Re(V) complexes with an open-chain quadridentate ligand containing two amine and two amido donors. Synthesis, characterization, and solution equilibria of Re2O3(dioxo-tetH(4))(2) and [ReO(H2O)(dioxo-tetH(4))]Cl (dioxo-tetH(6)=1,4,8,11-tetraazaundecane-5,7-dione)

We are interested in identifying mononuclear cationic [M(V)=O](3+) (M = Tc, Re) complexes for radiopharmaceutical applications. The open-chain ligand, 1,4,8,11-tetraazaundecane-5,7-dione (dioxo-tetH(6)) with two amine and two amide donors, was selected for investigation since the literature led us t o expect that a five-coordinate [Re(V)=O(dioxo-tetH(4))](+) cation would do minate. Instead, the neutral mu -oxo bridged dinuclear complex, Re2O3-(diox o-tetH(4)](2) (1), and a salt of the six-coordinate mononuclear cation, [Re O(H2O)(dioxo-tetH(4))](+) (2), were isolated; the structure of each was det ermined by X-ray crystallography. The cation (2) is unusual because it has a trans-oxo/aqua core. Such aqua compounds are rarely isolated, and the Re- OH2 distance is relatively short (2.185 Angstrom). The cation has two pK(a) values, 4.1 and 8.7, determined with visible spectroscopy. Since the Re-OH 2 bond is short, the coordinated water is likely to be acidic. Thus the two pK(a)'s are assigned to the stepwise deprotonation of the water ligand to give a trans-oxo/hydroxo neutral form and a trans-dioxo anion. Although 1 w as the first product isolated following ligand exchange of ReOCl3(Me2S)(OPP h3) with dioxo-tetH(6) under neutral conditions, it probably formed from th e hydroxo mononuclear complex. Under concentrated conditions (similar to 30 0 mM) the dinuclear complex deposited from solution, but the H-1 NMR spectr a of 2 (similar to 20 mM) were consistent with the presence of only monomer ic forms in D2O, pH 3-12. H-1 NMR experiments demonstrated that in DMSO-d(6 ) 2 converts to 1 upon addition of base, consistent with the proposal that two units of the hydroxo monomer condense to give the dinuclear form. In ad dition, all spectra of pure 1 dissolved in DMSO-d(6) included extra low int ensity signals that were characteristic of the monomer. Thus, although 1 is favored over the neutral monomer in DMSO-d(6), the two complexes exist as a mixture of equilibrating forms. Our results do not support the previous f indings for the Re(V) complex with a macrocyclic diamine-diamide ligand rel ated to dioxo-tetH(6). The data indicate that the ability of an amido group to donate electron density to a Re(V) center is moderately greater than th e donating ability of a neutral amine group.