Carbon dioxide reduction by an Al–O–P frustrated Lewis pair

The reaction of tBu2P(O)H with Bis2AlH (Bis = CH(SiMe3)2) afforded the adduct tBu2P(H)–O–Al(H)Bis2 (3). It slowly releases H2 to form the first oxygen-bridged geminal Al/P frustrated Lewis pair tBu2P–O–AlBis2. It is capable of reversibly binding molecular hydrogen to afford 3, shown by NMR and H/D scrambling experiments, and forms a 1,2-adduct with CO2. Importantly, the H2 adduct 3 reduces CO2 in a stoichiometric reaction leading to the formic acid adduct tBu2P(H)–O–Al(CO2H)Bis2. The formation of the different species was explored by density functional theory calculations which provide support for the experimental results. All products were characterized by NMR spectroscopy as well as X-ray diffraction experiments and elemental analyses.


Introduction
The persisting high level of interest in small molecule activation with frustrated Lewis pairs (FLPs) has led to an enormous variety of different combinations of acid and base functions, the chemistry and applications of which have been investigated in detail. 1 Most typically, boranes with electronegative C 6 F 5 substituents are used as Lewis acids.
Among the most intensely studied reactions of FLP are the activation of molecular hydrogen and carbon dioxide or a combination of both, that is a reduction of carbon dioxide. This was achieved with different systems containing boron compounds as Lewis acids. [2][3][4][5] One early example for this is the system TMP/B(C 6 F 5 ) 3 (TMP ¼ 2,2,6,6-tetramethylpiperidine), which cleaves molecular hydrogen to form [TMP-H] + [H-B(C 6 F 5 ) 3 ] À . 6 This ammonium borate complex reduces CO 2 to the formate stage when used in equimolar amounts and to methanolate when used in excess. 1c Despite its intrinsically higher Lewis acidity 7 the higher homologue aluminum was signicantly less frequently used. FLP systems containing aluminum demonstrated their great potential through the large variety of reaction types, 8 including C-H activation, 9,10 hydrogen activation, 11 and carbon dioxide reduction 12 or xation, 10,13 as well as their use as catalysts for polymerization reactions. 14 A convenient method to prepare geminal Al/P FLPs is the hydroalumination of alkynylphosphines. The obtained products tend to quench themselves by dimerization or the formation of adducts with the alane reagent, even though it was shown that dimeric species are also able to form adducts with CO 2 . 15 Geminal linker motifs other than carbon-based ones like alkylidene 10,16 or methylene 17 are rarely found for neutral group 13/P FLP. Köster et al. described the formation of different P-O-B species from reactions of diphenylphosphine oxide with trialkylboranes or dialkylhydridoboranes as early as 1987. However, they did not explicitly investigate adduct formations in the sense of FLP chemistry but only reported adducts with an additional hydridodialkylborane being coordinated to the phosphorus atom. 18 Only very recently were a few examples of neutral oxygen-bridged P-O-B systems presented, which show typical reactivity towards CO 2 and molecular hydrogen. However, these reactions require untypically harsh conditions or only reversibly form adducts despite the strongly electronegative substituents at boron. 19,20 Other reactivities of oxygenbridged FLP include the reversible binding of SO 2 by a P/B system derived from phosphinoboranes and the activation of B-Scheme 1 Synthesis of FLP 4 and reversible formation of the H 2 adduct 3.
H and O-H bonds by an N/B FLP which is formed in the reaction of TEMPO with HB(C 6 F 5 ) 2 . 21
The NMR spectra show the formation of compound 3 to be completed immediately aer mixing the substances. A doublet at 5.76 ppm in the 1 H NMR spectrum shows a characteristic P-H coupling constant of 457 Hz. The 31 P{ 1 H} NMR spectrum contains a signal at 72.6 ppm. The conversion of 3 into 4 under release of hydrogen is rather slow. It was monitored by NMR in a closed tube and is not completed even aer two days (conversion of about 92%). Complete conversion to 4 is achieved by removal of all volatiles in vacuo. The 31 P{ 1 H} NMR spectrum of 4 shows a singlet at 142.8 ppm; this corresponds well to the comparable P-O-B system tBu 2 P-O-Bcat (159.3 ppm). 20 Resonances of the methine protons of the CH(SiMe 3 ) 2 groups are very sensitive to the coordination number at aluminum; the signal for 4 at À0.38 ppm is in a typical range for tri-coordinated aluminum. 22 The determination of the solid-state structure by singlecrystal X-ray diffraction conrms the monomeric structure of 4 ( Fig. 1). The molecular structure exhibits a large Al(1)-O(1)-P(1) angle of 138.1(1) and an Al(1)/P(1) distance of 3.122(1)Å. The aluminum atom features a slightly distorted trigonalplanar coordination with O(1)-Al(1)-C(1/2) angles of 117.6(1) / 127.7(1) and a C(1)-Al(1)-C(2) angle of 114.6(1) . The P(1)-O(1) bond (1.635(1)Å) is clearly longer than the P-O bond in tBu 2 P(O)H (1.482(2)Å). 23 Reactivity towards hydrogen and carbon dioxide Strikingly, H/D-scrambling experiments with H 2 /D 2 mixtures at one atmosphere of pressure showed a reversible binding of molecular hydrogen to 4. However, this reaction is rather slow under the given conditions. At rst, the formation of small amounts of H 2 and D 2 adducts of 4 was observed aer ve days. Another week later, the characteristic triplet for HD was observed in the 1 H NMR spectrum at 4.57 ppm with a 1 J D,H coupling constant of 42.6 Hz. Density Functional Theory (DFT) calculations (PCM(hexane)-M06-2X/def2-TZVPP//PCM(hexane)-M06-2X/def2-SVP) conrmed for this binding of hydrogen a free energy difference between 3 and 4 + H 2 of only 1.6 kcal mol À1 (at 298 K), with 3 being the more energetically favorable species (Fig. 2). Considering a possible error in this number of a few kcal mol À1 , it is consistent with the experimental observation of a slow and incomplete formation of 4 in  the presence of hydrogen. This equilibrium indicates a reversible nature of this process and nds its conrmation by the computed interconversion barrier of DG ‡ ¼ 24.3 kcal mol À1 (from 3).
Interestingly, compared to the analogous systems containing the heavier elements of group 13, gallium and indium, the hydrogen adducts 3_b/c are thermodynamically favored and the barriers for the reactions 4_b/c + H 2 / 3_b/c are lower for the heavier FLPs.
The reaction of 4 with carbon dioxide afforded adduct 5 in nearly quantitative yield (Scheme 2). Unlike in previously reported P-O-B systems, the binding is stable at room temperature and an equilibrium is not formed. 19,20 Compared to the free FLP 4, the 31 P NMR signal at 64.8 ppm is clearly high eld shied, while the characteristic 13 C{ 1 H} NMR doublet for the bound CO 2 appears at 166.0 ppm with a 1 J P,C coupling constant of 98.6 Hz. According to our DFT calculations, the formation of 5 occurs in a concerted manner through the asynchronous transition state depicted in Scheme 2 24 and with an activation barrier of only 5.5 kcal mol À1 in a highly exergonic (DG R ¼ À22.9 kcal mol À1 ) transformation.

CO 2 reduction
The formation of the CO 2 adduct 5 in combination with the reversible binding of molecular hydrogen raised the question of how H 2 adduct 3 would react with CO 2 . Since 3 releases hydrogen slowly and is only formed in small amounts when 4 is treated with hydrogen, handling is only possible for a very limited time. We therefore mixed alane 1 and phosphine 2 to generate 3 which could be reacted directly before the release of hydrogen. Upon complete dissolution of the solids, this mixture was cooled to À196 C, and the reaction ask was degassed and backlled with CO 2 . By this, we achieved stoichiometric reduction of carbon dioxide to afford the formic acid adduct 6 (Scheme 3). The characteristic 1 H NMR signal for the formate moiety appears at 8.45 ppm, while the 31 P{ 1 H} NMR shi of 72.2 ppm is similar to that of 3. This unexpected reactivity makes FLP 4 a versatile tool with highly useful abilities. Being able to (reversibly) bind both CO 2 and H 2 individually on one hand and at the same time unite both reactivities to reduce carbon dioxide with previously activated molecular hydrogen almost instantly at ambient temperature and atmospheric pressure is a unique combination in the eld of FLP chemistry. As mentioned in the introduction, this reactivity has been observed for boron containing systems before. However, the reduction was only achieved at higher temperatures or elevated CO 2 pressure. 1c,2-5 However, the nature of the individual reactions of 3 with H 2 (equilibrium on the side of free H 2 ) and CO 2 (formation of a stable adduct) make the system unlikely to be used as a catalyst for the reduction of CO 2 .   Compound 6 forms an equilibrium in solution: small amounts of phosphine 2 and the alane Bis 2 Al(CO 2 H) (7) are observed by NMR spectroscopy (Scheme 3). Characteristic 1 H resonances are the doublet at 5.84 ppm for 2 and the signal of the formate group of 7 at 7.35 ppm.
The asymmetric unit of the unit cell of crystalline 6 contains two independent molecules, whose structural parameters differ mainly in the O-Al-O-P torsion angles (107.6(3) /41.6(12) ); the Al-O-P angles (168.8(1) /154.3(1) ) are noticeably wider than in 4. Moreover, the bond of aluminum to the bridging oxygen atom is signicantly longer (1.834(1)/1.814(1) vs. 1.708(1)Å) while the corresponding P-O bond is shorter (1.529(1)/1.519(1)Å vs. 1.635(1)Å). The aluminum atom exhibits a slightly distorted tetrahedral geometry, with a s 0 4 parameter 25 of 0.93 (Fig. 4). We observed that the formic acid adduct 6 is in equilibrium with small amounts of phosphine oxide 2 and alane 7 in solution. This raised the question of whether CO 2 could also be directly reduced with Bis 2 AlH and then treated with tBu 2 P(O)H to give 6. When alane 1 was reacted directly with CO 2 , the 1 H NMR spectrum of the dissolved solid obtained (7d, identied by XRD, see below) contained two sets of signals, those of the monomer 7 and dimer 7d. Signals for the formate protons are found at 8.84 (7d) and 7.33 ppm (7). Aer several days, practically only one species (7.33 ppm, 7) remained observable. Inconsistent with this experimental observations, the DFT calculations predict the dimerization of 7 to 7d to be rather exergonic (DG R ¼ À19.8 kcal mol À1 ).
The identication of these species is based on diffusion ordered spectroscopy (DOSY). Using the Stokes-Einstein equation, the diffusion coefficients of 4.65 Â 10 À10 m 2 s À1 (8.84 ppm, 7d) and 5.70 Â 10 À10 m 2 s À1 (7.33 ppm, 7) were converted into hydrodynamic radii and volumes: 7.21Å/1570Å 3 for 7d and 5.87 A/850Å 3 for 7. Compared to the empirically calculated values 26 of 6.29Å/1043Å 3 and 5.01Å/527Å 3 , the values in solution are increased due to a solvation shell. Still, the volume of the species at 8.84 ppm is about twice the size of the one at 7.33 ppm, which conrms that monomeric alane 7 forms from an initially dimeric species 7d upon dissolution (Scheme 4).
Solely the dimeric species was found in the solid state, as determined by X-ray diffraction studies of a single crystal (Fig. 5). It has an almost planar eight-membered ring with an inversion center. Each aluminum atom is coordinated by two oxygen atoms from different formate groups in a distorted tetrahedral manner ðs When adding phosphine oxide 2 to a solution of Bis 2 -Al(CO 2 H) (7), the formic acid adduct 6 is formed. However, as described above, the reaction remains incomplete, and small amounts of phosphine oxide 2 and alane 7 are detectable in solution, which further conrms the proposed equilibrium described above. DFT calculations were also carried out to gain more insight into the formation of 6 in solution (Fig. 6). We found that 1 and 2 form adduct 3 in a barrier-free and highly exergonic reaction (DG R ¼ À25.0 kcal mol À1 ). Subsequent reaction of 3 with CO 2 leads to the highly exergonic (DG R ¼ À27.0 kcal mol À1 ) formation of the formic acid adduct 6 via the Scheme 4 Reduction of CO 2 by Bis 2 AlH, equilibria between monomeric and dimeric alanes and formation of formic acid adduct 6.  transition state TS 3-6 , a saddle point mainly associated with the nucleophilic addition of the aluminum-hydride to the electrophilic carbon atom of CO 2 . Both the computed free energy of reaction and activation barrier (DG ‡ ¼ 19.3 kcal mol À1 ) are compatible with a process occurring at room temperature.
The alternative process involving the initial reaction of alane 1 with CO 2 was also explored. This transformation leads to the exergonic (DG R ¼ À44.0 kcal mol À1 ) formation of 7 with a low activation barrier of only 7.6 kcal mol À1 through the transition state TS 1-7 , a saddle point associated again with the concomitant formation of the new C-H and Al-O bonds. Subsequent barrier-free addition of phosphine oxide 2 leads to the formation of 6 again in an exergonic transformation (DG R ¼ À8.5 kcal mol À1 ).
The computed free energies of reaction and relatively low barriers therefore conrm the feasibility of both alternative processes, the formic acid adduct 6 being the most stable species in both computed proles. Our calculations also predict that the possible release of formic acid from 6 is highly endergonic (DG R ¼ +29.2 kcal mol À1 ) and thus highly unlikely (see Fig. S19 in the ESI †).

Conclusions
In essence, we have described herein the preparation, full characterization and reactivity of a rst geminal oxygen-bridged Al/P FLP tBu 2 P-O-AlBis 2 . The reactivity of this system includes the reversible binding of molecular hydrogen and adduct formation with carbon dioxide. Reduction of carbon dioxide was achieved with the FLP-H 2 addition product to the formate stage. Its reactivity is unique in the sense of being able to both, to reversibly split molecular hydrogen and to reduce carbon dioxide to the formate stage, with the H 2 addition product rapidly at ambient temperature and atmospheric pressure. Our ndings encourage us to further explore and investigate this reactivity. Theoretical results for analogous systems involving the heavier elements gallium and indium indicate the possibility of a facilitated uptake of hydrogen by a higher homologue of FLP 4. We also expect an easier release of formic acid from the higher homologues of 6. With this promising outlook we will focus on synthesizing these systems and conrming theoretical predictions with the aim to eventually achieve a catalytic reduction of carbon dioxide.

General considerations
All reactions and manipulations with air-and moisturesensitive compounds were carried out under conventional Schlenk techniques using nitrogen as inert gas or in a glove box using argon as inert gas. Volatile compounds were handled using a vacuum line. n-Hexane and [D 6 ]benzene were dried over a Na/K alloy, distilled and degassed prior to use. CDCl 3 was dried over 3Å molecular sieves, distilled and degassed prior to use. CO 2 (99.5%, Linde) was used without further purication. Bis 2 AlH (Bis ¼ CH(SiMe 3 ) 2 ) was prepared according to a literature procedure. 22 NMR spectra were recorded using a Bruker Avance III 300, Avance III 500 or Avance III 500 HD spectrometer at ambient temperature. Chemical shis were referenced to the residual proton or carbon signal of the solvent (C 6 D 6 : 1 H: 7.16 ppm, 13 C: 128.1 ppm; CDCl 3 : 1 H: 7.26 ppm) or externally ( 29 Si: SiMe 4 , 31 P: 85% H 3 PO 4 in H 2 O). Elemental analyses were carried out by co-workers of the University of Bielefeld using a EURO EA Elemental Analyzer.
Analytical Preparation of tBu 2 P(O)AlBis 2 $CO 2 (5). FLP (57 mg, 0.11 mmol) was placed in an ampoule with a greaseless tap and dissolved in n-hexane (5 mL). The solution was degassed (1Â freeze-pump-thaw) and CO 2 (0.46 mmol, 4.2 eq.) was condensed into the ampoule. The solution was stirred overnight and all volatiles were removed in vacuo to obtain FLP$CO 2 as a colorless solid (61 mg, 0.11 mmol, quant.). Crystals were obtained by slow evaporation of a solution of tBu 2 P(O)AlBis 2 $CO 2 in C 6 D 6 .  (6). Bis(bis-(trimethylsilyl)methyl)alane (139 mg, 401 mmol) and di-tertbutylphosphine oxide (65 mg, 0.40 mmol, 1 eq.) were placed in an ampoule with a greaseless tap and n-hexane (5 mL) was added. Right aer dissolution of all solids, the mixture was frozen with liquid N 2 and evacuated. CO 2 (800 mmol, 2 eq.) was condensed into the ampoule and the mixture was allowed to reach room temperature and stirred overnight. Aer removal of all volatiles, a colorless oil was obtained, which was recrystallized from n-hexane at À5 C to obtain tBu Preparation of Bis 2 Al(CO 2 H) (7)/(Bis 2 Al(CO 2 H)) 2 (7d). In an ampoule with a greaseless tap, bis(bis(trimethylsilyl)methyl)alane (105 mg, 303 mmol) was dissolved in n-hexane (4 mL) and degassed (1Â freeze-pump-thaw). CO 2 (0.61 mmol, 2 eq.) was condensed into the ampoule and the reaction mixture was allowed to reach room temperature, resulting in the formation of a colorless suspension which was stirred overnight. All volatiles were removed in vacuo and Bis 2 Al(CO 2 H) was obtained as a colorless solid (128 mg, 303 mmol, quant.). Crystals were obtained by slow evaporation of a solution of (Bis 2 Al(CO 2 H)) 2 in C 6 D 6 .

Conflicts of interest
There are no conicts to declare.