Preparation and reactivity of several alkylidene complexes of the type W(CHR')(N-2,6-C6H3-iso-Pr2)(OR)2 and related tungstacyclobutane complexes. Controlling metathesis activity through the choice of alkoxide ligand

Abstract

1423actual dissociation (AD2 -AD + D) in which the leaving molecule carries away the excess energy. SummaryThe emission properties of jet-cooled adducts between ethers and some aromatic hydrocarbons may be accounted for by assuming an excited state that is a superposition of a van der Waals locally excited (LE) state and a charge-transfer (CT) state. The direct optical transition from the ground state to the C T state is forbidden by either symmetry or Franck-Condon considerations. Thus, the excitation spectrum is dominated by the LE state, and emission from the CT state has a longer decay time. The C T character of the excited state is more prominent in anthracene and its derivatives than in perylene. This is particularly so when a hydrogen atom is bound to a C-9 or C-IO carbon. It is therefore proposed that this atom may serve as a bridge in transmitting the charge from the oxygen lone pair orbital of the ether. Geometric factors appear to be decisive in determining the interaction as shown by the fact that tetrahydrofuran, in which the oxygen atom is constrained to be away from the 9-or 10-position, does not show exciplex-type emission with anthracene. Abstract:The reaction between W(C-t-Bu)(dme)CI, and ArNH(TMS) (Ar = 2,6-C,H3-i-Pr2) yields W(C-t-Bu)(NHAr)(dme)CI,.

Direct observation of reaction intermediates for a well defined heterogeneous alkene metathesis catalyst

“…Observation of stable metallacyclobutanes in two types of geometries is fully consistent with previous experimental (in solution) (18)(19)(20)(21)(22)(23)(24)(25) and theoretical studies (39-43) for d 0 metal complexes. It is also noteworthy that in solution, the SBP/TBP isomer ratio and the reactivity in metathesis are correlated, for a series of isoelectronic metallacyclobutane intermediates of the general formula [(RO) 2 W(ϵNAr)(CH 2 CH 2 CH 2 )], to the electronic properties of the RO ligand as follows: (i) for RO ϭ (CF 3 ) 2 (CH 3 )CO (the most electron-withdrawing ligand), only the TBP isomer is observed, and the complex is very reactive; (ii) for RO ϭ (CF 3 )(CH 3 ) 2 CO, a mixture of TBP (a few %) and SBP isomers is observed, and they display intermediate reactivity; and (iii) for RO ϭ (CH 3 ) 3 CO, only the SBP isomer is observed, with very low reactivity because of the high stability of the metallacyclobutane (21,22).…”

“…In the specific case of alkene metathesis, it has been proposed that carbenes and metallacyclobutanes are key reaction intermediates (16). Although they have been unambiguously prepared and observed for homogeneous catalysts (17)(18)(19)(20)(21)(22)(23)(24)(25)(26)(27)(28)(29), observing them as heterogeneous catalysts has proved to be a formidable challenge (30)(31)(32). Note that because the surface can play such a crucial role, one should never assume that the intermediates are identical in heterogeneous and homogeneous systems (31,32).…”

(3-Chloro-1,2-propanediolato-O,O')bis(2,3-dimethyl-2,3-butanediolato-O,O')tungsten(VI)

“…This lengthening of the W---O bonds may be due to the electronegative chloro substituent. A similar phenomenon is clearly seen in tungsten complexes with fluoro-substituted alcoholato ligands (Schrock, DePue, Feldman, Schaverien, Dewan & Liu, 1988).…”

Activating Thiolate‐Based Imidoalkylidene Tungsten(VI) Metathesis Catalysts by Grafting onto Silica

“…The analysis of the structure reveals at ungsten atom coordinated by an alkylidene, two thiolates,a nd an imido moiety in ad istorded tetrahedral geometry.T he imido nitrogen, the tungsten, and the C43a nd C44 carbonso ft he alkylidene are coplanar (dihedral angle = 0.858) in the usual syn geometry.W hile the WÀSa nd WÀNb ond lengths in the complexa re usual, the WÀC ene bond distance (1.832(9) , C ene being the C43 carbon of the alkylidene moiety) is the shortest tungsten alkylidene bond distance described to our knowledge (ranging from 1.859(22) [14] to 2.163(3) [15] according to the Cambridge Crystallographic Data Centre). This discrepancy suggestst hat the WÀC ene bond has partial triple bond character or as trongera gostic interaction with the alkylidenep roton, as would suggest the relatively low J CÀH coupling constant [16] (J CÀH = 107 Hz), similarly to what has been observed previously for the surface species [( SiO)Re(CtBu)(CHtBu)(CH 2 tBu)].…”