TY - JOUR
T1 - Characterization of titanocene(III) complexes of β-diketonates by electrochemical, spectroscopic and crystallographic methods
T2 - stabilization of oxidized and reduced β-diketonate radicals by acetyl and titanocene derivatization, respectively
AU - Bond, Alan M.
AU - Colton, Ray
AU - Englert, Ulli
AU - Hügel, Helmut
AU - Marken, Frank
PY - 1995/7
Y1 - 1995/7
N2 - 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(η5-C5H5)2(O,O-3-acetyl-1-benzyl-2-hydroxyl-5-methoxyindolate) are described. The molecular structure of Ti(η5-C5H5)(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) A ̊, β = 106.9(1)°, U = 1362(3) A ̊3, Z = 4, T = 293 KR = 0.087, Rw = 0.093 based on 1015 independent reflections for I>2σ(I)). The structure was found to be slightly distorted and exhibited normal average TiO and TiC 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on 3-acetyl-1-benzyl-2-hydroxy-5-methoxyindole are compared to that from derivatives in which the hydroxy hydrogen has been replaced by an acetyl or a titanocene group.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(η5-C5H5)2(benzolacetone) the reduction process becomes reversible at low temperature to yield a moderately stable coordinated dianion radical.
AB - 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(η5-C5H5)2(O,O-3-acetyl-1-benzyl-2-hydroxyl-5-methoxyindolate) are described. The molecular structure of Ti(η5-C5H5)(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) A ̊, β = 106.9(1)°, U = 1362(3) A ̊3, Z = 4, T = 293 KR = 0.087, Rw = 0.093 based on 1015 independent reflections for I>2σ(I)). The structure was found to be slightly distorted and exhibited normal average TiO and TiC 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(η5-C5H5)2(benzolacetone) the reduction process becomes reversible at low temperature to yield a moderately stable coordinated dianion radical.
KW - Indole complexes
KW - Titanocene complexes
KW - Voltammetry
KW - β-Diketonate complexes
UR - http://www.scopus.com/inward/record.url?scp=0042570606&partnerID=8YFLogxK
U2 - 10.1016/0020-1693(95)90052-8
DO - 10.1016/0020-1693(95)90052-8
M3 - Article
AN - SCOPUS:0042570606
SN - 0020-1693
VL - 235
SP - 117
EP - 126
JO - Inorganica Chimica Acta
JF - Inorganica Chimica Acta
IS - 1-2
ER -