Spectroscopic and electrochemical study of dinuclear and mononuclear copper complexes with the bidentate ligand of the 2,2′-diquinoline series

Article abstract of DOI:10.1007/s11172-010-0153-9

The newly synthesized complex (2) of copper(I) chloride with di-n-hexyl 2,2'-biquinoline-4,4'-dicarboxylate (L) was spectroscopically and electrochemically characterized. The X-ray diffraction study showed that the crystals of complex 2 consist of the dinuclear moieties [L₂Cu ¹ ₂(μ-Cl)₂] containing Cu ₂(μ-Cl)₂ clusters. Spectrophotometric studies and ESI-mass spec-trometric measurements showed that after the dissolution of complex 2 in acetonitrile (AN) and N-methyl-2-pyrrolidone (NMP), the solution contained not only the dinuclear complexes [L₂Cu¹ ₂(μ-Cl)₂] but also [L₂Cu¹]Cl, [LCu¹Cl(Sol)], and [Cu¹Cl(Sol)] (Sol is the solvent). The electrochemical data also confirm the conclusion that bridged dinuclear chloride complex 2 dissociates both in NMP and AN to form the tetrahedral bis-biquinoline complex [L₂Cu¹]Cl. In solutions of complex 2 in alcohols and _N,_N-dimethylformamide (DMF), only [L₂Cu¹]Cl and [Cu¹Cl(Sol)] are present. In EtOH, AN, and DMF, [Cu¹Cl(Sol)] undergoes disproportionation to [Cu¹¹Cl(Sol)] and Cu⁰.

Full text of DOI:10.1007/s11172-010-0153-9

Russian Chemical Bulletin, International Edition, Vol. 59, No. 4, pp. 724—732, April, 2010
724
Spectroscopic and electrochemical study of dinuclear and mononuclear
copper complexes with the bidentate ligand of the 2,2´ꢀbiquinoline series
S. Z. Vatsadze,a A. V. Dolganov,a A. V. Yakimanskii,b M. Ya. Goikhman,b I. V. Podeshvo,b
K. A. Lyssenko,c A. L. Maksimov,a and T. V. Magdesievaa
aDepartment of Chemistry , M. V. Lomonosov Moscow State University,
Build. 3, 1 Leninskie Gory, 119992 Moscow, Russian Federation.
Eꢀmail: tvm@org.chem.msu.ru
bInstitute of Macromolecular Compounds, Russian Academy of Sciences,
31 Bolshoi prosp., 199004 St. Petersburg, Russian Federation
cA. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences,
28 ul. Vavilova, 119991 Moscow, Russian Federation
The newly synthesized complex (2) of copper(I) chloride with diꢀnꢀhexyl 2,2´ꢀbiquinolineꢀ
4,4´ꢀdicarboxylate (L) was spectroscopically and electrochemically characterized. The Xꢀray
diffraction study showed that the crystals of complex 2 consist of the dinuclear moieties
[L2CuI (μꢀCl)2] containing Cu2(μꢀCl)2 clusters. Spectrophotometric studies and ESIꢀmass specꢀ
2
trometric measurements showed that after the dissolution of complex 2 in acetonitrile (AN) and
Nꢀmethylꢀ2ꢀpyrrolidone (NMP), the solution contained not only the dinuclear complexes
[L2CuI (μꢀCl)2] but also [L2CuI]Cl, [LCuICl(Sol)], and [CuICl(Sol)] (Sol is the solvent). The
2
electrochemical data also confirm the conclusion that bridged dinuclear chloride complex 2
dissociates both in NMP and AN to form the tetrahedral bisꢀbiquinoline complex [L2CuI]Cl. In
solutions of complex 2 in alcohols and N,Nꢀdimethylformamide (DMF), only [L2CuI]Cl and
[CuICl(Sol)] are present. In EtOH, AN, and DMF, [CuICl(Sol)] undergoes disproportionation
to [CuIICl(Sol)] and Cu0.
Key words: crystal engineering, supramolecular chemistry, coordination compounds,
2,2´ꢀbiquinolines, Xꢀray diffraction study, mass spectrometry, electrochemistry, spectroꢀ
photometry.
Copper coordination compounds with 1,4ꢀdiimineꢀ
ligand bonds, it is possible to design systems, in which
reversible transitions between two or several states can
occur as a result of the replacement of anions by various
coordinating molecules, primarily, by solvents.
2,2´ꢀBiquinoline (biq) is the well known 1,4ꢀdiimineꢀ
type ligand. In the studies7—9 on the monovalent copper
complexes of biquinolines with different chemical strucꢀ
tures, it was noted that these systems hold promise as
catalysts for the polymerization, photocatalytic devices,
and biologically active compounds.
In terms of crystal engineering, copper(II) complexes
with 2,2´ꢀbipyridine (bipy), phenanthroline (phen), and
their derivatives related to 2,2´ꢀbiquinoline were studied
in most detail.10,11 In these systems, the chlorineꢀbridged
dinuclear complexes [L2CuII2(μꢀCl)2]}2+ (L = bipy10,12—18
or phen11,19—24) containing the metal atom in a squareꢀ
pyramidal coordination environment belong to the most
widespread structural type.
type ligands have attracted interest because they can be
used to study the relationship between the redox, magnetꢀ
ic, and spectroscopic properties of the complexes, on the
one hand, and the coordination geometry of the metal
atom, on the other hand.1 The ability to control the aboveꢀ
mentioned properties by varying the coordination enviꢀ
ronment of the metal atoms and to control the coordinaꢀ
tion of the complexes by changing the external conditions
allows one to design aesthetically attractive and functionꢀ
ally promising structures.2,3 In particular, the reversible
CuI/CuII redox transition can be used to influence the
←
macroscopic solution gel transitions. The ability to conꢀ
→
trol the solidꢀstate structures of the complexes is a key
concept of the soꢀcalled crystal engineering, whose main
aim is to synthesize desired crystal structures from approꢀ
priately chosen molecular components.5,6 The use of bridgꢀ
ing ligands as linkages between individual molecules of
solidꢀstate complexes is an equally important problem.
This mainly refers to simple acido ligands, such as halides
and carboxylates. Due to the lability of the copper—acido
Copper complexes with ligands of the 2,2´ꢀbiquinoline
series have not been studied in terms of crystal engineerꢀ
ing. We found the only characterized crystal structure of
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 710—718, April, 2010.
1066ꢀ5285/10/5904ꢀ0724 © 2010 Springer Science+Business Media, Inc.

Copper(II/I) 2,2´ꢀbiquinolyl complexes
Russ.Chem.Bull., Int.Ed., Vol. 59, No. 4, April, 2010
725
Scheme 1
the mononuclear complex of 2,2´ꢀbiquinoline with CuCl2,
in which the metal atom is in a distorted tetrahedral coorꢀ
dination.25 Based on numerous published data on the haloꢀ
genꢀbridged structures, it can be expected that biquinoline
complexes with CuCl can form dinuclear acido clusters
[L2CuI (μꢀCl)2] in crystals and, probably, in solution.
2
The presence of carboxy groups in a heterocyclic ligand
has a high potential for its further functionalization.
Among these transformations, the esterification is of parꢀ
ticular interest. Actually, a change in the lengths of the
alkyl groups in alcohols used for the esterification makes it
possible to control intermolecular interactions in solids.
The stacking interaction between the π systems of the quinꢀ
oline rings is another factor determining the mutual
arrangement of the molecules in solidꢀstate structures.
In turn, the interactions of complexes with solvent moleꢀ
cules can break interactions characteristic of crystals and,
as a consequence, can lead to substantial changes in the
coordination sphere of the metal atoms and the properties
of the metal complexes.
Several electrochemical studies26,27 of copperꢀcontainꢀ
ing complexes with biquinolines and bipyridines were pubꢀ
lished. These studies were focused primarily on the relaꢀ
tionship between the electrochemical oxidation potentials
of the complexes and their structures in solution, which
strongly depend on the nature of the solvent.28 The elecꢀ
trochemical data27 provide evidence on the possible strucꢀ
tures of the complexes in solution, in particular, on the
presence of chlorideꢀbridged dinuclear structures.
Table 1. Principal crystallographic parameters and the structure
refinement statistics for compounds 1 and 2
Parameter
1
2
Molecular formula
Molecular weight
T/K
C32H36N2O4 C64H72Cl2Cu2N4O8
512.63
100
1223.24
100
It should be noted that the structures and stability of
copper complexes with biquinoline ligands in the solid
state have not been compared with those in solution. The
main aims of the present study were to determine the crysꢀ
tal structure of the complex of copper(I) chloride with the
bidentate ligand dihexyl 2,2´ꢀbiquinolineꢀ4,4´ꢀdicarbꢀ
oxylate and to investigate the possible equilibria involving
copperꢀcontaining particles in different solvents by specꢀ
troscopic and electrochemical methods.
Crystal system
Space group
Z (Z´)
a/Å
b/Å
c/Å
α/deg
β/deg
Monoclinic
P21/c
Triclinic
Pꢀ1
1 (0.5)
2 (1´)
8.2391(14)
16.767(4)
9.618(3)
90.00
94.669(8)
90.00
1324.3(6)
1.286
0.85
8.3100(13)
13.484(2)
14.907(4)
113.011(6)
102.776(5)
99.724(4)
1437.4(5)
1.413
γ/deg
V/Å3
dcalc/g cm–3
μ/cm–1
Results and Discussion
8.93
F(000)
548
640
2θmax/deg
Number of measured
reflections
Number of independent
reflections
Number of reflections
with I > 2σ(I)
Number of refined
parameters
R1
54
6578
54
9708
Diꢀnꢀhexyl 2,2´ꢀbiquinolineꢀ4,4´ꢀdicarboxylate (1,
hereinafter denoted L) was synthesized in 55% yield
(Scheme 1).
2973
1464
173
6204
4143
363
Crystals of the ligand suitable for Xꢀray diffraction were
isolated from a solution of the complex [L2CuI (μꢀCl)2]
2
(2) in DMF (Table 1). In the crystal structure, centrosymꢀ
metric molecules 1 adopt a planar anti conformation (with
respect to the C(1)—C(1A) bond) due apparently to the fact
that there is a repulsion of the hydrogen atoms in the
possible syn conformation (Fig. 1). The ester group lies in the
plane of the aromatic ring; the C(2)—C(3)—C(11)—O(1)
torsion angle is 10.1°. A slight twist of the group is apꢀ
parently attributed to the rather strong intramolecular
C—H...O interaction (H...O, 2.12 Å) with the hydrogen
0.0565
0.1437
0.914
0.0482
0.1073
0.953
wR2
GOF
Residual electron
density
0.250/–0.241
0.459/–0.362
/e•Å–3, ρmax/ρ
min

726
Russ.Chem.Bull., Int.Ed., Vol. 59, No. 4, April, 2010
Vatsadze et al.
C(16A)
C(14A)
C(17A)
C(12A)
C(15A)
O(1A)
C(11A)
C(13A)
C(7)
C(8)
C(6)
O(2A)
C(2A)
N(1)
C(9)
C(3A)
C(1A)
C(4A)
C(5)
O(2)
C(1)
C(5A)
C(6A)
C(4)
C(3)
N(1A)
C(2)
C(9A)
C(8A)
C(11)
C(7A)
C(12)
C(14)
C(13)
O(1)
C(16)
C(15)
C(17)
Fig. 1. Molecular structure of compound 1 with thermal ellipsoids (p = 50%).
atom at the C(5) atom. The hexyl substituent has a planar
zigzag conformation; the C—C—C—C torsion angles vary
in the range of 174—177°. In the crystals, molecules 1 are
involved not only in weak intermolecular C—H...π and
C—H...O interactions but also in interactions between the
π system of the heterocycle and the C=O bond with the
C(6)...C(11) distance (1 – x, 1 – y, 2 – z) equal to 3.326(1) Å.
Complex 2 of ligand 1 with copper(I) chloride was
synthesized by refluxing the reagents in a ratio of 1 : 1 in
EtOH in the presence of an excess of hydroxylamine and
ammonia. The recrystallization of the product from EtOH
afforded small needleꢀlike crystals suitable for Xꢀray difꢀ
fraction. According to the Xꢀray diffraction data, the
crystal structure of 2 consists of the dinuclear complexꢀ
Fig. 3) to form chains along the crystallographic axis a.
In turn, the chains are linked together by intermolecuꢀ
lar C=O...π interactions like those observed in 1 and by
H...Cl and H...O interactions to form a threeꢀdimensional
framework.
A comparison of the singleꢀcrystal Xꢀray diffraction
data and the Xꢀray powder diffraction data for complex 2
showed that the single crystals and the major phase of
the product are identical in composition. Hence, the synꢀ
thesis according to the aboveꢀdescribed procedure affords
only complex 2.
Table 2. Selected bond lengths (d) and bond
angles (ω) in the structure of 2
es [L2CuI (μꢀCl)2] containing Cu2(μꢀCl)2 clusters (see
2
Tables 1 and 2, Fig. 2). Selected geometric parameters of
the biquinoline ligand in complex 2 are virtually equal to
those of 1. The fragments of two biquinoline ligands linked
together by the fourꢀmembered metallocycle Cu2Cl2 are
coplanar but are shifted with respect to each other in such
a way that they form a ladder motif with the distance
between the planes equal to 1.95 Å. This shift is apparentꢀ
ly associated with the tendency to reduce the steric repulꢀ
sion between the hydrogen atoms of adjacent ligands (analꢀ
ogous ladder structures were found in the complexes of biq
with nickel(II)29 and manganese(II)30 chlorides). The fiveꢀ
membered chelate ring adopts an envelope conformaꢀ
tion with the copper atom deviating from the mean
N(1)C(1)C(10)N(2) plane by 0.32 Å. As opposed to 1, the
ester groups form intramolecular C—H...O contacts not
only with the hydrogen atom at C(5) but also with the
hydrogen atom at C(11). In the crystals, molecules 2
are linked together through relatively strong stacking
interactions (the shortest C(7)...C(18) distance is 3.27 Å,
Parameter
Value
Bond length
d/Å
Cu(1)—N(1)
Cu(1)—N(2)
Cu(1)—Cl(1А)
Cu(1)—Cl(1)
2.054(2)
2.068(2)
2.2781(9)
2.3932(10)
Angle
ω/deg
N(1)—Cu(1)—N(2)
N(1)—Cu(1)—Cl(1А)
N(2)—Cu(1)—Cl(1А)
N(1)—Cu(1)—Cl(1)
N(2)—Cu(1)—Cl(1)
Cl(1А)—Cu(1)—Cl(1)
Cu(1А)—Cl(1)—Cu(1)
C(1)—N(1)—Cu(1)
C(9)—N(1)—Cu(1)
C(10)—N(2)—Cu(1)
C(18)—N(2)—Cu(1)
79.19(10)
128.24(7)
125.82(7)
108.76(7)
112.67(7)
101.30(3)
78.70(3)
114.40(2)
126.07(19)
114.00(2)
126.20(2)

Copper(II/I) 2,2´ꢀbiquinolyl complexes
Russ.Chem.Bull., Int.Ed., Vol. 59, No. 4, April, 2010
727
C(6)
C(5)
O(1)
C(7)
C(4)
C(19)
Cl(1)
C(3)
C(2)
C(8)
Cu(1A)
O(2)
C(21)
C(20)
Cu(1)
N(1)
C(1)
C(10)
C(23)
N(2)
Cl(1A)
C(18)
H(21A)
H(23A)
C(11)
O(3)
C(12)
C(25)
C(17)
C(13)
C(26)
O(4)
C(27)
C(16)
C(15)
C(14)
C(29)
C(31)
C(28)
C(30)
C(32)
Fig. 2. Molecular structure of 2.
The hydrophobic hexyl "tails" are responsible for the
good solubility of compound 2 in organic solvents. Howꢀ
ever, this factor is substantially reduced by the stacking
interactions between the aromatic rings. Thus, complex 2
is very poorly soluble in chlorineꢀcontaining solvents, acꢀ
etonitrile (AN), and alcohols and is slightly better soluble
in dimethylformamide (DMF) and Nꢀmethylꢀ2ꢀpyrꢀ
rolidone (NMP).
nating solvent with the bidentate ligands would be expectꢀ
ed to give rise to the mononuclear species [LCuICl(Sol)x]
(3•Sol) and even lead to the complete dissociation of the
complex to form solvated copper(I) chloride (4a•Sol) and
free ligand 1. For the copper complexes with biq and phen,
the ligand metathesis giving a twoꢀligand complex and
solvated copper(I) halide was documented.27,28 In some
solvents, monovalent copper chloride (4a•Sol) can unꢀ
dergo disproportionation to copper(II) chloride (4b•Sol)
and elemental copper Cu0. The oxidation of the copper(I)
complex [L2Cu]+ (5a) to the copper(II) complex [L2Cu]2+
(5b), which is not accompanied by a change in its composiꢀ
The dissolution of complexes of type 2 can occur in
several steps (Scheme 2). First, the dissolution of the comꢀ
plex [L2CuI (μꢀCl)2] is not accompanied by a change in
2
its composition. Second, the competition of the coordiꢀ
Scheme 2
Sol is the solvent

728
Russ.Chem.Bull., Int.Ed., Vol. 59, No. 4, April, 2010
Vatsadze et al.
N(1A)
Cl(1)
N(2A)
Cu(1A)
Cl(1A)
C(7)
C(16)
C(17)
C(8)
Cu(1)
N(1)
C(6)
N(1B)
C(18)
C(15)
C(9)
N(2B)
C(14)
N(2)
C(5)
C(13)
C(1)
C(4)
C(10)
C(11)
Fig. 3. Stacking interactions between the carbocyclic moieties of the quinoline ligands in the crystal structure of 2.
C(12)
C(3)
C(2)
tion, can occur both under electrochemical conditions and
in the presence of atmospheric oxygen (see Scheme 2).
In the present study, the behavior of complex 2 in
solution was investigated by UV—Vis spectroscopy, cyclic
voltammetry, and electrospray ionization mass spectroꢀ
metry (ESIꢀMS). The 1H NMR spectra provide little inforꢀ
mation because of the low solubility of the complex and
the presence of paramagnetic impurities.
the species responsible for this color is different in differꢀ
ent solvents. Thus, no noticeable changes in the spectra
are observed in alcohols during several hours. In the other
solvents, the color of the complex changes with time from
violet to paleꢀyellow (Fig. 4). The rate of the change in the
color decreases in the series NMP > DMF > AN.
The spectrophotometric titration of a solution of the
ligand with a solution of copper(I) chloride showed the
presence of several types of species (Fig. 5). In the presꢀ
ence of a ninefold excess of copper(I) chloride, the major
absorption band is observed at 445 nm, and the band at
580 nm has a low intensity. In the presence of an excess of
the ligand, these absorption bands are similar in intensity.
Based on the published data,9,28,31,32 the band at 580 nm
should be assigned to MLCT of the tetrahedral mononuꢀ
clear complex [L2Cu]Cl ([5a]Cl). Taking into account
Mass spectrometry. The ESIꢀMS data obtained in the
positive ion mode are presented in Table 3. The assignꢀ
ment of the peaks was made based on the character of the
molecular mass distribution of the corresponding ions.
The spectra of all solutions show the major peak at
m/z 1087.5 assigned to the [L2Cu]+ ion (5a). The involꢀ
vement of the solvent molecules in the equilibrium is
evidenced by the fact that the spectrum contains the
[LCu(Sol)]+ ions (3•Sol – Cl), whose intensity varies
from 10 to 20% of the major signal. In addition, the specꢀ
tra recorded in DMF and NMP contain ion peaks correꢀ
sponding to the protonated ligand, whose intensity inꢀ
creases with time. The signal of the [L2Cu2Cl]+ ion
(2 – Cl) assigned to the dinuclear complexes is observed
in a solution of the complex in acetonitrile and in a freshly
prepared solution in NMP. The intensity of this peak varꢀ
ies from 5 to 10% of the intensity of the major ion peak. It
should be noted that the peak corresponding to the comꢀ
plex with the composition [L2Cu]+ (5a) is absent in the
spectrum recorded after the mixing of acetonitrile soluꢀ
tions of ligand 1 and [Cu(BF4)2•(H2O)х], and the specꢀ
trum shows only the peak of the [LCuH2O]+ ion that is
formed as a result of the reduction of the complex under
the conditions of the analysis.
Table 3. Composition of the solutions of complex 2 determined
by ESIꢀMS
Solvent
NMP
m/z [Ion]
1187.4
1087.5
674.3
513.2
[L2Cu2Cl]+ [L2Cu]+ [LCu(NMP)]+ [LH]+
MeOH
AN*
—
1087.5
[L2Cu]+
1087.5
—
—
—
1187.4
616.2
[L2Cu2Cl]+ [L2Cu]+ [LCu(AN)]+
DMF
AN**
—
—
1087.5
[L2Cu]+
—
—
513.2
[LH]+
—
593.2
[LCuH2O]+
* After storage in air, a peak of the [LCuCl]+ ion corresponding
to the CuII complex appeared. This peak (along with the peak of
the monovalent copper complex [L2Cu]+) is the major peak in
the solution prepared by mixing L and CuCl2 in AN.
** In the study, a solution prepared by mixing solutions of
[Cu(BF4)2•(H2O)х] and ligand 1 was used.
Spectrophotometry. Solutions of complex 2 in EtOH,
MeOH, DMF, NMP, and AN are characterized by the
violet color, which is attributed to a relatively intense metꢀ
alꢀtoꢀligand chargeꢀtransfer band (MLCT,27,28 Table 4).
However, the stability of the color and, consequently, of

Copper(II/I) 2,2´ꢀbiquinolyl complexes
Russ.Chem.Bull., Int.Ed., Vol. 59, No. 4, April, 2010
A
729
A
580 nm
0.25
0.20
0.15
0.20
0.15
0.10
0.05
0.10
2
1
0.05
3
0
400 450 500 550 600 650 700 750
λ/nm
Fig. 4. Time evolution of the UV—Vis spectrum of a solution of
complex 2 in NMP (changes in the color during 10 min).
400 450 500 550 600 650 700
λ/nm
Fig. 5. Spectrophotometric titration of a solution of 1 with
a solution of copper chloride in AN at different ligandꢀtoꢀmetal
ratios: 1 : 4 (1), 1 : 1 (2), and 9 : 1 (3).
this fact, the band at 445 nm can be assigned to the dinuꢀ
clear complex [L2Cu2Cl2] (2). A similar value was reportꢀ
ed in the study27 for the dinuclear complex of CuCl with
phen. The absence of isosbestic points in Fig. 5 is indicaꢀ
tive of a complex system of equilibria in solution, which is
consistent with Scheme 2.
Cyclic voltammetry. The electrochemical behavior of
complex 2 was studied in two solvents (AN and NMP).
Figure 6, a shows the cyclic voltammogram of comꢀ
plex 2 in AN.
Two quasiꢀreversible peaks are observed in the anodic
scan at Е = 0.74 and 1.15 V (see Fig. 6, a). The oxidation
of the complex occurs at a relatively high potential
(0.74 V), which is characteristic of sterically crowded tetraꢀ
azaꢀcoordinated CuI complexes.26,31,33 The second oxiꢀ
a
I/μA
15
10
5
0
–5
Table 4. UV—Vis data for solutions of complex 2 and related
compounds
0
200 400 600 800 1000 1200 E/mV
Complex
Solvent
AN
λ
/nm
Reference
*
max
I/μA
b
2
445,
580
580
445 (sh)
580
580
580
524,
552
546
545
–50
–55
–60
–65
–70
EtOH
NMP
*
*
Me2CO
NMP
DMF
*
*
31
[L2Cu]BF4
[(biq)2Cu]BF4
[(biq)2Cu]ClO4
[(biq)2Cu]Х
(X = Cl, Br, I)
[(4ꢀbiqꢀCOONa)2Cu]Cl**
[(phen)CuCl]2
AN
9
28
Me2CO
EtOH
H2O
545
565
445
32
27
0
200
400
600
800
1000 E/mV
Me2CO
* The present study.
** 4ꢀbiqꢀCOONa is 2,2´ꢀbiquinolineꢀ4,4´ꢀdicarboxylic acid
disodium salt.
Fig. 6. Cyclic voltammograms of complex 2 in MeCN (a) and
NMP (b) (Pt, 0.05 М Bu4NBF4, 100 mV s–1, with respect to
Ag/AgCl/KCl).

730
Russ.Chem.Bull., Int.Ed., Vol. 59, No. 4, April, 2010
Vatsadze et al.
dation peak at Е = 1.15 V can be assigned to either the
oxidation of the second copper center in bridged dinuclear
complex 2 (similar examples were published in the literaꢀ
ture27) or the transformation products of 2 in solution (see
Scheme 2). Thus, the chlorideꢀbridged copper(I) phen
complexes in Me2CO exhibit two reversible peaks with the
same current intensities during electrochemical oxidꢀ
ation;27 the nature of the peaks was explained by the sucꢀ
cessive oxidation of the copper centers in the bridged diꢀ
nuclear complexes stable in this medium.
In our study, the current of the first oxidation peak is
two times higher than that of the second peak (see Fig. 6, a).
This experimental fact is inconsistent with the successive
oxidation of two copper atoms in the bridged dinuclear
complexes; otherwise, the peaks should be equal in height.
In our opinion, bridged dinuclear copper complex 2 unꢀ
dergoes dissociation upon dissolution in AN (see Scheme 2).
In the first step, the chloride bridges in complex 2 are
cleaved to form the complex 3•Sol, in which the chloride
ion and the solvent molecules are exchanged for the biquinꢀ
oline ligand (ligand metathesis) to form mononuclear
copper bisꢀbiquinoline complex 5a and the complex
4a•Sol. It is known34 that CuCl is unstable in AN and
readily undergoes disproportionation to copper metal
and copper(II) chloride. The control experiment on the
electrochemical behavior of CuCl2 in AN showed the
presence of a reversible oxidation peak at +1.15 V correꢀ
sponding to the CuII/CuIII redox transition. Hence, the
reversible oxidation of complex 5a to complex 5b occurs at
Е = +0.74 V (see Scheme 2), and the reversible oxidation
of CuCl2 that is formed in solution occurs at +1.15 V (4b,
see Fig. 6, a).
The stoichiometry of the aboveꢀconsidered steps sugꢀ
gests that the current of the second oxidation peak should
be two times smaller than that of the first peak, which was
confirmed experimentally (see Fig. 6, a). The ratio of the
heights of the direct and reverse peaks for the first redox
transition is close to the unity, which indicates that comꢀ
plex 5b is stable on the cyclic voltammetry time scale. The
potential difference between the direct and reverse oxidꢀ
ation peaks is 150 and 105 mV for the first and second
oxidation peaks, respectively. This difference can be atꢀ
tributed to a change in the geometry of the coordination
unit attendant on the redox transition.
The formation of the tetrahedral CuI complex conꢀ
taining two biquinoline ligands in the coordination enviꢀ
ronment of the copper ion (5a) in solution is in good
agreement with our previous data.35 The anodic dissoluꢀ
tion of copper in the presence of diester of 2,2´ꢀbiquinolylꢀ
4,4´ꢀdicarboxylic acid (4ꢀbiqꢀCOOR) affords the comꢀ
plex [(4ꢀbiqꢀCOOR)2CuI]BF4 (5a•BF4), whose structure
was established by different physicochemical methods
(UV—Vis, MALDIꢀTOF mass spectrometry, and cyclic voltꢀ
ammetry). The cyclic voltammogram of this complex
shows an oxidation peak in the anodic scan at the same
potential (Е = 0.75 V)35 as that observed for a solution
of 2 in AN.
The electrochemical behavior of complex 2 was studꢀ
ied also in NMP. The corresponding cyclic voltammoꢀ
gram is shown in Fig. 6, b.
In the anodic potential scan, only one quasiꢀreversible
peak is observed at E = 0.74 V, which corresponds to the
←
transition 5a 5b. The reverse reduction peak is seen on
→
the reverse scan (the current ratio of the direct and reverse
waves Ic/Ia = 0.7). This is indicative of the lower stability
of the oxidized form of the complex in NMP compared to
AN. The potential difference between the direct and
reverse oxidation peaks is relatively large (320 mV). This is
evidence that the electron transfer is followed by the chemꢀ
ical step or an essential structural reorganization of the
complex.36 Apparently, the oxidation of the copper comꢀ
plex in NMP is accompanied by the elimination of one of
the biquinoline ligands from complex 5b and the stabiliꢀ
zation of the resulting complex 3•Sol by NMP moleꢀ
cules. Previously, our calculations37 of the energies of
the formation of the complexes [(biq)2Cu]2+ and
[(biq)Cu(NMP)2]2+ by the DFT method have shown that
these energies are similar in the magnitude (–458.6 and
–454.8 kcal mol–1, respectively).
Therefore, the above data suggest that the dinuclear
structure of chlorideꢀbridged complex 2 is destroyed both
in NMP and AN to form tetrahedral bisꢀbiquinoline comꢀ
plex 5a as the major product (see Scheme 2). The main
difference in the electrochemical behavior of complex 2 in
NMP and AN is that the oxidation peak of CuCl2 is absent
in the former case. This is attributed to the fact that CuCl
that is formed in NMP in the course of the ligand metꢀ
athesis does not undergo disproportionation; instead, it
precipitates due to the low solubility in this solvent. The fact
that dinuclear complex 2 is not observed in electrochemiꢀ
cal experiments is associated with that the electrochemical
experiments were carried out in the presence of a 100ꢀfold
excess of the supporting electrolyte (tetrabutylammonium
tetrafluoroborate), which apparently additionally faciliꢀ
tates the cleavage of the chloride bridges in complex 2.
To summarize the results obtained with the use of the
aboveꢀdescribed three methods, the following conclusions
can be drawn.
(1) The L/Cu/Cl system crystallizes from EtOH and
AN only as dinuclear molecules 2.
(2) After the dissolution of complex 2 in AN and NMP,
the solutions contained the [L2Cu2(μꢀCl)2] (an absorpꢀ
tion band at 445 nm; the presence of the [L2Cu2Cl]+ ion
in the mass spectrum), [L2Cu]Cl (an absorption band at
580 nm; the presence of the [L2Cu]+ ion in the mass specꢀ
trum), [LCuCl(Sol)] (the presence of the corresponding
ions in the mass spectra), and [CuCl(Sol)] (observed only
in AN) species.
(3) Solutions of complex 2 in alcohols and DMF conꢀ
tain only [L2Cu]Cl and [CuCl(Sol)].

Copper(II/I) 2,2´ꢀbiquinolyl complexes
Russ.Chem.Bull., Int.Ed., Vol. 59, No. 4, April, 2010
731
solution (400 mL) were placed in a oneꢀneck roundꢀbottom flask.
The solution was refluxed for 24 h. The resulting salt was filtered
off, washed with an alkali solution, dissolved in water, and acidꢀ
ified with a weak hydrochloric acid solution. The crystals that
formed were filtered off, washed with water, dried, and purified
by repricipitation from a potassium hydroxide solution. The yield
was 78%, m.p. 300—301 °C (cf. lit. data38: m.p. 300—302 °C).
Dihexyl 2,2´ꢀbiquinolineꢀ4,4´ꢀdicarboxylate (1). 2,2´ꢀBiquinꢀ
olineꢀ4,4´ꢀdicarboxylic acid (1 g) and nꢀhexanol (10 mL) were
placed in a oneꢀneck roundꢀbottom flask equipped with a reflux
condenser, and then concentrated sulfuric acid (2 mL) was addꢀ
ed dropwise. The reaction mixture was refluxed for 1.5 h, cooled,
and poured into water (100 mL). The precipitate that formed
was filtered off, washed with water, dried, and twice recrystalꢀ
lized from EtOH. The yield was 55%, m.p. 110 °C (cf. lit. data32
:
m.p. 115 °C). Attempts to isolate ester 1 in the analytically pure
form failed, and it was used for the synthesis of complex 2 withꢀ
out additional purification.
100 nm
Reaction of compound 1 with copper(I) chloride. A solution of
hydroxylamine hydrochloride (3 g, 43 mmol) in water (15 mL)
was brought to pH = 5.5 with a 10% ammonia solution. Then
EtOH (50 mL) and CuCl (0.27 g, 2.7 mmol) were added. The
resulting solution was added to a solution of dihexyl 2,2´ꢀbiquinꢀ
olineꢀ4,4´ꢀdicarboxylate (1) (1.4 g, 2.9 mmol) in EtOH (150 mL),
and the mixture was refluxed for 0.5 h. Then EtOH was distilled
off and the solution was cooled. The precipitate that formed
(dark sparkling crystals) was filtered off, washed three times with
EtOH, and dried. The yield of complex 2 was 1.5 g (91%).
Found (%): C, 62.63; H, 6.01; N, 4.59. C32H36Cl2CuN2O4. Calꢀ
culated (%): C, 62.70; H, 5.88; N, 4.47.
Fig. 7. Electron photomicrograph of the sample prepared by the
evaporation of an ethanolic solution of 2.
(4) In EtOH, AN, and DMF, the [CuCl(Sol)] species
undergo disproportionation to [CuIICl(Sol)] and Cu0.
Copper nanoparticles in the samples prepared by the evapꢀ
oration of solutions of 2 can be observed with a transꢀ
mission electron microscope (Fig. 7). In NMP, the
[CuCl(Sol)] species are insoluble and precipitate without
disproportionation.
This study was financially supported by the Russian
Foundation for Basic Research (Project Nos 06ꢀ03ꢀ33077,
08ꢀ03ꢀ00142, and 07ꢀ03ꢀ90824).
Experimental
The electrochemical oxidation and reduction potentials were
measured on an IPC_Win digital potentiostat/galvanostat conꢀ
nected to a personal computer. The voltammograms were reꢀ
corded using 0.05 M Bun4NPF6 as the supporting electrolyte in
AN and NMP at 20 °C in a 8 mL electrochemical cell. Oxygen
was removed from the cell by purging with dry argon. The voltaꢀ
mmograms were recorded by cyclic voltammetry on a stationary
platinum electrode at different potential scan rates. Platinum
wire served as the auxiliary electrode, and a saturated silver chloꢀ
ride electrode was used as the reference electrode (the potential
vs. Fc/Fc+ is –0.48 V in NMP).
The Xꢀray diffraction studies of compounds 1 and 2 were
carried out on a SMART APEX II CCD diffractometer (MoꢀKα
radiation, graphite monochromator, 120(2) K, ωꢀscanning techꢀ
nique) The structures were solved by direct methods and refined
anisotropically by the fullꢀmatrix leastꢀsquares method based on
F2hkl. The hydrogen atoms were positioned geometrically and
refined isotropically using a riding model. The principal crystalꢀ
lographic parameters and the refinement statistics are given in
Table 1. All calculations were carried out with the use of the
SHELXTL PLUS program package.
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Received December 18, 2008;
in revised form December 11, 2009