Crystal structures and magnetic properties of acid–base molecular complexes, (p-pyridyl nitronylnitroxide)2X (X=hydroquinone, fumaric acid and squaric acid) Takeo Otsuka,a Tsunehisa Okuno,a Kunio Awaga*,a,c and Tamotsu Inabeb aDepartment of Basic Science, Graduate School of Arts and Sciences, T he University of T okyo, Komaba,Meguro, T okyo 153, Japan bDivision of Chemistry, Graduate School of Science, Hokkaido University, Sapporo 060, Japan cStructure and T ransformation, PRESTO, Japan Science and T echnology Corporation (JST), Japan The reactions of 2-(4-pyridyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-1-oxyl 3-N-oxide (or p-pyridyl nitronylnitroxide, abbreviated as p-PYNN) with the three dibasic organic acids, X [=fumaric acid (FA), squaric acid (SA) and hydroquinone (HQ)], result in the formation of hydrogen-bonding complexes of (p-PYNN)2 X composition.In their crystals, the organic acids make selective hydrogen bonds to the two kinds of hydrogen-bond accepting sites in p-PYNN; (a) the oxygen atom in the NO group and (b) the nitrogen atom of the pyridyl ring. p-PYNN2 HQ crystallizes in the monoclinic P21/n space group. The HQ molecule bridges two p-PYNNs, and selects site (a) in p-PYNN as the hydrogen bond acceptor [i.e.( p- PYNN)NO HO(HQ)OH ON( p-PYNN)]. p-PYNN2 FA crystallizes in the monoclinic P21/n space group. The FA molecule connects two p-PYNN molecules with an intermolecular hydrogen bond to site (b) [( p- PYNN)N HO(FA)OH N(p-PYNN)]. The 251 compound of p-PYNN and SA crystallizes with the crystal solvent, 1,4- dioxane (abbreviated as diox), in the formula for p-PYNN2 SA diox.The crystal belongs to the triclinic P19 space group. The SA molecule occupies the space between two p-PYNNs, making contact with site (b), as FA does in the p-PYNN2 FA crystal. However the structure of SA indicates that it is a dianion in which the two protons are missing and, thus, the hydrogen bond is ionic [(p-PYNN)NH+ O-(SA)O- H+N(p-PYNN)].The selectivity and features of the hydrogen bonds can be qualitatively understood in terms of competition between the electrostatic and charge-transfer terms in the hydrogen-bonding energy, which is governed by the acidity of the organic acids and the proton accepting abilities of the two sites in p-PYNN.The three molecular compounds exhibited diVerent antiferromagnetic properties, which depend on the mutual arrangement of p- PYNN in the crystals. The intermolecular interactions were interpreted based on the McConnell’s spin polarization mechanism. It is widely accepted that hydrogen bonding can influence intermolecular arrangements in the condensed state containing organic molecules and recently, this has been recognised as a key factor in supramolecular chemistry1 for designing and synthesizing molecular aggregates with fundamental structural, symmetry, topology, and network properties, and for applications,2 such as non-linear optics, ferromagnetism, ferroelectricity, liquid crystals, and electronics.Hydrogen bonding is not only the main factor in determining solid state structures but is also the microscopic origin of physical properties.3 It is important to study the patterns, topology, selectivity, and the strength of hydrogen bonds, that are dependent on the hydrogen-bonding abilities of organic functional groups and on the steric (size and shape) eVects of molecules.While most research has been based on the measurement of solution association constants under equilibrium conditions,4 extraction of the hydrogen-bonding patterns from a wide variety of crystallographic data was performed by Etter et al.5 Crystallographic studies are advantageous in the analysis of hydrogen-bond selectivity in complex systems such as the 2-aminopyrimidinecarboxylic acid compounds, which involve multiple hydrogen bonds.5 In the field of molecular magnetic materials, the idea of making use of intermolecular hydrogen bonds to get ferromagnetic intermolecular interactions, has been tested by Veciana6 and Sugawara7 et al.In addition, we have observed the coexistence of organic radicals and naked protons in the solid state: the reaction of 2-(3-pyridyl)- or 2-(4-pyridyl)- 4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-1-oxyl 3-N-oxide (abbreviated as m- or p-PYNN, respectively) and HBr gas leads to the 251 complex, (m-PYNN)2 HBr or (p-PYNN)2 HBr.8 Although single crystals were not obtained, IR spectra strongly indicated proton sharing between the two pyridyl rings, i.e.an intermolecular [NHN]+ hydrogen bond.This type of situation may lead to cooperative phenomena between magnetism and proton dynamics in the future. In this paper, we describe the crystal structures and the magnetic properties of three acid–base molecular compounds: p-PYNN2 HQ, p-PYNN2 FA and p-PYNN2 SA diox. We analyzed their crystal structures, focusing on selectivity and the features of the hydrogen bonds.The observed selectivity in the hydrogen bonds was interpreted in terms of competition between the electrostatic and the charge-transfer terms in the hydrogen bonding energy. In addition, the likelihood of molecular recognition in the hydrogen-bonding formation was analyzed. J O U R N A L O F C H E M I S T R Y Experimental p-PYNN was prepared according to the procedure previously described.9 HQ, FA and SA were of reagent grade and were used as commercially obtained.The molecular compounds, p- PYNN2 HQ and p-PYNN2 FA were prepared as follows: p- PYNN and HQ (or FA) in 251 proportion were dissolved in acetone at 40 °C, and their crystals were grown with slow evaporation of the solvent at 0 °C.The molecular compound of p-PYNN and SA was obtained following the same procedure using water–dioxane (251) instead of acetone. Crystallization took place with dioxane to give p-PYNN2 SA diox. Selected IR data n:/cm-1 (KBr): 3232 (m, br; OH st.), 1596 (m; pyridyl J. Mater. Chem., 1998, 8(5), 1157–1163 1157 Materialsring st.) for PYNN (m; pyridyl ring st.) for p-PYNN2 FA; 2574 (m, br; NH st.), 1630 (m; pyridyl ring st.) for p-PYNN 2 HQ; 2440 and 1933 (m, br; OH st.), 1603 2 SA diox.X-Ray diVraction data were collected on a Rigaku AFC-5S 2 FA) automatic four-circle diVractometer 2 HQ and p-PYNN2 SA diox, or using 26 reflections (p-PYNN2 HQ and p-PYNN2 SA diox) or a Mac Science MXC18 ( p-PYNN with graphite-monochromatized Mo-Ka radiation (l= 0.71073 A°). Unit cell dimensions were obtained by a leastsquares refinement, using 25 reflections with 20°<2h<25° for p-PYNN with 30°<2h<35° for p-PYNN2 FA.During data collection, the intensities of three representative reflections were measured as a check on crystal stability, and no loss was observed. No absorption correction was performed, because the influence of the absorption was negligible.The crystal structures were solved by direct methods (SHELXS-8610 and DIRDIF11). Block-matrix-diagonalization least-squares refinement (UNICS-III12) with anisotropic thermal parameters for all non-hydrogen atoms was employed for p-PYNN2 SA diox bond. The intermolecular distance between O(1) O(18) is bridged the NO groups of separate p-PYNNs with a hydrogen and p-PYNN2 HQ.The positions of the hydrogen atoms were 2.728(5) A °. This type of hydrogen bond has been observed in found by diVerential Fourier methods and they were refined the complex of phenylboronic acid and phenyl nitronylnitroxisotropically with a fixed thermal parameter in order to avoid ide: viz. the acid bridge in the NO groups of the nitronylnitroxan excessive number of parameters.A refinement for the nonide in the solid state.15 hydrogen atoms in p-PYNN2 FA was performed by using the Table 3 shows the atomic charges on the skeleton of pfull-matrix least-squares method with anisotropic thermal par- PYNN calculated using the geometry in the bulk crystal.16 ameters.The positions of the hydrogen atoms were calculated, Table 3 also shows the calculated spin densities. As a common and were refined with isotropic thermal parameters at a characteristic of the nitronylnitroxide radical family, the posigeometrically restrained position. Details in regards to the tive spin densities are concentrated on the NO groups since crystallographic parameters are given in Table 1.Full crystallothe oxygen atom in the NO group has the largest negative graphic details excluding structure factors have been deposited charge. The spin polarization slightly penetrates the aromatic at the Cambridge Crystallographic Data Centre (CCDC). See substituent at the a carbon (the pyridyl ring) and alternation Information for Authors, J.Mater. Chem., 1998, Issue 1. Any of the positive and negative spin densities is observed on request to the CCDC for this material should quote the full the skeleton. literature citation and the reference number 1145/85. Static magnetic susceptibilities and magnetization were measured in a 1 T field on a Faraday balance, which has been previously described.13 The temperature dependence of the magnetic susceptibility was examined at 3–280 K.Molecular orbital calculations were performed with the MOPAC package,14 in order to estimate the atomic charges and the spin densities on p-PYNN. PYNN2 HQ C30H36O6N6 578.67 monoclinic P21/n P21/n P19 26.24(1) 6.806(2) 8.330(3) 94.61(3) 1480.0(9)2 2 1 1.299 1.310 1.329 Mo-Ka (l=0.71073 A °, graphite monochromator) 3658 1542 0.0677 0.0602 Crystal structures 1/n space group.The structure of the p- expected to result in an antiferromagentic coupling. Since the stacking columns are separated from each other by the HQ Table 1 Crystal data and experimental conditions for p-PYNN2 HQ, p-PYNN2 FA and p-PYNN2 SA diox p-PYNN2 HQ The 251 complex of p-PYNN and HQ crystallizes in the monoclinic P2 PYNN2 HQ unit is shown in Fig. 1, in which half of the unit compound formula formula weight crystal system space group a/A ° b/A ° c/A ° a(°) J. Mater. Chem., 1998, 8(5), 1157–1163 b(°) c(°) V /A °3 Dc/g cm-3 radiation Z2h range (°) 55 measured observed W RR1158 Fig. 1 Molecular structures in the p-PYNN2 HQ unit is crystallographically independent. The selected bond distances and angles are listed in Table 2. The HQ molecule Fig. 2(a) and (b) show projections of the crystal structure of p-PYNN2 HQ parallel and perpendicular to the b axis, respectively. In Fig. 2(b) the HQ molecules are omitted for the sake of clarity. There is a one-dimensional stacking column of p-PYNN along the b axis, in which the molecules are arranged head-to-tail.In the intermolecular arrangement, the short intermolecular distances are 3.370(7) for O(1) C(16)i and 3.347(6) A °for N(5) C(6)i [symmetry operation; (i) 0.5-x, y+0.5, 0.5-z]. The former indicates a spatial overlap between the positive spin densities on the neighboring molecules and the latter between the negative spin densities.According to McConnell’s spin polarization mechanism,17 these kinds of overlaps between polarized spin densities of the same sign are PYNN2 SA diox PYNN2 FA C32H42O10N6 670.72 C28H36O8N6 584.63 triclinic monoclinic 14.970(5) 7.552(2) 13.090(5) 8.988(2) 13.011(4) 7.894(3) 93.69(3) 92.58(3) 102.58(3) 109.82(2) 837.9(5) 1478.5(8) 554176 3555 0.0531 0.0541 604809 2250 0.0691 0.0823Table 2 Selected bond lengths (A °) and angles (°) for p-PYNN2 HQ bond length/A ° 1.277(6) 1.343(5) 1.390(7) 1.385(7) 1.385(6) O(2)MN(4) C(6)MC(13) C(15)MN(5) C(13)MC(17) C(19)MC(21) O(1)MN(3) N(4)MC(6) C(14)MC(15) C(16)MC(17) C(19)MC(20) O(1)MN(3)MC(6) N(3)MC(6)MN(4) C(13)MC(14)MC(15) C(15)MN(5)MC(16) C(16)MC(17)MC(13) O(18)MC(19)MC(21) bond angle (°) 126.4(4) 107.8(4) 118.0(4) 115.7(4) 118.8(4) 123.4(4) 119.2(4) C(19)MC(20)MC(21)i (i ) Symmetry operation; x:-1, y:, z:-1.Table 3 Atomic charge and spin density distribution on p-PYNN as obtained by the PM3-UHF method H(9a)~(9c) O(1) H H H C(9) H(14) H(15) C(14) C(15) H(10a)~(10c) N(3) C(7) H H C(10) H C(6) C(13) N(5) N(4) C(17) C(16) H C(11) C(8) H H(11a)~(11c) H H(17) C(12) H H H H(16) spin densities 0.33102 0.43052 -0.18245 -0.50487 0.02126 0.00950 0.01297 0.26929 -0.25617 0.20464 -0.00069 -0.00079 0.01451 -0.01229 O(2) atomic charges -0.53200 0.68960 -0.04680 -0.43240 -0.14115 -0.06480 -0.08155 0.03010 -0.10165 -0.06480 0.04785 0.05150 0.12180 0.10065 H(12a)~(12c) atom(s) O(1), O(2) N(3), N(4) N(5) C(6) C(7), C(8) C(9), C(11) C(10), C(12) C(13) C(14), C(17) C(15), C(16) H(9a)~(9c), H(11a)~(11c) H(10a)~(10c), H(12a)~(12c) H(14), H(17) H(15), H(16) molecules, the magnetic interaction between the columns would be very weak.p-PYNN2 FA P2 The structure of p-PYNN2 FA belongs to the monoclinic 1/n space group. The geometry of p-PYNN2 FA is shown in Fig. 3. Selected bond distances and angles are listed in Table 4. The organic acid FA occupies the space between the pyridyl rings of p-PYNNs.There is an intermolecular hydrogen bond between the OH group of FA and the nitrogen atom [i.e. N(5)] on the pyridyl ring of p-PYNN, although N(5) is less electronegative than O(1) and O(2) in the NO groups (see Table 3). The charge-transfer term would contribute to the hydrogen bond contact here, as will be discussed later. The intermolecular, interatomic distance between N(5) O(18) is 2.634(2) A °, indicating the presence of a strong hydrogen bond.The asymmetric bond distances in the CO2 moiety of FA [1.317(4) A °for C(21)–O(18) and 1.208(4) A °for C(21)–O(19)] indicate that the hydrogen on the intermolecular N O distance remains on the side of FA (i.e. OMH N). This is further supported by the fact that the IR spectrum shows two broad n(OH) stretching bands at 2440 and 1933 cm-1.Usually, 1.350(6) 1.392(6) 1.321(6) 1.284(5) 1.456(6) 1.327(7) N(3)MC(6) C(13)MC(14) N(5)MC(16) 1.383(5) O(18)MC(19) 1.401(6) 1.377(6) 125.5(4) 117.2(4) 125.5(5) O(2)MN(4)MC(6) C(14)MC(13)MC(17) N(5)MC(15)MC(14) N(5)MC(16)MC(17) O(18)MC(19)MC(20) C(20)MC(19)MC(21) 124.8(4) 116.9(4) 119.7(4) 121.0(4) C(19)MC(21)MC(20)i (i) 0.5-x, y+0.5, 0.5-z] Fig. 2 Projection of the crystal structure of p-PYNN2 HQ (a) parallel to the b axis and (b) perpendicular to the b axis [symmetry operation: Fig. 3 Molecular structures in the p-PYNN2 FA unit 1159 J. Mater. Chem., 1998, 8(5), 1157–1163Table 4 Selected bond lengths (A °) and angles (°) for p-PYNN2 FA bond length/A ° 1.279(4) 1.353(3) 1.388(4) 1.386(4) 1.208(4) O(2)MN(4) C(6)MC(13) C(15)MN(5) C(13)MC(17) C(20)MC(21) O(1)MN(3) N(4)MC(6) C(14)MC(15) C(16)MC(17) O(19)MC(20) bond angle (°) 126.2(2) 108.3(2) 118.3(3) 117.5(3) 118.6(3) 112.6(2) 122.3(3) O(1)MN(3)MC(6) N(3)MC(6)MN(4) C(13)MC(14)MC(15) C(15)MN(5)MC(16) C(16)MC(17)MC(13) O(18)MC(20)MC(21) C(20)MC(21)MC(21)i (i ) Symmetry operation; 1-x, 1-y, 1-z .Fig. 4 Projection of the crystal structure of p-PYNN2 FA onto the ab plane [symmetry operation: (i) x, y-1, z] these bands are observed in a strongly asymmetric, doubleminimum hydrogen bond in complexes of carboxylic acids and pyridines.18 Fig. 4 shows a projection of the crystal structure on to the ab plane.The p-PYNN molecules show side-by-side and headto-head stacking along the b axis. The short distances in the stacking are 3.192(3) A °for O(1) C(16)i and 3.347(3) A °for O(1) C(17)i [symmetry operation; (i) x, y-1, z]. There are hydrogen bonds between O(1) in the NO group and the hydrogen atoms, H(16)i and H(17)i, on the pyridyl ring.The contact O(1) H(16)i is an overlap between the positive and negative spin densities, while the contact O(1) H(17)i is one between the positive spin densities. The magnetic interactions appear to be competing in this arrangement. The two-dimensional sheet shown in Fig. 4 is stacked along the c axis with a screw relationship. There was no significant contact in the intersheet arrangement. p-PYNN2 SA diox The adduct crystallizes in the triclinic P19 space group. Fig. 5 shows the structure of the p-PYNN2 SA diox unit. Selected sites: (a) the oxygen in the NO group and (b) the nitrogen on J. Mater. Chem., 1998, 8(5), 1157–1163 1160 1.351(3) 1.397(3) 1.334(4) 1.274(4) 1.465(3) 1.334(4) 1.317(4) 1.307(4) 1.398(3) 1.491(4) N(3)MC(6) C(13)MC(14) N(5)MC(16) O(18)MC(20) C(21)MC(21)i 126.1(3) 118.2(2) 123.8(3) 123.5(3) 124.2(3) 123.2(3) O(2)MN(4)MC(6) C(14)MC(13)MC(17) N(5)MC(15)MC(14) N(5)MC(16)MC(17) O(18)MC(20)MO(19) O(19)MC(20)MC(21) Fig. 5 Molecular structures in the p-PYNN2 SA diox unit [symmetry operation: (i ) x:, 1-y, 1-z] Fig. 6 shows the structure of p-PYNN bond distances and angles are listed in Table 5.The SA molecule bridges the gap between the pyridyl rings of the p- PYNNs, as well as FA in p-PYNN2 FA. The intermolecular, interatomic distance N(5) O(18) is 2.578(2) A °, which is shorter than that in p-PYNN2 FA. It is significant that the two oxygen atoms of the SA molecule which are attached to two separate p-PYNN molecules are diagonal to each other, and not adjacent to each other.In addition, the SA molecule is on a crystallographic inversion symmetry centre and has little bond alternation: the bond distances C(20)MO(18) and C(21)MO(19) are 1.241(3) and 1.211(2) A °, respectively, and C(20)MC(21) and C(20)MC(21)i are 1.433(3) and 1.492(3) A °, respectively [symmetry operation; (i) x:, 1-y, 1-z].The structure and coordination of SA indicate that the molecule is a squarate dianion in which the two protons are missing and the proton in the N O intermolecular contact is attached to p-PYNN (i.e. O- H+-N-). 2 SA diox. In the crystal, p-PYNN acts as a dimer with a short distance between the NO groups: the distances O(1) O(1)ii and O(1) N(3)ii are 3.667(2) and 3.658(2) A °, respectively [symmetry operation; (ii ) x:-1, y:, z-1].Since the positive spin densities are concentrated on the NO groups, there would be a large overlap between them, thereby resulting in antiferromagnetic coupling in the dimer. The arrangement resembles that of p-N-methylpyridinium nitronylnitroxide (abbreviated as p-MPYNN) in its iodide salt.19 p-MPYNN has a substituent (methyl group) at the pyridyl nitrogen atom and a positive charge on the pyridinium ring, as well as the N-protonated p-PYNN in p- PYNN2 SA diox.The arrangement of p-MPYNN in p- MPYNN I resulted in a strong antiferromagnetic coupling.19 Selectivity in hydrogen bonds of p-PYNN The crystal structures of the 251 molecular complexes of p- PYNN and three organic acids with diVerent acidity have been examined.The three acids formed diVerent hydrogen bonds with p-PYNN, which involved two hydrogen-bond acceptingTable 5 Selected bond lengths (A °) and angles (°) for p-PYNN2 SA dioxa bond length/A ° 1.274(2) 1.354(3) 1.371(3) 1.370(3) 1.211(2) 1.426(3) O(2)MN(4) C(6)MC(13) C(15)MN(5) C(13)MC(17) C(20)MC(21) O(22)MC(24) O(1)MN(3) N(4)MC(6) C(14)MC(15) C(16)MC(17) O(19)MC(21) O(22)MC(23) O(1)MN(3)MC(6) N(3)MC(6)MN(4) C(13)MC(14)MC(15) C(15)MN(5)MC(16) C(16)MC(17)MC(13) O(18)MC(20)M(21)i bond angle (°) 127.1(2) 108.4(2) 119.5(2) 120.7(2) 119.5(2) 134.5(2) 135.3(2) 135.3(2) 110.4(2) O(19)MC(21)MC(20) C(20)MC(21)MC(20)i O(22)MC(23)MC(24)iii a(i ) Symmetry operation; x:-1, 1-y, 1-z: (iii) Symmetry operation; 1-x, 1-y, z.ON+ ON+ NON+ Fig. 6 View of the crystal structure of p-PYNN2 SA diox [symmetry operation: (ii ) x:-1, y:, z:-1] the pyridyl ring. The features of the hydrogen bonds in the three are shown in Scheme 1. The acidic hydrogen of HQ attached to site (a) (OMH O), while FA and SA selected site (b).In addition, the hydrogen bond in p-PYNN p-PYNN2 SA diox is ionic (O- H+MN). The distance between the two atoms connected by the hydrogen bond becomes longer in the order p-PYNN2 HQ, p-PYNN2 FA, p-PYNN2 SA diox. The nature of the hydrogen bond [AMH B] can be expressed on the whole by the following five configurations:20 4 A- H+MB 5 A H-MB+ 2 A-MH+ B 1 AMH B 3 A+MH- B Structures 1, 2 and 3 produce the electrostatic stabilization energy EEL of the hydrogen bond, while structures 4 and 5 cause the charge transfer stabilization ECT.Since HQ is a very weak acid, electrostatic stabilization terms 1, 2 and 3 play a dominant role in the hydrogen bonds of HQ, and the contributions of 4 and 5 which are given by the dissociation of the AMH bond must be small.Therefore, the more electronegative oxygen in the NO group of p-PYNN would be chosen as the hydrogen-bond acceptor in the crystal of p-PYNN2HQ. On the other hand, FA and SA are much stronger acids than HQ, and the ECT in their hydrogen bonds which is produced by 2 FA is neutral (OMH N), while that in configurations 4 and 5 must be larger than that in the hydrogen bond of HQ.Here, the binding energies of the HMB bonds in the CT states govern the hydrogen bonds. With regard to the of the H+MN bond at site (b) would be larger than that of two hydrogen-bond acceptors in p-PYNN, the binding energy the HMON bond at (a). Therefore strong acids are considered to prefer site (b) to (a) as a hydrogen bond acceptor.This is why site (b) was chosen in the crystals of p-PYNN2 FA and p-PYNN2 SA diox. The hydrogen bond in p-PYNN2 FA is neutral and the acidic hydrogen remains in FA. This indicates that EEL is still eVective in p-PYNN2 FA, in addition to ECT . The hydrogen bond in p-PYNN2 SA diox is ionic, indicating the superiority of ECT over EEL .The pKa diVerence between the pyridinium ions and the carboxylic acids in aqueous solution, which is denoted as DpKa , was used to analyze the hydrogen bonds in various pyridine–carboxylic acid complexes in solution or even in the solid state.18 The hydrogen bond would be ionic when DpKa>3.75, and would be neutral when it is less than that value.The selectivity and features of the hydrogen bonds in the three molecular compounds of p-PYNN Scheme 1 J. Mater. Chem., 1998, 8(5), 1157–1163 1.353(2) 1.385(3) 1.335(3) 1.275(3) 1.459(3) 1.333(3) 1.241(3) 1.492(3) 1.503(3) 1.400(3) 1.433(3) 1.417(3) N(3)MC(6) C(13)MC(14) N(5)MC(16) O(18)MC(20) C(20)MC(21)i C(23)MC(24)iii 126.2(2) 118.1(2) 121.3(2) 120.9(2) 134.1(2) 91.4(2) O(2)MN(4)MC(6) C(14)MC(13)MC(17) N(5)MC(15)MC(14) N(5)MC(16)MC(17) O(18)MC(20)MC(21) C(21)MC(20)MC(21)i 136.1(2) 109.6(2) 111.0(2) O(19)MC(21)MC(20)i C(23)MO(22)MC(24) O(22)MC(24)MC(23)iii N N HO OH •O O• N N N HO O O O N N OH N+ OONH+ O O N O 2- O N O O +HN N+ ON+ O- 1161EL can be qualitatively explained in terms of the competition between E and ECT in the hydrogen bonds, which depends on the acidity of the organic acids and on the binding energy of hydrogen to the two hydrogen bond accepting sites in p-PYNN.(1) 0+a1x+a2x2 1+a3x+a4x2+a5x3 2 FA. The between the protonated p-PYNNs in p-PYNN2 SA diox is As mentioned previously, the intermolecular arrangement similar to that of p-MPYNN in p-MPYNN I.The meta derivative (m-MPYNN) exhibits a strong ferromagnetic intermolecular interaction on an unusual lattice named Kagome.23 Research on protonated m-PYNN is now in progress. Magnetic properties The temperature dependences of the magnetic susceptibilities of the three compounds were examined in the temperature range 3–280 K.The correction for the diamagnetic contribution was performed by using the diamagnetic susceptibilities that were evaluated assuming that the paramagnetic component follows the Curie law at high temperatures. The paramagnetic susceptibilities, xp , obtained are shown in Fig. 7, where the molar unit is defined as half of the p-PYNN2 X unit and a logarithmic scale was used in order to clarify the lowtemperature behavior.The open circles in Fig. 7 show the temperature dependence p p of x for p-PYNN2 HQ. The value of xp increases with a decrease in temperature to 5 K and, after reaching a maximum, it decreases with a further decrease in temperature. Below 5 K, x approaches a non-zero value at absolute zero.The behavior can be interpreted in terms of an antiferromagnetic onedimensional chain,21 using eqn. (1); x= 4C a Twith x=|J |/2kBT , B constants a0–a5 have been defined elsewhere,22 |J | is the ferromagnetic spin-coupler in the molecular compound of searchers found that phenylboronic acid acts as a magnitude of intrachain magnetic coupling, C the Curie con- (phenylboronic acid) (phenyl nitronylnitroxide) through an stant and k the Boltzmann constant.The solid curve in Fig. 7 NO HOMBMOH ON pathway.15 However, there is no which goes through the plots for p-PYNN2 HQ is the best suggestion of a magnetic interaction through the hydrogen theoretical fit for eqn. (1) with parameters |J |/kB=4.7 K and bonds in the three compounds, even though HQ bridges the C=0.376 emu mol-1 (fixed).As described in the previous NO groups in p-PYNN HQ. This could be responsible for section, the p-PYNN radicals form a one-dimensional stacking the fact that the distances between the acidic hydrogen atoms column along the b axis in p-PYNN2 HQ (see Fig. 2). The are much longer in the three acids than in the boronic acid.stack should be assigned to the antiferromagnetic chain. The squares in Fig. 7 show the results of p-PYNN value of xp gradually increases with a decrease in temperature in the temperature range studied. This behavior can be interpreted in terms of the Curie–Weiss law. The solid curve which Fig. 7 Temperature dependences of the paramagnetic susceptibilities xp in a log scale for (#) p-PYNN2 HQ, (%) p-PYNN2 FA and ($) p-PYNN2 SA diox. The solid curves are theoretical ones (see the text).J. Mater. Chem., 1998, 8(5), 1157–1163 1162 was fitted to the plots for p-PYNN2 FA is the best theoretical fit of the Curie–Weiss law with a Curie constant of C=0.376 emu (fixed) and a Weiss constant of h=-0.68 K.The weak magnetic interaction would be caused by the cancellation between the ferromagnetic and antiferromagentic contributions in the arrangement shown in Fig. 4. The closed circles in Fig. 7 show xp for p-PYNN2 SA diox. The value of xp increases with a decrease in temperature from 280 to 22 K; below that it shows a decrease. The increase of xp below 4 K would be caused by Curie spins on the lattice defects. Since p-PYNN exists as a dimer in the crystal of p- PYNN2 SA diox, the temperature dependence of xp can be interpreted, as follows, based on the singlet–triplet model, eqn.(2); Cdef (2) x= 4C exp(2J/kBT ) T 1+3 exp(2J/kBT ) + T where J is the intradimer antiferromagnetic coupling constant and Cdef is the Curie constant for the lattice defect.The solid curve that was fitted to the plots for p-PYNN2 SA diox in Fig. 7 is the best theoretical fit of eqn. (2) whose parameters are J/kB=-18.5 K, C=0.376 emu K mol-1 (fixed) and Cdef= 0.015 emu K mol-1. The theoretical curve can explain the observed behavior. The three molecular compounds exhibited diVerent magnetic properties. This reflects the diVerence in the molecular arrangements of p-PYNN in these compounds. Their intermolecular magnetic interactions can be understood in terms of the McConnell’s spin polarization mechanism. Other re- 2 Conclusion We analyzed the crystal structures and the magnetic properties of the three acid–base molecular complexes, p-PYNN2 HQ, p-PYNN2 FA and p-PYNN2S diox. The organic acids formed selective hydrogen bonds to the two kinds of hydrogenbond accepting sites in p-PYNN. The acidic hydrogen of HQ attached to the oxygen (OMO), while FA and SA selected the nitrogen on the pyridyl ring of p-PYNN. The hydrogen bond in p-PYNN2 FA was neutral (OMHN), while that in p-PYNN2 SA diox was ionic (O-H+MN). The hydrogen bonding patterns that we observed in the three crystals were qualitatively interpreted, in terms of the competition between the electrostatic stabilization and the charge-transfer stabilization in the hydrogen bond. This depends on the acidity of the organic acids and on the proton accepting abilities of the two sites in p-PYNN. The possibility of selective, controllable hydrogen-bond production is suggested. The three molecular compounds exhibit diVerent antiferromagnetic properties, which are dependent on the intermolecular arrangement of p-PYNN in the crystals. Their magnetic couplings are explained, based on the McConnell’s spin polarization mechanism. This work was supported by a Grant-in-aid for ScientificResearch from the Ministry of Education, Science, and Culture, of the Japanese government. References