Three polymorphic Cd(II) coordination polymers obtained from the solution and mechanochemical reactions of 3-cyanopentane-2,4-dione with Cd(II) acetate.

We previously reported that monomeric and polymeric metal complexes are obtained from solution and mechanochemical reactions of 3-cyano-pentane-2,4-dione (CNacacH) with 3d metal acetates (M = Mn(II), Fe(II), Co(II), Ni(II), Cu(II), and Zn(II)). A common feature found in all complexes was that their structural base is trans-[M(CNacac)(2)]. Here, we report that the reactions of CNacacH with Cd(II) acetate in the solution and solid states afford different coordination polymers composed of trans-[Cd(CNacac)(2)] and cis-[Cd(CNacac)(2)] units, respectively. From a methanol solution containing CNacacH (L) and Cd(OAc)(2)⋅2 H(2)O (M), a coordination polymer (Cd-1) in which trans-[Cd(CNacac)(2)] units are three-dimensionally linked was obtained. In contrast, two different coordination polymers, Cd-2 and Cd-3, were obtained from mechanochemical reactions of CNacacH with Cd(OAc)(2)⋅2 H(2)O at M/L ratios of 1:1 and 1:2, respectively. In Cd-2, cis-[Cd(CNacac)(2)] units are two-dimensionally linked, whereas the units are linked three-dimensionally in Cd-3. Furthermore, Cd-1 and Cd-2 converted to Cd-3 by applying an annealing treatment and grinding with a small amount of liquid, respectively, in spite of the polymeric structures. These phenomena, 1) different structures are formed from solution and mechanochemical reactions, 2) two polymorphs are formed depending on the M/L ratio, and 3) structural transformation of resulting polymeric structures, indicate the usability of mechanochemical method in the syntheses of coordination polymers as well as the peculiar structural flexibility of cadmium-CNacac polymers.