# A Robust C 3 -Symmetric Aluminate Hydride for CO 2 Hydroboration Catalysis: Mechanistic Insights and Countercation Influence on Catalytic Performance

Yuri C. A. Sokolovicz, Frédéric Hild, Satawat Tongdee, Christophe

Gourlaouen, João dos Santos, Henri Schrekker, Samuel Dagorne

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► To cite this version:

Yuri C. A. Sokolovicz, Frédéric Hild, Satawat Tongdee, Christophe Gourlaouen, João dos Santos, et al. A Robust C 3 -Symmetric Aluminate Hydride for CO 2 Hydroboration Catalysis: Mechanistic Insights and Countercation Influence on Catalytic Performance. Inorganic Chemistry, 2025, 64 (4), pp.2096-2105. 10.1021/acs.inorgchem.4c05070. [DOI: 10.1021/acs.inorgchem.4c05070](https://doi.org/10.1021/acs.inorgchem.4c05070)

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A Robust C3-symmetric Aluminate Hydride for CO2 Hydroboration Catalysis:
Mechanistic Insights and Counter-cation Influence on Catalytic Performance

Yuri C. A. Sokolovicz<sup>a,b,c</sup>, Frédéric Hild<sup>a</sup>, Satawat Tongdee<sup>a</sup>, Christophe

Gourlaouen<sup>d</sup>, João H. Z. dos Santos<sup>b</sup>, Henri S. Schrekker<sup>b,*</sup>, Samuel Dagorne<sup>a,*</sup>

<sup>a</sup>Institute of Chemistry, Université de Strasbourg, CNRS, 67000 Strasbourg, France.

<sup>b</sup>Chemistry Institute, Universidade Federal do Rio Grande do Sul, Porto Alegre, RS, 90650-01, Brazil.

<sup>c</sup>Department of Chemistry, Universidade Federal de Santa Maria, CCNE, Santa Maria, RS, 97105-900, Brazil.

<sup>d</sup>Molecular Modelling and Simulation Laboratory, Université de Strasbourg, CNRS, 67000, Strasbourg, France

<footnote>* Email: dagorne@unistra.fr, henri.schrekker@ufrgs.br</footnote>
**Abstract.** The present study details the synthesis and characterization of a robust, monomeric Al–H aluminate supported by a tridentate *tris*-phenolate ligand, isolated as [2][Li(THF)₄] and [2][N("Bu)₄] salts, which were then exploited as CO₂ hydroboration catalysts. As initial reactivity studies, it was observed that the nucleophilic Al-H anion in [2][C] (C = counter cation [Li(THF)₄]⁺ or [N("Bu)₄]⁺) reacts fast with CO₂, to afford the corresponding Al formate complexes [3][C], which were isolated and structurally characterized. Such anions were then exploited as potential CO₂ reduction catalysts. Salts [2-3][N("Bu)₄] are efficient and robust CO₂ hydroboration catalysts in the presence of pinBH or Me₂S-BH₃ as hydroborane sources to selectively afford formate-equivalent or methanol-equivalent products (TON up 1920), depending on reaction conditions and the nature of the counter-cation.

As deduced from detailed DFT calculations, the Al formate anion [3]⁻ acts as a nucleophilic catalyst (for borane activation) but also as an electrophile (through the AlOCO carbon) allowing CO₂ activation/functionalization and thus the reduction catalysis to occur, a process thermodynamically driven by the stability of the reduction products. The anionic nature of [2]⁻ and [3]⁻ aluminate, resulting in an enhanced nucleophilicity (vs. neutral analogues), may thus be crucial for catalytic activity. In contrast, according to DFT calculations performed with a model anion of [3]⁻ and pinBH, a CO₂ reduction processing *via* an Al-O/B-H σ-bond metathesis appears kinetically unfavored. The proposed mechanism involving an electrophilic/nucleophilic dual activation mode also rationalizes the importance of counter-cation [C]⁺ in [2-3][C] for catalytic activity and selectivity, as demonstrated by the higher performance of [2][N("Bu)₄] vs. [2][Li(THF)₄].
Introduction

The catalytic functionalization of carbon dioxide, a cheap and readily available C1 source, is currently of tremendous interest to access chemicals with added value, most notably methanol, in a sustainable manner.¹,² The high thermodynamic stability of CO2 requires the development of efficient catalysts for CO2 activation and functionalization catalysis. To this end, numerous metal-based homogeneous catalysts have been investigated over the past years using hydrogen,³ hydroboranes⁴ and hydrosilanes⁵ as reducting/functionalizing agents. Though CO2 reductive hydrogenation with H₂ is most desired for atom economy, only a few discrete metal catalysts may efficiently catalyze such a transformation given the relative inertness of H₂.³,⁶ Thus, the use of reductants such as hydroboranes and hydrosilanes, which are more reactive due to a polar H–E bond (E = B, Si), is particularly attractive for CO2 functionalization and has led to significant developments in metal-catalyzed CO2 reductions, including a number of product-selective catalytic systems (at the formate, ketal, methanol and methane reduction level) with transition as well as main group metal centers.⁴,⁵,⁷ In the area of CO2 hydroboration, various main group element species have been attracting significant attention because the frequent oxophilicity and Lewis acidity of those metal/heteroatom centers may permit CO2 activation and functionalization catalysis, most notably through FLP-type reactivity with the involvement of a two-component catalytic system.⁸ Because of their polar M–H bond and the presence of a nucleophilic hydride moiety, well-defined main group metal hydride complexes are also interesting candidates for CO2 functionalization catalysis, as CO2 may readily insert in the M–H bond as an initial reduction step.⁹ As a consequence, the use of main group species for CO2 hydroboration catalysis has emerged over the past few years.⁹,¹⁰

Group 13 metal hydride species are a currently fast-growing class of species for the catalytic reduction of various unsaturated polar/apolar substrates through hydrogenation and hydro-silylation/-boration catalysis, a research field particularly promoted by the earth-
abundance and low cost of Al metal.¹¹ Yet, discrete group 13 metal hydrides remain little explored as CO₂ hydroboration catalysts, with currently known systems (A-E) depicted in Figure 1.

<figure><img src="image_3.png" /><figcaption>**Figure 1.** Known well-defined group 13 metal hydrides for use in CO₂ hydroboration catalysis.</figcaption></figure>

The first example of CO₂ hydroboration catalysis by a group 13 metal hydride was reported by Aldridge in 2015 using a neutral Ga-H derivative (A, Figure 1).¹² A few other neutral Al-H species were also shown to catalyze CO₂ hydroboration since then, (B,¹³ C,¹⁴ E¹⁵; Figure 1) while Mézailles and So recently demonstrated that a cationic Al-H species may also effectively promote CO₂ hydroboration catalysis (D¹⁶; Figure 1). In contrast, to our knowledge, the use of discrete anionic Al-H complexes for CO₂ functionalization catalysis is not known to date, though the expected enhanced nucleophilicity of the M-H moiety in such anions (vs. neutral analogues) may promote effective CO₂ functionalization. In that regard, LiAlH₄ is
known to stoichiometrically reduce CO₂ since the 1950’s, and, more recently, Fedushkin and co-workers studied well-defined Al–H aluminates supported by BIAN-type ligands (BIAN = bis-iminoacenaphthene) for the stoichiometric insertion of CO₂.¹⁷ Of relevance to the subject, it should also be noted that the groups of Mulvey and Okuda reported on lithium diamidodihydridoaluminates operating as ketone hydroboration catalysts thanks to bimetallic cooperativity.¹⁸,¹⁹

We herein report on the synthesis and structure of a robust C₃-symmetric tridentate tris-phenolate Al hydride anion (as a Li⁺ or [N(nBu)₄]⁺ salt), its reactivity with CO₂ and use in CO₂ hydroboration catalysis. Combined experimental and computational studies led to a mechanistic rational accounting for the catalytic activity/selectivity and counter cation’s influence on catalysis.

## Results – Discussion

**Tris-phenolate Al–H aluminate salts [2][C]: structure and reactivity with CO₂.** The C₃-symmetric triphenol O,O,O-type ligand precursor **1** (Scheme 1) was chosen as a supporting ligand for coordination to Al(III) so that to provide appropriate electronic and steric stabilization to access robust four-coordinate mononuclear Al–H anions of the type [X₃Al–H]⁻. Besides, protio ligand **1** (Scheme 1) may be straightforwardly prepared through a one-step procedure from 2-tert-butyl-4-methylphenol triethyl orthoformate.²⁰ The coordination of tris-phenol ligand of type **1** (and related) with AlR₃ precursors has been previously studied and essentially exploited for the preparation of various multinuclear Al alkyl species and study of their reactivity.²¹ In contrast, the stoichiometric reaction of ligand **1** with 1 equiv of LiAlH₄ (THF, RT, 17 h) led to the isolation of well-defined mononuclear Al–H anion [2]⁻ as a [Li(THF)₄]⁺ salt as colorless powder (62 % yield, Scheme 1). The molecular structure of salt [2][Li(THF)₄] was established through X-ray crystallographic studies. Salt [2][Li(THF)₄]
crystallizes as discrete and fully dissociated [2]⁻ and [Li(THF)₄]⁺ ions. As shown in Figure 2, the molecular structure of anion [2]⁻ features a four-coordinate Al(III) center in a slightly distorted tetrahedral environment resulting from the κ³-O,O,O chelation of the tris-phenolate ligand and the presence of an Al–H hydride which was crystallographically located. In line with a symmetrical Al chelate, the Al–O distances (1.747 Å average) are nearly identical and in the normal range for Al–OPhO bonds,²² while the Al–H bond length (1.58(3) Å) also lies in the expected range for Al–H bonds (1.50–1.75 Å).²³

<figure><img src="image_4.png" /><figcaption>**Scheme 1.** Synthesis of the Al–H aluminate [2][Li(THF)₄]</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>**Figure 2.** Molecular structure (ORTEP view) of the Al–H anion [2]⁻ with partial atom labelling. The hydrogen atoms, except the Al–H hydride, are omitted for clarity. Selected bond distances (Å) and angles (°): Al1–O1 = 1.747(2), Al1–O2 = 1.750(2), Al1–O1 = 1.745(2), Al1–H1Al = 1.58(3), O1–Al1–O2 = 107.09(9), O1–Al1–O3 = 105.91(9), O2–Al1–O3 = 108.97(9), O2–Al1–H1AL = 115.5(11), O3–Al1–H1AL = 112.0(11).</figcaption></figure>
In solution, NMR data for [2][Li(THF)₄] agree with the solid state structure being retained in solution. In particular, the presence of only one set of ¹H and ¹³C NMR resonances for the tris-phenolate ligand is consistent with an effective C₃-symmetric structure for anion [2]⁻ under the studied conditions (C₆D₆ or CD₂Cl₂, RT). Salt [2][Li(THF)₄], soluble and stable for days in C₆D₆ or CD₂Cl₂ solution at room temperature, also showed no sign of decomposition upon heating in C₆D₆ at 120 °C for 48 h, demonstrated the thermal stability and robustness of anion of [2]⁻ in solution. IR analysis of [2][Li(THF)₄] further confirmed the presence of Al–H moiety with an Al–H absorption observed at 1805 cm⁻¹ in agreement with literature data on Al–H species.²³ While hydridoaluminate complexes frequently form multinuclear aggregates through hydrido/heteroatom bridges, an enhanced tendency compared to neutral analogues since incorporating more nucleophilic Al–H moieties, anion [2]⁻ lies among the few structurally characterized mononuclear hydridoaluminate species with a terminal Al–H group,²³,²⁴ thus reflecting an effective steric protection of the Al(III) center by the tris-phenolate ligand in [2]⁻.

To probe the potential influence of the counter-cation and possibly increase the reactivity/nucleophilicity of anion [2]⁻, salt [2][N(nBu)₄], incorporating the non-Lewis acidic [N(nBu)₄]⁺ cation, was prepared in a nearly quantitative yield (93% yield) via a cation exchange reaction between [2][Li(THF)₄] and Cl[N(n'Bu)₄]. All NMR data agree with the formation of salt [2][N(nBu)₄], with, in particular, NMR signals assigned to [2]⁻ being identical to those in [2][Li(THF)₄]. The Al–H IR stretch for [2][N(nBu)₄] (1804 cm⁻¹) is also nearly identical to that in [2][Li(THF)₄]. Unlike [2][Li(THF)₄], [2][N(nBu)₄] is only sparingly soluble in C₆D₆ at room temperature, but readily solubilizes in CD₂Cl₂. It also displays a similar thermal stability to [2][Li(THF)₄] (no decomposition after 48 h in C₆D₆, 48 h).

To study the potential usefulness of anion [2]⁻ for CO₂ functionalization, the reactivity of salts of [2][Li(THF)₄] and [2][N(n'Bu)₄] with CO₂ was initially evaluated. Both salts immediately react with CO₂ (1.5 atm) at room temperature to quantitatively afford within 30
min (not optimized) the corresponding formate products [3][Li(THF)₄] and [3][N(nBu)₄] (Scheme 2), arising from CO₂ insertion into the Al–H bond. Both salts were isolated in high yield (> 90%) as analytically pure colorless solids, as deduced from combustion analysis. ¹H and ¹³C NMR data for species [3][Li(THF)₄] and [3][N(nBu)₄] are consistent with the proposed formulation. In particular, NMR data are consistent with the presence of a formate moiety, including a ¹H NMR singlet (δ = 8.10 and 9.41 ppm in C₆D₆ for [3][Li(THF)₄] and [3][N(nBu)₄], respectively) assigned to the HCO₂–Al proton and a ¹³C NMR O₂C–H signal (δ = 169.4 and 164.0 ppm for [3][Li(THF)₄] and [3][N(nBu)₄], respectively). The significant ¹H NMR chemical shift difference for the HCO₂–Al proton in [3][Li(THF)₄] vs. [3][N(nBu)₄] possibly reflect different cation/anion close contacts in [3][N(nBu)₄] vs. [3][Li(THF)₄], though NOESY NMR experiments performed for both salts (C₆D₆, RT) were inconclusive on that matter.

<figure><img src="image_6.png" /><figcaption>**Scheme 2.** Synthesis of the Al formate complexes [3][Li(THF)₄] and [3][N(nBu)₄]</figcaption></figure>

Crystallization of a 1/10 dichloromethane/pentane solution of isolated [3][Li(THF)₄] led to the formation of single crystals of ions-associated [3-Li(THF)₃] instead, as established through XRD studies (Figure 3). Its formation thus arises from the coordination of the Lewis acidic Li⁺ cation (from [Li(THF)₄]⁺) to the Al-formate [3]⁻ to yield [3-Li(THF)₃] and release of a THF molecule. As shown in Figure 3, [3-Li(THF)₃] contains an Al-bonded formate anion
coordinated to a $[Li(THF)_3]^+$ moiety through O5 reflecting the Lewis basic character of the
terminal $[Al-OCHO^-]$ formate oxygen. The bonding and geometrical parameters at the
tetrahedral Al(III) and Li(I) centers are as expected, with Al–O and Li–O bond distances (1.721
and 1.920 Å average, respectively) in the normal range. When compared to the C–O bond length
of the free formate anion (fully π-delocalized, C–O = 1.251 Å), the C–O bond distances of the
Al-OCHO group in $[3-Li(THF)_3]$ (C35-O4 = 1.264(4), C35-O5 = 1.219(4) Å) indicate a
partially delocalized moiety, with the bridging formate acting essentially as a L-type ligand
towards $Li^+$ (through O5) and a X-type ligand (through O4) to Al.

<figure><img src="image_7.png" /><figcaption>**Figure 3.** Molecular structure (ORTEP view) of [κ³-O,O,O-{(C₁₁H₁₄O)₃CH}AlOCHO-Li(THF)₃] ([3-Li(THF)₃]) with partial atom labelling. The hydrogen atoms are omitted for clarity. Selected bond distances (Å) and angles (°): Al1-O1 = 1.727(2), Al1-O2 = 1.725(2), Al1-O3 = 1.718(2), Al1-O4 = 1.775(2), Li1-O5 = 1.921(7), Li1-O6 = 1.917(7), C35-O4 = 1.264(4), C35-O5 = 1.219(4), Li1-O5-C35 = 127.9(3), Al1-O4-C35 = 130.8(2).</figcaption></figure>

**CO₂ Hydroboration Catalysis.** The ability of Al–H species [2][Li(THF)₄] and [2][N(nBu)₄] to readily insert CO₂, a key step in CO₂ hydroboration catalysis by M–H species, prompted us to test them as CO₂ hydroboration catalysts. Initial tests (pinacolborane as a hydroborane source, PₐCO₂ = 1.5 atm, 10% mol Al species, C₆D₆) showed no reaction occurred after 24 h at room temperature. The reaction temperature was then set at 90 °C for the appropriate time with
various catalyst loadings and solvent conditions. All results are compiled in Table 1. Under these conditions, species [2][Li(THF)₄] (10% mol) is catalytically active and selectively catalyzes CO₂ hydroboration with a 71% pinBH conversion to the methoxyborane reduction product pinBOMe after 15 h (along with pinB-O-Bpin) according to ¹H and ¹¹B NMR data, while 93% pinBH conversion was achieved after 37 h (entries 1 and 2, Table 1). The NMR data for pinBOMe and pinB-O-Bpin match those reported in the literature.²⁹ In contrast, under identical conditions, the alkylammonium salt [2][N(nBu)₄] (10% mol) is a significantly faster CO₂ hydroboration catalyst and selectively affords the mono-reduction product pinBO₂CH, with 73% and 96% pinBH conversion within 2 h and 6 h, respectively (entries 3 and 4, Table 1). CO₂ hydroboration catalysis with isolated Al-formate [3][N(nBu)₄] (10 mol%) led to similar catalytic activity and selectivity to those of parent [2][N(nBu)₄], with 85% conversion of pinBH to formate product pinBO₂CH within 2 h (entry 5, Table 1).
<figure><img src="image_8.png" /><figcaption>**Table 1.** CO₂ hydroboration catalysis by pinBH catalyzed by anionic Al-H species [2-3][C] (C = Li(THF)₄, N(nBu)₄)ᵃ</figcaption></figure>

<table><thead><tr><th rowspan="2">Entry</th><th rowspan="2">Cat.</th><th rowspan="2">Cat.<br>(mol<br>%)</th><th rowspan="2">Time<br>(h)</th><th rowspan="2">HBpin<br>conv.<br>(%)<sup>b</sup></th><th colspan="2">Product selectivity</th><th rowspan="2">pinB-O-Bpin<br>form.<br>(%)<sup>b</sup></th></tr><tr><th>pinBO₂CH (%)<sup>b</sup></th><th>pinBOMe (%)<sup>b</sup></th></tr></thead><tbody><tr><td>1</td><td>[2][Li(THF)₄]</td><td>10</td><td>15</td><td>71</td><td>-</td><td>&gt; 98</td><td>22</td></tr><tr><td>2</td><td>[2][Li(THF)₄]</td><td>10</td><td>37</td><td>93</td><td>-</td><td>&gt; 98</td><td>30</td></tr><tr><td>3</td><td>[2][N("Bu)₄]</td><td>10</td><td>2</td><td>73</td><td>&gt; 98</td><td>-</td><td>24</td></tr><tr><td>4</td><td>[2][N("Bu)₄]</td><td>10</td><td>6</td><td>96</td><td>&gt; 98</td><td>-</td><td>32</td></tr><tr><td>5</td><td>[3][N("Bu)₄]</td><td>10</td><td>2</td><td>85</td><td>&gt; 98</td><td>-</td><td>27</td></tr><tr><td>6</td><td>[3-Li(THF)₃]</td><td>10</td><td>23</td><td>36</td><td>-</td><td>&gt; 98</td><td>10</td></tr><tr><td>7</td><td>[2][N("Bu)₄]</td><td>1</td><td>19</td><td>60</td><td>10</td><td>90</td><td>20</td></tr><tr><td>8</td><td>[2][N("Bu)₄]</td><td>1</td><td>48</td><td>95</td><td>4</td><td>96</td><td>31</td></tr><tr><td>9<sup>c</sup></td><td>[2][N("Bu)₄]</td><td>0.1</td><td>18</td><td>67</td><td>-</td><td>&gt; 98</td><td>21</td></tr><tr><td>10<sup>d</sup></td><td>[2][N("Bu)₄]</td><td>0.1</td><td>22</td><td>97</td><td>-</td><td>&gt; 98</td><td>33</td></tr><tr><td>11<sup>e</sup></td><td>[2][N("Bu)₄]</td><td>0.05</td><td>40</td><td>&gt; 98</td><td>-</td><td>&gt; 98</td><td>33</td></tr></tbody></table>

<footnote>ᵃ Conditions unless indicated otherwise: [Cat.]₀ = 0.015 M, P⁰CO₂ = 1.5 bar, C₆D₆, 90 °C. ᵇ Determined by ¹H and ¹¹B NMR spectroscopy. ᶜ [Cat.]₀ = 5x10⁻³ M, the NMR tube was re-charged once with CO₂ after 10 h of reaction. ᵈ Solvent free run, the NMR tube was re-charged once with CO₂ after 10 h of reaction. ᶠ Solvent free run, the NMR tube was re-charged once with CO₂ after 20 h of reaction.</footnote>
The lower catalytic activity of Al–H species [2][Li(THF)₄] is certainly due to the formation of the ions-associated CO₂ insertion product [3-Li(THF)₃] under catalytic conditions, thus indicating that Li⁺ coordination to the formate group is detrimental to catalytic activity. A reactive/nucleophilic formate moiety hence appears central for efficient catalysis. In line with the latter, performing the hydroboration catalysis using isolated Al-formate species [3-Li(THF)₃] as catalyst (10 % mol) only led to 36% pinBH conversion to pinBOMe after 23 h (C₆D₆, 90 °C), indicating that [3-Li(THF)₃] is even less active than pre-catalyst Al–H precursor [2][Li(THF)₄] (entry 6 vs. entry 1, Table 1). This suggests that, under catalytic conditions (i.e. excess pinBH), pinBH and [Li(THF)₄]⁺ are competing Lewis acidic entities for formate coordination, with pinBH coordination to the generated Al-O₂CH formate being required for the catalysis to occur (vide infra).

The selective formation of monoreduction product pinBO₂CH with catalyst [2][N(nBu)₄] (10% mol), i.e. at rather low feed in pinBH (entries 2 and 3, Table 1), is in line with a ready activation/functionalization of CO₂ by nucleophilic anion [3]⁻ in the presence of by pinBH. In contrast, under identical reaction conditions and catalyst loading, the selectivity in pinBOMe of catalyst [2][Li(THF)₄] agrees with a much slower CO₂ reduction to pinBO₂CH, competing with subsequent pinBO₂CH reduction to the most thermodynamical product pinBOMe.

For additional information on the relative catalysis rate of [2][Li(THF)₄] vs. [2][N(nBu)₄] in CO₂ hydroboration, kinetic data were acquired for both systems showing an observed first order dependence in pinBH concentration vs. time (see Supporting Information), with catalyst [2][N(nBu)₄] (kobs = 0.253 h⁻¹) being roughly five times faster than its [Li(THF)₄]⁺ analogue (kobs = 0.057 h⁻¹).

Next, the catalytic performances were further studied and optimized with the most active catalyst [2][N(nBu)₄]. Thus, using 1% mol of [2][N(nBu)₄] as catalyst led to 60% pinBH
conversion to a 1/9 pinBO₂CH/pinBOMe mixture (along with pinB-O-Bpin side product) after 19 h of reaction (entry 7, Table 1), while nearly complete conversion to pinBOMe (95% pinBH conversion, 96% selectivity in pinBOMe) occurs within 48 h (entry 8, Table 1). ¹H and ¹¹B NMR monitoring of the catalysis agree with the formation of pinBO₂CH as the major product at low pinBH conversion (15 % pinBH conversion after 3 h to a 5/1 pinBO₂CH/pinBOMe mixture). At longer reaction time, the formation of the more stable B-OMe methoxy product (along with pinB-O-Bpin) occurs at higher pinBH loading (1% vs. 10% mol [2][N(nBu)₄], entries 7 and 8 vs. entries 3 and 4, Table 1). Remarkably, catalyst [2][N(nBu)₄] retains an effective CO₂ hydroboration activity even at 0.1 mol % loading (C₆D₆, 90 °C) with 67 % conversion of pinBH to pinBOMe (and pinB-O-Bpin) after 18 h (entry 9, Table 1), consistent with a robust catalyst under the studied conditions. [2][N(nBu)₄] was also tested under solvent free conditions (90 °C) at 0.1 % mol catalyst loading, affording a quantitative conversion of pinBH to pinBOMe after 22 h (entry 10, Table 1). A further decrease in catalyst loading (0.05 % mol) required longer reaction time (40 h) under solvent free conditions for complete borane conversion to pinBOMe, yet affording a TON of 1920 (entry 11, Table 1).

Last, the more reactive and industrially produced hydroborane BH₃-SMe₂ was also tested for CO₂ hydroboration catalysis with [2][N(nBu)₄] at 1% mol loading. While neglectable catalytic activity was observed at RT after 24 h, complete consumption of BH₃-SMe₂ to a 18/82 [(MeOBO)₃]/[B(OMe)₃] mixture was achieved after 24 h at 60 °C (not optimized, Scheme 3), as deduced from ¹H and ¹¹B NMR spectra and their comparison to literature data.²⁵

<figure><img src="image_9.png" /><figcaption>**Scheme 3.** CO₂ hydroboration catalysis with Me₂S-BH₃ mediated by [2][N(nBu)₄].</figcaption></figure>
**Control experiments between [2-3][N("Bu)₄] and pinBH in the presence/absence of CO₂.**
The hydroboration of ketones/aldehydes catalyzed by neutral aluminum hydride species such as [(BDI)Al(H)(OTf)] (BDI = β-diketiminate; Tf = triflate) was proposed to occur through an insertion/metathesis mechanism, with an initial insertion of the carbonyl substrate into the Al-H bond followed by a B-H/Al-O σ-bond metathesis reaction with the hydroborane leading to the hydroboration product and re-generating the Al-H catalyst.²⁶ In contrast, a subsequent report by Aldridge and Goicoechea showed that species [(BDI)Al(H)(OTf)] are inactive in CO₂ hydroboration catalysis with various hydroborane sources, which was attributed to unfavored B-H/Al-O σ-bond metathesis in the studied system.⁷ᵃ

To better understand how CO₂ hydroboration catalysis proceeds with [2-3][Li(THF)₄] and [2-3][N(nBu)₄], the reactivity of the Al formate salts [3][Li(THF)₄] and [3][N(nBu)₄] with pinBH was studied through NMR scale control experiments. No apparent reaction occurred between [3][Li(THF)₄]/[3][N(nBu)₄] and pinBH (1 or 4 equiv) after 24 h upon heating (90 °C, C₆D₆ for [3][Li(THF)₄]; 80 °C, C₂D₄Cl₂ for [3][N(nBu)₄]), as deduced from ¹H and ¹¹B NMR data, suggesting that B-H/Al-O σ-bond metathesis is not favorable for the present systems.²⁷ In contrast, in the presence of CO₂ (1.5 bar), the reaction of [3][N("Bu)₄] with 4 equiv of pinBH (C₂D₄Cl₂, 80 °C) led to 55% conversion of pinBH to monoreduction product pinBO₂CH after 2 h, with species [3][N("Bu)₄] retaining its integrity all along the catalytic process according to ¹H and ¹¹B NMR data (see Supporting Information). Using the Al-H salt [2][N("Bu)₄] as starting catalyst instead led to an identical product outcome, in line with the immediate and quantitative formation of CO₂ insertion product [3][N(nBu)₄] prior to catalysis. Overall, besides suggesting that salt [3][N(nBu)₄] is a resting state in the present CO₂ hydroboration catalysis, the above experiments only led to the observation of the reaction products, which is consistent with CO₂
hydroboration catalysis proceeding through higher energy intermediates (not observable) and
with the catalysis being thermodynamically driven by the formation of the reduction products.

**DFT-estimated CO₂ hydroboration mechanism.** For further mechanistic insights, DFT calculations (ωB97XD theory level) were performed for CO₂ hydroboration catalysis in the presence of pinBH considering the anionic Al-H model [II]⁻ (model of the Al-H anion [2])²⁸ as pre-catalyst. All computed energies were evaluated in benzene using a PCM model of solvation, and the mechanistic profiles are depicted in Figures 4-8. Thus, in accordance with experimental results, the reaction of anion [II]⁻ with CO₂ was initially modeled and found to proceed through a nucleophilic attack by the Al-H hydride [II]⁻ of an incoming CO₂ molecule (TS1, ΔG = 23.2 kcal.mol⁻¹, Figure 4) to afford the more thermodynamically stable Al formate model [III]⁻ (ΔG = -25.3 kcal.mol⁻¹), thus with an energy barrier compatible with a CO₂ insertion occurring at room temperature as experimentally observed.

<figure><img src="image_10.png" /><figcaption>**Figure 4.** left: DFT-estimated (ωB97XD/Gaussian 09) reaction profile of CO₂ insertion into the Al-H model [II]⁻ (all energies are reported in benzene solution). Right: molecular structure of transition state **TS1**.</figcaption></figure>

From Al formate anion [III]⁻, a σ-bond metathesis/insertion mechanism was first considered to account for the catalytic formation of pinBO₂CH (Figure 5). From [III]⁻ and pinBH (ΔG = -25.3 kcal.mol⁻¹), the Al-O/B-H σ-bond metathesis reaction leading to the Al
hydride anion [II]⁻ and generating pinBO₂CH (ΔG = -8.9 kcal.mol⁻¹) is thermodynamically disfavored, being endergonic by 16.4 kcal.mol⁻¹. From this point, the re-generation of catalyst [III]⁻ requires a CO₂ insertion reaction with [II]⁻, proceeding through transition state TS1 (ΔG = 14.3 kcal.mol⁻¹) with thus an energy barrier of 23.2 kcal.mol⁻¹ from [II]⁻. Though thermodynamically feasible, such pathway appears kinetically unfavorable because the experimental barrier (ΔΔG = 39.6 kcal.mol⁻¹ from [III]⁻ + pinBH) is too high given the catalysis conditions. Therefore, for the present system, and in line with earlier studies with neutral Al species [(BDI)Al(H)(OTf)], a CO₂ hydroboration catalysis involving an Al-O/B-H σ-bond metathesis reaction seems unlikely.

<figure><img src="image_11.png" /><figcaption>**Figure 5.** DFT-estimated (ωB97XD/Gaussian 09) reaction profile of CO₂ hydroboration catalyzed by Al formate anion [III]⁻ considering an Al-O/B-H σ-bond metathesis and insertion sequence.</figcaption></figure>

Next, an alternative mechanism exploiting the Lewis basic/nucleophilic character of the Al formate anion [III]⁻ was computed (Figure 6). Anion [III]⁻ may readily coordinate to pinBH to form the corresponding Lewis pair [III-pinBH]⁻ (ΔG = -13.9 kcal.mol⁻¹), in which the B-H hydride is more nucleophilic than in pinBH. Though less thermodynamically stable, [III-pinBH]⁻ is only a fleeting intermediate that reacts fast with CO₂ through transition state TS2 (ΔG = 7.9 kcal.mol⁻¹) to form the CO₂ reduction intermediate [IV]⁻ incorporating a formate moiety (ΔG = -20.0 kcal.mol⁻¹). Interestingly, the molecular structure of TS2 shows that CO₂
functionalization occurs through a dual activation process: i.e. a nucleophilic attack of CO₂ by the B-*H* hydride *concerted* with an attack of the electrophilic Al-O₂CH carbon by the B-*H*-activated CO₂ (O8, molecular structure of **TS2**, Figure 6). The formate intermediate [*IV*]⁻ may then rearrange *via* formate transfer to the boron center, through **TS3** (ΔG = -7.8 kcal.mol⁻¹, see Supporting Information), to produce pinBO₂CH and re-generate catalyst [*III*]⁻ (ΔG = -30.6 kcal.mol⁻¹). Overall, the driving force of the reaction is the formation of the more thermodynamically stable reduction product pinBO₂CH. Also, the experimental barrier (ΔΔG = 33.2 kcal.mol⁻¹), though relatively high, remains compatible with the catalysis conditions (T = 90 °C for hours to days). In addition, the present dual nucleophilic/electrophilic activation mechanism is consistent with all experimental observations, in particular rationalizes the significantly lower catalytic activity of [*3*][Li(THF)₄] (*vs.* [*3*][N(*n*Bu)₄]). In [*3*][Li(THF)₄], the Lewis acidic Li⁺ may severely compete with pinBH for Al-formate coordination, thus the observed lower catalytic activity.
<figure><img src="image_12.png" /><figcaption>**Figure 6.** Top: DFT-estimated (ωB97XD/Gaussian 09) reaction profile of CO₂ hydroboration catalyzed by Al formate anion **III** to produce the mono-reduction production pinBO₂CH (all energies are reported in benzene solution). Bottom: molecular structure of transition state **TS2**.</figcaption></figure>

As depicted in Figure 7, mono-reduction product pinBO₂CH may then further be reduced in the presence of pinBH and anion [**III**]⁻, with adduct [**III-pinBH**]⁻ reacting with pinBO₂CH to produce formyl-equivalent Al species [**IV**]⁻ through **TS4** (ΔG = 7.5 kcal.mol⁻¹, see Supporting Information). Such reaction proceeds similarly to that with CO₂: i.e. *via* a nucleophilic attack of the pinBO₂CH C=O carbon by the B-*H* hydride concerted with the attack of the Al-OCO carbon by the activated carbonyl. Intermediate anion [**IV**]⁻ then intramolecularly reacts at low energy cost (**TS5**, ΔG = -11.6 kcal.mol⁻¹, Figure 7) to yield a 1/1 formaldehyde/pinB-O-Bpin mixture and re-generate catalyst [**III**]⁻ (ΔG = -38.8 kcal.mol⁻¹).
Besides being thermodynamically favored, the present pathway for pinBO₂CH reduction to formaldehyde occurs with an experimental energy barrier ($ΔΔG = 32.8$ kcal.mol⁻¹) compatible with reaction conditions.

<figure><img src="image_13.png" /><figcaption>**Figure 7.** Top: DFT-estimated (ωB97XD/Gaussian 09) reaction profile of CO₂ hydroboration catalyzed by Al formate anion [III]⁻ to produce the second reduction mixture formaldehyde and pinB-O-Bpin (all energies are reported in benzene solution). Bottom: molecular structure of transition state **TS5**.</figcaption></figure>

From formaldehyde, the third reduction process occurs following the same dual activation to that earlier computed with CO₂ and pinBO₂CH, as depicted in Figure 8. The reaction of [III-pinBH]⁻ with formaldehyde via **TS6** ($ΔG = -38.8$ kcal.mol⁻¹, Supporting
Information) affords intermediate [V]⁻ that reacts at little energy cost (**TS7**, ΔG = -24.7 kcal.mol⁻¹) via an intramolecular C-O/B-O σ-bond metathesis to produce the most thermodynamically stable product pinBOMe and re-generate catalyst [III]⁻ (ΔG = -58.5 kcal.mol⁻¹).

<figure><img src="image_14.png" /><figcaption>**Figure 8.** Top: DFT-estimated (ωB97XD/Gaussian 09) reaction profile of CO₂ hydroboration catalyzed by Al formate anion [III]⁻ to produce the pinBOMe (all energies are reported in benzene solution). Bottom: molecular structure of transition state **TS7**.</figcaption></figure>

Overall, the proposed DFT-computed mechanism for CO₂ hydroboration catalysis by Al formate model [III]⁻ (or pre-catalyst Al-H [II]⁻) is consistent with all experimental data, and
with all the reduction catalysis being thermodynamically driven. Importantly, it shows that both the nucleophilic nature of the Al formate anion [3]⁻ and its electrophilic character (AlOCO carbon) are crucial for CO₂, pinBO₂CH and formaldehyde activation. Such dual activation by [3]⁻ allows the hydroboration catalysis to proceed. In line with such mechanism, the countercation may directly impact catalytic performances by interaction/coordination with/to [3]⁻, as experimentally observed. In the present systems, the Al-O/B-H σ-bond metathesis and insertion sequence mechanism, reported to operate for the catalytic hydroboration of carbonyls by neutral Al species, appears unlikely from a kinetic point of view.

Conclusion

A robust, monomeric and sterically protected tetra-coordinate Al–H aluminate was readily prepared as [2][C] salts ([C]⁺ = [Li(THF)₄]⁺ and [N("Bu)₄]⁺), which were then successfully exploited as CO₂ hydroboration catalysts. The nucleophilic Al-H anion in [2][C] reacts fast with CO₂, despite steric hindrance at the Al(III) center, to afford the corresponding Al formate complexes [3][C], prompting their use in CO₂ reduction catalysis. Salts [2-3][N("Bu)₄] are indeed effective and robust CO₂ hydroboration catalysts in the presence of pinBH or Me₂S-BH₃ as hydroborane sources to selective afford formate-equivalent or a methanol-equivalent products (TON up 1920).

DFT calculations performed for mechanistic insights on the CO₂ hydroboration catalysis suggests that the Al formate anion [3]⁻ acts both as a nucleophile and electrophile permitting effective CO₂ activation/functionalization in a catalytic manner, with an overall process being thermodynamically driven by the higher stability of the reduction products. The dual electrophilic/nucleophilic activation of CO₂ (and subsequent reduction products) permitted by Al formate anion [3]⁻ in the presence of hydroborane appears crucial for an effective CO₂ hydroboration catalysis.
Experimental Section

**General procedures.** All reactions were performed under a dry nitrogen atmosphere using a MBraun Unilab glovebox. Dichloromethane, pentane and toluene were purchased from Sigma-Aldrich, dried in a MBraun SPS purification system, and stored over activated molecular sieves (4 Å) for at least 24 h prior to use. Tetrahydrofuran was purchased from Sigma-Aldrich, distilled with Na/benzophenone, and stored over activated molecular sieves (4 Å) for at least 48 h. Deuterated solvents C₆D₆, C₆D₅Br and were purchased from Sigma-Aldrich, distilled over CaH₂ and then dried over molecular sieves (4 Å) for at least 24 h. All other chemicals were purchased from Sigma-Aldrich and used as received. NMR spectra were recorded on Bruker AC 300, 400 or 500 MHz NMR spectrometers in Teflon-valved J-Young NMR tubes at ambient temperature. ¹H and ¹³C chemical shifts are reported *vs*. SiMe₄ and were determined by reference to the ¹H and ¹³C solvent peaks. Elemental analysis for all compounds were performed at the service de Microanalyse of Université de Strasbourg. The triphenol ligand precursor **1** was prepared according to a literature procedure.²⁰

[κ³-O,O,O-{(C₁₁H₁₄O)₃CH}AlH][Li(THF)₄] ([2][Li(THF)₄]): A pre-cooled 1M THF suspension of lithium aluminium hydride (1.00 mL, 1.00 mmol) was added drop-wise to a pre-cooled solution of ligand precursor **1** (0.50 g, 1.00 mmol) in THF (5 mL). The reaction mixture was stirred in a closed screw-capped flask at 25 °C for 17 h. From time to time the flask was opened to release the hydrogen pressure, and a colorless solution was obtained. Evaporation of the solvent afforded a colorless solid that was purified by precipitation in a 5:1 THF:pentane mixture, leading to salt [2][Li(THF)₄] as a colorless solid (0.45 g, 0.55 mmol, 55% yield) after drying under vacuum. Anal. calcd for C₅₀H₇₆AlLiO₇ (823.07): C, 72.96; H, 9.31; found: C, 72.54; H, 9.63. IR (Nujol): νAl-H = 1805 cm⁻¹. ¹H NMR (400 MHz, C₆D₆): δ 7.07 (d, ⁵JHH = 2.0 Hz, Ar, 3H); 7.00 (d, ⁵JHH = 1.6 Hz, Ar, 3H); 5.37 (s, CH, 1H); 3.29 (br, THF, 16H); 2.14 (s, CH₃, 9H); 1.59 (s, tBu, 27H); 1.28 (br, THF, 16H). ¹³C NMR (125 MHz, C₆D₆): δ 156.5 (Cipso); 140.0 (Cipso);
133.8 (C<sub>ipso</sub>); 131.3 (C<sub>ipso</sub>); 126.6 (C<sub>ipso</sub>); 126.1 (C<sub>ipso</sub>); 68.1 (THF); 59.9 (CH); 35.3 ('Bu); 30.7 (CH<sub>3</sub>); 25.5 (THF); 21.0 ('Bu). <sup>13</sup>C NMR (75 MHz, CD<sub>2</sub>Cl<sub>2</sub>): δ 156.0 (C<sub>ipso</sub>); 139.0 (C<sub>ipso</sub>); 133.3 (C<sub>ipso</sub>); 130.8 (C<sub>ipso</sub>); 125.8 (C<sub>ipso</sub>); 125.2 (C<sub>ipso</sub>); 68.3 (THF); 59.5 (CH); 34.7 ('Bu); 30.4 (CH<sub>3</sub>); 25.4 (THF); 20.4 ('Bu).

[κ³-O,O,O-{(C₁₁H₁₄O)₃CH}AlH][N(nBu)₄] ([2][N(nBu)₄]). In a nitrogen-filled glovebox and in a screw-capped vial, a pre-cooled solution of [N(nBu)₄]Cl (0.17 g, 0.63 mmol) in dichloromethane (2 mL) was added dropwise onto a precooled solution of the hydride salt [2][Li(THF)₄] (0.52 g, 0.63 mmol) in dichloromethane (5 mL). The reaction mixture subsequently got cloudy, and after 1 h of stirring at room temperature, the LiCl formed during the reaction was filtered and the filtrate was evaporated to dryness to yield the desired pure hydride complex [2][N(nBu)₄] as a colorless solid (0.45 g, 0.58 mmol, 93 % yield). Anal. Calcd for C₅₀H₈₀AlNO₃ (799.61): C, 77.98; H, 10.47; N, 1.82. Found: C, 77.63; H, 10.01; N, 1.72. IR (Nujol): ν<sub>Al-H</sub> = 1804 cm⁻¹. ¹H NMR (400 MHz, CD₂Cl₂): δ 6.88 (d, ⁵JₐH = 2.0 Hz, Ar, 3H), 6.84 (d, ⁵JₐH = 1.6 Hz, Ar, 3H), 5.07 (s, CH, 1H), 3.09 (t, ³JₐH = 7.6 Hz, CH₂, 8H), 2.22 (s, CH₃, 9H), 1.61 (m, CH₂, 8H), 1.46 (m, CH₂, 8H), 1.42 (s, 'Bu, 27H), 1.05 (t, ³JₐH = 6.8 Hz, CH₃, 12H). **Érreur ! Signet non défini.** ¹³C NMR (125 MHz, CD₂Cl₂): δ 158.0 (C<sub>ipso</sub>), 138.8 (C<sub>ipso</sub>), 134.3 (C<sub>ipso</sub>), 131.5 (C<sub>ipso</sub>), 125.5 (C<sub>ipso</sub>), 124.3 (C<sub>ipso</sub>), 61.0 (CH), 59.5 (CH₂), 35.2 ('Bu), 30.5 (CH₃), 24.3 (CH₂), 20.9 (CH₃), 20.1 (CH₂), 13.8 ('Bu).

[κ³-O,O,O-{(C₁₁H₁₄O)₃CH}AlOCHO][Li(THF)₄] ([3][Li(THF)₄]). A solution of [2][Li(THF)₄] (30.0 mg, 36.0 µmol) in C₆D₆ (0.7 mL) was placed into a J-Young NMR tube and cooled in liquid N₂. The tube was pressurized with CO₂ (1.5 bar). After 30 min at 25 °C, the Al-formate complex [3][Li(THF)₄] was quantitatively formed according to ¹H and ¹³C NMR data. Evaporation and subsequent pentane wash led to pure [3][Li(THF)₄] as a colorless solid (27.0 mg, 34.0 µmol, 93% yield). Anal. calcd for [3][Li(THF)₄], C₅₁H₇₆AlLiO₉ (876.08): C, 71.01; H, 8.62; found: C, 70.86; H, 8.53. ¹H NMR (300 MHz, C₆D₆): δ 8.10 (s, OCOH, 1H), 7.10 (d, ⁵JₐH = 2.1 Hz, Ar, 3H), 7.07 (d, ⁵JₐH = 2.0 Hz, Ar, 3H), 5.72 (s, CH, 1H), 3.41 (m, 16
H, THF); 2.16 (s, CH₃, 9H); 1.51 (s, tBu, 27H); 1.34 (m, 16 H). ¹³C NMR (126 MHz, C₆D₆): δ 169.4 (OCOH), 156.3 (Cipso), 138.5 (Cipso), 133.9 (Cipso), 132.8 (Cipso), 128.5, 126.4 (Cipso), 126.3, 68.1 (THF), 60.2 (CH), 35.2 (tBu), 30.7 (CH₃), 25.6 (THF), 21.1 (tBu). Crystallization of [3][Li(THF)₄] from a 1/10 CH₂Cl₂/pentane solvent mixture led the formation of single crystals of [3-Li(THF)₃] according to X-ray crystallographic analysis.

[κ³-O,O,O-{(C₁₁H₁₄O)₃CH}AlOCHO][N("Bu)₄] ([3][N("Bu)₄]). A solution of [2][N(nBu)₄] (30.0 mg, 39.0 µmol) in C₆D₆ (0.7 mL) was placed into a J-Young NMR tube and cooled in liquid N₂. The tube was pressurized with CO₂ (1.5 bar). After 1 h at 25 °C, the Alformate complex [3][N(nBu)₄] was quantitatively formed according to ¹H NMR data. Evaporation and subsequent pentane wash led to pure [3][N(nBu)₄] as a colorless solid (30.0 mg, 37.0 µmol, 95% yield). Anal. calcd for C₅₁H₈₀AlNO₇ (843.45): C, 75.24; H, 9.90; N, 1.72; found: C, 74.92; H, 9.76; N, 1.69. ¹H NMR (400 MHz, C₆D₆): δ 9.41 (s, 1H, OCOH, tBu), 7.24 (d, 7H, tBu) = 1.8 Hz, 3H, Ar), 7.15 (d, 5H, tBu) = 2.4 Hz, 3H, Ar), 5.82 (s, 1H, CH), 2.46 (m, CH₂, 8H, nBu), 2.25 (s, 9H, CH₃), 1.68 (s, tBu, 27H), 1.01 (m, 16H, nBu), 0.77 (m, 12H, nBu). ¹³C NMR (126 MHz, CD₂Cl₂): δ 164.0 (OCOH), 157.0 (Cipso), 139.0 (Cipso), 133.7 (Cipso), 131.8 (Cipso), 125.9 (Cipso), 125.5 (Cipso), 60.2 (CH), 59.4 (CH₂), 35.1 (tBu), 30.2 (CH₃), 24.3 (CH₂), 20.8 (CH₃), 20.1 (CH₂), 13.8 (tBu).

**General procedure for the reduction of CO₂ by hydroboration with aluminum salt complexes.**

In a nitrogen-filled glove box, a J-Young NMR tube was charged with a solution of the internal hexamethylbenzene standard in C₆D₆ (0.5 mL), the desired aluminium catalyst (6.1 µmol, 1.00 equiv) and the appropriate amount of the borane reagent (typically 10 or 100 equiv of HBpin or Me₂S-BH₃ vs. catalyst). For lower catalyst loadings (i.e., 0.1% and 0.05% mol), a 1M toluene solution of the catalyst was prepared and the proper amount introduced to the NMR tube, after which toluene was removed in vacuo before addition of the borane reagent and C₆D₆.
Next, the NMR tube was charged with CO₂ (1.5 bar) and was immersed into a pre-heated oil bath at 90 °C for the appropriate time. All catalysis runs were monitored by ¹H and ¹¹B NMR showing the formation of reduction products pinBO₂CH and/or pinBOMe, as deduced by comparison with literature data.²⁹ The residual solvent peak (C₆D₅H) was used as an internal standard.

**Supporting Information.** NMR spectra, DFT calculations details and selected structures drawings (PDF). Atom coordinates of models (XYZ)

CCDC 2395196 and 2395197 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/ cif, or by emailing data_request@ccdc.cam.ac.uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

**Acknowledgments.** The CNRS (Paris, France) and the University of Strasbourg are acknowledged for financial support. The Brazilian agency Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq; Science without Borders Special Visiting Scientist project 405784/2013-9; Doutorado Sanduíche no Exterior, 207466/2015-7; Chamada CNPq Nº 32/2023 - Pós-Doutorado Júnior - PDJ 2023, Processo 174545/2023-1) is acknowledged for financial support. The Agence Nationale de la Recherche [ANR, project Tu-Zin-Cat (ANR-19-CE07-0020-01)] is thanked for a PhD fellowship (S. T.).
For Table of Contents Only

<figure><img src="image_15.png" /></figure>

- Robust Al-H anionic catalyst

- Hydroboration
  via dual activation

- TON up to 1920
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(28) For simplicity, the methyl *para*-substituents of the phenol rings were replaced by hydrogens for the computation of all models.

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