Formation and stabilization of nanosized Pd particles in catalytic systems: Ionic nitrogen compounds as catalytic promoters and stabilizers of nanoparticles

A Dichotomy in Cross-Coupling Site Selectivity in a Dihalogenated Heteroarene: Influence of Mononuclear Pd, Pd Clusters, and Pd Nanoparticles—the Case for Exploiting Pd Catalyst Speciation

“…Astruc et al explored this electrosteric stabilization in the design of bespoke architectures for the stabilization of PdNPs. 57,58 This valuable prior knowledge underpins our hypothesis that electrosteric stabilization of PdNPs is critical to the site-selectivity switch seen in the cross-coupling reactions of 1. Thus, stabilized PdNPs, formed in situ from either precatalysts Pd(OAc) 2 and 1PPh 3 or Pd 3 Cl 2 by additive or in situ generated salts, are the catalyst species responsible for this atypical selectivity and relatively high activity, compared to that of the dominant mononuclear catalytic species generated from Pd(OAc) 2 and ≥3PPh 3 , Pd(PPh 3 ) 4 , or Pd 2 (dba) 3 •CHCl 3 /2PPh 3 .…”

Discovery of the N–NHC Coupling Process under the Conditions of Pd/NHC- and Ni/NHC-Catalyzed Buchwald–Hartwig Amination

“…Another interesting question concerns the potential reversibility of the N–NHC coupling. It is known that H–NHC and C–NHC couplings are reversible, ,, and experimental examples of the oxidative addition of [NHC–R]­X salts, where R = H, , or aryl and alkyl, , to Pd and Ni species were reported in the literature. We tried to observe the formation of Pd/NHC complexes by ESI-MS from compounds 10a and 11j in reactions with Pd­(PPh 3 ) 4 or Pd 2 dba 3 in the presence of Bu t OK; however, Pd/NHC complexes were not detected (Scheme S3a,b).…”

“…30,36,[39][40][41][42][43][44]46 However, despite the high strength of the metal−NHC bond, complexes M/NHC can suffer decomposition during catalysis. 26,29 The M−NHC bond cleavage reactions induce the deactivation of the so-called NHC-connected mode of catalysis; however, they can activate the NHC-disconnected mode of catalysis with "ligandless" metal species. 26 It was demonstrated recently that Pd/NHC precatalysts can transform into "cocktail"-type catalytic systems and, apparently, both NHC-connected and NHC-disconnected modes of catalysis can operate in the Buchwald−Hartwig amination of activated aryl bromides.…”

“…N-Heterocyclic carbenes (NHCs) are usually considered robust ligands, providing high performance of Pd and Ni catalysts in the Buchwald–Hartwig and other amination reactions. ,,− The enhanced stability of the metal–NHC bond, lower toxicity, high variability of electronic and steric parameters, and the relative ease of preparation and handling of M II /NHC precatalysts are the main benefits of NHCs over phosphines and many other ligands. ,,,− Some types of NHCs, especially N , N ′-diarylimidazol-2-ylidenes with bulky N -aryl substituents, demonstrate very high catalytic activity in the Buchwald–Hartwig amination, reduce Pd loadings up to 0.005–0.05 mol %, − , or decrease the reaction temperature up to 20 °C. ,,− , However, despite the high strength of the metal–NHC bond, complexes M/NHC can suffer decomposition during catalysis. , The M–NHC bond cleavage reactions induce the deactivation of the so-called NHC-connected mode of catalysis; however, they can activate the NHC-disconnected mode of catalysis with “ligandless” metal species . It was demonstrated recently that Pd/NHC precatalysts can transform into “cocktail”-type catalytic systems and, apparently, both NHC-connected and NHC-disconnected modes of catalysis can operate in the Buchwald–Hartwig amination of activated aryl bromides .…”

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

“…A crucial role for additive salts was delineated under both Suzuki–Miyaura and Kumada–Corriu cross-coupling conditions, indicating likely conversion to an active PdNP species under catalytic conditions, in line with previous literature precedents. , Taken together, this work showed that, when information regarding catalyst speciation is known, this information can be exploited to tune catalyst activity.…”