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 6

Cation effects on dynamics of

ligand-benzylated formazanate boron and

aluminium complexes

The dynamic processes present in ligand-benzylated formazanate boron and aluminium complexes are investigated. The variable temperature NMR experiments and subsequent NMR lineshape analyses allowed to determine the activation parameters for the nitrogen inversion processes present in these ligand-benzylated products. The activation parameters for the nitrogen inversion processes are compared between boron and aluminum complexes. The influence of counter-cations on the dynamics of these anionic B/Al complexes has been examined.

This chapter will be submitted for publication:

Ranajit Mondol and Edwin Otten* “Cation effects on dynamics of ligand-benzylated formazanate boron and aluminium complexes.”

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134

6.1 Introduction

Alkali-metal cations have important roles in biological chemistry1–3_{and synthetic chemistry}4– 6_{. The noncovalent interactions of organic moieties with alkali-metal cations could influence}

electronic structure of complexes in such an extent that the geometry,7,8_{redox potentials,}9_N₂

cleavage,10_{reaction rates,}11–14_{and even selectivity of chemical transformations}15–17_{are greatly}

affected. For example, in 2018, Holland and coworkers described a series of reduced iron complexes bearing a redox- active formazanate ligand having alkali-metal cations as counter cations.18_{In the presence of a crown ether which sequestered the alkali cation, the reduced iron}

complex was obtained in monomeric form, whereas in absence of a crown ether the reduced iron complex was isolated in dimeric form (Chart 6.1, A).18_{They demonstrated that in dimeric}

reduced iron complexes, the binding mode of counter cations to the ligand backbones is dependent on the nature of the alkali-metal (Chart 6.1).18_{Their further study revealed that in}

solution, the decay of these dimers into their corresponding monomers is also dependent on the binding mode and size of the alkali cation.18

Chart 6.1 N FeII _N N N Me3Si SiMe3 N _FeII N N N SiMe3 SiMe3 p-tol N N (THF)3 Na (THF)Na 3 p-tol N FeII N N N Me3Si Me3Si N FeII N N N SiMe3 SiMe3 p-tol N N p-tol (THF)2 M M(THF)n M = K; B Holland A M = Rb; C M = Cs; D

More than a decade ago, in 2007, Hicks and coworkers described the ligand-based redox-reactions in formazanate boron diacetate compound.19_{This study for the first time revealed the}

redox-active property of formazanate ligands. In 2014, our group has started to investigate the coordination chemistry, ligand-centered reductions and ligand-based reactivity of compounds with formazanate ligands.20–29_{Concurrent with our work, the Gilroy group}30–37_{and others}38–40

have synthesized variety of compounds with formazanate ligands, and studied their optical and electrochemical properties for their further applications. Previously, we described ligand-based sequential storage of “two-electron and one electrophile (i.e., 2e-_/E+_{, where E}+_{= Bn}+_{, H}+_{)” in}

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135 boron and aluminium compounds with formazanate ligands (Scheme 6.1).41–44_{Furthermore, we}

have demonstrated that the ligand-centered stored “2e-_/E+_{” could be converted to Bn}•_{and H}•

radicals via the homolytic cleavage of the N-C(Bn) and N-H bonds, respectively (Scheme 6.1).42_{Additionally, we have observed that at room temperature the}1_{H NMR (400MHz,}

THF-d8) spectrum of [Bn1][Na] has several broadened resonances, which is an indication for the

presence of chemical exchange process within [Bn_1][Na].42_{In this chapter, we present the}

detailed analysis of this chemical exchange process by variable temperature NMR experiment and NMR lineshape analysis, which reveals that nitrogen inversion is responsible for the observed dynamics in [Bn_{1][Na]. The effects on the nitrogen inversion process due to the} replacement of central element boron in [Bn_{1][Na] to aluminum will be described. Also, the} influence of the different counter cations on the nitrogen inversion has been studied.

N N N N Ph Ph p-tol Z Ph Ph N N N N Ph Ph p-tol Z Ph Ph 2e -2 [1/2][Na2] N N NN Ph Ph p-tol Z Ph Ph [Bn_1/Bn_2][Na] (E = CH2Ph) E-X, THF -NaBr Z = B; 1 Z = Al; 2 ligand-based strorage of 2e -ligand-based strorage of 2e-/E+ (THF)3Na _Bu 4NBr, THF, -NaBr [Bn1/Bn_2][NBu4] Bu4N E _N N N N Ph Ph p-tol Z Ph Ph Ph E-X = PhCH 2-Br, H-OH [H1][Na] (E = H) E = CH2Ph

Scheme 6.1 Ligand-centered storage of [2e-_/E+_{] in boron and aluminum compounds with}

formazanate ligands

6.2 Investigation of nitrogen inversion in ligand-benzylated formazanate

boron and aluminium complexes

6.2.1 Investigation of nitrogen inversion in [

Bn

_{1][Na] and [}

Bn

_1][NBu

₄

_]

In Chapter 3, the synthesis and characterization of the ligand-benzylated compound [Bn_LBPh₂_]

-([Bn_{1][Na], Scheme 6.1) was reported. The}1_{H NMR (400 MHz, THF-d}₈_{) spectrum of}_[Bn_1][Na] at room temperature showed that the resonances for the diastereotopic benzyl-CH2 as well as

for the BPh2 moiety are broadened, likely due to chemical exchange. To elucidate this dynamic

process a variable temperature NMR (VT-NMR) experiment (500 MHz, THF-d8) in the

temperature range between -5 and +75 °C has been performed. By cooling the NMR sample inside the NMR probe to -5 °C, two sharp doublets are observed for the diastereotopic benzyl-CH2 at 3.79 and 3.38 ppm with the mutual coupling constant of 2JHH = 15.3 Hz (Figure 6.1).

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136 -5 °C 15 °C 40 °C 25 °C 65 °C 75 °C

singlet at 3.69 ppm. With this temperature variations, similar changes are also observed for the resonances of the BPh2 moiety: Two distinct BPh resonances are observed at -5 °C, which are

also coalesced to a single set of resonances for the BPh2 moiety at ca. 65 °C (Figure 6.1).

Lineshape analysis has been performed for both the exchanging resonances of the benzyl-CH2

group as well as the ortho-protons of the two B-Ph groups in the temperature range between -5 and 75 °C. The rate of chemical exchange of both pairs is found to be identical across the entire temperature range, which means that a single dynamic process is responsible for the observed broadening in the resonances for the diastereotopic benzyl-CH2 and for the BPh2 moiety. The

subsequent Eyring analysis gives activation parameters for the exchange process in [Bn_1][Na] as ΔH‡_{= 80.4 (1.7) kJ/mol and ΔS}‡_{= 59.3 (5.6) J/mol/K (Figure 6.2 and see SI for details).}

3.2 3.6 4.0 4.4 4.8 5.2 5.6 6.0 6.4 6.8 7.2 7.6 8.0 1 2 3 4 5 6

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137

Figure 6.2 Eyring plot for the determination of activation parameters for the N-inversion in [Bn_1][Na]

The large positive ΔS‡ _{value indicates that rate determining step of this dynamic process could}

be the homolytic cleavage of N-C(Bn) bond, which generates benzyl radical (Bn•_{) and the}

planar boron formazanate radical (‘borataverdazyl’) species (1.-_{) (Scheme 6.2, (A)).}42_{Then, the}

generated Bn• _{radicals are recombined with}₁.-_{either from top or from bottom side of the ligand} backbone (i.e., NNCNN) of compound 1.-_{(Scheme 6.2, (A)). So, this recombination process} could be a possible explanation for the observed dynamics in [Bn_{1][Na]. But, our previous study} (Chapter 3) revealed that ΔH‡ _{value for the homolytic cleavage of N-C(Bn) bond in}_[Bn_1][Na] is = 121 (5) kJ/mol,42_{which is relatively larger than the obtained ΔH}‡ _{value for the dynamic}

process operating in [Bn_{1][Na]. Thus, on the basis of the discrepancy between these ΔH}‡ _values,

the homolytic cleavage of N-C(Bn) bond in [Bn_{1][Na] is ruled out as a reason for the observed} dynamics in [Bn_1][Na].

Alternatively, the observed dynamics in [Bn_{1][Na] could be due to the pyramidal inversion at} the benzylated N-atom in [Bn_{1][Na]. In order to invert, the pyramidal geometry around the} inverting N-atom in [Bn_{1][Na] should go through a trigonal planar transition state which then} reverts back to the lower energy pyramidal geometry (Scheme 6.2, (B)). Thus, during this entire

inversion process the inverting N-atom changes its hybridization from sp3 _{to sp}2_{to sp}3 _{and the}

orbital which contains the lone pair on the inverting N-atom changes its hybridization from sp3

to p to sp3_{(Scheme 6.2, (B)). Previously, we demonstrated that in the solid state structure of}

[Bn_{1][Na], the BPh}₂_{moieties and the benzyl group pointed toward opposite directions from the} ligand core backbone (i.e., NNCNN)44_{to maintain minimum steric repulsion between them.}

We anticipate that [Bn_{1][Na] maintains its solid state structure also in the solution. Due to the} inversion process, the benzyl group is forced to move from one side to other side of the ligand

ΔH‡_{= 80.4 (1.7) kJ/mol}

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138

core backbone via a trigonal planar transition state, which is depicted in scheme 6.2 (B). The observation that the rate constant for BPh2 exchange is identical to that for the diastereotopic

CH2Ph group indicates that the movement of the BPh2 part of the molecules occurs in a manner

that is correlated (‘geared’) to the N-inversion, and suggests that it occurs through a species with overall Cs-symmetry (Scheme 6.2, (B)).

N N N N Ph Ph p-tol Z Ph Ph Bn N N N N Ph Ph p-tol Z Ph Ph Bn ‡ N N N N Ph Ph p-tol Z Ph Ph Z = B; Bn₁ -Z = Al; Bn₂- 1 .-_/2 .-Bn Bn N N N N Ph Ph p-tol Z Ph Ph Bn bottom top Bn N N N N Ph Ph p-tol Z Ph Ph Bn Bn. attacks from bottom Bn. attacks from top or Bn₁-_/Bn₂ -(A) (B) N N N Ph Ph p-tol Z Ph Ph ‡ Z = B; Bn₁ -Z = Al; Bn₂ -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

Homolytic N-C(Bn) bond cleavage

Nitrogen inversion

Scheme 6.2 Plausible mechanistic pathways for the observed dynamics in Bn₁-_andBn₂- The nitrogen inversion process shown in Scheme 6.2 (B) is an intra-molecular process, and for an intra-molecular process ΔS‡ _{value is generally small or close to zero.}45,46_{Moreover, the}

suggested inversion transition state would be expected to be quite strained , and ΔS‡ _{should be}

negative.45,46_{But, the ΔS}‡_{determined experimentally is a large positive quantity (59.3 (5.6)}

J/mol/K), which indicates that in the rate determining step either the dissociation of coordinated sodium cation from the [Bn_LBPh₂_]-_{moiety, coordinated through one of the internal N-atom in}

the ligand backbone or the release of THF molecules from the Na(THF)3 moiety into the bulk

solution (or both of these processes) are involved. In order to investigate these possibilities, we have examined the nitrogen inversion process in [Bn_1][NBu₄_{], which was prepared following} the procedure described in chapter 5.44_{In this compound, the organic cation Bu}₄_N+_{does not}

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139 solvent-separated ion pair in solution; Scheme 6.1).44

The activation parameters (ΔH‡ _{and ΔS}‡_{) for the nitrogen inversion process in}_[Bn_1][NBu₄_{] have} been determined following the same methodology as before, which led to ΔH‡_{= 66.7 (1.6)}

kJ/mol and ΔS‡_{= -4.2 (5.0) J/mol/K (see SI for details). The small, negative value for ΔS}‡ _is

consistent with the intra-molecular nitrogen inversion process as in this process the molecule has to pass through a more congested trigonal planar transition state from its pyramidal ground state. The comparison of ΔS‡_{values for the nitrogen inversion process in}_[Bn_{1][Na] (ΔS}‡_{= 59.3}

(5.6) J/mol/K) and in [Bn_1][NBu₄_{] (ΔS}‡_{= -4.2 (5.0) J/mol/K) reveals that dissociation of the}

sodium cation from the [Bn_LBPh₂_]-_{moiety occurs in the rate determining step for the nitrogen}

inversion process in [Bn_{1][Na]. Thus, this finding also instigates us to further investigate the} influence of alkali-metal cation on this nitrogen inversion process, the details of which are

presented in the section 6.2.3.

This activation energy for the nitrogen inversion process in [Bn_1][NBu₄_{] (i.e., ΔG}‡ _{(298K) =}

63.1 kJ/mol) is larger than the activation energy for the nitrogen inversion process found in amines (in the range of 20.9-41.8 kJ/mol) by 21.3-42.2 kJ/mol,46–49_{and in}

2,3-diazabicyclo[2.2.2]octane, 2,3-dimethyl (ΔG‡ _{= 49.4 kJ/mol) by 13.7 kJ/mol. In case of}

[Bn_1][NBu₄_{], in the transition state of nitrogen inversion process, not only the inverting N-atom} but also the remaining part of the molecule adopts a planar geometry, which leads to a significant ring strain within six-membered chelate ring of [Bn_1][NBu₄_{]. In contrast, in case of} amines and 2,3-diazabicyclo[2.2.2]octane, 2,3-dimethyl, in the transition state of nitrogen inversion process, only the inverting N-atom has to move through a planar geometry from their pyramidal ground states. Thus, in case of open-chain amines there are no contribution to the activation barrier for the inversion process due to the ring strain, and due to this reason the activation barrier for the inversion process is significantly higher in [Bn_1][NBu₄_{] than in amines.} But, the differences in the activation energies for the nitrogen inversion processes between

[Bn_1][NBu₄_{] and 2,3-diazabicyclo[2.2.2]octane, 2,3-dimethyl is relatively smaller (13.7 kJ/mol)} than between [Bn_1][NBu₄_{] and amines (21.3-42.2 kJ/mol), which is due to the following} reason: As in case of 2,3-diazabicyclo[2.2.2]octane, 2,3-dimethyl two nitrogen invert simultaneously, in the transition state there is a eclipsed methyl-methyl interaction, which further increases the energy barrier for the inversion process.

6.2.2 Investigation of nitrogen inversion in [

Bn

_{2][Na] and [}

Bn

_2][NBu

₄

_]

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140

the less electropositive element boron in Bn₁-_{with the more electropositive aluminium,} ligand-benzylated formazanate aluminium compounds [Bn_{2][Na] and [}Bn_2][NBu₄_{] are synthesized} according to the published procedures (see chapter 5).44_{The VT-NMR experiments (500MHz,}

THF-d8) of [Bn2][Na] and of [Bn2][NBu4] in the temperature ranges of 40-75 °C and 35-85 °C, respectively, reveal that chemical exchange are also occurs in these compounds (Figure S6.10 and Figure S6.9). The NMR lineshape analyses and subsequent Eyring plots affords activation parameters for the nitrogen inversion processes in [Bn_{2][Na] as ΔH}‡ _{= 88.9 (2.5) kJ/mol and}

ΔS‡_{= 56.5 (7.3) J/mol/K. The corresponding values for}_[Bn_2][NBu₄_{] are ΔH}‡ _{= 68.4 (1.3) kJ/mol}

and ΔS‡_{= -19.7 (3.9) J/mol/K (Figure S6.3 and see SI for details).}_{Similar to what was observed}

in the boron analogues, the change in counter cation from Na+_{to Bu}₄_N+_{results in markedly}

different activation entropies for these two compounds: The ΔS‡_{is large/positive when the}

cation (Na+_{) is bound to the ligand backbone (i.e., for}_[Bn_{2][Na] ΔS}‡_{= 56.5 (7.3) J/mol/K),}

whereas it is small/negative when the cation (Bu4N+) does not interact with the ligand backbone

(i.e., for [Bn_{2][ NBu}₄_{] ΔS}‡_{= -19.7 (3.9) J/mol/K). Thus, these experimentally determined ΔS}‡

values for the nitrogen inversion processes in [Bn_{1][Na] (ΔS}‡_{= 56.5 (7.3) J/mol/K)}_{and in}

[Bn_1][NBu₄_{] (ΔS}‡_{= -19.7 (3.9) J/mol/K) indicates that similar to the boron analogues, the rate}

determining step for the nitrogen inversion process in [Bn_{2][Na] is also dissociation of the} sodium cation from the [Bn_LAlPh₂_]-_{moiety. A comparison of activation enthalpies for nitrogen}

inversion processes in [Bn_1][NBu₄_{] (ΔH}‡_{= 66.7 (1.6) kJ/mol}_{) and in [}Bn_2][NBu₄_{] (ΔH}‡ _{= 68.4}

(1.3) kJ/mol) indicates that the effect of the central element on nitrogen inversion is rather small.

6.2.3 Exploration of alkali-metal cations effects on the dynamic process

The effect of alkali-metal cations on the dynamic process in ligand-benzylated formazanate boron compounds has been explored. In this context at first, two-electron reduced formazanate boron compounds having potassium cation (K+_{) and rubidium cation (Rb}+_{) as counter cations}

(i.e., [1][K2] or [1][Rb2], Scheme 6.3) have been prepared by the reduction of neutral formazanate boron compound 1 by two equivalents of potassium graphite (KC8) and rubidium

graphite (RbC8), respectively (Scheme 6.3). The compounds [1][K2] and [1][Rb2] are characterized by 1_{H NMR (in THF-d}₈_{) and UV/Vis spectroscopy (in THF). The characteristic} 1_{H NMR and UV/Vis data of}_[1][K₂_{] and [1][Rb}₂_{] are similar to that of [1][Na}₂_]41_(see

experimental section, and Figure S6.1), suggesting that the structures of this series of compounds in solution does not depend on the nature of the cation. Then, both [1][K2] and

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141 expected color change from orange to green for both reactions,42,44_{and these observations}

primarily suggest the formation of ligand-benzylated products [Bn_{1][K] and [}Bn_1][Rb], respectively (Scheme 6.3). The formation of ligand-benzylated products [Bn_{1][K] and [}Bn_1][Rb] is further confirmed via NMR and UV/Vis spectroscopic characterization techniques. Similar to [Bn_1][Na],42_the1_{H NMR spectra of both of the isolated ligand-benzylated products}_[Bn_1][K] and [Bn_{1][Rb] at room temperature (400MHz, THF-d}₈_{) show two broad resonances for the} diastereotopic protonsof the benzyl-CH2 group at δ 3.46 and 3.77 ppm, and 3.46 and 3.76 ppm,

respectively (see experimental section). Furthermore, the 1_{H NMR spectrum of the}

ligand-benzylated product [Bn_{1][K] displays two resonances for the para-protons of N-Ph groups at δ} 6.10 and 6.16 ppm, respectively, and the 1_{H NMR spectrum of the ligand-benzylated product}

[Bn_{1][Rb] shows one overlapped resonance for the para-protons of N-Ph groups at 6.15 ppm} (see experimental section). The UV/Vis absorbance spectra of both [Bn_{1][K] and [}Bn_1][Rb] display absorption bands ca. 396 nm (Figure S6.1), and the compound [Bn_{1][Na] also showed} an absorbance band at ca. 396 nm,42_{which further supports the structural similarity for these}

compounds. N N NN _Ph Ph p-tol B Ph Ph 2 MC8 N N N N Ph Ph p-tol B Ph Ph M(THF)x x(THF)M THF, RT RT, THF BnBr - MBr N N NN _Ph Ph p-tol B Ph Ph Ph x(THF)M M = K, Rb [1][K2]/[1][Rb2] 1 [Bn1][K]/[Bn_1][Rb]

Scheme 6.3 Synthesis of two-electron reduced formazanate boron compounds as their

dipotassium and dirubidium salts ([1][K2]/[1][Rb2]) and their corresponding ligand-benzylated products ([Bn_1][K]/[Bn_1][Rb])

Finally, in order to investigate the influence of alkali-metal cations on the dynamics of ligand-benzylated formazanate boron compounds, the activation parameters for the exchange processes in [Bn_{1][K] (ΔH}‡ _{= 68.4 (2.3) kJ/mol and ΔS}‡_{= 59.3 (7.3) J/mol/K) and in}_[Bn_1][Rb] (ΔH‡ _{= 72.7 (5.5) kJ/mol and ΔS}‡_{= 42.3 (18.0) J/mol/K)}_{have been determined (see SI for}

details). Regardless of the alkali-metal cation (Na+_{, K}+_{or Rb}+_{), the activation entropy is large}

and positive, which we interpret as an indication for dissociation to the corresponding ion pairs being involved in the transition state. The results furthermore reveal that on going from

[Bn_{1][Na] to [}Bn_{1][K], the ΔH}‡_{value for the exchange process is decreased by ca. 12.0 kJ/mol}

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142

[Bn_{1][Na] to [}Bn_{1][K] indicates that the sodium cation interacts more strongly with the anionic} [Bn_LBPh₂_]-_{moiety than the potassium cation. This result is consistent with the literature which}

demonstrated that for the alkali-metal cations, the cation-anion interaction decreases in the order of Li+_{> Na}+_{> K}+ _{> Rb}+ _{≈ Cs}+_,50_{whereas, this order is reversed for the cation-π interaction}

(i.e., Li+_{< Na}+ _{< K}+ _{< Rb}+ _{≈ Cs}+_).50,51 _{The Holland group also described that for formazanate}

iron complexes containing alkali-metal cations as counter cations, the order of the cation-anion interaction and the cation-π interaction is similar as found in the cation-anionic benzyl-based derivatives.18_{Our group also observed that in the alkali-metal salts of formazanate ligands, the}

number of cation-anion interactions is decreased on going from sodium cation to potassium cation, whereas, this number is reversed for the cation-π interactions.32_{However, ΔH}‡_values

for this exchange process in [Bn_{1][K] and [}Bn_{1][Rb] are similar. From the above mentioned} literatures precedent,18,32,50,51_{it can be anticipated that the cation-anion interaction in [}Bn_1][Rb] is lower than in [Bn_{1][K], and the corresponding cation-π interaction in [}Bn_{1][Rb] is higher than} in [Bn_{1][K]. Thus, the similar ΔH}‡_{value for the exchange process on going from}_[Bn_{1][K] to}

[Bn_{1][Rb] indicates that the strength of the combined interaction (i.e., cation-anion interaction} and cation-π interaction) are similar in [Bn_{1][Rb] and in [}Bn_1][K].

6.3 Conclusions

In conclusion, our NMR study reveals that the observed dynamics in the 1_{H NMR spectra of}

ligand-benzylated formazanate boron and aluminium compounds (i.e., Bn₁- _andBn₂-_{) is due to} the nitrogen inversion processes present in these compounds rather than homolytic N-C bond cleavage. From this data, a plausible mechanism has been deduced to illustrate how nitrogen inversion processes are operating in Bn₁- _andBn₂-_{. In}_[Bn_{1][Na] and [}Bn_{2][Na], the counter cation} (Na+_{) is bound to the ligand backbone, and activation entropies (ΔS}‡_{) for the nitrogen inversion}

process in [Bn_{1][Na] and in [}Bn_{2][Na] are large/positive. In [}Bn_{1][ NBu}₄_{] and in [}Bn_{2][ NBu}₄_], the counter cation (Bu4N+) does not interact with the ligand backbone, and activation entropies

(ΔS‡_{) for the nitrogen inversion process in}_[Bn_{1][ NBu}₄_{] and in [}Bn_{2][ NBu}₄_{] are small/negative.} Thus, the remarkable changes in activation entropies for the nitrogen inversion process in Bn₁

-/Bn₂-_{due to the change in counter cation Na}+ _to_Bu₄_N+_{reveals that the dissociation of counter} cation (Na+_{) from the [}Bn_LBPh₂_]-_/[Bn_LAlPh₂_]-_{moiety occurs in a rate determining step that}

precedes nitrogen inversion process. Our study reveals that there is no effect on the nitrogen inversion process due to the replacement central element boron to aluminium in ligand-benzylated formazanate compounds. The variation of alkali-metal cations in Bn₁-_{show effects}

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143 on the exchange process, and reasons for this influence have been described.

6.4 Experimental section

6.4.1 General considerations

All manipulations were carried out under nitrogen atmosphere using standard glovebox, Schlenk, and vacuum-line techniques. The compounds 1, [Bn_{1][Na], [}Bn_1][NBu₄_],_[Bn_2][Na]_and [Bn_2][NBu₄_{] are prepared according to published procedures.}41,42,44 _{Potassium graphite (KC}₈₎

and Rubidium graphite (RbC8) were synthesized following the published procedures.18 THF

and hexane (Aldrich, anhydrous, 99.8%) were passed over columns of Al2O3 (Fluka). The

solvents (THF, hexane, THF-d8) used in this study were additionally dried on Na/K alloy and

subsequently vacuum transferred, degassed and stored under nitrogen. NMR spectra were recorded on Varian Mercury 400 or Inova 500 spectrometers. The 1_{H and}13_{C NMR spectra}

were referenced internally using the residual solvent resonances and reported in ppm relative to TMS (0 ppm); J is reported in Hz. Assignments of NMR resonances was aided by COSY, NOESY, HSQC and HMBC experiments using standard pulse sequences. UV-vis spectra were recorded in THF solution (~ 10-3_{M) in a quartz cuvette that was sealed under N}₂_atmosphere

using an AVANTES AvaSpec-2048 spectrometer.

6.4.2 Compounds synthesis and characterization

Synthesis of [LBPh2][K2(THF)2] ([1][K2]). Compound 1 (100.0 mg, 0.21 mmol) was dissolved in a 4 mL THF in a vial inside the glove box. To the dark pink color THF solution of

1 was added 2.2 equivalents of KC8 (62.5 mg, 0.46 mmol). The reaction mixture was stirred for

overnight, during which it changed color to orange. Then the orange color solution was filtered through a plug of celite. After concentrating the filtrate under vacuum, a layer of hexane (3 mL) was added onto the THF solution (filtrate), and allowed the two layers to diffuse slowly by keeping the mixture at -30 °C. Even after keeping for 7 days, no crystalline materials were afforded, only the orange color precipitates were obtained. The precipitates were washed with pentane (3 x 2 mL) and dried to give compound [1][K2] as a powder material (72.0 mg, 0.10 mmol, 48 %). 1_{H NMR (400 MHz, THF-d}₈_{, 25 °C) δ 8.21 (d, J = 7.8 Hz, 2H, p-tol o-H), 7.52}

(d, J = 7.1 Hz, 4H, BPh o-H), 6.91-6.96 (m, 10H, p-tol m-H, BPh m-H, NPh o-H ), 6.85 (t, J = 7.0 Hz, 2H, BPh p-H), 6.49 (t, J = 7.7 Hz, 4H, NPh m-H), 5.82 (t, J = 6.9 Hz, 2H, NPh p-H), 3.55-3.59 (m, 7H, THF), 2.23 (s, 3H, p-tol CH3), 1.71-1.75 (m, 7H, THF). 11B NMR (128.0

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144

Ph), 153.21 (N-C(ipso) Ph), 147.97 (NCN), 142.22 (p-tol C(ipso)-NCN), 135.29 (BPh o-CH),

134.41 (p-tol C-CH3), 127.78 (p-tol m-CH), 127.66 (NPh o-CH), 127.33 (BPh m-CH), 126.38

(p-tol o-CH), 124.24 (BPh p-CH), 115.34 (NPh m-CH), 110.16 (NPh p-CH), 68.27 (THF),

26.43 (THF), 21.31 (p-tol CH3).

Synthesis of [LBPh2][Rb2(THF)2] ([1][Rb2]). Compound 1 (100.0 mg, 0.21 mmol) was dissolved in a 4 mL THF in a vial inside the glove box. To the dark pink color THF solution of

1 was added 2.2 equivalents of RbC8 (83.5 mg, 0.46 mmol). The reaction mixture was stirred

for overnight, during which it changed color to orange. Then the orange color solution was filtered through a plug of celite. After concentrating the filtrate under vacuum, a layer of hexane (3 mL) was added onto the THF solution (filtrate), and allowed the two layers to diffuse slowly by keeping the mixture at -30 °C. Even after keeping for 7 days, no crystalline materials were afforded, only the orange color precipitates were obtained. The precipitates were washed with pentane (3 x 2 mL) and dried to give compound [1][Rb2] as a powder material (75.0 mg, 0.09 mmol, 43 %). 1_{H NMR (400 MHz, THF-d}₈_{, 25 °C) δ 8.28 (d, J = 8.0 Hz, 2H, p-tol o-H), 7.59}

(d, J = 6.9 Hz, 4H, BPh o-H), 7.08 – 6.92 (m, 10H, p-tol m-H, BPh m-H, NPh o-H ), 6.88 (t, J = 7.2 Hz, 2H, BPh H), 6.49 (t, J = 7.7 Hz, 4H, NPh m-H), 5.84 (t, J = 6.9 Hz, 2H, NPh p-H), 3.59-3.67 (m, 6H, THF), 2.28 (s, 3H, p-tol CH3), 1.75-1.83 (m, 6H, THF). 11B NMR (128.0

MHz, d8-THF, 25 °C) δ -1.61 (s). 13C NMR (100 MHz, THF-d8, 25 °C) δ 157.95 (B-C(ipso)

Ph), 153.10 (N-C(ipso) Ph), 147.17 (NCN), 142.21 (p-tol C(ipso)-NCN), 135.71 (BPh o-CH),

134.27 (p-tol C-CH3), 127.87 (p-tol m-CH), 127.63 (NPh o-CH), 127.34 (BPh m-CH), 126.23

(p-tol o-CH), 124.27 (BPh p-CH), 115.36 (NPh m-CH), 109.84 (NPh p-CH), 68.27 (THF),

26.43 (THF), 21.32 (p-tol CH3).

Synthesis of [Bn_LBPh₂_][K(THF)₁_{] ([}Bn_{1][K]). Compound [1][K}₂_]_{(50.0 mg, 0.07 mmol) was} dissolved in 3 ml of THF in a vial inside the glove box. To this was added 1 equiv of benzyl bromide, which caused the color to change from orange to green. After stirring the mixture for 30 minutes, all the volatiles were removed under reduced pressure and the crude product was washed with hexane (3 x 2ml). Subsequently, drying under vacuum gave compound [Bn_1][K] as an oily green material (33.0 mg, 0.05 mmol, 71 %). 1_{H NMR (400 MHz, THF-d}₈_{, 25 °C) δ}

7.82 (d, J = 7.8 Hz, 2H, p-tol o-H ), 7.66 (bs, 2H, B(1)Ph o-H), 7.04-6.97 (overlapped, 9H, (benzyl)Ph o-H, N(1)Ph o-H, B(2)Ph o-H, p-tol m-H and B(1)Ph p-H), 6.83-6.82 (m, 3H, (benzyl)Ph (m+p)-H), 6.60 (overlapped, 3H, N(1)Ph m-H and B(2)Ph p-H), 6.48 (overlapped, 4H, N(2)Ph (o+m)-H), 6.16 (p, J = 3.9 Hz, 1H, N(2)Ph p-H), 6.11 (t, J = 7.0 Hz, 1H, N(1)Ph

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145 p-H), 3.62 (m, 3H, THF), 2.29 (s, 3H, p-tol CH3), 1.77 (m, 3H, THF), at room temperature due

to the broadening in the NMR spectrum the resonances of benzyl-CH2 and B(1,2)Ph m-H could

not be assigned correctly. 11_{B NMR (128.3 MHz, THF-d}₈_{, 25 °C) δ 1.07 (s).} 13_{C NMR (100}

MHz, THF-d8, 25 °C) δ 158.27 (N(2)Ph ipso-C), 154.07 (N(1)Ph ipso-C), 143.56 (NCN),

141.30 ((benzyl)Ph ipso-C)), 137.53 (NCN-p-tol ipso-C), 137.06 (B(1,2)Ph o-CH), 135.76

(p-tol-CH3 ipso-C), 129.86 ((benzyl)Ph o-CH), 128.89 (p-tol m-CH), 127.64 (p-tol o-CH), 127.44

((benzyl)Ph p-CH), 126.96 (N(1) Ph CH), 126.35 (N(2) Ph CH), 125.80 ((benzyl)Ph m-CH), 123.40 ((N(2)Ph o-m-CH), 118.42 (N(1) Ph o-m-CH), 116.34 (N(2)Ph p-m-CH), 113.77 (N(1)Ph p-CH), 68.27 (THF), 58.57 (benzyl-CH2), 26.43 (THF), 21.35 (p-tol CH3), (B(1,2)Ph ipso-C)

and (B(1,2)Ph (m+p)-CH) could not be assigned correctly.

Synthesis of [Bn_LBPh₂_][Rb(THF)₁_{] ([}Bn_{1][Rb]). Compound [1][Rb}₂_{] (45.0 mg, 0.06 mmol)} was dissolved in 3 ml of THF in a vial inside the glove box. To this was added 1 equiv of benzyl bromide, which caused the color to change from orange to green. After stirring the mixture for 30 minutes, all the volatiles were removed under reduced pressure and the crude product was washed with hexane (3 x 2ml). Subsequently, drying under vacuum gave compound [Bn_1][Rb] as an oily green material (26.0 mg, 0.04 mmol, 67 %). 1_{H NMR (400 MHz, THF-d}₈_{, 25 °C) δ}

7.84 (d, J = 7.8 Hz, 2H, p-tol o-H ), 7.06-7.03 (overlapped, 6H, N(1)Ph o-H, B(2)Ph o-H and p-tol m-H), 6.98-6.96 (overlapped, 3H, (benzyl)Ph o-H and B(1)Ph p-H), 6.83-6.81 (m, 3H, (benzyl)Ph (m+p)-H), 6.65 (overlapped, 3H, N(1)Ph m-H and B(2)Ph p-H), 6.48 (overlapped, 4H, N(2)Ph (o+m)-H), 6.15 (t, J = 6.8 Hz, 2H, N(1,2)Ph p-H), 3.67-3.58 (m, 6H, THF), 2.30 (s, 3H, p-tol CH3), 1.81-1.76 (m, 6H, THF), (at room temperature due to the broadening in the

NMR spectrum the resonances of benzyl-CH2, B(1,2)Ph m-H and B(1)Ph o-H could not be assigned correctly). 11_{B NMR (128.3 MHz, THF-d}₈_{, 25 °C) δ xx (s).} 13_{C NMR (100 MHz,}

THF-d8, 25 °C) δ 157.82 (N(2)Ph ipso-C), 153.92 (N(1)Ph ipso-C), 144.43 (NCN), 140.87

((benzyl)Ph ipso-C)), 137.22 (NCN-p-tol ipso-C), 136.98 (B(1,2)Ph o-CH), 136.19 (p-tol-CH3

ipso-C), 130.03 ((benzyl)Ph o-CH), 128.99 (p-tol m-CH), 127.73 (p-tol o-CH), 127.47 ((benzyl)Ph p-CH), 127.39 (N(1) Ph CH), 126.53 (N(2) Ph CH), 125.97 ((benzyl)Ph m-CH), 122.78 ((N(2)Ph o-m-CH), 118.19 (N(1) Ph o-m-CH), 116.15 (N(2)Ph p-m-CH), 113.81 (N(1)Ph p-CH), 68.27 (THF), 58.63 (benzyl-CH2), 26.43 (THF), 21.35 (p-tol CH3), (B(1,2)Ph ipso-C)

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146

6.5 Supplementary information

6.5.1 UV/vis spectroscopy

Figure S6.1 Absorbance spectra of compounds [1][K2], [1][Rb2], [Bn1][K] and [Bn1][Rb] in THF

6.5.2 Determination of activation parameters for the dynamic process in

compounds [

Bn

_{1][Na], [}

Bn

_{1][K], [}

Bn

_{1][Rb], [}

Bn

_1][NBu

₄

_{], [}

Bn

_{2][Na] and}

[

Bn

_2][NBu

₄

_]

1_{H NMR spectra of compound [}Bn_{1][Na] were collected at 500 MHz NMR spectrometer in the} temperature range between -5 and 75 °C (Figure 6.6). The gCVT tool which was included in the gNMR50_{installation had been used to convert the Varian NMR data files to the gNMR file}

format. The chemical shifts of the peaks of interest (i.e., the benzyl-CH2 as well as BPh2 moiety)

were taken from the experimental spectrum and exchange between pairs (i.e., the two diasterotopic benzyl-CH2 as well as the ortho-H from each B-Ph group) were modelled; the

signal for the p-tol-methyl-H was included without exchange. The peak of p-tol-methyl-H was used to estimate the linewidth in the absence of chemical exchange (due to relaxation, inhomogeneity of the magnetic field etc.). Additional line broadening due to chemical exchange was then included, and the agreement between experimental and simulated spectrum was inspected visually. Due to the presence of additional peaks in the region of interest, attempts to perform least-squares fitting of the line shapes were unsuccessful. An estimate of the error in

-0.1 0.4 0.9 1.4 1.9 200 300 400 500 600 700 800 Abs or ba nc e Wavelengths (nm) [1][K2] [1][Rb2] [Bn1][K] [Bn1][Rb]

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147 the exchange rate constants was made visually by running simulations with different rate constants and evaluating in which range a satisfactory fit was still obtained. A comparison between experimental spectra and those with ‘best’ fit parameters are shown in Figure S6.12. The rate constants thus obtained were used for constructing an Eyring plot of Ln(k/T) vs. 1/T. The estimated errors were taken into account by giving each data point a weight that was proportional to 1/(σ(k)2_{). Fitting was performed using Wolfram Mathematica 11.3,}51_and

activation parameters are determined using standard procedures from the slope and intercept. Following the above mentioned same methodology, the activation parameters for the N-inversion in compounds [Bn_{1][K], [}Bn_{1][Rb], [}Bn_1][NBu₄_{], [}Bn_{2][Na] and [}Bn_2][NBu₄_{] were} determined.

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148

Figure S6.2 Eyring plot for the calculation of activation parameters for the nitrogen inversion

in compounds [Bn_{1][Na] and [}Bn_2][Na]

Figure S6.3 Eyring plot for the calculation of activation parameters for nitrogen inversion in

compounds [Bn_1][NBu₄_{] and [}Bn_2][NBu₄_]

Figure S6.4 Eyring plot for the calculation of activation parameters for nitrogen inversion in

compounds [Bn_1][NBu₄_{], [}Bn_{1][Na], [}Bn_{1][K] and [}Bn_1][Rb]

[Bn 1][Na] [Bn 2][Na] [Bn_1][Na] [Bn_2][Na] ΔH‡_{= 80.4 (1.7) kJ/mol} ΔS‡_{= 59.3 (5.6) J/mol/K} ΔH‡_{= 88.9 (2.5) kJ/mol} ΔS‡_{= 56.5 (7.3) J/mol/K} [Bn_1][NBu₄_] [Bn_2][NBu₄_] ΔH‡_{= 66.7 (1.6) kJ/mol} ΔS‡_{= -4.2 (5.0) J/mol/K} ΔH‡_{= 68.4 (1.3) kJ/mol} ΔS‡_{= -19.7 (3.9) J/mol/K} [Bn_1][NBu 4] [Bn 2][NBu4] [Bn_1][NBu₄_] [Bn_1][Na] ΔH‡_{= 66.7 (1.6) kJ/mol} ΔS‡_{= -4.2 (5.0) J/mol/K} ΔH‡_{= 80.4 (1.7) kJ/mol} ΔS‡_{= 59.3 (5.6) J/mol/K} [Bn_1][K] [Bn_2][Rb] ΔH‡_{= 68.4 (2.3) kJ/mol} ΔS‡_{= 59.3 (7.3) J/mol/K} ΔH‡_{= 72.7 (5.5) kJ/mol} ΔS‡_{= 42.3 (18.0) J/mol/K} [Bn_1][NBu₄_] [Bn_1][Na] [Bn_1][K] [Bn_2][Rb]

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149 3.4 3.8 4.2 4.6 5.0 5.4 5.8 6.2 6.6 7.0 7.4 7.8 1 2 3 4 5 6 7

Figure S6.5 1_{H NMR of compound}_[Bn_1][NBu₄_{] in THF-d}₈_{at various temperatures}

3.4 3.8 4.2 4.6 5.0 5.4 5.8 6.2 6.6 7.0 7.4 7.8 1 2 3 4 5 6 7 8 9 10 11 12 13 14

Figure S6.6 1_{H NMR of compound}_[Bn_{1][Na] in THF-d}₈_{at various temperatures} 60 °C -5 °C 20 °C 0 °C 5 °C 10 °C 15 °C 35 °C 55 °C 40 °C 25 °C 30 °C 65 °C 75 °C 35 °C 45 °C 25 °C 55 °C 65 °C 85 °C 75 °C

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150 3.2 3.6 4.0 4.4 4.8 5.2 5.6 6.0 6.4 6.8 7.2 7.6 1 2 3 4 5 6 7 8 9 10 11

Figure S6.7 1_{H NMR of compound}_[Bn_{1][K] in THF-d}₈_{at various temperatures}

3.2 3.6 4.0 4.4 4.8 5.2 5.6 6.0 6.4 6.8 7.2 7.6 8.0 1 2 3 4 5 6 7 8

Figure S6.8 1_{H NMR of compound}_[Bn_{1][Rb] in THF-d}₈_{at various temperatures.} 65 °C -5 °C 20 °C 5 °C 10 °C 15 °C 45 °C 55 °C 25 °C 35 °C 75 °C 5 °C 15 °C 25 °C 35 °C 45 °C 55 °C 65 °C -5 °C

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151 4.2 4.4 4.6 4.8 5.0 5.2 5.4 5.6 5.8 6.0 6.2 6.4 6.6 6.8 7.0 7.2 7.4 7.6 7.8 8.0 1 2 3 4 5 6

Figure S6.9 1_{H NMR of compound}_[Bn_2][NBu₄_{] in THF-d}₈_{at various temperatures.}

4.2 4.4 4.6 4.8 5.0 5.2 5.4 5.6 5.8 6.0 6.2 6.4 6.6 6.8 7.0 7.2 7.4 7.6 7.8 8.0 1 2 3 4 5 6 7

Figure S6.10 1_{H NMR of compound}_[Bn_{2][Na] in THF-d}₈_{at various temperatures.} 40 °C 45 °C 50 °C 55 °C 60 °C 65 °C 75 °C 35 °C 45 °C 55 °C 65 °C 75 °C 85 °C

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152

Figure S6.11 Comparison of selected experimental and simulated 1_{H-NMR spectra of}

[Bn_1][NBu₄_{] (for each temperature, top: experimental spectrum, bottom: simulated). Rate} constants used for the simulation including an estimate of the error are shown for each spectrum.

[Bn_{1][Na] (for each temperature, top: experimental spectrum, bottom: simulated). Rate} constants used for the simulation including an estimate of the error are shown for each spectrum.

298 K: estimated rate constant k = 7 ± 2 s-1

338 K: estimated rate constant k = 260 ± 75 s-1 _{358 K: estimated rate constant k = 950 ± 200 s}-1

273 K: estimated rate constant k = 2 ± 1 s-1 _{288 K: estimated rate constant k = 16 ± 1.5 s}-1

328 K: estimated rate constant k = 1300 ± 200 s-1

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153

Figure S6.13 Comparison of selected experimental and simulated 1_{H-NMR spectra of}_[Bn_1][K] (for each temperature, top: experimental spectrum, bottom: simulated). Rate constants used

for the simulation including an estimate of the error are shown for each spectrum.

[Bn_{1][Rb] (for each temperature, top: experimental spectrum, bottom: simulated). Rate} constants used for the simulation including an estimate of the error are shown for each spectrum.

268 K: estimated rate constant k = 4.5 ± 0.5 s-1 _{288 K: estimated rate constant k = 55 ± 5 s}-1

298 K: estimated rate constant k = 200 ± 20 s-1 348 K: estimated rate constant k = 8500 ± 1500 s-1

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154

308 K: estimated rate constant k = 1.5 ± 0.5 s-1 _{328 K: estimated rate constant k = 9 ± 1.5 s}-1

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155

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