Ligand-induced substrate steering and reshaping of [Ag2(H)]+ scaffold for selective CO2 extrusion from formic acid

Abstract

Metalloenzymes preorganize the reaction environment to steer substrate(s) along the required reaction coordinate. Here, we show that phosphine ligands selectively facilitate protonation of binuclear silver hydride cations, [LAg2(H)]+ by optimizing the geometry of the active site. This is a key step in the selective, catalysed extrusion of carbon dioxide from formic acid, HO2CH, with important applications (for example, hydrogen storage). Gas-phase ion-molecule reactions, collision-induced dissociation (CID), infrared and ultraviolet action spectroscopy and computational chemistry link structure to reactivity and mechanism. [Ag2(H)]+ and [Ph3PAg2(H)]+ react with formic acid yielding Lewis adducts, while [(Ph3P)2Ag2(H)]+ is unreactive. Using bis(diphenylphosphino)methane (dppm) reshapes the geometry of the binuclear Ag2(H)+ scaffold, triggering reactivity towards formic acid, to produce [dppmAg2(O2CH)]+ and H2. Decarboxylation of [dppmAg2(O2CH)]+ via CID regenerates [dppmAg2(H)]+. These gas-phase insights inspired variable temperature NMR studies that show CO2 and H2 production at 70 °C from solutions containing dppm, AgBF4, NaO2CH and HO2CH.

Synthesis, Characterization, and Reactivity of the Group 11 Hydrido Clusters [Ag6H4(dppm)4(OAc)2] and [Cu3H(dppm)3(OAc)2]

“…[1][2][3][4][5][6][7][8][9][10][11][12][13][14] In contrast, only a handful of examples of silver-catalyzed hydrogenation and hydrosilylation are known. [15][16][17][18][19][20] For example, AgOTf has been shown to catalyze the hydrosilylation of aryl aldehydes in the presence of Me 2 PhSiH and PEt 3 . 16 Similarly, the semihydrogenation of alkynes by a heterobimetallic Ag/Ru catalyst has been reported.…”

Models Facilitating the Design of a New Metal‐Organic Framework Catalyst for the Selective Decomposition of Formic Acid into Hydrogen and Carbon Dioxide

“…Although much progress has since been made in constructing a wide range of MOF catalysts, the vacuum like environment within MOFs suggested to us a tantalizing new concept for the design of novel MOF based catalysts, where gas‐phase studies are used to examine the likely reactivity of a catalytic metal site within a MOF (Scheme ). The use of gas‐phase models seemed plausible given that our previous reaction‐mechanisms approach, where gas‐phase studies using multistage mass spectrometry experiments (MS n ) in an ion trap are blended with DFT calculations, was successfully applied to the design of new catalysts from the ground up and the invention of new reactions for use in organic synthesis . In the former studies, through a sequence of iterations in which different ligands and metal sites were evaluated, we developed a series of ligated binuclear coinage metal hydride cationic catalysts for the selective decarboxylation of formic acid (Scheme a, b), a reaction of considerable interest for the use of formic acid in hydrogen storage applications .…”

“…In all cases a two‐step catalytic cycle operates (Scheme a) in which the metal hydride complex, 1 , reacts in Step I with formic acid to liberate hydrogen and form a coordinated metal formate, 2 , which in Step II liberates CO 2 under conditions of collision‐induced dissociation (CID). For these first generation catalysts the best ligand for the silver hydride was 1,1‐bis(diphenylphosphino)methane (dppm) to give complex 1 a (Scheme b), and solution phase studies confirmed the liberation of H 2 and CO 2 at a relatively low temperature of 65 °C . Although the exact nature of the stoichiometry of the solution phase catalysts is unknown, subsequent gas phase studies highlighted that tetranuclear silver hydride cluster complexes were unreactive towards formic acid .…”

How to Translate the [LCu2(H)]+‐Catalysed Selective Decomposition of Formic Acid into H2 and CO2 from the Gas Phase into a Zeolite.

“…Here we use a combination of theory and gas‐phase experiments to establish the requirements to “heterogenize” our recently designed phosphine ligated coinage metal hydrides for the catalytic and selective gas‐phase decomposition of formic acid into carbon dioxide and hydrogen [Eq. ], a reaction crucial for the use of formic acid in hydrogen storage applications …”

“…Decarboxylation of [(L)M1M2(O 2 CH)] + by collision‐induced dissociation (CID) regenerates the catalyst [(L)M1M2(H)] + (Scheme , step II). The nature of the bisphosphine ligand, metal and stoichiometry were all found to play key roles, with [(dppm)Cu 2 (H)] + (dppm=1,1‐bis(diphenylphosphino)‐methane) emerging as the most effective catalyst …”