摘要:

Self-assembly of a novel nanoscale giant cluster [Mo176O496(OH)32(H2O)80]† Chang-Chun Jiang Yong-Ge Wei Qun Liu Shi-Wei Zhang,* Mei-Cheng Shao and You-Qi Tang Department of Chemistry Peking University Beijing 100871 PR China. E-mail zsw@ipc.pku.edu.cn Reduction of an acidified solution of Na2MoO4·2H2O by iron powder results in the formation of the title compound [Mo176O16O480(OH)32(H2O)80]; the compound is tyre shaped and consists of sixteen {Mo8} subunits being linked by fortyeight MoO6 octahedra; remarkable features are the size and mass (which is of the order of a protein) and its nanodimensional cavity of ca. 3 nm in diameter. One of the challenges chemists are facing today is how to synthesize larger mesoscopic molecules from molecular fragments. The aim of such research is not only to improve the understanding of the extreme complexity of natural process but also to synthesize molecular materials with novel properties e.g.electric and magnetic which may be anticipated for mesoscopic compounds. Indeed considerable progress has been made in the field of polymetalate chemistry.1,2 We have evolved the so-called ‘reduction–reconstitution’ self-assembly process and synthesized a class of large mixed-valent nanoscale polyoxomolybdate anions3–7‡ constructed of {Mo8} or {Mo17} fragments:2 [Mo36O108(NO)4(H2O)16]122 1,3 [Mo57V6- O180(NO)6(OH)3(H2O)18]212 2,4 [Mo57FeIII 6O174(NO)6(OH)3- (H2O)24]152 3,5 [Mo57FeII 6O177(NO)6(OH)2(H2O)22- (MoO)2]182 46 and {{(H2O)MoO2.5[Mo36O108(NO)4- (H2O)16]O2.5Mo(H2O)}122}n 57 and some of them were also obtained later by M�uller’s group.2 By slightly changing the reaction conditions in the same system M�uller et al.succeeded in isolating a giant cluster [Mo154- (NO)14O420(OH)28(H2O)70]142 6.2,8 Now we have succeeded in synthesizing an even larger nanocompound [Mo176O496(OH)32(H2O)80] 7 which is also constructed of {Mo8} fragments. Compound 7 was prepared following the general method leading to ‘molybdenum blue’. A solution of Na2- MoO4·2H2O (5.0 g 20.7 mmol) in H2O (50 ml) was adjusted to a pH of ca. 1.0 with 36.5% hydrochloric acid. After addition of iron powder (50 mg 0.9 mmol) the mixture was left to stand for one month to crystallize. The preparation yields blue–black tetragonal-bipyramidal well defined crystals of 7 in ca. 30% yield. Reduction with elemental Al or Zn as well as N2H4·2HCl yields the same well defined crystals.The compound was characterized by IR and UV–VIS spectroscopy cerium(iv) sulfate redox titration and elemental analysis,§ as well as by single-crystal X-ray diffraction.¶ The rather intricate single-crystal X-ray structure analysis reveals that the ring-shaped compound 7 consists of 160 MoO6 octahedral and 16 pentagonal bipyramids of type MoO7 (Figs. 1 and 2). The structure has approximate D8d symmetry. Sixteen {Mo8} fragments are linked with sixteen Mo atoms in the equatorial plane and another sixteen {Mo2} groups.2 Each of the sixteen subunits has a pentagonal-bipyramidal MoO7 center about which seven other MoO6 octahedral are grouped by corner- and edge-sharing to form an {Mo8} fragment. This {Mo8} fragment also occurs in 1–6 which however possess a pentagonal bipyrimidal Mo(NO)O6 centre.The structure type of 1–5 is different from that of 6 and 7. In 1–5 two {Mo8} fragments above and below the equatorial plane are linked through one equatorial Mo atom and are transformed into each other by reflection across the equatorial mirror plane. Thus anion 1 has approximate C2h symmetry and anions 2 and 3 have D3h symmetry. In other words this kind of linking manner implies that there are {Mo17} fragments as building blocks in these anions. By contrast the arrangement of {Mo8} fragments in 7 (D8d symmetry) which is like that of 6 (D7d symmetry) are twisted relative to each other. This shift means that in 6 and 7 two equatorial Mo atoms are required to link two {Mo8} fragments and so there are sixteen Mo atoms in the equatorial plane in 7 (fourteen in 6).Two neighbouring {Mo8} fragments on the same side of the equatorial plane are linked Fig. 1 Ball-and-stick model of the nanoscale cluster (view parallel to the C8 axis) Fig. 2 Packing of the {Mo176} molecules in the crystal (view parallel to the c axis) Chem. Commun. 1998 1937 through two Mo atoms (or one {MoO2(H2O)(m2-O)- MoO2(H2O)} group) i.e. a {Mo2} unit instead of only one V or Fe center as in 2 and 3 respectively. However the central {MoNO}3+ group of the {Mo8} fragment of 6 is replaced by {MoO}4+ group in 7. Whereas the basic structure of compound can be determined unambiguosly it will probably never be possible to ascertain the exact number of crystallization waters by XRD owing to disorder of the lattice water molecules the tendency to lose solvent and the weak diffraction of the crystal to X-rays.According to elemental analysis and TGA the number of water is ca. 600 ± 50. A remarkable feature of 7 is its nanoscale cavity and corresponding host properties. The cavity itself which is like the inside of a tyre measures about 3 nm in diameter. A further characteristic of 7 is that it dissolves extremely well in water ethanol or acetone. This high solubility can be attributed to the large surface built up of a large number of H2O molecules and OH groups. It is noteworthy that crystals of 7 are obtained from ‘molybdenum blue’. ‘Molybdenum blue’ used to be regarded as amorphous but recently M�uller et al. have made some suggestions about its structure9 and they also succeeded in isolating a compound Na15{Mo144O409(OH)28(H2O)56} ca. 250 H2O 8 from it10 whose structure resembles 6 apart from some defects in 8.Now it can be proposed that ‘molybdenum blue’ is a mixture of compounds which have a similar basic ring-shaped structure. Polyoxoanions constitute a large class of inorganic compounds 11 however the number of different structural types is small. There are indications that 1–8 belong to the same kind structural type of compounds as they are all synthesized by a similar method and linked up via {Mo8} fragments in different ways. It can be presumed that under suitable reaction conditions a variety of novel nanoscale clusters with high complexity and multifunctionality could be synthesized by the reduction– reconstruction self-assembly process developed in our laboratory. Financial support from the National Natural Science Fondation of China No.29371004 and No. 29733080 is gratefully acknowledged. Notes and References † Just before this manuscript was mailed similar work by M�uller et al. was published (Angew. Chem. Int. Ed. Engl. 1998 37 1220). However they prepared their crystals by a different method and solved the structure in space group Cmcm. ‡ The charges given here are the values given by M�uller et al. However cluster anions with different charges are also obtained in our laboratory. § Elemental analysis. Calc. for [Mo176O16O480(OH)32(H2O)80]·(600 ± 50)H2O Mo 44.9; Found Mo 44.0; Na 0.2; Cl 0.2%. Characterization of 7 main IR bands (KBr disc) 1726s 1618m 974m 914w 668w 558s cm21; UV–VIS (H2O) 750 220 nm. Cerium sulfate titration 1 g of 7 reduces 0.830 mmol Ce(SO4)2·2(NH4)2SO4·4H2O corresponding to 32 MoV atoms in each molecule.¶ Crystal data for [Mo176O16O480(H2O)80(OH)32]·(600 ± 50) H2O M = 36 522.24 orthorhombic space group Amm2 (no. 38) a = 66.628(13) b = 53.760(11) c = 31.775(6) Å V = 113 816(39) Å3 F(000) = 71 024 Z = 4 Dc = 2.131 g cm23 Mo-Ka radiation l = 0710 69 Å m = 1.99 min21. Intensity data were collected on a Rigaku AFC6S diffractometer at 296 K and a total of 25 975 reflections were collected of which 12 706 reflections were observed the structure was solved with direct method and difference Fourier map using SHELXS97 and refined using SHELXL97. Owing to the poor ability of diffraction of the crystal and the limit of the number of reflections only the Mo atoms are refined anisotropically with R1 = 0.1050 for observed reflections and wR2 = 0.2685 forata.1 K. Wassermann M. H. Dickman and M. T. Pope Angew. Chem. Int. Ed. Engl. 1997 36 1445. 2 A. M�uller F. Peters M. T. Pope and D. Gatteschi Chem. Rev. 1998 98 239 and references therein. 3 S.-W. Zhang D.-Q. Liao M.-C. Shao and Y.-Q. Tang J. Chem. Soc. Chem. Commun. 1986 835. 4 S.-W. Zhang G.-Q. Huang M.-C. Shao and Y.-Q. Tang J. Chem. Soc. Chem. Commun. 1993 37. 5 G.-Q. Huang S.-W. Zhang and M.-C. Shao Polyhedron 1993 12 2067. 6 G.-Q. Huang S.-W. Zhang and M.-C. Shao Chin. Sci. Bull. 1995 40 1438. 7 S.-W. Zhang Y.-G. Wei Q. Yu M.-C. Shao and Y.-Q. Tang J. Am. Chem. Soc. 1997 119 6440. 8 A. M�uller E. Krickmeyer J. Meyer H. Bogge F. Peters W. Plass E. Diemann S. Dillinger F. Nonnenbruch M. Randerath and C. Menke Angew. Chem. Int. Ed. Engl. 1995 34 2122. 9 A. M�uller J. Meyer E. Krickemeyer and E. Diemann Angew. Chem. Int. Ed. Engl. 1996 35 1206. 10 A. M�uller E. Krickemeyer H. Bogge M. Schmidtmann F. Peters C. Menke and J. Meyer Angew. Chem. Int. Ed. Engl. 1997 36 484. 11 M. T. Pope Heteropoly and Isopoly Oxometalates Springer-Verlag New York 1983. Received in Cambridge UK 9th June 1998; 8/04358D 1938 Chem. Comm

ISSN:1359-7345    

DOI:10.1039/a804358b

出版商:RSC    

年代:1998

数据来源: RSC