Flexible bifunctional monoethylphosphonate/carboxylates of Zn(ii) and Co(ii) reinforced with DABCO co-ligand: paradigmatic structural organization with pcu topology

Abstract

Two novel isostructural phosphonate-monoethylcarboxylate MOFs with the structural formula of [M₂(EtBCP)₂(DABCO)_0.5]·2DMA (M = Zn (2), Co (3); H₂EtBCP = O-ethyl-P-(4-carboxyphenyl)phosphonic acid, DABCO = 1,4-diazabicyclo[2.2.2]octane, DMA = N,N-dimethylacetamide) were synthesized and characterized. The frameworks of 2 and 3 are sustained by {Zn₂(PO₂(OEt))₂}_n metal-phosphonate- and DABCO-extended {Zn₂(COO)₄(DABCO)}_n paddle-wheel carboxylate chain-SBUs. The chains providing connectivity in three, mutually orthogonal directions are running parallel and are combined in a framework, which could be interpreted as having a pcu topology. The simple structure-organization principle, which suggests the possibility of the elongation of the bifunctional ligand with scaling in two directions, allows to view the structures of 2 and 3 as prototypes for an isoreticular series. The porosity of both compounds, based on a relatively short ligand, is low: no adsorption of N₂ was registered, however, CO₂ is adsorbed readily allowing to estimate the surface area at ∼330 m² g⁻¹ (∼900–1060 m² g⁻¹ geometric estimate). The compounds demonstrate a two-step CO₂ adsorption isotherm both at 195 K (0–1 bar) and 298 K (0–20 bar). The adsorption isotherms are characterized by a gradual (type “F-I”), albeit still relatively steep onset of the second step, associated with structural flexibility/bistability. The estimated pore volumes at the start of the transformation (195 K) for 2 and 3 are ∼0.11 (0.08P/P₀) and 0.12 cm³ g⁻¹ (0.12P/P₀) respectively, which corresponds considerably well to the geometrically calculated accessible volume of ∼0.07 cm³ g⁻¹ for the experimental structure (3.3 Å probe diameter). The structural prerequisites of the observed flexibility of the framework, which might be associated with the non-planarity of the metal-phosphonate moieties, acting as ‘levers’ for propagating mechanical stress, are discussed.