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 Å. 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 Å. 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.