Photochemical behavior of (bisphosphane)(η(2)-tolane)Pt(0) complexes. Part B: An insight from DFT calculations

A series of various (bisphosphane)(η(2)-tolane)Pt(0) complexes, exhibiting a manifold of substitution patterns of the tolane ligand (5a-g) and different rigid bisphosphanes defining various P-Pt-P bite angles at the Pt center (9a-b) have been theoretically investigated using time-dependent density functional theory (TD-DFT). UV/Vis absorption spectra have been calculated in order to rationalize the photochemistry of the complexes. Metal-ligand charge transfer (MLCT) transitions from the Pt atom to the alkyne are assigned as the photochemical "active" states responsible for promoting the C(aryl)-C(ethynyl) bond activation. The steric, the electronic effects, as well as the P-Pt-P bite angle play an important role in determining the presence/absence of photochemical "active" states of d→π*(alk) character. Thus, electron-withdrawing substituted series and ortho-substituted complexes are best candidates to achieve C(aryl)-C(ethynyl) bond activation. C-Br bond cleavage is also theoretically rationalized. The observed photochemical C(aryl)-C(ethynyl) bond cleavage is, oppositely to C-Br bond activation, reversible under thermal conditions regaining the appropriate Pt(0) complexes by reductive elimination (see T. Weisheit, D. Escudero, H. Petzold, H. Görls, L. González and W. Weigand, Photochemical behavior of (bisphosphane)(η(2)-tolane)Pt(0) complexes in solution and in the solid state. Part A: Experimental considerations, Dalton Trans., 2010, 39, DOI: 10.1039/B925562a). In this part, we rationalize and clarify the thermal reductive elimination reactions via mechanistic DFT studies on the ground state.