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

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.