Novel bispidine ligands and their first-row transition metal complexes: trigonal bipyramidal and trigonal prismatic geometries.

Four very rigid second generation bispidine-based ligands (bispidine = 3,7-diazabicyclo[3.3.1]nonane; tetra-, penta- and hexadentate; exclusively tertiary amine donors except for one of the pentadentate ligands, where one of the donors is a pyridyl group) and their Co(II), Ni(II), Cu(II), and Zn(II) complexes are reported. The experimentally determined X-ray crystal structures and computational data, based on empirical force field (MM) and approximate density functional theory (DFT) calculations, indicate that these new ligands, which are based on a modular system and therefore allow for a wide range of donor sets and coordination geometries, have rather large cavities (i.e., lead to a preference for +II over +III oxidation states and induce relatively low ligand fields), enforce trigonal geometries (pentacoordinate systems: preference for trigonal bipyramidal, hexacoordinate complexes: preference for trigonal prismatic), and lead, especially for Cu(II), to very high complex stabilities.