Zwitterionic Alkali-Metal Silanides of Tripodal Ligand Geometry

Article pubs.acs.org/IC

Zwitterionic Alkali-Metal Silanides of Tripodal Ligand Geometry: Synthesis, Structure, and Lewis Acid−Base Chemistry Vidura D. Thalangamaarachchige, Hui Li, David B. Cordes, Daniel K. Unruh, and Clemens Krempner* Department of Chemistry & Biochemistry, Texas Tech University, Box 1061, Lubbock, Texas 79409-1061, United States S Supporting Information *

ABSTRACT: A unique family of zwitterionic alkali-metal silanides of general formula [Si(SiMe2OR)3M] (M-3), where M = Li, Na, K and R = CH2CH3 (M-3b), CH(CH3)2 (M-3c), CH 2 CH(CH 3 ) 2 (M-3d), CH 2 CH 2 OSCH 3 (M-3g), CH2CH2N(CH3)2 (M-3h), have been synthesized and their structures fully characterized. Compounds M-3 were prepared from reactions of the corresponding alkali-metal alkoxides, R′OM (R′ = But, Bui; M = Li, Na, K) with the silanes Si(SiMe2OR)4 (2a−h), where R = CH2CH3 (b), CH(CH3)2 (c), CH2CH(CH3)2 (d), CH2CH2OSCH3 (g), CH2CH2N(CH3)2 (h), via selective Si−Si bond cleavage. Analysis of the X-ray data of Li-3a−e,g, Li-h, Li-4, Na-3a−c,e,f,h, and K-3b,d,e,g revealed two major structural motifs in the solid state: infinite chain type and monomeric zwitterions. The reaction of Si(SiMe2OCH2CH3)3Li (Li-3b) with BPh3 in THF gave the zwitterionic Lewis acid−base complex Ph3BSi(SiMe2OCH2CH3)3Li· THF (Li-5b). Reactions of W(CO)6 with Li-3b−d and Na-3b,c resulted in the formation of the zwitterionic silyl tungstenate complexes (CO)5WSi[SiMe2OCH2CH3]3Li (Li-6b), (CO)5WSi[SiMe2OCH(CH3)2]3Li (Li-6c), (CO)5WSi[SiMe2OCH2CH(CH3)2)]3Li (Li-6d), (CO)5WSi(SiMe2OCH2CH3)3Na (Na-6b), and (CO)5WSi[SiMe2OCH(CH3)2]3Na (Na-6c), respectively. The solid-state structures of Li-5b and Na-6b were determined by X-ray crystallography.

1. INTRODUCTION Silyl anions of the alkali metals, also known as alkali-metal silanides, are of fundamental importance as synthetic intermediates, as reagents, and as anionic spectator ligands in organometallic and inorganic synthesis.1 In particular, sterically encumbered organosilyl-substituted silyl anions of general formula (R3Si)3Si− (R = alkyl, aryl) proved to be very useful σ-donating ligands, as the steric profile and the reactivity of these anions can easily be tailored by a proper choice of the SiR3 groups. The ease by which these silanides are synthesized has led to some exciting developments in group 14 chemistry ranging from the synthesis of disilenes, digermenes, germylenes, stannylenes, and plumbylenes2 to silyl radicals and battery materials,3 polysilane dendrimers,4 and novel group 14 clusters.5 Conversely, alkali-metal silanides that contain pendant organic donor groups have been far less developed than their nonfunctionalized counterparts.6 This is primarily due to the lack of suitable synthetic methods resulting from the incompatibility of most organic functional groups with highly localized anionic charges, as is the case for silyl anions.1 Nonetheless, there is increasing interest in these multidentate ligand systems, owing to their ability to coordinate to maingroup and transition-metal centers in various binding modes. The interaction of such a silicon-based monoanionic ligand scaffold with a metal cation can be described as follows: the silyl anion, a strong and relatively “soft” σ-donor, along with the pendant neutral donor groups bind to the metal cation, © 2017 American Chemical Society

resulting in a discrete metal/ligand framework with a largely polar covalent metal−silicon bond.7 Alternatively, the metal center is fully charge separated from the silyl anion via the pendant donor groups which exclusively coordinate to the metal cation, while insulating the metal cation from the silyl anion through the linker groups. This results in the formation of a zwitterionic metal silanide with a stereochemically active electron pair localized at the silyl anion (referred to as the “naked” silyl anion) and a metal cation that can adopt various coordination geometries depending on the number and electronic nature of the donor groups (Chart 1).8 Our laboratory has recently reported a synthetic and structural study of two types of alkali-metal silanides of formula [Si(SiMe2OMe)3M]n and [Si(SiMe2OCH2CH2OMe)3M]n, Chart 1. Structures of Discrete Zwitterionic Alkali-Metal Silanides

Received: June 1, 2017 Published: July 31, 2017 9869

DOI: 10.1021/acs.inorgchem.7b01227 Inorg. Chem. 2017, 56, 9869−9879

Article

Inorganic Chemistry Chart 2. Structures and Coordination Modes of Aggregated Donor-Substituted Alkali-Metal Silanidesa

a

Methyl groups at silicon and oxygen omitted for clarity.

where M = Li, Na, K.8a,c We noticed that these zwitterions have the propensity to aggregate with formation of dimers, tetramers, or even polymers. This is particularly the case when the corresponding cations are large and additional external donors (such as donor solvents) that can bind to the cation are not available (Chart 2). In an attempt to understand factors that govern the degree of aggregation and charge separation as well as the coordination geometry of organometallic zwitterions, we summarize herein our efforts in synthesizing discrete zwitterionic alkali-metal silanides that contain various pendant donor groups ranging from simple alkoxy groups of different steric profiles (OR = OMe, OEt, OPri, OBui) to polydonor groups containing various heteroatoms (OR = OCH2CH2OMe, OCH2CH2SMe, OCH2CH2NMe2).

through treatment of Si(SiMe2Cl)4 with HOCH2CH2OMe in the presence of NEt3 as base at room temperature.8c With this optimized synthetic protocol in hand, we treated Si(SiMe2Cl)4 with an excess of the primary alcohols HOCH3, HOCH2CH3, HOCH2CH2SMe, and HOCH2CH2NMe2, respectively, in the presence of 4.5 equiv of NEt3 as base and hexanes as solvent. Filtration of the reaction mixture followed by vacuum distillation afforded the alkoxy-substituted silanes 2a,b,e−h in good to excellent isolated yields as colorless liquids. The synthesis of 2c,d was considerably more challenging, as isopropyl alcohol and isobutyl alcohol are sterically more demanding nucleophiles in the reaction with Si(SiMe2Cl)4. Thus, under conditions similar to those mentioned above, a mixture of incompletely substituted alkoxysilanes was observed. Performing the reaction in neat alcohol (ca. 40-fold excess) and in the presence of 4.5 equiv of NEt3 as base, however, furnished after 3−4 days at room temperature the alkoxysilanes 2c,d as colorless liquids in excellent isolated yields upon distillation under vacuum. Compounds 2a−h are moisture-sensitive but air-stable liquids or semisolids that can be synthesized easily on a 5 g scale and stored under nitrogen indefinitely without any sign of decomposition. They were fully characterized by 1H, 13 C, and 29Si NMR spectroscopy and the results of elemental analysis. In analogy to our previous experiment, 2g,h underwent Si−Si bond cleavage9,10 in THF as solvent upon adding 1 equiv of LiOBut, NaOBut, and KOBut, respectively, as smoothly and selective as 2a,e,f. After a few hours at room temperature, THF solutions of the silanides M-3g,h were formed almost quantitatively as judged by 1H NMR spectroscopy. The Si−Si bond cleavage of 2b−d proved to be equally selective but significantly more time consuming, presumably as a result of the increased size of the alkoxy groups, preventing the tertbutoxide nucleophile from readily approaching the central silicon of Si(SiMe2OR)4. For example, with 2c,d 3−6 days at room temperature was required to achieve full conversion. Increasing the temperature did not improve the yields; instead, additional side products were observed. Interestingly, reaction of 2d with LiOBut in THF did not give the expected product Li-3d; rather, a mixture of lithium silanides resulting from partial nucleophilic exchange at SiMe2OBui side arms was observed. None of these compounds could be isolated from the reaction mixture; however, by crystallization from hexanes single crystals suitable for X-ray analysis were obtained, which were identified as Li-4 (for a structural discussion, see below).

2. RESULTS AND DISCUSSION 2.1. Synthesis. As mentioned above, we have recently reported a simple, two-step synthetic route to the alkali-metal trimethoxyhypersilanides M-3a (M = Li, Na, K) in high overall yields. The precursor silane Si(SiMe2Cl)4 was treated with HC(OMe)3 in the presence of catalytic amounts of AlCl3 to generate the silane Si(SiMe2OMe)4 (Scheme 1). After being Scheme 1. General Synthetic Scheme to the Alkali Metal Silanides M-3a−h (M = Li, Na, K)a

a

Legend: (i) excess ROH, 4.5 equiv of NEt3.

purified by distillation, Si(SiMe2OMe)4 was subsequently treated with MOBut (M = Li, Na, K) to furnish the silanides M-3a in excellent yields.8a Extension of this synthetic protocol to the preparation of sterically more demanding derivatives Si(SiMe2OR)4, where R = Et, Pri, failed. In part this is due to the very slow conversion with HC(OEt)3 and HC(OPri)3 as alkoxylation reagents (several weeks under reflux) and the required harsh conditions, resulting in partial decomposition. In a second communication we reported an optimized protocol toward the high-yield synthesis of Si(SiMe2OCH2CH2OMe)4 9870


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Inorganic Chemistry

X-ray data for Na-3c,g and K-3d was too low to allow for a detailed discussion of the distances and angles; connectivity, however, could clearly be confirmed. 2.2.1. Aggregation. With the exception of the tetrameric silanide K-3a,11 all structurally characterized alkali-metal silanides display polymeric or monomeric structures in the solid state, both featuring a central bicyclooctane unit, Si(SiO) 3M. They can be classified according to their aggregation behavior and the coordination number of the metal cation as structural types A−F (Chart 3). Compounds Li3a,b, Na-3b,c, and K-3b,d (type D) feature linear infinite chains in which the monomeric subunits are held together (head to tail) via intermolecular Si−M bonds, resulting in coordination number 4 for the metal cations. Slightly different is the sterically more demanding lithium silanide Li-3c, which still forms a linear infinite chain, but with negligible Si···Li contacts (4.17 Å). Compound Na-3a (type F), on the other hand, forms an infinite chain structure with zigzag conformation (head to head) with two structurally different sodium cations, one being tetracoordinated by two silyl anions and two THF molecules (for clarity, THF molecules are not shown in Figure 1). The second sodium cation is hexacoordinated and fully chargeseparated from the silyl anions via the methoxy donor groups.8a This coordination mode is quite unusual and so far has only been seen in the discrete divalent rare-earth-metal- and alkalineearth-metal-containing zwitterions of formula [Si(SiMe2OMe)3]2M, where M = Mg2+, Ca2+, Sr2+, Ba2+, Yb2+, Eu2+, Sm2+.8d,h Thus, the only truly monomeric alkali-metal silanides in the solid state are Li-3d and Li-4 (type E). Compounds Li-3e,g,h, Na-3e,f−h, and K-3g, all decorated with O,O-, O,S-, and O,Ncontaining polydonor groups, form discrete zwitterionic structures, except K-3e, which features a linear infinite chain structure.8c,12 Whether these pendant polydonor groups fully coordinate to the central metal cation or not appears to depend on their identity. For example, K-3e (type B) contains pendant MeO groups that fully coordinate, though it forms a linear infinite chain in the solid state. Strong intermolecular Si···K (3.66 Å) contacts lead to a coordination number of 7 for the potassium cation, which results in relatively long intramolecular K···Si (4.50 Å) distances. The MeS-substituted silanide K-3g (type A), on the other hand, is monomeric with the central potassium cation being hexacoordinated and consequently with a shorter intramolecular K···Si contact of 3.69 Å. The “softer” MeS donors appear to interact more strongly with the relatively “soft” Lewis acid K+ in an octahedral coordination environ-

However, to have access to lithium silanide Li-3d, silane 2d was treated with LiOBui (derived from deprotonation of isobutyl alcohol with n-BuLi at low temperature). Gratifyingly, after reaction for 3 days at room temperature, Li-3d was isolated as a highly hexanes soluble crystalline material, in almost quantitative yield as judged by 1H NMR spectroscopy (eq 1).

All isolated silanides are highly air and moisture sensitive solids that to our surprise showed very different solubilities in common organic solvents such as hexanes, benzene, THF, and ether. For example, while the silanides Li-3c−h, Li-4, Na-3e−h, and K-3e−h show moderate to good solubility in hexanes, benzene, or toluene, Li-3a,b, Na-3a−d, and K-4a−d are insoluble in hexanes and only sparingly soluble in benzene or toluene. It is also worth noting that reactions of Si(SiMe3)4 with NaOBu t and LiOBu t , respectively, do not give (Me3Si)3SiLi and (Me3Si)3SiNa under similar conditions; even after 3 days at room temperature no reaction occurred. Apparently, the alkoxy donor groups in 2a−h increase the electrophilicity of the silicon atoms of the SiMe2OR groups, facilitating selective Si−Si bond cleavage by nucleophilic attack of the tertiary butoxide anion. The increased rate by which the Si−Si bond is cleaved could also be attributed to the coordination of the SiMe2OR groups to the cation of the metal alkoxide, which would result in an increased nucleophilicity of the alkoxide anion. 2.2. Structural Characterization. All newly synthesized alkali-metal zwitterions were characterized in solution by 1H, 13 C, 29Si, 23Na, and 7Li NMR spectroscopy and were subjected to combustion analysis. The 29Si NMR chemical shifts of the “naked” silyl anion of M-3a−h are summarized in Table 1 along with the nonzwitterionic silanides, MSi(SiMe3)3. For M-3a−d, the silicon NMR signals are shifted to higher field in the order Na < K < Li, while for M-3e−h the order in chemical shift corresponds to K < Na < Li. Relatively narrow silicon NMR chemical shift ranges were noted for each countercation regardless of the nature of the pendant donor groups. In addition to being characterized by multinuclei NMR spectroscopy, Li-3b−d,g,h, Li-4, Na-3b,c,g,h and K-3b,d,g were structurally determined by single-crystal X-ray crystallography. For comparison the results are shown in Figures 1 and 2 along with the previously published structures Li-3a,e, Na-3a,e,f, and K-3e. Selected average bond lengths and angles of all relevant compounds can be found in Tables 2 and 3. The quality of the

Table 1. 29Si-NMR Chemical Shifts δ (ppm) of the Central Silyl Anion (in Boldface) of Selected Alkali-Metal Silanides at 300 K (C6D6 or THF-D8)

a

compound

M = Li

M = Na

M=K

Si[SiMe2OMe]3M (M-3a) Si[SiMe2OEt]3M (M-3b) Si[SiMe2OPri]3M (M-3c) Si[SiMe2OBui]3M (M-3d) Si[SiMe2O(CH2CH2OMe]3M (M-3e) Si[SiMe2O(CH2CH2CH2OMe]3M (M-3f) Si[SiMe2O(CH2CH2SMe]3M (M-3g) Si[SiMe2O(CH2CH2NMe2]3M (M-3h) (Me3Si)3SiM

−217.2 −214.5b −216.6b −220.1c −215.3c,d −215.4d,c −215.7c −214.1c −181.9a,b

−199.4 −199.7b −198.8b −199.7b −204.5c,d −200.3d,c −200.6c −203.4c −179.8a,b

−199.5a,c −200.5b −201.0b −201.0b −194.1c,d −194.7c,d −193.8c −194.6c −185.7a,b

a,b

a,b

Taken from ref 8a. bMeasured in THF-D8; cMeasured in C6D6; dTaken from ref 8c. 9871


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Inorganic Chemistry

Figure 1. Solid-state structures of Li-3a−d, Li-4, Na-3a,b, and K-3b.

ment, preventing the structure from aggregating via intermolecular K···Si contacts. The sodium-containing zwitterions Na-3e−h (type A) exclusively feature discrete structures with all three pendant polydonors coordinating to the sodium cation, whose coordination sphere is best described as distorted octahedral. Structurally more diverse are the lithium derivatives, as the lithium cation, a “hard” Lewis acid of small size, shows coordination numbers 4 and 6. For example, the lithium ion of MeOCH2CH2O-containing silanide Li-3e (type A) is hexa c o o r d i na ted , w h i le in t he M e SCH 2 CH 2 O- and

Me2NCH2CH2O-substituted silanides Li-3g,h (type C), lithium is in a tetrahedral coordination environment with only one of the three MeS or Me2N donors coordinating (Figure 1). Nonetheless, temperature-dependent 1H NMR spectroscopic investigations in solution (THF-D8 and toluene-D8) suggest all three Me2NCH2CH2O or MeSCH2CH2O donor groups to be equivalent. Even at −80 °C no spectral changes were detected, indicating either a fluxional process with an on−off coordination of the polydonor groups or equal binding of all three donor arms to the central lithium cation, resulting in a 9872


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Inorganic Chemistry

Figure 2. Solid-state structures of Li-3e,h,g, Na-3h,e,f, and K-3e,g.

Table 2. Selected Average Distances (Å) and Average Angles (deg) of Li-3a−e,g,h and Li-4 CN of Li Si−Si Si−O Li−O(Si) Si−Si−Si Si−O−Li O−Li−O(Si) Si−Si−O

Li-3a

Li-3b

Li-3c

Li-3d

Li-3e

Li-3g

Li-3h

Li-4

4 2.32 1.70 1.97 98 116 107 110

4 2.33 1.69 1.97 97 118 105 110

3 2.31 1.70 1.92 100 110 114 113

3 2.32 1.71 1.89 99 107 116 111

6 2.32 1.69 2.08 96 126 95 113

4 2.32 1.70 1.95 99 113 104 111

4 2.32 1.70 1.94 98 115 109 112

3 2.33 1.70 1.91 101 103 119 110

2.2.2. Bond Parameters. We analyzed the structural parameters of the central −Si(SiO)3M+ bicyclooctane unit; the corresponding average distances and angles are shown in Tables 2 and 3. Notably, the overwhelming majority of the structurally characterized silanides have relatively short average Si−Si distances of 2.32 Å, regardless of the identity and coordination number of the cation. Only a few of these structures have Si−Si distances deviating from 2.32 by ±0.01 Å. These distances are slightly shorter than the average values for the recently reported compounds (Me3Si)3SiM(donor)n (M = Li, Na, K; average 2.34 Å), Si(SiMe3)4 (average 2.36 Å), and HSi(SiMe3)2SiMe2R (2.34 Å).13 The Si−O bonds, on the other hand, are elongated, ranging from 1.67 to 1.71 Å, with the majority of the Si−O distances being within the range of 1.68− 1.70 Å. A shortening of the Si−Si bonds and an elongation of the Si−O bonds are in good agreement with stabilizing negative hyperconjugative interactions between the lone pair (LP) p orbital of the silicon and the relatively low lying σ* orbitals of the Si−OR bonds. The oxygen of the OR group is nearly in an antiperiplanar orientation relative to the LP (LP−Si−Si−OR torsion angles of ca. 165−170°), which strengthens orbital overlap and shortens the Si−Si bonds in Li-3e, Na-3e, and K3e. According to our previous DFT calculations using second-

Table 3. Selected Average Distances (Å) and Average Angles (deg) of Na-3a,b,e,f,h and K-3b,e,g Na-3a CN of Na Si−Si Si−O Na−O(Si) Si−Si−Si Si−O−Na O−Na−O(Si) Si−Si−O CN of K Si−Si Si−O K−O(Si) Si−Si−Si Si−O−K O−K−O(Si) Si−Si−O

6 2.32 1.67 2.42 103 125 94 113

Na-3b 4 2.32 1.67 2.31 102 116 100 111 K-3b 4 2.31 1.68 2.66 106 113 95 114

Na-3e 6 2.32 1.68 2.32 99 122 95 113 K-3e 7 2.32 1.68 2.74 100 125 80 113

Na-3f 6 2.32 1.68 2.35 101 124 93 113

Na-3h 6 2.31 1.69 2.35 100 121 95 114 K-3g 6 2.31 1.69 2.63 109 111 101 116

hexacoordinated zwitterion in solution, as is the case for the sodium species Na-3e−h. 9873


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Inorganic Chemistry Chart 3. Coordination Modes of Zwitterionic Alkali-Metal Silanides in the Solid Statea

a

Organic substituents at silicon and oxygen are omitted for clarity.

order perturbation theory analysis of the NBOs, these stabilizing hyperconjugative interactions are on the order of 35−40 kcal/mol.12 We also noticed that the lithium-containing silanides have Si−O bonds slightly longer than those of sodium and potassium (Tables 2 and 3). This is probably due to the ability of lithium to polarize the Si−O bonds more strongly than the larger cations sodium and potassium, resulting in stronger O→ Li donor−acceptor interactions in comparison to those of O→ Na and O→K. The Li−O distances of the lithium silanides were found to be a sensitive function of the coordination number of the lithium cations, with the Li−O distances steadily increasing as the coordination number of lithium increases from ca. 1.89 Å (CN = 3) for Li-3d to 2.08 Å (CN = 6) for Li-3e. Similar trends can be anticipated for the respective sodium and potassium silanides; however, there were not enough X-ray data available to confirm such a trend. 2.2.3. Inter- and Intramolecular Si−M Distances. Table 4 shows the intra- and intermolecular Si−M distances of all structurally characterized zwitterionic silanides M-3 along with Si(3,5-Me2-pz)3Li·THF, the only other structurally characterized zwitterionic alkali-metal silanide of tripodal ligand geometry.8b For comparison, the Si−M distances of selected alkali-metal silanides, [(Me3Si)3SiM(donor)x]y (M = Li, Na, K), are also included. As expected, these nonzwitterionic species have the metal cation and silyl anion in close proximity, owing to attractive electrostatic interactions, with Si−M distances being primarily a function of the size and the coordination number of the metal cation. Similarly, the aggregated structures Li-3a,b, Na-3a,b, and K-3b have relatively short intermolecular Si−M distances, suggesting significant intermolecular ionic bond interactions between the metal cation and silyl anion in the solid state. The silanides Li-3c,d,e,g,h, Li-4, Na-3e,f,h, and K-3g, on the other hand, do not exhibit intermolecular Si···M contacts; their intramolecular Si···M distances are fairly long, ranging from 2.97 Å (Si···Li) for Li-4 up to 4.00 Å (Si···Na) for Na-3e. Each of these values is clearly beyond those commonly expected to be found in compounds with Si−Li, Si−Na, and Si−K bonds, clearly confirming the zwitterionic nature of these discrete metal silanides. Note also that the intramolecular Si···M distances in both the aggregated and the monomeric structures appear to correlate relatively well with the coordination number of the respective metal cation, which is particularly apparent for the lithium series: that is, the higher the coordination number

Table 4. Inter- and Intramolecular M−Si Distances (Å) of Selected Alkali-Metal Silanides M−Si (intra) (But2MeSi)3SiLia (Me3Si)3SiLib (Me3Si)3SiLi(THF)2c (Me3Si)3SiLi(THF)3d Li-4 Li-3d Li-3c Si(3,5-Me2-pz)3Li·THFe Li-3g Li-3h Li-3a Li-3b Li-3e [(Me3Si)3Si(μ-Na)]2f Na-3b Na-3h Na-3a Na-3f Na-3e (Me3Si)3SiK(η6-C6H6)3f K-3g K-3b K-3e a

M−Si (inter)

2.53 2.57 2.60 2.64 2.97 3.15 3.20 3.26 3.38 3.45 3.49 3.59 3.92 2.94 3.70 3.95 3.96 3.98 4.00 3.34 3.69 3.87 4.50

4.17

2.77 2.86

2.98 3.02

3.41 3.66

CN of M 1 1 3 4 3 3 3 4 4 4 4 4 6 2 4 6 6 6 6 4 6 4 7

ref 14. bref 15. cref 16. dref 17. eref 8b. fref 18.

of Li, the longer the Si−M distances. That Li-3a,b have longer Si···Li distances than Li-3g,h and Breher’s Si(3,5-Me2-pz)3Li· THF8b can largely be attributed to the fact that the lithium cations in the former zwitterions are coordinated by a monoanionic silyl ligand known to be a strong σ donor, while in the latter zwitterions the lithium cations are exclusively coordinated by neutral donor groups. 2.3. Lewis Acid−Base Chemistry. We have recently demonstrated that zwitterionic alkali-metal silanides with pendant donor groups such as Li-3e, Na-3e, and K-3e are readily accessible for Lewis acid−base chemistry. We also reported in previous publications that reactions of Li-3e and K3e with B(C6F5)3 gave the zwitterionic Lewis acid−base complexes Li-5e8c and K-5e,8e respectively, with the latter being structurally characterized by X-ray crystallography (Scheme 2). The reaction of the carbon analogue C9874


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Inorganic Chemistry Scheme 2. Lewis Acid−Base Chemistry of K-3e and Li-3b

(SiMe2OCH2CH2OMe)3K19 with B(C6F5)3, on the other hand, resulted in a mixture of unidentified products and partial decomposition, and the sodium derivative C(SiMe2OCH2CH2OMe)3Na did not react with various boranes such as BPh3, BEt3, HBMes2, and FBMes2 to form the expected Lewis acid−base adducts. Reaction of these so-called frustrated Lewis pairs (FLPs) with dihydrogen resulted, however, in the heterolytic cleavage of dihydrogen to give borohydride salts.20,21 Therefore, we were interested in investigating whether or not the newly prepared alkali-metal silanides would react with selected moderately to weakly Lewis acidic organoboranes. Upon reaction of Li-3b with BPh3 in THF as solvent a crystalline product was isolated, which by multinuclei NMR spectroscopy, combustion analysis, and the results of a singlecrystal X-ray analysis was identified as the zwitterionic Lewis acid−base complex of formula Ph3BSi(SiMe2OEt)3Li·THF (Li5b). Its 11B{1H} NMR spectrum shows a distinct signal at −10.7 ppm clearly indicative of a tetrahedrally coordinated boron center. The signal of the central silicon in the 29Si NMR spectrum appeared at −155.5 ppm, significantly shifted to lower field relative to the respective signal for Li-3b (−214.5 ppm). The analysis of the X-ray data (Figure 3) further confirmed the structural integrity of Li-5b consistent with the assumed formula Ph3B−Si(SiMe2OEt)3Li·THF. The Si−B distance of Li-5b (2.15 Å) is somewhat shorter than that of K-5e (2.17 Å) but still longer than those of the structurally related adducts L→SiCl2→B(C6F5)3 (2.11 Å)22 and [Ph(NBut)CNBut]SiCl→B(C6F5)3 (2.11 Å).23 The greater Si−B distance in Li5b is perhaps the result of steric repulsion occurring between the metal silanide and BPh3 units, as the Si−B distance in the sterically less demanding [(Me3Si)3SiBH3K·THF]2 has been reported to be significantly shorter (1.99 Å).24 As a result of adduct formation the Si−Si distances (average 2.35 Å) have slightly increased, while the Si−O distances (average 1.70 Å) remained largely unchanged relative to those of Li-3b. The lithium cation of Li-5b is coordinated by the three ethoxy donors and one molecule of THF, which gives rise to a distorted-tetrahedral coordination environment for the lithium cation with O−Li−O angles and Li−O distances ranging from 104 to 120° and 1.97 to 2.00 Å, respectively. We next studied reactions of various alkali-metal silanides with the weak Lewis acid BEt3. In none of these cases was formation of Lewis base adducts observed and attempts to cleave hydrogen heterolytically with these FLPs failed. That the silanides do not engage in hydrogen cleavage with BEt3 appears to be due to their lower basicity in comparison to those of the more basic carbon-based zwitterion C(SiMe2OCH2CH2OMe)3Na.12,21 Finally, the bulkier and more Lewis acidic borane BMes3 was treated with various metal silanides, respectively, resulting in all cases in deeply colored solutions upon mixing at room temperature. NMR analysis of these solutions suggested radical species to be involved. It is very likely that a single electron transfer from the electron-rich silanide to BMes3 had occurred, presumably

Figure 3. Solid-state structure of Li-5b. Selected distances (Å) and angles (deg): Si1−B1 2.150(3), Si1−Si4 2.337(1), Si1−Si2 2.348(1), Si1−Si3 2.351(1), Si2−O2 1.699(2), Si3−O3 1.697(2), Si4−O4 1.694(2), O1−Li1 1.973(6), O2 Li1−1.998(5), O3−Li1 1.988(6), O4−Li1 1.972(6); B1−Si1- Si4 116.5(1), B1−Si1−Si2 117.4(1), B1− Si1−Si3 117.7(1), Si4−Si1−Si2 99.7(1), Si4−Si1−Si3 103.0(1), Si2− Si1−Si3 99.7(1), O2−Si2−Si1 104.5(1), O3−Si3−Si1 105.0(1), O4− Si4−Si1 105.7(1), Si2−O2- Li1 116.4(2), Si3−O3−Li1 115.9(2), Si4O4−Li1 116.6(2), O4−Li1−O1 104.3(2), O4−Li1−O3 108.4(3), O1−Li1−O3 112.1(3), O4−Li1−O2 106.2(3), O1−Li1−O2 119.8(3), O3−Li1−O2 105.3(2).

resulting in the formation of the radical anion •BMes3−.25 Note that persilylated silyl anions can be good single electron donors, as the corresponding silyl radicals are stabilized by the surrounding silyl groups.26 We recently reported the synthesis and NMR characterization of the zwitterionic silyl tungstenate complex Li-6e8c according to Scheme 3, but unfortunately we were not able to Scheme 3. Synthesis of the Zwitterionic Silyl Tungstenate Complexes M-6

grow crystals suitable for an X-ray analysis. Therefore, reactions of selected alkali-metal silanides with W(CO)6 were reinvestigated. They proceeded smoothly at room temperature with loss of one molecule of CO to quantitatively produce the complexes Li-6b−d and Na-6b,c, respectively (Scheme 2). Their structures were unambiguously confirmed by multinuclear NMR and IR spectroscopy as well as the results of elemental analysis. In the 29Si NMR spectra of Li-6b−d and Na-6b,c, the signals of the central anionic silicon nuclei showed significant 9875


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Figure 4. Network structure of Na-6b. Disordered tungsten atoms and all H atoms are omitted for clarity. Color scheme: green, silicon; black, carbon; orange, sodium; red, oxygen; purple, tungsten.

Figure 5. Solid-state structure of Na-6b (disordered tungsten W2B omitted for clarity). Selected distances (Å): W2A−Si5 2.69(2), W1−Si1 2.68(2), Si1−Si4 2.33(1), Si1−Si3 2.33(1), Si1−Si2 2.34(1), Si5−Si7 2.33(1), Si5−Si8 2.34(1), Si5−Si6 2.34(1), Si2−O1 1.68(1), Si3−O3 1.68(1), Si4−O2 1.69(1), Si6−O5 1.69(1), Si7−O4 1.66(1), Si8−O6 1.67(1), W1−C25 1.98(1), W1−C26 1.98(1), W1−C28 2.01(1), W1−C29 2.02(1), W1−C27 2.06(1), W2A−C34 1.94(1), W2A−C30 1.97(1), W2A−C31 1.98(1), W2A−C32 1.99(1), W2A−C33 2.02(1).

downfield shifts (−152.0 to −154.2 ppm) relative to the respective metal silanides. The 13C NMR spectra of Li-6b−d and Na-6b,c each showed two resonances in a 4:1 ratio ranging from 204.8 to 205.8 ppm due to the carbonyl groups in the cis position and from 206.4 to 207.5 ppm due to the carbonyl groups in the trans position. The presence of the sodium and lithium cations in these compounds was confirmed by 23Na and 7 Li NMR spectroscopy. The IR spectra (CH2Cl2) of the carbonyl region of Li-6b−d and Na-6b,c display four bands each ranging from 1868 to 2039 cm−1. These values are characteristic of M[W(CO)5X] species that do not have rigorous C4v geometry27 and are similar to the values for (CO)5WSi[Si(CH3)2(3-Butpz)]3K(18-crown-6),6i [Me11Si6W(CO) 5 ]Li(DME) 2 , 28 [Me 3 Sn) 3 SiW(CO) 5 ]NEt 4 , 29 and [Me3Si)3SiW(CO)5]NEt4.30 In addition, the solid-state structure of Na-6b was determined by X-ray crystallography. The results are shown in Figures 4 and 5 and reveal that Na-6b forms a twodimensional network through interconnected polymeric chains with a zigzag conformation. Those zigzag chains are composed of monomeric subunits of formula (CO) 5 WSi[Si(CH3)2OCH2CH3]3Na held together via bridging of the trans CO group of one subunit to the next sodium cation of another subunit (Figure 4). The chains are interconnected via

coordination of a cis CO group of every second subunit to the sodium cation of a neighboring chain. Each sodium cation is coordinated by the three ethoxy donors and additionally by either two CO molecules or one CO and one THF molecule, which gives rise to two different sodium cations, both having distorted-trigonal-bipyramidal coordination environments. The W−Si distances (2.68 and 2.69 Å) of Na-6b are close to those of (CO)5WSi[Si(CH3)2(3-But-pz)]3K(18-crown-6) (2.68 Å),6i [Me11Si6W(CO)5]Li(DIME)2, (2.67 Å),28 [(Me3Sn)3SiW(CO)5]NEt4 (2.66 Å),29 and [Me3Si)3SiW(CO)5]NEt4 (2.67 Å).30

3. CONCLUSION In this paper we have summarized our efforts on the synthesis and structural characterization of a unique family of zwitterionic alkali-metal silanides that contain various pendant donor groups. Regioselective, heterolytic Si−Si bond cleavage9,10 of Si[Si(CH3)2OR]4 via treatment with alkali-metal alkoxides MOBut (M = Li, Na, K) enabled quick and easy access to the alkali-metal zwitterions M-3 (M = Li, Na, K) in excellent isolated yields. In situ generated solutions of M-3 could be used for subsequent Lewis acid−base chemistry without the need for further purification. 9876


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With the exception of the tetrameric silanide K-3a, all structurally characterized alkali-metal silanides display polymeric or monomeric structures in the solid state, both featuring a central bicyclooctane unit, Si(SiO)3M, with the negative charge at silicon and the positive charge at the metal. Discrete structures are preferentially found in metal silanides that have polydonor groups OCH2CH2OR (R = Me, SMe, NMe2). With the exception of monomeric Li-3d, the incorporation of simple alkoxy donors such as OMe and OEt, OPri, and OBui gave rise to infinite chain type structures with the central metal cation being in a tetrahedral coordination environment. The naked silicon anion is available for additional metal coordination, as shown for example by reactions with BPh3 and W(CO)6 leading to the formation of hitherto unknown zwitterionic silyl borates and silyl tungstenates.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01227. Synthetic procedures, NMR spectra, and crystallographic data (PDF) Accession Codes

CCDC 1000502−1000505 and 1537986−1537993 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/ data_request/cif, or by emailing [email protected]. uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail for C.K.: [email protected]. ORCID

Clemens Krempner: 0000-0003-2596-591X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project was funded in part by the NSF (Grant No. 1407681; Project SusChEM: IUPAC) as part of the IUPAC International Funding Call on “Novel Molecular and Supramolecular Theory and Synthesis Approaches for Sustainable Catalysis”. The NSF is also thanked for purchase of a JEOL ECS-400 spectrometer (CRIF-MU CHE-1048553).

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REFERENCES

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Zwitterionic Alkali-Metal Silanides of Tripodal Ligand Geometry

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