Heterolytic Activation of Hydrogen by Transition Metal Complexes

Reactions of Monomeric [1,2,4-(Me3C)3C5H2]2CeH and CO with or without H2: An Experimental and Computational Study

“…Heterolytic activation of dihydrogen generally is invoked when the removal of two electrons from the metal center is not energetically possible. Heterolytic cleavage of dihydrogen is well-known where dipolar dihydrogen adds to a M−X bond where X is more electronegative than M and is a key feature of the selective hydrogenation of polar bonds in the Noyori ruthenium catalyst . In the present case, the carbon atom of the η 2 -acyl accumulates electron density while the cerium atom carries a positive charge.…”

Reversible Coordination of H2 by a Distannyne

“…The interaction of hydrogen with transition metals is a key process in organometallic chemistry. − Furthermore, reversible oxidative addition and reductive elimination reactions of H 2 with transition metal complexes constitute fundamental steps in many catalytic cycles. , Such processes have been widely studied not only in organometallic chemistry but also in surface chemistry − and hydrogenase enzymes , and in connection with their relevance to chemical hydrogen storage. , In effect, the binding, storage, and release of H 2 under mild conditions are of major importance, − but despite the investigations of several H 2 carrier species, e.g., amine borane, Mg, , or Al , systems, there have been relatively few instances where reversible absorption and release of hydrogen has been effected under mild conditions for main-group compounds. − Two examples involve the use of metal-free frustrated Lewis pair systems − and antiaromatic boron-containing organic rings . In addition, several main-group compounds have been shown to react with H 2 under ambient conditions to yield hydrides. − These include low-valent group 13 and 14 element compounds, which feature frontier orbitals with small energy separations and suitable symmetry to react with H 2 and whose reactivity can mimic that of transition metal complexes .…”

Cascade Reductive Friedel–Crafts Alkylation Catalyzed by Robust Iridium(III) Hydride Complexes Containing a Protic Triazolylidene Ligand

“…Many cascade reactions involve (reversible) hydrogen transfer to produce reactive intermediates in situ via hydrogen borrowing processes. , Direct hydrogenation has been applied in tandem with other reactions as an efficient and atom economical method for advanced substrate functionalization. − The activation of the H–H bond by transition-metal complexes has been dominated in the field for a long time. In many classical examples in homogeneous catalysis, such transformations occur at the metal center by oxidative addition, homolytic or heterolytic cleavage, while the ligands remain unchanged over the course of the reaction . More recent discoveries introduced the concept of metal–ligand bifunctional catalysis, where ligands participate directly in the bond activation step and undergo a reversible chemical transformation. , Such cooperation between the metal as the hydride acceptor and the ligand as the internal Lewis base proceeds synergistically and substantially lowers the otherwise high H–H bond dissociation energy (435.8 kJ/mol) and the unattractively high p K a (∼35 in tetrahydrofuran (THF)), even though the acidity changes drastically once hydrides are formed. , …”

“…In many classical examples in homogeneous catalysis, such transformations occur at the metal center by oxidative addition, homolytic or heterolytic cleavage, while the ligands remain unchanged over the course of the reaction. 10 More recent discoveries introduced the concept of metal−ligand bifunctional catalysis, where ligands participate directly in the bond activation step and undergo a reversible chemical transformation. 11,12 Such cooperation between the metal as the hydride acceptor and the ligand as the internal Lewis base proceeds synergistically 13 and substantially lowers the otherwise high H−H bond dissociation energy (435.8 kJ/mol) 14 and the unattractively high pK a (∼35 in tetrahydrofuran (THF)), 15 even though the acidity changes drastically once hydrides are formed.…”