Catalysis by metal colloids: Trajectories for atom assembling in Pd and Ptcolloids

The methods for the synthesis of giant clusters and the pathways of their f ormation from palladium and platinum complexes are discussed. The approache s to the study of these compounds, based on the concepts of both cluster ch emistry and physics of solids, are discussed. Giant clusters contain a posi tively charged, close-packed nucleus of metal atoms, whose number can be se veral hundreds and have discrete values, "magic numbers," for example, Pt-5 5, Pt-309, and Pd-561. The metal nucleus is stabilized by the coordinated l igands, for example, PPh3, phen, dipy, O2-, and Cl-, and the outer-sphere a nions like OAc- and PF6-. The structures of giant clusters are determined b y high-resolution electron microscopy, electron diffraction, tunneling micr oscopy, small-angle X-ray scattering, EXAFS, and other techniques combined with elemental microanalysis. The polynuclear low-valence complexes [Pd(4)p hen(oAc)(2)(H)(x)](n), [Pt(4)phen(OAc)(3)](n), and [Pt(8)phen(3)(OAc)(4)(OH )(4)(H2O)(6)](m) (n, m similar to 100), which are the precursors of giant c lusters, are isolated and characterized using the same techniques. Their pa thways of transformation to giant clusters are studied using temperature-pr ogrammed EXAFS combined with thermal analysis (DTA-TG). Giant clusters exhi bit high catalytic activity in the oxidation of alkenes, CO, and alcohols, the oxidative acetoxylation of alkenes and alkylarenes, the reduction of ni triles and nitroarenes, acetal formation, and the oxidative carbonylation o f phenol.