Indian Journal of Engineering & M~erial~ Sciences Vol. 6, April 1999, pp. 100-106

Synthesis and magnetic properties of 3D-metal oxalate networks as molecular-based magnets

B P Singh', B Singh'·, R A Singh' & P Delhaesb

• Department of Chemistry, Faculty of Science, Banaras Hindu University, Varanasi 221 005 , India

b CRPP, CNRS Avenue A. Schweitzer F- 33600 Pcssac, France

Received /6 April /998; accepted / January 1999

The reaction of oxalic acid, metal(lI) salt and tetra(n-butyl) ammonium bromide or dimethyl dioctadecyl ammonium bromide in a molar ratio of 1.5: I: I in water or ethanol-water at room temperature afforded polymetallic assemb lies with the formulae (NBu.MM2(OXU}. [M = Mn2'(1), Fe2+(2), Co2+(3)] and {(DMDOAMMiOXU} . [M = Mn!+(4), Fe2+(S), Coh (6)] . The magnetic susceptilbility dafa of 5 and 6 in the range 20 K < T < 300 K obey the Curie-Weiss law and on low­ering the temperature the effecti~e magnetic moment decreases gradually and reveals anti ferromagnetic phase transition . They have also been characterized by FAB mass, IR, electronic and ESR spectra.

Molecular magnetism emerged as a new field of re­search in mid 1980's. The first molecular based mag­nets were recorded a decade ago l

•2 and in the last few

years a number of new compounds of this type have been reportedJ.6. Magnets have captivated mankind since the discovery of the iron metal. Later the elec­trical conducting properties of the metals expanded man 's horizons and today both phenomena are crucial to high tech age. The materials that exhibit these phe­nomena are inorganic solids with bonding in three dimensions. Evolution from atom-based inorganic materials to molecule-based materials should allow both the modulation of magnetic properties by con­ventional organic chemistry and ease of fabrication often enjoyed by organic materials7

.

The molecular magnets can be used to store infor­mation as in a magnetic disc or tape, in a.c. motors, as 'smart ' material, to shield electromagnetic radiations and strong magnetic fie ld, in magnetic resonance im­aging eqUIpment, in genuine smart switches, sensors and transducers that a lmost certai nly wi ll contain magnetic material s. Recently, three types of molecu­lar-based systems exhibiting a spontaneous magneti­za tion have been fu ll y characterized from both struc­tura l and magnetic points of view, (i) one­dimensional charge- transfer complex by Miller et ([ I.R, (ii) one-dimensiona l oxamato bridged Mn(II)­Cu(IJ) complex by Kahn et a1. 9

, and (iii) one-

*For correspondcncc

dimensional Mn(II)-radical complex by Gatteschi et al. 10

The design of molecular-based ferromagnets is one of the stimulating challenges for the chemists. Ferro­magnets can be described as having magnetic vectors of constituting paramagnetic centers alinged parallel over the three dimensional lattice. They exhibit bulk 'properties such as long range magnetic ordering with a spontaneous magnetization below a critical tem­perature (TC> preferably around room temperature and bistability with hysterisis and large coercivity which confer the memory effects of the system. Molecular based ferromagnets reported so far are based on one dimensional chains as the constituents in which the magnetic vectors are assembled by ferromagnetic interactions. An attempt to obtain synthetic ferro­magnets, the molecular design should have answer to the questions: (i) how to achieve a ferromagnetic in­teraction between the nearest-neighbouring magnetic spins and (ii) how to extend parallel alignment of the magnetic vectors over the three dimensional lattice. The molecu lar magnets or polymetallic complexes possessing ferromagnetic properties may be synthe­sized by molecular assemblies through molecular en­gineering. In these molecular assoc iations, two or more metal ions which may be of the same kind or different types are bridged by ambidentate ligands of the appropriate dimensions so that they may result in co-operative magnetic effects-ferromagnetic and an­tiferromagnetic interactions. The oxalate and cyanide

SINGH et at.: 3D-METAL OXALATE NEIWORKS AS MOLECULAR-BASED MAGNETS 101

ions are most suitable bridging moieties resulting in two or three dimensional molecular assemblies showing spontaneous magnetization effects, and thus yield the molecular magnets.

The mixed metal systems whose magnetic orbitals are strictly orthogonal to each other can give the fer­romagnetic interaction between them. In the search of chemical correlation between structure and properties in this interesting class of compounds, an important aspects is to extend the range of examples from three points of view. First, since they form layer structures, systematically varying alkylammonium ion should modulate the separation between the layers and hence the physical properties. Second in view of the fact that three-dimensional as well as layer compounds of the type [Fe (bpy»)J [Mlox»)J have been reportedlt, it is important to delineate the extent to the template cation (A) which determines the crystal chemistry. Third, varying Mil gains access to a wide range of magnetic behaviour. It is, therefore, of great impor­tance to find out how wide-spread this phenomenon is and whether a connection can be drawn with the ca­tion (A). The synthetic possibility for building up 30-oxalate metal network is described and their charac­terization is also discussed. The magnetic properties are of paramount interest and the magnetic suscepti­bilities of Fe(II) and Co(ll) network are presented here.

Experimental Procedure Materials

Tetrabutyl ammonium bromide and dimethyl dioctadecyl ammonium bromide from Sigma, USA,

were used as obtained. FeS04 • 7H20, MnCI2.4H20 and CoCl26HzO were obtained from Aldrich, Ger­many.

(A) Synthesis of(N-BuJ:fMlOX)J (Mfl = Mn, Fe, Co)

To an aqueous solution (20 mL) of N-tetrabutyl ammonium bromide (l mmol) and metal (II) chloride or sulphate (I mmol) was added slowly a solution of oxalic acid (l.5 mmol) in water (l0 mL) with stirring. A mIcrocrystalline precipitate started to appear after few minutes . It was filtered off, washed with water and dried under reduced pressure.

(B) Synthesis of (DMDOA)] [M] (0)0 J (Nt' = Mn, Co)

To an ethanol solution (30 mL) of dimethyl dioc­tadecyl ammonium bromide (I mmol) and metal (II) chloride (I mmol) was added slowly a solution of oxalic acid (1.5 mmol) in ethanol (10 mL) with stir­ring. A microcrystalline precipitate started to appear after a few minutes. It was filtered off, washed with ethanol and dried under reduced pressure.

(C) Synthesis of(DMDOA) y [Fe](Ox)J To a solution of dimethyl dioctadecyl ammonium

bromide (Immol) in ethanol (50 mL) and FeS04 •

7H20 (I mmol) in water (10 mL) was added slowly a solution of oxalic acid (1.5 mmol) in ethanol (10 mL) . A microcrystalline precipitate started to appear after a few minutes. It was filtered off, washed with ethanol and water and dried under reduced pressure.

Table I-Analytical data and general properties of the complexes

S No. Complex Melting point Yield, % Magnetic moment Found (calculated), %

°C (8M) at RT M C H N

I. C38 Hn N20 'lMn2 340d 65 10.54 12.43 52.88 8. 21 3.76 (12.79) (53 .15) (8.44 ) (3.26)

2. C3s Hn N20' 2Fe2 350d 70 8.32 12.54 53 .27 8. 76 3.47 ( 12.97) (53 .03) (8.43) (3 .25 )

3. C,gHn N2O' 2C02 350d 62 8.05 13 . 12 52.89 8. 10 3.54 (13.59) (52 .65) (8.37) (3 .23)

4. C82 H'boN20 '2Mnl 125-130 74 9.40 7. 12 66.43 10.74 2.03 (7 .44) (66 .72) (1 0.92) ( 1.89)

5. C' 2H '6oN1O'2Fe2 210-215 68 6.25 7.32 66.35 II 13 2.21 (7.55) (66 .64) (10 .91 ) (1.89)

6. Cg2 H ' 60N20 '2C02 240-250 71 8.35 7. 58 66.72 10.57 1.67 (7 .94) (66 .36) (10.86) (1.88)

d = decomposition temperature.

102 INDIAN 1. ENG. MATER. seL, APRIL 1999

Analysis and physlca' measurements The iron content in the complexes was determined

as oxinate gravimetrically and manganese and cobalt contents were determined volumetrically by EDTA tritration after removing the organic moiety with aqua-regia followed by sulphuric acid. The com­pounds give satisfactory C, Hand N values (Table 1).

The infrared spectra of complexes were recorded in KBr disc on a JASCO-5300 spectrophotometer. Electronic spectra of the complexes were recorded on a Simadzu 160A spectrophotometer in nujol medium . . The F AB mass spectra of complexes were recorded on a Jeol SX-I02 mass spectrometer. The ESR spec­tra of the complexes wer.e recorded at liquid nitrogen temperature on a Varian X band E4 spectrometer us­ing TCNE as g marker.

Results and Discussion

The oxalate bridged bimetallic assemblies were readily obtained by a one pot reaction of a metal(II) salt, tetra alkyl ammonium salt and oxalic acid in aqueous and/or ethanol solution. The complexes are insoluble in common solvents.

The F AB mass spectra (Figs 1 and 2) of (DMDOA)2 [Fe2(OXU and (DMDOA)2 [COlOX)3] show base peak at mlz = 552, which corresponds to

+

the (C I8H37)2 N (CH3)2 ion . The presence of anions [Fe2(OX)3f and [Coz(oX)3f are shown by lines at mlz = 375 and mlz = 380 respectively. The fragmen-

+ tation of (C 18Hd2 N (CH3)2 ions is shown in Schemes 1 and 2, respectively. The presence of tetra butyl ammonium and dimethyl dioctadcyl ammonium in the complexes (NBU4)~ [Mz(OX)3] and (DMDOA)2 [M2(OX)3] is indicated by a band at 2950-2945 cm·1

and 2925-2920 cm-I in their IR spectra (Figs 3 and 4) respectively. The chelated oxalate groups generally show an antisymmetric v(CO) vibration at ca. 1700 cm-I and D(CO) at ca. 800 em-I. The IR spectra of the complexes, 1-6 the vibrations due oxalate group ap­pear at 1665-1600 and 810-765 em-I, indicating that all the oxalate ions function as bridging group. The bonding of oxalate to the metal ions is shown by the presence of bands at 530-490 and 465-410 em-I due to v(MO)+v(CC) and v(MO) + ring-deformation, re­spectively. A broad band centered at 3500 em-I in the spectra of the alkyl ammonium bromide and the complexes are attributed to v(OH) of HiO absorbed

100.--5-.-------------------,m5~1--------~

700 800

: f ·1 f

1200 1300 1400 1500 1600

M IZ

Fig. I-F AS mass spectrum of (DMDOA}z[Fei OX))]

100 551

SI

OJ 50

T 617

u ~ c .41 lSI 4~ 511 -4 .0 0 14114

" 0 c

" 0 100 200 300 400 500 600 700 : 100 1112 ..

10,4 I1U 1 :> 50 ~Ol (; eol IU III .. ~. . . . • L.

a: ~4 . 0 J 0

700 800 900 1000 1100 1200 1300 14000

MIZ

Fig. 2-F AS mass spectrum of ( DMDOA )2 [ CO2 ( OX))]

SCheme. 1.

C ~l- N- CH)

mlz=44

~- CH2 N - CH)

ml z = 30

SINGH et al.: 3D-METAL OXALATE NElWORKS AS MOLECULAR-BASED MAGNETS 103

40.0.0. 350.0. 30.0.0. 250.0. 20.0.0. 150.0. 10.0.0. 50.0. 40.0.

¥lovenumber (em-I)

Fig. 3-lnfrared spectrum of dimethyl dioctadecyl ammonium bromide (DMDOA)

Scheme. 2.

. u c :? E : o ~

4000 3000 2000 1500 1000 400

Fig. 4-lnfrared spectrum of (a) (NBu. h [Mn2(OX)3] and (b) (DMDOAMCoiOXU

• u C

" D (;

0. .220.

1: 0 .10.0.

'"

0..0.2

40.0. 50.0. 60.0. ·70.0. 80.0. Wave length (nm)

Fig. 5-Electronic spectrum of (DMDOAMCoiOXU

104 INDIAN 1. ENG. MATER. SCI., APRIL 1999

,..Id ut -2000 Scan ...... -Uoo

2200 2700 3200 3700 4200

Fig. 6-ESR spectrum of (DMDOAMFei OX )J]

by KBf. A band observed at 9485 and 9478 cm·) in the electronic spectra of 2 and 5 respectively is as­signed to 5Tzg ~ Eg (D) transition in an oCtahedral stereochemistry around the metal ion. Two bands at 16000 and 25000 cm·) in the spectrum of 1 and at 16667 and 18867 cm·) in that of 4 are attributed · to 6A 4T d 6A 4E . . . 1 )g ~ )g an Ig ~ g transItIons, respectIve y indicating octahedral geometry of complexes. The complex 3 exhibits two dod bands at 16806 and 18867 cm·), while 6 shows two bands at 16667 and 18887 cm·) in the electronic spectra (Fig. 5) which are due to 4T)g (F) ~ 4Azg and 4T)g (F) ~ ~)g (P) transitions, respectively. These features of electronic spectra are characteristic of octahedral bonding of C20/ with Coz

+ ion. The ESR spectrum (Fig. 6) of the powder (DMpOAMFez(OX)3] recorded at liquid nitrogen temperature shows a signal at 3040 G yielding a g value of 2.127. It also shows a half field signal at 1280G and yields g value of 5.210: This suggests magnetic interaction between the metal ions in bimetallic assembly.

Magnetic Properties The cryomagnetic properties of the bimetallic as- I

semblies (DMDOAMFe2(OX)3] and (DMDOA)2 [COlOX)3] were studied and the respective cryomag­netic behaviour is shown in Figs 7 and 8 in the form of XM-) vs T, X M VS T, XMT vs T and ~eff VS T plots, where XM is the magnetic susceptibility per M2 and I

~efT is the effective magnetic moment calculated by the equation ~e lT = [8XM T f2 .

The magnetic susceptibility of (DMDOA)2 [Fe2(OXU is found 4.69 cm3 mol·) Kat 300 K, which results in a magnetic moment value of 6.12 ~B' that is lower than the spin-only value of 8.94 ~B for the magnetically dilute Fe(II)-Fe(II) [ SFe = 2, SFe = 2) . When the temperature is lowered XMT and ~eff. con­tinuously decrease and reach a plateau around 20 K with XMT = 3.57 cm3 mol·) K and ~efT = 5.34 ~B. The magnetic susceptibility obeys the Curie-Weiss law [XM·) = (T-8) /C ] in the range of 20-300 K with Weiss constant - 16K. The observed magnetic behaviour suggests the operation of an anti ferromagnetic inter­action within the assembly. The same cryomagnetic behaviour is observed at both 6 and 8 kG magnetic fields.

The complex (DMDOAMC02(OX)3] exhibits magnetic susceptibility 8.69 cm3 mol·) K and mag­netic moment 8.34 ~B at 300 K. The magnetic mo­ment value is larger than the spin-only value of 6.92· ~B for the · magnetically dilute Co(II)-Co(II) (Sec =

3/2, Seo = 3/2) . On lowering the temperature XMT and ~efT continuously decrease and show values XMT =

2.23 cm3 mol·) K yielding the ~eff = 4.34 ~B at 20 K. The magnetic susceptibility obeys the Curie-Weiss law in the range of 20-300 K with a Weiss constant -24 K. The same cryomagnetic behaviour is observed at both 6 and 8 ~G magnetic fields which implies the

I-

SINGH el al.: 3D-METAL OXALATE NETWORKS AS MOLECULAR-BASED MAGNETS 105

4.9

4 .7

~ "0 ,

J e 4 .3 ..,

'e e u u 4 . 1

i ..: )(., 2 ......

"-

Temperature. ' K

Fig. 7a-Thermal dependence of molar magnetic susceptibili ty XM T(e) and inverse susceptibility, \ /XM(X) of complex (DMDOAMFe11i(OX)J

0 . 16

0 .14

., 0 . 12

0 0 .1 "0 e 40 e .., 0 .08

.., . E E u

0 .06 u

i ~ 20 >I..

"- ......

10

50 100 150 200 250

Temperature, K

Fig . 7b--Thermal variation of XM(e) and inverse susceptib i lit~ ,

l /XM(x) of complex (DMDOAMFe/ '(OX)Jl

70 6.4

6 , 2

"0 6 E

~ '? ai 5. 8 E

30 u

- 5 .6 2 .. >I.. "\. 20 "-

5 .2 10

50 100 150 200 250 Temperature. 'K

Fig. 7c-Plot of the temperature dcpendence of the rcciprocal magnetic susccptibility( x) and magnetic moment(e) per Fe(II)­Fe(lI) for (D MDOA)JFe/ '(OX)Jl

10 40

8 30 _

'" 0

25., .. 6 "0

20 e e u on 4 e 15

JI..~: u 10 ~ ......

2 ;A.2 5

00 0 50 100 150 200 250 300

Temperature, K

Fig. 8a-Thermal dependence of molar magnetic susceptibility XM T(e) and inverse susceptibil ity, \ /X~(x) of complex (DM DOAMC011i(OX)J

0.1 40

0 .08 30 "0 -; E

"0 0 .06 .., E I

20 E .., u E 0 .04 u 2

"'" "",2 10 "-0.02

0 0 0 50 100 150 200 250 300

Temperature. K

Fig . 8b--Thermal variation of XM(e) and inverse susceptibility, I/XM(X) of complex (DM DOAMCo/'(OX)Jl

40 8 .5

7.5 30 ~ "0 ai E

6.5 .... 20 . E :: u .. 5 .5 i ~ >I.... 10

4 .5 ~

3 .5 0 0 50 100 150 200 250 300

Temperature, K

Fig. 8c-Plot of the temperature dependence of the reciprocal magnetic susceptibilit y(x) and magnetic moment(e) per Co(l \)­Co(ll) for (DMDOAMCozli(OX)Jl

106 INDIAN J. ENG. MATER. SCI., APRIL 1999

Fig. 9-Schematic representation of the OX - bridged network: structure of Mz(OX)J [Mil = Mn, Fe, Co]

operation of an anti ferromagnetic interaction between Co(lI)-Co(lI) ions. The larger J.l.ff value of CO2 (OX)/ and lower value of Fe2 (OX)/ at room temperature than the spin-only value may be due to difference in the magnetic interaction. Particularly, the cobalt ox­alate network may show the ferromagnetic interaction at high temperature and this study is in progress. A tentative structure of these oxalate bridged molecular assembly may be proposed as shown in Fig. 9.

Acknowledgement We thank Head, Chemistry DepaItment, Banaras.

Hindu University, for providing laboratory facilities . The cryomagnetic measurement facility provided by Prof. R.N. Mukherjee, .Chemistry Department, lIT Kanpur is gratefully acknowledged. Recording of F AB mass spectra and C, H, and N analyses (CDRI, Lucknow) and ESR Spectra (RSIC, lIT Bombay) is also acknowledged. Weare thankful to the !FCP AR, New Delhi for the financial assistance.

References 1 Miller J S, Calabrese J C, Epstein A J, Bigelow R W, Zhang J

H & Reiff W M, J Chem Soc Chem Commurr, (1986) 1026-28.

2 Pei Y, Verdaguer M, Kahn 0, Sletten J & Renard J P, JAm Chem Soc, 108 (1986) 428.

3 Miller J S & Epstein A J, Angew Chem Irrt Ed Engl, 33 (1994) 385.

4 Kahn 0 , Molecular magnetism, (Verlag Chemie GmbH, New York), (1993).

5 Kahn 0, Adv Inorg Chem, 43 (1995) 179.

6 Nakozawa Y, Tamura M, Shirakawa N, Shiomi D, Takahashi M, Kinoshito M & Ishikawa M, Phys Rev, B46 (1992) 8906.

7 Miller J S, Epstein A J & Reiff W M, Acc Chem Res, 21 (1988) 114-120.

8 Miller J S, Epstein A J & Reiff W M, Chem Rev, 88 (1988) 201.

9 Kahn 0, Pei Y, Verdaguer M, Renard J P' & Sletten J, JAm Chem Soc, 110 (1988) 782.

10 Ganeschi A, Gatteschi D & Sessoli R, Acc Chem Res, 22 (1989) 392.

II Decurtins S, Schmal1e H W, Schneuwly P, Ensling J & Philipp Gutlich, JAm Chem Soc, 116 (1994) 9521-9528.