A chiral salen-based MOF catalytic material with high thermal, aqueous and chemical stabilities

Abstract

A highly stable chiral salen-based metal–organic framework [(Cu₄I₄)₂L₄]·20DMF·3CH₃CN (1) [L = (R,R)-N,N′-bis(3-tert-butyl-5-(4-pyridyl)salicylidene)-1,2-diphenylethylenediamine nickel(II)] has been synthesized and characterized by single crystal X-ray diffraction and other physicochemical methods. 1 exhibits a rare 8-fold interpenetrated 3D framework constructed by a 4-connecting Cu₄I₄ cluster and a 2-coordinating L ligand. Remarkably, in spite of 8-fold interpenetration, 1 still possesses two types of 1D chiral hydrophobic channels with pore window sizes of 6.77 × 8.64 Ų and 6.09 × 10.96 Ų along the crystallographic a axis. All Ni(salen) moieties of L lie inside the 1D channels and the empty coordination sites of Ni²⁺ are oriented to the cavities. PXRD and N₂ adsorption measurements confirmed that 1 is extremely stable under high temperature (>400 °C), in water vapor (90% relative humidity), in acid/base aqueous solution (pH 0–14), and in saturated NaOH solution at 100 °C, as well as in 30 wt% H₂O₂ and 70 wt% tert-butyl hydroperoxide solution. 1 was proved to be an excellent recycled heterogeneous catalyst for the conversion of simulated industrial CO₂ (that is, involving tiny amounts of water vapor and other acidic gases) with epoxides into cyclic carbonates under mild conditions for the first time. The synthesis of β-hydroxy-1,2,3-triazoles from the same epoxides, alkyne and sodium azide was also catalyzed by 1 in aqueous solution with high yield. Interestingly, the cycloaddition reaction of CO₂ to bulky epoxides shows a decrease in the activity with an increase in the alkyl chain length of the substrate because of confinement of the channel size of 1, showing size-dependent selectivity. The plausible catalytic mechanisms for these two reactions have also been proposed.