Syntheses and Characterization of Dicarbonyl−Nitrosyl Complexes of Technetium(I) and Rhenium(I) in Aqueous Media: Spectroscopic, Structural, and DFT Analyses

This work describes new synthetic routes to produce mixed carbonyl−nitrosyl complexes of technetium(I) and
rhenium(I) in aqueous media. NaNO2, NOHSO4, and NO2(g) have been used to produce in situ nitrous acid as the
primary source of NO+. Starting from the organometallic precursor fac-[MX3(CO)3]+, 1 (M = 99Tc, Re; X = Cl, Br),
the formation of mixed dicarbonyl−mononitrosyl complexes was observed in aqueous hydrochloric and hydrobromic
acid. Time-dependent analyses of the reactions by means of HATR-IR and 99Tc NMR spectroscopy in solution
revealed the almost quantitative substitution of one CO ligand by NO+ and, thus, the formation of complexes with
facial arrangement of the three π-acceptor ligands. In the case of technetium, the monomeric complex
(NEt4)[TcCl3(CO)2NO] (3a) and the dimeric, chloride-bridged, neutral complex [TcCl(μ-Cl)(CO)2NO]2 (4a) were
produced. In the case of rhenium, the monomeric species (NEt4)[ReBr2X(CO)2NO] (X = Br (3b), NO3 (5)) was
solely isolated. The X-ray structure of complexes 4a and 5 are discussed. The crystallographic analyses revealed
the coordination of the NO+ group trans to the terminal chloride (4a) or the bromide (5), respectively. Crystal data: 
complex 4a (C4Cl4N2O6Tc2), monoclinic, Cc, a = 18.82(3) Å, b = 6.103(6) Å, c = 12.15(2) Å, α = 90°, β =
105.8(2)°, γ = 90°, V = 1343(3) Å3, Z = 4; complex 5 (C10H20N3O6Br2Re), orthorhombic, P212121, a = 10.2054(5) Å, b = 12.5317(7) Å, c = 13.9781(7) Å, V = 1787.67(16) Å3, Z = 4. The isolated complexes and their
potential facial isomers have been further investigated by density functional theory (DFT) calculations. The energy
differences of the isomers are relatively small; however, the calculated energies are consistent with the formation
of the observed and isolated compounds. The calculated bond lengths and angles of complex 5 are in good
agreement with the data determined by X-ray diffraction. Experiments on the no-carrier-added level starting from
fac-[99mTc(H2O)3(CO)3]+ revealed the formation of the complex fac-[99mTcCl(H2O)2(CO)2NO]+ in reasonable good
yields. This aqueous-based, synthetic approach will enable the future evaluation of this novel, low-valent metal
precursor for potential use in radiopharmacy.