Les pratiques enseignantes et leurs effets sur les apprentissages des élèves

The chemistry of Au­(I) complexes with two types of cyclic (alkyl)­(amino)­carbene (CAAC) ligands has been explored, using the sterically less demanding dimethyl derivative ^Me2CAAC and the 2-adamantyl ligand ^AdCAAC. The conversion of (^AdCAAC)­AuCl into (^AdCAAC)­AuOH by treatment with KOH is significantly accelerated by the addition of <i>t</i>BuOH. (^AdCAAC)­AuOH is a convenient starting material for the high-yield syntheses of (^AdCAAC)­AuX complexes by acid/base and C–H activation reactions (X = OAryl, CF₃CO₂, N­(Tf)₂, C₂Ph, C₆F₅, C₆HF₄, C₆H₂F₃, CH₂C­(O)­C₆H₄OMe, CH­(Ph)­C­(O)­Ph, CH₂SO₂Ph), while the cationic complexes [(^AdCAAC)­AuL]⁺ (L = CO, CN^<i>t</i>Bu) and (^AdCAAC)­AuCN were obtained by chloride substitution from (^AdCAAC)­AuCl. The reactivity toward variously substituted fluoroarenes suggests that (^AdCAAC)­AuOH is able to react with C–H bonds with p<i>K</i>_a values lower than about 31.5. This, together with the spectroscopic data, confirm the somewhat stronger electron-donor properties of CAAC ligands in comparison to imidazolylidene-type N-heterocyclic carbenes (NHCs). In spite of this, the oxidation of ^Me2CAAC and ^AdCAAC gold compounds is much less facile. Oxidations proceed with C–Au cleavage by halogens unless light is strictly excluded. The oxidation of (^AdCAAC)­AuCl with PhICl₂ in the dark gives near-quantitative yields of (^AdCAAC)­AuCl₃, while [Au­(^Me2CAAC)₂]Cl leads to <i>trans</i>-[AuCl₂(^Me2CAAC)₂]­Cl. In contrast to the chemistry of imidazolylidene-type gold NHC complexes, oxidation products containing Au–Br or Au–I bonds could not be obtained; whereas the reaction with CsBr₃ cleaves the Au–C bond to give mixtures of [^AdCAAC-Br]⁺[AuBr₂]⁻ and [(^AdCAAC-Br)]⁺ [AuBr₄]⁻, the oxidation of (^AdCAAC)­AuI with I₂ leads to the adduct (^AdCAAC)­AuI·I₂. Irrespective of the steric demands of the CAAC ligands, their gold complexes proved more resistant to oxidation and more prone to halogen cleavage of the Au–C bonds than gold­(I) complexes of imidazole-based NHC ligands

Differentially Substituted Acyclic Diaminocarbene Ligands Display Conformation-Dependent Donicities

Complexes of the type [(L)Ir(COD)Cl] and [(L)Ir(CO)(2)Cl] (L = N,N&apos;-dimesityl-N,N&apos;-dimethylformamidin-2-ylidene (3) and N,N&apos;-bis(2,6-di-isopropylphenyl)-N,N&apos;-dimethylformamidin-2-ylidene (4); COD = cis,cis-1,5-cyclooctadiene) were synthesized and studied in solution as well as in the solid state. While the acyclic diaminocarbene (ADC) ligand in [(3)Ir(COD)Cl] adopted a conformation in which the N-aryl substituents were anti with respect to the coordinated metal, the respective Ir carbonyl complex was prepared as separable isomers ([(anti-3)Ir(CO)(2)Cl] and [(amphi-3)Ir(CO)(2)Cl]). The ADC ligands in [(4)Ir(COD)Cl] and [(4)Ir(CO)(2)Cl] adopted exclusively amphi conformations, where one N-aryl substituent was oriented toward the coordinated metal and the other was oriented away. The Tolman electronic parameter (TEP) for anti-3 (2047.8 cm(-1)) was derived from the carbonyl stretching energy (v(CO)) of the aforementioned Ir(CO)(2)Cl complex and was found to be larger than the TEPs calculated for amphi-3 (2044.4 cm(-1)) and 4(2044.0 cm(-1)). Likewise, the oxidation potential of [(anti-3)Ir(CO)(2)Cl], as measured by cyclic voltammetry, was found to be significantly higher (1.57 V) than the analogous oxidation potentials measured for [(amphi-3)Ir(CO)(2)Cl] (1.26 V) and [(4)Ir(CO)(2)Cl] (1.24 V)