Facile Activation of Dihydrogen by a Phosphinito-Bridged Pt(I)-Pt(I) Complex

摘要

The phosphinito-bridged Pt(I) complex [(PHCy_2)Pt(μ-Pcy_2){κ~2P,O-μ-P(O)Cy_2}Pt(PHCy_2)](Pt-Pt) (1) reversibly adds H_2 under ambient conditions, giving cis-[(H)(PHCy_2)Pt~1(μ-Pcy_2)(μ-H)R~2(PHCy_2){κP-P(O)Cy_2}](Pt-Pt) (2). Complex 2 slowly isomerizes spontaneously into the corresponding more stable isomer frans-[(PHCy_2)(H)R(μ-Pcy_2)(μ-H)Pt(PHCy_2){κP-P(O)Cy_2}](PftPt) (3). DFT calculations indicate that the reaction of 1 with H_2 occurs through an initial heterolytic splitting of the H_2 molecule assisted by the phosphinito oxygen with breaking of the Pt-O bond and hydrogenation of the Pt and O atoms, leading to the formation of the intermediate [(PHCy_2)(H)Pt(μ-Pcy_2)Pt(PHCy_2){κP-P(OH)Cy_2}](Pt-Pt) (D), where the two split hydrogen atoms interact within a six-membered Pt-H…H-O-P-Pt ring. Compound D is a labile intermediate which easily evolves into the final dihydride complex 2 through a facile (9-15 kcal mol~(-1), depending on the solvent) hydrogen shift from the phosphinito oxygen to the Pt-Pt bond. Information obtained by addition of para-H_2 on 1 are in agreement with the presence of a heterolytic pathway in the 1 → 2 transformation. NMR experiments and DFT calculations also gave evidence for the nonclassical dihydrogen complex [(PHCy_2)(η~2-H_2)R(μ-Pcy_2)R(PHCy_2){κP-P(O)Cy_2}](Pt-Pt) (4), which is an intermediate in the dehydrogenation of 2 to 1 and is also involved in intramolecular and intermolecular exchange processes. Experimental and DFT studies showed that the isomerization 2→3 occurs via an intramolecular mechanism essentially consisting of the opening of the Pt-Pt bond and of the hydrogen bridge followed by the rotation of the coordination plane of the Pt center with the terminal hydride ligand.