Aggregation control of Ru and Ir nanoparticles by tunable aryl alkyl imidazolium ionic liquidsElectronic supplementary information (ESI) available: TAAIL synthesis and analysis, IL viscosity data, IR-spectra, TEM images with particle size histograms, SAEDs, details of -ePC-SAFT calculations, surface atom calculations and additional activity-time plots for catalysis. See DOI: 10.1039/c8nr10286d

Metal-nanoparticles (M-NPs) were synthesized in a wet-chemical synthesis route in tunable aryl alkyl ionic liquids (TAAILs) based on the 1-aryl-3-alkyl-substituted imidazolium motif from Ru 3 (CO) 12 and Ir 4 (CO) 12 by microwave-heating induced thermal decomposition. The size and size dispersion of the NPs were determined by transmission electron microscopy (TEM) to an average diameter of 2.2(±0.1) to 3.9(±0.3) nm for Ru-NPs and to an average diameter of 1.4(±0.1) to 2.4(±0.1) nm for Ir-NPs. The TAAILs used contain the same bis(trifluoromethylsulfonyl)imide anion but differ in the substituents on the 1-aryl ring, e.g. 2-methyl-, 4-methoxy- and 2,4-dimethyl groups and in the 3-alkyl chain lengths (C 4 H 9 , C 5 H 11 , C 8 H 17 , C 9 H 19 , C 11 H 23 ). All used TAAILs are suitable for the stabilization of Ru- and Ir-NPs over months in the IL dispersion. Different from all other investigations on M-NP/IL systems which we are aware of the particle separation properties of the TAAILs vary strongly as a function of the aryl substituent. Good NP separation can be achieved with the 4-methoxyphenyl- and 2,4-dimethylphenyl-substituted ILs, irrespective of the 3-alkyl chain lengths. Significant aggregation can be observed for 2-methylphenyl-substituted ILs. The good NP separation can be correlated with a negative electrostatic potential at the 4-methoxyphenyl or 4-methylphenyl substituent that is in the para -position of the aryl ring, whereas the 2-( ortho -)methylphenyl group assumes no negative potential. -ePC-SAFT calculations were used to validate that the interactions between ILs and the washing agents (required for TEM analyses) do not cause the observed aggregation/separation behaviour of the M-NPs. Ru-NPs were investigated as catalysts for the solvent-free hydrogenation of benzene to cyclohexane under mild conditions (70 °C, 10 bar) with activities up to 760 (mol cyclohexane) (mol Ru) −1 h −1 and over 95% conversion in ten consecutive runs for Ru-NPs. No significant loss of catalytic activity could be observed. Small change-large effect: An added para-methyl group on the aryl ring in the 1-aryl-3-alkyl-imidazolium motif of TAAILs leads to well-separated metal nanoparticles.