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; 1,10phenanthroline, 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 [ 99 Tc(CO) 3 (bipy)(CNCH 2 COOEt)](ClO 4 ) (in the form of a solvate with 0.5CH 2 Cl 2 ) and [ 99 Tc(CO) 3 (phen)-(CNCH 2 COOEt)](ClO 4 ) (in the form of an adduct with an outersphere 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, 99 Tc; N ∧ N = bipy, phen; X = Cl − , ClO 4 − ) by CNCH 2 COOEt in ethanol were compared. The 99 Tc bipy complexes with X = ClO 4 − (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 99 Tc 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 [ 99m Tc(CO) 3 (bipy)(CNCH 2 COOEt)] + can be prepared under the conditions acceptable for nuclear medical applications, although higher isonitrile concentrations are required as compared to the chloride-free system.

Existence and Properties of [TcH(CO)5]

“…However, monitoring the reaction by IR spectroscopy evidenced that no reaction took place with these ligands in an inert atmosphere in toluene even at 110 °C, supporting the importance of one ligand with π-donation properties such as the halides. Thus, the reactivity of the technetium complex is different from [ReH­(CO) 5 ], which reacts with 2,2′-bipyridine in boiling toluene under inert atmosphere to form fac -Re­(bpy)­(CO) 3 H, despite the fact that rhenium carbonyls are usually less reactive than technetium carbonyls . As far as thermal stability and CO replacement in an inert atmosphere are concerned, 1 behaves as a relatively robust compound.…”

“…The formation of these substitution products is unexpected taking the previously described reactivity patterns of [ 99 TcX(CO) 5 ] or [ 99 TcH­(CO) 5 ] into account. On the one hand, the reactions of the former with aromatic heterocycles in air give [ 99 TcX­(CO) 3 Q 2 ] (X = Cl, Br; Q = pyridine, or Q 2 = bipy, phen) type complexes without formation of pertechnetate or carbonate. , On the other hand, the reaction of 1 in solution with atmospheric oxygen without diimine base leads merely to oxidative coupling to form 99 Tc 2 (CO) 10 6, or [ 99 Tc 3 H(CO) 14 ] depending on the conditions. We hypothesize that ambient oxygen attacks the Tc–H bond in 1 to form reactive oxygen species (ROS) (possible mechanisms involve, e.g., oxygen insertion to form hydroperoxide or hydrogen abstraction to form radical species).…”