Cobalt, nickel and iron embedded chitosan microparticles as efficient and reusable catalysts for Heck cross-coupling reactions.

Cobalt, nickel and iron are efficient catalysts for numerous chemical and biological reactions, but they are hardly immobilized on solid matrixes due to their relatively weak chelation with the surface molecules. We have employed a co-electrospraying coupling with glutaraldehyde cross-linking technique to prepare cobalt (Co2+), nickel (Ni2+) and iron (Fe2+) catalyst embedded chitosan microparticles (M@MicroCS) (M = Co, Ni and Fe). The Co@MicroCS beads exhibited excellent catalytic activity for the Heck cross-coupling reactions of aromatic iodides with alkenes. Moreover, the Co@MicroCS bead catalyst could be even reused at least for five times without significant loss of the catalytic activities. The average diameter of the free volume holes for the porous Co@MicroCS beads was determined to be much larger than the size of the reactant and product molecules, allowing them to freely migrate through the catalytic sites inside the chitosan beads. These results clearly indicate that the Heck reactions catalyzed by the Co@MicroCS beads primarily occurs inside the porous chitosan beads. Taken together, we have demonstrated a straightforward approach to prepare efficient and recyclable heterogeneous catalysts derived from the first-row transition metal catalysts by entrapping them inside the chitosan microparticles.