A MÖSSBAUER EFFECT STUDY OF RELAXATION IN TWO IRON(III) SCHIFF-BASE COMPLEXES

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A MÖSSBAUER EFFECT STUDY OF RELAXATION IN TWO IRON(III) SCHIFF-BASE COMPLEXES

G. Long, J. Wrobleski, G. Longworth

To cite this version:

G. Long, J. Wrobleski, G. Longworth. A MÖSSBAUER EFFECT STUDY OF RELAXATION IN

TWO IRON(III) SCHIFF-BASE COMPLEXES. Journal de Physique Colloques, 1979, 40 (C2), pp.C2-

358-C2-360. �10.1051/jphyscol:19792127�. �jpa-00218498�

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JOURNAL DE PHYSIQUE Colloque C2, suppl&ment au n ^O3, Tome 40, mars 1979, page C2-358

G.J. Long, J.T. Wrobleski and G. ~ o n g w o r t h ~

Department of Chemistry, University of Missouri-Rol la, Rolla, Missouri 65401, USA.

'~uclear Physics Division, Atomic Energy Research EstabZishment, HarwelZ, Didcot, Oxfordshire, OX1 1 ORA England.

Rdsum6.- La base de Schiff N-salicylidsne-DL-phenylalamine forme d e w complexes avec le fer(II1).

Dans le premier, les ions fer sont pontds par un ion 0x0- pour former un complexe antiferromagnd- t ique

.

pe(salphefl20, avec J = -89 cm-'- Dans le second, k s ions fers sont pontds par d e w ions hydroxy pour donne? ~e(salphe)0~2 avec J = -15 cm-I

.

Le spectre Mossbauer de ~ e ( s a l p h e ~ 2 0 entre 300 et 23.8K indique qu'il existe une relaxation magndtique relativement rapide et un pic large avec un ddplacement isomdrique variant de 0,37 1 0,46 m / s . A 4,2 K on observe les effets d'une relaxation de vitesse intermddiaire, sous la forme de cinq pics larges. L'application d'un champ magndtique de 6 T k 4.2 K fait apparartre un spectre de 6 pics fins

-

avec un champ hyperfin effectif de 490 kOe ce composd est caractdrisd par une relaxation lente 1 1.14 K et donne un spectre 3 6 pics avec un champ hyperfin effectif de 530 kOe et un ddplacement isom6rique de 0,50 m l s . Par contre, p e (salphe)0qz est paramagndtique 1 toute temperature avec S = 0,51 m / s . et AEQ = 0,83 mmls.

1.14 K. Pour ce compose, les effets d'une relaxation lente n'apparaissent qu'1 4.2 K et en dessous, comme le montre la presence d'une composante hyperfine magndtique dont l'intensitd ddcroet 1 1.10 K L'intensitd relative de cette composante, qui a un champ hyperfin effectif de 500 kOe, crort en prdsence d'un champ appliqud.

Abstract.- The Schiff-base, N-salicylidene-DL-phenylalanine, salphe, forms two complexes with iron (III).In the first, the iron ions are bridged by an 0x0-ion to form an antiferromagnetic complex,

~e(salphe)lT~O, with J = -89 cm-I. In the second, the iron ions are bridged by two hydroxy-ions to give pe(salphe)0q 2 with J = -15 cm-I

.

The ~Essbauer spectrum of pe(salphe)I120 between 300 and 23.8K exhibits relatively fast magnetic relaxation and a broad line with an isomer shift varying from 0.37 to 0.46 mmls. At 4.2 K the effects of an intermediate relaxation rate are observed in the form of five broad lines. The application of a 6T applied magnetic field at 4.2 K results in a sharp six line spectrum with an effective paramagnetic hyperfine field of 490 kOe. This compound exhibits slow relaxation at 1.14 K and gives a six-line s ectrum with an effective hyperfine field of 530 kOe and an isomer shift of 0.50 m l s . In contrast, $e(salphe)Og2 exhibits a paramagnetic component at all temperatures with 6 = 0.51 and AEQ = 0.83 m / s at 1.14 K. For this compound, the effects of slow relaxation are only apparent at 4.2 K and below as indicated by the presence of a hyperfine component whose intensity increases at 1.10 K. The relative intensity of this component, which has an effective hyperfine field of ca. 500 kOe, increases in the presence of an applied field.

The Schif f-base N-salicylidene-DL-phenylala- The ~ s s b a u e r effect spectrum of Ee(salphea 20 as nine, salphe, which results from the condensation of a function of temperature is shown in Figure 1 and salicylaldehyde and phenylalanine forms two different

complexes with iron(II1) /I/. These complexes, which may be reversibly interconverted by acid-base titra- tion, differ in the bridging ligand. In pe(sa1- phefl20, bridging occurs via an 0x0-ion to yield an antiferromagnetic dimer with an intramolecular exchange coupling constant, J, of -89 cm-' and a g value of 2.00. In ~e(salphe)0~2, bridging is ac- complished through two hydroxyl ions to give an antiferromagnetic dimer with J = -15 cm-'and g = 2.00.

In each compound the magnitude of the intramolecular exchange coupling is reasonable for the proposed structure 12-51 and the magnitude of the intermolecular exchange coupling is small and probably highly anisotropic. These conclusiocs are supported by spectral and magnetic results /I/.

the various varameters are resented in Table I.

Table I. MEssbauer Effect Data for be(salpheU 20.

T, Happ, 6 r a s Hint Ib

x2

/

^K ^T mmls m l s kOe ns

300 0 0.37 -1.4

- -

^-0.3

I

a~elative to natural a-iron foil. b~elative intensity of line two (and five) to line one (and six).

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19792127

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Fig. I : The zero-applied field Mzssbauer effect spectra of pe(salpheUn0 obtained at several temperatures.

From these results, it is apparent that this compound is undergoing relatively fast magnetic relaxation at temperatures of 23.8 K and above with a mean correlation life time, T, of ca. 0.4 nanose- conds. This life-time increases by about one order of magnitude between 23.6 and 4.2 K and another order of magnitude between 4.2 and 1.14K, such that,at 1.14 K, the mean correlation life-time is 34.0 na- noseconds and the compound is undergoing slow relaxation. The resulting spectrum exhibits an effective paramagnetic hyperfine field of 530 kOe and a small (+ 0.09 m/s) quadrupole shift. At all temperatures, the observed isomer shift is typical of a high-spin iron(II1) complex.

The mean life-times quoted in Table I for data above 1.14 K are only approximate. At these t e r peratures, only qualitative agreement could be obtained between theoretically calculated relaxation spectra, and the observed spectra 161. This difficulty may arise because the theoretical model 171, in which S 512 and the quadrupole interac- tion is zero, includes relaxation only between the Sz =

+

112 states. We are currently studying alter-

nate models for their applicability to these spectra. The observation of the large effective hyperfine field apparently indicates highly anisotropic intermolecular exchange coupling.

The MEssbauer spectra of Ee(salphea20 a t 4.2 K in several applied fields are shown in Figure 2 and numerical data are presented in Table I. As expected 171, the mean correlation life-time increases and the relaxation rate decreases with in- creasing applied field. At an applied field of 6T the mean correlation life-time is ca. 30 nanose- conds and the effective hyperfine field has decrea- sed to 490 kOe.

Fig. 2 : The MGssbauer effect spectra of Fe(sal7 phe)IfrO obtained at 4.2 K in several applied fields.

The behavior of pe(salphe)~a 2 is different in that only one, quadrupole split, line is observed at temperatures above 4.2 K. At 4.2 and 1.10 K this doublet is also observed, but an additional hyperfine component is present. Mgssbauer effect data for this compound are presented in Table 11

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c2-360 JOURNAL DE PHYSIQUE

and the spectra obtained at 4.2 K are illustrated in figure 3.

Table 11. : ~assbauer Effect Data for ~e(salphe)0~2.

Magnetic Component Paramagnetic Component

T, Happ, Hint, QS, '5 ,a

r a

^lb ^AEo, ^6,"

r ,

^Area

x 2

5elative to natural a-iron foil. b~elative intensity of line two (and five) to one (and six).

In this compound, with its weaker intramolecular antiferromagnetic coupling, the relaxation rate is much faster than that observed in pe(salpheg20.

Hence, much lower temperatures (i.e. 1.14 K) are necessary before the results of slow relaxation are observed in a zero applied field. The application of an external field once again increases the mean correlation life-time and decreases the relaxation rate as is observed in Figure 3.

Acknowledgement.-- This work was supported in part by the National Science Foundation (USA) through Grant CHE-75-20417.

References

/I/ Wrobleski, J.T., Doctoral Dissertation, Univer- sity of Missouri-Rolla. 1977.

/2/ Reiff. W.M., Baker Jr, W.A. and Erickson, N.E., J. Arner. Chem. Soc. 90 (1968) 4794; Reiff, W.M.

Long, G.J. and Baker Jr, W.A., 3. Amer. Chem.

SOC. 90 (1968) 6347.

/3/ Long, G.J.. Robinson, W.T., Tappmeyer, W.P. and Bridges, D., 3. Chem. Soc. Dalton (1973) 573.

/4/ Long, G.J., Inorg. Chem. 17 (1978) 2702.

/5/ Johnson,C.E., in Hyperfine interactions in Exci- ted Nuclei, ed. by G. Goldring and R. Kalish, (Gordon and Breach, N.Y.) 1971, p. 803.

/6/ Wickman, H.H.. in Mijssbauer Effect Methodology, Vo1.2, ed. by 1.3. Gruverman (Plenum Pres8,N.Y.)

1972, p. 39.

/7/ Wickman, H.H., Klein, M.P. and Shirley, D.A., Phys. Rev. 152 (1966) 345.

Fig. 3 : The ~zssbauer effect spectra of P(sa1- phe)~g ²obtained at 4.2 K and in two appl~ed fields at 4.2 K