Exposing the inadequacy of redox formalisms by resolving redox inequivalence within isovalent clusters

Abstract

In this report we examine a family of trinuclear iron complexes by multiple-wavelength, anomalous diffraction (MAD) to explore the redox load distribution within cluster materials by the free refinement of atomic scattering factors. Several effects were explored that can impact atomic scattering factors within clusters, including 1) metal atom primary coordination sphere, 2) M−M bonding, and 3) redox delocalization in formally mixed-valent species. Complexes were investigated which vary from highly symmetric to fully asymmetric by 57Fe Mössbauer and X-ray diffraction to explore the relationship between MAD-derived data and the data available from these widely used characterization techniques. The compounds examined include the all-ferrous clusters [nBu4N][(tbsL)Fe3(μ3–Cl)] (1) ([tbsL]6– = [1,3,5-C6H9(NC6H4-o-NSitBuMe2)3]6–]), (tbsL)Fe3(py) (2), [K(C222)]2[(tbsL)Fe3(μ3–NPh)] (4) (C222 = 2,2,2-cryptand), and the mixed-valent (tbsL)Fe3(μ3–NPh) (3). Redox delocalization in mixed-valent 3 was explored with cyclic voltammetry (CV), zero-field 57Fe Mössbauer, near-infrared (NIR) spectroscopy, and X-ray crystallography techniques. We find that the MAD results show an excellent correspondence to 57Fe Mössbauer data; yet also can distinguish between subtle changes in local coordination geometries where Mössbauer cannot. Differences within aggregate oxidation levels are evident by systematic shifts of scattering factor envelopes to increasingly higher energies. However, distinguishing local oxidation levels in iso- or mixed-valent materials can be dramatically obscured by the degree of covalent intracore bonding. MAD-derived atomic scattering factor data emphasize in-edge features that are often difficult to analyze by X-ray absorption near edge spectroscopy (XANES). Thus, relative oxidation levels within the cluster were most reliably ascertained from comparing the entire envelope of the atomic scattering factor data.

Revealing redox isomerism in trichromium imides by anomalous diffraction

“…However, the left-hand falling edge and right-hand rising edge, i.e., the overall energetic location of the 𝑓𝑓′ curve was found to shift to higher energy on oxidation. 28 As in the previous study, the uncertainty in the incident X-ray energy is roughly 1 eV, so a shift ≤1 eV is within that uncertainty.…”

“…Given that chromium has the lowest energy K-edge of the elements to which high resolution MAD has been applied, we note that 𝑓𝑓′ spectra were obtained with similarly low uncertainty values to those obtained for iron complexes following similar exposure times to synchrotron radiation (8-10 h per sample). 28 The results from our study benchmarking the use of highresolution MAD on a series of triiron complexes directly informed both this MAD study and our discussion and interpretation of its results, below. To summarize briefly, we determined that the local coordination geometry of each transition metal site in a polynuclear complex strongly impacts the appearance/features of the 𝑓𝑓′ curves, particularly in the pre-edge and in-edge regions.…”

“…Meanwhile, Cr IV imide 7 has a much sharper minimum at 5997 eV, corresponding to the on-edge feature in the K edge scan, and the on-edge feature in the edge scan of Cr(N(TMS) 2 ) 2 (thf) 2 translates into a doubleminimum in its 𝑓𝑓′ curve. As we have previously discussed for a related Fe system, 28 the absolute minimum in f' is strongly affected by in-edge features, which can arise from allowed 1s4p transitions or from "shake-down" transitions, depending on the local geometry of the metal site. 42,43 Due to these in-edge features the absolute minimum of f' should not be interpreted as directly related to oxidation state.…”

“…10,[23][24][25][26][27] A previous study by our research group revealed how cluster features (e.g., metal primary coordination sphere, M-M bonding, mixed valency) in a triiron system impact MAD measurements, allowing us to correlate site specific redox information against bulk measurement techniques (e.g., 57 Fe Mössbauer, cyclic voltammetry). 28 Alongside the two imide structure types 2 and 3, a homovalent structural analogue of 2, ( tbs L)Cr 3 (CNBn) (4), and the further oxidized bisimide ( tbs L)Cr 3 (µ 3 -NPh)(NPh) (5) were also investigated using MAD. The MAD results for this series of closely related trichromium complexes reveal distinct profiles of oxidation distribution for each of the three mixed-valent imide complexes, thus demonstrating the profound effects of geometric structure on redox load distribution even within complexes which are otherwise extremely similar.…”

Postbiosynthetic modification of a precursor to the nitrogenase iron–molybdenum cofactor

“…In these and other examples, the high cluster nuclearity is thought to facilitate efficient catalysis, but it also introduces practical challenges for mechanistic and spectroscopic studies. In particular, techniques that directly probe the metal centers (e.g., Mössbauer spectroscopy, which is exceptionally useful for mononuclear Fe enzymes) are often less insightful for high-nuclearity metalloclusters; with increasing cluster nuclearity it can be difficult or impossible to resolve the spectroscopic responses arising from many metal sites, and it is further challenging to correlate these spectroscopic features to specific sites in the three-dimensional structure (8)(9)(10)(11)(12)(13)(14)(15)(16). Thus, the polynuclear nature of complex metalloclusters both underpins their unique functions and, in many cases, precludes a molecular-level understanding of their electronic structures and reactivity.…”