ABSTRACT
Abstract Novel ionic liquid formylating agents were synthesized from alkylmethylimidazolium (RMIM+) chlorides and AlCl3. These intrinsically Lewis acids containing excess AlCl3 absorbed HCl to develop strong Brønsted acidity. Modeling by semi-empirical methods showed that HCl experienced three, local energy minimum positions in the IL. Ab initio predictions of the 1H-NMR spectra using Hartree-Fock methods showed that the three sites for the absorbed HCl had different chemical shifts (CSs). Additional modeling of the Al-species (27Al-NMR) showed that two distinct dinuclear aluminum chloride anions, having different CS’s could be stabilized in the IL. Indirect acidity measurements of the HCl protons by 13C-NMR of 2-13C-acetone confirmed the predictions of multiple, strong acid sites in the IL. Subsequent measurements of the 27Al-NMR confirmed the existence of two Al species in the intrinsically-Lewis acidic IL’s and most remarkable, the concentrations of these two species depended upon the length of the alkyl groups, R, present in RMIM+. The pseudo, first-order rate constants observed for the toluene carbonylation reaction decreased systematically in ILs having increasing chain length of R in a way that could be easily correlated by the mole fraction of the dinuclear Al chloride species: [Cl3AlClAlCl3]- observed by 27Al-NMR. Introduction Ionic liquids (IL's) are substances that form liquids at room temperature and lower at atmospheric pressure (1). The very hydrophilic, chloroaluminate Ils, O+[AlnCl3n+1], are said to exhibit Lewis acidity when n > 1 (2, 3) where O+ is the organic cation. These IL’s may be prepared using organic cations such as the pyridinium or the substituted imidazolium cations. Recently, it was reported that strong, Brønsted acidity could be created in EMIM+(Al2Cl7)- upon exposure to dry HCl (4). Moreover, it was shown (5) that arene carbocations were formed when combined with IL’s derived from trimethylsulfonium bromide-AlCl3/AlBr3 and exposed to HBr gas. We showed how a combination of HCl with EMIM+(Al2Cl7)- resulted in a potent conversion agent for toluene carbonylation (6).
