RESUMO
Thiocarboxamide chelates are known to assemble [2Mn2S] diamond core complexes via µ-S bridges that connect two MnI(CO)3 fragments. These can exist as syn- and anti-isomers and interconvert via 16-electron, monomeric intermediates. Herein, we demonstrate that reduction of such Mn2 derivatives leads to a loss of one thiocarboxamide ligand and a switch of ligand binding mode from an O- to N-donor of the amide group, yielding a dianionic butterfly rhomb with a short Mn0-Mn0 distance, 2.52 Å. Structural and chemical analyses suggest that reduction of the Mn(I) centers is dependent on the protonation state of the amide-H, as total deprotonation followed by reduction does not result in the reduction of the Mn2 core. Partial deprotonation followed by reduction suggests a pathway that involves monomeric Mn(CO)3(S-O) and Mn(CO)3(S-N) intermediates. Ligand modifications to tertiary amides that remove the possibility of amide-H reduction led to complexes that preserve the [2Mn2S] diamond core during chemical reduction. Further comparison with the tethered system, linking the Mn(CO)3(S-O) sites together, suggests that dimer dissociation is necessary for the overall reductive transformation. These results highlight organomanganese carbonyl chemistry to establish illustrations of peptide fragment binding modes in the uptake of low-valent metal carbonyls related to binuclear active sites of biocatalysts.
RESUMO
Occasioned by the discovery of a ligand transfer from M(N2S2) to MnI in Mn(CO)5Br, the resulting H2N2S2 ligand-tethered dimanganese complex, (µ4-N,N'-ethylenebis(mercaptoacetamide))[Mn2(CO)6], was found to have myriad analogues of the type (µ-S-E)2[Mn2(CO)6], making up an under-studied class containing Mn2S2 rhombs. The attempt to synthesize a nontethered version resulted in a solid-state structure in an anti-conformation. However, a direct comparison of the Fourier-transform infrared spectra of the tethered versus nontethered complexes in combination with theoretical frequency calculation suggested the coexistence of syn- and anti-isomers and their interconversion in solution. Analysis of the syn- versus anti-version of the dimanganese components led to the understanding that whereas the anti-form exists as centrosymmetric RS isomers, the syn-form is restricted by C2 symmetry to be either RR or SS. Molecular scrambling experiments indicated monomeric, pentacoordinate, 16-e- (S-O)Mn(CO)3 intermediates with lifetimes sufficiently long to sample R and S monomers. Density functional theory analysis of the mechanistic pathway and a kinetic study corroborated that the proposed isomerization involves the cleavage and reformation of the dimeric structures.