Rhenium diazenide ternary complexes with dithiocarbamate ligands: towards new rhenium radiopharmaceuticals

The reaction of the mono-diazenide core, ReCl2(NNC6H(4)-4-OCH3)(NCCH3)(PPh3)2], with four equivalents of the sodium or potassium salts of dithiocarbamate (dtc) ligands gives neutral complexes of the formula Re(NNC6H(4)-4-OCH3)(dtc)2(PPh3)]. It is possible to use a wide range of dithiocarbamate ligands (S2CNRR') with a variety of R groups (R=R'=methyl, phenyl or ethyl; R=methyl, R'=phenyl and R=R'=morpholino). Substitution reactions with dtc ligands on the mono-diazenide derived from 2-hydrazinopyridine, ReCl2(NNC5H4N)(PPh3)2, give the analogous complexes, Re(NNC6H5N)(dtc)2(PPh3)]. The new complexes have been characterised by a combination of NMR spectroscopy, mass spectrometry and X-ray crystallography. Cyclic voltammetry measurements in dimethyl formamide show that the rhenium diazenido-dtc complexes undergo a quasi-reversible oxidation tentatively assigned to a ReIII/ReIV oxidation. Since the parent complex, ReCl2(NNC6H(4)-4-R)(NCCH3)(PPh3)2] can be prepared directly from perrhenate and readily derivatised with dtc ligands these complexes have potential relevance to the development of new therapeutic rhenium radiopharmaceuticals.