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)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.