N-Heterocyclic Carbenes in Late Transition Metal Catalysis

We present a free web application for the calculation of the buried volume (% VBur) of NHC ligands. The web application provides a graphic and user-friendly interface to the SambVca program, developed for the calculation of % VBur values not only of NHC ligands but also of other classic organometallic ligands such as, for example, phosphanes and cyclopentadienyl-based ligands. To provide a reliable procedure for the calculation of % VBur values we tested our approach in the interpretation of the binding energies of NHC ligands in Cp*Ru(NHC)Cl complexes in terms of steric and electronic parameters.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

SambVca: A Web Application for the Calculation of the Buried Volume of N‐Heterocyclic Carbene Ligands

Determination of N-Heterocyclic Carbene (NHC) Steric and Electronic Parameters using the [(NHC)Ir(CO)2Cl] System

Catalytic homogeneous asymmetric hydrogenations of largely unfunctionalized alkenes.

This review describes the stabilization of various coordinatively unsaturated metal centers through the incorporation of N-heterocyclic carbene (NHC) ligands. Such species are more thermally stable compared to the more ubiquitous tertiary phosphane systems. Although NHCs can be considered as phosphane mimics it has become apparent that there are substantial differences between the two ligand families. In general, NHC ligands are much more electron-donating and sterically demanding than bulky phosphane ligands. We also discuss the various thermochemical and infrared studies of metal carbene species that have provided insight into NHC ligands properties, and at last allowed meaningful comparison to tertiary phosphane ligands. While NHCs are generally viewed as strong binding low reactive ligands, in some instances they have been found to be not so innocent and can undergo facile intramolecular C–H activation as well as abnormal ligand binding [C-5(4) vs. C-2]. Here we highlight such reactions with late-transition metal centers that have allowed the isolation of various unsaturated LTM species. These have so far eluded isolation in analogous phosphane-based systems. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

Stabilization of Organometallic Species Achieved by the Use of N‐Heterocyclic Carbene (NHC) Ligands

Catalytic olefin hydrogenation using N-heterocyclic carbene–phosphine complexes of iridium

The search for new hydrogenation catalyst motifs based on N-heterocyclic carbene ligands

Caffeine is a hydrate-forming polymorphic crystalline compound that can exist in α, β, and hydrate forms. Phase transitions between hydrate and anhydrous forms of a crystalline ingredient, and related water migration, can create product quality challenges. The objective of this study was to determine the relative humidity (RH)-temperature phase boundary between anhydrous β-caffeine and caffeine hydrate. The β-caffeine→caffeine hydrate and caffeine hydrate→β-caffeine RH-temperature transition boundaries were determined from 20 to 45 °C using a combination of water activity (aw ) controlled solution and vapor-mediated equilibration, moisture sorption, powder X-ray diffraction, and Fourier-transform infrared spectroscopy techniques. Two transition boundaries were measured: the β-caffeine→caffeine hydrate transition boundary (0.835 ± 0.027 aw at 25 °C) was higher than the caffeine hydrate→β-caffeine transition boundary (0.625 ± 0.003 aw at 25 °C). Moisture sorption rates for β-caffeine, even at high RHs (>84% RH), were slow. However, caffeine hydrate rapidly dehydrated at low RHs ( 65% RH) at lower temperatures (20 to 30 °C) resulted in full restoration to a 4/5 caffeine hydrate. This transitional anhydrous state was unstable and converted to a less hygroscopic state after annealing at 50 °C and 0% RH for 1 day. It was postulated that the caffeine hydrate→β-caffeine was the true β-caffeine↔caffeine hydrate phase boundary and that β-caffeine could be metastable above the caffeine hydrate→β-caffeine transition boundary. These caffeine RH-temperature transition boundaries could be used for selecting formulation and storage conditions to maintain the desired caffeine crystalline form. PRACTICAL APPLICATION: Caffeine can exist as either an anhydrous (without water) or hydrate (internalized water) crystalline state. The stability of each caffeine crystalline form is dictated by humidity (or water activity) and temperature, and these environmental stability boundaries for the caffeine crystalline forms are reported in this manuscript. Conversions between the two crystalline states can lead to deleterious effects; for example, the presence of caffeine hydrate crystals in a low water activity food (e.g., powder) could lead to the relocation of the water in caffeine to other ingredients in the food system, leading to unwanted water-solid interactions that could cause clumping and/or degradation.

Relative humidity-temperature transition boundaries for anhydrous β-caffeine and caffeine hydrate crystalline forms.

N-heterocyclic carbene–phosphine complexes of iridium have been synthesized and examined for their performance in the catalytic homogeneous hydrogenation of a range of olefins; the reaction was further explored using para-hydrogen induced polarization (PHIP) 1H NMR.

Bridget T. Owens

Papers

Catalytic olefin hydrogenation using N-heterocyclic carbene–phosphine complexes of iridium

The search for new hydrogenation catalyst motifs based on N-heterocyclic carbene ligands

Relative humidity-temperature transition boundaries for anhydrous β-caffeine and caffeine hydrate crystalline forms.

Catalytic Olefin Hydrogenation Using N‐Heterocyclic Carbene—Phosphine Complexes of Iridium.