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Ligand-Field Spectroscopy of Co(III) Complexes and the Development of a Spectrochemical Series for Low-Spin d6 Charge-Transfer Chromophores

Cite this: J. Am. Chem. Soc. 2022, 144, 27, 12488–12500
Publication Date (Web):June 24, 2022
Copyright © 2022 American Chemical Society

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    Abstract Image

    A study of a series of six-coordinate Co(III) complexes has been carried out to quantify spectroscopic parameters for a range of ligands that are commonly employed to realize strong charge-transfer absorptions in low-spin, d6 systems. Identification of any three ligand-field transitions allows for the determination of the splitting parameter (10 Dq) as well as the Racah B and C parameters for a given compound. The data revealed a relatively small spread in the magnitude of 10 Dq, ranging from ca. 23 000 cm–1 in the case of [Co(pyrro-bpy)3]3+ (where pyrro-bpy is 4,4′-dipyrrolidinyl-2,2′-bipyridine) to ca. 26 000 cm–1 for [Co(terpy)2]3+ (where terpy is 2,2′:6′,2″-terpyridine). Significantly, trends across the series suggest that polypyridyl ligands behave as net π-donors when interacting with Co(III), in contrast to the net π-accepting character they exhibit when bound to second- and third-row metals. The influence of strong σ donation associated with carbene-based ligands was evident from the data acquired for [Co(BMeImPy)2]3+ (where BMeImPy is 3,3′-(pyridine-2,6-diyl)bis(1-methyl-1H-3-imidazolium)), where a 10 Dq value of ca. 30 000 cm–1 was determined. Spectroscopic data were also analyzed for [Fe(bpy)3]2+ using the results on [Co(bpy)3]3+ as a reference point. A value for 10 Dq of 21 000 cm–1 was estimated, indicating a reduction in the ligand-field strength of ca. 3000 cm–1 upon replacing Co(III) with Fe(II). We suggest that this approach of taking advantage of the blueshift of the charge-transfer feature in Co(III) complexes to reveal otherwise obscured ligand-field bands can be a useful tool for the development of new ligand systems to expand the photofunctionality of first-row transition-metal-based chromophores.

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    • X-ray crystallography; UV–vis spectra of studied complexes; Tanabe–Sugano diagrams; and 1H NMR spectra of complexes (PDF)

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    CCDC 2157807, 2158019, and 2161073 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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    Cited By

    This article is cited by 9 publications.

    1. Micheal M. Alowakennu, Atanu Ghosh, James K. McCusker. Direct Evidence for Excited Ligand Field State-based Oxidative Photoredox Chemistry of a Cobalt(III) Polypyridyl Photosensitizer. Journal of the American Chemical Society 2023, 145 (38) , 20786-20791.
    2. Lei Wang, Zhu-Lin Xie, Brian T. Phelan, Vincent M. Lynch, Lin X. Chen, Karen L. Mulfort. Changing Directions: Influence of Ligand Electronics on the Directionality and Kinetics of Photoinduced Charge Transfer in Cu(I)Diimine Complexes. Inorganic Chemistry 2023, 62 (35) , 14368-14376.
    3. Roger Sanchis-Gual, Diego Hunt, Camilo Jaramillo-Hernández, Alvaro Seijas-Da Silva, Martín Mizrahi, Carlo Marini, Víctor Oestreicher, Gonzalo Abellán. Crystallographic and Geometrical Dependence of Water Oxidation Activity in Co-Based Layered Hydroxides. ACS Catalysis 2023, 13 (15) , 10351-10363.
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    6. Narayan Sinha, Oliver S. Wenger. Photoactive Metal-to-Ligand Charge Transfer Excited States in 3d6 Complexes with Cr0, MnI, FeII, and CoIII. Journal of the American Chemical Society 2023, 145 (9) , 4903-4920.
    7. Verónica García Rojas, Jhon Fredy Pérez Torres. Derivation of Dq/B and C/B from Electronic Spectra of Transition Metal Ions in Cubic Fields Using Auxiliary Tanabe–Sugano Diagrams. Journal of Chemical Education 2023, 100 (1) , 251-258.
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    9. Christian Marvelous, Lucas de Azevedo Santos, Maxime A. Siegler, Célia Fonseca Guerra, Elisabeth Bouwman. Cleaner and stronger: how 8-quinolinolate facilitates formation of Co( iii )–thiolate from Co( ii )–disulfide complexes. Dalton Transactions 2022, 51 (31) , 11675-11684.

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