Photophysical Integrity of the Iron(III) Scorpionate Framework in Iron(III)–NHC Complexes with Long-Lived ²LMCT Excited States

Abstract

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-methylimidazoline-2-ylidine)borate]–, [Fe(meophtmeimb)2]PF6 (meophtmeimb = [(4-methoxyphenyl)tris(3-methylimidazoline-2-ylidine)borate]–, and [Fe(coohphtmeimb)2]PF6 (coohphtmeimb = [(4-carboxyphenyl)tris(3-methylimidazoline-2-ylidine)borate]–. These were derived from the parent complex [Fe(phtmeimb)2]PF6 (phtmeimb = [phenyltris(3-methylimidazoline-2-ylidine)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.

Photosensitized activation of diazonium derivatives for C–B bond formation

“…[18][19][20] [Fe(Br-phtmeimb) 2 ] + , where Br-phtmeimb is tris(3-methylimidazolium-1-yl)(4-Brphenyl)borate, was synthesized by reacting FeBr 2 and Br-phtmeimb in anhydrous THF at room temperature for 48 h. Wa ¨rnmark et al very recently reported the synthesis of [Fe(Br-phtmeimb) 2 ] + , which occurred with similar yields but in much shorter reaction times in anhydrous DMF. 21 Br-phtmeimb was synthesized in two steps starting from (4bromophenyl)-trimethylsilane that first underwent reaction with BBr 3 in dichloromethane at À78 C following a related procedure reported by Kaufmann in 1987. 22 The resulting 4-bromo-dibromophenylborane was directly reacted with imidazole in toluene in the presence of TMSCl, similarly to the procedure reported by Smith et al 23 The use of TMSCl allowed for easier recovery of tris(3-methylimidazolium-1yl)(4-Br-phenyl)borate in 80% yield over two steps after ion metathesis using ammonium hexafluorophosphate in water.…”

“…[31][32][33][34] Finally, both Fe(III) PSs exhibited a doublet ligand-to-metal charge transfer band at 500 nm with a molar absorption coefficient of 2,950 M À1 cm À1 . 20,21 The introduction of Br groups had no significant impact on the UV-visible (UV-vis) absorption features.…”

“…Time-resolved measurements of the PL in argon-purged acetonitrile containing 0.1 M TBABF 4 were well described by first-order kinetics, which yielded excited-state lifetimes ranging from 2 to 2010 ns ( This yielded an excited-state lifetime of 2 ns for [Fe(phtmeimb)] + as well as for [Fe(Br-phtmeimb)] + , in agreement with literature values. 20,21 Cyclic voltammetry and differential pulse voltammetry (DPV) allowed determination of the reduction potentials of the different aryl diazoniums and PSs. In Table 2, the PSs are sorted according to their excited-state oxidation potential E ox *; i.e., according to their ability to release one electron from their excited state.…”

“…The values obtained from these equations matched those reported in the literature. 20,21,31,[38][39][40] The aryl diazonium reduction waves were irreversible, and the corresponding potentials were estimated by DPV. Reduction potentials ranged from À0.51 to 0.77 V versus NHE and trended linearly with their Hammett parameters (Figure 3A).…”

Electronic Structure and Geometry of the Lowest 2LMCT State of Fe(III) Potential Fluorescent Emitters

“…This general consensus has stimulated intense research efforts and is an actively pursued goal in photophysics, − photobiology, or photoredox catalysis . Focusing on iron, the most abundant transition metal in the Earth’s crust, the past few years have witnessed spectacular extensions in charge transfer excited-state lifetimes of iron complexes, which have jumped from sub-ps to ps − to ns timescales, − through the use of strongly donating ligands. These ligands, which destabilize otherwise low-lying metal-centered states but might favor the Fe­(III) oxidation state, display N-heterocyclic or mesoionic carbene donor sites, amido ligands, and/or cyclometallating ligands, combined with π-accepting polypyridine-type moieties.…”