# Coordination Behavior of 3-Ethoxycarbonyltetronic Acid towards Cu(II) and Co(II) Metal Ions

Authors: Giorgos Athanasellis; Georgia Zahariou; Stefanos Kikionis; Olga Igglessi-Markopoulou; John Markopoulos
Journal: Bioinorganic Chemistry and Applications
 (2008)

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## Abstract

Tetronic acids, 4-hydroxy-5H-furan-2-ones, constitute a class of heterocyclic compounds with potent biological and pharmacological activity. Theβ,β′-tricarbonyl moiety plays an integral role in biological systems and forms a variety of metal complexes. In this report, we present the complexation reactions of 3-ethoxycarbonyl tetronic acids with acetates and chlorides of Cu(II) and Co(II). These complexes have been studied by means of EPR spectroscopy and magnetic susceptibility measurements. From the obtained results, a preliminary complexation mode of the ligand is proposed.


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## Body

## 1. Introduction

The  chemistry of tetronic acids
is a field of continuing interest.  The  appreciable number of tetronic acids
found in nature [1, 2] and their very promising biological activities [3–5] prompted
many research groups to attempt new methods for the synthesis of this class of
heterocyclic compounds. Tetronic  acids and their derivatives are present in a
large number of natural products which exhibit a variety of biological and
pharmacological properties. This class of heterocyclic compounds includes
agglomerins A-D, ylidene tetronic acids [6], and the ATP-ase gastric inhibitors
A88696C and A88696F [7], as well as aspertetronins and gregatins isolated from
fungi which exhibit antibacterial and antifungal activities [8, 9].The
recent literature gives us a few examples of tetronic acids, both those
isolated from nature and those synthesized in the laboratory. Such compounds
are the CCK-B receptor antagonist tetronothiodin [10], the marine
furanosesterpene natural product (18S)-variabilin [11], and the antibiotic
abyssomicin C [12–14].The
coordination chemistry of tetronic acids has been investigated by many research
groups in the past. Studies of complexes
of oximidobenzotetronic acid complexed with Fe(II), Co(II), Ni(II), Cu(II),
Zn(II), Cd(II), and U(VI) by conductometric and by pH-metric titrations
revealed that the metals form 1:2 (metal:ligand) complexes with the exception
of Fe(II) and Co(II) complexes which form 1:3 ratios [15]. Processes for the
synthesis of a Pt(II) complex with a 3-acetyl tetronic acid [16] and Pd(II)
complexes involving tetronic acid derivatives [17] have been reported.  Furthermore, a series of 3-acyl tetronic
acids and their Cu(II) complexes, which possess a tricarbonylmethane structure,
were prepared and tested for antimicrobial activity [18]. In addition,
3-(1-iminoalkyl) tetronic acids and their Cu(II) complexes were prepared and
tested for inhibitory activity towards chlorophyll development of plants [18].
Finally, X-ray crystallographic studies of the complexes of a dinuclear
nitrogen bridged tetronic acid with Cu(II) and Ni(II) [19] showed that Cu(II)
coordinates by means of two nitrogen and two oxygen atoms of the ligand and one
water molecule on the top of a tetragonal pyramid. In contrast, the Ni(II)
complex, having an extra water molecule, forms a nearly regular octahedron
structure.In
the course of our research program on the synthesis of five membered
heterocyclic compounds, we have developed a new advantageous methodology for
the synthetic approach of functionalized tetronic [20] and thiotetronic acids [21].
The common feature of these heterocycles is the β,β′-tricarbonyl system which
provides them with sites available for metal complexation. Based upon the observation
that tetramic acid analogues with metal ions show increased biological
activities [22, 23], we have investigated the complexation of tetramic acids
with several metal ions [24–29].In this paper, we
examined the complexation of 3-ethoxycarbonyltetronic acid (HETA) (Scheme1)
with Cu(II) and Co(II) ions. We report herein our results based on the data
collected after EPR spectroscopy and magnetic susceptibility measurements, and
using these data, we propose structures for these complexes.Scheme 1
3-Ethoxycarbonyltetronic acid.


## 2. Experimental

### 2.1. Materials and Methods

Reagent grade chemicals and solvents (Fluka, Aldrich, Acros) were used
without further purification unless otherwise noted. Infrared spectra were
recorded in KBr in the range 4000–400 cm−1 on a Nicolet Magna 560R
FT-IR spectrophotometer. C, H, and N analyses were performed in the Organic
Chemistry Laboratory (NTUA) using a EuroVector EA 3000 elemental analyzer. 1H
and 13C NMR spectra were recorded on a Varian Gemini-2000 300 MHz
spectrometer. The magnetic susceptibility measurements were made using a Gouy
balance at room temperature using mercury tetrathiocyanatocobaltate (II), Hg[Co(NCS)4]  as calibrant. EPR measurements were obtained at 4.2 K
with an upgraded Bruker ER-200D spectrometer interfaced to a personal computer
and equipped with an Oxford ESR 900 cryostat, an Anritsu MF76A frequency
counter, and a Bruker 035M NMR gaussmeter. The perpendicular 4102ST cavity was
used, and the microwave frequency was 9.41 GHz. The samples used for the EPR
measurements were powders.


#### 2.1.1. 3-Ethoxycarbonyl Tetronic Acid (HETA) [20]

Powder(1.07 g,62%), m.p. 112–114C°
(Anal. Found: C,48.82; H,4.80.Calc. for C7H8O5:C,48.63;H,4.65%); vmax/cm−1(C=O)1761,1651, (C=C)1605;δH(DMSO‐d6)1.20 (3H, tJ=7.5Hz,COOCH2CH3),  4.14(2H,qJ=7.5Hz, COOCH2CH3), 4.68(2H,s,CH2ring); δC(DMSO‐d6)14.3(COOCH2CH3),   59.2(COOCH2CH3), 66.5(C‐5),
91.9(C‐3), 161.1(C‐6), 169.5(C‐2),   186.1(C‐4).


#### 2.1.2. [Cu(ETA)(OAc)] (1) (Ac = Acetyl)

A methanolic solution (12 mL) of the ligand (2.5 mmol) was
added to a refluxing methanol solution (30 mL) ofCu(OAc)2⋅H2O (2.5 mmol). The resulting solution was refluxed for 2 hours. The solution was
evaporated to a small volume and the deposited precipitate was collected by
filtration, washed with cold methanol, diethylether and dried in vacuo over P2O5.  Powder (0.53 g, 72%), μeff2.08μB; (Anal. Found: C,36.82;H,3.67. Calc.for C9H10O7Cu:C,36.70;H,3.41); vmax/cm−1(C=OandC=O)1724s, 1634s, 1554s, 1497s,1402m,     (Cu−O)525w,477w.


#### 2.1.3. [Cu(ETA)(OAc)·H2O]2 (2) (Ac = Acetyl)

A methanolic solution
(12 mL) of the ligand (1.9 mmol) was added to a refluxing methanol solution
(15 mL) ofCu(OAc)2⋅H2O (0.95 mmol). The resulting
solution was refluxed for 2 hours. The solution was evaporated up to a small
volume of the solvent and the deposited precipitate was collected by
filtration, washed with cold methanol, diethylether, and dried in vacuo over P2O5.  Powder (0.25 g, 90%), μeff1.63μB; (Anal. Found:  C,34.97;H,3.67.  Calc.forC18H24O16Cu2:   C, 34.67; H, 3.85); vmax/cm−1  (OH) 3542br, 3444br, (C=OandC=C)    1726s, 1635s, 1557s, 1499s, 1441m, 1403m,   (Cu−O)  523w, 475w.


#### 2.1.4. [Cu(ETA)2·(H2O)2] (3)

A methanolic
solution (12 mL) of the ligand (2.0 mmol) was added to a refluxing methanol
solution (10 mL) ofCuCl2⋅2H2O (1.0 mmol). The
resulting solution was refluxed for 2 hours. The solution was evaporated to a
small volume and the deposited precipitate was collected by filtration, washed
with cold methanol, diethylether, and dried in vacuo over P2O5. Light blue powder
(0.27 g, 64%), μeff2.17μB; (Anal. Found:  C,38.22;H,3.78.  Calc.forC14H16O11Cu:  C,38.04;H,4.07); vmax/cm−1 (OH)  3540br, 3436br, 3276br,  (C=OandC=C)  1724s, 1635s, 1558s, 1498s, (Cu−O)  526w,467w.


#### 2.1.5. [Co(ETA)(OAc)·CH3OH] (4) (Ac = Acetyl)

A methanolic solution (5 mL) of the ligand (1.3 mmol) was added to a refluxing methanol solution (10 mL) ofCo(OAc)2⋅4H2O (1.3 mmol). The resulting solution was refluxed for 2 hours. The solution was evaporated to a minimal volume and the deposited precipitate was collected by filtration, washed with cold methanol, diethylether, and dried in vacuo over P2O5.
Powder (0.26 g, 62%), μeff4.96μB; (Anal. Found: C,37.16;H,4.04. Calc.forC10H14O8Co:  C,37.38;H,4.36); vmax/cm−1(OH)  3466br, 3281br,  (C=OandC=C)  1711s, 1651s, 1567s, 1492s,  1441m, 1400m, (Co‐O)   528w, 432w.


## 2.1. Materials and Methods

Reagent grade chemicals and solvents (Fluka, Aldrich, Acros) were used
without further purification unless otherwise noted. Infrared spectra were
recorded in KBr in the range 4000–400 cm−1 on a Nicolet Magna 560R
FT-IR spectrophotometer. C, H, and N analyses were performed in the Organic
Chemistry Laboratory (NTUA) using a EuroVector EA 3000 elemental analyzer. 1H
and 13C NMR spectra were recorded on a Varian Gemini-2000 300 MHz
spectrometer. The magnetic susceptibility measurements were made using a Gouy
balance at room temperature using mercury tetrathiocyanatocobaltate (II), Hg[Co(NCS)4]  as calibrant. EPR measurements were obtained at 4.2 K
with an upgraded Bruker ER-200D spectrometer interfaced to a personal computer
and equipped with an Oxford ESR 900 cryostat, an Anritsu MF76A frequency
counter, and a Bruker 035M NMR gaussmeter. The perpendicular 4102ST cavity was
used, and the microwave frequency was 9.41 GHz. The samples used for the EPR
measurements were powders.


### 2.1.1. 3-Ethoxycarbonyl Tetronic Acid (HETA) [20]

Powder(1.07 g,62%), m.p. 112–114C°
(Anal. Found: C,48.82; H,4.80.Calc. for C7H8O5:C,48.63;H,4.65%); vmax/cm−1(C=O)1761,1651, (C=C)1605;δH(DMSO‐d6)1.20 (3H, tJ=7.5Hz,COOCH2CH3),  4.14(2H,qJ=7.5Hz, COOCH2CH3), 4.68(2H,s,CH2ring); δC(DMSO‐d6)14.3(COOCH2CH3),   59.2(COOCH2CH3), 66.5(C‐5),
91.9(C‐3), 161.1(C‐6), 169.5(C‐2),   186.1(C‐4).


### 2.1.2. [Cu(ETA)(OAc)] (1) (Ac = Acetyl)

A methanolic solution (12 mL) of the ligand (2.5 mmol) was
added to a refluxing methanol solution (30 mL) ofCu(OAc)2⋅H2O (2.5 mmol). The resulting solution was refluxed for 2 hours. The solution was
evaporated to a small volume and the deposited precipitate was collected by
filtration, washed with cold methanol, diethylether and dried in vacuo over P2O5.  Powder (0.53 g, 72%), μeff2.08μB; (Anal. Found: C,36.82;H,3.67. Calc.for C9H10O7Cu:C,36.70;H,3.41); vmax/cm−1(C=OandC=O)1724s, 1634s, 1554s, 1497s,1402m,     (Cu−O)525w,477w.


### 2.1.3. [Cu(ETA)(OAc)·H2O]2 (2) (Ac = Acetyl)

A methanolic solution
(12 mL) of the ligand (1.9 mmol) was added to a refluxing methanol solution
(15 mL) ofCu(OAc)2⋅H2O (0.95 mmol). The resulting
solution was refluxed for 2 hours. The solution was evaporated up to a small
volume of the solvent and the deposited precipitate was collected by
filtration, washed with cold methanol, diethylether, and dried in vacuo over P2O5.  Powder (0.25 g, 90%), μeff1.63μB; (Anal. Found:  C,34.97;H,3.67.  Calc.forC18H24O16Cu2:   C, 34.67; H, 3.85); vmax/cm−1  (OH) 3542br, 3444br, (C=OandC=C)    1726s, 1635s, 1557s, 1499s, 1441m, 1403m,   (Cu−O)  523w, 475w.


### 2.1.4. [Cu(ETA)2·(H2O)2] (3)

A methanolic
solution (12 mL) of the ligand (2.0 mmol) was added to a refluxing methanol
solution (10 mL) ofCuCl2⋅2H2O (1.0 mmol). The
resulting solution was refluxed for 2 hours. The solution was evaporated to a
small volume and the deposited precipitate was collected by filtration, washed
with cold methanol, diethylether, and dried in vacuo over P2O5. Light blue powder
(0.27 g, 64%), μeff2.17μB; (Anal. Found:  C,38.22;H,3.78.  Calc.forC14H16O11Cu:  C,38.04;H,4.07); vmax/cm−1 (OH)  3540br, 3436br, 3276br,  (C=OandC=C)  1724s, 1635s, 1558s, 1498s, (Cu−O)  526w,467w.


### 2.1.5. [Co(ETA)(OAc)·CH3OH] (4) (Ac = Acetyl)

A methanolic solution (5 mL) of the ligand (1.3 mmol) was added to a refluxing methanol solution (10 mL) ofCo(OAc)2⋅4H2O (1.3 mmol). The resulting solution was refluxed for 2 hours. The solution was evaporated to a minimal volume and the deposited precipitate was collected by filtration, washed with cold methanol, diethylether, and dried in vacuo over P2O5.
Powder (0.26 g, 62%), μeff4.96μB; (Anal. Found: C,37.16;H,4.04. Calc.forC10H14O8Co:  C,37.38;H,4.36); vmax/cm−1(OH)  3466br, 3281br,  (C=OandC=C)  1711s, 1651s, 1567s, 1492s,  1441m, 1400m, (Co‐O)   528w, 432w.


## 2.1.1. 3-Ethoxycarbonyl Tetronic Acid (HETA) [20]

Powder(1.07 g,62%), m.p. 112–114C°
(Anal. Found: C,48.82; H,4.80.Calc. for C7H8O5:C,48.63;H,4.65%); vmax/cm−1(C=O)1761,1651, (C=C)1605;δH(DMSO‐d6)1.20 (3H, tJ=7.5Hz,COOCH2CH3),  4.14(2H,qJ=7.5Hz, COOCH2CH3), 4.68(2H,s,CH2ring); δC(DMSO‐d6)14.3(COOCH2CH3),   59.2(COOCH2CH3), 66.5(C‐5),
91.9(C‐3), 161.1(C‐6), 169.5(C‐2),   186.1(C‐4).


## 2.1.2. [Cu(ETA)(OAc)] (1) (Ac = Acetyl)

A methanolic solution (12 mL) of the ligand (2.5 mmol) was
added to a refluxing methanol solution (30 mL) ofCu(OAc)2⋅H2O (2.5 mmol). The resulting solution was refluxed for 2 hours. The solution was
evaporated to a small volume and the deposited precipitate was collected by
filtration, washed with cold methanol, diethylether and dried in vacuo over P2O5.  Powder (0.53 g, 72%), μeff2.08μB; (Anal. Found: C,36.82;H,3.67. Calc.for C9H10O7Cu:C,36.70;H,3.41); vmax/cm−1(C=OandC=O)1724s, 1634s, 1554s, 1497s,1402m,     (Cu−O)525w,477w.


## 2.1.3. [Cu(ETA)(OAc)·H2O]2 (2) (Ac = Acetyl)

A methanolic solution
(12 mL) of the ligand (1.9 mmol) was added to a refluxing methanol solution
(15 mL) ofCu(OAc)2⋅H2O (0.95 mmol). The resulting
solution was refluxed for 2 hours. The solution was evaporated up to a small
volume of the solvent and the deposited precipitate was collected by
filtration, washed with cold methanol, diethylether, and dried in vacuo over P2O5.  Powder (0.25 g, 90%), μeff1.63μB; (Anal. Found:  C,34.97;H,3.67.  Calc.forC18H24O16Cu2:   C, 34.67; H, 3.85); vmax/cm−1  (OH) 3542br, 3444br, (C=OandC=C)    1726s, 1635s, 1557s, 1499s, 1441m, 1403m,   (Cu−O)  523w, 475w.


## 2.1.4. [Cu(ETA)2·(H2O)2] (3)

A methanolic
solution (12 mL) of the ligand (2.0 mmol) was added to a refluxing methanol
solution (10 mL) ofCuCl2⋅2H2O (1.0 mmol). The
resulting solution was refluxed for 2 hours. The solution was evaporated to a
small volume and the deposited precipitate was collected by filtration, washed
with cold methanol, diethylether, and dried in vacuo over P2O5. Light blue powder
(0.27 g, 64%), μeff2.17μB; (Anal. Found:  C,38.22;H,3.78.  Calc.forC14H16O11Cu:  C,38.04;H,4.07); vmax/cm−1 (OH)  3540br, 3436br, 3276br,  (C=OandC=C)  1724s, 1635s, 1558s, 1498s, (Cu−O)  526w,467w.


## 2.1.5. [Co(ETA)(OAc)·CH3OH] (4) (Ac = Acetyl)

A methanolic solution (5 mL) of the ligand (1.3 mmol) was added to a refluxing methanol solution (10 mL) ofCo(OAc)2⋅4H2O (1.3 mmol). The resulting solution was refluxed for 2 hours. The solution was evaporated to a minimal volume and the deposited precipitate was collected by filtration, washed with cold methanol, diethylether, and dried in vacuo over P2O5.
Powder (0.26 g, 62%), μeff4.96μB; (Anal. Found: C,37.16;H,4.04. Calc.forC10H14O8Co:  C,37.38;H,4.36); vmax/cm−1(OH)  3466br, 3281br,  (C=OandC=C)  1711s, 1651s, 1567s, 1492s,  1441m, 1400m, (Co‐O)   528w, 432w.


## 3. Results and Discussion

The complexesMu(OAc)x(ETA)y(H2O)z(MeOH)w  (wherein1,M=Cu,u=1,x=1, y=1,z=0,w=0;2, M=Cu, u=2,x=2,y=2,z=2, w=0;3,M=Cu,u=1,x=0, y=2,z=2,w=0;4, M=Co,u=2,x=2, y=2,z=0,w=2 (Scheme 2) were prepared by
reaction of the appropriate acetate salt M(OAc)2⋅xH2O
(x = 1, and 4 for compounds 1,2,4) or chloride salt MCl2⋅xH2O
(for compound 3,M=Cu,x=2) and HETA in MeOH under reflux by simply changing
the metal:ligand ratio.Scheme 2The complexes were
isolated as powders following evaporation of the mixture to a minimum volume. The
products were stable in the normal laboratory atmosphere and soluble in warm
MeOH. An interesting feature in the synthesis of complexes was the
impossibility of isolating a complex of the “core formula”M(ETA)2 when we used M(OAc)2xH2O:HETA in a 1:2 ratio; the only
isolable compounds were M(OAc)(ETA) complexes.The IR spectra of the metal
complexes1–4 show the ν(C=O) lactam and ν(C=O)
diketone characteristic bands shifted to lower wave numbers with respect to
those of the free ligands confirming that two oxygen atoms are involved in the
coordination sphere of the metal [30]. New bands at higher frequencies (~3550,~3450,~3300cm−1)  appeared when the ligands were complexed to the
metal ions (data not shown). These bands can be attributed to the stretching
vibrations of the OH group from coordinated water or methanol molecules.Since the IR spectra gave evidence
that the metals were successfully complexed with HETA via oxygen atoms, we
performed magnetic susceptibility studies to gain initial information about the spin states of the
metal centers.The magnetic moments
at room temperature of Cu(II) complexes1,2 and 3 (2.08 BM,
1.63 BM, and 2.17 BM, resp.) indicated that no reduction to Cu(I) had occurred,
whereas that of Co(II)
complex 4 (4.96 BM) was characteristic of octahedral stereochemistry.
The complexes 1 and 3 with d9 configuration of the
central atom are magnetically diluted systems and they have the expected values
[27]. However, for the complex 2,
with μeff=1.63 BM, the observed value was somewhat less
than the spin only value for an S=1/2 system and noticeably less than that
expected for a magnetically isolated Cu(II) system [31]. This behavior may be attributed
to the presence of weak intermolecular interactivity (possibly involving a
hydrogen bonding network), but variable temperature measurements will be required
to quantify such effects [32]. The study of these effects is currently in
progress in our laboratory.The EPR spectra  of compounds1,2 and 3 were recorded at 4.2 K (Figure 1). For compound 1, the spectrum exhibited two sets of
signals. The  first set consisted of four peaks at 1636G, 2170G, 3715G, and
4010G (indicated by asterisks, Figure 1), while the second signal defined an
asymmetric feature at the region 2507–3528G.Figure 1
X-band spectrum of compounds1,2,
and 3. EPR conditions: microwave
frequency 9.407 GHz, temperature 4.2 K, mod. ampl. 8 Gpp, microwave power 8.2 mW,
sweep time 200 seconds, t.c.: 300 milliseconds.  The asterisks
(∗) indicate unique peaks characteristic for compound 1.The presence of
the first set of signals indicates the population of anS = 1 triplet state
which is characteristic of dinuclear Cu(II) systems [33]. The spin  Hamiltonian
for a triplet state is given by the following equation [34]: H^=g⋅μB⋅H⋅S^+D⋅[Sz2−(1/3)⋅s⋅(s+1)]+E⋅(Sx2−Sy2).Here,D and E are the zero
field splitting parameters, μB is the Bohr magneton, and x,y,z are the principal axes. According to Wasson et al. [35] for
the case of a rhombic symmetry (E≠0), two transitions are allowed by the
transition rule (Δms=±1) along each principal direction, and therefore six resonance fields can be
determined. In axial symmetry (E=0), four Δms=±1 transitions are allowed.  These four resonance
fields are given by the following equations:(1)Hz1=(gegz)⋅(H0−D′),Hxy12=(gegxy)2⋅H0⋅(H0−D′),Hxy22=(gegxy)2⋅H0⋅(H0+D′),Hz2=(gegz)⋅(H0+D′), where H0=h⋅ν/g⋅μB  and  D′=D/g⋅μB.The
signals centered at 1636G, 2170G, 3715G, and 4014G, assigned to the resonance
fieldsHz1    Hxy1  Hxy2  Hz2,
respectively, are indicative of copper species in a tetragonally distorted
octahedral environment. From the above equations of four Δms=±1 transitions, the calculated parameters are
the following: D=0.13 cm−1,  gz=2.35,gxy=2.2. At higher
temperatures, the four resonance fields disappeared (data not shown). These
effects, together with the small magnitude of the D value, indicated weak interactions (mainly dipolar) between Cu⋯Cu
ions. However, the features at the region 2507–3528G consisted of a derivative
at g~2.13 and a shoulder at g~2.45. This set of signals was characteristic of a
spin doublet S = 1/2 with an axial symmetry. We assigned these signals to mononuclear
Cu(II) species in a tetragonally distorted octahedral environment with g//=2.45
and g⊥=2.13. The
fact that g//>g⊥ was
consistent with a dx2−y2 orbital ground state of the copper ion.The spectrum of compound2 consists mainly of a derivative line
at g~2.1, and an absorption peak at g~2.5. We assigned these features to an
axial S = 1/2 species with g//=2.5 and g⊥=2.1,
attributed to a monomer copper center. The fact that g//>g⊥ suggests
that the unpaired electron is localized in the dx2−y2 orbital.
Moreover, the presence of a weak signal at ~1600G, often encountered in several
copper complexes [36], is attributed to Δms=±2 transition, owing to S = 1 triplet state
population. This signal indicated the existence of the dinuclear species of
this copper compound.The EPR spectrum from compound3 exhibited an asymmetric feature at
the region 2330–3650G, which was assigned to a mononuclear copper complex. The
asymmetric form of this signal indicated g anisotropy, with g⊥=2.1. The
g// component is not well resolved. The absence of signals
characteristic for the S = 1 population indicated that dinuclear species of this
compound does not
exist.The EPR spectrum of compound4, recorded at 4.2 K, is dominated by one resonance derivative line at g2=3.5,
as well as signals at g1=8.3  and g3=2.17 (Figure 2).
These signals are characteristics of high-spin (S = 3/2) cobalt ion, whose
spectroscopic states are separated. The splitting of spectroscopic states of a
d7 configuration in coordination complexes results in two general
patterns, either in an orbitally nondegenerate ground state (4A2),
as may be found in tetra- and penta-coordinate sites, or in an orbitally
degenerated ground state (4T1) in which the orbital
levels are separated by spin-orbit coupling, as in cases of high symmetry
crystal field [37]. The magnetic moment of Co(II) for compound 4 (4.96μB) is characteristic ofoctahedral stereochemistry, suggesting
that its ground state is the orbitally degenerate 4T1.
The combined effects of spin-orbit coupling and distortion of the crystal field
from high symmetry lead to a series of Kramer doublets ms=±1/2,ms=±3/2.
The 4T1 ground state is split into a series of levels
approximately described by fictitious orbital angular momentum L = 1 and the
corresponding  J  values of 1/2(mj=±1/2),3/2(mj=±1/2,±3/2), and 5/2(mj=±1/2,±3/2,±5/2). The presence of the three g
values of the EPR spectrum indicated g anisotropy, which may be assigned to the
above combined effects.Figure 2
X-band spectrum of compound4.
EPR conditions: microwave frequency 9.407 GHz, temperature 4.2 K, mod.
ampl. 8 Gpp, microwave power 0.6 mW, sweep time 200 seconds, t.c.: 300 milliseconds.  The arrows indicate regions of g
anisotropy.


## 4. Conclusions

The isolated complexes of Cu(II) and
Co(II) acetates with HETA in 1:1 ratio have octahedral stereochemistry with
bidentate co-ordination through O(4) and O(6) of the tetronate ring and
structures of the general formulaM(OAc)x(ETA)y(H2O)z(MeOH)w.
The chloride Cu(II) complex 3  with HETA in 1:2 ratio has octahedral
stereochemistry and a structure of the formula M(ETA)2⋅2H2O.
Both Cu(II) complexes 1 and 2 showed the presence of two sets of EPR signals indicating an inhomogenity of
centers; some of them point to a mononuclear structure, while the others adopt
a dinuclear structure [38]. Moreover, EPR studies for compounds 3 and 4 showed mononuclear and dinuclear structures, respectively. In summary, we have
prepared a plausible model for the copper-cobalt β,β′-tricarbonyl coordination compounds. Our proposed model may help
define some of the unusual features associated with copper and cobalt
metallobiochemistry.



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*Source: 547915-2009-02-03.xml*