Towards an anthropogenic nitrogen cycle based on nitrite

The overall goal of this thesis was to investigate the feasibility of a new route to anthropogenic nitrogen fixation based on the oxidation of nitrogen (to give primarily nitrite), and then electrocatalytic conversion of nitrite to other N-containing species of interest, such as nitrate and nitric oxide (NO). In pursuit of this goal, the synthesis of metal-ligand coordination complexes that could act as electrocatalysts for the oxidation of nitrite to nitrate was attempted, as was the synthesis of metal-ligand coordination complexes that could act as electrocatalysts for the reduction of nitrite to NO. As a corollary to this, routes for the initial fixation reaction were also investigated, of which the ultrasonic generation of nitrite from aerated aqueous solutions was found to be the most promising. The work detailed in this thesis is organized in the following manner: In Chapter 1 we discuss coordination complexes that mimic the enzymes promoting the redox reactions of the nitrogen cycle involving nitrite as a substrate or product. During this introduction we will also give an overview of topics that are relevant to the following chapters, such as proton-coupled-electron transfer and basic kinetic treatment of catalytic reactions. Chapter 2 is a description of the different techniques used throughout this thesis. Once having set the bases, we shall start with the actual research, which corresponds to Chapters 3 to 6. Chapter 3 deals with the synthesis, characterization and catalytic properties of a copper coordination compound mimicking the active site of the copper nitrite reductase (CuNiR) class of enzymes. This chapter includes a detailed study of the kinetics and electrocatalytic properties of this complex towards the mono-electronic reduction of nitrite to nitric oxide. Chapters 4 and 5 deal with the unusual structures and spectroscopic properties of a number of new cobalt complexes that we isolated whilst trying to develop Mo(bis-dithiolene) coordination complexes that might act as analogues of the molybdenum nitrite oxidoreductase (MoNiOR), which oxidises nitrite to nitrate in nature. Our original Mo-containing targets proved impossible to obtain and are not discussed in this thesis. However, we found that cobalt readily makes coordination complexes with these bis-dithiolene ligands, which allowed us to isolate the compounds we present in Chapters 4 and 5. Hence in Chapter 4 we show the synthesis and the solvatochromic properties of mixed-ligand mono-nuclear Co-diimine o-catecholato complexes and compare these complexes with the analogous compounds prepared with o-benzenedithiolato ligands. Chapter 5 then discusses the synthesis and redox properties of a mixed-ligand di-cobalt coordination complex in which the two cobalt centres have (unprecedented) inequivalent metal coordination environments. Finally, in Chapter 6 we describe a much-underexplored way to fix nitrogen based on a sonochemical reaction. After a brief introduction we describe the optimisation of the procedure and comparisons with previous reports

Crystal structure of catena-poly[[(μ-6-{[bis­­(pyridin-2-ylmeth­yl)amino]­meth­yl}pyridine-2-carboxyl­ato)copper(II)] perchlorate aceto­nitrile monosolvate]

The crystal structure of the title compound, {[Cu(C19H17N4O2)]ClO4·C2H3N}n, is reported and compared to similar structures in the literature. The compound crystallizes in the monoclinic space group P21. The unit cell contains one complex mol­ecule in addition to perchlorate as the counter-ion and solvent (aceto­nitrile). The crystal packing evinces extended chains whereby the carboxyl­ate moiety on the 6-carboxyl­ato-2-(pyridyl­meth­yl)bis­(pyridin-2-ylmeth­yl)amine ligand bridges between two different copper centers in adjacent mol­ecules. This packing arrangement for the title compound appears to be unique when compared to allied structures in the literature. The perchlorate anion showed signs of disorder and its oxygen atoms were modelled over two sets of partially occupied sites, the occupancy of which was competitively refined to 0.564 (12)/0.436 (12). The crystal studied was refined as a two-component inversion twin

Attempted characterisation of phenanthrene-4,5-quinone and electrochemical synthesis of violanthrone-16,17-quinone. How does the stability of bay quinones correlate with structural and electronic parameters?

In bay quinones, two carbonyl moieties are forced into close proximity by their spatial arrangement, resulting in an interesting axially chiral and nonplanar structure. Two representatives of this little-explored class of compounds were investigated experimentally in this work. Electrochemical oxidation of 4,5-dihydroxyphenanthrene failed to provide evidence for the reversible formation of phenanthrene-4,5-quinone. Even at temperatures as low as T = 229 K, cyclic voltammograms did not show any evidence for reversibility, indicating that phenanthrene-4,5-quinone likely is a reactive intermediate even at low temperatures. Electrochemical oxidation of the larger homologue 16,17-dihydroxyviolanthrone, on the other hand, was reversible, and the quinone could be characterised by spectroelectrochemical means. The results of quantum chemical calculations confirm the experimental findings and indicate that a bay dicarbonyl moiety, also found in a number of angucycline antibiotics, does not necessarily have to confer extreme reactivity. However, in a series of phenanthrene quinones with an equal number (zero) of Clar sextets and a varying number of bay carbonyl groups (zero to two), there was a clear correlation between the triplet energy, taken as a measure of biradical character, and the number of bay carbonyl moieties, with the lowest triplet energy predicted for phenanthrene-4,5-quinone (two bay carbonyl moieties)

Proton-coupled electron transfer enhances the electrocatalytic reduction of nitrite to NO in a bioinspired copper complex

This work was supported by the EPSRC (Grant No. EP/ K031732/1) and the Royal Society (University Research Fellowship UF150104 to M.D.S.). The data which underpin this work are available at http://dx.doi.org/10.5525/gla. researchdata.590 and are available under a CC-BY licence.The selective and efficient electrocatalytic reduction of nitrite to nitric oxide (NO) is of tremendous importance, both for the development of NO-release systems for biomedical applications and for the removal of nitrogen oxide pollutants from the environment. In nature, this transformation is mediated by (among others) enzymes known as the copper-containing nitrite reductases. The development of synthetic copper complexes that can reduce nitrite to NO has therefore attracted considerable interest. However, there are no studies describing the crucial role of proton-coupled electron transfer during nitrite reduction when such synthetic complexes are used. Herein, we describe the synthesis and characterization of two previously unreported Cu complexes ( 3 and 4 ) for the electrocatalytic reduction of nitrite to NO, in which the role of proton-relaying units in the secondary coordination sphere of the metal can be probed. Complex 4 bears a pendant carboxylate group in close proximity to the copper center, while complex 3 lacks such functionality. Our results suggest that complex 4 is twice as effective an electrocatalyst for nitrite reduction than is complex 3 and that complex 4 is the best copper-based molecular electrocatalyst for this reaction yet discovered. The differences in reactivity between 3 and 4 are probed using a range of electrochemical, spectroscopic, and computational methods, which shed light on the possible catalytic mechanism of 4 and implicate the proton-relaying ability of its pendant carboxylate group in the enhanced reactivity that this complex displays. These results highlight the critical role of proton-coupled electron transfer in the reduction of nitrite to NO and have important implications for the design of biomimetic catalysts for the selective interconversions of the nitrogen oxides.Publisher PDFPeer reviewe

Ligand-directed synthesis of {MnIII5} twisted bow-ties

Two isostructural polymetallic complexes [MnIII5(μ3-O)2(CH3COO)4(L1)4]− and [MnIII5(μ3-O)2(CH3COO)4(L2)4]− have been synthesised by using two Schiff base ligands derived from 3,5-diamino-1,2,4-triazole, following two different preparative routes, either using the pre-formed ligand (for L1) or via a metal-mediated template synthesis (for L2). The {MnIII5} structure is unusual, being based on two corner-sharing perpendicular {Mn3} triangles forming a twisted bow-tie. The magnetic studies reveal antiferromagnetic coupling between Mn(iii) ions while electrochemical experiments are consistent with a quasi-reversible Mn(iii)↔Mn(iv) redox process at the central manganese ion

Ligand-directed synthesis of {MnIII5} twisted bow-ties

Two isostructural polymetallic complexes [MnIII5(μ3-O)2(CH3COO)4(L1)4]− and [MnIII5(μ3-O)2(CH3COO)4(L2)4]− have been synthesised by using two Schiff base ligands derived from 3,5-diamino-1,2,4-triazole, following two different preparative routes, either using the pre-formed ligand (for L1) or via a metal-mediated template synthesis (for L2). The {MnIII5} structure is unusual, being based on two corner-sharing perpendicular {Mn3} triangles forming a twisted bow-tie. The magnetic studies reveal antiferromagnetic coupling between Mn(III) ions while electrochemical experiments are consistent with a quasi-reversible Mn(III)↔Mn(IV) redox process at the central manganese ion

The Electronic and Solvatochromic Properties of [Co(L)(bipyridine)2]+ (L = o-catecholato, o-benzenedithiolato) Species: a Combined Experimental and Computational Study

Complexes of Co(III) containing mixed chelating diimine and o-quinone ligand sets are of fundamental interest on account of their fascinating magnetic and electronic properties. Whilst complexes of this type containing one diimine and two o-quinone ligands have been studied extensively, those with the reverse stoichiometry (two diimines and one o-quinone) are much rarer. Herein, we describe a ready route to the synthesis of the complex [CoIII(o-catecholate) (2,2'-bipyridine)2]+ (1), and also report the synthesis of [CoIII(o-catecholate)(5,5′-dimethyl-2,2′-bipyridine)2]+ (2) and [CoIII(o-benezenedithiolate)(5,5′-dimethyl-2,2′-bipyridine)2]+ (3) for the first time. Spectroscopic studies show that complex 2 displays intriguing solvatochromic behaviour as a function of solvent hydrogen bond donation ability, a property of this type of complex which has hitherto not been reported. Time-dependent density function theory (TD-DFT) shows that this effect arises as a result of hydrogen bonding between the solvent and the oxygen atoms of the catecholate ligand. In contrast, the sulfur atoms in the benzenedithiolate analogue 3 are much weaker acceptors of hydrogen bonds from the solvent, meaning that complex 3 is only very weakly solvatochromic. Finally, we show that complex 2 has some potential as a molecular probe that can report on the composition of mixed solvent systems as a function of its absorbance spectrum

The electronic and solvatochromic properties of [Co(L)(bipyridine)2]+ (L = o-catecholato, o-benzenedithiolato) species: a combined experimental and computational study

Complexes of Co(III) containing mixed chelating diimine and o-quinone ligand sets are of fundamental interest on account of their fascinating magnetic and electronic properties. Whilst complexes of this type containing one diimine and two o-quinone ligands have been studied extensively, those with the reverse stoichiometry (two diimines and one o-quinone) are much rarer. Herein, we describe a ready route to the synthesis of the complex [CoIII(o-catecholate) (2,2′-bipyridine)2]+ (1), and also report the synthesis of [CoIII(o-catecholate)(5,5′-dimethyl-2,2′-bipyridine)2]+ (2) and [CoIII(o-benezenedithiolate)(5,5′-dimethyl-2,2′-bipyridine)2]+ (3) for the first time. Spectroscopic studies show that complex 2 displays intriguing solvatochromic behaviour as a function of solvent hydrogen bond donation ability, a property of this type of complex which has hitherto not been reported. Time-dependent density function theory (TD-DFT) shows that this effect arises as a result of hydrogen bonding between the solvent and the oxygen atoms of the catecholate ligand. In contrast, the sulfur atoms in the benzenedithiolate analogue 3 are much weaker acceptors of hydrogen bonds from the solvent, meaning that complex 3 is only very weakly solvatochromic. Finally, we show that complex 2 has some potential as a molecular probe that can report on the composition of mixed solvent systems as a function of its absorbance spectrum

Synergy of Cobalt and Silver Microparticles Electrodeposited on Glassy Carbon for the Electrocatalysis of the Oxygen Reduction Reaction: An Electrochemical Investigation

The combination of two different metals, each of them acting on different steps of the oxygen reduction reaction (ORR), yields synergic catalytic effects. In this respect, the electrocatalytic effect of silver is enhanced by the addition of cobalt, which is able to break the O–O bond of molecular oxygen, thus accelerating the first step of the reduction mechanism. At the same time, research is to further reduce the catalyst’s cost, reducing the amount of Ag, which, even though being much less expensive than Pt, is still a noble metal. From this point of view, using a small amount of Ag together with an inexpensive material, such as graphite, represents a good compromise. The aim of this work was to verify if the synergic effects are still operating when very small amounts of cobalt (2–10 μg·cm−2) are added to the microparticles of silver electrodeposited on glassy carbon, described in a preceding paper from us. To better stress the different behaviour observed when cobalt and silver are contemporarily present in the deposit, the catalytic properties of cobalt alone were investigated. The analysis was completed by the Levich plots to evaluate the number of electrons involved and by Tafel plots to show the effects on the reaction mechanism