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