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

Abstract

The preparation and characterization of the titanocene(III)-β-diketonate complexes Ti( η 5-C 5H 5) 2(acetylacetonate), Ti( η 5-C 5H 5) 2(benzoylacetonate), Ti( η 5-C 5H 5) 2( ortho-hydroxy-acetophenonate) and Ti( η 5-C 5H 5) 2( O, O-3-acetyl-1-benzyl-2-hydroxyl-5-methoxyindolate) are described. The molecular structure of Ti( η 5-C 5H 5)(acetylacetonate) was determined by X-ray crystallography (crystal data: monoclinic, space group P2 1/ c, a = 8.338(6), b = 21.71(2), c = 7.86(1) A ̊ , β = 106.9(1)°, U = 1362(3) A ̊ 3 , Z = 4, T = 293 K R = 0.087, R w = 0.093 based on 1015 independent reflections for I>2 σ( 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 Å (average), respectively. Electrochemical data obtained from voltammetric studies on 3-acetyl-1-benzyl-2-hydroxy-5-methoxyindole are compared to that from derivatives in which the hydroxy hydrogen has been replaced by an acetyl or a titanocene group. Appropriate derivatization is shown to stabilize the organic oxidation (acetyl) and reduction (titanocene) 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 β-diketonate ligand. The product of this 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 derivative, 2-acetoxy-3-acetyl-1-benzyl-5-methoxyindole, undergoes a reversible one-electron oxidation to produce a moderately stable cation radical which is characterized spectroscopically 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 achieved. In the case of Ti( η 5-C 5H 5) 2(benzolacetone) the reduction process becomes reversible at low temperature to yield a moderately stable coordinated dianion radical.