Preparation and Characterization of Supported Bimetallic PdIV-CoIII Model Catalyst from Organometallic Single Source Precursor for Aerobic Oxidation of Alcohols

The bimetallic paddlewheel catalyst precursor, [(PdCoII)-Co-II(mu-OOCCH3)(4)] H2O center dot 2CH(3)COOH (1), prepared from [Pd-3(mu-OOCCH3)(6)] and [Co(OOCCH3)(2)], was used as a single source precursor to prepare, after binding to a surface-hydroxylated silicon wafer and oxidation, the bimetallic oxides of (PdCoIII)-Co-IV/SiO2 catalyst supported on a model planar (i.e., two-dimensional) silicon wafer. This catalyst catalyzes the aerobic oxidation of alcohols to its corresponding carbonyl compounds. The bimetallic tetracarboxylato catalyst precursor was bonded to the surface-hydroxylated silicon wafer by spin-coating and also by grafting. X-ray photoelectron spectroscopy (XPS) revealed that one of the four mu-acetato bridging ligands was substituted by Si-O fragments in a covalent bond formation process during grafting of 1 onto the wafer. In contrast, during the spin-coating process, all four acetato ligands remained intact during fixation on the silicon surface. Upon oxidation and workup, the grafted sample's Pd:Co ratio remained unchanged (1.0:1.3), whereas the spin-coated sample's Pd content decreased with respect to Co content. XPS determined binding energies were interpreted to imply that after oxidation in an oxygen/argon mixture of the grafted sample both Pd-II and Co-II were oxidized to produce PdO2 (337.5 eV) and (Co2O3)-O-III (781.1 eV) which most probably interacts with the silicon surface via Pd-IV-O-Si and Co-III-O-Si bonds. Solvent free aerobic oxidation of octadecanol to its corresponding carbonyl compound was achieved on this oxidized (PdCoIII)-Co-IV/SiO2 model catalyst using molecular oxygen as oxidant under solvent-free conditions. The use of the single source catalyst precursor, 1, resulted in a (PdCoIII)-Co-IV/SiO2 catalyst with superior catalytic activity toward the oxidation of octadecanol over a catalyst prepared from a physical mixture of the separate reactant compounds tripalladium(II) hexaacetate and cobalt(II) diacetate.