Chemistry of o-xylenediyl-metal complexes: Part 5. Silicon and tin metallacycles derived from o-C6H4([C with combining overline]HR)2(R = H or SiMe3)

Treatment of SnCl₂Ph₂ with [Mg(LL)(thf)] 6 or [Li(tmen)]₂[o-C₆H₄{CH(SiMe₃)} ₂] 7 in OEt₂ yields the stannacycle [Sn(LL)Ph₂] 1 or [Sn(L′L′-meso)Ph₂] 2 {(LL)^2- = o-C₆H₄(C̄H₂)₂, (L′L′)^2- = o-C₆H₄[C̄H(SiMe₃)]₂, thf = tetrahydrofuran, tmen = Me₂N[CH₂]₂NMe₂}. Likewise, SnCl₄ with 6 or 7 at 30 °C in OEt₂ affords exclusively the stannaspirobicycle [Sn(LL)₂] 3 or [Sn(L′L′-meso)₂] 4; by contrast SnCl₄ with 7 at -78 °C gives, together with 4 (52%), the yellow, diamagnetic tin(II) metallacycle [{Sn(L′L′-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 [Si(LL)Me₂] 8 is obtained by an in situ Grignard reaction from SiCl₂Me₂, o-C₆H₄(CH₂Cl)₂ and Mg in thf, while [Si(L′L′-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, 2.14₇ Å in 3) than the corresponding α,α′-bis(trimethylsilyl) derivatives (2.16₆ in 2, 2.17₄ Å 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 Sn^IVL′L′) complexes 2 and 4 than for the their Sn^IV(LL) 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.