Selective Oxidation and Reduction of Trinuclear Titanium(II) Hexaazatrinaphthylene ComplexesSynthesis, Structure, and Electrochemical Investigations

Titanocene complexes with chelating N-heterocyclic ligand bridges react with ferrocenium salts as selective oxidants to afford air-stable cationic complexes and allow the preparation of exceptional mixed valence hexaazatrinaphthylene complexes [(Cp₂Ti)₃(μ₃-HATNMe₆)]ⁿ⁺ (1ⁿ⁺) (n = 1, 2, 3, 4). Cyclic voltammograms (CV) and differential pulse voltammograms (DPV) show that nine oxidation states of 1 are generated without decomposition. Comproportionation constants K_c have been calculated in order to determine the extent of electronic communication between the titanium centers. The K_c values of the mixed valence states are indicative of uncoupled (1⁴⁺), moderately coupled (1⁵⁺), and strongly coupled (1^-, 1⁺, and 1²⁺) systems. Small but significant structural changes occurring upon oxidation of neutral 1 are observed by X-ray structural analysis on 1⁺−1⁴⁺. Anion−π interactions between the electron-deficient central ring of the HATNMe₆ moiety and PF₆^- and BF₄^- counterions, respectively, are found for 1²⁺, 1³⁺, and 1⁴⁺. The short cation−anion contacts cause interesting molecular allignments in terms of molecular architecture. For 1²⁺ the assembly of an one-dimensional (1D) polymer is observed. Electrochemical investigations on the mononuclear cationic titanocene complexes [(Cp₂Ti)(L)]⁺ (L = 2,2‘-biquinoline (2⁺), 4,4‘-dimethyl-2,2‘-biquinoline (3⁺), and 5,8‘-dimethyl-2,3‘-biquinoxaline (4⁺)) showed similar oxidation and reduction characteristics among each other. Conversion to monoanionic, neutral, and dicationic states is enabled. As found for the trinuclear compounds 1ⁿ⁺, the molecular structures of 2⁺−4⁺ reveal significant differences compared to their neutral parents.