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Logo of nihpaAbout Author manuscriptsSubmit a manuscriptHHS Public Access; Author Manuscript; Accepted for publication in peer reviewed journal;
 
J Org Chem. Author manuscript; available in PMC 2010 September 18.
Published in final edited form as:
PMCID: PMC2749078
NIHMSID: NIHMS140076

Facile Self-Assembly of Neutral Dendritic Metallocycles via Oxygen-to-Platinum Coordination

Abstract

A new approach for the fabrication of neutral dendritic metallocycles is described. By combining rigid 120° dicarboxylate donor linkers funtionalized with [G0]-[G3] Fréchet-type dendrons and complementary rigid 60° and 120° di-Pt(II) acceptor subunits, neutral rhomboidal metallodendrimers and hexagonal metallodendrimers, respectively, were prepared under mild conditions in high yields. The assemblies have well-defined shapes and sizes and were characterized by multinuclear NMR (1H and 31P), mass spectrometry (ESI (+)-TOF-MS and APPI(+)-TOF-MS), and elemental analysis. Isotopically resolved mass spectrometry data support the formation of the neutral [2+2] rhomboidal and [3+3] hexagonal metallodendrimers, and NMR data are consistent with the formation of all ensembles. The structures of the [G0] and [G1] neutral rhomboidal metallodendrimers (3a and 3b) were unambiguously confirmed via single-crystal X-ray crystallography. The shape and size of [G3] neutral hexagonal metallodendrimer 5d was established with MMFF force-field simulations.

Introduction

The directional-bonding approach of coordination-driven self-assembly1 has proven to be a particularly powerful method for the construction of supramolecular two-dimensional (2D) and three-dimensional (3D) structures with well defined shapes and sizes2. Dative metal-ligand bonds are highly directional and relatively strong compared to the weaker noncovalent interactions3 such as hydrogen-bonding, van der Waals, and solvophobic interactions. The directional-bonding coordination-driven approach allows for the combination of rigid electron-poor metal centers and complementary, rigid electron-rich organic donors to provide a wide variety of discrete polygonal and polyhedral coordination assemblies.4

In this area, square planar platinum and palladium metals have long been used in conjunction with neutral nitrogen-based organic building blocks such as substituted pyridines and nitriles. Upon formation the resulting supramolecular structures are positivly charged, bearing as many positive charges as Pt-N or Pd-N coordination bonds. The coordination of anionic ligands to metals such as Mo,5 Rh,6 Re,7 and Zn8 in order to produce neutral supramolecules has also been explored. Neutral supramolecular assemblies have the advantage that they are more readily soluble in organic solvents and are likely to be more suitable for the encapsulation of neutral organic guests. Recently, oxygen-to-platinum coordination has been shown to be a suitable means of constructing neutral supramolecular self-assemblies.9 For example, rigid or flexible dicarboxylate–based building blocks have been used to self-assemble neutral rectangles, rhomboids, and triangles with Pt(II)-based acceptors via Pt-O coordination-driven self-assembly methodology.9a,b In addition, various neutral functionalized polygons have also been prepared from dicarboxylate building blocks functionalized with ferrocene9c, carborane9d, and crown ether9f moities. However, research involving the self-assembly of neutral dendritic metallocycles formed via oxygen-to-platinum coordination is still a challenge. Noncovalent routes to the facile preparation of neutral dendritic supramolecules are especially attractive given the wide-ranging applications of dendrimers, the ease of noncovalent self-assembly, and the greater range of organic media that neutral supramolecules are miscible in.

Since the pioneering work of Newkome et al.10 and Balzani et al.11 in the early 1990s, metallodendrimers12 have received considerable attention because of their potential applications in catalysis,13 biological mimetics,14 and in photo- and electrochemistry15. Newkome and coworkers have previously reported the synthesis of isomeric neutral metallodendrimers based on bis(2,2′:6′,2″-terpyridine)ruthenium(II) connectivity16a,b as well as neutral metallodendrimers possessing adamantane termini16c. Very recently, we have developed a new strategy17 to prepare metallodendrimers18 with well-defined shapes and sizes through exo-functionalization, e.g. the covalent attachment of Fréchet-type dendrons to the periphery of supramolecules. For example, by combining predesigned 120° angular dendritic organic donors with 180° di-Pt(II) acceptors, “snowflake-shaped” [6+6] metallodendrimers have been prepared via coordination-driven self-assembly.18b

With the aim of developing the new self-assembly paradigm that utilizes Pt-O coordination interactions, our attention has turned to the construction of neutral dendritic metallocycles with well-defined shapes and sizes. Herein we report the design, self-assembly, and characterization of a new family of platinum(II)-based neutral dendritic metallocycles with predetermined shapes (rhomboid and hexagon) from 120° dendritic dicarboxylate donor tectons substituted with Fréchet-type dendrons19 (Figure 1). Characterization of these new neutral metallodendrimers by X-ray crystallography, multinuclear NMR, mass spectroscopy, and elemental analysis is discussed.

FIGURE 1
Schematic and chemical structures of 120° dendritic dicarboxylate donor subunits 1.

Results and Discussion

Self-Assembly of Neutral Rhomboidal Metallodendrimers 3a–d

In general, the shape of an individual two-dimensional polygon is determined by the value of the turning angle within its angular components. According to the “directional bonding” and “symmetry interaction” models,2a,e discrete rhomboidal entities can be self-assembled via the combination of 60° units with 120° linking components. Upon the addition of an aqueous solution of 120° dendritic disodium carboxylate 1a-d to an acetone solution of 60° angular acceptor 2,9-(trans-Pt(PEt3)2NO3)2-phenanthrene (2)20 in a 1:1 molar ratio a white precipitate of rhomboidal neutral metallodendrimers 3a-d was formed (Scheme 1). In each case the product was centrifuged and washed several times with water. The white solid was then dissolved in CD2Cl2 for 1H and 31P{1H} NMR studies.

SCHEME 1
Self-Assembly of [G0]–[G3] 120° Angular Dendritic Linkers 1ad with 60° Di-platinum Acceptor 2 to Afford Neutral Rhomboidal Metallodendrimers 3ad

Multinuclear NMR (1H and 31P) analysis of [G0]–[G3] assemblies 3ad exhibited very similar characteristics, each of which suggested the formation of discrete, highly symmetric rhomboidal dendritic metallocycles. The 1H and 31P{1H} NMR spectra of 3d are shown in Figure 2 as representative examples. The 31P{1H} NMR spectra of the [G0]–[G3] assemblies 3ad displayed a sharp singlet (ca. 19.0 ppm) shifted upfield from the starting platinum acceptor 2 by approximately 2.0 ppm. Compared to the charged rhomboidal metallodendrimers18b, where the corresponding shift is 6.4 ppm, this shift is noticeably smaller. This smaller shift can be attributed to the greater similarity between the newly formed platinum-oxygen bond and the Pt-ONO2 bond in the starting material. Examination of the 1H NMR spectrum of each neutral rhomboidal metallodendrimer 3a-d is indicative of the formation of highly symmetrical structure. For example, two sharp singlets at 8.27 ppm and 7.80 ppm, respectively, were assigned to the protons (Ha and Hb) on the benzene ring of the dendritic dicarboxylate in 3d. Moreover the singlet and doublet at 8.70 ppm and 7.74 ppm, respectively, were also observed in the 1H NMR spectrum of 3d, which can be attributed to the protons H4 and H2 on the phenanthrene ring (Figure 2). The sharp NMR signals in both the 31P and 1H NMR (see Supporting Information), along with the solubility of these species, ruled out the formation of oligomers in solution.

FIGURE 2
The partial 1H NMR (top) and 31P{1H} NMR (bottom) spectra of [G3] neutral rhomboidal metallodendrimer 3d (See Figure 1 and Scheme 1 for the structures of building blocks 1a-d and 2).

The structures of the neutral rhomboidal metallodendrimers 3a and 3b have also been confirmed by ESI-MS spectrometry. In the ESI mass spectra of the [G0] and [G1] assemblies, peaks attributable to [M + H]+ at m/z = 2618.8 for 3a and m/z = 3043.0 for 3b, where M represents the intact assemblies, were observed. These peaks were isotopically resolved and they agree very well with their respective theoretical distributions (Figure 3). The analysis of the signals observed in the full mass spectra confirmed that no other assembled species were formed during self-assembly.

FIGURE 3
Calculated (top) and experimental (bottom) ESI-MS spectra of [G0] rhomboidal metallodendrimer (A) and [G1] rhomboidal metallodendrimer (B).

X-ray crystallographic analysis unambiguously established the structures of 3a and 3b as discrete [G0] and [G1] neutral rhomboidal metallodendritic assembles (Figures 4 and and5).5). Crystals suitable for single-crystal X-ray analysis were grown by vapor diffusion of acetone into separate dichloromethane solutions of 3a and 3b at ambient temperatures for 2 days. Table 1 summarizes the data, structure solution, and refinement for 3a and 3b. At the molecular level, both structures feature a well-defined rhombus with an approximately 2.4×1.3 nm cavity, similar to that of the cationic rhomboidal metallodendrimers reported previously (2.3×1.3 nm).18b The rhomboidal structure of 3a has external dimensions of ca. 3.09 nm long and 1.89 nm wide, while 3b spreads out over an area of ca. 4.11×1.91 nm2. The geometry around each Pt metal center in both 3a and 3b is approximately square planar. For example, in 3a an O(9)-Pt(1)-P(1) angle of 87.1, O(9)-Pt(1)-P(2) angle of 94.6, and a P(1)-Pt(1)-P(2) angle of 173.06 are observed, which is consistent with a square planar geometry. Selected bond lengths and angles are presented in Table 2.

FIGURE 4
Crystal structure of [G0] neutral rhomboidal metallodendrimer 3a. Hydrogen atoms have been removed for clarity.
FIGURE 5
Crystal structure of [G1] neutral rhomboidal metallodendrimer 3b. Hydrogen atoms have been removed for clarity.
TABLE 1
Crystal Data and Structure Refinement for 3a and 3b
TABLE 2
Selected Bond Lengths (Å) and Angles (deg) for 3a and 3b

Self-Assembly of Neutral Hexagonal Metallodendrimers 5a–d

Discrete hexagonal entities can be self-assembled via the combination of two complementary ditopic building blocks A2 and X2, each incorporating 120° angles between their coordination sites, allowing for the formation of hexagonal structures of type A23X23.1d,2a Recently, three-component and six-component multi-cationic hexagonal metallodendrimers have been prepared by using this strategy.18 In order to extend the scope of this approach to the construction of neutral hexagonal metallodendrimers, we have investigated the self-assembly of 120° dendritic disodium carboxylates 1a-d with 120° di-platinum acceptor 421 (Scheme 2).

SCHEME 2
Self-Assembly of [G0]–[G3] 120° Angular Dendritic Linkers 1ad with 120° Di-platinum Acceptor 4 to Afford Neutral Hexagonal Metallodendrimers 5ad

Neutral hexagonal metallodendrimers 5a-d were prepared according to the same procedure used for in the self-assembly of neutral rhomboidal metallodendrimers 3a-d. 31P{1H} NMR analysis of each product is consistent with the formation of a single, highly symmetric species as indicated by the appearance of a sharp singlet (ca. 18.8 ppm) with concomitant 195Pt satellites, shifted upfield by ca. 1.9 ppm as compared to 4. As expected this upfield shift is also smaller than that in the case of the previously reported charged hexagonal ones (ca. 5.5 ppm)18a, which occurs for the similar reason discussed in the formation of neutral rhomboidal metallodendrimers. Likewise, the 1H NMR spectra of metallodendrimers 5ac displayed very simple and sharp resonances, indicative of the generation of the desired symmetric hexagonal products. For instance, the 1H NMR spectrum of 5d presented two sharp singlets at 8.18 ppm and 7.70 ppm, respectively, which can be assigned to the protons (Ha and Hb) on the benzene ring of the dendritic dicarboxylate (Figure 6). A doublet attributable to the proton (Hc) on the benzene ring attached to the platinum atom was found at 7.59 ppm in the 1H NMR spectrum of 5d as well. Upon stirring at 298 K for 72 h, the 31P{1H} and 1H NMR of assemblies 5a-c do not show any significant changes, demonstrating the stability of these novel supramolecular assemblies in solution. The formation of oligomers was ruled out by the sharp NMR signals in both the 31P{1H} and 1H NMR spectra (see Supporting Information) along with the solubility of these species.

FIGURE 6
The partial 1H NMR (top) and 31P{1H} NMR (bottom) spectra of [G3] neutral hexagonal metallodendrimer 5d (See Figure 1 and Scheme 2 for the structures of building blocks 1a-d and 4).

Mass-spectrometric studies of metallodendrimers 5ad were performed by ESI(+)-TOF-MS and APPI(+)-TOF-MS spectrometry, which allows the assemblies to remain intact during the analysis process in order to obtain the high resolution required for the unambiguous determination of their absolute molecular weight and molecularity. However, due to the high molecular weight and relatively weak Pt(II)-O bonds formed between a soft metal and a hard ligand, it is more difficult to get strong mass signals even under the ESI(+)-TOF-MS or APPI(+)-TOF-MS conditions. With considerable effort, however, two peaks corresponding to the charge states [M + Na] + and [M + 2Na]2+ of [G0] assembly 5a were observed in both ESI(+)-TOF-MS and APPI(+)-TOF-MS spectra: m/z =3963.2 and m/z =1993.6, respectively. These peaks were isotopically resolved (Figure 7) and agree well with their theoretically predicted distribution. The results of these mass studies provide strong support for the formation of neutral hexagonal metallodendrimers.

FIGURE 7
Calculated (top), experimental ESI(+)-TOF-MS (middle), and experimental APPI(+)-TOF-MS (bottom) spectra of [G0] neutral hexagonal metallodendrimer 5a (column A shows the [M + Na]+ peak and column B shows the [M + 2Na]2+ peak).

Close examination of the mass spectra of 5a revealed no peaks indicating the formation or existence of [2+2] rhomboidal or [4+4] octagonal structures. It should be noted that it is not possible to form polygons with an odd number of sides (i.e. pentagon, heptagon, etc.) by combining 120° donors with 120° acceptors as they would require the direct connection of either two acceptor or two donor moieties. The similar characteristics observed in each 1H and 31P NMR spectrum of [G0]-[G3] neutral assemblies 5a-d, e.g. the singularity of each 31P{1H} NMR signal, along with the lack of mass spectral peaks corresponding to other polygon architectures in 5a are supportive of the conclusion that [3+3] neutral hexagonal metallodendrimers are the products in each self-assembly. The elemental analysis obtained for neutral hexagonal metallodendrimer assemblies 5a-d all match their theoretical values.

Large supramolecular hexagonal structures and flexible, high generation dendrimers often prove difficult to crystallize. Hence, all attempts to grow X-ray quality single crystals of hexagonal metallodendrimers 5ac have proven unsuccessful to date. MMFF force-field simulations were employed to optimize the geometry of the [G3] neutral hexagonal metallodendrimer 5d. The space-filling model of the simulated structure (Figure 8) indicates that 5d has a roughly planar hexagonal ring at its core surrounded by flexible dendrons. The hexagonal ring-shaped metallodendrimer has an internal radius of approximately 1.1 nm and an average outer dendron radius of 3.5 nm.

FIGURE 8
Simulated molecular model of [G3] neutral hexagonal metallodendrimer 5d (C = grey, O = red, P = purple, Pt = yellow; hydrogen atoms have been removed for clarity).

Conclusion

The work presented here provides a simple yet effective approach to the construction of well-defined neutral metallodendrimers possessing predetermined sizes and shapes via oxygen-to-platinum coordination. [G0]–[G3] neutral rhomboidal metallodendrimers and hexagonal metallodendrimers were prepared by the combination of predesigned 120° dendritic dicarboxylate donor subunits and di-Pt(II) acceptors with appropriate complementary angles (60° and 120°, respectively) under mild conditions in high yields. Multinuclear NMR (1H and 31P) analysis of all assemblies displayed very similar characteristics that are indicative of the generation of discrete, highly symmetric species. All neutral metallodendrimers exhibit remarkable solubility in common organic solvents, such as dichloromethane and chloroform. The sharp NMR signals in both 31P{1H} and 1H NMR spectra along with the solubility of these species ruled out the formation of oligomers. The structures of metallodendrimers 3a, 3b, and 5a were further confirmed by mass spectrometry. The structures of [G0] and [G1] neutral rhomboidal dendrimers 3a and 3b were unambiguously established via X-ray crystallography.

We have provided a novel strategy for the design and synthesis of neutral dendritic metallocycles, in which the formation of Pt(II)-O bonds plays an essential role during the construction of the desired assemblies. This approach can be used to prepare a variety of neutral metallodendrimers with well-defined shapes and sizes through the proper choice of subunits with predefined angles and symmetry. For instance, the shape of the neutral dendritic metallocycles can be rationally designed to be either a rhomboid or a hexagon as described herein. This study offers a complementary approach to the synthesis of metallocycles that avoids the polycationic nature of analogous Pt-N based dendritic metallocycles. Extending this idea further to additional two-dimensional structures, such as squares, rectangles and triangles, and even three-dimensional architectures like trigonal prisms and trigonal bipyramids is currently under investigation.

Experimental Section

General Procedure for the Preparation of Neutral Rhomboidal Metallodendrimers 3a-c and Neutral Hexagonal Metallodendrimers 5a-c

To a 2 mL acetone solution containing 0.005 mmol of 60° di-Pt(II) acceptor 2 or 120° di-Pt(II) acceptor 4 was added an aqueous solution (for [G3] the solution was acetone/water 1:1) of the appropriate [G0] – [G3] dendritic disodium carboxylate (0.005 mmol) drop by drop with continuous stirring (10 min), whereupon the white product precipitated. The reaction mixture was centrifuged, washed several times with acetone and water, and dried in an oven at 80 °C. The product was collected and redissolved in CD2Cl2 for NMR analysis.

[G0] Neutral Rhomboidal Metallodendrimer 3a

Yield: 6.41 mg (white solid), 98%. 1H NMR (CD2Cl2, 300 MHz): δ 8.71 (s, 4H), 8.25 (s, 2H), 7.74-7.77 (m, 8H), 7.35-7.53 (m, 18H), 5.20 (s, 4H), 1.54-1.57 (m, 48H), 1.13-1.24 (m, 72H). 31P{1H} NMR (CD2Cl2, 121.4 MHz): δ 18.9 (s, 1JPt-P = 2908.8 Hz). Anal. Calcd for C106H156O10P8Pt4·2H2O: C, 47.96; H, 6.08; Found: C, 47.67; H, 5.98.

[G1] Neutral Rhomboidal Metallodendrimer 3b

Yield: 7.22 mg (white solid), 95%. 1H NMR (CD2Cl2, 300 MHz): δ 8.72 (s, 4H), 8.27, (s, 2H), 7.79 (s, 4H), 7.76 (d, J = 8.1 Hz, 4H), 7.35-7.47 (m, 28H), 6.80 (d, J = 2.1 Hz, 4H), 6.59 (t, J = 2.1 Hz, 2H), 5.16 (s, 4H), 5.10 (s, 8H), 1.54-1.57 (m, 48H), 1.14-1.27 (m, 72H). 31P{1H} NMR (CD2Cl2, 121.4 MHz): δ 19.0 (s, JPt-P = 2903.8 Hz). Anal. Calcd for C134H180O14P8Pt4·2H2O: C, 52.27; H, 6.02; Found: C, 51.99; H, 5.88.

[G2] Neutral Rhomboidal Metallodendrimer 3c

Yield: 9.33 mg (white solid), 96%. 1H NMR (CD2Cl2, 300 MHz): δ 8.71 (s, 4H), 8.27, (s, 2H), 7.80 (s, 4H), 7.75 (d, J = 7.8 Hz, 4H), 7.30-7.46 (m, 48H), 6.78 (s, 4H), 6.73 (d, J = 1.8 Hz, 8H), 6.57 (s, 6H), 5.15 (s, 4H), 5.07 (s, 16H), 5.03 (s, 8H), 1.52-1.55 (m, 48H), 1.13-1.26 (m, 72H). 31P{1H} NMR (CD2Cl2, 121.4 MHz): δ 18.9 (s, 1JPt-P = 2897.1 Hz). Anal. Calcd for C190H228O22P8Pt4: C, 58.63; H, 5.90; Found: C, 58.22; H, 6.10.

[G3] Neutral Rhomboidal Metallodendrimer 3d

Yield: 13.41 mg (white glassy solid), 96%. 1H NMR (CD2Cl2, 300 MHz): δ 8.70 (s, 4H), 8.27, (s, 2H), 7.80 (s, 4H), 7.74 (d, J = 7.8 Hz, 4H), 7.25-7.45 (m, 88H), 6.80 (d, J = 1.8 Hz, 4H), 6.72 (d, J = 1.8 Hz, 8H), 6.69 (d, J = 2.1 Hz, 16H), 6.61 (s, 2H), 6.55 (s, 12H), 5.14 (s, 4H), 5.00-5.07 (m, 56H), 1.52-1.55 (m, 48H), 1.11-1.21 (m, 72H). 31P{1H} NMR (CD2Cl2, 121.4 MHz): δ 19.0 (s, 1JPt-P = 2908.6 Hz). Anal. Calcd for C302H324O38P8Pt4: C, 64.89; H, 5.84; Found: C, 65.13; H, 5.86.

[G0]-Neutral Hexagonal Metallodendrimer 5a

6.23 mg (white solid), Yield: 95%. 1H NMR (CD2Cl2, 300 MHz): δ 8.16 (s, 3H), 7.68 (s, 6H), 7.61 (d, J = 7.2 Hz, 12H), 7.34-7.51 (m, 27H), 5.16 (s, 6H), 1.55-1.59 (m, 72H), 1.12-1.20 (m, 108H). 31P{1H} NMR (CD2Cl2, 121.4 MHz): δ 18.8 (s, 1JPt-P = 2859.1 Hz). Anal. Calcd for C156H234O18P12Pt6: C, 47.56; H, 5.99; Found: C, 47.58; H, 6.24.

[G1] Neutral Hexagonal Metallodendrimer 5b

7.32 mg (white solid), Yield: 96%. 1H NMR (CD2Cl2, 300 MHz): δ 8.19 (s, 3H), 7.70 (s, 6H), 7.62 (d, J = 7.8 Hz, 12H), 7.36-7.45 (m, 42H), 6.76 (s, 6H), 6.58 (s, 3H), 5.13 (s, 6H), 5.08 (s, 12H), 1.56-1.62 (m, 72H), 1.11-1.21 (m, 108H). 31P{1H} NMR (CD2Cl2, 121.4 MHz): δ 18.8 (s, 1JPt-P = 2856.8 Hz). Anal. Calcd for C198H270O24P12Pt6: C, 51.96; H, 5.95; Found: C, 51.92; H, 5.99.

[G2] Neutral Hexagonal Metallodendrimer 5c

9.45 mg (white solid), Yield: 97%. 1H NMR (CD2Cl2, 300 MHz): δ 8.18 (s, 3H), 7.70 (s, 6H), 7.60 (d, J = 7.5 Hz, 12H), 7.29-7.44 (m, 72H), 6.75 (s, 6H), 6.71 (d, J = 2.1 Hz, 12H), 6.56 (s, 9H), 5.11 (s, 6H), 5.05 (s, 24H), 5.01 (s, 12H), 1.54-1.56 (m, 72H), 1.09-1.19 (m, 108H). 31P{1H} NMR (CD2Cl2, 121.4 MHz): δ 18.8 (s, 1JPt-P = 2873.3 Hz). Anal. Calcd for C282H342O36P12Pt6·2H2O: C, 57.54; H, 5.93; Found: C, 57.71; H, 6.35.

[G3] Neutral Hexagonal Metallodendrimer 5d

13.29 mg (white glassy solid), Yield: 95%. 1H NMR (CD2Cl2, 300 MHz): δ 8.18 (s, 3H), 7.70 (s, 6H), 7.59 (d, J = 7.8 Hz, 12H), 7.28-7.40 (m, 132H), 6.76, (s, 6H), 6.70 (s, 12H), 6.67 (d, J = 1.8 Hz, 24H), 6.59 (s, 3H), 6.54 (s, 18H), 5.09 (s, 6H), 4.98-5.01 (m, 84H), 1.53-1.55 (m, 72H), 1.07-1.17 (m, 108H). 31P{1H} NMR (CD2Cl2, 121.4 MHz): δ 18.8 (s, 1JPt-P = 2868.3 Hz). Anal. Calcd for C450H486O60P12Pt6: C, 64.37; H, 5.83; Found: C, 64.76; H, 5.75.

Supplementary Material

1_si_001

2_si_002

3_si_003

Acknowledgments

H.-B. Y. thanks the Shanghai Pujiang Program (09PJ1404100) for financial support. P.J.S. thanks the NIH (Grant GM-057052) for financial support. B.H.N. thanks the NIH (Grant GM-080820) for financial support. We thank Dr. Jim Muller for his help with mass spectral measurements

Footnotes

Supporting Information Available: Copies of NMR spectra for compounds (3a-d and 5a-d) and crystallographic file (in CIF format) of 3a and 3b. This material is available free of charge via the Internet at http://pubs.acs.org.

References

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