Fundamental Studies of Molybdenum and Tungsten Methylidene and Metallacyclobutane Complexes

• Main Authors: Schrock, Richard Royce (Contributor), Jiang, Annie J. (Contributor), Marinescu, Smaranda C. (Contributor), Simpson, Jeffrey H. (Contributor), Muller, Peter (Contributor)
• Other Authors: Massachusetts Institute of Technology. Department of Chemistry (Contributor)
• Format: Article
• Language: English
• Published: American Chemical Society, 2012-03-08T17:10:50Z.
• Subjects: Article
• Online Access: Get fulltext

Addition of ethylene to Mo(NAr)(CHCMe2Ph)(OHIPT)(Pyr) (NAr = N-2,6-i-Pr2C6H3, OHIPT = O-2,6-(2,4,6-i-Pr3C6H2)2C6H3, Pyr = NC4H4) led to the trigonal-bipyramidal metallacyclobutane complex Mo(NAr)(C3H6)(OHIPT)(Pyr), in which the imido and aryloxide ligands occupy axial positions. Mo(NAr)(C3H6)(OHIPT)(Pyr) loses ethylene to give isolable Mo(NAr)(CH2)(OHIPT)(Pyr). W(NAr)(CH2)(OTPP)(Me2Pyr) (OTPP = O-2,3,5,6-Ph4C6H, Me2Pyr = 2,5-Me2NC4H2) was prepared similarly. Single-crystal X-ray studies of Mo(NAr)(CH2)(OHIPT)(Pyr) and W(NAr)(CH2)(OTPP)(Me2Pyr) show that they are monomers that contain an η1-pyrrolide ligand and a methylidene ligand in which the M−C−Hanti angle is smaller than the M−C−Hsyn angle, consistent with an agostic interaction between CHanti and the metal. Attempts to prepare analogous Mo(NAd)(CH2)(OHIPT)(Pyr) (Ad = 1-adamantyl) yielded only the ethylene complex Mo(NAd)(C2H4)(OHIPT)(Pyr). W(NArtBu)(CH2)(OTPP)(Me2Pyr) (ArtBu = 2-t-BuC6H4) was isolated upon loss of ethylene from W(NArtBu)(C3H6)(OTPP)(Me2Pyr), but decomposed in solution over a period of several hours at 22 °C. NMR studies of Mo(NAr)(C3H6)(OHIPT)(Pyr) and W(NAr)(C3H6)(OHIPT)(Pyr) species showed them both to be in equilibrium with ethylene/methylidene intermediates before losing ethylene to yield the respective methylidene complexes. Detailed NMR studies of Mo(NAr)(C3H6)(OBitet)(Me2Pyr) (OBitet is the anion derived from (R)-3,3'-dibromo-2'-(tert-butyldimethylsilyloxy)-5,5',6,6',7,7',8,8'-octahydro-1,1'-binaphthyl-2-ol) were carried out and compared with previous studies of W(NAr)(C3H6)(OBitet)(Me2Pyr). It could be shown that Mo(NAr)(C3H6)(OBitet)(Me2Pyr) forms an ethylene/methylidene intermediate at 20 °C at a rate that is 4500 times faster than the rate at which W(NAr)(C3H6)(OBitet)(Me2Pyr) forms an ethylene/methylidene intermediate. It is proposed that the stability of methylidene complexes coupled with their high reactivity accounts for the high efficiency of many olefin metathesis processes that employ monoaryloxidepyrrolide catalysts.
National Science Foundation (U.S.) (CHE-0841187)