Confronting Neutrality: Maximizing Success in the Analysis of Transition-Metal Catalysts by MALDI Mass Spectrometry

Abstract

The elucidation of molecular identity is a central challenge in homogeneous transition-metal catalysis. Most exacting, despite advances in electrospray-ionization mass spectrometry (ESI-MS), are neutral complexes, which encompass the vast majority of metal catalysts. Insight into molecular constitution, readily achieved in other areas of the chemical sciences, is commonly thwarted by decomposition of inorganic and organometallic compounds, particularly for highly reactive species. MALDI-MS, where coupled with charge-transfer ionization, stands out from all other current MS methods in its unique capacity to report on the molecular identity of metal complexes irrespective of their initial charge state. Identified in the present work are methods that enable routine, unambiguous identification of such complexes across a wide range of standard MALDI mass spectrometers. The origin of fragmentation during MALDI-MS analysis is explored on 13 different instruments at nine facilities, and across a range of mass analyzers, from TOF, TOF-TOF, and Q-TOF to Orbitrap. Selected as test analytes were the second-generation Hoveyda and Grubbs metathesis catalysts, and the Grubbs resting-state methylidene complex, which span a very broad range in terms of ligand lability, and hence susceptibility to fragmentation. Three critical parameters emerge: (1) Using the minimum applied laser energy necessary for sample volatilization; (2) Minimizing the absorption cross-section of the analyte at the laser wavelength: (a) by appropriate laser choice, where options exist; (b) by using a matrix that absorbs as strongly as possible at the laser wavelength, in significant excess relative to the analyte (e.g. 500-fold); (c) by prompt analysis, to limit matrix sublimation in the ion source; (3) Using a charge-transfer matrix devoid of reactive protic or donor sites. A final, forward-looking section highlights relevant advances in state-of-the-art instrumentation, and instrumental features that would contribute to the optimization of next-generation MALDI spectrometers for this emerging application. These include fast-firing, contoured-profile and, potentially, wavelength-tunable lasers; high-resolution mass analyzers; automatic plate rastering with software to support retroactive compilation of spectra from optimal sampling locations, and soft-vacuum or atmospheric-pressure sources, in conjunction with a standardized anaerobic interface.

Laser Desorption/Ionization Mass Spectrometry of Perovskite Solar Cells: Identification of Interface Interactions and Degradation Reactions

“…The detection of ionized CTCs in LDI‐MS is quite unusual. CTCs are often employed for MALDI‐MS analysis of neutral and/or photosensitive compounds with the help of charge‐transfer matrices . However, the analyte‐matrix CTCs are not stable under LDI conditions and dissociate into the cation radical of the electron donor, observed in the positive detection mode, and the anion radical of the electron‐acceptor, detectable in the negative detection mode.…”

Handling considerations for the mass spectrometry of reactive organometallic compounds

“…They also interfaced an inert atmosphere glove box with a matrix-assisted laser desorption ionization-time of flight (MALDI-TOF) to overcome problems regarding decomposition during sample transfer to the source, as seen in Figure 7. 45,46 Using the above-mentioned MALDI setup, an oxygen-sensitive titanium (III) ethylene polymerization catalyst was analyzed, as seen in box for laser desorption/ionization MS of air-sensitive solids can be used if the user is worried about contamination from the matrix. 47 When a purge box is used, the air-sensitive sample is always under inert gas while transferring to the source.…”

Ionization methods for the mass spectrometry of organometallic compounds

“…But this situation has been remedied by the introduction of matrices suitable for organometallic compounds by Fogg and co‐workers, who have shown that unfunctionalized polyarenes such as anthracene and pyrene perform well as matrices for fragile metal complexes . Fogg also pioneered the fully integrated combination of mass spectrometer and full‐size glovebox, which is an essential adjunct for MALDI of highly air‐ and moisture‐sensitive compounds given the necessity of exposure to the atmosphere during sample preparation . Figure shows the MALDI‐TOF mass spectrum of a variety of ruthenium complexes.…”