Technetium(III), technitium(II), and technetium(I) complexes with pyridine ligands. Can pyridine coordination stabilize the low oxidation states of technetium?

The substitution chemistry of TcCl3(PPh(3))(2)(CH3CN) is rather facile relative to the analogous rhenium complex, since both the chloride and phosphine ligands are easily substituted for various pyridine ligands. Consequently a series of Tc-III complexes with amine, pyridine, and polypyridyl ligands were prepared and characterized by H-1 NMR and cyclic voltammetry. In addition, the zinc reduction of TcCl4(py)(2) in the presence of pyridine results in TcCl2(py)(4). Structural and spectroscopic data indicate that this Tc-II complex exhibits strong metal-pyridine interactions characteristic of low-valent amine complexes of Re-II and Os-II. For example, a decrease of 0.04 and 0.06 Angstrom is observed for the trans-Tc-N bond length in TcCl2(py)(4) relative to mer-TcCl3(py)(3) and [TcCl2(py)(3)(PPh(3))](+), respectively. This ability of pyridine to function both as a strong a-donor and moderate Jc-acid ligand has resulted in the isolation of technetium complexes in various oxidation states with similar ligand environments. As a result, a structural comparison of [TcCl2(py)(3)(PPh(3))(3)](+), TcCl2(py)(4), TcCl(tpy)(py)(2), and other known Tc-III and Tc-II pyridine complexes is presented. Crystals of [TcCl2(py)(3)(PPh(3))]PF6 are triclinic, with space group <P(1)over bar >, Z = 2, and lattice parameters a = 12.677(4) Angstrom, b = 13.064(4) Angstrom, c = 13.103(5) Angstrom, alpha = 110.14(3)degrees, alpha = 101.12(3)degrees, gamma = 96.61 degrees, V = 1959 Angstrom(3), and R = 0.0615 (R(w) = 0.1148). Crystals of TcCl2(py)(4) are tetragonal, with space group I4(1)/acd, Z = 8, and lattice parameters a = 15.641(4) Angstrom, c = 16.845(6) Angstrom, V = 4121 Angstrom(3), and R = 0.0373 (R(w) = 0.0290). Crystals of TcCl(tpy)(py)(2) are orthorhombic, with space group C222(1), Z = 4, and lattice parameters a = 9.359(3) Angstrom, b = 16.088(6) Angstrom, c = 18.367(4) Angstrom, V = 2765 Angstrom(3), and R = 0.0499 (R(w) = 0.0599).