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.
