The new air-stable and moisture-insensitive Ir catalysts for efficient transfer hydrogenation of ketones contain a chelating bis(N-heterocyclic carbene) ligand. Most other hydrogen transfer catalysts show activity Rh > Ir, but we find Ir > Rh for these cases. Tuning of the ligand wingtip substituents, R, can greatly increase catalyst activity (R = neopentyl) or selectivity (R = isopropyl). Reactivity studies and isotopic labeling are consistent with a monohydride mechanism for the hydrogen transfer.

Chelating bis(imidazolium) salts having (CH 2 ) n chains of different lengths (n ) 1-4) linking the diazole rings show very large reactivity differences on metalation with [(cod)RhCl] 2 . Long linkers favor a square-planar Rh(I) product, while short linkers favor octahedral Rh(III). We ascribe the origin of the effect to the restricted rotation of the highly sterically anisotropic diazole rings and the different preferred orientations of these rings as n changes. Defining the x and y axes as the Rh-carbene bond directions, we find that with short linkers the diazole rings tend to be oriented close to the xy plane. This tends to favor Rh(III) because these complexes, [Rh(bis-carbene)I 2 (OAc)], have the lowest steric hindrance in the xy plane. With long linkers, the diazole rings tend to be aligned face to face along the (z axis. This tends to favor Rh(I) because these complexes, [(cod)Rh(bis-carbene)]PF 6 , have the lowest steric hindrance along the (z axis. Crystallographic studies are reported. Electrospray MS data provide evidence for strong metal-carbene binding.

Changing the counteranion along the series Br, BF4, PF6, SbF6 in their ion-paired 2-pyridylmethyl imidazolium salts causes the kinetic reaction products with IrH5(PPh3)2 to switch from chelating N-heterocyclic carbenes (NHCs) having normal C2 (N path) to abnormal C5 binding (AN path). Computational work (DFT) suggests that the AN path involves C-H oxidative addition to Ir(III) to give Ir(V) with little anion dependence. The N path, in contrast, goes by heterolytic C-H activation with proton transfer to the adjacent hydride. The proton that is transferred is accompanied by the counteranion in an anion-coupled proton transfer, leading to an anion dependence of the N path, and therefore of the N/AN selectivity. The N path goes via Ir(III), not Ir(V), because the normal NHC is a much less strong donor ligand than the abnormal NHC. PGSE NMR experiments support the formation of ion-pair in both the reactants and the products. 19F,1H-HOESY NMR experiments indicate an ion-pair structure for the products that is consistent with the computational prediction (ONIOM(B3PW91/UFF)).

Some neopentyl-substituted NHC iridium complexes catalyze the reduction of a variety of aldehydes and even enolizable aldehydes by hydrogen transfer from refluxing 2-propanol with alkali-metal carbonate bases. Imidazole and 1,2,4-triazole complexes had comparable activities. The aldehydes could also be reduced more slowly using a mild carbonate base and no iridium complex.

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In this article, the strategies and techniques used to successfully teach advanced inorganic chemistry, in the lecture and laboratory, to a legally blind student are described. At Fairfield University, these separate courses, which have a physical chemistry corequisite or a prerequisite, are taught for junior and senior chemistry and biochemistry majors. A student earns a separate grade in each the lecture (three credits) and the laboratory course (two credits). An overview of the course topics is given, followed by general accommodations and specific approaches that were used. Student assistants were very helpful and provided extra support for the blind student. Student assistants were utilized for the laboratory course, problem sets, and exams. Specific examples and detailed explanations of approaches that were helpful to the legally blind student throughout the entire course are provided. The legally blind student benefited from extensive, verbal description of complexes, figures, and diagrams. In addition, the student benefited from tactile description of figures and models. The student assistants and extra office hours were essential for the blind student to succeed and excel in advanced inorganic chemistry. The approaches discussed in this paper are the product of immediate and continual feedback from the student over the course of the semester. The student would frequently comment after class that he followed the lesson or was confused, and the latter comment elicited experimentation with different approaches.