J. Chil. Chem. Soc., 50, N° 3 (2005), págs: 575-580

CYCLIC VOLTAMMETRY OF 1-(n-HEXYL)-3-METHYL-5-PYRAZOLONE-BASED ENAMINES AND THEIR CHLOROMANGANESE(III) AND NITRIDOMANGANESE(V) COMPLEXES

F. R. PEREZ^a , L. BASÁEZ^*b , J. BELMAR ^b, and P. VANYSEK^c

^aFacultad de Ciencias de la Salud, Universidad Privada Antenor Orrego, Av. América Sur 3145, Monserrate, Trujillo, Perú.
^bFacultad de Ciencias Químicas, Universidad de Concepción, Casilla 160-C, Concepción, Chile, lbasaez@udec.cl
^cNorthern Illinois University, Department of Chemistry and Biochemistry, DeKalb, Illinois, USA

ABSTRACT

In this publication we are reporting for the first time on examples of the redox behavior of enamines and complexes derived from 1-alkylpyrazolones. Cyclic voltammetry experiment was performed on three enamines derived from 4-acyl-1-(n-hexyl)-3-methylpyrazol-5-ol and their chloromanganese(III) and nitridomanganese(V) complexes. The ligands have an irreversible oxidation potential of 1.1 V. Chloromanganese(III) complexes are quasi-reversible with an E_1/2 of ­ 0.1 V. Nitridomanganese(V) complexes are also quasi-reversible, with E_1/2 potentials of 0.95 V. However, for nitridomanganese(V) complexes reversibility increases as the scan speed raises from 100 to 3000 mV·s^-1.

INTRODUCTION

As a result of a number of interesting applications, the study of manganese complexes is still the main objective of vigorous research^1-4. In biological sciences such complexes are studied as models for enzymatic^5-8 and photochemical^9,10 reactions. In chemistry they have importance in catalytic oxidation,^11,12 nitrogen transfer reactions,¹³ or electrochemistry.^14,15 Since electron transfer is involved in all these reactions, the study of the electrochemical behavior of these compounds is very important to fully characterize understand the chemical reaction in which they may be involved. Schiff bases^16,17 and manganese complexes^18,19 in their different oxidation states have been already studied. It is important to note that the vast majority of those studies involves the salen-type ligands and complexes. Nitridomanganese(V) constitutes an interesting type of manganese complexes. It has been used as a stoichiometric reagent in nitrogen transfer reactions with olefinic substrates. In this case salen, porphyrin, and phtalocyanine ligands have been described.²⁰

Pyrazolones are an important group of organic compounds²¹ that has been also widely studied. They are interesting for both theoretical and practical reasons.²² Pyrazolone derivatives find numerous applications,^23-26 one of the more important being that they are chelating agents for many ions.^{27, 28}

The first synthesis of pyrazolone was reported in the nineteenth century²⁹ and the same procedure remains the most important to this day. The reaction is the condensation of a b-ketoesther with hydrazine. Literature in this area deals mainly with the N-1 unsubstituted or 1-arylpyrazolones, while the 1-alkyl homologues are seldom mentioned. This may be due to the fact that hydrazine and phenylhydrazine have long been commercially available at convenient prices. Alkylhydrazines, on the other hand, are not very common; only a few of them are commercially available and they are very expensive. Besides the commercial scarcity, there are no convenient procedures to synthesize them; likely a circular argument why they are not commercially accessible. This situation is quite unfortunate because it is observed that 1-alkylpyrazolone derivatives are more soluble than their 1-aryl counterparts, which would make them valuable chemicals. To ameliorate the scarcity of these compounds, a few years ago a research aimed to synthesize 1-alkylpyrazolones and its derivatives³⁰ has commenced. Special consideration was given to study tautomerism of these compounds. More recently^31,32 these efforts lead to the synthesis of enamines derived from 4-acyl-1-(n-hexyl)-3-methyl-5-pyrazolones. With the goal to exploit the usefulness of these new ligands, nitridomanganese(V) complexes were prepared.³³ Since they can be used in reactions that involve electron transfer it is important to study and understand their electrochemical behavior, which is the focus of this work.

Electrochemical studies using metal complexes with ligands based on 1-phenylpyrazolone Schiff bases have been seldom reported.³⁴ In this regard, to the best of our knowledge, there are no reports for those complexes or ligands derived from 1-alkylpyrazolones. Herein, preliminary results, using classical cyclic voltammetry to study tetradentate enamines derived from 1-(n-hexyl)-3-methyl-5-pyrazolone and their chloromanganese (III) and nitridomanganese(V) complexes, are reported.

EXPERIMENTAL

Cyclic voltammetry was carried out with the Voltammetry Analyzer BAS CV-50W in a classic cell equipped with three electrodes platinum working, platinum auxiliary and an Ag/AgCl reference electrodes. Dry acetonitrile (boiling to reflux with CaH₂ for 2 days),was the solvent in which 0.1 mol dm^-3 tetrabutylammonium perchlorate (TBAP) supporting electrolyte was used. Potential window of solvent: -1.2 ­ +1.2 V. The scan rate was 100 mV·s^-1, unless stated otherwise.

RESULTS AND DISCUSSION

The route to obtain the chloromanganese(III) (compound 2) and nitridomanganese(V) (compound 3) complexes is outlined in Scheme 1. The enamine ligands were synthesized from 1-(n-hexyl)-3-methyl-5-pyrazolone following the reported procedures.^31,33 The yields for complex 2 were 98% (2a), 91 % (2b) and 80 % (2c) and those for complex 3 were 72 % (3a), 69 % (3b) and 67 % (3c).

Reaction scheme 1. Pathway to synthesis of nitridocomplexes of manganese(V) (3) and chlorocomplexes of manganese(III) (2)

To avoid misinterpretation due to confusion between the redox processes of the ligands (1) and those of the metallic centers, the free ligands were first thoroughly studied. The voltammograms of these compounds show an irreversible oxidation wave (only) within potential range of 0.9 and 1.1 V vs. Ag/AgCl. As an example, the voltammogram of ligand 1b is shown in Figure 1. A voltammogram for four sequential scans was also recorded (Fig. 2), and it show very close behavior in all the scans. This result can be considered a proof and the stability of the process. The data corresponding to the three compounds in Scheme 1 are summarized in Table 1. According to the results, the ligands exhibit similar irreversible oxidation potentials around 1.1 V vs. Ag/AgCl, showing, qualitatively speaking, that the ligands have similar reactivity and similar electrochemical behavior. Apart from a small increase in the oxidation potential, the shape of the waves was unaffected by changing the scan rate (Fig. 3). Therefore, it can be concluded, the electrochemical oxidation of the ligand is not affected by the nature of the ligand; any observed change must therefore depend on the nature of the compounds in scheme 1.

Figure 1. Cyclic voltammogram of ligand 1b.

Figure 2. Voltammogram of ligand 1b during its fist sweep and after four cycles.

Figure 3. Voltammograms of ligand 1b at different sweep rates.

Chloromanganese(III) complexes (compound 2 in Scheme 1) showed in the cyclic voltammogram a behavior that remained unchanged over the course of several cycles. This result again indicates that the electrochemical process that give a rise to the voltammetry is stable. Figures 4 and 5 show the voltammograms of the first and fourth scans for complex 2a. It follows from the DE_p-p values and the i_a/i_c ratio (Table 2), which is not close to 1.0, that for the pair Mn^III/Mn^IV complex the redox processes are quasi-reversible. Similar results have been also reported for related compounds.^{35, 36} The E_1/2 values obtained for the three complexes 2 are small, implying that these compounds can be easily oxidized or reduced. This feature would make them valuable as catalysts in redox reactions. In addition, from the fact that the three E_1/2 values are close to each other, it is follows that the three chloromanganese complexes would exhibit nearly identical redox activity. In fact, the three were oxidized to the nitrides (3) with very similar yields, as mentioned before.

Figure 4. Cyclic voltammogram of complex 2a

Figure 5. Voltammogram of complex 2a shown in four subsequent cycles

The voltammograms of compounds 3 show a cyclic oxidation-reduction process that does not change after repeated scans. In Figure 6, the voltammogram of compound 3a is shown. The data for the peak potentials and related parameters are presented in Table 3. According to the DE_p-p values, the redox processes of these complexes can be considered quasi reversible. Furthermore the measured values, can be assigned to the pair Mn(V)/Mn(VI) in agreement with other reported studies.³⁷ The observed E_1/2 values for the complexes 3 are close to each other. Therefore, it can be expected that the three compounds will also exhibit similar redox reactivity. Furthermore, it can be deduced from the calculated E_1/2 that these compounds cannot be easily oxidized nor can they be reduced. Thus, in order to use them successfully in redox reactions, some structure modifications to decrease the E_1/2 would have been required. In fact, this requirement is documented in known practical application. For instance, in order to use manganese(V) nitridocomplexes in nitrogen transfer to olefins, the complexes must be activated first to an imido structure using an acylating agent .²⁰ In a reversible redox process, the transferred charge in the reduction and in the oxidation stages is the same and thus, their ratio equals the unity. However, in compounds 3, with scan rate of 100 mV·s^-1, this charge ratio is 2:1. This may be due to the fact that reduction takes place at a lower rate than oxidation and that some oxidized material may be adsorbed on the electrode's surface. On increasing the scan rate from 100 to 3000 mV·s^-1 (Figure 7), the reduction-oxidation waves become more reversible-like. Probably this change is due to increase of scan rate and diffusion problems occur. Although DE_p-p increased (Table 4), the i_a/i_c ratio diminished to 1.0. All these results are interpreted as a reversibility increase. The voltammogram remains unchanged during four oxidation-reduction cycles (Figure 8), an evidence of the stability of the complexes 3 in these processes.

Figure 6. Cyclic voltammogram of the nitridocomplex 3a

Figure 7. Voltammogram of the nitridocomplex 3a at different scan rates.

Figure 8. Four subsequent cycles of a cyclic voltammogram of the nitridocomplex 3a

CONCLUSIONS

In conclusion, the present study of cyclic voltammetry of tetradentated ligands derived from 1-(n-hexyl)-3-methyl-5-pyrazolone and its Mn complexes, showed that the ligands presented an irreversible oxidation behavior and the complexes a quasi-reversible redox behavior. Finally the low potential E1/2 that the Mn(III) chlorocomplexes presented predict that they may be used for catalysis proposals.

ACKNOWLEDGEMENTS

The authors are grateful with the University of Concepción for financial support (grants PDI: 203.023.032-1.0 and PDI: 203.021.017-1.0). The graduate scholarship for Mr. F. R. Peréz was provided the MECESUP Program of the Chilean Government.

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