Carbon−Sulfur Bond Cleavage in Bis(<i>N</i>-alkyldithiocarbamato)cadmium(II) Complexes:  Heterolytic Desulfurization Coupled to Topochemical Proton Transfer

Three bis(<i>N</i>-alkyldithiocarbamato)cadmium(II) complexes [Cd(S<sub>2</sub>CNHR)<sub>2</sub>] (<b>1</b>, R = <i>n</i>-C<sub>3</sub>H<sub>7</sub>; <b>2</b>, R = <i>n</i>-C<sub>5</sub>H<sub>11</sub>; <b>3</b>, <i>n</i>-C<sub>12</sub>H<sub>25</sub>) were prepared by metathesis of the corresponding lithium salt, Li[S<sub>2</sub>CNHR], with cadmium chloride. The crystal structures of <b>2</b> and <b>3</b> consist of planar molecular units of [Cd(S<sub>2</sub>CNHR)<sub>2</sub>] connected by intermolecular Cd···S interactions to give a one-dimensional chain. The chains are connected by a network of intermolecular N−H···S hydrogen bonds between the dithiocarbamato nitrogen atom and bridging sulfur atoms in neighboring chains. In solution, the <sup>113</sup>Cd NMR spectrum of <b>2</b> is dependent on concentration and temperature, indicative of a dimerization equilibrium mediated by similar Cd···S intermolecular bridging interactions. In the solid state, thermal gravimetric analyses show that all three complexes decompose smoothly via a heterolytic C−S bond cleavage reaction to give the corresponding alkyl isothiocyanate and cadmium sulfide as the primary products, with the formation of primary amine and CS<sub>2</sub> as coproducts. These products can result only from the net transfer of protons between <i>N</i>-alkyldithiocarbamato ligands in the solid state. Thus, the C−S bond cleavage reaction is interpreted in terms of the topochemical arrangement of molecular units in the crystalline state, which provides a pathway for proton transfer between ligands via N−H···S hydrogen bonds. Decomposition was also initiated by addition of a tertiary amine to a solution of [Cd(S<sub>2</sub>CNHR)<sub>2</sub>]. This confirms that C−S bond cleavage must be coupled to deprotonation of the −NH group, and explains why dialkylated derivatives [Cd(S<sub>2</sub>CNR<sub>2</sub>)<sub>2</sub>] are inert to this particular mode of C−S bond cleavage. This system thus constitutes an unusual example of heterolytic, nonoxidative C−S bond cleavage that appears to proceed by a topochemical transfer of protons, which has implications for C−S bond cleavage processes in single-source precursors for II−VI semiconductor materials.