Preparation and characterization of supported bimetallic Pd(IV)-Co(III) model catalyst from organometallic single source precursor for aerobic oxidation of alcohols.

The bimetallic paddlewheel catalyst precursor, [Pd(II)Co(II)(μ-OOCCH(3))(4)] H(2)O·2CH(3)COOH (1), prepared from [Pd(3)(μ-OOCCH(3))(6)] and [Co(OOCCH(3))(2)], was used as a single source precursor to prepare, after binding to a surface-hydroxylated silicon wafer and oxidation, the bimetallic oxides of Pd(IV)Co(III)/SiO(2) 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 μ-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 PdO(2) (337.5 eV) and Co(III)(2)O(3) (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 Pd(IV)Co(III)/SiO(2) model catalyst using molecular oxygen as oxidant under solvent-free conditions. The use of the single source catalyst precursor, 1, resulted in a Pd(IV)Co(III)/SiO(2) 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.