Axially asymmetric metal alkyls. Part 2. Synthesis of the di-Grignard reagent [{Mg(thf)nCl}2R][thf = tetrahydrofuran, R =(2-CH2C6H4)22–] and 6,7-dihydro-5H-dibenzo[c,e]-silepines and -stannepines; X-ray crystal structures of [SnPh2R] and δ-SS- and λ-RR-[SnPh2{[2-CH(SiMe3)C6H4]2}]

Abstract

Reactions of Mg and Li with 2,2′-bis(chloromethyl)biphenyl in tetrahydrofuran (thf) yield respectively a new di-Grignard reagent, [{Mg(thf)_nCl}2{(2-CH₂C₆H₄)₂}](5), for rather critical conditions, and the intramolecular cyclization product, 9,10-dihydrophenanthrene which further reacts to form a radical anion (g= 2.0037, hydrogen hyperfine coupling unresolved). Substituted 6,7-dihydro-5H-dibenzo[c,e]-silepines and -stannepines derived from (5) and the dilithium complexes [{Li(tmen)}₂{(2-CHRC₆H₄)₂}](R = H or SiMe₃; tmen =NNN′N′-tetramethylethylenediamine) are also reported. The crystal structures of the compounds [SnPh₂{(2-CHRC₆H₄)₂}][R = H (8) or SiMe₃(9)] have been determined. Sn–C (aromatic) distances are similar [2.139(8) and 2.149(9)Å in (8), cf. 2.136(4)Å in (9)] whereas the Sn–CHR distances are significantly different [2.157(8) and 2.159(9)Å in (8), cf. 2.184(4)Å in (9)]; the angles subtended by the bidentate ligands are 98.7(3)° in (8) and 99.4(2)° in (9) and the associated torsion angles along the biphenyl axis are respectively 66.0 and 70.8°. Compound (9) comprises a racemate of δ-SS and λ-RR isomers, in the solid, with the molecules lying on C₂ axes.