Preparation and reactivity of tungsten(VI) metallacyclobutane complexes. Square pyramids versus trigonal bipyramids

Article abstract of DOI:10.1021/om00159a024

Neopentylidene complexes of the type W(CH-t-Bu)(NAr)(OR)₂ (Ar = 2,6-C₆H₃-i-Pr₂; OR = O-t-Bu, OCMe₂(CF₃), OAr) react with ethylene to give trigonal-bipyramidal or square-pyramidal tungstacyclobutane complexes. A square-pyramidal form is observed when OR = O-t-Bu. Both forms are present when OR = OCMe₂(CF₃) or OAr, and they interconvert at a rate that is on the order of the NMR time scale. A complex of the type W[CH₂CH(R)CH₂](NAr)(OAr)₂ is a square pyramid when R = t-Bu but a trigonal bipyramid when R = SiMe₃. Square-pyramidal tungstacycles are characterized by comparable chemical shifts tor α- and β-protons, chemical shifts for α- and β-carbon atoms that differ by only ～25 ppm, and aliphatic J_CH values, as opposed to large differences in chemical shifts between α- and β-carbons and α- and β-hydrogens and olefinic J_CH values in TBP tungstacycles. Unsubstituted metallacycles react with excess neohexene to give square-pyramidal W[CH₂CH(t-Bu)CH₂](NAr)(OR)₂ complexes (OR = O-t-Bu, OCMe₂(CF₃), OAr). W[CH₂CH(t-Bu)CH₂](NAr)[OCMe₂(CF ₃)I₂ has been characterized crystauographically (space group P4₂/n, a = 25.26 (2) A?, c = 9.720 (5) A?, V = 6202 A?³, M_r = 711.48, ρ(calcd) = 1.524 g cm^-3, Z = 8, μ = 40.05 cm^-1, R = 0.056, R_w = 0.082). The most characteristic feature of the WC₃ ring in the SP complex is the relatively normal W-C_β distance of ～2.8 A? and W-C_α bond lengths of 2.15-2.20 A?, as opposed to a short W-C_β distance of ～2.4 A? and W-C_α bond lengths of 2.95-2.10 A? in TBP complexes. It is proposed that square-pyramidal complexes form in order to avoid competition between a relatively basic axial alkoxide and an axial imido ligand in a trigonal bipyramid. The reaction between W[CH₂CH(t-Bu)CH₂] (NAr)(OAr)₂ and ethylene to give W[CH₂CH₂CH₂](NAr)(OAr)₂ and tert-butylethylene is zero order in ethylene and first order in tungsten between 9 and 34°C with ΔH? = 19.7 kcal mol^-1 and ΔS? = -6 eu. Reactions between ethylene and trigonal-bipyramidal W[CH₂CH(Me₃Si)CH₂](NAr)(OAr)₂, W[CH₂CH(Me₃Si)CH₂](NAr)-[OCMe(CF ₃)₂]₂, and W[CH₂CH(Me₃Si)CH₂](NAr)[OC(CF₃) ₂(CF₂CF₂CF₃)]₂ are analogous but have positive values for ΔS? (11-23 eu). It is proposed that square-pyramidal tert-butoxide complexes are relatively stable toward loss of an olefin because the WC₃ ring is further from an "olefin/alkylidene" transition state than is the WC₃ ring in a trigonal bipyramid and in general for that reason complexes that contain relatively electron-withdrawing alkoxides will lose an olefin more readily than those that contain relatively electron-donating alkoxides.