Electrospray ionization mass spectrometric study of mercury complexes of N-heterocyclic carbenes derived from 1,2,4-triazolium salt precursors

Abstract

By mixing 1,2,4-triazolium salts (precursors of N-heterocyclic carbenes 1-6) with mercury acetate, a number of complexes have been obtained under electrospray ionization condition. Carbenes 1 and 2 contain one carbene center; therefore, they are able to bond only one mercury cation. Carbenes 3-5 contain two carbene centers; therefore, they can bond two mercury cations. Mercury complexes of 1-5 always contain an acetate anion attached to a mercury cation. Carbene 6 also contains two carbene centers; however, its structure allows formation of a complex containing mercury bonded simultaneously to both centers, therefore, the complex that does not contain an acetate anion. The MS/MS spectra taken for complexes of carbenes 1-5 have shown formation of a cation corresponding to N1 substituent (adamantyl or benzyl), and those of complexes of carbenes 3-5 (doubly charged ions) have also shown the respective complementary partner ions. Mercury complex of 2 has yielded some other interesting fragmentation pathways, e.g. a loss of the HHgOOCCH 3 molecule. The fragmentation pathway of the mercury complexes of 6 was found to be complicated.

Free N‐heterocyclic carbenes from Brønsted acidic ionic liquids: Direct detection by electrospray ionization mass spectrometry

“…Free NHCs can be obtained in the gas phase, as well as in solution, by adding strong bases 28,29 or eventually forcing a hydrogen transfer reaction by collisionally dissociating the ion pair of the parent IL 30 . Indirect evidence of the presence of NHCs in ILs was adduced by the addition of an aldehydic substrate to form the corresponding Breslow intermediate, 13 by trapping transient NHCs in stable metal complexes, 31,32 or by stabilizing this species through a hydrogen bond between the electron pair of the carbene and the C2‐H hydrogen of a surrounding imidazolium cation 33 . In the latter case, diazolium ILs were successfully employed in the gas phase as a source of incipient NHCs since the flexibility of the chemical linker connecting the two heterocyclic heads (one carbenic and the other cationic) allows the folding of the molecular structure and the formation of an intramolecular hydrogen bond 34,35 …”