Synthesis, Characterization, and Electrochemistry of Diferrocenyl β-Diketones, -Diketonates, and Pyrazoles

The synthesis of FcC(O)CH(R)C(O)Fc (Fc = Fe(eta(5)-C5H4)(eta(5)-C5H5); R = H, 5; Bu-n, 7; CH2CH2(OCH2CH2)(2)OMe, 9), [M(kappa O-2,O '-FcC(O)CHC(O)Fc)(n)] (M = Ti, n = 3, 10; M = Fe, n = 3, 11; M = BF2, n = 1, 12), and 1-R '-3,5-Fc(2)-(c) C3HN2 (R ' = H, 13; Me, 14; Ph, 15) is discussed. The solid-state structures of 5, 7, 9, 12, 13, 15, and 16 ([TiCl2(kappa O-2,O '-PhC(O)CHC(O)Ph)(2)]) show that 7 and 9 exist in their beta-diketo form. Compound 13 crystallizes as a tetramer based on a hydrogen bond pattern, including one central water molecule. The electrochemical behavior of 5-7 and 9-16 was studied by cyclic and square-wave voltammetry, showing that the ferrocenyls can separately be oxidized reversibly between -50 and 750 mV (5-7, 9, 12-15: two Fc-related events; 10, 11: six events, being partially superimposed). For complex 10, Ti-centered reversible redox processes appear at -985 (Ti-II/Ti-III) and -520 mV (Ti-III/Ti-IV). Spectro-electrochemical UV-Vis/NIR measurements were carried out on 5, 6, and 12, whereby only 12 showed an IVCT (intervalence charge-transfer) band of considerable strength (nu(max) = 6250 cm(-1), Delta nu(1/2) = 4725 cm(-1), epsilon(max) = 240 L center dot mol(-1)center dot cm(-1)), due to the rigid C3O2B cycle, enlarging the coupling strength between the Fc groups.