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Ruthenocene-containing β-diketones: Synthesis, pKa′ values, keto-enol isomerization kinetics, and electrochemical aspects

Claisen condensation of acetyl ruthenocene with the appropriate methyl ester, RCOOMe, under the influence of the hindered base lithium diisopropylamide gave the β-diketones (RcCOCH2COR) 1-ruthenocenyl-4,4,4- trifluorobutane-1,3-dione (ruthenocenoyltrifluoroacetone, 1, R = CF3; pKa′ = 7.36 ± 0.03), 1-ruthenocenylbutane-1,3-dione (ruthenocenoylacetone, 2, R = CH3; pKa′ = 10.22 ± 0.01), 1-phenyl-3-ruthenocenylpropane-1,3-dione (benzoylruthenocenoylmethane, 3, R = C6H5; pK a′ = 11.31 ± 0.03), 1-ferrocenyl-3-ruthenocenylpropane- 1,3-dione (ferrocenoylruthenocenoylmethane, 4, R = Fc = (C5H 5)Fe(C5H4) = ferrocenyl; pKa′ = ca. 12.8), and 1,3-diruthenocenylpropane-1,3-dione (diruthenocenoylmethane, 5, R = Rc = (C5H5)Ru(C5H4) = ruthenocenyl; pKa′ = ca. 12.7). The group electronegativity of the ruthenocenyl group, ΧRc = 1.89 on the Gordy scale, was obtained from the linear relationship between IR carbonyl stretching frequencies of a series of methyl esters, RCOOMe, and ΧR. A 1H NMR kinetic study of the enol-keto interconversion resulted in accurate equilibrium constants, Kc, for this equilibrium, as well as forward and reverse rate constants of the isomerization process. Cyclic voltammetry in CH3CN/N(nBu)4PF6 utilizing a glassy-carbon electrode showed the ruthenocene center exhibited, in contrast to the ferrocene center, irreversible electrochemistry. The multiple peak anodic (oxidation) potentials observed are a consequence of slow isomerization kinetics and made peak assignments for the keto and enol isomers possible. The kinetics of enol to keto isomerization for 4 was also studied by cyclic and Osteryoung square wave voltammetry; obtained rate constants were mutually consistent with those obtained by the 1H NMR technique. Kinetic rate constants, Kc, and pKa′ and Ep,a were related to ΧR values of the R groups in RCCOCH2COR.

Intramolecular communication and electrochemical observation of the 17-electron ruthenocenium cation in fluorinated ruthenocene-containing β-diketones; Polymorphism of C10H21 and C 10F21 derivatives

Ruthenocene-containing β-diketones, RcCOCH2COR with Rc = ruthenocenyl and R = C10F21 (1), CF3 (2), C6F5 (3), C10H21 (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, pKa′, of the new ruthenocene-containing β-diketones are 7.14(4) (1, R = C 10F21), 9.92(3) (3, R = C6F5) and 10.06(2) (4, R = C10H21). Peak anodic potentials of the ruthenocenyl group of 1-6, pKa′ values and the FTIR ν(CO) stretching frequencies of the precursor esters, RCOOCH3, correlated linearly with the Gordy scale group electronegativity, χR, of the C10F21 (χC10F21 = 3.04), C 6F5 (χC6F5 = 2.46), C 10H21 (χC10H21 = 2.43) and other R-groups. An electrochemical study in the non-interacting solvent and electrolyte system CH2Cl2/0.1 mol dm-3 [N( nBu4)][B(C6F5)4] revealed electrochemically irreversible one-electron transfer Rc/Rc+ couples in the potential range 650 pa +(C 5H5)(C5H4COCH2COR)], was clearly observed for all β-diketones possessing fluorinated R-groups, even at slow (100 mV s-1) scan rates. The enol isomer of the fluorinated β-diketones had >90% abundance under the conditions of study and the first order rate constant of enol to keto conversion varied between 220 and 50 000 s-1 depending on solvent (CDCl3 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).