Fundamental Studies of Molybdenum and Tungsten Methylidene and Metallacyclobutane Complexes

Addition of ethylene to Mo(NAr)(CHCMe 2 Ph)(OHIPT)(Pyr) (NAr = N-2,6-i-Pr 2 C 6 H 3, OHIPT = O-2,6-(2,4,6-i-Pr 3 C 6 H 2 ) 2 C 6 H 3, Pyr = NC 4 H 4 ) led to the trigonal-bipyramidal metallacyclobutane complex Mo(NAr)(C 3 H 6 )(OHIPT)(Pyr), in which the imido and aryloxide ligands occupy axial positions. Mo(NAr)(C 3 H 6 )(OHIPT)(Pyr) loses ethylene to give isolable Mo(NAr)(CH 2 )(OHIPT)(Pyr). W(NAr)(CH 2 )(OTPP)(Me 2 Pyr) (OTPP = O-2,3,5,6-Ph 4 C 6 H, Me 2 Pyr = 2,5-Me 2 NC 4 H 2 ) was prepared similarly. Single-crystal X-ray studies of Mo(NAr)(CH 2 )(OHIPT)(Pyr) and W(NAr)(CH 2 )(OTPP)(Me 2 Pyr) show that they are monomers that contain an η 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 CH anti and the metal. Attempts to prepare analogous Mo(NAd)(CH 2 )(OHIPT)(Pyr) (Ad = 1-adamantyl) yielded only the ethylene complex Mo(NAd)(C 2 H 4 )(OHIPT)(Pyr). W(NAr tBu )(CH 2 )(OTPP)(Me 2 Pyr) (Ar tBu = 2-t-BuC 6 H 4 ) was isolated upon loss of ethylene from W(NAr tBu )(C 3 H 6 )(OTPP)(Me 2 Pyr), but decomposed in solution over a period of several hours at 22 °C. NMR studies of Mo(NAr)(C 3 H 6 )(OHIPT)(Pyr) and W(NAr)(C 3 H 6 )(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)(C 3 H 6 )(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)(C 3 H 6 )(OBitet)(Me 2 Pyr). It could be shown that Mo(NAr)(C 3 H 6 )(OBitet)(Me 2 Pyr) forms an ethylene/methylidene intermediate at 20 °C at a rate that is 4500 times faster than the rate at which W(NAr)(C 3 H 6 )(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.