Ligand Cleavage Put into Reverse: PC Bond Breaking and Remaking in an Alkylphosphane Iron Complex

Abstract

Complex formation between FeX(2)6 H(2)O (X=BF(4) or ClO(4)) and the pyridine-derived tetrapodal tetraphosphane C(5)H(3)N[CMe(CH(2)PMe(2))(2)](2) (1) in methanol proceeds with solvent-induced cleavage of one PMe(2) group. Depending on the reaction temperature and the nature of the counterion, iron(II) is coordinated, in distorted square-pyramidal fashion, by the anionic remainder of the chelating ligand, C(5)H(3)N[CMe(CH(2)PMe(2))(2)][CMe(CH(2)PMe(2))(CH(2) (-))] (NP(3)C(-) donor set: X=BF(4), -50 degrees C: 2; X=ClO(4), RT: 4) or its protonated form C(5)H(3)N[CMe(CH(2)PMe(2))(2)][CMe(CH(2)PMe(2))(CH(3))], in which the methyl group is in agostic interaction with the metal centre (X=BF(4), RT: 3; X=ClO(4), +50 degrees C: 5). A monodentate phosphinite ligand Me(2)POMe, formed from the cleaved PMe(2) group and methanol, completes the coordination octahedron in both cases. Working in CD(3)OD (X=BF(4), RT) gives the deuterium-substituted analogue of 3, with ligands L(CH(2)D) (L=residual chelating ligand) and Me(2)POCD(3). A mechanism for the observed phosphorus-carbon bond cleavage is suggested. Complex 2, when isolated at -50 degrees C, is stable in the solid state even at room temperature. The reaction of 2 in methanol with carbon monoxide (10.5 bar) at elevated temperature forms, in addition to as yet unidentified side products, the carbonyl complex [(1)Fe(CO)](BF(4))(2) (7), in which the previous P--C bond cleavage has been reversed, reforming the original tetrapodal pentadentate NP(4) ligand 1. All compounds have been fully characterised, including X-ray structure analyses in most cases.