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

Abstract

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.