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  • Luecke, Ana-Luiza; Wiechmann, Sascha; Freese, Tyll; Nieger, Martin; Foeldes, Tamas; Papai, Imre; Gjikaj, Mimoza; Adam, Arnold; Schmidt, Andreas (2018)
    Unstable N-heterocyclic carbenes can be masked and stabilized as pseudo-cross-conjugated hetarenium-carboxylates which decarboxylate on warming. This study deals with the decarboxylation of carboxylates of mesoionic compounds to generate anionic N-heterocyclic carbenes. Lithium sydnone-4-carboxylates were therefore prepared via 4-bromosydnones by halogen-lithium exchange with nBuLi and subsequent treatment with carbon dioxide. Protonation gave the corresponding sydnone-4-carboxylic acids. Thermogravimetric measurements in addition to temperature dependent IR spectroscopy proved the decarboxylation of lithium sydnone-4-carboxylates and formation of the corresponding sydnone anions which can be represented as anionic N-heterocyclic carbenes. In DMSO-d6 solution, water favors the decarboxylation. Calculations have been performed to elucidate the mechanism of the decarboxylation in the absence and presence of water. (C) 2018 Elsevier Ltd. All rights reserved.
  • Freese, Tyll; Namyslo, Jan C.; Nieger, Martin; Schmidt, Andreas (2019)
    The sydnone imines (5-benzoylimino)-3-(2-methoxyphenyl)-sydnone imine and molsidomine were deprotonated at C4 to give sydnone imine anions which can be represented as anionic N-heterocyclic carbenes, respectively. Trapping reactions with sulfur gave unstable sydnone imine sulfides which were stabilized by the formation of methyl thioethers, methyl sulfoxides, gold complexes [(PPh3) Au-S-sydnone imine] and a bis(ligand) mercury(II) complex. The latter possesses a tetrahedral coordination of the mercury central atom to the sulfur atoms with the N6 nitrogen atoms coordinating as neutral ligands. Water converted the molsidomine anion into ethyl(2-morpholino-2-thioxoacetyl) carbamate. Mercury(II) chloride and triphenylborane were employed to trap the sydnone imine carbenes as mercury complexes as well as BPh3 adducts.
  • Joost, Maximilian; Nieger, Martin; Lutz, Martin; Ehlers, Andreas W.; Slootweg, Jacob Christiaan (Chris); Lammertsma, Koop (2020)
    O- and S-heterocyclic carbenes (OHCs, SHCs) are shown experimentally and computationally to be stronger pi acceptors than NHCs and lack, of course, substituents at the heteroatoms. These different electronic and steric characteristics make OHCs and SHCs interesting ligands for coordination chemistry. Convenient synthetic routes are presented to access their iridium(I), iridium(III), and coinage-metal(I) (Cu, Ag, Au) complexes in good yields by means of dissociation of olefins, deprotonation of precursor salts, and transmetalation from a silver carbene complex Molecular structures and detailed bonding analyses of these complexes are presented.
  • Zhang, Jiaxi; Pidlypnyi, Nazar; Nieger, Martin; Namyslo, Jan C.; Schmidt, Andreas (2014)