Gas-Phase Reactions of Carbon Dioxide with Copper Hydride Anions Cu₂H₂^–: Temperature-Dependent Transformation

Abstract

The hydrogenation of carbon dioxide into value-added chemicals is of great importance for CO 2 recycling. However, the underlying mechanism of CO 2 hydrogenation remains elusive owing to the lack of experimental evidence for the formation of the C−H bond. Herein, the gas-phase reaction of copper hydride anion Cu 2 H 2 − with CO 2 at variable temperatures (∼300−560 K) was investigated. Metal hydrides are the ideal models to study the nature of C−H bond formation in CO 2 hydrogenation, while the related studies are scarcely reported, particularly for the hydrogenation reactions at temperatures above 300 K. The generation of formate (HCO 2 − ) attached on product CuH 2 CO 2 − was identified by temperature-dependent mass spectrometric experiments and density functional theory calculations. Temperature played crucial roles to fine-tune the product selectivity, from Cu 2 H 2 CO 2 − that dominates the room-temperature reaction into CuH 2 CO 2 − at elevated temperatures. The nature behind the temperature-dependent product selectivity and the mechanism of CO 2 hydrogenation has been interpreted by using theoretical calculations. The combined experimental and computational studies have provided solid evidence for the formation of formate attached in CuH 2 CO 2 − .

Release of Formic Acid from Copper Formate: Hydride, Proton‐Coupled Electron and Hydrogen Atom Transfer All Play their Role

“…Reaction (5) is very interesting since the reverse reaction is formate formation, which so far was not observed with Cu(II) hydrides in the gas phase. O'Hair and co‐workers demonstrated it with Cu(I) species while He and co‐workers used Cu 2 H 2 − . With the reduced copper species, formate formation proceeds nearly barrierless, as corroborated in our calculated PES for the reverse of reaction (8), proceeding through TS12 in Figure c.…”

“…Although the mechanism for the transformation of carbon dioxide to formate with copper hydride is well understood, it is not clear how formic acid is ultimately released. Herein, we show how formic acid is formed in the decomposition of the copper formate clusters Cu(II)(HCOO) 3 À and Cu(II) 2 (HCOO) 5 À . Infrared irradiation resonant with the antisymmetric CÀ O stretching mode activates the cluster, resulting in the release of formic acid and carbon dioxide.…”

“…Copper formate has been chosen since copper is a versatile carboxylation catalyst, [8] and numerous gas phase studies have addressed various aspects of copper catalysis. [5,6,9] Surfacedeposited size-selected copper clusters have even been shown to catalyze the conversion of methane to methanol. [10] Gas phase models of CO 2 functionalization include studies with CO 2 À (H 2 O) n as well as Grignard-type metal complexes.…”

Decomposition of Copper Formate Clusters: Insight into Elementary Steps of Calcination and Carbon Dioxide Activation

“…Such a situation is typical for a hydride transfer [33] along with the fact that the CÀ HÀ Cu atoms are not oriented linearly in the transition state. [18] As all decarboxylation reactions observed here are endothermic, the reverse reactions with copper centers leading to formate seem to be a general feature in the activation of CO 2 by copper(I) hydrides. For the reaction of Cu(I)(HCO 2 )H À !Cu(I)H 2 À + CO 2 , calculations at the BMK/def2TZVP predict a transition state (TS2) for À leading to a) Cu(I)H 2 À and b) Cu(I) 2 H 3 À , respectively, calculated at the B3LYP/def2TZVP level of theory.…”

“…Efficient methanol synthesis was demonstrated on size-selected copper clusters deposited on aluminum oxide films. [17][18][19] Hydrated Cu 2 + clusters Cu 2 + (H 2 O) n undergo the charge separation reaction [20] CuOH + (H 2 O) m + H 3 O + (H 2 O) at a critical size [21] of n = 6, which is higher for copper than for most transition metals. [16] Copper hydride anions show reactivity towards CO 2 , leading to formate formation.…”