Design, Synthesis and Reactivity of New Zwitterionic Imidazolium-based Species

N-Heterocyclic Carbenes (NHCs) are an interesting family of ligands with donor properties comparable to phosphane ones. They can form complexes with transition elements and, among them, the coinage metals belonging to the 11th group of the periodic table. Metal-NHCs compounds have obtained a lot of interest in the last years because of their potential applications in catalysis, carbene transfer reactions and medicinal inorganic chemistry. In this study, the rational design and synthesis of novel N-(alkyl/aryl)imidazolium-borates compounds and the corresponding reactions towards the deprotonation have been studied. The imidazolium-trihydridoborate adducts have been obtained by the addition of BH3·THF to the starting materials N-benzyl- or N-mesityl-imidazole. The related imidazolium-triphenylborate species have been prepared by the reaction of ammonium tetraphenylborate with benzylimidazole or mesitylimidazole with the loss of one of the phenyl groups. All these new species have been fully characterized, both analytically and spectroscopically, to confirm their structures by means of CHN elemental analysis, 1H-NMR, 13C-NMR, 11B-NMR, FT-IR spectroscopy and electrospray ionization mass spectrometry (ESI-MS). Regarding the compounds (3-benzylimidazolium-1-yl)trihydridoborate, (HImBn)BH3, and (3-mesitylimidazolium-1-yl)trihydridoborate, (HImMes)BH3, single crystals suitable for the X-ray diffraction crystallography have been obtained by slow evaporation of CHCl3 and CHCl3/THF solutions, respectively. The reactivity of all novel compounds as carbene precursors has been evaluated with the aim to produce borate-NHC complexes and a new carbene-borate species has been synthesized via a microwaves-assisted method.