Determination of the Configuration Equilibrium of a Four Coordinate ...

Lab #2 Nickel Salicylalimine Magnetism - 2have different colors and hence different visible absorption spectra. You will record theelectronic absorption spectrum in several solvents to determine which bands correspond to eachgeometry.Two complexes are to be synthesized. The first, bissalicylaldehydenickel(II)dihydrate, 2, is typical of octahedralnickel with six oxygens in the first coordination sphere, beinggreen and paramagnetic. This complex will be used as startingmaterial in the synthesis of the more interesting compound bis-(N-isopropylsalicylaldimine) nickel(II), Ni( i PrSal) 2 , 1. The synthesis of 1 is an example of aSchiff base condensation reaction that is often used, with a metal ion as a template (Ni 2+ in thiscase) to prepare.The bis-(N-isopropylsalicylaldimine)nickel(II) is soluble in organic solvents and isunusual because it exists as an equilibrium mixture of a square-planar, diamagnetic form and atetrahedral, paramagnetic form. This equilibrium has been studied by magnetic andspectroscopic methods 1-4 and by nuclear magnetic resonance (NMR). 5 References to many otherstudies of this and similar Schiff-base complexes may be found in the literature cited. The NMRstudy is particularly interesting in that the complex contains two asymmetric carbons (one oneach chelate ring) and the NMR spectrum shows distinct peaks for the meso and dd-11 forms.The N-isopropyl complex to be synthesized is tetrahedral and fully paramagnetic in thecrystalline state with eff = 3.28 B.M. In solution in inert solvents (benzene, chloroform, etc.) themoment is considerably less since there is some planar form in equilibrium with the tetrahedralform.Preparation of Bis-salicylaldehyde)Ni(II)•2H 2 O, 2. Dissolve 5 g (0.0408 moles) ofsalicylaldehyde in 10 ml of 50/50 water/ethanol mixture and heat to boiling. To this add a hotsolution of 5.01 g (0.0205 moles) of nickel acetate tetrahydrate, Ni(C 2 H 3 0 2 ) 2 •4H 2 0 (FW = 248.9g/mol) dissolved in 10 mL water with stirring. Allow the solution to cool, filter in a Buchnerfunnel and wash successively with warm water, ethanol, and diethyl ether. Allow product to airdry. Weigh and compute yield.Preparation of Bis-(N-isopropyllsalicylaldimine)Ni(II), 1. Place your entire yield of thecomplex 2 in a round bottom flask and add 25 ml of ethanol and a 10% mole excess over 2 moles(CH 3 ) 2 CHNH 2 for one mole Ni(II) and reflux. After the reaction is complete, cool and filter the

Lab #2 Nickel Salicylalimine Magnetism - 3product. Wash with ethanol, weigh the product, and take a melting point. Compare to theliterature reported melting point in reference 4 .Determination of the Solid State Magnetic Moment. A Gouy balance will be used todetermine the solid state magnetic moment of compound 1. The following equations will allow acalculation of the effective magnetic moment, eff . The corrected molar susceptibility, X corr m , isX corr m = X m - X d , where X m is the molar magnetic susceptibility and X d is the molar diamagneticcontribution which results from all of the paired electrons in the molecule. X d is a negativenumber and is determined from Pascal's constants. 5 For Ni( i Pr-Sal) 2 , X d = -220 x 10 -6 cgs/mole.The effective magnetic moment, eff is eff = 2.84 (X corr m T) 1/2 (3)Determination of the Solution Magnetic Moment. Often the Gouy method is used; however,another method involves using nuclear magnetic resonance (NMR) spectroscopy. The keyequation for determining X g is due to D. F. Evans 6 and is for the 200 MHz NMR spectrometeryou will use:where is the chemical shift separation in Hz (see below), is the spectrometer frequency (200x 10 6 Hz), c is the concentration in grams/mL, and X 0 g is the gram susceptibility of the puresolvent. Make up a solution by dissolving 0.20 to 0.25 g of Ni( i PrSal 2 ) in CHC1 3 (not CDC1 3 !Why?) and dilute to the mark in a 10.0 mL volumetric flask. Transfer enough of the solutioninto a clean NMR tube to give ca. 2 cm column of solution. Carefully insert the inner concentricNMR tube which contains enough pure CHC1 3 to completely fill the narrow portion (see Figure1). Submit your NMR tube for analysis. An NMR spectrum of your sample focused on theCHC1 3 resonances, i.e., the inner and outer solvent peaks (Figure 2) will be returned to you.Measure the separation between the two peaks. Solve equation 4 for X g . X 0 g for CHC1 3 is -0.497x 10 -6 cgs/gram. Now calculate X corr m and eff as before.

Lab #2 Nickel Salicylalimine Magnetism - 4Figure 1.Figure 2. 300 MHz NMRDetermination of the UV-vis Absorbance Spectra. The UV-vis absorbance instrument in KSC440 will be used to collect the absorbance of compound 1. Before you begin, please carefullyread the instructions above the instrument so that you will know how to operate the spectrometerand save your data properly. Record the absorbance spectrum in the range 300 - 800 nm.Record the spectrum using CHC1 3 and toluene solvents. Determine the correct concentration tokeep the peaks on scale. Which of these solvents is more polar? Can you identify peaks due tothe planar and tetrahedral isomers? Note the color of the solid tetrahedral isomer. This shouldgive you a clue as to the peak assignments in solution. Assign the peaks for the tetrahedral andplanar isomers; identify the electron transitions involved for each form.In your report answer the following questions:A. Using simple crystal field theory, show why the tetrahedral complex has two unpairedelectrons while the planar isomer is diamagnetic.B. Ni( i PrSal) 2 is called a Schiffs base complex. Why?C. What other experiments can you think of which can verify that nickel is really undergoing ageometry change in this complex?D. Can you suggest an alternate explanation (i.e., other than a planar tetrahedral equilibrium) forthe magnetic results in this.

Lab #2 Nickel Salicylalimine Magnetism - 5References1. Holm, R. H., Journal of the American Chemical Society 1961, 83, 4683-90.2. Holm, R. H.; Chakravorty, A.; Dudek, G. O., Journal of the American Chemical Society1964, 86, (3), 379-87.3. Holm, R. H.; Swaminathan, K., Inorg. Chem. 1963, 2, 181-6.4. Sacconi, L.; Paoletti, P.; Ciampolini, M., Journal of the American Chemical Society1963, 85, 411-16.5. Pignolet, L. H.; Horrocks, W. D., Jr.; Holm, R. H., Journal of the American ChemicalSociety 1970, 92, (7), 1855-63.6. Evans, D. F., Journal of the Chemical Society 1959, 2003-5.Adapted with permission from Dr. Kent Mann, University of Minnesota.

Lab #2 Nickel Salicylalimine Magnetism - 6Pre-Lab Questions1. Using the Ligand Field or MO theory of bonding, predict the number of unpaired electronsfor Ni(II) complexes of tetrahedral, square planar, and octahedral geometry.2. What is a Schiff base condensation reaction?3. What is a spin-only magnetic moment? Why is that moment used for Ni(II) in this case?4. Why do you change solvents between preparation of the bis-(salicylaldehyde)Ni(II) • 2H 2 Oand the bis-(N-isopropylsalicyladimine)Ni(II)?