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 5

Structure and bonding in reduced boron

and aluminium complexes with

formazanate ligands

Group 13 complexes of the type [(PhNNC(p-tol)NNPh)ZPh2]2- (Z = B, Al) containing a highly

reduced, trianionic formazanate-derived ligand were studied and the differences in structure, bonding and reactivity between the B and Al compounds was investigated. The increased ionic character in the bonding of the Al complex is evident from enhanced charge delocalization onto the peripheral ligand substituents (N-Ph) via the π-framework, as shown by the rotation barrier around the N-C(Ph) bond. The electron-rich nature of these compounds allows facile benzylation at the ligand, and the structures of the products were analysed by X-ray crystallography. The products are inorganic analogues of 1-alkylated 1,2,3,4-tetrahydro-1,2,4,5-tetrazines (‘leucoverdazyls’). The six-membered heterocyclic cores of the B and Al compounds are shown to be different, having envelope- and boat-type conformations, respectively. Homolysis of the N-C(benzyl) bond in these compounds was studied by NMR spectroscopy under conditions that trap the organic radical as TEMPO-Bn. Analysis of the reaction kinetics affords activation parameters that approximate the N-C(benzyl) bond strength. The ionic Al compound has one of the weakest N-C bonds reported so far in this type of inorganic leucoverdazyl analogues.

This chapter has been published:

Ranajit Mondol and Edwin Otten* “Structure and bonding in reduced boron and aluminium complexes with formazanate ligands” Dalton Trans., 2019, 48, 13981-13988 (DOI:

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104

5.1 Introduction

The direct involvement of the ligands in redox-reactions by coordination complexes bearing non-innocent ligands is an emerging research area.1–3_{In this context, there is a growing interest}

in designing new type of redox-active (non-innocent) ligands. In 2007, Hicks and coworkers observed reversible ligand-based reductions in a formazanate boron acetate compound.4_This

demonstrated for the first time that the formazanate ligand can be considered a redox-active analogue of the well-known β-diketiminate ligands.5–8_{Taking inspiration from this work, our}

group has explored the coordination chemistry, redox-behavior and reactivity of complexes with formazanate ligands.9–16_{Along with our work, the Gilroy group}17–26_{and others}27–29_have

been involved in the synthesis and application of the new molecular complexes with formazanate ligands. Previously, we showed that the ligands in boron and aluminum compounds with formazanate ligands could be sequentially reduced by 1- and 2-electron at moderate reduction potentials (Scheme 5.1).30,31_{The 2-electron reduced formazanate boron}

compound (12-_{) subsequently reacted with electrophiles (E}+_{) such as benzyl bromide (BnBr)}

and water (H2O) to form ligand-benzylated and -protonated products (Bn1- and H1- in Scheme

5.1).32_{In these compounds, the formazanate ligands is modified by ‘storage’ of [2e}-_/E+_{], which}

could be converted to Bn•_{and H}•_{radicals by the homolytic cleavage of the N-C(Bn) and N-H}

bonds, respectively (Scheme 5.1).32_{These reactions occur readily, because the boron-containing}

radical that is generated (1-•₎_is

N N N N Ar Ph p-tol Z Ph Ph N N N N Ar Ph p-tol Z Ph Ph N N N N Ar Ph p-tol Z Ph Ph 1e -1e -2 (1/1Mes/2) 2-(1/1Mes_/2) N N N N Ph Ph p-tol B Ph Ph E [Bn1][Na] (E = CH2Ph) [H1][Na](E = H) E-X = PhCH2-Br H-OH E-X, THF -NaX TEMPO . TEMPO-E Z = B; Ar = Ph, 1 Z = B; Ar = Mes, 1Mes Z = Al; Ar = Ph, 2 ligand-based strorage of 2e -ligand-based strorage of 2e-/E+

Scheme 5.1 Ligand-based storage of [2e-_/E+_{], and subsequent conversion to E}•_(Bn•_/H•_{) radicals}

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105 relatively stable due the presence of a low-energy SOMO that is delocalized over all four N-atoms in the ligand backbone.30,32_{It is anticipated that the basicity (nucleophilicity), radical}

stability and N-C/N-H bond strength of compounds bearing functionalized formazanate ligands could be tuned either via ligand substituent effects or by incorporation of a different central element (main group or transition metal) in the formazanate chelate ring.

In order to investigate the effects of changing the central element (boron) in 1 to the more

electropositive aluminum, here we provide a detailed comparison of formazanate B and Al complexes with an identical ligand. The comparison includes an analysis of resonance delocalization in the two-electron reduced formazanate aluminum diphenyl compound (22-_{) via}

dynamic NMR spectroscopy. The synthesis of ligand-benzylated products Bn₂-_{is described, and}

crystallographic and spectroscopic characterization data is provided. Furthermore, the kinetics of homolytic N-C(benzyl) cleavage is studied.

5.2 The hindered rotation around the N-C(Ph) bonds in two-electron reduced

formazanate aluminium diphenyl compound (2

2-

₎

The two-electron reduced formazanate aluminium diphenyl compound [PhNNC(p-tol)NNPh]AlPh22- (22-) was synthesized as its disodium salt according to the previously

published procedure.31 _{The product}₂2-_{, which has an electron-rich, formally trianionic}

formazanate ligand, is highly air-sensitive, but stable at room temperature under inert conditions. The 1_{H NMR spectrum of}₂2-_{in THF-d}₈_{was shown to be temperature-dependent: at 233 K, the}

spectrum contains 5 inequivalent resonances due to the N-Ph groups. Increasing the temperature results in line broadening and ultimately coalescence into three distinct signals as expected for the ortho, meta and para-positions of a Ph group (Figure 5.1).‡_{These features are indicative of}

hindered rotation around the N-C(Ph) bond, which leads to inequivalent chemical environments for the two ortho- and meta-H positions, with exchange rates that are on the order of the NMR timescale.33,34_{Lineshape analysis was carried out for the pairs of exchanging resonances in the}

temperature range 233-303 K, which gave the activation parameters for the exchange process in 22-_{as ΔH}‡_{= 54.1 ± 1.7 kJ/mol and ΔS} ‡_{= -2.5 ± 6.0 J/mol/K (see SI for details). The activation}

enthalpy reflects a substantial amount of N-C(Ph) π-bonding character being broken upon going to the transition state, whereas there is little difference in entropy. A comparison with the related boron-containing compounds (12-_{) shows that the values are similar to those in the asymmetric}

derivative [MesNNC(p-tol)NNPh]BPh22- ([1Mes]2-; ∆H‡ = 57.4 ± 1.8 kJ·mol-1 and ∆S‡ = 1 ±

6 J·mol-1_·K-1_).30_{On the other hand, the B-analogue}₁2-_{, which has the same (symmetrical)}

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106

timescale:§_{no line-broadening is observed down to 233 K.}30_{We interpret the difference}

between the B and Al complexes with an identical formazanate ligand (12-_{and 2}2-_{) as a}

reflection of the difference in bonding within the boron and aluminium heterocycles. In particular, the largely ionic character in the Al-N bonds in comparison to the more covalent B-N interaction is responsible for increased accumulation of negative charge within the B-NB-NCB-NB-N framework in the Al compound. The increase in N-C(Ph) π-bonding due to resonance delocalization of the negative charge into the N-Ph group is also observed in the solid state structures obtained by X-ray crystallography: the N-C(Ph) bonds in 22-_{are 1.371(2)/1.375(3)}

Å,31_{whereas those in}₁2-_{are marginally larger at 1.379(3)/1.385(3) Å.}30

Figure 5.1 1_{H NMR spectra of}₂2-_{in THF-d}₈_{at various temperatures.}

To further probe the bonding differences between formazanate boron and aluminium complexes, a computational study was carried out at the DFT level (B3LYP functional and 6-311+G(d,p) basis set). The geometries were optimized in the gas phase starting from the crystallographic coordinates, with the Na(THF)x cations removed (the computational results for 12-calc were

described previously).30_{The metrical parameters of the optimized structures of}₁_2-calc_and₂_2-calc

are in good agreement with the experimental structures, albeit that the N-N bonds are somewhat shorter in the DFT models (see Table S2). Visual inspection of the frontier orbitals shows that

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107 the HOMO is primarily π-antibonding between the N-atoms in the formazanate ligand, and in addition, evidences the presence of π-bonding character in the N-C(Ph) fragment (Figure S5.12). Although the differences are small, the calculated Wiberg bond index35_{for the N-C(Ph) bonds}

is larger in the Al compound (22-calc: 1.24) than in the B compound (12-calc: 1.22), corroborating the trend for the strength of π-bonding that was obtained from the NMR study. In addition, a higher Wiberg bond index is found for the B-N bonds in 12-_{(0.68) in comparison to the Al-N}

bonds in 22-_{(0.40) and the natural charges indicate that the formazanate ligand bears a}

significantly higher negative charge in the Al complex (-2.40 e) than in the B analogue (-1.99 e).

Computational evaluation of the barrier to rotation around the N-C(Ph) bond in 22-calc was

carried out by scanning the dihedral angle between the Ph ring and the ligand backbone. As expected, a maximum was found at a dihedral angle of ca. 90°, and at this geometry a transition state optimization was carried out to arrive at a saddle point (22-TS, Nimag = 1). The computed

barrier for N-C(Ph) bond rotation was found to be 55.9 kJ·mol-1_{, which is in good agreement}

with the experimental value (~ 54.9 kJ·mol-1_{at 298 K). A comparison between the ground and}

transition state shows that π-bonding is lost in the N-Ph ring that is rotated (Wiberg bond index = 1.04), whereas a small increase is observed in the other N-C(Ph) bond (1.26). This is also reflected in the different N-C(Ph) bond lengths that are calculated for 22-TS (1.411 Å and 1.365 Å; cf. 1.370 Å in 22-calc).

5.3 Reactivity study of two-electron reduced formazanate aluminium

diphenyl compound (2

2-

_{) with benzyl bromide}

5.3.1 Synthesis of ligand-benzylated product

Bn

₂

-

_{, and its characterization}

by NMR spectroscopy

We subsequently evaluated the reactivity of compound 22-_{towards the electrophile benzyl}

bromide. Treatment of an orange THF-d8 solution of 22- with 1 equivalent of BnBr resulted in

immediate colour change from orange to yellowish-green. The 1_{H NMR spectrum of the}

reaction mixture at room temperature (400MHz, THF-d8) reveals diagnostic resonances for the

diastereotopic protonsof the benzyl-CH2 group at δ 4.29 and 4.45 ppm with a geminal coupling

constant of 2_J_HH_{= 12.5 Hz. Also, the}1_{H NMR spectrum shows two inequivalent resonances for}

the para-protons of the N-Ph rings at δ 6.09 and 6.34 ppm due to a descent in symmetry from the C2v-symmetric precursor 22-. The NMR data of the reaction product are similar to that of

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108

in the ligand backbone. Consequently, we formulate the product as the anionic, ligand-benzylated complex [PhNN(Bz)C(p-tol)NNPh]AlPh2- (Bn2-, Scheme 5.2). On a preparative

scale, the reaction between 22-_{and BnBr allowed isolation of the sodium salt [}Bn_2][Na]_{as a}

solid material in 52 % yield.

N N N N Ph Ph p-tol Al Ph Ph 2 2-RT, THF PhCH2Br N N N N Ph Ph p-tol Al Ph Ph [Bn2][Na] - NaBr Na(THF)3 (THF)3Na Ph (THF)3Na RT, THF Bu4NBr N N N N Ph Ph p-tol Z Ph Ph [Bn1][NBu4] [Bn_2][NBu_4] - NaBr Ph N N N N Ph Ph p-tol Z Ph Ph Ph (THF)3Na Bu4N Z = B, [Bn_1][Na] Z = Al, [Bn_2][Na]

Scheme 5.2 Synthesis of ligand-benzylated products [Bn_{2][Na], [}Bn_2][NBu₄_{] and [}Bn_1][NBu₄_]

5.3.2 Crystallographic characterization data of ligand-benzylated products

Bn

₁

-

_and

Bn

₂

-

Crystals of [Bn_{2][Na] were obtained by slow diffusion of hexane into the THF solution at -30 °C.}

Although the previously reported boron analogue [Bn_{1][Na] also crystallizes under these}

conditions, these crystals quickly melt at room temperature, thus thwarting a structure determination.32_{Treatment of both these sodium salts with Bu}₄_{NBr resulted in cation exchange}

and formation of the tetrabutyl ammonium salts [Bn_1][NBu₄_{] and [}Bn_2][NBu₄_{] in yields of ca.}

75% (Scheme 5.2). Gratifyingly, crystals of [Bn_1][NBu₄_{] had a much higher melting point, and}

X-ray structure determinations were successfully completed for both B- and Al-containing products. The molecular structures of [Bn_1][NBu₄_{] and [}Bn_{2][Na] shows that in both of these}

compounds, the benzyl group is attached to one of the internal N-atoms to retain the 6-membered chelate ring structures of the dianionic precursors (Figure 5.2). A closer inspection of the central six-membered heterocyclic rings in these compounds reveals some noteworthy differences. Compound [Bn_1][NBu₄_{] adopts an envelope conformation that is similar to the}

precursor 12-_,30_{with the formazanate backbone atoms (NNCNN) nearly coplanar and the B}

atom residing 0.568 Å above that plane. In contrast, the six-membered ring in the Al compound [Bn_{2][Na] is found to be present in a boat conformation in which the N1, Al1, N3 and C7 are}

coplanar, whereas and N2 and N4-atoms are displaced out from that plane by 0.502 and 0.507 Å, respectively (Figure 5.2). This is markedly different from the nearly planar conformation in the precursor 22-_.31_{A Cremer-Pople puckering analysis}36_{was carried out using PLATON,}37

which allowed a more quantitative distinction between the divergent geometrical parameters within the core of [Bn_1][NBu₄_{] vs. [}Bn_{2][Na]. In this analysis, a combination of the puckering}

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109 sphere. The following puckering parameters are obtained for these compounds: for [Bn_1][NBu₄_]

(Q = 0.44 Å; θ = 57.0°; φ = 332.2°) and [Bn_{2][Na] (Q = 0.58 Å; θ = 89.8°; φ = 300.2°). Of these}

parameters, the θ angles in six-membered rings distinguish between chair, envelope and boat conformations, whereas φ defines pseudorotation pathways that transverse twisted conformations as well. The angles θ found for both compounds are indicative of either an envelope (idealized: 54.7°; [Bn_1][NBu₄_{]: 57.0°) or boat conformation (idealized: 90°; [}Bn_2][Na]:

89.8°).

Figure 5.2 (a) Molecular structures of [Bn_{2][Na] (top, left) and [}Bn_1][NBu₄_{] (top, right), and}

showing 50% probability ellipsoids. Hydrogen atoms for [Bn_{2][Na] and [}Bn_1][NBu₄_{] are}

omitted for clarity. THF molecules (except for the O atoms bonded to Na) for [Bn_{2][Na] are}

omitted for clarity. (b) Showing six-membered chelate rings for [Bn_{2][Na] (bottom, left) and for}

[Bn_1][NBu₄_]_{(bottom, right).}

In the B and Al dianions (12-_and₂2-_{) the bonding in the six-membered core is delocalized, as}

shown by equivalent N-N and C-N bond lengths, whereas ligand benzylation leads to a more localized bonding picture. For example, the C-N bonds in the ligand backbone change from 1.332(2)/1.328(2) Å in 22-_{to 1.437(1)/1.292(1) Å in [}Bn_{2][Na]. Similar changes in bond lengths}

occur in the boron analogue (see Tables S5.1/S5.2 for a full comparison of metrical data). In both compounds, the benzylated N-atom is pyramidal as indicated by the sum of angles around N(2) being much smaller than 360° (∑∠(N(2)) = 348.1° and 334.7° for[Bn_{1][ NBu}₄_{] and}

distorted boat conformation

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(b)

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110

[Bn_{2][Na], respectively). In [}Bn_{2][Na] the Ph-substituted N-atoms are planar (sp}2_-hybridized)

with ∑∠(N) = 359.5° for N(1) and 354.5° for N(4). Conversely, in the boron analogue [Bn_1][NBu₄_{] the N(4) atom is almost planar (∑∠(N) = 357.5°), while N(1) is pyramidalized}

((∑∠(N) = 347.8°). It appears that these differences reflect a larger degree of directionality (covalency) in the B-N bonds in comparison to the more ionic Al congener.

Table 5.1 Selected bond lengths (Å) and bond angles (°) of [Bn_1][NBu₄_{] and [}Bn_2][Na]

[Bn_1a][NBu₄_] _[Bn_2][Na] _[Bn_1a][NBu₄_] _[Bn_2][Na]

N1-N2 1.428(2) 1.446(1) N1-C1 1.414(2) 1.387(1) N3-N4 1.393(2) 1.402(1) N4-C15 1.392(2) 1.399(1) N2-C7 1.396(2) 1.437(1) N2-C33 1.468(2) 1.486(1) C7-N3 1.289(2) 1.292(1) N1-B1-N4 105.78(1) Al1-N1 1.867(1) N1-Al1-N4 95.24(4) Al1-N4 1.914(1) N1-N2-C7 115.41(1) 112.27(8) B1-N1 1.575(2) N1-N2-C33 113.94(1) 110.33(8) B1-N4 1.584(2) C7-N2-C33 118.79(1) 112.13(8)

5.3.3 The evaluation of ligand-benzylated products

Bn

₁

-

_and

Bn

₂

-

_{by DFT}

calculations

The anions Bn₁-_andBn₂-_{were also evaluated by DFT calculations. Optimized geometries of} Bn₁_-calc_andBn₂_-calc_{were obtained starting from the crystallographic coordinates (counter cations}

were removed) and shown to be in good agreement with the empirical structures. Importantly, the dissimilarities in pyramidalization at the N-atoms of the B and Al compounds is reproduced in the computational models, suggesting that these are intrinsic and not due to packing or ion pairing effects. In both the experimental structures and the DFT models, there is a small but noticeable elongation of the N-C(Bn) bond in the Al compound Bn₂-_{(X-ray: 1.486(1) Å; DFT:}

1.492 Å) compared to that in the B congener Bn₁-_{(X-ray: 1.468(2) Å; DFT: 1.472 Å). An NBO}

analysis indicated that the bonding within the NNCNN ligand backbone is similar in both B and Al compounds and can be described by a normal σ-bonding framework with an additional

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111

Figure 5.3 (A) Selected natural bond orbitals for the σ-framework in the ligand backbone (top);

the N-C π-bond and those around the N-benzyl atom (bottom). (B) Comparison of the B-N (covalent) vs Al-N (ionic) bonding interactions.

localized N=C π-bond (Figure 5.3A). The hybridization of the benzyl-substituted nitrogen atom is intermediate between sp2_{and sp}3_{, with a lone pair that has a small but non-negligible amount}

of s-character (10% for Bn₁_-calc_{; 16% for}Bn₂_-calc_{). Importantly, the NBO analysis shows the}

N-B and N-Al interaction in both compounds to be quite different. For the boron compound Bn₁-_,

we find two natural bond orbitals that represent relatively covalent B-N σ-bonds (ca. 77% contribution from N; 23% from B). For the Al analogue Bn₂-_{, on the other hand, no 2-center}

Al-N bonds are obtained and instead the Al-NBO analysis indicates Al-N-based lone pairs that interact with an empty Al orbital (Figure 5.3). Similarly, the natural charges indicate the Al complex to be much more ionic (NPA charge for Al in Bn₂_-calc_{: 1.82; B in}Bn₁-_{: 0.74).}

5.3.4 Characterization of ligand-benzylated product

Bn

₂

-

_{by UV/Vis}

spectroscopy

A comparison of the UV/Vis spectra of the Na+_{and Bu}₄_N+_{salts of}Bn₁-_andBn₂-_{shows that the}

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compounds Bn₁- _{and the aluminium compounds}Bn₂-_{absorb at 395 and 389 nm, respectively}

(Figure S5.1). These absorbance maxima are most likely due to a π-π* transition of the localized N=C bonds present in these compounds, as shown by the crystallographic data (vide supra), and are blue-shifted compared to the delocalized precursors 12- _and₂2-_(λ_max_{= 486 nm).}†,31

5.4 Investigation of the cleavage of the N-C(Bn) bond in

Bn

₂

-The compounds Bn₁-_andBn₂-_are_{inorganic, anionic analogues of purely organic 1-alkylated}

tetrahydro-1,2,4,5-tetrazines (‘leucoverdazyls’).38 _{Leucoverdazyls (with N-H bonds) and the}

alkylated (N-C) derivatives are known to have weak N-H/N-C bonds because homolytic cleavage generates a stable verdazyl radical.38–41 _{The stability of this type of radicals extends to}

inorganic systems.4,9,10,14,17,26,30,31,42_{In order to examine the effect on the N-C(Bn) bond}

dissociation energy (BDE) due to the replacement of B to the more electropositive Al in [Bn_{2][Na], the cleavage of the N-C bond in [}Bn_{2][Na] was investigated. The N-C(Bn) bond}

dissociation enthalpy in [Bn_{2][Na] was obtained experimentally by NMR spectroscopic}

monitoring of the kinetics of benzyl radical transfer from [Bn_{2][Na] to TEMPO (present in}

excess) in the temperature range of 65-85 °C (Scheme 5.3). Clean exponential decay of the starting material and concomitant appearance of TEMPO-Bn was observed, which allowed the rate constants to be determined (Figure S5.8). An Eyring analysis afforded the activation parameters △H‡_{and △S}‡_{of 107 ± 4 kJ∙mol}-1_{and 17 ± 11 J∙mol}-1_∙K-1_{, respectively, for the}

N N N N Ph Ph p-tol Al Ph Ph Bn _N N N N Ph Ph p-tol Al Ph Ph Bn ‡ TEMPO N N N N Ph Ph p-tol Al Ph Ph + _N O Bn [Bn₂-_][Na] 2.- TEMPO-Bn

Scheme 5.3 Benzyl group transfer from [Bn_{2][Na] to TEMPO}

Figure 5.4 Eyring analysis for benzyl transfer from [Bn_{2][Na] to TEMPO}

∆H‡_{= 107 ± 4}

kJ∙mol-1

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113 benzyl transfer (Figure 5.4; see SI for details). Both these values are somewhat smaller than those found for the boron analogue [Bn_{1][Na] (△H}‡_{= 121 ± 5 kJ∙mol}-1_{; △S}‡_{= 77 ± 14 J∙mol} -1_∙K-1_).32_{A likely explanation for these differences is the ground-state destabilization of the}

N-C(Bn) bond that is indicated by the somewhat larger N-N-C(Bn) distance found by both experiment and theory (vide supra).

To investigate a possible influence of ion-pairing on the N-C(Bn) bond dissociation energy, the kinetics of benzyl transfer were also measured for the tetrabutyl ammonium salts [Bn_1][NBu₄_]

and [Bn_2][NBu₄_{] using the same methodology. The resulting rate constants are in good}

agreement with those of the sodium salts (see SI for details). Thus, even though in the solid state the sodium cation is bound to the ligand backbone, this weak interaction is likely broken in solution. This is further supported by the observation that the UV/Vis spectra in solution do not depend on the nature of the cation.

These data indicate that N-C(Bn) bond homolysis is modulated by the central element in these heterocyclic leucoverdazyl analogues: on going from relatively covalent, C-based parent structures (i.e., 1-benzyl-substituted 1,2,3,4-tetrahydro-1,2,4,5-tetrazines), the N-C(Bn) bond strength progressively decreases with an increase in the electropositive nature of the central element (i.e., C > B > Al).

5.5 Conclusions

In conclusion, this work addresses the differences in structure and bonding between dianionic formazanate boron (12-_{) and aluminum (}₂2-_{) complexes. Experimental (NMR, UV/Vis}

spectroscopy, X-ray crystallography) and computational studies (Wiberg bond indices, NBO analysis) reveal that the increased ionic character in the Al compounds results in a higher degree of resonance delocalization of the ligand negative charge into the periphery of the ligand (i.e., the N-Ph substituents), which is reflected in the rotation barrier around the N-C(Ph) bond. For both compounds, facile ligand benzylation occurs upon reaction with benzyl bromide to form

Bn₁-_andBn₂-_{as anionic analogues of carbon-based leucoverdazyls. X-ray diffraction studies of} Bn₂-_{and the boron congener}Bn₁-_{are reported and a comparison shows distinct differences in}

the solid state structures between these complexes that can be related to a different degree of ionic character in the bonding. The kinetics of benzyl transfer show that the N-C(Bn) bond homolysis is modulated by the nature of central element present in the six-membered heterocyclic rings of Bn₁-_andBn₂-_{, with the ionic Al-based compound}Bn₂-_{having the weakest}

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114

5.6 Notes

‡ The additional broadening observed above 283 K is attributed to the presence of a small amount of radical species (the paramagnetic monoanion 2-•_{) that engages in electron transfer}

with 22-_.

§ The estimated upper limit of the barrier for N-C(Ph) bond rotation is ca. 40 kJ·mol-1_{. See ESI}

for details.

† The UV/Vis spectrum of 12-_{reported in the ESI of reference 30 is incorrect, likely due to}

decomposition of this highly sensitive compound.

5.7 Experimental section

5.7.1 General considerations

All manipulations were carried out under nitrogen atmosphere using standard glovebox, Schlenk, and vacuum-line techniques. THF and hexane (Aldrich, anhydrous, 99.8%) were passed over columns of Al2O3 (Fluka). The compounds [Bn1][NBu4],[Bn2][Na]and [Bn2][NBu4]

are highly air-sensitive, and the solvents (THF and hexane) used for their preparation and characterization were additionally dried on Na/K alloy and subsequently vacuum transferred and stored under nitrogen. All solvents were degassed prior to use and stored under nitrogen. THF-d8 (Sigma-Aldrich) was vacuum transferred from Na/K alloy and stored under nitrogen.

The compounds [Bn_1][Na]32_and ₂2-_{(as its disodium salt,}

[(PhNNC(p-tol)NNPh)AlPh2][Na2(DME)4])31 were synthesized according to published procedures.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 using an AVANTES}

AvaSpec-2048. Samples for elemental analyses were sent to Kolbe Microanalytical Laboratory (Mülheim an der Ruhr, Germany). However, despite our best efforts, no satisfactory analysis data could be obtained for these compounds, which is likely due to their highly air-sensitive nature.

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5.7.2 Compounds synthesis and characterization

Synthesis of [Bn_LBPh₂_][NBu₄_{] ([}Bn_1][NBu₄_]_{). Compound [}Bn_{1][Na] (75 mg, 0.092 mmol) was}

dissolved in 2 ml of THF in a vial inside the glove box. To this was added 1 equiv of tetrabutyl ammonium bromide (Bu4NBr) and the mixture was stirred overnight. The NaBr that had

precipitated was allowed to settle and the supernatant was transferred to another vial. After concentration under vacuum, a layer of hexane (1 mL) was added on top of the THF solution and the layers were allowed to slowly diffuse at -30 °C. After two days, a crystalline material had precipitated, which was isolated and washed with hexane (3 x 2 mL). Subsequently, drying under vacuum gave compound [Bn_1][NBu₄_{] as a light green crystalline material (59 mg, 0.072}

mmol, 78 %). 1_{H NMR (400 MHz, THF-d}₈_,_{25°C) δ 7.83 (d, J = 8.1 Hz, 2H, p-tol o-H), 7.70}

(d, J = 7.1 Hz, 2H, B-Ph(1) o-H), 7.11-7.06 (m, overlapped, 4H, B-Ph(1) (m)-H and p-tol m-H), 7.03-6.98 (m, overlapped, 7H, B-Ph(1) p-H, B-Ph(2) o-H, N(1)Ph o-H and (benzyl)Ph o-H), 6.86 – 6.79 (m, 3H, (benzyl)Ph (m+p)-H), 6.64 – 6.53 (m, 5H, overlapped, 5H, N(1)Ph m-H and B-Ph(2) (m+p)-H), 6.46 (d, J = 4.2 Hz, 4H, N(2)Ph (o+m)-H), 6.17-6.13 (m, 1H, N(2)Ph p-H), 6.09 (t, J = 7.1 Hz, 1H, N(1)Ph p-H), 3.78 (d, J = 15.3 Hz, 1H, benzyl-CH2), 3.67 – 3.59

(m, 4H, THF), 3.44 (d, J = 15.3 Hz, 1H, benzyl-CH2 ), 3.12 (s, 8H, NBu4), 2.29 (s, 3H, p-tol

CH3), 1.58 (s, 8H, NBu4), 1.34-1.30 (m, 8H, NBu4), 0.95 (t, J = 6.8 Hz, 12H, NBu4).11B NMR

(128.3 MHz, THF-d8, 25 °C) δ 1.26 (s). 13C NMR (100 MHz, THF-d8, 25°C) δ 158.47 (N(2)Ph ipso-C), 155.16 (B-Ph(1,2) ipso-C), 154.14 (N(1)Ph ipso-C), 142.91 (NCN), 141.43 ((benzyl)Ph ipsC)), 137.70 (NCN-p-tol ipsC), 137.19 (B-Ph(1) CH), 137.08 (B-Ph(2) o-CH), 135.55 (p-tol-CH3 ipso-C), 129.82 ((benzyl)Ph o-CH), 128.86 (p-tol m-CH), 127.60

((benzyl)Ph p-CH), 127.44 (p-tol o-CH), 126.90 (B-Ph(2) m-CH), 126.43 (B-Ph(1) m-CH), 126.27 (N(2)Ph o-CH), 125.85 (B-Ph(2) p-CH), 125.77 ((benzyl)Ph m-CH), 124.16 (B-Ph(1) p-CH), 123.61 (N(2)Ph m-CH), 123.53 (N(1)Ph m-CH), 118.39 (N(1) Ph o-CH), 116.36 (N(2)Ph p-CH), 113.77 (N(1)Ph p-CH), 59.67 (NBu4), 58.55 (benzyl-CH2), 24.78 (NBu4),

21.37 (p-tol CH3), 20.71 (NBu4), 14.08 (NBu4).

Synthesis of [Bn_LAlPh₂_][Na(THF)₃_{] ([}Bn_{2][Na]). Compound 2}2-_{(500 mg, 0.543 mmol) was}

dissolved in 3 ml of THF in a small vial inside the glove box. To this was added 1 equiv of benzyl bromide, which caused the color to change from orange to yellowish-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_{2][Na] as an oily green material. The oil was dissolved in a minimal amount of}

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116

allowed to diffuse slowly at -30 °C. After three days, the light green crystals that had precipitated were isolated and washed with hexane (3 x 2 mL). The crystals were dried under vacuum to give compound [Bn_{2][Na] in 52 % yield (232 mg, 0.281 mmol).}1_{H NMR (400 MHz,}

THF-d8, 25°C) δ 7.94 (d, J = 7.8 Hz, 2H, p-tol o-H), 7.86 (d, J = 6.4 Hz, 2H, Al-Ph(1) o-H),

7.62 – 7.60 (m, 2H, Al-Ph(2) o-H ), 7.37 (d, J = 8.0 Hz, 2H, N(2)Ph o-H), 7.13-7.06 (overlapped, 4H, (benzyl)Ph H and Al-Ph(1) m-H), 7.04 (overlapped, 3H, (benzyl)Ph p-H and N(1)Ph o-H), 6.97 (d, J = 7.8 Hz, 2H, p-tol m-o-H), 6.90-6.84 (overlapped, 8H, (benzyl)Ph m-H, Al-Ph(1) p-H, Al-Ph(2) (m+p)-H and N(2)Ph m-H), 6.69 (t, J = 7.5 Hz, 2H, N(1)Ph m-H), 6.34 (t, J = 7.0 Hz, 1H, N(2)Ph p-H), 6.09 (t, J = 6.9 Hz, 1H, N(1)Ph p-H), 4.44 (d, J = 12.5 Hz, 1H, benzyl-CH2), 4.29 (d, J = 12.5 Hz, 1H, benzyl-CH2), 3.68-3.59 (m, 4H, THF), 2.26 (s, 3H, p-tol CH3),

1.80-1.75 (m, 4H, THF). 13_{C NMR (100 MHz, THF-d}₈_{, 25°C) δ 157.29 (N(1)Ph ipso-C), 155.70}

(N(2)Ph ipso-C), 154.24 (Al(1,2)Ph ipso-C), 146.35 (NCN), 139.97 (benzyl)Ph ipso-C), 139.65 (Al-Ph(1) o-CH), 139.56 (Al-Ph(2) o-CH),138.80 (NCN-p-tol ipso-C), 135.65 (p-tol-CH3

ipso-C), 131.17 (benzyl)Ph o-CH), 128.67 (p-tol m-CH), 128.39 (N(2)Ph m-CH), 128.15 (N(1)Ph m-CH), 127.90 (p-tol p-CH), 127.82 (Al-Ph(2) m-CH), 126.96 (Al-Ph(1) m-CH), 126.67 ((benzyl)Ph m-CH), 126.49 ((benzyl)Ph p-CH), 126.14 (Al-Ph(2) p-CH), 125.87 (Al-Ph(2) p-CH), 116.45 (N(2)Ph o-CH), 115.89 (N(2)Ph p-CH), 114.50 (N(1)Ph o-CH), 113.32 (N(1)Ph p-CH), 68.26 (THF), 58.60 (benzyl-CH2), 26.43 (THF), 21.34 (p-tol CH3).

Synthesis of [Bn_LAlPh₂_][NBu₄_{] ([}Bn_2][NBu₄_{]). Compound [}Bn_{3][Na] (50 mg, 0.062 mmol)}

was dissolved in 2 ml of THF in a vial inside the glove box. To this was added 1 equiv of tetrabutyl ammonium bromide (Bu4NBr) and the mixture was stirred overnight. The NaBr

that had precipitated was allowed to settle and the supernatant was transferred to another vial. After concentration under vacuum, a layer of hexane (1 mL) was added on top of the THF solution and the layers were allowed to slowly diffuse at -30 °C. After two days, a crystalline material had precipitated, which was isolated and washed with hexane (3 x 2 mL). Subsequently, drying under vacuum gave compound [Bn_][_NBu₄_{] as a light green crystalline material (37 mg,}

0.045 mmol, 73 %). 1_{H NMR (400 MHz, THF-d}₈_{, 25°C) δ 7.97 (d, J = 8.0 Hz, 2H, p-tol}

o-H), 7.81 (d, J = 7.1 Hz, 2H, Al-Ph(1) o-o-H), 7.70-7.60 (m, 2H, Al-Ph(2) o-H ), 7.41 (d, J = 7.8 Hz, 2H, N(2)Ph o-H), 7.13-7.02 (overlapped, 7H, (benzyl)Ph (o+p)-H, Al-Ph(1) m-H and N(1)Ph o-H), 6.99 (d, J = 8.0 Hz, 2H, p-tol m-H), 6.96-6.84 (overlapped, 8H, (benzyl)Ph m-H, Al-Ph(1) p-H, Al-Ph(2) (m+p)-H and N(2)Ph m-H), 6.74 (t, J = 7.8 Hz, 2H, N(1)Ph m-H), 6.37 (t, J = 7.1 Hz, 1H, N(2)Ph p-H), 6.14 (t, J = 7.1 Hz, 1H, N(1)Ph p-H), 4.44 (d, J = 12.4 Hz, 1H, benzyl-CH2), 4.27 (d, J = 12.4 Hz, 1H, benzyl-CH2), 2.95 – 2.81 (m, 8H, NBu4), 2.27

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117 J = 7.3 Hz, 12H, NBu4). 13C NMR (100 MHz, THF-d8, 25°C) δ 157.12 (N(1)Ph ipso-C), 155.60

(N(2)Ph ipso-C), 153.82 (Al(1,2)Ph ipso-C), 146.33 (NCN), 139.98 (benzyl)Ph ipso-C), 139.68 (Al-Ph(1) o-CH), 139.61 (Al-Ph(2) o-CH),138.90 (NCN-p-tol ipso-C), 135.93 (p-tol-CH3

ipso-C), 131.27 (benzyl)Ph o-CH), 128.79 (p-tol m-CH), 128.50 (N(2)Ph m-CH), 128.35 (N(1)Ph m-CH), 128.06 (p-tol p-CH), 127.86 (Al-Ph(2) m-CH), 127.03 (Al-Ph(1) m-CH), 126.80 ((benzyl)Ph m-CH), 126.70 ((benzyl)Ph CH), 126.27 (Al-Ph(2) CH), 126.21 (Al-Ph(2) CH), 116.61 (N(2)Ph o-CH), 116.0 (N(2)Ph CH), 114.61 (N(1)Ph o-CH), 113.60 (N(1)Ph p-CH), 59.14 (NBu4), 58.52 (benzyl-CH2), 24.75 (NBu4), 21.36 (p-tol CH3), 20.55 (NBu4), 14.08

(NBu4).

5.7.3 X-ray crystallography

Suitable crystals of compounds [Bn_1][NBu₄_{] and [}Bn_{2][Na] were mounted on top of a cryoloop}

and transferred into the cold (100 K) nitrogen stream of a Bruker D8 Venture diffractometer. Data collection and reduction was done using the Bruker software suite APEX2.43_Data

collection was carried out at 100 K using Cu radiation (1.54178 Å) (for [Bn_1][NBu₄_{]) and Mo}

radiation (0.71073 Å) (for [Bn_{2][Na]). The final unit cell was obtained from the xyz centroids}

of 9929 ([Bn_1][NBu₄_{]) and 3515 ([}Bn_{2][Na]) reflections after integration. A multiscan}

absorption correction was applied, based on the intensities of symmetry-related reflections measured at different angular settings (SADABS).43 _{The structures were solved by intrinsic}

phasing methods using SHELXT,44_{and refinement of the structure was performed using} SHELXL45_{. For [}Bn_1][NBu₄_{], the refinement indicated the presence of smeared out}

electron-density due to a disordered THF molecule that could not be modelled in a satisfactory manner. The contribution of this electron-density was removed using the PLATON/SQUEEZE routine,46

giving 4 solvent-accessible volumes in the unit cell, each containing 37 electrons (in agreement with THF). For [Bn_{2][Na], refinement was frustrated by a disorder problem: two of the THF}

molecules bound to the Na+_{cation showed unrealistic displacement parameters when refined}

freely. For one of these, a two-site occupancy model was applied for all the atoms of the THF molecule and the site occupancy factor was refined (major fraction: 0.82). A SAME instruction was applied for the two disorder components, such that the two disordered THF molecules were restrained to have a similar geometry as that found for the THF molecule without disorder. Some atoms in the disordered THF molecule showed non-positive definite displacement parameters when refined freely, and RIGU/DELU instructions were applied. One of the carbon atoms of the remaining Na-THF fragment was split into two disorder components, which refined to a s.o.f. of 0.73 for the major component. The hydrogen atoms were generated by

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geometrical considerations, constrained to idealized geometries and allowed to ride on their carrier atoms with an isotropic displacement parameter related to the equivalent displacement parameter of their carrier atoms. Crystal data and details on data collection and refinement are presented in Table 5.1.

Table 5.1 Crystallographic data for [Bn_1][NBu₄_{] and [}Bn_2][Na]

[Bn_1][NBu₄_] _[Bn_2][Na]

chem formula C55 H70 B N5 C51 H58 Al N4 Na O3

Mr 811.97 824.98

cryst syst Monoclinic Monoclinic

color, habit green, hexagonal yellow, block

size (mm) 0.21 x 0.19 x 0.06 0.36 x 0.33 x 0.33 space group P21/c P21/n a (Å) 11.0674(3) 14.8381(8) b (Å) 16.7937(4) 16.7498(10) c (Å) 27.9584(6) 18.1161(19) α (°) 90 90 β (°) 90.985(1) 96.559(4) γ (°) 90 90 V (Å3₎ _5195.7(2) _4473.0(6) Z 4 4 ρcalc, g.cm-3 1.038 1.225 Radiation [Å] 1.54178 0.71073 µ(Mo Kα), mm-1 0.102 µ(Cu Kα), mm-1 0.454 F (000) 1760 1760 temp (K) 100(2) 100(2) θ range (°) 3.07 - 75.01 2.958 - 29.686 Data collected (h,k,l) -13:11; -20:19; -33:32; -20:20; -23:23; -25:25; no. of rflns collected 51829 181202

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119

no. of indpndt reflns 9493 12588

observed reflns Fo ≥ 2.0 σ (Fo) 8215 10500 R(F) (%) 4.96 3.98 wR(F2_{) (%)} _15.67 _10.61 GooF 1.038 1.017 weighting a,b 0.0918, 2.0831 0.0471, 2.3119 params refined 555 598

min, max resid dens -0.343, 0.300 -0.276, 0.367

5.7.4 Computational studies

Calculations were performed with the Gaussian09 program47_{using density functional theory}

(DFT) in the gas phase. The geometries of the anions were fully optimized (after removing the counter cation from the crystallographically determined structure) using the B3LYP exchange-correlation functional with the 6-311+G(d,p) basis set. Optimizations were performed without (symmetry) constraints, and the resulting structures were confirmed to be minima on the potential energy surface by frequency calculations (number of imaginary frequencies = 0). The stationary points found for 22-calc, Bn1-calc and Bn2-calc closely resemble their crystallographically

determined structures, respectively. For the NBO analysis, single point calculations were performed on the final optimized geometries of 12-calc,30₂_2-calc_,_Bn₁_-calc_and_Bn₂_-calc_{by including}

keywords for NBO calculation.

To explore the chemical exchange process of 22-_{due to the rotation around the N-C(Ph) bond,}

we scanned the dihedral angle between the two adjacent N atoms in the ligand and the ipso- and ortho-carbons of the Ph group in steps of 10 degrees (all other degrees of freedom were optimized). Starting from the highest energy points along the scan coordinate, the transition state was optimized and confirmed by frequency analysis (number of imaginary frequencies for

22-calc-ts =1).

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5.8 Supplementary information

5.8.1 UV-Vis spectroscopy

Figure S5.1 UV-vis spectra of [Bn_1][Na],32_[Bn_1][NBu₄_{], [}Bn_{2][Na] and [}Bn_2][NBu₄_{] at room}

temperature in THF

5.8.2 NMR spectra

Figure S5.2 1_{H NMR spectra of compound}₂2-_{at various temperatures in THF-d}_8. 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 260 310 360 410 460 510 ε(L it. m ol -1.cm -1x 10000) Wavelength (nm) [Bn1][Na] [Bn1][NBu4] [Bn2][Na] [Bn2][NBu4]

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5.8.3 Estimation of upper limit of the barrier for N-C(Ph) bond rotation in

1

2-The 1_{H NMR spectra of}₁2-_{do not show decoalescence down to 213 K. Even though the}

frequency difference between the exchanging 1_{H resonances in}₁2-_{could thus not be measured}

experimentally, an upper limit for the activation free energy can be estimated as follows: The coalescence temperature found for 22-_{is ca. 283 K with a frequency difference at slow}

exchange of ca. 290 Hz; from this the exchange rate and activation free energy at the coalescence temperature can be determined as k = 644 s-1_{and ΔG}‡₂₈₃_{= 54 kJ/mol. Assuming}

that the frequency difference is similar in 1-2_{, this means that in this compound the exchange}

rate at 213 K is substantially larger than 644 s-1_{, meaning that the activation free energy at 213}

K (ΔG‡₂₁₃_{) is < 40 kJ/mol.}

5.8.4 Determination of exchange kinetics in compound 2

2-

1_{H NMR data for compound}₂2-_{were collected in the temperature range 233 – 303 K. The}

Varian NMR data files were converted to the gNMR49_{file format using the gCVT tool included}

in the gNMR installation. The chemical shifts of the peaks of interest (those of the N-Ph group) were taken from the experimental spectrum and exchange between pairs (the two ortho-H and meta-H) was modelled; the signal for the para-H was included without exchange. The latter peak 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 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 S5.4. 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 11.2,}50_and

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Figure S5.3 Eyring plot for the calculation of activation parameters for rotation around the

N-C(Ph) bond in compound 22-_. ∆H‡_{= 54.1± 1.7 kJ/mol} ∆S‡_{= -2.5 ± 6.0 J/mol/K}

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

273 K: estimated rate constant k = 170 ± 10 s-1 _{283 K: estimated rate constant k = 500 ± 100 s}-1 253 K: estimated rate constant k = 27 ± 3 s-1

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Figure S5.4 Comparison of experimental and simulated 1_{H-NMR spectra of}₂2-_{(for each}

temperature, top: experimental spectrum, bottom: simulated). Rate constants used for the

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

5.8.5 Analysis of reaction kinetics of [

Bn

_1][NBu

₄

_{] + TEMPO, [}

Bn

_{2][Na] +}

TEMPO and [

Bn

_2][NBu

₄

_{] + TEMPO}

A solution of [Bn_{2][Na] in THF-d}₈_{was prepared in a J. Young’s NMR tube and a few crystals}

of bibenzyl were added as internal standard. Subsequently, 20 equiv of TEMPO were added to efficiently trap the benzyl radical generated upon thermolysis. The tube was taken out from the glovebox and inserted into the probe of the NMR spectrometer, which was heated to the desired reaction temperature. The NMR probe was tuned and shimmed before insertion of the sample using another NMR tube with the same amount of THF-d8 at that temperature. After the sample

was inserted, an acquisition array was started that was set up to collect spectra at regular time intervals throughout the course of the reaction (see Figure S8 for example of a few traces of the reaction of [Bn_{2][Na] with TEMPO (20 equiv)). Integration of the TEMPO-Bn resonance (δ}

4.82 ppm in the 1_{H NMR spectrum) relative to the internal standard bibenzyl allowed analysis}

of the rate constant at each temperature by non-linear curve fitting (a+b(1-Exp[-kt]) in Mathematica 11.2.50_{To investigate the effects of counter cation, the kinetics of benzyl transfer}

to TEMPO from [Bn_1][NBu₄_{] and [}Bn_2][NBu₄_{] were measured using this procedure. For the}

kinetics of benzyl transfer to TEMPO from [Bn_1][NBu₄_{] and [}Bn_2][NBu₄_{], the residual proton}

resonances from THF-d8 (at 1.73 ppm for [Bn1][NBu4] and at 3.58 ppm for [Bn2][NBu4],

respectively) were used as an internal standard due to the overlapping nature of the bibenzyl resonance (at 2.9 ppm) with the one of the proton resonances from NBu4+ (see Figure S9 and

Figure S10 for example of a few traces of the reaction of [Bn_2][NBu₄_{] and [}Bn_1][NBu₄_{] with}

TEMPO (20 equiv), respectively).

303 K: estimated rate constant k = 2000 ± 200 s-1

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Figure S5.5 Selected 1_{H-NMR spectra from the 85}_{°C kinetics array for the reaction of [}Bn_2][Na]

with TEMPO (20 equiv) in THF-d8.

Figure S5.6 Selected 1_{H-NMR spectra from the 85}_{°C kinetics array for the reaction of}

[Bn_2][NBu₄_{] with TEMPO (20 equiv) in THF-d}₈_.

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125

Figure S5.7 Selected 1_{H-NMR spectra from the 85} °_{C kinetics array for the reaction of}

[Bn_1][NBu₄_{] with TEMPO (20 equiv) in THF-d}₈_.

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126

Figure S5.9 Eyring analysis for [Bn_{2][Na]; ∆H}‡_{= 107 ± 4 kJ∙mol}-1_{and ∆S}‡_{= 17 ± 11 J∙mol} -1_∙K-1

Figure S5.10 Kinetic traces for the reaction of [Bn_{2][Na] (Blue) and [}Bn_2][NBu₄_{] (Yellow) with}

TEMPO (20 equiv) in THF-d8 at 75 °C (left) and at 85 °C (right).

Figure S5.11 Kinetic traces for the reaction of [Bn_{1][Na] (Blue)}3_{and [}Bn_1][NBu₄_{] (Yellow) with}

TEMPO (20 equiv) in THF-d8 at 65 °C (left) and at 85 °C (right).

340 345 350 355 360 T 0.000 0.005 0.010 0.015 0.020k Eyringplot 0.00280 0.00285 0.00290 0.00295 12.0 11.5 11.0 10.5 10.0 1 T Ln k T linearEyringplot

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5.8.6 Computational and X-ray crystallographic characterization data

Figure S5.12 HOMOs of 22-calc (left) and the transition state for N-Ph rotation of 22-calc (2 2-calc-ts, right).

Table S5.1 Selected bond lengths (Å) for 12-_,8 ₁_2-calc_,8 _[Bn_1][NBu₄_],Bn₁_-calc_,₂2-_,₂_2-calc_,

[Bn_{2][Na] and}Bn₂_-calc_.

Compo-unds

12- ₁_2-calc _[Bn_1][NBu₄_] Bn₁_-calc ₂2- ₂_2-calc _[Bn_2][Na] Bn₂_-calc

B1-N1 1.578(3) 1.590 1.575(2) 1.585 Al1-N1 1.871(2) 1.896 1.866(9) 1.902 B1-N4 1.583(3) 1.590 1.584(2) 1.602 Al1-N4 1.873(2) 1.897 1.914(1) 1.946 B1-C21 1.637(4) 1.651 1.635(2) 1.645 Al1-C21 2.011(2) 2.03 1.991(1) 2.009 B1-C27 1.626(3) 1.645 1.624(3) 1.644 Al1-C27 2.008(2) 2.03 1.989(1) 2.015 N1-N2 1.428(3) 1.402 1.428(2) 1.426 N1-N2 1.432(2) 1.406 1.446(1) 1.441 N3-N4 1.433(3) 1.402 1.393(2) 1.356 N3-N4 1.432(2) 1.406 1.402(1) 1.364 N2-C7 1.325(3) 1.328 1.396(2) 1.413 N2-C7 1.332(2) 1.333 1.437(1) 1.443 C7-N3 1.326(3) 1.327 1.289(2) 1.293 C7-N3 1.328(2) 1.333 1.292(1) 1.293 N1-C1 1.379(3) 1.372 1.414(2) 1.412 N1-C1 1.375(2) 1.370 1.387(1) 1.386 N4-C15 1.384(3) 1.372 1.392(2) 1.404 N4-C15 1.371(3) 1.370 1.399(1) 1.403 N2-C33 1.468(2) 1.472 N2-C33 1.486(1) 1.493

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Table S5.2 Selected bond angles (°) for [Bn_1][NBu₄_],Bn₁_-calc_,_[Bn_{2][Na] and}Bn₂_-calc_.

Compounds [Bn_1][NBu₄_] Bn₁_-calc _[Bn_2][Na] Bn₂_-calc

N1-B1-N4 105.78(1) 104.58 N1-Al1-N4 95.24(4) 95.02 C21-B1-C27 115.82(1) 112.73 C21-Al1-C27 112.46(5) 112.33 C21-B1-N1 109.5(1) 111.07 C21-Al1-N1 109.15(4) 113.2 C21-B1-N4 110.28(1) 108.93 C21-Al1-N4 111.73(4) 110.16 C27-B1-N1 105.45(1) 108.8 C27-Al1-N1 114.96(4) 112.92 C27-B1-N4 109.44(1) 110.43 C27-Al1-N4 112.14(4) 112.04 N2-C7-N3 127.57(1) 125.17 N2-C7-N3 126.80(9) 125.12 B1-N1-N2 113.06(1) 114.43 Al1-N1-N2 119.07(6) 114.84 B1-N1-C1 124.31(1) 124.8 Al1-N1-C1 127.89(7) 129.65 C1-N1-N2 110.44(1) 111.82 C1-N1-N2 112.55(8) 114.23 N1-N2-C7 115.41(1) 114.88 N1-N2-C7 112.27(8) 111.09 N1-N2-C33 113.94(1) 114.11 N1-N2-C33 110.33(8) 111.74 C7-N2-C33 118.79(1) 116.19 C7-N2-C33 112.13(8) 113.33 N3-N4-B1 118.8(1) 120.96 N3-N4-Al1 117.27(7) 120.34 B1-N4-C15 127.75(1) 126.84 Al1-N4-C15 125.47(7) 124.24 N3-N4-C15 110.99(1) 112.19 N3-N4-C15 111.72(8) 112.44

Table S5.3 Wiberg bond indices for the selected bonds in 12-calc, 12-calc-ts, 22-calc, 22-calc-ts

12-calc 12-calc-ts 22-calc 22-calc-ts

Bonds Wiberg Bond Index

N-C(Ph) 1.2196/1.2196 1.2262/1.0049 1.2412/1.2415 1.2605/1.0365

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