Observation of Discrete Valence Tautomers in Crystalline Cyclopentadienyl Radicals

Exploiting Molecular Symmetry to Quantitatively Map the Excited-State Landscape of Iron-Sulfur Clusters

Cuboidal [Fe4S4] clusters are ubiquitous cofactors in biological redox chemistry. In the [Fe4S4]1+ state, pairwise spin coupling gives rise to six arrangements of the Fe valences ("valence isomers") among the four Fe centers. Because of the magnetic complexity of these systems, it has been challenging to understand how a protein's active site dictates both the arrangement of the valences in the ground state as well as the population of excited-state valence isomers. Here, we show that the ground-state valence isomer landscape can be simplified from a six-level system in an asymmetric protein environment to a two-level system by studying the problem in synthetic [Fe4S4]1+ clusters with solution C3v symmetry. This simplification allows for the energy differences between valence isomers to be quantified (in some cases with a resolution of <0.1 kcal/mol) by simultaneously fitting the VT NMR and solution magnetic moment data. Using this fitting protocol, we map the excited-state landscape for a range of clusters of the form [(SIMes)3Fe4S4-X/L]n, (SIMes = 1,3-dimesityl-imidazol-4,5-dihydro-2-ylidene; n = 0 for anionic, X-type ligands and n = +1 for neutral, L-type ligands) and find that a single ligand substitution can alter the relative ground-state energies of valence isomers by at least 103 cm-1. On this basis, we suggest that one result of "non-canonical" amino acid ligation in Fe-S proteins is the redistribution of the valence electrons in the manifold of thermally populated excited states.

Coordination polymers of 5-substituted 1,2,3,4-tetracyanocyclopentadienides: structural and electrochemical properties of complex compounds of 5-amino- and 5-nitro-tetracyanocyclopentadienide

Compounds of the 5 amino (ATCC) and 5 nitro 1,2,3,4 tetracyanocyclopentadienide (NTCC) ligand with iron(II) and iron(III), silver(I) and potassium(I) were prepared and characterized by electrochemical methods using EPR, cyclic voltammetry and Mößbauer spectroscopy as well as UV-Vis spectroscopy. The investigation sheds light on the free radical activity in non- as well as transition metal compounds, and shows signs of photoinduced electron transfer in transition metal compounds of the ligands. We characterized silver(I) and potassium compounds of the compounds via X-ray diffraction, with one of the structures exhibiting a porous coordination polymer.