Synthesis and isolation of cuboctahedral and icosahedral platinum nanoparticles. Ligand-dependent structures

The reaction of Pt(dba)(2) with CO (1 atm) in toluene affords a brown precipitate which can be isolated and redissolved in CH2Cl2 to give a colloidal solution of fee platinum particles with a large size dispersity (10-20 Angstrom, I). Redissolution of I in THF leads to stable 12 Angstrom fcc platinum particles (II) which call be isolated and characterized by high-resolution electron microscopy (HREM) and wide-angle X-ray scattering (WAXS). The original reaction in THF leads to the direct formation of II. Addition of 0.2 equiv of PPh(3) to II yields a new colloid, III, of the same size but of icosahedral structure. III was characterized by HREM and spectroscopic techniques. In particular, C-13 and P-31 NMR spectroscopy demonstrates the absence of a Knight shift for these particles. Addition of excess triphenylphosphine leads to another species, IV, displaying broader size distribution with a maximum at 17 Angstrom and an fee structure. The reaction of I with more than 0.2 equiv of PPh(3) affords mixtures containing colloids, clusters (predominantly Pt-5(CO)(6)(PPh(3))(4)), and mononuclear complexes. Both the formation of the colloids and their transformation into molecular species is rapid at room temperature. It is suggested that such processes mag be more frequent in organometallic chemistry than previously thought.