Technetium and Rhenium in Five-Coordinate Symmetrical and Dissymmetrical Nitrido Complexes with Alkyl Phosphino-thiol Ligands. Synthesis and Structural Characterization

The reactivity of bulky alkylphosphino-thiol ligands (PSH) toward nitride-M(V, VI) (M = Tc/Re) precursors was investigated. Neutral five-coordinate monosubstituted complexes of the type [M(N)(PS)Cl(PPh3)] (Tc 1-4, Re 1-2) were prepared in moderate to high yields. It was found that these [M(N)(PS)Cl(PPh3)] species underwent ligandexchange reactions under mild conditions when reacted with bidentate mononegative ligands having soft donor atoms such as dithiocarbamates (NaLn) to afford stable dissymmetrical mixed-substituted complexes of the type [M(N)(PS)(Ln)] (Tc 5,8-10, Re 5-9) containing two different bidentate chelating ligands bound to the [M-N]2+ moiety. In these reactions, the dithiocarbamate replaced the two labile monodentate ligands (Cl and PPh3) leavingthe [M(N)(PS)]+ building block intact. In the above reactions, technetium and rhenium were found to behave in a similar way. Instead, under more drastic conditions, reactions of PSH with [M(N)Cl2(PPh3)2] gave a mixture of monosubstituted [M(N)(PS)Cl(PPh3)] and bis-substituted species [M(N)(PS)2] (Tc 11-14) in the case of technetium, whereas only monosubstituted [M(N)(PS)Cl(PPh3)] complexes were recovered for rhenium. All isolated products were characterized by elemental analysis, IR and multinuclear (1H, 13C, and 31P) NMR spectroscopies, ESI MS spectrometry, and X-ray crystal structure determination of the representative monosubstituted [Tc(N)(PStbu)Cl(PPh3)] (Tc4) and mixed-substituted [Re(N)(PScy)(L3)] (Re7) and [Re(N)(PSiso)(L4)] (Re9) complexes. The latter rhenium complexes represent the first example of a square-pyramidal nitrido Re species with the basal plane defined by a PS3 donor set. Monosubstituted [M(N)(PS)Cl(PPh3)] species bearing the substitution-inert [M(N)(PS)]+ moieties act as suitable building blocks proposed for the construction of new classes of dissymmetrical nitrido compounds with potential application in the development of essential and target specific 99mTc and 188Re radiopharmaceuticals for imaging and therapy, respectively.