Bis(α,ω-dimethyltripyrrinato)cadmium(II) (Trpy2Cd): Serendipitous finding and structural characterization of an unexpected coordination compound

The treatment of the trifluoroacetatocadmium(II) complex of a sterically hindered α,ω-dimethyltripyrrin TrpyCdOAcF (1) with fluoride and other basic anions like hydroxide, phenoxide or t-butoxide does not result in the expected formation of the ligand exchange product nor in the breakdown of the sensitive tripyrrolic ligand framework, but yields the sterically congested 2:1 product, bis(tripyrrinato)cadmium(II) (Trpy2Cd) (2), as the only isolated compound in almost quantitative yield. Trpy2Cd (2) is the first tripyrrinate with a hexacoordinate metal centre observed so far. The compound was characterized by means of solution spectroscopy (1H/13C NMR, UV/Vis, MS), combustion analysis and single crystal X-ray diffraction. The result of the X-ray crystallographic analysis of the new compound demonstrates the chiral nature of single molecules of Trpy2Cd in the solid state. This chirality stems from the helical distortion of both tripyrrin ligands, which in turn is the structural answer to the presence of a steric constraint at the open face of this ligand. In solution, however, the compound racemizes rapidly.

Graphical abstractA ML2 complex is formed from the sterically hindered α,ω-tripyrrin ligand and the large CdII ion upon treatment of a simple trifluoroacetato cadmiumtripyrrin with basic ligands such as fluoride, hydroxide and the like. Due to the steric congestion the tripyrrin backbones are distorted, resulting in the observation of a chiral species in the solid.Figure optionsDownload full-size imageDownload as PowerPoint slide