When Does Catalysis with Transition Metal Complexes Turn into Catalysis by Nanoparticles?

Modeling Key Pathways Proposed for the Formation and Evolution of “Cocktail”-Type Systems in Pd-Catalyzed Reactions Involving ArX Reagents

“…As we have already mentioned, the observed catalytic activity of Pd in reactions involving ArX reagents is generally attributed to leached Pd species in solution. ,,− Pd nanoparticles with (7 3 0) and (2 2 1) high-index facets have been reported as highly active in cross-coupling and Heck reactions. They provide at least a 2-fold increase in activity (measured in turnover numbers) in comparison with nanoparticles that have (1 1 1) and (1 0 0) facets only. ,, The boosted activity is attributed to facile leaching of Pd atoms from (7 3 0) and (2 2 1) high-index facets, as they are composed of low-coordinated Pd atoms ( N broken = 4, 5, respectively). , Although Pd nanoparticles with (1 1 1) and (1 0 0) low-index facets ( N broken = 9, 8, respectively) can also act as catalysts, their activity may be due to leaching of edge and vertex Pd atoms, as well as Pd adatoms and the atoms located at surface defects.…”

“…Anionic mono- and dipalladium species reportedly form catalytic intermediates in cross-coupling and functionalization reactions ,− (see structures (174), (175), (185), (187), (189), and (191) in the Supporting Information). Thus, in active catalytic systems, the presence of anionic species in solution is expectable.…”

“…The first one is oligomerization; [ArPdX] may bind to another valence-unsaturated [ArPdX] molecule or to [ArPdX 2 ] − . Dimeric and monomeric [ArPdX] n and [ArPdX 2 ] − species were observed in cross-coupling, Heck, and amination reactions. ,− ,, [PhPdX] 2 dimers have three isomers, since phenyl rings may bind to transition-metal atoms in σ as well as in π binding mode (see Figure ). μ 2 binding of aryl groups in M­(II)–M­(II) species was observed in Ni-catalyzed Kumada-type coupling .…”

“…The cohesive energy of Pd 4 is −38.0 kcal/mol per atom, while the cohesive energies of the considered Pd 79 and Pd 140 nanoparticles are −69.1 and −72.2 kcal/mol per atom, respectively (DFT-PBE, plane-wave basis set, and PAW 54,[102][103][104][105][106][107][108][109]127,128 [PhPdX] 2 dimers have three isomers, since phenyl rings may bind to transition-metal atoms in σ as well as in π binding mode (see Figure 6). μ 2 binding of aryl groups in M(II)−M(II) species was observed in Ni-catalyzed Kumada-type coupling.…”

“…The activity of these catalytic systems is attributed to the “leached Pd” in the solution phase. At the same time, PdNPs are considered as a reservoir of active species. ,,− Palladium leaching is a reversible process, and its mechanistic counterpart is usually referred to as capture or redeposition. , All of these observations comprise the unified picture of Pd-catalyzed reactions with the central part of unclear chemical nature (Figure , left). Species, being intermediate to both Pd nanoparticles and Pd complexes, are the “leached Pd”.…”