Molecular Batteries Based on Carbon-Carbon Bond Formation and Cleavage in Titanium and Vanadium Schiff Base Complexes

The reduction of titanium(III) and vanadium(III) salophen complexes led to the reductive coupling of imino groups in the ligands, and thus to the formation of C-C-bonded dimers which released electrons on subsequent cleavage of the C-C bond. On reduction of [M(salophen)(Cl) (thf)] [M = Ti (1), V (2)] with sodium metal in a 1:1 molar ratio the dimers [M-2(salophen(2)) (thf)(2)] [M = Ti (3), V (4)] formed, in which two salophen units are joined by a single C-C bond. Further reduction resulted in the introduction of an additional C-C bridge between the two salophen units and thus to the formation of [M-2(*salophen(2)*)(Na)(2)(thf)(6)] [M = Ti (6), V (7)], in which *salophen(2)* is a dinucleating, octadentate, octaanionic ligand. In 6 and 7 the two metal centers are very close together [Ti ... Ti=2.518(l)Angstrom in 6; V ... V= 2.393(1) Angstrom in 7]. Complexes 3 and 4 reacted with 9,10-phenanthrenequinone to give [M(salophen)(9,10-phen)] [M = Ti (9), V (10)], using both the electrons stored at the C-C bond and those from the oxidation of M-III to M-IV, The reaction of 3 and 4 with O-2 yielded the oxometal(IV) complexes [M(salophen)(O)] [M=Ti (13),V (11)] with a four-electron oxidation of the starting dimers. The intermediate oxovanadium(III) complex [V(salophen) {mu-O-Na(DME)(2)}] (16), which can be subsequently oxidized to 11, was isolated from the reaction of 7 with O-2. The reaction became synthetically and mechanistically interesting when organic azides were used as oxidizing agents: reaction of 3 with PhN3 and Me3SiN3 led to the mu-phenylimido dimers [Ti-2(salophen(2))(mu-PhN)(thf)(2)] (17) and [Ti-2(salophen(2))(mu-Me3SiN)] (18), which was hydrolyzed to the analogous mu-oxo [{Ti-2-(salophen(2))(mu-O)}(2)] (19). The reaction of 3 with Ph3CN3 led to the phenylimido complex [Ti(salophen)(NCPh3)] (20), and 4 reacted with PhN3 to give [V(salophen)(NPh)] (21). Extended Huckel calculations enabled us to substantiate the electron-transfer process, which never involves the C-C site as a reactive center; it only functions as an electron reservoir.