In situ selective N-alkylation of pendant pyridyl functionality in mixed-valence copper complexes with methanol and copper(II) bromide.

The reactions of CuBr(2) with pyridyl 2,2':6',2''-terpyridine ligands in methanol yielded four copper complexes under solvothermal conditions. The self-assembly processes were accompanied by designing bitopic precursor ligands and increasing the stoichiometric metal-ligand ratio. In the four resulting complexes, the pendant pyridyl groups of pyridylterpyridine were selectively in situ N-methylated and yielded the 4'-(N-methylpyridinium)-2,2':6',2''-terpyridine cations, including the 2-position pyridyl group which is difficult to be N-alkylated due to the steric problem. Partial divalent copper atoms were reduced to cuprous ones in the solvothermal reactions, which made the mixed-valence copper atoms coexist in each compound. The mixed-valence complexes have a varied dimensionality (from 2D to 0D) and the Cu(I)Br cluster, which can be controlled by changing the metal-ligand ratio. Theoretical studies show that the nucleophilic attack of the nitrogen atom in the pendant pyridyl is more facile than others of terpyridine. A possible mechanism was also proposed.