Modular Ion Mobility Calibrants for Organometallic Anions Based on Tetraorganylborate Salts

Abstract

Organometallics are widely used in catalysis and synthesis. Their analysis relies heavily on mass spectrometric methods, among which traveling-wave ion mobility spectrometry (TWIMS) has gained increasing importance. Collision cross sections (CCS) obtainable by TWIMS significantly aid the structural characterization of ions in the gas phase, but for organometallics, their accuracy has been limited by the lack of appropriate calibrants. Here, we propose tetraorganylborates and their alkali-metal bound oligomers [M n–1(BR4) n ]− (M = Li, Na, K, Rb, Cs; R = aryl, Et; n = 1–6) as calibrants for electrospray ionization (ESI) TWIMS. These species chemically resemble typical organometallics and readily form upon negative-ion mode ESI of solutions of alkali-metal tetraorganylborates. By combining different tetraorganylborate salts, we have generated a large number of anions in a modular manner and determined their CCS values by drift-tube ion mobility spectrometry (DTIMS) (DTCCSHe = 81–585, DTCCSN2 = 130–704 Å2). In proof-of-concept experiments, we then applied these DTCCS values to the calibration of a TWIMS instrument and analyzed phenylcuprate and argentate anions, [Li n–1M n Ph2n ]− and [M n Ph n+1]− (M = Cu, Ag), as prototypical reactive organometallics. The TWCCSN2 values derived from TWIMS measurements are in excellent agreement with those determined by DTIMS (<2% relative difference), demonstrating the effectiveness of the proposed calibration scheme. Moreover, we used theoretical methods to predict the structures and CCS values of the anions considered. These predictions are in good agreement with the experimental results and give further insight into the trends governing the assembly of tetraorganylborate, cuprate, and argentate oligomers.

Ion Mobility Mass Spectrometry for the Separation and Characterization of Small Molecules

“…Due to the existence of radio frequency voltage, ions are not in a constant field in TWIMS mode, so that the CCS can be deduced from drift time only after calibration by reference materials with known CCS values. Calibration compounds and calibration methods have been reported in refs − . The TW CCS N2 database is based on a single laboratory measurement preferably cross-validated and then applied to peak annotations .…”

The Magnesium Dication and Water Synergistically Promote the Protonolysis of Two of the B–C Bonds in the Tetraphenylborate Anion

“…18−20 The oligomeric metal complexes of tetraphenylborate and related anions have recently been used as calibrants for traveling-wave ion mobility spectrometry experiments. 21 As part of an ongoing series of studies on the gas-phase chemistry of metal complexes of organoborates, 22−25 we were surprised to discover that, in the absence of CID, the ESI-MS/MS spectrum of the magnesium complex [Mg(BPh 4 )] + contains product ions resulting from IMRs with background water via two sequential protonolysis reactions of its B−C bonds. Here we used time-resolved MS/ MS experiments and DFT calculations to explore these unusual reactions.…”

“…Weakly coordinating anions such as the tetraphenylborate anion, [BPh 4 ] − , are widely used as counterions in inorganic and organometallic chemistry, with important applications in synthesis and industrial chemistry. − The oligomeric metal complexes of tetraphenylborate and related anions have recently been used as calibrants for traveling-wave ion mobility spectrometry experiments As part of an ongoing series of studies on the gas-phase chemistry of metal complexes of organoborates, − we were surprised to discover that, in the absence of CID, the ESI-MS/MS spectrum of the magnesium complex [Mg­(BPh 4 )] + contains product ions resulting from IMRs with background water via two sequential protonolysis reactions of its B–C bonds.…”