Syntheses and Structures of Arenethiolato Cobalt(II) Complexes Containing Acylamino Groups: Steric Effects of Bulky Ligands on Coordination and Geometry.

Bulky thiolato ligands have been developed for creating biomimetic model complexes of active sites in metalloenzymes. Herein, we report a series of di-ortho-substituted arenethiolato ligands containing bulky acylamino groups (RCONH; R = t-Bu-, (4-t-BuC6H4)3C-,{3,5-(Me2CH)2C6H3}3C-, and {3,5-(Me3Si)2C6H3}3C-) that were developed for biomimetics. Bulky hydrophobic substituents generate a hydrophobic space around the coordinating sulfur atom through the NHCO bond. This steric environment induces the formation of low-coordinate mononuclear thiolato Co(II) complexes. The well-positioned NHCO moieties in the hydrophobic space coordinate to the vacant sites of the cobalt center with different coordination modes, viz., the S,O-chelate of the carbonyl C═O or the S,N-chelate of the acylamido CON-. The solid (crystalline) and solution structures of the complexes were investigated in detail using single-crystal X-ray crystallography, 1H NMR, and absorption spectroscopic analyses. The spontaneous deprotonation of NHCO, which is commonly observed in metalloenzymes but requires a strong base in artificial systems, was simulated by forming a hydrophobic space in the ligand. This new ligand design strategy is advantageous for creating model complexes that have never been constructed artificially.