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, C 6 H 5 ] with [H(OEt 2 ) 2 ][BAr‘ 4 ] [BAr‘ 4 = tetrakis(3,5-trifluoromethylphenyl)borate] results in the formation of cationic platinum(II) η 2 -arene complexes [κ 2 -(HTp‘)Pt(η 2 -HAr)(R)][BAr‘ 4 ] [Ar = C 6 H 5, R = H ( 2 ); Ar = MeC 6 H 4, R = H ( 4 ); Ar = 3,6-Me 2 C 6 H 3, R = H ( 6 ); Ar = R = C 6 H 5 ( 8 )]. A barrier of 9.4 kcal/mol for platinum migration around the η 2 - 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 Δ H ⧧ = 11.7 ± 0.5 kcal mol - 1 and Δ S ⧧ = −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 η 2 -arene adducts with adjacent hydride ligands has allowed the barrier to oxidative addition for arene C−H bonds (Δ G ⧧ ) 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 η 2 -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.