Reduction–oxidation properties of organotransition-metal complexes. Part 9. Silver(I) adducts of cyclic polyeneone complexes, the X-ray crystal structure of [{Fe(CO)3[η5-C5Ph4OAg(OH2)]}2][PF6]2, and comments on the use of the silver(I) ion as a one-electron oxidant

Abstract

The tetraphenylcyclopentadienone complexes [M(CO)₂L(η⁴-C₅Ph₄O)][1; M = Fe, L = CO, PPh₃, or P(OMe)₃; M = Ru, L = CO] and [Rh(η⁵-C₅H₅)(η⁴-C₅Ph₄O)](2) react with Ag[PF₆] to give dimeric oxygen-bonded silver adducts [{M(CO)₂L(η⁵-C₅Ph₄OAg)}₂][PF₆]₂(3) and [{Rh(η⁵-C₅H₅)(η⁵-C₅Ph₄OAg)}₂][PF₆]₂(4). The X-ray crystal structure of (3; M = Fe, L = CO) confirms that the cation contains a central planar asymmetric Ag₂O₂ bridge (Ag–O, 2.27 and 2.65 Å) but there is a molecule of water co-ordinated to each Ag atom. The five-membered ring of the Fe(CO)₃(C₅Ph₄O) moiety is best regarded as a substituted cyclopentadienyl although the carbon atom bearing the oxygen is significantly further from the iron atom (Fe–C, 2.31 Å) than are the other four carbon atoms [Fe–C, 2.11 (mean)Å]. There are also weak interactions between the silver atom and the two phenyl rings adjacent to the CO group in the C₅ ring. The crystals were of poor quality, due to partial inclusion of molecules of CH₂Cl₂ the positions of which were not refined. This is reflected in the-relatively poor R value and high standard deviations. The crystals are monoclinic, a= 13.293(7), b= 14.128(6). c= 19.805(14)Å, β= 105.75(5)°, space group P2₁/n. The Structure was solved by heavy-atom methods and refined to R 0.11 5 for 2 847 reflections. The iron complexes (3) react with halide ions to regenerate (1). but form [FeX(CO)₂L(η⁴-C₅Ph₄O)][PF₆][5; X = Br or I, L = CO, PPh₃, or P(OMe)₃] with halogens. Reaction of Ag[BF₄] with [Cr(CO)₃(η⁶-tropone)] gives [Cr(CO)₃{η⁷-C₇H₆(OH)}][BF₄]via the decomposition of an adduct related to (3) and (4). A summary is given of the behaviour of Ag^I as a reagent in organometallic chemistry, and a brief comment made on its use as a one-electron oxidant.