Quo Vadis CO 2 Activation: Catalytic Reduction of CO 2 to Methanol Using Aluminum and Gallium/Carbon-based Ambiphiles

We report on so-called “hidden FLPs” (FLP: frustrated Lewis pair) consisting of a phosphorus ylide featuring a group 13 fragment in the ortho position of a phenyl ring scaffold to form five-membered ring structures. Although the formation of the Lewis acid/base adducts was observed in the solid state, most of the title compounds readily react with carbon dioxide to provide stable insertion products. Strikingly, 0.3–3.0 mol% of the reported aluminum and gallium/carbon-based ambiphiles catalyze the reduction of CO 2 to methanol with satisfactory high selectivity and yields using pinacol borane as stoichiometric reduction equivalent. Comprehensive computational studies provided valuable mechanistic insights and shed more light on activity differences.


Introduction
The use of fossil fuels to meet the world's energy needs and as a means of accessing chemical feedstocks is leading to a steady increase in carbon dioxide (CO 2 ) emissions.CO 2 levels in the atmosphere currently stand at 418.17 parts per million, as measured at the Mauna Loa Baseline Observatory on September 14, 2023. [1]CO 2 is considered a sustainable alternative C1 feedstock for fossil fuel-derived carbon monoxide (CO) because it is abundant, inexpensive, and non-toxic.A variety of compounds can be produced from CO 2 , including basic chemicals, polymers, and fuels. [2]However, the promising use of CO 2 as a C1 synthon faces significant practical challenges caused by its high thermodynamic stability.
The development of stable, low-cost, active, and selective catalysts that operate under mild conditions is of great interest to industry.On account of the significant interest in abundant main-group element catalysts in green chemistry and environmental protection, aluminum compounds gained a lot of attention in the last two decades. [3]This led to the increased use of aluminum compounds in catalytic reactions, as recently summarized in a review article by Roesky and co-workers. [4]5b] The reported aluminum hydride derivatives (Scheme 1) showed catalytic activities with catalyst loadings between 5 and 10 mol% after initial hydroalumination of CO 2 and further reaction with boranes. [5]Overviews of metal-free reductions of CO 2 were published by Fontaine and Stephan as well as Wirth and Melen. [6]Common FLP-systems consisting of B/P, [7] B/N, [8] Al/P [9] or Si/N [10] Lewis acid/Lewis base combinations reduce CO 2 in the presence of hydroboranes to methanol.Quite recently, Wang and Mo reported the geometrically constrained bis(silylene)-stabilized borylene, which activates C=O, NÀ H and PÀ P bonds in a cooperative manner.They further showed the catalytic activity in the reduction of CO 2 to the corresponding N-formamides in the presence of amines and HBpin. [11]he combination of a carbon-based Lewis base and an aluminum-based Lewis acid is uncommon in the field of FLP chemistry. [12]At variance, a whole series of hidden frustrated Lewis pairs with phosphorus or nitrogen bases are known (two selected examples are given in Scheme 1, top). [13]Interestingly, phosphorus ylides have proven to be valuable carbon Lewis bases for the stabilization of highly reactive compounds and small molecule activation chemistry. [14]Furthermore, we confirmed most recently that the cooperative action of an aluminum Lewis acid and a Lewis basic phosphorus ylide allows the activation of the NÀ H bond in NH 3 . [15]As already shown by Schlosser et al. and Sundermeyer et al., non-stabilized phosphorus ylides Ph 3 PC(R 1 )H (R 1 = alkyl, aryl) can be metallated in ortho-position on one of the phenyl groups. [16]This reactivity was exploited in this work to prepare ortho-aluminum-and ortho-gallium-substituted phosphorus ylides (referred to as o-AlCPs/o-GaCPs in the following) continuing our most recent work in that field of chemistry. [15]In this article, we report on the reactivity of these species with CO 2 and their successful application as catalysts for the reduction of CO 2 to methanol.
Crystals of all o-AlCPs 2 R were investigated by X-ray structure analysis.The molecular structure of 2 Me in the solid state is shown in Figure 1 (space group P2 1 /c), whereas the structures of the other compounds are compiled in the Supporting Information, Section S4.Selected bond lengths and angles of 2 R are listed in Table 1.
ized ylides Ph 3 PC(Me)BEt 2 (1.717(3) Å) or Ph 3 PC(R 1 )ER 2 2 Cl (E = Si, Ge with 1.682(2)-1.7055(15)14b,c] Comparing these with the average bond lengths for PÀ C single (1.87 Å) and P=C double bonds (1.67 Å), the P1À C1 bonds are closer to the value for a single bond. [17]This is confirmed by the calculation of the corresponding Wiberg Bond Indices (WBI), which are close to one (0.99 and 0.97, computed for 2 Me and 2 tBu , respectively).This observation is not surprising with regard to the Lewis-acidic character of the Al fragments.The observed C1À Al1 distances (2.112(3)-2.1429(12)Å), between the Lewis-basic carbon atom and the Lewis-acidic aluminum atom, and the shorter C5À Al1 distances of 1.988(2)-2.0363(11) Å to the bridging phenylene ring, further support the findings.The P1À C1À Al1 angles of 99.64(8)-103.47(5)°slightlydeviate from the ideal tetrahedral angle of 109.4°.The ylidic CMe 2 fragment at C1 is tilted by Φ = 26.8-40.0°from the plane spanned by the atoms Al1, C5, C4, and P1.Overall, the crystal structures clearly show that the closed fivemembered ring forms of the title compounds 2 R are the dominant resonance structures in the solid state.Now it was of interest whether the o-AlCPs react as socalled "hidden FLPs" under ring opening with small molecules, or whether the intramolecular saturation leads to the complete cancellation of the ambiphilic character.13b] The reaction of 2 R with CO 2 allowed the isolation of the CO 2 adducts of the o-AlCPs (R=Me (3 Me ), Et (3 Et ), t-Bu (3 tBu ), Mes (3 Mes ) and C 6 F 5 (3 C6F5 )) (Scheme 3).To this end, solutions of 2 R in benzene were gassed with CO 2 (1.1 bar) and then heated to 90 °C for 4-17 days.The course of the reaction was monitored by 31 P NMR spectroscopy.When 4 bars of CO 2 were applied, the reactions were completed after 4 days at 60 °C.
Crystals of 3 Me and 3 Mes , studied by X-ray structural analysis, formed directly from the benzene solution after cooling the reaction solution to room temperature (for 3 Me : Figure 2; space group P2 1 /n). 3 Et and 3 tBu crystallize by slow concentration of their benzene solutions (for 3 tBu : Supporting Information, Figure S52).Unfortunately, no complete data set could be obtained for the crystals of 3 Et and 3 Mes .
As can be seen from the molecular structures of 3 Me and 3 tBu in the solid state (Figure 2; Figure S52), a CO 2 molecule is inserted into the bond between the ylidic carbon atom and the aluminum fragment.The P1À C1 distance of 1.8593(13) Å in 3 tBu is elongated by almost 6 pm as compared to 2 tBu and is now very close to the value for a PÀ C single bond (1.87 Å) based on the atomic radii. [17]This can also be found in the P1À C1 distance (1.869(3) Å) in 3 Me .The angles O1À Al1À C5 = 107.08(11)°and C24À C1À P1 = 108.47(16)°for 3 Me and O1À Al1À C5 = 104.20(6)and C30À C1À P1 = 111.54(10) for 3 tBu , respectively, show an almost ideal tetrahedral coordination environment at the ylidic carbon and aluminum atoms.Comparing the angles C4À C5À Al1 (132.0(2)°) and C5À C4À P1 (119.1(2)°) in 3 Me and C4À C5À Al1 (132.55(11)°) and C5À C4À P1 (120.53(11)°) in 3 tBu , a clear widening (12°) of the angle between the bridging phenyl group and the aluminum atom can be seen.The respective Al1À O1 distances of 1.799(2) Å and 1.8075(12) Å, however, are almost identical and about 6 pm shorter than those in the CO 2 adduct of the geminal FLP published by Uhl et al. [18] By comparing the carbon-oxygen distances of O1À C24 = 1.286(3)Å and O2À C24 = 1.212(3)Å in 3 Me with the values for a CÀ O single (1.43 Å) and C=O double bond (1.19 Å), it becomes clear that the O2À C24 bond can be regarded as a slightly elongated double bond (computed WBIs of 1.66 and 1,68, for 3 Me and 3 tBu , respectively).The relatively short O1À C24 bond (WBIs of 1.19 and 1.16, respectively) indicates that the initial C=O double character in CO 2 is significantly reduced in the adduct, as expected.The corresponding bonds in 3 tBu show an almost identical picture.13b,18] The investigation in solution by 31 P NMR spectroscopy showed a high-field shift of the signals by 3-5 ppm (Table 2).This contrasts with the lengthening of the P1À C1 distances, for which a low-field shift would be expected due to the lower double bond character.The coordination of an oxygen atom to the aluminum atom reduces its electron-withdrawing character  Table 2. 31 P NMR chemical shifts of 2 R compared to the CO 2 adducts 3 R (ppm).Measured ν(C=O) IR stretching frequencies of 3 R (cm À 1 ).towards the bridging phenyl ring.The latter can thus provide more electron density for the phosphorus atom, which would explain the observed shift to the high-field.
To further substantiate the adduct formation o-AlCPs with CO 2 , especially of those where no crystals could be obtained, IR spectra of the isolated compounds 3 R were measured.The observed bands associated with the ν(C=O) stretching of 3 R are similar in all cases and appear in the usual range for C=O double bonds (see Table 2).
Moreover, the formation of the CO 2 adducts 3 R has been also studied computationally by means of Density Functional Theory (DFT) calculations at the PCM(benzene)-B3LYP-D3/def2-SVP level (see computational details in the Supporting Information, section S5).As depicted in Figure 3, which shows the computed reaction profiles for the reactions involving 2 Me and 2 tBu , the process occurs in a concerted manner leading to the formation of the new (P)CÀ C(=O) and AlÀ O(CO) bonds with concomitant rupture of the initial AlÀ C(P) bond in 2 R . [22]The process is exergonic by ΔG R of approx.À 14 to À 18 kcal/mol, which is in agreement with the observed stability of the CO 2 adducts under our conditions.In addition, the computed activation barriers of ΔG ¼ 6 � 36 kcal/mol are consistent with the observed low reaction rates at 90 °C (4-17 days).This CO 2 activation is therefore rather different from the analogous process involving the reversible, stepwise activation of NH 3 . [15]imilar to related intramolecular FLPs, [19] the cooperative action of the Lewis antagonists in 2 R can be revealed by applying the Natural Orbital for Chemical Valence (NOCV) method.As shown in Figure 4, the NOCV method indicates that two different orbital interactions take place simultaneously during the CO 2 activation mediated by 2 R , namely the donation from the ylide to the π*(C=O) molecular orbital (denoted as ρ1) and the donation from the terminal oxygen atom of the CO 2 to the vacant p-atomic orbital of the aluminum atom (denoted as ρ2).According to the associated stabilization energies (ΔE(ρ)) computed for the transition state involved in the 2 Me + CO 2 reaction, ρ1 is stronger than ρ2, which resembles the mode of action of related FLPs in the activation of CO 2 , [19d,e] therefore confirming the reactivity likeness of these species.
Next, we turned our attention to the catalytic reduction of CO 2 with HBpin as stoichiometric reduction equivalent.In a first attempt, we subjected the CO 2 adduct 3 tBu to the reaction with HBpin.Neither at RT nor at 60 °C a transformation was observed (see Supporting Information, Figure S39).This observation renders 3 tBu as a catalytically incompetent off-cycle intermediate.However, when 2 tBu was mixed with 30 equiv.HBpin in benzene-d 6 , degassed and subsequently exposed to 4 bar CO 2 full conversion of the borane was observed (Scheme 4).
After heating of the mixture to 60 °C for 6 days, 11 B NMR spectroscopy confirmed the catalytic conversion of CO 2 into boric esters.Furthermore, the metallacycle 2 tBu was fully regenerated as confirmed by 1 H and 31 P NMR spectroscopy, rendering the catalytic reduction of CO 2 most feasible.
Next, we investigated the impact of the R group in 2 R on the catalytic reduction of CO 2 with HBpin.Solutions of 2 R and 30 equiv. of HBpin in 0.6 ml benzene-d 6 were prepared, providing a 0.57 M solution with respect to HBpin and 3 mol% of catalyst.The reaction mixtures were degassed and subsequently exposed to 4 bar CO 2 , sealed and then heated to 60 °C.The formation of MeOBpin was monitored over a period  of 6 days by 1 H NMR spectroscopy using C 6 Me 6 as internal standard.
The reaction profiles in Figure 5 show that the t-Bu derivative 2 tBu (black datapoints) is the most efficient one and provides quantitative conversion of HBpin into MeOBpin.
Conversion with 2 Mes (yellow datapoints) reached 58 % after 6 days at 60 °C, whereas 2 Me (blue) and 2 Et (orange) reached a plateau at 52 % (after three days) and 43 % (after one day), respectively (details in Supporting Information Table S2).Qualitatively, the reaction profiles of 2 tBu , 2 Mes and 2 C6F5 show almost linear behaviour up to 60 % conversion.For 2 C6F5 (violet-blue) only slow formation of MeOBpin is observed, but even after 6 days a decrease of the catalyst's performance is not noted.This is in stark contrast to 2 Me and 2 Et , which are deactivated after 40-50 % conversion is reached.During these reactions, colorless precipitates were formed, which most likely corre-spond to the respective CO 2 adducts of 3 Me and 3 Et .These species are insoluble in benzene and do not react with HBpin at 60 °C, so that they are removed from the catalytic cycle.This finding is in full support of your initial experiment with 3 tBu / HBpin (vide supra) and is fully supported by our computational experiments (compare Figure 3 and Figure 6).
The catalyzed reaction of 2 tBu was stopped by the addition of water and subjected to NMR spectroscopy.A yield of 68 % of MeOH was determined by 1 H NMR spectroscopy using C 6 Me 6 as internal standard (Figure S42 in the Supporting Information).While the kinetic data presented in Figure 5 should be approached with caution, it is possible to make a careful comparison of catalytic performance with reported related systems.The TOFs of the Al-based catalysts 2 R vary between 0.03-0.27h À 1 (see Supporting Information, Table S2), which falls into the range of the square-planar Al complexes reported by Greb (TOF = 0.12 h À 1 ) and Riddlestone (TOF = 0.08). [20]Related group 2 and aluminum hydride complexes, which undergo hydrometallation in the CO 2 reduction, show comparable TOFs of 0.07-0.165a,21] To shed light on the mechanism of the catalytic reduction of CO 2 by HBpin mediated by 2 R , DFT calculations were conducted on the system involving 2 Me as a representative example and using a model borane where the methyl groups in HBpin were replaced by hydrogen atoms (HBeg).As can be seen from the computed reaction profile provided in Figure 6, the process begins with the insertion of one BÀ O entity of the borane into the AlÀ C bond of 2 Me through transition state TS1 (ΔG ¼ 6 = 32.1 kcal/mol) thus forming INT1.This reaction is kinetically favored over the alternative BÀ H activation reaction, which according to its computed barrier (ΔG ¼ 6 = 40.0kcal/mol) seems unfeasible, and over the initial CO 2 activation (ΔG ¼ 6 = 35.9kcal/ mol, see Figure 3).Subsequent exergonic (ΔG = À 10.1 kcal/mol)  activation reaction of CO 2 takes place to form INT2, a similar species to those reported for the process involving the abovementioned aluminum hydrides. [5]This reaction, which exhibits an accessible barrier of 28.1 kcal/mol (from INT1), occurs via TS2, a saddle point associated with the hydride addition to the electrophilic carbon atom of CO 2 with concomitant formation of the AlÀ O(C=O) bond.Subsequent slightly endergonic (ΔG = 6.1 kcal/mol) coordination of a second molecule of the borane produces INT3, which undergoes a hydroboration reaction through TS3 (ΔG ¼ 6 = 24.4kcal/mol) to afford intermediate INT4 in a highly exothermic reaction (ΔG = À 24 kcal/mol).Subsequent reaction with a new molecule of borane produces INT3 in a strongly exergonic reaction (ΔG = À 32.7 kcal/mol).The next reaction step takes place via TS4, a saddle point associated with the formation of the final CÀ H bond with concomitant release of (Beg)O(Beg), with a barrier of 32.6 kcal/mol.Final elimination reaction through TS5 (ΔG ¼ 6 = 29.1 kcal/mol) affords the fully reduced MeOBeg molecule with concomitant regeneration of the catalytic species 2 Me in a slightly exergonic transformation (ΔG = À 1.0 kcal/mol).
For completeness, we also computed the alternative profile starting from the CO 2 -activated product 3 Me (see Supporting Information Figure S56).Our calculations indicate that whereas the first hydroboration reaction is feasible (ΔG ¼ 6 = 35.0kcal/ mol), the second hydroboration reaction is unfeasible (barrier ca.60 kcal/mol), which is compatible with the lack of reactivity of 3 tBu observed experimentally (see above).
Having the HBpin (HBeg) activation involving the o-AlCPs and the capability of CO 2 reduction of the reported Ga hydride complexes of Aldridge and Goicoechea in mind, [22] we prepared the o-GaCPs analogues 4 R and investigated their activity as catalysts for the reduction of CO 2 .
The o-GaCPs (2-{GaR 2 }-C 6 H 4 )Ph 2 PCMe 2 (R=Et (4 Et ), t-Bu (4 tBu ), and C 6 F 5 (4 C6F5 )) were prepared in the same way as the o-AlCPs (Scheme 2).Recrystallization from hexane or cyclopentane afforded 4 R in pure form in yields of 15-59 %.Crystals of all o-GaCPs 4 R could be investigated by X-ray structure analysis.The molecular structure of 4 Et in the solid state is exemplarily shown in Figure 7 (space group P � 1).The structures of the other compounds are compiled in the Supporting Information, Section S4.Selected bond lengths and angles of 4 R are listed in Table 3, which, as expected, are quite similar to those found in their Al counterparts 2 R .
4 tBu showed the same stability towards air and moisture as reported for 2 tBu . [15]It can be stored on the bench for weeks and NMR spectra can be recorded in non-dried solvents without any signs of decomposition.In contrast, 4 Et decomposes slowly by applying high vacuum, in solution, and while storing the solid in the glovebox at ambient temperature.
Although our DFT calculations suggest that the formation of the CO 2 adduct (ΔG ¼ 6 = 32.4kcal/mol) starting from 4 R is related to their aluminum derivatives 2 R , the process is much less exergonic (ΔG R = À 7.7 kcal/mol for 4 tBu ) than for the corresponding Al derivatives (ΔG R = À 17.9 kcal/mol for 2 tBu , cf. Figure 3).Exposing of a solution of 4 tBu to CO 2 (1.1 bar) and heating for a week did neither furnish the adduct, nor other reactions did occur according to the 1 H and 31 P NMR spectra.Despite that, we successfully applied 4 tBu and 4 C6F5 in the catalytic reduction of CO 2 using HBpin (Table 4), which further strongly supports the insertion of the borane as the initial step in the CO 2 reduction.
4 tBu and 4 C6F5 showed catalytic activities that exceed that of the aluminum analogues 2 R by an order of magnitude.Even with lower catalyst loadings of 0.3 mol% (3 mol% for Al derivatives) based on HBpin, the reaction times are significantly reduced to 24-48 h.Surprisingly, the catalytic activity of the o-GaCPs 4 R surpass the activity of the reported Ga hydrides (TOF = 2.5 and 2.6 h À 1 ).Quenching the reaction mixture of 4 tBu with H 2 O gave 77 % of methanol according to 1 H NMR spectroscopic investigations using C 6 Me 6 as internal standard.
The significantly increased catalytic activity of the Ga derivatives compared to the Al analog may be accounted by two ways.First, the strongly exergonic off-cycle deactivation pathway involving the formation of the CO 2 adducts 3 R is absent for the Ga derivatives, leading to a higher concentration of catalytically competent species (Scheme 5).Second, a more feasible energetic profile (Figure 6) may be present for the Ga derivatives.Indeed, our calculations confirm that the process  3. [a] Φ = tilt angle of the CMe 2 fragment at C1, i. e. the measured angle between the planes spanned by the atoms Ga1, C5, C4, P1 and Ga1, C1, P1.
involving 4 Me proceeds through a rather similar reaction pathway where the final elimination step, leading to the fully reduced MeOBpin (MeOBeg in the calculations) and 4 Me , is more exergonic (ΔG = À 6.2 kcal/mol) and requires a lower activation barrier (ΔG ¼ 6 = 26.4kcal/mol, see Figure 6).

Conclusions
We report on a rare combination of an aluminum or gallium Lewis acid and a carbon Lewis base in the field of frustrated Lewis pair chemistry.Herein, we further elaborate on a new class of hidden FLPs consisting of a phosphorus ylide featuring an aluminum or gallium fragment in the ortho position of a phenyl ring scaffold.The o-AlCPs (2 R ) and o-GaCPs (4 R ) are accessible through salt metathesis of the lithiated ylide 1 with the corresponding dialkyl element halides.Although the formation of the intramolecular Lewis acid/base adduct was observed in the solid state, the o-AlCPs (2 R ) readily react with carbon dioxide (CO 2 ), forming stable adducts 3 R .The analogous gallium derivatives 4 R , however, were surprisingly unreactive towards CO 2 .Despite this, all title compounds are capable of catalyzing the reduction of CO 2 to methanol with satisfactory high selectivity, yields, and low catalyst loadings of 0.3-3 mol% using HBpin as stoichiometric reduction equivalent.The computed reaction profile suggests that the reduction is initiated by the insertion of one BÀ O entity of the borane into the EÀ C bond (E=Al, Ga) followed by the hydride addition to CO 2 and not by the adduct formation with CO 2 .Interestingly, o-GaCPs 4 tBu and 4 C6F5 exceed the o-AlCPs 2 R in orders of magnitude in their catalytic activity.It is assumed that the suppressed reaction pathway of the CO 2 adduct formation, on the one hand, leads to full conversion in both cases with 4 R , and on the other hand, makes the reduction of CO 2 more favorable.

Figure 1 .
Figure 1.Molecular structures of 2 Me in the solid state (ellipsoids with 30 % probability).The hydrogen atoms are omitted for clarity.For selected bond lengths (Å) and angles (°), see Table 1.

Figure 3 .
Figure 3. Computed profile for the reaction of CO 2 with 2 Me (black lines) and 2 tBu (grey lines).Relative energies (free energies, computed at 298 K, within parentheses) and bond distances in the transition states TS-CO2 are given in kcal/mol and angstroms, respectively.All data have been computed at the PCM(benzene)-B3LYP-D3/def2-SVP level.

Figure 5 .
Figure 5. Kinetic studies of the catalytic reduction of CO 2 with HBpin and 3.3 mol% 2 R .Plotted is the conversion of HBpin vs. reaction time.Catalyst concentration of 10 mol% (3.33 mol%) based on CO 2 (HBpin).

Figure 6 .
Figure 6.Computed reaction profiles of the catalytic reduction of CO 2 using model HBeg mediated by 2 Me (black values) or 4 Me (blue values).Relative free energies (ΔG, at 298 K) are given in kcal/mol.All data have been computed at the PCM(benzene)-B3LYP-D3/def2-SVP level.

Figure 7 .
Figure 7. Molecular structure of 4 Et in the solid state (ellipsoids with 30 % probability).The hydrogen atoms are omitted for clarity.For selected bond lengths (Å) and angles (°), see Table3.

Table 1 .
Selected bond lengths d (Å) and angles
31lvents were dried over Na/K or CaH 2 and rigorously degassed before use.NMR spectra were recorded on a Bruker Avance Neo 400 or an Avance 300 spectrometer operating at 1 H Larmor frequencies of 400 or 300 MHz in dry degassed deuterated solvents.For the kinetic studies we used a Migratek Spinsolve 80 Benchtop NMR spectrometer. 1 H, and 13 C{ 1 H} chemical shifts where reported against TMS,31P{ 1 H} against H 3 PO 4 and 19 F against BF 3 OEt 2 .Coupling constants (J) are given in Hertz as positive values, regardless of their real individual signs.IR spectra were measured on a Bruker Alpha spectrometer using the attenuated reflection technique (ATR) and the data are quoted in wavenumbers (cm À 1 ).Melting points were measured with a Thermo Fischer melting point apparatus and are not corrected.Elemental analyses were carried out in the institutional technical laboratories of the Karlsruhe Institute of Technology (KIT).