to PdIV. On the activities for the α-alkylation of ketones with alcohols with a
other hand, we were able to determine the whole Pd0/II/0 low catalyst loading (0.1 mol%) and base amount (10 mol%)
pathway with the DFT-B3LYP hydride functional in water. In under the air atmosphere. The Ir-hydride species were detected
1
the early stage of the mechanism, a stable PhBr-catalyst by H-NMR spectroscopy at −17.40 ppm. The palladium com-
complex forms and the activation barrier of the first step were plexes 6–9 were tested as active precatalysts for the Suzuki–
determined relative to this complex. Then, the Pd0 catalyst Miyaura cross-coupling reaction. The PEPPSI-type complex 6
concertedly breaks the C–Br bond of PhBr and Pd0 oxidize to catalyzed this reaction in a short time at room temperature.
PdII with an activation free energy of 19 kcal mol−1. In the next The DFT calculations indicated that the Pd0/II/0 pathway is
step, bromide anion leaves the complex with very low acti- more favorable than the other pathway which requires further
vation energy letting the formation of an int02-1 cation which oxidation of PdII to PdIV.
barrierlessly forms an int02-2 intermediate complex. The
addition of the PhB(OH)2 reagent initiates the transmetallation
step. After the formation of int02-2 and PhB(OH)2 complex, it
Experimental
readily converts into int03 complex. Then, the phenyl group
attached to the boron atom in int03 attacks to PdII center.
All experimental applications of organometallic compounds
were carried out under an inert atmosphere using standard
Schlenk line techniques. All complexes were synthesized in dry
acetonitrile. Analytical grade acetonitrile (Merck) was dried
over phosphorus(V) oxide and stored over molecular sieves.
[RuCl2(p-cymene)]2 was prepared according to the method
reported by Bennett and Smith through the reaction of ruthe-
nium(III) chloride with α-terpinene.27 [IrCl2Cp*]2 was syn-
thesized according to the published procedures by a reaction
of iridium(II) chloride and pentamethylcyclopentadiene.28 All
catalytic reactions were carried out under the inert atmosphere
on the Carousel 12 Plus Reaction Station system. 1H- and
13C-NMR spectra were recorded using Varian 400 MHz spec-
trometers and chemical shifts are recorded in ppm. J values
were given in Hz. NMR chemical shifts were referenced to the
solvent signal in CDCl3 and DMSO-d6. Reagents were pur-
chased from Aldrich or Merck and were used as received.
Infrared spectra were recorded on a PerkinElmer Spectrum 100
IR spectrometer using the KBr pellet technique between 4000
and 400 cm−1 in the solid state. Other reagents were commer-
cially available and used without further purification.
Since this Ph migration needs rather high energy, and this
step (transmetallation) becomes the rate-determining step
with an activation free energy of ∼37 kcal mol−1. In the last
step, the reduction of the PdII complex, reproduction of the
Pd0 catalyst, and the formation of a diaryl product occur simul-
taneously with moderate activation energy (16 kcal mol−1).
Also, the possible mechanism of the Suzuki−Miyaura C–C
coupling reactions is summarized (Scheme 5).
Synthesis of complex 1
[Ru(p-cymene)Cl2]2 (980 mg, 1.6 mmol) and nitron (1000 mg,
3.2 mmol) were dissolved in 30 ml of DCM. The mixture was
stirred overnight at room temperature, and then DCM was
evaporated in vacuo. The complex was purified by column
chromatography (DCM). The precipitate was recrystallized in
DCM/Et2O. Yield: 447 mg (24%), orange solid. M.
p. 242–244 °C. Anal. calcd for C30H29ClN4Ru (582.10): C, 61.90;
H, 5.02; N, 9.62. Found: C, 61.79; H, 5.01; N, 9.59. FT-IR νmax
/
cm−1 in KBr: 3255, 3047 (NH); 2955, 745 (C–H); 1579 (CvN).
1H-NMR (400 MHz, CDCl3): δ 8.11 (m, 1H, Ph–H), 7.74 (t, J =
4.0 Hz, 4H, Ph–H), 7.50 (m, 3H, Ph–H), 7.35 (m, 2H, Ph–H),
Scheme 5 Proposed mechanisms of Suzuki–Miyaura coupling
reactions.
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7.04 (m, 4H, Ph–H), 6.14 (s, 1H, N–H), 5.42 (d, J = 6.0 Hz, 1H, Synthesis of complex 5
p-cymene-CH), 5.20 (d, J = 6.0 Hz, 1H, p-cymene-CH), 4.67 (d, J
[Ir(COD)Cl2]2 (150 mg, 0.22 mmol) and nitron (139 mg,
0.44 mmol) were dissolved in 10 ml of toluene. The mixture
was stirred overnight at room temperature, and then toluene
was evaporated in vacuo. The product was purified by column
chromatography (DCM). The precipitate was recrystallized in
chloroform/pentane. Yield: 196.8 mg (69%), yellow solid, m.p:
213–215 °C. Anal. calcd for C28H28ClN4Ir (648.22): C, 51.80; H,
= 6.0 Hz, 1H, p-cymene-CH), 4.58 (d, J = 6.0 Hz, 1H, p-cymene-
CH), 2.14 (m, 1H, p-cymene-CH(CH3)2), 1.92 (s, 3H, p-cymene-
CH3), 0.84 (d, J = 6.8 Hz, 3H, p-cymene-CH(CH3)2), 0.69 (d, J =
6.8 Hz, 3H, p-cymene-CH(CH3)2). 13C-NMR (100 MHz, CDCl3):
δ 188.3, 160.4, 148.9, 145.3, 140.7, 138.1, 134.9, 130.4, 129.3,
124.6, 122.8, 122.5, 117.6, 112.7, 104.9, 99.7, 92.8, 90.2, 89.6,
82.6, 30.9, 23.3, 18.9.
4.35; N, 8.64. Found: C, 51.62; H, 4.35; N, 8.61. FT-IR νmax
/
cm−1 in KBr: 3259, 3072 (NH); 2920, 751 (C–H); 1585 (CvN).
1H-NMR (400 MHz, CDCl3): δ 8.64 (m, 2H, Ph–H), 7.91 (br, 2H,
Synthesis of complex 2
The complex 2 was prepared in the same manner as complex 1 Ph–H), 7.64 (m, 3H, Ph–H), 7.50 (t, J = 7.2 Hz, 3H, Ph–H), 7.43
using [Rh(Cp*)Cl2]2 (150 mg, 0.24 mmol) and nitron (152 mg, (m, 2H, Ph–H), 7.34 (m, 2H, Ph–H), 7.06 (t, J = 7.2 Hz, 1H, Ph–
0.48 mmol). The complex was purified by column chromato- H), 6.04 (s, 1H, N–H), 4.57 (m, 2H, COD–H), 2.64 (m, 1H,
graphy (DCM). The precipitate was recrystallized in DCM/Et2O. COD–H), 2.46 (m, 1H, COD–H), 2.01 (m, 1H, COD–H), 1.79 (m,
Yield: 131.9 mg, (47%), orange solid, m.p. 225–227 °C. Anal. 1H, COD–H), 1.60 (m, 2H, COD–H), 1.40 (m, 2H, COD–H), 1.27
calcd for C30H30ClN4Rh (584.94): C, 61.60; H, 5.17; N, 9.58. (m, 2H, COD–H). 13C-NMR (100 MHz, DMSO-d6): δ 181.3,
Found: C, 61.37; H, 5.18; N, 9.56. FT-IR νmax/cm−1 in KBr: 149.4, 139.6, 137.9, 134.2, 130.2, 129.8, 128.7, 127.5, 123.0,
3414, 3053 (NH); 2913, 749 (C–H); 1579 (CvN). 1H-NMR 122.9, 117.6, 86.2, 84.1, 53.1, 52.4, 34.1, 33.7, 32.0, 29.9, 28.5,
(400 MHz, CDCl3): δ 7.97 (br, 2H, Ph–H), 7.77 (t, J = 4.0 Hz, 22.3, 14.1.
1H, Ph–H), 7.58 (m, 6H, Ph–H), 7.33 (t, J = 7.6 Hz, 2H, Ph–H),
7.07 (m, 3H, Ph–H), 6.41 (s, 1H, N–H), 1.41 (s, 15H, C5(CH3)5).
Synthesis of complex 6
13C-NMR (100 MHz, CDCl3): δ 182.0 (d, J = 58 Hz, C–Rh),
Complex 6 was prepared in the same manner as complex 1
using Pd(Py)2Br2 (473 mg, 1.10 mmol) and nitron (350 mg,
1.10 mmol). The product was purified by column chromato-
graphy (DCM). The precipitate was recrystallized in DCM/Et2O.
Yield: 463.0 mg (64%), yellow solid. M.p.: 236–238 °C. Anal.
calcd for C25H21Br2N5Pd (657.70): C, 45.65; H, 3.22; N, 10.65.
Found: C, 45.48; H, 3.21; N, 10.60. FT-IR νmax/cm−1 in KBr:
3413, 3054 (NH); 2918, 753 (C–H); 1579 (CvN). 1H-NMR
(400 MHz, DMSO-d6): δ 9.01 (s, 1H, N–H), 8.62 (d, J = 7.6 Hz,
2H, Py–H) 8.54 (m, 2H, Py–H), 7.91 (m, 3H, Ph–H), 7.72 (m,
6H, Ph–H), 7.53 (m, 2H, Ph–H), 7.45 (t, J = 6.4 Hz, 2H, Ph–H),
7.31 (t, J = 7.6 Hz, 2H, Ph–H), 7.01 (t, J = 7.2 Hz, 1H, Ph–H).
13C-NMR (100 MHz, DMSO-d6): δ 154.7, 152.3, 151.6, 139.6,
139.4, 139.3, 134.5, 130.7, 130.0, 129.6, 129.3, 129.1, 125.6,
123.7, 123.0, 119.1.
156.3, 156.0, 149.3, 145.3, 138.1, 136.9, 134.3, 130.4, 130.0,
129.2, 125.7, 122.8, 117.7, 112.4, 98.0, 97.9, 9.3.
Synthesis of complex 3
Complex 3 was prepared in the same manner as complex 1
using [Ir(Cp*)Cl2]2 (150 mg, 0.19 mmol) and nitron (119 mg,
0.38 mmol). The complex was purified by column chromato-
graphy (DCM). The precipitate was recrystallized in DCM/Et2O.
Yield: 189.6 mg (74%), yellow solid, m.p: 232–234 °C. Anal.
calcd for C30H31ClN4Ir (674.26): C, 53.44; H, 4.48; N, 8.31.
Found: C, 53.32; H, 4.46; N, 8.29. FT-IR νmax/cm−1 in KBr:
3414, 3062 (NH); 2965, 748 (C–H); 1579 (CvN). 1H-NMR
(400 MHz, CDCl3): δ 7.89 (br, 2H, Ph–H), 7.75 (m, 1H, Ph–H),
7.67 (t, J = 7.6 Hz, 2H, Ph–H), 7.59 (m, 2H, Ph–H), 7.49 (m, 2H,
Ph–H), 7.34 (m, 2H, Ph–H), 7.06 (m, 3H, Ph–H) 6.20 (s, 1H, N–
H), 1.48 (s, 15H, C5(CH3)5). 13C-NMR (100 MHz, CDCl3): δ
199.1, 156.4, 149.3, 146.4, 138.1, 136.9, 134.0, 130.5, 130.0, Synthesis of complex 7
129.2, 125.8, 117.8, 112.5, 98.0, 9.39.
[(Nitron)Pd(Py)Br2] (150 mg, 0.23 mmol) was dissolved in
15 mL DCM and then PPh3 (60 mg, 0.23 mmol) was added.
The mixture was stirred at room temperature for 15 min. DCM
Synthesis of complex 4
Complex 4 was observed as a by-product of the synthesis of was evaporated in vacuo. The product was purified by column
complex 3. Complex 4 was also obtained as the second product chromatography (DCM). Yield: 174 mg (90%), yellow solid,
from the same column with complex 3. 1H-NMR (400 MHz, m.p.: 227–229 °C. Anal. calcd for C38H31Br2N4PPd (840.88): C,
CDCl3): δ 11.85 (s, 1H, N–H), 8.28 (d, J = 8.4 Hz, 2H, Ph–H), 54.28; H, 3.72; N, 6.66. Found: C, 54.13; H, 3.69; N, 6.67. FT-IR
8.01 (d, J = 8.4 Hz, 2H, Ph–H), 7.68 (s, 1H, Ph–H), 7.57 (m, 6H, νmax/cm−1 in KBr: 3410, 3052 (NH); 2923, 747 (C–H); 1584
Ph–H), 7.46 (m, 2H, Ph–H), 7.35 (t, J = 7.6 Hz, 2H, Ph–H), 7.12 (CvN). 1H-NMR (400 MHz, DMSO-d6): δ 9.05 (s, 1H, N–H),
(t, J = 7.2 Hz, 1H, Ph–H), 1.56 (s, 15H, C5(CH3)5). Yield: 8.5 mg 8.54 (t, J = 8.4 Hz, 2H, Ph–H), 7.87 (t, J = 4.0 Hz, 2H, Ph–H),
(3%), yellow solid. Anal. calcd for C30H32Cl3IrN4 (747.18): C, 7.76 (m, 3H, Ph–H), 7.60 (m, 6H, Ph–H), 7.38 (m, 15H, PPh3–
48.22; H, 4.32; N, 7.50. Found: C, 48.09; H, 4.29; N, 7.51. PhH), 7.00 (t, J = 7.2 Hz, 1H, Ph–H). 13C-NMR (100 MHz,
13C-NMR (100 MHz, CDCl3): δ 163.7, 149.6, 149.1, 145.3, 135.7, DMSO-d6): δ 139.7 (d, J = 24 Hz, C–Pd), 134.9, 134.8, 130.9,
130.6, 130.1, 129.9, 129.8, 129.2, 129.0, 126.0, 122.7, 122.3, 130.8, 130.3, 129.9, 129.5, 129.4, 129.3, 128.5, 128.4, 123.2,
120.6, 119.2, 117.6, 111.9, 91.6, 9.13.
119.0.
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Synthesis of complex 8
for Pd and Br atoms).31 Vibrational frequencies were computed
analytically at the same level of theory to confirm whether the
structures are minima (no imaginary frequencies) or transition
states (only one imaginary frequency). All transition state struc-
tures were confirmed to connect the corresponding reactants
and products by intrinsic reaction coordinate (IRC) calcu-
lations.32 Solvation effects were taken into account by the PCM
model.33
PdBr2(MeCN)2 (250 mg, 0.96 mmol) and nitron (300 mg,
0.96 mmol) were dissolved in 30 mL of DCM. The mixture was
stirred overnight at room temperature and then DCM was
evaporated in vacuo. The product was purified by column
chromatography (DCM : MeOH = 99 : 1). Yield: 205.2 mg (24%),
yellow solid, m.p.: 212–214 °C. FT-IR νmax/cm−1 in KBr: 3415,
1
3051 (NH); 2921, 755 (C–H); 1588 (CvN). H-NMR (400 MHz,
DMSO-d6): δ 9.16 (s, 2H, N–H), 8.25 (d, J = 8.0 Hz, 4H, Ph–H),
7.77 (m, 12H, Ph–H), 7.65 (m, 4H, Ph–H), 7.52 (d, J = 8.4 Hz,
4H, Ph–H), 7.31 (t, J = 7.6 Hz, 4H, Ph–H), 7.01 (t, J = 6.8 Hz,
2H, Ph–H). 13C-NMR (100 MHz, DMSO-d6): δ 159.2, 151.8,
139.3, 138.7, 133.8, 131.2, 130.6, 130.1, 130.1, 129.6, 129.3,
129.2, 128.8, 124.3, 123.8, 123.0, 119.0.
X-ray crystallography
Crystallographic data for the structural analysis have been de-
posited with the Cambridge Crystallographic Data Centre,
CCDC no. 1883006 for 1, 1883005 for 2, 1883008 for 3,
1883007 for 4 and 1841200 for 6.†
Synthesis of complex 9
PdCl2 (400 mg, 2.25 mmol), nitron (705 mg, 2.25 mmol) and Conflicts of interest
NaBr (696 mg, 6.76 mmol) were dissolved in 10 mL of dry
DMSO. The reaction mixture was stirred overnight at 90 °C
There are no conflicts to declare.
under argon atmosphere. Then, DMSO was vacuumed. The
residue was dissolved in DCM. The resulting salts were filtered.
Acknowledgements
The product was purified by column chromatography
(DCM : MeOH = 99 : 1). Yield: 781.1 mg (30%), orange solid,
Financial support from Ege University (Project FYL-2019-
decomposition: 197–199 °C. FT-IR νmax/cm−1 in KBr: 3413,
20423) is gratefully acknowledged. The numerical calculations
reported in this paper were fully performed at the TUBITAK
ULAKBIM, High Performance and Grid Computing Center
(TRUBA resources).
1
3059 (NH); 2924, 753 (C–H); 1586 (CvN). H-NMR (400 MHz,
DMSO-d6): δ 8.95 (s, 2H, N–H), 8.49 (d, J = 8.0 Hz, 4H, Ph–H),
7.72 (m, 14H, Ph–H), 7.50 (d, J = 8.4 Hz, 4H, Ph–H), 7.29 (t, J =
7.6 Hz, 4H, Ph–H) 6.99 (t, J = 6.8 Hz, 2H, Ph–H). 13C-NMR
(100 MHz, DMSO-d6): δ 151.6, 139.5, 139.4, 139.2, 138.8, 130.1,
130.0, 129.6, 129.5, 129.3, 129.2, 129.0, 124.4, 124.0, 123.1,
123.0, 119.1.
Notes and references
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General procedure for the alpha-alkylation of ketones with
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KOH (0.1 mmol, 5.6 mg) and toluene (1.0 mL) were added to a
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General procedure for the Suzuki–Miyaura C–C coupling
reaction
A Radley’s tube was charged with aryl halide (1.0 mmol),
phenylboronic acid (1.0 mmol), KOH (0.5 mmol) and catalyst
(0.1 mol%) in a 2-propanol–H2O mixture (2 mL, 0.5 : 1.5). The
mixture was stirred at room temperature for 30 min under an
air atmosphere. Ethyl acetate was added to the mixture and
the organic phase was monitored by gas chromatography.
Theoretical calculations
All of the DFT calculations were performed with the Gaussian
09 package.29 Geometry optimizations were carried out using
the B3LYP functional with the LANL2DZ basis set30 (effective
core potentials (ECP) of the LANL2DZ basis set were employed
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