University of Groningen A journey into the coordination chemistry, reactivity and catalysis of iron and palladium formazanate complexes Milocco, Francesca

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University of Groningen

A journey into the coordination chemistry, reactivity and catalysis of iron and palladium

formazanate complexes

Milocco, Francesca

DOI:

10.33612/diss.160960083

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Publication date:

2021

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Milocco, F. (2021). A journey into the coordination chemistry, reactivity and catalysis of iron and palladium

formazanate complexes. University of Groningen. https://doi.org/10.33612/diss.160960083

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Chapter 1

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1.1 Catalytic transformations

A long series of disruptive and creative events have made possible life on Earth as we know it. The stellar nucleosynthesis, the self-replication, the metabolism, the photosynthesis, the transition from a reducing sulfur-rich atmosphere to an oxidizing oxygen-rich atmosphere, just to name a few. All these events, as well as a lot of process and items in our daily life (such as digestion, cars, beers, plastic, bread, detergents, etc.), have one thing in common: catalysis.

Appreciation of the subtle and delicate balance required for our life, can motivate us to find sustainable way of living in order to preserve it. There is an urgent need of re-thinking catalytic transformations and, in this context, fundamental research on novel types of reactivity is essential.

Homogeneous catalytic processes can be classified as reductive, oxidative or redox-neutral processes. In many cases, even if the reaction is formally redox-neutral, two-electron transformations are required to promote bond-forming and bond-breaking events. In particular, oxidative addition and reductive elimination are key steps in several catalytic cycles and these reactions are typical of square-planar complexes with 2nd_{and 3}rd_{row transition metals. Noble metals are often the first choice in}

catalysis thanks to their accessible Mn+_{and M}(n+2)+_{oxidation states. A famous example of this kind of}

catalysis is the palladium-catalyzed cross couplings, for which Heck, Negishi and Suzuki won the Nobel Prize in Chemistry in 2010.1

However, if on the one hand noble metals give outstanding results in catalysis, on the other hand their use is accompanied by many drawbacks, such as the fact that they are expensive, rare and toxic. Therefore, the replacement of noble metals with cheap, abundant and less toxic2_{first-row transition}

metals is highly desirable. The supremacy of noble metals in homogeneous catalysis over base metals comes from the differences in electronic structure. Noble metals generally prefer two-electron redox events, while base metals usually favor one-electron redox events leading to odd-electron pathways that are difficult to control.3_{Therefore, one of the biggest challenges in order to employ more}

extensively earth-abundant first-row transition elements in homogeneous catalysis is either manage to achieve control and selectivity over odd-numbered redox steps, or find a way to confer nobility on base metals. Moreover, another advantage for the late metals is their high affinity to π-bonds, which is a relevant feature because the chemical industry is mainly oil-based having as feedstocks alkenes, arenes and alkynes. On the other hand, the future direction is towards renewable raw materials and this could favor the employment of base metals.4_{Regarding the need of the development of more}

sustainable post-fossil energy systems, a lot of effort has been addressed to convert readily accessible and abundant small molecules (e.g. N2, O2, CO2, CH4, H2O, H2) into high-value chemical feedstocks and

fuels. Therefore, the challenge is to find the right conditions to make these mostly stable and inert molecules reactive and to control the reaction pathways that often involve multielectron redox processes coupled with proton transfer.5_{To do so, it is fundamental to understand the bonding and}

coordination properties of transition metals to such small molecules, by investigating the metal-ligand interaction in different oxidation and spin state.6

1.1.1 CO

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conversion

While nature is based on circularity as exemplified by the relationship between photosynthesis and cellular respiration, the CO2 cycle has been broken by humankind since the industrial revolution.7 This

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ppm,8_{with drastic consequences for the climate change.}9_{Therefore, it is fundamental to reinvent}

industrial processes in order to close the loop of that cycle and bring it back to balance.

Carbon capture utilization and storage (CCUS) could play a critical role in reaching net-zero emissions.

10_{The most oxidized form of carbon is carbon dioxide and the conversion of this molecule is challenging}

due to its high thermodynamic and kinetic stability. Nevertheless, the thermodynamic stability can be overcome by providing an energy input in the form of heat, electric current or radiation or by combining CO2 with high free energy compounds, such as hydrogen, amines, or epoxides, resulting in

an exergonic reaction. The kinetic stability can be tackled by using a suitable catalyst and different catalytic methods have been explored for CO2 conversion: ranging from homogeneous,

heterogeneous, electrochemical and photochemical catalysis.7, 11_{For the sake of sustainability it is}

essential to consider the choice of source of energy supply (renewable or integrated heat-management systems), source of chemical reagents to couple with CO2; of catalyst (based on earth abundant

metal).7a, 12

Scheme 1.1 depicts some of the possible reactions that use carbon dioxide as a feedstock to synthesize various added-value chemicals.7, 11b, f, 12-13

Scheme 1.1. Examples of CO2 conversion into various chemicals.

Many of the possible transformations involving CO2 require proton-coupled multielectron steps, which

are typically challenging for earth-abundant metals (see Section 1.2), nevertheless promising results have been shown in this field.11d, 14_{Examples of CO}

2 conversion with non-noble metal are reported in Section 1.4.2.

1.2 Development of non-noble metal catalysts for 2-electron

redox-transformations

One possible approach to replace rare metals with earth abundant ones is to mimic the noble behavior with base elements and this can be achieve by bringing the ligand framework into play. In classical homogeneous catalysis, the so-called ancillary or spectator ligands remain unchanged during the chemical transformation, which take place at the metal center, and their role is to modulate the

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electronic and steric properties of the metal complex in order to achieve the desired reactivity. Conversely, actor ligands are those that engage in a chemical change. With the aim of developing novel catalytic reactivity, increasing attention has been addressed to the strategy of changing the role of the ancillary ligands making them actively participate at the transformation in several ways:15

i) cooperative ligands, which are directly involved in the bond activation reaction and they cooperate

with the metal in a synergistic approach called Metal-Ligand Cooperation (MLC).15b, c_{As a}

consequence, they can be chemically modified either at the 1st_{coordination sphere or at the 2}nd

coordination sphere. For example, one research line in our group is focused on the catalytic reactivity of ruthenium pincer (PNP and PNN) complexes, which is based on aromatization/dearomatization of the pyridine fragment of the ligand;16

ii) hemilabile ligands can be employed in order to create coordinative unsaturation at the metal

center;

iii) redox-non-innocent ligands, which can change their oxidation state and can be used to avoid

unstable oxidation steps during chemical redox transformations (see Section 1.4).

Many actor ligands combine more than one of the above strategies and perhaps that may be the origin of the confusion of terminologies adopted throughout the literature to address those ligands, e.g. "cooperative", "non-innocent", "redox-non-innocent", "redox-active" (see Section 1.3).

1 1.2.1 Iron

Iron is the Earth’s most abundant element by mass, fourth most abundant element of the crust and it is an essential trace element in almost all the living organisms (first most abundant transition metal in human body, a4 g).17_{Iron abundancy in the Earth is related to its copious production by nuclear fusion}

in high-mass stars, where it is the last chemical element to be formed accompanied by release of energy before the rapid and violent collapse of the massive star which scatters the chemical elements (including iron) into space.

This readily available, cheap and relatively nontoxic2_{metal is a highly desirable candidate in catalysis.}

In addition, its central location in the d-block could make it cover both early and late transition metal characters. Moreover, it is a redox active metal with its formal oxidation states going from –II to +VI, therefore, it can be useful both in reductive and oxidative chemistry.4

On the one hand, iron has made a breakthrough in heterogeneous catalysis (e.g. Haber-Bosh process to convert molecular nitrogen into ammonia and Fisher-Tropsch process to transform carbon monoxide into liquid fuel) and its vital role in biocatalysis is also well-established (e.g. heme group in the hemoglobin for oxygen transport or in the cytochrome P450 for oxygen activation), on the other hand, its employment in homogeneous catalysis has been for long time hampered by the established supremacy of noble metals in the field. Nevertheless, in the last 25 years the number of reports on this topic has significantly increased, showing the potentiality of iron catalysis.4, 17a, 18

As already mentioned in Section 1.1, the use of iron catalysts to mimic noble metals behavior has to deal with competing single electron transfer. Different strategies could be used to alter the inclination toward a certain type of redox action (1e−_{vs 2e}−_):4

i) modulating the splitting of the d-orbital energy levels;

ii) achieving metal-ligand cooperativity thanks to “non-innocent” ligands;

iii) developing metal-metal cooperativity by the combination of two metal centers.

This thesis focuses on the first two strategies and the following section aims to uncover how they can be seen as two sides of the same coin.

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1.3 Metal-ligand bonding: the electrons and orbitals dance

The electronic occupancy of the orbitals in a complex is controlled by the splitting (Δ) of the d-orbital energy levels, which can be modulated by the coordination geometry and the nature of the ligands. Commonly, the “normal ligand-field” splittings are two below three for the tetrahedral, three below two for the octahedral and four below one for the square-planar coordination (Figure 1.1).19

Figure 1.1. Most common ligand-field splittings.

In addition, strong field ligands (π-acceptor ligands) lead to larger splitting, whereas weak field ligands (π-donor ligands) result in smaller splitting. This feature is particularly relevant in the octahedral environment where ligand field theory can be used either to favor the electron pairing and therefore the formation of low-spin complexes or to encourage the occupancy of all the levels resulting in high-spin complexes.

When we construct the orbital interaction diagram for a transition metal ion interacting with the surrounding ligands (both for the σ-donor and π-donor character) we typically assume that the ligand orbitals are lower in energy compared to the metal d orbitals (Figure 1.2 left). This assumption is justified by the fact that the ionization potential of transition metal d orbitals is usually substantially smaller than those of typical Lewis base ligands.19

Figure 1.2. Ligand field molecular orbital for σ-bonding (in orange) and π-bonding (light blue) in different regimes:

classical Werner-type (left); covalent (middle) and inverted (right). Free-ion ΔTd t2g eg ΔTd= 4/9 ΔOh Octahedral Tetrahedral e t2 dxy dyz dxz dx2−y2 dz2 dx2−y2 dz2 dxy dyz dxz ΔOh Δ dx2−y2 dz2 dxy dyz dxz Square planar eg a1g b2g b1g

Classical Covalent Inverted

d (M−L)σorπ (M−L)* σorπ d d σorπ σorπ σorπ (M−L)σorπ (M−L)* σorπ (M−L)σorπ (M−L)* σorπ or or or or or or E

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However, we can imagine that if we raise the ligand levels close (Figure 1.2 middle) or above (Figure 1.2 right) the metal levels, we are able to alter the molecular orbital splitting and potentially obtain

inverted ligand-field splittings with respect to Figure 1.1 (i.e. three below two for the tetrahedral, two

below three for the octahedral and one below four for the square-planar).19-20_{In fact, cases of inverted}

ligand fields are being more and more recognized in the chemistry comunitry.19-21

In those “non classical” cases, often one question arises: are the resulting frontier orbitals mainly centered on the metal or on the ligands? The fact that it is not always straightforward to answer to this question causes ambiguity in the oxidation assignment.

Moreover, metal-ligand covalency also affects the pairing energy (i.e. increasing covalency means increasing molecular orbital size and, therefore, diminishing the electron repulsion through the so-called nephelauxetic effect), which together with Δ, determines the ground-state electron occupancy in the orbitals.22_{Clearly, the correct description of the spin state of a transition metal}

complex is fundamental given that different structures, chemical properties and reactivity are associated with different spin states.22-23

Metal-ligand covalency and ligand-field inversion can lead to bonding schemes where the frontier

orbitals are molecular orbitals that are ligand localized rather than metal-localized. This is the feature of non-innocent/redox active ligands, which possess higher energy HOMO, or alternatively lower-lying LUMO, compared to those of typical innocent/redox inactive ligands and therefore can participate in electron transfer.18c, 20d

Often the terms non-innocent ligand (NIL) and redox-active ligand (RAL) are used interchangeably. Some reports, however, shed light on the differences between these terms.23a, 243a_{Moreover, it is}

usually implicit that the first term, NIL, refers to redox non-innocent ligands.

The definition of the term "innocent" ligand goes back to 1966, when Jørgensen stated that “ligands are innocent when they allow oxidation states of the central atoms to be defined”.25_Conversely,

"non-innocent" ligands imply an uncertainty or ambiguity in the oxidation state assignment.23a, 24, 26

Chirik then pointed out in 201124_{the fact that modern experimental and computational methods can}

often identify the appropriate oxidation state of the metal. Therefore, the term "redox-active" ligand is more appropriately used in cases in which a well-defined redox process occurs at the ligands. As stated by Hofmann et al., "molecules do what comes naturally; it is we who, in our struggle to understand them, pigeonhole them into rigid categories and eventually run into trouble".19

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1.4 Redox active ligands

The idea of redox-active ligands comes from Nature, which has developed metalloenzymes based on earth-abundant metal, such as iron and copper, even if these metals preferentially react via one-electron redox steps.27 _{To overcome the problem that several biological transformations are}

multi-electron processes, redox-active ligands have been employed in the enzymatic active sites both to promote two-electron steps and to discourage potentially damaging radical reactions.3b, 24

In a complex with traditional innocent25_{spectator ligands the redox event occurs at the metal center}

because the oxidation or reduction of the ligands are too energetically demanding (Scheme 1.2 a). However redox-active ligands have energetically accessible levels for reduction or oxidation, therefore the changes in the oxidation state can take place either exclusively at the ligand (Scheme 1.2 b) or in a synergetic manner both at the ligand and the metal (Scheme 1.2 c).3_{As a result, redox-active ligands}

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structures, which could hopefully confer to first raw transition metals some noble catalytic properties.3b

Scheme 1.2. a) Innocent ligand; b) Redox-active ligand; c) Synergism between redox-active ligand and redox-active

metal.

One of the most famous examples of synergism between the metal and the ligand is the cytochrome P450, which is featured by a heme group in the active site and it is capable of selectively catalyzing monooxygenase reactions (Scheme 1.3).27_{The heme is constituted by a porphyrin ligand coordinated}

to a Fe2+_{center. The activation of O}₂_{requires its two-electron reduction to the peroxo state and}

subsequently the O–O bond cleavage takes place generating the so-called “Compound I”, which is a highly reactive intermediate considered responsible for the oxidation of the substrate. In 2010 Green

et al. were able to isolate and characterize Compound I, which resulted to be an iron(IV)oxo species

with a singly oxidized porphyrin radical ligand.28 _{Compound I promotes the hydroxylation of}

hydrocarbons to alcohols for which one oxidant equivalent comes from the Fe(IV) and another from the porphyrin radical ligand, recovering the iron(III)porphyrin complex.

Scheme 1.3. (Right) Catalytic cycle of the hydroxylation of hydrocarbons by Cytochrome P450 and (left) structure

of Compound I.

Fascinatingly, the heme group is also responsible for oxygen transport and storage in the hemoglobin and myoglobin proteins. This shows the elegant fine-tuned character of the heme molecule which is capable to modulate its electronic structure and function and engage either in O2-activation as well as

reversible O2 binding. In this regard, spin crossover properties of the heme are relevant both in the

globins as well as in the cytochrome P450. Moreover, modulation of the extent of π-back bonding into the O-O bond is fundamental to achieve either weak and reversible binding or strong binding, conferring to the heme moiety the dual abilities of O2-transport and activation.17b

In general a redox-active ligand can act in four main ways:3a, 29

a) The oxidation/reduction of the ligand modifies the Lewis acidity/basicity of the metal. b) + 2 e− a) + 2 e− c) + 2 e− Synergism

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b) The ligand acts as an electron reservoir preventing the metal to go in unstable oxidation states by delivering or accepting the electron density required from the multielectron process (e.g. Cytochrome P45027_).

c) Formation of reactive ligand-radicals, which are actively involved in bond-making and breaking.

d) Activation of the substrate via ligand-to-substrate single electron transfer, allowing odd-electron reactivity.

1 1.4.1 Redox active ligands acting as electron reservoir for catalytic reactions

The term “electron reservoir” has been introduced for the first time in 1979 for 19-electron sandwiches, such as [FeI_(η5_-C₅_H₅_)(η6_-C₆_Me₆_{)], and it has been defined by the author as “a highly}

reduced species which is one half of a totally reversible redox catalytic system in which both the oxidized and reduced forms are stable and isolable”.30_{This terminology has then been used to describe}

the ability of redox-active ligand to store electrons.3a, 29, 31_{Even if redox-active ligands can provide}

tremendous opportunities in homogeneous catalysis, this field started to be more systematically explored only in the last 15 years. Examples of redox-active ligands reported in literature are dithiolenes,32_dioxolenes,32_α-diimines,33_{bis(imino)pyridines,}31a, 34_{bis(imino)acenaphtene,}35

β-diketiminates,31c_formazanate36_{(see Section 1.5) and porphyrins,}37_{(Chart 1.1).}

Chart 1.1. Examples of redox-active ligands.

Examples of how redox active ligands are used to donate or accept electrons, enabling new (catalytic) reactivity with base-metals, are shown below.

Chirik and Wieghardt highlighted that the iron complex (iPr_PDI)Fe(N₂₎₂_{is a suitable precatalyst for}

several reactions,3b, 31a, 34a_{for instance for hydrogenation (Scheme 1.4 a) and hydrosilylation (Scheme}

1.4 b) of olefins,38_{cyclization of enynes and dynes (Scheme 1.4 c)}34b_{and [2π + 2π] cycloaddition of}

α,ω-dienes (Scheme 1.4 d).34c

Even if these reactions are overall two-electron processes, the iron(II) oxidation state is maintained and the required electrons are provide by the redox-active ligand, which acts as an electron reservoir.3b, 31a_{The complex [Fe(}iPr_PDI)(N₂₎₂_{] comes from the reduction of the [Fe(}iPr_PDI)(X)₂_{] (X = Br, Cl)}

(Scheme 1.5) and it was proved to be not the Fe(0) species with two neutral bis(imino)pyridine ligands, as initially described by the authors,38 _{but a Fe(II) species with the doubly reduced form of the}

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Scheme 1.4. Example of reactions catalyzed by the complex (iPr_PDI)Fe(N

2)2.34b, c, 38

However, in a later review about non-innocent ligands18c_{the authors ascribed the difficulties in}

assigning a definite oxidation state due to the high covalent character of the metal-ligand bond and they proposed that the complex (iPr_PDI)Fe(N₂₎₂_{is better represented as a mesomeric equilibrium. This}

way of describing the system also fits with the concept that this complex can be depicted in the framework of multi-state reactivities (MSR),39_{meaning that the intrinsic reactivity of the complex is}

related to different spin configurations. Hence, the bis(imino)pyridine ligands can act either as innocent ligand (e.g. Brookhart olefin polymerization catalyst40_{) or as non-innocent ligand with the}

same metal, proving also the adaptability in term of spin state of iron.18c

Scheme 1.5. Reduction of (iPr_{PDI)Fe(II)(Cl)}₂_{to (}iPr_PDI)Fe(N₂₎₂_{and mesomeric electronic configurations of}

(iPr_PDI)Fe(N₂₎₂_.18c, 31a

Another example of two-electron transformations which takes place without any changes in the oxidation state of the iron center is the reductive elimination of disulfide with an iron complex bearing the pincer redox-active ligand [ONO] (Scheme 1.6 a).41 _{Heyduk et al. reported a five coordinated}

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tert-butyldisulfide in the presence of 3 eq of pyridine (Scheme 1.6 b). The resulting complex is still an

Fe(III) species featuring a two-electron reduced ligand [ONOcat_]3-_.

Scheme 1.6. a) Redox active pincer ligands [ONO·]; b) Reductive elimination of a disulfide from [ONO]Fe complex.41

Considering the two above examples it has to be highlighted that redox-active [ONO] ligand was found to favor iron in its +3 oxidation state,41_{while redox-active bis(imino)pyridine ligand favors its +2}

oxidation state.3b, 31a_{Two possible reasons have been suggested:}31a _{first, the harder nature of the}

oxygen atoms and amide nitrogen of the [ONO] ligands compared to the one of pyridine and imine nitrogen of the diiminopyridine ligand should favor the iron(III) oxidation state; second, the oxidized forms of the [ONO] ligand (the radical semi-quinonate [ONOsq·_]2-_{and the quinonate [ONO}q_]-_{) are}

significantly stronger oxidants than the neutral diiminopyridine, therefore, they favor the iron(III) oxidation state.

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1.4.2 Redox active ligands: a useful tool for CO

2

conversion

Iron porphyrins are known powerful electrocatalysts for the reduction of CO2 to CO42 and typically the

iron(0) complex, [Fe(TPP)]2-_{, was proposed to be the active species. However, porphyrins can act as}

redox active ligands (as shown in Scheme 1.3 for the heme group), and in fact, spectroscopic and computational studies pointed out that [Fe(TPP)]2-_{, is best described as an intermediate spin iron(II)}

center antiferromagnetically coupled to a porphyrin diradical dianion (Scheme 1.7).43

Scheme 1.7. Ligand-based reductions of [Fe(TPP)].

Therefore, the reduction is ligand-centered and the two electrons occupy the porphyrin π*-orbitals. Furthermore, a mechanistic study for the reduction of CO2 to CO was reported for the iron porphyrin

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having four trimethyl ammonium groups in para position of the phenyl groups, where it was calculated that the Fe maintains its oxidation state of +2 during the entire catalytic cycle.44

Another example in which a redox active ligand is used to accommodate reducing equivalents, leading to interesting reactivity is the one reported by Peters et al. in 2014.45_{A cobalt complex bearing the}

redox active pyridyldiimine ligand, [CoIII_N₄_H(Br)₂_]+_{, was tested for electrocatalytic CO}₂_{reduction in wet}

MeCN and reduction of CO2 to CO was observed occurs near the formal CoI/0 redox couple (Scheme

1.8).45_{Subsequently, the precatalyst [CoN}

4H(MeCN)]+ has been isolated by protonation of the “CoI”

amido complex [CoN4], both of which were characterized by X-ray crystallography and DFT, which

elucidated that the pyridyldiimine moiety acts as an electron reservoir. In particular, it has been found that in both complexes the Co is not in the oxidation state +1, but they are low-spin CoII_ions

antiferromagnetically coupled to a ligand radical-anion (N4H•- and N4•-). Later a combined

spectroscopic-theoretical study57_{showed that CO}

2 reacts with [CoN4H(MeCN)]+ forming a η1-C adduct,

[CoN4H(CO2)]+, where the CO2 ligand is only partially charged and moreover it can also be formed

from the complex [CoN4H(MeCN)]2+ (Scheme 1.8 bottom). From this study it is suggested that [CoN4H]

is not necessarily part of the electrocatalytic cycle and that the 2e−_{species could also be directly}

generated from [CoN4H(CO2)]+. Nevertheless, both these two studies highlight the redox

non-innocence of the N4H ligand, which is actively involved in the CO2 activation in cooperation with

the metal center. 11b, 45-46

Scheme 1.8. Electrocatalytic CO2 reduction mediated by the complex [CoN4H(MeCN)]+.11b, 45-46

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1.5 Formazanate ligands

Formazans are a class of colorful nitrogen-rich compounds containing the characteristic chain of atoms R1_-_NH-N=CR3_-N=N-R5_{which have been discovered at the end of the 1800s}47_{and have been then}

reviewed by Nineham in 1955.48_{Thanks to their intense color, formazans have found applications as}

dyes,49_{in chemical biology as redox-based staining agents for cell-viability assays}50_{and they raised}

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The backbone of formazans allows structural isomerization (Chart 1.2 a) and the different isomers vary in colors, ranging from the blood red color typical of the “closed” form to the orange and yellow colors of the “open” and “linear” isomers, respectively. The structural isomers of formazans reflects also the possible coordination modes of the deprotonate formazanate ligand to an inorganic element, allowing formation of six-, five- and four-membered chelates (Chart 1.2 b).36_{While the coordination chemistry}

and the catalytic applications of structural analogues N-donor ligands such as β-diketiminates (Chart 1.3 a) are well established,52_{formazanate molecules have started to receive more attention as chelate}

ligands only in the last decade36, 53_{and examples of catalytic tranformations involving}

formazanate-based catalysts still remain extremely rare.54

Chart 1.2. a) Structural isomers of formazans. b) Common coordination modes of formazanate ligands to an

inorganic element (E).

Recently, β-diketiminates, which have been usually considered robust ancillary ligands and have been employed to stabilize both very high and low oxidation state metal centres, were found to be non-innocent ligands that actively participate in numerous redox processes.31c, 52b, 55_However,

β-diketiminate complexes show a limited stability upon changing oxidation state. On the other hand, formazanate ligands are featured by highly enhanced redox-active properties and increased coordinative flexibility, thanks to the presence of two additional nitrogen atoms in the backbone (NNCNN instead of NCCCN).31b, c, 36, 56_{Formazanate ligands are able to engage in both reductive and}

oxidative redox chemistry due to high lying HOMO and low-lying LUMO orbitals of π-symmetry (see

Chapter 5).36_{The stability of the reduced form of formazanate can be related to the known stability of}

the organic verdazyl radical.56-57_{In fact, there is an isolobal relationship between the neutral verdazyl}

radicals and the metallaverdazyl radical obtained from the reduction of the formazanate complex (Chart 1.3), where the unpaired electron can be delocalized over four nitrogen atoms.

Chart 1.3. Molecular structure of: a) β-diketiminate and formazanate ligands; b) verdazyl radical and

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1 1.5.1 Redox activity of formazanate ligands

The redox-active nature of formazanate ligands was described for the first time only in 2007 by Hicks

et al.56_{They reported that the complex [B(L1)(OAc)}₂_{] (L1 = PhNNC(p-Tol)NNPh) shows a}

quasi-reversible reduction wave in the cyclic voltammetry and they proved that is ligand-based by chemically synthesizing the single-reduced complex [B(L1)(OAc)2][Cp2Co] and characterizing it via EPR and UV-Vis

spectroscopy (Scheme 1.9 a).

Scheme 1.9. Selected examples of isolated single and double- formazanate-centered reduced compounds. 31b, 56, 58

The redox properties of formazanate ligands have been then established in our group by exploring the coordination chemistry of those ligands with the redox inactive metal Zn(II).31b, 59_{Taking advantage of}

the modular synthesis of the formazanate ligands, which allows the introduction of a variety of different substitution patterns, a set of bis(formazanate)zinc complexes have been synthesized and their redox behavior has been investigated by cyclic voltammetry.59_{In all the cases the ability of storing}

at least one electron in each formazanate moiety was observed, giving access to the redox series L2Zn0/1−/2−, and in some compounds even the second reduction of the ligand was feasible, extending

the series to L2Zn3−/4−.59 In addition, the single and double reduced compounds were chemically

synthesized (Scheme 1.9 b) and characterized by X-Ray, DFT, EPR and UV-Vis spectroscopy, proving that one or two electrons can be stored in the ligand framework.31b

Furthermore, our group has also studied the coordination chemistry of formazanate to group 13 elements (B and Al)58a, b, 60_{and for the first time a compound containing the formazanate moiety in}

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the trianionic form was structurally characterized first with boron58a_{and then with aluminum,}58b

[E(L1)(Ph)2][Na2] (E = B, Al), (Scheme 1.9 c).

1 1.5.2 Iron formazanate complexes

An interesting case is the one of iron formazanate complexes, where the redox active ligands are combined with a redox active transition metal. Two examples can be considered to illustrate how playing with the ligand-field splitting and pursuing metal-ligand cooperativity through the use of “non-innocent” ligands can be seen as two sides of the same coin (Section 1.3).

In 2016, our group reported an unusual pseudo-tetrahedral bis(formazanate) iron(II) complex, [Fe(L1)2]

(1), which undergoes thermal switching between a low-spin (S = 0) and a high-spin (S = 2) state.61

Typically, if we consider the case of Fe(II) d6_{in an octahedral environment, two possibilities can be}

depicted: a high spin complex in which ΔO is smaller than the pairing energy and the electrons prefer

to occupy also the higher orbitals, or a low spin complex where the splitting is higher and the electrons are paired in the lower orbitals. However, for a tetrahedral coordination the high spin state is always the favorite one, even with strong-field ligands, due to the decreased splitting of the d-orbitals (Δt =

4/9 ΔO). Octahedral complexes in which these two states are close in energy can exhibit the

phenomenon of spin-crossover (SCO); they can switch between low-spin (LS) and high-spin (HS) states using external stimuli such as temperature, light or pressure.62_{The fact that a complex has two}

accessible spin states, which typically have completely different structures, reactivity and spin density distributions,23a_{has a relevant impact on mechanisms, rates and selectivity in organometallic reaction}

steps relevant to catalysis, as highlighted by Schröder, Shaik and Schwarz with the two-state reactivity model (TSR).39b

However, four-coordinate complexes that show SCO are very rare. Few examples have been reported in the literature with iron: the three-fold symmetric phosphinimido iron complexes with a tris(carbene)borate ligand by Smith et al.,63_{the square-planar bis(imino)pyridines iron imidos by Chirik} et al.,64_{the pseudo-tetrahedral tris(phosphine)borate phosphiniminato iron complexes by Peters et} al.65_{and the pseudo-tetrahedral bis(formazanate) iron complex by our group}61_{(see Chart 2.1, Chapter} 2). These works are example of how the “normal ligand-field” splitting can be inverted19_{as the result}

of forced geometries and alteration of the electronic structure exerted by the ligands. Therefore, the deviation from the classical tetrahedral ligand-field splitting makes possible the switch between low and high spin. Specifically, the unusual occurrence of spin-crossover in the bis(formazanate) iron(II) complex (1) was ascribed to π-back interaction between the iron center and the low-lying formazanate π*-orbital, resulting in significant stabilization of one of the high-energy orbitals (t2).61 This leads to ligand-field inversion and a 2-over-3 splitting, reminiscent of that of six-coordinate octahedral

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Figure 1.3. Molecular orbital diagram for [Fe(L1)2] (1) in the LS (S=0) state and the HS (S=2) state compared to the

classical tetrahedral.Adapted from ref 61_{(https://pubs.acs.org/doi/10.1021/jacs.6b01552) with permission from} the American Chemical Society. Further permissions related to the material excerpted should be directed to the ACS.

Interestingly, chemical one-electron reduction of 1 allows the isolation of the anionic complex [Fe(L1)2][NBu4] and structural, spectroscopic and computational data demonstrated that the singly

reduced compound is best described as a low-spin (S = 1/2) Fe(I) complex featuring closed-shell, monoanionic formazanate ligands. Therefore, despite the redox-active nature of the formazanate ligands the first reduction in this case preferentially occurs at the iron center.

A different situation has been later reported by Holland et al. for a mono(formazanate) iron(II) amide complex [Fe(L1)(N(SiMe3)2)(THF)] and its one-electron reduction product

[Fe(L1)(N(SiMe3)2)(THF)][Na(12-crown-4)2] (Scheme 1.9 d), for which a variety of spectroscopic

techniques suggest formazanate-centered reduction. Multiconfigurational calculations identified a quartet ground state that reproduces the empirical spectroscopic data (Stotal = 3/2). Furthermore, two

configurations were found to be dominant, contributing of approximately 25% each. Therefore, according to the calculations, the ground state is best described as a virtually equally mix of two electronic configurations: a high spin Fe(II) center (SFe = 2) antiferromagnetically coupled to the

unpaired electron located in the formazanate π*-orbital (SL = 1/2) and a high-spin Fe(I) (SFe = 3/2)

without ligand participation.58c

It is important to underline that, despite the different involvement of the formazanate ligand in the reduction chemistry in the two iron complexes reported by Otten61_{and Holland,}58c_{in both cases the}

peculiar features described for those compounds arise from the presence of a low-lying empty π*-orbital, which in one case is involved in π-backdonation61_{and in the other case allows formazanate}

participation to the redox event.58c_{This emphasizes that is feasible for formazanate iron complexes to}

achieve a synergism between the redox-active ligand and the redox-active metal and that further fundamental research on the electronic structure of these compounds is needed.

dz2 dx2-y2 dyz dxz dxy π* π* dz2 dx2-y2 dyz dxy dxz Classical tetrahedral d6 LS S = 0 HS S = 2 1 Formazanate π-acceptor ligand e t2

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1

1.6 Thesis outline

In this thesis we present an in-depth study of the coordination chemistry of formazanate ligands to iron and palladium and we explore the (catalytic) reactivity of the resulting complex toward small molecules.

In Chapter 2, the synthesis and characterization of five novel bis(formazanate) iron complexes, [FeL2]

capable of undergoing spin-crossover is described. We demonstrate that is possible to tune the SCO properties of those compounds by electronic substituent effects.

In Chapter 3, we extend the study of how the spin-crossover properties of bis(formazanate) iron complexes may be modulated via modification of the ligand via different strategies: electronic effects, steric effects, π-stacking interactions and ligand denticity.

In Chapter 4, the reactivity of bis(formazanate) iron(I) and (II) complexes toward CO and isocyanide is investigated. Novel low spin 6-coordinate complexes, [Fe(L)2(CNAr)2], are synthesized and

characterized.

In Chapter 5, the redox behavior of bis(formazanate) iron complexes was examined via cyclic voltammetry. Oxidative addition on a bis(formazanate) iron(I) complex gave useful insight on the poor stability of these compounds in the Fe(III) state. We then show that isolation of a stable Fe(III) formazanate iron complex is possible with a tridentate NNO ligand, leading to an example of "hidden" non-innocence.

In Chapter 6, we report the synthesis of formazanate ferrate(II) dihalides complexes ([FeLX2][NBu4], X

= Br, Cl) via salt metathesis, which provide a straightforward synthetic pathway toward mono(formazanate) complexes. The halides in the resulting compounds are shown to be labile, opening up possibilities for further reactivity.

In Chapter 7, the aforementioned mono(formazanate) iron(II) dihalide complexes, are used as highly selective single component catalyst for the conversion of CO2 into cyclic carbonates starting from

various terminal and internal epoxides.

In Chapter 8, the synthesis and characterization of a mono(formazanate) alkyl palladate complex, [Pd(L)(CH3)(Cl][NBu4], is reported and ligand substitution and insertion reactions are explored.

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