Eclipsed- and Staggered-[Ge18Pd3{EiPr3}6]2– (E = Si, Sn): Positional

Article Cite This: J. Am. Chem. Soc. 2017, 139, 15176-15181

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Eclipsed- and Staggered-[Ge18Pd3{EiPr3}6]2− (E = Si, Sn): Positional Isomerism in Deltahedral Zintl Clusters Luis G. Perla,† Alvaro Muñoz-Castro,‡,§ and Slavi C. Sevov*,† †

Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556, United States Lab de Química Inorgánica y Materiales Moleculares, Universidad Autonoma de Chile, Llano Subercaceaux 2801, San Miguel, Santiago 8910060, Chile § Doctorado en FisicoQuimica Molecular, Universidad Andres Bello, Av. Republica 275, Santiago 8370146, Chile ‡

S Supporting Information *

ABSTRACT: We report the synthesis and characterization of the cluster anions [Ge18Pd3{SiiPr3}6]2− (1) with a core of facefused twinned icosahedra, Ge18Pd3, and two sets of three i Pr3Si-substituents positioned in “eclipsed” geometry. The new anion is a positional isomer of the recently reported “staggered” stannyl-ligated counterpart [Ge18Pd3{SniPr3}6]2− (2), showing the possibility to find such positional isomerism in Zintl clusters. Both anions are characterized by single-crystal X-ray diffraction, 1H and 13C NMR, and negative-ion ESI-MS. Using relativistic DFT calculations, we elucidate and discuss the reasons for the different positioning of the ligands in the stannyl- and silyl-functionalized species.

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larger face-fused [Sn14Ni(CO)]4−.24,25 In a similar way, oxidation by transition-metal salts has led to fusion of two [Ge9 {Hyp}3] − species to form the 18-atom aggregate [Ge18{Hyp}6] reported by Schnepf et al.10 In a different approach, we carried out cluster enlargements by insertion of an additional vertex atom into the 9-vertex [Ge9{Hyp}3R]0 to produce the 10-vertex closo-cluster [PPh3-PdGe9{Hyp}3R]0 (R = Et).12,13 We have shown that the hypersilyl ligands in the former are mobile, while they are static in the latter.12 More recently, we experimented with ligands smaller than hypersilyl (Hyp) and studied the transition-metal driven aggregation of the resulting species. This led to the recently reported [Ge18Pd3{SniPr3}6]4− (2),26 which has the core of a facefused (the Pd3-face) twinned icosahedron and two sets of three SniPr3 substituents positioned in a “staggered” geometry. In order to achieve this geometry, one of the sets of three ligands has had to move to positions different from those in the parent trisubstituted [Ge9R3]− cluster. This and some preliminary theoretical calculations suggested that other ligands with different bulkiness and mobility may result in the corresponding “eclipsed” geometry. We have achieved this now by the synthesis of an aggregate with the same core but with SiiPr3ligands instead, namely in [Ge18Pd3{SiiPr3}6]2− (1). Thus, 1 and 2 are positional pseudo-isomers where “eclipsed” and “staggered” geometries are realized with SiiPr3- and SniPr3ligands, respectively. Such isomerization, although common in organic chemistry, is unknown for deltahedral Zintl clusters. It

INTRODUCTION Deltahedral Zintl ions extracted in solutions from polar intermetallic precursors have been the subject of exponentially increasing interest for several decades.1,2 Numerous studies carried out on their reactivity in solutions revealed that organic groups and organometallic fragments can be attached to them, transition-metal atoms can be inserted inside their cages, and that they can be aggregated in various ways.3−5 More recently, we also demonstrated that functionalization with up to four exo-bonded substituents can be achieved by heterogeneous reactions using suspensions of the intermetallic precursor K4Ge9.6 Thus, acetonitrile suspensions generate the trisilylated species [Ge9{Hyp}3]− in very high yield when reacted with hypersilyl chloride HypCl (Hyp = Si(SiMe3)3).7 The same compound, but in much lower yield, was originally synthesized by the so-called subhalide approach wherein a metastable GeBr generated at 1550 °C is reacted with LiHyp at −78 °C.8 The hypersilylated species have been, in turn, studied for their reactivity to add a fourth substituent, to coordinate to transition metals, to aggregate, to add cluster atoms, etc.9−20 In addition, similar heterogeneous reactions have produced clusters that are trisubstituted with other silyl-based substituents, thus demonstrating the versatility and capabilities of the proposed synthetic approach.21−23 We have been interested in cluster enlargement for building novel larger species from known smaller ones and have approached this in a few different ways. Thus, ligand removal and mild oxidation by heating mixed main-group/transitionmetal species has led to the much larger [Bi12Ni7(CO)4]4− with three endohedral (nonligated) nickel atoms as well as to the © 2017 American Chemical Society

Received: August 11, 2017 Published: October 4, 2017 15176

DOI: 10.1021/jacs.7b08562 J. Am. Chem. Soc. 2017, 139, 15176−15181

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Journal of the American Chemical Society is also very rare in other deltahedral clusters such as the wellknown boranes, carboranes, and metallocarboranes. Only a handful of regioisomers have been structurally isolated, for example. 1,12-C2B10H10-n,m-I 2 (n,m = 2,3-, 2,7-, 2,9-), 1,2C2B10H11-n-(HS) (n = 1-, 9-), and Fe(2,3,5-C3B7H9-n-CH3)2 (n = 3-, 5-).27−29 Herein, we account for the differences between both isomers given by 1 and 2, discussing the factors leading to the possibility to find positional isomers based on the twinned icosahedron Ge18Pd3 core. In addition, theoretical calculations have been carried out to estimate the variety of isomers by using different ligands, ranging from smaller to bulkier examples (H3E-, Me3E-, Et3E-, iPr3E-, and Hyp3E- for E = Si and Sn).

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RESULTS AND DISCUSSION Synthesis, Structure, and Electronic Structure. Both the eclipsed [Ge 18 Pd 3 {Si i Pr 3 } 6 ] 2− (1) and staggered [Ge18Pd3{SniPr3}6]2− (2) aggregates are synthesized following exactly the same protocol using iPr3SiCl as a reagent in the former and the corresponding tin-based iPr3SnCl reagent in the latter. The reactions involve initial generation of the trisubstituted single-cage monoanions [Ge9{EiPr3}3]− (E = Si, Sn) by the same approach that was used previously for the synthesis of [Ge9{Hyp}3]−. This is accomplished by a reaction of an acetonitrile suspension of the Zintl phase precursor K4Ge9 with iPr3ECl. The resulting anions [Ge9{EiPr3}3]− were confirmed by ESI-MS (Figure S1A,B for [Ge9{SiiPr3}3]−; ref 26 for [Ge9{SniPr3}3]−). Although neither of the two anions has been structurally characterized, they are both most likely isostructural with the hypersilyl analogue [Ge9{Hyp}3]−,7,30 a tricapped trigonal prism of Ge9 with three ligands exo-bonded to the three capping atoms. In a second step of the synthesis of 1 and 2, the corresponding trisubstituted anions are reacted with [Pd(PPh3)4] in MeCN. Complete conversion to the fused cages is observed by ESI-MS (Figure S1C,D for 1; ref 26 for 2). 1 H and 13C NMR were used to confirm the presence of 1 in solution as an intact aggregate (Figures S2 and S3). Only single sets of iPr-signals were observed in 1 for the two nuclei, while the spectra for 2 show two sets of signals that correspond to the two different environments of the tin-based ligands in the staggered geometry (ref 26). In the solid state, 1 was isolated as [K(2.2.2-crypt)](1) salt from THF solution kept at −20 °C for three months (Figure 1, top). Similarly, crystals of [K(2.2.2crypt)](2)·iPr2O were previously isolated after layering an acetonitrile solution of 2 with iPr2O (Figure 1, bottom).26 Lastly, an anion with the same staggered core but with tricyclohexyl tin ligands, [Ge18Pd3{SnCy3}6]2− (3), was also synthesized and crystallized from THF as [K(2.2.2-crypt)]2(3)· 2THF. It is isostructural with 2, that is, the ligands are in the staggered geometry. Clearly, 1 is the eclipsed version of the already reported staggered 2 (Figure 1 and TOC). They both contain 21-atom cores of Ge18Pd3 with six ligands that are exo-bonded to six germanium vertices of the cores. Topologically, the two cores are identical and can be viewed as made of two face-fused icosahedra, also called a “twinned icosahedron” by Burdett and Canadell.31 The shared face is the Pd3-triangle that is “trapped” between two [Ge9R3]− moieties (Figure 1). The latter are wellknown for R = Hyp and are initially trigonal prisms made of atoms 1−2−3 and 7−8−9 that are tricapped by atoms 4, 5, 6 at the rectangular faces. In 1−3 they are found with an opened prismatic base, of atoms 7−8−9, in order to accommodate the Pd-triangle. The ligands in each [Ge9R3]− are initially exo-

Figure 1. Structures of the “eclipsed” 1 (top) and the “staggered” 2 (bottom) with six exo-bonded iPr3Si- and iPr3Sn-groups (magenta), respectively (Ge, gray; Pd, gold; all C- and H atoms are omitted for clarity; thermal ellipsoids shown at 50% probability).26

bonded to the capping atoms 4, 5, and 6. They stay at those positions for both halves in the eclipsed 1, but in the staggered 2 and 3, they move to positions 7′, 8′, and 9′ in one of the two halves. The Ge−Ge contacts in 1 range from 2.5737(14) Å to 2.8742(15) Å with the longest of the contacts being the former prismatic heights of each half (1−7, 2−8, 3−9), which is common in other trisubstituted clusters.30 The Ge−Si distances at the exo-bonded vertices range from 2.387(3) Å to 2.416(7) Å and compare well with the reported silylated germanium clusters.21,22 Similarly to 2, the Pd−Pd contacts in 1 are in a narrow range of 2.8389(8)−2.8438(7) Å, suggestive of little to no Pd−Pd interactions as previously reported.32−34 Interestingly and, as it turned out, importantly (see below), the intericosahedral contacts 7−7′, 8−8′, and 9−9′ are quite short, 2.8255(12)−2.8390(13) Å, shorter even than some of the intraicosahedral distances. The role of the Pd3 ring in the formation of 1 and 2 was evaluated by relativistic DFT computational studies. The molecular orbital diagram shown in Figure 2 indicates that the main bonding interactions between the Pd3-ring and the Ge-cages involve primarily the 5s orbitals of the Pd-atoms. The totally symmetric combination of the three 5s orbitals interacts with the totally symmetric radial π-combinations of the two Ge9 halves. This generates one significantly stabilized and populated molecular orbital of Ge9-Pd3-Ge9 bonding character as well as one destabilized and empty combination that becomes the LUMO of the aggregate (Figure 2). This is very similar to the previously reported anion [Sb3Au3Sb3]3− where the Au3-ring mediates the Sb3-Sb3 aggregation.35 The two degenerate antibonding 5s combinations of the Pd3-triangle also interact with a pair of degenerate orbitals on the two Ge9-halves. This results in one stabilized and occupied pair that becomes the degenerate HOMO of the molecule and an antibonding 15177

DOI: 10.1021/jacs.7b08562 J. Am. Chem. Soc. 2017, 139, 15176−15181

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mobility difference was eliminated as a factor, the only other factor left was the slightly different steric demands of those ligands due to the different distances for the Ge-SiiPr3 and GeSniPr3 exo-bonds. In order to elucidate this effect, we carried out calculations on optimized compounds with the much smaller and sterically noninterfering ligands H3Si- and H3Sn-. In both cases the staggered form turned out to be more stable by about 6 kcal/mol (Table 1). This indicated that the Table 1. Energy Differences in kcal/mol between the Eclipsed and Staggered Arrangements of the Ligand in [Ge18Pd3{ER3}6]2− Figure 2. Schematic representation of the contribution of the 5s-Pd levels to the overall electronic structure of [Ge18Pd3{SiiPr3}3]2−. Relevant levels are given in black. The isosurface value for the MO drawings is set at ±0.01 au.

combination that is significantly destabilized and empty. The resulting HOMO−LUMO gap of 1.31 eV is smaller than that of the parent [Ge9{SiiPr}3]− cluster mainly due to the formation of the LUMO with Ge9-Pd3-Ge9 antibonding character in the aggregate. Thus, the Pd3 ring mediates the cluster fusion without the need for oxidation of the [Ge9R3]− clusters prior to their coupling, as observed in the formation of the related [Ge18{Hyp}6] structure.10,36,37 This, in turn, allows each half of the aggregate to retain the same number of valence electrons as in the two starting [Ge9{EiPr3}3]− clusters. Isomerism. When the already reported anion 2 was initially synthesized, the staggered positioning of its iPr3Sn-ligands was attributed to the known mobility of Sn-based ligands on the surface of Ge9-clusters.6,13 It was assumed that the staggered geometry is preferred due to steric reasons, but no detailed studies were carried out in order to confirm or refute this. However, after the synthesis and characterization of 1 with eclipsed positioning of the ligands, the question of the stability of the two isomers became very important. We considered as possible both kinetic and thermodynamic types of stability. The former was considered in light of the already known mobility of tin-based ligands.6,13 Thus, one possibility is that the eclipsed geometry is less stable than the staggered one, but the less mobile Si-based ligands in 1 could not move from their original positions 4′, 5′, 6′ to the staggered positions 7′, 8′, 9′ (Figure 1). This would be the result of the stronger Ge−Si bonds than the Ge−Sn bonds. However, the DFT calculations of the optimized eclipsed 1 and staggered 2 showed that they are more stable (thermodynamically) than the optimized hypothetical staggered 1 and eclipsed 2 by 12.1 and 5.7 kcal·mol−1, respectively. This clearly indicates thermodynamic stability rather than kinetic stabilization due to different mobility of the ligands. This conclusion was also supported by the variabletemperature NMR spectroscopy carried out for 1. The 1H NMR spectra did not show a change of the eclipsed geometry upon heating up to 100 °C, and this indicates that the geometry is thermodynamically stable in that temperature range. Of course, mobility is essential for the tin-based ligands in order to form the staggered geometry observed for 2 and 3. Without sufficient mobility these ligands most likely would not form the aggregates at all. As it turns out, according to calculations, the eclipsed positioning is not an option for these ligands (details below). We were very puzzled by the fact that nearly identical ligands such as iPr3Sn- and iPr3Si- led to different isomers. Since the

i

Pr3Si-

t

H3Si-

Me3Si-

Et3Si-

eclipsed staggered

5.7 0.0 H3Sn-

3.3 0.0 Me3Sn-

5.7 0.0 Et3Sn-

0.0 12.1 i Pr3Sn-

Bu3Si-

0.0 25.2 t Bu3Sn-

eclipsed staggered

6.5 0.0

6.8 0.0

8.3 0.0

5.7 0.0

5.9 0.0

staggered core is intrinsically more stable and should be generally preferred. However, the two sets of three ligands are significantly closer in the staggered geometry than in the eclipsed one. Thus, the average distance between the exobonded Ge-atoms 4, 5, and 6 in one icosahedron to atoms 7′, 8′, and 9′ in the other icosahedron is ca. 4.48 Å in the staggered geometry, while the distance between the same atoms 4, 5, 6 and atoms 4′, 5′, 6′ is about 4.80 Å in the eclipsed form. Furthermore, each exo-bonded Ge-atom has two intericosahedral neighbors at 4.48 Å in the staggered conformation, for example, atom 4 is at that distance from atoms 7′ and 8′. At the same time, in the eclipsed form each exo-bonded atom has only one inter-icosahedral distance of 4.80 Å, for example, atoms 4−4′. What all this means is that steric repulsion will destabilize the staggered geometry more than the eclipsed for the same ligands. Calculations on species with increasingly larger ligands confirmed exactly that (Figure S4). Thus, the staggered form is preferred for the relatively small Si-based ligands H3Si-, Me3Si-, and Et3Si- (Table 1). However, the larger i Pr3Si- and tBu3Si- stabilize the eclipsed form better. The same trend is observed for the Sn-based ligands, but the switch to eclipsed position does not occur even for ligands as large as tBu, that is, the staggered form is preferred for all R3Sn- with R = H, Me, Et, iPr, and tBu (Table 1). Lastly, the formation of eclipsed and staggered structures was considered for a hypothetical aggregate with the significantly larger hypersilyl ligands, that is, [Ge18Pd3{Hyp}6]2−. The calculations showed that the steric hindrance results in repulsion of 13.0 and 14.3 kcal·mol−1, respectively, which clearly prevents the formation of such a hypothetical cluster. The different staggered-to-eclipsed switching for the Sn- and Si-based ligands is due to the different Sn−Ge and Si−Ge distances. Although the R3Sn- and R3Si-ligands themselves have the same bulkiness for the same R, the former are positioned further away from the Ge18Pd3-core, while the latter are pulled in closer. Thus, in the silyl cases there is larger steric repulsion between each other, as their steric cones are pulled closer to each other. This in turn causes greater destabilization of the staggered SiR3-ligated form where the ligands interact with more neighbors and at shorter distances. The above discussion explains why larger ligands favor the eclipsed over the staggered geometry. However, it does not answer why the staggered form is more stable than the eclipsed 15178

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large aggregate. If the six ligands are small enough, one set of them moves to positions 7′, 8′, and 9′ to form the staggered geometry which is thermodynamically preferable due to absence of repulsion between close lone-pairs. This is the case for 2 and 3. However, sterically more demanding ligands prevent such a move since it would bring them too close to each other in the staggered position. Thus, the eclipsed geometry is preferred for such ligands, as it is the case for 1. These examples, as discussed in the current report, suggest that steric factors can be used to control the formation of a preferred positional isomer in some deltahedral Zintl clusters and that the formation of other such isomers is possible as well.

form for small noninteracting ligands, that is, for R3E with R = H, Me, Et. We carried out calculations on the corresponding naked cores [Ge18Pd3]8− at frozen geometries taken from the overall optimized ligated clusters at both staggered and eclipsed structures, that is, the geometries of [Ge18Pd3{R3E}6]2− are optimized for staggered and eclipsed ligands, and then the ligands are removed. The calculated energies for R = H, Me, Et showed that the cores derived from the staggered isomers are more stable than the eclipsed ones by about 6.2−7.5 kcal/mol (Table S1). Careful analysis of the geometric differences between the two cores indicated one noticeable disparity, namely the shorter inter-icosahedral distances 7−7′, 8−8′, and 9−9′ in the eclipsed form (Figure 1). This distance in 1 is in the range 2.826−2.839 Å, while the range for 2 is 2.939−2.988 Å. The reason for this difference is in the pattern of Ge-atoms with exo-bonds and with lone pairs. It is typical that exobonded vertices are usually less pyramidal within a cluster compared to vertices with lone pairs. Therefore, in order to keep reasonable Ge−Ge distances within the cluster, exobonded Ge-vertices “push” away the neighboring cluster atoms. Thus, the exo-bonded atoms 4, 4′, 5, 5′, 6, 6′ in the eclipsed aggregate push the pairs 7−7′, 8−8′, and 9−9′ closer to each other. As a consequence of this, the lone-pair orbitals of the latter come close enough to interact with each other (Figure 3,

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EXPERIMENTAL SECTION

Materials. All reactions and materials were handled under inert atmosphere or vacuum using standard glovebox techniques. 2.2.2Crypt(4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo [8.8.8]hexacosane) (Sigma-Aldrich, 98%), acetonitrile (anhydrous, MeCN, EMD Millipore 99.8%) (stored under 4 Å molecular sieves), triisopropylsilyl chloride (SiiPr3Cl) (Sigma-Aldrich, 97%), tetrakis(triphenylphosphine) palladium (Strem, 98%) were used as received. Tetrahydrofuran (THF) (Alfa-Aesar, 99.8+%) was dried by passing over a copper-based catalyst and 4 Å molecular sieves and then stored in gastight ampule containing sodium metal under nitrogen. K4Ge9 was synthesized from the elements (K cubes: Sigma-Aldrich 99.5%, surfaces shaved before use, Ge powder: Alfa Aesar, 99.99%) at 950 °C for 2 days in sealed niobium tubes then evacuated within a fused silica jacket. Synthesis of K[Ge9{SiiPr3}3]. 335 mg (0.41 mmol) of freshly opened and finely ground K4Ge9 was loaded into a 20 mL vial and suspended with 15 mL MeCN. 235 mg (1.22 mmol) SiiPr3Cl were added directly and stirred for 18 h resulting in a brown-red solution. The mixture is then centrifuged, and the filtrate is used without further purification. ESI-MS (m/z), 1124 [Ge9{SiiPr3}3]−. To date we have not been successful in the structural isolation of [Ge9{SiiPr3}3]−. Synthesis of [K(2.2.2-crypt)]2[Ge18Pd3{SiiPr3}6] (1). To the solution described above, 330 mg (0.28 mmol) of [Pd(PPh3)4] was added and stirred for an additional day. The resulting mixture is centrifuged following concentration of the filtrate to half volume. In another vial containing 148 mg (0.39 mmol) 2.2.2-crypt, the concentrate is slowly added, stirred for 2 h, and then allowed to settle for another hour. Decanting a dark brown solution isolates an oily black residue, which is dried under vacuum. The resulting oil is then dissolved in THF, and the brown-red solution stored at −20 °C for three months resulting in the isolation of a few red plates (17 mg, 1.29% crystalline yield based on K4Ge9) alongside an unknown black oil. Crystal data: P1̅, a = 16.1039(6) Å, b = 17.0961(6) Å, c = 27.6983(10) Å, α = 95.184(2)°, β = 91.202(3)°, γ = 117.634(2)°, V = 6711.1(4) Å3, Z = 2, λ (Cu−Kα) = 1.54178 Å, R1/wR2 = 0.0599/0.1535 for the observed data and R1/ wR2 = 0.0821/0.1699 for all data. ESI-MS negative ion mode (m/z), 2569 [Ge18Pd3{SiiPr3}6]−. 1H NMR (25 °C, Pyridine-d5) δ 1.07 (d. CH3), δ 1.06 (overlapping. CH). 13C NMR (25 °C, Pyridine-d5) δ 18.30 (CH3), δ 13.61 (CH). 1H−13C HSQC (Figure S3). Synthesis of [K(2.2.2-crypt)]2[Ge18Pd3{SnCy3}6]·2THF (3). 430 mg (0.53 mmol) of freshly opened and finely ground K4Ge9 was loaded into a 20 mL vial and suspended with 15 mL MeCN. 296 mg (0.73 mmol) SnCy3Cl was added, stirred for 1 h, followed by a second addition of 300 mg (0.74 mmol) SnCy3Cl and stirred for 1 day resulting in a brown solution. The mixture is then centrifuged, and to the filtrate, 519 mg (0.44 mmol) [Pd(PPh3)4] is added and stirred for an additional day. The mixture is centrifuged, and 131 mg 2.2.2-crypt is added to the filtrate resulting in a black precipitate that is filtered and washed two times with 5 mL of MeCN. The solid is then dissolved in THF, and black blocks are isolated over the course of the next couple of days. (535 mg, 21% crystalline yield based on K4Ge9). Structure Determination. Single crystal X-ray diffraction data were obtained on a Bruker APEX-II diffractometer equipped with a curved graphite monochromator and a CCD area detector using Cu

Figure 3. Lone-pair orbitals of [Ge18Pd3(SiiPr3)]2− in the eclipsed geometry at atoms 7 (left) and 7′ (middle), and their antibonding overlap (red region) to the right (the lone pairs at 8−8′ and 9−9′ overlap similarly; see Figure S6).

left) producing bonding and antibonding combinations. Since they are both occupied, the net result is a relatively strong antibonding overlap (Figure 3, right). These types of interactions have been described and discussed before in both hypothetical and real cases.31 Depending on the strength of the interaction, some of the known species are stabilized by losing a pair of electrons (those at the antibonding combination)25 or even by vacating one of the two atoms, that is, generating a vacancy.38 For the staggered conformation, not only the atoms in the inter-icosahedral pairs 7−7′, etc., are further apart but also the interactions are between a lone-pair at one of them and an exobond at the other one. Such interactions are much less pronounced because the bonding orbital is much more localized and confined between the two atoms of the exobond and can not overlap sufficiently with the lone-pair orbital (Figures 3 and S5). This and the longer distance make the staggered conformation preferred for sterically nondemanding ligands.

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CONCLUSION Thus, the formation of [Ge 18Pd3{ER3}6]2− starts with monomeric [Ge9{ER3}3]− clusters where the ER3-ligands are exo-bonded at atoms 4, 5, and 6. Upon insertion of the Pd3triangle between two such clusters, they fuse together into one 15179

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Journal of the American Chemical Society Kα radiation at 120 K. The crystals were mounted on Mitegen micromount loops after selection from Paratone-N oil and positioned under a nitrogen cold stream. The structures were solved using SHELXT and refined on F2 against all reflections using SHELXL2014 on an OLEX-2 graphical interface.39,40 The tri-isopropyl groups on Si1 were modeled over two positions with occupancies of 52% and 48%. RIGU and DELU restraints were used for isopropyl groups with problematic displacement parameters. A solvent accessible void carrying residual electron density was found and modeled using the solvent mask function in OLEX-2. Computational Details. Relativistic density functional theory calculations were carried out by using the ADF code, incorporating scalar corrections via the ZORA Hamiltonian.41−43 We employed the triple-ξ Slater basis set plus two polarization functions (STO-TZ2P) for valence electrons, within the generalized gradient approximation (GGA) according to the Perdew−Burke−Ernzerhof (PBE) exchange− correlation functional because of its improved performance on longrange interactions and relatively low computational cost employed in clusters.44,45 The frozen core approximation was applied to the [1s23p6] shells for Ge, [1s2-4p6] shells for Sn and Pd, [1s2-2p6] for Si, and [1s2] for C, leaving the remaining electrons to be treated variationally. Geometry optimizations were performed without any symmetry restrains via the analytical energy gradient method implemented by Versluis and Ziegler. An energy convergence criterion of 10−4 Hartree, gradient convergence criteria of 10 −3 Hartree/Å, and radial convergence criteria of 10−2 Å were employed for obtaining the relaxed structures. In order to compensate the effects of polar solvents and counterions, the polarizable continuum model was incorporated into the calculations by considering a conductor-like screening model treatment via the COSMO module with THF as a solvent,46 as implemented in the ADF code. The representations of the lone-pair orbitals were obtained as localized molecular orbitals (LMO’s) via the Boys−Foster localization scheme of the molecular orbitals,47 as employed in related works before.48 Mass Spectrometry. ESI-MS spectra were collected on a Micromass Quattro-LC triple-quadrupole mass spectrometer (125 °C source temperature, 150 °C desolvation temperature, 2.0 kV capillary voltage, and 30 V cone voltage). Samples were directly injected for analysis. NMR Spectrometry. Deuterated pyridine-d5 (Cambridge Isotope Laboratories) was stored under 4 Å molecular sieves before use. 1H, 13 C, and 1H−13C HSQC were recorded on an Bruker 400 instrument. The deuterium signal was locked and referenced to tetramethylsilane for 1H and 13C spectra.

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mass spectrometry analysis. A.M.-C. thanks the support from FONDECYT 1140359 grant.

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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/jacs.7b08562. Additional spectroscopic and computational data and figures (PDF) X-ray crystallographic data for 1 (CIF) X-ray crystallographic data for 3 (CIF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

Alvaro Muñoz-Castro: 0000-0001-5949-9449 Slavi C. Sevov: 0000-0003-4425-868X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Center for Environmental Science and Technology (CEST) at the University of Notre Dame for the 15180

DOI: 10.1021/jacs.7b08562 J. Am. Chem. Soc. 2017, 139, 15176−15181

Article

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DOI: 10.1021/jacs.7b08562 J. Am. Chem. Soc. 2017, 139, 15176−15181