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Synthesis and MagneticExchange Interaction of µTetracarboxylato BinuclearCobalt(II) ComplexesYan-Tuan Li a & Cui-Wei Yan ba Department of Chemistry , Qufu NormalUniversity , Qufu, Shandong, 273165, P. R. Chinab Department of Biology , Qufu Normal University ,Qufu, Shandong, 273165, P. R ChinaPublished online: 28 Apr 2008.

To cite this article: Yan-Tuan Li & Cui-Wei Yan (2000) Synthesis and MagneticExchange Interaction of µ Tetracarboxylato Binuclear Cobalt(II) Complexes, Synthesisand Reactivity in Inorganic and Metal-Organic Chemistry, 30:6, 1069-1082, DOI:10.1080/00945710009351820

To link to this article: http://dx.doi.org/10.1080/00945710009351820

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S W . REAn. INORG. MET.-ORG. CHEh4.. 30(6), 1069-1082 (2000)

SYNTHESIS AND MAGNETIC EXCHANGE INTERACTION OF p- TETRACARBOXYLATO BINUCLEAR COBALT(I1) COMPLEXES

Y an-Tuan Li" Department of Chemistry, Qufu Normal University,

Qufu, Shandong, 273165, P. R. China

Cui-Wei Yan Department of Biology, Qufu Normal University,

Qufu, Shandong, 273165, P. R. China

ABSTRACT

Four new tetracarboxylato-bridged binuclear cobalt(I1) complexes have

been synthesized and characterized, namely [Co2(PMTA)L4], where L

denotes 4,7-diphenyl-l, 1 0-phenanthroline (Ph2-phen); 2,9-dimethyl-l,10-

phenanthroline (Me2-phen); diaminoethane (en); 1,3-diaminopropane (pn)

and PMTA represents the tetraanion of pyromellitic acid. Based on elemental

analyses, molar conductivity measurements, spectroscopic (electronic and R spectra) studies, it is proposed that these complexes have PMTA-bridged

structures and consist of two cobalt(I1) ions in a distorted octahedral

environment. The complexes [c@(PMTA)(Ph2-~hen)~] (1) and [Co2(PMTA)-

(Mez-phen)4] (2) were further characterized by variable temperature magnetic

susceptibility measurements (4-300 K) and the observed data were

successfully simulated by the equation based on the spin Hamiltonian

Copyright Q ZOO0 by Marcel Dekkcr. Inc.

1069

www.dckkcr.com

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1070 LI AND YAN

operator, H = -255, . i2, giving the exchange integrals J = -0.48 cm-’ for

(1) and -0.41 cm-’ for (2). The results indicate that there is a very weakly

antiferromagnetic spin-exchange interaction between the two Co(l1) ions

within each molecule.

INTRODUCTION

The study of magnetic exchange interactions of binuclear transition-

metal complexes propagated by multiatom bridges is a topic that has

attracted considerable interest in recent years, not only for gaining some

insight into the pathways of electron transfers in biologcal systems’, but also

for obtaining information about designmg and synthesizing molecule-based

magnet^^.^ and for investigating the spin-exchange mechanism between

paramagnetic metal ions4. So far, much effort has been devoted to the

development of multiatom bridging ligands that can afford long-distance

magnetic interactions5-l4. The tetraanion of pyromellitic acid (abbreviated as

PMTA), due to its peculiar structure, could be a good candidate in supporting

long-distance magnetic exchange interactions. Chaudhuri et a/. first utilized

PMTA as a multiatom bridge to synthesize the binuclear copper(l1) complex

[LCu(p-tetracarboxylato)CuL].4H20 (L = 1,4,7-lrimethyl- 1,4,7-triazacyclo-

nonane). It has been revealed by the single crystal X-ray and magnetic

analyses’ that the long-range antiferromagnetic coupling could occur between

the copper(I1) ions bridged by the PMTA ligand although the Cu.-.Cu

separation is 7 . 8 i . In order to provide more examples of PMTA-bridged

binuclear complexes and to understand better the magnetic properties of this

lund of complexes, quite recently we used PMTA as a multiatom bridge to

synthesize and characterize binuclear nickel(I1) complexes and to study their

magnetic properties”. As an extension of that investigation, this paper deals

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p-TETRACARBOXYLATO BINUCLEAR COBALT(I1) COMPLEXES 1071

with the synthesis, characterization and magnetism of four new cobalt(I1)

binuclear complexes using PMTA as a bridging ligand: [Cq(PMTA)L4],

where L denotes 4,7-&phenyl-l ,lo-phenanthroline (Ph2-phen); 2,9-dimethyl-

1,l O-phenanthroline (Mez-phen); diaminoethane (en) and 1,3-diaminopropane

(pn); and PMTA represents the tetraanion of pyromellitic acid.

EXPERIMENTAL

Materials

Hydrated cobalt(I1) perchlorate was prepared and purified by the

literature methodI6. Pyromellitic acid (WMTA), LiOHeH20 and the terminal

ligands 4,7-diphenyl-l,1 O-phenanthroline (Ph2-phen), 2,9-dimethyl-l,10-

phenanthroline (Me2-phen), daminoethane (en) and 1,3-daminopropane @n)

(analytical grade) were purchased fiom the Beijing Chemical Company.

Svnthesis of New Binuclear Cobalt(I1) Complexes

All four binuclear complexes were obtained in nearly the same way as

exemplified by the preparation of [Cq(PMTA)(Me2-phen)4]. To a solution of

254.3 mg (1 mmol) of pyromellitic acid in methanol (20 mL) was added

dropwise a methanol solution (20 mL) of 167.8 mg (4 mmol) of LiOH.H20

under stirring at room temperature. The stirring was continued until the

mixture became clear. To this solution was then added a methanol solution

(1 5 mL) of 73 1.8 mg (2 mmol) of Co(C10&.6H20 and 833.9 mg (4 mmol) of

Me2-phen in 20 mL methanol solution under N2. The color of the solution

changed immediately from reddish to orange and a small amount of

precipitate formed. Stirring was continued for 3 h; the precipitate formed was

filtered, washed with methanol, water and diethyl ether several times and

dried with P205 under reduced pressure. Recrystallization was canied out

fiom an acetonitrile/ethanol(1:2) mixture. Yield, 1056.9 mg.

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1072 L1 AND YAN

All analytical data, colors, yields and melting points of the complexes are

collected in Table I. In the solid state all the complexes are fairly stable in

air, thus allowing measurements to be made.

Phvsical Measurements

Carbon, hydrogen and nitrogen elemental analyses were performed with a

Model 240 Perlun-Elmer elemental analyzer. The cobalt content was

determined by EDTA titration. The infrared spectra were measured on a

Model 810 Shimadzu m a r e d spectrometer in Kl3r pellets. Reflectance

spectra were measured on a H~tachi-340 spectrophotometer. The melting

points of the complexes were determined on a Model XT 7-1 micro-melting

point apparatus. Molar conductances were measured (DMF solution) with a

Shanghai DDS-11 A conductometer. Variable temperature magnetic suscep-

tibilities (4-300 K) were measured at the Institute of Physics, Chinese

Academy of Sciences, using a Model CF-1 vibrating sample magnetometer

(sensitivity m = lo4 emu) made by Nee1 Laboratory de CNRS, France.

Diamagnetic corrections were made with Pascal's constants" for all the

constituent atoms and the effective magnetic moments were calculated by the

equation h n = 2.828(mT)'", where is the magnetic susceptibility per

molecule corrected for diamagnetism of the constituting atoms.

RESULTS AND DISCUSSION

Preaaration and Comnosition of the Comnlexes

The PMTA-bridged binuclear complexes were obtained by the reaction

of H2PMTA with Co(C10&.6H~0 and L (L = Ph2-phen, Mez-phen, en, pn) in methanol in the presence of a base. The use of LiOH.H20 as the base gave

good results because it and its salt (LiC104) formed in the reaction are all

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p-TETRACARBOXY LATO BINUCLEAR COBALT(I1) COMPLEXES 1073

fairly soluble in methanol and the products are little contaminated with these

inorganic materials. Indeed, elemental analyhc data for the newly prepared

complexes, listed in Table I, indicate that the reaction of PMTA with

Co(C10&.6HzO and L (L = Ph2-phen, Mez-phen, en, pn) yielded the

binuclear complexes of the general formula [Coz(PMTA)L4]. The synthetic

pathway for the complexation may be represented by the following equation.

hPMTA + 2LiOH + 2Co(C104)2 + 4L + [Cq(PMTA)L4] + 2LiC104 + 3H20

Based on the conductivity measurements, spectroscopic characterization

and magnetic studies (vide infra) these complexes are presumed to have the

coordination environment as shown in Fig. 1.

Solubilitv and Molar Conductance

All the binuclear complexes are sparingly soluble in water, ethanol,

carbon tetrachloride, chloroform and benzene, but soluble in acetone,

acetonitrile, DMF and DMSO to give stable solutions at room temperature.

For the four complexes, the molar conductance values in DMF solution (see

Table 11) show that all complexes are non-electrolytes'*. This is consistent

with the measured IR data.

Infrared SDectra

The IR spectra taken in the region 4000-400 cm-'provide some

information regarding the mode of coordination in the complexes and were

analysed in comparison with that of the fiee ligand (H2PMTA). The most

relevant IR absorption bands due to the complexes, together with their

assignments, are shown in Table 11. The IR spectrum of pyromellitic acid

shows a broad band near 1700 cm-', whch is attributed to u(C=O) of the

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c. 0 4

P

Tab

le I.

Ele

men

tal A

naly

ses,

Yie

lds,

Col

ors a

nd M

eltin

g Po

ints

of t

he C

ompl

exes

Com

plex

Em

piric

al F

orm

ula

Col

or

Yie

ld

M.p

. El

emen

tal

Ana

lvsis

Fo

und

(Cal

c.) C%

)

(1)

CIM

H~

~N

SO

~C

O~

O

rang

e 88

31

1 74

.88

3.80

6.

42

6.81

(For

mul

a Wei

ght)

(W

(“C)

C

H

N

c

o

(169

7.60

) (7

5.00

) (3

.92)

(6

.60)

(6

.94)

(2)

C&5o

N80

&02

Y

ello

w

80

323

65.9

1 4.

08

9.15

9.

68

(1 2

01.0

3)

(66.

00)

(4.2

0)

(9.3

3)

(9.8

1)

(3)

CIS

H~

~N

~O

SC

@

Yel

low

- 60

3

19

35.4

0 5.

51

18.1

9 19

.16

(608

.3 8

) G

reen

(3

5.54

) (5

.63)

(1

8.42

) (1

9.37

)

(4)

C22

h2N

80&

02

Dar

k-

75

286

39.5

2 6.

21

16.6

1 17

.51

(664

.48)

Y

ello

w

(39.

77)

(6.3

7)

(16.

86)

(17.

74)

(1) =

[Co2

(PM

TA)(P

h2-p

hen)

4],

(2) =

[C0~

(PM

TA)(

Me2

-phe

n)~l

, (3

) = [C

o2(P

MTA

)(en)

4],

(4) =

[Co2

(PM

TA)(p

n)4]

.

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p-TETRACARBOXYLATO BINUCLEAR COBALT(I1) COMPLEXES 1075

Fig. 1. Coordination Environment of the Binuclear Complexes (N-N = Phz-phen, Mez-phen, en, pn).

carboxylic group. However, in the IR spectra of all complexes, this band had

disappeared, accompanied by the appearance of two characteristic strong and

broad bands at ca. 1600 cm-’ and 1370 cm-’ attributed to uas(C02-)

(1 620- 1600 cm-’) and u,(C02-) (1 375-1 370 cm-’), stretching vibrations of

the coordinated carboxylate groups. The absence of any splitting of the

u,(C02-) and uS(C02-) bands strongly suggests the end-to-end linking of the

PMTA ligand in an equivalent way at both sites’. Moreover, the coordination

modes of carboxylate groups have often been diagnosed by the separation

between uas(Ca-) and us(C02-). That is, bidentate carboxylate groups show

a separation smaller than 200 cm-I, whereas unidentate carboxylate groups

show a separation larger than 200 cm-I. Thus, for the present complexes,

these two bands were separated by ca. 230 cm-’ (see Table II), suggesting an

unidentate coordination mode for the four carboxylato groups of the PMTA

ligand”. The unidentate coordination modes of the carboxylates in PMTA

were supported by the crystal structure of the analogous complex’ [LCu(p-

tetracarboxylato)CuL]~4H~O (L = 1,4,7-trimethyl- 1,4,7-triazacyclononane).

In addition, the -N=C- or -NH2 vibrations for the terminal ligands (Phz-

phen, Me2-phen, en, pn) are shlfied to higher fiequencies in corresponding

binuclear complexes (see Table II), suggesting that the N atoms of the

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Tab

le XI.

Phy

sica

l Dat

a fo

r the

Bin

ucle

ar C

ompl

exes

(1)

8.1

1620

vs

1375

vs

1548

s -

523

w

3550

0 38

500

(2)

7.3

2600

vs

1370

vs

1530

s -

520

w

3600

0 38

500

- 32

50vs

51

5 w

37

200

(3)

7.0

I610

vs

1372

vs

4 150

0

- 31

75vs

51

8 w

35

300

(4)

6.2

1615

vs

1380

vs

3920

0

vs: v

ery

stro

ng,

s: st

rong

, w

: wea

k

k

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v-TETRACARBOXY LATO BINUCLEAR COBALT(I1) COMPLEXES 1077

terminal ligands Lcoordinate with the metal ion. This view is further supported

by the appearance of a band corresponding to the metal-nitrogen stretching

vibration at 515-523 cm-' in the complexes. Furthermore, the band centered

at 1100 cm-', typical for u(C1-0) stretching of the perchlorate group2", was

not found for the complexes. Ths is consistent with the conductance

measurements and elemental analyses of the complexes.

Electronic SDectra

In order to obtain further mformation on the mode of bonding of the

Co(I1) ion to the ligand, the reflectance spectra of the four complexes were

measured at room temperature. For all four binuclear complexes, two strong

absorptions in the UV range were observed (see Table II), whch may be

assigned to the charge-transfer absorption bands21 and three weaker bands

(Table H I ) appearing in the 9000-9400, 19000-19800 and 21 100-21600

cm-' regions which may reasonably correspond to the 4T1,(F) + 4Tzs(F) (v I ) ,

4T~g(F) + 4A2e(F) ( V Z ) and 4T1,(F) -+ 4T,g(P) (v3) transitions, respectively,

consistent with the presence of an octahedral coordination geomet# around

the cobalt(I1) ion. According to Lever's m e t h ~ d ~ ~ . ~ ~ , some coordination field

parameters of the complexes may be obtained by using the two observed

bands u1 and u3 and the calculated results are summarized in Table 111. Thus,

it can be seen that the observed uzvalues are in agreement with the

calculated ones (see Table 111). Ths shows that the assignments are

reasonable and are additional evidence for the octahedral structure. In

addition, the values of p < 1 show that the bonding for cobalt(I1) is

predominately ionic with only a small percent of covalent character in these

cornp~exes~~.

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LI AND YAN 1078

Table 111. Electronic Spectral Data (cm-') of the Complexes

(1) 9000 19000 (19200)" 21100 1021.5 888.3 0.915

(2) 9300 195000(19800) 21200 1051.8 876.5 0.903

(3) 9400 19800(20100) 21300 1056.7 880.6 0.907

(4) 9200 19100(19700) 21600 1045.7 909.3 0.936

'The values in parentheses are the calculated values:

All our efforts to grow large crystals of these binuclear cobalt(I1)

complexes suitable for X-ray structure determination so far have been

unsuccessful. However, based on the composition of these complexes, the

infixed and electronic spectra, as well as conductivity measurements, the

results of variable-temperature susceptibilities, whch we will discuss later,

and the crystal structure of the analogous complex', it is reasonable to

suppose that these complexes have an extended PMTA-bridged structure, in

which each carboxylic group is bound to the cobalt(I1) ion in a monodentate

fashion through only one oxygen atom, yielding two seven-membered rings.

Each cobalt(I1) ion is in a hstorted octahedral environment (Fig. 1).

Mametic Properties

In order to obtain further mformation on the structure of the complexes,

variable-temperature susceptibility (4-300 K) data were collected for the

[Co2(PMTA)(Ph~-phen>4] (1) and [Co~(PMTA)(Me2-phen)4] (2) complexes,

by way of example, and are shown in Fig. 2 in the form of the m, versus

T plot, m being the molar magnetic susceptibility, p,=n the effective moment

and T the absolute temperature. The cobalt(I1) ion under Oh-symmetry is in

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p-TETRACARBOXYLATO BINUCLEAR COBALT(I1) COMPLEXES 1079

the 4T~, ground state whose magnetic moment is temperature-dependent.

Therefore, the magnetic susceptibility for a binuclear cobalt(I1)-cobalt(I1)

complex (S1 = & = 3/2) with cobalt(I1) in Oh-symmetry is dfficult to explain.

However, the configuration around the cobalt(I1) ion for the present case is

somewhat distorted fiom &-symmetry. Therefore, according to Sinn25, the

magnetic susceptibilities for the present cobalt(I1)-cobalt(I1) complexes may

be interpreted by the Heisenberg model, because the symmetry around the

metal is lower than for a regular octahedron. From Fig. 2 it is evident that the

magnetic behavior of the two complexes is similar. In the 30-300 K region,

the effective magnetic moments (k~) are virtually constant, but sharply

decrease below 30 K. This behavior is characteristic of weak antifeno-

magnetic spin-exchange interaction between cobalt(I1) ions through the

PMTA-bridge within each molecule.

In order to understand quantitatively the magrutudes of the spin-exchange

interaction between cobalt(I1) and cobalt(I1) ions, a magnetic analysis was

performed with the susceptibility equation derived from the Heisenberg spin

opeator H = -2Ji, . i2, where the exchange integral J is negative for anti-

ferromagnetic interaction and positive for ferromagnetic interaction. For the

Co(1I)-Co(I1) system (Sl = SZ = 3/2), the molar magnetic susceptibility is

given by the expressionz6 of equation (1):

1 (1) 14 + 5exp(-6J/KT) + exp(-lOJ/KT)

7 + 5exp(-6J/KT)+ 3exp(-lOJ/KT) + exp(-125/KT)

Where denotes the susceptibility per dinuclear complex and the remaining

symbols have their usual meanings. As shown in Fig. 2, good least-squares

fitting of the experimental data to equation (1) gave J = -0.48 cm-‘, g = 2.39

for (1) and J = -0.41cm-l, g = 2.41 for (2). The agreement factor F, defined

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1080 LI AND YAN

c

6 - [Co2(PMTA)(Me2-p hen),] - 9 - - -

7 4 -

E - 0

I?

E

Y x x

s 2 - - 3

- - O i l , , I I I ,

0 150 T/K 300

Fig. 2. Temperature Variation of (Lower Curve) and c(en (Upper Curve) for the Complexes [Co2(PMTA)(Ph,-phen)4] (1) and [Coz(PMTA)(Me2- phen)4] (2). The Curves are Based on Eq. (1) Using the Magnetic Parameters Given in the Text. (e), Experimental Data. (-), Calculated Curves as Described in the Text.

here as F = [C(m)obs.- C ( W ) ~ ~ . J ~ / c ( ~ ) ~ ~ . , is then equal to I. 1 x lo-’ for (1)

and 2 . 7 ~ 1 0 - ~ for (2). The results (negative and small J values) indicate that

the complexes exhibit weak antiferromagnetic spin-exchange interaction

between binuclear cobalt(I1) centres in the two complexes. These small J

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p-TETRACARBOXYLATO BINUCLEAR COBALT(I1) COMPLEXES 1081

values of the complexes (1) and (2) may arise mainly by the geometric

structures of the complexes and the properties of the bridged-ligand.2’

ACKNOWLEDGMENTS

This project was supported by the Natural Science Foundation of

Shandong Province and the National Natural Science Foundation of C h a .

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Received 23 October 1998 Referee I: K. Moedritzer Accept& 6 M i i r c h 2 0 Referec 11: D. E. Pennington

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