Chronoamperospectrometry of Redox Reactions of the TCNQ Adducts with a Cobalt Schiff-Base Complex in Dmf-Acetonitrile

ABSTRACT

By mixing the dmf (N,N-dimethylformamide) solution of [Co(II)(saloph)] with the acetonitrile solution of LiTCNQ, the two adducts 1 and 2 containing TCNQ(-II) moiety were formed. By the use of the spectrocyclic voltammetry (SCV) and the potential-step chronoamperospectrometry (PSCAS), the specral change was observed and the electron-transfer mechanism of the formation processes of the TCNQ(-II)-type adducts 1 and 2 were discussed.

INTRODUCTION

Remarkable electronic and magnetic properties of TCNQ and its donor-acceptor complexes in crystal have been widely known and investigated¹. These properties were discussed to relate closely to the intermolecular interaction between the TCNQ moieties and the facility of change between the various oxidation states of the TCNQ moiety². Neutral TCNQ and TCNQ(-I) anion were stable both in crystal and in solution. However, a few TCNQ(-II) moieties were reported in crystal^3,4 and most of them decomposed by the solvent molecules in solution. {(µ4-TCNQ) [Ru(NH3)5]4}⁸⁺ was only the stable species observed in solution⁵. We have investigated the reactions of the systems of TCNQ(0)/TCNQ(-I) as an electron acceptor and [Co(II)(saloph)] ([N,N’-o- phenylenebis (salicylideneaminato)]cobalt(II)) as an electron donor, using the UV-vis spectroscopy and the ¹H

Spectrocyclic Voltammetry and Potential-Step

Chronoamperospectrometry of Redox Reactions of the TCNQ Adducts with a Cobalt Schiff-Base Complex in Dmf-Acetonitrile

Solution [TCNQ= 7,7,8,8-Tetracyanoquinodimethane]

Yukako OHASHI*, Kazunori ISHIKAWA** and Masao KANEKO***

電気化学的吸収測定法を用いたコバルトシッフ塩基 - TCNQ 会合体 Dmf -アセトニトリル溶液の

酸化還元反応に関する研究

大橋ゆか子

・石川和紀

・金子正夫

＊   おおはし  ゆかこ    文教大学教育学部 

＊＊ いしかわ  かずのり  茨城大学理学部 

＊＊＊かねこ  まさお      茨城大学理学部 

NMR measurements. In the dmf-acetonitrile solutions of [Co(II)(saloph)] and LiTCNQ, we have found a stable TCNQ(-II)-type adduct 1, [Co(III)(saloph)]₂·TCNQ(-II) by the ¹H NMR experiments and an intermediate TCNQ(-II)-type adduct 2 by time-resolved spectroscopy⁶. Since TCNQ has the various oxidation states in crystal and in solution depending on the surrounding condition, various formation processes of the TCNQ(-II)-type adducts may be considered. The spectroscopic results are not sufficient to determine the process, so that we intended to examine the redox reactions occurred in the formation processes of the adducts 1 and 2 by using the spectrocyclic voltammetry (SCV) and the potential-step chronoamperospectrometry (PSCAS) in this work,.

EXPERIMENTALS

[Co(II)(saloph)] and LiTCNQ were synthesized following literatures⁶ and the solvents of spectroscopic grade were used. The thin layer electrochemical cell was constructed; a Pt gauze electrode (0.1mm thick) was sandwiched with two quartz plates, and a spiral Pt counter electrode and a Ag wire reference electrode were used. The potential of the Ag wire was -0.171V in acetonitrile, -0.202 in dmf, and -0.173V in acetonitrile/dmf=1/1 vs. SCE. The spectral change in the PSCAS and SCV methods was measured by combining a photodiode array spectrophotometer (Otsuka Electronics IMUC-7000) with a potentiostat and a function generator. The dmf-acetonitrile solutions of [Co(II)(saloph)] and LiTCNQ with 2:1 molar ratio of 10^-3 M concentrations containing 0.1M Bu4NClO4 were deaerated by bubbling argon gas for 30min before the measurements.

RESULTS and DISCUSSION

Previously⁶, we have reported in the dmf-acetonitrile solutions of [Co(II)(saloph)] and LiTCNQ with 2:1 molar ratio that the adduct 1 (Figures 1 and 2) was formed at 60 °C through a single rate- determining path with k = 9x10^-4 s^-1. The rate constant was determined by analyzing the

hand, when the mixing was carried out at 20 °C, the TCNQ(-I) peaks exhibited two-exponential decay with the rate constants of the order of 10^-3 s^-1 and 10^-4 s^-1. The final products at 20 °C and 60 °C gave the same spectra (Figure 2).

Figure 1. UV-vis absorption spectra of TCNQ and LiTCNQ in acetonitrile and the adducts 1 and 2 in dmf-acetonitrile.

Figure 2. The time-resolved absorption spectra (solid line) by 130s interval of the dmf-acetonitrile solutions of [Co(II)(saloph)] (0.6mM) and LiTCNQ(-I) (0.3mM) in 1 mm quartz cell at 20 °C and 60 °C; the dotted line is the additive absorption of two components, and the dotted-broken line is the spectra after 24 hours (adduct 1).

From these results, at 20 °C the formation of the adduct 1 via an intermediate adduct 2 was suggested. [Co(II)(saloph)] was reported to form a dimer in dmf at lower temperature⁷, so that the following mechanism is proposed at 20 °C;

[Co(II)L]₂ +LiTCNQ(-I) ➝ adduct 2 (1)

adduct 2 ➝ adduct 1, [Co(III)L]2·TCNQ(-II) (2),

where the saloph ion is abbreviated hereafter as L. At 0 °C the adduct 2 has longer lifetime, so the spectrum of the adduct 2 (Figure 1) was obtained by analyzing the spectral equilibrium at 0 °C.

The formation constant was determined as 5x10⁴ M^-1 at 0 °C. Since the adducts 1 and 2 have no absorption bands characteristic to TCNQ(-I), the TCNQ moiety in the adduct 2 is assigned to TCNQ(-II)-type. [Co(II)L] is easily oxidized to [Co(III)L], so that [Co(II)LCo(III)L]·LiTCNQ(-II) is proposed as the adduct 2. Taking into consideration of the observed slow decay of the TCNQ (-I) absorption, the step (2) is expressed as a redox reaction;

[Co(II)LCo(III)L]·LiTCNQ(-II) + M ➝ [Co(III)(saloph)]₂·TCNQ(-II) + LiM . (2)’

On the other hand, most of [Co(II)L] exist as monomer⁶ at 60 °C, so the reactions at 60 °C may be represented as,

[Co(II)L] +LiTCNQ(-I) ➝ [Co(II)L]·LiTCNQ(-I) (3) [Co(II)L]·LiTCNQ(-I) + [Co(II)L] + M ➝ adduct 1 + LiM (4).

With respect to the TCNQ(-I) adducts with metal fragments, it was reported that the 1:1 complexes were rapidly formed by electron-transfer autocatalytic process but the formation of the polynuclear complexes was considerably slow⁸. Then, the observed slow decay of the TCNQ(-I) peaks at 60 °C is possibly assigned to the step (4). [Co(III)L] moiety in the adduct 1 and the residual LiTCNQ(-I) have an ability as an electron acceptor, M, in the steps (2)’ and (4), but the role was not yet examined at present.

In order to confirm the above proposed redox reactions we carried out the experiments by spectrocyclic voltammetry and chronoamperospectrometry at 20 °C and 60 °C. First, the redox reactions of the each reactant, [Co(II)L] in dmf and TCNQ(-I) in acetonitrile, were measured by spectrocyclic voltammetry. The spectral change was reversible during the potential sweep from –0.6 to +0.8 V for [Co(II/III)L] (Figure 3a), and from 0 to 1.2 V for TCNQ(-I/0) (Figure 3b) with scan rate of 2 or 20 mV s^-1. The redox potentials of [Co(II/III)L] and TCNQ(-I/0) were determined as 0.24 V and 0.32 V, respectively.

Figure 3. Spectrocyclic voltammogram at 20 °C with 2 mV s^-1, (a) [Co(II)L] in dmf from -0.6 to 0.8 V and (b) LiTCNQ in acetonitrile from 0 to 0.7 V.

Figure 4a shows the spectral change during the potential sweep between 0V and 0.7 V of the dmf-acetonitrile solution of [Co(II)L] and TCNQ(-I) with 2:1 molar ratio at 20 °C. The observed results are as follows. (1) At the first positive scan from 0 (dotted-broken line) to 0.7 V (bold slid line), the absorption bands in the wavelength region shorter than 420nm and longer than 600nm decreased, and the 480nm band increased. Especially the 750nm peak of TCNQ(-I) completely disappeared within the first positive scan to 0.7 V (350s); the rate is much faster than the decay rate of LiTCNQ(-I) peaks shown in Figure 2. Although both adducts 1 and 2 have the absorption peaks at 350 nm and 480 nm (Figure 1), the strongest peak appears at 350 nm for the adduct 2 and at 480nm for the adduct 1. Therefore, Figure 4a indicates that the positive scan accelerated the formation of the adduct 1 via the adduct 2 (the step (1) and the step (2)). (2) After the subsequent negative scan from 0.7 V to 0 V (dotted line of Figure 4a), the decrease of the 480nm-intensity (adduct 1) and the increase of the 350nm-intensity (adduct 2) was observed but the TCNQ(-I) peaks did not appear.

Therefore, the reaction induced by reduction corresponds to the reverse reaction of the step (2)’.

The above experimental results by SCV suggest that the step (2)’ is the redox process in the formation of the adduct 1 at 20 °C.

Figure 4b shows the intensity-change of the TCNQ(-I) peaks (750nm and 410nm) during four cycles of oxidation and reduction at 60 °C. The intensity of the bands decreased roughly with the rate constant in the order of 10^-3 s^-1 and the repeated small intensity change by reduction/oxidation was observed additionaly. The reduction makes the TCNQ(-I) peaks increase, so that it corresponds to the reverse reaction of the step (4). The redox reaction to form the adduct 1 is an intermolecular redox reaction at 60 °C but an intra-adduct reaction at 20 °.

Figure 4. (a) Spectrocyclic voltammogram at 20 °C from 0.7 to 0 V with 2 mV s^-1, and (b) decay of the TCNQ(-I) peak-intensity at 60 °C during repeated scan from 0 to 0.7 V with 20 mV s^-1 of the 1/1 dmf-acetonitrile solutions of [Co(II)(saloph)] and LiTCNQ(-I) with 2:1 molar. The lines in (b) show time-dependence of the TCNQ(-I) peaks observed after mixing the dmf solution of [Co(II)(saloph)] and the acetonitrile solution of LiTCNQ(-I) with 2:1 molar ratio.

By the potential-step chronoamperospectrometry, the spectral change of the each reactant was first measured. [Co(II)L]/dmf was oxidized to [Co(III)L] at 0.8 V in dmf. For TCNQ (-I)/acetonitrile, the absorption in the 300 – 1000 nm range weakened as a whole with lifetimes of t_1/2

= 3000 s at 20 °C and 300s at 60 °C, by keeping at 1.2 V in acetonitrile. Figure 5 shows the results of PSCAS at 0.7 V of the dmf-acetonitrile solution of [Co(II)L] and TCNQ(-I) with 2:1 molar ratio at 20 °C. Figure 5a shows the time-dependence of the absorbance at 750nm (TCNQ(-I)), 480nm (adduct 1 and adduct 2) and 370nm (mainly adduct 2). Figure 5b shows the spectral change with time; first the TCNQ(-I) moiety changes to the adduct 2 (420 s) and subsequently the adduct 2 yields the adduct 1 (1300 s). Since the sensitivity is low in the region less than 350nm, the detailed comparison of the absorbance in the region may not be done between Figures 2 and 5b. On the other hand, at 60 °C the TCNQ(-I) peaks disappeared more rapidly (within 100s) compared with the case of free TCNQ(-I) (t1/2 = 300s) but the absorption of the adduct 1 was not observed. It means that the 1:1 intermediate adduct at 60 °C, [Co(II)L]·LiTCNQ(-I), was more unstable than LiTCNQ(-I) at oxidation and decomposed without formation of the adduct 1.

In the formation mechanism of the TCNQ adducts with metal fragments, it is generally difficult to assign the oxidative states of TCNQ moieties without ambiguity. This work suggests that the SCV and PSCAS methods can be effectively used for the identification of the TCNQ moiety in redox processes. In most cases TCNQ(-II) moiety has been assigned as the -II oxidative state from the

and OCN^- in polar solvent⁹. The present experimental results by SCV and PSCAS exhibited that the adduct 1 is reversibly reduced to adduct 2 at 20 °C (step 2) and to [Co(II)L]·LiTCNQ(-I) at 60 °C (step 4). When the adducts 1 or 2 decomposes to α,α-dicyano-p-toluoylcyanide(-I) and OCN^-, the above reversible reactions are impossible to occur. In the present system, the coordination of two [Co(III)L] to one TCNQ(-II) plays an important role to stabilize the TCNQ(-II) moiety and to prevent the TCNQ(-II) from decomposing to α,α-dicyano-p-toluoylcyanide(-I) by attack of the solvent molecules.

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Figure 5. Time dependence of absorption-intensity (a) and spectral change (b) of [Co(II)(saloph)]

and TCNQ(-I) in dmf-acetonitrile by potential step method at 0.7 V at 20 °C.