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 RuII 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 characterised by 1H NMR, 31P{1H} NMR, IR, FAB-MS, microanalysis and in some cases by Xray diffraction analysis on a single crystal. The ligands show a great variety of different coordinating behaviours such as ?3-(H)PNO, ?2-(H)PN, ?1-(H)P and ?3-PNO, depending on the ruthenium precursor and on the synthetic experimental conditions. The complexes trans-[Ru(?3-(H)PNO)(PPh3)Cl2] reacted with dmso to give the bis-chelate complex [Ru(?3-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 precatalysts 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 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)(PPh3)(C=CH-C4H9)Cl].