Bis(silylenyl)-substituted ferrocene-stabilized η6-arene iron(0) complexes: synthesis, structure and catalytic application

Abstract

Reaction of FeX₂(thf)_n (X = Cl n = 1.5, Br n = 2) with the chelating 1,1'-bis(silylenyl)-substituted ferrocene ligand SiFcSiA (Fc = ferrocendiyl, Si = PhC(N^tBu)₂Si:) furnishes the corresponding dihalido Fe(ii) complexes [(SiFcSi)FeX₂] (X = Cl, 1 and X = Br, 2) in high yields. Reduction of the latter with an excess of KC₈ in the presence of benzene and toluene leads to the unprecedented bis(silylene) stabilized Fe⁰ complexes [(SiFcSi)Fe-η⁶(C₆H₆)] 3 and [(SiFcSi)Fe-η⁶(C₇H₈)] 4, respectively. The ⁵⁷Fe Mössbauer spectrum of 3 at 13 K exhibits parameters (σ = 0.3676 mm s^-1; ΔE_Q = 1.334 mm s^-1) which are consistent with the presence of a pentacoordinated Fe⁰ atom in a pseudo trigonal-bipyramidal coordination environment, with two dative Si→Fe bonds and three coordination sites occupied by the η⁶-coordinated arene ligand. Results from DFT calculations, ⁵⁷Fe Mössbauer parameters and the diamagnetic NMR spectra confirm the redox-innocent nature of these ligands and the zero oxidation state of the iron center. The catalytic ability of 3 was investigated with respect to ketone hydrogenation. In all cases, good to excellent yields to the corresponding alcohols were obtained at 50 °C and 50 bar H₂ pressure. Electron-donating as well as -withdrawing substituents were tolerated with excellent to good yields. Conversions of bulkier ketones and unactivated aliphatic ketones lead merely to moderate yields. This represents the first example of a silylene-iron metal complex which has been utilized as a highly active precatalyst in the hydrogenation of ketones. The results underline the powerful ability of chelating bis(N-heterocyclic silylene) ligands acting as strong σ-donor ligands in stabilizing a new generation of low-valent, electron-rich transition metal complexes for catalytic transformations.