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Increasing interest in sustainable chemistry coupled with the quest to explore new reactivity has spurred research on first-row transition metal complexes for potential applications in a variety of settings. One of the more active areas of research is photoredox catalysis, where the synthetically tunable nature of their electronic structures provides a rich palette of options for tailoring their reactivity to a desired chemical transformation. Understanding the mechanism of excited-state reactivity is critical for the informed development of next-generation catalysts, which in turn requires information concerning the propensity of their electronic excited states to engage in the desired electron or energy transfer processes. Herein we provide direct evidence of the highly oxidizing nature of the lowest-energy ligand-field (LF) excited state of a first-row d6-low-spin Co(III) photosensitizer [Co(4,4′-Br2bpy)3]3+(where 4,4′-Br2bpy is 4,4′-dibromo-2,2′-bipyridine). The redox potential associated with the LF excited state of the Co(III) complex was bracketed by performing bimolecular quenching studies by using a series of simple organic electron donors. Time-resolved absorption spectroscopy confirmed a dynamic quenching process attributed to reductive quenching of the lowest-energy ligand-field excited state of the Co(III) chromophore. Analysis of the Stern–Volmer plots for each chromophore-quencher pair revealed a limiting value of Ered*∼ 1.25 V vs Fc/Fc+for the metal-centered excited state, which is significantly stronger than that of more commonly employed transition metal-based photoredox agents such as [Ru(bpy)3]2+(Ered*= 0.32 V vs Fc/Fc+) and [Ir(ppy)2(bpy)]+(Ered*= 0.27 V vs Fc/Fc+). These results suggest that this class of chromophores could find utility in applications requiring the activation of oxidatively resistant organic substrates for photoredox catalysis.

The development of chromophores based on earth-abundant transition metals whose photophysical properties are dominated by their charge-transfer excited states has inspired considerable research over the past decade. One challenge associated with this effort is satisfying the dual requirements of a strong ligand field and chemical tunability of the compound’s absorptive cross-section. Herein we explore one possible approach using a heteroleptic compositional motif that combines both of these attributes into a single compound. With the parent complex [Fe(phen)3]2+(1; where phen is 1,10-phenanthroline) as the starting material, replacement of one of the phen ligands for two cyanides to obtain Fe(phen)2(CN)2(2) allows for conversion to [Fe(phen)2(C4H10N4)]2+(3), a six-coordinate Fe(II) complex whose coordination sphere consists of two chelating polypyridyl ligands and one bidentate carbene-based donor. Ground-state absorption spectra of all three compounds exhibit 1A1→ 1MLCT transition(s) associated with the phen ligands that are relatively insensitive to the identity of the third counterligand(s). Ultrafast time-resolved electronic absorption measurements revealed lifetimes for the MLCT excited states of compounds 1and 2of 180 ± 30 and 250 ± 90 fs, respectively, values that are typical for iron(II)-based polypyridyl complexes. The corresponding kinetics for compound 3were substantially slower at 7.4 ± 0.9 ps; the spectral evolution associated with these dynamics confirms that these kinetics are tracking the MLCT excited state and, more importantly, are coupled to ground-state recovery from this excited state. The data are interpreted in terms of a modulation of the relative energies of the MLCT and ligand-field states across the series, leading to a systematic destabilization of metal-localized ligand-field excited states such that the low-energy portions of the charge-transfer and ligand-field manifolds are at or near an energetic inversion point in compound 3. We believe these results illustrate the potential for a modular, orthogonal approach to chromophore design in which part of the coordination sphere can be targeted for light absorption while another can be used to tune electronic-state energetics.

Photoredox catalysis employing ruthenium- and iridium-based chromophores have been the subject of considerable research. However, the natural abundance of these elements are among the lowest on the periodic table, a fact that has led to an interest in developing chromophores based on earth-abundant transition metals that can perform the same function. There have been reports of using FeII-based polypyridyl complexes as photocatalysts, but there is limited mechanistic information pertaining to the nature of their reactivity in the context of photoredox chemistry. Herein, we report the results of bimolecular quenching studies between [Fe(tren(py)3)]2+(where tren(py)3= tris(2-pyridyl-methylimino-ethyl)amine) and a series of benzoquinoid acceptors. The data provide direct evidence of electron transfer involving the lowest-energy ligand-field excited state of the Fe(II)-based photosensitizer, definitively establishing that Fe(II) polypyridyl complexes can engage in photoinduced redox reactions but by a mechanism that is fundamentally different than the MLCT-based chemistry endemic to their second- and third-row congeners.

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, d6systems. Identification of any three ligand-field transitions allows for the determination of the splitting parameter (10 Dq) as well as the Racah Band Cparameters for a given compound. The data revealed a relatively small spread in the magnitude of 10 Dq, ranging from ca. 23 000 cm–1in the case of [Co(pyrro-bpy)3]3+(where pyrro-bpy is 4,4′-dipyrrolidinyl-2,2′-bipyridine) to ca. 26 000 cm–1for [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–1was 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–1was estimated, indicating a reduction in the ligand-field strength of ca. 3000 cm–1upon 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.