Iron Fluoroanions and Their Clusters by Electrospray Ionization of a Fluorinating Ionic Liquid

Abstract

Metal fluoroanions are of significant interest for fundamental structure and reactivity studies and for making isotope ratio measurements that are free from isobaric overlap. Iron fluoroanions [FeF 4 ] - and [FeF 3 ] - were generated by electrospray ionization of solutions of Fe(III) and Fe(II) with the fluorinating ionic liquid 1-ethyl-3-methylimidazolium fluorohydrogenate [EMIm] + [F(HF) 2.3 ] -. Solutions containing Fe(III) salts produce predominately uncomplexed [FeF 4 ] - in the negative ion spectrum, as do solutions containing salts of Fe(II). This behavior contrasts with that of solutions of FeCl 3 and FeCl 2 (without [EMIm] + [F(HF) 2.3 ] -) that preserve the solution-phase oxidation state by producing the gas-phase halide complexes [FeCl 4 ] - and [FeCl 3 ] -, respectively. Thus, the electrospray-[EMIm] + [F(HF) 2.3 ] - process is oxidative with respect to Fe(II). The positive ion spectra of Fe with [EMIm] + [F(HF) 2.3 ] - displays cluster ions having the general formula [EMIm] + (n+1) [FeF 4 ] - n, and DFT calculations predict stable complexes, both of which substantiate the conclusion that [FeF 4 ] - is present in solution stabilized by the imidazolium cation. The negative ion ESI mass spectrum of the Fe-ionic liquid solution has a very low background in the region of the [FeF 4 ] - complex, and isotope ratios measured for both [FeF 4 ] - and adventitious [SiF 5 ] - produced values in close agreement with theoretical values; this suggests that very wide isotope ratio measurements should be attainable with good accuracy and precision when the ion formation scheme is implemented on a dedicated isotope ratio mass spectrometer. [Figure not available: see fulltext.]