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Single crystals of PbADC (1) and PbADC·H2O (2) formed at the phase boundary of an aqueous silica gel containing acetylenedicarboxylic acid (HOOC–C≡C–COOH, H2ADC) and an aqueous solution containing Pb(NO3)2. By choosing different concentrations of Pb(NO3)2, compounds 1 and 2 were obtained as phase pure products. Additionally, 1 was obtained by grinding Pb(CH3COO)2·3H2O with H2ADC resulting in a polycrystalline sample. The crystal structures of 1 (I41/amd, Z = 4; SrADC type structure) and 2 (P21/c, Z = 4, new structure type) were solved and refined from X‐ray single crystal data. Compound 1 exhibits a three‐dimensional framework structure: lead cations with a diamond‐like arrangement are interconnected by bridging ADC2– ligands. In 2 double‐layers are formed by lead cations, bridging ADC2– anions, and water molecules. These layers are held together by hydrogen bonds through water molecules and oxygen atoms of the ADC2– ligands. Suspending 1 for 24 h in water at ambient conditions leads to the formation of 2, which can be converted to 1 again by careful dehydration at approx. 400 K in vacuo. This reversible reaction can be structurally interpreted as a topochemical reaction, which transforms a 3D coordination network into a 2D network structure and vice versa, as both crystal structures show noticeable structural similarities.

Single crystals of Ba

The crystal structures of 3 ϱ [RE 2 (ADC) 3 (H 2 O) 6 ]·2H 2 O (RE = Pr, Nd, Sm, Eu, Tb, Dy) were solved and refined from X-ray single crystal data. They crystallize in a structure type already known for RE = La, Ce and Gd (P1, no. 2, Z = 2), which is characterized by REO 9 polyhedra forming dimeric units being the nodes of a 3D framework structure linked by ADC 2anions ( -O 2 C-CϵC-CO 2 -= acetylenedicarboxylate). From synchrotron powder diffraction data it was shown that isostructural coordination networks are formed for RE = Ho, Er, Y, whereas for RE = Tm, Yb, Lu a new structure type crystallizing in

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