Formation of N,N,N′,N′-tetramethylformamidinium disulphide from the chemical and electrochemical oxidation of tetramethylthiourea

Abstract

The N,N,N′,N′-tetramethylformamidinium disulphide (TMFDS) was prepared from either the electro-oxidation of tetramethylthiourea (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=S-S=C [N(CH₃)₂]₂Cl₂·2H₂O, 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) Å, β = 126.91(1)°, and Z = 8. The structure was solved from 2489 reflections with I > 2σ(I) and refined to an agreement R1-factor of 0.0405. The two molecular halves of the dimeric <math><mrow is="true"><mo stretchy="false" is="true">[</mo><mtext is="true">N</mtext><mo stretchy="false" is="true">(</mo><msub is="true"><mrow is="true"><mtext is="true">CH</mtext></mrow><mrow is="true"><mn is="true">3</mn></mrow></msub><msub is="true"><mrow is="true"><mo stretchy="false" is="true">)</mo></mrow><mrow is="true"><mn is="true">2</mn></mrow></msub><msub is="true"><mrow is="true"><mo stretchy="false" is="true">]</mo></mrow><mrow is="true"><mn is="true">2</mn></mrow></msub><mtext is="true"><mtext>C</mtext><mglyph src="https://sdfestaticassets-us-east-1.sciencedirectassets.com/shared-assets/16/entities/dbnd"></mglyph><mtext>S</mtext><mglyph src="https://sdfestaticassets-us-east-1.sciencedirectassets.com/shared-assets/16/entities/sbnd"></mglyph><mtext>S</mtext><mglyph src="https://sdfestaticassets-us-east-1.sciencedirectassets.com/shared-assets/16/entities/dbnd"></mglyph><mtext>C</mtext></mtext><mo stretchy="false" is="true">[</mo><mtext is="true">N</mtext><mo stretchy="false" is="true">(</mo><msub is="true"><mrow is="true"><mtext is="true">CH</mtext></mrow><mrow is="true"><mn is="true">3</mn></mrow></msub><msub is="true"><mrow is="true"><mo stretchy="false" is="true">)</mo></mrow><mrow is="true"><mn is="true">2</mn></mrow></msub><msubsup is="true"><mrow is="true"><mo stretchy="false" is="true">]</mo></mrow><mrow is="true"><mn is="true">2</mn></mrow><mrow is="true"><mn is="true">2</mn><mo is="true">+</mo></mrow></msubsup></mrow></math> ion are linked by a disulphide single bond [d(S-S) = 2.0454(9) Å] and related to each other by a non-crystallographic pseudo two-fold axis. DFT structure optimisation and normal mode frequencies were calculated using the 6-311G(d,f) basis set at DFT theory level. An estimation of the free energy for the dimer formation and rate ratio for the homogeneous and heterogeneous processes are presented. These data are consistent with the electrochemical mechanism of the anodic formation of TMFDS²⁺ from TMTU electro-oxidation in acid solutions.