Well-Defined Pd_n Clusters for Cross-Coupling and Hydrogenation Catalysis: New Opportunities for Catalyst Design

Abstract

In recent studies it has been demonstrated that the privileged reactivity of higher-order metal clusters can be exploited in widely applied catalytic processes, particularly cross-coupling reactions and hydrogenative transformations. Relatively small, well-defined Pd n clusters have been known since the 1960s. Unique reactivity, reaction (product) selectivity, and catalyst behavior have been recently uncovered, from which there is much potential in catalyst design and application. Ligated Pd n clusters of a smaller size (where n is less than 6) may form upon degradation of mononuclear Pd species en route to larger particulate Pd (from <5 nm particles to large moribund forms in the >1 μm range). This review presents the catalytic applications of Pd n clusters. We pay particular attention to the underlying structure of the Pd n clusters, linked to their reactivity. A hypothesis that ligated Pd n clusters may constitute a mechanism by which higher-order Pd species may form (as a bridging point for monoligated Pd species through to PdNPs) is further discussed. Where appropriate, we mention other catalytic reaction processes that complement the discussion focused on cross-coupling and hydrogenation processes.

Opening a Pandora’s Flask on a Prototype Catalytic Direct Arylation Reaction of Pentafluorobenzene: The Ag2CO3/Pd(OAc)2/PPh3 System

“…For Xphos and PPh 3 , there is experimental evidence that Pd(0)­L n species can form from mixtures of Pd­(OAc) 2 /phosphine ligand . We recognize that dinuclear Pd species (leading to other higher-order Pd clusters) can also form, as shown by stoichiometric experiments reported by Fairlamb et al (for PPh 3 ), Jutand et al (for Xphos), and others. − For the purposes of this paper, we refer to the active catalyst species being Pd(0)­L n , which is supported by our global findings (see later).…”

“…Recent studies have demonstrated the formation of Pd I clusters with bridging phosphide ligands in the reactions of Pd­(OAc) 2 and PPh 3 in THF. , We were concerned to establish whether these species are formed under the conditions of our cross-coupling reactions. We therefore examined the nature of the Pd species formed in the absence of substrates by conventional 31 P­{ 1 H} NMR spectroscopy, searching for characteristic phosphide resonances in the δ 200 region.…”

Indomuscone-Based Sterically Encumbered Phosphines as Ligands for Palladium-Catalyzed Reactions

“…Figure shows the kinetic results for the telomerization reaction under optimized conditions, either adding Pd­(OAc) 2 and ligand 2a by separating from the beginning in a 1:2 molar ratio or adding the isolated complex Pd­( 2a ) 2 (OAc) 2 . The results show an induction time when the complex is prepared in situ and the disappearance of this induction time when the isolated complex is used, supporting that Pd­( 2a ) 2 (OAc) 2 is the active pre-catalyst of the reaction . It has been proposed that the Pd species in the catalytic cycle are in reduced form, and that they just have one or none phosphine ligands; thus, the observed acceleration by preforming Pd­( 2a ) 2 (OAc) 2 could be due to a facile reductive elimination by the phosphine and OAc ligands.…”

Fluorescent organometallic dyads and triads: establishing spatial relationships

“…78,79 In Pd-catalyzed cross coupling reactions di-and polynuclear precatalysts and intermediates appear to be highly signicant. 80 The small amount of catalyst used in such reactions and/or the small amount of binuclear species formed, renders their direct observation difficultespecially in the absence of a unique spectroscopic signature of such bimetallic species. The methodology presented here, should enable the direct observation of such intermediates via FRET and should facilitate the determination of the respective kinetics.…”