Two Homochiral Copper(I) Complexes with Strong Luminescence

Jun 9, 2005 - ABSTRACT: N-3-Cyanobenzylcinchonidinium bromide (N-3-CBC) (1) as a homochiral spacer reacts with CuBr under hydrothermal conditions ...
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CRYSTAL GROWTH & DESIGN

Two Homochiral Copper(I) Complexes with Strong Luminescence Based on Chiral Phase Transfer Catalysts (PTCs) as Building Blocks

2005 VOL. 5, NO. 4 1603-1608

Yu-Mei Song, Qiong Ye, Yun-Zhi Tang, Qian Wu, and Ren-Gen Xiong* Coordination Chemistry Institute, State Key Laboratory of Coordination Chemistry, Nanjing University, Nanjing, 210093, P. R. China Received March 4, 2005;

Revised Manuscript Received May 8, 2005

ABSTRACT: N-3-Cyanobenzylcinchonidinium bromide (N-3-CBC) (1) as a homochiral spacer reacts with CuBr under hydrothermal conditions, resulting in the formation of discrete CuBr2(N-3-CBC)(CH3OH) (2). Similarly, a chiral phase transfer catalyst (CPTC) building block, [bis(N,N′-cinchoninium)-p-xylene dibromide (NCXD)] (3), was employed as an asymmetric bridging linker to construct a one-dimensional (1D) photoluminescence coordination polymer, Cu2Br4(NCXD)(H2O)3 (4). Compounds 2 and 4 exhibit bright luminescent emissions at room temperature upon irradiation by UV light. Crystal data for 2: space group P21 with a ) 8.3596(7) Å, b ) 20.1496(17) Å, c ) 8.7591(8) Å, R ) γ ) 90°, β ) 110.517(2)°, V ) 1381.8(2) Å3, Z ) 2, R1 [I > 2σ(I)] ) 0.0539, wR2 (all) ) 0.1336, Flack value ) 0.011(14); crystal data for 4: space group P212121 with a ) 21.4407(18) Å, b ) 24.779(2) Å, c ) 9.2166(8) Å, R ) β ) γ ) 90°, V ) 4896.6(7) Å3, Z ) 4, R1 [I > 2σ(I)] ) 0.1051, wR2 (all) ) 0.2760, Flack value ) 0.11(3). Introduction Luminescent spectroscopy of copper(I) complexes continues to attract much attention not only because of the unusually large number of emitting excited states of these complexes but also because of the challenges in identifying the nature of the emitting states.1 Metalto-ligand charge transfer is probably the most common transition,2-6 and the synthesis and rational design of suitable organic ligands are a great challenge to date. The special physical property can be achieved through conjugation of the ligand, with a combined crystal engineering strategy and hydro(solvo)thermal synthesis method.7 Furthermore, the ligand conjugation size and steric hindrance have a strong effect on the quantum yields of the copper(I) compounds. The coordination around the copper is frequently, but not always, based on a four-coordinate tetrahedral geometry. Species ranging from monomers, dimers, and tetranuclear cubic clusters to polymers have been identified. In addition, the multinuclear components range from a single bridging group (e.g., pyrazine) through multiple bridging ions (e.g., halides) to multiple ligation in the cubic structures.8-14 So our interest in the luminescence spectroscopy of copper(I) complexes has focused on the synthesis and rational design of the suitable organic ligand, chiral phase transfer catalysts (CPTCs). N-3-Cyanobenzylcinchonidinium bromide (N-3-CBC) (1) as a homochiral ligand reacts with CuBr under hydrothermal conditions, resulting in the formation of CuBr2(N-3-CBC)(CH3OH) (2) (Scheme 1). Similarly, the CPTC [bis(N,N′cinchoninium)-p-xylene dibromide (NCXD)] (3) was also employed as asymmetric bridging ligand to construct a one-dimensional (1D) luminescent coordination polymer, Cu2Br4(NCXD)(H2O)3 (4) (Scheme 2). To the best of our knowledge, the homochiral luminescent copper(I) compounds are relatively rare. Herein we report their syntheses, solid-state structures, and photoluminescence properties. * To whom correspondence should be addressed. E-mail: Xiongrg@ netra.nju.edu.cn.

Scheme 1

Scheme 2

Experimental Section Chemicals and solvents in this work were purchased from Aldrich and used as received. CD spectra were measured on a JASCO J-720W spectrophotometer at room temperature. KBr discs were prepared as for IR measurements except that the particle size was much smaller in the case of the CD experiments. Typically, 80-100 mg of dried KBr and an appropriate amount of a sample were ground extensively in an agate motor, and the mixture was pressed as usual. The FT-IR spectra were recorded as a solid in the KBr matrix in the range 400-4000 cm-1 on a Nicolet Inpact 170S FT-IR spectrometer. Preparation of N-3-Cyanobenzylcinchonidinium Bromide (N-3-CBC) (1). A solution of 3-cyanobenzyl bromide (10

10.1021/cg050081g CCC: $30.25 © 2005 American Chemical Society Published on Web 06/09/2005

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mmol) in CHCl3 (10 mL) was added to a stirred solution of cinchonidine (10 mmol) in CHCl3 (20 mL). The mixture was refluxed for 4 h. The resulting precipitate was isolated by filtration, washed, dried, and recrystallized from ethanol. The colorless crystalline of 1 was obtained in about 83% yield. IR spectrum (KBr, cm-1): 3417(m), 3185(s), 2949(s), 2235(s), 1638(w), 1589(w), 1570(w), 1508(m), 1462(w), 1407(s), 1326(s), 1303(w), 1232(s), 1212(m), 1157(w), 1132(w), 1097(w), 1059(m), 780(m), 759(w), 743(m), 635(w), 572(w). Anal. Calcd for 1: C, 66.06; N, 8.56; H, 5.71. Found: C, 66.14; N, 8.45; H, 5.80%. Preparation of CuBr2(N-3-CBC)(CH3OH) (2). CuBr (2.5 mmol, 0.36 g) and N-3-CBC (1 mmol, 0.49 g) were placed in a thick Pyrex tube (ca. 20 cm long). After addition of 2 mL of HBr (4 N) and 1 mL of MeOH, the tube was frozen with liquid N2, evacuated under vaccum, and sealed with a torch. The tube was heated at 50 °C for 2 days to give red block crystals (pure phase) in 40% yield based on N-3-CBC. IR spectrum (KBr, cm-1): 3285(br, s), 2936(w), 2247(s), 1657(w), 1590(m), 1059(m), 780(m), 759(w), 743(m), 635(w), 572(w). Anal. Calcd for 2: C, 51.22; N, 6.3; H, 4.8. Found: C, 51.54; N, 6.05; H, 4.5%. Preparation of [Bis(N,N′-cinchoninium)-p-xylene Dibromide (NCXD)] (3). A solution of 4,4′-dibromo-p-xylene (10 mmol) in DMF (20 mL) was added to a stirred solution of cinchonidine (20 mmol) in DMF (20 mL). The mixture was refluxed for 14 h. The resulting precipitate was isolated by filtration, washed, dried, and recrystallized from ethanol. The colorless crystalline of 3 was obtained in about 72% yield. IR spectrum (KBr, cm-1): 3405(br, s), 2951(w), 1665(s), 1059(m), 1063(w), 778(m), 552(w). Anal. Calcd for 3: C, 64.73; N, 6.58; H, 6.10. Found: C, 64.54; N, 6.65; H, 6.15%. Preparation of Cu2Br4(NCXD)(H2O)3 (4). CuBr (2.5 mmol, 0.36 g) and NCXD (1 mmol, 0.85 g) were placed in a thick Pyrex tube (ca. 20 cm long). After addition of 2 mL of HBr (4 N) and 1 mL of 2-butanol, the tube was frozen with liquid N2, evacuated under vaccum, and sealed with a torch. The tube was heated at 80 °C for 4 days to give pale yellow block crystals (pure phase) in 40% yield based on NCXD. IR spectrum (KBr, cm-1): 3345(br, s), 2957(w), 2359(w), 1628(w), 1590(m), 1239(w), 944(w), 770(m), 558 (w). Anal. Calcd for 4: C, 46.22; N, 4.69; H, 4.86. Found: C, 46.08; N, 4.82; H, 5.01%. Single-Crystal Structure Determination. A suitable yellow single crystal of 2 or 4 was carefully selected under a polarizing microscope and glued to a thin glass fiber with cyanoacrylate (superglue) adhesive. The data were collected on a Siemens SMART CCD area detector diffractometer equipped with Mo KR radiation (λ ) 0.71073 Å) using ω-scan mode. The data collection covered over a hemisphere of reciprocal space by a combination of three sets of exposures; each set had a different φ angle (0°, 88°, and 180°) for the crystal, and each exposure of 30 s covered 0.3° in ω. The crystal-to-detector distance was 4 cm, and the detector swing angle was -35°. Coverage of the unique set is over 99% complete. Crystal decay was monitored by repeating 30 initial frames at the end of data collection and analyzing the duplicate reflections and was found to be negligible. The unit cell parameters were determined using SMART.15 The three sets of data collected were reduced using the program SAINT.16 The structure was solved with direct methods using the program SHELXTL.17 All the non-hydrogen atoms were located from the trial structure and then refined anisotropically with SHELXTL using full-matrix least-squares procedure. The hydrogen atom positions were fixed geometrically at calculated distances and allowed to ride on the parent carbon atoms. Crystal data and structure refinement parameters for CuBr2(N3-CBC) (CH3OH) (2) and Cu2Br4 (NCXD) (H2O) 3 (4) are listed in Table 1. Measurement of SHG Responses. Approximate estimations of the second-order nonlinear optical intensity were obtained by comparison of the results obtained from a pellet (Kurtz powder test) of powdered sample (80-150 µm diameter), with that obtained for urea. A pulsed Q-switched Nd: YAG laser at a wavelength of 1064 nm was used to generate the SHG signal. The backward scattered SHG light was

Song et al. Table 1. Crystal Data and Structure Refinement Parameters for 2 and 4 structure parameters empirical formula fw cryst syst space group cryst size (mm) a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) vol (Å3) Z Fcalc (g cm-3) µ (mm-1) θ range (deg) total no. of data collcd no. of unique data R indexesa [I > 2σ(I)] R (all data)a goodness-of-fit on F2 largest diff map hole and peak, e Å-3 a

2 C28H32Br2CuN3O2 665.93 monoclinic P2(1) 0.08 × 0.1 × 0.3 8.3596(7) 20.1496(17) 8.7591(8) 90 110.517(2) 90 1381.8(2) 2 1.600 3.712 2.02 to 29.44 10083 5678 R1 ) 0.0539, wR2 ) 0.1135 R1 ) 0.0968, wR2 ) 0.1336 0.980 0.635 and -0.355

4 C46H58Br4Cu2N4O5 1193.68 orthorhombic P2(1)2(1)2(1) 0.1 × 0.2 × 0.3 21.4407(18) 24.779(2) 9.2166(8) 90 90 90 4896.6(7) 4 1.619 4.180 2.07 to 26.00 30010 9624 R1 ) 0.1051, wR2 ) 0.2536 R1 ) 0.1641, wR2 ) 0.2760 1.054 0.809 and -0.939

R1 ) ∑||Fo| - |Fc||/∑|Fo|; wR2 ) [∑ w(Fo2 - Fc2)2/∑ w(Gο2)2]1/2.

Figure 1. ORTEP diagram (50% probability ellipsoids) showing the solid-state structure for 2. collected using a spherical concave mirror and passed through a filter that transmits only 532 nm radiation. Thus, the SHG responses of 2 and 4 are equal to that of urea, respectively.

Results and Discussion Initial Characterization. Elemental analysis, CD (circular dichroism), IR, and photoluminescence were used to characterize the compounds 2 and 4. A very strong peak at ca. 2235 cm-1 in the IR spectrum of 1 suggests the presence of a cyan group. In the IR spectrum of 2, there is a very strong peak at ca. 2247 cm-1 that also suggests the presence of a cyan group. There are no typical peaks in IR spectra of both 3 and 4; however, elemental analysis results support the formation of them. Structure of CuBr2(N-3-CBC)(CH3OH) (2). The structure of 2 has 31 non-hydrogen atoms in the asymmetric unit, of which there is only one Cu atom. The Cu atom has a planar-trigonal coordination with two bromide atoms and a nitrogen atom of the quinoline ring. Thus, ligand 1 only acts as a monodentate, and bromides are terminal group to lead in the formation of a discrete monomer, as shown in Figure 1. Few monomeric neutral bromocopper(I) complexes with ni-

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Crystal Growth & Design, Vol. 5, No. 4, 2005 1605

Figure 3. Solid state structures of 4. Hydrogen atoms and water molecules were omitted for clarity. Color codes: C black, N blue, O red, and Cu cyan. Table 4. Hydrogen Bond Distances (Å) and Angles (°) in 4

Figure 2. A perspective view of the 3D network of 2 through hydrogen bonds. Table 2. Selected Bond Distances (Å) and Angles (°) for 2 Br(1)-Cu(2) Cu(2)-Br(2) O(1W)-C(30) C(4)-C(5) N(3)-Cu(2)-Br(1) Br(1)-Cu(2)-Br(2) N(1)-C(1)-C(20)

2.3612(14) 2.3764(13) 1.281(14) 1.538(8) 124.37(18) 115.31(5) 178.2(10)

Cu(2)-N(3) C(1)-N(1) N(2)-C(9) O(1)-C(4) N(3)-Cu(2)-Br(2) C(15)-N(2)-C(9) C(8)-C(9)-N(2)

1.993(5) 1.133(10) 1.517(7) 1.408(7) 120.20(18) 110.0(5) 114.3(4)

Table 3. Hydrogen Bond Distances (Å) and Angles (°) in 2 D-H

d d (D-H) (H‚‚‚A) ∠DHA

A

symmetry

O(1)-H1A 0.8202 2.1609 119.69 O(1W) C(9)-H(9B) 0.9703 2.5259 172.68 O1(W) C(10)-H(10A) 0.9299 2.9108 161.21 Br(2) 2 - x, -1/2 + y, 1-z C(14)-H(14A) 0.9307 2.3278 103.62 O(1) C(15)-H(15A) 0.9707 2.7581 154.39 Br(1) 1 - x, -1/2 + y, 1-z C(16)-H(16A) 0.9701 2.4919 125.32 O(1)

trogen donor ligands have as yet been reported,18-21 and to the best of our knowledge 2 is the first example of a structurally determined homochiral metal complex containing N-3-cyanobenzyl-cinchonidinium anion as a ligand. It is worth noting that the structure packs in such a way that there are no significant π-π interactions (Figure 2). Selected bond distance and angles are listed in Table 2. The Cu-N distance is 1.993(5) Å, shorter than that found in bis ((µ2-bromo)-bis (quinolineN)-copper(I)) quinoline solvate (2.011 Å).22 The Cu-Br distances in the range of 2.3612(14)-2.3764(13) Å (av 2.3688 Å) are comparable to those of bis ((µ3-bromo)bis (µ2-bromo)-tetrakis(quinoline-copper(I)) (2.385 Å),23 but much shorter than those observed in bis((µ2-bromo)bis (quinoline-N)-copper(I)) quinoline solvate (2.5586 Å). The N/Br-Cu-N/Br bond angles range from 115.31 (5)° to 124.37 (18)° (av 119.96°). A hydrogen bond was found between methanol molecule and bromide atoms in the lattices to generate an interesting hydrogen-bonding network in the solid state. The O-H of the methanol molecules, the H atoms of the hydroxyl group of the ligand, and the bromide atoms as well as the H atoms of the aromatic ring are all involved in hydrogen-bond formation. The C-H hydrogen bonds to Br(1) and Br(2) among layers are the major ones, resulting in the formation of a three-dimensional (3D) network (Figure 2). Selected hydrogen-bonding interactions are listed in Table 3.

d (D-H)

d (H‚‚‚A)

∠DHA

A

symmetry

C3-H3A

0.9271

2.8686

155.37

Br4

3/2 - x, 2 - y, -3/2 + z

C9- H9B C13-H13A

0.9696 0.9311

2.3567 2.7271

130.07 172.10

O2 Br2

C14-H14A C15-H15B C16-H16B

0.9300 0.9700 0.9698

2.4500 2.3267 2.8512

136.28 125.20 162.21

O1 O1 Br4

C16-H16A

0.9695

2.5922

157.33

O2W

D-H

x, -1 + y, -1 + z x, y, 1 + z 3/2 - x, 2 - y, -1/2 + z

Structure of Cu2Br4(NCXD)(H2O)3 (4). The crystal structure of 4 reveals a cation infinite polymeric chain bridged alternately by two bromides and NCXD′ cation (Figure 3). In the structure, the copper(I) centers are also bridged by bromide, and the two central copper(I) atoms have a different coordination environment, which are unusual in halocopper(I) complexes containing nitrogen donors.18,24 One of the copper(I) atoms coordinated with three bromide atoms and one nitrogen atom of the quinoline ring. The coordination geometry of this metal center is thus a distorted tetrahedral. Another copper(I) atom has a different coordination geometry and is bonded to two bromide atoms and a nitrogen atom to display a nonplanar trigonal as that in 2. The cation charges of this highly unusual Cu(I)-Br polymer are balanced by a bromide anion. Three highly ordered water molecules are present in the lattice. They also participate in hydrogen bonds. There are intramolecular and intermolecular hydrogen bonds in the structure. The C13-H13A hydrogen bonds to Br2 are intramolecular. The C3-H3A hydrogen bonds to Br4 and C16-H16B hydrogen bonds to Br4 are intermolecular. Similar to compound 2, the C-H proton hydrogen bonds to bromide atoms among layers are the major ones resulting in the formation of a 3D network. Selected hydrogen-bonding interactions are depicted in Table 4. Since the distances of aromatic rings are all much larger than 3.8 Å, there are also no significant π-π interactions in the structure. The Cu-Br bond distances, compared with those in 2, are longer in the range of 2.398(4)-2.629(4) Å (av 2.527 Å). This can be ascribed to the fact that the bromide in 2 is only coordinated to one copper(I) atom, while in 4, the bromide atoms are linked to two copper(I) atoms. The Cu-N (1.996(12) Å) distance is similar to that found in 2 (1.993(5) Å). The Cu-Cu distance is unusually short, only 2.467 Å, which is much shorter than that found in bis ((µ2-bromo)bis(quinoline-N)-copper(I)) quinoline solvate (3.140 Å) and bis((µ3-Bromo)-bis (µ2-bromo)-tetrakis (quinoline-

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Song et al.

Figure 4. A 1D chain representation of [Cu2Br4(NCXD)(H2O)3]n (4) highlighting Cu tetrahedral.

Figure 6. Luminescent spectrum of CuBr2(N-3-CBC)(CH3OH) (2) in solid at room temperature (λex ) 415 nm).

Figure 5. A perspective view of the 3D network of 4 through hydrogen bonds. Table 5. Selected Bond Distances (Å) and Angles (°) for 4 Br(4)-Cu(1) 2.506(4) Br(3)-Cu(1) 2.398(4) Br(2)-Cu(2) 2.505(4) Cu(1)-Cu(2) 2.476(3) N(2)-C(32) 1.487(16) Cu(1)-Br(4)-Cu(2) 58.05(10) N(1)-Cu(2)-Cu(1) 177.1(5) Br(2)-Cu(2)-Br(4) 106.00(12) Cu(1)-Cu(2)-Br(2) 72.76(16) Br(2)-Cu(2)-Br(3) 98.16(12)

Br(4)-Cu(2) 2.595(4) Br(3)-Cu(2) 2.629(4) Cu(2)-N(1) 1.996(12) N(3)-C(16) 1.515(17) C(18)-C(5) 1.332(5) Cu(1)-Br(3)-Cu(2) 58.82(10) N(1)-Cu(2)-Br(2) 108.8(6) Br(4)-Cu(2)-Br(3) 97.33(12) N(1)-Cu(2)-Br(4) 117.9(5) Cu(1)-Cu(2)-Br(3) 55.93(11)

copper(I)) (3.986 Å),22,23 suggesting there may be a strong Cu(I)-Cu(I) interaction. The N/Br-Cu-N/Br bond angles range from 97.33(12)° to 106.00(12)° (av 100.482°) is smaller than those found in 2. As expected, the C-C, C-N, and C-O bonds are unexceptional as it can be seen in Table 5 which lists selected bond distance and angles. Finally, in contrast with the structures of 2 and 4, it is found that the rigidity of the ligand 3 is much harder than that of ligand 1 after its coordination to a metal ion, suggesting that the catalytic action may be more effective than that of 2 because the more the rigidity of chiral phase transfer ligand, the more effective its catalytic action.25 Luminescent Spectra of 2 and 4. The complexes of copper(I) always display two maximum emissions: a high-energy (HE) band (430-540 nm) and a low energy (LE) band (620-698 nm).26a The HE band was attributed to metal-to-ligand charge transfer (MLCT) or halide-to-ligand charge transfer (XLCT). The HE band energies are quite sensitive to substituents on the

Figure 7. Luminescent spectrum of [Cu2Br4(NCXD)(H2O)3]n (4) in solid at room temperature (λex ) 415 nm).

ligand, but electron-withdrawing substituents shift them to lower values.26b-26d The LE band was observed for complexes with Cu-Cu distances less than the summed Cu (I) van der Waals radii (2.8 Å) and considered as a property of the Cu4X4 core and involves extensive interactions between the copper(I) centers owning to the σ bonding nature of the LUMO for the clusters with short metal-metal distances.26 The luminescent spectra of 2 and 4 in the solid state at room temperature are shown in Figures 6 and 7, respectively. The emission spectrum of 2 with a maximum at ca. 586 nm is similar to those found in Cu4I4 (pyridine) 4 (λmax ) 580 nm), [Cu (3,4-bipyridine)(Br)]n (λmax ) 580 nm), [(Cu(I))3(4-PYA)2(H2O)(BF4)], and [Cu(3-PYA)]n as well as {[(2-PYA)Cu(I)]×e2(H2O)}n (λmax ) 580 nm, PYA ) pyridylacrylate).27,28 Thus, the emission at 586 nm in 2 can be tentatively assigned to MLCT because a maximum emission peak of the free ligand was observed at ca. 518 nm. Since in the structure of 4 the Cu-Cu distance is only 2.476(3) Å which was much shorter than 2.8 Å, there must be an LE band in the luminescent spectra of 4. As shown in Figure 7, we can see the maximum emission of compound 4 exactly appears at ca. 618 nm while the maximum emission peak about

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Crystal Growth & Design, Vol. 5, No. 4, 2005 1607

symmetric space group (P21 and P212121, respectively) their optical properties were studied. Preliminary examinations of powdered samples indicate that both 2 and 4 are SHG-active with an approximate response similar to that of urea.29,30 Conclusions

Figure 8. CD spectra using a KBr disc containing solid CuBr2(N-3-CBC)(CH3OH) (2).

Figure 9. CD spectra using a KBr disc containing solid [Cu2Br4(NCXD)(H2O)3]n (4).

its free ligand occurred at ca. 525 nm but the intensity is very weak.28d CD Spectra of 2 and 4. A single crystal of 2 or 4 was cut into two pieces. One piece was submitted to X-ray crystallographic analysis followed by absolute configuration determination. The second piece was used to produce a solid-state CD spectrum (as shown in Figures 8 and 9, respectively). They were all CD-active in exactly the same manner. In both of the CD spectra, there were excision-coupled split Cotton effects (negative to positive in going from shorter to longer wavelengths). This indicates that the two Cu(I) complexes are both homochiral.29 The origin of optical activity in the crystalline state is not yet understood. Even though the chiral origin of the CD spectra is difficult to explain, the microcrystalline method is simple and tells us immediately whether crystals belong to the same handedness or not. In the future, the determination of absolute configuration and chirality of a crystal by the CD spectral pattern will become possible by the accumulation of a large number of experimental data and theoretical considerations.29a Powder Second Harmonic Generation Results. Given that 2 and 4 both crystallize in the noncentro-

In summary, we have demonstrated the synthesis, structure, and photoluminescence properties of two homochiral copper(I) complexes. In contrast to the previously reported structures, formed predominantly by CPTCs, the present compounds have ligation only by the nitrogen atom of the quiniline ring without coordination with the olefin group. This increases the structural diversity and variety of the known metal coordination complexes. Furthermore, the two compounds can be excited directly to emit strong luminescence at room temperature. It is likely that other new optical materials can be prepared by the careful choice of ligating CPTC molecules. Solid-state spectroscopy such as CD spectroscopy is a promising research area, which can provide valuable information about molecular interactions in the solid state together with X-ray diffractometry. Further work will be necessary to obtain true solid-state CD and to understand the origin of crystal CD spectra. Acknowledgment. The authors acknowledge financial support from the Major State Basic Research Development Program (Grant No. G2000077500), National Natural Science Foundation of China, Distinguished Young Scholar Funds to R.G.X. (2025103) and Overseas Outstanding Young Scholar Fund (Z.-L.X.) (20028101) from the National Natural Science Foundation of China, EYTP of MOE (PRC), BK2003204 (PRC) and the National Science Foundation (CHE-0212137). We thank Prof. Z. Xue for his revision of this manuscript. Supporting Information Available: X-ray data in CIF format. This material is available free of charge via the Internet at http://pubs.acs.org.

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