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].

structure 1

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.

Scheme
scheme 1

1.

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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).

Fig. 1.
figure 1

(a) 1H and (b) 13C NMR spectra of macrocycle 3 in CDCl3 at 22°C. The inserts show the shapes of the (c) CH2 signal at 50°C, (d) CH3 signal at –50°C, and (e) 3-H/4-H and 7-H/8-H signals at 50°C.

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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).

Fig. 2.
figure 2

(a) 1H and (b) 13C NMR spectra of macrocycle 4 in CDCl3 at 22°C.

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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.