Stereoregular coordination polymers formed on binding of peptide-based polydentate ligands to silver(I) and copper(I). X-ray structure of ([Ag{N-[N-((5-methyl-2-thienyl)methylidene)-L-methionyl]histamine}]+[O3SCF3]-.MeOH).infin. and a solution structure study

Abstract

From the 1:1 metal salt to ligand reactions of AgIO3SCF3 or CuIO3SCF3 with the polydentate donor ligand system N-[N-((5-R-thienyl)methylidene)-L-methionyl]histamine (= (5R)Th-Met-Histam; R = H, Me, Me3Si) stable coordination complexes have been obtained. The solid-state structure of [Ag{(5Me)Th-Met-Histam}](OTf) was determined. ([C16H22N4S2Ag]+[O3SCF3]-.CH4O) infinity crystallizes in the orthorhombic space group P2(1)2(1)2(1), with a = 11.339 (1) angstrom, b = 13.122 (1) angstrom, and c = 17.451 (1) angstrom. The complex cation has a polymeric structure that results from each ligand molecule stretching out over three different Ag(I) cations. Each Ag(I) center is sp2 hybridized and trigonally coordinated by an imidazole N [2.218 (4) angstrom], an imine N [2.378 (5) angstrom], and a methionine S atom [2.509 (2) angstrom]. There is an additional weak interaction with an amide O atom [2.568 (4) angstrom], which is situated above the trigonal plane. A more distant thiophene S atom lies on the other side of the plane [3.081 (2) angstrom]. Viewed from S to O along an axis perpendicular to the trigonal plane the configuration of all Ag(I) centers is R; this defines a DELTA-helix structure for the polymeric cation. By use of a combination of spectroscopic techniques (F-19, Ag-109, and H-1 NMR, UV, CD, and optical rotation) and other measurements (vapor pressure osmometry and conductivity), it is established that the complexes [Ag{(5R)Th-Met-Histam}](OTf) (R = H, Me, Me3Si) in solution have an oligomeric structure that is similar to the polymeric structure of [Ag{(5Me)Th-Met-Histam}](OTf) in the solid state. The average chain length of the oligomer is a function of complex concentration and temperature; for the (5Me)Th-Met-Histam complex at 318 K the chain length varies from 1 unit in dilute solution to at least 11 units at a concentration of 0.14 mol.L-1. Comparison of the X-ray structure conformations of (5Me)Th-Met-Histam as a free molecule and as ligand in this Ag(I) complex shows that, apart from one rotation, the changes are minimal. The self-assembly process, which affords the polycationic Ag(I) coordination complexes through a simultaneous, configurationally unique arrangement of different ligating sites from three separate sources around each cationic nucleus, manifests a degree of self-organization that has so far not often been encountered in synthetic coordination chemistry. The Cu(I) complexes have structures that are comparable to those of their Ag(I) counterparts, although the average chain length under identical conditions may be less. An intramolecular electron shift equilibrium mechanism, involving Cu(I)-imidazole and Cu(II)-imidazole-transitions, is proposed to explain the line broadening present in the H-1 NMR spectra of the Cu(I) complexes.