Design, Synthesis, and Reactivity of Multidentate Ligands with Rhenium(I) and Rhenium(V) Cores

Synthetic pathways to a range of potentially N3O‐tetradentate ligands designed to coordinate to rhenium cores, as well as their coordination behaviors towards different rhenium cores (oxidation states +I and +V) are investigated. Two functionalized N‐{[1‐(4‐R)‐1H‐1,2,3‐triazol‐4‐yl]methyl}‐2‐(pyridin‐2‐ylmethoxy)aniline derivatives L1H (R = methyl acetate) and L2H (R = 4‐nitrophenyl) act exclusively as bidentate ligands and lead to the formation of mononuclear tricarbonylrhenium(I) complexes of the general formula [(LH)Re(CO)3Cl] with L = L1 or L2. Both complexes are characterized by 1H NMR and 13C NMR, FTIR spectroscopy, electrospray ionization mass spectrometry, and in the case of [(L2H)Re(CO)3Cl], single‐crystal X‐ray diffraction. The rhenium is coordinated by three carbonyl groups, a chlorine atom and two nitrogen atoms of a triazole group, and a nitrogen of the aniline ring of the ligand, respectively. A theoretical study shows complex [(L2H)Re(CO)3Cl] is the most stable structural isomer. In addition, the oxorhenium(V) complex [(L3)ReO] is isolated and fully characterized after the reaction of the ReV precursor [ReOCl3(PPh3)2] with L3H3 [methyl 2‐(4‐{[2‐(2‐hydroxyphenylamino)‐2‐oxoethylamino]methyl}‐1H‐1,2,3‐triazol‐1‐yl)acetate]. Its corresponding 99mTc complex was achieved with a good radiochemical yield (> 90 %). The convenient synthesis of this ligand, coupled with its high affinity for [ReO]3+ and [99mTcO]3+ cores, make it a promising chelator for biomedical applications. The coordination behavior of three potentially N3O‐tetradentate ligands towards different rhenium cores (oxidation states +I and +V) are investigated. L1H and L2H lead to the formation of unexpected mononuclear tricarbonylrhenium(I) complexes of the general formula [(LH)Re(CO)3Cl] (L = L1 or L2), whereas L3H3 gives the mononuclear complex [(L3)ReO] and its corresponding 99mTc complex in good yield.