Low coordinate iron derivatives stabilized by a β-diketiminate mimic. Synthesis and coordination chemistry of enamidophosphinimine scaffolds to generate diiron dinitrogen complexes

Abstract

In an effort to mimic N-diaryl-β-diketiminate ligands (Nacnac), we have converted N-arylimine phosphine ligands to enamine-phosphinimines via the Staudinger reaction. By varying the aryl azide, one can access enamine-phosphinimine ligands with the same or different N-aryl substituents on both the enamine and phosphinimine units, which allows this intrinsically unsymmetrical bidentate donor set to present variable steric effects at the metal center. The enamine-phosphinimine was deprotonated and used in metathetical reactions with Fe(II) bromide precursors to generate low coordinate complexes of the empirical formula [^CY5NpN^Ar,Ar′]FeBr (where CY5 = cyclopentenyl; Ar,Ar′ = 2,6-diisopropylphenyl, 2,6-dimethylphenyl, 2,4,6-trimethylphenyl). Depending on the substituents, these bromide derivatives can be monomeric or dimeric via bromide bridges. Reduction under dinitrogen using potassium graphite generates the dinitrogen complexes ([^CY5NpN^Ar,Ar′]Fe)₂(μ-N₂) for Ar,Ar′ = 2,6-diisopropylphenyl, and Ar = 2,6-dimethylphenyl, Ar′ = 2,4,6-trimethylphenyl. However, for the former, a unsymmetrical side product can be isolated that has a bridging N-2,6-diisopropylphenylimide unit with one enamido-phosphine ligand bound to one iron and the other iron stabilized with an intact enamido-phosphinimine. When the steric bulk is reduced on both nitrogen donors, a complicated product mixture is obtained after reduction from which a small amount of [^CY5NpN^Ar,Ar′]Fe[^CY5NP^Ar] (Ar,Ar′ = 2,6-dimethylphenyl) could be isolated. All of these complexes are paramagnetic and have been characterized by elemental analysis, magnetic studies and X-ray crystallography.