Introducing Ion Mobility Mass Spectrometry to Identify Site-Selective C–H Bond Activation in N‑Heterocyclic Carbene Metal Complexes

• Published in: Journal of the American Society for Mass Spectrometry Vol. 33; no. 12; pp. 2291 - 2300
• Main Authors: Mollar-Cuni, Andrés, Ibáñez-Ibáñez, Laura, Guisado-Barrios, Gregorio, Mata, Jose A., Vicent, Cristian
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 07.12.2022
• ISSN: 1044-0305; 1879-1123; 1879-1123
• DOI: 10.1021/jasms.2c00257

The activation of C–H bonds in a selective manner still constitutes a major challenge from a synthetic point of view; thus, it remains an active area of fundamental and applied research. Herein, we introduce ion mobility spectrometry mass spectrometry-based (IM-MS) approaches to uncover site-selective C–H bond activation in a series of metal complexes of general formula [(NHC)­LMCl]+ (NHC = N-heterocyclic carbene; L = pentamethylcyclopentadiene (Cp*) or p-cymene; M = Pd, Ru, and Ir). The C–H bond activation at the N-bound groups of the NHC ligand is promoted upon collision induced dissociation (CID). The identification of the resulting [(NHC-H)­LM]+ isomers relies on the distinctive topology that such cyclometalated isomers adopt upon site-selective C–H bond activation. Such topological differences can be reliably evidenced as different mobility peaks in their respective CID-IM mass spectra. Alternative isomers are also identified via dehydrogenation at the Cp*/p-cymene (L) ligands to afford [(NHC)­(L-H)­M]+. The fragmentation of the ion mobility-resolved peaks is also investigated by CID-IM-CID. It enables the assignment of mobility peaks to the specific isomers formed from C­(sp2)–H or C­(sp3)–H bond activation and distinguishes them from the Cp*/p-cymene (L) dehydrogenation isomers. The conformational change of the NHC ligands upon C–H bond activation, concomitant with cyclometalation, is also discussed on the basis of the estimated collision cross section (CCS). A unique conformation change of the pyrene-tagged NHC members is identified that involves the reorientation of the NHC ring accompanied by a folding of the pyrene moiety.