Diverse Cooperative Reactivity at a Square Planar Aluminium Complex and Catalytic Reduction of CO 2

: The use of a sterically demanding pincer ligand to prepare an unusual square planar aluminium complex is reported. Due to the constrained geometry imposed by the ligand scaffold, this four-coordinate aluminium centre remains Lewis acidic and reacts via differing metal-ligand cooperative pathways for activating ketones and CO 2 . It is also a rare example of a single-component aluminium system for the catalytic reduction of CO 2 to a methanol equivalent at room temperature.


Introduction
[3] In cooperative reactions, the metal centre typically functions as a Lewis acid whilst acting with an electron-rich ligand site serving as a nucleophile.Pincer ligands are ideally suited to MLC as they allow for both coordinative control to stabilise a Lewis acidic metal centre and incorporate suitable functional groups to reversibly form the nucleophilic ligand site. [4]This process is often carried out through a secondary amine that can be de/(re)protonated, for example Macho-type scaffolds, [5] or a pyridine residue that can undergo dearomatisation/(re)aromatisation, for example Milstein's PNNand PNP-scaffolds. [6][9] Despite these successes in transition metal chemistry, metal (element) ligand cooperativity in main group chemistry remains rare. [10]This perhaps reflects the difficulties associated with the preparation of ambiphilic complexes featuring a Lewis acidic metal centre with a nucleophilic ligand site.
[22][23][24][25] We proposed that using an appropriate pincer ligand could induce Lewis acidity at aluminium by structural constraint and the formation of a nucleophilic ligand site to promote MLC reactivity.
We now report the use of a sterically demanding pincer ligand to impose structural constraint at an aluminium centre resulting in the formation of an unusual square planar aluminium complex.This square planar complex remains Lewis acidic and undergoes differing modes of metal-ligand cooperative reactivity with carbonyl-containing substrates, including CO 2 .Furthermore, in the presence of pinacolborane (HBpin), it catalyses the room temperature reduction of CO 2 to the methanol equivalent MeOBpin.

Results and Discussion
The reaction of the sterically demanding NNN-pincer ligand 1 with AlMe 3 in toluene at low temperature, followed by warming to room temperature, results in the elimination of methane and the formation of the singly metalated complex 2 (Figure 1).Complex 2 has been characterised spectroscopically and by single-crystal X-ray diffraction (see Supporting Information).The elimination of the second equivalent of methane to form the doubly metalated complex 3 requires heating the toluene reaction mixture at 80 °C.Complex 3 was isolated as the toluene solvate by crystallisation at low temperature from toluene, and the molecular structure of 3•0.5 toluene is shown in Figure 2.
The geometry at the aluminium centre of 3 is close to square planar, reflected by a τ 4 value of 0.20.This deviates from 0 predominantly due to the ligand-imposed distortion in the N(1)-Al(1)-N(3) angle (157.87(5)°)within the plane defined by the N(1), N(2), N(3), and C(1) centres.There are limited systems for comparison in the literature.However, the doubly reduced bis(imino) aryl-supported Ph I 2 PAl-X complexes reported by Berben (Figure 1) have τ 4 values of 0.22, 0.21 and 0.13 for X = Cl, I, and H, respectively. [15,16]The calix [4]pyrrolato aluminate system reported by Greb has a τ 4 value of 0. [17,18] The difference in electronic structure between the redox non-innocent [ Ph I 2 P] 2À ligand in Ph I 2 PAlÀ X and the precise electron bonding present in 3 is clearly illustrated by the comparison of AlÀ N bond distances.In 3, the AlÀ N amide bond distances (1.865(1) Å) are statistically indistinguishable and are considerably shorter than the AlÀ N Py (2.008(1) Å).In contrast, in Ph I 2 PAlÀ X complexes, the AlÀ N Py bond distance is shorter than the AlÀ N imine (cf.AlÀ N Py 1.820(1)-1.8331(9)Å; AlÀ N imine 1.930(1)-1.965(5)Å) reflecting the reduction of the ligand scaffold and the amide donor character of the formerly pyridyl nitrogen. [15,16][28] Coordination of Et 3 P=O to 3 results in a Δδ P = 17.0 ppm indicating that 3 is a moderate Lewis acid, weaker than the Ph I 2 PAl-X complexes (Δδ = 23.2 and 25.2 ppm for X = H and Cl respectively) [16] but similar to the calix [4]pyrrolato aluminate (Δδ = 17.0 ppm). [17,29]The increased 31 P{ 1 H} NMR shift of the PhI 2 PAlÀ X complexes is consistent with the calculated fluoride ion affinities, as discussed below.
To gain further insight into complex 3, DFT calculations were performed.A plot of the Kohn-Sham frontier orbitals at the B3LYPÀ D3(BJ)/def2-TZVP level of theory shows that the HOMO predominantly relates to the N1 and N3 electron lone pairs (Figure 3) and the LUMO and LUMO + 1 are mostly ligandbased (see Supporting Information).However, the low-lying LUMO + 2 (À 0.04 eV) shows significant localisation at aluminium consistent with 3 functioning as an aluminium based Lewis acid (Figure 3).
Whilst gas phase fluoride ion affinity (FIA) values for the calix [4]pyrrolato aluminate have been reported at the PW6B95-D3(BJ)/def2-TZVPP level of theory, [17] values for Ph I 2 PAlÀ X, have so far not been reported.The FIAs of 3 and Ph I 2 PAl-H were calculated using isodesmic reactions, [30] at this same level of theory, to be -370 and -390 kJ mol -1 respectively.They are significantly larger than those reported for the calix [4]pyrrolato aluminate (À 128 kJ mol À 1 ), presumably reflecting their lack of negative charge.
Consistent with the presence of a vacant orbital at aluminium identified by computational studies, 3 functions as an aluminium-centred Lewis acid (natural charge Al 1.94 j e j).In contrast to typical aluminium-based organometallics, the AlÀ Me bond does not react with ketones (cf.AlMe 3 ).In fact, probing the coordination of substrates to 3 with ketones identified alternative reactivity consisting of differing modes of MLC (Scheme 1).The addition of the non-enolisable ketone benzophenone to a solution of 3 in C 6 D 6 immediately results in the oxidation of the ligand scaffold and transfer of a hydride from  the methylene group of the ligand to the benzophenone. [31]The resulting imine and corresponding aluminium-bound alkoxide are readily identified from the 1 H NMR spectrum of 4. Reactivity with the more sterically demanding fluorenone proceeded more slowly but gave the analogous product.
In contrast, the addition of the enolisable ketone acetophenone to a C 6 D 6 sample of 3 results in the immediate deprotonation by one of the aluminium amides and formation of the aluminium enolate 5 (Scheme 1).This is evidenced by the 1 H NMR spectrum, where the alkenyl protons are observed as singlets, each integrating to 1 at @ H = 4.69 and 3.91 ppm.In addition, the secondary amine proton and the adjacent methylene protons are mutually coupled, resulting in an ABX splitting pattern.The crystallisation of 4 and 5 proved elusive, but further confirmation of this reactivity was achieved through controlled degradation studies (see Supporting Information).
Exposing a degassed solution of 3 in dichloromethane to CO 2 (1 bar) resulted in the formation of a small amount of precipitate and a pale-yellow solution.Filtration and subsequent crystallisation resulted in the characterisation of the dimer 6. CO 2 inserts into the AlÀ N amide bond, forming a carbamate (6 M in Figure 4).Two of these units dimerise, resulting in a thermodynamically favoured pentacoordinate aluminium centre with distorted trigonal bipyramidal geometry (Figure 5) (ΔΔG°= À 33.6 kcal mol À 1 ).
Whilst the insertion of CO 2 into AlÀ N bonds has been reported, [32][33][34] comparisons can be drawn between the reactivity of 3 towards CO 2 and that of transition metal pincer complexes featuring an amide donor. [35,36]The dimeric nature of 6 in the solid state highlights the greater oxophilicity of aluminium which drives dimerisation and bridging, rather than k 2 -N,Obound carbamate groups.
To gain greater insight into the differing modes of reactivity between carbonyl-containing substrates, we turned to DFT.The  energy profiles for the reactions of 3 with benzophenone, acetophenone, and CO 2 were computed at the B3LYP-D3(BJ)/ def2-TZVP level of theory.The reaction profiles, including solvent effects, are shown in Figure 4.The Lewis acidity of aluminium is central to this reactivity, and the first step in each reaction is the coordination of the substrate oxygen atom to aluminium.For acetophenone (INT1) and benzophenone (INT3), these intermediates are stabilised by À 21.5 and À 25.1 kcal mol À 1 , and the binding of CO 2 (INT4) is only thermodynamically uphill by + 5.2 kcal mol À 1 .Transition states were located for the reactions with acetophenone (TS1), benzophenone (TS3), and CO 2 (TS4), corresponding to overall reaction barriers of 13.9, 13.5, and 24.3 kcal mol À 1 , respectively.Although the Gibbs free energy of activation for TS4 is relatively high, attempts to identify alternative pathways involving a dimer of 3 or concerted CO 2 activation were unsuccessful.Interestingly, the difference in reaction barrier for the reduction of acetophenone (INT2 to TS2) was only marginally higher in energy (14.2 kcal mol À 1 ) than that for the experimentally observed deprotonation (13.9 kcal mol À 1 ).
With the coordination of the carbonyl fragment to the Lewis acidic aluminium centre proving crucial to the reactivity of 3, its potential toward hydroboration catalysis was investigated.Whilst 3 functions as a modest hydroboration catalyst for ketones and esters (TOFs of 0.38-2.10h À 1 ) (see Supporting Information), the addition of CO 2 (1 bar) to a CD 2 Cl 2 solution of 3 (5 mol %) and HBpin results in the room temperature catalytic reduction of CO 2 .Monitoring of the reaction by multinuclear NMR spectroscopy identifies the final products of the reduction as the methanol equivalent MeOBpin and pinBOBpin (Scheme 2).
Although complete conversion of HBpin does not occur even at extended reaction times (5 days) 3 has a TOF of 0.08 h À 1 (see Supporting Information).It therefore falls within the range of other reported main group systems for CO 2 hydroboration (0.07-14.5 h À 1 ), [37][38][39][40][41][42] but is a rare example of a single component system [43] operating at room temperature with HBpin as the reducing agent. [23,39,42]To our knowledge, the only other example based on aluminium is the square planar calix [4]pyrrolato aluminate reported by Greb (TOF = 0.12 h À 1 ). [23]he catalytic hydroboration reactivity of 3 contrasts the properties of the β-diketiminate aluminium hydride complexes reported by Aldridge, for which catalytic turnover is unfavourable. [44]

Conclusion
In conclusion, the use of the sterically demanding pincer ligand 1 allows for the synthesis of the structurally constrained compound 3 as an unusual square-planar Al III complex.Due to its geometry, 3 is Lewis acidic at Al and this, in combination with the electron-rich ligand scaffold, results in multiple metalligand cooperative bond activation modes with carbonylcontaining small molecules.This reactivity can be exploited by using the borane HBpin to allow for the catalytic hydroboration of CO 2 , reducing it to a methanol equivalent.Further studies on the mechanism of catalysis and extension of this cooperative reactivity are ongoing.

Experimental Section
General considerations: All manipulations were carried out under an atmosphere of argon or dinitrogen using standard Schlenk line and glove box techniques.Prior to use all solvents were dried and stored over activated molecular sieves or a potassium mirror.2,6diisopropylaniline was vacuum distilled from CaH 2 and stored over activated molecular sieves.Acetophenone was vacuum transferred from CaH 2 and stored over activated molecular sieves.Benzophenone and 9-fluorenone were dried in vacuo with P 2 O 5 for 3 days.All other reagents were used as received from the supplier.NMR spectra were recorded on Bruker Ascend 400 MHz and Bruker Avance III 500 MHz spectrometers and referenced to residual solvent signals for 1 H and 13 C spectra for C 6 D 6 (δ 7.16, 128.06),CD 2 Cl 2 (δ 5.32, 53.84) and C 7 D 8 (δ 2.08).Detailed experimental and computational methods can be found in the Supporting Information.
General procedure for CO 2 hydroboration: In a J Young's NMR tube, HBpin (0.4 mmol) and 1,3,5-trimethoxybenzene (25 mol%) were dissolved in CD 2 Cl 2 (0.3 cm 3 ).A solution of 3 (5 mol%) in CD 2 Cl 2 (0.4 cm 3 ) was further added to the NMR tube.The solution was freeze-pump-thaw degassed four times and then filled with CO 2 (1 bar).The 1 H and 11 B NMR spectra were recorded immediately and then periodically whilst maintained at room temperature.Deposition Numbers 2211657 (2), 2211658 (3), and 2211659 (6) contain the supplementary crystallographic data for this paper.These data are provided free of charge by the joint Cambridge Crystallographic Data Centre and Fachinformationszentrum Karlsruhe Access Structures service.
DFT calculations: Calculations were carried out using either Orca 4.2.0 [45][46][47] or the Gaussian16 [48] software packages.Geometries were fully optimised and confirmed to be true minima on the respective potential energy surfaces.

RESEARCH ARTICLE
Structural constraint imparted by a sterically demanding pincer ligand results in an unusual tetracoordinate aluminium Lewis acid that undergoes multiple modes of metal-ligand cooperative reactivity with ketones and

Figure 4 .
Figure 4. Computed reaction profile (free energy, kcal mol À 1 ) for the reactions of 3 with acetophenone, benzophenone and CO 2 .Computed geometries for key transition states are shown with bond distances in Å.