Experimental and Theoretical Investigations for Revealing the Influence of Cyclometalated Ligand Type on Protonolysis of Pt–Carbon Sites

This study investigates possible pathways arising from the reaction of anionic K[Pt(C^N)(<i>p</i>-MeC₆H₄)(CN)] complexes, C^N = 2-phenylpyridinate (ppy) and 7,8-benzo[h]quinolate (bzq), with trifluoroacetic acid (TFA), which has been employed in both experimental and computational approaches. Experimental studies clarify that the products of the protonolysis reaction can vary in the K[Pt(C^N)(<i>p</i>-MeC₆H₄)(CN)] complex depending on the type of the cyclometalated ligand. In the cyclometalated complex with ppy, only one product was observed, resulting from the cleavage of the Pt-C_ppy bond of the cyclometalated ligand. Notably, when K[Pt(bzq)(<i>p</i>-MeC₆H₄)(CN)] reacts with trifluoroacetic acid, the protonolysis of both Pt-C_{<i>p</i>-tolyl} and Pt-C_bzq occurs in nearly equal proportions. The results indicate that the S_E2 mechanism plays a primary role in the emergence of the products. Additionally, the experimental measurements did not detect any evidence for HCN creation, which is rooted in the high energy barrier and complex mechanism of protonation of the Pt-C_sp(CN) in contrast to Pt-C_sp²(<i>p</i>-MeC₆H₄) and Pt-C_sp²(C^N) bonds. Comparison of the C-H bond protonolysis reaction on the C_sp, C_sp², and C_sp³ atoms in the investigated complexes has been carried out by substitution of the <i>p</i>-MeC₆H₄ ligand with a CH₃ ligand to form a [Pt(ppy)(CH₃)(CN)]^- complex. According to our density functional theory (DFT) calculations, this substitution leads to protonolysis of the Pt-C_C^N bond as the main product. The absence of the CH₄ product is due to the increase of the reaction barrier for the Pt-C_Me bond protonolysis and a decrease in steric hindrance by the presence of a CH₃ ligand.