Assembling mono-, di- and tri-nuclear coordination complexes with a ditopic analogue of 2,2′:6′,2′′-terpyridine: syntheses, structures and catalytic studies

Abstract

4-Phenyl-2,6-bis(2′-pyrazinyl)pyridine (L) as a closely related ditopic analogue of 2,2′:6′,2′′-terpyridine, was synthesized through a facile one-pot Kröhnke condensation, along with the structural determination of transition metal coordination assemblies of this ditopic ligand. A variety of metal complexes including mono-, di- and tri-nuclear structures 1–6 were revealed, dependent upon the metal salts and reaction conditions employed. Although reactions of L with copper(II) or zinc(II) nitrates and zinc(II) iodide gave only simple mononuclear mono-ligand complexes, that using cobalt thiocyanate led to a dinuclear dimeric complex. The potential of outer N-donors of the ligand backbone in extending mononuclear complexes into polynuclear assemblies was further investigated. The solution reaction of [Fe(L)₂]²⁺ species with Co^II gave an ionic complex containing [Co(SCN)₄]²⁻ counterions, leaving the outer pyrazinyl-N non-coordinated. However, the solvothermal assembly of L with copper(II) chloride facilitates the formation of a discrete trinuclear complex via outer-N coordination, indicating the promising applications of L in constructing complicated supramolecular structures. Catalytic tests were carried out on copper(II)- and cobalt(II)-containing complexes 1 and 4–6 for the aerobic alcohol oxidation. Up to 99% conversion of benzylic alcohol to benzaldehyde was observed when 1 (1 mol%) was used as a catalyst, in the presence of TEMPO (2,2,6,6-tetramethylpiperidinyl-1-oxyl) and DMAP (4-dimethylaminopyridine) in acetonitrile, while the trinuclear 6 was a better catalyst when the reaction was conducted in water.