Axially asymmetric metal alkyls. Part 1. Lithium alkyls of 2,2′-dimethylbiphenyl and its trimethylsilylmethylated compounds: Crystal structures of [{Li(Me₂NCH₂CH₂NMe₂)} ₂{(2-CHRC₆H₄)₂}] for R = H (polymeric) and R = SiMe₃ (monomeric)

Metallation of (2-CH₂RC₆H₄)₂, with excess Li(tmen)Buⁿ (tmen = NNN′N′-tetramethylethylenediamine) yields the crystalline compounds [{Li(tmen)}₂{(2-CHRC₆H₄)₂}][R = H (7) or SiMe₃ (9)] whereas for 2-methyl-2′-trimethylsilylmethylbiphenyl, although selectivity is maintained, the crystalline product is a monolithiated species. This difference has been rationalised on the basis of the results of the crystal structure determination of the lithium complexes (7) and (9). The structure of (7) is polymeric (cf. dimeric in benzene by ebullioscopy) with a 'metallacycle' lithium and a lithium bridging methylene groups of different 'metallacycles' (electron-deficient bonding): Li-C (mean) 2.30 Å; Li-C-Li (mean) 159.5, C-Li-C (mean) 106.5°. Compound (9) is best described as a 'contact ion-pair' structure; Li-C(H)SiMe₃ contacts, 2.22(1) and 2.19(2) Å. Lithiation of 2-[bis(trimethylsilyl)methyl]-2′-methylbiphenyl results in the formation of (9), via an anionic 1,6-silyl migration.