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

2007-07-18T00:00:00Z (GMT) by Ahmet Gunay William D. Jones
Carbon−carbon bond activation of diphenylacetylene and several substituted derivatives has been achieved via photolysis and studied. Pt<sup>0</sup>−acetylene complexes with η<sup>2</sup>-coordination of the alkyne, along with the corresponding Pt<sup>II</sup> 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<sup>II</sup> complexes was not observed. However, the reverse reaction, C−C reductive coupling (Pt<sup>II</sup> → Pt<sup>0</sup>), did occur under thermal conditions, allowing the determination of the energy barriers for C−C bond formation from the different Pt<sup>II</sup> complexes. For the reaction (dtbpe)Pt(−Ph)(−C⋮CPh) (<b>2</b>) → (dtbpe)Pt(η<sup>2</sup>-PhC⋮CPh) (<b>1</b>), Δ<i>G</i><sup>⧧</sup> 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(C<sub>6</sub>F<sub>5</sub>)(C⋮CC<sub>6</sub>F<sub>5</sub>) (<b>6</b>) → (dtbpe)Pt(η<sup>2</sup>-bis(pentafluorophenyl)acetylene) (<b>5</b>), 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(η<sup>2</sup>-(<i>p</i>-fluorophenyl-<i>p</i>-tolylacetylene) (<b>9</b>), both (dtbpe)Pt(<i>p</i>-fluorophenyl)(<i>p</i>-tolylacetylide) (<b>10</b>) and (dtbpe)Pt(<i>p</i>-tolyl)(<i>p</i>-fluorophenylacetylide) (<b>11</b>) were obtained. Kinetic studies of the reverse reaction confirmed that <b>10</b> was more stable toward the reductive coupling [the term “reductive coupling” is defined as the formation of (dtbpe)Pt(η<sup>2</sup>-acetylene) complex from the Pt<sup>II</sup> complex] than <b>11</b> by 1.22 kcal/mol, under the assumption that the transition-state energies are the same for the two pathways. The product ratio for <b>10</b> and <b>11</b> was 55:45, showing that the electron-deficient C−C bond is only slightly preferentially cleaved.