Well-Defined Aryl-Fe^II Complexes in Cross-Coupling and C–H Activation Processes

Abstract

Herein we explore the intrinsic organometallic reactivity of iron embedded in a tetradentate N 3 C macrocyclic ligand scaffold that allows the stabilization of aryl-Fe species, which are key intermediates in Fe-catalyzed cross-coupling and C−H functionalization processes. This study covers C−H activation reactions using Me L H and FeCl 2 , biaryl C−C coupling product formation through reaction with Grignard reagents, and crosscoupling reactions using Me L Br or H L Br in combination with Fe 0 (CO) 5 . Synthesis under light irradiation and moderate heating (50 °C) affords the aryl-Fe II complexes [Fe II (Br)( Me L)(CO)] (1 Me ) and [Fe II ( H L)(CO) 2 ]Br (1 H ). Exhaustive spectroscopic characterization of these rare low-spin diamagnetic species, including their crystal structures, allowed the investigation of their intrinsic reactivity.

Cyclometalated Iron and Ruthenium Complexes Supported by a Tetradentate Ligand Scaffold with Mixed O, N, and C Donor Atoms: Synthesis, Structures, and Excited-State Properties

“…Herein is described a panel of Fe­(II/III) and Ru­(II/III) [O^N^C^N] complexes and their structural, photophysical, and excited-state properties. In the literature, most examples of cyclometalated Fe­(II/III) complexes containing σ-aryl carbanion(s) such as the ones with bidentate/tridentate/tetradentate [C Aryl ^N], [C Aryl ^C Acyl ], [C Aryl ^P], [C Aryl ^O], [N^C Aryl ^N], [C Aryl ^N^N], [C Aryl ^N^C Aryl ], [C Aryl ^N^C Vinyl ], [S^C Aryl ^N], [P^C Aryl ^P], [C NHC ^C Aryl ^C NHC ], and [N 3 C Aryl ] ligand(s) are known for their reactivity in cross-coupling and C–H functionalization reactions (Chart ).…”

“…We recently reported a series of cationic bis­(tridentate) cyclometalated Fe­(II) complexes [Fe II (C Aryl ^N^N)­(L)] + (HC Aryl ^N^N = 6-(phenyl)-2,2′-bipyridine; L = neutral tridentate [N^N^N]/[C NHC ^N^N]/[P^N^N] ligand) revealing the enhancement of photophysical properties of Fe­(II) complexes by introducing a cyclometalating unit (i.e., panchromatic absorption by bathochromatically shifting 1 MLCT bands and higher molar attenuation coefficient (ε) values in the visible region) . One of the obstacles in the research on cyclometalated Fe complexes is their limited synthetic accessibility and relatively low stability; for example, decomposition via reductive elimination of σ-aryl group(s) mediated by Fe­(III) has usually been observed. , Our recent work showed that dicationic [Fe III (C Aryl ^N^N)­(C NHC ^N^N)]­(PF 6 ) 2 is not stable enough for isolation due to the facile reductive coupling of two tridentate units to form a C­(sp 2 ) Aryl –C­(sp 2 ) NHC bond . Similarly, Le Lagadec, Giaddon, and co-workers reported the formation of bis­(2-(pyridine-2-yl)­phenyl)­methanone and ferric oxide (Fe III 2 O 3 ) upon irradiation of [ cis -Fe II (C Aryl ^N) 2 (CO) 2 ] (HC Aryl ^N = 2-phenylpyridine) with visible light in the presence of air, possibly via reductive elimination of a photogenerated σ-acyl Fe­(II/III) species formed by an insertion of CO into an Fe σ-aryl bond .…”

Synthesis of Cytotoxic Iron‐Containing Macrocycles Through Unexpected C(sp2)−C(sp2) Bonds Formation

“…[19] We relied on the transmetalation reaction between [Fe 3 (CO) 12 ] and the mercurated derivative of 1,3-di(pyridin-2-yl)benzene reported for the preparation of the [Fe(NCN) 2 ] + cyclometalated complexes [17] (Scheme 1a), as well as on the oxidative addition of brominated pincer derivatives on iron(0) precursors previously described for the synthesis of various iron(II) metalacycles (Scheme 1). [20][21][22] However, instead of obtaining the expected cyclometalated iron complexes, compounds derived from the formation of C(sp 2 )À C(sp 2 ) bonds between two pincer units were isolated (Scheme 1d). These unexpected compounds were fully characterized and their cytotoxicity towards two gastric cancer cell lines was evaluated.…”