Mn(II) complexes containing the polypyridylic chiral ligand (-)-pinene[5,6]bipyridine. Catalysts for oxidation reactions

A series of mononuclear and dinuclear chiral manganese(II) complexes containing the neutral bidentate chiral nitrogen ligand (-)-pinene[5,6]bipyridine, (-)-L, were prepared from different manganese salts. The chirality in these complexes arises from the pinene ring that has been fused to the 5,6 positions of one pyridine group of the bipyridine ligand. These complexes have been characterized through analytical, spectroscopic (IR, UV/Vis, ESI-MS) and electrochemical techniques (cyclic voltammetry). Single X-ray structure analysis revealed a five-coordinated Mn(II) ion in [{MnCl((-)-L)}₂(μ-Cl)₂] (2), [{Mn((-)-L)}₂(μ-OAc)₃](PF₆) (3) and [MnCl₂(H₂O)((-)-L)] (4) and a six-coordinated one in [MnCl₂((-)-L)₂] (5), [Mn(CF₃SO₃)₂((-)-L)₂] (6) and [Mn(NO₃)(H₂O)((-)-L)₂)](NO₃) (7). The magnetic properties of the binuclear compounds 2 and 3 have been studied. Both compounds show a weak antiferromagnetic coupling (2, J = -0.22 cm^-1; 3, J = -0.85 cm^-1). The catalytic activity of the whole set of complexes has been tested with regard to the epoxidation of aromatic alkenes with peracetic acid. In the particular case of styrene, good selectivities and moderate enantioselectivities were obtained. Furthermore, total retention of the initial cis configuration was achieved when epoxidizing cis-β-methylstyrene with the chloride complexes. In general, the epoxidation activity of these manganese complexes is strongly dependent on the steric encumbrance of the substrates employed.