Syntheses of Molybdenum Oxo Benzylidene Complexes

The reaction between Mo­(O)­(CHAro)­(ORF6)2(PMe3) (Aro = ortho-methoxy­phenyl, ORF6 = OCMe­(CF3)2) and 2 equiv of LiOHMT (OHMT = O-2,6-(2,4,6-Me3C6H2)2­C6H3) leads to Mo­(O)­(CHAro)­(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(CArp)­(OR)3(THF)2 (Arp = para-methoxyphenyl, OR = ORF6 or OC­(CF3)3 (ORF9)) leads to formation of {Mo­(CArp)­(OR)2­(μ-OH)­(THF)}2­(μ-THF) complexes. Addition of 1 equiv of a phosphine (L) to Mo­(CArp)­(ORF9)3(THF)2 in THF, followed by addition of 1 equiv of water, all at room temperature, yields Mo­(O)­(CHArp)­(ORF9)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)­(CHArp)­(ORF9)2(PEt3) and 2 equiv of LiOHMT proceeds smoothly at 90 °C in toluene to give Mo­(O)­(CHArp)­(OHMT)2, a four-coordinate syn alkylidene complex. Mo­(O)­(CHArp)­(OHMT)2 reacts with ethylene (1 atm in C6D6) to give (in solution) a mixture of Mo­(O)­(CHArp)­(OHMT)2, Mo­(O)­(CH2)­(OHMT)2, and an unsubstituted square pyramidal metallacyclobutane complex, Mo­(O)­(CH2CH2CH2)­(OHMT)2, along with ethylene and ArpCHCH2. Mo­(O)­(CHArp)­(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