Synthetic, spectral, structural and catalytic activity of infinite 3-D and 2-D copper(ii) coordination polymers for substrate size-dependent catalysis for CO2 conversion

Abstract

Two copper(II) coordination polymers, viz. [Cu₂(OAc)₄(μ₄-hmt)_0.5]_n (1) and [Cu{C₆H₄(COO⁻)₂}₂]_n·2C₉H₁₄N₃ (2), have been synthesized solvothermally and characterized. The solid-state structure reveals that 1 is an infinite three-dimensional (3D) motif with fused hexagonal rings consisting of Cu(II) and hmt in a μ₄-bridging mode, while 2 is an infinite two dimensional (2D) motif containing Pht⁻² in a μ₁-bridging mode. CP 1 has a two-fold interpenetrated diamondoid network composed of 4-connected sqc6 topology with the point symbol of {6⁶}, while 2 has a Shubnikov tetragonal plane network possessing a 4-connected node with an sql topology with a point symbol of {4⁴·.6²}-VS [4·4·4·4·*·*]. Both CPs 1 and 2 serve as efficient catalysts for CO₂-based chemical fixation. Moreover, 1 demonstrates one of the highest reported catalytic activity values (%yield) among Cu-based MOFs for the chemical fixation of CO₂ with epoxides. 1 shows high efficiency for CO₂ cycloaddition with small epoxides but its catalytic activity decreases sharply with the increase in the size of epoxide substrates. The catalytic results suggested that the copper(II) motif-catalyzed CO₂ cycloaddition of small substrates had been carried out within the framework, while large substrates could not enter into the framework for catalytic reactions. The high efficiency and size-dependent selectivity toward small epoxides on catalytic CO₂ cycloaddition make 1 a promising heterogeneous catalyst for carbon fixation and it can be used as a recoverable stable heterogeneous catalyst without any loss of performance. The solvent-free synthesis of the cyclic carbonate from CO₂ and an epoxide was monitored by in situ FT-IR spectroscopy and an exposed Lewis-acid metal site catalysis mechanism was proposed.