Molecular Structure, Optical and Magnetic Properties ...

Communications

DOI: 10.1002/slct.201800739 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57

z Organic & Supramolecular Chemistry

Molecular Structure, Optical and Magnetic Properties of Dianionic Free-Base Tetrapyrazinoporphyrazine Macrocycle Dmitri V. Konarev,*[a] Maxim A. Faraonov,[a] Alexey V. Kuzmin,[b] Alexey M. Fatalov,[a, c] Nikita G. Osipov,[a, c] Salavat S. Khasanov,[b] Alexander F. Shestakov,[a] and Rimma N. Lyubovskaya[a] Free-base tetrapyrazinoporphyrazine (H2TPyzPz) has been synthesized and the reduction of this macrocycle by an excess of sodium fluorenone ketyl in the presence of 2.2.2-cryptand yields {Na(2.2.2-cryptand)}2(H2TPyzPz)·2C6H4Cl2 (1) salt containing H2TPyzPz2- dianions. Blue shift of both Soret and Q-bands is observed in the spectrum of 1 and a new band is observed at 822 nm. Reduction is accompanied by alternation of the C-Nimine bonds in the macrocycle. The difference between short and ˚ is essentially higher than that for freelong bonds of 0.092 A base phthalocyanine radical anions (0.040 A˚). This difference can be explained by population of LUMO of H2TPyzP by two electrons in singlet ground state. According to the calculations distortion almost disappears in triplet excited state since in this state two additional electrons occupy orbitals which bonding and antibonding properties are interchanged. Salt 1 shows a weak narrow EPR signal indicating diamagnetism of H2TPyzPz2-. Calculations also show singlet ground state for the dianions whereas the excited triplet state is positioned by 1130 K higher than the ground state and is not populated at room temperature.

Macrocycles present a large family of organic compounds including porphyrins, phthalocyanines, corroles, tetrapyrazinoporphyrazines and many other macrocycles able to form complexes with metals thus expanding a range of compounds of such type.[1,2] Some of these macrocycles and their metal complexes are known to show biological activity.[3] Others are known as dyes and promising materials for optics, organic electronics and solar cells.[4,5] An unpaired electron generated at oxidation or reduction of these macrocycles can participate in realization of high conductivity or magnetic ordering of spins. For example, it has been shown that under partial oxidation of [a] Dr. D. V. Konarev, Dr. M. A. Faraonov, A. M. Fatalov, N. G. Osipov, Prof. Dr. A. F. Shestakov, Prof. Dr. R. N. Lyubovskaya Institute of Problems of Chemical Physics RAS Chernogolovka, Moscow region, 142432 Russia E-mail: [email protected] [b] A. V. Kuzmin, Dr. S. S. Khasanov Institute of Solid State Physics RAS Chernogolovka, Moscow region, 142432 Russia [c] A. M. Fatalov, N. G. Osipov Moscow State University Leninskie Gory, 119991 Moscow, Russia Supporting information for this article is available on the WWW under https://doi.org/10.1002/slct.201800739

ChemistrySelect 2018, 3, 4339 – 4343

Wiley VCH 1816 / 111207

phthalocyanine macrocycle in H2Pc, MIIPc or the [MIII(CN)2Pc]anions, conductivity is realized through the p-system of the macrocycle.[6] In spite of that oxidation chemistry of macrocycles has been developed for a long time,[6] reduced macrocyclic compounds have been obtained only recently and their crystal structures, optical and magnetic properties are not studied well.[7–11] Selection of appropriate reductants is required to obtain compounds with radical anions of macrocycles. Free-base phthalocyanine H2Pc has the first reduction potential of 0.66 vs SCE in DMF, whereas the second reduction potential of H2Pc is 1.06 V.[12] The use of relatively strong reductants such as sodium fluorenone ketyl, decamethylchromocene or decamethylcobaltocene allows one to generate H2Pc - radical anions in solution and precipitate compounds with the H2Pc - radical anions in a crystalline form.[7–9] However, the H2Pc2- dianions cannot be generated even with an excess of sodium fluorenone ketyl.7 Substituted porphyrins and corroles (Cor) have even more negative reduction potentials: 1.23 V (vs SCE in CH2Cl2) for H2TPP (TPP is tetraphenylporphyrin)13 and from 0.98 to 1.36 V (vs SCE in benzonitrile) for substituted H3TRCor,[14] where R can be different aryl substituents. Their reduced species can be obtained if very strong reductants such as decamethylchromocene or potassium graphite (KC8) are used.[10,11] It was shown for triphenylcorrole that the reduction is accompanied by the deprotonation of the macrocycle and the formation of the radical H2TPCor 2- dianions.[10] Electrochemistry of metal macrocycles shows that acceptor properties of the macrocycles can be enhanced by introducing acceptor substituents or adding nitrogen atoms into the macrocycle. For example, zinc tetrapyrazinoporphyrazine and tetra(pyrazinopyrazino)porphyrazines have essentially more positive first and second reduction potentials in comparison with zinc phthalocyanine.[15] It should be noted that more positive oxidation potentials of radical anions and dianions provide higher stabilization of their radical anion salts in the air. In spite of that substituted tetrapyrazinoporphyrazines are synthetically available and preparation of free-base and metal-containing tetrapyrazinoporphyrazines is well developed,[16] no crystalline anionic salts of tetrapyrazinoporphyrazine are known so far. In this work we synthesized free-base tetrapyrazinoporphyrazine (H2TPyzPz) and crystalline salt {Na(2.2.2-cryptand)}2 (H2TPyzPz)·2C6H4Cl2 (1) containing the H2TPyzPz2- dianions. The reduction effect on molecular structure, optical and magnetic properties of this macrocycle can be elucidated for the first *

*

*

4339

2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Montag, 23.04.2018 [S. 4339/4343]

1

Communications 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57

time. The study of macrocycle dianions in solid state is generally unavailable due to their very negative reduction potentials. 2,3-Dicyanopyrazine is the most suitable precursor for preparation of unsubstituted tetrapyrazinoporphyrazine (H2TPyzPz). Free-base macrocycle can be obtained either by fusion of corresponding dinitrile or by demetallation of MTPyzPz (M = magnesium(II), dilithium or disodium) complexes.[16] We synthesized free-base tetrapyrazinoporphyrazine (H2TPyzPz) by demetallation of MgIITPyzPz complex obtained by tetramerization of 2,3-dicyanopyrazine in the presence of magnesium butoxide in n-butanol in 82% yield (Scheme 1).

reduction. Weak bands at 1264 and 1294 cm 1 strongly increase in intensity. There are two hydrogen atoms bonded to pyrrole nitrogen atoms in pristine H2TPyzPz. The band corresponding to the N–H bond vibrations is observed in the spectrum of neutral H2TPyzPz at 3286 cm 1. This band is shifted by 54 cm 1 to larger wavenumbers. Thus, reduction of H2TPyzPz leads to the noticeable shortening of the N–H bonds in the center of the macrocycle. Earlier it was shown that the formation of the H2Pc - radical anions is also accompanied by shortening of the N–H bonds.[9–11] The spectra of pristine H2TPyzPz and salt 1 in KBr pellets in the UV-visible-NIR range are shown in Figure 1. Free-base H2Pc *

Scheme 1. Synthesis of salt 1.

Free-base phthalocyanine can be reduced by sodium fluorenone ketyl to form the H2Pc - radical anion salts.[7–9] According to redox potentials of metal macrogeterocycles,[15,16] tetrapyrazinoporphyrazine is stronger acceptor than phthalocyanine. For example, zinc complex of TPrzPz has a more positive reduction potential (-0.76 V vs SCE in THF) in comparison with ZnPc (-0.85 V vs SCE in THF).[15] H2TPyzPz was reduced by an excess of sodium fluorenone ketyl in the presence of two equivalents of 2.2.2-cryptand. That leads to dissolution of H2TPyzPz in o-dichlorobenzene in spite though pristine H2TPyzPz is insoluble in this solvent. Slow mixing of the obtained solution with n-hexane leads to the formation of crystalline dianionic salt {Na(2.2.2-cryptand)}2 (H2TPyzPz)·2C6H4Cl2 1 (Scheme 1). It should be noted that freebase phthalocyanine yields only known radical anion salt {Na(2.2.2-cryptand)}(H2Pc)·1.5C6H4Cl2 in these experimental conditions.[7] Reduction of tetrapyrazinoporphyrazine macrocycle to H2TPyzPz2- dianions affects optical properties. IR spectra of starting compounds and salt 1 measured in KBr pellet are shown in Fig. S1, and absorption bands are listed in Table S1. The IR spectrum of 1 is a superposition of absorption bands of pristine porphyrazine (Pz), 2.2.2-cryptand and solvent molecules (Fig. S1, Table S1), but some absorption bands of Pz are shifted essentially or change in intensity at the formation of the salt (Table S1). For example, strong bands at 800 and 1364 cm 1 in the spectrum of parent macrocycle are shifted to 789 and 1356 cm 1, respectively. The bands at 726, 752 and 1198 cm 1 are shifted by 5–8 cm 1 to smaller wavenumbers at the *


Wiley VCH 1816 / 111207

Figure 1. UV-visible-NIR spectra of pristine H2TPyzPz and salt 1 in KBr pellets prepared for 1 in anaerobic conditions. The band in the NIR range for H2TPyzPz2- is shown by dotted line.

shows the Soret band maximum at 336 nm and the split Qband with maxima at 636 and 702 nm.7 The Soret band has close position in the spectrum of pristine H2TPyzPz with maximum at 330 nm. Maximum of the Q-band of H2TPyzPz is blue shifted to 607 and 644 nm in comparison with the spectrum of H2Pc. The formation of 1 is accompanied by strong blue shift of both Soret and Q bands as compared with the spectrum of pristine H2TPyzPz, and these bands are observed at 296 nm and 542, 646 nm, respectively. Splitting of the Q band in the spectrum of H2TPyzPz2- is essentially more pronounced than that in the spectrum of parent macrocycle. The most significant difference in the spectra of negatively charged and neutral macrocycles is observed in the NIR range. No absorption in this range is observed in the spectrum of neutral H2TPyzPz, whereas the H2TPyzPz2- dianions show an intense absorption band with maximum at 822 nm. Since absorption band in the NIR range is manifested in the spectrum of {Na(2.2.2-cryptand)(H2Pc)·1.5C6H4Cl2 with H2Pc - at 1033 nm,[9] it can be concluded that the H2TPyzPz2- band in the NIR range is blue shifted in comparison with that of H2Pc -. The structure of salt 1 contains four independent halves of H2TPyzPz, four independent Na(2.2.2-cryptand) cations and *

*

4340


Montag, 23.04.2018 [S. 4340/4343]

1

Communications 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57

Figure 2. The length of the C N and C C bonds in the tetrapyrazinoporphyrazine macrocycle in 1, the length of bonds is averaged over four independent H2TPyzPz2- units. The shortened and elongated bonds are shown by red and blue color, respectively (a). Molecular structure of the H2TPyzPz2- dianion according to the DFT calculations (b).

solvent o-C6H4Cl2 molecules. Geometry of the H2TPyzPz2dianion was studied for the first time. It is seen from Figure 2a that hydrogen atoms are statistically disordered between two positions. As a result, all four nitrogen atoms in the center of the macrocycle are linked to the hydrogen atoms having the 0.5 occupancy. The tetrapyrazinoporphyrazine macrocycle has four types of C N bonds. Among them there are shorter bands with imine (im) nitrogen atoms and longer bands with pyrrole (pyr) nitrogen atoms in the central 24-atom macrocycle plane. There is no alteration of the C-Nim and C-Npyr bonds in neutral substituted metal tetrapyrazinoporphyrazines.[17,18] We calculated the length of these bonds in the macrocycle of 1 by averaging over four independent H2TPyzPz units. The most essential effect is observed for the C-Nim bands. It is seen that four C-Nim bonds are elongated (1.376(2) A˚, blue bonds in ˚ , red Figure 2a) and four other bonds are shortened (1.284(2) A bonds in Figure 2a). The average difference between short and long bonds is 0.092 A˚. It should be noted that short and long C–Nim bonds belong to two oppositely located pyrrolopyrazine units as shown in Figure 2a. The formation of H2TPyzPz2- is also accompanied by alternation of other bonds in the macrocycle. ˚ , red bonds in The difference between short (1.411(2) A ˚ Figure 2a) and long (1.470(2) A, blue bonds in Figure 2a) C C bonds in the pyrrole rings which are not involved in the pyrazine substituents is 0.059 A˚. The macrocycle also contains four short (1.370(2) A˚, red bonds in Figure 2a) and four long (1.398(2) A˚, blue bonds in Figure 2a) C–Npyr bonds but with essentially slighter difference between short and long bonds of 0.028 A˚. It is obvious that this alternation is not associated with the position of hydrogen atoms attached to the Npyr atoms ChemistrySelect 2018, 3, 4339 – 4343

Wiley VCH 1816 / 111207

since they are disordered between four positions in the macrocycle (Figure 2a). It is seen that changes in lengths of the C-Nim bonds are opposite within one pyrrolopyrazine unit to those of the C C bonds in the pyrrole ring and the C-Npyr bonds (Figure 2a). Earlier it was found that single reduction of phthalocyanine, corrole and porphyrin macrocycles is also accompanied by alternation of the C-Nim or C-Cmeso bonds (the difference between short and long bonds is 0.02-0.04 A˚).[7–11] However, alternation of bonds is essentially more pronounced for the dianionic macrocycles such as H2TPyzPz2-. DFT calculations using PBE functional were carried out for pristine H2TPyzPz (Figure S3a), and the H2TPyzPz2- dianions in singlet ground (Figures 2b and S3b) and triplet excited states (Figure S3c). For all species the optimized geometry has D2h symmetry. The pristine macrocycle has only weak alternation of bonds since the difference between short and long C-Nim bonds is only 0.018 A˚, and the corresponding difference for the C C and C-Npyr bonds in the pyrrole ring is also small (0.012 A˚ and 0.013 A˚, respectively). According to the calculations alternation of bonds increases by 2–3 times in the H2TPyzPz2- dianions since the difference between short and long bonds is 0.064, 0.026 and 0.025 A˚, respectively (Figure 2b). The lowest unoccupied (LUMO) orbital and LUMO + 1 orbital of H2TPyzPz of b2g and b1g symmetry have a slight difference in energy, 0.070 eV only. They show bonding and antibonding properties for two sets of the C-Nim bonds providing their shortening and elongation at population of b1g orbital and vice versa at population of b1g orbital. This is seen from the geometry of H2TPyzPz2- dianion in singlet state with electronic configuration (b1g)2 (Figure S3b). Therefore, population of both orbitals in the 4341


Montag, 23.04.2018 [S. 4341/4343]

1

Communications 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57

H2TPyzPz2- dianion in triplet state (b1g)1(b2g)1 3B3g leads to a small difference between short and long bonds of 0.014, 0.013, and 0.015 A˚ (Fig. S3c), respectively. This difference is very close to that for pristine H2TPyzPz (Figure S3a). We can assume that population of LUMO and LUMO + 1 orbitals has an additive effect on bond lengths. Disruption of aromaticity due to the formation of less stable 20 p-electron system of the dianion (in comparison with stable aromatic 18 p-electron system of pristine H2TPyzPz) and the Jahn-Teller distortions[19,20] can also contribute to the distortion of the H2TPyzPz macrocycle. Macrocycles have almost flat conformation in 1. Some pyrazine groups only slightly come out of 24-atom porphyrazine plane. Main building block of the complex is shown in Figure 3a. Salt 1 has structure with isolated H2TPyzPz2- dianions,

2.0037 and the linewidth (DH) of 0.58 mT, g3 = 2.0019 and DH = 0.61 mT, g2 = 1.9999 and DH = 0.63 mT) at 221 K (Figure S2b). The g-factors and DH of the lines remain almost unchanged down to 4.2 K (Figure S2a). However, integral intensity of the signal indicates that less than 2% of spins from the total amount of H2TPyzPz2- contributes to this signal and this signal can be attributed to paramagnetic impurities. Therefore, the H2TPyzPz2- dianions are diamagnetic and EPR silent. That is in a good agreement with the DFT calculations which show singlet ground state for the dianions, whereas excited triplet state is located by about 1125 K higher than the ground state and should have very low population at room temperature. Moreover, the calculations were made for gas phase, and in real crystals the effect of strong Coulomb field of surrounding cations can lead to additional distortions of the macrocycles, as it follows from the comparison of experimental and theoretical geometry. That can even more increase the singlet-triplet gap in the H2TPyzPz2- dianions in a crystal. For comparison, vertical S T splitting in H2TPyzPz2- is twice as higher than adiabatic one. Thus, free-base tetrapyrazinoporphyrazine (H2TPyzPz) was obtained. Reduction of H2TPyzPz and crystallization in the form of salt allow one to study molecular structure and properties of the H2TPyzPz2- dianions. The formation of dianions is accompanied by blue shift of both Soret and Q-bands and the appearance of a new band at 822 nm in the NIR range. Essential distortions of macrocycles 1 provide alternation of the C-Nim, CNpyr bonds and the C C bonds in the pyrrole rings with the appearance of four short and four long bonds. The H2TPyzPz2dianions are diamagnetic and EPR silent. Triplet state is not populated at room temperature due to rather wide singlettriplet energy gap. We suppose that the radical anions of freebase tetrapyrazinoporphyrazine can provide more closelypacked structures with essential magnetic interactions between the paramagnetic macrocycles. Work for preparation of these salts is now in progress.

Supporting information summary Experimental section and X-ray diffraction data, IR spectra of referring compounds and salt 1, EPR spectra of 1 at 221 and 4.2 K, results of theoretical calculations and calculated frequencies of IR vibrations.

Acknowledgements Figure 3. Crystal structure of 1: (a) main building block containing two Na(2.2.2-cryptand) cations, the H2TPyzPz2- dianion and solvent o-C6H4Cl2 molecule; (b) View on the crystal structure approximately along the b axis. Hydrogen atoms are not shown.

The work was supported by Russian Science Foundation (project No 17-73-10199).

Conflict of Interest The authors declare no conflict of interest. and there are no van der Waals (vdW) C, N···C, N contacts between H2TPyzPz2-. Large voids between macrocycles are filled by the Na(2.2.2-cryptand) cations (Figure 3b). The EPR spectrum of salt 1 contains a weak asymmetric signal which can be simulated well by three narrow components with g–factor value close to free-electron value (g1 = ChemistrySelect 2018, 3, 4339 – 4343

Wiley VCH 1816 / 111207

Keywords: crystal structure · DFT calculations · optical and magnetic properties · reduction · tetrapyrazinoporphyrazine

4342


Montag, 23.04.2018 [S. 4342/4343]

1

Communications 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57

[1] The Porphyrin Handbook, V. 1; V. 2, Chapters: 10–13; V. 3, Chapter 25 (Eds. K. M. Kadish, K. M. Smith; R. Guilard), Academic Press, London 2000. [2] L. R. Milgrom in The colors of life: an introduction to the chemistry of porphyrins and related compounds, Oxford University Press, Oxford 1997, pp. 249. [3] T. Mashiko, D. Dolphin in Porphyrins, hydroporphyrins, azaporphyrins, phthalocyanines, corroles, corrins and related macrocycles, in Comprehensive Coordination Chemistry, V. 2 (Eds. G. Wilkinson, R. Guilard, S. A. McCleverty, Pergamon Press, Oxford 1987, pp. 813-898. [4] C. G. Claessens, W. J. Blau, M. Cook, M. Hanack, R. J. M. Nolte, T. Torres, D. Wçhrle, Monat. Chem. 2001, 132, 3-11. [5] C. M. Drain, A. Varotto, I. Radivojevic, Chem. Rev. 2009, 109, 1630–1658. [6] T. Inabe, H. Tajima, Chem. Rev. 2004, 104, 5503-5534. [7] D. V. Konarev, L. V. Zorina, S. S. Khasanov, A. L. Litvinov, A. Otsuka, H. Yamochi, G. Saito, R. N. Lyubovskaya, Dalton Trans. 2013, 42, 6810–6816. [8] D. V. Konarev, A. V. Kuzmin, M. A. Faraonov, M. Ishikawa, Y. Nakano, S. S. Khasanov, A. Otsuka, H. Yamochi, G. Saito, R. N. Lyubovskaya, Chem. Eur. J. 2015, 21, 1014–1028. [9] D. V. Konarev, S. S. Khasanov, M. Ishikawa, A. Otsuka, H. Yamochi, G. Saito, R. N. Lyubovskaya, Dalton Trans. 2017, 46, 3492–3499. [10] D. V. Konarev, D. R. Karimov, S. S. Khasanov, A. Otsuka, H. Yamochi, H. Kitagawa, R. N. Lyubovskaya, Dalton. Trans. 2017, 46, 13994–14001. [11] D. V. Konarev, A. V. Kuzmin, S. S. Khasanov, A. Otsuka, H. Yamochi, H. Kitagawa, R. N. Lyubovskaya, J. Org. Chem. 2018, 83, 1861–1866.


Wiley VCH 1816 / 111207

[12] D. W. Clack, N. S. Hush, I. S. Woolsey, Inorg. Chim. Acta 1976, 19, 129– 132. [13] K. M. Kadish, G. Royal, E. van Caemelbecke, L. Gueletti in The Porphyrin Handbook, V. 9 (Eds. K. M. Kadish, K. M.Smith, R. Guilard), Academic Press, San Diego 2000, pp. 1–219. [14] J. Shen, J. Shao, Z. Ou, W. E. B. Koszarna, D. T. Gryko, K. M. Kadish, Inorg. Chem. 2006, 45, 2251–2265. [15] V. Novakova, P. Reimerova, J. Svec, D. Suchan, M. Miletin, H. Rhoda, V. N. Nemykin, P. Zimcik, Dalton Trans. 2015, 44, 13220–13233. [16] M. P. Donzello, C. Ercolani, V. Novakova, P. Zimcik, P. A. Stuzhin, Coord. Chem. Rev. 2016, 309, 107–179. [17] E. Viola, M. P. Donzello, S. Ciattini, G. Portalone, C. Ercolani, Eur. J. Inorg. Chem. 2009, 1600–1607. [18] K. A. Ustimenko, D. V. Konarev, S. S. Khasanov, R. N. Lyubovskaya, Macroheterocycles 2013, 6, 345–352. [19] J. Tbik, E. Tosatti, J. Phys. Chem. A 2007, 111, 12570–12576. [20] M. G. Cory, H. Hirose, M. C. Zerner, Inorg. Chem. 1995, 34, 2969–2979.

Submitted: March 13, 2018 Revised: April 13, 2018 Accepted: April 16, 2018

4343


Montag, 23.04.2018 [S. 4343/4343]

1