Boronated Cyanometallates
Abstract
Fourteen boronated cyanometallates ([M(CN-BR₃)₆]^(3/4/5–) (M = Cr, Mn, Fe, Ru, Os, R = BPh₃, B(2,4,6,-F₃C₆H₂)₃, B(C₆F5)₃) have been characterized by X-ray crystallography and spectroscopy [UV-vis-NIR, NMR, IR, spectroelectrochemistry, and magnetic circular dichroism (MCD)]; CASSCF+NEVPT2 methods were employed in calculations of electronic structures. For (t_(2g))⁵ electronic configurations, the lowest energy ligand-to-metal charge transfer (LMCT) absorptions and MCD C terms in the spectra of boronated species have been assigned to transitions from cyanide σ+π + B-C borane σ orbitals. CASSCF+NEVPT2 calculations including t1u and t2u orbitals reproduced t_(1u)/t_(2u) → t_(2g) excitation energies. All ([M(CN-BR3)6]3/4− complexes exhibited highly electrochemically reversible redox couples. Notably, the formal potentials of all five [M(CN-B(C₆F₅)₃)₆]³⁻ anions scale with LMCT energies; and Mn(I) and Cr(II) compounds, (K(18-crown-6))₅[Mn(CN-B(C6F₅)₃)₆] and (TBA)₄[Cr(CN-B(C₆F₅)₃)₆], are surprisingly stable. Continuous wave and pulsed electron paramagnetic resonance (hyperfine sublevel correlation) spectra were collected for all Cr(III) complexes; as expected, ¹⁴N hyperfine splittings are greater for (TBA)₃[Cr(NC-BPh₃)₆] than for (TBA)₃[Cr(CN-BPh₃)₆]. Using (TBA)₄[Fe(CN-B(C₆F₅)₃)₆] and (TBA)₃[Fe(CN)₆], a model flow battery was constructed and found to have an 80% energy efficiency.
Additional Information
The content is available under CC BY NC ND 4.0 License. We dedicate this paper to the memory of Bob Grubbs, a great scientist and dear friend, who urged (ordered!) three of us (H.B.G., B.J.M., E.D.A.) to develop new redox complexes for use in nonaqueous redox flow batteries. After considering various options, we began work on boronated cyanometallates. We acknowledge the X-ray Crystallography Facility in the Beckman Institute at Caltech, and the Dow Next Generation Instrumentation Grant for X-ray structure collection. R.G.H. gratefully acknowledges financial support from Caltech and the Dow Next Generation Educator Fund. EPR spectroscopy was performed in the Caltech EPR facility, which is also supported by the Beckman Institute and the Dow Next Generation Educator Fund. We thank David van der Velde for assistance in interpreting NMR data. We thank Wesley Kramer and Brian Sanders for helpful discussions. The computations presented here were conducted in the Resnick High Performance Computing Center, a facility supported by Resnick Sustainability Institute at the California Institute of Technology. This work was supported by the National Science Foundation (CHE-1763429). Additional funding was provided by two Arthur A. Noyes SURF Fellowships (C. N. and A. A.) and the Beckman Institute Laser Resource Center supported by the Arnold and Mabel Beckman Foundation. Author Contributions. All authors have given approval to the final version of the manuscript.Attached Files
Submitted - boronated-cyanometallates.pdf
Supplemental Material - boronated-cyanometallates-supporting-information.pdf
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Additional details
- Eprint ID
- 115868
- Resolver ID
- CaltechAUTHORS:20220726-998148000
- CHE-1763429
- NSF
- Dow Next Generation Educator Fund
- Caltech Beckman Institute
- Caltech Summer Undergraduate Research Fellowship (SURF)
- Created
-
2022-07-27Created from EPrint's datestamp field
- Updated
-
2023-03-24Created from EPrint's last_modified field
- Caltech groups
- Resnick Sustainability Institute