One- and Two-Step Self-Assembly Processes in Zn(II) Supramolecular Frameworks with Ditopic Bis(pyrazolyl)methane Ligands. Chiral Recognition and Formation of Cyclic Helicates

The
new ligands bis­(pyrazol-1-yl)­(pyridine-3-yl)­methane (bpzm3py, 1) and bis­(3,5-dimethylpyrazol-1-yl)­(pyridine-3-yl)­methane
(bpz*m3py, 2) have been synthesized. Box-like cyclic
dimers have been obtained by reaction of the ligands with Zn­(NO3)2, namely, [Zn­(NO3)2L]2 (L = 1, complex 3; L = 2, complex 4), after the self-assembly of two metal centers
and two ligands in a head-to-tail fashion. In 3 and 4 the pyridine rings exhibit planar chirality and the two
pR,pR and pS,pS enantiomers coexist in the crystal. A two-step self-assembly
process using 3 as an SBU and sodium 1,4-benzenedicarboxylate
(Na2BDC) as a linker leads to the formation of hexanuclear
cyclic helicates with a distorted trigonal prism form [Zn2(BDC)­(1)2(H2O)4]3(BDC)3 (5) constituted by cyclic dimers
and μ2,κ2-BDC connectors that are
oriented diagonally. The chiral recognition that is observed means
that the three dimers of the helicate have the same chirality. In
the 3-D structure, chains of helicates of the same chirality are formed
and these interdigitate with chains of opposite chirality connected
by hydrogen bonds and through a column of π–π stacking
interactions involving alternate pyridine and benzene rings.