Gallium(III) Catecholate Complexes as Probes for the Kinetics and Mechanism of Inversion and Isomerization of Siderophore Complexes1

The catechol siderophore analog K3[Ga(3)3], 4 (H23 = 2,3-dihydroxy-N,N‘-diisopropylterephthalamide), is D3-symmetric in aqueous solution, and exists in two enantiomeric forms, Δ-4 and Λ-4. Variable temperature NMR experiments demonstrate that the inversion of the enantiomers of 4 in D2O is facile. The rate of inversion is independent of pH above pH 8. The mechanism is intramolecular. From line-shape analysis the free energy of activation ΔG⧧298 = 67.4(9) kJ mol-1 in D2O at pD 12.1, with ΔH⧧ = 58.5(6) kJ mol-1 and ΔS⧧ = −0.030(9) kJ mol-1 K-1. Below pD 8 the rate of inversion for 4 is pD dependent and initially first order in [D+]. Potentiometric titrations reveal that 4 protonates in two one-proton steps with log KHML3 = 4.66(4) and log KH2ML3 = 3.99(7). In DMSO-d6, formation of a tight contact ion pair between K+ and [Ga(3)3]3- ions increases the free energy barrier to inversion by ∼7 kJ mol-1. The complex K3[Ga(9)3], 10 (H29 = 2,3-dihydroxy-N-tert-butyl-N‘-benzylterephthalamide), was prepared to elucida...