Single-strand helical complexes constructed from 2-pyridinyl-3-pyridinylmethanone: tuning the helical pitch length by variation of metal cation and/or counter anion

Abstract

The new ligand 2-pyridinyl-3-pyridinylmethanone (L) proves to be an excellent building block for the construction of single-strand helical architectures. A series of helical complexes have been synthesized by the reaction of L with various metal salts, in which L exhibits three kinds of coordination modes involving two kinds of bridging conformations, resulting in four types of single-strand helical chains. The counter anions in the series of 2₁ helical silver(I) complexes {[Ag(L)]X}_∞ (X = NO₃, 1; PF₆, 2; BF₄, 3; ClO₄, 4; CF₃CO₂, 5; CF₃SO₃, 6) are fully or partially embedded inside the cylindrical helix, and the pitch length corresponds not only to the size of the anion but also to its manner of docking into the groove of the helix. Formation of the helical structure in {[Cu(L)(CH₃CN)(H₂O)(ClO₄)]ClO₄}_∞ (7) is driven by Ow–H⋯O (perchlorate) hydrogen bonding that leads to a stable triangular motif which rigidly fixes the configuration of the helix. In {[Co(L)(H₂O)₃](ClO₄)₂·2H₂O}_∞ (8) and {[Zn(L)(H₂O)₃](CF₃SO₃)₂·H₂O}_∞ (9), similar helical chains without anion embedment suggest that the pitch length can be tuned by the size of metal cations. Notably, complex {[Ag(L)]CF₃SO₃}_∞ (10), a conformational polymorph of 6, has a 4₁ helix induced by argentophilic interaction.