Platinum(II) 1,2,4-Triazolin-5-ylidene Complexes: Stereoelectronic Influences on Their Catalytic Activity in Hydroelementation Reactions

Five platinum­(II) 1,2,4-triazolin-5-ylidene (tazy) complexes of the general formula [PtCl2(DMSO)­(R-tazy)] (1–5), bearing different N4 substituents (R = Dipp (1), Mes (2), Ph (3), Nap (4), and Bn (5)), have been successfully synthesized. The compounds have been fully characterized by means of ESI-MS mass spectrometry, NMR spectroscopy, elemental analysis, and X-ray diffraction analysis. The presence of two rotamers in complex 4 was elucidated using multinuclear magnetic resonance spectroscopy (1H, 13C NMR) spectroscopy and theoretical calculations. The five complexes were tested for their catalytic activities in the hydration and hydrosilylation of phenylacetylene. Significant differences in catalytic performance were observed. In the hydration of phenylacetylene, only Markovnikov products were formed with yields ranging from poor to moderate with compound 3 performing the best. In the hydrosilylation reaction, both Markovnikov (α) and anti-Markovnikov (β-E) products were obtained. While complex 1 was not very active, reactions catalyzed by complex 5 show complete conversions. A detailed study on the steric and electronic properties of the five triazolinylidenes have been carried out to rationalize the different catalytic performances of their complexes.