A reconstructed porous copper surface promotes selectivity and efficiency toward C2 products by electrocatalytic CO2 reduction

Electrocatalytic synthesis of multicarbon (C₂₊) products from CO₂ reduction suffers from poor selectivity and low energy efficiency. Herein, a facile oxidation–reduction cycling method is adopted to reconstruct the Cu electrode surface with the help of halide anions. The surface composed of entangled Cu nanowires with hierarchical pores is synthesized in the presence of I⁻, exhibiting a C₂ faradaic efficiency (FE) of 80% at −1.09 V vs. RHE. A partial current density of 21 mA cm⁻² is achieved with a C₂ half-cell power conversion efficiency (PCE) of 39% on this electrode. Such high selective C₂ production is found to mainly originate from CO intermediate enrichment inside hierarchical pores rather than the surface lattice effect of the Cu electrode.

The Cu electrode surface is reconstructed by a halide anion assisted method for promoting CO₂ reduction.     

Soluble,crystalline, and thermally stable alkali CO2− and carbonite (CO22−) clusters supported by cyclic(alkyl)(amino) carbenes

The mono- and dianions of CO₂ (i.e., CO₂⁻ and CO₂²⁻) have been studied for decades as both fundamentally important oxycarbanions (anions containing only C and O atoms) and as critical species in CO₂ reduction and fixation chemistry. However, CO₂ anions are highly unstable and difficult to study. As such, examples of stable compounds containing these ions are extremely limited; the unadulterated alkali salts of CO₂ (i.e., MCO₂, M₂CO₂, M = alkali metal) decompose rapidly above 15 K, for example. Herein we report the chemical reduction of a cyclic (alkyl)(amino) carbene (CAAC) adduct of CO₂ at room temperature by alkali metals, which results in the formation of CAAC-stabilized alkali CO₂⁻ and CO₂²⁻ clusters. One-electron reduction of CAAC–CO₂ adduct (1) with lithium, sodium or potassium metal yields stable monoanionic radicals [M(CAAC–CO₂)]_n (M = Li, Na, K, 2–4) analogous to the alkali CO₂⁻ radical, and two-electron alkali metal reduction affords dianionic clusters of the general formula [M₂(CAAC–CO₂)]_n (5–8) with reduced CO₂ units which are structurally analogous to the carbonite anion CO₂²⁻. It is notable that crystalline clusters of these alkali–CO₂ salts may also be isolated via the “one-pot” reaction of free CO₂ with free CAAC followed by the addition of alkali metals – a process which does not occur in the absence of carbene. Each of the products 2–8 was investigated using a combination of experimental and theoretical methods.

The direct chemical reduction of CAACCO2 adducts by alkali metals to yield multinuclear clusters is reported. The mono- and dianions of CO₂ have been studied for decades and are fundamentally important oxycarbanions and critical species in CO₂ fixation chemistry.     

An amino acid based system for CO2 capture and catalytic utilization to produce formates

Herein, we report a novel amino acid based reaction system for CO₂ capture and utilization (CCU) to produce formates in the presence of the naturally occurring amino acid l-lysine. Utilizing a specific ruthenium-based catalyst system, hydrogenation of absorbed carbon dioxide occurs with high activity and excellent productivity. Noteworthy, following the CCU concept, CO₂ can be captured from ambient air in the form of carbamates and converted directly to formates in one-pot (TON > 50 000). This protocol opens new potential for transforming captured CO₂ from ambient air to C1-related products.

A novel amino acid based reaction system for CO₂ capture and utilization (CCU) to produce formates is presented applying a ruthenium-based catalyst. Noteworthy, CO₂ can be captured from ambient air and converted to formates in one-pot (TON > 50 000).     

Facilitated inversion complicates the stereodynamics of an SN2 reaction at nitrogen center

Bimolecular nucleophilic substitution (S_N2) reactions at carbon center are well known to proceed with the stereospecific Walden-inversion mechanism. Reaction dynamics simulations on a newly developed high-level ab initio analytical potential energy surface for the F⁻ + NH₂Cl nitrogen-centered S_N2 and proton-transfer reactions reveal a hydrogen-bond-formation-induced multiple-inversion mechanism undermining the stereospecificity of the N-centered S_N2 channel. Unlike the analogous F⁻ + CH₃Cl S_N2 reaction, F⁻ + NH₂Cl → Cl⁻ + NH₂F is indirect, producing a significant amount of NH₂F with retention, as well as inverted NH₂Cl during the timescale within the unperturbed NH₂Cl molecule gets inverted with only low probability, showing the important role of facilitated inversions via an FH…NHCl⁻-like transition state. Proton transfer leading to HF + NHCl⁻ is more direct and becomes the dominant product channel at higher collision energies.

Multiple-inversion, the analogue of the double-inversion pathway recently revealed for S_N2@C, is the key mechanism in S_N2 at N center undermining stereospecificity.     

Modulation of the superficial electronic structure via metal–support interaction for H2 evolution over Pd catalysts

Electronic interactions can radically enhance the performance of supported metal catalysts and are critical for fundamentally understanding the nature of catalysts. However, at the microscopic level, the details of such interactions tuning the electronic properties of the sites on the metal particle''s surface and metal–support interface remain obscure. Herein, we found polarized electronic metal–support interaction (pEMSI) in oxide-supported Pd nanoparticles (NPs) describing the enhanced accumulation of electrons at the surface of NPs (superficial Pd^δ−) with positive Pd atoms distributed on the interface (interfacial Pd^δ+). More superficial Pd^δ− species mean stronger pEMSI resulting from the synergistic effect of moderate Pd–oxide interaction, high structural fluxionality and electron transport activity of Pd NPs. The surface Pd^δ− species are responsible for improved catalytic performance for H₂ evolution from metal hydrides and formates. These extensive insights into the nature of supported-metal NPs may open new avenues for regulating a metal particle''s electronic structure precisely and exploiting high-performance catalysts.

A new type of electronic effect, polarized metal-support interaction (pEMSI), in oxide-supported Pd nanoparticles describing the enhanced accumulation of electrons at the superficial surface is responsible for improved catalytic H₂ evolution.     

Coordination polymer glass from a protic ionic liquid: proton conductivity and mechanical properties as an electrolyte

High proton conducting electrolytes with mechanical moldability are a key material for energy devices. We propose an approach for creating a coordination polymer (CP) glass from a protic ionic liquid for a solid-state anhydrous proton conductor. A protic ionic liquid (dema)(H₂PO₄), with components which also act as bridging ligands, was applied to construct a CP glass (dema)_0.35[Zn(H₂PO₄)_2.35(H₃PO₄)_0.65]. The structural analysis revealed that large Zn–H₂PO₄⁻/H₃PO₄ coordination networks formed in the CP glass. The network formation results in enhancement of the properties of proton conductivity and viscoelasticity. High anhydrous proton conductivity (σ = 13.3 mS cm⁻¹ at 120 °C) and a high transport number of the proton (0.94) were achieved by the coordination networks. A fuel cell with this CP glass membrane exhibits a high open-circuit voltage and power density (0.15 W cm⁻²) under dry conditions at 120 °C due to the conducting properties and mechanical properties of the CP glass.

A proton-conducting coordination polymer glass derived from a protic ionic liquid works as a moldable solid electrolyte and the anhydrous fuel cell showed I–V performance of 0.15 W cm⁻² at 120 °C.     

Nitric oxide monooxygenation (NOM) reaction of cobalt-nitrosyl {Co(NO)}8 to CoII-nitrito {CoII(NO2−)}: base induced hydrogen gas (H2) evolution

Here, we report the nitric oxide monooxygenation (NOM) reactions of a Co^III-nitrosyl complex (1, {Co-NO}⁸) in the presence of mono-oxygen reactive species, i.e., a base (OH⁻, tetrabutylammonium hydroxide (TBAOH) or NaOH/15-crown-5), an oxide (O²⁻ or Na₂O/15-crown-5) and water (H₂O). The reaction of 1 with OH⁻ produces a Co^II-nitrito complex {3, (Co^II-NO₂⁻)} and hydrogen gas (H₂), via the formation of a putative N-bound Co-nitrous acid intermediate (2, {Co-NOOH}⁺). The homolytic cleavage of the O–H bond of proposed [Co-NOOH]⁺ releases H₂via a presumed Co^III-H intermediate. In another reaction, 1 generates Co^II-NO₂⁻ when reacted with O²⁻via an expected Co^I-nitro (4) intermediate. However, complex 1 is found to be unreactive towards H₂O. Mechanistic investigations using ¹⁵N-labeled-¹⁵NO and ²H-labeled-NaO²H (NaOD) evidently revealed that the N-atom in Co^II-NO₂⁻ and the H-atom in H₂ gas are derived from the nitrosyl ligand and OH⁻ moiety, respectively.

Base-induced hydrogen (H₂) gas evolution in the nitric oxide monoxygenation reaction.

As a radical species, nitric oxide (NO) has attracted great interest from the scientific community due to its major role in various physiological processes such as neurotransmission, vascular regulation, platelet disaggregation and immune responses to multiple infections.¹ Nitric oxide synthase (NOS),² and nitrite reductase (NiR)³ enzymes are involved in the biosynthesis of NO. NOSs produce NO by the oxidation of the guanidine nitrogen in l-arginine.⁴ However, in mammals and bacteria, NO₂⁻ is reduced to NO by NiRs in the presence of protons, i.e., NO₂⁻ + e⁻ + 2H⁺ → NO + H₂O.⁵ Biological dysfunctions may cause overproduction of NO, and being radical it leads to the generation of reactive nitrogen species (RNS), i.e., peroxynitrite (PN, OONO⁻)⁶ and nitrogen dioxide (˙NO₂),⁷ upon reaction with reactive oxygen species (ROS) such as superoxide (O₂˙⁻),⁸ peroxide (H₂O₂),⁹ and dioxygen (O₂).¹⁰ Hence, it is essential to maintain an optimal level of NO. In this regard, nitric oxide dioxygenases (NODs)¹¹ are available in bio-systems to convert excess NO to biologically benign nitrate (NO₃⁻).¹²NO₂⁻ + Fe^II + H⁺ ↔ NO + Fe^III + OH⁻1[M–NO]ⁿ + 2OH⁻ → [M–NO₂]⁽ⁿ⁻²⁾ + H₂O2NOD enzymes generate NO₃⁻ from NO;^11b,12−13 however, the formation of NO₂⁻ from NO is still under investigation. Clarkson and Bosolo reported NO₂⁻ formation in the reaction of Co^III-NO and O₂.¹⁴ Nam and co-workers showed the generation of Co^II-NO₂⁻ from Co^III-NO upon reaction with O₂˙⁻.¹⁵ Recently, Mondal and co-workers reported NO₂⁻ formation in the reaction of Co^II-NO with O₂.¹⁶ Apart from cobalt, the formation of Cu^II-NO₂⁻ was also observed in the reaction of Cu^I-NO and O₂.¹⁷ For metal-dioxygen adducts, i.e., Cr^III-O₂˙⁻ and Mn^IV-O₂²⁻, NOD reactions led to the generation of Cr^III-NO₂⁻ (ref. 18) and Mn^V O + NO₂⁻,¹⁹ respectively. However, the NOD reaction of Fe^III-O₂˙⁻ and Fe^III-O₂²⁻ with NO and NO⁺, respectively, generated Fe^III-NO₃⁻via Fe^IV O and ˙NO₂.²⁰ Ford suggested that the reaction of ferric-heme nitrosyl with hydroxide leads to the formation of NO₂⁻ and H⁺.¹² Lehnert and co-workers reported heme-based Fe-nitrosyl complexes²¹ showing different chemistries due to the Fe^II-NO⁺ type electronic structures. On the other hand, Bryan proposed that the one-electron reduction of NO₂⁻ to NO in ferrous heme protein is reversible (eqn (1)).²² Also, it is proposed that excess NO in biological systems is converted to NO₂⁻ and produces one equivalent of H⁺ upon reaction with ˙OH.²³ Previously reported reactivity of M–NOs of Fe²⁴ with OH⁻ suggested the formation of NO₂⁻ and one equivalent of H⁺, where H⁺ further reacts with one equivalent of OH⁻ and produces H₂O (eqn (2)).²⁵Here in this report, we explore the mechanistic aspects of nitric oxide monooxygenation (NOM) reactions of the Co^III-nitrosyl complex, [(12TMC)Co^III(NO⁻)]²⁺/{Co(NO)}⁸ (1),^15,26 bearing the 12TMC ligand (12TMC = 1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclododecane) with mono-oxygen reactive species (O²⁻, OH⁻ and H₂O) (Scheme 1). Complex 1 reacts with the base (OH⁻, tetrabutylammonium hydroxide (TBAOH)/or NaOH in the presence of 15-crown-5 as the OH⁻ source) and generates the corresponding Co^II-nitrito complex, [(12TMC)Co^II(NO₂⁻)]⁺ (3), with the evolution of hydrogen gas (H₂) via the formation of a plausible N-bound Co-nitrous acid intermediate ([Co-NOOH]⁺, 2) in CH₃CN at 273 K (Scheme 1, reaction (I)). Also, when 1 reacts with the oxide (O²⁻ or Na₂O in the presence of 15-crown-5), it generates the Co^II-nitrito complex (3) via a probable Co^I-nitro, [(12TMC)Co^I(NO₂⁻)] (4), intermediate (Scheme 1, reaction (II)); however, 1 does not react with water (Scheme 1, reaction (III)). Mechanistic investigations using ¹⁵N-labeled-¹⁵NO, D-labeled-NaOD and ¹⁸O-labelled-¹⁸OH⁻ demonstrated, unambiguously, that the N and O-atoms in the NO₂⁻ ligand of 3 resulted from NO and OH⁻ moieties; however, the H-atoms of H₂ are derived from OH⁻. To the extent of our knowledge, the present work reports the very first systematic study of Co^III-nitrosyl complex reactions with H₂O, OH⁻ and O²⁻. This new finding presents an alternative route for NO₂⁻ generation in biosystems, and also illustrates a new pathway of H₂ evolution, in addition to the reported literature.^12,27Open in a separate windowScheme 1Nitric oxide monooxygenation (NOM) reactions of cobalt-nitrosyl complex (1) in the presence of a base (OH⁻), sodium oxide (Na₂O) and water (H₂O).To further explore the chemistry of [(12TMC)Co^III(NO⁻)]²⁺ (1),^15,26 and the mechanistic insights of NOM reactions, we have reacted it with a base (OH⁻), an oxide (O²⁻), and water (H₂O). When complex 1 was reacted with TBAOH in CH₃CN, the color of complex 1 changed to light pink from dark pink. In this reaction, the characteristic absorption band of 1 (370 nm) disappears within 2 minutes (Fig. 1a; ESI, Experimental section (ES) and Fig. S1a^†), producing a Co^II-nitrito complex, [(12TMC)Co^II(NO₂⁻)]⁺ (3), with H₂ (Scheme 1, reaction (Ib)), in contrast to the previous reports on base induced NOM reactions (eqn (2)).^12,25,28 The spectral titration data confirmed that the ratio-metric equivalent of OH⁻ to 1 was 1 : 1 (ESI, Fig. S1b^†). 3 was determined to be [(12TMC)Co^II(NO₂⁻)](BF₄) based on various spectroscopic and structural characterization experiments (vide infra).^15,26bOpen in a separate windowFig. 1(a) UV-vis spectral changes of 1 (0.50 mM, black line) upon addition of OH⁻ (1 equiv.) in CH₃CN under Ar at 273 K. Black line (1) changed to red line (3) upon addition of OH⁻. Inset: IR spectra of 3-¹⁴NO₂⁻ (blue line) and 3-¹⁵NO₂⁻ (red line) in KBr. (b) ESI-MS spectra of 3. The peak at 333.2 is assigned to [(12TMC)Co^II(NO₂)]⁺ (calcd m/z 333.1). Inset: isotopic distribution pattern for 3-¹⁴NO₂⁻ (red line) and 3-¹⁵NO₂⁻ (blue line).The FT-IR spectrum of 3 showed a characteristic peak for nitrite stretching at 1271 cm⁻¹ (Co^II-14NO₂⁻) and shifted to 1245 cm⁻¹ (Co^II-15NO₂⁻) when 3 was prepared by reacting ¹⁵N-labeled NO (Co^III-15NO) with OH⁻ (Inset, Fig. 1a and Fig. S2^†). The shifting of NO₂⁻ stretching (Δ = 30 cm⁻¹) indicates that the N-atom in the NO₂⁻ ligand is derived from Co^III-15NO. The ESI-MS spectrum of 3 showed a prominent peak at m/z 333.2, [(12TMC)Co^II(¹⁴NO₂⁻)]⁺ (calcd m/z 333.2), which shifted to 334.2, [(12TMC)Co^II(¹⁵NO₂⁻)]⁺ (calcd m/z 334.2), when the reaction was performed with Co^III-15NO (Inset, Fig. 1b; ESI, Fig. S3a^†); indicating clearly that NO₂⁻ in 3 was derived from the NO moiety of 1. In addition, we have reacted 1 with Na¹⁸OH (ES and ESI^†), in order to follow the source of the second O-atom in 3-NO₂⁻. The ESI-MS spectrum of the reaction mixture, obtained by reacting 1 with Na¹⁸OH, showed a prominent peak at m/z 335.2, [(12TMC)Co^II(¹⁸ONO⁻)]⁺ (calcd m/z 335.2), (SI, Fig. S3b^†) indicating clearly that NO₂⁻ in 3 was derived from ¹⁸OH⁻. The ¹H NMR spectrum of 3 did not show any signal for aliphatic protons of the 12TMC ligand, suggesting a bivalent cobalt center (Fig. S4^†).^26b Furthermore, we have determined the magnetic moment of 3, using Evans'' method, and it was found to be 4.62 BM, suggesting a high spin Co(ii) metal center with three unpaired electrons (ESI^† and ES).²⁹ The exact conformation of 3 was provided by single-crystal X-ray crystallographic analysis (Fig. 2b, ESI, ES, Fig. S5, and Tables T1 and T2^†) and similar to that of previously reported Co^II-NO₂⁻/M^II-NO₂⁻.^15,26b Also, we have quantified the amount of nitrite (90 ± 5%), formed in the above reaction, using the Griess reagent (ESI, ES, and Fig. S6^†).Open in a separate windowFig. 2Displacement ellipsoid plot (20% probability) of 3 at 100 K. Disordered C-atoms of the TMC ring, anion and H-atoms have been removed for clarity.As is known from the literature, a metal-nitrous acid intermediate may form either by the reaction of a metal-nitrosyl with a base²⁷ or by the metal-nitrite reaction with an acid (nitrite reduction chemistry);^26b however, the products of both the reactions are different. Here, for the first time, we have explored the reaction of Co^III-nitrosyl (1) with a base. In this reaction, it is clear that the formation of Co^II-nitrito would be accomplished by the release of H₂ gas via the generation of a transient N-bound [Co-(NOOH)]⁺ intermediate (Scheme 2, reaction (II)). The formation of Co^II-NO₂⁻ (3) from the [Co-(NOOH)]⁺ intermediate is likely to proceed by either (i) homolytic cleavage of the O–H bond and release of H₂via the proposed Co^III-H transient species (Co^III-H = Co^II + 1/2H₂)³⁰ (Scheme 2, reaction (III)), as reported in previous literature where the reduced cobalt, in a number of different ligand environments, is a good H⁺ reduction catalyst and generates H₂ gas via a Co^III-H intermediate³¹ or (ii) heterolytic cleavage of the O–H bond and the formation of Co^I-NO₂⁻ + H⁺.²⁷ In the present study, we observed the formation of 3 and H₂via the plausible homolytic cleavage of the NOO–H moiety of 2 as shown in Scheme 2, in contrast to the previous reports on base-induced reactions on metal-nitrosyls (eqn (3)).²⁷ Taking together both possibilities, (i) is the most reasonable pathway for the NOM reaction of complex 1 in the presence of a base (as shown in Scheme 2, reaction (III)). And the reaction is believed to go through a Co^III-H intermediate as reported previously in Co^I-induced H⁺ reduction in different ligand frameworks and based on literature precedence, we believe that complex 1 acts in a similar manner.³¹Open in a separate windowScheme 2NOM reaction of complex 1 in the presence of OH⁻, showing the generation of Co^II-nitrito (3) and H₂via a Co(iii)-hydrido intermediate.In contrast to an O-bound Co^II-ONOH intermediate, where N–O bond homolysis of the ON-OH moiety generates H₂O₂ (Scheme 2, reaction (IV)),^26b the N-bound [Co-(NOOH)]⁺ intermediate decomposes to form NO₂⁻ and a Co(iii)-H transient species, arising from β-hydrogen transfer from the NOO–H moiety to the cobalt-center (Scheme 2, reaction (II)).^30a,c,32 The Co(iii)-hydrido species may generate H₂ gas either (a) by its transformation to the Co(ii)-nitrito complex (2) and H₂ gas as observed in the case of Co^III-H intermediate chemistry^30a,c,e−g as proposed in the chemistry of the Co^I complex with H⁺ reduction³¹ and other metal-hydrido intermediates³² and also explained in O₂ formation in PN chemistry^17,33 or (b) by the reacting with another [Co-(NOOH)]⁺ intermediate (Scheme 2, reaction (III)).Furthermore, we have confirmed the H₂ formation in the NOM reaction of 1 with OH⁻ by headspace gas mass spectrometry (Fig. 3a). Also, carrying out the reaction of 1 with NaOD leads to the formation of the [Co-(NOOD)]⁺ intermediate, which then transforms to a Co^III-D transient species. Further, as described above, the Co^III-D species releases D₂ gas, detected by headspace gas mass spectrometry (Fig. 3b), which evidently established that H₂ gas formed in the reaction of 1 with OH⁻. In this regard, we have proposed that in the first step of this reaction, the nucleophilic addition of OH⁻ to {Co-NO}⁸ generates a transient N-bound [Co-(NOOH)]⁺ intermediate that is generated by an internal electron transfer to Co^III (Scheme 2, reaction (I)). By following the mechanism proposed in the case of Co^III-H,^30a−c O₂,¹⁵ and H₂O₂(ref. 26b) formation, we have proposed the sequences of the NOM reaction of 1, which leads to the generation of Co^II-nitrito and H₂ (Scheme 2, reaction (I)–(III) and Scheme 3). In the second step, O–H bond homolytic cleavage generates a Co^III-H transient species + NO₂⁻via a β-hydrogen elimination reaction of the [Co-(NOOH)]⁺ intermediate.³² The Co^III-H intermediate may undergo the following reactions to generate H₂ gas and Co^II-nitrito either (a) by the natural decomposition of the Co^III-H transient species to generate H₂,^30a,c,e−g or (b) by the H-atom abstraction from another [Co-(NOOH)]⁺ intermediate (Scheme 3). Also, to validate our assumption that the reaction goes through a plausible N-bound [Co-(NOOH)]⁺ intermediate followed by its transformation to the Co^III-H species (vide supra), we have performed the reaction of 1 with NaOH/NaOD (in 1 : 1 ratio). In this reaction, we have observed the formation of a mixture of H₂, D₂, and HD gases, which indicates clearly that the reaction goes through the formation of Co^III-H and Co^III-D transient species via the aforementioned mechanism (Fig. 3c). This is the only example where tracking of the H atoms has confirmed the H₂ generation from an N-bound NOO–H moiety as proposed for H₂ formation from Co^III-H.³⁰Open in a separate windowFig. 3Mass spectra of formation of (a) H₂ in the reaction of 1 (5.0 mM) with NaOH (5.0 mM), (b) D₂ in the reaction of 1 (5.0 mM) with NaOD (5.0 mM), (c) D₂, HD, and H₂ in the reaction of 1 (5.0 mM) with NaOD/NaOH (1 : 1), and (d) H₂ in the reaction of 1 (5.0 mM) with NaOH in the presence of 2,4 DTBP (50 mM).Open in a separate windowScheme 3NOM reaction of complex 1 in the presence of OH⁻, showing the different steps of the reaction.While, we do not have direct spectral evidence to support the formation of the transient N-bound [Co-(NOOH)]⁺ intermediate and its decomposition to the Co^III-H transient species via β-hydrogen transfer from the NOOH moiety to the cobalt center, support for its formation comes from our finding that the reactive hydrogen species can be trapped by using 2,4-di-tert-butyl-phenol (2,4-DTBP).³⁴ In this reaction, we observed the formation of 2,4-DTBP-dimer (2,4-DTBP-D, ∼67%) as a single product (ESI, ES, and Fig. S7^†). This result can readily be explained by the H-atom abstraction reaction of 2,4-DTBP either by [Co-(NOOH)]⁺ or Co^III-H, hence generating a phenoxyl-radical and 3 with H₂ (Fig. 3d and Scheme 2, reaction (a)). Also, we have detected H₂ gas formation in this reaction (ESI,^† ES, and Fig. 3d). In the next step, two phenoxyl radicals dimerized to give 2,4-DTBP-dimer (Scheme 2c, reaction (II)). Thus, the observation of 2,4-DTBP-dimer in good yield supports the proposed reaction mechanism (Scheme 2, reaction (a) and (b)). Further, the formation of 2,4 DTBP as a single product also rules out the formation of the hydroxyl radical as observed in the case of an O-bound nitrous acid intermediate.^26bFurthermore, we have explored the NOM reactivity of 1 with Na₂O/15-crown-5 (as the O²⁻ source) and observed the formation of the Co^II-nitrito complex (3) via a plausible Co^I-nitro (4) intermediate (Scheme 1, reaction (IIa); also see the ESI^† and ES); however, 1 was found to be inert towards H₂O (Scheme 1, reaction (III); also see the ESI, ES and Fig. S8^†). The product obtained in the reaction of 1 with O²⁻ was characterized by various spectroscopic measurements.^15,26b The UV-vis absorption band of 1 (λ_max = 370 nm) disappears upon the addition of 1 equiv. of Na₂O and a new band (λ_max = 535 nm) forms, which corresponds to 3 (ESI, Fig. S9^†). The FT-IR spectrum of the isolated product of the above reaction shows a characteristic peak for Co^II-bound nitrite at 1271 cm⁻¹, which shifts to 1245 cm⁻¹ when exchanged with ¹⁵N-labeled-NO (¹⁵N¹⁶O) (ESI, ES, and Fig. S10^†), clearly indicating the generation of nitrite from the NO ligand of complex 1.^26b The ESI-MS spectrum recorded for the isolated product (vide supra) shows a prominent ion peak at m/z 333.1, and its mass and isotope distribution pattern matches with [(12-TMC)Co^II(NO₂)]⁺ (calc. m/z 333.1) (ESI, Fig. S11^†). Also, we quantified the amount of 3 (85 ± 5%) by quantifying the amount of nitrite (85 ± 5%) using the Griess reagent test (ESI, ES, and Fig. S6^†).In summary, we have demonstrated the reaction of Co^III-nitrosyl, [(12-TMC)Co^III(NO⁻)]²⁺/{CoNO}⁸ (1), with mono-oxygen reactive species (O²⁻, OH⁻ and H₂O) (Scheme 1). For the first time, we have established the clear formation of a Co^II-nitrito complex, [(12TMC)Co^II(NO₂⁻)]⁺ (3), and H₂ in the reaction of 1 with one equivalent of OH⁻via a transient N-bound [Co-(NOOH)]⁺ (2) intermediate. This [Co-(NOOH)]⁺ intermediate undergoes the O–H bond homolytic cleavage and generates a Co^III-H transient species with NO₂⁻, via a β-hydrogen elimination reaction of the [Co-(NOOH)]⁺ intermediate, which upon decomposition produces H₂ gas. This is in contrast to our previous report, where acid-induced nitrite reduction of 3 generated 1 and H₂O₂via an O-bound Co^II-ONOH intermediate.^26b Complex 1 was found to be inert towards H₂O; however, we have observed the formation of 3 when reacted with O²⁻. It is important to note that H₂ formation involves a distinctive pathway of O–H bond homolytic cleavage in the [Co-(NOOH)]⁺ intermediate, followed by the generation of the proposed Co^III-H transient species (Co^II + 1/2H₂)³⁰ prior to H₂ evolution as described in Co^I chemistry with H⁺ in many different ligand frameworks.³¹ The present study is the first-ever report where the base induced NOM reaction of Co^III-nitrosyl (1) leads to Co^II-nitrito (3) with H₂ evolution via an N-bound [Co-(NOOH)]⁺ intermediate, in contrast to the chemistry of O-bound Co^II-ONOH^26b, hence adding an entirely new mechanistic insight of base induced H₂ gas evolution and an additional pathway for NOM reactions.     

Capture and displacement-based release of the bicarbonate anion by calix[4]pyrroles with small rigid straps

Two-phenoxy walled calix[4]pyrroles 1 and 2 strapped with small rigid linkers containing pyridine and benzene, respectively, have been synthesized. ¹H NMR spectroscopic analyses carried out in CDCl₃ revealed that both of receptors 1 and 2 recognize only F⁻ and HCO₃⁻ among various test anions with high preference for HCO₃⁻ (as the tetraethylammonium, TEA⁺ salt) relative to F⁻ (as the TBA⁺ salt). The bound HCO₃⁻ anion was completely released out of the receptors upon the addition of F⁻ (as the tetrabutylammonium, TBA⁺ salt) as a result of significantly enhanced affinities and selectivities of the receptors for F⁻ once converted to the TEAHCO₃ complexes. Consequently, relatively stable TEAF complexes of receptors 1 and 2 were formed via anion metathesis occurring within the receptor cavities. By contrast, the direct addition of TEAF to receptors 1 and 2 produces different complexation products initially, although eventually the same TEAF complexes are produced as via sequential TEAHCO₃ and TBAF addition. These findings are rationalized in terms of the formation of different ion pair complexes involving interactions both inside and outside of the core receptor framework.

The inherent selectivity of anion receptors can be reversed by ion pairing occurring both inside and outside of the receptor cavity.     

The in situ study of surface species and structures of oxide-derived copper catalysts for electrochemical CO2 reduction

Oxide-derived copper (OD-Cu) has been discovered to be an effective catalyst for the electroreduction of CO₂ to C2+ products. The structure of OD-Cu and its surface species during the reaction process are interesting topics, which have not yet been clearly discussed. Herein, in situ surface-enhanced Raman spectroscopy (SERS), operando X-ray absorption spectroscopy (XAS), and ¹⁸O isotope labeling experiments were employed to investigate the surface species and structures of OD-Cu catalysts during CO₂ electroreduction. It was found that the OD-Cu catalysts were reduced to metallic Cu(0) in the reaction. CuO_x species existed on the catalyst surfaces during the CO₂RR, which resulted from the adsorption of preliminary intermediates (such as *CO₂ and *OCO⁻) on Cu instead of on the active sites of the catalyst. It was also found that abundant interfaces can be produced on OD-Cu, which can provide heterogeneous CO adsorption sites (strong binding sites and weak binding sites), leading to outstanding performance for obtaining C2+ products. The Faradaic efficiency (FE) for C2+ products reached as high as 83.8% with a current density of 341.5 mA cm⁻² at −0.9 V vs. RHE.

CuO_x species were shown to exist on OD-Cu during the CO₂RR, which resulted from the adsorption of preliminary intermediates (such as *CO₂ and *OCO⁻) on Cu instead of on the active sites of the catalyst.     

Proton-conductive coordination polymer glass for solid-state anhydrous proton batteries

Designing solid-state electrolytes for proton batteries at moderate temperatures is challenging as most solid-state proton conductors suffer from poor moldability and thermal stability. Crystal–glass transformation of coordination polymers (CPs) and metal–organic frameworks (MOFs) via melt-quenching offers diverse accessibility to unique properties as well as processing abilities. Here, we synthesized a glassy-state CP, [Zn₃(H₂PO₄)₆(H₂O)₃](1,2,3-benzotriazole), that exhibited a low melting temperature (114 °C) and a high anhydrous single-ion proton conductivity (8.0 × 10⁻³ S cm⁻¹ at 120 °C). Converting crystalline CPs to their glassy-state counterparts via melt-quenching not only initiated an isotropic disordered domain that enhanced H⁺ dynamics, but also generated an immersive interface that was beneficial for solid electrolyte applications. Finally, we demonstrated the first example of a rechargeable all-solid-state H⁺ battery utilizing the new glassy-state CP, which exhibited a wide operating-temperature range of 25 to 110 °C.

Melt-quenched coordination polymer glass shows exclusive H⁺ conductivity (8.0 × 10⁻³ S cm⁻¹ at 120 °C, anhydrous) and optimal mechanical properties (42.8 Pa s at 120 °C), enables the operation of an all-solid-state proton battery from RT to 110 °C.     

Dual Ag/Co cocatalyst synergism for the highly effective photocatalytic conversion of CO2 by H2O over Al-SrTiO3

Loading Ag and Co dual cocatalysts on Al-doped SrTiO₃ (AgCo/Al-SrTiO₃) led to a significantly improved CO-formation rate and extremely high selectivity toward CO evolution (99.8%) using H₂O as an electron donor when irradiated with light at wavelengths above 300 nm. Furthermore, the CO-formation rate over AgCo/Al-SrTiO₃ (52.7 μmol h⁻¹) was a dozen times higher than that over Ag/Al-SrTiO₃ (4.7 μmol h⁻¹). The apparent quantum efficiency for CO evolution over AgCo/Al-SrTiO₃ was about 0.03% when photoirradiated at a wavelength at 365 nm, with a CO-evolution selectivity of 98.6% (7.4 μmol h⁻¹). The Ag and Co cocatalysts were found to function as reduction and oxidation sites for promoting the generation of CO and O₂, respectively, on the Al-SrTiO₃ surface.

Deposition Ag and Co dual cocatalysts onto Al-SrTiO₃ significantly improves its activity for photoreduction of CO₂ by H₂O, with extremely high selectivity to CO evolution (99.8%), in which Ag and Co enable CO₂ reduction and H₂O oxidation, respectively.     

H2/CO2 separations in multicomponent metal-adeninate MOFs with multiple chemically distinct pore environments

Metal–organic frameworks constructed from multiple (≥3) components often exhibit dramatically increased structural complexity compared to their 2 component (1 metal, 1 linker) counterparts, such as multiple chemically unique pore environments and a plurality of diverse molecular diffusion pathways. This inherent complexity can be advantageous for gas separation applications. Here, we report two isoreticular multicomponent MOFs, bMOF-200 (4 components; Cu, Zn, adeninate, pyrazolate) and bMOF-201 (3 components; Zn, adeninate, pyrazolate). We describe their structures, which contain 3 unique interconnected pore environments, and we use Kohn–Sham density functional theory (DFT) along with the climbing image nudged elastic band (CI-NEB) method to predict potential H₂/CO₂ separation ability of bMOF-200. We examine the H₂/CO₂ separation performance using both column breakthrough and membrane permeation studies. bMOF-200 membranes exhibit a H₂/CO₂ separation factor of 7.9. The pore space of bMOF-201 is significantly different than bMOF-200, and one molecular diffusion pathway is occluded by coordinating charge-balancing formate and acetate anions. A consequence of this structural difference is reduced permeability to both H₂ and CO₂ and a significantly improved H₂/CO₂ separation factor of 22.2 compared to bMOF-200, which makes bMOF-201 membranes competitive with some of the best performing MOF membranes in terms of H₂/CO₂ separations.

Tailorable multicomponent MOFs and MOF membranes for efficient H₂/CO₂ separation.     

The impact of surface Cu2+ of ZnO/(Cu1−xZnx)O heterostructured nanowires on the adsorption and chemical transformation of carbonyl compounds

The surface cation composition of nanoscale metal oxides critically determines the properties of various functional chemical processes including inhomogeneous catalysts and molecular sensors. Here we employ a gradual modulation of cation composition on a ZnO/(Cu_1−xZn_x)O heterostructured nanowire surface to study the effect of surface cation composition (Cu/Zn) on the adsorption and chemical transformation behaviors of volatile carbonyl compounds (nonanal: biomarker). Controlling cation diffusion at the ZnO(core)/CuO(shell) nanowire interface allows us to continuously manipulate the surface Cu/Zn ratio of ZnO/(Cu_1−xZn_x)O heterostructured nanowires, while keeping the nanowire morphology. We found that surface exposed copper significantly suppresses the adsorption of nonanal, which is not consistent with our initial expectation since the Lewis acidity of Cu²⁺ is strong enough and comparable to that of Zn²⁺. In addition, an increase of the Cu/Zn ratio on the nanowire surface suppresses the aldol condensation reaction of nonanal. Surface spectroscopic analysis and theoretical simulations reveal that the nonanal molecules adsorbed at surface Cu²⁺ sites are not activated, and a coordination-saturated in-plane square geometry of surface Cu²⁺ is responsible for the observed weak molecular adsorption behaviors. This inactive surface Cu²⁺ well explains the mechanism of suppressed surface aldol condensation reactions by preventing the neighboring of activated nonanal molecules. We apply this tailored cation composition surface for electrical molecular sensing of nonanal and successfully demonstrate the improvements of durability and recovery time as a consequence of controlled surface molecular behaviors.

Unexpected features of surface Cu²⁺ on ZnO/(Cu1−xZnx)O nanowires for molecular transformation and electrical sensing of carbonyl compounds were found.     

Quasi-square-shaped cadmium hydroxide nanocatalysts for electrochemical CO2 reduction with high efficiency

Powered by a renewable electricity source, electrochemical CO₂ reduction reaction is a promising solution to facilitate the carbon balance. However, it is still a challenge to achieve a desired product with commercial current density and high efficiency. Herein we designed quasi-square-shaped cadmium hydroxide nanocatalysts for CO₂ electroreduction to CO. It was discovered that the catalyst is very active and selective for the reaction. The current density could be as high as 200 mA cm⁻² with a nearly 100% selectivity in a commonly used H-type cell using the ionic liquid-based electrolyte. In addition, the faradaic efficiency of CO could reach 90% at a very low overpotential of 100 mV. Density functional theory studies and control experiments reveal that the outstanding performance of the catalyst was attributed to its unique structure. It not only provides low Cd–O coordination, but also exposes high activity (002) facet, which requires lower energy for the formation of CO. Besides, the high concentration of CO can be achieved from the low concentration CO₂via an adsorption-electrolysis device.

Quasi-square cadmium hydroxide nanocrystals (Cdhy-QS) showed outstanding performance for electroreduction CO₂ to CO.     

Chemo-bio catalysis using carbon supports: application in H2-driven cofactor recycling

Heterogeneous biocatalytic hydrogenation is an attractive strategy for clean, enantioselective C X reduction. This approach relies on enzymes powered by H₂-driven NADH recycling. Commercially available carbon-supported metal (metal/C) catalysts are investigated here for direct H₂-driven NAD⁺ reduction. Selected metal/C catalysts are then used for H₂ oxidation with electrons transferred via the conductive carbon support material to an adsorbed enzyme for NAD⁺ reduction. These chemo-bio catalysts show improved activity and selectivity for generating bioactive NADH under ambient reaction conditions compared to metal/C catalysts. The metal/C catalysts and carbon support materials (all activated carbon or carbon black) are characterised to probe which properties potentially influence catalyst activity. The optimised chemo-bio catalysts are then used to supply NADH to an alcohol dehydrogenase for enantioselective (>99% ee) ketone reductions, leading to high cofactor turnover numbers and Pd and NAD⁺ reductase activities of 441 h⁻¹ and 2347 h⁻¹, respectively. This method demonstrates a new way of combining chemo- and biocatalysis on carbon supports, highlighted here for selective hydrogenation reactions.

Heterogeneous chemo-bio catalytic hydrogenation is an attractive strategy for clean, enantioselective C X reduction.     

Modulating the ground state,stability and charge transport in OFETs of biradicaloid hexahydro-diindenopyrene derivatives and a proposed method to estimate the biradical character

Biradicaloid compounds with an open-shell ground state have been the subject of intense research in the past decade. Although diindenoacenes are one of the most developed families, only a few examples have been reported as active layers in organic field-effect transistors (OFETs) with a charge mobility of around 10⁻³ cm² V⁻¹ s⁻¹ due to a steric disadvantage of the mesityl group to kinetically stabilize compounds. Herein, we disclose our efforts to improve the charge transport of the diindenoacene family based on hexahydro-diindenopyrene (HDIP) derivatives with different annelation modes for which the most reactive position has been functionalized with (triisopropylsilyl)ethynyl (TIPS) groups. All the HDIP derivatives show remarkably higher stability than that of TIPS-pentacene, enduring for 2 days to more than 30 days, which depends on the oxidation potential, the contribution of the singlet biradical form in the ground state and the annelation mode. The annelation mode affects not only the band gap and the biradical character (y₀) but also the value of the singlet–triplet energy gap (ΔE_S–T) that does not follow the reverse trend of y₀. A method based on comparison between experimental and theoretical bond lengths has been disclosed to estimate y₀ and shows that y₀ computed at the projected unrestricted Hartree–Fock (PUHF) level is the most relevant among those reported by all other methods. Thanks to their high stability, thin-film OFETs were successfully fabricated. Well balanced ambipolar transport was obtained in the order of 10⁻³ cm² V⁻¹ s⁻¹ in the bottom-gate/top-contact configuration, and unipolar transport in the top-gate/bottom-contact configuration was obtained in the order of 10⁻¹ cm² V⁻¹ s⁻¹ which is the highest value obtained for biradical compounds with a diindenoacene skeleton.

Biradicaloid HDIP derivatives show that the ΔE_S–T gap does not follow the reverse trend of the biradical character but depends more on the delocalization of the radical centres at the outer rings.     

An amide-based second coordination sphere promotes the dimer pathway of Mn-catalyzed CO2-to-CO reduction at low overpotential

The [fac-Mn(bpy)(CO)₃Br] complex is capable of catalyzing the electrochemical reduction of CO₂ to CO with high selectivity, moderate activity and large overpotential. Several attempts have been made to lower the overpotential and to enhance the catalytic activity of this complex by manipulating the second-coordination sphere of manganese and using relatively stronger acids to promote the protonation-first pathway. We report herein that the complex [fac-Mn(bpy-CONHMe)(CO)₃(MeCN)]⁺ ([1-MeCN]⁺; bpy-CONHMe = N-methyl-(2,2′-bipyridine)-6-carboxamide) as a pre-catalyst could catalyze the electrochemical reduction of CO₂ to CO with low overpotential and high activity and selectivity. Combined experimental and computational studies reveal that the amide NH group not only decreases the overpotential of the Mn catalyst by promoting the dimer and protonation-first pathways in the presence of H₂O but also enhances the CO₂ electroreduction activity by facilitating C–OH bond cleavage, making [1-MeCN]⁺ an efficient CO₂ reduction pre-catalyst at low overpotential.

The amide NH group decreases the overpotential of Mn-based CO₂ reduction catalysts by promoting the dimer and protonation-first pathways in the presence of H₂O and enhances the CO₂ electroreduction activity by facilitating C–OH bond cleavage.     

Silyloxymethanesulfinate as a sulfoxylate equivalent for the modular synthesis of sulfones and sulfonyl derivatives

An efficient protocol for the modular synthesis of sulfones and sulfonyl derivatives has been developed utilizing sodium tert-butyldimethylsilyloxymethanesulfinate (TBSOMS-Na) as a sulfoxylate (SO₂²⁻) equivalent. TBSOMS-Na, easily prepared from the commercial reagents Rongalite™ and TBSCl, serves as a potent nucleophile in S-alkylation and Cu-catalyzed S-arylation reactions with alkyl and aryl electrophiles. The sulfone products thus obtained can undergo the second bond formation at the sulfur center with various electrophiles without a separate unmasking step to afford sulfones and sulfonyl derivatives such as sulfonamides and sulfonyl fluorides.

An efficient protocol for the modular synthesis of sulfones and sulfonyl derivatives has been developed utilizing sodium tert-butyldimethylsilyloxymethanesulfinate (TBSOMS-Na) as a sulfoxylate (SO₂²⁻) equivalent.     

Electrochemical C–N coupling with perovskite hybrids toward efficient urea synthesis

Electrocatalytic C–N coupling reaction by co-activation of both N₂ and CO₂ molecules under ambient conditions to synthesize valuable urea opens a new avenue for sustainable development, while the actual catalytic activity is limited by poor adsorption and coupling capability of gas molecules on the catalyst surface. Herein, theoretical calculation predicts that the well-developed built-in electric field in perovskite hetero-structured BiFeO₃/BiVO₄ hybrids can accelerate the local charge redistribution and thus promote the targeted adsorption and activation of inert N₂ and CO₂ molecules on the generated local electrophilic and nucleophilic regions. Thus, a BiFeO₃/BiVO₄ heterojunction is designed and synthesized, which delivers a urea yield rate of 4.94 mmol h⁻¹ g⁻¹ with a faradaic efficiency of 17.18% at −0.4 V vs. RHE in 0.1 M KHCO₃, outperforming the highest values reported as far. The comprehensive analysis further confirms that the local charge redistribution in the heterojunction effectively suppresses CO poisoning and the formation of the endothermic *NNH intermediate, which thus guarantees the exothermic coupling of *N N* intermediates with the generated CO via C–N coupling reactions to form the urea precursor *NCON* intermediate. This work opens a new avenue for effective electrocatalytic C–N coupling under ambient conditions.

The local charge redistribution in BiFeO₃/BiVO₄ hybrids promotes the targeted adsorption and activation of inert gas molecules and guarantees the exothermic coupling of *N N* with generated CO via C–N coupling reactions to form *NCON* precursor.     

The inorganic cation-tailored “trapdoor” effect of silicoaluminophosphate zeolite for highly selective CO2 separation

Functional nanoporous materials are widely explored for CO₂ separation, in particular, small-pore aluminosilicate zeolites having a “trapdoor” effect. Such an effect allows the specific adsorbate to push away the sited cations inside the window followed by exclusive admission to the zeolite pores, which is more advantageous for highly selective CO₂ separation. Herein, we demonstrated that the protonated organic structure-directing agent in the small-pore silicoaluminophosphate (SAPO) RHO zeolite can be directly exchanged with Na⁺, K⁺, or Cs⁺ and that the Na⁺ form of SAPO-RHO exhibited unprecedented separation for CO₂/CH₄, superior to all of the nanoporous materials reported to date. Rietveld refinement revealed that Na⁺ is sited in the center of the single eight-membered ring (s8r), while K⁺ and Cs⁺ are sited in the center of the double 8-rings (d8rs). Theoretical calculations showed that the interaction between Na⁺ and the s8r in SAPO-RHO was stronger than that in aluminosilicate RHO, giving an enhanced “trapdoor” effect and record high selectivity for CO₂ with the separation factor of 2196 for CO₂/CH₄ (0.02/0.98 bar). The separation factor of Na-SAPO-RHO for CO₂/N₂ was 196, which was the top level among zeolitic materials. This work opens a new avenue for gas separation by using diverse silicoaluminophosphate zeolites in terms of the cation-tailored “trapdoor” effect.

The sodium form of silicoaluminophosphate RHO zeolite exhibits a pronounced cation-tailored “trapdoor” effect, showing an unprecedented selectivity adsorption separation performance for CO₂/CH₄ and CO₂/N₂.