Adsorption and Dissociation of Ammonia Borane Outside and Inside

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Adsorption and Dissociation of Ammonia Borane Outside and Inside Single-Walled Carbon Nanotubes: A Density Functional Theory Study Chenghua Sun,*,† Aijun Du,† Xiangdong Yao,‡ and Sean C. Smith*,† †

Centre for Computational Molecular Science, Australia Institute for Bioengineering and Nanotechnology, The University of Queensland, Brisbane, Queensland 4072, Australia ‡ Queensland Micro- and Nanotechnology Centre, Griffith University, Nathan, Queensland 4111, Australia ABSTRACT: Amonia borane (AB) has been identified as a potential candidate highcapacity hydrogen storage material. This work probes the adsorption and dissociation of AB inside and outside single-walled carbon nanotubes (SWCNTs) within the framework of density functional theory. The dissociation barriers of AB have been calculated and compared with that of the isolated AB molecule. On the basis of the present calculations, no notable improvement results from SWCNT confinement; on the contrary, the dissociation barrier slightly increases with respect to isolated AB.

1. INTRODUCTION Developing highly efficient materials for hydrogen storage is one of the key steps for hydrogen economy. In recent years, ammonia borane (AB), NH3BH3, has been identified as a promising candidate due to its high storage capacity (up to 19.6 wt %), moderate operating temperature, exothermic nature of decomposition, etc.1
6 Moreover, its adsorption/desorption performance can be further improved through metallic doping or catalysis,7
16 functionization,17
19 or nanosized confinement.20
24 For those approaches, nanosized confinement is particularly interesting since the chemical component of AB has no change while both the kinetics and the dynamics of hydrogen release can be improved greatly.20
22 For instance, the operation temperature can be dramatically reduced through confinement within mesoporous silica or carbon.20,21 When AB is inserted into mesoporous silica, the energy barrier for the release of hydrogen molecules (H2) can be reduced by ∼120 kJ/mol with respect to neat AB.20 Similar improvement has been also obtained with mesoporous carbon (CMK-3) in our group:21 7 wt % hydrogen can be released from AB loaded in the 5% Li-doped CMK-3 framework at ∼60 °C. More recently, it was found that confinement within a metal organic framework can dramatically improve the kinetics of hydrogen release from AB and effectively eliminate the generation of ammonia.22 Meanwhile, the physical/chemical origin of such improvements is not yet clear. Generally, when AB is confined by nanoscaffolds, two typical factors should be considered, including a possible confinement effect and a possible functional group effect. In the former case, the reaction is confined in a limited space; therefore, the distributions of electrons and the movement of atoms are constrained, which may change the energetics of reactants, products, and transition states and thus lead to novel changes of reaction kinetics/dynamics. Nanoscaffolds can provide such r 2011 American Chemical Society

confinement for AB dissociation, and as mentioned above, it is true that lower-temperature hydrogen release can be obtained if nanosized confinement is applied.20,21 In the latter case, specific functional groups/ions play an intrinsically chemical role. For instance, the H2 binding affinity can be enhanced by allowing H2 to interact with coordinatively unsaturated or partially charged light-weight metal centers that are characterized by electrostatic charge
quadrupole and charge
induced dipole interactions, and sometimes, charge-transfer effects like forward donation of the s electron density of H2 to the metal.7
10 A typical example is alkali-metal amidoboranes,7
10 LiNH2BH3 and NaNH2BH3, both of which can release ∼10.9 and ∼7.5 wt % hydrogen, respectively, at ∼90 °C, much better than neat AB (∼12% at 150 °C). In the literature, researchers often claim that the improvement associated with nanoscaffold confinements is a combination of functional groups and nanosized confinement.20
24 For instance, the confinement of platinum nanoparticles by singlewalled carbon nanotubes (SWCNTs) can dramatically reduce the dehydrogenation temperature of AB, and it is believed that the combination of platinum nanoparticle and nanotube confinement plays the crucial role.24 In our early work, it was also believed that both the functional groups of mesoporous carbon (CMK-3) and the pores play key roles on AB dehydrogenation.21 In fact, the catalytic effect of carbon-based nanostructures for hydrogen storage or release has been recognized for several years.25
28 For example, it has been reported that SWCNTs can dramatically improve the hydrogen release from MgH225 and LiBH4.26 Nanoporous carbon, graphene, and C60 have also been Received: April 18, 2011 Revised: May 18, 2011 Published: May 26, 2011 12580

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Figure 1. Structural models of AB@SWCNT: (a) SWCNT with open ends, and (b) SWCNT periodic supercell of SWCNT. Carbon, nitrogen, boron, and hydrogen are represented as gray, blue, pink, and white spheres.

tested and similar improvement with SWCNTs has been found.28 It seems that SWCNTs are particularly effective, probably due to the confinement associated with the tube shape or the unique hybridization of carbon.28 But in most cases, SWCNTs are added through ball-milling,28 which may destroy the tubular shape completely. So it is difficult to be certain that the catalytic role results from the nanotube confinement. Another example is borane nitrogen nanotubes, which have been shown to facilitate the release of hydrogen from AB at lower temperature, with minimal induction time and less exothermicity.29 However, the above improvement was obtained with AB being only adsorbed on the surface, rather than inside the nanotube.29 Moreover, ballmilled hexagonal BN additive can also decrease the onset temperature for hydrogen release,30 suggesting that the observed improvement may result from the unique B
N bonding (i.e., a partial chemical effect), although it does not rule out the possibility that borane nitrogen nanotube confinement might improve the dehydrogenation dynamics of AB. Hence, the effect of nanotube confinement on hydrogen release from AB is not yet clear. To clarify the confinement effect, the major challenge comes from two aspects: (i) the AB molecule, which tends to form large clusters due to intramolecular hydrogen bonding, may fail to get into the nanotube channels under its decomposition temperature; and (ii) nanotubes often contain various functional groups at the open ends or on the sidewall, and thus it is difficult to eliminate the effect of those functional groups and isolate the confinement effect. Theoretical modeling, however, may offer a model to distinguish those effects from each other. For example, perfect SWCNTs can be employed to hold AB molecules, with the sidewalls (without any functional groups) as a “pure” confining framework, in which case the confinement effect can be discussed. The present study is formulated with this goal in mind. We discuss the interaction of AB with SWCNTs, including adsorption and dissociation. The kinetics and dynamics for the release of hydrogen from AB both adsorbed on the sidewall of a SWCNT and within its channel are described in terms of the reaction energies and barriers, using isolated AB as a reference. We will show that almost no improvement can be expected from simply confining by SWCNTs.

2. COMPUTATIONAL METHOD Spin-polarized DFT calculations have been carried out using the DMol3 code.31,32 The generalized gradient approximation (GGA) with the functional of Perdew
Burke
Ernzerhof functional (PBE)33 was utilized for all geometric optimization and single-point energy calculations. Effective core potentials with double-ζ plus polarization function basis set (DNP) have been employed for the description of core electrons during the

optimization and the energy calculation, respectively. During our calculations, the convergence criteria for structure optimizations were set to (1) energy tolerance of 2.7  10
5 eV, (2) maximum force tolerance of 2.7  10
3 eV/Å, and (3) maximum displacement tolerance of 1.0  10
3 Å. K-space is sampled by gamma point due to the big size of the employed models. The efficiency and reliability of the numerical basis set can be found in Delley’s work,31,32 in which the estimated errors from the PBE functional with the DNP basis set were supposed to be lower than those with the hybrid B3LYP/6-31G functional. In this work, the key data is the energy barrier of AB dissociation and thus a particular test has been performed. For individual AB, the calculated barrier (PBE/DNP) is 1.47 eV. After zero-point energy (ZPE) correction, it is reduced to 1.27 eV, which is obviously lower than the benchmark data calculated with CCSD(T)/CBS, 1.58 eV (36.4 kcal/mol34). This is not surprising given the well-known limit of pure DFT using semilocal exchangecorrelating functionals and can be significantly improved with screened hybrid density functionals.35,36 Unfortunately, hybrid functionals bring significant computational cost and are not affordable for large systems, like AB-inserted SWCNTs in this work. Therefore, we still use PBE-based pure DFT below and keep in mind that the calculated reaction barriers are underestimated according to several representative assessments of semilocal and hybrid functionals.37
39 Based on above tests, total energies employed below are not corrected by ZPE because the errors associated with PBE functionals can be partly canceled through this treatment. In addition, AB powders are widely used in real experimental tests, and thus the energy barrier for hydrogen release from AB clusters (4 AB molecules) has been calculated, Ea = 1.48 eV (without ZPE correction), which is almost the same as for individual AB (Ea = 1.47 eV), indicating that it is acceptable to use individual AB as the reference. To investigate the energetics associated with the insertation of AB into SWCNTs, a finite length tube of ∼20 Å is employed, with two open ends terminated by hydrogen as shown in Figure 1a. For comparison, an infinitely long SWCNT is also considered and modeled by a periodic tube with an axial length of ∼18 Å, as shown in Figure 1b, under which the distance between any two AB molecules is larger than 15 Å. AB inserted in the SWCNT channel is labeled as AB@SWCNT. In our calculations, a vacuum of 15 Å is employed to eliminate the van der Waals interaction between two neighboring SWCNTs. During the geometric optimization, no atom is fixed, except that lattice parameters were fixed to experimental values. The transition state (TS) for AB dissociation was searched based on a constrained minimization technique, in which all the degrees of freedom of the system are relaxed except only the reaction coordinate (H
H bonds in our case). The TS was identified when both total energies and atomic force are converged to the 12581

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Figure 2. Optimized geometries for the adsorption of single AB: (a) on the sidewall (outside); (b) at the open end; and (c) inside the tube.

criteria described above. Although only finite images have been considered during the above search, the error for the H
H bond length associated with the TS is less than 0.02 Å (corresponding to an energy error of 0.01 eV based on the test of individual AB), which is smaller than the typical error of DFT energy calculations. To confirm that the achieved state is the transition state, the vibration frequency is computed by diagonalizing the massweighted second-derivative matrix after one DFT self-consistent field run, which has been embedded in the DMol3 module. TS is confirmed and employed for the calculation of reaction barriers when there is one and only one imaginary frequency; otherwise, it is further searched along the reaction path. Given AB is only slightly adsorbed, rather than strongly bonded, by the SWCNT framework, only the vibrication of AB has been considered during the frequency analysis. It is necessary to point out that the state with the maximum energy identified from a series of limited images is an approximate TS, and it can be further improved through inserting more images along the reaction path; the error of energy barrier resulting by such TS searching is less than 0.02 eV in the following calculations according to our tests, which does not affect the major points made below. The adsorption energy (Eads) and insertion energy (Eins) are defined by, Eads ¼ ðnEðABÞ þ EðSWCNTÞ
EðnAB
SWCNTÞÞ=n ð1aÞ Eins ¼ ðnEðABÞ þ EðSWCNTÞ
EðnAB@SWCNTÞÞ=n ð1bÞ where n is the total number of adsorbed AB molecules, and E(AB), E(SWCNT), E(AB
SWCNT), and E(AB@SWCNT) are respectively the energies of isolated AB, isolated SWCNT, AB adsorbed onto the SWCNT, and AB inserted into the SWCNT channel. By definition, positive Eads and Eins indicate that such adsorption and insertation are energetically preferable.

3. RESULTS AND DISCUSSION 3.1. Adsorption of Single AB. For SWCNTs with finite lengths, there are three possible adsorption sites, including the exterior sidewall surface, the two ends, and the inside channel, as shown in Figure 2a
c, respectively. In the case of single AB, adsorptions onto the sidewall and into the channel are very weak, with Eads = 0.09 and 0.28 eV, respectively. The most stable site is at the open ends, with N heading to the tube (see Figure 2b), leading to an adsorption energy of 0.49 eV. If all terminating

Figure 3. Optimized geometries of AB molecules adsorbed on (6,6) SWCNT: (a) AB molecules adsorbed on the two ends with Eads = 0.54 eV per AB molecule; (b) AB molecules adsorbed at one end and on the sidewall with Eads = 0.47 eV per AB molecule; and (c) AB molecules adsorbed on the sidewall with Eads = 0.44 eV per AB molecule.

hydrogen atoms are removed, Eads will reduce by 0.12 eV, suggesting that the C
H groups play an additional stabilization role. 3.2. Adsorption of 16 AB Molecules around SWCNT. Given AB molecules have strong intramolecular hydrogen bonding, it is desirable to depict the interaction of AB clusters with SWCNT; thus, 16 AB molecules were put around SWCNT initially on the sidewall (outside), or at the two ends, or mixed on the sidewall (8 AB molecules) and at the open ends (8 AB molecules). The optimized geometries are shown in Figure 3a
c, and the averaged adsorption energies, defined by eq 1a, are 0.54, 0.47, and 0.44 eV, respectively. Accordingly, three features can be summarized: (i) consistent with the case of a single AB, staying at the open ends is slightly preferred (Eads = 0.54 eV); (ii) no AB molecule partially gets into the channel as we found in the single 12582

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Figure 4. Energy profiles for the insertion of one AB into (6,6) SWTN with (a) a B-headed configuration and (b) a N-headed configuration. All energies are presented using the total energy of the initial state as the zero energy point in unit of eV. Local states and transition states are indicated as “LS” and “TS”, respectively.

Figure 5. AB dissociation on the sidewall of (6,6) SWCNT: (a) energy profile with Ea = 1.44 eV; (b) optimized geometry of transition state, with characteristic atomic distances in angstroms. The numbers in parentheses are for the TS of isolated AB.

AB case (see Figure 2b), which may result from the intramolecular interactions; and (iii) no AB molecule completely gets in the channel, suggesting that there is an energy barrier for AB insertion, in line with the case of single AB. Therefore, it may reasonably be expected that AB can be physically adsorbed around the SWCNT, but additional energy is needed to get AB molecules into the SWCNT channel, which has been verified below. 3.3. Insertion of AB into SWCNT Channels. Before we study AB dissociation inside SWCNTs, it is essential to answer how much energy is needed to insert AB into SWCNTs. Figure 4, a and b, shows the energy profiles for the insertion of AB into the SWCNT with the N-head and the B-head leading, respectively. The zero energy is set as the sum of E(AB) and E(SWCNT). According to the energy profiles, a local state (LS) with minimum energy is identified, as shown in Figure 4, a and b, which corresponds to the physical adsorption at the open ends. Clearly, approach of AB from the initial state (IS) to the LS can occur without a barrier. To move into the SWCNT channel, however, AB has to get over a small barrier of 0.23 eV (N-headed) or 0.34 eV (B-headed). As shown in Figure 4, the energy profile for B-headed configurations is more complex than that of N-headed ones and a larger barrier is indicated. From detailed comparison of all images, it is found that the B-headed AB molecule rotates at the open ends and gets into the channel via N-headed configurations, and the larger barrier, 0.11 eV, actually comes from such rotation. 3.4. AB Dissociation on the Sidewall of SWCNT. Figure 5a shows the energy profile for AB dissociation (elimination of H2) on the sidewall of the SWCNT, leading to an energy barrier of

Figure 6. AB dissociation inside of (6,6) SWCNT: (a) energy profile and (b) geometry of transition state with bond lengths in angstroms; the numbers in parentheses are from the TS of neat AB.

1.44 eV, slightly smaller than that for isolated AB (Ea = 1.47 eV). Figure 5b shows the optimized geometry of the TS, with characteristic atomic distances shown in angstroms. Two features can be summarized: (i) the nearest AB
SWCNT distance is up to 3.30 Å, meaning that the AB
SWCNT interaction is weak; and (ii) with respect to the TS of isolated AB, the major change is the distance between dissociated hydrogen and B and N, while the changes for H
H and B
N bonds are only 0.06 and 0.04 Å, respectively. Based on the above information, it is clear that the SWCNT has very little effect on the dissociation of externally adsorbed AB. 3.5. AB Dissociation Inside SWCNTs. Figure 6a shows the energy profile for AB dissociation inside the (6,6) SWCNT channel. The geometry of the corresponding TS is shown in Figure 6b and again characteristic atomic distances are shown in angstroms. From Figure 6a, an energy barrier of 1.58 eV is derived, which is higher than that of isolated AB by 0.11 eV, implying that confinement within the SWCNT does not facilitate hydrogen release from AB. Compared with that for isolated AB, the geometry of the TS in Figure 6b for hydrogen elimination has no notable change. It is curious to find the reason why SWCNT confinement results in a larger energy barrier. It is well-known that electrons energetically prefer to locate outside of SWCNT due to the curvature effect. As a result, the inner sidewall of SWCNT is positively charged. In the AB molecule, hydrogen atoms bonded with N are positively charged (with 0.255 e per atom according to Mulliken charge analysis) due to the strong electronegativities of nitrogen. Hence it could be speculated that the SWCNT sidewall may apply a repulsive effect on the movement of hydrogen off the nitrogen atom, raising slightly the overall elimination barrier. 3.6. AB Dissociation Inside Other SWCNTs. All of the above results are obtained using a (6,6) SWCNT with open ends 12583

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H bonds apparently show a slight effect on the adsorption characteristics of AB (see, e.g., Figure 2), a periodical supercell without terminating hydrogen has also been employed. For this case, the dissociation barrier inside the SWCNT is 1.54 eV, which is slightly smaller than that obtained for the H-terminated SWCNT and still larger than that computed for isolated AB. This supports the conclusion that the elimination of H2 from AB confined within finite or infinitely long SWCNTs is slightly more difficult than that it is for AB alone. Finally, we have tested another three zigzag SWCNTs, including (7,0), (8,0), (9,0), and two armchair SWCNTs, including (5,5) and (7,7). In all cases, periodic supercells were employed. The calculated dissociation barriers for AB inside those SWCNTs all fall in the range of 1.51
1.60 eV, being consistent with that obtained with (6,6) SWCNT (Ea = 1.54 eV). The curvature effect reduces for SWCNTs with larger diameters, and correspondingly the confinement effect becomes gradually smaller. 3.7. AB f BH3 þ NH3. Experimentally, AB may dissociate and release BH3 and NH3,22 which has already been investigated at the CCSD(T) level and it is found that (i) no TS has been identified and (ii) the total energy of (BH3 þ NH3) is slightly lower than the TS for hydrogen release from AB by 0.45 eV (10.4 kcal/mol34), indicating that BH3/NH3 will be released even before hydrogen elimination. Our calculations comfirmed the above results, except that the energy difference for two dissociations (AB f BH3 þ NH3 vs AB f BH2NH2 þ H2) is 0.05 eV, mainly due to the underestimation of the energy barrier for hydrogen release (note: for AB f BH3 þ NH3, no TS is found and thus what we calculated is reaction energy, not reaction barrier). With AB inserted into SWCNTs, the difference increases to 0.08 eV, suggesting that the confinement does not bring significant effect on the dissociation of single AB. We should be cautious that the dissociation reactions in this work are studied with only one AB involved. If two or more are considered, the catalytic role of BH3 should be taken into account. As revealed in ref 34, the energy barrier for AB f BH2NH2 þ H2 can be reduced from 1.58 to 0.26 eV with the presence of BH3. The same group also identified multiple pathways for hydrogen elimination assisted by diborane (BH3BH3), which again illustrates that BH3 or BH3BH3 may play a key role for AB dissociation.40 Given that nanosized pores can effectively confine gas molecules in a limited space, BH3 molecules released from AB dissociation may easily form BH3BH3 and further react with AB, leading to easier pathways for hydrogen elimination from AB. Therefore, our following work is to perform dynamic simulation to clarify such reactions involving AB, BH3, NH3, and BH3BH3 in limited spaces.

4. CONCLUSION In this work, the interaction of ammonia borane with SWCNTs has been investigated using the DFT approach in order to assess the question as to whether SWCNT confinement alone—independent of any chemically related functional group effect—should play a role in facilitating release of hydrogen from AB. Our conclusion is that confinement per se does not reduce the barrier for elimination of H2 from AB to any significant extent in comparison with that predicted for an isolated AB molecule. In fact, we predict dissociation barriers for AB confined within SWCNT channels to be marginally higher. These results shed

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light on the confinement effect of nanostructures on the release of hydrogen from ammonia borane and will help to inform subsequent discussion. In addition, advanced simulations are required to investigate the catalytic role of BH3 and BH3BH3 predicted by early calculations.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] (S.C.S.); [email protected] (C.S.).

’ ACKNOWLEDGMENT This work is inspired by collaborations within the Australian Research Council Centre of Excellence for Functional Nanomaterials and has been financially supported by The University of Queensland (Research Excellence Award for C.S.), the Australian Research Council, and the Queensland State Government (Smart Future Fellowship for C.S.). We also appreciate the generous grants of CPU time from both The University of Queensland and the Australian National Computational Infrastructure Facility. ’ REFERENCES (1) Wolf, G.; van Miltenburg, J. C.; Wolf, U. Thermochim. Acta 1998, 317, 111. (2) Wolf, G.; Bauman, J.; Baitalow, F.; Hoffmann, F. P. Thermochim. Acta 2000, 343, 19. (3) Baitalow, F.; Baumann, J.; Wolf, G.; Jaenicke-R€ossler, K.; Leitner, G. Thermochim. Acta 2002, 391, 159. (4) Stephens, F. H.; Pon, V.; Baker, R. T. Dalton Trans. 2007, 25, 2613. (5) Lin, Y.; Mao, W. L.; Mao, H.-K. Proc. Natl Acad. Sci. U.S.A. 2009, 106, 8113. (6) Liu, C.; Li, F.; Ma, L. P.; Cheng, H. M. Adv. Mater. 2010, 22, E28. (7) Xiong, Z. T.; Yong, C. K.; Wu, G. T.; Chen, P.; Shaw, W.; Karkamkar, A.; Autrey, T.; Jones, M. O.; Johnson, S. R.; Edwards, P. P.; David, W. I. F. Nat. Mater. 2008, 7, 138. (8) Li, W.; Scheicher, R. H.; Araujo, C. M.; Wu, G. T.; Blomqvist, A.; Wu, C. Z.; Ahuja, R.; Feng, Y. P.; Chen, P. J. Phys. Chem. C 2010, 114, 19089. (9) Zhang, Y.; Shimoda, K.; Ichikawa, T.; Kojima, Y. J. Phys. Chem. C 2010, 114, 14662. (10) Kang, X. D.; Fang, Z. Z.; Kong, L. Y.; Cheng, H. M.; Yao, X. D.; Lu, G. Q.; Wang, P. Adv. Mater. 2008, 20, 2756. (11) Li, L. L.; Peng, B.; Tao, Z. L.; Cheng, F. Y.; Chen, J. Adv. Funct. Mater. 2010, 20, 1894. (12) He, T.; Xiong, Z.; Wu, G.; Chu, H.; Wu., C.; Zhang, T.; Chen, P. Chem. Mater. 2009, 21, 2315. (13) Diyabalanage, H. V. K.; Shrestha, R. P.; Semelsberger, T. A.; Scott, B. L.; Bowden, M. E.; Davis, B. L.; Burrell, A. K. Angew. Chem., Int. Ed. 2007, 46, 8995. (14) Denney, M. C.; Pons, V.; Hebden, T. J.; Heinekey, D. M.; Goldberg, K. I. J. Am. Chem. Soc. 2006, 128, 12048. (15) Blaquiere, N.; Diallo-Garcia, S.; Gorelsky, S. I.; Black, D. A.; Fagnou, K. J. Am. Chem. Soc. 2008, 130, 14034. (16) Kalidindi, S. B.; Sanyal, U.; Jagirdar, B. R. Phys. Chem. Chem. Phys. 2008, 10, 5870. (17) Bowden, M. E.; Brown, I. W. M.; Gainsford, G. J.; Wong, H. Inorg. Chim. Acta 2008, 361, 2147. (18) Chen, Y. S.; Fulton, J. L.; Linehan, J. C.; Autrey, T. J. Am. Chem. Soc. 2005, 127, 3254. (19) Sun, C. H.; Yao, X. D.; Du, A. J.; Li, L.; Smith, S.; Lu, G. Q. Phys. Chem. Chem. Phys. 2008, 10, 6104. (20) Gutowska, A.; Li, L.; Shin, Y. S.; Wang, C. M.; Li, X. H. S.; Linehan, J. C.; Smith, R. S.; Kay, B. D.; Schmid, B.; Shaw, W.; Gutowski, M.; Autrey, T. Angew. Chem., Int. Ed. 2005, 44, 3578. 12584

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