CO₂ Reduction by Lanthanum Borohydride Complexes and Hydroboration Reactivity

Although lanthanide borohydride precursors are useful building blocks to access molecular lanthanide complexes, the nucleophilic reactivity of the attached [BH₄]^- unit has not been thoroughly investigated. We herein report the reaction of [La(Cp^ttt)₂(BH₄)] (1, Cp^ttt = 1,2,4-tris(tert-butyl)cyclopentadienyl) toward CO₂, leading to the formation of a formate complex, which exists as an equilibrium mixture between the dimeric and monomeric forms in aromatic solvents. ¹H DOSY NMR experiments allowed a convenient analysis of the reaction mixture and provided an estimation of the hydrodynamic radii for both species. Hydroborane reagents (BH₃, HBPin, H(9-BBN)) further react with the formate complex, leading to the formation of borylated methanol and the corresponding lanthanide boroxide complexes. Preliminary investigations were performed to investigate the viability of 1 as a precatalyst in the hydroboration of CO₂. This work highlights the potential application of readily available rare-earth borohydride complexes in the transformation and valorization of CO₂.