Controlling and Predicting Alkyl-Onium Electronic Structure

X-ray photoelectron spectroscopy (XPS) and ab initio calculations show that fully alkylated onium cation electronic structure can be tuned using both the alkyl chains and the central onium atom. The key for tuning the central onium atom is methyl versus longer alkyl chains, allowing selection of the optimum cation for a wide range of applications, including catalysis and biocides.


X-ray photoelectron spectroscopy (XPS) and ab initio calculations
show that fully alkylated onium cation electronic structure can be tuned using both the alkyl chains and the central onium atom.The key for tuning the central onium atom is methyl versus longer alkyl chains, allowing selection of the optimum cation for a wide range of applications, including catalysis and biocides.
Electrostatic interactions are key for onium-anion and onium-neutral molecule interactions, as there are no -bonds and only -bonds on fully alkylated onium cations.In synthesis, hydrogen bonding catalysis is based on electrostatic interactions between onium cations and substrates, 2, 3 and phase-transfer catalysis relies on anion-onium cation electrostatic interactions 4 .[14][15] Moreover, a single calculable electronic descriptor that can capture through-bond effects has been used to understand the impact of substituents on [N n,n,n,n ] + , 16,17 and a range of calculated electronic structure properties (e.g.orbital energies) have been used to predict the impact of [N n,n,n,n ] + supporting electrolytes on electrochemical performance 8 .Despite these extensive examples, there is still a large knowledge gap in the electronic structure of fully alkylated onium cations that underpins electrostatic interactions in the wide variety of applications.Using a combination of experimental X-ray photoelectron spectroscopy (XPS) and loneion-SMD (Solvation Model based on Density) density functional theory (DFT) calculations we provide both experimental and computational evidence on liquid-phase onium cations.
Element-specific XPS core-level binding energies, E B (core), are capable of capturing multiple electronic structure descriptors for each onium cation, e.g.E B (N 1s).Moreover, E B (core) and the electrostatic potential at a nucleus, V n , have been found to be linearly correlated for ionic liquids (ILs), 18,19 meaning that E B (core) can be readily interpreted unlike e.g. chemical shifts in NMR.Furthermore, V n is an excellent localised non-covalent bonding/reactivity strength descriptor; 20 atoms with large E B (core) and large V n will attract electrons relative to atoms with small E B (core) and small V n .
ILs with relatively weakly interacting anions, e.g.[NTf 2 ] - (bis[(trifluoromethane)sulfonyl]imide) or [FSI] - (bis(fluorosulfonyl)imide), can be used to probe intrinsic cation properties, as cations are effectively in a sea of anions that give non-specific interactions. 18,19 dditionally, lone-cation-SMD  The range of onium cations studied in the liquid-phase using XPS to date is limited, with the main focus on onium cations in ILs [21][22][23][24][25] -which have sufficiently low vapour pressure to be used with standard XPS apparatus 26 .A key finding for XPS of [N n,n,n,n ] + -based ILs was the impact of cations with linear (e.g.[N 6,6,6,14 ] + ammonium cation) versus ring (e.g.[C 8 C 1 Pyrr] + pyrrolidinium cation) alkyl on the IL electronic structure; cationanion interactions were linked to the conformational flexibility of the cation. 22,23 owever, the number of different onium cations studied was relatively limited, leaving open questions, e.g.what is the influence of the alkyl substituent on onium electronic structure?Herein, we measured E B (core) for 12 [onium cation][NTf 2 ] and two [onium cation][FSI] ILs using XPS and performed lone-ion-SMD DFT calculations (ESI Section 3 for details) for 31 onium cations to address these questions.

ChemComm Accepted Manuscript
Varying the alkyl chain length of [N n,n,n,n ] + from methyl to longer has a significant and predictable impact on E B (N cation 1s).N 1s XPS for the two ILs gave a relatively large E B (N cation 1s) of -0.36 eV (Figure 2a, Figure 3a and ESI Table S8), representing E B (N cation 1s) going from three CH 3 to three longer alkyl chains.Each successive change of one CH 3 to one longer alkyl gives E B (N cation 1s) of ~-0.12 eV (Figure 2a), as demonstrated by 2a, Figure 3a and ESI Table S8).This effect of the alkyl chain lengths on cation electronic structure is independent of the anion identity, as the same E B (N cation 1s) effect is observed when the S17 and ESI Table S8).
Changing the alkyl substituent from ethyl to longer had no discernible impact on E B (N cation 1s), with clear evidence from three cases: (i) [  2a and ESI Table S8).
Calculated E B (N cation 1s) match the experimental E B (N cation 1s) very well, both visually (Figure 2a, Figure 2b and ESI Figure S18), and an excellent linear correlation for calculated versus experimental E B (N cation 1s) (ESI Figure S20).Going from [N 1,1,1,1 ] + to [N 2,2,2,2 ] + , each time a CH 3 is changed to a longer alkyl chain gives E B (N cation 1s) ~-0.08 eV (Figure 2b and Figure 3a), which is slightly smaller than the experimental E B (N cation 1s) of ~-0.12 eV.This difference may arise from the choice of functional/basis set combinations; the wB97XD functional was selected as it has minimal self-interaction error and correctly describes longrange electron-electron interactions (important in determining ionisation energies).An alternative explanation is the failure of the SMD model to capture local interactions (switching from IL Please do not adjust margins Please do not adjust margins SMD to water SMD had a small impact on E B (N cation 1s), Figures S19c and 19d and Table S9).
The excellent matches for the experimental and calculated E B (X cation core) (where X = N, S or P) demonstrate that the changes in the experimental E B (X cation core) are due to ground state effects and can be related directly to the molecular electrostatic potential at the central onium atom nucleus, V X (Figure 5).The differences in E B (X cation core) for fully alkylated onium cations are explained by the alkyl group inductive effect, where the additive strength is -CH 2 CH 2 CH 3  -C 2 H 5 < -CH 3 . 27Four CH 3 substituents gives the largest E B (X cation core) = largest V X = strongest X atom electrostatic interaction.Conversely, four longer alkyl substituents gives the smallest E B (Ncation 1s) = smallest V X = weakest X atom electrostatic interaction.
Changing the alkyl groups from linear to branched (i.e.going from each α-C atom having one C and two H to each α-C atom having two C and one H) gives a small impact on E B (N cation 1s) and V N .Changing the cation from [N 2,2,2,2 ] + to [N i3,i3,i3,i3 ] + (i3 = isopropyl, ESI Table S3) gave E B (N cation 1s) = -0.09eV (ESI Figure S22 and ESI Table S9).[N i3,i3,i3,i3 ] + is unlikely to be stable in solution, but adding one branched alkyl group to give e.g.
[N i3,2,2,2 ] + will give stable cations and will allow small fine tuning of E B (N cation 1s) and V N .
The electronic environment of the carbon atoms in onium cations can be finely controlled by changing X cation (Figure 4 and ESI Section 14), as demonstrated by using our second spectroscopic handle, E B (C 1s).E B (C α-C 1s) and therefore V C can be tuned using the central X atom identity.The experimental order for E B (C α-C 1s) is N > S > P (Figure 4 upper), which matches literature E B for N versus P (S was not included in that publication) and also partial charge calculations. 22, 28E B (C α-C 1s) following N > S > P matches the order of the central atom electronegativity, i.e.N > S > P, so nitrogen withdraws the most electron density from α-C to X cation (Figure 5).
V X for X cation can be inferred from E B (C α-C 1s), given that all onium cations will have approximately the same overall charge of +1, so any change in V α-C will have the opposite effect on V X .E B (C α-C 1s) and V α-C trend N > S > P, so V X (relative to a neutral X   Please do not adjust margins Please do not adjust margins atom) trends P > S > N.
Both E B (X cation core) and E B (C α-C 1s) can be tuned using the counteranion identity. 19,21,22 Canging from Cl -to [NTf 2 ] -had the same effect (within experimental uncertainty) of +0.4 eV on E B (C α-C 1s), E B (N cation 1s) and E B (P cation 2p). 19,21,22 Tis effect of the anion is similar in magnitude to changing from four alkyl substituents to four longer alkyl substituents (Figure 3).
Given that the onium cation headgroup will dominate any electrostatic interaction with a substrate, both V X and V -C are clearly vital descriptors.We have demonstrated that both V X and V -C can be predictably controlled for onium cations by the choice of alkyl chain length and central X atom identity.Furthermore, for certain applications where cation-anion are likely to be bound together (e.g.ion pairing in solvents with low relative permittivity, in ionic liquids), the counteranion identity can also contribute strongly to both V X and V -C .
We envisage this information being useful in two areas.Firstly, V X and V -C are expected to be very useful descriptors for developing models using both quantitative structure-property relationships (QSPR) and machine learning.Such descriptors could be calculated using the same DFT methods demonstrated here, or a cheaper but cruder method would be to use the linear relationships developed here to make predictions of V X and V - C .Secondly, a semi-quantitative judgement of the optimum onium cation to be used in any application can be made using our results.The largest V X (i.e.strongest X atom electrostatic interaction) would be [X 1,1,1,1 ][FAP] (the very weakly electrostatically interacting anion [FAP] - = tris(pentafluoroethyl)trifluorophosphate 19 ), and the smallest V X (i.e.weakest X atom electrostatic interaction) would be e.g.[X 2,2,2,2 ]Cl.For biocides for example, a strong electrostatic interaction is expected to be desirable.Therefore, focusing on electronic effects (and ignoring steric effects) [X 1,1,1,1 ] + would be better than [X 2,2,2,2 ] + .However, such a selection is challenging as both V X and V -C are affected in opposite directions by the central X atom identity, and a single site interaction model cannot be assumed.Essentially, the electrostatic interactions are too complicated for a single metric to be used for judgement.
Overall, we have presented new experimental and calculated electronic descriptors for judging electrostatic interaction strengths in fully-alkylated onium cations (Figure 5).X cation , α-C and C alkyl can be predictably tuned using a combination of the alkyl chain lengths, the central X atom identity and the counteranion.In future work we will aim to demonstrate further control onium electronic structure using heteroatom substituents (e.g.O atoms) or X-H (protic cations).

Figure 5 .
Figure 5.Effect of the onium cation structure on V X and V α-C .
DOI: 10.1039/D4CC03388DThis journal is © The Royal Society of Chemistry 20xxPlease do not adjust margins Please do not adjust margins calculations (i.e. with no counteranions) capture the inherent cation properties.