Liselotte Siegfried

Modified electrode surfaces for catalytic reduction of carbon dioxide

Ni(II) complexes with substituted tetraazacyclotetradecanes were incorporated into Nafion films or assembled in monolayers on electrodes in order to obtain electrocatalytic systems for the reduction of carbon dioxide. Reproducible and stable loading of the Nafion films with the complexes has been observed. Systematic increase of N-methyl substitution of the catalytic [Ni(cyclam)]2+ complex leads to a shift of the Ni(II)L/Ni(I)L formal potential to more positive values, however, at the same time to a decrease of the catalytic current of CO2 reduction. Monolayer techniques — the Langmuir–Blodgett and self-assembly methods — were found advantageous for the preparation of electrode surfaces active in the catalytic reduction of CO2 compared to Nafion coatings containing the catalyst. Glassy carbon electrode was suitable for the transfer of Langmuir–Blodgett monolayers. Catalysis was observed only when the electrode was touching the monolayer at the air–water interface or was covered in the dipping mode, hence when the catalyst molecules were oriented with their alkyl chains towards the substrate and macrocyclic head-group towards the solution. Most efficient catalysis was found using electrodes coated by the self-assembly procedure with monolayers of the cyclam complex modified with a pyridine side group. The role of the pyridine moieties was to anchor the catalytic monolayer to the electrode surface.

Bicyclam [6,6'-bi(1,4,8,11-tetra-azacyclotetradecane)]: a ditopic receptor for homo- and hetero-bimetallic complexes

Bicyclam [6,6′-bi(1,4,8,11-tetra-azacyclotetradecane)](L2) was prepared by reducing 6,6′-bi(1,4,8,11-tetra-azacyclotetradecan-5,7-dione)(L1) with B2H6 in bis(2-methoxyethyl) ether. Ligand L2 forms homo- and hetero-binuclear complexes with Cu2+ and Ni2+, the spectral properties and the electrochemistry of which have been studied. Whereas the visible spectra are very similar to those of the mononuclear complexes with cyclam (1,4,8,11-tetra-azacyclotetradecane), the electrochemistry indicates a weak interaction between the metal ions co-ordinated by bicyclam, as shown by a shift of the second oxidation potential M2+→ M3+ to more positive values.

Effects of ligand structure on the adsorption of nickel tetraazamacrocyclic complexes and electrocatalytic CO2 reduction

N-methyl substituted 1,4,8,11-tetraazacyclotetradecanes were used as ligands for Ni(II) in order to study the relationship between the structure of the complex and its catalytic activity in the electrochemical reduction of carbon dioxide. Systematic increase of N-methyl substitution in [Ni(cyclam)]2+ was found to increase the adsorption of the Ni(II) complex on the mercury electrode and at the same time decreased the stability of the reduced Ni(I) form. Binding of CO2 requires the catalyst to be in the stable, adsorbed and reduced trans I (or RSRS) Ni(I) form. For the tetramethylated derivative the strong adsorption of the Ni(II) complex was found to make its reduction to the catalytically active Ni(I) form impossible. This explains the lack of CO2 catalytic reduction when the tetraazamacrocyclic ligand is fully substituted by methyl groups.

H+- and CN−-induced dissociation of Ni2+-complexes with tetra-N- substituted 1,4,8,11-tetraazacyclotetradecanes

Abstract The H + and the CN − induced dissociation kinetics of the Ni 2+ complexes with a series of tetra- N - substituted 1,4,8,11-tetraazacyclotetradecanes have been studied by spectrophotometry at 25°C. The pH dependence of the H + induced reaction follows the rate law ν 1 , = κ 1 [NiL 2+ ] + κ 2 [NiLH 3+ ], in which NiLH 3+ is a protonated species present in equilibrium with NiL 2+ at pH values below 2. The rate constant κ2 is larger than k 1 by a factor of 50–120, indicating that, when the complex is protonated, its tendency to dissociate is greatly increased. Both κ 1 and κ2 are relatively insensitive to the nature of the N -alkyl groups. The CN − induced process is described by ν2 = κ 3 [NiL(CN) + ][CN − ], where NiL(CN)+ is a ternary species rapidly and fully formed under the experimental conditions. The values of κ3 also do not depend on the nature of the N-alkyl groups. A mechanisms considering different attack possibilities of CN − is discussed.

Electrochemical behaviour of isomers of a tetraazamacrocyclic Ni(II) complex in solutions saturated with argon, CO and CO2

Abstract Isomers of [Ni(CRH)] 2+ have been isolated and studied by d.c. and a.c. voltammetry on mercury electrodes in aqueous solution. The configurational isomers of [Ni(CRH)] 2+ were found to differ in their stability in the solution and the isomerization process was monitored voltammetrically. The adsorptivity of the isomer A on mercury electrodes was found to be stronger than that of the isomer B, while no differences were seen for the adsorbed Ni(I) complexes. The solution resident [Ni(CRH)] + complexes are more labile than the corresponding Ni(II) compounds, hence upon reduction the isomer B undergoes fast transformation into the more stable isomer A. The electroreduction of [Ni(CRH)] 2+ in the presence of CO and CO 2 is described and the influence of the isomeric form used in the processes is discussed.

The dual role of self-assembled monolayers of a tetraazamacrocyclic copper(II) complex in ascorbate oxidation catalysis

Abstract The copper(II) complex with 1,4,8-trimethyl-11-(2-thioethyl)-1,4,8,11-tetraazacyclotetradecane was adsorbed on the surface of glassy carbon (GCE) and gold electrodes using the self-assembly method. More stable coverage of the electrode was obtained using a gold substrate when the complex is anchored to the electrode through its thioethyl substituent. The amount of the complex on the electrode surface based on the charge of the Cu 2+ /Cu + peak points to the nearly monolayer coverage of the electrode surface. The catalytic effect of the monolayer towards ascorbic acid oxidation was observed in neutral and alkaline solutions, and ascribed to the electrostatic interaction of ascorbate anion with the positively charged copper complex bound to the electrode surface. In dioxygenated solutions, the copper complex adsorbed on the electrode catalyses both the electrooxidation and auto-oxidation of ascorbate. The copper-catalyzed autoxidation of ascorbic acid leads to a decrease of the ascorbate electrooxidation peak with time. The progress of this reaction was monitored based on the changes of the electrooxidation current. Copper catalyzed autoxidation of ascorbate is known to require turnover between Cu 2+ and Cu + ions. This indicates that the molecules of the copper complex anchored through the thiol groups to the gold electrode, and tightly packed on the surface play a dual role in the ascorbate oxidation process. The role of the monolayer in enhancing the autoxidation of ascorbic acid resembles the function of copper in some biological complexes catalyzing in vivo ascorbate oxidation.

Kinetics and Mechanism of the Cu2+ Induced Hydrolysis of Nitrile Groups in the Side Chain of Tetraazamacrocycles. Models for Nitrilases

Abstract A series of mono-N-functionalized tetraaza macro-cycles having a nitrile group in their side chain have been synthesized and the kinetics and mechanism of the Cu2+ induced hydrolysis has been studied. Two factors were systematically varied: the length of the side chain and thus the distance between Cu2+ and the nitrile group, as well as the rigidity of the macro-cycle by introducing an additional ethylene bridge. The mechanism of the hydrolysis proceeds by an intramolecular attack of a coordinated OH− onto the nitrile group in a five or six center transition state. The intramolecular nature of the reaction has been proven (a) by the pH dependence of the hydrolysis, which in some cases has a plateau at high pH values, (b) by the competitive inhibition with SCN−, and (c) by the spectral changes observed at high pH. The sequence of Cu2+ induced hydrolysis rates is the following: flexible macrocycle with a short chain > rigid macrocycle with a short chain > flexible macrocycle with a longer chain < rigid macro-cycle with a longer chain. The length of the side chain, which determines whether a five or six center transition state is formed, is the most important factor. The fastest hydrolysis has a half-life time of about 50 ms at pH 12.5 and 25°C and indicates the efficiency of the metal ion. The rigidity of the macrocycle also influences the reactivity since in the rigid complexes on one side the Cu2+ ion is less accessible for OH− to give the reactive intermediate and on the other side the transition state is less reactive because of topological aspects.

Electroreduction of CO2 catalyzed by Ni (II) tetraazamacrocyclic complexes-reasons of poisoning of the catalytic surfaces

Abstract Mechanism of electrode poisoning in the reduction of CO 2 catalyzed by [Ni(CRH)] 2+ and [Ni(cyclam)] 2+ is studied by cyclic and stripping dc voltammetry. In both cases the product of CO 2 reduction is CO, which undergoes further reaction with the Ni(I) form of the catalyst. Comparison of the process in solutions saturated with CO 2 and CO proved that the decay of catalytic efficiency was due to the formation of Ni(0) carbonyl compound blocking the electrode surface. Stripping voltammetry in diluted solutions of catalyst allowed to neglect bulk reactions of catalyst and revealed the transformation of adsorbed Ni(I) complex catalyst into the catalytically inactive Ni(0) carbonyl deposit. The latter is oxidized yielding a well developed anodic stripping peak. This signal was used to quantify the extent of harmful deposit formation and was shown to be distinctly larger in case of the [Ni(CRH)] 2+ catalyst.

Kinetic studies of the on/off reaction of the amino group in the side chain of Cu(II), Ni(II), and Co(II) complexes with 14-membered tetraazamacrocycles

The kinetics of the on/off reaction of the amino group in the side chain of tetraazamacrocyclic Cu2+, Ni2+ and Co2+ complexes has been measured. The rate law k(obs)=k(0)+k(H)[H+]+k(OH)/[H+], the sum of the forward and reverse reaction, gives rise to u-shaped pH dependences from which the three rate constants can be determined. k(H) describes the proton assisted dissociation of the amino group bound to the metal ion and is roughly correlated to the equilibrium constant of the reaction. k(OH) is determined by the protonation constant of the free amino group and the rate constant describing the binding of the amino group to the metal ion. k(0) is composed of the rate constant for the opening of the chelate ring without proton assistance and the rate for the reactivity of the ammonium group in the formation of the chelate ring. Our results show that the rates of the opening and closing of the chelate ring are very little dependent on the nature of the metal ion.

A fully automated pH–NMR titration set-up for protonation studies

A fully automated pH–NMR titration set-up, consisting of a Bruker 250 MHz NMR instrument and a potentiometric titration unit, has been built so that pH titrations with simultaneous recording of 1H and 31P NMR spectra at each titration point can be run. The new set-up has been tested by studying the protonation of three diazacrown ethers having dangling phosphonate groups. From the fitting of the pH dependences of the chemical shifts of the 31P and 1H signals the protonation constants as well as the chemical shifts of the individual protonated species were obtained using the program HYPNMR. The main advantages of the new set-up are the relatively small amount of substance (0.05 mmol) needed for a single titration and the fact that once started the system needs no operator during the whole titration time (about 20 h).