Chemistry of o-xylenediyl–metal complexes. Part 5. Silicon and tin metallacycles derived from o-C6H4(HR)2(R = H or SiMe3)

Abstract

Treatment of SnCl₂Ph₂ with [Mg(LL)(thf)]6 or [Li(tmen)]₂[o-C₆H₄{CH(SiMe₃)}₂]7 in OEt₂ yields the stannacycle [[graphic omitted])Ph₂]1 or [[graphic omitted]′-meso)Ph₂]2{(LL)^2–=o-C₆H₄(H₂)₂, (L′L′)^2–=o-C₆H₄[H(SiMe₃)]₂, thf = tetrahydrofuran, tmen = Me₂N[CH₂]₂NMe₂}. Likewise, SnCl₄ with 6 or 7 at 30 °C in OEt₂ affords exclusively the stannaspirobicycle [[graphic omitted])₂]3 or [[graphic omitted]′-meso)₂]4; by contrast SnCl₄ with 7 at –78 °C gives, together with 4(52%), the yellow, diamagnetic tin(II) metallacycle [{[graphic omitted]′-meso)}₄](Sn–Sn)5(14%). Compound 5 is more conveniently prepared by treating Sn(OC₆H₂Bu^t₂-2,6-Me-4)₂ with 7; it is tetrameric in the solid and in solution, but monomeric in the gas phase and has a low first ionisation energy (7.6 eV). The compound [[graphic omitted])Me₂]8 is obtained by an in situ Grignard reaction from SiCl₂Me₂, o-C₆H₄(CH₂Cl)₂ and Mg in thf, while [[graphic omitted]′-meso)Me₂]9 is prepared from SiCl₂Me₂ and 7. Each of the metallacycles derived from the ligand (L′L′)^2– was obtained stereospecifically as the meso diastereoisomer. X-Ray structure determinations of complexes 1–5 have been performed. The α,α′-unsubstituted metallacycles have shorter 〈Sn–C_α〉 distances (2.14₄ in 1, 2.14₇Å in 3) than the corresponding α,α′-bis(trimethylsilyl) derivatives (2.16₆ in 2, 2.17₄Å in 4). The C₆C_αC_α′o-xylenediyl plane is folded relative to the MC_αC_α′ plane, the ‘fold angle’θ being 8.5 (1), 19.8 (2), 0.8 and 11.9 (3), 24.1 and 23.0 (4) and 20.8 °(5). The larger θ values for the [graphic omitted]L′) complexes 2 and 4 than for the their [graphic omitted]) counterparts 1 and 3(as for comparable Sn–C_α distances) is attributable to greater interligand strain and crowding around the metal centre in 2 and 4. Molecules of 5 have 4 symmetry, with Sn–Sn 2.852(3), Sn–C_α 2.15(3) and Sn–C_α′ 2.09(4)Å.