Binuclear methylaminobis(difluorophosphine) iron carbonyls: phosphorus–nitrogen bond cleavage in preference to iron–iron multiple bond formation

Abstract

The readily available CH3N(PF2)2 ligand has been shown experimentally to form the binuclear iron carbonyl complexes CH3N(PF2)2Fe2(CO)n (n = 8, 7) and [CH3N(PF2)2]2Fe2(CO)n (n = 6, 5) in which the central Fe2 units are bridged by the CH3N(PF2)2 ligands. In the CH3N(PF2)2Fe2(CO)n series the lowest energy CH3N(PF2)2Fe2(CO)8 structure has Fe(CO)4 moieties bonded to each phosphorus atom in the CH3N(PF2)2 ligand with all terminal CO groups and no iron–iron bond. Loss of one CO group from this structure to give CH3N(PF2)2Fe2(CO)7 converts one of the remaining terminal CO groups into a bridging CO group and introduces a formal Fe–Fe single bond of length ∼2.7 A. Further loss of a CO group from this CH3N(PF2)2Fe2(CO)7 structure retains the Fe–Fe single bond to give a Fe2(CO)6(μ-CH3NPF2)(μ-PF2) structure with all terminal CO groups but separate bridging PF2 and CH3NPF2 groups arising from P–N bond cleavage of the CH3N(PF2)2 ligand. Still further decarbonylation of this Fe2(CO)6(μ-CH3NPF2)(μ-PF2) structure results in simple loss of one of the terminal CO groups while retaining the central Fe2(μ-CH3NPF2)(μ-PF2) unit including the formal Fe–Fe single bond of length ∼2.7 A. The lowest energy structures of the [CH3N(PF2)2]2Fe2(CO)n (n = 6, 5, 4, 3) series can be derived from the corresponding CH3N(PF2)2Fe2(CO)n+2 structures by replacing one terminal CO group on each iron atom by the second bridging CH3N(PF2)2 ligand.