The reaction of phosphines with [Fe2(CO)4L2(η5: η′5-C16H16)]2+[L = CO or P(OPh)3]; cyclopropane ring-opening of the bicyclo[5.1.0]octadienyl groups, and the X-ray crystal structure of [Fe2(CO)6{η4:η′4-C16H16(PPh3)2}][PF6]2·0.5C6H5Me

Abstract

Tributylphosphine and [Fe₂(CO)₄L₂(η⁵:η′⁵-C₁₆H₁₆)][PF₆]₂(1; L = CO) form the symmetrical bis(phosphonium)salt [Fe₂(CO)₆{η⁴:η′⁴-C₁₆H₁₆(PBuⁿ₃)₂}][PF₆]₂(2) in which one phosphine ligand has added to the terminal carbon atom of each of the two dienyl units of the bis(bicyclo[5.1.0]octadienyl) carbon skeleton of (1; L = CO). By contrast, PPh₃ and (1; L = CO) yield [Fe₂(CO)₆{η⁴:η′⁴-C₁₆H₁₆(PPh₃)₂}][PF₆]₂(3), X-ray studies of which show that the dication comprises two open C₇ rings joined by a trans ethylenic bridge of four coplanar carbon atoms, and that the two phosphine ligands are bonded to the two carbon atoms adjacent to the bridgehead of one C₇ ring. The remaining carbon atoms of this ring are η⁴-bonded to an Fe(CO)₃ group. In the other C₇ ring, however, four atoms adjacent to the bridgehead are η⁴-bonded to another Fe(CO)₃ group while the other two atoms form an ethylenic moiety not bonded to the metal. The complex crystallises in the monoclinic space group P2₁/c, and the asymmetric unit comprises two molecules of the dication, four [PF₆]^– counter ions, and two half-molecules of solvent (toluene), one of which is disordered. The structure has been refined to R= 0.142 for 6 436 reflections [l 2.0σ(l)]. The high R value is a consequence of poor crystal quality: of 22 019 unique reflections measured, 15 583 were rejected. Cyclopropane ring opening also occurs on reacting [1; L = P(OPh)₃] with PBuⁿ₃ or PPh₃; the hydrocarbon ligand of the resulting complex [Fe₂(CO)₄{P(OPh)₃}₂(η⁴:η′⁵-C₁₆H₁₆L′)][PF₆]₂(4; L′= PBuⁿ₃ or PPh₃) is related to that of (3). NaBH₄ and (2) react to give [Fe₂(CO)₆(η⁴:η′⁴-C₁₆H₁₈)] but (3) is reduced to [Fe₂(CO)₆(η⁴:η′⁴-C₁₆H₁₆)](5), a complex of trans-1,2-di(cycloheptatrienyl)ethylene.