Synthesis of mixed-ligand copper(II) complexes containing bis(pyrazol-1-yl)methane ligands

Article abstract of DOI:10.1016/j.poly.2006.03.016

Complexes of copper(II) nitrate and chloride with several bis(pyrazol-1-yl)methane derivatives have been prepared and characterized by spectroscopic (IR, UV-Vis) and electrochemical (solution conductivity measurements, cyclic voltammetry) techniques. The

Full text of DOI:10.1016/j.poly.2006.03.016

Polyhedron 25 (2006) 2683–2690
www.elsevier.com/locate/poly
Synthesis of mixed-ligand copper(II) complexes containing
bis(pyrazol-1-yl)methane ligands
*
Andrei S. Potapov, Andrei I. Khlebnikov
Department of Chemistry, Altai State Technical University, 46 Lenin Street, Barnaul 656038, Altai Region, Russia
Received 1 February 2006; accepted 19 March 2006
Available online 5 April 2006
Abstract
Complexes of copper(II) nitrate and chloride with several bis(pyrazol-1-yl)methane derivatives have been prepared and characterized
by spectroscopic (IR, UV–Vis) and electrochemical (solution conductivity measurements, cyclic voltammetry) techniques. The complexes
react with bidentate (2,20-bipyridyl and acetylacetone) and monodentate (triphenylphosphine, 1H-benzimidazole, 1H-benzotriazole,
3,5-dimethyl-1H-pyrazole) ligands forming mixed-ligand complexes.
ꢀ 2006 Elsevier Ltd. All rights reserved.
Keywords: Bis(pyrazol-1-yl)methane complexes; Iodo-derivatives; Copper(II) complexes; Mixed-ligand complexes; 2,20-bipyridyl; Acetylacetone
1. Introduction
ligand ratio M:L = 1:2. According to IR- and UV–Vis spec-
troscopic data, the Cu2+ ion in this complex is octahedrally
surrounded, and both of the nitrate ions are monodentate
[2].
Bis(pyrazol-1-yl)methane (L1, Fig. 1) and its derivatives
are bidentate chelating ligands forming complexes with
most transition metal ions and some main-group elements
[1]. The coordination properties of bis(pyrazol-1-yl)meth-
anes may be varied in a rather wide range by introducing
substituents into the pyrazole rings. Hence, the donor ability
of the ligands is determined by the presence of electron-with-
drawing or electron-donating functional groups, and steric
factors altered by the introduction of bulky alkyl groups
into the rings and the methylene bridge between them.
Several copper(II) complexes of bis(pyrazol-1-yl)meth-
ane derivatives are known so far. In 1979 Jan Reedijk
et al. reported the preparation of bis(3,5-dimethylpyrazol-
1-yl)methane (L2) complexes with copper(II) halides and
nitrate [2]. These authors found CuBr2 to react with L2
(molar ratio 1:2) in ethanol to give the Cu(L2)2Br2 complex,
while in the case of copper(II) chloride an adduct Cu3(L2)4-
Cl6 Æ EtOH of undetermined structure was obtained [2].
Copper(II) nitrate also forms a complex with a metal to
Complexes of L1 and L2 ligands with copper(II) chloride
and nitrate of 1:1 composition were also reported [3]. IR-
spectroscopy suggests the nitrate ions to be monodentate,
so the Cu2+ ions in these complexes are tetrahedral.
The reactivity of bis(pyrazol-1-yl)methane complexes
with transition metals towards other ligands is scarcely
investigated. Mesubi [4] reported the reactions of the 2,2-
bis(pyrazol-1-yl)propane complex Cu(L3)Cl2 with pseudo-
halides in aqueous solution leading to the substitution of
chloride ions with the formation of Cu(L3)X2 (X = SꢀCNꢀ,
CNOꢀꢀ, N3ꢀ) complexes. A similar reaction with PF6 and
BPh4 anions yields [Cu(L3)2Cl]PF6 and [Cu(L3)2Cl]BPh4
species [4].
Displacement of the L3 ligand from the coordination
sphere was observed when the complex Cu(L3)Cl2
was reacted with bidentate anions (acetylacetone, dialkyl-
dithiocarbamate, bis(pyrazol-1-yl)borate and tris(pyrazol-
1-yl)borate) [4]. The first example of a 3d-transition metal
mixed-ligand complex Zn(L4)(Qꢀ)2, containing bis-
(3,4,5-trimethylpyrazol-1-yl)methane (L4) and anionic 1-
phenyl-3-methyl-4-benzoyl-5-pyrazolone enolate (Qꢀ) was
*
Corresponding author. Tel.: +7 3852 245513/522436; fax: +7 3852
367864.
E-mail address: aikhl@nm.ru (A.I. Khlebnikov).
0277-5387/$ - see front matter ꢀ 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2006.03.016

2684
A.S. Potapov, A.I. Khlebnikov / Polyhedron 25 (2006) 2683–2690
R1
R1
chloride. The reactivity of the complexes obtained with
various mono- and bidentate ligands is investigated.
N
N
N
N
2. Experimental
R2
R2
2.1. Syntheses
Commercial reagents and solvents were used as received.
Bis(pyrazol-1-yl)methane (L1) was prepared as previ-
ously described [12]. Briefly, 5.00 g (73.5 mmol) of 1H-pyr-
azole were dissolved in 30 ml of DMSO, and 16.46 g
(294.0 mmol) of finely powdered KOH were added. The
suspension was vigorously stirred for 1 h at 60 ꢁC, and
6.40 g (2.6 ml, 36.8 mmol) of dibromomethane in 20 ml of
DMSO were added dropwise over 30 min. Stirring at
60 ꢁC was continued for an additional 4 h, and the reaction
mixture was poured into 300 ml of water and extracted with
chloroform (6 · 30 ml). The extract was washed with water
(2 · 20 ml) and dried over anhydrous calcium chloride.
Removal of the solvent produced 4.45 g (yield 82%) of solid
product. M.p. 107–107.5 ꢁC (benzene–hexane, 1:1, lit. m.p.
108 ꢁC [13]). Anal. Calc. for C7H8N4: C, 56.74; H, 5.44; N,
37.81. Found: C, 56.36; H, 5.67; N, 38.05%. 1H NMR spec-
R1
R1
R3
R3
L1: R1=H; R2=H; R3=H
L5: R1=H; R2=CH3; R3=H
L6: R1=H; R2=I; R3=H
L7: R1=CH3; R2=I; R3=H
L2: R1=CH3; R2=H; R3=H
L3: R1=H; R2=H; R3=CH3
L4: R1=CH3; R2=CH3; R3=H
Fig. 1. Bis(pyrazol-1-yl)methane derivatives.
prepared by Pettinari et al. [5]. It was noted that in the case
of the complexes of CdI2 and HgCl2 displacement of the L4
ligand is observed under analogous conditions. Neverthe-
less, L4 reacts with mercury(II) acetate in the presence of
saccharine (o-sulfobenzoic acid imide, SacH) in aqueous
solution with the formation of a mixed-ligand complex
Hg(L4)(Sacꢀ)2 [5].
trum corresponds to literature data [14]. IR bands, cmꢀ1
:
The same authors [5] also report that ligand L4 is dis-
placed from the complexes Zn(L4)X2 (X = Clꢀ, Brꢀ) and
Cd(L4)Br2 upon their reaction with various monodentate
ligands (pyrazole, N-methylimidazole, 4-phenylimidazole)
and 2,20-bipyridyl. Similarly, when Hg(L5)Cl2 was reacted
with triphenylphosphine (PPh3) and 2,20-bipyridyl (bipy),
only Hg(PPh3)2Cl2 and Hg(bipy)Cl2 compounds were iso-
lated [6].
A considerable quantity of 3d-transition-metal mixed-
ligand complexes with bis(pyrazol-1-yl)methane derivatives
involves copper(I) ions. Thus, [Cu(NO3)(PPh3)2] reacts
with L1 [7] or L5 [8] to form mixed-ligand adducts [Cu(L)-
(PPh3)2](NO3) (L = L1, L5). It is interesting to note that
the same reaction was observed even in the case of strongly
electron-deficient bis(4-nitropyrazol-1-yl)methane [7]. Simi-
larly, complexes containing other counter-ions, namely
[Cu(L)(PPh3)X] (L = L1, L2; X = Clꢀ, Brꢀ) [8], [Cu(L)-
(PPh3)2](ClO4) (L = L1, L2, L5) [9], [Cu(L2)(PPh3)](ClO4)
[10] and [Cu(L1)(MeCN)n](ClO4) (n = 1, 2) [11] have been
reported.
A binuclear mixed-ligand copper(I) complex with a
bridging bidentate heterocyclic ligand, [Cu2(L2)2(MeCN)2-
(l-Prz)](ClO4)2, was prepared by the reaction of [Cu(L2)-
(MeCN)](ClO4) with pyrazine (Prz) in dichloromethane
solution [10].
As it is seen from the literature, no copper(II) mixed-
ligand complexes containing bis(pyrazol-1-yl)methane
derivatives together with any other organic ligands are
known.
1610, 1510, 1450 (mring – pyrazole ring stretching vibrations);
1355 (bCH – in-plane C–H bending vibrations); 1045 (pyr-
azole ring breathing); 940, 740 (dC–H). UV–Vis bands
(EtOH), kmax, nm (e, dm3 cmꢀ1 molꢀ1): 222 (8860), 203 (sh).
Bis(3,5-dimethylpyrazol-1-yl)methane (L2) was prepared
similarly. Yield 92%, m.p. 105–105.5 ꢁC (benzene–hexane,
1:1, lit. m.p. 105 ꢁC [13]). Anal. Calc. for C11H16N4: C,
64.68; H, 7.89; N, 27.43. Found: C, 63.88; H, 8.03; N,
1
26.82%. H NMR spectrum corresponds to literature data
[14]. IR bands, cmꢀ1: 1550, 1450 (mring); 1345 (bCH); 1125;
1020 (pyrazole ring breathing); 955, 765, 700. UV–Vis
bands (EtOH), kmax, nm (e, dm3 cmꢀ1 molꢀ1): 222 (6790),
206 (5900).
Bis(4-iodopyrazol-1-yl)methane (L6) was prepared as
previously described [15]. Briefly, a suspension of 3.50 g
(23.6 mmol) of L1, 4.80 g (18.9 mmol) of powdered iodine,
1.65 g (9.4 mmol) of HIO3 in 30 ml of glacial acetic acid
and 3 ml 30% aqueous H2SO4 was vigorously stirred for
15 min at 70 ꢁC until the reaction mixture became color-
less. The mixture was then poured into 250 ml of water,
the precipitate formed was filtered, washed with H2O,
and dried to yield 9.25 g (98%) of compound L6. M.p.
165–165.5 ꢁC (benzene–hexane, 4:1), Anal. Calc. for
C7H6N4I2: C, 21.02; H, 1.51; N, 14.01. Found: C, 21.47;
1
H, 1.93; N, 13.99%. H NMR (CDCl3): d, ppm: 6.15 (s,
2H, CH2), 7.49 (s, 2H, H3), 7.62 (s, 2H, H5). IR bands,
cmꢀ1: 1520 (mring); 1395 (bCH); 1310; 1000 (pyrazole ring
breathing). UV–Vis bands (EtOH), kmax
,
nm (e,
dm3 cmꢀ1 molꢀ1): 238 (8410), 202 (8410).
Bis(3,5-dimethyl-4-iodopyrazol-1-yl)methane (L7) was
prepared similarly. Yield 94%, m.p. 183–184 ꢁC (EtOH).
Anal. Calc. for C11H14N4I2: C, 29.27; H, 3.09; N, 12.28.
In this contribution we report the preparation of
complexes of L1, L2 ligands and novel bis(4-iodopyrazol-
1-yl)methane (L6) and bis(3,5-dimethyl-4-iodopyrazol-
1-yl)methane (L7) ligands with copper(II) nitrate and
1
Found: C, 29.34; H, 3.04; N, 12.08%. H NMR (CDCl3):

A.S. Potapov, A.I. Khlebnikov / Polyhedron 25 (2006) 2683–2690
2685
d, ppm: 2.12 (s, 6H, 3-CH3), 2.40 (s, 6H, 5-CH3), 6.08 (s,
2H, CH2). IR bands, cmꢀ1: 1525, 1460 (mring); 1370; 1335
(bCH); 1050 (pyrazole ring breathing). UV–Vis bands
(EtOH), kmax, nm (e, dm3 cmꢀ1 molꢀ1): 238 (6790), 204
(6560).
(m3 mode of NO3ꢀ); 1455 (mring); 1370 (bCH); 1280 (m3
mode of NO3ꢀ); 1040 (pyrazole ring breathing); 995
(m1 mode of NO3ꢀ); 820 (m2 mode of NO3ꢀ); 730, 690
(m4 mode of NO3ꢀ). UV–Vis bands (ethanol), kmax, nm
(e, dm3 cmꢀ1 molꢀ1): 935 (sh); 685 (120); 338 (sh); 256
(sh); 219 (16580). E1/2 = ꢀ137 mV, DE = 112 mV.
2.1.1. Bis(pyrazol-1-yl)methanedinitratocopper(II)
monohydrate (1)
2.2.3. Bis(4-iodopyrazol-1-yl)methanedinitratocopper(II) (3)
Cu(L6)(NO3)2. Yield 77%; m.p. 229–230 ꢁC. Anal. Calc.
for C7H6CuI2N6O6: C, 14.31; H, 1.03; N, 14.30. Found: C,
14.77; H, 1.30; N, 14.00%. IR bands, cmꢀ1: 1742, 1703
(m1 + m4 of NO3ꢀ); 1521 (mring); 1454 (m3 mode of NO3ꢀ);
1433 (mring); 1329 (bCH); 1293 (m3 mode of NO3ꢀ); 1032
(pyrazole ring breathing); 1003 (m1 mode of NO3ꢀ); 827
(m2 mode of NO3ꢀ); 782, 735 (m4 mode of NO3ꢀ).
A solution of 0.752 g (4.0 mmol) of L1 in 6 ml of acetone
was added to a solution of 0.592 g (4.0 mmol) of
Cu(NO3)2 Æ 3H2O in 3 ml of the same solvent. The immedi-
ately formed light-blue precipitate was filtered after 30 min,
thoroughly washed with acetone and dried in vacuo at
40 ꢁC. The yield of compound 1 was 0.800 g (57%).
Complexes 2–5, 7, 9, 10 were obtained similarly, whilst a
2:1 ligand to metal ratio was used in case of compounds 6
and 8.
2.2.4. Bis(3,5-dimethyl-4-iodopyrazol-1-yl)methane-
dinitratocopper(II) dihydrate (4)
2.1.2. Bis(pyrazol-1-yl)methane-2,20-bipyridyl-
nitratocopper(II) nitrate (11)
Cu(L7)(NO3)2 Æ 2H2O. Yield 94%; m.p. 222–223 ꢁC;
k = 14.5 Xꢀ1 cm2 molꢀ1 (acetone). Anal. Calc. for C11H18-
CuI2N6O8: C, 19.44; H, 2.67; N, 12.37. Found: C, 19.82;
A solution of 0.078 g (0.5 mmol) 2,20-bipyridyl in 1 ml
of acetone was added to a warmed-up (40 ꢁC) solution of
0.168 g (0.5 mmol) of the complex [Cu(L1)(O2NO)2] Æ H2O
(1) in 15 ml of acetone. The precipitated light-blue crystals
of compound 11 (yield 0.170 g, 69%) were filtered, washed
with acetone and dried in vacuo.
H, 2.95; N, 12.09%. IR bands, cmꢀ1: 3400br (mO–H
,
H2O); 1620 (dO–H, H2O); 1520 (m3 mode of NO3ꢀ); 1470
(mring); 1370 (bCH); 1265 (m3 mode of NO3ꢀ); 1070 (pyra-
zole ring breathing); 1000 (m1 mode of NO3ꢀ); 835 (m2 mode
of NO3ꢀ); 725, 700 (m4 mode of NO3ꢀ). UV–Vis bands (eth-
anol), kmax, nm (e, dm3 cmꢀ1 molꢀ1): 1030 (sh); 722 (50.0);
342 (sh); 234 (13860); 204 (27650). E1/2 = ꢀ176 mV,
DE = 183 mV.
Compounds 12–14 were prepared similarly from copper
complexes 2 or 3 and bidentate ligands (2,20-bipyridyl or
acetylacetone) in a 1:1 ratio. Mixed-ligand complexes 15–
18 were obtained upon mixing one equivalent of complex
2 with two equivalents of the corresponding monodentate
ligand in ethanol (compound 15), acetone (complexes 16,
17) or isopropyl alcohol (compound 18).
2.2.5. Bis(pyrazol-1-yl)methanedichlorocopper(II) (5)
Cu(L1)Cl2. Yield 93%; m.p. 266–267 ꢁC (lit. m.p. 264–
265 [3]); k = 15.0 Xꢀ1 cm2 molꢀ1 (acetone). Anal. Calc.
for C7H8Cl2CuN4: C, 29.75; H, 2.85; N, 19.82. Found: C,
29.55; H, 2.60; N, 20.10%. IR bands, cmꢀ1: 1500 (mring);
1260 (bCH); 1050 (pyrazole ring breathing). UV–Vis bands
(ethanol), kmax, nm (e, dm3 cmꢀ1 molꢀ1): 940 (sh); 765 (73);
334 (sh); 260 (sh); 215 (11630). E1/2 = ꢀ169 mV,
DE = 128 mV.
2.2. Analytical data
2.2.1. Bis(pyrazol-1-yl)methanedinitratocopper(II)
monohydrate (1)
Cu(L1)(NO3)2 Æ H2O. Yield 57%; m.p. 207 ꢁC; molar
conductivity k = 27.5 Xꢀ1 cm2 molꢀ1 (acetone). Anal. Calc.
for C7H10CuN6O7: C, 23.77; H, 2.85; N, 23.76. Found: C,
2.2.6. Bis{bis(pyrazol-1-yl)methane}copper(II) dichloride
dihydrate (6)
24.04; H, 2.86; N, 24.10%. IR bands, cmꢀ1: 3400br (mO–H
,
H2O); 2354, 2167 (m1 + m3 of NO3ꢀ); 1788, 1757 (m1 + m4 of
NO3ꢀ); 1618 (dO–H, H2O); 1514 (mring); 1464 (m3 mode of
NO3ꢀ); 1398 (mring); 1321 (bCH); 1285 (m3 mode of NO3ꢀ);
1045 (pyrazole ring breathing); 1017 (m1 mode of NO3ꢀ);
809 (m2 mode of NO3ꢀ); 760, 729 (m4 mode of NO3ꢀ). UV–
Vis bands (ethanol), kmax, nm (e, dm3 cmꢀ1 molꢀ1): 935
(sh), 685 (120), 338 (sh), 256 (sh), 219 (16580). Cyclic vol-
tammetry, E1/2 = ꢀ214 mV, DE = 182 mV.
Cu(L1)2Cl2 Æ 2H2O. Yield 90%; m.p. 180–182 ꢁC;
k = 11.7 Xꢀ1 cm2 molꢀ1 (acetone). Anal. Calc. for C14H20-
Cl2CuN8O2: C, 36.02; H, 4.32; N, 24.00. Found: C,
36.03; H, 4.16; N, 23.60%. IR bands, cmꢀ1: 3400br (mO–H
,
H2O); 1630 (dO–H, H2O); 1500, 1450 (mring); 1390 (bCH);
1045 (pyrazole ring breathing). UV–Vis bands (ethanol),
k
max, nm (e, dm3 cmꢀ1 molꢀ1): 1040 (sh); 765 (82); 337
(sh); 319 (163); 268 (2175); 220 (12120). E1/2 = ꢀ111 mV,
DE = 45 mV.
2.2.2. Bis(3,5-dimethylpyrazol-1-yl)methane-
dinitratocopper(II) (2)
2.2.7. Bis(3,5-dimethylpyrazol-1-yl)methane-
dichlorocopper(II) (7)
Cu(L2)Cl2. Yield 78%; m.p. 200–202 ꢁC; k = 52.3 Xꢀ1
cm2 molꢀ1 (acetone). Anal. Calc. for C11H16Cl2CuN4: C,
39.00; H, 4.76; N, 16.54. Found: C, 38.60; H, 4.78; N,
Cu(L2)(NO3)2. Yield 93%; m.p. 230–231 ꢁC; k =
19.1 Xꢀ1 cm2 molꢀ1 (acetone). Anal. Calc. for C11H16Cu-
N6O6: C, 33.72; H, 4.12; N, 21.45. Found: C, 34.20; H,
4.45; N, 20.98%. IR bands, cmꢀ1: 1540 (mring); 1500

2686
A.S. Potapov, A.I. Khlebnikov / Polyhedron 25 (2006) 2683–2690
16.90%. IR bands, cmꢀ1: 1540 (mring); 1260 (bCH); 1015
H, 4.45; N, 20.93%. IR bands, cmꢀ1: 2392, 2283 (m1 + m3
of NO3ꢀ); 1744, 1713 (m1 + m4 of NO3ꢀ); 1601 (mring of pyr-
idine); 1567 (mring); 1477 (m3 mode of NO3ꢀ); 1334 (bCH);
1288 (m3 mode of NO3ꢀ); 1159 (bCH of pyridine ring);
1041 (pyrazole ring breathing); 1018 (m1 mode of NO3ꢀ);
902 (cCH of pyridine ring); 833, 808 (m2 mode of NO3ꢀ);
(pyrazole ring breathing). UV–Vis bands (ethanol), kmax
,
nm (e, dm3 cmꢀ1 molꢀ1): 950 (sh); 740 (102); 335 (sh);
268 (2433); 222 (10260). E1/2 = ꢀ179 mV, DE = 140 mV.
2.2.8. Bis{bis(3,5-dimethylpyrazol-1-yl)methane}-
copper(II) dichloride dihydrate (8)
732, 705 (m4 mode of NO3ꢀ). UV–Vis bands (ethanol), kmax
,
Cu(L2)2Cl2 Æ 2H2O. Yield 73%; m.p. 174–175 ꢁC;
k = 52.1 Xꢀ1 cm2 molꢀ1 (acetone). Anal. Calc. for C22H36-
Cl2CuN8O2: C, 45.63; H, 6.27; N, 19.35. Found: C,
nm (e, dm3 cmꢀ1 molꢀ1): 910 (sh); 685 (74.4); 341 (sh); 313
(12410); 302 (13060); 243 (sh); 210 (35740). E1/2
ꢀ190 mV, DE = 157 mV.
=
45.62; H, 5.86; N, 19.20%. IR bands, cmꢀ1: 3430br (mO–H
,
H2O); 1610 (dO–H, H2O); 1550, 1460 (mring); 1370 (bCH);
2.2.13. Bis(4-iodopyrazol-1-yl)methane-2,20-bipyridyl-
nitratocopper(II) nitrate (13)
1020 (pyrazole ring breathing). UV–Vis bands (ethanol),
max, nm (e, dm3 cmꢀ1 molꢀ1): 945 (sh); 715 (168); 351
Cu(L6)(bipy)(NO3)2. Yield 46%; m.p. 203–206 ꢁC;
k = 76.5 Xꢀ1 cm2 molꢀ1 (acetone). Anal. Calc. for C17-
H14CuI2N8O6: C, 27.45; H, 1.90; N, 15.07. Found: C,
27.74; H, 2.07; N, 14.96%. IR bands, cmꢀ1: 2371, 2282
(m1 + m3 of NO3ꢀ); 1743, 1716 (m1 + m4 of NO3ꢀ); 1610,
1601 (mring of pyridine); 1567 (mring); 1497 (m3 mode of
NO3ꢀ); 1445 (mring); 1331 (bCH); 1293 (m3 mode of
NO3ꢀ); 1160 (bCH of pyridine ring); 1032 (pyrazole ring
breathing); 1014 (m1 mode of NO3ꢀ); 902 (cCH of pyri-
dine ring); 831, 812 (m2 mode of NO3ꢀ); 734, 706 (m4
mode of NO3ꢀ). UV–Vis bands (ethanol), kmax, nm (e,
dm3 cmꢀ1 molꢀ1): 910 (sh); 705 (107); 336 (sh); 311
(26460); 308 (30370); 290 (sh); 232 (sh); 210 (53520).
E1/2 = ꢀ150 mV, DE = 127 mV.
k
(sh); 265 (sh); 225 (30950). E1/2 = ꢀ202 mV, DE = 140 mV.
2.2.9. Bis(4-iodopyrazol-1-yl)methanedichlorocopper(II) (9)
Cu(L6)Cl2. Yield 93%; m.p. 267–269 ꢁC; k = 18.6 Xꢀ1
cm2 molꢀ1 (acetone). Anal. Calc. for C7H8Cl2CuI2N4: C,
15.73; H, 1.13; N, 10.48. Found: C, 15.80; H, 1.19; N,
10.90%. IR bands, cmꢀ1: 1500, 1430 (mring); 1360 (bCH);
1005 (pyrazole ring breathing). UV–Vis bands (ethanol),
k
max, nm (e, dm3 cmꢀ1 molꢀ1): 870 (38.7); 228 (15800).
2.2.10. Bis(3,5-dimethyl-4-iodopyrazol-1-yl)methane-
dichlorocopper(II) dihydrate (10)
Cu(L7)Cl2 Æ 2H2O.
Yield
88%;
m.p.
229 ꢁC;
k = 22.7 Xꢀ1 cm2 molꢀ1 (acetone). Anal. Calc. for
C11H18Cl2CuI2N4O2: C, 21.09; H, 2.90; N, 8.94. Found:
2.2.14. Bis(3,5-dimethylpyrazol-1-yl)methaneacetyl-
acetonatonitratocopper(II) (14)
C, 21.33; H, 3.10; N, 8.49%. IR bands, cmꢀ1: 3300br (mO–H
,
H2O); 1690 (dO–H, H2O); 1585, 1520, 1470 (mring); 1345
(bCH); 1070 (pyrazole ring breathing). UV–Vis bands (eth-
anol), kmax, nm (e, dm3 cmꢀ1 molꢀ1): 820 (67); 341 (sh); 238
(14460). E1/2 = ꢀ180 mV, DE = 164 mV.
Cu(L2)(acac)(NO3). Yield 67%; m.p. 177–178 ꢁC; k =
67.9 Xꢀ1 cm2 molꢀ1 (acetone). Anal. Calc. for C16H23-
CuN5O5: C, 44.80; H, 5.40; N, 16.33. Found: C, 44.35; H,
5.12; N, 16.78%. IR bands, cmꢀ1: 2392, 2283 (m1 + m3 of
NO3ꢀ); 1744, 1709 (m1 + m4 of NO3ꢀ); 1556 (mring); 1478 (m3
mode of NO3ꢀ); 1475 (mring); 1335 (bCH); 1288 (m3 mode
of NO3ꢀ); 1041 (pyrazole ring breathing); 1018 (m1 mode
of NO3ꢀ); 836, 807 (m2 mode of NO3ꢀ); 741, 704 (m4 mode
of NO3ꢀ). UV–Vis bands (ethanol), kmax, nm (e, dm3
cmꢀ1 molꢀ1): 1020 (sh); 680 (54.1); 340 (sh); 295 (5970);
245 (sh); 219 (sh). E1/2 = ꢀ187 mV, DE = 114 mV.
2.2.11. Bis(pyrazol-1-yl)methane-2,20-bipyridyl-
nitratocopper(II) nitrate (11)
Cu(L1)(bipy)(NO3)2. Yield 69%; m.p. 194–196 ꢁC; k =
52.6 Xꢀ1 cm2 molꢀ1 (acetone). Anal. Calc. for C17H16Cu-
N8O6: C, 41.51; H, 3.28; N, 22.78. Found: C, 41.90; H,
2.90; N, 22.30%. IR bands, cmꢀ1: 2371, 2278 (m1 + m3 of
NO3ꢀ); 1743, 1712 (m1 + m3 of NO3ꢀ); 1610, 1601 (mring of
pyridine); 1567 (mring); 1475 (m3 mode of NO3ꢀ); 1343
(bCH); 1279 (m3 mode of NO3ꢀ); 1160 (bCH of pyridine
ring); 1032 (pyrazole ring breathing); 1016 (m1 mode of
NO3ꢀ); 902 (cCH – out-of-plane C–H bending vibrations
of pyridine ring); 831, 812 (m2 mode of NO3ꢀ); 731, 705
(m4 mode of NO3ꢀ). UV–Vis bands (ethanol), kmax, nm (e,
dm3 cmꢀ1 molꢀ1): 880 (sh); 695 (65.3); 336 (sh); 312
2.2.15. Bis(3,5-dimethylpyrazol-1-yl)methane-
bis(triphenylphosphine)nitratocopper(II) nitrate (15)
Cu(L2)(PPh3)2(NO3)2. Yield 77%; m.p. 184–185 ꢁC; k =
88.1 Xꢀ1 cm2 molꢀ1 (acetone). Anal. Calc. for C47H46Cu-
N6O6P2: C, 61.60; H, 5.06; N, 9.17. Found: C, 61.65;
H, 5.07; N, 8.75%. IR bands, cmꢀ1: 1557 (mring); 1479
(m3 mode of NO3ꢀ); 1465 (mring); 1435 (mC@C); 1332
(bCH); 1281 (m3 mode of NO3ꢀ); 1097; 1049 (pyrazole
ring breathing); 1022 (m1 mode of NO3ꢀ); 836, 810 (m2
mode of NO3ꢀ); 743, 705 (m4 mode of NO3ꢀ); 694
(d-mode of PPh3); 521 (mC–P); 503 (y-mode of PPh3).
UV–Vis bands (methanol), kmax, nm (e, dm3 cmꢀ1 molꢀ1):
(16270); 300 (17370); 242 (sh); 213 (32850). E1/2
ꢀ153 mV, DE = 121 mV.
=
2.2.12. Bis(3,5-dimethylpyrazol-1-yl)methane-2,20-
bipyridylnitratocopper(II) nitrate (12)
Cu(L2)(bipy)(NO3)2. Yield 81%; m.p. 229–231 ꢁC; k =
81.6 Xꢀ1 cm2 molꢀ1 (acetone). Anal. Calc. for C21H24-
CuN8O6: C, 46.03; H, 4.41; N, 20.45. Found: C, 45.76;
700 (16.8); 340 (sh); 256 (sh); 215 (80910). E1/2
ꢀ171 mV, DE = 145 mV; E1/2 = ꢀ829 mV, DE = 244 mV.
=

A.S. Potapov, A.I. Khlebnikov / Polyhedron 25 (2006) 2683–2690
2687
2.2.16. Bis(3,5-dimethylpyrazol-1-yl)methane-
NMR spectra were recorded on a Bruker AV300
instrument.
bis{1H-benzimidazole}nitratocopper(II) nitrate (16)
Cu(L2)(BImH)2(NO3)2. Yield 84%; m.p. 312 ꢁC;
k = 55.6 Xꢀ1 cm2 molꢀ1 (ethanol). Anal. Calc. for
C25H28CuN10O6: C, 47.81; H, 4.49; N, 22.30. Found: C,
47.50; H, 4.08; N, 22.11%. IR bands, cmꢀ1: 3380br (mN–H
in BImH); 2388 (m1 + m3 of NO3ꢀ); 1712 (m1 + m4 of
NO3ꢀ); 1624 (mC–C); 1595, 1557 (mring); 1479 (m3 mode
of NO3ꢀ); 1465 (mring); 1332 (bCH); 1290 (m3 mode of
NO3ꢀ); 1041 (pyrazole ring breathing); 1012 (m1 mode
of NO3ꢀ); 835, 808 (m2 mode of NO3ꢀ); 748, 705 (m4 mode
Cyclic voltammograms were obtained on a TA-4 voltam-
metric analyzer (Tomanalyt, Tomsk, Russia). The three-
electrode electrochemical cell consisted of a mercury-film
working electrode and saturated Ag/AgCl reference and
auxiliary electrodes. The concentration of the sample in
the cell was 1.0 · 10ꢀ4 mol/dm3, and 0.1 mol/dm3 KCl
was used as a supporting electrolyte. The potential was var-
ied in the range 0 to ꢀ1.3 V at 50 mV sꢀ1 scan rate. Oxygen
was removed from the solutions by bubbling nitrogen
through immediately before any measurements. Formal
potentials were calculated as E1/2 = (Ep.c. + Ep.a.)/2.
of NO3ꢀ). UV–Vis bands (ethanol), kmax
, nm (e,
dm3 cmꢀ1 molꢀ1): 1060 (sh); 670 (68.2); 339 (sh); 279
(14710); 273 (13320); 267 (sh); 250 (12720); 244 (13720);
213 (26440). E1/2 = ꢀ171 mV, DE = 159 mV; E1/2
=
3. Results and discussion
ꢀ509 mV, DE = 294 mV.
3.1. Synthesis of the ligands
2.2.17. Bis(3,5-dimethylpyrazol-1-yl)methane-
bis{1H -benzotriazole}nitratocopper(II) nitrate (17)
Bis(pyrazol-1-yl)methanes L1 and L2 were first prepared
by Trofimenko [14]. We have developed a new procedure
for double alkylation of azoles via the reaction of 1H-pyr-
azole or 3,5-dimethyl-1H-pyrazole with dibromomethane
in a superbasic medium (DMSO-KOH). Iodo-derivatives
L6 and L7 were obtained by oxidative iodination of L1
and L2 by an I2–HIO3–H2SO4 mixture in acetic acid.
Cu(L2)(BtaH)2(NO3)2. Yield 43%; m.p. >330 ꢁC. Anal.
Calc. for C23H26CuN12O6: C, 43.84; H, 4.16; N, 26.68.
Found: C, 43.86; H, 3.87; N, 27.00%. IR bands, cmꢀ1
:
3380br (mN–H in BtaH); 1763, 1720 (m1 + m4 of NO3ꢀ);
1560 (mring); 1470 (m3 mode of NO3ꢀ); 1465 (mring); 1332
(bCH); 1283 (m3 mode of NO3ꢀ); 1195, 1153; 1049 (pyra-
zole ring breathing); 1011 (m1 mode of NO3ꢀ); 839, 830
(m2 mode of NO3ꢀ); 692 (m4 mode of NO3ꢀ).
3.2. Synthesis and characterization of bis(pyrazol-1-yl)-
methane complexes
2.2.18. Bis(3,5-dimethylpyrazol-1-yl)methanebis-
The complexes were prepared by the reaction of the
ligands with copper(II) nitrate or chloride in acetone solu-
tion. In the case of Cu(NO3)2, independent of the metal to
ligand ratio used (1:1 or 1:2), complexes 1–4 of 1:1 compo-
sition were formed. The same feature was observed for the
reaction of copper(II) chloride with the ligands L6 and L7
(complexes 9 and 10 were obtained). When CuCl2 reacted
with the ligands L1 and L2, 1:1 complexes 5 and 7, as well
as 1:2 complexes 6 and 8 were isolated depending on the
reagent ratio.
{3,5-dimethyl-1H-pyrazole}nitratocopper(II) nitrate (18)
Cu(L2)(Me2PyrH)2(NO3)2. Yield 50%; m.p. 237–238 ꢁC;
k = 52.5 Xꢀ1 cm2 molꢀ1 (ethanol). Anal. Calc. for C21H32-
CuN10O6: C, 43.18; H, 5.52; N, 23.98. Found: C, 42.95; H,
5.70; N, 24.02%. IR bands, cmꢀ1: 3300br (mN–H in Me2-
PyrH); 2393, 2283 (m1 + m3 of NO3ꢀ); 1747, 1713 (m1 + m4
of NO3ꢀ); 1557 (mring); 1479 (m3 mode of NO3ꢀ); 1467 (mring);
1333 (bCH); 1288 (m3 mode of NO3ꢀ); 1042 (pyrazole ring
breathing); 1017 (m1 mode of NO3ꢀ); 835, 809 (m2 mode of
NO3ꢀ); 733, 705 (m4 mode of NO3ꢀ). UV–Vis bands (etha-
nol), kmax nm (e, dm3 cmꢀ1 molꢀ1): 1030 (sh); 685 (59.6);
340 (sh); 280 (sh); 215 (26150). E1/2 = ꢀ193 mV, DE =
162 mV; E1/2 = ꢀ559 mV; E1/2 = ꢀ722 mV, DE = 41 mV.
All of the coordination compounds obtained gave satis-
factory elemental analysis and were characterized by molar
conductivity, infra-red and electronic spectroscopy.
The complexes of copper(II) nitrate, 1, 2 and 4, are weak
ꢀ
2.3. Physical measurements
electrolytes in acetone, therefore both of the NO3 ions are
bound to the central ion. The solubility of complex 3 was
insufficient for conductivity measurements.
Conductivities of solutions (c = 1.0 · 10ꢀ3 mol/dm3 in
acetone or ethanol) were measured in a temperature-con-
trolled cell with stainless steel electrodes, cell constant
Bis(pyrazol-1-yl)alkanes are bidentate in most of their
complexes, so the same coordination mode is to be
expected for the investigated compounds with L1, L2, L6
and L7. Bidentate coordination of bis(pyrazol-1-yl)meth-
ane derivatives in the prepared complexes can be assumed
from the absence of the splitting of pyrazole ring vibra-
tional bands in the IR spectra. This splitting should have
occurred if only one of pyrazole rings in bis(pyrazol-1-
yl)methane ligand was coordinated to the copper(II) ion.
The possibility of formation of polymeric complexes, in
which metal ions are connected by bridging ligands, can
u = 0.251 cmꢀ1
.
Elemental analyses were carried out on a Carlo Erba
analyzer.
IR spectra of solid samples as KBr pellets were recorded
on Specord 71 IR (in the range of 4600–650 cmꢀ1) and
Nicolet 5700 (4000–400 cmꢀ1) spectrophotometers.
Electronic absorption spectra were registered on Perkin–
Elmer 124 (in the range of 200–800 nm) and SF-26 (800–
1200 nm) spectrophotometers.

2688
A.S. Potapov, A.I. Khlebnikov / Polyhedron 25 (2006) 2683–2690
ꢀ
be ruled out by sharp melting points of the compounds
obtained. In the copper(II) nitrate complexes, the NO3
free NO3 is symmetry-forbidden, so this band does not
appear in IR spectra or has a very minute intensity [16].
Furthermore, weak combination bands (m1 + m4) and
(m1 + m3) with the splitting values D(m1 + m4) and
D(m1 + m3) in accord with those expected for bidentate
nitrate ion coordination [18] were detected in the spectra
of the complexes 1 and 3.
Therefore, in nitrate complexes 1–4 the bidentate
organic ligands occupy two coordination places, also each
of the nitrate ions occupies two places. Consequently, the
copper(II) ions in these complexes are hexacoordinated.
Locations of absorption bands in the region of d–d*
transitions in the electronic spectra of the complexes (a
shoulder in the 930–1030 nm area and a band around
685–720 nm) correspond very well to the literature values
for octahedrally surrounded Cu2+ [20]. For example, the
already mentioned [Cu(L2)2(O2NO)](NO3) exhibits an
absorption at 694 nm with a shoulder at 1042 nm [2].
ꢀ
ions may be coordinated in a mono- or bidentate fashion.
The coordination modes of nitrate ions can be established
by means of IR spectroscopy [16–19].
Bands corresponding to all of the vibrational modes of
the nitrate ion were detected in the IR spectra of complexes
1–4 (Table 1). Two intense bands in the 1455–1500 and
1265–1295 cmꢀ1 regions were assigned to stretching vibra-
ꢀ
tions of N–O bonds in a NO3 ion (m3) belonging to the C2v
symmetry group [17]. The bands of in-plane bending vibra-
tions (m4) appear at 690–735 and 725–780 cmꢀ1. Large
splitting of these bands (D(m3) > 160 and D(m4) > 25 cmꢀ1
)
confirms the presence of bidentate nitrate ions in all of
the four complexes. For monodentate coordination of
ꢀ
NO3 the splitting of the m3 band is not higher than
120 cmꢀ1 and that of m4 is less than 20 cmꢀ1 [17].
Single bands around 820 cmꢀ1 (out-of-plane bending
vibrations m2) as well as bands of medium intensity at about
1000 cmꢀ1 (symmetric stretching vibrations m1) were
observed in IR spectra of complexes 1–4 [17]. The occur-
rence of the latter band gives further evidence for the nitrate
ions coordination, since the vibrational transition m1 in the
3.3. Preparation of mixed-ligand complexes
We have also studied the reactivity of the synthesized
complexes towards various mono- and bidentate ligands,
Table 1
Characteristic frequencies of nitrate ion vibrations
Compound
Formula
m1
m2
m3
D(m3)
m4
D(m4)
m1 + m4
D(m1 + m4)
m1 + m3
D(m1 + m3)
1
Cu(L1)(NO3)2 Æ H2O
1017
809
1464
1285
179
760
729
31
1788
1757
31
2354
2167
187
2
3
Cu(L2)(NO3)2
1040
1003
1000
1016
1018
1014
1018
1022
1012
1011
1017
820
827
835
1500
1280
220
161
255
196
199
204
190
198
189
187
191
730
690
40
47
25
26
27
28
37
38
43
Cu(L6)(NO3)2
1454
1293
782
735
1742
1703
39
4
Cu(L7)(NO3)2 Æ 2H2O
Cu(L1)(bipy)(NO3)2
Cu(L2)(bipy)(NO3)2
Cu(L6)(bipy)(NO3)2
Cu(L2)(acac)(NO3)
Cu(L2)(PPh3)2(NO3)2
Cu(L2)(BImH)2(NO3)2
Cu(L2)(BtaH)2(NO3)2
Cu(L2)(Me2PyrH)2(NO3)2
1520
1265
725
700
11
12
13
14
15
16
17
18
831
812
1475
1279
731
705
1743
1712
31
31
27
35
2371
2278
93
109
89
833
808
1477
1288
732
705
1744
1713
2392
2283
831
812
1497
1293
734
706
1743
1716
2371
2282
836
807
1478
1288
741
704
1744
1709
2392
2283
109
836
810
1479
1281
743
705
835
808
1479
1290
748
705
1712
2388
839
830
1470
1283
692
1763
1720
43
34
835
809
1479
1288
733
705
28
1747
1713
2393
2283
110

A.S. Potapov, A.I. Khlebnikov / Polyhedron 25 (2006) 2683–2690
2689
namely 2,20-bipyridyl (bipy), acetylacetone (Hacac), triphe-
nylphosphine (PPh3), benzimidazole (BImH), benzotri-
azole (BtaH) and 3,5-dimethylpyrazole (Me2PyrH). These
compounds represent four different types of ligands – phos-
phorus-containing, O,O-bidentate, N,N-bidentate and
monodentate nitrogen-containing heterocycles, which are
widely spread among important biologically active sub-
stances. Mixed-ligand complexes with such compounds
may serve as models for metalloenzymes [21] and as build-
ing blocks for supramolecular systems [22].
We have found that the complexes of copper(II) nitrate
1–3 react with 2,20-bipyridyl with the formation of mixed-
ligand complexes 11–13, comprising of coordinated mole-
cules of bis(pyrazol-1-yl)methane derivatives as well as
those of 2,20-bipyridyl. Moreover, complex 2 reacts with
other above-mentioned compounds also forming mixed-
ligand complexes 14–18. At the same time, copper(II) chlo-
ride complexes 5–10, as well as the nitrate complex 4
undergo a displacement of bis(pyrazol-1-yl)methanes from
the coordination sphere yielding homoligand complexes.
These complexes are identical by melting points and spec-
tral characteristics to the compounds prepared under the
same conditions using copper(II) chloride or nitrate instead
of bis(pyrazol-1-yl)methane-copper(II) complexes.
plexes 11–13 spectra correspond to pyridine ring stretching
vibrations [24,25]. Triphenylphosphine characteristic bands
at 503 and 694 cmꢀ1 [26] were detected in the spectra of the
PPh3 derivative 15. Broad bands in the area of 3300–
3500 cmꢀ1 corresponding to N–H stretching vibrations
were detected in the spectra of the complexes 16–18, com-
prising of nitrogen-unsubstituted heterocycles. Strong
broadening of these bands may be explained by intra-
molecular hydrogen-bonding with coordinated nitrate ions.
The presence of these bands indicates that the heterocycles
are coordinated in their neutral, not anionic forms.
Acetylacetonate and 2,20-bipyridyl, as well as bis(pyra-
zol-1-yl)methane derivatives are certainly bidentate
ligands. It can be assumed that the coordination sphere
of the complexes 11–14 consists of two organic bidentate
ligands and one nitrate ion is also coordinated in a biden-
tate fashion as follows from the IR data (see example in
Fig. 2). The central copper(II) ion in these complexes is
therefore hexacoordinated, and the second nitrate ion
remains uncoordinated. This is consistent with the detec-
tion of two bands in the out-of-plane bending vibrations
area (800–830 cmꢀ1, Table 1). Presumably, one of them is
ꢀ
related to bound NO3 ions and the other is due to the free
nitrate ions. Apparently, the remaining bands of the free
nitrate ions overlap with the bands of organic ligands
and were not registered.
The same conclusions about hexacoordination of the
copper(II) ion can be made for the complexes 15–18 con-
taining two monodentate molecules of triphenylphosphine
or nitrogen-containing heterocycle, one bidentate molecule
of L2ꢀligand and one bidentate nitrate-ion, while the second
NO3 ion remains in the outer sphere (Fig. 3).
Bands due to bis(pyrazol-1-yl)methane ligand vibrations
as well as due to additional ligands were observed in the
IR-spectra of the mixed-ligand complexes 11–18. Strong
bands in the area of 1550–1560 cmꢀ1 account for pyrazole
ring stretching vibrations and bands at 1030–1050 cmꢀ1
correspond to ring breathing vibrations in bis(pyrazol-1-
yl)methane derivatives [23]. Absorption in the 1600 and
1610 cmꢀ1 regions of 2,20-bipyridyl mixed-ligand com-
+
N
Cu
H
N
-
NO3
C
N
N
N
O
H
O
N
O
N
+bipy
N
O
O
O
H
H
C
Cu
12
N
N
O
N
N
O
N
O
+Hacac
O
H
H
O
2
C
Cu
N
N
O
N
N
O
N
O
14
Fig. 2. Synthesis of bis(pyrazol-1-yl)methane-copper(II) mixed-ligand complexes with bidentate ligands.

2690
A.S. Potapov, A.I. Khlebnikov / Polyhedron 25 (2006) 2683–2690
+
O
N
L
O
L
H
H
H
H
O
+2L
-
Cu
Cu
NO3
C
C
N
N
N
N
N
N
O
O
N
N
O
O
N
N
O
O
(1)
15 (L=PPh3);16 (L=BImH);17(L=BtaH); 18 (L=Me2PyrH)
Fig. 3. Synthesis of bis(pyrazol-1-yl)methane-copper(II) mixed-ligand complexes with monodentate ligands.
3.4. Cyclic voltammetry
mixed-ligand complexes with these ligands have been
reported previously.
We have also carried out a cyclic voltammetric study of
the synthesized homo- and mixed-ligand coordination
compounds. Complexes of copper(II) nitrate and chloride
1–10 as well as mixed-ligand complexes 11–18 undergo a
quasi-reversible (or reversible in case of the complex 6)
reduction on a mercury-film electrode in the range of
potentials from ꢀ111 to ꢀ214 mV. Furthermore, cyclic
voltammograms of several mixed-ligand complexes demon-
strate additional peaks which are probably associated with
the electrochemical activity of the organic ligands. Thus,
the complex [Cu(L2)(PPh3)2(O2NO)](NO3) (15) undergoes
a quasi-reversible reduction at a formal potential of
ꢀ829 mV (relative to the saturated Ag/AgCl electrode)
and the complex [Cu(L2)(BImH)2(O2NO)](NO3) (16) is
reduced at ꢀ509 mV. In the case of [Cu(L2)(Me2PyrH)2-
(O2NO)](NO3) (18) an irreversible reduction wave at
ꢀ559 mV and a reversible one at a formal potential of
ꢀ722 mV were observed.
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Many copper complexes can mimic metalloenzymes,
superoxide dismutase (SOD) being one of them. Li et al.
pointed out that in order to act as a SOD mimic, the cop-
per complex should have a redox potential in the range
ꢀ0.36 . . . +0.69 V (vs. the saturated Ag/AgCl electrode)
[27]. Furthermore, the higher the redox potential E1/2 and
the greater the reversibility of the electrode process (char-
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antioxidant activity of a given complex. As is seen from
our cyclic voltammetry data, all of the complexes meet
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4. Conclusions
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24 (2003) 6071.
In summary, novel coordination compounds of copper(II)
with bis(pyrazol-1-yl)methane derivatives are reported in
this paper. The complexes synthesized reacted with a num-
ber of mono- and bidentate ligands yielding mixed-ligand
complexes. No examples of bis(pyrazol-1-yl)methane