An Isolable Bis(silanone-borane) Adduct.

Abstract The reaction of bis(silylenyl)‐substituted ferrocene 1 with two molar equivalents of BPh3 yields the corresponding bis(silylene–borane) Lewis adduct 2. The latter is capable to activate CO2 to furnish the borane‐stabilized bis(silanone) 3 through mono‐oxygenation of the dative SiII→B silicon centers under release of CO. Removal of BPh3 from 3 with PMe3 affords the corresponding 1,3,2,4‐cyclodisiloxane and the Me3P−BPh3 adduct. All isolated new compounds were characterized and their molecular structures were determined by single‐crystal X‐ray diffraction analyses.

ocyclic silylene in 1994 by Denk and West, the formation of a silylene-borane adduct was reported two years later,w hich, however,s lowly rearranges to as ilylborane through Si II insertion into the BÀCb ondo fB (C 6 F 5 ) 3 . [8] Since then, an increasing number of compounds containing ad ative Si II !B III bond with four-and five-coordinate Si II centers have been isolated and structurally characterized. [9] Due to al arge polarization of the Si=Ob ond and the remarkably weak SiÀO p bond (58.5 kJ mol À1 )c ompared to the SiÀO s-bond strength (119.7 kJ mol À1 ), compounds with aS i =O bond are intrinsically susceptible to auto-oligomerization to the corresponding polysiloxanes. [10] Thus, introductiono fa n electron donor at the Si atom or/and an acceptor at the O atom are needed to disfavor head-to-tail oligomerization of the polar Si=Ob ond. [11] This led to the first Lewis acid-base supported silanonec omplex, the silaformamide-borane A (Scheme 1), which wasr eportedb yu si n2 007, starting from a silylene and H 2 O·B(C 6 F 5 ) 3 . [12] Roesky et al. described in 2011t he isolation of the acid anhydride B generated from the reaction of ac hlorosilylenew ith H 2 O·B(C 6 F 5 ) 3 in the presence of NHC. [13] Similarly,R oesky et al. reported also the silaformyl chloride complex C,r esulting from an NHC-stabilized silylene and H 2 O·B(C 6 F 5 ) 3 . [14] In 2019, the isolation of the first donor-acceptor-supported silaaldehyde D was accomplished by the Inoue group. [15] Remarkably,K ippings dream of isolable genuine silanones was realized in 2014 with the isolation of the first metallosilanoneb yF ilippou [16] and 2017 by the groupso fI noue and Rieger. [17] Very recently,asilicon analogue of ak etone with an unperturbed Si=Ob ond was synthesized by Iwamoto and coworkers. [18] Starting from an in situ generated silylene-borane adduct, Te ng et al. reportedi n2 016 on the activation of THF leading to the isolation of ac orresponding ring-opening product. [20] Recently,B raun and co-workers used as ilylene-borane Lewis adduct as atool for trapping asingle water molecule, affording az witterionic silanol stabilized by intramolecular hydrogen bonds. [23] In 2017, our group reported the first intramolecular silylene-borane FLP which activatesH 2 ,O 2 ,CO 2 and even dehydrogenates water yielding ab orane-stabilized silanone E with ad ative Si=O!Bb ond. [19] Herein, we present the synthesis of the bis(silylene-borane) adduct 2 with the ferrocenes pacer and its mild oxidation with CO 2 yielding the first borane-stabilized bis(silanone) adduct 3.R emoval of BPh 3 from 3 by addition of PMe 3 leads to the corresponding 1,3,2,4-cyclodisiloxane through intramolecular Si=Oh ead-to-tail dimerization. Moreover,t he reactiono f2 with elemental sulfur yields ab is(silathione) with two 'borane-free' Si=Smoieties.
The reaction of the ferrocene-derived bis(silylene) [24] 1 with two molar equivalents of triphenylborane in toluene at room temperature leads to the formation of the bis(silylene-borane) adduct 2 whichw as isolatedi n7 4% yields as ar ed crystalline solid (Scheme 2). The identity of 2 was provenb ye lemental analysis, single-crystal X-ray diffraction analysis and multinuclear NMR spectroscopy in the solid state and in solution. Crystals suitable for an X-ray diffraction analysis were obtained in a concentrated toluene solution of 2 at À30 8C, the crystals are a mixture of the two rotational conformers (Figure 1; see also the Supporting Information).
To remove the boranes from the bis(silanone-borane) complex 3,t rimethylphosphane (PMe 3 ,5equiv) wasa dded. This resulted in the clean formation of the correspondingL ewis pair Me 3 P!BPh 3 ( 31 PNMR: À15.3 ppm) and the 1,3,2,4-cyclodisiloxane 4 (head-to-tail dimer of Si=Om oieties). The latter is identical with the isolated product from the reaction of 1 with CO 2 in 76 %y ields (Scheme 2). Single crystals of 4 suitablef or X-ray diffractionanalysiswere obtained from aconcentrated solution in a1 :1 benzene/hexane mixture at room temperature ( Figure 3). The formation of Me 3 P!BPh 3 was additionally confirmed by as ingle-crystal X-ray analysis obtainedi nt he reaction mixture of 3 and PMe 3 in THF solutions (see the Supporting Information).
Interestingly,r eaction of the bis(silylene-borane) 2 with 10 equivalents of PMe 3 led to the formation of an ew species 2' in the course of borane-deprotection of oneS i II moiety in 2 (Scheme 3, see the Supporting Information). This process is reversible because removal of the solvent and PMe 3 in vacuum and re-dissolving of the residue in C 6 D 6 furnishes compound 2 as shown by NMR spectroscopy.
In contrastt ot he oxygenation of 2 with CO 2 ,t reatment of 2 with elemental sulfur in toluene at room temperature leads to the selective formation of the 'borane-free' bis(silathione) 5. Compound 5 is identical with the product from the reaction of bis(silylene) 1 with elementals ulfur in toluene at room temperature, which was isolated in 54 %y ield (Scheme 4). Similar to the product of an intramolecular silylene-borane FLP with elemental sulfur reported by our group, [19] no Si=S!Bi nteraction was observed. The structure of 5 ( Figure 4) features two Si=S bonds with al ow-field shifteds inglet 29 Si NMR signal at d = 12.1 ppm. The Si=Sd istances of 1.9867(13) and 1.9858(13) are consistent with relateds ilathiones with four-coordinates ilicon atoms [{PhC(NtBu) 2 }Si(S)Cl]( 2.079 (6) )a nd as reported for aS i =Sp roduct from sulfuration of an intramolecular silyleneborane FLP with elementals ulfur (1.9795 (10) ). [19,26] Bis(silathione) 5 is stable in C 6 D 6 solutions over ap eriod of several weeksw hich can be explained by al ess polarized Si=Sb ond (DEN = 0.7) compared to the Si=Ob ond (DEN = 1.7) based on their electronegativities(EN).