Characterization of selected organometallic compounds by electrospray ionization‐ and matrix‐assisted laser desorption/ionization‐mass spectrometry using different types of instruments: Possibilities and limitations

Rationale Organometallic compounds are becoming increasingly important in their industrial application as catalysts. Mass spectrometry is an essential tool for the structural confirmation of such organometallics. Because the analysis of this class of molecules can be challenging, the ionization behavior and structural confirmation of selected transition metal catalysts are described in this work. Methods The transition metal catalysts investigated were analyzed using classical vacuum MALDI reflectron TOF‐MS as well as intermediate pressure matrix‐assisted laser desorption/ionization quadrupole time‐of‐flight mass spectrometry (MALDI QTOF‐MS). Obtained mass spectra were compared with electrospray ionization MS (ESI‐MS) already established for organometallic compounds, utilizing a QTOF mass spectrometer here. In addition, various sample preparations, including two selected MALDI matrices (trans‐2‐[3‐(4‐tert‐butylphenyl)‐2‐methyl‐2‐propenylidene]malononitrile and 2,2′:5′,2″‐terthiophene) with different solvent combinations for MALDI‐MS measurements, were investigated in detail with respect to their correct isotope distribution of the molecular ions observed. Results All investigated organometallic compounds were successfully identified by vacuum and intermediate pressure MALDI‐MS. Accurate masses of ions related to molecular ion species (e.g., [M‐Cl]+, [M]+, and [M + Na]+) could be determined by MALDI QTOF‐MS measurements with a mass error of less than ±5 ppm for all compounds. Both investigated MALDI matrices performed equally on both instruments. The impact of the analyte/matrix solvent mixtures turned out to be crucial for a successful analysis of the investigated compounds. In contrast, ESI QTOF‐MS yielded masses of ions related to molecular ion species in favorable cases. Conclusions The use of MALDI‐MS for the structural confirmation of organometallic compounds is still not widely used. Nevertheless, this work showed that this analytical technique does have some benefits. The analysis of neutral catalysts proves to be quite useful, concluding that this technique provides a complement and/or an alternative to ESI‐MS.


| INTRODUCTION
The development of efficient and sustainable organometallic catalysts for chemical transformations of simple/small organic molecules is an important part of current research. 1,2 There is a wide selection of literature dealing with the structural elucidation and analysis of these coordination compounds. [3][4][5] In the field of mass spectrometry (MS), electrospray ionization (ESI) MS, which is considered a soft ionization technique, has already been established in the analysis of this substance class. 3,4,6,7 Despite convincing advantages such as fast and easy sample preparation, the analysis of neutral organometallic complexes with ESI-MS is still a challenging task. 8 Over the past few decades, matrix-assisted laser desorption/ionization in combination with reflectron time-of-flight mass spectrometry (MALDI TOF-MS) became a well-established analytical technique in the field of biochemistry and bioanalysis. 9,10 It is known for its ability to study particularly large complex molecules (>300 kDa). 11 Research performed using MALDI-MS in the area of organometallic compounds, which are typically rather small molecules, is still rare. [12][13][14] Therefore, the analysis with MALDI-MS has some advantages that could compensate for the disadvantages of ESI-MS: neutral compounds can be detected more easily. 8 The current work investigates the ionization behavior of selected

| Chemicals and materials
All the analytes used in this work were synthesized and provided by our research group. [15][16][17] These are bench-stable compounds, where no special treatment under an inert gas atmosphere is required. The chemical structures of the organometallic catalysts selected for mass spectrometric investigation are shown in Figure 1, and the corresponding notations are presented in Table 1. Further information on the nomenclature of pincer complexes is available in the supporting information ( Figure S1). Red phosphorus, which was used for calibration in MALDI-MS for both types of instruments, was obtained from Sigma-Aldrich. 18 ES tuning mix, used for calibration of the 6545 QTOF, was purchased from Agilent Technologies (Santa Clara, CA, USA).

| Preparation of samples and solutions
The different solvent mixtures used for MALDI-MS experiments are presented in Table 2. Both matrices, DCTB and TTP, were dissolved in the mentioned solvent mixtures resulting in a matrix concentration of 10 mg/mL. (Information on the chemical structure of the matrices used can be obtained from the supporting information [ Figure S2].) The prepared matrix solution was then added to the solid analyte and mixed in a vortexer with an analyte concentration of 10 mg/mL. All final matrix/analyte solutions were doped with sodium chloride and sodium bromide (0.6 mg/mL), respectively. The type of salt to be added depends on the chemistry of the ligand(s) of the investigated sample. If a bromine atom is present as a ligand, sodium bromide was added, and for complexes with a chloride ligand or no halide ligand, sodium chloride was used. An aliquot (0.7 μL) of these sodium-doped solutions was then applied onto the MALDI target by using the drieddroplet method. 19 For ESI-MS sample preparations, solvent mixtures of ACN/dichloromethane, methanol/dichloromethane, and methanol/ chloroform (v/v, 70:30) were prepared. All analytes were dissolved in these solvent mixtures resulting in a final analyte concentration of 0.1 mg/mL. A sodium salt was then added to the solution according to the selection criterion mentioned earlier.

| RESULTS AND DISCUSSION
Analyzing coordination compounds by either MALDI or ESI-MS, the identity of the elemental composition related to their chemical structure can be identified based on not only high-resolution/accurate mass determination but also their specific isotope pattern. Many transition metals (chromium, iron, and nickel), but also ligands such as halogens (chlorine and bromine), have a specific isotope pattern whose presence in the mass spectrum is decisive for the identification of a particular substance. All analytes investigated and shown in Figure 1 constitute first-row transition metal pincer/bidentate complexes, with bromine or chlorine as co-ligands except for 3 and 6.
In the course of this work, isotope patterns of the identified molecular species were compared with the simulated ones. This is explained in detail here using 1 [V(Triaz NMe2 -iPr)Cl 3 ] as an example.   (Table S1).

| Intermediate-pressure MALDI QTOF-MS, high-vacuum MALDI RTOF-MS, and ESI QTOF-MS-a comparison
To provide a direct comparison between the performance of different  (Table S2).

ACKNOWLEDGMENT
Financial support by the Austrian Science Fund (FWF) is gratefully acknowledged (project no. P 33016).

DATA AVAILABILITY STATEMENT
The data that support the findings of this study are available from the corresponding author upon reasonable request.