Naphthyl Amines as Novel Organoimido Ligands for Design of POM

Moreover, cyclic voltammetry studies on compound 1 show the reversible reduction wave at -0.53 V, which is .... completeness to theta ) 25.00°. 99.7%...
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Naphthyl Amines as Novel Organoimido Ligands for Design of POM-Based Organic-Inorganic Hybrids: Synthesis, Structural Characterization, and Supramolecular Assembly of (Bu4N)2[Mo6O18N(Naph-1)]

CRYSTAL GROWTH & DESIGN 2006 VOL. 6, NO. 7 1620-1625

Yi Zhu,† Zicheng Xiao,† Ning Ge,† Na Wang,† Yongge Wei,*,†,‡ and Yuan Wang*,† Department of Chemistry, Tsinghua UniVersity, Beijing 100084, China, and Department of Chemistry, State Key Laboratory for Structural Chemistry of Unstable and Stable Species, College of Chemistry and Molecular Engineering, Peking UniVersity, Beijing 100871, PR China ReceiVed February 10, 2006; ReVised Manuscript ReceiVed April 29, 2006

ABSTRACT: A novel naphthylimido derivative of hexamolybdate, (Bu4N)2[Mo6O18N(Naph-1)] (1), was synthesized for the first time by the reaction of octamolybdate ion and 1-naphthyl amine hydrochloride with DCC (N,N′-dicyclohexylcarbodiimide) as a dehydrating agent. The ultraviolet-visible absorption spectrum shows the lowest energy electronic transition of 1 occurs at 383 nm, which is dramatically red-shifted compared to that of [Mo6O19]2- and other monosubstituted phenylimido hexamolybdates reported in the literature. Moreover, cyclic voltammetry studies on compound 1 show the reversible reduction wave at -0.53 V, which is cathodically shifted relative to that of [Mo6O19]2-. Another interesting characteristic of 1 is that the cluster anions form supramolecular 1D chains via C-H‚‚‚O hydrogen bonding interactions. Introduction Polyoxometalates (POMs) occupy a significant position in quite diverse disciplines, including catalysis, medicine, and materials science, because their molecular and electronic structures are diverse.1-5 These species also play an important role in the design of new materials with novel electronic, magnetic, catalytic, and topological properties.6 Additionally, the chemical modification of POMs (i.e., the replacement of one or several oxo ligands by other ligands) with organic species has attracted increasing attention7 in recent years because it affords a convenient and effective way to generate novel organic-inorganic hybrid materials with their fascinating properties,2,8 which result from the possible synergistic effects between the organic and inorganic components. Among the organically derived POMs, organoimido derivatives9 are especially important inasmuch as the organic π electrons may extend their conjugation to the inorganic framework, remarkably modifying the electronic structure and redox properties of the parent POM components.10 Moreover, they can be applied as building blocks to construct more complicated POM-based organic-inorganic hybrids, providing a route to applications in supramolecular chemistry. This modular building block approach enables the synthesis of organic-inorganic hybrids more rational and controllable. At present, there have been three types of reactions to synthesize the organoimido derivatives of POMs. These reactions include reactions with phosphinimines,11,12 isocyanates,3,13 and aromatic amines.14 A few years ago, based on the latter type of reaction, a reaction protocol was reported by Peng and co-workers15 that allows the modification of POMs with aromatic amines derived from aniline using N,N′-dicyclohexylcarbodiimide (DCC) as the dehydrating agent under mild conditions and in high yields. A number of aryl imido derivatives of POMs have been successfully synthesized via this * To whom all correspondence should be addressed. Telephone: +8610-62797852. Fax: +86-10-62757497. E-mail: [email protected]; [email protected]; [email protected]. † Peking University. ‡ Tsinghua University.

reaction protocol.11-13b,14b,15-19 Moreover, iodo- and ethynylfunctionalized phenylimido hexamolybdates have recently been discovered that they can undergo Pd-catalyzed carbon-carbon coupling reactions,20a which have successfully been exploited to assemble hybrid nanodumbbells20b and polymers18,21 in a more controllable manner. Despite the many phenylimido derivatives above, non-phenyl arylimido derivatives have never been reported. Herein we hope to report such an organoimido derivative, (Bu4N)2[Mo6O18N(Naph-1)] (1), which was synthesized by the metathesis reaction of (Bu4N)4[Mo8O26] and 1-naphthyl amine hydrochloride with DCC as a dehydration agent. It can be expected that such a POM-based inorganicorganic hybrid would have interesting photophysical and electrochemical properties different from those of previously reported phenylimido hexamolybdates due to the more extended naphthalene ring, which has a conjugated structure larger than a benzene ring, promising the closer interaction of delocalized π electrons in the naphthyl group with the empty d-π orbitals of the POM skeleton. Indeed, as will be seen in the text, the lowest energy electronic transition of 1 is dramatically redshifted compared to those of phenylimido hexamolybdates reported in the literature.15a,16 The remarkable reduction of the HOMO-LUMO band gap in such hybrids provides us a potential chance to make unique molecular semiconductors based on POM-organic hybrid materials.22 Experimental Procedures 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 structure of (Bu4N)4[Mo8O26] was confirmed by elemental analysis and X-ray single-crystal structure determination.23 1-Naphthyl amine hydrochloride was prepared by adding 1 equiv of hydrochloric acid to an ethanol solution of 1-naphthyl amine and collecting the precipitates. Acetonitrile was dried by refluxing in the presence of CaH2 and was distilled prior to use. Elemental analysis was performed on a Vario EL elemental analyzer. IR spectra were recorded with a Nicolet MagnaIR 750 spectrometer using KBr pellets. 1H NMR spectra were taken on a Bruker ARX 400 NMR spectrometer at 298 K using d6-DMSO as solvent. The mass spectra were recorded using an ion trap mass spectrometer (Thermo Finnigan LCQ Deca XP Plus). Negative mode

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Naphthyl Amines as Novel Organoimido Ligands Scheme 1.

Synthesis of Compound 1

Crystal Growth & Design, Vol. 6, No. 7, 2006 1621 Table 1. Summary of Crystallographic Data for Compound 1 empirical formula formula weight temperature wavelength crystal system space group unit cell dimensions

was used for the experiment (capillary voltage 4500 V). Sample solution (in acetonitrile) was infused using a syringe pump into the ESI source at a flow rate of 5 µL/min. Cyclic voltammetry was performed with a CHI750A electrochemical working station (CHI Instruments) in oxygenand moisture-free acetonitrile with a scan rate of 20 mV s-1. Solutions were 0.1 M for the supporting electrolyte, [n-Bu4N][PF6], and ∼10-3 M for compound 1. A standard three-electrode cell was used, which consisted of a glassy carbon working electrode, an auxiliary platinum electrode, and Ag/AgCl (in saturated KCl aqueous solution) as the reference electrode. Synthesis of 1. A mixture of (Bu4N)4[Mo8O26] (6.46 g, 3.0 mmol), DCC (1.24 g, 6.0 mmol), and 1-naphthyl amine hydrochloride (0.72 g, 4.0 mmol) was refluxed in anhydrous acetonitrile (20 mL) at 110 °C for about 6 h. When the reaction mixture dissolved instantly in anhydrous acetonitrile at room temperature, 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 precipitated from the filtrate as a red solid. The product was washed with ethanol using the ultrasonic method and then recrystallized twice from the mixture of acetone and EtOH (1:1); the product deposited as orange crystals (3.67 g, yield 62%). Elemental anal. Calc (%) for C42H79Mo6N3O18: C, 33.86; N, 2.82; H, 5.35. Found: C, 33.78; N, 2.65; H, 5.35. 1H NMR (d6-DMSO, 298 K): δ 0.93 (t, 24H, -CH3, [Bu4N]+), 1.30 (m, 16H, -CH2-, [Bu4N]+), 1.56 (m, 16H, -CH2-, [Bu4N]+), 3.16 (t, 16H, N-CH2-, [Bu4N]+), 7.40 (m, 1H, aromatic), 7.56 (t, 1H, aromatic), 7.65 (t, 1H, aromatic), 7.71 (m, 2H, aromatic), 7.76 (d, 1H, aromatic), 7.98 (d, 1H, aromatic), 8.61 (d, 1H, aromatic). IR (KBr pellet, major absorbances, cm-1): 2963, 2875, 1482, 1393, 1380, 953 (shoulder at 976 is diagnostic for mono-organoimido substituted hexamolybdate12,14b), 881, 797. UV/ vis (MeCN, nm): λmax ) 383. ESI-mass spectrometry (MeCN, m/z): 1245.3 (100%), 1006.2 (10%), and 504.6 (37%) were assigned to [Bu4N][Mo6O18N(Naph-1)]-, [HMo6O18N(Naph-1)]-, and [Mo6O18N(Naph-1)]2-, respectively. Cyclic voltammetry (MeCN): the reversible reduction wave E1/2 ) -0.53 V. X-ray Crystallography. One suitable single-crystal having approximate dimensions of 0.30 × 0.20 × 0.20 mm3 for 1 was mounted on a glass fiber. The measurements were made on a Rigaku R-axis Rapid IP diffractometer. The data collection was performed at 296 K with graphite-monochromated Mo ΚR radiation (λ ) 0.710 73 Å) operating at 50 kV and 30 mA. Absorption corrections were applied based on the symmetry-equivalent reflections using the ABSCOR program.24 Data reduction was performed by the teXsan for Windows version 1.06.25 Both structures were solved by direct methods and refined by the full-matrix least-squares method on F2 with the SHELXTL software package of version 5.01.26 The naphthylimido group is disorderly distributed at the same occupancy on Mo1 and Mo6, respectively, which was treated with ideal rigid models. All of the nonhydrogen atoms were refined anisotropically, and the hydrogen atoms were included at their idealized positions.

Results and Discussion Preparation of the Compound. Scheme 1 shows the synthesis of compound 1. When a mixture of (Bu4N)4[Mo8O26], the hydrochloride salt of 1-naphthyl amine, and DCC is refluxed in anhydrous acetonitrile, the corresponding monofunctionalized naphthylimido derivative of [Mo6O19]2- is formed, and this reaction is usually completed in about 6 h. In fact, the hydrochloride salt is a proton carrier that introduces the proton into the reaction mixture. The likely role of the proton is to complex with DCC and, hence, to increase the electrophilic ability of DCC to attack the oxo group of Mo-O. Additionally,

volume Z density (calculated) absorption coefficient F(000) crystal size θ range for data collection index ranges reflections collected independent reflections completeness to theta ) 25.00° absorption correction max. and min. transmission refinement method data/restraints/parameters goodness-of-fit on F2 final R indices [I > 2σ(I)] R indices (all data) largest diff peak and hole

C42H79Mo6N3O18 1489.72 296(2) K 0.71073 Å monoclinic P21/c a ) 12.840(3) Å b ) 22.688(5) Å c ) 19.722(4) Å β ) 104.54(3)° 5561(2) Å3 4 1.779 mg/m3 1.379 mm-1 2992 0.30 × 0.20 × 0.20 mm3 2.31-25.00° 0 e h e 15, 0 e k e 26, -23 e l e 22 35 685 9761 [R(int) ) 0.0562] 99.7% semiempirical from equivalents 0.7700 and 0.6824 full-matrix least-squares on F2 9761/188/664 0.635 R1 ) 0.0352, wR2 ) 0.0514 R1 ) 0.1321, wR2 ) 0.0607 0.394 and -0.429 e‚Å-3

Table 2. Selected Bond Lengths/Å and Angles/° for the Two Conformational Isomers of the Cluster Anion of Compound 1 Mo(1)-N(1A) Mo(1)-O(6B) Mo(1)-O(18) Mo(1)-O(15) Mo(2)-O(2) Mo(2)-O(9) Mo(2)-O(8) Mo(3)-O(3) Mo(3)-O(8) Mo(3)-O(13) Mo(4)-O(4) Mo(4)-O(15) Mo(4)-O(10) Mo(5)-O(5) Mo(5)-O(14) Mo(5)-O(9) Mo(6)-O(6A) Mo(6)-N(1B) Mo(6)-O(18) Mo(6)-O(7)

C(1A)-N(1A)-Mo(1)

1.704(4) 1.704(4) 1.921(4) 1.962(4) 1.680(3) 1.906(3) 1.921(3) 1.669(4) 1.927(3) 1.942(3) 1.688(4) 1.903(3) 1.959(3) 1.675(4) 1.913(4) 1.942(3) 1.671(4) 1.671(4) 1.922(4) 1.926(3)

159.7(5)

Mo(1)-O(12) Mo(1)-O(16) Mo(1)-O(1)

1.914(4) 1.940(4) 2.273(3)

Mo(2)-O(10) Mo(2)-O(7) Mo(2)-O(1) Mo(3)-O(11) Mo(3)-O(12) Mo(3)-O(1) Mo(4)-O(13) Mo(4)-O(14) Mo(4)-O(1) Mo(5)-O(16) Mo(5)-O(17) Mo(5)-O(1) Mo(6)-O(17) Mo(6)-O(11) Mo(6)-O(1)

1.882(3) 1.917(3) 2.347(3) 1.903(4) 1.931(4) 2.345(3) 1.898(3) 1.924(4) 2.328(3) 1.909(4) 1.929(4) 2.303(3) 1.915(4) 1.925(4) 2.286(3)

C(1B)-N(1B)-Mo(6)

142.5(7)

it also promotes the conversion of the octamolybdate into the hexamolybdate through a degradation and reassembly process, since, in an acidic organic solvent, the hexamolybdate and its derivatives are much more stable than an octamolybdate.18 The key role of DCC here is to act as a special dehydrating agent with an activating effect on the terminal Mo-O bond, which is similar to its activating effect on the carboxyl group in the synthesis of amide or peptides.15a In our tests, the amount of DCC is equivalent to, or slightly higher than, that of the water needed to be removed from the reaction system because excess DCC usually results in unexpected side reactions and reduces the yield of the products.15a Structures Description. A summary of the crystallographic data and structural determination for compound 1 is provided in Table 1. Selected bond lengths and angles are listed in Table 2. Compound 1 crystallizes in the monoclinic space group P21/ c, with an asymmetric unit containing one cluster anion and two counter tetrabutylammonium cations. Its molecular structure has been confirmed by single-crystal X-ray diffraction. The naphthylimido group is bound to the hexamolybdate skeleton, disorderly with equivalent probability of 50%, at two cis Mo atoms (Mo1 and Mo6) in a terminal fashion, which originate from two possible orientations, perpendicular to each other, of

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Figure 1. ORTEP viewing and atomic labeling scheme for the two conformational isomers of the cluster anion of compound 1.

Figure 2. Two different 1D supramolecular chains, formed from conformations A and B of the cluster anion of 1, respectively, via C-H‚‚‚O hydrogen bonds.

the cluster anion in cell packing, forming two different conformational isomers. As a matter of convenience, only the two conformational isomers A and B of the cluster anion of compound 1 are shown in Figure 1 with the atomic labeling scheme. Generally speaking, the molecular structure of the cluster anion of 1 has similar features to those of phenylimido derivatives. For instance, the Mo-N bond distance and MoN-C bond angle (Mo1-N1A, 1.704(4) Å, C1A-N1A-Mo1, 159.7(5)°; Mo1-N1B, 1.671(4) Å, C1B-N1B-Mo1, 142.5(7)°) are within the region of phenylimido derivatives reported before, in accord with a substantial degree of MotN triple bond character. Due to the weaker trans-effect of the naphthylimido group than a terminal oxo group, the central oxygen atom, O1, encapsulated in the cluster cage is closer to the imido-bearing

Mo atom with a distance of 2.273(3) and 2.286(3) Å, in conformational isomers A and B, respectively. Different from the cases of the reported phenylimido derivatives, no π-π stacking interaction between naphthalene rings is observed in the cell packing of the cluster anion of 1. However, there are interesting C-H‚‚‚O hydrogen bonding interactions (C9A-H9AA‚‚‚O6Ai, 3.513(10) Å, 162.8°; C9BH9BA‚‚‚O16ii, 3.466(19) Å, 150.2°; C10B-H10C‚‚‚O6Bii, 3.413(15) Å, 146.0°. Symmetric codes: (i) -x, 0.5 + y, 0.5 z; (ii) -x, y - 0.5, 0.5 - z), which lead to two different supramolecular 1D chains along the b axis for conformations A and B, respectively (Figure 2). In addition, in conformation A, there is also an intramolecular C-H‚‚‚O hydrogen bond (C7A-H7AA‚‚‚O16, 3.684(11) Å, 157.8°), which affords an

Naphthyl Amines as Novel Organoimido Ligands

Crystal Growth & Design, Vol. 6, No. 7, 2006 1623

Figure 3. IR spectrum of compound 1.

intramolecular seven-membered hydrogen-bonding ring. It is such a different hydrogen-bonding mode between A and B that results in the difference of Mo-N-C bond angle in the two conformational isomers, which also causes the Mo-N-C bond to bend and deviate significantly from 180°, especially in the case of conformation B, compared to the cases of the reported phenylimido derivatives. Spectroscopic Characterization. The IR spectrum of compound 1 (see Figure 3) resembles that of the parent hexamolybdate anion. In the low wavenumber region (ν˜ < 1000 cm-1), compound 1 displays a pattern characteristic of the Lindqvist structure: two very strong bands of the Mo-Ot and Mo-ObMo asymmetric stretching vibrations at ca. 953 and 796 cm-1. The band near 976 cm-1 usually appears as a strong shoulder peak around the Mo-Ot stretching, which is diagnostic for the monosubstituted organoimido hexamolybdates12,14b deriving from the Mo-N bond stretching vibration.27 However, the IR bands of the naphthyl amine ligand in this region (mostly γ(C-C and C-H)) are of low intensity with respect to those of the polyoxometalate framework. In the high-frequency region, the aromatic γ(Ar-H) bands (ν˜ > 3000 cm-1) are hardly visible, due to their low intensity, and the complex pattern around 2900 cm-1 is aliphatic γ(C-H) bands of the tetrabutylammonium cation. In the medium-frequency region (1650-1000 cm-1), there are characteristic peaks from γ(CsN); the bands at 1585 and 1482 cm-1 were shown to be associated with γ(CdC) of the naphthalin mode; the 1380 and 1393 cm-1 bands were assigned to δ(CsH) of the tetrabutylammonium cation; and the 1321 cm-1 band was thought to be associated with γ(CsN) of the naphthyl group. Figure 4 shows the UV-vis absorption spectra of the tetrabutylammonium salt of [Mo6O19]2- and compound 1. The lowest energy electronic transition at 325 nm in [Mo6O19]2was assigned to a charge-transfer transition from the oxygen π-type HOMO to the molybdenum π-type LUMO, which is bathochromically shifted by more than 50 nm and becomes considerably more intense in 1 (383 nm), originating from the charge-transfer transition of the coordinated N atom to the molybdenum atom (LMCT) and indicating that the Mo-N π-bond is formed and the delocalization of organic conjugated π-electrons has extended from the naphthalene ring to the hexamolybdate skeleton. In other words, there is a strong electronic interaction between the metal-oxygen cluster and the organic conjugated ligands. In addition, compared to the cases of previously reported monosubstituted phenylimido hexamolybdates,15a,16 there is also a significant red-shift of more than ca. 30 nm, in accord with Mu¨lliken theory,28 which implies that there is a stronger electronic interaction between the metal-

Figure 4. UV/vis absorption spectrum of compound 1 in CH3CN.

oxygen cluster and the organic conjugated segment in compound 1 due to a larger conjugated naphthyl group being incorporated. The second band near 254 nm in [Mo6O19]2-, originating from the n-π transition from the oxygen π-type nonbonding orbitals to the molybdenum π-type LUMO, keeps no obvious variation in compound 1, which implies that the incorporated naphthylimido ligands have few effects on the skeleton of the hexmolybdate and the energy levels of the oxygen π-type nonbonding orbitals in these related cluster anions are almost identical. The 1H NMR spectrum (in d6-DMSO) of compound 1 (see Figure 5) shows seven obviously resolved signals of the naphthyl protons, which can be unambiguously assigned. The integration matches well with the proposed structure. Compared to the 1H NMR spectrum of the corresponding free amine ligand, the aromatic protons in compound 1 all exhibit significantly downfield chemical shifts, indicating the much weaker shielding nature of the [Mo5O18(MoN)]2- group than the amino group NH2- and reflecting the electron-withdrawing nature of the hexamolybdate cluster. Compound 1 has also been characterized by ESI mass spectroscopy. There are three major isotopic clusters, centered at m/z 1245.3 (100%), 1006.2 (10%), and 504.6 (37%), confirming the functionalization. These signals are all welldefined. The signal at m/z 1245.3 is due to the aggregate between the cluster anion and the [Bu4N]+ cation, [Bu4N][Mo6O18N(Naph-1)]- (calcd m/z: 1247.32), while the signal at m/z 1006.2 is assigned to [HMo6O18N(Naph-1)]- (calcd m/z: 1005.92). Additionally, the signal at m/z 504.6 can be attributed to the naked cluster anion [Mo6O18N(Naph-1)]2- (calcd m/z: 502.46). Electrochemical Studies. Figure 6 shows the voltammogram of compound 1. In the range from +2.5 V to -2.0 V (vs Ag/ AgCl), there are two reversible reduction waves (E1/2) at -0.34 V and -0.53 V. Considering the E1/2 value of -0.34 V for the first reduction wave of an authentic sample of (Bu4N)4[Mo6O19], recorded in acetonitrile, the other wave at -0.53 V can be unambiguously attributed to [Mo6O18N(Naph-1)]2-. The cathodic shift in the reduction potential of compound 1 compared to [Mo6O19]2- is consistent with the trend in π-donor ability of corresponding ligands, i.e. O2- < RN2-. In other words, the electron-donating nature of naphthylimido group is stronger than that of the oxo group.

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Figure 5. (Top) 1H NMR spectrum of the naphthalene ring in compound 1. (Bottom) 1H NMR spectrum of compound 1.

Acknowledgment. This work is sponsored by NFSC No. 20373001 and 20201001, SRF for ROCS of SEM, and MOST No. TG2000077503. Supporting Information Available: X-ray crystallographic CIF file for compound 1. This material is available free of charge via the Internet at http://pubs.acs.org.

References

Figure 6. Cyclic voltammogram of compound 1.

Conclusions A novel naphthylimido derivative of hexamolybdate, (nBu4N)2[Mo6O18(NC10H7)], has been synthesized for the first time, which belongs to the family of Linqvist-type polyoxometalates. Compared to the cases of monosubstituted phenylimido hexamolybdates, the significant red-shift of its lowest energy band, which occurs at 383 nm, implies that there is a stronger electronic interaction between the metal-oxygen cluster and the naphthalene ring in compound 1. In addition, there are interesting 1D hydrogen bond chains formed via C-H‚‚‚O hydrogen bonding interactions in solid compound 1. Such supramolecular assemblies and the stronger electronic interaction between the organic segments and inorganic clusters imply that such organoimido derivatives have potential applications in polyoxometalate-based organic-inorganic hybrid semiconducting materials.

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