Lower-valent complexes of early transition metals
In general, the only complexes of the early transition metals (Sc, Ti, V) that are stable to oxygen and water are those with the metals in the highest oxidation states. These transition metals have a high affinity for oxygen. For example, the most common and air-stable complexes of vanadium incorporate the vanadyl ion, VO2+. The most common titanium material is titanium dioxide, which is used as a whitener in paints.
However, lower-valent complexes of these transition metals are interesting as well. Care must be taken to exclude oxygen and water in the synthesis of the complexes because the starting materials and products will react with oxygen and/or water to give unwanted oxidized products. The complex [Ti(urea)6]I3 is unique in that it is reasonably air stable if kept dry. [It is believed to be stabilized by hydrogen bonding between adjacent urea molecules. This stability is kinetic rather than thermodynamic (Why?)]
I. Synthesis of[Ti(urea)6]I3
TiCl3 (air and moisture
solution of 25g urea in 25 mL H2O
solution of 50g KI in 30 mL H2O Prepare the two solutions needed on the day of the synthesis. Warming of the mixtures to 40-50 degrees C may be necessary. Prepare a glove bag containing the stock bottle of TiCl3, a balance, vials with caps, spatulas and scoopulas. Purge the glove bag for 30 min with flowing nitrogen gas. Weigh out 4.00g TiCl3 into a vial, tightly cap the vial and set aside for removal when use of the glove bag is complete.
Perform the next step in a properly vented fume hood!
Add the TiCl3 to the solution of urea in a beaker quickly and carefully with stirring. Anhydrous TiCl3 fumes in moist air. The reaction of TiCl3 with water is very exothermic, and the resulting warm mixture (about 45-50 degrees C) should be filtered to remove any TiO2 solid. Add the solution of KI with stirring. Cool the reaction mixture in an ice bath and collect the deep blue crystals of [Ti(urea)6]I3 by filtration. Dry the crystals by continuous suction and transfer them to a beaker. Store the beaker overnight in a dessicator filled with fresh dessicant, then transfer the crystals to a vial. The vial should be filled completely with tightly packed product and capped to exclude oxygen. There is a pronounced color change upon decomposition.
II. Characterization of[Ti(urea)6]I3
- Determine the IR spectra of the reactants and product.
- Determine the UV-Vis spectra of the product dissolved in KI solution and in a freshly prepared urea solution. Compare both spectra to that of Ti(H2O)63+ (available in most standard inorganic chemistry textbooks).
- Determine the weight percent of titanium in the product. Heat a weighed amount of the product in a dry, weighed crucible to red heat in the fume hood. All components except titanium are decomposed to volatile products, and titanium is left behind as TiO2. Decomposition proceeds with the evolution of a very unpleasant smoke. Weigh the residue.
- Determine the magnetic moment of the complex using the Guoy balance.
III. Computational Chemistry
This component is designed for the CAChe molecular modeling program.
- Build the structures [Ti(water)6]3+ and [Ti(urea)6]3+. Remember to designate the titanium atom as a cation with d2sp3 hybridization, +3 charge and a radius of 0.745 Å.
- Minimize the structures in Mechanics.
- Optimize the geometry of both molecules in ZINDO. Specify the state as doublet (one unpaired electron on the Ti3+).
- Do a configuration interaction calculation in ZINDO. Specify the state as doublet (one unpaired electron on the Ti3+), and choose the SCF type as ROHF.
- In Tabulator, create an electron density map of each molecule using the density gradient.
- Report the yield, percent yield and characterization of [Ti(urea)6]I3.
- Report the obtained and theoretical percent titanium in the product. Using this information, determine the purity of the product.
- Identify and assign pertinent bands in the IR spectra of urea and [Ti(urea)6]I3. They should be bands useful for distinguishing between urea bonded to Ti through N or O.
- Using the results of the UV-Vis and IR spectra, determine the coordination sphere about the Ti ion and the ease with which the urea ligands are replaced. Is I- coordinated to Ti3+?
- Report the magnetic moment of the complex. Compare your value with the spin-only value for a d1 ion (2.73 B.M.) and the published value for [Ti(urea)6]I3. Is your result consistent with the assignment of the complex as a Ti3+ complex?
- Analyze the computed spectra of the two molecules. How do they compare to the experimental results? Based on the modeling results of the urea complex, can you explain why there are two bands in the UV-Vis spectrum when only one is expected?
- Look at the electron density maps of the two molecules. Based on these, can you explain why the urea complex is kinetically stable?