Hemihelices on molecular scale: extended metal atom chains with helical perversions in ligands

Abstract

Hemihelices, which contain helical perversions, are found infrequently alongside helices in nature. Recently a synthesized penta-nuclear metal atom chain, Mo₂NiMo₂(tpda)₄(NCS)₂, showed the existence of helical perversions in its surrounding ligands, and became a unique hemihelix on the molecular scale, or a metal string complex with meso-conformation from the organic chemistry point of view. This experimental discovery raises the question of the origin and conditions of its formation. By using DFT calculations, we demonstrate that the formation of helical ligands is not only the result of steric repulsion of β-hydrogens on pyridyls but is also affected by the mismatch of bond lengths of central metal–metal bonds and peripheral carbon–nitrogen bonds. Moreover, as two Mo–Ni bond lengths in the center of the chain are increased, the hemihelical conformation in surrounding ligands is stabilized gradually relative to the helical conformations.