Selectivity in metal–carbon bond protonolysis in p-tolyl- (or methyl)-cycloplatinated(ii) complexes: kinetics and mechanism of the uncatalyzed isomerization of the resulting Pt(ii) products

Abstract

Reaction of each of the known starting complexes [PtR(C^N)(SMe₂)], 1, in which R = Me or p-MeC₆H₄ and C^N is either ppy (deprotonated 2-phenylpyridine) or bhq (deprotonated benzo[h]quinoline), with one equivalent of CF₃CO₂H, gave the complexes [Pt(C^N)(CF₃CO₂)(SMe₂)], 3 (C^N = ppy, 3a; bhq, 3b). The bis-chelate complexes [Pt(C^N)(P^P)](CF₃CO₂), 4, were obtained by reaction of complexes 3 with one equivalent of either of the P^P bisphosphine reagents, dppf = 1,1′-bis(diphenylphosphino)ferrocene or dppe = bis(diphenylphosphino)ethane. Complexes 4 were alternatively made by reaction of the complexes [PtMe(κ¹C-C^N)(P^P)], 2, with one equivalent of CF₃CO₂H. When the complex 3b was reacted with 0.5 equivalents of dppe, 0.5 equivalents of the related bis-chelate product, 4d, formed along with 0.5 equivalents of the unreacted starting complex 3b. In contrast, when the complex 3b was reacted with 0.5 equivalents of dppf, then the dimeric complex [Pt₂(bhq)₂(CF₃CO₂)₂(μ-dppf)], 5, formed in pure form. In all the above-mentioned acid reactions, the M–R bond rather than the M–C bond of the cycloplatinated complex is cleaved. When the PPh₃ analogues of complexes 1, i.e. the complexes [PtR(C^N)(PPh₃)], 6, in which C^N is ppy or tpy = deprotonated 2-p-tolylpyridine, were reacted with one equivalent of CF₃CO₂H, the course of the reaction reversed and the M–C bonds of the cycloplatinated complexes are cleaved rather than the M–R bonds. The latter reaction gave [PtR(κ¹N-HC^N)(PPh₃)(CF₃CO₂)], as an equilibrium mixture of two isomers 7 and 8. Crystal structures of the typical complexes show a variety of extensive intermolecular hydrogen bonding involving C–H bonds from the different ligands and electronegative atoms (O or F) from the CF₃CO₂ moiety. On the basis of data obtained from kinetic studies (using ¹H NMR spectroscopy), a dissociative mechanism is proposed for the case of the 7c/8c isomerization process, involving dissociation of the κ¹N-Htpy neutral ligand, rather than the alternative route of PPh₃ or CF₃CO₂ ligand dissociation.