Synthesis and characterization of NOHC organometallic complexes

N-heterocyclic carbene ligands in transition metal complexes have been widely studied in the last decades. Surprisingly, N-oxy-heterocyclic carbene species (abbreviated NOHC) have not raised as much interest in scientific community. After the earliest researches on Ni(II), Pd(II), Rh(I), Ag(I) and Au(I) complexes with these types of ligands in 2007 and 2010, no new progresses have been reported until 2021. In this master thesis, a collaboration between Università degli Studi di Padova and Technische Universität Dresden, we focused on the synthesis of Ag(I), Au(I) and Pt(II) NOHC complexes in order to explore different synthesis pathways for extended aromatic motifs and to evaluate the photoluminescence properties of the Pt(II) compounds. The project can be summarized in these sections: i) silver(I) and gold(I) complexes with symmetrically substituted N-O-heterocyclic carbene ligands; ii) metal complexes with phenyl-substituted NOHC ligands; iii) deoxygenation experiments in order to expand the type of NOHC ligands. i) Silver(I) and gold(I) complexes with symmetrically substituted NOHC ligands. Two bis-carbene silver(I) complexes and their gold(I) analogues, with the general formula [ML2]+(PF6)- (L=NOHC ligand), have been synthesized. The Au(I) compounds were obtained through transmetalation from sensitive Ag(I) precursors. The symmetrically substituted benzyl-NOHC was already reported in the literature, whereas the 2-methylnaphtalenyl analogue was not. The proligand with the larger substituent contains small amounts of impurities (the monosubstituted compound) in both the proligand and complexes samples. That proved that for large aromatic substituents the synthesis via nucleophilic attack is not satisfactory. ii) Metal complexes with phenyl-substituted NOHC ligands. Four Pt(II) cyclometalated complexes with a general formula [Pt(C^C*)(L^L)] (C^C*=cyclometalated NOHC ligand; L^L= β-diketonate ligand) have been obtained and fully characterized. Part of the structure of complex 13 was not defined by the obtained data. Single crystals of compounds 11 and 14 were analysed and the structure geometry have been confirmed. Photophysical characterization of the four compounds and comparison with results reported in the literature highlighted that the addition of an oxygen-bearing moiety on the nitrogen atom of the imidazole ring has a limited influence. Cyclic voltammetries and quantum chemical calculations concerning the electron density distribution and the free energies of the ground singlet and the first excited triplet states confirmed the emission features and gave an estimation of the HOMO-LUMO gap. The Au(I) complex 15 was synthesized both via weak base assisted metalation and through transmetalation route. Nonetheless, this last procedure affords a less impurities-containing product as demonstrated by 1H NMR spectrum. The crystal structure of complex 15 was determined as well. iii) Deoxygenation experiments. To further modulate the properties of NOHC complexes, the cleavage of one of the N-O bonds could be considered as the synthesis of 1-hydroxyimidazole-3-oxide is easier and more efficient than the singly-oxygenated imidazolium rings. Pd/C hydrogenation of 1-hydroxyimidazole-3-oxide proved to be unproductive as imidazole, the main side product, is barely separable from the mono-deoxygenated one. Raney-Nickel did not give the expected result as well, since no product was detected. A polymerization of the resulting compound can be hypothesized from the HRMS analysis.