COMPRENDAMOS

The synthesis of coordination complexes [1] is an extremely important area of both inorganic and organometallic chemistry. However, in many instances it is difficult to obtain high yields and/or to apply conventional methods when large-scale preparations are involved.

Electrochemical reactions have not been fully exploited as useful, simple and clean process alternatives in spite of their enormous potential [2]. They make it possible to add or remove electrons from an electroactive substance at room temperature, with high selectivity and obviating the need to add oxidizing or reducing agents.

Organic chemists are now aware of this enormous potential, having used it to synthesize some compounds wich could not have been obtained otherwise [2].

However, inorganic and organometallic chemists are just starting to realize the usefullness of this new tool, which is hard to explain since a wealth of information is available to them concernig the interaction of many different chemical species with electrodes.

On the other hand, coordination complexes are usually synthesized by reacting a metal salt (chloride, sulfate, acetate, etc.) solution with some ligand precursor. Since a metal salt is obtained from a metal the direct (one-step) synthesis of the complexes from the metal must be preferred on energetic and even economic grounds.

In addition, since the common metals are made on a huge scale by some of the oldest and best developed chemical reactions know to man, such elements are convenient to use, easily stored and available in almost any desired degree of purity.

Now, it is a well-know fact that most direct synthesis experiments using very pure massive metals are notoriously sluggish, requiring acceleration by one or more of the means discussed, for instante, by Garnovskii et al [5]. However, there is an additional -and radically different- way to prod a metal into reaction, namely by using it as a "sacrificial" anode; the first work based on this approach were conducted as far back as 1882 by B. Gerdes and in 1905 by Chugaev [3].

Nine decades after Gerdes’s pioneering experiments, Lehmkuhl in Germany and Garnovskii in Russia rediscovered "direct electrosynthesis"; soon thereafter, Tuck in Canada gave it impetus through the develpment of simple electrochemical methods employing relatively unsophisticated apparatus, and non-aqueous solvents.

It must be pointed out that very few Mexican researchers have consider use direct electrosynthesis as a tool for the preparation of coordination compounds. The group of Dr. Boris Kharisov in Universidad Autonoma de Nuevo Leon, and the Dr. Gojon Zorrilla and Dr. Quiroz in Universidad de las Americas-Puebla, are the two exceptions.

Summarizing, direct electrosynthesis of metal complexes possesses the following advantages:

    a) It proceeds at room temperature.
    b) It is energy-efficient.
    c) It is clean, with a high "environmetal quotient".
    d) It avoids the use of metal salts with a potential to contribute undesirable anions which mus somehow be eliminated in a subsequent purification step.
    e) "bulk" metals are satisfactory as starting materials (plate, rod, wire, etc.), these being cheaper than powders or foil, which must be used in other direct-synthetic methods.
    f) It allows unique access to some metal complexes.
    g) Yields are usually high.
    h) Separation and purification are easily accomplished.
    i) The use of non-aqueous solvents helps prevent hydrolysis and/or hydratation.
    j) The solvent can be recycled and the process may be rather easily sealed up.
    k) Very simple electrolysis cells may be used (usually undivided).
    l) The method is highly versatile, allowing the researcher to prepare specific complexes.
    m) Sometimes the products are more active (possess higher specific surface areas) than those obtained by conventional methods.

However, the direct electrosynthesis could present some disvantages at some cases. Because we need use some kind of supporting electrolyte in order to mantain the system conductivity, many times supporting electrolyte form part of the final product (as anions, cations or ligand), difficulting a simple synthesis between the ligand and the metal. Some researchers think that is possible discart this problem using low concentrations of supporting electrolyte or employing "inert supports", but at many cases, the products are contaminated by the electroactive species.

We have developed an alternative for the obtention of pure ligand-metal complexes in non aqueous media, named "in situ generation of supporting electrolyte". This technique employs the chemical characteristics or the ligand as an electroactive specie, and can be succesfull used at some electrosynthesis as we have showed.

The REDOX reactions involved during a direct electrosynthes, in general, are the next:

Anode: 2L- + M ----> ML2 + 2e-
Catode: 2HL + 2e- -----> H2 + 2L-

We have observed that the ligand as anion, can be employed as supporting electrolyte in some cases, so we have generated chemicaly this specie previously the electrochemical reaction. In our particular case, we have formed the enolate species of oxicams (LH) molecules by direct reduction with metallic sodium or potasium in dry acetonitrile. The reaction can be expressed as following:

2HL + 2Na -----> H2 + 2Na+ + 2L-

The concentration of this electroactive specie (L-), is fixed at the beginning of the reaction and it is employed as the supporting electrolyte during the direct electrosynthesis, regenerating it by the ligand reduction on the catode.

This technique owing us to obtain the ML2.nCH3CN (n = 1, 2) complexes, where LH is the oxicam and M is the metal dissolved, with high yields and pure. We attempt unsuccesfully the direct electrosynthesis previously, employing as supporting electrolyte tetraammine tetrafluoroborate and tetrammine bromide in 0.1 and 0.01 M concentrations. The products in these cases, were the bromide complexes or tetraammine salts of the complexes, not the desire complexes.

The in situ generation of support electrolyte is an attractive alternative to obtain metal complexes with ligands as phenols, amines, carboxilic acids, alcohols, enols, tiols and many other compounds with susceptible functional groups to form electroactive species in non aqueous media.