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Facile Access to Dative, Single, and Double Silicon−Metal Bonds

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Facile Access to Dative, Single, and Double Silicon−Metal Bonds Full Paper Chemistry—A European Journal doi.org/10.1002/chem.202000866 & Transition Metals |Hot Paper| Facile Access to Dative, Single, and Double Silicon Metal Bonds Through M Cl Insertion Reactions of Base-StabilizedÀ SiII Cations À PhilippFrisch,[a] Tibor Szilvµsi,[b] and Shigeyoshi Inoue*[a] Abstract: Silicon(II) cationscan offer fascinating reactivity 1,2-migration of asilicon-bound NHC to the transition metal patterns due to their unique electronic structure:alone pair takes place after formation of an initial silyliumylidene transi- of electrons,two empty porbitals and apositive charge tion-metal complex.The mechanism could be verified exper- combined on asingle silicon center.Wenow reportthe imentally through characterization of the intermediate com- facile insertion of N-heterocycliccarbene(NHC)-stabilized si- plexes. Furthermore, the obtained chlorosilylene complexes lyliumylidene ions into M Cl bonds (M =Ru, Rh), forming a can be conveniently utilized as synthons to access Si Mand À À series of novel chlorosilylene transition-metal complexes. Si=Mbonding motifs bondsthrough reductive dehalogena- Theoretical investigations revealed areactionmechanism in tion. which the insertioninto the M Cl bond with concomitant À Introduction Hence, both amount anddiversity of reported reactivities lag behind those of silylenes, where common reactivity pat- The presence of alone pair of electrons, two vacant orbitals terns include insertion reactions into varioustypes of (strong) and apositivechargeonthe silicon centermakes silyliumyli- bonds. Astaggering number of examples for the insertion into dene ions an incredibly versatile and promising class of low- E H(E= H, N, O, S, C, B, …) and E Halogen bonds have been À À valent silicon compounds.[1] They offer alarge, yet untapped reported in recent years.[6] Similarly,the coordination chemistry synthetic potential in organosilicon chemistry with the possibil- of silyleneswith transition metals is acontinuously expanding ity to form up to three new bonds in asingle reaction.[2] They researchfield with variouscatalytic applications.[6d, 7] are promising candidates for the activation of small molecules, In contrast, even as the number of isolable base-stabilized si- transition metal free catalysis[3] and can act as synthons for lyliumylidenes continues to grow,[4a,8] reportedreactivities novel (low-valent) silicon compounds. Further, with the pres- remain scarce.[5,9] Only few reactivity studies with small mole- ence of astereochemically active lone pair,they can also func- cules[10] have been found and E Hbond activation reactions À tion as ligands in transition metal complexes. So far,noone- are limited to S H, O Hand acidic C Hbonds.[8g,11] À À À coordinate SiII cation has been isolated[4] and most reported The chemistry of SiII cations as transition metal ligands has examples are three-coordinate and utilize two Lewis bases for seen some progress in recent years.[12] Reported examples in- their stabilization (e.g. NHCs).[5] This brings the drawback of a clude complexes with coinage metals[12d] and group 6and 8 generally reduced reactivity by blocking the empty p-orbitals. metal carbonyls,[12b, e] butnofurther reactivity of these com- plexeshas been reported to date. Importantly,the synthesis of new types of complexes with silicon-basedligands and sub- stituentsisofhigh interestfor the development of improved [a] P. Frisch, Prof. Dr.S.Inoue catalysts.[7a–c,13] With their intriguing synthetic potential, silyliu- Department of Chemistry WACKER-Institute of Silicon Chemistry and Catalysis Research Center mylidenes are uniquely suited for the facile synthesis of various Technische UniversitätMünchen, Lichtenbergstraße 4, 85748 Garching bei types of Si M(multiple) bonds (e.g. through salt metathesis or À München (Germany) formation of coordination complexes followedbyabstraction/ E-mail:[email protected] migrationofstabilizing Lewis bases). This was elegantly dem- [b] Dr.T.Szilvµsi onstrated by Filippou et al. with the direct synthesis of amo- Department of Chemical and Biological Engineering [12c] University of Wisconsin-Madison, 1415 Engineering Drive lybdenum silylidyne complex. Madison, Wisconsin 53706-1607 (USA) For silylene complexes,avariety of follow-up chemistry is Supporting information and the ORCID identification number(s) for the au- known.[6d,7] For instance, multiple insertion reactions into thor(s) of this article can be found under: metal-chloride bonds of acoordinated transition metal frag- https://doi.org/10.1002/chem.202000866. ment have been reported. For example, Jutzi and co-workers 2020 The Authors. Published by Wiley-VCH Verlag GmbH&Co. KGaA. disclosed the insertion of Decamethylsilicoceneinto aHgX This is an open access article under the terms of the Creative Commons At- À tributionLicense, whichpermits use, distributionand reproduction in any bond, furnishing silyl-substituted Hg compounds (I, medium, provided the original work is properly cited. Scheme1A).[14] The group further reported analogous insertion Chem. Eur.J.2020, 26,6271 –6278 6271 2020 The Authors. Published by Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim Full Paper Chemistry—A European Journal doi.org/10.1002/chem.202000866 Scheme2.Synthesis of chlorosilylene ruthenium complex 2. tion of cold acetonitrile to amixture of 1a and the precursor at 408Cled to an immediate color change of the solution to À deep red. At about 20 to 158C, the color of the solution À À rapidly changed to orange. Even at 408C, acolor change to À orange can be observed within 2hours. The 29Si NMR of the orange solution displays one resonance with an expected Scheme1.(A) Examples for silyleneinsertion reactionsinto M Cl bonds. [8g] II À downfield shift at 17.6 ppm (from 69.5 ppm (1a) ), indicat- (B) Formationofchlorosilylenes via insertion of Si cations into M Cl bonds. À À ing the formation of asingle coordination product. Interesting- ly,the corresponding 1HNMR (cf. Supporting Information, Fig- reactions into Ni Cl and Au Cl bonds[15] and related reactivi- ure S8) showed ahighly asymmetricspecieswith four separate À À ties with Pt Cl bonds were also reported by Lappert et al.[16] septets and eight doublets(corresponding to four chemically À Recently,Kato,Baceiredo, and co-workersreported the inser- unique iso-propyl groups) and two distinctsignal sets for the tion of achlorosilylene ligand into the Rh Cl bond of acoordi- NHCs. The 13CNMR showed two resonances for the carbene À nated [RhCl(COD)] fragment (II), forming the corresponding carbon atoms at 169.9 and 154.8 ppm, indicating the possible RSiCl Rh(COD) compound III.[17] For silyliumylideneions and migration of one NHC to the transition metal. 2À their transition-metal complexes, no analogous reactivity has The complex rapidly decomposesatroomtemperature in been observed so far.Infact, insertion reactions into E Halo- solution to amixture of products, making further investigation À gen bonds have not been reported at all. and functionalization difficult. Nevertheless, crystals suitable Herein,wenow report the first reactivity studies regarding for SC-XRD analysiscouldbeobtained by storing aconcentrat- insertionreactions of aSiII cation into transition metal-chloride ed solution of 2 in MeCN at 35 8C. Figure 1shows the solid- À bonds. Reactions of NHC-stabilized silyliumylidene ions with di- state structure of 2,unambiguously confirming the asymmetric meric, chloro-bridged transition-metal precursors lead to coor- natureofthe complex and the shift of one NHCtothe metal. dination of the SiII cation to the metal fragment, followed by The half-sandwich complex with apiano-stool configuration insertionofthe silyliumylideneligand into the M Cl (M=Ru, À Rh) bond,furnishingNHC-stabilized transitionmetal silylene complexes (Scheme 1B). The complexes have been fully char- acterized by multinuclear NMR spectroscopy and SC-XRD (single crystal X-ray diffraction) and the insertion mechanism has been investigated theoretically and verified experimentally. Furthermore, we present afacile access route to Si Mand À Si=Mbonds through stepwise reduction of the isolated com- plexes with KC8,initially furnishing silyl-substituted complexes, followed by the formation of the corresponding Si=Ru double bond through additional reductive dehalogenation.Important- ly,while thesetypes of insertionreactions are generally accom- panied by an increase of the silicon oxidation state from II to IV,nosuch change occurs for silyliumylidene ions (cf. Scheme 1). Results and Discussion Insertion of aSiII cation into aRu Cl Bond Figure 1. Ellipsoid plot (50%) of the molecularstructure of 2.Hydrogen À atomsand the anion are omitted.The Tipp substituent is simplified as a wireframe for clarity.Selectedbond lengths []and angles [8]: Si1 Ru1 While exploring the coordination chemistry of NHC-stabilized À II 2.409(1), Si1 Cl1 2.167(1), Si1 C1 1.941(4), Si1 C16 1.970(4), Ru1 Cl2 Si cations, we investigated the reaction of the Tipp-substituted À À À À 2.404(1), Ru1 C23 2.077(4),Ru1 p-cym 1.770(1), C1 Si1 Ru1 123.3(1), Si1 [8g] À À ? À À À silyliumylidene ion 1a with the transition metal precursor Ru1 p-cym 131.1(1),Cl1 Si1 Ru1 Cl2 173.1(1),C16 Si1 Ru1 C23 À ? À À À À À À À [RuCl (p-cym)] (Scheme 2, p-cym=1-Me-4-iPr-benzene). Addi- 11.1(2). 2 2 À Chem. Eur.J.2020, 26,6271 –6278 www.chemeurj.org 6272 2020 The Authors. Published by Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim Full Paper Chemistry—A European Journal doi.org/10.1002/chem.202000866 features an NHC- stabilized aryl-chlorosilylene ligand with atet- sertion reaction of achlorosilyleneligand into aRh Cl bond À rahedral coordination sphere aroundthe silicon center and a (II III,Scheme 1).[17] However,akey distinction to the inser- ! Si1 Ru1bond length (2.409(1) )typical for Si Ru bonds.[18] tion reactions of silylenes is that in the case of the SiII cation, À À The Si C (1.970(4) )and Ru C (2.077(4) )bond lengths the formal oxidation state of the silicon center does not À NHC À NHC are in the typical range for Si C and Ru C bonds.
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