Binding, activation, and oxidative addition of dihydrogen using Ir-pincer systems and implications in hydrogenation catalysis

includes bibliographical references and index

PhD; 2023; Inorganic and Physical Chemistry

Binding of unreactive small molecules, such as H2, N2, CO2, CH4, etc. on a transition metal
center and their subsequent activation are fundamental problems that have been extensively
studied in organometallic chemistry for the past four decades. Towards this direction,
activation of the H–H bond in H2, has been a topical area of research interest. The ground
breaking discovery of Kubas’ complex, [W(η2
–H2)(CO)3(PR3)2] in 1984,
2
laid the foundation
in treating other electronically saturated molecules with strong sigma bonds such as the C–H
bond in alkanes in a similar manner for their activation and functionalization. From the
discovery of the first dihydrogen complex to a few recently reported sigma methane complexes,
the advancement in the binding and activation of either H2 or CH4 using metal complexes is
not only restricted to academic research pursuits but also to pave the way for futuristic concepts
such as “hydrogen economy” or “methane economy”.
In the pursuit of developing a single organometallic system (catalyst) showcasing versatile
reactivity towards H2, both in terms of binding and activation, an iridium pincer complex with
PPh3 as a co-ligand, [Ir(H)(Cl)(iPr)4(POCOP)(PPh3)] has been synthesized. Two different
complexes derived from the same iridium pincer backbone: a weakly coordinated iridium
dinitrogen cationic complex, trans-[Ir(H)(N2)
(iPr)4(POCOP)(PPh3)]+
,
3
and a four coordinate,
neutral iridium complex, [Ir(iPr)4(POCOP)(PPh3)] ,4 with two vacant sites favoring controlled
binding of H2 and the subsequent scission of the H–H bond on the iridium center, has been
elucidated. An unusual intramolecular H-atom site exchange occurring between H2 and the
hydride ligands trans-disposed to one another, in a trans-[Ir(H)(η2
–H2)
(iPr)4(POCOP)(DMAP)]+
has been investigated in detail.
5
In addition to the binding of H2 to give a σ-H2 complex and its cleavage to form a classical
dihydride complex, attempts were made to study the systematic elongation of the H–H bond
along the continuum for the oxidative addition of H2 to a metal center. A study of an
equilibrium between cis-[Ir(H)(η2
–H2)
(iPr)4(POCOP)(PPh3)]+ to its tautomeric form
[Ir(H)3
(iPr)4(POCOP)(PPh3)]+ as a function of pressure of H2, has also been investigated.6 These
studies were carried out to design and develop better performance catalysts for hydrogenation.
The results of these studies will be presented.