SYNTHESIS OF MOLYBDENUM IMIDO ALKYLIDENE COMPLEXES AND SOME REACTIONS INVOLVING ACYCLIC OLEFINS

The reaction between Mo(C-t-Bu)(dme)Cl3 (dme = 1,2-dimethoxyethane) and Me3SiNHAr (Ar = 2,6-diisopropyl phenyl) yields Mo(C-t-Bu)(NHAr)Cl2(dme) (1), which upon treatment with a catalytic amount of NEt3 is transformed into Mo(CH-t-Bu)(NAr)Cl2(dme) (2). Complexes of the type Mo(CH-t-Bu)(NAr)(OR)2 (OR = OCMe(CF3)2, OCMe2(CF3), O-t-Bu, or OAr) have been prepared from 2. Complexes of the type Mo(C-t-Bu)(NHAr)(OR)2 (OR = OCMe(CF3)2 or OAr) have been prepared from 1, but they cannot be transformed into Mo(CH-t-Bu)(NAr)(OR)2 complexes. A precursor to imido alkylidene complexes that is related to 2 has been prepared by the sequence MoO2 → MoO2Cl2 → Mo(NAr)2Cl2 → Mo-(NAr)2(CH2R’)2 → Mo(CHR’)(NAr)(OTf)2(dme) (R’ = t-Bu or CMe2Ph; OTf = OSO2CF3). Mo(CH-t-Bu)(NAr)(OTf)2(dme) crystallizes in the space group PĪ with a = 17.543 Å, b = 19.008 Å, c = 9.711 Å, α = 91.91°, β = 99.30°, γ = 87.27°, Z = 4, Mr = 729.60, V = 3191.1 Å3, p(calcd) = 1.518 g cm–3. It is a pseudooctahedral species in which the imido and alkylidene ligands are cis to one another, the triflate ligands are mutually trans, and the Zerr-butyl group points toward the imido ligand (syn orientation). Neophylidene complexes, Mo(CHCMe2Ph)(NAr)(OR)2 (OR = O-t-Bu, OAr, or O-2-C6H4-t-Bu), have been prepared from Mo(CHCMe2Ph)(NAr)(triflate)2(dme). Activity for the metathesis of cis-2-pentene by Mo(CHR’)-(NAr)(OR)2 complexes roughly correlates with the electron-withdrawing ability of OR, being rapid when OR = OCMe(CF3)2 and slow to negligible when OR = O-t-Bu. In several cases it is clear from proton NMR studies that the alkylidene ligand can rotate on the NMR time scale; in Mo(CHSiMe3)(NAr)(OAr)2 it has been shown that ΔG*298 = 16.0 kcal mol–1 for this process. Mo[CH(SiMe3)CH(SiMe3)CH2](NAr)[OCMe2(CF3)]2 has been observed and found to be ~3 orders of magnitude less stable than the analogous tungsten complex. Trigonal-bipyramidal Mo(CH2CH2CH2)(NAr)[OCMe(CF3)2]2 can be prepared at 25 °C in high yield, but it decomposes over a period of 12 h. Instability of OCMe(CF3)2 metallacyclobutane complexes has been traced to reduction by β -hydride rearrangement to give an olefin. In one case a complex containing the olefin product, Mo(NAr)(Me3SiCH=CH2)[OCMe(CF3)2]2, was isolated. © 1990, American Chemical Society. All rights reserved.