Ferrocene-containing β-diketones derived from lactones: synthesis, complexation with rhodium(I), electrochemistry and substitution kinetics

New ferrocene-containing β-diketones of the form FcCOCH2CO(CH2)nOH (Fc = ferrocenyl) where n = 3 [pKa' = 5.97], 4 [pKa' = 8.01] and 5 [pKa' = 10.44] were prepared by the Claisen condensation of acetyl ferrocene and the appropriate cyclic ester under the influence of lithium diisopropylamide. The rate of conversion of FcCOCH2CO(CH2)5OH from the enol to the keto form and vice versa was studied and the kinetic parameters determined. All β-diketones were also reacted with [Rh2(COD)2Cl2] to yield the [Rh(β-diketonato)(COD)] complexes [Rh(FcCOCHCO(CH2)nOH)(COD)] where n = 3, 4 and 5. The group electronegativity of the alcohol side chains was determined by the linear relationship between the methyl or ethyl ester (RCOOMe or RCOOEt) infrared carbonyl stretching frequencies and the group electronegativities of known R groups, R = CF3, CCl3, CH3, C6H5 and Fc. A cyclic voltammetry study of the free β-diketones showed chemical and quasi-electrochemically reversible behaviour for the iron core in the free β-diketones with side-chain lengths of 3, 4 and 5 carbons with E0ˊ = 197 mV, 174 mV and 151 mV respectively. Electrochemical and chemical reversibility were observed for the ferrocene moiety during the study of the [Rh(β-diketonato)(COD)] complexes. It was found that the rhodium centre of the rhodium complexes exhibited two coordination numbers. A 4-coordinate rhodium redox centre was observed with ipc/ipa. >1 and ΔE < 100 mV. Electrochemical evidence of a 5-coordinate rhodium centre by virtue of interaction between the OH-endgroups of the side-chain of the β-diketonato ligand and the rhodium centre, was observed. Substitution of the β-diketonato ligand from the [Rh(β-diketonato)(COD)] complexes with 1,10- phenanthroline in methanol was also studied and the kinetic parameters determined. Large negative activation entropy values were obtained; these suggested an associative substitution mechanism. All substitution reactions had observable mechanistic solvent pathway contributions.