Preparation, structure and vibrational spectrum of the dimethylmethyleniminium ion, including the role of cationic polymers in its formation

Abstract

Crystalline [(CH₃)₂NCH₂]X (1X, where X = Br or I) results from the reaction of N,N,N′,N′-tetra-methylmethylenediamine with CH₂Br₂ or CH₂I₂ at room temperature. The structure of 1Br has been determined by X-ray crystallography. The crystals are orthorhombic, a= 6.262(2)Å, b= 7.428(3)Å, c= 5.651(2)Å, z= 2, space group Pmmn. The final R= 0.037 for 484 observed reflections. The dimethylmethyleniminium cation, 1, has crystallographically imposed mm2 symmetry, with the CN bond distance 1.263(5)Å, C—N bond distance 1.474(4)Å, and the CH₃—N—CH₃ angle 114.4(4)°. The Raman spectrum of 1, investigated as the bromide and iodide salts, and supplemented by IR data for 1₂SnBr₆, has been fully assigned with the aid of ab initio calculations using the 3-21G** basis at the SCF and MP2 levels. Isotopic labelling experiments which generate [(CH₃)₂NCD₂]⁺ from CD₂l₂ have been used in conjunction with NMR and Raman measurements to investigate the reactions leading to the iminium salt. The intermediate [Me₂NCH₂NMe₂CH₂NMe₂]⁺ and a cationic polymer [Me₂N(CH₂NMe₂)_nCH₂NMe₂]ⁿ⁺ are implicated in the reactions which form the basis of the synthesis of [(CH₃)₂NCH₂]l.