Selective inclusion of cesium ion in a cryptand-type Ti(IV) complex derived from a tripodal tris-2,3-dihydroxynaphthalene ligand

Enhancement of Alkali Metal Ion Recognition by Metalation of a Tris(saloph) Cryptand Having Benzene Rings at the Bridgeheads

A cryptand derivative, H₆L, which has three H₂saloph arms connected by two benzene ring bridgeheads, was synthesized and converted into the trinuclear metallocryptand, LNi₃. The nonmetalated host, H₆L, was found to bind to alkali metal ions (Na⁺, K⁺, Rb⁺, Cs⁺; logK_a = 3.37-6.67) in its well-defined cavity in DMSO/chloroform (1:9). The binding affinity was enhanced by 1-2 orders of magnitude upon the conversion into the metallocryptand, LNi₃, which can be explained by the more polarized phenoxo groups in the [Ni(saloph)] arms. The guest binding affinity of Na⁺ < K⁺ < Rb⁺ ≈ Cs⁺ was clearly demonstrated by the ¹H NMR competition experiments. The DFT calculations suggested that the Rb⁺ ion most suitably fit into the benzene-benzene spacing with a cation-π interaction and that only the largest Cs⁺ ion can almost equally interact with all six phenoxo oxygen donor atoms. The metallocryptand, LNi₃, also showed a strong binding affinity to Ag⁺ by taking advantage of cation-π interactions, which was confirmed by spectroscopic titrations and crystallographic analysis as well as DFT calculations. Thus, the well-defined three-dimensional cavity of LNi₃ was found to be suitable for strong binding with alkali metal ions as well as Ag⁺.

How Many O-Donor Groups in Enterobactin Does It Take to Bind a Metal Cation?

The E. coli siderophore enterobactin, the strongest Fe^III chelator known to date, forms hexacoordinate complexes with Si^IV , Ge^IV , and Ti^IV . Synthetic protocols have been developed to prepare non-symmetric enterobactin analogues with varying denticities. Various benzoic acid residues were coupled to the macrocyclic lactone to afford a diverse library of ligands. These enterobactin analogues were bound to Si^IV , Ge^IV , and Ti^IV , and the complexes were investigated through experimental and computational techniques. The binding behavior of the synthesized chelators enabled assessment of the contribution of each of the phenolic hydroxy groups in enterobactin to metal-ion complexation. It was found that at least four O-donors are needed for enterobactin derivatives to act as metal binders. Density functional theory calculations indicate that the strong binding behavior of enterobactin can be ascribed to a diminished translational entropy penalty, a common feature of the chelate effect, coupled with the structural arrangement of the three catechol moieties, which allows the triseryl base to be installed without distorting the preferred local metal-binding geometry of the catecholate ligands.

Synthesis and structural characterization of hexacoordinate silicon, germanium, and titanium complexes of the E. coli siderophore enterobactin

The E. coli siderophore enterobactin, one of the strongest Fe(III) chelators known to date, is also capable of binding Si(IV) under physiological conditions. We report on the synthesis and structural characterization of the tris(catecholate) Si(IV) -enterobactin complex and its Ge(IV) and Ti(IV) analogues. Comparative structural analysis, supported by quantum-chemical calculations, reveals the correlation between the ionic radius and the structural changes in enterobactin upon complexation.

An enantiomerically pure siderophore type ligand for the diastereoselective 1 : 1 complexation of lanthanide(III) ions

A facile synthesis of a highly preorganized tripodal enterobactine-type ligand 1a-H₃ consisting of a chiral C₃-symmetric macrocyclic peptide and three tridentate 2-amido-8-hydroxyquinoline coordinating units is presented. Complex formation with various metal ions (Al(³+), Ga(³+), Fe(³+), La(³+) and Eu(³+)) was investigated by spectrophotometric methods. Only in the case of La(³+) and Eu(³+) were well defined 1 : 1 complexes formed. On the basis of CD spectroscopy and DFT calculations the configuration at the metal centre of the La(³+) complex was determined to show Lambda helicity. The coordination compounds [(1a)Ln] presented should be prototypes for further lanthanide(III) complexes with an enterobactine analogue binding situation.