In coordination chemistry, the simplest supramolecular architecture is the two-metal-center helicate. 1 As a result of a spontaneous, self-assembling process in which ligands wrap around two or more metal centers, helicates can be viewed as simple models of more complex natural structures. 2 Recently, we have described how the formation of the helical cluster is driven by the geometry of the ligand and the incommensurate coordination numbers of the ligand and metal. 3 Perhaps the earliest triple helicate4-7 to be characterized, however, is that formed with rhodotorulic acid, the dihydroxamate siderophore produced by the yeast Rhodotorula pilimanae. At neutral pH, a complex of Fe2L3 stoichiometry is formed and was characterized as an enantiomerically pure Δ-cis helicate. 8, 9 Subsequently, we published the X-ray structure of a related synthetic iron (III) trihelicate based on 1, 2-hydroxypyridinone binding groups. 10 This paper reports the synthesis, structure, and inversion mechanism of a new class of dinuclear biscatechol triple helicates11, 12 in which the two coordination centers are coupled such that the chirality at the first metal center (Λ or Δ) is replicated at the second. For the first time, the coupling of the two metal centers of such a system is investigated with respect to the kinetics of inversion, using nuclear magnetic resonance as a probe. The crystal structure and solution reaction dynamics provide a clear picture of how the two coordination sites interact, both in determining the static ground-state geometry and the dynamic transition state for inversion. Molecular mechanics calculations indicated that the biscatecholamide ligand 2 would form a racemic mixture of the homochiral, Λ, Λ-, Δ, Δ-Ga2 (2) 3 6-anions rather than the heterochiral meso or Λ, Δ-Ga2 (2) 3 6-species. 13 Triple-stranded helicates containing either the biscatecholamide 2 or the bisterephthalamides 3-6 were obtained by the stoichiometric reaction of the doubly deprotonated ligands, K2H2L, 14 with Ga-(acac) 3 or Fe (acac) 3 in CH3OH at room temperature (Scheme 1). 15 Recently, it has been suggested that a trans influence in the catecholamide moiety is relevant to the formation of the dinuclear helicates. 12 Our results demonstrate that terephthalamide-based ditopic ligands, which have no potential for a trans influence, form helicates equally well. Attempts to study the structure and mechanism of inversion in dinuclear metallohelicates of the type Ga2 (L) 3 6-focused on ligands 3-6. The CD spectrum of the chiral iron complex K6-Fe2 (6) 3 shows a negative Cotton effect at 443 nm, which supports the assignment of homochiral Λ, Λ-and Δ, Δ-configurations of the Ga2 (L) 3 6-anions in solution. Final corroboration came from single-crystal X-ray diffraction of complexes, K6Ga2 (3) 3 and K6Ga2 (4) 3, which contain nonchiral ligands. 16 These are racemic mixtures of dinuclear triple helical complexes of D3 molecular symmetry and Λ, Λ-or Δ, Δ-configuration. 17 At ambient temperature, solutions of K6Ga2 (L) 3, L) 2, 4, and 6, or (N (CH3) 4) 6Ga2 (3) 3 in CD3OD or DMSO-d6 each display one discrete, static set of 1H NMR resonances which are slightly shifted when compared to the spectra of the free ligands. The chirality at the metal centers of Ga2 (4) 3 6-and Ga2 (5) 3 6-was confirmed by the proton resonance splitting of the diastereotopic methyl groups of the isopropyl substituents, which appear as a A3B3X system. Since ligand 5 is unsymmetrical, the complexation reaction (Scheme 1) yields a mixture of cis-K6Ga2 (5) 3 and trans-K6Ga2-(5) 3 isomers. From integration of the respective resonances, the cis: trans ratio was determined to be 0.35 (1), slightly higher than the value of 0 …