β-Diketones containing a ferrocenyl group: synthesis, structural aspects, pKa1 values, group electronegativities and complexation with rhodium(I)

Abstract

1-Ferrocenyl-4,4,4-trifluorobutane-1,3-dione (ferrocenoyltrifluoroacetone, Hfctfa, pK_a¹ = 6.53 ± 0.03), 4,4,4-trichloro-1-ferrocenylbutane-1,3-dione (trichloroferrocenoylacetone, Hfctca, pK_a¹ = 7.15 ± 0.02), 1-ferrocenylbutane-1,3-dione (ferrocenoylacetone, Hfca, pK_a¹ = 10.01 ± 0.02), 1-ferrocenyl-3-phenylpropane-1,3-dione (benzoylferrocenoylmethane, Hbfcm, pK_a¹ = 10.41 ± 0.02) and 1,3-diferrocenylpropane-1,3-dione (diferrocenoylmethane, Hdfcm, pK_a¹ = 13.1 ± 0.1) were prepared by Claisen condensation of acetylferrocene with an appropriate ester under the influence of sodium amide, sodium ethoxide or lithium diisopropylamide. The group electronegativity of the ferrocenyl group is 1.87 (Gordy scale) as inferred from a linear β-diketone pK_a¹–group electronegativity relationship as well as from a linear methyl ester IR carbonyl stretching frequency–group electronegativity relationship. Complexes [Rh(β-diketone)(cod)] were obtained in yields approaching 80% by treating the β-diketones with [Rh₂Cl₂(cod)₂], while the copper(II) chelates form just as readily. Treatment of all [Rh(β-diketone)(cod)] complexes with 1,10-phenanthroline (phen) and some of its derivatives resulted in substitution of the β-diketone ligand to form [Rh(cod)(phen)]⁺. The uncomplexed β-diketones are increasingly stable towards the OH^– nucleophile in the order Hdfcm (apparent most unstable) < Hfctfa < Hbfcm < Hfctca < Hfca (most stable). Asymmetric enolisation in the direction furthest from the ferrocenyl group was observed for all β-diketones. This finding is considered to be the result of resonance driving forces rather than inductive electronic effects of substituents on the pseudo-aromatic β-diketone core.