Fundamental Studies of Molybdenum and Tungsten Methylidene and Metallacyclobutane Complexes

Addition of ethylene to Mo(NAr)(CHCMe2Ph)(OHIPT)(Pyr) (NAr = N-2,6-i-Pr2C6H3, OHIPT = O-2,6-(2,4,6-i-Pr3C6H2)(2)C6H3, 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)(Me(2)Pyr) (OTPP = O-2,3,5,6-Ph4C6H, Me(2)Pyr = 2,5-Me2NC4H2) was prepared similarly. Single-crystal X-ray studies of Mo(NAr)(CH2)(OHIPT)(Pyr) and W(NAr)(CH2)(OTPP)(Me(2)Pyr) show that they are monomers that contain an eta(1)-pyrrolide ligand and a methylidene ligand in which the M-C-H-anti angle is smaller than the M-C-H-syn 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)(Me(2)Pyr) (Ar-tBu = 2-t-BuC6H4) was isolated upon loss of ethylene from W(NArtBu)(C3H6)(OTPP)(Me(2)Pyr), but decomposed in solution over a period of several hours at 22 degrees 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)(Me(2)Pyr) (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)(Me(2)Pyr). It could be shown that Mo(NAr)(C3H6)(OBitet)(Me(2)Pyr) forms an ethylene/methylidene intermediate at 20 degrees C at a rate that is 4500 times faster than the rate at which W(NAr)(C3H6)(OBitet)(Me(2)Pyr) 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.