The phthalhydroxamate ligand and its divalent transition metal complexes: Synthesis, characterization, and spectral and thermal studies

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THE PHTHALHYDROXAMATE LIGAND AND ITS DIVALENT
TRANSITION METAL COMPLEXES: SYNTHESIS,
CHARACTERIZATION, AND SPECTRAL AND THERMAL
STUDIES
Fatih Yilmaz a & Veysel T. Yilmaz b
a Department of Chemistry, Faculty of Arts and Sciences , Ondokuz Mayis University , 55139,
Kurupelit, Samsun, Turkey
b Department of Chemistry, Faculty of Arts and Sciences , Ondokuz Mayis University , 55139,
Kurupelit, Samsun, Turkey
Published online: 09 Dec 2011.
To cite this article: Fatih Yilmaz & Veysel T. Yilmaz (2001) THE PHTHALHYDROXAMATE LIGAND AND ITS DIVALENT TRANSITION
METAL COMPLEXES: SYNTHESIS, CHARACTERIZATION, AND SPECTRAL AND THERMAL STUDIES, Synthesis and Reactivity in
Inorganic and Metal-Organic Chemistry, 31:8, 1421-1432, DOI: 10.1081/SIM-100107207
To link to this article: http://dx.doi.org/10.1081/SIM-100107207
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SYNTH. REACT. INORG. MET.-ORG. CHEM., 31(8), 1421±1432 (2001)
THE PHTHALHYDROXAMATE LIGAND
AND ITS DIVALENT TRANSITION METAL
COMPLEXES: SYNTHESIS,
CHARACTERIZATION, AND SPECTRAL
AND THERMAL STUDIES
Fatih Yilmaz andVeysel T. Yilmaz *
Department of Chemistry, Faculty of Arts and Sciences,
Ondokuz Mayis University, 55139 Kurupelit,
Samsun, Turkey
ABSTRACT
The potassium salt of phthalhydroxamic acid (PHA) (alter-
natively named o-carboxybenzohydroxamic acid) was syn-
thesized and its coordination behaviour with Mn(II), Co(II),
Ni(II), Zn(II) and Cd(II) was studied. The hydroxamate
ligand reacts with the transition metal ions to form poly-
meric complexes. The ligand is coordinated to the metal ions
through the deprotonated O and carbonyl O of the hydro-
xamate group, and the carboxylate group, behaving as
a tetradentate ligand. KPHA and its solid metal complexes
were characterized by elemental analyses, electronic and
*Corresponding author. E-mail: vtyilmaz@omu.edu.tr
1421
Copyright # 2001by Marcel Dekker, Inc.
www.dekker.com

1422
YILMAZ AND YILMAZ
IR spectroscopy, and magnetic moment measurements.
The thermal decomposition behaviour of the ligand and the
metal complexes was also studied. On the basis of
the experimental data, structures of the complexes are
proposed.
INTRODUCTION
The biological and medical importance of hydroxamic acids are now
well recognised due to their involvement in processes such as microbial iron
transport1, inhibition of the nickel-dependent urease enzymes2,3 and the
zinc-dependent matrix metalloproteinases4. This activity is largely due to the
presence of the hydroxamate group at the active sites of some biological
enzymes and the ability of the hydroxamate group to form very stable
chelates with metal ions.
The complexing and physico-chemical properties of a wide range of
hydroxamic acids in solution and the solid state were reported5719. Chat-
terjee20 reviewed the donor properties of hydroxamic acids. In metal com-
plexes of simple hydroxamic acids, the ligands behave as bidentate O,O0
donors. However, if the ligand contains other donor sites adjacent to the
hydroxamate group, a range of coordination possibilities exists. For ex-
ample, benzohydroxamic acid acts as a bidentate ligand through the car-
bonyl and deprotonated oxygens, and acetylation of the ligand results in the
formation of acetylsalicylhydroxamic acid which behaves as a tridentate
ligand with O,O0,O00 donor atoms17.
In this work, we report the synthesis, characterization, spectral and
thermal analysis of the title novel hydroxamate ligand (Fig. 1) and its
solid complexes with manganese(II), cobalt(II), nickel(II), zinc(II) and
cadmium(II).
Figure 1. The structure of KPHA.

PHTHALHYDROXAMATE COMPLEXES
EXPERIMENTAL
Reagents andInstruments
1423
Phthalic acid, hydroxylamine hydrochloride and metal salts were
purchased from Merck. All other reagents were of the purest available
grade. Electronic spectra were measured using a Unicam UV2 spectro-
photometer in N-methylformamide in the 2007800 nm range. IR spectra
were recorded on a Mattson FTIR spectrophotometer as KBr pellets. Room
temperature magnetic susceptibility measurements were carried out ac-
cording to the Evans method using a Sherwood Scienti®c MXI model Gouy
_
magnetic balance. C, H and N analyses were carried out at the TUBITAK
Marmara Research Centre (Gebze). The simultaneous TG, DTG and DTA
curves were obtained using a Rigaku TG8110 thermal analyser combined
with a TAS100 thermogravimetric analyser. The experiments were carried
out in a dynamic nitrogen atmosphere with a ¯ow rate of 80 mL minÀ1 in the
temperature range 2071000 ꢀC, using platinum crucibles. Sample sizes of
5710 mg and a heating rate of 10 ꢀC minÀ1 were used. Sintered a-alumina
was used as the reference material.
Synthesis of Potassium Phthalhydroxamate (KPHA)
Diethyl Phthalate
Phthalic acid (49.8 g, 300 mmol) was dissolved in 150 mL of ethanol.
The solution was acidi®ed with H2SO4 (98%, 6.50 mL) and re¯uxed with
stirring for 6 h. The excess ethanol was removed on a rotary evaporator and
H2SO4 was neutralized with a 5% aqueous NaHCO3 solution until for-
mation of CO2 ceased. The formed diethyl phthalate was washed with H2O
and separated in a separatory funnel. Yield, 45.2 g (67.3%).
Potassium Phthalhydroxamate
NH2OHÁHCl (1.74 g, 25 mmol) dissolved in methanol (20 mL) was
mixed with a methanol solution (20 mL) containing 1.40 g (25 mmol) of
KOH. KCl precipitated from the solution was removed by suction ®l-
tration and then, a methanol solution (20 mL) with 1.40 g (25 mmol) KOH
was added to the ®ltrate. Subsequently, diethyl phthalate (4.90 g, 25 mmol)
dissolved in 20 mL methanol was added to the NH2OH solution and
stirred for 1h at room temperature. A colour change to yellow occurred

1424
YILMAZ AND YILMAZ
during the reaction. The volume of the resulting solution was reduced to
15 mL by evaporating at 50 ꢀC and adding 25 mL acetone to the solution
precipitated the potassium salt of PHA. The pale yellow-orange pre-
cipitate was recrystallized in H2O and dried in a desicator. Yield, 3.5 g
(72.5%).
Synthesis of Metal Complexes of PHA
The metal salts (MnSO4ÁH2O, CoSO4Á7H2O, NiSO4Á7H2O,
Zn(NO3)3Á8H2O or CdCl2ÁH2O) (3 mmol) dissolved in distilled H2O
(30 mL) were added with stirring to a solution of KPHA (0.66 g, 3 mmol)
in H2O (30 mL). The Mn(II), Co(II), Zn(II) and Cd(II) complexes pre-
cipitated immediately after mixing of the solutions of the metal salt and
the ligand. The Ni(II) complex crystallized after standing at room tem-
perature for several days. The solid complexes were ®ltered by suction,
washed with H2O and dried in a desicator. The Cr(III), Fe(II), Fe(III) and
Cu(II) complexes were formed in solution. However, attempts to isolate
solid chromium, iron and copper complexes were not successful due to co-
precipitation of the metal complexes, starting materials and other reaction
products.
RESULTS AND DISCUSSION
Synthesis
The preparation of KPHA is shown in eqs (1) and (2). The structure
proposed for the ligand is consistent with the elemental analysis data
(Table I), electronic and IR spectral data (Table II). KPHA is air-stable, but
somewhat hygroscopic when exposed to air for a long time and highly
soluble in water. KPHA melts in the temperature range 1337137 ꢀC.
The analytical data and some physico-chemical properties of the iso-
lated solid metal complexes are given in Table I. These complexes are air-
stable and were obtained in high yield. They are insoluble in common
solvents, but sparingly soluble in N-methylformamide. As will be discussed
later, thermal analysis data show that all of the metal complexes, except the
Mn(II) and Cd(II) complexes, contain a molecule of water of crystallization
in addition to coordinated water. The metal complexes do not have melting
points, but are thermally stable up to the decomposition points given in
Table I.

PHTHALHYDROXAMATE COMPLEXES
1425
ꢁ1†
ꢁ2†
Spectroscopic andMagnetic Properties
KPHA displays a broad and complex band in the UV region. This
band exhibits mainly eight maxima in the 2157300 nm wave length range
due to n ! pà and p ! pà transitions. Since the metal complexes are only
sparingly soluble in N-methylformamide, their saturated solutions were used
to record the UV-VIS spectra. In addition to the intraligand transitions
between 200 and 300 nm, the Mn(II) complex exhibits several comparatively
narrow d-d bands with very low intensity and the Co(II) and Ni(II) com-
plexes show broad bands with low intensity in the 4007800 nm range, which
are assigned to d-d transitions of octahedral coordination21 (Table II). The
Zn(II) and Cd(II) complexes display only transitions of PHA in the UV
region. The room temperature magnetic moments of the metal complexes
are in good agreement with octahedral coordination (Table I). However, the
corresponding zinc(II) and cadmium(II) complexes are diamagnetic as
expected.
Table II summarizes the most signi®cant frequencies in the IR spectral
data of KPHA and its corresponding metal complexes. A broad band
centered at 3400 cmÀ1 is observed in the IR spectra of the free ligand and is
assigned to the n(OH) absorption of the carboxyl group with the strong
hydrogen bond. This band also appears in the metal complexes at a higher
frequency range and con®rms the presence of both coordinated and lattice
water. The typical N-H stretching bands in the free ligand and the metal
complexes appear as medium bands at 320073350 cmÀ1. The N-H frequency
of the free ligand is somewhat lower than those of the metal complexes and
this also indicates its participation in intramolecular hydrogen bonding,
probably with the carbonyl oxygen of the hydroxamate group. The IR
spectrum of KPHA has a strong adsorption band at 1645 cmÀ1 due to the

1426
YILMAZ AND YILMAZ

PHTHALHYDROXAMATE COMPLEXES
1427

1428
YILMAZ AND YILMAZ
CˆO vibration of the hydroxamate carboxyl group. Upon complexation
with the metal ions, the absorption band of the hydroxamate carbonyl of
KPHA shifted signi®cantly to lower frequencies in the spectra of the metal
complexes (Table II); such behaviour clearly provides evidence for co-
ordination through the carbonyl oxygen in addition to the OÀ site of the
same group11717. The carboxyl bands of hydroxamic acids are shifted to
lower wave numbers when coordinated to a metal20. The strong bands at
1560 and 1385 cmÀ1 are attributed to the asymmetric (nasym) and symmetric
vibrations (nsym) of the carboxylate group of KPHA, respectively. The Dn
(nasymÀnsym) values observed for the metal complexes are usually used to
determine the mode of coordination of the carboxylate ligands, comparing
them with the Dn values of parent carboxylate ligands22. The Dn values
indicate bidentate coordination of the carboxylate group of KPHA
(Table II). The C-N vibrations occurred at ca. 1320 cmÀ1 for KPHA and the
metal complexes, while the stretching vibrations of the M-O bonds were
observed in the range 3007500 cmÀ1 as weak bands22.
Thermal Analysis
Thermoanalytical data (mass losses, temperature ranges, peak tem-
peratures and residues) of the ligand and the metal complexes are given in
ꢀ
Table III. KPHA melts at ca. 1 35 C and begins to decompose at 175 ꢀC.
The highly exothermic peak at 183 ꢀC corresponds to elimination of CO2
and subsequent formation of benzohydroxamic acid (BHA) as intermediate,
based on the IR spectra and stoichiometric calculations (mass loss, found
20.5%; calcd. 20.1%). This product is stable up to 284 ꢀC and decomposes
in several steps to give potassium oxide as the end product.
The metal complexes lose water in the ®rst stage of decomposition,
giving dehydrated metal complexes. The second stage usually corresponds
to removal of CO2. Mass loss calculations suggest formation of BHA
adducts of Mn(II) and Co(II) as solid intermediates, whereas the inter-
mediates of the Ni(II), Zn(II) and Cd(II) complexes do not exhibit a de®nite
chemical compositions. Although the literature suggests that N-hydroxy-
lactams are common intermediates in the thermal decomposition of metal
hydroxamates6,7,18, the relatively few studies do not consider this conclusion
as a general rule8. According to the present work, it seems that the de-
composition of the metal chelates of hydroxamic acids depends on the
chemical and ligation nature of the hydroxamic acids. The degradation of
the organic residues takes place in the subsequent stages with a series of
endotherms and exotherms to give the respective metals (Table III). The
®nal decomposition products indicate a reductive character for all the metal

PHTHALHYDROXAMATE COMPLEXES
1429

1430
YILMAZ AND YILMAZ
complexes in nitrogen atmosphere. Although the nature of the degradation
reactions is not known in detail, it is assumed that volatile products such CO
formed during degradation may act as reducing agent for the M(II) ions.
The formation of the metals as the end products was detected by their
lustrous shiny appearance and supported by mass loss calculations.
Structures of Metal Complexes
Transition metal complexes of hydroxamic acids are frequently used as
bioinorganic models to understand enzymatic interactions and binding
properties of these bioligands. Hydroxamic acids usually behave as bi-
dentate ligands by using carbonyl oxygens and hydroxamic hydroxyl oxy-
gens with the release of hydrogen ions11720. The addition of the carboxyl
group to BHA resulted in the formation of an additional donor atom, so
that, in these complexes, PHA acts as a tetradentate ligand (O,O0,O00,O000).
The ligand in the metal complexes is a dianion resulting from carboxylate
and hydroxamate group ionization. This ®nding is supported by the mi-
croanalyses and spectral data. The insolubility of the complexes in common
solvents suggests a polymeric structure through carboxylate and hydro-
xamate groups. It was reported that dihydroxamic acids with additional
donor groups have a tendency to form polynuclear complexes16. The ligand
coordinates to the metal ions to form a tetrahedral coordination with a
metal to ligand ratio of 1:1. On the basis of the observed spectral and
physico-chemical properties, the structures shown in Fig. 2 are proposed for
the metal complexes of PHA.
Figure 2. The proposed structure of the Mn(II), Co(II), Ni(II), Zn(II), and Cd(II)
complexes of PHA.

PHTHALHYDROXAMATE COMPLEXES
ACKNOWLEDGMENT
1431
This work was supported by the Research Fund of Ondokuz Mayis
University, Samsun, Turkey.
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Received November 9, 2000
Accepted June 29, 2001
Referee II: R. D. Pike
Referee III: B. Durham