ARTICLE Received 15 Mar 2013 | Accepted 15 May 2013 | Published 14 Jun 2013 DOI: 10.1038/ncomms3014 A ladder polysilane as a template for folding palladium nanosheets Yusuke Sunada1,2, Ryohei Haige2, Kyohei Otsuka3, Soichiro Kyushin3 & Hideo Nagashima1,2 Although discrete nano-sized compounds consisting of a monolayer sheet of multiple atoms have attracted much attention, monolayer transition metal nanosheets are difficult to access. Here we report a template synthesis of the folding metal nanosheet (2) consisting of 11 palladium atoms by treatment of a ladder polysilane, decaisopropylbicyclo[2.2.0]hexasilane t (1), with Pd(CN Bu)2. Crystallographic analysis reveals that the compound is composed of two monolayer Pd7 sheets sharing three palladium atoms at the junction. Each Pd atom is stabilized by Pd–Si s-bonds, Pd–Pd bonds and coordination of isocyanides. Ligand exchange t of 2 from CN Bu to CN(2,4,6-Me3-C6H2) is accompanied by structural rearrangement, leading to the formation of another folding Pd11 nanosheet (3) consisting of two edge-sharing Pd7 sheets. The shapes of the Pd7 sheets as well as the dihedral angle between the two Pd7 sheets are dependent on the substituent of the isocyanide ligand. 1 Institute for Materials Chemistry and Engineering, Kyushu University and CREST, Japan Science and Technology Agency (JST), Kasuga, Fukuoka 816-8580, Japan. 2 Graduate School of Engineering Sciences, Kyushu University, 6-1 Kasugakoen, Kasuga, Fukuoka 816-8580, Japan. 3 Department of Chemistry and Chemical Biology, Graduate School of Engineering, Gunma University, Kiryu, Gunma 376-8515, Japan. Correspondence and requests for materials should be addressed to H.N. (email: [email protected]). NATURE COMMUNICATIONS | 4:2014 | DOI: 10.1038/ncomms3014 | www.nature.com/naturecommunications 1 & 2013 Macmillan Publishers Limited. All rights reserved. ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms3014 he isolation of graphene from graphite, typically by and an additional four Pd atoms participate in the construction of scratching away multilayers of graphite, has made it the folding nanosheet structure. No intermediary species were Tpossible to access a new form of carbon with nanosheet visible in the reaction from 1 to 2. Indeed, treatment of 1 with 1–4 t structures . Several metal oxides and sulphides having seven equivalents of Pd(CN Bu)2 resulted in exclusive formation multilayered structures also form nanosheets by peeling off of 2 (65%) with recovery of 1 (35%). their monolayers5. These nanosheets show profoundly different physical properties from their precursors, eliciting much interest Molecular structure of 2. The molecular structure of 2 is in nanoscience. These nanosheets suggest the possible depicted in Fig. 2a,b and Supplementary Figs S1 and S2. The preparation of transition metal nanosheets; however, the lack of molecular structure of Pd11 cluster 2 can be regarded as one large precursors to which to apply mechanical separation suitable for folding nanosheet containing eleven palladium atoms or two isolation of monolayers prevents easy access to these compounds. small nanosheets (Pd7 sheet I and Pd7 sheet II in Fig. 2a) con- The synthesis of relatively small transition metal nanosheets sisting of seven palladium atoms each, sharing three palladium has attained some success in organometallic and inorganic atoms at the junction. The dihedral angle between sheets I and II chemistry6–9. Among them, a novel methodology to synthesize 10–16 is 37.7° (Fig. 2c and Supplementary Fig. S3). Each Pd7 sheet Pd3 to Pd5 nanosheets was developed by Murahashi et al. contains two silylene moieties, SiiPr (iPr ¼ i-propyl), which are using (poly)cyclic aromatic hydrocarbons as a template for the 2 located in the plane of the Pd7 sheet, whereas one silylyne group, arrangement of Pd atoms in a two-dimensional sheet structure. i 2 þ Si Pr, is located out of the plane as described later in more detail. The sandwich compound, [Pd5(naphthacene)2] , is the largest 10 Four palladium atoms at the edge of the Pd7 sheet are bonded to palladium nanosheet reported to date . Success of this template the terminal isocyanide ligands. Three linearly assembled Pd synthesis of transition metal nanosheets suggests a strategy to atoms, Pd(5), Pd(6) and Pd(7), lie on the junction (Fig. 2d), and access new nanosheets using templates that force the metals into a two CNtBu ligands bridge the Pd(5)–Pd(6) and Pd(6)–Pd(7) planar arrangement owing to metal–metal bonding interactions. bonds to enhance the Pd–Pd interactions. In this paper, we report a ladder polysilane, decaisopropylbi- cyclo[2.2.0]hexasilane (1)17–20, that serves as a template for two Pd clusters, 2 and 3, which are considered to be folding metal Synthesis of another folding Pd11 nanosheet 3. To our surprise, 11 t nanosheets having two planar Pd7 units. The array of Pd and Si the CN Bu ligands were reactive towards ligand exchange by atoms and the dihedral angle of the two Pd7 sheets are dependent other isocyanide ligands, for example, CN(2,4,6-Me3-C6H2). on the substituent of the isocyanide ligand. Treatment of 2 with ten equivalents of CN(2,4,6-Me3-C6H2)in toluene at À 35 °C for 12 h resulted in quantitative displacement t Results of all of the CN Bu ligands with CN(2,4,6-Me3-C6H2) to produce the new Pd11 cluster 3 in 55% yield after recrystallization from Synthesis of folding Pd11 nanosheet 2. Insertion of a Pd(CNR)2 species between a Si–Si bond of oligosilanes to give complexes of ether (Fig. 1). X-ray structure analysis revealed that compound 3 also has a folding Pd11 nanosheet structure similar to 2 (Fig. 2e,f the type (RNC)2Pd(SiR3)2 has been investigated extensively by Ito and coworkers21,22. These complexes serve as intermediates for and Supplementary Figs S4 and S5). However, the dihedral angle catalytic transformations of oligosilanes. However, only a few (47.7°) between the two Pd7 sheets (Fig. 2g and Supplementary cyclic oligosilanes were subjected to the study, and they were Fig. S6) is significantly different from that in 2. Similar to 2, each reportedly less reactive than their linear analogues even at Pd7 nanosheet is supported by two silylene units in the plane and elevated temperatures. Our new discovery, a ladder polysilane one silylyne moiety out of the plane; however, the array of Pd and having seven Si–Si bonds in a molecule, decaisopropyl- Si atoms is different from that in 2 (vide infra). Three palladium bicyclo[2.2.0]hexasilane (1), is highly reactive towards atoms at the edge of the Pd7 sheet are bonded to terminal iso- Pd(CNtBu) (tBu ¼ t-butyl), and the reaction unexpectedly leads cyanide ligands, whereas five Pd atoms, Pd(1), Pd(5), Pd(6), Pd(7) 2 and Pd(8), aligning in a slightly curved line are bound to four to the formation of the Pd11 cluster 2. As shown in Fig. 1, t bridging isocyanide ligands (Fig. 2h). treatment of 1 with 11 equivalents of Pd(CN Bu)2 in toluene at room temperature resulted in complete consumption of 1 after 18 h. The product 2 was isolated as dark green crystals suitable for Detailed molecular structures of 2 and 3. Several unique points crystallography in 65% yield by recrystallization from toluene/ should be noted in the molecular structures of 2 and 3. First, both pentane at À 35 °C. The molecular structure of 2 (vide infra) 2 and 3 consist of eleven palladium atoms, six organosilyl moi- t i suggests that oxidative addition of Pd(CN Bu)2 moieties into eties and ten isocyanide ligands with formula Pd11(Si Pr2)4 i seven Si–Si bonds in 1 explains the origin of seven Pd atoms in 2, (Si Pr)2(CNR)10. Second, both 2 and 3 have a pseudo twofold axis i i i Pr L L Pr2 i L' L' iPr Pr 2 Pr2 2 i i Si Si Pr Si Si Pr i Si Si 2 Si 2 Pd Pd CN Pr Pd Pd i Pd Pd 2 Pr i L L 2 i Si Si Pr L L Si L' L' Pr2Si i 2 Pd Pd Pd Si Pr Pd Pd Pd (10 equiv.) Pd Pd (1) Si Si Si Si Si Si toluene iPr i i iPr toluene Pd i i Pd 2 Pr Pr 2 L' Pd Pr Pr L' + r.t., 18 h Pd Pd Pd –35°C, 12 h Pd Pd Pd(CNtBu) L L 2 N N N N N (11 equiv.) N Mes Mes tBu tBu Mes Mes i i t i i Pd11(Si Pr)2(Si Pr2)4(CN Bu)10 (2) Pd11(Si Pr)2(Si Pr2)4{CN(2,4,6-Me3-C6H2)}10 (3) (L = CNtBu) (L' = CN(2,4,6-Me3-C6H2)) Figure 1 | Synthesis of 2 and 3. Synthesis of 2 by using a ladder polysilane (1) as the template, and synthesis of 3 by ligand exchange and skeletal rearrangement. r.t., room temperature. 2 NATURE COMMUNICATIONS | 4:2014 | DOI: 10.1038/ncomms3014 | www.nature.com/naturecommunications & 2013 Macmillan Publishers Limited. All rights reserved. NATURE COMMUNICATIONS | DOI: 10.1038/ncomms3014 ARTICLE a Side view e Side view Si(5) Si(5) Pd(3) Si(2) Si(2) Pd(10) Pd(10) Pd(3) Sheet II Sheet I Pd(4) Pd(11) Sheet III Pd(1) Sheet IV Si(4) Pd(2) Pd(8) Pd(4) Pd(11) Pd(2) Pd(9) Si(1) Si(6) Si(3) Pd(9) Pd(8) Pd(1) Pd(5) Si(4) Si(6) Si(3) Si(1) Pd(7) Pd(6) Pd(5) Pd(7) Pd(6) b f c g 47.7 o 37.7 o Sheet II Sheet I Sheet IV Sheet III Front view Front view = Pd = Si (silylene) = Si (silylyne) d Pseudo 2-fold axis h Pseudo 2-fold axis Si(2) Si(5) Si(2) Si(5) Sheet I Sheet II Sheet IV Sheet III Pd(10) Pd(3) Pd(10) Pd(3) Pd(8) Pd(4) Pd(11) Pd(1) Pd(4) Pd(11) Pd(9) Pd(2) Si(4) Pd(2) Pd(9) Si(1) Si(6) Si(3) Si(6) Si(3) Si(4) Si(1) Pd(8) Pd(1) Pd(7) Pd(6) Pd(5) Pd(7) Pd(6) Pd(5) t Figure 2 | Molecular structures of 2 and 3.