Synthesis of [MoS ₄] ^2--M (M = Cu and Cd) clusters: Potential NMR spectroscopic structural probes for the orange protein

Two synthetic strategies of tetrathiomolybdate-metal clusters with the potential to be used as NMR structural probes for the location of the metal cofactor in the orange protein (ORP) are described. The first strategy is based on the substitution reaction in which small organic ligands bind directly to the metal centre in a molybdenum-copper hetero-dinuclear cluster. Interaction between [PPh ₄] ₂[MoS ₄CuCl] and either aliphatic [β-mercaptoethanol (b-me)] or aromatic [o-aminobenzenethiol (abt)] thiols in the presence of a strong base resulted in the formation of [Ph ₄P] ₂[S ₂MoS ₂Cu(b-me)] (1a) and [Et ₄N] ₂[S ₂MoS ₂Cu(abt)]·H ₂O·0.25DMF (1b), which can be used to obtain intermolecular NOEs. The compound 1a readily hydrolyzed to [Ph ₄P] ₂[OSMoS ₂Cu(b-me)] (1a ^hydro) in contact with a protic solvent. The second strategy consisted of the incorporation of cadmium into tetrathiomolybdate ([MoS ₄] ^2-), which gives rise to the trinuclear cluster compound [PPh ₄] ₂[(MoS ₄) ₂Cd] (2). All clusters were characterized spectroscopically and their structure determined by X-ray diffraction. The NMR spectroscopic data are consistent with the formation of a complex with a 1:1 ratio of {MoS ₄Cu} and thiol. The ¹¹³Cd NMR chemical shift of compound 2 is consistent with the cadmium having a tetrahedral geometry and coordinated by four sulfur ligands. The tetraphenylphosphonium cation in compound 1a was replaced by a tetramethylammonium countercation originating in the water-soluble compound [Me ₄N-1a]. Solubility in aqueous buffers is a requirement for incorporating this cluster into apo-ORP. These compounds will be used to identify the exact location of the ORP heterometallic cluster using NMR methodologies.