The Chemistry of Phenylimidotechnetium(V) Complexes with Isocyanides: Steric and Electronic Factors.

Organometallic approaches are of ongoing interest for the development of novel functional ^99m Tc radiopharmaceuticals, while the basic organotechnetium chemistry seems frequently to be little explored. Thus, structural and reactivity studies with the long-lived isotope ⁹⁹ Tc are of permanent interest as the foundation for further progress in the related radiopharmaceutical research with this artificial element. Particularly the knowledge about the organometallic chemistry of high-valent technetium compounds is scarcely developed. Here, phenylimido complexes of technetium(V) with different isocyanides are introduced. They have been synthesized by ligand-exchange procedures starting from [Tc(NPh)Cl ₃ (PPh ₃ ) ₂ ]. Different reactivity patterns and products have been obtained depending on the steric and electronic properties of the individual ligands. This involves the formation of 1:1 and 1:2 exchange products of Tc(V) with the general formulae [Tc(NPh)Cl ₃ (PPh ₃ )(isocyanide)], cis - or trans -[Tc(NPh)Cl ₃ (isocyanide) ₂ ], but also the reduction in the metal and the formation of cationic technetium(I) complex of the formula [Tc(isocyanide) ₆ ] ⁺ when p-fluorophenyl isocyanide is used. The products have been studied by single-crystal X-ray diffraction and spectroscopic methods, including IR and multinuclear NMR spectroscopy. DFT calculations on the different isocyanides allow the prediction of their reactivity towards electron-rich and electron-deficient metal centers by means of the empirical SADAP parameter, which has been derived from the potential energy surface of the electron density on their potentially coordinating carbon atoms.