Variety of polymorphic forms contrasted with uniform crystal packing in sparteine ML2 complexes: Crystal structure, spectroscopic and magnetic properties of (−)-α-isosparteine and (−)-sparteine complexes with CuBr2

This paper reports the synthesis, thermogravimetric and magnetic measurements for (−)-α-isosparteine CuBr2 complex (1) and the existence of the fourth polymorph (2) of (−)-sparteine CuBr2 complex. The two CuBr2 complexes have been structurally characterized by X-ray crystallography. A diverse supramolecular aggregation of (−)-sparteine CuBr2 complex has been contrasted with strong tendency for isostructuralism observed in a series of ML2 complexes (M = metal, L = halogen or pseudohalogen) with both sparteine (3) and α-isosparteine (4). The incidence of polymorphism in a group of compounds which readily form the isomorphic series is quite unusual. Moreover, the reported crystal structure of 2 constitutes the first example showing that the organization of metal complexes of sparteine isomers into similar packing architecture is possible, although in general configurational changes within the sparteine ligand differentiate packing of otherwise similar (−)-sparteine-ML2 or (−)-α-isosparteine-ML2 molecules.