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

Bulkythiolato ligands have been developed for creating biomimeticmodel complexes of active sites in metalloenzymes. Herein, we reporta series of di-ortho-substituted arenethiolato ligandscontaining bulky acylamino groups (RCONH; R = t-Bu-,(4-t-BuC6H4)(3)C-,{3,5-(Me2CH)(2)C6H3}(3)C-,and {3,5-(Me3Si)(2)C6H3}(3)C-) that were developed for biomimetics. Bulky hydrophobicsubstituents generate a hydrophobic space around the coordinatingsulfur atom through the NHCO bond. This steric environment inducesthe formation of low-coordinate mononuclear thiolato Co-(II) complexes.The well-positioned NHCO moieties in the hydrophobic space coordinateto the vacant sites of the cobalt center with different coordinationmodes, viz., the S,O-chelate of the carbonyl C=O or the S,N-chelateof the acylamido CON-. The solid (crystalline) andsolution structures of the complexes were investigated in detail usingsingle-crystal X-ray crystallography, H-1 NMR, and absorptionspectroscopic analyses. The spontaneous deprotonation of NHCO, whichis commonly observed in metalloenzymes but requires a strong basein artificial systems, was simulated by forming a hydrophobic spacein the ligand. This new ligand design strategy is advantageous forcreating model complexes that have never been constructed artificially. Arenethiolato ligands containing bulkyacylamino groupswere synthesized, and topographic studies revealed the generationof hydrophobic pockets. The ligands afforded mononuclear cobalt-(II)thiolate with various coordination modes of NHCO moieties dependingon the bulkiness of hydrophobic pockets. The extremely buried environmentaround the metal center induced a unique reaction, which is observedin a pocket of metalloenzymes. The results indicate that the designof ligands is valuable for the creation of novel model complexes.