Multifunctional Catalysis by a One-Dimensional Copper(II) Metal Organic Framework Containing Pre-existing Coordinatively Unsaturated Sites: Intermolecular C-N, C-O, and C-S Cross-Coupling; Stereoselective Intramolecular C-N Coupling; and Aziridination Reactions.

Abstract

The coordinatively unsaturated sites (CUS) are vital in metal-centered catalysis. Metal-organic frameworks (MOFs) provide a unique opportunity to generate and stabilize CUS due to their robust structure. Generally, the generation of CUS in MOFs needs prior activation under heat and high vacuum to remove labile molecules occupying the catalytic sites. Herein, we report a solvothermal synthesis of a ready-to-use copper MOF containing accessible pre-existing CUS that does not need activation. The single crystal X-ray diffraction structure reveals a square planar Cu(II) center with two N-methylimidazoles (Mim) and one benzenedicarboxylic acid (BDC) with the formula unit [CuII(BDC)(Mim)2]n (Cu-1D) forming an infinite one-dimensional (1D) chain along the c axis. The 1D chains are stabilized by noncovalent π-π, CH···π, and H-bonding interaction to give 2D (sheet-like) and 3D networks in the solid state. The quantification of non-covalent interaction is studied by Hirshfeld surface analysis, and the formation of a higher architecture in the solid state is confirmed by SEM analysis. The reported Cu-1D MOF acts as a solid heterogeneous catalyst and exhibits efficient catalytic activity in intermolecular and intramolecular cross-coupling reactions. Intermolecular C-heteroatom cross-coupling of a variety of N-heterocycles, aliphatic, aromatic, alicyclic amines and amides (C-N), phenols (C-O), and thiols (C-S) with aryl halides (halide = I, Br) was achieved with 70 to 95% yield, better than the state-of-the-art Cu-based homogenous system. The C-N coupling catalytic cycle is initiated by the in situ reduction of Cu(II) by KOH/DMSO to Cu(I) species. Subsequently, Cu(I) undergoes oxidative addition followed by reductive elimination to form a cross-coupled product. High stereoselectivity was found for the intramolecular C-N coupling reaction to give tetrahydroquinoxalines with an enantiomeric excess (ee) of more than 99%. For a broader application, Cu-1D was applied as the catalyst for the synthesis of a library of aziridines that gives yields of up to 99% with more than 93% recyclability for each cycle.