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

Abstract

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