N-Centered Tripodal Phosphine Re(V) and Tc(V) Oxo Complexes:Revisiting a [3+2] Mixed-Ligand Approach br

N-Triphos derivatives (NP3R, R = alkyl, aryl) andasymmetric variants (NP2RXR ',R '= alkyl, aryl, X = OH, NR2,NRR ') are an underexplored class of tuneable, tripodal ligands inrelation to the coordination chemistry of Re and Tc for biomedicalapplications. Mixed-ligand approaches are aflexible synthetic routeto obtain Tc complexes of differing core structures andphysicochemical properties. Reaction of the NP3Phligand withthe Re(V) oxo precursor [ReOCl3(PPh3)2] generated the bidentatecomplex [ReOCl3(Kappa 2-NP2PhOHAr)], which possesses an unusualAA'BB'XX'spin system with a characteristic second-order NMRlineshape that is sensitive to the bi- or tridentate nature of thecoordinating diphosphine unit. The use of the asymmetricNP2PhOHArligand resulted in the formation of both bidentateand tridentate products depending on the presence of base. The tridentate Re(V) complex [ReOCl2(Kappa 3-NP2PhOAr)] has provided thebasis of a new reactive"metal-fragment"for further functionalization in [3 + 2] mixed-ligand complexes. The synthesis of [3 + 2]complexes with catechol-based pi-donors could also be achieved under one-pot, single-step conditions from Re(V) oxo precursors.Analogous complexes can also be synthesized from suitable99Tc(V) precursors, and these complexes have been shown to exhibithighly similar structural properties through spectroscopic and chromatographic analysis. However, a tendency for the {MVO}3+coreto undergo hydrolysis to the {MVO2}+core has been observed both in the case of M = Re and markedly for M =99Tc complexes. Itis likely that controlling this pathway will be critical to the generation of further stable Tc(V) derivatives.