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.