Synthesis, Substitution Kinetics, and Electrochemistry of the First Tetrathiafulvalene-Containing β-Diketonato Complexes of Rhodium(I)

The synthesis of the first rhodium(I) cyclooctadiene complexes containing tetrathiafulvalene (TTF) groups substituted on a beta-diketonato ligand in either the methine position (3 position), [Rh(cod)(H3CCOC {S-TTF-(MeS)(3)}COCH3)] (3), or terminal position (1 position), [Rh(cod) (Me-3-TTF)COCHCOCH3H}] (4), is reported. The effect of the beta-diketonato substitution position on the kinetics of substitution of the TTF-containing beta-diketonato ligand with 1,10-phenanthroline from 3 and 4 to give [Rh(cod)(phen)](+), as well as on the electrochemical properties of 3 and 4, was investigated, Second-order substitution rate constants, k(2), in methanol were found to be almost independent of the substitution position, with 4 (k(2) = 2.09 x 10(3) dm(3) mol(-1) s(-1)) reacting only about twice as fast as 3. An appreciable solvent pathway in the substitution mechanism was only observed for 4 with k(2) = 42 s(-1). A complete mechanism for both substitution reactions is proposed. The electrochemistry of 3 and 4 in CH2Cl2/0.10 mol dm(-3) [N(Bu-n)(4)][B(C6F5)(4)] showed three redox processes. Two of these were electrochemically reversible and are associated with the redox-active TTF group. For 3, TTF-based formal reduction potentials, E degrees', were observed at 0.082 and 0.659 V vs Fc/Fc', respectively; 4 exhibited them at -0.172 and 0.703 V vs Fc/Fc(+) at a scan rate of 100 mV s(-1). A Rh-II/Rh-I redox couple was observed at E degrees' = 0.89 V for 3, after both TTF oxidations were completed, and at 0.51 V for 4; this is between the two TTF redox processes. The more difficult oxidation of the Rh-I center of 3 indicates more effective electron-withdrawing from the Rh-I center to the first-oxidized TTF+ group at the methine position of the beta-diketonato ligand of 3(+) than to the terminal-substituted TTF+ group in 4(+).