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.