Synthesis, characterization and stereochemistry of condensation products between (1R)-3-hydroxymethylenebornane-2-thione and diamines and their metal complexes

Abstract

Tetradentate ligands derived from the condensation of (1R)-3-hydroxymethylenebornane-2-thione and symmetric diamines were synthesised together with their zinc(II) complexes. As deduced from cumulative spectral evidence, freshly prepared samples of the free ligands consist exclusively of the thioxoenamine tautomer, while binding to zinc(II) occurs through the enethiolate imine form. The circular dichroism (CD) spectrum of the ligand bearing an ethylene bridging chain shows very intense Cotton effects characteristic of exciton-coupled transitions. The pattern of CD bands associated with the thioxoenamine transitions suggests a gauche structure for the dominant conformer. The NMR spectra of zinc(II) complexes are indicative of the presence of several species resulting from geometric isomerism at the metal atom and conformational mobility of the flexible part of the ligands in the chelates. Lengthening of the carbon chain in the diamine moiety from two to four carbon atoms favours the adoption of tetrahedral structures at zinc(II) as is evidenced by the CD spectral behaviour of the complexes. The absolute configuration of the dominant structure is assigned on the basis of the CD spectrum.