Abstract
In polynuclear biological active sites, multiple electrons are needed for turnover, and the distribution of these electrons among the metal sites is affected by the structure of the active site. However, the study of the interplay between structure and redox distribution is difficult not only in biological systems but also in synthetic polynuclear clusters since most redox changes produce only one thermodynamically stable product. Here, the unusual chemistry of a sterically hindered trichromium complex allowed us to probe the relationship between structural and redox isomerism. Two structurally isomeric trichromium imides were isolated: asymmetric terminal imide (tbsL)Cr3(NDipp) and symmetric, μ3-bridging imide (tbsL)Cr3(μ3–NBn) ((tbsL)6− = (1,3,5-C6H9(NC6H4-o-NSitBuMe2)3)6−). Along with the homovalent isocyanide adduct (tbsL)Cr3(CNBn) and the bisimide (tbsL)Cr3(μ3–NPh)(NPh), both imide isomers were examined by multiple-wavelength anomalous diffraction (MAD) to determine the redox load distribution by the free refinement of atomic scattering factors. Despite their compositional similarities, the bridging imide shows uniform oxidation of all three Cr sites while the terminal imide shows oxidation at only two Cr sites. Further oxidation from the bridging imide to the bisimide is only borne at the Cr site bound to the second, terminal imido fragment. Thus, depending on the structural motifs present in each [Cr3] complex, MAD revealed complete localization of oxidation, partial localization, and complete delocalization, all supported by the same hexadentate ligand scaffold.
Highlights
While THF-adduct 1 is an all-CrII complex, 1 is less structurally analogous to 2 than 4, as the Cr1ÀCr2 distances in both the terminal imido 2 (2.7034(7) A) and the isocyanide 4 (2.5541(6) A) are elongated relative to the THF-adduct (2.439(1) A). Both 2 and 3 represent two-electron oxidations of the trichromium cluster (e.g., [Cr3]8+, where [Cr3]n+ denotes the cumulative oxidation state of the trichromium core) compared to 1 ([Cr3]6+), but we have previously shown that the highlyreducing [Cr3] core of 1 is capable of further substrate reduction to form four-electron oxidized (e.g., [Cr3]10+) bisimido productsCr3(m3–NPh)(NMes) andCr3(m3– NBn)(NBn).[20]
We have previously reported a protocol for multiple-wavelength anomalous diffraction (MAD) which consistently resulted in well-resolved f0 curves for three unique sites of triiron complexes on the platform.[28]
We assessed the distribution of oxidation in three imido-bound isomers of a trichromium complex
Summary
Challenging to decipher due to the possible interplay of through-ligand superexchange, orbital-mediated direct exchange, and structural exibility. Changing the azide substrate size and reaction solvent provided us a path to isolate both the terminal imido complex (tbsL)Cr3(NDipp) (2) and the bridging imido complex (tbsL)Cr3(m3–NBn) (3) (Dipp 1⁄4 (2,6-diisopropyl) phenyl, Bn 1⁄4 benzyl, Scheme 1) In addition to their structural isomerism, mixed-valency, and shared overall molecular oxidation state, these complexes feature bridging ligands capable of mediating superexchange and have metal–metal distances within the van der Waals radius of chromium, allowing for direct exchange between the metal centers. The MAD results for this series of closely related trichromium complexes reveal distinct pro les of oxidation distribution for each of the three mixed-valent imide complexes, demonstrating the profound effects of geometric structure on redox load distribution even within complexes which are otherwise apparently similar
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