University of Groningen Biomimetic metal-mediated reactivity Wegeberg, Christina

University of Groningen

Biomimetic metal-mediated reactivity

Wegeberg, Christina

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BIOMIMETIC METAL-MEDIATED

REACTIVITY

Design, synthesis and characterization (NMR, IR, UV/vis, EPR, MS, SCXRD, CV) of the iron compounds described in this thesis together with reactivity studies and catalysis were performed at the Department of Physics, Chemistry and Pharmacy, University of Southern Denmark, Denmark. Head-space gas detection and additional characterization (rRaman, UV/vis) were completed at the Molecular Inorganic Chemistry Department of the Stratingh Institute for Chemistry, University of Groningen, The Netherlands.

The work reported in this thesis was supported financially by The Danish Council for Independent Research | Natural Sciences (grant 4181-00329).

Printed by: Ipskamp Printing, The Netherlands

Cover artwork made by Mie Thorborg Pedersen. Enzyme: Taurine/α-ketoglutarate dioxygenase from Escherichia coli. PDB ID: 1GQW. Crystal structure: [Fe(tpena)](ClO4)2(MeCN)1.5. CCDC reference code: CUXPAO.

ISBN: 978-94-034-1166-8 (printed version) ISBN: 978-94-034-1165-1 (electronic version)

BIOMIMETIC METAL-MEDIATED REACTIVITY

PhD thesis

to obtain the degree of PhD at the University of Groningen

on the authority of the Rector Magnificus Prof. E. Sterken

and in accordance with the decision by the College of Deans

and

to obtain the degree of PhD at the University of Southern Denmark

on the authority of the Dean Prof. Martin Zachariasen

Double PhD degree

This thesis will be defended in public on Friday January 11th _{2019 at 16.15 hours}

and

Monday January 14th _{2019 at 14.00 hours}

by

Christina Wegeberg

born on the 18th _{of February 1989}

Supervisors

Prof. C. J. McKenzie

Prof. W. R. Browne

Assessment Committee

Prof. V. McKee

Prof. E. Otten

Prof. T. D. Waite

Prof. M. Swart

Table of Contents

List of Publication ... i Abbreviations ... ii Abstract ... iv Resumé ... vi Samenvatting ... viii Acknowledgements ... x 1. Introduction 1.1 Inspiration from Nature ... 1

1.2 Mononuclear Non-Heme Systems ... 5

1.3 Generation of Mononuclear Iron(III) Peroxo Species ... 6

1.4 Generation of Mononuclear Iron(IV)oxo Species ... 11

1.5 The Hunt for a Mononuclear Iron(V)oxo Species ... 16

1.6 The Nature of the Active Oxidant ... 19

1.7 Conclusions ... 22

2. Summary ... 23

Bibliography ... 28

3. Paper I: Halogen-Bonding-Assisted Iodosylbenzene Activation by a Homogenous Iron Catalyst ... 37

4. Paper II: Reduction of Hypervalent Iodine by Coordination to Iron(III) and the Crystal Structures of PhIO and PhIO2 ... 49

5. Paper III: Noncovalent Halogen Bonding as a Mechanism for Gas-Phase Clustering ... 59

6. Paper IV: Directing a Non-Heme Iron(III)-Hydroperoxide Species on a Trifurcated Reactivity Pathway ... 69

7. Paper V: Catalytic Alkyl Hydroperoxide and Acylperoxide Disproportionation by a Nonheme Iron Complex ... 83

8. Paper VI: Photoinduced O2-dependent Stepwise Oxidative-Deglycination of a Nonheme Iron(III) Complex ... 109

9. Paper VII: The Reactivities of Non-heme Iron(III)peroxo and Iron(IV)oxo Complexes are Tuned by Presence of a Cis Carboxylato, Alkoxido or Pyridine Donor ... 137

10. Paper VIII: The Oxidizing Power of Iron(IV)oxo Complexes of a Series of Rtpen Ligands in Water Correlates with the Increasing Energy of the LMCT ... 151

Appendix A ... 176

Appendix B ... 177

i

List of Publication

This thesis is based on the following publications:

I. D. P. de Sousa, C. Wegeberg, M. S. Vad, S. Mørup, C. Frandsen, W. A. Donald and C. J. McKenzie. Halogen-Bonding Assisted Iodosylbenzene Activation by a Homogenous Iron

Catalyst. Chem., Eur. J. 2016, 22, 3810-3820. DOI: 10.1002/chem.201503112

II. C. Wegeberg, C. G. Frankær, C. J. McKenzie. Reduction of Hypervalent Iodine by

Coordination to Iron(III) and the Crystal Structures of PhIO and PhIO2. Dalton Trans.

2016, 45, 17714 – 17722. DOI: 10.1039/C6DT02937J

III. C. Wegeberg, W. A. Donald, C. J. McKenzie, Non-Covalent Halogen Bonding as a

Mechanism for Gas Phase Clustering. J. Am. Soc. Mass Spectr. 2017, 28, 2209-2216. DOI:

10.1007/s13361-017-1722-z

IV. C. Wegeberg, F. R. Lauritsen, C. Frandsen, S. Mørup, W. R. Browne, C. J. McKenzie.

Directing a Non-Heme Iron(III)-Hydroperoxide Species on a Trifurcated Reactivity Pathway. Chem. Eur. J. 2018. DOI: 10.1002/chem.201704615

V. C. Wegeberg, W. R. Browne, C. J. McKenzie. Catalytic Alkyl Hydroperoxide and

Acylperoxide Disproportionation by a Nonheme Iron Complex. ACS Catalysis 2018, 8,

9980-9991. DOI: 10.1021/acscatal.8b02882

VI. C. Wegeberg, V. M. Fernández-Alvarez, A. de Aguirre, C. Frandsen, W. R. Browne, F. Maseras, C. J. McKenzie. Photoinduced O2-dependent Stepwise Oxidative-Deglycination

of a Nonheme Iron(III) Complex. J. Am. Chem. Soc. 2018, 140, 14150-14160 DOI:

10.1021/jacs.8b07455

VII. C. Wegeberg, W. R. Browne, C. J. McKenzie. The Reactivities of Non-heme Iron(III)peroxo

and Iron(IV)oxo Complexes are Tuned by Presence of a Cis Carboxylato, Alkoxido or Pyridine Donor, in preparation

VIII. C. Wegeberg, A. L. Gonzalez, W. R. Browne, C. J. McKenzie. The Oxidizing Power of

Iron(IV)oxo complexes of a Series of Rtpen Ligands in Water Correlates with the Increasing Energy of the LMCT, in preparation

List of publications not included in this thesis:

IX. C. Wegeberg, V. McKee, C. J. McKenzie, A Coordinatively Flexible Hexadentate Ligand

Gives Structural Isomeric Complexes M2(L)X3 (M = Cu, Zn; X = Br, Cl). Acta Cryst. 2016,

C72, 68-74. DOI: 10.1107/S2053229615023773

X. C. Henriksen, C. Wegeberg, D. Ravnsbæk, Phase Transformation Mechanism of Li-ion

Storage in Iron(III) Hydroxide Phosphates. J. Phys. Chem. C 2018, 122, 1930-1938. DOI:

10.1021/acs.jpcc.7b10352

XI. K. Vejlegaard, C. Wegeberg, V. McKee, J. Wengel, Novel Conformationally Constrained

2’-C-Methylribonucleosides: Synthesis and Incorporation into Oligonucleotides. Org.

ii

Abbreviations

Ac acetate BLM Bleomycin BPMCN N,N’-bis(2-pyridylmethyl)-N,N’-dimethyltrans-1,2-diaminocyclohexane bppa bis(6-pivalamido-2-pyridylmethyl)(2-pyridylmethyl)amine bpy 2,2'-bipyridine H3buea tris[(N’-tert-butylureaylato)-N-ethyl]aminato Bz benzyl bzbpena N-benzyl-N,N’-bis(2-pyridylmethyl)ethylenediamine-N’-acetate bztpen N-benzyl-N,N’,N’-tris(2-pyridylmethyl)-1,2-diaminoethane

CAN ceric ammonium nitrate

CCDC The Cambridge Crystallographic Data Centre CID collision-induced dissociation

cryptand 4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane DFT density functional theory

DNA deoxyribonucleic acid

EPR electron paramagnetic resonance

ESI-MS electrospray ionization mass spectrometry

Et ethyl

EXAFS extended X-ray absorption fine structure HAT hydrogen atom transfer

IR infrared

m-CPBA meta-chloro peroxy benzoic acid

Me methyl

MeN4Py 1,1-di(pyridin-2-yl)N,N-bis(pyridin-2-ylmethyl)ethan-1-amine metpen N-methyl-N,N’,N’-tris(2-pyridylmethyl)-1,2-diaminoethane

MIMS membrane inlet mass spectrometry

N4Py N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine

NMO N-Methylmorpholine N-oxide

NMR nuclear magnetic resonance NIR near infrared

OAT oxygen atom transfer OTf triflate phen phenanthroline PhIO iodosylbenzene Py pyridyl PyNMe3 3,6,9-trimethyl-3,6,9-triaza-1(2,6)-pyridinacyclodecaphane Hqn quinaldic acid rRaman resonance Raman

TACNPy2 1-di(2-pyridyl)methyl-4,7-dimethyl-1,4,7-triazacyclononane

TAML 3,4,8,9-tetrahydro-3,3,6,6,9,9-hexamethyl-1H-1,4,7,10-benzotetraazacyclo- tridocane-2,5,7,10-(6H,11H)tetrone

TAML* substituted TAML derivative

TLA tris[(6-methyl-2-pyridyl)methyl]amine

TMC 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane TMCO 4,8,12-trimethyl-1-oxa-4,8,12-triazacyclotetradecane TMG3tren 1,1,1-tris{2-[N2-(1,1,3,3-tetramethylguanidino)]ethyl}amine

TPA tris(2-pyridylmethyl)amine

tpen N,N,N’,N’-tetrakis(2-pyridylmethyl)ethane-1,2-diamine

iii H3TPAPh tris-(5-phenyl-1H-pyrrol-2-ylmethyl)-amine

UV/vis ultraviolet/visible

XAS X-ray absorption spectroscopy XANES X-ray absorption near edge structure

iv

Abstract

The work presented in this thesis focuses on the activation of terminal oxidants (PhIO, NMO, H2O2, tBuOOH, cumylOOH, m-CPBA, ClO-) in organic and aqueous solutions by the mononuclear

non-heme iron complex [Fe(tpena)]2+_{, i.e., detection and characterization of transient}

[Fe(tpena)]2+_{-based oxidants (scheme A) as well as elucidation of mechanisms and reactivity}

patterns important for the use in oxidation catalysis. (tpena = N,N,N’-tris(2-pyridylmethyl)ethylene diamine-N’-acetate)

Scheme A. Simplified and unified schematic illustration of the iron chemistry presented in this PhD thesis. Changes of the oxidant (XO- _{or PhIO) and/or the ligand (L) around the iron centre in non-heme iron complexes control the}

formation of the possible iron-based oxidants and hence the catalytic activity. X = OH, Ot_{Bu, Ocumyl, m-CBA, Cl,}

NM(O). L = ethylenediamine backboned ligand: N-R-N,N’,N’-tris(2-pyridylmethyl)ethane-1,2-diamine, (R = CH3

(metpen), CH2CH3 (ettpen), CH2C6H5 (bztpen), CH2C6H4N (tpen), CH2CH2OH (tpenOH) and CH2COOH (tpenaH)).

[Fe(tpena)]2+ _{is a germane biomimetic system for iron non-heme O}

2 activating enzymes due to

the presence of a carboxylate donor in the first coordination sphere and a second coordination sphere base. The carboxylate donor induces a significantly lower FeII_/FeIII _{reduction potential for}

[Fe(tpena)]2+ _{compared to the many non-heme iron complexes without this functional group}

reported over the past three decades. As a consequence an iron(III) resting state rather than an iron(II) resting state is stabilized, which creates a catalyst with a remarkable diversity: the reactivity is controlled by the choice of terminal oxidant and can be switched between the paradigms of HAT- and OAT-based oxidations.

The HAT-mediated reactivity of the iron-tpena system is ascribed to the iron(IV)oxo species [FeIV_O(Htpena)]2+ _{generated upon homolytic bond cleavage of [FeO-X(tpenaH)]}2+_{. The}

combination of enhanced lability of the FeO-X bond and greater oxyl radical character of [FeIV_O(Htpena)]2+ _{is identified as the key reason for a more aggressive reactivity compared to}

other non-heme iron model complexes, which is demonstrated through rapid hydrogen, alkyl and acylperoxide disproportionation, greater second order constants in C-H abstraction and larger catalytic product yields. The drawback of using peroxides is that free and promiscuous radicals, X•, are subsequently formed alongside [FeIV_O(Htpena)]2+_{. The radicals can also work as}

oxidants, and thereby decrease selectivity of the substrate oxidations and cause ligand degradation, if favourable experimental design has not been made. This loss of selectivity can however be avoided by the direct generation of the iron(IV)oxo species from its iron(III)

v precursor with a one electron acceptor in aqueous solutions. Within the series of ethylenediamine based iron(IV)oxo species, [FeIV_O(Htpena)]2+ _{and [Fe}IV_O(HtpenO)]2+ _indeed

perform best in oxidation of C-H (both in aqueous and organic solutions) and O-H bonds, respectively.

In contrast to the use of peroxides, radical chemistry is not observed when the oxidant PhIO is employed. Rather selective and catalytic oxygenations are demonstrated suggesting an OAT mechanism catalysed by a metal-based oxidant, e.g., the detectable [FeIII_{(OIPh)(tpena)]}2+_,

{[FeIII_{(OIPh)(tpena)]}}

24+ or undetected iron(V)oxo species generated through heterolytic O-I

bond cleavage. Halogen bonding and the different nature of the FeIII_{O-X bond for PhIO compared}

to peroxides are believed to play central roles for the observations of the different reactivity patterns (OAT vs. HAT).

[Fe(tpena)]2+ _{undergoes irreversible, light-promoted O}

2-dependent N-deglycination to generate

an iron(II) complex under ambient conditions. The transformation includes a mass loss equivalent to a glycyl group involving consecutive C-C and C-N cleavages documented by the quantitative measurement of the sequential production of CO2 and formaldehyde, respectively.

Time-resolved spectroscopy has allowed for the spectroscopic characterization of two iron-based transients along the reaction pathway.

vi

Resumé

Denne afhandling fokuserer på aktivering af terminale oxidationsmidler (PhIO, NMO, H2O2, t_{BuOOH, cumylOOH, m-CPBA, ClO}-_{) i organiske og vandige opløsninger af det mononukleare}

non-hæm jern kompleks [Fe(tpena)]2+ _{dvs. detektering og karakterisering af transiente [Fe(tpena)]}2+

-baserede oxidanter (skema B) samt belysning af mekanismer og reaktivitetsmønstre vigtige for brugen i oxidation katalyse.

Skema B. Forenklet skematisk illustration af jern-kemien rapporteret i denne ph.d.-afhandling. Ændringer af oxidationsmidlet (XO- _{eller PhIO) og/eller liganden (L) omkring jern centeret i non-hæm komplekser kontrollerer}

dannelsen af de mulige jern-baserede oxidanter og som konsekvens den katalytiske aktivitet. X = OH, Ot_{Bu, Ocumyl,}

m-CBA, Cl, NM(O). L = ethylenediamin-baseret ligand: N-R-N,N’,N’-tris(2-pyridylmethyl)ethan-1,2-diamin, (R = CH3

(metpen), CH2CH3 (ettpen), CH2C6H5 (bztpen), CH2C6H4N (tpen), CH2CH2OH (tpenOH) and CH2COOH (tpenaH)).

[Fe(tpena)]2+ _{er et relevant system til bioefterligning af non-hæm jern O}

2 aktiverende enzymer

grundet tilstedeværelsen af en carboxylat donor i den første koordinationssfære og en base i den anden koordinationssfære. Carboxylat donoren forårsager et betydelig mindre FeII_/FeIII

reduktionspotentiale for [Fe(tpena)]2+ _{sammenlignet med de mange non-hæm jern komplekser}

uden denne funktionelle gruppe rapporteret de sidste tre årtier. En konsekvens deraf er stabilisering af et jern(III) oxidationstrin til forskel fra et jern(II) oxidationstrin, som herved skaber en katalysator med en bemærkelsesværdig mangfoldighed: reaktiviteten kontrolleres af den valgte terminale oxidant, hvilket gør det muligt at skifte mellem paradigmerne af HAT- og OAT-baserede oxidationer.

Reaktiviteten af jern-tpena systemet tilskrives jern(IV)oxo forbindelsen [FeIV_O(Htpena)]2+_{, som}

dannes via homolytisk kløvning af [FeO-X(tpenaH)]2+_{. Helt afhørende grunde til en mere}

aggressiv reaktivitet sammenlignet med andre non-hæm jern model komplekser er kombinationen af øget labilitet af FeO-X bindingen og større oxyl radikal karakter af [FeIV_O(Htpena)]2+_{. Dette giver sig til udtryk i hydrogen, alkyl og acylperoxid disproportionation}

samt større anden ordens reaktionskonstanter i C-H abstraktion og katalytiske udbytter. Ulempen ved brugen af peroxider som oxidationsmiddel er dog dannelsen af frie og promiskuøse radikaler, X•, som dannes samtidig med [FeIV_O(Htpena)]2+_{. Radikalerne kan også}

virke som oxidanter, og som konsekvens vil selektiviteten i oxidation af substrater sænkes og ligand-nedbrydning kan forekomme, hvis ikke det rette eksperimentelle design er blevet benyttet. Tab i selektivitet kan dog blive forhindret ved at generere jern(IV)oxo forbindelsen

vii direkte med en en-elektron acceptor fra dens jern(III) forgænger. I serien af ethylenediamin-baserede jern(IV)oxo forbindelser er [FeIV_O(Htpena)]2+ _{og [Fe}IV_O(HtpenO)]2+ _{overlegne i hhv.}

oxidation af C-H (både vandige og organiske opløsninger) og O-H bindinger.

Modsat brugen af peroxider, observeres der ingen radikale kemi, når oxidanten PhIO benyttes. Selektive og katalytiske oxidationer er derimod demonstreret, hvilket tyder på en OAT mekanisme katalyseret af en metal-baseret oxidant f.eks. den detekterbare [FeIII_{(OIPh)(tpena)]}2+_,

{[FeIII_{(OIPh)(tpena)]}}

24+ eller en ikke detekterbar jern(V)oxo forbindelse genereret via heterolytisk

kløvning af O-I bindingen. Halogen binding og forskellighederne af FeIII_{O-X bindingen for PhIO}

sammenlignet med peroxiderne tilordnes en helt afgørende betydning for observationen af de forskellige reaktionsmønstre (OAT vs. HAT).

[Fe(tpena)]2+ _{gennemgår under ambiente forhold irreversibel, lys-induceret O}

2-afhængig

N-deglycinering under dannelse af et jern(II) kompleks. Denne proces inkluderer et masse tab svarende til en glycyl gruppe. Fortløbende C-C og C-N kløvninger er dokumenteret ved kvantitative målinger samtidig med sekventiel produktion af hhv. CO2 og formaldehyd.

Tidsafhængig spektroskopi har muliggjort spektroskopisk karakterisering af to jern-baserede intermediater på reaktionsvejen til nedbrydningsproduktet.

viii

Samenvatting

Het werk gepresenteerd in dit proefschrift richt zich primair op de activatie van terminale oxidanten (PhIO, NMO, H2O2, tBuOOH, cumylOOH, m-CPBA, ClO-) in organische oplossingen en in

water door het mononucleaire niet-heem ijzer complex [Fe(tpena)]2+ _{(tpena =}

N,N,N’-tris(2-pyridylmethyl)ethylene diamine-N’-acetate), bijvoorbeeld voor de detectie en karakterizatie van kortlevende op [Fe(tpena)]2+ _{gebaseerde oxidanten (Schema C) zowel als de verheldering van}

mechanismen en patronen in reactiviteit die belangrijk zijn voor gebruik in katalytische oxidaties.

Schema C. Versimpeld schematisch overzicht van de reactiviteit van het ijzercomplex dat centraal staat in dit proefschrift. Veranderingen in de oxidant (XO- _{or PhIO) en/of het ligand (L) rond het ijzer-centrum in niet-heem ijzer}

complexen beïnvloeden de formatie van mogelijke op ijzer gebaseerde oxidanten, en daarmee de katalytische functie. X = OH, Ot_{Bu, Ocumyl, m-CBA, Cl, NM(O). L = op ethyleendiamine gebaseerde ligand (R = CH}

3 (metpen), CH2CH3

(ettpen), CH2C6H5 (bztpen), CH2C6H4N (tpen), CH2CH2OH (tpenOH) and CH2COOH (tpenaH)).

[Fe(tpena)]2+ _{is een bloedeigen biomimetisch systeem voor ijzer niet-heem O}

2 activerende

enzymen dankzij de aanwezigheid van een carboxylaat donor in de eerste coördinatiesfeer en een tweede coördinatiesfeer base. De carboxylaat donor brengt een significant lager FeII_/FeIII

reductiepotentiaal teweeg voor [Fe(tpena)]2+ _{vergeleken met de meeste niet-heem ijzer}

complexen van de afgelopen tientallen jaren zonder deze functionele groep. Hierdoor is een ijzer(III) ruststaat in plaats van een ijzer(II) ruststaat gestabiliseerd, wat een katalytisch systeem met uitzonderlijke eigenschappen creëert: de reactiviteit is beïnvloed door de keuze van terminale oxidant en kan verwisseld worden tussen het paradigma van HAT- en OAT-bemiddelde oxidaties.

De HAT-bemiddelde reactiviteit van het ijzer-tpena systeem is toegeschreven aan het ijzer(IV)oxo soort [FeIV_O(Htpena)]2+ _{dat ontstaat onder homolytische dissociatie van de bond in}

[FeO-X(tpenaH)]2+_{. De combinatie van verlaagde stabiliteit van de FeO-X bond en het verhoogde}

radicaal-karakter van het oxyl van [FeIV_O(Htpena)]2+ _{is geïdentificeerd als de primaire reden voor}

een versterkte reactiviteit in vergelijking tot de andere niet-heem complexen, wat is gedemonstreerd met snelle waterstof, alkyl en acylperoxide disproportionering, hogere tweedegraads constanten in C-H abstractie en grotere katalytische opbrengsten. Het nadeel van het gebruik van peroxiden is dat vrije radicalen, X•, vervolgens naast [FeIV_O(Htpena)]2+

gegenereerd worden. De radicalen kunnen ook als oxidanten fungeren, en verlagen daarmee de selectiviteit van de oxidaties op de substraten en veroorzaken tevens degradatie van de

ix liganden, wanneer de experimentele condities hier niet op zijn aangepast. Het verlies in selectiviteit kan echter vermeden worden door het direct creëren van het ijzer(IV)oxo soort van zijn ijzer(III) voorganger met een één elektron acceptor in water. Binnen de series van de op ethyleendiamine gebaseerde ijzer(IV)oxo soorten vertonen [FeIV_O(Htpena)]2+ _en

[FeIV_O(HtpenO)]2+ _{inderdaad verbeterde oxidatie van respectievelijk C-H bonden (zowel in water}

als in organische oplosmiddelen) en O-H bonden.

In tegenstelling tot bij het gebruik van peroxide is radicaal-chemie niet geobserveerd wanneer de oxidant PhIO is gebruikt. In plaats daarvan wordt selectieve en katalytische oxygenatie waargenomen, wat suggereert dat een OAT-mechanisme wordt gekatalyseerd door een op metaal gebaseerde oxidant, namelijk de geobserveerde [FeIII_{(OIPh)(tpena)]}2+_,

{[FeIII_{(OIPh)(tpena)]}}

24+ of het niet geobserveerde ijzer(IV)oxo soort dat gecreëerd wordt door

heterolytische O-I bond dissociatie. Halogeen binding en de afwijkende aard van de FeIII_{O-X bond}

voor PhIO vergeleken met peroxides worden geacht een centrale rol te spelen in de observaties van afwijkende reactiviteit (OAT versus HAT).

[Fe(tpena)]2+ _{ondergaat onomkeerbare, licht-gestimuleerde O}

2-afhankelijke N-deglycinatie om

een ijzer(III) complex te genereren onder atmosferische omstandigheden. De verandering omvat een verlies in massa equivalent aan de glycyl groep, wat betekent dat successieve C-C en C-N dissociaties voorvallen, verder ondersteund door de kwantitatieve detectie van respectievelijk de daaruit volgende productie van CO2 en formaldehyde. Tijd-afhankelijke spectroscopie maakte

bovendien de spectroscopische karakterizatie van twee op ijzer gebaseerde kortlevende intermediaire soorten mogelijk tijdens het reactieverloop.

x

Acknowledgements

As this thesis marks the end of my PhD studies, there are several people I would like to thank. First and foremost I would like to thank my supervisor Prof. Christine McKenzie: I still think back to the day when I entered your office in 2012 and told you that I wanted to go to Australia, and if you could basically help me skipping the dark Danish winter in return for an Australian summer with a scuba diving tank on my back. You laughed and told me that everyone wants to go to Australia; I felt like a fool, but an adventure certainly started! Jeg har haft det som blommen i et

æg the past several years, and I deeply value the professional and personal relationship we have

developed. I admire your optimistic attitude to science and life in general, and you have certainly taught me the importance of small details. I will miss our marathon meetings in front of your computer discussing a sentence word for word or tiny aesthetics in a picture, while a “duck” or two occasionally interfere. In the beginning your mad ChemBioDraw schemes were a mystery to me; but now, several years later, I realize that I actually also enjoy the puzzle of drawing a catalytic cycle, which somehow is rather scary! I appreciate the friendly and humorous atmosphere during our many(!) discussions, and I have loved that it is not all about science; so, whenever you need a fashion advice for a talk in the future – just send a picture beforehand, I will be happy to comment. I also want to thank you for letting me develop on my own and inviting me into your network all over the world, which leads me to thank my supervisor Prof. Wesley Browne. It is funny that it all started with just one visit, which turned into numerous visits and now years later I in fact feel at home in Groningen. Wes, thank you for improving my skills with spectroscopy and experimental setups, your hospitality and many great discussions over the past years. The combination of both of you as supervisors has been perfect for me: you have very different mindsets, approaches and forces, which have challenged me to make up my own opinion and become my own boss. I have no doubt that the double degree agreement has made me a stronger and more independent researcher – thank you!

My sincere gratitude goes out to the reading committee Prof. Vickie McKee, Prof. Edwin Otten, Prof. Marcel Swart and Prof. T. David Waite for approving this thesis and(!) for taking the time out of your schedules to come to both Groningen and Odense for evaluation of my PhD thesis. Also, I want to thank Vickie for your enormous patience while teaching me crystallography – I still remember the day, where we spent several hours going through a cif-file line for line! It has been great having you visiting Odense so often, and I have enjoyed our many walks with ice cream.

Prof. Feliu Maseras and his group are thanked for performing DFT calculations on the light-promoted O2-activation presented in Paper VI, Prof. Cathrine Frandsen for assistance with

collection of Mössbauer data in Paper IV and Paper VI, Dr. Christian G. Frankær for assistance with PXRD and XAS experiments in Paper II, Dr. W. Alex Donald for calculations on the gas-phase clusters in Paper III, Andrea L. Gonzalez for the initial characterization on the iron(IV)oxo species in Paper VIII and of course Nina Stiesdal and Mie Thorborg Pedersen are thanked for their graphical expertise making the inside cover of Chem. Eur. J. for Paper IV and the front cover of this thesis, respectively.

xi I would like to thank my administrative helpers Tanja Løvgren and Cristina D’Arrigo for taking care of all of the extra work with legal details in connection with the double degree doctorate. Without your willingness and persistency, I am not sure a contract would ever have formed. Also, the Head of the PhD school at SDU Prof. Jacob Kongsted is specially thanked for being so helpful and calming in the entire process.

Former and present members of the McKenzie Group are thanked for a great working environment over the years. Especially I want to thank David de Sousa for discussions on the iron-tpena system, Claire Deville for proofreading parts of this thesis and Morten Liljedahl for assisting with ligand synthesis. Charlotte Damsgaard is thanked for all of her assistance in the lab. At SDU I also must thank the entire TAP group for always being so helpful! A special thanks to Danny Kyrping for assistance with the X-ray diffractometer and the EPR spectrometer – the fact that you are always ready to help whenever an instrument is not working is highly appreciated. Pia Haussmann is thanked for being so helpful with my many MS samples and Lars Duelund for teaching me the EPR spectrometer.

Huge thanks to all previous and current Brownies! Whenever I have left my so-called holiday in Denmark to spend time Groningen, I have never felt like a visitor, but rather as a full-time member of the group, so basically: thanks for adopting me in your family! A special thanks to Sandeep Padamati for babysitting me during my first visits at RUG. Also, I want to thank the Otten group for introducing me to the Borrel and always being up for a fun Friday evening. Particularly, I want to thank Francesca Milocco. I am so happy that I happened to place myself next to you at that CARISMA meeting in Ljubljana back in 2015. I have indeed enjoyed our many talks, fits of laughter and beers all over Europe. Francesca and Jorn Steen are specially thanked for doing me the honour of being my paranymphs.

During the past years at SDU, I have always enjoyed the friendly atmosphere during lunch breaks

under the chandelier. Thanks to former and present members of the groups of TUG, DBR, UGN,

theoretical chemistry as well as experimental physics for making these friendly links among the research groups possible.

I am probably what one could call “a child of the ECOSTBio action”, and neither can I imagine my PhD time without being involved in both this COST action and the CARISMA COST action, hence I want to thank both actions for financial support to attend conferences, scientific short-term missions and summer schools. Involvement in these COST actions has certainly expanded my scientific network and I have now great friends and colleagues all over Europe. Furthermore, I would like to specially thank Dr. Eckhard Bill for agreeing on organizing a scientific school with me on EPR and Mössbauer spectroscopy. It was 10 wonderful days in Mülheim and I must admit: I still deeply admire how d-orbitals seem to simply just split up for your inner eye.

Finally, I want to thank my family for all of your support over the past years. Constantly you are trying to understand what I am doing; I am well aware that it is not an easy task. I highly appreciate that you are still trying so hard. The fact that you have learned buzzwords such as