Following the In Situ Pathway of Photoactivated Cyclopentadienone–NHC Iron Complexes as Ammonia–Borane Dehydrocoupling Bifunctional Catalysts

Abstract

Carbonyl iron complexes, bearing cyclopentadienone/hydroxycyclopentadienyl ligands and N-heterocyclic carbene as noninnocent and ancillary ligands, are active as ammonia–borane dehydrocoupling catalysts under photoactivation conditions. Dehydrogenation occurs at room temperature and is complete (in NMR tube) after 2 h with 10 mol% of cationic complex 2, and after 4 h at lower catalytic loading (2 mol%) with the preactivated form 3. Reaction coproducts have been identified by ¹¹B-NMR. The reaction crudes reveal mixture of borazine and corresponding crosslinked polymer, together with other linear/branch polyaminoboranes for 1 and 2, and insoluble polyaminoboranes under even milder conditions (no irradiation) for 3. Scale-up of the reaction in a photoreactor then allows to quantify the catalysts activation, measuring CO release, as well as their productivity in term of H₂ produced. The in situ IR analysis sheds light on the different mechanistic pathways followed by the neutral 1 and the cationic 2 precatalysts. As a general statement, 2 is both easier to be activated and more efficient than the neutral counterpart 1. Preactivation of catalyst 2, replacing a CO with a more labile CH₃CN leading to complex 3, is exploited to improve the reaction speed and further explore the reaction mechanism.