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Boron, aluminum, gallium, and indium complexes of 1,3-bis(2-pyridylimino)isoindoline (BPI)
Inorganic Chemistry Communications 30 (2013) 147–151
Contents lists available at SciVerse ScienceDirect
Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
Boron, aluminum, gallium, and indium complexes of 1,3-bis(2-pyridylimino) isoindoline (BPI) Jeremy D. Dang a, Timothy P. Bender a, b, c,⁎ a b c
Department of Chemical Engineering and Applied Chemistry, University of Toronto, 200 College Street, Toronto, Ontario, Canada, M5S 3E5 Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario, Canada, M5S 3H6 Department of Materials Science and Engineering, University of Toronto, 184 College Street, Toronto, Ontario, Canada M5S 3E4
a r t i c l e
i n f o
Article history: Received 12 October 2012 Accepted 16 November 2012 Available online 30 January 2013 Keywords: 1,3-Bis(2-pyridylimino)isoindoline Coordination Boron Aluminum Gallium Indium
a b s t r a c t Numerous metal complexes of 1,3-bis(2-pyridylimino)isoindoline (BPI) are known, but coordination to this ligand has been limited to transition metals. Herein, we report on the synthesis of group 13 (boron, aluminum, gallium, and indium) complexes of BPI and their basic spectroscopic properties. In its deprotonated monoanionic form, the BPI ligand chelates to boron via a bidentate N^N interaction, forming a non-symmetric molecule. Coordination to aluminum, gallium, and indium occurs via a tridendate N^N^N interaction, forming a symmetric molecule. Absorption spectroscopy shows multiple absorption bands in the UV–visible region for the boron, gallium, and indium complex. A single broad absorption in the UV region was noted for the aluminum complex. Fluorescence emission spectroscopy using an excitation wavelength of 352 nm showed small differences in emission behavior among the complexes. However, with excitation at the λmax and longest λ of absorption, the aluminum complex of BPI was found to be much more emissive than the complexes of boron, gallium and indium; The latter complexes all had emission intensities on the same order of magnitude, each having a low fluorescence quantum yield. © 2013 Elsevier B.V. All rights reserved.
Introduction. 1,3-Bis(2-pyridylimino)isoindoline (BPI, Fig. 1) is a conjugated compound consisting of two pyridyl groups attached to an isoindoline core via an imine bridging system [1]. In its deprotonated monoanionic form, BPI can serve as a tridendate N^N^N ligand [2] and has been shown to complex with many first and second row transition metals such as manganese [3], iron [4], cobalt [5], nickel [6], copper [7], zinc [8], and palladium [9] to form a 1:1 or 2:1 (ligand:metal) complex. Metal complexes of BPI have previously been employed as catalysts for several organic transformation processes [10–12], as model compounds for probing metal coordination environments of biological systems [5,13], and as a mediator for controlled radical polymerization of acrylates [14]. Despite BPI's rich coordination chemistry, to the best of our knowledge there is no precedent for their complexation with group 13 metals/metalloids. Our motivation to pursue this investigation is due to the structural similarity of BPI (i.e. isoindoline-based scaffold) to phthalocyanines (Pcs, Fig. 1) and their analogs. Pcs, which are porphyrin-like compounds with isoindoline subunits bridged by imines [15–17], have been extensively studied for their ⁎ Corresponding author at: Department of Chemical Engineering and Applied Chemistry, University of Toronto, 200 College Street, Toronto, Ontario, Canada M5S 3E5. Tel.: +1 416 978 6140; fax: +1 416 978 8605. E-mail address: [email protected] (T.P. Bender). 1387-7003/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.inoche.2012.11.020
metal-binding properties. This has led to the synthesis and study of Pcs of nearly all metal elements of the periodic table [18]. Some of these metallophthalocyanines have been investigated in applications that include non-linear optics [19], catalytic reactions [20], gas sensing [21], and photovoltaic cells [22]. An analogue of Pcs called the hemiporphyrazines (HPs), which have two of the isoindoline subunits of Pc substituted for other aromatic moieties like pyridines (Fig. 1) [23], benzenes [24], and azoles [25], have recently emerged as a target ligand of interest. Ziegler and Durfee have recently demonstrated the coordinating versatility of hemiporphyrazines towards a series of metals. For example, dicarbahemiporphyrazines, a Pc analog with two substituted benzene units, was shown to coordinate with Ag [26,27], Cu(I) [26,27], Fe(II) [27,28], Mn(II) [27,28], Co(II) [26–28], Ni(II) [29], Zn [30], and Li [31] while benziphthalocyanines, a Pc analog with one substituted benzene unit, was shown to coordinate with Co(II) [26,32], Co(III) [26,32], Ni(II) [29], Zn [30], and Li [31]. We are similarly interested in Pc analogs. To date our group focused on boron subphthalocyanines (BsubPcs, Fig. 1), a class of macrocyclic compounds which are ring contracted analogs to Pcs and are made up of three isoindoline units that chelate a single boron atom [33]. BsubPcs have been explored as functional materials in a wide range of organic electronic applications, such as non-linear optics [34], organic field effect transistors [35,36], organic light-emitting diodes [37,38], and organic solar cells [39–42]. In this communication,
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J.D. Dang, T.P. Bender / Inorganic Chemistry Communications 30 (2013) 147–151
N
N
N
NH N
N
HN
NH N
N
N
N
N
NH N
N
Cl
N
N
N
HN N
N NB N
N
N N
Fig. 1. From left to right: structure of 1,3-bis(2-pyridylimino)isoindoline (BPI), phthalocyanine (Pc), hemiporphyrazine (HP), and boron subphthalocyanine (BsubPc).
starting materials used for the synthesis of BPI (Scheme 1). This direct approach produced BPI·GaCl2 in 9% yield after two days of refluxing in n-hexanol. Although the yield is lower than the aforementioned synthetic method (9% vs. 17%), this approach bypasses the formation of the BPI intermediate. Optimization of the complexation reaction in an aim to improve the isolated yields of BPI·BF2, BPI·AlCl2, BPI·GaCl2, and BPI·InCl2 is the subject of future investigation. Compounds BPI·BF2, BPI·AlCl2, BPI·GaCl2, and BPI·InCl2 were characterized by 1H NMR, 13C NMR, and HRMS. In each case other than BPI·BF2, 1H and 13C NMR spectra (Figs. S1–S8 in Supplementary data) confirmed the symmetry of each complex by exhibiting 6 1H and 9 13C resonances. This was supported by a single crystal X-ray diffraction analysis of BPI·GaCl2 (Fig. 2, CCDC deposition number: 904845), which showed the gallium metal binding to the nitrogen atom of each pyridyl ring and to the nitrogen atom of the isoindoline ring (i.e. N^N^N interaction). For BPI·BF2, the 1H and 13C NMR spectra showed a unique chemical shift for each proton (12 resonances) and carbon (18 resonances) atom, suggesting that the molecule is not symmetric. This was confirmed by the single crystal structure of BPI·BF2, where the boron atom was found to bind to a nitrogen atom of one pyridyl ring and to the nitrogen atom of the isoindoline ring (N^N interaction, Fig. 3, CCDC deposition number: 904844). The B1–N4 distance (2.961 Å) was found to be nearly twice as long as the B1–N1 (1.517 Å) and B1–N3 (1.583 Å) bond lengths. It is worth noting that the coordination mode of BPI adopted in the BPI·BF2 compound has only been observed in one other compound — a dimolybdenum complex of BPI. In this compound, one of the two molybdenum centers is bound to an isoindoline nitrogen and to a pyridyl nitrogen while the second molybdenum center is bound to a bridging imino nitrogen [43]. Moreover, the coordination motif in BPI·BF2 strongly resembles that found in the highly fluorescent 4,4-difluoro-4bora-3a,4a-diaza-s-indacene (BODIPY) dye, where the boron atom is bound to two fluorine atoms and to two pyrrole nitrogen atoms [44,45]. Suitable crystals of BPI·AlCl2 and BPI·InCl2 for X-ray diffraction analysis could not be grown by slow evaporation from dichloromethane,
we report on the synthesis and basic spectroscopic properties of the boron complex of BPI (BPI·BF2). We also report the synthesis and characterization of the remaining group 13 complexes of BPI namely its aluminum (BPI·AlCl2), gallium (BPI·GaCl2), and indium (BPI·InCl2) complexes. Results and discussion. The BPI·BF2, BPI·AlCl2, BPI·GaCl2, and BPI·InCl2 complexes were synthesized by heating the BPI ligand, formed from the condensation of o-phthalonitrile and 2-aminopyridine, with the appropriate group 13 halide in the presence of triethylamine base (Scheme 1). The products were isolated in low yields, which also varied from batch-to-batch. The highest isolated yield obtained for BPI·BF2, BPI·AlCl2, BPI·GaCl2, and BPI·InCl2 was 19, 25, 17, and 27%, respectively. The poor yields were largely caused by the need to remove the triethylamine salt and unreacted group 13 halide from the crude product using water. The aqueous wash not only dissolved and removed the unwanted side products, but it also partially dissolved and removed the desired compound. Efforts to recover the product via liquid–liquid extraction with a variety of organic solvents were ineffective; the distribution coefficient of BPI·BF2, BPI·AlCl2, BPI·GaCl2, and BPI·InCl2 clearly favors the aqueous phase over the organic phase. Purification via silica gel column chromatography was also ineffective as the salt product and the desired product stayed fixed on the column and thus, could not be separated. Additional efforts were made to optimize the work-up procedure, where triethylamine was replaced with tri-n-butylamine. The motivation behind the substitution for this fatty organic base was to avoid an aqueous wash work-up and to facilitate the removal of the side products via washing of the crude product with an organic solvent that would not solubilise the target compound (e.g. hexane). The approach was unsuccessful as rinsing with water was still required to produce a pure product. Since the yield of BPI·GaCl2 was the lowest among the complexes, an attempt was made to determine whether BPI·GaCl2, and perhaps the other group 13 complexes, could be prepared more readily through a direct approach. This was carried out by adding GaCl3 into a reaction mixture of o-phthalonitrile and 2-aminopyridine,
CN
+
CN
N
N
(i)
N
NH N
N H2N
N (ii)
N
N BF2 N BPI•BF2
N
N (iii) (iv)
N N
N MCl2 N
M = Al; BPI•AlCl2 M = Ga; BPI•GaCl2 M = In; BPI•InCl2
Scheme 1. Synthesis of BPI·BF2, BPI·AlCl2, BPI·GaCl2, and BPI·InCl2. Reagents and conditions: (i) calcium chloride, n-hexanol, reflux, 12 h; (ii) boron trifluoride diethyl etherate, toluene, 100 °C, 24 h; (iii) MCl3 (M = aluminum, gallium(III), indium(III)), toluene, 100 °C, 24 h; and (iv) gallium(III) chloride, n-hexanol, reflux, 2 days.
J.D. Dang, T.P. Bender / Inorganic Chemistry Communications 30 (2013) 147–151
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Fig. 2. Ellipsoid plot (50% probability) showing the structure and atom numbering scheme for the BPI·GaCl2 complex (CCDC deposition number: 904845). Hydrogen atoms have been omitted for clarity.
methanol, toluene, or benzene, by slow cooling from hot toluene or n-hexanol, by vapor diffusion of pentane into dichloromethane or heptane into benzene, or by layering hexane onto dichloromethane. The UV–visible absorption spectra for BPI, BPI·BF2, BPI·AlCl2, BPI·GaCl2, and BPI·InCl2 were acquired in degassed dichloromethane solutions at room temperature (Fig. 4). With the exception of BPI·AlCl2, which showed a single broad absorbing band at a λmax = 301 nm, the spectra for the complexes were broad and consisted of multiple absorption bands in the UV and short wavelength visible region. Similar findings were reported for the nickel(II) and platinum(II) complexes of BPI [6,46,47]. In comparison with the absorption spectrum of BPI (λmax = 386 nm), the spectra of BPI·GaCl2 (λmax = 408 nm) and BPI·InCl2 (λmax = 411 nm) were red shifted while the spectra of BPI·BF2 (λmax = 370 nm) and BPI·AlCl2 (λmax = 301 nm) were blue shifted. A clear trend in the UV–vis absorption profiles for the group 13 complexes was not observed. Fluorescence emission spectra were also acquired for the group 13 complexes in degassed dichloromethane solutions at room temperature at three excitation wavelengths (λex): (1) a common wavelength of 352 nm, (2) the λmax of absorption, and (3) the longest λ of absorption (Fig. 5, Table S13 in Supplementary data). When excited at 352 nm, a small red shift was observed in the spectra of BPI·BF2 (λmax = 399 nm) and BPI·AlCl2 (λmax = 399 nm) while a blue shift was observed in the spectra of BPI·GaCl2 (λmax = 396 nm) and
BPI·InCl2 (λmax = 398 nm) relative to the emission spectrum of BPI (λmax = 398 nm). Since the highest and the longest λmax of absorption were different for each complex, comparisons of the emission spectra at these two λex could not be made. However, a trend is noted where the maximum emission intensity decreases with higher λex. For example, the maximum emission intensity is 13.9, 10.8, and 5.4 (arbitrary units) at a λex of 352, 370, and 388 nm, respectively, for BPI·BF2. Although the concentration of each complex was not kept constant, the maximum absorbance value for each sample was kept in the range of 0.04 and 0.05. This allowed for qualitative comparisons to be made between the different compounds. At the common λex of 352 nm, the maximum emission intensity for all complexes was on the same order of magnitude. At their λmax and longest λ of excitation, BPI·AlCl2 was found to be orders of magnitude more emissive than BPI and the other group 13 complexes, which were all found to have similar emission intensities at both λex. Moreover, the shapes of the emission curves were all similar to one another with the exception of BPI·AlCl2. From the UV–vis absorption and fluorescence spectra, it is apparent that BPI·AlCl2 differs significantly from the rest of the other complexes. Their dissimilar spectroscopic properties warrant further investigation. The fluorescence quantum yields (φ) for all compounds other than BPI·AlCl2 were measured to be in the range of 0.7–2.7% relative to a standard of 9,10-diphenylanthracene (Table S14 and Eq. S1 in
Fig. 3. Ellipsoid plot (50% probability) showing the structure and atom numbering scheme for the BPI·BF2 complex (CCDC deposition number: 904844). Hydrogen atoms have been omitted for clarity.
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J.D. Dang, T.P. Bender / Inorganic Chemistry Communications 30 (2013) 147–151
Fig. 4. UV/Vis absorption spectra of BPI, BPI·BF2, BPI·AlCl2, BPI·GaCl2, and BPI·InCl2 in degassed dichloromethane solutions at room temperature.
Supplementary data). The very low values indicate that these compounds are essentially non-emissive. This finding is in accordance with reported quantum yields for a platinum(II) complex of BPI and its post-metallated derivatives, which are all in the range of 0.03– 3.79% in degassed dichloromethane solutions at ambient temperature [47]. It is interesting to note that BPI·BF2 is non-emissive although it shares a similar coordination motif to that found in the highly fluorescence BODIPYs [44,45]. An exploration into this difference is worthy of a future investigation. Quantum yields (φ) could not be measured for BPI·AlCl2 using a relative approach due to our inability to find a well characterized standard with absorption and emission bands that closely match BPI·AlCl2. We can however surmise that given its higher emission intensity (Fig. 5), its quantum yield is likely to be significantly higher. Conclusions. We have reported on the synthesis of boron (BPI·BF2), aluminum (BPI·AlCl2), gallium (BPI·GaCl2), and indium (BPI·GaCl2) complexes of 1,3-bis(2-pyridylimino)isoindoline (BPI). Their two-step synthesis began with the condensation reaction of o-phthalonitrile
Fig. 5. Fluorescence emission spectra (λex = 352 nm, highest and longest λmax of absorption as indicated) for BPI, BPI·BF2, BPI·AlCl2, BPI·GaCl2, and BPI·InCl2 in degassed dichloromethane solutions at room temperature.
J.D. Dang, T.P. Bender / Inorganic Chemistry Communications 30 (2013) 147–151
and 2-aminopyridine to form the BPI ligand, followed by complexation with the appropriate group 13 halide in the presence of an organic base. For BPI·AlCl2, BPI·GaCl2, and BPI·InCl2, the group 13 centers were found to coordinate with BPI in a tridentate N^N^N manner, preserving the symmetric nature of the BPI ligand. In the case of BPI·BF2, the boron atom was bound to BPI in a N^N manner, breaking the symmetry of the BPI ligand. The UV–visible absorption spectra for BPI·BF2, BPI·GaCl2, and BPI·nCl2 were broad and consisted of multiple absorption bands in the UV–visible (short wavelength) region, while a single broad peak in the UV region was observed for BPI·AlCl2. Fluorescence emission spectra were acquired at three wavelengths of excitation. Small differences in the emission spectra of BPI and its group 13 complexes were observed when excited at 352 nm. At the λmax and longest λ of absorption, BPI·AlCl2 was found to be much more emissive than the other complexes, which all had fluorescence emission intensities on the same order of magnitude. Very low fluorescence quantum yields in the range of 0.7–2.7% were measured for BPI, BPI·BF2, BPI·GaCl2, and BPI·InCl2. These values are in line with reported quantum yields of platinated BPIs. Optimization of the BPI complexation reaction, an examination into the spectroscopic differences of BPI·AlCl2 and the other group 13 complexes, and an investigation into the fluorescence dissimilarities of BPI·BF2 to BODIPY are the subject of future study. Acknowledgments The authors wish to acknowledge funding for this research from the Natural Sciences and Engineering Research Council (NSERC) of Canada. Appendix A. Supplementary material Supplementary data associated with this article including complete experimental methods, synthetic procedures, NMR spectra, detailed crystallographic data, fluorescence emission data, and fluorescence quantum yields can be found, in the online version, at http://dx.doi. org/10.1016/j.inoche.2012.11.020. References [1] J.A. Elvidge, R.P. Linstead, J. Chem. Soc. (1952) 5000–5007. [2] W.O. Siegl, J. Org. Chem. 42 (1977) 1872–1878. [3] J. Kaizer, G. Barath, G. Speier, M. Reglier, M. Giorgi, Inorg. Chem. Commun. 10 (2007) 292–294. [4] E. Balogh-Hergovich, G. Speier, M. Reglier, M. Giorgi, E. Kuzmann, A. Vertes, Inorg. Chem. Commun. 8 (2005) 457–459. [5] P.T. Selvi, H. Stoeckli-Evans, M.J. Palaniandavar, Inorg. Biochem. 99 (2005) 2110–2118. [6] R.J. Letcher, W. Zhang, C. Bensimon, R.J. Crutchley, Inorg. Chim. Acta 210 (1993) 183–191.
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Contents lists available at SciVerse ScienceDirect
Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
Boron, aluminum, gallium, and indium complexes of 1,3-bis(2-pyridylimino) isoindoline (BPI) Jeremy D. Dang a, Timothy P. Bender a, b, c,⁎ a b c
Department of Chemical Engineering and Applied Chemistry, University of Toronto, 200 College Street, Toronto, Ontario, Canada, M5S 3E5 Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario, Canada, M5S 3H6 Department of Materials Science and Engineering, University of Toronto, 184 College Street, Toronto, Ontario, Canada M5S 3E4
a r t i c l e
i n f o
Article history: Received 12 October 2012 Accepted 16 November 2012 Available online 30 January 2013 Keywords: 1,3-Bis(2-pyridylimino)isoindoline Coordination Boron Aluminum Gallium Indium
a b s t r a c t Numerous metal complexes of 1,3-bis(2-pyridylimino)isoindoline (BPI) are known, but coordination to this ligand has been limited to transition metals. Herein, we report on the synthesis of group 13 (boron, aluminum, gallium, and indium) complexes of BPI and their basic spectroscopic properties. In its deprotonated monoanionic form, the BPI ligand chelates to boron via a bidentate N^N interaction, forming a non-symmetric molecule. Coordination to aluminum, gallium, and indium occurs via a tridendate N^N^N interaction, forming a symmetric molecule. Absorption spectroscopy shows multiple absorption bands in the UV–visible region for the boron, gallium, and indium complex. A single broad absorption in the UV region was noted for the aluminum complex. Fluorescence emission spectroscopy using an excitation wavelength of 352 nm showed small differences in emission behavior among the complexes. However, with excitation at the λmax and longest λ of absorption, the aluminum complex of BPI was found to be much more emissive than the complexes of boron, gallium and indium; The latter complexes all had emission intensities on the same order of magnitude, each having a low fluorescence quantum yield. © 2013 Elsevier B.V. All rights reserved.
Introduction. 1,3-Bis(2-pyridylimino)isoindoline (BPI, Fig. 1) is a conjugated compound consisting of two pyridyl groups attached to an isoindoline core via an imine bridging system [1]. In its deprotonated monoanionic form, BPI can serve as a tridendate N^N^N ligand [2] and has been shown to complex with many first and second row transition metals such as manganese [3], iron [4], cobalt [5], nickel [6], copper [7], zinc [8], and palladium [9] to form a 1:1 or 2:1 (ligand:metal) complex. Metal complexes of BPI have previously been employed as catalysts for several organic transformation processes [10–12], as model compounds for probing metal coordination environments of biological systems [5,13], and as a mediator for controlled radical polymerization of acrylates [14]. Despite BPI's rich coordination chemistry, to the best of our knowledge there is no precedent for their complexation with group 13 metals/metalloids. Our motivation to pursue this investigation is due to the structural similarity of BPI (i.e. isoindoline-based scaffold) to phthalocyanines (Pcs, Fig. 1) and their analogs. Pcs, which are porphyrin-like compounds with isoindoline subunits bridged by imines [15–17], have been extensively studied for their ⁎ Corresponding author at: Department of Chemical Engineering and Applied Chemistry, University of Toronto, 200 College Street, Toronto, Ontario, Canada M5S 3E5. Tel.: +1 416 978 6140; fax: +1 416 978 8605. E-mail address: [email protected] (T.P. Bender). 1387-7003/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.inoche.2012.11.020
metal-binding properties. This has led to the synthesis and study of Pcs of nearly all metal elements of the periodic table [18]. Some of these metallophthalocyanines have been investigated in applications that include non-linear optics [19], catalytic reactions [20], gas sensing [21], and photovoltaic cells [22]. An analogue of Pcs called the hemiporphyrazines (HPs), which have two of the isoindoline subunits of Pc substituted for other aromatic moieties like pyridines (Fig. 1) [23], benzenes [24], and azoles [25], have recently emerged as a target ligand of interest. Ziegler and Durfee have recently demonstrated the coordinating versatility of hemiporphyrazines towards a series of metals. For example, dicarbahemiporphyrazines, a Pc analog with two substituted benzene units, was shown to coordinate with Ag [26,27], Cu(I) [26,27], Fe(II) [27,28], Mn(II) [27,28], Co(II) [26–28], Ni(II) [29], Zn [30], and Li [31] while benziphthalocyanines, a Pc analog with one substituted benzene unit, was shown to coordinate with Co(II) [26,32], Co(III) [26,32], Ni(II) [29], Zn [30], and Li [31]. We are similarly interested in Pc analogs. To date our group focused on boron subphthalocyanines (BsubPcs, Fig. 1), a class of macrocyclic compounds which are ring contracted analogs to Pcs and are made up of three isoindoline units that chelate a single boron atom [33]. BsubPcs have been explored as functional materials in a wide range of organic electronic applications, such as non-linear optics [34], organic field effect transistors [35,36], organic light-emitting diodes [37,38], and organic solar cells [39–42]. In this communication,
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N
N
N
NH N
N
HN
NH N
N
N
N
N
NH N
N
Cl
N
N
N
HN N
N NB N
N
N N
Fig. 1. From left to right: structure of 1,3-bis(2-pyridylimino)isoindoline (BPI), phthalocyanine (Pc), hemiporphyrazine (HP), and boron subphthalocyanine (BsubPc).
starting materials used for the synthesis of BPI (Scheme 1). This direct approach produced BPI·GaCl2 in 9% yield after two days of refluxing in n-hexanol. Although the yield is lower than the aforementioned synthetic method (9% vs. 17%), this approach bypasses the formation of the BPI intermediate. Optimization of the complexation reaction in an aim to improve the isolated yields of BPI·BF2, BPI·AlCl2, BPI·GaCl2, and BPI·InCl2 is the subject of future investigation. Compounds BPI·BF2, BPI·AlCl2, BPI·GaCl2, and BPI·InCl2 were characterized by 1H NMR, 13C NMR, and HRMS. In each case other than BPI·BF2, 1H and 13C NMR spectra (Figs. S1–S8 in Supplementary data) confirmed the symmetry of each complex by exhibiting 6 1H and 9 13C resonances. This was supported by a single crystal X-ray diffraction analysis of BPI·GaCl2 (Fig. 2, CCDC deposition number: 904845), which showed the gallium metal binding to the nitrogen atom of each pyridyl ring and to the nitrogen atom of the isoindoline ring (i.e. N^N^N interaction). For BPI·BF2, the 1H and 13C NMR spectra showed a unique chemical shift for each proton (12 resonances) and carbon (18 resonances) atom, suggesting that the molecule is not symmetric. This was confirmed by the single crystal structure of BPI·BF2, where the boron atom was found to bind to a nitrogen atom of one pyridyl ring and to the nitrogen atom of the isoindoline ring (N^N interaction, Fig. 3, CCDC deposition number: 904844). The B1–N4 distance (2.961 Å) was found to be nearly twice as long as the B1–N1 (1.517 Å) and B1–N3 (1.583 Å) bond lengths. It is worth noting that the coordination mode of BPI adopted in the BPI·BF2 compound has only been observed in one other compound — a dimolybdenum complex of BPI. In this compound, one of the two molybdenum centers is bound to an isoindoline nitrogen and to a pyridyl nitrogen while the second molybdenum center is bound to a bridging imino nitrogen [43]. Moreover, the coordination motif in BPI·BF2 strongly resembles that found in the highly fluorescent 4,4-difluoro-4bora-3a,4a-diaza-s-indacene (BODIPY) dye, where the boron atom is bound to two fluorine atoms and to two pyrrole nitrogen atoms [44,45]. Suitable crystals of BPI·AlCl2 and BPI·InCl2 for X-ray diffraction analysis could not be grown by slow evaporation from dichloromethane,
we report on the synthesis and basic spectroscopic properties of the boron complex of BPI (BPI·BF2). We also report the synthesis and characterization of the remaining group 13 complexes of BPI namely its aluminum (BPI·AlCl2), gallium (BPI·GaCl2), and indium (BPI·InCl2) complexes. Results and discussion. The BPI·BF2, BPI·AlCl2, BPI·GaCl2, and BPI·InCl2 complexes were synthesized by heating the BPI ligand, formed from the condensation of o-phthalonitrile and 2-aminopyridine, with the appropriate group 13 halide in the presence of triethylamine base (Scheme 1). The products were isolated in low yields, which also varied from batch-to-batch. The highest isolated yield obtained for BPI·BF2, BPI·AlCl2, BPI·GaCl2, and BPI·InCl2 was 19, 25, 17, and 27%, respectively. The poor yields were largely caused by the need to remove the triethylamine salt and unreacted group 13 halide from the crude product using water. The aqueous wash not only dissolved and removed the unwanted side products, but it also partially dissolved and removed the desired compound. Efforts to recover the product via liquid–liquid extraction with a variety of organic solvents were ineffective; the distribution coefficient of BPI·BF2, BPI·AlCl2, BPI·GaCl2, and BPI·InCl2 clearly favors the aqueous phase over the organic phase. Purification via silica gel column chromatography was also ineffective as the salt product and the desired product stayed fixed on the column and thus, could not be separated. Additional efforts were made to optimize the work-up procedure, where triethylamine was replaced with tri-n-butylamine. The motivation behind the substitution for this fatty organic base was to avoid an aqueous wash work-up and to facilitate the removal of the side products via washing of the crude product with an organic solvent that would not solubilise the target compound (e.g. hexane). The approach was unsuccessful as rinsing with water was still required to produce a pure product. Since the yield of BPI·GaCl2 was the lowest among the complexes, an attempt was made to determine whether BPI·GaCl2, and perhaps the other group 13 complexes, could be prepared more readily through a direct approach. This was carried out by adding GaCl3 into a reaction mixture of o-phthalonitrile and 2-aminopyridine,
CN
+
CN
N
N
(i)
N
NH N
N H2N
N (ii)
N
N BF2 N BPI•BF2
N
N (iii) (iv)
N N
N MCl2 N
M = Al; BPI•AlCl2 M = Ga; BPI•GaCl2 M = In; BPI•InCl2
Scheme 1. Synthesis of BPI·BF2, BPI·AlCl2, BPI·GaCl2, and BPI·InCl2. Reagents and conditions: (i) calcium chloride, n-hexanol, reflux, 12 h; (ii) boron trifluoride diethyl etherate, toluene, 100 °C, 24 h; (iii) MCl3 (M = aluminum, gallium(III), indium(III)), toluene, 100 °C, 24 h; and (iv) gallium(III) chloride, n-hexanol, reflux, 2 days.
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Fig. 2. Ellipsoid plot (50% probability) showing the structure and atom numbering scheme for the BPI·GaCl2 complex (CCDC deposition number: 904845). Hydrogen atoms have been omitted for clarity.
methanol, toluene, or benzene, by slow cooling from hot toluene or n-hexanol, by vapor diffusion of pentane into dichloromethane or heptane into benzene, or by layering hexane onto dichloromethane. The UV–visible absorption spectra for BPI, BPI·BF2, BPI·AlCl2, BPI·GaCl2, and BPI·InCl2 were acquired in degassed dichloromethane solutions at room temperature (Fig. 4). With the exception of BPI·AlCl2, which showed a single broad absorbing band at a λmax = 301 nm, the spectra for the complexes were broad and consisted of multiple absorption bands in the UV and short wavelength visible region. Similar findings were reported for the nickel(II) and platinum(II) complexes of BPI [6,46,47]. In comparison with the absorption spectrum of BPI (λmax = 386 nm), the spectra of BPI·GaCl2 (λmax = 408 nm) and BPI·InCl2 (λmax = 411 nm) were red shifted while the spectra of BPI·BF2 (λmax = 370 nm) and BPI·AlCl2 (λmax = 301 nm) were blue shifted. A clear trend in the UV–vis absorption profiles for the group 13 complexes was not observed. Fluorescence emission spectra were also acquired for the group 13 complexes in degassed dichloromethane solutions at room temperature at three excitation wavelengths (λex): (1) a common wavelength of 352 nm, (2) the λmax of absorption, and (3) the longest λ of absorption (Fig. 5, Table S13 in Supplementary data). When excited at 352 nm, a small red shift was observed in the spectra of BPI·BF2 (λmax = 399 nm) and BPI·AlCl2 (λmax = 399 nm) while a blue shift was observed in the spectra of BPI·GaCl2 (λmax = 396 nm) and
BPI·InCl2 (λmax = 398 nm) relative to the emission spectrum of BPI (λmax = 398 nm). Since the highest and the longest λmax of absorption were different for each complex, comparisons of the emission spectra at these two λex could not be made. However, a trend is noted where the maximum emission intensity decreases with higher λex. For example, the maximum emission intensity is 13.9, 10.8, and 5.4 (arbitrary units) at a λex of 352, 370, and 388 nm, respectively, for BPI·BF2. Although the concentration of each complex was not kept constant, the maximum absorbance value for each sample was kept in the range of 0.04 and 0.05. This allowed for qualitative comparisons to be made between the different compounds. At the common λex of 352 nm, the maximum emission intensity for all complexes was on the same order of magnitude. At their λmax and longest λ of excitation, BPI·AlCl2 was found to be orders of magnitude more emissive than BPI and the other group 13 complexes, which were all found to have similar emission intensities at both λex. Moreover, the shapes of the emission curves were all similar to one another with the exception of BPI·AlCl2. From the UV–vis absorption and fluorescence spectra, it is apparent that BPI·AlCl2 differs significantly from the rest of the other complexes. Their dissimilar spectroscopic properties warrant further investigation. The fluorescence quantum yields (φ) for all compounds other than BPI·AlCl2 were measured to be in the range of 0.7–2.7% relative to a standard of 9,10-diphenylanthracene (Table S14 and Eq. S1 in
Fig. 3. Ellipsoid plot (50% probability) showing the structure and atom numbering scheme for the BPI·BF2 complex (CCDC deposition number: 904844). Hydrogen atoms have been omitted for clarity.
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Fig. 4. UV/Vis absorption spectra of BPI, BPI·BF2, BPI·AlCl2, BPI·GaCl2, and BPI·InCl2 in degassed dichloromethane solutions at room temperature.
Supplementary data). The very low values indicate that these compounds are essentially non-emissive. This finding is in accordance with reported quantum yields for a platinum(II) complex of BPI and its post-metallated derivatives, which are all in the range of 0.03– 3.79% in degassed dichloromethane solutions at ambient temperature [47]. It is interesting to note that BPI·BF2 is non-emissive although it shares a similar coordination motif to that found in the highly fluorescence BODIPYs [44,45]. An exploration into this difference is worthy of a future investigation. Quantum yields (φ) could not be measured for BPI·AlCl2 using a relative approach due to our inability to find a well characterized standard with absorption and emission bands that closely match BPI·AlCl2. We can however surmise that given its higher emission intensity (Fig. 5), its quantum yield is likely to be significantly higher. Conclusions. We have reported on the synthesis of boron (BPI·BF2), aluminum (BPI·AlCl2), gallium (BPI·GaCl2), and indium (BPI·GaCl2) complexes of 1,3-bis(2-pyridylimino)isoindoline (BPI). Their two-step synthesis began with the condensation reaction of o-phthalonitrile
Fig. 5. Fluorescence emission spectra (λex = 352 nm, highest and longest λmax of absorption as indicated) for BPI, BPI·BF2, BPI·AlCl2, BPI·GaCl2, and BPI·InCl2 in degassed dichloromethane solutions at room temperature.
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and 2-aminopyridine to form the BPI ligand, followed by complexation with the appropriate group 13 halide in the presence of an organic base. For BPI·AlCl2, BPI·GaCl2, and BPI·InCl2, the group 13 centers were found to coordinate with BPI in a tridentate N^N^N manner, preserving the symmetric nature of the BPI ligand. In the case of BPI·BF2, the boron atom was bound to BPI in a N^N manner, breaking the symmetry of the BPI ligand. The UV–visible absorption spectra for BPI·BF2, BPI·GaCl2, and BPI·nCl2 were broad and consisted of multiple absorption bands in the UV–visible (short wavelength) region, while a single broad peak in the UV region was observed for BPI·AlCl2. Fluorescence emission spectra were acquired at three wavelengths of excitation. Small differences in the emission spectra of BPI and its group 13 complexes were observed when excited at 352 nm. At the λmax and longest λ of absorption, BPI·AlCl2 was found to be much more emissive than the other complexes, which all had fluorescence emission intensities on the same order of magnitude. Very low fluorescence quantum yields in the range of 0.7–2.7% were measured for BPI, BPI·BF2, BPI·GaCl2, and BPI·InCl2. These values are in line with reported quantum yields of platinated BPIs. Optimization of the BPI complexation reaction, an examination into the spectroscopic differences of BPI·AlCl2 and the other group 13 complexes, and an investigation into the fluorescence dissimilarities of BPI·BF2 to BODIPY are the subject of future study. Acknowledgments The authors wish to acknowledge funding for this research from the Natural Sciences and Engineering Research Council (NSERC) of Canada. Appendix A. Supplementary material Supplementary data associated with this article including complete experimental methods, synthetic procedures, NMR spectra, detailed crystallographic data, fluorescence emission data, and fluorescence quantum yields can be found, in the online version, at http://dx.doi. org/10.1016/j.inoche.2012.11.020. References [1] J.A. Elvidge, R.P. Linstead, J. Chem. Soc. (1952) 5000–5007. [2] W.O. Siegl, J. Org. Chem. 42 (1977) 1872–1878. [3] J. Kaizer, G. Barath, G. Speier, M. Reglier, M. Giorgi, Inorg. Chem. Commun. 10 (2007) 292–294. [4] E. Balogh-Hergovich, G. Speier, M. Reglier, M. Giorgi, E. Kuzmann, A. Vertes, Inorg. Chem. Commun. 8 (2005) 457–459. [5] P.T. Selvi, H. Stoeckli-Evans, M.J. Palaniandavar, Inorg. Biochem. 99 (2005) 2110–2118. [6] R.J. Letcher, W. Zhang, C. Bensimon, R.J. Crutchley, Inorg. Chim. Acta 210 (1993) 183–191.
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