Design and construction of polyaromatic group containing Cd(ii)-based coordination polymers for solvent-free Strecker-type cyanation of acetals

Abstract

In the present work, we have synthesized and characterized two novel Cd(II) coordination polymers, [Cd₄(L1)₄(DMF)₆]_n·3n(DMF) (1) and [Cd₂(L2)₂(DMF)₃]_n·2n(DMF) (2), and studied their catalytic application. They were synthesized via solvothermal reaction of the polyaromatic group-containing carboxylic acid pro-ligands 5-{(anthracen-9-ylmethyl)amino}isophthalic acid (H₂L1) and 5-{(pyren-1-ylmethyl)amino}isophthalic acid (H₂L2) and Cd(NO₃)₂·6H₂O in the presence of DMF : MeOH and DMF : EtOH, respectively. They were characterized by ATR-FTIR, elemental, thermogravimetry, BET surface area, powder and single-crystal X-ray diffraction analyses. The single-crystal X-ray diffraction analysis revealed that CP 1 has a one-dimensional tube-like structure, and the anthracene groups of the ligands are present outside the tube. In CP 1 two types of dinuclear {Cd₂(COO)₄(DMF)₃} units act as secondary building units (SBU); the assembly of two of each dinuclear SBU led to the formation of rings that are interconnected via isophthalate groups to produce a 1D hollow tube-like structure having 11.9 Å × 13.5 Å channels, which are occupied by the coordinated DMF molecules. CP 2 also holds dinuclear {Cd₂(COO)₄(DMF)₃} SBU units that form a two-dimensional grid structure based on adjacent {Cd₂(isophthalate)}₄ rings with the pyrene groups directed outside the grid. The topological analysis showed that CP 1 has a 2,2,4-connected trinodal net, whereas the 2D framework 2 exhibits a 2,4-connected binodal net. We have also tested the solvent-free Strecker-type cyanation reactions of different acetals, with trimethylsilyl cyanide (TMSCN), catalysed by our synthesized coordination polymers under heterogeneous conditions. To our knowledge, this is the first example of such a reaction catalysed by coordination polymers under heterogeneous conditions. CP 1 is more effective (yield 96% within 4 h of reaction time) than CP 2, possibly due to the lower dimensionality and higher porosity (BET surface areas of 24.1 m² g⁻¹ for 1 and 10.9 m² g⁻¹ for 2), with the corresponding higher accessibility of the metal centres. The recyclability and heterogeneity of these CPs were also investigated, proving that they remain active for at least five recyclings without losing their activity significantly.