Reactions of Gold(III) Oxo Complexes with PPh3 and Alkenes. Crystal Structure of [Au(bipyip)(h2-CH2=CHPh)][PF6] (bipyip = 6-iso-propyl-2,2Õ-bipyridine).
Giovanni Minghetti, Deborha Bacciu, Sergio Stoccoro, Antonio Zucca, Mario Manassero,
Two different kinds of gold(III) oxo complexes, [Au2(N,N)2(m-O)2][X]2 and [Au2(C,N,N)2(m-O)][X]2 (X = BF4, PF6), both supported by chelating nitrogen ligands, have been reported few years ago (1). In the first series, characterized by a Au2(m-O)2 core, each gold atom is bound to a bidentate 6-alkyl-bipyridine, bipyR (R = Me, CH2Me, CHMe2, CMe3, CH2CMe3) or to 2,2Í-bipyridine, bipy. The second one features a cyclometallated 6-benzyl-2,2Í-bipyridine, bipyRÍ (RÍ = CHMeC6H5, CMe2C6H5), and an unsupported Au-O-Au bridge. Both series have been characterized, inter alia, by X-ray analysis.
Some aspects of their reactivity have been also studied. For example, treatment of the complexes with water converts the cyclometallated derivatives [Au2(C,N,N)2(m-O)][PF6]2 and the unsubstituted bipyridine complex [Au2(bipy)2(m-O)2][PF6]2 into the water soluble monomeric hydroxo species [Au(C,N,N)(OH)][PF6] and [Au(bipy)(OH)2][PF6], respectively. These hydroxo species, likely the intermediates in the formation of the oxo complexes, are quantitatively reverted to the latter ones by reflux in anhydrous solvents.
The oxo complexes were tested in oxygen transfer reactions. Reaction of [Au2(bipyR)2(m-O)2][PF6]2 with PPh3 (1:6) gave OPPh3, the gold(I) complex [Au(PPh3)2][PF6] and free bipyR. At variance, from the reaction of the mono-bridged cyclometallated complexes [Au2(C,N,N)2(m-O)][PF6]2 with PPh3 (1:5), besides OPPh3 the free ligand bipyRÍ and [Au(PPh3)2][PF6], the gold(III) derivative [Au(C,N,N)(PPh3)][PF6]2 was obtained.
Another aspect of the reactivity of these species we deemed of interest is the oxygen transfer to unsaturated substrates. Alkene and alkyne oxidation by oxygen atom transfer is a particularly desirable reaction for the late transition metal oxo complexes.
Here we present the reaction of the bis m-oxo complexes [Au2(bipyR)2(m-O)2][PF6]2 (R = Me, CH2Me, CHMe2, CMe3, CH2CMe3) with different alkenes. The reaction with styrene has been studied with all the complexes. The reaction, carried out in MeCN at room temperature, allows in all cases to isolate unexpected styrene adducts [Au(bipyR)(h2-CH2=CHPh][PF6] which have been characterized by spectroscopic and, in one case, X-ray diffraction analyses.
The reaction is not quantitative and most of the starting material is recovered from the reaction mixture; small amounts of metallic gold and organic products are also formed, phenylacetaldehyde being the main product. Addition of small amounts of water to the reaction mixture improves the yields in the olefine complexes. Analogous adducts have been obtained with a-methylstyrene and norbornene.
1 M.A. Cinellu, G. Minghetti, M.V. Pinna, S. Stoccoro, A. Zucca, M. Manassero and M. Sansoni, J. Chem. Soc., Dalton Trans., 1998, 1735; M.A. Cinellu, G. Minghetti, M.V. Pinna, S. Stoccoro, A. Zucca and M. Manassero, Chem. Commun., 1998, 2397.
nitrogen ligands, oxo complexes, alkene complexes