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

With the goal of generating anionic analogues to MN 2 S 2 ∙ Mn(CO) 3 Br we introduced metallodithiolate ligands, MN 2 S 2 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, (H 2 N 2 (CH 2 C=O(µ-S)) 2 [Mn(CO) 3 ] 2 resulted from loss of M originally residing in the N 2 S 2 4- pocket, replaced by protonation at the amido nitrogens, generating H 2 ema 2- . Accordingly, the ema ligand has switched its coordination mode from an N 2 S 2 4- cavity holding a single metal, to a binucleating H 2 ema 2- with bridging sulfurs and carboxamide oxygens within Mn-µ-S-CH 2 -C-O, 5-membered rings. In situ metal-templating by zinc ions gives quantitative yields of the Mn 2 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 H 2 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.