The mystery of how 1,3-substituted imidazolium-based ionic liquids (ILs) can provide high stabilization for transition-metal(0) nanoclusters, that is, in the absence of the usual strongly coordinating anions, has been probed. 2H NMR product and kinetic studies of 1,3-substituted imidazolium ILs under D2 reveal that nanocluster-catalyzed H/D exchange occurs at the 2- (as well as at the 4-, 5-, and 8-) C-H positions of the imidazolium cation. The results (i) provide compelling evidence that N-heterocyclic carbene formation and ligation of nanoclusters is occurring in ILs; and (ii) argue that N-heterocyclic carbenes merit further investigation as heretofore unappreciated stabilizers of transition-metal nanoclusters.

Following a brief introduction to the nanocluster stabilization literature and DLVO (Derjaugin-Landau-Verwey-Overbeek) theory of colloidal stability, F-, CI-, Br-, and I- are evaluated for their efficacy in the formation and stabilization of prototype Ir(O)n nanoclusters prepared from a [(1,5-COD)lr(CH3CN)2][BF4] precursor in both acetone and propylene carbonate solvent. First, under conditions utilized previously for establishing an anion stabilization series ("Standard Conditions," 1.2 mM Ir precursor concentration at 22 degrees C in acetone solvent), the 5 criteria developed in 2002 for ranking nanocluster stabilizers are evaluated for each halide (each with 1 equiv BF4 present from the Ir precursor). Under Standard Conditions, bulk metal is the final product (i.e., no stable nanoclusters) in the presence of each of the four halides, as well as for BF4 in the absence of any halide. Next, each halide, again in the presence of 1 equiv BF4, is evaluated under "Improved Conditions" (0.24 mM Ir precursor concentration at 60 degrees C in propylene carbonate solvent), propylene carbonate being known to be a preferred nanocluster solvent in the presence of anionic (electrostatic) stabilizers. Nanocluster syntheses under the Improved Conditions did, as expected, yield Ir(O)n nanoclusters for each of the four halide plus BF4 systems as well as BF4 alone, although none of these nanoclusters are isolable from solution. Importantly, even the traditionally weakly coordinating BF4 is shown to contribute significantly to nanocluster stability in the high dielectric constant solvent propylene carbonate. Hence, the importance of anions in conjunction with a high dielectric constant solvent for nanocluster formation and stabilization is illustrated.