2 + 1 Tricarbonyl Complexes of Technetium(I) with a Combination of N, N-Bidentate Ligands and Ethyl Isocyanoacetate: How Strong Is the Interfering Effect of Chloride Ions on Their Formation?

Abstract

Technetium(I) 2 + 1 tricarbonyl complexes with a combination of N,N-bidentate ligands (2,2'-bipyridine, bipy; 1,10-phenanthroline, phen) and ethyl isocyanoacetate were prepared and characterized by NMR, IR, UV/visible, and luminescence spectroscopies and by high-performance liquid chromatography (HPLC). The crystal structures of [⁹⁹Tc(CO)₃(bipy)(CNCH₂COOEt)](ClO₄) (in the form of a solvate with 0.5CH₂Cl₂) and [⁹⁹Tc(CO)₃(phen)(CNCH₂COOEt)](ClO₄) (in the form of an adduct with an outer-sphere phen molecule) were determined by single-crystal X-ray diffraction. To evaluate the interfering effect of chloride ions on the formation of the 2 + 1 complexes, the kinetics of the replacement of labile monodentate ligand X in the complexes [MX(CO)₃(N^∧N)] (M = Re, ⁹⁹Tc; N^∧N = bipy, phen; X = Cl^-, ClO₄^-) by CNCH₂COOEt in ethanol were compared. The ⁹⁹Tc bipy complexes with X = ClO₄^- (according to the IR data, perchlorate anion in ethanol is displaced from the coordination sphere by the solvent molecule) and X = Cl^- are characterized by close ligand replacement rates. In the case of the ⁹⁹Tc complexes with phen and Re complexes with both phen and bipy, the chloride complexes are appreciably less reactive than the chloride-free complexes. The technetium complexes are considerably more reactive in ligand replacement than their rhenium analogues. In the chloride-containing medium (saline), the complex [^99mTc(CO)₃(bipy)(CNCH₂COOEt)]⁺ can be prepared under the conditions acceptable for nuclear medical applications, although higher isonitrile concentrations are required as compared to the chloride-free system.