Synthesis and physico-chemical studies of some mixed-ligand complexes of bis(benzoylacetonato)copper(II) with some biologically active heterocyclic chelating donors

Article abstract of DOI:10.1081/SIM-120021938

Six new mixed-ligand complexes of copper(II) of the compositions [Cu(bzac)₂(L)₂], where bzacH = benzoylacetone and L = 3-methyl-2-pyrazoline-5-one (MP) (1), 2,3-dimethyl-1-phenyl-3-pyrazoline-5-one (DMPHP) (3), benzimidazole (BZL) (4

Full text of DOI:10.1081/SIM-120021938

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Synthesis and Physico‐Chemical Studies
of Some Mixed‐Ligand Complexes of
bis(Benzoylacetonato)copper(II) with Some Biologically
Active Heterocyclic Chelating Donors
R. C. Maurya a , R. Verma a & H. Singh a
a Coordination Chemistry Laboratory , Department of P. G. Studies and Research in
Chemistry , R. D. University , Jabalpur, 482 0019, India
Published online: 15 Feb 2007.
To cite this article: R. C. Maurya , R. Verma & H. Singh (2003) Synthesis and Physico‐Chemical Studies of Some Mixed‐Ligand
Complexes of bis(Benzoylacetonato)copper(II) with Some Biologically Active Heterocyclic Chelating Donors, Synthesis and
Reactivity in Inorganic and Metal-Organic Chemistry, 33:6, 1063-1080
To link to this article: http://dx.doi.org/10.1081/SIM-120021938
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SYNTHESIS AND REACTIVITY IN INORGANIC AND METAL-ORGANIC CHEMISTRY
Vol. 33, No. 6, pp. 1063–1080, 2003
Synthesis and Physico-Chemical Studies of Some
Mixed-Ligand Complexes of
bis(Benzoylacetonato)copper(II) with Some
Biologically Active Heterocyclic Chelating Donors
*
R. C. Maurya, R. Verma, and H. Singh
Coordination Chemistry Laboratory, Department of P. G. Studies and
Research in Chemistry, R. D. University, Jabalpur, India
ABSTRACT
Six new mixed-ligand complexes of copper(II) of the compositions
[Cu(bzac)2(L)2], where bzacH = benzoylacetone and L = 3-methyl-2-
pyrazoline-5-one (MP) (1), 2,3-dimethyl-1-phenyl-3-pyrazoline-5-one
(DMPHP) (3), benzimidazole (BZL) (4) or benzothiazole(BZT) (5) and
[Cu(bzac)2(L–L)], where L–L = 4-amino-2,3-dimethyl-1-phenyl-3-pyr-
azoline-5-one (ADMPHP) (2) or 2-aminobenzothiazole (ABZT) (6), have
been synthesized by the interaction of bis(benzoylacetonato)copper(II)
with the ligands L and L–L. The compounds so obtained were charac-
terized by analytical data, molar conductances, magnetic measurements,
*
Correspondence: R. C. Maurya, Coordination Chemistry Laboratory, Department of
P. G. Studies and Research in Chemistry, R. D. University, Jabalpur 482 001, India;
E-mail: rcmaurya@sancharnet.in.
1063
DOI: 10.1081/SIM-120021938
0094-5714 (Print); 1532-2440 (Online)
www.dekker.com
Copyright D 2003 by Marcel Dekker, Inc.

1064
Maurya, Verma, and Singh
ESR, IR, and electronic spectral studies. Monomeric structures are sug-
gested for these compounds.
Key Words: Keto-enol tautomerism; Mixed-ligand b-diketone
complexes; Heterocyclic chelating donors; cis-Octahedral.
INTRODUCTION
b-Diketones and related compounds capable of exhibiting keto-enol
tautomerism[1] react with metal cations to form complexes, in which the
metal replaces the enolic hydrogen and a six-membered chelate ring is pro-
duced. Since the enolate ion carries a single negative charge, divalent metal
atoms reacts with two enolate ions to give neutral molecules, ML2 (LH = b-
diketone), in which there are four donor oxygen atoms to each metal atom. If
the normal coordination number of the metal towards ligand oxygen atoms is
four, the result is an inner complex, or neutral molecule. The larger divalent
ions of transition metals usually have coordination numbers towards oxygen
ligands which are greater than four. The neutral molecules, ML2, then
behave as Lewis acids and form addition compounds with Lewis bases.[2,3]
The chemistry of b-diketone complexes has aroused much interest in
many areas, viz., spectral studies, gas chromatography, solvent extraction,
column and thin layer chromatography, NMR shift reagents, laser
technology and in the polymer industry.[4,5]
In the past few years there has been continuing interest in the synthesis
and characterization of some mixed-ligand b-diketone complexes[2,3,6–13] of
Co(II), Ni(II), Cu(II) and Zn(II) involving tetra-, penta- and hexa-coor-
dination. Mahapatra and Das[14] have recently reported the synthesis and
characterization of some mixed-ligand chelates of Co(II), Zn(II), Cd(II) and
Hg(II) with a monobasic bidentate oxygen donor (acetoacetanilide) and a
neutral monodentate N-donor (isoquinoline). In earlier communications
Maurya et al.[9–13] have described the isolation and characterization of
hexa-coordinated mixed-ligand Co(II), Ni(II) and Zn(II) derivatives of
b-diketones or b-diketoesters with heterocyclic nitrogen donors and 2- or
3-pyrazoline-5-one derivatives. A series of new mixed-ligand complexes
of Co(II) of the composition [Co(Cl)(A–A)(L)(H2O)2], where (A–
A)H = acetoacetanilide, o-acetoacetanisidide or o-acetoactotoluidide and
L = 3-methyl-1-phenyl-2-pyrazoline-5-one, 2,3-dimethyl-4-dimethylamino-
methanesulphonatesodium-1-phenyl-3-pyrazoline-5-one, 2,3-dimethyl-
1-phenyl-3-pyrazoline-5-one have been synthesized and characterized by
Maurya et al.[15] In a recent communication[16] from our coordination
chemistry laboratory, we have reported the synthesis, magnetic and spectral
studies of some mixed-ligand chelates of bis(2-hydroxyacetophenonato)-

Mixed-Ligand Complexes of Copper(II)
1065
copper(II) with 2- or 3-pyrazoline-5-one derivatives. The crystal structure of
the trans form of bis(benzoylacetonato)copper(II)has been determined[17] by
three-dimensional X-ray diffraction data. However, there is no previous
report on mixed-ligand complexes of bis(benzoylacetonato)copper(II) with
biologically active heterocyclic chelating molecules. We, therefore, have
examined the ligational behaviour of some heterocylic chelating donors,
viz., 3-methyl-2-pyrazoline-5-one (MP), 4-amino-2,3-dimethyl-1-phenyl-3-
pyrazoline-5-one (ADMPHP), 2,3-dimethyl-1-phenyl-3-pyrazoline-5-one
(DMPHP), benzimidazole (BZL), benzothiazole (BZT), 2-aminobenzothia-
zole (ABZT) towards bis(benzoylacetonato)copper(II). The present paper
describes the results of such a study. The benzoylacetone and heterocyclic
chelating donors are structurally shown in Figure 1.
Figure 1. Structures of ligands.

1066
Maurya, Verma, and Singh
EXPERIMENTAL
Materials Used
Copper(II) sulfate pentahydrate (B. D. H. Chemicals, Bombay),
benzoylacetone and 2-aminobenzothiazole (Sisco-Chem. Industry, Bom-
bay), 3-methyl-2-pyrazoline-5-one and 4-amino-2,3-dimethyl-1-phenyl-3-
pyrazoline-5-one (Aldrich Chemical Co., U. S. A.), 2,3-dimethyl-1-phenyl-
3-pyrazoline-5-one (B. D. H. Chemicals, England), benzimidazole (Wilson
Laboratories, Bombay) and benzothiazole (E. Merck, Germany) were used
as supplied.
Preparation of bis(Benzoylacetonato)copper(II)
This compound was prepared by the method of Hon et al.,[17] and used
as the parent compound.
Synthesis of Mixed-Ligand Complexes
A stoichiometric quantity of the appropriate heterocyclic organic
donors (0.02 mole for MP (1.96 g), DMPHP (3.76 g), BZL (2.36 g) or BZT
(2.70 g), and 0.01 mole for ADMPHP (2.03 g) or ABZT (1.50 g) dissolved
in an excess of ethanol ( ꢀ 20 mL) was added to the solution of parent
compound, bis(benzoylacetonato)copper(II) (0.01 mole, 3.85 g) dissolved in
ethanol ( ꢀ 25 mL). The resulting mixture was refluxed for 3–4 hours at
80 °C over a hot plate equipped with a magnetic stirrer. During this time
coloured solids separated. In the case of ADMPHP and DMPHP separa-
tion of the coloured solids took place immediately. The precipitates were
filtered by suction, washed several times with ethanol and dried in vacuo to
constant weight.
Analyses and Physical Methods
Carbon, hydrogen and nitrogen were determined microanalytically at
the Central Salt and Marine Chemicals Research Institute, Bhavnagar. Cop-
per was determined as copper salicylaldoximate.[18] Conductance measure-
ments of the synthesized complexes were measured in analytical grade DMF
(10–3 M solution) on an APX 185 digital conductivity meter, using a dip-
type cell (cell constant: 1.0 cm–1). Magnetic susceptibility measurements
were made at room temperature by the Gouy method. The apparatus was
calibrated using mercury(II) tetrathiocyanatocobaltate(II), Hg[Co(NCS)4], as
the standard. Infrared spectra (4000–450 cm–1) of the uncoordinated ligands

Mixed-Ligand Complexes of Copper(II)
1067
and of the synthesized complexes were recorded in Nujol mulls supported
between sodium chloride plates on a FT-IR spectrophotometer. Electronic
spectra of the complexes were recorded in 10–3 molar dimethylformamide
solutions on a Shimadzu model UV-160 spectrophotometer at the Central
Salts and Marine Chemical Research Institute, Bhavnagar. The X-band EPR
spectra of the complexes were measured at room temperature on a Bruker
ESP X-band spectrometer using powdered samples at the microwave
frequency of 9.46–9.47 GHz at the Regional Sophisticated Instrumentation
Center, Indian Institute of Technology, Madras. The 1,1-diphenyl-2-
picrylhydrazyl (DPPH) free radical was used as an internal standard.
Thermogravimetric analysis was done by heating the sample at the rate of
15 °C min–1 up to 800 °C on a thermal analyzer at the Regional
Sophisticated Instrumentation Center, Nagpur University Campus, Nagpur.
The decomposition temperatures of the parent compound as well as the
synthesized mixed-ligand complexes were recorded by an electrically
operated Kumar Industries, Bombay, melting point apparatus with a heating
capacity up to 360 °C.
RESULTS AND DISCUSSION
Bis(benzoylacetonato)copper(II) reacted with heterocyclic chelating
donors in ethanol to yield stable hexa-coordinated complexes. The cop-
per(II) mixed-ligand complexes were prepared according to the follow-
ing equations:
Ethanol
!½CuðbzacÞ2ðLÞ2Š
½CuðbzacÞ2Š þ 2L
Reflux
L ¼ MP ð1Þ; DMPHP ð3Þ; BZL ð4Þ or BZT ð5Þ
and
Ethanol
!½CuðbzacÞ2ðLLފ
½CuðbzacÞ2Š þ LL
Reflux
L-L ¼ ADMPHP ð2Þ or ABZT ð6Þ
The resulting compounds are air-stable coloured solids (see Table 1 for
colours). The analytical data, percentage yields and decomposition
temperatures of these compounds are given in Table 1. The solubilities
of these complexes in different solvents are recorded in Table 2.

1068
Maurya, Verma, and Singh

Mixed-Ligand Complexes of Copper(II)
1069

1070
Maurya, Verma, and Singh

Mixed-Ligand Complexes of Copper(II)
1071
The compounds so obtained were characterized by various physico-
chemical techniques, viz., IR, electronic, ESR, TGA, magnetic and con-
ductance measurements as described below in detail.
Conductance Measurements
The molar conductances of the synthesized complexes in 10–3 molar
DMF solutions are in the range 3.12–18.1 ohm–1 cm2 mol–1 (Table 3).
These results are indicative of the non-electrolytic nature of these
complexes. The non-zero molar conductances of these complexes are most
probably due to the strong donor capacity of DMF which may lead to the
displacement of bzac and change of electrolyte type.[19]
Magnetic Measurements
Copper(II) has a d9 electronic configuration and its complexes contain
one unpaired electron in the d-shell. The majority of complexes are formed
by involvement of d-orbitals and they are square-planar or distorted-
octahedral. The Jahn-Teller distortion contributes a major role in the
distortion of the geometry of the complexes. The formation of tetrahedral
complexes of Cu(II) without using d-orbital was also reported to be
formed.[20] When the Cu(II) complexes display magnetic moment values
equivalent to one unpaired electron, the complexes are referred to as
magnetically dilute. In such complexes the paramagnetic metal centers are
situated apart and metal ions are surrounded by ligand molecules in such a
way that unpaired spins to neighbouring metal ions remain unaffected. In
case of tetrahedral or distorted-octahedral Cu(II) complexes the room
temperature magnetic moment values are usually observed in the range of
1.8–2.2 B.M., and are not affected appreciably by the temperature and
magnetic field. In case of square-planar dimeric or polynuclear species, the
complexes display subnormal magnetic moments. In practice, compounds
whose geometry approaches octahedral usually exhibit magnetic moments
at the lower end of the range, while those approaching tetrahedral geometry
are at the higher end. In the present investigation the Cu(II) complexes
exhibit magnetic moments in the range 1.77–1.83 B.M. (Table 2). These
data suggest octahedral geometry for these compounds.
Infrared Spectra
The important IR spectral bands and their tentative assignments are
presented in Table 3. The IR spectra of the parent compound and the
mixed-ligand complexes derived from it do not show characteristic band(s)
due to lattice or coordinated water.

1072
Maurya, Verma, and Singh
The significant absorption bands due to coordinated benzoylacetone
are n(C=O) and n(C=C), and these have been observed in the regions
1588–1600 and 1533–1566 cm–1, respectively, in all of the synthesized
complexes. These results are comparable to the data reported elsewhere.[8,9]
The ligands MP and DMPHP used in the present study contain three
potential donor sites, 1) the ring carbonyl oxygen, 2) and 3) the ring N1 and
N2, while the ligand ADMPHP possesses four potential donor sites, 1) the
cyclic carbonyl oxygen, 2) and 3) the ring N1 and N2, and 4) the exocyclic
amino nitrogen.
The ligand benzimidazole used in this study contains two potential
donor sites, 1) the imidazole NH nitrogen and 2) the imidazole tertiary
nitrogen. Benzothiazole has two potential donor sites, 1) the benzothiazole
‘S’ and 2) the benzothiazole ‘N’, whereas the 2-aminobenzothiazole pos-
sesses three potential donor sites, 1) and 2) are the same as in benthiazole,
and 3) the amino nitrogen. However, considering the planarity of the latter
ligand, ABZT, it is unlikely that this ligand could be tridentate on a single
metal. Hence, this ligand is potentially bidentate: 1) the benzothiazole-N or
-S, and 2) the amino nitrogen as the donor sites. For the sake of con-
venience the present discussion is divided in to three parts.
Complexes of MP, ADMPHP, and DMPHP
The coordination of the ring N1 in the ligand MP is unlikely because of
possible zwitterion formation[21] due to the presence of cyclic amide and
thereby reduction of electron density. The n(C=N2) band in the MP ap-
pearing at 1585 cm–1 remains almost unchanged and merged with n(C=O)
(carbonyl group of benzoylacetone) and thus suggests that the ring nitrogen
N2 is not taking part in coordination.
The coordination of N1 and N2 in ADMPHP and DMPHP is also
unlikely because of possible zwitterion formation[21] due to cyclic amide
alone (plus the steric demand of the phenyl ring), and zwitterions for-
mations due to the presence of conjugation only, respectively.
The n(N–H) bands of free ADMPHP observed at 3430 and 3320 cm–1,
respectively, are shifted to lower[22] wave numbers and then appear as a
broad band at 3300–3200 cm–1. This indicates coordination of the amino
nitrogen of ADMPHP to copper upon complex formation.
For carbonyl donors,[23] a significant shift to lower wave numbers of
n(C=O) takes place because of the coordination through carbonyl oxygen.
A comparison of the IR spectra of MP, ADMPHP and DMPHP and their
complexes shows that the n(C=O) bands observed at 1620, 1670 and 1665
cm–1, respectively, in uncoordinated MP, ADMPHP and DMPHP, respec-
tively, are shifted to lower wave numbers in the respective complexes (see

Mixed-Ligand Complexes of Copper(II)
1073
Table 3). This suggests the bonding of the cyclic carbonyl oxygen to cop-
per(II) in the complexes (1)–(3).
BZL Complex
The n(N–H) band occurring at 3050 cm–1 in benzimidazole remains
unchanged in its complex. This suggests that the NH-nitrogen of the
imidazole nucleus is not involved in coordination. However, the n(C=N)
mode occurring at 1605 cm–1 in benzimidazole, undergoes a shift to higher
frequency in the complex (4) and then appears at 1633 cm–1. This suggests
coordination of the benzimidazole via the imidazole tertiary nitrogen.[24,25]
Complexes of BZT and ABZT
It is found that the CSC band of BZT and ABZT appearing at ꢀ720
cm–1 is almost unchanged in the respective complexes, indicating that the
thiazole-S is not involved in the bonding. However, the appearance of a
band at 1616–1648 cm–1 attributable n(C=N) plus n(C=C) vibrations,[26]
suggests the involvement of the thiazole nitrogen in the coordination to
copper(II) in the complexes (5) and (6). The nas(N–H) and ns(N–H) bands
of the amino group in free ABZT, observed at 3455 and 3220 cm–1, re-
spectively, are shifted to lower wave numbers and then appeared at 3300
and 3200 cm–1 in the complex (6); this suggests coordination of the amino
nitrogen to copper.
Electronic Spectra
The electronic spectra of two representative compounds, [Cu-
(bzac)2(ADMPHP)] (2) and [Cu(bzac)2(BZT)2 (5) (Figures 2 and 3), were
recorded in 10–3 M DMF solutions. The electronic spectra of these com-
plexes show two spectral peaks at 371–375 and 652–690 nm. The second
broad peak at 652–690 nm (e = 59–94 L M–1 cm–1) is most probably due to
2
the T2g 2Eg transition, suggesting distorted-octahedral geometry for these
complexes.[27] The first spectral peak at 371–375 nm of relatively higher
intensity (e= 1316–1426 L M–1 cm–1) is due to the M L charge transfer
transition.
Electron Paramagnetic Resonance Spectra
The room temperature X-band EPR spectra of two compounds,
[Cu(bzac)2(ADMPHP)] (2) and [Cu(bzac)2(ABZT)] (6) (Figures 4 and 5),
were recorded using powdered samples at the microwave frequency

1074
Maurya, Verma, and Singh
Figure 2. Electronic spectrum of [Cu(bzac)2(ADMPHP)] (2).
Figure 3. Electronic spectrum of [Cu(bzac)2(BZT)2] (5).

Mixed-Ligand Complexes of Copper(II)
1075
Figure 4. ESR spectrum of [Cu(bzac)2(ADMPHP)] (2).
9.46–9.47 GHz. The g?, gk and gav values for these complexes were
observed at 2.062/2.068, 2.196/2.178 and 2.106/2.105, respectively.
The fact that both parallel and perpendicular features of 63Cu are re-
solved in the spectrum is characteristic of axial symmetry. Nuclear hyper-
fine coupling is observed in the gk region with only one component clearly
visible. It appears, therefore, that the second, third and fourth components
are obscured by the g? component.[28]
Figure 5. ESR spectrum of [Cu(bzac)2(ABZT)] (6).

1076
Maurya, Verma, and Singh
The trend gk (2.196/2.178) > g? (2.062/2.068) > ge (2.0036, free spin
value) observed in these two complexes shows that the unpaired electron
is in the dx2 ꢁ y2 orbital of Cu(II).[29] The gk and g? values in the com-
plexes deviate considerably from the free-spin value, and are close to those
reported for a number of distorted-octahedral Cu(II) complexes.[29]
Figure 6. Proposed cis-octahedral structures of the complexes.

Mixed-Ligand Complexes of Copper(II)
Thermogravimetric Analysis
1077
The thermogravimetric curve of a representative compounds, complex
(6), was recorded in the temperature range from ambient to 600 °C. An
observation of this curve shows that there is no mass loss of the compound
up to 170 °C. It does show a mass loss of 58.08% in the temperature range
180 to 228 °C corresponding to the loss of one molecule of benzoylacetone
and one molecule of 2-aminobenzothiazole per molecule of the complex
(calc. 58%). Further decomposition occurs in the temperature range 230–
480 °C corresponding to the elimination of one molecule of benzoylace-
tone. The final residue was found to be CuO. Thus the thermogram of this
compound corroborates some of the assumptions made on the basis of the
infrared spectral studies (vide supra).
CONCLUSIONS
The satisfactory analytical data and the physical studies presented
above suggest that the complexes are of the general formulas [Cu(b-
zac)2(L)2] (where L = MP, DMPHP, BZL or BZT), and [Cu(bzac)2(L–L)]
(where L–L = ADMPHP or ABZT). Keeping in view the normal magnetic
moments of all the complexes, ESR results of [Cu(bzac)2(ADMPHP)] and
[Cu(bzac)2(ABZT)] (involving bidentate ligands, and wherein a singlet-
triplet transition for a dimeric structure near half-field is absent in each
spectrum), and the well established cis-octahedral structure[30] of [Cu(hfa)2
(py)2] and [Cu(hfa)2(bpy)] (hfaH = hexafluoroacetylacetone and bpy = bipy-
ridine), it is reasonable to propose a similar structure for these complexes
as shown in Figure 6. The inclusion of the bidentate ligands (ADMPHP
or ABZT) spanning the trans-position of the monomeric octahedron, is
very unlikely.
ACKNOWLEDGMENTS
We are thankful to Professor S. D. Tripathi, Vice-chancellor of this
University, for encouragement, and Professor K. K. Mishra, Head, De-
partment of Chemistry, for laboratory facilities and helpful discussion.
Analytical facilities provided by the Central Drug Research Institute,
Lucknow, India, and the Regional Sophisticated Instrumentation Centre,
Indian Institute of Technology, Chennai, India, are gratefully acknowl-
edged. Thanks are also due to Referee I for constructive suggestions
regarding structures of the complexes.

1078
Maurya, Verma, and Singh
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Received May 14, 1998
Accepted March 6, 2003
Referee I: R. P. Hotz
Referee II: K. Moedritzer