The Full d3–d5 Redox Series of Mononuclear Manganese Complexes: Geometries and Electronic Structures of [Mn(dgpy)2]n+

East, Nathan; Förster, Christoph; Carrella, Luca M.; Rentschler, Eva; Heinze, Katja

doi:10.1021/acs.inorgchem.2c01680

Cited by 9 publications

(26 citation statements)

References 74 publications

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“…5). 54 [Mn(dgpy)2] 4+ even oxidizes chloride ions in CH3CN in the dark (Supplementary Fig. 6), similar to the solid state material MnO2, 55 while light-induced chloride oxidation had been reported using designed ruthenium(II) and organic photooxidants using one-and two-photon excitation mechanisms.…”

Section: Ground State Properties Of [Mn(dgpy)2] 4+supporting

confidence: 58%

“…1a) with a d 3 electron configuration yielding a 4 A2 ground state (term symbol given for octahedral symmetry) is a strong oxidant in the ground state with E½(Mn IV /Mn III ) = +0.94 V vs. SCE (0.56 V vs. ferrocene) in acetonitrile (dgpy = 2,6diguanidylpyridine). 54 One-electron reduction gives the yellow colored high-spin manganese(III) complex [Mn(dgpy)2] 3+ (Supplementary Fig. 5).…”

Section: Ground State Properties Of [Mn(dgpy)2] 4+mentioning

confidence: 99%

“…1a) peaking at 514 nm (6860 M -1 cm -1 ) and 775 nm (sh, 3320 M -1 cm -1 ) and extending into the near-infrared spectral region. 54 On the other hand, the manganese(III) complex [Mn(dgpy)2] 3+ absorbs only weakly in the visible spectral region with a weak absorption band at 450 nm ( = 600 M -1 cm -1 , Supplementary Fig. 5).…”

Section: Ground State Properties Of [Mn(dgpy)2] 4+mentioning

confidence: 99%

“…5). 54 According to time-dependent Density Functional Theory (TD-DFT) calculations and charge transfer number analysis, the strong absorption bands of [Mn(dgpy)2] 4+ arise from symmetry and spin allowed 4 LMCT transitions in particular from the guanidine moieties of the ligands to the high-valent manganese center. 54…”

Section: Ground State Properties Of [Mn(dgpy)2] 4+mentioning

confidence: 99%

“…Two short Mn … N(CH3CN) distances of 4.91 and 5.73 Å are present in the solid state structure (CCDC-2171816). 54 The molecular dynamics simulation of [Mn(dgpy)2][PF6]4 in CH3CN over 1 µs resulted in shells of CH3CN molecules around the tetracation with Mn … [CH3CN] (center-of-mass) distances peaking at ca. 3.9, 6.0, 8.5, 12.3 and 16.8 Å (Supplementary Fig.…”

Section: Ultrafast Spectroscopy and The Excited State Reactivity Of A...mentioning

confidence: 99%

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Oxidative Two-State Photoreactivity of a Manganese(IV) Complex using NIR Light

East

Naumann

Förster

et al. 2023

Preprint

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Highly reducing or oxidizing photocatalysts are a fundamental challenge in the field of inorganic and organic photochemistry. Only a few transition metal complexes with earth-abundant metal ions have so far advanced to excited state oxidants, including chromium, iron and cobalt. All these photocatalysts require high energy light for excitation and their oxidizing power has not been fully exploited due to significant energy dissipation before reaching the photoactive state. Herein we demonstrate that the complex [Mn(dgpy)2]4+ based on earth-abundant manganese can be excited with low-energy NIR light (850 nm, 1.46 eV) to yield a luminescent mixed 2LMCT/2MC excited state (1435 nm, 0.86 eV) with a lifetime of 1.6 ns. The dissipated energy amounts to 0.60 eV. In spite of this energy loss, *[Mn(dgpy)2]4+ with its excited state redox potential Ered* of 1.80 V vs SCE outcompetes the strongest reported precious metal photooxidant (iridium(III)). *[Mn(dgpy)2]4+ oxidizes naphthalene (Eox 1.31 1.54 V vs. SCE) to its radical cation giving the manganese(III) complex [Mn(dgpy)2]3+ in a clean outer-sphere electron transfer process. Unexpectedly, mesitylene, toluene, benzene and nitriles with even extremely high oxidation potentials up to Eox = 2.4 V provoke the [Mn(dgpy)2]4+/3+ reduction under photolysis. A higher energy short-lived 4LMCT excited state with a lifetime of 0.78 ps is made responsible for these demanding oxidations, which proceed by static rather than dynamic quenching. This dual excited state reactivity from 2LMCT/2MC and 4LMCT states is linked to the 4LMCT 2LMCT/2MC intersystem crossing process. These unique findings demonstrate how the design of manganese complexes (i) expands the absorption cross section to 400 850 nm, (ii) increases the 2LMCT/2MC state lifetime to the nanosecond range allowing luminescence and classical dynamic photoredox processes and (iii) enables non-classical static quenching of an extremely oxidizing 4LMCT excited state by the solvent. This conceptually novel approach of static quenching by the solvent minimizes free energy losses, harnesses the full photooxidizing power and thus allows even oxidation of nitriles and benzene using earth-abundant elements and low-energy light.

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Section: Ground State Properties Of [Mn(dgpy)2] 4+supporting

confidence: 58%

Section: Ground State Properties Of [Mn(dgpy)2] 4+mentioning

confidence: 99%

Section: Ground State Properties Of [Mn(dgpy)2] 4+mentioning

confidence: 99%

Section: Ground State Properties Of [Mn(dgpy)2] 4+mentioning

confidence: 99%

Section: Ultrafast Spectroscopy and The Excited State Reactivity Of A...mentioning

confidence: 99%

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Oxidative Two-State Photoreactivity of a Manganese(IV) Complex using NIR Light

East

Naumann

Förster

et al. 2023

Preprint

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The Nephelauxetic Effect Becomes an Important Design Factor for Photoactive First‐Row Transition Metal Complexes

2023

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The expansion of d‐orbitals as a result of metal‐ligand bond covalence, the so‐called nephelauxetic effect, is a well‐established concept of coordination chemistry, yet its importance for the design of new photoactive complexes based on first‐row transition metals is only beginning to be recognized. Until recently, much focus has been on optimizing the ligand field strength, coordination geometries, and molecular rigidity, but now it becomes evident that the nephelauxetic effect can be a game changer regarding the photophysical properties of 3d metal complexes in solution at room temperature. In CrIII and MnIV complexes with the d3 valence electron configuration, the nephelauxetic effect was exploited to shift the well‐known ruby‐like red luminescence to the near‐infrared spectral region. In FeII and CoIII complexes with the low‐spin d6 electron configuration, charge‐transfer excited states were stabilized with respect to detrimental metal‐centered excited states, to improve their properties and to enhance their application potential. In isoelectronic (3d6) isocyanide complexes of Cr0 and MnI, the nephelauxetic effect is likely at play as well, enabling luminescence and other favorable photoreactivity. This minireview illustrates the broad applicability of the nephelauxetic effect in tailoring the photophysical and photochemical properties of new coordination compounds made from abundant first‐row transition metals.

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Nickel(II) Analogues of Phosphorescent Platinum(II) Complexes with Picosecond Excited‐State Decay

Ogawa,

Wenger

2023

Angewandte Chemie

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Square‐planar Ni(II) complexes are interesting as cheaper and more sustainable alternatives to Pt(II) luminophores widely used in lighting and photocatalysis. We investigated the excited‐state behavior of two Ni(II) complexes, which are isostructural with two luminescent Pt(II) complexes. The initially excited singlet metal‐to‐ligand charge transfer (1MLCT) excited states in the Ni(II) complexes decay to metal‐centered (3MC) excited states within less than 1 picosecond, followed by non‐radiative relaxation of the 3MC states to the electronic ground state within 9 ‐ 21 ps. This contrasts with the population of an emissive triplet ligand‐centered (3LC) excited state upon excitation of the Pt(II) analogues. Structural distortions of the Ni(II) complexes are responsible for this discrepant behavior and lead to dark 3MC states far lower in energy than the luminescent 3LC states of Pt(II) compounds. Our findings suggest that if these structural distortions could be restricted by more rigid coordination environments and stronger ligand fields, the excited‐state relaxation in four‐coordinate Ni(II) complexes could be decelerated such that luminescent 3LC or 3MLCT excited states become accessible. These insights are relevant to make Ni(II) fit for photophysical and photochemical applications that relied on Pt(II) until now.

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The Full d³–d⁵ Redox Series of Mononuclear Manganese Complexes: Geometries and Electronic Structures of [Mn(dgpy)₂]ⁿ⁺

Cited by 9 publications

References 74 publications

Oxidative Two-State Photoreactivity of a Manganese(IV) Complex using NIR Light

Oxidative Two-State Photoreactivity of a Manganese(IV) Complex using NIR Light

The Nephelauxetic Effect Becomes an Important Design Factor for Photoactive First‐Row Transition Metal Complexes

Nickel(II) Analogues of Phosphorescent Platinum(II) Complexes with Picosecond Excited‐State Decay

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