A Novel Salicylate-Based Macrobicycle with a “Split Personality”

Abstract

Two new ligands (the tripodal TRENIAM and macrobicyclic TRENSAM) which use 2-hydroxyisophthalic acid as a salicylato-type of binding group have been prepared, and their coordination chemistry has been investigated. The structures of several metal complexes have been determined in both solution and the solid state. The ferric and gallium complexes of TRENIAM are isostructural and crystallize in the triclinic space group P1̄, with Z = 4. For Fe[TRENIAM]: a = 12.0203(4) Å, b = 12.6996(4) Å, c = 24.6435(8) Å, α = 83.146(1)°, β = 88.531(1)°, γ = 85.282(1)°. For Ga[TRENIAM]: a = 11.9780(2) Å, b = 12.6417(3) Å, c = 24.5404(6) Å, α = 83.324(1)°, β = 88.488(1)°, γ = 85.038(1)°. The metal cations are bound by three phenolic oxygens and three carbonyl oxygens in a ligand structure analogous to that of salicylamide. The ferric and gallium complexes of macrobicyclic TRENSAM are essentially isosteric and crystallize in the monoclinic space group P2₁/n, with Z = 4. For Fe[macrobicyclic TRENSAM]⁺: a = 10.7156(2) Å, b = 23.10490(10) Å, c = 19.8373(3) Å, β = 98.829(1)°. For Ga[macrobicyclic TRENSAM]⁺: a = 11.1144(2) Å, b = 22.8382(4) Å, c = 19.4525(4) Å, β = 99.247(1)°. The metal complexes of macrobicyclic TRENSAM impose an unusual conformational distortion on the macrobicycle, breaking the C₂ symmetry axis of the parent ligand. This results in a macrocyclic complex with two acyclic analogues, TRENSAM and TRENIAM, giving the cryptate a “split personality” with structural features of both acyclic ligands. ¹H NMR shows that the metal complexes of macrobicyclic TRENSAM are kinetically inert and retain an asymmetric structure in solution. Cyclic voltammetry experiments show that the ferrous complexes are strongly stabilized by the macrocyclic structure.