Octahedral RuII 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(PPh3)3Cl2], [Ru(dmso)4Cl2] and [Ru(p-cymene)Cl2]2. The complexes were characterized by 1H-NMR, 31P{1H}-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 κ3-(H)PNO, κ2-(H)PN, κ1-(H)P and κ3-PNO, depending on the ruthenium precursor and on the synthetic experimental conditions. The complexes trans-[Ru(κ3-(H)PNO)(PPh3)Cl2] reacted with dmso to give the bis-chelate complex [Ru(κ3-PNO)2], [Ru(dmso)4]Cl2, OPPh3, HCl and Me2S, through an oxygen transfer reaction from dmso to PPh3. A catalytic version of this reaction was also developed. The complexes obtained from [Ru(PPh3)3Cl2] 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 1H-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 allowed to ascertain that the enol esters formation occurs through an intermolecular attack of an external carboxylate anion onto a vinylidene-Ru intermediate of the type [Ru(PNO)(PPh3)(C=CH-C4H9)Cl].