Introduction

The purpose of this research project was to find the equilibrium constant for hydrogen bonding between Ru(bipy)2(di-carb)Cl2 and potassium oxalate molecules by using capillary electrophoresis. Capillary electrophoresis can be used because it excites and detects fluorescing molecules such as Ru(bipy)2(di-carb)Cl2. The original goal of this study was to synthesize and then hydrogen bond Ru(bipy)2(di-carb)Cl2 to potassium oxalate, a substrate that shows hydrogen bonding capacity. The synthesis of Ru(bipy)2(di-carb)Cl2 was not successful so [Ru(bipy)2(amino-phen)]Cl2 (Figure 1) was synthesized instead and hydrogen bonded to potassium oxalate.

Hypothesis:

The hypotheses for this project were: first, a Ru(bipy)2(di-carb)Cl2 molecule would be an effective molecule for this experiment because of its hydrogen-bonding capacity. Second, capillary electrophoresis, which uses an argon laser to excite fluorescing molecules for detection, could be used effectively to find the equilibrium constant because the Ru(bipy)2(di-carb)Cl2 is a fluorescing molecule.

Background:

The 2-2’-bipyridine molecule is a hetero-cyclic aromatic molecule (Figure 2). The bipyridines are uncharged molecules with the chlorines acting as counter-ions. The unshared pair of electrons on the nitrogens bond nicely to the ruthenium during the synthesis of the Ru(bipy)2(di-carb)Cl2 (Boyd).

The Ru(bipy)2(di-carb)Cl2 molecule is shaped like a ship’s propeller with each nitrogen in the bipyridine bonding to the ruthenium. The bipyridines are spaced evenly around the central ruthenium molecule at a 90º bond angle. The dicarboxlic acids are at 90º angles to the bipyridines, forming hydrogen-bonding sites for the substrates. The structure of the Ru(bipy)2(di-carb)Cl2 molecule is important because the hydrogen bonding sites are spaced far enough apart so that the substrates bonded to them are not attracted each other, so as to create an undesirable effect such as polymerization (Boyd).

The capillary electrophoresis instrument (Figure 3) separates substances by size and charge. Platinum wires are used as electrodes as they are inserted into two buffer vials. Electricity is then run through the wires, creating a circuit in the small bore (50µ) capillary. The electric field running through the capillary attracts positively charged ions to the negatively charged electrode, therefore separating the different substances (Figure 4) (Mabott).

The capillary electrophoresis instrument measures time for molecules to travel from one end of the capillary to the other by detecting the release of photons by the molecules after a laser excites the molecules. At the end of the capillary, there is a window in which a laser beam excites the sample. As the sample returns to its ground state, it emits photons that are detected by an optical sensor. The capillary electrophoresis instrument will only detect fluorescing molecules because they stay in excited states long enough to be detected by the sensor (Boyd).

Ruthenium chloride was used for its fluorescing nature in an aqueous solution. Ruthenium chloride absorbs photons at 400 to 500 nm (Figure 5) and emits at 600 nm. The absorbance and emissions meet the criteria needed for detection in the capillary electrophoresis instrument (Mabott).

Procedure:

Synthesis of Ru(bipy)2Cl2: (Procedure from Sprintschnik)

In a 25mL round bottom flask fitted with a reflux condenser, 2 g of ruthenium(III) chloride trihydrate was mixed with 2.2 g of LiCl; 2.4 g of 2,2’-bipyridine; and 12 mL of N,N-dimethylformamide (DMF). The solution was then refluxed for eight hours and cooled to room temperature. The solution was then poured into 40 mL of stirring acetone and cooled to 0º C overnight. The resulting crystals were filtered in a fine glass frit and washed with three portions of 5 mL of water and three portions of 5 mL of diethyl ether.

Synthesis of [Ru(bipy)2(di-carb)]Cl2:

A 1.2 mmol ratio of Ru(bipy)2Cl2 to 2,2’-bipy-4,4’-dicarboxylic acid was mixed in a 50 mL three-neck, round bottom flask which had a gas adapter, reflux condenser, and a rubber stopper. The reaction was performed under a nitrogen pure atmosphere using a Schlenk line. A minimum of 50/50 by volume ethanol-water solution was passed through a canula until all of the solid was dissolved. The mixture was then allowed to reflux for three hours, then cooled for 30 minutes. The solid was precipitated out of the mixture by dripping saturated NH4PF6 into the solution. The solid was then filtered in a fine glass frit, washed with water, and dried with diethyl ether.

Synthesis of [Ru(bipy)2(amino-phen)]Cl2:

A three neck round bottom flask was fitted on a heating mantle with a reflux condenser on one of the necks and a magnetic stir bar. Then a 1.2 mmol ratio of 10-10’-amino-phenanthroline to [Ru(bipy)2]Cl2 was dissolved in a minimum of acetonitrile. The mixture was refluxed for 30 minutes and then cooled at room temperature. Next a saturated solution of NH4PF6 in acetonitrile was made. By dripping the NH4PF6 solution into the solution, a [Ru(bipy)2(amino-phen)]PF6 precipitate crashed out. The [Ru(bipy)2(amino-phen)]PF6 solid was dissolved in acetone. A saturated solution of tetrabutyl-ammonium-chloride in acetone was made and dripped into the [Ru(bipy)2(amino-phen)]PF6 solution to precipitate [Ru(bipy)2(amino-phen)]Cl2.

Capillary Electrophoresis Procedure:

The wavelength filter was taken out of the capillary electrophoresis instrument. A time program was written that performed a high pressure rinse of

1 M HCl for five minutes, then a high pressure rinse of 1 M NaOH for five minutes, followed by a high pressure rinse with acetic acid buffer for five minutes. The sample was then injected into the capillary with high pressure for two seconds. The separation time was set for 20 minutes at 10 Kv with the separation buffer. The time program and method were saved. The program was run and the data file saved.

NMR Procedure:

The sample was run through NMR. The peaks were then analyzed using Techmag software.

Results:

The successful synthesis of [Ru(bipy)2(amino-phen)]Cl2 (Figure 6) was supported by proton NMR spectroscopy (Figure 7). The [Ru(bipy)2(amino-phen)]Cl2 has 25 protons that show up on a NMR spectrograph. As seen in Figure 7 of a NMR spectrograph, one integrated peak equals one proton detected. When a NMR spectrograph of the [Ru(bipy)2(amino-phen)]Cl2 molecule was analyzed, a total of 25 integrated peaks were seen. This is substantial proof that the [Ru(bipy)2(amino-phen)]Cl2 molecule was successfully synthesized. Any deviation from the expected 25 integrated points would have indicated a different molecule was formed; this was not the case.

In the capillary electrophoresis, a larger molecule takes longer to pass the detector. As seen in Figure 8, the [Ru(bipy)2(amino-phen)]Cl2 bonded to the potassium oxalate in the capillary electrophoresis because the run time of the unbonded [Ru(bipy)2(amino-phen)]Cl2 and the run times of the [Ru(bipy)2(amino-phen)]Cl2/potassium oxalate mixture were four minutes and six minutes respectively

Conclusion:

The synthesis of [Ru(bipy)2(amino-phen)]Cl2 was successful and proven by proton NMR spectroscopy seen in Figure 7. The hypothesis that the capillary electrophoresis would provide the necessary data was proven by the fact that the [Ru(bipy)2(amino-phen)]Cl2 was excited by the argon laser and the fact that the [Ru(bipy)2(amino-phen)]Cl2/ oxalate mixture moved through the capillary slower than the [Ru(bipy)2(amino-phen)]Cl2 alone (Figure 8). Although no equilibrium data was obtained, it was proven that the capillary electrophoresis will provide the necessary information to find the hydrogen bonding equilibrium constant. The next step for this project is already in progress. A graduate student is continuing my research by modifying the [Ru(bipy)2(amino-phen)]Cl2 molecule and bonding it to different substrates.

Appendix

Figure 1 (Diagram by Shroff after Boyd).

Figure 2. 2-2’-bipyridine (Diagram by Shroff)

 

Figure 3. Capillary electrophoresis parts (Diagram by Shroff after Mabott)

Figure 4. The migrating molecules (Diagram by Shroff after Mabott).

 

Figure 5. Absorbance spectra for Ru(bipy)3Cl2 (Diagram by Shroff).

 

 

 

Figure 6. [Ru(bipy)2(amino-phen)]Cl2 showing 25 protons. (Diagram by Shroff)

Figure 7. 1H NMR spectrograph of [Ru(bipy)2(amino-phen)]Cl2

Figure 8. Electropherogram showing the [Ru(bipy)2(amino-phen)]Cl2 and potassium oxalate mixture moving slower than the plain [Ru(bipy)2(amino-phen)]Cl2

 

Bibliography

Bentor, Yinon. Chemical Element.com - Ruthenium. Nov. 17, 1998

<http://www.chemicalelements.com/elements/ru.html>.

Boyd, Dr. David C. Personal Interview. 25 June 1998

Giordiano, Paul J., et al. Journal of American Chemical Society, 1978, 100:22, p. 6960

Goss, Charles A. and Héctor D. Abruña. Inorganic Chemistry, 1985, 24, 4263

Mabott, Dr. Gary letter to Shroff, Gaurav. 19 June 1998. [An unpublished letter summarizing the use of the capillary electrophoresis instrument]

Mabott, Dr. Gary. Personal Interview. 19 July, 1998.

Mabott, Dr. Gary letter to Boyd, Dr. David. 24 July 1998. [An unpublished letter summarizing the progress of the project]

Sprintschnik, Gerhard, et al. Journal of American Chemical Society, 1977, 99, p.4947