New Palladium(II) Complex of P,S-Containing Hybrid Calixphyrin. Theoretical Study of Electronic Structure and Reactivity for Oxidative Addition

The palladium complex of P,S-containing hybrid calixphyrin 1 was investigated with the DFT method. There are two kinds of valence tautomer in 1: one is a Pd(II) form in which the calixphyrin moiety possesses -2 charges and the Pd center takes +2 oxidation state, and the other is a Pd(0) form in which the calixphyrin is neutral and the Pd center takes zero oxidation state. Complex 1 takes the Pd(II) form in the ground state. Though the Pd center takes +2 oxidation state, DFT computations clearly show that the oxidative addition of phenyl bromide (PhBr) to 1 occurs with moderate activation enthalpy, as experimentally proposed. The first step of the oxidative addition is the coordination of PhBr with the Pd center to form intermediate 1INTa, in which the Pd center and the calixphyrin moiety are neutral; in other words, the valence tautomerization from the Pd(II) form to the Pd(0) form occurs in the palladium calixphyrin moiety. The activation enthalpy is 22.5 kcal/mol, and the enthalpy change of reaction is 20.3 kcal/mol. The next step is the C-Br sigma-bond cleavage of PhBr, which occurs with activation enthalpy of 2.0 kcal/mol relative to 1INTa. On the other hand, the oxidative additions of PhBr to palladium complex of P,S-containing hybrid porphyrin 2 and that of conventional porphyrin 3 need much larger activation enthalpies of 49.1 and 74.4 kcal/mol, respectively. The differences in the reactivity among 1, 2, and 3 were theoretically investigated; in 1, the valence tautomerization occurs with moderate activation enthalpy to afford the Pd(0) form which is reactive for the oxidative addition. In 2, the tautomerization from the Pd(II) form to the Pd(0) form needs very large activation enthalpy (43.3 kcal/mol). In 3, such valence tautomerization does not occur at all, indicating that the Pd(II) must change to the Pd(IV) in the oxidative addition of PhBr to 3, which is a very difficult process. These differences are interpreted in terms of the pi* orbital energies of P,S-containing hybrid calixphyrin, hybrid porphyrin, and conventional porphyrin and the flexibility of their frameworks.