silanes and Their PdCl2 Complexes: Pd-Capped Tetrahedral Silicon

Article pubs.acs.org/Organometallics

7‑Azaindol-1-yl(organo)silanes and Their PdCl2 Complexes: PdCapped Tetrahedral Silicon Coordination Spheres and Paddlewheels with a Pd−Si Axis Sven Wahlicht,† Erica Brendler,‡ Thomas Heine,§ Lyuben Zhechkov,§ and Jörg Wagler*,† †

Institut für Anorganische Chemie, TU Bergakademie Freiberg, D-09596 Freiberg, Germany Institut für Analytische Chemie, TU Bergakademie Freiberg, D-09596 Freiberg, Germany § School of Engineering and Science, Theoretical Physics - Theoretical Materials Science, Jacobs University Bremen gGmbH, D-28759 Bremen, Germany ‡

S Supporting Information *

ABSTRACT: Supported by excess triethylamine, 7-azaindole (HL) and dichlorodimethylsilane (Me2SiCl2), methyltrichlorosilane (MeSiCl3), and tetrachlorosilane (SiCl4) react, respectively, with formation of the 7-azaindol-1-yl-substituted silanes Me2SiL2, MeSiL3, and SiL4. In these compounds the silicon atom adopts [4+2], [4+3], and [4+4] coordination, respectively, with the pyridine N atoms of the 7-azaindol-1-yl groups capping the tetrahedral Si coordination sphere from distances of >3 Å. Two pyridine nitrogen atoms of these silanes replace the acetonitrile ligands of [PdCl2(NCMe)2], thus forming the complexes Me2Si(μ-L)2PdCl2, MeSiL(μ-L)2PdCl2, and SiL2(μ-L)2PdCl2. In addition to capping of the silicon coordination sphere by the pyridine N atoms of the dangling 7-azaindol-1-yl groups (in MeSiL(μ-L)2PdCl2 and SiL2(μ-L)2PdCl2), these three palladium complexes exhibit capping of one tetrahedral face of the Si coordination sphere by the palladium atom (with Si···Pd separations of 3.34, 3.43, and 3.31 Å, respectively). According to computational analyses, the paddlewheel complex ClSi(μ-L)4PdCl should be energetically favored over its isomer SiL2(μ-L)2PdCl2; however, isomerization into this paddlewheel compound requires higher temperatures (150 °C) or the addition of a Lewis acid (such as GaCl3).

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INTRODUCTION The chemistry of heterodinuclear complexes of transition metal (TM) and main group metal or metalloid elements (E), which exhibit formal σ-lone-pair donation from d10 or d8 metal complexes or other late transition metal compounds (TM → E), has been explored for a variety of TM/E combinations during the past decade. Investigations of the so-called metallaboratranes (e.g., I),1 which accommodate a TM→B bond within a cage structure of bridging bidentate ligands with TM and B atoms in bridgehead positions, pioneered these endeavors, thus initiating research on TM/E combinations across the periodic table.2 The selection of compounds shown in Chart 1 clearly demonstrates the fundamental characteristics of the buttressing ligand, which is a monoanionic heterobidentate 1,3-donor ligand in most cases (e.g., in II,2v III,2d and IV2i). The heterobidentate nature of the buttresses with the © 2014 American Chemical Society

inherent coordinative preference for a soft and a hard central atom clearly supports the architecture of those heterodinuclear complexes. A set of such ambidentate buttresses can be combined within a tripodal ligand (e.g., in V2b), as shown by Lu et al. Ambidentate ligands devoid of a net charge (e.g., in VI2q) can also serve the same purpose. Examples of related TM−E complexes with symmetric buttresses are rare, for instance the carboxylate-bridged Rh−Bi system VII2o and the naphthalene1,8-diyl-bridged Hg−Sb compound VIII.2m The 7-azaindol-1-yl moiety thus fills an intermediate gap between the highly symmetric bidentate ligands and those with two different donor atoms, as it bears two nitrogen donor atoms but in chemically different environments. In reactions with hydroborates 7Received: December 20, 2013 Published: May 12, 2014 2479

dx.doi.org/10.1021/om401220m | Organometallics 2014, 33, 2479−2488

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Chart 1

Scheme 2. Generic Scheme for the Syntheses of 7-Azaindol1-ylsilanes Me4−nSiLn and Their Reaction with [PdCl2(NCMe)2] with Formation of Complexes Me4−nSiLn−2(μ-L)2PdCl2 (n = 2, 3, 4)

azaindole has been shown to be capable of forming 7-azaindol1-yl- and 7-azaindol-7-yl-substituted products, where the coordination can be retained in transition metal complexes obtained from these borates (IX and X, Scheme 1).3 In the

the form of ClSiL3·SiL4. The compounds MeSiL3 and Me2SiL2 crystallized from acetonitrile without solvent of crystallization. The above-mentioned 7-azaindolylsilanes were characterized by single-crystal X-ray diffraction (Figure 1) and allow for insights

Scheme 1

context of the diagonal relationship of boron and silicon and in the context of our own investigations2v−x of TM→E coordination compounds with E = Si we noticed a lack of structurally characterized 7-azaindol-(1-or-7)-yl-substituted silanes in the literature and became interested in exploring the 7azaindolylsilanes as starting materials for heterodinuclear complexes with TM→Si coordination.

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Figure 1. Molecular structures of SiL4 (in the solvent-free crystal structure), ClSiL3 (in the crystal structure of ClSiL3·SiL4), MeSiL3 (one of the two crystallographically independent molecules), and Me2SiL2 (one of the two crystallographically independent molecules).

RESULTS AND DISCUSSION Supported by triethylamine as a sacrificial base, 7-azaindole and methylchlorosilanes Me4−nSiCln (n = 2, 3, 4) react with formation of the corresponding 7-azaindol-1-yl-substituted silanes Me4−nSi(L)n (n = 2, 3, 4, L = 7-azaindol-1-yl) (Scheme 2). Following our previously published procedure for the syntheses of methimazolylsilanes,4 we used small amounts of Nmethylimidazole (NMI) as a catalyst to promote complete substitution in the case of the synthesis of SiL4. Depending on crystallization conditions, we found the two compositions of SiL4 and SiL4·(MeCN) to crystallize from acetonitrile. Also aiming at the synthesis of the partially substituted azaindolylsilane ClSiL3, our investigations indicated the facile formation of SiL4 even in the absence of NMI and even upon using the desired stoichiometric amounts of the starting materials. Thus, we have not been able to isolate ClSiL3 as a pure compound; however we were able to characterize it crystallographically in

into their intramolecular coordination patterns. The solution NMR spectra (1H, 13C, 29Si) always exhibit only one set of signals characteristic of the proposed compound with chemically equivalent 7-azaindolyl moieties. Thus, we can exclude significant 7-azaindol-1-yl to 7-azaindol-7-yl isomerization in solution. (For related aluminum compounds such an isomerization has been proposed.5) The silicon coordination spheres are [4+n]-capped tetrahedral, n being the number of 7azaindolyl groups. The capping Si···N separations, which range between 3.09 and 3.22 Å, are slightly shorter than the corresponding Al···N separations in related aluminum compounds (e.g., 3.26 Å in AlL3(LH)),5 and they are similar to those observed for a 4-fold-capped SiO4-coordination sphere with diazobenzene N atoms as the capping lone-pair donors 2480


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(3.07−3.12 Å).6 Interestingly, crystal packing effects seem to dominate the molecular conformation of the 7-azaindolylsilanes, because cappings of tetrahedral faces opposite Si−N (in MeSiL3) and Si−C bonds (in Me2SiL2) are observed. Furthermore, the similarity of Si−N−C angles of each 7azaindolyl group (e.g., Si1−N1−C1 127.5(1)°, Si1−N1−C7 126.5(1)° in Me2SiL2) hints at only marginal interactions between silicon and the vacant N atoms. Accordingly, the 29Si NMR shifts of SiL4 (−50.5 CDCl3, −49.9 solid state), ClSiL3 (−42.9 CDCl3), MeSiL3 (−17.9 CDCl3, −16.1 solid state), and Me2SiL2 (+2.9 CDCl3, −1.9 and −2.8 solid state) clearly reflect tetracoordination of the silicon atoms. Furthermore, comparison of the 29Si NMR shifts of SiL4 (−50.5) and tetrakis(indol1-yl)silane (−51.3),7 which lacks the potential additional donor function, underlines the apparent absence of noticeable N−Si coordination in the 29 Si NMR features of these 7azaindolylsilanes. The silanes Me2SiL2, MeSiL3, and SiL4 are capable of replacing acetonitrile ligands in [PdCl2(NCMe)2] (whereas in related batches [PdCl2(PPh3)2] did not react with those silanes) and forming (respectively) the palladium complexes Me2Si(μ-L)2PdCl2, MeSiL(μ-L)2PdCl2, and SiL2(μ-L)2PdCl2. These compounds are practically insoluble in organic solvents such as acetonitrile, chloroform, or THF and were thus characterized by solid-state methods. Whereas crystals suitable for single-crystal X-ray diffraction analyses (Figure 2) were

obtained from the reaction mixtures, agreement of the gross product compositions with the compounds found in the single crystals was established with 29Si solid-state NMR spectroscopy and elemental analysis. In general, the silicon atoms in Me2Si(μ-L)2PdCl2, MeSiL(μ-L)2PdCl2, and SiL2(μ-L)2PdCl2 can be regarded as tetracoordinate, because the 29Si NMR shifts of these compounds do not reveal an upfield shift characteristic of a significantly enhanced silicon coordination number (δ 29Si for the couples of silanes SiL4, MeSiL3, and Me2SiL2 and their PdCl2 complexes are −49.9/−51.0; −16.1/−18.8; −1.9, −2.8/ +6.1). Especially in the case of the transition from Me2SiL2 to Me2Si(μ-L)2PdCl2 the downfield shift of the 29Si resonance by about 8 ppm indicates formation of a silicon compound with lower coordination number. In the context of the previously mentioned [4+2] coordination of the Si atom in Me2SiL2 we can rationalize this lowering of the coordination number as a transition toward [4+1] coordination, i.e., removal of two capping N atoms and addition of a Pd atom, which now caps one of the tetrahedral faces. (Similar coordination patterns (Lewis acidic site at the apex of a square pyramidal transition metal coordination sphere) have been previously published for combinations such as Pd−Te2l and Pd−B.2aa) Even though this kind of lowering of the Si coordination number proceeds in the related transformations of SiL4 and MeSiL3 into SiL2(μL)2PdCl2 and MeSiL(μ-L)2PdCl2 as well (from [4+3] and [4+4] to [4+2] and [4+3], respectively), replacement of the remote donor moieties has less net impact on the 29Si shift in these latter cases. From the combination of these results and the observation that the 29Si NMR shift of SiL4 is similar to that of tetrakis(indol-1-yl)silane, we can conclude that in these compounds steric effects (bond angles near the Si coordination sphere, orientation of aromatic moieties, etc.) exert far more influence on the 29Si NMR shifts than the action of the remote lone-pair donor moieties. In the solid-state structures of MeSiL(μ-L)2PdCl2, Me2Si(μL)2PdCl2, and SiL2(μ-L)2PdCl2 (in this order) we observe widening of the bond angles associated with the Pd-capped tetrahedral face (sum of angles is 330.1°, 337.1°, and 339.8°), which correspond to 5%, 27%, and 36% progress along the coordinate of the transition from a tetrahedral coordination sphere (sum of angles 328.5°) toward trigonal bipyramidal (sum of angles 360°). This progress of widening of angles is systematically related to the shortening of the Pd−Si separation in the same order (3.43, 3.34, and 3.31 Å). Steric repulsion between the Pd-bound chlorine atoms and one of the Si-bound substituents might contribute substantially to this angle widening, because repulsion of the PdCl2 moiety away from the Si atom is reflected in the smaller Pd1−N2−C6 and Pd1− N4−C13 angles (which range between 114.7° and 118.3°) versus wider Pd1−N2−C7 and Pd1−N4−C14 angles (122.8− 128.3°). Interestingly, the Pd−Si separations are only 0.8 Å longer than the sum of the covalent radii (rcov) with d(SiPd)/ ∑rcov of 1.37, 1.34, and 1.32 for MeSiL(μ-L)2PdCl2, Me2Si(μL)2PdCl2, and SiL2(μ-L)2PdCl2, respectively (according to the covalent radii data published by Alvarez et al.).8 Noteworthily, the weak Si←N coordination in some silatranes and oxinato complexes (with Si−N separations of 2.75 Å, which exceed the sum of the covalent radii even more, by about 0.9 Å) causes more pronounced angle widening (sum of angles 344.5° and 348°, respectively).9 This allows for the conclusion that the Pd−Si separations of about 3.3−3.4 Å found in our complexes should in principle be sufficient for weak Pd−Si coordination, but in the herein reported complexes the Pd atoms exert less

Figure 2. Molecular structures of (from top) SiL2(μ-L)2PdCl2, MeSiL(μ-L)2PdCl2, and Me2Si(μ-L)2PdCl2 in their crystal structures. 2481


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Figure 3. Gas phase optimized molecular structures of SiL2(μ-L)2PdCl2, SiL2(μ-L)2PdClF, and their paddlewheel isomers ClSi(μ-L)4PdCl and FSi(μ-L)4PdCl, respectively.

lone-pair donor action than nitrogen donors in silatranes and oxinato silicon complexes with weak Si←N coordination. An additional interaction between the Pd atom and the 7azaindolylsilane is found in the solid-state structures of SiL2(μ-L)2PdCl2 and MeSiL(μ-L)2PdCl2. That is, the nitrogen atom N5 (see Figure 2) is slightly pyramidalized toward the Pd atom (sum of angles about N5 is 356° in SiL2(μ-L)2PdCl2 and 355° in MeSiL(μ-L)2PdCl2) and thus indicates a weak N→Pd donor−acceptor interaction (N−Pd separations are 3.19 and 3.04 Å, respectively). This feature, however, disappears upon optimization of the gas-phase molecular structure (as shown for SiL2(μ-L)2PdCl2, vide inf ra) and thus appears to be inferior with respect to the also weak Pd→Si interaction. For the SiL2(μ-L)2PdCl2 compound we had expected isomerization into the paddlewheel-shaped (4,0)-ClSi(μL)4PdCl. In some first experiments, neither spontaneous isomerization was observed (upon heating in chloroform) nor did the addition of a soluble chloride salt (as chloride transfer catalyst from Pd to Si) result in the isomerization of SiL2(μL)2PdCl2 (i.e., no rearrangement observed in the presence of Et3NHCl in a chloroform suspension of SiL2(μ-L)2PdCl2). As confirmed by 29Si solid-state NMR spectroscopy, we always recovered the starting material. As this observation was in contradiction to our expectations, which are based on the facile formation of (4,0)-ClSi(μ-mt)4PdCl-type paddlewheel complexes with mt = methimazolide, the energetic difference between the isomers SiL2(μ-L)2PdCl2 and (4,0)-ClSi(μL)4PdCl was analyzed by computational methods. For comparison, the molecular structures of the fluoro-analogous compounds SiL2(μ-L)2PdClF (two isomers) and (4,0)-FSi(μL)4PdCl were optimized as well and analyzed for their energetics. Figure 3 shows the molecular structures of the gas phase optimized complexes, and Table 1 lists their relative energies and Pd−Si separations. We find that the isomerization of compound SiL2(μ-L)2PdCl2 into its (4,0)-paddlewheel isomer should be energetically favorable. As expected, formation of a Si−F bond should furthermore support the formation of such kind of complex. Thus, SiL2(μ-L)2PdCl2 was

Table 1. Relative Energies [kcal/mol] and Pd−Si Separations [Å] of the Gas Phase Optimized Molecular Structures of SiL2(μ-L)2PdCl2, SiL2(μ-L)2PdClF, and Their Paddlewheel Isomers ClSi(μ-L)4PdCl and FSi(μ-L)4PdCl, Respectivelya

SiL2(μL)2PdCl2 ClSi(μL)4PdCl

SiL2(μL)2PdClF (a) SiL2(μL)2PdClF (b) FSi(μ-L)4PdCl

Pd−Si

E(PBE0/ TZ2P)

E(B3LYP/ TZ2P)

E(PBE/ TZ2P)

E(BP/ TZ2P)

3.31

21.3

11.2

29.3

27.1

2.62

0

0

0

0

Pd−Si

E(PBE0/ TZ2P)

E(B3LYP/ TZ2P)

E(PBE/ TZ2P)

E(BP/ TZ2P)

3.39

58.3

49.2

63.0

61.4

3.16

58.0

49.3

61.4

59.9

2.62

0

0

0

0

a

Optimized with PBE, TZ2P for Si and Pd, DZP for other atoms, single-point energy analyses with different methods.

treated in chloroform with Bu4NBF4, which proved to be an effective fluoride source for the generation of Si−F-functionalized metallasilatranes.2x Once again, we could not observe formation of any new Pd−Si compound; only the starting material was recovered. Apparently, the formation of the paddlewheel-type compound ClSi(μ-L)4PdCl (and its F−Si analogue) is hindered by a kinetic barrier, which cannot be overcome easily by the above-mentioned methods. Alternative strategies include treatment of SiL2(μ-L)2PdCl2 at higher temperatures and the use of a Lewis acid for chloride abstraction from Pd to initialize cage closure and paddlewheel formation. These two strategies eventually were successful. Whereas SiL2(μ-L)2PdCl2 did not isomerize at 120 °C (odichlorobenzene, 5 days), slow isomerization was observed in odichlorobenzene at 150 °C (as shown in Scheme 3 for X = Cl). As indicated by 29Si solid-state NMR spectroscopy (see Supporting Information), after 3 h the solid was composed of the starting material (δ −51, predominant component) and a 2482


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disorder by symmetry (space group I4/m, the 4-fold axis Cl− Si−Pd−Cl of the molecule is perpendicular to a bisecting plane, which generates the alternative orientation of the molecule with opposite direction of the Cl−Si−Pd−Cl axis and the 7azaindolyl ligands). Thus, various restraints had to be used for the refinement, and the structural parameters are therefore of limited quality for the discussion of molecular features. With the aid of GaCl3 as a chloride scavenger SiL2(μL)2PdCl2 could be transformed into a paddlewheel complex at room temperature in chloroform (Scheme 4). In a first reaction a 1:1 stoichiometric mixture of SiL2(μ-L)2PdCl2 and GaCl3 was layered with chloroform (aiming at slow crystallization of the product). As we found a fine powdery solid in the Schlenk tube even after 3 weeks, the solvent was removed in a vacuum (trap condensation) and the solid residue was recrystallized from acetonitrile. Within a few days a crystalline solid formed, which contained tetragonal plates suitable for X-ray analysis, and 29Si solid-state NMR spectroscopy confirmed the presence of only one Si species in the solid product with a chemical shift (δ −143) similar to that observed with ClSi(μ-L)4PdCl. As proven by single-crystal X-ray analysis, the gallium chloride had captured a chloride, and the cation of the product is a paddlewheel complex with a Cl−Si−Pd axis and four 7azaindolyl bridges (Figure 5, top). In this cation, the axial coordination site at palladium is occupied by an acetonitrile molecule to furnish the compound (ClSi(μ-L)4PdNCMe)(GaCl4). In order to probe whether GaCl3 would isomerize SiL2(μ-L)2PdCl2 into ClSi(μ-L)4PdCl if added in substoichiometric amounts, a mixture of SiL2(μ-L)2PdCl2 (0.36 mmol) and GaCl3 (0.11 mmol) was stirred in chloroform at room temperature for 1 day. Thereafter, upon filtration, some solid SiL2(μ-L)2PdCl2 (0.14 mmol) was recovered, and 29Si solidstate NMR spectroscopy proved this solid to consist of SiL2(μL)2PdCl2 only. Hence, 0.22 mmol of the starting material had reacted with the 0.11 mmol of GaCl3 added. Gas phase diffusion of n-pentane into the filtrate led to the formation of a multicrystalline solid, and by crystal picking two different compounds could be identified by single-crystal X-ray diffraction analysis. One of them is the ionic compound (ClSi(μ-L)4Pd)2(Cl)(GaCl4)(CHCl3)2 (Figure 5, bottom),

Scheme 3

new compound, which produced a significantly upfield-shifted resonance (δ −143). The latter is at the expected chemical shift for compound ClSi(μ-L)4PdCl, because the 29Si NMR shifts calculated for the solid-state structure of SiL2(μ-L)2PdCl2 (as a benchmark) and for the optimized molecular structure of ClSi(μ-L)4PdCl are −55.3 and −142.2, respectively. Thus, another sample of SiL2(μ-L)2PdCl2 in o-dichlorobenzene was kept at 150 °C for 15 h to afford a solid, which contained only the rearrangement product. The solid product contained thin tetragonal crystalline plates, which were suitable for X-ray diffraction analysis (Figure 4, left). The crystal structure

Figure 4. Molecular structures of ClSi(μ-L)4PdCl in (from left) the solvent-free crystal structure and in the crystal structure of the chloroform solvate (ClSi(μ-L)4PdCl)2(CHCl3)3.

analysis confirms the identity of the product as the paddlewheel complex ClSi(μ-L)4PdCl. Unfortunately, the structure suffers Scheme 4

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∑rcov = 1.05, calculated for ClSi(μ-L)4PdCl, 2.64(1) and 2.642(3) Å, d(SiPd)/∑rcov = 1.06, found in the crystal structures), whereas in the methimazolyl-bridged complex ClSi(μ-mt)4PdI the S···N separation (2.74 Å) is wider than the Pd−Si bond (2.56 Å). Thus, propeller tilt of methimazolyl moieties versus straight alignment of 7-azaindolyl moieties can be interpreted as structural features that contribute to the accommodation of the Pd−Si bond and parallel SiN4 and PdN4 (or PdS4) planes in a paddlewheel fashion. As shown in Scheme 5, Si−Cl versus Si−F substitution can be expected to have only Scheme 5. Selected Interatomic Separations [Å] in the Gas Phase Optimized Paddlewheel Complexes ClSi(μ-L)4PdCl and FSi(μ-L)4PdCl

Figure 5. Molecular structures of (from top) the cations [ClSi(μL)4PdNCMe]+ and [(ClSi(μ-L)4Pd)2(Cl)]+ in the crystal structures of their tetrachlorogallate salts (ClSi(μ-L)4PdNCMe)(GaCl4) and (ClSi(μ-L)4Pd)2(Cl)(GaCl4)(CHCl3)2, respectively. In the latter, Cl3 bridges Pd1 and Pd2 in a rather symmetric linear manner (Pd1−Cl3 2.95 Å, Pd2−Cl3 2.91 Å, Pd1−Cl3−Pd2 174°).

little influence on the Pd−Si bond length. In a more pronounced manner, intramolecular H-contacts with the halide atoms correspond to this substitution with shortening of the Si−N bonds upon transition to the Si−F derivative. Similar Si− N bond length shortening has been observed with related methimazolyl-bridged complexes.2x The cationic paddlewheel complexes [(ClSi(μL)4PdNCMe)]+ and [(ClSi(μ-L)4Pd)2(Cl)]+ exhibit slightly longer Pd−Si bonds (2.662(2), d(SiPd)/∑rcov = 1.06, and 2.683(1)/2.684(1) Å, d(SiPd)/∑rcov = 1.07, respectively). The slightly weaker Pd−Si coordination most likely arises from the different trans-disposed donor moieties (acetonitrile and bridging chloride, respectively, versus chloride in ClSi(μL)4PdCl). The Si−N bond lengths (1.883(3) and 1.863(3)− 1.893(3) Å, respectively) are slightly shorter than those calculated for ClSi(μ-L)4PdCl (1.91 Å) and found in the crystal structures of this compound (1.902(1) and 1.898(5)− 1.939(11) Å for the solvent-free and the chloroform-solvated form, respectively). Also, the Si−Cl bonds (2.139(3) and 2.105(1)/2.090(1) Å, respectively) are shorter than those calculated for ClSi(μ-L)4PdCl (2.16 Å). These observations are in accord with the slightly weaker donor strength of the Pd atom in these cationic complexes. For a more quantitative discussion of the different donor strengths of the Pd atom, natural localized molecular orbital (NLMO) analyses were carried out for SiL2(μ-L)2PdCl2 (coordinates from optimized structure), ClSi(μ-L)4PdCl (coordinates from optimized structure), and [ClSi(μL)4PdNCMe]+ (coordinates from partially optimized structure) as representatives of the herein presented classes of compounds with capping (weak) Pd donor action and strong Pd donor function in neutral and cationic paddlewheel complexes with a Pd−Si bond, respectively (Table 2). In addition to the NLMO representative of the Pd−Si interaction the electron localization function (ELF) is shown in Figure 6. In SiL2(μ-L)2PdCl2 we find marginal contributions of the Si atom to this NLMO (1%), which thus has Pd lone-pair character. Visualization of the NLMO shows a polarization of

and the other compound is the chloroform solvate of (ClSi(μL)4PdCl)2(CHCl3)3 (Figure 4, right). 29Si solid-state NMR spectroscopy of the solid product revealed the presence of two different hexacoordinate silicon compounds (δ −135, −143). The more upfield signal was assigned to (ClSi(μL) 4PdCl) 2(CHCl3) 3 and the other signal to (ClSi(μL)4Pd)2(Cl)(GaCl4)(CHCl3)2. Thus, from the 2:1 stoichiometry of the reaction observed (i.e., from the amount of recovered starting material) we conclude that in chloroform SiL2(μ-L)2PdCl2 and GaCl3 (substoichiometric) react with formation of [(ClSi(μ-L)4Pd)2(Cl)]+ and [GaCl4]−, and upon addition of pentane this compound crystallizes and/or undergoes dissociation into ClSi(μ-L)4PdCl (which also crystallizes as a chloroform solvate) and other compounds. Unfortunately, despite the different packing in the chloroform solvate, the molecule of ClSi(μ-L)4PdCl again suffers severe disorder problems. Thus, for closer inspection of the structural features of ClSi(μ-L)4PdCl the results of the computationally predicted molecular structure could be more reliable. In the case of the ionic compounds (ClSi(μL)4PdNCMe)(GaCl4) and (ClSi(μ-L)4Pd)2(Cl)(GaCl4)(CHCl3)2 the crystal structures allow for reasonable discussion of molecular features. The optimized molecular structures of ClSi(μ-L)4PdCl and FSi(μ-L)4PdCl show the straight alignment of the 7-azaindolyl bridges parallel to the X−Si−Pd−Cl (X = Cl, F) axis, which clearly distinguishes these complexes from related methimazolyl bridged (4,0)-type paddlewheel complexes such as ClSi(μmt)4PdI,2x which exhibit noticeable propeller tilt of the bridging ligands. This feature is in general reflected by the crystal structures of ClSi(μ-L) 4 PdCl and (ClSi(μ-L) 4 PdCl) 2 (CHCl3)3. The apparent reason for this different positioning of the bridging moieties is the different separation of the lonepair donor atoms. The 7-azaindolyl N···N separation (2.40 Å) is noticeably shorter than the Pd−Si bond (2.62 Å, d(SiPd)/ 2484


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Table 2. Selected Data of the NLMO Analyses of the Pd−Si Interaction in SiL2(μ-L)2PdCl2, ClSi(μ-L)4PdCl, and [ClSi(μL)4PdNCMe]+ Pd hybrid SiL2(μ-L)2PdCl2 ClSi(μ-L)4PdCl [ClSi(μ-L)4PdNCMe]+

Si hybrid

% Pd orbital contribution

% Si orbital contribution

%s

%p

%d

%s

%p

%d

97.5 82.6 89.1

1.0 15.2 7.6

1.8 3.2 2.7

0.1 0.1 0.1

98.1 96.6 97.2

7.5 44.7 28.4

88.9 53.8 68.9

3.0 1.4 2.6

Figure 6. Color representations of the NLMO of the Pd−Si interaction (top) and of the ELF along the PdSi axis (bottom) of SiL2(μ-L)2PdCl2, ClSi(μ-L)4PdCl, and [ClSi(μ-L)4PdNCMe]+ (color scale for ELF diagrams: red 0.0, green 0.5, dark blue 0.9).

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CONCLUSION In the herein presented 2-, 3-, and 4-fold substituted 7azaindolylsilanes the 7-azaindol-1-yl substitution pattern was encountered and the nitrogen atoms in 7-position establish only weak cappings of tetrahedral faces of the Si coordination sphere. These silanes can serve as chelating ligands, as shown by formation of their PdCl2 complexes. Even though they replace the N-donor ligand acetonitrile from [PdCl2(NCMe)2], they did not replace the phosphine ligands in [PdCl2(PPh3)2], which is in sharp contrast to the reactivity of methimazolylsilanes such as Si(mt)4.2v In the compounds Me2Si(μ-L)2PdCl2, MeSiL(μ-L)2PdCl2, and SiL2(μ-L)2PdCl2 the Pd atom is always capping a tetrahedral face of the silicon coordination sphere. Even though computational analyses predict the isomer ClSi(μ-L)4PdCl to be energetically more favorable than SiL2(μ-L)2PdCl2, the formation of paddlewheel-shaped compounds of the type ClPd(μ-ligand)4SiX (X = Cl, F) with Pd→ Si donor−acceptor interaction is hampered in the case of ligand = 7-azaindolyl; once again this is in contrast to compounds with ligand = methimazolyl. Transformation into the paddlewheelshaped isomer ClSi(μ-L)4PdCl required elevated temperatures (150 °C) or the use of a chloride scavenger (GaCl3), whereas related methimazolyl-bridged paddlewheel complexes formed at room temperature and without addition of a chloride scavenger.

this lone pair toward the Si atom, thus hinting at a weak donor−acceptor interaction. Furthermore, the ELF map also indicates a weak electron density shift toward Si (green area on the Pd−Si axis). For the paddlewheel complexes ClSi(μL)4PdCl and [ClSi(μ-L)4PdNCMe]+ we find noticeably higher contributions of the Si atom to this NLMO (15.2% and 7.6%, respectively), and the graphical representations of both the NLMO and the ELF map show reasonable electron density localization on the Pd−Si bond axis. The Pd−Si bond in ClSi(μ-L)4PdCl is similar to that in the methimazolyl-bridged analogue ClSi(μ-mt)4PdCl, which has contributions of 84% Pd and 12% Si to the corresponding NLMO.2w Comparison of the NLMO and ELF of the two paddlewheel complexes ClSi(μL)4PdCl and [ClSi(μ-L)4PdNCMe]+ confirms that in the latter Pd is indeed a weaker donor. Both the noticeably smaller Si contribution to the NLMO and the less pronounced electron density shift toward Si (blue area on the Pd−Si axis in ClSi(μL)4PdCl versus green area for the corresponding part in [ClSi(μ-L)4PdNCMe]+) allow for this interpretation. In the order of decreasing lone-pair donor strength of the Pd atom (i.e., in the order ClSi(μ-L) 4 PdCl, [ClSi(μL)4PdNCMe]+, SiL2(μ-L)2PdCl2) we find systematic variation of the hybrid orbital contributions to this NLMO. That is, the already high d character of the Pd hybrid increases (at the expense of s character), and the Si contribution shifts from a close to ideal sp hybrid in ClSi(μ-L)4PdCl to a predominant p contribution in SiL2(μ-L)2PdCl2.

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EXPERIMENTAL SECTION

General Considerations. Chemicals commercially available were used as received without further purification. [PdCl2(NCMe)2] was prepared by stirring commercially available PdCl2 (3.00 g, 16.9 mmol)

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in acetonitrile (15 mL) at room temperature for 2 weeks, filtration, washing with acetonitrile (5 mL), and drying in a vacuum (yield: 4.30 g, 16.6 mmol, 98%). THF, hexane, and toluene were distilled from sodium benzophenone and were stored over sodium wire (hexane, toluene) or activated 3 Å molecular sieves under an argon atmosphere (THF). Chloroform, acetonitrile, and o-dichlorobenzene were stored over 3 Å molecular sieves. All reactions involving chlorosilanes and/or 7-azaindolylsilanes were carried out under an atmosphere of dry argon utilizing standard Schlenk techniques. Solution NMR spectra (1H, 13C, 29Si) were recorded on a Bruker DPX 400 MHz spectrometer (Me4Si as internal standard). 29Si solidstate (CP/MAS) NMR spectra were recorded on a Bruker Avance 400 WB spectrometer with 7 mm ZrO2 rotors and KelF inserts. 1H and 13 C NMR signal assignments adhere to the labels shown in Scheme 6.

122.9 (3), 128.3 (1), 130.1 (4), 142.9 (6), 153.5 (7). 29Si{1H} NMR (CDCl3, 79.5 MHz): δ 2.9. 29Si{1H} NMR (CP/MAS, 79.5 MHz): δ −1.9, −2.8. Anal. Found (%): C 65.1, H 5.8, N 19.1. Calcd for C16H16N4Si (M = 292.42): C 65.72, H 5.52, N 19.16. Synthesis of (7-Azaindol-1-yl)3SiMe (MeSiL3). A solution of 7azaindole (2.01 g, 17.0 mmol) and triethylamine (2.43 g, 24.0 mmol) in THF (50 mL) was stirred at room temperature, and methyltrichlorosilane (0.85 g, 5.7 mmol) was added dropwise via syringe. The white suspension thus obtained was heated to and stirred at 50 °C (for 2 h) and then cooled to room temperature. Thereafter, the triethylamine hydrochloride precipitate was filtered off and washed with THF (3 mL), and from the combined filtrate and washings the solvent was removed under reduced pressure (condensation into a cold trap). The residue was recrystallized from acetonitrile (4 mL), and the crystalline product was separated by decantation, washed with acetonitrile (2 × 1 mL), and dried under vacuum. Yield: 1.50 g (3.80 mmol, 67%). Mp: 153−155 °C. 1 H NMR (CDCl3, 400.1 MHz): δ 1.91 (s, 3 H, SiCH3), 6.50 (d, 3.6 Hz, 3 H, b), 6.65 (d, 3.6 Hz, 3 H, a), 7.09 (dd, 8.0 Hz, 4.8 Hz, 3 H, d), 7.90 (dd, 8.0 Hz, 1.6 Hz, 3 H, c), 8.24 (dd, 4.8 Hz, 1.6 Hz, 3 H, e). 13 C{1H} NMR (CDCl3, 100.6 MHz): δ −0.9 (SiCH3), 105.1 (2), 117.1 (5), 123.1 (3), 128.6 (1), 130.1 (4), 143.4 (6), 153.4 (7). 29 Si{1H} NMR (CDCl3, 79.5 MHz): δ −17.9. 29Si{1H} NMR (CP/ MAS, 79.5 MHz): δ −16.1. Anal. Found (%): C 67.0, H 4.6, N 21.2. Calcd for C22H18N6Si (M = 394.50): C 66.98, H 4.60, N 21.30. Synthesis of (7-Azaindol-1-yl)4Si (SiL4). A solution of 7-azaindole (2.04 g, 17.3 mmol) and triethylamine (2.14 g, 21.2 mmol) in THF (60 mL) was stirred at room temperature, and N-methylimidazole (10 drops) and tetrachlorosilane (0.73 g, 4.3 mmol) were added dropwise via syringe. The white suspension thus obtained was heated to and stirred at 50 °C (for 2.5 h) and then stored at 6 °C for 12 h. Thereafter, the triethylamine hydrochloride precipitate was filtered off and washed with THF (3 × 3 mL), and from the combined filtrate and washings the solvent was removed under reduced pressure (condensation into a cold trap). The residue was recrystallized from acetonitrile (10 mL) by heating in an oil bath and slow cooling of the Schlenk flask (which was stored in the oil bath), and after 12 h the crystalline product was separated by decantation, washed with acetonitrile (2 × 1 mL), and dried under vacuum. Yield: 1.27 g (2.56 mmol, 59%). Mp: decomposition without melting. 1 H NMR (CDCl3, 400.1 MHz): δ 6.59 (d, 3.6 Hz, 4 H, b), 6.93 (dd, 8.0 Hz, 4.8 Hz, 4 H, d), 7.49 (d, 3.6 Hz, 4 H, a), 7.80 (d, 8.0 Hz, 4 H, c), 7.94 (d, 4.8 Hz, 4 H, e). 13C{1H} NMR (CDCl3, 100.6 MHz): δ 105.6 (2), 117.1 (5), 123.1 (3), 128.3 (1), 132.0 (4), 143.0 (6), 153.5 (7). 29Si{1H} NMR (CDCl3, 79.5 MHz): δ −50.5. Anal. Found (%): C 67.5, H 4.8, N 22.5. Calcd for C28H20N8Si (M = 496.60): C 67.72, H 4.06, N 22.56. [Note: In a similar preparation, i.e., recrystallization of 5.3 mmol of crude product from 14 mL of acetonitrile, but with different cooling rate, because the hot Schlenk flask was removed from the oil bath, SiL4(MeCN) crystallized. Yield: 2.01 g (3.74 mmol, 71%).] Anal. Found (%): C 67.0, H 4.3, N 23.5. Calcd for C30H23N9Si (M = 537.65): C 67.02, H 4.44, N 22.83. 1H, 13C, and 29Si NMR data (CDCl3 solution) are consistent with SiL4 (see above) and contain the signals of MeCN in the expected intensity. 29Si{1H} NMR (CP/MAS, 79.5 MHz): δ −49.9. Synthesis of (ClSiL3)·(SiL4). A solution of 7-azaindole (1.00 g, 5.0 mmol) and triethylamine (1.12 g, 11.1 mmol) in THF (30 mL) was stirred at room temperature, and tetrachlorosilane (0.49 g, 2.9 mmol) was added dropwise via syringe. The white suspension thus obtained was heated to and stirred at 45 °C (for 2 h) and then cooled to room temperature. Thereafter, the triethylamine hydrochloride precipitate was filtered off and washed with THF (2 × 3 mL), and from the combined filtrate and washings two-thirds of the solvent was removed under reduced pressure (condensation into a cold trap). The remaining solution was layered with diethyl ether (20 mL) and stored at 6 °C. After 3 days the crystalline product was separated from the supernatant by decantation. Yield: 0.64 g. As confirmed by X-ray crystallography, this product contains crystals of (ClSiL3)·(SiL4). Furthermore, the 29Si NMR spectrum exhibited two peaks of similar

Scheme 6

Elemental analyses were performed using an Elementar Vario MICRO cube. Single-crystal X-ray diffraction data were collected on a STOE IPDS-2T diffractometer using Mo Kα-radiation. The structures were solved by direct methods using SHELXS-97 and refined with the fullmatrix least-squares method of F2 against all reflections with SHELXL97.10 All non-hydrogen atoms were anisotropically refined. Hydrogen atoms were isotropically refined in idealized positions (riding model). Graphics of molecular structures (Figures 1−5) were generated with ORTEP-311 and POV-Ray 3.62.12 Computational analyses were performed with the ADF2012.01 package.13 The molecular structures of SiL2(μ-L)2PdCl2, SiL2(μL)2PdClF (two isomers), ClSi(μ-L)4PdCl, and FSi(μ-L)4PdCl were optimized using the PBE14 functional in combination with a mixed basis set (TZ2P for Si and Pd, DZP for all the other elements) including scalar relativistic corrections. The single-point energies have been calculated with TZ2P for all the atoms, using the following levels of theory: PBE,14 BP,15 B3LYP,16 PBE0.14 For NLMO17 and ELF18 analyses, the optimized molecular structures of SiL2(μ-L)2PdCl2 and ClSi(μ-L)4PdCl (see above) and the partially optimized (H atoms only) structure of the cation [ClSi(μL)4PdNCMe]+ were used. For 29Si NMR calculations19 the reference molecule SiMe4 has been optimized using scalar-relativistic approximation (same level of theory as for the previously mentioned optimizations). 29Si NMR shifts were calculated with DFT PBE (ZORA20 QZ4P(Si, Pd), TZ2P(other atoms)) including spin−orbit approximation. Synthesis of (7-Azaindol-1-yl)2SiMe2 (Me2SiL2). A solution of 7azaindole (1.01 g, 8.6 mmol) and triethylamine (1.25 g, 12.4 mmol) in THF (30 mL) was stirred at room temperature, and dimethyldichlorosilane (0.55 g, 4.2 mmol) was added dropwise via syringe. The white suspension thus obtained was heated to and stirred at 50 °C (for 3 h) and then cooled to room temperature. Thereafter, the triethylamine hydrochloride precipitate was filtered off and washed with THF (2 × 2 mL), and from the combined filtrate and washings the solvent was removed under reduced pressure (condensation into a cold trap). The residue was recrystallized from acetonitrile (4 mL), and the crystalline product was separated by decantation, washed with acetonitrile (2 × 1 mL), and dried in a vacuum. Yield: 0.85 g (2.91 mmol, 69%). Mp: 86−87 °C. 1 H NMR (CDCl3, 400.1 MHz): δ 1.19 (s, 6 H, Si(CH3)2), 6.52 (d, 3.5 Hz, 2 H, b), 7.06 (dd, 7.9 Hz, 4.7 Hz, 2 H, d), 7.10 (d, 3.5 Hz, 2 H, a), 7.88 (dd, 7.9 Hz, 1.5 Hz, 2 H, c), 8.27 (d, 4.7 Hz, 2 H, e). 13C{1H} NMR (CDCl3, 100.6 MHz): δ −0.8 (Si(CH3)2), 103.9 (2), 116.5 (5), 2486


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intensity at δ −42.9 (ClSiL3) and −50.5 (SiL4), which hints at the gross product composition (ClSiL3)·(SiL4). The 1H NMR spectrum revealed the presence of large amounts of triethylamine hydrochloride in the product, and therefore melting point and elemental analysis have not been performed. As this product did not serve the purpose of our further investigations, we have not pursued its further purification yet. Synthesis of Me2Si(μ-L)2PdCl2. In a Schlenk flask Me2SiL2 (0.23 g, 0.80 mmol) and [PdCl2(NCMe)2] (0.21 g, 0.80 mmol) were stirred in acetonitrile (6 mL) for 12 h at room temperature. Thereafter, the light yellow solid was filtered, washed with acetonitrile (2 × 1 mL), and dried under vacuum. Yield: 0.34 g (0.72 mmol, 90%). Decomposition without melting occurred above 250 °C. Solution NMR spectroscopy was hampered by the very poor solubility of this compound in solvents such as chloroform, DMSO, acetonitrile, and THF. 29 Si{1H} NMR (CP/MAS, 79.5 MHz): δ 6.1. Anal. Found (%): C 40.7, H 3.3, N 12.0. Calcd for C18H16N4SiCl2Pd (M = 469.37): C 40.91, H 3.43, N 11.93. Crystals for X-ray diffraction analysis were grown by layering stoichiometric amounts of Me2SiL2 and [PdCl2(NCMe)2] with acetonitrile and keeping the Schlenk flask undisturbed for 1 day. Synthesis of MeSiL(μ-L)2PdCl2. In a Schlenk flask MeSiL3 (0.26 g, 0.65 mmol) and [PdCl2(NCMe)2] (0.17 g, 0.66 mmol) were stirred in acetonitrile (6 mL) for 12 h at room temperature. Thereafter, the light yellow solid was filtered, washed with acetonitrile (2 × 1 mL), and dried under vacuum. Yield: 0.34 g (0.59 mmol, 91%). Decomposition without melting occurred above 200 °C. Solution NMR spectroscopy was hampered by the very poor solubility of this compound in solvents such as chloroform, DMSO, acetonitrile, and THF. 29 Si{1H} NMR (CP/MAS, 79.5 MHz): δ −18.8. Anal. Found (%): C 45.9, H 3.6, N 14.7. Calcd for C22H18N6SiCl2Pd (M = 571.83): C 46.21, H 3.17, N 14.70. Crystals for X-ray diffraction analysis were grown by layering stoichiometric amounts of MeSiL3 and [PdCl2(NCMe)2] with acetonitrile and keeping the Schlenk flask undisturbed for 1 day. From the filtrate, upon storage for 2 weeks, an orange crystal of a decomposition product (L′PdCl2) was obtained, which is a PdCl2 complex of a bidentate ligand formed by addition of the NH group of 7-azaindole across the CN bond of acetonitrile. Details of this crystal structure can be found in the Supporting Information. Synthesis of SiL2(μ-L)2PdCl2. In a Schlenk flask SiL4·(MeCN) (0.19 g, 0.39 mmol) and [PdCl2(NCMe)2] (0.094 g, 0.39 mmol) were stirred in acetonitrile (10 mL) for 1 d at room temperature. Thereafter, the light yellow solid was filtered, washed with acetonitrile (2 × 1 mL), and dried under vacuum. Yield: 0.23 g (0.34 mmol, 94%). Decomposition without melting occurred above 290 °C. Solution NMR spectroscopy was hampered by the very poor solubility of this compound in solvents such as chloroform, DMSO, acetonitrile, and THF. 29 Si{1H} NMR (CP/MAS, 79.5 MHz): δ −51.0. Anal. Found (%): C 49.6, H 3.3, N 16.8. Calcd for C28H20N8SiCl2Pd (M = 673.92): C 49.90, H 2.99, N 16.63. Crystals for X-ray diffraction analysis were grown by layering stoichiometric amounts of SiL4·(MeCN) and [PdCl2(NCMe)2] with acetonitrile and keeping the Schlenk flask undisturbed for 1 day. Synthesis of ClSi(μ-L)4PdCl. In a sealed ampule SiL2(μ-L)2PdCl2 (0.19 g, 0.28 mmol) and o-dichlorobenzene (0.7 mL) were stored at 150 °C for 15 h and then stored at room temperature for 1 d. The resultant suspension was filtered, and the solid was washed with odichlorobenzene (1 mL), diethyl ether/o-dichlorobenzene, 3:1 (2 mL), and diethyl ether (2 mL) and dried in vacuo. Yield: 0.105 g (0.156 mmol, 56%). 29 Si{1H} NMR (CP/MAS, 79.5 MHz): δ −143.1. Anal. Found (%): C 50.2, H 3.1, N 16.5. Calcd for C28H20N8SiCl2Pd (M = 673.92): C 49.90, H 2.99, N 16.63. Crystals for X-ray diffraction analysis were found in the ampule and were isolated for analysis before filtration of the product suspension. Synthesis of [ClSi(μ-L)4PdNCMe][GaCl4]. In a Schlenk tube SiL2(μ-L)2PdCl2 (0.20 g, 0.30 mmol) and GaCl3 (53 mg, 0.30 mmol) were layered with chloroform (1 mL) and stored at room

temperature for 3 weeks. Thereafter, the solvent was removed in vacuo (trap condensation) and the residue was recrystallized from acetonitrile (9 mL). After storage at room temperature for 5 d the resultant solid was filtered off, washed with acetonitrile (1 mL), and dried in vacuo. Yield: 128 mg (0.144 mmol, 48%). 29 Si{1H} NMR (CP/MAS, 79.5 MHz): δ −143.0. Anal. Found (%): C 41.6, H 2.8, N 13.7. Calcd for C30H23N9SiCl5PdGa (M = 891.06): C 40.44, H 2.60, N 14.15. Crystals for X-ray diffraction analysis were found in the Schlenk tube and were isolated for analysis before filtration of the product suspension. Reaction of SiL2(μ-L)2PdCl2 with GaCl3 in Chloroform. In a Schlenk tube SiL2(μ-L)2PdCl2 (246 mg, 0.36 mmol) was stirred in chloroform (3 mL), and a solution of GaCl3 (20 mg, 0.11 mmol) in chloroform (1 mL) was added. The resultant suspension was stirred at room temperature for 26 h. Thereafter, the remaining yellow solid was filtered off and dried in vacuo. Yield: 98 mg (0.14 mmol) of the starting material SiL2(μ-L)2PdCl2. The Schlenk flask with the filtrate was then equipped with a bridge and a flask with n-pentane for gas phase diffusion of pentane into the filtrate. Within 2 d a mixture of crystalline solids formed, which were identified as (ClSi(μ-L)4Pd)2(Cl)(GaCl4)(CHCl3)2 and (ClSi(μ-L)4PdCl)2(CHCl3)3 by single-crystal X-ray analyses. The crystals were separated from the supernatant by decantation, washed with pentane (0.5 mL), and briefly dried in vacuo. Yield: 84 mg of product mixture. 29 Si{1H} NMR (CP/MAS, 79.5 MHz): δ −135.3; −143.4.

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ASSOCIATED CONTENT

S Supporting Information *

An ORTEP diagram of the decomposition product L′PdCl2, CIF files with crystallographic details of the crystal structures reported in this paper, tables with atomic coordinates of the optimized molecular structures of SiL2(μ-L)2PdCl2, SiL2(μL) 2PdClF (two isomers), ClSi(μ-L)4PdCl, and FSi(μL)4PdCl, and 29Si CP/MAS NMR spectra of the products obtained from SiL2(μ-L)2PdCl2 upon heating in o-dichlorobenzene and upon reaction with GaCl3 in chloroform. This material is available free of charge via the Internet at http:// pubs.acs.org. The crystallographic data for Me2SiL2 (CCDC 977928), MeSiL3 (CCDC 977921), SiL4 (CCDC 977924), SiL4·(MeCN) (CCDC 977922), ClSiL3·SiL4 (CCDC 977923), [(L′)PdCl2]2·MeCN (CCDC 977927), Me2Si(μ-L)2PdCl2 (CCDC 977929), MeSiL(μ-L)2PdCl2 (CCDC 977926), SiL2(μ-L)2PdCl2 (CCDC 977925), ClSi(μ-L)4PdCl (CCDC 995082), (ClSi(μ-L)4PdCl)2(CHCl3)3 (CCDC 995083), (ClSi(μ-L)4PdNCMe)(GaCl4) (CCDC 995084), and (ClSi(μ-L)4Pd)2(Cl)(GaCl4)(CHCl3)2 (CCDC 995081) may also be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request.cif.

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AUTHOR INFORMATION

Corresponding Author

*Fax: +49 3731 39 4058. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Dedicated to the memory of Professor Daniel Kost, a precious colleague and friend and a marvelous researcher in silicon coordination chemistry. We thank the reviewers for their helpful suggestions, which added significant value to the content of this paper. 2487


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