Magnetic and spectroscopic characterization of copper(II) toluate adducts with substituted piperidines

Article abstract of DOI:10.1081/SIM-120020321

New copper(II) toluate complexes with saturated monoheterocyclic bases, substituted piperidines, of the type Cu(OOCR)₂L (where R = 2-, 3-, or 4-CH₃C₆H₄ and L = 1-methyl-, 1-ethyl- or 2,6-dimethylpiperidine) have been synthesized and characterized by elemental analyses, IR, electronic reflectance and EPR spectral and magnetic susceptibility measurement studies. All of these complexes show antiferromagnetic behaviour. The spin exchange parameter, - 2J, for three of these complexes has been evaluated from EPR measurements at different temperatures, which corresponds to axial binuclear copper(II) complexes with S = 1.

Full text of DOI:10.1081/SIM-120020321

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Magnetic and Spectroscopic Characterization of
Copper(II) Toluate Adducts with Substituted Piperidines
B. S. Manhas a , A. K. Sardana a & S. B. Kalia b
a Department of Chemistry , Punjabi University , Patiala, 147‐002, India
b Department of Chemistry , Himachal Pradesh University , Shimla, 171‐005, India
Published online: 15 Feb 2007.
To cite this article: B. S. Manhas , A. K. Sardana & S. B. Kalia (2003) Magnetic and Spectroscopic Characterization of
Copper(II) Toluate Adducts with Substituted Piperidines, Synthesis and Reactivity in Inorganic and Metal-Organic Chemistry,
33:4, 519-531, DOI: 10.1081/SIM-120020321
To link to this article: http://dx.doi.org/10.1081/SIM-120020321
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SYNTHESIS AND REACTIVITY IN INORGANIC AND METAL-ORGANIC CHEMISTRY
Vol. 33, No. 4, pp. 519–531, 2003
Magnetic and Spectroscopic Characterization
of Copper(II) Toluate Adducts with
Substituted Piperidines
B. S. Manhas,1 A. K. Sardana,1 and S. B. Kalia2,
*
1Department of Chemistry, Punjabi University, Patiala, India
2Department of Chemistry, Himachal Pradesh University,
Shimla, India
ABSTRACT
New copper(II) toluate complexes with saturated monoheterocyclic
bases, substituted piperidines, of the type Cu(OOCR)2L (where R = 2-,
3-, or 4-CH3C6H4 and L = 1-methyl-, 1-ethyl- or 2,6-dimethylpiper-
idine) have been synthesized and characterized by elemental analyses,
IR, electronic reflectance and EPR spectral and magnetic susceptibility
measurement studies. All of these complexes show antiferromagnetic
behaviour. The spin exchange parameter, ꢀ 2J, for three of these com-
plexes has been evaluated from EPR measurements at different
temperatures, which corresponds to axial binuclear copper(II) complexes
with S = 1.
*
Correspondence: S. B. Kalia, Department of Chemistry, Himachal Pradesh Univer-
sity, Shimla 171 005 India; E-mail: sanjay263@rediffmail.com.
519
DOI: 10.1081/SIM-120020321
0094-5714 (Print); 1532-2440 (Online)
www.dekker.com
Copyright D 2003 by Marcel Dekker, Inc.

520
Manhas, Sardana, and Kalia
INTRODUCTION
A literature survey reveals that there has been considerable interest in
the chemistry of metal(II) carboxylates and their complexation properties
with nitrogen and oxygen donor ligands[1–7] and only some workers have
examined the role of ‘‘steric hindrance’’ or electronic nature of the
substituent group on pyridine type ligands[3,5–10] in their complexation
behaviour with cobalt(II), nickel(II) and copper(II) carboxylates. No such
systematic efforts have yet been made for saturated heterocyclic bases.
Thus, in the present work a study of the complexation properties of
substituted piperidines with copper(II) toluates has been undertaken.
EXPERIMENTAL
The bases, 1-methylpiperidine, 1-ethylpiperidine and 2,6-dimethylpi-
peridine (Aldrich Chemicals or K and K Laboratories) were dried by
refluxing over sodium hydroxide beads. The colourless liquids obtained
after distillation were stored over sodium hydroxide beads.
Preparation of Copper(II) Toluates
(ortho-, meta- and para-)
To the solution of toluic acid (Sisco) (5 g, 36.72 mmol) in the
minimum quantity of acetone (30 mL) was added drop-wise with stirring at
room temperature a solution of sodium hydroxide (1.469 g, 36.72 mmol) in
distilled water (8 mL). The pH of the resulting solution of sodium toluate
was then adjusted to 5 by adding an appropriate amount of toluic acid. A
filtered solution of copper(II) sulfate pentahydrate (4.585 g, 18.73 mmol) in
distilled water (30 mL) was then added to the stirred solution of sodium
toluate. The precipitate, thus obtained, was filtered, washed with distilled
water and dried in air. It was then dried at 110 °C for two hours.
Preparation of Complexes
Copper(II) toluate (1 g, 2.99 mmol) was suspended in diethyl ether
(30 mL) in a round-bottom flask. A solution of the stoichiometric amount
or 50% excess (Table 1) of heterocyclic base in the same solvent (15 mL)
was added drop-wise after successive intervals of 10 min. in a total period
of about 2.5 h to the continuously stirred suspension of copper(II) toluate.
Coloured solutions of the adducts (Table 1) were obtained as the solid
copper(II) toluate dissolved. The solid adducts of the bases separated after

Copper(II) Toluate Adducts
521

522
Manhas, Sardana, and Kalia
the contents had been stirred for another two hours and then cooled. The
separated product was filtered in a G4 filtration unit, washed with acetone
and diethyl ether, dried under vacuum and placed in a desiccator containing
anhydrous calcium chloride.
Elemental Analyses and Physical Measurements
Copper in the complexes was determined volumetrically by EDTA tit-
ration using xylenol orange as indicator. Carbon and hydrogen analyses were
performed on an automatic Coleman-33 analyser, while nitrogen was deter-
mined by Kjeldahl’s method in the departmental microanalytical laboratory.
Infrared spectra of the copper(II) toluate (ortho-, meta- and para-)
complexes with the heterocyclic bases, and their anhydrous sodium toluates
were recorded as KBr pellets on a Pye Unicam SP3-300 infrared spec-
trophotometer in the 4000–200 cm–1 region. Electronic reflectance spectra
of the complexes (either neat or diluted with an appropriate amount of
MgCO3) were recorded on a Unicam SP-700 UV-Vis. spectrophotometer
having a SP-735 diffuse reflectance attachment (50,000–4000 cm–1). The
X-band EPR measurements (9.439–9.442 GHz) of powdered samples at
room temperature were made on a JES-FE3XG electron paramagnetic re-
sonance spectrometer while the X-band EPR measurements (8.916–9.190
GHz) of powdered samples of three complexes at different temperatures
were made on a Varian E-4 electron paramagnetic resonance spectrometer.
Diphenylpicrylhydrazyl (DPPH) was used as calibrant. The room tempe-
rature magnetic susceptibility of all of the complexes was measured on a
Gouy balance using Hg[Co(NCS)4] as calibrant.
RESULTS AND DISCUSSION
Copper(II) toluates were prepared by reacting aqueous solution of
copper(II) sulfate with stoichiometric amounts of sodium salts of the cor-
responding toluic acids in 1:1 water–acetone mixture. The synthesis of the
adducts of copper(II) toluates with substituted piperidines was performed by
reacting their diethyl ether solutions in 1:05; 1:1 and 50% excess of
stoichiometric amounts of the bases whereby only complexes of the 1:1
stoichiometry were obtained by the following equations.
n-CH3C6H4COOH þ NaOH ! n-CH3C6H4COONa þ H2O
ð1Þ
2ðn-CH3C6H4COONaÞ þ CuSO4Á5H2O
! CuðOOCC6H4CH3-nÞ2 þ 5H2O þ Na2SO4
ð2Þ

Copper(II) Toluate Adducts
523
CuðOOCC6H4CH3-nÞ2 þ L ! CuðOOCC6H4CH3-nÞ2L
where n = 2, 3 or 4 and L = 1-PePip, 1-EtPipd and 2,6-Me2Pipd.
ð3Þ
The complexes are insoluble in organic solvents and do not melt, but
decompose above 250 °C. The interest in the present investigations was to
see the role of pKa of the parent carboxylic acid on the structure and the
effect of steric and electronic properties of the six-membered saturated
monoheterocyclic bases on the stoichiometry, magnetic behaviour and
structure of the complexes. Chemical formulae based upon elemental ana-
lyses, colour and particulars of the method of preparation of complexes are
listed in Table 1.
IR Spectral Studies
The structural possibilities for complexes of the stoichiometry
Cu(OOCR)2L depend upon the mode of coordination of the carboxylate
group and the monoheterocyclic bases, the substituted piperidines. IR
studies are quite useful in determining the mode of coordination of these
ligands.[5] Examination of the positions and direction of the shifts of the
na(OCO) and ns(OCO) bands of the carboxylate group in the complexes as
compared to their positions in sodium toluates (Table 2), suggests a
bridging bidentate mode of coordination of the carboxylate group for all of
the complexes.[11] Additional bands in the complexes, by comparison with
the IR spectra of uncomplexed copper(II) toluates and free bases in the
regions 410–465 cm–1 and 275–320 cm–1, tentatively have been assigned
to n(Cu–O) and n(Cu–N) modes, respectively.[12–16]
Table 2. Some coordinatively diagnostic features of infrared spectra (cm–1) of the
sodium salts of carboxylic acids and their copper(II) complexes.
Compound
na(OCO) ns(OCO) n(Cu–O) n(Cu–N)
Cu(OOCC6H4CH3-4)2(1-MePipd)
Cu(OOCC6H4CH3-4)2(1-EtPipd)
Cu(OOCC6H4CH3-4)2(2,6-Me2Pipd)
Cu(OOCC6H4CH3-3)2(1-MePipd)
Cu(OOCC6H4CH3-3)2(1-EtPipd)
Cu(OOCC6H4CH3-3)2(2,6-Me2Pipd)
Cu(OOCC6H4CH3-2)2(1-EtPipd)
4-CH3C6H4COONa
1562 s
1598 s
1580 s
1573 s
1585 s
1580 s
1560 s
1545 s
1560 s
1515 s
1408 vs
1408 vs
1408 vs
1415 vs
1402 vs
1410 vs
1420 vs
1406 vs
1405 vs
1403 vs
450 m
458 m
450 m
420 m
410 m
430 m
465 m
305 s
315 s
300 s
275 s
290 s
320 s
305 s
3-CH3C6H4COONa
2-CH3C6H4COONa

524
Manhas, Sardana, and Kalia
Table 3. Electronic reflectance spectral data (cm–1) and room temperature meff
values of the antiferromagnetic copper(II) toluates with substituted piperidines.
Absorption II
ligand ! metal
Absorption I
d–d transitions
charge transfer
transitions
Meff
(B.M.)
Compound
Cu(OOCC6H4CH3-4)2(1-MePipd)
Cu(OOCC6H4CH3-4)2(1-EtPipd)
Cu(OOCC6H4CH3-4)2(2,6-Me2Pipd)
Cu(OOCC6H4CH3-3)2(1-MePipd)
Cu(OOCC6H4CH3-3)2(1-EtPipd)
Cu(OOCC6H4CH3-3)2(2,6-Me2Pipd)
Cu(OOCC6H4CH3-2)2(1-EtPipd)
13,890
13,795
13,795
13,605
13,515
13,985
13,335
35,715
37,035
37,035
32,260
33,335
33,335
33,335
1.353
1.493
1.432
1.387
1.516
1.452
1.638
Electronic Reflectance Spectral Studies
Two absorptions (Table 3), one as a broad band in the region 13,985–
13,335 cm–1 (band I) and the other as a shoulder around 32,260–37,035
cm–1 (band II), are observed for all of the complexes under study. Band I
may be due to the absorption corresponding to the dxy !dx2ꢀy2 and dxz,
dyz ! dx2ꢀy2 electronic transitions for the copper(II) ion in tetragonal
environments.[17] The band II can be assigned to the 2pp(O) ! (dx2ꢀy2)*.
*The star is put to indicate the antibonding nature of the dx2ꢀy2 orbital
charge-transfer transition; the appearance of this band is considered to
be characteristic of the dimeric structure for copper(II) carboxylate
complexes.[17–19]
EPR Studies
The X-band room temperature EPR spectra of powdered samples of all
the complexes under discussion exhibit three electron paramagnetic
resonance absorption lines at the field strengths 600 G, 4700 G and 6100
G, which are characteristic of the spin-triplet state (S = 1) in the axial
symmetry of binuclear complexes and correspond, respectively, to the
resonance fields Hz1, H and Hz2 with D > hv and E = 0,[20,21] the zero
?
field splitting parameters for a triplet state. That these complexes do not
seem to undergo rhombic distortion, is confirmed by the absorption line at
4700 G (H ) not being split even at low temperatures. The range of the
?
electron paramagnetic resonance parameters g or gz (2.41–2.5), g (2.10–
k
?
2.12) and D (0.37–0.39) (Table 4), as evaluated from the EPR spectra, are

Copper(II) Toluate Adducts
525

526
Manhas, Sardana, and Kalia
Figure 1. Electron paramagnetic resonance spectrum of [Cu(OOCC6H4CH3-4)2-
(1-EtPipd)] at 113 K.
comparable with the earlier reported values for binuclear copper(II) car-
boxylate complexes.[4–7] In the complex Cu(OOCC6H4CH3-4)2(1-EtPipd) at
low temperature (113 K) the electron paramagnetic resonance absorption
band around 600 G (Hz1) is split into seven lines having an intensity ratio
of 1:2:3:4:3:2:1 due to hyperfine coupling of the nuclear spin of two copper
ions with the electron spin (2nI + 1 lines; n = 2 and I = 3/2 for 63Cu)
(Figure 1). The observed value of A is found to be about one half
k
(0.006968 cm–1) of that generally observed for magnetically dilute cop-
per(II) complexes in similar environments. The value of A lies close to the
k
values reported earlier[21,22] for binuclear copper(II) carboxylate complexes.
This observation further supports the magnetic exchange-taking place
between two metal ions[21,22] (in spin–spin coupled complexes). In addition
to the above-mentioned absorptions, an absorption line around 3200 G is
also observed. The intensity of this line increases while the intensity of the
lines due to the triplet state, as expected, decreases on lowering of the
temperature. This line is, therefore, attributed to the magnetically dilute

Copper(II) Toluate Adducts
527
copper(II) impurity which is commonly present in copper(II) arylcarbox-
ylate complexes.[20,23,24]
Magnetic Susceptibility Studies
The room-temperature meff values for all of the copper(II) complexes
lie in the range 1.35–1.64 B.M. (Table 3) which is less than the spin-only
value for one unpaired electron (1.73). These values are, however, close to
the values reported for dinuclear or polynuclear copper(II) carboxylate
complexes in which the carboxylate group is acting as a bridge between
two metal ions[6,7,13] and spin–spin interaction exists. For three complexes
the value of the spin exchange parameter (ꢀ2J) has been calculated from
the variation of the intensity (I) of the EPR lines at the temperatures 223,
173 and 113 K using the relation of Eq. 4.[20,24]
T2ð3 þ eꢀ2J=kT
T1ð3 þ eꢀ2J=kT
Þ
Þ
2
A1
A2
I1
I2
¼
¼
ð4Þ
1
The relative areas of the curves [A1/A2] at two different temperatures (T1
and T2) were calculated by the reported methods.[25] Uncertainties in the
values of ꢀ2J by this method are 11 cm–1. The EPR method, although
it is free from the presence of the magnetic impurity as compared to
the magnetic susceptibility method for calculating ꢀ2J, suffers from the
limitation in evaluating the accurate areas under the EPR curves. The
values of ꢀ2J (Table 3) are comparable to those for the earlier reported
copper(II) carboxylate complexes with terminal ligands of low basicity, e.g.
pyridine, etc.
For all of the complexes under study which exhibit antiferromagnetic
behaviour, a copper(II) acetate monohydrate-like dinuclear structure[26]
(Figure 2) may be assigned.
The literature[6,7] reveals that the variation of ꢀ 2J values with the
properties of the terminal ligand is not a simple function of the donor
strength of the ligand or any other readily recognizable parameter. The
study of antiferromagnetic interactions in the copper(II) arylcarboxylate
complexes had been considered to be of special interest since a predictable
variation in the electron donor properties of the arylcarboxylate group can
be effected much more easily by varying the substituent and its position on
the aryl ring. However, attempts at arriving at a firm correlation between
ꢀ2J values and the donor ability of the arylcarboxylate anion or the pKa of
the parent acid have been disappointing which, in view of the earlier
reports, is not surprising.[5] This is partly because, in general, ꢀ2J values

528
Manhas, Sardana, and Kalia
Figure 2. Proposed structure for the complexes Cu(OOCR)2L.
for arylcarboxylates are lower than those of alkylcarboxylates and the data
for reasons like limitations in the availability of resources are imprecise.[4,5]
Consequently, the variation in ꢀ 2J values for the complexes for which
these have been determined is small and the differences may well be of the
order of the experimental error.
The general lack of correlation between the extent of antiferromagnetic
interaction and the pKa’s and the donor strength of the ligands (terminal
ligand and the bridging carboxylate) may be explained as follows: Firstly,
in view of the well-known concept of soft and hard acids and bases[26] and
Drago and Wayland’s[27] two-parameter model for acids and bases, it is not
proper to assume that the donor ability toward a particular acceptor of
different types of ligands will follow the trend expected from the pKa
values of the ligands or the parent acids. Since the pKa values are
determined by the ability of the donor atoms for protons only, only ligands
having similar donor atoms and an electronic charge on them, with similar
steric and other electronic characteristics, can be compared as regards the
interactions with a particular metal in a complex. The reason for this is that
here the acceptor (metal ion) has particular electronic characteristics and
stringent steric requirements. Secondly, even if it is assumed that the donor
ability of the ligands is determined only by their pKa’s, the larger amount
of charge transferred to the copper ions by the ligands considered to be
stronger donors on this basis may not lead to larger antiferromagnetic
interactions. While expecting a correlation between donor strength and ꢀ2J
values in copper(II) complexes, it is assumed that a greater electronic
charge transferred by a stronger donor would result in a smaller residual

Copper(II) Toluate Adducts
529
positive charge on copper ions and a greater expansion and overlap of the d
orbitals containing unpaired electrons in a metal–metal direct interaction.
However, these days, indirect exchange is considered to be the predominant
mechanism, which does not seem to require expansion of d orbitals for
adequate overlap with ligand orbitals. Moreover, the stronger linkage of a
bridging ligand will push dx2ꢀy2 orbitals on copper(II) to a higher energy
level and thus cause the degree of mixing of dxy* orbitals into dx2ꢀy2
orbitals (which contain unpaired electrons) to be diminished. This will lead
to a reduction in the indirect exchange interaction and consequent dimi-
nution in antiferromagnetic interaction, since the indirect exchange takes
place largely through interaction of the delocalized p-bonding framework of
the bridging carboxylates with dxy* orbitals of the metal ions.[28] The
smooth correlations observed/claimed in a few cases, especially for the
bridging ligand, could just be fortuitous.
CONCLUSIONS
Our interest in the present work was to study the complexation
behaviour of saturated monoheterocyclic bases of different but high ba-
sicities and different steric characteristics with copper(II) carboxylates.
An additional interest was to see whether these bases form magnetically
dilute or antiferromagnetic complexes. In the present study all the com-
plexes have been found to be antiferromagnetic. The results of analysis
show that only complexes with 1:1 stoichiometry were obtained and that
the steric hindrance of the amine ligands plays an important role in
determining the stoichiometry, structure and, hence, the magnetic pro-
perties of the complexes.
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Received: November 15, 2001
Accepted: November 28, 2002
Referee I: M. A. Green
Referee III: J. D. Zubkowski