Defining active catalyst structure and reaction pathways from ab initio molecular dynamics and operando XAFS: Dehydrogenation of dimethylaminoborane by rhodium clusters

We present the results of a detailed operando XAFS and density functional theory (DFT)-based ab initio molecular dynamics (AIMD) investigation of a proposed mechanism of the dehydrogenation of dimethylaminoborane (DMAB) by a homogeneous Rh4 cluster catalyst. Our AIMD simulations reveal that previously proposed Rh structures, based on XAFS measurements, are highly fluxional, exhibiting both metal cluster and ligand isomerizations and dissociation that can only be accounted for by examining a finite temperature ensemble. It is found that a fluxional species Rh₄(H₂BNMe ₂)₈²⁺ is fully compatible with operando XAFS measurements, suggesting that this species may be the observed catalyst resting state. On the basis of this assignment, we propose a mechanism for catalytic DMAB dehydrogenation that exhibits an energy barrier of approximately 28 kcal/mol.