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

The phosphinito-bridged Pt(I) complex [(PHCy2)Pt(μ-PCy2){κ2P,O-μ-P(O)Cy2}Pt(PHCy2)](Pt−Pt) (1) reversibly adds H2 under ambient conditions, giving cis-[(H)(PHCy2)Pt1(μ-PCy2)(μ-H)Pt2(PHCy2){κP-P(O)Cy2}](Pt−Pt) (2). Complex 2 slowly isomerizes spontaneously into the corresponding more stable isomer trans-[(PHCy2)(H)Pt(μ-PCy2)(μ-H)Pt(PHCy2){κP-P(O)Cy2}](Pt−Pt) (3). DFT calculations indicate that the reaction of 1 with H2 occurs through an initial heterolytic splitting of the H2 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 [(PHCy2)(H)Pt(μ-PCy2)Pt(PHCy2){κP-P(OH)Cy2}](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-H2 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 [(PHCy2)(η2-H2)Pt(μ-PCy2)Pt(PHCy2){κP-P(O)Cy2}](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.