Synthetic, Structural, Spectroscopic, Electrochemical Studies and Self

Synthetic, Structural, Spectroscopic, Electrochemical Studies and Self-assembly of Nanoscale Polyoxometalate-Organic Hybrid Molecular Dumbbells Yi Zhu,† Longsheng Wang,‡ Jian Hao,‡ Zicheng Xiao,† Yongge Wei,*,†,‡ and Yuan Wang*,†

CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 8 3509–3518

Department of Chemistry, State Key Laboratory for Structural Chemistry of Unstable and Stable Species, College of Chemistry and Molecular Engineering, Peking UniVersity, Beijing 100871, P. R. China, and Department of Chemistry, Tsinghua UniVersity, Beijing 100084, China ReceiVed March 1, 2009; ReVised Manuscript ReceiVed April 6, 2009

ABSTRACT: Three rigid-rod conjugated organic-inorganic hybrid nanodumbbells (Bu4N)4[O18Mo6N(C6H4)NMo6O18] (1), (Bu4N)4[O18Mo6N(C12H8)NMo6O18] (2), and (Bu4N)4[O18Mo6N(C14H12)NMo6O18] (3) bearing terminal polyoxometalate (POM) cages, with different lengths of approximately 2-3 nm and different substituents on the rod, have been synthesized via one-pot reaction of octamolybdate and the appropriate aromatic diamine dihydrochloride with N,N′-dicyclohexylcarbodiimide (DCC) as a dehydrating agent. These complexes have been characterized with UV-vis, IR, 1H NMR, ESI mass spectrometry, cyclic voltammetry, differential pulse voltammetry, and X-ray single-crystal diffraction techniques. The influence of the rod length and substituents on properties of these complexes has been investigated. In addition, the self-assembly of these nanorods has also been explored. Introduction In recent years, there have been considerable efforts on synthesis and property studies of nanosized materials, such as nanoparticles, nanowires, nanotubes, and so on.1,2 These materials, with particular size-dependent properties and resulting applications, serve as building blocks for the bottom-up approach to materials of wide uses. However, most of these materials are quantum dots or nanocrystals which are assemblies of small molecules or simple ions, and individual nanoscale macromolecules as nanobuilding blocks are rarely reported, especially nanoscale molecular rods with two active centers connected by a medium whose length and properties can be controlled on purpose. The motivation for synthesis and study of the properties of nanoscale molecular rods arises from interest in areas including long-distance interaction phenomena such as electron and energy transfer, magnetic coupling of transition-metal atoms, and use of molecular rods for the construction of supramolecular assemblies and giant molecules, which might be able to perform functions that small molecules do not, primarily in biochemical and material science applications.3 To make full use of molecular nanorods, chemists have to attach the desired terminals, adjust the length of the rod to a desired value, and secure sufficient rigidity. Hence, there have been increasing efforts toward the preparation of various nanorods, many of which are nanodumbbells.3 Among diverse rods for such dumbbell-shaped molecules, rigid rods have received much attention because of their unique behavior. Compared to flexible rod nanodumbbells, those with rigid rods are strongly anisotropic both in shape and in properties.3 The π-conjugated unit especially p-phenylene is usually considered as the most typical monomer for the construction of rigid rods, suggesting a high stiffness for these molecules. On the one hand, unlike the other rigid monomers such as oligoynes and cage modules (e.g., cubane),3 it only has * Authors to whom all correspondence should be addressed. Tel: +86-1062797852. Fax: +86-10-62757497. E-mail: [email protected] (Y. Wei); [email protected] (Y. Wang). † Peking University. ‡ Tsinghua University.

Scheme 1. Synthesis of Compounds 1-3

a 2-fold symmetry axis along the rod direction and is strongly anisotropic. On the other hand, nanodumbbells with p-phenylene as the monomer of the rigid rod may exhibit interesting electrochemical, optical, and electric properties since p-phenylene is an excellent conjugated system. As a result, a variety of such conjugated rigid rodlike molecules have been synthesized, the terminals of which are usually organic or metal-organic substituents including fullerene4 and metal complexes;3 however, nanodumbbells with pure inorganic substituents, especially POMs as terminals, are rarely explored.5 Polyoxometalates (POMs), clusters of early transitional metals, have potential applications in quite diverse disciplines, including catalysis, medicine, and materials science, due to their unusual structure, physical properties, and chemical reactivity.6-16 Recently, some POMs have been discovered to show remarkable self-assembly in solution17 or on surfaces.18 Also, the application of magnetic POMs in quantum computing has been confirmed.19,20 Therefore, POM-terminal molecular rods or nanodumbbells might have interesting potentials in the design of new materials with respect to their unique electrochemical, magnetic, catalytic, antimicrobial, and antitumor properties. The nature of the two parts, POMs and the rigid rod constructed by p-phenylene monomer, attracts much attention to generating such nanodumbbells in which two POM cages are covalently connected by extended π-conjugated rods consisting of p-phenylene monomer units. In addition to the resulting fine-tuned properties of the two parts, possible novel synergistic effects between inorganic POM clusters and organic π-conjugated segments must also be brought about, resulting from the strong interplay between the POM cluster and the organic delocalized segment.21 In the present study, we report

10.1021/cg9002516 CCC: $40.75  2009 American Chemical Society Published on Web 05/22/2009

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Table 1. Summary of Crystallographic Data for Compounds 2 and 3a empirical formula formula weight crystal system space group unit cell dimensions

volume (Å3) Z density (calculated) (Mg/m3) absorption coefficient (mm-1) F(000) crystal size (mm3) theta range for data collection index ranges reflections collected independent reflections completeness to theta ) 24.00° (%) data/restraints/parameters goodness-of-fit on F2 final R indices [I > 2σ(I)] largest diff peak and hole (e · Å-3)

C76H152Mo12N6O36

C78 H156Mo12N6O36

2877.31 monoclinic P21/c a ) 23.109(5) Å b ) 25.207(5) Å c ) 19.314(4) Å β ) 95.53(3)° 11198(4) 4 1.712 1.367 5788 0.30 × 0.20 × 0.05 3.00-24.00° -26 e h e 26, -28 e k e 28, -22 e l e 22 73949 17226 [Rint ) 0.0888] 98.0 17226/748/1181 1.066 R1 ) 0.0592, wR2 ) 0.1408 0.752 and -0.708

2905.37 monoclinic P21/c a ) 23.374(5) Å b ) 25.243(5)Å c ) 19.130(4) Å β ) 96.30(3)° 11219(4) 4 1.720 1.365 5832 0.40 × 0.30 × 0.04 2.99-24.00° -26 e h e 26, -28 e k e 28, -21 e l e 21 71214 17270 [Rint ) 0.0916] 98.0 17270/991/1093 1.234 R1 ) 0.0655, wR2 ) 0.1686 1.228 and -0.930

a R ) Σ|Fo| - |Fc|/Σ|Fo| and Rw ) [Σ[w(Fo2 - Fc2)2]/Σw(Fo2)2]1/2 with w ) 1/[σ2(Fo2) + (aP)2 + bP], where P ) (Fo2 + 2Fc2)/3 2: a ) 0.0018, b ) 98.9889; 3: a ) 0.0010, b ) 27.2375.

three dumbbell-shaped molecules with POM cages as terminals, bridged by p-phenylene and p-biphenylene units via Mo≡N triple bond, which easily form in a one-pot reaction of (Bu4N)4[Mo8O26] and an appropriate aromatic diamine dihydrochloride via well-established imido functionalization methods,22 of which the synthesis of 1 using a harsher procedure has been communicated before.5b On the basis of systematic studies on the properties of these compounds, the influence of the rod length and substituents on their properties has been fully discussed. Moreover, their self-assembly has also been investigated, which may help them toward more complex systems such as molecular devices in material science applications. Results and Dicussion Synthesis. Considerable attention has been drawn to functionalizing POM clusters with organic species, owing to the pioneering work of Klemperer,23 Pope,24 and Knoth.25 Moreover, in the light of the discoveries of Maatta,26 Errington,27 and Proust28 to prepare organoimido derivatives of POMs, a convenient “DCC protocol” was reported to generate organoimido-substituted hexamolybdates with aromatic amines as the imido-donating reagents by removal of the water formed during the reaction progress.22a And the one-pot reaction of (Bu4N)4[R-Mo8O26], aromatic amine hydrochlorides, and DCC in refluxing anhydrous acetonitrile was established as a foremost means to afford corresponding monosubstituted aromatic amine derivatives of hexamolybdates.22b However, one-pot reactions for the synthesis of organic-segment-bridged hexamolybdates, which essentially belong to monofunctionalized derivatives, has never been developed. In an attempt to prepare these complexes and in the light of reaction described above, aromatic diamine dihydrochlorides were employed to replace aromatic monoamine hydrochlorides as imido-releasing reagents to provide dumbbellshaped backbone and the stoichiometric amount of DCC was accordingly increased to remove water produced by another protonized amino group to ensure both amino groups were functionalized by hexamolybdates (Scheme 1). Indeed, diimidobridged hexamolybdates were obtained by this method. Compared with the two-step synthesis of similar hybrid POM-organic dumbbells via a Pd-catalyzed coupling reaction between dif-

Figure 1. ORTEP viewing and atomic labeling scheme of the cluster anions within compounds 2 and 3. Selected bond lengths/Å and angles/° for compound 2: C(1)-N(1) 1.390(13), Mo(1)-N(1) 1.714(9), Mo(1)O(1) 2.204(6), Mo(2)-O(2) 1.694(8), Mo(2)-O(1) 2.325(7), Mo(3)-O(3) 1.684(8), Mo(3)-O(1) 2.330(7), Mo(4)-O(4) 1.670(8), Mo(4)-O(1) 2.341(7), Mo(5)-O(5) 1.677(8), Mo(5)-O(1) 2.335(7), Mo(6)-O(6) 1.695(8), Mo(6)-O(1) 2.366(6), C(1)-N(1)-Mo(1) 168.3(10), N(1)Mo(1)-O(1) 175.2(4), N(1)-Mo(1)-O(17) 105.2(4), N(1)-Mo(1)O(10) 101.5(4), N(1)-Mo(1)-O(15) 102.1(4), N(1)-Mo(1)-O(16) 97.8(4). Selected bond lengths /Å and angles /° for compound 3: N(1)-C(1) 1.377(12), Mo(1)-N(1) 1.730(8), Mo(1)-O(1) 2.196(6), Mo(2)-O(2) 1.679(7), Mo(2)-O(1) 2.324(6), Mo(3)-O(3) 1.695(8), Mo(3)-O(1) 2.345(7), Mo(4)-O(4) 1.672(7), Mo(4)-O(1) 2.313(7), Mo(5)-O(5) 1.703(7), Mo(5)-O(1) 2.325(6), Mo(6)-O(6) 1.677(7), Mo(6)-O(1) 2.345(6), C(1)-N(1)-Mo(1) 174.4(9), N(1)-Mo(1)O(1) 177.0(4), N(1)-Mo(1)-O(17) 104.7(4), N(1)-Mo(1)-O(10) 101.7(4), N(1)-Mo(1)-O(15) 101.8(4), N(1)-Mo(1)-O(16) 98.6(4).

ferent presynthesized monofunctionalized derivatives of hexamolybdate,5a such a one-step reaction is much more convenient. This reaction can be applied to specially synthesize hybrid organic-hexamolybdate dumbbells.

Nanoscale POM-Organic Hybrid Molecular Dumbbells

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Figure 2. The basic hydrogen bond building blocks of compounds 2 and 3.

X-ray Structural Studies. A summary of X-ray crystal data for compounds 2 and 3 is provided in Table 1. Compound 2 crystallized in monoclinic space group P21/c. The asymmetric unit contains one crystallographically independent anion [O18Mo6N(C12H8)NMo6O18]4- and two [Bu4N]+ cations. The ORTEP drawing of the anion cluster is shown in Figure 1. Two monosubstituted hexamolybdates are bridged by one diimido ligand of p-diaminodiphenyl, in which the two phenyl rings are slightly torsional with a dihedral angle about 27.1° because of the steric repulsion between the ortho hydrogen atoms. Each hexamolybdate cluster shows typical monosubstituted imidohexamolybdates structural character: one terminal oxo ligand of Lindqvist-type hexamolybdates is replaced by one imido ligand; the terminal imido ligand demonstrates a typical sp hybridized mode with an approximately linear Mo≡N-C angle; the central oxo atom slightly shifts toward the imido-containing Mo atom as the result of a so-called “trans effect” which derives from the weaker electronegativity of the nitrogen atom in the imido ligand than that of the oxo ligand. Therefore, the whole anion cluster can be viewed as a nanoscale dumbbell with a length of ca. 2.560 nm.

Besides the interesting dumbbell structure, another interesting structural feature is their C-H · · · O hydrogen bonding, which has also been found to play an important role in cell packing of some organically derived Anderson or Lindqvist clusters.29 The basic hydrogen bond building block is shown in Figure 2. C11, C3, O17A, and O4A formed one nine-member ring containing two hydrogen bonding donors and two acceptors, which is defined as a symbol of R22(9), according to hydrogen bonding graph set.30 While O22 and O34 form a complicated four central hydrogen bond system containing three C-H · · · O hydrogen bonds, which can be represented by one total graph set N(2) containing two subsets of R21(7) and R12(4). By basic hydrogen bond rings R22(9), R21(7), and R12(4), the dimer of neighboring dumbbells are further connected into one 1D zigzag infinite double chain along the c-axis as shown in Figure 3. The structure of compound 3 is similar to that of compound 2, except that the linking ligand is replaced by o-tolidine. The asymmetric unit contains one crystallographically independent anion [O18Mo6N(C14H12)NMo6O18]4- and two [Bu4N]+ cations. As shown in Figure 1, it can be viewed as a nanodumbbell of ca. 2.562 nm long. Introduction of methyl into the nanodumbbell

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Figure 3. One 1D zigzag infinite double chains of compounds 2 and 3 formed from the basic hydrogen bond building blocks.

results in an interesting change of linkage mode of hydrogen bond in its packing mode. The basic hydrogen bond building block is shown in Figure 2. In this building block, O22, C10A, C11A, C4A, and C5A forms one seven-member ring through two C-H · · · O hydrogen bonds (R21(7)), while C14, H14a, O17A, Mo4A, O4A, H3a, C3, C4, C11, C12, and C13 are connected into one 11-member ring through two C-H · · · O

hydrogen bonds (R22(11)). Those basic building blocks are further linked into one 1-D zigzag infinite double chain just like that of compound 2 (as shown in Figure 3). The hydrogen bond data for compounds 2 and 3 are summarized in Table 2. The crystals of 1 are so thin that they are not of X-ray quality, and its X-ray structure has not been confirmed yet. However, its molecular structure should be similar to those of compound



Table 2. Hydrogen Bonds of Compounds 2 and 3 [Å and °] D-H · · · A

d(H · · · A)

d(D-H)

d(D · · · A)

∠(DHA)

3.303(15) 3.456(15) 3.452(15) 3.296(16) 4.224(14) 3.936(14) 3.513(14)

156.4 163.7 130.1 118.7 153.3 133.3 135.1

3.332(13) 3.380(14) 3.827(14) 4.221(19) 3.719(19) 4.283(18) 3.734(17) 3.255(14) 4.267(15) 3.979(15) 3.882(15) 3.460(17) 3.857(16)

163.4 172.0 121.9 144.3 130.1 165.1 109.4 118.1 146.0 130.9 139.2 154.5 120.2

a

Compound 2 C(3)-H(3) · · · O(4)#1 C(5)-H(5) · · · O(22)#2 C(5)-H(5) · · · O(34)#2 C(9)-H(9) · · · O(22)#2 C(9)-H(9) · · · O(28)#2 C(11)-H(11) · · · O(4)#1 C(11)-H(11) · · · O(17)#1

0.93 0.93 0.93 0.93 0.93 0.93 0.93

2.43 2.55 2.78 2.75 3.37 3.24 2.79 Compound 3b

C(3)-H(3) · · · O(4)#1 C(5)-H(5) · · · O(22)#2 C(5)-H(5) · · · O(34)#2 C(7)-H(7A) · · · O(22)#2 C(7)-H(7A) · · · O(34)#2 C(7)-H(7A) · · · O(36)#2 C(7)-H(7C) · · · O(10) C(10)-H(10) · · · O(22)#2 C(10)-H(10) · · · O(28)#2 C(12)-H(12) · · · O(4)#1 C(12)-H(12) · · · O(17)#1 C(14)-H(14A) · · · O(17)#1 C(14)-H(14B) · · · O(34)

0.93 0.93 0.93 0.96 0.96 0.96 0.96 0.93 0.93 0.93 0.93 0.96 0.96

2.43 2.46 3.25 3.40 3.03 3.35 3.30 2.71 3.46 3.31 3.13 2.57 3.28

a Symmetry transformations used to generate equivalent atoms: #1 x, -y + 3/2, z - 1/2; #2 x, -y + 3/2, z + 1/2. b Symmetry transformations used to generate equivalent atoms: #1 x, -y + 3/2, z - 1/2; #2 x, -y + 3/2, z + 1/2.

Table 3. The Lowest Energy Electronic Transition Peaks of Nanodumbbells and Monofunctionalized Hexamolybdate Derivatives of Aniline and o-Methyl Aniline

2 and 3 except for the replacement of a biphenyl by a phenyl group. Correspondingly, its size is estimated to be ca 2.134 nm. Electronic Spectroscopy. UV-vis electronic spectra data are collected in Table 3. The lowest energy electronic transition at 325 nm in [Mo6O19]2- was assigned to a charge-transfer transition from the oxygen π-type HOMO to the molybdenum π-type LUMO, which is bathochromically shifted significantly to 424, 410, and 420 nm, in 1, 2, and 3, respectively, due to the charge-transfer transition of the coordinated N atom to the

molybdenum atom (LMCT), indicating that the Mo-N π bond is formed and the delocalization of organic conjugated π-electrons has extended from the benzene or biphenyl group to the hexamolybdate skeletons, in accord with Mu¨lliken theory.31 In other words, there is a strong electronic interaction between the metal-oxygen clusters and the organic conjugated rigid rod. Compared to the corresponding monoarylimido hexamolybdates (compounds 2 and 3 could be regarded formally as their dehydrogenative coupling products),28,32 these lowest energy electronic transitions in 2 and 3 are both dramatically red-shifted by 68-70 nm, suggesting that the two terminal POM clusters rather than only one both conjugate with the organic conjugated π system. Additionally, compared to 1, there is some extent blue-shift of the lowest energy electronic transition in 2 and 3, respectively, which results from the fact that the planes of the adjacent phenyl rings of phenylene moieties in the two compounds form an angle of about 27.1° and 37.7° because of the steric repulsion between the ortho hydrogen atoms, as confirmed in respective by their crystal structure investigations. This twist reduces π-orbital delocalization within the whole conjugated π system, finally leading to the blue shift. Therefore, the degree of conjugation in the organic rod has a significant influence on their photophysical properties. Moreover, relative to the case of 2, there is a slight red-shift of the lowest energy electronic transition in 3, due to the electron-donating effects of methyl on the organic conjugated π system. However, compared to compounds 1-3, two previously reported POM-organic dumbbells (the last two entries in Table 3),5a show LMCT bands at 426 and 438 nm, respectively. The larger red-shift should result from their larger conjugated π system between POM terminals and better coplanarity between adjacent phenyl rings (connecting adjacent phenyl rings with linear ethynyl reduces their dihedral angle and hence facilitates the coplanarity). IR Spectroscopy. The IR spectra of these compounds, showing similar bands in the Mo-O stretching region of 1000-700 cm-1, resemble each other and have characteristics of both hexamolybdate and its mono-organoimido derivatives. The very strong peaks located at ca. 952 and 794 cm-1, as in

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Figure 4. 1H NMR spectra of compounds 1-3 in DMSO-d6.

the case of the parent hexamolybdate anion, are respectively ascribed to the stretching vibration of the Mo-Ot and the asymmetric stretching vibration of the Mo-Ob-Mo, indicating the Lindqvist structures are preserved. Furthermore, they also show a strong shoulder peak near 975 cm-1 around the Mo-Ot stretching, which is typical for mono-organoimido substitution,22b deriving from the Mo-N bond stretching vibration,

implying these compounds essentially belong to mono-organoimido derivatives. Compared to those of the hexamolybdate framework, the IR bands of the aromatic diamine ligands in this region (mostly γ-(C-C and C-H)) are of low intensity. Take compound 1 as an example: in the high-frequency region, the aromatic γ(Ar-H) bands (γ > 3000 cm-1) are hardly visible, due to their low intensity, and the complex pattern around 2900



Table 4. Chemical Shift of Aromatic Protons of Compounds 1-3 and Corresponding Free Diamine Ligands

cm-1 is aliphatic γ(C-H) bands of the tetrabutylammonium cations. In the medium-frequency region (1650-1000 cm-1), different from (Bu4N)2[Mo6O19], there are bands characteristic for phenylimido groups at 1639, 1591, 1482, and several weak bands from 1330 to 1100 cm-1. As for compounds 2 and 3, all of those characteristic vibrations can be found within 10 cm-1 shifts. 1 H NMR Spectroscopy. Figure 4 shows the 1H NMR spectrum (in DMSO-d6) of these compounds, in which obviously resolved signals of protons in the organic conjugated π-system could be unambiguously assigned, and the integration matches well with the proposed structure. Compared to the 1H NMR spectrum of the corresponding free diamine ligands, the aromatic protons in these compounds all exhibit significant downfield chemical shifts (see Table 4), reflecting the greater electronwithdrawing nature of the hexamolybdate cluster compared with the amino group NH2-. Electrochemical Studies. Cyclic voltammetry (CV) and differential pulse voltammetry (DPV), were used to study the electrochemistry of parent hexamolybdate anion and compounds 1-3. In the potential range from 0 to -1.1 V vs SCE, a reversible one-electron reduction wave is observed at -0.553 V for hexamolybdate, whereas a seeming reversible one-electron reduction wave is at -0.754 V, -0.743 V, -0.756 V vs SCE, respectively, for compounds 1-3 (see Figure S1, Supporting Information). The remarkable cathodic shifts in the reduction potentials of these compounds are consistent with the trend in π-donor ability of corresponding ligands, that is, O2- < RN2-, resulting from the stronger electron-donating nature of arylimido group than that of the oxo atom. Noticeably, the seeming reversible one-electron reduction wave for compound 1 is more complicated. The DPV curves for this potential window reveal that this reduction wave is actually the result of the superposition of two relatively close potentials (-0.714 and -0.808 V vs SCE, see Figure S1, Supporting Information), which correspond to two successive one-electron reduction processes: the first electron is added to one cluster, and delocalizing within the whole inorganic-organic conjugated system including two clusters; afterward, the second electron is added to the system instead of the second cluster. All these indicate that the two POM clusters interact and electronically communicate through the organic bridge. Nevertheless, compared with the case of compound 1, the DPV curves of compounds 2 and 3 show the

reduction waves are respectively only one peak (see Figure S1, Supporting Information), which should have been the superposition of two relatively close potentials in theory. The most likely reason is that the two potentials are so close that they could not be separated. The nearness of the two potentials probably originates from the noncoplanarity of the organic bridge in the two compounds. The dihedral angle of the rings of adjacent phenylene moieties reduces conjugation between the two POM cages, resulting in the delocalization of the first electron being too weak to obviously influence the second reduction, and thus both electrons are added in a similar manner as if two clusters are reduced individually. It implies that the electronical communication between the two clusters via the organic conjugated rod is too weak to be detected. Self-Assembly. Some nanoscale POMs exhibit interesting self-assembling behaviors, which tend to self-associate into spherical structures in very dilute solution.17 Relative to these POMs, the POM-organic hybrid molecular nanodumbbells have the tendency to assemble into spherical aggregates, via natural evaporation of organic polar solvent such as acetone. Figure 5a shows a typical scanning electron microscopy (SEM) image of the assemblies, which mainly range in size from 0.5 to 1 µm. Figure 5b shows their atomic force microscopy (AFM) image on a highly oriented pyrolytic graphite (HOPG) surface, indicating their horizontal sizes are much larger than the corresponding vertical heights (Figure 5c). Figure 5d shows their transmission electron microscopy (TEM). Figure 5e is the energy-dispersive X-ray (EDX) spectrum (single aggregate spot-resolved EDX spectrume) of the aggregates. The presence of Mo confirms they are indeed aggregates of such nanodumbbells. Compared with the self-association process of some POM macroions17 and surfactant-encapsulated POMs33,34 in solution demonstrated by the laser light scattering techniques (LLS), no assembling of such nanorodlike POM-organic hybrid compounds in solution is observed. This finding suggests that the spherical aggregates are not constructed in solution but during evaporation of solvent. Formation of aggregates on different substrates (silicon surface, HOPG surface, and carbon-coated copper TEM grid) reveals that the substrate has little influence on this selfassembly. A rough proposed self-assembly process for them is that the anions and cations of these compounds, existing

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Figure 5. (a) SEM image of spherical assemblies of compound 2. (b) AFM height image. (c) Analysis of the spherical assemblies on highly oriented pyrolytic graphite (HOPG) surface. (d) TEM image of spherical associates of compound 2. (e) EDX spectrum of associates of compound 2.

as single ions in polar solvent, attract each other via electrostatic interaction during natural volatilization of solvent.

and electronic communication between the two clusters of each compound. Their self-assembly implies that they are potentially constructed into more complicated system with applications in material science.

Conclusions Three nanoscale POM-organic hybrid molecular dumbbells, synthesized via only one-step reaction, have been characterized by UV-vis, IR, 1H NMR, ESI, CV, DPV, and single-crystal X-ray diffraction, and their self-assembly has also been investigated with scanning electron microscopy (SEM) and transmission electron microscopy (TEM). These studies reveal that rod length and substituents have an important influence on their properties, especially the lowest energy electronic transitions

Experimental Section All syntheses and manipulations were performed in the open air. DCC was used as received. (Bu4N)4[Mo8O26] was conveniently prepared by the addition of NBu4Br to an aqueous solution of (NH4)6Mo7O24 · 4H2O, from which the product immediately precipitates. The aromatic diamine dihydrochlorides were prepared by adding 2 equiv of hydrochloric acid to an acetonitrile solution of appropriate aromatic diamine and collecting the precipitates. Acetonitrile was dried by refluxing in the presence of CaH2 and was distilled prior to use.

Nanoscale POM-Organic Hybrid Molecular Dumbbells UV-vis spectra were measured using a Lambda45 UV-vis spectrometer (Perkin-Elmer Intruments). Elemental analysis was performed on a Vario EL elemental analyzer. IR spectra were recorded with a Nicolet Magna-IR 750 spectrometer using KBr pellets. 1H NMR spectra were taken on a Bruker ARX 400 NMR spectrometer at 298 K using DMSO-d6 as solvent. The mass spectra were recorded using an ion trap mass spectrometer (Thermofisher LTQ). Negative mode was used for the experiment (capillary voltage 33 V). Sample solution (in acetonitrile) was infused into the ESI source at a flow rate of 300 µL/ min. Cyclic voltammetry and differential pulse voltammetry were performed with a CHI750A electrochemical working station (CHI Instruments) in DMF. Solutions were 0.05 M for the supporting electrolyte, [n-Bu4N][PF6], and ∼10-3 M for these compounds. A standard three-electrode cell was used, which consisted of a platinum working electrode, an auxiliary platinum electrode, and SCE (in saturated KCl aqueous solution) as the reference electrode. SEM experiments were performed at 10 kV using a FEI Strata DB235 SEM. Atomic force microscopy (AFM) studies were performed with a Nanoscope IIIa microscope (Extended Multimode, Digital Instruments, Santa Barbara, CA). All experiments were carried out with tapping mode at ambient temperature. A silicon cantilever was used with a resonance frequency of 323 kHz. TEM experiments were performed at 120 kV using a JEOL JEM-200CX TEM. Energy-dispersive analysis by X-rays (EDX) was performed on a FEI Tecnai F30 HRTEM. For SEM experiments, a droplet (ca. 10 µL) of compound 2 in acetone solution (1 mg/mL) was put on a freshly cleaned silicon surface and allowed to evaporate naturally. For AFM experiments, a droplet (ca. 1 µL) of compound 2 in acetone solution (1 mg/mL) was dropped on the freshly cleaved HOPG surface. AFM images were taken when all solvent evaporated. For TEM experiments, a droplet (ca. 10 µL) of compound 2 in acetone solution (1 mg/mL) was spread onto the carboncoated copper TEM grid and allowed to evaporate naturally. Compound 1: A mixture of (Bu4N)4[Mo8O26] (6.5 g, 3.0 mmol), DCC (1.34 g, 6.5 mmol), and p-phenylenediamine dihydrochlorides (0.363 g, 2 mmol) was refluxed in anhydrous acetonitrile (20 mL) at 110 °C for about 18 h. While the reaction mixture dissolved in anhydrous acetonitrile with the temperature rising, the solution turned red. During the course of the reaction, its color changed to red-brown, and some white precipitates (N,N′-dicyclohexylurea) formed. When the resulting dark-red solution was cooled to room temperature, the white precipitates were removed by filtration. While the acetonitrile evaporated slowly in the open air, the product was collected from the filtrate as red precipitates (1.53 g, yield 27%) after the removal of unreacted white (Bu4N)4[Mo8O26] precipitates. The product was washed successively with benzene and Et2O for several times, and then dissolved in acetone. After the addition of EtOH (VEtOH/Vacetone ) 1:3), the solution was centrifuged and red oily precipitates were collected. This step was repeated three times. Crystals of 1 were obtained by diffusion of Et2O into their solution in acetonitrile. Elemental Anal. Calc (%) for C70H148Mo12N6O36: C, 30.01; N, 3.00; H, 5.33. Found: C, 29.65; N, 2.95; H, 5.35. 1H NMR (DMSO-d6, 298 K): δ 0.93 (t, 48H, -CH3, [Bu4N]+), 1.30 (m, 32H, -CH2-, [Bu4N]+), 1.56 (m, 32H, -CH2-, [Bu4N]+), 3.16 (t, 32H, N-CH2-, [Bu4N]+), 7.20 (s, 4H, aromatic). ESI mass spectrometry (MeCN, m/z): 457.72 (100%), 690.15 (12%), and 1178.47(6%) were assigned to [O18Mo6N(C6H4)NMo6O18]4-, [Bu4N][O18Mo6N(C6H4)NMo6O18]3-, and [Bu4N]2[O18Mo6N(C6H4)N Mo6O18]2-, respectively. IR (KBr pellet, major absorbances, cm-1): 2963, 2875, 1482, 1393, 1380, 953 (shoulder at 976 is diagnostic for mono-organoimido substituted hexamolybdate22b), 881, 797. UV-vis (MeCN, nm): λmax ) 424. Cyclic voltammetry (DMF): the reversible reduction wave E1/2 ) -0.754 V. Compound 2: The synthetic method was similar to that for 2 by using benzidine dihydrochlorides instead of p-phenylenediamine dihydrochlorides. The product was collected from the filtrate as red precipitates (1.39 g, yield 25%) after the removal of unreacted (Bu4N)4[Mo8O26] precipitates. Crystals of 2 were obtained by diffusion of Et2O into their solution in acetonitrile. Elemental anal. calc (%) for C76H152Mo12N6O36: C, 31.72; N, 2.92; H, 5.32. Found: C, 31.28; N, 2.65; H, 5.35. 1H NMR (DMSO-d6, 298 K):δ 0.93 (t, 48H, -CH3, [Bu4N]+), 1.30 (m, 32H, -CH2-, [Bu4N]+), 1.56 (m, 32H, -CH2-, [Bu4N]+), 3.16 (t, 32H, N-CH2-, [Bu4N]+), 7.29 (d, 4H, aromatic), 7.82 (d, 4H, aromatic). ESI mass spectrometry (MeCN, m/z): 477.04 (100%), 716.52 (20%), and 1195.16(4%) were assigned to [O18Mo6N(C12H8)NMo6O18]4-, [Bu4N][O18Mo6N(C12H8)N Mo6O18]3-,and [Bu4N]2[O18Mo6N(C12H8)NMo6O18]2-, respectively. IR (KBr pellet, major absor-

Crystal Growth & Design, Vol. 9, No. 8, 2009 3517 bances, cm-1): 2963, 2875, 1482, 1393, 1380, 953 (shoulder at 976 is diagnostic for mono-organoimido substituted hexamolybdate22b), 881, 797. UV-vis (MeCN, nm): λmax ) 410. Cyclic voltammetry (DMF): the reversible reduction wave E1/2 ) -0.743 V. Compound 3: The synthetic method was similar to that for 3 by using o-tolidine dihydrochlorides instead of p-phenylenediamine dihydrochlorides. The product was collected from the filtrate as red precipitates (1.73 g, yield 30%) after the removal of unreacted (Bu4N)4[Mo8O26] precipitates. Crystals of 3 were obtained by diffusion of Et2O into their solution in acetonitrile. Elemental anal. calc (%) for C78H158Mo12N6O36: C, 32.24; N, 2.89; H, 5.48. Found: C, 31.88; N, 2.65; H, 5.35. 1H NMR (DMSO-d6, 298 K): δ 0.93 (t, 48H, -CH3, [Bu4N]+), 1.30 (m, 32H, -CH2-, [Bu4N]+), 1.56 (m, 32H, -CH2-, [Bu4N]+), 3.16 (t, 32H, N-CH2-, [Bu4N]+), 2.60 (s, 6H, -CH3), 7.23 (d, 2H, aromatic), 7.62 (t,2H, aromatic), 7.70 (s, 2H, aromatic). ESI mass spectrometry (MeCN, m/z): 483.69 (100%), 646.72 (54%), and 723.98 (15%) were assigned to [O18Mo6N(C14H14)N Mo6O18]4-, [HO18Mo6N(C14H14)N Mo6O18]3-, and [Bu4N][O18Mo6N(C14H14)N Mo6O18]3-, respectively. IR (KBr pellet, major absorbances, cm-1): 2963, 2875, 1482, 1393, 1380, 953 (shoulder at 976 is diagnostic for mono-organoimido substituted hexamolybdate22b), 881, 797. UV-vis (MeCN, nm): λmax ) 420. Cyclic voltammetry (DMF): the reversible reduction wave E1/2 ) -0.756 V. X-ray Crystallography. Summaries of crystal data, data collection, and refinement parameters are given in Table 1. A suitable single crystal having approximate dimensions 0.30 × 0.20 × 0.05 mm3 for 2, 0.40 × 0.30 × 0.04 mm3 for 3, was mounted on a glass fiber, respectively. All measurements were made on a Bruker Smart Apex CCD diffractometer. Data collection was performed at 293 K by using graphitemonochromated Mo KR radiation (λ ) 0.71073 Å). The raw frame data was processed using Rigaku RAPID AUTO Ver2.30 to yield the reflection data. Subsequent calculations were carried out using SHELXTL9735 program. Structure was solved by direct methods. Refinement was performed by full-matrix least-squares analysis. The non-hydrogen atoms were refined anisotropically. Hydrogen atoms were included at their calculated positions without refinement.

Acknowledgment. This work is sponsored by NFSC No 20871073 and 20373001, MOST No G2006CB806102, and THSJZ. Supporting Information Available: CV and DPV curves of compounds 1-3. These materials are available free of charge via the Internet at http://pubs.acs.org.

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CG9002516

Synthetic, Structural, Spectroscopic, Electrochemical Studies and Self

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