Polymorphism in lithium amides: A structural and theoretical study. Synthesis, mechanism, and NMR studies of the lithiation of N,N'-Di-tert-butylethylenediamine

The lithiation of N,N'-di-tert-butylethylenediamine by MeLi in benzene has been shown by ¹H NMR spectroscopy to proceed via the partially lithiated species [cis-{Li[μ-N(t-Bu)CH₂CH₂N(H)t-Bu]}₂], 2, and [{Li[N(t-Bu)CH₂-CH₂N(H)t-Bu]}₂Li{N(t-Bu)CH ₂CH₂Nt-Bu}Li], 3, prior to the formation of the dilithiated species (Li[N(t-Bu)CH₂CH₂Nt-Bu]Li}, 4. The solid state structures of 2, 3, and a dimeric form of 4 (4a) have been determined. A sparingly soluble form of 4 (4b) has also been isolated which has a proposed polymeric ladder structure. These structures are discussed with respect to the alternatives available for the aggregation of the dilithiated species; stacking to form dimeric Li₄N₄ cages and laddering to form Li_nN_n ladders. Ab initio molecular orbital calculations give insight into the energetics of these aggregates and the possible structures adopted by solvated and unsolvated dilithium ethylenediamide complexes. Crystals of 2 are monoclinic, of space group C2/c (No. 15), a = 19.222(7), b = 8.734(2), c = 17.149(5) Å, β= 119.40(1)°, Z = 4. Crystals of 3 are monoclinic, of space group P2₁/c (No. 14), a = 9.836(8), b = 17.821(3), c = 21.78(2) Å, β = 101.57(4)°, 2 = 4. Crystals of 4a are monoclinic, of space group P2₁/c (No. 14), a = 15.990(7), b = 10.0162(9), c = 16.42(1) Å, β = 104.49(2)°, Z = 4. Crystals of 6 are monoclinic, of space group P2₁/c (No. 14), a = 10.124(8), b = 17.861(3), c = 22.21(2) Å, β = 102.05(4)°, Z = 4.