Design, Synthesis, and Structural Characterization of a New Class of Ferrocene-Containing Heterometallic Triple-Stranded Helicates

Abstract

The new ditopic organoiron ligand, [3,5-bis(1-ferrocenyl-prop-3-enol-1-one)(pyridine)] (H2L3,5), has been prepared and the reactions of its dianion (Na2L3,5) with M3+ ions (M = Ga or In) yield a new class of “3d-np block” heterometallic triple-stranded helicates, M2(L3,5)3, by the self-assembly process. The X-ray structural analysis of the new ligand shows that it is in the enolic form with each enolic carbon bonded to the pyridine ring and each carbonyl carbon connected to a ferrocene moiety; overall, the nonferrocenyl part of the molecule is nearly planar. The M2(L3,5)3 (M = Ga or In) complexes are helicates with three ligand strands, each of which is twisted into an S-shape, coordinating to two metal ions, each of which is in a distorted octahedral geometry. The new helicates are observed as a racemic mixture in the solid state by single-crystal X-ray analysis, and in solution by NMR, with both the left-handed Λ,Λ- and the right-handed Δ,Δ-isomers present. Variable-temperature 1H NMR study of the Ga2(L3,5)3 helicate indicates that the right-handed Δ,Δ-isomer and left-handed Λ,Λ-isomer equilibrate through a heterochiral Λ,Δ-intermediate by a concerted twist motion of one-half of the dinuclear complex through a trigonal prismatic transition state, according to the Bailar twist mechanism. Electrochemical properties of the ligand (H2L3,5) and the M2(L3,5)3 helicates were investigated through cyclic voltammetry, and the results indicate the lack of communication between the ferrocene units, because the separation between any two ferrocene units is greater than the 5–6 Å range in both the free ligand and the helicates.