I.Synthesis of (μ₄-O)(denc)₄Cu₂(Ni(H₂O))₂Cl₆ [denc = N,N-diethylnicotinamide, core structure = Cu₂Ni₂], (μ₄-O)(denc)₄Cu₃CoCl₆^.H₂O [Cu₃Co] and (μ₄-O)(denc)₄Cu₂Co₂Cl₆ [Cu₂Co₂] Complexes

At the initial stage of the project our primary objective was to organize the laboratory, and introduce the students with the various experimental skills necessary for the synthesis and purification of the complexes. To achieve this goal, we performed previous transmetallation works on Cu₂Ni₂, Cu₃Co, and Cu₂Co₂ complexes.^1-3 The purity of the complexes were checked using their electronic spectra.

Cu₂Ni₂ was synthesized in methylene chloride by transmetallation according to

(µ₄-O)(denc)₄Cu₄Cl₆ + 2Ni(NS) ₂ → (μ₄-O)(denc)₄Cu₂(Ni(H₂O))₂Cl₆ + 2Cu (NS) ₂ (1)

where NS = S-methyl isopropylidenehydrazinecarbodithioate.^1-3 An additional water molecule is coordinated to the Ni center of Cu₂Ni₂ during its purification. The Cu(NS)₂ was separated from Cu₂Ni₂ by gel permeation chromatography.

(µ₄-O)(denc)₄Cu₄Cl₆ [Cu₄] was prepared by treating a solution of CuCl₂^.2H₂O with a solution of denc, both in methanol, and refluxing the reaction mixture and adding sodium methoxide solution in methanol.⁴ Ni(NS)₂ was obtained by treating a solution of HNS with a solution of Ni(acetate)₂^.4H₂O, both in ethanol, and refluxing the reaction mixture.⁵

Cu₃Co and Cu₂Co₂ were synthesized and purified following the procedure of Cu₂Ni₂ using Co(NS)₂ as a transmetallator. Co(NS)₂ was prepared using the same procedure to prepare Ni(NS)₂ except that all the solvents used were flushed with nitrogen prior to use.⁵ Further studies on Cu₂Co₂ were not conducted due to a very limited amount of sample obtained from the synthesis.

II. Electrochemical Studies of the Complexes

One of the main objectives of this project is to extend the electrochemical studies conducted on the (μ₄-O)(denc)₄Cu_4-x(Ni(H₂O))_xCl₆ [x = 0 to 4] complexes to other types of heteropolymetallic complexes (μ₄-O)(denc)₄M'_4-xM_xCl₆ [M' and M = Cu, Zn, or Co, and x = 0 to 4]. In this work, we have shown that Cu₃Co complex is electrochemically active and at higher cathodic potential the complex forms a deposit on the electrode surface. To acquaint the students with the electrochemical studies, we also conducted previous cyclic voltammetric (CV) studies on the Cu₄ and Cu₂Ni₂ complexes.⁶

All CV studies were carried using PAR Model 263A Potentiostat – Galvanostat (PerkinElmer) with three electrodes system consisting of Pt working (3 mm diameter, CH instrument Co.), Pt wire counter, and Ag/(0.01 M) silver hexafluorophosphate (AgPF₆) -CH₃CN reference electrodes. The Pt working electrode was polished with 0.05 μm alumina (Buehler), washed with deionized water, sonicated for about 5 minutes, and dried before use. All experiments were carried out using 1 mM solutions of the complexes with 0.20 M tetrabutylammonium hexafluorophosphate as the supporting electrolyte in dimethylsulfoxide (DMSO). The solutions were bubbled with N₂ prior to the electrochemical measurement and blanketed with N₂ while conducting the experiments. CV studies as a function of switching potentials and pause time were conducted with the first scan after polishing the electrode while the rest of the scans were continuous.

  CVs of the Cu₃Co shown in Figure 1 exhibit the appearance of multiple anodic and cathodic peaks at higher cathodic switching potential. Switching at – 0.60 V produces single well-defined and reproducible reduction and oxidation peaks, Figure 2. CV studies at a higher cathodic switching potential as a function of pausing time clearly demonstrated an increase of two anodic peaks with pause time increase, Figure 3. This indicates that the peaks are stripping peaks of an electrodeposited film on the electrode. Similar trends were observed from the CV studies of Cu₄ and Cu₂Ni₂ in agreement with reference 6. Further works will be conducted to study the property of the film of Cu₃Co and extend the project to other types of heteropolymetallic complexes.

References

1. Davies, G.; El-Sayed, M. A.; El-Toukhy, A., Chem. Soc. Rev., 1992, 101.

2. Caulton, K. G.; Davies, G.; Holt, E. M., Polyhedron, 1990, 9, 2319.

3. El-Toukhy, A.; Cai, G. Z.; Davies, G.; Gilbert, T.; Onan, K.; Veidis, M., J. Amer.

Chem. Soc., 1984, 106, 4596.

4. Dieck, H. T., Inorg. Chim. Acta, 1973, 7, 397.

5. El-Toukhy, A.; El-Essawi, M.; Tawfik, M.; El-Sayed, L.; Iskander, M. F., Trans. Met.

Chem., 1982, 7, 158.

6. Workie, B.; Dube, C. E.; Aksu, L.; Kounaves, S. P.; Robbat, A. Jr.; Davies, G., J.

Chem. Soc., (Dalton Trans.), 1997, 10, 1739 and refs therein.

Figure 1. The effect of switching potential on CV of 1 mM of the Cu₃Co core complex

in 0.20 M TBAPF₆-DMSO at a scan rate of 0.02 Vs^-1.

Figure 2. CV of 1 mM Cu₃Co core complex in 0.20 M TBAPF₆-DMSO at a scan rate of

0.02 Vs^-1.

Figure 3. CVs of 1 mM Cu₃Co core complex in 0.20 M TBAPF₆-DMSO at a scan rate of 0.02 Vs^-1. Increasing peak currents correspond with pause time of 0, 10,

20, 40 and 100 s.