Near-critical water, a cleaner solvent for the synthesis of a metal–organic framework

Abstract

The microporous metal–organic framework {[Zn₂(L)]·(H₂O)₃}_∞ (H₄L = 1,2,4,5-tetrakis(4-carboxyphenyl)benzene) has been synthesised using near-critical water (300 °C) as a cleaner alternative to toxic organic solvents. A single crystal X-ray structure determination confirms that the complex incorporates tetrahedral Zn(II) centres bridged through the carboxylate anions to form a binuclear building block, which extends into a one dimensional chain along the c axis. Four L⁴⁻ ligands bind to each Zn(II) centre and cross-link the one dimensional chains along both a and b axes to afford a three dimensional network structure incorporating pores of ca. 4.3 Å in diameter. The complex shows high thermal stability up to 425 °C by gravimetric thermal analysis, and on desolvation, displays a high adsorption enthalpy of 11.0 kJ mol⁻¹ for H₂ uptake at zero coverage, consistent with the narrow pore diameter for the framework.