A hexadentate bis(thiosemicarbazonato) ligand: rhenium(v), iron(iii) and cobalt(iii) complexes

Abstract

A new 1,3-diaminopropane bridged bis(thiosemicarbazone) ligand (H₄L) has been synthesised. The new hexadentate ligand is capable of forming six coordinate complexes with rhenium(V), iron(III) and cobalt(III). In the case of the iron(III) and cobalt(III) complexes the ligand doubly deprotonates to give the monocations [Fe^III(H₂L)]⁺ and [Co^III(H₂L)]⁺ in which the metal ion is in a distorted octahedral environment. In the rhenium(V) complex the ligand loses four protons by deprotonation of both secondary amine nitrogen atoms to give [Re^V(L)]⁺ with the metal ion in a distorted trigonal prismatic coordination environment. [Re^V(L)]⁺ represents a rare example of a rhenium(V) complex that does not contain one of the ReO³⁺, ReN²⁺ or Re(NPh)²⁺ cores. The new ligand and metal complexes have been characterised by a combination of NMR spectroscopy, X-ray crystallography, mass spectrometry and microanalysis. The electrochemistry of [Fe^III(H₂L)]⁺, [Co^III(H₂L)]⁺ and [Re^V(L)]⁺ has been investigated by cyclic voltammetry with each complex undergoing a single electron reduction event. It is possible to prepare the rhenium(V) complex from ReOCl₃(PPh₃)₂ or directly from [ReO₄]⁻ with the addition of a reductant, which suggests the new ligand may be of interest in the development of rhenium radiopharmaceuticals.