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

N-Triphos derivatives (NP ₃ ^R , R = alkyl, aryl) and asymmetric variants (NP ₂ ^R X ^R' , R' = alkyl, aryl, X = OH, NR ₂ , NRR') are an underexplored class of tuneable, tripodal ligands in relation to the coordination chemistry of Re and Tc for biomedical applications. Mixed-ligand approaches are a flexible synthetic route to obtain Tc complexes of differing core structures and physicochemical properties. Reaction of the NP ₃ ^Ph ligand with the Re(V) oxo precursor [ReOCl ₃ (PPh ₃ ) ₂ ] generated the bidentate complex [ReOCl ₃ (κ ² -NP ₂ ^Ph OH ^Ar )], which possesses an unusual AA'BB'XX' spin system with a characteristic second-order NMR lineshape that is sensitive to the bi- or tridentate nature of the coordinating diphosphine unit. The use of the asymmetric NP ₂ ^Ph OH ^Ar ligand resulted in the formation of both bidentate and tridentate products depending on the presence of base. The tridentate Re(V) complex [ReOCl ₂ (κ ³ -NP ₂ ^Ph O ^Ar )] has provided the basis of a new reactive "metal-fragment" for further functionalization in [3 + 2] mixed-ligand complexes. The synthesis of [3 + 2] complexes with catechol-based π-donors could also be achieved under one-pot, single-step conditions from Re(V) oxo precursors. Analogous complexes can also be synthesized from suitable ⁹⁹ Tc(V) precursors, and these complexes have been shown to exhibit highly similar structural properties through spectroscopic and chromatographic analysis. However, a tendency for the {M ^V O} ³⁺ core to undergo hydrolysis to the {M ^V O ₂ } ⁺ core has been observed both in the case of M = Re and markedly for M = ⁹⁹ Tc complexes. It is likely that controlling this pathway will be critical to the generation of further stable Tc(V) derivatives.