文摘
The reaction of thiophene with a zerovalent platinum bisalkylphosphine fragment yields a highly stable thiaplatinacycle derived from cleavage of the C−S bond. Calculations on the [Pt(dmpe)] model system using Density Functional Theory are consistent with experimental results obtained with [Pt(dippe)] in that the reaction is exothermic overall and furthermore predict that the initial η2-coordination of thiophene through the Cmg src="http://pubs.acs.org/images/entities/dbd_2.gif">C double bond is energetically more favorable than coordination through the sulfur atom (ΔG = 9.3 kcal/mol). There are three well-defined transition states along the pathway to the oxidative addition product from both of these coordination modes. Two of these lead to a higher energy η2-C,S-coordinated intermediate, while the third one leads to cleavage of the C−S bond from the η2-C,S complex. As the reaction was carried out in a polar solvent (THF), the effect of solvation was taken into account by using the polarizable continuum model. The thermodynamic preference for the initial coordination of thiophene through the Cmg src="http://pubs.acs.org/images/entities/dbd_2.gif">C bond is found to be greater in THF (ΔG = 11.4 kcal/mol). More importantly, the total free energy of the transition state from the Cmg src="http://pubs.acs.org/images/entities/dbd_2.gif">C coordinated complex is now lower than that of the S-coordinated complex in solution. Therefore, the initial η2-coordination of thiophene through the Cmg src="http://pubs.acs.org/images/entities/dbd_2.gif">C double bond results in the kinetically preferred pathway. Molecular orbital analyses were carried out to rationalize the results.