Formation and Characterization of (dppf)ZnCl2 From Ferrocene.Eleanor R. Goblirsch, Danielle L. Millin

Department of Chemistry, Lawrence University, 711 E Boldt Way, Appleton, Wisconsin 54911, United StatesABSTRACT: The synthesis of bis-diphenylphosphino ferrocene zinc dichloride was done to confirm the predicted geometry at the zinc metal center. The characterization of the compound was done using cyclic voltammetry, UV visible spectroscopy, 1H NMR, 31P NMR, and IR Spectroscopy. These tests verified the creation of (dppf)ZnCl2 and revealed certain properties including color, bond type, and oxidative state.

INTRODUCTIONOrganometallic compounds and their geometries ex-hibit many different chemical properties that are in-formative when starting to study inorganic chemistry. The lab incorporates many new techniques such as NMR, cyclic voltammetry, and magnetic susceptibil-ity measurements. The reaction in question is the lithination of fer-rocene, production of bis-diphenylphosphino fer-rocene (dppf), and chelation of a zinc ligand to the phenyl groups (Scheme 1). The ferrocene contains a metal center with aromatic rings attached and can be functionalized by lithium which enables bidentate lig-ands, like the phenyl groups, to attach where the rings were lithinated. From this, a zinc complex is added to form a heterobimetallic complex. Scheme 1. Reaction Scheme of Ferrocene to (dppf) ZnCl2

A concept applicable to finding the geometry of a molecule through hypothesis is doing a d-orbital split-ting diagram for the metal. From the complex draw-

ing, zinc has a +4 oxidation state and a d8 electron count, as seen in figure 1. With a d8 electron count, the geometry around zinc should be square planar. The complex has all its electrons paired so it should be diamagnetic.

Figure 1. D-orbital splitting diagram of zinc. Although we can predict the shape of the complex, it needs to be tested in lab. To characterize the complex and the geometry of the complex, multiple tests can determine what is in the compound, different types of bonds, or the energy potentials. The quickest way to determine the purity of the product will be to do 31P NMR because of the small amount of phosphorus in the compound. A drawback within the lab is the handling of two reagents. The TMEDA, used to enhance the metalat-ing ability of the lithium, is pyrophoric. The n-butylithium is also pyrophoric and can ignite with ex-posure to air and water. It must be done through a sy-ringe in a vacuum setting under nitrogen.

METHODS

Safety

Caution. N-butylithium is pyrophoric and should be used under vacuum in nitrogen.

ZnCl2

Lithination of Ferrocene

A three-necked flask was flushed with nitrogen gas via Schelnk technique and fitted with a reflux con-denser. 2.6 mL of TMEDA and 5.0 mL of dry hex-anes were combined. 7.2 mL of n-butylithium was sy-ringe-transferred while under constant stirring. The complex formed after ten minutes and was a yellow color. 1.7 g of ferrocene in 40 mL of hexane was added into the flask via syringe with continuous stir-ring. The reaction was stirred for a week until next lab; however, reference c. stirred for only 4 1/2 hours.

Bis-Diphenylphosphinoferrocene

5.5 mL of hexane was added to the dilithioferrocene. 3.2 mL of diphenylchlorophosphine was added drop-wise through syringe. The complex turned a red-or-ange color and the mixture was stirred for about two hours. 3.4 mL of water was added to the ferrocene so-lution to quench any excess reactants and then filtered out. 15.41 mL of dioxane was heated and the hot dioxane was added to a flask to dissolve the product.

Diphenylphosphinoferrocene Chelation to Zinc Dichloride

The crystallized dppf was rinsed out the flask with cold dioxane and filtered. The dppf was analyzed us-ing 31P NMR. The solvent used for the NMR was CDCl3. The dppf did not show conclusive results for the right compound so other dppf from another group was used. 16 mL of 2-propanol was heated. 0.23 g of dppf was added to the hot propanol and stirred to dis-solve. 2 mL of 2-propanol and 1 mL of methanol were combined. .0562 g of ZnCl2 was added to the 2-propanol and methanol solution. The ZnCl2 solution was added to the hot propanol and dppf solution. It was boiled in a thermal well for three hours and then take off stirring. It was allowed to cool and crystal-lize.

Characterization of Diphenylphosphinoferrocene Zinc Dichloride

1, 2 dichloroethane was used as the solvent for doing ultra-violet visible spectroscopy. A solution of 0.003 M was used. Infrared spectroscopy was also done. 31P NMR and 1H NMR was done using CDCl3 as a sol-vent. A magnetic susceptibility balance was going to be used, but there was a bubble in the capillary tube so there were not good readings to determine the dipole moment. Cyclic voltammetry was done with the dppf in a 3 M 1, 2 dichloroethane solution.

RESULTS

The lab resulted in a flawed preparation of the dppf compound and a formation of a zinc ligand to the dppf.

Observations

The final (dppf)ZnCl2 product formed through our methods was made up of two distinct components. One was an orange-yellow needle like crystal struc-ture which could be dissolved in 1,2 dichloroethane. The other was composed of rusty red opaque flakes which did not dissolve in 1,2 dichloroethane. All analyses of liquid product were done using solutions composed of a homogenous mixture of these compo-nents with the red flaks filtered out, besides IR.

Several analyses of the final (dppf)ZnCl2 complex were used in order to verify and characterize the com-pound. These analyses included CV, 31P NMR, 1H NMR, IR, and UV Vis Spectroscopy.

Cyclic Voltammetry

Figure 2. Cyclic Voltammogram. Performed on 3mM solution of (dppf)ZnCl2 sample in 1,2-dichloroethane, o.1M tetrabutylammonium hexafluorophosphate. Ag was reference electrode and glassy carbon was work-ing electrode.As is seen in Figure 2, (dppf)ZnCl2 exhibits a small anodic peak but no corresponding cathodic peak. The (dppf)ZnCl2 had an energy potential of 0.437 V. When compared to the reference electrode of silver and has an E˚ of 0.799 V gives us a potential of 1.236 V in reference to the NHE [d].

31P NMR 31P NMR spectra for the dppf intermediates and final dppf products were obtained in CDCl3. Each peak ob-served in Table 1 appeared in singlet.

31P NMR peaks observed for the dppf complex formed did not correspond to peaks expected from a pure dppf product. A compound synthesized by col-leagues did have appropriate peaks and was substi-tuted for the remainder of the experiment.

Final (dppf)ZnClz product did display a singlet peak in the expected region.

Table 1. 31P NMR Peaks Observed vs. Peaks Expected

Sample Peaks Ob-served (ppm)

Expected peaks (ppm)

dppf made 38.18, 67.95 -16.82a

dppf used -17.42 -16.82a

(dppf)ZnCl2 -22.19 -21.34a

1H NMR

Proton NMR in CDCl3 revealed several groups of peaks including a triplet peak around 0 ppm, a doublet around 1.2 pmm, and a singlet around 1.54 ppm. A fi-nal singlet can be observed in the 7.2 ppm range (See Table 2).

Table 2. 1H NMR Peaks

UV Vis Spectroscopy

Figure 3. UV Vis graph depicting spectrum for (dppf)ZnCl2.

A λmaz for the UV absorbance spectrum was identified at 451.04 nm. The absorbance at this point falls around 0.29. The solution absorbs light in the violet spectrum; therefore, a yellow-colored solution is seen. The noise seen before 300 nm could be a result of the glass cuvette used or the solvent.

IR Spectroscopy

IR Spectroscopy displayed key peaks at 1480 cm-1, 1434 cm-1, 896 cm-1, and 830 cm-1. Additional peaks were found in the fingerprint region that were not found to be easily identifiable.

Dipole Moment of (dppf)ZnCl2 Using a Magnetic Sus-ceptibility Balance

The test of the dipole moment could not be accom-plished because there was a bubble within the capil-lary tube that continuously altered the reading so the readings would not stay at a steady point and a dipole moment could not be found.

DISCUSSIONThe initial goal was to study the geometry of the mol-ecule. However, unforeseen errors in the creation and characterization of the geometry of the compound made us rethink our goal to just being the characteri-zation of the compound and reasons for the errors seen in lab.

After the dppf formation, it was hard to tell which product in the flask was the product needed. There was a large amount of solid left along the sides of the glass which could have been unreacted product be-cause the stir bar was not large enough or been the dppf product needed. However, the 31P NMR showed that the dppf was contaminated with other product or

another compound was accidently made. Smaller glassware and larger stir bars should be used to make sure all the reactants come into contact with each other and fully react.

After a dppf compound with an anticipated peak num-ber in the 31P NMR was chelated to a zinc ligand, many tests were run to see if the desirable product was made. The results from this experiment were also compared with reference a. because of the similar ex-periment and same metal ligand.

The cyclic voltammetry shows that once the com-pound was oxidized, it could not be reduced again. The anode attracted the electrons from the (dppf)ZnCl2 into the compound into [(dppf)ZnCl2]+. The cation of this oxidation results in an instability of the complex; therefore, quickly decomposing and not showing a cathodic peak.

31P NMR displayed a singular peak in the expected re-gion, likely meaning we successfully created our de-sired product.

1H NMR showed a variety of peaks. The lowest a 0 ppm can represent the TMS of the NMR. The next peaks around 1.20 ppm and 1.54 ppm represented 2-propanol and water, respectively. The peak at 7.24 ppm can represent hydrogens around a phenyl group as seen in reference b. This is consistent with the ex-pected structure of our compound.

The UV Vis spectrum gave us a λmaz at 451.04 nm which is consistent with reference a. Using Beer’s law of ε=alc, ε can be determined to be 96.66 M-1cm-1. This value is not consistent with reference a. which have found ε to be approximately 160 M-1cm-1. This difference in ε can be attributed to the inaccuracy of our concentration of the sample. The total mass recorded was not completely dissolved in the 1, 2 dichloroethane and solid material was filtered out. However, from the detectable shift in energy of the electrons, there is a transition within the d-orbitals of Zn, possibly from the t2g orbital set to the eg set, if the d-orbital splitting diagram holds being a square planar molecule.

IR spectroscopy displayed specific bond peaks can show what types of bonds are in the molecule. The pertinent bonds are at 1480 cm-1 and 1430 cm-1 and describe the C=C stretching bonds within the phenyl group. Additionally, peaks at 830 cm-1 and 896 cm-1

could be indicative to hydrogen attached to aromatic carbons [e]. The peaks in the fingerprint region were not characterized due to available research on the compound. We hypothesized that there were many

impurities in this compound that were not filtered out for this test.

Although the magnetic susceptibility constant could not be determined, the predicted value should be 0 μ because all the electrons are paired; therefore; the magnetic field would not affect the compound.

CONCLUSION Despite errors in the synthesis process, the 1, 2 dichloroethane soluble portion of the final product was verified through NMR to be the target product of (dppf)ZnCl2. UV Vis analysis did display the ex-pected absorption peak and therefore the correct color. This compound was determined to have an un-stable cation as predicted in previous research but other analyses could not be adequately performed due to the impurity of the product.

ASSOCIATED CONTENT Supplemental materials such as full NMR and IR spectra can be found in supplemental materials.

ACKNOWLEDGMENTSThe authors thank Professor Sazama for his mentor-ship in lab and Daniel Martin for providing stockroom assistance. Thank you to Trent and Imran for the loan of dppf.

ABBREVIATIONSDppf, 1,1‘-Bis(diphenylphosphino)ferrocene; TMEDA, tetramethylethylediamine; NMR, Nuclear Magnetic Resonance Spectroscopy; IR, Infrared Spec-troscopy; CV, Cyclic voltammetry

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d. Standard electrode potential (data page). (2016, October 24). Retrieved November 15, 2016, from https://en.wikipedia.org/wiki/Stan-dard_electrode_potential_(data_page)

e. IR Absorption Table. (n.d.). Retrieved Novem-ber 15, 2016, from http://webspectra.chem.u-cla.edu/irtable.html