Water Content-Controlled Formation and Transformation of Concomitant Pseudopolymorph Coordination Polymers

Two concomitant pseudopolymorph coordination polymers
{[Cd2L2(OAc)4]·2DMSO}n (1) and {[CdL­(OAc)2]·2.75H2O}n (2) were synthesized
by self-assembly of 1,4-bis­(4-pyridyl)-2,3-diaza-1,3-butadiene (L) and cadmium acetate in DMSO. Single-crystal X-ray diffraction
confirmed that 1D ladder structural motifs exist for pseudopolymorphs 1 and 2 which contain DMSO and water guest molecules,
respectively. Our study illustrated the active role of solvent water
content in obtaining compound 2. We find that the presence
of water as an impurity in the DMSO solvent creates the possibility
of formation of concomitant pseudopolymorph coordination polymers
which is a unique event. Furthermore, our analyses showed the effect
of environmental humidity on the transformation of unstable compound 1. 1D ladder pseudopolymorphic compound 1 could
be transformed to guest-free 1D linear compound [CdL­(OAc)2(H2O)]n (3′) (the powder form of single crystals of 3) through
a scarce case of water absorption from air. Also, the crystalline
material of coordination polymer 3 was transformed to
coordination polymer 2 through the dissolution–recrystallization
structural transformation process in DMF or DMSO. Our study clarified
that the amount of water in the reaction container can control the
formation of one of the compounds 2 or 3. In the presence of a significant amount of water, compound 3 (coordinated water) will be produced, whereas if a small
amount of water is present, compound 2 (uncoordinated
water) is prepared as an exclusive product.