Magnetic and electrochemical properties in new Mn(III) and Mn(IV) compounds

UNIVERSIDADE DE LISBOA

FACULDADE DE CIÊNCIAS

DEPARTAMENTOD DE QUÍMICA E BIOQUÍMICA

Magnetic and Electrochemical Properties in new Mn(III) and

Mn(IV) compounds

César Augusto Pifre Reis

Mestrado em Química

Especialização em Química

Dissertação orientada por:

Doutor Paulo Nuno Barradas Pereira Martinho

Doutora Liliana Maria Pires Ferreira

This year was incredible regarding the scientific and social skills that I acquired with this fantastic lab group. Most of all I would like to highlight the good environment lived in the lab.

First of all, I would like to thank to my supervisor Doctor Paulo Nuno Martinho for welcoming me in the lab, for all the opportunities given that I would, probably, have never had in another group. I would like to highlight your amazing scientific background and capacity to motivate all of us, which made me see investigation with another perspective, giving me the will to continue this area. I also want to thank my supervisor Doctor Liliana Ferreira for the time dedicated for guiding me, your kindness, availability and for all help in this specific area that I so interesting in. I am sure that Professor Maria de Deus must be proud of your excellent supervising and knowing that I ended up with the right people.

A special thank to Doctor Sara Realista that was almost my supervisor as well. I would like to thank her for letting me learn so much from her and for the patience she had with me. Was a pleasure to follow your way of thinking and your incredible energy. This will help me a lot in the future.

I would like to thank Professor Maria José Calhorda for having me in this group and for the opportunities given.

I want to thank all my lab colleagues Doctor Ana Vicente for your good spirit and shared jokes, Frederico for your sense of humour that contributed to the good environment in the lab, Janaína for your friendship and good spirit, Marcos for your craziness that made me laugh so much, André for your friendship and shared music, Rafaela for your kindness, Marta for your good mood and kindness, Mohammed for being funny, Ana Elisa for your contribution to the good environment in the lab and Olga also for your sweetness.

I would like to thank Nuno Bandeira for the preliminary theoretical calculations.

Last but not least, to my friends, and specially to my family that always believed in me and to Bárbara Oliveira for all love, patience and help given during this year. Was awesome to share the same lab, colleagues and ideas with you.

O manganês é um metal de transição da série 3d e apresenta a importante característica de poder adotar vários estados de oxidação, permitindo que possa ter um papel relevante em diferentes aplicações tecnológicas, de que são bons exemplos o magnetismo molecular e a catálise. Nesta dissertação de mestrado foram investigados compostos em que o manganês assumiu dois estados de oxidação, manganês(III) e manganês(IV), levando a que os resultados dos estudos realizados fossem naturalmente apresentados em dois capítulos separados, diferenciados pelo estado de oxidação do metal. De facto, as propriedades eletrónicas e magnéticas do manganês podem alterar-se de forma a participar em centros ativos com diferente número de coordenação e na presença de diferentes intensidades do campo de ligandos (série espectroelectroquímica), conferindo propriedades próprias aos compostos que integra, e justificando, assim, que a organização do trabalho que aqui se apresenta reflita a participação do manganês em duas áreas distintas de aplicação. Quanto aos compostos de manganês (IV), eles terão interesse em aplicações catalíticas, visto ter-se verificado que estes complexos dão resultados interessantes quanto à captura e ativação/conversão de pequenas moléculas como, por exemplo, dióxido de carbono (CO2). Relativamente aos complexos de manganês (III), dadas as

propriedades magnéticas encontradas nos compostos estudados, é de considerar a exploração destes complexos, por exemplo, para aplicação em dispositivos magnéticos de armazenamento de informação.

A síntese dos ligandos utilizados na preparação dos compostos de manganês(IV) e de manganês(III) foi conseguida fazendo reagir uma amina com um aldeído através de uma reação de condensação Schiff base.

Para obtenção dos dois complexos de manganês(IV), foram sintetizados dois ligandos tridentados Schiff base, L1 e L2, e caracterizados isoladamente por espectroscopia UV-Vis, FTIR e Ressonância Magnética Nuclear (RMN). Esta última técnica permitiu identificar a presença dos sinais esperados dos protões presentes nos ligandos. Os complexos de manganês(IV), C1 e C2, depois de sintetizados, foram caraterizados por espectroscopia de infravermelho com transformada de Fourier (FTIR), espectroscopia ultravioleta-visível, análise elementar e voltametria cíclica. Com a análise por FTIR pretendeu-se identificar os modos vibracionais característicos das iminas presentes nos ligandos e compará-las com os mesmos picos presentes no espectro dos ligandos antes da sua coordenação ao metal. A espectroscopia ultravioleta-visível foi utilizada para identificar as bandas correspondentes às transições eletrónicas d-d, bem como as bandas π-π* _{dos ligandos e as bandas de transferência de carga entre o metal e o ligando (MLCT}

e LMCT). Os espectros UV-vis de C1 e C2 foram comparados aos espectros dos ligandos isolados, permitindo identificar a presença das suas bandas características nos complexos formados. Os estudos por voltametria cíclica permitiram a identificação dos processos de oxidação-redução e a confirmação do estado de oxidação +4 do manganês em ambos os complexos.

Explorou-se depois a capacidade dos complexos C1 e C2 capturarem/ativarem dióxido de carbono. Para tal, as soluções dos complexos foram saturadas com CO2 e sujeitas novamente

a métodos eletroquímicos de verificação de processos de redução do CO2. Por voltametria cíclica

verificou-se que ambos os complexos têm capacidade para capturar CO2 e a respetiva conversão

foi estudada através de eletrólises efetuadas com vista à obtenção de espécies gasosas provenientes da eletroredução do CO2. Foi efetuada cromatografia gasosa (gas chromatography-

GC) do headspace do produto da eletrólise recorrendo a um cromatógrafo com detetor de condutividade térmica (thermal conductivity detector- TCD), com o objetivo de detetar essas

de GC-TCD. Como tal, sabendo que haveria redução de CO2, recorreu-se à espectroscopia de

RMN usando uma extração feita a partir da solução da eletrólise e, de facto, foram detetados sinais correspondentes ao protão do grupo C-H presente no ácido fórmico e proveniente da redução de CO2. Tendo em conta esta informação e as implicações na estrutura e propriedades

eletrónicas de complexos que capturam CO2, foi necessário recorrer a novos estudos, tendo sido

combinadas as técnicas de UV-vis e FTIR com a eletroquímica –espectroeletroquímicos (IR-SEC e UV-Vis-SEC) – em solução, e efetuados estudos de ambas as técnicas espectroscópicas com aplicação de potencial controlado. Através desta nova caracterização, foi possível detetar a formação de um composto intermediário (M-COOH) proveniente da redução de CO2. Na tentativa

de compreender o ciclo catalítico e a formação da espécie ativa na electroredução de CO2 foram

efetuados cálculos teóricos preliminares. No futuro próximo, será importante aprofundar este estudo teórico, de forma a obter informação acerca da reatividade e seletividade dos dois complexos de manganês(IV) estudados quanto à captura/ativação de CO2.

Seis complexos de manganês(III), C3 a C8, foram sintetizados pelo método one-pot, através do qual os complexos são gerados in-situ (sem isolar os ligandos), tendo-se utilizado ligandos tridentados (C7), tetradentados (C8) e hexadentados (C3 a C6), numa esfera de coordenação com átomos dadores ONO, N2O2 e N2O4, respetivamente. BF4- ou Cl- foram

utilizados como contra-iões. A configuração eletrónica do manganês(III), d4_{, neste tipo de}

complexos, pode conduzir a fenómenos de transição de spin, ou seja, a transições entre estados em que a energia de emparelhamento dos eletrões d é superior ao delta octaédrico (Δoct),

apresentando-se o ião metálico em spin alto, e estados em que se dá a situação oposta e o metal se encontra no estado de spin baixo. O estudo do comportamento magnético destes complexos foi feito usando um dispositivo supercondutor de interferência quântica (superconducting quantum

interference device- SQUID), variando-se a temperatura da amostra entre 10 K e 370 K, enquanto

se mantém aplicado um campo magnético externo. A partir dos valores retirados diretamente destas medidas, e que correspondem ao momento magnético total da amostra inserida no SQUID, é posteriormente calculada a suscetibilidade magnética molar do complexo (χ𝑀) e apresentado o

resultado na forma do produto da suscetibilidade magnética pela temperatura (χ𝑀𝑇). No caso do

manganês(III), os valores característicos para χ𝑀𝑇 são: 1 cm3.K.mol-1 quando o ião está no estado

de spin baixo e 3 cm3_.K.mol-1 _{quando está no estado de spin alto.}

De entre os complexo de manganês(III) sintetizados, o complexo C3 tem o comportamento magnético mais complexo, apresentando uma transição de spin incompleta e com dupla histerese à volta da temperatura ambiente. O complexo C4 tem um comportamento magnético com transição de spin quase completa, em degrau. O complexo C7 apresenta um valor de χ𝑀𝑇 acima da temperatura ambiente que é característico de um estado em spin baixo. Contudo,

esse valor decresce à medida que a temperatura baixa e a variação de χ𝑀 sugere um alinhamento

antiparalelo dos spins (comportamento de tipo antiferromagnético) a baixas temperaturas. Os restantes complexos, C5, C6 e C8, apresentam valores de χ𝑀𝑇 razoavelmente constantes e

próximos do que seria esperado para o estado de spin alto do manganês(III).

À semelhança dos complexos de manganês(IV), os seis complexos de manganês(III) foram também caraterizados por espectroscopia de infravermelho com transformada de Fourier (Fourier transform infrared spectroscopy- FTIR), por espectroscopia ultravioleta-visível, por análise elementar (exceto C5 e C6) e por voltametria cíclica (exceto C8). A técnica de FTIR permitiu verificar a presença dos picos referentes às iminas dos ligandos, bem como dos picos correspondentes às vibrações da ligação B-F do contra-ião BF4-. A espetroscopia

ultravioleta-caso dos complexos de manganês(III), a voltametria cíclica é usada para confirmar o estado de oxidação do metal, através da identificação de processos de oxidação-redução. Complexos semelhantes ao complexo C8, com número de coordenação 5, foram já publicados como tendo aplicações mais direcionadas para a área da catálise homogénea. No futuro próximo, será interessante efetuar testes à possível atividade catalítica deste complexo tendo em conta as aplicações já publicadas de compostos semelhantes (hidroformilação de alcenos). Uma vez que o comportamento magnético de um complexo é afetado pelas interações intramoleculares e intermoleculares, para uma compreensão ao nível mais fundamental do comportamento magnético destes compostos, será essencial recorrer a técnicas de determinação da estrutura cristalina de todos os complexos, nomeadamente, raio-X de monocristal (single crystal X-ray

diffraction- XRD).

Palavras-chave:

The main goal of the research project that gave rise to this thesis was to develop new Mn(III) complexes with the property of single molecular magnet. During the course of the project, however, six Mn(III) and two Mn(IV) complexes were obtained. This thesis will show that, according to the oxidation state, the ligands effect (spectrochemical series) or the coordination environment of the metal ion, the compounds obtained will be suitable for different applications.

The syntheses of the ligands used in the preparation of the Mn(III) and Mn(IV) complexes were done reacting an amine with an aldehyde through a condensation reaction forming the Schiff base ligands.

The Mn(IV) Schiff base complexes synthesised, as well as the ligands used, were isolated and characterised by UV-vis, FTIR spectroscopy, Nuclear Magnetic Resonance (NMR) and SQUID magnetometry. The two different Mn(IV) complexes reported, both have tridentate ligands and are useful to be used as catalyst for CO2 electroreduction. Electrolysis, gas

chromatography (GC-TCD) and spectroelectrochemical studies (IR-SEC and UV-Vis-SEC) were performed after the characterisation by cyclic voltammetry in N2 and CO2 atmosphere. In order

to get a deeper understanding of the catalytic cycle, preliminary computational studies were done to know which active specie is responsible for the electroreduction of the CO2.

The syntheses of six Mn(III) complexes were done in one-pot method with tridentate and hexadentate ligands in a ONO and N4O2 donor atoms, respectively. The metal coordination

environment of these complexes is balanced with Cl- _{or BF}

4- as counter ion. A Mn(III) complex

with coordination number of five was also synthesised with a tetradentate ligand. The characterisation of the compounds were performed by elemental analysis, cyclic voltammetry, UV-Vis and FTIR spectroscopy. This type of complexes can display a spin crossover (SCO) phenomenon because its electronic distribution (d4_{) can change between high spin (HS)}

configuration and low spin (LS) configuration with temperature stimulation. The magnetic properties of all Mn(III) complexes were therefore investigated using SQUID magnetometry and the six complexes gave rise to a variety of magnetic behaviours. In fact, one of the complexes displayed an unusual hysteretic magnetic behaviour around room temperature accompanied by an incomplete spin transition; another one presented a stepped and slow SCO; one was in equilibrium at HS state over all temperature range and another one was in the LS state at 300 K and displayed an antiferromagnetic behaviour at lower temperatures. These different magnetic properties indicate that the complexes worth deeper structural characterization to explore their suitability in applications such as for sensors or data storage devices

Keywords:

1 Introduction ... 1

1.1 Manganese: coordination metal... 1

1.1.1 Crystal-field theory ... 1

1.1.2 Crystal-field stabilisation energy (CFSE) ... 2

1.2 Schiff base reaction ... 3

1.3 Magnetic properties ... 4

1.4 Mn(III) complexes... 6

1.4.1 Jahn-Teller effect ... 6

1.4.1.1 Spin crossover (SCO) ... 6

1.4.1.2 Single molecule magnets ... 8

1.5 Application of manganese complexes ... 9

1.5.1 Magnetism ... 9 1.5.2 Catalysis ... 11 1.5.2.1 CO2 electroreduction ... 12 1.6 Scope ... 16 2 Mn(IV) complexes ... 17 2.1 Synthesis ... 18 2.2 Characterisation ... 19

2.2.1 Fourier-Transform Infrared Spectroscopy ... 19

2.2.2 Ultraviolet-visible Spectroscopy ... 19

2.2.3 1_{H and} 13_{C APT NMR Spectroscopy ... 21}

2.2.4 SQUID Magnetometry ... 23

2.3 Electrochemical Studies ... 24

2.3.1 Cyclic Voltammetry ... 24

2.3.1.1 In N2 atmosphere ... 24

2.3.1.2 In CO2 atmosphere ... 25

2.3.2 Controlled potential electrolysis ... 27

2.3.3 Spectroelectrochemistry studies ... 28

2.3.3.1 UV-vis-SEC ... 28

2.3.3.2 IR-SEC ... 30

3 Mn(III) complexes... 32

3.1 Synthesis ... 33

3.1.1 Complexes with hexadentate ligands ... 33

3.2.1 Fourier-Transform Infrared Spectroscopy ... 35 3.2.2 Ultraviolet-visible Spectroscopy ... 36 3.2.3 Cyclic Voltammetry ... 38 3.3 SQUID magnetometry... 40 3.3.1 Complex 3 (C3) ... 41 3.3.2 Complex 4 (C4) ... 42 3.3.3 Complex 5 (C5) ... 43 3.3.4 Complex 6 (C6) ... 44 3.3.5 Complex 7 (C7) ... 44 3.3.6 Complex 8 (C8) ... 45 4 Experimental section ... 46 4.1 Synthesis ... 47 4.1.1 Mn(IV) ... 47 4.1.2 Mn(III) ... 49 4.2 Reagents ... 52 4.3 Instrumentation... 52

5 Final remarks and perspectives ... 57

6 References ... 59 Appendix ... I

Figure 1.1. Mn complexes with two (left) four (middle) and six coordination number (right) .... 1

Figure 1.2. Degeneracy lift of d orbitals in an octahedral environment ... 2

Figure 1.3. d orbital energies in an octahedral environment ... 2

Figure 1.4. Saltmen (left) and naphtmen (right) Schiff base ligands ... 3

Figure 1.5. Tridentate Schiff base ligand (left) and Hexadentate Schiff base ligand (middle and right) ... 4

Figure 1.6. Typical alignment of the atomic magnetic moments, at a certain temperature and under zero applied magnetic field, for different classes of bulk magnetism [22] _{... 4}

Figure 1.7 Typical plots of the magnetic susceptibility vs temperature (a) and of the product of the magnetic susceptibility and temperature vs temperature (b) for paramagnetic, ferromagnetic, antiferromagnetic and ferrimagnetic materials[22] _{... 5}

Figure 1.8. Jahn-Teller effect in Mn(III) with the eg orbitals elongation at (a) dz2 orbital (b) dx2-dy2 orbital[10] _{... 6}

Figure 1.9. Spin states of a Mn(III) SCO ... 7

Figure 1.10. Different types of spin transition: (a) gradual and abrupt; (b) gradual incomplete; (c) with hysteresis warming/cooling; (d) two-step spin transition ... 7

Figure 1.11. Energies of the d levels in octahedral and tetragonal symmetry ... 8

Figure 1.12. Mn(III) S=2 splitting by the negative axial zero-field splitting ... 9

Figure 1.13. [Mn(TRP)] complex ... 9

Figure 1.14. [Mn(TRP)] magnetic profile ... 10

Figure 1.15. Schematic [Mn12O12(CH3COO)16(H2O)4].2CH3COOH.4H2O cluster (acetate bridging ligands the water molecules are not represented) ... 10

Figure 1.16. Jacobsen catalyst ... 11

Figure 1.17. Mn(amp)2 Schiff base complex used for OER applications ... 11

Figure 1.18. Latimer-Frost diagram of CO2 reduction in homogeneous aqueous solution at pH 7 considering multi-electron and multi-proton processes ... 12

Figure 1.19. Catalytic cycle with CO and HCO2- release through a macrocyclic metal complex ... 13

Figure 1.20. Catalytic cycle with a solvent decoordination ... 14

Figure 1.21. A Co pincer ([Co(t-Bu)2PEPyEP(t-Bu)2(CH3CN)2][BF4]2), a Rh cluster [(RhCp*)3(3 -S)2]2+ in the middle and Fe porphyrin complex (Fe(TPP)) at right ... 14

Figure 1.22. [Mn(bipyridyl)(CO)3Br] complex (left) and [Mn(bipyridyl)(CO)3]2 dimer (right) 15 Figure 2.1. FTIR spectra of L1 (black line) and C1 (red line) at left (a) and L2 (black line) and C2 (red line) at right (b) in KBr pellets ... 19

Figure 2.2. Tanabe-Sugano diagram for a d3 _{electron metal-ion configuration in octahedral} environment[11] _{... 20}

Figure 2.3. UV-Vis Spectra of L1 and C1 (a) and L2 and C2 (b) 0.02 mM in dichloromethane ... 20 Figure 2.4. 1_{H NMR of L1 in CDCl} 3 ... 21 Figure 2.5. 13_{C-APT of L1 in CDCl} 3 ... 22 Figure 2.6. 1_{H NMR of L2 in DMSO-d} 6 ... 22

Figure 2.7. 13_{C-APT of L2 in DMSO-d} 6 ... 23

respectively. 0.10 M Bu4NPF6 was used as supporting electrolyte. υ = 100 mV/s ... 25

Figure 2.10. Cyclic voltammograms of dimethylformamide solutions of C1 (left) and C2 (right) under CO2 atmosphere (blue line) and N2 (red line). Glassy carbon, SCE and Pt wire were used as working, reference and counter electrodes, respectively. 0.10 M Bu4NPF6 was used as supporting electrolyte. υ = 100 mV/s ... 26

Figure 2.11. Cyclic voltammograms of dimethylformamide solution of C1 under CO2 atmosphere adding some water concentration 0.0M H2O (black), 0.2M H2O (red), 0.5M H2O (blue), 1.0M H2O (green) and 3.0M H2O (pink) Glassy carbon, SCE and Pt wire were used as working, reference and counter electrodes, respectively. 0.10 M Bu4NPF6 was used as supporting electrolyte. υ = 100 mV/s ... 26

Figure 2.12. Current (i) vs time graphs and Charge (Q) vs time and of the C1 electrolysis ... 27

Figure 2.13. 1_{H NMR spectrum showing the proton signal of the formic acid produced at δ=8.11} ppm in D2O. Before electrolysis (blue line) and after electrolysis (black line). The liquid phase of the bulk electrolysis experiment was extraction from the bulk solution of C1 with CH2Cl2 adding a drop of HCl after electrolysis ... 28

Figure 2.14. UV-Vis-SEC difference absorbance spectra of C1 in a saturated N2 solution over time with an applied potential of -1.50 V vs NHE. Left: in dimethylformamide (a) and right: in dichloromethane (b) ... 29

Figure 2.15. UV-Vis-SEC difference absorbance spectra of C1 in a saturated CO2 dimethylformamide solution over time at applied potential (-1.50 V vs NHE). ... 29

Figure 2.16. UV-Vis-SEC difference absorbance spectra of a dimethylformamide over time with an applied potential of -1.50 V vs NHE. Left: under N2 (a) and right: under CO2 (b) ... 30

Figure 2.17. IR-SEC experiments at -1.40 vs. Ag pseudo-reference using 5 mM of C1 and 0.1 M TBAPF6 in a CO2 saturated ACN solution, Pt grids as working and counter electrodes and Ag wire as pseudo-reference electrode ... 31

Figure 2.18. IR-SEC experiments at -1.40 vs. Ag pseudo-reference using 5 mM of C2 and 0.1 M TBAPF6 in a CO2 saturated ACN solution, Pt grids as working and counter electrodes and Ag wire as pseudo-reference electrode ... 31

Figure 3.1. FTIR plots of C3 to C5 (left) and C6 to C8 (right) ... 36

Figure.3.2. Tanabe-Sugano diagram for a d4 _{electron configuration in octahedral environment} [11] ... 36

Figure 3.3. UV-vis spectra of C3 0.1mM (left) and C4 (right) 0.05 mM in DMF ... 37

Figure 3.4. UV-vis spectra of C5 (left) and C6 (right) 0.1 mM in DMF ... 37

Figure 3.5. UV-vis spectra of C7 0.1 mM in DMF ... 38

Figure 3.6. Cyclic voltammograms of 1 mM dimethylformamide solutions of C3(black line) and C4 (red line) at left and C5(black line) at right under N2. Glassy carbon, SCE and Pt wire were used as working, reference and counter electrodes, respectively. 0.10 M Bu4NPF6 was used as supporting electrolyte. υ = 100 mV/s ... 39

Figure 3.7. Cyclic voltammograms of 1 mM dimethylformamide solutions of C6 (red line) and C7 (black line) in under N2. Glassy carbon, SCE and Pt wire were used as working, reference and counter electrodes, respectively. 0.10 M Bu4NPF6 was used as supporting electrolyte. υ = 100 mV/s ... 39

Figure 3.8. 𝜒𝑀𝑇 vs T graph of C3 ... 41

Figure 3.9. 𝜒𝑀𝑇 vs T graph of C3 between 200K and 370 K ... 42

Figure 3.10. 𝜒𝑀𝑇 vs T graph of C4 ... 42

Figure 3.13. 𝜒𝑀𝑇 vs T graph of C7 ... 44 Figure 3.14. 𝜒𝑀𝑇 vs T graph of C8 ... 45 Figure 4.1. Cyclic voltammetry experiment; glassy carbon, SCE and Pt wire were used as working, reference and counter electrodes, respectively in a 4-neck electrochemical cell. ... 53 Figure 4.2. Gas chromatograph (TCD) Agilent Technologies 7820A. ... 54 Figure 4.3. 1 mm Thin Layer Quartz Glass Spectroelectrochemical cell from IJ Cambria Scientific Ltd with a platinum gauze working electrode, platinum counter electrode and a SCE reference electrode ... 55 Figure 4.4. Optical Transparent Thin Layer Electrochemical (OTTLE) Cell from LabOmak Science Instruments and Accessories. Pt grids as working and counter electrodes and Ag wire as pseudo-reference electrode ... 55 Figure 4.5. SQUID magnetometer system components (parts numbered 6, 8 and 9 are the superconducting elements) ... 56 Figure A 1. COSY of L1 in CDCl3 ... II

Figure A 2 HMBC of L1 in CDCl3 ... II

Figure A 3. HSQC of L1 in CDCl3 ... III

Figure A 4. COSY of L2 in DMSO-d6 ... III

Figure A 5. HSQC of L2 in DMSO-d6 ... IV

Figure A 6. HMBC of L2 in DMSO-d6... IV

Figure A 7. Cyclic voltammograms of 1 mM dichloromethane solutions of L1 under N2. Glassy

carbon, SCE and Pt wire were used as working, reference and counter electrodes, respectively. 0.10 M Bu4NPF6 was used as supporting electrolyte. υ = 100 mV/s ... V

Figure A 8. Cyclic voltammograms of 1 mM dichloromethane solutions of L2 under N2. Glassy

carbon, SCE and Pt wire were used as working, reference and counter electrodes, respectively. 0.10 M Bu4NPF6 was used as supporting electrolyte. υ = 100 mV/s ... V

Figure A 9. Cyclic voltammograms of 1 mM dichloromethane solutions of C1 (left) and C2 (right) under N2 until +1.75 V vs NHE. Glassy carbon, SCE and Pt wire were used as working, reference

and counter electrodes, respectively. 0.10 M Bu4NPF6 was used as supporting electrolyte. υ = 100

mV/s ... V Figure A 10. GC-TCD chromatograph after bulk electrolysis with C1 ... VI Figure A 11 FTIR of C3 (left) and C4 (right) in KBr pellets... VI Figure A 12. FTIR of C5 (left) and C6 (right) in KBr pellets... VI Figure A 13. FTIR of C7 (left) and C8 (right) in KBr pellet ... VII Figure A 14. Cyclic voltammograms of 1 mM dimethylformamide solutions of C3 (left) and C4 (right) under N2. Glassy carbon, SCE and Pt wire were used as working, reference and counter

electrodes, respectively. 0.10 M Bu4NPF6 was used as supporting electrolyte. υ = 100 mV/s .. VII

Figure A 15. 𝜒𝑀 vs T graph of C3 ... VII Figure A 16. 𝜒𝑀 vs T graph of C4 ... VIII Figure A 17. 𝜒𝑀 vs T graph of C5 ... VIII Figure A 18. 𝜒𝑀 vs T graph of C6 ... VIII Figure A 19. 𝜒𝑀 vs T graph of C7 ... IX Figure A 20. 𝜒𝑀 vs T graph of C8 ... IX

Scheme 1.1. Schiff base formation reaction (R1 ≠ H) ... 3

Scheme 2.1. General synthesis of ligands (L1 and L2) and manganese(IV) Schiff-base complexes (C1 and C2). i) MeOH; ii) KOH; iii) MnCl2.4H2O, air,  ... 18

Scheme 3.1. General reactions of Manganese(III) Schiff-base complexes C3, C4, C5 and C6/ i) MeOH; ii) KOH, iii) MnCl2.4H2O, air, iv) NH4BF4 ... 34

Scheme 3.2. General reactions of Manganese(III) Schiff-base complexes (C7) / i) MeOH; ii) MnCl2.4H2O, air;  ... 34

Scheme 3.3. General reactions of Manganese(III) Schiff-base complexes (C8) / i) MeOH; ii) KOH, iii) MnCl2.4H2O, air;  ... 35

Table 2.1 Potentials values (V vs NHE) for C1 and C2 redox processes ... 25

Table 3.1. Wavenumber values of the vibrational stretching for C3 to C8 ... 35

Table 3.2 Wavelength of the d-d transitions observed by UV-vis ... 38

𝜒𝑀 Magnetic susceptibility

A Microampere

ACN Acetonitrile

amp N-(2-hydroxyphenyl)salicylaldimine

bpy bipyridine

CFSE Crystal-field Stabilisation Energy

Cp Cyclopentadienyl

DCM Dichloromethane

DSC Differential scanning calorimetry DMF Dimethylformamide

DMSO Dimethyl sulfoxide

en ethylenediamine

EPR Electron Paramagnetic Resonance FTIR Fourier Transform Infrared Spectroscopy

GC-TCD Gas Chromatography-Thermal Conductivity Detector

H External magnetic field

HS High Spin

IR-SEC Infrared Spectroelectrochemistry LMCT Ligand-Metal Charge Transfer

LS Low Spin

MLCT Metal-Ligand Charge Transfer

mM Milimol

NHE Normal Hydrogen Electrode NMR Nuclear Magnetic Resonance OCP Open Circuit Potential

Oe Oersted

OTTLE Optical Transparent Thin Layer Electrochemical

ox oxalate

P Electron-pairing energy ppm parts-per-million phen phenanthroline

Salen N, N′-ethylenebis(salicylimine)

SCE Standard electrode reference

SCO Spin Crossover

SMM Single Molecule Magnet SOC Spin-Orbit Coupling

SQUID Superconducting Quantum Interference Device TMS Tetramethylsilane

TPP Tetraphenylporphyrin tren Tris(2-aminoethyl)amine

TRP [tris[1-(2-azolyl)-2-azabuten-4-yllamine] UV-vis Ultraviolet-visible

XRD X-ray powder diffraction ZFS Zero-field Splitting

1.1 Manganese: coordination metal

Manganese is a brittle metal with several possible oxidation states and can be found in nature under different forms, such as oxides, silicates, carbonates, etc.[1] _{Manganese is a transition}

metal from the 7th _{group and 3}rd _{row with 7 valence electrons (3d}7_{) and is an important}

coordination metal used in distinct applications, such as catalysis[2]_{, luminescent materials for}

LED applications[3] _{and molecular magnetism}[4]_{, specifically for data storage devices and}

quantum computing[5]_{. Being an ion with different possible oxidation states, consequently}

allowing changes on its electronic properties, manganese can influence its coordination environment, therefore forming complexes with different geometries. Most of the manganese complexes are found in octahedral geometry. However, examples of manganese complexes with two [6]_{, four}[7] _{(Figure 1.1) and five coordination number}[8] _{are also reported in the literature.}

Figure 1.1. Mn complexes with two (left) four (middle) and six coordination number (right)

1.1.1 Crystal-field theory

The crystal-field theory is a model to predict the degeneracy lift of the orbitals energy (d orbitals, in the present case), and the geometry stabilisation [9] _{(Figure 1.2).}

In octahedral geometry, the splitting between the t2g and eg orbital subgroups is

denominated by Δoct. It is related with the interaction between the ligands and the metal centre

and is a measure of the ligands’ strength and oxidation state of the metal centre. A high Δoct value,

for instance, means a strong ligand field. The ligand’s strength is organised in the spectrochemical series by ascending order from the weaker to the stronger ligands: I- _{< Br}- _{< NCS} - _{< Cl}- _{< F}- _<

OH- _{< ox}2- _{≈ H}

2O < NCS- < NH3< en < bpy < phen < CN- ≈CO. Additionally, the higher the metal

atomic number, the easier it is to increase the Δoct. Likewise, a higher oxidation state of a certain

Figure 1.2. Degeneracy lift of d orbitals in an octahedral environment

1.1.2 Crystal-field stabilisation energy (CFSE)

For octahedral manganese complexes, d4 _{to d}7 _{electron configurations with high and low}

spin states are possible. The spin state of the metallic centre is influenced by the oxidation state of the metal ion itself, by the ligands or by the intermolecular interactions, A higher Δoct value

promotes low spin states where the d electrons first occupy the lowest energy orbitals (t2g),

whereas a lower Δoct value provides high spin states, as the electron distribution follows the

Hund’s rule.

Figure 1.3. d orbital energies in an octahedral environment

As shown in Figure 1.3, in an octahedral crystal field there are two different orbital energies: the t2g orbitals, where each electron contributes with -0.4Δoct to the total energy value,

and the eg orbitals, with each electron adding +0.6Δoct to the total energy. Therefore, a lower

(negative) energy leads to a higher crystal-field stabilisation.

Considering Mn(III) complexes, there are only two possible spin states, the high spin state (S=2) and the low spin state (S=1) with the respective CFSE being:

-CFSE (high spin) = (−0.4 x 3Δoct) + 0.6Δoct = −0.6Δoct -CFSE (low spin) = (−0.4 x 4Δoct) + 𝑃 = −1.6Δoct + 𝑃 where P is the electron-pairing energy.

For Mn(IV) complexes there is only one possible spin state, S=3/2, because it is a d3

configuration. The corresponding CFSE calculated is then equal to: -CFSE = (−0.4 x 3Δoct) = −1.2Δoct

The crystal-field theory is easier to predict for simple ligands, as those belonging to the spectrochemical series. The prediction of CFSE for Schiff base ligands would be very useful, however, it is not straightforward.[9–11]

1.2 Schiff base reaction

Schiff base ligands are characterised by the presence of an imine group where the nitrogen atom is bonded to an alkyl group. These ligands are synthesised by reacting a primary amine with an aldehyde or a ketone, as can be seen in Scheme 1.1. The Schiff bases syntheses are condensation reactions wherein water molecules are released during the reaction giving place to C=N bonds.

Scheme 1.1. Schiff base formation reaction (R1 ≠ H)

These reactions are frequently reported in the literature for the synthesis of ligands in coordination chemistry, due to the presence of a donor atom in the imine group that is able to coordinate to the metal centre. Additionally, most of the ligands are chelates, easy to synthesize ,[12,13] _{and the products are obtained with good yields}[14]_.

In coordination chemistry many research groups aim to obtain chelate ligands inspired on the salen-type ligands[15] _{(N,N′-ethylenebis(salicylimine)) to produce other, more tuned, ligands.}

This kind of compounds has been reported for catalysis[8] _{and molecular magnetism}

application[16]_.

Figure 1.4. Saltmen (left) and naphtmen (right) Schiff base ligands

In Figure 1.4, two tetradentate salen-type ligands, saltmen (N,N`-(1,1,2,2-tetramethylethylene)bis(salicylideneiminato)), at left, and naphtmen (N,N`-(1,1,2,2-tetramethylethylene)bis(naphthylideneiminato)), at right, are represented. Both compounds are synthesised by reacting salicylaldehyde or naphthaldehyde with a chiral diamine in a 2:1 stoichiometric ratio. These tetradentate ligands in octahedral compounds allow other molecules (e.g. solvents) to coordinate to the metal centre, sometimes resulting in the formation of dimeric species to stabilise the geometry[17]_{. The dimerisation process depends on how bulky the ligands}

are and how the steric effects condition the enantioselectivity. Another similar interesting Schiff base ligand was reported by Jacobsen and co-workers and will be discussed further as an example of a catalyst used in homogeneous catalysis[18]_.

Additionally, Schiff-base ligands can be used in transition metal chemistry to promote the spin crossover (SCO) phenomenon.[19] _{Most of the ligands that promote SCO have N}

2O donor

sets for tridentate ligands and N4O2 donor sets for hexadentate ligands. The synthesis of the

some example of ligands are shown in Figure 1.5. The SCO phenomenon will be discussed in more detail later in this thesis.

Figure 1.5. Tridentate Schiff base ligand (left) and Hexadentate Schiff base ligand (middle and right) The tridentate ligands can adopt either a meridional (mer) or a facial (fac) spatial arrangement greatly contributing to geometry’s stability. Furthermore, the hexadentate ligands (N4O2) may also condition the coordination in relation to the ligand’s flexibility and donor’s set

spatial arrangement. Therefore, the phenolate oxygens can be found in the cis or trans configurations depending on the ligand.[10]

1.3 Magnetic properties

Figure 1.6 shows the orientation of the atomic magnetic moments for the main types of the magnetic behaviour

Figure 1.6. Typical alignment of the atomic magnetic moments, at a certain temperature and under zero applied magnetic field, for different classes of bulk magnetism [22]

At a certain temperature and without an applied magnetic field, if all the atoms or ions of the material have their magnetic moments aligned in the same direction (parallel moments), the material behaves like a large permanent magnet and is known as a ferromagnet. It can be characterized by a spontaneous magnetization (M), which is the total magnetic moment normalized by the material’s mass, for example.

If the magnetic moments are oriented equally in two opposite directions (antiparallel moments), then the sum of the total moments of the material adds to zero and the compound is known as an antiferromagnet. In the case that these opposite moments have different values, then

the total sum gives rise to a small moment but not to a zero value and the material is known as a ferrimagnet.

These types of magnetic materials have a certain temperature above which the thermal energy is higher that the spontaneous magnetic interaction energy between the moments. Above this transition temperature, therefore, the magnetic moments lose their spontaneous alignment. However, there is a type of magnetic behaviour where, independently of the temperature, the atomic magnetic moments are never spontaneously aligned, they are always randomly distributed. These materials are known as paramagnetic and, as can be seen in figure 1.6, the random distribution of the magnetic moment’s orientations, imply that an applied magnetic field is needed to align them (in the field direction), and to give rise to a non-zero value of the magnetization.

Independently of the type compound, one of the ways to characterize its magnetic behaviour is to measure the magnetisation (M) as a function of the temperature (T), under the influence of a constant applied magnetic field (H). The magnetic susceptibility (𝜒) can then be evaluated by the ratio:

𝜒 =𝑀

𝐻 (1.1)

Many complexes with d unpaired electrons have a paramagnetic behaviour and their molar magnetic susceptibility (𝜒𝑀) follows the Curie law defined by eq. 1.2, where C is known

as the Curie constant, which is characteristic of the complexes: 𝜒𝑀=

𝐶 𝑇 (1.2)

Figure 1.7 Typical plots of the magnetic susceptibility vs temperature (a) and of the product of the magnetic susceptibility and temperature vs temperature (b) for paramagnetic, ferromagnetic, antiferromagnetic and

ferrimagnetic materials[22]

The magnetic response of the compounds to the applied magnetic field as temperature varies can be evaluated through different graphical representations. Figures 1.7 (a) and (b) show typical plots of the magnetic susceptibility vs temperature (𝜒𝑀 𝑣𝑠 T plot) and of the magnetic

susceptibility and temperature product vs temperature (𝜒𝑀𝑇 𝑣𝑠 𝑇 plot), respectively, for the

1.4 Mn(III) complexes

1.4.1 Jahn-Teller effect

One of the most interesting manganese oxidation states is +3 and the d4 _{Mn complexes}

are usually found in the high spin state (HS).[23] _{In this case, the t}

2g orbitals of the metal centre are

half filled and a single electron occupies the eg orbitals. This electronic configuration results in a

permanent distortion of the ion geometry which is known as Jahn-Teller effect[24] _{(Figure 1.8).}

This effect leads to a distortion in the octahedral geometry promoting a tetrahedral geometry through a dz2 or a dx2-y2 orbital elongation. This phenomena leads to a degeneracy split between

the orbitals and it can influence the magnetic behaviour of the complexes [10,11]

Figure 1.8. Jahn-Teller effect in Mn(III) with the eg orbitals elongation at (a) dz2 orbital (b) dx2-dy2 orbital[10]

1.4.1.1 Spin crossover (SCO)

The first compound showing the SCO phenomenon was observed by Cambi and Szegö in 1931 through the temperature dependence of the magnetic moment in an Fe(III) complex with dithiocarbamate ligands[25]_.

In the crystal-field theory subchapter previously presented, it was described that, in an octahedral environment, transition metal ions with an electronic configuration between d4 _{and d}7

can display two different spin states: depending if the Δoct value is higher or lower than the

electron-pairing energy (P), the metal ion is in the low spin (LS) or the high spin (HS) state, respectively(Figure 1.7).[10,26]

Spin transition can be promoted by temperature, light, pressure or exposure to an external magnetic field.[27] _{Figure 1.9 shows the example of a spin transition for a Mn(III) complex. As a}

sample is warmed or cooled at a fixed external magnetic field (H), the metal-ligand (M-L) bonds change their length, generally increasing their value during the warming cycle. In this case, as opposed, the M-L bonds decrease their length during the cooling cycle. [28]_.

Figure 1.9. Spin states of a Mn(III) SCO

A Mn(III) complex has a d4 _{electron configuration and can benefit from the spin crossover}

(SCO) phenomena where a low spin state (S=1) with a 3_T

1g ground state changes to a high spin

state (S=2) 5_E

g or vice versa.

In Figure 1.10 it is possible to observe the most common SCO profiles reported in the literature. These depend on the metal used, the packing effects and the intermolecular interactions between neighbouring molecules, such as hydrogen bonding, π –π stacking, and electrostatic interactions promoted by the ligands[19]_{. The plots in Figure 1.10 represent the total conversion of}

the low spin to high spin state (XHS) with temperature.[29] The scope of applications of these compounds depend on the type of magnetic profile: (a) complete abrupt SCO and incomplete gradual SCO; (b) two incomplete gradual SCO; (c) abrupt with hysteresis SCO; (d) incomplete two-step SCO. The abrupt magnetic behaviour is more adequate for complexes with application molecular switching devices and complexes with a gradual SCO are more useful to be applied as sensors.[30,31]_{. The two-step SCO behaviour can be explained by the existence of an intermediate}

spin state or a mixture of spin states among the metal ions in a dinuclear complex [31]_{. The}

occurrence of hysteresis during SCO is an interesting behaviour and the complexes that show this behaviour can be used in molecular memory devices.[32] _{However, more research in this field is}

necessary in order to understand which type of parameters are relevant to reach a specific magnetic behaviour, as well as a room temperature SCO,.[33]

Figure 1.10. Different types of spin transition: (a) gradual and abrupt; (b) gradual incomplete; (c) with hysteresis warming/cooling; (d) two-step spin transition

Most of the magnetic profiles are reported through the MT or eff values in

temperature-dependent plots. The M is the magnetic molar susceptibility and the effective magnetic moment,

eff , is related to the MT product . by the equation (1.3), in cgs units:

𝜇𝑒𝑓𝑓 = √8𝜒𝑀𝑇 (1.3)

The MT theoretical values for a Mn(III) ion in the low spin state is 1.00 cm3 K mol-1 and

for the high spin state is 3.00 cm3 _{K mol}-1_{. These are only values, considering that the}

spin-orbit or spin-spin coupling are inexistent or can be neglected as a first approximation.

1.4.1.2 Single molecule magnets

Single molecule magnets (SMM) are molecules that exhibit slow relaxation of the magnetisation [34] _{and are also known for their weak intermolecular interactions and low}

spin-orbit coupling (SOC) constants[35,36]_.

The electronic configuration of the high spin Mn(III) ion in an elongated Jahn–Teller distortion is known to be (dxy)1, (dyz)1, (dxz)1 and (dz2)1 with a 5B1 ground state.[15] Figure 1.8 shows

a schematic representation of the Jahn -Teller effect and how it affects the geometry of the complex. Depending if the forth electron occupies a dz2 or dx2- y2 orbital, it gives rise to different

splitting in the d orbitals (Figure 1.11).

Figure 1.11. Energies of the d levels in octahedral and tetragonal symmetry

Figure 1.11 represents two possible splittings of the d-orbitals in octahedral geometry. This effect is induced by the spin-orbit interaction of metallic ions with S≥1 and orbital momentum quenched in first order, and is known as zero-field splitting (D) of the ground state. A compressed tetragonal geometry is obtained when the zero-field splitting is greater than 0 (D>0) and an elongated tetragonal geometry is obtained when D<0. This axial distortion is a characteristic of SMM.[34]

A parameter that relates the zero-field splitting and the spin state is the energy barrier Ueff,

defined as:

Ueff = |𝐷|. 𝑆2 or Ueff = |𝐷|. (𝑆2−

1

4) (1.4)

Figure 1.12 shows the double-well diagram for negative zero-field splitting (D). The Ueff

parameter is the energy barrier to relaxation of the magnetisation and is denotated as the difference between the spin quantum number, M =0 and M=±2 for Mn(III) complexes. Despite this, also

some complexes can evidence zero-field splitting through the dz2 orbital compression leading to

a positive D value[37]_.

Figure 1.12. Mn(III) S=2 splitting by the negative axial zero-field splitting

Ideally, the main goal will be to increase the D and S parameters at the same time, however the zero-field splitting is usually inversely proportional to the spin.[38] _{Nevertheless there}

are still so many researchers developing work in order to obtain compounds with both higher values.[39]

The Jahn-Teller effect is characterised by distortions on the geometry. This effect could cause an increase of the zero-field splitting influencing the break of symmetry leading to the magnetic anisotropy and enhance the D and S parameters[4,40]_.

In the beginning, SMM was described as a molecule with slow relaxation of the magnetisation and the relaxation time is related the Ueff by the equation (1.5)

ln 𝜏 = ln 𝜏0+ 𝑈𝑒𝑓𝑓

𝑘𝐵𝑇 (1.5)

where the 𝜏 is the relaxation time, the 𝜏0 the relaxation rate and 𝑘𝐵the Boltzmann constant.

Considering these equations, it is possible to conclude that with the increase of Ueff value

it is possible to obtain slow relaxation times of magnetisation (𝜏) and consequently obtain a SMM behaviour[41]_.

1.5 Application of manganese complexes

1.5.1 Magnetism

The first Mn(III) compound displaying the SCO phenomenon was first reported in 1980 for the [Mn(TRP)] complex (Figure 1.13).[42]

Figure 1.13. [Mn(TRP)] complex

TRP (tris(1-(2-azolyl)-2-azabuten-4-yl)amine)) is a chelate ligand that coordinates as a hexadentate ligand by N6 donor atoms and it shows the right ligand field strength to promote SCO

in Mn(III). The ligand can be synthesised by a Schiff base condensation reaction through reaction between tren (tris(2-aminoethyl)amine) and three equivalents of pyrrole-2-carboxaldehyde yielding the TRP ligand.[43]

The spin transition was observed by a change of the magnetic moment from 3.2 to 4.9μB

between 40-50K that corresponds to a transition of S=1 to S=2. It is possible to observe a spin-orbit coupling contribution in low spin state corresponding to the 3_T

1 ground state split and

distortion from octahedral symmetry, as can be demonstrated in Figure 1.14.

Figure 1.14. [Mn(TRP)] magnetic profile

In the Figure 1.15 is depicted the first single molecule magnet composed by a Mn cluster with a mixed of Mn( III) and Mn(IV) reported in 1993 with the molecular formula of [Mn12O12(CH3COO)16(H2O)4].2CH3COOH.4H2O synthesised by Sessoli, R. and her research

group[44]_{. The cluster contains 12 Mn atoms in which 8 of them are represented by the largest}

white circles that correspond to Mn(III) (S=2) and the other 4 Mn ions are the shaded circles with S=3/2, Mn(IV).

Figure 1.15. Schematic [Mn12O12(CH3COO)16(H2O)4].2CH3COOH.4H2O cluster (acetate bridging ligands the water

molecules are not represented)

The mixed spin states lead to a total spin value S=10 caused by the antiferromagnetic interaction between the Mn(III) and the Mn(IV) ions.[45] _{The cluster provides a high magnetic}

anisotropy due to Jahn-Teller effect of the Mn(III) ions. Combining the zero-field splitting, promoted by the anisotropy, with the total spin value (S=10), it is possible to reach a slow relaxation of the magnetisation.

1.5.2 Catalysis

As previously mentioned, Mn(III) Schiff base complexes (e.g. Jacobsen catalyst, Figure 1.16) had a huge impact in catalysis, in particular in the homogeneous asymmetric epoxidation of olefins.[46]

Figure 1.16. Jacobsen catalyst

The Jacobsen catalyst was reported in 1990 as an enantioselective catalyst for the epoxidation of olefins due to the steric effect created by the tert-butyl group increasing the bite angle, (angle between two donor atoms of a chelate ligand), and consequently the selectivity of this catalyst.[47] _{This is a Mn complex with a N}

2O2 donor ligand and a chloride anion coordinated

in an axial position. Therefore, the labile chloride ligand (weak ligand) can give rise to a vacant binding site in the metal centre and influence the active species and an open olefin binding site[48]

Moreover, solvent molecules could decoordinate in electrocatalysis to facilitate the catalyst activation[49]_.

Mn(IV) Schiff base complexes have also been used as catalysts in the oxygen evolving reaction (OER) in the photo-oxidation of water[50]_{. The Mn(IV) Schiff base complex with}

N-(2-hydroxyphenyl)salicylaldimine (amp) ligands, gives rise to Mn(amp)2 (in Figure 1.17) which was

then used in catalytic applications.

Figure 1.17. Mn(amp)2 Schiff base complex used for OER applications

Mononuclear Mn(IV) complexes with N2O4 donor atoms can also be used in

electrocatalytic applications.[51–53] _{However, the electrocatalytic activity of these compounds are}

still scarcely reported and only few cases were studied over the last years.[50,54,55] _{Although these}

type of complexes have been reported for OEC reactions, they can be used for CO2

electroreduction, since they can reduce themselves during this process, as will be discussed during the thesis.[56,57]

1.5.2.1 CO

electroreduction

CO2 has been a very explored compound by the scientific community as one of the main

gases responsible for the greenhouse effect. Since CO2 emissions are to avoid, its capture and

conversion into other added-value products are alternatives to balance the climate changes of our planet [58–60]_.

Considering these environmental aspects, many groups are currently working on CO2

capture and conversion and researchers have been using first-row transition metal complexes as catalysts for CO2 reduction. These first-row metal ions are a more sustainable alternative when

compared to the second and third-row transition metal ions due to their abundance and low cost. An example is the substitution of Re(I) with Mn(I), such as the complex [Mn(bpy-tBu)(CO)3Br][52]. CO2 is reduced to other added-value products such as CO, formic acid,

formaldehyde or methanol[61]_.

In the reactions from (1.6) to (1.11) are reported the possible products from the CO2

electroreduction and in equation (1.12) the competitive reaction of the proton reduction, as well as the potentials in homogeneous aqueous solution at pH 7 conditions [62,63]_.

CO2 + e- → CO2•- E0 = -1.90 V (1.6) CO2 + 2H+ + 2e- → CO + H2O E0 = -0.53 V (1.7) CO2 + 2H+ + 2e- → HCO2H E0 = -0.61 V (1.8) CO2 + 4H+ + 4e- → HCHO + H2O E0 = -0.48 V (1.9) CO2 + 6H+ + 6e- → CH3OH + H2O E0 = -0.38 V (1.10) CO2 + 8H+ + 8e- → CH4 + 2H2O E0 = -0.24 V (1.11) 2H+ _{+ 2e}- _{→ H} 2 E0 = -0.41 V (1.12)

All these reactions are driven by proton-coupled electron transfer, protons and electrons are both needed for the conversion to any of the previously referred products.[64]

In Figure 1.18 is demonstrated the Latimer-Frost diagram for CO2 reduction in a

homogeneous aqueous solution at pH 7 and represents the energy needed, depending on the number of electrons transferred, for each possible CO2 reduction product. The red lines show that

CO and HCOOH are easier to obtain considering the lower relative free energy, whereas the CO2

conversion into CH3OH and CH4 requires more energy. The blue dots represent the formate anion

(COOH-_{) and H}

2C(OH)2 intermediates that can be formed during CO2 electroreduction.[62]

Figure 1.18. Latimer-Frost diagram of CO2 reduction in homogeneous aqueous solution at pH 7 considering

The electrochemical reduction of CO2 raises doubts regarding the number of electrons

transferred during the conversion of CO2. The lower the number of electrons involved in the

reduction, the higher potential is needed to convert it. This leads to the important fact that the medium may play an important role because of the proton donation involved in the reaction of CO2. This is why in aqueous media, the applied potentials are lower than in anhydrous media.[65].

The mechanism proposed by Saveant applies to when CO2 is reduced directly at the

surface of an inert electrode at very negative potentials giving place to a CO2•- intermediate

species. This may not apply to catalytic processes that involve complexes in solution which could stabilise reaction intermediates avoiding the formation of these intermediate species, reactions (1.13) to (1.15)[66]_:

2CO2•- → CO + 3CO22- (1.13)

2CO2•- → C2O42- (1.14)

2CO2•- H+ e- → HCO22- (1.15)

If the free intermediate in reaction (1.6) is not formed, the reactions in (1.13) and (1.14) become less favourable and the formate intermediate should be formed by a different reaction that in (1.15).

For example, in Figure 1.19 is demonstrated a general catalytic cycle using a macrocycle metal complex. In the production of formate is involved an interaction between carbon dioxide and a hydride complex, followed by reduction of the complex and release of the formate. Likewise, the CO conversion by the same compound can be promoted through a proton migration after the CO2 coordination to the metal (Figure 1.19).[48,62]

Figure 1.19. Catalytic cycle with CO and HCO2- release through a macrocyclic metal complex

Another possible explanation for the CO2 conversion into CO can be demonstrated in

Figure 1.20 in which is suggested that the catalytic cycle starts from the complex reduction, following the promotion of a vacant site by a decoordination of a ligand or a solvent molecule

[48,49] _{and the metal-formate intermediate (M-COOH) forms after the CO}

2 coordination[65,67]. The

Figure 1.20. Catalytic cycle with a solvent decoordination

Therefore, the catalyst must have energy levels that match the reduction potential of CO2

to the desired species in order to reduce overpotential and increase efficiency.[68]

Electrochemical characterisation is needed, such as cyclic voltammetry or bulk electrolysis to understand which species are active, what potential to use and how many electrons are involved in the mechanism[65,69]_{. Combining these techniques with theoretical calculations and}

even spectroelectrochemical studies it becomes possible to predict the active species or the catalytic cycle for the CO2 electroreduction[70].

Compounds used as electrocatalysts for CO2 reduction, such as organometallic[67],

pincers[71]_{, clusters}[72]_{, cyclams, macrocyclic complexes and porphyrins}[54]_{, (Figure 1.21), etc are}

reported in the literature.

Figure 1.21. A Co pincer ([Co(t-Bu)2PEPyEP(t-Bu)2(CH3CN)2][BF4]2), a Rh cluster [(RhCp*)3(3-S)2]2+ in the middle

and Fe porphyrin complex (Fe(TPP)) at right

However, none of these compounds include Mn ions neither Mn Schiff base complexes, such as those that will be discussed during the thesis.

Twenty-one years after the Jacobsen catalyst, a Mn complex (Figure 1.22.) with a good selectivity, efficiency and stability for CO2 electroreduction was developed by Deronzier’s

Figure 1.22. [Mn(bipyridyl)(CO)3Br] complex (left) and [Mn(bipyridyl)(CO)3]2 dimer (right)

In the figure above is demonstrated two different structures, at the right is shown the [Mn(bipyridyl)(CO)3Br] complex that was subject to an electroreduction of CO2 and it pass

through a dimeric intermediate after one e- _{reduction, [Mn(bipyridyl)(CO)}

3]2. This dimer

undergoes into a second e- _{reduction leading to the formation of the active species}

-[Mn(bipyridyl)(CO)3]- - which is responsible for the CO2 reduction into CO due to its own easier

1.6 Scope

The main objective of this dissertation was to synthesise new Manganese(III) Schiff base complexes with properties of single molecular magnets. However, in the course of the research work not only Mn(III) but also Mn(IV) complexes were synthesized. Therefore, the obtained compounds were investigated exploring their suitability for applications in molecular magnetism (Mn(III)) and in catalysis (Mn(IV)).

The Mn(IV) complexes (C1 and C2), adequate to electrocatalytic applications, specifically to CO2 electroreduction, were synthesised using new tridentate Schiff base ligands

(L1 and L2). The isolated ligands and the complexes were characterised by NMR, FTIR and UV-vis spectroscopy. Electrochemical studies (cyclic voltammetry, UV-UV-vis-Spectroelectrochemistry (SEC) and IR-SEC) were performed in N2 and CO2 atmospheres to understand the electrocatalytic

ability of the complexes. Moreover, one of the main goals of this part of the dissertation was to correlate the CO2 capture/conversion with electronic properties changes, as well as the

qualitatively evaluation about the species formed by the electroreduction through IR-SEC experiments.

In addition, six Mn(III) Schiff base complexes (C3 to C8) were synthesised. These complexes were characterised by FTIR, UV-vis spectroscopy and elemental analysis. Cyclic voltammetry was also performed to confirm their redox peaks. The magnetic behaviour of the complexes was studied using a Superconducting Quantum-Interference Device (SQUID) magnetometer. These measurements were performed aiming to observe the spin crossover phenomenon in these Mn(III) complexes, or other type of magnetic behaviours, depending on different intermolecular interactions promoted by the ligands.

2.1 Synthesis

This chapter reports the synthesis of two new Mn(IV) Schiff-base complexes with N2O4

donor atoms. Starting with a Schiff-base condensation reaction between an aldehyde (4-(diethylamino)salicylaldehyde) and two different primary amines (2-amino-2-methyl-1-propanol and 2-aminophenol), two tridentate ligands were obtained. The syntheses are illustrated in Scheme 2.1. The ligands were isolated by precipitation and characterised by NMR, FTIR and UV-vis spectroscopy.

After the imine synthesis, KOH is added to deprotonate the hydroxyl groups of both ligands and the reaction mixture heated to reflux for 3 hours with a methanolic solution of MnCl2.4H2O

using a 1:2 stoichiometry (metal:ligand) to obtain the desired complexes (Scheme 2.1).[73]

Scheme 2.1. General synthesis of ligands (L1 and L2) and manganese(IV) Schiff-base complexes (C1 and C2). i) MeOH; ii) KOH; iii) MnCl2.4H2O, air, 

These types of complexes are normally sterically favourable in the meridional conformation as the bulky ligands promote an higher steric effect.[73,74]

The complexes C1 and C2 were synthesised with tridentate ligands L1 and L2, respectively. These ligands are chelating, providing different bite angles when coordinated to the metal. This fact could also influence the reactivity of the complexes, depending on the angle formed.

2.2 Characterisation

The ligands (L1 and L2) as well as the Mn(IV) complexes (C1 and C2) were characterised

by both spectroscopic techniques (NMR, FTIR and UV-vis) and electrochemical techniques (cyclic voltammetry for all and spectroelectrochemistry for the complexes). The complexes were also characterised by elemental analysis and these results are presented in the experimental section.

Complex C1 and C2 were also measured by SQUID magnetometry.

2.2.1 Fourier-Transform Infrared Spectroscopy

In the FTIR spectra (Figure 2.1) it is possible to observe the overlapped spectra for both

C1 and C2 with L1 and L2, respectively. The ʋC=N vibrational modes for both complexes at 1603

cm-1 _{and 1609 cm}-1 _{confirm the imine formation with a negative shift compared to the ligand due}

to the coordination to the metal centre. For L1 and C1 (Figure 2.1 (a)) it was possible to identify the ʋCH vibrational mode from the methyl groups of the ligand between 2965-2822 cm-1. These

ʋCH stretching modes were also observed for L2 and C2 (Figure 2.1 (b)) with lower intensities

as less aliphatic groups are presented in these compounds.

The fact that there is no counter ion in the final complex providing strong vibrational modes (e.g.: ClO4-) makes it dificult to diferentiate the ligand from the complex using this

technique. However, further characterisation studies were perfomed to prove the successful syntheses of the complexes.

Figure 2.1. FTIR spectra of L1 (black line) and C1 (red line) at left (a) and L2 (black line) and C2 (red line) at right (b) in KBr pellets

2.2.2 Ultraviolet-visible Spectroscopy

UV-vis spectroscopy was acquired for the ligands to identify the π-π* transitions. For the complexes ligand-to-metal charge transfer (LMCT), d-d transitions as well as the π-π* transitions of the ligand were identified. The Tanabe-Sugano diagram for a d3 _{metal ion shows three spin}

allowed (same multiplicity) d-d transtions (Figure 2.2), 4_A

2g → 4T2g, 4A2g → 4T1g (F) and 4A2g → 4_T

Figure 2.2. Tanabe-Sugano diagram for a d3 _{electron metal-ion configuration in octahedral environment}[11]

Figure 2.3 shows the overlapped spectra of L1 and L2 with C1 and C2, respectively, obtained at room temperature from 233 to 1000 nm. The ligands spectra (black line, Figure 2.3 a and b) only show one band, as expected. For L1 (Figure 2.3 (a)) this band is observed at 338 nm and for L2 (Figure 2.3 (b)) at 387 nm. For C1 and C2 (red line, Figure 2.3 (a) and (b)) the band correspondent to the phenolate π-π* transition is also present, with a slight red shift when compared to the ligand. These transitions were detected at 345 nm and 419 nm respectively, due to binding to the metal centre. For the complexes (red line, Figure 2.3 (a) and (b) additional bands are observed. For C1, one of the LMCT bands can be identified at 405 nm while the other is possibly overlapped with a band from a probably d-d transition of the metal centre at 582 nm corresponding to the 4_A

2g → 4T1g (F) transition. The large value of the extinction molar coefficient

could be a consequence of the broad and intense tail band[73,75,76]_{. For C2, the LMCT bands can}

be found between 500 nm and 600 nm. The bands correspondent to the d-d transitions in the metal appear around 619 nm and 880 nm, corresponding to the transition between terms 4_A

2g → 4T1g

(F) and 4_A

2g → 4T2g, respectively.[77]

2.2.3

_{H and}

_{C APT NMR Spectroscopy}

Both ligands (L1 and L2) were characterised by NMR spectroscopy to prove the Schiff base formation. Figures 2.4 and 2.6 display the 1_{H NMR spectra and Figures 2.5 and 2.7 show the} 13_{C APT spectrafor L1 and L2. In the appendix the COSY, HSQC and HMBC (Figures A1 to}

A6) of both ligands, are also reported.

For L1 in CDCl3, the assignment of the protons by the 1H NMR spectrum and of the

carbons by 13_{C APT spectrum is shown in Figures 2.4 and 2.5, respectively.}

Figure 2.4. 1_{H NMR of L1 in CDCl} 3

The imine formation is supported by the presence of the imine proton signal (H9) observed at the typical chemical shift of 7.91 ppm, with a singlet multiplicity (s) and an integration value of 1. The H1 and H2 protons are from the -NEt2 group and were found at 3.36 ppm and 1.18

ppm as a quartet and a triplet, respectively. A singlet was assigned at 1.32 ppm as the H11 signal with an integration of 6, corresponding to the protons of the two methyl groups. Another singlet signal was observed at 3.54 ppm shown in figure 2.4 and it was identified as H12 with an integration of 2. The signal at 5.98 ppm corresponds to H4 that is an aromatic proton giving rise to a singlet signal. The assignment of the aromatic protons H5 and H6 was confirmed by 2D NMR techniques, such as HMBC and COSY. The 2D COSY experiment showed the correlation between H5 and H6 and, to distinguish between these signals, the HMBC technique had an important role as it revealed a correlation between C4 and H5.

H1 H2 H11 H12 H9 H4 H5 H6

Figure 2.5. 13_{C-APT of L1 in CDCl} 3

For L2 in DMSO-d6, the assignment of the protons by the 1H NMR spectrum and of the

carbons by 13_{C APT spectrum is shown in Figures 2.6 and 2.7, respectively.}

C11 C2 C1 C10 C12 C4 C5 C7 C6 C3 C9 C8 H1 H2 H9 H4 H5 H13, H11, H12 H14, H6 OH OH

As previously observed for L1, the Schiff base ligand formation was proved by the presence of the imine proton signal (H9) at 8.63 ppm. Again, the H1 and H2 protons from the -NEt2 group were found at 3.38 and 1.11 ppm as a quartet and a triplet signal, respectively.

However, the H1 signal integration is not 4 dues to an overlap with the protons signal of the water present in the solvent. At 5.96 ppm, it can be observed the signal correspondent to the aromatic proton H4 as a singlet multiplicity. By HMBC, it was possible to assign the doublet signal H5 at 6.25 ppm due to a correlation between the C4 and H5. The signals corresponding to the protons H6 and H14 are overlapped with a total integration of 2. These two signals were detected by HSQC, with closer carbon chemical shifts and the assignment of the aromatic proton H6 at 7.23 ppm was obtained through the correlation with C9 in the HMBC experiment. To distinguish between C10 and C15, HMBC was performed as only C10 may have a correlation with H9. After assigning C10, the HMBC experiment was used to found H11 and H12. These signals are a doublet (H11) and a triplet (H12) at 6.89 ppm and 6.83 ppm, respectively. Finally, the triplet proton signal H13 was assigned at 7.01 ppm due to the correlation detected by the COSY experiment with H14 doublet.

Figure 2.7. 13_{C-APT of L2 in DMSO-d} 6

2.2.4 SQUID Magnetometry

Using SQUID Magnetometry, the variation of the magnetization as a function of temperature, between 10 K and 300 K, was measured under an applied magnetic field of 1000 Oe. The corresponding molar paramagnetic susceptibility, 𝜒𝑀, was then calculated after

subtraction of the diamagnetic component. The diamagnetic contribution was roughly estimated based on the molar mass of the complex, M (g.mol-1_{), through the empirical formula}

𝜒𝑑𝑖𝑎(cm3mol−1) = −0.5 × 10−6𝑀.[26] C2 C1 C4 C5 C7 C6 C3 C9 C8 C15 C10 C13 C12 C14 C11