New heterobidentate N-heterocyclic carbene-triazolyl ligands and several of their palladium(II) complexes have been synthesized in a modular fashion using click chemistry. These complexes are the first examples where triazolyl-substituted NHCs exhibit bidentate behavior, which was confirmed by NMR and X-ray diffraction studies. The synthesis of the complexes could be achieved in relatively few steps by introducing the diversity at a late stage in the synthesis. The complexes are active precatalysts in the transfer semihydrogenation of alkynes to Z-alkenes, with activity and selectivity depending on the triazolyl substituent and the NHC nitrogen substituent. Selectivities as high as 99% were observed.

Direct evidence of the conformation of a Pd-N heterocyclic carbene (NHC) moiety imbedded in a hybrid material and of the Pd-NHC bond were obtained by dynamic nuclear polarization surface-enhanced NMR spectroscopy (DNP SENS) at natural abundance in short experimental times (hours). Overall, this silica-based hybrid material containing well-defined Pd-NHC sites in a uniform environment displays high activity and selectivity in the semihydrogenation of alkynes into Z-alkenes (see figure).

A convenient and easy-to-use protocol for the Z-selective transfer semihydrogenation of alkynes was developed, using ammonium formate as the hydrogen source and the easily prepared and commercially available, highly stable complex PdCl­(η3-C3H5)­(IMes) (1) as the (pre)­catalyst. Combined with triphenyl posphine as an additional ligand, this system provides a robust catalytic synthetic method that shows little to no over-reduction or isomerization after full substrate conversion. The system allows the direct use of solvents and reagents, as received from the supplier without drying or purification, thus providing a practical method for semihydrogenation of a broad range of alkynes. The mechanism behind these high and enhanced selectivities was determined through a set of kinetic experiments.

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 [PdII(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 [PdII(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.

Perovskite-type oxides, ABO(3), can be successfully applied as solid "oxygen reservoirs" in redox reactions such as selective hydrogen combustion. This reaction is part of a novel process for propane oxidative dehydrogenation, wherein the lattice oxygen of the perovskite is used to combust hydrogen selectively from the dehydrogenation mixture at 550 degrees C. This gives three key advantages: it shifts the dehydrogenation equilibrium to the side of the desired products, heat is generated, thus aiding the endothermic dehydrogenation, and it simplifies product separation (H(2)O vs H(2)). Furthermore, the process is safer since it uses the catalysts' lattice oxygen instead of gaseous O(2). We screened fourteen perovskites for activity, selectivity and stability in selective hydrogen combustion. The catalytic properties depend strongly on the composition. Changing the B atom in a series of LaBO(3) perovskites shows that Mn and Co give a higher selectivity than Fe and Cr. Replacing some of the La atoms with Sr or Ca also affects the catalytic properties. Doping with Sr increases the selectivity of the LaFeO(3) perovskite, but yields a catalyst with low selectivity in the case of LaCrO(3). Conversely, doping LaCrO(3) with Ca increases the selectivity. The best results are achieved with Sr-doped LaMnO(3), with selectivities of up to 93 % and activities of around 150 mumol O m(-2). This catalyst, La(0.9)Sr(0.1)MnO(3), shows excellent stability, even after 125 redox cycles at 550 degrees C (70 h on stream). Notably, the activity per unit surface area of the perovskite catalysts is higher than that of doped cerias, the current benchmark of solid oxygen reservoirs.

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.[a] Molecular Scheme 3. Synthesis of symmetrically substituted benzylic histidinium bromides. The Cyclic Urea ApproachSubsequently, we employed the cyclic urea approach, which allows the synthesis of dissymmetrically substituted histidinium salts and further tuning of the properties of the NHC metal complex. This is not straightforward because histidine has two tautomeric forms, which means that the δ-and the ε-nitrogen of the imidazole possess both imine and amine character (compound 4, Scheme 5). [20] Therefore, mixtures of regioisomers are obtained when the imidazole is reacted with one equivalent of an electrophile. To obtain the desired dissymmetrically substituted histidinium salts, we applied a route based on the report by Hodges and Chivikas, [17] and was improved by Brégeon et al. [30] (Scheme 4). Scheme 4. The cyclic urea route toward dissymmetrically substituted histidinium salts.This cyclic urea route induces regioselectivity because only the six-membered cyclic urea 5 can be formed. The second nitrogen atom can then be functionalized selectively through nucleophilic substitution. Subsequently, the R 1functionalized urea compounds can be ring-opened by reaction with an alcohol, which liberates the δ-nitrogen atom of the imidazole, and provides a carbamate-protected amine. The use of tBuOH affords the Boc-protected histidine, Eur. J. Inorg. Chem. 2015, 982-996

C 24 H 19 NO 3 PRh, triclinic, P¯ (no. 2), a = 9.1734(16) Å, b = 9.5433(18) Å, c = 12.342(2) Å, α = 92.400(6)°, β = 105.460 (5) CCDC no.: 1463131The gure shows the asymmetric unit of the title structure. Tables 1-3 contain details of the methods used and a list of the atoms including atomic coordinates and displacement parameters.

C7H4NO4Rh, monoclinic, P21/c (no. 14), a = 3.5780 (12)