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

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Abstract
Three bis(N-alkyldithiocarbamato)cadmium(II) complexes [Cd(S2CNHR)2] (1, R = n-C3H7; 2, R = n-C5H11; 3, n-C12H25) were prepared by metathesis of the corresponding lithium salt, Li[S2CNHR], with cadmium chloride. The crystal structures of 2 and 3 consist of planar molecular units of [Cd(S2CNHR)2] 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 113Cd NMR spectrum of 2 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 CS2 as coproducts. These products can result only from the net transfer of protons between N-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(S2CNHR)2]. This confirms that C−S bond cleavage must be coupled to deprotonation of the −NH group, and explains why dialkylated derivatives [Cd(S2CNR2)2] 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.

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