Synthesis, characterization, electrochemical properties and conversions of carbon dioxide to cyclic carbonates mononuclear and multinuclear oxime complexes using as catalyst

Article abstract of DOI:10.1016/j.ica.2012.09.020

Unsymmetrical dioxime ligand and its mono- and multinuclear metal complexes were synthesized and characterized by ¹H and ¹³C NMR spectra, FT-IR, UV-Vis, elemental analysis, melting point measurements, LC-MS spectroscopy, molar conductivity measurements and magnetic susceptibility and cyclic voltammetry techniques. We firstly prepared new unsymmetrical dioxime ligand (1) and its mononuclear [Ni(dioxime)₂] complex (2). Then, the intramolecular O-H?O bridges are replaced with Cu(II)(N-N) complex to synthesize multinuclear [Ni(dioxime)₂Cu₂(N-N) ₂](ClO₄)₂ oxime complexes (3-7) where N-N = 2,2′-bipyridine (bpy), 1,10-phenanthroline (phen), 3,3′-dicarboxy-2, 2′-bipyridine (dcbpy), 4,5-diazafluoren-9-one (dafo) and 1,10-phenanthroline-5,6-dione (dione). The mononuclear complex [Ni(dioxime) ₂] (2) was used as a precursor for building multinuclear oxime complexes (3-7). In this paper, our goal is to study spectroscopic, electrochemical and catalytic properties of this new ligand and its metal complexes.

Full text of DOI:10.1016/j.ica.2012.09.020

Inorganica Chimica Acta 394 (2013) 635–644
Contents lists available at SciVerse ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Synthesis, characterization, electrochemical properties and conversions
of carbon dioxide to cyclic carbonates mononuclear and multinuclear oxime
complexes using as catalyst
a
a
b
a
Ahmet Kilic a, , Ahmet Arif Palali , Mustafa Durgun , Zeynep Tasci , Mahmut Ulusoy ,
⇑
Metin Dagdevren c, Ismail Yilmaz c
a Harran University, Department of Chemistry, 63190 Osmanbey Campus, Sanliurfa, Turkey
b Mug˘la University, Department of Chemistry, 48000 Kötekli, Mug˘la, Turkey
c Istanbul Technical University, Department of Chemistry, 34469 Istanbul, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history:
Unsymmetrical dioxime ligand and its mono- and multinuclear metal complexes were synthesized and
characterized by 1H and 13C NMR spectra, FT-IR, UV–Vis, elemental analysis, melting point measure-
ments, LC–MS spectroscopy, molar conductivity measurements and magnetic susceptibility and cyclic
voltammetry techniques. We firstly prepared new unsymmetrical dioxime ligand (1) and its mononu-
clear [Ni(dioxime)2] complex (2). Then, the intramolecular OꢀHꢁ ꢁ ꢁO bridges are replaced with
Cu(II)(N–N) complex to synthesize multinuclear [Ni(dioxime)2Cu2(N–N)2](ClO4)2 oxime complexes (3–
7) where N–N = 2,20-bipyridine (bpy), 1,10-phenanthroline (phen), 3,30-dicarboxy-2,20-bipyridine
(dcbpy), 4,5-diazafluoren-9-one (dafo) and 1,10-phenanthroline-5,6-dione (dione). The mononuclear
complex [Ni(dioxime)2] (2) was used as a precursor for building multinuclear oxime complexes (3–7).
In this paper, our goal is to study spectroscopic, electrochemical and catalytic properties of this new
ligand and its metal complexes.
Received 28 March 2012
Received in revised form 14 September
2012
Accepted 20 September 2012
Available online 5 October 2012
Keywords:
Oxime complexes
Mass spectra
Electrochemistry
Carbon dioxide
Cyclic carbonate
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
dioxime ligands and their mono and multinuclear complexes have
been studied for a long time as medicine [19–21], catalysis
The coupling reaction of carbon dioxide (CO2) with epoxides
using different metal complexes as catalyst has intensively been
studied in recent decades as one of the most promising processes
for fixation of CO2 [1–6]. The carbon dioxide fixation has received
much attention in last decades, since carbon dioxide is the most
inexpensive and infinite carbon resource [7,8]. A majority of these
studies has involved the reaction of CO2 with epoxides to generate
polycarbonates and/or cyclic carbonates. Cyclic carbonates are
being used industrially as polar aprotic solvent, substrate for small
molecule synthesis, additive, anti-foam agent for antifreeze, and
plasticizer [9,10]. For metal complex catalysts, salen-type metal
complexes have been of significant interest in conjunction with Le-
wis base or organic salts as co-catalysts [11–13]. The most promi-
nent advantages of these metal complexes are easy synthesis and
excellent stability against moisture and air [14–18]. To the best
of our knowledge, so far there is no literature report that uses
mono- and multinuclear oxime metal complexes as a catalyst for
coupling reaction of carbon dioxide (CO2) with epoxides. The
[22–24], electro optical sensors [25], liquid crystals [26], trace me-
tal analysis [27] and hydrogen production from water as catalyst
[28–30].
In the structure of multinuclear metal complexes, the Ni(II) ion
is centered into the main oxime core by the coordination of the
imino groups while the two Cu(II) ions are coordinated as dianionic
oxygen donors of the oxime groups, and linked to the ligands such
as 2,20-bipyridine, 1,10-phenanthroline, 3,30-dicarboxy-2,20-bipyri-
dine, 4,5-diazafluoren-9-one and 1,10-phenanthroline-5,6-dione.
The development of new efficient catalysts that can be easily pre-
pared, recovered and reused without losing their activities has re-
ceived much attention from a practical and environmental point of
view [31]. In this study, Unsymmetrical dioxime ligand and its
mono- and multinuclear metal complexes were synthesized and
characterized by 1H and 13C NMR spectra, FT-IR, UV–Vis, elemental
analysis, melting point measurements, LC–MS spectroscopy, molar
conductivity measurements, magnetic susceptibility techniques
and cyclic voltammetry techniques. The aim of this study is to
investigate spectroscopic, electrochemical and catalytic properties
of the new ligand and its metal complexes. The best of our knowl-
edge, we have herein for the first time reported novel catalysts of
mono- and multinuclear oxime metal complexes in conjunction
⇑
Corresponding author. Tel.: +90 4143183587; fax: +90 4143183541.
E-mail address: kilica63@harran.edu.tr (A. Kilic).
0020-1693/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.ica.2012.09.020

636
A. Kilic et al. / Inorganica Chimica Acta 394 (2013) 635–644
with [bmim]PF6 or other Lewis bases for the conversion of CO2 to
cyclic carbonates.
under Ar atmosphere. Followed by the addition of the yellow
product (2.7 g, 15 mmol) synthesized in the first step to the
solution. The stirred mixture was then heated to the reflux tem-
perature for 4 h. The mixture was stirred for 1 h at room tem-
perature and the precipitation formed after 30 min. The solvent
was slowly evaporated at room temperature, and pale yellow
crystals were obtained from EtOH/H2O (1:2). After filtration,
the crystals were washed with EtOH and diethylether. Then,
the crystals were dried under vacuum. Color: pale yellow, yield
(%): 77, mp: 112 °C, Anal. Calc. for [C10H12N2O2] (F.W: 192 g/
mol): C, 62.50; H, 6.25; N, 14.58. Found: C, 62.48; H, 6.28; N,
2. Experimental
2.1. Materials and measurements
All reagents and solvents were of reagent-grade quality and ob-
tained from commercial suppliers. Elemental analysis was carried
out on a LECO CHNS model 932 elemental analyzer. 1H NMR spec-
tra were recorded on a Varian AS-400 MHz instrument at room
temperature for catalytic measurements. 1H and 13C NMR spectra
were recorded on a Bruker Avance 300 MHz NMR spectrometer
for spectroscopic characterization. FT-IR spectra were recorded
on a Perkin Elmer Spectrum RXI FT-IR Spectrometer by KBr pellets
in the wavenumber range of 4000–400 cmꢀ1. Magnetic Suscepti-
bilities were determined on a Sherwood Scientific Magnetic Sus-
ceptibility Balance (Model MK1) at room temperature (20 °C)
using Hg[Co(SCN)4] as a calibrant; diamagnetic corrections were
calculated from Pascal’s constants [32,33]. Electronic spectral stud-
ies were conducted on a Perkin-Elmer model Lambda 25 UV–Vis
spectrophotometer in the wavelength range from 200 to
1100 nm. Melting points were measured in open capillary tubes
with an Electrothermal 9100 melting point apparatus and were
uncorrected. Molar conductivities (KM) were recorded on an Inolab
Terminal 740 WTW Series. Mass Spectra results were recorded on
an Agilent LC/MSD LC–MS/MS spectrometer. Cyclic voltammetric
(CV) measurements were carried out with an instrument (Prince-
ton Applied Research Model 2263 potentiostat controlled by an
external PC, using the computer program, Power CV) utilizing a
three electrode configuration at 25 °C. A platinum wire served as
the counter electrode. Ag/AgCl electrode was employed as the ref-
erence electrode. The working electrode was a platinum plate with
an area of 0.2 cm2. The working electrode was polished with Al2O3
prior to each experiment. Throughout the experiment, oxygen free
nitrogen was bubbled through the solution for 10 min. Electro-
14.52%. FT-IR (KBr pellets,
m m(O–H),
max/cmꢀ1): 3505–3102
3025
1405
m
m
(Ar–CH), 2966–2871
m(Aliph–CH), 1609 m(C@N), 1461–
(C@C), and 1285
m
(N–O). 1H NMR (DMSO-d6, TMS,
300 MHz,
d
ppm): 11.86 (d, 2H, OH, J = 8.7 Hz), 11.69 and
11.47 (s, 2H, OH) for cis/trans-isomer, 8.48 and 7.88 (s, 1H,
HC@N) for cis/trans-isomer, 7.49 (d, 2H, J = 8.1 Hz, Ar-CH), 7.23
(d, 2H, J = 7.4 Hz, Ar–CH), 2.70–2.58 (q, 2H, CH3–CH2), 1.19 (t,
3H, J = 15 Hz; CH3–CH2), 13C NMR (DMSO-d6, TMS, 75 MHz, d
ppm): C1(151.26 and 145.02), C2(153.48 and 148.46),
C3(131.95), C4(129.48 and 127.74), C5(128.96 and 127.38),
C6(141.28), C7(28.53 and 28.43), and C8(16.06 and 16.00). UV–
Vis (kmax, nm, ⁄ = shoulder peak): 233 and (in CH3OH); 270 (in
DMSO). LC–MS (Scan ES+): m/z (%) 192.2 (100) [M]+, 148.3 (45)
[MꢀCH@NꢀOH]+, and 122.1 (5) [MꢀC8H9]+.
2.3. Synthesis of mononuclear [Ni(dioxime)2] (2) complex
A solution of ligand (LH2) (3.0 g, 15.6 mmol) in absolute ethanol
(50 mL) dissolved at room temperature and then NiCl2ꢁ6H2O (1.9 g,
7.8 mmol) dissolved in absolute ethanol (30 mL) was added by syr-
inge and the solution turned red. Nitrogen was bubbled through
the resulting red solution for 30 min. Then, a decrease in pH of
the solution was observed. The pH of the solution was ca. 1.5–3.0
and was adjusted to 4.5–5.5 by the addition of 1% NaOH solution
in EtOH. After refluxing the mixture for 5 h in a water bath, the pre-
cipitate was filtered off; washed with H2O and diethyl ether several
times, and then dried in vacuo at 35 °C. Color: red, yield: (67%),
mp = 286 °C. Anal. Calc. for C20H22N4O4Ni (MW: 441 g/mol): C,
54.46; H, 5.03; N, 12.70. Found: C, 54.52; H, 5.05; N, 12.68%.
chemical
grade
tetrabutylammoniumperchlorate
(TBAP)
(0.1 mol dmꢀ3) was employed as the supporting electrode. Origin
7.5 graphing program was used to evaluate Power CV data, to draw
voltammograms and analyze them. Catalytic tests were performed
in a PARR 4843 50 mL stainless pressure reactor. 4,5-Diazafluoren-
9-one (dafo) [34], 3,30-dicarboxy-2,20-bipyridine (dcbpy) [35] and
1,10-phenanthroline-5,6-dione [36] were prepared according to
the literature procedures.
KM = 13
3558–3297
1606 (C@N), 1501–1417
X
ꢀ1 cm2 molꢀ1
,
l
eff = Dia, IR (KBr pellets,
m
max/cmꢀ1):
(Aliph–H),
(N–O), and 514 (Ni–N).
m(O–Hꢁ ꢁ ꢁO) 3066
m(Ar–H), 2967–2865 m
m
m
(C@C), 1275
m
m
1H NMR (DMSO-d6, TMS, 300 MHz, d ppm): 18.12 (s, 2H, O–
Hꢁ ꢁ ꢁO), 18.29 and 18.01 (s, 2H, O–Hꢁ ꢁ ꢁO) for cis/trans-isomer, 8.20
and 8.14 (s, 2H, HC=N) for cis/trans-isomer, 8.03 and 8.01 (d, 4H,
J = 3.3 Hz, Ar–CH), 7.35 and 7.32 (d, 4H, J = 3.0 Hz, Ar–CH) for cis/
trans-isomer, 2.69–2.61 (q, 4H, CH3–CH2), 1.20 (t, 3H, J = 15 Hz,
CH3–CH2). 13C NMR (DMSO-d6, TMS, 75 MHz, d ppm) C1(147.30),
C2(147.16 and 146.24), C3(129.31 and 129.16), C4(128.47),
C5(126.24), C6(142.47 and 141.88), C7(28.62), and C8(15.71). UV–
Vis (kmax, nm, ⁄ = shoulder peak): 242⁄, 278, 382, and 402 (in CH3-
OH); 297, 351, 409 and 522⁄ (in DMSO). LC–MS (Scan ES+): m/z (%)
441.3 (15) [M]+, 413.3 (100) [MꢀCH2CH3ꢀH]+, 409.2 (55) [MꢀCH2-
CH3ꢀ3H]+ and 261.2 (20) [MꢀC16H21O3]+.
2.2. Synthesis of the ligand (LH2) (1)
The unsymmetrical ligand (LH2) was synthesized following
the procedure with some modifications [37,38]. Sodium metal
(0.8 g, 35 mmol) was dissolved in absolute ethanol (150 mL) at
ꢀ5 °C temperature. Followed by addition n-butyl nitrite (3.2 g,
31 mmol), the mixture was cooled to room temperature and
kept at this temperature. To this solution, 4-ethyl acetophenone
(0.4 g, 31 mmol) was added dropwise under N2 atmosphere with
continuous stirring during 1 h. The mixture was stirred for more
than 2 h, and the temperature was raised to 20 °C. Then, the
mixture was left to precipitate up to 2 days. The red–yellow pre-
cipitate was collected by vacuum filtration. The product was
washed three times with diethyl ether. The collected product
was dissolved in a little water, and precipitated by adding acetic
acid dropwise. The yellow product was washed with water and
recrystallized from EtOH/H2O (1:2). Then, hydroxylamine hydro-
chloride (1.1 g, 15 mmol) was dissolved in absolute ethanol
(70 mL) at room temperature. To this solution, sodium acetate
(2.2 g, 15 mmol) was added portionwise with continuous stirring
2.4. Synthesis of the [Ni(dioxime)2Cu2(N–N)2](ClO4)2 (3–7) complexes
All of multinuclear [Ni(dioxime)2Cu2(N–N)2](ClO4)2 (N–N = 2,
20-bipyridine (bpy), 1,10-phenanthroline (phen), 3,30-dicarboxy-
2,20-bipyridine (dcbpy), 4,5-diazafluoren-9-one (dafo) and 1,10-
phenanthroline-5,6-dione (dione)] complexes were synthesized
according to the reported analogous procedure with some modifi-
cations. The mononuclear [Ni(dioxime)2] complex (0.4 g,
0.91 mmol) was added to Et3N (0.30 mmol) in absolute ethanol
(70 ml) and the mixture was stirred for 2 h. A solution of Cu(ClO4)2-

A. Kilic et al. / Inorganica Chimica Acta 394 (2013) 635–644
637
ꢁ6H2O (0.69 g, 1.82 mmol) in absolute ethanol (20 ml) was added
to the stirred ethanol mixture of [Ni(dioxime)2] (2) complex and
Et3N. Then an ethanolic solution (15 mL) of the N–N ligand
[2,20-bipyridine (0.29 g, 1.82 mmol), 1,10-phenanthroline monohy-
drate (0.36 g, 1.82 mmol), 3,30-dicarboxy-2,20-bipyridine (0.44 g,
1.82 mmol), 4,5-diazafluoren-9-one (0.35 g, 1.82 mmol) or 1,10-
phenanthroline-5,6-dione (0.38 g, 1.82 mmol)] was added and
stirred under reflux for 5 h. After boiling under reflux for 5 h, the
mixture was left to precipitate up to 2 days. The solvent was slowly
evaporated at room temperature, and the products were recrystal-
lized from EtOH/H2O (1:2). Different color crystals obtained were
filtered, washed with EtOH, MeOH and Et2O, and dried in air.
2965–2873
1456–1430
m
(Aliph–H), 1697
m
(C@O) 1604 and 1577
m
(C@N),
m
(C@C), 1273 (N–O), 1085 and 626
m
m(ClO4), 504
m(Ni–N), and 483
m
(Cu–O). UV–Vis (kmax, nm, ⁄ = shoulder peak):
230, 241⁄, 272, 298 and 412⁄ (in CH3OH); 278, 283, 296, and
421⁄ (in DMSO). LC–MS (Scan ES+): m/z (%) 1185.6 (18) [M]+,
744.1 (45), 566.2 (100), 372.9 (35) and 176.1 (20).
2.5. General procedure for the cycloaddition of epoxides to CO2
A 50 mL steel pressure reactor was charged with mononuclear
[Ni(dioxime)2] complex or multinuclear [Ni(dioxime)2Cu2
(N–N)2](ClO4)2 complexes (1.125 ꢂ 10ꢀ5 mol), epoxide (1.125 ꢂ
10ꢀ2 mol), and DMAP or other Lewis base (2.25 ꢂ 10ꢀ5 mol). The
reaction vessel was placed under a constant pressure of carbon
dioxide for 2 min to allow the system to equilibrate. Carbon diox-
ide was charged into the autoclave with desired pressure then
heated to the desired temperature. The pressure was kept constant
during the reaction. The vessel was then cooled to 5–10 °C in an ice
bath after the expiration of the desired time of reaction. The
pressure was released, and the excess gases were vented. The
conversions of epoxides to corresponding cyclic carbonates were
determined by comparing the ratio of product to substrate in the
1H NMR spectrum of an aliquot of the reaction mixture.
2.4.1. For [Ni(dioxime)2Cu2(bpy)2](ClO4)2 (3)
Color: dark green; Yield (%): 65, m.p: 240 °C, Anal. Calc. for [C40-
H36N8O12Cl2Cu2Ni] (F.W: 1077 g/mol): C, 44.59; H, 3.37; N, 10.40.
Found: C, 44.54; H, 3.42; N, 10.43%. KM = 146
eff = 1.41 [B.M]. FT-IR (KBr pellets,
max/cmꢀ1): 3066 and 3055
(Ar–H), 2964–2877 (Aliph–H), 1602 and 1568 (C@N), 1494–
(N–O), 1089 and 626 (ClO4), 509 (Ni–N),
X ,
ꢀ1 cm2 molꢀ1
l
m
m
m
m
1445
m
(C@C), 1270
m
m
m
and 470 m
(Cu–O). UV–Vis (kmax, nm, ⁄ = shoulder peak): 244, 298,
308, 382, 445⁄ and 646⁄ (in CH3OH); 265, 302, 312 and 465⁄(in
DMSO). LC–MS (Scan ES+): m/z (%) 1078.1 (16) [M+H]+, 1030.1
(30), 902.0 (100), 770.0 (14) and 708.2 (10).
2.4.2. For [Ni(dioxime)2Cu2(phen)2](ClO4)2 (4)
3. Results and discussion
Color: green; Yield (%): 68, m.p: 236 °C, Anal. Calc. for [C44H36-
N8O12Cl2Cu2Ni)] (F.W: 1125.5 g/mol): C, 46.96; H, 3.22; N, 9.96.
3.1. Synthesis and spectral properties
Found: C, 46.88; H, 3.26; N, 9.92%. KM = 116
eff = 1.56 [B.M]. FT-IR (KBr pellets,
max/cmꢀ1): 3058
2961–2856 (Aliph-H), 1605 and 1558 (C@N), 1495–1428
(N–O), 1088 and 626 (ClO4), 506 (Ni–N), and
X
ꢀ1 cm2 molꢀ1
(Ar-H),
,
l
t
m
The unsymmetrical ligand (LH2) (1) was prepared in moderate
yield by the procedure described in literature with some modifica-
tions [37,38] (Scheme 1). The mononuclear [Ni(dioxime)2] (2) com-
plex (Scheme 2) was synthesized by treating NiCl2.6H2O with one/
two equivalents of the corresponding unsymmetrical ligand (1) in
absolute ethanol at reflux temperature. For the multinuclear
[Ni(dioxime)2Cu2(N–N)2](ClO4)2 (3–7) complexes, the mononu-
clear complex [Ni(dioxime)2] (2) was used as precursor and 2,20-
bipyridine, 1,10-phenanthroline, 3,30-dicarboxy-2,20-bipyridine,
4,5-diazafluoren-9-one or 1,10-phenanthroline-5,6-dione were
used as linked ligands. To confirm the identity of the pre-catalysts
prepared in the present work, a variety of techniques including 1H
and 13C spectra, FT-IR, UV–Vis, elemental analysis, melting point
measurements, LC–MS spectroscopy, molar conductivity measure-
ments, magnetic susceptibility techniques and cyclic voltammetry
techniques were used for determination of the mononuclear and
multinuclear oxime complexes. For mononuclear [Ni(dioxime)2]
(2) complex, the Ni(II) ion is tetra coordinated and has a square
planar arrangement with four Ni N bonds. In the multinuclear
complexes (3–7), the Ni(II) ion is coordinated to the imino nitrogen
atoms of the oxime core and the two Cu(II) ions are coordinated to
oxygen donors of oxime groups and the N–N ligand (2,20-bipyri-
dine, 1,10-phenanthroline, 3,30-dicarboxy-2,2’-bipyridine, 4,5-
diazafluoren-9-one or 1,10-phenanthroline-5,6-dione). We have
attempted to prepare single crystals of ligand (LH2) (1) and mono-
and multinuclear metal complexes (2–7) in different solvents and
techniques, but we could not prepare convenient single crystals
of ligand and its metal complexes. However, the spectroscopic
and analytical results supported the proposed structures for these
complexes.
m
m
m
471
(C@C), 1272
m
m
m
m
(Cu–O). UV–Vis (kmax, nm, ⁄ = shoulder peak): 226, 269, 294,
453⁄ and 664⁄ (in CH3OH); 282, 302, 347, 469⁄and 670⁄ (in DMSO).
LC–MS (Scan ES+): m/z (%) 1125.3 (12) [M]+, 1032.1 (30), 902.0
(100), 770.0 (15) and 708.2 (10).
2.4.3. For [Ni(dioxime)2Cu2(dcbpy)2](ClO4)2 (5)
Color: pale green; Yield (%): 68, m.p: 236 °C, Anal. Calc. for [C44-
H36N8O20Cl2Cu2Ni] (F.W: 1253.5 g/mol): C, 42.16; H, 2.90; N, 8.94.
Found: C, 42.09; H, 2.86; N, 8.91%. KM = 138
eff = 1.54 [B.M]. FT-IR (KBr pellets,
max/cmꢀ1): 3573–3100
(COOH), 3079 and 3053 (Ar–H), 2968–2869 (Aliph–H), 1615
(C@O) 1589 and 1570 (C@N), 1461–1435 (C@C), 1276 (N–O),
(ClO4), 522 (Ni–N), and 478 (Cu–O). UV–Vis
X ,
ꢀ1 cm2 molꢀ1
l
m
m
m
m
m
m
m
m
1089 and 626
t
m
t
(kmax, nm, ⁄ = shoulder peak): 238, 255, 298 and 471⁄ (in CH3OH);
264, 269, 281 and 368⁄ (in DMSO). LC–MS (Scan ES+): m/z (%)
1253.2 (14) [M]+, 1207.1 (8), 669.1 (100), 373.0 (28) and 102.2 (76).
2.4.4. For [Ni(dioxime)2Cu2(dafo)2](ClO4)2 (6)
Color: dark green; Yield (%): 70, m.p: 220 °C, Anal. Calc. for [C42-
H32N8O14Cl2Cu2Ni] (F.W: 1129.4 g/mol): C, 44.66; H, 2.86; N, 9.92.
Found: C, 44.59; H, 2.83; N, 9.95%. KM = 127
eff = 1.55 [B.M]. FT-IR (KBr pellets,
max/cmꢀ1): 3056
(C@O) 1589 and 1561
(N–O), 1090 and 626 (ClO4), 526
X
ꢀ1 cm2 molꢀ1
(Ar–H),
(C@N),
,
l
m
m
m
2966–2872
1471–1432
m
m
(Aliph–H), 1723
(C@C), 1270
m
m
m
m
(Ni–N), and 482
m
(Cu–O). UV–Vis (kmax, nm, ⁄ = shoulder peak):
241, 268, 303, and 316 (in CH3OH); 272, 281, 303, 317 and 710⁄
(in DMSO). LC–MS (Scan ES+): m/z (%) 1130.2 (16) [M+H]+, 764.5
(15), 684.0 (30), 303.2 (70) and 102.2 (100).
The FT-IR spectra of the mono- and multinuclear oxime com-
plexes (2–7) were compared with those of the free ligand (1) in or-
der to determine the coordination sites that could be involved in
chelation. There are some guide peaks in the spectrum of the li-
gand, which are of significant help for achieving this goal. The po-
sition and/or the intensities of these peaks are expected to be
changed upon chelation. Coordination of the unsymmetrical oxime
2.4.5. For [Ni(dioxime)2Cu2(dione)2](ClO4)2 (7)
Color: dark green; Yield (%): 72, m.p: 285 °C, Anal. Calc. for [C44-
H32N8O16Cl2Cu2Ni] (F.W: 1185.5 g/mol): C, 44.58; H, 2.72; N, 9.45.
Found: C, 44.54; H, 2.69; N, 9.41%. KM = 130
X
ꢀ1 cm2 molꢀ1
(Ar–H),
,
l
eff = 1.42 [B.M]. FT-IR (KBr pellets, m
m
max/cmꢀ1): 3078

638
A. Kilic et al. / Inorganica Chimica Acta 394 (2013) 635–644
Scheme 1. The structure of the proposed ligand (LH2) (1).
Scheme 2. The structure of the proposed [Ni(dioxime)2] (2) and [Ni(dioxime)2Cu2(N–N)2](ClO4)2 (3–7) complexes.
ligand (1) to the Ni(II) ion through the nitrogen atom is expected to
reduce the electron density in the azomethine link and lower the
nickel complex where the copper ions encapsulated to form multi-
nuclear metal complexes, namely Cu2(bpy)2, Cu2(phen)2, Cu2(-
dcbpy)2, Cu2(dafo)2 or Cu2(dione)2 [40–42]. However, the
m
(C@N) absorption frequency. The remarkably strong and sharp
bands located at 1609 cmꢀ1
between 1606–1589 and
1577–1558 cmꢀ1, were assigned to the
(C@N) stretching vibra-
,
disappearance of
m(O–H) stretching bands of free ligand together
m
with the existence of H-bridge
m
(O–Hꢁ ꢁ ꢁO) between 3558–
tions of the azomethine of the oxime ligand (1), the mono- and
multinuclear complexes (2–7), respectively. Some shifts to a lower
wavenumbers were clearly observed by the formation of the
mono- and multinuclear complexes. Hence, these shifts suggested
the participation of the azomethine group of this ligand in binding
to the Ni(II) ion [39,40]. In the FT-IR spectrum of the ligand and
mononuclear [Ni(dioxime)2] (1) complex, the stretching vibrations
3297 cmꢀ1 and the shift of –C@N and –N–O stretches in the FT-
IR spectra of the mononuclear metal complexes provided support
for MN4-type coordinations in the metal complexes (2–7) [43].
The FT-IR spectra showed a strong peak at 1615 cmꢀ1 for complex
(5), 1723 cmꢀ1 for complex (6), and 1697 cmꢀ1 for complex (7),
due to carbonyl or carboxylic acid group. Perchlorate salts showed
strong antisymmetric stretching band observed between 1089–
of the intramolecular hydrogen bonds
m
(O–Hꢁ ꢁ ꢁO) were observed
1085 cmꢀ1 and sharp antisymmetric stretching band observed at
,
between 3558–3102 cmꢀ1, whereas in the FT-IR spectra of multi-
626 cmꢀ1
,
an indication of uncoordinated perchlorate anions
nuclear metal complexes (3–7), these bonds were not observed
[44]. The coordination mode of the ligand (1) was further sup-
as a result of disappearing the H-bonds
m(O–Hꢁ ꢁ ꢁO) of mononuclear
ported by new frequencies occurring in the range 526–504 cmꢀ1

A. Kilic et al. / Inorganica Chimica Acta 394 (2013) 635–644
639
Fig. 1. 1H NMR spectrum of ligand (LH2) (1).
electronic
p ?
p⁄ or n ? p⁄ transitions of the organic ligand. Addi-
tionally, the new broad bands with the lowest energies and in
lower intensities between 646–664 (in CH3OH) and 522–710 (in
DMSO) nm are attributed to d–d transitions for mononuclear
[Ni(dioxime)2] (2), and multinuclear [Ni(dioxime)2Cu2(N–
N)2](ClO4)2 (3–7) complexes. These modifications in shifts and
intensities for the absorption bands supported coordination of
the ligand to the Ni(II) and Cu(II) ion in the proposed compounds.
1H and 13C NMR chemical data for the ligand (LH2) (1) and
mononuclear [Ni(dioxime)2] (2) complex in DMSO-d6 were re-
ported in the experimental section and Figs. 1–3. The chemical
shifts were observed as one doublet (11.86 ppm, 2H) or two singlet
(11.69 and 11.47 ppm, 1H each) as a double resonance for the deu-
terium exchangeable protons of the C@N–OH groups of oxime in
the 1H NMR spectra of unsymmetrical free ligand (1) (Fig. 1). The
existence of intramolecular deuterium exchangeable H-bridge
(O–Hꢁ ꢁ ꢁO) protons was characterized by a new different singlet sig-
nals at low field, (18.29 and 18.01 ppm, 1H each) or (18.12 ppm,
2H) as a double resonance (Fig. 3). The 1H NMR resonances with
expected integrated intensities were observed as two doublet
peaks at 8.48 and 7.88 ppm corresponding to the HC@N as a double
resonance, 7.49 and 7.23 ppm as doublet, corresponding to the Ar–
CH, 2.70–2.58 ppm as quartet, corresponding to the CH3–CH2, and
1.19 ppm as triplet, corresponding to the CH3–CH2, respectively for
free ligand. In the 1H NMR spectrum of [Ni(dioxime)2] (2) complex,
these resonances were observed at 8.20 and 8.14 ppm as doublet,
corresponding to the HC@N as a double resonance, 8.03 and
8.01 ppm with 7.35 and 7.32 ppm as doublet, corresponding to
the Ar–CH as a double resonance, 2.69–2.61 ppm as quartet, corre-
sponding to the CH3–CH2, and 1.20 ppm as triplet, corresponding
to the CH3–CH2, respectively. The azomethine group has shifted
to the different field region indicating coordination of –C@N to
the nickel ion. The other chemical shift values of [Ni(dioxime)2]
(2) complex are extremely close to that of ligand (1) protons.
More detailed information about the structure of free ligand (1)
and its mononuclear [Ni(dioxime)2] (2) complex was provided by
the 13C NMR spectrum (Fig. 2 and experimental section). In the
13C NMR spectrum, the carbon resonances of oxime groups were
observed at 151.26 and 145.02 ppm with 153.48 and 148.46 ppm
as a double resonance for free ligand and 147.30 and 146.80 ppm
Fig. 2. 13C NMR spectrum of ligand (LH2) (1).
due to
m
m
(Ni N) stretching vibrations and 483–470 cmꢀ1 due to
(Cu–O) stretching vibrations that were not observed in the infra-
red spectra of the ligand.
Electronic spectra of free ligand and mono- and multinuclear
metal complexes (1–7) have been recorded on the 200–1100 nm
range in CH3OH and DMSO solvents, and their corresponding data
were given in the experimental part. The position of absorption
bands was strongly influenced by the structure of the compounds
and the electronic and steric nature of the N–N ligand (2,20-bipyr-
idine, 1,10-phenanthroline, 3,30-dicarboxy-2,20-bipyridine, 4,5-
diazafluoren-9-one or 1,10-phenanthroline-5,6-dione) substituted
on the conjugated system of multinuclear metal complexes. The
reflectance UV–Vis spectra of the complexes (1–7) have contained
absorption bands in the 226–471 (in CH3OH) and 264–465 (in
DMSO) nm region which are attributable to ligand-to-Ni(II) metal
or the linked ligands to the Cu(II) metal CT transition and the

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A. Kilic et al. / Inorganica Chimica Acta 394 (2013) 635–644
Fig. 3. 1H NMR spectrum of [Ni(dioxime)2] (2) complex.
Fig. 5. The LC–MS spectra of [Ni(LH)2] (2).
to be 70% cis-isomer and 30% trans-isomer from the 1H NMR and
13C NMR data. Since multinuclear [Ni(dioxime)2Cu2(N–
N)2](ClO4)2 oxime complexes have paramagnetic properties, their
NMR spectra could not be obtained.
Fig. 4. The LC–MS spectra of ligand (LH2) (1).
with 147.16 and 146.24 ppm as a double resonance for [Ni(diox-
ime)2] (2) complex. However, the other some carbon resonances
have a double resonance both for free ligand (1) and its mononu-
clear [Ni(dioxime)2] (2) complex. Consequently, both in the 1H
and 13C NMR spectra of free ligand (1) and its mononuclear
[Ni(dioxime)2] (2) complex, some proton and carbon resonances
have a double resonance indicating that free ligand (1) and its
mononuclear [Ni(dioxime)2] (2) complex have cis–trans isomerism
[43,45]. In this work, the cis-isomer refers to the two hydroxy
groups sitting on the same side to each other in both ligand (1)
and mononuclear [Ni(dioxime)2] (2) complex, whereas hydroxy
groups sit on opposite sides to each other at trans-isomer. The for-
mation of both isomers has been reflected by NMR spectra as the
difference in intensities of proton and carbon signals, correspond-
ing to both isomers. There was a significant difference between the
intensities of the two peaks, hence one of the isomers was formed
in higher intensities than the other one. The isomer ratio was found
Magnetic susceptibility measurements provided sufficient data
to characterize the structure of the metal complexes. Magnetic mo-
ment measurements of compounds were carried out at room tem-
perature. The effective magnetic moments
(leff) of all the
complexes were measured on samples at room temperature, there-
after necessary diamagnetic corrections were done using Pascal’s
table. The results showed that the mononuclear [Ni(dioxime)2]
(2) complex was diamagnetic, indicating the low-spin (S = 0)
square planar d8-systems, whereas magnetic moments of the mul-
tinuclear Ni(dioxime)2Cu2(N–N)2](ClO4)2 (3–7) complexes were
found between 1.56–1.41 B.M. The observed effective magnetic
moments were comparable to the expected spin-only value
(1.73 lB) for one unpaired electron and confirmed the formal +2
oxidation state of the copper centers. However, it is obvious that
the multinuclear oxime complexes (3–7) possess antiferromag-
netic properties at room temperature by strong intramolecular

A. Kilic et al. / Inorganica Chimica Acta 394 (2013) 635–644
641
Fig. 6. The LC–MS spectra of [Ni(dioxime)2Cu2(bpy)2](ClO4)2 (3) complex.
6
3
IIIpa
40
20
0
Ipa
IIpa
Ia
IIIpc
0
IIpc
I
I
-20
Ic
Ic
-40
IIc
-3
Pt elektrode
GC electrode
LH2
[Ni(dioxime)2]
-60
Ic
-1.5
-1.0
-0.5
0.0
0.5 1.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
Potential/V vs. Ag/AgCl
Potential/V vs. Ag/AgCl
Fig. 7. Cyclic voltammograms of the ligand (1) and [Ni(dioxime)2] (2) complex in
Fig. 8. Cyclic voltammograms of the [Ni(dioxime)2Cu2(dafo)2](ClO4)2 (6) complex
DMSO/0.1 M TBAP at 0.1 V sꢀ1 scan rate.
on Pt electrode and GC electrode in DMSO/0.1 M TBAP at 0.1 V sꢀ1 scan rate.
antiferromagnetic spin exchange interaction as reported previ-
ously for the intramolecular O–Hꢁ ꢁ ꢁO bridges replaced Cu(II) con-
taining metal complexes with oximate bridge ligands [40,46].
The conductivity measurements have frequently been used in
structural elucidation of metal chelates within the limits of their
solubility. They provide a method of testing the degree of ionization
of the complexes, the molecular ions that a complex liberates in
solution (in case of presence of anions outside the coordination
sphere), the higher will be its molar conductivity and vice versa.
The molar conductivity values indicate that the anions may be pres-
ent outside the coordination sphere or inside or absent [47]. With a
view to studying the electrolytic nature of the mononuclear
[Ni(dioxime)2] (2) complex and multinuclear Ni(dioxime)2Cu2(N–
N)2](ClO4)2 (3–7) complexes, their molar conductivities were mea-
sured in DMF (10ꢀ3 M). The molar conductivity (KM) values of the
(N–N)2](ClO4)2 (3–7) complexes are more soluble than [Ni(diox-
ime)2] (2) complex, due to the presence of Cu2(bpy)2, Cu2(phen)2,
Cu2(dcbpy)2, Cu2(dafo)2 or Cu2(dione)2 bridged groups in the oxime
moieties. Although the mononuclear [Ni(dioxime)2] (2) complex is
soluble in DMSO, DMF and slightly soluble in CH2Cl2, CHCl3 All com-
plexes are stable in the solvents reported in this study at room
temperature.
LC–MS was used to obtain the molecular masses of the ligand
(1) and its mono- and multinuclear (2–7) oxime complexes. The
LC–MS spectra of the ligand (1) and their mono- and multinuclear
(2–7) oxime complexes were taken as evidence for the formation
of the proposed structures (Figs. 4–6). The values of molecular
weights supplied by mass spectrometer are presented in the exper-
imental section. Mass spectrometric analysis of the resulting solu-
tion revealed that a peak at m/z = 192.2 (100%) for ligand (1), 441.3
(15%) for complex (2), 1078.1 (16%) for complex (3), 1125.3 (12%)
for complex (4), 1253.2 (14%) for complex (5), 1130.2 (16%) for
complex (6) and 1185.6 (%18) for complex (7), respectively that
could be assigned to the [M]+ or [M+H]+ molecular ion fragment,
supported the proposed structure of the ligand (1) and its mono-
and multinuclear (2–7) oxime complexes.
mononuclear [Ni(dioxime)2] (2) complex is found 13
X
ꢀ1 cm2 -
molꢀ1 at room temperature, indicating its almost non-electrolytic
nature. The result indicates that this complex (2) is poor in molar
conductivity due to the non-free ions in complex (2), whereas mul-
tinuclear Ni(dioxime)2Cu2(N–N)2](ClO4)2 (3–7) complexes are in
the range of 146–116
X
ꢀ1 cm2 molꢀ1 at room temperature, indicat-
ing 1:2 electrolytes or existence of three ionic species in solution
[48]. The higher values of multinuclear Ni(dioxime)2Cu2(N–
N)2](ClO4)2 (3–7) complexes than the mononuclear [Ni(dioxime)2]
(2) complex, indicated the presence of ClO4 ions as counter ion.
The ligand (1) is soluble in common organic solvents such as THF,
C2H5OH, CH2Cl2, and DMSO Multinuclear Ni(dioxime)2Cu2
3.2. Electrochemical properties
The electrochemical properties of the metal complexes were
investigated using cyclic voltammetric techniques in DMSO

642
A. Kilic et al. / Inorganica Chimica Acta 394 (2013) 635–644
Table 1
Table 2
Synthesis of styrene carbonate from styrene oxide and CO2 catalyzed by complexes
Coupling of CO2 and various epoxides catalyzed by complex (2).
(2–7).
Entries
1
Product
Yield (%)
91
TON
910
TOF (hꢀ1
455
)
O
O
O
O
O
2
52
520
260
O
Entry
Catalyst
Additive
Yielda
TOFb
1
2
3
4
5
6
7
8
2
3
4
5
6
7
–
2
2
–
2
–
2
–
3
4
5
6
7
3
4
5
6
7
3
4
5
6
7
DMAP
DMAP
DMAP
DMAP
DMAP
DMAP
DMAP
–
55
8
5
275
40
25
10
50
60
25
–
320
70
455
260
345
125
75
65
25
85
100
230
215
110
250
275
135
110
50
2
CH
Cl
3
10
12
5
3
4
99
65
990
650
495
325
O
O
trace
64
14
91
52
69
25
15
13
5
17
20
46
43
22
50
55
27
22
10
28
31
O
O
O
O
9
[bmim]I
[bmim]I
[bmim]PF6
[bmim]PF6
NBu4Br
NBu4Br
[bmim]I
[bmim]I
[bmim]I
[bmim]I
[bmim]I
[bmim]PF6
[bmim]PF6
[bmim]PF6
[bmim]PF6
[bmim]PF6
NBu4Br
NBu4Br
NBu4Br
NBu4Br
NBu4Br
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
CH
3
Reaction conditions: 2 (1.125 ꢂ l0ꢀ5 mol), [bmim]PF6 (2.25 ꢂ l0ꢀ5 mol), epoxide
(1.125 ꢂ l0ꢀ2 mol), CO2 (1.5 Mpa), 100 °C, 2 h.
Table 3
Voltammetric data for complexes in (2–7) DMSO-TBAP.
140
155
MetalComplexes
L/Lꢀ
Ni2+/Ni+
Cu2+/Cu+
Cu2+/Cu3+
E1/2 (V)a
Epc (V)
Epa (V)
Epa (V)
Catalyst (1.125 ꢂ l0ꢀ5 mol), DMAP (2.25 ꢂ l0ꢀ5 mol), styrene oxide (1.125 ꢂ l0ꢀ2
(2)
(3)
(4)b
(5)
(6)
(7)
ꢀ0.941
ꢀ1.246
ꢀ1.161
ꢀ0.928
ꢀ0.956
ꢀ0.928
ꢀ1.302
–
–
mol), CO2 (1.5 Mpa), 2 h.
–
–
–
–
–
ꢀ0.005
0.010
0.217
0,040
ꢀ0.128
0.321
0.806
0.490
0.522
0.569
a
Yield of epoxides to corresponding cyclic carbonates was determined by com-
paring the ratio of product to substrate in the 1H NMR spectrum of an aliquot of the
reaction mixture.
b
Moles of cyclic carbonate produced per mole of catalyst.
a
E1/2 = (Epc + Epa)/2.
GC electrode.
b
containing 0.1 M TBAP. The data obtained in this work are listed in
Table 3. Fig. 7 represents the cyclic voltammetry of the ligand (1)
and [Ni(dioxime)2] (2) complex. As seen from Fig. 7, the ligand
has one cathodic peak in Ag/AgCl electrode system. The cathodic
complex showed reversibility at a scan rate of 100 mV sꢀ1
. DEp val-
ues of [Ni(dioxime)2Cu2(dafo)2](ClO4)2 (6) increased with the scan
rate but remained in the range of reversible systems. In addition
differently on the other complexes, glassy carbon (GC) electrode
measurements showed better results than Pt electrode. Pt elec-
trode showed only reversible character of the ligand based reduc-
tion. Cu2+/Cu+ and Cu2+/Cu3+ couples were not clearly seen.
However, the glassy carbon electrode showed the reduction and
oxidation of all species.
peak potential of this reduction process was observed at Epc
=
ꢀ0,825 V. The cathodic peak corresponding to the ligand was also
seen in [Ni(dioxime)2] (2) complex, but it negatively shifted and
appeared at E1/2 = ꢀ0,864 V. The ligand (1) has an irreversible
reduction–oxidation process because of the high value for the ano-
dic-to-cathodic peak separation and the smaller anodic-to-catho-
dic peak current ratio. Second reduction peak could be assigned
to the Ni2+/Ni+ species [49]. All the complexes (except [Ni(diox-
ime)2] (2) complex) gave the reduction and oxidation couple with-
in the positive potential window, which were attributed to the
Cu2+/Cu+ and Cu2+/Cu3+ processes [50,51]. These redox couples
have irreversible character with high peak separation values and
lower anodic-to-cathodic peak current ratios (Table 3). Fig. 8
showed cyclic voltammograms of the [Ni(dioxime)2Cu2(dafo)2]
(ClO4)2 (6) complex on Pt and GC electrodes in DMSO/0.1 M TBAP
at 0.1 V sꢀ1 scan rate where the complex clearly exhibited a revers-
ible reduction process based on the ligand. Although the ligand
reduction processes were seen in all of the complexes but only dafo
3.3. Catalytic properties
Catalytic efficiencies of metal complexes for the cycloaddition
reaction of CO2 were experimented (Tables 1 and 2). Catalytic
experiments were carried out at optimized conditions, which were
determined at our previous studies [4–6]. It was obvious that
whereas, monometallic Ni(dioxime)2 complex (2) gave moderate
conversion (55%) bimetallic complexes (3–7) did not work well
for this catalytic reaction when DMAP was chosen as a Lewis base
(Table 1). While the catalytic efficiency is expected to increase in
the presence of bimetallic complexes in accordance with the

A. Kilic et al. / Inorganica Chimica Acta 394 (2013) 635–644
643
literature [52,53], a lower catalytic efficiency was exhibited by our
bimetalic complexes (3–7). This unexpected result was also seen at
Zn-cluster compounds of Kleij and Co-workers [54]. They sug-
gested that reactivity of the outer Zn ions was determined by the
axial substrate binding ability, steric factors, and flexibility of the
bridging fragment in the salen ligand. By the way, in this study
the lower catalytic efficiencies of bimetalic complexes may be
due to the insolubilities of them in the epoxide (styrene oxide). It
was observed that before and after the catalytic reaction, bimetal-
lic complexes (3–7) were in a solid form in the reaction vessel
while the complex 2 was soluble.
Instead of DMAP, other organic additives [tetrabutylammonium
bromide (NBu4Br), butylmethylimidazolium iodide (bmimI) or
butylmethylimidazolium hexafluorophosphate (bmimPF6)] were
used with the complex 2. Conversion of styrene carbonate was
found increasing by the order of DMAP < [bmim]I < NBu4-
Br < [bmim]PF6. This order did not change with the other com-
plexes (Table 1, entries 15–29). Without any additive,
Ni(dioxime)2 complex (2) did not work at all (entry 8). It was
remarkable that [bmim]PF6 was efficient alone in the ratio of
0.1 mol%. Ni(dioxime)2/[bmim]PF6 catalytic system which was
examined to survey the applicability of other terminal epoxides
(Table 2). Among the epoxides surveyed, the epichlorohydrin was
the most reactive epoxide, and the reaction finished perfectly in
2 h with a 99% conversion.
[57–59]. The disappearance of H-bonding (O–Hꢁ ꢁ ꢁO) in the
[Ni(dioxime)2Cu2(N–N)2](ClO4)2 metal complexes showed that
the Cu(II)-capped groups attached to the main oxime core. Also,
these complexes have enabled us a systematically probe the phe-
nomenon of proton-coupled electron transfer that occurs when
oxime ligand is coordinated to a redox active metal center [60].
Acknowledgements
This work was supported by the Technological and Scientific
Research Council of Turkey TUBITAK (TBAG Project No. 111T944)
and the Research Fund of Harran University (HUBAK Projects Nos.
1055 and 1042, Sanliurfa, Turkey).
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In this study, the unsymmetrical dioxime ligand (1), its the
mononuclear complex [Ni(dioxime)2] (2) and the multinuclear me-
tal complexes [Ni(dioxime)2Cu2(N–N)2](ClO4)2 (3–7) were synthe-
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The intramolecular O–Hꢁ ꢁ ꢁO bridges on the mononuclear complex
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thesize the multinuclear complexes [Ni(dioxime)2Cu2(N–
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NMR spectrum, which indicate that the free ligand (1) and its the
mononuclear [Ni(dioxime)2] (2) complex have cis–trans isomerism.
All the complexes (except [Ni(dioxime)2] (2) complex) gave the
reduction and oxidation couple within the positive potential
window, which were attributed to the Cu2+/Cu+ and Cu2+/Cu3+
processes. Additionally, the measurements carried out by glassy
carbon (GC) working electrode showed better results than those
by Pt working electrode. The nature of the ligands around the me-
tal has dramatically been found to affect the energy conversion
process. Particularly, the introduction of electronic effects via elec-
tron-donor substituents on 2,20-bipyridine, 1,10-phenanthroline,
3,30-dicarboxy-2,20-bipyridine, 4,5-diazafluoren-9-one and 1,10-
phenanthroline-5,6-dione ligands have notably improved the
absorption in the visible region for efficient sunlight collection
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