The synthesis, structure, and catalytic properties of Cl-substituted tridentate ONO Schiff base molybdenum(VI) complexes: Expeditious synthesis of 1,8-dioxo-octahydro-xanthene derivatives

A family of molybdenum complexes with a general formula of [MoO2L](2) have been synthesized and fully characterized, where L is a tridentate ONO Schiff base ligand featuring a Cl substituent in different positions on the aromatic ring of salicylidene-2-aminophenolato backbone. The solvent molecule stabilized the monomeric structure of [MoO2LD] (D = DMF or DMSO) allowing single-crystal X-ray diffraction of the three new complexes. The dimeric complexes are utilized to catalyze the reaction of aldehydes/ketones with dimedone, leading to 1,8-dioxo-octahydro-xanthene derivatives in N,N-dimethylformamide. We found that their catalytic activity is enhanced by the electron-withdrawing group of the Cl substituent on the ONO ligand. In addition, the position of the Cl substituent can also positively influence the catalysis performance: (ortho (L-1) > para (L-3) > meta (L-2) approximate to meta (L-4)). The high efficiency of the catalysts promises high yields and selectivity with a low catalyst loading and allows a wide substrate scope of aldehydes/ketones for the reaction. In addition, the method employs recrystallization for purifying products and can be easily scaled up to gram levels, indicating a strong potential for industrial utilization.