Kinetic and mechanistic study of the reaction between dithiomalonamides and diiodine. Crystal structure of the compounds 3,5-bis(ethylamino)-1,2-dithiolylium triiodide and 3,5-bis(morpholino)-1,2-dithiolylium iodide monohydrate

Abstract

The reaction of N,N′-diethyldithiomalonamide with I₂ has been investigated spectrophotometrically. A kinetic study carried out with the stopped-flow technique has shown the transient formation of a 1 : 1 N,N′-diethyldithiomalonamide·I₂ charge-transfer complex having a formation constant K₁= 2.3(3)×10³ dm³ mol^–1 at 25 °C. This is followed by an oxidation–reduction reaction leading to 3,5-bis(ethylamino)-1,2-dithiolylium iodide and Hl_n(n= 1 or 3), through a first- and a second-order process with respective rate constants at 25 °C k₁= 0.84(7) s^–1 and k₂= 2.0(3)× 10³ dm³ mol^–1 s^–1, consisting of an unassisted or I₂ assisted heterolytic fission of the I–I bond as the slow step. Further associative equilibria of 3,5-bis(ethylamino)-1,2-dithiolylium iodide and HI with free I₂ leading to the corresponding triiodides have also been observed, having formation constants, respectively, K₂= 6.2(7)× 10⁵ dm³ mol^–1 and K₃= 8.9(5)× 10⁶ dm³ mol^–1. All the kinetic and thermodynamic parameters were obtained by an expressly devised computational method based on nonlinear least-squares procedures. An X-ray crystal structure analysis of the title compounds, obtained as products of the reaction between I₂ and RNR′C(S)CH₂C(S)R′NR, showed that the crystals obtained in the RNR′= EtNH and I₂ excess case are monoclinic, space group C2/m, with a= 13.643(6), b= 9.749(5), c= 11.879(6)Å, β= 99.76(2)° and Z= 4. The structure consists of chains running parallel to [010], where the 3,5-bis(ethylamino)-1,2-dithiolylium cations and triiodide anions are connected through two strong interactions involving the terminal iodide atom of the unsymmetrical I₃^–(I₂⋯I^– anion) and both the sulphur atoms of the disulphide group. In the RNR′= OC₄H₈N and 1:1 reagents ratio case the crystals are monoclinic, space group P2₁/c, with a= 9.256(4), b= 20.257(6), c= 8.483(4)Å, β= 90.91 (2)° and Z= 4. The structure consists of layers, approximately parallel to (201), determined by contacts involving [OC₄H₈NC(S)CHC(S)NC₄H₈O] cations, iodide anions and water molecules.