Disilane Cleavage with Selected Alkali and Alkaline Earth Metal Salts

Abstract The industry‐scale production of methylchloromonosilanes in the Müller–Rochow Direct Process is accompanied by the formation of a residue, the direct process residue (DPR), comprised of disilanes MenSi2Cl6‐n (n=1–6). Great research efforts have been devoted to the recycling of these disilanes into monosilanes to allow reintroduction into the siloxane production chain. In this work, disilane cleavage by using alkali and alkaline earth metal salts is reported. The reaction with metal hydrides, in particular lithium hydride (LiH), leads to efficient reduction of chlorine containing disilanes but also induces disproportionation into mono‐ and oligosilanes. Alkali and alkaline earth chlorides, formed in the course of the reduction, specifically induce disproportionation of highly chlorinated disilanes, whereas highly methylated disilanes (n>3) remain unreacted. Nearly quantitative DPR conversion into monosilanes was achieved by using concentrated HCl/ether solutions in the presence of lithium chloride.


Introduction
Methylchlorosilanes Me n SiCl 4-n (n = 1-3) are produced in the Direct Process (DP) [1] at large scales by reaction of elemental silicon with chloromethane. The production of the main product Me 2 SiCl 2 , [2] however, is accompanied by the formation of an unwanted residue (the DPR) comprisedo fm ethylchlorodisilanes, Me n Si 2 Cl 6-n (n = 1-6) and, in minor amounts, of carbodisilanes, which accumulate in tens of thousands of tons annually. [3] Owing to the fact that enormous amountso fs ilicon are consumed for DPR formation,g reat efforts have been made in the past to devise preparative protocols for the conversion of the DPR into the corresponding monosilanes. [3a, 4] Specifically, the Lewis-base induced disproportionation of disilanes under moderate reaction conditions and disilane splitting can be achieved by use of catalytic amountso fp hosphines, [5] amines, [6] as well as phosphonium or ammonium chlorides. [7] These reactions afford transformation of the DPR constituents into silane monomers along with oligo-or polysilanes as side products. [8] Based on suggestions put forth in the literature [6a, 9] the course of disilanec leavage involves nucleophilic attack of the Lewis base at the disilane moiety with subsequent extrusion of aL ewis-base-stabilized silylenea nd formation of a monosilane. Silylenei nsertion into as econd disilane equivalent gives rise to trisilanef ormation.R eiteration of this step results in high molecular weight, in some cases insoluble, polysilanes. [9b] To reduce oligosilane formation, hydrogen chloride (HCl) is usually added as in situ trapping agent for the silylenes formed. [10] In any case, the workup of these oligosilanes, or their disposal by incineration, reduces the economic benefit of the overall DP significantly.
Recently,w eh ave reported on the competitive chlorination and cleavage of methylhydridodisilanes with ether/HCl solutions to yield bifunctionalm onosilanes in excellent yields. [11] The methylhydridodisilanes used there weres ynthesized by reductiono ft he corresponding chloro-substituted precursors with LiAlH 4 as the hydrogenation agent. In search of alternative hydrides ources, we reacted methylchlorodisilane mixtures mimicking the DPR with lithium hydride. To our surprise, we found that most disilanes are efficiently cleaved, resulting in formation of mostly bifunctional monosilanes in high yields (see below). These findings prompted us to conduct further investigationso nt he disilane hydrogenation and cleavage reactions with alkali-and alkaline earth hydrides. The resultso f these studies are reportedi nt he following.
An umber of earlier studies by others have shown that the hydrogenation of chlorosilanesc an efficiently be achieved by using complex reducing agents, such as LiAlH 4 , [12] NaBH 4 [13] and LiBH 4 . [14] Also the alkali metal hydrides LiH [15] and NaH [16] as well as alkaline earth metal hydrides such as MgH 2 [17] have been used for chlorosilane reduction. Polyether solvents have been often used to activate LiBH 4 and NaBH 4 for reduction of, for example, SiCl 4 ,M e 2 SiCl 2 or GeCl 4 . [18] Chlorosilane reductions are generally performed at ambient temperatures (20-25 8C) as temperatures above 100 8Co ften cause decomposition of the reducing agents or the desired product. [18] Moreover, calcium and titanium hydrides, [19] or mixtures of NaH/NaBH 4 [20] have been found effective in reduction reactions, but all synthetic routes reported thusf ar are lacking selectivity and yield the perhydrido-substituted derivatives as main products.
Bifunctionalm onosilanes represent fundamentally important buildingb locks in silicone technology.U tilizing 1) the SiÀH functionality for hydrosilylation reactions to create siliconcarbon bonds [21] and 2) the SiÀCl functions for hydrolysis or alcoholysis provides access to the corresponding silanols or alkoxysilanes employed in condensation reactions to form the siloxane SiÀOÀSi bondingm otif. [4k, 22] For the synthesis of bifunctional monosilanes, somep reparative protocols have been reported:A ss hown by D'Erricoa nd Sharp for av ariety of halosilanes, the selective reduction of as ingle SiÀCl bond is possible by using alkyltin hydrides. [23] Further,t he Roewerg roup converted Me 2 SiCl 2 to Me 2 SiHCl with organotin hydrides in the presenceo fphosphoniumc hlorides or amineb ases. [24] More recently,I r-mediated synthetic protocols utilizing H 2 as hydrogen source have been reported. [25] Alternatively, efficient access to monosilanes R 2 SiHCl andR SiHCl 2 has been established by selectivec hlorination of hydridomonosilanes with HCl in the presence of catalytic amountso fL ewis acids [26] or Lewis-bases such as ethers. [27] The efficient cleavage of silicon-silicon bonds with alkali metal saltsh as been first reported by Ring and co-workers. [28] This group studied reactions of Si 2 H 6 with alkali metal chlorides and hydrides to yield SiH 4 ,-(SiH 2 ) n -p olymers and metal silanides, [29] and also the cleavage of some alkyldisilanesw as investigated. [30] Furthermore, the pertinent patent literature reports on metal-salt-catalyzed cleavage of different disilanes present in the DPR and disclosed alkali metal halidest of orm complexes with various tertiary amines, which are effective in cleavage reactions. [31] We here report the cleavage reactions of differentm ethylchlorodisilanes with alkali and alkaline earth metal salts to give monosilanesi nh igh yields. We focus in particular on the synthesis of bifunctionalm onosilanes, bearing both hydrido and chloro substituents, formed by simultaneous cleavage and reduction of the disilanes presenti nt he DPR. [28][29][30]
Ta ble 1l ists the numbering scheme of startingm aterials and products relevant in this study,p rocedures as well as NMR spectroscopic data are provided as Supporting Information. Disilanes weres eparated from authentic industrial DPR samples and hydridodisilanes and hydridocarbodisilanes were obtained by reduction of chlorinated precursors with LiAlH 4 maintainingt he SiÀSi and the SiÀCÀSi backbone.
To study the reduction and cleavage of disilanes with lithium hydride, we chose tetramethyldichlorodisilane (3)a sarepresentativem odel compound for the "uncleavable" fraction of the DPR. 3 was quantitatively reduced at room temperature (RT) to tetramethyldisilane 18 with two equivalents of lithium hydride [Eq. (1)].
The reaction of 18 with excessL iH was studied further in variable-temperatureN MR experiments:M e 2 SiH 2 was, apart from traces of oligosilanes, the only product detectable up to 140 8C. [37] Based on earlier detailed studies on the chloride-induced aufbau of higherp erchlorinated oligosilanes from Si 2 Cl 6 , [35] we devised at entative reactionm echanism for the cleavage of disilane 18 (Scheme 1). As the initial step, we assumet he formation of as ilicate DH À by attachment of ah ydride ion, released from the LiH solid, to one of the silicon centers in disilane 18 (D), which subsequently undergoes SiÀSi bond cleavage to give Me 2 SiH 2 (M)a nd the silanidea nionH Me 2 Si À (A À ). [35a, 38] The silanide A À can then abstract ap roton [39] from another equivalent of D to yield monosilane Me 2 SiH 2 (M)a long with the higher silanide anion HMe 2 SiÀSiMe 2 À (B À À ). Aq uantum chemical assessment of this step at the SMD(THF)-M062X/6-31 + G(d,p) level reveals am oderate exoergicity( D R G = À4kcal mol À1 )a nd an activation barriero fD°G = 28 kcal mol À1 (Scheme 1), which is in line with ar eaction efficientlyt aking place only at elevated temperature. Alternatively, A À À can add to D to yield the higher silicate T À À .This speciescan then either undergo hydride migration to the terminal silyl group followed by SiÀSi bond cleavage to yield M and B À À ,o rr elease ah ydride ion back to the LiH solid, whichr esults in formationo ft he trisilane T.W ith T undergoing the same reactionc ascade the formation of higher oligosilanes HMe 2 SiÀ(SiMe 2 ) n ÀSiMe 2 Hr esults, which eventually become insoluble and escapeN MR spectroscopic identification (for characterized speciesw ith n = 1-4 see Supporting Information). Overall,t his scenarioi si nl ine with the previous work of Ring and co-workers [30] who showed that disilane cleavage with lithium hydride resultsi nm onosilanes, oligosilanes and/orl ithium silanides. At variance with Ring's experiments conducted at RT we do observe, however,c leavage of multiply methylated disilanesw ith lithium hydride at elevated reactiont emperatures. We note in passing that neither 3 nor 18 react with LiCl, even at temperatures as high as 220 8C.
Much to our surprise, dimethyltetrachlorodisilane (1), chosen as ar epresentative model fort he "cleavable" fraction of the DPR, does not form dimethyltetrahydridodisilane 16 upon reaction with 4equiv LiH but undergoes quantitative cleavage into monosilanes. Most notably,t he industrially important bifunctional silanes MeSiHCl 2 and MeSiH 2 Cl comprise almost 80 %o ft he product mixture obtained at RT.W eo ptimized their yield by using substoichiometric amountso fL iH:T he reaction with 1.3 equiv LiH yields 8 and 9 in almost 90 %, with 8 in significant excess ( Table 2).
In contrast to our observations for 18 detailed above,t he analogousr eduction of disilane 1 to yield 16 is not possible with LiH. Instead, SiÀSi bond cleavage interferes and 1 is quantitativelyconverted into monosilanes already at RT (Table2;oli-Scheme1.Suggestedmechanism of the LiH induced formation of HMe 2 Si À (A À )and Me 2 SiH 2 with concomitant Aufbau of higher oligosilanes. The inset shows the transition state structure TS À computedf or the proton abstraction step with selected structuralp arameters. gosilanes necessarily formed in this process are not NMR visible). Evidently,p artial reduction of 1 has taken place already with 1.3 equiv LiH at RT giving rise to the formation of LiCl, which mightt rigger chloride-induced disilaned isproportionation under thesec onditions. [35] This supposition was corroborated in further experiments:t reatment of 1 with catalytic amountso fL iCl at RT in polar solvents, such as glymes,T HF or 1,4-dioxane, resulted in MeSiCl 3 formation, comprising 50 %o f the reactionm ixture along with unreacted 1 and oligosilanes according to 29 Si NMR analysis. Full consumption of 1 is observed at longerr eaction times andh igher temperatures (Table 3). [34d] This observationc ontrasts the inability of LiCl to induce cleavage of highlym ethylated disilane 3;a lso the fully reduced dimethyldisilane 16 shows no signo fS i ÀSi bond cleavage in the presence of LiCl (cf. the Supporting Information). We thus conclude that reaction of disilane 1 with LiH initially leads to partial reduction and kinetically favored SiÀSi bond cleavage sets in, once sufficient amounts of LiCl have formed. The resulting monosilanes,i nt urn, are then partially reduced by LiH to yield the bifunctionalm onosilanes observed in the experiments. These findings complement our related study on the disilane cleavage with phosphonium chlorides alts [40] and will be addressed again in the next section.
Generally,t he disilanef raction of the DPR is contaminated with carbodisilanes. Ar epresentativem ixture of 30 (45 %), 31 (31 %), 32 (14 %), 34 (10 %) and 35 (1 %) was reacted with excess LiH (suspendedi nd iglyme in as ealed NMR tube, cf. section 6i nt he Supporting Information). Heating the sample to 180 8Cl ed to carbodisilane reduction and Si-C cleavaget o give MeSiH 3 (37 %) and Me 2 SiH 2 (31 %) as main products, along with the hydridocarbodisilanes 36-39 (32 %). [41] Scheme2illustrates at entative mechanistic suggestion that involves initial hydride-induced SiÀCb ond cleavage resulting in formationo f methylsilanes together with lithium silanides. The latter undergo couplingw ith chlorinated monosilanes to form disilanes, [42] which are subsequently cleaved in the presence of excess LiH. [43] The suitabilityo fo ther alkali and alkaline earth metal chlorides to induced isilaned isproportionation was investigated in reactions with an industrial DPR mixture (Table S29, Supporting Information). Although disilanec onversion was found moste ffectivew ith LiCl in diglyme, the use of NaCl, KCl, CaCl 2 ,a nd MgCl 2 is impeded by their lower solubility.A cceptable reaction rates, however,w ere found in tetraglymea t1 40 8Ca nd above. In exemplary reactions performed at ap reparative scale with both, LiCl and KCl, the DPR mixture was efficiently converted: with am aximum theoretical yield of 50 %t he disproportionation led to 42 %m onosilanes (predominantly MeSiCl 3 and Me 2 SiCl 2 ,c f. Supporting Information). The residue remaining after distillationo ft he monosilanes consistso fh ighly methylated disilanes, carbodisilanesa nd oligosilanes. Ab road signal at + 35 ppm in the 29 Si NMR spectrumo ft he sample wasa ssignedt ob ranched oligosilanes with terminal Cl 2 MeSi groups. Slightly higher conversion ratios were obtained with LiCla t 220 8C. [44] Conclusions In summary,w eh ave shown that chlorosilane reduction is possible with lithium hydride, which thereby is established as economically favorable alternative to LiAlH 4 .W eh ave further shown that LiCl, formed in the course of the reduction of chlorinated disilanes with LiH, acts as an efficient catalyst to trigger disproportionation of disilanes bearing SiMeX 2 groups (X=H, Cl) into the corresponding mono-a nd higher oligosi- lanes. [45] SiÀSi bond cleavage of highly methylated as well as perhydrogenated disilanes was not observed with lithiumchloride. We found, however,t hat lithium hydride efficiently triggers disproportionation of perhydrogenated disilanesi nto MeSiH 3 ,Me 2 SiH 2 ,and Me 3 SiH and oligosilanes.

Experimental Section
General procedure for disilane cleavagereactions For the elucidation of the reaction conditions, disilanes Me n Si 2 Cl 6-n (n = 2-6) were isolated from the DPR by fractional distillation and investigated as pure model compounds or in complex mixtures. The reactants for example, HCl/ether solutions, catalysts and solvents were placed in an NMR tube under nitrogen atmosphere and cooled to À196 8C, subsequently the disilanes were added and frozen. Then the NMR tube was evacuated (at À196 8C) and sealed in vacuo to avoid losses of low boiling monosilanes, such as MeSiH 3 (b.p. À58 8C), Me 2 SiH 2 (b.p. À20 8C), MeSiH 2 Cl (b.p. À46 8C), MeSiHCl 2 (b.p. 41 8C) and Me 2 SiHCl (b.p. 35 8C). After warming the mixture to RT the reaction temperatures were increased, and the course of reaction was followed by NMR spectroscopy,e specially by 29 Si NMR. The molar ratios of products formed were determined by integration of product specific NMR signals of the resulting mixtures. According to the optimum reaction conditions evaluated from the NMR investigations, upscaling was performed with larger amounts of starting materials in closed reaction ampules. Filling of reactants was similar as described for the experiments in sealed NMR tubes. Alternatively,u pscaling was performed in open systems. This procedure is described in the Supporting Information. [40]