Controlled Modification of Triaminoguanidine-Based μ<sub>3</sub> Ligands in Multinuclear [V<sup>IV</sup>O]/[V<sup>V</sup>O<sub>2</sub>] Complexes and Their Catalytic Potential in the Synthesis of 2-Amino-3-cyano-4<i>H</i>-pyrans/4<i>H</i>-chromenes
Mannar R. Maurya, Naveen Kumar, Fernando Avecilla
Abstract
Reaction of tris(2-hydroxybenzylidene)-triaminoguanidinium chloride ( I ·HCl) and tris(5-bromo-2-hydroxybenzylidene)-triaminoguanidinium chloride ( II ·HCl) with [V IV O(acac) 2 ] (1:1 molar ratio) in refluxing methanol resulted in mononuclear [V IV O] complexes, [V IV O(H 2 L 1′ )(MeOH)] ( 1 ) and [V IV O(H 2 L 2′ )(MeOH)] ( 2 ), respectively, where I and II undergo intramolecular triazole ring formation. Aerial oxidation of 1 and 2 in MeOH in the presence of Cs 2 CO 3 gave corresponding cis -[V V O 2 ] complexes Cs[(VO 2 )(H 2 L 1′ )] ( 3 ) and Cs[(VO 2 )(H 2 L 2′ )] ( 4 ). However, reaction of an aerially oxidized methanolic solution of [V IV O(acac) 2 ] with I ·HCl and II ·HCl in the presence of Cs 2 CO 3 (in 1:1:1 molar ratio) gave mononuclear complexes Cs[(VO 2 )(H 3 L 1 )] ( 5 ) and Cs[(VO 2 )(H 3 L 2 )] ( 6 ) without intramolecular triazole ring formation. Similar anionic trinuclear complexes Cs 2 [(VO 2 ) 3 ( L 1 )] ( 7 ) and Cs 2 [(VO 2 ) 3 ( L 2 )] ( 8 ) were isolable upon increasing the amounts of the vanadium precursor and Cs 2 CO 3 to 3 equiv to the reaction applied for 5 and 6 . Keeping the reaction mixture of 1 in MeOH under air gave [V V O(H 2 L 1’ )(OMe)] ( 9 ). Structures of 3, 7, 8, and 9 were confirmed by X-ray crystal structure study. A permanent porosity in the crystalline metal–organic framework of 7 confirmed by single-crystal X-ray investigation was further verified by the BET study. Along with a suitable reaction mechanism, these synthesized compounds were explored as effective catalysts for the synthesis of biomolecules 4 H -pyran/4 H -chromenes.