2(4)-Aminopyridines as Ligands in the Coordination and Extraction Chemistry of Platinum Metals

Abstract

The specific behavior of aromatic amines in the coordination and extraction processes of isolation and separation of platinum and other metals is discussed using the example of 2(4)-aminopyridines (2(4)-AP). As intrasphere ligands, 2(4)-AP have a high electron-donor capacity due to the pumping of an easily polarizable π-electron density. The chemistry of the extraction of platinum metals, iridium in particular, is considered: depending on the conditions, ion associates, coordination-solvated compounds or compounds containing an amine in the inner and outer coordination sphere of the metal are extracted. In the extraction of simple singly charged anions, there is a violation of the exchange-extraction series established for a large set of aliphatic amines. Soft anions (according to Pearson), for example, SCN- and I-, are best extracted, while for aliphatic amines such an anion is hard СlO4−. In the coordination compounds of platinum metals, 2(4)-AP acts as an electron donor, is coordinated by heterocyclic nitrogen with a redistribution of electron density not only to the accepting metal-complexing agent, but also further along the N-Me-X chain (X is an acido ligand in the composition of the complex), which leads to even greater covalence of the molecule as a whole.

1. Introduction

In recent years, interest has increased in the study of the extraction properties of high-molecular-weight aromatic amines, primarily because 2(4)-octylaminopyridines turned out to be good extractants for the isolation and separation of platinum metals [1, 2, 3]. Particularly interesting is the question of the specificity with respect to platinum metals of aromatic amines, as ligands, which differ from aliphatic amines in that the lone pair of electrons of the nitrogen atom largely acquires an π-donor character. Compared to aliphatic amines, aromatic amines demonstrate a number of new properties in coordination and extraction chemistry [2, 4]. All this determined the interest in this class of extractants, typical representatives of which are 2(4)-octylaminopyridines (2(4)-OAP).

Research carried out by the authors [5, 6, 7, 8, 9], allow us to get an idea of ​​the specifics of the behavior of 2(4)-aminopyridines in the coordination and extraction processes of isolation and separation of metals.

2. Specificity 2(4)-aminopyridinesas ligands

The specificity of 2(4)-aminopyridines as ligands is due to the nature of the nitrogen atom in aromatic amines, which can be judged from the results of studies of halides [8] and coordination compounds of 2-OAP with nickel, palladium, and platinum [9]. In metal complexes, 2-OAP, as an intrasphere ligand, has a high electron-donating capacity due to the pumping of an easily polarizable π-electron density. The mobility of the electron density in the 2-OAP molecule depending on the requirements of the acceptor is evidenced by the delocalization of the positive charge of the proton in the cation (outer sphere ligand), which is the higher, the greater the polarizability of the anion [8].

An idea of the mobility of the electron density in a 2(4)-OAP molecule can be obtained within the framework of the theory of limiting structures (the theory of resonance) [10], giving an account of a certain formalism of this theory. The conclusions obtained in the framework of the theory of resonance and the theory of perturbations of molecular orbitales (PMO), which have a deep quantum-chemical substantiation [11], are quite adequate.

Conventionally, the 2-OAP molecule (Figure 1) [2], as well as the 4-OAP molecule (Figures 2 and 3) [5], can be represented as an average between the amine and pyridonimine limiting structures.

Figure 1.

Limiting (resonant) structures of the neutral and the protonated 2-OAP molecule.

Figure 2.

Limiting (resonance) structures of 4-OAP molecules and the effective charges on nitrogen atoms calculated for them.

Figure 3.

Limiting (resonant) structures in protonated 4-OAP and effective charges on nitrogen atoms calculated for them.

The contribution of the pyridoniminine structure increases the electron density on the heterocyclic nitrogen and decreases the electron density on the amine nitrogen. This contribution can be estimated if the energy of the N1s level of heterocyclic and amine nitrogen atoms is known. Figure 4 shows, as an example, the experimental X-ray electron spectra of the N1s level of 4-OAP with band separation obtained on a Riber SIA-200 X-ray photoelectron spectrometer. It can be seen that the nitrogen atoms are not equivalent, the lower level refers to heterocyclic nitrogen.

Figure 4.

X-ray spectrum of 4-OAP. Peak area: 1–50.63, 2–49.37%.

The energy of the N1s level correlates with the effective charge on the nitrogen atom. Satisfactory correlation of these values for a large group of nitrogen-containing compounds of various structures was obtained in [12]. Effective charges on nitrogen atoms for limiting structures can be calculated using the concept of ionic nature (Figures 2 and 4) [10]. From the charge balance equations for nitrogen atoms, the contribution pyridoniminine structure in 2-OAP and 4-OAP molecules: 9 [2] and 48.6%, respectively.

Thus, in the first approximation, it can be assumed that 2(4)-OAP molecules represent a resonant structure with a contribution from the pyridoniminine component. This leads to an increased basicity of heterocyclic nitrogen compared to pyridine due to the pumping of electron density from the amino group in the ortho and para positions of the pyridine ring and partial delocalization of the charge in the cation. Since the “depth” of the resonance is higher in the case of 4-OAP, its basicity exceeds that of 2-OAP by two orders of magnitude.

All this points to the “soft” nature of 2(4)-AP as ligands. If we use Pearson’s classification [13], then free 2(4)-AP should be attributed to “soft” bases (inner sphere ligand), protonated − to “soft” acids (outer sphere ligand). Soft and intermediate bases include other aromatic amines, while aliphatic amines are “hard” bases.

Factors such as the energetic and spatial arrangement of the top donor orbital of the nitrogen atom are thought to be responsible for the “soft” or “hard” behavior of the amine. From the standpoint of the quantum theory of perturbed molecular orbitals (PMO), one can consider the energy of the metal–ligand interaction and the resulting extraction chemistry depending on the nature of the amine, metal, and extraction conditions [14].

The formation of a coordination-solvated compound or associate, where the metal is present in the composition of the acid complex, depends on the result of the competitive process of complexation of the amine and proton, on the one hand, and the metal, on the other. In the first approximation, the quantitative side of this process is expressed by the main equation of the PMO theory [14]. An ionic associate or a coordination-solvated compound is formed depending on the relative contribution of the Coulomb or covalent component to the metal-nitrogen interaction energy.

If the contribution of the covalent component is much greater than that of the Coulomb component, then the coordination of the amine by the metal in the presence of a proton is possible. The higher the energy of the donor orbital and the lower the energy of the acceptor orbital of the amine and metal, the greater the contribution of the covalent component. These energy parameters of the interacting orbitals within the PMO are characterized by the orbital electronegativity of the donor and acceptor according to Klopman [13]. In addition, the covalent component increases with the length of the amine donor orbital.

If we talk about the nature of the amine, then in the presence of a proton, only amines with a low orbital electronegativity of the lone electron pair (OELEP), which depends on the valence state of the nitrogen atom in the amine molecule, can be coordinated by the metal. The OELEP of nitrogen decreases with an increase in the ρ- and π-character of an unshared pair of electrons, that is, with a decrease in the energy of the donor orbital and with an increase in its population [13]. Consequently, the OELEP of nitrogen decreases as one goes from aliphatic amines to anilines and further to heteroaromatic amines. In the same series, the softness of amines and their ability to extract platinum metals in the form of coordination-solvated compounds increase.

Of no less interest is the behavior of protonated 2(4)-AP, which acts as an outer-sphere ligand with respect to acid complexes of platinum and other rare metals.

3. Chemistry of metal extraction

2(4)-OAP extract metals from acidic and slightly acidic solutions. The extraction of iridium and other platinum metals has been studied most fully [2, 6].

Iridium (III) is extracted with a 0.1 M solution of 2-OAP in chloroform from dilute hydrochloric acid solutions with distribution coefficients D = 100–200, however, a nonequilibrium minimum appears on the curve D = f (рН) at pH 2 (the contact time of the phases is 30 min). During extraction from 1 to 6 M HCl equilibrium is established slowly: the value of D increases by almost an order of magnitude with an increase in the duration of phase contact up to 50 hours. Iridium (IV) is reduced to iridium (III) during extraction. 4-OAP extracts iridium (IV) with high distribution coefficients from more acidic solutions [3].

When 2-OAP is introduced directly into the aqueous phase (0.1 M solution in acetone) and the solution is heated to boiling in the presence of a tin (II) chloride catalyst for 30–40 min followed by extraction of the resulting compounds with chloroform (“heterogeneous” extraction), iridium is extracted with an unusually high partition coefficient for this element. The maximum extraction is observed from 1 to 2 M HC1 and reaches 99.9% for a single extraction.

Other platinum metals, as well as Au, under conditions optimal for the extraction of iridium, are extracted much worse (Table 1), and gold is quantitatively, and silver and palladium are partially concentrated at the phase boundary. The distribution coefficient of non-ferrous metals and iron, from which iridium usually needs to be separated, under these conditions by 3–5 orders of magnitude lower than iridium. Of these elements, only copper in the form of Cu (II) passes into the organic phase in a noticeable amount. At a high concentration of SnCl₂ in the aqueous phase, the organic phase contains tin.

Metal	HCl, M		Metal	HCl, M
	one	3		one	3
Ir	800–1000	300–400	Fe	0.004	0.002
Rh	68	140	Ni	<0.001	<0.001
Pt	62	111	co	<0.004	<0.002
Pd	—	—	Zn	0.03	0.004
Ag	—	—	Sn (II)	0.03	0.014
Au	—	—	Sn (IV)	0.01	0.008
Cu	0.44	0.53

Table 1.

Distribution coefficient of some metals in the extraction of 2-OAP under iridium extraction conditions: Ir, Pt, Au, Ag − 1·10⁻⁴-1·10⁻⁵ M; Rh, Pd − 5·10⁻⁴; 0.05 M 2-OAP, 0.1 M SnCl₂, heating for 40 min at 100°C, phase contact for 15 min, organic phase – Chloroform.

Since alkylated 2(4)-AP − strong organic bases, they are able to extract halide and other metal acid complexes in the form of ion associates.

On the other hand, 2(4)-OAP can be considered as a potentially coordinating-active reagent due to the presence of heterocyclic aromatic nitrogen. In addition, during extraction, the formation of chelates due to the NH₂ group in α-position to the heterocyclic nitrogen. In the course of extraction, one or another mechanism is realized depending on the conditions [13].

Extraction of ion associates. In the form of ionic associates, platinum metals are extracted from HC1 solutions at a certain ratio of the concentration of components and the duration of phase contact [2]. Palladium (II) is extracted by 2-OAP predominantly in the form of an associate (OAPH⁺)₂[PdCl4] only from concentrated solutions of HC1 and when organic diluents are used solvents with a strong proton-donating ability. Platinum is extracted in the form of such a complex already from acidic solutions of HC1. Ir (III, IV) are predominantly extracted in the form of ionic associates of the composition (OAPH⁺)₂[IrCl₆] and (OAPH⁺)₃[IrCl₆] from 1 to 6 M HC1, especially with a short duration of phase contact.

According to this mechanism, Pd (II) is extracted from salicylate solutions with 4-heptylaminopyridine [15], and from oxalate solutions with 4-dodecylaminopyridine [16]. Ro (III) is extracted from citrate solutions with 2-dodecylaminopyridine [17], and from Ru (III) succinate solutions with 2-OAP [18]. From acetate solutions, 2-OAP extracts Ir (III) [19] from malonate – Au (III) [20]. 2-OAP and other metals are extracted in the form of ionic associates from chloride, malonate, succinate, salicylate, citrate media: To (IV) [21], Zr (IV) [22], V (V) [23], Mo (VI) [24], Cr (VI) [25], Bi (III) [26], Ga (III) [27], Tl (III) [28], Sm (III) [29], Hg (II) [30].

Anions of inorganic acids are also extracted as ionic associates [8].

Extraction of coordination-solvated compounds. Сu (II) is extracted with a solution of 2-OAP in chloroform from a neutral medium in the presence of at least 1 g-ion/l chloride ion, apparently in the form of a neutral coordination-solvated complex. In the case of Pd (II), a neutral diamine complex of the composition Pd(OAP)₂C1₂ is formed during extraction from solutions with a concentration of HC1 ≤ 3 M, Pt (II) − in the form of Pt(OAP)₂Cl₂ from weakly acid solutions of HC1 (pH >1.5); the phase contact duration is 30 min [2]. Most often, the organic phase contains compounds with 2(4)-OAP in the inner and outer coordination spheres of the metal (“mixed” extraction mechanism).

Mixed extraction mechanism. Iridium, under conditions optimal for its extraction, is extracted in the form of compounds containing 2-OAP in the inner and outer coordination spheres of the metal [7]. In addition to 2-OAP, the extractable compounds include SnCl₂, which is added to overcome the kinetic inertness of the initial complex iridium chlorides [6]. Complexation in the aqueous phase and subsequent extraction of the resulting compounds are described by the following Equations [7]:

$${{\left[{\text{IrСl}}_6\right]}^{3-}}_{\mathrm{w}}+\mathrm{m}{{\left[{\text{SnСl}}_3\right]}^{-}}_{\mathrm{w}}\to {{\left[\mathrm{Ir}{\left({\text{SnСl}}_3\right)}_{\mathrm{m}}{\mathrm{Сl}}_{6-\mathrm{m}}\right]}^{3-}}_{\mathrm{w}}+{\mathrm{mСl}}_{\mathrm{w}}$$E1

$$\begin{array}{l}{{\left[\mathbf{Ir}{\left({\mathbf{\text{SnСl}}}_{\mathbf{3}}\right)}_{\mathbf{m}}{\mathbf{Сl}}_{\left(\mathbf{6}\mathbf{-}\mathbf{m}\right)}\right]}^{\mathbf{3}\mathbf{-}}}_{\mathbf{w}}+{{\text{хОАПН}}^{+}}_{\mathrm{w}}\to {{\left[\mathbf{Ir}{\left({\mathbf{\text{SnСl}}}_{\mathbf{3}}\right)}_{\mathbf{m}\mathbf{-}\mathbf{x}}{\left(\mathbf{ОАП}\right)}_{\mathbf{х}}{\mathbf{Сl}}_{\left(\mathbf{6}\mathbf{-}\mathbf{m}\right)}\right]}^{\mathbf{x}\mathbf{-}\mathbf{3}}}_{\mathbf{w}}\\ +{\mathrm{хН}}_{\mathrm{w}}+\mathrm{хС}1\mathrm{в}+\mathrm{х}{{\left[{\mathbf{\text{SnСl}}}_{\mathbf{3}}\right]}^{\mathbf{-}}}_{\mathbf{w}}\end{array}$$E2

$$\begin{array}{l}{{\left[\mathbf{Ir}{\left({\mathbf{\text{SnСl}}}_{\mathbf{3}}\right)}_{\mathbf{m}\mathbf{-}\mathbf{x}}{\left(\mathbf{ОАП}\right)}_{\mathbf{х}}{\mathbf{Сl}}_{\left(\mathbf{6}\mathbf{-}\mathbf{m}\right)}\right]}^{\mathbf{x}\mathbf{-}\mathbf{3}}}_{\mathbf{w}}\mathbf{+}\left(\mathbf{x}\mathbf{-}\mathbf{3}\right)\mathrm{ОАП}·{\mathrm{НCl}}_{\mathrm{o}}\\ \to {\left({\text{ОАПН}}^{+}\right)}_{\mathrm{x}-3}{{\left[\mathbf{Ir}{\left({\mathbf{\text{SnСl}}}_{\mathbf{3}}\right)}_{\mathbf{m}\mathbf{-}\mathbf{x}}{\left(\mathbf{ОАП}\right)}_{\mathbf{х}}{\mathbf{Сl}}_{\left(\mathbf{6}\mathbf{-}\mathbf{m}\right)}\right]}^{\mathbf{x}\mathbf{-}\mathbf{3}}}_{\mathbf{o}}\mathbf{+}\left(\mathbf{3}\mathbf{-}\mathbf{x}\right){{\mathbf{Cl}}^{\mathbf{-}}}_{\mathbf{w}}\end{array}$$E3

Here x = (0–2), m = 1–6; component concentration interval: 1·10⁻⁵-1·10⁻³ g-at/l Ir; 0.05–0.2 M; SnСl₂ ≤ 0.1 M 2-OAP in acetone; 1–6 M HCl.

The ratio of ligands in the inner coordination sphere of iridium is determined by the concentration of the components in the specified range, as well as the temperature and duration of heating the solution before extraction. At a low concentration of 2-OAP, along with coordination-solvated compounds, anionic iridium chlorotin complexes are extracted that do not contain 2-OAP in the inner coordination sphere of the metal. The ratio between these two types of extractable compounds under these conditions can be estimated from the results of a physicochemical study of the extraction of iridium in the presence and absence of OAP in the aqueous phase upon heating [7].

The given chemistry of iridium extraction is confirmed by the study of extracts by high-voltage electrophoresis on paper and iridium compounds isolated from the extract using physicochemical and spectral methods of analysis [7]. These compounds are a dark brown pasty substance. The total content of the organic component (C, H, N), according to elemental analysis, is 41.95%, which indicates a high molecular weight of the anionic part of the associate; indirectly indicates the presence of tin. Direct evidence for the presence of tin in the complex is the Mossbauer spectrum of the compound on ¹¹⁹Sn nuclei, which is characteristic of the [SnCl₃] − ligand in the iridium coordination sphere (chemical shift 1.65 mm/s, quadrupole splitting 2.33 mm/s). Significant quadrupole splitting in the Mossbauer spectrum of the compound indicates the presence of 2-OAP in the inner coordination sphere of the metal.

This conclusion most convincingly follows from the data of PMR spectroscopy of substances before and after electrophoresis: in the spectrum of the substance after the separation of the cationic part, signals from the protons of the hetero ring and the octyl radical are clearly recorded; 2-OAP is indeed part of the anionic part of the associate and is coordinated by iridium. If we take into account the results of elemental analysis (32.38% C, 4.70% H, 4.87% N), then the probable composition of the compound is (OAPH⁺)[Ir(OAP)₂(SnCl₃)₃C1]⁻, possibly impurity of the complex (OAPH⁺)₂[Ir(OAP)(SnCl₃)₂Cl₃]²⁻.

Compounds containing OAP in the inner and outer coordination spheres of the metal can be extracted without preliminary heating of the metal solution with OAP in the aqueous phase, if its kinetic inertness is relatively low. In particular, the results of the study of platinum extracts using electron spectroscopy and thin layer chromatography [2] can be explained if the presence of the associate (OAPH⁺)[Pt(OAP)C1₃] – is assumed in the organic phase.

In principle, more than two molecules of 2(4)-AP can enter into the coordination sphere of a metal. In this case, the formation of complexes containing the metal in the cationic form is not excluded. Under the conditions of extraction of iridium, such compounds should precipitate at the phase boundary, which is observed in the extraction of palladium, as well as gold and silver, and only if 2-OAP is present during heating in the aqueous phase [2].

Another proof of the possibility of the formation of cationic complexes are the results of conductometric and spectrophotometric studies of complex formation 2(4)-AP with Pd (II), Pt (II) in aqueous solutions at concentrations of reagents 1·10⁻⁵ - 1·10⁻⁴ M, simulating the extraction conditions (Figures 5 and 6). The conductometric curves χ = f (CAm/CMe) show breaks at C_Am/CMe = 2 and 4.

Figure 5.

Electrical conductivity of 1∙10⁻⁴ M aqueous solution of K₂[PdCl₄] depending on the metal/amine molar ratio with the addition of: 1–2-AP; 2–4-AP.

Figure 6.

Electrical conductivity of 1∙10–4 M aqueous solution of K₂[PtCl₄] depending on the metal/amine molar ratio with the addition of: 1–2-AP; 2–4-AP.

Thus, the chemistry of 2(4)-AP metal extraction can be quite complex. Depending on the nature of the metal and extraction conditions, associates containing 2(4)-AP only in the cationic part, and the metal in the anionic part, associates with OAP in the inner and outer coordination spheres of the metal, neutral coordination-solvated compounds can pass into the organic phase; the formation of cationic complexes is also not excluded.

4. Interionic interactions in 2(4)-AP associates

Extraction of hydrochloric acid with a solution of 2(4)-OAP in chloroform according to the neutralization mechanism is described by the equation: К_ex = K_АmH+Cl^-(K_aК_D)⁻¹, where К_ex is the HCl extraction constant, K_АmH+Cl^- is the chloride distribution constant, K_a is the ionization constant of the protonated amine, К_D is the amine distribution constant.

2-OAP chloride with an intramolecular hydrogen bond passes into the organic phase and contains practically no water molecules, which apparently explains the low over stoichiometric extraction only from 12 M HCl [1]. On the contrary, for 4-OAP, a high over stoichiometric extraction is observed already from 6 M HCl, since in this case the chelate cycle based on the intramolecular hydrogen bond is not formed [5].

In the extraction of simple singly charged anions, there is a violation of the exchange-extraction series established for a large set of aliphatic amines. This conclusion follows from the data on the distribution constants of 2-aminopyridine [8] and 4-OAP salts between chloroform and water (Table 2), according to which, according to the extractability of 2(4)-OAP, singly charged anions are arranged in a row:

Anion, X^−	lgKAmH+X-		-ΔНh, kcal/mol	-∆Sh, kcal/mol·grad	-ΔGh, kcal/mol	R, A^o
	2AP	4-OAP
I^−	−2.32 ± 0.05	3.84	67	8.05	64	—
SCN^−	−2.74 ± 0.06	3.63	74	20*	68	1.95
Br^−	−3.24 ± 0.05	3.12	76	13.42	72	—
ClO4^−	−3.13 ± 0.10	3.53	54	13.30	50	2.36
NO3^−	−3.27 ± 0.04	2.99	74	16.90	69	1.89
Cl^−	−3.31 ± 0.07	2.48	84	17.10	79	—
F^−	−3.35 ± 0.09	1.63	116	30.70	107	—

Table 2.

Distribution constants of 2-AP and 4-OAP salts between chloroform and water (25 ± 2° С, μ ≈ 1) and thermodynamic characteristics of anion hydration in infinitely dilute solutions at 298°K (* − calculated by correlation dependence ∆Sh = f (R, А^o), where R− radius of ions in water.

$${\mathrm{F}}^{-}<{\mathrm{Сl}}^{-}<{{\mathrm{NO}}_3}^{-}<{\mathrm{Br}}^{-}<{{\mathrm{СlO}}_4}^{-}<{\mathrm{SCN}}^{-}<{\mathrm{I}}^{-}$$

Soft anions (according to Pearson) are best extracted: SCN⁻ and I⁻, while for aliphatic amines such an anion is hard СlO⁴⁻. In addition, it is well known that for aliphatic amines there is a linear correlation between the exchange constants of singly charged anions and the extraction constants of monobasic acids with the heat of hydration of the anion or the free energy of hydration. In the case of 2(4)-OAP, such a correlation is observed separately in the series Br⁻ < SCN⁻ < I⁻ and F⁻ < Сl⁻ < NO₃⁻ < СlO₄⁻, but not for the entire series as a whole (Figures 7 and 8).

Figure 7.

Dependence of distribution constants of 2-aminopyridine salts on free energy anion hydration.

Figure 8.

Dependence of distribution constants of 4-OAP salts on free energy anion hydration.

The study of 2-OAP halides by PMR, IR and X-ray electron spectroscopy showed [8] that they all have a structure similar to chloride (Figure 9):

Figure 9.

3D structure of 2-OAP chloride.

The specificity of the interionic interaction in 2(4)-OAP associates manifests itself in a decrease in the polarization of the n-electron cloud of the aromatic cation, depending on the nature of the anion, on the one hand, and the formation of a chelate cycle based on hydrogen bonds in the case of 2-OAP − with another. The data of IR spectroscopy indicate that the strength of the chelate ring in the case of 2-OAP decreases on passing to an anion with better extractability [8]. Consequently, the selectivity of the extraction of soft anions is due to the redistribution of the electron density in the aromatic cation, depending on the nature of the anion. Degree of indignation π-electron cloud of an aromatic cation can be quantified by the degree of charge delocalization (α) in the cation according to the data of X-ray electron or NMR spectroscopy [8]. For 2-OAP it is 90, 56, 54 and 48% in the series I⁻, Br⁻, Cl⁻, [GaCl₄]⁻, and for 4-OAP it is 90, 79, 65% in the series I⁻, Br⁻, Cl⁻, respectively.

The distribution constants of 2-aminopyridine halides increase with increasing α.

Other processes involving aromatic cations show similar phenomena. In particular, on the surface of micelles RPy⁺X⁻ (RPy⁺ − long chain alkyl pyridinium ion; X⁻ − anions of different nature) in an aqueous solution, an interaction with charge transfer was found for soft anions, which increases in the series Br⁻ < SO₃²⁻ < N₃⁻ < I⁻ < S₂O₃²⁻ according to an increase in the softness of the anion. Similarly, the interaction in the series Cl⁻ < Br⁻ < I⁻ is observed for ion pairs in chloroform and is absent in the case of hard СlO₄⁻. Charge transfer in ion pairs and on the surface of micelles is absent in the case of hard tetraalkyl- and tetraphenylammonium cations with soft Br⁻ and I⁻ [31]. The charge transfer is due to the mixing of the wave functions of nearby excited ones with the wave function of the ground state. Since in an ion pair the ground state is charged, and the excited − neutral, then it should be recognized that in the associates of a soft cation, for example, an OAPH⁺ or RPy⁺ cation with a soft anion, there is a covalent contribution (delocalization energy in terms of MO). This contribution is absent in associates with a hard cation, for example, the cation of an aliphatic amine. This is also confirmed by the results of the study of 2(4)-OAP associates.

5. Structure of coordination compounds

The extraction of platinum metals by 2(4)-OAP in the form of coordination-solvated compounds is highly selective with respect to non-ferrous metals, in particular with respect to nickel. Therefore, it is of interest to study the structure of coordination compounds of the isovalent and isoelectronic series of metals with the composition MeCl₂(OAP)₂, where Me = Ni, Pd, Pt, i.e. complexes that pass into the organic phase during the coordination extraction of Pd and Pt with a solution of 2-OAP in chloroform.

The complexes were synthesized according to specially developed procedures [9]. Their composition was confirmed by the results of elemental analysis and the properties of the complexes. The formal oxidation state of the central atom of the complexes is +2, which follows from X-ray electron spectroscopy data from the ionization energies of the Ni 2p_3/2, Pd 3d_5/2 and Pt 4f_7/2 levels (Table 3).

Compound	Ni 2p3/2, Rd 3d5/2, Rt 4f7/2	Cl 2p3/2	N 1s
2-OAP			399.2
2-OAP·HCl	—	199.7	399.9; 401.0
NiCl2(OAP)2	856.0	198.4	400
PdCl2(OAP)2	338.4	198.4	399.7
PtCl2(OAP)2	73.2	198.5	399.6

Table 3.

Binding energy (eV ± 0.1) of internal electrons of metal and ligands in Ni, Pd, Pt complexes with 2-OAP.

The chlorine ion is a part of the coordination sphere of the central atom, which is due to the relatively high energy of the 2p_3/2 level C1 in comparison with the corresponding energy values for ionically bound chlorine in the 2-OAP·HC1.

Formally, 2-OAP and 2-AP are ambidentate ligands with two donor nitrogen atoms, the heterocyclic nitrogen of the pyridine ring and the nitrogen of the amine group located in the α-position. The results of electron, IR, and NMR spectroscopy testify to the mode of coordination of 2-OAP by the metal [9]. 2-OAP and 2-AP are coordinated by Pd and Pt at the nitrogen of the heterocycle; The α-amino group does not interact directly with the metal. However, the spatial arrangement of the amino group, as well as the fact that coordinated chlorine has an excess negative charge, contribute to the formation of a chelate cycle due to the intramolecular H-bond, as in the case of associates (Figures 10 and 11A). The chelate cycle is absent in Pd and Pt complexes with 4-AP (Figure 11B).

Figure 10.

3D structure of the Pd complex with 2-OAP.

Figure 11.

3D structures of Pt(II) complexes: A – 2-AP, B – 4-AP.

The extraction of coordination-solvated complexes can be considered from the point of view of the formation of electron-donor-acceptor complexes by neutral halide complexes with the electron-donor OAP molecule. The results of X-ray electron spectroscopy indeed show that the binding energy of the N1s electrons of the nitrogen atom decreases upon passing from free 2-OAP to the complex for all the studied metals; 2-OAP is primarily an electron donor, and the energies of the N1s electrons of the aromatic and aliphatic nitrogen atoms are equalized during complexation. Based on the change in the energy of the N 1 s level of OAP during complex formation, the acceptor ability of Ni is significantly higher than the acceptor ability of Pd and Pt in the corresponding halides.

It is interesting to compare the electron ionization energy from the 2p_3/2 level of chlorine in the compounds MeCl₂(OAP)₂ and MeCl₂(NH₃)₂, where Me = Pd, Pt [32], in compounds in which the central atom in one case forms a bond with a heterocyclic nitrogen 2-OAP, and in another − with ammonia nitrogen (the most rigid aliphatic amine). In complexes with 2-OAP, these values are much smaller; the electron density initially localized on the donor nitrogen atom is not only and not so much directly redistributed to the accepting complexing metal, but also further along the N—Me—C1 chain, which leads to an even greater covalence of the molecule as a whole. It is noteworthy that, in this respect, the complexes of palladium with OAP are similar to the complexes with triphenylphosphine and diphenylthiourea [33] − other soft ligands.

6. Conclusion

The specific behavior of aromatic amines is considered.in coordination and extraction processes for the isolation and separation of platinum and other metals on the example of 2(4)-aminopyridines (2(4)-AP). Toas intrasphere ligands2(4)-APhave a high electron-donating capacity due to the pumping of an easily polarizable π-electron density. In a protonated amine, electron density mobility is accompanied by delocalization of the positive proton charge over the ligand molecule, depending on the requirements of the acceptor. The degree of delocalization is the higher, the greater the polarizability of the anion. Chemistry of extraction of platinum metals2(4)-AP, iridium in particular, can be quite complex. Depending on the nature of the metal and the extraction conditions, associates containing 2(4)-AP only in the cationic part, and the metal in the anionic part, associates with 2(4)-octylaminopyridine in the inner and outer coordination spheres of the metal, coordination neutral - solvated compounds; the formation of cationic complexes is also not excluded.

In the extraction of simple singly charged anions, the exchange-extraction series established for a large set of aliphatic amines is violated. Mild anions (according to Pearson), SCN- and I-, for example, are extracted best. For aliphatic amines, this anion is hard СlO4-. In coordination compounds of platinum metals, 2(4)-APact as an electron donor, coordinate on heterocyclic nitrogenwith the redistribution of the electron density not only to the accepting metal-complexing agent, but also further along the chain N—Me—X (X-acid ligand in the complex), which leads to an even greater covalence of the complex.