Highly active mesoionic chalcogenone zinc(II) derivatives for C-S cross-coupling reactions

Article abstract of DOI:10.1039/c9nj04440j

The first mesoionic heavier chalcogenones, L¹-L³ [L¹ = 1-(2-mesitylene)-3-methyl-4-phenyltriazolin-5-selone; L² = 1-(2-mesitylene)-3-methyl-4-phenyltriazolin-5-thione; L³ = 1-(benzyl)-2-3(methyl)-4-phenyltriazolin-5-selone], were isolated and characterised. Density functional theory was used to obtain insights into the σ donor and π accepting nature of mesoionic chalcogenones. Using these new ligands, a series of the first zinc(ii) mesoionic chalcogenone complexes were isolated. Three mono nuclear zinc(ii) chalcogenone complexes, [(L¹)Zn(Cl)₂(HOMe)] (1), [(L²)Zn(Cl)₂(HOMe)] (3) and [(L²)Zn(Br)₂(HOMe)] (4), and two dinuclear zinc complexes, [(L¹)Zn(Br)(μ²-Br)]₂ (2) and [(L³)Zn(Br)(μ²-Br)]₂ (5), containing mesoionic thione and selone ligands were synthesized and characterised. These new complexes 1-5 represent the first structurally characterized mesoionic chalcogenone supported metal derivatives. Furthermore, all zinc complexes were characterized by thermogravimetric analysis and UV-vis spectroscopy. The solid-state structures of all zinc complexes were determined by single-crystal X-ray diffraction. The catalytic activities of the zinc(ii) complexes in thioetherification reactions were investigated without scrubbing of oxygen. The scope of the catalytic reactions was explored with a wide range of thiophenols and aryl halides. The diaryl thioethers were obtained in very good yield under mild conditions. The present protocol furnishes a synthetic route for the C-S cross-coupling of thiophenols and aryl halides without scrubbing oxygen and moisture.

Full text of DOI:10.1039/c9nj04440j

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Highly active mesoionic chalcogenone zinc(II)
derivatives for C–S cross-coupling reactions†
Cite this: New J. Chem., 2020,
44, 129
Moulali Vaddamanu,a Kavitha Velappanb and Ganesan Prabusankar
*
a
The first mesoionic heavier chalcogenones, L1–L3 [L1 = 1-(2-mesitylene)-3-methyl-4-phenyltriazolin-5-
selone; L2 1-(2-mesitylene)-3-methyl-4-phenyltriazolin-5-thione; L3
1-(benzyl)-2-3(methyl)-4-
=
=
phenyltriazolin-5-selone], were isolated and characterised. Density functional theory was used to obtain
insights into the s donor and p accepting nature of mesoionic chalcogenones. Using these new ligands, a
series of the first zinc(II) mesoionic chalcogenone complexes were isolated. Three mono nuclear zinc(II)
chalcogenone complexes, [(L1)Zn(Cl)2(HOMe)] (1), [(L2)Zn(Cl)2(HOMe)] (3) and [(L2)Zn(Br)2(HOMe)] (4), and
two dinuclear zinc complexes, [(L1)Zn(Br)(m2-Br)]2 (2) and [(L3)Zn(Br)(m2-Br)]2 (5), containing mesoionic
thione and selone ligands were synthesized and characterised. These new complexes 1–5 represent the
first structurally characterized mesoionic chalcogenone supported metal derivatives. Furthermore, all
zinc complexes were characterized by thermogravimetric analysis and UV-vis spectroscopy. The solid-
state structures of all zinc complexes were determined by single-crystal X-ray diffraction. The catalytic
activities of the zinc(II) complexes in thioetherification reactions were investigated without scrubbing of
oxygen. The scope of the catalytic reactions was explored with a wide range of thiophenols and aryl
halides. The diaryl thioethers were obtained in very good yield under mild conditions. The present
protocol furnishes a synthetic route for the C–S cross-coupling of thiophenols and aryl halides without
scrubbing oxygen and moisture.
Received 27th August 2019,
Accepted 19th November 2019
DOI: 10.1039/c9nj04440j
rsc.li/njc
Begtrup et al. in 1975.11 Begtrup isolated the first mesoionic
oxide (I) through short-lived 1H-1,2,3-triazol-5-ylidene by depro-
Introduction
The application of mesoionic carbene (MIC) or ‘‘abnormal’’ tonation of the triazolium salt with NaH at 0 1C in the presence
N-heterocyclic carbene (aNHC) supported metal derivatives has of oxygen (Scheme 1a).11 Later Enders used triazolylidenes as
continued to expand at a rapid rate in catalysis compared to organocatalysts in 1995.12 Albrecht reported the synthesis of
N-heterocyclic carbene (NHC) metal derivatives.1–4 The better s 1,2,3-triazolylidenes using a Huisgen cycloaddition between
donor and poor p accepting nature of MICs along with low azides and alkynes in 2008.13 Thus the first 1,2,3-triazolylidene
steric hindrance compared to NHCs provide greater control on based MIC-Pd/Ag/Rh/Ir molecules were reported.13 Later, Bertrand
the metal centers for improved catalytic performance.7–9 The succeeded in isolating and characterizing the first free
first MIC carbenes and their transition metal derivatives were triazolylidene.14 Notably, the triazole framework is versatile
reported by Crabtree and co-workers.10 Subsequently, several for introducing substituents and electronically unique as the
MIC–metal complexes have subsequently been reported with carbenic carbon in triazolylidenes is flanked by only one
potential roles in synthetic and catalytic applications.1–9 heteroatom.15 The most recent review by Bertrand summarizes
Though the MICs and related chemistry have been known for the advancement of triazolylidene chemistry.9
a while, the triazolylidene type of MIC chemistry is relatively
Though Begtrup accounted for the first mesoionic oxide (I) via
new. The triazolylidene family of MICs was discovered by short-lived 1H-1,2,3-triazol-5-ylidene, thirty-seven years later,
Albrecht reported the first solid-state structure of mesoionic
oxide II–IV (Scheme 1b).16a II–IV were derived via the reaction
a Department of Chemistry, Indian Institute of Technology Hyderabad, Kandi, TS,
between Cu(I) triazolylidenes or triazolium salts and CsOH.
Also, the in situ generation of IIꢀCuCl was used as a catalyst in
[3+2] dipolar cycloaddition of alkynes and azides (a ‘click’
reaction). Notably, the catalytic efficiency of IIꢀCuCl was com-
parable with the corresponding copper(I) 1,2,3-triazol-5-ylidene
complex.16b,c However, the heavier congeners of 1,2,3-triazole
based mesoionic chalcogenone have not been isolated yet.
502285, India. E-mail: prabu@iith.ac.in; Fax: +914023016032;
Tel: +914023016089
b Department of Chemistry, GITAM University, Hyderabad, TS, 502329, India
† Electronic supplementary information (ESI) available: 1H and 13C NMR spectra
of the synthesized complexes. Solid state packing. Computational data. CCDC
1893235 (L1), 1893237 (L2), 1893236 (L3), 1893242 (1), 1893241 (2), 1893240 (3),
1893238 (4), 1893239 (5) and 1895992 (E14). For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/c9nj04440j
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herein, we report the first examples of heavier mesoionic
chalcogenones, L1–L3 [L1 = 1-(2-mesitylene)-3-methyl-4-phenyl-
triazolin-5-selone; L2
=
1-(2-mesitylene)-3-methyl-4-phenyl-
triazolin-5-thione; L3 = 1-(benzyl)-2-3(methyl)-4-phenyltriazolin-5-
selone], through a one-pot synthesis. Besides, L1–L3 were used
as ligands in isolating the mononuclear zinc(II) metal com-
plexes [(L1)Zn(Cl)2(HOMe)] (1), [(L2)Zn(Cl)2(HOMe)] (3), and
[(L2)Zn(Br)2(HOMe)] (4) and dinuclear zinc(II) metal complexes
[(L1)Zn(Br)(m2-Br)]2 (2), and [(L3)Zn(Br)(m2-Br)]2 (5). Subsequently,
1–5 were used as catalysts for C–S cross-coupling reactions
between thiophenols and aryl halides. These catalytic reactions
can be carried out at 80 1C and less reaction time without
scrubbing of oxygen, comprising a substantial advantage over
known ZnEt2-mediated cross-coupling reactions.
Results and discussion
Recently we have performed a comparative study between Ph3P,
NHC, and IMC supported copper(I) complexes for C–N and
C–Si bond formation reactions.24 Based on the s-donor and
p-accepting strength, we assumed that [(IMesQS)CuI] [IMesQS =
1,3-bis(2,4,6-trimethylphenyl)imidazoline-2-thone] was more
active than [(IMes)CuCl] [IMes = 1,3-bis(2,4,6-trimethylphenyl)-
imidazol-2-yilidine] for the synthesis of 1,2,3-triazoles and for
the cross-dehydrogenative coupling of alkynes with silanes.
However, the s-donor and p-accepting nature of NHCs vs. IMCs
remains unclear. In particular, the chemistry of the mesoionic
Scheme 1 Synthesis of mesoionic chalcogenones.
However, despite the growing number of examples docu- type of heavier chalcogenones has never been investigated.
menting imidazole chalcogenone (IMC) chemical reactivity, Therefore Density Functional Theory (DFT) calculations were
over the last ten years, IMCs have proven to be pivotal supporting carried out at the B3LYP level using an all-electron basis set
ligands for a variety of catalytically active metal complexes. which includes polarization and diffuse functions (6-311++G**)
We also noted the seminal contribution of Nolan, Son, Sing, for 1,3,4-trimethyl-1,2,3-triazolin-5-ylidene (I), 1,3,4-trimethyl-
and others, who have reported a broad range of imidazole 1,2,3-triazolin-5-thione (II) and 1,3,4-trimethyl-1,2,3-triazolin-5-
chalcogenones (IMCs), such as imidazole thione (IMT) and selone (III) to understand the s-donor and p-accepting nature
imidazole selone (IMSe). The IMC Pd(II), Ir(III), Au(I), Au(III), (Fig. 1). Notably, the HOMO energy of II and III is considerably
Cu(I), Zn(II), Cd(II), Bi(III), Ni(II), Ru(II) and Rh(III) derivatives were higher than that of I, while the LUMO energy of II and III is
used as catalysts in the Heck coupling reaction,17 Suzuki– lower than that of I. Thus, mesoionic or abnormal NHC analogs
Miyaura coupling,18 Sonogashira coupling,18b oxidative coupling II and III can become a good s-donor and p-acceptor ligand
of benzylamine to imine under visible-light,19 regioselective bory- compared to NHC. As a result, II or III can be considered as a
lation of internal alkynes,20,21 hydroamination reactions,22,23 click potential option to compete with the catalytic properties of
chemistry,23,24 nitroarene reduction to aromatic amines,25 Barbier mesoionic or abnormal NHC metal derivatives.
reactions in aqueous media,26 o-acylative cleavage of cyclic
The triazolium salts were prepared via a copper(I)-catalyzed
ethers,27 triarylmethane synthesis,28 norbornene polymeri- azide–alkyne cycloaddition reaction followed by methylation
zation,29 oxidation of alcohols,30 catalytic transfer hydrogenation of the triazole at the N3-position.16 The direct thionation or
reactions,31–33 N-alkylation of anilines,33 and cross- seleniation of N-alkylated-1,3 disubstituted 1,2,3 triazolium
dehydrogenative coupling of alkynes with silanes.24 In most cases, iodides with elemental sulfur or selenium powder in the
the catalytic efficiency of IMC–metal complexes is comparable presence of potassium carbonate gave corresponding mesoio-
or even much more efficient compared to the corresponding nic chalcogenone ligands L1, L2 and L3 in excellent yield.34,35 L1,
metal–NHC derivatives.20,23,24
L2, and L3 are soluble in common nonpolar solvents and
In contrast, the literature reveals a dearth of studies about mid-polar solvents. These compounds were characterized by
1
the chemistry of mesoionic chalcogenone (MIC) metal complexes. FT-IR, UV-vis, and multinuclear NMR techniques. In H NMR,
Thus, we believe that the heavier mesoionic type of triazole the formation of the chalcogenone derivatives (L1, L2, and L3)
chalcogenone (TAC) complexes are expected to have great was supported by the disappearance of an acidic triazolium
potential for the evolution of new catalysts with unprecedented proton (dH(range) 9.52 ppm).16 In 13C NMR, the chemical shift
ligand-induced catalytic control. On the basis of this evolution, value of the carbon attached to the chalcogenone in L1 or L3 is
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Fig. 1 The comparison of the HOMO and LUMO energy differences
between 1,3,4-trimethyl-1,2,3-triazolin-5-ylidene (I), 1,3,4-trimethyl-1,2,3-
triazolin-5-thione (II) and 1,3,4-trimethyl-1,2,3-triazolin-5-selone (III).
Density Functional Theory (DFT) calculations were carried out at the
B3LYP level using an all electron basis set which includes polarization
and diffuse functions (6-311++G**).
Fig. 2 Top: Molecular structure of L1. Selected bond lengths (Å) and bond
angles (1): C(1)–Se(1) 1.840(4), N(1)–C(1) 1.377(5), N(3)–C(2) 1.365(5),
C(1)–C(2) 1.404(5), N(1)–N(2) 1.338(4), N(2)–N(3) 1.328(5), N(1)–N(2)–N(3)
103.20 (3), C(2)–C(1)–N(1) 103.30(3). Middle: Molecular structure of L2.
Selected bond lengths (Å) and bond angles (1): C(1)–S(1) 1.687(2), N(1)–C(1)
1.385(3), N(3)–C(2) 1.359(3), C(1)–C(2) 1.400(3), N(1)–N(2) 1.339(2),
N(2)–N(3) 1.324(3), N(1)–N(2)–N(3) 103.43 (17), C(2)–C(1)–N(1) 102.9(2).
Bottom: Molecular structure of L3. Selected bond lengths (Å) and bond
angles (1): C(1)–Se(1) 1.841(4), N(1)–C(1) 1.361(6), N(3)–C(2) 1.360(5),
C(1)–C(2) 1.415(7), N(1)–N(2) 1.325(5), N(2)–N(3) 1.307(6), N(1)–N(2)–N(3)
103.50 (4), C(2)–C(1)–N(1) 105.8(4).
about 10 ppm upfield shifted compared to L2. The 13C NMR
chemical shift value of the carbon attached to the chalcogenone
in L1 or L2 is comparable with that of IMesSe (IMesSe =
1,3-dimesitylimidazole-2-selone).28 The solid-state structures
of L1, L2, and L3 were determined by single-crystal X-ray
diffraction analysis. The molecular structures of L1, L2, and L3
are depicted in Fig. 2. L1, L2, and L3 are the first structurally
characterized mesoionic NHC analogs of 1,2,3-triazolin-5-
chalcogenones. The CQS bond distance found in L2 (1.687(2) Å)
is 0.017 Å longer than that of IPrS (1.670(3) Å) (IPrS = 1,3-bis(2,6- isolate the mono nuclear zinc(II) chalcogenone complexes
diisopropylphenyl)imidazole-2-thione).28 The CQSe bond dis- [(L1)Zn(Cl)2(HOMe)] (1), [(L2)Zn(Cl)2(HOMe)] (3) and [(L2)Zn(Br)2-
tance found in L1 (1.840(4) Å) and L3 (1.841(5) Å) is marginally (HOMe)] (4). Under similar reaction conditions, the reaction
longer than that of IMesSe (1.827(6) Å) (IMesSe = 1,3-dimesityl- between ZnBr2 and L1 or L3 gave dinuclear zinc complexes
imidazole-2-selone).28 The C(2)–C(1)–N(1) bond angles found in [(L1)Zn(Br)(m2-Br)]2 (2) and [(L3)Zn(Br)(m2-Br)]2 (5), respectively.
L1, L2, and L3 are comparable. The C(2)–C(1)–N(1) bond angles
The mesoionic chalcogenone supported zinc complexes
found in L1, L2 and L3 are comparable to that of IPrS (104.6(2)1) (1–5) were characterized by FT-IR, UV-vis, TGA, and multi-
and IMesSe (104.8(5)1).28 Like 1,2,3-triazolin-5-ylidene,9 the N–N nuclear NMR studies and X-ray diffraction techniques. In the
or C–N or C–C bond distances in L1, L2, and L3 are nearly FT-IR spectra, the
uꢀ
stretching frequency for 1–5 appeared
CQE
comparable, which supports the aromatic nature of 1,2,3- at a 50 cmꢁ1 lower wavenumber (1122–1126 cmꢁ1) compared to
triazolin-5-chalcogenones.
the corresponding uncoordinated mesoionic thione ligand,
The coordination capabilities of these new ligands were L2 (1173 cmꢁ1), or uncoordinated mesoionic selone ligand
investigated with zinc halides in toluene (Schemes 2 and 3). (L1 and L3) (1160–1170 cmꢁ1).
L1 or L2 was treated with zinc(II) dihalides followed by recrys-
L1, L2, and L3 and 1–5 show an absorption band around
tallization from a mixed solvent of methanol and chloroform to 310 nm and 370 nm (Fig. 3, see the ESI†). The UV-vis absorption
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Fig. 3 Solution-state UV-vis spectra of L1, L2, and L3 and 1–5 in DMSO at
RT (1.04 ꢂ 10ꢁ4 M).
Scheme 2 Synthesis of 1–4.
Fig. 4 The thermal stability of compounds 1–5 (10 1C per minute) under a
nitrogen atmosphere.
Scheme 3 Synthesis of 5.
pattern of 1–5 is comparable with that of L1, L2, and L3. The a nitrogen atmosphere. 1, 3 and 4 showed two steps of weight
absorption band observed around 310 nm could be attributed loss due to loss of methanol in step 1 till 100 1C then the
to the electron transition associated with p to p*. The absorp- organic moiety in step 2 till 500 1C. 5 shows gradual weight loss
tion band at 370 nm can be attributed to the n to p* transitions. till 225 1C (8% weight loss) and then gradual weight loss till
1 shows both hyperchromic and bathochromic shifts compared 560 1C (85% weight loss) due to the loss of organic moieties,
to free ligand L1. 3 and 4 depict a hypochromic shift compared while 2 shows three-step weight loss to yield the residue. The
to free ligand L2. Besides, the 310 nm zone absorption bands of unchanged residue of 1–5 must be the corresponding zinc
3 and 4 show a bathochromic shift compared to free ligand L2. chalcogenides.
The dinuclear zinc(II) complex 2 shows a hypochromic shift
The solid-state structures of 1–5 were confirmed by single-
compared to L1, while 5 exhibits a hyperchromic shift compared crystal X-ray diffraction studies. X-ray quality crystals were
to L3. Based on the molar extinction coefficient, it is clear that isolated from a chloroform and methanol mixture for 1–4,
the good s donor capacity of the mesoionic chalcogenone was while from toluene for 5. The molecular structures of 1–5 are
noticed with Zn(II)Cl2 compared to Zn(II)Br2.
reported in Fig. 5–7. The crystallographic data for 1–5 are listed
The thermal stability of 1–5 was analyzed by TGA. Fig. 4 in Table S1 (see the ESI†). Molecules 1, 3, and 4 are mono-
presents the amount and rate of change in the weight of the nuclear zinc(II) complexes, while 2 and 5 are dinuclear zinc(II)
material as a function of temperature (10 1C per minute) under complexes. 1, 3, and 4 are isostructural molecules; thus, the
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Fig. 6 Molecular structure of 2. Selected bond lengths (Å) and bond
angles (1): C(1)–Se(1) 1.884(5), Zn(1)–Se(1) 2.416(8), Zn(1)–Br(1) 2.509(9),
Zn(1)–Br(2) 2.343(9), N(1)–N(2) 1.320(6), N(2)–N(3) 1.344(5), C(1)–Se(1)–
Zn(1) 99.33(15), Br(2)–Zn(1)–Se(1) 103.31(3), Br(1)–Zn(1)–Br(2) 114.84(4),
N(1)–N(2)–N(3) 103.20(4).
Fig. 5 Molecular structures of 1, 3 and 4. The N(1) substituent is omitted
for clarity. Selected bond lengths (Å) and bond angles (1) of 1: C(1)–Se(1)
1.879(4), Zn(1)–Se(1) 2.436(6), Zn(1)–Cl(1) 2.209(11), Zn(1)–Cl(2) 2.253(13),
Zn(1)–O(1) 2.032(3), N(1)–N(2) 1.339(4), N(2)–N(3) 1.320(4), Se(1)–Zn(1)–
Cl(1) 95.78(10), Se(1)–Zn(1)–Cl(2) 113.64(4), Se(1)–Zn(1)–O(1) 108.45(10),
N(1)–N(2)–N(3) 103.00(3). Selected bond lengths (Å) and bond angles (1) of
3: C(1)–S(1) 1.719(3), Zn(1)–S(1) 2.326(9), Zn(1)–Cl(1) 2.209(10), Zn(1)–Cl(2)
2.250(9), Zn(1)–O(1) 2.029(3), N(1)–N(2) 1.334(3), N(2)–N(3) 1.318(4),
S(1)–Zn(1)–Cl(1) 112.46(3), S(1)–Zn(1)–Cl(2) 113.44(4), S(1)–Zn(1)–O(1)
108.96(10), N(1)–N(2)–N(3) 103.40(2). Selected bond lengths (Å) and bond
angles (1) for 4: C(1)–S(1) 1.717(6), Zn(1)–S(1) 2.326(18), Zn(1)–Br(1)
2.383(11), Zn(1)–Br(2) 2.341(9), Zn(1)–O(1) 2.029(4), N(1)–N(2) 1.342(6),
N(2)–N(3) 1.328(6), S(1)–Zn(1)–Br(1) 113.94(5), S(1)–Zn(1)–Br(2) 112.85(5),
S(1)–Zn(1)–O(1) 109.10(2), N(1)–N(2)–N(3) 102.50(4).
B0.042 Å shorter then that of dichloridobis(1,3-diisopropyl-4,5-
dimethyl-2H-imidazole-2-thione)zinc(II) (2.366(9) and 2.370(10) Å).37
Molecules 2 and 5 consist of a dinuclear zinc(II) selone core.
The core of 2 and 5 is composed of [Zn2(Br)2(m2-Br)2(Se)2]. The
coordination environment around the zinc center is satisfied by
three bromide ions and one selone ligand, which leads to a
tetrahedral geometry. Two tetrahedrons are fused through the
Br–Br edge to result in the core [Zn2(Br)2(m2-Br)2(Se)2].
The bond length and bond angle parameters of 2 and 5 are not
comparable. The Zn–Se bond distance in 2 is 0.012 Å shorter than
that of 5. The bridging Zn–Br bond distances in 2 are considerably
structural features of 1, 3, and 4 are discussed collectively. The shorter than the terminal Zn–Br bond distances, while in 5 the
Zn(II) center in 1, 3 and 4 is tetra coordinated by one chalco- terminal Zn–Br bond distances are considerably shorter than the
genone ligand and two halides and a methanol molecule. The bridging Zn–Br bond distances. The C–Se–Zn bond angle in 2 is
geometry of the zinc(II) center in 1, 3, and 4 can be described as slightly wider than that of 5. The Br–Zn–Br angles in 2 are much
tetrahedral. The sums of angles around the zinc(II) center in 1, wider than those of 5. The N–N, N–C and C–C bond distances
3, and 4 are comparable. The Zn–Se bond distance found in 1 within the five-membered heterocyclic ring are nearly comparable,
(2.436(6) Å) is 0.042 Å shorter than that of dichlorobis[1,3- which supports the existence of resonances within the ring system.
dimethyl-2(3H)-imidazoleselone]zinc(II) (2.469 and 2.487 Å)36
Catalyst 1–5 mediated C–S bond formation reactions
oxoethyl)-imidazoline zinc(II) (2.463(6) Å).26 Similarly, the Zn–S Thioethers are important motifs found in organic synthesis,
and perchlorate tetra-1-(2,6-diisopropylphenyl)-3-(2-methoxy-2-
bond distance found in 3 (2.326(9) Å) and 4 (2.326(18) Å) is biological chemistry, and materials science. The synthesis of
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Scheme 4 Optimization of the solvent and base for the catalyst
1
mediated cross-coupling reaction between 1-bromo-4-nitro benzene
and thiophenol.
Fig. 8 Optimization of the solvent and bases in catalyst 1 mediated
thioetherification of aryl halides. Reaction conditions: 1-bromo-4-nitro
benzene (1 mmol), thiophenol (1 mmol), base (2 equiv.), catalyst
1
(10 mol%), L-proline (10 mol%), 80 1C, 15 h.
Fig. 7 Molecular structure of 5. Selected bond lengths (Å) and bond
angles (1): C(1)–Se(1) 1.881(5), Zn(1)–Se(1) 2.428(9), Zn(1)–Br(2) 2.498(8),
Zn(1)0–Br(2) 2.500(9), Zn(1)–Br(1) 2.347(7), N(1)–N(2) 1.315(5), N(2)–N(3)
1.319(5), C(1)–Se(1)–Zn(1) 97.56(15), Br(2)–Zn(1)–Se(1) 108.74(3),
Br(1)–Zn(1)–Br(20) 111.20(3), N(1)–N(2)–N(3) 103.50(4).
while ZnBr2 with NaOtBu (2 equiv.) in MeCN gave a poor yield
(for more information see the ESI,† Tables S5 and S6).46 The
best results were obtained in acetonitrile and NaOtBu at 80 1C
thioethers can be achieved through cross-coupling of thio- within 15 h. Thus, the desired product was isolated and
phenols and aryl halides using Pd(II), Cu(I), Cu(II), Ir(I), Ni(II), conveniently separated by column chromatography.
Fe(III), Mn(II), Co(II), In(III), Au(I), Rh(I), Ag(I), Bi(III) and La(III).38–46
The cross-coupling reaction between 1-bromo-4-nitro benzene
Recently, the C–S coupling protocol was made attractive from an and thiophenol can also be catalyzed by ZnCl2 or ZnBr2 or strong
economic standpoint with the use of Et2Zn under anaerobic bases.46 However, the isolated yield was very poor (5% to 25%)
conditions as an alternative to expensive methods.39a,46 Notably under very high catalyst loading with a high temperature in more
Zn(II) based catalysts for cross-coupling of thiophenols and aryl than 24 h.
halides under aerobic conditions are not known.39–46 Thus mole-
To compare the catalytic efficiency of 1–5, catalytic reactions
cules 1–5 were used as catalysts to investigate the cross-coupling were carried out between 1-bromo-4-nitrobenzene and thiophenol
of thiophenols and aryl halides under aerobic conditions. For using the zinc catalyst (10 mol%) and L-proline (10 mol%)
example, we have screened catalyst 1 with different bases and (Scheme 5). Catalysts 1–5 are very active and yielded the corres-
solvents at different temperatures to optimize the coupling ponding thioether in excellent yield within 15 h (Fig. 9). Among
reaction.
The cross-coupling reaction between 1-bromo-4-nitro benzene
catalysts 1–5, catalyst 1 is found to be very active. As reported,39,46
and thiophenol was investigated with selected solvents and bases
using catalyst 1 as a model system (Scheme 4). Polar to mid-polar
solvents such as toluene, MeCN, and THF and bases such as
t
K2CO3, Cs2CO3, and BuONa were screened (Fig. 8). The cata-
lytic reactions in solvents toluene and THF were not fruitful.
Similarly, bases K2CO3 and Cs2CO3 were not suitable for the
catalytic conversion. The control reactions using only ZnX2 or
ZnX2ꢀ6H2O with NaOtBu (2 equiv.) in MeCN were unsuccessful,
Scheme 5 Catalyst 1–5 mediated cross-coupling reaction between
1-bromo-4-nitro benzene and thiophenol.
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Fig. 9 The catalyst 1–5 mediated cross-coupling reaction between
1-bromo-4-nitro benzene and thiophenol. Reaction conditions: 1-bromo-
4-nitro benzene (1 mmol), thiophenol (1 mmol), NaOtBu (2 equiv.), catalyst
(10 mol%), L-proline (10 mol%), 80 1C, 15 h.
more than 50% of known catalysts were used in 10% catalyst
loading, while up to 20% catalyst loading has been utilized in
most cases. Therefore, the catalyst loading in the present report is
well within the reported range for C–S cross-coupling reactions.
This is essential to decrease the reaction time (for more informa-
tion see the ESI,† Table S6).
Scheme 6 Scope of catalyst 1 mediated thioetherification of electron
withdrawing group substituted aryl halides.
Thus, we have assessed the scope of catalyst 1 with a different
combination of electron-deficient aryl halides or thiophenols
(Scheme 6). The cross-coupling reactions between thiophenol
and 1-halo-EWG-benzene (EWG = 4-NO2 (entry 1), 2-NO2 (entry 2),
4-CN (entry 3), 4-COMe (entry 4)) gave excellent yields. Similarly,
excellent cross-coupling conversion was achieved between 4-chloro
thiophenol and 4-nitro/2-nitro/4-cyano aryl halide (Scheme 6,
entries 5–7)
The coupling reaction between an aryl halide such as
1-bromo-3-methoxy-benzene and thiophenol under the same
conditions gave about 30% less yield compared to much more
electron-deficient aryl halides (Scheme 7, entry 8). However, the
combination of both 4-nitro-1-bromo benzene and 4-methyl
and 4-methoxy aryl thiol gave a very good yield (Scheme 7,
entries 9 and 10).
Scheme 7 The influence of an electron-rich aryl halide or aryl thiol in
catalyst 1 mediated thioetherification reactions.
Similarly, the cross-coupling reaction between 2-bromo-3-
nitropyridine and 4-methoxy thiophenol gave a cross-coupled
product with good yield (Scheme 8). Hence, the electron-rich
aryl halides are compatible with these conditions to yield cross-
coupled products in excellent yield.
2-(Arylthio)benzothiazoles are considered to be a potential
class of bioactive ingredients in the pharmaceutical industry.47
Thus, the catalytic efficiency of 1 was investigated for the
thioetherification between 2-mercaptobenzothiazole and 4-nitro
thiophenol (Scheme 9). 2-(4-Nitrophenylthio)benzothiazole was
isolated in excellent yield (86%). The catalyst 1 mediated synthesis
Scheme 8 The catalyst 1 mediated thioetherification reaction between
2-bromo-3-nitropyridine and 4-methoxy thiophenol.
of 2-(4-nitrophenylthio)benzothiazole appears to be more efficient reaction at 135 1C for 24 h (83%),49 enaminone ligand assisted
than the CuBr mediated tandem one-pot synthesis at 80 1C for copper(II)-catalyzed at 100 1C for 12 h (51%),50 and copper(I)-
15 h (70%),48 the FeCl3–N,N0-dimethylethylenediamine meditated catalyzed at 100 1C for 24 h (83%).51
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Scheme 9 The catalyst 1 mediated thioetherification of 2-mercapto-
benzothiazole with an electron withdrawing group substituted aryl halide,
such as 4-nitro thiophenol.
Also, we have attempted to isolate polyaromatic scaffolds,
such as anthracene thioether derivatives. Similar molecules have
shown promising features in anti-counterfeiting technology,52
medicinal chemistry,53 and the electronics industry as soft
materials,54 to name a few. Under the above-optimized reaction
conditions with catalyst 1, the C–S cross-coupling strategy was
explored with 9-bromo anthracene. The C–S bond formation
reaction between 9-bromo anthracene and 4-halo aryl thiols in
the presence of catalyst 1 gave 9-(4-halo phenylthio)-anthracene
in very good yield (74–70%) (Scheme 10, entries 13–15).
It is noteworthy that the 9-(4-fluoro phenylthio)-anthracene,
9-(4-chloro phenylthio)-anthracene and 9-(4-bromo phenylthio)-
anthracene derivatives have never been isolated. The present
strategy results the formation of these rare molecules in pure
form. The solid-state structure of 9-(4-chloro phenylthio)-
anthracene has been characterised by the single-crystal X-ray
diffraction technique (Fig. 10). The C(9)–S(1) and C(3)–S(1) bond
distances are nearly comparable. The C(9)–S(1)–C(3) angle is
102.16(7)1. In the molecular packing of 9-(4-chloro phenylthio)-
anthracene, pairs of 9-(4-chloro phenylthio)-anthracene molecules
are arranged orthogonal to each other without any van der Waals
interaction (see the ESI†). The present protocol seems to be
competitive enough with the recently developed most efficient
C–S cross-coupling reaction to isolate an unfunctionalized
Fig. 10 Molecular structure of E14. Selected bond lengths (Å) and bond
angles (1): C(3)–S(1) 1.770(17), C(9)–S(1) 1.776(17), C(9)–S(1)–C(3) 102.16(7).
9-(phenylthio)-anthracene using an air and moisture stable bime-
tallic catalyst [(tBu3P)Pd(I)]2, 89%).55
Besides, the isolated yield under the above synthetic
methodology is more commodious and economical under mild
reaction conditions compared to the known methodologies.39
It is important to note that the addition of L-proline is very
much essential to activate the catalyst. The catalyst is inactive
in the absence of L-proline or the presence of less than 10 mol%
L-proline. Furthermore, the blank reaction under optimized
conditions gave zero progress in the catalytic conversion.
Besides, the reaction mechanism and scope of the metal choice
are yet to be explored further. Currently, we are working in this
direction.
Experimental section
Materials and methods
All manipulations were carried out under an argon atmosphere
using standard Schlenk techniques. The solvents were purchased
from commercial sources and purified according to standard
procedures and freshly distilled under an argon atmosphere
before use. Phenyl acetylene, methyl iodide, sulfur powder,
selenium powder, zinc chloride, and zinc bromide were pur-
chased from commercial sources. Aromatic azides, CuBr(PPh3)3,
1-mesityl-3-methyl-4-phenyl-1H-1,2,3-triazol-3-ium iodide, and
1-benzyl-3-methyl-4-phenyl-1H-1,2,3-triazol-3-ium iodide, were
prepared as described in the literature.13,16
Caution! Organic azides and sodium azide are potentially-
explosive substances. Azide salts tend to be highly heat and
shock-sensitive explosives. Avoid strong acids, which can lead
to the formation of hydrazoic acid, which is highly toxic, volatile,
and explosive. Handle sodium azide powder or solutions 45%
Scheme 10 The catalyst 1 mediated thioetherification of 4-halo thio-
phenol with anthracene bromide.
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only in a properly functioning fume hood. Wear respiratory 1523(w), 1445(s), 1319(w), 1285(w), 1170(s) (CQSe), 1113(m),
protection. Wear personal protective equipment. Avoid dust 1069(s), 1020(m), 855(s), 763(m), 693(s), 539(s) cmꢁ1
.
formation. Do not get in the eyes, or on the skin, or clothing.
Use only under a chemical fume hood. Do not breathe vapors/
dust. Do not ingest.
Synthesis of 1-mesityl-3-methyl-4-phenyl-1,2,3-triazole
thione (L2)
FT-IR measurements (neat) were carried out on a Bruker L2 was prepared by a procedure analogous to that of L1 starting
Alpha-P Fourier transform spectrometer. The UV-vis spectra from 1-mesityl-3-methyl-4-phenyl-1H-1,2,3-triazol-3-ium iodide
were recorded on a T90+UV-vis spectrophotometer. The thermo- (1.6 g, 3.95 mmol) and sulfur powder (0.25 g, 7.36 mmol). Yield:
1
gravimetric analysis (TGA) was performed using a TASDT Q600. 80% (based on the triazolium salt). M.p.: 170–175 1C. H NMR
The NMR spectra were recorded on Bruker Ultrashield-400 (CDCl3, 400 MHz): d 7.81 (d, J = 8.12 Hz, 2H, HAr) 7.49 (m, 3H,
spectrometers at 25 1C unless otherwise stated. Chemical shifts HAr), 7.02 (s, 2H, HMes), 4.11 (s, 3H, N-CH3), 2.34 (s, 3H, Ar-CH3),
are given relative to TMS and were referenced to the solvent 2.11 (s, 6H, Ar-CH3). 13C NMR (CDCl3, 100 MHz): d 158.76
resonances as internal standards. Single crystals of complexes (CQS), 140.55 (Ctrz–Mes), 137.10 (Ctrz–Ph), 135.55 (CAr), 131.94
suitable for single-crystal X-ray analysis were obtained from their (CAr), 129.76 (CAr), 129.26 (CAr), 128.91 (CAr), 126.12 (CAr), 38.80
ꢀ
reaction mixture in a chloroform and methanol mixture (1 : 1 (N-CH3), 21.30 (Ar-CH3), 17.96 (Ar-CH3). FT-IR (neat, u): 2915(w),
ratio) at room temperature. Suitable crystals of L1–L3, 1–5 and E14 2853(w), 1602(m), 1523(w), 1461(s), 1400(w), 1336(s), 1286(m),
were selected and mounted on a SuperNova, Dual, Cu at zero, Eos 1173(s) (CQSe), 1116(m), 1063(m), 1020(s), 861(s), 797(m),
diffractometer. The crystal was kept at 298 K during data collec- 759(s), 697(s), 546(s) cmꢁ1
tion. Using Olex2,56 the structure was solved with the olex2.solve57
.
Synthesis of 1-benzyl-3-methyl-4-phenyl-1,2,3-triazole
selone (L3)
structure solution program using direct methods and refined
with the olex2.refine58 refinement package using Gauss–Newton
minimization. Absorption corrections were performed based on L3 was prepared by a procedure analogous to that of L1 starting
multiple scans. Non-hydrogen atoms were anisotropically refined. from 1-benzyl-3-methyl-4-phenyl-1H-1,2,3-triazol-3-ium iodide
Hydrogen atoms were included in the refinement in the calcu- (1.5 g, 3.97 mmol) and selenium powder (0.31 g, 3.97 mmol).
lated positions riding on their carrier atoms. L1 and L3 gave one Yield: 84% (based on the triazolium salt). M.p.: 152–155 1C.
‘‘A’’ level alert due to a negligible difference in the measured 1H NMR (DMSO-d6, 400 MHz): d 7.64 (m, 4H, HAr), 7.48 (m, 3H,
fraction theta full value (0.861 for L1 and 0.925 for L3). Attempts to HAr), 7.35 (m, 3H, HAr), 5.84 (s, 2H, N-CH2), 3.97 (s, 3H, N-CH3).
rectify these issues were not fruitful. Molecule 5 gave one ‘‘B’’ level 13C NMR (DMSO-d6, 100 MHz): d 147.04 (CQSe), 141.12 (Ctrz
–
alert due to the disordered carbon atoms of the benzene/toluene Ar), 134.28 (CAr), 130.16 (CAr), 130.02 (CAr), 129.40 (CAr), 128.92
molecule. Though the reaction was carried out and crystallized in (CAr), 128.83 (CAr), 128.70 (CAr), 126.15 (CAr), 53.42 (N-CH2)
ꢀ
toluene, the lattice solvent molecule in 5 was fixed as benzene due 38.21 (N-CH3). FT-IR (neat, u): 3034(w), 2931(w), 1600(w),
to the heavily disordered nature of the lattice solvent molecule. 1524(w), 1470(w), 1416(m), 1367(m), 1310(s), 1246(m),
Our efforts to locate the toluene methyl group were not successful. 1213(w), 1160(s) (CQSe), 1069(s), 1018(m), 838(m), 761(m),
However the disordered lattice solvent molecule in 5 has no 729(m), 690(s), 576(s) cmꢁ1
implication for the structural properties of metal complex 5.
.
Synthesis of [(L1)Zn(Cl)2(HOMe)] (1)
L1 (0.12 g, 0.34 mmol) was added to a solution of ZnCl2 (0.05 g,
0.33 mmol) in toluene under an argon atmosphere at ambient
Synthesis of 1-mesityl-3-methyl-4-phenyl-1,2,3-triazole
selone (L1)
To dry potassium carbonate (1.36 g, 9.87 mmol), 1-mesityl-3- temperature. Subsequently, the reaction mixture was stirred at
methyl-4-phenyl-1H-1,2,3-triazol-3-ium iodide (1.6 g, 3.95 mmol), 100 1C for 24 h to give a colourless precipitate. The solvent
Se powder (0.4 g, 5.19 mmol) and methanol (20 mL) were added was decanted, and the solid residue was washed with hexane
under an argon atmosphere. The reaction mixture was stirred (2 ꢂ 5 mL) and dried under a high vacuum. The resulting
for 36 h at 68 1C. The progress of the reaction was monitored by powder was dissolved in a chloroform and methanol mixture
TLC. After completion of the reaction, water was added to (1 : 1 ratio). Colourless crystals of 1 were obtained at room
the reaction mixture and extracted with dichloromethane temperature. Yield: 80% (based on ZnCl2). M.p.: 200–205 1C
(3 ꢂ 15 mL). The organic extract was washed with brine (decomposed). 1H NMR (DMSO-d6, 400 MHz): d 7.91 (d, J =
solution, and dried over anhydrous Na2SO4. The organic 7.28 Hz, 2H, HAr), 7.56 (m, 3H, HAr), 7.09 (s, 2H, HMes), 4.19
solvent was evaporated under reduced pressure to isolate (s, 3H, N-CH3), 3.17 (d, 3H, OCH3), 2.34 (s, 3H, Ar-CH3), 1.98
analytically pure L1. Yield: 77% (based on the triazolium salt). (s, 6H, Ar-CH3). 13C NMR (DMSO-d6, 100 MHz): d 146.65
M.p.: 190–195 1C. 1H NMR (CDCl3, 400 MHz): d 7.82 (d, J = (CQSe), 140.38 (Ctrz–Mes), 139.85 (Ctrz–Ph), 135.07 (CAr),
7.4 Hz, 2H, HAr), 7.52 (m, 3H, HAr), 7.03 (s, 2H, HMes), 4.16 132.75 (CAr), 130.11 (CAr), 129.70 (CAr), 128.68 (CAr), 128.37
(s, 3H, NCH3), 2.36 (s, 3H, Ar-CH3), 2.09 (s, 6H, Ar-CH3). (CAr), 126.19 (CAr), 48.59 (N-CH3), 38.91 (OCH3), 20.71
13C NMR (CDCl3, 100 MHz): d 148.95 (CQSe), 141.42 (Ctrz–Mes), (Ar-CH3), 17.41 (Ar-CH3). FT-IR (neat, uꢀ): 3300(m), 2920(s),
140.67 (Ctrz–Ph), 135.28 (CAr), 132.81 (CAr), 130.13 (CAr), 129.25 2853(m), 2512(w), 1608(s), 1541(w), 1446(s), 1377(m), 1320(w),
(CAr), 128.92 (CAr), 126.06 (CAr), 38.62 (N-CH3), 21.32 (ArCH3), 1285(m), 1196(s) (CQSe), 1122(w), 1072(m), 1001(s), 857(s),
ꢀ
18.09 (ArCH3). FT-IR (neat, u): 2920(m), 2855(w), 1600(m), 778(s), 694(s), 546(m) cmꢁ1
.
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Synthesis of [(L1)Zn(Br)(m2-Br)]2 (2)
129.83(CAr), 129.22 (CAr), 126.26 (CAr), 125.78 (CAr), 118.35 (CAr),
ꢀ
56.12 (N-CH2), 39.55 (N-CH3). FT-IR (neat, u): 3354(s), 2332(w),
2 was prepared by a procedure analogous to that of 1, starting
from L1 (0.11 g, 0.30 mmol) and ZnBr2 (0.07 g, 0.29 mmol).
Yield: 74% (based on ZnBr2). M.p.: 200–205 1C (decomposed).
1H NMR (DMSO-d6, 400 MHz): d 7.90 (d, J = 6.32 Hz, 2H, HAr),
7.57 (m, 3H, HAr), 7.09 (s, 2H, HMes), 4.20 (s, 3H, N-CH3), 2.34
(s, 3H, Ar-CH3), 1.98 (s, 6H, Ar-CH3). 13C NMR (DMSO-d6,
2198(w), 2102(w), 2008(w), 1611(s), 1486(m), 1444(m), 1314(s),
1250(m), 1160(m) (CQSe), 1081(s), 1018(s), 807(m), 759(m),
694(s), 639(m), 568(s), 528(m) cmꢁ1
.
1–5 catalysed C–S cross-coupling reactions
100 MHz):
d 147.34 (CQSe), 140.17 (Ctrz–Mes), 139.77
The catalytic reactions were performed using a standard
Schleck technique using zinc complexes 1–5 as catalysts for
the C–S bond coupling reactions of aryl halides and thiols as
reported.39,46 A dry Schlenk tube was charged with 1-bromo-4-
nitro benzene (0.20 g, 1 mmol) and thiophenol (0.1 g, 1 mmol),
L-proline (10 mol%, 0.005 g), and NaOtBu (0.09 g, 0.10 mmol) in
CH3CN (3 mL). The Schlenk was heated in an oil bath at 80 1C
for 15 h. The reaction mixture was then cooled and extracted
with ethyl acetate (3 ꢂ 15 mL), and the ethyl acetate layer was
washed with a saturated aqueous NaCl solution. The organic
layer was dried over anhydrous Na2SO4, and the solvent
was removed under reduced pressure in a rotary evaporator.
The crude residue was purified by column chromatography
using EtOAc–hexane as the eluent to obtain the product as a
yellow solid.
(Ctrz–Ph), 135.07 (CAr), 132.85 (CAr), 130.09 (CAr), 129.61 (CAr),
128.63 (CAr), 128.32 (CAr), 126.35 (CAr), 48.56 (N-CH3), 20.66
(Ar-CH3), 17.39 (Ar-CH3). FT-IR (neat, u): 3374(s), 2319(w),
ꢀ
1611(s), 1472(m), 1378(w), 1320(w), 1284(m), 1194(m) (CQSe),
1122(w), 1071(w), 1016(w), 851(m), 686(s) cmꢁ1
.
Synthesis of [(L2)Zn(Cl)2(HOMe)] (3)
3 was prepared by a procedure analogous to that of 1, starting
from L2 (0.11 g, 0.35 mmol) and ZnCl2 (0.05 g, 0.35 mmol).
Yield: 70% (based on ZnCl2). M.p.: 210–215 1C (dec.). H NMR
(DMSO-d6, 400 MHz): d 7.90 (d, J = 7.04 Hz, 2H, HAr), 7.56
(m, 3H, HAr), 7.07 (s, 2H, HMes), 4.15 (s, 3H, NCH3), 2.32 (s, 3H,
ArCH3), 1.99 (s, 6H, ArCH3). 13C NMR (DMSO-d6, 100 MHz):
d 157.00 (CQS), 139.76 (Ctrz–Mes), 136.02 (Ctrz–Ph), 135.32
(CAr), 131.97 (CAr), 129.71 (CAr), 129.36 (CAr), 128.66 (CAr),
128.37 (CAr), 126.26 (CAr), 48.59 (NCH3), 20.68 (ArCH3), 17.31
1
ꢀ
(ArCH3). FT-IR (neat, u): 3319(s), 1615(s), 1541(w), 1442(s),
Conclusion
1368(m), 1326(s), 1289(s), 1203(s), 1126(m) (CQS), 1071(m),
1001(s), 860(s), 790(w), 753(w), 692(s), 545(s) cmꢁ1
.
In summary, the first mesoionic heavier chalcogenones, L1–L3,
were synthesized and characterized. The s donor and p accepting
nature of the mesoionic chalcogenones were investigated using
density functional theory. Compared to mesoionic triazolin-5-
ylidene, mesoionic heavier chalcogenones are likely to become a
good s-donor and p-acceptor ligand. Utilizing these ligands, five
monomeric and dimeric zinc complexes were prepared in good
yields. Compounds 1–5 were characterized in solution by NMR
and UV-vis spectroscopy as well as in the solid-state by TGA,
FTIR, and single-crystal X-ray diffraction. The UV-vis absorp-
tion, thermal, and structural properties of the zinc complexes
were significantly influenced by varying the N-substituents of
the mesoionic ligands. Single crystal X-ray studies show that
the zinc(II) centers in 1–5 are four-coordinated and exhibit
distorted tetrahedral geometry. Subsequently, the catalytic
application of 1–5 in cross-coupling reactions between thio-
phenols and aryl halides was demonstrated. 1–5 were active
towards C–S bond formation reactions under mild reaction
Synthesis of [(L2)Zn(Br)2(HOMe)] (4)
4 was prepared by a procedure analogous to that of 1, starting
from L2 (0.11 g, 0.34 mmol) and ZnBr2 (0.08 g, 0.33 mmol).
Yield: 73% (based on ZnBr2). M.p.: 215–220 1C (decomposed).
1H NMR (DMSO-d6, 400 MHz): d 7.96 (d, J = 7.02 Hz, 2H, HAr)
7.60 (m, 3H, HAr), 7.13 (s, 2H, HMes), 4.21 (s, 3H, N-CH3), 3.21
(d, 3H, OCH3), 2.38 (s, 3H, Ar-CH3), 2.05 (s, 6H, Ar-CH3).
13C NMR (DMSO-d6, 100 MHz): d 157.11 (CQS), 139.74
(Ctrz–Mes), 135.97 (Ctrz–Ph), 135.32(CAr), 131.99 (CAr),
129.70(CAr), 129.34 (CAr), 128.66 (CAr), 128.36 (CAr), 126.29
(CAr), 48.58 (N-CH3), 39.02 (OCH3), 20.69 (Ar-CH3), 17.32
(Ar-CH3). FT-IR (neat, uꢀ): 3363(s), 2195(w), 2120(w), 1610(s),
1473(m), 1365(w), 1328(w), 1289(s), 1194(s) (CQS), 1122(m),
1069(m), 1017(s), 851(s), 688(s), 549(m) cmꢁ1
.
Synthesis of [(L3)Zn(Br)(m2-Br)]2 (5)
L3 (0.10 g, 0.31 mmol) was added to a solution of ZnBr2 (0.07 g, conditions, and this reveals the potential of easily accessible
0.31 mmol) in toluene under an argon atmosphere at ambient mesoionic chalcogenone Zn(II) compounds in catalysis.
temperature. Subsequently, the reaction mixture was stirred at Notably, mononuclear zinc(II) complex 1 was more active
100 1C for 24 h to give a colourless clear solution. Colourless than 2–5. The introduction of the heavier chalcogenone ligand
crystals of 5 were obtained at room temperature. Yield: 77% in the zinc(II) salt led to increasing catalytic activity, presumably
(based on ZnBr2). M.p.: 110–115 1C (decomposed). 1H NMR due to its optimized steric and electronic support with the
(CD3CN, 400 MHz): d 7.61 (m, 5H, HAr), 7.51 (d, J = 5.82 Hz, 2H, metal center. This strategy contrasts with the previous use of
HAr), 7.44 (m, 3H, HAr), 7.23 (d, J = 6.40 Hz, 2H, HAr), 5.86 (s, 2H, highly reactive diethyl zinc under an inert atmosphere. The
N-CH2) 4.15 (s, 3H, N-CH3). 13C NMR (CD3CN, 100 MHz): present catalysts may thus render a cheap yet effective alter-
d 145.47 (CQSe), 138.90 (Ctrz–Ar), 136.47 (CAr), 134.43 (CAr), native for expensive late transition metal catalysts in addition
131.88 (CAr), 131.66 (CAr), 131.50 (CAr), 130.01(CAr), 129.92 (CAr), to a broad substrate scope.
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H.-X. Li, D. J. Young, Y. Wang and J.-P. Lang, Inorg. Chem.,
2017, 56, 11230–11243.
Conflicts of interest
There are no conflicts to declare.
19 J. Jin, H.-W. Shin, J. H. Park, J. H. Park, E. Kim, T. K. Ahn,
D. H. Ryu and S. U. Son, Organometallics, 2013, 32, 3954–3959.
20 H. R. Kim, I. G. Jung, K. Yoo, K. Jang, E. S. Lee, J. Yun and
S. U. Son, Chem. Commun., 2010, 46, 758–760.
21 K. Srinivas, C. N. Babu and G. Prabusankar, Dalton Trans.,
2015, 44, 15636–15644.
Acknowledgements
We gratefully acknowledge the DST-SERB (EMR/2017/001211)
for financial support. VM thanks UGC for the fellowship. VK
thanks DST-WOS-A (WOS-A/CS-65/2016) for the fellowship.
22 E. Alvarado, A. C. Badaj, T. G. Larocque and G. G. Lavoie,
Chem. – Eur. J., 2012, 18, 12112–12121.
23 K. Srinivas and G. Prabusankar, Dalton Trans., 2017, 46,
16615–16622.
24 K. Srinivas and G. Prabusankar, RSC Adv., 2018, 8,
32269–32282.
25 H.-N. Zhang, W.-G. Jia, Q.-T. Xu and C.-C. Ji, Inorg. Chim.
Acta, 2016, 450, 315–320.
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