We have studied the use of amino acid histidine as a precursor for N-heterocyclic carbene (NHC) ligands. This natural amino acid possesses an imidazole substituent, which makes it an interesting NHC precursor that contains both an acid and an amino functionality. These functionalities may be used for further tuning of NHC complexes. We have developed routes for the synthesis of symmetric and dissymmetric alkyl, benzyl, and aryl-substituted histidinium salts. Subsequently, the corresponding Ag and Pd histidylidenes were synthesized and the palladium complexes were tested in the Z-selective transfer semihydrogenation of alkynes. Histidylidene palladium complexes that contain additional donor functionalities were found to display good selectivities. The best catalytic results were obtained with a Pd-histidylidene complex that contains two picolyl functional groups.

Despite the prolific use of (di-)NHC complexes in homogeneous catalysis, there are relatively few reports on their successful application in asymmetric transformations. In this work the atropisomeric binaphthyl backbone was combined with readily obtainable 1,2,3-triazolylidenes to develop a strongly electron-donating C₂-symmetric ligand. The ligand was efficiently synthesized in a three-step procedure in an overall yield of 91 % starting from commercially available materials. Strategies for the synthesis of the corresponding di-NHC silver(I), palladium(II), rhodium(I), and iridium(I) complexes have been developed. The rhodium(I) complex was employed in the catalytic asymmetric hydrosilylation of ketones, providing good conversions at catalyst loadings as low as 0.2 mol-% and giving chiral inductions of up to 51 % ee.

A protocol was developed to distinguish between well-defined molecular and nanoparticle-based catalysts for the Pd-catalyzed semihydrogenation reaction of alkynes to Z-alkenes. The protocol applies quantitative partial poisoning and dynamic light scattering methods, which allow the institution of additional validation experiments. For the quantitative partial poisoning method, tetramethylthiourea (TMTU) was developed as an alternative for the standard poison ligand CS₂, and was found to be superior in its applicability. The protocol and the TMTU poison ligand were validated using the well-described [Pd^II(phenanthroline)]-catalyzed copolymerization of styrene and CO, confirming that this system is clearly operating as a well-defined molecular catalyst. The protocol was subsequently applied to three catalyst systems used for the semihydrogenation of alkynes. The first was proposed to be a molecular [Pd⁰(IMes)] catalyst that uses molecular hydrogen, but the data gathered for this system, following the new protocol, clearly showed that nanoparticles (NPs) are catalytically active. The second catalyst system studied was an N-heterocyclic carbene (NHC) Pd system for transfer semihydrogenation using formic acid as the hydrogen source, which was proposed to operate through an in situ generated molecular [Pd⁰(IMes)] catalyst in earlier studies. The investigations showed that only a small fraction of the Pd added becomes active in the catalytic reaction and that NPs are formed. However, despite these findings, a clear distinction between catalytic activity of NPs versus a molecular catalyst could not be made. The third investigated system is based on a [Pd^II(IMes)(η³-allyl)Cl] precatalyst with additive ligands. The combined data gathered for this system are multi-interpretable, but suggest that a partially deactivated molecular catalyst dominates in this reaction.

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 synthesis of an air-stable series of Pd⁰ complexes with unsymmetric bidentate N-pyridine N-heterocyclic carbene ligands has been described. The carbenes were generated by synthesis of the silver(I) complexes from the imidazolium salts, followed by transmetallation of the C-N ligands to obtain the target electron-rich zerovalent palladium compounds. The bidentate coordination behaviour of the ligands was confirmed by ¹H, ¹³C NMR and X-ray spectroscopy. The complexes are active precatalysts for the selective transfer semihydrogenation of alkynes to Z-alkenes, with selectivities up to 99%. Copyright © 2011 John Wiley & Sons, Ltd.

We present a straightforward protocol for making (phosphanyl-carbene)Pd^II complexes. These complexes have bidentate ligands containing an acyclic diamino- or aminooxy-carbene and a phosphane. The synthesis gives good yields (typically 70–90 %) for a variety of complexes (22 compounds). Moreover, it does not require the synthesis of imidazolium salts nor the a priori generation of free carbenes. Three of the new complexes were tested as catalysts for Sonogashira and Hay coupling reactions, with good yields and selectivities. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)