<i>C</i>₂‑Symmetric Iron(II) Diphosphine–Dialkoxide Dicarbonyl and Related Complexes

Reaction of Fe­(bda)­(CO)₃ (bda = benzylideneacetone) and Ph₂P-2-C₆H₄CHO (PCHO) affords the bis­phosphine bisalkoxide complex Fe­[(Ph₂PC₆H₄)₂C₂H₂O₂]­(CO)₂ (<b>1</b>) arising from the head-to-head coupling of two formyl groups concomitant with oxidation of Fe(0) to Fe­(II). Crystallographic studies show that <b>1</b> features <i>cis</i> alkoxide ligands that are <i>trans</i> to CO; the two phosphine groups are mutually <i>trans</i> with a P–Fe–P angle of 167.44(4)°. The pathway leading to <b>1</b> was examined, starting with the adduct Fe­(PCHO)­(CO)₄ (<b>2</b>), which was obtained by addition of PCHO to Fe₂(CO)₉. Compound <b>2</b> decarbonylates to give tricarbonyl Fe­(κ¹,η²-PCHO)­(CO)₃ (<b>3</b>), which features a π-bonded aldehyde. Photolysis of <b>2</b> gives a mixture of <b>3</b> and isomeric hydride HFe­(κ²-PCO)­(CO)₃. Complex <b>3</b> reacts with an additional equivalent of PCHO to afford <b>1</b>, whereas treatment with PPh₃ afforded the substituted product Fe­(κ¹,η²-PCHO)­(PPh₃)­(CO)₂ (<b>4</b>). In <b>4</b>, the phosphine ligands are <i>trans</i> and the aldehyde is π-bonded. The geometry around Fe is pseudo trigonal bipyramidal. To gain insights into the mechanism and scope of the C–C coupling reaction, complexes were prepared with the imine Ph₂PC₆H₄CHNC₆H₄Cl (abbreviated as PCHNAr), derived by condensation of 4-chloroaniline and PCHO. PCHNAr reacts with Fe₂(CO)₉ and with Fe­(bda)­(CO)₃ to afford the tetra- and tricarbonyl compounds Fe­(PCHNAr)­(CO)₄ (<b>5</b>) and Fe­(PCHNAr)­(CO)₃ (<b>6</b>), respectively. Treatment of <b>6</b> with PCHO gave the unsymmetrical C–C coupling complex Fe­[(Ph₂PC₆H₄)₂CH­(O)­CH­(NAr)]­(CO)₂ (<b>7</b>). Compound <b>7</b> was also prepared by the reaction of <b>3</b> and PCHNAr. The solid-state structure of <b>7</b>, as established by X-ray crystallography, is similar to that of <b>1</b> but with an amido group in place of one alkoxide. The deuterium-labeled phosphine aldehyde PCDO was prepared by the reaction of <i>ortho</i>-lithiated phosphine Ph₂PC₆H₄-2-Li with DMF-<i>d</i>₇. Reaction of <b>6</b> with PCDO gave <b>7</b>-<i>d</i>₁ with no scrambling of the deuterium label. Attempted oxidation of <b>1</b> with FcBF₄ (Fc⁺ = ferrocenium) gave the adduct Fe­[(Ph₂PC₆H₄)₂C₂H₂O₂(BF₃)₂]­(CO)₂ (<b>8</b>). The structures of <b>1</b> and <b>8</b> are almost identical. Compound <b>8</b> was independently synthesized by treating <b>1</b> with BF₃OEt₂ via the intermediacy of the 1:1 adduct, which was detected spectroscopically. Qualitative tests showed that <b>1</b> also reversibly protonates with HOSO₂CF₃ and binds TiCl<sub>4