Technetium and rhenium pentacarbonyl complexes with C₂ and C₁₁ ω-isocyanocarboxylic acid esters

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?

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.

Technetium Complexes with an Isocyano-alkyne Ligand and Its Reaction Products

The attachment of an ethyne substituent in the para position of phenylisocyanide, CNPh^pC≡CH, enables the isocyanide to replace carbonyl ligands in the coordination sphere of common technetium(I) starting materials such as (NBu₄)[Tc₂(μ-Cl)₃(CO)₆]. The ligand exchange proceeds under thermal conditions and finally forms the corresponding hexakis(isocyanide)technetium(I) complex. The product undergoes a copper-catalyzed cycloaddition ("Click" reaction), e.g., with benzyl azide, which gives the [Tc(CNPh^azole)₆]⁺ cation. The free, uncoordinated "Click" product is obtained from a reaction of the corresponding tetrakis(CNPh^azole)copper(I) complex and NaCN. It readily reacts with mer-[Tc(CO)₃(tht)(PPh₃)₂](BF₄) (tht = tetrahydrothiophene) under exchange of the thioether ligand. Alternatively, [Cu(CNPh^azole)₄]⁺ can be used as a transmetalation reagent for the synthesis of the hexakis(isocyanide)technetium(I) complex, which is the preferable approach for the synthesis of the technetium complex with the short-lived nuclear isomer ^99mTc, and a corresponding protocol for [^99mTc(CNPh^azole)₆]⁺ is reported. The ⁹⁹Tc and copper complexes have been studied by single-crystal X-ray diffraction and/or spectroscopic methods including IR and multinuclear NMR spectroscopy.

Mixed-Isocyanide Complexes of Technetium under Steric and Electronic Control

Reactions of the alkyl isocyanide fac-[Tc(CO)₃(CNR)₂Cl] complexes (2) (CNR = CNⁿBu or CN^tBu) with the sterically encumbered isocyanide CNp-FAr^DarF2 [DArF = 3,5-(CF₃)₂C₆H₃] allow a selective exchange of the carbonyl ligands of 2 and the isolation of the mixed-isocyanide complexes mer,trans-[Tc(CNp-FAr^DarF2)₃(CNR)₂Cl] (3). Depending on the steric requirements of the residues R, the remaining chlorido ligand can be replaced by another isocyanide ligand. Cationic complexes such as mer-[Tc(CNp-FAr^DarF2)₃(CNⁿBu)₃]⁺ (4a) or mer,trans-[Tc(CNp-FAr^DarF2)₃(CNⁿBu)₂(CN^tBu)]⁺ (6) have been prepared in this way and isolated as their PF₆^- salts. mer,trans-[Tc(CNp-FAr^DarF2)₃(CNⁿBu)₂(CN^tBu)](PF₆) represents to the best of our knowledge the first transition-metal complex with three different isocyanides in its coordination sphere. Since the degree of the ligand exchange seems to be controlled both by the electronic and steric measures of the incoming isocyanides, we undertook similar reactions with the sterically less demanding p-fluorophenyl isocyanide, CNPh^pF, which indeed readily led to the hexakis(isocyanide)technetium(I) cation through an exchange of all ligands in the staring materials [Tc₂(CO)₆(μ-Cl)₃]^- or fac-[Tc(CO)₃(CNR)₂Cl]. The influence of the substituents at the isocyanide ligands in such reactions has been reasoned with the density functional theory-derived electrostatic potential at the accessible surface of the corresponding isocyanide carbon atoms.

Technetium(I) Carbonyl Chemistry with Small Inorganic Ligands

[Tc(OH₂)(CO)₃(PPh₃)₂](BF₄) has been used as a synthon for reactions with small inorganic ligands with relevance for the treatment of nuclear waste solutions such as nitrate, nitrite, pseudohalides, permetalates (M = Mn, Tc, Re), and BH₄^-. The formation of bond isomers and/or a distinct reactivity has been observed for most of the products. [Tc(NCO)(CO)₃(PPh₃)₂], [Tc(NCS)(CO)₃(PPh₃)₂], [Tc(CN)(CO)₃(PPh₃)₂], [Tc(N₃)(CO)₃(PPh₃)₂], [Tc(NCO)(OH₂)(CO)₂(PPh₃)₂], [Tc(η²-OON)(CO)₂(PPh₃)₂], [Tc(η¹-NO₂)(CO)₃(PPh₃)₂], [Tc(η²-OONO)(CO)₂(PPh₃)₂], [Tc(η¹-ONO₂)(CO)₃(PPh₃)₂], [Tc(η²-OO(CCH₃))(CO)₂(PPh₃)₂], [Tc(η²-SSC(SCH₃))(CO)₂(PPh₃)₂], [Tc(η²-SSC(OCH₃))(CO)₂(PPh₃)₂], [Tc(η²-SSC(CH₃))(CO)₂(PPh₃)₂], [Tc(η²-SS(CH))(CO)₂(PPh₃)₂], [Tc(OTcO₃)(acetone)(CO)₂(PPh₃)₂], [Tc(OTcO₃)(CO)₃(PPh₃)₂], and [Tc(η²-HHBH₂)(CO)₂(PPh₃)₂] have been isolated in crystalline form and studied by X-ray crystallography. Additionally, the typical reactivity patterns (isomerization, thermal decomposition, hydrolysis, or decarbonylation) of the products have been studied by spectroscopic methods. ⁹⁹Tc NMR spectroscopy has proved to be a particularly useful tool for the evaluation of such reactions of the diamagnetic technetium(I) compounds in solution.

[Tc(OH2)(CO)3(PPh3)2]+: A Synthon for Tc(I) Complexes and Its Reactions with Neutral Ligands

A scalable synthesis of the novel and highly reactive [Tc(OH₂)(CO)₃(PPh₃)₂]⁺ cation is described. The ligand-exchange chemistry of this compound with neutral ligands coordinating through C, N, O, S, Se, and Te has been explored systematically. The complexes either retain the original mer-trans tricarbonyl core under exclusive exchange of the aqua ligand or form dicarbonyl complexes by thermal decarbonylation. Ligand exchange reactions starting from [Tc(OH₂)(CO)₃(PPh₃)₂]⁺ proceed under mild conditions and are generally almost quantitative. Some of the formed complexes are remarkably stable and inert, while others provide products with one labile ligand for further reactions. The derived complexes of the type [Tc(L)(CO)₃(PPh₃)₂]⁺ and [Tc(L)₂(CO)₂(PPh₃)₂]⁺ represent an interesting opportunity for the development of ^99mTc complexes with potential use in radiopharmacy. The ready displacement of the aqua ligand highlights the synthetic value of [Tc(OH₂)(CO)₃(PPh₃)₂]⁺ as a reactive entry point for further studies in the little explored field of the organometallic chemistry of technetium.