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

Abstract

The synthesis and characterization of two high-valent vanadium-cyclo-P3 complexes, (nacnac)V(cyclo-P3)(Ntolyl2) (1) and (nacnac)V(cyclo-P3)(OAr) (2), and an inverted sandwich derivative, [(nacnac)V(Ntolyl2)]2(μ2-η(3):η(2)-cyclo-P3) (3), are presented. These novel complexes are prepared by activating white phosphorus (P4) with three-coordinate vanadium(II) precursors. Structural metrics, redox behavior, and DFT electronic structure analysis indicate that a [cyclo-P3](3-) ligand is bound to a V(V) center in monomeric species 1 and 2. A salient feature of these new cyclo-P3 complexes is their significantly downfield shifted (by ∼300 ppm) (31)P NMR resonances, which is highly unusual compared to related complexes such as (Ar[(i)Pr]N)3Mo(cyclo-P3) (4) and other cyclo-P3 complexes that display significantly upfield shifted resonances. This NMR spectroscopic signature was thus far thought to be a diagnostic property for the cyclo-P3 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 (31)P NMR spectroscopic signature by relating the nuclear magnetic shielding with the electronic structure of the molecule, especially with the characteristics of metal-cyclo-P3 bonding.

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“…99 For example, the 31 P NMR chemical shift of triple bonded phosphorous has been found to correlate very well with the σ–π* energy gap as well as we also reported the significance of energy gap of coupling orbitals in determining the chemical shielding of phosphorous nuclei in metal- cyclo -P 3 complexes. 98 The very small differences in energy gaps (Δ E in Fig. 10), however, cannot account for the qualitatively different paramagnetic shielding effects of σ–π* coupling in the studied systems.…”

“…In a recent study we discussed the basic relationships of such density fluctuations with MOs, shielding tensors and chemical shifts and, by scrutinizing vanadium- and molybdenum- cyclo -P 3 complexes, we showed how a thorough analysis of shielding tensors of heavy nuclei can lead to a conceptual understanding of the bonding characteristics of unusual transition metal–ligand interactions. 98 …”

“…As we demonstrated recently, 98 the thorough analysis of the paramagnetic contribution to the shielding can be challenging, because one has to decompose the sum of a great number of individual magnetic fields originating from occupied–unoccupied orbital pairs that couple through the angular momentum operator. In addition, the magnetic fields generated by what is most spontaneously understood as electron density fluctuations are hard to visualize or imagine.…”

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“…Structural investigations revealed the cyclo-P 3 ligand adopted a triply negatively charged state. 37 In contrast to the analogous [HMo(κ 2 -i PrCvNAr){N( i Pr)Ar} 2 ] complex reported by Cummins 39 or the in situ generated vanadium(II) complex reported by Cloke, 40 44 failed to activate dinitrogen. 37 It is worth mentioning that the stoichiometry of white phosphorous used in this transformation is crucial.…”

“…37 It is worth mentioning that the stoichiometry of white phosphorous used in this transformation is crucial. Structural investigations revealed the cyclo-P 3 ligand adopted a triply negatively charged state.…”