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The α-functionalisation of N-containing compounds is an area of broad interest in synthetic chemistry due to their presence in biologically active substances among others. Visible light-induced generation of nucleophilic α-aminoalkyl radicals as reactive intermediates that can be trapped by electron-deficient alkenes presents an attractive and mild approach to achieve said functionalisation. In this work, [Fe(III)(phtmeimb)₂]PF₆ (phtmeimb = phenyl(tris(3-methylimidazol-2-ylidene))borate), an N-heterocyclic carbene (NHC) complex based on Earth-abundant iron, was used as photoredox catalyst to efficiently drive the formation of α-aminoalkyl radicals from a range of different α-trimethylsilylamines and their subsequent addition to a number of electron-deficient alkenes under green light irradiation. Mechanistic investigations elucidated the different reaction steps of the complete photocatalytic cycle. In terms of yields and substrate scope, we show that [Fe(III)(phtmeimb)₂]PF₆ can compete with noble metal photoredox catalysts, for instance outcompeting archetypal [Ru(bpy)₃]Cl₂ under comparable reaction conditions, illustrating that iron photocatalysts can efficiently facilitate photoredox reactions of synthetic value.

Rhenium bipyridine tricarbonyl complexes, fac-[Re(bpy)(CO)(3)X](n+), are highly effective in selectively converting CO2 to CO under electrochemical and photochemical conditions. Despite numerous mechanistic studies aimed at understanding its CO2 reduction reaction (CO2RR) pathway, the intermediates further into the catalytic cycle have escaped detection, and the steps leading to product release remained elusive. In this study, employing stopped-flow mixing coupled with time-resolved infrared spectroscopy, we observed, for the first time, the reduced Re-tetracarbonyl species, [Re(bpy)(CO)(4)](0), with a half-life of approximately 55 ms in acetonitrile solvent. This intermediate is proposed to be common in both electrochemical and photochemical CO2RR. Furthermore, we directly observed the release of the product (CO) from this intermediate. Additionally, we detected the accumulation of [Re(bpy)(CO)(3)(CH3CN)](+) as a byproduct following product release, a significant side reaction under conditions with a limited supply of reducing equivalents mirroring photochemical conditions. The process could be unambiguously attributed to an electron transfer-catalyzed ligand substitution reaction involving [Re(bpy)(CO)(4)](0) by simultaneous real-time detection of all involved species. We believe that this side reaction significantly impacts the CO2RR efficiency of this class of catalysts under photochemical conditions or during electrocatalysis at mild overpotentials.

Two iron complexes featuring the bidentate, nonconjugated N-heterocyclic carbene (NHC) 1,1′-methylenebis(3-methylimidazol-2-ylidene) (mbmi) ligand, where the two NHC moieties are separated by a methylene bridge, have been synthesized to exploit the combined influence of geometric and electronic effects on the ground- and excited-state properties of homoleptic Fe^III-hexa-NHC [Fe(mbmi)₃](PF₆)₃ and heteroleptic Fe^II-tetra-NHC [Fe(mbmi)₂(bpy)](PF₆)₂ (bpy = 2,2′-bipyridine) complexes. They are compared to the reported Fe^III-hexa-NHC [Fe(btz)₃](PF₆)₃ and Fe^II-tetra-NHC [Fe(btz)₂(bpy)](PF₆)₂ complexes containing the conjugated, bidentate mesoionic NHC ligand 3,3′-dimethyl-1,1′-bis(p-tolyl)-4,4′-bis(1,2,3-triazol-5-ylidene) (btz). The observed geometries of [Fe(mbmi)₃](PF₆)₃ and [Fe(mbmi)₂(bpy)](PF₆)₂ are evaluated through L–Fe–L bond angles and ligand planarity and compared to those of [Fe(btz)₃](PF₆)₃ and [Fe(btz)₂(bpy)](PF₆)₂. The Fe^II/Fe^III redox couples of [Fe(mbmi)₃](PF₆)₃ (−0.38 V) and [Fe(mbmi)₂(bpy)](PF₆)₂ (−0.057 V, both vs Fc^+/0) are less reducing than [Fe(btz)₃](PF₆)₃ and [Fe(btz)₂(bpy)](PF₆)₂. The two complexes show intense absorption bands in the visible region: [Fe(mbmi)₃](PF₆)₃ at 502 nm (ligand-to-metal charge transfer, ²LMCT) and [Fe(mbmi)₂(bpy)](PF₆)₂ at 410 and 616 nm (metal-to-ligand charge transfer, ³MLCT). Lifetimes of 57.3 ps (²LMCT) for [Fe(mbmi)₃](PF₆)₃ and 7.6 ps (³MLCT) for [Fe(mbmi)₂(bpy)](PF₆)₂ were probed and are somewhat shorter than those for [Fe(btz)₃](PF₆)₃ and [Fe(btz)₂(bpy)](PF₆)₂. [Fe(mbmi)₃](PF₆)₃ exhibits photoluminescence at 686 nm (²LMCT) in acetonitrile at room temperature with a quantum yield of (1.2 ± 0.1) × 10^–4, compared to (3 ± 0.5) × 10^–4 for [Fe(btz)₃](PF₆)₃.

Thiele's Hydrocarbons (THs) featuring a 9,10-anthrylene core with switchable geometric and electronic configurations offer exciting possibilities in advanced functional materials. Despite significant advances in main group-based diradicaloids in contemporary chemistry, main group THs containing an anthrylene cores have remained elusive, primarily due to the lack of straightforward synthetic strategies and the inherent high reactivity of these species. In this study, we utilize an anthracene-based phosphine synthon to demonstrate, for the first time, a facile and high-yielding synthetic strategy for robust P-functionalized overcrowded ethylenes (OCEs) within the TH family. These OCEs feature a non-symmetric environment, incorporating (thio) xanthyl and phosphaalkene termini. We systematically probe the electronic structures of these derivatives to illustrate the impact of the isolobal phosphaalkene motif on the quinoidal/diradicaloid character. Notably, the compounds exhibit dynamic redox behavior, leading to orthogonally twisted conformational changes upon oxidation, with a kinetically locked redox-couple.

We here report the synthesis of the homoleptic iron(II) N-heterocyclic carbene (NHC) complex [Fe(miHpbmi)(2)](PF6)(4) (miHpbmi = 4-((3-methyl-1H-imidazolium-1-yl)pyridine-2,6-diyl)bis(3-methylimidazol-2-ylidene)) and its electrochemical and photophysical properties. The introduction of the pi-electron-withdrawing 3-methyl-1H-imidazol-3-ium-1-yl group into the NHC ligand framework resulted in stabilization of the metal-to-ligand charge transfer (MLCT) state and destabilization of the metal-centered (MC) states. This resulted in an improved excited-state lifetime of 16 ps compared to the 9 ps for the unsubstituted parent compound [Fe(pbmi)(2)](PF6)(2) (pbmi = (pyridine-2,6-diyl)bis(3-methylimidazol-2-ylidene)) as well as a stronger MLCT absorption band extending more toward the red spectral region. However, compared to the carboxylic acid derivative [Fe(cpbmi)(2)](PF6)(2) (cpbmi = 1,1 '-(4-carboxypyridine-2,6-diyl)bis(3-methylimidazol-2-ylidene)), the excited-state lifetime of [Fe(miHpbmi)(2)](PF6)(4) is the same, but both the extinction and the red shift are more pronounced for the former. Hence, this makes [Fe(miHpbmi)(2)](PF6)(4) a promising pH-insensitive analogue of [Fe(cpbmi)(2)](PF6)(2). Finally, the excited-state dynamics of the title compound [Fe(miHpbmi)(2)](PF6)(4) was investigated in solvents with different viscosities, however, showing very little dependency of the depopulation of the excited states on the properties of the solvent used.

Iron N-heterocyclic carbene (FeNHC) complexes with long-lived charge transfer states are emerging as a promising class of photoactive materials. We have synthesized [Fe-II(ImP)(2)] (ImP = bis(2,6-bis(3-methylimidazol-2-ylidene-1-yl)phenylene)) that combines carbene ligands with cyclometalation for additionally improved ligand field strength. The 9 ps lifetime of its (MLCT)-M-3 (metal-to-ligand charge transfer) state however reveals no benefit from cyclometalation compared to Fe(II) complexes with NHC/pyridine or pure NHC ligand sets. In acetonitrile solution, the Fe(II) complex forms a photoproduct that features emission characteristics (450 nm, 5.1 ns) that were previously attributed to a higher ((MLCT)-M-2) state of its Fe(III) analogue [Fe-III(ImP)(2)](+), which led to a claim of dual (MLCT and LMCT) emission. Revisiting the photophysics of [Fe-III(ImP)(2)](+), we confirmed however that higher ((MLCT)-M-2) states of [Fe-III(ImP)(2)](+) are short-lived (<10 ps) and therefore, in contrast to the previous interpretation, cannot give rise to emission on the nanosecond timescale. Accordingly, pristine [Fe-III(ImP)(2)](+) prepared by us only shows red emission from its lower (LMCT)-L-2 state (740 nm, 240 ps). The long-lived, higher energy emission previously reported for [Fe-III(ImP)(2)](+) is instead attributed to an impurity, most probably a photoproduct of the Fe(II) precursor. The previously reported emission quenching on the nanosecond time scale hence does not support any excited state reactivity of [Fe-III(ImP)(2)](+) itself.

Symmetry-breaking charge separation in molecular materials has attracted increasing attention for optoelectronics based on single-material active layers. To this end, Fe(III) complexes with particularly electron-donating N-heterocyclic carbene ligands offer interesting properties with a (LMCT)-L-2 excited state capable of oxidizing or reducing the complex in its ground state. In this Communication, we show that the corresponding symmetry-breaking charge separation occurs in amorphous films of pristine [Fe(III)L-2]PF6 (L = [phenyl(tris(3-methylimidazol-2-ylidene))borate](-)). Excitation of the solid material with visible light leads to ultrafast electron transfer quenching of the (LMCT)-L-2 excited state, generating Fe(II) and Fe(IV) products with high efficiency. Sub-picosecond charge separation followed by recombination in about 1 ns could be monitored by transient absorption spectroscopy. Photoconductivity measurements of films deposited on microelectrode arrays demonstrated that photogenerated charge carriers can be collected at external contacts.

With the price-competitiveness of solar and wind power, hydrogen technologies may be game changers for a cleaner, defossilized, and sustainable energy future. H-2 can indeed be produced in electrolyzers from water, stored for long periods, and converted back into power, on demand, in fuel cells. The feasibility of the latter process critically depends on the discovery of cheap and efficient catalysts able to replace platinum group metals at the anode and cathode of fuel cells. Bioinspiration can be key for designing such alternative catalysts. Here we show that a novel class of iron-based catalysts inspired from the active site of [FeFe]-hydrogenase behave as unprecedented bidirectional electrocatalysts for interconverting H-2 and protons efficiently under near-neutral aqueous conditions. Such bioinspired catalysts have been implemented at the anode of a functional membrane-less H-2/O-2 fuel cell device.

Herein we report the first high turnover photocatalytic hydrogen formation reaction based on an earth-abundant Fe-III-NHC photosensitiser. The reaction occurs via reductive quenching of the (LMCT)-L-2 excited state that can be directly excited with green light and employs either Pt-colloids or [Co(dmgH)(2)pyCl] as proton reduction catalysts and [HNEt3][BF4] and triethanolamine/triethylamine as proton and electron donors. The outstanding photostability of the Fe-III-NHC complex enables turnover numbers >1000 without degradation.

Fe(III) complexes with N-heterocyclic carbene (NHC) ligands belong to the rare examples of Earth-abundant transition metal complexes with long-lived luminescent charge-transfer excited states that enable applications as photosensitizers for charge separation reactions. We report three new hexa-NHC complexes of this class: [Fe(brphtmeimb)2]PF6 (brphtmeimb = [(4-bromophenyl)tris(3-methylimidazol-2-ylidene)borate]–, [Fe(meophtmeimb)2]PF6 (meophtmeimb = [(4-methoxyphenyl)tris(3-methylimidazol-2-ylidene)borate]–, and [Fe(coohphtmeimb)2]PF6 (coohphtmeimb = [(4-carboxyphenyl)tris(3-methylimidazol-2-ylidene)borate]–. These were derived from the parent complex [Fe(phtmeimb)2]PF6 (phtmeimb = [phenyltris(3-methylimidazol-2-ylidene)borate]– by modification with electron-withdrawing and electron-donating substituents, respectively, at the 4-phenyl position of the ligand framework. All three Fe(III) hexa-NHC complexes were characterized by NMR spectroscopy, high-resolution mass spectroscopy, elemental analysis, single crystal X-ray diffraction analysis, electrochemistry, Mößbauer spectroscopy, electronic spectroscopy, magnetic susceptibility measurements, and quantum chemical calculations. Their ligand-to-metal charge-transfer (2LMCT) excited states feature nanosecond lifetimes (1.6–1.7 ns) and sizable emission quantum yields (1.7–1.9%) through spin-allowed transition to the doublet ground state (2GS), completely in line with the parent complex [Fe(phtmeimb)2]PF6 (2.0 ns and 2.1%). The integrity of the favorable excited state characteristics upon substitution of the ligand framework demonstrates the robustness of the scorpionate motif that tolerates modifications in the 4-phenyl position for applications such as the attachment in molecular or hybrid assemblies.