Configurational Flexibility of a Triaryl-Supported SBS Ligand with Rh and Ir: Structural Investigations and Olefin Isomerization Catalysis

Abstract

Here, we report Rh and Ir complexes containing a triaryl SBS pincer ligand flanked by neutral thiomethyl donor groups. Oxidative addition of the B–H bond in the diazaborole proligand H(MeSBSMe) to [Ir(COD)Cl]2 or [Rh(COE)Cl]2 yielded the chloride-bridged dimers [(MeSBSMe)MH(μ-Cl)]2, where M = Ir (1) or Rh (2). Addition of CO to 1 and 2 yielded unstable products that were difficult to isolate, but crystals of monomeric (MeSBSMe)IrH(CO)Cl (3) recovered from the reactions with 1 revealed a change in the MeSBSMe coordination mode from mer to fac. Treating 1 and 2 with LiN(SiMe3)2 yielded mer-(MeSBSMe)IrH[N(SiMe3)2] (4) and the unusual RhI–RhIII dimer (MeSBSMe)Rh(μ-H)[(MeS(μ-B)(μ-SMe)]Rh[N(SiMe3)2] (5) with both mer and fac MeSBSMe. 4 and 5 did not exhibit any alkane transfer hydrogenation reactivity when tested with tert-butylethylene and cyclooctane, but they are highly active for alkene isomerization with 1-hexene. Optimized isomerization reactions showed the highest turnover number (TON) with 4 at 60 °C after 16 h (TON = 10 000), and both catalysts are effective even when tested at room temperature with similar loadings (TON = 600). Collectively, these data highlight the reactivity and inherent coordinative flexibility of the MeSBSMe ligand for comparison to more well-established PBP complexes.