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.