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

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University of Groningen

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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English Summary

In recent years, redox-active ligands have gained much more attention. These ligands are actively involved in bond-breaking/making processes in the catalytic reactions performed by their metal complexes. The extensive utilization of redox-active ligands in the development of catalysts based on less toxic, more abundant, and hence cheaper first row transition metals shows the potential applicability of these redox-active ligands in homogeneous catalysis. In this thesis, this strategy is extended to main group complexes. We describe the synthesis of compounds based on the inactive group 13 elements B and Al in combination with redox-active formazanate ligands. Their reduction chemistry and ligand-based reactivity has been investigated. The fundamental aspects such as bonding, structural and electronic properties of these complexes has been described. The results presented in this thesis provides deeper understanding of redox-behavior and reactivity of main group complexes with formazanate ligands.

Chapter 1 provides a background into redox-active ligands. Two organic transformations

catalyzed by galactose oxidase and cytochrome P450, respectively, has been chosen to demonstrate how these metalloenzymes enable challenging multi-electron processes by involving redox-active organic scaffolds. Examples of synthetic redox-active ligands are discussed and applications of base metal catalyzed processes are presented to highlight how these perform multi-electron processes by utilizing redox-active ligands. Some representative redox-reactions performed by main group complexes are presented to showcase the recent advancement towards catalyst development based on main group complexes, including strategies based on redox-active ligands. Finally, the general features of our ligand design based on anionic formazanate ligands is discussed. An overview of known formazanate complexes, including their redox-active, optical and electronic properties, and their applications is provided.

In Chapter 2, the synthesis and characterization of stable boron complexes bearing formazanate

ligands in mono-, di-, and trianionic forms is described. The neutral mono(formazanate)boron diphenyl complexes (LBPh2; 2, Scheme 1) were synthesized. The cyclic voltammetry of THF

solution of 2 shows two (quasi)reversible redox couples which suggests that the ligand could

be reduced to its dianionic (12-_{) and trianionic (}₁3-_{) forms (Figure 1). The one- and two-electron}

reduced products 2.-_{and [}_2][Na₂_]_{were prepared by the treatment of}_{2 with one equivalent of}

decamethyl cobaltocene (Cp*₂_{Co) and two equivalents of sodium naphthalenide (Na/Np),}

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in the solid state, whereas it is dimeric for [2b][Na2]. Crystallographic data reveal continuous

elongation of the N-N bonds along the redox-series (i.e., 2a→2a.-_→_2a2-_),_{consistent with}

ligand-based reduction which populates the ligand N-N π*-orbitals. The dynamics of [2b][Na2]were

studied by NMR lineshape analysis, which revealed appreciable double bond character for the N-C(Ph) bond due to delocalization of the negative charge into the aromatic substituents. The crystallographic, spectroscopic (NMR or EPR, UV-vis), and computational data of the redox-series 20/.-/2-_{are in agreement with ligand-based reduction. The dianionic compounds [}_2][Na₂_]

are the first examples of isolated and fully characterized two-electron reduced formazanate complexes.

Scheme 1. (left) Synthesis of (formazanate)boron diphenyl compounds and their reduction

products, and (right) crystal structure of two-electron reduced product [2][Na2].

Figure 1. Cyclic voltammograms of 2a and 2b showing the presence of reduction waves

corresponding to one-electron reduction (12-_{, reduction I) and two-electron reduction (1}3-_,

reduction II) of a formazanate ligand (1-_).

In Chapter 3, the ligand-based reactivity of two-electron reduced formazanate boron diphenyl complex [2a][Na2] is investigated. The treatment of [2a][Na2] with the electrophiles BnBr and

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173 ([Bn_{3][Na] and [}H_{3][Na] in Scheme 2). [}Bn_{3][Na] and [}H_3][Na]_{are anionic boron analogues of}

1-alkylated 1,2,3,4-tetrahydro-1,2,4,5-tetrazines (‘leucoverdazyls’). The 2D NOESY spectrum

of [H_3][Na]_{reveals the presence of chemical exchange process due to a net H-atom transfer}

between the two ‘internal’ nitrogens in the ligand backbone. The exchange kinetics are followed by 2D EXSY NMR spectroscopy, and a plausible mechanistic pathway of H-atom transfer is provided (associative). The N-C(Bn) bond cleavage in [Bn_{3][Na] is investigated by following}

the kinetics of benzyl transfer from [Bn_{3][Na] to TEMPO, and shown to follow a dissociative}

pathway (Scheme 3). This study reveals that the formazanate ligand can sequentially storage two electrons (2e-_{) and an electrophile (E}+_{= Bn}+_{, H}+_{). Additionally, by this study we have}

shown that the [2e-_/E+_{] equivalent ‘stored’ in these systems can be easily converted to E}• _(1e

-/1E+_{) radicals. This type of reactivity has relevance to the energy storage applications such as}

hydrogen evolution. E-X, THF _N N N N Ph Ph p-tol B Ph Ph E [Bn3][Na] (E = CH2Ph) [H3][Na] (E = H) - NaX E-X = PhCH2-Br H-OH N N N N Ar Ph p-tol B Ph Ph [2][Na2] Na Na (THF)x (THF)x Na (THF)3

Scheme 2. Synthesis of compounds [Bn_{3][Na] and [}H_3][Na].

N N N N Ph Ph p-tol B Ph Ph Bn N N N N Ph Ph p-tol B Ph Ph Bn ‡ TEMPO _N N N N Ph Ph p-tol B Ph Ph + _N O Bn

Scheme 3. Proposed dissociative mechanism of benzyl transfer from [Bn_{3][Na] to TEMPO.}

In Chapter 4, several aluminum complexes bearing redox-active formazanate ligands are synthesized and fully characterized, including bis(formazanate)aluminum chloride (4),

mono(formazanate)aluminum diphenyl (7) and diiodide (8) complexes. The reduction

chemistry of 4, 7 and 8 are established by cyclic voltammetry experiments and chemical

reductions methods. The combined crystallographic, spectroscopic and computational data for the reduced products 4.-_,₇.-_,₇2- _and₈.-_{disclose that the reductions in}_{4, 7 and 8 are exclusively}

ligand-centered. Despite several attempts to obtain a formazanate aluminum(I) carbenoid (9)

by two-electron reduction of the diiodide 8, such a species remains elusive. A computational

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LUMO in 9 is highly stabilized, and due to this the computed singlet-triplet energy separation

is very small (11.5 kcal/mol). The synthesis and characterization data for formazanate aluminum complexes presented in this chapter provide scope for further exploration of the reactivity of these compounds by utilizing both the electrophilic properties of Al and the redox-active features of formazanate ligands.

N N NN p-tol Ph X Ph AlPh3, 90 °C, N N AlN N p-tol Ph Ph Ph Ph 7 2hrs, toluene, N N_AlN N p-tol Ph Ph I I 2 I2, RT, 3hrs, toluene, 8 1 eq AlCl3, Ph Ph p-tol N N N N Ph Ph p-tol NN Al N N RT, THF 4 Cl -2 KCl - Ph-H - 2Ph-I X = K, LK X = H, 1a

Scheme 4. Synthesis of compounds 4, 7 and 8 N N N N Ph Ph p-tol Al Ph Ph Cp2Co N N N N Ph Ph p-tol Al Ph Ph Cp2Co N N N N Ph Ph p-tol Al Ph Ph Na Na (DME)2 (DME)2 2 eq Na/Hg THF, RT 7 [7][Na2] 7-. THF, RT _{crystallize from} DME/hexane N N _Al N N Ph Ph Ph 9 Scheme 5. (left) Synthesis of one-electron and two-electron reduced derivatives of 7, and (right)

the chemdraw structure of formazanate aluminum(I) carbenoid 9.

In Chapter 5, the differences in structure, bonding and reactivity between two-electron reduced formazanate boron and aluminum complexes [(PhNNC(p-tol)NNPh)ZPh2][Na2](where Z = B,

[2a][Na2]; and Z = Al, [7][Na2]) containing a highly reduced, trianionic formazanate-derived

ligand is presented. Crystallographic, spectroscopic and computational data indicate that the bonding in [7][Na2] is more ionic than in [2a][Na2]. The increase in the ionic character for the

Al compound results in an enhanced resonance delocalization of negative charge into the periphery of the ligand (i.e., the N-Ph substituents), which is reflected by the higher barrier of rotation around the N-C(Ph) bond. The reaction of [7][Na2] leads to the facile formation of the

corresponding ligand-benzylated product [Bn_{7][Na] (Scheme 6). The differences between this}

compounds and its boron analogue are addressed by comparing crystallographic, computational and reactivity data.

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175 N N N N Ph Ph p-tol Al Ph Ph [7][Na2] RT, THF PhCH2-Br N N N N Ph Ph p-tol Al Ph Ph - NaBr Na Na Ph Na RT, THF Bu4NBr N N N N Ph Ph p-tol Z Ph Ph [Bn3][NBu4] [Bn7][NBu4] - NaBr Ph N N N N Ph Ph p-tol Z Ph Ph Ph Na Bu4N Z = B, [Bn_3][Na] Z = Al, [Bn_7][Na] (THF)3 _(THF)₃ _(THF)3 _(THF)3 [Bn_7][Na]

Scheme 6. Synthesis of ligand-benzylated products [Bn_{7][Na], [}Bn_7][NBu₄_{] and [}Bn_3][NBu₄_]

Figure 2. Molecular structures of [Bn_{7][Na] (left) and [}Bn_3][NBu₄_{] (right), and showing 50%}

probability ellipsoids. Hydrogen atoms for [Bn_{7][Na] and [}Bn_3][NBu₄_{] are omitted for clarity.}

THF molecules (except for the O atoms bonded to Na) for [Bn_{7][Na] are omitted for clarity.}

InChapter 6 a detailed analysis of the dynamic processes present in ligand-benzylated boron

and aluminum complexes Bn₃- _andBn₇-_{is described. NMR lineshape analysis reveals that the}

observed dynamics in these complexes is due to the nitrogen inversion, and mechanistic differences are described between compounds containing the organic cation Bu4N+ and alkali

metal cations. Our study reveals that in [Bn_{3][Na] and in [}Bn_{7][Na], the counter cation Na}+

dissociates in a rate determining step which precedes the actual pyramidal nitrogen inversion process.

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176 N N N Ph Ph p-tol Z Ph _Ph ‡ Z = B; Bn₃ -Z = Al; Bn₇ -Ph N N N N Ph Ph p-tol Z Ph Ph Ph H H N N N N Ph Ph p-tol Z Ph Ph Ph N H H H H sp3 sp3 sp2 p sp3 sp3 pyramidal trigonal planar

transition state (TS) pyramidal

Scheme 7. Schematic representation of nitrogen inversion in Bn₃-_andBn₇-

In Chapter 7 the mechanism of H-abstraction from H₃-_{to TEMPO is elucidated by intrinsic}

bond orbital (IBO) analyses (computational method). Analysis of the changes of IBOs along the intrinsic reaction coordinate (IRC) for this reaction reveal that the unpaired electron on the O-atom of TEMPO and an electron of a doubly-occupied (polarized) N=C π-bond in the formazanate fragment combine to form the new O-H bond in the product TEMPOH. No electrons from the N-H σ-bond are involved in the formation of O-H bond. Thus, the IBO analysis depicts that the proton and the electron do not travel together. Moreover, IBO analysis shows that the transfer of the electron and the proton is continuous, which means that the proton and the electron are transferred in a concerted fashion. Thus, our computational studies reveal that the H-abstraction from H₃-_{to TEMPO occurs by concerted proton-coupled electron transfer}

(cPCET) rather than hydrogen atom transfer (HAT). The result presented in this chapter provides a better insight into the electronic factors that underpin the reactivity of H₁-_{, the}

knowledge of which could help to steer away from the observed [1e-_/1H+_{] (i.e., H-atom) transfer}

reaction to multi-electron transfer reactions (e.g., hydrogen evolution reaction: [2e-_/2H+_{] = H}₂_).

N N N N Ph Ph p-tol B Ph Ph cPCET 2a .-N N N Ph Ph p-tol B Ph Ph H₃ -H N N O + + _O _N H TEMPO TEMPOH