Polymer Supported Nitrogen‐Bridged Symmetrical Binuclear Dioxidomolybdenum(VI) Complexes and Their Homogeneous Analogues as Potential Catalysts for Efficient Synthesis of 2‐Amino‐3‐Cyano‐4H‐Chromenes/Pyrans

Abstract

AbstractReaction of 2‐chloromethyl‐1H‐benzimidazole with known intermediates (i–iii), prepared from diaminoguanidine hydrochloride with salicylaldehyde, 5‐bromosalicylaldehyde or 3,5‐di‐tert‐butylsalicylaldehyde, in the presence of triethylamine (NEt3) led to the formation of benzimidazole appended new ligands, H4L1‐H4L3 (I–III). The homogeneous nitrogen‐bridged symmetrical binuclear complexes, [(MoVIO2)2(L1)(H2O)2] (1), [(MoVIO2)2(L2)(H2O)2] (2) and [(MoVIO2)2(L3)(MeOH)2] (3) have been isolated by reacting these ligands with [MoVIO2(acac)2] in a 1 : 2 molar ratio in refluxing methanol. Using 1 : 1 (ligand to Mo precursor) molar ratio under above reaction conditions resulted in the corresponding mononuclear complexes, [MoVIO2(H2L1)(MeOH)] (4), [MoVIO2(H2L2)(H2O)] (5) and [MoVIO2(H2L3)(MeOH)] (6). The binuclear heterogeneous compounds [(MoVIO2)2(L1)(DMF)2]@PS (PS‐1), [(MoVIO2)2(L2)(DMF)2]@PS (PS‐2) and [(MoVIO2)2(L3)(DMF)2]@PS (PS‐3) have been obtained by immobilization of 1–3 onto chloromethylated polystyrene (PS) beads. All synthesized ligands, homogeneous as well as supported compounds have been characterized by elemental analyses and various spectroscopic methods. Single crystal X‐ray diffraction study of complexes 1 and 3 confirms their nitrogen‐bridged symmetrical binuclear structures while 4 is mononuclear. Heterogeneous compounds (PS‐1‐PS‐3) have further been studied by microwave plasma atomic emission spectroscopy, X‐ray photoelectron spectroscopy, and field emission scanning electron microscopy along with energy dispersive spectroscopy. These compounds (homogeneous and heterogeneous) were explored for catalytic applications to one‐pot multicomponent reactions (MCRs) for efficient synthesis of biologically active 2‐amino‐3‐cyano‐4H‐chromenes/pyrans (21 examples). Optimising various reaction parameters helped in achieving as high as 97 % yields of products. Though, only half equivalent of the binuclear complexes (1‐3) was required compared to mononuclear analogues (4‐6) to achieve comparable yields, heterogeneous catalysts have an added advantage due to their stability and recyclability. Suitable reaction mechanism has also been proposed based on isolated intermediates.