Pyrrolyl-based pincer complexes of iron – Synthesis and electronic structure

Abstract

Four- and five-coordinate high-spin (S=2) Fe(II) complexes bearing the pyrrolyl-derived tBuPNP pincer ligand [tBuPNP=2,5-(tBu2PCH2)2(C4H2N)] have been prepared. All three complexes were characterized by X-ray diffraction, zero-field 57Fe Mössbauer spectroscopy and solid-state magnetic susceptibility studies. Furthermore, the experimentally determined Mössbauer parameters of compounds 4–6 can be modeled satisfactorily by density functional theory (DFT) computations at the B97D level of theory. Introduction of the sterically demanding bis(trimethylsilyl)amido moiety induces significant distortions from ideal tetrahedral geometry in [(tBuPNP)FeN(SiMe3)2] (4), featuring an Fe(II) high-spin state with large negative axial zero-field splitting and slow paramagnetic relaxation at low temperature, as evidenced by measurements of the solid-state magnetic susceptibility and zero-field 57Fe Mössbauer spectroscopy. Replacing the chlorido ligand in [(tBuPNP)FeCl] (3) by the bidentate acetylacetonato (acac) ligand enforces a trigonal–bipyramidal structure in [(tBuPNP)Fe(acac)] (5), in which the Fe(II) atom also adopts a high-spin configuration. Reaction of 3 with adamantyl azide allowed us to confirm the inherent lability of the phosphine coordination, since the azide inserts into the FeP bond to form [(tBuPNPN3Adamantyl)FeCl] (6), in which the tBuPNP ligand is transformed to a phosphino-pyrrolyl-phosphazide ligand framework (tBuPNPN3Adamantyl). This motif can be considered as a trapped Staudinger intermediate along the reaction pathway of free phosphines with organic azides to form phosphinimines. Bidentate binding of the phosphazide sidearm in κN:κN-fashion to the Fe(II) atom enforces an s-anti coordination of the P1–N2–N3–N4 moiety, which raises the barrier for the thermodynamically favorable N2 elimination and allows this intermediate to be trapped. Complex 6 contains a high-spin Fe(II) atom with a square-pyramidal structure.