Cleavage of Unstrained C(sp²)—C(sp²) Single Bonds with Ni⁰ Complexes Using Chelating Assistance

Abstract

The reaction of Ni(COD) 2 with a ligand containing a biphenyl subunit (P( i Pr) 2 -O-Ph-Ph-O-P( i Pr) 2, L1 ) resulted in a mixture of compounds, the main product being [ L1 ]Ni(COD), which could be quantitatively transferred to product 2 using bis( p -toluyl)acetylene. The possible activation products, cis - and trans - 3 ( 3 c and 3 t, respectively), which would result from the cleavage of the bridging C—C single bond were synthesized independently. The mechanism of the cis / trans -isomerization was examined, activation parameters determined and trapping tests performed. Reaction of (PPh 3 ) 2 Ni(CO) 2 with L1 gave a novel complex 4 by a ligand exchange reaction. This compound, when heated to 95 °C under a 5 bar atmosphere of CO for several days, slowly incorporated CO into the bridging C—C single bond. This behavior as well as isotopic labeling experiments with 13 CO indicated that, following a dissociation of CO in complex 4, an interaction between the metal center and the biphenyl moiety is induced, leading to activation. The reaction of Ni(COD) 2 with P( i Pr) 2 -O-Ph-CO-Ph-O-(P( i Pr) 2 ( L3 ), which contains a benzophenone moiety, resulted in the quantitative activation of the PhC—CO bond at 20 °C, leading to complexes 3 c/t ( trans / cis 80/20 at 20 °C). The mechanism was examined and intermediates of it were identified as well as activation parameters for their further reaction determined. Reaction of (PPh 3 ) 2 Ni(CO) 2 with L3 gave complex 5, which is stable at 20 °C. Complexes 3 c/t were shown to react quantitatively to give compound 5 under an atmosphere of CO. Selective isotopic labeling with 13 CO proved that an activation similar to that observed with Ni(COD) 2 occurs in complex 5 at 95 °C but much more slowly and is accompanied by the formation of complex 4 . Molecular structures of the novel complexes 2, 3 c, 4, and 5 have been determined by single crystal X-ray diffraction studies.