Understanding cyclotricatechylene and cryptophanes ligands for forming supramolecular assemblies that selectively coordinate heavy metal cations by mass spectrometry

In order to better understand their supramolecular chemistry, especially regarding encapsulation of heavy metal ions in water, two different types of host molecules, cyclotricatechylene (CTC) and a series of water-soluble cryptophanes were reacted with alkali metal ions (Na+, K+, Rb+, Cs+) and alkaline earth metal ions (Mg2+, Ca2+, Sr2+) in MeOH/H2O. The resulting host-guest complexes were analysed in detail using advanced mass spectrometry and ion mobility. Host-guest complexes of interest were identified by nanoelectrospray ionization mass spectrometry (nESI-MS) within ±7.8 ppm mass error. Using travelling wave-ion mobility spectrometry mass spectrometry combined with ESI (ESI-TW-IMS-MS) in N2, any isomeric complexes were separated, and collision-cross sections (CCS) were measured for each ion of interest. Potential candidate structures of these host-guest complexes were generated for comparison and structural assignment by density functional theory (DFT). These were optimized in the gas phase at the wB97X-D3BJ/def2-TZVP level of theory, then their CCS was calculated using the trajectory method in N2 with the IMoS CCS calculators. Comparing the experimental and computational CCS, it was determined all CTC-metal complexes formed clam and bowl stacked structures (ΔCCS% ≤ 6.3%), except for Mg2+; all cryptophanes encapsulated metal ions inside the cavity (ΔCCS% ≤ 6.7%), which indicates the cation-π interactions dominated between these host-metal complexes. The studies of hosts with alkaline earth metal ions provided the foundation to understand the potential for Cd2+ remediation in water.