Technetium(I) Carbonyl Chemistry with Small Inorganic Ligands

[Tc(NO)Cl2(PPh3)2(CH3CN)] and Its Reactions with 2,2'-Dipyridyl Dichalcogenides

The sparingly soluble technetium(I) complex [TcI(NO)Cl2(PPh3)2(CH3CN)] (1) slowly dissolves during reactions with 2,2'-dipyridyl ditelluride, (2-pyTe)2, 2,2'-dipyridyl diselenide, (2-pySe)2, or 2,2'-dipyridyl disulfide, (2-pyS)2, under formation of deeply colored solutions. Blue (Te compound) or red solids (Se compound) of the composition [{TcI(NO)Cl2(PPh3)2}2{µ2-(2-pyE)2}], E = Te (3), Se (4), precipitate from the reaction solutions upon addition of toluene. They represent the first technetium complexes with dichalcogenides. While [{TcI(NO)Cl2(PPh3)}2{µ2-(2-pyTe)2}] (3) is the sole product, a small amount of a second product, [TcII(NO)Cl2(PPh3)(2-pySe)] (5), was obtained from the respective mother solution of the reaction with the diselenide. From the corresponding reaction between 1 and (2-pyS)2, the technetium(II) compound, [TcII(NO)Cl2(PPh3)(2-pyS)] (6), could be isolated exclusively. The products were studied by single-crystal X-ray diffraction and spectroscopic methods including 99Tc NMR for the technetium(I) products and EPR spectroscopy for the Tc(II) complexes. The experimental results are accompanied by DFT considerations, which help to rationalize the experimental observations.

Pivotal role of 99Tc NMR spectroscopy in solid-state and molecular chemistry

The radioelement Technetium (element 43) pertains to various domains including the nuclear enterprise (i.e., spent nuclear fuel (SNF) reprocessing and nuclear waste remediation) and nuclear medicine (i.e., development of new imaging agents) as well as to the fundamental science of transition metals (i.e., chemical trends in catalytic properties). One method that can provide critical information to improve knowledge in these domains is 99Tc nuclear magnetic resonance (NMR) spectroscopy. The review, presented here, summarizes the pivotal role of 99Tc NMR spectroscopy over the past two decades and presents prospects of the method to tackle challenges in Tc chemistry.

[Tc(NO)(Cp)(PPh3)Cl] and [Tc(NO)(Cp)(PPh3)(NCCH3)](PF6), and Their Reactions with Pyridine and Chalcogen Donors

Reactions of the technetium(I) nitrosyl complex [Tc(NO)(Cp)(PPh3)Cl] with triphenylphosphine chalcogenides EPPh3 (E = O, S, Se), and Ag(PF6) in a CH2Cl2/MeOH mixture (v/v, 2/1) result in an exchange of the chlorido ligand and the formation of [Tc(NO)(Cp)(PPh3)(EPPh3)](PF6) compounds. The cationic acetonitrile complex [Tc(NO)(Cp)(PPh3)(NCCH3)]+ is formed when the reaction is conducted in NCCH3 without additional ligands. During the isolation of the corresponding PF6- salt a gradual decomposition of the anion was detected in the solvent mixture applied. The yields and the purity of the product increase when the BF4- salt is used instead. The acetonitrile ligand is bound remarkably strongly to technetium and exchange reactions readily proceed only with strong donors, such as pyridine or ligands with 'soft' donor atoms, such as the thioether thioxane. Substitutions on the cyclopentadienyl ring do not significantly influence the ligand exchange behavior of the starting material. 99Tc NMR spectroscopy is a valuable tool for the evaluation of reactions of the complexes of the present study. The extremely large chemical shift range of this method allows the ready detection of corresponding ligand exchange reactions. The observed 99Tc chemical shifts depend on the donor properties of the ligands. DFT calculations support the discussions about the experimental results and provide explanations for some of the unusual findings.

Induced fac-mer rearrangements in {M(CO)3}+ complexes (M = Re, 99(m)Tc) by a PNP ligand

The fac-mer rearrangements in [MX3(CO)3]2- (M = Re, 99Tc) induced by a pincer-type ligand (PNP) and a "halide scavenger" are reported. The reactions of fac-[99mTc(CO)3(OH2)3]+ or [99mTcO4]- in saline both yield mer-[99mTc(PNP)(CO)3]+, the first example of a mer-{99mTc(CO)3}+ type complex. In contrast, reactions with terpyridine (terpy) only gave the facial κ2-terpy complexes with Re and 99Tc.

Unlocking Air- and Water-Stable Technetium Acetylides and Other Organometallic Complexes

The first technetium complexes containing anionic alkynido ligands in an end-on coordination mode have been prepared by using the nonprotic, cationic precursor mer,trans-[Tc(SMe₂)(CO)₃(PPh₃)₂]⁺. This cation acts as a functional analogue of the highly reactive 16-electron metallo Lewis acid {Tc(CO)₃(PPh₃)₂}⁺ in reactions with alkynes, acetylides, and other organometallic reagents. Such reactions give a variety of organometallic technetium complexes in excellent yields and enable the preparation of [Tc(CH₃)(CO)₃(PPh₃)₂], [Tc(Ph)(CO)₃(PPh₃)₂], [Tc(Cp)(CO)₂(PPh₃)], [Tc(═CCH₂CH₂CH₂O)(CO)₃(PPh₃)₂]⁺, [Tc(═CCH₂CH₂CH₂CH₂O)(CO)₃(PPh₃)₂]⁺, [Tc(C≡C-H)(CO)₃(PPh₃)₂], [Tc(C≡C-Ph)(CO)₃(PPh₃)₂], [Tc(C≡C-^tBu)(CO)₃(PPh₃)₂], [Tc(C≡C-ⁿBu)(CO)₃(PPh₃)₂], [Tc(C≡C-SiMe₃)(CO)₃(PPh₃)₂], and [Tc{C≡C-C₆H₃(CF₃)₂}(CO)₃(PPh₃)₂]. The bonding situation in the alkynyl complexes is compared to that in corresponding alkyl- and arylnitrile and -isonitrile complexes. [Tc(N≡C-Ph)(CO)₃(PPh₃)₂](BF₄), [Tc(C≡N-Ph)(CO)₃(PPh₃)₂](BF₄), [Tc(N≡C-^tBu)(CO)₃(PPh₃)₂](BF₄), and [Tc(C≡N-^tBu)(CO)₃(PPh₃)₂](BF₄) were prepared in high yields by ligand exchange reactions starting from mer,trans-[Tc(OH₂)(CO)₃(PPh₃)₂](BF₄). The novel complexes were characterized by single-crystal X-ray diffraction and spectroscopic methods. In particular, ⁹⁹Tc nuclear magnetic resonance spectroscopy proved to be an invaluable and sensitive tool for the characterization of the complexes. Density functional theory calculations strongly suggest similar bonding situations for the related alkynyl, nitrile, and isonitrile complexes of technetium.