Addressing Reversibility of R–NHC Coupling on Palladium: Is Nano-to-Molecular Transition Possible for the Pd/NHC System?

Abstract

It has recently been shown that palladium-catalyzed reactions with N-heterocyclic carbene (NHC) ligands involve R− NHC coupling accompanied by transformation of the molecular catalytic system into the nanoscale catalytic system. An important question appeared in this regard is whether such a change in the catalytic system is irreversible. More specifically, is the reverse nanoto-molecular transformation possible? In view of the paramount significance of this question to the area of catalyst design, we studied the capability of 2-substituted azolium salts to undergo the breakage of C−C bond and exchange substituents on the carbene carbon with corresponding aryl halides in the presence of Pd nanoparticles. The study provides important experimental evidence of possibility of the reversible R−NHC coupling. The observed behavior indicates that the nanosized metal species are capable of reverse transition to molecular species. Such an option, known for phosphine ligands, was previously unexplored for NHC ligands. The present study for the first time demonstrates bidirectional dynamic transitions between the molecular and nanostructured states in Pd/NHC systems. As a unique feature, surprisingly small activation barriers (<18 kcal/mol) and noticeable thermodynamic driving force (−5 to −7 kcal/mol) were calculated for C−C bond oxidative addition to Pd(0) centers in the studied system. The first example of NHCmediated Pd leaching from metal nanoparticles to solution was observed and formation of Pd/NHC complex in solution was detected by ESI-MS.

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).…”

The key role of R–NHC coupling (R = C, H, heteroatom) and M–NHC bond cleavage in the evolution of M/NHC complexes and formation of catalytically active species

“…Reversibility of the C–NHC coupling was confirmed experimentally on an example of the CH 3 –NHC bond activation catalyzed by palladium nanoparticles. 61 Bulky N-substituents in NHC ligands usually increase the activation barriers; 58 however, the effects of bulkiness can be more complex, as a significant increase in steric bulkiness can induce dissociation of the stabilizing co-ligands. For example, DFT calculations of Ph–NHC coupling in (NHC)Pd(Ph)(I)DMF complexes predicted lower Δ E ≠ for the bulky IPr ligand (19.2 kcal mol –1 ) than for the non-bulky IMe ligand (20.9 kcal mol –1 ) owing to the splitting of DMF molecule from the complex with IPr ligand.…”

“…The heating of [NHC–Me] + salt with aryl halides in the presence of palladium nanoparticles, copper or nickel salts leads to formation of [NHC–Ar] + X – salts, that is a vivid example of the C–C bond oxidative addition-driven leaching. 61 …”

Mechanistic Study of Pd/NHC‐Catalyzed Sonogashira Reaction: Discovery of NHC‐Ethynyl Coupling Process

“…[55] Under regularr eaction conditions, ligandless palladium species should agglomerate and form nanoparticles. [40,56,57] Thereby, we have studied both reaction conditions:( i) With Cs 2 CO 3 as base at 80 8Cand (ii)with Et 3 Na sb ase at r.t. nanoparticles were clearly detected as well as some aggregates, which are usually expected for ad ynamic system in solution. When the mixture was heatedu pi nt he presenceo f cesium carbonate the mean diameter of palladium nanoparticles was 3.2 AE 0.8 nm, whileu nder mild conditions smaller 0.9 AE 0.3 nm nanoparticles were observed.U nder studied conditions, NHC-ethynyl and NHC-Ph couplings mayc ontributet o Pd nanoparticles formation.A lthough it is difficult to determine which process contributes more, the observed formation of nanoparticles is in good agreementw itht he conclusions made about the nature of the studied catalytic system.…”