PREPARATION, STRUCTURE AND VIBRATIONAL-SPECTRUM OF THE DIMETHYLMETHYLENIMINIUM ION, INCLUDING THE ROLE OF CATIONIC POLYMERS IN ITS FORMATION

Crystalline [(CH3)(2)N=CH2]X (1X, where X=Br or I) results from the reaction of N,N,N',N'-tetramethylmethylenediamine 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) Angstrom, b = 7.428(3) Angstrom, c = 5.651(2) (A)ngstrom, 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 C=N bond distance 1.263(5) Angstrom, C-N bond distance 1.414(4) Angstrom and the CH3-N-CH3 angle 114.4(4)degrees. The Raman spectrum of 1, investigated as the bromide and iodide salts, and supplemented by IR data for 1(2)SnBr(6) has been fully assigned with the aid of ab initio calculations using the 3-21G** basis at the SGF and MP2 levels. Isotopic labelling experiments which generate [(CH3)(2)N=CD2](+) from CD2I2 have been used in conjunction with NMR and Raman measurements to investigate the reactions leading to the iminium salt. The intermediate [Me(2)NCH(2)NMe(2)CH(2)NMe(2)](+) and a cationic polymer [Me(2)N(CH(2)NMe(2))(n)CH(2)NMe(2)](n+) are implicated in the reactions which form the basis of the synthesis of[(CH3)(2)N=CH2]I.