Structure correlation in bispidine coordination compounds—isomerism and tuneable reactivity

The published structural data of pseudo-square-pyramidal and pseudo-octahedral transition metal complexes with tetra- and pentadentate bispidine ligands (2,4-, 2,3,4- and 2,4,7-substituted 3,7-diazabicyclo[3.3.1]nonane, pyridine or 6-methylpyridine substituents), primarily with copper(I), copper(II) and iron(II) is reviewed, together with few molecular mechanics and DFT calculations, and in the light of the specific and uncommon molecular properties of bispidine coordination compounds. One of the main features is that, while the ligands are very rigid, there is a high degree of elasticity of the coordination geometries. That is, the potential energy surfaces of the complexes are flat; in addition, there are various shallow minima, and this leads to uncommon types of isomerism. These structural features, the emerging reactivities and possibilities to tune them are discussed in detail.