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

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.