Oxalate production via oxidation of ascorbate rather than reduction of carbon dioxide

Abstract

In the previous publication, some of us reported the conversion of a copper(I) complex to a copper(II) oxalate complex, and claimed that this conversion involved a reduction of CO 2 to oxalate (C 2 O 4 2− ). Herein, we show that the oxalate is produced not by reduction of CO 2 , but by reaction of ascorbate with oxygen. We also present new results that explain in a more comprehensive way the behaviour of these copper compounds under O 2 and CO 2 .Selective reduction of carbon dioxide to C ≥2 compounds using homogeneous metal complexes is a challenging transformation. Only a limited number of examples have been reported over the past decades [1][2][3][4][5][6][7][8][9][10][11][12] . In contrast, there has been a vast increase in reported catalysts for selective CO 2 reduction to C 1 compounds [13][14][15] . Among the examples reported for the reductive coupling of CO 2 to oxalate is a dinuclear Cu complex introduced by some of us in 2014 (ref. 16 ). The in situ generated Cu(I) complex [Cu 2 (m-xpt) 2 ](PF 6 ) 2 (3) formed by reduction of the Cu (II) precursor (1) with sodium ascorbate generated an oxalatebridged dinuclear complex (4), proposed to occur via reductive coupling of atmospheric CO 2 (Fig. 1). Release of the oxalate by addition of mineral acids was described, potentially enabling stepwise conversion of CO 2 into oxalic acid using sodium ascorbate as a comparatively mild reductant. Interestingly, oxidation of ascorbic acid by transition metal compounds, especially those of copper, has been well-known for more than a century 17,18 . Since then, the reaction mechanisms for such oxidations have been intensely studied [18][19][20][21][22] . More specifically, oxidative degradation of ascorbic acid by (a) inorganic oxidants (sodium periodate 23 , sodium hypoiodite 24 ); (b) oxygen 25,26 ; and (c) O 2 in the presence of Gd 27,28 , Co 27 , Pd 29 , Pt 29 , Cd 30 , Fe 31 , or Cu 32 compounds is reported to yield oxalate as a degradation product (see Supplementary Fig. 21 for a typical reaction sequence).We now report that the true origin of the oxalate in the communication published in 2014 is not CO 2 , as it was described, but oxidative degradation of sodium ascorbate.

Revisiting Reduction of CO2 to Oxalate with First-Row Transition Metals: Irreproducibility, Ambiguous Analysis, and Conflicting Reactivity

“…Besides irreproducibility and ambiguous analysis , conflicting reactivity causing the formation of oxalate without incorporation of CO 2 can be misleading and constitutes another major pitfall for investigations on the reductive dimerization of CO 2 . , This is showcased by the third selected example based on Cu α-ketocarboxylate complexes. In 2014, Fujisawa and co-workers reported the unusual formation of a dinuclear oxalate complex 3 ligated by Tp i Pr, i Pr which was isolated from the reaction of [Cu­(Tp i Pr, i Pr )­(O 2 CC­(O)­CH­(CH 3 ) 2 )] ( 9 ) with air, O 2 , or CO 2 /O 2 .…”

“…72 Similarly, in a joint cooperation with Maverick and co-workers, the oxidative decomposition of ascorbate, utilized as reductant for the formation of a Cu I species, was identified as the source of the observed oxalate. 66 In this case, isolation of a dinuclear Cu oxalate complex in combination with a misleading difference IR spectrum and air contamination of reactions under CO 2 atmosphere caused misinterpretation of the experimental data. Such conflicting reactions provide the basis for incorrect interpretation of data and exacerbate the unsteady progress within this field.…”

“…Likewise, the reaction mechanism for C–C bond formation appears to be rather specific for a respective system. Moreover, analysis of oxalate in the presence of additional CO 2 reduction products via NMR or IR spectroscopy can be challenging and common analytic techniques for oxalate quantification − have scarcely been adopted within this field. These obstacles hamper systematic development of novel and refinement of existing protocols for the coupling of CO 2 .…”

“…Hence, the small number of reports on CO 2 reductive coupling compared to well-established CO 2 reduction to CO, formic acid, and MeOH − comes as no surprise. Further complication arises from undesired side reactions and irreproducibility of experimental results. ,, As an example, in situ analysis by Knope et al on the formation of a Nd-based oxalate coordination polymer suggested that oxalate formation under hydrothermal conditions arose from oxidative decomposition of 2,3-pyrazinedicarboxylic acid without previously suggested incorporation of CO 2 . Similarly, in a joint cooperation with Maverick and co-workers, the oxidative decomposition of ascorbate, utilized as reductant for the formation of a Cu I species, was identified as the source of the observed oxalate .…”

Towards Sustainable Oxalic Acid from CO2and Biomass

“…In 2014, Maverick and co‐workers presented a copper complex for CO 2 transformation to oxalate but have retracted that article recently [154,155] …”