Imine-Centered Reactions in Imino-Phosphine Complexes of Iron Carbonyls

Fundamental reactions of imino-phosphine ligands were elucidated through studies on Ph₂PC₆H₄CH=NC₆H₄-4-Cl (PCHNAr^Cl) complexes of iron(0), iron(I), and iron(II). The reaction of PCHNAr^Cl with Fe(bda)(CO)₃ gives Fe(PCHNAr^Cl)(CO)₃ (1), featuring an η²-imine. DNMR studies, its optical properties, and DFT calculations suggest that 1 racemizes on the NMR time scale via an achiral N-bonded imine intermediate. The N-imine isomer is more stable in Fe(PCHNAr^OMe)(CO)₃ (1^OMe), which crystallized despite being the minor isomer in solution. Protonation of 1 by HBF₄·Et₂O gave the iminium complex [1H]BF₄. The related diphosphine complex Fe(PCHNAr^Cl)(PMe₃)(CO)₂ (2), which features an η²-imine, was shown to also undergo N protonation. Oxidation of 1 and 2 with FcBF₄ gave the Fe(I) compounds [1]BF₄ and [2]BF₄. The oxidation-induced change in hapticity of the imine from η² in [1]⁰ to κ¹ in [1]⁺ was verified crystallographically. Substitution of a CO ligand in 1 with PCHNAr^Cl gave Fe[P₂(NAr^Cl)₂](CO)₂ (3), which contains the tetradentate diamidodiphosphine ligand. This C-C coupling is reversed by chemical oxidation of 3 with FcOTf. The oxidized product of [Fe(PCHNAr^Cl)₂(CO)₂]²⁺ ([4]²⁺) was prepared independently by the reaction of [1]⁺, PCHNAr^Cl, and Fc⁺. The C-C scission is proposed to proceed concomitantly with the reduction of Fe(II) via an intermediate related to [2]⁺.