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

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.

Selectively ² H-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.

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, [C₁₁ H₁₇ ]⁺ and C₁₁ H₁₆ , from M²⁺ was determined, although the former was strongly preferred. In contrast, when M²⁺ was complexed with CB7, the neutral loss of the C₁₁ H₁₆ 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.

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.