A comparative study on NHC-functionalized ternary Se/Te-Fe-Cu compounds: Synthesis, catalysis, and the effect of chalcogens

Article abstract of DOI:10.1039/c9nj02051a

A novel family of N-heterocyclic carbene (NHC)-incorporated Se-Fe-Cu compounds, bis-1,3-dimethylimidazol-2-ylidene (bis-Me₂-imy)-containing compound [(μ₄-Se)Fe₃(CO)₉{Cu(Me₂-imy)}₂] (2), bis-N-methyl- or bis-N-isopropyl-substituted benzimidazol-2-ylidene (bis-Me₂-bimy or bis-ⁱPr₂-bimy)-incorporated compounds [(μ₄-Se)Fe₃(CO)₉{Cu(Me₂-bimy)}₂] (3) or [(μ₄-Se)Fe₃(CO)₉{Cu(ⁱPr₂-bimy)}₂] (4), and a bis-1,3-dimethyl-4,5-dichloroimidazol-2-ylidene (bis-Me₂-Cl₂-imy)-containing compound [(μ₃-Se)Fe₃(CO)₉{Cu(Me₂-Cl₂-imy)}₂] (5), were synthesized in moderate yields in facile one-pot reactions of the ternary pre-designed compound [(μ₃-Se)Fe₃(CO)₉{Cu(MeCN)}₂] (1) with the corresponding imidazolium salts and KO^tBu in THF in an ice-water bath. Single-crystal X-ray analyses revealed that the Me₂-imy compound 2 or the Me₂-bimy compound 3 each exhibited a trigonal bipyramidal SeFe₃(CO)₉Cu geometry with an Fe₂Cu plane further capped by a Cu(Me₂-imy) or Cu(Me₂-bimy) fragment, respectively, with one long Cu-Cu covalent bond. In addition, compound 4 also comprised a trigonal bipyramidal SeFe₃(CO)₉Cu core structure, but the second Cu(ⁱPr₂-bimy) group bridged the equatorial Fe-Fe edge with two unbonded Cu atoms, due to the presence of a sterically bulky ⁱPr₂-bimy fragment. On the other hand, the strong electron-withdrawing chloro-containing NHC compound 5 showed a comparatively open tetrahedral SeFe₃(CO)₉ metal core, where two Fe-Fe edges each were further bridged by a Cu(Me₂-Cl₂-imy) fragment. Due to the nonclassical C-H?O(carbonyl) hydrogen bonds between the CO groups of the SeFe₃(CO)₉Cu₂ core and CH moieties of the neighboring NHC ligands, both compounds 2 and 3 comprised a one-dimensional network, while compounds 4 and 5 each were made up of a two-dimensional framework in the solid state, which efficiently enhanced the stability of these Se-Fe-Cu NHC compounds. Importantly, all of these synthesized Se-Fe-Cu NHC compounds 2-5 had pronounced catalytic activities for the homocoupling of arylboronic acids with high catalytic yields. Finally, these Se-containing Fe-Cu NHC compounds further represented excellent models for studying chalcogen effects in comparison to their Te analogs, as demonstrated by their catalytic performances and electrochemical behaviors, and by DFT calculations.

Full text of DOI:10.1039/c9nj02051a

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A comparative study of NHC-functionalized ternary Se/Te−Fe−Cu
compounds: synthesis, catalysis, and the effect of chalcogens
Received 00th January 20xx,
Accepted 00th January 20xx
Minghuey Shieh,* Yu-Hsin Liu, Chih-Chin Wang, Si-Huan Jian, Chien-Nan Lin, Yen-Ming Chen and
Chung-Yi Huang
DOI: 10.1039/x0xx00000x
A novel family of N-heterocyclic carbene (NHC)-incorporated Se−Fe−Cu compounds, bis-1,3-dimethylimidazol-2-ylidene
(bis-Me2-imy)-introduced compound [(μ4-Se)Fe3(CO)9{Cu(Me2-imy)}2] (2), bis-N-methyl- or bis-N-isopropyl-substituted
benzimidazol-2-ylidene (bis-Me2-bimy or bis-iPr2-bimy)-incorporated compounds [(μ4-Se)Fe3(CO)9{Cu(Me2-bimy)}2] (3) or
[(μ4-Se)Fe3(CO)9{Cu(iPr2-bimy)}2] (4), and a bis-1,3-dimethyl-4,5-dichloroimidazol-2-ylidene (bis-Me2-Cl2-imy)-containing
compound [(μ3-Se)Fe3(CO)9{Cu(Me2-Cl2-imy)}2] (5), were synthesized in moderate yields in facile one-pot reactions of the
ternary pre-designed compound [(μ3-Se)Fe3(CO)9{Cu(MeCN)}2] (1) with the corresponding imidazolium salts and KOtBu in
THF in an ice-water bath. Single-crystal X-ray analyses revealed that the Me2-imy compound 2 or the Me2-bimy compound
3 each exhibited a trigonal bipyramidal SeFe3(CO)9Cu geometry with an Fe2Cu plane further capped by a Cu(Me2-imy) or
Cu(Me2-bimy) fragment, respectively, with one long Cu−Cu covalent bond. In addition, compound 4 also was comprised of
a trigonal bipyramidal SeFe3(CO)9Cu core structure, but the second Cu(iPr2-bimy) group bridged the equatorial Fe−Fe edge
i
with the two unbonded Cu atoms, due to the presence of a sterically bulky Pr2-bimy fragment. On the other hand, the
strong electron-withdrawing chloro-containing NHC compound 5 showed a comparatively open tetrahedral SeFe3(CO)9
metal core, where two Fe−Fe edges each was further bridged by a Cu(Me2-Cl2-imy) fragment. Due to the nonclassical
C−H···O(carbonyl) hydrogen bonds between the CO groups of the SeFe3(CO)9Cu2 core and CH moieties of the neighboring
NHC ligands, compounds 2 and 3 both were comprised of a one-dimensional network, while compounds 4 and 5 each
were made up of a two-dimensional framework in the solid states, which efficiently enhanced the stability of these
Se−Fe−Cu NHC compounds. Importantly, all of these synthesized Se−Fe−Cu NHC compounds 2−5 had pronounced catalytic
activities for the homocoupling of arylboronic acids with high catalytic yields. Finally, these Se-containing Fe−Cu NHC
compounds further represented excellent models for chalcogen effects in comparison with their Te analogs, as
demonstrated by their catalytic performances and electrochemical behaviors, as well as by DFT calculations.
our previous studies, a series of NHC Te−Fe−Cu-based carbonyl
compounds showed pronounced catalytic activities toward the
Suzuki homocoupling reaction, due to the incorporation of the
1. Introduction
The chemistry of heterometallic carbonyl clusters has
attracted widespread interest, due to their versatile structures,
distinct reactivities, and numerous applications in magnetism,
effective electron-donating and bulky inorganic ligand
[TeFe3(CO)9]2− 5
In spite of several reported NHC-containing
heterometallic carbonyl telluride compounds,5 only one
example of NHC-stabilized heterometallic selenide
.
catalysis,
conductivity,
and
nanotechnology.1-5
The
a
introduction of chalcogen atoms into heterometallic
complexes not only stabilize the resulting clusters but also can
be used to fine tune their physical and chemical properties.2-5
For example, it has been reported that E−Mn−Cr complexes (E
= S, Se, Te) had rich electrochemical behaviors,3 and Te−Fe−Cu-
based coordination polymers were reported to display
semiconducting properties with ultra-narrow energy gaps.4 In
compound has been reported,6 presumably due to the lack of
the systematic syntheses in the Se system. It should also be
noted that no examples of NHC heterometallic selenide
compounds were used as catalysts for organic reactions.
Prompted by these facts, we attempted to synthesize a
new class of Se-containing dimetallic NHC compounds. For this
purpose, we were drawn back to our previous research and
found that the pre-designed [(μ3-Te)Fe3(CO)9{Cu(MeCN)}2]
compound acted as an important starting material for
Department of Chemistry, National Taiwan Normal University, Taipei 11677,
Taiwan (Republic of China). E-mail: mshieh@ntnu.edu.tw
Electronic Supplementary Information (ESI) available: Compounds
characterization and supplementary data. CCDC 1909685, 1909691, 1909692,
1909693, and 1909694. See DOI: 10.1039/x0xx00000x
preparing
a
series of monodentate-, bidentate-, and
†
functionalized-NHC-containing Te−Fe−Cu compounds.5 In this
work, we not only successfully isolated an unstable tenrary
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pre-designed [(μ3-Se)Fe3(CO)9{Cu(MeCN)}2] cluster but also
provided three types of functionalized-NHC SeFe3Cu2-based
compounds, i.e. the structurally analogous Cu(Me2-imy)- or
Cu(Me2-bimy)-capped trigonal bipyramidal SeFe3(CO)9Cu-
based compounds, [(μ4-Se)Fe3(CO)9{Cu(Me2-imy)}2] (2) or [(μ4-
View Article Online
DOI: 10.1039/C9NJ02051A
Se)Fe3(CO)9{Cu(Me2-bimy)}2]
dimethylimidazol-2-ylidene;
(3)
Me2-bimy
(Me2-imy
=
1,3-
1,3-
=
dimethylbenzimidazol-2-ylidene), the Cu(iPr2-bimy)-bridged
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trigonal bipyramidal SeFe3(CO)9Cu-based compound, [(μ4-
Se)Fe3(CO)9{Cu(iPr2-bimy)}2]
(4)
(iPr2-bimy
=
1,3-
diisopropylbenzimidazol-2-ylidene), and the bis-Cu(Me2-Cl2-
imy)-bridged tetrahedral SeFe3(CO)9-based compound, [(μ3-
Se)Fe3(CO)9{Cu(Me2-Cl2-imy)}2] (5) (Me2-Cl2-imy
=
1,3-
dimethyl-4,5-dichloroimidazol-2-ylidene), from facile one-pot
reactions of compound 1 with the corresponding imidazolium
salts and KOtBu/THF in moderate yields. Importantly, these
synthesized compounds 2−5 could be used as efficient
catalysts in the homocoupling of 4-bromophenylboronic acid
and showed high turnover numbers (TONs). In addition, the
chalcogen effects of the structurally analogous E−Fe−Cu NHC
compounds (E = Se, Te) were further studied in terms of their
catalytic performances and electrochemical behavior,
supported by DFT calculations.
Fig. 1 An ORTEP diagram (30% thermal ellipsoids) showing the
structure and atom labeling for 1.
decomposition and to stabilize the thermally unstable cluster
[(μ3-Se)Fe3(CO)9{Cu(MeCN)}2]. After considerable efforts, we
were able to successfully synthesize the new Se−Fe−Cu cluster
[(μ3-Se)Fe3(CO)9{Cu(MeCN)}2] (1) in a moderate yield of 71% by
reacting [SeFe3(CO)9]2− 8 with 3 equiv. of [Cu(MeCN)4]+ in THF
at 0 oC (Scheme 1). It was notable that compound 1 was very
temperature-sensitive, hence, the reaction, purification, and
crystallization must be carried out at 0 oC. X-ray analysis
2. Results and discussion
2.1 One-pot synthesis of a novel family of the NHC-incorporated
showed that compound
1
possessed
a distorted (μ3-
Se)Fe3(CO)9Cu(MeCN)-based trigonal bipyramidal core with
the Fe3 ring residing in the equatorial plane, and the Fe2Cu
plane was further capped by a Cu(MeCN) fragment with two
Cu atoms covalently bonded (2.6682(5) Å) (Fig. 1). Although
the structure of compound 1 was similar to its Te analog, the
SeFe3(CO)9Cu(MeCN) metal core in 1 was more extensively
distorted than its Te congener, mainly due to the larger atomic
size difference between the Se, Fe, and Cu atoms. In addition,
the Cu−Cu bond distance in 1 (2.6682(5) Å) was much longer
than that of [(μ3-Te)Fe3(CO)9{Cu(MeCN)}2] (2.605(1) Å)4a and also
much longer than those of most previously reported Se−Fe−Cu
ternary Se−Fe−Cu carbonyl compounds
Only a few studies of the NHC heterometallic chalcogenide
compounds have been reported,5-7 probably owing to a lack of
methods for the systematic syntheses. In our previous study, a
series of TeFe3(CO)9 dicopper NHC compounds were
synthesized and found to act as efficient catalysts in the Suzuki
homocoupling reactions of arylboronic acid.5 Given our
interest
in
chalcogen-containing
metal
carbonyl
compounds,2c,2f,2i,2k,3-5 this prompted us to wonder whether
the electronic demands and steric hindrances of chalcogens
and NHCs in these chalcogen-iron-copper NHC compounds
might play an important factor in influencing their catalytic
performance. For this purpose, by mimicking previously
reported synthetic routes in the Te system,4a we attempted to
synthesize the pre-synthesized Se-containing Fe−Cu starting
material, [(μ3-Se)Fe3(CO)9{Cu(MeCN)}2], but this method was
complicated and was accompanied with a large quantity of
decomposition, presumably due to the steric bulk between the
Fe3(CO)9 ring and the incoming Cu(MeCN) fragments caused by
the smaller-sized Se atom. To address this issue, the reaction
complexes,
Cu2(µ2-η2-fcSe2)(PiPr3)2
(2.6155(8)
(2.60(3)
Å),9a
Å),9a
Å),9a
Cu20Se6(fcSe2)4(PnPr3)10
Cu36(fcSe2)6Se12(PnPr3)10(Ph2P(CH2)3SH)2
(2.62(6)
was carried out at
0
oC to decrease the extent of the
Chart 1. Structures and abbreviations of the N-heterocyclic
carbenes reported in this work.
Scheme 1 Synthesis of the ternary Se−Fe−Cu cluster.
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Scheme 2 One-pot synthesis of a novel family of SeFe3Cu2-based NHC compounds 2−5.
Cu36Se12(fcSe2)6(PnPr2Ph)12 (2.65(8) Å) (fc
=
FeCp2),9a metal core, the equatorial Fe3 ring in 1 was converted into the
[{SeFe3(CO)9}2Cu4Cl2]2− (2.583(1) Å),9b [{SeFe3(CO)9}2Cu4Br2]2− SeFe2 ring in 2 and 3 to produce
a μ4-Se atom, which
(2.5930(7) Å),9b and Cu40Se12(Se2fc)8(PPh3)9 (2.532(5) Å),9c presumably weakens the repulsion between the COs and the
indicating that the Cu−Cu bond in 1 was weak and much more incoming bulky Me2-imy and Me2-bimy ligands. It should be
sterically hindered.
noted, however, that the two Me2-imy- or Me2-
With the reactive Se−Fe−Cu precursor 1 in hand, we
treated
1 with various imidazolium salts, including 1,3-
dimethylimidazolium
dimethylbenzimidazolium
iodide
iodide
(Me2-imy·HI),
(Me2-bimy·HI),
1,3-
1,3-
diisopropylbenzimidazolium iodide (iPr2-bimy·HI), and 1,3-
dimethyl-4,5-dichloro-imidazolium iodide (Me2-Cl2-imy·HI)
were carried out. As shown in Scheme 2, four bis-NHC
Se−Fe−Cu compounds, [(μ4-Se)Fe3(CO)9{Cu(Me2-imy)}2] (2),
[(μ4-Se)Fe3(CO)9{Cu(Me2-bimy)}2] (3), [(μ4-Se)Fe3(CO)9{Cu(iPr2-
bimy)}2] (4), and [(μ3-Se)Fe3(CO)9{Cu(Me2-Cl2-imy)}2] (5), were
obtained in moderate yields directly via one-pot stoichiometric
reactions of 1, imidazolium salts, and KOtBu in THF in an ice-
water bath. As shown in Figs. 2 and 3, compounds 2 and 3 both
exhibited a trigonal bipyramidal SeFe3(CO)9Cu geometry with
an Fe2Cu plane capped by one Cu atom, in which each Cu atom
was coordinated by
a Me2-imy or Me2-bimy fragment.
Although compounds 2 and 3 and their parent compound 1 Fig. 2 An ORTEP diagram (30% thermal ellipsoids) showing the
exhibited the similar trigonal pyramidal SeFe3(CO)9Cu-based structure and atom labeling for 2.
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Fig. 5 An ORTEP diagram (30% thermal ellipsoids) showing the
structure and atom labeling for 5.
Wade’s rule. While SeFe3(CO)9Cu2-based clusters 1−4 each had
six skeletal electron pairs for
a trigonal bipyramidal
SeFe3(CO)9Cu structure with a capping fragment Cu(MeCN) or
Cu(NHC) (NHC = Me2-imy and Me2-bimy) or a bridging Cu(iPr2-
bimy) group, cluster 5 is a standard nido-type of SeFe3(CO)9-
based tetrahedral cluster with six skeletal electron pairs. Until
now, no other NHC-incorporated ternary Se−Fe−Cu carbonyl
compounds have been reported to date. In this study, we
discovered a new effective synthon 1 for constructing a novel
family of SeFe3(CO)9Cu2-based NHC compounds.
Fig. 3 An ORTEP diagram (30% thermal ellipsoids) showing the
structure and atom labeling for 3.
bimy-coordinated Cu atoms in 2 and 3 still remained covalently
bonded (2.869(1) Å for 2; 2.832(1) Å for 3), suggesting that the
(μ4-Se)Fe3(CO)9Cu2 metal core could tolerate the introduction
of the additional NHC ligands.
On the other hand, compound 4 also displayed a (μ4-
Se)Fe3(CO)9Cu-based trigonal bipyramidal metal core, where
the equatorial Fe−Fe edge was bridged by the second Cu(iPr2-
bimy) with two nonbonded Cu atoms (3.385(2) Å), owing to
the sterically bulky iPr2-bimy fragment (Fig. 4). In addition,
compound 5 was composed of a comparatively wide-open
tetrahedral SeFe3(CO)9 core with two Fe−Fe edges each of
which was linked by a Cu(Me2-Cl2-imy) fragment, where two
Cu(Me2-Cl2-imy) groups were oriented in a cis configuration
(Fig. 5). It is important to note that these NHC Se−Fe−Cu
compounds 2−5 were not temperature-sensitive, indicating
that the introduction of the NHC ligands into the
SeFe3(CO)9Cu2 cores efficiently enhanced their thermal
stability. In addition, all of these structures basically obeyed
2.2 X-ray structural comparison of compounds 2−5
For further a comparison of the steric effect of the introduced
NHC ligands of compounds 2−5, the distances between the
two Cu atoms in these compounds are listed in Table 1. As
shown in Table 1, when MeCN ligands in 1 were substituted by
the incoming Me2-imy ligands, the Cu−Cu bond in 2 became
elongated compared that in 1 due to steric effects. In addition,
when 1,3-dimethylbenzimidazol-2-ylidene (Me2-bimy) was
introduced, the distance between the Cu−Cu bond in 3 was
found to be shorter than that of 2, implying that the slightly
electron-withdrawing Me2-bimy fragments may have reduced
the extent of repulsion between the two Cu atoms. In addition,
the average Cu−Ccarbene bond distances of compounds 2−5
(1.91−1.96 Å) (Table 1) were within the range of the Cu−Ccarbene
bond (ca. 1.80−2.20 Å) in the related NHC-containing Cu(I)
complexes.5,10 It also was interesting to note that the bond
distance of the average Cu−Ccarbene bond in 5 (1.96(2) Å) was
the longest, followed by 3 (1.94(1) Å), 2 (1.927(1) Å), and 4
(1.918(1) Å), suggesting that the strong electron-withdrawing
chloro-containing NHC in 5 efficiently weakens the Cu−Ccarbene
bonds, even though compound 5 exhibited a comparatively
i
open tetrahedral structure. In contrast, the bulkiest Pr2-bimy-
containing compound
4 displayed the shortest Cu−Ccarbene
bond, probably because of the relief of steric restraint caused
by the two unbound Cu atoms.
In addition, we hypothesized that intra-/intermolecular
interactions within these structures might help stabilize these
bulky NHC-introduced highly strained SeFe3Cu2-based
compounds. In compounds 2−5, the distances between the
copper atoms and carbon atoms of the adjacent COs were less
than the sum of their van der Waals radii (3.10 Å) and
Fig. 4 An ORTEP diagram (30% thermal ellipsoids) showing the
structure and atom labeling for 4.
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Table 1 Average bond distances (Å) for 1, 2, 3, 4, and 5
Compound
Se−Fea
2.31(2)
Fe−Fea
2.70(6)
Fe−Cua
2.51(4)
Cu−Cu
Cu−X
1.926(2)b,g
Se−Cu
[SeFe3(CO)9{Cu(MeCN)}2] (1) (X = N)
2.6682(5)
1.891(2)c,f
1.926(7)b,e
1.928(6)c,f
1.946(6)b,e
1.929(6)c,f
1.919(6)c,e
1.917(6)d,h
1.96(2)a,d,h
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[SeFe3(CO)9{Cu(Me2-imy)}2] (2) (X = C)
[SeFe3(CO)9{Cu(Me2-bimy)}2] (3) (X = C)
2.34(3)
2.34(2)
2.65(3)
2.64(5)
2.6(1)
2.6(1)
2.869(1)
2.832(1)
2.429(1)
2.469(1)
2.501(2)
[SeFe3(CO)9{Cu(iPr2-bimy)}2] (4) (X = C)
[SeFe3(CO)9{Cu(Me2-Cl2-imy)}2] (5) (X = C)
a Average bond distances.
b Five-coordinated Cu atom.
c Four-coordinated Cu atom.
d Three-coordinated Cu atom.
2.33(1)
2.65(6)
2.65(3)
2.54(5)
2.53(4)
2.313(9)
f The Cu-NHC fragment capped the SeFe2 triangle.
g The Cu-NHC fragment capped the Fe2Cu triangle.
h The Cu-NHC fragment capped the Fe3 triangle.
i The Cu-NHC fragment bridged the Fe2 edge.
significantly longer than the sum of their covalent radii (2.01 temperature, and importantly, with no additive agents such as
Å).11a,11b The corresponding Fe−C−O angles were slightly bent ligands, bases, or other oxidants (Table 2).
from 180o, indicative of weak Cu/C interactions in these
[SeFe3(CO)9]2− was initially examined as a catalyst in this
ternary Se−Fe−Cu NHC clusters (Figs. 2−5). Although the COs homocoupling reaction. However, [SeFe3(CO)9]2− exhibited no
had slightly different environments in the solid states, only one activity toward 4-bromophenylboronic acid, even though the
resonance of the COs in these compounds was observed in the time for this catalytic reaction was extended to 1 day (Table 2).
13C NMR, due to the fast exchange of the COs.
Furthermore, [Cu(MeCN)4][BF4] and [Cu2(MeIm(CH2)2Im-
Notably, the 1D networks of 2 and 3 and 2D frameworks of Me)2][PF6]2 both showed catalytic activity,5a revealing that Cu
and are revealed in their solid-state packings, via was essential for catalytic performance. However, the turnover
nonclassical C−H···O(carbonyl) hydrogen bonds11c,11d between frequency (TOF) of [Cu2(MeIm(CH2)2ImMe)2][PF6]2 (TOF = 10.6
4
5
the COs of the SeFe3(CO)9Cu2 core and the CH moieties of the h−1
) was significantly higher than that of the non-NHC-
neighboring NHC ligands (Figs. S1 and S2 and Table S1, ESI†). containing Cu(I) complex [Cu(MeCN)4][BF4] (TOF = 0.8 h−1),
With these nonclassical C−H···O(carbonyl) forces11c,11d within indicating the importance of the NHC ligands in catalysis.
their infinite 1D- or 2D-frameworks, compounds 2−5 would be Surprisingly, these SeFe3(CO)9-based dicopper NHC catalysts
predicted to have very great stability and to be stored for 2−5 possessed even much higher TOFs (TOF = 18.2 ~ 21.8 h−1
)
)
extended times in the solid state. To further examine the than that of [Cu2(MeIm(CH2)2ImMe)2][PF6]2 (TOF = 10.6 h−1
stability of these compounds, 2 was examined for its air- and (Table 2). These results demonstrate that [SeFe3(CO)9]2− acts as
water-stability. Compared with the slightly air-sensitive an excellent and suitable ligand to efficiently enhance the
compound 1, compound 2 was stable in air or water for more catalytic performance of these NHC-dicopper complexes.
than 5 days, as confirmed by powder X-ray diffraction (PXRD) For further comparison, compound 2 (TOF = 21.8 h−1) had a
(Fig. S3, ESI†), indicating that these nonclassical slightly higher TOF compared to for the N-methyl- or N-
C−H···O(carbonyl) forces11c,11d between the COs and NHC isopropyl-substituted benzimidazol-2-ylidene compound
3
ligands further stabilized these SeFe3(CO)9Cu2-based NHC (TOF = 21.2 h−1) or 4 (TOF = 20.0 h−1), indicating that the
catalysts.
presence of benzimidazol-2-ylidene slightly retarded the
catalytic reaction (Table 2). In addition, the TOF of the Me2-Cl2-
imy compound
5 (TOF = ) was lower than the
18.2 h−1
2.3 Suzuki homocoupling reactions using 2−5 as catalysts
corresponding values for compounds 2−4 (TOF = 20.0−21.8 h−1
)
With these synthesized Se−Fe−Cu NHC compounds in hand, we
but much higher than that of the parent compound 1 (TOF =
13.3 h−1), indicating that the catalytic performance of the most
structurally open and strong electron-withdrawing NHC-
containing, namely, compound 5 was least effective compared
to compounds 2−4 but still better than that of the non-NHC-
containing compound 1.
examined the roles of the incorporated cluster, [SeFe3(CO)9]2−
and NHC ligands in catalytic reactions. For this purpose, they
were examined as catalysts in the Suzuki homocoupling
reaction12 of 4-bromophenylboronic acid, in which the reaction
condition was controlled so as to include 1.0 mol% of Cu
loading, a MeOH solvent system, an O2 atmosphere, ambient
,
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DOI: 10.1039/C9NJ02051A
Table 2 Homocoupling of 4-bromophenylboronic acid with different catalystsa
5
6
7
8
9
Entry
Catalyst
[Et4N]2[SeFe3(CO)9]
Cu (mol%)
Time (h)
Yield (%)b,c
TONd
TOF (h-1)e
Ref.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
1
2
0g
24
60
8
0
−
−
f
[Cu(MeCN)4][BF4]
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
47
85
80
85
87
88
85
82
80
80
82
81
47
85
80
85
87
88
85
82
80
80
82
81
0.8
5a
5a
f
3
[Cu2(MeIm(CH2)2ImMe)2][PF6]2
[SeFe3(CO)9{Cu(MeCN)}2] (1)
[TeFe3(CO)9{Cu(MeCN)}2]
10.6
13.3
28.3
21.8
44.0
21.2
18.2
20.0
26.7
18.2
36.8
4
6
5
3
5a
f
6
[SeFe3(CO)9{Cu(Me2-imy)}2] (2)
[TeFe3(CO)9{Cu(Me2-imy)}2]
[SeFe3(CO)9{Cu(Me2-bimy)}2] (3)
[TeFe3(CO)9{Cu(Me2-bimy)}2]
[SeFe3(CO)9{Cu(iPr2-bimy)}2] (4)
[TeFe3(CO)9{Cu(iPr2-bimy)}2]
[SeFe3(CO)9{Cu(Me2-Cl2-imy)}2] (5)
[TeFe3(CO)9{Cu(Me2-Cl2-imy)}2]
4
7
2
5a
f
8
4
9
4.5
4
5b
f
10
11
12
13
3
5b
f
4.5
2.2
5b
Reaction conditions: 4-bromophenylboronic acid (1.0 mmol), MeOH (3.0 mL), 25 oC, O2 (1 atm, balloon). The isolated yield as an
average of three runs. All reactions were monitored by TLC. Turnover number per copper for the 4-bromophenylboronic acid
consumed. e TON per hour. f This work. g 1.5 mol% of Fe loading.
a
b
c
d
To further understand the chalcogen effect of compounds performance, however, the electronic demand of the NHC
2−5 in the catalytic reaction, we were drawn back to the ligands of these Se-containing catalysts was considered to be a
catalytic performance of our previously reported Te−Fe−Cu key factor in affecting their catalytic efficiency.
NHC compounds.5 As shown in Table 2, the catalytic
To expand the scope of the catalytic reaction, the
performance of these EFe3Cu2-based catalysts (E = Se, Te) was homocoupling of two different para-substituted arylboronic
all high (80−88%), however, the TOFs for the Se−Fe−Cu NHC acids, 4-nitrophenylboronic acid and 4-methoxyphenylboronic
compounds (18.2−21.8 h−1) were somehow lower than those acids, by catalyst
2 was examined and the results are
of the Te−Fe−Cu NHC compounds (18.2−44 h−1),5 suggesting summarized in Table 3. As shown in Table 3, the catalytic yield
that these Se-containing Fe−Cu NHC compounds would be less (66%) from the 4-nitrophenylboronic acid bearing a stronger
efficient for a transmetalation with 4-bromophenylboronic electron-withdrawing nitro group was higher than that of
acid through
addition, it was also interesting to note that while the more (32%). However, the yield of 4-nitrophenylboronic acid was
a
proposed Cu(I)/Cu(III) catalytic cycle.13 In phenylboronic acid with an electron-donating methoxy group
structurally open Me2-Cl2-imy compound
5 had a lower still lower than that of phenylboronic acid with a weak
catalytic efficiency than that of the bulky Me2-bimy or iPr2- electron-withdrawing bromo group (87%). These results reveal
bimy group-containing compounds 3 or 4 in the Se system, the that the SeFe3(CO)9-based NHC-dicopper 2 exhibited catalytic
analogous
Te)Fe3(CO)9{Cu(Me2-Cl2-imy)}2], had the highest TOF compared phenylboronic acid with the weak electron-withdrawing Br
to those of [(μ4-Te)Fe3(CO)9{Cu(Me2-bimy)}2] and [(μ4- group showed the highest catalytic yields.
Te)Fe3(CO)9{Cu(iPr2-bimy)}2] in the Te cases.5b Based on the
catalytic efficiency of these E−Fe−Cu NHC catalysts (E = Se, Te), 2.4 Computational studies and electrochemical properties of 2−5
the steric hindrance of the NHC ligands of the Te-containing
catalysts appeared to play an important role in their catalytic
Me2-Cl2-imy
Te
compound,
[(μ3- selectivity toward these arylboronic acids, in which
To better understand the nature of these Se−Fe−Cu NHC
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Table 3 Substrate scope of arylboronic acids using catalyst 2a
Table 4 Results of natural population analyses of 1, 2, 3, 4, and
5
View Article Online
DOI: 10.1039/C9NJ02051A
Natural charge
Compound
5
6
Se
Fea
Cua
SeFe3(CO)9Cu2
7
8
9
1
2
3
4
5
−0.013
−0.593
0.665
0.636
0.638
0.657
0.642
−0.086
−0.374
−0.341
−0.319
−0.363
Entry
R
Cu (mol%)
1.0
Time (h)
4
Yield (%)b,c
−0.238 −0.537
−0.242 −0.538
−0.210 −0.555
−0.039 −0.544
1
2
3
para-Br
para-OMe
para-NO2
87
32
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
1.0
4.5
a Average natural charge.
1.0
52.5
66d
a
Reaction conditions: arylboronic acid (1.0 mmol), MeOH (3.0
According to the Cu(I)/Cu(III) catalytic cycle,13 the ease of
oxidation of these compounds should be related to their
catalytic performances. As a consequence, the ease of the first
oxidation of these Se−Fe−Cu NHC compounds 2−5 was found
to be perfectly parallel to their catalytic efficiency. In addition,
the redox patterns of 2−5 were also similar to those of their
analogous Te compounds,5 but the first oxidation in the Se
cases (0.042−0.192 V) was shifted more anodically in
comparison to the Te cases (0.015−0.084 V),5 due to the more
electronegative Se vs. Te, indicating that compounds 2−5 were
somehow more difficult to oxidize compared to their Te
congeners, which is consistent with the lower HOMO energy
levels found for the Se cases. Lastly, we not only discovered a
facile synthetic route for constructing a series of the NHC-
functionalized ternary Se−Fe−Cu compounds but also provided
excellent models for the comprehensive comparison of
chalcogen effects in the E−Fe−Cu NHC systems (E = Se, Te) in
catalytic performance for the homocoupling reactions of 4-
bromophenylboronic acid.
mL), 25 oC, O2 (1 atm, balloon). The isolated yield as an
b
c
d
average of three runs. All reactions were monitored by TLC.
An addition−elimination side product 4-nitrophenol was
isolated in 10% yield due to the presence of H2O in the reaction
and the major product 4,4′-dinitrobiphenyl was obtained in
56% yield. Therefore, the yield of 4,4′-dinitrobiphenyl was
considered as 66%.
compounds and their catalytic performance, density functional
theory (DFT) calculations were carried out at the B3PW91
functional14 with a Def2-TZVP basis set.15 The details of the
natural population analysis (NPA) and natural bond orbital
(NBO) analysis are summarized in Table 4 and Fig. 6.
As shown in Table 4, the natural charges of the
SeFe3(CO)9Cu2 in the NHC-incorporated compounds 2−5 were
significantly more negative than that of their parent
compound 1, revealing that the electron density of these
SeFe3(CO)9Cu2 cores was dramatically increased as the result
of introducing the NHC ligands. This result explains our
experimental results showing that compounds 2−5 all had
much better TOFs than 1 in the catalytic reaction, mainly
because the more electron-rich metal core in 2−5 was
predicted to be more efficient to proceed the Cu(I)/Cu(III)
catalytic processes.13 Although the natural charges of the
metal cores in compounds 2−5 were similar to those of their
Te analogues,5 the natural charges of the chalcogen atoms
were very clearly different. The selenium atoms in 2−5 were
negatively charged (−0.039~−0.242), in contrast, the tellurium
atoms in these Te congeners were positively charged
(0.283~0.570), indicating that the more electronegative Se
atom significantly reduced the electron density on the Fe and
Cu atoms thus resulting in the less easier oxidation of our Cu(I)
catalysts with a somehow lower efficiency.
To further understand the oxidative behavior of these
synthesized Se−Fe−Cu NHC compounds 2−5, differential pulse
voltammetry (DPV) experiments were conducted in MeCN
under N2. The DPVs revealed that compounds 2−5 all exhibited
two quasi-reversible oxidations and one quasi-reversible
reduction, as evidenced by the detection of two oxidation
potential peaks between 0.042 and 0.270 V vs. SCE (Fig. S4 and
Table S2, ESI†). In addition, their HOMO and LUMO each
received a major contribution from the d and p orbitals of the
three Fe atoms with a significant contribution from the d
orbitals of Cu atoms contained in the SeFe3(CO)9Cu2 core.
3. Conclusions
In conclusion, we report on the facile synthesis of a novel
series of the functionalized-NHC Se−Fe−Cu compounds starting
from the pre-synthesized cluster [(μ3-Se)Fe3(CO)9{Cu(MeCN)}2]
(1) with different types of the imidazolium salts and KOtBu in
stoichiometric amounts. In the solid state, these Se−Fe−Cu-
NHC compounds are comprised of 1D cluster-chain polymers
or 2D planar-like frameworks, stabilized by C−H···O hydrogen
bonds between the COs moieties of the SeFe3(CO)9Cu2-based
clusters and CH moieties of the neighboring NHC ligands,
which enhance their stability and long time storage
characteristics. Importantly, these SeFe3(CO)9Cu2-based NHC
compounds were all found to efficiently catalyze the
homocoupling of arylboronic acids with high turnovers (80−87)
under mild conditions. The TONs for these Se−Fe−Cu NHC
compounds were similar to those of their Te analogs but their
catalytic affinities were dependent on the functionalized NHC
ligand. In the highly strained Se system, the electronic
properties of the NHC ligands appear to play an important role
in tuning catalytic performance, while in the Te system, the
steric hindrance of the NHC ligands was a key factor in
controlling their catalytic activities. Finally, the catalytic
efficiency of both the Se and Te systems can be perfectly
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1
2
3
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
LUMO, −2.12 eV
LUMO, −1.84 eV
LUMO, −2.07 eV
HOMO, −5.26 eV
HOMO, −5.25 eV
HOMO, −5.35 eV
4
5
LUMO, −2.03 eV
LUMO, −2.29 eV
HOMO, −5.44 eV
HOMO, −5.58 eV
Fig. 6 The spatial graphs (isovalue = 0.03−0.04) of the frontier molecular orbitals and their associated calculated energies of 1, 2, 3, 4, and 5.
probed in terms of the ease of their first oxidation potential. In [Cu(MeCN)4][BF4],17 and the imidazolium salts, 1,3-
this work, we not only discovered a new strategy for achieving dimethylimidazolium
a novel family of the SeFe3(CO)9Cu2-based NHC compounds dimethylbenzimidazolium
iodide
iodide
(Me2-imy·HI),18a
(Me2-bimy·HI),18b 1,3-
1,3-
but also carried out a comprehensive study of the chalcogen diisopropylbenzimidazolium iodide (iPr2-bimy·HI),18c and 1,3-
effects of the E−Fe−Cu NHC systems (E
investigating their catalytic and electrochemical behaviors, the were prepared according to previously published methods.
findings of which were further supported by DFT calculations.
=
Se, Te) via dimethyl-4,5-dichloro-imidazolium iodide (Me2-Cl2-imy·HI),18d
Compounds were purified by chromatography on Merck 60
silica gel (40−63 mm) and analytical thin layer chromatography
(TLC) was performed on Silica Gel 60 F254 precoated plates.
Infrared spectra were recorded on a Perkin-Elmer Paragon
1000 or 500 IR spectrometer as solutions in CaF2 cells. The
NMR spectra were obtained on a Bruker AV 400 at 400.13 MHz
for 1H and 100.61 MHz for 13C or on a Bruker AV 500 at 500.13
MHz for 1H and 125.76 MHz for 13C. 1H and 13C chemical shifts
are reported in parts per million and were calibrated relative
to DMSO-d6 (1H: 2.49 ppm, 13C: 39.51 ppm) as the internal
4. Experimental section
4.1 General procedures
All synthetic reactions were performed under an atmosphere
of pure nitrogen using standard Schlenk techniques.16 Solvents
were purified, dried, and distilled under nitrogen prior to use.
KOtBu (ACROS) and 4-bromophenylboronic acid (Lancaster)
were
used
as
received.
[Et4N]2[SeFe3(CO)9],8
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DOI: 10.1039/C9NJ02051A
Table 5 Crystallographic data for 1, 2, 3, 4, and 5
5
6
7
8
1
2
3
4
5
empirical formula
formula weight
crystal system
space group
crystal size, mm
a, Å
b, Å
c, Å
, deg
, deg
C13H6Cu2Fe3N2O9Se
707.79
C19H16Cu2Fe3N4O9Se C27H20Cu2Fe3N4O9Se C35H36Cu2Fe3N4O9Se C19H12Cl4Cu2Fe3N4O9Se
817.95
918.06
1030.27
triclinic
Pī
955.72
triclinic
Pī
monoclinic
monoclinic
monoclinic
9
P21/n
P21/n
P21/c
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
0.57  0.23  0.12
8.8011(9)
10.775(1)
12.165(1)
85.346(2)
79.733(2)
68.143(2)
1053.4(2)
2
0.40  0.38  0.22
13.8056(3)
13.7351(3)
14.3112(4)
0.16  0.12  0.10
10.211(2)
18.962(4)
16.852(3)
0.58  0.52  0.34
12.115(5)
12.288(6)
15.442(7)
107.607(5)
99.965(6)
108.614(4)
1982(2)
2
0.18  0.08  0.02
8.3228(5)
23.245(2)
15.4952(7)
101.793(1)
105.308(6)
103.387(3)
, deg
V, Å3
2656.4(1)
4
2.045
3147(1)
4
1.938
2916.3(3)
4
2.177
Z
D (calc), g/cm–3
, mm–1
2.231
5.777
1.726
3.102
4.599
3.893
4.560
color, habit
diffractometer
radiation (), Å
temperature, K
black, prism
Apex II CCD
0.71073
black, prism
Kappa CCD
0.71073
293(2)
black, prism
Apex II CCD
0.71073
200(2)
black, prism
Apex II CCD
0.71073
200(2)
black, prism
Apex II CCD
0.71073
200(2)
200(2)

range for data
2.52–25.00
2.04–25.03
2.30–21.56
2.58–25.61
2.52–24.97
collection, deg
Tmin/Tmax
0.14/0.54
0.30/0.51
0.06/0.70
0.27/0.42
0.49/0.91
no. of independent
reflections (I > 2(I))
no. of parameters
goodness of fit
3211 (Rint = 0.0217)
4008 (Rint = 0.1211)
3334 (Rint = 0.0833)
4493 (Rint = 0.0619)
3919 (Rint = 0.0326)
273
1.040
343
1.359
415
0.907
487
0.988
379
1.092
a
R1a/wR2a (I > 2(I))
0.022/0.046
0.027/0.048
0.055/0.143
0.072/0.167
0.041/0.088
0.086/0.099
0.050/0.106
0.089/0.124
0.080/0.215
0.100/0.230
R1 /wR2a (all data)
a The functions minimized during least-squares cycles were R1 = Σ||Fo||Fc||/Σ|Fo| and wR2 = {[w(Fo –Fc2)2]/[w(Fo ) ]}1/2
.
2
2 2
standard. Elemental analyses for C, H, and N were performed on a A 20 mL of a THF solution of 1 (0.40 g, 0.57 mmol), Me2-imy·HI
Perkin-Elmer 2400 analyzer at the MOST Regional Instrumental (0.25 g, 1.13 mmol), and KOtBu (0.13 g, 1.13 mmol) was stirred
Center at National Taiwan University, Taipei, Taiwan.
in an ice-water bath overnight. The resulting solution was
filtered, and the solvent was removed under a vacuum. The
residue was extracted with Et2O to give a purplish-brown
solution and the solvent was removed under a vacuum. The
residue was washed with deionized water several times and
then extracted with CH2Cl2 and then recrystallized from n-
hexane/MeOH/CH2Cl2 to give
[SeFe3(CO)9{Cu(Me2-imy)}2] (2) (0.24 g, 0.30 mmol) (52% based
on 1). IR (νCO, CH2Cl2): 2036 (m), 1983 (vs), 1923 (w) cm−1 1H
NMR (500 MHz, DMSO-d6, 300 K, ppm): δ 7.31 (br, 4H; NCH),
3.77 (br, 12H; NCH3). 13C NMR (125 MHz, DMSO-d6, 300 K,
ppm): δ 217.98 (Fe-CO), 177.72 (Ccarbene), 122.06 (NCH), 37.21
(NCH3). Anal. Calcd for 2: C, 27.90; H, 1.97; N, 6.86. Found: C,
27.80; H, 2.21; N, 7.06. Mp: 128 oC dec. Crystals of 2 suitable
for X-ray diffraction were grown from n-hexane/MeOH/CH2Cl2.
4.2 Synthesis of [SeFe3(CO)9{Cu(MeCN)}2] (1)
A 20 mL of a THF solution was added to a mixture of
[Et4N]2[SeFe3(CO)9] (1.00 g, 1.30 mmol) and [Cu(MeCN)4][BF4]
(1.24 g, 3.90 mmol), and the mixture was stirred at 0 C for 3 h.
o
a purplish-black sample of
The resulting reddish-purple solution was filtered and removed
under a vacuum. The residue was washed with de-ionized
water and n-hexane several times and then extracted with
Et2O and then recrystallized from Et2O/n-hexane to give a
purplish sample of [SeFe3(CO)9{Cu(MeCN)}2] (1) (0.65 g, 0.92
mmol) (71% based on [Et4N]2[SeFe3(CO)9]). IR (νCO, THF): 2035
.
(w), 2000 (vs), 1985 (s), 1952 (m), 1911 (s) cm−1 1H NMR (500
.
MHz, DMSO-d6, 300 K, ppm): δ 2.08 (s, 3H; NCH3). 13C NMR
(125 MHz, DMSO-d6, 300 K, ppm): δ 217.97 (Fe-CO), 117.83
(NCCH3), 1.00 (NCCH3). Anal. Calcd for 1: C, 22.06; H, 0.85; N,
3.99. Found: C, 21.97; H, 0.82; N, 4.07. Mp: 78 oC dec. Crystals
4.4 Synthesis of [SeFe3(CO)9{Cu(Me2-bimy)}2] (3)
A 20 mL of a THF solution of 1 (0.40 g, 0.57 mmol), Me2-
bimy·HI (0.31 g, 1.13 mmol), and KOtBu (0.13 g, 1.13 mmol)
was stirred in an ice-water bath overnight. The resulting
solution was filtered, and the solvent was removed under a
of
1 suitable for X-ray diffraction were grown from n-
hexane/Et2O/THF.
4.3 Synthesis of [SeFe3(CO)9{Cu(Me2-imy)}2] (2)
vacuum. The residue was extracted with Et2O to give
a
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purplish-brown solution and the solvent was removed under a 4.7 Homocoupling of 4-bromophenylboronic acid with catalysts
vacuum. The residue was washed with deionized water several 1−5
View Article Online
DOI: 10.1039/C9NJ02051A
times and then extracted with CH2Cl2 and then recrystallized
from n-hexane/MeOH/CH2Cl2 to give a purplish-brown sample
of [SeFe3(CO)9{Cu(Me2-bimy)}2] (3) (0.16 g, 0.17 mmol) (30%
A 3 mL of a MeOH solution of catalyst 1, 2, 3, 4, or 5 and 4-
bromophenylboronic acid (1.00 mmol) was stirred at 25 oC in
an O2 atmosphere. After the reaction reached completion, as
determined by the disappearance of the starting material by
based on 1). IR (νCO, CH2Cl2): 2032 (m), 1993 (vs), 1981 (vs),
1959 (m), 1924 (w) cm−1 1H NMR (500 MHz, DMSO-d6, 300 K,
.
TLC, 4-bromophenylboronic acid, the resulting reaction
mixture was evaporated to dryness under a vacuum. The
residue was dissolved in CH2Cl2 and purified by column
chromatography on silica gel using n-hexane as the eluent to
afford the biaryl product which was confirmed by NMR
spectroscopy. To compare the activities of all of the catalysts,
all of the homocoupling reactions described herein were
carried out repeatedly and carefully to ensure accuracy.
ppm): δ 7.69 (dd, 4H; Ar-H), 7.43 (dd, 4H; Ar-H), 4.03 (s, 12H,
CH3). 13C NMR (125 MHz, DMSO-d6, 300 K, ppm): δ 218.06 (Fe-
CO), 186.89 (Ccarbene), 133.68, 123.46, 111.34 (Ar-C), 34.46
(CH3). Anal. Calcd for 3: C, 35.32; H, 2.20; N, 6.10. Found: C,
35.30; H, 2.21; N, 6.27. Mp: 142 oC dec. Crystals of 3 suitable
for X-ray diffraction were grown from n-hexane/MeOH/CH2Cl2.
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4.5 Synthesis of [SeFe3(CO)9{Cu(iPr2-bimy)}2] (4)
A 20 mL of a THF solution of 1 (0.40 g, 0.57 mmol), iPr2-bimy·HI 4.8 Homocoupling of 4-methoxyphenylboronic acid with catalyst 2
(0.37 g, 1.13 mmol), and KOtBu (0.13 g, 1.13 mmol) was stirred
A
solution of catalyst
2
(0.0050 mmol) and 4-
in an ice-water bath overnight. The resulting solution was
filtered, and the solvent was removed under a vacuum. The
residue was extracted with Et2O to give a purplish-brown
solution and the solvent was removed under a vacuum. The
residue was washed with deionized water several times and
then extracted with CH2Cl2 and then recrystallized from n-
hexane/MeOH/CH2Cl2 to give a purplish-brown sample of
[SeFe3(CO)9{Cu(iPr2-bimy)}2] (4) (0.42 g, 0.41 mmol) (72%
based on 1). IR (νCO, CH2Cl2): 2037 (m), 1982 (vs), 1961 (m),
methoxyphenylboronic acid (1.00 mmol) in MeOH (3 mL) was
stirred at 25 C. The resulting reaction mixture was evaporated
o
to dryness under a vacuum. The residue was purified by flash
chromatography on a silica gel column (petroleum ether/ethyl
acetate, 5: 1) to afford biaryl derivatives. To compare the
activities of the catalysts, all the homocoupling reactions
described herein were carried out repeatedly and carefully to
ensure accuracy.
1924 (w), 1900 (w) cm−1 1H NMR (500 MHz, DMSO-d6, 300 K,
.
4.9 Homocoupling of 4-nitrophenylboronic acid with catalyst 2
ppm): δ 7.88 (dd, 4H; Ar-H), 7.39 (dd, 4H; Ar-H), 5.20 (m, 4H,
NCH(CH3)2), 1.67 (d, 24H, CH3). 13C NMR (125 MHz, DMSO-d6,
300 K, ppm): δ 218.25 (Fe-CO), 182.23 (Ccarbene), 132.31, 123.37,
112.78 (Ar-C), 51.94 (NCH(CH3)2), 21.99 (CH3). Anal. Calcd for 4:
C, 40.80; H, 3.52; N, 5.44. Found: C, 40.85; H, 3.54; N, 5.50. Mp:
108 oC dec. Crystals of 4 suitable for X-ray diffraction were
grown from n-hexane/MeOH/CH2Cl2.
A
solution of catalyst
2
(0.0050 mmol) and 4-
nitrophenylboronic acid (1.00 mmol) in MeOH (3 mL) was
stirred at 25 C. The resulting reaction mixture was evaporated
o
to dryness under a vacuum. The residue was purified by flash
chromatography on a silica gel column (petroleum ether/ethyl
acetate, 5: 1) to afford 4,4'-dinitrobiphenyl (0.27 mmol, 56 %)
and 4-nitrophenol (0.09 mmol, 10 %). To compare the
activities of the catalysts, the homocoupling reaction described
herein was carried out repeatedly and carefully to ensure
accuracy.
4.6 Synthesis of [SeFe3(CO)9{Cu(Me2-Cl2-imy)}2] (5)
A 20 mL of a THF solution of 1 (0.40 g, 0.57 mmol), Me2-Cl2-
imy·HI (0.33 g, 1.13 mmol), and KOtBu (0.13 g, 1.13 mmol) was
stirred in an ice-water bath overnight. The resulting solution
was filtered, and the solvent was removed under a vacuum.
The residue was extracted with Et2O to give a reddish-brown
solution and the solvent was removed under a vacuum. The
residue was washed with deionized water several times and
then extracted with CH2Cl2 and then recrystallized from n-
4.10 X-ray structural characterization of 1−5
Selected crystallographic parameters for 1, 2, 3, 4, and 5 are
given in Table 5. All crystals were mounted on glass fibers with
epoxy cement. Data collection for 1, 3, 4, and 5 were carried
out on a Bruker Apex II CCD diffractometer with graphite-
monochromated MoKα radiation. Compound 2 was examined
on a Bruker-Nonius Kappa CCD diffractometer with graphite-
monochromated MoKα radiation. An empirical absorption
correction by the multi-scan method was applied to the data
using SADABS.19 All structures were solved by a combination of
direct methods and difference Fourier syntheses and refined
with the SHELXL-2014 program.20 All of the non-hydrogen
atoms for 1, 2, 3, 4, and 5 were refined with anisotropic
temperature factors. The selected distances and angles for 1, 2,
3, 4, and 5 are listed in Table S3 in ESI†. CCDC reference
numbers 1909685 for 1, 1909691 for 2, 1909692 for 3,
hexane/MeOH/CH2Cl2 to give
a reddish-brown sample of
[SeFe3(CO)9{Cu(Me2-Cl2-imy)}2] (5) (030 g, 0.31 mmol) (56%
based on 1). IR (νCO, CH2Cl2): 2034 (m), 1992 (s), 1981 (vs),
1
1960 (m), 1925 (w), 1900 (w) cm−1. H NMR (500 MHz, DMSO-
d6, 300 K, ppm): δ 3.79 (s, 12H; NCH3). 13C NMR (125 MHz,
DMSO-d6, 300 K, ppm): δ 217.89 (Fe-CO), 179.39 (Ccarbene),
116.06 (CCl), 36.36 (NCH3). Anal. Calcd for 5: C, 23.88; H, 1.27;
N, 5.86. Found: C, 24.17; H, 1.31; N, 5.97. Mp: 114 oC dec.
Crystals of 5 suitable for X-ray diffraction were grown from n-
hexane/MeOH/CH2Cl2.
10 | New J. Chem., 2019, 00, 1-12
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1909693 for 4, and 1909694 for 5 contain the supplementary
crystallographic data for this paper.
Notes and references
View Article Online
DOI: 10.1039/C9NJ02051A
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5
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4.11 Computational Details
The calculations reported in this study were performed via
density functional theory (DFT) using the Gaussian 09 series of
packages.21 The geometries of compounds 1, 2, 3, 4, and 5
were taken from single-crystal X-ray diffraction data. All
calculations were performed using the hybrid B3PW91
functional14 with the large basis set Def2-TZVP.15 Natural
charges22 were evaluated using the Weinhold NBO method.23
For orbital contributions, the molecular orbital compositions
were analyzed using the AOMix program.24 Graphical
representations of the molecular orbitals were obtained using
Avogadro.25
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4.12 Electrochemical Studies
Electrochemical measurements were performed at room
temperature under a nitrogen atmosphere and recorded using
a CHI 621D electrochemical potentiostat. A glassy carbon
working electrode, a platinum wire auxiliary electrode, and a
non-aqueous Ag/Ag+ electrode were used in a three-electrode
configuration. Tetra-n-butylammonium perchlorate (TBAP)
was used as the supporting electrolyte, and the solute
concentration was ~10−3 M. The redox potentials were
calibrated with a ferrocenium/ferrocene (Fc+/Fc) couple in the
working solution and were referenced to SCE. Electrochemical
measurements of 2, 3, 4, and 5 were carried out by differential
pulse voltammetry (DPV). Because of the interference of
irreversible desorption of Cu (−0.33 to −0.38 V)26 and the
reported one-electron oxidation of Cu(I) (0.45−0.36 V),27 the
DPV profiles were only discussed between 0.30 and −0.20 V.
The DPV data are summarized in Table S2 in ESI†. For the DPV
analysis, the peak width at half height (W1/2) was used to
determine electron stoichiometry.28 Compounds 2−5 and their
Te congeners5 each showed two one-electron quasi-reversible
oxidations (0.042 ~ 0.192 V and 0.252 ~ 0.270 V) and a one-
electron quasi-reversible reduction (−0.090 ~ −0.126 V) (Table
4
5
S2, ESI†). The widths of the DPV peaks at the half-height (W1/2
)
of these compounds were a bit greater or lower than the
expected value (W1/2 = 90 mV) for one-electron reversible
redox couples, indicating that these DPV responses were
quasi-reversible.28
Conflicts of interest
The authors declare no conflicts of interest.
6
7
Acknowledgements
This work was supported by the Ministry of Science and
Technology of Taiwan (Grant No. 107-2113-M-003-006 to M.
Shieh). We wish to thank the NCHC of Taiwan for providing the
computer time and the facilities. Our gratitude also goes to the
Academic Paper Editing Clinic, NTNU.
8
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New Journal of Chemistry
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View Article Online
DOI: 10.1039/C9NJ02051A
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A novel family of highly efficient Se−Fe−Cu-NHC catalysts was synthesized, which
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were suitable models for studies of the chalcogen effect.
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