J. MATER. CHEM., 1994, 4(9), 1377-1385 Magneto-Structural Correlation in a Series of Iodide Salts of p-N-Alkylpyridinium Nitronyl Nitroxided Dependence of the lodide- Pyridinium Ring Interaction on the Length of the N-Alkyl Chain Kunio Awaga,a Akira Yamaguchi," Tsunehisa Okuno: Tamotsu Inabe," Takayoshi Nakamura,c Mutsuyoshi Matsumoto" and Yusei Maruyamad a Department of Pure and Applied Sciences, The University of Tokyo, Komaba, Meguro-ku, Tokyo 753, Japan Department of Chemistry, Faculty of Science, Hokkaido University, Sapporo 060, Japan National Institute of Materials and Chemical Research, Tsukuba, lbaraki 305, Japan Institute for Molecular Science, Myodaiji, Okazaki 444, Japan Magnetic measurements and X-ray crystal analyses were carried out on iodide salts of p-N-alkylpyridinium a-nitronyl nitroxides [4-(4,4,5,5-tetramethyl-l -oxido-3-oxyl-4,5-dihydro-3H-imidazol-2'-yl)-l -R-pyridinium, with R =methyl (1+), ethyl (2+),n-propyl (3+)and n-butyl (4+)].The strongly antiferromagnetic crystal of 1+.I-consists of a radical dimer and the iodide ion is out of the plane of the pyridinium ring.2+4-,which is weakly antiferromagnetic, includes two crystallographically independent molecules, 2A+ and 2B', each of which forms a centrosymmetric dimer. In the pyridinium ring of 2A+ the iodides are 'out-of-plane' while for 26' they are 'in-plane'. The ferromagnetic 3+.1-and 4'V have similar structures: the crystal consists of a two-dimensional (2D) layer formed by a contact between the pyridinium ring and in-plane iodides.In this series, the iodide ion changes position from out-of-plane to in-plane and the magnetism varies from antiferromagnetic to ferromagnetic. It is found that the nitronyl nitroxide with an out-of- plane iodide has a short intermolecular contact between the NO groups (type I), while that with an in-plane iodide forms a contact between the NO group and the pyridinium ring (type 11). The observed magnetic behaviour can be interpreted in terms of an antiferromagnetic interaction for the type I contact and a ferromagnetic interaction for type II. There is currently rapid development in the field of molecular- based magnetic materials involving pure organic crystals, organic/organometallic polymers, metal-organic radical sys- tems and transition-metal complexes.' Recent advances include reports of bulk ferromagnetism in some pure organic crystal^,"^ a carbene with the largest spin quantum number of S=9,6 multi-dimensional crystal structures of some transition-metal complexes with T,> 10 K,7,8 T,>300 K in V(TCNE),.xCH,Cl, .9 It is also notable that experimental and theoretical reports on the ferromagnetic properties of nitronyl nitroxide radicals now appear quite freq~ently."-'~ The electronic structure of the nitronyl nitroxide has been examined with EPR,20 ultraviolet photoelectron spectroscopy (UVPS)21 and neutron diffraction.,, This radical family pos- sesses a strong spin polarization effect, mainly because of the proximity of the unpaired rc electron and the non-bonding electrons (n-n exchange interaction).The spin polarization effect stabilizes triplet charge transfer (CT) excited states, and the admixture of these states results in a ferromagnetic inter- molecular intera~tion.,~ This mechanism was originally pro- andposed by M~Connell,~~ the importance of the spin polarization effect has been illustrated theoretically by Yamaguchi et aLZ5 From reviewing the ferromagnetic nitronyl nitroxides that have been studied so far,''-'* we note that there is always an aromatic substituent at the a-position and that, in the crystal, there is a short intermolecular distance between the NO group and the aromatic substituent or the a-carbon, which is a bridge between the two NO groups. These observations can be understood as follows.The magnetic orbital (SOMO) of the nitronyl nitroxide is localized on the two NO groups, making a node on the central a-carbon, and has little popu- lation in the aromatic substituent, while the other frontier non-magnetic orbitals (NLUMO, NHOMO, etc.) are distrib- uted on both the nitronyl nitroxide group and the substitu- t The use of the term 'nitroxide' is discouraged by IUPAC; the preferred term is 'aminoxyl'. ent.26 Therefore, a short contact between the NO groups usually means an overlap between the magnetic orbitals, which always makes the intermolecular interaction antiferro- magnetic. An intermolecular contact between the NO group and the aromatic substituent or the r-carbon, on the other hand, means an interaction between the magnetic orbital and the non-magnetic orbitals.The non-magnetic orbi? als are naturally orthogonal to the magnetic orbital in the adjacent molecule. Ferromagnetic coupling can be expected -hrough the [magnetic or bi tall -[non-magnetic or bit all -[n iagnetic orbital] superexchange pathway. Recently, we initiated a study of N-alkylpyridinium nitronyl nitroxide cation radicals. They are designed to produce an intermolecular arrangement which satisfies the empirical con- dition for ferromagnetic coupling. Since the oxygen dtom in the NO group is equipped with a large negative charge, resulting from an electronic polarization in the NO bond, namely Ndf06-, a shorter intermolecular contact between the NO group and the pyridinium ring is expected in the solid state, owing to the Coulombic attraction force between the negative charge on the oxygen and the positive charge on the pyridinium ring (see Scheme 1).In this paper we describe the crystal structures and the magnetic properties of iodide salts of p-N-alkylpyridinium a-nitronyl nitroxides in detail. Scheme 2 shows the atom numberings. Both the mag- netic and the structural properties depend largely on the Q. R-,N+%;+x 0-Scheme 1 2 0-13 14 15 16 -CH2CH2CH2CH3 (4*) Scheme 2 length of the N-alkyl chain. Although we have reported the crystal structures and magnetic properties of 1+.I- and 2+.I- as short communication^,^^^^^ a new interpretation of their properties is given in this work from a systematic point of view in terms of the magneto-structural correlation in the + +series 1 .I --4 .I-.Experimental Materials The nitronyl nitroxide cation radicals, 1+-4+, were prepared by N-alkylation of p-pyridyl nitronyl nitroxide, as reported in ref. 27: the iodide salts of l+-4+ were precipitated in the corresponding alkyl iodide solutions of p-pyridyl nitronyl nitroxide, in the form of crystals (2+-1-) or microcrystalline powder (lf.I-, 3'01-and 4+.I-). 2+.I- crystallized with water, which could be from the air and/or the solvent, as (2+.I-),.Hz0. Single crystals of l+-I-, 3+.I-and 4+.I- were obtained by slow evaporation of their acetone solutions. Elemental analyses (Found: C, 41.35; H, 5.00; N, 11.29.Calc. for l+-I-: C, 41.50; H, 5.09; N, 11.17). [Found: C, 42.51; H, 5.32; N, 10.59. Calc. for (2+.T-),.H,O: C, 42.12; H, 5.55; N, 10.52.1 (Found: C, 44.82; H, 5.69; N, 10.53. Calc. for 3+-I-: C, 44.57; H, 5.73;N, 10.39.) (Found: C, 45.69; H, 5.94; N, 10.00. Calc. for 4+.I-: C, 45.94; H, 6.02; N, 10.05.) X-Ray Structure Determination X-Ray diffraction data were collected on a Rigaku AFC-5 (l+.I-, 3+.I- and 4+.I-) or an Enraf Nonius CAD4 (2+.1-) automatic four-circle diffractometer with graphite monochro- matized Mo-Kr radiation at room temperature. Unit-cell dimensions were obtained by a least-squares refinement using 25 reflections with 20 <2Q/degrees<25. During data collection the intensities of three representative reflections were measured as a check on crystal stability, and no loss was shown.The intensities for l+.I-, 2+.1- and 3+.1-were corrected for absorption, but not those for 4+.I-, because the influence of the absorption was found to be negligibly small. The crystal structures of l+.I-, 2+-I- and 4+.I- were solved by direct methods and the positions of hydrogen atoms were obtained by subsequent difference Fourier syntheses or by calculations. A block-diagonal least-squares technique (UNICS 111) was employed for the structure refinement, in which the positions of the non-hydrogen atoms were treated with anisotropic thermal parameters and those of the hydrogens were treated with isotropic parameters. In the analysis of the structure of 3+.I-, the iodide positions were obtained from a Patterson function and the other atoms were found via subsequent Fourier syntheses.The unit cell of 3+.1-was found to include four independent molecules, and the structure refinement with anisotropic parameters was carried out for all atoms except the hydrogens, in order to avoid excessive parametrization. The difference Fourier synthesis aft?r the refinement indicated no peak with an intensity >0.74 e AP3. Details of the crystal- J. MATER. CHEM., 1994, VOL. 4 lographic parameters are given in Table 1. Final positional parameters for 1+.1--4+-1-are listed in Tables 2-5, respectively.? Magnetic Measurements Static magnetic susceptibility and magnetization were meas- ured with a Faraday balance whose details were described previou~ly.~~The temperature dependence of the magnetic susceptibility was examined in the range 3-250K in a field of 1 T.Corrections for the diamagnetic contribution were carried out, using diamagnetic susceptibilities evaluated by assuming that paramagnetic susceptibilities follow the Curie law at high temperatures. Molecular Structures Two and four molecules are crystallographically independent in the crystals of 2+.I- and 3+.I-, respectively, while there is only one in l+.I- or 4+-I-. There are no signiiicant differences in the bond lengths and angles of the nitronyl nitroxide group or in those of the pyridinium ring, among the molecules 1+-4+. Furthermore, the N-alkyl chains in 1+-4+ take all- trans conformations. However, the dihedral angle between the nitronyl nitroxide group, 0-N-C-N-0, and the pyridin- ium ring tends to decrease with the extension of the N-alkyl chain, as shown in Table 6.The angle is known26 to be much affected by intermolecular contacts of the aromatic substituent, and is considered to be determined by the relative position of the iodide ion to the pyridinium ring, as is discussed later. MO calculations were performed on 1' -4+ with their atomic coordinates determined experimentally. There is little difference in either spin or atomic charge distribution among the molecules: most of the spin densities are distributed in the nitronyl nitroxide group, in contrast to the positive charge, which is localized in the pyridinium ring.Spin-charge separ- ation is characteristic of the N-alkylpyridinium nitronyl nitroxide cation radical. The electronic structure is negligibly affected by the length of the N-alkyl chain. Crystal structures We define two kinds of intermolecular arrangement between a pyridinium ring and an iodide ion, which are actually observed in crystals of N-methylpyridinium iodide.30 In this crystal, the N-methylpyridinium molecule interacts with two iodide ions, one of which lies in the plane of the pyridinium ring with short distances to the hydrogens on the ring ('in- plane' structure), and the other of which is located just above the pyridinium ring with a short distance to the nitrogen on it ('out-of-plane' structure). The positive charge in an N-alkylpyridinium ring is known to be distributed mainly on the nitrogen and the hydrogens of the pyridinium ring.30 Therefore, both the in-plane and the out-of-plane structure could be caused by Coulombic attraction forces.The former can be thought of as a CH--.I-hydrogen bond. Interestingly, the positions of the iodide ions in the crystals of 1+.1--4+.1- can be classified as in-plane or out-of-plane. 1+.I-The structure crystallizes into the triclinic Pi space group with 2=2. Fig. 1 (a)shows a projection of the structure along the [liO] direction. The iodide ions I(A)- and I(B)-occupy Further crystallographic data (atomic coordinates, hydrogen atom coordinates, bond lengths and angles, isotropic and aniso- tropic thermal parameters) are deposited with the Cambridge Crystallographic Data Centre.Details available from the Editorial Office. J. MATER. CHEM., 1994, VOL. 4 Table 1 Crystal data and experimental conditions for 1' -IP-4+-I 1 + -1-2+ -1-3+-1-4+-1 C13H1902N311 formula weight 376.22 399.25 404.27 418.30 crystal system triclinic triclinic triclinic monclclinic spcce group Pi pi P1 p2 L In a/'+ 11.843( 7) 12.582(6) 13.644( 3) 12.3I 1(2) biA 12.695( 7) 13.633(8) 14.798( 1) 13.6-13(2) CIA 9.532( 2) 11.120( 6) 9.873(2) 11.7J9( 2) rldegrees 95.53( 5) 93.31(4) 91.56( 1) Pldegrees 90.55( 5) 115.47(2) 113.11(1) 100. '5(2) ?/degrees 146.89(2) 88.91(4) 92.23( 1) VIA3 768.3(8) 1719(2) 1830.3( 5) 1945.3(6) Z 2 4 4 4 D(calc)/g cm-3 1.626 1.543 1.467 1.428 radiation Mo-Kz (jb=0.71073 A)graphite monochromator 20 range degrees 4.0-55.0 2.0-60.0 4.0-55.0 4.0- 55.0 no.collected 3852 5241 9238 4988 no. obsd 3056 4866 5571 2$65w0i>3.04~~1)R 0.0346 0.086 0.06 16 0.0552 Rw 0.0344 0.085 0.06 16 0.0548 Table 2 Atomic cpordinates ( x lo4)and equivalent isotropic thermal parameters ( lo2 A) for 1+ -1-Table 3 Atomic coordinates (x lo4) and equivalent isotropic thermal parameters ( lo2 A)for 2+.1--0.5H20O(0) O(0) O(0) 4.0 O(0) O(0) 5000(0) 4.0 4174(5) 9038( 5) 7615( 4) 3.0 816(5) 6040(5) 7007 (4) 2.8 4155(6) 5030( 5) 7620( 4) 3.2 9272( 1) 2258( 1) 5265(1) 5.4 6151(5) 10486( 4) 7991 (4) 4.5 4973( 1) 2788( 1 j 9746( 1) 6.3 -886(4) 41 84(4) 6550(4) 4.1 940( 7) 2979(5) 1469(8) 4.2 2756(6) 7134( 6) 7276(4) 2.7 1490( 7) 4528(5) 1751 (9) 4.6 3192(6) 9342(6) 7426( 5) 2.9 -2100( 9) 4661(8) 2793( 11) 7.1 861(6) 7236(6) 7451(5) 2.8 424( 7) 2191(5) 1465(9) 6.7 4196(8) 10920(7) 8628( 6) 4.6 1460(7) 5466( 5) 1907( 10) 7.3 3604( 7) 9973 (7) 5987( 5) 3.8 701(8) 3892(6) 1823 (9) 3.7 278(8) 6903 (7) 8948(5) 4.1 l882(8) 2982( 6) 973(9) 3.8 -719(7) 6552(7) 6419(6) 4.1 2457(8) 4009(6) 1555( 10) 3.8 3205(6) 6369( 6) 7326 (4) 2.7 1203( 11) 2893(8) -550( 11) 5.5 5138(7) 7484(6) 7077( 5) 3.3 271 3( 9) 2110(7) 1493( 11) 5.2 1749(6) 4532(6) 7671(5) 3.0 2794( 10) 4582(7) 618(11) 5.1 5567(7) 6778(7) 7215(5) 3.5 3498(9) 3989(7) 2928( 10) 5.3 2262( 7) 3901 (6) 7820( 5) 3.3 -242(8) 4155(6) 2179(8) 3.6 -1219(9) 3527(7) 1817( 11) 5.04708( 8) 4355(8) 7830(7) 4.6 -218(9) 5052( 7) 2931 (10) 4.7 -2119(10j 3814(8) 2139( 12) 5.8 -11 64( 1 1) 5283(8) 3192( 12) 6.1 the special positions (O,O,O) and (0,0,1/2), respectively.Each -3322( 14) 5070( 12) 2892( 15) 9.2 -3183( 17) 4639( 13) 3996( 19) 11.9pyridinium ring makes contacts with four out-of-plane iodide ions, resulting in a 2D network parallel to the o(liO) 6078(6) 879( 5 j 7793 (7) 3.7 4761 (6) 1591(5) 6075 (7) 3.7plane, where the shortest djstances are 3.606(5) A for 2225(7) -221(5) 7791(8) 4.1C(ll)(i).;.I(A)-(ii), 3.984(5) A for C(12)(i).-.I(A)-(iii), 6557(6) 389(5) 8832( 7) 5.3 3.829(5) A for C(ll)(i)-.-I(B)-(iv) and 3.742(5) A for 3766(6) 1847(5) 5149( 7) 5.5 C(12)(i).,.I(B)-(i) [symmetry operations; (i) x, y, z;(ii) x+ 1, 4907(7) 1012(6) 7068(8) 3.2 y+ 1, z + 1; (iii) x, y, z+ 1; (iv) x+ 1, y+ 1, 21.Fig. l(b) shows 6801(8) 13 12( 7) 7169(9) 3.9 the structure projected along the [1lo] direction; it consists 5899(8) 1948(7) 6116(9) 3.9 7825( 10) 1860( 9) 8208(12 6.7of a radical dimer whose top view is shown in Fig. 2(u). The 721 1 ( 11) 428(8) 6556( 13 5.9 two molecules are related by an inversion symmetry yith a 5976( 10) 3038( 7) 6525( 12 5.8 short distance between the NO groups; 3.383(8) A for 5829( 10) 1806(9) 4717(11 6.0 0(2)(i)...N(2)(v) [symmetry operation; (v) -x, -y+ 1, 3997(8) 599( 6) 7348(9) 3.7 4200(8) -203(6) 8115(9) 3.8-z + 11, which could be due to the intermolecular Coulombic attraction of N6+ 06-,reflecting a large charge polarization 2867(9) 983(8) 6805( 12) 5.7 3291(8) -617(7) 8319(10) 4.1on the NO bond.In this arrangement, the overlap between 1999( 9) 567(8) 7058( 12) 5.7 the TC orbitals on the NO groups appears very large. The 1257(10) -695( 7) 8027( 12) 5.4 interdimer arrangemept also has a short distance between the 614( 12) -10( 9) 8521( 16) 7.7 NO groups: 3.159(8) A for O(l)(i)-.-O(2)(vi) [symmetry oper- 1721(9) 3057( 7) 4701( 10) 9.3 ation; (vi) x + 1, y+ 1, z] (not shown). This is shorter than that in the intradimer arrangement, but the interdimer inter- J. MATER. CHEM., 1994, VOL. 4 Table 4 Atomic coordinates (x lo4) and equivalent isotropic thermal parameters (lo2 A)for 3' .I-atom x Y Z B eq atom X Jl Z B eq 408( 1) 2738( 1) 1916(2) 7.0 8721(19) 10294( 17) 8?33( 30) 9.5 9587( 1) 7268( 1 ) 8090(2) 7.6 9727( 16) 10629( 16) 7979(29) 9.6 3469(1) 7784( 1) 3198(2) 5.7 7588( 13) 7399( 12) 2214(20) 8.1 6529( 1) 2216( 1) 6802(2) 7.1 7554( 12) 6166( 11) 3466( 18) 6.5 6009( 15) 2510(11) 2318( 17) 7.2 10963( 11) 5644( 10) 2617( 16) 5.8 6647( 11) 1188(9) 2961( 17) 5.6 7900(9) 7874(9) 1293( 14) 6.7 2679( 12) 765( 10) 1990( 19) 6.7 7816(9) 5469(8) 41 15( 13) 5.6 5394(9) 3177(7) 1942( 14) 6.1 8140( 15) 6523( 12) 2753(22) 6.3 6680( 12) 355(9) 3346( 19) 8.8 6810( 14) 7653( 11 ) 2700( 19) 5.2 5731( 15) 1705(11) 2674( 17) 5.5 6492(14) 6572( 13) 3 143( 23) 6.6 7204( 13) 2687( 12) 2529( 18) 5.3 5878( 13) 8009(15) 1m7( 22) 6.9 7511( 16) 1639( 13) 2612( 25) 7.3 7370( 16) 8252( 14) 4011(22) 6.9 7224( 21 ) 3070( 16) 1087(28) 9.6 5675( 14) 6079( 12) 1746( 23) 6.7 7819( 17) 3207( 12) 4003 (21) 6.6 6409(17) 6683( 14) 4654( 22) 6.9 7273( 19) 1182( 15) 1022( 24) 7.7 9109( 12) 6245( 12) 2687(20) 5.5 8578( 13) 1385( 14) 3842( 25) 7.6 9757( 14) 6808( 10) 2O65( 20) 5.4 4743( 16) 1377( 13) 2604( 22) 6.4 9374( 17) 5386( 12) 31 21(21) 6.5 3940( 16) 1984( 17) 2224( 27) 8.7 10730( 17) 6622( 14) 2043 (24) 7.6 4515(16) 483( 13) 2630( 3 1) 8.6 10388( 14) 5106( 13) 3006(23) 6.4 2914( 16) 1563( 14) 2188(25) 7.3 11994( 14) 5420( 13) 2525(21) 5.7 3472( 16) 161( 14) 2297( 30) 8.6 11746( 18) 5083( 17) 929( 25) 8.0 1571( 16) 351( 15) 1630( 26) 7.5 12843( 17) 4737( 15) 891 (26) 7.7 1368( 18) 98(20) 2950( 3 1 ) 9.9 2345(9) 2654( 8) 7843( 12) 3.7 190( 18) -347( 20) 2421 (3 1) 10.0 2433 (9) 3797(8) 6617( 13) 4.0 3977(8) 7461 (7) 7670( 12) 3.4 -963 (9) 4242(8) 7504( 14) 4.2 3389( 11) 8783(9) 7002( 14) 5.O 2118( 11) 2046( 10) 8538(17) 8.0 7135( 12) 9317( 10) 7211(20) 7.1 2213( 12) 4542( 10) 5841( 18) 8.5 4595( 11) 6808( 9) 8000(15) 7.O 1880( 10) 3356(9) 7274(15) 3.4 3278 (9) 9597(8) 6572( 15) 6.5 3265( 13) 2496( 11) 7250( 18) 5.0 4197( 11) 8315( 10) 7320( 16) 4.0 3450( 13) 3381( 12) 6854( 17) 5.2 2830( 14) 7370( 11) 7437( 19) 5.2 4251 (15) 2052( 13) 8569(20) 6.2 2501( 12) 8343(11) 7376( 16) 4.6 2826( 18) 1747( 14) 5884( 21) 7.5 2768(13) 6801( 11) 8721( 15) 4.6 4360( 15) 4045( 15) 8077 (22 ) 6.8 2294( 14) 6755( 14) 5969(20) 6.4 3780( 15) 3538( 15) 5843( 21 ) 6.7 2293( 15) 8754( 12) 8797( 21) 6.2 901(11) 3659( 9) 7364( 16) 4.0 1420( 14) 8427( 13) 6 102( 20) 6.0 320( 11) 3114(11) 7830( 18) 4.8 5255( 11) 8623(10) 7347( 16) 3.9 533( 12) 4556( 10) 6934( 20) 5.1 6161( 12) 8134( 11) 7817( 18) 4.7 -609( 10) 3493(9) 7913( 16) 3.8 5292( 18) 9477( 14) 6550( 30) 8.6 -360( 14) 4821( 12) 69S1( 19) 5.5 7148( 17) 8422( 12) 8053( 19) 6.2 -2003( 15) 4651( 13) 7444(21) 6.0 6256( 15) 9777( 13) 6508( 26) 7.1 -1872( 13) 4945( 12) 8977( 19) 5.4 8177( 14) 9710( 13) 7013( 19) 5.9 -2873( 16) 5450( 15) 8838(22) 6.9 Table 6 Dihedral angles (degrees) between the pyridinium ring and Table 5 Atomic coordinates (x lo4)and equivalent isotropic thermal the nitronyl nitroxide group for 1' -4' parameters (lo2A)for 4+ .I-1+ 2+ 3+ 4i atom x V Z B eq 3 1.7( 3) A 22.1(4) A 16.9(14) 8.7( 2) 7934(0) 1324(0) 6041(0) 5.9 B 20.2(3) B 13.2(6) 4755(4) -23 10( 4) 2905 (4) 5.3 C 14.9(10) 4030( 5) -1945(4) 1129( 5) 5.9 D 10.5(6) 5080(4) 1419 (4) 2983(5) 5.4 5202(4) -2252( 4) 3970(4) 7.2 3761(7) -1469(4) 181(4) 9.5 action seems smaller than the intradimer one, because the 4491(5) -1565(4) 2167(5) 4.9 4554( 6) -3300( 5) 2335 (6) 5.8 two molecules, l+(i) and l+(iv), are arranged side by side 3760( 6) -3018( 5) 1195(6) 5.6 without a large n-orbital overlap.5684(8) -3662( 7) 2166(9) 8.8 4057(8) -3971 (6) 3126(7) 7.4 +3989(9) -3532(6) 127(7) 8.2 2 -1-.0.5H20 2550( 7) -3095( 7) 1260( 7) 7.3 The iodide salt of 2' takes an intermediate structure between 4697(5) -536( 5) 2444( 5) 4.9 5306(7) -238(5) 3500(7) 6.5 that of l+.I-and that of 3+.I-.The structure crystallizes in 4294( 6) 198(5) 1671(6) 6.2 the triclinic Pi space group with Z=4, consisting of two 548 l(6) 717(6) 3747(7) 6.7 crystallographically independent molecules, 2A and 2Bf .+ 4489(6) 1162(5) 1945( 6) 6.1 Fig. 3 shows a view of the crystal structure. The molecular 5321(7) 2456(6) 3316(7) 7.1 planes of 2Af(i) and 2B+(i) are oriented nearly perpendicular 6427( 7) 2747(6) 3 109 (8) 7.2 to each other [symmetry operation; (i) x,y,z].The pyridinium 6663(9) 3819(7) 3512( 12) 10.4 7725( 10) 4172( 10) 3390( 13) 12.4 ring of 2A+ makes contact with two out-of-plane iodide ions, while that of 2B+ does so with two in-plane iodide ions. This crystal includes both out-of-plane and in-plane iodide ions. The intermolecular, interatomic distances between the mol- J.MATER. CHEM., 1994, VOL. 4 :u + 0U" ,? -Fig. 1 Projection of the crystal structure of l+.I-:(a)along the [lTO] direction; (b) along the [1103 direction. For the symmetry operations, see text. ecule 2A+ and the two out-of-planeo iodide ions are 3.89( 1) A for C(llA)(i)-a-(A)(ii) and 3.72( 1)A for C( llA)(i)...I(B)(iii) [symmetry operations: (ii) x-1, y, z; (iii) x-1, y, z-11 and those between theomolecule 2B+ and the two in-plane i2dide ions are 3.822(9) A for C( llB)(i)...I(B)(iv) and 3.93( 1)A for C( 12B)(i)...I(A)(ii) [symmetry operation; (iv) --x+ 1, -y, -z+2]. The unit cell includes the two radical dimers, 2A+(i)...2A+(v) (dimer a) and 2B+(i)...2B+(iv) (dimer b) whose arrangements are shown in Fig.2(b) [symmetry oper- ation; (v) -x, -y+ 1, -z]. The intermolecular overlap in the dimer a is not so large, but tbe NO groups are not arranged far from each other; 4.16(1) A for 0(2A)(i)---N(2A)(v). The dimer b is formed by a contact between the NO group and the pyridinium ring with shqrt distances; 3.09( 1)A for O(lB)(i)-.-C( 11B)(iv), 3.40( 1)A for O(lB)(i)..-C( 13B)(iv) and 3.41( 1) A for O(lB)(i).--N(3B)(iv). 3+*I-The crystal of 3+.I- belongs to the triclinic P1 space group. The unit cell consists of four crystallographically independent molecules, 3A+-3D+, although 3A' and 3B+, and 3C+and 3D + are related by a pseudo-inversion symmetry. The analysis that assumes the Pi space group in which two molecules are independent, leads to a structure involving orientational dis- order of the pyridinium ring, in contrast to the analysis with the P1 space group which resulted in a structure without the disorder. If the orientational disorder is intrinsic, the latter structure should also include it.For these, we adopted the P1 space group. The large standard deviations of the atomic coordinates and thermal parameters given in Table 4 are presumably due to the pseudo-centrosymmetry. Q 6 dimer a dimer b , D Fig. 2 Intermolecular arrangements in the crystals of l+-I- -4'01~. For the symmetry operations and the labelling, see text. Fig.3 View of the crystal structure of 2+.I-. For the symmetry operations, see text. Fig. 4(u) shows a 2D layer parallel to the ub plane, consisting of 3A+, 3Cf, I(A)- and I(C)- .The molecular planes of the organic radicals are oriented parallel to the layer and the iodide ions lie in plane with respect to the pyridinium rings. The shortest distances bFtween the iodide ions and the pyridiq- ium rings are 3.65(3) A for C(l?A)(i)...I(C)-(ii), 3.98(3) A for C( 13A)(i)...I(A)-(i), 3.82(3) A for C( llA)(i)...IO(A)-(i), 3.87(2)A for C( 13C)(i)...I(S)-(iii), 3.78(2) A for C( llC)(i)...I(C)'-(iii) and 3.64(2) A for C( 12C)(i)...I(A)-(iii) [symmetry operations; (i) x, y, z; (ii) x,y-1, z; (iv) x+ 1, y,21. The molecules, 3B' and 3D+, and iodide ions form a similar 2D layer parallel to the ub plane. Fig. 4(b) shows a projection of the structure along the a axis.The two layers appear alternately along the c axis. There are short contacts between 3Af and 3B+, and between 3C+ and 3D+, in the interlayer molecular arrangement. Their arrangements shown in Fig. 2(c) are formed by short contacts between the NO group and the pyridinium ring. The sbortest distances between 3Af(i) oand 3B+(ii) are 3.57(3) A for 0(2A)...C( 13B), 3.51(3) A for 0(2A)...C(12B), 3.98(3) A for 0(2A)...N(3B) and 3.62(3) A for C( 1tA).-.0(2B) .Those between 3C+(i] and 3D+(iii) are 3.45(2) A fvr 0(2C)-..C( 13D), 3.98(2) A for 0(2C).-.C( 10D), 3.13(2)b for 0(2C)...C( 12D), 3.67(2) A for 0(2C).-.N(3D), 3.46(3) A for C(13C)-.-0(2D), 3.09(3) A for C( 12C)...0(2D) and 3.46(2) A for N(3C).-.0(2D). In the interdimer arrangements, interactions of the NO groups are protected by the methyl groups in the nitronyl nitroxide (not shown) .J. MATER. CHEM., 1994, VOL. 4 4+-I-The iodide salt of 4+ crystallizes into the monoclinic P&/n space group with Z=4. The crystal consists of a 2D layer parallel to the (lOi) plane, which resembles those in 3+.I-. Fig. 5(u) shows a projection of the layer along the [lOi] direction. The iodide ion in 4+-I- stands in plane with respect to the pyridinium ring, where the stortest distances are 3.751(8) A for C( ll)(i)a.-I-(i), 3.975(8) A for C( 12)(i)e.-I-(ii) and 3.957(9) A for C( 13)(i)-*.I-(ii) [symmetry operations; (i) x, y, z; (ii) x-1/2, -y+1/2, 2-1/21. Fig. 5(b) shows a projection of the structure along the b axis. The nitronyl nitroxide 4' exhibits a 1D alternating stacking along the c axis, whose geometry is shown in Fig.2(4. The radical molecules are arranged head to tail, and are connected by short contacts between the NO group and the pyridinium ring. The intermolecular, interatomic distances are 3.63( 1)A for O(l)(i)...C( ll)(iii), 3.39( 1)A for O(l)(i)...C( 13)(iii), C 3B+(ii) Fig. 4 Crystal structure of 3+.I-:(a) 2D layer projected along the c Fig,5 Crystal structure of 4+.I-:(a) 2D layer projected along the axis; (b)projection along the a axis (side view of the layers). For the [loll direction; (b) projection along the b axis (side view of the symmetry operations, see text. layers). For the symmetry operations, see text. J. MATER. CHEM., 1994, VOL.4 3.62(1)A for C(9)(i)...C( ll)(iii) and 3.62(1)A for 0(2)(i)..-C( 12)(iv) [symmetry operations; (iii) --x+ 1, -y, -z + 1; (iv) -x + 1, -y, -z].There is little n-orbital overlap between the neighbouring chains. Summary of the Solid-state Structures The position of the iodide ion in 1+.1--4+.1-changes from out-of-plane to in-plane with the length of the N-alkyl chain, which could cause the sequential decrease in the dihedral angle seen in Table 4. Analogously, the crystal of N-methyl- pyridinium iodide includes both in-plane and out-of-plane iodide ions,30 while that of N-butylpyridinium chloride includes only in-plane chloride ions.31 This may be because in the process of crystallization, free rotation of a longer N-alkyl chain prevents the iodide ions from approaching the nitrogen with a large share of the positive charge on the pyridinium ring.In this case, the iodide ions prefer the in-plane structure in which a Coulombic stabilization energy can be gained by making contacts with the hydrogens on the pyridin- ium ring. Fig. 2 shows the nearest-neighbour molecular arrangements observed in the crystals of 1'.1--4+*I-. The intermolecular contacts in this figure can be classified into the following two groups. The arrangements of 1 and 2+ (dimer a) are formed + by a contact between the NO groups (type I), while those of 2+ (dimer b), 3+ and 4+ are formed by a contact between the NO group and the pyridinium ring (type 11). It is the type I1 contact that we expected in the crystal of the N-alkylpyridin- ium nitronyl nitroxide.Note that the nitronyl nitroxide with an out-of-plane iodide ion has a type I nearest-neighbour arrangement, while that with an in-plane iodide ion has a type I1 arrangement. Fig. 6 shows space-filling views of the nearest-neighbour arrangement of 1 with the out-of-plane + iodide ions and 4+ with the in-plane iodide ions. The out-of- plane iodide ion appears to block the nitrogen on the pyridin- ium ring from approach by the neighbouring molecule. This could be why the out-of-plane iodide ion is involved in type I rather than type I1 intermolecular interactions. According to the discussion above, an antiferromagnetic interaction is expected for type I contacts, and ferromagnetic for type 11.Magnetic Properties Fig. 7(u) shows the temperature dependence of the paramag- netic susceptibilities, xp,of 1+-1--4+.1-, where xpT is plotted as a function of temperature. Fig. 7(b) shows the low-temperature behaviour of 3+-I-and 4+-I-on an enlarged OC N Fig. 6 Space-filling views of the intermolecular arrangements of l+-I-(a) and 4+.I-(b) I I I ' (a) I I I I I L O 10 20 30 40 500 0.1 I/ 0.0 100 150 200 250 0 50 TIK Fig. 7 (a)Temperature dependence of the paramagnetic susceptibility of: A, l+.I-; 0, 2+*I-;e, 3+-1-; 0, 4+.I-. (b) Low-temperature behaviour of 3+-I-and 4+*I-on an enlarged scale scale. The xpT values of l+*I-and 2+.I-decrease with decreasing temperature, indicating antiferromagnetic inter- molecular interactions, although the antiferromagnetic inter- action in 2+.I-is much weaker than that in l+.I-.xpT of 3+.I-shows an increase with decreasing temperature down to cu. 10K, and then a rapid decrease after passing through a maximum. This behaviour indicates the coexistence of a stronger ferromagnetic interaction and a weaker antiferromag- netic coupling between the ferromagnetic units. xpTof 4+.1-increases with decreasing temperature over the range 3-250K. In conclusion, the magnetic interaction of the p-N-alkylpyridinium nitronyl nitroxide changes from anti- ferromagnetic to ferromagnetic, as the N-alkyl chain length increases. Hereafter, we describe quantitative analyses of the magnetic properties of lf.I--4+.I-, in terms of an antiferromagnetic interaction in the type I contact and a ferromagnetic inter- action in the type I1 contact.1+*1-The crystal consists of a dimer whose molecular arrangement is of type I, suggesting an antiferromagnetic coupling. An antiferromagnetic interaction is also predicted for the inter- dimer arrangement, because it has a short contact between NO groups. In fact, the magnetic behaviour is well interpreted in terms of the modified singlet-triplet model, 4c xp = T[3+exp(-2J/k,T)] -6 where J is the intradimer coupling constant, 6 is the Weiss constant caused by the weak interdimer interaction, C is the Curie constant and kB is the Boltzmann constant. The deri- vation of eqn. (1)is described elsewhere.32 When J is positive, the ground state is a triplet, while a negative J means a ground singlet state.The solid curve fitted to the plots for l+-I-in Fig. 7 is the theoretical best fit of eqn. (1) with the parameters, J/kB= -74 K, 6= -4.7 K and C=0.376 emu 1384 K mol-' (1emu=4n x lop5m3). The intradimer strong anti- ferromagnetic coupling is caused by the large overlap between the SOMOs in the intradimer molecular arrangement shown in Fig. 2(a). 2+-1-*0.5H20 The crystal includes both type I and type I1 dimers. The observed weak antiferromagnetic interaction could result from cancellation of the antiferromagnetic contribution from the type I dimer by the ferromagnetic one from the type I1 dimer. The temperature dependence of xp is hence interpreted, assuming the two kinds of magnetic dimers, using 2c 2c xp = T[3 +exp(-2Jl/kBT)] + T[3+exp(-2J2/kBT)] (2) where J1 is the antiferromagnetic coupling constant for the type I dimer a, and J2is the ferromagnetic coupling constant for the type I1 dimer b.The theoretical best fit is obtained with J1/kB= -6.0 K, J2/kB =1.7 K and C = 0.376 emu K mol-'. Ferromagnetic interactions are predictable in the two type I1 dimers in Fig. 2(c), which have a very similar geometry. The rapid decrease of xpT below 10K indicates an interdimer antiferromagnetic interaction, although the origin in the crys- tal structure is not clear. We interpret the observed tempera- ture dependence using eqn. (1)with a positive J and a negative 8. The best fit is obtained with J/kB=2.4K, 8= -0.7 K and C =0.376 emu K mol-'.The radical moleculer 4' forms 1D alternating stacking chains, in which the two intermolecular arrangements are both of type 11, suggesting ferromagnetic interactions. Since the alternation appears very weak, the temperature depen- dence of xp is interpreted in terms of a 1D ferromagnetic chain,33 using xp= (C/T)[( 1+5.7979916K+16.902653K2 +29.37688~~+29.832959K4 +14.036981K5)/(1+2.7979916K+7.0086780K2 +8.6538644K3+4.57431 14K4)I2l3 (3) with K = J/2kBT). The theoretical best fit to the experimen- tal data is obtained with J/kB=0.30 K and C= 0.374 emu K mol-l. Table 7 shows the shortest intermolecular, interatomic distances in the type I1 arrangements of 2+, 3' and 4+, and the ferromagnetic coupling constants obtained.Nitronyl nitroxides 2+ and 3+, with distances of ca. 3.1 A, have ferromagnetic coupling constants of J/kBz2 K, while 4+, in which the distance is longer, has a much smaller constant. Comparison with m-N-Alkylpyridinium Nitronyl Nitroxides We have already reported that the magnetic properties of the iodide salts of m-N-R-pyridinium nitronyl nitroxides, with R=methyl, ethyl and n-propyl, can be interpreted in terms of a ferromagnetic intradimer interaction and an antiferromag- netic interdimer intera~tion.~~ The magnetic behaviour of the meta derivatives depends little on the length of the N-alkyl chain, in contrast to the variety of dependences exhibited by J. MATER. CHEM., 1994, VOL.4 Fig. 8 Intermolecular arrangement of rn-N-methylpyridinium nitronyl nitroxide (from ref. 17) the para derivatives. Fig. 8 shows the intradimer molecular arrangement of the rn-N-methyl derivative, which belongs to type I1 and, in fact, results in a ferromagnetic interaction such that J/kB z10 K.17 The intermolecular overlap of the meta derivatives appears much larger than those of the type I1 arrangements of the para derivatives shown in Fig. 2. If the para derivatives take an intermolecular arrangement such as that in Fig. 8, there should be a steric repulsion between the N-alkyl chain and the methyl groups of the nitronyl nitroxide. This implies that the type I1 arrangement, which is expected to be realized in the N-alkylpyridinium nitronyl nitroxide, is intrinsically unstable in the para derivatives, because of steric repulsion.In other words, the variety observed in the structure and magnetism of the para derivatives originated from a balance between the steric repulsion and the Coulombic attraction in the expected type I1 arrangement. Conclusion We have described the crystal structures and magnetic proper- ties of iodide salts of p-N-alkylpyridinium a-nitronyl nitrox- ides. We found interesting correlations between the length of the N-alkyl chain, the position of the iodide ion, the nearest- neighbour molecular arrangement and the magnetic inter- action. The iodide ion in the crystal of l+.I-is located out of the plane of the pyridinium ring, while the iodide ions in 3+.I-and 4+.I-are located in the plane of their rings.Both out-of-plane and in-plane iodide ions are observed in 2+.T-. The iodide ion in 1+.1--4+-1-changes position from out-of- plane to in-plane with increasing N-alkyl chain length, which can be understood in terms of a steric effect of the N-alkyl chain. The dihedral angle between the pyridinium ring and the nitronyl nitroxide group decreases with length of the chain, presumably reflecting the positions of the iodide ions. The out-of-plane iodide ion causes a type I nearest-neighbour molecular arrangement of the nitronyl nitroxide in which there is a short distance between NO groups, while the in-plane iodide ion produces a type I1 arrangement in which there is a contact between the NO group and the pyridinium ring.The former situation means that there is an overlap between the SOMOs, while latter results in an overlap between the SOMO and the other frontier orbitals. The magnetic behaviour of 1 +-1--4+-1- can be quantitatively interpreted Table 7 Shortest intermolecular, interatomic distances in type I1 contacts and ferromagnetic coupling constants 2' 3+ (dimer b) 3.09(1) 3.09(3) 1.7 2.4 4+ 3.39(1) 0.3 J. MATER. CHEM., 1994, VOL. 4 in terms of an antiferromagnetic intermolecular interaction in the type I contact and a ferromagnetic interaction in the type I1 contact. This work was supported by a Grant-in-aid for Scientific Research (No. 05453051) on Priority Area ‘Molecular Magnetism’ (area no.228/04242103) from the Ministry of Education, Science and Culture, Japan. Support from New Energy and Industrial Technology Development Organization (NEDO)is also acknowledged. References 1 Research Frontiers in Magnetochemistry, ed. Charles J. O’Connor, World Scientific, Singapore, 1993. 2 M. Kinoshita, P. Turek, M. Tamura, K. Nozawa, D. Shiomi, Y. Nakazawa, M. Ishikawa, M. Takahashi, K. Awaga, T. Inabe and Y. Maruyama, Chem. Lett., 1991, 1225; M. Takahashi, P. Turek, Y. Nakazawa, M. Tamura, K. Nozawa, D. Shiomi, M. Ishikawa and M. Kinoshita, Phys. Rev. Lett., 1991, 67, 746; M. Tamura, Y. Nakazawa, D. Shiomi, K. Nozawa, Y. Hosokoshi, M. Ishikawa, M. Takahashi and M. Kinoshita, Chem. Phys. Lett., 1991, 186, 401; L. P. Le, A.Keren, G. M. Luke, W. D. Wu, Y. J. Uemura, M. Tamura, M. Ishikawa and M. Kinoshita, Chem. Phys. Lett., 1993, 206,405. 3 P-M. Allemand, K. C. Khemani, A. Foch, F. Wudl, K. Holczer, S. Donovan, G. Gruner and J. Thompson, Science, 1991,253,301; K. Tanaka, A. A. Zakhidov, K. Yoshizawa, K. Okahara and T. Yamabe, Phys. Rev. B, 1993,47,7554. 4 R. Chiarelli, M. A. Novak, A. Rassat and J. L. Tholence, Nature (London), 1993,363,147. 5 T. Nogami, K. Tomioka, T. Ishida, H. Yoshikawa, M. Yasui, F. Iwasaki, H. Iwamura, N. Takeda and M. Ishikawa, Chem. Lett., in the press. 6 N. Nakamura, K. Inoue and H. Iwamura, Angew. Chem., Int. Ed. Engl., 1993,32,872. 7 H. 0.Stumpf, L. Ouahab, U. Pei, D. Grandjean and 0. Kahn, Science, 1993, 261, 447. 8 H. Tamaki, Z.J. Zhong, N. Matsumoto, S. Kida, M. Koikawa, K. Achiwa, Y. Hashimoto and H. Okawa, J. Am. Chem. SOC., 1992, 114,6974. 9 J. M. Manriquez, G. T. Yee, R. S. McLean, A. Epstein and J. S. Miller, Science, 1991, 252, 1415; P. Zhou, B. G. Morin, J. S. Miller and A. Epstein, Phys. Rev. B, 1993,48, 1325. 10 K. Awaga, T. Inabe and Y. Maruyama, Chem. Phys. Lett., 1992, 190, 349. 11 P. Turek, K. Nozawa, D. Shiomi, K. Awaga, T. Inabe, Y. Maruyama and M. Kinoshita, Chem. Phys. Lett., 1991, 180, 327. 12 P-M. Allemand, C. Fite, P. Canfield, G. Srdanov, N. Keder and F. Wudl, Synth. Met., 1991,41-43, 3291. 13 T. Sugano, M. Tamura, M. Kinoshita, Y. Sakai and Y Ohashi, Chem. Phys. Lett., 1992,200,235. 14 E. Hernandez, M. Mas, E. Molins, C. Rovira and J.Veciana, Angew. Chem., Int. Ed. Engl., 1993,32,882. 15 F. L. Panthou, D. Luneau, J. Laugier and P. Rey, J. Am. Chem. SOC.,1993, 115, 9095. 16 K. Awaga, T. Inabe, T. Nakamura, M. Matsumoto and Y. Maruyama, Chem. Phys. Lett., 1992,195,21. 17 M. Tamura, D. Shiomi, Y. Hosokoshi, N. Iwasawa, K. Vozawa, M. Kinoshita, H. Sawa and R. Kato, Mol. Cryst. Liq. Crj yt., 1993, 232, 45. 18 K. Inoue and H. Iwamura, Chem. Phys. Lett., 1993,207, :!51. 19 M. Okumura, K. Yamaguchi, M. Nakano and W. Moii, Chem. Phys. Lett., 1993, 207, 1. 20 D. G. B. Boocock and E. F. Ullman, J. Am. Chem. SOC., 1968,90, 6873; E. F. Ullman, J. H. Osiecki, D. G. B. Boocock and K. Darcy, J. Am. Chem. SOC.,1972,194,7049. 21 K. Awaga, T. Yokoyama, T. Fukuda, S. Masuda, Y. Harada and Y. Maruyama, Mol. Cryst. Liq. Cryst., 1993,232,27. 22 E. Ressouche, J-X. Boucherle, B. Gillon, P. Rey and J. Sc hweizer, J. Am. Chem. SOC.,1993,115,3610. 23 K. Awaga, T. Sugano and M. Kinoshita, Chem. Phys. Lett., 1987, 141, 540. 24 H. M. McConnell, J. Chem. Phys., 1963,39,1910. 25 K. Yamaguchi, T. Fueno, K. Nakasuji and I. Murata, Cht m. Lett., 1986,629. 26 K. Awaga, T. Inabe, U. Nagashima and Y. Maruyama, I. Chem. SOC., Chem. Commun., 1989,1617; 1990,520. 27 K. Awaga, T. Inabe, U. Nagashima, T. Na kamura, M. Matsumoto, Y. Kawabata and Y. Maruyama, Chew. Lett., 1991,1777. 28 A. Yamaguchi, K. Awaga, T. Inabe, T. Nakamura, M. Ma tsumoto and Y. Maruyama, Chem. Lett., 1993,1443. 29 K. Awaga and Y. Maruyama, Chem. Muter., 1990,2,535. 30 R. A. Lalancette, W. Furey, J. N. Costanzo, P. R. Hemines and F. Jordan, Acta Crystallogr., Sect. B, 1978,34,2950. 31 D. L. Ward, R. R. Rhinebarger and A. Popov, Acta Cry:,tallogr., Sect. C, 1986,42, 1771. 32 K. Awaga, T. Okuno, A. Yamaguchi, M. Hasegawa, I.Inabe, Y. Maruyama and N. Wada, Phys. Rev. B, 1994,49,3975. 33 D. D. Swank, C. P. Landee and R. D. Willett, Phys. Rev. B, 1979, 20,2154. 34 K. Awaga, T. Inabe, T. Nakamura, M. Matsumcto and Y. Maruyama, Mol. Cryst. Liq. Cryst., 1993,232,69. Paper 4/01395F; Received 9th March, 1994