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

With the goal of generating anionic analogues to MN₂S₂ ⋅ Mn(CO)₃Br we introduced metallodithiolate ligands, MN₂S₂²⁻ prepared from the Cys-X-Cys biomimetic, ema⁴⁻ ligand (ema=N,N′-ethylenebis(mercaptoacetamide); M=Ni^II, [V^IV≡O]²⁺ and Fe^III) to Mn(CO)₅Br. An unexpected, remarkably stable dimanganese product, (H₂N₂(CH₂C=O(μ-S))₂)[Mn(CO)₃]₂ resulted from loss of M originally residing in the N₂S₂⁴⁻ pocket, replaced by protonation at the amido nitrogens, generating H₂ema²⁻. Accordingly, the ema ligand has switched its coordination mode from an N₂S₂⁴⁻ cavity holding a single metal, to a binucleating H₂ema²⁻ 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 H₂ema²⁻ that is the optimal arrangement for binucleation. XRD molecular structures show extensive H-bonding at the amido-nitrogen protons in the solid state.