Towards multicomponent MOFs via solvent-free synthesis under conventional oven and microwave assisted heating

Abstract

Herein we prove the efficiency of the oven heating solvent-free synthesis, based on the acid–base reaction between a metal oxide/hydroxide, adenine (HAde) and monocarboxylic acids, to afford otherwise not accessible new MBioFs of formula [M₂(μ₃-Ade)₂(μ₂-OOC(CH₂)_xCH₃)₂]_n expressed as 1_M@monocarboxylate [M(II): Ni or Zn; monocarboxylate: butanoato (But) and propanoato (Prop)]. Additionally, a microwave assisted solvent-free procedure has been carried out, leading to products with a somewhat lower adsorption performance but with the advantages of reducing the reaction times to the minute scale and incorporating randomly distributed additional meso/macropores generated during the release of the water vapour by-product. Both heating techniques provide the resulting products in a monolithic form. The N₂ (77 K) and CO₂ (273 K) adsorption isotherms indicate a great selectivity towards CO₂ for the 1_Ni@But compound. On the other hand, a careful control over the solvent-free conditions provided good-quality single crystals of three new compounds based on the metal/nucleobase/carboxylate system: [Zn₃(μ₃-Ade)₂(μ-OOCCH₃)₄]_n·3H₂O (2), [Zn(μ-Hypo)(μ-OOCCH₃)]_n (3) (Hypo: hypoxanthinate) and [Ni₂(μ-HAde)₂(μ-OOCH)₂(OOCH)₂(OH₂)₂]·2{(H₂Ade)(HCOO)}·2HCOOH (4). Compounds 2 and 3 show lamellar structures without accessible voids. Compound 4 represents an intermediate stage between the initial reagent mixture and the final extended coordination polymers that depicts an insight into the reaction mechanism of the solvent-free approach. It also shows the difficulties in stabilizing the porous structure of 1 when short aliphatic monocarboxylic acids, such as acetic and formic acids, are employed.