PREPARATION AND REACTIVITY OF TUNGSTEN(VI) METALLACYCLOBUTANE COMPLEXES - SQUARE PYRAMIDS VERSUS TRIGONAL BIPYRAMIDS

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 of the complex 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 for 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(CF3)]2 has been characterized crystallographically (space group P42/n, a = 25.26 (2) Å, c = 9.720 (5) Å, V = 6202 Å3, 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 Å and W-Cα bond lengths of 2.15-2.20 Å, as opposed to a short W—Cβ distance of ~2.4 Å and W-Cα bond lengths of 2.05–2.10 Å 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 ΔH4 = 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(CF3)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. © 1990, American Chemical Society. All rights reserved.