Seven-Membered Cyclic Diamidoalumanyls of Heavier Alkali Metals: Structures and C–H Activation of Arenes

Like the previously reported potassium-based system, rubidium and cesium reduction of [{SiNDipp}AlI] ({SiNDipp} = {CH2SiMe2NDipp}2) with the heavier alkali metals [M = Rb and Cs] provides dimeric group 1 alumanyl derivatives, [{SiNDipp}AlM]2. In contrast, similar treatment with sodium results in over-reduction and incorporation of a formal equivalent of [{SiNDipp}Na2] into the resultant sodium alumanyl species. The dimeric K, Rb, and Cs compounds display a variable efficacy toward the C–H oxidative addition of arene C–H bonds at elevated temperatures (Cs > Rb > K, 110 °C) to yield (hydrido)(organo)aluminate species. Consistent with the synthetic experimental observations, computational (DFT) assessment of the benzene C–H activation indicates that rate-determining attack of the Al(I) nucleophile within the dimeric species is facilitated by π-engagement of the arene with the electrophilic M+ cation, which becomes increasingly favorable as group 1 is descended.


■ INTRODUCTION
−35 Like compound 1, the majority of subsequently reported alumanyl compounds (e.g., [{(NON)AlK] 2 (NON = O(SiMe 2 NDipp) 2 ; 2) have employed elemental potassium to effect reduction to the formal Al(I) oxidation state.As well as providing the necessary charge balance, the K + cation has been shown to deliver a variety of dimeric, monomeric, or charge-separated structures and to leverage contrasting chemical behaviors that are dependent either on the basicity of the solvent employed or through the introduction of a crown ether or cryptand cocomplexant. 5,7espite the initial pre-eminence of potassium alumanyl species, more recent attention has turned to derivatives comprising alternative lighter (Li and Na 12,26,35,36 ) or heavier (Rb and Cs 36,37 ) counter cations.Coles, Mulvey, and coworkers' extension of the NON-derived species to a contiguous series of contact pair alumanyls comprising all of the available alkali metals [{(NON)Al}M] 2 M = Li (3), 12 Na (4), 12 K (2), 9 Rb (5), 37 and Cs (6) 37 has highlighted a structural transition from so-termed "slipped" (3 and 4) through "symmetric" (2)  to "twisted" (5 and 6) modes of N-aryl•••M-based dimerization. 3While this solid-state feature may be attributed to the differing size and polarizability of the group 1 cations, of perhaps greater significance is the contrasting reactivity of 2−6 toward benzene solvent.The initial study of compound 1 disclosed that a C−H bond of benzene underwent oxidative addition to afford the potassium (hydrido)(phenyl)aluminate, [K{Al(H)(C 6 H 5 )( xanth NON)}] 2 , over 4 days at 57 °C. 1 Although several other subsequently reported potassium alumanyls have displayed similar or related C(sp 2 )−H reactivity, 6,15,22,38 only the cesium derivative (6) among Coles' NON-supported derivatives, 2−6, provided comparable behavior, yielding [Cs{Al(H)(C 6 H 5 )(NON)}] after 5 days of heating at 80 °C. 37Density functional theory (DFT) calculations attributed this limitation to a likely example of synergistic alkali metal mediation (AMM), 39 in which the heavier alkali metal exerts a greater (i.e., more exergonic) influence over the formation of an aluminum nucleophileinduced Meisenheimer intermediate prior to the C−H activation transition state. 37In an even more recent advance, Harder and co-workers have devised synthetic routes to a further complete series of dimeric group 1 alumanyls, [{(BDI Dipp -H)Al}M] 2 (BDI Dipp -H = [DippNC(Me)•C(H)C-(�CH 2 )(NDipp)] 2− ; M = Li (7), Na (8), K (9), Rb (10), and Cs (11)]. 36Although these compounds again adopt alkalimetal-dependent "slipped" (7, 8) or "symmetric" (9−11)  structures in the solid state, each of the potassium, rubidium, and cesium derivatives was shown to induce a similar, but 2fold, para-C−H activation of benzene.
Prompted by an initial intention to elaborate alumanyl chemistry to the heavier elements of group 2, we have previously reported the seven-membered cyclic potassium diamidoalumanyl, [({SiN Dipp }Al)K] 2 (12, {SiN Dipp } = {CH 2 SiMe 2 NDipp} 2 ), 29 which itself activates a methyl C-(sp 3 )−H bond of the solvent when heated in toluene at 110 °C for 24 h to provide the benzylaluminum hydride product, [K{SiN Dipp }Al(H)(CH 2 Ph)]. 17In this contribution, we report our efforts to extend the chemistry of the [{SiN Dipp }Al] − alumanyl anion to a broader range of alkali metals and the reactivity of the resultant contact pair dimers toward the C−H bonds of arene solvents.

■ RESULTS AND DISCUSSION
Synthesis of {SiN Dipp }-Ligated Sodium, Rubidium, and Cesium Alumanyls.Reduction of [{SiN Dipp }AlI] (13) either with a sodium mirror or with 5 wt.%Na/NaCl 40 in hexane resulted in the complete consumption of the iodide reagent and the generation of a pale-yellow solution and gray suspension within 3 days at room temperature.Filtration and storage of a concentrated solution at −10 °C afforded compound 14 as a colorless crystalline powder in a good (>75%) yield.In contrast to the 1 H and 13 C nuclear magnetic resonance (NMR) spectra provided by compound 12, which were diagnostic of a single chelated {SiN Dipp } environment and local C 2 symmetry about aluminum, the corresponding spectra of 14 were more complex and, irrespective of the physical form of the group 1 reducing agent or the reaction stoichiometry, consistent with the presence of multiple dianilide ligand resonances.Notably, the specificity of the production of compound 14 was demonstrated by a further reaction performed between 13 and 5 wt.%Na/NaCl in C 6 D 6 in a J Young NMR tube, monitoring of which evidenced the emergence of a 1 H NMR spectrum over 5 days that was virtually identical to that provided by an isolated and crystallized sample.Figure 2. Displacement ellipsoid (30% probability) plot of the structure of compound 14.For clarity, solvent has been omitted, and hydrogen atoms have been removed.Dipp iso-propyl substituents are also presented as wireframes for visual ease.Symmetry operation: 1 1 − x, 1 − y, and 1 − z.

Organometallics
The origin of these observations was ultimately resolved by X-ray diffraction analysis of a single crystal of compound 14 (Figure 2 and Table 1), which may be considered a product of over-reduction of the target sodium alumanyl, [({SiN Dipp }Al)-Na] 2 .Compound 14 is a heterobimetallic species, the asymmetric unit of which comprises half of the molecule, with the remainder generated via a crystallographic inversion center.The two symmetry-related aluminum atoms are N,N′coordinated by still-chelated {SiN Dipp } ligands and display close contacts to two sodium cations [Al1−Na1 3.1251(10); Al1−Na2 3.2368 (11) Å].The sodium atoms in each of the resultant trimetallic units are bonded by polyhapto interactions with the Dipp substituents of the chelated {SiN Dipp } ligands but are differentiated by their binding to an additional diamide dianion that now adopts a {Na2-μ−κ 1 -N,μ−κ 1 -N′-Na2′} bridging mode.This latter bonding situation and the gross structure of 14 are, thus, strongly reminiscent of the recently reported and isomorphous sodium magnesiate [{(SiN Dipp )Mg-(H)Na 2 } 2 {μ-(SiN Dipp )}] (15). 41Although charge balance in 15 was necessarily maintained by the trigonal encapsulation of a hydride by each MgNa 2 array, the Al−N bond lengths to the {SiN Dipp } chelate [Al1−N1 1.8886(18); Al1−N2 1.8625 (17)  Å] in 14 are consistent with an assignment of the Al(I) oxidation state. 17,29This latter observation leads us to discount the possible presence of hydride anions in 14 and to continue to assign each ({SiN Dipp }Al) as alumanyl units with a high degree of confidence.
Although we have no specific rationale for this outcome, we have previously noted the tendency of seven-membered chelated magnesium derivatives of the {SiN Dipp } dianion to undergo ring opening and the adoption of similar bimetallic bridged structures. 41,42These observations, in conjunction with the notable specificity of the formation of 14, lead us to suggest that the reaction stoichiometry presented in eq 1 for the synthesis of 14 is realistic and justifiable.
In contrast to these observations, reduction of 13 with Rb and Cs metals in hexane proceeded to completion at 30 °C in 3 and 2 days, respectively, to provide bright-yellow solutions of the heavier alkali metal alumanyls, compounds 16 and 17 (Scheme 1).Filtration and removal of volatiles at this point provided effectively pure samples of both compounds as yellow crystalline solids.Both 16 and 17 presented 1 H and 13 C NMR spectra that were strongly reminiscent of those of the analogous potassium derivative (12), indicative of a symmetrical N,N-chelated disposition of the dianilide ligand about aluminum.This inference was shown to be correct through the isolation of single crystals of both compounds, which were obtained by slow evaporation of hexane solutions at room temperature.The results of the subsequent X-ray analyses (Figure 3a,b) confirmed that, like 12, both compounds crystallize as contact ion pairs in which the two, formally anionic, [(SiN Dipp )Al] − subunits are solely connected by 2-fold polyhapto-Rb/Cs•••π-arene bridging interactions.The mean M•••centroid distances of 3.099 (16: M = Rb) and 3.247 (17 M = Cs) Å are comparable to those displayed by the previously reported rubidium (5 and 9) and cesium (6 and 10) alumanyl derivatives and reflect the differing radii of the respective M + cations. 43As was highlighted by Coles and Mulvey, 37 the enhanced conformational freedom accorded to the dimeric structure by increasing alkali metal cation radius is also reflected in the dihedral angles subtended by the N1-A1-N2 and N3−Al2−N4 least-squares planes, which increase in the order 12 (M = K: 48.11°) 29 < 16 (M = Rb: 62.33°) < 17 (M = Cs: 64.52°).The two latter values are reminiscent of those reported for the comparable metrics in compounds 5 and 6 such that 16 and 17 may also be described as presenting similarly "twisted" dimeric structures.
Activation of Arene Solvents.We have previously reported that compound 12 initiates the formal oxidative addition of a toluene-methyl C−H bond when heated at 110 °C for 24 h. 17In an attempt to extend this chemistry to the heavier alkali metal congeners, samples of the rubidium and cesium alumanyls, compounds 16 and 17, were similarly heated in d 8 -toluene at 110 °C.Monitoring of the reactions over the course of 2 days by NMR spectroscopy provided evidence for the consumption of both alumanyl reagents and the generation of a complex mix of products.Although the cesium-containing reaction mixture proved intractable toward further purification, storage of the rubidium-derived solution at low temperature provided a small number of colorless single

Scheme 1. Synthesis of Compounds 16 and 17
Organometallics crystals of compound 18, which could be separated mechanically from the mixture of otherwise unidentifiable products formed.X-ray diffraction analysis of 18 allowed its identification as the rubidium (hydrido)(benzyl)aluminate, [{(SiN Dipp )Al(H)(CH 2 Ph)}Rb], resulting from reductive activation of a toluene C(sp 3 )−H bond (Scheme 2).The results of this analysis, depicted in Figure 4, with selected bond lengths and angles summarized in Table 2, demonstrated that 18 is isostructural to its previously reported potassium analogue, 17 crystallizing as an array of 4-coordinate diamido aluminate anions, which propagate along the crystallographic b axis as a 1-dimensional polymer via a series of Al−H−Rb, Rbη 6 -Dipp, and Rb-η 6 -benzyl interactions.In related observations, Aldridge and co-workers have previously reported that heating compound 1 in toluene for 2 days at 80 °C provides two products, which cocrystallize in a 3:1 ratio and result from meta-aryl and benzylic C−H bond activation, respectively. 6In a subsequent report, Yamashita and co-workers observed that toluene solutions of a potassium dialkylalumanyl variant provided the corresponding potassium (hydrido)(m-tolyl)aluminate as the sole product when simply allowed to stand at room temperature. 15These observations were ascribed to the operation of cooperative S N Ar processes, in which attack of the aluminum nucleophiles is facilitated by simultaneous complexation of the arene π-system by potassium.In this manner, the observed high levels of meta-C−H discrimination could be rationalized on the basis of the preferred charge distribution associated with the resultant Meisenheimer-type intermediates.While the behavior of compounds 16 and 17 toward toluene prompts us to suggest that similar mechanisms may be operable, further meaningful comment is precluded by the apparently reduced levels of discrimination provided by the {SiN Dipp }-derived systems.

Organometallics
The potassium diamido-and dialkylalumanyl species described by Aldridge and Yamashita have also been reported to effect similar C(sp 2 )−H alumination of benzene under relatively mild conditions (57 °C and room temperature, respectively).In contrast, the sole representative example of analogous reactivity among Coles and co-workers' NONsupported systems (2−6) was provided by the cesium species (7), which also required more forcing conditions (5 days at 80 °C).With these observations in mind, therefore, we assessed the thermal stability of the respective potassium (12), rubidium (16), and cesium (17) alumanyl dimers in benzene solution.Although all three solutions required heating to 110 °C for the reactions to proceed at an appreciable rate, a gradual decolorization was observed in each case.Monitoring by 1 H NMR spectroscopy indicated that the reactions were complete after 14 days (12), 5 days ( 16), and 12 h (17), while the disappearance of the alumanyl starting materials was accompanied by the simultaneous deposition of colorless crystals of the alkali metal (hydrido)(phenyl)aluminates, compounds 19 (75%), 20 (67%), and 21 (60%), resulting from the potassium-, rubidium-, and cesium-based reactions, respectively (Scheme 2).Although all three compounds proved to be insufficiently soluble in benzene for further mechanistic/kinetic analysis of their formation, their characterization by NMR spectroscopy was readily achieved by redissolution in THF-d 8 .While the resultant data provided convincing corroborative evidence for the formation of the mooted phenylaluminum products of benzene C−H activation, single-crystal X-ray diffraction analysis again delivered a definitive demonstration of the constitution of all three compounds.
Selected bond length and angle data for all three compounds are presented in Table 2, and the asymmetric units of the potassium and rubidium (hydrido)(phenyl)aluminates, 19 and 20, are illustrated in Figure 5a,b, respectively.Although only the triclinic structures of 20 and 21 are crystallographically isomorphous, any variation in the unit cell dimensions and volume is traceable to the adjustment in the ionic radii of the K + , Rb + , and Cs + cations, 43 an observation that is also reflected in the various M−C bonds observed across the three structures (Table 2).Like 18, all three compounds crystallize as onedimensional polymers, with propagation provided by encapsulation of the M + ions, which span between adjacent pairs of aluminate anions via an alternating sequence of η 6 -M + -phenyl/ η 6 -M + -Dipp-N and Al−H-M + /η 6 -M + -Dipp-N interactions.This is illustrated for cesium derivative 21 in Figure 5c.Despite the changing identity of the group 1 counter cations, the aluminate anions of all three structures show only minor variations across the various Al−N and Al−C bonds and within the chelated {SiN Dipp } ligands.
DFT Calculations.Although Roesky's monometallic βdiketiminate derivative, [HC{C(Me)NDipp) 2 Al], 44 has been widely employed as a highly reducing and hydrocarbon-soluble source of aluminum(I), 45−64 unless promoted by palladium catalysis, 65 it has been reported as inert toward arene solvents. 66This behavior contrasts significantly with many of the alkali metal alumanyl derivatives summarized in Figure 1.It is notable, however, that benzene C−H activation, whether single 1,6,15,37,38 or 2-fold, 22,36 has been limited to alumanyl derivatives comprising a heavier (K−Cs) alkali metal cocation.In accord with earlier calculations on potassium alumanyls reported by Aldridge 6 and Yamashita, 15 Mulvey, Coles, McMullin, and co-workers attributed benzene activation by [{(NON)AlCs] 2 (6) to the operation of a synergistic AMM effect, in which the softer, heavier Cs + cation is best disposed to engage the benzene π system toward nucleophilic alumanyl attack. 37,67Although, among the series of [{(NON)AlM] 2 [M = Li − Cs] derivatives, the acquisition of experimental evidence was unique to 6, DFT calculations ascribed only a marginally diminished kinetic aptitude (ca. 5 kcal mol −1 ) for benzene C−H activation arising from its Rb analogue (5) and irrespective of whether a mono-or dimeric pathway was computed (vide infra).
The isolation of compounds 19−21, therefore, and the variable facility of their formation provide a coherent series with which to further assess the consequences of varying alkali metal identity on the arene reactivity of dimeric alumanyl derivatives.Accordingly, and in a comparable manner to the recent computational analysis of the reaction of benzene with [(NON)AlM] 2 (M = Rb and Cs), 37 two pathways (described herein as "dimeric" and "monomeric") were explored for benzene C−H activation by the series of [{SiN Dipp }AlM] 2 (where M = Na, K, Rb, and Cs) (see Supporting Information; Figures S25 and S26).Although similar profiles were calculated and the subsequent discussion relates to all four alkali metal species, for illustrative purposes, Figures 6 and 7 are restricted to the data arising from the respective dimeric and monomeric pathways for the rubidium-derived reaction only.
The dimeric pathway achieves benzene activation without any requirement for the initially dimeric alumanyl to dissociate into monomeric units.The association of one benzene molecule to the dialumanyl complex (A) forms species B, which then undergoes the first C−H activation via a Meisenheimer-like transition state, TS(B−C).Notably, and reminiscent of several recent reports in which a benzene derivative is activated through its polyhapto engagement with a heavier s-block or Yb(II) cation, 1,6,15,36,38,68−72 the assembly of TS(B−C) is facilitated by similar π engagement and resultant rear-side nucleophilic attack by one of the low oxidation state aluminum centers.Although this process disrupts the previously symmetrical structure of the alumanyl dimer (A), the resultant (hydrido)(phenyl)aluminate does not completely dissociate from the remaining alumanyl unit.Rather, contact between both [(SiN Dipp )Al] chelate structures is maintained in C through a persistent Dipp-M•••Dipp bridging interaction.A second benzene molecule is, however, able to interact with the now more exposed group 1 metal, which remains coordinated via the phenyl ligand of the newly formed phenylaluminate in C•C 6 H 6 .In a manner similar to the initial C−H activation process, attack of the second alumanyl proceeds via the formation of D and TS(D-E), the magnitude of which is closely comparable to TS(B-C) (Table 3).The thermodynamic viability of the overall transformation is then ensured by the stability of the ultimate (dihydrido)(diphenyl)dialuminate complex (E).
The alternative monomeric pathway first requires the cleavage of the dimer (A), which proceeds via species F through the insertion of a single benzene molecule into the Al 2 M 2 diamond.This process increases the separation between the Al centers to over 10 Å and ultimately develops into species G, a monomeric alumanyl in which benzene displays an η 6 -interaction with the group 1 metal cation.From G, Alinduced C−H activation occurs in a somewhat analogous fashion to that outlined for the previously described dimeric pathway.Attack of the Al(I) nucleophile at benzene provides an initial intermediate H via TS(G-H), whereupon cleavage of the C−H bond via TS(H-I) yields a monomeric (hydrido)-(phenyl)aluminate, I.The final step of this pathway involves a  small rotation of the phenyl group at aluminum through TS(I-J) to give the more stable conformer J. 73 The free energies arising from both computed pathways (Table 3) show the anticipated trend, where the ultimate products, E K /J K , E Rb /J Rb and E Cs /J Cs , represent monomeric variants of the polymeric (hydrido)(phenyl)aluminate species, 19, 20, and 21, respectively.Although experimentally unavailable for assessment, the highest activation barriers presented by both pathways are associated with the sodiumderived species and then decrease incrementally as group 1 descends.In every case, however, the sequential dimeric pathway is favored mechanistically, providing barrier heights that are congruent with the necessary experimental conditions and predicting that the first C−H activation step via TS(B-C) is rate-determining for all three reactions available for experimental assessment.

■ CONCLUSIONS
Reduction of [{SiN Dipp }AlI] with the alkali metals K, Rb, and Cs provides similarly dimeric group 1 alumanyl derivatives.Although the analogous reaction with sodium also results in the isolation of an alumanyl species, a consistent level of overreduction leads to the incorporation of a formal equivalent of [{SiN Dipp }Na 2 ] into the resultant structure.The dimeric (K, Rb, and Cs) species all react with arene solvents at elevated temperatures to yield (hydrido)organoaluminate species, while computational assessment of the benzene C−H activation indicates that the rate-determining attack of the Al(I) nucleophile is facilitated by π-engagement of the arene with the electrophilic cation, which becomes increasingly favorable as group 1 is descended.

■ EXPERIMENTAL SECTION
Unless stated otherwise, all of the experiments were conducted using standard Schlenk line and glovebox techniques under an inert atmosphere of argon.NMR spectra were recorded with a Bruker AVANCE III spectrometer ( 1 H at 400 MHz; 13 C at 101 MHz).The spectra are referenced relative to residual protio solvent resonances.Elemental analyses were performed at Elemental Microanalysis Ltd., Okehampton, Devon, UK.Solvents were dried by passage through a commercially available solvent purification system and stored under argon in ampoules over 4 Å molecular sieves.Benzene-d 6 and THF-d 8 were purchased from Sigma-Aldrich and dried over a potassium mirror before distilling and storage over molecular sieves.[{SiN Dipp } - AlI] ( 13) and [{SiN Dipp }AlK] 2 (12) were prepared according to the reported procedures. 29All other chemicals were purchased from Merck and used without further purification.

■ COMPUTATIONAL METHODOLOGY
DFT calculations were run with Gaussian 16 (C.01). 74The Na, Al, Si, K, Rb, and Cs centers were described with the Stuttgart RECPs and associated basis sets, 75 and the 6-31G** basis set was used for all other atoms (BS1). 76A polarization function was also added to Al (ζ d = 0.190), Si (ζ d = 0.284), K (ζ d = 1.000),Rb (ζ d = 0.491), and Cs (ζ d = 0.306).Initial BP86 optimizations were performed using the "grid = ultrafine" option, 77 with all stationary points being fully characterized via analytical frequency calculations as minima or transition states (all positive eigenvalues or one imaginary eigenvalue, respectively).All energies were recomputed with a larger basis set featuring 6-311++G** basis sets on all atoms with the exceptions of Rb and Cs, which used def2-TVZP (BS2).Corrections for the effect of benzene (ε = 2.2706) solvent were run using the polarizable continuum model and BS1, 78 using the keyword "scrf = benzene" within Gaussian.Single-point dispersion corrections to the BP86 results employed Grimme's D3 parameter set with Becke-Johnson damping, as implemented in Gaussian. 79ASSOCIATED CONTENT

Figure 3 .
Figure 3. Displacement ellipsoid (30% probability) plots of the structures of (a) compound 16 and (b) compound 17.For clarity, the solvent has been omitted, and with the exception of methyl groups that display close C−H•••M contacts, hydrogen atoms are not shown.Most Dipp iso-propyl substituents are presented as wireframes, also for visual ease.Scheme 2. Synthesis of Compounds 18−21

Figure 5 .
Figure 5. Displacement ellipsoid plots (30% probability) of the asymmetric units of (a) 19 and (b) 20.(c) Polymeric structure of compound 21.Symmetry operations (21): 1 x, 1 + y, z; 2 x, − 1 + y, z.For clarity, hydrogen atoms have been removed with the exception of those which are aluminum-bound or involved in C−H•••M interactions.Most Dipp iso-propyl substituents are presented as wireframes, and disorder has also been omitted from 19 for visual ease.