Syntheses of Molybdenum Oxo Benzylidene Complexes

The reaction between Mo(O)(CHAr_o)(OR_F6)₂(PMe₃) (Ar_o = ortho-methoxyphenyl, OR_F6 = OCMe(CF₃)₂) and 2 equiv of LiOHMT (OHMT = O-2,6-(2,4,6-Me₃C₆H₂)₂C₆H₃) leads to Mo(O)(CHAr_o)(OHMT)₂, an X-ray structure of which shows it to be a trigonal bipyramidal anti benzylidene complex in which the o-methoxy oxygen is coordinated to the metal trans to the apical oxo ligand. Addition of 1 equiv of water (in THF) to the benzylidyne complex, Mo(CAr_p)(OR)₃(THF)₂ (Ar_p = para-methoxyphenyl, OR = OR_F6 or OC(CF₃)₃ (OR_F9)) leads to formation of {Mo(CAr_p)(OR)₂(μ-OH)(THF)}₂(μ-THF) complexes. Addition of 1 equiv of a phosphine (L) to Mo(CAr_p)(OR_F9)₃(THF)₂ in THF, followed by addition of 1 equiv of water, all at room temperature, yields Mo(O)(CHAr_p)(OR_F9)₂(L) complexes in good yields for several phosphines (e.g., PMe₂Ph (69% by NMR), PMePh₂ (59%), PEt₃ (69%), or P( i-Pr)₃ (65%)). The reaction between Mo(O)(CHAr_p)(OR_F9)₂(PEt₃) and 2 equiv of LiOHMT proceeds smoothly at 90 °C in toluene to give Mo(O)(CHAr_p)(OHMT)₂, a four-coordinate syn alkylidene complex. Mo(O)(CHAr_p)(OHMT)₂ reacts with ethylene (1 atm in C₆D₆) to give (in solution) a mixture of Mo(O)(CHAr_p)(OHMT)₂, Mo(O)(CH₂)(OHMT)₂, and an unsubstituted square pyramidal metallacyclobutane complex, Mo(O)(CH₂CH₂CH₂)(OHMT)₂, along with ethylene and Ar_pCH═CH₂. Mo(O)(CHAr_p)(OHMT)₂ also reacts with 2,3-dicarbomethoxynorbornadiene to yield syn and anti isomers of the "first-insertion" products that contain a cis C═C bond.