Reactions of doubly deprotonated 2,6-naphthalenedicarboxylic acid with alcohols: Proton transfer versus solvation☆
Graphical abstract
Introduction
Solvents play a critical role in influencing structure and reactivity in both chemistry and biology [1]. They serve as a medium, function as passive participants in chemical transformations, but often play a more active role. For example, solvents can be involved in proton transfer reactions, serve as nucleophiles or ligands to metal centers, solvate reactants, products, reactive intermediates and transition structures, and influence aggregation states of salts and organometallic reagents [2].
In the gas phase solvent effects can be eliminated, and this has led to surprising findings and new chemical insights. A landmark example is the reversal of the acidity ordering of alcohols upon alkyl substitution at the hydroxyl bearing carbon (i.e., CH3OH < CH3CH2OH < (CH3)2CHOH < (CH3)3COH, where tert-butyl alcohol is the strongest gas-phase acid in this series but the weakest one in solution) [3]. Mass spectrometry based methods also can be used to gain an understanding of ion microsolvation, thereby providing a means to explore nucleation processes, reaction intermediates and the transition from an isolated species to its bulk phase behavior [4]. Small cluster ions are consequently of general interest and have been extensively investigated [5].
High pressure mass spectrometers and flowing afterglow devices can be used to synthesize clusters via association processes, but this method generally is not effective in low pressure environments. In 1973 Riveros et al. addressed this problem by reporting that COCl−, generated by electron ionization of phosgene, transfers Cl− to a wide variety of organic compounds under high vacuum conditions [6]. Anionic nucleophiles subsequently were found to react with alkyl formates predominantly via an α-elimination pathway leading to mono solvated anions (Eq. (1)).
This latter transformation and several variants of it have come to be known as the Riveros reaction [7]. It enables a wide range of cluster ions to be prepared and subsequently studied. These adducts are invariably less basic than their naked ion counterparts, and are also typically less reactive and can undergo different reactions [8]. This is because solvation is a stabilizing phenomenon, and the interaction of an ion with a neutral species via a single hydrogen bond such as in BH • −OR or B− • HOR is worth ∼15–25 kcal mol−1 whereas the analogous non-charged adduct HB • HOR has a weak association energy of ∼2–5 kcal mol−1 [9]. As a result, monosolvated anions usually have proton affinities that are ∼15–20 kcal mol−1 smaller than their free analogs (Scheme 1).
The advent of electrospray ionization (ESI) has enabled multiply charged ions to be routinely produced and provides another versatile approach for preparing solvated ions. This has enabled IR [10] and photoelectron spectroscopy [11] to be used to interrogate their structures, while unimolecular dissociations can be used to probe the competition between solvent evaporation and dissociative proton transfer [12], and electron capture experiments enabled an absolute electrochemical scale to be determined [13]. A new class of electrostatically defying cation–cation [14] and anion–anion [15] gas-phase clusters also has been proposed, and these species have strong parallels in a variety of biological systems including oligomers of HIV capsid proteins, the human rhinovirus capsid envelope, the HY-HEL5 Fab-lysozyme complex and many protein structures [16]. These studies raise questions such as can one generate like-ion complexes via proton transfer reactions and what happens to the Coulomb barrier in multiply charged ions upon solvation [17]? To address these issues, here we explore how doubly deprotonated 2,6-naphthalenedicarboxylic acid reacts with various alcohols via sequential solvation. The resulting structures were probed experimentally and computationally, and potential energy surfaces were calculated to explore the Coulombic barriers.
Section snippets
Experimental section
Commercial reagents were used as supplied. Gas phase ion-molecule reactions were carried out using a Finnigan LTQ FT hybrid linear ion trap (Finnigan, Bremen, Germany) fitted with an electrospray ionization source and modified as described previously to allow the introduction of gaseous neutral reagents into the ion trap [18]. Accurate mass measurements using the FT-ICR allowed unambiguous assignment of the observed ions. Doubly deprotonated 2,6-naphthalenedicarboxylic acid was generated by ESI
2,6-Naphthalenedicarboxylate (12−)
Aqueous methanol solutions of 2,6-naphthalenedicarboxylic acid containing a small amount of base were sprayed into a modified Finnigan LTQ ion trap at ∼10−3 Torr and strong signals of the (M − 2H+)2− anion (12−) at m/z 107 were produced. This ion was mass isolated and allowed to react with a series of alcohols of varying acidity. Ethanol, propanol, 2,2,2-trifluoroethanol (TFE) and background water solvate 12−to afford 12−• HOR (See supporting Fig. S1 for reaction with TFE) but do not protonate the
Conclusion
Ion solvation is a stabilizing phenomenon and the resulting interactions are especially strong in multiply charged species. This leads to sequential reductions in the proton affinities with each added solvent molecule, but proton transfer to these cluster ions can be facilitated nevertheless. This kinetic enhancement was found to be largest when both anionic centers are solvated and an incipient proton transfer leading towards an anion-anion complex results. In other words, proton abstraction
Acknowledgements
Support from the National Science Foundation and the Minnesota Supercomputer Institute for Advanced Computational Research are gratefully acknowledged. We thank the Australian Research Council (ARC) for financial support through the ARC CoE program (RAJO) and grants DP1096134 (to GNK) and DP150101388 (RAJO).
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Dedicated to our friend Prof. Jose Riveros in celebration of his being named Emeritus Professor and in recognition of his important contributions to gas-phase ion chemistry.