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, MN₂S₂^2? prepared from the Cys‐X‐Cys biomimetic, ema^4? 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 N₂S₂^4? pocket, replaced by protonation at the amido nitrogens, generating H₂ema^2?. Accordingly, the ema ligand has switched its coordination mode from an N₂S₂^4? cavity holding a single metal, to a binucleating H₂ema^2? 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” ema^4? to ema^4? frozen in the “tight‐loop” around single metals, and to the “looser” fold possible for H₂ema^2? that is the optimal arrangement for binucleation. XRD molecular structures show extensive H‐bonding at the amido‐nitrogen protons in the solid state.