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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.

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.

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.