Toward Organometallic 99mTc Imaging Agents: Synthesis of Water-Stable 99Tc–NHC Complexes

99TcVO2–NHC complexes containing monodentate and bidentate N-heterocyclic carbenes (NHCs) have been prepared by the reactions of [TcO­(glyc)2]− (glyc = ethyleneglycolato) with 1,3-dimethylimidazoline-2-ylidene (L1), 1,1′-methylene-3,3′-dimethyl-4,4′-diimidazoline-2,2′-diylidene (L2), and 1,1′-methylene-3,3′-diethyl-4,4′-diimidazoline-2,2′-diylidene (L3) in THF. The resulting complexes were fully characterized and their stabilities investigated. While complexes with monodentate NHCs only are hydrolytically unstable, complexes containing bidentate NHCs are water-stable over a broad pH range. The high water stability allows interconversion of the {99TcVO2}+ core into {99TcVOCl}2+ with HCl as the H+ and Cl– source. An alternative procedure to obtain 99TcVO2–NHC complexes is the in situ deprotonation of imidazolium salts, enabling the preparation of 99TcVO2–NHC compounds without free NHCs, thus increasing the scope of NHC ligands drastically. The remarkable stability and pH-controllable reactivity of the new complexes underlines the potential of NHCs as stabilizing ligands for 99Tc complexes and paves the way for the first 99mTc–NHC complexes in the future.