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

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.