Copper(ii) complexes with 3-(2-pyridyl)-4,5-dihydro-1H-pyrazoles: Synthesis, structural and electrochemical studies

Article abstract of DOI:10.1007/s11172-014-0488-8

Three new copper(ii) complexes with the 3-(2-pyridyl)-4,5-dihydro-1H-pyrazole ligands were obtained by reactions of appropriate organic ligands with CuCl₂·2H₂O. The structures of the complexes were determined by X-ray diffraction. Th

Full text of DOI:10.1007/s11172-014-0488-8

Russian Chemical Bulletin, International Edition, Vol. 63, No. 3, pp. 657—661, March, 2014
657
Copper(II) complexes with 3ꢀ(2ꢀpyridyl)ꢀ4,5ꢀdihydroꢀ1Hꢀpyrazoles:
synthesis, structural and electrochemical studies
N. I. Vorozhtsov, A. G. Majouga, E. K. Beloglazkina, A. A. Moiseeva, G. A. Golubeva,
I. V. Evstaf´ev, L. A. Sviridova, and N. V. Zyk
Department of Chemistry, M. V. Lomonosov Moscow State University,
1
Leninskie Gory, 119991 Moscow, Russian Federation.
Eꢀmail: bel@org.chem.msu.ru
Three new copper(II) complexes with the 3ꢀ(2ꢀpyridyl)ꢀ4,5ꢀdihydroꢀ1Hꢀpyrazole ligands
were obtained by reactions of appropriate organic ligands with CuCl •2H O. The structures of
2
2
the complexes were determined by Xꢀray diffraction. The molecular formula of all the comꢀ
plexes can be written as 2(L•MCl ) (L is an organic ligand). The coordination polyhedron of
2
either copper ion is a distorted trigonal bipyramid made up of two N atoms (coming from the
pyrazoline and pyridine rings) and one axial and two bridging chloride anions. The electroꢀ
II
chemical behavior of 1,5ꢀdiphenylꢀ3ꢀ(2ꢀpyridyl)ꢀ4,5ꢀdihydroꢀ1Hꢀpyrazole and its Cu comꢀ
plex was studied by cyclic voltammetry; the mechanisms of their electrooxidation and elecꢀ
troreduction were proposed.
Key words: 3ꢀ(2ꢀpyridyl)ꢀ4,5ꢀdihydroꢀ1Hꢀpyrazoles, copper(II) complexes, Xꢀray diffracꢀ
tion, cyclic voltammetry.
II
Dinuclear complexes of transition metals are attracꢀ
tive for researchers because of their growing interest in
compounds containing closely spaced metal centers magꢀ
netically interacting with each other.1 Being crucial for
the functions performed by dinuclear copperꢀcontaining
enzymes, such interactions give impetus to investigations
of lowꢀmolecularꢀweight analogs of metalloproteins caꢀ
pable of promoting various redox reactions, including acꢀ
Cu complexes with pyridylpyrazolines have not been isoꢀ
lated in the individual state, yet being detected by elecꢀ
1
1
tronic absorption spectroscopy.
The goal of the present work was to obtain and strucꢀ
turally characterize dinuclear Cu complexes with 3ꢀ(2ꢀ
pyridyl)ꢀ4,5ꢀdihydroꢀ1Hꢀpyrazoles and study their elecꢀ
trochemical behavior.
—3
II
4
—8
tivation of molecular oxygen.
Results and Discussion
9
According to the criteria formulated earlier, dinuclear
metal complexes can form if (1) the coordination sphere
of a metal is incompletely filled with electronꢀdonating
groups of a polydentate organic ligand and (2) either an
organic ligand or an additional inorganic anion contains
electronꢀdonating atoms capable of bridging metal cenꢀ
ters. From this viewpoint, reactions of organic N,N´ꢀtype
bidentate ligands (bipyridyl analogs) with metal chlorides
are promising for the synthesis of dinuclear complexes.
1
,5ꢀDisubstituted 3ꢀ(2ꢀpyridyl)ꢀ4,5ꢀdihydroꢀ1Hꢀpyrꢀ
azoles 1 and 2 were obtained from appropriate chalcones
according to a modified procedure (Scheme 1).
12
Scheme 1
3
ꢀ(2ꢀPyridyl)ꢀ4,5ꢀdihydroꢀ1Hꢀpyrazoles are heteroꢀ
cyclic N,N´ꢀtype ligands that potentially can form monoꢀ,
diꢀ, and polynuclear complexes with transition metals.
However, complexation reactions of 2ꢀpyrazoline derivaꢀ
tives have remained poorly studied until recently. Strucꢀ
turally characterized complexes with such ligands include
only two documented examples: a mononuclear ruthꢀ
Ar = Ph (1), 4ꢀMeOC H4 (2)
6
9
10
enium(II) complex and a dinuclear zinc(II) complex.
Organic ligands of the 3ꢀ(2ꢀpyridyl)ꢀ4,5ꢀdihydroꢀ1Hꢀ
Reagents: PhNH NH , NaOH, EtOH.
2
2
pyrazole series can be employed as selective fluorescent
sensors for Zn ions. At the same time, Ni , Co , and
3,5ꢀDisubstituted 1ꢀbenzylpyrazoline 3 was syntheꢀ
sized as described earlier (Scheme 2).
II
11
II
II
13
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 0657—0661, March, 2014.
066ꢀ5285/14/6303ꢀ0657 © 2014 Springer Science+Business Media, Inc.
1

658
Russ.Chem.Bull., Int.Ed., Vol. 63, No. 3, March, 2014
Vorozhtsov et al.
Scheme 2
Reagents: i. 1) NH NH •H O, 2) AcOH; ii. 1) HBF , 2) PhCHO; iii. 1)NaBH , EtOH, 2) HCl, H O.
2
2
2
4
4
2
Compounds 1—3 were used as ligands in complexꢀ
Scheme 3
ation reactions with copper(II) salt. For this purpose,
a solution of CuCl in MeOH was made to slowly diffuse
2
into solutions of organic ligands 1—3 in CH Cl . The
2
2
resulting complexes 4—6 (Scheme 3) were examined by
elemental analysis, electronic absorption spectroscopy,
and Xꢀray diffraction.
In the UVꢀVis spectra of complexes 5 and 6, the abꢀ
sorption peaks at 320—380 nm are slightly shifted, and
their optical density is considerably increased, compared
to the spectra of the starting ligands 2 and 3. These peaks
have been attributed11 to the —*ꢀtransitions in the orꢀ
Complex
Ar
Ph
R
Ph
Ph
Yield (%)
4
5
6
65
72
53
ganic ligand and are characteristic of complexes with a
metal : ligand ratio of 1 : 1. The spectra do not show abꢀ
sorption bands at 400—600 nm that would be indicative of
4ꢀMeOC H4
6
Ph
CH Ph
2
Reagents: CuCl •H O, CH Cl , MeOH.
1
1
2
2
2
2
the formation of complexes ML in solution.
2
Xꢀray diffraction study of complexes 4—6. Structures
—6 were confirmed by Xꢀray diffraction (Fig. 1). These
4
trigonal bipyramid made up of two N atoms of the pyrazoꢀ
line and pyridine rings and one axial and two bridging
chloride anions. The copper—copper distance (~3.3 Å) is
longer than the double van der Waals radius of the Cu
atom, thus suggesting that the metal ions do not interact
directly in the crystal structures of these complexes.
Electrochemical study of ligand 1 and its complex 4.
Ligand 1 and dinuclear copper(II) complex 4 were studied
complexes are dinuclear clusters of the formula Cu L Cl
2
2
4
(
L = 1—3). Crystallographic parameters and the data colꢀ
lection and refinement statistics for complexes 4—6 are
summarized in Table 1. Selected bond lengths and bond
angles in these structures are given in Table 2. According
to the Xꢀray diffraction data, the coordination polyhedron
of either copper atom in complexes 4—6 is a distorted
a
b
c
Cl(1)
N(2)
N(1)
N(2)
Cl(1)
Cu(1)
Cu(1)
N(3)
N(1)
Cl(1)
Cl(2)
Cu(1)
N(1)
Cl(2)
Cl(2)
Fig. 1. The molecular structures of complexes 4 (a), 5 (b), and 6 (c) (Xꢀray diffraction data).

Copper(II) complexes with pyridylpyrazolines
Russ.Chem.Bull., Int.Ed., Vol. 63, No. 3, March, 2014
659
Table 1. Crystallographic parameters and the data collection and refinement statistics for complexes 4—6
Parameter
4
5
6
Molecular formula
Molecular weight
Space group
a/Å
b/Å
c/Å
C H Cl Cu N
C H C Cu N O
C42H Cl Cu N
6
4
0
40
4
2
6
42 38 l4
2
6
2
38
4
2
873.66
P21/c
14.900(10)
7.467(6)
16.096(6)
90.00
927.66
895.68
P21/c
15.5750(19)
8.7391(8)
15.994(2)
90.00
–
P1
9.8740(9)
9.9050(9)
12.2010(11)
87.120(10)

/deg
/deg
/deg
V/Å3

94.77(5)
90.00
89.470(10)
60.530(10)
1037.41
1
1.485
1.327
108.107(9)
90.00

1784.6(19)
2
1.626
1.532
896
2069.16
2
1.438
1.324
916
Z
dcalc/g cm–3

/mm–1
F(000)
474

scan range/deg
Ranges of h, k, l
2.3—23.7
2.37—26.00
—8  h  12
–12  k  11,
–14  l  15
3932/707
245
2.60—17.09
–19  h  18
–3  k  10,
–19  l  19
4047/1678
209
–20  h  20,
10  k  10,
21  l  21
–
–
Number of measured/unique reflections
Number of refined parameters
GOOF on F2
4727/3526
235
1.505
0.586
0.847
R factors (I > 2(I))
R
0.0530
0.1212
0.0591
0.0999
0.0661
0.1439
1
wR2
R factors (for all reflections)
R1
wR2
0.0775
0.1317
0.2702
0.1398
031634
0.1857
by cyclic voltammetry (CV) at a glassy carbon electrode in
a
DMF with 0.1 M Bu NClO in DMF as a supporting
4
4
electrolyte. Ligand 1 is reduced in two steps and is oxiꢀ
dized in a single step. The first redox peaks correspond to
oneꢀelectron transfer (Table 3, Fig. 2), which becomes
evident when comparing the heights of these peaks with
Table 2. Selected bond lengths and bond angles in complexes 4—6
Parameter
Bond
4
5
6
b
d/Å
Cu(1)—Cl(1)
Cu(1)—Cl(2)
Cu(1)—Cl(2)
Cu(1)—N(1)
Cu(1)—N(2)
2.2307(15)
2.2882(14)
2.6184(17)
2.088(3)11
2.041(3)11
2.209(3)
2.279(3)
2.327(3)
2.000(8)
2.52(1)1
2.224(2)
2.288(2)
2.648(2)
2.038(6)
2.048(5)
Bond angle
/deg
N(1)—Cu(1)—N(2)
N(1)—Cu(1)—Cl(1)
N(2)—Cu(1)—Cl(2)
Cl(1)—Cu(1)—Cl(2)
Cu(1)—Cl(2)—Cu(1)
78.66(12)
91.08(9)1
92.37(9)1
93.60(6)1
90.63(5)1
74.2(4)1
93.4(3)1
87.4(3)1
84.70(9)
95.30(9)
78.9(2)11
93.49(18)
93.24(8)1
90.18(7)1
89.82(7)1
Fig. 2. The frontier orbitals of ligand 1 from quantum chemical
computations at the PM3 level: (a) HOMO and (b) LUMO.

660
Russ.Chem.Bull., Int.Ed., Vol. 63, No. 3, March, 2014
Vorozhtsov et al.
Table 3. The oxidation (EOx) and reduction potentials (ERed) of
ligand 1 and its complex 4a
at 0.37/0.50 V (see Table 3, Fig. 3) due to the transition
II
I
Cu  Cu . Both the copper ions of the dinuclear comꢀ
plex are simultaneously reduced at the same potenꢀ
tial; hence, they do not interact with each other. The
copper(I)ꢀcontaining intermediate is stable on the CV time
EpRed
EpOx
Compound
1
–2.03 (–1.98),–0.70;
1.01
II
scale and undergoes no disproportionation into Cu and
–
2.56 (–0.70), –0.17
0
b
b
Cu complexes. This is confirmed by the reverse scan
4
0.37 (0.50) ;
0.98 ; 1.16
–
2.00 (–1.95); –2.27
(down to Epc  –2.0 V) revealing no peak due to the oxidaꢀ
tive desorption of metallic copper from the electrode surꢀ
face (see Fig. 3). Since the potential of the second cathodꢀ
ic peak of complex 4 is close to the reduction potential of
the free ligand, further reduction of the complex seems to
occur at the pyrazolylpyridine fragment. Therefore, we
can propose a sequential scheme for the first two reducꢀ
tion steps of complex 4 (Scheme 4).
a
DMF, glassy carbon electrode. The peak potentials on the reꢀ
verse scan are given in parentheses.
The initial potential is 0.7 V.
b
the heights of the peaks for oneꢀelectron oxidation of ferꢀ
rocene used in equal concentration. The results of semiemꢀ
pirical (PM3)1 quantum chemical calculations (see Fig. 2)
suggest that the HOMO is mainly contributed by the conꢀ
jugated ꢀsystem of the phenylpyrazoline fragment (this
system is most liable to oxidation in the molecule) and
that the LUMO is mainly contributed by the ꢀsystem
consisting of the C=N bond of the pyrazoline ring and the
pyridine fragment (this system will accept an electron in
the first reduction step; it should be noted that the first
reduction potential is close to the reduction potential of
4
Scheme 4
In contrast to free ligand 1, the CV curve of complex 4
shows an additional oxidation peak at Epa = 1.16 V correꢀ
sponding to the oxidation of coordinated chloride anions.
To sum up, we demonstrated that the 3ꢀ(2ꢀpyridyl)ꢀ
pyridine1 (E = –2.07 V)).
In contrast to the CV curve of free ligand 1, that of
dinuclear complex 4 shows a quasireversible reduction peak
5
pc
4
,5ꢀdihydroꢀ1Hꢀpyrazoles react with cupric chloride
to give dinuclear complexes that are stable in both the
crystalline state and in solution. The results of the elecꢀ
trochemical study of a dinuclear copper complex with
a
1,5ꢀdiphenylꢀ3ꢀ(2ꢀpyridyl)ꢀ4,5ꢀdihydroꢀ1Hꢀpyrazole sugꢀ
gest the independent reduction of either metal center in this
complex as well as the stability of its reduced form in DMF.
5
A
Experimental
The course of the reactions was monitored and the reaction
1
products were identified by TLC on Silufol plates. H NMR
spectra were recorded on a Bruker Avance instrument (400 MHz).
IR spectra were recorded on URꢀ20 instruments (Nujol). UVꢀVis
spectra were recorded on a Specord instrument in EtOH. Xꢀray
diffraction studies were performed on a STOE STADI VARI
Pilatusꢀ100K diffractometer (MoꢀK radiation,  = 0.71073 Å,
T = 295(2) K) purchased in conformity with the development
plan of the M. V. Lomonosov Moscow State University.
b
5
A
Cyclic voltammograms were recorded using a PIꢀ50ꢀ1.1 poꢀ
tentiostat connected to a PRꢀ8 programmer. The potentials of
the working electrode (glassy carbon disk, d = 2 mm) were meaꢀ
sured versus Ag/AgCl/KCl(sat.) as a reference electrode; 0.1 M
Bu NClO in DMF was employed as a supporting electrolyte.
4
4
A platinum plate served as an auxiliary electrode. Prior to the
measurements, the working electrodes were polished with powꢀ
dered alumina (particle size <10 m, SigmaꢀAldrich). The poꢀ
tential scan rate was 200 mV s– . The potentials are corrected for
iRꢀcompensation. For all measurements, dry argon and deaeratꢀ
ed solvents were used.
1
1
.0
0
–1.0
–2.0
E/V
Fig. 3. Cyclic voltammograms (C = 10–3 mol L–1, glassy carbon
electrode, DMF, 0.1 M Bu NClO ) of ligand 1 (a) and comꢀ
plex 4 (b).
Pyridylꢀcontaining chalcones for the synthesis of compounds
4
4
1
6
1—3 were prepared according to a modified procedure.

Copper(II) complexes with pyridylpyrazolines
Russ.Chem.Bull., Int.Ed., Vol. 63, No. 3, March, 2014
661
1
,5ꢀDiphenylꢀ3ꢀ(2ꢀpyridyl)ꢀ4,5ꢀdihydroꢀ1Hꢀpyrazole (1).
interface by carefully pouring it down the wall of the reaction
vessel. The resulting mixture was kept in the dark for precipitaꢀ
tion. The black crystalline complex that formed was filtered off
and dried in air.
A solution of phenylhydrazine (0.74 g, 6.8 mmol) in EtOH
10 mL) was added to a suspension of NaOH (0.27 g) in EtOH
20 mL). The mixture was stirred for 5 min. Then 3ꢀphenylꢀ1ꢀ(2ꢀ
(
(
pyridyl)propꢀ2ꢀenꢀ1ꢀone (1.44 g, 6.8 mmol) was added for 10 min.
The reaction mixture was stirred for 30 min. The yellow precipiꢀ
tate that formed was washed with cold EtOH and recrystallized
from aqueous EtOH (1 : 1). The yield of compound 1 was 1.4 g
Diꢀꢀchloroꢀbis{chloro[1,5ꢀdiphenylꢀ3ꢀ(2ꢀpyridyl)ꢀ4,5ꢀdiꢀ
hydroꢀ1Hꢀpyrazole]copper(II)} (4). Yield 0.28 g (65%), m.p.
236—240 C. IR, /cm– : 1620 (C=N). Calculated (%): C, 55.31;
H, 3.92; N, 9.68. C H Cl Cu N . Found (%): C, 55.10;
1
4
0
40
4
2
6
(
70%), m.p. 114—116 C (cf. Ref. 12: m.p. 115—117 C).
ꢀ(4ꢀMethoxyphenyl)ꢀ1ꢀphenylꢀ3ꢀ(2ꢀpyridyl)ꢀ4,5ꢀdihydroꢀ
Hꢀpyrazole (2) was obtained from 3ꢀ(4ꢀmethoxyphenyl)ꢀ1ꢀ(2ꢀ
H, 3.91; N, 9.58.
5
Diꢀꢀchloroꢀbis{chloro[5ꢀ(4ꢀmethoxyphenyl)ꢀ1ꢀphenylꢀ3ꢀ(2ꢀ
pyridyl)ꢀ4,5ꢀdihydroꢀ1Hꢀpyrazole]copper(II)} (5). Yield 0.34 g
(72%), m.p. 205—210 C. IR, /cm– : 1610 (C=N). UVꢀVis,
/nm (lg): 340 (4.65). Calculated (%): C, 54.38; H, 4.13;
N, 9.06. C H Cl Cu N O . Found (%): C, 55.12; H, 4.08;
1
1
pyridyl)propꢀ2ꢀenꢀ1ꢀone (2.39 g, 10 mmol) as described above
for compound 1. Yield 1.52 g (54%), m.p. 129 C (cf. Ref. 12: m.p.
1
1
30 C). H NMR (CDCl ), : 3.32 (dd, 1 H, H(4), J = 12.0 Hz,
3
1
42 38
4
2
6
2
J = 4.0 Hz); 3.80 (s, 3 H, OMe); 3.97 (dd, 1 H, H(4), J = 18.0 Hz,
N, 9.49.
2
1
J2 = 12.0 Hz); 5.32—5.36 (m, 1 H, H(5)); 6.81—6.88 (m, 3 H,
Ar); 7.12—7.29 (m, 7 H, Ar); 7.72 (m, 1 H, Ph); 8.16 (t, 1 H,
H(3), pyridyl, J = 4.6 Hz); 8.56 (d, 1 H, H(6), pyridyl, J = 3.0 Hz).
UV, /nm (lg): 231 (3.63), 250 (3.56), 379 (3.42).
Bis{[1ꢀbenzylꢀ5ꢀphenylꢀ3ꢀ(2ꢀpyridyl)ꢀ4,5ꢀdihydroꢀ1Hꢀpyrꢀ
azole]ꢀꢀchloroꢀchlorocopper(II)} (6). Yield 0.23 g (53%), m.p.
253—255 C. IR, /cm– : 1610 (C=N). UVꢀVis, /nm (lg):
231 (4.30), 379 (4.30). Calculated (%): C, 56.26; H, 4.24;
N, 9.38. C H Cl Cu N . Found (%): C, 56.01; H, 4.45; N, 9.30.
1
1
ꢀBenzylꢀ5ꢀphenylꢀ3ꢀ(2ꢀpyridyl)ꢀ4,5ꢀdihydroꢀ1Hꢀpyrazole (3).
A hot solution of 3ꢀphenylꢀ1ꢀ(2ꢀpyridyl)propꢀ2ꢀenꢀ1ꢀone (4.18 g,
.02 mol) in EtOH (13 mL) was added in portions at 50—60 C
to a solution of hydrazine hydrate (3 g, 0.02 mol) in EtOH
4.8 mL). After the formation of a precipitate, acetic acid (2.4 mL)
was added dropwise and the reaction mixture was refluxed for
.5 h. The resulting solution was cooled and the precipitate that
42
38
4
2
6
0
This work was financially supported by the Russian
Foundation for Basic Research (Project No. 13ꢀ03ꢀ00399ꢀa).
(
References
1
formed was filtered off, washed with water, and squeezed. The
yield of 5ꢀphenylꢀ3ꢀ(2ꢀpyridyl)ꢀ4,5ꢀdihydroꢀ1Hꢀpyrazole was
1
2
3
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2
hyde (2.02 g, 0.020 mol) was added dropwise with stirring. The
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7
8
9
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3
0, 940.
1
1
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4
2, 9199.
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1
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(
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1
1
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005, 1558].
4. J. J. P. Stewart, J. Comput. Chem., 1989, 10, 209.
H(4), J = 6.0 Hz, J = 5.0 Hz); 3.64 (dd, 1 H, H(4), J = 6.0 Hz,
1
2
1
(
2
J = 5.0 Hz); 4.03 (d, 1 H, CH Ph, J = 12.0 Hz); 4.39 (dd, 1 H,
2
2
H(5), J = 6.0 Hz, J = 5.0 Hz); 4.62 (d, 1 H, CH Ph, J = 12.0 Hz);
1
2
2
7
.17 (m, 1 H, Ar); 7.20—7.45 (m, 10 H, Ar); 7.68 (t, 1 H, pyridyl,
–
1
15. W. M. Clark, OxidationꢀReduction Potentials of Organic Sysꢀ
tems, Williams and Wilkins, Baltimore (MD), 1960, 584 pp.
1
J = 3.5 Hz); 8.00 (d, 1 H, pyridyl, J = 5.0 Hz). IR, /cm : 1570
(
C=N). UV, /nm (lg): 243 (4.92), 324 (3.84).
6. C. Engler, A. Engler, Ber., 1902, 35, 4061.
Synthesis of complexes from pyridylpyrazolines 1—3 and CuCl2
(
general procedure). Methanol (2 mL) was slowly added to
a solution of pyridylpyrazoline 1—3 (1 mmol) in CH Cl (2 mL)
2
2
so that the mixture became separated in layers. A solution of
CuCl •2H O (1 mmol) in MeOH (2 mL) was delivered to the
Received June 3, 2013;
in revised form September 3, 2013
2
2