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Edited by Edward I. Solomon, Stanford University, Stanford, CA, and approved June 25, 2019 (received for review May 3, 2019)
Significance
Identifying the redox states within polynuclear cofactors in biology is critical for elucidating substrate binding sites. Multiple-wavelength, anomalous X-ray diffraction (MAD) has been used to address the redox states within the polynuclear cofactors in nitrogenase. While MAD provides a site-specific analytical technique to resolve differences in relative redox states within molecular species, we note that rigorous application of this technique on well-defined synthetic iso- and mixed-valent clusters has not been performed. Our report illustrates the large variation observed for nominally isovalent sites, thus providing a much-needed calibration on what to expect for resolving redox load distribution within cluster materials.
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.
Footnotes
↵1A.K.B. and J.J.T. contributed equally to this work.
↵2Present address: Department of Chemistry, Columbia University, New York, NY 10027.
↵3Present address: Department of Chemistry, University of Pittsburgh, Pittsburgh, PA 15260.
↵4Present address: Chemistry and Biochemistry, University of California, Santa Barbara, CA 93106.
↵5Present address: Department of Chemistry, University of California, Berkeley, CA 94720.
- ↵6To whom correspondence may be addressed. Email: betley{at}chemistry.harvard.edu.
Author contributions: A.K.B., J.J.T., R.H.S., B.J.M., and T.A.B. designed research; A.K.B., J.J.T., R.H.S., B.J.M., C.E.J., G.M., W.B., D.A.I., A.A.M., R.S., S.G.W., and Y.-S.C. performed research; A.K.B., J.J.T., R.H.S., C.E.J., and G.M. contributed new reagents/analytic tools; A.K.B., J.J.T., R.H.S., B.J.M., S.-L.Z., and T.A.B. analyzed data; and A.K.B., J.J.T., R.H.S., and T.A.B. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
Data deposition: The atomic coordinates and structure factors have been deposited in the Cambridge Crystallographic Data Centre (accession nos. 1897925–1897928).
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1907699116/-/DCSupplemental.
Published under the PNAS license.
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Exposing the inadequacy of redox formalisms by resolving redox inequivalence within isovalent clusters
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