Synthesis , structural studies and ligand in fl uence on the stability of aryl-NHC stabilised trimethylaluminium complexes †

Treatment of a series of aromatic NHCs (IMes, SIMes, IPr and SIPr) with trimethylaluminium produced their corresponding Lewis acid–base adducts: IMes·AlMe3 (1), SIMes·AlMe3 (2), IPr·AlMe3 (3), and SIPr·AlMe3 (4). These complexes expand the few known examples of saturated NHC stabilised Group 13 complexes. Furthermore, compounds 1–4 show differential stability depending on the nature of the NHC ligand. Analyses of topographic steric maps and NHC %VBur were used to explain these differences. All the compounds have been fully characterised by multinuclear NMR spectroscopy, IR and single crystal X-ray analysis together with computational studies.


Introduction
Since the discovery of the first stable N-heterocyclic carbene (NHC) by Arduengo in 1991, 1 these compounds have been extensively used as ligands in the chemistry of transition metals. 2,33][4] As NHCs are highly nucleophilic Lewis bases, they have also been used to stabilise many Group 13 complexes. 5,6Our interest in NHC-Group 13 complexes arises from the discovery that their properties and reactivities have not been thoroughly studied.However, their potential has been demonstrated for a diverse range of applications; for example, sterically demanding NHC ligands have been used to synthesise neutral B-B double and triple bonded species, 7 as well as stabilising a neutral aromatic Ga 6 octahedron cluster. 8In addition, NHCs that do not form stable Lewis acid-base adducts, forming frustrated Lewis pair (FLP) systems, have shown interesting properties in the acti-vation of small molecules. 9However, there is still much to be explored in terms of their properties and reactivity.The majority of NHC-aluminium complexes reported comprise hydride and halide groups (AlX n H 3−n , n = 0, 1, 2). 5 In contrast, there are only a few examples of aluminium alkyl complexes. 6n particular, in the case of the simplest alkyl substituent, trimethylaluminium, only five complexes have been fully characterised: IiPrMe (IiPrMe = 1,3-isopropyl-4,5-dimethyl-imidazol-2-ylidene, A); 6h ItBu (ItBu = 1,3-di-tert-butylimidazol-2-ylidene, B); 6d IMes (IMes = 1,3-bis (2,4,6-trimethylphenyl)imidazol-2ylidene, C); 6e a bidentate amino ligand (D) 6e and a chiral imidazolium sulfonate (E) 6f (Fig. 1).Furthermore, to the best of our knowledge, there are only a few known examples of other saturated NHC stabilised Group 13 metals that have been fully characterised.5b Here, we report the synthesis, characterisation and theoretical studies of a series of NHC aluminium alkyl complexes.
Compounds 1-4 are highly air-and moisture-sensitive; traces of decomposition were consistently observed during their characterisation, making their characterisation tedious.This was particularly pronounced in the case of complexes 3 and 4 where peaks corresponding to the imidazolylidenes were always present on the 1 H and 13 C NMR spectra.Moreover, this was also observed in the solid state, where argon-gas-stored samples of 3 and 4 slowly decomposed to imidazolylidene and imidazolinylidene respectively and other unidentified side-products at room temperature (see ESI †), whereas solids 1 and 2 can be stored over long periods of time without any observable decomposition.

Crystallographic studies of complexes 1-4
Single-crystal X-ray structures of complexes 1-4 are shown in Fig. 3-6.Complexes 2 and 3 crystallised out as two crystallographically independent but chemically equivalent molecules; hence only one molecule will be described herein (Table 1).Compounds 1-4 adopt a distorted tetrahedral geometry at the Al centre, with Al-C carbene bond lengths ranging from 2.098 to 2.127 Å, which are consistent with previously reported trimethylaluminium complexes (cf.2.124(6) Å, 6h 2.162(2) Å, 6d 2.097(2) Å, 6e 2.074(2) Å 6e and 2.078(3) Å 6f for A-E respectively).Interestingly, the Al-C carbene bond distance of SIPr (4) was similar to the less sterically bulky IiPrMe (A) (2.127(2) and 2.124(6) Å respectively).Moreover, Huynh et al. using an NHC-NMR spectroscopic probe reported that saturated NHC (sNHC) moieties are marginally more basic (i.e., stronger σ-donors) than their unsaturated (uNHC) counterparts (decreasing σ-donor strength SIPr ∼ SIMes > IPr > IMes).In our case the 1 H NMR chemical shift of the methyl groups on the aluminium centre also supports Huynh's observations.In addition, a slight bond lengthening consistent with this property is expected for 2 and 4 with respect to 1 and 3 (containing sNHC and uNHC respectively).4g,10,11 However, clear bond lengthening is only observed between 3 and 4, since the difference between 1 and 2 could be attributed to statistical error range (3σ).Complex B (i.e., ItBu) has the longest reported Al-C carbene bond length reported to date, mainly due to additional steric hindrance introduced by the large tert-butyl groups (vide infra), 36.9%VBur , resulting in the complex being susceptible towards isomerization or decomposition depending on the experimental conditions (solvent dependent).6d

Spectroscopic studies of complexes 1-4
The 1 H and 13 C NMR spectra obtained for complexes 1-4 were consistent with the low temperature X-ray crystallographic analysis.The 1 H and 13 C NMR spectra for these compounds display singlets at δ H −0.78 to −0.91 and at δ C −7 ppm respectively.This is indicative of the presence of methyl groups on the aluminium centre.The IR spectra of these complexes show relatively strong stretching signals at around 620 cm −1 , confirming the presence of these methyl groups. 12Moreover, the formation of the complex is further indicated by the upfield shifting of the C carbene signal that is consistent with a carbenemetal bond (Table 2). 13he optimised geometrical parameters, bond lengths and angles for complexes 1-4 calculated using PBE0/6-311G(d,p) model chemistry are in good agreement with the experimental values obtained from the single-crystal X-ray diffraction studies.Furthermore, the calculated 1 H and 13 C NMR spectra using B972/6-311+G(2d,p) on the optimised geometries were consistent with the experimental data obtained, which provided further validation of the identity of the complexes synthesised (see ESI †).

Lewis acid-Lewis base properties
Comparison between NHCs and phosphines has been carried out to assess the relative donor abilities (Lewis basicity) of this important family of ligands.For this reason, NHC-Al complexes 1-4 were compared to selected phosphine-Al counterparts.Similarly to what Barron et al. reported with trimethylaluminium phosphine complexes, 14 the lengths of the Al-C bonds increase (cf.1.956 Å for AlMe 3 , 1.985 Å, 1.987 Å 1.993 Å and 1.986 Å for compounds 1-4 respectively) and the C-Al-C angles decrease (ca.120°for AlMe 3 and respective average angles 112.6°, 111.6°, 112.1°, 111.7°for 1-4) upon coordination to the NHC.Both changes indicate increased p-character in the Al-C bonds on changing from planar to tetrahedral geometries.The greater distortion from planarity observed for NHC complexes compared with their phosphine counterparts (see Table 3), indicates higher Lewis basicity of the former.This is further evidenced by the 1 H NMR chemical shift of the methyl groups on the aluminium centre.Complexes 1-4 show signals at higher fields (δ H −0.78 to −0.91) than previously reported basic trimethylaluminium phosphine    3).
The Lewis acidity of trihalide and trihydride aluminium centres within NHC-aluminium complexes has been previously discussed in the literature.5a,d In the case of complexes 1-4, the trimethylaluminium moiety is found to be a poorer Lewis acid as compared to hydrides and halides.This was evident from the carbenic carbon to aluminium bond distances observed in the IMes (1) and IPr (3) complexes.The Lewis acidity trend, AlMe 3 < AlH 3 < AlX 3 , can be illustrated by Al-C carbene bond distances: 2.034(3) Å for IMes•AlH 3 ; 5t 2.017(2) Å for IMes•AlCl 3 ; 5h 2.056(2) Å for IPr•AlH 3 , 5n and 2.031(2) Å for IPr•AlI 3 .5g The same tendency was also observed in the mixed alane gallane halide complexes.5d,f In the case of indium and thallium complexes, Jones et al. also observed the same Lewis acidic behaviour during the synthesis of bis-NHC (i.e., NHC-(CH 2 ) 2 -NHC) group 13 complexes.Their studies showed monometallic pentacoordinate indium and thallium halide complexes containing chelating bis-NHC moieties, whereas hydride counterparts formed monodentate tetra-coordinate bimetallic species (i.e., R 3 E←NHC-(CH 2 ) 2 -NHC→ER 3 ) indicating the higher Lewis acidity of the former.5m,o,q Furthermore, the relative Lewis acidity can also be assessed using 13 C NMR spectroscopy, despite the fact that many Al-C carbene signals have not been reported in the literature due to the quadrupolar nature of the aluminium metal centre to which they are attached.Nevertheless, the chemical shifts observed for complexes 1-4 show that trimethylaluminium is a poorer electron acceptor compared with AlH 3 and AlX 3 since the corresponding 13  ).

Stability studies
Unstable NHC-AlMe 3 complexes have previously been reported; for example, the tert-butyl NHC complex B isomerised to an 'abnormal' NHC-AlMe 3 species in THF or toluene.6d We will use complex B as a benchmark throughout our comparative studies.Since the isomerization/decomposition of B was attributed to steric factors, and a standard parameter for quantifying the steric properties of NHCs is the percent buried volume, %V Bur , this parameter was used to compare complexes 1-4 with other NHC•AlR 3 species previously reported in the literature (Table 4).4b,c The %V Bur for each complex was calculated with the Al-NHC bond distance fixed at the experimental value obtained by X-ray diffraction studies and also at 2.0 Å, in order to provide a point of comparison independent of the Al-NHC distances.
Calculations revealed that the buried volume of the new NHC complexes was 4 > 3 > 2 > 1.In order to provide a meaningful assessment of the steric influence of the NHC moiety on the overall stability of the NHC-AlMe 3 complexes, the %V Bur values of previously characterised counterparts were included.With this inclusion, the overall order is 4 It can be noted that complex B occupies a larger volume than that calculated for 1-2, and is comparable to that of 3 but is surprisingly lower than that of 4 (cf.36.9% in B).Since the %V Bur of compound 3 is larger than that of 1 and 2 and no decomposition was observed for either of the latter, the onset of decomposition may be attributed to the larger volume occupied by the isopropylphenyl groups as compared to the mesityl groups.The lower stability exhibited by the sterically encumbered complex B was previously rationalised by Dagorne et al. using the congested nature of the NHC present (36.9%VBur ).Consequently, the %V Bur calculated for 3 (36.2%,comparable to B) and for 4 (38.5%,greater than B) rationalises their lower stability (cf. 1 and 2).To gain insight into the molecular level of the steric impact of the different NHCs, the topographic steric maps for compounds 1-4 and A-C were calculated (see ESI †).A comparative analysis of the topographic maps of complexes 2 and 4, chosen as representatives of a stable and of an unstable system, is reported in Fig. 7.The steric contour maps reveal that the distribution of the steric bulk of the ligand in 2 is quite symmetrical around the metal, with large grooves between the two mesityl rings.As expected, the enhanced steric hindrance in 4 is mainly localised around the bulkier  ortho isopropyl groups, blocking the grooves between the two N-substituents.The difference in the nature of the distribution of the NHC ligands around the metal centre (similar maps are found for 1 and 3, see ESI †) can be related to the lower stability of 3 and 4 as compared to 1 and 2.
At this stage, it is also worth doing a comparative analysis of the topographic steric map of B, as the only reported unstable NHC-AlMe 3 complex, with that of 4 (Fig. 8).The topographic steric map of complex B shows the two top quadrants being slightly more sterically hindered.However, this topographical asymmetry is lower compared to 4, where the distribution of the steric bulk is much more localised in the top left and top right quadrants.This difference is even more evident looking at the %V Bur representative of each single quadrant, i.e. 39.6-40.2%for B vs. 43.1-50.7%for 4. Once again, the greater localization of the ligand steric hindrance in one or two quadrants around the metal centre may be the reason for the lower stability of the complexes, in the case of 4 as compared to B.
In addition to the %V Bur and topographic steric maps, bond dissociation energies were also evaluated to further rationalise the stability differences observed.4f DFT calculations show that the bond dissociation energy of complexes 1-4 decreases with increasing steric volume of the corresponding NHC: 1 > 2 > 3 > 4, which further corroborated the observation that complexes 1 and 2 were less susceptible to dissociation as compared to 3 and 4 (114.47(1), 104.76 (2), 97.14 (3), and 79.82 (4) kJ mol −1 for 1-4 respectively).With the inclusion of the dissociation energy calculated for all NHC trimethylaluminium complexes, the order is as follows: 5).It is worth noting that the %V Bur calculated for complex 4 is higher than that calculated for B; however its E diss is lower.This discrepancy may be explained by the differing electronic properties of the SIPr and ItBu NHCs moieties.On the one hand, going from the unsaturated (uNHC) to saturated (sNHC) NHCs contributes to an increased donor ability of the latter (sNHC > uNHC) (vide supra).On the other hand, the presence of withdrawing aryl substituents in the NHC leads to a decreased donor ability (alkyl-NHC > aryl-NHC).The opposite electronic effects present in both SIPr and ItBu (i.e., the donating effect of the sp 3 backbone and withdrawing effects of the aryl groups in SIPr vs. the less donating sp 2 backbone combined with more donating alkyl groups in ItBu) make the relative NHC→metal donation properties difficult to predict. 10 However experimental evidence suggests that the SIPr N-heterocyclic carbene moiety present in 4 is a better donor ligand than ItBu since the 1 H NMR chemical swift of the methyl group on 4 (δ H −0.91) is more upfield than that found for B (δ H −0.73).This is also supported by 11 B NMR studies on NHC-BX 3 species, where the chemical shift for the ItBu-BCl 3 complex is more downfield than its IPr analogue. 19However, the overall stability of these complexes is a concomitant balance between the electronic and steric properties of the NHC moieties present.4f A plot of the calculated %V Bur (R = 2.0 Å) versus the calculated E diss for all the crystallographically characterised structures is shown in Fig. 9.The linear correlation between the steric bulk of the NHC ligand and the dissociation energy of these complexes (R 2 = 0.7057) shows that as the steric bulk increases, the dissociation energy decreases (see ESI †).
On inspection of the calculated %V Bur for all NHC•AlMe 3 complexes, it is observed that all stable complexes fall within or below a calculated %V Bur of 34%, whereas B, 3 and 4 have %V Bur values exceeding 36%.Therefore, the difference in the %V Bur observed between the stable and the unstable complexes is only 2-4% (Table 5).Despite the observed differences in %V Bur between 1 and 4 being minor and concentrated in small areas (as indicated by the topographic maps) they exhibit profound effects on the stability and dissociation energies of   these complexes (the asymmetry underlined by the maps adds value to this 2-4%).
To further test the proposed stability threshold of %V Bur of 36% and in order to complete the series of trimethylaluminium complexes, we attempted to synthesise SItBu•AlMe 3 (the saturated counterpart of B).Unfortunately, in all our synthetic attempts, only complex mixtures of products were obtained.The slurry formed in the reaction mixture was insoluble in most aprotic solvents ( pentane, hexane, ether, THF, benzene, and toluene) which made the isolation of any viable product unsuccessful.To allow for comparison, the optimised geometry for SItBu•AlMe 3 was calculated using DFT methods (see ESI †).The corresponding %V Bur and the dissociation energy calculated are shown in Tables 4 and 5. From the theoretical values obtained and in comparison with the rest of the isolated NHC trimethylaluminium complexes, the %V Bur for SIt-Bu•AlMe 3 falls within the range observed for the unstable complexes (37.6%), which may help explain our lack of success in its synthesis.
By-product obtained from SIPr•AlMe 3 (4)   As discussed previously, compounds 3 and 4 were shown to be susceptible towards the formation of the imidazolylidenes and other unidentified decomposition products.Efforts were made to isolate and identify some of these side-products.Since the observed rate of decomposition was temperature dependentand in order to accelerate this processthe reaction mixture, initially used to produce complex 4 (at RT), was refluxed overnight instead.Crystalline solids from this reaction proved to be remarkably air and moisture sensitive, and difficult to separate from the complex mixture of products obtained from the reaction.However, solid 5 was obtained when the reaction mixture was extracted in THF.Suitable single crystals for X-ray diffraction studies were grown in a THF-hexane mixture (Fig. 10).
Complex 5 crystallised out as a methylated imidazolium salt containing a formate counter ion and an acetic acid lattice molecule (1 : 1 : 1 ratio).Despite the extreme care taken to ensure inert atmosphere conditions, presumably trace impuri-ties of water, oxygen or carbon dioxide were present in the reaction mixture.Therefore, in the presence of these impurities, the formation of compound 5 could be considered closely related to the reaction proposed by Rogers et al. that describes the generation of carboxylate zwitterion species with acetate ionic liquids due to their relative structural similarity. 20urther mechanistic studies are being conducted in order to rationalise the formation of 5.

Conclusions
In conclusion, the work presented here describes the synthesis and characterization of a series of new aromatic N-substituted NHC trimethylaluminium species.These complexes exhibit differing stabilities, which is attributed to differences in steric bulk of the NHCs used during their synthesis.Our studies demonstrate that the mesityl substituted NHC complexes (1 and 2) are more robust than their isopropylphenyl counterparts (3 and 4).In addition, comparison with previously characterised trimethylaluminium complexes showed that small variations (2-4%) in the steric bulk of the NHC substituent (%V Bur ) exert a profound effect on the overall stability of the complex formed.The results obtained indicate that all the reported stable NHC•AlMe 3 complexes fall within or below a %V Bur of 34%.The unstable nature of complexes with %V Bur higher than 36% is illustrated by the new complexes 3 and 4 and the previously reported complex B. Mechanistic studies are currently underway to gain a better understanding of the reactivity of these trimethylaluminium complexes and to rationalise their decomposition pathways.

General method
All manipulations were carried out using standard Schlenk and glove-box techniques under a dried argon atmosphere and

Fig. 7
Fig. 7 Topographic steric maps of the SIMes and SIPr ligands in 2 and 4. The iso-contour curves of the steric maps are in Å.The maps have been obtained starting from the crystallographic data of the Al-NHC complexes (CIF), with the Al-C carbene distance fixed at 2.0 Å.The xz plane is the mean plane of the NHC ring, whereas the yz plane is the plane orthogonal to the mean plane of the NHC ring, and passing through the C carbene atom of the NHC ring.

Fig. 8
Fig. 8 Topographic steric maps of the ItBu and SIPr ligands in B and 4. The iso-contour curves of the steric maps are in Å.The maps have been obtained starting from the crystallographic data of the Al-NHC complexes (CIF), with the Al-C carbene distance fixed at 2.0 Å.The xz plane is the mean plane of the NHC ring, whereas the yz plane is the plane orthogonal to the mean plane of the NHC ring, and passing through the C carbene atom of the NHC ring.

Fig. 9
Fig. 9 Plot of calculated %V Bur vs. calculated E diss for NHC trimethylaluminium complexes.

Table 1
Al-C carbene bond length

Table 2
15lected 1 H and 13 C NMR chemical shifts for complexes 1-4 H −0.02-0.41)15indicative of a stronger donation from the NHC to the aluminium center (see ESI † and Table a 13 C chemical shift obtained from ref. 13.Paper Dalton Transactions 15168 | Dalton Trans., 2015, 44, 15166-15174 This journal is © The Royal Society of Chemistry 2015 complexes (cf.δ C NMR signals for Al-C carbene , shown in Table 2, were more downfield shifted with respect to hydride and halide counterparts (Al-C carbene signals at δ C 174.3 for ItBu (B), 6d δ C 175.3 for IMes•AlH 3 , 5t 153.9 for IMes•AlI 3 , and δ C 153.3 for IPr•AlI 3 5g

Table 3
Average Al-Me bond length and C-Al-C angles for selected complexes a Average values were taken for both bond lengths and angles.

Table 4
Al-C carbene bond lengths, %V Bur and dissociation energies for selected complexes

Table 5
%V Bur and dissociation energies for selected NHC•AlMe 3 complexes in increasing order of stability Structure was optimised using PBE0/6-311G(d,p) model chemistry. a