Copper complexes of 1,4-diazabutadiene ligands: Tuning of metal oxidation state and, application in catalytic C-C and C-N bond formation

Article abstract of DOI:10.1016/j.ica.2019.119228

Reaction of 1,4-diazabutadiene (p-RC₆H₄N = C(H)(H)C = NC₆H₄R-p; R = OCH₃, CH₃, H and Cl; abbreviated as L-R) with CuCl₂·2H₂O in methanol at ambient temperature (25 °C) affords a group of doubly chloro-bridged dicopper complexes of type [{Cu^I(L-R)Cl}₂], designated as 1-R. Similar reaction carried out in acetonitrile furnishes a family of doubly chloro-bridged dicopper complexes of type [{Cu^II(L-R)Cl₂}₂], designated as 2-R. Molecular structures of 1-OCH₃ and 2-OCH₃ have been determined by X-ray crystallography. While copper(I) is having a nearly tetrahedral N₂Cl₂ coordination sphere in 1-OCH₃, the N₂Cl₃ coordination sphere around copper(II) is distorted square pyramidal in nature in 2-OCH₃. Isolated 2-R complexes, on dissolution in methanol, are found to undergo facile reduction of the metal center to generate the corresponding 1-R complexes. The 1-R and 2-R complexes show intense absorptions in the visible and ultraviolet regions. Cyclic voltammetry on the 1-R and 2-R complexes shows both metal-centered and ligand centered redox responses. The 1-R complexes are found to efficiently catalyze C-N cross-coupling reactions between arylboronic acids and aryl amines; while the 2-R complexes display notable catalytic efficiency for nitroaldol reactions.

Full text of DOI:10.1016/j.ica.2019.119228

InorganicaChimicaActa500(2020)119228
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Research paper
Copper complexes of 1,4-diazabutadiene ligands: Tuning of metal oxidation
state and, application in catalytic C-C and C-N bond formation
Aparajita Mukherjee, Semanti Basu1, Samaresh Bhattacharya⁎
Department of Chemistry, Inorganic Chemistry Section, Jadavpur University, Kolkata 700 032, India
A R T I C L E I N F O
A B S T R A C T
Keywords:
Reaction of 1,4-diazabutadiene (p-RC6H4N = C(H)(H)C = NC6H4R-p; R = OCH3, CH3, H and Cl; abbreviated as
L-R) with CuCl2·2H2O in methanol at ambient temperature (25 °C) affords a group of doubly chloro-bridged
dicopper complexes of type [{CuI(L-R)Cl}2], designated as 1-R. Similar reaction carried out in acetonitrile
furnishes a family of doubly chloro-bridged dicopper complexes of type [{CuII(L-R)Cl2}2], designated as 2-R.
Molecular structures of 1-OCH3 and 2-OCH3 have been determined by X-ray crystallography. While copper(I) is
having a nearly tetrahedral N2Cl2 coordination sphere in 1-OCH3, the N2Cl3 coordination sphere around copper
(II) is distorted square pyramidal in nature in 2-OCH3. Isolated 2-R complexes, on dissolution in methanol, are
found to undergo facile reduction of the metal center to generate the corresponding 1-R complexes. The 1-R and
2-R complexes show intense absorptions in the visible and ultraviolet regions. Cyclic voltammetry on the 1-R
and 2-R complexes shows both metal-centered and ligand centered redox responses. The 1-R complexes are
found to efficiently catalyze C-N cross-coupling reactions between arylboronic acids and aryl amines; while the
2-R complexes display notable catalytic efficiency for nitroaldol reactions.
1,4-Diazabutadiene ligands
Dicopper(I) and dicopper(II) complexes
Tunability of copper oxidation state
Catalytic C-N and C-C coupling reactions
1. Introduction
R = OCH3, CH3,
H and Cl
H
N
H
N
H
N
H
N
The present work has originated from our continued interest in the
synthesis of designed transition metal complexes, and their utilization
in catalysis [1–8]. Though complexes of the platinum group of metals
are undoubtedly most favorite catalysts for majority of the reactions,
complexes of the first row transition metals are gradually finding im-
portance in this field of catalysis, primarily owing to their ease of for-
mation and inexpensive metal sources [9–13]. We have also been active
along this direction, and prepared few copper and nickel based systems
that showed notable catalytic properties towards C-C and C-N bond
formation reactions [14–16]. Accessibility of different oxidation states
of copper (viz. +1 to +3), associated with possible variation of co-
ordination number around the metal center, has endowed copper based
systems as favorite candidates for exploring their catalytic potential.
Encouraged by our own observations, as well as by the works of others,
on the development of copper based molecular systems and their uti-
lization as catalyst in useful reactions [16,17–20], we planned to syn-
thesize copper complexes of a group of 1,4-diazabutadiene ligands,
which
R
R
R
R
M
I
L-R
are abbreviated in general as L-R, where R depicts the para-substituent
in the aryl ring. Four different substituents (R = OCH3, CH3, H and Cl),
with different electron withdrawing properties, were chosen to study
their influence, if any, on the redox properties of the complexes. The
selected ligands are known to bind to metal centers as bidentate N,N-
donors forming five-membered chelate ring (I) [21–25]. The α-diimine
fragment in these ligands has considerable π-acidity, and hence they are
capable of stabilizing metals in relatively low oxidation states [26,27].
Though copper complexes with some diimine ligands are reported
[28–35], and catalytic application of some of them is known
[29,31–34], a comprehensive study on copper complexes of the 1,4-
diazabutadiene ligands, with particular reference to their catalytic ap-
plication in C-N and C-C bond formation reactions which is our final
goal, seems to have remained unexplored. The initial target of this work
⁎ Corresponding author.
E-mail address: samaresh_b@hotmail.com (S. Bhattacharya).
1 Present address: Department of Chemistry, Bejoy Narayan Mahavidyalaya, Itachuna, Hooghly, West Bengal 712147, India.
https://doi.org/10.1016/j.ica.2019.119228
Received 19 July 2019; Received in revised form 18 October 2019; Accepted 23 October 2019
Availableonline24October2019
0020-1693/©2019ElsevierB.V.Allrightsreserved.

A. Mukherjee, et al.
InorganicaChimicaActa500(2020)119228
was to synthesize a group of mixed-ligand copper complexes containing
1,4-diazabutadiene as neutral ligand and chloride ion as anionic ligand.
In order to achieve this target we selected cupric chloride as the source
of copper, as it is capable of providing the CuCl fragment. In this paper
we describe our observation on the interaction of the 1,4-diazabuta-
diene ligands with cupric chloride towards affording the mixed-ligand
complexes, their characterization, and exploration of their catalytic
efficiency for C-C and C-N bond formation.
acetonitrile (25 ml) was added to it. The solution was stirred at ambient
temperature for 1 h, whereby a yellowish-brown solution was obtained.
The solution volume was reduced to half via evaporation under reduced
pressure, and the concentrated solution was kept in a refrigerator for
12 h. The 2-OCH3 complex separated as a yellowish-brown crystalline
solid, which was collected by filtration, washed with ether, and dried in
air. Yield: 427 mg, 90%. Anal. Calcd. for C32H32N4O4Cl4Cu2: C, 47.70;
H, 3.98; N, 6.96. Found: C, 47.45; H, 3.95; N, 6.99%. Magnetic moment,
μ
eff = 2.43 μB. IR (cm−1): 494, 550, 797, 839, 851, 902, 1022, 1172,
2. Experimental
1259, 1299, 1371, 1459, 1503, 1519, 1591 and 1627.
2-CH3: Yield: 375 mg, 86%. Anal. Calcd. for C32H32N4Cl4Cu2: C,
51.82; H, 4.32; N, 7.56. Found: C, 52.05; H, 4.21; N, 7.43%. Magnetic
moment, μeff = 2.49 μB. IR (cm−1): 502, 559, 645, 763, 807, 847, 867,
1020, 1039, 1062, 1103, 1170, 1210, 1278, 1347, 1380, 1466, 1499
and 1634.
2.1. Materials
Copper(II) chloride dihydrate (CuCl2·2H2O) was procured from
Merck, Mumbai, India. Glyoxal was obtained from SD Fine Chem,
Mumbai, India, and the 4-R-anilines (R = OCH3, CH3, H and Cl) were
procured from Merck, Mumbai, India. The 1,4-diazabutadiene ligands
(L-R) were prepared by condensation of glyoxal with the 4-R-anilines in
hot methanol. Tetrabutylammonium hexafluorophosphate (TBHP)
procured from Aldrich and AR-grade acetonitrile procured from Merck
(India) were used in electrochemical work. All other chemicals and
solvents were reagent grade commercial materials and were used as
received.
2-H: Yield: 322 mg, 80%. Anal. Calcd. for C28H24N4Cl4Cu2: C, 49.05;
H, 3.50; N, 8.18. Found: C, 49.45; H, 3.55; N, 8.17%. Magnetic moment,
μeff = 2.41 μB. IR (cm−1): 521, 585, 604, 623, 696, 759, 836, 852, 881,
916, 1002, 1028, 1071, 1175, 1208, 1300, 1312, 1342, 1447, 1483,
1584 and 1628.
2-Cl: Yield: 426 mg, 88%. Anal. Calcd. for C28H20N4Cl8Cu2: C,
40.83; H, 2.43; N, 6.80. Found: C, 40.64; H, 2.40; N, 6.85%. Magnetic
moment, μeff = 2.47 μB. IR (cm−1): 535, 677, 715, 752, 819, 833, 874,
1014, 1091, 1167, 1208, 1345, 1412, 1456, 1482, 1590 and 1635.
2.2. Synthesis of complexes
2.3. Physical measurements
2.2.1. [{CuI(L-R)Cl}2] complexes
The [{CuI(L-R)Cl}2] (1-R; R = OCH3, CH3, H and Cl) complexes
were prepared by following a general procedure. Specific details are
given below for a particular complex.
Microanalyses (C, H, N) were performed using a Heraeus Carlo Erba
1108 elemental analyzer. IR spectra were obtained on a Perkin Elmer
Spectrum Two IR spectrometer with samples prepared as KBr pellets.
Magnetic susceptibilities were measured using a Sherwood MK-1 bal-
ance. 1H NMR spectra were recorded in CDCl3 solutions on a Bruker
Avance DPX 500 NMR spectrometer using TMS as the internal standard.
Electronic spectra were recorded on a JASCO V-570 spectrophotometer.
Electrochemical measurements were made using a CH Instrument
model 600A electrochemical analyzer. A platinum disc working elec-
trode, a platinum wire auxiliary electrode and an aqueous saturated
calomel reference electrode (SCE) were used in a three electrode con-
figuration. Electrochemical measurements were made under a dini-
trogen atmosphere. All electrochemical data were collected at 298 K
and are uncorrected for junction potentials. GC-MS analyses were per-
formed using a Perkin Elmer CLARUS 680 instrument.
1-OCH3: CuCl2·2H2O (200 mg, 1.17 mmol) was dissolved in me-
thanol (25 ml), and a solution of L-OCH3 (318 mg, 1.18 mmol) in me-
thanol (25 ml) was added to it. The solution was stirred at ambient
temperature for 1 h, whereby a brown solution was obtained. The so-
lution volume was reduced to half via evaporation under reduced
pressure, and the concentrated solution was kept in a refrigerator for
12 h. The 1-OCH3 complex separated as a greenish-brown crystalline
solid, which was collected by filtration, washed with ether, and dried in
air. Yield: 380 mg, 88%. Anal. Calc. for C32H32N4O4Cl2Cu2: C, 52.32; H,
4.36; N, 7.63. Found: C, 52.64; H, 4.34; N, 7.66%. 1H NMR (500 MHz,
CDCl3) [36]: 3.74 (OCH3); 6.87 (d, 2H, J = 8.5); 7.47 (d, 2H, J = 9.0);
8.27 (s, 1H). IR (cm−1): 497, 528, 634, 719, 797, 825, 1024, 1110,
1166, 1252, 1296, 1360, 1458, 1503, 1585 and 1611.
1-CH3: Yield: 335 mg, 85%. Anal. Calc. for Anal. Calcd. for
2.4. X-Ray crystallography
C
32H32N4Cl2Cu2: C, 57.31; H, 4.78; N, 8.36. Found: C, 57.44; H, 4.53;
N, 8.61%. 1H NMR (500 MHz, CDCl3): 2.21 (CH3); 7.04 (d, 2H,
J = 8.0); 7.32 (d, 2H, J = 8.5); 8.22 (s, 1H). IR (cm−1): 499, 530, 670,
759, 797, 844, 988, 1110, 1176, 1252, 1300, 1362, 1458, 1503 and
1611.
Single crystals of [{CuI(L-OCH3)Cl}2], (1-OCH3) and [{CuII(L-
OCH3)Cl2}2], (2-OCH3) were obtained by slow evaporation of solvent
from solutions of the complexes in methanol and acetonitrile respec-
tively. Selected crystal data and data collection parameters are given in
Table 1. Data on both the crystals were collected on a Bruker SMART
CCD diffractometer. X-ray data reduction and, structure solution and
refinement were done using the SHELXS-97 and SHELXL-97 packages
[37]. The structures were solved by the direct methods.
1-H: Yield: 278 mg, 77%. Anal. Calc. for C28H24N4Cl2Cu2: C, 54.72;
H, 3.91; N, 9.12. Found: C, 54.50; H, 4.01; N, 9.11%. 1H NMR
(500 MHz, CDCl3): 7.10 (t, 2H, J = 7.5); 7.28 (t, 1H, J = 7.0); 7.34 (d,
2H, J = 8.0); 8.26 (s, 1H). IR (cm−1): 524, 613, 694, 759, 846, 954,
1024, 1075, 1208, 1275, 1345, 1447, 1485, 1536, 1591 and 1625.
1-Cl: Yield: 362 mg, 82%. Anal. Calc. for C28H20N4Cl6Cu2: C, 44.68;
H, 2.66; N, 7.45. Found: C, 44.64; H, 2.66; N, 7.48%. 1H NMR
(500 MHz, CDCl3): 7.19 (d, 2H, J = 8.5); 7.40 (d, 2H, J = 8.5); 8.29 (s,
1H). IR (cm−1): 540, 597, 683, 715, 750, 823, 1013, 1091, 1170, 1211,
1285, 1345, 1405, 1450, 1480, 1539, 1588 and 1635.
2.5. Application as catalysts
2.5.1. General procedure for C-N coupling reactions
In a typical run, an oven-dried 10 ml round bottom flask was
charged with a known mole percent of catalyst and base, phenylboronic
acid (1.2 mmol) and aniline (1.0 mmol) with ethanol (5 ml). The mix-
ture was stirred at ambient temp (25 °C). After the specified time (5 h)
stirring was stopped, water (20 ml) was added, and extraction with
ether (2 × 10 ml) was done. The combined ether extract was washed
with water (3 × 10 ml), dried over anhydrous Na2SO4, and filtered.
Solvent was removed under vacuum. The residue was dissolved in
hexane and analyzed by GC-MS.
2.2.2. [{CuII(L-R)Cl2}2] complexes
The [{CuII(L-R)Cl2}2] (2-R; R = OCH3, CH3, H and Cl) complexes
were prepared by following a general procedure. Specific details are
given below for a particular complex.
2-OCH3: CuCl2·2H2O (200 mg, 1.17 mmol) was dissolved in acet-
onitrile (25 ml), and a solution of L-OCH3 (318 mg, 1.18 mmol) in
2

A. Mukherjee, et al.
InorganicaChimicaActa500(2020)119228
Table 1
Crystallographic data for 1-OCH3 and 2-OCH3.
Complex
1-OCH3
2-OCH3
Empirical formula
C32H32N4O4Cl2Cu2
734.62
C32H32N4O4Cl4Cu2
805.52
Formula weight
Crystal system
Space group
a (Å)
Monoclinic
P21/c
Triclinic
Pī
8.0130(14)
11.717(2)
34.489(7)
90
8.1280(8)
10.0921(10)
11.5611(11)
81.617(3)
71.377(3)
69.912(3)
843.32(14)
1
b (Å)
c (Å)
α(°)
β(°)
95.018(11)
90
γ(°)
V (Å3)
Z
3225.7(10)
4
D
calcd.(mg m−3
)
1.513
1.586
F(0 0 0)
λ (Å)
1504
410
0.71073
0.16 × 0.18 × 0.25
296
0.71073
0.07 × 0.08 × 0.10
296
Crystal size (mm3)
T (K)
µ (mm−1
)
1.527
1.621
Collected reflections
Rint
38,422
27,872
0.101
0.102
Fig. 1. Crystal structure of the [{CuI(L-OCH3)Cl}2] complex. Hydrogen atoms
Independent reflections
7781
3746
a
R1
0.0633
0.0479
are omitted for clarity.
b
wR2
0.1833
0.1507
GOFc
0.92
0.90
Table 2
a
b
c
Selected bond lengths (Å) and bond angles (°) for [{CuI(L-OCH3)Cl}2] and
R1 = Σ ||Fo|- |Fc ||/Σ|Fo|
wR2 = [Σ[w(Fo – Fc2)2]/Σ [w(Fo2)2]]1/2
2
[{CuII(L-OCH3)Cl2}2].
GOF = [Σ[w(Fo – Fc2)2]/(M-N)]1/2, where M is the number of reflections
2
[{CuI(L-OCH3)Cl}2]
Bond distance (Å)
and N is the number of parameters refined.
Cu1-Cl1
Cu1-Cl2
Cu1-N1
Cu1-N2
C1-N1
2.3421(18)
2.2909(18)
2.072(5)
2.094(4)
1.281(7)
1.436(8)
1.291(7)
1.426(7)
1.415(8)
Cu2-Cl1
Cu2-Cl2
Cu2-N3
Cu2-N4
C17-N3
C17-C18
C18-N4
C19-N3
C26-N4
2.2775(18)
2.3423(16)
2.086(5)
2.063(4)
1.280(7)
1.449(9)
1.290(7)
1.418(8)
1.424(8)
2.5.2. General procedure for nitroaldol reactions
In a typical run, an oven-dried 10 ml round bottom flask was
charged with
a known mole percent of catalyst, arylaldehyde
(1.0 mmol) and nitroalkane (5.0 mmol) with acetonitrile (5 ml). The
flask was placed in a preheated oil bath at required temp. After the
specified time the flask was removed from the oil bath, water (20 ml)
was added, and extraction with ether (4 × 10 ml) was done. The
combined organic layers were washed with water (3 × 10 ml), dried
over anhydrous Na2SO4, and filtered. Solvent was removed under va-
cuum. The residue was dissolved in toluene and analyzed by GC-MS.
C1-C2
C2-N2
C3-N1
C10-N2
Bond angle (°)
N1-Cu1-N2
Cl1-Cu1-Cl2
Cl1-Cu1-N1
Cl1-Cu1-N2
Cl2-Cu1-N1
Cl2-Cu1-N2
80.55(18)
104.07(6)
115.79(14)
112.01(13)
121.23(14)
122.60(14)
N3-Cu2-N4
Cl1-Cu2-Cl2
Cl1-Cu2-N3
Cl1-Cu2-N4
Cl2-Cu2-N3
Cl2-Cu2-N4
80.50(18)
104.49(6)
121.07(14)
117.49(14)
111.61(13)
121.27(13)
3. Results and discussion
[{CuII(L-OCH3)Cl2}2]
Bond distance (Å)
Cu1-Cl1
3.1. Synthesis and characterization
2.2479(14)
2.2929(11)
2.3465(13)
2.004(3)
C1-N1
C1-C2
C2-N2
C3-N1
C10-N2
1.282(5)
1.459(6)
1.276(5)
1.433(5)
1.431(5)
Cu1-Cl2
As delineated in the introduction, the initial goal of the present
study was to synthesize a group of mixed-ligand complexes of copper
having both 1,4-diazabutadiene (L-R) and chloride ion in the co-
ordination sphere. Accordingly reactions of the 1,4-diazabutadiene li-
gands were carried out with CuCl2·2H2O in methanol under stirring
condition at ambient temperature (25 °C), and from each of these re-
actions a greenish-brown complex, depicted in general as 1-R, was
obtained in good yield. Preliminary characterization of these complexes
showed that they are diamagnetic, and their composition agrees well
with the general formula Cu(L-R)Cl. In order to ascertain the compo-
sition of these complexes, as well as coordination mode of the 1,4-
diazabutadiene ligand in them, structure of a selected member of this
family, viz. 1-OCH3, was determined by X-ray crystallography. The
structure is shown in Fig. 1 and some selected bond parameters are
presented in Table 2. The structure reveals that 1-OCH3 is actually a
doubly chloro-bridged dicopper complex, in which copper is in +1
oxidation state. Each copper is surrounded by a 1,4-diazabutadiene li-
gand coordinated in the usual mode (I, M = Cu), and two chloride ions.
Disposition of the N,N-donor ligand and the two chlorides around each
copper center is grossly tetrahedral in nature, as expected in four-co-
ordinated copper(I) complexes, with a dihedral angle between the Cu
Cu1-Cl2a
Cu1-N1
Cu1-N2
2.276(3)
Bond angle (°)
N1-Cu1-N2
Cl1-Cu1-Cl2
Cl1-Cu1-N1
Cl1-Cu1-N2
77.75(12)
93.49(4)
Cl1-Cu1-Cl2a
Cl2-Cu1-Cl2a
Cl2-Cu1-N1
Cl2-Cu1-N2
166.28(5)
85.21(4)
89.15(10)
100.01(9)
169.17(11)
112.04(8)
(N-N) plane and the CuCl2 plane close to 87.5°. The bond distances
within the five membered Cu(N-N) chelate are normal, and are in-
dicative of usual delocalization of the π-cloud over the N-C-C-N frag-
ment. One Cu-Cl distance is found to be about 0.5 Å longer than the
other, a feature quite common in doubly chloro-bridged dicopper
complexes [38–42]. As all the four 1-R complexes were synthesized
similarly and they show similar properties (vide infra), the remaining
three 1-R (R = CH3, H and Cl) complexes are assumed to have similar
structures as 1-OCH3. Molecular formula of the 1-R complexes is hen-
ceforth represented in general as [{CuI(L-R)Cl}2]. Out of these four 1-R
complexes, synthesis and crystal structure of 1-OCH3 was reported
3

A. Mukherjee, et al.
InorganicaChimicaActa500(2020)119228
earlier by Diez-Gonzalez and co-workers [30]. They used cuprous
chloride as the source of copper(I) and carried out the synthesis in di-
chloromethane under stirring condition at room temperature for over-
night. While we started the synthesis taking L-OCH3, cupric chloride as
the copper source and methanol as the solvent, which provided the
copper(I) complex 1-OCH3 after 1 h of stirring at ambient temperature
via in situ reduction of the metal center (vide infra).
It is interesting to note here that though a copper(II) source, viz.
CuCl2·2H2O, was used in the synthesis, copper(I) complexes of type
[{CuI(L-R)Cl}2] were obtained as the end product, which indicates that
the metal center has undergone one-electron reduction during the
course of the synthetic reaction. In the absence of any recognized re-
ducing agent during the synthesis, methanol appears to have served as
the reductant. In order to verify it further, similar reactions between the
same two reagents, viz. CuCl2·2H2O and the 1,4-diazabutadiene ligands,
were carried out in acetonitrile under stirring condition at ambient
temperature (25 °C), which afforded a group of reddish-brown com-
plexes, represented as 2-R, in decent yields. The 2-R complexes are
paramagnetic, and their microanalytical data found to be consistent
with Cu(L-R)Cl2 formulation. For an unambiguous characterization of
these complexes, the crystal structure of 2-OCH3 was determined, and
the structure (Fig. 2) shows that 2-OCH3 is also a doubly chloro-bridged
dicopper complex. However, unlike in 1-OCH3, here each copper is
linked to three chloride ions, two of which are bridging and one is
terminal. A 1,4-diazabutadiene ligand is coordinated in chelating mode
to both the metal centers. Thus each copper is existing in +2 oxidation
state in this complex. The N2Cl3 coordination sphere around each
copper is distorted square pyramidal in nature. Besides the axial Cu-Cl
length being slightly shorter, the observed bond parameters (Table 2)
are found to compare well with those in 1-OCH3. In view of the simi-
larity in synthetic procedure and properties (vide infra), the other three
2-R (R = CH3, H and Cl) complexes are believed to have similar
structures as 2-OCH3. Molecular formula of these 2-R complexes is
represented in general as [{CuII(L-R)Cl2}2]. It is interesting to note that
by simple variation of the solvent in synthesis, copper(I) or copper(II)
complexes can be prepared predictably. It is also worth mentioning
here that the isolated [{CuII(L-R)Cl2}2] (2-R) complexes, on dissolution
in methanol, are readily and quantitatively converted to the corre-
sponding [{CuI(L-R)Cl}2] (1-R) complexes [43]. As methanol is unable
to reduce copper(II) directly in common
Fig. 2. Crystal structure of the [{CuII(L-OCH3)Cl2}2] complex. Hydrogen atoms
are omitted for clarity.
paramagnetic, and the observed magnetic moment (μeff = 2.41 – 2.49
μB) is slightly lower than expected the spin-only value for two isolated
copper(II) centers. This lowering of magnetic moment is attributable to
antiferromagnetic interaction between the two copper(II) centers,
which is well known in similar doubly chloro-bridged dicopper(II)
systems [44].
Infrared spectra of the 1-R and 2-R complexes show many bands of
varying intensities within 4000–450 cm−1. Spectral data are presented
in the experimental section. Besides small shifts in band positions,
spectrum of each 1-R complex is similar to that of the uncoordinated L-
R ligand. Infrared spectra of the 2-R complexes are also found to be
grossly similar to those of the corresponding 1-R analogue.
The 1-R complexes are soluble in common organic solvents like
methanol, ethanol, acetonitrile and dichloromethane, producing in-
tense greenish-brown solutions. Electronic spectra of the 1-R complexes
were recorded in dichloromethane solutions. Spectral data are pre-
sented in Table 3. Each complex shows several intense absorptions
spanning the visible and ultraviolet regions. The absorptions in the
CuCl2
CH3OH
[{CuII(L-R)Cl2}2]
[{CuI(L-R)Cl}2]
+ HCHO + 2HCl
L-R
(1)
cupric salts [44], we believe that [{CuII(L-R)Cl2}2] is generated first,
even when methanol is used as the solvent for synthesis, which then
undergoes one-electron reduction at each copper(II) center to afford the
corresponding [{CuI(L-R)Cl}2] complexes (eq. 1). This hypothesis has
been testified by identification of formaldehyde in the solution obtained
from the interaction of [{CuII(L-R)Cl2}2] with methanol [45]. Similar
reduction of copper(II) species to corresponding copper(I) complex is
known in the literature [32].
visible region are believed to be due to metal-to-ligand charge-transfer
transitions, while those in the ultraviolet region are attributable to
transitions within the diimine-ligand orbitals. The 2-R complexes are
found to be insoluble in dichloromethane, but soluble in methanol,
ethanol and acetonitrile, producing yellowish-brown solutions. In me-
thanol or ethanol solution, reduction of copper(II) to copper(I) was
observed, associated with rapid change in solution color. Hence elec-
tronic spectra of the 2-R complexes were recorded in acetonitrile so-
lution. Each complex shows three intense absorptions (Table 3); one
near 460 nm assignable to ligand-to-metal charge-transfer transition,
and two in the ultraviolet region within 260 – 340 nm due to transitions
within the diimine-ligand orbitals.
3.2. Spectral properties
Magnetic susceptibility measurements show that the [{CuI(L-
R)Cl}2] (1-R) complexes are diamagnetic, as expected. 1H NMR spectra
of the 1-R complexes, recorded in CDCl3 solutions, show all the ex-
pected signals arising from the coordinated L-R ligands. 1H NMR data
are given in the experimental section. For 1-OCH3, 1-CH3 and 1-H four
signals are observed while for 1-Cl three signals are found, which is
consistent with the composition and stereochemistry of these com-
plexes. The [{CuII(L-R)Cl2}2] (2-R) complexes are found to be
3.3. Electrochemical properties
Electrochemical properties of the dicopper(I) complexes (1-R) were
studied by cyclic voltammetry in dichloromethane solution (0.1 M
TBHP). Each complex showed three oxidative responses on the positive
side of SCE, and two reductive responses on the negative side. The
4

A. Mukherjee, et al.
InorganicaChimicaActa500(2020)119228
Table 3
Electronic spectral and cyclic voltammetric data of the complexes.
Complex
Solvent
Electronic spectral data λmax (nm) (ε (M−1cm−1))
Cyclic voltammetric dataa E, V vs SCE
1-OCH3
1-CH3
1-H
dichloromethane
dichloromethane
dichloromethane
dichloromethane
acetonitrile
700b (1400), 595 (4400), 559b (3600), 506 (6100), 440b (40900), 419 (42700), 248 (66700)
699b (1200), 552 (3100), 525 (2600), 471 (4700), 410b (40300), 400 (42000), 284 (54300)
698b (1300), 530 (2200), 507 (1900), 462 (4100), 401b (40900), 395 (42500), 288 (52800)
709b (1100), 570 (1900), 545b (1500) 486 (3600), 412b (38800), 400 (40700), 301 (50400)
468 (3900), 317b (16200), 272 (25000)
1.87c, 0.90 (2 6 6), 0.69 (2 8 1), −0.86d, −1.36d
1.90c, 0.89 (2 6 1), 0.72 (2 7 4), −0.97d, −1.45d
1.88c, 0.92 (2 6 8), 0.70 (2 8 4), −0.84d, −1.40d
1.91c, 0.98 (2 6 6), 0.76 (2 8 1), −0.86d, −1.37d
1.50c, 0.55 (1 1 2), −0.98d,
−1.57d
1-Cl
2-OCH3
2-CH3
2-H
acetonitrile
acetonitrile
acetonitrile
458 (4200), 339b (15700), 261 (22000)
452 (4000), 321b (15100), 260 (19300)
473 (4500), 340b (16900), 281 (23100)
1.54c, 0.61 (1 1 4), −0.94d,
−1.46d
1.49c, 0.70 (1 1 6), −0.80d,
−0.97d, −1.40d
2-Cl
1.55c, 0.79 (1 1 7), −0.71d,
−0.96d, −1.40d
a
b
c
Supporting electrolyte, TBHP; scan rate 50 mV s−1
Shoulder.
.
Epa (anodic peak potential) value.
Epc (cathodic peak potential) value.
d
Table 4
C-N cross-coupling reaction of arylboronic acids with aryl amines.a
H
N
1-OCH3
NEt3
B(OH)2
H2N
+
Ethanol
Stir, 5 h
X
Y
X
Y
Entry
X
Y
Yeildb, %
1
H
H
H
H
H
OCH3
CH3
H
99
98
98
96
95
98
94
99
92
91
97
94
92
93
90
98
92
12c
2
3
4
Cl
5
NO2
OCH3
CH3
H
6
CH3
7
CH3
CH3
CH3
CH3
Cl
8
9
Cl
10
11
12
13
14
15
16
17
18
NO2
OCH3
CH3
H
Cl
Cl
Cl
Cl
Cl
NO2
*
H
H
**
H
***
a
Reaction conditions: catalyst (1.0 mol%), arylamine (1.0 mmol), arylboronic acid (1.2 mmol), solvent
(5 ml).
b
Determined by GCMS.
c
The O-C coupled product was obtained in 73% yield.
NH2
NH2
NH2
*
**
***
OH
N
cyclic voltammetric data are presented in Table 3, and representative
voltammograms are shown in Fig. S1 (Supplementary material). The
two successive oxidations observed near 0.70 V and 0.90 V are assign-
able respectively to Cu(I)-Cu(II) and Cu(II)-Cu(III) oxidations. The third
irreversible oxidation near 1.90 V may be attributed to oxidation of the
coordinated diimine ligand. The two reductions are assignable to suc-
cessive one-electron reductions of the diimine fragment of the N,N-
donor ligand. The diimine moiety is capable of undergoing two one-
electron reductions producing a radical anion and a dianion respec-
tively (eq. 2). These two reductions are observed in each complex as
irreversible responses near −0.90 V and −1.40 V.
Ar
N
Ar
N
Ar
N
e-
e-
.
N
N
N
(2)
Ar
Ar
Ar
Cyclic voltammetry on the dicopper(II) complexes (2-R) were car-
ried out in acetonitrile solution (0.1 M TBHP), which showed two oxi-
dative responses on the positive side of SCE, and two-to-three reductive
5

A. Mukherjee, et al.
InorganicaChimicaActa500(2020)119228
N
N
Cl
Cl
N
N
CuI
CuI
H
N
N
N
X
Y
CuI
Cl
Y
(A)
¼ O2
½ H2O
X
NH
N
N
N
N
OH
Cl
CuIII
CuII
(E)
(B)
Cl
½ H2O
(HO)2B
X
¼ O2
Y
B(OH)3
X
X
NH2
N
N
N
N
CuII
CuII
Cl
Cl
(D)
(C)
H2N
Y
Scheme 1. Probable mechanism for the observed C-N cross-coupling reactions. N-N represents the coordinated 1,4-diazabutadiene ligand.
responses on the negative side (Table 3, Fig. S2 (Supplementary ma-
terial)). The first oxidative response, observed within 0.55 – 0.79 V, is
assigned to Cu(II)-Cu(III) oxidation. The second irreversible oxidation
near 1.50 V is tentatively assigned to oxidation of the diimine ligand.
The first reduction in the 2-OCH3 and 2-CH3 complexes, observed as an
irreversible response near −0.96 V, has the profile indicating two
overlapping reductions. This has become very apparent in the 2-H and
2-Cl complexes, where these two reductions are observed as two close
but separate reductive responses within −0.71 to −0.97 V. These two
reductions are attributed to Cu(II)-Cu(I) reduction and first reduction of
the diimine ligand respectively. The second reduction of the diimine
ligand is observed as an irreversible response within −1.40 to −1.57 V.
optimized condition the other three dicopper(I) complexes, viz. 1-CH3,
1-H and 1-Cl, were found to exhibit similar catalytic efficiency (entries
14–16, Table S1).
The scope of the reaction is shown in Table 4. As all the 1-R com-
plexes have shown comparable catalytic efficiency, the results obtained
with 1-OCH3 are only highlighted here. Using the optimized reaction
conditions, C-N coupling between three different arylboronic acids (p-
X-C6H4-B(OH)2; X = H, CH3 and Cl) and five para-substituted anilines
(p-Y-C6H4-NH2; Y = OCH3, CH3, H, Cl and NO2) were attempted, and
the corresponding C-N coupled products (p-X-C6H4-N(H)-C6H4-Y-p)
were obtained in excellent (≥90%) yield (entries 1–15). When a
bulkier arylamine, viz. 1-naphthylamine, was used, the yield remained
comparable (entry 16). When 2-aminopyridine, having recognized
pyridine-nitrogen donor site was used as substrate, the C-N coupled
product was obtained in 92% yield (entry 17), indicating no visible
influence of presence of donor site in the substrate. When 2-amino-
phenol was used, the expected C-N coupled product was obtained in
poor (12%) yield, however a C-O coupled product was obtained in good
(73%) yield (entry 18). In view of the relatively milder reaction con-
ditions and lower catalyst loading, the observed catalytic efficiency of
the dicopper(I) complexes is superior to majority of the copper catalysts
[54–64]. For example, similar coupling using molecular copper(II)
complexes are known to require higher catalyst loading and longer
reaction time [55,60–63], and in few cases higher temperature [62,63],
to afford the C-N coupled products in comparable yields.
3.4. Catalytic activity
Copper complexes are well known to catalyze a wide variety of
reactions [46–53]. Herein we have a family of dicopper(I) and a similar
family of dicopper(II) complexes, and we have explored catalytic po-
tential of these two group of complexes towards two different reactions.
Catalytic activity of the dicopper(I) complexes (1-R) was examined
towards Chan-Lam type C-N cross-coupling reactions between ar-
ylboronic acids and arylamines. We first examined C-N coupling be-
tween phenylboronic acid and p-toluidine using 1-OCH3 as the catalyst.
Table S1 (Supplementary material) provides information on the impact
of various reaction parameters on the efficiency of this process. After
several trials for optimization, it was observed that the best result is
obtained with 1.0 mol% of catalyst, NEt3 as base, ethanol as solvent,
and stirring at 25 °C temperature for 5 h (entry 11, Table S1). Under the
In copper-catalyzed Chan-Lam type C-N cross-coupling reaction, the
catalytic cycle is known to involve three different oxidation states of
copper, viz. +1, +2 and +3. However, in majority of the reported
6

A. Mukherjee, et al.
InorganicaChimicaActa500(2020)119228
Table 5
three dicopper(II) complexes, viz. 2-CH3, 2-H and 2-Cl, was found to be
Nitroaldol reaction between arylaldehydes and nitroalkanes.a
comparable with that of 2-OCH3 (entries 10–12, Table S2).
The scope of the nitroaldol reaction, with 2-OCH3 as the catalyst, is
presented in Table 5. Using the optimized reaction conditions, C-C
coupling between seven different arylaldehydes and three nitroalkanes
were attempted, and in each case the corresponding C-C coupled pro-
duct was obtained in good yield (entries 1–21). When para-substituted
benzaldehydes or 1-naphthaldehyde were used as substrate, the ex-
pected nitroaldols were obtained in very good (74–93%) yield (entries
1–9 and 13–15). However, when arylaldehydes with recognized donor
sites were chosen as substrate, the corresponding nitroaldols were ob-
tained in relatively lower (50–68%) yield (entries 10–12 and 16–21),
probably due to catalyst inhibition through coordination. The observed
copper(II) catalyzed nitroaldol reactions are believed to proceed
through sequences that are reported in the literature [67–75]. The
catalytic activity of the dicopper(II) complexes is found to be compar-
able to that of the many other copper(II) species utilized for similar
nitroaldol reactions [67,68,70–74]. For example, catalyst loading for
nitroaldol reactions using molecular copper(II) complexes is found to be
either similar to [70,74], or slightly higher [71–73] than, that of our
dicopper(II) complexes for furnishing the coupled products in com-
parable yields. Significantly higher catalyst loading is also required in
case of few other molecular copper(II) complexes [69,75]. However, in
view of reaction time, our dicopper(II) complexes appear to be more
efficient than others. As nitroaldol reaction is also known to be cata-
lyzed by copper(I) species [78–81], we tried our dicopper(I) complex 1-
OCH3 as catalyst which afforded the expected product, but in very poor
yield (entry 13, Table S2).
Entry
X
Y
R
R'
Yeildb, %
1
H
H
H
H
79
85
81
74
79
76
88
93
87
61
56
67
83
76
87
50
54
55
68
59
63
2
H
CH3
Cl
H
H
3
H
H
H
4
H
H
CH3
CH3
CH3
CH3
CH3
CH3
H
H
5
H
CH3
Cl
H
6
H
H
7
H
H
CH3
CH3
CH3
H
8
H
CH3
Cl
9
H
10
11
12
13
14
15
16
17
18
19
20
21
OCH3
H
OCH3
H
CH3
CH3
H
H
OCH3
H
CH3
H
1-naphthaldehyde
1-naphthaldehyde
1-naphthaldehyde
CH3
CH3
H
H
CH3
H
pyridine-2-aldehyde
pyridine-2-aldehyde
pyridine-2-aldehyde
salicylaldehyde
CH3
CH3
H
H
CH3
H
salicylaldehyde
CH3
CH3
H
salicylaldehyde
CH3
a
Reaction conditions: catalyst (4.0 mol%), arylaldehyde (1.0 mmol), ni-
troalkane (5.0 mmol), solvent (5 ml).
b
4. Conclusions
Determined by GCMS.
The present study shows that the 1,4-diazabutadiene ligands (L-R)
undergo facile reaction with CuCl2·2H2O, and afford a group of doubly
chloro-bridged dicopper complexes where oxidation state of copper is
found to depend on the nature of solvent chosen for the synthesis. When
methanol is used as solvent copper(I) complexes of type [{CuI(L-R)Cl}2]
(1-R) complexes are obtained; and methanol is found to serve as re-
ducing agent for the metal center. When a solvent like acetonitrile is
used that does not have reducing capacity like methanol, the +2 oxi-
dation state of copper is retained in the resulting [{CuII(L-R)Cl2}2] (2-
R) complexes. Quantitative conversion of any [{CuII(L-R)Cl2}2] to the
corresponding [{CuI(L-R)Cl}2] can be achieved by simple dissolution of
the former in methanol. This study also demonstrates that the dicopper
(I) complexes (1-R) can efficiently catalyze Chan-Lam type C-N cross-
coupling reactions between arylboronic acids and arylamines. The di-
copper(II) complexes (2-R) display notable catalytic efficiency for ni-
troaldol type C-C bond formation reaction between arylaldehydes and
nitroalkanes.
studies, copper(II) species were taken as the catalyst-precursor [54–64].
Our present work is, to our knowledge, the first instance where a mo-
lecular complex of copper(I) has been utilized as the pre-catalyst.
Moreover, our dicopper(II) complexes are readily converted to the
corresponding dicopper(I) complexes in ethanol solution, as in me-
thanol [43], which vitiates their use as catalyst in the present reaction.
A plausible mechanism for the observed C-N coupling reaction, that is
essentially drawn from the one reported by Rao and Wu [54], is illu-
strated in Scheme 1. The dimeric copper(I) complex (1-R) is believed to
provide the tri-coordinated
copper(I) species (A) [65], which marks the entry into the catalytic
cycle. A undergoes aerial oxidation in moist solvent to generate the
four-coordinated copper(II) species B. Transmetallation takes place
upon interaction with arylboronic acis in the next step producing spe-
cies C. Coordination of aniline generates five-coordinated copper(II)
species D, which undergoes aerial oxidation to generate the copper(III)
species E. Reductive elimination of the C-N coupled product from E
regenerates the initial copper(I) species A, and thus the catalytic cycle
continues. It may be noted here that in strictly unaerobic condition the
coupling does not happen, and in the absence of base (NEt3) the C-N
coupling occurs but yield of the product decreases significantly, pre-
sumably due to consumption of aniline as base by boric acid generated
along with conversion of B to C.
Acknowledgements
We thank the reviewers for their constructive comments, which
have been very helpful in preparing the revised manuscript. Financial
assistance received from the UGC-CAS program and the DST-PURSE
program of the Department of Chemistry, Jadavpur University, Kolkata
700032, is gratefully acknowledged. A.M. thanks the Council of
Scientific and Industrial Research (CSIR), New Delhi, for her fellowship
[grant number 09/096(0962)/2019-EMR-I]. The authors thank Prof.
Saurabh Das (Department of Chemistry, Jadavpur University) for his
help in recording the electronic spectra.
Catalytic activity of the dicopper(II) complexes (2-R) was examined
towards nitroaldol reaction, which is a C-C bond formation reaction
between arylaldehydes and nitroalkanes, and is also known as Henry
reaction [66]. The nitroaldol reaction is a well respected C-C bond
formation reaction that is usually base-catalyzed, but copper-catalyzed
nitroaldol reactions are also gaining importance [67–76]. We began this
type of C-C coupling reaction with benzaldehyde and nitromethane as
substrates, and 2-OCH3 as the catalyst. The experimental method was
optimized (Table S2; Supplementary material), and the best result was
obtained with 4.0 mol% of catalyst, acetonitrile as solvent, and heating
at 50 °C for 1 h (entry 5, Table S2) [77]. Catalytic efficiency of the other
Appendix A. Supplementary material
CCDC 1941330 (for 1-OCH3) and 1941331 (for 2-OCH3) contain
the supplementary crystallographic data for this paper. Screening of
experimental conditions for C-N coupling reactions (Table S1),
7

A. Mukherjee, et al.
InorganicaChimicaActa500(2020)119228
screening of experimental conditions for nitroaldol reactions (Table
[37] G.M. Sheldrick, SHELXS-97 and SHELXL-97, Fortran programs for crystal structure
solution and refinement, University of Gottingen, Gottingen, Germany, 1997.
[38] M. Al-Shakban, P.D. Matthews, G. Deogratias, P.D. McNaughter, J. Raftery,
I. Vitorica-Yrezabal, E.B. Mubofu, P. O’Brien, Inorg. Chem. 56 (2017) 9247.
[39] A.M. Willcocks, T. Pugh, S.D. Cosham, J. Hamilton, S.L. Sung, T. Heil, P.R. Chalker,
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S2), and cyclic voltammograms of 1-OCH3 and 2-OCH3 (Figs. S1 and
S2).
Appendix B. Supplementary data
Supplementary data to this article can be found online at https://
doi.org/10.1016/j.ica.2019.119228.
[42] G. Groger, U. Behrens, F. Olbrich, Organometallics 19 (2000) 3354.
[43] Facile conversion of the isolated [{CuII(L-R)Cl2}2] (2-R) complexes to the corre-
sponding [{CuI(L-R)Cl}2] (1-R) complexes in methanol is recognizable visually by
color change from yellowish-brown to greenish-brown, and has been verified
spectrophotometrically.
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8