Quadruply Metal-Metal Bonded Paddlewheels
Source: Corey Burns, Tamara M. Powers, Department of Chemistry, Texas A&M University
Paddlewheel complexes are a class of compounds comprised of two m…
1. Synthesis of Ligand ArN(H)C(H)NAr, Where Ar = p-(MeO)C6H4 (Figure 5)2
- Combine 6.0 g (0.050 mol) of p-anisidine and 4.2 mL (0.025 mol) of triethylorthoformate in a 100 mL round bottom flask with a magnetic stir bar.
- Attach a distillation head to the reaction flask.
- With stirring, heat the reaction in an oil bath to reflux (120 °C). Once reflux is achieved, the byproduct ethanol should begin to distill from the reaction. Collect the ethanol in a beaker placed at the end of the distillation head.
- Heat the reaction until the distillation of ethanol ceases (at least 1.5 h).
- Remove the flask from the oil bath and allow the reaction mixture to cool to room temperature. A precipitate should form. If the product does not precipitate, place the flask in an ice bath and scratch the bottom of the flask with a spatula to encourage crystallization.
- Recrystallize the product from a minimal amount of boiling toluene (for a more detailed procedure, please review the "Purifying Compounds by Recrystallization" video in the Essentials of Organic Chemistry series).
- Collect the product by filtration through a fritted funnel and wash with 10 mL of hexanes.
- Isolate the white product and allow it to dry in the air.
- Collect a 1H NMR of the solid using CDCl3.
Figure 5. Synthesis of ArN(H)C(H)NAr, where Ar = p-MeOC6H4.
2. Setup of the Schlenk Line
NOTE: For a more detailed procedure, please review the "Schlenk Lines Transfer of Solvent" video in the Essentials of Organic Chemistry series. Schlenk line safety should be reviewed prior to conducting this experiment. Glassware should be inspected for star cracks before use. Care should be taken to ensure that O2 is not condensed in the Schlenk line trap if using liquid N2. At liquid N2 temperature, O2 condenses and is explosive in the presence of organic solvents. If it is suspected that O2 has been condensed or a blue liquid is observed in the cold trap, leave the trap cold under dynamic vacuum. Do NOT remove the liquid N2 trap or turn off the vacuum pump. Over time the liquid O2 will sublime into the pump; it is only safe to remove the liquid N2 trap once all of the O2 has sublimed.
- Close the pressure release valve.
- Turn on the N2 gas and the vacuum pump.
- As the Schlenk line vacuum reaches its minimum pressure, prepare the cold trap with either liquid N2 or dry ice/acetone.
- Assemble the cold trap.
3. Synthesis of Mo2(ArNC(H)NAr)4 (Figure 6)2
CAUTION: The molybdenum source used in the synthesis of Mo2(ArNC(H)NAr)4 is Mo(CO)6, which is highly toxic and may be fatal if inhaled, absorbed through the skin, or swallowed. CO is generated during the reaction. Therefore, the synthesis must be conducted in a well-ventilated hood.
- Use standard Schlenk line techniques for the synthesis of Mo2(ArNC(H)NAr)4 (see the "Synthesis of a Ti(III) Metallocene Using Schlenk line Technique" video).
- Add 0.34 g (1.3 mmol) Mo(CO)6 and 1.0 g (3.9 mmol) of ArN(H)C(H)NAr to a 100 mL Schlenk flask and prepare the Schlenk flask for the cannula transfer of solvent.
- Add 20 mL of degassed o-dichlorobenzene to the Schlenk flask via cannula transfer.
- Fit the Schlenk flask with a condenser connected to the N2 gas line.
- Reflux the reaction for 2 h (180 °C) in a silicone-oil bath.
NOTE: Mo(CO)6 is volatile and will condense on the sides of the Schlenk flask during the reaction. To obtain higher yields, periodically re-dissolve any sublimed Mo(CO)6 by pulling the flask out of the oil bath and gently swirling the solvent in the flask. - Remove the Schlenk flask from the oil bath and allow the mixture to cool to room temperature.
- Filter the brown solution through a fritted funnel and wash the yellow precipitate with 10 mL of hexanes, followed by 5 mL of reagent grade acetone. The Mo2(ArNC(H)NAr)4 does decompose slowly in solution when O2 is present. Therefore, the filtration should be done promptly once the reaction is removed from N2.
- Collect the solid yellow Mo2(ArNC(H)NAr)4 and allow it to air dry.
- Using CDCl3 measure the 1H NMR spectrum of the product.
Figure 6. Synthesis of Mo2(ArNC(H)NAr)4, where Ar = p-MeOC6H4.
4. Single Crystal Growth
NOTE: Mo2(ArNC(H)NAr)4 oxidizes slowly in solution. The crystallization solvent should be degassed before use, but rigorous air-free conditions are not necessary to obtain X-ray quality crystals for single crystal X-ray diffraction.
- Degas 10 mL of dichloromethane (CH2Cl2) by bubbling N2 gas through the solution for 10 min (see the "Synthesis of a Ti(III) Metallocene Using Schlenk line Technique" video for a more detailed procedure on purging liquids).
- Make a saturated solution of Mo2(ArNC(H)NAr)4 by dissolving 20 mg of the solid in 2 mL of the degassed CH2Cl2.
- Make a pipette Celite plug by inserting a small piece of Kimwipe into a pipette. Add a small amount of Celite to the pipette.
- Filter the CH2Cl2 solution through the pipette Celite plug into a small 5 mL vial. Help to push the solution through the Celite using a pipette bulb.
- Using tweezers, insert the 5 mL vial into a 10 mL scintillation vial.
- In the outer scintillation vial, add 2 mL of hexanes.
- Tightly cap the scintillation vial and place it on a shelf where it will be undisturbed.
- Allow at least 24 h for single crystal growth (see the "X-ray Crystallography" video in the Essentials of Organic Chemistry series for a more detailed procedure on how to grow single crystals).
- Collect single crystal X-ray data on the sample (see the "Single Crystal and Powder X-ray Diffraction" video for a more detailed procedure on how to collect X-ray data).
Paddlewheel complexes are a class of compounds comprised of two metal ions held in proximity to each other by four bridging ligands. Depending on their properties, paddlewheel complexes are used as catalysts or building blocks for metal-organic frameworks, also known as MOFs.
The M-M bonding in a paddlewheel complex affects the structure and reactivity of the compound, and can be further modified by variation of the metal ion and ligands.
In order to understand these properties, it is crucial to comprehend the electronic structure of the M-M bond in a given paddlewheel complex.
This video will illustrate the principles of M-M?bonding, the synthesis and analysis of a dinuclear molybdenum complex, and various applications of paddlewheel complexes.
The M-M bond in a paddlewheel complex can be explained using molecular orbital theory.
When d-orbitals of two transition metals overlap, a M-M bond is formed. Depending on the orbital symmetry, three types of bonds can be created: ?, ?, and ? bonds.
If the z axis is assigned to the M-M bond, both dz2 orbitals overlap head-on to form a ? bond. Overlap between two lobes of the dxz or dyz orbitals creates a ? bond. Overlap between all four lobes of the dxy or dx2-y2 orbitals creates a ? bond.
The dx2-y2 orbital forms strong M-L bonds and usually does not contribute to M-M bonding. Hence, the maximum bond order achievable in many complexes is four.
Now, let's take a look at the M-M bond in a dimolybdenum complex. First, assign the axes and highest available symmetry.
The z-axis describes the highest rotational symmetry, which is the C4 axis lying along the Mo-Mo bond. Next, assign the x- and y-axis, which lie along the Mo-N bonds.
As seen, the dx2-y2 orbital on each Mo atom is involved in M-L bonding, leaving the dxy, dxz, dyz, and dz2 orbitals for M-M bonding. This can be further described with an MO diagram.
Linear combination of the dz2 orbital on each metal atom results in ? and ?* molecular orbitals, while dxz and dyz orbitals form ? and ?* MOs. Finally, linear combination of dxy atomic orbitals creates the ? and ?* MOs. Filling the MOs with the d electrons of the Mo centers results in a quadruple bond.
M-M bonds can be measured using X-ray crystallography. To normalize for atomic radius, the formal shortness ratio is calculated with this equation. The FSR describes the ratio of the bond distance in the solid state to the sum of the atomic radii of the individual atoms, and is used to analyze and compare bonds in different metal complexes.
Now that you understand what quadruple bonds are and how to analyze them, let's use this knowledge in a real example.
To begin, combine 6.0 g of p-anisidine and 4.2 mL of triethylorthoformate in a 100 mL round bottom flask with a magnetic stir bar. Attach a distillation head to the reaction flask, and place a beaker at the end of it.
Turn on the stirrer and hot plate. Collect the distilling byproduct ethanol in the beaker, and turn off the heat when ethanol distillation ceases.
Remove the flask from the oil bath and allow the reaction mixture to cool to room temperature. A precipitate should form. If the product does not precipitate, place the flask in an ice bath and scratch the bottom of the flask with a spatula to encourage crystallization.
Recrystallize the product from a minimal amount of boiling toluene. Collect the product by filtration through a fritted funnel and wash with 10 mL of hexanes.
Isolate the white product and allow it to dry in air in recrystallization dish. Lastly, using CDCl3, obtain a 1H NMR of the solid.
Before you start the synthesis, set up the Schlenk Line, ensuring N2 flow and a filled cold trap.
Familiarize yourself with the safety precautions using Mo(CO)6, which is highly toxic, and the Schlenk line techniques.
First, add 1.0 g of the freshly synthesized ligand and 0.34 g Mo(CO)6?to a 100 mL Schlenk flask and prepare the Schlenk flask for the cannula transfer of solvent.
Next, using cannula transfer add 20 mL of degassed o-dichlorobenzene to the Schlenk flask. Fit the Schlenk flask with a condenser connected to N2, and place the flask into a silicone-oil bath. Reflux the reaction for 2 h at 180 ?C.
When finished, remove the Schlenk flask from the oil bath and allow the mixture to cool to room temperature. Once cooled, promptly filter the brown solution through a fritted funnel, to reduce the rate of product oxidation in presence of air.
Wash the yellow precipitate with 10 mL of hexanes, followed by 5 mL of reagent grade acetone. Collect the yellow, solid product and allow it to dry on air. Using CDCl3, measure the 1H NMR spectrum of the product.
First, degas the 20 mL of CH2Cl2?to minimize the rate of product oxidation by bubbling N2 through it for 10 minutes. Then, dissolve 20 mg of the product in 2 mL of degassed CH2Cl2?to make a saturated solution.
Next, insert a small piece of a low-lint wipe into a pipette to make a Celite plug. Add a small amount of Celite to the pipette. Filter the saturated solution of product in CH2Cl2?through the plug into a 5 mL vial. Use a pipette bulb to carefully push the solution through the plug.
Using tweezers, insert the 5 mL vial into a 10 mL scintillation vial. Add 2 mL of hexanes to the outer scintillation vial. Cap it tightly and place it on a shelf where the scintillation vial will not be disturbed.
Wait at least 24 hours to allow for single crystal growth, then collect single crystal X-ray data on the sample. Now that all the data is collected, let's take a look at the results.
The ligand exhibits a characteristic peak for the NHC-HN?bond at 8.02 ppm. The aromatic peaks integrate to 8H, and the two methoxy groups integrate to 6H total at 3.80 ppm.
In comparison, the singlet for the NHC-HN?bond in the product occurs at 8.37 ppm and integrates to 4H. The doublets from the aromatic hydrogens are located at 6.49 and 6.16 ppm with a total integration of 32H. Lastly, the methoxy-groups are found at 3.70?ppm with an integration of 24H.
The two signals in the aromatic region indicate the 4-fold symmetry of the product. Additionally, the solid-state structure is consistent with the D4 point group and features a short Mo-Mo bond of 2.0925(3) ?.
Using the atomic radius of Mo, the FSR value for the M-M bond is calculated to be 0.72, which is consistent with the presence of a M-M quadruple bond.
Paddlewheel complexes, such as the dinuclear molybdenum complex synthesized in this video, display a wide range of properties and thus find application in diverse areas of chemistry.
For example, M-M bonds play an important role in catalysis. The paddlewheel complex Rh2(OAc)4 is a known catalyst for C-H bond functionalization via carbene and nitrene transfer reactions.
In a typical carbene transfer reaction, Rh2(OAc)4 reacts with a diazo compound to generate a Rh2 carbene intermediate. Subsequent insertion of the carbene into a C-H bond generates the product of C-H functionalization and regenerates the Rh2(OAc)4 catalyst.
Metal-organic frameworks, also known as MOFs, are porous compounds made of metal clusters linked together by organic ligands. This type of compound is a subclass of coordination polymers and can form one-, two-, or three-dimensional superstructures.
MOFs are used in many fields. Due to their high porosity and their large surface area per volume, MOFs find applications ranging from catalysts to gas storage and separation.
You've just watched JoVE's introduction to quadruply M-M?bonded complexes. You should now understand what quadruple M-M bonds are, how to synthesize paddlewheel complexes, and how to analyze them. Thanks for watching!
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Q1: What are paddlewheel complexes and how are they structured?
Paddlewheel complexes are compounds containing two transition metal ions held in close proximity by four bridging ligands, typically formamidinates or carboxylates. This arrangement enables metal-metal bonding between the two metal centers. The structure allows for diverse electronic configurations, making paddlewheel complexes useful as catalysts and building blocks for metal-organic frameworks.
Q2: How does molecular orbital theory explain M-M bonding in paddlewheel complexes?
When d-orbitals of two transition metals overlap, M-M bonds form based on orbital symmetry. The dz2 orbitals create sigma bonds, dxz and dyz orbitals form pi bonds, and dxy and dx2-y2 orbitals generate delta bonds. Since dx2-y2 typically engages in metal-ligand bonding, most paddlewheel complexes achieve a maximum quadruple bond order through sigma, pi, and delta contributions.
Q3: What is the formal shortness ratio and why is it used to analyze M-M bonds?
The formal shortness ratio (FSR) normalizes metal-metal bond distances by dividing the solid-state bond distance by the sum of atomic radii of the two metal atoms. This dimensionless value allows chemists to compare and analyze M-M bonds across different metal complexes, providing insight into bond strength and electronic structure without being affected by atomic size differences.
Q4: What experimental techniques are used to characterize paddlewheel complexes?
Paddlewheel complexes are characterized using NMR spectroscopy to analyze ligand structure and symmetry, and single crystal X-ray diffraction to determine solid-state structure and measure metal-metal bond distances. X-ray crystallography provides direct visualization of the complex geometry and enables calculation of the formal shortness ratio to confirm bond order.
Q5: How are dimolybdenum paddlewheel complexes synthesized in the laboratory?
Dimolybdenum complexes are synthesized by combining a freshly prepared ligand with Mo(CO)6 in degassed solvent under inert atmosphere using a Schlenk line. The mixture is refluxed at elevated temperature to promote metal-metal bond formation and ligand coordination. The product is isolated by filtration and purified through recrystallization to obtain pure material for characterization.
Q6: What are the main applications of paddlewheel complexes in chemistry?
Paddlewheel complexes serve as catalysts for organic transformations, such as carbene and nitrene transfer reactions for C-H bond functionalization. Additionally, they function as building blocks for metal-organic frameworks (MOFs), which are porous materials with high surface area used in catalysis, gas storage, and gas separation applications across diverse chemical fields.
Q7: Why is the D4 point group symmetry important in paddlewheel complex analysis?
The D4 point group symmetry describes the four-fold rotational symmetry of paddlewheel complexes along the metal-metal bond axis. This symmetry is reflected in NMR spectroscopy, where equivalent ligands produce single signals, and in X-ray crystallography, where the symmetric structure confirms the presence of a quadruple bond and validates the complex's electronic structure.