Exploiting synthetic conditions to promote structural diversity within the scandium(III)/pyrimidine-4,6-dicarboxylate system

An exhaustive study of the factors governing the hydro- and solvothermal reaction of ScCl₃ with pyrimidine-4,6-dicarboxylic acid (H₂pmdc) has led to five Sc(III)-organic architectures, namely {[Sc(μ-pmdc)(μ-ox)_0.5(H₂O)₂]·3H₂O}_n (1), {[Sc(μ-pmdc)(μ-OH)(H₂O)]·H₂O}_n (2), {(NH₄)[Sc(μ-ox)₂]·2H₂O}_n (3), {(tma)₂[ScK(μ-form)₆]}_n (4), and [Sc₂(pmdc)(OH)₃Cl]·DMF·2H₂O (5) (where ox = oxalate, tma = tetramethylammonium, form = formate, DMF = dimethylformamide). In this system, the pmdc anion has proven to be particularly sensitive to changes in the synthetic conditions in the presence of Sc(III), generating in situ byproducts that are incorporated into the final compounds. In fact, oxalate anions join the Sc-pmdc arrays into 2D sheets in 1 whereas ammonium ions fill the channels of the open 3D scandium oxalate architecture of 3. Making the reagent mixture a bit more basic prevents such decomposition and allows hydroxides to replace oxalate as ligands for the Sc, thus modifying the topology of layers in 2. Formate ions coming from the hydrolysis of DMF prevent the coordination of pmdc and give rise to the framework of 4. Structure 5 is an as yet unknown scandium pyrimidine-4,6-dicarboxylate framework which displays permanent porosity. CO₂ adsorption isotherms on 5 reveal an uptake of 3 mmol g^–1 and strong adsorbate···framework interactions. ⁴⁵Sc MAS NMR has afforded additional information on the metal environments, providing further characterization of compound 5.