Comparative Study of Homogeneous and Silica Immobilized N N and N O Palladium(II) Complexes as Catalysts for Hydrogenation of Alkenes, Alkynes and Functionalized Benzenes

This work reports the use of homogeneous and silica immobilized palladium(II) complexes of ligands (2-phenyl-2-((3(triethoxysilyl)propyl)imino)ethanol) (L1), (4-methyl-2-((3(triethoxysilyl)propyl)imino)methyl)phenol) (L2), [L1-MCM-41] (L1im), and [L2-MCM-41] (L2im) as catalysts in molecular hydrogenation of alkenes, alkynes and functionalized benzenes. The homogeneous complexes [Pd(L1)(2)] (Pd1), [Pd(L2)(2)] (Pd2), [Pd(L1)(Cl-2)] (Pd3), and [Pd(L2)(Cl-2)] (Pd4), and their respective silica immobilized complexes [Pd(L1)(2)]-MCM-41] (Pd1im), [Pd(L2)(2))-MCM-4] (Pd2im), [Pd (L1)(Cl-2)-MCM-41] (Pd3im) and [Pd(L2)(Cl-2)]-MCM-41] (Pd4im) formed active catalysts in the molecular hydrogenation of these substrates. The catalytic activities and product distribution in these reactions were largely dictated by the nature of the substrate. The kinetic studies revealed a pseudo-first order dependence on styrene substrate for both the homogeneous and immobilized catalysts. Significantly, the selectivity of both homogeneous and immobilized catalysts were comparable in the hydrogenation of both alkynes and multi-functionalized benzenes. The supported catalysts could be recycled up to four times with minimum loss of catalytic activity and showed absence of any leaching from hot filtration experiments. Kinetics and poisoning studies established that complexes Pd1-Pd4 were largely homogeneous in nature, while the immobilized complexes Pd1im-Pd4im formed Pd(0) nanoparticles as the main active species. Graphic Abstract [GRAPHICS] .