Controlled Modification of Triaminoguanidine-Based μ₃ Ligands in Multinuclear [VⁱᵛO]/[VᵛO₂] Complexes and Their Catalytic Potential in the Synthesis of 2-Amino-3-cyano-4H-pyrans/4H-chromenes

Abstract

[Abstract] Reaction of tris(2-hydroxybenzylidene)-triaminoguanidinium chloride (I·HCl) and tris(5-bromo-2-hydroxybenzylidene)-triaminoguanidinium chloride (II·HCl) with [VⁱᵛO(acac)₂] (1:1 molar ratio) in refluxing methanol resulted in mononuclear [VⁱᵛO] complexes, [VⁱᵛO(H₂L¹′)(MeOH)] (1) and [VⁱᵛO(H₂L²′)(MeOH)] (2), respectively, where I and II undergo intramolecular triazole ring formation. Aerial oxidation of 1 and 2 in MeOH in the presence of Cs₂CO₃ gave corresponding cis-[VᵛO₂] complexes Cs[(VO₂)(H₂L¹′)] (3) and Cs[(VO₂)(H₂L²′)] (4). However, reaction of an aerially oxidized methanolic solution of [VⁱᵛO(acac)₂] with I·HCl and II·HCl in the presence of Cs₂CO₃ (in 1:1:1 molar ratio) gave mononuclear complexes Cs[(VO₂)(H₃L¹)] (5) and Cs[(VO₂)(H₃L²)] (6) without intramolecular triazole ring formation. Similar anionic trinuclear complexes Cs₂[(VO₂)₃(L¹)] (7) and Cs₂[(VO₂)₃(L²)] (8) were isolable upon increasing the amounts of the vanadium precursor and Cs₂CO₃ to 3 equiv to the reaction applied for 5 and 6. Keeping the reaction mixture of 1 in MeOH under air gave [VᵛO(H₂L¹’)(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 4H-pyran/4H-chromenes.