Hybrid organic-inorganic framework structures: Influence of cation size on metal-oxygen-metal connectivity in the alkaline earth thiazolothiazoledicarboxylates

We report the synthesis of four organic-inorganic frameworks of alkaline earth cations with the organic ligand 2,5-thiazolo[5,4-d]thiazoledicarboxylate (C6N2S2O42-, Thz(2-)). Structures with remarkably different connectivities result when Mg2+, Ca2+, Sr2+, and Ba2+ react with Thz(2-)center dot Mg(Thz)(H2O)(4) (1) forms a 1-D coordination polymer in which one carboxylate oxygen on each terminus of the ligand connects individual MgO6 octahedra from their axial positions, while the remaining equatorial sites are coordinated by water molecules. Ca-2(Thz)(2)(H2O)(8) (II) forms a 1-D coordination polymer in which dimeric clusters with 7-fold Ca coordination are connected via the ligand in a linear fashion, with a second, uncoordinated Thz(2-) providing charge balance. Sr(Thz)(H2O)(3) (III) has 1-D infinite inorganic connectivity built from edge-sharing SrO7N polyhedra having one carboxylate oxygen and one water molecule acting as M-O-M bridges. Ba-2(Thz)(2)(H2O)(7) (IV) has 2-D inorganic connectivity based upon face- and edge-sharing BaO9N polyhedra. One carboxylate oxygen and all water molecules act as bridges between each Ba2+ and its three neighbors. We shall discuss the manner in which the increasing coordination requirements of the cations MgO6 < CaO7 < SrO7N < BaO9N) lead to an increase in inorganic connectivity through the series.