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)] (H2 L 3,5 ), has been prepared and the reactions of its dianion (Na2 L 3,5 ) with M3+ ions (M = Ga or In) yield a new class of “3d-np block” heterometallic triple-stranded helicates, M2(L 3,5 ) 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(L 3,5 ) 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(L 3,5 ) 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 (H2 L 3,5 ) and the M2(L 3,5 ) 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.

Recent Developments in the Preparation and Chemistry of Metallacycles and Metallacages via Coordination

“…Raja and co-workers designed a ditopic chelating ligand terminating with ferrocenyl groups that could be combined with Ga(III) or In(III) precursors to form M 2 L 3 triple-stranded helicates. 293 Coordination occurred at the enolate sites of the ligand, completing the distorted octahedral environments at the two metal centers. Cyclic voltammetry revealed a single oxidation wave corresponding to the Fc/Fc + couple, with no observed electronic communication between the ligands, or within a given ligand.…”

New coordination features; a bridging pyridine and the forced shortest non-covalent distance between two CO32−species

“…[1][2][3][4] The good chelating ability of b-diketonates together with a particular distribution throughout a given organic scaffold, in combination or not with additional donor groups has led to novel features in coordination chemistry. Some examples are; a whole category of oxygen based metallohelicates, [4][5][6][7] an entire family of molecular platforms for the construction of supramolecular edices, 3,8 or a novel type of paddle wheel complexes. 9 One subclass of this kind of ligands exhibits two b-diketone groups separated by an m-pyridinediyl spacer (Scheme 1A).…”

Competitive Transmetalation of First-Row Transition-Metal Ions between Trinuclear Triple-Stranded Side-by-Side Helicates

“…Among them metallo-helicates are particularly important as sensors, , DNA binders, ,, and as anticancer agents. , Several bi- and trinuclear homo- and heterometallic helicates for both transition metals − and f-block elements ,,− have been reported following the isolation of the first homometallic helicates by Lehn et al in 1987 . In addition, there are mesocates composed of an even number of metal centers with alternating chirality having a mirror plane at the middle. − In those cases, in which an odd number of metal centers with alternating chirality are present, the ligands bind in a parallel zigzag fashion instead of wrapping around the helix. , Such assemblies are known as side-by-side helicates. Recently, we have introduced a neutral hybrid tris-bidentate ligand ( L ), in which a central 2,2′-bipyridine and two terminal triazolyl-pyridine chelating coordination units are connected by methylene spacers.…”