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2007, 111, 5031-5033 Published on Web 04/14/2007
Influence of the Substituted Side Group on the Molecular Structure and Electronic Properties of TPP and Related Implications on Organic Zeolites Use Godefroid Gahungu,†,‡ Bin Zhang,§ and Jingping Zhang*,† Faculty of Chemistry, Northeast Normal UniVersity, Changchun 130024, China, Faculte´ des Sciences, UniVersite´ du Burundi, Departement de Chimie, B.P. 2700, Bujumbura, Burundi, and Organic Solid Laboratory, CMS, Institute of Chemistry, Beijing, 100080, China ReceiVed: January 5, 2007; In Final Form: February 14, 2007
Tris(o-phenylenedioxy)cyclotriphosphazene (TPP) became a compound of choice to investigate the structural features of organic zeolite and their potential applications. Different TPP-like materials are studied in this Letter from the electron-donor (E-D) capacity viewpoint, since this was reported as a stabilizing parameter of the TPP-Lewis acid inclusion compound up to high temperatures. On the basis of DFT-PBE0/6-31G(d,p) calculations, the results reported herein show a tight dependence of the E-D of the entire molecule on that of the side group. It was shown that both the O/NH substitution and the extension of the phenylenedioxyl group with an aromatic ring significantly enhance the E-D. As a result, the corresponding clathrates, including some reported ones, may also be exploited for the same issue, with an even wider range of operating temperatures when trapping compounds of Lewis acidity character comparable to that of I2. Furthermore, it was concluded that these two strategies may significantly enhance the E-D capacity without altering the tolerance of TPP-like host materials to the guest molecules.
1. Introduction Boosted by strategic industrial and environmental applications such as gas storage, selective gas recognition, and separation, the adsorption properties of materials are emerging as a forefront issue of present-day research.1 As a result of their unique features, molecular self-assembled materials and organic zeolites (OZ)2,3 seem to constitute a competing alternative in this field, and are thus still to be explored extensively. Originally studied by Allcock,4 tris(o-phenylenedioxy)cyclotriphosphazene (1a) became a compound of choice to investigate the structural features of OZ and their potential applications. Studies focused on the stability of the hexagonal modification compared to compact guest-free monoclinic,5 the investigation of gas storage or aromatic guest insertion by advanced NMR techniques,6 the confinement of I2 molecules by several crystallization procedures,7 and the insertion of dipolar molecules.8 From 1a to some of its derivatives (3a and 4a), the available space for absorbates can be modulated by the choice of the side group, which substitutes the dioxyphenylene in the former, the key factor of the tunnel formation being reported to be the rigid “paddle wheel” molecular shape and the requirements of the crystal state.9 Many differences including the absence of inclusion adducts for 2a with other compounds were reported.4,10 These differences were attributed to the greater molecular rigidity of 1a compared to 2a and to the stabilizing influence of a seven-membered ring at phosphorus (2a).11 * To whom correspondence should be addressed. E-mail: zhangjp162@ nenu.edu.cn. † Northeast Normal University. ‡ Universite ´ du Burundi. § Institute of Chemistry.
10.1021/jp070101k CCC: $37.00
With 1a and some of its derivatives, clathration was characterized as a pure mechanical phenomenon.11 Some of its relevant applications, however, may be based on physicochemical properties. Within tris(o-phenylenedioxy)cyclotriphosphazene (TPP) zeolite, which shows a strong affinity to include gaseous CH4, CO2,6b I2, and Xe,6a,7,12 specific host-guest interactions of the donor-acceptor type are expected for channels.6b A recent report by T. Hertzsch has shown that 1a may be used to remove radioactive I2, even from a humid environment or water.13 The stability of the inclusion compound TPP(I2)0.75 to temperatures up to 420 K7 was interpreted on the basis of the Lewis acidity of I2 and the electron-donor (E-D) capacity of the TPP-phenylenedioxy rings. It appears then clear that the E-D capacity may play a certain role in the trapping process of some compounds within TPP OZ, which may provide some potential applications in environmental chemistry. From this viewpoint, different TPP-like materials are studied in this Letter. We focused our interest on the relationship between the E-D capacity of the free side fragment and that for the entire © 2007 American Chemical Society
5032 J. Phys. Chem. B, Vol. 111, No. 19, 2007
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Figure 1. Optimized geometries for 1a-4a (for clarity, hydrogen atoms are not shown).
molecule. Three questions served us as a guideline: How does an extension of the side group of 1a, with one aromatic ring, linearly (3a) or laterally (4a) affect the E-D capacity? What may be the corresponding implications on their application for OZ use? May the O/NH substitution induce a significant E-D capacity change, which can be exploited for the same issue, without altering the molecular shape early reported to be a key factor of the tunnel formation9 and, consequently, the key factor of OZ use of TPP? Although a number of theoretical works on phosphazene containing systems can be found in the literature,14 very few were devoted to related OZ and none of these has addressed these problems. This contribution may provide some helpful insights toward the understanding of TPP-like OZ uses and the design as well. All of the calculations reported in this Letter were carried out using the density functional theory (DFT) based on the PBE0 functional15 coupled to the 6-31G(d,p)16 basis set. More details about this choice are provided in the Supporting Information. 2. Results, Discussion, and Conclusion In Figure 1, we show the optimized structures of 1a-4a (see Supporting Information Tables S1-S3 for details). Interestingly, the structural differences that were thought to be responsible for the fact that 1a readily forms channel inclusion clathrates whereas 2a does not11b are also well predicted. In 2a, the twist angle between phenyl units within each biphenyl group is ca. 42.9° vs 41° (experimental11b) and each biphenyl group is twisted by 45.3° vs 47.5°11b to the phosphazene ring plane, with the o-phenylenedioxy side groups in 1a being almost perpendicular to it. The total energies of 2a at different levels of theory (see Supporting Information Table S3c) favor the C3 conformation, which is predicted to be energetically more stable than the C2 conformation by only 1-1.4 kcal‚mol-1. This feature agrees well with the experimental report according to which conformations other than the experimentally observed C2 may be awaited in solution or the molten state.11b These results give credit to the computational approach currently used. Since no tunnel formation was observed for 2a, our further discussion will be focused on 1a-c, 3a-c, and 4a-c. In general, a good agreement was found between the calculated structures and available crystal data.5,11a A comparison of 1a, 3a, and 4a to the O/NH substituted derivatives shows that a half O/NH substitution (1b-4b) induces a chair conformation (by ∼14°) within the phosphazene ring which is reconverted into the planar conformation upon the second O/NH substitution (1c-4c). Due to the -NH- bonding environment,
Figure 2. Total and partial density of states (TDOS and PDOS) around the HOMO-LUMO gap for 1a, 3a, and 4a (dashed vertical lines indicate the HOMO and LUMO energies, respectively).
some slight deviations from the planarity of the side fragment are predicted for 1b,c, 3b,c, and 4b,c, without a profound alteration of the paddle wheel molecular shape, especially by a total O/NH substitution. For a more detailed comparative study of the electronic structures, the total density of states (TDOS) and projected partial density of states (PDOS) are analyzed. As plotted in Figure 2, these properties reveal that the molecular orbitals (MOs) around the highest occupied molecular orbital (HOMO)lowest unoccupied molecular orbital (LUMO) gap are fairly localized on the phosphazene ring with the main contributions coming from the side fragment. Although ultraviolet photoemission spectroscopy (UPS) measurement can provide similar valence band information (which has not been reported yet), the projection of TDOS to individual atoms explicitly reveals the contributions to the TDOS from each atom (impossible from UPS). For 1a, 3a, and 4a, a careful analysis shows that the HOMOs are mainly made of atomic orbitals of O and C atoms of the side fragments (Supporting Information Figure S3). Those atoms are more likely to react with other atoms and may be responsible for the reactivity of the molecules including the donor-acceptor behavior when a Lewis acid such as I2 is adsorbed in TPP crystals. The common DOS pattern of the series of derivatives described herein shows that the O/NH substitution does not profoundly affect this property, implying that many of the relating properties may also be preserved. In Table 1, the calculated EHOMO and ELUMO at both the BPE0 and HF levels are listed. From these results, it may be observed that the magnitudes of the numerical values and their relative differences (Eg) are different within the two approaches, with the main feature being, however, their relative trend. The HF EHOMO are -8.79, -8.02, -7.53, -8.00, 7.66, 7.46, 7.68, 7.26, and -6.91 eV for 1a, 1b, 1c, 3a, 3b, 3c, 4a, 4b, and 4c, respectively. Within PBE0/6-31(d,p) calculation results, the corresponding values are -6.60, -5.93, -5.49, -6.25, -5.90, -5.69, -5.76, -5.55, and -5.20 eV. For the ELUMO, 3.54 (-0.15), 3.88 (0.18), 4.06 (0.37), 2.44 (-1.04), 2.71 (-0.77), 2.84 (-0.64), 2.48 (-1.12), 2.72 (-0.78), and 2.91 (-0.58) eV were obtained with HF (PBE0) in the same order. As clearly summarized in Figure 3, the results suggest that (i) the O/NH substitution increases both the EHOMO and ELUMO energies in the sequence Na < Nb < Nc (with N ) 1, 3, and
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J. Phys. Chem. B, Vol. 111, No. 19, 2007 5033
TABLE 1: Electronic PropertiessIonization Potentials and Frontier Molecular Orbital (FMO) Energiesa 1a IPKTb IPvc IPad
1b
8.14 7.87 7.63
IPKTb 8.79 8.02 IPvc 7.70 7.01 IPad 7.63 ELUMO -0.15 0.18 EHOMO -6.60 -5.93
1c
3a
3b
3c
Free Side Groups 7.44 7.62 6.93 7.17 7.09 6.67 6.73 6.95 6.42
4a 7.20 7.45 7.30
4b
4c 6.74 6.62 6.41
TPP and TPP-like 7.53 8.00 7.66 7.46 7.68 7.26 6.91 6.57 7.15 6.80 6.62 6.90 6.49 6.15 6.44 7.11 6.51 6.85 6.44 6.08 0.37 -1.04 -0.78 -0.64 -1.12 -0.78 -0.58 -5.49 -6.25 -5.90 -5.69 -5.76 -5.55 -5.20
a Energies are given in eV. b Based on Koopman’s theorem. c Vertical IP. d Adiabatic IP.
cationic species for 1a, 3a, and 4a only revealed small structural changes. On the basis of these results, it was concluded that the total O/NH substitution may significantly enhance the electron-donor capacity without altering the tolerance of TPPlike host materials to the guest molecules. Acknowledgment. Financial support from the NSFC (nos. 50473032 and 20473095) and NCET-06-0321 is gratefully acknowledged. Supporting Information Available: Details of the computational procedure, absolute energies, and Cartesian coordinates for relevant structures and a figure of the DOS for the O/NH substituted derivatives. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes
Figure 3. PBE0 frontier molecular orbital (FMO) energy diagram for 1a-c, 3a-c, and 3a-c (L ) LUMOs, H ) HOMOs).
4) and (ii) π-conjugation increases the EHOMO, while decreasing the ELUMO in the sequence of 1i < 3i < 4i (i ) a, b, and c). From these results, it may be concluded that comparatively to 1a, extending the side group with an aromatic ring destabilizes the HOMO, which becomes more stabilized by the O/NH substitution. A comparison of 3i to 4i shows that the HOMO is even more destabilized by a lateral extension. E-D capacity is then increased within the same order as confirmed by ionization potential (IP) calculations whose results are summarized in Table 1. The results show a tight dependence of the E-D capacity of the TPP-like molecules on that of the free side group, resulting in some interesting implications for some aspects of OZ use: (i) The stability of the inclusion compound, TPP(I2)x, and the operating temperatures may be improved by using 1c, 3a, and 4a whose clathrates with many other molecules are already known, with the OZ-I2 inclusion compound based on 3a being expected to be less stable than that based on 4a. (ii) The E-D capacity of TPP side groups appears to be tunable, allowing predictions to be made about the stability of the inclusion compounds of OZ and molecules of Lewis acidity comparable to that of I2. (iii) The optimized structures for the neutral and
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