Metal-Templated, Tight Loop Conformation of a Cys-X-Cys Biomimetic Assembles a Dimanganese Complex.

With the goal of generating anionic analogues to MN2 S2 ⋅Mn(CO)3 Br we introduced metallodithiolate ligands, MN2 S2 2- prepared from the Cys-X-Cys biomimetic, ema4- ligand (ema=N,N'-ethylenebis(mercaptoacetamide); M=NiII , [VIV ≡O]2+ and FeIII ) to Mn(CO)5 Br. An unexpected, remarkably stable dimanganese product, (H2 N2 (CH2 C=O(μ-S))2 )[Mn(CO)3 ]2 resulted from loss of M originally residing in the N2 S2 4- pocket, replaced by protonation at the amido nitrogens, generating H2 ema2- . Accordingly, the ema ligand has switched its coordination mode from an N2 S2 4- cavity holding a single metal, to a binucleating H2 ema2- with bridging sulfurs and carboxamide oxygens within Mn-μ-S-CH2 -C-O, 5-membered rings. In situ metal-templating by zinc ions gives quantitative yields of the Mn2 product. By computational studies we compared the conformations of "linear" ema4- to ema4- frozen in the "tight-loop" around single metals, and to the "looser" fold possible for H2 ema2- that is the optimal arrangement for binucleation. XRD molecular structures show extensive H-bonding at the amido-nitrogen protons in the solid state.