Heterodinuclear Arene Ruthenium Complexes Containing a Glycine-Derived Phosphinoferrocene Carboxamide: Synthesis, Molecular Structure, Electrochemistry, and Catalytic Oxidation Activity in Aqueous Media

Three series of heterodinuclear ruthenium–iron
complexes
have been synthesized from (η6-arene)ruthenium dichloride
dimers and phosphinoferrocene ligands containing glycine-based carboxamido
substituents. The neutral complexes [(η6-arene)­RuCl2(Ph2PfcCONHCH2CO2Me-κP)] (4, arene = benzene (a), p-cymene (b), hexamethylbenzene (c); fc = ferrocene-1,1′-diyl) were obtained by the bridge cleavage
reaction of [(η6-arene)­RuCl2]2 with Ph2PfcCONHCH2CO2Me (1) in chloroform solution. The complex [(η6-p-cymene)­RuCl2(Ph2PfcCONHCH2CONH2-κP)] (6b) was synthesized in the same way from Ph2PfcCONHCH2CONH2 (3); the preparation of [(η6-p-cymene)­RuCl2(Ph2PfcCONHCH2CO2H-κP)]
(5b), featuring the ferrocene ligand in the free acid
form (2), failed due to side reactions and isolation
problems. The salts [(η6-arene)­RuCl­(MeCN)­(1-κP)]­[PF6] (7a–c) and [(η6-arene)­Ru­(MeCN)2(1-κP)]­[PF6]2 (8a–c) were prepared from 1 and the acetonitrile precursors [(η6-arene)­RuCl­(MeCN)2]­[PF6] and from 4a–c via halide removal with Ag­[PF6] in acetonitrile solution,
respectively. Alternative synthetic routes to 7b and 8b were also studied. The compounds were fully characterized
by elemental analysis, multinuclear NMR, IR, and electrospray ionization
mass spectra, and their electrochemical properties were studied by
cyclic voltammetry at a Pt-disk electrode. The single-crystal X-ray
analyses of two representatives (4b and 8b) revealed a pseudotetrahedral coordination geometry at the ruthenium
centers and eclipsed conformations of the ferrocene moieties, with
the substituents at the two cyclopentadienyl rings being anti with
respect to each other. All complexes showed high activity for the
catalytic oxidation of secondary alcohols with tert-butyl hydroperoxide to give ketones in aqueous media. The most active
catalyst was obtained from the neutral p-cymene complex 4b, showing a catalytic turnover frequency of 13 200
h–1 at room temperature for the oxidation of 1-phenylethanol
at a substrate/catalyst ratio of 100 000.