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Activation, Deactivation and Reversibility in a Series of Homogeneous Iridium Dehydrogenation Catalysts
Author(s) -
Sharninghausen Liam S.,
Crabtree Robert H.
Publication year - 2017
Publication title -
israel journal of chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.908
H-Index - 54
eISSN - 1869-5868
pISSN - 0021-2148
DOI - 10.1002/ijch.201700048
Subject(s) - dehydrogenation , chemistry , catalysis , catalytic cycle , iridium , combinatorial chemistry , homogeneous , homogeneous catalysis , decomposition , photochemistry , organic chemistry , physics , thermodynamics
Dehydrogenation is a sustainable form of oxidation catalysis, as it avoids any primary oxidant and the waste that would accompany it. Homogeneous catalyst design for this reaction is delicate: the catalyst must have a sufficient H 2 affinity to abstract two hydrogen atoms from the substrate but not so much affinity as to fail to release them as free H 2 . N‐Heterocyclic carbene (NHC) ligands achieve this balancing act in a series of iridium catalysts based on the Ir(IMe) 2 motif (IMe=N,N′‐dimethylimidazol‐2‐ylidene). Catalyst design also needs to account for mechanistic points outside of the catalytic cycle itself. To enter the cycle, there is often an initial precatalyst activation process, and elucidating this can lead to optimization of both precatalyst and reaction conditions. To leave the cycle, we can have reversible formation of off‐cycle species as well as an irreversible catalyst deactivation step. In our Cp*Ir(IMe) 2 Cl precatalyst for dehydrogenative oxidation, Cp* is lost in the activation step. This suggested moving to a catalyst precursor lacking the Cp*, [Ir(IMe) 2 (CO) 2 ] + . Catalyst deactivation produces a series of unusual cluster cations such as [Ir 6 (IMe) 8 (CO) 2 H 14 ] 2+ . Among applications, dehydrogenative oxidation can be useful in upgrading of biomass glycerol by conversion to lactate, as a part of hydrogen borrowing chemistry and in reversible hydrogenation/dehydrogenation for H 2 storage.