J. MATER. CHEM., 1994, 4(8), 1219-1226 Magnetic Properties and Crystal Structure of the p-Fluorophenyl Nitronyl Nitroxidet Radical Crystal: Ferromagnetic Intermolecular Interactions'leading to a Three-dimensional Network of Ground Triplet Dimeric Molecules Yuko Hosokoshi, Masafumi Tamura, Minoru Kinoshita,* Hiroshi Sawa, Reizo Kato, Youko Fujiwara* and Yutaka Ueda Institute for Solid State Physics, University of Tokyo, Roppongi, Minato-ku, Tokyo 106,Japan The crystal of p-fluorophenyl nitronyl nitroxide [2-(4-fluorophenyl)-4,4,5,5-tetramethyl-4,5-dihydro-l H-imidazol-1 -oxyl 3-oxide, abbreviated as p-FPNN] radical has been found to possess a dimeric structure. Each dimer is related to adjacent ones by four-fold screw symmetry to form a three-dimensional network.The temperature dependence of the paramegnetic susceptibility is explained by the formation of a triplet state within the dimer and additional interdimer ferromagnetic interactions. The intra- and inter-dimer exchange interactions are estimated to be J/k, =5.0 K and J'/kB= 0.02 K, respectively. The molecular arrangement in the dimer suggests that the intermolecular interactions between the NO group and the phenyl ring are favourable to ferromagnetic coupling. Slight structural changes below ca. 100 K have been studied by use of electron paramagnetic resonance and powder X-ray diffraction measurements. In 1991 it was found that P-p-nitrophenyl nitronyl nitroxide (p-p-NPNN) undergoes a ferromagnetic transition at 0.6 K.'T~ This is the first example of a purely organic radical ferromag- net consisting only of light elements such as C, H, N and 0.Following this, a second organic ferromagnet with T,=1.48 K has been rep~rted.~ These findings have initiated investigations into the magnetic properties of molecular materials. However, the detailed mechanism of the ferromagnetic intermolecular interactions in these materials is still open to research. Understanding the origin of the ferromagnetic interactions would help to improve the rational design of molecular magnets in terms of molecular and crystal structures, which are not fully available yet. In order to elucidate the physics behind the magnetism of molecular materials, knowledge of the relation between the molecular packing and the intermol- ecular magnetic interactions is of primary importance.Some other organic radical crystals, such as galvinoxyl [2,6-di-tert- butyl-4-( 3,5-di-tert-butyl-4-oxocyclohexa-2,5-di-enylidenemethyl) phenoxyl] ,4 TANOL suberate { 4,4'-[1,8-dioxoctane- 1,8-diyl) bis (oxy)] bis( 2,2,6,6-tetramethylpiperidin-1-yloxyl)},5 4-methacryloyloxy-2,2,6,6-tetramethylpiperidin-1-yloxyl (MOTMP)6 and Y-P-NPNN,~ are known to possess dominant ferromagnetic intermolecular interactions as sug- gested by the positive Weiss constants. However, the last three compounds have been found to undergo antiferromagnetic transitions owing to antiferromagnetic secondary interactions. As typically demonstrated by these facts, the molecular pack- ing that leads to three-dimensional (3D) ferromagnetic inter- actions in the crystal is crucial to bulk ferromagnetism.Hence, the control of the crystal structures is one of the most important factors in designing molecular magnets. The series of phenyl a-nitronyl nitroxides afford satisfactory crystallinity and stability. Many derivatives of this series can be readily obtained. These characteristics are suitable for the purpose of surveying the interrelations between the molecular form, crystal structure and the magnetism in organic radical crystals. By classifying the molecular packing of these com- pounds in the crystals, useful information can be found for the control of molecular magnetism. ?The use of the term nitroxide is discouraged by IUPAC; the preferred term is aminoxyl.$ Present address: Department of Chemistry, Faculty of Science, Gakushuin University, Mejiro 1-5-1, Toshima-ku, Tokyo 171, Japan. In this paper, an example of a 3D network of ferromagnetic intermolecular interaction is presented for p-fluorophenyl nitronyl nitroxide (p-FPNN). The crystal structure of this compound is first described, with emphasis on its dimeric character and its three-dimensionality. From the analysis of the static magnetic results, it can be seen that both intra- and inter-dimer interactions are ferromagnetic and that the system is thus ferromagnetic in three dimensions. The molecular packing is compared with that of other related compounds. Intermolecular interactions between the NO group and the phenyl ring are suggested as favourable for ferromagnetic coupling.It is found from electron paramagnetic resonance (EPR) measurements that structural distortion occurs below ca. 100 K. The change in crystal symmetry and lattice param- eters were analysed by means of powder X-ray diffraction techniques. Experimental Sample Preparation p-FPNN was prepared basically by following the route given in the literature (Scheme l).7,8 Slight modifications to the firstg and the second procedures'O in Scheme 1afforded higher yields (see Appendix). Dark-blue single crystals of p-FPNN were obtained by slow evaporation of a concentrated solution. The crystals from the solutions of various organic solvents, hexane, diethyl ether, benzene etc., were examined by means of the X-ray Weissenberg photograph.No polymorphism was found for room temperature crystal growth. Crystals obtained from a hexane solution were used for all the experiments. X-Ray Crystallographic Analysis X-Ray intensity data at 298 K were collected on an MAC Science automated four-circle diffractometer with graphite monochromatized Cu-Ka radiation by 8-20 scans up to 20 =130". Unit-cell parameters were determined from the least-squares refinement for 20 reflections within 56" <28 < 60". The independent 1782 reflections with (F,I >4a((F, ) were used for structural analysis. The structure was solved by the direct method and refined by the full-matrix least-squares procedure with an anisotropic approximation for non- J.MATER. CHEM., 1994, VOL. 4 Scheme 1 hydrogen atoms. An analytical absorption correction was performed." All the procedures were carried out with the 'Crystan' program package of MAC Science. Static Magnetic Measurements The magnetization below 10 K for the field up to 55 kOe and the susceptibility from 1.8 to 300K were measured using a Quantum Design MPMS SQUID magnetometer. The field was applied parallel (HIlc) or perpendicular (Hlc) to the c-axis. The susceptibility below 10 K was measured under a small applied field (500 Oe) to avoid saturation effects as far as possible. The diamagnetic contribution was estimated by fitting the data above 200 K to the Curie law. EPR Measurements EPR spectra of single crystals were measured by using a JEOL JES-FElXG EPR spectrometer (X band) with rotation of the crystal around the crystallographic axes.The sample was placed at the centre of a cylindrical cavity in the TEoll mode. The microwave magnetic field, H,, was always in the vertical direction and the static field, H, was in the horizontal direction. The sample was rotated around the vertical axis. An Air Products LTR-3-110 helium flow type cryostat was used for low-temperature measurements of down to 2.2K. The spectra were recorded at various temperatures on the heating process. The crystallographic axes were checked by means of the X-ray Weissenberg photograph every time before and after the low-temperature experiments. Powder X-Ray Diffraction Measurements Powder X-ray diffraction data were collected using an MAC Science MXP18 system with a rotating anode generator and a monochromator of single crystalline graphite for Cu-Ka radiation. A CIT closed cycle helium gas refrigerator model 22C was used for low-temperature measurements of down to 10 K.Powdered silicon was added as an internal standard for diffraction angles. Results and Discussion Crystal Structure p-FPNN crystallizes in the tetragonal system (space group 141/a). The crystallographic data and the final positional parameters are summarized in Tables 1 and 2. The unit cell contains 16 crystallographically equivalent molecules. Fig. 1 shows an ORTEP12 drawing of the molecule. The O(l)-N(3)-C(5)-N(4)-0(2) moiety is planar within 0.014 A.Table 1 Crystallographic data formula formula weight crystal size/mm crystal system spaoce group vik3 a/+ CIA z DJg cm -radiation scan mode total reflections measured unique reflections reflections used (IFol>4o(lF,I)) residuals: R; R," goodness of fit: S C,,H,,N,O,F251.30 0.25 x 0.40 x 0.30 tetragonal 14,la 5343 [21 21.931 [3] 11.110 [2] 16 1.25 Clu-Kx(i= 1.54178 A 8-28 2566 2214 1782 0.061; 0.086 2.32 "The function minimized was sum [W(IF~~~-(F~(~)~],in which w = [(olFo +0.00061F012] -1)' Theodeviationof C(12) and C(13) from this plane is about 0.1 A. The dihedral angle between the best planes of the ONCNO moiety and the phenyl ring is 36.0".According to EPRI3 and nuclear magnetic resonance (NMR)14 measurements, most of the spin densities of 2-nitronyl nitroxide radicals are concentrated on the ONCNO moiety. Therefore, in the following, attention is directed mainly toward the structural features related to the ONCNO moieties. Fig. 2 and 3 display the molecular packing in the crystal viewed along the c and b axes, respectively. Short inter- molecular atomic distances are selected in Table 3, in which interatomic distances concerning the methyl groups are omitted. As can be seen in Fig. 2, there is a noticeable dimeric structure. Short intermolecular contacts linking the NO group and the phenyl ring are found between the two molecules in the dimer. The relevant intermolecular atomic distancfs are 0(2)..-C(lO), 3.117[4] A and 0(2)...H(10), 2.23[4] A, the latter being shorter than the sum of the van der Waals radii.The intermolecular contacts are doubled as a result of the inversion symmetry at the centre of each dimer. A dimer is surrounded by four nearest-neighbours. Between the neighbouring dimers, ,a relatively short intermolecular atomic distance, 3.538[3] A, is found, which is between the terminal oxygen, O(l), of molecule i (x,y, z) and the bridge- head carbon atom of the ONCNO moiety, C(5), of molecule iii (1/4+7, -1/4+x, -1/4+z). As shown in Fig. 3, each partner of the dimers arrayed along a four-fold screw axis can interact with one another by the above relation. J. MATER. CHEM., 1994, VOL. 4 1221 Table 2 Positional parameters and equivalent isotropic thermal parameters with standard deviations in square brackets atom x Y Z 0(1) 0.25328 [8] 0.0985 [l] -0.0046 [2] 7.42 [7] O(2) 0.46249 [9] 0.1094 [11 0.0131 [3] 9.15 [9] N(3) 0.30725 [9] 0.1177 [l] 0.0171 [2] 5.17 [6] N(4) 0.4059 [l] 0.1224 [l] 0.0277 [a] 5.80 [6] C(5) 0.3593 [1] 0.0898 [1] -0.0139 [2] 4.69 [6] C(6) 0.3640 [l] 0.0325 [l] -0.0803 [2] 4.61 [6] C(7) 0.3219 [1] 0.0180 [l] -0.1696 [2] 5.16 [7] C(8) 0.3251 [2] -0.0368 [l] -0.2303 [31 6.01 [8] C(9) 0.3698 [l] -0.0766 [l] -0.1987 [3] 6.10 [8] C(10) 0.4123 [1] -0.0646 [l] -0.1123 [3] 6.45 [9] C(11) 0.4098 [l] -0.0094 [l] -0.0527 [3] 5.62 [8] C(12) 0.3177 [1] 0.1729 [l] 0.0936 [31 5.79 [S] ~(13) 0.3876 [ij 0.1796 [i] 0.0879 [31 6.57 [9] C(14) 0.2942 [2] 0.1561 [2] 0.2179 [4] 9.0 [l] ~-X a C(15) 0.2822 [2] 0.2256 [2] 0.0390 [61 9.3 c21 C(16) 0.4100 [3] 0.2306 [2] 0.0011 [7] 12.6 [2] Fig.2 Crystal structure of p-FPNN viewed along the c axis, showing C(17) 0.4200 [3] 0.1857 [5] 0.2056 [6] 14.0 [3] the relation between the dimers. Symmetry operations: i (.x, y, z),F( 18) 0.3723 [11 -0.13187 [9] -0.2558 [2] 8.97 [7] ii( 1+X,F,Z), iii (1/4+J, -1/4 +x,-1/4+z). The closest intradimer H(7) 0.290 [l] 0.047 [2] -0.185 [3] spacing it indicated by the dotted line together with the interiitomic H(8) 0.296 [2] -0.048 [2] -0.293 [3] distance/A.H(10) 0.444 [2] -0.094 [2] -0.088 [3] H(11) 0.437 [2] 0.002 [2] 0.011 [3]H(14A) 0.305 [2] 0.188 [2] 0.267 [4]H(14B) 0.250 [2] 0.151 [2] 0.210 [4]H(14C) 0.314 [2] 0.114 [2] 0.252 [4]H(15A) 0.292 [2] 0.267 [2] 0.090 [4]H(15B) 0.240 [2] 0.210 [2] 0.063 [4] H(15C) 0.303 [2] 0.231 [2] -0.047 [5] H(16A) 0.407 [3] 0.258 [3] 0.078 [6] H(16B) 0.382 [3] 0.223 [3] -0.052 [7] H(16C) 0.459 [3] 0.229 [3] -0.015 [6] H(17A) 0.405 [3] 0.212 [3] 0.251 [7] H(17B) 0.405 [3] 0.144 [3] 0.247 [6] H(17C) 0.459 [3] 0.191 [3] 0.184 [16] 0 Fig.3 Crystal structure of p-FPNN projected onto the bc plane. The closest intra- and inter-dimer spacing are indicated by the dotted an9 broken lines, respectively, together with the interatomic distarrces/A. The symmetry operation notation is the same as that in Fig. 2 Table 3 Short intermolecular atomic distances with standard devi- Fig.1 ORTEP drawing of the p-FPNN molecule showing the atom- ations in square brackets‘ numbering scheme. For simplicity, the hydrogen atoms of methyl ib ... groups are not shown. 11 distance/A 1 111 distmce/W ~~~ ~ ~ O(2) C(10) 3.117 [4] 0(1) C( 5) 3.538 [3] Fig. 4 is a schematic view of the unit cell, where the open O(2) C(11) 3.583 [4] O(1) C(11) 3.576 [4] C(7) C( 6) 3.587 [3] circles and the ‘bonds’ represent the dimers and the interdimer C(7) C( 7) 3.606 [2] interactions, respectively. It can be seen that dimers form a C(7) C(11) 3.706 [4] 3D network by the four-fold coordination of the interdimer C(l) C( 6) 3.718 [3] interaction. “Atomic distances concerning the methyl groups are omitted. bi (x, y, z);ii (l+X, -6 3;iii (1/4+Y, -1/4+x, -1/4+z).Static Magnetic Measurements The molar diamagnetic susceptibility was estimated to be The temperature dependence of the paramagnetic suscepti- -1.70 x emu mol-’ (= -2.14 x m3 mol-l) for Hllc bility is shown in Fig. 5. The Curie-Weiss law reproduces the and -1.50x lop4emu mol-’ for Hlc. observations above 25 K with the Curie constant, C,= The paramagnetic susceptibility (x,) for Hlc was slightly 0.375 emu K mol-’ and the Weiss constant, 0,=2.5 K. The (about 0.3%) larger than that for HJlc above 10 K. The Curie constant agrees with the existence of 1 mol of S= 1/2 anisotropy gradually increased below 10 K with decreasing species. The positive Weiss constant indicates the ferromag- temperature. For Hlc, xp was larger than that for Hllc by netic interactions between these spins. Below ca.lOK, the only about 1% even at the lowest temperature. Since this value of the susceptibilities are close to the prediction by the anisotropy plays no substantial role in the following dis- Curie law with C =0.5 emu K mol-’, suggesting the formation cussion, we refer only to the Hl\c data in this paper. of 1/2 mol of S =1 (triplet) species. A 0 a Fig. 4 Schematic display of the 3D network of the dimers. The circles and ‘bonds’ stand for the dimers and the interdimer interactions, respectively. The hatched circles and the filled circles are correspond- ing to the dimers depicted in Fig. 2 and 3. ’,I. s = 112 Curie-Weiss law e =2.5 K I-E E’ %* 1o4 1 10 100 TIK Fig.5 Paramagnetic susceptibility (x,) plotted against temperature The formation of triplet species at low temperature is also supported by the magnetization data. The magnetization curves below 10K are shown in Fig. 6, together with the theoretical curves for non-interacting S =1 and 1/2 species calculated on the basis of the expression, WHIT)=(N4/2S)gP,SBs(x) (1) where B,(x)is the Brillouin function and x=gSpBH/kBT.It is evident that the curves at 1.8 and 4.0 K are explained by eqn. (1)with S=l. Taking account of the dimeric structure of this material, we surmise that the two S= 1/2 spins form a triplet state within 6000F T 4000 E J. MATER. CHEM., 1994, VOL. 4 each dimer below ca. 4 K. Namely, the intradimer intermolecu- lar interactions are considered to be ferromagnetic. The plot of xpTversus T (Fig.7) is a convenient tool to find magnetic interactions; xpT is proportional to the Curie constant or to the square of the so-called effective moment. Starting from the value for non-interacting S =1/2 spins, 0.375 emu K mol-’, xpTincreases as T decreases. Below ca. 5 K, the increase in xPT is suppressed, suggesting that the triplet formation completes around this temperature. However, xpT continues to increase even at the lowest tem- peratures, and there is a large excess over the value for the S= 1 dimers (0.5 emu K mol-l) below about 3 K. It follows that there exist additional ferromagnetic interactions between the dimers from these observations. In view of the molecular arrangement in the crystal, a model can now be introduced to describe the whole tempera- ture dependence of the susceptibility. The mean-field treatment of the weak interdimer interactions yields xp=7T-8‘i3+exp(-2J/k,T) -1 where J denotes the intradimer interactions and C =0.5 emu K mol-’.The factor, 3/[3 +exp(-2J/kBT)], describes the temperature dependence of the number of the triplet species. The mean-field parameter, 8’, is related to the interdimer exchange coupling, J’, by 8’=2zJ’S(S+1)/3kB (3) where S =1 and z is the number of the nearest-neighbour dimers. As each dimer is surrounded by four dimers in the crystal, it is reasonable to assume z =4. In Fig. 7, the fit of this model to the experimental data is shown by the solid curve.The best fit is obtained, when J/kB= 5.0 K and 8’=0.1 K are used. The interdimer inter- action is weak enough to justify the application of this model to the present case; the validity of the mean-field approxi- mation holds when T>>8’and J/kB>>@. By putting 8’=O.l K into eqn. (3), the interdimer interaction, J’/kB,is estimated to be about 0.02 K. Only this assignment of the interactions to the intra- and inter-dimer ones is consistent with the molecular arrangement at 298 K. Let us now briefly summarize the magnetic state of the system as a function of temperature. As all the intermolecu- lar magnetic couplings are thermally disturbed above ca. 100 K, the system behaves as an assembly of independent S= 1/2 spins.The triplet species are formed within the dimers to raise xpTon cooling from 100 K, as a result of the intradimer interactions (J/kB=5.0K). At ca. 4 K, almost all the dimers are triplets. Further cooling makes the triplets interact ferro- 0.55~ 0 0 10 20 30 HT -l/koeK-l Fig. 7 Temperature dependence of xpT.The solid curve representsFig. 6 Magnetization curves. Solid curves are theoretical ones based the values calculated on the basis of eqn. (2) with J/k, =5.0 K and on the Brillouin function. 0, 1.8 K; 0,4 K; 0,10 K. 8‘=0.1 K. J. MATER. CHEM., 1994, VOL. 4 magnetically with each other, which affords an extra increase in x,T. It should be noted that the interdimer interactions form a 3D network as can be seen from the crystal structure (Fig.4). This would allow the system to exhibit ferromagnetism at very low temperatures. The mean-field parameter, 8’=0.1 K, is the upper limit of the transition temperature.? Magneto-structural Correlation Let us consider the correlation between the magnetic inter- actions and the molecular arrangements. The present data show that the dimer is responsible for the ferromagnetic exchange coupling, J/k, =5.0 K. This interaction is charac- terized by the close spacing between the terminal oxygen of the nitroxide group in the one molecule and the phenyl-side hydrogens and carbons in the other. Hereafter, we write this type of interaction as the NO..-Ar interaction, where Ar stands for an aryl group. We have also shown that the interdimer interaction is responsible for the additional ferromagnetic exchange coup- ling, J’/k, =0.02 K.The close spacing of the terminal oxygen of the nitroxide group in the one molecule and the bridgehead central carbon in the other is characteristic of this interaction. This conformation is expected to be stable from the viewpoint of electrostatic energy, which is potentially useful in designing the molecular form preferable to ferromagnetic interaction. Similar molecular arrangements have also been found in the crystals of p-~hlorophenyl,’~ p-iodophenyl16p-br~mophenyl,’~ and ~yrimidinyl’~ nitronyl nitroxides. However, the sign and amplitude of the exchange inter- actions related to this arrangement is very sensitive to the detailed structure.For example, the p-chloro derivative shows an antiferromagnetic interaction, whereas the p-bromo and p- iodo derivatives exhibit ferromagnetic couplings. This suggests that the observed net interactions are determined by the competition between the interactions of different signs; the interaction between the terminal oxygen and the central carbon and that between the oxygen and other atoms in the phenyl ring may compensate each other. Therefore, more improvement in the design of the molecular structure is needed in order to control this type of interaction efficiently. The NO-..Ar interactions are also found in the 8-,18,19y-,’ and of p-NPNN. All of them are known to exhibit ferromagnetic interactions.A 8-p-NPNN crystal consists of layers of dimers with the dimer structure similar to that of p- FPNN. A 7-p-NPNN crystal is composed of molecular chains along the [Oll] direction and the molecular contacts along the chain are also similar to those in the p-FPNN dimer. All these NO.. .Ar interactions are accompanied by the inversion symmetry at the centre of the dimer. The NO.--Ar interaction is also observed in a P-p-NPNN crystal, but there is no inversion symmetry in the P-phase ~rysta1.l~ These three phases of p-NPNN have another common structural feature, i.e. the intermolecular interactions between the nitroxide (NO) and nitro (NO,) groups. The ferromagnetic couplings in the p-hydroxyphenyl nitronyl nitroxide,,’ is also related to the interaction between the NO group and the para substituent (OH group).Therefore, the role of each molecular packing in affording the ferromagnetic couplings in the three phases cannot be uniquely concluded. However, the definite corre- spondence between the dimeric packing and the triplet forma- tion in p-FPNN enables us to conclude that the NO--.Ar interactions should bring ferromagnetic couplings. This t No magnetic phase transition was detected by heat capacity measurements down to 0.4K. The authors acknowledge Dr Y. Nakazawa and Prof. M. Ishikawa for the heat capacity measurements. conclusion ensures the three-dimensionality of P-p-NPNN, which is required for its bulk ferromagnetism,1,2,18 without the NO.-.Ar interactions P-p-NPNN is a two-dimensional ~ystern.’~ The close intermolecular spacing between the NO group and the aromatic ring has also been found in p-PYNN2, (p-pyridyl nitronyl nitroxide), which is reported to evhibit ferromagnetic intermolecular interactions.We thus point out that the NO..-Ar interaction is one of the key factors for the ferromagnetic interactions in a series of aryl nitronyl nitroxide derivatives. The magnitude of the ferromagnetic coupling due to the NO.-.Ar interaction ranges from J/k,=0.27 K in p- PYNN2, to J/k,=5.0 K in the present case; it is sensitive to subtle change in the geometry in the crystal. A sizeable enhancement of this type of ferromagnetic interaction has been achieved by use of cation radical salts based on pyridin- ium derivatives of nitronyl nitr~xide.,~ These finding would be useful in molecular designing for the search of an organic ferromagnet and should be chvcked by further investigations.EPR Measurements The principal values of the EPR g factor at 298 K were gaa(=gbb)=2.0080 and g,, =2.0034. A Lorentzian shaped spectrum was observed at 298 K. The peak-to-peak line width, AH,,, was about 4G, being almost independent of the direction of the static field. The temperature dependence of the EPR spectra was exam- ined down to 2.2 K. Observed were only Am= 1 transirions. The directions of the three principal axes were retaincd at any temperature. Fig. 8 shows an example of the temperature dependence of the g factor. The inequivalence between the HIJa’and H Ilb’ results clearly indicates that the tetragonal symmetry is no longer retained below about 100 K.As discussed in the next section, evidence for the lowering of the crystal symmetry is given by the powder X-ray diffraction measurements. The low-temperature shifts of the g factor are different from sample to sample. The line shape always becomes asymmetric below 10 K. Most puzzling are the H(lc results; the g shift goes upwards or downwards even for the same specimen on different runs. These features are due probably to twinning in the crystal at low temperatures. Nevertheless, it is certain for any crystals examined that the departure of the g factor for Hlla’ from that of Hllb’ starts at ca. 100K. Fig. 9 shows the temperature dependence of AH,, obtained from the spectra for the g values in Fig.8. All the data have similar temperature dependence, so that AHpp is not con-sidered to be seriously influenced by the structural change. Below 100 K, AHpp gradually increased with decreasing T down to ca. 20 K, where AH,, becomes about twice as large as that at room temperature, followed by a rapid increase below 10K. The temperature dependence of AHpp is thus characterized by the two anomalies around 100 and 10 K. By comparison with the susceptibility results, the first anomaly can be related to the beginning of the triplet formation Blithin each dimer, while the second anomaly possibly reflect$ the growth of short-range magnetic order as a result of interdimer interactions.Powder X-Ray Diffraction Measurements As was indicated by the EPR measurements, the tetragonal symmetry is lost at low temperature. This structural change was investigated by assessing the temperature dependence of the powder X-ray diffraction patterns. The diffraction patterns at 298 and at 10K are shown in J. MATER. CHEM., 1994, VOL. 4 2.020"'-4b 2ob 30 2.0001d 4 0 50 100 150 300 0 50 100 150 300 t 9 2.00017;:oO po; , o1000,, o1500,:: ,200I 0 50 12.0034 0 50 100 150 )*O 2.020 ' ' ' ' ' 'I ' '1 " ' 1 ' .t30!72ok 2*010-0. 0 0. 00 0 0 . . 2.0080 ~,0@1008800 000 0 0 0 2.0034 0 50 100 150 300 TIK Fig. 8 Temperature dependence of the principal values of the EPR g factor within the ab, bc and ca planes of the same specimen; Hlla' (@), Hllb' (A)and Hllc (0).The a' and b' axes are the principal axes within the plane perpendicular to the c axis at low temperatures; the minimum and maximum of the g factor were observed for Hila' and Hljb', respectively.Fig. l0.t Splitting of peaks was clearly observed at 10 K, indicating the breaking of the tetragonal symmetry. However, no marked extra peaks were detected; they are possibly too weak to be observed. No peak contradicts the extinction rule for body-centred (I-) lattice. And the splitting does not seem to affect the intensity of each peak. These suggest that the change in the structure is not drastic. The cell parameters were refined by means of the peak- profile fitting24 of the whole diffraction pattern using the data within 7"<20<35". In order to index all the observed peaks including the split ones, /i'# 90" is required.Among the subgroups of 14,/a, only the triclinic ones satisfy this require- ment. Therefore, it is appropriate to assume the triclinic lattice for the low-temperature structure. The final parameters obtained from the data at 10 K are reported in Table4(a), together with those at 298 K. The solid lines in Fig. 10 represent the fitting results. Fig. 11 shows the single peak profiles of the 211 diffraction at 298 K and that at 10K, which shows the splitting into the 211 and 121 diffractions. The structural change is not abrupt but gradual, as we had anticipated from the EPR results. In Fig.12 the full widths of t All the diffraction patterns for the sample kept for 2 h after puri- fication are indexed on the basis of the room-temperature crystal structure. However, standing of samples for a few days often gives rise to impurity peaks, whose positions are not reproducible. OOd0 50 100 150 300 T/K Fig.9 EPR linewidth (AHpp) as a function of temperature. The notation of the axes is the same as that in Fig. 8. half maxima (fwhm) versus temperature are plotted for some diffraction peaks. The broadening of these peaks below about 100 K is obvious. Some of them shows splitting at 20 K. The comparison of the cell parameters at 298 and 10 K is given in Table 4(b). In addition to the smallness of the changes in the cell parameters, it is also recognized that almost all the peaks obey the extinction rules for the I-lattice.From this result, we surmise that the characteristic dimeric structure of p-FPNN established at 298 K basically holds at low tempera- tures. We conclude that the essence of the correspondence between the structure and the magnetic interactions in p-FPNN is not influenced by the structural change. Conclusions The crystal structure and the magnetic properties of p-FPNN have been investigated. The molecules are arranged in a dimeric fashion in order to afford a 3D network. Two types of ferromagnetic intermolecular interactions have been found from the static magnetic measurements. The intradimer inter- actions are responsible for the formation of the triplet species at low temperatures.In addition, the interdimer interactions bring about the 3D ferromagnetic network below 4 K. From the temperature dependence of the EPR signals and the powder X-ray diffraction patterns we have found a slight structure deformation below 100 K. The assignment of the couplings to the molecular packing has lightened a magneto-structural correlation. The inter- actions between the NO group and the phenyl ring found J. MATER. CHEM., 1994, VOL. 4 15000 10000 5000 u)c. t 035! 10 15 20 15 20 2Bldegrees Fig. 10 Powder X-ray diffraction patterns at (a) 298 K and (b) 10 K for the same specimen. Solid curves represent the fitting results. Ranges of 7.56" d 28 d 7.64" at 298 K and 7.58" d 28 d 7.68" at 10 K were excluded from the final refinement, because of impurity peaks appeared at 7.58" and 28= 7.62', respectively, see the footnote to the text.Table 4 (a) Cell parameters determined from powder X-ray diffraction data with standard deviations in square brackets. (b) Temperature variation of cell parameters between 298 K and 10 K (4 cell parameter 298 K 10 K 44 21.927 [2] 21.795 [3] hi+ -21.888 [3] CIA 11.1113 [7] 10.860 [11 xldegrees -90.10 [l] Pldegrees -89.24 [l] ?;/degrees __ 89.86 [2] X Ax" IAxl/x(198 K) -0.132 A 6.0 x 10-3 -0.039 4 1.8 x 10-3 -0.251 A 22.6 x 10-3 0.10" 1.1 x 10-3 -0.76" 8.4x 10-3 -0.14" 1.6 x 10-3 ~ "Ax=x(~OK)-x(298 K). within the dimer are suggested to be a key factor for bringing ferromagnetic couplings.The architecture of p-FPNN crystal would give insight into construction of a new type of 3D network of ferromagnetic intermolecular interactions. The authors thank a referee for his profound and constructive comments, which helped us to clarify the symmetry of the low-temperature crystal structure. This work was supported by the Grant-in-Aid for Scientific Research on Priority Area 'Molecular Magnetism' (Area No. 228/04242103) from the 2Wdegrees Fig. 11 Peak profiles of the 21 1 diffractions at (a) 298 K and (h)10 K 0 0 50 100 300 TIK Fig. 12 Temperature dependence of the full widths of half maxima (fwhm) of peaks 101 (O), 21 1 (0)and 222 (A).For split peaks, the values of the sum of the actual fwhm and the difference in the diffraction angles for the hkl and kh 1 are used.Ministry of Education, Science and Culture, Japaii. The authors are grateful to Dr. D. Shiomi and Mr. K. Kozawa for valuable discussions. Appendix Preparation of Compounds 2 and 3 Compound 2 can be prepared by a one-pot operation \x ithout isolating the oily intermediate 2-iodo-2-nitropropane. To a solution of 80 g of sodium hydroxide (2.0 mol) in water-ethanol (900 cm3+250 cm3), 180 cm3 of 2-nitropropane (1) (2.0mol), 200 g of iodine (0.79 mol) and 1OOg of sodium iodide (0.67mol) were added. The mixture was refluxed for 1 h and then cooled to 0°C to yield a white crybtalline 1226 J. MATER. CHEM., 1994, VOL. 4 precipitate of compound 2.The precipitate was filtered off, washed with water until the washings became colourless and then dried. The typical yield was 85%. It is possible to extract compound 3 directly from the aqueous slurry resulting from the zinc reduction of 2 with a 5 6 T. Sugano and M. Kinoshita, J. Chem. Phys., 1986,57,453; Chem. Phys. Lett., 1987, 141, 540. G. Chouteau and C1. Veyret-Jeandey, J. Phjs. (Paris), 1981, 42, 1441. M. Kamachi, H. Sugimoto, A. Kajiwara, A. Harada, Y. Morishima, W. Mori, N. Ohmae, M. Nakano, M. Sorai, satisfactory yield, although a route through the hydro-chloride salt of 2 has been de~cribed.~ In a solution of 54g of ammonium chloride (1.0 mol) in water-methanol (180 cm3+450 cm3), 90 g of 2 (0.51mol) was suspended. The mixture was kept below 20 "C with vigorous stirring during slow addition of 200 g of zinc powder (3.1 mol) during ca.7 8 T. Kobayashi and K. Amaya, Mol. Cryst. Liq. Cryst., 1993, 232, 53; T. Kobayashi, M. Takiguchi, K. Amaya, H. Sugimoto, A. Kajiwara, A. Harada and M. Kamachi, J. Phys. SOC.Jpn., 1993, 62, 3239. E. F. Ullman, J. H. Osiecki, D. G. B. Boocock and R. Darcy, J. Am. Chem. SOC., 1972,94,7049. J. Goldman, T. E. Petersen and K. Torssell. Tetrahedron, 1973, 3 h. After the temperature ceased to increase, the stirring was continued at room temperature for an additional 3 h. The mixture was then filtered. The separated zinc compound was washed with methanol (3 x 100 cm3). The washings were combined with the filtrate and concentrated under reduced pressure, until the methanol was entirely removed.Further removal of water by evaporation is usually accompanied by 9 10 11 12 13 14 29, 3833. L. W. Seigle and H. B. Hass, J. Org. Chem., 1940, 5, 100. M. Lamchen and T. W. Mittag. J. Chem. SOC.C, 1966,2300. C. Katayama, Acta. Crystallogr., Sect. A, 1986,42, 19. C. K. Johnson, ORTEPII, Report ORNL5138, Oak Ridge National Laboratory, Oak Ridge, TN, 1976, vol. 22, p. 833. J. A. D'anna and J. H. Wharton, J. Chem. Phjs., 1970,53,4074. J. W. Neely, G. F. Hatch and R. W. Krilick, J. Am. Chem. Soc., appreciable sublimation of 3. Instead, the residual slurry was moderately dried by adding anhydrous sodium carbonate; the loss of 3 due to sublimation was thus avoided as far as possible. From this, 3 was extracted with dichloromethane using a Soxhlet apparatus for 2 days.The extract was concen- trated to yield crude 3 as a pale-yellow creamy material. 15 16 17 1974,96,652. M. Tamura, D. Shiomi, Y. Hosokoshi, N. Iuasawa, K. Nozawa, M. Kinoshita, H. Sawa and R. Kato, Mol. Crjst. Liq. Cryst., 1993, 232,45. Y. Hosokoshi, M. Tamura, H. Sawa, R. Kato and M Kinoshita, in preparation. F. L. de Panthou, D. Luneau, J. Laugier and P. Rey, J. Am. Chem. Triturating this in cold diethyl ether left sufficiently pure 3 as a white solid, with the impurity, amines, washed off. The typical yield was 40%. 18 19 SOC.,1993, 115,9095. M. Kinoshita, Mol. Cryst. Liq. Cryst., 1993,232, 1. K. Awaga, T. Inabe, Umpei. Nagashima and Y. Maruyama, J. Chem. SOC., Chem. Commun., 1989,1617; 1990,520. 20 P. Turek, K. Nozawa, D. Shiomi, K. Awaga, T. Inabe, . References Y. Maruyama and M. Kinoshita, Chem. Phys. Lett., 1991, 180, 327. 1 M. Tamura, Y. Nakazawa, D. Shiomi, K. Nozawa, Y. Hosokoshi, 21 E. Hernandez, M. Mas, E. Molins, C. Rovira and J. Veciana, M. Ishikawa, M. Takahashi and M. Kinoshita, Chem. Phys. Lett., 1991,186,401. 22 Angew. Chem., Int. Ed. Engl., 1993,32,882. K. Awaga, T. Inabe and Y. Maruyama, Chem. Phys. Lett,, 1992, 2 Y. Nakazawa, M. Tamura, N. Shirakawa, D. Shiomi, 190, 349. M. Takahashi, M. Kinoshita and M. Ishikawa, Phys. Rev. B: Condens. Matter., 1992,46, 8906. 23 K. Awaga, T. Inabe, Y. Maruyama, T. Nakamura and M. Matsumoto, Chem. Phys. Lett., 1992,195,31. 3 R. Chiarelli, M. A. Novak, A. Rassat and J. L. Tholence, Nature 24 H. Toraya, J. Appl. Crystallogr.. 1986, 19,440. (London), 1993,363,147. 4 K. Mukai, Bull. Chem. SOC. Jpn., 1969, 42, 40; K. Awaga, Paper 3/06820J; Received 15th Nouember, 1994