Boosting Low‐Valent Aluminum(I) Reactivity with a Potassium Reagent

Abstract The reagent RK [R=CH(SiMe3)2 or N(SiMe3)2] was expected to react with the low‐valent (DIPPBDI)Al (DIPPBDI=HC[C(Me)N(DIPP)]2, DIPP=2,6‐iPr‐phenyl) to give [(DIPPBDI)AlR]−K+. However, deprotonation of the Me group in the ligand backbone was observed and [H2C=C(N‐DIPP)−C(H)=C(Me)−N−DIPP]Al−K+ (1) crystallized as a bright‐yellow product (73 %). Like most anionic AlI complexes, 1 forms a dimer in which formally negatively charged Al centers are bridged by K+ ions, showing strong K+⋅⋅⋅DIPP interactions. The rather short Al–K bonds [3.499(1)–3.588(1) Å] indicate tight bonding of the dimer. According to DOSY NMR analysis, 1 is dimeric in C6H6 and monomeric in THF, but slowly reacts with both solvents. In reaction with C6H6, two C−H bond activations are observed and a product with a para‐phenylene moiety was exclusively isolated. DFT calculations confirm that the Al center in 1 is more reactive than that in (DIPPBDI)Al. Calculations show that both AlI and K+ work in concert and determines the reactivity of 1.

The first known low-valent Al I complex, (Cp*Al) 4 (I), [1] is in equilibrium with monomeric Cp*Al. This complex features a high-lying HOMO and a low-lying LUMO, a combination that leads to high reactivity and transition-metal-like properties. [2,3] Bulkier Cp ligands can direct the equilibrium exclusively to the more reactive monomer. [4,5] A kinetically stabilized monomeric Al I complex, ( DIPP BDI)Al (II), was isolated by the group of Roesky. [6] Despite its protective bulky DIPP BDI ligand, ( DIPP BDI)Al is like an N-heterocyclic carbene, [7] a good s-donor and p-acceptor exhibiting an unusually high reactivity that has been exploited in XÀH and CÀF bond activation. [8] Recent years have seen the development of even morereactive anionic Al I complexes. Given the theoretical prediction that the anionic N-heterocyclic aluminenes [C(H)N-(R)] 2 Al À are isolobal to Arduengo carbenes and should be stable, [9] the groups of Aldridge and Goicoechea presented the first anionic Al I complex, which crystallized as a dimer with bridging K + ions (III). [10] They demonstrated its unique reactivity as exemplified by the first oxidative addition of a benzene CÀH bond to a single main-group metal center and, even more spectacular, CÀC bond cleavage in benzene. [11] Subsequent work of the groups of Coles and Hill led to similar dimeric bis-amido Al I complexes, IV and V, respectively. [12,13] Most recently, Yamashita and co-workers isolated a highly reactive monomeric anionic bis-alkyl Al I complex (VI), while the group of Kinjo reported a monomeric Al I anion analogue to the CAAC-carbene ligand. [14] Some of these anionic Al I complexes are considerably more reactive than ( DIPP BDI)Al (II), which does not react with benzene. Jain et al. calculated that 1,4-addition of ( DIPP BDI)Al to benzene, giving the Al III complex ( DIPP BDI)Al(C 6 H 6 ), is only slightly endergonic [DG-(298 K) =+ 4.8 kcal mol À1 ] [15] but there is a high transition state. [16] Our group reported two methods to activate ( DIPP BDI)Al for reaction with benzene. In combination with a strong "naked" Lewis acid like [( DIPP BDI)Ca + ][B-(C 6 F 5 ) 4 me 1 a). [17] Or, in cooperation with catalytic quantities of a Lewis base-free calcium hydride complex [18] oxidative addition to the benzene C À H bond was observed (Scheme 1 b). [19] DFT calculations point to a mechanism in which donation of the hydride to the empty p-orbital of Al leads to increased nucleophilicity of the sp 2 lone pair of Al. The proposed intermediate in this reaction shows parallels to earlier reported anionic Al I complexes (III-VI). Along similar lines, we now propose a general concept for activation of ( DIPP BDI)Al by simple addition of a polar s-block metal reagent, R À M, which may generate an anionic Al complex of considerably higher reactivity (Scheme 1 c). This principle is in line with the well-known reactivity increase of low-valent late main-group complexes by neutral donors. [20,21] While synergy between s-and p-block metal reagents have led to spectacular deprotonating reagents, [22] mixing multimetallic cocktails to boost redox activity of low-valent main-group metal complexes is rather unexplored.
As all hitherto reported anionic Al complexes contain K + counter cations, we chose to test this concept by addition of a K reagent that is soluble in aromatic solvents. ( DIPP BDI)Al is unstable in THF. [8a] Addition of KCH(SiMe 3 ) 2 to an orange solution of ( DIPP BDI)Al in benzene at 5 8C led, after a few hours, to precipitation of a bright-yellow powder (1, yield: 73 %). NMR data and the crystal structure are conclusive for loss of a proton in the Me group of the DIPP BDI backbone (Scheme 1 d), which implies that the potassium reagent reacted as a base instead as a nucleophile. Repeating the experiment with the significantly weaker base KN(SiMe 3 ) 2 at room temperature also gave 1. This facile ligand backbone deprotonation is surprising: ( DIPP BDI)AlMe 2 does not react with KN(SiMe 3 ) 2 (even not after five days at 80 8C; see Figure S1 in the Supporting Information).
Similar to the anionic Al complexes III-V, 1 forms a dimer in which formally negatively charged Al centers are bridged by K + ions that show strong K + ···DIPP interactions (Figure 1 a). The ligand in 1 should be regarded as a dianionic bis-amide, 2À , and, although disordered, a suitable disorder model shows the typical CÀC/C=C bond alternation in the ligand backbone. Compared to III-V, 1 features the shortest Al-N and Al-K distances, and a considerably smaller N-Al-N' bite angle ( Table 1). The shortest Al-K contact of 3.499(1) is only slightly longer than the Al-K bond of 3.4549(5) in monomeric VI and consequently also the Al···Al' distance of 5.2357 (5) is significantly shorter than in the other dimers. Therefore, 1 should be considered a tightly bound dimer.
The complex 1 dissolves poorly in C 6 D 6 . 1 H NMR analysis in this solvent reveals two septets and four doublets for the iPr groups, signifying that it retains its highly symmetric dimeric   Table 1 for bond distances/angles (the backbone CH 2 and CH 3 substituents are disordered and only one of two positions is shown), and b) complex 2; solid-state structure. This is confirmed by DOSY which gave a MW of 855 Daltons (MW dimer: 966 g/mol). The complex 1 dissolves very well in [D 8 ]THF but at 20 8C slowly decomposes. DOSY measurements indicate a MW of 635 Daltons, which fits a monomeric structure solvated by two THF ligands (MW: 627 g/mol).
The reaction of 1 with benzene at 20 8C is very slow, however, heating a bright-yellow suspension of 1 in C 6 H 6 to 35 8C gave, after 7 days, a batch of colorless crystals of the product 2. The crystal structure of 2 shows double CÀH bond activation of benzene by oxidative addition exclusively in para-positions (Figure 1 b). The coordination polymer shows two ( DIPP BDI)AlH units connected by a para-phenylene bridge. While one of the K + ions resides over the phenylene bridge and is embedded in the p systems of DIPP and a cocrystallized benzene ligand, the second K + ion bridges these fragments with K + ···ligand and K + ···H-Al contacts. The complex 2 is insoluble in benzene but dissolves in [D 8 ]THF. The 1 H NMR spectrum in this solvent consists of two sets of signals. This can be explained by chirality at the Al centers, chirality which originates from asymmetry in the ligand backbone. Given the large spatial separation of the ligands, the two diastereomers are hardly different in energy and coexist in a 1:1 ratio. At higher temperature fast epimerization of chirality at Al is observed (T coal = 60 8C, DG z = 18.1 kcal mol À1 ). The inversion of chirality at Al likely proceeds by a reversible hydride transfer from Al to K (the short K···hydride distance in 2 supports this proposal). The 1 H NMR signal for the Al-H units at d = 4.2 ppm is extremely broad. Heating 2 in the presence of excess C 6 D 6 did not result in exchange of the C 6 H 4 2À unit for C 6 D 4

2À
, which means that its formation is irreversible.
NMR data are in agreement with the exclusive formation of the para-phenylene product. This formation is confirmed by reaction with I 2 which gave, after work-up, only paradiiodobenzene (see the Supporting Information). Considering that benzene is present in large excess, the preference for double versus single C À H bond activation is intriguing. Formation of 2 may be explained by a template effect that shows parallels to Mulveys mixed-metal base chemistry. [22a,b] Synergy in these heterobimetallic systems enable challenging multiple deprotonations, selectively giving highly charged species. As in Mulveys "inverse crown ethers", the metals in the Al/K complexes III-V and 1 are also arranged in a ring. Similar to the multiply charged anions in inverse crown ether complexes, the C 6 H 4 2À ion in 2 is also stabilized by several metal contacts. The importance of K + for the reactivity of 2 was investigated by incorporation of this cation in [2.2.2]cryptand. While addition of this cryptand to III changed the course of the reaction from CÀ H to C À C bond cleavage, [11] addition to 1 led to single C À H bond activation. This reactivity was confirmed by quenching the reaction mixture with I 2 which gave, after work-up only, iodobenzene instead of para-diiodobenzene (see the Supporting Information).
DFT calculations (wB97XD/6-311 + G**//wB97XD/6-31 + G**) gave further insight into the formation and electronic structure of 1. Although we never isolated our target complex [( DIPP BDI)AlN-(SiMe 3 ) 2 ] À K + , calculations show that its formation is slightly exergonic (Scheme 2 a). In agreement with experiment, the barrier for subsequent intramolecular backbone deprotonation is low. Since BDI backbone deprotonation is generally not easy, prior formation of intermediate [( DIPP BDI)AlN-(SiMe 3 ) 2 ] À K + may be the key to this process. Consistent with this reactivity is the observation that addition of neutral Lewis bases to ( DIPP BDI)Al also led to proton loss of the backbone Me group. [20a-d, 21] The anion in 1 is isolobal to neutral silylene and germylene complexes with the same deprotonated DIPP BDI ligand. [23] The calculated geometry of 1 compares well to that of the crystal structure (see Figure S30). Atoms-in-molecules (AIM) analysis shows the high electron densities at the Al centers which combined with a short Al···Al distance gives rise to a bond-critical-point (bcp), but the electron density of 0.004 e is rather low (Scheme 2 b). The bcp electron densities and Wiberg bond indices in the ligand backbone are consistent with CÀC/C=C bond alternation. Low electron densities at the Al···K bcps are in agreement with highly ionic contacts. The HOMO energy of the full dimeric complex 1 lies at À4.290 eV (B3LYP/6-31 + G*, Figure S31) and fits previously calculated values at this level: À4.035 eV (III), À4.205 eV (IV), and À3.630 eV (VI). [14a] Thus, the nucleophilic character of 1 should be similar to that of IV but lower than that of either III or VI. Compared to ( DIPP BDI)Al (HOMO at B3LYP/6-31 + G*: À4.458 eV), the reactivity of 1 should be clearly higher. This reactivity is illustrated by comparison of the calculated energy profiles for benzene C À H activation by ( DIPP BDI)Al and the anion in 1 (Scheme 2 c). Formation of a loosely bound benzene complex is for both slightly endergonic (+ 7.6/ + 4.1 kcal mol À1 ) while product formation is similarly exergonic (À27.8/À25.0 kcal mol À1 ). The main difference between the energy profiles lies in the activation barriers for C À H activation. Although these DG values are quite high because of overestimation of entropic factors, [24] the barrier for the anionic Al species (+ 27.5 kcal mol À1 ) is much lower than for ( DIPP BDI)Al (+ 43.4 kcal mol À1 ). The activation free energy for benzene CÀH bond cleavage by the anion in 1 compares well to that calculated for the anion in III (+ 28.7 kcal mol À1 [11] or + 31.8 kcal mol À1 [25] ). Incorporation of countercation K + led to a slightly lower activation free energy of + 26.4 kcal mol À1 . Interestingly, in the presence of K + an intermediate reminiscent of a Meisenheimer anion was found. The species originates from nucleophilic attack of Al I at benzene resulting in a strongly buckled [a] AlÀC bond distance.

Angewandte Chemie
Communications aromatic ring. NPA charges show that Al (+ 2.10) is formally + III while the ring (À1.53) carries a high negative charge which is stabilized by K + (+ 0.96). As has been noted earlier, [11,12] the presence or absence of K + can clearly change the course of anionic Al reactivity and should be considered in modelling its reactions. Most recently, a similar Al/K cooperativity was observed in the reaction of an aluminoxane potassium salt with CO 2 . [26] DFT calculations may also shed light on the para-selective double CÀH bond activation. Calculations on a simple model system like [H 3 Al-C 6 H 4 -AlH 3 ] 2À confirm a thermodynamic preference. Relative DG (kcal mol À1 ): 0.0 (para), + 3.5 (meta), + 14.0 (ortho). However, single alumination would thermodynamically still be preferred: [H 3 Al-C 6 H 4 -AlH 3 ] 2À + C 6 H 6 ! 2 [H 3 Al-C 6 H 5 ] À is exergonic by DG = À28.0 kcal mol À1 . Also the transition state for para-alumination of [H 3 Al-C 6 H 5 ] À is more than 10 kcal mol À1 higher than that for monoalumination of C 6 H 6 (see Figures S38 and S42). Given the important role of K + in the mechanism, preference for double alumination may be explained by a template effect. This effect is supported by calculations on a full dimeric model system in which the DIPP groups in 1 were replaced by Ph. The activation barriers for the first alumination (DG =+ 31.8 kcal mol À1 ) and the challenging second alumination (DG =+ 34.3 kcal mol À1 ) are high but of similar magnitude (see Figure S44). Reminiscent of "inverse crown ethers", the second transition state displays multiple metal-Ph contacts (Scheme 2 d). The Ph ring is sandwiched between K + ions that help stabilize its transformation into C 6 H 4

2À
, again underscoring the importance of K + cations for the reactivity of 1.
In summary, the reactivity of Roeskys well-known Al I complex ( DIPP BDI)Al (II), which is kinetically stabilized by a chelating b-diketiminate ligand, is easily boosted by the addition of a simple potassium base. Although we anticipated formation of the aluminate complex [( DIPP BDI)RAl À ]K + , deprotonation of a backbone Me group led to a dimeric anionic Al I complex (1). This complex, which dissolved in benzene is also dimeric, carries considerable electron density on Al, and is highly reactive. It activates two CÀH bonds in benzene by oxidative addition giving exclusively the paraaluminated product which in reaction with iodine forms paradiiodobenzene. Encapsulation of the K + ions in 1 by a cryptand changed its reactivity and at most single C À H bond activation in benzene was observed. Calculations support the idea that the reactivity of 1 is determined by close cooperation of Al I and K + centers. We will continue to study synergistic effects between covalent low-valent p-block metal species and ionic s-block metal reagents.