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Solution stability constants of some metal complexes of derivatives of catechol
J. inorg, nu¢l. Chem., 1966, Vol. 28, pp. 1237 to 1249. Pergamon Press Ltd. Printed. in Northern Ireland
SOLUTION STABILITY CONSTANTS OF SOME METAL COMPLEXES OF DERIVATIVES OF CATECHOL V. T. ATHAVALE,L. H. PRABHU and D. G. VARTAK Analytical Division, Atomic Energy Establishment Trombay, Bombay, India (Received 18 March 1965; in revised form 11 August 1965)
Abstract--The formation constant for the metal chelates of copper, zinc, nickel, cadmium and magnesium with catechol, photocatechuic acid, homoprotocatechuic acid and 3:4-dihydroxyhydrocinnamic acid have been determined. The Calvin-Bjerrumtitration technique has been adopted for the determination of stability constants of the proton ligand complexesand the metal li~nd chelates. The relation between the log of the stability constants of the metal chelates and the log of the protonligand formation constants of the ligands is discussed. The metal-ligand chelate stability constants follow the order Zn < Cu > Ni > Cd > Mg. CATECHOL, an o-hydroxy phenol, forms metal chelates with a number of metal ions. Catechol complexes of several metals such as Cr, Cu, Zn, Co, Ni, etc., have been studied by WEIh*LANDand co-workers. ¢1) They found that aU these complexes contained two or three catechol molecules attached to one metal ion with either alkali or alkaline earth metal ions or organic bases, fike pyridine, in the outer sphere. Rare earth, (z) alkaline earth, (a"L) rhenium (§) and ferric iron (6) catechol complexes are also known. The formation constants of the metal catechol complexes of a few metal ions have been studied but without much reference to the relation between the metal-ligand formation and proton-ligand formation constants. In the present investigation, the formation of complexes of Cu(II), Zn(II), Ni(II), Cd(II), Mg(II) with catechol, protocatechuic acid, homoprotocatechuic acid and 3:4-dihydroxyhydrocinnamic acid together with the dissociation constants of the respective reagents have been investigated by employing a potentiometric titration (Calvin--Bjerrum) technique. A few of the factors affecting the ligand dissociation constants and the chelate stability constants are discussed. EXPERIMENTAL Chemicals. All the chemicalsincluding the metal salts, sodium hydroxide, perchloric acid, potas-
sium biphthalate, borax, ethylenediaminetetraaceticacid (disodium salt) etc., used were of analytical reagent grade. Sodium hydroxide, free from carbonate, was prepared by a standard method,cv~ Chelating agents. Catechol(Ligand I): The reagent supplied by Eastman Kodak Co. (practical ~1~R. WFaNLANDet al., Z. anorg, al~. Chem. 126, 141 (1923): Chem. Abstr 17, 2542 (1923);/b/d,/d~m, 150, 69 (1925); Chem. Abstr 20, 717 (1926). ~l) L. FV.RNAND~,Gazz. Chim. ital. 55, 424 (1925): Chem. Abstr. 20, 536 (1926); /b/d, idem, 56, 682 (1926); Chem. Abstr. 21, 866 (1927). ts~R. SCHOLD~and M. WOL~,Z. anorg, allg. Chem. 210¢184 (1923); Chem. Abstr. 27, 2106 (1933). ~4~R. SCHOLDERand E. SCHLitZ,Z. anorg, allg. Chem., 211, 161 (1933); Chem. Abstr. 27, 2645
(1933). ~5)D. SEN and F. RAY, i. l~d'_-m Chem. Soc. 30, 253 (1953). ~e)A. K. BAnKO,J.gen. Chem. U.S.S.R. 16, 968 (1946); Chem Abstr 41, 2654e (1947). ~7~A.I. VOGEL,Text Book o f Quantitative Inorganic Analysis, p. 242, Longmans,Green,London (1961).
1237
1238
V.T. ATHAVALE,L. H. PRAnHUand D. G. VARTAK
grade) was distilled under reduced pressure ~s~ to obtain a pure white product (m.p. 105°C). The compound was preserved in a vacuum desiccator without exposure to light. Protocatechuic acid (Ligand II): BDH Laboratory grade acid was twice recrystallized from double distilled water in a dark room and dried in a vacuum desiccator. The product was white needles with one molecule of water of crystallization o~ (m.p. 199°C). Homoprotocatechuic acid (Ligand III) and 3:4-dihydroxyhydrocinnamic acid (Ligand IV): The reagents were supplied by H.M. Chemical Company, Santa Monica, California, U.S.A. These were used after purification. The stock solutions of all the reagents were of 0.04 M concentration except the last which was 0.032 M. All solutions were prepared in carbon dioxide free double distilled water. Metal perchlorates were prepared by dissolving the respective metal carbonate or oxide in a known volume of standard perchloric acid. The concentration of metal ions in each of the metal perchlorates of Cu, Zn, Ni and Cd, was estimated by potentiometric titrations as described by SCnWARZEr,raACn,cx°~ using solutions of Naa-EDTA and standard sodium hydroxide. Magnesium was estimated by titrating it against standard Na2-EDTA solution using Eriochrome Black-T as indicator. Calvin-Bjerrum titration. Potentiometric titrations, against 1 N NaOH of (i) the free mineral acid (0.03 M perchlorie acid) (ii) free mineral acid plus the reagent (0.004 M) under investigation, and (iii) free mineral acid, reagent (0.004 M) and metal salt (0.001 M), were carried out using a bench type Cambridge pH meter operating at 12 V d.c. The titration assembly consisted of a 5 ml micro burette containing 1.0 N NaOH fitted with a soda lime guard tube at the top and a glass reaction beaker of 150 ml capacity fitted with a rubber bung. The bung had holes to accommodate a Cambridge Universal glass electrode, a dip type calomel electrode a glass tubing for bubbling nitrogen and the delivery end of the burette. The titrating solution was made up to 100 ml with Sodium perchlorate and water in the reaction beaker which was immersed in a water bath maintained at 30 4- 0.1°C. Sodium perchlorate was added to keep the ionic strength constant at 0.1 M. The pH of the solution was noted after every addition of a small volume (say 0.02 ml) of~-l.0 N sodium hydroxide from the burette until pH 11"5 was reached. The maximum reading which was stable after each addition of alkali was recorded. The titration solution was kept free from carbon dioxide and oxygen by continuously bubbling purified nitrogen presaturated by passing it through 0.1 M sodium perchlorate solution. CALCULATIONS
Calculation of proton ligand formation constants The calculation o f the p r o t o n ligand formation constants is accomplished in two steps. (i) The formation curves are obtained f r o m fix, p H data. (ii) The curves are analysed to arrive at the formation constants. The values o f ax at various p H values (Table 1) are obtained f r o m the titration curves o f free acid and free acid plus reagent using the electroneutrality equation at each step as adapted by IRVlN~ and Rossorri. m) (v" - - v') ( N -k- E °)
ax= y--
(V °-k v') T°L
(1)
Where v" and v' denote the volume o f alkali required to reach the same p H in the titrations o f the free acid plus reagent and the free acid alone respectively, N, the normality o f alkali, E °, the initial concentration o f the free acid, T°L, the total concentration o f the ligand added and y, the total n u m b e r dissociable p r o t o n s attached to
cs~H. Gu_sc~N,Organic Synthesis, Collective Vol. 1, p. 143, J. Wiley, New York (1932). ~9~I. HvataRONand H. M. Btrroonv, Dictionaryof Organic Compounds,Vol. 3, p. 539, London (1943). c10~G. SCHWARZENnACH,Complexometric Titration, p. 60, Methuen, London (1957). m~ H. IRVINGand H. S. RossoT'n, J. chem. Soc. 2904 (1954).
Solution stability comtants of some metal complexes of derivatives of catechol
1239
TABLE 1 Catechol B nA B
8-00 8-25 8-50 1 " 9 7 9 6 1"9490 1.8914 10"00 10"25 10.50
Rx
1.1704
1.1027
1.0535
3.50
8.75 9.00 9.25 9.50 1"8077 1"6802 1"5377 1-3847 10"75 11.00 1 . 0 3 8 7 1-0194
9.75 1"2657
Protocatechuic acid B
3.00
3.25
t~A B Rx B nx B t~x
2.9949 5.00 2.2128 7-00 1"9918 9"00 1.3977
2 . 9 5 4 1 2.8995 5.25 5.50 2.1397 2.0854 7"25 7.50 1"9768 1 . 9 5 9 7 9-25 9.50 1.2711 1 . 1 7 1 7
3.75
2.8317 5.75 2.0514 7"75 1-9274 9"75 1.1026
4.00
4.25
4.50
2-7279 2.5851 6.00 6.25 2.0277 2.0140 8.00 8"25 1.8800 1"7916 10.00 10"25 1.0589 1.0354
4.75
2 - 4 4 2 5 2.3167 6.50 6.75 2.0040 1.9954 8-50 8"75 1 . 6 6 5 9 1-5385 10"50 10"75 1.0140 0.9880
Homoprotocatechuic acid B nA B nA B RA B ~A
3-00 2"9256 5"00 2"1481 7"00 10960 9.00 1.7325
3"25 2"8848 5-25 2.0941 7"25 1.9925 9-25 1.6041
3-50 2.8260 5.50 2"0553 7.50 1.9895 9.50 1.4641
3-75 4.00 4"25 4.50 4"75 2"7497 2"6333 2"4949 2"3644 2"2394 5-75 6.00 6"25 6"50 6"75 2"0333 2"0196 2"0145 2.0127 2"0062 7"75 8.00 8"25 8"50 8"75 1 . 9 8 0 8 1"9791 1.9520 1 . 8 9 8 9 1-8255 9.75 10-00 10.25 10.50 10-75 1 . 3 2 0 8 1 . 2 0 2 5 1 . 1 1 5 2 1.0520 0.9834
3:4 dihydroxyhydrocinnamic acid B RA ~x
3"00 2.9826 5.00 2.2538
B RA B
7.00 2.0002 9.00
2-1468 7.25 1.9936 9.25
7.50 1.9874 9.50
2-0221 2.0178 7.75 8.00 8.25 1 . 9 8 0 5 1 . 9 5 8 9 1.9280 9.75 10.00 10.25
RA
1.6961
1.5652
1.4209
1-2678
B
3"25 2"9606 5"25
3-50 2"9215 5"50 2.0835
3-75 2"8621 5"75 2.0461
4.00 2"7876 6.00
1.1586
4"25 2"6830 6"25
1.0716
4-50 2.5340 6.50 2.0133
4"75 2"3807 6"75
2.0047 8.50 8-75 1 . 8 7 9 8 1.7971 10.50 10.75 0.9738
ligand. ~A, the m e a n n u m b e r o f p r o t o n s b o u n d p e r l i g a n d ion, in w h a t e v e r form, is defined b y t h e e q u a t i o n . [ H A ] + 2[H~A] + ' . . + j [H~A] t~x ---- [A] + [ H A ] + [HsA] + " + [HjA]
(2)
o r in t e r m s o f K1~r, K~Tr etc., the respective f o r m a t i o n c o n s t a n t s , it can b e represented as
ri~
rlH[H] + 2K K H[H] + . . K,H[HIJ ----- 1 + K i n [ H ] -4- KI~K~[Hj ~ + " " -4- Klrlg~~ ' ' " K , ~ [ H ] 1
(3)
W h e r e j is the m a x i m u m n u m b e r o f p r o t o n s a s s o c i a t e d with the h g a n d . I n t h e case o f catcchol, E q u a t i o n (3) can b e w r i t t e n
K1HK~
(2 - - nA) [HI
1)
fix
1)[HI
= 0
(4)
1240
V.T. ATrIAVAt~,L. H. PRASHUand D. G. VARTAK
and for protocatechuic acid, Homoprotocatechuic acid and 3:4 dihydroxy hydrocinnamic acid, it can be written as
K UKU{(3--~A)tHI'K3 H (a_x_- 2) [__HIt (ax--1)' -- (~A--1) J
,qx (rix - 1 ) [ H ]
KXr r = 0
(5)
The proton ligand formation constants are calculated from Equations (4) or (5) as the case may be by the least square method, ¢12)using the experimental values of nx and [H]. The contribution of the term {(3 -- ax) [H]ZKsrr/(~x -- 1)} is almost negligible in the higher pH range. The values ofax nearer 1.0 are omitted for calculations by the least squares method. The final values, expressed as log VKxH, log ~K~H and log PKsrr, are practical proton-ligand formation constants and are given in Table 3. It may be mentioned here that the values of log PKxrr are obtained from the intercept on the { a x / ( a x - 1)[HI} axis (Equation 4), calculated by the least squares method. The titrations are carried out under identical conditions of temperature and ionic strength. The ionic product of water, PKw, is assumed to be constant. In the case of the tribasic acids (reagents II, III and IV) since the dissociation of one of the phenolic-OH starts at much higher pH than the dissociation of carboxylic group, the two steps in the formation of these can be treated separately and the presence of HA S- and A 3- can be neglected in the lower pH range. The equation for ~x can be written 2[H2A] + 3[HaA] ax = (6) [H~A] + [HaA ] and since KaH= [HaAI (7) [Hg.A] [H] We have from Equations (6) and (7) (3 -
hA)
log (nx -- 2-------)= B -- log Kau
(8)
where B is the practical pH unit. The plot of log (3 -- ~x/ax -- 2) against B, the experimental pH values gives a straight line. The intercept on the pH axis represents log/Ca H.
Calculation of metaMigand stability constants The metal ligand stability constants have been obtained from formation function curves. These have been calculated from a, pL data (Table 2), obtained by CalvinBjerrum titrations adopting the method of IRVING and ROSSOTTI.ill} The value of ~i, the mean number ofligands attached or bound per metal ion present in whatever form is calculated by the following simplified electro-neutrality equation. (v" -- vu) (N + E °) a = (V o + v')axT°M
(9)
Where v'# is the volume of alkali required, in case of metal titration, to reach the same pH at which v~ and v' are measured and T°M is the concentration of metal salt added. cxt~H. IRVINOand H. S. Rosso'm, J. Chem. Soc. 3397 (1953).
Solution stability constants of some metal complexes of derivatives of catechol
1241
TAm-~ 2 Cat~..hol-copl~r B pL B pL
4.50 4.75 5.00 5.25 5.50 5,75 6-00 6.25 0.1746 0 . 1 9 4 4 0 . 3 2 3 9 0"5145 0"7206 0 , 8 5 9 3 0 . 9 5 0 0 1"0794 15.0425 14.5450 14-0610 13.5857 13.1139 12.6343 12.1479 11.6685 6"50 6.75 7.00 7.25 7.50 7.75 8.00 8.25 1"2151 1 . 3 7 7 5 1 . 6 0 0 0 1 ' 8 1 6 3 1 . 9 6 1 7 2 . 0 3 9 7 2 . 0 9 1 7 2.1056 11.1909 10.7198 10.2627 9 . 8 0 9 4 9 . 3 4 5 2 8 . 8 6 8 8 8 . 3 9 0 3 7.9086
Catechol-zinc B pL B pL
7.00 0.0631 104)311 9.00 1.8310 6.4402
7.25 0.2347 9.5513 9.25 1'9047 6.0697
7.50 7.75 8.00 8.25 8.50 0 . 5 1 5 7 0.7930 1.0469 1.2875 1.4908 94)867 8.6265 8.1694 7.7195 7.2767 9.50 9.75 10.00 10.25 10.50 1 . 9 5 0 0 2 . 0 3 2 5 2 . 1 5 0 3 2.2434 2.3221 5 . 6 9 0 9 5..3728 5 . 0 9 2 0 4 . 8 2 9 6 4.5801
7.50 0.2347 9.0539 9.50 1"2366 5.5674
7.75 8.00 8.25 0.4141 0 - 6 1 3 3 0.8420 8 . 5 7 9 4 8 . 1 1 2 0 7"6560 9-75 10.00 10"25 1-4064 1"6656 1"8417 5"2589 4"9967 4"7455
8"75 1.6959 6.8530
Catechol-nickel B pL B pL
7.25 0.0874 9.5350 9"25 1-1950 5"9490
8.50 0.9740 7.1987 10.50 1"9538 4"4993
8"75 1.0897 6.7560 10"75 2"1292 4.2802
8.75 1.0479 6"7543
9.00 1.1605 6"3334
9.00 1.1527 6-3273 11"00 2"3333 4-0792
Catechol--cadmium B fi pL
7.50 7.75 8.00 8.25 0,0235 0 . 1 1 9 6 0 . 2 8 9 0 0"5630 9 " 0 3 1 1 8 " 5 4 6 4 8"0745 7 . 6 2 2 4
8-50 0"8295 7"1821
Catechol-magnesium B pL B t~ pL
8.75 9.00 0"0453 0"1328 6 . 6 2 6 4 6"1979 10-75 1.0223 4"0795
9"25 0.2780 5.8285
9"50 0-4069 5.4547
4"50 0"3651 15.1610 6"50 1"3972 11.0693
4"75 0"4397 14"5875 6"75 1"5979 10'6075
9"75 10.00 10"25 10"50 0"5460 0"6830 0"7826 0.8435 5"1354 4 . 8 4 5 6 4"5740 4"3135
Protocatechuic acid-copper B pL .B pL B pL
4.00 0-2500 16"4378 6.00 1.0735 12.0200 8.00 2.0928 8.2727
4"25 0"3026 15"7816 6.25 1"2017 11.5370 8"25 2.1174 7.8207
5-00 5"25 0-6401 0"7912 14"0414 13.5230 7'00 7.25 1'7524 1"8744 10"1410 9 . 6 7 2 2
5"50 5"75 0 . 9 0 0 7 0"9938 13"0149 12"5153 7"50 7"75 1"9987 2"0325 9-2095 8"7326
1242
V. T. ATHAVALE,L. H. PgAaHU and D. G. VARTAK TABLE 2
(cont.)
Protocateehuie acid-zinc B ri pL B fi pL B ti pL
6-50 0.1329 10-8834 8.50 1"6269 7"2497 10"50 2-2326 4"8874
6.75 0.2599 10-3978 8.75 1.8062 6.8786 10.75 2"3421 4-6621
7.00 0.4547 9.9220 9.00 1-9343 6"5349 I1.00 2"5180 4"4600
7"25 0.6993 9"4569 9.25 2'0177 6"2178
7"50 0.9113 8-9930 9"50 2"0518 5"9192
7.75 1.0820 8.5318 9.75 2.0480 5.6340
8.00 1.2932 8.0881 10-00 2"0811 5"3707
8-25 1.4679 7.6576 10-25 2"1584 5"1262
7.25 0-2945 9.4080 9.25 1"2209 6"0781
7"50 0.5393 8.9452 9.50 1 "3064 5'7855
7"75 0.7494 8.4864 9.75 1"3758 5-5120
8-00 0.9069 8.0324 10.00 1"5063 5-2630
8.25 0-9594 7.5811 10-25 1 "6697 5"0294
8"50 0.9974 7"1516 10.50 1"8248 4"8025
8.00 0.5353 7.9865 10.00 1"2009 5.2201
8.25 0.7227 7.5524 10.25 1 "3848 4"9883
8.50 0.8596 7.1364 10.50 1"5644 4.7637
8.75 0.9831 6.7502 10.75 1.7032 4"5381
9.00 1-0299 6-3886 I1.00 2"0390 4"3593
9.25 1.0683 6.0603
8.25 0-2113 7.4872 10.25 0-8990 4.9041
8.50 0.2610 7.0577 10.50 1.0176 4"6638
8.75 0.3703 6.6662 10.75 1.1434 4-4294
9.00 0.4818 6.3100 11.00 1.3182 4-2062
5"25 0.2605 13"3517 7'25 1 "4689 9.4940
5"50 0"4864 12"8628 7.50 1.7275 9"0663
5.75 0.7122 12.3832 7.75 1-8562 8.5777
6.00 0"8581 11"8984 8.00 1 "9525 8"1059
6"25 0"9549 11"4096 8"25 1 "9920 7"6270
6"50 1"0251 10-9187 8"50 2.0058 7"1501
7.25 0.2540 9-3107
7.50 0.5061 8"8418
7.75 0-7325 8.3735
8-00 0.9554 7"7094
8.25 1.0916 7-4401
8.50 1"2201 6"9788
Protoeateehuic acid-nickel B fi pL B ~i pL B pL
6.75 0.0527 10.3700 8.75 1-0800 6.7600 10"75 1"9424 4"5742
7-00 0.1196 9.8841 9.00 I q 501 6.4017 11"00 2.2054 4"3825
Protoeatechuic acid--cadmium B pL B pL
7.50 0.1164 8.8963 9.50 1"0840 5-7570
7-75 0.3008 8.4330 9.75 1.1029 5.4755
Protocatechuic acid-magnesium B pL B pL
7-75 0.0347 8.4023 9-75 0.7519 5.4176
8.00 0.1268 7.9368 10.00 0.7974 5.1529
9-25 9.50 0.65760.7361 5"9963 5.7001
Homoprotocatechuic acid--copper B pL B pL
4"75 0-0842 14.4084 6-75 1.0982 10"4296
5.00 0.1266 13"8656 7-00 1.2590 9.9561
Homoprotocatechuic acid-zinc B fi pL
6-75 0.0564 10.2880
7.00 0.0845 9-7911
Solution stability constants of some metal complexes of derivatives of catechol TABLE 2 (cont.) Homoprotocatechuic acid-zinc B pL B •~ pL
8.75 9-00 1.3724 1 . 6 3 5 6 6-5357 6.1331 10.75 2.2653 3.9817
9-25 1.8309 5.7500
9.50 1.9286 5.3852
9.75 10-00 10.25 10.50 1 . 9 6 7 9 2 . 0 1 4 2 2 . 0 7 8 3 2.1895 5 . 0 4 5 3 4.7392 4"4633 4.2178
Homoprotocatechuic acid-nickel B pL B pL
7.25 0.0635 9.2900 9.25 1.0860 5.6293
7-50 0.1878 8.8053 9.50 1.1587 5.2562
7.75 8.00 8.25 8.50 8.75 0"3555 0 . 5 8 7 0 0 . 7 8 3 6 0 . 9 2 7 5 1 . 0 1 4 9 8 . 3 2 7 7 7 . 8 6 1 9 7 . 3 9 8 2 6 . 9 3 7 4 6"4830 9.75 10.00 10.25 10.50 10-75 1 - 2 3 4 0 1 . 3 2 5 0 1 . 5 2 8 9 1 . 6 3 6 8 1.8150 4 . 9 1 9 7 4 - 6 1 8 0 4 . 3 6 1 3 4 . 1 1 0 5 3.8890
9-00 1.0632 6.0437
Homoprotocatechuic acid--cadmium B pL B pL
7.50 0.0278 8.7881 9.50 1"1257 5"2563
7.75 8.00 8-25 8.50 0 - 1 1 7 5 0 . 3 2 6 6 0.5181 0 . 7 0 9 7 8.3016 7.8323 7.3672 6.9117 9-75 10.00 10.25 10.50 1 . 1 9 8 9 1"2873 1-4357 1"5903 4 . 9 1 9 7 4 " 6 1 8 0 4 - 3 5 3 0 4"1106
8.75 0.8666 6.4663 10.75 1.7634 3.8890
9.00 0.9738 5.9281
9.25 1.0550 5.6293
Homoprotocatechuic acid-magnesium B pL B pL
8.75 9.00 0.0287 0 - 1 2 8 9 6.3638 5.9256 10-75 11.00 0"9307 1"0170 3 . 7 4 3 2 3.5159
9"25 9.50 0.2643 0.4224 5-5242 5.1577
9.75 10-00 10.25 10.50 0 . 5 5 2 3 0 . 6 3 0 5 0 . 6 5 5 9 0.7413 4 . 8 3 7 3 4 . 5 1 8 4 4-2321 3.9731
3:4 dihydroxyhydrocinnamic acid-copper B pL B
pL B pL
4"75 5.00 5"25 5"50 5"75 6.00 6"25 6-50 0-1387 0"2101 0"3437 0"5478 0 . 7 6 0 0 0 - 8 7 5 2 0"9723 1-0794 14"1939 13.6208 13"0852 12"5771 12"0867 11"5922 11"1000 10-6138 6"75 7.00 7"25 7"50 7"75 8.00 8"25 8"50 1"2029 1 - 3 6 2 4 1"5950 1"8217 1"9907 2 . 0 9 8 1 2 - 1 8 5 9 2.2090 10.1322 9-6583 8.75 9.00 2.2155 2 " 2 3 3 0 6.4409 6.0076
9 . 2 0 4 4 8-7543 8 . 2 9 7 8 7 . 8 3 2 6 7 . 3 6 9 6 6"8998 9"25 9"50 9"75 10.00 10-25 10"50 2"2837 2"3793 2 . 3 9 8 8 2 . 4 5 8 2 2'5830 2"6775 5 " 6 1 5 2 5 - 2 7 2 0 4"9424 4"6574 4 - 4 2 1 8 4"2000
3 : 4 dihydroxyhydrocinnamic acid-zinc B si pL B pL B pL
6-75 0-0989 9"9762 8"75 1.4121 6-2430 10"75 2"8431 3"9394
7"00 0"1747 9-4900 9.00 1.6813 5-8492
7"25 0"3158 9.0000 9'25 1"9366 5.4904
7-50 0"5116 8"5263 9"50 2-1694 5-1679
7-75 8.00 8"25 8"50 0 - 7 4 1 6 0"9227 1.0911 1.2247 8"0595 7 . 5 6 5 2 7"1288 6-6726 9"75 10.00 10"25 10"50 2.2530 2.3271 2"4934 2.6073 4 - 8 5 1 6 4 . 5 6 5 1 4 " 3 2 9 0 4.1032
1243
1244
V.T. AT/-IAVALE,L. H. P~BHU and D. G. VARTAK
TAnt~ 2 (cont.) 3:4 dihydroxyhydroeinnamie acid-nickel B ti pL B ri pL
7.50 7"75 8.00 8"25. 8.50 8.75 0.1133 0 . 2 7 7 7 0 . 4 8 6 8 0"7134 0 . 8 6 5 7 0 . 9 7 2 6 8.4804 8.0029 7.5356 7.0778 6.6214 6.1743 9.50 9.75 10-00 10.25 10-50 10-75 1"1066 1"1433 1"2760 1"5698 1"7382 1"9781 4 " 9 7 7 9 4"6476 4"3655 4 " 1 3 2 6 3"9047 3"7131
9.00 1.0447 5.7478
9.25 1.0668 5-3436
3:4 dihydroxyhydroeinnamic acid--cadmium B ~i pL B li pL
7.50 7.75 0.0488 0 . 1 7 2 3 8-4737 7.9918 9.50 1.1251 4.9854
8.00 0.3392 7.5193
8-25 0.5300 7.0565
8"50 0.7295 6"6063
8.75 9.00 0.9115 1.0668 6-1706 5.7554
9.25 1.1205 5.3562
3:4 dihydroxydrocirmamic acid-magnesium B pL B !i pL
8"50 0.0414 6.5228 10.50 1-1240 3"8110
8'75 0"0770 6"0660 10-75 1"1650 3-5793
9.00 0"1838 5.6406
9"25 9.50 0"3220 0 . 5 1 4 2 5"2497 4 . 9 0 1 9
9"75 10.00 10-25 0 . 6 6 8 8 0 . 8 2 6 2 1"0505 4 . 5 8 6 4 4"3036 4-0573
The value of pL or log 1/[L] is calculated using the following equation:
Pflia[H]l V ° + v" pL = log LT--~ -~ fi--~M × ~ V
(10)
where pflrr is the practical overall proton ligand stability constant. The stoichiometric metaMigand stability constants for 1 : 2 complexes in the range 0 < fi < 2 are determined by the method of half integral values as well as by the least squares method, substituting the experimental values of a and (L) in the equation n
(a-
1)[L]
=
(2 - - h) [L]
(t~- 1)
X'xK, - - X:~
(11)
The results obtained by least squares method are given in Table 5. DISCUSSION
Proton-ligandsystem It is seen from Table 1 that except for catechol, the formation curves extend over the range 1 < ax < 3 and their shape indicates the presence of the species HzA- and HA s-. This shows that the H + of the carboxylic group and one of the phenolic H + are easily dissociable in completely separable steps. Further, in all the cases, including eateehol, the lowest hA values were approximately equal to one in the pH ranges used for calculations. This indicates that either the second OH is little ionized or that the steps leading to the formation of HA ~ and A s-- in case of the ligands (II), (III), (IV) and H A - and A ~- in case of catechol are incomplete and not separated.
Solution stability constants of some metal complexes of derivatives of catechol
1245
Proton-ligand stability constants Table 3 gives the values obtained for the practical proton-figand stability constants represented by log PKH. It can be seen from the table that the log P K : value of the compound (II) is much lower than for the compounds (I), (III) and (IV). The anomalous value for compound (II) may be due to the electron withdrawing action of the nuclear substituted COOH group. The recent data of BrO,UDEand NACHOD~ ) is given in Table 4 for comparison, from which it can be seen that the electron withdrawing action of the COOH group TABLE 3
(1) (IF) aid (IV)
Method
from B; fix = 1
least square Eqn. (4) and (5)
(Eqan. 8)
Reagent
Log PK1R
Log PKIR
Log PK| 1I
Catechol Protocatechuic acid Homoprotocatechuic acid 3: 4-dihydroxyhydrocinnamic acid
,~12.6" 12.6 12.0 11"6
9.34 8.83 9-44 9-36
-4.40 4.25 4"56
I
* The value is approximate since t~A does not fall below one.
is very high due to resonance interaction, so that the 4-hydroxy group dissociates first in the case of the protocatechuic acid. In the case of compounds (III) and (IV) the effect of the --COOH group is decreased due to the linking through - - C H 2 and --CHs--CHI groups respectively. It may be observed that the introduction of alkyl groups into the ring of the phenol decreases its acid strength, ot~ the effect being most pronounced for ortho- and para- substituents since the electron releasing effect of the alkyl groups in the ortho- and para-positions to the hydroxyl group can be transmitted to the oxygen of the hydroxyl group, so that proton removal becomes more difficult. This effect cannot be transmitted when the substituents are in the rectaposition and for this reason, according to GOLUMn]Cet al. ~15~ the meta-substituted phenols are the strongest acids. They concluded that in the ethyl phenolic series, for which no exact ionization constants are available, the calculated l'Ka values of the meta-substituent groups are the lowest of the series. On further examination of Tables 3 and 4, it can be seen that the log I~K2Hvalues obtained in the present investigation follow the para-order and the log 1~K1~follow TABLE4.--PK II VALUESOF SOMEMONOSUBSlTrUTV.D pHENOLS Phenol
Meta-
Para-
--H ---CHs ---CIH5 --COOH
9"94 10"08 9-90 9'94
9"94 10'19 10.00 9"39
c1,~E. A. B~UDE and F. C. HACHOD, Determination of Organic Structure by Physical Method~, pp. 567-662, Academic Press, New York (1955). tl,~ G. W. W ~ . ~ I D , The Theory of Resonance, p. 185, J. Wiley, Hew York (1944). tls~ C. GOLUMmC,M. ORCH~ and S. W~I.Lsg, J. Am. chem. Soc. 71, 2624 (1949).
1246
V . T . A'rrlAVALE, L. H. Pr.Anrro and D. G. VARTAK
the meta order in case of compounds (II), (III) and (IV). Hence for these compounds the electron pull (meta effect) of the relevant --CH2COOH and --C~H4COOH groups act in order of the log PKxrr values. The log PKaa values i.e., the negative logarithm of the dissociation constant of the COOH group in the acid substituted catechols, fit the relation log PKarr = a -(b/d) as suggested by Rum'F and SADETre) where d is the distance along the chain, from the first aromatic carbon to the atom which attracts and fixes the hydrogen ion. The constants a and b are calculated to be 5"45 and 5" 1 respectively. The value of the constant b, which measures the acidifying effect of a given substituent is compatible with that found by the above authors, tm
Metal-ligand system The formation of complexes under the experimental conditions employed and the absence of hydrolysis, polynudear complexes and protonated complexes is dearly indicated by the following observations: (i) The departure of all metal ligand curves from the pure ligand curve at B values much lower than the B of hydrolysis of these metals. (ii) Appearance of distinctive colours well below the B of hydrolysis (Cu and Ni). (iii) The low concentration of the metal ions present (iv) Negligibly small contribution to ~ values due to metal ion hydrolysis at the B of hydrolysis of the metal. These ligands react in the following way" X-
,CoO
X--OH
X~
+ M X-
• X~
M + 2H + X-
where X represents b H , --COOH, --CH2COOH, or --(CH2)~. ---COOH. In the case of the copper titrations a volume of alkali equal to four equivalents of copper ions present in the solution, in excess of that required to neutralise the free perchloric acid, is required to reach pH 8 at which the llgand curve departs from the acid curve. This is also true in the case of the acid derivatives of catechol indicating that the COO-group does not take part in the chelate formationJ 1~) The metal ligand curves for zinc, nickel, cadmium and magnesium follow the same path as that of the ligands up to pH 6-5. For all the chelating agents the relative positions of the metal curves indicate the relative stabilities of the metal chelates, follow the order Zn < Cu :> Ni > Cd > Mg.
Metal-ligand stability constants The metal ligand stability constants are obtained from the ~, pL data. The values of pL at ~ = 0.5 and 1.5 correspond to the logs of the first and the second stability constants respectively. In the case of the cadmium and magnesium complexes a break ~t6}p. RUMPF and J. SADET,Bull. Soc. chim. Fr. 450 (1958). ~17)A. E. MARTELL and M. CALVIN, Chemistry of Metal Chelate Compounds, p. 156, Prentice-Hall, New York (1956).
Solution stability constants of some metal complexes of derivatives of catechol
1247
at ti = 1 indicates the formation of 1 : 1 chelates only. The stability constants were calculated graphically using the equation log
1--~ t~ = p L - - l o g K
1
for the points below the point of precipitation. All four ligands show a tendency to f o r m higher complexes with zinc, ligands I and I I with nickel and Ligand IV with copper. The portion of the curve which lies in the range 0 < a < 2 was treated by the least square method for obtaining the stability constants of the 1 : 2 complex. The determination of log Ka was not attempted by the least square method since its formation starts, invariably, at much higher p H and the difference between the p L TABLE 5.--MI~rAL-LIGAND STABILITYCONST.MqTS
I
Compound Catechol
II
Protocatechuic acid
III
IV
Homoprotocatechuic acid
3:4-dihydroxyhydrocinnamic acid
Metal Cu *+ Zn I+ Ni 2+ Cd s+ Mg*+ Cu *+ Zn ~+ Ni 2+ CAa+ Mg2+ Cu 2+
Log Kx 13"58 9"08 8"36 7"70 5"24 14"53 9"84 8"96 7"97 6"30 12"82
Log K~ 10'49 7'24 5"15 --10"76 7"55 5-38 4"75
Z n *+ Ni a+
8-80 8"04
4"35
Cd *+ Mg~+ Cu s+ Zn *+ Ni 2+ Cd 2+ Mg*+
7"35 4"94 12"74 8"64 7"45 7"14 4"90
4"28 -9"42 6"15 4"08 ---
-
-
9"50 6"37
values at ri = 1.5 and ~ ---- 2.5 indicate the formation of the third complex only after complete formation of the 1 : 2 complex. Further support to this argument is the wave in the formation curve near ~ = 2. Order o f stability constants or log K values
The log K values calculated for copper, zinc, nickel, cadmium and magnesium are given in Table 5. It can be seen that the order Zn < Cu > Ni Cd > Mg is in agreement with that of IRVING and WILLIAMS.(ls) The order Zn > Cd > M g is also found to agree with the published data for practically all the ligands studied so far. Log K for copper differs considerably from log K for the other metals and Zn is greater than Ni. The enormously higher log K value for the copper chelate may be due to a square planar structure. The order, Zn > Ni has been observed in a few cases, c19,2°~ where the relatively low value o f nickel is attributed to steric hindrance preventing the formation of a square planar structure. Again the hydrolysis constant ~xa~H. IRVINO and R. J. P. WILlOws, J. Chem. Soc. 3192 (1953). c],J W. D. JomcsroN and H. ~ R , Analytica chim Acta 11, 201 (1954). ~2o~W. D. JOHNSTONand H. F'gEIS~R,Analytica chim Acta 11, 301 (1954).
1248
V. T. ATHAV~m, L. H. PRAeHU and D. G. VARTAK
14.0! F3.C
II-C 12.C I0.0 Jl.(] --
9.0 100
-~80
J
Cd
70 I
~
6-C 5C
@
6£
Mg o ~ I
''''~'~ I
120
H4
4.0
rr
f
I~
'30
I14
llI I
I
IT
I
,20
log PK~r
130
log PK~ Fla. 1
150
log K~K2(Zn) 16.0
ologIK(Ni)vs. log K,(Izn) olog KhKz(Ni)vs.log K~Kz(Zn)/
IZ.O ] ~ -- 14.0
/ 9.0
z
z
-- 13.O
8.0
2.0
7"(
0
7"
f 9.0
I I0.0
log
K~ ( Z n )
l~o. 2
I1.0
Solution stability constants of some metal complexes of derivatives of cateehol
1249
of zinc is greater than that of nickel indicating a greater affinity of zinc for w O H ions. The catechol ligand resembles - - O H group carrying two negatively charged oxygen ions. As the bonds are purely ionic, the smaller zinc ion is expected to form a stronger complex than nickel as observed. The solubility of the catechol complexes in water supports this view.
Variation of log K values with the nature of the ligands A comparison of the metal ligand stability constants with the proton ligand stability constants is shown in Fig. 1. The plots are seen to be mostly straight lines showing that the general relation3 ~1~ Log K = apK + B is obeyed. A comparison of the metal ligand stability constants of different metals having the same co-ordination number, suggested by IRVING and ROSSOTTI(22) and FRIESERand co-workerstz3~is shown in Fig. 2. The values of stability constants follow a line with slope close to unity showing that the general relation Log fl~ri = 1"13 log fl2zn + C is obeyed in within ~0.3 log units. ~21~j. BJERRUM, Chem. Rev. 46, 381 (1950). (*~ H. I. IRVING and H. S. ROSSOTTI,Acta chem. stand. 10, 72 (1956). t~8~ H. FREISER,Q. FERNANDOand G. E. CHENI~Y,J. phys. Chem. 63, 250 (1959).
SOLUTION STABILITY CONSTANTS OF SOME METAL COMPLEXES OF DERIVATIVES OF CATECHOL V. T. ATHAVALE,L. H. PRABHU and D. G. VARTAK Analytical Division, Atomic Energy Establishment Trombay, Bombay, India (Received 18 March 1965; in revised form 11 August 1965)
Abstract--The formation constant for the metal chelates of copper, zinc, nickel, cadmium and magnesium with catechol, photocatechuic acid, homoprotocatechuic acid and 3:4-dihydroxyhydrocinnamic acid have been determined. The Calvin-Bjerrumtitration technique has been adopted for the determination of stability constants of the proton ligand complexesand the metal li~nd chelates. The relation between the log of the stability constants of the metal chelates and the log of the protonligand formation constants of the ligands is discussed. The metal-ligand chelate stability constants follow the order Zn < Cu > Ni > Cd > Mg. CATECHOL, an o-hydroxy phenol, forms metal chelates with a number of metal ions. Catechol complexes of several metals such as Cr, Cu, Zn, Co, Ni, etc., have been studied by WEIh*LANDand co-workers. ¢1) They found that aU these complexes contained two or three catechol molecules attached to one metal ion with either alkali or alkaline earth metal ions or organic bases, fike pyridine, in the outer sphere. Rare earth, (z) alkaline earth, (a"L) rhenium (§) and ferric iron (6) catechol complexes are also known. The formation constants of the metal catechol complexes of a few metal ions have been studied but without much reference to the relation between the metal-ligand formation and proton-ligand formation constants. In the present investigation, the formation of complexes of Cu(II), Zn(II), Ni(II), Cd(II), Mg(II) with catechol, protocatechuic acid, homoprotocatechuic acid and 3:4-dihydroxyhydrocinnamic acid together with the dissociation constants of the respective reagents have been investigated by employing a potentiometric titration (Calvin--Bjerrum) technique. A few of the factors affecting the ligand dissociation constants and the chelate stability constants are discussed. EXPERIMENTAL Chemicals. All the chemicalsincluding the metal salts, sodium hydroxide, perchloric acid, potas-
sium biphthalate, borax, ethylenediaminetetraaceticacid (disodium salt) etc., used were of analytical reagent grade. Sodium hydroxide, free from carbonate, was prepared by a standard method,cv~ Chelating agents. Catechol(Ligand I): The reagent supplied by Eastman Kodak Co. (practical ~1~R. WFaNLANDet al., Z. anorg, al~. Chem. 126, 141 (1923): Chem. Abstr 17, 2542 (1923);/b/d,/d~m, 150, 69 (1925); Chem. Abstr 20, 717 (1926). ~l) L. FV.RNAND~,Gazz. Chim. ital. 55, 424 (1925): Chem. Abstr. 20, 536 (1926); /b/d, idem, 56, 682 (1926); Chem. Abstr. 21, 866 (1927). ts~R. SCHOLD~and M. WOL~,Z. anorg, allg. Chem. 210¢184 (1923); Chem. Abstr. 27, 2106 (1933). ~4~R. SCHOLDERand E. SCHLitZ,Z. anorg, allg. Chem., 211, 161 (1933); Chem. Abstr. 27, 2645
(1933). ~5)D. SEN and F. RAY, i. l~d'_-m Chem. Soc. 30, 253 (1953). ~e)A. K. BAnKO,J.gen. Chem. U.S.S.R. 16, 968 (1946); Chem Abstr 41, 2654e (1947). ~7~A.I. VOGEL,Text Book o f Quantitative Inorganic Analysis, p. 242, Longmans,Green,London (1961).
1237
1238
V.T. ATHAVALE,L. H. PRAnHUand D. G. VARTAK
grade) was distilled under reduced pressure ~s~ to obtain a pure white product (m.p. 105°C). The compound was preserved in a vacuum desiccator without exposure to light. Protocatechuic acid (Ligand II): BDH Laboratory grade acid was twice recrystallized from double distilled water in a dark room and dried in a vacuum desiccator. The product was white needles with one molecule of water of crystallization o~ (m.p. 199°C). Homoprotocatechuic acid (Ligand III) and 3:4-dihydroxyhydrocinnamic acid (Ligand IV): The reagents were supplied by H.M. Chemical Company, Santa Monica, California, U.S.A. These were used after purification. The stock solutions of all the reagents were of 0.04 M concentration except the last which was 0.032 M. All solutions were prepared in carbon dioxide free double distilled water. Metal perchlorates were prepared by dissolving the respective metal carbonate or oxide in a known volume of standard perchloric acid. The concentration of metal ions in each of the metal perchlorates of Cu, Zn, Ni and Cd, was estimated by potentiometric titrations as described by SCnWARZEr,raACn,cx°~ using solutions of Naa-EDTA and standard sodium hydroxide. Magnesium was estimated by titrating it against standard Na2-EDTA solution using Eriochrome Black-T as indicator. Calvin-Bjerrum titration. Potentiometric titrations, against 1 N NaOH of (i) the free mineral acid (0.03 M perchlorie acid) (ii) free mineral acid plus the reagent (0.004 M) under investigation, and (iii) free mineral acid, reagent (0.004 M) and metal salt (0.001 M), were carried out using a bench type Cambridge pH meter operating at 12 V d.c. The titration assembly consisted of a 5 ml micro burette containing 1.0 N NaOH fitted with a soda lime guard tube at the top and a glass reaction beaker of 150 ml capacity fitted with a rubber bung. The bung had holes to accommodate a Cambridge Universal glass electrode, a dip type calomel electrode a glass tubing for bubbling nitrogen and the delivery end of the burette. The titrating solution was made up to 100 ml with Sodium perchlorate and water in the reaction beaker which was immersed in a water bath maintained at 30 4- 0.1°C. Sodium perchlorate was added to keep the ionic strength constant at 0.1 M. The pH of the solution was noted after every addition of a small volume (say 0.02 ml) of~-l.0 N sodium hydroxide from the burette until pH 11"5 was reached. The maximum reading which was stable after each addition of alkali was recorded. The titration solution was kept free from carbon dioxide and oxygen by continuously bubbling purified nitrogen presaturated by passing it through 0.1 M sodium perchlorate solution. CALCULATIONS
Calculation of proton ligand formation constants The calculation o f the p r o t o n ligand formation constants is accomplished in two steps. (i) The formation curves are obtained f r o m fix, p H data. (ii) The curves are analysed to arrive at the formation constants. The values o f ax at various p H values (Table 1) are obtained f r o m the titration curves o f free acid and free acid plus reagent using the electroneutrality equation at each step as adapted by IRVlN~ and Rossorri. m) (v" - - v') ( N -k- E °)
ax= y--
(V °-k v') T°L
(1)
Where v" and v' denote the volume o f alkali required to reach the same p H in the titrations o f the free acid plus reagent and the free acid alone respectively, N, the normality o f alkali, E °, the initial concentration o f the free acid, T°L, the total concentration o f the ligand added and y, the total n u m b e r dissociable p r o t o n s attached to
cs~H. Gu_sc~N,Organic Synthesis, Collective Vol. 1, p. 143, J. Wiley, New York (1932). ~9~I. HvataRONand H. M. Btrroonv, Dictionaryof Organic Compounds,Vol. 3, p. 539, London (1943). c10~G. SCHWARZENnACH,Complexometric Titration, p. 60, Methuen, London (1957). m~ H. IRVINGand H. S. RossoT'n, J. chem. Soc. 2904 (1954).
Solution stability comtants of some metal complexes of derivatives of catechol
1239
TABLE 1 Catechol B nA B
8-00 8-25 8-50 1 " 9 7 9 6 1"9490 1.8914 10"00 10"25 10.50
Rx
1.1704
1.1027
1.0535
3.50
8.75 9.00 9.25 9.50 1"8077 1"6802 1"5377 1-3847 10"75 11.00 1 . 0 3 8 7 1-0194
9.75 1"2657
Protocatechuic acid B
3.00
3.25
t~A B Rx B nx B t~x
2.9949 5.00 2.2128 7-00 1"9918 9"00 1.3977
2 . 9 5 4 1 2.8995 5.25 5.50 2.1397 2.0854 7"25 7.50 1"9768 1 . 9 5 9 7 9-25 9.50 1.2711 1 . 1 7 1 7
3.75
2.8317 5.75 2.0514 7"75 1-9274 9"75 1.1026
4.00
4.25
4.50
2-7279 2.5851 6.00 6.25 2.0277 2.0140 8.00 8"25 1.8800 1"7916 10.00 10"25 1.0589 1.0354
4.75
2 - 4 4 2 5 2.3167 6.50 6.75 2.0040 1.9954 8-50 8"75 1 . 6 6 5 9 1-5385 10"50 10"75 1.0140 0.9880
Homoprotocatechuic acid B nA B nA B RA B ~A
3-00 2"9256 5"00 2"1481 7"00 10960 9.00 1.7325
3"25 2"8848 5-25 2.0941 7"25 1.9925 9-25 1.6041
3-50 2.8260 5.50 2"0553 7.50 1.9895 9.50 1.4641
3-75 4.00 4"25 4.50 4"75 2"7497 2"6333 2"4949 2"3644 2"2394 5-75 6.00 6"25 6"50 6"75 2"0333 2"0196 2"0145 2.0127 2"0062 7"75 8.00 8"25 8"50 8"75 1 . 9 8 0 8 1"9791 1.9520 1 . 8 9 8 9 1-8255 9.75 10-00 10.25 10.50 10-75 1 . 3 2 0 8 1 . 2 0 2 5 1 . 1 1 5 2 1.0520 0.9834
3:4 dihydroxyhydrocinnamic acid B RA ~x
3"00 2.9826 5.00 2.2538
B RA B
7.00 2.0002 9.00
2-1468 7.25 1.9936 9.25
7.50 1.9874 9.50
2-0221 2.0178 7.75 8.00 8.25 1 . 9 8 0 5 1 . 9 5 8 9 1.9280 9.75 10.00 10.25
RA
1.6961
1.5652
1.4209
1-2678
B
3"25 2"9606 5"25
3-50 2"9215 5"50 2.0835
3-75 2"8621 5"75 2.0461
4.00 2"7876 6.00
1.1586
4"25 2"6830 6"25
1.0716
4-50 2.5340 6.50 2.0133
4"75 2"3807 6"75
2.0047 8.50 8-75 1 . 8 7 9 8 1.7971 10.50 10.75 0.9738
ligand. ~A, the m e a n n u m b e r o f p r o t o n s b o u n d p e r l i g a n d ion, in w h a t e v e r form, is defined b y t h e e q u a t i o n . [ H A ] + 2[H~A] + ' . . + j [H~A] t~x ---- [A] + [ H A ] + [HsA] + " + [HjA]
(2)
o r in t e r m s o f K1~r, K~Tr etc., the respective f o r m a t i o n c o n s t a n t s , it can b e represented as
ri~
rlH[H] + 2K K H[H] + . . K,H[HIJ ----- 1 + K i n [ H ] -4- KI~K~[Hj ~ + " " -4- Klrlg~~ ' ' " K , ~ [ H ] 1
(3)
W h e r e j is the m a x i m u m n u m b e r o f p r o t o n s a s s o c i a t e d with the h g a n d . I n t h e case o f catcchol, E q u a t i o n (3) can b e w r i t t e n
K1HK~
(2 - - nA) [HI
1)
fix
1)[HI
= 0
(4)
1240
V.T. ATrIAVAt~,L. H. PRASHUand D. G. VARTAK
and for protocatechuic acid, Homoprotocatechuic acid and 3:4 dihydroxy hydrocinnamic acid, it can be written as
K UKU{(3--~A)tHI'K3 H (a_x_- 2) [__HIt (ax--1)' -- (~A--1) J
,qx (rix - 1 ) [ H ]
KXr r = 0
(5)
The proton ligand formation constants are calculated from Equations (4) or (5) as the case may be by the least square method, ¢12)using the experimental values of nx and [H]. The contribution of the term {(3 -- ax) [H]ZKsrr/(~x -- 1)} is almost negligible in the higher pH range. The values ofax nearer 1.0 are omitted for calculations by the least squares method. The final values, expressed as log VKxH, log ~K~H and log PKsrr, are practical proton-ligand formation constants and are given in Table 3. It may be mentioned here that the values of log PKxrr are obtained from the intercept on the { a x / ( a x - 1)[HI} axis (Equation 4), calculated by the least squares method. The titrations are carried out under identical conditions of temperature and ionic strength. The ionic product of water, PKw, is assumed to be constant. In the case of the tribasic acids (reagents II, III and IV) since the dissociation of one of the phenolic-OH starts at much higher pH than the dissociation of carboxylic group, the two steps in the formation of these can be treated separately and the presence of HA S- and A 3- can be neglected in the lower pH range. The equation for ~x can be written 2[H2A] + 3[HaA] ax = (6) [H~A] + [HaA ] and since KaH= [HaAI (7) [Hg.A] [H] We have from Equations (6) and (7) (3 -
hA)
log (nx -- 2-------)= B -- log Kau
(8)
where B is the practical pH unit. The plot of log (3 -- ~x/ax -- 2) against B, the experimental pH values gives a straight line. The intercept on the pH axis represents log/Ca H.
Calculation of metaMigand stability constants The metal ligand stability constants have been obtained from formation function curves. These have been calculated from a, pL data (Table 2), obtained by CalvinBjerrum titrations adopting the method of IRVING and ROSSOTTI.ill} The value of ~i, the mean number ofligands attached or bound per metal ion present in whatever form is calculated by the following simplified electro-neutrality equation. (v" -- vu) (N + E °) a = (V o + v')axT°M
(9)
Where v'# is the volume of alkali required, in case of metal titration, to reach the same pH at which v~ and v' are measured and T°M is the concentration of metal salt added. cxt~H. IRVINOand H. S. Rosso'm, J. Chem. Soc. 3397 (1953).
Solution stability constants of some metal complexes of derivatives of catechol
1241
TAm-~ 2 Cat~..hol-copl~r B pL B pL
4.50 4.75 5.00 5.25 5.50 5,75 6-00 6.25 0.1746 0 . 1 9 4 4 0 . 3 2 3 9 0"5145 0"7206 0 , 8 5 9 3 0 . 9 5 0 0 1"0794 15.0425 14.5450 14-0610 13.5857 13.1139 12.6343 12.1479 11.6685 6"50 6.75 7.00 7.25 7.50 7.75 8.00 8.25 1"2151 1 . 3 7 7 5 1 . 6 0 0 0 1 ' 8 1 6 3 1 . 9 6 1 7 2 . 0 3 9 7 2 . 0 9 1 7 2.1056 11.1909 10.7198 10.2627 9 . 8 0 9 4 9 . 3 4 5 2 8 . 8 6 8 8 8 . 3 9 0 3 7.9086
Catechol-zinc B pL B pL
7.00 0.0631 104)311 9.00 1.8310 6.4402
7.25 0.2347 9.5513 9.25 1'9047 6.0697
7.50 7.75 8.00 8.25 8.50 0 . 5 1 5 7 0.7930 1.0469 1.2875 1.4908 94)867 8.6265 8.1694 7.7195 7.2767 9.50 9.75 10.00 10.25 10.50 1 . 9 5 0 0 2 . 0 3 2 5 2 . 1 5 0 3 2.2434 2.3221 5 . 6 9 0 9 5..3728 5 . 0 9 2 0 4 . 8 2 9 6 4.5801
7.50 0.2347 9.0539 9.50 1"2366 5.5674
7.75 8.00 8.25 0.4141 0 - 6 1 3 3 0.8420 8 . 5 7 9 4 8 . 1 1 2 0 7"6560 9-75 10.00 10"25 1-4064 1"6656 1"8417 5"2589 4"9967 4"7455
8"75 1.6959 6.8530
Catechol-nickel B pL B pL
7.25 0.0874 9.5350 9"25 1-1950 5"9490
8.50 0.9740 7.1987 10.50 1"9538 4"4993
8"75 1.0897 6.7560 10"75 2"1292 4.2802
8.75 1.0479 6"7543
9.00 1.1605 6"3334
9.00 1.1527 6-3273 11"00 2"3333 4-0792
Catechol--cadmium B fi pL
7.50 7.75 8.00 8.25 0,0235 0 . 1 1 9 6 0 . 2 8 9 0 0"5630 9 " 0 3 1 1 8 " 5 4 6 4 8"0745 7 . 6 2 2 4
8-50 0"8295 7"1821
Catechol-magnesium B pL B t~ pL
8.75 9.00 0"0453 0"1328 6 . 6 2 6 4 6"1979 10-75 1.0223 4"0795
9"25 0.2780 5.8285
9"50 0-4069 5.4547
4"50 0"3651 15.1610 6"50 1"3972 11.0693
4"75 0"4397 14"5875 6"75 1"5979 10'6075
9"75 10.00 10"25 10"50 0"5460 0"6830 0"7826 0.8435 5"1354 4 . 8 4 5 6 4"5740 4"3135
Protocatechuic acid-copper B pL .B pL B pL
4.00 0-2500 16"4378 6.00 1.0735 12.0200 8.00 2.0928 8.2727
4"25 0"3026 15"7816 6.25 1"2017 11.5370 8"25 2.1174 7.8207
5-00 5"25 0-6401 0"7912 14"0414 13.5230 7'00 7.25 1'7524 1"8744 10"1410 9 . 6 7 2 2
5"50 5"75 0 . 9 0 0 7 0"9938 13"0149 12"5153 7"50 7"75 1"9987 2"0325 9-2095 8"7326
1242
V. T. ATHAVALE,L. H. PgAaHU and D. G. VARTAK TABLE 2
(cont.)
Protocateehuie acid-zinc B ri pL B fi pL B ti pL
6-50 0.1329 10-8834 8.50 1"6269 7"2497 10"50 2-2326 4"8874
6.75 0.2599 10-3978 8.75 1.8062 6.8786 10.75 2"3421 4-6621
7.00 0.4547 9.9220 9.00 1-9343 6"5349 I1.00 2"5180 4"4600
7"25 0.6993 9"4569 9.25 2'0177 6"2178
7"50 0.9113 8-9930 9"50 2"0518 5"9192
7.75 1.0820 8.5318 9.75 2.0480 5.6340
8.00 1.2932 8.0881 10-00 2"0811 5"3707
8-25 1.4679 7.6576 10-25 2"1584 5"1262
7.25 0-2945 9.4080 9.25 1"2209 6"0781
7"50 0.5393 8.9452 9.50 1 "3064 5'7855
7"75 0.7494 8.4864 9.75 1"3758 5-5120
8-00 0.9069 8.0324 10.00 1"5063 5-2630
8.25 0-9594 7.5811 10-25 1 "6697 5"0294
8"50 0.9974 7"1516 10.50 1"8248 4"8025
8.00 0.5353 7.9865 10.00 1"2009 5.2201
8.25 0.7227 7.5524 10.25 1 "3848 4"9883
8.50 0.8596 7.1364 10.50 1"5644 4.7637
8.75 0.9831 6.7502 10.75 1.7032 4"5381
9.00 1-0299 6-3886 I1.00 2"0390 4"3593
9.25 1.0683 6.0603
8.25 0-2113 7.4872 10.25 0-8990 4.9041
8.50 0.2610 7.0577 10.50 1.0176 4"6638
8.75 0.3703 6.6662 10.75 1.1434 4-4294
9.00 0.4818 6.3100 11.00 1.3182 4-2062
5"25 0.2605 13"3517 7'25 1 "4689 9.4940
5"50 0"4864 12"8628 7.50 1.7275 9"0663
5.75 0.7122 12.3832 7.75 1-8562 8.5777
6.00 0"8581 11"8984 8.00 1 "9525 8"1059
6"25 0"9549 11"4096 8"25 1 "9920 7"6270
6"50 1"0251 10-9187 8"50 2.0058 7"1501
7.25 0.2540 9-3107
7.50 0.5061 8"8418
7.75 0-7325 8.3735
8-00 0.9554 7"7094
8.25 1.0916 7-4401
8.50 1"2201 6"9788
Protoeateehuic acid-nickel B fi pL B ~i pL B pL
6.75 0.0527 10.3700 8.75 1-0800 6.7600 10"75 1"9424 4"5742
7-00 0.1196 9.8841 9.00 I q 501 6.4017 11"00 2.2054 4"3825
Protoeatechuic acid--cadmium B pL B pL
7.50 0.1164 8.8963 9.50 1"0840 5-7570
7-75 0.3008 8.4330 9.75 1.1029 5.4755
Protocatechuic acid-magnesium B pL B pL
7-75 0.0347 8.4023 9-75 0.7519 5.4176
8.00 0.1268 7.9368 10.00 0.7974 5.1529
9-25 9.50 0.65760.7361 5"9963 5.7001
Homoprotocatechuic acid--copper B pL B pL
4"75 0-0842 14.4084 6-75 1.0982 10"4296
5.00 0.1266 13"8656 7-00 1.2590 9.9561
Homoprotocatechuic acid-zinc B fi pL
6-75 0.0564 10.2880
7.00 0.0845 9-7911
Solution stability constants of some metal complexes of derivatives of catechol TABLE 2 (cont.) Homoprotocatechuic acid-zinc B pL B •~ pL
8.75 9-00 1.3724 1 . 6 3 5 6 6-5357 6.1331 10.75 2.2653 3.9817
9-25 1.8309 5.7500
9.50 1.9286 5.3852
9.75 10-00 10.25 10.50 1 . 9 6 7 9 2 . 0 1 4 2 2 . 0 7 8 3 2.1895 5 . 0 4 5 3 4.7392 4"4633 4.2178
Homoprotocatechuic acid-nickel B pL B pL
7.25 0.0635 9.2900 9.25 1.0860 5.6293
7-50 0.1878 8.8053 9.50 1.1587 5.2562
7.75 8.00 8.25 8.50 8.75 0"3555 0 . 5 8 7 0 0 . 7 8 3 6 0 . 9 2 7 5 1 . 0 1 4 9 8 . 3 2 7 7 7 . 8 6 1 9 7 . 3 9 8 2 6 . 9 3 7 4 6"4830 9.75 10.00 10.25 10.50 10-75 1 - 2 3 4 0 1 . 3 2 5 0 1 . 5 2 8 9 1 . 6 3 6 8 1.8150 4 . 9 1 9 7 4 - 6 1 8 0 4 . 3 6 1 3 4 . 1 1 0 5 3.8890
9-00 1.0632 6.0437
Homoprotocatechuic acid--cadmium B pL B pL
7.50 0.0278 8.7881 9.50 1"1257 5"2563
7.75 8.00 8-25 8.50 0 - 1 1 7 5 0 . 3 2 6 6 0.5181 0 . 7 0 9 7 8.3016 7.8323 7.3672 6.9117 9-75 10.00 10.25 10.50 1 . 1 9 8 9 1"2873 1-4357 1"5903 4 . 9 1 9 7 4 " 6 1 8 0 4 - 3 5 3 0 4"1106
8.75 0.8666 6.4663 10.75 1.7634 3.8890
9.00 0.9738 5.9281
9.25 1.0550 5.6293
Homoprotocatechuic acid-magnesium B pL B pL
8.75 9.00 0.0287 0 - 1 2 8 9 6.3638 5.9256 10-75 11.00 0"9307 1"0170 3 . 7 4 3 2 3.5159
9"25 9.50 0.2643 0.4224 5-5242 5.1577
9.75 10-00 10.25 10.50 0 . 5 5 2 3 0 . 6 3 0 5 0 . 6 5 5 9 0.7413 4 . 8 3 7 3 4 . 5 1 8 4 4-2321 3.9731
3:4 dihydroxyhydrocinnamic acid-copper B pL B
pL B pL
4"75 5.00 5"25 5"50 5"75 6.00 6"25 6-50 0-1387 0"2101 0"3437 0"5478 0 . 7 6 0 0 0 - 8 7 5 2 0"9723 1-0794 14"1939 13.6208 13"0852 12"5771 12"0867 11"5922 11"1000 10-6138 6"75 7.00 7"25 7"50 7"75 8.00 8"25 8"50 1"2029 1 - 3 6 2 4 1"5950 1"8217 1"9907 2 . 0 9 8 1 2 - 1 8 5 9 2.2090 10.1322 9-6583 8.75 9.00 2.2155 2 " 2 3 3 0 6.4409 6.0076
9 . 2 0 4 4 8-7543 8 . 2 9 7 8 7 . 8 3 2 6 7 . 3 6 9 6 6"8998 9"25 9"50 9"75 10.00 10-25 10"50 2"2837 2"3793 2 . 3 9 8 8 2 . 4 5 8 2 2'5830 2"6775 5 " 6 1 5 2 5 - 2 7 2 0 4"9424 4"6574 4 - 4 2 1 8 4"2000
3 : 4 dihydroxyhydrocinnamic acid-zinc B si pL B pL B pL
6-75 0-0989 9"9762 8"75 1.4121 6-2430 10"75 2"8431 3"9394
7"00 0"1747 9-4900 9.00 1.6813 5-8492
7"25 0"3158 9.0000 9'25 1"9366 5.4904
7-50 0"5116 8"5263 9"50 2-1694 5-1679
7-75 8.00 8"25 8"50 0 - 7 4 1 6 0"9227 1.0911 1.2247 8"0595 7 . 5 6 5 2 7"1288 6-6726 9"75 10.00 10"25 10"50 2.2530 2.3271 2"4934 2.6073 4 - 8 5 1 6 4 . 5 6 5 1 4 " 3 2 9 0 4.1032
1243
1244
V.T. AT/-IAVALE,L. H. P~BHU and D. G. VARTAK
TAnt~ 2 (cont.) 3:4 dihydroxyhydroeinnamie acid-nickel B ti pL B ri pL
7.50 7"75 8.00 8"25. 8.50 8.75 0.1133 0 . 2 7 7 7 0 . 4 8 6 8 0"7134 0 . 8 6 5 7 0 . 9 7 2 6 8.4804 8.0029 7.5356 7.0778 6.6214 6.1743 9.50 9.75 10-00 10.25 10-50 10-75 1"1066 1"1433 1"2760 1"5698 1"7382 1"9781 4 " 9 7 7 9 4"6476 4"3655 4 " 1 3 2 6 3"9047 3"7131
9.00 1.0447 5.7478
9.25 1.0668 5-3436
3:4 dihydroxyhydroeinnamic acid--cadmium B ~i pL B li pL
7.50 7.75 0.0488 0 . 1 7 2 3 8-4737 7.9918 9.50 1.1251 4.9854
8.00 0.3392 7.5193
8-25 0.5300 7.0565
8"50 0.7295 6"6063
8.75 9.00 0.9115 1.0668 6-1706 5.7554
9.25 1.1205 5.3562
3:4 dihydroxydrocirmamic acid-magnesium B pL B !i pL
8"50 0.0414 6.5228 10.50 1-1240 3"8110
8'75 0"0770 6"0660 10-75 1"1650 3-5793
9.00 0"1838 5.6406
9"25 9.50 0"3220 0 . 5 1 4 2 5"2497 4 . 9 0 1 9
9"75 10.00 10-25 0 . 6 6 8 8 0 . 8 2 6 2 1"0505 4 . 5 8 6 4 4"3036 4-0573
The value of pL or log 1/[L] is calculated using the following equation:
Pflia[H]l V ° + v" pL = log LT--~ -~ fi--~M × ~ V
(10)
where pflrr is the practical overall proton ligand stability constant. The stoichiometric metaMigand stability constants for 1 : 2 complexes in the range 0 < fi < 2 are determined by the method of half integral values as well as by the least squares method, substituting the experimental values of a and (L) in the equation n
(a-
1)[L]
=
(2 - - h) [L]
(t~- 1)
X'xK, - - X:~
(11)
The results obtained by least squares method are given in Table 5. DISCUSSION
Proton-ligandsystem It is seen from Table 1 that except for catechol, the formation curves extend over the range 1 < ax < 3 and their shape indicates the presence of the species HzA- and HA s-. This shows that the H + of the carboxylic group and one of the phenolic H + are easily dissociable in completely separable steps. Further, in all the cases, including eateehol, the lowest hA values were approximately equal to one in the pH ranges used for calculations. This indicates that either the second OH is little ionized or that the steps leading to the formation of HA ~ and A s-- in case of the ligands (II), (III), (IV) and H A - and A ~- in case of catechol are incomplete and not separated.
Solution stability constants of some metal complexes of derivatives of catechol
1245
Proton-ligand stability constants Table 3 gives the values obtained for the practical proton-figand stability constants represented by log PKH. It can be seen from the table that the log P K : value of the compound (II) is much lower than for the compounds (I), (III) and (IV). The anomalous value for compound (II) may be due to the electron withdrawing action of the nuclear substituted COOH group. The recent data of BrO,UDEand NACHOD~ ) is given in Table 4 for comparison, from which it can be seen that the electron withdrawing action of the COOH group TABLE 3
(1) (IF) aid (IV)
Method
from B; fix = 1
least square Eqn. (4) and (5)
(Eqan. 8)
Reagent
Log PK1R
Log PKIR
Log PK| 1I
Catechol Protocatechuic acid Homoprotocatechuic acid 3: 4-dihydroxyhydrocinnamic acid
,~12.6" 12.6 12.0 11"6
9.34 8.83 9-44 9-36
-4.40 4.25 4"56
I
* The value is approximate since t~A does not fall below one.
is very high due to resonance interaction, so that the 4-hydroxy group dissociates first in the case of the protocatechuic acid. In the case of compounds (III) and (IV) the effect of the --COOH group is decreased due to the linking through - - C H 2 and --CHs--CHI groups respectively. It may be observed that the introduction of alkyl groups into the ring of the phenol decreases its acid strength, ot~ the effect being most pronounced for ortho- and para- substituents since the electron releasing effect of the alkyl groups in the ortho- and para-positions to the hydroxyl group can be transmitted to the oxygen of the hydroxyl group, so that proton removal becomes more difficult. This effect cannot be transmitted when the substituents are in the rectaposition and for this reason, according to GOLUMn]Cet al. ~15~ the meta-substituted phenols are the strongest acids. They concluded that in the ethyl phenolic series, for which no exact ionization constants are available, the calculated l'Ka values of the meta-substituent groups are the lowest of the series. On further examination of Tables 3 and 4, it can be seen that the log I~K2Hvalues obtained in the present investigation follow the para-order and the log 1~K1~follow TABLE4.--PK II VALUESOF SOMEMONOSUBSlTrUTV.D pHENOLS Phenol
Meta-
Para-
--H ---CHs ---CIH5 --COOH
9"94 10"08 9-90 9'94
9"94 10'19 10.00 9"39
c1,~E. A. B~UDE and F. C. HACHOD, Determination of Organic Structure by Physical Method~, pp. 567-662, Academic Press, New York (1955). tl,~ G. W. W ~ . ~ I D , The Theory of Resonance, p. 185, J. Wiley, Hew York (1944). tls~ C. GOLUMmC,M. ORCH~ and S. W~I.Lsg, J. Am. chem. Soc. 71, 2624 (1949).
1246
V . T . A'rrlAVALE, L. H. Pr.Anrro and D. G. VARTAK
the meta order in case of compounds (II), (III) and (IV). Hence for these compounds the electron pull (meta effect) of the relevant --CH2COOH and --C~H4COOH groups act in order of the log PKxrr values. The log PKaa values i.e., the negative logarithm of the dissociation constant of the COOH group in the acid substituted catechols, fit the relation log PKarr = a -(b/d) as suggested by Rum'F and SADETre) where d is the distance along the chain, from the first aromatic carbon to the atom which attracts and fixes the hydrogen ion. The constants a and b are calculated to be 5"45 and 5" 1 respectively. The value of the constant b, which measures the acidifying effect of a given substituent is compatible with that found by the above authors, tm
Metal-ligand system The formation of complexes under the experimental conditions employed and the absence of hydrolysis, polynudear complexes and protonated complexes is dearly indicated by the following observations: (i) The departure of all metal ligand curves from the pure ligand curve at B values much lower than the B of hydrolysis of these metals. (ii) Appearance of distinctive colours well below the B of hydrolysis (Cu and Ni). (iii) The low concentration of the metal ions present (iv) Negligibly small contribution to ~ values due to metal ion hydrolysis at the B of hydrolysis of the metal. These ligands react in the following way" X-
,CoO
X--OH
X~
+ M X-
• X~
M + 2H + X-
where X represents b H , --COOH, --CH2COOH, or --(CH2)~. ---COOH. In the case of the copper titrations a volume of alkali equal to four equivalents of copper ions present in the solution, in excess of that required to neutralise the free perchloric acid, is required to reach pH 8 at which the llgand curve departs from the acid curve. This is also true in the case of the acid derivatives of catechol indicating that the COO-group does not take part in the chelate formationJ 1~) The metal ligand curves for zinc, nickel, cadmium and magnesium follow the same path as that of the ligands up to pH 6-5. For all the chelating agents the relative positions of the metal curves indicate the relative stabilities of the metal chelates, follow the order Zn < Cu :> Ni > Cd > Mg.
Metal-ligand stability constants The metal ligand stability constants are obtained from the ~, pL data. The values of pL at ~ = 0.5 and 1.5 correspond to the logs of the first and the second stability constants respectively. In the case of the cadmium and magnesium complexes a break ~t6}p. RUMPF and J. SADET,Bull. Soc. chim. Fr. 450 (1958). ~17)A. E. MARTELL and M. CALVIN, Chemistry of Metal Chelate Compounds, p. 156, Prentice-Hall, New York (1956).
Solution stability constants of some metal complexes of derivatives of catechol
1247
at ti = 1 indicates the formation of 1 : 1 chelates only. The stability constants were calculated graphically using the equation log
1--~ t~ = p L - - l o g K
1
for the points below the point of precipitation. All four ligands show a tendency to f o r m higher complexes with zinc, ligands I and I I with nickel and Ligand IV with copper. The portion of the curve which lies in the range 0 < a < 2 was treated by the least square method for obtaining the stability constants of the 1 : 2 complex. The determination of log Ka was not attempted by the least square method since its formation starts, invariably, at much higher p H and the difference between the p L TABLE 5.--MI~rAL-LIGAND STABILITYCONST.MqTS
I
Compound Catechol
II
Protocatechuic acid
III
IV
Homoprotocatechuic acid
3:4-dihydroxyhydrocinnamic acid
Metal Cu *+ Zn I+ Ni 2+ Cd s+ Mg*+ Cu *+ Zn ~+ Ni 2+ CAa+ Mg2+ Cu 2+
Log Kx 13"58 9"08 8"36 7"70 5"24 14"53 9"84 8"96 7"97 6"30 12"82
Log K~ 10'49 7'24 5"15 --10"76 7"55 5-38 4"75
Z n *+ Ni a+
8-80 8"04
4"35
Cd *+ Mg~+ Cu s+ Zn *+ Ni 2+ Cd 2+ Mg*+
7"35 4"94 12"74 8"64 7"45 7"14 4"90
4"28 -9"42 6"15 4"08 ---
-
-
9"50 6"37
values at ri = 1.5 and ~ ---- 2.5 indicate the formation of the third complex only after complete formation of the 1 : 2 complex. Further support to this argument is the wave in the formation curve near ~ = 2. Order o f stability constants or log K values
The log K values calculated for copper, zinc, nickel, cadmium and magnesium are given in Table 5. It can be seen that the order Zn < Cu > Ni Cd > Mg is in agreement with that of IRVING and WILLIAMS.(ls) The order Zn > Cd > M g is also found to agree with the published data for practically all the ligands studied so far. Log K for copper differs considerably from log K for the other metals and Zn is greater than Ni. The enormously higher log K value for the copper chelate may be due to a square planar structure. The order, Zn > Ni has been observed in a few cases, c19,2°~ where the relatively low value o f nickel is attributed to steric hindrance preventing the formation of a square planar structure. Again the hydrolysis constant ~xa~H. IRVINO and R. J. P. WILlOws, J. Chem. Soc. 3192 (1953). c],J W. D. JomcsroN and H. ~ R , Analytica chim Acta 11, 201 (1954). ~2o~W. D. JOHNSTONand H. F'gEIS~R,Analytica chim Acta 11, 301 (1954).
1248
V. T. ATHAV~m, L. H. PRAeHU and D. G. VARTAK
14.0! F3.C
II-C 12.C I0.0 Jl.(] --
9.0 100
-~80
J
Cd
70 I
~
6-C 5C
@
6£
Mg o ~ I
''''~'~ I
120
H4
4.0
rr
f
I~
'30
I14
llI I
I
IT
I
,20
log PK~r
130
log PK~ Fla. 1
150
log K~K2(Zn) 16.0
ologIK(Ni)vs. log K,(Izn) olog KhKz(Ni)vs.log K~Kz(Zn)/
IZ.O ] ~ -- 14.0
/ 9.0
z
z
-- 13.O
8.0
2.0
7"(
0
7"
f 9.0
I I0.0
log
K~ ( Z n )
l~o. 2
I1.0
Solution stability constants of some metal complexes of derivatives of cateehol
1249
of zinc is greater than that of nickel indicating a greater affinity of zinc for w O H ions. The catechol ligand resembles - - O H group carrying two negatively charged oxygen ions. As the bonds are purely ionic, the smaller zinc ion is expected to form a stronger complex than nickel as observed. The solubility of the catechol complexes in water supports this view.
Variation of log K values with the nature of the ligands A comparison of the metal ligand stability constants with the proton ligand stability constants is shown in Fig. 1. The plots are seen to be mostly straight lines showing that the general relation3 ~1~ Log K = apK + B is obeyed. A comparison of the metal ligand stability constants of different metals having the same co-ordination number, suggested by IRVING and ROSSOTTI(22) and FRIESERand co-workerstz3~is shown in Fig. 2. The values of stability constants follow a line with slope close to unity showing that the general relation Log fl~ri = 1"13 log fl2zn + C is obeyed in within ~0.3 log units. ~21~j. BJERRUM, Chem. Rev. 46, 381 (1950). (*~ H. I. IRVING and H. S. ROSSOTTI,Acta chem. stand. 10, 72 (1956). t~8~ H. FREISER,Q. FERNANDOand G. E. CHENI~Y,J. phys. Chem. 63, 250 (1959).