Electrochemical epoxidation of olefins using water as an oxygen atom source is emerging as an alternative approach for an atom economic and sustainable method towards a highly selective synthesis of epoxides. We report an electrochemical procedure for epoxidation of cyclooctene using water as the sole oxygen atom source over a sodium dodecyl sulfonate (SDS) modified nickel hydroxide Ni(OH)2 catalyst directly grown on Ni foam. The SDS modification facilitates the mass transfer of cyclooctene towards the anode, thus achieving a 2.5-fold higher conversion with more than 90 % selectivity towards the corresponding epoxide compared with pure Ni(OH)2 catalyst.
Renewable electricity-powered nitrate (NO3 - ) reduction reaction (NO3 RR) offers a net-zero carbon route to the realization of high ammonia (NH3 ) productivity. However, this route suffers from low energy efficiency (EE, with a half-cell EE commonly <36%), since high overpotentials are required to overcome the weak NO3 - binding affinity and sluggish NO3 RR kinetics. To alleviate this, a rational catalyst design strategy that involves the linear assembly of sub-5 nm Cu/Co nanophases into sub-20 nm thick nanoribbons is suggested. The theoretical and experimental studies show that the Cu-Co nanoribbons, similar to enzymes, enable strong NO3 - adsorption and rapid tandem catalysis of NO3 - to NH3 , owing to their richly exposed binary phase boundaries and adjacent Cu-Co sites at sub-5 nm distance. In situ Raman spectroscopy further reveals that at low applied overpotentials, the Cu/Co nanophases are rapidly activated and subsequently stabilized by a specifically designed redox polymer that in situ scavenges intermediately formed highly oxidative nitrogen dioxide (NO2 ). As a result, a stable NO3 RR with a current density of ≈450 mA cm-2 is achieved, a Faradaic efficiency of >97% for the formation of NH3 , and an unprecedented half-cell EE of ≈42%.
A bimetallic triply fused copper(II) porphyrin complex (1) was prepared, comprising two monomeric porphyrin units linked through ß-ß, meso-meso, ß'-ß' triple covalent linkages and exhibiting remarkable catalytic activity for the electrochemical hydrogen evolution reaction in comparison to the analogous monomeric copper(II) porphyrin complex (2). Electrochemical investigations in the presence of a proton source (trifluoroacetic acid) confirmed that the catalytic activity of the fused metalloporphyrin occurred at a significantly lower overpotential (≈320â mV) compared to the non-fused monomer. Controlled potential electrolysis combined with kinetic analysis of catalysts 1 and 2 confirmed production of hydrogen, with 96 and 71 % faradaic efficiencies and turnover numbers of 102 and 18, respectively, with an observed rate constant of around 107 â s-1 for the dicopper complex. The results thus firmly establish triply fused porphyrin ligands as outstanding candidates for generating highly stable and efficient molecular electrocatalysts in combination with earth-abundant 3d transition metals.
Metalloporphyrins , Porphyrins , Porphyrins/chemistry , Copper/chemistry , Hydrogen , Kinetics , Metalloporphyrins/chemistry
Metal complexes of multi-porphyrins and multi-corroles are unique systems that display a host of extremely interesting properties. Availability of free meso and ß positions allow formation of different types of directly linked bis-porphyrins giving rise to intriguing optical and electronic properties. While the fields of metalloporphyrin and corroles monomer have seen exponential growth in the last decades, the chemistry of metal complexes of bis-porphyrins and bis-corroles remain rather underexplored. Therefore, the impact of covalent linkages on the optical, electronic, (spectro)electrochemical, magnetic and electrocatalytic activities of metal complexes of bis-porphyrins and -corroles has been summarized in this review article. This article shows that despite the (still) somewhat difficult synthetic access to these molecules, their extremely exciting properties do make a strong case for pursuing research on these classes of compounds.
Coordination Complexes , Metalloporphyrins , Porphyrins , Coordination Complexes/chemistry , Metalloporphyrins/chemistry , Porphyrins/chemistry
Alkenes are known to undergo oxidation to radical cations and dications. The radical cations are often highly reactive and not stable under air. Herein, we report the synthesis, isolation, characterization, and molecular structure of an alkene-derived radical cation A, which is stable in air both in the solid state and in solution. The access to this compound was facilitated from E-diamino tri-substituted alkene B as a synthon for the synthesis of A through one-electron oxidation. The E-diamino tri-substituted alkene B was synthesized by the two-electron reduction of N,N'-1,2-propylene-bridged bis-2-phenyl-pyrrolinium cation C. Under two-electron oxidation, alkene B transforms back to cation C involving a double carbocation rearrangement.
Herein, we report diamidocarbene (DAC)-based Thiele and Tschitschibabin hydrocarbons, diradicaloids that contain four carbonyl/amido functional groups. The impact of two different π-conjugated spacers, p-phenylene vs p,p'-biphenylene, has been realized. The quantum chemical calculations suggest diamidocarbene (DAC)-based Thiele hydrocarbon (p-phenylene bridged) closed-shell singlet is the ground state, whereas for the diamidocarbene (DAC)-based Tschitschibabin hydrocarbon (p,p'-biphenylene bridged), open-shell singlet is the ground state. The influence of two different π-conjugated spacers also has been reflected in their UV-vis spectra. To gain more information on the diamidocarbene (DAC)-based Thiele and Tschitschibabin hydrocarbons, we have also carried out cyclic voltammetry investigations along with UV-vis-NIR-spectroelectrochemical studies of their corresponding 2-e oxidized product.
The systematic combination of N-heterocyclic olefins (NHOs) with fluoroarenes resulted in twisted push-pull alkenes. These alkenes carry electron-donating cyclicdiamino substituents and two electron-withdrawing fluoroaryl substituents in the geminal positions. The synthetic method can be extended to a variety of substituted push-pull alkenes by varying the NHO as well as the fluoroarenes. Solid-state molecular structures of these molecules reveal a notable elongation of the central C-C bond and a twisted geometry in the alkene motif. Absorption properties were investigated with UV-vis spectroscopy. The redox properties of the twisted push-pull alkenes were probed with electrochemistry as well as UV-vis/NIR and EPR spectroelectrochemistry, while the electronic structures were computationally evaluated and validated.
Herein, we report the synthesis, characterization, and reactivity of α,α'-diamino-p-tetrafluoroquinodimethane, a p-tetrafluorophenylene-bridged monosubstituted carbene-based Thiele's hydrocarbon A. The compound exhibits a reversible two-step one-electron oxidation with a marginally stable radical cation state B. The in situ formation of the radical cation could be confirmed by electron paramagnetic resonance spectroscopy. Interestingly, α,α'-diamino-p-tetrafluoroquinodimethane fixates atmospheric oxygen to form a 16-membered peroxide-bridged macrocyclic compound C.
Carbene-based radicals are important for both fundamental and applied chemical research. Herein, extensive electrochemical investigations of nine different 1,2,3-triazolylidene selenium adducts are reported. It is found that the half-wave potentials of the first reduction of the selones correlate with their calculated LUMO levels and the LUMO levels of the corresponding triazolylidene-based mesoionic carbenes (MICs). Furthermore, unexpected quasi-reversibility of the reduction of two triazoline selones, exhibiting comparable reduction potentials, was discovered. Through UV/Vis/NIR and EPR spectroelectrochemical investigations supported by DFT calculations, the radical anion was unambiguously assigned to be triazoline centered. This electrochemical behavior was transferred to a triazolylidene-type MIC-gold phenyl complex resulting in a MIC-radical coordinated AuI species. Apart from UV-Vis-NIR and EPR spectroelectrochemical investigations of the reduction, the reduced gold-coordinated MIC radical complex was also formed in situ in the bulk through chemical reduction. This is the first report of a monodentate triazolylidene-based MIC ligand that can be reduced to its anion radical in a metal complex. The results presented here provide design principles for stabilizing radicals based on MICs.
α-Diketones are an important class of building blocks employed in many organic synthetic reactions. However, their coordination chemistry has rarely been explored. In light of this, our earlier report on [(acac)2RuII(µ-2,2'-pyridil)RuII(acac)2] (acac = acetylacetonate) showcased the sensitivity of a diketone fragment towards oxidative C-C cleavage. Following the lead, the synthesis of similar but stable diketo fragments containing diruthenium compounds was attempted. Three diruthenium compounds with the bridge 1,2-bis(2-hydroxyphenyl)ethane-1,2-dione (L) were prepared: diastereomeric [(acac)2RuIII(µ-L2-)RuIII(acac)2], 1a(rac)/1b(meso), [(bpy)2RuII(µ-L2-)RuII(bpy)2](ClO4)2, [2](ClO4)2 and [(pap)2RuII(µ-L2-)RuII(pap)2](ClO4)2, [3](ClO4)2 with ancillary ligands of different donating/accepting characteristics. The metal is stabilised in different oxidation states in these complexes: Ru(iii) is preferred in 1a/1b when σ-donating acac is used as the co-ligand whereas electron rich Ru(ii) is preferred in [2](ClO4)2 and [3](ClO4)2 when co-ligands of moderate to strong π-accepting properties are employed. The oxidative chemistry of these systems is of particular interest with respect to the participation of varying bridging-ligands which contain phenoxide groups. On the other hand, the reduction processes primarily resulting from the metal or the ancillary ligands are noteworthy as the normally reducible 1,2-diketo- group remains unreduced. These results have been rationalised and outlined from thorough experimental and theoretical investigations. The results presented here shed light on the stability of metal coordinated α-diketones as a function of their substituents.
Two Co(III) complexes (1Py2 and 2Py2) of new corrole ligands H3L1 (5,15-bis(p-methylcarboxyphenyl)-10-(o-methylcarboxyphenyl)corrole) and H3L2 (5,15-bis(p-nitrophenyl)-10-(o-methylcarboxyphenyl)corrole) with two apical pyridine ligands have been synthesized and thoroughly characterized by cyclic voltammetry, UV-vis-NIR, and EPR spectroscopy, spectroelectrochemistry, single-crystal X-ray diffraction studies, and DFT methods. Complexes 1Py2 and 2Py2 possess much lower oxidation potentials than cobalt(III)-tris-pentafluorophenylcorrole (Co(tpfc)) and similar corroles containing pentafluorophenyl (C6F5) substituents, thus allowing access to high oxidation states of the former metallocorroles using mild chemical oxidants. The spectroscopic (UV-vis-NIR and EPR) and electronic properties of several oxidation states of these complexes have been determined by a combination of the mentioned methods. Complexes 1Py2 and 2Py2 undergo three oxidations within 1.3 V vs FcH+/FcH in MeCN, and we show that both complexes catalyze water oxidation in an MeCN/H2O mixture upon the third oxidation, with kobs (TOF) values of 1.86 s-1 at 1.29 V (1Py2) and 1.67 s-1 at 1.37 V (2Py2). These values are five times higher than previously reported TOF values for C6F5-substituted cobalt(III) corroles, a finding we ascribe to the additional charge in the corrole macrocycle due to the increased oxidation state. This work opens up new possibilities in the study of metallocorrole water oxidation catalysts, particularly by allowing spectroscopic probing of high-oxidation states and showing strong substituent-effects on catalytic activity of the corrole complexes.
Herein, we report the rational design, synthesis, and characterization of α,α'-diamino-substituted-p-quinodimethanes, which are a group of partially substituted p-quinodimethanes. These exhibit two reversible one-electron redox steps and electrochromism in the ultraviolet, visible, and near-infrared regions. We were able to isolate the crystalline compounds of all three oxidation states: neutral, radical cation, and dication. The obtained results not only create the bridge between p-quinodimethane and α,α,α',α'-tetrasubstituted-p-quinodimethane, but also demonstrate the straightforward modular approach for the synthesis of π-conjugated open-shell compounds.
Bridging quinonoid ligands are important platforms for generating metal-based switchable optoelectronic and magnetic materials. A possible sound way of influencing the properties of the aforementioned materials is to modify the direct metal-ligand interface. We present herein a series of dinuclear RuII complexes where the set of donor atoms at the bridging quinonoid ligands range from [O,O,O,O], [O,O,O,N], [O,N,O,N] and [O,N,O,N']. Additionally, the substituents on the N-donors were varied as well (a total of eight different quinonoid bridges are compared). We also present a mononuclear RuII complex for comparison purposes. The dinuclear complexes act as switchable NIR dyes, absorbing in the NIR region in their mixed-valent RuII/RuIII form but not in the neighboring RuII/RuII and RuIII/RuIII states. The switching potentials (the potentials at which NIR absorptions appear) and the λmax of the NIR band can be fine-tuned by varying the donor atoms as well as the electron-donating ability of the substituents on the nitrogen atoms (tuning E by ca. 0.4 V and λmax by ca. 450 nm). Introducing more electron-rich substituents at the nitrogen atoms of the bridge results in higher band energies and more cathodic redox potentials. Unsymmetrical bridging ligands increase the thermodynamic stability of the mixed-valent state. Whereas almost all of the mixed-valent species presented here belong to the delocalised type III of the Robin-Day classification, the most unsymmetrical complex 2O,N(Mes) shows characteristic signs of a borderline Class-II-III compounds. This comprehensive study thus establishes the lesser used unsymmetrically substituted quinones as excellent bridges for generating and tuning a series of properties in their corresponding metal complexes.
The coulombic repulsion between two adjacent cation centres of 1,2-carbodications is known to decrease with π- and/or n-donor substituents by a positive charge delocalization. Here we report the delocalization of the positive charge of transient 1,2-carbodications having one H-substituent by an intramolecular base-coordination. N-heterocyclic olefin (NHO) derived 2-pyrrolidinyl appended trisubstituted geminal diazaalkenes were used for the generation of transient 1,2-carbodications through a 2-e chemical oxidation process. We have also studied the 1-e oxidation reaction of trisubstituted geminal diazaalkenes (electrochemically and chemically) and also studied them using in situ EPR spectroscopy.
Highly symmetric enneanuclear copper(ii) phosphates [Cu9(Pz)6(µ-OH)3(µ3-OH)(ArOPO3)4(DMF)3] (PzH = pyrazole, Ar = 2,6-(CHPh2)2-4-R-C6H2; R = Me, 2MeAr; Et, 2EtAr; iPr, 2iPrAr; and Ar = 2,6-iPr2C6H3, 2Dip) comprising nine copper(ii) centers and pyrazole, hydroxide and DMF as ancillary ligands were synthesized by a reaction involving the arylphosphate monoester, 1, copper(i)chloride, pyrazole, and triethylamine in a 4 : 9 : 6 : 14 ratio. All four complexes were characterized by single crystal structural analysis. The complexes contain two distinct structural motifs within the multinuclear copper scaffold: a hexanuclear unit and a trinuclear unit. In the latter, the three Cu(ii) centres are bridged by a µ3-OH. Each pair of Cu(ii) centers in the trinuclear unit are bridged by a pyrazole ligand. The hexanuclear unit is made up of three dinuclear Cu(ii) motifs where the two Cu(ii) centres are bridged by an -OH and a pyrazole ligand. The three dinuclear units are connected to each other by phosphate ligands. The latter also aid the fusion of the trinuclear and the hexanuclear motifs. Magnetic studies reveal a strong antiferromagnetic exchange between the Cu(ii) centres of the dinuclear units in the hexanuclear part and a strong spin frustration in the trinuclear part leading to a degenerate ground state.
A N-heterocyclic olefin (NHO), a terminal alkene selectively activates aromatic C-F bonds without the need of any additional catalyst. As a result, a straightforward methodology was developed for the formation of different fluoroaryl-substituted alkenes in which the central carbon-carbon double bond is in a twisted geometry.
Thiele, Chichibabin and Müller hydrocarbons are considered as classical Kekulé diradicaloids. Herein we report the synthesis and characterization of acyclic diaminocarbene (ADC)-based Thiele, Chichibabin, and Müller hydrocarbons. The calculated singlet-triplet energy gaps are ΔE S-T = -27.96, -3.70, -0.37 kcal mol-1, respectively, and gradually decrease with the increasing length of the π-conjugated spacer (p-phenylene vs. p,p'-biphenylene vs. p,p''-terphenylene) between the two ADC-scaffolds. In agreement with the calculations, we also experimentally observed the enhancement of paramagnetic diradical character as a function of the length of the π-conjugated spacer. ADC-based Thiele's hydrocarbon is EPR silent and exhibits very well resolved NMR spectra, whereas ADC-based Müller's hydrocarbon displays EPR signals and featureless NMR spectra at room temperature. The spacer also has a strong influence on the UV-Vis-NIR spectra of these compounds. Considering that our methodology is modular, these results provide a convenient platform for the synthesis of an electronically modified new class of carbon-centered Kekulé diradicaloids.
Reports on aerobic oxidation of amines to amides are rare, and those reported suffer from several limitations like poor yield or selectivity and make use of pure oxygen under elevated pressure. Herein, we report a practical and an efficient ruthenium-catalyzed synthetic protocol that enables selective oxidation of a broad range of primary aliphatic, heterocyclic and benzylic amines to their corresponding amides, using readily available reagents and ambient air as the sole oxidant. Secondary amines instead, yield benzamides selectively as the sole product. Mechanistic investigations reveal intermediacy of nitriles, which undergo hydration to afford amide as the final product.
A ligand controlled catalytic system for the aerobic oxidation of 1° amines to nitriles and imines has been developed where the varying π-acidic feature of BIAN versus phen in the frameworks of ruthenium catalysts facilitates switchable selectivity.
Oxidative transformations utilizing molecular oxygen (O2 ) as the stoichiometric oxidant are of paramount importance in organic synthesis from ecological and economical perspectives. Alcohol oxidation reactions that employ O2 are scarce in homogeneous catalysis and the efficacy of such systems has been constrained by limited substrate scope (most involve secondary alcohol oxidation) or practical factors, such as the need for an excess of base or an additive. Catalytic systems employing O2 as the "primary" oxidant, in the absence of any additive, are rare. A solution to this longstanding issue is offered by the development of an efficient ruthenium-catalyzed oxidation protocol, which enables smooth oxidation of a wide variety of primary, as well as secondary benzylic, allylic, heterocyclic, and aliphatic, alcohols with molecular oxygen as the primary oxidant and without any base or hydrogen- or electron-transfer agents. Most importantly, a high degree of selectivity during alcohol oxidation has been predicted for complex settings. Preliminary mechanistic studies including (18) O labeling established the in situ formation of an oxo-ruthenium intermediate as the active catalytic species in the cycle and involvement of a two-electron hydride transfer in the rate-limiting step.
