Modern Electrospray Ionization Mass Spectrometry Techniques for the Characterization of Supramolecules and Coordination Compounds

Abstract

Mass spectrometry is routinely used for myriad applications in clinical, industrial, and research laboratories worldwide. Developments in the areas of ionization sources, high-resolution mass analyzers, tandem mass spectrometry, and ion mobility have significantly extended the repertoire of mass spectrometrists; however, for coordination compounds and supramolecules, mass spectrometry remains underexplored and arguably underappreciated. Here, the reader is guided through different tools of modern electrospray ionization mass spectrometry that are suitable for larger inorganic complexes. All steps, from sample preparation and technical details to data analysis and interpretation are discussed. The main target audience of this tutorial is synthetic chemists as well as technicians/mass spectrometrists with little experience in characterizing labile inorganic compounds.

Figure 1

Mass spectrometric analysis of supramolecules and coordination compounds, including sample preparation, ionization conditions, ion transfer, and analysis. Abbreviations are as follows: (n)ESI, (nano)-electrospray ionization; MALDI, matrix-assisted laser desorption ionization; CSI, coldspray ionization; DC, direct current; RF, radio frequency; m/z, mass to charge ratio. Designed with BioRender.

Figure 2

Experimental (blue) and predicted (gray) isotopic patterns of (a) [Cr₆Gd₂F₈(O₂C^tBu)₁₆NH₂ⁿPr₂]⁺ and (b) [Cr₁₂Gd₄F₂₁(O₂C^tBu)₂₈(NH₂ⁿPr₂)₂]⁺. Example a) illustrates how the charge state can be determined by quantifying the m/z distance between two neighboring isotopic peaks, leading to d = z = 1. The predicted isotopic pattern agrees well with the experiment, and so does the accurate mass of the most intense peak. Case b) is more difficult, and the unexperienced reader might consider the agreement between the experiment and prediction sufficient. While the digits of the accurate mass are in good agreement with simulation, the experimental maximum and the whole distribution is shifted to lower m/z. The agreement is not sufficient to confidently assign this peak to the proposed sum formula. Based on X-ray crystallography data, we found that some of the fluoride atoms have likely been substituted for hydroxyl groups, which suggests an overlap of ions with different numbers of F atoms and OH groups present. Predicted isotopic patterns were simulated with enviPat based on a ThermoFisher QExactive UHMR at a resolution of 12 500 (as experiment). Adapted with permission from ref (37). Copyright 2023, The Authors.

Figure 3

Fragmentation of the singly charged [AB]⁺ and the doubly charged [AB]²⁺. For [AB]⁺, the fragment ions [A]⁺ and [B]⁺ are found at a lower m/z than the precursor. The dissociation of [AB]²⁺ can result in [A]⁺ or [B]⁺ at a higher m/z than the precursor, due to the loss of charge, whereas [A]²⁺ and [B]²⁺ are always at a lower m/z than [AB]²⁺.

Figure 4

(a) Fragmentation of CDEF with either a cyclic or linear structure. The presence or absence of CF can inform on the connectivity of CDEF. (b) Fragmentation of the chain XYX to the fragments XY + X at lower and 2X + Y at higher collision energies, respectively. The relative abundances of X, Y, and XY can inform on the precursor structure.

Figure 5

(a) MS² data of [Ring_Mn]⁻ at E_lab = 110 eV. Inset: structure of [Ring_Mn]⁻ (Cr, green, Mn, cyan, F, yellow, O, red, C, gray). Hydrogen atoms in the tert-butyl groups were omitted for clarity. (b) Normalized survival yield vs E_com for [Ring_M]⁻ fitted to a sigmoidal function (M = Mn, cyan; Fe:, purple; Co, orange; Ni, black; Cu, green; Zn, red; Cd, blue). E_com is the collision energy in the center-of-mass frame and is a more precise representation of the energy that is transferred during single ion-gas collisions. Reproduced from ref (88). Copyright 2022, The Authors.

Figure 6

(a) CCS_N2 Distributions of [Ring_Mn]⁻ and fragments (1–3) at E_lab = 110 eV. (b) Fragmentation of [Ring_Mn]⁻ to 3 including structural assignments of 3C to closed heptametallic rings and 3E to conformationally dynamic, open structures. Reproduced from ref (88). Copyright 2022, The Authors.