Barriers for Arene C−H Bond Activation in Platinum(II) η2-Arene Intermediates

Protonation of platinum(IV) complexes of the type Tp‘Pt(Ar)(H)(R) [Tp‘ = hydridotris(3,5-dimethylpyrazolyl)borate; Ar = aryl; R = H, C6H5] with [H(OEt2)2][BAr‘4] [BAr‘4 = tetrakis(3,5-trifluoromethylphenyl)borate] results in the formation of cationic platinum(II) η2-arene complexes [κ2-(HTp‘)Pt(η2-HAr)(R)][BAr‘4] [Ar = C6H5, R = H (2); Ar = MeC6H4, R = H (4); Ar = 3,6-Me2C6H3, R = H (6); Ar = R = C6H5 (8)]. A barrier of 9.4 kcal/mol for platinum migration around the η2-p-xylene ring was calculated for complex 6 at 183 K following variable-temperature NMR measurements. Eyring analysis of hydrogen exchange between the bound benzene and the hydride ligand for cationic benzene complex 2 indicates ΔH ⧧ = 11.7 ± 0.5 kcal mol-1 and ΔS ⧧ = −3.8 ± 2 cal mol-1 K-1. This process is assumed to proceed via arene C−H oxidative addition to give the five-coordinate platinum(IV) phenyl dihydride intermediate 2a. Isolation of ground state η2-arene adducts with adjacent hydride ligands has allowed the barrier to oxidative addition for arene C−H bonds (ΔG ⧧) to be measured as 13.3 kcal/mol for the p-hydrogen and 13.6 kcal/mol for the m-hydrogens in the toluene adduct 4 and 14.2 kcal/mol for the p-xylene adduct 6. Averaging of the environments for two Tp‘ arms is an NMR observable feature for the C 1 symmetric η2-benzene phenyl adduct 8. This dynamic process, which presumably occurs via a five-coordinate platinum(IV) diphenyl hydride intermediate 8a with C s symmetry, allows the barrier for arene C−H oxidative addition to be calculated as 12.9 kcal/mol. Kinetic isotope effects for oxidative addition of the arene C−H(D) bond in the benzene hydride adduct 2 and the benzene phenyl adduct 8 were determined to be k H/k D = 3.0 at 259 K and 4.7 at 241 K, respectively. These data provide insight into the energetics of arene C−H bond activation.