Cesium Reduction of a Lithium Diamidochloroberyllate

Room temperature reaction of elemental cesium with the dimeric lithium chloroberyllate [{SiNDipp}BeClLi]2 [{SiNDipp} = {CH2SiMe2N(Dipp)}2, where Dipp = 2,6-di-isopropylphenyl, in C6D6 results in activation of the arene solvent. Although, in contrast to earlier observations of lithium and sodium metal reduction, the generation of a mooted cesium phenylberyllate could not be confirmed, this process corroborates a previous hypothesis that such beryllium-centered solvent activation also necessitates the formation of hydridoberyllium species. These observations are further borne out by the study of an analogous reaction performed in toluene, in which case the proposed generation of formally low oxidation state beryllium radical anion intermediates induces activation of a toluene sp3 C–H bond and the isolation of the polymeric cesium benzylberyllate, [Cs({SiNDipp}BeCH2C6H5)]∞.


■ INTRODUCTION
Following a period of relative dormancy, 1 the molecular chemistry of beryllium has experienced something of a renaissance during the past decade.−17 Notably, and very recently, the long sought 18,19 and predicted 20,21 isolation of a Be−Be bonded species has been realized through Aldridge and co-workers' isolation of CpBeBeCp. 22,23−30 Attempting to apply a similar heterobimetallic motif to magnesium's lighter congener, we have observed that Li or Na reduction of the chelated beryllium dianilide, [{SiN Dipp }Be] (II), results in C−H activation of the benzene solvent and isolation of phenylberyllate species, [M({SiN Dipp }BePh)] (III M , where M = Li or Na; Figure 1a). 31Although no definitive spectroscopic or structural evidence could be obtained, we tentatively suggested that this chemistry invoked the generation of transient Be(I) radical anions, onward reaction of which with the solvent provided a mixture of III M and a presumed hydridoberyllate species, [M({SiN Dipp }BeH)], which was unstable under the applied reaction conditions (100 °C).
In an extension to this research, we have very recently reported that reduction of the dimeric lithium chloroberyllate (IV Li ) with either sodium, potassium, or rubidium metal provides no discernible beryllium reduction but results in exclusive replacement of the lithium cations to provide, [{SiN Dipp }BeClM] 2 (IVM, where M = Na, K, Rb; Figure 1b). 32Although similar treatment of IV Li with Cs metal also enabled the isolation of a cesium analogue, IV Cs , this reaction required immediate low temperature workup to suppress the formation of further products.Prompted by the supposition that this onward reactivity invokes subsequent berylliumcentered reduction of IV Cs by the most electropositive available group 1 metal, we now report our further observations of this reaction system.

■ EXPERIMENTAL SECTION
General Considerations.CAUTION: Beryllium and its compounds are extremely toxic.Suitable precautions (e.g., use of protective clothing, a breathing apparatus, and a well-ventilated fume cupboard) should be taken for all manipulations involving these species.Our general policy when handling beryllium compounds and reagents is to limit the potential for ambient proliferation by working on a microscale.
All manipulations were carried out using standard Schlenk line and glovebox techniques under an inert atmosphere of argon.NMR experiments were conducted in J. Young tap NMR tubes prepared in a glovebox.NMR spectra were recorded on a Bruker BioSpin GmbH spectrometer operating at 400. 13  BeClLi] (IV Li ) were synthesized according to literature procedures. 31,33ynthesis of [{SiN Dipp } 2 BeHCs 2 N(H)Dipp] 2 (1).{SiN Dipp }Li-(OEt 2 ) 5 (71 mg, 0.1 mmol) and BeCl 2 (10 mg, 0.125 mmol) were introduced into a Young's NMR tube and dissolved in protio-benzene (0.6 cm 3 ), whereupon the sample was agitated and left to sit overnight before being filtered into a fresh NMR tube containing Cs metal.Upon contact with Cs an orange coloration and black precipitate was instantly observed.The reaction mixture was sonicated for 2 h and left to settle overnight before being filtered.Several colorless crystals of [{SiN Dipp } 2 Be−H−Cs 2 -N(H)Dipp] 2 (1) were grown from the paleorange solution at ambient temperature, though not enough for spectroscopic data.This compound was found to be unstable when exposed to vacuum.This product was also observed when reacting Cs with [{SiN Dipp } 2 BeClLi] 2 in protio-benzene.

■ RESULTS AND DISCUSSION
An initial reaction performed in protiobenzene at room temperature between IV Li and an excess of Cs metal resulted in the generation of an orange solution and a black precipitate.Ultrasonication for 2 h, filtration, and crystallization at ambient temperature resulted in the isolation of several colorless single crystals of compound 1, which, while insufficient in quantity to allow further spectroscopic characterization, were suitable for X-ray diffraction analysis.Although the resultant solid-state structure (Figure 2) identified compound 1 as a cesium hydridoberyllate comprising the previously suggested [{SiN Dipp }BeH] − anion (Figure 1a), each molecule also incorporates a formal equivalent of the cesium anilide,  3) Å] close contacts to the anilide anions.Dimer propagation is also achieved via these latter anions, which act as bridging units through polyhapto engagement of Cs2/Cs2 1 , with the remaining unsaturation of the large Cs + cations satisfied by C−H•••Cs close contacts to the isopropyl groups of the various Dipp substituents.The beryllium {N 2 BeH} coordination environment of 1 is unambiguously three-coordinate and, although a number of molecular beryllium hydrides have now been structurally characterized, 34 reports have been sporadic and the sole precedent with a similarly low coordination number was reported only very recently. 35lthough 1 H NMR analysis of the supernatant collected after the isolation of the crystalline sample of 1 evidenced a complex reaction that frustrated confident interpretation (Figure S1), its structure confirms that reduction of the lithium cation of IV Li can ensue alongside beryllium hydride formation.The observation of the free anilide anion within its structure, however, indicated that competitive degradation of the {SiN Dipp } ligand structure had also occurred, despite the mild room temperature reaction conditions.In an attempt to obtain more meaningful diagnostic analysis of these processes by NMR spectroscopy, therefore, a further reaction between 1 and Cs metal was undertaken in C 6 D 6 .Although a similar outcome was anticipated, the resultant 1 H NMR spectrum was consistent with significantly enhanced kinetic discrimination and the generation of a predominant new product (2, Scheme Organometallics 1).Notably, the similarity of this spectrum to the analogous data provided by all five of the previously described chloroberyllate species, IV M (M = Li, Na, K, Rb, Cs), was indicative of a comparably symmetrical structure.Similarly, the single resonant frequency presented by the corresponding 9 Be NMR experiment (δ = 11.8 ppm), and the breadth of this signal (ω 1/2 = 353 Hz), were consistent with the maintenance of a similar 3-coordinate and significantly asymmetric beryllium binding environment. 36The origin of these observations was again resolved through the isolation and Xray diffraction analysis of single crystals of compound 2 (Scheme 1, Figure 3).This experiment further confirmed the validity of our previous assumption of hydridoberyllate formation under such reductive reaction conditions.Although its gross structure demonstrates a close comparison to those of the similarly dimeric but chloride-bridged species, IV M , the 7membered chelate dimer halves of both independent molecules within the asymmetric unit display a notably more twisted disposition of the {SiN Dipp }-chelated Be-containing units, presumably a consequence of the lower steric demands and directionality of binding to the hydride substituent.The relevant N−Be−N least-squares planes consequently subtend dihedral angles of 69.87 and 68.11°for the Be1/Be2-and Be3/ Be4-containing molecules, respectively.
Additional solution analysis of the isolated crystals of compound 2 by 2 H NMR spectroscopy in C 6 D 6 identified a singlet resonance at δ 1.29 ppm, which we assign to the beryllium-bound deuteride signal arising from activation of the C 6 D 6 solvent.The in situ solution analysis of the reaction leading to the isolation of compound 2, however, provided no evidence for the simultaneous generation of a cesium species (III Cs ) analogous to the previously reported lithium and sodium phenylberyllate derivatives, III Li and III Na (Figure 1a). 31Notably, the solid-state structure of III Na was observed to be a 1-dimensional polymer propagated by intra-and intermolecular polyhapto engagement of the Na + cations with the beryllium-bound phenyl substituent and the N-Dipp πsystem of an adjacent SiN Dipp spectator ligand.On this occasion, therefore, we suggest that potential for similar polymerization is exacerbated by the significantly larger radius of the cesium cation of the supposed [Cs({SiN Dipp }BePh)] ∞ (III Cs ), 37 such that this compound most likely comprises a significant component of the insoluble material formed during the reaction.In a further attempt to provide corroborative evidence for the simultaneous generation of both hydrido-and organoberyllium products of arene solvent activation, the reaction of IV Li and Cs metal was repeated in d 8 -toluene.This process resulted in the familiar observation of a black precipitate and orange solution, analysis of which by 1 H NMR spectroscopy presented signals strongly reminiscent of the initial data provided by compound 2. A singlet resonance that could also be discriminated at δ 1.35 ppm in the corresponding 2 H NMR spectrum was tentatively assigned as the Be-D signal of a deuteride species analogous to that assigned for 2, albeit with a slight discrepancy in chemical shift induced by the differing deuterated solvents.
Although filtration and crystallization of the reaction solution did not allow definitive solid-state authentication of the formation of 2, the resultant crystals were identified by Xray diffraction analysis as a further product of berylliumcentered activation of the arene solvent (3-d, Figure 4).In this case, C−D bond cleavage occurs at the thermodynamically preferred methyl substituent to provide the perdeuterobenzylberyllate species, [Cs({SiN Dipp }BeCD 2 C 6 D 5 )] ∞ .The resultant solid-state structure of compound 3-d is, thus, reminiscent of that previously observed for III Na in comprising a 1dimensional polymeric array.In the current case, the larger cesium cations of the molecular [Cs({SiN Dipp }BeCD 2 C 6 D 5 )] units are coordinated not only by the pi-system of each beryllium-bound benzylic substituent but also by a molecule of toluene solvent.The cesium coordination sphere is completed by further intermolecular Cs•••arene interactions with a Dipp substituent of each adjacent [Cs({SiN Dipp }BeCD 2 C 6 D 5 )] moiety to generate a polymeric helix, the individual strands of which pack as supramolecular sheets stacked along the b axis defined by the orthorhombic unit cell (Figure S20).
Mindful of the kinetic discrimination provided by the otherwise identical reactions performed in perdeutero-and protio-benzene, compound 1 was also reacted with cesium metal in protio-toluene.Although the course of the reaction in this case appeared to proceed in a fashion analogous to that observed in the deuterated medium, analysis of the resultant crystals of compound 3-h in C 6 D 6 enabled assignment of the respective benzylic methylene and aromatic resonances by 1 H NMR spectroscopy (Figure S15).

■ CONCLUSIONS
In conclusion, we have observed that a dimeric lithium chloroberyllate reacts at room temperature with cesium metal to provide cesium hydrido-and organoberyllate products resulting from C−H activation of the benzene or toluene solvent.These observations lend further credence to our earlier hypothesis that such arene activation is best rationalized as a result of highly reactive and short-lived Be(I) radical anion intermediates.We are thus continuing to study this and related reactivity with a view toward achieving the more direct observation or isolation of relevant lower oxidation state group 2 species.

Figure 1 .
Figure 1.(a) Beryllium-centered benzene activation induced by attempted reduction of II; (b) reduction of Li + resulting from reaction of IV Li by Na, K, Rb, or Cs metal.

Scheme 1 . 3 Figure 3 .
Scheme 1. Proposed Course of Reaction of Compound IV Li with Cesium Metal Leading to the Isolation of Compounds 2 and 3