Highly efficient and recyclable pre-catalysts based on mono- and dinuclear heteroleptic Cu(I) dithio- PPh3 complexes to produce variety of glycoconjugate triazoles

Article abstract of DOI:10.1016/j.mcat.2019.03.009

Highly efficient and reusable pre-formed mono- and dinuclear heteroleptic copper(I) dithiocarbamate and dithiocarbimate complex based catalysts, [Cu(PPh₃)₂(L)] and [Cu₂(PPh₃)₄(L)] (L = N-(4-methylpyri

Full text of DOI:10.1016/j.mcat.2019.03.009

MolecularCatalysis470(2019)152–163
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
Highly efficient and recyclable pre-catalysts based on mono- and dinuclear
heteroleptic Cu(I) dithio- PPh3 complexes to produce variety of
glycoconjugate triazoles
Avadhesh K. Singha,1, Chote Lal Yadava, Kunj Bihari Mishrab,1, Santosh K. Singhc, Ajit N. Guptaa,
⁎
Vinod Kumar Tiwaria, Michael G.B. Drewd, Nanhai Singha,
a Department of Chemistry, Banaras Hindu University, Varanasi 221005, India
b Department of Chemistry, Indian Institute of Technology, Banaras Hindu University, Varanasi 221005, India
c Department of Chemistry, Hari Prasad Sah College, Nirmali, Bihar 847452, India
d Department of Chemistry, University of Reading, Whiteknights, Reading RG6 6AD, UK
A R T I C L E I N F O
A B S T R A C T
Keywords:
Highly efficient and reusable pre-formed mono- and dinuclear heteroleptic copper(I) dithiocarbamate and di-
thiocarbimate complex based catalysts, [Cu(PPh3)2(L)] and [Cu2(PPh3)4(L)] (L = N-(4-methylpyridyl)-N-(3-
methylpyridyl)dithiocarbamate− L1 1, N-methylfuryl-N-methylthiophenedithiocarbamate- L2 2; 4-chlor-
obenzenesulfonyl dithiocarbimate2- L3 3, 4-bromobenzenesulfonyldithiocarbimate2- L4 4) have been utilized in
the cycloaddition reactions of azide and alkyne to form a variety of glycoconjugate triazoles in Click chemistry.
These new pre-catalysts have been characterized by elemental (C, H, N) analysis, IR, UV–vis., 1H, 13C{1H}, and
31P{1H} NMR spectroscopy and their structures have been revealed by X-ray crystallography. In the structures of
(1,2)/(3,4) the copper atoms are situated within a four coordinate (P2S2)/(P2NS) distorted tetrahedral geometry.
Notably in the dinuclear complexes 3 and 4, the dithiocarbimate ligands are bonded in a S, S- chelating mode to
one copper atom and simultaneously bridge the other copper centre via N, S- donor atoms. 1-4 are strongly
luminescent in CH2Cl2 solution at room temperature.
Heteroleptic Cu(I) dithiocarbamates/
dithiocarbimates
Click reactions
Catalytic activity
Glycoconjugates
Luminescent properties
Crystal structures
1. Introduction
copper centres together with the ligands play key roles in the catalytic
transformations. The best developed systems are those with hetero-
Novel catalytic activities of low valent electron rich transition metal
phosphine complexes have been extensively explored. The introduction
of the Click concept by Sharpless [1] and Meldal [2] in which copper(I)
assisted cycloaddition reactions of azides and alkynes produce triazoles
has revolutionized research in diverse chemical disciplines, such as
drug discovery, synthetic polymers, fluorescent imaging and material
science [3–5]. In comparison to the in situ generation of copper(I)
species by the treatment of Cu(II) salts with sodium ascorbate reductant
[6], use of pre-formed copper(I) complexes is more advantageous be-
cause it facilitates the reaction under mild conditions and avoids the
addition of reducing agents. Here the ligating moieties protect the
oxidation/ reduction of Cu(II) centres and modify the activity of the Cu
(I) centres [7]. Among many copper(I) complexes used for this reaction,
the N-heterocyclic carbenes (NHCs) [8], mild basic amines and other O/
S and P donor ligands [9–10] have been frequently reported. The
cyclic carbene and phosphine ligands.
Many catalytic transformations using metal thiolato complexes have
been extensively studied [11]. By comparison research involving
copper(I) complexes assisted Click chemistry for the cycloaddition re-
actions of azides and alkynes with the O/S donor ligands is rare [12]. It
has been reported that in this reaction dinuclear Cu(I) complexes have
more potential than mononuclear complexes because of the synergic
effect due to the appropriate proximity of the metal centres. This has
been corroborated by DFT calculations [13].
Generally, the copper(I) catalyzed 1, 3-dipolar cycloaddition reac-
tion is considered to be a step wise process dissimilar from [3 + 2]-
cycloaddition reactions and does not proceed through concerted way.
Previously, this stepwise mechanistic study was confirmed by the DFT
calculations and kinetic evidences [13a–c]. However, recently the
previous predicted π,σ- biscopper acetylide and a 3,5-bismetalated
⁎ Corresponding author.
E-mail address: nsingh@bhu.ac.in (N. Singh).
1 A.K.S and K.B.M. contributed equally to this work.
https://doi.org/10.1016/j.mcat.2019.03.009
Received 28 November 2018; Received in revised form 6 March 2019; Accepted 8 March 2019
2468-8231/©2019ElsevierB.V.Allrightsreserved.

A.K. Singh, et al.
MolecularCatalysis470(2019)152–163
Scheme 1. Potassium salts of the dithiocarbamate and dithiocarbimate ligands used in this work.
triazole complex intermediates were isolated by Bertand research group
[14] and supported the previous reports.
2. Experimental
The chemistry of transition metal dithiocarbamates including that
with Cu(II) has been extensively investigated due to their myriad of
diverse applications in magnetic, optical, agricultural and industrial
areas [15–17]. Heteroleptic Cu(I) complexes [Cu(phosphine)(L)] have
attracted considerable attention because of their important optical
properties and their use as sensitizers in solar cells and biological
imaging [18], but despite synthetic versatility and structural simplicity,
to the best of our knowledge heteroleptic copper(I) dithiocarbamate
and dithiocarbimate complexes have not yet been investigated under
the Click approach. Motivated by these facts, in this contribution the
synthesis, crystal structures, luminescent characteristics and catalytic
efficiency for the synthesis of glycoconjugate triazoles, new mono- and
dinuclear Cu(I) complexes 1-4 utilizing dithiocarbamate/dithiocarbi-
mate (Scheme 1) in conjunction with PPh3 ligands were undertaken.
The present work was pursued considering the following points : (i) the
modification of the ligand framework by varying the substituents on the
dithio unit and using of sterically demanding PPh3 which may sub-
stantially modify electronic and steric properties together with the
chemical reactivity of the complexes which are crucial for the devel-
opment of promising Click catalysts, (ii) the incorporation of electron
rich moieties, Py(N) in the 3- and 4- positions may enhance conjugation
in the molecule, hence affect the catalytic activities and luminescent
characteristics of the complexes, (iii) in spite of some common features,
the monoanionic dithiocarbamate and dianionic dithiocarbimate li-
gands differ significantly in their structure and properties. Generally
heteroleptic Cu(I) dithiocarbamate complexes bearing bulky phosphine
ligands are mononuclear [16a,b] in which identical S,S- donor atoms
are bonded to the metal in a bidentate mode while, though rare, the
analogous dithiocarbimate complexes are dinuclear [17a] where the
dithiocarbimate group is attached to the metal via the S,S-/S,N- donor
atoms in a chelating as well as bridging mode. This difference in
bonding behavior is attributed to the dominant resonance forms,
2.1. Materials and measurements
The synthesis of the complexes was performed in aerobic condi-
tions. Solvents dichloromethane, methanol and dimethylformamide
(DMF) were distilled and dried according to known methods. The
chemicals, Cu(NO3)2·3H2O (SDFCL), para-Cl or Br substituted benze-
nesulfonamide, 4-pyridine carboxyaldehyde, 3-picolylamine (all sigma-
Aldrich), KOH, NaBH4 and carbon disulfide (Merck), triphenylpho-
sphine (Spectrochem) and 2-thiophenecarboxyaldehyde (Avra) were
used as received. The ligands N-(4-methylpyridyl)-N-(3-methylpyridyl)
dithiocarbamate (KL1), N-methylfuryl-N-methylthiophene dithiocarba-
mate (KL2) / 4-chlorobenzenesulfonyl dithiocarbimate (K2L3) and 4-
bromobenzenesulfonyl dithiocarbimate (K2L4) were prepared by the
reaction of appropriate secondary amines/sulfonamide, KOH and
carbon disulfide in THF/ DMF according to literature procedure
[16,19]. The starting compound [Cu(PPh3)2NO3] was synthesized by
the reported method [20].
The melting points of the complexes were measured with
a
Gallenkamp apparatus in open capillaries and are uncorrected. The
experimental details regarding elemental (C, H, N) analysis, IR as KBr
pellet, UV–vis and photoluminescence spectra are the same as described
earlier [12c]. NMR (1H, 13C{1H},31P{1H}) spectra were recorded on a
JEOl ECZ500 MHz FT NMR in DMSO-d6 /CDCl3. Chemical shifts are
noted in ppm downfield from internal TMS and J values in Hz. Thin-
layer chromatography (TLC) was accomplished using 60 F254 silica gel,
pre-coated on aluminum plates and revealed with either a UV lamp
(λmax =254 nm) or a specific color reagent (iodine vapors) or by
spraying with methanolic H2SO4 solution and subsequent charring by
heating at 100 °C.
2.2. Crystallography
R2N+=CS2
and RSO2N = CS2 exhibited respectively in their
2−
2-
Single crystal X-ray data for complexes 1-4 were collected with an
Oxford Diffraction X-calibur CCD diffractometer using Mo-Kα radiation
at temperatures of 150(2) K for 1, 2, 4 293(2) K for 3. Data reduction
for 1–4 was performed using the CrysAlis program [21]. The structures
were solved by direct methods using SHELXS-97 [22] and refined on F2
by full-matrix least-squares technique using SHELXL 2016-6 [23]. Non-
hydrogen atoms were refined anisotropically and hydrogen atoms were
geometrically fixed with thermal parameters equivalent to 1.2 times
that of the atom to which they were bonded. Diagrams for the com-
plexes were prepared using Olex2 [24] and Mercury [25] software. In 2,
one five-membered ring is identified as furan with the oxygen dis-
ordered between two positions in the ratio 0.89(2), 0.11(2), while the
other five-membered ring is disordered between furan and thiophene
with the ratio of S/CH 0.47(2), 0.53(2). In 3 and 4, some phenyl rings of
complexes. Furthermore, in the dithiocarbamate complexes electron
delocalization occurs via CN, CS2 and MS2 bonds whereas in the di-
thiocarbimates it is present via CS2, CN and aromatic SO2 groups. The
difference in their bonding and electronic properties may significantly
influence the catalytic activities and luminescent properties of their
complexes, (iv) Cu(I), d10 dithio-ligand complexes with Cu-S bonds
formed with the soft Cu(I) and S donors are generally thermo-
dynamically more stable and less labile in comparison to the Cu-N bond
formed with hard N donors. This fact also increases their hydrolytic
stability hence improves the purity, reproducibility and catalytic ac-
tivities and (v) often dinuclear Cu(I) complexes display better catalytic
activity than mono-nuclear complexes due to the synergic effect of two
Cu(I) ions.
153

A.K. Singh, et al.
MolecularCatalysis470(2019)152–163
PPh3 ligands are disordered. In 3, four of the phenyl rings were dis-
ordered over two orientations. These were refined as rigid groups with
occupation factors of x and 1-x, with x values of 0.49(3), 0.62(4),
0.53(4), 0.64(4). In 4, three of the phenyl rings were disordered over
two orientations. 3 and 4 are isomorphous and as the data for 4 was
poor, details of the structure are provided in the ESI. Complexes 1-4
crystallized with 0.5 CH2Cl2, 0.5H2O; CH3OH, 1.5H2O; 0.25CH2Cl2,
H2O and 0.5CH2Cl2, 2.5H2O solvent molecules respectively. The crystal
data of 1-4 have been deposited at the Cambridge Crystallographic Data
Centre, the reference numbers CCDC1867082-1867085.
133.67, 133.56, 128.51, 128.41 (BrC6H4), 128.77–132.09 (C6H5). 31P
{1H} (202.47 MHz, CDCl3, ppm): δ -0.083 [Cu2(PPh3)4(L4)], 29.813 [Cu
(PPh3)]+
,
43.944 [Cu(PPh3)3]+
. UV–vis. (CH2Cl2, λmax (nm), ε
(M−1 cm−1)): 255 (1.06 × 104), 290 (1.05 × 104), 310 (1.02 × 104),
345 (0.96 × 104).
2.5. General procedure for synthesis of diverse glycoconjugates from pre-
catalysts 1–4
A solution of sugar azides 5 (1. 0 equivalent), alkyne 7 (1.2
equivalent) and pre-catalysts 1–4 (10 mol%) in the presence /absence
of triethylamine in normal CH2Cl2 was stirred at room temparature in a
closed vessel for 2–8 h. The reaction was monitored by TLC, and then
after completion the mixture was concentrated in vacuo to obtain a
crude residue which was further purified by silica gel (100–200 mesh)
column chromatography to afford compound 8.
2.3. Synthesis of mono-/dinuclear complexes [Cu(PPh3)2(L)]and
[Cu2(PPh3)4(L)] (L = L1(1), L2(2), L3(3) and L4(4))
All four heteroleptic complexes were synthesized by adopting si-
milar procedures. To a 15 ml stirred methanol solution of the ligand KL1
(0.313 g, 1 mmol), KL2 (0.307 g, 1 mmol), K2L3(0.172 g, 0.5 mmol) or
K2L4(0.194 g, 0.5 mmol) was added gradually a 10 ml CH2Cl2 solution
of [Cu(PPh3)2NO3] (0.650 g, 1 m mol) and the reaction was stirred for
5 h at room temperature in each case. The dark yellow solid products
formed were filtered off, washed with methanol followed by diethyl
ether and vacuum-dried. The light yellow needle shaped crystals were
obtained within 2–3 weeks by the recrystallisation of the solid products
in dichloromethane/methanol mixture.
3. Results and discussion
3.1. Synthesis and methods
The heteroleptic mono-/dinuclear dithiocarbamate and dithio-
carbimate complexes [Cu (PPh3)2(L)] and [Cu2(PPh3)4(L)] were pre-
pared in good yield (Scheme S1), by treatment of a CH2Cl2 solution of
[Cu(PPh3)2NO3] with the methanolic solution of KL1-KL2/K2L3-K2L4 in
1:1/2:1 M ratio and characterized by spectroscopy and X-ray crystal-
lography. The pre-formed complexes 1-4 have been investigated as
catalysts for the synthesis of a variety of glycoconjugate triazoles under
Click approach. Their photoluminescent characteristics have been stu-
died.
2.4. Characterisation
1. Yield: (0.707 g, 82%). M.p.: 152–156 °C.
C49H42N3P2S2Cu
(862.52): calc. C 68.23, H 4.91, N 4.87%; found C 67.85, H 4.98, N
4.62%. IR (KBr, cm–1): 1435 (νC–N), 1095 (νC–S). 1H NMR (500.15 MHz,
DMSO-d6, ppm): δ 8.61-8.50 (d, 4H, C5H4N), 7.75–7.30 (m, 34 H, Ar-H
+4H, C5H4N), 5.15 (s, 4H, −CH2-C5H4N).13C{1H} NMR (125.76 MHz,
DMSO-d6, ppm): δ 211.00 (CS2), 149.26, 148.97, 137.07 123.98
(C5H4N), 135.61, 134.08, 132.86, 130.4, 129.00, 127.87 (C6H5), 54.3
(−CH2-C5H4N), 51.8 (−CH2-C5H4N). 31P{1H} (202.46 MHz, DMSO-d6,
ppm): δ -0.449 [Cu(PPh3)2(L1)], 26.377 [Cu(PPh3)]+, 42.946 [Cu
3.2. Spectroscopy
The IR spectra of dithiocarbamate complexes (1,2) display the
ν(CeN) and ν(CeS) vibrations at 1435–1455 and 1095 cm−1 whereas
in dithiocarbimate complexes (3,4) the appearance of ν(C]N),
νasym(SO2), νsym(SO2) and νasym(CS2) frequencies at 1480, 1306–1383,
1147–1158 and 998-1023 cm-1 respectively are diagnostic of ligand
coordination in the complexes. Notably a perceptible increase in the
ν(CeN) frequency of the complexes (1,2) as compared to the potassium
salts of ligands (KL1, KL2) 1362-1398 cm−1 is concomitant with the
(PPh3)3]+
.
UV–vis. (CH2Cl2, λmax (nm),
ε
(M−1 cm−1)): 260
(1.54 × 104), 310 (0.48 × 104).
2. Yield: (0.736 g, 86%). M.p.:148-152 °C. C47H40S3P2NOCu
(856.52): calc. C 65.91, H 4.71, N 1.64%; found C 65.83, H 4.78, N
1.62%. IR (KBr, cm–1): 1455 (νC–N), 1095 (νC–S). 1H NMR (500.15 MHz,
DMSO-d6, ppm): δ 6.88–7.50 (m, 30 H, C6H5), 6.35–6.40 (m, 3H,
C4H3S), 6.23–6.34 (m, 3H, C4H3O), 5.20 (s, −CH2-C4H3S), 5.28 (s,
−CH2-C4H3O).13C{1H} NMR (125.76 MHz, DMSO-d6, ppm): δ 203.23
(CS2), 107.73, 110.07, 125.37, 152.26 (C4H3O), 110.36, 110.76,
125.68, 141.69 (C4H3O), 126.27–127.01 (C6H5), 48.4 (CH2C4H3O),
46.1 (CH2C4H3S). 31P{1H} (202.47 MHz, DMSO-d6, ppm): δ -0.997 [Cu
2-
dominant contribution of the resonance form R2N+=CS2 indicating
enhancement of the C–N bond order (vide infra, crystal structures).
In the 1H and 13C {1H} NMR spectra, complexes 1-4 show re-
sonances characteristic of ligand functionalities and integrate well to
the corresponding hydrogens. The 13C NMR spectra of complexes (1,2)
displayed a single up field resonance signal at δ(203.24, 211.00 ppm)
for the NCS2 carbon in comparison to the free dithiocarbamate ligand at
(PPh3)2(L2)], 29.868 [Cu(PPh3)]+
,
43.971 [Cu(PPh3)3]+
. UV–vis.
(CH2Cl2, λmax (nm),
ε
(M−1 cm−1)): 290 (1.10 × 104), 350
δ(215.92, 218.48 ppm) because of the dominant contribution of
(0.43 × 104).
R2N+=CS2
resonance form in the complexes. On the contrary the
2−
3. Yield: (0.490 g, 68%), M.p. 118–122 °C. C79H64NS3O2P4ClCu2
(1441.99): calc. C 65.80, H 4.47, N 0.97%; Found: C 65.65, H 4.52, N
0.93%. IR (KBr, cm−1): 1478 ν(C = N), 1306 νasym(SO2), 1147
νsym(SO2), 961 νasym(CS2). 1H NMR (500.15 MHz, DMSO-d6, ppm): δ
7.67 (d, 2H,ClC6H4), 7.28 (d, 2H,ClC6H4), 7.54-7.65(m, 60 H, C6H5).
13C{1H} NMR (125.76 MHz, DMSO-d6, ppm):δ 205.12 (CS2), 133.99,
133.83, 129.30, 129.06 (ClC6H4), 129.40–133.21(C6H5).31P{1H}
(202.47 MHz, DMSO-d6, ppm): δ -3.297 [Cu2(PPh3)4(L3)], 26.211 [Cu
dithiocarbimate complexes (3, 4) showed a downfield resonance at δ
(200.67, 205.12 ppm) in comparison to the free dithiocarbimate ligands
at δ (189.15, 197.41 ppm). This may be attributed to greater shielding
of the NCS2 carbon of dithiocarbamate complexes compared to free
ligands while in dithiocarbimate complexes the NCS2 carbon is less
shielded than in the free ligands. In the 31P NMR spectra of (1, 2) the
occurrence of a single resonanace signal at δ (-0.449, -0.997 ppm) is
indicative of the equivalent nature of the PPh3 ligands bonded to the
copper atom. It is to be noted that the dinuclear dithiocarbimate
complexes 3 and 4 show only one 31P resonance at δ -3.297 and
-0.083 ppm respectively instead of the expected two signals due to
different coordination environments about the two copper centers,
presumably the fluxional behavior of the ligand PPh3 causes signal
overlap at room temperature which results in only one broad signal.
(PPh3)]+
,
42.919 [Cu(PPh3)3]+
. UV–vis. (CH2Cl2, λmax (nm), ε
(M−1 cm−1)): 250 (1.02 × 104), 305 (1.07 × 104), 350 (1.02 × 104).
4. Yield: (0.476 g, 64%), M.p. 120–124 °C. C79H64NS3O2P4BrCu2
(1486.45): calc. C 63.83, H 4.34, N 0.94%; Found: C 63.55, H 4.48, N
0.90%. IR (KBr, cm−1): 1480 ν(C = N), 1383 νasym(SO2), 1158
νsym(SO2), 1023 νasym(CS2). 1H NMR (500.15 MHz, CDCl3, ppm): δ 7.72
(d, 2H, BrC6H4), 7.24(d, 2H,BrC6H4),7.25–7.68 (m, 60 H,
The appearance of two additional sharp resonances at
δ
C6H5).13C{1H} NMR (125.76 MHz, CDCl3, ppm):
δ 200.66 (CS2),
(26.211–29.868 ppm) and (42.919–43.971 ppm) in all four complexes
154

A.K. Singh, et al.
MolecularCatalysis470(2019)152–163
Fig. 1. Electronic absorption spectra in CH2Cl2 solution for 1-2 (a) and 3-4 (b).
shows the formation of [Cu(PPh3)]+ and [Cu(PPh3)3]+ cationic species
respectively via association/dissociation in solution. Cu(I) thiocarba-
mate [26] and thiophene-2-thiocarboxylate- phosphine complexes
[26e] have been reported to form an equilibrium mixture of different
cationic/anionic species via associative /dissociative process.
420–500 nm (Fig. 2) emanating most likely from the admixture of ILCT
and MLCT states with a remarkable Stokes shift of 120- 200 nm because
the free ligands are non- emissive/very weakly emissive. The compar-
able luminescent properties of the complexes may be attributed to only
slight changes in their luminescent chromophores and minimal struc-
tural rearrangements in the ground and excited states of complexes
which is probably fixed by the rigid ligand environments. Notably,
highly intense photoluminescent characteristics of 1 and 4 arise due to
Py-(N) on the 3- and 4 positions and p-bromo substituents of the dithio
unit which enhance electron cloud hence conjugation in the molecule.
3.3. Electronic absorption and photoluminescence spectra
The d10 coinage metal complexes including clusters are becoming
more important due to their intriguing optical and luminescent prop-
erties. The absorption spectra of the ligands (KL1, KL2) and (K2L3, K2L4)
recorded in methanol (Fig. S1) show two absorptions near 250–260 nm
3.4. Crystal structures
(ε
=
0.75–0.79 × 105M–1 cm–1
)
and 300–325 nm (ε
=
0.48–0.49 × 105M–1 cm–1) while PPh3 shows an absorption at 262 nm
[27] which is assigned to intraligand, π–π* charge transfer transitions.
The spectra of corresponding Cu(I) dithiocarbamate (1,2) and dithio-
carbimate (3,4) complexes collected in CH2Cl2 (Fig. 1) show two ab-
sorptions at somewhat longer wavelengths near 260–290 nm (ε =
Slow evaporation of a dichloromethane/methanol solution of the
compound yielded single crystals of complexes 1-4. The crystal-
lographic details, selected bond distances and angles are given in Tables
1–3. The ORTEP drawings of a mononuclear 1 and dinuclear complex 3
are illustrated in Figs. 3 and 4 and those of 2 and 4 in ESI figure S2.
Dimensions for 1-3 are given in Table 2. As the data for 4, which is
isomorphous with 3, were not so good, dimensions are given in the ESI.
In the mononuclear complexes (1, 2) the Cu(1) atoms are bonded to
two sulphur atoms of a dithiocarbamate and phosphorus atoms of two
PPh3 ligands with Cu-S and Cu-P bond lengths in the range 2.399(2)-
2.413(2) Å and 2.2447(14)-2.2595(19) Å which are well within the
expected range [28]. In these structures the geometry around the
copper atoms is distorted tetrahedral as is evident from the angles listed
in Table 2. The small S(11)-Cu(1)-S(13) bite angles observed at
1.10–1.54 × 104M–1 cm–1), 310–350 nm (ε
=
0.43–0.48 × 104
M–1 cm–1) and three to four bands near 250 nm (ε = 1.02–1.06 × 104
M–1 cm–1), 290–310 nm (ε = 1.02–1.07 × 104 M–1 cm–1) and 350 nm (ε
= 0.96–1.02 × 104 M–1 cm–1) respectively, the higher energy absorp-
tions are assigned to ILCT transitions and those near 300 nm and above
arise from a mixture of ILCT, π–π* and MLCT, d- π* transitions.
Under UV-irradiation all the complexes emit intense green light.
When excited at 300 nm in CH2Cl2 solution at room temperature,
complexes 1-4 show an intense broad unstructured emission band near
Fig. 2. Emission spectra (λexc=300 nm) in CH2Cl2 solution for 1-2 (a) and 3-4 (b).
155

A.K. Singh, et al.
MolecularCatalysis470(2019)152–163
Table 1
Crystallographic data and structure refinements for complexes.
Compound
1.0.5CH2Cl2.0.5H2O
2.CH3OH.1.5H2O
3.0.25CH2Cl2.H2O
4.0.5CH2Cl2.2.5H2O
Chemical formula
C49.5 H44 Cl Cu N3O0.5 P2 S2'
913.92
C48H47CuNO3.98P2S2.53
907.89
C79.25 H65.5 Cl1.5 Cu2 N O3 P4 S3
C79.5 H69 Br Cl Cu2 N O4.5 P4 S3
Formula weight
1480.14
Monoclinic
P21/c
1572.85
Monoclinic
P21/c
Crystal system
Space group
a (Å)
Triclinic
Triclinic
P -1
P -1
12.4838(11)
12.8385(10)
16.3550(12)
109.858(7)
91.866(7)
114.915(8)
2188.0(3)
2
12.4657(8)
12.7303(9)
16.8683(12)
101.227(6)
97.177(6)
115.990(7)
2292.0(3)
2
19.9784(14)
16.8728(9)
24.5716(12)
(90)
19.817(2)
16.9319(13)
24.204(4)
(90)
b (Å)
c (Å)
α(o)
β(o)
92.488(5)
(90)
93.332(12)
(90)
γ(o)
V (Å3)
Z
8275.1(8)
4
8107.7(17)
4
ρcalc (g cm−3
T (K)
)
1.387
1.3164
1.216
1.289
150(2)
150(2)
295(2)
150(2)
μ (Mo Kα) (mm−1
F(000)
)
0.769
0.704
0.758
1.253
948
946
3120
3228
Reflections collected
10687
13633
39348
38738
Independent reflns
7581
10251
14443
13602
Reflections with I > 2σ(I)
Final indices [I > 2σ(I)] R1a, wR2
R1[a], wR2[b] [all data]
GOF
5065
7762
6044
6649
b
0.0965, 0.1750
0.1463, 0.1988
1.102
0.0860, 0.2236
0.1107,0.2410
1.056
0.1227, 0.3126
0.2314, 0.3740
0.963
0.1494, 0.3567
0.2465, 0.4031
1.071
Residual electron density e/Å3
0.858, -1.070
2.555, -0.578
1.195, -0.483
a
b
R1 = Σ||Fo| − |Fc||/Σ|Fo|. R2 = {[Σw(Fo2 − Fc2)/Σw(Fo2)2]}1/2, w = 1/[σ2(Fo2) + (xP)2], where P = (Fo2 + 2Fc2)/3.
75.09(7)°-75.36(5)° are primarily responsible for the distortion from
ideal tetrahedral geometry. The P(1)-Cu(1)-P(2) angles at 124.08(8)°
and 125.86(5)° are similar, whereas P-Cu(1)-S angles are observed in
the range 104.88(7)°-117.38(7)° and 106.37(5)°-119.08(5)° respec-
tively. In the complexes 1-2 the two planes are nearly perpendicular to
each other as is evident from the angles between the two least square
planes formed by Cu(1), S(11), S(13), C(12) and P(1), Cu(1), P(2) which
are 83.97(8)°, 82.26(6)° respectively. The r.m.s. deviations from pla-
narity of the atoms Cu(1), S(11), S(13) and C(12), defining the chelate
rings, are 0.030 and 0.008 Å respectively. In the structure of 2, one five
membered ring is identified as furan with the oxygen disordered be-
tween two positions while the other five-membered ring is disordered
between furan and thiophene.
Table 3
Selected bond lengths(Å) and bond angles(°) for dithiocarbimate complex 3.
Bond lengths(Å)
3
Bond angles(o)
3
Cu(1)-P(2)
Cu(1)-P(1)
Cu(1)-S(11)
Cu(1)-S(13)
Cu(2)-N(14)
Cu(2)-P(3)
Cu(2)-P(4)
Cu(2)-S(13)
C(12)-S(11)
C(12)-S(13)
C(12)-N(14)
Cu(1)-Cu(2)
2.262(3)
2.252(3)
2.415(3)
2.436(3)
2.153(7)
2.241(3)
2.254(3)
2.464(3)
1.706(9)
1.744(9)
1.293(11)
4.850(2)
P(1)-Cu(1)-P(2)
P(2)-Cu(1)-S(11)
P(1)-Cu(1)-S(11)
P(2)-Cu(1)-S(13)
P(1)-Cu(1)-S(13)
S(11)-Cu(1)-S(13)
P(3)-Cu(2)-N(14)
P(4)-Cu(2)-N(14)
P(3)-Cu(2)-P(4)
S(13)-Cu(2)-N(14)
P(3)-Cu(2)-S(13)
P(4)-Cu(2)-S(13)
120.52(10)
119.49(10)
112.02(11)
106.05(9)
115.04(10)
74.22(9)
112.9(2)
118.4(2)
123.91(11)
65.9(2)
115.24(10)
105.22(11)
In the structure of 3 [Cu2(PPh3)4(L3)] the p-chlorobenzenesulphonyl
dithiocarbimate ligand is bonded in S,S –chelating mode to one of the
Cu(1)(PPh3)2 units and simultaneously bridges the other Cu(2)(PPh3)2
through the N,S- donor atoms. Both Cu(1) and Cu(2) atoms have dis-
torted tetrahedral geometries with CuP2S2 and CuP2NS cores respec-
tively. The Cu(1)- Cu(2) distance at 4.850(2) Å is large enough to rule
out any metal-metal bonding. The dimer contains a central planar core
containing Cu(1), Cu(2), S(11), C(12), S(13), N(14) and S(15) with an
r.m.s deviation of 0.046 Å. The two PPh3 ligands attached to Cu(1) and
Cu(2) atoms complete the tetrahedral coordination of each metal with
Cu-P distances in the range 2.241(3) Å -2.262(3) Å. The bridging Cu-S
(13) distance at 2.464(3) Å is longer than that to the chelating Cu-S(11)
at 2.415(3) Å. The Cu(2)-N(14) bond length at 2.153(7) Å is well within
the expected values for a Cu(2)-N bond [18a].
(1.27(1)Å) bond lengths and are significantly longer than that observed
for 3 at 1.293(11) Å, a difference which is associated to the contribution
of resonating structures (Scheme S2). The C–S bond lengths at 1.700(6)
Å -1.744(9) Å in 1-3 are significantly smaller than the C–S single bond
(ca. 1.81 Å) which indicate the delocalization of the π electrons over
NCS2 unit.
The important role of CeH···π(MCS2, chelate) interactions in metal
bis(1,1-dithioligand) complexes have been described in the formation
of supramolecular assembly of varying dimensionality [29]. It was
suggested that for homoleptic bis-dithio complexes the values of
α < 20, β ranging from 110-180° and d between 2.4 and 3.6 Å were
crucial to assesses the interactions where α is the angle between the
perpendicular to the ring and the CG⋯H vector, β, the CG…H-C angle
As expected the C(12)-N(14) bond distances at 1.35(1) Å in (1,2) are
intermediate between the single C–N (1.47 Å) and double C = N
Table 2
Selected bond lengths (Å) and bond angles(°) for dithiocarbamate complexes 1-2.
Bond lengths(Å)
1
2
Bond angles(°)
1
2
Cu(1)-P(2)
Cu(1)-P(1)
Cu(1)-S(11)
Cu(1)-S(13)
C(12)-S(11)
C(12)-S(13)
C(12)-N(14)
2.248(2)
2.260(2)
2.413(2)
2.399(2)
1.721(7)
1.709(7)
1.355(9)
2.2447(14)
2.2481(14)
2.4008(14)
2.4112(14)
1.700 (6)
1.718(5)
P(1)-Cu(1)-P(2)
P(2)-Cu(1)-S(11)
P(1)-Cu(1)-S(11)
P(2)-Cu(1)-S(13)
P(1)-Cu(1)-S(13)
S(11)-Cu(1)-S(13)
124.08(8)
112.44(7)
104.88(7)
111.60(7)
117.38(7)
75.09(7)
125.86(5)
119.08(5)
106.37(5)
106.41(5)
112.46(5)
75.36 (5)
1.352(7)
156

A.K. Singh, et al.
MolecularCatalysis470(2019)152–163
Fig. 3. ORTEP diagram of 1 with thermal ellipsoids set to 30% probability level. Solvent molecules and hydrogen atoms are omitted for clarity.
with α, β and d values of 6, 114°, 3.165 Å and 7, 144°, 2.956 Å in 3 (Fig.
S3c).
and d, the CG⋯H distance where CG is the centre of gravity of the
chelate ring. For 1 the CeH···π(CuCS2, chelate) interaction involving H
(96) of the PPh3 group to CG of the CuCS2 ring is found with α 0, β 136°
and d 2.707 Å (Fig. S3a). On the other side of the chelate ring H(52)
shows values of 7, 121°, 3.055 Å. Also, in 2 two CeH···π(Cu1-CS2,
chelate) interactions are found between hydrogen atoms of PPh3 moi-
eties H(42) and H(92) on opposite sides of the ring with parameters, 4,
133°, 2.798 Å and 1, 121°, 2.872 Å respectively (Fig. S3b). In 3 similar
interactions with the Cu(1)CS2, chelate are found for H(56) and H(76)
with parameters 9, 113°, 3.319 Å and 11, 144°, 3.136 Å respectively
(Fig. S3c).
The weak inter molecular H···H interactions are found in complexes
1 and 3 with an H (31B)···H(54) distance of 2.37 Å in 1 (Fig. S4a) and an
H(134)···H(135) distance of 2.37 Å in 3 (Fig. S4b). The intermolecular
C–H···π interactions sustain the supramolecular structures of 1 and 3.
3.5. Catalytic activity of the pre-formed Cu(I) complexes in click chemistry:
synthesis of diverse glycoconjugates triazoles
We investigated the developed mono-/ dinuclear Cu(I) dithio-
carbamate / dithiocarbimate- PPh3 complexes 1-4 for azide-alkyne
cycloaddition reaction of diverse sugar azides (5a-m) with the vaniline
Complex 3 also contains a C–H⋯π(Cu(2)CNS, chelate) ring which
forms similar interactions with H(92), H(136) of the two PPh3 moieties
Fig. 4. ORTEP diagram of 3 with thermal ellipsoids set to 30% probability level. Solvent molecules and hydrogen atoms are omitted for clarity.
157

A.K. Singh, et al.
MolecularCatalysis470(2019)152–163
Table 4
Optimization of catalytic efficiency of developed Cu(I) pre-catalyst 1–4 in the synthesis of vanillin glycoconjugates 8a.
Entry
Catalyst
Solvent
Base (1 equiv)
Time (h)
Yield (%)d
1a
1 (5 mol%)
2 (5 mol%)
3 (5 mol%)
4 (5 mol%)
1 (5 mol%)
2 (5 mol%)
1 (10 mol%)
2 (10 mol%)
1 (10 mol%)
2 (10 mol%)
1 (10 mol%)
1 (10 mol%)
1 (10 mol%)
1 (10 mol%)
CuI(10 mol%)
CuSO4/NaAsc
DCM
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
–
4
4
4
4
4
4
2
2
2
2
2
2
2
2
2
2
89
91
25
55
87
90
94
92
93
Nd
28
32
45
70
48
43
2a
DCM
3a
DCM
4a
DCM
5b
DCM
6b
DCM
7b
DCM
8b
DCM
9c
DCM
10c
11c
12c
13c
14c
15
16
DCM
–
MeOH
EtOH
CH3CN
THF
–
–
–
–
DCM
Et3N
–
THF/H2O
a
b
c
Reaction proceeded with 5 (1 equiv), 7 (1 equiv) and Et3N (1 equiv) in dry DCM under argon atmosphere.
Reaction proceeded with 5 (1 equiv), 7 (1 equiv) and Et3N (1 equiv) in normal DCM in air.
Reaction proceeded with 5 (1 equiv) and 7 (1 equiv) in air.
d
Yield after purification (SiO2).
based alkyne 7 to synthesis of relatively rare biologically relevant va-
reduced the reaction time, with completion within two hours (Table 4,
entries 7, 8). Next, to make the catalytic system more precise and
economic for the synthesis of medicinal valued triazolyl glycoconju-
gates, the reactions were performed in the absence of base. Surprisingly
the reaction proceeded well in the presence of catalyst 1 and afforded
excellent yield (Table 4, entry-9) but by contrast complex 2 did not
react and no product was formed. Presumably this may be ascribed to
the presence of two pyridine moieties in complex 1 thereby enhancing
the basicity of the reaction medium that was needed to initiate the
reaction. Finally, it was established that complex 1 behaves as a highly
efficient catalyst that enables the reaction to proceed smoothly in air
without a base, with no need of dry solvent so that the reaction was
completed within two hours. Indeed, pre-catalyst 1 showed high ac-
tivity in different reaction media such as methanol, ethanol, acetonitrile
and tetrahydrofuran. However because of the low solubility of 1 in
methanol and ethanol, the reactions proceeded sluggishly. Although,
reactions performed in tetrahydrofuran displayed better results than in
acetonitrile but poorer results than in dichloromethane (Table 4, entries
13–14). The performance of 1 was also compared with at traditional Cu
(I) source using CuI (10 mol%) in dichloromethane and also by using a
combination of CuSO4.5H2O(5 mol %)/sodium ascorbate (10 mol%) in
THF/H2O for the synthesis of vanillin-glycoconjugate derivatives
(Table 4, entries 1516).
nillin-glycoconjugates (8a-m). We also investigated the efficiency and
recyclability of homo-dinuclear 3-4 complexes in click protocol with 5n
and phenylacetylene 9 for synthesis of triazolyl glycoconjugate.
The synthetic work began with the synthesis of diverse sugar azides
5a-m from readily available monosaccharides using various high
yielding steps of protections and modifications [30]. The synthesis of
vanillin based terminal alkynes 7/9 was accomplished through the
substitution reaction at the free hydroxyl position of p-vanillin with
propargyl bromide and potassium carbonate. Initially, the catalytic
activity of pre-catalyst 1–4 in the azide-alkyne cycloaddition reaction
with the vanillin based terminal alkyne 7 and sugar azide 5a was in-
vestigated. In the preliminary examination, we treated 7 (1 equiv) with
5a (1 equiv) in dry dichloromethane under inert atmosphere in the
presence of triethylamine (1 equiv) with a set up of four separate re-
actions for each catalytic system in which the initial loading of catalysts
was 5 mol%. Consumption of starting material was observed after four
hours. These results all showed the catalytic efficiency of complexes
1–4. In particular the mono nuclear Cu(I) complexes (1,2) exhibited
virtually similar efficacy (Table 4, entries 1,2) thus yielding product 8a
in 89% and 91% respectively. By contrast the di-nuclear Cu(I) com-
plexes 3 and 4 showed sluggish reaction and yielded 8a in 25% and
55% respectively (Table 4, entries 3,4), albeit they completed the re-
action in good yield within 8–10 hours. Furthermore, the eco-friendly
character of pre-catalysts was checked by performing the reactions in
air and in simple dichloromethane (non dried DCM) but oxidation of
copper (I) complexes to copper (II) during the course of reaction was
not observed as evident by the fact that no perceptible change in color,
i.e. blue to – green, appeared in solution and thus the product was
isolated with similar efficiency (Table 4, entries 5, 6). Out of the four Cu
(I) complexes the mononuclear complexes (1, 2) were found to provide
excellent results, therefore, they were investigated further. The effect of
the amount of catalyst used on reactivity was also observed and it was
found that the loading of catalyst with double quantities (10 mol %),
Further synthesis were explored by applying these optimized reac-
tion conditions for clicking the vanillin derived alkynes with diverse
sugar azides and azido alcohols (Table 5). The reactions of 5c-k with 7
in presence of pre-catalyst 1 (10 mol%) in dichloromethane in air were
also studied with appropriate reaction conditions. This afforded a li-
brary of biological relevant vanillin glycoconjugates. The catalytic
system was found suitable for all reactions and exhibited wide func-
tional group tolerance. All synthesized compounds were characterized
by NMR spectroscopy (Table S2).
The recyclability of catalysts 1-4 was investigated. All complexes
were recovered from the crude reaction product via different
158

A.K. Singh, et al.
MolecularCatalysis470(2019)152–163
Table 5
Synthesis of triazole-linked p-vanillin glycoconjugates 8c-k using pre-catalyst 1.
Entrya
1
Azideb (5)
Productc (8)
Time (h)
2.5
Yieldd (%)
88
5c
8c
2
3
3
87
84
5d
5e
8d
2
8e
8f
4
5
2
3
85
89
5f
5g
8g
6
3
86
5h
8 h
(continued on next page)
159

A.K. Singh, et al.
MolecularCatalysis470(2019)152–163
Table 5 (continued)
Entrya
7
Azideb (5)
Productc (8)
Time (h)
Yieldd (%)
90
2
5i
8i
8j
8
9
2.5
2.5
85
82
5j
5k
8k
a
b
c
Molar ratio of reagents Azide (1.0 equiv), Alkyne (1.2 equiv) and pre-catalyst (10 mol%).
developed glycosyl azides.
developed triazolyl glycoconjugates.
d
isolated yield of product after purification (SiO2).
techniques. Thus the recyclability of complex 1 by clicking azide 5a and
necessary in the case of 6-azido sugar 5a (Table 1). Both the dinuclear
complexes 3 and 4 were also observed to be reusable catalysts (Table 8)
and recovered using column chromatography during purification of the
product. Catalyst loading of 10 mol% for these complexes was found
sufficient for catalyzing the reaction and obtaining recyclability
(Table 8, entries 1–5).
alkyne 7 was evaluated under optimized reaction conditions. We set up
three reactions sequentially, in which each reaction was initiated with
the catalyst recovered from the former reaction and the reaction crude
was extracted with 2 ml of methanol three times where the product was
soluble in methanol and decayed in another pot. The remaining solid
was identified as the catalyst which was reused in the next reaction for
azide-alkyne cycloaddition.
4. Conclusions
Recyclability of complex 2 was also observed and recovered in
slightly less amount than used initially in the first reaction using
column chromatography during purification of the product. For this, we
proceeded three different reactions sequentially, in which each reaction
was catalyzed with recovered catalyst 2 from the former reaction
mixture (Table 6). First, the reaction was catalyzed with 10 mol% of
catalyst in the presence of base Et3N (1 equiv) which completed the
reaction within 2 h. Likewise, the other reactions were performed with
similar equivalents of reactants and recovered quantity of the catalyst
(Table 7).
New mono- and homo-dinuclear Cu(I) dithiocarbamates (1,2) and
dithiocarbimates (3,4) complexes bearing PPh3 ligands were synthe-
sized, structurally characterized and their catalytic activities under the
Click protocol for the transformation of azides and alkynes into glyco-
conjugate triazoles have been investigated. Among these pre-catalysts
the mono-nuclear Cu(I) dithiocarbamate complexes (1,2) are highly
efficient and reusable; in particular 1 is equally efficient under base free
conditions due to the presence of the pyridyl group which provides the
basicity in the reaction medium. By comparison under similar mild
reaction conditions and low catalytic loadings, the observed activity of
dinuclear dithiocarbimate complexes (3,4) is rather sluggish; their
weaker efficiency may be attributed to the varied environment about
the two copper(I) centres, which provide electronic characteristics that
Further, we explored the scope of catalysts 3 and 4 with anomeric
azides 5m-n and alkyne 7, 9 (Table 8, entries 1, 6–7). We observed that
both catalysts were found efficient for the synthesis of glycoconjugates
8n-p; though, they required slightly longer reaction times 6–8 h than
160

A.K. Singh, et al.
MolecularCatalysis470(2019)152–163
Table 6
Recyclability of pre-catalyst 1 in the synthesis of triazole-linked p-vanillin glycoconjugates 8a.
Entrya
Catalyst
10 mol%
Temp
Time (h)
Yield (%)b
1
2
3
Rt
Rt
Rt
2
2
2
94%
89%
86%
c
-
d
-
a
b
c
Molar ratio: 5a (1 equiv.) and 7 (1.2 equiv.) in DCM (2 mL).
Yields reported after purification by column chromatography (SiO2).
Pre-catalyst recovered from entry 1.
d
Pre-catalyst recovered from entry 2.
might be affect their activity. The development of Cu(I) dithiocarba-
mate based complexes bearing phosphine ligands may show wider and
efficient utility in Click chemistry under eco-friendly conditions. The
synthesis of these catalysts is easy and the chemicals involved are rather
cost effective. 1-4 emits green luminescence in solution at room tem-
perature.
Acknowledgements
The funding from the UGC-BSR (ref. No. F. 18-1/2011) Faculty
Fellowship (NS), SRF (CLY) and JRF (AKS) the University Grant
Commission (UGC), NPDF (KBM) (SERB No. PDF/2016/001709) New
Delhi and the Department of Chemistry, Institute of Science, Banaras
Table 7
Synthesis of triazole-linked p-vanillin glycoconjugates 8 using pre-catalyst 2.
Entry
1b
Azide (5)
Product (8)
Time (h)
2
Yieldd(%)
92
5b
7
8b
2b
2.5
2.5
87
83
5l
5l
7
9
8l
3c
8m
a
Reaction proceeded with 5 (1 equiv), 7 (1 equiv), Et3N and pre-catalyst 2 (10 mol%).
b
Reaction proceeded with recovered pre-catalyst 2 from entry 1.
c
Reaction proceeded with recovered pre-catalyst 2 from entry 2.
d
Isolated yield after purification.
161

A.K. Singh, et al.
MolecularCatalysis470(2019)152–163
Table 8
Synthesis of N-glycosides through triazole formation using pre-catalyst 3 and 4.
Entrya
Azide
Alkyne
Product
Catalyst (10 mol%)
Time (h)
Yieldf (%)
Y1 and Y2
1
5m
5m
5m
5m
5m
5m
9
9
9
9
9
7
8n
8n
8n
8n
8n
8o
3
8
8
6
6
6
8
Y1 = 75
Y1 = 69
Y2 = 92
Y2 = 87
Y2 = 82
Y1 = 76
Y2 = 88
Y1 = 72
Y2 = 85
2b
3c
4d
5e
6
–
4
–
–
3 or 4
7
5n
7
8p
3 or 4
8
a
Reaction proceeded with alkyne (1.0 equiv), azide (1.0 equiv), Et3N (1 equiv) and pre-catalyst 3.
Reaction proceeded with recovered pre-catalyst 3 from entry 1.
b
c
d
e
f
Reaction proceeded with alkyne (1 equiv), azide (1 equiv), Et3N (1 equiv) and pre-catalyst 4.
Reaction proceeded with recovered pre-catalyst 4 from entry 3.
Reaction proceeded with recovered pre-catalyst 4 from entry 4.
Y1 represented yield for catalyst 3 and Y2 for catalyst 4.
Hindu University, UGC CAS-II for infrastructural facilities. We also
(e) D. Wang, N. Li, M. Zhao, W. Shi, C. Ma, B. Chen, Green. Chem. 12 (2010)
2120–2123.
thank the University of Reading and the EPSRC (UK) for funds for the
[11] (a) D.A. Evans, F.E. Michael, J.S. Tedrow, K.R. Campos, J. Am. Chem. Soc. 125
(2003) 3534–3543;
diffractometer used for the data collections of structures 1-4.
(b) J.W. Faller, J.C. Wilt, J. Parr, Org. Lett. 6 (2004) 1301–1304;
(c) B.S. Mandimutsira, J.L. Yamarik, T.C. Brunold, W. Gu, S.P. Cramer,
C.G. Riordan, J. Am. Chem. Soc. 123 (2001) 9194–9195;
(d) X. Verdaguer, M.A. Perica`s, A. Riera, M.A. Maestro, J. Mahı´a,
Organometallics 22 (2003) 1868–1877;
Appendix A. Supplementary data
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.mcat.2019.03.009.
(e) O.G. Mancheño, R.G. Arrayás, J.C. Carretero, Organometallics 24 (2005)
557–561.
[12] (a) N. Sareen, A.S. Singh, V.K. Tiwari, R. Kant, S. Bhattacharya, Dalton Trans. 46
(2017) 12705–12710;
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