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Electro and photo-assisted carbon dioxide reduction on
molecular catalysts
Matthew Stanbury
To cite this version:
Matthew Stanbury. Electro and photo-assisted carbon dioxide reduction on molecular catalysts. Inor-ganic chemistry. Université Grenoble Alpes, 2016. English. �NNT : 2016GREAV094�. �tel-01685572�
THÈSE
Pour obtenir le grade de
DOCTEUR DE LA COMMUNAUTE UNIVERSITE
GRENOBLE ALPES
Spécialité : Chimie inorganique et bio-inorganique Arrêté ministériel : 7 août 2006
Présentée par
Matthew STANBURY
Thèse dirigée par Sylvie CHARDONpréparée au sein du Département de Chimie Moléculaire dans l'École Doctorale Chimie et Sciences du vivant
Réduction photo et
électro-assistée sélective du
dioxyde de carbone sur des
catalyseurs moléculaires
Thèse soutenue publiquement le 1 décembre 2016 devant le jury composé de :
M. Gareth WILLIAMS
Professeur à l’université de Durham, Royaume-Uni Rapporteur
M. Marc ROBERT
Professeur à l’université Paris Diderot, Paris 7 Rapporteur
M. Christophe BUCHER
Directeur de recherche CNRS, Lyon Président
Mme Pascale DELANGLE
Ingénieure de recherche CEA, Grenoble Examinatrice
M. Alan LE GOFF
As my time in Grenoble has drawn to a close I feel like it is the end of a major chapter in my life and soon to be the opening of a new one, wherever that may be. I look back with great fondness on the last four years spent in France and fully recognise that this comes from a combination of the experiences of life and work that I was fortunate enough to undergo. Over the course of my Ph.D. there have been many people that have contributed to its successful undertaking and completion that I would like to recognise and thank for their contributions to this journey and its experiences. I begin with many thanks to Marc Robert and Gareth Williams for having examined my thesis and for their participation at the final defence of my work. Likewise at this defence, thank you to Pascale Delangle and Alan Le Goff for their attendance and for their support and discussions over the years as members of my thesis follow-up committee. Thank you also to Christophe Bucher for attending and presiding over the defence as well.
This thesis could not have been achieved in such a satisfactory and fruitful way without the support and guidance of my thesis director, Sylvie Chardon. Thank you for the opportunity that you gave me in offering to take me on as your student after my masters project. Your patience, availability and guidance over the years allowed me to fully develop the skills and competences needed to complete a successful thesis and I appreciate all that you have done for me and all that we have achieved together. In addition we have had the good fortune to have worked with several unique individuals who have contributed greatly to this work. One of the most prominent figures to me among this group would have to be Jean-Daniel Compain, whom effectively mentored me over my first two years. His guidance in the lab, the fruitful discussions and this inspiring, engaging and passionate attitude towards research was a great aid as I began to meander through the intense and challenging first year, and a good support during the productive second year. Particular mention also has to go to Florian Molton, who was an enormous help in the lab, and was always patiently on-hand whenever there were tricky technical difficulties to overcome. In addition, I want to thank the others who have contributed to this thesis, including Monica Trejo, Parker Smith and Eric Goure.
The time and experience in the lab was shaped by the people that I shared it with. I was blessed to have excellent and friendly colleagues in the lab such as fellow PhD students Bertrand, Rajaa, Deborah and more recently Daniela, Jamie, Robin and Long. They, and others colleagues, really contributed to a feeling of belonging and companionship over the course of the PhD, and made the overall experience more valuable and meaningful to me.
In more general terms I would like to thank the DCM for taking me on a Ph.D. student and to the Universite Grenoble Alpes for financing my time here. I also fortunate and thankful to the Durham University Chemistry department for enabling me to join the DCM on an Erasmus exchange
programme. Without these foundations, I probably would not have ended up in this place and I would most likely have missed out on all that it has to offer.
Lastly, and very generally, I want to thank my parents for starting me on this course towards a life in France, they gave me the initial desire to combine my studies with an international experience and I am very grateful for it, as well as their unending help and support over the years.
Résumé de thèse en Francais
Le travail de recherche décrit dans ce manuscrit de thèse s’inscrit dans le contexte général de la valorisation du CO2 et relate une recherche fondamentale, visant à trouver de nouveaux catalyseurs
moléculaires pour réaliser la conversion du CO2. Les résultats obtenus apportent des connaissances
au vu de développer un processus efficace et sélectif de catalyse pour la réduction du CO2.
L’un des plus grands défis scientifique, auquel est confrontée l'humanité, est une conséquence du changement climatique principalement lié à l’activité humaine. Le réchauffement climatique, provoqué par des émissions anthropiques de gaz à effet de serre (GES), pourrait avoir des effets dramatiques sur l’environnement à long terme si rien n’est fait pour tenter, par exemple, d'atténuer et de contrôler ses émissions. Il est donc primordial de réduire les quantités importantes de GES rejetées et présentes dans l’atmosphère, et en particulier le dioxyde de carbone (CO2). Ceci pourrait
être en partie réalisé en trouvant, par exemple, des substituts aux combustibles fossiles (source d'énergie), issus du CO2.
Capturer le CO2 et le transformer en produits chimiques utilisables ou en molécules à valeur
énergétique (stockage d'énergie) est un moyen envisageable qui pourrait apporter une solution pour utiliser le CO2. Nous, mais aussi les générations futures, aurons à l’avenir encore besoin de produits
à base de carbone, et si ces derniers pouvaient être issus du CO2 atmosphérique cela serait une
avancée primordiale. Compte tenu des technologies actuelles, les applications les plus prometteuses de capture/utilisation du CO2 sont directement liées aux rejets de gaz de combustion des
installations industrielles. Dans ces effluents gazeux, le CO2 peut être concentré efficacement et
converti, contrairement au CO2 de l'atmosphère dont la capture directe reste encore à ce jour un très
grand défi. Le processus de transformation du CO2 devra mettre en œuvre des sources d'énergie et
des catalyseurs renouvelables et être économiquement viable. Cela signifie que le développement de catalyseurs est un élément essentiel de cette recherche.
Les principaux objectifs de cette thèse sont le développement de nouveaux catalyseurs moléculaires à base de manganèse, métal non noble, et la compréhension des mécanismes catalytiques mis en jeux, pour la réduction du CO2 en produits chimiques à valeur ajoutée ou en produits riches en
énergie.
les catalyseurs développés dans notre équipe), le manganèse, qui est un métal non noble et plus abondant, a donc été un métal de choix évident pour le développement d'un processus catalytique moléculaire de conversion de CO2 aussi renouvelable que possible.
Chapitre I
Le premier Chapitre présente d’abord des généralités sur l'utilisation du CO2 et fait un point
bibliographique sur les catalyseurs moléculaires pour l’électro et la photo-réduction du CO2. Cette
courte revue montre la grande variété de complexes de métaux de transition utilisés comme catalyseurs, en particulier ceux à base de rhénium. Ensuite une analyse ciblée sur l'état de l’art de la réduction du CO2 par des catalyseurs moléculaires du type complexes carbonyle de manganèse est
relatée. Il est montré que ce domaine de recherche est en croissance, qu’il est dynamique et gagne de nouvelles contributions rapidement et progressivement.
Réduire les quantités de CO2 émises est un défi majeur auquel l’ensemble de la planète est
confronté et les processus permettant d'utiliser le CO2 pourraient apporter des solutions utilisables
pour relever ce défi. Les connaissances acquises lors de recherches passées et en cours sont relatives à des complexes métalliques qui, pour certains, sont efficaces pour la catalyse de la réduction du CO2, comme par exemple les complexes Re et Mn-carbonyle. L'utilisation récente du
Mn comme centre métallique, pour les catalyseurs moléculaires de réduction de CO2, a permis une
avancée significative vers de nouvelles découvertes, et a contribué à l'introduction de complexes alternatifs moins chers et plus renouvelables pour cette réaction catalytique. Cette progression de la recherche scientifique vers des procédés plus renouvelables est particulièrement importante et marquée dans ce domaine de l'utilisation du CO2.
Profitant des diverses connaissances du domaine relatives aux complexes Re-carbonyle, les complexes équivalents de Mn, non étudiés jusque-là pour leur application à la réduction du CO2,
étaient donc des candidats idéaux pour de nouvelles études. Il avait été décrit, que ces complexes pouvaient être activés par réduction, électro- ou/et photo-induite, et ainsi sous leur forme réduite, activer la catalyse de la réduction du CO2 de façon similaire à leurs analogues de Re. A ce jour,
l’étude des complexes de Mn continue activement et s'est élargie à de nouvelles familles d’espèces, qui apportent de nouvelles propriétés liées à leurs structures (nature des ligands) mais aussi à leur environnement direct (e.g. solvant). Cette recherche présente un large attrait pour la communauté des chimistes, avec de nombreux travaux expérimentaux et/ou théoriques et avec un objectif final qui est d'acquérir une meilleure compréhension des phénomènes catalytiques dans ce domaine. Parmi les études de la réduction catalytique du CO2 à l'aide de complexes Mn, la recherche la plus
développée est celle liée à leurs propriétés rédox. Les premières études relatives à leurs propriétés catalytiques vis-à-vis de l’électroréduction du CO2, datent de 2011. Dans tous les cas, la vitesse et la
variété avec lesquelles cette recherche a été publiée depuis cette date, témoignent du grand intérêt que cette recherche suscite.
Les chapitres suivants relatent les résultats obtenus durant cette thèse à savoir un travail de recherche mené sur de nouveaux catalyseurs moléculaires à base de complexes de Mn pour la réduction électrochimique et photochimique du CO2. Ils abordent de nombreux aspects allant de la
synthèse aux applications. Ils développent les propriétés électrochimiques et photochimiques et les méthodes utilisées pour appliquer ces systèmes à la réduction catalytique de CO2. Dans tous les
exemples discutés, les ligands coordinés au centre métallique de Mn jouent un rôle important et influencent les propriétés des complexes, et par conséquent leurs comportements catalytiques. En plus de cela, dans certains cas, l'influence de facteurs externes (par exemple la source de protons, le solvant ou la longueur d’onde de la lumière irradiante) sur les propriétés des complexes est analysée afin d'acquérir une meilleure compréhension des interactions, entre catalyseur et CO2, et des
mécanismes de catalyse. Les résultats des travaux que nous avons menés sont en adéquation avec les résultats obtenus en parallèle par d'autres groupes de recherche, et fournissent ensemble une quantité importante de données pour la compréhension de ces nouveaux systèmes.
Chapitre II
Dans le Chapitre II, la discussion débute par un bref aperçu de la recherche développée relative à de nouveaux complexes contenant des dérivés du ligand ambidentate terpyridine de formule générale: [Mn(L)(CO)3(X)]n+ (X = Br, n = 0 ou CH3CN, n = 1; L = terpyridine et dérivés). L'intérêt de ces
complexes est double, car leurs propriétés physico-chimiques leur confèrent des applications potentielles en catalyse de réduction du CO2, et dans le domaine des molécules pour le relargage
contrôlé de CO. Dans ce contexte, de nouveaux complexes de Mn ont été synthétisés. Pour les complexes [Mn(L)(CO)3(MeCN)]PF6 (L = tolyl-terpy et tolyl-terpy(Me2) une étude détaillée des
propriétés électrochimiques et photochimiques, ainsi que de leur capacité à être des catalyseurs pour la photo- et l’électrocatalyse de réduction du CO2 sont détaillées. Les résultats, comparés entre eux,
et avec ceux obtenus avec le complexe de référence [Mn(terpy)(CO)3(MeCN)]PF6 et les complexes
décrits dans la littérature, conduisent à plusieurs conclusions intéressantes sur leur fonctionnement respectif. Leur propriété à relarguer des molécules de CO est également discutée et des comparaisons sont faites entre les différents complexes en fonction du type de ligand terpy coordinné. Le résultat le plus remarquable, provient de la capacité de ces espèces à libérer de façon contrôlée l'un des ligands carbonyle, ce qui a conduit à la découverte de complexes originaux de Mn à deux ligands carbonyle, catalyseurs sélectifs pour la réduction du CO2, mais aussi de
[Mn(L)(CO)3(MeCN)]PF6 (L = tolyl-terpy et terpy) ont des propriétés spectro-électrochimiques
similaires. Leurs réactivités électrochimiques et photochimiques sont semblables, ce qui indique que les changements structuraux et électroniques sont comparables. Lors de la première réduction mono-électronique la formation d'un dimère, lié par deux centres métalliques (Mn0-Mn0), a été mise en évidence et caractérisée. Une seconde réduction mono-électronique conduit, après rupture de la liaison Mn-Mn du dimère, à un complexe monomère contenant un centre métallique Mn-I. Lors des
expériences de réduction électrocatalytiques du CO2, les voltammogrammes cycliques (VC) en
présence de CO2 et de H2O (source de protons) montrent dans tous les cas que l'espèce catalytique
active est le complexe de Mn doublement réduit.
Nous avons montré que la réduction efficace et sélective du CO2 en CO, par le complexe
Mn-tolyl-terpy, implique un intermédiaire qui est un complexe dicarbonyle où la terpyridine est tridenté au Mn : [Mn(terpy)(CO)2(MeCN)]PF6. Ceci est dû à une réaction de décarbonylation auto catalysée
qui se produit in situ au début de la première réduction à un électron du complexe.
La capacité de cette famille de catalyseurs à influencer la réaction de réduction du CO2, en
modifiant ou en substituant le ligand terpy, a été démontrée dans ce chapitre. Des améliorations, en termes de sélectivité en produit formé et de durée de vie de la catalyse ont été obtenues en allant du complexe terpy le plus simple aux complexes Mn-tolyl-terpy plus ou moins substitué. Le substituant tolyl a permis d’améliorer l’efficacité de la catalyse. Le complexe tolyl-terpy méthylé (2 substituants méthyle), a conduit quant à lui à une amélioration supplémentaire de la sélectivité du procédé électrocatalytique, en permettant une réduction du CO2 sélective et quantitative en CO à un
potentiel moins négatif, ce qui constitue un gain en énergie avantageux pour l’électrolyse. En général, les mêmes tendances se retrouvent dans les résultats obtenus avec l’approche par réduction photocatalytique où la modification du ligand terpy du complexe permet une amélioration notoire du nombre total de cycle catalytique (TON). La nature de l’espèce catalytique active a été caractérisée comme étant un complexe dicarbonyle de Mn formé in situ par une réaction de décarbonylation du complexe induite par irradiation.
Cette propriété de décarbonylation sous irradiation a été mise à profit et étudiée dans le but de proposer un nouveau type de complexes pour le relargage contrôlé de petites quantités de molécules de CO (CO-RMs). L'étude de ces CO-RMs, et l’influence que les divers substituants sur le ligand terpy confèrent à la cinétique de libération de CO, a conduit à un résultat important pour la recherche de nouveaux complexes à propriété de CO-RMs.
Chapitre III
Le chapitre III couvre l'étude d’un nouveau complexe [Mn(phen-dione)(CO)3(X)]n+ (X = Br, n = 0
ses propriétés électrochimiques, photochimiques et son intérêt pour l’électro- et la photocatalyse de réduction du CO2. L'objectif initial de la synthèse de ces complexes était de développer des
catalyseurs moléculaires solubles dans l'eau pour activer la réduction du CO2. Bien que soluble dans
l’eau, le comportement unique de ces complexes, dû aux groupes phen-dione rédox actifs, est rapidement devenu la motivation de cette étude.
Les résultats obtenus apportent des compléments d’informations utiles sur le comportement et les propriétés des complexes Mn-tricarbonyle bipyridyle et leurs dérivés. La caractéristique la plus marquée, est directement liée aux propriétés rédox du ligand phen-dione, qui affecte les propriétés électro- et photochimiques du complexe correspondant. Une différence significative est observée sur les courbes de VC du complexe [Mn(phen-dione)(CO)3X] comparées aux courbes obtenues
avec un complexe équivalent contenant la phénanthroline simple. L'effet de différentes conditions expérimentales d’étude (solvant, pH), sur la nature du complexe, et plus particulièrement des différentes formes que peuvent prendre les cétones présentes sur le ligand phen-dione, a également été mis en évidence et a permis d’expliquer la difficulté rencontrée pour caractériser ce type de complexe en solution.
Par ailleurs, la réduction électrocatalytique du CO2 avec ce complexe de Mn-phen-dione a été
obtenue avec une sélectivité de 100%, et les résultats de la réduction photocatalytique du CO2 ont
été remarquables. Le nombre de cycle catalytique (TON) obtenu sont élevés et la sélectivité pour la production d’acide formique sur une longue période d'irradiation sont comparables au complexe de référence [Mn(bpy)(CO)3Br] publié. Ces complexes ont donc apporté, eux aussi, une pierre à
l’édifice pour la catalyse moléculaire de la réduction du CO2. Même si ces ligands rédox actifs
conduisent à des systèmes plus complexes à étudier, ils offrent néanmoins des propriétés intéressantes, conduisant à de nouveaux comportements électrochimiques, photochimiques et catalytiques.
Chapitre IV
Les résultats présentés dans le chapitre IV sont basés sur l’étude d’une autre famille de nouveaux complexes carbonyle de Mn, ils résultent d’études réalisées à la fin de ma thèse, ce sont des résultats préliminaires qui nécessiteraient un complément de travail. L’étude a été concentrée principalement sur les complexes [Mn(L)(CO)3(MeCN)]PF6 (L = pyperNH, pybzimNH et
pyimNH). Après une partie dédiée à la synthèse et à la caractérisation, ce chapitre consacre une part importante au rôle que peuvent jouer différents paramètres expérimentaux sur l’efficacité de la réaction catalytique de la réduction du CO2. Les ligands L, de par leur système aromatique et leur
provoque la perte du proton N-H et l'électrogénération d'un ligand déprotoné anionique ([Mn(pybzimN-)(CO)
3(MeCN)]) qui présente de nouvelles propriétés électrochimiques par rapport
au complexe initiale. Pour les complexes avec les ligands pybzimNH et pyimNH, il n’y a pas de déprotonation. Le comportement de l’ensemble de ces complexes vis-à-vis de la réduction électrocatalytique du CO2, et malgré la présence dans certains cas de ligand R-N- in-situ
électrogénérés, est tout à fait similaire, en termes de sélectivité et d'efficacité pour la production de CO que celui de [Mn(bpy)(CO)3Br].
Pour le complexe [Mn(pyperNH)(CO)3(MeCN)]PF6, des résultats remarquables ont été obtenus.
Cette espèce est capable de catalyser l’électroréduction du CO2 sélectivement en acide formique
avec un meilleur rendement, une meilleure sélectivité à un potentiel plus faible que le meilleur catalyseur de Mn décrit actuellement dans la littérature. Les rares cas de production électrocatalytique d'acide formique à partir du CO2, en utilisant des complexes de Mn, font de ce
comportement une découverte importante. Ces résultats préliminaires sont originaux et prometteurs et ouvrent de nouvelles perspectives. En outre, ce même complexe est également très actif dans un système homogène photocatalytique de réduction du CO2. Il constitue le meilleur catalyseur
découvert durant cette thèse. Les TON importants et la bonne sélectivité pour le CO vis à vis de l'acide formique ainsi que la stabilité de ce complexe sous irradiation lumineuse (lumière bleue) sont des résultats importants pour l'amélioration des performances des complexes Mn-carbonyle. L'étude de ces complexes dans lesquels la fonction N-H a été convertie en N-Me a également été conduite afin d'évaluer l'impact de ce groupe sur la réaction catalytique de réduction du CO2. Une
étude spectroélectrochimique détaillée a montré que [Mn(L)(CO)3(MeCN)]PF6 (L = pybzimNMe et
pyimNMe) se comportent de manière très semblable à [Mn(bpy)(CO)3Br], avec la formation d'un
dimère Mn0-Mn0 après une première réduction mono électronique, et d'un monomère Mn-I après
une seconde réduction à un électron. Ce complexe est très efficace pour catalyser la réduction du CO2 en CO et montre une activité catalytique qui surpasse certains des meilleurs catalyseurs de Mn
pour la réduction du CO2, bien que cette électrocatalyse soit réalisée à un sur-potentiel plus élevé.
Le point clé de ce chapitre, et de cette thèse en générale, est lié à la compréhension des propriétés physicochimiques fondamentale de ces complexes en utilisant des méthodes de caractérisation spectro-électrochimiques et en mettant en œuvre des expériences d’électrolyses de réduction du CO2 à l'échelle préparative. Nous démontrons ainsi, que la synthèse de complexes ciblés a un intérêt
majeur, pour approfondir la compréhension des phénomènes observés lors de la caractérisation et des résultats obtenus lorsque ces systèmes sont appliqués à la catalyse de réduction du CO2.
Table of Contents
General Introduction ____________________________________________________________ 1
Chapter I Activation of carbon dioxide using Mn carbonyl complexes ___________________ 3
1. Background ____________________________________________________________________ 3 2. Electrochemical CO2 activation using manganese tricarbonyl complexes __________________ 7
3. Photochemical CO2 activation ____________________________________________________ 26
4. Photo-electrochemical CO2 activation ______________________________________________ 29
5. Conclusion ____________________________________________________________________ 31 6. References ____________________________________________________________________ 32
Chapter II Mn tricarbonyl complexes with substituted terpyridine ligands ______________ 39
1. Introduction ___________________________________________________________________ 39 2. Synthesis, X-ray characterisations and summary of previous work _____________________ 42 3. Spectroelectrochemical properties and electrocatalytic CO2 reduction using
[Mn(tolyl-terpy)(CO)3(MeCN)]PF6 ________________________________________________ 49
4. Photochemical properties and photocatalytic CO2 reduction using
[Mn(tolyl-terpy)(CO)3(MeCN)]PF6 ________________________________________________ 59
5. Spectroelectrochemical properties and electrocatalytic CO2 reduction using
[Mn(Me2-tolyl-terpy)(CO)3(MeCN)]PF6 ____________________________________________ 66
6. Photochemical properties and photocatalytic CO2 reduction using
[Mn(Me2-tolyl-terpy)(CO)3(MeCN)]PF6 ____________________________________________ 73
7. Additional Mn tricarbonyl complexes with various substituted terpy ligands _____________ 75 8. Properties of Mn tricarbonyl terpyridyl complexes as photo carbon monoxide
release molecules (photo-CORMs) _________________________________________________ 78 9. Conclusion ____________________________________________________________________ 83 10. References _____________________________________________________________________ 84
Chapter III Mn tricarbonyl complexes with 1,10-phenanthroline-5,6-dione ______________ 89
1. Introduction ___________________________________________________________________ 89 2. Synthesis and X-ray characterisations _____________________________________________ 91 3. Spectroelectrochemical properties and electrocatalytic CO2 reduction using
[Mn(phendione)(CO)3Br] and [Mn(phendione)(CO)3(MeCN)]PF6 ______________________ 93
4. Photochemical properties and photocatalytic CO2 reduction using
6. Conclusion ___________________________________________________________________ 107 7. References ___________________________________________________________________ 107
Chapter IV Mn tricarbonyl complexes with heterocyclic diimine ligands _______________ 109
1. Introduction __________________________________________________________________ 109 2. Synthesis and X-ray characterisations ____________________________________________ 111 3. Spectroelectrochemical properties of [Mn(pyperNH)(CO)3(MeCN)]PF6 ________________ 114
4. Electrocatalytic CO2 reduction using [Mn(pyperNH)(CO)3(MeCN)]PF6 ________________ 124
5. Photochemical properties and photocatalytic CO2 reduction using
[Mn(pyperNH)(CO)3(MeCN)]PF6 ________________________________________________ 135
6. Spectroelectrochemical properties and electrocatalytic CO2 reduction using
[Mn(pyperNMe)(CO)3(MeCN)]PF6 _______________________________________________ 139
7. Other heterocyclic diimine ligand Mn-carbonyl complexes ___________________________ 143 8. Conclusion ___________________________________________________________________ 164 9. References ___________________________________________________________________ 166
General Conclusion and perspectives ____________________________________________ 169
Chapter V Experimental section ________________________________________________ 171
1. Instrumentation ________________________________________________________________ 171 2. Crystallography ________________________________________________________________ 171 3. Electrochemistry _______________________________________________________________ 172 4. Photochemistry ________________________________________________________________ 173 5. Synthesis and characterisation _____________________________________________________ 174 6. References ____________________________________________________________________ 183
General Introduction
The greatest challenge faced by humanity comes in the form of climate change caused principally by human activity in the world. Global warming due to anthropogenic greenhouse gas emissions is very likely to have drastic long-term environmental effects if attempts are not made to mitigate or halt its potential consequences. It is therefore of the utmost importance for society to work towards reducing greenhouse gas emissions, in particular the vast quantities of CO2 released into the
atmosphere every year. This can be done by finding renewable substitutes to fossil fuels as a source of energy as well as attempting to reduce the already high levels of CO2 in the atmosphere.
Capturing CO2 and chemically transforming it to useful chemical products or energy storage
molecules is one way that can be envisioned to help solve the issue. Ours and future generations will still have a demand for carbon based products, and by deriving them from atmospheric CO2 as
the carbon source is an interesting pursuit. Practically, given current technologies, the most promising applications of CO2 capture and utilisation lie in flue gases, where CO2 can be more
efficiently concentrated and converted, as opposed to the greater challenge of capture and utilisation directly from the atmosphere. The CO2 transformation process would require a renewable energy
source and a catalyst to become an economically viable pursuit. This means that catalyst development is an essential part of the overall goal.
The major focus of this thesis is on the development and understanding of new, non-noble manganese based catalysts for CO2 reduction to carbon based chemicals or energy rich products.
Basing our studies on research already developed over decades on CO2 reduction, based on
molecular catalysts containing rare noble metals such as Re and Ru (including catalyst developed recently developed in our laboratory), the more abundant non-noble metal manganese was therefore a natural choice for the development of a CO2 conversion process which is as renewable in nature
as possible.
In the first part of this thesis some of the foundational work concerning molecular CO2 reduction
catalysts will be touched upon, before an in-depth review of the current state of affairs using manganese-based molecular catalysts is presented. Subsequently, the core of the research undertaken during this thesis will be discussed in the Chapters II to IV.
The thesis begins with a look at novel manganese tricarbonyl terpyridyl complex derivatives that show promise as CO2 reduction catalysts, as well as a promising alternative use as CO release
In Chapter III, a new complex containing a highly redox-active ligand is discussed. In the study of CO2 reduction catalysis the coordinated ligands are being recognised as important players in
catalytic mechanisms, and that the development of these types of ligands, which have active roles in catalysis, may have beneficial consequences for CO2 reduction. In this study we found very strong
involvement of the ligand in the intrinsic properties of the complex.
The family of complexes studied in Chapter IV somewhat brings the ideas of the two previous chapters together. The ligands on the metallic manganese centre show modifications to the previously studied ones. They contain larger aromatic cyclic ring systems which create redox-active properties in the complexes. This series of complexes was therefore an entirely novel system to study for CO2 catalytic reduction.
Overall, the synthesis of new complexes, the study of their physicochemical properties, as well as their electro- and photocatalytic behaviour towards CO2 reduction in various conditions, led to
deeper understanding of manganese carbonyl based molecular catalysts. The variation found in the properties of the different complexes studied showed how important a role the ligands have, and demonstrate how highly dynamic and fascinating this research is towards CO2 reduction chemistry.
Chapter I
Activation of carbon dioxide using Mn carbonyl complexes
1. Background
One of the most pressing matters concerning humanity at the moment are the effects of human induced climate change and the potentially catastrophic implications it will on have on life. This makes mitigating or reducing its effects one of the most important challenges that we have ever faced. The largest contributing factor to this climate change comes from the vast quantities of CO2
released into the atmosphere by human activities (anthropogenic CO2 emissions). There is now no
doubt in the scientific community that anthropogenic CO2 emissions will continue to cause drastic
climate changes to our planet and will lead to increased causal weather phenomena that will affect humanity in overwhelmingly negative ways. There are currently major efforts on-going to tackle this challenge, with governments, corporations and individuals working towards solutions, be it through government policies to reduce greenhouse gas emissions, increasing spending and investment in renewable sources of energy or attempting to change the habits of individuals to become more energy conscience and efficient. We will not be discussing all the various ways that are underway to reduce emissions, as most fall out of the scope of this thesis, but we will focus on one particular aspect, namely, CO2 utilisation.
This introduces the idea of Carbon Capture and Utilisation (CCU). In this approach CO2 from
combustion is captured and then, with the input of renewable energy sources, is transformed into value-added chemicals or energy-rich products.
In general CO2 generated from human activity is sometimes seen as a waste product of industrial
processes, one that has no intrinsic value and that is usually discarded into the atmosphere. However CO2 is indeed a source of carbon that can potentially be valorised and utilised directly or
with the right processes and conversion methods. The greatest obstacle to efficient CO2
transformation is the energy input needed to effect conversion to more useful products. This can be overcome with the adoption and expansion of increased renewable energy generation sources. The carbon in CO2 is in its highest possible oxidation state (+IV), therefore all conversions focus on
decreasing this oxidation state. This can lead to the production of a wide variety of useful chemical products (Table I.1.1).[1]
The difficulty arises from the fact that the CO2 molecule is very stable, with strong C=O bond
energies of 803 kJ mol-1. Therefore any CO
2 reduction process inevitably requires an input of
energy and, in order to minimise the total energy, a catalyst is also an essential component of the process.
The idea of using renewable energy and catalysts for CO2 conversion to useful chemical products is
not a new one. In nature, the photosynthetic process, which occurs in various forms of life such as plants and algae, is able to capture energy from sunlight (a renewable energy source) and carry out the conversion of CO2 and H2O into C6H12O6 (glucose: a useful organic product) and O2. This
process can be seen as a direct storage of solar energy into a chemical form.
Inspired by nature, CO2 may be converted via an artificial synthetic process into value-added
chemicals or energetic chemical products (a form of direct energy storage). These days, there are many different ways of transforming CO2.[2–5]
They include:
Hydrogenation reactions to produce hydrocarbons such as ethanol
Non-hydrogenation reactions to produce carbamates, acids, isocyanates, polycarbonates… Biological conversions to ethanol, sugars…
Reforming reactions to produce CO and H2 for Fischer Tropsch chemistry
Industrial CO2 transformations to products such as urea, methanol, carbonates or salicylic
acid (1% usage of total anthropogenic CO2 emissions).
However, the main focus and key interest of this chapter is on electro- and photo-assisted chemical reduction of CO2 to small molecule products such as CO, HCOOH, MeOH, CH4.
Electro- and photochemical reductions are very promising approaches for CO2 transformation,
although they do have their drawbacks. Renewable sources of energy can be employed to drive the reaction, as opposed to indirect energy sources often used for other transformations. However, Table I.1.1 Examples of chemical products from proton assisted CO2 reduction (including proton reduction to H2); pH 7; aqueous
solution E vs. NHE. Standard corresponding reduction potentials.[1]
CO2+ 2H++ 2e–→ CO + H 2O E0’ = – 0.52 V CO2+ 2H++ 2e–→ HCO 2H E0’ = – 0.61 V CO2+ 4H++ 4e–→ HCHO + H 2O E0’ = – 0.48 V CO2+ 6H++ 6e–→ CH3OH + H2O E0’ = – 0.38 V CO2+ 8H++ 8e–→ CH4+ 2 H2O E0’ = – 0.24 V CO2+ 2e–→ CO2•– E0’ = – 1.90 V 2H++ 2e–→ H 2 E0’ = – 0.41 V
problems are encountered when it comes to product selectivity and conversion efficiency. Simple uncatalysed but proton assisted CO2 reduction can lead to a wide range of products and can also
engage in the undesirable H+ reduction that occurs at a similar reduction potential if the catalyst is
not sufficiently selective for CO2 (Table I.1.1). In addition, the reduction occurs at a large, energy
costly overpotential, even with proton assistance. Thus a catalyst is a key component needed to diminish the overpotential. This is an aspect of vital importance if an economically viable and large scale operation is to be developed.
Consequently research on catalysts for CO2 electro- and photoreduction is an ever expanding field
with many catalytic heterogeneous and homogeneous systems being researched and developed (Figure I.1.1).[6] Going forwards we will now focus primarily on the past and present research for
CO2 electrochemical and photochemical reduction using molecular catalysts.
Photo- and electrocatalytic CO2 activation using Re tricarbonyl and transition-metal based complexes
The selective catalytic reduction of CO2 via electrochemical and photochemical means, using
molecular catalysts, began with tricarbonyl rhenium complexes containing a bidentate bipyridyl ligand. The first study on selective photochemical CO2 reduction to CO was reported by Lehn et al.
with a report published in 1983 using the complex [Re(bpy)(CO)3X] (bpy = 2,2’-bipyridine,
X = Cl, Br) in a solution of TEA:DMF (TEA = triethylamine).[7] In this system the Re complex acted as both the catalyst and photosensitiser, with the TEA acting as an electron and proton donor. This work was swiftly followed in 1984 with the discovery of selective electrocatalytic CO2
reduction using [Re(bpy)(CO)3Cl] by the same group.[8]
From that period onwards, research on electro- and photocatalytic CO2 reduction using Re
Figure I.1.1 Number of publications concerning the electrochemical and photoelectrochemical CO2 reduction per year, since the
research groups. Deronzier et al. investigated platinum electrodes coated with poly(pyrrole) films of [Re(bpy)(CO)3Cl] for heterogeneous electrocatalytic CO2 reduction,[9] as well as a similar report by
Sullivan, Meyer et al. using polyvinylbipyridine (v-bpy) functionalised by Re-carbonyl films.[10]
During the 90s research was continued by various groups[11–14] including work by Kaneko et al. who studied the incorporation of [Re(bpy)(CO)3Br] into a Nafion® membrane in order to achieve
CO2 electroreduction in aqueous electrolytes.[15]
To this day, work on Re complexes, for electrochemical CO2 reduction, is still on-going. In 2006,
Cecchet et al.[16] described the electropolymerisation of [Re(v-bpy)(CO)
3Cl] on TiO2 electrodes that
gave enhanced CO2 electroreduction efficiency. This type of work, along with studies by groups
such as Kubiak et al.,[17] Carter et al.[18] and Portenkirchner et al.[19] between 2013 and 2014
demonstrate the prolonged and continued interest in these types of Re complexes for electrocatalytic CO2 reduction with studies continuing to be published in the past couple of
years.[20–22]
Research on photocatalytic CO2 reduction using Re tricarbonyl complexes also follows a similar
history of very early work by Lehn et al.[7] and Kutal, Ferraudi et al.[23] in which efficient and
selective photocatalytic CO2 reduction was studied. This was followed by more studies such as
those by Gibson et al.[24] and Fujita et al.[25] which delved into the CO
2 reduction mechanism using
these Re complexes. Finally, like with the electrocatalytic work, there has been a renewed surge in interest in studying photochemical CO2 reduction, with studies by Ishitani et al.,[26–28] for example,
looking at binuclear complexes of Re and Ru for improved photocatalytic efficiency, greater durability and higher CO2 reduction product turnover frequencies (TOF) for CO. In addition Kubiak
et al.[29] have worked on developing a better understanding of the kinetics and structures that
contribute to the selectivity observed with [Re(tBu2-bpy)(CO)3Br], towards CO formation. The
work of Murata et al.[30] has also helped to demonstrate that this type of Re complex can also
undergo CO2 photocatalytic reduction in aqueous solution in the form of vesicles.
Catalytic CO2 reduction has also been demonstrated using molecular complexes containing Os, Ru
and Fe metal centres. Ru complexes with various ligands, such as bpy and CO, have shown CO2
photocatalytic reduction.[31–33] In addition electrocatalytic CO
2 reduction using [Ru(bpy)(CO)2]n
films (Deronzier et al.[34]) as well as polymers of [Os(bpy)(CO)
2]n (Chardon-Nobalt, Deronzier,
Hartl et al.[35]) were shown to be efficient at CO
2 reduction to CO or formate (selectivity controlled
through the bpy substituents). More recently greater interest has been paid to complexes that contain earth abundant metal centres as they are cheap and a more sustainable alternative to expensive and rare noble metals. Fe has therefore also been studied for CO2 reduction since the
initial investigation by Saveant et al.[36] that looked at electrochemical reduction using Iron (0)
al.[38] have developed new complexes containing Fe (and Co and Ni) as the metal centre for the purpose of efficient CO2 electrocatalytic reduction.
Many more examples of CO2 reduction catalysts exist with various other metal centres, such as
Ni,[39–43] Co,[38,44–46] and Cu[47] as well as Cr,[48] Ir,[49][50][51][52][53] and Rh.[54][55] The description of
catalysts for CO2 reduction above is far from exhaustive but was limited to a few example studies.
A more extensive and detailed set of information for CO2 reduction catalysts, including more on Re
and Fe complexes can be found in two separate reviews for photochemical CO2 reduction[56] and
electrochemical CO2 reduction.[57]
2. Electrochemical CO2 activation using manganese tricarbonyl complexes
The focus of this section will be on the electrochemical activation of CO2 using molecular Mn
complex catalysts. In recent times there has been a great increase in this area of research, as new and interesting properties of Mn complexes for CO2 electroreduction are discovered. As will be
discussed, several different factors govern their outcome and behaviour; these include aspects such as the nature of the ligands and the properties they may impart to the complex as a whole, and the importance of an external proton source to drive the electrochemical reaction. In addition to this, research has also been focussing on attempting to elucidate catalytic reaction mechanisms, either by experimental observations or by performing simulation calculations which can aid in forming a more complete picture of the catalytic process as a whole.
The section is separated into several parts which are focussed on a single complex or a family of closely related complexes, and the investigations that were performed on them by a variety of different research groups.
bpy and dmbpy complexes
The work in the domain of electrochemical CO2 activation using Mn carbonyl complexes is
constantly evolving and expanding, therefore we commence with the work described below and have a cut-off point of relevant research up to the date of July 31st 2016.
The first reported case of CO2 reduction using a Mn tricarbonyl complex, equivalent to the Re
complexes previously discussed, was developed in the CIRe/DCM research group, the group in which I performed my Ph.D. research, in 2011 with the two bpy complexes shown in Scheme I.2.1.[58] This seminal work has since been cited in the literature over 130 times,
In this original work, the electrochemical properties of both Mn complexes were studied under Ar and CO2, as well as under CO2 with added H2O as a proton source for the electrocatalytic reaction.
The CVs of the complexes showed two redox systems that were investigated with exhaustive electrolysis. The first reduction peak showed the formation of a Mn-Mn dimer identified using in situ UV-Vis spectroscopy (Eq. I.2.1-3, Figure I.2.2) on the basis of the appearance of d-d transition bands in the spectrum of the newly generated species, which agreed well with previously reported dimeric species generated electrochemically[59] and photochemically[60] in THF. A comparison can be made with the behaviour of the equivalent Re complex which did not form a dimer but instead a ligand radical species.[13] Exhaustive electrolysis on the second reduction peak then transformed the dimer into a Mn anionic complex, which was also evidenced with UV-Vis spectroscopy (Eq. I.2.4, Figure I.2.3) and by comparison of the new bands with previously reported similar doubly reduced Mn bisphosphine equivalent complexes in THF.[61]
[Mn(L)(CO)3Br] + e- [Mn(L)(CO)3Br]- Eq. I.2.1
[Mn(L)(CO)3Br]- [Mn(L)(CO)3] + Br- Eq. I.2.2
[Mn(L)(CO)3] ½ [Mn(L)(CO)3]2 Eq. I.2.3
½ [Mn(L)(CO)3]2 + e- [Mn(L)(CO)3]- Eq. I.2.4
Under a CO2 atmosphere these redox systems remained relatively unchanged, indicating no
catalytic response (Figure I.2.4). However after addition of a small quantity of H2O there was a
large increase in the cathodic current on the second reduction system with the complex 1, which therefore linked the reduced Mn dimer (Eq. I.2.4) to the active catalytic species generated in solution.
Scheme I.2.1 Chemical structures of Mn complexes.[58]
Figure I.2.1 Citation report of [58] from Web of Science, July 2016 - Number of published items in each year since 2012.
Chardon-Noblat, Deronzier et al. came to the conclusion that a proton source, such as H2O, was an
essential component in generating catalytic activity. Preparative scale electrolysis was performed at -1.70 V using 1 in MeCN:H2O (95:5) electrolyte under a CO2 atmosphere. For the first 4 h of
catalysis the Faradaic yield (FY) was at its optimum with 100% CO and no competitive H2
formation, with the produced CO having a direct proportionality to the number of electrons consumed (TON = 13). 2 was even more effective with 100% CO produced after 138 C of charge was passed through the solution (TON = 34 after 18 h). In addition, 2 also displayed catalytic currents at both of its reduction peaks (Figure I.2.5), which indicated that CO2 reduction could be
Figure I.2.2 CVs of 2, 1 mM in MeCN + 0.1 M TBAP at a VC electrode (diam. 3 mm) = 100 mV s-1; a) initial scan; b) after
exhaustive one-electron reduction at -1.70 V; inset: UV-vis changes accompanying the reduction of 2 (1 mm probe).[58]
Figure I.2.3 UV-vis changes accompanying the bulk reduction at – 1.95 V of [Mn2(dmbpy)2(CO)6]
(0.5 mM in MeCN + 0.1 M TBAP) recorded using a 1 mm probe. Full lines: before and after the electrolysis; dotted lines: during the electrolysis; dashed line: 10 min after electrolysis.[58]
Figure I.2.4 CVs of 1, 1 mM in MeCN with 0.1 M TBAP on a VC electrode (3mm diam.), = 100 mV s-1, a) under Ar and under
previous reports on CO2 electrocatalytic reduction using Re complexes.[62] It should be emphasised
that the Mn complex reduced CO2 at a less negative potential than their Re equivalents, with no loss
of selectivity or durability.
A subsequent follow-up paper from Orio, Chardon-Noblat et al. from the CIRe/DCM, investigated the CO2 reduction mechanism with complex 2, following the publication just described.[63] The
primary focus of this paper was to study the catalytic enhancement observed in the CV of 2 (Figure I.2.5) at the first reduction potential, where the Mn-Mn dimer was formed (Eq. I.2.1-3). This species appeared to be reactive in the presence of CO2 and added H2O (Scheme I.2.2), which
was a surprising finding, as dimers are deactivating products for the equivalent Re complexes; nevertheless CO catalytic production from CO2 was achieved.
In this work, pulsed-EPR measurements were able to detect the presence of a MnII with a
coordinated hydroxycarbonyl ligand at the potential of the first reduction system. During the catalysis, carbon from CO2 interacted with the Mn0 dimer, and H+ (from H2O) interacted with the
newly formed MnII hydroxyl carbonyl monomer species. A direct metal-carbon bonding and
protonation was characterised experimentally with in situ UV-Vis and EPR spectroscopies, performed during the CO2 reduction experiment.
DFT calculations were able to identify the mer-isomer form of the complex in solution as the lower energy one. It was believed that the reaction, overall, proceeds via a first reduction to form a dimer, the dimer then reacted with CO2 via a two-electron oxidative addition and a H+ capped the moiety,
Figure I.2.5 CVs of [Mn(dmbpy)(CO)3Br], 1 mM in MeCN + 0.1 M TBAP at a VC electrode (diam. 3 mm);
= 100 mV s-1; a) under Ar; b) under CO
accompanied by isomerisation of the species. It was then reduced by two electrons, liberating H2O
and CO and regenerated the starting complex in the catalytic cycle.
This exceptional case demonstrated that CO2 reduction occurred using the Mn0-Mn0 dimer
(a mono-reduced species) as catalyst.
Going even further, in terms of the reduction mechanism of these Mn complexes compared to their Re equivalents, Carter et al.[64] published an in-depth computational study on the CO2
electroreduction pathway taken by Mn and Re metal complexes upon applying the requisite potential. This study effectively clarified and supported the reduction pathways taken by the Mn complex, by calculating free energies of different reaction pathways and activation barriers at each step of the process, and, using these calculated values, justified the experimentally observed results. At the first one-electron reduction to form the Mn-Mn dimer (Eq. I.2.1-3) there was no evidence for a singly reduced monomer (Eq. I.2.1),[58,65] it was calculated that the preferred pathway for the Mn complex to take, was a reduction and simultaneous loss of X- (halide) ligand, as the five-coordinate complex (2 in Scheme I.2.3) was more stable than a six-coordinate singly reduced one (2X). It was then predicted to undergo very fast radical-radical recombination to form the dimer (step (2) to (2D)). It could be reduced to the Mn monomer anion complex (3) and active catalytic species after dimer cleavage. There were significant differences when compared to the Re analogue complex, which was predicted to favour a six-coordinate singly reduced complex and has been shown experimentally,[14] as well as having a relatively slower rate of dimerisation (an unfavourable process).
In general, the reasoning for these results comes from the fact that Mn will have its 3dz2 orbital
lower in energy than the ligand π* orbital, whereas for Re its 5d
z2 will be higher in energy; which
are that ligand coordination to an electron enriched metal centre, was less likely than coordination to a metal with the electron density on the ligand. This increased electron density on the Mn metal centre compared to the Re one was shown experimentally by Hartl et al. a few years ago, with similar complexes of Mn and Re containing diazabutadiene type ligands.[66,67]
Continuing with this study, Carter et al. investigated the CO2 reduction process with the Re and Mn
complexes. In almost all instances, when discussing these kinds of Mn complexes, the doubly reduced Mn anion species was the catalytically active one, and therefore the one to which CO2 will
bind (Figure I.2.6, (3)). To start with, the two different binding mechanisms that can occur under a CO2 atmosphere in the presence of H+ were compared for their relative favourability. The two
pathways either started with the binding of CO2 (steps 35 in Figure I.2.6) or with H+ (34). In
general H+ binding had a higher activation barrier than CO
2, making this process unlikely to occur
in any significant quantity (H+ addition vs. CO2 addition in Figure I.2.6). However, although CO2
binding was more favourable than H+, it was an endergonic process for Mn, compared to exergonic for Re, and only a barrierless activation energy for protonation of the bound CO2 could
thermodynamically drive the reaction to the carboxylic acid Mn complex (56). The Re complexes did not require a source of protons but protonation did greatly improve catalysis, as the carboxylic acid form of the complex was far more favourable than the carboxylate one. For Mn complexes, these calculations therefore justify and confirm the necessity of H+ for CO2 reduction, which was
observed experimentally.[58][65]
Subsequently, the step after carboxylic acid Mn species formation was calculated. There could either be a protonation-first pathway or a reduction-first one (Figure I.2.7). For the protonation-first
pathway the neutral complex [Mn(NN)(CO)3(COOH)] was protonated a second time to cleave the
C-O bond and released a H2O molecule (steps in 67 in Figure I.2.7), which left a cationic
tetracarbonyl species. This was then reduced (72CO or 2) and created a complex that spontaneously lost a CO ligand (2CO or 23), and so the active catalyst complex was regenerated. The other pathway involved reduction first to form an anionic [Mn(NN)(CO)3(COOH)]-complex
(68), which was protonated, lost H2O and formed a neutral tetracarbonyl complex (82CO or 2),
before losing a CO ligand and reforming the active catalyst (2CO or 23).
The pathway preferences came down to the relative stability of the neutral and anionic carboxylic acid species. Calculations indicated that for Mn, the neutral complex was more favourable and will therefore exist at higher concentrations in an equilibrium process at an applied potential of -1.45 V vs. SCE. As the barrier for C-O cleavage was almost the same for both species this meant that the protonation-first pathway was the more favourable path for the Mn complex. In contrast, as the anion complex was more stable for Re, it followed the reduction first pathway. This was also in good agreement with previously reported work on proton-dependent electrocatalytic CO2 reduction
using a [Re(bpy)(CO)3Cl] complex.[18]
Figure I.2.6 Potential energy surface for H+ or CO
2 addition to active catalysts (Mn or Re). Reaction intermediates are depicted
Another study by Carter et al.[68] used DFT calculations, in addition to continuum solvation and microkinetics simulations to investigate the effect that different Brønsted acids have on the turnover frequencies (TOF) for CO2 reduction to CO, using the electrocatalysts [Re(bpy)(CO)3]- and
[Mn(bpy)(CO)3]-. They calculated the difference between using phenol, (2,2,2-trifluoroethanol)
TFE, H2O and MeOH as proton source, as well as catalysis with no proton source where protons are
scavenged from the MeCN solvent or the electrolyte. They found that, in general, the computed TOF follow the trends observed experimentally by Kubiak et al.[65] using a tBu substituted bpy ligands of Re and Mn catalysts. For the Re catalyst the TOF order came to TFE > phenol > MeOH > H2O and for the Mn one it was TFE > phenol > H2O > MeOH. Using calculations on the effect of
acid strength with respect to CO2 binding and applied potentials for catalysis, they found that
(especially with Mn catalysts) more negative applied potentials are needed when weak acids are used, as they are too weak to protonate CO2 or stabilise its binding; alternatively a stronger acid
could be used. The group also looked at how catalysis can be effected without the presence of an external proton source, with the Re catalyst able to be protonated by the solvent acetonitrile. On the other hand, calculations showed that the Mn catalyst was not protonated by acetonitrile. This work further developed the research into the effect that different Brønsted acids have on CO2 reduction
catalysis, and the very important role that they fulfil.
Keeping in the field of computational research a report by Nielsen et al.[69] undertook work using density functional theory (DFT) calculations, looking closely at the catalytic mechanism of electrochemical CO2 reduction to CO using TFE as a proton source and Mn bipyrimidine and bpy
type complexes as catalyst. During electrocatalysis the CO2 coordinated to the anionic complex
[Mn(L)(CO)3]- (L = bis-alkyl-substituted bipyridine) and after protonation of the carboxylate,
formed the [Mn(L)(CO)3(COOH)] species. This then reacted via two different pathways, either TFE
Figure I.2.7 Potential energy surface for reduction- and protonation-initiated C−O bond cleavage of the protonated M−CO2 adduct.
Reaction intermediates are depicted with solid lines, transition states with dashed lines. Arrows indicate electron transfer steps. These calculations are performed with applied potentials of −1.45 and −1.76 V vs SCE for Mn and Re, respectively.[64]
helped to dehydroxylate the complex and left [Mn(L)(CO)4]+ followed by a reduction, or vice versa,
where reduction occurred first followed by dehydroxylation. Another complex with bipyrimidine, was also studied computationally (although it has not been studied experimentally) and was shown to reduce CO2 at less negative overpotentials than their bpy equivalents.
In general, these Mn bpy type complexes often behave in a similar fashion electrochemically. Therefore, there have also been studies on similar Mn complexes that attempted to reveal differences in electrochemical behaviour by altering the ligand environment around the metal centre. One such approach was undertaken by Kubiak, Agarwal et al.[70] in which they investigated
[Mn(bpy)(CO)3(CN)]. The CN- ligand is a more strongly binding ligand than Br- and so may
influence the one-electron reduction pathway and X ligand loss. Experimental results did indeed show a first reversible one-electron reduction system, with no loss of the CN- ligand. Upon second
reduction, CN- loss did occur and the 5-coordinated active catalytic species was generated. Using
infrared spectroelectrochemistry (IR-SEC) it was also observed that at the potential of the first reduction peak, the doubly reduced catalytic species was generated via a disproportionation reaction between the two singly reduced complexes (Scheme I.2.4).
Controlled potential electrolysis (CPE) at both the bulk and preparative scale was performed under CO2 with an added H+ source (PhOH). IR-SEC was used to monitor the electrogenerated species.
No evidence of [Mn(bpy)(CO)3]- was observed, indicating rapid consumption of the species upon
generation at the first reduction potential (-1.90 V vs. Fc/Fc+*). When the applied potential was decreased (-2.10 V) the anionic species was observed as it was formed at a faster rate than it was consumed. At the first reduction peak, bulk scale electrolysis was performed while monitoring the CO and H2 quantities produced, which came out to be an average FY of 98% for CO after around
3 C of charge was passed through the solution.
An interesting example of electrocatalytic CO2 reduction performed in an aqueous environment,
using a membrane supported [Mn(bpy)(CO)3Br] complex, was published by Cowan et al.[71] In
general, as will be discussed, high proton environments tend to favour higher catalytic currents for CO2 reduction; therefore aqueous environments are of great interest. However, the competitive H+
reduction is often favoured in high H+ conditions, so complexes that are selective and that have low
reduction overpotentials are essential. The group of Cowan proposed that, although immobilised catalysts for CO2 reduction on Nafion® membranes often have low current densities, in their
situation Mn complexes might benefit due to the high concentration of protons.† When reduced, these complexes form a dimer, due to existing in local clusters, and a subsequent reduction generates the Mn anionic active catalyst.
CPE of the Nafion®/[Mn(bpy)(CO)
3Br] films under CO2 produced CO with a CO:H2 ratio of 2 and
CO TON of around 14. In practice this TON should be larger as most of the deposited complex was inactive, making the real value around 450 TONs of CO. The slight catalytic increase in current on the first reduction peak was also observed which agreed with the previous study by Oria, Chardon-Noblat et al.[63] The issue of low complex deposition on Nafion® was addressed using
multi-walled carbon nanotubes (MWCNTs), which have very large surface areas and were found to also have good -interactions with this Mn complex. Up to 11% loading was observed as opposed to the 0.25% coverage found using just the Nafion®. A ten-fold current enhancement was therefore
achieved with a CO:H2 ratio of 1:2, at a CPE of -1.4 V vs. SCE at pH 7.
In a follow-up paper, Cowan et al.[72] tested other Mn complexes such as, [Mn(tBu
2-bpy)(CO)3Br],
a complex which will be discussed in more detail in the next section, [Mn((COOH)2-bpy)(CO)3Br]
and [Mn((OH)2-bpy)(CO)3Br], in an aqueous environment using Nafion®/MWCNT electrodes. The
complex with the tBu2-bpy ligand showed improved CO:H2 ratio of around 1 at the same
overpotential of -1.4 V vs. SCE at pH 7. In an attempt to improve product selectivity by reducing the overpotential needed for the second electron reduction, the two (COOH)2-bpy and (OH)2-bpy
complexes should have been suitable, as the bpy substituents on the 4,4’-positions are electron withdrawing groups (EWGs). This improvement in selectivity using EWGs was achieved by Kubiak et al.[73] using a [Re(tBu
2bpy)(CO)3Cl] complex. This was indeed the case for Mn
complexes as they had less negative reduction potentials than the tBu2-bpy and bpy complexes.
However, although the (OH)2-bpy ligand is nominally an EWG it was found that it undergoes
reductive deprotonation (much like a previously studied Re equivalent complex)[74] and the
subsequent oxyanion acts as a strong electron donor. These complexes ended up having no significant CO2 reduction ability on Nafion®/MWCNT electrodes.
†Nafion®
tBu2-bpy complexes
The group of Kubiak et al.[65] also started to investigate the use of Mn complex catalysts for CO 2
electroreduction instead of Re ones. Their first major contribution to this work involved identifying that, in addition to H2O, different weak acids (proton sources) such as MeOH and
TFE, led to electrocatalytic CO2 reduction; they synthesised a new Mn complex slightly modified
from the previous ones ([Mn(bpy-tBu2)(CO)3X], X = CH3CN, Br). The electrochemical properties
of this new complex were very similar to those observed with the bpy derivatives, with two irreversible redox systems observed on the reductive scan of its CV. The first reduction corresponded to the formation of a Mn0 dimer, and the second to the formation of the catalytically
active anionic complex [Mn-I(bpy-tBu
2)(CO)3]-, at a less negative overpotential (= 300 mV) than
the Re analogue, which implied lower overpotentials during catalysis. Indeed, compared to Re electrocatalytic conditions, the anodic shift may be a reason why added protons are needed for catalysis. Proton coordination to a {Mn-I-CO
2} moiety may be required to facilitate bond cleavage
during the CO2 reduction reaction; this relates to the computational work later described by Carter
in collaboration with Kubiak et al.[64], precisely on these processes. Electrocatalytic CO
2 reduction
using this complex was investigated in a variety of proton sources (H2O, MeOH, TFE). An in-depth
analysis looked at the difference in catalytic activity that they would give to both of these Mn complexes and their comparable Re analogues. In general, as acid strength increased the catalytic current in the CV of the complex also increased. Experiments using 1.4 M TFE gave FY of 100% for CO2 reduction to CO (TOF = 340 s-1).
This work therefore introduced the major influence of proton donors into the field of electrocatalytic CO2 reduction using Mn tricarbonyl complexes.
Later on in this section, other complexes will be discussed which have modified bpy ligands that bring a proton into the vicinity of the metal centre (local proton source) which would be able to access protons via an intramolecular reaction.
Kubiak et al.[75] developed a method to monitor the two one-electron reductions observed for the
complex [Mn(tBu2-bpy)(CO)3X] (X = Br or MeCN) using infrared spectroelectrochemistry
(IR-SEC). They note the initial IR spectra contained three CO stretches 2028 (symmetric), 1933 and 1923 cm-1 (asymmetric). The dimer was formed after exhaustive reduction and the new IR spectrum
corresponded well with analogous Re dimer complexes[13] that were previously described, as well
as tetramethyl biphosphine based Mn dimers reported with IR-SEC by Hartl, Foch et al.[61] in early
2003. The subsequent second reduction formed the Mn anionic complex whose IR spectrum were comparably similar to the equivalent Re complex.[13] Additional evidence for the generation of this
