Week 7: Measuring Ligand Stability

This week I remade each of the manganese ligands that I’ve studied so far this summer and tested their stabilities over time. After going through my lab notebook record I picked the method for making each one that had worked the best. I made the manganese oxalate and manganese malate by using the acid version of the ligand combined with ammonium bicarbonate, degasing with argon, and adding manganese acetate. The manganese oxalate reaction did not work this time, most likely because I made up the oxalic acid/ammonium bicarbonate solution too far in advance and it was no longer stable when I added manganese acetate. Next week I am going to try the reaction again with a fresh solution. For the manganese pyrophosphate and manganese citrate ligands, I added the sodium version of the ligand to degased water and then added manganese acetate, adjusting the pH before and after addition. Making up the manganese DFOb allowed me to use manganese oxides like the ones I made up last week. In an effort to add more ligands to the list, I tried L-glutamic acid, ascorbic acid, and succinic acid with ammonium bicarbonate and manganese acetate. None of them reacted, but I am hoping to try reacting them with manganese chloride (MnCl₂) instead of manganese acetate, to see if a different oxidation state of manganese for the reactant will work better. Immediately after I made each ligand solution I did a scan from 260 to 748 nm to see where the compound absorbed light. I also measured the absorbance at a specific wavelength, such as 428 nm for manganese citrate, so I could use the molar absorptivity constants of each compound to calculate the concentration of the solution. Then, at a variety of time points through the week I measured the reactivity of the ligands with leucoberbolin blue (LBB) and calculated the rate constants of the reactions to see how they changed over time.