(2010). The influence of electron delocalization upon the stability and structure of potential N-heterocyclic carbene precursors with 1,3-diaryl-imidazolidine-4,5-dione skeletons. New Journal of Chemistry, 34 (7), 1295-1308. doi:10.1039 Aiming to develop synthetic protocols for the preparation of N-heterocyclic carbenes (NHCs) with increased π-acceptor character featuring N-pentafluorophenyl substituents, the molecular 2-chloro-1,3-bis(pentafluorophenyl)imidazolidine-4,5-dione (1a) was isolated from the condensation reaction of the corresponding formamidine with o xalyl chloride, instead of the expected, ionic 1,3-10 bis(pentafluorophenyl)-4,5-dioxo-4,5-dihydro-3H-imidazolium chloride. The nature of the extraannular N-substituents does not appear to exercise a significant influence on the structure of the products of this reaction, as evidenced by the similar structures observed with 2,6-dimethylphenyl (1b) and 2,6-diisopropylphenyl (1c) N-substituted analogues. These formal adducts of NHCs with hydrogen chloride demonstrate reactivity akin to that expected of alkyl halides. For instance, 1,3,1',3'-tetrakis(2,6-dimethylphenyl)-[2,2']diimidazolidinyl-4,5,4',5'-tetraone (2b) is formed via the reductive coupling of two units of 1b, while 1,3-15 bis(2,6-diisopropylphenyl)-4,5-dioxoimidazolidin-2-yl acetate (3c) is formed as the result of a metathesis reaction with mercury acetate. Chloride abstraction results in the formation of unstable imidazolium-4,5-dione salts (4a-c) that decompose rapidly, except in the case of the kinetically-stabilized 1,3-bis(2,6-diisopropylphenyl)imidazolium-4,5-dione hexafluorophosphate 4c, which could be observed spectroscopically. All imidazolium-4,5-dione hexafluorophosphate salts decompose to neutral 2-fluoro-1,3-bis(aryl)imidazolidine-4,5-diones (5a-c) via fluoride abstraction, demonstrating a clear preference for a four-coordinate 20 geometry about the diaminocarbon center in these compounds. 2-Methoxy-1,3-di(aryl)imidazolidine-4,5-diones (6a-c) were also prepared and they proved to be highly stable, failing to undergo thermolysis and yield the free NHCs. Computational analyses revealed that the instability of NHCs with an oxalamide skeleton, as well as that of their precursors, imidazolium-4,5-diones, results from a π-framework which extends over both carbonyl moieties and gives rise to a very low energy LUMO, rendering the compounds in question highly electrophilic.25

A series of rhodium complexes, [Rh(cod)(NHC-F(x))(OH(2))] (cod = 1,5-cyclooctadiene; NHC = N-heterocyclic carbene), incorporating anionic N-heterocyclic carbenes with 2-tert-butylmalonyl backbones and 2,6-dimethylphenyl (x = 0), 2,6-difluorophenyl (x = 4), 2,4,6-trifluorophenyl (x = 6), and pentafluorophenyl (x = 10) N,N'-substituents, respectively, has been prepared by deprotonation of the corresponding zwitterionic precursors with potassium hexamethyldisilazide, followed by immediate reaction of the resulting potassium salts with [{RhCl(cod)}(2)]. These complexes could be converted to the related carbonyl derivatives [Rh(CO)(2)(NHC-F(x))(OH(2))] by displacement of the COD ligand with CO. IR and NMR spectroscopy demonstrated that the degree of fluorination of the N-aryl substituents has a considerable influence on the σ-donating and π-accepting properties of the carbene ligands and could be effectively used to tune the electronic properties of the metal center. The carbonyl groups on the carbene ligand backbone provided a particularly sensitive probe for the assessment of the metal-to-ligand π donation. The ortho-fluorine substituents on the N-aryl groups in the carbene ligands interacted with the other ligands on rhodium, determining the conformation of the complexes and creating a pocket suitable for the coordination of water to the metal center. Computational studies were used to explain the influence of the fluorinated N-substituents on the electronic properties of the ligand and evaluate the relative contribution of the σ- and π-interactions to the ligand-metal interaction.

Enhancement of water droplet evaporation by added infrared radiation was modeled and studied experimentally in a vertical laminar flow channel. Experiments were conducted on droplets with nominal initial diameters of 50 μm in air with relative humidities ranging from 0% to 90% RH. A 2800 nm laser was used with radiant flux densities as high as 4 × 105 W/m2. Droplet size as a function of time was measured by a shadowgraph technique. The model assumed quasi-steady behavior, a low Biot number liquid phase, and constant gas–vapor phase material properties, while the experimental results were required for model validation and calibration. For radiant flux densities less than 104 W/m2, droplet evaporation rates remained essentially constant over their full evaporation, but at rates up to 10% higher than for the no radiation case. At higher radiant flux density, the surface-area change with time became progressively more nonlinear, indicating that the radiation had diminished effects on evaporation as the size of the droplets decreased. The drying time for a 50 μm water droplet was an order of magnitude faster when comparing the 106 W/m2 case to the no radiation case. The model was used to estimate the droplet temperature. Between 104 and 5 × 105 W/m2, the droplet temperature changed from being below to above the environment temperature. Thus, the direction of conduction between the droplet and the environment also changed. The proposed model was able to predict the changing evaporation rates for droplets exposed to radiation for ambient conditions varying from dry air to 90% relative humidity.

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