Molybdenum and Tungsten Alkylidene and Metallacyclobutane Complexes That Contain a Dianionic Biphenolate Pincer Ligand

Molybdenum imido alkylidene and tungsten oxo alkylidene complexes that contain a tridentate “pincer” [ONO]^2– ligand have been prepared and treated with ethylene to give unsubstituted metallacyclobutane complexes that have a 16e count. Both Mo and W metallacyclobutane complexes exchange C₂D₄ into the metallacyclobutane ring at 22 °C at a rate that is first order in metal and zero order in C₂D₄. These metallacycles lose ethylene at least 10⁴–10⁵ times slower than reported 14e unsubstituted Mo and W metallacyclobutane complexes that have been explored in the literature that have a TBP geometry with the metallacyclobutane ring bound in the equatorial positions. Our studies suggest that breaking up the metallacyclobutane ring in these 16e d⁰ Mo or W complexes is slow because a 14e TBP metallacyclobutane complex cannot be accessed readily.