Alkyne complexes are known for transition metals across the d-block with exception of the radioelement technetium despite considerable synthetic efforts. DFT calculations suggest that this is not inherent to the transition metal but a consequence of the overall ligand sphere. The arsenic-based tolane ligand 1, 2-bis (2- (diisopropylarsaneyl) -4- (trifluoromethyl) phenyl) ethyne (L^iPr) forces a coordination of the central alkyne moiety onto the metal through ligand design. The stable, crystalline Tc (III) and Tc (V) alkyne complexes mer-Tc^IIICl₃ (κ⁴-As, CC, As-L^iPr), mer-TcVNX₂ (κ⁴-As, CC, As-L^iPr) (X = Cl, Br) and cis, trans, mer-TcVN (CN) Cl (κ⁴-As, CC, As-L^iPr) alongside their rhenium homologs mer-ReVCl₃ (κ⁴-As, CC, As-L^iPr) and mer-ReVNCl₂ (κ⁴-As, CC, As-L₈ₑ) have been prepared and fully characterized. According to spectroscopic and DFT analyses, the technetium complexes represent robust, classical 2e^− alkyne complexes, while a different situation was found for mer-ReVCl₃ (κ⁴-As, CC, As-L^iPr) with a formally oxidized metal ion and reduced 4e^− donor ligand. This has general implications for π-ligand coordination in group 7 and potentially for neighboring elements. Successful translation to the medicinally relevant nuclear isomer ^99mTc proves the viability of alkyne donors as building blocks for stable chelation of technetium at the tracer level.
Jungfer et al. (Thu,) studied this question.