In one word, how would you describe your research?Dynamic.
How did this research begin?During our work on dynamic processes in Pd/NHC catalytic systems, one of the key techniques we utilized was mass spectrometry.E lectrospray ionization mass spectrometry was applied to understand possible reduction pathways of the studied Pd/NHC complexes. Surprisingly,s everal NHC-free species were detected, unstabilized palladium hydride being one of them. This elusive intermediate caught our attention and we delved into the study of the observed phenomena.What is the most significant result of this study?Formation of palladium complexes, non-stabilized by direct metal bonding with strong NHC ligand, makes av ery important point. Dynamic nature of the catalytic system allows ligand-free ionic palladium complexes to be formed. These ionic species do not agglomerate while being in Pd 0 oxidation state and, notably,t hey can be isolated from the reaction mixture and recycled.
What was the inspiration for this cover design?Thinking about an artwork that might describe dynamic nature of the studied system, we wanted to highlight the ease with which the catalytic system can transform. This brought us to the sketch of palladium complexes on as eesaw.I nt he image we created, alternative palladium complexes play together as friends, seesawed easily up and down by the possibility of dynamic transformations.Invited for the cover of this issue are ValentineP .A nanikov and co-workers. The image depicts the dynamic behaviour of a Pd/NHCc atalytic systemw ith easy transition from molecular to ionic complex. Read the fullt ext of the article at
The product of a revealed transformation—NHC‐ethynyl coupling—was observed as a catalyst transformation pathway in the Sonogashira cross‐coupling, catalyzed by Pd/NHC complexes. The 2‐ethynylated azolium salt was isolated in individual form and fully characterized, including X‐ray analysis. A number of possible intermediates of this transformation with common formulae (NHC)nPd(C2Ph) (n=1,2) were observed and subjected to collision‐induced dissociation (CID) and infrared multiphoton dissociation (IRMPD) experiments to elucidate their structure. Measured bond dissociation energies (BDEs) and IRMPD spectra were in an excellent agreement with quantum calculations for coupling product π‐complexes with Pd0. Molecular dynamics simulations confirmed the observed multiple CID fragmentation pathways. An unconventional methodology to study catalyst evolution suggests the reported transformation to be considered in the development of new catalytic systems for alkyne functionalization reactions.
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.
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