The crude product was then recrystallized to form our pure product. This was done by adding the crude product into a 250mL beaker and gradually adding dichloromethane (50.0mL) to dissolve. With the addition of dichloromethane, a white and grayish-black precipitate. This was then filtered again via vacuum filtration with the same conditions as before, except in this case the precipitate was discarded and the filtrate collected. The filtrate was transferred to a 250mL erlenmeyer flask where it was to be crystallized. The solution was placed on an ice bath for 45 minutes and ethanol(30mL) was added to lower the solubility of the dichloromethane to help crystallization occur. This was not enough to produce visible crystals, so the solution was placed under a steady stream of nitrogen gas to reduce the amount of dichloromethane and making a better ethanol : dichloromethane ratio. After about 30 minutes, lots of visible, white precipitate was formed. Once more, this was collected with vacuum filtration and stored in a small vial with a plastic stopper. The final mass of the product was 0.480 grams, which was a 19.7% yield from the possible 100%.
A melting point was then taken of the product. The observed melting point was 152°C to 162°C, with a theoretical melting point of 167°C1. Afterwards, an infrared spectroscopy was taken by making a pellet with KBr and a small amount of product. Infrared data was also taken for the decomposed product residue and filtrate1, and a triphenylphosphine stock solution.
1 SFU Chem 236 Lab manual, pp24-25.
Results and Data:
Harvey, B. K; A. A; McQuaker, N. R. Infrared and Ramen Spectra of Lithium Borohydride[Online] 1971, 49, 3282
2Bruice, P.K, Folchetti, N., Eds. Organic Chemistry; Pearson Education, Inc.: Upper Saddle River, NJ, 2007.
3Nakamoto, K,. Eds. Infrared and Raman Spectra of Inorganic and Coordination Compounds, 5th Edition, John Wiley and Sons: New York, 1997.
4 Kurita E.; Matsumoto, S.; Tomanaga, T. Quantum chemical calculations and vibrational analysis of compounds containing carbon-phosphorus multiple and single bonds [Online] 2003, 636, pp 53-67.
Balanced Equation:
CuSO4*5H2O + 2(C6H5)3P + NaBH4 [(C6H5)3P]2CuBH4
Theoretical Yield:
% Yield:
1Bruice, P.K, Folchetti, N., Eds. Organic Chemistry; Pearson Education, Inc.: Upper Saddle River, NJ, 2007.
2Kurita E.; Matsumoto, S.; Tomanaga, T. Quantum chemical calculations and vibrational analysis of compounds containing carbon-phosphorus multiple and single bonds [Online] 2003, 636, pp 53-67.
3Frisch, M. A; Heal, H. G; Mackle, H; Madden, I. O. Bonding and reactivity in Triphenylphosphineborane [Online] 1965, pp 899 - 907.
4SFU Chem 236 Lab manual, pp24.
Discussion:
The results show that our triphenylphosphine copper(I) tetrahybridoborate was successfully synthesized with pretty good purity. The limiting reagent in this experiment was calculated to be the copper sulphate pentahydrate. When dissolved with the triphenylphosphine in ethanol, there was a blue precipitate left at the bottom - this was the product of the unreacted copper and sulphate which contributed to a low product yield (see below). The addition of sodium borohydrate to the decanted solution formed a vigorous, exothermic reaction. At this time, there were many things happening in the reaction pot. This is where the formation of the three-center-two-electron bond is formed. To do this, the borane breaks its bonds with the sodium, to bond with copper. Since this bonding is electron deficient and usually unstable, it causes effervescence seen until an equilibrium is reached and the molecule can stabilize. To reduce the loss of product at this point, the sodium borohydrate was added slowly at first to prevent overflow. Once the bubbling seamed to have calmed down, indicating that almost all of the copper and triphenylphosphine had reacted, the rest of the sodium borohydrate could be added safely.
Some of the peaks obtained on the IR spectra of the product is also present in the spectra of triphenylphosphine and correspond quite nicely to its literature references. The spectrum obtained has all the necessary peaks to show that the product is as predicted.
For the triphenylphosphine, Aromatic (C-H) and (C-C) stretches/bends to show the presence of the benzene, and a (C-P) region which indicates the bonding between the aromatic rings and phosphorus as shown in the figure 1.
Figure 1: Triphenylphosphine - Trigonal planar geometry with three benzene rings branching off of one sp2 phosphorus atom
There was a terminal (B-H) stretch at ~2500cm-1, a bridging (B-H) stretch, and ν1 ν2 ν3 which correspond to that of a bridging borate ligand show in figure 2.
Figure 2: Copper (I) tetrahybridoborate - Tetrahedral, sp3 hybridized boron atom with 2 bridging trans-hydrogen atoms connecting to a single copper atom
From comparing the vibrational frequencies of the triphenylphosphine copper(I) tetrahybridoborate to the literature values obtained, we can conclude that the bridging between the copper and boron is bidentate and has the three-center-two-electron bonding. The three-center-two-electron bonding is possible due to the two bridging hydrogen’s sharing one lone pair of electrons while only occupying one bonding oribital. If the compound were tridentate there would have been significantly different peaks for the ν1 ν2 and ν3, and the mono dentate would have had only two of the three peaks. There was one noticeable impurity in the compound which would have lead to the lower than expected melting point, and the presence of a unwanted peak in the IR spectra. This was due to having not dried the compound well enough and quick crystallization, leaving excess water in the system and giving a large peak around 3500cm-1 on the IR spectra.
Though the experiment was a success, the yield was still really low. At one point in the synthesis, there were issues recrystallizing the product. This lead to having to go outside the guidelines of the procedure to try to induce crystallization in another fashion. The product was placed under a stream of nitrogen gas to try to evaporate some of the excess dichloromethane since it was the most viscous substance in the mixture. With doing this there was hope to reduce the solubility of the solvent to get some crystals forming. This worked fine, but due to lack of time, it could not be done for an extended period of time which would have been needed to obtain a much higher yield. Also, while doing this, the temperature of the solution reduced significantly, really quickly. With doing this, the crystals formed really fast, which would help give some impurities to the compound (H2O) seen on the IR and in the melting point.
Conclusion:
Triphenylphosphine copper(I) tetrahybridoborate was successfully synthesized by combining copper sulphate pentahydrate, triphenylphosphine sodium borohydrate. An infrared spectrum was then taken of the product, as well as a melting point to compare to literature references. These showed the relative purity, composition, and bonding types of the final product. With analysis from the infrared data, the product has a bridging bidentate bonds and an electron deficient three-center-two-electron structure.
