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

Treatment of [Fe(BH4)((PNP)-P-Cy)] ((PNP)-P-Cy = bis-(dicyclohexylphosphinomethyl)pyrrole) with 2 or 3 equiv of phosphine afforded the octahedral low-spin iron hydride compounds [FeH(PMe2Ph)(N-2)((PNP)-P-Cy)] and [FeH(PMe3)(2)((PNP)-P-Cy)], respectively. These new hydrides, as well as the previously reported [FeMe((PNP)-P-Cy)], react with silanes to form a new class of square planar, intermediate-spin (S = 1) iron silyl complexes, [Fe(SiRPh2)((PNP)-P-Cy)] (R = H, Me, F). The more sterically encumbered iron hydride [FeH ((PNP)-P-tBu)] (tBupN,-.f = bis(di-tert-butylphosphinomethyl)pyrrole) was also employed to isolate [Fe(SiH2Ph)((PNP)-P-tBu)] via treatment with PhSiH3. [Fe(SiRPh2)((PNP)-P-Cy)] exists in an equilibrium with low-spin [Fe(N-2)(SiRPh2)((PNP)-P-Cy)] under an atmosphere of molecular nitrogen, whereas [Fe(SiH2Ph)((PNP)-P-tBu)] does not. Introducing an atmosphere of CO to [Fe(SiHPh2)((PNP)-P-Cy)] yielded the octahedral transdicarbonyl species [Fe(CO)(2)(SiHPh2)((PNP)-P-Cy)], but removal of that atmosphere resulted in loss of one CO to afford the square-pyramidal complex [Fe(Co)(SiHPh2)((PNP)-P-Cy)]. 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(SiHPh2)((PNP)-P-Cy)] with phenylacetylene was found to generate the square-planar, intermediate-spin vinylsilane complex [Fe(C(SiHPh2)C(H)Ph)((PNP)-P-Cy)], 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(SiHPh2)Me)((PNP)-P-Cy)]. Under similar conditions, [Fe(SiHPh2)((PNP)-P-Cy)] reacted with p-trifluoromethylbenzaldehyde to generate a new intermediate-spin complex assigned as [Fe(CH(OSiHPh2)Ar)((PNP)-P-Cy)] (Ar = 4-CF3C6H4), corresponding to a 2,1-insertion of the aldehyde. Finally, the reaction of CO2 with [Fe(SiHPh2)((PNP)-P-Cy)] resulted in reduction of CO2 to CO and formation of [Fe(CO)(SiHPh2)((PNP)-P-Cy)]. The stoichiometric reactivity of this new class of iron silyl complexes indicates diverse insertion behavior applicable to iron-mediated hydrosilylation catalysis.