Forcing the Side‐on π‐Coordination of a C≡C Triple Bond to Technetium Using the As,$CC$,As Alkyne Pincer Ligand 1,2‐Bis(2‐(diisopropylarsaneyl)‐4‐(trifluoromethyl)phenyl)Ethyne

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$^{III}$Cl$_3$(κ$^4$-As,CC,As-L$^{iPr}$)], mer-[Tc$^V$NX$_2$(κ$^4$-As,CC,As-L$^{iPr}$)] (X = Cl, Br) and cis,trans,mer-[Tc$^V$N(CN)Cl(κ$^4$-As, CC, As-L$^{iPr}$)] alongside their rhenium homologs mer-[Re$^V$Cl$_3$(κ$^4$-As,CC,As-L$^{iPr}$)] and mer-[Re$^V$NCl$_2$(κ$^4$-As,CC,As-L$_{iPr}$)] 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-[Re$^V$Cl$_3$(κ$^4$-As,CC,As-L$^{iPr}$)] with a formally oxidized metal ion and reduced 4e$^−$ donor ligand. ... mehrThis has general implications for π-ligand coordination in group 7 and potentially for neighboring elements. Successful translation to the medicinally relevant nuclear isomer $^{99m}$Tc proves the viability of alkyne donors as building blocks for stable chelation of technetium at the tracer level.