- Email: [email protected]
Synthesis and structural characterization of two complex tantalum(V) siloxides
Inorganic Chemistry Communications 74 (2016) 82–85
Contents lists available at ScienceDirect
Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
Short communication
Synthesis and structural characterization of two complex tantalum(V) siloxides☆ Sophie Ehle, Volker Lorenz, Phil Liebing, Liane Hilfert, Frank T. Edelmann ⁎ Chemisches Institut der Otto-von-Guericke-Universität Magdeburg, 39106 Magdeburg, Germany
a r t i c l e
i n f o
Article history: Received 16 September 2016 Accepted 6 November 2016 Available online 9 November 2016 Keywords: Tantalum Disiloxanediolate Silsesquioxane Metallasilsesquioxane X-ray structure
a b s t r a c t The reaction of tantalum(V) ethoxide, Ta(OEt)5, with 1 equiv. of 1,1,3,3-tetraphenyl-disiloxane-1,3-diol, (HO)SiPh2OSi-Ph2(OH) (1), afforded the dinuclear tantalum(V) disiloxanediolate complex [{μ(Ph2OSiO)2O}Ta(OEt)2(μ-OEt)]2 (3). Similarly, Ta(OEt)5 reacted with the incompletely condensed silsesquioxane precursor Cy7Si7O9(OH)3 (2, Cy = cyclohexyl) in a 1:1 molar ratio to afford the dinuclear tantalasilsesquioxane derivative [Cy7Si7O12Ta(OEt)(μ-OEt)]2 (4) in 80% yield. Both new complexes were structurally characterized by single-crystal X-ray diffraction studies. © 2016 Elsevier B.V. All rights reserved.
Well-defined molecular metal siloxides (=metallasiloxanes) comprising M-O-Si functionalities attract significant interest due to their diverse potential applications [1]. Metal siloxides are not only excellent molecular models for silica-supported heterogeneous metal catalysts [2], but they can also serve as precursors for new materials [3], nanoparticles [4], well-defined surface species [5], and homogeneous catalysts [6]. Particularly useful and versatile precursors for a large variety of complex metal siloxides are disilanols like 1,1,3,3-tetraphenyl-disiloxane1,3-diol, (HO)SiPh2OSi-Ph2(OH) (1), which forms complexes with virtually all metallic elements across the Periodic Table [1,7]. Even more complex molecular metal siloxides (“metallasilsesquioxanes”) can be designed using an incompletely condensed silsesquioxane derivative such as Cy7Si7O9(OH)3 (2, Cy = cyclohexyl) as precursor [8]. We report here the straightforward preparation and structural characterization of two new complex tantalum(V) siloxides derived from 1 and 2. Our synthetic protocol involved treatment of commercially available tantalum(V) ethoxide Ta(OEt)5, with equimolar amounts of the free silanol precursors 1 and 2. This salt-free route has been frequently employed successfully for the synthesis of titanium(IV) and zirconium(IV) siloxides [1,8]. To our knowledge, there is only one report in the literature describing the use of tantalum(V) ethoxide as precursor for cyclic tantalum(V) siloxides. Pannell and co-workers prepared cyclic di- and trisiloxanediolato tantalum(V) complexes by treatment of Ta(OEt)5 with 1,1,3,3-tetramethyl-1,3disiloxanediol, (HO)SiMe2OSiMe2(OH) [9]. A major advantage of these protolytic reactions between transition metal alkoxides and the free ☆ Dedicated to Professor Kenneth N. Raymond on the occasion of his 75th birthday. ⁎ Corresponding author. E-mail address: [email protected] (F.T. Edelmann).
http://dx.doi.org/10.1016/j.inoche.2016.11.002 1387-7003/© 2016 Elsevier B.V. All rights reserved.
silanol precursors is the formation of lower alcohols as the only by-products. In our hands, reaction of Ta(OEt)5 with 1 in a molar ratio of 1:1 according to Scheme 1 afforded a clear, colorless solution from which colorless, prism-like crystals of [{μ-(Ph2OSiO)2O}Ta(OEt)2(μ-OEt)]2 (3) could be isolated as the sole product after concentration to a small volume [10]. The fairly low isolated yield (34%) could be traced back to the very high solubility of compound 3 in all common organic solvents. The compound is moisture-sensitive in solution as well as in the crystalline state. The NMR data of 3 showed the replacement of two ethoxide ligands by the formally dianionic disiloxanediolate fragment. Both the 1H and 13C NMR spectra clearly indicated the presence of both terminal and bridging ethoxide groups. This was confirmed by a single-crystal X-ray diffraction study which revealed a centrosymmetric dinuclear complex as shown in Fig. 1. The molecular structure of 3 [11] is very similar to that observed for the previously described 1,1,3,3-tetramethyl-1,3-disiloxanediolate analogue [9]. Thus the two Ta atoms are not only linked by two μ-κO1:O3bridging disiloxanediolato ligands, but also by two μ-bridging ethoxide moieties. By two additional ethoxide ligands per Ta atom, a slightly distorted octahedral coordination is achieved (e.g. O1-Ta-O3′ 175.6(1)° and O4-Ta-O4′ 70.5(1)°). The Si1-O2-Si2 angle within the disiloxanediolato ligand is 139.8(2)° and therefore much larger than the ideal tetraeder angle. The Ta-O(Si) distances are virtually equal with 193.4(3) (Ta-O1) and 193.8(3) pm (Ta-O3′) and are therefore marginally longer than in the appropriate methyl-substituted complex (Ta-O(Si) 191.8(5) and 192.5(5) pm) [9]. This can possibly be attributed to the higher sterical demand of the Si-attached phenyl groups compared to methyl groups. The Ta-O(Et) bonds in compound 3 are slightly shorter in the case of the terminal EtO− ligands (186.3(3) and
S. Ehle et al. / Inorganic Chemistry Communications 74 (2016) 82–85
83
EtO OEt Ph 2 Ph 2 Si O Ta O Si Ph 2Si
O
O
SiPh 2
EtO
OEt
O
Si O Ta O Si Ph 2 Ph 2 EtO OEt 3
OH OH 1 Ta(OEt)5 Cy7Si7O9(OH) 3 2
O
Cy Si
O
Cy Si
Et O
O O O Cy CySi O SiCy Si O Ta O O O OEt Et O O Si Si O Cy Cy 4
Cy Si
OEt O Ta O
O
Cy Si
O O O Cy Si O SiCy SiCy O O O O Si Cy O Si Cy
Scheme 1. Synthesis of the title compounds 3 and 4.
188.1(1) pm), while the same are much longer in the case of the μbridging EtO− ligands (211.6(3) and 212.4(3) pm). All these values are very similar to those observed in the related literature compound [9]. The same applies to the geometric parameters of the disilanediolate ligand (Si-O distances, O-Si-O and Si-O-Si angles). There are only very few reports in the literature on metallasilsesquioxane derivatives of tantalum [12–15], although these have some significance as components of unusual polyhedral oligosilsesquioxane polymers [12] or as molecular models for silicagrafted tantalum surface species [13]. However, it took until 2013 that the first tantalum silsesquioxane complexes have been structurally characterized. All these are mononuclear complexes in which the open corner of the cube-like silsesquioxane framework in 2 is capped by a TaCp⁎ moiety (Cp⁎ = η5-pentamethylcyclopentadienyl) [15]. A reaction of tantalum(V) ethoxide with the incompletely condensed silsesquioxane precursor Cy7Si7O9(OH)3 (2, Cy = cyclohexyl) in a 1:1 molar ratio was carried out in toluene (cf. Scheme 1) [16]. Just like in the case of 3, the only by-product in this reaction would be ethanol.
Fig. 1. Molecular structure of compound 3 in the crystal. Thermal ellipsoids of Ta, Si and O drawn at the 50% probability level, H atoms omitted for clarity. Both phenyl substituents at Si2 as well as the ethoxide ligands containing O4 and O6 are disordered over each two orientations (only one orientation is each represented). Selected bond lengths [Å] and angles [°]: Ta-O1 193.4(3), Ta-O3′ 193.8(3), Ta-O4 212.3(3), Ta-O4′ 211.7(3), Ta-O5 186.4(3), Ta-O6 188.1(3), O1-Ta-O3′ 175.6(1), O4-Ta-O4′ 70.5(4), Ta-O4-Ta′ 109.5(1), Si1-O1 161.2(4), Si1-O2 162.5(3), Si2-O2 162.7(3), Si2-O3 160.9(3), O1-Si1-O2 109.8(2), O2-Si2-O3 110.4(2), Si1-O2-Si2 139.8(2). Symmetry operator to generate equivalent atoms: ‘1–x, 4–y, 3–z’.
Simple cooling of the concentrated reaction mixture afforded the dinuclear tantalum(V) complex [Cy7Si7O12Ta(OEt)(μ-OEt)]2 (4) as colorless, block-like single-crystals in 80% isolated yield. The NMR data of the product showed replacement of three ethoxide groups by the formally trianionic silsesquioxane ligand. The appearance of only three resonances in the 29Si NMR spectrum (δ = − 66.3, − 68.7, − 69.4 ppm) indicated the formation of a rather symmetrical molecule. Different from the previously reported monomeric tantalum(V) silsesquioxides [iBu7Si7O12Ta(Cp⁎)X] (X = Cl, Me, OTf) and [iBu7Si7O12Ta(Cp*)][B(C6F5)4] [15], the single-crystal X-ray structure determination of compound 4 revealed the presence of a dimeric complex as shown in Fig. 2 [17]. This can be attributed to the absence of a bulky co-ligand such as (Cp⁎)−, allowing the interconnection of two [Cy7Si7O12Ta(OEt)]+ fragments by two μ-bridging ethoxide ligands. As a result, an octahedral coordination of the Ta atom is achieved by means of the tridentate silsequioxide ligand, two μ-bridging EtO− ligands and one terminal EtO− ligand. The Ta-O(Si) distances are in a narrow range of 188.6(1)– 195.1(1) pm and therefore similar to those observed in the previously described tantalum(V) silsequioxides (Ta-O 184.3(4)–196.9(2) pm) [15], as well as the corresponding Ta-O(Si) distances in compound 3. The coordination octahedron around the Ta atom is distorted with OTa-O(cis) angles from 70.19(6)° (O13-Ta-O13’) to 101.31(6)° (O1-TaO12). The Si-O bond lengths and O-Si-O angles within the silsequioxide ligand are in a typical range of 160.8(1)–163.1(1) pm and 107.8(1)– 110.5(1)°, respectively. Although the smallest Si-O-Si angle (Si1-O7Si7 139.6(1)°) is virtually equal to that in compound 3, the majority of the Si-O-Si angles in compound 4 is enlarged up to 159.0(1)° (Si3-O9Si7). Due to this almost linear oxygen coordination, a considerable steric strain can be ascribed to the silsesquioxide framework, accounting for the tendency of those compounds to undefined fragmentation reactions. As it was figured out previously, the conformation of the cube-like Si7Ta body is strongly affected by the coordination sphere around the Ta atom [15]. Thus an almost regular Si7Ta cube has been observed for the cationic complex [iBu7Si7O12Ta(Cp*)]+ (both Si-Si-Si and Si-Si-Ta angles in a narrow range around 90°; Fig. 3b), while the same is irregularly distorted in the case of [iBu7Si7O12Ta(Cp*)Cl] with highly coordinated tantalum (broad Si-Si-Si and Si-Si-Ta angle range; Fig. 3c). In compound 4, the silsequioxide ligand displays a fairly regular conformation just as in [iBu7Si7O12Ta(Cp*)]+, but the Ta edge is considerably moved out of the cube-like framework along the body diagonal (Si-SiSi angles in a narrow range around 90° and average Si-Si-Ta angle larger than 90°; Fig. 3a). This might be traced back to the spatial proximity of two sterically demanding [Cy7Si7O12]3 − ligands in the dimeric compound 4, while the related literature compounds are monomeric.
84
S. Ehle et al. / Inorganic Chemistry Communications 74 (2016) 82–85
Fig. 2. Molecular structure of 4 in the crystal. Thermal ellipsoids of Ta, Si and O drawn at the 50% probability level, H atoms and peripheral C atoms of the cyclohexyl substituents omitted for clarity. Selected bond lengths [Å] and angles [°]: Ta-O1 188.6(1), Ta-O6 195.1(1), Ta-O12 193.0(1), Ta-O13 211.8(1), Ta-O13′ 209.2(1), Ta-O14 190.0(2), O1-Ta-O6 91.89(6), O1-Ta-O12 101.31(6), O6-Ta-O12 88.20(6), O13-Ta-O13′ 70.19(6), Ta-O13-Ta′ 109.81(6), Ta-O1-Si1 168.0(1), Ta-O6-Si5 156.4(1), Ta-O12-Si6 149.7(1), Si1-O1 162.1(2), Si5-O6 160.8(1), Si6-O12 162.0(1), Si-O(Si) 161.2(2)–163.1(2), O-Si-O 107–8(1)–110.5(1), Si-O-Si 139.6(1)–159.0(1). Symmetry operator to generate equivalent atoms: 1–x, 2–y, 1–z.
Fig. 3. Representation of the Si7Ta framework in compound 4 (a) compared to those in the related complexes [iBu7Si7O12Ta(Cp*)]+ (b) and [iBu7Si7O12Ta(Cp*)Cl] (c) [15].
In summarizing the results reported here, we succeeded in the facile preparation and structural characterization of two new complex tantalum(V) siloxides, [{μ-(Ph2OSiO)2O}Ta(OEt)2(μ-OEt)]2 (3) and [Cy7Si7O12Ta(OEt)(μ-OEt)]2 (4).
[3]
Acknowledgements This work was financially supported by the Otto-von-GuerickeUniversität Magdeburg.
[4]
Appendix A. Supplementary material
[5]
Crystallographic data for the crystal structures reported in this paper can be obtained from the Cambridge Crystallographic Data Center, 12 Union Road, Cambridge CB21EZ, UK (fax: +44-1223-336-033; e-mail: [email protected]; URL: http://www.ccdc.cam.ac.uk/) by referring to the depository numbers CCDC 1504270 (3) and 1414157 (4). Supplementary data to this article can be found online at http://dx.doi. org/10.1016/j.inoche.2016.11.002. [6]
References [1] Recent reviews on metal siloxides: (a) M. Veith, Adv. Organomet. Chem. (2006) 53, 101–153 (b) M.M. Levitskii, B.G. Zavin, A.N. Bilyachenko, Russ. Chem. Rev. 76 (2007) 847–866; (c) T.J. Boyle, L.A.M. Ottley, Chem. Rev. 108 (2008) (1896); (d) M.M. Levitskii, V.V. Smirnov, B.G. Zavin, A.N. Bilyachenko, A.Y. Rabkina, Kinet. Catal. 50 (2009) 490–507; (e) V. Lorenz, A. Edelmann, S. Gießmann, C.G. Hrib, S. Blaurock, F.T. Edelmann, Z. Anorg. Allg. Chem. 636 (2010) 2172–2191; (f) C. Krempner, Eur. J. Inorg. Chem. 1689–1698 (2011). [2] (a) R. Murugavel, M. Walawalkar, G. Prabusankar, P. Davis, Organometallics 20 (2001) 2639–2642;
[7]
(b) R. Fandos, A. Otero, A. Rodriguez, M.J. Ruiz, P. Terreros, Angew. Chem. Int. Ed. 40 (2001) 2884–2887; (c) K.L. Fujdala, R.L. Brutchey, T.D. Tilley, Top. Organomet. Chem. 16 (2005) 69–115; (d) J. Jarupatrakorn, M.P. Coles, T.D. Tilley, Chem. Mater. 17 (2005) 1818–1828. (a) R. Murugavel, M.G. Walawalkar, M. Dan, H.W. Roesky, C.N.R. Rao, Acc. Chem. Res. 37 (2004) 763–774; (b) L.G. Hubert-Pfalzgraf, N. Touati, S.V. Pasko, J. Vaissermann, A. Abrutis, Polyhedron 24 (2005) 3066–3073; (b) A. Abrutis, L.G. Hubert-Pfalzgraf, S. Pasko, N. Touati, V. Kazlauskiene, Vacuum 81 (2006) 13–17; (b) S. Ehle, V. Brüser, V. Lorenz, C.G. Hrib, K. Saulich, S. Müller, A. Quade, F.T. Edelmann, Eur. J. Inorg. Chem. (2013) 1451–1457. M. Veith, J. Freres, P. König, O. Schütt, V. Huch, J. Blin, Eur. J. Inorg. Chem. 3699–3710 (2005). (a) J.-M. Basset, R. Psaro, D. Roberto, R. Ugo (Eds.), Modern Surface Organometallic Chemistry, Wiley-VCH, Weinheim, 2009; (b) O. Michel, S. König, K.W. Törnroos, C. Maichle-Mössmer, R. Anwander, Chem. Eur. J. 17 (2011) 11857–11867; (c) B. Marciniec, S. Rogalski, M.J. Potrzebowski, C. Pietraszuk, ChemCatChem 3 (2011) 904–910; (d) E. Le Roux, Y. Liang, K.W. Törnroos, F. Nief, R. Anwander, Organometallics 31 (2012) 6526–6537; (e) P.J. Cordeiro, P. Guillo, C.S. Spanjers, J.W. Chang, M.I. Lipschutz, M.E. Fasulo, R.M. Rioux, T.D. Tilley, ACS Catal. 3 (2013) 2269–2279; (f) P. Laurent, L. Veyre, C. Thieuleux, S. Donet, C. Copéret, Dalton Trans. 42 (2013) 238–248. (a) I. Kownacki, B. Marciniec, K. Szubert, M. Kubicki, Organometallics 24 (2005) 6179–6183; (b) E. Mieczynska, A.M. Trzeciak, J.J. Ziolkowski, I. Kownacki, B.J. Marciniec, Mol. Catal. A: Chem. 237 (2005) 246–253; (c) S.M. Bruno, B. Monteiro, M.S. Balula, C. Lourenco, A.A. Valente, M. Pillinger, P. Ribeiro-Claro, I.S. Goncalves, Molecules 11 (2006) 298–308; (d) I. Kownacki, B. Marciniec, A. Macina, S. Rubinsztajn, D. Lamb, Appl. Catal. A: General 317 (2007) 53–57. (a) S. Gießmann, S. Blaurock, V. Lorenz, F.T. Edelmann, Inorg. Chem. 46 (2007) 10383–10389; (b) S. Gießmann, S. Blaurock, V. Lorenz, F.T. Edelmann, Inorg. Chem. 46 (2007) 10956–10958; (c) A. Edelmann, S. Blaurock, C. Hrib, F.T. Edelmann, J. Organomet. Chem. 695 (2010) 1026–1030; (d) V. Lorenz, A. Edelmann, S. Blaurock, C. Hrib, F.T. Edelmann, Compt. Rend. Chim. 13 (2010) 577–583;
S. Ehle et al. / Inorganic Chemistry Communications 74 (2016) 82–85
[8]
[9] [10]
[11]
(e) B. McNerney, S. Mummadi, F. Hung-Low, D.B. Cordes, D.K. Unruh, C. Krempner, Inorg. Chem. Commun. 70 (2016) 103–106. Recent reviews on metallasilsesquioxanes: (a) V. Lorenz, F. T. Edelmann, Adv. Organomet. Chem. 53 (2005) 101–153 (b) E.A. Quadrelli, J.-M. Basset, Coord. Chem. Rev. 254 (2010) 707–728; (c) A.J. Ward, A.F. Masters, T. Maschmeyer, Adv. Silicon Sci. 3 (2011) 135–166; (d) A. Mehta, G. Tembe, M. Bialek, P. Parikh, G. Mehta, Polym. Adv. Technol. 24 (2013) 441–445; (e) M.M. Levitsky, A.N. Bilyachenko, Coord. Chem. Rev. 306 (2016) 235–269. R.N. Kapoor, F. Cervantes-Lee, C.F. Campana, C. Haltiwanger, K. Abney, K.H. Pannell, Inorg. Chem. 45 (2006) 2203–2208. Synthesis of [{μ-(Ph2OSiO)2O}Ta(OEt)2(μ-OEt)]2 (3): Neat Ta(OEt)5 (4.13 g, 10.0 mmol) was added dropwise via syringe to a stirred solution of 1 (4.17 g, 10.0 mmol) in 120 ml of dry THF and the mixture was stirred for 48 h at r.t. The volume of the resulting clear solution was reduced to ca. 20 ml. Colorless, prism-like crystals (2.5 g, 34%) formed upon standing at r.t. for several days. Anal. Calc. For C60H70O12Si4Ta2 (1457.42 g mol−1): C 49.45%, H 4.84%. Found: C 49.05%, H 4.45%. M.p. 222 °C (dec.). IR (ATR, ν (cm−1)): 3068w, 3048w, 3026w, 3002w, 2974m, 2922w, 2874w, 2707vw, 1962vw, 1893vw, 1828vw, 1776vw, 1714vw, 1664vw, 1592w, 1569vw, 1486w, 1442w, 1430m, 1381w, 1359vw, 1334vw, 1307vw, 1278vw, 1265vw, 1188w, 1152m, 1125s, 1101m, 1067m, 1048s, 1027s, 1002s, 956vs, 879m, 743m, 7217w, 716m, 701s, 556m, 519m, 499m, 478m, 436w. 1H NMR (400 MHz, THF-d8, 25 °C): δ = 7.73 (m, 8H, p-CH Ph); 7.29 (m, 16H, CH Ph); 7.22 (m, 16H, p-CH Ph); 4.32 (q, 4H, CH2 of μ-OEt); 4.20 (q, 6H, CH2 of terminal OEt); 1.08 (t, 12H, CH3 of terminal OEt); 0.99 ppm (t, 6H, CH3 of terminal OEt). 13C NMR (101 MHz, THF-d8): δ = 138.9 (ipso-C); 135.5, 130.1, 128.0 (CH Ph); 70.5, 19.0 (terminal OCH2CH3); 69.6, 17.6 ppm (μ-OCH2CH3). 29Si NMR (79.5 MHz, THF-d8): −45.3 ppm. The crystallographic data of compound 3 were collected on a STOE IPDS 2T diffractometer using graphite-monochromated Mo-Kα radiation. Details on data collection
[12] [13] [14] [15] [16]
[17]
85
and structure refinement are summarized in Table S1, while atomic coordinates, bond lengths and angles are listed in Tables S2–S6 in the SI. T.S. Haddad, J.D. Lichtenhan, Polym. Prepr. 35 (1994) 708–709. M. Chabanas, E.A. Quadrelli, B. Fenet, C. Coperet, J. Thivolle-Cazat, J.-M. Basset, A. Lesage, L. Emsley, Angew. Chem. Int. Ed. 40 (2001) 4493–4496. Z. Fei, S. Busse, F.T. Edelmann, J. Chem, Soc. Dalton Trans. (2002) 2587–2589. P. Guillo, M.E. Fasulo, M.I. Lipschutz, T.D. Tilley, Dalton Trans. 42 (2013) 1991–1995. Synthesis of [Cy7Si7O12Ta(OEt)(μ-OEt)]2 (4): 0.86 g (2.1 mmol) Ta(OEt)5 were added to a solution of 2.0 g (2.05 mmol) 2 in toluene (100 ml; the initially formed suspension was slightly heated until a clear solution had formed). The reaction mixture was stirred for 24 h, followed by refluxing for additionally 2 h. One third of the solvent was evaporated in vacuum; cooling of the solution to 5 °C for 1 week afforded 4 as colorless, block-like crystals. Yield: 2.0 g (80%). Anal. Calc. For C92H174O28Si14Ta2 (2483.46 g mol−1): C 44.49%, H 7.06%. Found: C43.94%, H 6.93%. Decomposition N 130 °C. IR (ATR, ν (cm−1)): 2920s, 2848s, 1651m, 1625m, 1592m, 1447m, 1341m, 1308m, 1284w, 1269w, 1214m, 1193s, 1156s, 1100vs, 1077vs, 1013m, 981s, 923m, 884m, 849m, 825w, 760s, 729w, 692w, 669w. 1H NMR (400 MHz, THF-d8, 25 °C): δ = 4.60 (br s, 4H, CH2 of μ-OEt), 4.20 (q., 4H; CH2 of terminal OEt) 1.90–1.60 (br m, 70H; CH2 of c-C6H11), 1.40–1.15 (br m, 70H; CH2 of c-C6H11 + 12H, CH3 of OEt), 0.80–0.60 (br m, 14H, CH of cC6H11) ppm. 13C NMR (101 MHz, THF-d8, 25 °C): δ = 67.5 (br, OCH2CH3), 28.5, 28.4, 28.1, 27.8, 27.7, 27.7 (CH2, c-C6H11), 25.3, 24.3, 24.3 (CH of c-C6H11), 19.0 (br, OCH2CH3) ppm. 29Si NMR (79.5 MHz, THF-d8, 25 °C): δ = − 66.3, − 68.7, −69.4 ppm. The crystallographic data of compound 4 were collected on a Xcalibur Atlas Nova diffractometer equipped with a Nova Cu-Kα X-ray source. Details on data collection and structure refinement are summarized in Table S1, while atomic coordinates, bond lengths and angles are listed in Tables S2–S6 in the Supplementary Information.
Contents lists available at ScienceDirect
Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
Short communication
Synthesis and structural characterization of two complex tantalum(V) siloxides☆ Sophie Ehle, Volker Lorenz, Phil Liebing, Liane Hilfert, Frank T. Edelmann ⁎ Chemisches Institut der Otto-von-Guericke-Universität Magdeburg, 39106 Magdeburg, Germany
a r t i c l e
i n f o
Article history: Received 16 September 2016 Accepted 6 November 2016 Available online 9 November 2016 Keywords: Tantalum Disiloxanediolate Silsesquioxane Metallasilsesquioxane X-ray structure
a b s t r a c t The reaction of tantalum(V) ethoxide, Ta(OEt)5, with 1 equiv. of 1,1,3,3-tetraphenyl-disiloxane-1,3-diol, (HO)SiPh2OSi-Ph2(OH) (1), afforded the dinuclear tantalum(V) disiloxanediolate complex [{μ(Ph2OSiO)2O}Ta(OEt)2(μ-OEt)]2 (3). Similarly, Ta(OEt)5 reacted with the incompletely condensed silsesquioxane precursor Cy7Si7O9(OH)3 (2, Cy = cyclohexyl) in a 1:1 molar ratio to afford the dinuclear tantalasilsesquioxane derivative [Cy7Si7O12Ta(OEt)(μ-OEt)]2 (4) in 80% yield. Both new complexes were structurally characterized by single-crystal X-ray diffraction studies. © 2016 Elsevier B.V. All rights reserved.
Well-defined molecular metal siloxides (=metallasiloxanes) comprising M-O-Si functionalities attract significant interest due to their diverse potential applications [1]. Metal siloxides are not only excellent molecular models for silica-supported heterogeneous metal catalysts [2], but they can also serve as precursors for new materials [3], nanoparticles [4], well-defined surface species [5], and homogeneous catalysts [6]. Particularly useful and versatile precursors for a large variety of complex metal siloxides are disilanols like 1,1,3,3-tetraphenyl-disiloxane1,3-diol, (HO)SiPh2OSi-Ph2(OH) (1), which forms complexes with virtually all metallic elements across the Periodic Table [1,7]. Even more complex molecular metal siloxides (“metallasilsesquioxanes”) can be designed using an incompletely condensed silsesquioxane derivative such as Cy7Si7O9(OH)3 (2, Cy = cyclohexyl) as precursor [8]. We report here the straightforward preparation and structural characterization of two new complex tantalum(V) siloxides derived from 1 and 2. Our synthetic protocol involved treatment of commercially available tantalum(V) ethoxide Ta(OEt)5, with equimolar amounts of the free silanol precursors 1 and 2. This salt-free route has been frequently employed successfully for the synthesis of titanium(IV) and zirconium(IV) siloxides [1,8]. To our knowledge, there is only one report in the literature describing the use of tantalum(V) ethoxide as precursor for cyclic tantalum(V) siloxides. Pannell and co-workers prepared cyclic di- and trisiloxanediolato tantalum(V) complexes by treatment of Ta(OEt)5 with 1,1,3,3-tetramethyl-1,3disiloxanediol, (HO)SiMe2OSiMe2(OH) [9]. A major advantage of these protolytic reactions between transition metal alkoxides and the free ☆ Dedicated to Professor Kenneth N. Raymond on the occasion of his 75th birthday. ⁎ Corresponding author. E-mail address: [email protected] (F.T. Edelmann).
http://dx.doi.org/10.1016/j.inoche.2016.11.002 1387-7003/© 2016 Elsevier B.V. All rights reserved.
silanol precursors is the formation of lower alcohols as the only by-products. In our hands, reaction of Ta(OEt)5 with 1 in a molar ratio of 1:1 according to Scheme 1 afforded a clear, colorless solution from which colorless, prism-like crystals of [{μ-(Ph2OSiO)2O}Ta(OEt)2(μ-OEt)]2 (3) could be isolated as the sole product after concentration to a small volume [10]. The fairly low isolated yield (34%) could be traced back to the very high solubility of compound 3 in all common organic solvents. The compound is moisture-sensitive in solution as well as in the crystalline state. The NMR data of 3 showed the replacement of two ethoxide ligands by the formally dianionic disiloxanediolate fragment. Both the 1H and 13C NMR spectra clearly indicated the presence of both terminal and bridging ethoxide groups. This was confirmed by a single-crystal X-ray diffraction study which revealed a centrosymmetric dinuclear complex as shown in Fig. 1. The molecular structure of 3 [11] is very similar to that observed for the previously described 1,1,3,3-tetramethyl-1,3-disiloxanediolate analogue [9]. Thus the two Ta atoms are not only linked by two μ-κO1:O3bridging disiloxanediolato ligands, but also by two μ-bridging ethoxide moieties. By two additional ethoxide ligands per Ta atom, a slightly distorted octahedral coordination is achieved (e.g. O1-Ta-O3′ 175.6(1)° and O4-Ta-O4′ 70.5(1)°). The Si1-O2-Si2 angle within the disiloxanediolato ligand is 139.8(2)° and therefore much larger than the ideal tetraeder angle. The Ta-O(Si) distances are virtually equal with 193.4(3) (Ta-O1) and 193.8(3) pm (Ta-O3′) and are therefore marginally longer than in the appropriate methyl-substituted complex (Ta-O(Si) 191.8(5) and 192.5(5) pm) [9]. This can possibly be attributed to the higher sterical demand of the Si-attached phenyl groups compared to methyl groups. The Ta-O(Et) bonds in compound 3 are slightly shorter in the case of the terminal EtO− ligands (186.3(3) and
S. Ehle et al. / Inorganic Chemistry Communications 74 (2016) 82–85
83
EtO OEt Ph 2 Ph 2 Si O Ta O Si Ph 2Si
O
O
SiPh 2
EtO
OEt
O
Si O Ta O Si Ph 2 Ph 2 EtO OEt 3
OH OH 1 Ta(OEt)5 Cy7Si7O9(OH) 3 2
O
Cy Si
O
Cy Si
Et O
O O O Cy CySi O SiCy Si O Ta O O O OEt Et O O Si Si O Cy Cy 4
Cy Si
OEt O Ta O
O
Cy Si
O O O Cy Si O SiCy SiCy O O O O Si Cy O Si Cy
Scheme 1. Synthesis of the title compounds 3 and 4.
188.1(1) pm), while the same are much longer in the case of the μbridging EtO− ligands (211.6(3) and 212.4(3) pm). All these values are very similar to those observed in the related literature compound [9]. The same applies to the geometric parameters of the disilanediolate ligand (Si-O distances, O-Si-O and Si-O-Si angles). There are only very few reports in the literature on metallasilsesquioxane derivatives of tantalum [12–15], although these have some significance as components of unusual polyhedral oligosilsesquioxane polymers [12] or as molecular models for silicagrafted tantalum surface species [13]. However, it took until 2013 that the first tantalum silsesquioxane complexes have been structurally characterized. All these are mononuclear complexes in which the open corner of the cube-like silsesquioxane framework in 2 is capped by a TaCp⁎ moiety (Cp⁎ = η5-pentamethylcyclopentadienyl) [15]. A reaction of tantalum(V) ethoxide with the incompletely condensed silsesquioxane precursor Cy7Si7O9(OH)3 (2, Cy = cyclohexyl) in a 1:1 molar ratio was carried out in toluene (cf. Scheme 1) [16]. Just like in the case of 3, the only by-product in this reaction would be ethanol.
Fig. 1. Molecular structure of compound 3 in the crystal. Thermal ellipsoids of Ta, Si and O drawn at the 50% probability level, H atoms omitted for clarity. Both phenyl substituents at Si2 as well as the ethoxide ligands containing O4 and O6 are disordered over each two orientations (only one orientation is each represented). Selected bond lengths [Å] and angles [°]: Ta-O1 193.4(3), Ta-O3′ 193.8(3), Ta-O4 212.3(3), Ta-O4′ 211.7(3), Ta-O5 186.4(3), Ta-O6 188.1(3), O1-Ta-O3′ 175.6(1), O4-Ta-O4′ 70.5(4), Ta-O4-Ta′ 109.5(1), Si1-O1 161.2(4), Si1-O2 162.5(3), Si2-O2 162.7(3), Si2-O3 160.9(3), O1-Si1-O2 109.8(2), O2-Si2-O3 110.4(2), Si1-O2-Si2 139.8(2). Symmetry operator to generate equivalent atoms: ‘1–x, 4–y, 3–z’.
Simple cooling of the concentrated reaction mixture afforded the dinuclear tantalum(V) complex [Cy7Si7O12Ta(OEt)(μ-OEt)]2 (4) as colorless, block-like single-crystals in 80% isolated yield. The NMR data of the product showed replacement of three ethoxide groups by the formally trianionic silsesquioxane ligand. The appearance of only three resonances in the 29Si NMR spectrum (δ = − 66.3, − 68.7, − 69.4 ppm) indicated the formation of a rather symmetrical molecule. Different from the previously reported monomeric tantalum(V) silsesquioxides [iBu7Si7O12Ta(Cp⁎)X] (X = Cl, Me, OTf) and [iBu7Si7O12Ta(Cp*)][B(C6F5)4] [15], the single-crystal X-ray structure determination of compound 4 revealed the presence of a dimeric complex as shown in Fig. 2 [17]. This can be attributed to the absence of a bulky co-ligand such as (Cp⁎)−, allowing the interconnection of two [Cy7Si7O12Ta(OEt)]+ fragments by two μ-bridging ethoxide ligands. As a result, an octahedral coordination of the Ta atom is achieved by means of the tridentate silsequioxide ligand, two μ-bridging EtO− ligands and one terminal EtO− ligand. The Ta-O(Si) distances are in a narrow range of 188.6(1)– 195.1(1) pm and therefore similar to those observed in the previously described tantalum(V) silsequioxides (Ta-O 184.3(4)–196.9(2) pm) [15], as well as the corresponding Ta-O(Si) distances in compound 3. The coordination octahedron around the Ta atom is distorted with OTa-O(cis) angles from 70.19(6)° (O13-Ta-O13’) to 101.31(6)° (O1-TaO12). The Si-O bond lengths and O-Si-O angles within the silsequioxide ligand are in a typical range of 160.8(1)–163.1(1) pm and 107.8(1)– 110.5(1)°, respectively. Although the smallest Si-O-Si angle (Si1-O7Si7 139.6(1)°) is virtually equal to that in compound 3, the majority of the Si-O-Si angles in compound 4 is enlarged up to 159.0(1)° (Si3-O9Si7). Due to this almost linear oxygen coordination, a considerable steric strain can be ascribed to the silsesquioxide framework, accounting for the tendency of those compounds to undefined fragmentation reactions. As it was figured out previously, the conformation of the cube-like Si7Ta body is strongly affected by the coordination sphere around the Ta atom [15]. Thus an almost regular Si7Ta cube has been observed for the cationic complex [iBu7Si7O12Ta(Cp*)]+ (both Si-Si-Si and Si-Si-Ta angles in a narrow range around 90°; Fig. 3b), while the same is irregularly distorted in the case of [iBu7Si7O12Ta(Cp*)Cl] with highly coordinated tantalum (broad Si-Si-Si and Si-Si-Ta angle range; Fig. 3c). In compound 4, the silsequioxide ligand displays a fairly regular conformation just as in [iBu7Si7O12Ta(Cp*)]+, but the Ta edge is considerably moved out of the cube-like framework along the body diagonal (Si-SiSi angles in a narrow range around 90° and average Si-Si-Ta angle larger than 90°; Fig. 3a). This might be traced back to the spatial proximity of two sterically demanding [Cy7Si7O12]3 − ligands in the dimeric compound 4, while the related literature compounds are monomeric.
84
S. Ehle et al. / Inorganic Chemistry Communications 74 (2016) 82–85
Fig. 2. Molecular structure of 4 in the crystal. Thermal ellipsoids of Ta, Si and O drawn at the 50% probability level, H atoms and peripheral C atoms of the cyclohexyl substituents omitted for clarity. Selected bond lengths [Å] and angles [°]: Ta-O1 188.6(1), Ta-O6 195.1(1), Ta-O12 193.0(1), Ta-O13 211.8(1), Ta-O13′ 209.2(1), Ta-O14 190.0(2), O1-Ta-O6 91.89(6), O1-Ta-O12 101.31(6), O6-Ta-O12 88.20(6), O13-Ta-O13′ 70.19(6), Ta-O13-Ta′ 109.81(6), Ta-O1-Si1 168.0(1), Ta-O6-Si5 156.4(1), Ta-O12-Si6 149.7(1), Si1-O1 162.1(2), Si5-O6 160.8(1), Si6-O12 162.0(1), Si-O(Si) 161.2(2)–163.1(2), O-Si-O 107–8(1)–110.5(1), Si-O-Si 139.6(1)–159.0(1). Symmetry operator to generate equivalent atoms: 1–x, 2–y, 1–z.
Fig. 3. Representation of the Si7Ta framework in compound 4 (a) compared to those in the related complexes [iBu7Si7O12Ta(Cp*)]+ (b) and [iBu7Si7O12Ta(Cp*)Cl] (c) [15].
In summarizing the results reported here, we succeeded in the facile preparation and structural characterization of two new complex tantalum(V) siloxides, [{μ-(Ph2OSiO)2O}Ta(OEt)2(μ-OEt)]2 (3) and [Cy7Si7O12Ta(OEt)(μ-OEt)]2 (4).
[3]
Acknowledgements This work was financially supported by the Otto-von-GuerickeUniversität Magdeburg.
[4]
Appendix A. Supplementary material
[5]
Crystallographic data for the crystal structures reported in this paper can be obtained from the Cambridge Crystallographic Data Center, 12 Union Road, Cambridge CB21EZ, UK (fax: +44-1223-336-033; e-mail: [email protected]; URL: http://www.ccdc.cam.ac.uk/) by referring to the depository numbers CCDC 1504270 (3) and 1414157 (4). Supplementary data to this article can be found online at http://dx.doi. org/10.1016/j.inoche.2016.11.002. [6]
References [1] Recent reviews on metal siloxides: (a) M. Veith, Adv. Organomet. Chem. (2006) 53, 101–153 (b) M.M. Levitskii, B.G. Zavin, A.N. Bilyachenko, Russ. Chem. Rev. 76 (2007) 847–866; (c) T.J. Boyle, L.A.M. Ottley, Chem. Rev. 108 (2008) (1896); (d) M.M. Levitskii, V.V. Smirnov, B.G. Zavin, A.N. Bilyachenko, A.Y. Rabkina, Kinet. Catal. 50 (2009) 490–507; (e) V. Lorenz, A. Edelmann, S. Gießmann, C.G. Hrib, S. Blaurock, F.T. Edelmann, Z. Anorg. Allg. Chem. 636 (2010) 2172–2191; (f) C. Krempner, Eur. J. Inorg. Chem. 1689–1698 (2011). [2] (a) R. Murugavel, M. Walawalkar, G. Prabusankar, P. Davis, Organometallics 20 (2001) 2639–2642;
[7]
(b) R. Fandos, A. Otero, A. Rodriguez, M.J. Ruiz, P. Terreros, Angew. Chem. Int. Ed. 40 (2001) 2884–2887; (c) K.L. Fujdala, R.L. Brutchey, T.D. Tilley, Top. Organomet. Chem. 16 (2005) 69–115; (d) J. Jarupatrakorn, M.P. Coles, T.D. Tilley, Chem. Mater. 17 (2005) 1818–1828. (a) R. Murugavel, M.G. Walawalkar, M. Dan, H.W. Roesky, C.N.R. Rao, Acc. Chem. Res. 37 (2004) 763–774; (b) L.G. Hubert-Pfalzgraf, N. Touati, S.V. Pasko, J. Vaissermann, A. Abrutis, Polyhedron 24 (2005) 3066–3073; (b) A. Abrutis, L.G. Hubert-Pfalzgraf, S. Pasko, N. Touati, V. Kazlauskiene, Vacuum 81 (2006) 13–17; (b) S. Ehle, V. Brüser, V. Lorenz, C.G. Hrib, K. Saulich, S. Müller, A. Quade, F.T. Edelmann, Eur. J. Inorg. Chem. (2013) 1451–1457. M. Veith, J. Freres, P. König, O. Schütt, V. Huch, J. Blin, Eur. J. Inorg. Chem. 3699–3710 (2005). (a) J.-M. Basset, R. Psaro, D. Roberto, R. Ugo (Eds.), Modern Surface Organometallic Chemistry, Wiley-VCH, Weinheim, 2009; (b) O. Michel, S. König, K.W. Törnroos, C. Maichle-Mössmer, R. Anwander, Chem. Eur. J. 17 (2011) 11857–11867; (c) B. Marciniec, S. Rogalski, M.J. Potrzebowski, C. Pietraszuk, ChemCatChem 3 (2011) 904–910; (d) E. Le Roux, Y. Liang, K.W. Törnroos, F. Nief, R. Anwander, Organometallics 31 (2012) 6526–6537; (e) P.J. Cordeiro, P. Guillo, C.S. Spanjers, J.W. Chang, M.I. Lipschutz, M.E. Fasulo, R.M. Rioux, T.D. Tilley, ACS Catal. 3 (2013) 2269–2279; (f) P. Laurent, L. Veyre, C. Thieuleux, S. Donet, C. Copéret, Dalton Trans. 42 (2013) 238–248. (a) I. Kownacki, B. Marciniec, K. Szubert, M. Kubicki, Organometallics 24 (2005) 6179–6183; (b) E. Mieczynska, A.M. Trzeciak, J.J. Ziolkowski, I. Kownacki, B.J. Marciniec, Mol. Catal. A: Chem. 237 (2005) 246–253; (c) S.M. Bruno, B. Monteiro, M.S. Balula, C. Lourenco, A.A. Valente, M. Pillinger, P. Ribeiro-Claro, I.S. Goncalves, Molecules 11 (2006) 298–308; (d) I. Kownacki, B. Marciniec, A. Macina, S. Rubinsztajn, D. Lamb, Appl. Catal. A: General 317 (2007) 53–57. (a) S. Gießmann, S. Blaurock, V. Lorenz, F.T. Edelmann, Inorg. Chem. 46 (2007) 10383–10389; (b) S. Gießmann, S. Blaurock, V. Lorenz, F.T. Edelmann, Inorg. Chem. 46 (2007) 10956–10958; (c) A. Edelmann, S. Blaurock, C. Hrib, F.T. Edelmann, J. Organomet. Chem. 695 (2010) 1026–1030; (d) V. Lorenz, A. Edelmann, S. Blaurock, C. Hrib, F.T. Edelmann, Compt. Rend. Chim. 13 (2010) 577–583;
S. Ehle et al. / Inorganic Chemistry Communications 74 (2016) 82–85
[8]
[9] [10]
[11]
(e) B. McNerney, S. Mummadi, F. Hung-Low, D.B. Cordes, D.K. Unruh, C. Krempner, Inorg. Chem. Commun. 70 (2016) 103–106. Recent reviews on metallasilsesquioxanes: (a) V. Lorenz, F. T. Edelmann, Adv. Organomet. Chem. 53 (2005) 101–153 (b) E.A. Quadrelli, J.-M. Basset, Coord. Chem. Rev. 254 (2010) 707–728; (c) A.J. Ward, A.F. Masters, T. Maschmeyer, Adv. Silicon Sci. 3 (2011) 135–166; (d) A. Mehta, G. Tembe, M. Bialek, P. Parikh, G. Mehta, Polym. Adv. Technol. 24 (2013) 441–445; (e) M.M. Levitsky, A.N. Bilyachenko, Coord. Chem. Rev. 306 (2016) 235–269. R.N. Kapoor, F. Cervantes-Lee, C.F. Campana, C. Haltiwanger, K. Abney, K.H. Pannell, Inorg. Chem. 45 (2006) 2203–2208. Synthesis of [{μ-(Ph2OSiO)2O}Ta(OEt)2(μ-OEt)]2 (3): Neat Ta(OEt)5 (4.13 g, 10.0 mmol) was added dropwise via syringe to a stirred solution of 1 (4.17 g, 10.0 mmol) in 120 ml of dry THF and the mixture was stirred for 48 h at r.t. The volume of the resulting clear solution was reduced to ca. 20 ml. Colorless, prism-like crystals (2.5 g, 34%) formed upon standing at r.t. for several days. Anal. Calc. For C60H70O12Si4Ta2 (1457.42 g mol−1): C 49.45%, H 4.84%. Found: C 49.05%, H 4.45%. M.p. 222 °C (dec.). IR (ATR, ν (cm−1)): 3068w, 3048w, 3026w, 3002w, 2974m, 2922w, 2874w, 2707vw, 1962vw, 1893vw, 1828vw, 1776vw, 1714vw, 1664vw, 1592w, 1569vw, 1486w, 1442w, 1430m, 1381w, 1359vw, 1334vw, 1307vw, 1278vw, 1265vw, 1188w, 1152m, 1125s, 1101m, 1067m, 1048s, 1027s, 1002s, 956vs, 879m, 743m, 7217w, 716m, 701s, 556m, 519m, 499m, 478m, 436w. 1H NMR (400 MHz, THF-d8, 25 °C): δ = 7.73 (m, 8H, p-CH Ph); 7.29 (m, 16H, CH Ph); 7.22 (m, 16H, p-CH Ph); 4.32 (q, 4H, CH2 of μ-OEt); 4.20 (q, 6H, CH2 of terminal OEt); 1.08 (t, 12H, CH3 of terminal OEt); 0.99 ppm (t, 6H, CH3 of terminal OEt). 13C NMR (101 MHz, THF-d8): δ = 138.9 (ipso-C); 135.5, 130.1, 128.0 (CH Ph); 70.5, 19.0 (terminal OCH2CH3); 69.6, 17.6 ppm (μ-OCH2CH3). 29Si NMR (79.5 MHz, THF-d8): −45.3 ppm. The crystallographic data of compound 3 were collected on a STOE IPDS 2T diffractometer using graphite-monochromated Mo-Kα radiation. Details on data collection
[12] [13] [14] [15] [16]
[17]
85
and structure refinement are summarized in Table S1, while atomic coordinates, bond lengths and angles are listed in Tables S2–S6 in the SI. T.S. Haddad, J.D. Lichtenhan, Polym. Prepr. 35 (1994) 708–709. M. Chabanas, E.A. Quadrelli, B. Fenet, C. Coperet, J. Thivolle-Cazat, J.-M. Basset, A. Lesage, L. Emsley, Angew. Chem. Int. Ed. 40 (2001) 4493–4496. Z. Fei, S. Busse, F.T. Edelmann, J. Chem, Soc. Dalton Trans. (2002) 2587–2589. P. Guillo, M.E. Fasulo, M.I. Lipschutz, T.D. Tilley, Dalton Trans. 42 (2013) 1991–1995. Synthesis of [Cy7Si7O12Ta(OEt)(μ-OEt)]2 (4): 0.86 g (2.1 mmol) Ta(OEt)5 were added to a solution of 2.0 g (2.05 mmol) 2 in toluene (100 ml; the initially formed suspension was slightly heated until a clear solution had formed). The reaction mixture was stirred for 24 h, followed by refluxing for additionally 2 h. One third of the solvent was evaporated in vacuum; cooling of the solution to 5 °C for 1 week afforded 4 as colorless, block-like crystals. Yield: 2.0 g (80%). Anal. Calc. For C92H174O28Si14Ta2 (2483.46 g mol−1): C 44.49%, H 7.06%. Found: C43.94%, H 6.93%. Decomposition N 130 °C. IR (ATR, ν (cm−1)): 2920s, 2848s, 1651m, 1625m, 1592m, 1447m, 1341m, 1308m, 1284w, 1269w, 1214m, 1193s, 1156s, 1100vs, 1077vs, 1013m, 981s, 923m, 884m, 849m, 825w, 760s, 729w, 692w, 669w. 1H NMR (400 MHz, THF-d8, 25 °C): δ = 4.60 (br s, 4H, CH2 of μ-OEt), 4.20 (q., 4H; CH2 of terminal OEt) 1.90–1.60 (br m, 70H; CH2 of c-C6H11), 1.40–1.15 (br m, 70H; CH2 of c-C6H11 + 12H, CH3 of OEt), 0.80–0.60 (br m, 14H, CH of cC6H11) ppm. 13C NMR (101 MHz, THF-d8, 25 °C): δ = 67.5 (br, OCH2CH3), 28.5, 28.4, 28.1, 27.8, 27.7, 27.7 (CH2, c-C6H11), 25.3, 24.3, 24.3 (CH of c-C6H11), 19.0 (br, OCH2CH3) ppm. 29Si NMR (79.5 MHz, THF-d8, 25 °C): δ = − 66.3, − 68.7, −69.4 ppm. The crystallographic data of compound 4 were collected on a Xcalibur Atlas Nova diffractometer equipped with a Nova Cu-Kα X-ray source. Details on data collection and structure refinement are summarized in Table S1, while atomic coordinates, bond lengths and angles are listed in Tables S2–S6 in the Supplementary Information.