Oxidation of ruthenium(II) to ruthenium(IV) eta(4)-diene complexes: Swing mechanism and diene-allyl conversion

This paper reports on the synthesis and reactivity of Ru(IV) eta(4)-diene complexes of the type Ru(eta(5)-C(5)Me(5))(eta(4)-diene)Br-2(+) obtained by oxidative bromine additions to Ru(II) eta(4)-diene complexes. Mechanistic details are derived from the reaction products varying with the oxidizing agent and the leaving ligand and are backed up by extended Huckel molecular orbital calculations. The diene fragment in Ru(IV) is prone to gauche deformation and is therefore extremely susceptible to nucleophilic attack of even weak anionic bases generating the corresponding Ru(IV) eta(3)-allyl complexes. Accordingly, Ru(IV) eta(4)-diene complexes are stabilized either by using 2,3-disubstituted diene ligands in which case gauche deformation is highly unfavorable or else by excluding nucleophilic agents. For the latter method, two convenient routes to affording Ru(IV) eta(4)-diene complexes are (i) the reaction of Br+CF3SO3- with Ru(eta(5)-C(5)Me(5))(eta(4)-diene)Br and (ii) the reaction of dibromine with labile Ru(eta(5)-C(5)Me(5))(eta(4)-diene)(CF3SO3). The conversion of the Ru(II) eta(4)-diene to the Ru(IV) eta(4)-diene complex is suggested to proceed via a swing of the diene ligand by changing the dihedral angle between the C(5)Me(5) and diene ligands from about +20 to about -70 degrees concomitantly pushing the leaving ligand (Br- or CF3SO3-) out.