Low-symmetry, mixed-valent, μ-oxo technetium complexes with pyridine and halide ligands

Dinuclear, asymmetric, and dissymmetric μ-oxo complexes of technetium in which the two Tc atoms are in intimate electronic communication, have been synthesized and structurally characterized. The two types of complexes are isomers differing in their arrangement of equatorial ligands around each technetium, with the configurations being X4 and (pyridine)4 for the dissymmetric case and X3(pyr) and (pyr)3X for the asymmetric. A compound representative of the asymmetric complexes, [(Pic)Cl3(Pic)Tc-O-TcCl(Pic)3Cl] (III), crystallizes in the monoclinic space group P21n with the following unit cell parameters: a = 12.421 A, b = 15,471 A, c = 18.764 A, β = 93.1740, and Z = 4. The geometry is essentially octahedral around each Tc, with the equatorial ligands on the opposite metal atoms being staggered relative to one another. Dissymmetric complexes of the type [X(L)4Tc-O-TcX4L], where X = Cl or Br and L = pyridine, 4-picoline, and 3,5 lutidine, are also formed by spontaneous self-assembly but are immediately derived from the asymmetric species. In the solid state, both types of compounds exhibit a small magnetic moment (μ/sub eff/ ∼ 0.9-1.3 β). EPR spectra of the dissymmetric complex, [Cl(Pic)4Tc-O-TcCl4(Pic)] (I), whose structure has been previously reported, are consistent with an axial, binuclear species, containing splitting by two I = 92 metal nuclei. Infrared Tc-O stretching frequencies are in the range 698-726 cm-1, while Tc-Cl bands appear in the range 305-317 cm-1 and Tc-Br at at 235-266 cm-1. Tissue distributions of these complexes in animals are reported and discussed