Full-Range Interconversion of Nanocrystals and Bulk Metal with a Highly Selective Molecular Catalyst

Abstract

Catalysis by soluble metal complexes often encompasses the occurrence of nanoparticles and aggregated inactive states, but the role of the different species is unclear. For the generation of highly active catalysts, it is crucial to know the relations between active and inactive states and the reversibility of such interconversions. This is further complicated by the question of the true nature of the active species, in particular for reactions that use dispersed nanoparticles as a catalyst (precursor), which can interconvert to soluble species. We show that a molecular catalyst can interconnect completely up to the step of bulk metal in an isomerizing methoxycarbonylation converting an unsaturated fatty acid to a linear diester with a very characteristic reactivity. The active species, a diphosphine-coordinated Pd hydride, decomposes to the diprotonated diphosphine ligand and a Pd⁰ species that agglomerates and precipitates as Pd black. This reaction is completely reversible, as shown for several examples of Pd⁰ precursors. Precise Pd nanocrystals enabled imaging of the dissolution process with the diprotonated diphosphine ligand via transmission electron microscopy. The characteristic selectivity of the isomerizing methoxycarbonylation is a clear indicator of the molecular nature of the active species. This was further demonstrated by NMR spectroscopy via capture of the active Pd hydride formed from Pd nanocrystals. CO, often a problematic reductant, acts as a stabilizer of the molecular oxidized catalyst species. The activation of Pd⁰ was also realized from macroscopically separated bulk species, like Pd black, sponge, and wire, evidencing that even highly agglomerated states of Pd can be converted to the molecular catalytically active species.