From [2Mn2S] Diamond Cores to Butterfly Rhombs: Transformations\nThat Highlight Alternating Peptide Binding Sites

Thiocarboxamide chelates\nare known to assemble [2Mn2S] diamond\ncore complexes via μ-S bridges that connect two Mn<sup>I</sup>(CO)<sub>3</sub> fragments. These can exist as <i>syn-</i> and <i>anti</i>-isomers and interconvert via 16-electron,\nmonomeric intermediates. Herein, we demonstrate that reduction of\nsuch Mn<sub>2</sub> derivatives leads to a loss of one thiocarboxamide\nligand and a switch of ligand binding mode from an O- to N-donor of\nthe amide group, yielding a dianionic butterfly rhomb with a short\nMn<sup>0</sup>–Mn<sup>0</sup> distance, 2.52 Å. Structural\nand chemical analyses suggest that reduction of the Mn(I) centers\nis dependent on the protonation state of the amide-H, as total deprotonation\nfollowed by reduction does not result in the reduction of the Mn<sub>2</sub> core. Partial deprotonation followed by reduction suggests\na pathway that involves monomeric Mn(CO)<sub>3</sub>(S–O) and\nMn(CO)<sub>3</sub>(S–N) intermediates. Ligand modifications\nto tertiary amides that remove the possibility of amide-H reduction\nled to complexes that preserve the [2Mn2S] diamond core during chemical\nreduction. Further comparison with the <i>tethered</i> system,\nlinking the Mn(CO)<sub>3</sub>(S–O) sites together, suggests\nthat dimer dissociation is necessary for the overall reductive transformation.\nThese results highlight organomanganese carbonyl chemistry to establish\nillustrations of peptide fragment binding modes in the uptake of low-valent\nmetal carbonyls related to binuclear active sites of biocatalysts.