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Structural and spectroscopic characterizations of aluminum phenoxide
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 157 (2016) 238–243
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Structural and spectroscopic characterizations of aluminum phenoxide Gholamhossein Mohammadnezhad a,⁎, Mostafa M. Amini b, Hamid Reza Khavasi b, Winfried Plass c a b c
Department of Chemistry, Isfahan University of Technology, Isfahan 84156-83111, Iran Faculty of Chemistry, Shahid Beheshti University G.C., Tehran 1983963113, Iran Institute of Inorganic and Analytical Chemistry, Chair of Inorganic Chemistry II, Friedrich Schiller University Jena, Humboldtstr. 8, 07743 Jena, Germany
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Article history: Received 8 August 2015 Received in revised form 30 December 2015 Accepted 6 January 2016 Available online 7 January 2016 Keywords: Aluminum phenoxide Crystal structure Spectroscopy 27 Al NMR
a b s t r a c t A synthetic route to obtain crystalline aluminum phenoxide was established. Its molecular structure in solid-state and solution is unambiguously determined by single-crystal X-ray diffraction and 1H, 13C and 27Al NMR spectroscopy. The single-crystal X-ray analysis revealed the presence of the dimeric THF adduct [Al(OPh)3·THF]2 with a disordered trigonal bipyramidal geometry at the aluminum atom which is bonded to a THF ligand, two terminal and two bridging phenoxy groups (OPh). The solution behavior of the title compound was investigated by 27Al NMR in non-coordinating (CDCl3) as well as coordinating (THF) solvents at different temperatures. The obtained results indicate the presence of four- and five-coordinate species in solution. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Metal alkoxides are of general interest as promising materials in the synthesis of advanced functional materials [1–7]. In this regard, great interest is devoted to study the chemistry of this class of compounds [3, 4,8–12]. In particular, the determination of their chemical structures has attracted considerable attention, as this is an important starting point for the elucidation of reaction mechanisms of a wide variety of chemical processes in which they are involved [4,13,14]. Among metal alkoxides, aluminum alkoxides have been the subject of extensive studies within a wide range of applications, including catalysis, catalysis support, optics, and electronics [15–19]. Due to the importance of the molecular structure of aluminum alkoxides, many investigations on their preparation and structural characterization have been carried out [3,4]. Various coordination numbers and geometries are reported for aluminum alkoxides, which is controlled by factors such as electronic and steric effects of the ligands [3, 4]. In the presence of sterically demanding alkyl groups such as tertbutoxide, dimeric structure with a coordination number of four at the aluminum center is observed [20]. Whereas for aluminum ethoxide a polymeric structure is observed, in which the aluminum centers possess an increased coordination number of five, as the steric demand of the ligand is reduced [21]. For aluminum iso-propoxide, which has extensively been studied in earlier days, a trimeric structure was observed for a freshly prepared sample, in contrary to a tetrameric aggregate for aged samples [22–24]. ⁎ Corresponding author. E-mail addresses: [email protected], [email protected] (G. Mohammadnezhad).
http://dx.doi.org/10.1016/j.saa.2016.01.009 1386-1425/© 2016 Elsevier B.V. All rights reserved.
Despite the long-lasting utilization of aluminum aryloxides, and specifically metal derivatives of phenol in various industrial processes such as catalysts for synthesis of fragrance and flavoring materials, synthesis of new class of antioxidants, selective alkylation of phenols, lubricants, surfactants, disinfectants, fungicides, and insecticides, its molecular structure is still not unambiguously determined [25–28]. Aluminum phenoxide was synthesized and used differently in several chemical processes. It has been utilized in isolated or in situ generated forms [27,29–31]. In fact, the synthesis of aluminum phenoxide was first reported over 130 years ago [32]. In 1984 Kriz et al. based on 27Al NMR spectra and molecular weight measurements concluded the presence of dimeric and trimeric aggregates in solution [33]. Further characterization was provided by Gilje and coworkers [34] with 1H and 13 C NMR spectra, which showed that either the OPh groups are chemically equivalent or in fast exchange. Moreover, the mass spectrum revealed peaks that suggest the presence of at least trimeric aggregates. Based on these results an oligomeric structure of [Al(OPh)3]n has been proposed for aluminum phenoxide. Nevertheless, attempts to crystallize or purify aluminum phenoxide have not been successful, as it decomposes upon heating during distillation or sublimation [35]. However, the presence of tri- and tetranuclear aggregates in the solid state could be elucidated by 27Al MAS NMR [36]. To continue our exploration of aluminum compounds, we report here the preparation, purification, single-crystal structure determination, and solution behavior of aluminum phenoxide. 2. Experimental All reactions and manipulations were carried out under dry nitrogen atmosphere, using standard Schlenk techniques. Phenol was purchased from Merck Chemical Co. Solvents were dried with appropriate reagents
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prior to use. All other reagents obtained commercially and used as received. 2.1. Synthesis of [Al(OPh)3·THF]2 Aluminum phenoxide was prepared based on the modified procedure reported previously [34]. In a typical synthesis, a mixture of aluminum turnings (4.00 g, 148 mmol) and phenol (41.86 g, 448.8 mmol) was refluxed in the presence of Hg2Cl2 (0.10 g) in 200 mL of THF for four days (caution: the amount of evolved gas during the first few hours is substantial). Upon cooling to room temperature a large amount of crystals was formed. Unreacted aluminum turnings were removed by dissolution of the crystals in an excess amount of THF and subsequent filtration. Suitable crystals for single-crystal structure study were formed upon recrystallization of a concentrated solution at −10 °C. Crystals separated with frit filter and washed with toluene several times and dried under vacuum. Yield: 52%. 1H NMR (300 MHz, CDCl3, δ, ppm): 6.85 (d, Jd = 7.5 Hz, 12 H, Hortho), 6.95 (t, Jt = 7.2 Hz, 6 H, Hpara), 7.27 (t, Jt = 7.5 Hz, 12 H, Hmeta). 1H NMR (600 MHz, THF-d4, 27 °C, ppm): 6.00–7.50 (m, 30 H, Ph). 1H NMR (200 MHz, THF-d4, 60 °C, ppm): 6.70 (t, Jt = 7.2 Hz, 6 H, Hpara), 6.93 (d, Jd = 7.5 Hz, 12 H, Hortho), 6.11 (t, Jt = 7.8 Hz, 12 H, Hmeta).13C NMR (75 MHz, CDCl3, δ, ppm): 115.3 (Cortho), 120.8 (Cpara), 129.7 (Cmeta), 155.5 (Cipso). 27Al NMR (78 MHz CDCl3, ppm): 71.4. 27Al NMR (156 MHz, THF-d4, 27 °C, ppm): 31.5 and 77.6. 2.2. Analytical procedures
Table 1 Crystallographic data and structure refinement for [Al(OPh)3·THF]2. Empirical formula
C44H46Al2O8
Formula weight Temperature (K) Wavelength (Å) Crystal system Space group a (Å) b (Å) c (Å) α (°) β (°) γ (°) V (Å3) Z Absorption coefficient (mm−1) F(000) Crystal size (mm) Dc (g cm−3) θ Range for data Reflections collected Unique reflections [R(int)] Data/restraints/parameters Goodness-of-fit on F2 Final R indices [I N 2σ(I)]a
756.77 120 (2) 0.71073 Triclinic Pī 10.9890(7) 12.5160(7) 14.4604(8) 85.672(5) 80.611(5) 80.370(5) 1932.2 2 0.130 800 0.50 × 0.40 × 0.35 1.301 2.13–29.15 21,446 10,334 (0.1088) 10,334/0/487 1.067 R1 = 0.0894 wR2 = 0.1947 R1 = 0.1422 wR2 = 0.2199 1059013
R indices (all data) CCDC a
1
13
239
R1 = Σ||Fo | − |Fc || / Σ|Fo |, wR2 = [Σ(w(F2o − F2c )2) / Σw(F2o)2]1/2.
27
The H, C, and Al NMR spectra were obtained in CDCl3 and THFd4 (vs. Me4Si in ppm) using a Bruker AVANCE 200, 300, and 600 MHz spectrometers. 2.3. X-ray crystallography X-ray diffraction data were collected using a STOE IPDS-II diffractometer with graphite monochromated Mo-Kα radiation (λ = 0.71073 Å). A colorless block crystal with the dimensions 0.50 × 0.40 × 0.35 mm was mounted on a glass fiber and used for the data collection. The cell constants and orientation matrices for the data collection were obtained by least-squares refinement of diffraction data from 10,334 unique reflections, using the Stoe X-AREA software package [37]. Data were collected at 120 K in a series of ω scans in 1° oscillations to a maximum θ value of 29.15. A numerical absorption correction was applied in each case using the X-RED [38] and X-SHAPE software package [39]. The structures were solved by direct methods and subsequent difference Fourier maps, and then refined on F2 by full-matrix least-squares procedure using anisotropic displacement parameters [40]. Atomic structure factors were obtained from the International Tables for X-ray Crystallography [41]. All refinements were performed using the X-STEP32 crystallographic software package [42]. Molecular graphics were drawn by ORTEP and DIAMOND programs [43,44]. Crystal data and refinement details of the X-ray analyses are listed in Table 1. 3. Results and discussion Aluminum phenoxide can be prepared by reaction of metallic aluminum and phenol in the presence of a catalytic amount of mercuric chloride or iodine in appropriate solvents such as THF and subsequent evaporation of solvents [34,45]. Distillation of the product was not possible due to decomposition upon heating. Therefore, the solvent was not evaporated after completion of reflux, instead crystals of aluminum phenoxide were isolated by cooling the reaction mixture to room temperature. Suitable crystals for X-ray analysis were formed upon recrystallization of the filtered crystals from THF. Aluminum phenoxide was crystallized in the triclinic crystal system with space group Pī as a dimer with one coordinating THF molecule at each aluminum center leading to the formula of [Al(OPh)3·THF]2. An
ORTEP drawing of the complex is illustrated in Fig. 1. Selected bond lengths and angles are presented in Table 2. The aluminum atoms are five coordinates which four of the coordination sites were occupied by two bridging and two terminal phenoxy groups. The fifth coordinating position is occupied by a THF molecule. The asymmetric unit consists of two independent molecules. These centrosymmetric binuclear molecules consist of edge-sharing distorted trigonal-bipyramidal units that are linked by two phenoxo ligands (Fig. 2). Two terminal phenoxy groups along with one of the bridging phenoxy groups are positioned at equatorial sites, whereas the other bridging phenoxy and a THF molecule are positioned at the axial sites of the trigonal bipyramid. The angle between the axial ligands is about 166° resulting in a τ parameter of 0.75 for the distorted five-coordinate polyhedral (Table 2). 27 Al NMR spectroscopy has proven to be a suitable method to determine the structures and the degree of aggregation in solution, as it usually gives rise to resonances with suitable width and strength [11,33,45]. Due to quadruple moment of the 27Al nucleus the widths of 27Al NMR signals strongly depend on the electrical-field gradient at aluminum center which is related to the coordination environment. Aluminum compounds with the coordination number of six have the least gradient leading to sharp signals, while this gradient, along with signal width, increases gradually as the coordination number alters to four, five, and three. Moreover, possible dynamic effects and bulkiness of the ligands have a considerable impact on the broadening of the signals. Consequently, for lower molecular symmetries such as four and five-coordinate aluminum compounds higher magnetic fields are favorable. The spectral data for lower symmetries obtained at relatively low magnetic fields made it difficult to observe very broad signals or even assign their position correctly [33]. In 1984, Kriz and co-workers presented 27Al NMR spectral data for aluminum phenoxide which measured at a spectrometer frequency of 52.13 MHz [33]. They report two signals at 29 and 35 ppm at 22 °C, and two signals at 30 and 44 ppm at 70 °C. However, 1H and 13C NMR data in the literature were only obtained for grayish amorphous samples in which collected by complete evaporation of the reaction mixture and washing with toluene [34]. Moreover, due to solubility problems, these spectra could only be measured for solutions in a mixture of methanol (a non-favorable protic solvent) and DMSO. Therefore, we reinvestigated
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Fig. 1. Labeling scheme and thermal ellipsoids at the 50% probability level for [Al(OPh)3·THF]2.
the NMR spectroscopic data using the colorless crystalline material obtained by our modified synthetic procedure. Fig. 3 presents the 27Al NMR spectra (78 MHz, CDCl3) of [Al(OPh)3·THF]2 at 20 and 50 °C, respectively. At 20 °C the presence of one broad signal at 70 ppm is an indication of four-coordinate aluminum centers. However, the signal is not symmetric and has a shoulder on the upfield side, which probably can be assigned to a second overlapping signal with lower intensity. This could be evidenced by measuring the spectrum at a higher temperature of 50 °C, for which the second
Table 2 Selected bond lengths (Å) and angles (°) for [Al(OPh)3·THF]2. Bond lengths C(1)–O(1) C(7)–O(2) C(13)–O(3) C(19)–O(4) C(22)–O(4) C(23)–O(5) C(29)–O(6) C(35)–O(7) C(41)–O(8) C(44)–O(8) Al(1)–O(2)
1.340(4) 1.348(4) 1.394(4) 1.473(4) 1.463(4) 1.344(4) 1.393(4) 1.348(4) 1.469(4) 1.462(4) 1.734(3)
Al(1)–O(1) Al(1)–O(3) Al(1)–O(3)#1 Al(1)–O(4) Al(2)–O(7) Al(2)–O(5) Al(2)–O(6) Al(2)–O(6)#2 Al(2)–O(8) Al(2)–Al(2)#2 O(3)–Al(1)#1 O(6)–Al(2)#2
1.741(2) 1.833(2) 1.936(2) 1.958(2) 1.734(2) 1.735(2) 1.825(2) 1.936(2) 1.949(2) 2.9497(18) 1.936(2) 1.937(2)
O(7)–Al(2)–O(6)#2 O(5)–Al(2)–O(6)#2 O(6)–Al(2)–O(6)#2 O(7)–Al(2)–O(8) O(5)–Al(2)–O(8) O(6)–Al(2)–O(8) O(6)#2–Al(2)–O(8) Al(1)–O(3)–Al(1)#1 Al(2)–O(6)–Al(2)#2 C(1)–O(1)–Al(1) C(7)–O(2)–Al(1) C(23)–O(5)–Al(2) C(35)–O(7)–Al(2)
99.92(11) 90.35(11) 76.73(11) 89.73(10) 92.46(11) 90.33(10) 166.42(10) 104.46(10) 103.27(11) 138.7(2) 140.7(2) 134.0(2) 131.5(2)
signal generating the shoulder becomes more pronounced at a position of around 36 ppm, which corresponds to aluminum centers with a coordination number of five. To probe the influence of the coordinating THF molecules, crystals were placed under high vacuum for 1 h to remove all volatiles and the 27 Al NMR spectra were measured for this sample at two different temperatures. The 27Al NMR spectrum measured at 20 °C is depicted in Fig. 4. It shows a symmetric signal at 71 ppm which is at a position similar to what was observed in the spectrum of the untreated sample (Fig. 3). Increasing the temperature to 50 °C did not significantly change the 27Al NMR spectrum (see Fig. 5). This is in contrasts with the observed behavior for the untreated sample for which two signals were observed at a higher temperature (Fig. 3). This result is consistent with the presence of four-coordinate aluminum centers and indicates the absence of five-coordinate species under these conditions. In particular, the symmetric shape of the signals indicates the absence of any signal with significant intensity that could be attributed to four-coordinate species.
Bond angles O(2)–Al(1)–O(1) O(2)–Al(1)–O(3) O(1)–Al(1)–O(3) O(2)–Al(1)–O(3)#1 O(1)–Al(1)–O(3)#1 O(3)–Al(1)–O(3)#1 O(2)–Al(1)–O(4) O(1)–Al(1)–O(4) O(3)–Al(1)–O(4) O(3)#1–Al(1)–O(4) O(7)–Al(2)–O(5) O(7)–Al(2)–O(6) O(5)–Al(2)–O(6)
118.77(12) 120.34(12) 120.69(12) 100.64(11) 89.70(11) 75.54(10) 90.21(11) 93.23(11) 90.98(10) 165.70(10) 122.62(13) 118.08(12) 119.23(11)
Symmetry transformations used to generate equivalent atoms: #1 −x + 1, −y + 2, −z. #2 −x, −y + 1, −z + 1.
Fig. 2. Polyhedral perspective view for [Al(OPh)3·THF]2.
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Fig. 5. 27Al NMR spectrum of vacuum dried of aluminum phenoxide (78 MHz, CDCl3, 50 °C).
reduced overlap of the signals due to somewhat sharper signals at higher temperatures. Fig. 3. 27Al NMR spectra of [Al(OPh)3·THF]2 measured at 78 MHz in CDCl3 a) at 20 °C and b) at 50 °C.
To further elucidate the solution behavior of [Al(OPh)3·THF]2 the 27 Al NMR spectrum was measured in THF-d8 as a coordinating solvent. The spectrum is obtained at room temperature at a magnetic field of 156 MHz, and the result is depicted in Fig. 6. Two distinct signals at 29 and 65 ppm are observed, which can be assigned to five and fourcoordinate aluminum, respectively. However, overlap of the two signals hampers the exact determination of the relative ratio of the signals. Therefore, a deconvolution of overlapped signals was performed by subsequent integration of the obtained signal areas utilizing the topspin NMR software. The deconvolved 27Al NMR signals are depicted in Fig. 7 together with their superposition. The integration leads to a ratio of about 3:2 for the five to four-coordinate aluminum centers present in solution. For the solution of aluminum phenoxide in THF also the temperature dependency of the 27Al NMR spectra was explored and the results are illustrated in Fig. 8. At first glance, the intensity of the signal at 65 ppm seems to increase with increasing temperature, whereas the intensity of the signal at 29 ppm seems to decrease. This might suggest a change in the ratio between the four- and five-coordinate species in equilibrium. However, a detailed analysis, including the deconvolution of the signals revealed no significant changes, neither as the position of the signals nor as their integration ratio is concerned. The overall observed changes upon increasing the temperature can solely be attributed to the
Fig. 4. 27Al NMR spectrum of vacuum dried aluminum phenoxide (78 MHz, CDCl3, 20 °C).
4. Conclusion The molecular structure of the THF adduct of aluminum phenoxide has been unambiguously determined in solid state and solution. In the solid state, it possesses a dimeric structure in which each aluminum center has trigonal bipyramidal geometry. In solution, four- and fivecoordinate aluminum species are present. In non-coordinating solvents, such as chloroform, the species containing four-coordinate aluminum are dominant, while in coordinating solvents such as THF, the fivecoordinate species are favored. Temperature dependency of the equilibrium present in THF was investigated and the results show that within the accessible temperature range the ratio between four- and fivecoordinate species is not affected. The observed differences can solely be attributed to sharper signals leading to a less overlap. Supplementary material Crystallographic data of aluminum phenoxide has been deposited with the Cambridge Crystallographic Data Centre, CCDC no. 1059013. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via http://www.ccdc.cam.ac.uk/data-request/cif. Acknowledgments The authors gratefully acknowledge the Iran National Science Foundation (INSF), Isfahan University of Technology, and the VicePresident's of Office for Research Affairs Shahid Beheshti University for
Fig. 6. 27Al NMR spectrum of [Al(OPh)3·THF]2 at 156 MHz in THF-d8.
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Fig. 7. Deconvolution result for the 27Al NMR spectrum of [Al(OPh)3·THF]2 depicted in Fig. 6.
Fig. 8. Variable temperature 27Al NMR spectra for [Al(OPh)3·THF]2 measured at 156 MHz in THF-d8 in the range of 27 to 57 °C in intervals of 10 °C.
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Structural and spectroscopic characterizations of aluminum phenoxide Gholamhossein Mohammadnezhad a,⁎, Mostafa M. Amini b, Hamid Reza Khavasi b, Winfried Plass c a b c
Department of Chemistry, Isfahan University of Technology, Isfahan 84156-83111, Iran Faculty of Chemistry, Shahid Beheshti University G.C., Tehran 1983963113, Iran Institute of Inorganic and Analytical Chemistry, Chair of Inorganic Chemistry II, Friedrich Schiller University Jena, Humboldtstr. 8, 07743 Jena, Germany
a r t i c l e
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Article history: Received 8 August 2015 Received in revised form 30 December 2015 Accepted 6 January 2016 Available online 7 January 2016 Keywords: Aluminum phenoxide Crystal structure Spectroscopy 27 Al NMR
a b s t r a c t A synthetic route to obtain crystalline aluminum phenoxide was established. Its molecular structure in solid-state and solution is unambiguously determined by single-crystal X-ray diffraction and 1H, 13C and 27Al NMR spectroscopy. The single-crystal X-ray analysis revealed the presence of the dimeric THF adduct [Al(OPh)3·THF]2 with a disordered trigonal bipyramidal geometry at the aluminum atom which is bonded to a THF ligand, two terminal and two bridging phenoxy groups (OPh). The solution behavior of the title compound was investigated by 27Al NMR in non-coordinating (CDCl3) as well as coordinating (THF) solvents at different temperatures. The obtained results indicate the presence of four- and five-coordinate species in solution. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Metal alkoxides are of general interest as promising materials in the synthesis of advanced functional materials [1–7]. In this regard, great interest is devoted to study the chemistry of this class of compounds [3, 4,8–12]. In particular, the determination of their chemical structures has attracted considerable attention, as this is an important starting point for the elucidation of reaction mechanisms of a wide variety of chemical processes in which they are involved [4,13,14]. Among metal alkoxides, aluminum alkoxides have been the subject of extensive studies within a wide range of applications, including catalysis, catalysis support, optics, and electronics [15–19]. Due to the importance of the molecular structure of aluminum alkoxides, many investigations on their preparation and structural characterization have been carried out [3,4]. Various coordination numbers and geometries are reported for aluminum alkoxides, which is controlled by factors such as electronic and steric effects of the ligands [3, 4]. In the presence of sterically demanding alkyl groups such as tertbutoxide, dimeric structure with a coordination number of four at the aluminum center is observed [20]. Whereas for aluminum ethoxide a polymeric structure is observed, in which the aluminum centers possess an increased coordination number of five, as the steric demand of the ligand is reduced [21]. For aluminum iso-propoxide, which has extensively been studied in earlier days, a trimeric structure was observed for a freshly prepared sample, in contrary to a tetrameric aggregate for aged samples [22–24]. ⁎ Corresponding author. E-mail addresses: [email protected], [email protected] (G. Mohammadnezhad).
http://dx.doi.org/10.1016/j.saa.2016.01.009 1386-1425/© 2016 Elsevier B.V. All rights reserved.
Despite the long-lasting utilization of aluminum aryloxides, and specifically metal derivatives of phenol in various industrial processes such as catalysts for synthesis of fragrance and flavoring materials, synthesis of new class of antioxidants, selective alkylation of phenols, lubricants, surfactants, disinfectants, fungicides, and insecticides, its molecular structure is still not unambiguously determined [25–28]. Aluminum phenoxide was synthesized and used differently in several chemical processes. It has been utilized in isolated or in situ generated forms [27,29–31]. In fact, the synthesis of aluminum phenoxide was first reported over 130 years ago [32]. In 1984 Kriz et al. based on 27Al NMR spectra and molecular weight measurements concluded the presence of dimeric and trimeric aggregates in solution [33]. Further characterization was provided by Gilje and coworkers [34] with 1H and 13 C NMR spectra, which showed that either the OPh groups are chemically equivalent or in fast exchange. Moreover, the mass spectrum revealed peaks that suggest the presence of at least trimeric aggregates. Based on these results an oligomeric structure of [Al(OPh)3]n has been proposed for aluminum phenoxide. Nevertheless, attempts to crystallize or purify aluminum phenoxide have not been successful, as it decomposes upon heating during distillation or sublimation [35]. However, the presence of tri- and tetranuclear aggregates in the solid state could be elucidated by 27Al MAS NMR [36]. To continue our exploration of aluminum compounds, we report here the preparation, purification, single-crystal structure determination, and solution behavior of aluminum phenoxide. 2. Experimental All reactions and manipulations were carried out under dry nitrogen atmosphere, using standard Schlenk techniques. Phenol was purchased from Merck Chemical Co. Solvents were dried with appropriate reagents
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prior to use. All other reagents obtained commercially and used as received. 2.1. Synthesis of [Al(OPh)3·THF]2 Aluminum phenoxide was prepared based on the modified procedure reported previously [34]. In a typical synthesis, a mixture of aluminum turnings (4.00 g, 148 mmol) and phenol (41.86 g, 448.8 mmol) was refluxed in the presence of Hg2Cl2 (0.10 g) in 200 mL of THF for four days (caution: the amount of evolved gas during the first few hours is substantial). Upon cooling to room temperature a large amount of crystals was formed. Unreacted aluminum turnings were removed by dissolution of the crystals in an excess amount of THF and subsequent filtration. Suitable crystals for single-crystal structure study were formed upon recrystallization of a concentrated solution at −10 °C. Crystals separated with frit filter and washed with toluene several times and dried under vacuum. Yield: 52%. 1H NMR (300 MHz, CDCl3, δ, ppm): 6.85 (d, Jd = 7.5 Hz, 12 H, Hortho), 6.95 (t, Jt = 7.2 Hz, 6 H, Hpara), 7.27 (t, Jt = 7.5 Hz, 12 H, Hmeta). 1H NMR (600 MHz, THF-d4, 27 °C, ppm): 6.00–7.50 (m, 30 H, Ph). 1H NMR (200 MHz, THF-d4, 60 °C, ppm): 6.70 (t, Jt = 7.2 Hz, 6 H, Hpara), 6.93 (d, Jd = 7.5 Hz, 12 H, Hortho), 6.11 (t, Jt = 7.8 Hz, 12 H, Hmeta).13C NMR (75 MHz, CDCl3, δ, ppm): 115.3 (Cortho), 120.8 (Cpara), 129.7 (Cmeta), 155.5 (Cipso). 27Al NMR (78 MHz CDCl3, ppm): 71.4. 27Al NMR (156 MHz, THF-d4, 27 °C, ppm): 31.5 and 77.6. 2.2. Analytical procedures
Table 1 Crystallographic data and structure refinement for [Al(OPh)3·THF]2. Empirical formula
C44H46Al2O8
Formula weight Temperature (K) Wavelength (Å) Crystal system Space group a (Å) b (Å) c (Å) α (°) β (°) γ (°) V (Å3) Z Absorption coefficient (mm−1) F(000) Crystal size (mm) Dc (g cm−3) θ Range for data Reflections collected Unique reflections [R(int)] Data/restraints/parameters Goodness-of-fit on F2 Final R indices [I N 2σ(I)]a
756.77 120 (2) 0.71073 Triclinic Pī 10.9890(7) 12.5160(7) 14.4604(8) 85.672(5) 80.611(5) 80.370(5) 1932.2 2 0.130 800 0.50 × 0.40 × 0.35 1.301 2.13–29.15 21,446 10,334 (0.1088) 10,334/0/487 1.067 R1 = 0.0894 wR2 = 0.1947 R1 = 0.1422 wR2 = 0.2199 1059013
R indices (all data) CCDC a
1
13
239
R1 = Σ||Fo | − |Fc || / Σ|Fo |, wR2 = [Σ(w(F2o − F2c )2) / Σw(F2o)2]1/2.
27
The H, C, and Al NMR spectra were obtained in CDCl3 and THFd4 (vs. Me4Si in ppm) using a Bruker AVANCE 200, 300, and 600 MHz spectrometers. 2.3. X-ray crystallography X-ray diffraction data were collected using a STOE IPDS-II diffractometer with graphite monochromated Mo-Kα radiation (λ = 0.71073 Å). A colorless block crystal with the dimensions 0.50 × 0.40 × 0.35 mm was mounted on a glass fiber and used for the data collection. The cell constants and orientation matrices for the data collection were obtained by least-squares refinement of diffraction data from 10,334 unique reflections, using the Stoe X-AREA software package [37]. Data were collected at 120 K in a series of ω scans in 1° oscillations to a maximum θ value of 29.15. A numerical absorption correction was applied in each case using the X-RED [38] and X-SHAPE software package [39]. The structures were solved by direct methods and subsequent difference Fourier maps, and then refined on F2 by full-matrix least-squares procedure using anisotropic displacement parameters [40]. Atomic structure factors were obtained from the International Tables for X-ray Crystallography [41]. All refinements were performed using the X-STEP32 crystallographic software package [42]. Molecular graphics were drawn by ORTEP and DIAMOND programs [43,44]. Crystal data and refinement details of the X-ray analyses are listed in Table 1. 3. Results and discussion Aluminum phenoxide can be prepared by reaction of metallic aluminum and phenol in the presence of a catalytic amount of mercuric chloride or iodine in appropriate solvents such as THF and subsequent evaporation of solvents [34,45]. Distillation of the product was not possible due to decomposition upon heating. Therefore, the solvent was not evaporated after completion of reflux, instead crystals of aluminum phenoxide were isolated by cooling the reaction mixture to room temperature. Suitable crystals for X-ray analysis were formed upon recrystallization of the filtered crystals from THF. Aluminum phenoxide was crystallized in the triclinic crystal system with space group Pī as a dimer with one coordinating THF molecule at each aluminum center leading to the formula of [Al(OPh)3·THF]2. An
ORTEP drawing of the complex is illustrated in Fig. 1. Selected bond lengths and angles are presented in Table 2. The aluminum atoms are five coordinates which four of the coordination sites were occupied by two bridging and two terminal phenoxy groups. The fifth coordinating position is occupied by a THF molecule. The asymmetric unit consists of two independent molecules. These centrosymmetric binuclear molecules consist of edge-sharing distorted trigonal-bipyramidal units that are linked by two phenoxo ligands (Fig. 2). Two terminal phenoxy groups along with one of the bridging phenoxy groups are positioned at equatorial sites, whereas the other bridging phenoxy and a THF molecule are positioned at the axial sites of the trigonal bipyramid. The angle between the axial ligands is about 166° resulting in a τ parameter of 0.75 for the distorted five-coordinate polyhedral (Table 2). 27 Al NMR spectroscopy has proven to be a suitable method to determine the structures and the degree of aggregation in solution, as it usually gives rise to resonances with suitable width and strength [11,33,45]. Due to quadruple moment of the 27Al nucleus the widths of 27Al NMR signals strongly depend on the electrical-field gradient at aluminum center which is related to the coordination environment. Aluminum compounds with the coordination number of six have the least gradient leading to sharp signals, while this gradient, along with signal width, increases gradually as the coordination number alters to four, five, and three. Moreover, possible dynamic effects and bulkiness of the ligands have a considerable impact on the broadening of the signals. Consequently, for lower molecular symmetries such as four and five-coordinate aluminum compounds higher magnetic fields are favorable. The spectral data for lower symmetries obtained at relatively low magnetic fields made it difficult to observe very broad signals or even assign their position correctly [33]. In 1984, Kriz and co-workers presented 27Al NMR spectral data for aluminum phenoxide which measured at a spectrometer frequency of 52.13 MHz [33]. They report two signals at 29 and 35 ppm at 22 °C, and two signals at 30 and 44 ppm at 70 °C. However, 1H and 13C NMR data in the literature were only obtained for grayish amorphous samples in which collected by complete evaporation of the reaction mixture and washing with toluene [34]. Moreover, due to solubility problems, these spectra could only be measured for solutions in a mixture of methanol (a non-favorable protic solvent) and DMSO. Therefore, we reinvestigated
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Fig. 1. Labeling scheme and thermal ellipsoids at the 50% probability level for [Al(OPh)3·THF]2.
the NMR spectroscopic data using the colorless crystalline material obtained by our modified synthetic procedure. Fig. 3 presents the 27Al NMR spectra (78 MHz, CDCl3) of [Al(OPh)3·THF]2 at 20 and 50 °C, respectively. At 20 °C the presence of one broad signal at 70 ppm is an indication of four-coordinate aluminum centers. However, the signal is not symmetric and has a shoulder on the upfield side, which probably can be assigned to a second overlapping signal with lower intensity. This could be evidenced by measuring the spectrum at a higher temperature of 50 °C, for which the second
Table 2 Selected bond lengths (Å) and angles (°) for [Al(OPh)3·THF]2. Bond lengths C(1)–O(1) C(7)–O(2) C(13)–O(3) C(19)–O(4) C(22)–O(4) C(23)–O(5) C(29)–O(6) C(35)–O(7) C(41)–O(8) C(44)–O(8) Al(1)–O(2)
1.340(4) 1.348(4) 1.394(4) 1.473(4) 1.463(4) 1.344(4) 1.393(4) 1.348(4) 1.469(4) 1.462(4) 1.734(3)
Al(1)–O(1) Al(1)–O(3) Al(1)–O(3)#1 Al(1)–O(4) Al(2)–O(7) Al(2)–O(5) Al(2)–O(6) Al(2)–O(6)#2 Al(2)–O(8) Al(2)–Al(2)#2 O(3)–Al(1)#1 O(6)–Al(2)#2
1.741(2) 1.833(2) 1.936(2) 1.958(2) 1.734(2) 1.735(2) 1.825(2) 1.936(2) 1.949(2) 2.9497(18) 1.936(2) 1.937(2)
O(7)–Al(2)–O(6)#2 O(5)–Al(2)–O(6)#2 O(6)–Al(2)–O(6)#2 O(7)–Al(2)–O(8) O(5)–Al(2)–O(8) O(6)–Al(2)–O(8) O(6)#2–Al(2)–O(8) Al(1)–O(3)–Al(1)#1 Al(2)–O(6)–Al(2)#2 C(1)–O(1)–Al(1) C(7)–O(2)–Al(1) C(23)–O(5)–Al(2) C(35)–O(7)–Al(2)
99.92(11) 90.35(11) 76.73(11) 89.73(10) 92.46(11) 90.33(10) 166.42(10) 104.46(10) 103.27(11) 138.7(2) 140.7(2) 134.0(2) 131.5(2)
signal generating the shoulder becomes more pronounced at a position of around 36 ppm, which corresponds to aluminum centers with a coordination number of five. To probe the influence of the coordinating THF molecules, crystals were placed under high vacuum for 1 h to remove all volatiles and the 27 Al NMR spectra were measured for this sample at two different temperatures. The 27Al NMR spectrum measured at 20 °C is depicted in Fig. 4. It shows a symmetric signal at 71 ppm which is at a position similar to what was observed in the spectrum of the untreated sample (Fig. 3). Increasing the temperature to 50 °C did not significantly change the 27Al NMR spectrum (see Fig. 5). This is in contrasts with the observed behavior for the untreated sample for which two signals were observed at a higher temperature (Fig. 3). This result is consistent with the presence of four-coordinate aluminum centers and indicates the absence of five-coordinate species under these conditions. In particular, the symmetric shape of the signals indicates the absence of any signal with significant intensity that could be attributed to four-coordinate species.
Bond angles O(2)–Al(1)–O(1) O(2)–Al(1)–O(3) O(1)–Al(1)–O(3) O(2)–Al(1)–O(3)#1 O(1)–Al(1)–O(3)#1 O(3)–Al(1)–O(3)#1 O(2)–Al(1)–O(4) O(1)–Al(1)–O(4) O(3)–Al(1)–O(4) O(3)#1–Al(1)–O(4) O(7)–Al(2)–O(5) O(7)–Al(2)–O(6) O(5)–Al(2)–O(6)
118.77(12) 120.34(12) 120.69(12) 100.64(11) 89.70(11) 75.54(10) 90.21(11) 93.23(11) 90.98(10) 165.70(10) 122.62(13) 118.08(12) 119.23(11)
Symmetry transformations used to generate equivalent atoms: #1 −x + 1, −y + 2, −z. #2 −x, −y + 1, −z + 1.
Fig. 2. Polyhedral perspective view for [Al(OPh)3·THF]2.
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Fig. 5. 27Al NMR spectrum of vacuum dried of aluminum phenoxide (78 MHz, CDCl3, 50 °C).
reduced overlap of the signals due to somewhat sharper signals at higher temperatures. Fig. 3. 27Al NMR spectra of [Al(OPh)3·THF]2 measured at 78 MHz in CDCl3 a) at 20 °C and b) at 50 °C.
To further elucidate the solution behavior of [Al(OPh)3·THF]2 the 27 Al NMR spectrum was measured in THF-d8 as a coordinating solvent. The spectrum is obtained at room temperature at a magnetic field of 156 MHz, and the result is depicted in Fig. 6. Two distinct signals at 29 and 65 ppm are observed, which can be assigned to five and fourcoordinate aluminum, respectively. However, overlap of the two signals hampers the exact determination of the relative ratio of the signals. Therefore, a deconvolution of overlapped signals was performed by subsequent integration of the obtained signal areas utilizing the topspin NMR software. The deconvolved 27Al NMR signals are depicted in Fig. 7 together with their superposition. The integration leads to a ratio of about 3:2 for the five to four-coordinate aluminum centers present in solution. For the solution of aluminum phenoxide in THF also the temperature dependency of the 27Al NMR spectra was explored and the results are illustrated in Fig. 8. At first glance, the intensity of the signal at 65 ppm seems to increase with increasing temperature, whereas the intensity of the signal at 29 ppm seems to decrease. This might suggest a change in the ratio between the four- and five-coordinate species in equilibrium. However, a detailed analysis, including the deconvolution of the signals revealed no significant changes, neither as the position of the signals nor as their integration ratio is concerned. The overall observed changes upon increasing the temperature can solely be attributed to the
Fig. 4. 27Al NMR spectrum of vacuum dried aluminum phenoxide (78 MHz, CDCl3, 20 °C).
4. Conclusion The molecular structure of the THF adduct of aluminum phenoxide has been unambiguously determined in solid state and solution. In the solid state, it possesses a dimeric structure in which each aluminum center has trigonal bipyramidal geometry. In solution, four- and fivecoordinate aluminum species are present. In non-coordinating solvents, such as chloroform, the species containing four-coordinate aluminum are dominant, while in coordinating solvents such as THF, the fivecoordinate species are favored. Temperature dependency of the equilibrium present in THF was investigated and the results show that within the accessible temperature range the ratio between four- and fivecoordinate species is not affected. The observed differences can solely be attributed to sharper signals leading to a less overlap. Supplementary material Crystallographic data of aluminum phenoxide has been deposited with the Cambridge Crystallographic Data Centre, CCDC no. 1059013. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via http://www.ccdc.cam.ac.uk/data-request/cif. Acknowledgments The authors gratefully acknowledge the Iran National Science Foundation (INSF), Isfahan University of Technology, and the VicePresident's of Office for Research Affairs Shahid Beheshti University for
Fig. 6. 27Al NMR spectrum of [Al(OPh)3·THF]2 at 156 MHz in THF-d8.
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Fig. 7. Deconvolution result for the 27Al NMR spectrum of [Al(OPh)3·THF]2 depicted in Fig. 6.
Fig. 8. Variable temperature 27Al NMR spectra for [Al(OPh)3·THF]2 measured at 156 MHz in THF-d8 in the range of 27 to 57 °C in intervals of 10 °C.
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