Preparation, Characterization, and Catalytic Reactions of NCN Pincer Iron Complexes Containing Stannyl, Silyl, Methyl, and Phenyl Ligands

Ito, Junichi; Hosokawa, Satomi; Khalid, Hairuzana Binti; Nishiyama, Hisao

doi:10.1021/acs.organomet.5b00082

Cited by 39 publications

(22 citation statements)

References 119 publications

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“…However, the complex species involved feature low-spin iron(II) centers with ligands, which participate in heterolytic hydrogen cleavage by providing a basic site as a proton acceptor. Especially Morris’ group contributed major advances in understanding the mechanistic details of those transformations including the precatalyst activation. − Next to hydrogenations and transfer hydrogenations, hydrosilylations have been studied extensively in the context of iron catalysis, but without the same level of mechanistic understanding to date. − Besides the identification of the reaction manifold underlying the catalytic transformation and the role of the enabling ligand(s), the reactions involved in the activation of catalyst precursors provide the key to a more rational development of highly active catalysts.…”

Section: Introductionmentioning

confidence: 99%

Mechanism of the Iron(II)-Catalyzed Hydrosilylation of Ketones: Activation of Iron Carboxylate Precatalysts and Reaction Pathways of the Active Catalyst

Bleith

Gade

2016

J. Am. Chem. Soc.

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A detailed mechanistic study of the catalytic hydrosilylation of ketones with the highly active and enantioselective iron(II) boxmi complexes as catalysts (up to >99% ee) was carried out to elucidate the pathways for precatalyst activation and the mechanism for the iron-catalyzed hydrosilylation. Carboxylate precatalysts were found to be activated by reduction of the carboxylate ligand to the corresponding alkoxide followed by entering the catalytic cycle for the iron-catalyzed hydrosilylation. An Eyring-type analysis of the temperature dependence of the enantiomeric ratio established a linear relationship of ln(S/R) and T(-1), indicating a single selectivity-determining step over the whole temperature range from -40 to +65 °C (ΔΔG(‡)sel, 233 K = 9 ± 1 kJ/mol). The rate law as well as activation parameters for the rate-determining step were derived and complemented by a Hammett analysis, radical clock experiments, kinetic isotope effect (KIE) measurements (kH/kD = 3.0 ± 0.2), the isolation of the catalytically active alkoxide intermediate, and DFT-modeling of the whole reaction sequence. The proposed reaction mechanism is characterized by a rate-determining σ-bond metathesis of an alkoxide complex with the silane, subsequent coordination of the ketone to the iron hydride complex, and insertion of the ketone into the Fe-H bond to regenerate the alkoxide complex.

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Section: Introductionmentioning

confidence: 99%

Mechanism of the Iron(II)-Catalyzed Hydrosilylation of Ketones: Activation of Iron Carboxylate Precatalysts and Reaction Pathways of the Active Catalyst

Bleith

Gade

2016

J. Am. Chem. Soc.

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“…The first example of an iron silyl compound was synthesized by Wilkinson in 1956, employing the CpFe(CO) 2 fragment . Unfortunately this compound and many other reported examples of iron silyl species to date feature coordinatively and electronically saturated metal centers, mitigating their usefulness in subsequent chemistry. − Silyls have also been employed in the construction of supporting ligands, − although in this capacity they are not primed for reactivity with incoming substrates. Consequently, there are relatively few examples of the synthesis and application of iron silyl compounds as hydrosilylation catalysts. ,, Further convoluting the role of silyl species in catalysis is the fact that they are often generated by oxidative addition of a Si–H bond to a reduced metal center.…”

Section: Introductionmentioning

confidence: 99%

Square-Planar Iron(II) Silyl Complexes: Synthesis, Characterization, and Insertion Reactivity

2019

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Treatment of [Fe(BH 4 )( Cy PNP)] ( Cy PNP = bis-(dicyclohexylphosphinomethyl)pyrrole) with 2 or 3 equiv of phosphine afforded the octahedral low-spin iron hydride compounds [FeH(PMe 2 Ph)(N 2 )( Cy PNP)] and [FeH(PMe 3 ) 2 ( Cy PNP)], respectively. These new hydrides, as well as the previously reported [FeMe( Cy PNP)], react with silanes to form a new class of squareplanar, intermediate-spin (S = 1) iron silyl complexes, [Fe(SiRPh 2 )-( Cy PNP)] (R = H, Me, F). The more sterically encumbered iron h y d r i d e [ F e H ( t B u P N P ) ] ( t B u P N P = b i s ( d it e r tbutylphosphinomethyl)pyrrole) was also employed to isolate [Fe(SiH 2 Ph)( tBu PNP)] via treatment with PhSiH 3 . [Fe(SiRPh 2 )-( Cy PNP)] exists in an equilibrium with low-spin [Fe(N 2 )(SiRPh 2 )-( Cy PNP)] under an atmosphere of molecular nitrogen, whereas [Fe(SiH 2 Ph)( tBu PNP)] does not. Introducing an atmosphere of CO to [Fe(SiHPh 2 )( Cy PNP)] yielded the octahedral transdicarbonyl species [Fe(CO) 2 (SiHPh 2 )( Cy PNP)], but removal of that atmosphere resulted in loss of one CO to afford the square-pyramidal complex [Fe(CO)(SiHPh 2 )( Cy PNP)]. The insertion chemistry of the silyl complexes with unsaturated molecules was examined to shed light on possible reaction pathways relevant to iron-catalyzed hydrofunctionalization protocols involving silanes. The reaction of [Fe(SiHPh 2 )( Cy PNP)] with phenylacetylene was found to generate the square-planar, intermediate-spin vinylsilane complex [Fe(C(SiHPh 2 )C(H)Ph)( Cy PNP)], corresponding to a net 1,1-insertion of the phenylvinylidene tautomer, whereas reaction with 2-butyne afforded the expected 1,2-insertion product [Fe(C(Me)C-(SiHPh 2 )Me)( Cy PNP)]. Under similar conditions, [Fe(SiHPh 2 )( Cy PNP)] reacted with p-trifluoromethylbenzaldehyde to generate a new intermediate-spin complex assigned as [Fe(CH(OSiHPh 2 )Ar)( Cy PNP)] (Ar = 4-CF 3 C 6 H 4 ), corresponding to a 2,1-insertion of the aldehyde. Finally, the reaction of CO 2 with [Fe(SiHPh 2 )( Cy PNP)] resulted in reduction of CO 2 to CO and formation of [Fe(CO)(SiHPh 2 )( Cy PNP)]. The stoichiometric reactivity of this new class of iron silyl complexes indicates diverse insertion behavior applicable to iron-mediated hydrosilylation catalysis.

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“…The majority of reported Fe pincer complexes make use of charge neutral ligands that feature central donor moieties based on pyridines and simple amines. , Examples of iron complexes containing anionic pincer ligands are known and include both PCP and PNP variants containing hydrocarbyl and amido donors, respectively. − Despite this precedent, anionic pincers are underdeveloped in the chemistry of iron, and the applications of such compounds to catalysis are few in comparison to their neutral analogues, which have enjoyed a measure of success. Moreover, among the iron complexes of anionic pincer ligands reported to date, the majority concern low-spin iron in the 2+ and 0 oxidation states.…”

Section: Introductionmentioning

confidence: 99%

A Pyrrole-Based Pincer Ligand Permits Access to Three Oxidation States of Iron in Organometallic Complexes

2017

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Treatment of FeCl2(thf)1.5 with the sodium salt of bis(dicyclohexylphosphinomethyl)pyrrole (NaCyPNP) in the presence of pyridine (py) affords the high-spin, five-coordinate iron(II) complex [FeCl(py)(CyPNP)]. The chloride complex serves as a starting point for a variety of organometallic iron(II) compounds including S = 1 square planar alkyls, [FeR(CyPNP)] (R = Me, Et, Bn, Ph). Treatment of [FeCl(py)(CyPNP)] with NaBEt3H does not generate an analogous square-planar hydride complex, but rather a bimetallic high-spin iron(II) species containing bridging hydride ligands. Analogous transmetalation reactions in the presence of carbon monoxide produced six-coordinate low-spin species [FeR(CO)2(CyPNP)] (R = Et, Ph, and H). Reduction of [FeCl(py)(CyPNP)] under CO afforded the low-spin iron(I) carbonyl complex [Fe(CO)2(CyPNP)]. This species could be further reduced with KC8 to afford the anionic iron(0) complex K[Fe(CO)2(CyPNP)]·thf. The breadth of coordination numbers, spin states, and valencies supported by this pyrrole-based PNP ligand demonstrates the flexibility afforded by this pincer framework.

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Preparation, Characterization, and Catalytic Reactions of NCN Pincer Iron Complexes Containing Stannyl, Silyl, Methyl, and Phenyl Ligands

Cited by 39 publications

References 119 publications

Mechanism of the Iron(II)-Catalyzed Hydrosilylation of Ketones: Activation of Iron Carboxylate Precatalysts and Reaction Pathways of the Active Catalyst

Mechanism of the Iron(II)-Catalyzed Hydrosilylation of Ketones: Activation of Iron Carboxylate Precatalysts and Reaction Pathways of the Active Catalyst

Square-Planar Iron(II) Silyl Complexes: Synthesis, Characterization, and Insertion Reactivity

A Pyrrole-Based Pincer Ligand Permits Access to Three Oxidation States of Iron in Organometallic Complexes

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