[ビス(サリシルアルデヒド) -o- フェニレンジイミナト]コバルト(II)の薄層分光法および電気化学的研究

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[ビス(サリシルアルデ.ヒド)一〇一フェニレンジィ.ミナト]・コバルト(II)の薄層分光法および電気化学的研究

大橋ゆか子

Thin-Layer

Spectroelectrochemical

Studies of

[Bis(salicylaldehyde)-o-phenylenediiminato]cobalt

(II)

Yukako Ohashi Introduction

The four-coordinated planar Schiff-base complexes of cobalt(II) have been in-vestigated on the basis of the interest of the excellent catalytic behavior for oxidative reactions. In sevseral solutions of these cobalt(II) complexes axial coordianation of the solvent molecule and oxidation to the corresponding cobalt(III) complexes were reported. In order to examine the relation between the catalytic behaviors and the solvent effect on the complexes, we have studied the spectroscopic properties of [bis(salicylaldeyde)-o-phenyrenediiminato]cobalt(II), [Co(saloph)] in various solventsl). In this paper we report the simultaneous measurements of the UV-VIS absorption spectra and the cyclic voltammograms in the dimethylformamide(DMF) solutions of [Co(saloph)]. In the DMF solutioi, s, we observed several molecular species by spec-troscopic measurements. In this solvent, axial solvations to give the five- and the six-coordinated complexes and an oxidation to the Co(III) complex are possible. The present results identified the spectra of the cobalt(II) and the cobalt(III) comlexes and gave an important information to the assignment of the molecular species in DMF.

Experimental Section

[Co(saloph)] was synthesized by the literature2) and stored under nitrogen atmoshere. DMF was of luminescent grade (DOTITE luminazol) and the solution con-tained 2.1x10-3mol/l [Co(saloph)] and 0.5mol/l tetraethyl ammonium perchlor-ate(TEAP). The optical path of the thin layer cell was about 0.2mm. The

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pic measurements were carried out by PHOTAL IMU-7000(OTSUKA ELECTRO-NICS). The working electrode was a platinum net made by .80 mesh wires and a plati-num wire served as auxiliary electrode, and a silver wite was a reference electrode.

Results and Discussions

Figure 1 shows the cyclic voltammogram of the DMF solution of [Co(saloph)]. The reported oxidation reduction potential of Co(II)/Co(III) of [Co(saloph)] inDMF was + 0.085V3) and the peak separation between its oxidation and reduction waves (145mV) depended on factors inherent in the cell and to the quasi-reversibility of the redox process. Therefore, the observed peaks of 0.25V (oxidation wave) and 0 V (reduction wave) can be assigned to the process of Co(II)/Co(III). The scan speed was 2mV/S.

The change of the absorption spectra from 0.4V to + 0.8V was shown in Fig. 2. The absorbance change was almost reversible when a scan was in the negative direction. The figure reveals that there are two steps of spectroscopic equilibria. Fig. 3 shows the spectral change from - 0.6V to + 0.5V, the insosbestic points being at 451 nm and 513 nm. Fig. 4 corresponds to the second equilibrium observed by scan-ning form + 0.5V to + 0.8V, the isosbestic points of which are at 450 nm and 500 rim. When a scan started from +0.8V in the negative direction, the latter spectrosco-pic equilibrium was observed to - 0.4V. In the region from - 0.4.V to - 0.6V the former equilibrium appeared. From the above-mentioned results, the following pro-cesses are presumed.

[Co(II)(saloph)] ' X [Co(II)(saloph)]

Figure 5 shows the absorbance change of ICo(II)(saloph)] in dichloromethane af-ter an addition of a small amount of dimethylamine vapor. The amine causes an ox-idation of Co(II) to Co(III). In this case the observed spectra show an analogous two-step oxidation. From these similarities of the oxidation processes of [Co(II)(sa-loph) in DMF and in dichloromethane, it can be concluded that the oxidation of [Co(II)(saloph)] to [Co(III)(saloph)] proceeds through an intermediate(X).

We would like to express thanks for the measurements of the cyclic voltammetry and the spectra to Dr. Masao Kaneko of the Institute of Physical and Chemical Re-search

References

1) Y. Ohashi, Bulletin of Bunkyo University, 23, 88(1989) : ibid., 24, 96 (1990). 2) D. Chen and A. E. Martell, Inorg. Chem., 26, 1026 (1987).

3) D. F. Rohrbach. W. R. Heineman and E. Deutsch, 18, 2536 (1979).

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. . Figure1.Cyclicvoltammogramof2.1x10-3mol/1[Co(saloph)]andO.5mo1/1TEAPinDMF. Figure2.Spectralchangeof[Co(saloph)]inDMFinth駱otentialregionfrom-0.4Vto+0.8Vper O.05V 一116一

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Figure3.Spectralchangeof.【Co(saloph)linDMFinthepotentialregionfro血一〇.6Vto十〇.らV ..

Figure4.Spectralchangeof[Co(saloph)linDMFinthepotentialregionfrom十〇:5Vto十〇.8V.

Figure5.Timedependenceofspectralof[Co(saloph)Jindichloromethanaper3minutesafteran additionofdimethylamine.