Abstract
New macrocyclic 1,10-phenanthroline-2,9-dicarboxamide derivatives containing two phenanthroline cores were synthesized, and their structure was confirmed by NMR spectroscopy and other methods. The new macrocyclic phenanthrolines showed a complex stereodynamic behavior in solution, which was studied by 1H NMR spectroscopy at different temperatures.
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INTRODUCTION
1,10-Phenanthroline-2,9-dicarboxamides constitute a large class of N,N′,O,O′-tetradentate ligands capable of forming strong complexes with transition metal cations whose radius exceeds 1 Å. In recent years, 1,10-phenanthroline-2,9-dicarboxamide derivatives have been especially actively studied as extractants for the separation of lanthanides (Ln) and actinides (An) in modern technologies for reprocessing and disposal of spent nuclear fuel [1–6]. The obvious advantages of these ligands are their high hydrolytic and radiation resistance and the ability to form in strongly acidic medium with high selectivity complexes with Ln and An cations soluble in organic solvents. By varying the structure of the amide fragments and introducing substituents into the phenanthroline fragments, their extraction properties can be adjusted to the requirements of a particular task. Although the 1,10-phenanthroline core is one of the most widely used building blocks for the construction of macrocyclic molecules [7], there are only a few reports in the literature on macrocyclic 1,10-phenanthroline-2,9-dicarboxamides [8, 9]. We recently synthesized first representatives of macrocyclic phenanthrolinedicarboxamides 1 and 2 containing two phenanthroline fragments in the molecule [9].
Unlike open-chain 1,10-phenanthroline-2,9-dicarboxamides [1, 10–12], compounds 1 and 2 turned out to lose the ability to extract Ln3+ and An3+ cations from acidic media, though they formed with these cations 1:1 and 1:2 complexes in organic solvents. On the other hand, these compounds were able to extract Ln3+ and An3+ cations from neutral and alkaline aqueous solutions. It is important that macrocycles 1 and 2 are conformationally rigid, which imposes certain restrictions on the possibility of adjusting their coordination cavities to metal cations.
RESULTS AND DISCUSSION
In this work, we extended the series of such ligands by synthesizing two new macrocyclic phenanthrolinedicarboxamides containing two 1,10-phenanthroline fragments in the molecule. Macrocycles 3 and 4 were obtained in up to 48% yield by reacting the corresponding 1,10-phenantroline-2,9-dicarbonyl chlorides with N,N′-dimethylethylenediamine in the presence of triethylamine (Scheme 1). The synthetic procedure and analytical data for compounds 3 and 4 are given in Experimental.
Macrocyclic compounds 3 and 4 are white powders decomposing without melting above 400°C and sparingly soluble in chloroform, methylene chloride, DMSO, and DMF. Their structures were determined on the basis of 1H and 13C NMR, IR, and mass (HRMS and MALDI) spectra. In the IR spectra of 3 and 4, the carbonyl stretching band appeared at 1627 and 1634 cm–1, respectively.
Macrocycles 3 and 4 contain ethylenediamine units. Due to restricted rotation about the amide bonds, compounds 3 and 4 showed a complex stereodynamic behavior in solution. We previously observed a similar behavior of 1,10-phenanthroline-2,9-dicarboxamides with an open structure [10, 13, 14]. The results of quantum chemical calculations indicated that macrocycles 3 and 4 can have numerous conformations whose energies differ by about 1 kcal/mol. This was clearly seen when their stereodynamic behavior in solution was studied by NMR spectroscopy.
The 1H and 13C NMR spectra of compound 3 in CDCl3 (Figs. 1a, 1b) at 22°C suggested fast (on the NMR time scale) exchange between the conformers. Only the signals of the CH2 groups of the ethylenediamine fragments were broadened, but this broadening disappeared on heating (Fig. 1c). Characteristically, the chemical shifts of 3-H/4-H and 7-H/8-H at 22°C coincided with each other, but they become different at 50°C, and AB coupling was observed (Fig. 1e). Exchange broadening of the methyl proton signal appeared at low temperature (Fig. 1d).
The 1H and 13C NMR spectra of 4 were similar to the spectra of 3. At 22°C, exchange broadening was clearly observed not only for the CH2 signal but also for the 3-H/8-H signal (Fig. 2).
EXPERIMENTAL
All syntheses were carried out in an argon atmosphere. Methylene chloride was purified according to known procedure [15]. Triethylamine was kept over sodium hydroxide for 24 h, followed by distillation. The 1H and 13C NMR spectra were recorded on an Agilent 400-MR spectrometer at 400.1 and 100.6 MHz, respectively. Preliminarily, the 1H NMR spectra were recorded on a Magritek Spinsolve 60 spectrometer at 60 MHz. The IR spectra were recorded on a Thermo Nicolet IR 200 spectrometer with Fourier transform (resolution 4 cm–1, scan number 20). The high-resolution mass spectra (electrospray ionization) were recorded on Bruker Daltonics MicroTof and Orbitrap Elite instruments.
General procedure for the synthesis of macrocycles 3 and 4. A 2-L flask was charged with 500 mL of anhydrous methylene chloride, and solutions of N,N′-dimethylethylenediamine (2 mmol, 176 mg) and triethylamine (5 mmol, 0.7 mL) in 200 mL of anhydrous methylene chloride and of the corresponding 1,10-phenanthroline-2,9-dicarbonyl dichloride (2 mmol) in 200 mL of anhydrous methylene chloride were simultaneously added dropwise with stirring at room temperature under argon. When the addition was complete, the mixture was stirred at room temperature for 72 h and concentrated under reduced pressure (10 mm Hg) to 1/10 of the initial volume. The residue was washed with distilled water (3×100 mL), the organic phase was dried over anhydrous Na2SO4, and the solvent was removed under reduced pressure (10 mm Hg). The residue was purified by chromatography using methylene chloride–ethanol (3:1) as eluent.
3,6,10,13-Tetramethyl-3,6,10,13-tetraaza-1,8(2,9)-diphenanthrolinacyclotetradecaphane-2,7,9,14-tetraone (3). Yield 307 mg (48%), white powder decomposing above 400°C. IR spectrum, ν, cm–1: 3066, 2928 (C–H), 1627 (C=O), 1548, 1477, 1447 (C=C, C=N). 1H NMR spectrum (400 MHz, CDCl3), δ, ppm: 3.32 m (12H, CH3), 4.35–5.00 m (8H, CH2), 7.34 s (4H, 5-H, 6-H), 7.94 s (8H, 3-H, 4-H, 7-H, 8-H). 13C NMR spectrum (101 MHz, CDCl3), δC, ppm: 36.8 (CH3), 51.4 (CH2), 123.5 (C3, C8), 126.7 (C5, C6), 128.1 (C4a, C6a), 136.4 (C4, C7), 143.2 (C10a, C10b), 152.3 (C2, C9), 168.0 (C=O). Mass spectrum (HRMS, ESI-TOF): m/z 641.2605 [M + H]+. C36H33N8O4. Calculated: M + H 641.2619.
14,17,84,87-Tetrachloro-3,6,10,13-tetramethyl-3,6,10,13-tetraaza-1,8(2,9)-diphenanthrolina-cyclotetradecaphane-2,7,9,14-tetraone (4). Yield 280 mg (36%), white powder decomposing above 400°C. IR spectrum, ν, cm–1: 3089, 3061, 2931 (C–H), 1634, 1610 (C=O), 1532, 1463, 1447 (C=C, C=N). 1H NMR spectrum (400 MHz, CDCl3), δ, ppm: 3.35 s (12H, CH3), 4.15–5.02 m (8H, CH2), 7.90 s (4H, 3-H, 8-H), 8.07 s (4H, 5-H, 6-H). 13C NMR spectrum (101 MHz, CDCl3), δC, ppm: 37.4 (CH3), 51.2 (CH2), 123.4 (C3, C8), 124.6 (C4a, C6a), 126.2 (C5, C6), 143.6 (C10a, C10b), 152.4 (C2, C9), 166.4 (C=O). Mass spectrum (HRMS, ESI-TOF): m/z 777.1063 [M + H]+. C36H29Cl4N8O4. Calculated: M + H 777.1060.
CONCLUSIONS
Two new macrocyclic phenanthrolinedicarboxamides containing two 1,10-phenanthroline fragments linked through mobile N,N′-dimethylethylenediamine spacers were synthesized, and their stereodynamic behavior was studied by NMR spectroscopy. The new macrocyclic ligands were shown to be more conformationally labile than their piperazine analogs reported previously. The results of our study provide prospects for more efficient adjustment of the geometry of new macrocycles for binding and separating f-elements.
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ACKNOWLEDGMENTS
The authors acknowledge the support from the program for the development of Lomonosov Moscow State University. Preliminary NMR spectra were recorded using a Magritek Spinsolve 60 MHz spectrometer (Istina MGU ID 545023931).
Funding
This study was financially supported by the Russian Science Foundation (project no. 21-73-10067, “Macrocyclic Phenanthroline Ligands for the Separation of f-Elements in Nuclear Energy”).
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Translated from Zhurnal Organicheskoi Khimii, 2023, Vol. 59, No. 10, pp. 1357–1362 https://doi.org/10.31857/S0514749223100063.
Dedicated to Full Member of the Russian Academy of Sciences B.A. Trofimov on his 85th anniversary.
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Ustynyuk, Y.A., Petrov, V.S., Lemport, P.S. et al. New Macrocyclic Bis-1,10-phenanthroline-2,9-dicarboxamides. Synthesis and Stereodynamics in Solution. Russ J Org Chem 59, 1709–1713 (2023). https://doi.org/10.1134/S1070428023100056
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DOI: https://doi.org/10.1134/S1070428023100056