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 (PCHNArCl) complexes of iron(0), iron(I), and iron(II). The reaction of PCHNArCl with Fe(bda)(CO)3 gives Fe(PCHNArCl)(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(PCHNArOMe)(CO)3 (1OMe), 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(PCHNArCl)(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 PCHNArCl gave Fe[P2(NArCl)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(PCHNArCl)2(CO)2]2+ ([4]2+) was prepared independently by the reaction of [1]+, PCHNArCl, and Fc+. The C-C scission is proposed to proceed concomitantly with the reduction of Fe(II) via an intermediate related to [2]+.