Cleavage of Carbon−Carbon Bonds of Diphenylacetylene and Its Derivatives via Photolysis of Pt Complexes:  Tuning the C−C Bond Formation Energy toward Selective C−C Bond Activation

Carbon−carbon bond activation of diphenylacetylene and several substituted derivatives has been achieved via photolysis and studied. Pt⁰−acetylene complexes with η²-coordination of the alkyne, along with the corresponding Pt^II C−C activated photolysis products, have been synthesized and characterized, including X-ray crystal structural analysis. While the C−C cleavage reaction occurs readily under photochemical conditions, thermal activation of the C−C bonds or formation of Pt^II complexes was not observed. However, the reverse reaction, C−C reductive coupling (Pt^II → Pt⁰), did occur under thermal conditions, allowing the determination of the energy barriers for C−C bond formation from the different Pt^II complexes. For the reaction (dtbpe)Pt(−Ph)(−C⋮CPh) (2) → (dtbpe)Pt(η²-PhC⋮CPh) (1), ΔG^⧧ was 32.03(3) kcal/mol. In comparison, the energy barrier for the C−C bond formation in an electron-deficient system, that is, (dtbpe)Pt(C₆F₅)(C⋮CC₆F₅) (6) → (dtbpe)Pt(η²-bis(pentafluorophenyl)acetylene) (5), was found to be 47.30 kcal/mol. The energy barrier for C−C bond formation was able to be tuned by electronically modifying the substrate with electron-withdrawing or electron-donating groups. Upon cleavage of the C−C bond in (dtbpe)Pt(η²-(p-fluorophenyl-p-tolylacetylene) (9), both (dtbpe)Pt(p-fluorophenyl)(p-tolylacetylide) (10) and (dtbpe)Pt(p-tolyl)(p-fluorophenylacetylide) (11) were obtained. Kinetic studies of the reverse reaction confirmed that 10 was more stable toward the reductive coupling [the term “reductive coupling” is defined as the formation of (dtbpe)Pt(η²-acetylene) complex from the Pt^II complex] than 11 by 1.22 kcal/mol, under the assumption that the transition-state energies are the same for the two pathways. The product ratio for 10 and 11 was 55:45, showing that the electron-deficient C−C bond is only slightly preferentially cleaved.