Dinitrosyl Iron Complex [K‐18‐crown‐6‐ether][(NO)₂Fe(^MePyrCO₂)]: Intermediate for Capture and Reduction of Carbon Dioxide

Abstract

Continued efforts are made for the utilization of CO 2 as a C 1 feedstock for regeneration of valuable chemicals and fuels. Mechanistic study of molecular (electro-/photo-)catalysts disclosed that initial step for CO 2 activation involves either nucleophilic insertion or direct reduction of CO 2. In this study, nucleophilic activation of CO 2 by complex [(NO) 2 Fe(m-Me Pyr) 2 Fe(NO) 2 ] 2À (2, Me Pyr = 3-methylpyrazolate) results in the formation of CO 2-captured complex [(NO) 2 Fe-(Me PyrCO 2)] À (2-CO 2 , Me PyrCO 2 = 3-methyl-pyrazole-1-carboxylate). Single-crystal structure, spectroscopic, reactivity, and computational study unravels 2-CO 2 as a unique intermediate for reductive transformation of CO 2 promoted by Ca 2+. Moreover, sequential reaction of 2 with CO 2 , Ca(OTf) 2 , and KC 8 established a synthetic cycle, 2 ! 2-CO 2 ! [(NO) 2 Fe(m-Me Pyr) 2 Fe(NO) 2 ] (1) ! 2, for selective conversion of CO 2 into oxalate. Presumably, characterization of the unprecedented intermediate 2-CO 2 may open an avenue for systematic evaluation of the effects of alternative Lewis acids on reduction of CO 2. Scheme 1. Electrochemical/metal-mediated process, synthetic cycle, and (electro)catalysis for CO 2-to-oxalate conversion.

Selectivity and activity modulation of electrocatalytic carbon dioxide reduction by atomically dispersed dual iron catalysts

“…2 Conclusively, both the strongest orbital interaction (Cs–O(CO 2 ) electrostatic coordination) and dual Fe 2 -site stabilization work in concert to contribute the smallest electrochemical activation energy (23.7 kJ mol −1 ). 2,7,8,21 The distinct dynamics of [Fe–μ-C(O)O–Fe] formation that occurs concurrently with [*COOCs]˙/[*COOCs] − formation supports the negative shift of the overpotential (273 mV) to drive 10 mA cm −2 of CO partial current density in CO 2 -saturated 0.5 M CsHCO 3 electrolyte (Fig. S11d†).…”

“…In qualitative terms, CO 2 activation is triggered by π backdonation (σ bond interaction) between a filled d z 2 orbital of the transient Fe I center and the empty CO 2 π* orbital (in plane) together with π bonding interaction (MLCT, metal-to-ligand charge transfer) involving a filled Fe I/II d xz /d yz orbital and the empty CO 2 π* orbital (out of plane), resulting in [CO 2 ]˙ − /[CO 2 ] 2− intermediates coordinating to the Fe II center. 2,6–9,50,51 Owing to the lowest hydration energy of Cs + cation (soft hydration shell), the partial desolvated Cs + cation could polarize CO 2 molecule through strong Cs + ⋯O 2 δ − C 2 δ + electrostatic interaction, which enables facile CO 2 adsorption on the Fe I site and then promotes electron transfer from the Fe I site to CO 2 for the formation of Fe II -bound [CO 2 ]˙ − /[CO 2 ] 2− intermediates. Meanwhile, the partial desolvated Cs + cation could stabilize Fe II -bound [CO 2 ]˙ − /[CO 2 ] 2− intermediates via Cs–O(CO 2 ) coordination interaction.…”

“…The sufficient overlap (strong Cs–O(CO 2 ) electrostatic coordination) between the Cs + 6s orbital and the [CO 2 ]˙ − /[CO 2 ] 2− empty π* orbital enhances the π-accepting ability of the activated CO 2 ligand, strengthening the interaction between the Fe II site and the activated CO 2 molecule for the thermodynamic stabilization of the [CO 2 ]˙ − /[CO 2 ] 2− intermediates. 7–9,52,53 In contrast, the sluggish CO 2 -to-CO electrokinetics in LiHCO 3 solution originates from the weak Li–O(CO 2 ) electrostatic coordination, attributed to the small size (2s orbital) and hard hydration shell of Li + cation. This speculation could be corroborated by time-resolved IR analysis of the [*COOM] − intermediate (Fig.…”

Selective Transformation of Nickel‐Bound Formate to CO or C−C Coupling Products Triggered by Deprotonation and Steered by Alkali‐Metal Ions

“…The formation of IV from complex II is remarkable, as the direct transformation of formate to oxalate so far has only been achieved via thermal decomposition, calcination or at elevated temperatures and high pressures from bulk CO 3 2− (via formate intermediates) but is unknown in coordination chemistry [40–45] . There are only a few examples for oxalate formation directly from CO 2 [22, 46–52] and one precedent case, where the reaction of formate derived CO 2 2− with CO 2 led to oxalate [19] …”

“…(via formate intermediates) but is unknown in coordination chemistry. [40][41][42][43][44][45] There are only af ew examples for oxalate formation directly from CO 2 [22,[46][47][48][49][50][51][52] and one precedent case, where the reaction of formate derived CO 2 2À with CO 2 led to oxalate. [19] Thus,h aving clarified the stoichiometry of the reaction between [L tBu NiOOCH], II,a nd K[N(SiMe 3 ) 2 ], the mechanism of oxalate formation shifted into our focus.Asdiscussed at the end of the last but one section ac onceivable intermediate is,f or instance,[ L tBu Ni I (OCO)C À ]K, A,n amely the product of an intramolecular electron transfer within [L tBu Ni II (CO 2 ) 2À ]K (see Figure 7).…”