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

[摘要] English: 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 ofacetyl ferrocene and the appropriate cyclic ester under the influence of lithium diisopropylamide. Therate of conversion of FcCOCH2CO(CH2)5OH from the enol to the keto form and vice versa wasstudied 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 relationshipbetween the methyl or ethyl ester (RCOOMe or RCOOEt) infrared carbonyl stretching frequenciesand 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-electrochemicallyreversible behaviour for the iron core in the free β-diketones with side-chain lengths of 3, 4 and 5carbons with E0ˊ = 197 mV, 174 mV and 151 mV respectively. Electrochemical and chemicalreversibility 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 twocoordination 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 betweenthe 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 negativeactivation entropy values were obtained; these suggested an associative substitution mechanism. Allsubstitution reactions had observable mechanistic solvent pathway contributions.

[发布机构] University of the Free State