Philip H. Merrell Bowdoin College Brunswick, Maine 0401 1 F. L. Urbach and M i c h a e l Arnold Case Western Reserve University Cleveland. Ohio 44106
I
Synthesis and Characterization of a Macrocyclic Nickel Complex
I I
An experihRnt to conclude an advanced inorganic or analytical course
I
A well-known nickel (11) macrocvclic comnlex is nrenared - . and characterized and is p r e s e n t e d a s a g o o d l a b experiment for an upperclass undergraduate analytical o r inorganic -. . - lab
and on a stir plate. (Aless complex set-up using s beaker on a hot plate or steam bath and a glass rod as a stirrer also works well.) The crude lirand is weiehed. and an eouimolar amount of Ni(OAcb4H.O is .~ dikdved m iil ~ I ' M ~ O H . T iignnd & ia then added ond thesolution course. T h e complex 5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraa- i i hentpd for nbaut 1 hr. After the heating perlod is over, the solution ir filtered, and the volume redured on a hot waterorswam bath until zacyclotetradeca-4,ll-dieneatonickel(11) iodide (see below) the crystals 1,egin t t ~ form It is then cooled for on hour ur more. is prepared i n t w o distinct steps. In the next lal, period the yellow crystalline material 1s filtered from soiutiun and rrrrvslallizrd from ethanol. tlf the filtration is diffirult. filter just a littleat a time or fish the crystals out of solution and air drv them.) The reervstallization should he done fairlv raoidlv usine H hot wnter bath to k d u r e the ~ . o i u m e o f ~ t ~ ~ i f ~ e he e ~ rs c~ r y . rrptalleed pmduet is then dried and stored in a desirratur until the phys~mlmeasurements are to be made. Yield 7.7g. 34'- of a bright yellow crystalline substance. ~~
~
~~
.~ ~
Ni[4,1l dieneNJ,
Firstly, the macrocyclic ligand is prepared by the Schiff's Base condensation of one mole of ethylenediamine monohydroiodide with two moles of acetone after the manner of Curtis and Hay (I). This experiment can he carried out easily in a three-hour lab period. The second step involves the reaction of the macrocyclic ligand dihydroiodide with nickel (11) acetate forming the nickel iodide complex. This complex is well known and well characterized in the literature (2). Macrocyclic metal complexes have been studied extensively ( 3 , 4 ) because of their similarity to the macrocyclic metal complexes found in biological systems. The metal complexes of porphin and corrin provide a variety of compounds with manifold biological functions; and because of their complexity and high molecular weights, model compounds are necessary in order to study biological mechanisms. We think that the synthesis and the characterization of this complex will give the student agood idea of both the organic and the inorganic chemistry involved in synthesizing a rather complex ligandmetal system. I t will also give him a chance t o use several common analytical tools in order to fully characterize the complex obtained. The complex will be characterized by elemental analysis of the nickel and the iodide, by conductivity, hy infrared, uv-visible, and nuclear magnetic resonance spectroscopies and the results compared t o those in the literature.
Results a n d Discussion The mechanism of the formationof the ligand appears t o bequite complex; however, in reality it essentially involves only three steps shown in Figure 1. Firstly, a self condensation of the acetone in the presence of acid forms a small amount of mesityl oxide. Secondly, a Michael addition of the amine to the 4 - u n s a t u r a t e d ketone produces a substituted 0-amino-ketone (5).Further reaction with the second amino-group on the ethylenediamine is blocked by the acid (H'). The third step is a Schiffs Base condensation of the amine with the keto-group of the other molecule forming the cyclized product. The order of the three steps has not been confirmed experimentally, but it is apparent that this mechanism or one similar to this forms the product shown. The reaction takes place equally as well beginning with mesityl oxide (a) which would indicate that the first step could be the one shown here ( I ) . The formation of the nickel (11) complex with this protonated ligand depends upon the fact that the acetate ion is the conjugate base of a weak acid and thus deprotonates the macrocyclic ligand allowing the metal complex to form. Since the iodide ion is a better ligand than the acetate ion, the final product formed is the Ni([l4]-7,ll-dieneN4)12.
Experimental Reagents used are reagent grade. The ethylenediamine was not purified, however, and it contained a slight brown impurity. Preparation o f 5,7,7,12,14,14-Hexamethyl-1,4,8,11tetraazacvclotetradeca-4.1l d i e n e dihvdrooen iodide t 1141 > 4.1 ldiene N42HI) 2
-
.&
The ligand, 4.11-dime NcZHI, was prepared using a method similar to that reported by Curtis and Hay (11. A 0.2 mole sample of ethylenediamine (13.2 ml) is put into 10 mlEtOH and cooled in an ice hath. A 0.20 m (36.2 ml of 47%) sample of colorless hydroiodic acid is slowly added to the cool ethylenediamine solution being careful not to let the solution boil over as thisstrong acid strong base reaction isquite exothermic. After the HI is added, 30 ml of acetone is added (an excess of 0.4 m is required) and the solution is allowed to cool in the ice bath far several minutes. After the solution has cooled, the white crystalline material is filtered from the solution. It is dried crudely by pullingair through it for 20 min while it is on the filter paper. Yield: 21.15 g; 19.5%. (Smaller amounts can he used; up t o one fourth the amounts reported seemed to work fairly well.) Preparation 01 5,7,7,12,14, 14-hexamethyl- 1.4.8.1 1tetraazac~clotetradeca-4. 1 ld~enatonickek'tlliodide. Ni .. ([I41 4.1 i d i e n e - ~ ~ ) ~ ~ During the 20-min drying period, a 50-ml round bottom flask with a reflux condenser and a magnetic stir bar is set up in a heating mantel 'M.A. wishes t o thank Project SEED for a summer stipend. 580 / Journal of Chemical Education
d Figure 1. A proposed mechanism for the formation of 4.1 ldiene N4.2HI.
Nickel Analysis. The n.icke1 determination was done by the standard dimethylglyoxime (DMG) method (6). Between 0.1 g a n d 0.2 g of the complex is decomposed by adding about 10 ml eon HCI and 5 ml can H N 0 3 and heating to dryness. It may he necessary to repeat this step as this compound issometimesdifficult to decompose. The typical laboratory results were: Th:9.92%; Fd:9.8-10.0%. Iodide Analysis. There are several methods for determining the %I in the complex. Two were used by our students with good results. The first method of iodide determination is the iodate oxidation method using standardized KIOa as the oxidant (7). KI03 is dried in an oven and standard solutions are made by weighing KIOx into a volumetric flask. Between 0.1 and 0.2 g of the complex are accurately weighed into beakers and dissolved in 6 M HCI. Five milliliters of CHCh is added and this mixture is titrated with standard KIOJ, slowly adding with vigorous shaking to allow the aqueous and the organic layers to equilibrate. A bright orange crystalline ppt may appear and must he broken up with a stirring rod to get it heck into solution. The endpoint is reached when the purple color has left the CHCL layer. Typical results were; Th: 42.8%; Fd: 41.&43.5%. The second method utilized was a potentiometric titration of the iodide with a standard solution of AgNOs (8).A plot is made of ml versus potential and a first derivative curve is also made (A ml versus potential). A second derivative plot [A(A ml versus potential)] can also be utilized. This method proved a little more difficult than the iodate oxidation hut gwd results were obtained here also. Th: 42.8%; Fd: 41.3-43.5%. Conduetiuity. A crude conductivity was run in water. An approximately 1 0 V molar solution is made and put in a normal conductivity cell. The values obtained are about 215-230 mhos which is the value of a 2:l electrolyte. This number can allow the student t o begin thinking about the structure of the complex, i.e. if i t is a 2:l electrolyte the wmpound can be either a square planar complex in solution or an octahedral complex with water molecules in the axial positions. Melting Point. The melting point of the ligand can he determined and most of the students got between 135-145'C as the melting point hut a few got as high as 148% which is the accepted value. Infrared Spectra. Only a few specific features of the infrared spectra will be mentioned here. (See Figs. 2 and 3.) Both spectra show strong hands centered around 3500 cm-I which are due to moisture that iipwkrd upduring thegrinding d t h e r o m p , d u,ith K B r The hand rhar shvuld he prewnt tin tlwdr)~cwnpuuncllat :3180cm-'due 118 thp N - H stretrh is shifted downiield due w the hydrogen handing ( I ) and is not observed. This phenomenon is well known and cannot he prevented easily in the undergraduate lab. A characteristic feature of the ligand spectrum is the quaternary amine hands that caused the sloping baseline in the 2000-2800 cm-I region with a fairlyprominent hand a t about 2300 em-'. This is typical of all quaternary aminesandshould be totally absent in thespectrum of the nickel complex.
3000 2000 1500 1200 1000 900 800 Figure 2. The infrared spectrum of the ligand 4.11diene N1.2HI.
Another characteristic band to look for in both spectra is the imine stretch a t 1665 cm-'. I t is i m ~ o r t a nto t note that there are no hands in the 1700- IXUO cm-'region uhich uould indicate the presence 01 the awmrr ion. If a hand r h d d appear in thi- rtyion, r r e r y s t a l l ~ z ~ t ~ m of the complex is necessary. Proton Magnetic Resonance Spectrum. There are seven magnetically different types of protons in this complex (one of which IN-HI is not observed due to the quadrupole moment ofthe nitrogen). (See Fig. 4.) Since there is no overlapping of the peaks, the spectrum is discernible. The nmr spectrum that we obtained in D20 shows clearly the six magnetically different protons that are observable. The strongest peaksare those due to the three non-equivalent methyl groups which appear as sharp singlets a t 1.25,2.02, and 2.14 ppm downfield from DDS. A study by Warner,Rose, andBuschshowed that by these peaks the different diastereoisomers could be distinguished (9). Diastereoisomers in these complexes arise from variations in the configurations of the asvmmetric amine nitroeren atom. In our svnthesis we have not separated the isomers and a racemate-mesa equilibrium of 6:l is established. The peaks due to the methyl groups in the meso (less predominate) configuration are nearly equidistant; while in the predominate species the peaks due to the axial methyl (11) and the methyl adjacent to the imine bond (I) are magnetically clwe due t o t h e shifting of the axial methyl peak downfield. A small peak a t 1.71 ppm is due to the axial methyl group in the less abundant meso isomer and a peak a t 2.18 ppm is due to the imine methyl of the meso isomer. There are three distinct methylene groups which appear a t three distinct areas along the baseline. The lowest field methylene group appears as a broad multiplet a t -3.46 ppm downfield and is due to the imine dimethylene protons broadened by the quadrupale moment of the nitrogen atoms. The next methylene resonance appears as a sharp peek a t 2.84 ppm and is attributed t o the methylene protons adjacent to two carbon groups. The peaks attributed to the aminomethylene protons falls a t 2.60 ppm and is again rather broad and appears to he a multiplet. UV-Visible Spectrum. The ultraviolet region shows a charge transfer hand a t 220 nm; 45,450 cm-' (18,000) with a strong shoulder t o the low energy side and a band a t 283 nm; 35,300 cm-' (6000) associated with a charge transfer transition of the a electrons of the imine group (this band disappears upon hydrogenation) (10). There is asingle hand in thevisible region a t 437 nm; 22,880 cm-' (90) which is typical of singlet ground state Ni(I1) complexes in 4N square planar coordination (10). The value of the extinction coefficient can give some idea of the geometry of the complex in solution. A value of 90 is considered high for an octahedral complex (although some d-d bands in octahedral complexes have been known t o have a value this high) and low for a tetrahedral complex. It is suitable for a square planar complex.
Conclusion T h i s laboratory experiment can be adapted to a two or three hr laboratory period o r an open-end laboratory experiment. It gives the s t u d e n t practical experience i n synthesizing a compound a n d then in moving. that t h e c o m ~ o u n dis the o n e expicted. It c a n help h i m f i t together m u c h information t h a t he h a s learned from books o r in o t h e r courses a n d allows h i m
700
Y'
3000
2000 IS00 1300 1000 900 800 700 Figwe 3. f he infrared spectrum of the complex Ni(4.l ldiene N3 1.
Flgure 4. The nmr spectrum of the complex Ni(4.11-diene N,) I,. Volume 54. Number 9, September 1977 1 581
to have hands on experience with the ordinary lab equipment of the practicing chemist. Literature Cited (1) Curtis, N. F., and Hay, R. W., Chem. Commun.. 524 (19661. (2) Curtis, N. L a n d Housc,D.A.,Chem.Ind. (London), 1708.(19611.
582 1 Journal of Chemlcal Education
Buseh, D. H..Helu. Ckim. Aefo., 174 (1967). Curtis, N. F., Coord. Chem. Re"., 3,3 (19Mo. Smifh,M.E.,andAdkinr,H.,J Amer Cham. Soe., 60,407 (19381. Skws, D. A,, and Wet, D. M.."Fundsmental8of Analytical Chemistry," 2nd Ed..Holt Rinehart, and Winston, Inc., New York. New York, 1969, p. 195. (7) Ref. (61pp. 43W41. (8) Wheatland. D A., J. CHEM EDUC., 8S4 (1973). (9) Warner, L. Q.,Roae, N. J., and Busch, D.H., J. Amen Chom. Sor., 90,6938. (10) Curtis, N. F.. Curtis, Y. M., an Powell, H. K., J. Chom. Soe A. 1015 (19661. (3) (41 (5) (6)
a
