Exploring the Limits of Dative Boratrane Bonding: Iron as a Strong Lewis Base in Low-Valent Non-Heme Iron-Nitrosyl Complexes
Abstract
We previously reported the synthesis and preliminary characterization of a unique series of low-spin (ls) {FeNO}⁸⁻¹⁰ complexes supported by an ambiphilic trisphosphineborane ligand, [Fe(TPB)(NO)]^(+/0/−). Herein, we use advanced spectroscopic techniques and density functional theory (DFT) calculations to extract detailed information as to how the bonding changes across the redox series. We find that, in spite of the highly reduced nature of these complexes, they feature an NO+ ligand throughout with strong Fe−NO π-backbonding and essentially closed-shell electronic structures of their FeNO units. This is enabled by an Fe−B interaction that is present throughout the series. In particular, the most reduced [Fe(TPB)(NO)]− complex, an example of a ls-{FeNO}¹⁰ species, features a true reverse dative Fe → B bond where the Fe center acts as a strong Lewis-base. Hence, this complex is in fact electronically similar to the ls-{FeNO}⁸ system, with two additional electrons "stored" on site in an Fe−B single bond. The outlier in this series is the ls-{FeNO}⁹ complex, due to spin polarization (quantified by pulse EPR spectroscopy), which weakens the Fe−NO bond. These data are further contextualized by comparison with a related N₂ complex, [Fe(TPB)(N₂)]⁻, which is a key intermediate in Fe(TPB)-catalyzed N₂ fixation. Our present study finds that the Fe → B interaction is key for storing the electrons needed to achieve a highly reduced state in these systems, and highlights the pitfalls associated with using geometric parameters to try to evaluate reverse dative interactions, a finding with broader implications to the study of transition metal complexes with boratrane and related ligands.
Additional Information
© 2020 American Chemical Society. Received: June 8, 2020; Published: September 29, 2020. This work was supported by grants from the National Science Foundation (CHE-1608331 and CHE-2002855 to NL) and the National Institutes of Health (GM 070757 to JCP). HTD acknowledges support from the Eastman Summer Research Fellowship and the Robert W. Parry Scholarship. MJC acknowledges support from the Resnick Sustainability Institute at Caltech. The Caltech EPR facility was supported by the Dow Next Generation Educator Fund. The authors declare no competing financial interest.Attached Files
Accepted Version - nihms-1638627.pdf
Supplemental Material - ic0c01686_si_001.pdf
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Additional details
- PMCID
- PMC7640944
- Eprint ID
- 105634
- Resolver ID
- CaltechAUTHORS:20200929-105404784
- NSF
- CHE-1608331
- NSF
- CHE-2002855
- NIH
- GM 070757
- Eastman Summer Research Fellowship
- Robert W. Parry Scholarship
- Resnick Sustainability Institute
- Dow Next Generation Educator Fund
- Created
-
2020-09-29Created from EPrint's datestamp field
- Updated
-
2022-02-12Created from EPrint's last_modified field
- Caltech groups
- Resnick Sustainability Institute