Highly Electron-Rich Pincer-Type Iron Complexes Bearing Innocent Bis(metallylene)pyridine Ligands: Syntheses, Structures, and Catalytic Activity

Abstract

The first neutral bis(metallylene)pyridine pincer-type [ENE] ligands (E = SiII, GeII) were synthesized, and their coordination chemistry and reactivity toward iron was studied. First, the unprecedented four-coordinate complexes κ2E,E′-[ENE]FeCl2 were isolated. Unexpectedly and in contrast to other related pyridine-based pincer-type Fe(II) complexes, the N atom of pyridine is reluctant to coordinate to the Fe(II) site due to the enhanced σ-donor strength of the E atoms, which disfavors this coordination mode. Subsequent reduction of κ2Si,Si′-[SiNSi]FeCl2 with KC8 in the presence of PMe3 or direct reaction of the [ENE] ligands using Fe(PMe3)4 produced the highly electron-rich iron(0) complexes [ENE]Fe(PMe3)2. The reduction of the iron center substantially changes its coordination features, as shown by the results of a single-crystal X-ray diffraction analysis of [SiNSi]Fe(PMe3)2. The iron center, in the latter, exhibits a pseudosquare pyramidal (PSQP) coordination environment, with a coordinative (pyridine)N→Fe bond, and a trimethylphosphine ligand occupying the apical position. This geometry is very unusual for Fe(0) low-spin complexes, and variable-temperature 1H and 31P NMR spectra of the [ENE]Fe(PMe3)2 complexes revealed that they represent the first examples of configurationally stable PSQP-coordinated Fe(0) complexes: even after heating at 70 °C for >7 days, no changes are observed. The substitution reaction of [ENE]Fe(PMe3)2 with CO resulted in the isolation of [ENE]Fe(CO)2 and the hitherto unknown κ2E,E′-[ENE]Fe(CO)2L (L = CO, PMe3) complexes. All complexes were fully characterized (NMR, MS, XRD, IR, and 57Fe Mössbauer spectroscopy), showing the highest electron density on the iron center for pincer-type complexes reported to date. DFT calculations and 57Fe Mössbauer spectroscopy confirmed the innocent behavior of these ligands. Moreover, preliminary results showed that these complexes can serve as active precatalysts for the hydrosilylation of ketones.