CHARACTERIZATION OF TITANOCENE(III) COMPLEXES OF BETA-DIKETONATES BY ELECTROCHEMICAL, SPECTROSCOPIC AND CRYSTALLOGRAPHIC METHODS - STABILIZATION OF OXIDIZED AND REDUCED BETA-DIKETONATE RADICALS BY ACETYL AND TITANOCENE DERIVATIZATION, RESPECTIVELY

The preparation and characterization of the titanocene(III)-beta-diket onate complexes Ti(eta(5)-C5H5)(2)(acetylacetonate), Ti(eta(5)-C5H5)(2 )(benzoylacetonate), Ti(eta(5)-C5H5)(2)(ortho-hydroxy-acetophenonate) and Ti(eta O,O-3-acetyl-1-benzyl-2-hydroxy-5-methoxyindolate) are desc ribed. The molecular structure of Ti(eta(5)-C5H5)(2)(acetylacetonate) was determined by X-ray crystallography (crystal data: monoclinic, spa ce group P2(1)/c, a = 8.338(6), b = 21.71(2), c = 7.86(1) Angstrom, be ta = 106.9(1)degrees, U = 1362(3) Angstrom(3), Z = 4, T = 293 K, R = 0 .087, R(w) = 0.093 based on 1015 independent reflections for I > 2 sig ma(I)). The structure was found to be slightly distorted and exhibited normal average Ti-O and Ti-C bond lengths of 2.07 and 2.37 Angstrom ( average), respectively. Electrochemical data obtained from voltammetri c studies on 3-acetyl-1-benzyl-2-hydroxy-5-methoxyindole are compared to that from derivatives in which the hydroxy hydrogen has been replac ed by an acetyl or a titanocene group. Appropriate derivatization is s hown to stabilize the organic oxidation (acetyl) and reduction (titano cene) processes. In the case of the titanocene derivative, both metal and ligand based processes are observed. The titanocene(III) fragment can be reversibly oxidized in a one-electron metal based process which occurs at a potential which is essentially independent of the beta-di ketonate ligand. The product of this process is the expected Ti(IV) mo iety. The ligand coordinated to the titanocene is oxidized at a consid erably more positive potential. In contrast, the acetyl derivative, 2- acetoxy-3-acetyl-1-benzyl-5-methoxyindole undergoes a reversible one-e lectron oxidation to produce a moderately stable cation radical which is characterized spectroscopically in an in situ electrochemical EPR e xperiment. The reduction of 3-acetyl-1-benzyl-2-hydroxy-5-methoxyindol e leads to reductive deprotonation with the resulting anion not being reducible. However, voltammetric reduction after coordination to a tit anocene(III) fragment can be achieved, In the case of Ti(eta 5-C5H5)(2 )(benzoylacetonate) the reduction process becomes reversible at low te mperature to yield a moderately stable coordinated dianion radical.