Octahedral RuII complexes with [PNO] hydrazonic ligands:: Synthesis, structure, reactivity and catalytic activity in the addition of benzoic acid to alkynes

A series of Run complexes containing [(H)PNO] hydrazonic ligands were synthesised using different ruthenium sources such as [Ru(PPh3)(3)Cl-2], [Ru(dmsO)(4)Cl-2] and [Ru(p-cymene) Cl-2](2). The complexes were characterised by H-1 NMR, P-31(H-1) 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 kappa(3)-(H)PNO, kappa(2)-(H)PN, K-1-(H)P and K-3-PNO, depending on the ruthenium precursor and on the synthetic experimental conditions. The complexes trans-[Ru(kappa(3)-(H)PNO) (PPh3)Cl-2] reacted with dmso to give the bis-chelate complex [Ru(kappa(3)-PNO)(2)], [Ru(dmso)(4)]Cl-2, 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)(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 H-1 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]