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

Abstract

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.

Intermolecular Hydroarylation of Unactivated Olefins Catalyzed by Homogeneous Platinum Complexes

“…[16] Replacement of the bound arene of F with an olefin releases the product giving A. Displacement of the arene in D by the olefin would lead to the formation of dialkylated arene products, and b-hydride elimination from B would lead to styrene formation. Scrambling of H,D between Ptbound Ph and arene groups has been previously reported.…”

Hydrocarbon C—H Bond Activation with Tp'Pt Complexes

“…Isolation of the perdeuterio benzene hydride adduct (17-d) and benzene phenyl adduct (20-rf) permitted direct measurement of the kinetic isotope effects for intramolecular arene C-H(D) bond activation. The kn/k D value for complexes 17 and 20 were 3.0 at 259 Κ and 4.7 at 241 K, respectively, consistent with significant C-H(D) bond cleavage to reach the transition state (84).…”

“…For R = S1R3, five-coordinate Pt(IV) complexes of the type [κ - (HTp^PtiHJiiSiRa^tBAr^] have been isolated (72). In addition, isolation of a Pt(II) arene adduct prior to replacement of the Ar-H ligand was achieved in the form of Pt(II) t| 2 -arene adduct (83,84). By utilizing R = SiR 3 and G>R 5 rather than R = alkyl in the low temperature protonation of Tp'PtR(H)2, isolation of analogs of proposed intermediates in C-H bond activation by Pt(II) complexes was possible (Scheme 3).…”

“…These T| 2 -arene platinum(II) adducts exhibit dynamic *H NMR spectra which can be attributed to equilibration of the T) 2 -arene adduct with a fivecoordinate platinum aryl dihydride intermediate via arene C-H oxidative addition (eq 8) (83,84). Scrambling of the two hydride ligands in the fivecoordinate intermediate allows for exchange to occur.…”

Intermolecular Hydroarylation of Unactivated Olefins Catalyzed by Homogeneous Platinum Complexes

“…Scrambling of H,D between Ptbound Ph and arene groups has been previously reported. [16] Replacement of the bound arene of F with an olefin releases the product giving A. Displacement of the arene in D by the olefin would lead to the formation of dialkylated arene products, and b-hydride elimination from B would lead to styrene formation. In summary, Pt-catalyzed intermolecular hydroarylation of unactivated olefins has been rationally developed.…”