Nitric oxide reactivity of Cu(II) complexes of tetra- and pentadentate ligands: structural influence in deciding the reduction pathway

Four Cu(II) complexes, 1, 2, 3 and 4, are synthesized with ligands, L1, L2, L3 and L4 [L1 = N1,N2-bis((pyridin-2-yl)methyl)ethane-1,2-diamine; L2 = N1,N3-bis((pyridin-2-yl)methyl)propane-1,3-diamine; L3 = N1,N1,N2-tris((pyridin-2-yl)methyl)ethane-1,2-diamine; L4 = N1-((1-methyl-1H-imidazol-2-yl)methyl)-N1,N2-bis((pyridin-2-yl)methyl)ethane-1,2-diamine], respectively, as their perchlorate salts. The complexes were characterized by various spectroscopic techniques as well as single crystal X-ray structure determination. Nitric oxide reactivities of the complexes were studied in acetonitrile as well as methanol solvent. It has been found that the ligand frameworks have a considerable effect in controlling the mechanism of the reduction of a Cu(II) center by nitric oxide. The flexibility of the ligand/s for a Cu(II) complex to attain a trigonal bipyramidal geometry after NO coordination is found to be the most important parameter in dictating the pathway for their interaction. In the present study, all the four compounds, because of structural constraints, were found to follow a deprotonation pathway for the reduction of a Cu(II) center by nitric oxide rather than [Cu(II)-NO] intermediate formation. All the ligands were found to yield an N-nitrosoamine product along with the reduction of Cu(II) centers by nitric oxide.