Synthesis and structure of trans-[O 2(Im) 4Tc]Cl·2H 2O, trans-[O 2(1-meIm) 4Tc]Cl·3H 2O and related compounds

Abstract

The synthesis, spectra and HPLC behavior of a series of trans-dioxotechnetium(V) complexes with imidazole and bidentate nitrogen ligands are reported. Crystals of trans-[O 2(Im) 4Tc]Cl·2H 2O were found to form in the monoclinic space group C2/ c with unit cell parameters a = 13.249 (3) Å, b = 11.239 (2) Å, c = 14.358 (3) Å and β = 115.56 (2)°. The cell volume is 1929 (1) Å 3 with Z = 4. The similar compound trans-[O 2(1-melm) 4Tc]Cl·3H 2O crystallized in the same space group with a = 20.980 (5) Å, b = 8.599 (3) Å, c = 16.105 (3) Å and β = 124.22 (2)°. The cell volume is 2402 (1) Å 3 with Z = 4. The unweighted R-factors for the two structures are 0.091 and 0.122, respectively. The TcO and TcN bond distances are essentially identical in the two structures and are 1.71 Å and 2.15 Å, respectively. The imidazole ligands are canted slightly by an average angle of 15° off a plane perpendicular to that of the technetium and nitrogens. Infrared absorbances assigned to the asymmetric OTcO stretch occur in the range 810–830 cm −1. The compounds are all diamagnetic due to the strong tetragonal distortion exerted by the oxo ligands. Proton NMR resonances are deshielded slightly relative to the free ligands. The imidazole complexes are unstable in aqueous solution and the bidentate amine complexes are unstable at low pH. HPLC capacity factors by reverse-phase ion-pair chromatography varied in an unexpected manner and, in general, decreased with increasing aliphatic chain length of the eluant anion.