Lithium monoxide anion: A ground-state triplet with the strongest base to date

Abstract

Lithium monoxide anion (LiO ؊ ) has been generated in the gas phase and is found to be a stronger base than methyl anion (CH 3 ؊ ).This makes LiO ؊ the strongest base currently known, and it will be a challenge to produce a singly charged or multiply charged anion that is more basic. The experimental acidity of lithium hydroxide is ⌬H°a cid ‫؍‬ 425.7 ؎ 6.1 kcal⅐mol ؊1 (1 kcal ‫؍‬ 4.184 kJ) and, when combined with results of high-level computations, leads to our best estimate for the acidity of 426 ؎ 2 kcal⅐mol ؊1 .computations ͉ mass spectrometry ͉ super base T he gas-phase acidities of the hydrogen halides were first reported via the application of a thermodynamic cycle (Eqs. 1-5) in 1942 by Briegleb (1, 2).In subsequent years, the acidities of thousands of compounds have been measured by using a variety of techniques (3), and the acidity scale currently spans a 125 kcal⅐mol Ϫ1 (1 kcal ϭ 4.184 kJ) range from CH 4 (⌬H°a cid ϭ 416.8 Ϯ 0.7 kcal⅐mol Ϫ1 ) (4, 5) to HN(SO 2 C 4 F 9 ) 2 (⌬H°a cid ϭ 291.1 Ϯ 2.2 kcal⅐mol Ϫ1 ) (6) [see supporting information (SI) Text]. Methyl anion is the strongest base currently known, which is a position it has occupied for the past 30 years. This raises the question as to whether a more basic species can be made. In this article, we use sophisticated experimental techniques and state-of-the-art theoretical calculations to show that the lithium monoxide anion (LiO Ϫ ) is in fact more basic than methyl anion, and that it will be a challenge to produce a species that is still more basic. Alkyl groups are polarizable but also are generally electronreleasing and, depending on which influence is larger, can destabilize anions. Kinetic measurements indicate that ethane and the secondary position of propane [(CH 3 ) 2 CH 2 ] are 2-3 kcal⅐mol Ϫ1 less acidic than methane (7), but their conjugate bases have never been observed (8, 9). This is not surprising because the electron affinity of methyl radical is only 1.8 Ϯ 0.7 kcal⅐mol Ϫ1 (4), and CH 3 CH 2 Ϫ and (CH 3 ) 2 CH Ϫ are predicted to be unbound with respect to electron detachment (7). Electronegative substituents stabilize negative ions and increase acidities, as reflected by the first-row hydrides [i.e., HF (most acidic) Ͼ H 2 O Ͼ NH 3 Ͼ CH 4 (least acidic)]. To decrease the acidity of a compound and make a stronger base, one might employ an electropositive substituent such as lithium. However, the conjugate bases of lithiated compounds are difficult to prepare in the gas phase, and almost nothing is known about them because the neutral acids tend to be involatile, moisture sensitive, and pyrophoric.A general method for producing metal-containing anions that overcomes these practical problems was developed by Bachrach, Hare, and Kass (10) and subsequently exploited by . In this approach, metal salts of dicarboxylates are produced by electrospray ionization (ESI) and fragmented via energetic collisions (CID), thereby leading to the sequential expulsion of two molecules of carbon dioxide but retention of the metal ion. For example, the con...

Old acid, new chemistry. Negative metal anions generated from alkali metal oxalates and others

“…Oxalic acid ( Figure 1) and lithium oxalate [18] anions readily decompose by loss of carbon dioxide to produce a stable hydrogen (lithium) carbon dioxide complex, respectively. A secondary decay channel for loss of carbon monoxide is seen with oxalic acid and lithium oxalate and produces an MCO 2 O Ϫ ion; it is not seen with the other metal oxalates.…”

“…However, the generation of alkali metal and silver anions via collision-induced dissociation of the metal oxalate anion has not been previously been reported, though Tian and coworkers recently investigated the dissociation of lithium oxalate [18]. The exothermic decomposition of alkali metal oxalate anion to carbon dioxide in the collision cell of a triple quadrupole mass spectrometer leaves no place for the electron to reside, resulting in a double electron-transfer reaction to produce an alkali metal anion.…”

“…In the gas phase, metal oxalate clusters can be readily generated and detected with both negative and positive electrospray mass spectrometry [15][16][17]. In 2008, Tian and coworkers generated the lithium oxalate anion and reported the loss of CO 2 and CO by CID mass spectrometry, producing the strongest base known to date, LiO Ϫ [18].The decomposition of oxalic acid and various oxalates has been studied both experimentally and theoretically [19]. Oxalic acid has been noted to decompose Address reprint requests to Professor P. M. Mayer,…”

Gas Phase Ligand Fragmentation to Unmask Reactive Metallic Species

“…The lithium oxide anion has been synthesized in the gas phase via two stages of CID on the lithium oxalate anion [93]. In the first stage, decarboxylation occurs to give the lithium salt of doubly deprotonated formic acid (Eq.…”

Gas‐phase synthesis of hydrodiphenylcyclopropenylium via nonclassical Favorskii rearrangement from alkali‐cationized α,α′‐dibromodibenzyl ketone

“…A gas‐phase synthesis strategy could help us to understand the chemical nature and reactivity of the target ions without any solvent and counterion effects and could also help to study some transformations which are difficult to achieve in solution. The groups of O'Hair and Kass have exploited a decarboxylation strategy for the gas‐phase synthesis of many active organometallic species 11–17. Our group has studied the acid‐induced Smiles rearrangement in the gas phase and successfully applied it to the reaction in solution 18.…”