Transition metal pincer catalysts for formic acid dehydrogenation: a mechanistic perspective

Abstract

The storage and transportation of hydrogen gas, a non-polluting alternative to carbon-based fuels, have always been challenging due to its extreme flammability. In this regard, formic acid (FA) is a promising liquid organic hydrogen carrier (LOHC), and over the past decades, significant progress has been made in dehydrogenating FA through transition metal catalysis. In this review, our goal is to provide a detailed insight into the existing processes to expose various mechanistic challenges associated with FA dehydrogenation (FAD). Specifically, methodologies catalyzed by pincer-ligated metal complexes were chosen. Pincer ligands are preferred as they provide structural rigidity to the complexes, making the isolation and analysis of reaction intermediates less challenging and consequently providing a better mechanistic understanding. In this perspective, the catalytic activity of the reported pincer complexes in FAD was overviewed, and more importantly, the catalytic cycles were examined in detail. Further attention was given to the structural modifications, role of additives, reaction medium, and their crucial effects on the outcome.

Keywords: decarboxylation; formic acid dehydrogenation; ligand cooperativity; pincer complexes; transition metal formate; transition metal hydride.

SCHEME 1

Formic acid as a hydrogen carrier for CO₂ hydrogenation/dehydrogenation.

FIGURE 1

Formic acid dehydrogenation cycle involving metal–hydride and metal–formate complexes.

SCHEME 2

Various pathways to activate formic acid (left) and decarboxylate the metal–formate complex (right).

FIGURE 2

Ruthenium pincer catalysts for the dehydrogenation of FA with base additives.

SCHEME 3

Proposed mechanism of (PNP)Ru-catalyzed FA dehydrogenation.

SCHEME 4

Ruthenium pincer catalysts for base-free FAD (left). Proposed mechanistic cycle involving ligand cooperation (right).

FIGURE 3

Bis(phosphine)-based non-aromatic (PNP) Ru pincer complexes applied in FAD.

SCHEME 5

FAD cycles by catalysts 5Aʹ (left) and 5A (right).

FIGURE 4

Acridine-based (PNP) ruthenium catalysts for solvent-less, additive-less FA decomposition.

SCHEME 6

Proposed mechanistic cycle of 6A-catalyzed dehydrogenation of FA.

FIGURE 5

Different iron pincer complexes for FA decomposition.

SCHEME 7

FAD catalytic cycles by iron pincers involving additive-assisted decarboxylation.

SCHEME 8

Tertiary amine-based (PNP)iron hydrides (above). Parallel catalytic cycles in FAD by (PPP)iron pincer complexes (stereochemistry is not confirmed for all the intermediates) (below).

FIGURE 6

Iridium pincer complexes studied in FAD and the (^tBuP^HNN^HNP)Ir trihydride immobilized on silica (above). Rhodium and cobalt pincer complexes studied in FAD (below).

SCHEME 9

Mechanistic cycle of (^tBuPNC)Rh(CO)-catalyzed FAD (left). Analogous rhodium pincer complexes attempted in FAD (right).

SCHEME 10

Proposed catalytic cycle of FA dehydrogenation by cobalt pincer 14A (above, cycle 10a). DFT calculated catalytic pathway by cobalt pincer 15A (below, cycle 10b).

FIGURE 7

Palladium and nickel pincer complexes for FAD.

SCHEME 11

Various heterobimetallic complexes of palladium and molybdenum/tungsten in FAD (left) and the proposed catalytic cycle (right).

SCHEME 12

Base-free formic acid dehydrogenation by the rhenium hydride complex.

FIGURE 8

Manganese pincer catalysts for dehydrogenating formic acid.

SCHEME 13

Proposed resting state of 21A-catalyzed FAD and efficiency comparison of various manganese pincer complexes (above). Plausible base-free formic acid dehydrogenation pathway by the (PNNNP)Mn complex (below).

SCHEME 14

Proposed FA dehydrogenation pathway by (PN^ArNNP)Mn complexes in the presence of lysine as additive (above). Manganese pincer catalysts and their reactivity in formic acid dehydrogenation in the presence of lysine (below).

SCHEME 15

Formic acid dehydrogenation by aluminum pincer complexes.