Experimental and Theoretical Investigations for Revealing\nthe Influence of Cyclometalated Ligand Type on Protonolysis of Pt–Carbon\nSites

This study investigates possible\npathways arising from the reaction\nof anionic K[Pt(C^N)(<i>p</i>-MeC<sub>6</sub>H<sub>4</sub>)(CN)] complexes, C^N = 2-phenylpyridinate (ppy) and 7,8-benzo[h]quinolate\n(bzq), with trifluoroacetic acid (TFA), which has been employed in\nboth experimental and computational approaches. Experimental studies\nclarify that the products of the protonolysis reaction can vary in\nthe K[Pt(C^N)(<i>p</i>-MeC<sub>6</sub>H<sub>4</sub>)(CN)]\ncomplex depending on the type of the cyclometalated ligand. In the\ncyclometalated complex with ppy, only one product was observed, resulting\nfrom the cleavage of the Pt–C<sub>ppy</sub> bond of the cyclometalated\nligand. Notably, when K[Pt(bzq)(<i>p</i>-MeC<sub>6</sub>H<sub>4</sub>)(CN)] reacts with trifluoroacetic acid, the protonolysis\nof both Pt–C<sub><i>p</i>‑tolyl</sub> and\nPt–C<sub>bzq</sub> occurs in nearly equal proportions. The\nresults indicate that the S<sub>E</sub>2 mechanism plays a primary\nrole in the emergence of the products. Additionally, the experimental\nmeasurements did not detect any evidence for HCN creation, which is\nrooted in the high energy barrier and complex mechanism of protonation\nof the Pt–C<sub>sp</sub>(CN) in contrast to Pt–C<sub>sp</sub><sup>2</sup>(<i>p</i>-MeC<sub>6</sub>H<sub>4</sub>) and Pt–C<sub>sp</sub><sup>2</sup>(C^N) bonds. Comparison\nof the C–H bond protonolysis reaction on the C<sub>sp</sub>, C<sub>sp</sub><sup>2</sup>, and C<sub>sp</sub><sup>3</sup> atoms\nin the investigated complexes has been carried out by substitution\nof the <i>p</i>-MeC<sub>6</sub>H<sub>4</sub> ligand with\na CH<sub>3</sub> ligand to form a [Pt(ppy)(CH<sub>3</sub>)(CN)]<sup>−</sup> complex. According to our density functional theory\n(DFT) calculations, this substitution leads to protonolysis of the\nPt–C<sub>C^N</sub> bond as the main product. The absence of\nthe CH<sub>4</sub> product is due to the increase of the reaction\nbarrier for the Pt–C<sub>Me</sub> bond protonolysis and a decrease\nin steric hindrance by the presence of a CH<sub>3</sub> ligand.