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

Abstract

The reaction of the mono-diazenide core, [ReCl₂(NNC₆H₄-4-OCH₃)(NCCH₃)(PPh₃)₂], with four equivalents of the sodium or potassium salts of dithiocarbamate (dtc) ligands gives neutral complexes of the formula [Re(NNC₆H₄-4-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 Re^III/Re^IV oxidation. Since the parent complex, [ReCl₂(NNC₆H₄-4-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.