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

Abstract

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(sp 2 )−H or C(sp 3 )−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.

Ion mobility mass spectrometry uncovers regioselectivity in the carboxylate-assisted C–H activation of palladium N-heterocyclic carbene complexes

“…Complementary CID experiments registered after the ion mobility separation, the so-called CID-IM-CID experiment, 43 were performed as a means to obtain structural information of the three mobility-resolved peaks and assign them to specific isomers. In this mode of operation, the ion mobility-resolved isomers are subjected to a second fragmentation step.…”

“…Complementary CID experiments registered after the ion mobility separation, the so-called CID-IM-CID experiment, 43 were performed as a means to obtain structural information of the three mobility-resolved peaks and assign them to specific…”

“…In this sense, ion mobility spectrometry mass spectrometry (IM-MS) could serve to identify these isomers as long as the site-selective C-H bond activation affords products of different topology (size and shape). 43 The use of the IM-MS technique for gas-phase ion chemistry studies was pioneered in 2011, 44,45 but mechanistic reports that rely on the use of IM-MS are still scarce. [46][47][48][49][50][51][52] Herein, we investigate the C-H activation at the N-bound groups of NHC palladium complexes (see Scheme 1a).…”

Visible light harvesting alkyne hydrosilylation mediated by pincer platinum complexes

“…Reaction monitoring by 1 H NMR analysis allowed us to confirm the advance of the reaction by identifying the initial and final products and provides evidence of catalyst resting state but gives insufficient information about the reaction intermediates formed along the reaction process. However, this limitation is mitigated in combination with ESI-MS, which is one of the most sensitive and powerful techniques to identify reaction intermediates in catalysis, [61][62][63][64] including also photochemical transformations. [65] Reaction monitoring by 1 H NMR spectroscopy in the photohydrosilylation of internal alkynes clearly shows the characteristic signals corresponding to Si-H disappearance and C-H formation of  and β addition products (Figure S31).…”