The First Chelate-Free Crystal Structure of a Silicide Transition Metal Complex [K0.28Rb7.72Si9Ni(CO)2]2·16NH3
Institute of Inorganic Chemistry, University of Regensburg, 93040 Regensburg, Germany
*
Author to whom correspondence should be addressed.
Crystals 2015, 5(3), 275-282; https://doi.org/10.3390/cryst5030275
Submission received: 10 April 2015
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Revised: 3 June 2015
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Accepted: 5 June 2015
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Published: 24 June 2015
Abstract
:Single-crystals X-ray structure analysis of very thermally labile and moisture sensitive ammoniate crystals of [K0.28Rb7.72Si9Ni(CO)2]2·16NH3 show the presence of the very rarely observed nickel-silicide complex [{Ni(CO)2}2(µ-Si9)2]8−, which was up to now only known in the 18-crown-6 involving solid [Rb@18-crown-6)]2[K@18-crown-6)]2Rb4[{Ni(CO)2}2(µ-Si9)2]·22NH3. This shows that, like already known for the heavier homologues, the presence of 18-crown-6 (1,4,7,10,13,16-hexaoxacyclooctadecane) is no precondition for the stabilization of the silicide cluster anion in solid state and the absence of the alkali metal ligand even allows for the crystallization in the higher symmetric monoclinic space group C2/c compared to the triclinic space group P-1 when 18-crown-6 is present.
1. Introduction
Homoatomic polyanions of group 14 elements represent naked, anionic elemental building blocks and are an intriguing field of main group chemistry due to their versatile reaction possibilities. They can be used as promising starting materials for the synthesis of new compounds, and especially the monocapped square antiprismatic shaped nine atom cluster anions E94− show a wide synthetic potential [1,2,3,4]. For the heavier homologues of this group, germanium, tin and lead, an abundance of beautiful examples can be found in literature, reaching from coupled clusters [5] over alkylated derivatives [6,7] ending at the involvement of transition metals, where the cluster is acting as a ligand (sometimes under rearrangement of the atoms yielding new cluster shapes) [8] and even endohedral metalloids are formed [9,10] (for a recent overview see [1]). The case is different for the lighter homologue silicon, of which a solution chemistry has only been known since 2004 [11,12,13,14] and very few examples of reactions are known up to now. Therefore, only a diminutive number of products, a total of three, have been characterized. In addition to [Si9ZnPh]3− [15] (Ph = phenyl = C6H5) and [(MesCu)2Si4]4− [16] (Mes = mesitylene = C6H2(CH3)3)) (Figure 1a,b), where organometallic precursors have been used during synthesis, we previously could show that the Si94− cluster anions act as ligands in a simple ligand-exchange reaction of triphenylphosphine in the transition metal complex Ni(CO)2(PPh3)2 (PPh3 = triphenylphosphine = P(C6H5)3) yielding the new complex [{Ni(CO)2}2(µ-Si9)2]8− [17] (Figure 1c). All hitherto known crystal structures have the involvement of chelating additives in common, which are used during synthesis for the enhancement of the solubility of the Zintl educt material A12Si17 [18,19]. The question arose whether 18-crown-6 (1,4,7,10,13,16-hexaoxacyclooctadecane) or [2.2.2]-cryptand (4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane) are imperatively necessary for the stabilization of the complex anion in the crystal structures. We here present the fourth crystal structure at all of a silicide complex anion in the crystal structure of [K0.28Rb7.72Si9Ni(CO)2]2·16NH3, which proves the stability of the transition metal complex in a rigid cation-anion-ammonia network without any further chelating additives.
2. Results and Discussion
The reaction of the ternary Zintl material K6Rb6Si17 and Ni(CO)2(PPh3)2 in the presence of 18-crown-6 resulted in the first chelate free crystal structure of the silicide transition metal complex [{Ni(CO)2}2(µ-Si9)2]8− and proves the stability of the complex in the solid state, independently from 18-crown-6. Although 18-crown-6 is absent in the crystal structure of the here discussed compound, an even larger excess was used during the synthesis, compared to the synthesis of the 18-crown-6 involving material. This might indicate the basic necessity of the chelating additive for a better solubility of the precursor material but disproves its stabilizing effects in solid state. In the following, the new compound [K0.28Rb7.72Si9Ni(CO)2]2·16NH3 (1) is compared to the known crystal structure of [Rb@18-crown-6]2[K@18-crown-6]2Rb4[{Ni(CO)2}2(μ-Si9)2]·22NH3 (2) in order to analyze similarities and discrepancies in the crystal structures of both compounds. As the mixed-cationic compound K6Rb6Si17 was used during synthesis, the positions of all cations were carefully investigated for mixed occupancy during the refinement using the derived electron density maps. A mixed cationic site could only be resolved for one cation site (Rb4/K4 0.725(7):0.275(7)), the remaining four crystallographically independent cation positions turned out to be exclusively occupied by rubidium. One cationic site (Rb5) is located beside an inversion center within the glide plane (site coordinates 0.25, 0.75, 0.5, Wyckoff position 4c) and therefore the s.o.f. (site occupancy factor) was fixed at 0.5. The crystal data and details of the structure refinement are given in Table 1.
| Item | Numerical Value |
|---|---|
| CCDC No. | 1043014 |
| Chemical formula | [K0.28Rb7.72Si9Ni(CO)2]2·16NH3 |
| Mr (g·mol−1) | 1678.63 |
| Temperature (K) | 123(2) |
| Crystal system | monoclinic |
| Space group | C2/c |
| a (Å) | 30.669(6) |
| b (Å) | 9.919(2) |
| c (Å) | 19.894(4) |
| α (°) | 90 |
| β (°) | 110.18(3) |
| γ (°) | 90 |
| V (Å3) | 5680(2) |
| Z | 4 |
| ρcalc (g/cm3) | 1.963 |
| μ (mm−1) | 7.676 |
| F(000) (e) | 3260.0 |
| Crystal size (mm3) | 0.18 × 0.12 × 0.1 |
| Radiation | MoKα (λ = 0.71073) |
| 2Θ range for data collection (°) | 4.636 to 55.28 |
| Index ranges | −39 ≤ h ≤ 39, −12 ≤ k ≤ 12, −25 ≤ l ≤ 25 |
| Reflections collected | 46,537 |
| Independent reflections | 6534 [Rint = 0.0729, Rsigma = 0.0523] |
| Data/restraints/parameters | 6534/6/253 |
| Goodness-of-fit on F2 | 0.954 |
| R1, wR2 (I ≥ 2σ (I)) | R1 = 0.0349, wR2 = 0.0667 |
| R1, wR2 (all data) | R1 = 0.0568, wR2 = 0.0711 |
| Δρmax, Δρmin (e Å−3) | 0.90/−0.46 |
In both compounds, the complex anion [{Ni(CO)2}2(µ-Si9)2]8− is located on an inversion center of space group C2/c (1), respectively P-1 (2), and this results in very similar dimensions of the latter in both crystal structures (Figure 2, Table 2). This proves that the geometry of the anion is not affected by packing effects or cation-anion interaction.
Figure 2.
[{Ni(CO)2}2(µ-Si9)2]8− complex anion in 1 and 2; distances are given in Table 2.
Figure 2.
[{Ni(CO)2}2(µ-Si9)2]8− complex anion in 1 and 2; distances are given in Table 2.
Table 2.
Distances within the complex anion [{Ni(CO)2}2(µ-Si9)2]8− in the two known crystal structures 1 and 2; atom assignment is given in Figure 2.
| 1 | 2 | ||||
|---|---|---|---|---|---|
| Atom 1 | Atom 2 | Distance (Å) | Atom 1 | Atom 2 | Distance (Å) |
| Ni1 | Si2 | 2.3052(12) | Ni1 | Si2 | 2.3033(17) |
| Ni1 | Si7 | 2.3044(13) | Ni1 | Si7 | 2.3001(18) |
| Ni1 | C1 | 1.738(4) | Ni1 | C1 | 1.743(8) |
| Ni1 | C2 | 1.746(4) | Ni1 | C2 | 1.729(6) |
| C1 | O1 | 1.167(5) | C1 | O1 | 1.169(8) |
| C2 | O2 | 1.159(5) | C2 | O2 | 1.183(8) |
In 2, the complex anion shows direct contacts to six rubidium cations, two of which are symmetrically independent. A very similar arrangement is found for 1, but the absence of cation chelating crown ether molecules allows for more direct cation contacts (Figure 3). The anionic moiety in 2 exclusively shows contacts to rubidium, and the potassium cations are only coordinated by 18-crown-6 molecules and ammonia molecules, agreeing with the HSAB (hard and soft acids and bases) principle.
Figure 3.
The comparison of the cation coordination of the complex anion [{Ni(CO)2}2(µ-Si9)2]8− in 1 (a) shows the presence of additional contacts (given in bright green) compared to the cation coordination of the complex anion [{Ni(CO)2}2(µ-Si9)2]8− in 2 (b). The position of Rb4 is mixedly occupied by rubidium and potassium (see text and Figure 3). Displacement ellipsoids are drawn at the 50% probability level. (Symmetry codes: #1: 1 − x, 2 − y, 1 − z; #2: x, 1 + y, z; #3: 1 − x, 1 − y, 1 − z; #4: 0.5 − x, 1.5 − y, 1 − z; #5: 2 − x, 1 − y, −z; #6: 1 − x, 1 − y, z).
Figure 3.
The comparison of the cation coordination of the complex anion [{Ni(CO)2}2(µ-Si9)2]8− in 1 (a) shows the presence of additional contacts (given in bright green) compared to the cation coordination of the complex anion [{Ni(CO)2}2(µ-Si9)2]8− in 2 (b). The position of Rb4 is mixedly occupied by rubidium and potassium (see text and Figure 3). Displacement ellipsoids are drawn at the 50% probability level. (Symmetry codes: #1: 1 − x, 2 − y, 1 − z; #2: x, 1 + y, z; #3: 1 − x, 1 − y, 1 − z; #4: 0.5 − x, 1.5 − y, 1 − z; #5: 2 − x, 1 − y, −z; #6: 1 − x, 1 − y, z).
The shortest Rb-O distances to the oxygen atoms of the carbonyl group of the complex anion in 2 are found at 3.275(5) Å (Rb2-O2#2), whereas the lack of 18-crown-6 in 1 results in short Rb-O distance of the carbonyl oxygen atoms O1 and O2 to the additional cationic site K4/Rb4 (see Table 3). Interestingly, the mixed occupancy perfectly agrees with the HSAB principle, as the smaller and therefore harder potassium prefers to coordinate to the small and hard oxygen atoms of the carbonyl ligand and short K/Rb-O distances can only be realized for this cationic site. The coordination of the cations is completed by contacts to the silicon atoms of the complex anion and ammonia molecules (Figure 4, Table 3). Altogether, a dense network of cations, anions and ammonia molecules is observed, which is also reflected by the calculated density of 1.963 g·cm−3 for 1 compared to the value 1.517 g·cm−3 for 2.
Figure 4.
Coordination spheres of all five crystallographically independent cationic sites. The mixedly occupied cationic site K4/Rb4 shows only contacts to oxygen atoms of the carbonyl group and ammonia molecules, which agrees with the HSAB principle. The according distances are given in Table 2. Displacement ellipsoids are drawn at the 50% probability level, hydrogen atoms are omitted for clarity (Symmetry codes: #1: 1 − x, 2 − y, 1 − z; #3: 1 − x, 1 − y, 1 − z; #4: 0.5 − x, 1.5 − y, 1 − z; #7: x, −1 + y, z; #8: 1 − x, y, 0.5 − z; #9: 0.5 − x, 0.5 + y, 0.5 − z; #10: x, 3 − y, −0.5 + z; #11: x, 1 − y, −0.5 + z).
Figure 4.
Coordination spheres of all five crystallographically independent cationic sites. The mixedly occupied cationic site K4/Rb4 shows only contacts to oxygen atoms of the carbonyl group and ammonia molecules, which agrees with the HSAB principle. The according distances are given in Table 2. Displacement ellipsoids are drawn at the 50% probability level, hydrogen atoms are omitted for clarity (Symmetry codes: #1: 1 − x, 2 − y, 1 − z; #3: 1 − x, 1 − y, 1 − z; #4: 0.5 − x, 1.5 − y, 1 − z; #7: x, −1 + y, z; #8: 1 − x, y, 0.5 − z; #9: 0.5 − x, 0.5 + y, 0.5 − z; #10: x, 3 − y, −0.5 + z; #11: x, 1 − y, −0.5 + z).
Table 3.
Distances for the coordination of the cations according to Figure 3.
| Atom 1 | Atom 2 | Distance (Å) | Atom 1 | Atom 2 | Distance (Å) |
|---|---|---|---|---|---|
| Rb1 | Si7 | 3.9366(14) | Rb4 | N2 | 3.300(4) |
| Si9#7 | 3.7493(15) | N3 | 3.007(4) | ||
| Si2#7 | 3.8338(14) | O1 | 2.832(3) | ||
| Si4 | 3.6312(14) | O2#3 | 2.855(4) | ||
| Si7 | 3.9366(14) | K4 | O1 | 2.70(3) | |
| Si8 | 4.0366(15) | O1#8 | 2.99(3) | ||
| N2 | 3.116(4) | O2#3 | 2.79(2) | ||
| N1 | 3.322(4) | O2#11 | 2.98(2) | ||
| O2 | 3.853(4) | N2 | 3.04(3) | ||
| O1#3 | 3.241(4) | N2#8 | 3.58(2) | ||
| Rb2 | Si7#1 | 3.5962(15) | N3 | 2.776(17) | |
| Si6#1 | 3.5971(15) | N3#8 | 3.246(19) | ||
| Si6 | 3.8961(13) | Rb5 * | Si5 | 3.619(4) | |
| Si3 | 3.4674(13) | Si5#4 | 3.639(4) | ||
| Si9#1 | 3.5273(17) | Si4 | 4.049(3) | ||
| Si8#1 | 3.8988(17) | Si8#4 | 3.744(4) | ||
| Si7#1 | 3.5961(15) | Si8 | 3.988(3) | ||
| N3 | 3.406(4) | N7 | 3.234(6) | ||
| N3#8 | 3.367(4) | N7#4 | 3.222(6) | ||
| N4#8 | 3.157(4) | N1 | 3.258(5) | ||
| N4 | 3.090(4) | ||||
| O1 | 3.263(4) | ||||
| O2#1 | 3.702(4) | ||||
| Rb3 | Si5 | 3.7988(13) | |||
| Si1#10 | 3.7609(14) | ||||
| Si8 | 3.6490(13) | ||||
| Si3#10 | 4.0429(18) | ||||
| Si9 | 3.7252(14) | ||||
| N5 | 2.935(5) | ||||
| N7#9 | 3.227(5) | ||||
| N6 | 3.000(4) |
Note: * Rb5 is located beside a twofold axis; the distances for the Rb5 generated by symmetry are given in the cif file (Supplementary materials).
3. Experimental Section
All manipulations were performed under inert atmosphere in an argon filled MBraun Labmaster 130 G glove box. Schlenk tubes were used as reaction vessels. Ni(CO)2(PPh3)2 (ABCR) was used without further purification. 18-crown-6 (1,4,7,10,13,16-hexaoxacyclooctadecane; Sigma-Aldrich Chemie Gmbh, Munich, Germany) was sublimated and then stored under argon. The solvent liquid ammonia was stored over potassium to remove traces of water before it was condensed on the reactands. For the synthesis of the precursor material K6Rb6Si17 0.294 g (7.5 mmol) K, 0.644 g (7.5 mmol) Rb and 0.5 g (17.8 mmol) Si were placed in a duran glass ampoule, which had been dried in vacuo before, and sealed under argon. The ampoule was placed in a second quartz glass ampoule for safety and heated to 460 °C using a heating rate of 25 °C/h. The temperature was kept for 72 h, afterwards the product was cooled down using a cooling rate of 20 °C/h. 0.12 g (0.098 mmol) of the blue black precursor phase, 0.063 g (0.098 mmol) of Ni(CO)2(PPh3)2 and 0.08 g (0.30 mmol) 18-crown-6 were weighed in a Schlenk tube. Approximately 15 mL of dry ammonia were condensed on the reactands at −78 °C. The solution immediately changed color from yellow to red after shaking. The reaction vessel was kept at −40 °C, where an increase in color could be observed. After four weeks red, rod shaped crystals suitable for X-ray structure analysis could be obtained
4. Conclusions
The here presented crystal structure of [K0.28Rb7.72Si9Ni(CO)2]2·16NH3 proves the stability of the silicide complex anion [{Ni(CO)2}2(µ-Si9)2]8− without any chelating additives in solid state, but the presence of 18-crown-6 for solubility enhancement might be a necessity.
Supplementary Information
Supplementary materials can be accessed at: https://www.mdpi.com/2073-4352/5/3/0275/s1.
Author Contributions
Markus Hamberger and Stefanie Gärtner carried out experimental work (synthesis, crystallization and X-ray structure determination), Stefanie Gärtner prepared the manuscript, Nikolaus Korber designed and conceived the study.
Conflicts of Interest
The authors declare no conflict of interest.
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Gärtner, S.; Hamberger, M.; Korber, N. The First Chelate-Free Crystal Structure of a Silicide Transition Metal Complex [K0.28Rb7.72Si9Ni(CO)2]2·16NH3. Crystals 2015, 5, 275-282. https://doi.org/10.3390/cryst5030275
AMA Style
Gärtner S, Hamberger M, Korber N. The First Chelate-Free Crystal Structure of a Silicide Transition Metal Complex [K0.28Rb7.72Si9Ni(CO)2]2·16NH3. Crystals. 2015; 5(3):275-282. https://doi.org/10.3390/cryst5030275
Chicago/Turabian StyleGärtner, Stefanie, Markus Hamberger, and Nikolaus Korber. 2015. "The First Chelate-Free Crystal Structure of a Silicide Transition Metal Complex [K0.28Rb7.72Si9Ni(CO)2]2·16NH3" Crystals 5, no. 3: 275-282. https://doi.org/10.3390/cryst5030275
