Ni–NiO heterojunctions: a versatile nanocatalyst for regioselective halogenation and oxidative esterification of aromatics

Herein, we report a facile method for the synthesis of Ni–NiO heterojunction nanoparticles, which we utilized for the nuclear halogenation reaction of phenol and substituted phenols using N-bromosuccinimide (NBS). A remarkable para-selectivity was achieved for the halogenated products under semi-aqueous conditions. Interestingly, blocking of the para-position of phenol offered ortho-selective halogenation. In addition, the Ni–NiO nanoparticles catalyzed the oxidative esterification of carbonyl compounds with alcohol, diol or dithiol in the presence of a catalytic amount of NBS. It was observed that the aromatic carbonyls substituted with an electron-donating group favoured nuclear halogenation, whereas an electron-withdrawing group substitution in carbonyl compounds facilitated the oxidation reaction. In addition, the catalyst was magnetically separated and recycled 10 times. The tuned electronic structure at the Ni–NiO heterojunction controlled selectivity and activity as no such para-selectivity was observed with commercially available NiO or Ni nanoparticles.