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. Pt-0-acetylene complexes with eta(2)-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 -> Pt-0), 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)(-CCPh) (2) -> (dtbpe)Pt(eta(2)-PhCCPh) (1), Delta G 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)(CCC6F5) (6) -> (dtbpe)Pt(eta(2)-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(eta(2)-(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(eta(2)-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.