Chemical Science View Article Online EDGE ARTICLE View Journal | View Issue Preparation of an ion with the highest calculated proton affinity: ortho-diethynylbenzene dianion† Cite this: Chem. Sci.,2016,7,6245 Berwyck L. J. Poad,ab Nicholas D. Reed,b Christopher S. Hansen,b Adam J. Trevitt,b Stephen J. Blanksby,a Emily G. Mackay,c Michael S. Sherburn,c Bun Chan‡d and Leo Radomd Owing to the increased proton affinity that results from additional negative charges, multiply-charged anions have been proposed as one route to prepare and access a range of new and powerful “superbases”. Paradoxically, while the additional electrons in polyanions increase basicity they serve to diminish the electron binding energy and thus, it had been thought, hinder experimental synthesis. We report the synthesis and isolation of the ortho-diethynylbenzene dianion (ortho-DEB2À) and present observations of this novel species undergoing gas-phase proton-abstraction reactions. Using a theoretical model based on Marcus–Hush theory, we attribute the stability of ortho-DEB2À to the Received 20th April 2016 presence of a barrier that prevents spontaneous electron detachment. The proton affinity of 1843 kJ Creative Commons Attribution 3.0 Unported Licence. Accepted 17th June 2016 molÀ1 calculated for this dianion superbase using high-level quantum chemistry calculations significantly DOI: 10.1039/c6sc01726f exceeds that of the lithium monoxide anion, the most basic system previously prepared. The ortho- www.rsc.org/chemicalscience diethynylbenzene dianion is therefore the strongest base that has been experimentally observed to date. Introduction solvent interaction, provides an ideal way to investigate the fundamental basicity of these highly reactive systems.7 Exploration of the fundamental thermochemistry of acids and In the gas phase, the proton affinity of an anion is equivalent D bases informs our understanding of chemical transformations to the enthalpy of deprotonation ( acidH298) of the conjugate This article is licensed under a À ¼ D and can drive innovation in the design of new reactions and acid (i.e., PA[X ] acidH298[XH]). The strongest base prepared À 8 reagents.1–3 The hydroxide anion has the largest proton affinity to date is the lithium monoxide anion (LiO ). With an esti- À À possible in an aqueous environment, since any base with mated proton affinity of 1782 Æ 8 kJ mol 1, LiO supplanted the Open Access Article. Published on 20 June 2016. Downloaded 10/5/2021 7:08:50 AM. À À a larger PA will abstract a proton from H2O (PA[OH ] ¼ 1633.14 methide anion (CH3 ) at the top of the basicity scale in 2008, À Æ 0.04 kJ mol 1).4 To generate stronger bases in solution, non- exceeding the proton affinity of the carbanion by approximately À 4,9,10 aqueous solvents are required. For example lithium diisopro- 40 kJ mol 1. More recently, computational studies have pylamide, which is oen employed in organic synthesis as proposed extending this framework to even more basic ions À 11 a deprotonating agent, must be used in an aprotic solvent such such as OLi3O . However no clear synthetic route to form as tetrahydrofuran.5 Such extremely strong bases are referred to these ions in the gas phase has been demonstrated. Anionic 6 À À as superbases. Owing to these environmental factors, investi- superbases such as LiO and CH3 necessarily satisfy two gation and comparison of the intrinsic basicity of compounds essential requirements: they are the conjugate bases of very in solution is limited. Probing the reactivity of high proton weak gas-phase acids and their neutral radicals have low elec- affinity species in the gas phase, an environment free from any tron affinities (EAs). Multiply-charged anions can also full these thermochemical requirements, as the gas-phase acidity of ffi aCentral Analytical Research Facility, Institute for Future Environments, Queensland an anion is inherently low while the electron a nity of an anion University of Technology, Brisbane, QLD 4001, Australia. E-mail: berwyck.poad@ (i.e., the affinity for addition of a second electron to produce qut.edu.au a dianion) can be low or even negative. Despite their potential bSchool of Chemistry, University of Wollongong, Gwynneville, NSW 2522, Australia instability, such dianion systems have been observed because of cResearch School of Chemistry, Australian National University, Canberra, ACT 2601, a repulsive Coulomb barrier (RCB) that arises from the inter- Australia action between the local bound-potential of the functional dSchool of Chemistry, University of Sydney, Sydney, NSW 2006, Australia † group carrying the charge (e.g., a carboxylate group) and the Electronic supplementary information (ESI) available: Synthesis of precursor 12,13 compounds, detailed experimental and theoretical procedures, 3 supporting repulsive Coulomb potential between like charges. This RCB gures, 4 supporting tables, NMR spectra of synthesised compounds. See DOI: can stabilise multiply-charged anions and, in some cases, 10.1039/c6sc01726f allows for the generation and isolation of polyanions despite ‡ Present address: Graduate School of Engineering, Nagasaki University, Bunko their negative electron binding energies.14,15 Based on these 1-14, Nagasaki 852-8521, Japan. This journal is © The Royal Society of Chemistry 2016 Chem. Sci.,2016,7,6245–6250 | 6245 View Article Online Chemical Science Edge Article À considerations, the 1,3-diethynylbenzene dianion (meta-DEB2 ) was postulated to be a gas-phase superbase with a calculated PA À À of 1796.6 kJ mol 1, approximately 15 kJ mol 1 greater than that of the lithium monoxide anion.8 Thus far, no experimental studies have been reported on this gas-phase dianion – presumably due to the expectation that Coulomb repulsion between the proximate negative charges would destabilise the dianion and thus pose a challenge to its generation and isolation. À In this article, we outline the synthesis of meta-DEB2 in the gas phase along with the isomeric 1,2- and 1,4-dieth- À ynylbenzene dianions (ortho- and para-DEB2 , respectively). Observation of proton-transfer reactions between these dia- nions and a number of weak acids demonstrates their behav- Fig. 1 Mass spectra illustrating the synthesis of the ortho-DEB dianion ffi iour as gas-phase bases. The calculated proton a nity of each of base. The mass-isolated dicarboxylate anion at m/z 106 (a) is observed À the DEB2 isomers exceeds that of the lithium monoxide anion, to decarboxylate under CID to yield m/z 84 (b). Subsequent isolation À with ortho-DEB2 representing the strongest gas-phase base and activation of this m/z 84 ion yields a second decarboxylation m z meta synthesised to date. product at / 62 and associated reaction products (c). The - and para-DEB dianions were synthesised using the same approach. Results and discussion À diacids (i.e., ortho, meta and para-DEB2 ). This is strongly sup- 2À Synthesis of the ortho-DEB dianion at a mass-to-charge ratio ported by the differences in product ions and product ion Creative Commons Attribution 3.0 Unported Licence. (m/z) of 62 was performed using tandem mass spectrometry in abundances observed in the mass spectra at each step of the À À a linear quadrupole and followed the process outlined in gas-phase preparation (cf. ortho-DEB2 in Fig. 1 and para-DEB2 Scheme 1 and Fig. 1. Negative ion electrospray ionisation of the in ESI Fig. S1†). Moreover, the pseudo rst-order decay of diacid precursor generated the dicarboxylate dianion (m/z 106), each of the m/z 62 ion populations is consistent with only which was mass-selected and subjected to successive collisional a single isomer in each instance (Fig. 2). Full experimental activation steps to remove the carboxylate groups while retaining both charges. Such decarboxylation processes that are accom- panied by retention of charge have previously been noted for several organolithium compounds.8,16 The same method was This article is licensed under a deployed successfully for the generation of both meta-andpara- À DEB2 , using the appropriate isomeric diacid precursor. In À addition to the DEB2 dianion (m/z 62) and its associated proton- Open Access Article. Published on 20 June 2016. Downloaded 10/5/2021 7:08:50 AM. transfer product (m/z 125) observed in Fig. 1c, the main product ions following the nal collisional activation step arise from loss of CO2 and loss of an electron from m/z 84 (m/z 124), accompa- nied by a small amount of C2 loss from this ion (m/z 100). Decarboxylation of carboxylate anions upon collision- induced dissociation has previously been demonstrated as an effective means to prepare regiospecic anions in the gas phase.17 Such precedent strongly suggests that regiochemistry of the three m/z 62 dianions is retained from their precursor À Fig. 2 (a) Evidence for proton abstraction by ortho-DEB2 . Mass Scheme 1 Gas-phase synthesis of the ortho-DEB isomer. Negative spectra acquired by isolating ortho-diethynylbenzene dianion (m/z 62) ion electrospray ionisation produces the dicarboxylate dianion and monitoring the production of the proton-transfer product (m/z 106). Subjecting this ion to successive stages of collisional (m/z 125) for increasing trapping times in the presence of background activation results in the loss of two carbon dioxide molecules, with water show that the ion signal intensity growth for the proton-transfer retention of both negative charges, yielding the ortho-DEB2À dia- product is clearly coupled to the decay of the dianion superbase ion nion (m/z 62). Synthesis of the meta-andpara-DEB2À isomers signal. (b) Decay plots showing the decrease in integrated ion signal proceeds in an analogous manner, using the appropriate diacid intensity with increased trapping time for m/z 62 for all three DEB2À precursor. dianions. 6246 | Chem. Sci.,2016,7,6245–6250 This journal is © The Royal Society of Chemistry 2016 View Article Online Edge Article Chemical Science details, including preparation of the diacid precursors, are l DG 2 DG* ¼ 1 þ presented in the ESI.† 4 l (1) À Mass selection allowed isolation of the ortho-DEB2 dia- nion within the ion trap and an investigation of its fate over D * time.