Octahedral Ru II Complexes with [PNO] Hydrazonic Ligands: Synthesis, Structure, Reactivity and Catalytic Activity in the Addition of Benzoic Acid to Alkynes

A series of Ru II complexes containing [(H)PNO] hydrazonic ligands were synthesised using different ruthenium sources such as [Ru(PPh 3 ) 3 Cl 2 ], [Ru(dmso) 4 Cl 2 ] and [Ru( p ‐cymene)Cl 2 ] 2 . The complexes were characterised by 1 H NMR, 31 P{ 1 H} NMR, IR, FAB‐MS, microanalysis and in some cases by X‐ray diffraction analysis on a single crystal. The ligands show a great variety of different coordinating behaviours such as κ 3 ‐(H)PNO, κ 2 ‐(H)PN, κ 1 ‐(H)P and κ 3 ‐PNO, depending on the ruthenium precursor and on the synthetic experimental conditions. The complexes trans ‐[Ru(κ 3 ‐(H)PNO)(PPh 3 )Cl 2 ] reacted with dmso to give the bis‐chelate complex [Ru(κ 3 ‐PNO) 2 ], [Ru(dmso) 4 ]Cl 2 , OPPh 3 , HCl and Me 2 S, through an oxygen‐transfer reaction from dmso to PPh 3 . A catalytic version of this reaction was also developed. The complexes obtained from [Ru(PPh 3 ) 3 Cl 2 ] were tested as homogeneous pre‐catalysts for the coupling between benzoic acid and terminal alkynes to give the corresponding enol esters. High stereo‐ and regio‐selectivity, up to 100 % (determined by 1 H NMR), in favour of the ( Z )‐ anti ‐Markovnikov products ( Z )‐alk‐1‐en‐1‐yl benzoate was observed. An ESI‐MS monitoring of the catalytic couplings revealed that the enol ester formation occurs through an intermolecular attack of an external carboxylate anion onto a vinylidene–Ru intermediate of the type [Ru(PNO)(PPh 3 )(C=CH–C 4 H 9 )Cl]. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)