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

Abstract

We report the use of solution and solid-state ³¹P Nuclear Magnetic Resonance (NMR) spectroscopy combined with Density Functional Theory calculations to benchmark the covalency of actinide-phosphorus bonds, thus introducing ³¹P NMR spectroscopy to the investigation of molecular f-element chemical bond covalency. The ³¹P NMR data for [Th(PH₂)(Tren^TIPS)] (<b>1</b>, Tren^TIPS = {N(CH₂CH₂NSiPrⁱ₃)₃}^3-), [Th(PH)(Tren^TIPS)][Na(12C4)₂] (<b>2</b>, 12C4 = 12-crown-4 ether), [{Th(Tren^TIPS)}₂(μ-PH)] (<b>3</b>), and [{Th(Tren^TIPS)}₂(μ-P)][Na(12C4)₂] (<b>4</b>) demonstrate a chemical shift anisotropy (CSA) ordering of (μ-P)^3- > (═PH)^2- > (μ-PH)^2- > (-PH₂)^1- and for <b>4</b> the largest CSA for any bridging phosphido unit. The B3LYP functional with 50% Hartree-Fock mixing produced spin-orbit δ_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 <b>1</b>-<b>4</b> via Natural Bond Orbital and Natural Localized Molecular Orbital analyses. Shielding analysis revealed that the ³¹P δ_iso values are essentially only due to the nature of the Th-P bonds in <b>1</b>-<b>4</b>, 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 δ_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 ³¹P δ_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.