University of Groningen Redox-behavior and reactivity of formazanate ligands Mondol, Ranajit

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Redox-behavior and reactivity of formazanate ligands

Mondol, Ranajit

DOI:

10.33612/diss.107969043

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Publication date: 2019

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Mondol, R. (2019). Redox-behavior and reactivity of formazanate ligands: Boron and aluminum chemistry. University of Groningen. https://doi.org/10.33612/diss.107969043

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Chapter 1

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1.1 General introduction

Elementary steps involved in the catalytic cycles for organic transformations are often two-electron processes (e.g., oxidative addition and reductive elimination reactions). These reactions are predominantly carried out by the noble metals (e.g., Pd, Pt, Rh, Ru, etc.) as they have (n) and (n+2) oxidation states that are close in energy and therefore can be accessed in a reversible manner. But, the use of noble metals is not sustainable as they are rare, expensive and often toxic, and above all the resources of noble metals are limited.1,2 _{That is why there is a}

considerable interest in the community to move from noble metal catalysts to ones based on earth-abundant (hence cheaper) and less toxic elements (base metal or main group elements). But this presents several challenges.

For example, base metals (first row transition metals) usually change their oxidation states by one-electron due to their relatively stable (n) and (n+1) oxidation states, which limits their utility in the aforementioned two-electron processes. The utilization of main group complexes as catalysts is even much more challenging. In contrast to the noble metal complexes, where close-lying filled and empty d-orbitals interact synergistically to make/break bonds via (reversible) two-electron changes in the metal oxidation state (reductive elimination / oxidative addition), main group complexes have large separation between filled (s) and empty (p) orbitals does generally not allow for (reversible) redox-reactions with these compounds. Now, the question is how to impose noble metal characteristics to complexes with base metals or main group elements so that these could perform above mentioned two-electron processes. A possible answer to this question is the incorporation of redox-active ligands in base metal3–8_{or main}

group9–11_{complexes. Redox-active ligands can donate or accept electrons to or from substrates}

during chemical transformations, and could provide access to two-electron redox-reactions by stabilizing coordination compounds in unusual (formal) oxidation states. Thus, base metal or main group complexes having redox-active ligands may perform catalytic reactions that are not possible with conventional ligands.

1.2 Redox-active ligands

Conventionally, transition metal catalysts change their oxidation state during the catalytic cycle, and the coordinated ligands are not involved in the redox reactions. These coordinated ligands give stabilization to the metal complex during catalysis by acting as a supporting ligand, and also, these ligands allow to tune properties of catalysts by changing the steric and electronic properties via ligand substituent effect. In contrast, when the ligand actively participates in the redox-reaction by accepting or donating redox-equivalents into its backbone, the ligand is

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defined as a ‘redox non-innocent’ ligand.12–14_{Particularly, when the oxidation states of metal}

and ligand in the complex are clearly defined, then the ligand is identified as ‘redox-active’. Due to the presence of energetically accessible frontier orbitals, redox non-innocent/redox-active ligands can accept or donate electrons during chemical redox transformations, and allow the metal to avoid going through unstable oxidation states, and thus, enables challenging chemical transformations.

The following section describes how natural and non-natural catalysts enable challenging chemical transformations with base metal complexes by utilizing the redox-active ligands.

1.2.1 Utilization of redox-active ligands by Nature

Nature is a pioneer in the use of redox-active ligands in organic transformations and catalysis. Nature uses base metals in the active sites of several metalloenzymes (e.g., Fe in P-450,15_Cu

in galactose oxidase,16_{etc.) in which either the redox-active organic moiety is in very close}

proximity to, or directly bound to the active metal center. In these metalloenzymes, both the active site and the redox-active moiety actively participate as redox-equivalents in the organic transformation. This synergistic effect between metal and ligand allow challenging multi-electron redox reactions. For example, galactose oxidase oxidase16_{oxidizes primary alcohols}

to aldehyde, which is a two-electron process.

(Inactive) - e -OH2 CuII O (581H)N (496H)N (495Y)O (Active) RCH2OH _{- H} 2O2 RCHO S (C228) (T272) OH2 CuII O (581H)N (496H)N (495Y)O (C228)S (T272) O CuII O (581H)N (496H)N (495Y)O (C228)S (T272) CuI O (581H)N (496H)N S (C228) (T272) CuII O (581H)N (496H)N S (C228) (T272) R OH H2O R HH R H O O2 H2O2 Galactose Oxidase H net reaction: + O2 H (495Y)O O R HH H (495Y)O H

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During substrate oxidation, one electron is stored on the Cu(II) active site by reduction to Cu(I) and a second electron is taken up by the tyrosine, redox-active ligand scaffold (Scheme 1.1). Thus, copper maintains an energetically favorable Cu(II)/Cu(I) redox-cycle (without going to the energetically unfavorable Cu(II)/Cu(0) redox-cycle).

Another example where Nature uses redox-active ligands for catalytic transformation is cytochrome P450s, which perform monooxygenation and have been found in all domains of life.17_{The active site of cytochrome P450 contains a heme cofactor which is constituted by a}

Fe+2_{center and a porphyrin ligand (Chart 1.1).}18_{In 2010, Green and co-workers captured and}

characterized a highly reactive species, an intermediate of the catalytic cycle of hydrocarbon to alcohol conversion, named as P450 compound I (P450-I).15_{The P450-I contains an}

iron(IV)-oxo species and a singly oxidized porphyrin radical ligand (Chart 1.1). The P450-I oxidizes hydrocarbons to alcohols, which is a two-electron process. During this oxidation process, one electron is stored on the iron center (reduces Fe(IV) to Fe(III)) and another electron is taken up by the ligand to convert the porphyrin radical cation into the parent porphyrin.

Chart 1.1 Active site of cytochrome p450 (left) and compound I (right)

N _N N N FeIV HOOC HOOC Cytochrome P450 O N _N N N FeIII HOOC HOOC P450 Compound I

1.2.2 Bio-inspired and artificial redox-active ligands

Taking inspiration from these enzymatic systems, several efforts have been made for the development of other classes of redox-active ligands. For examples, oxygen-based catecholate (OO) ligands7,19–27_{and its nitrogen (NN)}27–32_{-, nitrogen-oxygen (NO)}7,26,27,33,34_{-, sulfur (SS)}35– 37_{- derivatives, and the porphyrin}38–44_{type ligands are well known (Chart 1.2). In recent years,}

several pincer type tridentate redox-active ligands [i.e., (ONO), (NNN), (SNS)- types] have been developed.27,45–51_{Also, two new nitrogen-rich pyridine-based redox-active ligands, such}

as pyridine imino ligands (IP) and pyridine diimino ligands (PDI), have gained much attention in recent years.4,7,9–11,26,52–61_{Structures and various oxidation states of IP and PDI}

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ligands are shown in Chart 1.2. Redox-active properties of these newly synthesized redox-active ligands have been utilized in catalyst development with base metals bearing these redox-active ligands.3–8,62,63

Chart 1.2 General structure (top, left), and different oxidation states (middle) of the catecholate

type ligands, porphyrin type ligands (top, right), various oxidation states of pyridine imino ligands (IP) (bottom, left) and various oxidation states of pyridine diimino ligands (PDI) (bottom, right) O -O -O -N -N -N -R R S -S -N N N N M R R R R R X -X -X -X X X - e- - e -L L2- _L

-a) cat b) o-O,N c) o-N,N d) o-S,S

X = O, N, S, P N N R R NR R N N R R - e- N N R R - e- N N R R - e- - e -N N R R NR R N N R R NR R IP IP IP

2-PDI PDI PDI 2-+ e- + e

-+ e- + e- _{+ e}

-+ e

-In the next paragraphs, among the several examples one representative example is presented to show the progress of catalyst development with the base metals bearing redox-active ligands.

1.2.2.1 Terminal alkene hydrosilylation catalyzed by iron complexes

containing redox-active ligands

The Chirik group has been involved in the investigation of redox-active properties of nitrogen based tridentate pyridine(diimine) ligands (PDI).4,14,64,65_{They have utilized redox-active}

properties of PDI ligands coordinated to several base metals, which mediated unusual catalytic chemical transformations. Examples include iron catalyzed hydrogenation and hydrosilylation of olefins,6,64_{iron and cobalt catalyzed [2+2] cycloaddition of dienes,}4,58,66,67_{iron catalyzed}

hydrogenative cyclization of enynes and diynes,3_{cobalt catalyzed asymmetric alkene}

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metal catalysts can perform multi-electron processes by utilizing redox-active ligands. Here is an example of catalytic hydrosilylation of alkenes catalyzed by a [Fe(PDI)(N2)]2(μ-N2) complex,

which has a FeII_{center coordinated by a triplet diradical [PDI]}2-_{ligand and dinitrogen molecules}

(Scheme 1.2).6_{In the first step of this catalytic cycle, the alkene is coordinated to the vacant}

site at iron. In the second step, the Si-H bond of Et3Si-H is added via oxidative addition to the

iron center. In this step, two electrons are supplied by the reduced ligand backbone, not by the iron center, and thus the iron center maintains its stable Fe(II) oxidation state without going through the unstable Fe(IV) oxidation state. Then, the migratory insertion of hydride into the internal position of the coordinated alkene occurs, and is followed by reductive elimination to form the product and regenerate the active catalyst. During the reductive elimination process, two electrons are stored on the ligand backbone, not on the metal center, and thus again, the iron center maintains its stable Fe(II) oxidation state without going through the unstable Fe(0) oxidation state. Thus, throughout the catalytic cycle the iron center maintains its stable Fe(II) oxidation state and all the redox-processes are enabled by the redox-active ligand.

N N N Ar Ar Fe N2 N2 N N N Ar Ar FeII N N N Ar Ar FeII N N N Ar Ar FeII Ar = 2,6-(Me)2-C6H3 C5H11 C5H11 C5H11 Et3Si Et3Si H C5H11 N N N Ar Ar FeII Et3Si Et3Si-H C5H11 Alkene Hydrosilylation net reaction: C5H11 + Et3Si-H C5H11 Et3Si cat [Fe] neat, 23° C cat [Fe] Scheme 1.2 Proposed catalytic cycle for the Fe-catalyzed hydrosilylation of alkene

1.2.2.2 Redox reactions in main group complexes

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show the recent advancement towards catalyst development based on main group elements. The reactivity of main group compounds and their application in catalysis is mostly based on the availability of an empty orbital (i.e., Lewis acidic character) or a lone pair of electrons (Lewis base). In main group complexes, the large separation between filled (s) and empty (p) orbitals does not generally allow for (reversible) redox-reactions with these complexes. In spite of this drawback, in the past decades, new strategies have been developed to stabilize main group complexes in low-valent states, in particular for the heavier congeners, and this has led to the development of reactions that can be carried out under mild conditions and are reminiscent of transition metal chemistry (e.g., small-molecule activation).70_Low-valent

compounds of carbon (N-heterocyclic carbenes, NHCs) have gained prominence as ligands in coordination chemistry and as organocatalysts,71_{because of their singlet electronic ground state}

that results from a large HOMO-LUMO gap. In 2007, Bertrand and co-workers reported seminal work on the reactivity of related alkyl amino carbenes (AACs), which were shown to cleave strong bonds in H2 and NH3 at a single carbon center (Chart 1.3 a).72 The increased

reactivity of AACs in comparison to the (unreactive) NHCs was rationalized based on the significant lowering of the LUMO energy level to bring it close in energy to the HOMO (a hallmark of the d-orbitals in transition metal compounds). The resulting combination of the nucleophilic and electrophilic characters at the carbene C-atom allows (formal) oxidative addition of H2 and NH3. Following this discovery, activation of small molecules with low-valent

main-group compounds has been studied extensively.70,73–77_{For example, the oxidative}

addition of NH3 to stannylene followed by reductive elimination to form B-N and B-H bonds

was reported by Aldridge and co-workers (Chart 1.3 b), opening the possibility to develop catalytic transformations based on a Sn(II)/Sn(IV) redox cycle.78_{A recently developed}

Si(II)/Si(IV) redox-cycle reported by Inoue and co-workers is worthy to mention.79_{In this study,}

they showed that a highly reactive acyclic iminosilylsilylene underwent intramolecular insertion into a C=C bond to form silacycloheptatriene, which converted back to Si(II) at elevated temperatures (Chart 1.3 c).79_{As an example of related chemistry in group 15,}

Radosevich and co-workers developed P(III)/P(V) redox cycles to cleave O-H and N-H bonds in a reversible manner (Chart 1.3 d).80_{In recent years, oxidative addition of strong bonds to}

Roesky’s β-diketiminate aluminum(I) species, (DippNC(Me)CHC(Me)NDipp)Al (Dipp = 2,6-diisopropylphenyl, Chart 1e)81_{has been explored in detail.}74,77_{The reaction between the}

aluminum(I) complex and the corresponding dihydride was shown to form an equilibrium between the starting materials and the mixed dimer (Chart 1.3 f), indicating that (formal)

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oxidative addition / reductive elimination of the Al-H bonds is reversible in this system.82_In

2019, Aldridge and co-workers showed the reversible oxidative addition of C-C bond of benzene to a single aluminum center of isolated monomeric aluminyl compound [K(2.2.2-

Chart 1. 3 Redox reactions in the main group complexes

B Sn BN N N N R R R R BN N _R R NH2 BN N _R R H reduced Sn species NAl N Dipp Dipp N Al N Dipp Dipp + H H N Al N Dipp Dipp H NAl N Dipp Dipp H NMe N MeN P NMe N MeN P OR H (b) (e) (d) RT ∆ + N Dipp Bertrand (a) Aldridge Nikonov Radosevich B Sn BN N N N R R R R H NH2 NH3 NH3 N Dipp H2N H RO-H R = Dipp O t_Bu t_Bu N N Al Dipp Dipp O t_Bu t_Bu N N Al Dipp Dipp [K(2,2,2-Crypt)] [K(2,2,2-Crypt)] Benzene, room temp, 48 h Benzene-d6, 80 °C Benzene, 80 °C O t_Bu t_Bu N N Al Dipp Dipp [K(2,2,2-Crypt)] D D D D D D (f) Aldridge N Al N N Ph Ph _Dipp Dipp THF Cl 2H+/1e -1e -H2 L L N Al N N Ph Ph _Dipp Dipp L L H H Cl L = 4-Dimethylaminopyridine (DMAP) Berben _N N (g) N NDipp Dipp NSiBr3 2 KSiTMS3 -2 KBr -TMS3SiBr N N C N Si (TMS)3Si C N N N (c) -78°C, toluene C Si C (TMS)3Si Si(II) _Si(IV) Inoue

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crypt)][(NON)Al] (Chart 1.3 g) at room temperature, which further adds a significant contribution to the field of catalyst development based on main group compounds.73,83_{In this}

context, small molecule activation and catalysis by frustrated Lewis pairs (FLPs) is an another sub-field of main group chemistry, which has gained much attention since its inception in 2006 and a staggering progress has been achieved. Several recent reviews describe the advances in FLP chemistry,84–94_{and the reader is referred to these articles for an in-depth discussion of this}

chemistry.

Despite the advances in stabilization of low-valent main group species and their use to cleave strong bonds via oxidative addition, the reverse reaction (i.e., reductive elimination) still presents considerable challenges in main group chemistry due to the stability of the highest oxidation state in particular for the lighter (2nd_{and 3}rd_{row) elements. As a consequence,}

catalytic turnover involving the oxidative addition / reductive elimination sequences are rarely observed in main group chemistry. In this respect, similar to base metal complexes, the incorporation of redox-active ligands in main group complexes may allow to perform chemical transformations catalytically. In this context, the Berben group has reported a series of reduced Al(III) and Ga(III) complexes with redox-active imino pyridine (IP) and diimino pyridine (PDI) ligands.9–11,52–55,60,61_{The Berben group has utilized the metal-ligand cooperative property and}

the redox-active property of IP and PDI ligands for several chemical transformations, ranging from dehydrogenative coupling of aniline and benzyl amine, catalytic conversion of CO2 into

MgCO3 or CaCO3, conversion of formic acid into CO2 and H2, electrocatalytic hydrogen and

ammonia production etc.10,11,53,54_{From these above mentioned examples, ligand-centered}

electrocatalytic hydrogen production is one of the most promising achievements towards catalyst development based on main group complexes bearing redox-active ligands. For this catalytic hydrogen evolution reaction, the locus of protonation and reduction is the redox-active PDI ligand, and the metal center (Al) is not involved (Chart 1.3 f).10

1.3 Formazanate ligands

1.3.1 General features of formazans

Formazanates are monoanionic (deprotonated form of formazan) bidentate N-chelating ligands, which are nitrogen-rich (having NNCNN backbone) analogues of β-diketiminate ligands (also known as NacNac ligands). The coordination chemistry of β-diketiminate ligands has been explored extensively during the past decades. Due to easy accessibility and high

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stereoelectronic tunability, a variety of metal complexes of the β-diketiminate ligands have been synthesized, characterized, and several of these metal complexes have found applications as catalysts.95–103_{Apart from being used as supporting ligands, in the last few years, their}

redox-active properties have also been investigated and well established.104

Formazan has a long history of being used as dye molecules.105,106,107_{In modern chemical}

research, it has been used in the biochemical research field.108,109_{Formazan has been used as a}

precursor of thermally stable verdazyl radicals (Chart 1.4).110_{Due to the presence of four}

nitrogen atoms in the backbone, the verdazyl radical has a low-energy SOMO, which has π anti-bonding character between N-N bonds (the SOMO is shown in Chart 1.4). The unpaired electron of the verdazyl radical is delocalized over all four nitrogen atoms of this heterocyclic ring, which gives exceptional stability to these verdazyl radicals.111,112_{As verdazyl radicals have}

a low-energy SOMO and a high-energy HOMO, they can be reversibly oxidized to cations and reduced to anions. Due to the exceptional stability of verdazyl radicals to air and moisture, these radicals have found several applications, such as in the preparation of molecular magnets,113_as

ESR spin labels114_{and inhibitors for polymerization,}115_{or mediators in living radical}

polymerizations.116

Chart 1.4 Verdazyl radical (left) and SOMO of verdazyl radical (right)

In spite of the structural similarity of formazanate ligands with β-diketiminate ligands, only in recent years attention has been given to the exploration of their coordination chemistry. The presence of two extra nitrogen atoms in the formazanate ligand backbone compared to the structurally related β-diketiminate ligand, introduces a significant amount of flexibility in the coordination chemistry of formazanate ligands. Due to the increased flexibility in the geometry of formazanate ligands, these ligands not only form common 6-membered chelate rings in their metal complexes, but also 5-membered as well as 4-membered chelate rings are accessible (Scheme 1.3).117,118 N N N N R5 R1 R3 R6 Verdazyl radical R5 R1 R3 R6 SOMO

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Processed on: 21-11-2019 PDF page: 23PDF page: 23PDF page: 23PDF page: 23 11 N N R N N N N Zn N N N N p-tol Mes C6F5 C6F5 Mes p-tol N N N N Zn N N N N p-tol Mes C6F5 C6F5 Mes p-tol N N M N R Ar M = Na, K N Ar N N Ar Ar M or

Scheme 1.3 Various coordination modes of the formazanate ligands117,118

1.3.2 Redox-active properties of formazanate ligands

In 2007, the first example of ligand-based reduction of a formazanate ligand was reported by Hicks and co-workers in the formazanate boron diacetate complex LB(OAc)2 (Scheme 1.4).119

Cyclic voltammetry experiment on LB(OAc)2 shows a quasi-reversible redox event at -0.84 V

vs Fc0/+1_{, which indicates the possibility to access the reduced complex of LB(OAc)}₂_{. They have}

reduced LB(OAc)2 with cobaltocene to afford the reduced product [LB(OAc)2][Cp2Co]. This

reduced product was characterized by EPR and UV-vis absorption spectroscopic techniques. This study not only reveals the redox-active property of formazanate ligands, but also indicates the easy accessibility of the reduced product of LB(OAc)2 by using mild reducing agents

(reduction potential of LB(OAc)2 = -0.84 V vs Fc0/+1).

N N NHN Ph Ph p-tol N N N N Ph Ph p-tol B(OAc)3 B AcO OAc Cp2Co N N N N Ph Ph p-tol B AcO OAc Cp2Co

Scheme 1.4 Ligand-based reduction in a formazanate boron complex (reported by Hicks and

co-workers)119

Inspired by the work from Hicks, Gilroy and co-workers have prepared a variety of boron complexes with redox-active formazanate ligands.120–131_{They have extensively studied the}

optical and electrochemical properties of these newly generated materials. All of these complexes absorb strongly in the visible region due to the π-π* transition within the ligand frameworks.120,121,123–127_{Cyclic voltammetry experiments reveal that most of the formazanate}

boron complexes show two sequential one-electron (quasi)reversible redox events that are ligand-centered.120,121,124–127_{The first redox event corresponds to the formation of radical}

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the optical (e.g., absorption and emission maxima, fluorescence, etc.) and the electrochemical (e.g., reduction potentials) properties of these (formazanate)boron complexes could be modulated via the systematic variation of substituents as well as via the extension in π conjugation within the ligand framework.120,121,124–127_{Very recently, they have expanded the}

study on formazanate complexes by incorporating heavier analogues of group 13 elements (e.g., aluminum)132_{and higher congeners of group 14 elements (e.g., Si, Ge and Sn)}133_{. The}

formazanate complexes of aluminum and group 14 elements also disclose redox-active properties of formazanate ligands. Furthermore, their studies not only reveal the tunable optical and electrochemical properties of the (formazanate)boron complexes but also some of these complexes have photoluminescent properties121,124,125_{which has led to applications as}

cell-imaging agents,125,128_{electrochemiluminescence emitters,}126,134_{multifunctional polymers,}131,135

and precursors to a variety of BN heterocycles.136

Concurrent with the Gilroy group, our group also started a research program to explore the coordination chemistry and the redox properties of formazanate ligands. In 2014, our group has studied the ligand-based redox chemistry of bis(formazanate)zinc complexes.137_Cyclic

voltammetry experiments indicate four sequential one-electron redox waves for bis(formazanate)zinc complexes (Figure 1.1). The first and second redoxevents occurred at -1.54 and -1.84 V vs Fc0/+1_{, respectively. These two redox-events correspond to the}

ligand-centered one-electron reduction of each ligand, which leads to the formation of radical anions and radical dianions, respectively, which were isolated and structurally characterized. The stability of the reduced products of bis(formazanate)zinc complexes are due to their “metalaverdazyl”-type structure.137_{Further redox-events that are observed at more negative}

potentials (< -2.5 V vs Fc0/+_{) are due to the two-electron reduction of each ligand (Figure}

1.1).117,137 N N N N Ph Ph p-tol L -N N N N Ph Ph p-tol 2 L 2-N N N N Ph Ph p-tol L 3-e -I/I' and II/II' e -III/III' and IV/IV' I' I II III IV IV' III' II'

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In the same year, reduction chemistry of the mono(formazanate)boron difluoride complexes (LBF2) was investigated.138 Cyclic voltammetry of the complex LBF2 shows two sequential

(quasi)reversible one-electron reduction waves at -0.98 and -2.06 V Fc0/+1_{, respectively.}138_This

indicates the possibility of two-electron reduction in a single formazanate ligand. Though the one-electron reduction product [LBF2]- was isolated and characterized, the isolation of the

two-electron reduced product [LBF2]2- was unsuccessful due to the elimination of NaF, which led to

the formation of N-heterocyclic boron carbenoid species, which react further and become incorporated in a series of unusual BN heterocycles.139

In our group, including the above examples, several other formazanate boron and zinc complexes have been prepared and their optical properties (e.g., absorption and emission spectra) and redox properties have been studied.117,137–140_{In 2016, our group has described the}

synthesis of a series of (formazanate)boron complexes and their ligand-centered one-electron reduced products. The neutral complexes are fluorescent, whereas the reduced products do not show any fluorescent properties.140_{This study shows the possibility to use (formazanate)boron}

complexes as redox-switchable dyes. Recently, our group also expanded the coordination chemistry and redox chemistry of formazanate complexes by employing iron and palladium metals.141–144_{Due to the π-acceptor properties of formazanate ligands, the}

bis(formazanate)iron complex obtains an unusual electronic structure and shows switchable magnetism (spin-crossover).141

In recent years, the group of Holland has also reported ligand-based reductions in several low-coordinate iron complexes with the formazanate ligand.145,146_{In 2016, Sundermeyer and}

co-workers have described the synthesis and characterization of series of formazanate complexes incorporating heavier analogues of group 13 elements (LMMe2, M = Al, Ga and In).147

In the last decade, several transition metal (Ru,148_Co,149_Ni,150–152_Pd,153_Pt,154–156_Cu,157_and

other158_{) complexes with redox-active formazanate ligands have been reported. Despite the fact}

that a wide range of reported formazanate complexes is known, only few complexes are active catalysts for chemical transformations, which includes H2O2 decomposition159 and ethylene

oligomerization.153,150,160_{But, in both these chemical transformations formazanates act as}

‘innocent’ supporting ligands without engaging in redox-activity. Thus, in order to utilize the redox-active properties of formazanate ligands in chemical reactivity, there is a need to further enhance the understanding of bonding and electronic structural features of neutral and reduced formazanate complexes.

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1.4 Overview of the thesis

The overall aim of the research presented in this thesis is the exploration of the redox-active properties and the study of ligand-centered reactivity of formazanate complexes. In order to achieve this goal, we have synthesized boron and aluminum complexes with formazanate ligands and their redox chemistry has been studied. For the first time, two-electron reduced mono(formazanate) boron and aluminum complexes were isolated and fully characterized, and subsequently, ligand-based reactivity with these reduced complexes is explored.

In chapter 2, the synthesis and characterization of the redox-series of boron diphenyl

complexes with formazanate ligands (i.e., [LBPh2]0/-1/-2) is presented. First, we have established

the redox-active features of the formazanate ligands in mono(formazanate) boron diphenyl complexes (LBPh2) by cyclic voltammetry. For the first time, ligand-based two-electron

reduced products of formazanate complexes ([LBPh2]-2) are isolated. The redox-series LBPh2 0/-/2-_{is fully characterized by single crystal structure determination and spectroscopic techniques.}

The experimental data are corroborated by a computational study (density functional theory). In chapter 3, we have explored the ligand-based reactivity of the two-electron reduced

formazanate boron diphenyl complex (i.e., [LBPh2]2-). The reactions of nucleophilic [LBPh2]2-

with the electrophiles BnBr and H2O lead to the formation of benzylated and

ligand-protonated products, respectively, which are anionic boron analogues of leucoverdazyls. N-C and N-H bond homolysis of these systems is studied by the exchange NMR spectroscopy and kinetic experiments. This study reveals that not only formazanate ligands can store two electrons and an electrophile (E+_{= Bn}+_{, H}+_{) on their backbones, but these can react as source}

of E•_{radicals. This type of reactivity is very important in energy storage applications such as}

hydrogen evolution.

In chapter 4, the synthesis of a series of aluminum complexes with redox-active formazanate

ligands is described. The cyclic voltammograms of mono(formazanate)aluminum diphenyl and diiodide and bis(formazanate)aluminum chloride complexes are presented. Chemical reduction of all the newly synthesized complexes shows that ligand-based redox reactions in these compounds are accessible. The two-electron reduction of mono(formazanate)aluminum diiodide was performed in order to investigated the possibility to obtain a formazanate aluminum(I) carbenoid species. A computational study further sheds light on the electronic structure and the stability of sub-valent formazanate aluminum(I) carbenoid species.

In chapter 5, the differences in structure, bonding and reactivity between two-electron reduced

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containing a highly reduced, trianionic formazanate-derived ligand is described. Due to the substantial delocalization of excess electron density from the ligand core (i.e., NNCNN) onto the peripheral ligand substituents (N-Ph) via the π-framework, partial double bond character is built up in the N-C(Ph) bonds which leads to the restricted rotation around N-C(Ph) bonds. The barrier of this restricted rotation around N-C(Ph) bonds in the two-electron reduced formazanate aluminum complex has been determined by variable temperature NMR experiments (VT-NMR) and NMR lineshape analysis, and is compared to the boron analogues. The crystallographic data of ligand-benzylated products of [(PhNNC(p-tol)NNPh)ZPh2]2- (Z = B, Al), which are anionic

B and Al analogues of leucoverdazyls, are presented and the main differences are addressed. The kinetics of benzyl transfer from the ligand-benzylated products to TEMPO reveal that the more ionic Al compound has one of the weakest N-C bonds reported so far in this type of inorganic leucoverdazyl analogues.

In chapter 6, a detailed analysis of the observed dynamics present in ligand-benzylated

formazanate boron and aluminium complexes ([Bn_LZPh₂_]-_{, where}Bn_{L =}

(PhNN(Bn)C(p-tol)NNPh), and Z = B, Al) is presented. The activation parameters for the dynamic process present in these ligand-benzylated products have been determined by variable temperature NMR experiments and subsequent NMR lineshape analyses, which indicates that the observed dynamics in these complexes is due to nitrogen inversion. On the basis of these activation parameters, we have proposed a plausible mechanism for the nitrogen inversion processes present in these ligand-benzylated products. Our study reveals that the counter cation which is bound to the ligand backbone of [Bn_LBPh₂_]-_/[Bn_LAlPh₂_]-_{moiety, dissociates in a rate}

determining step that precedes nitrogen inversion. The effects of counter-cations on the dynamic processes present in these anionic B/Al complexes has been investigated. The comparison of activation parameters for the nitrogen inversion processes between boron and aluminum complexes indicates that the central element has little influence on the nitrogen inversion. In chapter 3, we described sequential ligand-based storage of two electrons (2e-_{) and one proton}

(H+_{) (i.e., [2e}-_/H+_{] equivalent) in the formazanate boron diphenyl compound LBPh}₂_{, which led}

to the formation of ligand-protonated product [H_LBPh₂_]-_{. In addition, we showed a net H-atom}

transfer from [H_LBPh₂_]-_{to TEMPO. This net H-atom transfer from [}H_LBPh₂_]-_{to TEMPO could}

occur either by hydrogen atom transfer (HAT) or by concerted proton-coupled electron transfer (cPCET) mechanism. In chapter 7, a detailed computational analysis of the mechanism of this

reaction is described by analysis of the changes in intrinsic bond orbitals (IBOs) along the reaction coordinate.

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