Aryl-substituted BIAN complexes of iron dibromide: Synthesis, X-ray and electronic structure, and catalytic hydrosilylation activity

Abstract

Anhydrous iron dibromide complexes bearing bidentate α-diimine ligands Ar N=C(Me)-(Me)C=N Ar and Ar BIAN (BIAN = bis(imino)acenaphthene; Ar = dpp and Mes; dpp = 2,6diisopropylphenyl; Mes = 2,4,6-trimethylphenyl) have been prepared and characterized by 1 H NMR spectroscopy. The aryl-substituted BIAN complexes were structurally characterized by single-crystal X-ray diffraction, and their metrical parameters are consistent with a redoxinnocent chelating ligand. A high-spin iron(II) electronic structure description for the Ar BIAN iron complexes is supported by Mössbauer spectroscopy, solution state magnetic measurements, and quantum-chemical calculations. Upon reduction, the iron complexes promote catalytic hydrosilylation of 1-hexene with phenylsilane at 22 o C.

Synthesis and Catalysis of Redox‐Active Bis(imino)acenaphthene (BIAN) Iron Complexes

“…In contrast, the minor component of the reduction of 2 a with n BuLi, which was obtained from extraction with toluene and washing with hexane, showed identical catalytic activity to the in situ generated catalyst mixture ((dipp 2 BIAN)FeCl 2 , n BuLi). In accordance with recent literature, we postulate a three‐electron reduction of the (BIAN)FeCl 2 complex to a low‐valent (BIAN)Fe species, which possibly contains the BIAN ligand in the radical anion or dianion state (Scheme ) . Attempts to disclose the chemical identity of this fraction have not yet been successful.…”

“…Most of these reports involve modification of the general ligand structure with a pendant donor arm with the aim of mimicking the PDI behavior . Recently, Fe complexes with sterically hindered dipp 2 BIAN (dipp: 2,6‐diisopropylphenyl) and mes 2 BIAN (mes: mesityl) have been successfully applied as precatalysts for hydrosilylations of carbonyl compounds and olefins . These studies documented only moderate activity of the complexes and involved no full electrochemical characterization of the complexes, despite the strongly reducing reaction conditions and the postulation of an active catalyst species in lower oxidation states.…”

Ferrous complexes supported by sterically encumbered asymmetric bis(arylimino)acenaphthene (BIAN) ligands: synthesis, characterization and screening for catalytic hydrosilylation of carbonyl compounds

“…This is similar to the observation for previously reported analogous BIAN Fe(II) halide complexes. 20,21 The new ferrous complexes were further characterized by IR spectroscopy. As summarized in Table 1 frequencies of the iron compounds are lower than those of the corresponding neutral BIAN ligands, indicating the presence of bonding interactions between imine groups and the metal centers.…”

“…18 It was discovered that the solid-state structures of these tridentate BIAN metal complexes exhibited a range of coordination modes depending on the crystallization solvent as well as the nucleophilicity of the third donor atoms. 19 Other known BIAN-Fe compounds usually involve identical aryl N-substituents (Mes or Dipp), featuring mononuclear, 20,21 dinuclear 22 or multinuclear structural motifs. 23 It was discovered that the radical anionic form of BIAN ligands could be installed to Fe(II) centers through the treatment of FeI 2 with various molar ratio of potassium salt of the radical anion.…”

“…20 This compound was shown to be an efficient precatalyst for the hydrosilylation of aldehydes and ketones using Ph 2 SiH 2 at 70 C. Very recently, similar ( Ar BIAN)FeBr 2 complexes (Ar ¼ Mes or Dipp) were reported by Hoyt and co-workers, and the catalytic hydrosilylation of 1hexene was investigated. 21 Reduction of carbonyl compounds is among the most valuable synthetic routes in both academia and industry. Transition-metal-catalyzed hydrosilylation followed by hydrolysis workup has emerged as a useful protocol for the production of alcohols.…”