31P Nuclear Magnetic Resonance Spectroscopy as a Probe of Thorium-Phosphorus Bond Covalency: Correlating Phosphorus Chemical Shift to Metal-Phosphorus Bond Order

We report the use of solution and solid-state P-31 Nuclear Magnetic Resonance (NMR) spectroscopy combined with Density Functional Theory calculations to benchmark the covalency of actinide-phosphorus bonds, thus introducing P-31 NMR spectroscopy to the investigation of molecular f-element chemical bond covalency. The P-31 NMR data for [Th(PH2)(Tren(TIPS))] (1, Tren(TIPS) = {N(CH2CH2NSiPr3i)(3)}(3-)), [Th(PH)(Tren(TIPS))][Na(12C4)(2)] (2, 12C4 = 12-crown-4 ether), [{Th(Tren(TIPS))}(2)(mu-PH)] (3), and [{Th(Tren(TIPS))}(2)(mu-P)][Na(12C4)(2)] (4) demonstrate a chemical shift anisotropy (CSA) ordering of (mu-P)(3-) > (=PH)(2-) > (mu-PH)(2-) > (-PH2)(1-) and for 4 the largest CSA for any bridging phosphido unit. The B3LYP functional with 50% Hartree-Fock mixing produced spin-orbit delta(iso) values that closely match the experimental data, providing experimentally benchmarked quantification of the nature and extent of covalency in the Th-P linkages in 1-4 via Natural Bond Orbital and Natural Localized Molecular Orbital analyses. Shielding analysis revealed that the P-31 delta(iso) values are essentially only due to the nature of the Th-P bonds in 1-4, with largely invariant diamagnetic but variable paramagnetic and spin-orbit shieldings that reflect the Th-P bond multiplicities and s-orbital mediated transmission of spin-orbit effects from Th to P. This study has permitted correlation of Th-P delta(iso) values to Mayer bond orders, revealing qualitative correlations generally, but which should be examined with respect to specific ancillary ligand families rather than generally to be quantitative, reflecting that P-31 delta(iso) values are a very sensitive reporter due to phosphorus being a soft donor that responds to the rest of the ligand field much more than stronger, harder donors like nitrogen.