Gas-phase fragmentation of 1-adamantylbisimidazolium salts and their complexes with cucurbit[7]uril studied using selectively 2H-labeled guest molecules

Abstract

Rationale: Bisimidazolium salts (BIMs) represent an interesting family of ditopic ligands that are used in the construction of supramolecular systems with hosts based on cyclodextrins or cucurbit[n]urils. Understanding the fragmentation mechanism of individual BIMs and how this mechanism changes after complexation with cucurbit[n]urils can bring new insight into the intrinsic host-guest relationship, thereby allowing utilization of mass spectrometry to describe binding behavior. Methods: Selectively 2H-labeled bisimidazolium salts were prepared and fully characterized by spectroscopic methods. All MSn experiments were conducted in the positive-ion mode using an electrospray ionization (ESI) ion-trap mass spectrometer. The structures of the proposed fragments were supported by theoretical optimizations performed at the B3LYP/6-31G(d) level of density functional theory (DFT) using the Spartan'14 program. Results: Using selectively deuterium-labeled isotopologues of two adamantylated bisimidazolium salts and DFT calculations, we describe the fragmentation pathways of bisimidazolium salts. The release of two important adamantane moieties, [C11H17]+ and C11H16, from M2+ was determined, although the former was strongly preferred. In contrast, when M2+ was complexed with CB7, the neutral loss of the C11H16 fragment was favored. The fragmentation pattern strongly depended on the steric hindrance of the M2+ guest against slippage of the CB7 unit over the guest molecular axle. Conclusions: The structures of two adamantane-based fragments and the mechanisms of their formation were rationalized. Two distinct geometric arrangements for the adamantane cage inside the CB7 cavity were hypothesized to explain the differences in the fragmentation patterns for guests with minimal, moderate, and high steric hindrance. This finding brings new insight into the understanding of intrinsic behavior of the adamantane-based guests inside the CB7 cavity. Copyright © 2017 John Wiley & Sons, Ltd.