A highly efficient antioxidant based on boron and a Schiff base bridged phenolic diphenylamine: synthesis, crystal structure and thermal and antioxidant properties.

Antioxidants can effectively protect vegetable-oil-based lubricants against oxidative degradation and prolong their service life. A novel compound, (E)-4-({3,5-di-tert-butyl-2-[(diphenylboranyl)oxy]benzylidene}amino)-N-phenylaniline (BSPD) or {2-[(4-anilinophenyl)iminomethyl-κN]-4,6-di-tert-butylphenolato-κO}diphenylboron, CHBNO, was synthesized by a one-pot reaction of the Schiff base bridged phenolic diphenylamine 2,4-di-tert-butyl-6-{N-[4-(phenylamino)phenyl]carboximidoyl}phenol (SPD) with phenylboronic acid. The corresponding single-crystal X-ray study revealed that the B atom in BSPD has a typical tetrahedral geometry, and the N,O-chelated six-membered ring adopts a chair conformation. According to thermogravimetric analysis, the 5% weight-loss temperature of BSPD increases to 328 °C and the 95% weight-loss temperature is up to 414 °C. More importantly, the antioxidant capability of BSPD in vegetable oil is much better than that of commercial antioxidants [diphenylamine (DPA) and 2,6-di-tert-butyl-4-methylphenol (BHT)] and unmodified SPD.