Calcium‐Catalyzed Arene C−H Bond Activation by Low‐Valent AlI

Abstract The low‐valent ß‐diketiminate complex (DIPPBDI)Al is stable in benzene but addition of catalytic quantities of [(DIPPBDI)CaH]2 at 20 °C led to (DIPPBDI)Al(Ph)H (DIPPBDI=CH[C(CH3)N‐DIPP]2, DIPP=2,6‐diisopropylphenyl). Similar Ca‐catalyzed C−H bond activation is demonstrated for toluene or p‐xylene. For toluene a remarkable selectivity for meta‐functionalization has been observed. Reaction of (DIPPBDI)Al(m‐tolyl)H with I2 gave m‐tolyl iodide, H2 and (DIPPBDI)AlI2 which was recycled to (DIPPBDI)Al. Attempts to catalyze this reaction with Mg or Zn hydride catalysts failed. Instead, the highly stable complexes (DIPPBDI)Al(H)M(DIPPBDI) (M=Mg, Zn) were formed. DFT calculations on the Ca hydride catalyzed arene alumination suggest that a similar but more loosely bound complex is formed: (DIPPBDI)Al(H)Ca(DIPPBDI). This is in equilibrium with the hydride bridged complex (DIPPBDI)Al(μ‐H)Ca(DIPPBDI) which shows strongly increased electron density at Al. The combination of Ca‐arene bonding and a highly nucleophilic Al center are key to facile C−H bond activation.


Introduction
Efficient and selective CÀHbond activation is one of the longstanding Holy Grails in chemistry. [1] Being the subject of numerous reviews, [2] it is ac rucial technology towards the effective use of low-cost feedstocks like simple alkanes or aromatics.D irect manipulation of the C À Hb ond enables synthetic pathways that avoid unnecessary functionalization steps,m aking it an attractive step-economical goal in green chemistry. [3] However,t here are several major challenges, among which is the low reactivity of the strong, hardly polarized CÀHb ond. This explains why often harsh conditions are needed. In addition, the many C À Hbonds present in organic molecules pose strict requirements on control over selectivity.
There are several different classifications of CÀHb ond cleavage reactions but the major pathways (Scheme 1) are divided between s-bond metathesis (SBM) and oxidative addition/reductive elimination (OA/RE). [4] Tr ansition between these extremes is smooth and depends on the strength of the metal···H interaction in the transition state or intermediate.F or the SBM pathway as one extreme,i ti s non-existent, while it is dominant for the OA/RE pathway as the other extreme.
First examples of C À Hb ond activation dealt with aromatic substrates using precious platinum group metals like Ru or Pd. [5] These pioneering studies have been fundamental to the development of Ir-o rR h-mediated catalytic routes for borylation of arenes. [6] Although CÀH activation of unactivated arenes has gone al ong way, examples of selective C À Hb ond activation without using directing groups are still limited and often need higher catalyst loadings. [7] Apart from that, there is an increasing interest to replace the precious metals that are central to CÀH bond activation for more abundant, less toxic, 3d-metals. [8] In this respect, main group metals would be highly advantageous.Directed or non-directed, arene deprotonation by any strong s-block metal base could be considered C À Hb ond activation through aS BM pathway (Scheme 1). Selectivity control in such metallation chemistry developed to adiscipline of its own. [9] It is,h owever,d isputable whether the wellestablished deprotonation reaction should be labeled CÀH activation. [10] Bond activation by oxidative addition, which is the general reaction found for most transition metals,i sl ess common in main group chemistry.H owever,d uring the last decade it became clear that low-valent main group metal complexes,d espite their lack of d-orbitals,c an display transition metal-like behavior in bond activation (Sche-me 2a). [11] This is especially true for Al I complexes,w hich show avery rich chemistry for activation of XÀHbonds (X = H, B, Al, C, Si, Ge,Sn, N, P, As,O ,S). [12] Examples for C À Hbond activation, however,are limited: Cp*H reacts under forced conditions (70 8 8C, 3days) with ( DIPP BDI)Al, [12b] aß-diketiminate complex with arather high singlet-triplet gap (30-40 kcal mol À1 ) [13] introduced by Roesky and co-workers (Scheme 2b, [DIPP BDI = CH[C(CH 3 )N-DIPP] 2 ,D IPP = 2,6-diisopropylphenyl). [14] Similar,S chnçck-elsC p*Al [15] has been shown to undergo reversible OA/RE with the activated diene C À Hb ond in Cp*H giving Cp* 2 AlH. [16] Similar to this reactivity,C rimmin described cleavage of the activated allylic CÀHb ond in propene by ( DIPP BDI)Al in benzene (80 8 8C, 14 h). [17] Reaction of ad ialumene with PhCCH also resulted in CÀHa ctivation (Sche-me 2c), apart from [2+ +2]-cycloaddition. [18] While similar dialuminene complexes react by [4+ +2]-cycloaddition with benzene, [19] calculations support the experimental verification that mononuclear ( DIPP BDI)Al does not parallel such reactivity. [20] Only after Lewis-acid activation of benzene by ac ationic Ca complex dearomatization is observed (Scheme 2d). [21] Despite this reactivity,C ÀHb ond alumination of unactivated arenes is highly challenging.The Crimmin group recently reported the Pd-catalyzed oxidative addition of ( DIPP BDI)Al to C À Hb onds in benzene,t oluene,a nd xylenes and proposed amechanism in which the C À Hbond is activated at the Pd center (Scheme 2e). [22,23] Hitherto,o nly ahighly unusual nucleophilic Al anion was shown to activate the CÀHbond in benzene (Scheme 2f) [24] but higher temperatures and long reaction times are needed. Most recently,i t was shown that this reactivity can be topped by addition of aK + selective cryptand and even cleavage of the C À Cbond in benzene was achieved. [25]

Results and Discussion
Herein we report aserendipitous Ca hydride catalyzed C À Hbond activation of several unactivated arenes by oxidative addition of ( DIPP BDI)Al at room temperature (Scheme 3). As af ollow-up of our studies on the potential synthesis of heterobimetallic AlÀCa complexes (Scheme 2d), [21] we reacted ( DIPP BDI)Al in C 6 D 6 with the Ca hydride dimer [( DIPP BDI)CaH] 2 in a2 /1 ratio.T he latter hydride complex introduced by Hill and co-workers [26] is aL ewis base-free form of the well-established Ca hydride complex [( DIPP BDI)CaH·THF] 2 . [27] Being more Lewis-acidic, AlÀCa  coordination could be envisioned (Scheme 3). Similar to earlier observed reactivity of ( DIPP BDI)Al with metal-hydride complexes, [12] this may be followed by oxidative insertion of Al in the Ca À Hb ond. However,t he reaction followed ac ompletely different course.
While the Ca hydride complex was not converted, oxidative addition of ( DIPP BDI)Al to the unactivated CÀH bond of benzene was observed (Scheme 3). This reaction is smooth and, at room temperature,f ull conversion to ( DIPP BDI)Al(Ph)H was reached within one hour.T he latter Al III complex was fully characterized and its crystal structure ( Figure S21 in the Supporting Information) equals that reported by the Crimmin group. [22] Since it is well-known that ( DIPP BDI)Al does not react with benzene [20] and the Ca hydride complex remained fully intact, it seems reasonable that the latter is acatalyst for the observed oxidative addition. Indeed, using ac atalyst loading of 5mol %[ ( DIPP BDI)CaH] 2 at room temperature led to full conversion within 8hours (Table 1, entry 1) or within 3hours at 60 8 8C( entry 2). Doubling the reaction time,the catalyst loading could be halved to 2.5 mol %( entry 3). In all cases only mono-alumination was observed.
Thes mooth conversion of an unactivated aromatic substrate like benzene,p oses the question whether electronrich methyl-substituted arenes would be reactive towards Ca hydride catalyzed CÀHa lumination. Using 5mol %c atalyst loading, toluene could be converted equally efficient (entries 4-5), giving high selectivity for C À Hb ond activation in the meta-position especially at room temperature.T he raw product mixture did not show any conversion in the paraposition but asmall quantity of ortho-alumination is observed (meta/ortho = 90/10). Thecrystal structure of ( DIPP BDI)Al(mtolyl)H is shown in Figure S22. Theh igh selectivity for CÀH alumination in the meta-position is unusual. Functionalization of mono-substituted arenes is dominated by sterics and generally takes place in meta-a nd para-positions roughly in a2:1 ratio with at most trace amounts of ortho-products. [28] Using directing groups can give very high selectivity for orthofunctionalization by the complex-induced-proximity-effect (CIPE). [29] Although in some cases the selectivity can be steered towards the para-position by controlling electronics [30] and/or sterics, [31] selective meta-functionalization remains very challenging [32] and often needs catalysts with sophisticated directing ligands [32d] or templates. [32e] Thehigh selectivity for meta-alumination of toluene here observed also differs strongly from Pd catalyzed toluene alumination (ortho/meta/ para = 42/46/12). [22] In the latter case,t he unusually high percentage of ortho-tolyl product has been explained by aweak attractive C À H···p interaction between the Me group of toluene and the aromatic ring of ( DIPP BDI)Al.
para-Xylene,anaromatic substrate with two Me-substituents,w as also fully converted albeit slightly slower (entry 6-7). It gave exclusively the fully characterized mono-functionalized product ( DIPP BDI)Al(2,5-dimethylphenyl)H (see Figure S23 for crystal structure). Considering the meta-directing influence of the Me group,s trong activation of the meta-CH bond in meta-xylene was expected. However, meta-aswell as ortho-xylene did not show Ca hydride catalyzed CÀHb ond activation. Similar to the Pd catalyzed alumination as observed previously, [22] the trisubstituted substrate mesitylene could not be converted (entry 8).
Although not catalytically,w ew ere able to run the reaction in astoichiometric cycle by reaction of the alumination product with I 2 (Scheme 4). TheA l À Hb ond in ( DIPP BDI)Al(m-tolyl)H is considerably more reactive than the Al-tolyl bond and reacts very fast at room temperature with 0.5 equivalent of I 2 to give ( DIPP BDI)Al(m-tolyl)I and H 2 , apparent from vigorous gas development and 1 HNMR monitoring ( Figure S15). In as econd, much slower step, ( DIPP BDI)Al(m-tolyl)I reacts with I 2 to give m-tolyl iodide and ( DIPP BDI)AlI 2 ( Figure S16). To close the cycle,t he latter Al iodide complex can be reduced with potassium to give ( DIPP BDI)Al. Demonstration of this stoichiometric cycle is af irst step towards the long-term goal of making CÀHb ond activation aredox-catalytic process that is also truly catalytic in the Al reagent. Replacing catalyst [( DIPP BDI)CaH] 2 with ( DIPP BDI)CaN-(SiMe 3 ) 2 (10 mol %) did not give any conversion, also not after heating to 60 8 8C( entry 9), implying the importance of ahydride functionality.Inorder to assess metal influences,we investigated the catalytic activity of the recently published Sr hydride complex [( DIPeP BDI)SrH] 2 . [33]D IPeP BDI is ab ulkier version of the DIPP BDI ligand in which iPr groups have been replaced by isopentyl substituents.T his highly reactive Sr hydride complex, which was previously shown to be active in H/D exchange with C 6 D 6 , [33] gave full conversion in catalytic quantities but the reaction took one week for completion (entry 10). Thec onsiderable retardation of the catalytic reaction upon increasing the bulk of the BDI ligand underlines the importance of an open coordination site at the group 2metal but could also be due to the lower Lewis-acidity of Sr 2+ vs.Ca 2+ .
Further influences of the catalyst metal have been studied by exchange of Ca for Mg or Zn, metals that show strong similiarities in their chemistry but are clearly more Lewis acidic than Ca. However,s toichiometric reaction of ( DIPP BDI)Al with [( DIPP BDI)MgH] 2 [34] or ( DIPP BDI)ZnH [35] in benzene followed adifferent course.Instead of benzene C À H alumination, insertion of Al in the metal hydride bond gave clean formation of ( DIPP BDI)Al(H)M( DIPP BDI) complexes (M = Mg, Zn), which both have been fully characterized. While the crystal structure of the Zn complex showed major disorder ( Figure S25), that of the Mg complex is well behaved (Figure 1). 1 Ha nd 13 CNMR spectra for the Al À Zn complex show au nique signal for each Ca nd H, 8h eptets and 16 doublets are observed for the iPr-substituents.T he Al À Mg complex is more dynamic.Only after cooling of a[D 8 ]toluene solution to À15 8 8C, asimilar set of NMR signals was observed.
These more pronounced dynamics are likely related to the weaker Al À Mg bond (vide infra).
Ther eactivity and crystal structures observed here show strong similarities with the insertion of ( DIPP BDI)Al in the M-Rb ond of (BDI)M-R species (M-R = Mg-Me or Zn-Et) reported earlier. [36] TheM g ÀAl (2.7687(5) )a nd ZnÀAl (2.488(1) )bond lengths in these complexes compare well to those depicted in Figure 1. Thei nsertion of ( DIPP BDI)Al in Mg À Ho rZ n À Hb onds also shows parallels to the oxidative insertion of Cp*Al in the Zn À Nb onds of Zn[N(SiMe 3 ) 2 ] 2 , which gave Zn[Al(Cp*)N(SiMe 3 ) 2 ] 2 with Zn À Al bonds of 2.448(2) . [37]  Further experimental investigation of the mechanism and the role of the Ca hydride catalyst in benzene alumination was found to be difficult. 1 HNMR monitoring of the conversion of ( DIPP BDI)Al into ( DIPP BDI)Al(C 6 H 5 )H over time gave an untypical curve revealing an induction time indicative for an intermediate species (Figure S19-20). Indeed, during catalysis as et of signals for an unidentified species builds up and disappears towards the end of the reaction. Under all circumstances,w en ever detected the presence of H 2 , indicating that ad eprotonation mechanism is unlikely. Deuterium labeling studies gave only limited insight. A solution of ( DIPP BDI)Al and [( DIPP BDI)CaH] 2 in C 6 D 6 reacted to ( DIPP BDI)Al(C 6 D 5 )D but H/D scrambling in the hydride position was found. Since as olution of [( DIPP BDI)CaH] 2 and ( DIPP BDI)Al(C 6 D 5 )D in C 6 H 6 also led to rapid H/D exchange ( Figure S12-13), no further conclusions can be drawn. We also found that as olution of [( DIPP BDI)CaH] 2 in C 6 D 6 gave H/D exchange.T his reactivity parallels the Et/D exchange found by Hill and co-workers for as olution of [( DIPP BDI)CaEt] 2 in C 6 D 6 [26] but is much slower.While the origin of the hydride in the product remains questionable,i ti sc lear that the CÀH bond activation process is not reversible:asolution of ( DIPP BDI)Al(C 6 D 5 )D in C 6 H 6 in the presence of the Ca hydride catalyst gave at most hydride D/H scrambling but no C 6 D 5 /C 6 H 5 exchange was observed.
To provide ar easonable explanation for the herein reported facile CÀHb ond activation at room temperature, the reaction of ( DIPP BDI)Al with benzene was investigated by DFT calculation (wB97XD/6-311 + G**//wB97XD/6-31 + G**, DG values in kcal mol À1 at 298 Kand 1bar are corrected for solvent effects in benzene using the PCM method). Since the reaction runs equally well in the dark and the singlettriplet gap in ( DIPP BDI)Al is larger than 30 kcal mol À1 , [13] we did not consider radical mechanisms but evaluated three different closed shell processes in which, for simplicity and reduction of computation time,t he Ca hydride catalyst was modeled by CaH 2 (Scheme 5): A direct oxidative addition, B the Meisenheimer anion route and C the C 6 H 6 2À route. Pathway A:T he direct oxidative addition of low-valent ( DIPP BDI)Al to the CÀHb ond in benzene yielding ( DIPP BDI)Al(Ph)H is exergonic by À25.0 kcal mol À1 .W ithout ac atalyst, the activation energy for this process is 51.0 kcal mol À1 ( Figure S27). However,t he presence of CaH 2 reduces this barrier significantly.F irst, a( DIPP BDI)Al(m-H)CaH complex (A1)i sf ormed in which ah ydride ligand binds to Al along its empty p-orbital. An alternative minimum in which the hydride is fully transferred to Al under formation of an AlÀCa bond (A2)is0.4 kcal mol À1 lower in energy.Complexation of A1 with benzene also leads to hydride transfer and AlÀCa bond formation (A3). Thelatter AlÀCa bond is weak and not present in transition state A4*,i nw hich Ca is fully bound to benzene.T he low barrier for insertion (A3-A4*: 21.5 kcal mol À1 )i si na greement with the room temperature reaction and originates from activation by Ca-C 6 H 6 pcoordination and strong hydride-to-Al interaction. This hydride-Al contact increases the nucleophilicity of the Al sp 2 lone-pair significantly and, similar to the Al anion in Scheme 2f,C ÀHb ond activation is facilitated.
Pathway B:The Meisenheimer anion route starts with the formation of ab enzene-CaH 2 complex (B1), which is followed by an unusual nucleophilic aromatic attack (B2*). This reactivity is reminiscent to Et/D exchange in asolution of [( DIPP BDI)CaEt] 2 in C 6 D 6 ,r ecently reported by Hill and coworkers, [26] or to H/D exchange in as olution of [( DIPeP BDI)SrH] 2 in C 6 D 6 published by us. [33] Assisted by Cabenzene complexation, the energy barrier for this reaction is only 20.6 kcal mol À1 and formation of Meisenheimer complex B3 is slightly endergonic.S tarting from B3 there are two possibilities:( DIPP BDI)Al attacks the C 6 H 7 À ion either from the Ca-bound side or form the opposite direction. Thel atter leads to precomplexation (B4)a nd an unrealistically high Scheme 5. Three potentialm echanisms calculated for the Ca hydride catalyzed alumination of benzene (wB97XD/6-311 + G**(PCM = benzene)// wB97XD/6-31 + G**, relative DG values at 298 Ka nd 1bar are given in kcal mol À1 ).
transition state (B5*), after which conversion via B6-B8 is essentially barrier-free.Attack from the Ca-bound side leads to facile hydride transfer from the C 6 H 7 À ion to Al (B9*). Due to as tabilizing Al-Ca interaction, the barrier for this transition state is considerably lower. However,t he product of this pathway is the Al-Ca complex A3.T his means that pathway B represents an alternative,but more difficult route for formation of A3, which further follows pathway A3! A4*!A5.
Pathway C:T he route that involves an anti-aromatic C 6 H 6 2À intermediate is reminiscent of our earlier work on Ca/ Al assisted benzene reduction (Scheme 2d). [21] Theu ncatalyzed reaction of ( DIPP BDI)Al with benzene to give the Al III complex ( DIPP BDI)Al(C 6 H 6 )isendergonic by + 6.5 kcal mol À1 and an activation energy of 33.0 kcal mol À1 is required (Figure S28). In contrast, the highest barrier calculated for nucleophilic addition to ab enzene-CaH 2 complex (B1)i s only 23.4 kcal mol À1 (C2*)a nd formation of C3 is exergonic. Thef urther course of the reaction is ar earrangement of the weakly bound CaH 2 to the bridgehead carbon (C4)w hich further rearranges to C5,acomplex in which CaH 2 bridges between Al and one of the C = Cbonds.Starting from here,the concerted double H-transfer process via C6* is essentially barrier-free.
Using D-labeling, one should be able to discriminate between pathways A/B and C.H owever,f acile hydride exchange between Ca and Al (vide supra) does not allow for any further experimental verification. Although the model system chosen for this computational study may be simple,it indicates that route A seems to be the most likely pathway.It is also in agreement with the experimentally observed induction time and is in line with the formation of as table intermediate (A1-A2)a sw ell as its non-reversibility:c onversion A5!A4* would require 31.0 kcal mol À1 .I nterestingly,r ecalculating pathway A with Mg instead of Ca gave am uch higher activation barrier of + 37.4 kcal mol À1 for the CÀHbond braking step (Scheme 6). This explains the stability of ( DIPP BDI)Al(H)Mg( DIPP BDI) in refluxing benzene.W e therefore believe that the key to Ca-catalyzed benzene alumination is the very weak Ca···Al interaction which precluded isolation of am ixed Ca/Al species.A lthough labile,w ew ere able to compute the minimum for ( DIPP BDI)Al(H)Ca( DIPP BDI) which shows an extraordinary long Ca···Al contact of 3.120 ( Figure 2). It has to be note that this Ca···Al contact is of similar length as that in the model system ( DIPP BDI)Al(H)CaH (A2)of3.165 ,confirming that CaH 2 ,a lthough small, is ar easonable substitute for ( DIPP BDI)CaH in the computational study.
It  Zn 2.491(1) ,M g2 .681 and Ca 3.120 .W hile the calculated values for the Zn À Al and Mg À Al bonds compare reasonably well with experimental values,t he increments from Zn to Mg (0.19 )a nd Mg to Ca (0.439 )a re much larger than the differences in ionic radii (Zn 2+ 0.74 ,M g 2+ 0.72 ,Ca 2+ 1.00 ). [38] Thelong Ca···Al contact is likely due to its substantial electrostatic character.According to Allred-Rochow electronegativities (Al 1.47, Zn 1.66, Mg 1.23 and Ca 1.04) [39] bond polarities increase along the row Zn À Al < Mg À Al < Ca À Al. Bond weakening along the row Zn À Al > Mg À Al > Ca À Al is also clear from metal exchange energies given in Figure 2. TheZ n ÀAl bond is more than 10 kcal mol À1 more stable than the CaÀAl bond. Atoms-In-Molecules (AIM) analyses of the complexes show increasing bond-critical-point electron densities,and therefore increasing bond strength and covalency, along the row Ca À Al < Mg À Al < Zn À Al. The lability of the Ca À Al bond is demonstrated by localization of as econd minimum with ab ridging hydride,( DIPP BDI)Al(m-H)Ca( DIPP BDI), which is only 2.6 kcal mol À1 higher in energy. Although the CaÀAl bond distance of 2.958 is considerably shorter than the 3.120 distance in the CaÀAl bound species, there is no bond-critical-point along the Ca···Al axis.T he contour plot of the Laplacian of the electron density clearly shows that the electron pair is located at Al. Similar minima for Zn(m-H)Al or Mg(m-H)Al species could not be located.
Thel argely different nature of the ZnÀAl, MgÀAl and CaÀAl bonds is also clear from NPAcharges (Figure 2). The most remarkable difference is found for the NPAcharges:the Al center in the Ca À Al complex is clearly less positive,t he charge of + 0.37 is unusually low and also considerably less than that on Al in ( DIPP BDI)Al of + 0.82. Therefore,the CaÀ Al complex could be regarded as the ion-pair [( DIPP BDI)Ca] + [( DIPP BDI)Al(H)] À .T his is supported by NPAc harges of + 0.769/À0.769 calculated for these ions.T he aluminyl anion [( DIPP BDI)Al(H)] À in this ion pair is reminiscent of the C À H bond activation of the anionic Al complex reported by the Aldrich and Goicoechea groups. [24,25] It is proposed that the [( DIPP BDI)Ca] + cation and [( DIPP BDI)Al(H)] À anion operate in concert. Thec ombination of Ca···C 6 H 6 complexation and strong nucleophilicity of the aluminyl anion are crucial to the herein presented facile C À Hbond activation.

Conclusion
Cleavage of the sp 2 CÀHb ond in unactivated arenes (benzene,t oluene,x ylene) by the low valent Al I complex ( DIPP BDI)Al has been achieved at room temperature.I n contrast to previously reported Pd-catalyzed benzene alumination, [22,23] this challenging reaction can also be mediated by the early main group metal hydride complex [( DIPP BDI)CaH] 2 .This observation contributes to the growing awareness that d-orbital participation is not an essential requirement for oxidative addition to unactivated C À H bonds.
Fortoluene,aremarkable selectivity for meta-functionalization has been observed. Functionalization with elemental I 2 gave conversion to m-tolyl iodide.Although this reaction is not catalytic in Al, ap rotocol for recycling to the starting material ( DIPP BDI)Al is presented. However,the challenge to use both, the Ca hydride and low-valent Al I complexes,i n atrue catalytic sense still remains along-term goal.
Themechanism for the Ca hydride catalyzed alumination of benzene has been investigated by experiment and theory. Conversion vs.t ime does not follow simple kinetics but suggests the formation of an intermediate.F rom D-labeling studies,itisunclear whether the source of the hydride in the product is the arene or the catalyst. Since the product ( DIPP BDI)Al(C 6 D 5 )D and the [( DIPP BDI)CaH] 2 catalyst show D/H scrambling,n of urther conclusions could be drawn. Replacing Ca for Mg or Zn fully deactivated the catalyst and the mixed metal products ( DIPP BDI)Al(H)M( DIPP BDI) (M = Mg, Zn) have been characterized by single crystal X-ray diffraction. Thel atter are extremely stable and do not react with benzene,e ven under reflux conditions. Three different pathways have been evaluated by DFT calculations.This study indicates that the first reaction step is the formation of al oosely bound CaÀAl complex, ( DIPP BDI)Al(H)Ca( DIPP BDI), which is in equilibrium with the hydride bridged complex ( DIPP BDI)Al(m-H)Ca( DIPP BDI). Bonding of the hydride along Alsempty p-orbital axis leads to increased nucleophilicity of the Al center. This is supported by calculation of an unusually low positive charge on Al. Similar as for the first aluminyl anions, [24,25] the mixed metal CaÀAl species is activated for facile oxidative addition to aC ÀHb ond in an arene.S ince it is the combined action of anucleophilic Al center and arene activation by p-coordination to aLewis acidic Ca center, parallels to frustrated Lewis pair chemistry may be drawn. [40] Theh erein observed activation of Al I centers by metal hydride interaction is unique.R eplacing Pd with aC a-based catalyst for ac hallenging reaction like the CÀHb ond activation in unactivated arenes represents another jewel in the crown of the rapidly developing field of s-block metal catalysis. [41] Thec oncept of activation of ( DIPP BDI)Al by addition of ap olar reagent can likely be extended much further and is currently comprehensively investigated.