1,4-Bis(trimethylsilyl)but-2-ene-1,4-diyl Complexes of Lithium, Magnesium, Aluminum, and Gallium

(Z)-1,4-Dilithio-1,4-bis(trimethylsilyl)but-2-ene has been prepared as a THF (≡tetrahydrofuran) adduct, [{(CH(Me₃Si)CH)₂Li₂(THF)₂}₂] (4), via demetalation of the (E)-but-2-ene precursor using n-BuLi in THF and by reduction of (E,E)-1,4-bis(trimethylsilyl)buta-1,3-diene by lithium in THF. The complex is dimeric in the solid state with two lithium atoms bridging the (Z)-but-2-ene-1,4-diyl units; the other two lithium atoms are external to the dianion moieties and are solvated by two THF molecules, forming a Li(dianion)Li₂(dianion)-Li inverted double-decker sandwich structure. Treatment of the (Z)-1,4-dilithio-1,4-bis-(trimethylsilyl)but-2-ene adduct of TMEDA (≡N,N,N',N'-tetramethylethylenediamine), [(CH-(Me₃Si)CH)₂{Li(TMEDA)}₂] (3), with (a) MgCl₂ or i-PrMgCl in diethyl ether results in formation of the TMEDA adduct of the magnesacyclopent-3-ene, isolated as [{meso-(CH-(Me₃Si)CH)₂Mg}(TMEDA)] (5) or (b) MeAlCl₂, Et₂AlCl, or GaCl₃ in diethyl ether results in stereospecific formation of the corresponding metallacyclopent-3-enes, isolated as [{meso-HC(Me₃Si)HC)₂MR}₂(TMEDA)] (M = Al, R = Me (6a), Et (6b); M = Ga, R = Cl (7)). Assignment of the meso isomers for 5–7 is based on NMR data and X-ray crystal structure determination for 5, 6b, and 7. Crystals of 4 are monoclinic, space group P2₁/n (No. 14), with a = 9.874(5) Å, b = 19.691(5) Å, c = 12.563(7) Å, β = 94.46(3)°, V = 2435(2) ų, and Z = 2. Crystals of 5 are monoclinic, space group P2₁/c (No. 14), with a = 13.535(5) Å, b = 12.464(5) Å, c = 16.128(7) Å, β = 101.69(3)°, V = 2660(10) ų, and Z = 4. Crystals of 6b are orthorhombic, space group Pbca (No. 61), with a = 17.689(2) Å, b = 16.743(2) Å, c = 14.091(5) Å, V = 4173(1) ų, and Z = 4. Crystals of 7 are monoclinic, space group P2₁/n (No. 14), with a = 14.118(5) Å, b = 17.309(2) Å, c = 16.000(6) Å, β = 91.96(2)°, V = 3908(2) ų, and Z = 4.