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

Treatment of FeCl2(thf)(1.5) with the sodium salt of bis(dicyclohexylphosphinomethyl)pyrrole ((NaPNP)-P-cY) in the presence of pyridine (py) affords the high-spin, five-coordinate iron(II) complex [FeCl(py)((PNP)-P-cY)]. The chloride complex serves as a starting point for a variety of organometallic iron(II) compounds including S = 1 square planar alkyls, [FeR((PNP)-P-cY)] (R = Me, Et, Bn, Ph). Treatment of [FeCl(py)((PNP)-P-cY)] 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)((PNP)-P-cY)] (R = Et, Ph, and H). Reduction of [FeCl(py)((PNP)-P-cY)] under CO afforded the low-spin iron(I) carbonyl complex [Fe(CO)(2)((PNP)-P-cY)]. This species could be further reduced with KC8 to afford the anionic iron(0) complex K[Fe(CO)(2)((PNP)-P-cY)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.