Intramolecular communication and electrochemical observation of the 17-electron ruthenocenium cation in fluorinated ruthenocene-containing β-diketones; polymorphism of C10H21 and C10F21 derivatives

Abstract

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, pK_a′, of the new ruthenocene-containing β-diketones are 7.14(4) (1, R = C₁₀F₂₁), 9.92(3) (3, R = C₆F₅) and 10.06(2) (4, R = C₁₀H₂₁). Peak anodic potentials of the ruthenocenyl group of 1–6, pK_a′ values and the FTIR ν(CO) stretching frequencies of the precursor esters, RCOOCH₃, correlated linearly with the Gordy scale group electronegativity, χ_R, of the C₁₀F₂₁ (χ_C10F21 = 3.04), C₆F₅ (χ_C6F5 = 2.46), C₁₀H₂₁ (χ_C10H21 = 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 < E_pa < 1110 mV. Strikingly, reduction of the unstable monomeric 17 electron species, [Rc⁺(C₅H₅)(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).