Characterization of titanocene(III) complexes of β-diketonates by electrochemical, spectroscopic and crystallographic methods: stabilization of oxidized and reduced β-diketonate radicals by acetyl and titanocene derivatization, respectively

Article abstract of DOI:10.1016/0020-1693(95)90052-8

The preparation and characterization of the titanocene(III)-β-diketonate complexes Ti(η5-C5H5)2(acetylacetonate), Ti(η5-C5H5)2(benzoylacetonate), Ti(η5-C5H5)2(ortho-hydroxy-acetophenonate) and Ti(η5C5H5)2(O,O-3-acetyl-1-benzyl-2-hydroxy-5-methoxyindolate) are described. The molecular structure of Ti(η5-C5H5)2(acetylacetonate) was determined by X-ray crystallography (crystal data: monoclinic, space group P21/C, a=8.338(6), b=21.71(2), c=7.86(1) ?, β=106.9°, V=1362(3) ?**3, Z=4, T=293 K, R=0.087, R(w)=0.093 based on 1015 independent reflections for I>2σ(I)). The structure was found to be slightly distorted and exhibited normal average Ti-O and Ti-C bond lenghts of 2.07 and 2.37 ? (average), respectively. Electrochemical data obtained from voltammetric studies on 3-acetyl-1-benzyl-2-hydroxy-5-methoxyindole are compared to that from derivates in which the hydroxy hydrogen has been replaced by an acetyl or a titanocene group. Appropriate derivatization is shown to stabilize teh organic oxidation (acetyl) and reduction (titanocene) processes. In the case of the titanocene derivative, both metal and ligand based process is the expected Ti(IV) moiety. The ligand coordinated to the titanocene is oxidized at a considerably more positive potential. In contrast, the acetyl derivate, 2-zcetoxy-3-acetyl-1-benzyl-5-methoxyindole, undergoes a reversible one-electron oxidation toproduce a moderately stable cation radical which is characterized spect roscopially in an in situ electrochemical EPR experiment. The reduction of 3-acetyl-1-benzyl-2-hydroxy-5-methoxyindole leads to reductive deprotonation with the resulting anion not being reducible. However, voltammetric reduction after coordination to a titanocene(III) fragment can be recieved. In the case of Ti(η5-C5H5)2(benzoylacetonate) the reduction process becomes revrsible at low temperature to yield a moderately stable coordinated dianion radical.