Liberation of carbon monoxide from formic acid mediated by molybdenum oxyanions

Abstract

Multistage mass spectrometry experiments, isotope labelling and DFT calculations were used to explore whether selective decarbonylation of formic acid could be mediated by molybdate anions [(MoO₃)_x(OH)]⁻ (x = 1 and 2) via a formal catalytic cycle involving two steps. In step 1, both molybdate anions undergo gas-phase ion-molecule reactions (IMR) with formic acid to produce the coordinated formates [(MoO₃)_x(O₂CH)]⁻ and H₂O. In step 2, both coordinated formates [(MoO₃)_x(O₂CH)]⁻ undergo decarbonylation under collision-induced dissociation (CID) conditions to reform the molybdate anions [(MoO₃)_x(OH)]⁻ (x = 1 and 2), thus closing a formal catalytic cycle. In the case of [MoO₃(O₂CH)]⁻ an additional decarboxylation channel also occurs to yield [MoO₃(H)]⁻, which is unreactive towards formic acid. The reaction between [Mo¹⁸O₃(¹⁸OH)]⁻ and formic acid gives rise to [Mo¹⁸O₃(O₂CH)]⁻ highlighting that ligand substitution occurs without ¹⁸O/¹⁶O exchange between the coordinated ¹⁸OH ligand and HC¹⁶O₂H. The reaction between [(MoO₃)_x(OD)]⁻ (x = 1 and 2) and DCO₂H initially produces [(MoO₃)_x(OH)]⁻ (x = 1 and 2), indicating that D/H exchange occurs. DFT calculations were carried out to investigate the reaction mechanisms and energetics associated with both steps of the formal catalytic cycle and to better understand the competition between decarbonylation and decarboxylation, which is crucial in developing a selective catalyst. The CO and CO₂ loss channels from the monomolybdate anion [MoO₃(O₂CH)]⁻ have similar barrier heights which is in agreement with experimental results where both fragmentation channels are observed. In contrast, the dimolybdate anion is more selective, since the decarbonylation pathway of [(MoO₃)₂(O₂CH)]⁻ is both kinetically and thermodynamically favoured, which agrees with experimental observations where the CO loss channel is solely observed.