Solid-State NMR as a Spectroscopic Tool for Characterizing Phosphane–Borane Frustrated Lewis Pairs

Abstract

Frustrated Lewis pair (FLP) chemistry has provided a new strategy for small molecule binding and/or catalytic activation. It is based on the cooperative reaction behavior of Lewis acid and Lewis base centers that are in close proximity to each other (e.g., within the same molecule) but cannot form a direct bond because of geometrical constraints. The most prominent FLPs are based on intramolecular phosphane-borane adducts, whose catalytic properties can be tailored over wide ranges of reactivity and selectivity. For the structural and chemical design of such systems, a fundamental understanding needs to be developed on how structure, dynamics and covalent interactions between the Lewis centers influence the reactivity profile. Advanced solid-state nuclear magnetic resonance (NMR) spectroscopic techniques afford new opportunities for addressing this challenge. Following a general introduction into the fundamentals of NMR spectroscopy, this review discusses the different types of internal interactions - magnetic shielding, nuclear electric quadrupolar coupling, indirect spin-spin interactions, and "through-space" dipole-dipole couplings - influencing NMR spectra in the solid state. As discussed in detail, each type of interaction bears specific informational content with regard to structural issues in FLP chemistry. One of the most attractive features of solid-state NMR is the possibility of tailoring the effective Hamiltonian by manipulations in either physical space or spin space. Using such "decoupling" or "recoupling" techniques certain types of interactions can be selectively turned off for spectral simplification or turned on for selective evaluation. The present review summarizes the most important selective averaging techniques that have found applications in the characterization of FLPs. In a second step the interaction parameters need to be connected with structure and bonding information. As illustrated in this chapter, ab initio calculations using density functional theory (DFT) methods have become indispensable for this task. Based on this comprehensive strategy including advanced NMR methodology, computer simulations, and ab initio calculations, the present review illustrates the utility of (31)P and (11)B NMR chemical shifts, (11)B electric field gradient tensors, and (31)P-(11)B indirect and direct dipole-dipole interactions for characterizing intramolecular borane-phosphane FLPs, illustrating the potential of this method to (1) quantify the extent of boron-phosphorus bonding interactions (and hence the "degree of frustration") and (2) reveal specific structural details (i.e., boron-phosphorus distances and other local geometry aspects) relating to the catalytic activities of these exciting materials.