Department of Chemistry, Bu-Ali Sina University, Hamadan, Iran. Kevin P. Wainwright. School of Chemistry Physics and Earth Sciences, The Flinders University ...
Transition Metal Chemistry 28: 425–429, 2003. 2003 Kluwer Academic Publishers. Printed in the Netherlands.
425
Potentiometric determination of the formation constants for complexes of 3,2¢,2¢¢-triaminopropyldiethylamine with cobalt(II), nickel(II), copper(II), zinc(II) and cadmium(II) Hassan Keypour*, Masoud Dehdari and Sadegh Salehzadeh Department of Chemistry, Bu-Ali Sina University, Hamadan, Iran Kevin P. Wainwright School of Chemistry Physics and Earth Sciences, The Flinders University of SA, GPO Box 2100, Adelaide, SA 5001, Australia Received 14 June 2002; accepted 08 August 2002
Abstract The tripodal tetraamine ligand N{(CH2)3NH2}{(CH2)2NH2}2 (pee), has been investigated as an asymmetrical tetraamine chelating agent for CoII, NiII, CuII, ZnII and CdII. The protonation constants for this ligand and the formation constants for its complexes have been determined potentiometrically in 0.1 M KCl at 25 C. The successive protonation constants (log Kn) are: 10.22, 9.51, 8.78 and 1.60 (n ¼ 1–4). One complex with formula M(pee)2+ (M ¼ Co, Ni, Cu, Zn and Cd) is common to all five metal ions and the formation constant (log bML) is: 12.15, 14.17, 16.55, 13.35 or 9.74, respectively. In addition to the simple complexes, CoII, CuII and ZnII also give hydroxo complexes, and CuII and NiII give complexes with monoprotonated pee. [Zn(pee)](ClO4)2 and [Cd(pee)Cl](ClO4) complexes were isolated and are believed to have tetrahedral and trigonal-bipyramidal structures, respectively.
Introduction The most common branched quadridentate chelating agents are the tripodal tetraamine ligands, tren, tpt, ppe and ppb, shown in Figure 1, in which branching from the unique nitrogen atom results in non-planarity of the donor atom set. The chemistry of the ligands tren and tpt has been reviewed by Zipp et al. [1]. Potentiometric determination of the formation constants for their cobalt(II), nickel(II), copper(II) and zinc (II) complexes has also been reported [2–4]. The ligand ppe, together with the crystal structure of its cobalt(III) complex, [Co(ppe)(O2NO)](ClO4)2, and the reactivity of [Co(ppe)(OH2)2]3+ towards phosphate esters has been reported by Fanshawe and Blackman [5]. We have reported the synthesis and formation constants of its cobalt(II), nickel(II), copper(II), zinc(II) and cadmium(II) complexes [6, 7]. We have also reported a simple synthesis for ppb and the crystal structure of its fivecoordinate copper(II) complex, [Cu(ppbH)Cl2]ClO4, [8] together with the formation constants for its cobalt(II), nickel(II), copper(II) and zinc(II) complexes [9]. The purpose of the present work was to measure the protonation constants of the asymmetrical tripodal ligand, pee, shown in Figure 2, [6], and the formation constants of its cobalt(II), nickel(II), copper(II), zinc(II) * Author for correspondence
and cadmium(II) complexes at 25 C in 0.1 M KCl. These data form the missing link in the sequence of data for the tripodal ligands shown in Figure 1, and completion of the work now enables us to make a complete comparison of pKa and formation constants, measured in the same ionic medium, across the whole series of symmetrical and asymmetrical tripodal ligands (Tables 1 and 2). Protonation constants for pee and the formation constants for its copper(II) complex species at 25 C were recently reported, [10] but these results were obtained in 0.15 M NaCl and are thus not strictly comparable with the results for the remainder of the series. The molecular structure of [Cu(pee)(NH3)](PF6)2 was also reported, showing the complex to be trigonal bipyramidal with pee in the expected tripodal configuration and the coordinated amine trans to the tertiary nitrogen atom [11].
Experimental KCl (0.1 M ), used as supporting electrolyte, was obtained as reagent grade material in ampoules from BDH. Carbonate-free solutions of 0.1 M KOH were prepared by dilution of the ampoule content and standardized against potassium hydrogen phthalate which was used as the primary standard. Metal chloride stock solutions were standardized by EDTA titration
426
Fig. 1. Structure of the tripodal tetraamine ligands; tren, tpt, ppe and ppb.
Fig. 3. The proposed molecular structure of (a) [Cd(pee)Cl](ClO4)2 and (b) [Zn(pee)](ClO4)2 complexes.
Fig. 2. Structure of Pee Æ 4HCl, along with n.m.r numbering.
using xylenol orange [12]. I.r. spectra and n.m.r. spectra were measured on Shimadzu IR-435 and Bruker DPX 300 NMR spectrometers, respectively. Ligand synthesis The ligand pee was prepared as its tetrahydrochloride salt (Figure 2) by the literature method [6]. Yield: 5.7 g (43%) (Found: C, 24.8; H, 7.9; N, 16.5; C7H24N4Cl4 Æ 1.5H2O calcd.: C, 25.2; H, 8.1; N, 16.8%). FAB MS (positive FAB in nitrobenzyl alcohol): m/z 161 (peeH+, 100%). 1H-n.m.r. dH (D2O, p.p.m.) 2.31 (2 H, m, 2-H ), 3.25 (2 H, t, 1-H ) 3.7 (10 H, m, 3-H, 4-H and 5-H ). 13Cn.m.r. dC (D2O, p.p.m.) 24.11 (C-2), 36.36 (C-4), 38.98 (C-1), 52.29 (C-5), 53.46 (C-3). Complex synthesis All complexes were readily prepared by the following method. Pee Æ 4HCl Æ 1.5H2O (1 mmol) and NaOMe (ca. 6 mmol) were refluxed for 20 min in MeOH solution
with the metal salt (1 mmol, usually the perchlorate). The solvent was evaporated to a small bulk and cooled; crystalline products were usually obtained by slow diffusion of Et2O into the solution. [Cd(pee)Cl]ClO4 (Figure 3a): (Found: C, 19.7; H, 5.4; N, 13.1; C7H20N4Cl2O4Cd calcd.: C, 19.4; H, 4.5; N, 12.8%.) I.r. (Nujol mull, cm)1): 3346.9, 3294.8, 1589.5s, 1100vs. [Zn(pee)](ClO4)2 (Figure 3b): (Found: C, 20.8; H, 5.0; N, 13.4; C7H20N4Cl2O8Zn calcd.: C, 19.8; H, 4.7; N, 13.2%.) I.r. (Nujol mull, cm)1): 3310, 3270, 1587.6, 1100vs. 13C-n.m.r. dC (CD3CN, p.p.m.): 24.60, 36.60, 42.9, 52.1, 56.2. 1H-n.m.r. dH (CD3CN, p.p.m.) 1.75 (2H, m), 2.62 (4H, m), 2.97 (8H, m), 3.25 (4H, s, N-H), 3.33 (2H, s, N-H). Potentiometric measurements The potentiometric titrations were carried out under an inert atmosphere of H2O-saturated N2 in a 100 cm3 water-jacketed vessel maintained at 25.0 C. Data were obtained from 50.0 cm3 aliquots of solution containing 0.01 M HCl, 0.1 M KCl and ca. 0.001 M ligand titrated with 0.1 M KOH. A 10.0 cm3 capacity Mettler DV11 piston burette was used to deliver the titrant and the potential was measured with a 713 METROHM pH meter using a glass combination electrode. The electrode was calibrated daily by titration in the absence of the ligand. The pKa and stability constants were determined using the program BEST [13, 14]. The pKw for H2O under these conditions was found to be 13.78.
Table 1. Protonation constants for pee and related polyamines measured at 298.2 K I = 0.100 mol dm3 KCl trena
peeb
ppec
tptd
ppbe
Log K1
10.140 9.430
Log K3
8.410
Log K4
– –
10.378 10.4(1)f 9.682 9.87(7)f 8.952 8.85(5) 3.808 3.86(5)f
10.511 10.58(2)f 9.824 9.92(4)f 9.129 9.28(4)f 5.615 5.80(8)f
10.690
Log K2
10.22 10.14(9)f 9.52 9.72(6)f 8.78 8.40(4)f 1.61 2.1(3)f
10.120 9.490 6.720
2,2¢,2¢¢-triaminotriethylamine, Ref. [2]; b this work; c 3,3¢,2¢¢-triaminodipropylethylamine, Ref. [7]; d 3,3¢,3¢¢-triaminotripropylamine, Ref. [3]; e f 3,3¢,4¢¢-triaminodipropylbutylamine, Ref. [9]; Ref. [10], 3 I = 0.15 mol dm NaCl. a
Results and discussion The tripodal tetraamine ligand pee was prepared by the literature method [6] and was characterized by the 1Hn.m.r., 13C-n.m.r., mass spectrometry (FAB) and by elemental analysis. Zinc(II) and cadmium(II) complexes of this ligand were also prepared and were characterized. While the 13C-n.m.r. spectra of both the ligand and its complexes are similar and show, as we expected, five distinct methylene carbons, the 1H-n.m.r. spectra are slightly different. The difference is due to the nature of the coordination of the ligand to the metal [15]. It seems that, in the case of cadmium(II), the complex has one
427 Table 2. Formation constants at 298.2 K for metal complexes of the set of N4 tripodal ligandsa Overall reaction M2+ + L + H+ « MLH+
M2+ + L « ML2+
M2+ + L « ML-H+ + H+
Derived reactions M + LH+ « MLH+
pKa: ML2+ « ML-H+ + H+
Ligand
log 10b Co2+
Ni2+
Cu2+
Zn2+
Cd2+
trenb pee
c c
19.1 19.78
c c
c c
ppef
c
18.04
c
c
tptg
c
15.78
c
h
ppbi tren
14.94 12.8
16.09 14.8
17.24 14.65
h 11.7
pee
12.15
14.17
13.35
9.74
ppe
9.26
11.95
12.01
8.33
tpt
6.36
8.70
10.70
i
ppb tren pee
6.70 c 1.14
6.79 c c
9.37 c 1.61
i c c
ppej
–0.26
c
0.58
c
tpt
–4.43
c
22.08d 23.38 23.44e 23.83 23.90e 21.27 21.50e 22.20 18.8 18.45d 16.55 {17.10 [16.6]}e 15.98 16.1e 13.12 {13.21 [12.8]}e 12.98 9.36d 6.74 {7.7 [7.0]}e 6.51 6.3e 3.33 {3.3 [2.9]}e
c
i
tren pee
– –
9 9.56
– –
– –
ppe
–
7.66
–
–
tpt
–
5.27
–
–
ppb tren pee
4.25 – 11.01
5.40 – –
6.55 – 11.74
– – –
ppe
9.52
–
11.43
–
tpt
10.79
–
11.94d 13.16 13.3e 13.45 13.5e 10.76 10.92e 11.51 9.09d 9.81 {9.4 [9.6]}e 9.47 9.8e 9.79 {9.9 [9.9]}e
–
–
a I = 0.1 mol dm3 KCl. A value of 13.78 was obtained for the apparent pKw under these conditions; b Ref. [2] measured at 293.2 K; c reaction not observed; d Ref. [17] measured at 310.2 K, I = 0.15 mol dm3 KNO3; e Ref [10] measured at 298.2 K, I = 0.15 mol dm3 NaCl, values in square brackets are determined from visible spectra; fRef. [7]; g Ref. [3]; h not measured; i Ref. [9]; j Ref. [18].
chloride ion coordinated to the metal ion, and its geometry is probably trigonal bipyramidal (Figure 3a), but in the case of the smaller zinc(II) ion the complex has a tetrahedral structure (Figure 3b). Tetrahedral structures were found for Zn(ppe)2þ and Zn(tpt)2+ where the zinc(II) ion was completely encapsulated by the tripodal tetraamine ligand [7, 16]. Protonation constants Potentiometric equilibrium curves for pee and for the 1:1 M2+:pee system, and the distribution curve for the protonated pee species, as a function of pH, are shown in Figures 4 and 5. Analysis of the curve yields the successive protonation constants [defined in Equations (1) and (2)], which are shown in Table 1 along with values for the related tripodal polyamines.
Hn1 Lðn1Þþ þ Hþ ) * Hn Lnþ
ð1Þ
Kn ¼ ½Hn Lnþ =f½Hn1 Lðn1Þþ ½Hþ g
ð2Þ
Comparison with corresponding constants for ppb, tpt and tren shows that the value for each stage is less than in the previous stage. This is to be expected on the basis of both statistical factors and electrostatic repulsion between the hydrogen ion and the ligand molecule as it becomes progressively more positively charged. Thus, over the four stages of neutralization, the summation of the constants gives the order of basicity as: ppb > tpt > ppe > pee > tren. Within the pH range of the titration, protonation of the tertiary nitrogen is readily observed for ppb and tpt, barely so for ppe and pee, and not at all for tren. This is obviously due to the
428
Fig. 6. Formation constants for M2+–pee and related polyamines.
Fig. 4. Potentiometric equilibrium curves for pee (L) and for the 1:1 M2þ:pee system; 25 C, I ¼ 0.100 mol dm)3 (KCl).
constants (b) to be derived for the ML, MLH, and MLOH complexes. The data for ML complexes are presented in Figure 6, where they are set against those for complexes of the other tripodal ligands. Comparisons of log b for the formation of the ML complexes with ppb, tpt, ppe, pee and tren indicate that, in spite of the lower basicity of tren, relative to ppb, tpt, ppe and pee it forms most stable complexes. In fact the order of stability of complexes with the exception of cobalt(II) is: M(tren)2+ > M(pee)2+ > M(ppe)2+ > M(tpt)2+ > M(ppb)2+. The reason for this order must be that tren complexes have three five-membered chelate rings, pee complexes have two five- and one six-membered chelate rings, ppe complexes have one five- and two six-membered chelate rings, tpt complexes have three six-membered chelate rings and ppb complexes have two six- and one seven-membered chelate rings. It is well known that seven-membered chelate rings are less stable than sixmembered rings and both are less stable than fivemembered chelate rings [19].
Fig. 5. Distribution curves for the protonated pee (L) species as a function of pH. [pee] ¼ 1.01 mM, 25 C, I ¼ 0.100 mol dm)3 (KCl).
smaller electrostatic repulsion exerted by the three positively charged atoms as the number of trimethylene linkages to the tertiary nitrogen atom increases. The magnitude of the pKa values clearly indicate that the triprotonated form of pee dominates over a wide central pH range.
Acknowledgements We are grateful to the Department of Chemistry, Bu-Ali Sina University, and Hamadan province, for financial support.
Formation constants
References
The distribution curves for species originating from 1:1 molar ratios of M2þ:pee [M ¼ cobalt(II), nickel(II), copper(II), zinc(II), and cadmium(II)] can be produced using the data tabulated in Tables 1 and 2. From these it can be seen that complex formation begins in the pH 4–8 region. As usual, the order of binding strength is cobalt(II) < nickel(II) < copper(II) > zinc(II) > cadmium(II), with strong discrimination in favor of copper(II) (Table 2). Inspection of the curves shows that the ML species is common to all five metal ions. In addition to the simple complexes, cobalt(II), copper(II) and zinc(II) give hydroxo species and nickel(II) and copper(II) bind to the monoprotonated ligand as well. Analysis of the titration curves allowed formation
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429 11. A.M. Dittler-Klingemann and F.E. Hahn, Inorg. Chem., 35, 1996 (1996). 12. A.I. Vogel, Textbook of Quantitative Inorganic Analysis, 4th Edit., Longman, London, 1978. 13. R.J. Motekaitis and A.E. Martell, Can. J. Chem., 60, 2403 (1982). 14. A.E. Martell and R.J. Motekaitis, Determination and Use of Stability Constants, VCH, Berlin, 1992. 15. H. Keypour, S. Salehzadeh, R.G. Pritchard and R.V. Parish, Polyhedron, 19, 1633 (2000).
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TMCH 5398
