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.