Rhenium(V) and Technetium(V) Nitrido Complexes with Mixed 2 Tridentate pigreco-Donor and Monodentate pigreco-Acceptor Ligands

Mixed-ligand [M(N)(SNS)(PPh3)] complexes (M = Tc, Re) (1, 2) were prepared by reaction of the precursor [M(N)Cl2(PPh3)2] with ligand 2,2′-dimercaptodiethylamine [H2SNS = NH(CH2CH2SH)2] in refluxing dichloromethane/ethanol mixtures. In these compounds, 2,2′-dimercaptodiethylamine acts as a dianionic tridentate chelating ligand bound to the [M≡N]2+ group through the two π-donor deprotonated sulfur atoms and the protonated amine nitrogen atom. Triphenylphosphine completes the coordination sphere, acting as a monodentate ligand. [M(N)(NS2)(PPh3)] complexes can assume two different isomeric forms depending on the syn and anti orientations of the hydrogen atom bound to the central nitrogen atom of the SNS ligand with respect to the M≡N moiety. X-ray crystallography of the syn isomer of complex 2 demonstrated that it has a distorted trigonal bipyramidal geometry with the nitrido group and the two sulfur atoms defining the equatorial plane, the phosphorus atom of the monophosphine and the protonated amine nitrogen of the tridentate ligand spanning the two reciprocal trans positions along the axis perpendicular to the trigonal plane. Synthesis of the analogous Tc derivatives with tris(2-cyanoethyl)phosphine, [Tc(N)(SNS)(PCN)] [(PCN = P(CH2CH2CN)3], required the preliminary preparation of the new precursor [Tc(N)(PCN)2Cl2]2 (3), which was prepared by reacting [n-NBu4][Tc(N)Cl4] with a high excess of PCN. The crystal structure of compound 3 consists of a noncrystallographic centrosymmetric dimer of Tc(V) nitrido complexes having an octahedral geometry. In this arrangement, the apical positions are occupied by two tris(2-cyanoethyl)phosphine groups and the equatorial positions by the nitrido group whereas the two Cl– anions and one cyano ligand belong to the other octahedral component of the dimer. By reacting the new precursor [Tc(N)(PCN)2Cl2]2 with the ligand H2SNS the complex [Tc(N)(SNS)(PCN)] (5) was finally obtained in acetonitrile solution. The new Tc(III) complex trans-[Tc(PCN)2Cl4][n-NBu4] (4) was also isolated from the reaction solution used for preparing complex 3 as side product and characterized by X-ray diffraction. The crystal structure of 4 consists of independent trans-[TcCl4(PCN)2]− anions situated on crystallographic centers of symmetry and tetrabutylammonium cations in general positions.