摘要:

Dc and ac magnetic properties of the two-dimensional molecular-based ferrimagnetic materials A2M2[Cu(opba)]3 nsolv [A+=cation, MII=MnII or CoII, opba=orthophenylenebis( oxamato) and solv=solvent molecule] Olivier Cador,a Daniel Price,a Joulia Larionova,a Corine Mathonie`re,a Olivier Kahn*a and J. V. Yakhmib aL aboratoire des SciencesMole�culaires, Institut de Chimie de la Matie`re Condense�e de Bordeaux, UPR CNRS no 9048, 33608 Pessac, France bChemistry Division, Bhaba Atomic Research Center, 400 085 Bombay, India This paper is devoted to a thorough study of the magnetic properties, both in the dc and ac modes, of two-dimensional ferrimagnetic materials.The general formula of the compounds, abbreviated as A2M2Cu3, is A2M2[Cu(opba)]3 nsolv where A+ stands for a counter cation (radical cation, alkali-metal or tetraalkylammonium), opba is ortho-phenylenebis(oxamato), MII is MnII or CoII , and solv is a solvent molecule (Me2SO or H2O).The dc magnetic susceptibility data for all compounds down to ca. 50 K are characteristic of two-dimensional ferrimagnets. In the A2Mn2Cu3 series, three classes of compounds have been distinguished. Class I has a unique representative, AV2Mn2Cu3, where AV+ is the radical cation 2-(1-methylpyridinium-4-yl)-4,4,5,5- tetramethylimadozolin-1-oxyl-3-oxide, the structure of which has already been solved.This compound is a magnet with Tc= 22.5 K. Class II corresponds to compounds with large cations (tetraethyl- and n-tetrabutylammonium). They also behave as magnets with Tc around 15 K. Class III corresponds to compounds with small cations (alkali-metal ions and tetramethylammonium).They behave as metamagnets with a long-range antiferromagnetic ordering in zero field around 15 K, and a field-induced ferromagnetic state. The critical fields are of the order of 0.15 kOe. All the A2Co2Cu3 compounds are magnets with Tc around 30 K. Furthermore, the cobalt derivatives show a very strong coercivity, with coercive fields of several kOe at 5 K.They also display a pronounced magnetic after-eect in the ordered phase. In the course of this work several peculiar features have been observed. In particular, the A2Mn2Cu3 compounds have been found to present weak but significant negative out-of-phase ac magnetic signals at temperatures just above Tc. All the observed phenomena are discussed and, in particular, a mechanism for the long-range ordering in these two-dimensional compounds has been proposed.For more than a decade, one of our main fields of research where opba is ortho-phenylenebis(oxamato), with a divalent metal ion, MII, in 151 stoichiometry. Two-dimensional com- has dealt with the design of molecular-based magnets and the pounds may be also obtainedby reactingthe dianionic precursor study of their physical properties.The very first compounds of with a divalent metal ion, but in 253 stoichiometry and in the this kind were described in 1986 by Miller et al.,1,2 and by our presence of two equivalents of a non-coordinating cation A+. group.3,4 Since these pioneering results, quite a few research This aords compounds of formula A2M2[Cu(opba)]3 nsolv groups have initiated some activity along this line, and a large where solv is a solvent molecule.Many types of cations A+ number of molecular-based materials exhibiting a spontaneous may be utilized, including alkali-metal, simple organic cations magnetization below a certain critical temperature have been such as tetraalkylammonium, as well as radical cations.In one reported.3–10 In many cases these compounds contain two case, involving the radical cation 2-(1-methylpyridinium-4-yl)- kinds of spin carriers, either two dierent metal ions,11–20 or a 4,4,5,5-tetramethylimidazolin-1-oxyl-3-oxide (AV+), the crystal metal ion and an organic radical.21–28 In at least two cases structure of the compound was solved.29,30 This structure, shown three spin carriers were involved.29–31 Other very interesting in Fig. 1, consists of two perpendicular and fully interlocked magnetic compounds, at the frontier between molecular and networks of honeycomb-like layers. In this compound the solid-state chemistry, have also been reported.32–34 interlocking arises from the bridging ability of the radical cation Furthermore, several conferences have been devoted to this which bridges two CuII ions belonging to perpendicular layers.subject in recent years.35–37 Since all the other A+ cations used in this work lack any The syntheticapproach we have advocated to design molecu- coordination ability, their structure is probably that of a single lar-based magnets is via heterobimetallic compounds.This network of parallel honeycomb layers, with the A+ cations approach consists of designing low-dimensional heterobimetal- situated between the layers. lic species showing ferrimagnetic behaviour, and of assembling In preceding papers we reported on the magnetic properties them within the crystal lattice in a ferromagnetic fashion. The of some of these two-dimensional compounds, exclusively in ferrimagnetic behaviour arises from antiferromagnetic inter- the dc mode.12,29–31 Here, we present a detailed investigation actions between dierent spin carriers.We first focused on of the magnetic properties in both dc and ac modes. We will one-dimensional,19,38,39 then on two-dimensional ferrimag- focus on two series of compounds, with MII=MnII and CoII, nets.12,20 The one-dimensional compounds are synthesized by respectively.For convenience, all the compounds of formula reacting a dianionic copper(II) precursor such as [Cu(opba)]2-: A2M2[Cu(opba)]3 nsolv are abbreviated to A2M2Cu3 . The nature and the number of solvent molecules present in the structure, as deduced from chemical analyses, are specified in Tables 1 and 2. Experimental Syntheses Solvents and reagents were obtained commercially, and were used without further purification.The ligand ortho-phenylene- J. Mater. Chem., 1997, 7(7), 1263–1270 1263Fig. 1 Structure of AV2Mn2Cu3 . (Top) view of a honeycomb-like layer. (Bottom) view showing the interpenetration of the two perpendicular networks (reproduced with permission from ref. 29 and 30). 1264 J. Mater. Chem., 1997, 7(7), 1263–1270Table 1 Some characteristic temperatures (K) for the magnetic proper- tives the eect of concentration upon the reaction is crucial. If ties of the A2Mn2Cu3 materials the solution is too concentrated, Me2SO solvated manganese( II) acetate appears in addition to the expected compound ac mode whereas if the solution is too dilute, no compound is obtained, dc mode MnII being slightly base sensitive, with decompositions before xMT a x¾Mb xMb or during the crystallization, producing an insoluble brown class I impurity.For cobalt derivatives, the eect of concentration is A 2Mn2[Cu(opba)]3 2Me2SO 2H2O 115 23 22.5 not so crucial,although if the solutionis too dilute precipitation class II will take longer. (NEt4)Mn2[Cu(opba)]3 4Me2SO 2H2O 125 15.5 15.5 Chemical analyses of all elements, except oxygen, were (NBu4)Mn2[Cu(opba)]3 4Me2SO 2H2O 125 15 15 carried out and satisfactory results were obtained.In particular, class III the ratio Cu/M (M=Mn or Co) was found to be 1.5±0.04 Na2Mn2[Cu(opba)]3 11Me2SO 10H2O 125 15 for all the compounds. On heating the solvent molecules are 5.5 5.5 removed, which usually leads to an increase of the critical K2Mn2[Cu(opba)]3 11Me2SO 10H2O 120 14 temperatures.12 However, the resulting compounds seem to be 7.5 6 poorly crystallized, so we discuss them no further.(NMe4)2Mn2[Cu(opba)]3 4Me2SO 2H2O 125 14 8 Magnetic measurements aTemperature for minimum of xMT . bTemperature for maximum of x¾M These were carried out with a Quantum Design MPMS-5S or xM .SQUID magnetometer, working in both dc and ac modes between 2 and 300 K, and from 0 to 50 kOe. Table 2 Some characteristic temperatures (K) for the magnetic properties of the A2Co2Cu3 materials A2Mn2 [Cu(opba)]3 nsolv compounds ac mode We can distinguish three classes of A2Mn2Cu3 compounds, dc mode namely: class I containing a unique representative with AV+, xMT a x¾Mb xMb class II where A+ is a large cation (NEt4+ or NBu4+) and Na2Co2[Cba)]3 6Me2SO 4H2O —c 31 31 class III where A+ is a small cation (Na+, K+ or NMe4+).K2Co2[Cu(opba)]3 12Me2SO 3H2O —c 32.5 32.5 Independently of their magnetic properties, it became evident Cs2Co2[Cu(opba)]3 5Me2SO 4H2O —c 32.5 32.5 over the course of this work that not all these materials had (NMe4)2Co2[Cu(opba)]3 7Me2SO H2O —c 33 32.5 the same stability. While compounds of classes I and II were (NBu4 )2Co2[Cu(opba)]3 6Me2SO H2O 95 29 28.5 stable for over a year at room temperature compounds of aTemperature for minimum of xMT .bTemperature for maximum of x¾M class III were found to gradually decompose. At room tempera- or xM. cThe magnetic susceptibility data were not measured ture this decomposition is noticeable within a few hours of above 70 K.their removal from the mother-liquor, and was apparently complete within a matter of a day and was detected as a bis(ethyloxamate), H2Et2opba, was synthesized as previously colour change from blue to purple. Storing the samples in the described.12,39 The synthesis of AV2Cu(opba) H2O and cold, however, extends their lifetime suciently for physical AV2Mn2[Cu(opba)]3 2Me2SO 2H2O were also performed as studies to be performed.The study of the magnetic properties previously described.29,30 On the other hand, the syntheses of of the compounds after decomposition very strongly suggests the other precursors A2Cu(opba) nsolv and ferrimagnetic that a mixture of a chain compound MnCu(opba) nsolv12,39 materials A2M2[Cu(opba)]3 nsolv were slightly modified.A and the isolated precursor A2Cu(opba) was obtained. It is typical synthesis of a precursor, (NBu4)2Cu(opba), is as follows: sucient to point out that compounds of class III seem to be an aqueous solution of NBu4OH (8.77 g, 40% m/m, 13.5 mmol) thermodynamically unstable with respect to the breakdown of was added dropwise, with rapid stirring, to a solution of the planar structure into chains and residual non-coordinated H2Et2opba (2.92 g, 3.25 mmol) and CuCl2 2H2O (0.55 g, copper precursors.The Rb+ and Cs+ compounds were found 3.23 mmol) in methanol (100 ml). Initially, a green precipitate to be so unstable that they could not be studied. was formed, but upon continued addition of the base this redissolved to give a blue–violet solution.The reaction mixture Dc magnetic properties. Both the temperature and field dependences of the dc magnetic responses were investigated. was stirred for 5 min, filtered, and the solvent removed under reduced pressure to give the crude product as an oil, which The temperature dependences may be represented in the form of the xMT vs.T plots, where xM is the dc molar magnetic slowly crystallized. Recrystallization from acetonitrile gave violet needles of (NBu4)2Cu(opba), which were collected by susceptibility and T the temperature. The xMT vs. T plot for AV2Mn2Cu3 has already been investigated in detail.29,30 All filtration and dried under vacuum (1.95 g, 76%). The preparation of A2Cu(opba) precursors with other organic cations the other compounds behave in a similar way down to ca. 30 K. At room temperature, xMT is ca. 8.1 emu K mol-1, which is (NMe4+ and NEt4+) was the same but using the appropriate hydroxides. For the alkali-metal cations, Na+, K+ and Cs+, slightly lower than expected for isolated spin carriers. Then xMT decreases smoothly as the temperature is lowered, reaches water was added as a co-solvent to aid the solubility of the precursors (methanol–water, 80/20 v/v).a rounded minimum around 120 K, then increases very abruptly at lower T to a very large maximum value around A typical example of the synthesis of a ferrimagnetic material, that of (NBu4)2Mn2[Cu(opba)] 4Me2SO 2H2O, is as follows: 15 K. This maximum value of xMT is roughly twice as large for compounds of class II than for compounds of class III.A (NBu4)2Cu(opba) (0.2469 g, 0.449 mmol) was dissolved in Me2SO (2 ml), which gives a violet solution to which was typical example, that of (NBu4)2Mn2Cu3, is shown in Fig. 2. The minimum in the xMT vs. T plot reveals antiferromagnetic added Mn(MeCO2)2 4H2O (0.059 g, 0.241 mmol). The resulting blue solution was filtered, and allowed to stand.After interactions between nearest-neighbour metal ions without correlation length, and the huge increase of xMT at low one day a microcrystalline product had formed. This was collected by filtration, washed with Me2SO, dried under temperature is due to the increase of the correlation length with the SMn spins of MnII ions aligned along the field direction vacuum for 4 h, and stored at-18 °C.All the other derivatives were synthesized by a similar procedure. It is probably worth and the SCu spins of CuII ions aligned along the opposite direction. specifying here that for the A2Mn2[Cu(opba)]3 nsolv deriva- J. Mater. Chem., 1997, 7(7), 1263–1270 1265above ca. 0.15 kOe, which is characteristic of metamagnetic behaviour (Fig. 3). The field dependence of the magnetization, M=f (H), at 2 K for AV2Mn2Cu3 has already been reported.29,30 The magnetization first increases very rapidly, as expected for a magnet, then instead of reaching a saturation value at low field, increases smoothly as H increases, and does not reach saturation even at 200 kOe.This peculiar behaviour was attributed to the progressive decoupling of the radical spin with respect to the Mn2[Cu(opba)]3 skeleton spin.For all the compounds of classes II and III the magnetization at 2 K first increases very rapidly, then reaches a saturation value of ca. 7 mB. This value corresponds well to that expected if all the SMn spins are oriented along the field direction, and all the SCu spins oriented along the opposite direction. An interesting feature is that .M/.H is not at a maximum at exactly zero field, but at a field which depends on temperature.For instance, for (NBu4)2Mn2Cu3, the maximum of .M/.H is observed at 60 and 100 Oe at 5 and 10 K, respectively. This behaviour implies that a small coercive field is required to orient the domains, and therefore that some anisotropy is present in the system. For all the A2Mn2Cu3 materials, the coercive fields are Fig. 2 xMT vs. T plot for (NBu4 )2Mn2Cu3. Black dots represent the experimental data, and the full line represents the calculated curve in extremely weak being of the order of 10 Oe. These magnets the 50–300 Ktemperature range, using the high-temperature expansion do not exhibit significant hystereses, in striking contrast with approach (see text).The insert emphasizes the minimum of xMT the A2Co2Cu3 derivatives. characterizing the ferrimagnetic behaviour. Ac magnetic properties. The ac magnetic properties also reveal dierences between the three classes of the A2Mn2Cu3 Recently, we developed two theoretical approaches of the compounds. In the following we define x¾M and xM as the in- thermodynamical properties of A2Mn2Cu3 two-dimensional and out-of-phase molar magnetic susceptibilities, respectively.compounds, the former based on a high-temperature expansion Let us first consider the unique class I compound, and the latter on a Monte Carlo simulation.40 In both AV2Mn2Cu3. Fig. 4 shows the in-phase, x¾M, and out-of-phase, approaches SMn=5/2 is treated as a classical spin and SCu= xM, responses as a function of temperature at a frequency of 1/2 is treated as a quantum spin.The high-temperature expan- 125 Hz and a drive amplitude of 3 Oe. x¾M shows a sharp sion was developed up to eleventh order in J/kT where J is maximum at 23 K, which reveals a long-range magnetic the interaction parameter occurring in the spin-Hamiltonian ordering. xM also shows a maximum near this temperature, of the form -JSSMn SCu, which led to the following at 22.5 K.The non-zero value of xM reveals that in the expression: magnetically ordered state the local momenta do not compensate. xM reflects the magnetization lag with respect to the ac xMT= Nb2 3k C2gMn2S2+ 9 4 gCu2-6gMngCuSK field, and depends on both the relaxation time and the drive frequency. Furthermore, x¾M and xM are strongly correlated and represent two components of the same quantity.Assuming +2AgMn2S2+gCu2BK2- 2 3 gMngCuSK3 that the relaxation time in the magnetically ordered phase does not vary dramatically with temperature and that the drive frequency is fixed results in similar temperature depen- - 3 5gCu2K4- 8 45 gMngCuSK5+A2 25gMn2S2+ 533 945 gCu2BK6 + 2 945 gMngCuSK7- A 4 315 gMn2S2+ 5683 14175 gCu2BK8 - 4 525 gMngCuSK9+ A 524 99225 gMn2S2+ 19912 66825 gCu2BK10 + 7108 5457375 gMngCuSK11D with: S=[SMn(SMn+1)]D and K=-JS/2kT where k is the Boltzmann constant and b the electron Bohr magneton.An excellent agreement, as shown in Fig. 2, was found between the experimental data down to 50 K and calculated data for J=-33.1 cm-1, and the local Zeeman factors gMn=2.0 and gCu=2.2. Below ca. 30 K the xM vs. T curves dier between classes I–III. AV2Mn2Cu3 shows a break in the xM vs. T plot at Tc= 22.5 K, corresponding to a long-range ferromagnetic ordering. Compounds of class II behave qualitatively in a similar way, but with Tc around 15 K. On the other hand, the magnetic behaviour of class III compounds is more complex. At very low field xM shows a maximum around 15 K, suggesting that Fig. 3 xM vs. T plot for Na2Mn2Cu3 at various fields. The full lines the long-range orderingis antiferro- rather than ferro-magnetic. are guides for the eye. H=0.05 (#), 0.10 ($), 0.15 (+), 0.20 ('), 0.40 kOe (%). This maximum of xM vanishes when applying a magnetic field 1266 J. Mater. Chem., 1997, 7(7), 1263–1270Fig. 4 In-phase, x¾M ($), and out-of-phase, xM (#), ac molar magnetic susceptibilities for AV2Mn2Cu3 Fig. 6 In-phase, x¾M ($), and out-of-phase, xM (#), ac molar magnetic susceptibilities for Na2Mn2Cu3. The full line is a guide for the eye. dences of x¾M and xM. The rapid decrease of both x¾M and xM below Tc may have two origins: (i) for microcrystalline powders the particles may form monodomains and Landau theory for and II compounds, the xM signal remains very weak, even at second-order phase transitions predicts such behaviour;41,42 5.5 K.It follows that at 15 K the material exhibits a transition (ii) measurements are carried out under a very weak ac field, toward a magnetically ordered state without spontaneous and the hysteresis loop starts to open at temperatures lower magnetization.This transition is therefore antiferromagnetic. than Tc. The very weak coercivity observed for the A2Mn2Cu3 Another magnetic transition takes place around 5.5 K toward compounds of classes I and II might be sucient to give rise a magnetically ordered state, apparently with spontaneous to the observed behaviour.41,43 magnetization. A weak negative xM signal is observed in the We can consider (NBu4)2Mn2Cu3 as a typical example of a temperature range 9–20 K, with surprisingly a minimum value compound belonging to class II.The temperature dependences around 15 K. The metamagnetic behaviour of Na2Mn2Cu3 is of the x¾M and xM responses are shown in Fig. 5. x¾ shows a nicely demonstrated by the temperature dependences of xM very sharp peak at 15 K, and xM an even sharper peak at the under various dc fields.While xM is very slightly negative same temperature, which can be regarded as the critical around 15 K in zero field, it shows a positive peak at 100 Oe. temperature. In other respects, the high-temperature side of the xM curve reveals a weak but significant negative out-of- A2Co2 [Cu(opba)]3 nsolv compounds phase response.This behaviour was seen for dierent samples arising from dierent synthetic procedures, and may be con- All the cobalt derivatives present similar magnetic properties sidered as an intrinsic property. We note that negative out-of- and are more stable than the manganese derivatives, those phase responses have already been observed by several with large cations (NEt4+ and NBu4+) being perhaps slightly authors.44,45 more stable than those with small cations (Na+, K+, Cs+ Finally, the ac response for Na2Mn2Cu3 as an archetype of and NMe4+).class III compounds is examined (Fig. 6). The in-phase signal shows two peaks, a well pronounced one at 15 K and a Dc magnetic responses. Fig. 7 shows a typical example of rounded one around 5.5 K. The out-of-phase signal, on the a xMT vs.T curve for an A2Co2Cu3 material; i.e. of other hand, shows a single peak at 5.5 K; compared to class I (NBu4)2Co2Cu3. At room temperature xMT is ca. 7 emu K mol-1, and decreases as the temperature is lowered, revealing again a ferrimagnetic behaviour. A minimum of xMT is observed around 95 K. As the temperature is lowered further, xMT increases dramatically, and reaches a maximum around 30 K.In the present case, there is no adequate theoretical model to interpret quantitatively the magnetic susceptibility data. The field dependences of the magnetization are particularly interesting. First, the maximum of .M/.H in the first magnetization curve is observed for a rather large field, and is temperature dependent. For K2Co2Cu3 this maximum occurs at 2 kOe at 5 K, and 1 kOe at 25 K.This behaviour reveals a large magnetic anisotropy. Secondly, even at 50 kOe, saturation is not reached. The magnetization value for K2Co2Cu3 at 50 kOe is 1.9 mB while a saturation value >3 mB may be anticipated. Most remarkably, the A2Co2Cu3 compounds present extremely broad magnetic hysteresis loops, as shown in Fig. 8 for K2Co2Cu3. The coercive field was 4.20 kOe at 5 K, 2.58 kOe at 15 K, and 0.58 kOe at 25 K. For the A2Co2Cu3 materials the coercive field tends to decrease slightly as the size of A+ increases. For instance, the coercive field for (NBu4 )2Co2Cu3 Fig. 5 In-phase, x¾M ($), and out-of-phase, xM (#), ac molar magnetic susceptibilities for (NBu4 )2Mn2Cu3. The full line is a guide for the eye.is found to be 1.40 kOe at 5 K. J. Mater. Chem., 1997, 7(7), 1263–1270 1267Fig. 7 xMT vs. T plot for (NBu4)2Co2Cu3. The insert emphasizes the minimum of xMT characterizing the ferrimagnetic behaviour. Fig. 8 Magnetic hysteresis loops for K2Co2Cu3 at 5 (2), 15 ($) and Fig. 9 Time dependences of the magnetization under 50 Oe for 25 K (#). The full lines are guides for the eye.(NBu4)2Co2Cu3 at (a) T=5 K, (b) T=15 K, (c) T=25 K. The full lines represent a fitting of the experimental data, using logarithmic laws. In the course of this study we made an interesting observation concerning the magnetic relaxation in the ordered state latter temperature is taken as the critical temperature. No as illustrated for (NBu4)2Co2Cu3. The sample was first cooled negative out-of-phase signal was observed.below the critical temperature Tc=29 K in zero field, then a In order to fully characterize the nature of the long-range field of 50 Oe was applied and the time dependence of the magnetic ordering in the A2Co2Cu3 materials, we checked that magnetization was measured. Fig. 9 shows the results at three the maxima of both x¾M and xM did not depend on the dierent temperatures, 5, 15 and 25 K.In order to compare frequency within experimental uncertainty between 10 and the three curves, the magnetization was set at zero at t=0. 1000 Hz and firmly excluded superparamagnetic behaviour.47 The magnetization exhibits an after-eect for >1 h. At the three temperatures the M=f (t) curve was found to follow Discussion logarithmic behaviour, which suggests that the system presents a distribution of energy barriers.46 In this section we discuss (i) the possible structures of the The temperature dependence of the relaxation demonstrates compounds, (ii) the dierences between the magnetic properties that the process is thermally activated.The magnetization of A2Mn2Cu3 compounds belonging to class I and II on the increases faster as the system is closer to the critical tempera- one hand, and to class III on the other, (iii) the mechanism of ture.This behaviour is similar to the temperature dependence the long-range magnetic ordering, and finally (iv) the origin of of the M=f (H) hysteresis loops. In all probability, the hyster- the huge coercivity observed for the A2Co2Cu3 compounds.esis loops for the A2Co2Cu3 compounds are time dependent. Structures of the compounds Ac magnetic responses. A typical example of an ac magnetic response is shown in Fig. 10 for (NBu4)2Co2Cu3. x¾M displays Only the crystal structure of AV2Mn2Cu3 has been solved.29,30 However, for all the A2Mn2Cu3 compounds the dc magnetic a peak at 29 K, and xM displays a peak at 28.5 K and this 1268 J.Mater. Chem., 1997, 7(7), 1263–1270The crucial point concerning the dierences between the class II and III compounds is that the long-range magnetic ordering in zero field is ferromagnetic for the former and antiferromagnetic for the latter, which brings important insights on the mechanism of the long-range ordering in these compounds (see below). Mechanism of long-range ordering The long-range magnetic ordering in two-dimensional compounds may have two origins, namely in-plane magnetic anisotropy and/or interplane interactions.It is well established that there is no long-range ordering for a pure two-dimensional array of isotropic spins.48 The fact that Tc is roughly twice as large for A2Co2Cu3 than for A2Mn2Cu3 compounds suggests that magnetic anisotropy plays a significant role.Even for the manganese derivatives some magnetic anisotropy is present as revealed by the fact that .M/.H is at a maximum for a nonzero applied field. This weak anisotropy which is not sucient to give a hysteresis loop might contribute to long-range ordering. Fig. 10 In-phase, x¾M ($), and out-of-phase, xM (#), ac molar Let us now examine the role of the interlayer interactions.magnetic susceptibilities for (NBu4 )2Co2Cu3. The full line is a guide For A2Mn2Cu3 we have seen that apparently the nature of for the eye. these interactions depends on the size of A+. Such a crossover between antiferro- (AF) and ferromagnetic (F) interactions susceptibility data above ca. 30 K are very similar. These data might result from a competition between exchange (or orbital) are very well described by a two-dimensional model of edge- eects favouring the AF interactions when the layers are close sharing hexagons with MnII ions at the corners of the hexagons to each other and dipolar eects favouring F interactions when and CuII ions at the middles of the edges.It follows that it is the layers are farther apart from each other.If so, then except quite reasonable to assume that these honeycomb like layers for A2Mn2Cu3 compounds of class III, dipolar eects would are present in all compounds. What, as yet, remains unknown dominate. The dipolar interactions are expected to decrease as is the relative position of the layers as well as the position of the interlayer distance becomes very large.This is observed A+ cations between the layers. The A2Co2Cu3 compounds are when using very bulky cations such as [Ru(bipy)3]2+ or obtained as microcrystalline powders with very small grains, [Ru(phen)3]2+.49,50 and attempts to grow single crystals suitable for X-ray dirac- To summarize, we can conclude that long-range ordering tion appear to be fruitless. On the other hand, the situation is arises from the combined eect of in-plane anisotropy and not so severe for A2Mn2Cu3 compounds.The gel technique interplane interactions. Those latter have both an exchange aords hexagonal plate-shaped single crystals albeit too small. and dipolar origins. However, for (NBu4 )2Mn2Cu3 we succeeded in obtaining the lattice parameters, a=9.65 A° , b=34.54 A° , c=15.21 A° , b= Origin of the coercivity in A2Co2Cu3 compounds 101.3°, V=4970 A° 3 in a monoclinic crystal system.It is worth The A2Co2Cu3 compounds to the best of our knowledge are noting that the cell volume is roughly half that of AV2Mn2Cu3, molecular-based magnets exhibiting the strongest coercivity and that the lattice parameters would be compatible with a reported so far and we have already discussed the origin of two-dimensional honeycomb like structures and a basal spac- this.The value of the coercive field at a given temperature for ing of ca. 9.5 A° . a polycrystalline magnet depends on both chemical and struc- We have already pointed out the instability of the A2Mn2Cu3 tural factors such as the size and the shape of the grains within compounds belonging to class III.This instability might be the sample. As far as chemical factors are concerned, the key due to the fact that the A+ cations in class III materials are role is played by the magnetic anisotropy of the spin carriers too small to fit properly the available space both inside the which prevents the domains from rotating freely when applying Mn6Cu6 hexagons and between the layers.the field.31 For A2Co2Cu3 compounds the large coercivity arises from the magnetic anisotropy of the CoII ion in octa- Class II vs. class III A2Mn2Cu3 compounds hedral surroundings. One of the striking results of this work concerns the dierences between class II A2Mn2Cu3 compounds with large A+ cations Conclusion and class III A2Mn2Cu3 compounds with small A+ cations.While the former behave as normal magnets with Tc around What is particularly appealing in the field of molecular materials is to observe physical properties which dier from those 15 K, the latter present a more complex magnetic behaviour, which can be analysed as follows: down to ca. 30 K they of classical materials which is usually a consequence of elaborate molecular structure.Here we have described the magnetic exhibit two-dimensional ferrimagnetic behaviour with a minimum of xMT at the same temperature as for class II compounds, properties of a series of two-dimensional ferrimagnetic compounds, focusing on the long-range ordering. In all cases, the and a rapid increase of xMT below this temperature. Around 15 K, xM in low field and x¾M show a maximum, but not xM.magnetic properties above ca. 50 K are characteristic of the ferrimagnetic regime. As the temperature is lowered, each layer This can be attributed to a long-range antiferromagnetic ordering. A magnetic field of ca. 0.15 kOe is sucient to acquires a very large magnetic momentum resulting from the non-compensation of the MII (MnII or CoII) spins on the one overcome the antiferromagnetic interlayer interactions.These class III compounds may be defined as metamagnets. What hand and the CuII spins on the other. The momenta of the ferrimagnetic layers can couple within the lattice either in an remains unclear is the nature of the transition revealed by the maximum of xM around 5.5 K. A possible hypothesis is the AF or in a F fashion.The former situation is encountered for the compounds A2Mn2Cu3 of class III. The AF interlayer onset of a canted antiferromagnetism (or weak ferromagnetism) at that temperature. interactions, however, are very weak, and can be overcome by J. Mater. Chem., 1997, 7(7), 1263–1270 126920 Y. Pei, S. S. Turner, L. Fournes, J. S. Miller and O. Kahn, J. Mater. applying a magnetic field of 0.15 kOe.These compounds are Chem., 1996, 6, 1521. therefore metamagnets. The latter situation is encountered for 21 J. S. Miller, J. C. Calabrese, D. A. Dixon, A. J. Epstein, all the other compounds. The mechanism of the long-range R. W. Bigelow, J. H. Zhang and W. M. Rei, J. Am. Chem. Soc., ordering probably involves both in-plane anisotropy and 1987, 109, 769.interplane interaction (of both exchange and dipolar origin). 22 W. E. Broderick, J. A. Thompson, E. P. Day and B. M. Homan, Science, 1990, 249, 410. The role of the magnetic anisotropy is further evidenced by 23 G. T. Yee, J. M. Manriquez, D. A. Dixon, R. S. McLean, the very strong coercivity of the CoII containing materials. A D. M. Groski, R. B. Flippen, K. S. Narayan, A.J. Epstein and puzzling result arises from this work, namely the negative out- J. S. Miller, Adv. Mater., 1991, 3, 309. of-phase signals observed for the A2Mn2Cu3 compounds. So 24 W. E. Broderick and B. M. Homan, J. Am. Chem. Soc., 1991, far, we have no convincing interpretation of this behaviour. In 113, 6334. 25 D. M. Eichhorn, D. C. Skee, W. E. Broderick and B. M. Homan, this connection some peculiarities concerning molecular-based Inorg.Chem., 1993, 32, 491. ferrimagnets have already been reported.17,51,52 26 J. S. Miller, J. C. Calabrese, R. S. McLean and A. J. Epstein, Adv. Molecular-based magnetic materials have already shown Mater., 1992, 4, 498. surprising behaviour from chemical, physical and even theoreti- 27 K. Inoue and H. Iwamura, J. Am. Chem.Soc., 1994, 116, 3173. cal perspectives. We are convinced that further new phen- 28 A. Caneschi, D. Gatteschi, R. Sessoli and P. Rey, Acc. Chem. Res., 1989, 22, 392. omena, concerning magnetic as well as other physical 29 H. O. Stumpf, L. Ouahab, Y. Pei, D. Grandjean and O. Kahn, properties, will be observed and we are pursuing our work Science, 1993, 261, 447. along this line. 30 H. O. Stumpf, L. Ouahab, Y. Pei, P. Bergerat and O. Kahn, J. Am. Chem. Soc., 1994, 116, 3866. 31 H. O. Stumpf, Y. Pei, C. Michaut, O. Kahn, J. P. Renard and L. Ouahab, Chem. Mater., 1994, 6, 657. References 32 T. Mallah, S. Thiebaut, M. Verdaguer and P. Veillet, Science, 1993, 262, 1554. 1 J. S. Miller,J. C. Calabrese, A. J. Epstein, R. W. Bigelow, J.H. Zang 33 W. R. Entley and G.S. Girolami, Science, 1995, 268, 397. and W. M. Rei, J. Chem. Soc., Chem. Commun., 1986, 1026. 34 S. Ferlay, T. Mallah, R. Ouahe`s, P. Veillet and M. Verdaguer, 2 J. S. Miller, J. C. Calabrese, H. Rommelman, S. R. Chittipedi, Nature (L ondon), 1995, 378, 701. J. H. Zang, W. M. Rei and A. J. Epstein, J. Am. Chem. Soc., 1987, 35 Proceedings of the Symposium on Ferromagnetic and High Spin 109, 769.Molecular Based Materials, ed. J. S. Miller and D. A. Dougherty, 3 Y. Pei, M. Verdaguer, O. Kahn, J. Sletten and J. P. Renard, J. Am. Mol. Cryst. L iq. Cryst., 1989, 176. Chem. Soc., 1986, 108, 428. 36 Proceedings of the Symposium on the Chemistry and Physics of 4 O. Kahn, Y. Pei, M. Verdaguer, J. P. Renard and J. Sletten, J. Am. Molecular Based Magnetic Materials, ed.H. Iwamura and Chem. Soc., 1988, 110, 782. J. S. Miller,Mol. Cryst. L iq. Cryst., 1993, 323–233. 5 J. S. Miller and A. J. Epstein, Angew. Chem., Int. Ed. Engl., 1994, 37 Proceedings of the Fourth International Conference on Molecule- 33, 385. Based Magnets, ed. J. S. Miller and A. J. Epstein, Mol. Cryst. L iq. Cryst., 1995, 271–274. 6 O. Kahn,MolecularMagnetism, VCH, New York, 1993. 38 H. O. Stumpf, Y. Pei, L. Ouahab, F. Le Berre, E. Codjovi and 7 O. Kahn, Adv. Inorg. Chem., 1995, 43, 179. O. Kahn, Inorg. Chem., 1993, 32, 5687. 8 Y. Nakazawa, M. Tamura, N. Shirakawa, D. Shiomi, 39 V. Baron, B. Gillon, J. Sletten, C. Mathonie`re, E. Codjovi and M. Takahashi, M. Kinoshita and M. Ishikawa, Phys. Rev. O. Kahn, Inorg. Chim. Acta, 1995, 235, 69. (L ondon), 1992, B46, 8906. 40 J. Leandri, Y. Leroyer, S. V. Meshkov, Y. Meurdesoif, O. Kahn, 9 R. Chiarelli, M. A. Nowak, A. Rassat and J. L. Tholence, Nature B. Mombelli and D. Price, J. Phys.: Condens.Matter, 1996, 8, L271. (L ondon), 1993, 363, 147. 41 D. W. Carnegie, C. J. Tranchita and H. Claus, J. Appl. Phys., 1979, 10 D. Gatteschi, Adv. Mater., 1994, 6, 635. 50, 7318. 11 K. Nakatani, J. Y. Carriat, Y.Journaux, O. Kahn, F. Lloret, 42 C. Bellitto, Mol. Cryst. L iq. Cryst., 1989, 176, 465. J. P. Renard, Y. Pei, J. Sletten and M. Verdaguer, J. Am. Chem. 43 A. J. Epstein and J. S. Miller,Mol. Cryst. L iq. Cryst., 1989, 176, 359. Soc., 1989, 111, 5739. 44 W. Fujita and K. Awaga, Inorg. Chem., 1996, 35, 1915. 12 H. O. Stumpf, Y. Pei, O. Kahn, J. Sletten and J. P. Renard, J. Am. 45 A. Bhattacharjee, S. Ijima, F. Mizutani, H. Hayakawa, Chem. Soc., 1993, 115, 6738. M. Hagiwara and K. Katsumata, Mol. Cryst. L iq. Cryst., 1996, 13 H. Tamaki, Z. J. Zhong, N. Matsumoto, S. Kida, M. Koikawa, 286, 141. N. Achiwa, Y. Hashimoto and H. Okawa, J. Am. Chem. Soc., 1992, 46 J. M. Hernandez, X. X. Zhang, F. Luis, J. Bartolome, J. Tejada and 114, 6974. R. Ziolo, Europhys. L ett., 1996, 35, 301. 14 H. Okawa, M. Mitsumi, M. Ohba, M. Kodera and N. Matsumoto, 47 F. Palacio, F. J. Lazaro and A. J. van Duyneveldt, Mol. Cryst. L iq. Bull. Chem. Soc. Jpn., 1994, 67, 2139. Cryst., 1989, 176, 289. 15 S. Decurtins, H. W. Schmalle, H. R. Oswald, A. Linden, J. Ensling, 48 R. L. Carlin, Magnetochemistry, Springer-Verlag, Berlin, 1986. P. Gu�tlich and A. Hauser, Inorg. Chim. Acta, 1994, 216, 65. 49 S. S. Turner, C. Michaut, O. Kahn, L. Ouahab, A. Lecas and 16 S. Decurtins, H. W. Schmalle, P. Schneuwly, J. Ensling and E. Amouyal, New J. Chem., 1995, 19, 773. P. Gu�tlich, J. Am. Chem. Soc., 1994, 116, 9521. 50 J. Larionova, S. S. Scott and O. Kahn, unpublished work. 17 C. Mathonie`re, C. J. Nuttall, S. G. Carling and P. Day, Inorg. 51 S. A. Chavan, R. Ganguly, V. K. Jain and J. V. Yakhmi, J. Appl. Chem., 1996, 35, 1201. Phys., 1996, 79, 5260. 18 M. Ohba, N. Maruono and H. Okawa, J. Am. Chem. Soc., 1994, 52 N. Re, E. Gallo, C. Floriani, H. Miyasaka and N. Matsumoto, Inorg. Chem., 1996, 35, 5964; 6004. 116, 11566. 19 S. Turner, O. Kahn and O. Rabardel, J. Am. Chem. Soc., 1996, 116, 6428. Paper 7/00272F; Received 13th January, 1997 1270 J. Mater. Chem., 1997, 7(7), 1263–

ISSN:0959-9428    

DOI:10.1039/a700272f

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Structural studies of molecular-based nanoporous materials. Novel networks of silver(I ) cations assembled with the polydentate N-donor bases hexamethylenetetramine and 1,3,5-triazine Marco Bertelli, Lucia Carlucci, Gianfranco Ciani,* Davide M. Proserpio and Angelo Sironi Dipartimento di Chimica Strutturale e Stereochimica Inorganica and Centro CNR, V ia G. Venezian 21, 20133 Milano, Italy Novel polymeric materials have been isolated from the reactions of silver(I) salts of poorly coordinating anions with the polydentate bases hexamethylenetetramine (hmt) and 1,3,5-triazine (trz), the one-dimensional polymer[Ag(hmt)] SbF6 H2O 1, and the three-dimensional networked compounds [Ag11(hmt)6][PF6]11 14H2O 2 and [Ag6(trz)8][BF4]6 H2O 3, characterized by single-crystal X-ray analysis.Compound 1 is monoclinic, space group P21/n, a=14.252(4), b=11.061(8), c=16.450(8) A° , b= 90.17(3)°. The structure contains a novel type of ribbon formed by hexagonal meshes of alternate AgI ions and hmt molecules. The water molecules coordinated to the silver ions form hydrogen bonds which generate a three-dimensional network of unprecedented topology. Crystals of 2 are monoclinic, space group I2, a=19.552(6), b=10.856(6), c=25.749(11) A° , b=92.83(3)°. It consists of an open three-dimensional network of complex topology with all the hmt molecules tetraconnected.Out of the eleven independent AgI ions per formula unit, nine are biconnnected and two triconnected to the hmt bases. The large channels in the network contain many water molecules.Compound 3 is monoclinic, space group P21 /c, a=12.535(8), b=29.517(8), c= 14.704(6) A° , b=93.95(4)°. The structure contains six independent Ag ions and eight independent triazine molecules, which display varied connectivities to give a puzzling three-dimensional network. The design of new nanoporous materials consisting of two- trigonal 1,3,5-triazine (trz). We have already described a threedimensional polymer from AgPF6 and hmt, namely [Ag(hmt)] and three-dimensional networks from the assembly of metal ions and suitable polydentate ligands is of great current PF6 H2O, containing a (103)-a framework.18 Here, we report on the isolation and characterization of two novel Ag–hmt interest.1 Hoskins and Robson2 have suggested useful criteria for the self-assembly of metallic centres of dierent coordi- coordination polymers, namely [Ag(hmt)]SbF6 H2O 1 and [Ag11(hmt)6][PF6]11 14H2O 2, and of the triazine derivative nation geometries and molecular rods of variated nature and length.Many examples of three-dimensional super-diamond [Ag6(trz)8][BF4]6 H2O 3. The latter two compounds contain three-dimensional coordination networks which represent nets are presently known,3 exhibiting up to nine-fold interpenetration, 4 and networked coordination polymers related to model examples of molecular-based nanoporous materials.other topological archetypes, such as simple cubic a-polonium,5 PtS,6 quartz,7 rutile,8 SrSi2 9 and a-ThSi210 are also known. Results and Discussion Some studies on the exchange ability of guest molecules11 and on the catalytic properties12 of this type of materials have In our recent investigations on the self-assembly of AgI cations with dierent bidendate N-donor bases (e.g.pyrazine, 4¾,4¾- recently appeared. Dierent synthetic strategies can be employed in the bipyridyl and similar ligands), we have observed some general trends: (i) the reactions lead often to mixtures of products and, assembly of these species, leading to frameworks containing nodes of dierent nature, i.e., (i) using bidentate ligands, nets moreover, on leaving the solutions to stand, the first product containing only metallic nodes are obtained, while (ii) polydentate ligands can give nets with alternate metallic and organic centres.Moreover, another class of polymers could be obtained, in principle, from suitable polydentate molecules and metallic connectors, represented by simple ions [such as digonal AgI or CuI] or metal complexes with two transoid sites available for coordination, acting only as spacers (ligand,metal,ligand synthons), 13 with the geometry of the supramolecular array driven by the organic units.We have recently reported some examples of three-dimensional coordination polymers derived from the self-assembly of AgI ions and dierent bidentate N-donor rigid ligands.Networks topologically related to the prototypal frames of diamond,14 a-polonium,15 and the triconnected SrSi2 [(103)-a type]16 and a-ThSi2 [(103)-b type]17 have been discovered (see Fig. 1). In these species interpenetration of the nets is a very common feature, an interesting phenomenon from the structural point of view, but to be avoided, or at least reduced, in order to obtain larger channels and internal cages.We are now reacting silver(I) and copper(I) salts of poorly coordinating anions with dierent polydentate N-donor bases, in particular with the potentially tetradentate tetrahedral hexa- Fig. 1 Prototypal structural types found in some silver(I ) polymers with bidentate ligands methylenetetramine (hmt) and with the potentially tridentate J.Mater. Chem., 1997, 7(7), 1271–1276 1271formed (favoured by kinetic factors) can give further reactions; (ii ) the role of counter ions19 and the solvent system are important in orienting the self-assembly; (iii ) the silver ions show a great variety of coordination modes.Indeed, the observed geometries with these ligands include, besides the more usual digonal, trigonal and tetrahedral, also Tshaped, 17,20 square-planar,15 saw-horse,21,22 square-pyramidal15 and octahedral15 stereochemistries. This versatility can represent a problem in the deliberate design of nanoporous materials, but it has oered to us the possibility to characterize frameworks with novel topologies, useful as prototypal models.On passing to polydentate bases, we have reacted a number of silver(I ) salts with hexamethylenetetramine and 1,3,5- triazine, using the methods of slow diusion for single-crystal growth already employed in our previous works. Polycrystalline samples or materials not suitable for singlecrystal X-ray analyses were obtained in many cases.Single crystals were isolated for compounds 1–3 described below. Fig. 2 One-dimensional ribbon in compound 1 The one-dimensional coordination polymer [Ag(hmt)]SbF6 H2O 1 2.374 A° ], while the external ones are biconnected [mean Ag(2)MN 2.303 A° ]. However, each metal ion interacts also This species is the dominant product of the reaction of AgSbF6 with hmt in a 151 molar ratio in ethanol–CH2Cl2 and, notably, with a water molecule (mean AgMO 2.507 A° ), and the external Ag(2) ions show an additional AgMF(SbF5) contact of in spite of sharing the same stoichiometry as [Ag(hmt)]PF6 H2O, it has a completely dierent structure, 2.428(6) A° .The resulting coordination geometries are quite distorted tetrahedral for both types of silver ions (Table 1).consisting of ribbons of condensed hexagons of alternating AgI ions and hmt molecules, as shown in Fig. 2. The one-dimensional structural motif observed in 1 is, to our knowledge, unprecedented. It can be described also in There are two dierent types of hmt molecules and AgI ions: those located in the central part of the ribbons and the external terms of a double chain (two vierer chains) of vertex sharing tetrahedra (Fig. 3, top), reminiscent of the stuctures of double ones. Considering only the Ag–hmt interactions, the internal silver ions and hmt ligands are triconnected [mean Ag(1)MN chain inosilicates, but not related to known species.23 Table 1 Main bond lengths (A° ) and selected bond angles (degrees) within compounds 1–3 compound 1 Ag(1)MN(11) 2.360(6) Ag(1)MO(1) 2.510(6) Ag(2)MO(2) 2.504(7) Ag(1)MN(12) 2.408(6) Ag(2)MN(13) 2.287(6) Ag(2)MF(16) 2.428(6) Ag(1)MN(21) 2.355(6) Ag(2)MN(22) 2.319(6) NMAg(1)MN 114.6(2)–125.7(2) N(22)MAg(2)MN(13) 129.9(2) NMAg(1)MO(1) 91.0(2)–94.2(2) NMAg(2)MO(2) 92.9(2)–102.6(3) compound 2 Ag(1)MN(11) 2.36(2) Ag(3)MN(21) 2.18(2) Ag(5)MN(32) 2.31(2) Ag(1)MO(W1) 2.48(7) Ag(3)MN(34) 2.22(2) Ag(5)MO(W5) 2.48(4) Ag(1)MO(W2) 2.54(7) Ag(4)MN(12) 2.26(2) Ag(6)MN(14) 2.31(2) Ag(2)MN(22) 2.28(3) Ag(4)MN(23) 2.25(3) Ag(6)MN(31) 2.44(2) Ag(2)MN(24) 2.22(2) Ag(4)MF(62) 2.47(7) Ag(6)MN(33) 2.39(2) Ag(2)MO(W3) 2.48(4) Ag(5)MN(13) 2.21(2) Ag(6)MO(W6) 2.49(3) Ag(2)MO(W4) 2.45(5) N(11)MAg(1)MN(11) 169.1(11) N(32)MAg(5)MN(13) 143.8(8) N(22)MAg(2)MN(24) 140.2(9) N(14)MAg(6)MN(33) 120.1(8) N(21)MAg(3)MN(34) 171.2(9) N(14)MAg(6)MN(31) 115.4(8) N(12)MAg(4)MN(23) 161.0(8) N(31)MAg(6)MN(33) 119.2(8) compound 3 Ag(1)MN(1A) 2.33(2) Ag(3)MN(3G) 2.25(2) Ag(5)MN(5B) 2.30(2) Ag(1)MN(1B) 2.28(2) Ag(3)MN(1H) 2.39(2) Ag(5)MN(5F) 2.39(2) Ag(1)MN(1C) 2.35(2) Ag(3)MF(44) 2.67(3) Ag(5)MO(1A)a 2.38(4) Ag(1)MN(3E) 2.54(2) Ag(4)MN(3C) 2.35(2) Ag(5)MO(1B)a 2.55(6) Ag(2)MN(3B) 2.33(2) Ag(4)MN(3D) 2.77(2) Ag(6)MN(5D) 2.34(2) Ag(2)MN(5C) 2.30(2) Ag(4)MN(5E) 2.43(2) Ag(6)MN(1E) 2.30(2) Ag(2)MN(1D) 2.33(2) Ag(4)MN(1F) 2.30(2) Ag(6)MN(3F) 2.34(2) Ag(2)MN(3H) 2.65(2) Ag(4)MF(62B) 2.57(5) Ag(6)MN(1G) 2.29(2) Ag(3)MN(3A) 2.24(2) Ag(5)MN(5A) 2.67(3) NMAg(1)MN 85.5(7)–128.7(7) N(5E)MAg(4)MN 93.7(7)–117.9(7) NMAg(2)MN 85.6(7)–130.1(7) N(5E)MAg(4)MF(62B) 79.9(13) N(3A)MAg(3)MN(3G) 147.2(8) N(5A)MAg(5)MN(5F) 142.1(8) N(1H)MAg(3)MN 93.5(8)–117.4(7) N(5B)MAg(5)MO(1A) 153.7(11) NMAg(3)MF 81.4(8)–98.8(8) N(5B)MAg(5)MN 76.0(8)–103.7(7) N(3C)MAg(4)MN(1F) 146.4(7) NMAg(6)MN 95.2(7)–123.1(7) N(3D)MAg(4)MF(62B) 158.7(14) aWater molecule statistically disordered on two close positions. 1272 J. Mater. Chem., 1997, 7(7), 1271–1276because the water molecules also interact (using the other hydrogen atoms) with the uncoordinated SbF6- anion.Attention should be devoted in the near future to the possibility of preparing useful nanoporous materials with the concurrent contribution of dierent types of interactions to link the units. The results here described are of relevance for the newborn area of crystal engineering concerning the building of networks via both coordinative and hydrogen bonds.Investigations based on dierent strategies are in progress, involving either the self-assembly (direct or mediated by molecular spacers) of suitable mononuclear24 or polynuclear3,25 coordination compounds, or the insertion of molecular crosslinkers to join (via hydrogen bonds) one-dimensional coordination polymers.26 Fig. 3 Two schematic views of the ribbon in 1: (top) front view showing the double chain of tetrahedral units defined by the N atoms (#) of The three-dimensional networked polymer the hmt ligands and by the coordination spheres of the Ag atoms (#); [Ag11 (hmt)6][PF6]11 14H2O 2 (bottom) lateral view showing the conformation of the hexagons ($, baricentres of the hmt molecules) Compound 2 has been isolated from the reactions of AgPF6 and hmt in ethanol–CH2Cl2 , which gave mixtures of polymeric species.Examination under the microscope of samples con- The ribbons are undulated, as schematically shown in Fig. 3 taining as the dominant product the already described (bottom), and are all disposed parallel to the b crystallographic [Ag(hmt)]PF6 H2O18 has also revealed the presence of minor axis.However, they are not isolated and display an interesting amounts of small flat hexagonal crystals of 2, investigated by supramolecular chemistry, in the sense that are cross-linked single-crystal X-ray analysis. by hydrogen bonds to give a three-dimensional network. The The structure consists of an open three-dimensional cationic coordinated water molecules form OMH,N hydrogen bonds frame, with all the hmt molecules acting as tetradentate ligands, with the hmt ligands of adjacent ribbons [O,N 2.837(10), while nine silver cations are biconnected and two are tricon- 2.971(10) A° ; OMH,N 163.7, 157.8°], as illustrated in Fig. 4. nected to hmt molecules per formula unit (note that six Ag These interactions generate a three-dimensional net (sche- and three hmt molecules are crystallographically independent, matically shown in Fig. 5) of uncommon topological type, with one Ag lying on a two-fold axis). From the point of view based on three-connected (Ag and hmt external in the ribbons) of the topology of the network, the nine biconnected AgI ions and four-connected (Ag and hmt internal in the ribbons) are only spacers (metallic synthons).13 Many interactions of centres in the ratio 151.However, the net is more complex the AgI ions with water molecules are also present, which have influence on the coordination geometries of the individual metal ions, but do not change their topological role. A rationalization of the structure can be achievedconsidering that it is comprised of layers, as illustrated in Fig. 6, which are normal to the [100] direction, and are joined together only by biconnected silver ions [Ag(1), Ag(3)], to give the overall three-dimensional network.These layers contain the hmt ligands and the other four independent metal ions [Ag(2), Ag(4), Ag(5), Ag(6)], and are unusual, showing two alternate types of one-dimensional structural motifs extending in the [010] direction, i.e.ribbons of condensed hexagons and helical MAgMhmtMAgMhmtM chains, interconnected by metallic spacers. The ribbons are quite similar to those in compound 1 and contain the biconnected Ag(5) and the triconnected Fig. 4 View of the hydrogen bonds linking the adjacent ribbons in 1 Ag(6) ions (mean AgMN 2.26 and 2.38 A° , respectively), each Fig. 5 Schematic view of the three-dimensional net in compound 1 (0, water molecules, other symbols as in Fig. 3). Empty bonds represent Fig. 6 A two-dimensional layer (down a ) contained in compound 2 the hydrogen bonds, dashed bonds involving the SbF6- anions. J. Mater. Chem., 1997, 7(7), 1271–1276 1273interacting also with a water molecule (mean AgMO 2.48 A° ). The helixes involve the Ag(2) ions, which also bear two water molecules [mean Ag(2)MN and Ag(2)MO 2.25 and 2.46 A° , respectively].The Ag(4) ions, which join the two one-dimensional motifs, interact with the fluorine atom of one anion [mean Ag(4)MN 2.26 A° , Ag(4)MF 2.47(7) A° ]. The main bond parameters, illustrating the coordination geometries of the independent metal ions, are collected in Table 1. The Ag(1) and Ag(3) ions graft the layers of Fig. 6. Two water molecules complete the coordination of Ag(1) (mean AgMO 2.51 A° ), while Ag(3) shows an almost regular digonal coordination. The (3,4)-connected network, schematically shown in Fig. 7 (down b), is comprised of three-connected centres [Ag(6)] and four-connected ones (the hmt ligands) in the ratio 153; its topological type is complex and quite probably unique, because the four-connected centres are all dierent.The view in Fig. 7 reveals the presence of two types of parallel channels running down b. These channels host the anions and the 14 water molecules (per formula unit), both coordinated (ten) and the solvated ones (four). Interestingly, from the results for 1 and 2 we see that the same one-dimensional structural motif (the ribbons of con- Scheme 1 densed hexagons) can participate in the construction of threedimensional nets using dierent kinds of synthons:13 trz, has appeared.19 It contains an interesting network based AgMOMH,N(hmt) hydrogen bonds and (hmt)NMAgM on three-connected silver ions and trz ligands, of the (103)-d N(hmt) bridges, respectively.In reacting hmt and silver salts topological type, according to Wells.28 we had hoped to isolate a supertetrahedral [Ag2(hmt)] net, In contrast with the simplicity of this species, compound 3 based on hmt centres only and Ag spacers (Scheme 1, M= presents a complicated structure which is dicult to razional- metallic synthon), a strict coordination analogue of the ize.This is due to the presence of six AgI ions and eight trz extended diamondoid network formed by the interaction of ligands which are crystallographically independent and show carboxyl groups in pairs in 1,3,5,7-adamantanetetracarboxylic varied connectivities.acid.27 Though we have failed our objective, both the stoichi- The coordination geometries of the six independent silver ometry and the nature of the centres in 2 make this species ions are shown in Fig. 8. The environments of the Ag(1), Ag(2) close to what expected for that hypothetical net. However, the and Ag(6) ions are distorted tetrahedral, each being connected presence of the two triconnected silver(I) ions greatly complito four N atoms of trz ligands. Also Ag(3) and Ag(5) are cates the structure. tetracoordinated: Ag(3) is bound to three trz ligands and to a BF4- anion in a saw-horse like geometry, while Ag(4) is The complex three-dimensional network of coordinated to three trz ligands and a water molecule (dis- [Ag6(trz)8][BF4]6 H2O 3 ordered on two close positions), in a flattened tetrahedral Many attempts were made to obtain polymeric complexes fashion.Finally, Ag(4) shows a distorted square-pyramidal with 1,3,5-triazine (trz) using dierent silver(I) salts and vari- coordination, involving four trz ligands and one equatorial ated reaction conditions.All were unsuccesful, except for the anion. A list of the relevant bond lengths and angles is given reaction with AgBF4 in acetone–isopropyl alcohol, leading to in Table 1. compound 3. The simple potentially tridentate trigonal ligand As usual with networked compounds, it is convenient for trz seems particularly attractive for the preparation of two- topological purposes to ignore atoms and groups which are dimensional graphite-like nets or three-dimensional frames of the (103)-a or (103)-b type (Fig. 1). Indeed, very recently the structure of [Ag(trz)]- CF3SO3 H2O, the first coordination polymer assembled with Fig. 8 Coordination geometries of the six independent silver ions in Fig. 7 Schematic view of the three-dimensional net in 2 (down b), with bold bonds within and empty bonds across the layers 3. The triazine rings are labelled A–H. 1274 J. Mater. Chem., 1997, 7(7), 1271–1276Conclusions We have described three novel polymeric silver(I) species with the polydentate bases hmt and trz. The use of these ligands instead of bidentate ones has introduced another variable into the system, related to the possibility of an incomplete and varied employment of the base donor functions, as observed in 1 and in 3, which can lead to unusual or complex stuctures.Compound 1 has oered to us the interesting opportunity to study a three-dimensional frame with mixed bonding interactions. In compounds 2 and 3 the three-dimensional nets are not interpenetrated, in contrast to what is observed in many previously studied networks with bidentate bases.The tetradentate hmt seems to be more promising than trz in giving networked materials with large channels. Considering the complicated structure of 3 we can perhaps conclude that trz should be better employed with a metal ion having a less variable coordination geometry than silver.Experimental All the reagents and solvents employed were commercially Fig. 9 A stereoview of the network in compound 3 (down c). The available high-grade purity materials (Aldrich Chemicals), used BF4- anions and the water molecules are omitted for clarity. as supplied, without further purification. Elemental analyses were carried out at the Microanalytical Laboratory of this University.simple appendages of the net (dead ends). Considering, therefore, only the Ag–trz interactions we have four tetraconnected Preparation of compounds 1–3 [Ag(1), Ag(2), Ag(4), Ag(6)] and two triconnected [Ag(3), Ag(5)] silver ions. All the trz ligands are triconnected but two Synthesis of [Ag(hmt)]SbF6 H2O 1. On a solution of hexamethylenetetramine (0.0451g, 0.32 mmol) in CH2Cl2 (3 ml) (G and H), which are simple spacers. A stereoview of the cell content (without the anions and the was layered 1 ml of pure ethanol and then a solution of AgSbF6 (0.106 g, 0.31 mmol) in ethanol (3 ml).The solution water molecules) is shown in Fig. 9. The shortest circuits are six-membered rings of alternate Ag and trz centres, like that was left to slowly evaporate for many days in the dark, aording elongated crystals, with small amounts of impurities.in Fig. 8, involving Ag(1), Ag(2) and Ag(6) and the B, D and E rings. The crystals were recovered by filtration, dried in air, and, after having mechanically removed the impurities, submitted This puzzling (3,4)-connected net (with a 251 ratio of the two types of centres) shows also large channels, extending to elemental analysis (yield ca. 50%). Anal. Found: C, 14.02; H, 2.98; N, 11.30. Calc. for C6H14AgF6N4OSb: C, 14.36; H, down c. A topological rationalization and classification is fraught with diculty and is of little practical interest. 2.81; N, 11.17%. Table 2 Crystallographic data for compounds 1–3 formula C6H14AgF6N4OSb 1 C36H100Ag11F66N24O14P11 2 C24H24Ag6B6F24N24O 3 formula weight 501.83 3874.64 1832.75 space group (no.) P21/n (14) I2 (5) P21/c (14) a/A° 14.252(4) 19.552(6) 12.535(8) b/A° 11.061(8) 10.856(6) 29.517(8) c/A° 16.450(8) 25.749(11) 14.704(6) b/degrees 90.17(3) 92.83(3) 93.95(4) V /A°3 2593(2) 5459(4) 5428(4) Z 8 2 4 Dc/g cm-3 2.571 2.357 2.243 absorption coecient/mm-1 3.664 2.260 2.257 crystal size/mm 0.45×0.1×0.07 0.20×0.15×0.05 0.50×0.35×0.30 scan interval/degrees 1.3+0.35 tan h 1.4+0.35 tan h 1.1+0.35 tan h max time per reflection/s 60 60 40 h range/degrees 3–25 3–24 3–23 index ranges hkl -16 to 16, 0–13, 0–19 -22 to 22, 0–12, 0–29 -13 to 13, 0–25, 0–16 reflections collected 4773 4489 5317 independent reflections (Rint) 4514 4489 5137 crystal decay (%) 0 0 9 min., max.transmission 0.76, 1.0 0.55, 1.0 0.85, 1.0 data/restraints/parameters 3870/0/344 2942/631/397 2970/351/387 R indices (observed data)b R1 0.0335, wR2 0.0887 R1 0.0856, wR2 0.2192 R1 0.0717, wR2 0.1752 weighting A,Bc 0.0605, 8.2507 0.1694, 0.0 0.0827, 77.3044 goodness-of-fit GOFd 1.038 1.070 1.081 aDetails in common: monoclinic, v scans, Rint=0.0000, reflections I>2s(I) considered observed.bR1=S||Fo|-|Fc||/S|Fo|, wR2=[S (Fo2-Fc2)2/S wFo4]1/2. cWeighting: w=1/[s2(Fo2)+(AP)2+BP] where P=(Fo2+2 Fc2 )/3. dGOF=[Sw(Fo2-Fc2 )2/(n-p)]1/2 where n is the number of reflections and p is the number of refined parameters. J. Mater. Chem., 1997, 7(7), 1271–1276 12751546. (b) R. Robson, B. F. Abrahams, S. R. Batten, R. W. Gable, Isolation of [Ag11 (hmt)6][PF6]11 14H2O 2.Compound 2 B. F. Hoskins and J. Liu, Supramolecular Architecture, ACS publi- was isolated as a minor product from the reaction of AgPF6 cations, Washington, DC, 1992, ch. 19. and hmt in ethanol–CH2Cl2, together with the already 3 See for example: M. J. Zaworotko, Chem. Soc. Rev., 1994, 283. described [Ag(hmt)]PF6 H2O.18 Slow evaporation (almost to 4 K. A. Hisch, D.Venkataraman, S. R. Wilson, J. S. Moore and dryness) of a system consisting of an ethanolic solution (5 ml) S. Lee, J. Chem. Soc., Chem. Commun., 1995, 2199. 5 See for example: (a) S. C. Abrahams, J. L. Bernstein, R. Liminga of AgPF6 (0.074 g, 0.29 mmol) layered on a solution of hmt and E. T. Eisenmann, J. Chem. Phys., 1980, 73, 4585; (0.041 g, 0.29 mmol) in CH2Cl2 (3 ml) aorded a crystalline (b) B.F. Hoskins, R. Robson and N. V. Y Scarlett, J. Chem. Soc., product containing mainly tetrahedral or octahedral crystals Chem. Commun., 1994, 2025; (c) T. Soma, H. Yuge and T. Iwamoto, of [Ag(hmt)]PF6 H2O (ca. 80%), together with a minor Angew. Chem., Int. Ed. Engl., 1994, 33, 1665. amount of small flat hexagonal crystals of 2. Both compounds 6 B. F. Abrahams, B.F. Hoskins, D. M. Michail and R. Robson, are air- and light-stable and the nature of 2 has been established Nature (L ondon), 1994, 369, 727. 7 (a) S. C. Abrahams, L. E. Zyontz and J. L. Bernstein, J. Chem. by single-crystal X-ray analysis. Phys., 1982, 76, 5485; (b) B. F. Hoskins, R. Robson and N. V. Y. Scarlett, Angew. Chem., Int. Ed. Engl., 1995, 34, 1203. Synthesis of [Ag6(trz)8][BF4]6 H2O 3.A solution of triazine 8 S. R. Batten, B. F. Hoskins and R. Robson, J. Chem. Soc., Chem. (0.035g; 0.43 mmol) in isopropyl alcohol (7 ml) was layered on Commun., 1991, 445. a solution of AgBF4 (0.762g, 0.39 mmol) in acetone (6 ml). 9 (a) S. Decurtins, H. W. Schmalle, P. Schneuwly and H. R. Oswald, After several days in the dark a white precipitate together with Inorg.Chem., 1993, 32, 1888; (b) S. Decurtins, H. W. Schmalle, needle shaped crystals were formed. This material was reco- R. Pellaux, P. Schneuwly and A. Hauser, Inorg. Chem., 1996, 35, 1451. vered by filtration, washed with a small amount of isopropyl 10 G. B. Gardner, D. Venkataraman, J. S. Moore and S. Lee, Nature alcohol and dried in the air (yield ca. 70%). The elemental (L ondon), 1995, 374, 792.analyses of the precipitate and of the crystals were identical. 11 (a)D. Venkataraman, G. B. Gardner, S. Lee and J. S.Moore, J. Am. Anal. Found: C, 15.33; H, 1.68; N, 18.01. Calc. for Chem. Soc., 1995, 117, 11600; (b) G. B. Gardner, Y.-H. Kiang, C24H26Ag6B6F24N24O:C, 15.71; H, 1.43; N, 18.33%. S. Lee, A. Asgaonkar and D. Venkataraman, J. Am. Chem. Soc., 1996, 118, 6946.Single-crystal X-ray analysis 12 M. Fujita, Y. J. Kwon, S. Washizu and K. Ogura, J. Am. Chem. Soc., 1994, 116, 1151. Colourless crystals of 1–3 were mounted on glass fibres in the 13 G. R. Desiraju, Angew. Chem., Int. Ed. Engl., 1995, 34, 2311. air at room temperature, on an Enraf-Nonius CAD4 auto- 14 L. Carlucci, G. Ciani, D. M. Proserpio and A. Sironi, J. Chem.Soc., Chem. Commun., 1994, 2755. mated diractometer, and 25 intense reflections (17°<2h<23°) 15 L. Carlucci, G. Ciani, D. M. Proserpio and A. Sironi, Angew. were centred using graphite-monochromated Mo-Ka radiation Chem., Int. Ed. Engl., 1995, 34, 1895. (0.710 69 A° ). Least-squares refinement of their setting angles 16 L. Carlucci, G. Ciani, D. M. Proserpio and A. Sironi, Chem.resulted in the unit-cellparameters reported in Table 2, together Commun., 1996, 1393. with other details associated with data collection and refine- 17 L. Carlucci, G. Ciani, D. M. Proserpio and A. Sironi, J. Am. Chem. ment. The diracted intensities were corrected for Lorentz, Soc., 1995, 117, 4562. 18 L. Carlucci, G. Ciani, D. M. Proserpio and A. Sironi, J. Am. Chem. polarization and decay eects.An empirical absorption correc- Soc., 1995, 117, 12861. tion based on y-scans was applied to all data. The structures 19 D. Venkataraman, S. Lee, J. S. Moore, P. Zhang, K. A. Hisch, were solved by direct methods with SIR9229 and refined by G. B. Gardner, A. C. Covey and C. L. Prentice, Chem.Mater., 1996, full-matrix least-squares on Fo2. Anisotropic thermal displace- 8, 2030.ments were assigned to all non-hydrogen atoms for compound 20 See for example: (a) F. Robinson and M. J. Zaworotko, J. Chem. 1 and only to the heavy atoms Ag, P for 2 and Ag for 3. All Soc., Chem. Commun., 1995, 2413; (b) O.M. Yaghi and H. Li, J. Am. Chem. Soc., 1996, 118, 295. the hydrogen atoms were placed in calculated positions; for 2 21 L. Carlucci, G.Ciani, D. M. Proserpio and A. Sironi, Inorg. Chem., and 3 the water hydrogen atoms were ignored. The final 1995, 34, 5698. dierence electron density maps showed no significant features. 22 D. Venkataraman, G. B. Gardner, A. C. Covey, S. Lee and All calculations were performed using SHELX-93.30 J. S. Moore, Acta Crystallogr., Sect. C, 1996, 52, 2416. Compound 1 showed a monoclinic unit cell [b=90.17(3)°] 23 F.Liebau, Structural Chemistry of Silicates, Springer, Berlin, 1985. emulating orthorhombic symmetry, and a considerable 24 See for example: M. M.Chowdhry, D. M. P. Mingos, A. J. P. White and D. J. Williams, Chem. Commun., 1996, 899. improvement of the final agreement factor was obtained with 25 S. B. Copp, S. Subramanian and M. J. Zaworotko, J. Chem. Soc., the TWIN refinement directive of SHELX-93 with twin matrix Chem.Commun., 1993, 1078. [1 0 0, 0 -1 0, 0 0 -1] and parameter 0.1811(8). The 26 S. Kawata, S. Kitagawa, H. Kumagai, C. Kudo, H. Kamesaki, assignement of the absolute structure for compound 2 was T. Ishiyama, R. Suzuki, M. Kondo and M. Katada, Inorg. Chem., confirmed by the statistics and the refinement of the absolute 1996, 35, 4449. structure parameter to a value of 0.07(13). Some disordered 27 O. Ermer, J. Am. Chem. Soc., 1988, 110, 3747. 28 (a)A. F.Wells, Acta Crystallogr., 1954, 7,535; (b) T hree-dimensional water molecules and anions were refined as 50% over two Nets and Polyhedra, Wiley, New York, 1977; (c) Further studies of positions in compounds 2 and 3. Crystal drawings were T hree-dimensional Nets, ACS Monogr., No. 8, Pittsburgh, PA, produced with SCHAKAL.31 Atomic coordinates, thermal 1979. parameters, and bond lengths and angles have been deposited 29 A. Altomare, G. Cascarano, C. Giacovazzo, A. Guagliardi, at the Cambridge Crystallographic Data Centre (CCDC). See M. C. Burla, G. Polidori and M. Camalli, J. Appl. Crystallogr., Information for Authors, J. Mater. Chem., 1997, Issue 1. Any 1994, 27, 435. 30 G. M. Sheldrick, SHELX-93: program for structure refinement, request to the CCDC for this material should quote the full University of Go�ttingen, 1994. literature citation and the reference number 1145/36. 31 E. Keller, SCHAKAL 92: a computer program for the graphical representation of crystallographic models, University of Freiburg, References 1992. 1 C. L. Bowes and G. A. Ozin, Adv.Mater., 1996, 8, 13. 2 (a) B. F. Hoskins and R. Robson, J. Am. Chem. Soc., 1990, 112, Paper 7/00020K; Received 2nd January, 1997 1276 J. Mater. Chem., 1997, 7(7), 1271–12

ISSN:0959-9428    

DOI:10.1039/a700020k

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Lamellar polymer–LixMoO3 nanocomposites via encapsulative precipitation Lei Wang,a Jon Schindler,b Carl R. Kannewurfb and Mercouri G. Kanatzidis*a aDepartment of Chemistry and the Center for FundamentalMaterials Research,Michigan State University, East L ansing,Michigan 48824, USA bDepartment of Electrical Engineering and Computer Science, Northwestern University, Evanston, Illinois 60208, USA With LixMoO3 (x=0.31–0.40) as a host material, a new family of polymer–molybdenum bronze nanocomposites has been synthesized using an exfoliation/encapsulation methodology.Nanocomposites with poly(ethylene oxide), poly(ethylene glycol), poly(propylene glycol), poly(vinylpyrrolidinone), methyl cellulose, polyacrylamide, and nylon-6 were prepared and characterized by thermal gravimetric analysis (TGA), dierential scanning calorimetry (DSC), powder X-ray diraction, FTIR spectroscopy, UV–VIS spectroscopy, variable-temperature 7Li and 13C solid-state NMR spectroscopy and magnetic susceptibility measurements.The electrical conductivity of these materials lies in the range from 10-2 to 10-7 S cm-1, and decreases as the interlayer separation increases.The intercalated polymer imparts both mechanical strength and ease of processing to these materials. The water-soluble polymer–LixMoO3 nanocomposites can be cast into films and other shapes, which may provide opportunities for applications. Factors aecting the intercalation reaction and the structure of nanocomposites, such as variations in the preparation procedure, the polymer molecular mass and the annealing behaviour of the products are discussed.The investigation of polymer–inorganic nanophase composites able-temperature solid-state 7Li and 13C NMR spectroscopy, magnetic susceptibility measurements and electrical conduc- is motivated by many reasons, including the need for novel tivity measurements. electronic anisotropic materials, better performing battery cathode materials, functionalized structural materials with superior mechanical properties, hierarchical materials, and systems in Experimental which to study polymer orientation, epi- and endo-taxy and polymer/inorganic surface interactions.1–28 Polymer-based Materials and methods nanocomposites have been reported with layered silicates Reagents.PEO (5000000), PEO (100000), PEG (10000), (e.g.montmorillonite, hectorite, etc.),1–9 FeOCl,10 V2O5,11–13 PEG (2000), PEG (400), PPG (1000), PVP (10000), MCel MoO3,14,15,26,27 layered metal phosphates,16–20 MS2 (M= (63000), PAM (5000000), PA-6 (10000) and LiBH4 were Mo,21–24,26 Ti24), layered metal phosphorus chalcogenides purchased from Aldrich Chemical Company, Inc. After the (MPS3)25–27 and layered double hydroxides.28 The most polymers were dissolved, the polymer solutions were filtered to common methods of preparing polymer–inorganic nanocomremove insoluble polymer residues.MoO3 (99.95%) was pur- posites are (a) by monomer intercalation followed by polymer- chasedfrom JohnsonMatthey.Anhydrousdiethyl ether(99.0%), ization, (b) by in situ intercalative polymerization, (c) by direct 2,2,2-trifluoroethanol (99%), acetonitrile (99.5%) isopropyl insertion and (d) by encapsulative precipitation from solutions alcohol (99.9%) and absolute ethanol were from Columbus of exfoliated lamellar solids.The last two methods give poly- Chemical Industries Inc., Lancaster Synthesis Inc., EM Science mer–inorganic nanocomposites in which the molecular masses Inc., Mallinckrodt Chemical Inc.and Quantum Chemical and nature of the polymers can be decided before intercalation. Company respectively. No further purification was applied to Encapsulative precipitation has been applied with V2O5,11a–c,12 the chemicals above. Water used in the reactions was distilled MoO3,14a–c,26 MoS2,22b,c,23,24,26 WS2 ,29 and TiS2,24 in combi- water provided by the Department of Chemistry of MSU, and nation with various polymers. was degassed by bubbling nitrogen for 30 min before use.MoO3 is one of the layered metal oxides which shows reversible Li ion insertion properties which are relevant to rechargeable Li batteries.30,31 In intercalative electrodes of Synthesis of LixMoO3 ( 0.30<x<0.40). Commercial MoO3, rechargeable batteries, ion conductivity is very important.used as the starting material, was fired in an open quartz vial Other solid-state ionic applications such as electrochromic in the air at 600 °C for 36 h during which the crystallite size devices also need good ion conductivity. Polymer–inorganic of MoO3 remarkably increased. This MoO3 was used to react nanocomposites should exhibit fast ion conduction8 and intro- with LiBH4 to prepare the lithium molybdenum bronze.32 In duction of a polymer with anity for Li ions between the a typical reaction, 0.1 mol of MoO3 was reacted with 0.04 mol sheets of MoO3 could improve its performance as an inter- of LiBH4 in 80 ml diethyl ether for 24 h, under a nitrogen calative electrode.atmosphere. The product was collected by filtration, washed Polymer insertion into MoO3 has been reported previously, with ether and dried in vacuo.The yield was >98%. The namely, with a polymeric ionomer,14a,b with PEO14c,26 and lithium molybdenum bronze was thereafter stored in a nitrogen with polyaniline.14d,15,27 This research develops further the dry-box. The X-ray powder diraction pattern can be indexed polymer intercalation chemistry associated with MoO3 and on the basis of an orthorhombic cell similar to that of MoO3 introduces a new family of polymer–LixMoO3 nanocomposites.but expanded along the stacking (a-axis) direction, with a= Nanocomposites with poly(ethylene oxide) (PEO), poly(ethyl- 16.528 A° , b=3.775 A° and c=3.969 A° . The dhkl-spacings (A° ) are: ene glycol) (PEG), poly(propylene glycol) (PPG), poly(vinyl- 8.27200 (vs), 4.132400 (s), 3.578201 (m), 3.434210 (m), 2.755600 (w), pyrrolidinone) (PVP), methylcellulose (MCel), polyacrylamide 2.453311 (m) and 2.372411 (m).(PAM) and nylon-6 (PA6) are reported here. These nanocom- The amount of lithium in the bronze was analysed by TGA posites were characterized by thermal gravimetric analysis under oxygen flow, heating up to 650°C, and by elemental (TGA), dierential scanning calorimetry (DSC), powder X-ray analysis which was accomplished by Oneida Research Services, Inc., Whitesboro, New York.Elemental analysis of Li in the diraction, FTIR and solid-state UV–VIS spectroscopy, vari- J. Mater. Chem., 1997, 7(7), 1277–1283 1277molybdenum bronze was done by ion chromatography, while with a Quantum Design MPMS2 SQUID magnetometer.Samples were sealed in low-density polyethylene (LDPE) bags Mo was measured by X-ray fluorescence. under a nitrogen atmosphere. Room-temperature conductivity measurements were performed on pressed sample pellets with Preparation of polymer–LixMoO3 nanocomposites. The LixMoO3 was exfoliated in degassed water by 5 min of sonic- a four-probe detector connected to a Keithley-236 source measuring unit.Variable-temperature dc electrical conductivity ation, to form a suspension with a concentration of 5 g l-1. This suspension was added dropwise into a stirred polymer measurements were performed on compacted powders in pellet form with 60 and 25 mm gold wires used for the current and solution of the same volume, which contained five times excess of polymer (per repeat unit) to MoO3 and the mixture was voltage electrodes, respectively.Measurements of the pellet cross-sectional area and voltage probe separation were made stirred for 2 days under a nitrogen atmosphere. The nanocomposites formed were isolated in dierent ways according to with a calibrated binocular microscope. Electrical conductivity data were obtained with a computer-automated system their behaviour in solution.Nanocomposites of methylcellulose, polyacrylamide and nylon-6 precipitated and were col- described elsewhere.33 lected by filtration. Those containing MCel and PAM were washed with water, while the nanocomposites of PA6 were Results and Discussion washed with trifluoroethanol. Nanocomposites with poly- (ethylene oxide), poly(ethylene glycol), poly(propylene glycol) Synthesis and characterization of LixMoO3 and poly(vinylpyrrolidinone) remained in colloidal form and The lithium molybdenum bronze exfoliates readily in water to were isolated by pumping o the water to dryness.The dried form stable colloidal solutions, and this makes it an appealing material was stirred in an appropriate solvent for several hours candidate for polymer intercalation. The LiBH4 route to to dissolve the extraneous polymer.The solid product was LixMoO3 is the best one to date in providing the material filtered and was washed again with the solvent. The solvents conveniently and in high yield. The previously reported method used to process the products are listed in Table 1.The products for Lix(H2O)yMoO334 involves one step to prepare were pumped to dryness and stored in a nitrogen atmosphere. Nax(H2O)yMoO3 and two steps to accomplish ion exchange and gives only 26% yield.† An additional advantage of the Instrumentation LiBH4 method is that it is conducted in diethyl ether and so provides the anhydrous form of the molybdenum bronze. The X-Ray powder diraction patterns were obtained on a Rigaku Ru-200B X-ray diractometer equipped with graphite-mon- LixMoO3 product prepared in this fashion, though still crystalline, shows broader diraction maxima than the precursor ochromated Cu-Ka radiation.A scanning speed of 1° min-1 was chosen. TGA was performed with a Shimadzu TGA-50 MoO3. The Li insertion into MoO3 is topotactic as evidenced by our ability to fully index the X-ray powder diraction instrument under a nitrogen or oxygen flow at a rate of 46 ml min-1 and the heating rate was 10°C min-1.DSC was carried pattern of the product. The amount of Li in LixMoO3 was determined both by out on a Shimadzu DSC-50 instrument under a nitrogen flow at a rate of 20 ml min-1. The heating and cooling rates were TGA and by direct elemental analysis.When the material is heated in an oxygen atmosphere to 650 °C, it gains 1.71 mass%, 5°C min-1. Sample cells were made of aluminium and were annealed at 450 °C in nitrogen after they were cleaned. Samples owing to a change from LixMoO3 to Li2O and MoO3. This gain of mass is reproducible and corresponds to an x value of were sealed in cells under a nitrogen atmosphere before measurement.IR spectra were collected on a Nicolet IR/42 0.31(3). On the other hand, the elemental analysis showed that the molybdenum bronze consisted of 1.64% Li and 56.39% FTIR spectrometer at a resolution of 2 cm-1. Generally 64 scans were obtained for samples as KBr pellets. Electronic Mo. This gives a ratio of Li to Mo of 0.451 corresponding to the molar ratio of LiBH4 used in the lithiation reaction.It is transmission spectra were recorded with a Shimadzu UV- 3101PC UV–VIS–NIR scanning spectrophotometer. Samples of course possible that x varies slightly from sample to sample in the range 0.3<x<0.4. were dried as thin films on quartz slides. Variable-temperature solid-state 7Li and 13C NMR measure- The lithium molybdenum bronze is metastable and undergoes an intense, irreversible and exothermic phase change ments were taken on a 400 MHz Varian NMR instrument.Samples were loaded in a glove box under a nitrogen atmosphere. Magnetic susceptibility measurements were performed † Yield obtained by reproducing the experiment in ref. 34. Table 1 Some chemical and structural characteristics of polymer–LixMoO3 nanocomposites washing expansion of coherence nanocomposite polymer MWa solubility solvent d-spacing/A° gallery/A° length/A° PEO–LixMoO3 5000 000 yes MeCN 16.6 9.7 64 PEO–LixMoO3 100 000 yes MeCN 16.6 9.7 121 PEG–LixMoO3 10 000 yes MeCN 16.8 9.9 108 PEG–LixMoO3 2000 yes MeCN 13.8 6.9 72 12.6–14.7 5.7–7.8 —b PEG–LixMoO3 400 yes MeCN 13.5 6.6 111 12.6 5.7 97 PPG–LixMoO3 1000 yes EtOH 17.2 10.3 113 11.8 4.9 108 18.0, 11.5 — —b MCel–LixMoO3 63 000 no H2O 27.6 20.7 92 PVP–LixMoO3 10 000 yes PriOH 38.6 31.7 154 PAM–LixMoO3 5 000 000 no H2O 38.0 31.1 69 33.8 26.9 80 41.2 34.2 —b PA6–LixMoO3 10 000 no CF3CH2OH 22.1 15.2 39 16.8 9.9 26 aPolymer dissolved in water, except for PA6-10 000, dissolved in 2,2,2-trifluoroethanol.bPeaks too broad to obtain estimate. 1278 J. Mater. Chem., 1997, 7(7), 1277–1283at 356 °C, as detected by DSC (Fig. 1) and X-ray powder diraction. The product appears to be a mixture of at least two unknown phases. As this mixture is heated to higher temperatures it undergoes additional phase changes yielding other new phases. The details of this reaction are currently under investigation. Exfoliation and polymer encapsulation chemistry The lithium molybdenum bronze described above can be readily exfoliated in water after several minutes of sonication.The exfoliated product forms supramolecular complexes with most water-soluble polymers. The complexes form solutions or precipitates in water, depending on the type and molecular mass of the polymers.We have encapsulated PEO, PEG, PPG, PAM, MCel and PVP inside the lithium molybdenum bronze to obtain lamellar nanocomposite materials.Poly(ethyleneimine) (PEI) could not be successfully intercalated. Instead, the blue Lix(H2O)yMoO3 monolayer suspension decolorized and totally dissolved in the aqueous PEI solution. The same phenomenon occurred when ammonia was introduced in the Lix(H2O)yMoO3 suspension, suggesting that the PEI solution is too basic and attacks the MoO3 lattice. Waterinsoluble polymers were also tried, however of these, only PA6 was successfully intercalated.Details of the reactions are given in Table 1. The existence of polymer chains between the layers of the lithium molybdenum bronze was verified by IR spectroscopy Fig. 2 IR spectra of (a) poly(vinylpyrrolidinone), (b) Lix(H2O)y(PVP- and X-ray powder diraction.Fig. 2 shows a comparison of 10000)zMoO3 and (c) hydrated lithium molybdenum bronze the IR spectra of a nanocomposite Lix(H2O)y(PVP)zMoO3 and its components; from this, it is obvious that the vibrational spectrum of Lix(H2O)y(PVP)zMoO3 is a combination of the vibrational spectrum of PVP and that of Lix(H2O)yMoO3. The positions of the vibrationalpeaks arising from the encapsulated PVP are close to those of pure PVP, while the positions of the peak due to the MoNO stretch [for Lix(H2O)yMoO3] is shifted to higher wavenumbers, suggesting that the MoO3 layers in the nanocomposite are slightly more oxidized.The optical transmission absorption spectra of these macromolecular intercalates were examined. The dark-blue colour of these systems arises from the intense intervalence transitions associated with the Mo5+–Mo6+ couple.These electronic transitions are broad and range from the IR region to the visible (Fig. 3) and are responsible for the electrical conductivity of these materials. The absorption at 286 nm arises from excitations across the band-gap from the O2- p band to the Mo6+ d band and is present in all compounds including pristine MoO3 and Lix(H2O)yMoO3 .This is consistent with the expectation that host metal oxide structure is not disturbed upon intercalation. The encapsulation of polymers inside the interlayer galleries Fig. 3 Solid-state optical absorption spectra of the polymer–LixMoO3 nanocomposites: (a) MoO3, (b) Lix(H2O)yMoO3, (c) Lix(H2O)y(PEO- 100000)zMoO3, (d) Lix(H2O)y(PAM-5 000000)zMoO3 of MoO3 is also demonstrated by X-ray powder diraction, which shows an expansion of the gallery space.Fig. 4 and 5 show typical XRD patterns of some of the nanocomposites. The sharp and intense (001) reflections indicate that the MoO3 layers are well stacked. X-Ray scattering coherence lengths, which are calculated from the Scherrer formula L hkl= Kl/bcosh,35 and the gallery spacings are given in Table 1.The basal spacing of some nanocomposites depends on the preparation procedure. For example, Lix(H2O)y(PVP)zMoO3, which is water soluble, showed a d-spacing of 59.0 A° before Fig. 1 DSC diagram of LixMoO3 washing with isopropyl alcohol and 38.3 A° after washing. J. Mater. Chem., 1997, 7(7), 1277–1283 1279to control, as for example in PEO(5000 000), PEG(2000) and PPG(1000).The Lix(H2O)y(PEO-5 000 000)zMoO3 had a consistent d-spacing of ca. 16.6 A° , but the peaks were broad. Lix(H2O)y(PEG-2000)zMoO3 showed broad peaks in the range 12.6–14.7 A° . Lix(H2O)y(PPG-1000)zMoO3 sometimes showed a peak in the range 11.8–17.2 A° , while other samples showed two peaks in this range suggesting a mixture of phases.Evidently, for low molecular masses the polymers are mobile enough in the galleries to form several dierent arrangements leading to multiple phases. The eect of polymer molecular mass on product formation was examined, particularly with PEO and PEG, and was found to be significant. The high molecular mass PEO(5000 000) immediately formed a precipitate with lithium molybdenum bronze in water upon mixing while this phenomenon did not occur with PEO of lower molecular mass.The molecular mass also aects the structure of the nanocomposites. Table 1 shows that the Lix(H2O)y(PEO- 5000 000)zMoO3 sample has a lower coherence scattering length than its lower molecular mass analogues, which is attributed to the fact that it is kinetically unfavourable for extremely long polymer chains to align in an ordered structure.When the molecular mass is extremely low, i.e. in oligomer range, the gallery expansion of the intercalate is lower, almost one half of that of the long polymers. For PEO and PEG, a molecular mass of 2000 is about the upper limit of this Fig. 4 Typical XRD patterns of nanocomposites with poly(ethylene situation.The data listed in Table 1 show that the glycol) and poly(ethylene oxide) of dierent molecular masses. The Lix(H2O)y(PEG-2000)zMoO3 sample has a much shorter polymer and its molecular mass are indicated on each spectrum. coherence length than its analogues with higher or lower molecular masses. An analogous Lix(H2O)y(PEG-2000)zMoO3 sample, prepared under similar conditions, had a broad X-ray basal peak which corresponded to d-spacings varying from 12.6 to 14.7 A° , suggesting that the local conformation of the polymer is important.Annealing the Lix(H2O)y(PEG)zMoO3 and the Lix- (H2O)y(PEO)zMoO3 samples at 150°C and then gradually cooling them to room temperature tends to improve their lamellar order, especially when the starting coherence length is short.For example, after annealing, the Lix(H2O)y(PEG- 2000)zMoO3 sample whose XRD pattern had a broad peak centred at 13.4 A° gave a pattern with a sharp peak at 12.7 A° , (Fig. 6). The water-insoluble nanocomposite Lix(H2O)y(PAM- 5000 000)zMoO3 gave samples with d-spacings of 38.0, 33.8 Fig. 5 Typical XRD patterns of the various polymer–LixMoO3 nanocomposites. The polymer and its molecular mass are indicated on each spectrum.Washing these materials may not only remove extra-lamellar polymer, but could lead to polymer loss from the galleries changing their composition. This behaviour makes it dicult to determine at what polymer loading we begin to saturate the intralamellar space. Similar phenomena were described in polymer–V2O5 xerogel systems.11a Nevertheless, the observed d-spacings were consistent to within ±1 A° , as long as the preparation procedure was not altered significantly from batch to batch.The d-spacings and the degrees of lamellar stacking Fig. 6 XRD patterns of Lix(H2O)y(PEO-2000)zMoO3 showing the of products which contained polymers at the very extremes of eect of annealing on the stacking regularity of the layered structure of a nanocomposite the molecular mass range (very high or very low) were hard 1280 J.Mater. Chem., 1997, 7(7), 1277–1283and 41.2 A° . The Lix(H2O)y(PA6-10 000)zMoO3 showed d-spac- of the solid LiCl standard. At -80 °C the lineshapes in the two spectra dier, with that of Lix(H2O)y(PEO-100 000)zMoO3 ings of 22.1 and 16.8 A° . Occasionally, in the intercalation of PA6 a mixed-phase material with basal spacings of 12.8 and being slightly more asymmetric, Fig. 7. This suggests that the presence of PEO causes a distribution of Li ions over several, 9.8 A° was obtained. This shows the diculty in controlling the reaction when quick precipitation is used to obtain a specific slightly dierent sites in the gallery. Some of the sites may involve coordination of water molecules while others are phase.To prepare nanocomposites of this type, a very dilute Lix(H2O)yMoO3 suspension of <0.5 mass% is recommended. associated with the ether-like oxygen atoms of PEO or even those in the MoO3 layers. The linewidth (width at half maxi- The polymer compositions of the nanocomposites were determined by TGA in an oxygen atmosphere and are listed mum) of the resonance peak is greater in the PEO intercalated sample than in the host LixMoO3 material and this too is in Table 2.The water in the nanocomposites was estimated by the mass loss step observed below 230 °C, and the amount of consistent with a distribution of the Li ions over several sites in the former. The data in the low-temperature region suggest polymer was determined by the mass loss steps observed at higher temperatures.Despite drying under vacuum, the a more well defined coordination environment for the Li ion in LixMoO3 as would be expected in a crystalline solid. nanocomposites retain some water in the galleries. The water-soluble nanocomposites, Lix(H2O)y(PEO)zMoO3, The linewidth of 2300 Hz in LixMoO3 at 23°C is much narrower than that of ca. 12000 Hz observed for Li2Mo2O4 Lix(H2O)y(PEG)zMoO3, Lix(H2O)y(PPG)zMoO3 and Lix- (H2O)y(PVP)zMoO3, usually contain 2–4 mass% water which indicating a substantial degree of ion motion in the lattice of LixMoO3 relative to that of Li2Mo2O4.38 This is rationalized is very dicult to remove. This water is thought to be coordinated to Li+ ions. The water-insoluble nanocomposites by the fact that in the latter the Li ions fully occupy well defined crystallographic positions in the lattice39 while the Lix(H2O)y(MCel)zMoO3 and Lix(H2O)y(PAM)zMoO3, however, contain much less water.non-stoichiometric nature of LixMoO3 gives rise to Li mobility via vacant crystallographic sites. Compared to the corresponding polymer–MoS2 intercalates, 22b most polymer–LixMoO3 intercalates have much higher The resonance peak in both samples narrows dramatically as the temperature is increased from -80 to 100 °C, Fig. 8. polymer contents and larger gallery spacings. As in the polymer –MoS2 case, PVP and MCel give the largest expansions. This is attributed to rapid motion of Li ions between the MoO3 layers. At 100 °C the peak linewidth in the spectra of A marked dierence is found in PAMwhich gives an expansion as large as 34.3 A° , while PAM–MoS2, a hybrid prepared by Lix(H2O)y(PEO-100 000)zMoO3 is comparable to that of Li0.5(H2O)1.3Mo2O4.38 The onset temperature of the transition us, has only an expansion of 9.1 A° .36 This confirms that multiple layers of this polymer can enter the accessible space from a wide to a narrow peak in Lix(H2O)y(PEO- 100000)zMoO3 and LixMoO3 is similar. The spectra of both of LixMoO3.The water-soluble polymer–LixMoO3 nanocomposites can be cast into films and other shapes, which may provide opportunities for applications. The nanocomposites with high molecular mass polymers are strong, though their mechanical properties depend on the polymer. For example, the nanocomposite of PEO(5000000) is tough, while that of PAM(5000 000) is hard.Lix(H2O)y(PEO-5000 000)zMoO3 can be swollen by acetonitrile and becomes resilient and plastic with the consistency of unsulfurized rubber. When the Lix(H2O)y(PAM-5 000 000)zMoO3 is swollen by water, it is not as plastic, but is stronger and tougher. Precise mechanical measurements have not been taken yet.37 Solid-state NMR spectroscopy In order to probe the eect of the polymer on the behaviour of the lithium ions in the gallery, variable-temperature 7Li solid-state NMR static spectra were measured for LixMoO3 Fig. 7 Static 7LiNMR spectra of (a)LixMoO3 and (b) Lix(H2O)y(PEO- and Lix(H2O)y(PEO-100 000)zMoO3. In both cases a broad 100000)zMoO3 at -80°C. The broader asymmetric line in the spectrum of the nanocomposite is evident in the upfield region.peak was observed with a chemical shift very similar to that Table 2 Composition and physicochemical properties of the polymer–LixMoO3 nanocomposites limit of limit of thermal thermal electronic polymer d-spacing/ composition stability stability conductivity/ nanocomposite MW A° (according to TGA) in N2/°C inO2/°C Scm-1 pure LixMoO3 8.27 1.3×10-2 pure MoO3 6.93 3.3×10-5 PEO–LixMoO3 5×106 16.6 Li0.25(H2O)0.20(PEO)0.83MoO3 260 220 2.4×10-5 PEO–LixMoO3 100 000 16.6 Li0.25(H2O)0.32(PEO)1.04MoO3 260 220 2.9×10-5 PEG–LixMoO3 10 000 16.8 Li0.25(H2O)0.29(PEG)0.75MoO3 260 220 5.2×10-5 PEG–LixMoO3 2000 13.8 Li0.25(H2O)0.28(PEG)0.57MoO3 260 220 2.2×10-4 PEG–LixMoO3 400 13.5 Li0.25(H2O)0.38(PEG)0.33MoO3 260 220 3.1×10-4 PPG–LixMoO3 1000 17.2 Li0.25(H2O)0.18(PPG)0.99MoO3 220 200 — 11.8 Li0.25(H2O)0.52(PPG)0.14MoO3 5.3×10-4 MCel–LixMoO3 63 000 27.6 Li0.25(MCel)0.70MoO3 180 170 2.0×10-6 PVP–LixMoO3 10 000 38.6 Li0.25(H2O)0.43(PVP)1.17MoO3 220 220 1.8×10-7 PAM–LixMoO3 5×106 38.0 Li0.25(PAM)3.2MoO3 150 150 6.3×10-7 PA6–LixMoO3 10 000 22.1 Li0.25(H2O)0.44(PA6)0.32MoO3 280 270 — 16.8 Li0.25(H2O)0.48(PA6)0.32MoO3 2.1×10-4 J.Mater. Chem., 1997, 7(7), 1277–1283 1281of LixMoO3 are more similar to those of Li0.9Mo6O1740 than of other molybdenum bronzes such as the blue bronze A0.3MoO3 (A=K, Tl) and purple bronzes A0.9Mo6O17 (A=K, Na, Tl).41–44 In the latter bronzes the temperature dependence of the magnetic susceptibility shows transitions at low temperature associated with charge density waves. Such phenomena were not observed in the LixMoO3 samples reported here.Electrical conductivity Electrical conductivity values for all the nanocomposites are listed in Table 2. The room-temperature electrical conductivity of LixMoO3 was 0.013 S cm-1 which is slightly lower than values reported in related materials.45,46 As shown in Table 2, the conductivity of the nanocomposites is significantly lower than that of LixMoO3 and decreases with increasing layer Fig. 8 Temperature dependence of the linewidth of the resonance peak expansion. For materials with extremely large gallery spacing, in the solid-state 7Li NMR spectra of (a) LixMoO3 and the electrical conductivity is lower than that of MoO3 itself. (b) Lix(H2O)y(PEO-100000)zMoO3 When the measurements are performed under vacuum the conductivity decreases concomitantly with the loss of water materials end up with an equally narrow peak at 100°C from the galleries.This suggests that water is contributing to indicating comparable rates of hopping of Li ions between charge transport in these materials probably via proton dierent positions in the interior of the two materials.mobility. Fig. 10 shows the temperature dependence of the Preliminary solid-state, cross-polarization, magic angle spin- electrical conductivity of LixMoO3 and Lix(H2O)y(PEO- ning 13C NMR spectra, as a function of temperature, suggest 100000)zMoO3, which is thermally activated. Mixed ionic/ a substantial degree of motion in the polymer backbone as electronic conducting nanocomposites are of current interest.12 well. At room temperature a single broad resonance is observed for the MCH2M group at d 71.6, with a linewidth of 1380 Hz.Conclusion As the temperature rises, the resonance does not shift but it narrows dramatically and at 80°C it is only 200 Hz, consistent Anew family of polymer–molybdenum bronze nanocomposites with rapid thermal motion of the polymer chains.This motion has been synthesized. The host material, LixMoO3 , was should facilitate the hopping of Li ions in the gallery and synthesized via a LiBH4 route which is dierent from the contribute to a high ionic conductivity for the material. conventional approach. This material exfoliates in water and has anity for a large variety of polymers. Polymers such as Magnetism PEG, PEO, PPG, PVP, MCel, PAM and PA6 give well stacked lamellar nanocomposites.Depending on the nature of Because the materials are formally mixed-valence compounds and exhibit intense intervalence Mo5+–Mo6+ optical transitions, we expect unpaired electrons to be delocalized over the d orbitals of the Mo atoms. Magnetic susceptibility measurements were carried out as a function of temperature for LixMoO3 and Lix(H2O)y(PEO-100 000)zMoO3 and the data are displayed in Fig. 9. Surprisingly, the susceptibility of both these compounds is rather small with substantial contributions from temperature independent paramagnetism (xTIP). Correcting for the latter, 1/(xmolar-xTIP) vs. T plots for LixMoO3 and Lix(H2O)y(PEO-100000)zMoO3 are linear in the temperature ranges 5–120 K and 2–300 K, respectively.The Curie constants estimated from the slope of the plots yield meff of 0.15 and 0.09 respectively, however, the weak susceptibilities and the large diamagnetic and xTIP corrections make meff values unreliable. It is interesting that the magnetic properties Fig. 10 Variable-temperature electrical conductivity measurements for Fig. 9 Temperature dependence of magnetic susceptibility of LixMoO3 (#) and Lix(H2O)y(PEO-100000)zMoO3 (').The measurements were pressed pellet samples of (a) LixMoO3 and (b) Lix(H2O)y(PEO- 100000)zMoO3 conducted at 1000 G with powder samples. 1282 J. Mater. Chem., 1997, 7(7), 1277–1283M. G. Kanatzidis, Chem. Mater., 1996, 8, 525; (d) C.-G. Wu, the polymer and its molecular mass, some of the nanocom- M.G. Kanatzidis, H. O. Marcy, D. C. 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J.Fisher, NATO ASI Ser., Plenum, 1993, p. 63. 45 C. Julien and G. A. Nazri, Solid State Ionics, 1994, 68, 111. 11 (a) Y.-J. Liu, D. C. DeGroot, J. L. Schindler, C. R. Kannewurf and 46 J. O. Besenhard, J. Heydecke, E.Wudy and H. P. Fritz, Solid State M. G. Kanatzidis, Chem. Mater., 1991, 3, 992; (b) Y.-J. Liu, Ionics, 1983, 8, 61. D. C. DeGroot, J. L. Schindler, C. R. Kannewurf and M. G. Kanatzidis, Adv. Mater., 1993, 5, 369; (c) Y.-J. Liu, J. L. Schindler, D. C. DeGroot, C. R. Kannewurf, W. Hirpo and Paper 7/00202E; Received 8th January, 1997 J. Mater. Chem., 1997, 7(7), 1277–1283

ISSN:0959-9428    

DOI:10.1039/a700202e

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Registered growth of mesoporous silica films on graphite Hong Yang,a Neil Coombs,b Igor Sokolova and Georey A. Ozin*a aMaterials Chemistry Research Group, L ash Miller Chemical L aboratories, University of T oronto, 80 St. George Street, T oronto, Ontario, Canada M5S 3H6 bImagetek Analytical Imaging, 32 Manning Avenue, T oronto, Ontario, Canada M6J 2K4 It has recently been demonstrated that hexagonal mesoporous silica films can be grown at mica/water interfaces. In this report it is established that the films can also be synthesized on cleaved pyrolytic graphite.Diraction and microscopy studies of the growth, structure and form of the films indicate that the channels are aligned along the hexagonal symmetry axes of the graphite surface. The registry of the mesoporous silica film with the underlying substrate may be facilitated by an organized hemicylindrical surfactant monolayer assembly that is observed at the boundary between the graphite and water.Growth of the mesoporous silica film most likely begins within a liquid-crystal surfactant/silicate film that is aligned with the hemimicelle–graphite overstructure. The recently reported surfactant-based syntheses of hexagonal a horizontal position.The film forming process commenced within minutes under static conditions in an oven at 80 °C. mesoporous silica films at air/water, oil/water and solid/water interfaces1–5 raises a number of issues. These include the Film growth was allowed to proceed for a period of one day to a week. The films so formed were transferred from the vessel structure of the interfacial surfactant, details of the nucleation and growth processes, the factors that determine the alignment using tweezers and washed with deionized water.The calcination of graphite-supported films was conducted in air and of the channels, and the domain and defect architecture of the films. in a furnace attached to an Omega CN-2010 programmable temperature controller. The temperature ramp was Here, we present experimental details relating to the growth, structure and form of a hexagonal mesoporous silica film <1°C min-1 and typically the sample was held at 450 or 540°C for 4 h.deposited onto the surface of freshly cleaved pyrolytic graphite. A combination of powder X-ray diraction (PXRD), highresolution scanning electron microscopy (HRSEM), atomic Characterization force microscopy (AFM) and transmission electron microscopy Powder X-ray diraction.PXRD data was obtained on a (TEM), provide converging evidence that the channels of the Siemens D 5000 diractometer using Ni-filtered Cu-Ka radi- film appear to be aligned with the hexagonal symmetry axes ation with l=1.54178 A° . Samples of the graphite-supported of the graphite surface.A well ordered hemimicellar surfactant films were mounted on top of a home-made low-background assembly is shown, by pre-contact electrical double layer quartz cell for recording PXRD data. (EDL) AFM, to pre-exist at the graphite/water interface. The polymerization of a pre-organised liquid-crystal surfactant/ Scanning electron microscopy.SEM images were obtained silicate assembly located at the surface of graphite is believed on a Hitachi S-4500 field emission microscope using a low to direct the nucleation and growth of the mesoporous silica acceleration voltage of 2–5 kV to minimize the charging of film. film surfaces. Samples were generally imaged directly except on one occasion when the sample was coated with a very thin layer of gold before imaging. Experimental Transmission electron microscopy.TEM images of the films Synthesis were recorded on a Philips 430 microscope operating at an The graphite used in this work is taken from a 12 mm×12 mm×2 mm pyrolytic graphite monochromator (Grade ZYB). This was a generous gift donated by the Advanced Ceramics Corporation. Thin sheets of graphite substrate were obtained by peeling a uniform section from the graphite block with Scotch tape and then cutting it into the desired size.Tetraethylorthosilicate (TEOS, 99+%, Aldrich), cetyltrimethylammonium chloride (CTACl, 29 mass% aqueous solution, Pflatz & Bauer) and hydrochloric acid (36.5–38 mass% aqueous solution, BDH) were used as received. The synthesis of mesoporous silica films at the graphite/water interface was conducted under quiescent acidic conditions.1,2,6 The reactant mole ratios used were 100H2O57HCl50.11 CTACl50.1TEOS The CTACl surfactant solution was mixed with the TEOS source of silica and stirred for ca. 2 min at room temperature and then transferred into a polypropylene bottle and allowed to achieve a stable air/water interface.A ca. 10 mm×10 mm freshly cleaved pyrolytic graphite substrate was then positioned Fig. 1 A representative PXRD pattern for an as-synthesized mesoporous silica film grown on freshly cleaved pyrolytic graphite at the surface of the synthesis solution and allowed to float in J. Mater. Chem., 1997, 7(7), 1285–1290 1285accelerating voltage of 100 kV. In order to get ultrathin sections the tapping mode was engaged, the tip was elevated manually by the motor for the vertical displacements.Normally the (100–300 A° ) of the mesoporous silica film grown on graphite, a gold-coated film was embedded in epoxy, heated to form a elevation needed was about 100–200 nm. The jump into precontact mode was clearly evident every time. block and cut at dierent angles using a diamond knife following the standard ultramicrotome procedure.The gold coating was used both to identify the film position in the Results epoxy block and the sides of the film, since the adhesion forces between the film and the graphite substrate are not suciently The PXRD pattern of the film grown on graphite shows only two low-angle peaks assigned to the (100) and (200) reflections strong to withstand the microtome cutting force.Consequently the film is easily cleaved from the graphite upon sectioning. of a hexagonal symmetrymesoporous silica, Fig. 1. The absence of the (110) reflection for the film, compared to that of a randomly oriented powdered sample,7 confirms that the chan- Atomic force microscopy. AFM experiments were conducted on a NanoScope III microscope (Digital Instruments, CA) nel axis is aligned parallel to the graphite surface.Calcination of the film was performed at 450 °C in air for 4 h. The using silicon integrated tip cantilevers (Park Scientific Instruments, CA) for height mode scanning of the films, and high-temperature treatment causes the PXRD d100-spacing to contract by 1–3 A° with concomitant changes in the intensity special silicon cantilevers (Digital Instruments, CA, FESP type, 227 mm cantilever, resonance frequency 70–100 KHz) for tap- of the PXRD pattern.These eects are associated with the polymerization of residual SiOH groups. The integrity of the ping mode scanning in fluids. The cantilevers were used as received. Images of mesoporous silica film surfaces were film and its mesostructure are well maintained throughout this thermal process.obtained by using direct contact (dc) scanning mode. Images of CTACl assemblies on graphite were obtained by using two SEM images of the films grown on the graphite substrate are shown in Fig. 2. The low-magnification images reveal that dierent methods. The first employed a pre-contact EDL height imaging mode.The second utilized a combined EDL the films are essentially continuous, Fig. 2(a). Intriguing equilateral triangle-shaped pits are observed in the films, pre-contact and tapping imaging mode. This method is a new way to image soft structures with minimal disturbance of their Fig. 2(b)–(d). Also, the growth fronts of the films typically have parallel filamentous extensions, terminated with edges inclined integrity. It was applied successfully in this work to image surfactant assemblies adsorbed at the graphite/water interface.at 60° or 120°, and which sometimes display sharp 60° or 120° bends, Fig. 2(a), (b). The triangle-shaped pits display both CTACl aqueous solution with concentrations of 4–18 mM was used. The frequency of the cantilever tapping in the surfactant straight and convex edges, Fig. 2(b)–(d). It is noteworthy that the edges of the triangles are exclusively aligned with respect solution was about 33 KHz. To achieve pre-contact mode during tapping, the following procedure was adopted. After to each other as well as in alignment with the edges of the Fig. 2 SEM images of an as-synthesized mesoporous silica film grown on freshly cleaved pyrolytic graphite: (a) a large area and a growth front of the film; (b) a growth front showing the filaments and equilateral triangular pits (note that this sample has been coated with gold before imaging); (c), (d) close-up views of the triangular pits 1286 J.Mater. Chem., 1997, 7(7), 1285–1290filaments at the growing front of the film, Fig. 2(b).A closer AFM soft imaging technique.5,8–10 We have found in this study that the perturbation of the surfactantassembly by the scanning examination of the inner walls of the triangular pits byHRSEM reveals a multi-layer topology with step features aligned with tip can be further reduced and the quality of the AFM images enhanced by using the tapping mode in conjunction with the the sides of the triangles, Fig. 2(c), (d). TEM images of the mesoporous silica films grown on pre-contact (EDL) technique. This can be understood by recalling that the EDL consists graphite show that the channels are hexagonally close-packed with a centre-to-centre distance of about 40–45 A° and run of a bilayer of oppositely charged surfactant cations and counter anions adsorbed on the AFM tip.The result is an parallel to the graphite/water growth boundary, Fig. 3(a). The TEM estimated unit-cell dimensions agree well with the PXRD additional EDL force of repulsion between the tip and the surfactant assembly, adsorbed on graphite, being imaged. Thus, d100-spacings of 37–39 A° . A TEM image of a cross-section of the film cut along the channel axis is shown in Fig. 3(b). The the apex of the tip floats over the fragile surfactant assembly with a gentler type of interaction. This type of pre-contact observed parallel lines have a repeat distance of 40–45 A° which corresponds well with the separation of the walls of the scanning mode, in conjunction with the tapping action of the probe, will cause less deformation of the assembly being imaged mesopores and confirms that the channels grow parallel to the graphite surface. The smooth bottom surface of the films because of a lack of friction force. According to previous experiments, and our own work, the shown in Fig. 3 implies that it corresponds to the side of the mesoporous silica film that has grown from the graphite direct pre-contact mode scanning along the boundary of the EDL provides an image of the adsorbed hemicylindrical surfac- surface.The mesoporous silica films can be grown on graphite to a thickness of around 0.5–0.6 mm. tant monolayer assembly on the graphite surface. The evidence for this being a hemimicellar monolayer comes from the AFM is an eective probe of the topology of the films grown on graphite, Fig. 4. AFM direct contact (dc) mode images of thickness measurement of the surfactant assembly.This is achieved by increasing the scanning force, whereby the EDL the outer surface of the film reveal a mottled structure at the 50–200 nm length scale. A representative example of this repulsion is overcome and the tip jumps through the film.5,8–10 Pre-contact (EDL) tapping mode AFM images of an aque- texture on the surface of the film surrounding a triangular pit is shown in Fig. 4(a). Overall, these surface images portray the ous solution of CTACl adsorbed on graphite have been obtained over the concentration range of 4–18 mM. One can occurrence of a multi-layer step structure in the central regions of the films, Fig. 4(b), as well as on the walls of the triangular discern well defined parallel stripes with a periodicity of ca. 53 A° , Fig. 5(a), and a thickness up to 500 A° , Fig. 5(b). The film pits, Fig. 4(a), and on the filamentary growth fronts of the films, Fig. 4(c). thickness was estimated by measuring the height dierence when the tip jumps from the pre-contact tapping to the contact The organization of adsorbed surfactants at the boundary between water and silica, mica and graphite substrates has tapping mode.The collapse of the surfactant assembly was either spontaneous or induced by the change in the setpoint recently been observed using the pre-contact mode (EDL) of the AFM scanning. This implies the existence of up to 10 layers of 50 A° diameter surfactantcylindrical micelles organized as a liquid-crystal film at the boundary between water and graphite.The AFM images also reveal the presence of domains within which parallel arrays of cylindrical micelles meet at boundaries with either 60° or 120° angles, Fig. 5(c). This suggests that the director of the liquid-crystal overlayer is aligned with the hexagonal unit-cell axes of the graphite surface as well as the axis of the adsorbed hemicylindrical micellar monolayer.The ability to observe, for the first time, such surfactant multilayer assemblies without their destruction apparently stems from the lack of friction force in the AFM tapping mode compared to that of the direct contact mode. Discussion The above results provide a basis for beginning to understand the origin of the preferred orientation and apparent registry of the channels of the mesoporous silica film with the hexagonal symmetry axes of the graphite surface.The assembly process in the absence of TEOS probably begins with adsorption of CTACl on the graphite surface. The hydrophobic interaction between the alkane chain and the graphite causes the surfactants to lie flat on the surface.8,9 Geometrical matching of the methylene groups in the all-trans alkane chain with the aromatic carbon six-rings in the planar graphite surface, favours a head-to-head and tail-to-tail packing arrangement of the surfactant along the hexagonal symmetry axes of graphite.This geometryis driven by hydrophobic, electrostatic ion-pair and image dipole forces between the CTACl and the electrically conducting graphite surface.8,9 The result is 50 A° parallel stripes of surfactants running orthogonally to the graphite hexagonal symmetry axes.This Fig. 3 TEM images of an as-synthesized mesoporous silica film show- organized surfactant monolayer assembly then serves as a ing (a) the hexagonally closed-packed channel structure with a centre- template for the further adsorption of CTACl from solution to-centre spacing of ca. 45 A° ; (b) the channels running parallel to the and the formation of a monolayer of 50 A° diameter hemi- graphite surface.Bottom (smooth side), grown at the graphite/water cylindrical micelles with a thickness of ca. 25 A° .8,9 Using interface; top (rough side), grown in solution. (Note, imaging the entire film compromises visualization of the channels.) pre-contact AFM imaging mode we have reproduced this J.Mater. Chem., 1997, 7(7), 1285–1290 1287nm nm nm nm nm nm 400.00 500.00 600.00 ( a ) (b ) ( c ) Fig. 4 AFM dc mode images of the outer surface of an as-synthesized mesoporous silica film on graphite: (a) mottled texture on the film surrounding a triangular pit; (b) steps on the edges of the triangular pits; (c) terraces on the filaments at a growth front result over the 4–18 mM surfactant concentration range investi- selves in alignment with the edges of the filaments that comprise the growth fronts of the film.These structural features likely gated in this study. However, for a surfactant concentration above roughly 9 mM and when the AFM imaging is changed arise from the ability of the surfactant/silicate/graphite assembly to control the alignment of the channels in the to the pre-contact tapping mode, we still observe 50 A° parallel stripes but now with a film thickness of about 500 A° , Fig. 5(b). growth of the mesoporous silica film, along the three symmetry equivalent hexagonal axes of graphite. Growth fronts are This implies that continued accretion of CTACl from solution leads to the development of a multilayer of close-packed 50 A° expected to meet at 60° in the body of the mesoporous silica film to form triangular features and to terminate at the diameter cylindrical micelles.These are presumably organized in the form of a liquid-crystal film registered with the extremities of the film to create filaments displaying 60° and 120° angular features, Fig. 2. graphite surface. In the presence of TEOS, nucleation of the mesoporous The stepped textures observed for the films, the triangular pits and the filaments, Fig. 4, most likely originate from silica film is initiated by polymerization of charge-balancing silicate anions in the headgroup region of a liquid-crystal film polymerization and growth of cylindrical surfactant/silicate seeds11 in the liquid-crystal film adsorbed on the graphite that has its director axis registered with the hexagonal symmetry axes of the underlying graphite substrate, Plate 1.surface. Further studies will be required to elucidate the details of the nucleation and growth processes that give rise to these Growth of the mesoporous silica film is likely to be determined by the charge and structure encoded in the surfactant/silicate/ mesoporous silica films.graphite film. This process could produce a mesoporous silica film in which the channels are aligned with the hexagonal Conclusion symmetry axes of the graphite surface. Subsequent deposition and polymerization of surfactant/silicate micellar assemblies In this paper, we have demonstrated that mesoporous silica films can be synthesized on the hydrophobic surface of freshly results in the continuous growth and thickening of the mesoporous silica film up to the observed value of about half a cleaved pyrolitic graphite.Our combined PXRD, HRSEM, TEM and AFM data provide converging evidence that the micrometre. This templating model for the polymerization of a surfactant/ channels of the film are probably registered with the hexagonal symmetry surface structure of the underlying graphite.This silicate liquid-crystal film on graphite is consistent with the observation of mutually aligned triangular-shaped pits in the arrangement can be understood in terms of the growth of the mesoporous silica film within a liquid-crystal surfactant/silicate body of the resulting mesoporous silica film, which are them- 1288 J.Mater. Chem., 1997, 7(7), 1285–1290Fig. 5 Pre-contact (EDL) tapping mode AFM images (using phase detection) of CTACl assemblies adsorbed on graphite showing (a) parallel stripes with a periodicity of ca. 53A° , and (b) a thickness of ca. 500 A° ; (c) domains of surfactants with parallel arrays of cylindrical micelles that meet at sharp boundaries with an angle of either 60° or 120°.The surfactant concentration was in the range of 9–18 mM. film that is aligned with the symmetry axes of graphite. Overall, the synthesis of mesoporous silica films on hydrophilic mica and hydrophobic graphite surfaces, as well as free-standing ones formed at the boundary between air and water, augurs well for their use in a range of applications, such as large-molecule catalysis, membrane separations and chemical sensing.Financial support from Mobil Technology Company is deeply appreciated. H.Y. is grateful for an Ontario Graduate Scholarship and a University of Toronto Open Scholarship. H.Y. also thanks Mr S. Boccia and Mr F. Neud for the helpful technical tutorial on using HRSEM. We would also like to express our deepest gratitude to Dr Grant Henderson for the use of his AFM equipment and for valuable technical discussions.References 1 H. Yang, A. Kuperman, N. Coombs, S. Mamiche-Afara and G. A. Ozin, Nature (L ondon), 1996, 379, 703. 2 H. Yang, N. Coombs, I. Sokolov and G. A. Ozin, Nature (L ondon), 1996, 381, 589. 3 G. A. Ozin, D. Khushalani and H. Yang, Proceedings of the NAT O Plate 1 Graphical illustration of proposed model for the formation of Advanced Research Workshop on Self Assembly, ed. J. Wuest, Val Morin, May 1996. a mesoporous silica film on graphite. Red, surfactant tail; yellow, surfactant headgroup; light blue, silicate building-block; dark blue, 4 S. Schacht, Q. Huo, I. G. Voigt-Martin, G. D. Stucky and F. Schu�th, Science, 1996, 273, 768. silica; black, graphite. J. Mater. Chem., 1997, 7(7), 1285–1290 12895 I. A. Aksay, M. Trau, S. Manne, I. Honma, N. Yao, L. Zhou, 8 S. Manne, J. P. Cleveland, H. E. Gaub, G. D. Stucky and P. K. Hansma, L angmuir, 1994, 10, 4409. P. Fenter, P. M. Eisenberger and S. M. Gruner, Science, 1996, 273, 892. 9 S. Manne and H. E. Gaub, Science, 1995, 270, 1480. 10 E. J. Wanless and W. A. Ducker, J. Phys. Chem., 1996, 100, 3207. 6 Q. Huo, D. I. Margolese, U. Ciesla, P. Feng, T. E. Geir, P. Sieger, R. Leon, P. M. Petro, F. Schu�th and G. D. Stucky, Nature 11 O. Regev, L angmuir, 1996, 12, 4940. (L ondon), 1994, 368, 317. 7 C.T. Kresge, M. Leonowicz, W. J. Roth, J. C. Vartuli and Paper 6/08171A; Received 3rd December, 1996 J. C. Beck, Nature (L ondon), 1992, 359, 710. 1290 J. Mater. Chem., 1997, 7(7), 1285–

ISSN:0959-9428    

DOI:10.1039/a608171a

出版商:RSC    

年代:1997

数据来源: RSC