The question of whether palladium nanoclusters are the actual catalysts in the so-called cluster -catalyzed Sonogashira cross-coupling is investigated, using the coupling of phenylacetylene with 4-bromobenzonitrile as a model reaction. By combining a detailed kinetic analysis with transmission electron microscopy (TEM), we show that a soluble species must be present in the system when Pd nanoclusters are used as catalysts. Various Pd clusters show similar kinetic profiles to that of a homogeneous Pd(dba) 2 complex. Most importantly, TEM analysis of samples taken before, during, and after the reaction shows that the cluster size decreases during the reaction. Based on these findings, we present a possible two-path mechanism for Sonogashira cross-coupling reactions in the presence of Pd nanoclusters.

The power of natural selection through survival of the fittest is nature's ultimate tool for the improvement and advancement of species. To apply this concept in catalyst development is attractive and may lead to more rapid discoveries of new catalysts for the synthesis of relevant targets, such as pharmaceuticals. Recent advances in ligand synthesis using combinatorial methods have allowed the generation of a great diversity of catalysts. However, selection methods are few in number. We introduce a new selection method that focuses on the stability of catalytic intermediates measured by mass spectrometry. The stability of the intermediate relates inversely to the reactivity of the catalyst, which forms the basis of a catalyst-screening protocol in which less-abundant species represent the most-active catalysts, 'the survival of the weakest'. We demonstrate this concept in the palladium-catalysed allylic alkylation reaction using diphosphine and IndolPhos ligands and support our results with high-level density functional theory calculations.

We herein report on the application and structural investigation of a new set of complexes that contain bidentate N-heterocyclic carbenes (NHCs) and primary amine moieties of the type [M(arene)Cl(L)] [M=Ru, Ir, or Rh; arene=p-cymene or pentamethylcyclopentadienyl; L=1-(2-aminophenyl)-3-(n-alkyl)imidazol-2-ylidine]. These complexes were tested and compared in the hydrogenation of acetophenone with hydrogen. Structural variations in the chelate ring size of the heteroditopic ligand revealed that smaller chelate ring sizes in combination with ring conjugation in the ligand are beneficial for the activity of this type of catalyst, favoring an inner-sphere coordination pathway. Additionally, increasing the steric bulk of the alkyl substituent on the NHC aided the reaction, showing almost no induction period and formation of a more active catalyst for the n-butyl complex relative to complexes with smaller Me and Et substituents. As is common in hydrogenation reactions, the activity of the complexes decreases in the order Ru>Ir>Rh. The application of [Ru(p-cym)Cl(L)]PF6 , which outperforms its reported analogues, has been successfully extended to the hydrogenation of more challenging biomass-inspired substrates.

The development of a novel set of complexes bearing an NHC-amine ligand (C NHC -NH 2 ) is described. M(cod) complexes (M = Ir, Rh) and a Ru complex have been synthesized in which three different coordination modes of the ligand were established: monodentate, neutral bidentate, and anionic bidentate. The anionic bidentate coordination mode of the anionic C NHC -NH − ligand arises from deprotonation of the amine moiety of the neutral C NHC -NH 2 ligand. Ligand deprotonation proved to be reversible for the Rh and Ir complexes, as was shown by subsequent treatment of the complexes with base and acid. The structural parameters of these differently coordinated ligands were examined, and it was shown that the conjugation of the aniline ring plays a major role in determining the ligand properties. ■ INTRODUCTIONThe use of carbenes as ligands is well developed in the field of transition-metal complex chemistry.1 In particular, the Nheterocyclic carbene (NHC) class of imidazol-2-ylidines, first reported in the 1960s, 2 have received their fair share of attention. Many successful examples of the application of NHCs in transition-metal-catalyzed transformations such as hydrogenations and metathesis reactions 3−5 have been reported. We recently reported on the synthesis of a conjugated NHC-amine ligand species where an NHC is conjugated with an aniline (C NHC -NH 2 ). 6 In this species, the primary amine is tethered to the NHC via an aromatic ring. The amine moiety of our C NHC -NH 2 ligand A (see Figure 1) can coordinate in a neutral or in an anionic fashion (when deprotonated). Monoanionic (bidentate) ligands have found widespread application in homogeneous catalysis. The most recognized are (pentamethyl)cyclopentadienyl (Cp, Cp*), acetylacetonate (acac), and diketimine (nacnac, where the two oxygen atoms of acac have been replaced by nitrogen-based moieties of the form NR). 7,8 Well-known examples of bifunctional ligands are ligands containing hydroxyl or oxime 9 groups and pincer-type ligands containing an anionic donor arm. 10,11 Previous work by Morris, involving a benzylic amine tethered to an NHC, has shown the ability of this ligand scaffold to display cooperativity, but this depended greatly on other factors such as the ancillary ligand (p-cym vs Cp*) and solvent assistance.12 Morris also explored the potential of iridium(I)-cod complexes containing neutral bidentate NHC-benzylic amine ligands, coordinated as seven-membered chelate rings to the metal center. Anilinebased C NHC -NH 2 ligands, forming neutral complexes containing conjugated anionic C NHC -NH − ligands bound to Rh, Ir, or