The Formation of N,N,N',N',Tetramethylformamidinium Disulphide from the Chemical and Electrochemical Oxidation of Tetramethylthiourea. The Vibrational Spectra and Crystal Structure of the Chloride Dihydrate Salt.

A. E. BOLZÁN, J. A. GÜIDA, R. C. V. PIATTI, A. J. ARVIA, O. E. PIRO, J. R. SABINO Y E.E. CASTELLANO

JOURNAL OF MOLECULAR STRUCTURE

Año: 2007 vol. 871 p. 131 - 139

0022-2860

The N,N,N0,N0-tetramethylformamidinium disulphide (TMFDS) was prepared from either the electro-oxidation of tetramethylthiou- rea (TMTU) on platinum electrodes in aqueous perchloric acid solution or the chemical oxidation of TMTU with hydrogen peroxide in  hydrochloric acid-containing absolute ethanol. The electrochemical formation of TMFDS at different potentials was followed by in situ  FTIRRAS spectroscopy. The IR and Raman spectra of TMFDS chemically formed as a chloride dihydrate were determined and compared to the product produced electrochemically. The crystal structure of the chloride salt [N(CH3)2]2C@SAS@C [N(CH3)2]2Cl2Æ2H2O,  as determined by X-ray diffraction, crystallises in the monoclinic C2/c space group with a = 23.267(1), b = 10.824(1), c = 17.774(1)A ˚ , b = 126.91(1), and Z = 8. The structure was solved from 2489 reflections with I > 2r(I) and refined to an agreement R1-factor of0.0405. The two molecular halves of the dimeric ½NðCH3Þ22C@SAS@C½NðCH3Þ22þ 2 ion are linked by a disulphide single bond[d(SAS)A2.0454(9)A ˚ ] and related to each other by a non-crystallographic pseudo two-fold axis. DFT structure optimisation and normalmode frequencies were calculated using the 6-311G(d,f) basis set at DFT theory level. An estimation of the free energy for the dimerformation and rate ratio for the homogeneous and heterogeneous processes are presented. These data are consistent with the electro-chemical mechanism of the anodic formation of TMFDS2+ from TMTU electro-oxidation in acid solutions.