Rhenium and Technetium Tricarbonyl Complexes of N‑Heterocyclic Carbene Ligands

A strategy
for the conjugation of N-heterocyclic carbene (NHC)
ligands to biomolecules via amide bond formation is described. Both
1-(2-pyridyl)­imidazolium or 1-(2-pyridyl)­benzimidazolium salts functionalized
with a pendant carboxylic acid group were prepared and coupled to
glycine benzyl ester using 1-ethyl-3-(3-(dimethylamino)­propyl)­carbodiimide.
A series of 10 rhenium­(I) tricarbonyl complexes of the form [ReX­(CO)3(ĈN)] (ĈN is a bidentate NHC ligand, and X is
a monodentate anionic ligand: Cl–, RCO2–) were synthesized via a Ag2O transmetalation
protocol from the Re­(I) precursor compound Re­(CO)5Cl. The
synthesized azolium salts and Re­(I) complexes were characterized by
elemental analysis and by 1H and 13C NMR spectroscopy,
and the molecular structures for one imidazolium salt and seven Re­(I)
complexes were determined by single-crystal X-ray diffraction. 1H NMR and mass spectrometry studies for an acetonitrile-d3 solution of [ReCl­(CO)3(1-(2-pyridyl)-3-methylimidazolylidene)]
show that the monodentate chloride ligand is labile and exchanges
with this solvent yielding a cationic acetonitrile adduct. For the
first time the labeling of an NHC ligand with technetium-99m is reported.
Rapid Tc-99m labeling was achieved by heating the imidazolium salt
1-(2-pyridyl)-3-methylimidazolium iodide and Ag2O in methanol,
followed by the addition of fac-[99mTc­(OH2)3(CO)3]+. To confirm the
structure of the 99mTc-labeled complex, the equivalent 99Tc complex was prepared, and mass spectrometric studies showed
that the formed Tc complexes are of the form [99m/99Tc­(CH3CN)­(CO)3(1-(2-pyridyl)-3-methylimidazolylidene)]+ with an acetonitrile molecule coordinated to the metal center.