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, ema^4- ligand (ema=N,N'-ethylenebis(mercaptoacetamide); M=Ni^II , [V^IV ≡O]²⁺ and Fe^III ) 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 ema^2- . Accordingly, the ema ligand has switched its coordination mode from an N₂ S₂^4- cavity holding a single metal, to a binucleating H₂ ema^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 H₂ ema^2- that is the optimal arrangement for binucleation. XRD molecular structures show extensive H-bonding at the amido-nitrogen protons in the solid state.