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 (11)B electric field gradient tensor relative to the (11)B(…31)P internuclear vector of (11)B-(31)P spin pairs. It is based on the anisotropic dephasing of (11)B spins in the dipolar field of (31)P nuclei via (11)B{(31)P} 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).