A post-cyclotetramerization strategy towards novel binuclear phthalocyanine dimers

Abstract

A new and efficient post-cyclotetramerization strategy was developed for the synthesis of binuclear phthalocyanine dimers sharing one common pyrazine moiety, for the first time, paving a new way towards the design and synthesis of novel conjugated oligomeric phthalocyanine derivatives with various application potentials.

---

Inspired by the successful isolation of the initial binuclear porphyrin dimers connected by alkenyl, alkynyl, furandiyl, and pyrazino[2,3-g]quinoxaline conjugated linkages,^1a,2a a large number of porphyrin oligomers with a conjugated electronic structure showing superior electronic and photonic properties over their mononuclear counterparts and special applications in molecular wires and photosynthetic antenna models have been fabricated from different monomeric porphyrin units.^1–3 Phthalocyanines are the most important synthetic analogues of porphyrins. However, different from the porphyrin chromophore, the bridging atoms of meso-positions are four nitrogen atoms rather than four carbon atoms and four benzene moieties are fused onto the pyrrole periphery in phthalocyanine, which lead to the very harder functionalization of phthalocyanine ligands in comparison with the porphyrin chromophore, rendering the construction of π-conjugated phthalocyanine oligomers only through the cyclotetramerization pathway with 1,2,4,5-tetracyanobenzene as the necessary precursor, Scheme S1 (ESI†).⁴ Nevertheless, random cyclotetramerisation of 1,2,4,5-tetracyanobenzene with another phthalonitrile induces the formation of a statistical mixture including mononuclear phthalocyanine as the major product and various oligomers as the minor product. This in turn leads to the problem of difficult isolation and purification of the aimed π-conjugated phthalocyanine oligomers. Actually, only homoleptic binuclear phthalocyanine dimers and few examples of homoleptic trinuclear phthalocyanine trimers have been isolated thus far.⁴ The synthesis of heteroleptic phthalocyanine dimers is even more challenging since the monomeric unsymmetric ABBB-type phthalocyanine analogue with two cyano groups has to be prepared in the first stage, which then reacts with the second phthalonitrile precursor, giving the target heterobinuclear compound in quite low yield via twice statistically-driven cyclocondensation.⁵

Obviously, developing a new synthesis strategy besides cyclotetramerization towards π-conjugated phthalocyanine oligomers becomes highly desired in this field. In the present paper, a new and efficient post-cyclotetramerization strategy was developed for the first time towards the synthesis and characterization of unprecedented binuclear phthalocyanine dimers sharing one common pyrazine moiety M₁Pc(OR₁)₆–M₂Pc(OR₁)₆ (M₁–M₂ = 2H–2H, Zn–2H, Zn–Zn, OR₁ = 2,6-dimethylphenoxy) (1–3), H₂Pc(SR₂)₆–H₂Pc(SR₂)₆ (4), and H₂Pc(OR₁)₆–H₂Pc(SR₂)₆ (5) (SR₂ = hexylthio), Scheme 1, in an unexpected good reaction yield as high as 72%, paving a new way towards novel conjugated oligomeric phthalocyanine derivatives with various application potentials.

Scheme 1 Molecular structure of binuclear phthalocyanine dimers 1–5.

For the initial purpose of synthesizing a mixed phthalocyanine–porphyrin-fused dimer,⁶ unsymmetrical ABBB-type diamino-Pc 6 ⁷ and dione porphyrin^6b monomers were prepared according to the published procedures. Surprisingly, a reaction between 6 and the dione porphyrin monomer in CH₂Cl₂ at room temperature in the presence of 0.1% trifluoroacetic acid (TFA) as a catalyst afforded the metal free homobinuclear phthalocyanine H₂Pc(OR₁)₆–H₂Pc(OR₁)₆ (1) in the yield of ca. 10% as the main product and only a trace amount of the targeted phthalocyanine-porphyrin-fused dimer, suggesting the effective self-condensation reaction of diamino-Pc 6 under the present reaction conditions. With diamino-Pc 6 as the sole mononuclear precursor, metal free binuclear phthalocyanine 1 was isolated in the yield of 12% under exactly the same reaction conditions, Scheme 2. More interestingly, the increase in the proportion of TFA from 0.1% led to a significant increase in the yield of 1 with the highest value of 72% being achieved in the CH₂Cl₂/TFA ratio of 80 : 20, Fig. 1. In order to further understand the role of TFA in the formation of 1, other acid species including acetic acid, propanoic acid, H₂SO₄, and HCl instead of TFA were employed to carry out the above-mentioned self-condensation reaction, however, all of which failed to afford the dimer 1. Nevertheless, only a trace amount of 1 could be detected by MALDI-TOF spectrometry for the self-condensation reaction of 6 in the absence of TFA. These results clearly reveal the key role of the TFA catalyst in the formation of phthalocyanine dimer 1.

Scheme 2 Synthesis of the metal-free homobinuclear phthalocyanine 1 in CH₂Cl₂ in the presence of TFA at room temperature.

Fig. 1 Change of the yield of 1 as a result of the proportion of TFA in the mixed solvent of CH₂Cl₂ and TFA at room temperature.

To reveal the generality of this newly developed synthetic strategy, the zinc complex of diamino-Pc 7 instead of the metal free species 6 was utilized as the precursor to carry out the self-condensation reaction, resulting in the isolation of the homobinuclear phthalocyanine zinc dimer ZnPc(OR₁)₆–ZnPc(OR₁)₆ (3) also in good yield, 56%. In addition, condensation between the metal free diamino-Pc 6 and its zinc complex 7 led to the isolation of the heterobinuclear phthalocyanine dimer ZnPc(OR₁)₆–H₂Pc(OR₁)₆ (2) in the yield of 41% in addition to the homobinuclear phthalocyanine dimers 1 and 3 in the yield of 10 and 8.0%, respectively, Scheme S2 (ESI†). Nevertheless, mixed condensation of metal free diamino-Pc precursor 6 with another metal free diamino-Pc precursor 8 bearing different peripheral substituents induced the isolation of the heterobinuclear phthalocyanine dimer H₂Pc(OR₁)₆–H₂Pc(SR₂)₆ (5), 7.4%, in addition to the homobinuclear phthalocyanine dimers 1 and H₂Pc(SR₂)₆–H₂Pc(SR₂)₆ (4) in the yield of 30 and 21%, Scheme 3. Obviously, it seems strange at the first glance for the significantly lower yield of the heterobinuclear phthalocyanine dimer 5 in comparison with that for the analogue 2. This, however, could be rationalized on the basis of the very much different reaction activity between the two different monomeric diamino-Pc precursors 6 and 8 bearing different peripheral substituents. In contrast, despite having the different central metal ions, the same peripheral substituents in the two monomeric diamino-Pc precursors 6 and 7 endow them with quite similar reaction activity. As a result, random condensation reaction between these two precursors led to the isolation of heterobinuclear phthalocyanine dimer 2 in obviously higher yield than those for the homoleptic counterparts 1 and 3. Conversely, both the diamino-Pc precursors 6 and 8 were inclined to carry out self-condensation rather than cross-condensation reaction, resulting in the isolation of the heterobinuclear dimer 5 in lower yield than their homobinuclear phthalocyanine dimers 1 and 4. Actually, to the best of our knowledge, only phthalocyanine dimers/trimers sharing a common benzene/naphthalene/anthracene moiety have been reported thus far.^4,5 As a consequence, the phthalocyanine dimeric compounds 1–5 sharing a common pyrazine moiety described in the present work represent the unprecedented type of binuclear phthalocyanine derivatives, affording new members in the family for further functional investigations.

Scheme 3 Reaction between diamino-Pcs 6 and 8 in CH₂Cl₂/TFA (80 : 20) at room temperature (OR₁ = 2,6-dimethylphenoxy, SR₂ = hexylthio).

The newly prepared binuclear phthalocyanine dimers 1–5 gave satisfactory elemental–analytical results. Their MALDI-TOF mass spectra clearly show intense signals for the corresponding molecular ion [M]⁺. The isotopic pattern closely resembled the simulated one as exemplified by the spectrum of 1 given in Fig. S1 (ESI†). These compounds (except for 4 due to its intensive aggregation nature in common organic solvents including CH₂Cl₂, CHCl₃, THF, DMF, toluene, and chlorobenzene) were further characterized with a range of spectroscopic methods including NMR, electronic absorption, and magnetic circular dichroism (MCD) spectroscopy. Fig. S2–S5 (ESI†) show the ¹H NMR spectra of 1–3 and 5, and all the signals could be unambiguously assigned with the result as tabulated in Table S1 (ESI†).

Fig. 2 and S6 (ESI†) show the electronic absorption and MCD spectra of 1–3 and 5 in either CHCl₃ or CHCl₃–pyridine (100 : 1) with the corresponding data summarized in Table S2 (ESI†). As can be seen, the dimeric compounds 1–3 with the same peripheral substituents display an intense near IR Q band at ca. 830 nm and an intense Soret band at ca. 360 nm. In comparison with the monomeric Pcs (which usually exhibit the Q band at ca. 680 nm),^4c the Q band of 1 is significantly red-shifted by ca. 150 nm due to the extended π-electron system of the conjugated binuclear phthalocyanine dimer, which can be explained by the exciton coupling type interaction of the two constituent Pc units.^4b,i,j However, compared to the homobinuclear phthalocyanine analogue linked by a benzene moiety,^4c,i the Q band of 1 is blue-shifted by ca. 20 nm owing to the stabilized highest occupied molecular orbital and a slight decrease of the practical π-system due to the two nitrogen atoms in the pyrazine moiety.⁸ This is also true for 2 and 3, Fig. S6 and Table S2 (ESI†). From the Faraday B MCD terms of opposite sign, the long- and short-axis polarized Q transitions of 1 lie at around 830 and 720 nm, while those of 5 lie at ca. 840 and 685 nm, Fig. 2, revealing the characteristic electronic transitions to the nondegenerate excited states.^4f This is also true for 2 and 3, Fig. S6 (ESI†).

Fig. 2 Electronic absorption and MCD spectra of 1 (top) and 5 (bottom) in CHCl₃.

Single crystals of ZnPc(OR₁)₆–ZnPc(OR₁)₆ (3) suitable for X-ray diffraction analysis were obtained by slow diffusion of methanol into the solution of this compound in CHCl₃ with a drop of pyridine. 3 crystallizes in the orthorhombic system with a P_bca space group containing four homobinuclear phthalocyaninato zinc molecules coordinated with two pyridine molecules per unit cell. Detailed crystal and structural data are listed in Table S3 (ESI†). Fig. 3 shows the molecular structure of 3, disclosing its binuclear phthalocyanine nature in an unambiguous manner. As can be seen, in the dimeric molecule each zinc ion locates in the center of the tetrapyrrole ring coordinated by four isoindole N atoms and one pyridine N atom, forming a five-coordinate pentagonal pyramid geometry around the zinc ion. This leads to two N₄ planes in the binuclear phthalocyaninato zinc molecule slightly domed with the average dihedral angle of the individual isoindole ring with respect to the corresponding N₄ mean plane being 4.33°. In addition, owing to the extended π-electron system, the binuclear phthalocyaninato zinc molecule adopts a slightly ruffled conformation with a mean deviation of 0.16 Å from planarity for the 74 atoms of the binuclear phthalocyanine core.

Fig. 3 Molecular structure of ZnPc(OR₁)₆–ZnPc(OR₁)₆ (3) in top-view and side-view with all the hydrogen atoms omitted for clarity.

In conclusion, a new post-cyclotetramerization strategy was developed for the first time towards the synthesis of an unprecedented type of π-conjugated binuclear phthalocyanine dimeric compound sharing one common pyrazine moiety, paving a new pathway towards the design and synthesis of novel π-conjugated oligomeric phthalocyanine derivatives with various application potentials.

Financial support from the National Key Basic Research Program of China (Grant No. 2013CB933402), the Natural Science Foundation of China (Grant No. 21290174 and 21401009), JSPS (Grant No. JP15H00910), the Beijing Municipal Commission of Education, and the University of Science and Technology Beijing is gratefully acknowledged.

Notes and references

1. (a) D. P. Arnold, Synlett, 1999, 296–305; (b) D. P. Arnold and L. J. Nitschinsk, Tetrahedron, 1992, 48, 8781–8792; (c) D. P. Arnold and G. A. Heath, J. Am. Chem. Soc., 1993, 115, 12197–12198; (d) D. P. Arnold, D. A. James, C. H. L. Kennard and G. Smith, J. Chem. Soc., Chem. Commun., 1994, 2131–2132; (e) D. P. Arnold and D. A. James, J. Org. Chem., 1997, 62, 3460–3469; (f) D. P. Arnold and D. A. James, J. Org. Chem., 1998, 63, 4856–4856.
2. (a) M. J. Crossley and P. L. Burn, J. Chem. Soc., Chem. Commun., 1987, 39–40; (b) M. J. Crossley and P. Thordarson, Angew. Chem., Int. Ed., 2002, 41, 1709–1712; (c) M. J. Crossley and L. A. Johnston, Chem. Commun., 2002, 1122–1123; (d) R. Paolesse, L. Jaquinod, F. D. Sala, D. J. Nurco, L. Prodi, M. Montalti, C. D. Natale, A. D'Amico, A. D. Carlo, P. Lugli and K. M. Smith, J. Am. Chem. Soc., 2000, 122, 11295–11302; (e) H. Uoyama, K. S. Kim, K. Kuroki, J.-Y. Shin, T. Nagata, T. Okujima, H. Yamada, N. Ono, D. Kim and H. Uno, Chem. – Eur. J., 2010, 16, 4063–4074.
3. (a) A. Tsuda and A. Osuka, Science, 2001, 293, 79–82; (b) T. Sakurai, K. Shi, H. Sato, K. Tashiro, A. Osuka, A. Saeki, S. Seki, S. Tagawa, S. Sasaki, H. Masunaga, K. Osaka, M. Takata and T. Aida, J. Am. Chem. Soc., 2008, 130, 13812–13813; (c) S. Ito, S. Hiroto, S. Lee, M. Son, I. Hisaki, T. Yoshida, D. Kim, N. Kobayashi and H. Shinokubo, J. Am. Chem. Soc., 2015, 137, 142–145; (d) T. Kamada, N. Aratani, T. Ikeda, N. Shibata, Y. Higuchi, A. Wakamiya, S. Yamaguchi, K. S. Kim, Z. S. Yoon, D. Kim and A. Osuka, J. Am. Chem. Soc., 2006, 128, 7670–7678; (e) Y. Rao, T. Kim, K. H. Park, F. Peng, L. Liu, Y. Liu, B. Wen, S. Liu, S. R. Kirk, L. Wu, B. Chen, M. Ma, M. Zhou, B. Yin, Y. Zhang, D. Kim and J. Song, Angew. Chem., Int. Ed., 2016, 55, 6438–6442; (f) J. Song, S. Y. Jang, S. Yamaguchi, J. Sankar, S. Hiroto, N. Aratani, J.-Y. Shin, S. Easwaramoorthi, K. S. Kim, D. Kim, H. Shinokubo and A. Osuka, Angew. Chem., Int. Ed., 2008, 47, 6004–6007.
4. (a) C. C. Leznoff, H. Lam, S. M. Marcuccio, W. A. Nevin, P. Janda, N. Kobayashi and A. B. P. Lever, J. Chem. Soc., Chem. Commun., 1987, 699–701; (b) N. Kobayashi, H. Lam, W. A. Nevin, P. Janda, C. C. Leznoff, T. Koyama, A. Monden and H. Shirai, J. Am. Chem. Soc., 1994, 116, 879–890; (c) S. Makarov, C. Litwinski, E. A. Ermilov, O. Suvorova, B. Rçder and D. Wöhrle, Chem. – Eur. J., 2006, 12, 1468–1474; (d) K. Wang, D. Qi, H. Wang, W. Cao, W. Li, T. Liu, C. Duan and J. Jiang, Chem. – Eur. J., 2013, 19, 11162–11166; (e) Y. Asano, J. Sato, T. Furuyama and N. Kobayashi, Chem. Commun., 2012, 48, 4365–4367; (f) C. Huang, K. Wang, J. Sun and J. Jiang, Dyes Pigm., 2014, 109, 163–168; (g) J. Yang and M. Mark, Tetrahedron Lett., 1993, 34, 5223–5226; (h) M. Handa, N. Kataoka, Y. Ito, T. Tonomura, I. Hiromitsu, T. Sugimori, K. Sogabe and K. Kasuga, Bull. Chem. Soc. Jpn., 2004, 77, 1647–1648; (i) N. Kobayashi, T. Fukuda and D. Lelievre, Inorg. Chem., 2000, 39, 3632–3637; (j) N. Kobayashi and H. Ogata, Eur. J. Inorg. Chem., 2004, 906–914; (k) T. Ikeue, N. Sawada, N. Matsumoto, A. Miyazaki, T. Sugimori, M. Koikawa, I. Hiromitsu, K. Yoshino, M. Mikuriya, Y. Kataoka and M. Handa, J. Porphyrins Phthalocyanines, 2014, 18, 708–714; (l) T. V. Dubinina, N. E. Borisova, M. V. Sedova, L. G. Tomilova, T. Furuyama and N. Kobayashi, Dyes Pigm., 2015, 117, 1–6.
5. (a) K. Ishii, N. Kobayashi, Y. Higashi, T. Osa, D. Lelièvre, J. Simon and S. Yamauchi, Chem. Commun., 1999, 969–970; (b) M. Calvete and M. Hanack, Eur. J. Org. Chem., 2003, 2080–2083; (c) N. Kobayashi, Y. Higashi and T. Osa, J. Chem. Soc., Chem. Commun., 1994, 1785–1786; (d) N. Kobayashi and H. Ogata, Eur. J. Inorg. Chem., 2004, 906–914; (e) N. Kobayashi, Y. Higashi and T. Osa, Chem. Lett., 1994, 1813–1816; (f) M. J. F. Calvete, D. Dini, S. R. Flom, M. Hanack, R. G. S. Pong and J. S. Shirk, Eur. J. Org. Chem., 2005, 3499–3509.
6. (a) Y. Zhang, J. Oh, K. Wang, C. Chen, W. Cao, K. H. Park, D. Kim and J. Jiang, Chem. – Eur. J., 2016, 22, 4492–4499; (b) K. Wang, D. Qi, J. Mack, H. Wang, W. Li, Y. Bian, N. Kobayashi and J. Jiang, Chin. J. Inorg. Chem., 2012, 28, 1779–1789.
7. (a) D. Wöhrle, M. Eskes, K. Shigehara and A. Yamada, Synthesis, 1993, 194–196; (b) H. Pan, C. Chen, K. Wang, W. Li and J. Jiang, Chem. – Eur. J., 2015, 21, 3168–3173; (c) T. Ikeue, T. Fukahori, T. Mitsumune, K. Ueda, K. Fujii, S. Kurahashi, M. Koikawa, T. Sugimori, I. Hiromitsu, K. Yoshino, M. Mikuriya and M. Handa, Inorg. Chim. Acta, 2014, 409, 433–440.
8. P. Zimcik, V. Novakova, K. Kopecky, M. Miletin, R. Z. U. Kobak, E. Svandrlikova, L. Váchová and K. Lang, Inorg. Chem., 2012, 51, 4215–4223.

---

Footnote

† Electronic supplementary information (ESI) available: Experimental section, NMR and IR results, electrochemical properties, spectroscopic and electrochemical data, and crystallographic data for 3 (CIF). CCDC 1476231. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6qi00408c

---