Self-Assembly of Silver(I) and Ditopic Heteroscorpionate Ligands. Spontaneous Chiral Resolution in Helices and Sequence Isomerism in Coordination Polymers

The self-assembly
of the ligands bis­(pyrazol-1-yl)­(pyridine-4-yl)­methane
(L1) and bis­(3,5-dimethylpyrazol-1-yl)­(pyridine-4-yl)­methane
(L2) with AgBF4 leads
to three types of species: L1 gives rise to the formation
of a boxlike cyclic dimer (1) while L2 leads to two different coordination polymers
that are sequence isomers. With dependence on the crystallization
conditions, either a homochiral helix (2) with spontaneous
resolution or a zigzag polymer is formed (3). By means
of solid-state circular dichroism studies, the presence of both enantiomers
in 2 is observed, enriched locally in colonies of crystals.
The homochiral motif is stable, even upon removal of guest solvent
molecules. The noncovalent interactions in the crystalline structure
are clearly affected by the presence of the methyl groups on the pyrazolyl
rings. The behavior of the compounds in solution is discussed.