Solid-state NMR studies for the determination of super(11)B 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 Theta specifying the orientation of the principal component of the super(11)B electric field gradient tensor relative to the super(11)B super(...) super(31)P internuclear vector of super(11)B- super(31)P spin pairs. It is based on the anisotropic dephasing of super(11)B spins in the dipolar field of super(31)P nuclei via super(11)B{ super(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 Theta , which can be measured with an estimated precision of plus or minus 5 degree , offers a clear spectroscopic distinction between classical Lewis-acid/base adducts and active Frustrated Lewis pairs (FLPs).