"Synthesis of Ruthenium-Dithiocarbamate Chelates Bearing Diphosphine Ligands and their Use as Latent Initiators for Atom Transfer Radical Additions"

Mohammed Zain Aldin, Guillermo Zaragoza, and Lionel Delaude

journal cover 
          source: Journal of Organometallic Chemistry
            year: 2021
          volume: 950
         article: 121993
           pages: 12
             doi: 10.1016/j.jorganchem.2021.121993
graphical abstract

Abstract: Nine representative [Ru(S2CNEt2)2(diphos)] complexes were prepared in almost quantitative yields (91-97%) from [RuCl2(p-cymene)]2, a diphosphine (dppm, dppe, dppp, dppb, dpppe, dppen, dppbz, dppf, or DPEphos), and sodium diethyldithiocarbamate trihydrate using a novel, straightforward, one-pot procedure. The recourse to a monomodal microwave reactor was instrumental in reaching the thermodynamic equilibria favoring the targeted monometallic trichelates. All the products were fully characterized by using various analytical techniques and the molecular structures of seven of them were determined by X-ray crystallography. NRM, XRD, and IR spectroscopies evidenced a significant contribution of the thioureide resonance form Et2N+=CS22- to the electronic structure of the 1,1-dithiolate ligand. MS/MS spectrometry showed the formation of phosphine-free [Ru(S2CNEt2)2]+ cations in the gas phase, except when starting from [Ru(S2CNEt2)2(dppbz)]. The activity of the nine complexes was probed in three different catalytic processes, viz., the cyclopropanation of styrene with ethyl diazoacetate, the synthesis of vinyl esters from benzoic acid and 1-hexyne, and the atom transfer radical addition (ATRA) of carbon tetrachloride and methyl methacrylate. In the first two reactions, the saturated trichelates were poorly efficient. This was most likely due to their high stability, which prevented the formation of coordinatively unsaturated species. Contrastingly, with a turnover number of 2000 and an initial turnover frequency of 2080 h-1 for a 0.05 mol% catalyst loading, the [Ru(S2CNEt2)2(dppm)] complex emerged as a very robust, latent ATRA initiator, whose activity matched or outperformed those displayed by the most efficient ruthenium catalysts described so far.

Keywords: Homogeneous Catalysis, Ligand Effects, Microwave Synthesis, Radical Reactions, Reaction Mechanisms

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