112481-70-6Relevant academic research and scientific papers
Preparation and reactivity of tungsten(VI) metallacyclobutane complexes. Square pyramids versus trigonal bipyramids
Feldman, Jerald,Davis, William M.,Thomas, Julie K.,Schrock, Richard R.
, p. 2535 - 2548 (2008/10/08)
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 JCH values, as opposed to large differences in chemical shifts between α- and β-carbons and α- and β-hydrogens and olefinic JCH 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, Mr = 711.48, ρ(calcd) = 1.524 g cm-3, Z = 8, μ = 40.05 cm-1, R = 0.056, Rw = 0.082). The most characteristic feature of the WC3 ring in the SP complex is the relatively normal W-Cβ distance of ～2.8 A? and W-Cα bond lengths of 2.15-2.20 A?, as opposed to a short W-Cβ distance of ～2.4 A? and W-Cα 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.
