Nanostructures in ionic liquids: correlation of iridiumnanoparticles’ size and shape with imidazolium salts’ structural organization and catalytic properties

Abstract

Hydrogen reduction of cationic or neutral Ir(I) compounds, namely [Ir(COD)₂]BF₄ and [Ir(COD)Cl]₂respectively. in the ionic liquid (IL) 1-alkyl-3-methylimidazolium tetrafluoroborate affords either irregularly sized spherical (from 1.9 ± 0.4 to 3.6 ± 0.9 nm) or worm-like metal nanoparticles, depending on the nature of the imidazolium alkyl group and the type of iridium precursor. The ionic Ir(I) precursor tends to be dissolved and concentrated on the IL polar domains (populated by the imidazolium nucleus and tetrafluoroborate anions) while the neutral precursor dissolves preferentially in the non-polar region of the IL (populated mainly by N-alkyl side chains). The size, or volume, of the nano-region where the Ir(I) precursor is dissolved and reduced, determines the size and, probably, the shape of the formed nanoparticles. The HR-TEM image shows that the Ir(0) with worm-like shape are polycrystalline and formed from aggregation individual “spherical” nanoparticles of around 1.9 nm. The catalytic activity of Ir(0) NPs on the hydrogenation of cyclohexene (0.01 mol L⁻¹ of Ir atoms in IL, 75 °C, 8 bar of H₂, 500 rpm stirring, 1/1000 Ir(0)/cyclohexene ratio) is always greater in C₁C₁₀I·BF₄ than C₁C₄I·BF₄, regardless of the nature of Ir(I) precursor. Moreover, the cyclohexene hydrogenations performed with Ir(0) nanocatalysts made from ionic Ir(I) precursor are approximately twice faster than those NPs obtained from the neutral Ir(I) precursor, in the same IL.