Solid-state NMR studies for the determination of 11B electric field-gradient tensor orientations in P/B Frustrated Lewis Pairs and related systems

A solid state NMR method is described for measuring the angle Θ specifying the orientation of the principal component of the 11B electric field gradient tensor relative to the 11B…31P internuclear vector of 11B–31P spin pairs. It is based on the anisotropic dephasing of 11B spins in the dipolar field of 31P nuclei via 11B{31P} Rotational Echo DOuble Resonance (REDOR) experiments. The method is applied to four solid borane–phosphane compounds related to Frustrated Lewis Pair (FLP) chemistry. Results determined by numerical line shape simulations are found in excellent agreement with theoretically calculated values using advanced DFT methods. The angle Θ, which can be measured with an estimated precision of ±5°, offers a clear spectroscopic distinction between classical Lewis-acid/base adducts and active Frustrated Lewis pairs (FLPs). [Display omitted] •Orientation effects in spectra measured by 11B{31P} REDOR.•Experimental and theoretical determination of the orientation between dipolar vector and quadrupole tensor.•Classification of Frustrated Lewis pairs based on this orientation.