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We demonstrate that a copper cubane cluster [Cu₄Cl₄(LH)₄] can be converted into an octanuclear cluster [Cu₈L₈] through deprotonation of the triisopropylacetylene ligands. The former displays exclusively side-on coordination of the terminal acetylene, while the latter exhibits both side-on and end-on coordination of the deprotonated acetylide moiety. The octanuclear complex shows solid-state emission around 640 nm, while the cubane displays a thermochromic emission shifting from 563 nm at r.t. to 608 nm upon cooling to 77 K.

An Fe(III)-NHC complex has been employed for the green light driven catalysis of base-promoted homolytic aromatic substitution (BHAS) reactions. Tributylamine was used as a sacrificial electron donor, together with potassium carbonate as base in dimethyl sulfoxide as solvent. In contrast to previously studied photocatalysts, the excited Fe(III)-NHC complex is not reducing the arylhalide substrates. Instead, the latter are activated by α-aminoalkyl radicals formed upon reductive quenching of the photocatalyst by tributylamine. Avoiding strongly reducing photocatalysts as well as strong base, these mild reaction conditions allowed for the expansion of the substrate scope to accommodate also aldehyde and ester substituents. 100 % conversion was obtained after 48 h of irradiation. In this way a wide variety of cyclized products and their corresponding hydrodehalogenated products were obtained as isolated and pure compounds, in the vast majority of cases.

The design of iron complexes with long-lived charge transfer states suitable for applications as photosensitizers remains a formidable challenge. Here, we investigated the effect of an extended ligand pi-system on the ground- and excited-state properties of iron(II) complexes with N-heterocyclic carbene (NHC) ligands. For this purpose, three iron complexes based on the established [Fe(II)(pbmi)2]2+ motif (pbmi = (1,1 '-(pyridine-2,6-diyl)bis(3-methylimidazole-2-ylidene))) have been modified with phenylethynyl moieties attached to the pyridine part of the ligand. In general, the introduction of the phenylethynyl units served to red shift the main absorption band, as well as to increase the extinction coefficient of the same, compared to the parent complex. The lowered MLCT energies are in line with the electrochemical data that revealed substantially easier reduction of the phenylethynyl-modified ligands, while the potentials of the Fe(III/II) couple are only moderately increased. Only minor modifications of the electronic effect intrinsic to the phenylethynyl moieties could be implemented with bromide and dimethylamino substituents on the phenylene units. As a result, all three complexes experience similar stabilization of their 3MLCT states, about 0.3 eV compared to the parent complex, and feature transient absorption data in line with ES dynamics that are dominated by a moderately long-lived (similar to 17 ps) 3MLCT state. These values exceed the 3MLCT lifetimes reported for the parent complex (up to 9 ps) and resemble the results for carboxylic acid and imidazolinium derivatives with comparable 3MLCT energies and lifetimes.

A ferrous complex bearing tris(carbene)borate ligands with imidazol-2-ylidene donors has been characterized by experimental and computational methods. Despite the pronounced destabilization of metal centered states by the exceptionally σ-donating ligand, the high-energy 3MLCT state of [Fe(II)(phtmeimb)2] is rapidly deactivated by the barrierless conversion to the 3MC state.

Fe^III complexes based on the [Fe^III(ImP)₂]⁺ motif (ImP = bis(2,6-bis(3-methylimidazol-2-ylidene-1-yl)phenylene)), where the ligand contains both carbene and cyclometalated moieties, are a promising class of photoactive materials made from this abundant metal. In this work, it is shown that bromo or furanyl substituents attached to the cyclometalating moiety of the ImP ligands stabilize the ²LMCT excited state to very different extent resulting in opposing effects on the ²LMCT lifetime. For [Fe^III(ImPBr)₂]⁺, the lifetime (255 ps) of its moderately stabilized ²LMCT state (1.85 eV) is slightly increased compared to the parent complex (1.90 eV, 240 ps) pointing to an increased barrier for deactivation via the ⁴MC state and enabling applications as photoredox catalyst. In contrast, the ²LMCT energy of [Fe^III(ImPFur)₂]⁺ is lowered substantially to a value of 1.63 eV due to the extended π-system of the ligands and the reduced energy gap favors internal conversion directly to the ground state resulting in a considerably reduced ²LMCT lifetime of 59 ps. These findings have general implications for design of ligand modifications aiming at extended LMCT lifetimes and/or modified ground and excited state potentials.

Manganese bipyridine tricarbonyl complexes show high efficiency and selectivity in electrochemical CO2 reduction (e-CO2RR) to CO. Efforts to shift selectivity toward HCOOH have been made by introducing second-sphere hydroxyl or amine functional groups and using amines or proton-coupled electron transfer (PCET) mediators. However, the direct spectroscopic evidence for the bifurcation pathways leading to CO and HCOOH remained elusive. Using stopped-flow mixing with decamethyl cobaltocene reductant and time-resolved infrared (TRIR) spectroscopy, we identified, for the first time, the key intermediates in this bifurcation pathway for an Mn complex with second-sphere hydroxyl groups in real time under catalytic conditions. The measured rate constants align with reported TOF values from electrochemical studies, validating the relevance of the results to e-CO2RR conditions. Our findings reveal that HCOOH production involves proton transfer from hydroxyl groups to the doubly reduced Mn center, forming the Mn-hydride intermediate, followed by CO2 insertion, leading to the Mn-formate intermediate. However, the inability of the resulting phenolate to rebind protons from weak acids like water leads to rapid catalyst degradation, limiting sustained catalysis. This work provides mechanistic insights and paves the way for designing molecular catalysts with enhanced selectivity and stability for HCOOH production during e-CO2RR.

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.