Dec., 19581 Ih’ PLANTS BY ATOMIC-ABSORPTION SPECTROSCOPY 661 4: 4’- Substituted 2: 2’-Dipyridyls in Chelation Reactions With Ferrous Iron BY G. FREDERICK SMITH AND WM. M. BANICK (,Voyes Chemical Laboratories, University of Illinois, Urbana, Illinois, U.S.A .) The molecular extinction coefficients and wavelengths of maximum absorption in the visible and near ultra-violet part of the spectrum have been determined for the ferrous complexes of eleven new 4 : 4’- derivatives of 2 : 2’-dipyridyl, viz., those with -CH,, -C,H,, -Rr, -C1, -OCH,, -OC,H,, -OC,H,, -COOC,H,, -CONH,, -COOH and -KO, as substituent. Division of these complexes into three groups according to the shape of their absorption spectra is suggested. In general, spectrophotometric absorption by the uncomplexed ligands is negligible a t the wavelengths of maximum absorption in the ultra-violet region for the ferrous complexes; this is in contrast with the substituted 1 : 10- phenanthrolines as a class.Possible applications to analysis are discussed. THIS paper continues the study of the iron11 complexes formed by chelating agents containing the grouping =N-C-C-N= and their use in analysis. Substitution in the 3-, 4-, 5- and 6- positions in both rings of the 2:2’-dipyridyls changes the properties of the chelate formed with ironII. These changes are not the same as those brought about by similar substitu- tions in the 3-, 4-, 5-, 6- and 7- positions in the 1:lO-phenanthrolines, The results of substitutions in the 6 : 6’- positions in dipyridyls and the 2 : 9- positions in 1 : 10-phenanthro- lines are comparable; chelation with iron11 to form a colour is inhibited in both cases.The most consistent modifications in properties for the whole series of dipyridyls, diquinolyls, tripyridyls and 1 : 10-phenanthrolines are brought about by substitutions in the 4- and 4 : 7 - positions (para to ring nitrogens). The ferroine and cuproine reactions (chelation with ironII and copper1) differ markedly and in a predictable manner. II I / NEW 4 : 4‘- DISUBSTITUTED 2 : 2’-DIPYRIDYLS STUDIED The formation of the iron11 complex cations and the way their physical constants vary -CH,, --C,H, -Br, -C1, -OCH,, -OC,H,, -OC,H,, -COOC,H,, -CONH,, -COOH and -NO, I, 11, 111, IV, v, VI, VII, VIII, IX, X and XI have been investigated for dipyridyls substituted in the 4 : 4’- positions with- PREVIOUS STUDIES OF SUBSTITUTED DIPYRIDYLS The ferroine reaction of 3-methyl-2 : 2’-dipyridyl yields a complex of lower stability than the unsubstituted 2 : 2’-dipyridyl, and 3 : 3’-dimethyl-2 : 2’-dipyridyl forms a complex stable only in a narrow range of pH.I 3 : 3’-Dicarboxyl substitutions completely inhibit the formation of a coloured chelate with ir0nII.29~ Single substituent groups in the 6- position diminish, and in the 6 : 6’- positions completely inhibit, the ferroine reaction, but permit the cuproine reaction.19496 Dipyridyls substituted in the 5 : 5’- positions with -NO,, -Br or -C1 groups do not give a ferroine reactiom6662 SMITH AND BANICK: 4 : 4’-SUBSTITGTED 2 : 8‘-DIPTRIDYLS [Vol.83 Substituent groups such as -C,H, in the 4 : 4’- positions of 2 : 2’-dipyridyls (para sub- stitutions to nitrogen atoms) produce marked changes in property,‘ and this is also true for the three other types of organic ligand, 1 : 10-phenanthrolines, 2 : 2’-diquinolyl and 2 : 2‘ : 2”-tripyridyl.l97,8 Further 4 : 4‘- substitutsed 2 : 2’-dipyridyls with a variety of sub- stituent groups have been studied in an endeavour to provide data for the selection of the groupings most likely to produce improved reagents when substituted into other types of ligand containing the cuproine or ferroine functional group, such as the tripyridyls.Such compounds might then be synthesised. REAGENTS- of solution. copper by treatment with bathophenanthroline and extraction with isoamyl alcohol. EXPERIMENTAL Standard iron solzttiolz-A solution of ferric chloride containing 0.1108 mg of iron per g Hydroxylamine hydrocizloride--A 10 per cent.aqueous solution freed from iron and Ethanol, 95 per cent.-Analytical-reagent grade, for preparing solutions. isoAmyl alcohol-Analytical-reagent grade, for extractions. Buffer solutions-Solutions 1.0 M in the first named of the following pairs of substances- Potassium chloride - hydrochloric acid Potassium chlorilcle - hydrochloric acid Acetic acid - sodium acetate Acetic acid - sodium acetate Sodium acetate - acetic acid Ammonium chloride - ammonium hydroxide Ammonium chloride - ammonium hydroxide Ammonium chloride - ammonium hydroxide Ammonium chloride - sodium hydroxide PH 1 pH 1.9 pH 3.1 pH 4.0 pH 5.7 pH 7.1 Ammonium acet,ate pH 8.3 pH 8.8 pH 9.4 pH 9.4 to pH 12 DIPYRIDYL SOLGTIONS- 0.01 M Solutions in ethanol of unsubstituted 2 : 2’-dipyridyl and of 2 : 2‘-dipyridyls substituted in the 4:4‘- positions by -CH, (I), -C,H, (11), -Br (111), -C1 (IV), -OCH, (V), -OC,H, (VI) and -OC,H, (VII).This last was least soluble in ethanol and had to be treated with sufficient hydrochloric acid to form the hydrochloride. 0.01 M Solutions in 50 per cent. ethanol made 1.5 M in hydrochloric acid of the -COOC,H, and -CONH, derivatives (VIII and IX). A 0.005 M solution of the -COOH derivative (X) in water to which sufficient ammonium hydroxide was added to complete dissolution. -4 0.0025 M solution of the -NO, derivative (XI) in distinctly acidified water. OPTIMUM pH FOR FORMATION OF CHELA.TE COMPLEX AND ITS EXTRACTION Before the spectrophotometric constants of the iron11 complexes could be evaluated, it was necessary to find the pH for maximum colour development.To each of a series of test-tubes containing 5 0 ml of buffer solution, 1.0 ml of hydroxyl- amine hydrochloride solution and 0.25 ml of iron111 solution was added 0.5 ml of a 0.01 M solution of the ligand. (The ligand solutions that were 0.005 M were added in 1.0-ml portions.) Those ligands prepared in 1.5 M hydrochloric acid were treated by addition of 0.10 M sodium hydroxide in sufficient amount to neutralise the acid. If the complex or the excess of ligand was precipitated, enough 95 per cent. ethanol was added to re-dissolve it. The colours produced at the various pH values were noted. The solution of the complex in each tube was extracted, if possible, by adding 2.0 ml of isoamyl alcohol and then shaking the tube (one or two extractions only were applied).Because only a limited amount of ligand XI, the -NO, derivative, was available, it was not included in these tests. No reducing agent other than hydroxylamine was tested. In the buffer solutions of pH 8.8 to 9.4, for three of the ligands studied, there was interference by ammonium hydroxide; when sodium hydroxide was used instead, there was no interference and the maximum colour intensity was attained. In the course of obtaining the results in Table I, it was noted that the narrow range of maximum colour formation with the dibromo and dichloro derivatives was due to their low instability constants. This same effect The results are shown in Table I.Dilution with ethanol augmented this effect.Dec., 19581 IN CHELATION REACTIONS WITH FERROUS IRON 663 has previously been noted for some chloro derivatives of 1 : lO-phenanthr~line.~ The -COOC,H, derivative, which also gave a complex whose colour was stable only over a narrow range of pH, was affected by atmospheric oxidation when extracted into isoamyl alcohol. Although the -C2.H5 derivative gives its maximum colour over a wider range of pH than does the -CH, derivative, it is noteworthy that for the -OCH, and -OC,H, deriva- tives this effect is reversed. Only the -OC,H, derivative gave a ferrous complex that was insoluble in aqueous solution. TABLE I INFLUENCE OF pH ON FORMATION AND COLOUR INTENSITY OF IROKII COMPLEXES ~SOAMYL ALCOHOL OF 4 : 4'- DISUBSTITUTED 2 : 2'-DIPYRIDYLS AND THEIR EXTRACTABILITY INTO Disubstituted pH range for pH range for Ligand with complex formation maximum colour Unsubstituted ___ 1.0 to 12 3.1 t o 7.1 I -CH.1.9 to 12 3.1 to 8.3 I1 -C& 111 -Br IV -C1 1.9 to 12 1.0 to 12 1.0 to 12 3.1 to 12 1.9 to 3.1 1.0 to 3.1 V -OCH8 1.9 to 12 3.1 to 8.3 VI -OC,H, 3.1 to 12 4.0 to 8.3 VII -OC,H5 1.0 to 12 3.1 to 8.3 VIII -COOC2H5 1.0 to 12 3.1 to 4.0 4.0 t o 12 X -COOH 1.0 to 13 3.1 to 12 IX -CONH, 1.0 to 13 * Extractability: = extracted; 0 = not extracted Extractability* 0 0 0 0 + + + + 0 0 T DETERMINATION OF SPECTROPHOTOMETRIC CONSTAKTS Ligands I, 11, V and VI and the unsubstituted dipyridyl were prepared for spectro- photometric examination as follows.Five millilitres of a buffer solution having a pH within the range that gave maximum colour intensity (pH 3.1 to 5.0; see Table I), 2 ml of hydroxyl- amine hydrochloride solution and 2.0ml of 0.01 M ligand solution were placed in 25-ml calibrated flasks with transfer pipettes. A weighed amount of standard iron solution was added, and the contents of the flasks were made up to volume with water. The same procedure was followed for ligand VII, except that the dilution to volume was with 95 per cent. ethanol. For ligand VIII, the procedure was the same as for ligand VII, except that 3.0 ml of the 0.005 M ligand solution were taken and a few drops of ammonium hydroxide were added to neutralise the excess of hydrochloric acid. For ligand X, the procedure used for ligand VII was followed, except that 3.0ml of 0.005 M solution of ligand were taken.For ligand IX, the procedure used was that for ligand VIII, except that the dilution was with water and the excess of reagent that was precipitated was filtered off. The slight tendency for the colour of this complex to diminish on dilution with ethanol made this mandatory. For ligands I11 and IV, 5.0 ml of buffer solution (pH 3.1), 2.0 ml of hydroxylamine hydrochloride solution and 5.0 ml of 0.01 M ligand solution were transferred by pipette to 10-ml calibrated flasks. Weighed amounts of iron solution were then added, the complexes were allowed to form, and the contents of the flasks were diluted to volume with 95 per cent. ethanol. The excess of ligand that was precipitated was filtered off before spectro- photometric observations were made.The iron" complex of ligand XI was prepared by taking 5 ml of the 0.0025 M solution of the derivative and adding 2.0 ml of hydroxylamine hydrochloride solution and weighed amounts of iron, in a 50-ml calibrated flask. Dilute ammonium hydroxide solution was added until the colour developed, and the flask contents were then diluted to volume with water. As soon as they had been prepared, the solutions were examined spectrophotometrically with a Cary recording instrument, model 14M. Matched 10-mm silica cells were used for all measurements. A wavelength range of 340 to 650mp was covered, and appropriately prepared blank solutions were used for all measurements.664 [Vol. 83 Calculations of the molecular extinction coefficient were based upon the amount of iron added, and each value is the average of a series with increasing amounts of iron.(That for ligand XI is based upon a single observation.) All complexes gave colours that conformed to Beer's law over the range 1 to 6 p.p.m. of iron. SMITH AND BANICK : 4 : 4'-SUBSTITUTED 2 : 2'-DIPYRIDYLS The spectrophotometric results are shown in Table 11. TABLE I1 SPECTROPHOTOMETRIC Disubstituted Ligand with Unsubstituted _- I -CH, I1 -C,H, I11 -Br VI -c1 V -0CH. VI -oc,€f, VIII -OC&I, VIII -COOC,H6 IX -CONH, X -COOH- XI -NO, DATA FOR THE 4 : 4'- DISUBSTITUTED 2 : 2I-DIPYRIDYLS AS FERROUS COMPLEXES Wave1e:ngth Average Wavelength Average No. of of maximum molecular of maximum molecular deter- absorpt.ion, extinction absorption, extinction minations mP coefficient mP coefficient 4 3491 6430 522 8710 4 354. 7860 528 9340 4 356, 8410 529 9880 2 357 4960 534 5550 2 3561 7500 532 8300 3 35" 7750 525 6680 4 35Gl 9100 525 7660 5 3631 10,180 540 8060 5 384.10,380 541 14,150 4 384. 11,120 540 14,940 5 378 10,990 540 14,760 1 - - 525 9190 No diminution in colour intensity on storage for 30 days in glass-stoppered containers was noted for these solutions, except for ligands I11 and XI. The colours of these solutions, which had lost much of their original intensity, were not restored by the addition of another portion of reducing agent. From Table I1 it will be seen that the absorption peak in the near ultra-violet region (349 to 384mp) is common to all these complex cations. This peak was also found by Schilt and Smith' for 4 : 4'-diamino- and 4 : 4'-diphenyl-2 : 2'-dipyridyls.There is no comparable absorption peak for 1 : 10-phenanthrolines, and this serves as a differentiating characteristic. All the 4 : 4'- disubstitutions led to increase:; in the wavelength of maximum absorption as compared with the unsubstituted ligand at both absorption maxima (Tables I1 and 111). TABLE I11 CHANGE IN WAVELENGTH OF MAXIMUM ABSORPTION (Ah,,,) AND MOLECULAR EXTINCTION COEFFICIENT (A€) FOR THE 4 : 4'- DISUBSTITUTED 2 : 2'-DIPYRIDYL - IRONII COMPLEXES Relative to Am=. = 6430 at 349 mp and E =- 8710 at 522 mp for the unsubstituted 2 : 2'-dipyridyl- iron11 complex Ultra-violet absorption maximum Disubstituted (---, Ligand with AX,,,., A€, mP % I -CH, +5 + 22 I1 -C2H6 + 5 +31 I11 -Br $8 - 23 IV -c1 + 6 -C 17 V -0CH.1 8 , - VI -oczz, -10 VII -0CaHS + I 1 VIII -COOC,H. 35 IX -C0NHo " + 35 + 21 + 41 + 58 + 61 + 73 Visible region absorption maximum t 3 + 3 + 18 + 19 + 18 - 23 - 12 - 7 + 62 i 72 X -COOH" + 29 + 7 1 + 18 + 70 4 : 4'-diamino* + 30 + 112 + 47 -5 4 : 4'-diphenyl* +37 +216 + 30 + 144 * Taken from the values reported for these ligands by Schilt and Smith.' Ligands V, VI and VII differ from the remainder in that their iron11 complex cations have higher molecular extinction coefficients in the near ultra-violet than they possess inDec., 19581 IN CHELATION REACTIONS WITH FERROUS IRON 665 the visible portion of the spectrum. To this group should be added 4:4'-diamino-2:2'- dipyridyl. The molecular extinction coefficients for the iron11 chelate of unsubstituted 2 : 2'-dipyridyl (8710 at 522 mp and 6430 at 349 mp) found in this work are in good agreement with values reported by Busch and Bailar,lo who gave values of 8700 at 522 mp and 6500 at 348 mp.The re-determination of the molecular extinction coefficient of ligand I gave a value 10 per cent. higher than that reported at 528 mp by Cagle and Smith.2 In Table I11 are shown the changes in wavelength of maximum absorption and in molecular extinction coefficient brought about by the substitutions in the various ligands. It can be seen that- (a) the spectra for all the complex cations exhibit bathochromic shifts in the maximum absorption in both the ultra-violet and visible regions ; ( b ) diphenyl substitutions produce the greatest alteration of A,,,.and E values (compare Table 11), an effect that also occurs for similarly positioned phenyl-group substi- tutions in diquinolyls, tripyridyls and phenanthrolines ; (c) in general, increase in A,,,, values is accompanied by an increase in E values, and this trend is most consistent for the near ultra-violet absorption; and (d) the last five groups listed in Table 111, when substituted with dipyridyl, have the greatest influence on the spectrophotometric constants, a finding that is in agreement with similar experimental results for the corresponding 1 : 10-phenanthrolines, diquinolyls and tripyridyls.' To these five substituent groups (-COOC,H,, -CONH,, -COOH, -NH, and -C,H,) the hydroxyl group should be added, as exem- plified by Synder's reagent (4 : 7-dihydroxy- 1 : 10-phenanthroline), as shown by Schilt, Smith and Heimbuch.ll GROUPING OF REAGENTS BY SPECTROPHOTOMETRIC DATA Spectrophotometricaly, unsubstituted 2 : 2'-dipyridyl and ligands I, 11, I11 and IV are similar.Absorption peaks, both in the ultra-violet and visible (349 to 357 mp and 522 to 534 mp) regions, are pronounced, and are sharper than those of the similarly substituted 1 : 10-phenanthrolines, although less sharp than those of the similarly substituted tripyridyls.8 U .- 5 0 ' 6 p - J $ 0.4 ; 0 ' 6 R 5 0.4 - ,; 0.2 8 '5 0.2 0 0" '350 450 550 650 Wavelength, mp Wavelength, rnp Fig. 1. Absorption spectrum Fig. 2. Absorption spectrum of the 4: 4'-dimethyl-2: 2'-dipy- of the 4 : 4'-diethoxy-2: 2'-dipy- ridyl - iron11 complex 350 450 550 650 ridyl - iron11 complex Wavelength, mp Fig.3. Absorption spectrum of the 4: 4'-dicarboxy-2: 2'-dipy- ridyl - iron11 complex All four ligands have a peak absorption in the ultra-violet region that is 10 t o 15 per cent. Fig. 1 shows an example of this type of absorption lower than that in the visible region. spectrum.666 [Vol. 83 The ultra-violet absorption peak is sharp, while that in the visible region is broad. For this group the ultra- violet absorption is the more intense by 17 to :26 per cent. Fig. 2 shows an example of this type. Ligands VIII, IX and X (see Fig. 3) have similar absorption characteristics to those shown in Fig. 1. They are distinguished by the absence of a shoulder on the short-wavelength peak and by having only a slight indication of a shoulder on the long-wavelength peak, THE POSSIBLE DETERMINATION OF IRON B Y ULTRA-VIOLET SPECTROPHOTOMETRY Attempts to use the 1 : 10-phenanthrolines - iron11 complexes for the determination of iron by ultra-violet absorption, with possibly increased sensitivity, have failed owing to interference by absorption at practically the sa:me wavelength from the necessary excess of ligand present.Similar difficulties occur with the substituted dipyridyls, although in certain circum- stances it might be possible to use ligands 11, VI, VII and VIII (and 4:4'-diphenyl-2:2'- dipyridyl), which absorb negligibly at 340, 330, 340 and 350 (and 356) mp, respectively, whereas their respective iron11 complexes absorb at 355, 359, 363 and 384 (and 386) mp. This makes the proposed scheme of analysis less attractive, except when special conditions owing to other colour interferences are met with. The syntheses of the newly prepared 2 : 2'-dipyridyls herein described were carried out by Professor F. H. Case and G. Maerker of Temple University in Philadelphia and will be described elsewhere. Only through this valued assistance in a series of difficult and carefully developed synthetic reaction techniques has thir; study been made possible. FEIGL AND JUNGREIS: SPOT TESTS FOR PHENOLS AND Ligands V, VI and VII are again similar in a.bsorption characteristics. All the ferrous complexes as well as the excesses of ligand are extractable. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. Smith, G. Frederick, Anal. Chem., 1954, 26, 1534:. Cagle, F. W., and Smith, G. Frederick, J. Amer. Chem. SOC., 1947, 69, 1860. Richter, F. P., Doctorate Dissertation, University of Illinois, 1941. Burstall, F. H., J . Chem. SOC., 1938, 1664. Gaines, X., jun., Hammett, L. P., and Walden, C;. H., jun., J. Amer. Chem. SOC., 1936, 58, 1668. Case, F. H., I b i d . , 1946, 68, 2574. Schilt, A. A,, and Smith, G. Frederick, Anal. Ch;:m. Acta, 1957, 16, 401. -,- , Ibid., 1956, 15, 567. Schilt, A. A,, Doctorate Dissertation, University of Illinois, 1956. Busch, D. H., and Bailar, J . C., jun., J . Amer. Chem. SOC., 1956, 78, 1137. Schilt, A. X., Smith, G. Frederick, and Heimbucli, X., Anal. Chem., 1956, 28, 809. First received November 26th, 1957 Amended, July 16th, 1958