Directed Synthesis of Noncentrosymmetric Molybdates Eric A. Muller,‡ Robert J. Cannon,‡ Amy Narducci Sarjeant,† Kang Min Ok,§ P. Shiv Halasyamani,§ and Alexander J. Norquist*,‡ Department of Chemistry, Haverford College, Haverford, Pennsylvania 19041, Department of Chemistry, University of Houston, Houston, Texas 77204-5641, and Department of Chemistry, Johns Hopkins University, Baltimore, Maryland 21218
CRYSTAL GROWTH & DESIGN 2005 VOL. 5, NO. 5 1913-1917
Received April 26, 2005
ABSTRACT: The directed synthesis of two noncentrosymmetric sulfated molybdates has been achieved through the use of chiral organic amines. Reaction gels containing either (R)-2-methylpiperazine or (S)-2-methylpiperazine, MoO3, H2SO4, and H2O were subjected to mild hydrothermal conditions, resulting in the growth of single crystals of [(R)-C5H14N2][(MoO3)3(SO4)]‚H2O and [(S)-C5H14N2][(MoO3)3(SO4)]‚H2O. These materials crystallize in the noncentroysmmetric space group P212121 (No. 19), which exhibits the enantiomorphic crystal class 222 (D2). The second harmonic generation activities were measured on sieved powders. Introduction Materials that possess crystallographic noncentrosymmetry are of great interest to researchers because they have some or all of the following desirable physical properties:1 enantiomorphism, optical activity (circular dichroism), piezoelectricity, pyroelectricity, and secondorder nonlinear optical activity (second-order harmonic generation (SHG)). The presence of these technologically advantageous properties is solely dependent upon symmetry.2 Unfortunately, the a priori synthesis of noncentrosymmetric materials is extremely difficult. Several approaches are currently employed to facilitate the formation of noncentrosymmetric materials. First, the incorporation of metal centers with acentric coordination environments in inorganic materials can be achieved through the use of second-order Jahn-Teller active cations, specifically, d0 transition metals.3-6 Early transition metals are of specific interest owing to the high polarizabilities of the M-O bonds, which are the suspected source of the unusually high responses in materials such as KTiOPO47-9 and LiNbO3.10 Second, main group cations with nonbonded electron pairs also exhibit local acentric coordination environments, with examples including selenites,11,12 tellurites,13 and iodates.14,15 The inclusion of a cation from either of these two classes alone is insufficient to force noncentrosymmetry because adjacent sites can possess antiparallel geometries, which can result in increased symmetry.16,17 A third route involves the formation of organicinorganic hybrid materials using tetrahedral nodes.18,19 Network interpenetration can however result in the creation of extraframework centers of inversion and subsequent centrosymmetry. In this study, we report the effects from inclusion of chiral organic amines into systems containing second* To whom correspondence should be addressed. Department of Chemistry, Haverford College, 370 Lancaster Avenue, Haverford PA, 19041. Phone: (610) 896-2949. Fax: (610) 896-4963. E-mail: anorquis@ haverford.edu. ‡ Haverford College. § University of Houston. † Johns Hopkins University.
order Jahn-Teller cations and comment upon this technique as an approach toward the synthesis of noncentrosymmetric materials. The structures and SHG activities of [(S)-C5H14N2][(MoO3)3(SO4)]‚H2O and [(R)C5H14N2][(MoO3)3(SO4)]‚H2O20 are discussed. Experimental Section Materials. MoO3 (99.5%, Aldrich), H2SO4 (98%, Aldrich), and (S)-2-methylpiperazine (99%, Adrich) were used as received. Deionized water was used in this synthesis. Synthesis. [(S)-C5H14N2][(MoO3)3(SO4)]‚H2O was synthesized through the reaction of 0.485 g (3.37 × 10-3 mol) of MoO3, 0.141 g (1.41 × 10-3 mol) of (S)-2-methylpiperazine, 0.135 g (1.41 × 10-3 mol) of H2SO4, and 0.974 g (5.41 × 10-2 mol) of deionized water. All reactants were placed in a 23 mL poly(fluoro-ethylene-propylene) lined pressure vessel. Reactions were heated to 180 °C over 30 min and allowed to soak for 24 h. The reactions were then cooled to room temperature at a rate of 6 °C h-1. Autoclaves were opened in air, and products were recovered through filtration. Colorless needles were recovered in approximately 15% yield (based upon Mo). Single-Crystal X-ray Diffraction. Data were collected using a CrysAlis CCD diffractometer with graphite monochromated Mo KR radiation (λ ) 0.71073 Å). A single crystal was mounted on a glass fiber using N-paratone oil and cooled insitu to 153(2) K for data collection. Frames were collected, indexed, and processed and the files were scaled using CrysAlis RED.21 The heavy atom positions were determined using SIR92.22 All other non-hydrogen sites were located from Fourier difference maps. All non-hydrogen sites were refined using anisotropic thermal parameters using full matrix least squares procedures on Fo2 with I > 3σ(I). Hydrogen atoms were placed in geometrically idealized positions. All calculations were performed using Crystals.23 Relevant crystallographic data are listed in Table 1. Powder X-ray Diffraction. Powder diffraction patterns were recorded on a GBC-Difftech MMA powder diffractometer. Samples were mounted on aluminum plates. Calculated powder patterns were generated from single-crystal data using ATOMS v. 6.0.24 Infrared Spectroscopy. Infrared measurements were obtained using a Perkin-Elmer FT-IR Spectrum 1000 spectrophotometer. Samples were diluted with spectroscopic grade KBr and pressed into a pellet. A scan was run over the range of 400-4000 cm-1. Nonlinear Optical Measurements. Powder SHG measurements were conducted using a modified Kurtz-NLO system, with a 1064 nm light source.25,26 Polycrystalline [(S)-
10.1021/cg050184z CCC: $30.25 © 2005 American Chemical Society Published on Web 07/26/2005
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Table 1. Crystallographic Data for [(S)-C5H14N2][(MoO3)3(SO4)]‚H2O [(S)-C5H14N2][(MoO3)3(SO4)]‚H2O formula fw space group a/Å b/Å c/Å V/Å3 Z Dc/g cm-3 λ/Å T/°C µ/mm-1 R1a wR2b a
C5H16N2Mo3O14S 648.07 P212121 (No. 19) 8.5398(6) 10.7500(9) 18.1077(11) 1662.3(2) 4 2.589 0.71069 153(2) 2.429 0.0231 0.0524
R1 ) ∑||Fo| - Fc||/∑|Fo|. b wR2 ) [∑w(Fo2 - Fc2)2/[∑w(Fo2)2]1/2. Table 2. Selected Bond Lengths (Å) in [(S)-C5H14N2][(MoO3)3(SO4)]‚H2O bonds
Mo1-O1 Mo1-O2 Mo1-O3 Mo1-O4 Mo1-O5 Mo1-O6 Mo2-O4 Mo2-O5 Mo2-O6 Mo2-O7 Mo2-O8 Mo2-O9
1.697(4) 1.698(4) 2.343(4) 2.326(5) 1.939(4) 1.949(5) 1.953(4) 1.954(4) 2.294(3) 1.695(3) 1.717(3) 2.182(3)
Mo3-O4 Mo3-O5 Mo3-O6 Mo3-O10 Mo3-O11 Mo3-O12 S1-O3 S1-O9 S1-O12 S1-O13
1.944(4) 2.321(4) 1.967(5) 1.706(4) 1.691(4) 2.330(4) 1.476(4) 1.528(3) 1.484(4) 1.453(4)
C5H14N2][(MoO3)3(SO4)]‚H2O and [(R)-C5H14N2][(MoO3)3(SO4)]‚ H2O were ground and sieved into distinct particle size ranges: < 20 µm, 20-45 µm, 45-63 µm, 63-75 µm, 75-90 µm and 90-120 µm. Crystalline R-quartz was ground and sieved into identical particle size ranges to compare the SHG properties of [(S)-C5H14N2][(MoO3)3(SO4)]‚H2O and [(R)-C5H14N2][(MoO3)3(SO4)]‚H2O with known materials. All powders were placed in separate capillary tubes; no index-matching fluid was used in any experiment. The SHG, i.e., 532 nm light, was collected in reflection and detected using a photomultiplier tube. A 532-nm narrow-band-pass interference filter was attached to the tube to detect only the SHG light.
Results [(S)-C5H14N2][(MoO3)3(SO4)]‚H2O. This compound crystallizes in the noncentrosymmetric space group P212121 (Flack parameter ) -0.08(6)). Three distinct molybdenum sites are observed in [(S)-C5H14N2][(MoO3)3(SO4)]‚H2O, each of which is coordinated by six oxide ligands in a distorted octahedral geometry. Two “short”, two “intermediate”, and two “long” Mo-O bonds are observed. The short bonds range between 1.691(4) and 1.717(3) Å, the intermediate bonds range between 1.939(4) and 1.967(5) Å, and the long bonds range between 2.182(3) and 2.343(4) Å. Selected bond lengths are listed in Table 2. Three oxide ligands in each MoO6 coordination octahedron bridge between adjacent molybdenum centers in a µ3 fashion, forming MoO3/1O3/3 chains that exhibit a structure analogous to R-molybdena.27 Three additional oxide ligands, O3, O9, and O12, bridge from three adjacent molybdenum centers to the same sulfur center, which is at the center of a SO4 tetrahedron, forming [(MoO3)3(SO4)]n2n- chains. The SO4 orientations
Figure 1. Chains in [(S)-C5H14N2][(MoO3)3(SO4)]‚H2O, viewed along the (a) [1 0 0] and (b) [1 0 1] directions. Red octahedra and blue tetrahedra represent MoO6 and SO4, respectively.
alternate in a “up-down-up” motif (Figure 1). This chain connectivity has been previously reported in the compound Rb2SMo3O13.28 S1 exhibits three long bonds to bridging oxides, with a range of distances from 1.476(4) to 1.528(3) Å, while the S-Oterminal bond is significantly shorter at 1.453(4) Å. The presence of SO4 tetrahedra on the MoO3 chains results in sinusoidal ruffling in the [(MoO3)3(SO4)]n2nchains that exhibits a period length of 10.750(6) Å, caused by contraction of the MoO6-MoO6 tilt angle by the SO4 tetrahedra to 145.8°. This is analogous to [MoO3(C2N3H3)].29 [(MoO3)3(SO4)]n2n- chains are aligned along the [0 1 0] direction (Figure 2). The space between chains is occupied by [C5H14N2]2+ cations and occluded water molecules, which form the basis of an extensive, threedimensional hydrogen-bonding network. Bond valence sums30 were used to quantify both the relative strength and residual charges on each bond and respective ligand, using parameters compiled by Brese and O’Keeffe.31 The valence of each Mo-O, S-O and O-H bond was calculated (Table 3). The overall charge on each Mo and S site can be calculated by adding the appropriate bond valences. In each case, the Mo and S valences are close to 6+, the expected value. The relative residual negative charge on each oxide ligand can be calculated by adding the valences of each Mo-O, S-O, and O-H bond in which a given oxide participates. The total bond valence (∑Si) is subtracted from the predicted valence of an oxide ligand, giving the residual negative charge on each oxide. The propensity to accept a hydrogen bond from either an organic cation or occluded water molecule is directly related to this nucleophilicity. A wide variation in these charges is observed. The small positive values of O4, O5, and O6 are not surprising as each of these ligands acts as a µ3 bridge between adjacent molybdenum centers. Calculation of the valences for O-H bonds in occluded water molecules is difficult because hydrogen atoms were placed in geometrically idealized positions with fixed bond lengths; therefore, accurate hydrogen atomic positions are not known. Approximations are included because water molecules are an important part of the hydrogen bond structure in [(S)-C5H14N2][(MoO3)3(SO4)]‚ H2O, and neglecting the effects of the O-H valences on the oxide nucleophilicities leads to unreasonable values.
Directed Synthesis of Noncentrosymmetric Molybdates
Crystal Growth & Design, Vol. 5, No. 5, 2005 1915 Table 3. Bond Valence Sumsa for [(S)-C5H14N2][(MoO3)3(SO4)]‚H2O Si
Mo1
O1 O2 O3 O4 O5 O6 O7 O8 O9 O10 O11 O12 O13 O14
1.76 1.76 0.31 0.32 0.92 0.89
∑Si
5.96
Mo2
Mo3
S1
Η atomsc
∑Si
V - ∑Si
0.8 × 2
1.76 1.76 1.80 2.11 2.13 2.09 1.77 1.67 1.77 1.72 1.79 1.78 1.59 1.6
-0.24 -0.24 -0.20 0.11 0.13 0.09 -0.23b -0.33b -0.23b -0.28b -0.21 -0.22 -0.41b -0.4b
1.49 0.88 0.88 0.35 1.77 1.67 0.48
0.91 0.33 0.85 1.30 1.72 1.79 0.32
6.04
5.92
1.46 1.59 5.84
a Valence sums calculated with the formula S ) exp[(R - R )/ i 0 i B], where Si is the bond valence of bond “i”, R0 is a constant dependent upon the bonded elements, Ri is the bond length of bond “i”, and B equals 0.37. ∑Si is the bond valence sum for each atom. V is the predicted valence for a site. R0 (MoVI-O) ) 1.907, R0 (SVIO) ) 1.624. b Hydrogen-bond acceptor (determined by N-O and O-O distances). c O-H bond valences on occluded water molecules are approximated using R0 ) 0.939.
Figure 2. Packing in [(S)-C5H14N2][(MoO3)3(SO4)]‚H2O, viewed along the (a) [0 1 0] and (b) [1 0 0] directions. Red octahedra and blue tetrahedra represent MoO6 and SO4, respectively. White, blue, and red spheres represent carbon, nitrogen, and oxygen atoms, respectively. Hydrogen atoms have been removed for clarity.
Three components comprise the hydrogen-bonding network in [(S)-C5H14N2][(MoO3)3(SO4)]‚H2O. [C5H14N2]2+ cations act as hydrogen bond donors, occluded water molecules act as both donors and acceptors, and the [(MoO3)3(SO4)]n2n- chains are acceptors. Each [C5H14N2]2+ cation donates four hydrogen bonds through its four acidic protons to O8, O9, and O10, each of which resides upon a different [(MoO3)3(SO4)]n2n- chain, and to the occluded water molecule containing O14. This water molecule in turn donates two hydrogen bonds to O7 and O13, each of which are part of two different [(MoO3)3(SO4)]n2n- chains. This results in the formation of an extensive three-dimensional network. The anions that accept hydrogen bonds closely match those with the highest calculated nucleophilicities. The four most nucleophilic oxides each accept a hydrogen bond, while the six least nucleophilic oxides do not (Table 3). The presence of the template is confirmed using infrared spectroscopy. N-H and C-H bands are observed at 3017 and 1612, and 1465 cm-1, respectively. S-O bands are observed at 1104 and 1199 cm-1.
Figure 3. Particle size vs SHG intensity for (a) [(S)-C5H14N2][(MoO3)3(SO4)]‚H2O and (b) [(R)-C5H14N2][(MoO3)3(SO4)]‚H2O. The curves drawn are to guide the eye and are not fits to the data.
Elemental microanalysis for [(S)-C5H14N2][(MoO3)3(SO4)]‚H2O, obsd (calcd): C 9.31% (9.26%), H 2.71% (2.47%), N 4.12% (4.32%), S 4.40% (4.93%), Mo 40.38% (44.41%). The SHG intensity vs particle size is shown in Figure 3a. Measurements were made on ground and sieved powders of [(S)-C5H14N2][(MoO3)3(SO4)]‚H2O, with the 532 nm SHG light quantified as a function of particle
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size. The SHG response for [(R)-C5H14N2][(MoO3)3(SO4)]‚ H2O is approximately 5 × R-SiO2. [(R)-C5H14N2][(MoO3)3(SO4)]‚H2O. The synthesis, structure, and characterization of [(R)-C5H14N2][(MoO3)3(SO4)]‚H2O has been previously reported.20 The structures of [(S)-C5H14N2][(MoO3)3(SO4)]‚H2O and [(R)C5H14N2][(MoO3)3(SO4)]‚H2O are approximately inverses of one another, with small variations in atomic positions and bond lengths and angles. The SHG intensity vs particle size is shown in Figure 3b. Measurements were made on ground and sieved powders of [(R)-C5H14N2][(MoO3)3(SO4)]‚H2O, with the 532 nm SHG light quantified as a function of particle size. The SHG response for [(R)-C5H14N2][(MoO3)3(SO4)]‚ H2O is approximately 5 × R-SiO2.
Muller et al.
Figure 4. [(MoO3)3(SO4)]n2n- chains in (a) Rb2SMo3O13 and (b) [(S)-C5H14N2][(MoO3)3(SO4)]‚H2O. Red, green, and yellow spheres represent oxygen, molybdenum, and sulfur atoms, respectively. Selected symmetry elements are shown.
Discussion Rb2SMo3O13, [(S)-C5H14N2][(MoO3)3(SO4)]‚H2O and [(R)-C5H14N2][(MoO3)3(SO4)]‚H2O each contain [(MoO3)3(SO4)]n2n- chains that exhibit the same connectivity. However, their symmetries differ considerably. Rb2SMo3O13 crystallizes in the centrosymmetric space group P21/n (No. 14), while [(S)-C5H14N2][(MoO3)3(SO4)]‚ H2O and [(R)-C5H14N2][(MoO3)3(SO4)]‚H2O crystallize in the noncentrosymmetric space group P212121 (No. 19), which has the enantiomorphic crystal class 222 (D2).1,32 The differences in both the structure of the [(MoO3)3(SO4)]n2n- chains in Rb2SMo3O13 versus [(S)-C5H14N2][(MoO3)3(SO4)]‚H2O and [(R)-C5H14N2][(MoO3)3(SO4)]‚ H2O are a result of the respective countercation. Rb2SMo3O13 contains spherical Rb+ cations that balance the charge of the [(MoO3)3(SO4)]n2n- chains, while in [(S)C5H14N2][(MoO3)3(SO4)]‚H2O and [(R)-C5H14N2][(MoO3)3(SO4)]‚H2O either [(S)-C5H14N2]2+ or [(R)-C5H14N2]2+ cations are present. Rb+ acts as a space filler and charge balancer, while the role of the 2-methylpiperazinium cations is 2-fold. First, they assume the well-established role of protonated amines in hydrothermal metal oxide chemistry, which includes acting as a source of charge balance for the anionic inorganic architectures, donating hydrogen bonds that stabilize the three-dimensional structure and filling space. The second, which is less well established, involves the effects of cation chirality on the extended symmetry. [(S)-C5H14N2][(MoO3)3(SO4)]‚H2O and [(R)-C5H14N2][(MoO3)3(SO4)]‚H2O were synthesized from reaction gels in which a single enantiomer of 2-methylpiperazine was present, in the form of either [(S)-C5H14N2]2+ or [(R)C5H14N2]2+. The use of such cations precludes the cancellation of the local distortions via formation of extraframework inversion centers because the relation of a [(S)-C5H14N2]2+ cation through an inversion center requires a corresponding [(R)-C5H14N2]2+ cation. The presence or absence of each enantiomer can be chemically controlled; if [(S)-C5H14N2]2+ cations alone are present in a structure, they can never be related to oneanother through centers of inversion because the required [(R)-C5H14N2]2+ cations are absent. Therefore, the formation of any inversion centers is prohibited, and the space group of the material is constrained to be noncentrosymmetric. The chirality of the organic cations is reflected in the enantiomorphic crystal class of these two compounds.
Figure 5. [(MoO2/1O1/2O3/3)3(SO1/1O3/2)]2- clusters in (a) Rb2SMo3O13 and (b) [(S)-C5H14N2][(MoO3)3(SO4)]‚H2O. Red, green, and yellow spheres represent oxygen, molybdenum, and sulfur atoms, respectively. Atomic labels and selected bond lengths (Å) and angles (°) are included.
The reduction of symmetry in Rb2SMo3O13 to that present in [(S)-C5H14N2][(MoO3)3(SO4)]‚H2O and [(R)C5H14N2][(MoO3)3(SO4)]‚H2O is manifested in the structure of the [(MoO3)3(SO4)]n2n- chains. Rb2SMo3O13 contains [(MoO3)3(SO4)]n2n- chains that are aligned along the [0 0 1] direction, reside at (0, 0, z) and (1/2, 1/2, z), and are coincident with a series of inversion centers (Figure 4). The [(MoO3)3(SO4)]n2n- chains in [(S)-C5H14N2][(MoO3)3(SO4)]‚H2O and [(R)-C5H14N2][(MoO3)3(SO4)]‚ H2O contain a 21 screw axis but no inversion centers. Differences can also be observed through investigation of the bond lengths and angles in the
Directed Synthesis of Noncentrosymmetric Molybdates
[(MoO2/1O1/2O3/3)3(SO1/1O3/2)]2- clusters from which the extended chains are formed (Figure 5). These differences are most clearly demonstrated between the Mo1-O bond lengths and O-Mo3-O angles in Rb2SMo3O13 (Figure 5a) and the Mo3-O bond lengths and O-Mo1-O angles in [(S)-C5H14N2][(MoO3)3(SO4)]‚H2O (Figure 5b). The magnitude of the C2 distortion in Mo3 in [(S)C5H14N2][(MoO3)3(SO4)]‚H2O is greater than that in the corresponding molybdenum site in Rb2SMo3O13, Mo1. The Mo3-O5 and Mo3-O10 bond lengths in [(S)C5H14N2][(MoO3)3(SO4)]‚H2O are markedly longer than their counterparts in Rb2SMo3O13, 2.321(4) and 1.706(4) Å versus 2.26(2) and 1.67(2) Å, respectively. This is accompanied by a contraction of the Mo3-O4 and Mo3O6 bond lengths with respect to Rb2SMo3O13 analogues, 1.944(4) and 1.967(5) Å versus 1.95(2) and 1.98(2) Å, respectively, resulting in an elongation of the Mo3 coordination octahedron in [(S)-C5H14N2][(MoO3)3(SO4)]‚ H2O with respect to Mo1 in Rb2SMo3O13. The coordination octahedron surrounding Mo1 in [(S)C5H14N2][(MoO3)3(SO4)]‚H2O is also more distorted than the analogous octahedron in Rb2SMo3O13. The position of the intrachain oxides remains approximately constant between compounds, with bond angles of 72.19(14) and 74.28(15)° versus 71.3 and 72.1° for [(S)-C5H14N2][(MoO3)3(SO4)]‚H2O and Rb2SMo3O13, respectively. However, the two extrachain bond angles differ greatly. O1Mo1-O5 and O1-Mo1-O6 bond angles of 105.11(18) and 102.47(17)° are observed in [(S)-C5H14N2][(MoO3)3(SO4)]‚H2O, while their corresponding bond angles in Rb2SMo3O13 are 102.7 and 108.8°. This is a result of a distortion in the position of extrachain oxide, O1, in [(S)C5H14N2][(MoO3)3(SO4)]‚H2O, with respect to Rb2SMo3O13. The possibility of pseudosymmetry within the inorganic chains was investigated. The organic cations were removed from the crystallographic models in both [(S)C5H14N2][(MoO3)3(SO4)]‚H2O and [(R)-C5H14N2][(MoO3)3(SO4)]‚H2O and Platon33 was used to probe for missing symmetry. None was found, confirming ability of chiral cations to impart noncentrosymmetry in inorganic structures. The noncentrosymmetry of [(S)-C5H14N2][(MoO3)3(SO4)]‚H2O and [(R)-C5H14N2][(MoO3)3(SO4)]‚H2O is manifested in the nonlinear optical activity of these two compounds. Both [(S)-C5H14N2][(MoO3)3(SO4)]‚H2O and [(R)-C5H14N2][(MoO3)3(SO4)]‚H2O are SHG active. Their respective low SHG responses, approximately 5 × R-SiO2, are a result of their crystallizing in a nonpolar enantiomorphic crystal class, (222). Conclusion The directed synthesis of noncentrosymmetric metal oxide materials is possible through the incorporation of enantiomerically pure chiral amines. The inclusion of a single isomer precludes the formation of inversion centers and forces crystallization in a noncentrosymmetric space group. This directed synthesis resulted in the formation of two new function materials, each of which is SHG active.
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Acknowledgment. A.J.N. thanks the Camille and Henry Dreyfus Foundation and the David and Lucile Packard Foundation. P.S.H. thanks the Robert A. Welch Foundation and the NSF-Career Program (DMR0092054) for support. Supporting Information Available: A thermal ellipsoid plot (50% probability) of [(S)-C5H14N2][(MoO3)3(SO4)]‚H2O and an X-ray crystallographic file (CIF), containing complete lists of crystallographic data, atomic positions and anisotropic thermal parameters, and bond lengths and angles, are available free of charge via the Internet at http://pubs.acs.org.
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