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

⁹⁹Tc^VO₂–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 {⁹⁹Tc^VO₂}⁺ core into {⁹⁹Tc^VOCl}²⁺ with HCl as the H⁺ and Cl^– source. An alternative procedure to obtain ⁹⁹Tc^VO₂–NHC complexes is the in situ deprotonation of imidazolium salts, enabling the preparation of ⁹⁹Tc^VO₂–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.