Synthesis, Characterisation and Structural Analysis of Rhenium and Technetium Nitride Complexes With Tridentate...

The reactivity of two tridentate salicylthiosemicarbazones (H₂L1 = 3-methoxysalicylaldehyde thiosemicarbazone; H₂L2 = 3-methoxysalicylaldehyde N,N-dimethyl-thiosemicarbazone), with the [M^V ≡ N]²⁺ core (M = Re, ^99mTc), is here described, leading to the formation of heteroleptic complexes of the general composition [MN(L)P] (P = triphenylphosphine, PPh₃; tris(2-cyanoethyl)phosphine, PCN). The [ReN(L)PPh₃] complexes were prepared in good yield starting from [Re^VNCl₂(PPh₃)₂] precursor and in lower quantity from (Bu₄N)[ReN^VICl₄]. Complexes were fully characterised by elemental analyses, spectroscopic, spectrometric techniques and X-ray diffraction. Single-crystal X-ray diffraction analysis of [ReN(L1)PPh₃] and [ReN(L2)PPh₃] showed the formation of a distorted square base pyramid geometry with the rhenium atom located 0.57 Å above the pyramid base and the nitride ion in apical position. The basal plane is occupied by the S,N,O-thiosemicarbazonate and the triphenylphosphine ligands. Superimposable complexes were obtained in high yield at tracer level with technetium-99m and under carried-added conditions. In technetium complexes, triphenylphosphine can be successfully replaced with PCN, forming corresponding complexes in high yield; this substitution was unfeasible with cold rhenium. Based on the collected preliminary results, it is reasonable to assume that [^99mTc][TcN(L)P] (L = bisdeprotonated ligand; P = monophosphine) could be a promising platform for developing new potential technetium-99m-based radiotracers. However, improvements in labelling efficiency and stability must be addressed to fully realise its potential.