Study of temperature dependent three component dynamic covalent assembly via Hantzsch reaction catalyzed by dioxido- and oxidoperoxidomolybdenum(vi) complexes under solvent free conditions

Abstract

Tridentate ONO donor ligands derived from heterocyclic compound 4-acetyl-3-methyl-1-phenyl-2-pyrazoline-5-one (Hap) and aromatic hydrazides {benzoyl hydrazide (Hbhz), isonicotinoyl hydrazide (Hinh), nicotinoyl hydrazide (Hnah) and furoyl hydrazide (Hfah)} react with [Mo^VIO₂(acac)₂] (Hacac = acetylacetone) in equimolar ratio in methanol to give dioxidomolybdenum(VI) complexes, [MoO₂(ap-bhz)(MeOH)] 1, [MoO₂(ap-inh)(MeOH)] 2, [MoO₂(ap-nah)(MeOH)] 3 and [MoO₂(ap-fah)(MeOH)] 4. Reaction of these ligands with in situ generated oxidoperoxidomolybdenum(VI) precursor results in the formation of oxidoperoxidomolybdenum(VI) complexes, [MoO(O₂)(ap-bhz)(MeOH)] 5, MoO(O₂)(ap-inh)(MeOH)] 6, MoO(O₂)(ap-nah)(MeOH)] 7 and MoO(O₂)(ap-fah)(MeOH)] 8. These complexes have been characterized by elemental analysis, spectroscopic techniques (infrared, UV-vis, ¹H and ¹³C NMR) and thermogravemetric analysis. The structures of complexes [Mo^VIO₂(ap-bhz)(H₂O)] 1a (water coordinated), [Mo^VIO₂(ap-bhz)(DMSO)] 1b (DMSO coordinated), [Mo^VIO₂(ap-nah)(DMF)] 3a (DMF coordinated), [Mo^VIO(O₂)(ap-bhz)(MeOH)] 5 (methanol coordinated) and [Mo^VIO(O₂)(Hap-nah)(OMe)]·MeOH 7a (methoxy coordinated) have been confirmed by single crystal X-ray studies. X-ray diffraction study also reveals that tridentate ligands bind to the metal center through enolic oxygen (of pyrazolol), azomethine nitrogen and enolic oxygen (of hydrazide) atoms. In complex 7a, pyridinic nitrogen is protonated. These complexes [dioxidomolybdenum(VI) as well as oxidoperoxidomolybdenum(VI)] have been tested as catalysts for temperature dependent one pot three component (methylacetoacetate, benzaldehyde and ammonium acetate) dynamic covalent assembly, via Hantzsch reaction, using 30% H₂O₂ as a green oxidant under solvent free conditions. Various parameters such as the amount of catalyst, oxidant and temperature of the reaction mixture have been taken into consideration to optimize the reaction conditions. In the Hantzsch reaction, the temperature and oxidant control the conversion and selectivity of the desired product.