Study of Challenging Calcium Alkoxides in Solution by Electrospray Ionization Mass Spectrometry (Part 1)

Abstract

Electrosprayionization was applied on calcium altkoxides studying the mostsuitableoperativeconditions for their detection/identification. To reach this aim, calcium methoxide and ethylate were obtained by two different synthetic pathways, in order to understand their possible different aggregation states. The reaction mixture shows the presence of a supernatant and of a precipitate poorly soluble in most organic solvents. The purpose of this preliminary study is to understand the qualitative differences between the precipitated species and the ones in solution by analyzing both of them with the same analytical method. The electrospray ionization (ESI) operating conditions (voltages, temperatures, solvents) allow not only the detection of single species but also the study of clusters present in solution. Particular attention was paid to establishing the role of ESI conditions in the formation of the detected species. Experiments performed at different sprayer voltages (1 kV, 2kV, 3 kV and 4 kV) proved that ESI does not perturbate the equilibria present in the original solution, demonstrating that the technique can be a useful tool to achieve information on this class of compounds.

Study of Challenging Calcium Alkoxides in Solution by Electrospray Ionisation Mass Spectrometry (Part 2)

“…As it has recently been reported, 10 the formation of oligomeric species is not strongly affected by the instrumental conditions and only by changing the chemistry of the solution there is a significant change in the spectrometric results.…”

“…Furthermore, it must be considered that the dissolution of calcium alkoxides in an alcoholic medium can give rise to alcoholysis reactions, as was observed by the addition of EtOH to [Ca(OMe) 2 ] and vice versa by the addition of MeOH to [Ca(OEt) 2 ]. 3,10 The alcoholysis process (Ca(OR) 2 +2R¢OH ® Ca(OR) 2 -n (OR¢) n +nROH) can have different rates and different equilibria depending on the alcohol used as solvent, on the ratio between alkoxide and added alcohol, on the pH of the system and on the stability of the starting complex. 3 The retention of the original alcoholate ligand is a clear indication of the stability of the original complex.…”

“…3 The retention of the original alcoholate ligand is a clear indication of the stability of the original complex. The alcoholic solvent, in spite of the alcoholysis process, is however necessary according to previous experimental data, 10 because of (i) the enhancement of the solubility by solvation phenomena, (ii) the possible formation of new species which are easily detectable and (iii) the easy ionisation in this medium (see later).…”

“…As far as the ESI conditions are concerned, high spraying capillary voltages and high entrance capillary temperatures are required for the ESI-mass analyses of Si and Ti alkoxides 11,12 considered calcium alkoxides, which decompose under high temperatures. High spraying voltages are not necessary, as the detection of the species is not influenced by this parameter; 10 however, they lead to a significant increase in the signal intensity. Interestingly, the addition of NH 4 OAc proved to be a powerful tool for the complex stabilisation and for the consequent detection of the ionic species present in the sprayed solution.…”

“…1,2 These complexes find applications in many fields, such as in material science, homogeneous catalysis and, more recently, in cultural heritage. [3][4][5][6] In a previous study 10 on the synthesis and ESI-MS characterisation of [Ca(OMe) 2 ] and [Ca(OEt) 2 ], it was shown that: (i) the synthetic approach can influence cluster formation, (ii) clustering does not necessarily give rise to insoluble products and (iii) small differences in the ligand structure generate large differences in the detected species, either from the qualitative or quantitative point of view.…”