Structure-Directing Effect of Organic Cations in the Assembly of Anionic In(III)/Diazinedicarboxylate Architectures

We report herein the synthesis and physicochemical characterization of seven new indium–pyrazine-2,5-dicarboxylato (pzdc) and pyridazine-3,6-dicarboxylato (pddc) compounds: (dma)6[In6(μ-pzdc)12]·xH2O (1), (dea)6[In6(μ-pzdc)12]·xH2O (2), {(tma)­[In­(μ-pzdc)2]·2H2O}n (3), {(dea)­[In­(μ-pzdc)2]·2H2O}n (4), {(dma)­[In­(μ-pddc)2]·xH2O}n (5), {(dma)­[In­(μ-pddc)2]}n (6), and (dma)4[In8(μ-pddc)12(H2O)8(OH)4]·xH2O (7) (where dma = dimethylammonium, dea = diethylammonium, tma = tetramethylammonium). Two types of In­(III)/pzdc structures have been obtained. The first one (1 and 2) is comprised of discrete hexanuclear anionic assemblies held together by hydrogen bonding interactions through the organic cations generated by the thermal hydrolysis of the amide solvents. The second one (3 and 4) consists of an anionic three-dimensional (3D) framework with channels that are occupied by the counterions and solvent molecules. The first type of structure seems to be the kinetically preferred one since it is obtained when using relatively soft solvothermal conditions (120 °C) and counterions that are able to establish relatively strong hydrogen bonding interactions. The 3D frameworks crystallize when the organic counterion is unable to establish hydrogen bonding interactions or when employing a higher temperature (150 °C). The metal–organic assemblies obtained in the In­(III)/pddc system range from two-dimensional (2D) sheets (5 and 6) to discrete octameric entities (7) depending on the amount of water in the reaction mixture. It is worth noting that the open lamellar crystal structure of compound 5 undergoes a solid state transformation accompanied by the release of water molecules, rendering the solvent free 2D architecture of 6 that exhibits a different connectivity. Surprisingly, a prolonged exposure of 6 to a water saturated atmosphere does not revert to 5 but promotes a partial and reversible transformation to give a new unidentified In-pddc compound.