Solid-State NMR and Theoretical Studies Illuminate Lanthanum Borohydride C-H Borylation Catalysts Confined Within a Zeolite

Abstract

Zeolite-supported single-site lanthanum borohydride catalyzes C–H borylation of hydrocarbons while the related silica-supported complex is inactive under comparable conditions. The identical composition of support–La(BH₄)₂(THF)₂ sites in the two materials implies that the support bestows underlying structural features onto La that are required for bond activation catalysis, yet the nature of those effects, which could include confinement effects and/or electronic modulation of the site itself, remain to be identified. We used solid-state nuclear magnetic resonance (SSNMR) spectroscopy and molecular dynamics simulations with machine-learning potentials (ML-MD) to analyze the electronic and steric effects imparted by the faujasite support on precatalyst structure to correlate with catalytic activity. DFT calculations show that THF dissociates from La under the influence of confinement, leading to coordinatively unsaturated sites in the zeolite pores. Then, the La complex grafts on Brønsted acid sites (La^BAS) or isolated silanols (La^SiO) or remains physisorbed in the zeolite pores. Catalytic studies comparing compounds supported on faujasite zeolites containing or lacking BAS and/or silanols show that only the former complexes lead to active sites, ruling out confinement as the sole requirement for catalysis. The DFT calculations and ML-MD simulations also reveal that the surface-lanthanum coordination number is two (bidentate) for La^BAS, with the metal center forming long, flexible bonds to two of the oxygen bridging Si and Al, but only one oxygen (monodentate) for La^SiO. The structure-activity relationship identifies confined, BAS-grafted species as active sites and provides important guidance for the design of enhanced atom-efficient catalysts.