Heterobimetallic Catalysis: E-Selective Semi-Hydrogenation via Spectroscopic and Theoretical Studies

The stepwise reaction development of a unique cooperative H2 activation reaction by heterobimetallic (NHC)M’-MCp(CO)2 complexes (NHC = N-heterocyclic carbene, M’ = Cu or Ag, M = Fe or Ru) to yield a catalytic E-selective semi-hydrogenation transformation of alkynes is presented. Late-late heterobimetallic complexes featuring polar metal-metal bonds, introduced by the Mankad group, are utilized and improved in this study. To design catalysts methodically, it is important to gain insight into the structure and function of these heterobimetallic complexes. Therefore, an experimental and a theoretical analysis is pursued first. Initially, the electronic structure and the function of heterobimetallic complexes are investigated using XANES, IR and Mössbauer spectroscopy. Bimetallic oxidative addition is studied extensively and the individual roles of the metals and ligands are elucidated. Reactivity is investigated further using theoretical calculations. A DFT analysis of a bimetallic oxidative addition and a bimetallic N2O activation reveal transition states and thermodynamic parameters comparable to experimental values. The key components of the catalysts are identified, and a series of novel complexes are synthesized by the Mankad group. A theoretical analysis of these complexes is presented and structural trends are deduced. The actual implementation of the heterobimetallic strategy into reaction development is carried out next. The initial catalyst screening is performed by thermodynamic energy calculations using DFT. A robust heterobimetallic catalyst that utilizes H2 under atmospheric pressure for E-selective semi-hydrogenation of alkynes, a rare selectivity mode in high yields, is developed. The mechanistic features of this reaction are also explored. Experimentally, the proposed mechanism is examined using model complexes. Theoretically, the transition state of the bimetallic H2 activation step, bond orders and fragment charges along the reaction coordinate, and key orbital interactions, are calculated. A kinetic study motivated by calculations from a collaborating group is also presented. Finally, the future directions inspired by this work are discussed. Preliminary work on the development of bifunctional catalytic reactions by trapping the intermediates of the initial hydrogenation reaction to introduce new substrates, is presented, revealing a gateway for many future developments.