Carbohydrate-Appended 2,2‘-Dipicolylamine Metal Complexes as Potential Imaging Agents

Three discrete carbohydrate-appended 2,2‘-dipicolylamine ligands were complexed to the {M(CO)3}+ (M = 99mTc/Re) core: 2-(bis(2-pyridinylmethyl)amino)ethyl-β-d-glucopyranoside (L 1 ), 2-(bis(2-pyridinylmethyl)amino)ethyl-β-d-xylopyranoside (L 2 ), and 2-(bis(2-pyridinylmethyl)amino)ethyl-α-d-mannopyranoside (L 3 ). An ethylene spacer is used to separate the carbohydrate moiety and the dipicolylamine (DPA) function in all three ligands. The Re complexes [Re(L1-3)(CO)3]Br were characterized by 1H and 13C 1D/2D NMR spectroscopies, which confirmed the pendant nature of the carbohydrate moieties in solution. NMR measurements also established the long-range asymmetric effect of the carbohydrate functions on the chelating portion of the ligand. One analogue, [Re(L 1 )(CO)3]Cl, was characterized in the solid state by X-ray crystallography. Further characterization was provided by IR spectroscopy, elemental analysis, conductivity, and mass spectrometry. Radiolabeling of L 1 −L 3 with [99mTc(H2O)3(CO)3]+ afforded high yield compounds of identical character to the Re analogues. The radiolabeled compounds were found to be stable toward ligand exchange in the presence of a large excess of either cysteine or histidine over a 24-h period.