Synthesis of Tri-aryl Imidazolium Tethered Ligands for Catalysis within Ionic Liquid Media
Richard J. Rosso, Department of Chemistry, St. John’s College of Liberal Arts and Sciences
N. Nawaz, Z. Aranbayev, J. Singavaropu, N. Harsouni, and C. Gandham
Abstract
One of the most common methods of fine-tuning any catalytic reaction is to alter the ligands around the metal complex. Leeching of the metal complex into the organic media is a common problem for reactions conducted within these systems. It has been recently shown in the literature that ligands having a positively charged moiety remain within the ionic liquid media better than their neutral and anionic analogs. However, the rate of these reactions is still slower than heterogeneous analogs since much of the reaction chemistry only occurs at the interface between the ionic and organic phases. We have synthesized and fully characterized several imidazolium analogs of diphenylalkylphosphines with different carbon chain lengths between the phosphorus atom and the imidazolium group(s). The separation of the cationic group from the phosphine by a large carbon chain will result in an increase in substrate solubility and may promote self-assembly of the phosphines into a micellular or monolayer structures increasing catalytic activity. Such effects have been reported with analogous anionic phosphine compounds within aqueous media systems. We intend to determine the ideal chain length of these catalysts with a variety of olefins in hydrogenation and hydroformylation reactions. This work was initiated early last year and a manuscript detailing the synthesis of these compounds is in its final revision. We are currently exploring a new synthetic method to produce tri-aryl substituted ligands with similar groups tethered on one or more of the aryl rings. Such ligands would have similar electronic and steric properties to triphenyl-phosphine, and therefore show promise to be highly efficient.