- Email: [email protected]
base additions
Tetrahedron Letters 54 (2013) 7049–7052
Contents lists available at ScienceDirect
Tetrahedron Letters journal homepage: www.elsevier.com/locate/tetlet
Monomeric and dimeric red/NIR-fluorescent dipyrrin–germanium complexes: facile monomer–dimer interconversion driven by acid/base additions Masaki Yamamura, Hiroyuki Takizawa, Naoya Sakamoto, Tatsuya Nabeshima ⇑ Graduate School of Pure and Applied Sciences, Tsukuba Research Center for Interdisciplinary Materials Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8571, Japan
a r t i c l e
i n f o
Article history: Received 14 September 2013 Revised 8 October 2013 Accepted 15 October 2013 Available online 23 October 2013 Keywords: Dipyrrin Fluorescence Dimerization Supramolecule Germanium
a b s t r a c t Highly coordinate germanium complexes of the N2O2-type tetradentate dipyrrin ligand have been synthesized. X-ray crystallographic analysis revealed the pentacoordinate structure of dimeric germanium complex 2 (=(Ge)2O) and the hexacoordinate structure of monomeric complex 3 (=Ge(OMe)(HOMe)). The dimer 2 was easily hydrolyzed in a solution to give monomer 4, though the corresponding siloxane (Si)2O did not react under the same conditions. The addition of DBU to a solution of 4 gave dimer 5, and neutralization by adding acetic acid regenerated the monomer 4, providing the facile and reversible interconversion between the monomer and dimer. The dipyrrin germanium complexes showed an intense absorption and fluorescence near the NIR region, which is more red-shifted than the silicon complexes. Ó 2013 Elsevier Ltd. All rights reserved.
Highly coordinate heavier group 14 compounds (Si, Ge, and Sn) have been extensively studied because they are important intermediates in useful synthetic reactions1 due to their easier bond dissociation than the tetracoordinate ones.2 We reported the pentacoordinate silicon complexes of the N2O2-type tetradentate dipyrrin ligands, and their unique optical properties and reactivities;3 the dipyrrin silicon complexes showed a strong luminescence as seen in the dipyrrin–boron complexes, the BODIPYs,4 that are often used as a luminescent dye. One of the most important features of our complexes is their strong luminescence in the near-infrared (NIR) region because NIR luminescent dyes are attractive for biological applications due to the low emission background and high transparency of biological tissues and living cells in the NIR region. Furthermore, interconversion between the monomer, silanol SiOH, and the dimer, disiloxane (Si)2O, reversibly occurred by hydrolysis and condensation, which is characteristic of the highly coordinate siloxane (Scheme 1).5 The reversible bond dissociation and formation led to the interconversion between the monomer and dimer of the dipyrrin–silicon complexes associated with changes in the NIR luminescent properties. However, these reactions occurred under reflux conditions each in a different solvent. Consequently, a time-consuming process involving evaporation of the solvent and redissolution was neces-
⇑ Corresponding author. Tel./fax: +81 29 853 4507. E-mail address: [email protected] (T. Nabeshima). 0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.tetlet.2013.10.073
Scheme 1.
sary for the interconversion. An even more reactive dipyrrin complex is desired for a more facile interconversion system driven only by an additive, heat, or other external stimuli. We now report the synthesis of a more reactive germanium analogue of the silicon complexes, germoxane, and the interconversion and regulation of the NIR luminescence based on reversible bond dissociation of the germoxane. The dipyrrin–germanium complex 2 (=(Ge)2O) was synthesized in a way similar to that of (Si)2O. Reaction of the dipyrrin ligand 16 with tetrachlorogermane in the presence of diisopropylethylamine followed by quenching with methanol gave the dipyrrin–germanium complex 2 in 92% yield (Scheme 2).7
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M. Yamamura et al. / Tetrahedron Letters 54 (2013) 7049–7052
Scheme 2.
Figure 2. Molecular structure of 3 (50% probability). Hydrogen atoms and a solvent molecule were omitted for clarity.
log(m 2/s)
Ge–O–Ge but via a pseudorotation mechanism, as shown in the siloxane (Si)2O. Upon the addition of water to the C6D6 solution of 2, the NMR signals of 2 disappeared and the signals assigned to a new dipyrrin complex appeared. In the diffusion-ordered NMR spectroscopic (DOSY) experiment of a mixture of 2 and the hydrolyzed product, the diffusion peaks of two species were clearly separated (Fig. 3). The diffusion constants of 2 and the other were determined to be 6.06 1010 m2/s and 7.42 1010 m2/s, respectively. These values are quite similar to those of siloxane (Si)2O (5.99 1010 m2/ s) and silanol SiOH (7.30 1010 m2/s), respectively. Thus the signals with the larger diffusion constant were assigned to monomeric germanium complex 4 (=GeLn) formed by the hydrolysis of 2 (Scheme 3), although the coordination number and substituents of the germanium atom, n and L, are unclear in solution.10 The
– 9.2
2
– 9.1
In the crystal structure, 2 has the germoxane structure having two germanium atoms bridged by an oxygen atom (Fig. 1). The Ge atoms have a slightly distorted trigonal-bypramidal (TBP) geometry, which is typical of the pentacoordinate germanium complexes. The dipyrrin-ligand part has an asymmetrical structure in which the N2 and O1 atoms are located at the apical positions on Ge1, and the other three atoms (N1, O2, and O3) are located at the equatorial positions. The sum of the three equatorial-to-equatorial angles (O(2)–Ge(1)–O(3), O(2)–Ge(1)–N(1), and O(3)–Ge(1)–N(1)) are almost 360° (359.99°), indicative of the ideal TBP structure. The slight distortions are indicative of the contribution of the square-pyramidal (SP) character. In order to evaluate the SP contributions, the s values were calculated; s = 1 for an idealized TBP and s = 0 for an SP.8 The s values on Ge1 and Ge2 were calculated to be 0.89 and 0.77, respectively, which indicated the high TBP character of (Ge)2O. The structure of the germoxane is almost the same as that of the silicon complex other than the bond angle of Ge–O– Ge (120.62(11)°),9 which is much sharper than that of the Si–O– Si of siloxane (180°). This difference in the bond angles should result from the longer bond length of Ge–O (1.767(2)) than Si–O (1.613(2) Å), which decreases the repulsion between the two dipyrrin complex moieties. Interestingly, the recrystallization of 2 from methanol affords the hexacoordinate germanium complex 3 (=Ge(OMe)(HOMe)) via Ge–O–Ge bond cleavage by solvolysis. Complex 3 exhibited a slightly distorted octahedral structure (bond angles around Ge: 84.44(8)–95.41(8)°), in which methoxide and neutral methanol are bound to the germanium atom (Fig. 2). The bond lengths on the Ge atom of the hexacoordinate 3 (1.8273(17)–2.0166(16) Å) are longer than those of the pentacoordinate 2 (1.767(2)– 1.965(3) Å). The 1H NMR spectrum of dimeric complex 2 in C6D6 showed that the two pyrrole units are equivalent, although these units are unsymmetrically located at the apical and equatorial positions in the crystal state. The symmetry in solution is due to the fast structural exchange on the germanium atom. The two nonequivalent o-methyl groups of the meso-mesityl moiety revealed that the exchange reaction does not proceed via bond dissociation of the
–9.0
4
7.8
7.6
7.4
7.2
– 8.9
CDCl3 7.0
1
6.8
ppm
Figure 3. H DOSY of dimer 2 (=(Ge)2O) and monomer 4 (=GeLn) (400 MHz, CDCl3).
Figure 1. Molecular structure of 2 (50% probability). Hydrogen atoms, mesityl groups, and a solvent molecule were omitted for clarity.
Scheme 3.
M. Yamamura et al. / Tetrahedron Letters 54 (2013) 7049–7052
chemical shifts of all the aromatic protons of 4 are shifted downfield compared to 2 due to the shielding effect from the closely located dipyrrin moiety. Similar chemical-shift changes were observed in (Si)2O and SiOH.3 In contrast to 2, the silicon analogue (Si)2O did not react under the same conditions, revealing the higher reactivity of germoxane 2. Electrospray ionization mass spectrometry (ESI-MS) of a methanol solution of 2 also confirmed the dissociation into monomeric species by the observation of monomeric species [Ge(OMe)2] (=[3–H]) without any dimeric species, while the ESI-MS of (Si)2O exhibited an ion peak of a dimer.11 One of the explanations for the higher reactivity of 2 is the lower steric hindrance around the germanium atom caused by the longer bond lengths. The formation of 2 from 4 is difficult due to the fast hydrolysis of 2 even in the presence of a small amount of water. To accomplish the reversible interconversion in solution, the regeneration of 2 from 4 was tried using various additives, such as MgSO4, N,N0 -dicyclohexylcarbodiimide (DCC), and 1,4-diazabicyclo[2.2.2] octane (DABCO), to a CDCl3 solution of 4. However, no reaction took place. Significant NMR spectral changes were observed only upon the addition of 1,8-diaza[5.4.0]bicycloundecene (DBU) (Fig. 4). As the amount of added DBU increased, the signals were first broadened, then finally turned sharp. In the DOSY experiments, the product showed a diffusion constant, 6.48 1010 m2/ s, smaller than the monomer 4 and as large as the dimer 2. The diffusion constant suggested the formation of a dimer. In addition, all the 1H NMR signals of the product in the aromatic region shifted upfield upon the addition of DBU. Considering that the chemical shifts of the dimer 2 were observed upfield, a germoxane-like product should be formed. However, the chemical shifts of the product are slightly different from those of 2. This difference is explained by the assumption that the product was germoxane 5 (=(GeL0 m)2O) with the coordination states different from 2 in basic condition.12 At a higher concentration, 5 was formed by the addition of a smaller amount of DBU. This concentration dependence is due to the condensation reaction favorable at a higher concentration. Neutralization by adding acetic acid regenerated the monomer 4 (Fig. 4d). As a result, the interconversion between the monomer and dimer was achieved by the addition of the base and acid. A measurement of the UV–vis spectrum of a CHCl3 solution of 2 was attempted. However, it is difficult to measure the spectrum of 2 due to the hydrolysis of 2 to 4 in a diluted concentration for opti-
Figure 4. 1H NMR spectra of 4 with (a) none, (b) 1 equiv, and (c) 3 equiv of DBU, and (d) 4 with 3 equiv of DBU and 9 equiv of AcOH (400 MHz, [4] = 1 mM, CDCl3).
7051
Figure 5. (a) UV–vis and (b) emission spectroscopic titration of 4 upon the addition of DBU ([4] = 20 lM, CHCl3). kex = 520 nm.
cal measurement. The UV–vis spectrum of 4 (Fig. 5a) exhibited an intense absorption maximum at 615 nm (e = 30,700 M1 cm1).13 Upon the addition of DBU to the solution, the absorbance at 615 nm decreased, and a new band at 626 nm appeared. Isosbestic points in the spectral titration indicated the existence of only two species, the monomer 4 and dimer 5. The dimerization process of 4 was saturated by the addition of over 70 equiv of DBU. The germanium complexes showed a significant red-shift in the absorption maxima compared to the corresponding silicon complexes (kmax = 601 nm for both SiOH and (Si)2O). In the emission spectrum of 4 (Fig. 5b), a strong fluorescence was observed at 669 nm. The fluorescence maximum is longer than that of SiOH (621 nm), and comes close to the NIR region. The quantum yield of 4 (Uful = 71%) is extremely high compared to those of the well-known NIR luminescent dyes containing a cyanine unit and a rhodamine-type unit.14 Upon the addition of DBU, the intensity of the fluorescence slightly decreased and the maximum was blue shifted by 5 nm. Neutralization by adding acetic acid regenerated the spectrum of 4 (see Supporting information). This switch in the optical properties was repeated at least three times. In conclusion, the dimeric and monomeric dipyrrin–germanium complexes 2 (=(Ge)2O) and 3 (=Ge(OMe)(HOMe)) were synthesized and characterized. The dimer 2 was easily hydrolyzed to give the monomer 4 in solution. The addition of DBU to 4 produced the dimer 5 in solution, providing the facile and reversible interconversion between the monomer and dimer, which is superior to the silicon complexes interconvertible by refluxing and solvent exchange. The dipyrrin germanium complexes showed an intense absorption and fluorescence near the NIR region, which is more red-shifted than the silicon complexes. In addition, these optical properties were controlled by the structural change between the monomeric and dimeric complexes, which can be utilized as an excellent method for the regulation of the NIR fluorescence.
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Acknowledgments This research was financially supported by Grants-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science, and Technology of Japan. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.tetlet.2013. 10.073. References and notes 1. (a) Corriu, R. J. P.; Young, J. C. In The Chemistry of Organic Silicon Compounds; Patai, S., Rappoport, Z., Eds.; John Wiley & Sons: Britain, 1989; p 1241; (b) Hosomi, A. Acc. Chem. Res. 1988, 21, 200–206; (c) Holmes, R. R. Chem. Rev. 1990, 90, 17–31; (d) Chuit, C.; Corriu, R. J. P.; Reye, C.; Young, J. C. Chem. Rev. 1993, 93, 1371–1448; (e)Molecular Chemistry; Atwood, J. L., Steed, J. W., Eds.; Marcel Dekker Inc.: New York, 2004; pp 31–41. 2. (a) Kost, D.; Kalikhman, I. Acc. Chem. Res. 2009, 42, 303–314; (b) Wagler, J.; Doert, T.; Roewer, G. Angew. Chem., Int. Ed. 2004, 43, 2441–2444; (c) Kano, N.; Yamamura, M.; Kawashima, T. J. Am. Chem. Soc. 2004, 126, 6250–6251; (d) Lippe, K.; Gerlach, D.; Kroke, E.; Wagler, J. Organometallics 2009, 28, 621–629.
3. Sakamoto, N.; Ikeda, C.; Yamamura, M.; Nabeshima, T. J. Am. Chem. Soc. 2011, 133, 4726–4729. 4. Loudet, A.; Burgess, K. Chem. Rev. 2007, 107, 4891–4932. 5. a) Tacke, R.; Burschka, C.; Richter, I.; Wagner, B.; Willeke, R. J. J. Am. Chem. Soc. 2000, 122, 8480–8485; b) Theis, B.; Weiß, J.; Lippert, W. P.; Bertermann, R.; Burschka, C.; Tacke, R. Chem. Eur. J. 2012, 18, 2202–2206. 6. Ikeda, C.; Ueda, S.; Nabeshima, T. Chem. Commun. 2009, 2544–2546. 7. Only one example of a dipyrrin–germanium complex: Nakano, K.; Kobayashi, K.; Nozaki, K. J. Am. Chem. Soc. 2011, 133, 10720–10723. 8. Addison, A. W.; Rao, T. N.; Reedijk, J.; van Rijn, J.; Verschoor, G. C. J. Chem. Soc., Dalton Trans. 1984, 1349. 9. Ge–O–Ge angles are 135.3–157.6° in hypercoordinate digermoxanes: (a) Livant, P.; Northcott, J.; Webb, T. R. J. Organomet. Chem. 2001, 620, 133–138; (b) Mastroianni, M.; Zhu, W.; Stefanelli, M.; Nardis, S.; Fronczek, F. R.; Smith, K. M.; Ou, Z.; Kadish, K. M.; Paolesse, R. Inorg. Chem. 2008, 47, 11680–11687; (c) Doyle, D. J.; Hitchcock, P. B.; Lappert, M. F.; Li, G. J. Organomet. Chem. 2009, 694, 2611–2617. 10. The co-ordination number n is in the range of 1–2 (pentacoordinate or hexacoordinate state). A candidate for the substituent L is OH or H2O. 11. [3-H] was only observed in the negative-mode ESI-MS. In the positive-mode ESI-MS, no peak assigned to a germanium compound was observed. 12. The co-ordination number m is in the range of 0–1 (pentacoordinate or hexacoordinate state). A candidate for the substituent L0 is OH or H2O. 13. The complete hydrolysis of 2 was clarified by 1H NMR. 14. McCann, T. E.; Kosaka, N.; Koide, Y.; Mitsunaga, M.; Choyke, P. L.; Nagano, T.; Urano, Y.; Kobayashi, H. Bioconjugate Chem. 2011, 22, 2531–2538.
Contents lists available at ScienceDirect
Tetrahedron Letters journal homepage: www.elsevier.com/locate/tetlet
Monomeric and dimeric red/NIR-fluorescent dipyrrin–germanium complexes: facile monomer–dimer interconversion driven by acid/base additions Masaki Yamamura, Hiroyuki Takizawa, Naoya Sakamoto, Tatsuya Nabeshima ⇑ Graduate School of Pure and Applied Sciences, Tsukuba Research Center for Interdisciplinary Materials Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8571, Japan
a r t i c l e
i n f o
Article history: Received 14 September 2013 Revised 8 October 2013 Accepted 15 October 2013 Available online 23 October 2013 Keywords: Dipyrrin Fluorescence Dimerization Supramolecule Germanium
a b s t r a c t Highly coordinate germanium complexes of the N2O2-type tetradentate dipyrrin ligand have been synthesized. X-ray crystallographic analysis revealed the pentacoordinate structure of dimeric germanium complex 2 (=(Ge)2O) and the hexacoordinate structure of monomeric complex 3 (=Ge(OMe)(HOMe)). The dimer 2 was easily hydrolyzed in a solution to give monomer 4, though the corresponding siloxane (Si)2O did not react under the same conditions. The addition of DBU to a solution of 4 gave dimer 5, and neutralization by adding acetic acid regenerated the monomer 4, providing the facile and reversible interconversion between the monomer and dimer. The dipyrrin germanium complexes showed an intense absorption and fluorescence near the NIR region, which is more red-shifted than the silicon complexes. Ó 2013 Elsevier Ltd. All rights reserved.
Highly coordinate heavier group 14 compounds (Si, Ge, and Sn) have been extensively studied because they are important intermediates in useful synthetic reactions1 due to their easier bond dissociation than the tetracoordinate ones.2 We reported the pentacoordinate silicon complexes of the N2O2-type tetradentate dipyrrin ligands, and their unique optical properties and reactivities;3 the dipyrrin silicon complexes showed a strong luminescence as seen in the dipyrrin–boron complexes, the BODIPYs,4 that are often used as a luminescent dye. One of the most important features of our complexes is their strong luminescence in the near-infrared (NIR) region because NIR luminescent dyes are attractive for biological applications due to the low emission background and high transparency of biological tissues and living cells in the NIR region. Furthermore, interconversion between the monomer, silanol SiOH, and the dimer, disiloxane (Si)2O, reversibly occurred by hydrolysis and condensation, which is characteristic of the highly coordinate siloxane (Scheme 1).5 The reversible bond dissociation and formation led to the interconversion between the monomer and dimer of the dipyrrin–silicon complexes associated with changes in the NIR luminescent properties. However, these reactions occurred under reflux conditions each in a different solvent. Consequently, a time-consuming process involving evaporation of the solvent and redissolution was neces-
⇑ Corresponding author. Tel./fax: +81 29 853 4507. E-mail address: [email protected] (T. Nabeshima). 0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.tetlet.2013.10.073
Scheme 1.
sary for the interconversion. An even more reactive dipyrrin complex is desired for a more facile interconversion system driven only by an additive, heat, or other external stimuli. We now report the synthesis of a more reactive germanium analogue of the silicon complexes, germoxane, and the interconversion and regulation of the NIR luminescence based on reversible bond dissociation of the germoxane. The dipyrrin–germanium complex 2 (=(Ge)2O) was synthesized in a way similar to that of (Si)2O. Reaction of the dipyrrin ligand 16 with tetrachlorogermane in the presence of diisopropylethylamine followed by quenching with methanol gave the dipyrrin–germanium complex 2 in 92% yield (Scheme 2).7
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M. Yamamura et al. / Tetrahedron Letters 54 (2013) 7049–7052
Scheme 2.
Figure 2. Molecular structure of 3 (50% probability). Hydrogen atoms and a solvent molecule were omitted for clarity.
log(m 2/s)
Ge–O–Ge but via a pseudorotation mechanism, as shown in the siloxane (Si)2O. Upon the addition of water to the C6D6 solution of 2, the NMR signals of 2 disappeared and the signals assigned to a new dipyrrin complex appeared. In the diffusion-ordered NMR spectroscopic (DOSY) experiment of a mixture of 2 and the hydrolyzed product, the diffusion peaks of two species were clearly separated (Fig. 3). The diffusion constants of 2 and the other were determined to be 6.06 1010 m2/s and 7.42 1010 m2/s, respectively. These values are quite similar to those of siloxane (Si)2O (5.99 1010 m2/ s) and silanol SiOH (7.30 1010 m2/s), respectively. Thus the signals with the larger diffusion constant were assigned to monomeric germanium complex 4 (=GeLn) formed by the hydrolysis of 2 (Scheme 3), although the coordination number and substituents of the germanium atom, n and L, are unclear in solution.10 The
– 9.2
2
– 9.1
In the crystal structure, 2 has the germoxane structure having two germanium atoms bridged by an oxygen atom (Fig. 1). The Ge atoms have a slightly distorted trigonal-bypramidal (TBP) geometry, which is typical of the pentacoordinate germanium complexes. The dipyrrin-ligand part has an asymmetrical structure in which the N2 and O1 atoms are located at the apical positions on Ge1, and the other three atoms (N1, O2, and O3) are located at the equatorial positions. The sum of the three equatorial-to-equatorial angles (O(2)–Ge(1)–O(3), O(2)–Ge(1)–N(1), and O(3)–Ge(1)–N(1)) are almost 360° (359.99°), indicative of the ideal TBP structure. The slight distortions are indicative of the contribution of the square-pyramidal (SP) character. In order to evaluate the SP contributions, the s values were calculated; s = 1 for an idealized TBP and s = 0 for an SP.8 The s values on Ge1 and Ge2 were calculated to be 0.89 and 0.77, respectively, which indicated the high TBP character of (Ge)2O. The structure of the germoxane is almost the same as that of the silicon complex other than the bond angle of Ge–O– Ge (120.62(11)°),9 which is much sharper than that of the Si–O– Si of siloxane (180°). This difference in the bond angles should result from the longer bond length of Ge–O (1.767(2)) than Si–O (1.613(2) Å), which decreases the repulsion between the two dipyrrin complex moieties. Interestingly, the recrystallization of 2 from methanol affords the hexacoordinate germanium complex 3 (=Ge(OMe)(HOMe)) via Ge–O–Ge bond cleavage by solvolysis. Complex 3 exhibited a slightly distorted octahedral structure (bond angles around Ge: 84.44(8)–95.41(8)°), in which methoxide and neutral methanol are bound to the germanium atom (Fig. 2). The bond lengths on the Ge atom of the hexacoordinate 3 (1.8273(17)–2.0166(16) Å) are longer than those of the pentacoordinate 2 (1.767(2)– 1.965(3) Å). The 1H NMR spectrum of dimeric complex 2 in C6D6 showed that the two pyrrole units are equivalent, although these units are unsymmetrically located at the apical and equatorial positions in the crystal state. The symmetry in solution is due to the fast structural exchange on the germanium atom. The two nonequivalent o-methyl groups of the meso-mesityl moiety revealed that the exchange reaction does not proceed via bond dissociation of the
–9.0
4
7.8
7.6
7.4
7.2
– 8.9
CDCl3 7.0
1
6.8
ppm
Figure 3. H DOSY of dimer 2 (=(Ge)2O) and monomer 4 (=GeLn) (400 MHz, CDCl3).
Figure 1. Molecular structure of 2 (50% probability). Hydrogen atoms, mesityl groups, and a solvent molecule were omitted for clarity.
Scheme 3.
M. Yamamura et al. / Tetrahedron Letters 54 (2013) 7049–7052
chemical shifts of all the aromatic protons of 4 are shifted downfield compared to 2 due to the shielding effect from the closely located dipyrrin moiety. Similar chemical-shift changes were observed in (Si)2O and SiOH.3 In contrast to 2, the silicon analogue (Si)2O did not react under the same conditions, revealing the higher reactivity of germoxane 2. Electrospray ionization mass spectrometry (ESI-MS) of a methanol solution of 2 also confirmed the dissociation into monomeric species by the observation of monomeric species [Ge(OMe)2] (=[3–H]) without any dimeric species, while the ESI-MS of (Si)2O exhibited an ion peak of a dimer.11 One of the explanations for the higher reactivity of 2 is the lower steric hindrance around the germanium atom caused by the longer bond lengths. The formation of 2 from 4 is difficult due to the fast hydrolysis of 2 even in the presence of a small amount of water. To accomplish the reversible interconversion in solution, the regeneration of 2 from 4 was tried using various additives, such as MgSO4, N,N0 -dicyclohexylcarbodiimide (DCC), and 1,4-diazabicyclo[2.2.2] octane (DABCO), to a CDCl3 solution of 4. However, no reaction took place. Significant NMR spectral changes were observed only upon the addition of 1,8-diaza[5.4.0]bicycloundecene (DBU) (Fig. 4). As the amount of added DBU increased, the signals were first broadened, then finally turned sharp. In the DOSY experiments, the product showed a diffusion constant, 6.48 1010 m2/ s, smaller than the monomer 4 and as large as the dimer 2. The diffusion constant suggested the formation of a dimer. In addition, all the 1H NMR signals of the product in the aromatic region shifted upfield upon the addition of DBU. Considering that the chemical shifts of the dimer 2 were observed upfield, a germoxane-like product should be formed. However, the chemical shifts of the product are slightly different from those of 2. This difference is explained by the assumption that the product was germoxane 5 (=(GeL0 m)2O) with the coordination states different from 2 in basic condition.12 At a higher concentration, 5 was formed by the addition of a smaller amount of DBU. This concentration dependence is due to the condensation reaction favorable at a higher concentration. Neutralization by adding acetic acid regenerated the monomer 4 (Fig. 4d). As a result, the interconversion between the monomer and dimer was achieved by the addition of the base and acid. A measurement of the UV–vis spectrum of a CHCl3 solution of 2 was attempted. However, it is difficult to measure the spectrum of 2 due to the hydrolysis of 2 to 4 in a diluted concentration for opti-
Figure 4. 1H NMR spectra of 4 with (a) none, (b) 1 equiv, and (c) 3 equiv of DBU, and (d) 4 with 3 equiv of DBU and 9 equiv of AcOH (400 MHz, [4] = 1 mM, CDCl3).
7051
Figure 5. (a) UV–vis and (b) emission spectroscopic titration of 4 upon the addition of DBU ([4] = 20 lM, CHCl3). kex = 520 nm.
cal measurement. The UV–vis spectrum of 4 (Fig. 5a) exhibited an intense absorption maximum at 615 nm (e = 30,700 M1 cm1).13 Upon the addition of DBU to the solution, the absorbance at 615 nm decreased, and a new band at 626 nm appeared. Isosbestic points in the spectral titration indicated the existence of only two species, the monomer 4 and dimer 5. The dimerization process of 4 was saturated by the addition of over 70 equiv of DBU. The germanium complexes showed a significant red-shift in the absorption maxima compared to the corresponding silicon complexes (kmax = 601 nm for both SiOH and (Si)2O). In the emission spectrum of 4 (Fig. 5b), a strong fluorescence was observed at 669 nm. The fluorescence maximum is longer than that of SiOH (621 nm), and comes close to the NIR region. The quantum yield of 4 (Uful = 71%) is extremely high compared to those of the well-known NIR luminescent dyes containing a cyanine unit and a rhodamine-type unit.14 Upon the addition of DBU, the intensity of the fluorescence slightly decreased and the maximum was blue shifted by 5 nm. Neutralization by adding acetic acid regenerated the spectrum of 4 (see Supporting information). This switch in the optical properties was repeated at least three times. In conclusion, the dimeric and monomeric dipyrrin–germanium complexes 2 (=(Ge)2O) and 3 (=Ge(OMe)(HOMe)) were synthesized and characterized. The dimer 2 was easily hydrolyzed to give the monomer 4 in solution. The addition of DBU to 4 produced the dimer 5 in solution, providing the facile and reversible interconversion between the monomer and dimer, which is superior to the silicon complexes interconvertible by refluxing and solvent exchange. The dipyrrin germanium complexes showed an intense absorption and fluorescence near the NIR region, which is more red-shifted than the silicon complexes. In addition, these optical properties were controlled by the structural change between the monomeric and dimeric complexes, which can be utilized as an excellent method for the regulation of the NIR fluorescence.
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Acknowledgments This research was financially supported by Grants-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science, and Technology of Japan. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.tetlet.2013. 10.073. References and notes 1. (a) Corriu, R. J. P.; Young, J. C. In The Chemistry of Organic Silicon Compounds; Patai, S., Rappoport, Z., Eds.; John Wiley & Sons: Britain, 1989; p 1241; (b) Hosomi, A. Acc. Chem. Res. 1988, 21, 200–206; (c) Holmes, R. R. Chem. Rev. 1990, 90, 17–31; (d) Chuit, C.; Corriu, R. J. P.; Reye, C.; Young, J. C. Chem. Rev. 1993, 93, 1371–1448; (e)Molecular Chemistry; Atwood, J. L., Steed, J. W., Eds.; Marcel Dekker Inc.: New York, 2004; pp 31–41. 2. (a) Kost, D.; Kalikhman, I. Acc. Chem. Res. 2009, 42, 303–314; (b) Wagler, J.; Doert, T.; Roewer, G. Angew. Chem., Int. Ed. 2004, 43, 2441–2444; (c) Kano, N.; Yamamura, M.; Kawashima, T. J. Am. Chem. Soc. 2004, 126, 6250–6251; (d) Lippe, K.; Gerlach, D.; Kroke, E.; Wagler, J. Organometallics 2009, 28, 621–629.
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