Manganese(II), lead(II) and cadmium(II) coordination complexes containing a tetrazole‐based acylamide ligand: synthesis and the influence of the metal ions on the structures

Three new manganese(II), lead(II) and cadmium(II) coordination complexes have been prepared by reaction of N‐(1H‐tetrazol‐5‐yl)cinnamamide (HNTCA) with divalent metal salts (MnCl₂, PbCl₂ and CdCl₂) in a mixed‐solvent system, affording mononuclear to trinuclear structures namely, bis(methanol‐κO)bis[5‐(3‐phenylprop‐2‐enamido)‐1H‐1,2,3,4‐tetrazol‐1‐ido‐κ²N¹,O]manganese(II), [Mn(C₁₀H₈N₅O)₂(CH₃OH)₂], (1), bis[μ‐5‐(3‐phenylprop‐2‐enamido)‐1H‐1,2,3,4‐tetrazol‐1‐ido]‐κ³N¹,O:N²;κ³N²:N¹,O‐bis{aqua[5‐(3‐phenylprop‐2‐enamido)‐1H‐1,2,3,4‐tetrazol‐1‐ido‐κ²N¹,O]lead(II)}, [Pb₂(C₁₀H₈N₅O)₄(H₂O)₂], (2), and hexakis[μ₂‐5‐(3‐phenylprop‐2‐enamido)‐1H‐1,2,3,4‐tetrazol‐1‐ido‐κ³N¹,O:N²]tricadmium(II), [Cd₃(C₁₀H₈N₅O)₆], (3). The structures of these three compounds reveal that the nature of the metal ions and the side groups of the organic building blocks have a significant effect on the structures of the coordination compounds formed. Intermolecular hydrogen bonds link the molecules into two‐dimensional [complex (1)] and three‐dimensional hydrogen‐bonded networks. Complexes (2) and (3) show significant fluorescence, while complex (1) displays no fluorescence.     

Cleavage of Carbon Dioxide C=O Bond Promoted by Nickel-Boron Cooperativity in a PBP-Ni Complex

The synthesis and characterization of (^tBuPBP)Ni(OAc) ( 5 ) by insertion of carbon dioxide into the Ni−C bond of (^tBuPBP)NiMe ( 1 ) is presented. An unexpected CO₂ cleavage process involving the formation of new B−O and Ni−CO bonds leads to the generation of a butterfly-structured tetra-nickel cluster (^tBuPBOP)₂Ni₄(μ-CO)₂ ( 6 ). Mechanistic investigation of this reaction indicates a reductive scission of CO₂ by O-atom transfer to the boron atom via a cooperative nickel-boron mechanism. The CO₂ activation reaction produces a three-coordinate (^tBuP₂BO)Ni-acyl intermediate ( A ) that leads to a (^tBuP₂BO)−Ni^I complex ( B ) via a likely radical pathway. The Ni^I species is trapped by treatment with the radical trap (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) to give (^tBuP₂BO)Ni^II(η²-TEMPO) ( 7 ). Additionally, ¹³C and ¹H NMR spectroscopy analysis using ¹³C-enriched CO₂ provides information about the species involved in the CO₂ activation process.     

Stereoselective Activation of Small Molecules by a Stable Chiral Silene

The reaction of the dilithium salt of the enantiopure (S)-BINOL (1,1’-bi-2-naphthol) with two equivalents of the amidinate-stabilized chlorosilylene [L^PhSiCl] (L^Ph=PhC(NtBu)₂) led to the formation of the first example of a chiral cyclic silene species comprising an (S)-BINOL ligand. The reactivity of the Si=C bond was investigated by reaction with elemental sulfur, CO₂ and HCl. The reaction with S₈ led to a Si=C bond cleavage and concomitantly to a ring-opened product with imine and silanethione functional groups. The reaction with CO₂ resulted in the cleavage of the CO₂ molecule into a carbonyl group and an isolated O atom, while a new stereocenter is formed in a highly selective manner. According to DFT calculations, the [2+2] cycloaddition product is the key intermediate. Further reactivity studies of the chiral cyclic silene with HCl resulted in a stereoselective addition to the Si=C bond, while the fully selective formation of two stereocenters was achieved. The quantitative stereoselective addition of CO₂ and HCl to a Si=C bond is unprecedented.     

Computational Design of Iron Diphosphine Complexes with Pendant Amines for Hydrogenation of CO2 to Methanol: A Mimic of [NiFe] Hydrogenase

Inspired by the active‐site structure of the [NiFe] hydrogenase, we have computationally designed the iron complex [P^tBu₂N^tBu₂)Fe(CN)₂CO] by using an experimentally ready‐made diphosphine ligand with pendant amines for the hydrogenation of CO₂ to methanol. Density functional theory calculations indicate that the rate‐determining step in the whole catalytic reaction is the direct hydride transfer from the Fe center to the carbon atom in the formic acid with a total free energy barrier of 28.4 kcal mol^?1 in aqueous solution. Such a barrier indicates that the designed iron complex is a promising low‐cost catalyst for the formation of methanol from CO₂ and H₂ under mild conditions. The key role of the diphosphine ligand with pendent amine groups in the reaction is the assistance of the cleavage of H₂ by forming a Fe?H^δ????H^δ+?N dihydrogen bond in a fashion of frustrated Lewis pairs.     

Alternating Metal-Ligand Coordination Improves Electrocatalytic CO2 Reduction by a Mononuclear Ru Catalyst**

Molecular electrocatalysts for CO₂-to-CO conversion often operate at large overpotentials, due to the large barrier for C−O bond cleavage. Illustrated with ruthenium polypyridyl catalysts, we herein propose a mechanistic route that involves one metal center that acts as both Lewis base and Lewis acid at different stages of the catalytic cycle, by density functional theory in corroboration with experimental FTIR. The nucleophilic character of the Ru center manifests itself in the initial attack on CO₂ to form [ Ru -CO₂]⁰, while its electrophilic character allows for the formation of a 5-membered metallacyclic intermediate, [ Ru -CO₂CO₂]^0,c, by addition of a second CO₂ molecule and intramolecular cyclization. The calculated activation barrier for C−O bond cleavage via the metallacycle is decreased by 34.9 kcal mol⁻¹ as compared to the non-cyclic adduct in the two electron reduced state of complex 1 . Such metallacyclic intermediates in electrocatalytic CO₂ reduction offer a new design feature that can be implemented consciously in future catalyst designs.     

Ligand‐Controlled CO2 Activation Mediated by Cationic Titanium Hydride Complexes, [LTiH]+ (L=Cp2, O)

CO₂ activation mediated by [LTiH]⁺ (L=Cp₂, O) is observed in the gas phase at room temperature using electrospray‐ionization mass spectrometry, and reaction details are derived from traveling wave ion‐mobility mass spectrometry. Wheresas oxygen‐atom transfer prevails in the reaction of the oxide complex [OTiH]⁺ with CO₂, generating [OTi(OH)]⁺ under the elimination of CO, insertion of CO₂ into the metal–hydrogen bond of the cyclopentadienyl complex, [Cp₂TiH]⁺, gives rise to the formate complex [Cp₂Ti(O₂CH)]⁺. DFT‐based methods were employed to understand how the ligand controls the observed variation in reactivity toward CO₂. Insertion of CO₂ into the Ti?H bond constitutes the initial step for the reaction of both [Cp₂TiH]⁺ and [OTiH]⁺, thus generating formate complexes as intermediates. In contrast to [Cp₂Ti(O₂CH)]⁺ which is kinetically stable, facile decarbonylation of [OTi(O₂CH)]⁺ results in the hydroxo complex [OTi(OH)]⁺. The longer lifetime of [Cp₂Ti(O₂CH)]⁺ allows for secondary reactions with background water, as a result of which, [Cp₂Ti(OH)]⁺ is formed. Further, computational studies reveal a good linear correlation between the hydride affinity of [LTi]²⁺ and the barrier for CO₂ insertion into various [LTiH]⁺ complexes. Understanding the intrinsic ligand effects may provide insight into the selective activation of CO₂.     

Bimetallic Cooperativity and Hydrogen Bonding Allow Efficient Reduction of CO2

Reducing CO₂ selectively to one of the several C1 products is challenging, as the thermodynamic reduction potentials for the different n e⁻/n H⁺ reductions of CO₂ are similar and so is the reduction potential for H⁺ reduction. Recently, Halime, Aukauloo, and co-workers have taken inspiration from the active site of nickel CO dehydrogenase (Ni-CODH) to design bimetallic iron porphyrins bridged by a urea moiety. These complexes show fast and selective reduction of CO₂ to CO and the results suggest a Ni-CODH type mechanism at play where one of the two metals binds and reduces the CO₂ while the other stabilizes the reduced species by forming a bridged complex, facilitating the C−O bond cleavage.     

A Light/Ketone/Copper System for Carboxylation of Allylic C−H Bonds of Alkenes with CO2

A photo‐induced carboxylation reaction of allylic C?H bonds of simple alkenes with CO₂ is prompted by means of a ketone and a copper complex. The unique carboxylation reaction proceeds through a sequence of an endergonic photoreaction of ketones with alkenes forming homoallyl alcohol intermediates and a thermal copper‐catalyzed allyl transfer reaction from the homoallyl alcohols to CO₂ through C?C bond cleavage.     

Kinetics and mechanism of the oxidation of 4‐methyl‐3‐thiosemicarbazide by acidic bromate

The oxidation of 4‐methyl‐3‐thiosemicarbazide (MTSC) by bromate and bromine was studied in acidic medium. The stoichiometry of the reaction is extremely complex, and is dependent on the ratio of the initial concentrations of the oxidant to reductant. In excess MTSC and after prolonged standing, the stoichiometry was determined to be H₃CN(H)CSN(H)NH₂ + 3BrO₃^? → 2CO₂ + NH₄⁺ + SO₄^2? + N₂ + 3Br^? + H⁺ (A). An interim stoichiometry is also obtained in which one of the CO₂ molecules is replaced by HCOOH with an overall stoichiometry of 3H₃CN(H)CSN(H)NH₂ + 8BrO₃^? → CO₂ + NH₄⁺ + SO₄^2? + HCOOH + N₂ + 3Br^? + 3H⁺ (B). Stoichiometry A and B are not very different, and so mixtures of the two were obtained. Compared to other oxidations of thiourea‐based compounds, this reaction is moderately fast and is first order in both bromate and substrate. It is autocatalytic in HOBr. The reaction is characterized by an autocatalytic sigmoidal decay in the consumption of MTSC, while in excess bromate conditions the reaction shows an induction period before autocatalytic formation of bromine. In both cases, oxybromine chemistry, which involves the initial formation of the reactive species HOBr and Br₂, is dominant. The reactions of MTSC with both HOBr and Br₂ are fast, and so the overall rate of oxidation is dependent upon the rates of formation of these reactive species from bromate. Our proposed mechanism involves the initial cleavage of the C? N bond on the azo‐side of the molecule to release nitrogen and an activated sulfur species that quickly and rapidly rearranges to give a series of thiourea acids. These thiourea acids are then oxidized to the sulfonic acid before cleavage of the C? S bond to give SO₄^2?, CO₂, and NH₄⁺. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 237–247, 2002     

Evidence for the synchronous cleavage of C—C and C—H bonds bis(2,2′-bipyridyl)copper(II) permanganate oxidation of cobalt(III) bound and unbound α-hydroxy acids

The kinetics of bis(2,2-bipyridyl)copper(II) permanganate oxidation of Co^III bound and unbound -hydroxy acids such as mandelic, lactic and glycolic acids have been studied in aqueous MeCO₂H. The reaction exhibits second order kinetics: first order in each reactant. The formation of Co^II, PhCHO and CO₂ to the extent of 24% [Co^III]_initial indicate C—bonds;H cleavage occurring to the extent of 24% and ca. 76% yield of the phenylglyoxylato-pentaamminecobalt(III) complex indicate C—H cleavage occurring to the extent of 76%.     

Dual activity of electrocatalytic activated CO2 toward pyridine for synthesis of isonicotinic acid: An EC′C′C mechanism

The present study demonstrates the indirect electrocatalytic synthesis of isonicotinic acid using a Ni complex, [Ni^II(Me₄-(NO₂Bzo)₂[14]tetraeneN₄)], in an acetonitrile solution at room temperature. The complex was used as an excellent electrocatalyst for the reduction of carbon dioxide. The results indicate that the electrocatalytic reduction product of CO₂ (CO₂⁻) has a dual role in the electrosynthesis of isonicotinic acid. The dual activity of CO₂⁻ involved indirect electrocatalytic reduction of pyridine as well as its radical reaction with pyridine radical anion to form isonicotinic acid. Finally, EC′C′C mechanism was proposed for the synthesis of isonicotinic acid. In contrast, the reaction of pyridine with CO₂ in the absence of the complex follows an EC′C mechanism, and the final product is 4,4′-bipyridine.     

Facile C=O Bond Splitting of Carbon Dioxide Induced by Metal–Ligand Cooperativity in a Phosphinine Iron(0) Complex

New iron complexes [Cp*Fe L ]^? ( 1‐σ and 1‐π , Cp*=C₅Me₅) containing the chelating phosphinine ligand 2‐(2′‐pyridyl)‐4,6‐diphenylphosphinine ( L ) have been prepared, and found to undergo facile reaction with CO₂ under ambient conditions. The outcome of this reaction depends on the coordination mode of the versatile ligand L . Interaction of CO₂ with the isomer 1‐π , in which L binds to Fe through the phosphinine moiety in an η⁵ fashion, leads to the formation of 3‐π , in which CO₂ has undergone electrophilic addition to the phosphinine group. In contrast, interaction with 1‐σ —in which L acts as a σ‐chelating [P,N] ligand—leads to product 3‐σ in which one C=O bond has been completely broken. Such CO₂ cleavage reactions are extremely rare for late 3d metals, and this represents the first such example mediated by a single Fe centre.     

Mechanistic studies on the N-formylation of amines with CO2 and hydrosilane catalyzed by a Cu–diphosphine complex

The reaction mechanism, reaction intermediates, and catalytically active species of the Cu–diphosphine-catalyzed N-formylation of amines (R¹R²NH) with CO₂ and hydrosilane were investigated. The NMR and kinetic experiments show that the catalytically active species is a Cu-hydride–diphosphine complex, which was generated from the Cu precursor, diphosphine ligand, and hydrosilane. Isotopic experiments using ¹³CO₂ and deuterated hydrosilane revealed the incorporation of the carbonyl group of CO₂ and the H atom of Si–H moiety into the formamide (R¹R²NCHO) product. The formation of a Cu-formate species as an intermediate of the reaction was clarified by in situ ¹H and ¹³C NMR studies.     

Diagnostic cyclisation reactions which follow phosphate transfer to carboxylate anion centres for energised [M-H]- anions of pTyr-containing peptides

The low‐energy negative ion phosphoTyr to C‐terminal ‐CO₂PO₃H₂ rearrangement occurs for energised peptide [M–H]^– anions even when there are seven amino acid residues between the pTyr and C‐terminal amino acid residues. The rearranged C‐terminal ‐CO₂PO₂H(O^–) group effects characteristic S_Ni cyclisation/cleavage reactions. The most pronounced of these involves the electrophilic central backbone carbon of the penultimate amino acid residue. This reaction is aided by the intermediacy of an H‐bonded intermediate in which the nucleophilic and electrophilic reaction centres are held in proximity in order for the S_Ni cyclisation/cleavage to proceed. The ΔG_reaction is +184 kJ mol^?1 with the barrier to the S_Ni transition state being +240 kJ mol^?1 at the HF/6‐31 + G(d)//AM1 level of theory. A similar phosphate rearrangement from pTyr to side chain CO₂^– (of Asp or Glu) may also occur for energised peptide [M–H]^– anions. The reaction is favourable: ΔG_reaction is ?44 kJ mol^?1 with a maximum barrier of +21 kJ mol^?1 (to the initial transition state) when Asp and Tyr are adjacent. The rearranged species R¹‐Tyr‐NHCH(CH₂CO₂PO₃H^–)COR² (R¹ = CHO; R² = OCH₃) may undergo an S_Ni six‐centred cyclisation/cleavage reaction to form the product anion R¹‐Tyr(NH^–). This process has a high energy requirement [ΔG_reaction = +224 kJ mol^?1, with the barrier to the S_Ni transition state being +299 kJ mol^?1]. Copyright © 2011 John Wiley & Sons, Ltd.     

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.     

Complexation and abstraction channels in the Al + CO2 reaction

The AI + CO₂ reaction is studied in the gas phase at 296 K by laser-induced fluorescence monitoring of Al and AlO. Pressure dependence of the effective bimolecular rate constant in the range 10–600 Torr (Ar+CO₂) indicates a complex formation channel yielding a stable Al·CO₂ adduct. Observation of AlO confirms the presence of an abstraction channel. A simple chemical activation mechanism is used to interpret the pressure dependence of the effective bimolecular rate constant. The activation energy for Al·CO₂ complex formation is estimated at ⪢ 1.0 kcal mol⁻¹, and the binding energy is estimated at ⪢ 9 kcal mol⁻¹.     

A Water-Soluble Sodium Pectate Complex with Copper as an Electrochemical Catalyst for Carbon Dioxide Reduction

A selective noble-metal-free molecular catalyst has emerged as a fruitful approach in the quest for designing efficient and stable catalytic materials for CO₂ reduction. In this work, we report that a sodium pectate complex of copper (PG-NaCu) proved to be highly active in the electrocatalytic conversion of CO₂ to CH₄ in water. Stability and selectivity of conversion of CO₂ to CH₄ as a product at a glassy carbon electrode were discovered. The copper complex PG-NaCu was synthesized and characterized by physicochemical methods. The electrochemical CO₂ reduction reaction (CO₂RR) proceeds at −1.5 V vs. Ag/AgCl at ~10 mA/cm² current densities in the presence of the catalyst. The current density decreases by less than 20% within 12 h of electrolysis (the main decrease occurs in the first 3 h of electrolysis in the presence of CO₂). This copper pectate complex (PG-NaCu) combines the advantages of heterogeneous and homogeneous catalysts, the stability of heterogeneous solid materials and the performance (high activity and selectivity) of molecular catalysts.     

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

Continued efforts are made for the utilization of CO₂ as a C₁ feedstock for regeneration of valuable chemicals and fuels. Mechanistic study of molecular (electro‐/photo‐)catalysts disclosed that initial step for CO₂ activation involves either nucleophilic insertion or direct reduction of CO₂. In this study, nucleophilic activation of CO₂ by complex [(NO)₂Fe(μ‐^MePyr)₂Fe(NO)₂]^2? ( 2 , ^MePyr=3‐methylpyrazolate) results in the formation of CO₂‐captured complex [(NO)₂Fe(^MePyrCO₂)]^? ( 2‐CO₂ , ^MePyrCO₂=3‐methyl‐pyrazole‐1‐carboxylate). Single‐crystal structure, spectroscopic, reactivity, and computational study unravels 2‐CO₂ as a unique intermediate for reductive transformation of CO₂ promoted by Ca²⁺. Moreover, sequential reaction of 2 with CO₂, Ca(OTf)₂, and KC₈ established a synthetic cycle, 2 → 2‐CO₂ → [(NO)₂Fe(μ‐^MePyr)₂Fe(NO)₂] ( 1 ) → 2 , for selective conversion of CO₂ into oxalate. Presumably, characterization of the unprecedented intermediate 2‐CO₂ may open an avenue for systematic evaluation of the effects of alternative Lewis acids on reduction of CO₂.     

Electride-Sponsored Radical-Controlled CO2 Reduction to Organic Acids: A Computational Design

Converting CO₂ into high-value chemicals has been regarded as an important solution for a sustainable low-carbon economy. In this work, we have theoretically designed an innovative strategy for the absorption and activation of CO₂ by the electride N3Li, that is, 1,3,5(2,6)-tripyridinacyclohexaphane (N3) intercalated by lithium. DFT computations showed that the interaction of CO₂ with N3Li leads to the catalytic complex N3Li(η²-O₂C), which can initiate the radical-controlled reduction of another CO₂ to form organic acids through radical reactions in the gas phase. The CO₂ reduction consists of four steps: (1) The formation of N3Li(η²-O₂C) through the combination of N3Li and CO₂, (2) hydrogen abstraction from RH (R=H, CH₃, and C₂H₅) by N3Li(η²-O₂C) to form the radical R^. and N3Li(η²-O₂C)H, (3) the combination of CO₂ and the radical R^. to form RCOO^., and (4) intermolecular hydrogen transfer from the intermediate N3Li(η²-O₂C)H to RCOO^.. In the whole reaction process, the CO₂ moiety in the complex N3Li(η²-O₂C) maintains a certain radical character at the carbon atom of CO₂ and plays a self-catalyzing role. This work represents the first example of electride-sponsored radical-controlled CO₂ reduction, and thus provides an alternative strategy for CO₂ conversion.     

Photo-thermal Cooperative Carbonylation of Ethanol with CO2 on Cu2O-SrTiCuO3-x

Carbonylation of ethanol with CO₂ as carbonyl source into value-added esters is of considerable significance and interest, while remains of great challenge due to the harsh conditions for activation of inert CO₂ in that the harsh conditions result in undesired activation of α-C−H and even cleavage of C−C bond in ethanol to deteriorate the specific activation of O−H bond. Herein, we propose a photo-thermal cooperative strategy for carbonylation of ethanol with CO₂, in which CO₂ is activated to reactive CO via photo-catalysis with the assistance of *H from thermally-catalyzed dissociation of alcoholic O−H bond. To achieve this proposal, an interfacial site and oxygen vacancy both abundant SrTiCuO_3-x supported Cu₂O (Cu₂O-SrTiCuO_3-x) has been designed. A production of up to 320 μmol g⁻¹ h⁻¹ for ethyl formate with a selectivity of 85.6 % to targeted alcoholic O−H activation has been afforded in photo-thermal assisted gas-solid process under 3.29 W cm⁻¹ of UV/Vis light irradiation (144 °C) and 0.2 MPa CO₂. In the photo-driven activation of CO₂ and following carbonylation, CO₂ activation energy decreases to 12.6 kJ mol⁻¹, and the cleavage of alcoholic α-C−H bond has been suppressed.