Crystal structure and catalytic activity of a copper(II) complex based on a tetradentate bis-benzimidazole diamide ligand

A dimeric copper(II) complex bridged via a new tetra dentate bis benzimidazole diamide ligand [N,N′-bis(benzimidazolyl-2-yl)(methyl)pentane diamide](GBGA) with the composition [Cu₂(GBGA)₂(NO₃)₂](NO₃)₂ has been isolated and characterized. The X-ray structure of the above complex reveals that the unit cell consists of two centrosymmetric, crystallographically independent molecules, but differing in the coordination mode of ion. In one case ion is symmetric bidentate while in the other case it is monodentate. The coordination around Cu(II) is either a trigonally distorted octahedron (where the N2–O2 equatorial plane is formed by two benzimidazole N atoms and two carbonyl O atoms) or a distorted square pyramidal. The copper(II) complex carries out the selective oxidation of cinnamyl alcohol (allylic), geraniol (aliphatic-allylic) and 3-pyridyl carbinol (hetero aryl alcohol) to their respective aldehydes in the presence of tertiary butyl hydroperoxide as an alternative source of oxygen. The catalytic efficiency has been found to be much higher for the analogous copper(II) complex formed with the corresponding N-octylated ligand (O-GBGA). The percentage yield of the products viz geranial, cinnamyl aldehyde and 3-pyridyl carbinal varies between 34% and 57%. While the respective turnovers are 13-, 19- and 32-fold with respect to the copper(II) catalyst. A higher turnover in the case of 3-pyridyl carbinol is due to the transformation of the parent Cu(II) catalyst (having a N2–O2 type equatorial plane) to a more active Cu(II) species which have been shown to have a 4N donor equatorial plane as identified by low temperature EPR spectroscopy. Such a switch from a carbonyl O donor to an amine N donor of the peptidic link in the ligand may be important for the redox functioning of copper(II) bound to small peptides.