Spectroscopic and electrochemical studies on the chromotropism of ternary copper(II) complexes

Article abstract of DOI:10.1039/b101292b

Ternary complexes of copper(II) with β-ketoesters [β-keto = ethyl acetoacetate (HETAA) and ethyl benzoylacetate (HETBA)] and diamines [diam = N,N,N′,N′-tetramethylethylenediamine (Me₄en), N,N,N′-trimethylethylenediamine (Me₃en), N,N,N′-triethylethylenediamine (Et₃en) and N-methyl-1,4-diazacycloheptane (medach)] of general formula Cu(β-keto)(diam)X, whereX^- = ClO₄, BPh₄, NO₃, Cl or Br, have been prepared. Their structure and chromotropicity have been characterized using spectral analyses, electrochemical and magnetic susceptibility measurements. Complexes having perchlorate, tetraphenylborate and nitrate as anions show a remarkable change from reddish violet to green in different solvents and anions with increasing donor strength of the solvent or anion. Complexes having chloride and bromide as anions are highly influenced by the acceptor property rather than the donor property of the solvent. The spectral studies revealed the possibility of using these complexes as Lewis acid-base color indicators for both solvent and anion donor strength. Cyclic voltammetric measurements on the complexes in different solvents showed that the reduction process is mainly diffusion controlled and quasi-reversible or irreversible. Such behavior has been explained according to the ECE mechanism (E = electrochemical step, C = chemical step). The solvent effect has been rationalized in terms of the thermodynamics and kinetics of the redox processes. A correlation has been found between the copper(II) reduction potential and the spectral data in different solvents.

Full text of DOI:10.1039/b101292b

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Spectroscopic and electrochemical studies on the chromotropism of
ternary copper(II) complexes
A. Taha¤
Department of Chemistry, Faculty of Education, Ain Shams University, Roxy, Cairo, Egypt
Received (in Montpellier, France) 7th February 2001, Accepted 22nd March 2001
First published as an Advance Article on the web 2nd May 2001
Ternary complexes of copper(II) with b-ketoesters [b-keto \ ethyl acetoacetate (HETAA) and ethyl benzoylacetate
(HETBA)] and diamines [diam \ N,N,N@,N@-tetramethylethylenediamine (Me en), N,N,N@-trimethylethylene-
4
diamine (Me en), N,N,N@-triethylethylenediamine (Et en) and N-methyl-1,4-diazacycloheptane (medach)] of general
3
3
formula Cu(b-keto)(diam)X, where X~ \ ClO , BPh , NO , Cl or Br, have been prepared. Their structure and
4
4
3
chromotropicity have been characterized using spectral analyses, electrochemical and magnetic susceptibility
measurements. Complexes having perchlorate, tetraphenylborate and nitrate as anions show a remarkable change
from reddish violet to green in di†erent solvents and anions with increasing donor strength of the solvent or anion.
Complexes having chloride and bromide as anions are highly inÑuenced by the acceptor property rather than the
donor property of the solvent. The spectral studies revealed the possibility of using these complexes as Lewis
acidÈbase color indicators for both solvent and anion donor strength. Cyclic voltammetric measurements on the
complexes in di†erent solvents showed that the reduction process is mainly di†usion controlled and
quasi-reversible or irreversible. Such behavior has been explained according to the ECE mechanism
(E \ electrochemical step, C \ chemical step). The solvent e†ect has been rationalized in terms of the thermo-
dynamics and kinetics of the redox processes. A correlation has been found between the copper(II) reduction
potential and the spectral data in di†erent solvents.
without further puriÐcation. The solvents, 1,2-dichloroethane
Introduction
(DCE), nitromethane (MeNO ), acetonitrile (MeCN), acetone
2
The ternary complexes of copper(II) with b-diketones and
methylated diamines have attracted great interest as color
indicators for solvent and anion donor strength.1h10 However,
b-ketoesters have not attracted the same attention as b-
diketones in this Ðeld. In our previous work spectral and
molecular orbital calculations on copper(II) complexes with
thenoyltriÑuoroacetone and diamine derivatives were re-
ported.11 As expected from the behaviors of the analogous
copper(II)Èb-diketone complexes, the ternary complexes of
copper(II) with the b-ketoesters ethyl acetoacetate (HETAA)
and ethyl benzoylacetate (HETBA) would be expected to be
strongly solvato- and anionochromic.
(Me CO), methanol (MeOH), formamide (FA), dimethyl-
2
formamide (DMF), dimethyl sulfoxide (DMSO) and hexa-
methylphosphotriamide (HMPA), were puriÐed using
standard methods.12,13 The e†ect of anions was studied using
their tetrabutylammonium salts (Bu NX) where X~ \ ClO ,
4
4
CF SO , I, SCN, Br, N or Cl, which were prepared as
3
3
3
described earlier,11,14 except for the CH CO ~ and CO 2~
3
2
3
anions which were ammonium salts.
The visible absorption spectra for Cu(b-keto)(diam)X
(5 ] 10~3 mol dm~3) solutions in various organic solvents,
and anions in nitromethane, and for solid complexes in Nujol
mulls were recorded with a Hitachi U-2000 spectrophotom-
eter using 10 mm quartz cells at 25 ¡C. The infrared spectra in
KBr (400È4000 cm~1) were recorded using a Shimadzu FTIR
8101 spectrometer. Magnetic moments were calculated from
the molar susceptibility values measured using the Gouy
method and a Johnson Matthey Alfa Products magnetic
balance MSB-MK I. Cyclic voltammetric measurements were
performed using a standard polarographic and potentiostatÈ
galvanostat system (Princeton Applied Research, Texas) based
on a three electrode system as described elsewhere.15 Cyclic
voltammograms were recorded at a potential scan rate of 100
mV s~1 for solutions of 0.1 mol dm~3 tetrabutylammonium
perchlorate as supporting electrolyte. Then 1 ] 10~3 mol
dm~3 of the investigated complex and the internal reference
were added, respectively. In the current study, two internal
references, ferroceneÈferrocenium (FcÈFc`) and bis(bi-
phenyl)chromium(I)/(0)16 have been employed due to the
restrictions on the solubility of both in the solvents used.
All potentials were then re-scaled to bis(biphenyl)chrom-
ium(I)/(0)17,18 due to its higher solubility in most solvents
used.
Based on the above, the present work reports the synthesis
of seven new ternary complexes: Cu(b-keto)(diam)X (b-
keto \ ETAA or ETBA; diam \ diamine \ N,N,N@,N@-tetra-
methylethylenediamine (Me en), N,N,N@-trimethylethylenedi-
4
amine (Me en), N,N,N@-triethylethylenediamine (Et en) and
3
3
N-methyl-1,4-diazacycloheptane
(medach);
X~ \ ClO ,
4
BPh , NO , Cl or Br). The e†ects of anions, solvents and
4
3
substituents on both b-ketoester and diamine ligands on the
structure have been investigated. These e†ects are correlated
with the spectral and electrochemical data of these complexes.
Experimental
Materials and methods
All the reagents and solvents used were from either Merck
or Aldrich. b-Ketoester and diamine ligands were used
¤ Present address: UAE University, Faculty of Science, Chemistry
Department, Al-Ain, P.O. Box 17551, United Arab Emirates. E-mail:
A.Taha=uaeu.ac.ae.
DOI: 10.1039/b101292b
New J. Chem., 2001, 25, 853È858
853
This journal is ( The Royal Society of Chemistry and the Centre National de la Recherche ScientiÐque 2001

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Syntheses of Cu(b-keto)(diam)X complexes
These complexes were prepared by adding a mixture of b-
ketoester, ethyl benzoylacetate or ethyl acetoacetate (1.78 and
1.30 g, 10 mmol, respectively) in 30 mL ethanol and solid
anhydrous Na CO (1 g, 10 mmol) to an ethanolic solution of
2
3
CuX É nH O (2.42 and 3.70 g, 10 mmol) for X \ NO ~ and
2
2
3
ClO ~, respectively) with continuous stirring for about 30
min. A solution of diamine (10 mmol) in 10 mL EtOH was
4
added dropwise with continuous stirring for 25 min. The
resulting solution was Ðltered and left to stand overnight and
the complexes obtained were recrystallized from nitro-
methane.
Preparation of Cu(ETAA)(Me en)BPh
4
4
1.85 g (5 mmol) of Cu(ETAA)(Me en)NO was dissolved in 25
4
3
mL 1,2-dichloroethane and then added, with vigorous stirring,
to a suspension of 3.10 g (10 mmol) NaBPh in 20 mL DCE.
The resulting mixture was Ðltered and the solution kept at
60 ¡C for an hour then Ðltered. The pale reddish violet precipi-
tate obtained was Ðltered o† then washed with DCE.
4
Scheme
1
Position of the substituents on the b-keoester and
diamine-ligands in the investigated Cu(b-keto)(diam)X complexes.
bands are assigned in comparison with related copper(II) com-
plexes.3,11,19,20 The coordination modes of the anions have to
be inferred from the IR data.
Results and discussion
Perchlorate anion shows two IR stretching vibrational
bands at 1100 and 630 cm~1. Weak splitting of the strong
Table 1 lists the colors, analytical data, and magnetic
moments of the prepared complexes. Elemental analyses
conÐrm the proposed formulae, Cu(b-keto)(diam)X (Scheme
1). The magnetic moment values are found in the range 1.67È
broad band l
at 1100 cm~1 indicates that the ClO ~ is
~
ClO4
4
partially involved in coordination.19,20 In the current study
weak splitting of this band was observed for
Cu(ETBA)(Me en)ClO and Cu(ETBA)(medach)ClO com-
1.98 k , indicating the absence of CuÈCu interaction,11 and
3
4
4
B
plexes which indicates that ClO ~ is partially contributing to
the coordination sphere of copper(II). In contrast, the other
perchlorate complexes do not show any splitting, which indi-
cates that the perchlorate anion is mainly ionic.19
The nitrate complex Cu(ETAA)(Me en)NO shows three
vibrational modes at 1464, 1273 and 1024 cm~1 which corre-
spond to l (A ) ] l(N2O), l (B ) ] l (NO ) and l (A )
suggest a square planar geometry similar to that of CuN O
4
2
2
complexes.6
Infrared spectra
4
3
The main characteristic IR absorption frequencies of the
investigated complexes are given in Table 2. The observed
1
1
5
2
asym
2
2
1
Table 1 Colors, analytical data, and magnetic moments of Cu(b-keto)(diam)X complexes
Found (Calc.)
C%
Complex/
Empirical formula
Formula
weight
Color
Yield(%)
H%
N%
5.72
(6.16)
5.56
(5.78)
6.41
(6.33)
5.84
(6.14)
6.73
(6.85)
4.27
(4.53)
11.48
(11.30)
k
/k
eff
B
1
2
3
4
5
6
7
Cu(ETBA)(medach)ClO
454.23
484.40
442.23
456.25
408.55
617.92
370.75
Reddish violet
Reddish violet
Reddish violet
Reddish violet
Reddish violet
Violet
73
58
53
74
79
59
62
41.85
(42.27)
44.78
(44.59)
41.06
(40.70)
41.89
(42.08)
34.95
(35.25)
70.14
(69.91)
39.23
4.86
(5.10)
5.96
(6.04)
5.53
(5.24)
5.65
(5.52)
5.95
(6.17)
7.19
(7.50)
6.39
1.86
1.84
1.69
1.72
1.68
1.76
1.98
4
C
H
N O ClCu
16 23
2 7
Cu(ETBA)(Et en)ClO
3
4
C
H
N O ClCu
18 29
2 7
Cu(ETBA)(Me en)ClO
3
4
4
C
H
N O ClCu
15 23
2 7
Cu(ETBA)(Me en)ClO
4
C
H
N O ClCu
16 25
2 7
Cu(ETAA)(Me en)ClO
4
4
C
H
N O ClCu
12 25
2 7
Cu(ETAA)(Me en)BPh
4
4
C
H
N O Cu
36 46
2 3
Cu(ETAA)(Me en)NO
Bluish violet
4
3
C
H
N O Cu
(38.80)
(6.80)
12 25
3 6
Table 2 Wavenumber/cm~1 of the main absorptions in the infrared spectra of Cu(b-keto)(diam)X complexes; vs \ very strong, s \ strong,
m \ medium, w \ weak, b \ broad
Complex
l(NH)
l(C2O)
l(C2C)
l(ClO ~)
l(CuÈO)
4
1
2
3
4
5
6
7
Cu(ETBA)(medach)ClO
Cu(ETBA)(Et en)ClO
3285 w
3229 m
3252 w
È
È
È
1619 s
1613 vs
1613 vs
1619 s
1615 vs
1601 vs
1620
1527 m
1528 s
1520 s
1525 m
1518 s
1516 s
1520
1380 m
1389 m
1369 m
1377 m
1385 m
1366 m
1381
1123 s
1107 vs
1111 vs
1105 b
1103 vs
È
1091 s
È
1089 s
È
È
È
632 s
625 s
625 s
630 s
625 s
È
4
559 w
556 w
3
4
Cu(ETBA)(Me en)ClO
3
4
4
Cu(ETBA)(Me en)ClO
4
Cu(ETAA)(Me en)ClO
525 m
471 w
440 w
4
4
Cu(ETAA)(Me en)BPh
4
4
Cu(ETAA)(Me en)NO
È
È
È
È
4
3
854
New J. Chem., 2001, 25, 853È858

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] l (NO ), respectively. The separation of the two highest-
s
2
frequency bands (191 cm~1) indicates that the nitrate anion
might act as a bidentate ligand to the copper(II) in the solid
state. Additional evidence was obtained from the separation of
the combination bands, l ] l , which appear at 1763 and
1
4
1700 cm~1. The separation of 63 cm~1 conÐrms the presence
of a bidentate NO ~ anion.19
Moreover, the IR absorptions corresponding to the C2O
3
stretching frequencies of the Cu(ETAA)(diam)X complexes are
found at 1601, 1615 and 1620 cm~1 for X~ \ BPh , ClO
4
4
and NO , respectively. This Ðnding indicates that this stretch-
3
ing frequency is inÑuenced by the coordination ability of the
anion toward the central copper(II), i.e. the l
values are
C/O
shifted to a higher frequency as the donor strength value of
the anion (DN ) increases.2 This means that coordination of
X
the anion with the central metal ion weakens the CuÈO bond
Fig. 1 Electronic absorption spectra of Cu(ETAA)(Me en)ClO
(see Table 2), as expected from the extended donorÈacceptor
concept.21 This interpretation is further conÐrmed by the
negative slope of the linear correlation of the CuÈO stretching
4
4
(5 ] 10~3 mol dm~3) in MeNO , MeCN, Me CO, DMF and DMSO
2
2
solutions at 25 ¡C.
frequencies with the donor strength of the anions (DN
),
X, NM
l
\ 564 [ 6.46 DN
, r \ 0.999. This negative slope
yield linear relationships with both of the stretching fre-
CuhO
X, NM
indicates that the strength of the CuÈO bond decreases as the
donor strength of the coordinating anion increases.
quencies of CuÈO (l
) of these complexes and DN
of
\
CuhO
X, NM
, r \ 0.997 and l
CuhO max
the anions, l \ 10966 [ 11.835 l
max
Further evidence supporting the view mentioned above
(e†ect of the anion on the strength of the CuÈO bond) was
17854 [ 76.30 DN
, r \ 0.99, respectively. This linearity
X, NM
reÑects the extent of the anionÏs coordination toward the
central metal ion and its e†ects on the CuÈO bond strength.
In contrast, the dÈd visible absorption bands of the bromide
obtained by correlating the stretching frequencies l
with
CuhO
the dÈd visible absorption band of the Nujol mull of these
complexes; a linear correlation was obtained (vide infra).
and chloride complexes, Cu(b-keto)(Me en)X (prepared by
4
adding Bu NBr or Bu NCl to a nitromethane solution of the
4
4
corresponding perchlorate complex in the molar ratio 2 : 1),
are weakly a†ected by the donor strength of the solvent inter-
action, indicating that the bromide and chloride anions coor-
dinate more strongly with the central metal ion than most of
the solvents used in the current study.
Electronic spectra
Fig.
1
shows the visible absorption spectra of the
Cu(ETAA)(Me en)ClO complex in a number of organic sol-
4
4
vents. As was the case for the well-known [Cu(acac)(diam)]`
complexes,1 the dÈd band observed exhibits a red shift with
the increase of the solventÏs donor number (DN).21 Similar
spectral changes are found for the complexes in the present
work. The changes in their dÈd visible absorption spectral
data in di†erent solvents are given in Table 3. In general, the
The j
values of the dÈd visible absorption band of the
present complexes in DCE (DN \ 0) are remarkably higher
max
than those in MeNO (DN 2.7). This trend was observed for
similar complexes and ascribed to the formation of ion pairs
2
in DCE.5,6,11 This conclusion is further supported by the
j
values of the tetraphenylborate complex in di†erent
linear correlation of the l
values of perchlorate complexes
max
max
in DCE solution vs. their IR CuÈO (l
CuhO
solvent solutions are lower than those of the corresponding
perchlorate and nitrate complexes. On the other hand, in the
weak or intermediate donor solvents the dÈd visible absorp-
tion band of the nitrate complex was shifted to higher wave-
lengths than those of the corresponding perchlorate
complexes. These observations are consistent with the coordi-
nation ability of anion toward the central metal ion as con-
cluded from the IR studies.
) stretching fre-
quencies in the solid state,
r \ 0.97. It was also found that the j
l
\ 25048 [ 14.27
l
,
max
CuhO
values in Me CO
max
2
(DN 17.0) are slightly lower than in MeCN (DN 14.1) solu-
tion. This might be attributed to steric and p-bonding e†ects
in acetone and acetonitrile, respectively, since they are acting
in opposite directions.11
Table 4 shows that the anion e†ect on the dÈd visible
absorption bands of perchlorate complexes in nitromethane
solution is similar to that found for the solvent e†ect, i.e. the
same red-shift was observed as the donor strength of the
Moreover, the dÈd visible absorption bands of Nujol mulls
of Cu(ETAA)(Me en)X (X~ \ BPh , ClO or NO ) complex-
4
4
4
3
es are found at 569, 580 and 619 nm, respectively. These data
Table 3 Absorption maxima j /nm (absorption coefficients are in the range 100È120 dm3 mol~1 cm~1) for the Cu(b-keto)(diam)X complexes
max
in various organic solutions at 25 ¡C
Complex
HMPA
DMSO
DMF
FA
MeOH
Me CO
MeCN
MeNO
DCE
2
2
1
Cu(ETBA)(medach)ClO
Cu(ETBA)(Et en)ClO
655
699
657
659
665
656
667
645
690
721
725
622
663
633
646
644
624
642
645
665
696
719
611
647
627
649
633
619
634
665
690
723
744
609
630
609
625
624
609
626
625
613
646
641
596
617
608
610
609
601
610
621
624
645
639
581
602
599
593
593
583
610
664
703
733
712
585
611
601
604
600
593
602
672
708
748
701
577
572
570
568
551
551
568
680
697
737
710
585
588
584
580
570
565
575
718
702
740
736
4
2
3
4
3
Cu(ETBA)(Me en)ClO
3
4
4
4
Cu(ETBA)(Me en)ClO
4
5
Cu(ETAA)(Me en)ClO
4
4
6
Cu(ETAA)(Me en)BPh
4
4
7
Cu(ETAA)(Me en)NO
4
3
8a
9
Cu(ETAA)(Me en)Br
4
Cu(ETBA)(Me en)Br
3
Cu(ETBA)(Et en)Br
3
10
11
Cu(ETBA)(medach)Cl
a Bromide and chloride complexes are formed in solution by mixing Bu NX and the complex in a 2 : 1 ratio in nitromethane solution.
4
New J. Chem., 2001, 25, 853È858
855

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Table 4 Absorption maxima j /nm (absorption coefficients are in the range 110È130 dm3 mol~1 cm~1) of Cu(b-keto)(diam)X complexes with
max
various anions in nitromethane solutions at 25 ¡C
Complexa
ClO ~
NO ~
OAc~
CO 2~
I~
Br~
SCN~
N ~
Cl~
4
3
3
3
1
2
3
4
5
577
572
570
568
551
615
617
599
630
613
623
639
604
644
648
617
586
581
608
620
631
676
654
669
646
669
737
697
712
680
698
700
657
683
681
696
715
680
685
669
710
729
694
721
721
a Numbers as in Table 1.
solvent or anion increases. Therefore, these complexes are
highly solvato- and aniono-chromic, respectively.4
than that of the corresponding b-diketonate complexes. This
could be ascribed to the electron attracting properties of the
OR group on the b-keto ligand which weakens the coordi-
nation ability of the carbonyl group toward Cu(II) compared
with the alkyl group on the corresponding b-diketonate ligand
(electron-donating properties). This explanation is further sup-
The chromotropicity of the present complexes could quanti-
tatively be expressed by linear regression analysis, using the
equation lc \ l0 ] a (DN), where, lc \ the measured wave-
number, l0 \ the extrapolated wavenumber and a \ the slope
(chromotropicity).11 Linearity indicates stabilization of the
ground state of these complexes achieved by coordination
with solvent molecules or anions.11 The chromotropic data
given in Table 5 reveal that the relative stability of the four-
ported by the linearity of the l0 values with TaftÏs p param-
s
eter, l0/103 \ 18.80 [ 0.35 p, R \ 0.98.
s
However, the chromotropicity, a values, are found to be in
the order ETAA [ ETBA P acac [ bzac complexes which is
s
coordinate species of the Cu(ETBA)(diam)ClO complexes
opposite to the order of the relative stability of the square
planar species. This indicates that, as the electron density
around the central metal ion increases, the chromotropicity of
the complex decreases which is contrary to the e†ect of
diamine ligand.
4
according to the l0 or l0 values follow the order: Me en [
s
x
4
Et en [ Me en [ medach. This order is opposite to the
ability to form Cu(II)É É ÉClO ion-pair attractions perpendicu-
lar to the CuN O complex plane. This interpretation is
3
3
4
2
2
further conÐrmed by the weak splitting of the l
IR
Chromotropic data, l0 and a, in Table 5, indicate that the
relative stability and chromotropicity of the perchlorate com-
plexes are more highly a†ected by the anion than the solvent.
Once again, the perchlorate and tetraphenylborate com-
~
ClO4
stretching bands at 1100 cm~1 which was found for
Cu(ETBA)(medach)ClO and Cu(ETBA)(Me en)ClO com-
4
3
4
plexes (see Table 2).
Likewise, the order of chromotropicity, a, is found to be:
plexes in most solvents exhibit lower j
values than do the
corresponding halide complexes (see Table 3). This indicates
that the former complexes in non- or very weak coordinating
max
Et en P Me en A Me en [ medach. This reveals that increas-
3
4
3
ing the relative stability of the square planar species will make
the environment of Cu(II) more planar such that the coordi-
nated solvent or anion can easily attack the square-planar
species in the same order.5
solvents (DCE or MeNO ) retain their four-coordinate
2
geometry since the coordinating ability of tetraphenylborate
and perchlorate anion to the central metal ion is very weak as
expected from their donor number (DN ).2 The dÈd band of
the latter complexes is shifted strongly to higher wavelengths,
at about 700 nm, which could be a result of distorted square-
pyramidal geometry.3,12,22
The e†ect of solvents on the halide complexes Cu(b-dik)
(diam)X (X \ Br or Cl) is rather complicated as it depends
upon both the donor and acceptor properties of the solvent.
Therefore, a multiple linear regression analysis was employed
to analyze the spectral data of these complexes in di†erent
solvents (see Table 3) which resulted in the following
It is now known that these spectral changes are mainly due
to the changes in the degree of axial solvation or anation of
the complex. This solvation, which tends to shift the dÈd band
to the red, increases with the donor strength (DN) of the
solvent, but is reduced according to the steric hindrance of the
diamine ligand as indicated from the low values of a and a
X
s
x
for Cu(ETBA)(medach)ClO , where the medach ligand pro-
vides steric hindrance due to its cyclic structure.3,11
4
Comparison of the chromotropic data of the present com-
plexes with those of analogous b-diketonate complexes,
Cu(acac)(Me en)ClO and Cu(bzac)(Me en)ClO (acac \
relationships: l /103 cm~1 \ 13.58 ] 40.20 AN ] 38.7 DN,
4
4
4
4
max
acetylacetonate and bzac \ benzoylacetonate), is interesting in
r \ 0.91; 12.91 ] 67.9 AN ] 22.1 DN, r \ 0.95; 12.19 ] 67.4
this connection. l0 values were found to be in the order:
AN ] 24.7 DN, r \ 0.94 and 12.58 ] 69.5 AN ] 9.48 DN,
r \ 0.93, for Cu(ETAA)(Me en)Br, Cu(ETBA)(Me en)Br,
s
acac [ bzac [ ETAA [ ETBA. This indicates that the rela-
4
3
tive stability of the square planar b-keto complexes is lower
Cu(ETBA)(Et en)Br and Cu(ETBA)(medach)Cl, respectively.
3
Table 5 Chromotropicity parameters of the Cu(b-keto)(diam)X complexes, according to the equation lc \ l0 ] a(DN)
10~3 Intercept/cm~1
Slope/cm~1
ra
Complex
l0
l0
a
a
Solvent
Anion
s
x
s
x
1 Cu(ETBA)(medach)ClO
2 Cu(ETBA)(Et en)ClO
17.80
17.86
17.71
17.74
17.70
18.25
17.73
18.61
18.75
18.52
18.70
18.57
18.80
18.74
È
61.73
91.60
64.14
69.87
70.58
77.66
73.4
119.0
146.0
122.0
139.0
128.0
È
È
È
È
0.99
0.99
0.97
0.99
0.98
0.97
0.99
0.99
0.98
0.93
0.96
0.94
0.93
0.95
È
4
3
4
3 Cu(ETBA)(Me en)ClO
3
4
4
4 Cu(ETBA)(Me en)ClO
4
5 Cu(ETAA)(Me en)ClO
4
4
6 Cu(ETAA)(Me en)BPh
4
4
7 Cu(ETAA)(Me en)NO
È
È
4
3
Cu(bzac)(Me en)ClO b
78.8
83.11
4
4
Cu(acac)(Me en)ClO b
È
È
4
4
a r \ Correlation coefficient. b Data taken from ref. 20.
856
New J. Chem., 2001, 25, 853È858

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These relationships indicate that the solution of the halide
complex is mainly a†ected by the acceptor properties (AN)
rather than the donor properties (DN) of the solvent.
The dÈd transition band in the visible region of the elec-
tronic absorption spectrum of the current complexes is com-
pletely resolved, which allows an accurate determination of
the bandÏs oscillator strength ( f ). The expression
f \ 4.6 ] 10~9 e
l
, where e
is the absorption coeffi-
cient at the maximum band height in dm3 mol~1 cm~1 and
max 1@2
max
l
the bandwidth at half band height, was used to calculate f
1@2
values.23 The oscillator strength, f, is a dimensionless quantity
that is used to express the electronic transition probabil-
ity.11,24 Table 6 collects the f values of the present complexes,
in several solvents. These values indicate that the dÈd visible
transition probability for these complexes decreases as the
Fig. 2 Cyclic voltammogram recorded with a platinum microelec-
trode from
a
DMSO solution for Cu(ETBA)(medach)ClO
4
(1.0 ] 10~3 mol dm~3), 0.1 mol dm~3 Bu NClO as supporting elec-
4
4
trolyte and a scan rate of 100 mV s~1.
donor strength of the solvent interaction increases. The Et en
3
complex is the most sensitive of the series towards solvent.
and has been explained according to the ECE mechanism
(E \ electrochemical step, C \ chemical step) (Scheme 2).
It must be taken into account that the expulsion of
copper(I) ions from the complex at the potential of peak A is
thermodynamically followed by reduction to copper metal at
the potential of peak B. No anodic peak is observed in MeCN
for Cu(ETBA)(medach)ClO or Cu(ETBA)(Me en)ClO in the
Ðrst reduction process. In contrast, in the other solvents the
anodic peak is always observed even at a slow scan rate. This
process is accompanied by a partial loss of the complex solu-
tion color and the deposition of copper metal on the working
electrode during the bulk-phase constant potential electrolysis
of the complexes.
Plotting the f values vs. the donor number of solvent (DN)
displays a linear relationship f \ f ] b DN and the linear
0
regression data are collected in Table 6. The intercept ( f ) rep-
0
resents the extrapolated oscillator strength, which was found
to be in the order: Et en [ Me en [ Me en [ medach. This
trend is similar to that found for the chromotropicity (a). For
3
4
3
4
4
4
the same complex cation, Cu(ETAA)(Me en)`, as the coordi-
4
nation ability of the anion increases the f value decreases.
0
The negative slope, b (see Table 6), indicates that the dÈd
absorption band becomes broader as the donor strength of
the solvent or anion increases.
Table
7 shows the potential values vs. bis(biphenyl)-
Electrochemical studies
chromium(I)/(0) of the redox peaks (E
Cu(ETBA)(medach)ClO ,
Cu(ETAA)(Me en)ClO complexes. These potentials depend
upon the donor properties of the solvent similar to what is
and E ) for
pc
4
pa
Cu(ETBA)(Me en)ClO
and
Electrochemical properties of some of the present complexes
were investigated by cyclic voltammetry in Ðve solvents, i.e.
4
4
4
4
MeNO , MeCN, FA, DMF and DMSO, with a stationary
2
observed for Cu(CF SO ) in di†erent solvents.18 The solvent
platinum microelectrode. The complexes undergo a reduction
3
3 2
inÑuences both the thermodynamics and kinetics of the redox
processes of the complexes under study.26 It is clear that there
is a relationship between the copper(II) reduction potential
and the solvent donor number (DN). This might be explained
on the basis of the assumption that the complex ions are sol-
vated, i.e. the solvent molecules attack the axial sites of the
copper(II), complexes as concluded from the electronic spectral
studies.
localized, evidently, on the metal atom. The reduction process
is mainly di†usion controlled as was concluded from the
linear dependence of the peak current on the square root of
the scan rate (the range of V \ 50 to 100 mV s~1).23
FerroceneÈferrocenium
(FcÈFc`)
and
bis(biphenyl)-
chromium(I)/(0) were used as internal standards for assignment
of the potential values as well as evaluation of the number of
transferred electrons. The character of the reduction pathway
and the peak currents at E are almost the same for the
studied complexes.
The voltammetric properties of the complexes are consistent
with their visible absorption spectra as indicated by the
linear correlation between the redox potential and the l
pc
Fig. 2 shows the cyclic voltammetric response, obtained at a
platinum microelectrode from
max
a
DMSO solution of
Cu(ETBA)(medach)ClO as a typical example for the present
4
complexes. At a scan rate of 100 mV s~1 a rather complicated
quasi-reversible or irreversible copper(II) reduction is
observed, the cathodic peak A being accompanied, in the
reverse scan, by the directly associated re-oxidation anodic
peak C. A more cathodic peak B and other anodic peaks D
and E were observed. At decreasing scan rates C tended to
disappear. This Ðnding is clearly due to the re-oxidation of
some electrodeposited copper metal. Indeed this voltammetric
picture is quite common to many copper(II) complexes,25h27
Scheme 2 Suggested mechanism of the reduction and re-oxidation
processes for Cu(b-keto)(diam)ClO complexes.
4
Table 6 Oscillator strength ( f ) of the dÈd transition absorption bands of the Cu(b-keto)(diam)X complexes
f ] 103
f \ f ] b DN
0
Complex
DMSO
DMF
MeOH
Me CO
MeCN
MeNO
f ] 103
b ] 105
r
2
2
0
1 Cu(ETBA)(medach)ClO
2 Cu(ETBA)(Et en)ClO
6.38
7.89
6.24
6.34
5.94
8.54
5.23
6.49
8.16
6.31
6.59
6.22
8.65
6.40
6.51
8.65
6.49
6.92
6.63
9.01
6.57
6.87
8.93
7.01
7.12
6.97
9.24
6.73
6.73
8.63
6.71
7.05
6.60
9.06
6.56
7.05
9.75
7.33
7.48
8.82
10.2
8.28
7.12
9.85
7.35
7.68
8.93
10.30
8.48
2.45
6.51
4.0
4.15
10.60
6.23
10.90
0.92
0.97
0.98
0.97
0.98
0.99
0.95
4
3
4
3 Cu(ETBA)(Me en)ClO
3
4
4
4 Cu(ETBA)(Me en)ClO
4
5 Cu(ETAA)(Me en)ClO
4
4
6 Cu(ETAA)(Me en)BPh
4
4
7 Cu(ETAA)(Me en)NO
4
3
New J. Chem., 2001, 25, 853È858
857

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Table 7 Electrochemical data for the reduction and re-oxidation processes of Cu(b-keto)(diam)ClO complexes in various solvents; potentials
4
E/V vs. bis(biphenyl)chromium(I)/(0) reference electrode
1 Cu(ETBA)(medach)ClO
4 Cu(ETBA)(Me en)ClO
5 Cu(ETAA)(Me en)ClO
4
4
4
4
4
Solvent
MeNO
E
/V
E
/V
E
/V
E
/V
E
/V
E
/V
E
/V
E
/V
E
/V
pc
pa1
pa2
pc
pa1
0.26
pa2
pc
pa1
pa2
0.55
0.93
0.56
0.64
0.82
0.87
0.79
0.91
0.18
0.89
0.41
0.93
0.64
0.65
1.02
0.89
0.81
0.51
0.22
2
MeCN
FA
0.10
0.75
0.68
0.29
0.34
0.58
È
0.67
0.55
1.00
0.83
0.73
0.91
È
[0.31
0.006
0.11
[0.06
0.24
DMF
DMSO
0.02
0.24
0.07
0.06
0.59
0.40
0.10
[0.06
[0.04
12 G. B. Porter and V. Hanten, J. Inorg. Nucl. Chem., 1979, 18,
2053.
values of the dÈd absorption band of Cu(ETAA)(Me en)ClO
4
4
in various solvents, E \ 3.36 [ 170 l , r \ 0.97.
pc
max
13 Organikum, 16th edn., VEB Deutscher Verlag der Wissenschaften,
The combination of electrochemical and spectroscopic
studies enables one to investigate the axial ligation of the
chromotropic complexes in detail and can be utilized to char-
acterize the solvent coordination ability. In the absence of
X-ray single crystal data for the current complexes, the pro-
posed structures in Scheme 1, based on the above physi-
cochemical studies, are tentative.
Berlin, 1986.
14 V. Gutmann and U. Mayer, Monatsh. Chem., 1968, 99, 1383.
15 W. Linert, A. Taha and R. F. Jameson, J. Coord. Chem., 1992, 25,
29.
16 G. Gritzner and Kuta, J. Appl. Chem., 1982, 54, 1527; G. Gritzner
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17 G. Gritzner, J. Phys. Chem., 1986, 90, 5478, and references cited
therein.
18 G. Gritzner, Inorg. Chim. Acta, 1977, 24, 5.
19 K. Nakamoto, Infrared and Raman Spectra of Inorganic and
Coordination Compounds, 4th edn., Wiley-Interscience, New
York, Chichester and Brisbane, 1986.
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New J. Chem., 2001, 25, 853È858