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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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 7

Mechanism of H-abstraction from

ligand-protonated formazanate boron complex: a

computational study

In chapter 3, we described ligand-based sequential storage of two electrons (2e-_{) and one proton}

(H+_{) in formazanate boron diphenyl compound}_{1 and subsequent transfer of a net H-atom from}

the resulting ligand-protonated product H₁-_{to TEMPO. In this chapter, we elucidate the reaction}

mechanism of this reaction by computational methods (IBO analysis).

This chapter will be submitted for publication:

Mondol, R., Klein, J.E.M.N.,* and Otten, E.* “Elucidation of mechanism of net H-atom transfer from ligand-protonated formazanate boron complex to TEMPO.”

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160

7.1 Introduction

Proton-coupled electron transfer (PCET) events play an important role in several chemical and biological processes, examples are ranging from the solar energy conversion,1_hydrocarbon

combustion,2_{photosynthesis and respiration,}1,3,4_{and nitrogen fixation in the biosphere.}5

Proton-coupled electron transfer (PCET) is a process by which an electron and a proton are transferred.5–7_{In the PCET reaction, the transfer of an electron and a proton can occur either in}

a single kinetic step or in a stepwise manner - proton transfer (PT) followed by electron transfer (ET) or electron transfer (ET) followed by proton transfer (PT) (Scheme 7.1 (a)).5–7_{In the}

concerted PCET (cPCET) mechanism, an electron and a proton are transferred between different sites or different orbitals in a single kinetic step. In the phenoxyl/phenol system during the self-exchange of a H-atom, a proton and an electron are simultaneously transferred to the oxygen atom of the phenoxyl radical. But, the proton and the electron transfer engages different orbitals: The proton is transferred between the σ-orbitals, whereas, the electron transfer occur via the π-type orbitals. Thus, this is an example of a (cPCET) (Scheme 7.1 (b)).8_{In a hydrogen}

atom transfer (HAT) reaction, an electron and a proton are transferred together to the same site, a subclass of cPCET process. In the benzyl/toluene system during the self-exchange of a H-atom, a proton and an electron of C-H σ-bond of toluene travel together to the σ-orbital on the carbon atom of benzyl radical to generate a new C-H bond (Scheme 7.1 (b)).8_{Thus, this is an}

example of a HAT process.

X-H + Y PT X- + HY+ ET ET XH+ + Y- X + H-Y PCET PT (a) O O cPCET: H H₂C vs HAT: (b) H H H+ e

-Scheme 7.1 Square scheme representation of proton-coupled electron transfer (PCET)

Given the fundamental importance of proton-coupled electron transfer reactions across chemistry, an in-depth understanding of the underlying mechanisms will provide guidelines for further development of this type of reactivity. However, distinguishing between HAT and cPCET mechanisms is not straightforward. Recently, Gerald Knizia has developed the intrinsic bond orbital (IBO) method as a means to extract a chemically intuitive bonding picture based on Lewis structures from (Kohn-Sham) wavefunctions.9_{In this method, the delocalized}

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161 molecular orbitals are transformed into a new set of localized orbitals by using IboView,10,11

without perturbing the exact representation of the Kohn-Sham wave function. By analyzing the changes of key IBOs along the IRC (IBO analysis), detailed insight into reaction mechanisms can be obtained. For example, in the oxidation of C(sp3_{)-H bonds by both lipoxygenase and the}

high-valent oxoiron(IV) intermediate TauD-J, a net H-atom is transferred from the C(sp3_)-H

bond. By employing the IBO method, Klein and Knizia were able to unambiguously show that for the oxidation of C(sp3_{)-H bond by lipoxygenase a cPCET mechanism is operational,}

whereas this oxidation by the high-valent oxoiron(IV) intermediate TauD-J is mechanistically distinct and occurs by a HAT process.12

In chapter 3, we showed that the ligand in formazanate boron diphenyl compound 1 could

sequentially store two electrons (2e-_{) and one proton (H}+_{) (i.e., [2e}-_/H+_{] equivalent), and led to}

formation of ligand-protonated product H₁-_{(Scheme 7.2).}13,14_{In addition, we showed that the}

reaction of H₁-_{with TEMPO resulted in formation of boron formazanate radical}

(‘borataverdazyl’) species (1.-_{) and TEMPO-H by the net H-atom transfer from}H₁-_{to TEMPO}

(Scheme 7.2).14_{This reaction revealed that the [2e}-_/H+_{] stored in}H₁-_{could be converted to [1e}

-/H+_{], i.e. a H}•_{radical, and this type of reaction has potential relevance in energy storage}

applications such as hydrogen evolution reaction (HER). On the other hand, this net H-atom transfer from H₁-_{to TEMPO can occur by two distinct classes of mechanisms, either by}

hydrogen atom transfer (HAT) or by concerted proton-coupled electron transfer (cPCET). The

N N N N Ph Ph p-tol B Ph Ph N N N N Ph Ph p-tol B Ph Ph 2e -2 1 2-N N N Ph Ph p-tol B Ph Ph H₁ -H 1

ligand-based 2e- storage ligand-based [2e-/H+] storage

N N N N N Ph Ph p-tol B Ph Ph 1 .-TEMPO .-TEMPOH HAT or cPCET? H+

Scheme 7.2 Sequential ligand-based storage of [2e-_/H+_{] in}_{1, and subsequent net H-atom}

transfer from H₁-_{to TEMPO.}

knowledge of mechanistic details (HAT or cPCET) by which net H-atom is transferred may allow to gain better insights into the electronics of H₁-_{and to provide clue/s for tuning the}

electronic properties of H₁-_{. In this chapter, we present the results of IBO analysis on the}

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162

7.2 Mechanistic details of net H-atom transfer from

H

₁

-

_{to TEMPO by}

intrinsic bond orbital (IBO) method

7.2.1 Computational details

Geometry optimizations were performed using the electronic structure code Gaussian 16, Revision B.01.15_{The transition state of net H-atom transfer from}H₁- _{to TEMPO was computed}

using the M06-L functional16_and_{def2-SVPD basis set.}17_{The SMD solvation model has been}

used for tetrahydrofuran (THF) solvent.18_{The transition state structure was confirmed by}

frequency analysis (number of imaginary frequencies = 1). Intrinsic reaction coordinates (IRCs) were computed using M06-L/def2-SVP/SMD level of theory, and by employing Hessian based predictor–corrector algorithm (HPC)19–21_{for up to 100 steps in each direction. The HPC}

algorithm can predict very accurately and efficiently the IRC of those reactions for which the previously optimized transition state (TS) geometry is used as a starting point to trace back the reactants and products (two energy minima on the potential energy surface) by following forward and reverse reaction pathways. For all calculations an ultrafine integration grid was used.

For the IBO analysis, Kohn-Sham wavefunctions for all structures along the IRC were computed at the M06-L16_/def2-SV(P)22_/SMD18_{level of theory using the electronic structure}

code ORCA 4.1.2.23,24_{Grid 7 was employed in all calculations. Then, from these delocalized}

Kohn-Sham wave functions, intrinsic bond orbitals (IBOs)9_{(iboexp = 2) were generated using}

IboView.11,10

7.2.2 Intrinsic bond orbital (IBO) analysis to elucidate the mechanism of net

H-atom transfer from

H

₁

-

_{to TEMPO}

To elucidate the mechanism of the net H-atom transfer from ligand-protonated formazanate boron compound H₁-_{to TEMPO,}14_{by IBO method,}9_{first we have generated IBOs for the α and}

β spin manifold (see computational details section for the generation of IBOs). Next, we have identified the localized orbitals of the N-H σ-bond, and then followed the changes that they undergo along the IRC. Also, the plot of root mean square deviation of the orbital partial charge distribution of IBOs along the IRC is analyzed which immediately provides information to estimate by how much each IBO changes and information about the IBOs which are participating in bond making and bond breaking along the reaction path. Two scenarios can be envisioned (HAT and cPCET) that can be distinguished as follows. If hydrogen-atom

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163 abstraction occurs by HAT, in which the electron and proton are transferred together,5–7_then

one electron of the IBO that represents the N-H σ-bond should be transferred and incorporated into the newly formed O-H bond in the product TEMPO-H. The other electron of this O-H bond should derive from the unpaired electron on oxygen atom of TEMPO. On the other hand, if N-H bond breaking occurs by a cPCET mechanism (i.e., the electron and proton do not travel together)5–7_{then none of the electrons forming the product O-H bond should originate from the}

N-H σ-bond in the starting material.

Keeping these characteristic features of HAT and cPCET mechanisms in mind, we have analyzed the changes of IBOs along the IRC for this net H-atom transfer from H₁-_{to TEMPO.}

The changes of the α and β spin IBOs of the N-H bond along the IRC are shown in Figure 7.1 (a), which reveal that neither α nor β electron of the N-H bond is transferred: both electrons remain on the formazanate ligand as a lone pair on N in the product 1•-_{. The changes of the}

O-centered unpaired electron of TEMPO along IRC (Figure 7.1 (b)) indicate that, as expected, this becomes part of the newly formed O-H bond. Monitoring the changes for the IBO that represents a doubly-occupied (polarized) N=C π-bond in the formazanate fragment (Figure 7.1 (c)) shows that one of these electrons gets transferred to form the O-H bond, whereas the other remains on the formazanate ligand in 1•-_{. Thus, analysis of the changes of the relevant IBOs}

along the IRC suggest that the proton and the electron do not travel together, and, thus, this reaction takes place by cPCET rather than HAT. Furthermore, the plot shown in Figure 7.2 (left) reveals that the electron flow for this PCET process is continuous, which also suggests that the proton and the electron are transferred in a concerted fashion.11,12

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164

Figure 7.1 Net H-atom transfer from H₁-_{to TEMPO. a) Changes of N-H IBO along IRC (α IBO}

purple and β IBO golden). b) Changes of IBO of O-centered unpaired electron of TEMPO along IRC (blue) which reveals the participation of this unpaired electron in the formation of newly formed O-H bond. c) Changes of IBO of lone paired electrons of the ligand-protonated N-atom along IRC (α IBO red and β IBO green), which shows the transfer of an electron from the lone paired electrons of the ligand-protonated N-atom for the formation of newly O-H bond. All hydrogen atoms except for N-H hydrogen, are omitted for clarity. Colors used for the elements as follows: N, cyan; C, gray; B, green; O, red.

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165

Figure 7.2 Left: Plot of changes of energies along the IRC (black circles), and plot of the root

mean square deviation of the orbital partial charge distribution of N-H (purple and golden colors line circles), of O-centered unpaired electron of TEMPO (blue line circles), and of lone paired electrons of the ligand-protonated N-atom (green and red line circles) IBOs along the IRC. Right: Plot of changes of energies (black circles), and of the magnitude of change in the dipole moment (red line circles) along the IRC.

N N N N Ph Ph p-tol B Ph Ph cPCET 1 .-N N N Ph Ph p-tol B Ph Ph H₁ -H N N O + + _O _N H TEMPO TEMPOH

Scheme 7.3 Representation of a net H-atom transfer from H₁-_{to TEMPO via cPCET mechanism.}

In 2010, Hammes-Schiffer and coworkers demonstrated that the plot of variation in the dipole moment along the H-atom donor axis could be useful to tool for distinguishing between cPCET and HAT mechanisms.8_{They utilized the self-exchange reactions in the phenoxyl/phenol and}

benzyl/toluene systems. They described that the change in the magnitude of dipole moment along the H-atom donor axis is significantly larger for the phenoxyl/phenol system than that for the benzyl/toluene system. They interpreted this outcome as follows: For the cPCET process, a proton (i.e., a positively charged hydrogen) and an electron travel simultaneously, but following separate paths, and thus, during this process a significant change in the electronic charge on the donor and acceptor molecules occurs. In contrast, for the HAT mechanism, a proton and an electron travel together as H-atom (i.e., a neutral hydrogen), and thus, for the HAT process there is no significant change in the electronic charge on the donor and acceptor molecules. Thus, the

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166

change in the magnitude of dipole moment along the H-atom donor axis is much larger for cPCET than that for the HAT. Based on this result, they had further confirmed that a cPCET and a HAT mechanism is operational in the phenoxyl/phenol and benzyl/toluene system, respectively.8

In order to further support the H-abstraction from H₁-_{to TEMPO via cPCET, we have also}

analyzed, and plotted the variation of dipole moment along the N-H bond (Figure 7.2 (right)), which shows that similar to the phenoxyl/phenol system,8_{a significant change in the dipole}

moment during reaction. Thus, the above observation indicates that a cPCET mechanism is the most appropriate description for the H-abstraction from H₁-_{to TEMPO.}

The transfer of a net H-atom from H₁-_{to TEMPO via cPCET is schematically presented in}

scheme 7.3.

As mentioned in the introduction section, PCET has fundamental importance in variety of chemical, industrial and biological processes, there are numerous transition metal complexes known in the literature that are involved in this type of reactions.1–5_{In recent years, a few}

complexes having non-innocent ligands which show H-atom transfer reactivity have been reported.25–27_{In contrast to our system, in those complexes the locus of reduction and}

protonation are metal centers and ligand backbones, respectively. Also, only for few of these complexes the actual mechanism (HAT or cPCET) has been addressed or investigated.25–27_For

example, in 2018 Waymouth and coworkers showed that the dicationic Co(III) complex bearing non-innocent azopyridine ligand could be reduced by two electrons and subsequently protonated to give a Co(I) complex with a protonated azopyridine ligand.25,28_{In this example,}

reduction and protonation occurs at the metal center and ligand backbone, respectively. They described that the product, a ligand-protonated cationic Co(I) complex, can act as a proton, hydrogen atom, and hydride donor.25_{For this H-atom transfer reaction, whether the mechanism}

is HAT or PCET has not been investigated yet.

7.3 Conclusions.

Our computational studies (IBO analysis) on the mechanism of a net H-atom transfer from H₁

-to TEMPO reveal that independent transfer of a pro-ton and an electron transfer occurs in a concerted manner. Thus, our result describes that the H-abstraction from H₁-_{to TEMPO occur}

via concerted PCET (cPCET). This result may provide a better insight into the electronics of

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167 functionalized formazanate ligands in such a way so that instead of [1e-_/1H+_{] (i.e., H-atom)}

transfer reaction, we can facilitate multi-electron transfer reactions (e.g., hydrogen evolution reaction: [2e-_/2H+_{] = H}₂_{). These reactions have direct implication in the energy storage}

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168

7.4 References

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