A microporous Cu2+ MOF based on a pyridyl isophthalic acid Schiff base ligand with high CO2 uptake

Abstract

A new Cu²⁺ complex that was isolated from the initial use of 5-((pyridin-4-ylmethylene)amino)isophthalic acid (PEIPH₂) in 3d metal–organic framework (MOF) chemistry is reported. Complex {[Cu₃(PEIP)₂(5-NH₂-mBDC)(DMF)]·7DMF}_∞ denoted as Cu-PEIP·7DMF was isolated from the reaction of Cu(NO₃)₂·2.5H₂O with PEIPH₂ in N,N-dimethylformamide (DMF) at 100 °C and contains both the PEIP²⁻ ligand and its 5-NH₂-mBDC²⁻ fragment. After the structure and properties of Cu-PEIP were known an analogous complex was prepared by a rational synthetic method that involved the reaction of Cu(NO₃)₂·2.5H₂O, 5-((pyridin-4-ylmethyl)amino)isophthalic acid (PIPH₂ – the reduced analogue of PEIPH₂) and 5-NH₂-mBDCH₂ in DMF at 100 °C. Cu-PEIP comprises two paddle-wheel [Cu₂(COO)₄] units and exhibits a 3D-framework with a unique trinodal underlying network and point symbol (4.5²)₄(4²·5⁴·6⁴·8³·9²)₂(5²·8⁴). This network consists of pillared kgm-a layers containing a hexagonal shaped cavity with a relatively large diameter of ∼8–9 Å surrounded by six trigonal shaped ones with a smaller diameter of ∼4–5 Å and thus resembles the structure of HKUST-1. Gas sorption studies revealed that Cu-PEIP exhibits a 1785 m² g⁻¹ BET area as well as high CO₂ sorption capacity (4.75 mmol g⁻¹ at 273 K) and CO₂/CH₄ selectivity (8.5 at zero coverage and 273 K).