An Application of the NMR Method for Determination of Magnetic Moments to Study Anomalous Paramagnetism in some lron[lll) Chelates Jeffrey G. Hughes' and Peter J. Lawson Capricornia Institute, Rockhampton, Queensland 4702, Australia Mametic Data for Fellll~1S.CNR.h Comolexes at 28 "C
The use of the Nuclear Magnetic Resonance technique to determine magnetic moments in solution has been described by Crawford and Swanson in this J o ~ r n a l The . ~ method relies on measuring the separation (Afl in the resonance positions of two identical protons in two solutions. One of the solutions contains the paramagnetic material and the other contains pure solvent. The separation, Af (in hertz) is related to the mass susceptibility, x,, of the dissolved paramagnetic substance by the following relationship
where f is the frequency of operation of the machine, m is the concentration of naramaenetic substance (e/mL) and r, is the mass suscepcbility 02 the pure solve;. he mag&c moment is then calculated using eqs 2 and 3. where x~ is the molar,susceptibility and M is the molar weight of the complex. xhl isohtainedfrom XM by includinga diamagnetic correction for the ligands. This is done by summing the diamagnetic corrections for each ligand a t o m 3
Magnetic Moments (EM) in Chloroform Substituents Student Data Literahrre value^"^ N,Ndibenryl
3.7-3.9
N.Kdiethy1 N,Udimethyl Pyrmlidyl
4.4-4.6 4.3-4.6 5.7-5.9 2.2-2.5
N,Ndicyclohexyl
3.66 4.41 4.20
5.90 2.62
The cwnplexes are prepared by mixing alkaline s d u t i o n ~ of the ligand with sroichiometrir amounts of F d l , iolution:
FeC13+ 3 R,NCS,Na
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Fe(R,NCS,),
+ 3NaCI
The complexes precipitate readily from aqueous solution and can be recrystallized from chloroform/ethauol to give lustrous black or brown crystals. Recrystallization is necessary to remove semihydroxide impurities. Some typical student data for the magnetic moments of these complexes are shown in the table.
where p is the magnetic moment in Bohr Magnetons (BM) and T is the temperature of the NMR probe. In this experiment the technique is applied to the study of the anomalous nararnaenetism of iron(II1) NN-dialkvldithiocarbamnte;. These complexes are anomalous in that their Preparation of the Ligands beha\.ior is neither "hirh s ~ i n "nor "low suin".','De~endin~ on the nature of the ilk$ substituents 6n the ligand, the NaOH !,\ value of the magnetic moments can be pure low spin, pure R2N: + CSz Na+ + $C-NRz high spin, or intermediate between these values. s The explanation is that the ligand field energies for these complexes lie close to the crossover between the high-spin, where Rz = pyrrolidyl, R = CH3, CHaCH&Hs,CH2, C~HII. weak-field (GA1ground state configuration, t&ei). and lowSolutionsof sodium salts of the ligands can he prepared by adding spin, strong-field (2Tz, t&.ez) states. Thus the spin pairing 0.1 ma1 of CS2to a solution of 0.1 mol of the amine in 100 mL of energy for these complexes must he close to the ligand field ethanol. Twenty milliliters of 6 M NaOH is then added with stirring. strength. The high-spin configuration is asextet ( S = 5/2)and For the pyrrolidyl ligands a precipitate forms and should be filtered t h e low-spin configuration a doublet ( S = %). For off.This precipitate can he redissolved in water for the preparation F ~ ( R z N C S Zcomplexes, )~ the low-spin ( S = %) case occurs of the complexes. For the other ligands, the solution can he used directly without needing to isolate the ligands. for R = isopropyl, cyclohexyl, and isobutyl, and high spin ( S = 612) for 2R = pyrrolidyl. Intermediate magnetic moments are observed for R = methyl, ethyl, henzyl. A spin equilibrium ( S = 1' 2 e S = 512) is suggested for these c o m p l e ~ e s . ~ ~ ~ Thus the energy separation between 2Tz and GA1must he Present address: Royal Melbourne Institute of Technology, Mele k T for the spin equilibrium case. bourne, Victoria 3001, Australia. The iron dithiocarbamates also have the advantage of Crawford,T. H.; Swanson, J. JChem. Educ. 1971, 48,382. being easy to prepare and purify and of having good solubiliFiggis, B. N.; Lewis. J. Modern Coordination Chemistr): Lewis: ty in solvents such as chloroform. As the paramagnetic shift, Wilkins. Eds.: Interscience: New York, 1960; Chapter 6. Af, in eq 1depends on concentration, i t is a n advantage to White, A. H.; Roper, R.; Kokot, E.; Waterman, H.: Martin, R. L. have as high a concentration as possible for accurate meaAust. J Chem. 1964. 17, 294. surement of the shift. For these complexes, shifts of 5-40 Hz Ewald, A. H.; Martin, R. L.: Sinn, E.: White. A. H. Inorg. Chem. 1969, 8, 1387. are observed for 0.02 g/mL chloroform solutions.
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Volume 64
Number 11 November 1987
973
Preparation of Fe(S&NR&
Corn~lexes~~~
The complexes are prepared by mixing 0.033 mol of 60% wIvFeC13 solution with the solution from the ligand preparation. A blackhrown orecioitate immediatelv forms. This should be recovered bv . . vacuum filtration..the orecioitate washed with ethanol and air drf. . cd. The tcmpltx is re
