Ignacy Cukrowski

A polarographic method of speciation for labile metal-ligand systems based on mass-balance equations. A differential pulse polarographic study at fixed ligand to metal ratio and varied pH

Abstract A new equation for speciation of labile species by polarography is derived. It relates the observed shift in a peak potential and a decrease in a peak height with a change in the concentration of a free metal ion in a solution. The free metal ion concentration is controlled by the formation of complexes of the metal ion with ligands and is calculated from mass-balance equations written for the metal-ligand system. This allows an incorporation into the ligand-metal system any number of complexes, those which are known and those which are thought to be formed and for which formation constants are calculated. As a test of this method, complexes of Cd(II) with the ligand N , N , N ′, N ′-tetramethylethylenediamine are studied by differential pulse polarography (DPP) at fixed ligand-to-metal ratio and varied pH. All known hydroxide species of cadmium, including polynuclear species, were incorporated into the metal-ligand system. A good agreement was found between formation constants calculated in this work from DPP and those reported previously from potentiometry. In addition, a new complex, viz. CdL 2 (OH) + , was found and its formation constant, as log β, was estimated to be 8.94 ± 0.03.

The effect of chelate ring size on metal ion size-based selectivity in polyamine ligands containing pyridyl and saturated nitrogen donor groups

Abstract Ligands containing pyridyl groups and saturated nitrogens as donor groups, DPA-2 ( N , N ′-di-2-picolylethylenediamine), DPA-3 ( N , N ′-di-2-picolyl-1,3-propanediamine), and TPEN ( N , N , N ′, N ′-tetra-2-picolylethylenediamine) were studied with large metal ions in 0.1 M NaNO 3 at 25 °C. The pair of ligands DPA-2 and DPA-3 which form five and six-membered chelate rings, respectively, between two central nitrogen donor atoms was studied with Cd II , Pb II , and Bi III by glass electrode potentiometry, by differential pulse polarography (DPP), and by differential pulse voltammetry (DPV). The complexes of TPEN with Cd II , Pb II , and Bi III were studied by DPP, and with Hg II by metal electrode potentiometry. Glass electrode potentiometry could not be used for metal-TPEN systems because precipitation of complexes occurs during titration and complexes are formed much below pH 2. Protonation constants and complex formation constants for these ligands and metal ions are reported. It has been shown that change in complex stability, Δ log K 1 , on passing from DPA-2 to DPA-3, plotted as a function of a metal ion radius, supports the rule that the change of chelate ring size from five to six-membered stabilizes the complexes of small relative to large metal ions. The effect of chelate ring size on complex stability obtained for DPA-2 and DPA-3 ligands is compared with corresponding relations for the pairs of ligands 2,2,2-tet and 2,3,2-tet, and py-12-aneN 4 and py-14-aneN 4 . The possible steric effects of the bulky pyridyl groups of DPA-2 and DPA-3 were analyzed by Molecular Mechanics calculation in relation to steric aspects of complex formation. The results obtained here and elsewhere for TPEN are discussed in relation to the affinity of each metal ion for nitrogen donor ligands.

Experimental and calculated complex formation curves for a labile metal-ligand system a differential pulse polarographic study of the Pb(II)-(N,N,N′,N′-tetramethylethylenediamine-OH system at fixed ligand to metal ratio and varied pH

Experimental and calculated complex formation curves (ECFC and CCFC) for a labile metal-ligand system are defined and used for the speciation study by differential pulse polarography (DPP) at fixed total ligand (LT) to total metal (MT) concentration ratio and varied pH. The CCFC is used for the modeling of species formed in a solution and optimization of formation constants for these species. The CCFC can be generated for any metal-ligand model, including polynuclear metal species, for any LT:MT ratio, and for more than one ligand competing in the complex formation reaction. The metal-ligand model and formation constants are optimized by solving mass-balance equations written for the assumed model and by fitting the CCFC into the ECFC. The methodology of building up the metal-ligand model with initial values for the formation constants, used for further refinement, is described. Lead(II) complexes with the ligand N,N,N′,N′-tetramethylethylenediamine (TMen) are studied by DPP, at 298 K and μ = 0.5 moll−1, at large LT: MT ratios and varied pH. Three lead complexes with the ligand TMen, viz. PbL2+, PbL22+ and PbL2(OH)+, were found and their overall stability constants, as log β, were estimated to be 2.56 ± 0.03, 4.86 ± 0.03 and 9.93 ± 0.02 respectively (all known hydroxide species of lead, including polynuclear species, were incorporated into the PbTMenOH system). The results obtained for PbTMen seem to be reasonable when compared with those for Pb-ethylenediamine reported previously [I. Cukrowski et al., Polyhedron 14 (1995) 1661].

Metal-ion speciation in blood plasma incorporating the bisphosphonate, 1-hydroxy-4-aminopropilydenediphosphonate (APD), in therapeutic radiopharmaceuticals.

In the quest for more effective pain palliation radiopharmaceuticals for metastatic bone cancer, this paper relates results obtained with 166Ho complexed to the bone-seeking bisphosphonate, 1-hydroxy-4-aminopropililydenediphosphonate (APD). APD is itself a bone cancer pain palliation agent and this work was therefore driven by the idea that the energetic beta-particle emitter, 166Ho, coupled with APD could afford a highly effective radiopharmaceutical in the treatment of bone cancer. Complex-formation constants for important blood plasma metal-ions were measured by potentiometry or polarography at 37 degrees C and I = 150 mmol dm-3. The latter technique was used for systems where precipitates formed at ligand-to-metal ratios appropriate for potentiometry. For trivalent lanthanides, neither electrochemical technique could be used. Animal tests showed that the 166Ho-APD complex was taken up primarily by the liver due to precipitation or colloid formation.

Formation constant calculation for non-labile complexes based on a labile part of the metal-ligand system. A differential pulse polarographic study at fixed ligand to metal ratio and varied pH: application to polarographically inactive complexes

Abstract A new method of speciation by differential pulse polarography (DPP) is described. It is focused on non-labile complexes on the DPP time scale. By solving mass-balance equations - which are written for the labile part of a metal-ligand system - free metal, free ligand and non-labile complex concentrations are calculated. These are used for calculation of formation constants of non-labile complexes for each datum point. Each datum point is obtained during titration of a single sample solution by an addition of a standard sodium hydroxide solution. Where possible, the determinations were carried out in such a way that results from polarography and glass electrode potentiometry have been simultaneously obtained from a single experiment. The theoretical and analytical procedures as well as computer program (written to calculate formation constants directly from the polarographic data) were verified by experiment. Three metal ions, viz. PbII, CdII, and BiIII, were studied with the ligand N,N′,N″-tris(hydroxyethyl)triazacyclononane and formation constants, expressed as log K1, were found to be 12.22, 10.61 and 16.36 from polarography, respectively. Results obtained for lead and cadmium are compared with those obtained from potentiometry. The validity of the proposed method and the application of DPP for polarographically inactive complexes are demonstrated.

THE NEUTRAL OXYGEN DONOR IN COMPLEXES OF LEAD AND CADMIUM: A DIFFERENTIAL PULSE POLAROGRAPHIC, POTENTIOMETRIC AND CALORIMETRIC STUDY

Abstract Differential pulse polarography has been used to determine the formation constants of Pb II with ethylenediamine (en) at ionic strength 0.1 M and 5, 25 and 50°C. The equilibria between Pb 2+ and the ligands en and OH − are established rapidly on the polarographic time scale for all complexes studied, so that one peak is observed, that for the metal ion and its labile complexes. At 5°C the following complexes were found: M(HL), ML, ML 2 and ML 2 (OH), for which the stability constants (logβ) were found to be 12.97, 5.62, 9.70 and 13.45, respectively. At 25 and 50°C, the complexes ML, ML 2 , ML 2 (OH) and ML 2 (OH) 2 were found and their stability constants were found to be 5.05 and 4.43, 8.67 and 7.37, 12.6 and 11.75, 15.29 and 14.8, respectively. The Lingane equation was modified to allow calculation of formation constants for labile complexes of lead with the two ligands, en and OH − . The two stepwise protonation constants of en at an ionic strength of 0.1 M and temperatures of 5 and 50°C are reported and their values were found to be log K 1 = 10.44 and 8.98 and log K 2 = 7.39 and 6.53, respectively. From the polarographic data, thermodynamic quantities were derived for the overall complex formation reactions of Pb II with en and the values of ΔH ° (kJ mol −1 ) for ML, ML 2 , ML 2 (OH) and ML 2 (OH) 2 were found to be −44.7, −90.6, −65.7 and −39.1, respectively. The stepwise protonation constants of N , N , N ′, N ′-tetrakis(2-hydroxypropyl)ethylenediamine (THPED) at an ionic strength of 0.5 M and 25°C are reported and their values (log K ) wer found to be 8.85 and 4.38. At 25°C and an ionic strength of 0.5 M, the complexes ML, M(HL) and ML(OH) of Cd II and Pb II with THPED were found and their cumulative stability constants (log β) were found to be 7.98 and 7.51, 11.75 and 11.3, 11.14 and 13.00, respectively. The values of ΔH ° (kJ mol −1 ) for ML complexes of Cd II and Pb II with THPED were found to be −48.0. Values of ΔG °, ΔH ° and ΔS ° for the ligand displacement reaction in which en is displaced by THPED in ML complexes of Pb II and Cd II are also reported and discussed. It is suggested that the coordination numbers of Pb II in the complexes Pb(en) 2+ and Pb(THPED) 2+ in solution probably differ.

Complexation of BiIII by nitrogen donor ligands. A polarographic study

Abstract Differential pulse polarography was used to determine the formation constants of Bi 3+ at ionic strenght 0.5 and 25°C with the ligands DIEN (1,4,7-triazaheptane), TETREN (1,4,7,10,13-pentaazatridecane), DPA [bis(2-pyridyl)amine], AMPY [2-(amino-methyl)pyridine], and THPED [N,N,N′,N′-tetrakis(2-hydroxypropyl)-1,2-diaminoethane]. The equilibria between Bi 3+ and these ligands were mostly established slowly on the polarographic timescale, so that separate peaks occurred in the differential pulse polarograms for the free metal ion and complexes, simplifying calculation of the formation constants. Values obtained for Bi 3+ were: DIEN, log K 1 = 17.4; log K (ML + H = MLH) = 3.9; log K (ML + OH = MLOH) = 8.1; TETREN, log K 1 = 23.9; log K (ML + OH = MLOH) = 6.9; AMPY, log K 1 = 9.6; THPED, log K 1 = 12.0; log K ML + OH = MLOH) = 12.0; DPA, log K 1 = 9.0, log K 2 = 7.4. These results are shown to be reasonable in terms of the formation constant log K 1 (NH 3 ) = 5.0 for Bi 3+ predicted by a dual basicity equation developed previously. A good linear free energy relationship between log K 1 values for Bi III complexes and log K 1 for analogous complexes of the isoelectronic Pb II ion was found, which may be useful in predicting Bi III solution chemistry.

A potentiometric and differential pulse polarographic study of CdII with 1-hydroxyethylenediphosphonic acid

The complexation of the ligand 1-hydroxyethylenediphosphonic acid (HEDP) with Cd II was studied by differential pulse polarography (DPP) and glass electrode potentiometry (GEP) at fixed total ligand to total metal concentration ratios and varied pH values. Labile and non-labile metal complexes were analysed simultaneously by the use of a polarographic experimental complex formation curve (ECFC) and the calculated complex formation curve (CCFC). Complex formation curves were used for modelling of the metal‐ligand system and the refinement of stability constants. The ECFC, in which experimental parameters of only labile DPP peak are included (a shift in the peak potential and a variation in the peak height) appears to be a characteristic function for a full metal‐ligand model (labile and non-labile parts of a metal‐ligand system). The CCFC is a theoretical curve calculated for the assumed metal‐ligand model from mass-balance equations. The final model of metal species formed is the one, which is confirmed by these two experimental techniques, and for which stability constants of metal complexes obtained from DPP and GEP differ the least. Six cadmium complexes M(H4L), M(H2L), M2L, ML (all labile), ML2 (non-labile), and ML(OH) (labile) and their stability constants as log found from DPP 25.040.06, 19.560.02, 12.670.03, 7.260.02, 10.390.06 and 10.930.04, respectively, are reported. Results obtained from GEP are also reported and they differ from polarographic results only within the experimental errors typical for these techniques. # 1999 Elsevier Science B.V. All rights reserved.

Study of protonation of 1,4,7-tris(2-hydroxyethyl)-1,4,7-triazacyclononane, and its complexes with metal ions, by crystallography, polarography, potentiometry, molecular mechanics and NMR

The unusually high first protonation constant (pK1) of 1,4,7-tris(2-hydroxyethyl)-1,4,7-triazacyclononane (L) is analysed. A, 1H NMR study of the protonation of L in D2O confirms the high value of pK1 reported previously (C.M. Madeyski, J.P. Michael and R.D. Hancock, Inorg. Chem., 23 (1984) 1487). The crystal structure of L·HBr is reported: monoclinic, space group P21n, a=8.511(5), b = 13.301(5), c = 13.604(4) A, β = 90.73(4)°, Z= 4, R = 0.0548. The structure shows that the proton, attached to one nitrogen of the macrocyclic ring, is hydrogen bonded to the other two nitrogens of the ring, plus an oxygen of one pendant 2-hydroxyethyl donor group. Molecular mechanics (MM) calculations suggest that this hydrogen bonding is probably not a major cause of the high first protonation constant of L. Rather, the MM analysis suggests that (pK1) for [9]-aneN3 (1,4,7-triazacyclononane) itself is low because in the free ligand one of the NH hydrogens is strongly hydrogen bonded within the macrocyclic cavity. This means that when the ligand is protonated, the proton is added to the outside of [9]-aneN3 and is not held within the macrocyclic cavity. Derivatives of [9]-aneN3 which have three N-alkyl groups, such as L, or Me3-[9]-aneN3 (N,N′,N″-trimethyl-1,4,7-triazacyclononane) will all have high first protonation constants because they have no NH hydrogens to occupy the macrocyclic cavity in the free ligand, so when the proton is added it is added within the macrocyclic cavity. MM calculations on free and monoprotonated forms of [9]-aneN3 and Me3-[9]-aneN3 are used to support this suggestion. The synthesis and structure of the quaternary salt, N,N,N′,N″-tetrakis(2-hydroxyethyl)-[9]aneN3 bromide, is reported: monoclinic, space group, P21c, a = 20.036(2), b = 19.075(5), c = 9.470(3) (A), β = 101.52(2)°, Z = 8, R = 0.0725. The structure shows that the fact that the 2-hydroxyethyl groups of the salt cannot occupy the macrocyclic cavity means that the ring is forced to adopt an irregular structure unlike that adopted by all known [9]aneN3 rings. The structure of the Fe(III) complex of L, [Fe2(L)2H3](ClO4)3, is reported: orthorhombic, space group Pnnm, a = 13.566(3), b = 14.981(5), c = 9.517(4) A, Z = 4, R = 0.0855. The structure consists of hydrogen-bonded dimers of Fe(III)LH−1.5 complex units, which between them have lost three protons from the oxygens of the alcoholic oxygens of L. The pairs of partly deprotonated Fe(III)LH−1.5 units are held together face-to-face, by three short OHO hydrogen bonds between the faces formed by the three alcoholic oxygens of each complex cation. The hydrogen-bonding OO distances are short at 2.42 A. The coordination geometry around the Fe(III) is distorted to close to trigonal prismatic rather than octahedral. The three oxygens donors are twisted, relative to the three nitrogens, around the C3 axis of the complex, by 45°, relative to the positions expected for regular octahedral coordination. The role of dπ−pπ π-bonding in shortening FeO bonds, by up to 0.23 A, in the examples considered, is discussed. It is shown that the trans effect that the shortened FeO bonds exert on the FeN bonds leads to an inverse relationship between FeO and FeN bond length in complexes of Fe(III) with N3O3 donor sets. Near normal FeO and FeN bond lengths are found for ligands where N is sp2 hybridized and capable of π-bonding in competition with the oxygen donors. The formation constants of L with Cd(II), Bi(III), and Pb(II) are reported, determined by both polarography and glass electrode potentiometry. For these large metal ions, log K1 for L is higher than for [9]aneN3 the analogue without 2-hydroxyethyl donors. It had previously been shown for the small Cu(II) and Zn(II) ions that log K1 for L is not significantly higher than for [9]aneN3 (C.M. Madeyski, J.P. Michael and R.D. Hancock, Inorg. Chem., 23 (1984) 1487). This is as expected from the effect that neutral oxygen donors have on increasing log K1 for larger metal ions relative to smaller ions (R.D. Hancock, in A.P Williams, C. Floriani and A.E. Merbach (eds.), Perspectives in Coordination Chemistry, VCH, Weinheim, 1992, p. 129).

Evaluation of a Peak Potential of an Uncomplexed Metal Ion in Speciation Study of Labile Metal–Ligand Systems by Polarography. Stability Constant Determination of CdII and PbII with Picolinic Acid at a Fixed LT:MT Ratio and Varied pH

The ligand picolinic acid was studied with CdII and PbII by differential pulse polarography (DPP) at fixed total ligand (LT) to total metal (MT) concentration ratios, varied pH at 25 °C and an ionic strength of μ = 0.1 and 0.5 mol dm−3. The corrected peak potential of an uncomplexed metal ion Ep(MFree) is defined for the first time and used, instead of experimentally available value, to evaluate the shift in the peak potential. The polarographic experimental and calculated complex formation curves (ECFC and CCFC) were used for modeling of the metal–ligand systems and the refinement of stability constants. It has been shown that with the use of the corrected value of Ep(MFree) together with the ECFC and CCFC it is possible to reproduce values of stability constants obtained by glass electrode potentiometry. Three cadmium complexes CdL, CdL2 and CdL3 and their stability constants as log β′ found from DPP at μ = 0.1 mol dm−3 4.34 ± 0.04, 8.01 ± 0.04, 10.79 ± 0.03, and at μ = 0.5 mol dm−3 4.29 ± 0.03, 7.89 ± 0.03, 10.49 ± 0.02, respectively, are reported. Four lead complexes PbL, PbL2, PbL3 and PbL2(OH) and their stability constants as log β′ found from DPP at μ = 0.5 mol dm−3 4.49 ± 0.02, 7.58 ± 0.03, 9.59 ± 0.02 and 11.46 ± 0.03, respectively, are reported. All stability constants at μ = 0.5 mol dm−3 and two lead complexes PbL3 and PbL2(OH) are reported for the first time, whereas the formation of the complex CdL3 has been confirmed by DPP. Results obtained in this work are compared with the literature data.