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

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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 Hln(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.

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