Polymorphism in Lithium Amides: A Structural and Theoretical Study. Synthesis, Mechanism, and NMR Studies of the Lithiation of N,N‘-Di-tert-butylethylenediamine

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Abstract
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)CH2CH2N(H)t-Bu]}2Li{N(t-Bu)CH2CH2Nt-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 LinNn 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)°, Z = 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.

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