We present the high-temperature synthesis of intermetallic precursors K4Ge9, their dissolution in ethylenediamine to form Ge94- deltahedral Zintl ions, and the reaction of the clusters with alkynes to form organo-Zintl ions. The latter are characterized by electrospray mass spectrometry in solutions and by single-crystal X-ray diffraction in the solid state.
Although the first studies of Zintl ions date between the late 1890’s and early 1930’s they were not structurally characterized until many years later.1,2 Their redox chemistry is even younger, just about ten years old, but despite this short history these deltahedral clusters ions E9n- (E = Si, Ge, Sn, Pb; n = 2, 3, 4) have already shown interesting and diverse reactivity and have been at the forefront of rapidly developing and exciting new chemistry.3-6 Notable milestones are the oxidative coupling of Ge94- clusters to oligomers and infinite chains,7-19 their metallation,14-16,20-25 capping by transition-metal organometallic fragments,26-34 insertion of a transition-metal atom at the center of the cluster which is sometimes combined with capping and oligomerization,35-47 addition of main-group organometallic fragments as exo-bonded substituents,48-50 and functionalization with various organic residues by reactions with organic halides and alkynes.51-58
This latter development of attaching organic fragments directly to the clusters has opened up a new field, namely organo-Zintl chemistry, that is potentially fertile for further synthetic explorations, and it is the step-by-step procedure for the synthesis of germanium-divinyl clusters described herein. The initial steps outline the synthesis of an intermetallic precursor of K4Ge9 from which the Ge94- clusters are extracted later in solution. This involves fused-silica glass blowing, arc-welding of niobium containers, and handling of highly air-sensitive materials in a glove box. The air-sensitive K4Ge9 is then dissolved in ethylenediamine in the box and then alkenylated by a reaction with Me3SiC≡CSiMe3. The reaction is followed by electrospray mass spectrometry while the resulting solution is used for obtaining single crystals containing the functionalized clusters [H2C=CH-Ge9-CH=CH2]2-. For this purpose the solution is centrifuged, filtered, and carefully layered with a toluene solution of 18-crown-6. Left undisturbed for a few days, the so-layered solutions produced orange crystalline blocks of [K(18-crown-6)]2[Ge9(HCCH2)2]•en which were characterized by single-crystal X-ray diffraction.
The process highlights standard reaction techniques, work-up, and analysis towards functionalized deltahedral Zintl clusters. It is hoped that it will help towards further development and understanding of these compounds in the community at large.
1. Preparing Niobium Tubes
2. Loading Niobium Tubes: Preparing K4Ge9
3. Preparing Fused Silica Tube via Glass-Blowing
4. Sealing Fused Silica Tube using a High Vacuum Line
5. Heating the Reaction Mixtures in a Furnace
6. Dissolving Precursor in Ethylenediamine
7. Reacting Ge9-clusters with Me3SiC≡CSiMe3
8. Running ES-MS of Reaction Solution
9. Crystallizing Ge9-divinyl Ions with a Sequestering Agent
10. Checking Crystals Unit Cell on a D8-Diffractometer
11. Representative Results
The unique isotope pattern of the anionic clusters allows them to be easily detected in the negative ion-mode (Fig 1). Also noteworthy is that reduced singly charged species, in addition to pairing with a potassium ion is a common phenomenon of this soft-ionization technique.59
The crystal structure with relevant bond lengths and angles of [Ge9(CH=CH2)2]2– in [K(18-crown-6)]2[Ge9(HCCH2)2]•en, 1, can be seen in Figure 2 .
Figure 1. ES-MS spectra (negative-ion mode) of ethylenediamine solutions of the reactions of Ge9 clusters with Me3SiC≡CSiMe3. Shown are also the theoretical isotope distributions below the experimental distribution. (Sevov et. al. Inorg. Chem. 2007, 46, 10953.)
Figure 2. A view of [K(18-crown-6)]2[Ge9(HCCH2)2]•en, 1. Color scheme: = Ge, = C, = H. Selected bond lengths and angles: Ge-C 1.961 and 1.950 Å, C=C 1.318 and 1.316 Å, Ge-C-C 123 and 127 °. (Sevov et. al. Inorg. Chem. 2007, 46, 10953.)
Figure 3. Schematic representation of Preparing Niobium Tubes: (a) cutting Nb tubes; (b) cleaning Nb tubes in a Nb acid solution; (c) using vise-grips to crimp and bend Nb tube.
Figure 4. Schematic representation of Preparing Niobium Tubes: (a) diagram of arc welder; (b) staggered Nb tubes in arc welder holder and (c) welding tip above Nb tubes.
Figure 5. Schematic representation of Loading Niobium Tubes : (a) inside the drybox and (b) Nb tubes: (i) before welding, (ii) after using vise-grips to crimp one edge, (iii) after welding one edge, (iv) after loading and then welding the Nb tube closed, (v) after opening the Nb tube to take out the K4Ge9 precursor.
Figure 6. Schematic representation of Preparing Fused-Silica Tube by Glass-Blowing in (a) and (b)(i) large and small quartz tubing, (ii) body and neck sealed together, (iii) neck with ball joint, (iv) neck and ball joint sealed together, (v) Nb tubes sealed inside quartz tube, (vi) after the fused silica tube is sealed.
Figure 7. Schematic representation of Sealing Fused Silica Tubes on a High Vacuum Line in (a) and (b) after the Nb tubes is sealed showing etching of the quartz tube from Nb acid solution.
Figure 8. Schematic representation of Placing Loaded Fused Silica Tubes in Furnace.
Figure 9. Schematic representation of Reacting K4Ge9 with Me3SiC≡CSiMe3 inside the drybox (a) (i) unopened Nb tube, (ii) one edge of the Nb tube cut with the (iii) cutting pliers, (iv) crushed precursor and (b) (i) precursor dissolved in ethylenediamine, (ii) immediately after Me3SiC≡CSiMe3 is added (oily droplets on top of test-tube walls seen).
Figure 10. Schematic representation of Running ES-MS of Reaction Solution in (a) mass spectrometer syringe prepared in dry box, (b) Bruker Microtof-II.
Figure 11. Schematic representation of Crystallizing Ge9-divinyl with Sequestering Agents in (a) reverse layering and (b) several hours later.
Figure 12. Schematic representation of Checking Crystals Unit Cell on a D8-Diffractometer: (a) selecting crystals under the microscope and (b) collecting a unit cell.
It is important to clean well the partially oxidized Nb tubes. However, if the tubes are left too long in the Nb cleaning solution, this will severely compromise the thickness of the tube. Thus, 10 – 15 seconds are imperative and the tubes should be very lustrous at the end (Fig 3). After the tubes are sealed inside the fused silica jacket they should be cleaned again with a dilute Nb acid solution. This should result in mild effervescence, cleaning any oxidized areas on Nb tubes that occurred during welding or glass-blowing. However, be careful not to leave too long as the acid solution etches the glass (Fig 7b).
It is important to ensure that the Alkali metal is inserted at the bottom of the precursor tube and none is at the top sides or opening of the Nb tube. This will make welding easier as it heats the tube and prevent the metal from melting and then bursting out. (Tip: Use kimwipes wrapped around a spatula to clean completely.)
When welding the loaded Nb tubes, do not allow the welding tip to dwell on the Nb tube too long before it is completely sealed. (Tip: If this happens (a) the Nb tube and tip will be welded together; (b) K(s) will melt and leak (burst) from tube).
There are a few key tips and pointers to be aware of when preparing the fused-silica jacket via glass-blowing (Fig 6). First, be sure to have your hair pulled back and no baggy sleeves on. Always use quartz safety glasses before inserting any tube into the flame and remember that it is extremely hot and be careful of burns. Using the Nb tubes wet with water in the flame will cause steam to travel and can lead to burns. If the tube is wet with acetone, this will be carbonized and should be discarded. When looking through the safety glasses, a white hot area on the tube indicates that the glass is pliable and easier to manipulate. The width of the flame can always be adjusted by increasing or decreasing the gas flow and should be appropriate per area working with. Flaring the edges of the tube before sealing a joint thickens and opens the edges and allows for a quicker and easier attachment. Ensure that the tubes are white hot before attaching together. Blowing too hard through the mouthpiece will create holes. However, one can use the shaping rod to add glass to the hole and then work it into the tube. Make sure not to overwork the glass as it can collapse or become too thin. Most importantly, it does not have to be pretty, just functional (Fig 6). [It usually takes about a month of practice to achieve the skill.]
It is important to reach high vacuum in the quartz ampule when flame-sealing it because niobium reacts with oxygen at high temperatures, becomes brittle, and the tube contents become exposed.
When the tesla coil is used to check for leaks, a purple arc will be seen if there is a hole. In this case, repeat step 3 before continuing. Heating in the presence of a small hole, under vacuum, will result in oxidized, black, brittle and unusable Nb tubes.
It is important to use very dry ethylenediamine and not to stir the solution of K4Ge9 too long before functionalization or the clusters will be oxidized from a red color to a green color. If this is the case, the product yield will be very low.3-6 It should be mentioned that Ge94- clusters can be extracted not only in ethylenediamine but also in liquid ammonia.
If the mass spectrometer is not attached to the drybox, the PEEK tubing is filled with anhydrous solvent to act as a barrier to air to prevent the decomposition of functionalized clusters. The clusters are air and moisture sensitive and their decomposition will lead to clogging of the mass spectrometer (Fig 10).
One draw-back towards dissolution of the clusters is the limited choice of solvents that they are soluble in. Without the use of sequestering agents such as 18-crown-6 (1,4,7,10,13,16-hexaoxacyclooctadecane) or 2,2,2-crypt (4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane), K4Ge9 is only soluble in ethylenediamine and liquid ammonia and sparingly so in pyridine. With the addition of sequestering agents, these clusters are soluble in anhydrous pyridine, DMF (dimethylformamide), DMSO (dimethylsulfoxide) and MeCN (acetonitrile).
We have shown before that the alkenylation of Ge9 clusters by a reaction with alkynes is a nucleophilic addition of the clusters to the triple bond:55,57
K4Ge9 + 2TMS-C≡C-TMS + 6H2NR → [K]2[Ge9-(CH=CH2)2] + 4TMS-NHR + 2K-NHR
where H2NR represents the solvent ethylenediamine. In the reaction, one of the germanium lone pairs attacks the empty π* orbitals of the triple bond and supplies a pair of electrons to it. This breaks one of the C-C π bonds and, instead, a C-Ge bond is formed. The second carbon atom becomes an anion and deprotonates an ethylenediamine molecule from the solvent. The resulting ethylenediamine anions attack the Si-atoms of the TMS groups (SN2 reaction) and bond to them to form TMS-NHR. The resulting carbon anions get protonated again from more ethylenediamine molecules to form the final product [H2C=CH-Ge9-CH=CH2]2-.
The methods described above are applicable to (a) synthesis of a variety of intermetallics and (b) functionalization of deltahedral Zintl ions with a variety of pendant groups. At the same time they are attractive for their potential use as building block in cluster-assembled nanoparticles, larger aggregates, and metastable bulk compounds.60-66 With these techniques at hand, the stage is set for further developments of the chemistry and establishing fundamentals in the field.
The authors have nothing to disclose.
The authors would like to thank the National Science Foundation for the continuous financial support (CHE-0742365) and for the purchase of a Bruker APEX II diffractometer (CHE-0443233) and a Bruker Microtof-II mass spectrometer (CHE-0741793). The authors would also like to thank CEST facility for their use of the Micromass Quattro-LC mass spectrometer.
Name of the reagent | Company | Catalogue number | Comments |
D8-Xray diffractometer | Bruker | Bruker APEX II | |
Electrospray mass spectrometer | Bruker | Microtof-II | |
Electrospray mass spectrometer | Micromass | Quattro-LC | triple -quadropole |
Drybox | Innovative Technology | S-1-M-DL | IT-Sys1 model |
Inert Gas/Vacuum Shielded Arc Welding Arrangement |
LDS Vacuum Products | Special Order | |
Arc Welder Power Source | Miller | Maxstar-91 | |
Welding Rubber Gloves | The Home Depot | KH643 | |
Electric Engraver | Burgess Products | 74 | Vibro-Graver |
Circular Glass Saw | Pistorius Machine Co. Inc | GC-12-B | |
Tube Furnace | Lindberg/Blue M | TF55035 | Minimite Laboratory Tube Furnace, Moldatherm (1100 °C) |
Glass Drying Oven | Fisher Scientific | 13-247-650G | |
High Vacuum Hg Schlenk-Line | Special Order | Univ Of Notre Dame | Alternative: Edwards E050/60; VWR International; Cat. No. EVB302-07-110 |
Large Torch | Victor | JT100C | Welding torch, tip: Victor 5-W-J |
Small Torch | Veriflo Co. | 3A | Blow-pipe |
Tesla Coil | VWR International | KT691550-0000 | Leak detector |
Stirrer/Hot -Plate | VWR International | 12620-970 | VWR HOT PLATE STR DY-DUAL120V |
Balance | Denver Instrument Co. | 100A | XE Series |
Centrifuge | LW Scientific | E8C-08AV-1501 | Variable speed |
Graphite Reamer, (flaring) | ABR Imagery, Inc. | 850-523 B01 | Open holes in Glass Blowing and flaring edges |
Striker | Fisher Scientific | 12-007 | |
Vise-Grips | The Home Depot | 0902L3SM | |
Pipe-Cutter | The Home Depot | 32820 | |
Cutting Pliers | The Home Depot | 437 | |
Plastic Beaker | VWR International | 13890-046 | |
Measuring Cylinder | VWR International | 65000-006 | Careful, HF etches glass (if using a glass one) |
Large Plastic Bottle | VWR International | 16128-542 | |
13 x 100 Test-Tubes | VWR International | 47729-572 | CULTURE TUBE 13X100 CS1000 |
Laboratory (Rubber) Stoppers | Sigma Chemical Co. | Z164437-100EA | Size 00 |
Test-Tube Rack | VWR International | 60196-702 | 10-13 mm tube OD |
Stir-Bars | StirBars.com/Big Science Inc. | SBM-0803-MIC | PTFE 8×3 mm Micro |
Glass Pipettes | VWR International | 14673-043 | VWR PIPET PASTEUR 9IN CS1000 |
Rubber Bulbs | VWR International | 56311-062 | Latex, thin walled |
Glass Wool | Unifrax I LLC | 6048 | Fiberfrax Bulk Fiber Insulation, Ceramic fiber |
Glass Slides | VWR International | 16004-422 | 75x25x1mm, Microscope Slides |
Paratone-N oil | Hampton Research | Parabar 10312 | Known as: Paratone-N, Paratone-8277, Infineum V8512 |
High Vacuum Silicone Grease | VWR International | 59344-055 | Dow Corning |
Liquid Nitrogen | Univ. of Notre Dame | ||
Argon Gas Cylinder | Praxair Distribution Inc. | TARGHP | |
Nitrogen Gas Cylinder | Praxair Distribution Inc. | QNITPP | |
Oxygen Gas Cylinder | Praxair Distribution Inc. | OT | 337 cf CYL |
Hydrogen Gas Cylinder | Praxair Distribution Inc. | HK | 195 cf CYL |
Propane Gas Cylinder/source | Univ. of Notre Dame | UND | |
Quartz tubing, Lg | Quartz Scientific Inc. | 100020B | 20 mm id x 22mm od x 48″ clear fused quart tubing |
Quartz tubing, Md | Quartz Scientific Inc. | 100007B | Clear Fused Quartz Tubing,7mm id x 9mm od x 48″ |
Round Bottom Quartz Joint | Quartz Scientific Inc. | 6160189B | Ball joint |
Quartz Safety Glasses | Wale Apparatus | 11-1127 | waleapparatus.com |
Pyrex Safety Glasses | Wale Apparatus | 11-2125-B3 | For clear and color borosilicate glass |
Blow Hose Kit | Glass House | BH020 | glasshousesupply.com |
Niobium Tubes | Shaanxi Tony Metals Co., Ltd | Niobium Tube, 50 ft | Seamless Niobium Tube Outside diameter: 0.375 (±0.005) inches. Wall thickness: 0.02(±0.003) Inches Niobium should be annealed. |
PEEK Starter Kit for Mass Spect | Waters | PSL613321 | PEEK (PolyEtherEtherKetone) tubing, nuts, ferrule, fits |
Mass Spect Needle Set | VWR International | 60373-992 | Hamilton Manufacturer (81165) |
H2SO4 | VWR International | BDH3072-2.5LG | ACS Grade |
HNO3 | VWR International | BDH3046-2.5LPC | ACS Grade |
HF | VWR International | BDH3040-500MLP | ACS Grade |
Distilled Water | Univ. of Notre Dame | UND | |
Acetone | VWR International | BDH1101-4LP | |
Ethylenediamine | VWR International | AAA12132-0F | 99% 2.5 L |
Toluene | VWR International | 200004-418 | 99.8 %, anhydrous |
Mercury | Strem Chemicals, Inc. | 93-8046 | |
Potassium (K) metal | Strem Chemicals, Inc. | 19-1989 | Sealed in glass ampoule under Ar |
Germanium (Ge) powder | VWR International | AA10190-18 | GERM PWR -100 MESH 99.999% 50G |
Bistrimetylsilylacetylene, (Me3SiC≡CCSiMe3) |
Fischer Scientific | AC182010100 | |
18-crown-6 (1,4,7,10,13,16-Hexaoxacyclooctadecane) |
VWR International | 200001-954 | 99%, 25 gm |
2,2,2-crypt (4,7,13,16,21,24-Hexaoxa-1,10 diazabicyclo[8.8.8]hexacosane) |
Sigma Aldrich | 291110-1G | 98% |