An investigation of beryllium coordination chemistry using electrospray ionisation mass spectrometry

Abstract

The electrospray ionisation mass spectrometric behaviour of various complexes of beryllium have been investigated in this thesis. These beryllium complexes were prepared in situ on a small scale by preparing appropriate molar mixtures of the Be²⁺ ion with ligands in a range of solvent systems. In view of the toxicity of beryllium compounds, this combinatorial type screening, involving miniscule amounts of material in solution, proved to be a safe strategy to pursue the coordination chemistry of beryllium. Starting from simple beryllium compounds which include the metal salts (Chapter 2), the speciation and hydrolysis of beryllium ions in aqueous solution have been studied over a pH range of 2.5-6.0 using electrospray ionisation mass spectrometry (ESI-MS). Ions observed by ESI-MS revealed that the speciation of beryllium with hydroxide ligands in solution was preserved into the gas phase via charge reduction by ion pairing with the salt anion. The pH-dependent hydrolytic tendencies of the Be²⁺ cation presented as an ESI-MS speciation diagram for beryllium hydrolysis is in good agreement with previous speciation data which indicates the ability of ESI-MS as a quick, sensitive and safe screening technique for observing beryllium speciation with ligands of interest at low concentrations. Collision induced dissociation patterns further confirmed that the trimer [Be₃(OH)₃]³⁺ is the most stable beryllium hydroxido-aggregate arrangement while the pronounced ion pairing of the beryllium cation with the sulfato ligand yielded additional beryllium ion cluster arrangements such as Be₃ (μ₃-O) aggregate and mixed sulfato-/hydroxido- species. These ions were further investigated using computational techniques simulated in the gas and aqueous environment which allowed for the first time, the molecular dynamics simulations of the ligand exchange processes of the Be²⁺ cation (Chapter 3). A major finding from the ab initio molecular simulations was that hydrogen bonding was relevant in stabilising the tetraaquaberyllium complex in beryllium solutions. Therefore, in the gas phase where the solvation shell of such beryllium species is very much compromised, the formation of inner-sphere complexes is commonplace as observed from the ion signal assignment in the ESI mass spectra. While these extraneous species may not be an exact representation of the solution state, these ions provided insight into plausible modes of beryllium aggregation that are not otherwise easily investigated. Building on these results, a variety of beryllium complexes was generated with various ligands in solutions and subjected to detailed characterisation by ESIMS. These ligands containing functional groups or architecture of interest, varied from simple monodentate ligands such as the acetate ion to more common beryllium chelators including 1,3-diketone, hydroxy keto, malonic acid, chromotropic acid and crown ethers (Chapter 4). Generally, there was an excellent correlation between the species observed in the mass spectrum and those confirmed to exist in solution by other techniques. This lent strong credence to the ESI-MS methodology used as an efficient analytical technique for the easy screening of a diverse range of potential ligands for the divalent beryllium ion. A fundamental issue in beryllium research is the search for suitable chelating ligands for environmental and biomedical application. Therefore, the ESIMS methodology was further employed to investigate several multidentate aminopolycarboxylic acids which are well-known commercial and biomedical chelating agents (Chapter 5). Notable among these are the nitrilotripropionic acid (NTP) and related tetradentate ligands designed towards the full encapsulation of the Be²⁺ cation. Stoichiometric information which was readily obtained from the ESI mass spectra was found to be effective for the preliminary screening of potential encapsulation by tetradentate coordination from a single ligand. Lastly, to corroborate ESI-MS speciation results, beryllium complexes with these class of ligands were synthesised on a larger scale and characterised by ⁹Be NMR and single-crystal X-ray crystallography.