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

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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(OC6H2But2-2,6-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–Cα〉 distances (2.144 in 1, 2.147A in 3) than the corresponding α,α′-bis(trimethylsilyl) derivatives (2.166 in 2, 2.174A in 4). The C6CαCα′o-xylenediyl plane is folded relative to the MCαCα′ 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–Cα 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–Cα 2.15(3) and Sn–Cα′ 2.09(4)A.