C2‑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 (1) arising from the head-to-head coupling of two formyl groups concomitant with oxidation of Fe(0) to Fe­(II). Crystallographic studies show that 1 features cis alkoxide ligands that are trans to CO; the two phosphine groups are mutually trans with a P–Fe–P angle of 167.44(4)°. The pathway leading to 1 was examined, starting with the adduct Fe­(PCHO)­(CO)4 (2), which was obtained by addition of PCHO to Fe2(CO)9. Compound 2 decarbonylates to give tricarbonyl Fe­(κ1,η2-PCHO)­(CO)3 (3), which features a π-bonded aldehyde. Photolysis of 2 gives a mixture of 3 and isomeric hydride HFe­(κ2-PCO)­(CO)3. Complex 3 reacts with an additional equivalent of PCHO to afford 1, whereas treatment with PPh3 afforded the substituted product Fe­(κ1,η2-PCHO)­(PPh3)­(CO)2 (4). In 4, the phosphine ligands are trans 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 (5) and Fe­(PCHNAr)­(CO)3 (6), respectively. Treatment of 6 with PCHO gave the unsymmetrical C–C coupling complex Fe­[(Ph2PC6H4)2CH­(O)­CH­(NAr)]­(CO)2 (7). Compound 7 was also prepared by the reaction of 3 and PCHNAr. The solid-state structure of 7, as established by X-ray crystallography, is similar to that of 1 but with an amido group in place of one alkoxide. The deuterium-labeled phosphine aldehyde PCDO was prepared by the reaction of ortho-lithiated phosphine Ph2PC6H4-2-Li with DMF-d7. Reaction of 6 with PCDO gave 7-d1 with no scrambling of the deuterium label. Attempted oxidation of 1 with FcBF4 (Fc+ = ferrocenium) gave the adduct Fe­[(Ph2PC6H4)2C2H2O2(BF3)2]­(CO)2 (8). The structures of 1 and 8 are almost identical. Compound 8 was independently synthesized by treating 1 with BF3OEt2 via the intermediacy of the 1:1 adduct, which was detected spectroscopically. Qualitative tests showed that 1 also reversibly protonates with HOSO2CF3 and binds TiCl4.