Intramolecular communication and electrochemical observation of the 17-electron ruthenocenium cation in fluorinated ruthenocene-containing p-diketones; polymorphism of C_(10)H_(21) and C_(10)F_(21) derivatives

摘要

Ruthenocene-containing β-diketones, RcCOCH2COR with Rc = ruthenocenyl and R = C_(10)F_(21) (1), CF3 (2), C6F5 (3), C_(10)H_(21) (4), CH3 (5) and C6H5 (6), were synthesised by Claisen condensation of the appropriate methyl ester with acetylruthenocene, and their spectroscopic, electrochemical and thermal properties were compared. A new synthetic route utilising 1,2,3-benzotriazol-1-ylethanone (9) or 1,2,3-benzotriazol-1-yl(ruthenocenyl)methanone (10) as a reactant, rather than the conventional esters, was found to be more efficient for β-diketone synthesis. The apparent acid dissociation constants, pK_a', of the new ruthenocene-containing p-diketones are 7.14(4) (1, R = C_(10)F_(21)), 9.92(3) (3, R = C6F5) and 10.06(2) (4, R = C_(10)H_(21)). Peak anodic potentials of the ruthenocenyl group of 1-6, pK_a' values and the FTIR ν(C=O) stretching frequencies of the precursor esters, RCOOCH3, correlated linearly with the Gordy scale group electronegativity, χr, of the C_(10)F_(21) (χC_(10)F_(21) = 3.04), C6F5 (χC6F5 = 2.46), C_(10)H_(21) (χC_(10)H_(21) = 2.43) and other R-groups. An electrochemical study in the non-interacting solvent and electrolyte system CH2CI2/0.1 mol dm~(-3) [N(~nBu4)][B(C6Fs)4] revealed electrochemically irreversible one-electron transfer Rc/Rc~+ couples in the potential range 650 < E_(pa) < 1110 mV. Strikingly, reduction of the unstable monomeric 17 electron species, [Rc~+(C5H5)(C5H4COCH2COR)], was clearly observed for all p-diketones possessing fluorinated R-groups, even at slow (100 mV s~(-1)) scan rates. The enol isomer of the fluorinated p-diketones had >90% abundance under the conditions of study and the first order rate constant of enol to keto conversion varied between 220 and 50000 s~(-1) depending on solvent (CDCI3 or CD3CN) and R-groups. Thermal analysis (DSC) of 1 and 4 showed no liquid crystalline mesophase behaviour but definite polymorphism was observed, β-diketones 1 and 4 exist as low temperature polymorphs below 42 °C or 12 °C respectively. The high temperature polymorphs converted to isotropic liquids at 83 °C (compound 1) or 52 °C (compound 4).