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Comprehensive Experimental and Computational Study of η6‑Arene Ruthenium(II) and Osmium(II) Complexes Supported by Sulfur Analogues of the β‑Diketiminate Ligand

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journal contribution
posted on 07.06.2018, 16:36 by Crystal O’Connor, Darren C. Lawlor, Conor Robinson, Helge Müller-Bunz, Andrew D. Phillips
In comparison to β-diketiminates, a highly exploited class of N,N-chelating ligands, the corresponding β-thioketoiminates, monothio-substituted analogues, have received only minor attention. β-Thioketoiminates are straightforwardly prepared through treatment of an appropriate β-ketoiminate with Lawesson’s reagent. Employing standard synthetic techniques for η6-arene Ru­(II) and Os­(II) β-diketiminate complexes, an analogous series of chlorido-metal complexes supported by different sized N-aryl substituted β-thioketoiminate ligands is reported. However, metal ligation of a β-thioketoiminate bearing an electron-withdrawing CF3 group was not possible. The metal–chlorine bond in these complexes is readily activated by various sodium or silver salts of weakly coordinating anions, affording coordinately unsaturated cationic formally 16-electron species. All η6-C6H6 metal β-thioketoiminate complexes were characterized by NMR and in the solid state using single crystal X-ray diffraction techniques. Structural studies reveal that incorporation of a thio-group induces substantial bond angle distortion within the metallocycle. The reactivity of the cationic η6-C6H6 Ru­(II) β-thioketoiminate complexes toward alkynes and isonitriles is analogous to that of the β-diketiminate species. Specifically, the reaction with 1-hexyne results in a [4 + 2] cycloaddition involving the metal and β-C sites, while reaction with isonitrile completely displaces the η6-C6H6 ligand. A comprehensive DFT study employing charge decomposition analysis (CDA) reveals a strong covalent metal–sulfur bond which dominates the metal β-thioketoiminate interaction. The M–S bond (M = Ru or Os) is strengthened by charge transfer from metal to sulfur, in contrast to the β-diketiminate species where back electron donation from the metal to the nitrogen centers is negligible. The first reported β-selenoketoiminate was prepared by reacting a β-ketoiminate with the Woolins’ reagent. However, this seleno-analog demonstrated significant instability with respect to hydrolysis, and coordination to an η6-arene Ru­(II) or Os­(II) moiety proved unsuccessful.