2019 Volume 2 Issue 4

^a Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, ul. Vavilova 28, Moscow, 119991 Russia
^b Chemistry Department, Moscow State University, Leninskie Gory 1, str. 3, Moscow, 119991 Russia

Corresponding author:  D. N. Zarubin, e-mail: zaroubine@ineos.ac.ru 
Received 27 August 2019; accepted 27 September 2019

Abstract

Diamides of 1,10-phenanthroline-2,9-dicarboxylic acid (PDCA) and its derivatives comprise a large class of N,N',O,O'-tetradentate ligands that readily form complexes with transition and post-transition metal cations. Over recent years, they have assumed particular importance as promising extracting agents for processing of highly radioactive nuclear wastes [1–3]. Owing to the rigid prearranged structures, the presence of different coordination sites (hard amide oxygen and soft heterocyclic nitrogen atoms according to the Pearson's acid–base concept), and the high coordinating ability at relatively low basicity, these compounds can efficiently extract actinides and lanthanides from aqueous nitric acid solutions into organic solvents. Important advantages of PDCAs are high oxidative and hydrolytic stabilities. Furthermore, some of these compounds exhibited high selectivity factors for separation of An/Ln [2, 4–8] and Ln/Ln' [9] pairs. At the same time, the extensive use of PDCA in extraction technologies is seriously restricted by the tedious synthesis of compounds that have functional substituents in a phenanthroline core and, in some cases, also by their low stability under radiolysis conditions. For example, recent investigations revealed that N,N'-diethyl-N,Nʹ-di(4-n-hexyl-phenyl)-4,7-dichloro-1,10-phenanthroline-2,9-dicarboxamide [5, 9] (compound 1, Fig. 1), which showed high selectivity for separation of actinides and lanthanides, readily undergoes hydrolysis/alcoholysis in a water–octanol medium under action of γ-radiation [10]. An additional factor that restricts the use of PDCAs in extraction technologies is their low solubility in saturated hydrocarbons, which are widely used as the cheapest solvents with appropriate technological characteristics. Our studies [2, 3, 5, 9–11] and the results obtained by other research groups [1, 4, 6–8, 12] indicate that the introduction of substituents at different positions of PDCA and related N-donor heterocyclic ligands allows one to significantly vary their solubility and extracting properties. The introduction of lipophilic alkyl substituents С_nH_2n+1 and alkoxy groups ОС_nH_2n+1 (n ≥ 4) is often used to increase the solubility of organic compounds in nonpolar solvents. Recently, Borisova et al. [12] synthesized two PDCA derivatives with n-pentoxy groups at positions 4 and 7 of a phenanthroline core; however, no data were provided on their solubility in saturated hydrocarbons. In this report, we describe an alternative method for introduction of alkoxy groups at the mentioned positions of a phenanthroline core, which is independently developed by our research group and seems to be more facile than the previously reported approach [12] from the preparative point of view. Using the suggested method, we synthesized N,Nʹ-diethyl-N,Nʹ-di(4-n-hexylphenyl)-4,7-di-n-butoxy-1,10-phenanthroline-2,9-dicarboxamide (compound 2, Fig. 1) and its complex with lanthanum nitrate. A structure of the latter was elucidated by XRD.

Figure 1. 4,7-Disubstituted 1,10-phenanthroline-2,9-dicarboxamides.

The previously reported synthetic approach for the related diamide, namely, N,Nʹ-diethyl-N,Nʹ-diphenyl-4,7-di-n-pentoxy-1,10-phenanthroline-2,9-dicarboxamide was based on a nucleophilic substitution of the chlorine atoms in 4,7-dichloroneocuproine, which was accomplished under action of amyl alcohol in the presence of dimsyl sodium. Subsequent oxidation of 4,7-di-n-pentoxyneocuproine with selenium dioxide and nitric acid followed by the conventional conversion of resulting 4,7-di-n-pentoxyphenanthroline-2,9-dicarboxylic acid into the corresponding dicarboxamide allowed the authors to isolate the target product in 25% overall yield.

The approach suggested by our research group avoids a labor-consuming step of production of 4,7-dichloroneocuproine from known heterocyclic precursor 3 [13, 14] under action of a large excess of phosphoryl chloride. In our opinion, it seemed more rational to accomplish the direct О-alkylation of ambident dione 3 followed by the oxidation of resulting 4,7-di-n-alkoxyneocuproine 4 into dicarboxylic acid 6 via the intermediate formation of dicarboxaldehyde 5 (Scheme 1).

We found that the alkylation of both of the oxygen atoms in dione 3 with n-butyl iodide proceeds selectively in a polar aprotic medium and affords 4,7-di-n-butoxyneocuproine 4 in a good yield. Note that in this case potassium carbonate, utilized as a base, in the presence of a phase-transfer catalyst (TEBAC) provides the same efficiency as conventionally used, but less convenient sodium hydride [15–18]. The oxidation of 4 into dicarboxaldehyde 5 under action of selenium dioxide can be accomplished upon heating in a water–dioxane mixture. An attempt to convert 5 to 4,7-di-n-butoxy-1,10-phenanthroline-2,9-dicarboxylic acid 6 under action of nitric acid, which had been used in the synthesis of the related 1,10-phenanthroline-2,9-dicarboxylic acid [4, 19], led to the formation of a complex mixture of products. However, this oxidation can be carried out quantitatively upon treatment of dicarboxaldehyde 5 with hydrogen peroxide in ethanol at room temperature. Furthermore, compound 5 can be used without chromatographic purification from selenium-containing admixtures, since they do not interfere and, presumably, even catalyze the following oxidation. Interestingly, under these conditions, the heterocyclic nitrogen atoms remain intact, allowing one to avoid the potential formation of N-oxides. The conversion of dicarboxylic acid 6 to dicarboxamide 2 was carried out by the conventional method via intermediate generation of dichloride 7. Product 2 was isolated as a viscous oil and was characterized by IR and NMR spectra. As it was expected, it readily mixes with aliphatic, aromatic and chlorine-substituted hydrocarbons almost in any ratio.

The ¹Н NMR spectrum of diamide 2 exhibits pronounced temperature dependence. The signals in the spectrum of a solution of 2 in deuterotoluene registered at room temperature are very broad (Fig. 2а). A temperature rise to 80 °С leads to the narrowing of the signals, which acquire the expected multiplicities (Fig. 2b). This type of a temperature dependence was already observed in the spectra of PDCA [6, 9] and the related diamides of pyridine-2,6-dicarboxylic acid [11]. In particular, they are observed in the ¹H NMR spectrum of diamide 1 [9]. These effects stem from the formation of diamide associates in solutions, which decompose with a temperature rise. This association is supposed to stem from π-stacking of PDCA molecules, although there is no strong evidence. For cyclic dilactams derived from pyridine-2,6-dicarboxylic acid, π-stacking was detected in crystals [11].

Figure 2. Aromatic part of the ¹Н NMR spectrum of 0.05 M solution of 4,7-di-n-butoxy-N,N'-diethyl-N,N'-di(4-n-hexylphenyl)-1,10-phenanthroline-2,9-dicarboxamide 2 in toluene-d₈ at 19 °C (a) and 80 °C (b). Asterisks (*) indicate residual proton signals of the solvent.

Heating of equimolar amounts of 2 and La(NO₃)₃(H₂O)₆ in acetonitrile afforded complex 8 with 1:1 composition in a yield close to the quantitative one (Scheme 1). The resulting complex is moderately soluble in acetonitrile and is readily crystallized at room temperature as pale-yellow polyhedral crystals. The ¹Н and ¹³С{¹Н} NMR spectra recorded at room temperature show a characteristic set of narrow singlets and multiplets with well-resolved fine structures. This evidences the retention of an initial planar symmetry of the ligand after binding with the metal ion.

The IR spectrum of the complex contains absorption bands characteristic for the stretching vibrations of nitrate ions coordinated by the metal ions, which are manifested as two very intensive bands at 1467 and 1274 cm^–1. The absorption band associated with the C=O bond stretching vibrations is detected at 1615 cm^–1 and is shifted to the lower frequencies compared to the spectrum of the corresponding dicarboxamide, where it appears at 1650 cm^–1. This is typical for coordination by the metal ion. The molecular structure of complex 8 obtained by XRD is depicted in Fig. 3.