Emerging Implications of the Concept of Hydricity in Energy‐Relevant Catalytic Processes

Abstract

The “hydricity” of a species refers to its hydride‐donor ability. Similar to how the pKa is useful for determining the extent of dissociation of an acid, the hydricity plays a vital role in understanding hydride‐transfer reactions. A large number of transition‐metal‐catalyzed processes involve the hydride‐transfer reaction as a key step. Among these, two key reactions—proton reduction to evolve H2 and hydride transfer to CO2 to generate formate/formic acid—represent a promising solution to build a sustainable and fossil‐fuel‐free energy economy. Therefore, it is imperative to develop an in‐depth relationship between the hydricity of transition‐metal hydrides and its influencing factors, so that efficient and suitable hydride‐transfer catalysts can be designed. Moreover, such profound knowledge can also help in improving existing catalysts, in terms of their efficiency and working mechanism. With this broad aim in mind, some important research has been explored in this area in recent times. This Minireview emphasizes the conceptual approaches developed thus far, to tune and apply the hydricity parameter of transition‐metal hydrides for efficient H2 evolution and CO2 reduction/hydrogenation catalysis focusing on the guiding principles for future research in this direction.

Cobalt Complexes Supported by Phosphinoquinoline Ligands for the Catalyzed Hydrosilylation of Carbonyl Compounds

“…Such results are in agreement with the involvement of a Co−H species. The Co−H complex supported by L should present a better hydride donor ability [31] than the one formed from [L’ 2 CoBr](PF 6 ) . As M−H complexes involving first row metals are generally less reactive than their noble congeners, the use of electron rich ligands are particularly important to enhance their reactivity [32] .…”

“…NMR spectra were recorded on a Bruker AC-300 SY spectrometer at 300 MHz for 1 H, 120 MHz for 31 P and 75 MHz for 13 C. Solvent peaks were used as internal references for 1 H and 13 C chemical shifts (ppm). 31 P{ 1 H} NMR spectra are relative to an 85 % H 3 PO 4 external reference. Unless otherwise mentioned, NMR spectra were recorded at 300 K. The abbreviations used to indicate the multiplicity of signals are: s (singlet), d (doublet), t (triplet), q (quadruplet), quint (quintuplet), m (multiple) or a combination of the above.…”

A Proton‐Responsive Pyridyl(benzamide)‐Functionalized NHC Ligand on Ir Complex for Alkylation of Ketones and Secondary Alcohols

“…The catalysts bearing these ligands effectively promote the reactions that involve hydride and proton management, such as (de)hydrogenation [1–11] and borrowing‐hydrogenation [12–16] . The nature (acid‐base properties) and the position/orientation of proton‐responsive unit, along with the metal‐hydricity, [17] are the key considerations to enable metal‐ligand cooperation (MLC) [18–20] for efficient hydrogen delivery/acceptance [21–24] . Among the various designs explored, the protic catalysts based on the reversible (de)protonation of −OH/=O, −CH 2 /=CH and −NH/=N motifs on pyridine, [25–27] bipyridine, [28–32] bipyrimidine [33] and azole‐pyridine/pyrimidine [34–37] are particularly effective.…”

Designing Catalytic Systems Using Binary Solvent Mixtures: Impact of Mole Fraction of Water on Hydride Transfer

“…Tabulating these hydricities based on their magnitude therefore provides a ranked list of hydride donors listed from most hydridic (small value, powerful donor) to least hydridic (large value, weak donor). The hydricities of metal hydrides have been shown to be significantly influenced by solvation. − The significant solvent dependence of hydricities is expected because the release of the hydride from the donor creates two charged species, LM + and H – , and the stabilities of these charge separated species is influenced substantially by differences in solvation. , A general trend of hydride donors becoming increasingly hydridic upon switching to more polar solvents has also been established. ,,, …”