Cyclo-P3 Complexes of Vanadium: Redox Properties and Origin of the 31P NMR Chemical Shift

The synthesis and characterization of two high-valent vanadium-cyclo-P-3 complexes, (nacnac)V(cyclo-P-3) (Ntolyl(2)) (1) and (nacnac)V(cyclo-P-3) (OAr) (2), and an inverted sandwich derivative, [(nacnac)V(Ntolyl(2))](2)(mu(2)-eta(3):eta(2)-cyclo-P-3) (3), are presented. These novel complexes are prepared by activating white phosphorus (P-4) with three-coordinate vanadium(II) precursors. Structural metrics, redox behavior, and DFT electronic structure analysis indicate that a [cyclo-P-3](3-) ligand is bound to a V(V) center in monomeric species 1 and 2. A salient feature of these new cyclo-P-3 complexes is their significantly downfield shifted (by similar to 300 ppm) P-31 NMR resonances, which is highly unusual compared to related complexes such as (Ar[Pr-i]N)(3)Mo(cyclo-P-3) (4) and other cyclo-P-3 complexes that display significantly upfield shifted resonances. This NMR spectroscopic signature was thus far thought to be a diagnostic property for the cyclo-P-3 ligand related to its acute endocyclic angle. Using DFT calculations, we scrutinized and conceptualized the origin of the unusual chemical shifts seen in this new class of complexes. Our analysis provides an intuitive rational paradigm for understanding the experimental P-31 NMR spectroscopic signature by relating the nuclear magnetic shielding with the electronic structure of the molecule, especially with the characteristics of metal-cyclo-P-3 bonding.