Kevin LOVELOCK1, Frances TOWERS TOMPKINS1, Lewis PARKER1, Najaat ALBLEWI1, Coby CLARKE2, David GRINTER3, Roger BENNETT1
1University of Reading, Reading, United Kingdom
2University of Nottingham, Nottingham, United Kingdom
3Diamond Light Source, Didcot, United Kingdom
Boron is a main group, non-metallic, earth-abundant element with growing potential in the field of organic synthesis.1 Boron’s wide ranging reactivity as a reagent, low cost and toxicity is useful to bulk manufacturing. The ability of organoboron compounds to mediate C-C bond formation through the Suzuki reaction (Nobel Prize for Chemistry, 2010) means it is of great importance to pharmaceutical and agrochemical industries.2 Traditionally, three coordinate boron reagents are used.1 However, four coordinate trifluoroborate anion reagents, [RBF3]-, have in recent years become very popular, due mainly to their ability to be thermochemically stable but reactive after photoactivation. Electrochemical oxidation potential has been used as a parameter to judge the reactivity of [RBF3]- reagents, with a variation with different R groups of ~0.75 V.1 [RBF3]--based ionic liquids (ILs) have become popular recently due to their relative ease of synthesis,3 with K[RBF3] salts widely available as a starting material for IL synthesis. However, no measurements of electronic structure of [RBF3]- have been made; therefore, there is limited understanding of the link between geometric structure and electronic structure.
We have recorded synchrotron non-resonant and resonant X-ray photoelectron spectroscopy (XPS) for both [C4C1Im][BuBF3] and [C4C1Im][BF4] on beamline B07 at Diamond Light Source, along with performing density functional theory calculations. We measured a range of electronic structure descriptors for [BuBF3]- and [BF4]-: (a) boron + carbon + fluorine atomic charges, (b) ionisation energy, (c) highest occupied valence state identity (i.e. which part of the anion is most readily ionised). We found that the C-BF3 carbon in [BuBF3]- is negatively charged, a hugely rare occurrence for carbon, only previously observed by XPS for metal carbides and silicon carbide. This finding matches well to 1H NMR spectroscopy. Furthermore, we found that the ionisation energy of [BuBF3]- is ~2 eV smaller than the ionisation energy for [BF4]-, due to the presence of the Bu chain. These electronic structure descriptors will be used to understand and interpret photo-oxidation potentials.
References
1. Marotta et al., Angew. Chem. Int. Ed., 2022, 61, e202207067.
2. Suzuki, Angew. Chem. Int. Ed., 2011, 50, 6722.
3. Clarke et al., ACS Sustain. Chem. Eng., 2020, 8, 16386.