Anion Modulated Structural Diversification in the Assembly of Cd(II) Complexes Based on a Balance-like Dipodal Ligand

Reaction of a balance-like dipodal ligand 2,6-bis­(pyridiyl)
hexahydro-4,8-ethenopyrrolo [3,4-f]­isoindole-1,3,5,7-tetrone (3-pybtd)
with various Cd­(II) salts afforded eight complexes, namely, [Cd2(3-pybtd)2(NO3)4(C2H5OH)2(H2O)2] (1), [Cd2(3-pybtd)2(SiF6)2(DMF)4(H2O)2]­(H2O)4·(DMF)2 (2), {[Cd­(3-pybtd)2(H2O)4]­(ClO4)2}n (3), {[Cd­(3-pybtd)2(OTf)2]·THF}n (4), {[Cd­(3-pybtd)2(SCN)2]·(H2O)2}n (5), [Cd­(3-pybtd)­(OTs)2(DMF)2]n (6), [Cd­(3-pybtd)2(OTs)2]n (7), and {[Cd2(3-pybtd)2Cl10/3]­[CdCl8/3]·(H2O)3}n (8). Complexes 1 and 2 are zero-dimensional
(0D) square-like or olive-like dimeric M2L2 metallacycles,
showing a pair of shape-modified molecular rectangles due to different
conformations of the ligands and coordination orientation of the metal
centers. Complexes 3–5 are one-dimensional
(1D) looplike chains composed of olive-like M2L2 metallacycle building units as in 2, showing 0D →
1D dimension increase via ligand addition, while complex 8 is a three-dimensional (3D) framework retaining the same olive-like
M2L2 metallacycle, showing 0D → 3D dimension
increase via linkage of μ3-Cl bridged Cd­(II) clusters. Complex 6 is a
wave-like MnLn chain, possessing the same ML building units as in 1 but showing 0D → 1D dimension increase via ring-opening polymerization.
Replacement of DMF molecules from the coordination sphere in 6 by the ligands resulted in a two-dimensional (2D) (4, 4)
network of 7, showing 1D → 2D dimension increase
from 6 via ligand addition or 1D → 2D dimension
increase from 3–5 via ring-opening
polymerization. Complexes 3–5 also
represent a series of supramolecular isomorphs displaying anion exchange
properties. Electrospray ionization mass spectrometry (ESI-MS) studies
in solution suggest that the discrete and infinite structures in 1, 6, and 7 are assembled from the
same monomeric ML building blocks, which crystallize in a different
way to lead to structural diversification via dimerization or polymerization
during the crystallization.