Molecule-Based Magnetic Materials - American Chemical Society

Chapter 22

Downloaded by UNIV MASSACHUSETTS AMHERST on October 8, 2012 | http://pubs.acs.org Publication Date: October 24, 1996 | doi: 10.1021/bk-1996-0644.ch022

Design and Magnetism of New Bimetallic Assemblies with Fe(III)-CN-Ni(II) Linkages Hisashi Okawa and Masaaki Ohba Department of Chemistry, Faculty of Science, Kyushu University, Hakozaki 6-10-1, Higashi-ku, Fukuoka 812, Japan

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Three types of bimetallic FeNi assemblies comprised of [Fe(CN)6] and [Ni(diamine) ] (diamine = en, pn) are described: (1) PPh [Ni(pn) ][Fe(CN) ]·H2O, (2) [Ni(en) ] [Fe(CN) ] ·2H O and (3) [Ni(pn) ] [Fe(CN) ]X·2H O (X = ClO -, BF -, PF -). PPh [Ni(pn) ][Fe(CN)6] forms a 1-D chain of the alternate array of trans-[Ni(pn) ] and [Fe(CN) ] -. [Ni(en) ] [Fe(CN) ] has a rope-ladder structure where two 1-D chains, formed by the alternate array of cis-[Ni(en) ] and [Fe(CN) ] -. are combined by trans-[Ni(en) ] . [Ni(pn) ] [Fe(CN) ]ClO4·2H O have a 2-D sheet of a square structure. Type 1 and type 2 of 1-D structures show no magnetic ordering over balk whereas type 3 of a 2-D structure shows a long-range magnetic ordering. 2+

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The design of highly ordered molecular systems using paramagnetic constituents is a current subject with the aim to obtain magnetic materials exhibiting spontaneous magnetization (1-2). A family of bimetallic assemblies comprised of hexacyanometallate [M(CN)6l and a simple metal ion have been extensively studied (3-6) and significantly high magnetic phase-transition temperatures are reported for some of them. Those bimetallic assemblies are presumed to have a face-centered cubic structure extended by M-CN-M' linkage, but structurally characterized assemblies are very few. To understand magnetostructural correlations of those high Tc ferromagnets, it is of great value to study analogous bimetallic assemblies that contain a metal complex instead of the simple metal ion. In this study three types of bimetallic assemblies comprised of [Fe(CN)6] ' and [Ni(diamine)2p are described: PPh4[Ni(pn) ][Fe(CN)6]-H20, [Ni(en) ] [Fe(CN) ]2-2H20 and [Ni(pn) ] [Fe(CN) lX - 2 H 0 (X=C10 , B F " , PF "). X-ray crystallography for those bimetallic assemblies has proved diverse bridging modes of [Fe(CN)6] " to give different network structures. Magnetic properties of the assemblies are discussed together with their network structures. +

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PPh4[Ni(pn)2][Fe(CN) ]-H20 6

In our preliminary study this assembly was obtained as microcrystals by the 1 :1 reaction of [Ni(pn>2]Cl2 and K [Fe(CN)6l in aqueous solution in the presence of 3

0097-6156/96/0644-0319$15.00/0 © 1996 American Chemical Society

In Molecule-Based Magnetic Materials; Turnbull, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.

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MOLECULE-BASED MAGNETIC MATERIALS

P H u C l , but eventually we found that the use of [Ni(pn)3JCl2 gave the same compound as large crystals. In the latter reaction, the slow dissociation of [Ni(pn)3] into [Ni(pn>2] in aqueous solution may lead to the growing of large crystals of the assembly. The assembly shows two v(CN) modes at 2130 and 2110 cm . In comparison in IR with K [Fe(CN)6] the weaker band at 2130 cm- is attributed to bridging C N group. 2+

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Crystal structure. Crystal data: Formula = C36H42NidOFFeNi, formula weight = 388.16, crystal system = monoclinic, space group = P2/c(#13), a = 12.958(3), b = 8.437(3), c = 17.250(2) A,j3 = 99.96(1)\ V = 1857.4(7) A*, Z = 2, Dc = 1.388 g cnr , /x(Mo-Ka) = 9.84 cm- , R = 0.081, Rw = 0.076. Figure la shows the ORTEP view of the asymmetric unit that consists of each one-half of [Fe(CN)6] ' anion, trans-[Ni(pn)2] cation, tetraphenylphosphate ion, and water molecule. The N i resides at the inversion center (0, 0, 0) and Fe (0, 0.3262, 0.25) and P(l) (0.5, 0.0691, 0.25) ions reside on the same mirror plane. The [Fe(CN)6] " coordinates to two adjacent /ra/w-[Ni(pn)2] molecules with its two C N nitrogens in cis, providing an 1-D zigzag chain with the alternate array of [Fe(CN)6] and /ra/w-[Ni(pn)2] ions, running along c axis (Figure lb). The Fe ion retains the octahedral surrounding with the Fe-C bond lengths ranging from 1.995(10) to 1.976(8) A. The Ni-N(l) bond distance is 2.146(6) A and the C(l)-N(l)-Ni angle is 158.3(8)°. The N i has a trans six-coordinate geometry with the Ni-N bonds of 2.067(8)-2.146(6) A. An projection of the chains on a plane is shown in Figure lb. The nearest interchain Fe-Fe(x, y+1, z), Ni-Ni(x, y+1, z) and Fe-Ni(-x, -y+1, -z) separations along b axis are 8.437, 8.437 and 7.135 A, respectively. The corresponding interchain separations along a axis are 12.958, 12.958 and 13.220 A, respectively. The PPh4 cation and lattice water reside among the chains.


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Magnetism. Cryomagnetic property of this assembly, determined on powdered sample, is shown in Figure 2 in the form of XMT vs. T plot. The XMT value at room temperature is 1.94 cm K mol (3.94 \i ) per FeNi that is larger than the value expected for non-coupled Fe(HI)(S=l/2)-Ni(II)(S=l). The ZMT gradually decreases with decreasing temperature to the round minimum of 1.75 cm K mol" (3.74 ji ) at 50 K and then increases up to the maximum of 2.34 cm K mol" (4.33 \i ) at 4.3 K . The decrease in XtJ from 290 to 50 K can be attributed to the temperature-dependence of the T 2 term of the low-spin Fe(III) under pseudo Oh symmetry. The magnetic behavior in the temperature range below 50 K indicates a ferromagnetic interaction between the adjacent Fe(III) and Ni(II) ions through cyanide bridge. The ferromagnetic coupling between the metal ions can be rationalized in terms of 'strict orthogonality of magnetic orbitals' (tfc^CFe) 1 eg (Ni)) (9-11). The maximal XMT is slightly larger than the value for Sr=3/2 of ferromagnetically coupled Fe(III)(S=l/2)Ni(II)(S=l). But no long-range magnetic ordering occurs in this assembly, as anticipated from the large interchain separation. 3

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[Ni(en) ] [Fe(CN) ]2 - 2 H 0 2

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This assembly was obtained as black crystals by the 3:2 reaction of [Ni(en)3]Cl2 and K3[Fe(CN)6] in an aqueous solution (7). A similar reaction using trans[NiCl2(en)2] gave the same product but in a polycrystalline form. Three v(CN) modes appear at 2150, 2130 and 2110 cm , indicating a lowered symmetry about [Fe(CN) ] - in the lattice. 1

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Crystal Structure. The asymmetric unit consists of each one [Fe(CN)6] ' and cis[Ni(en)2] , one half of /ra/w-[Ni(en)2] and one water molecule, with the inversion 3

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Design and Magnetism of New Bimetallic Assemblies


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Figure 1. (a) ORTEP drawing of the asymmetric unit and (b) a projection of 1-D zigzag chains of PPh4[Ni(pn) ][Fe(CN)6]'H20 (PPli4 cations are omitted for clarity). +

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Figure 3. A projection of 1-D chains of [N^enfeMFeCCNfeh-ZIfeO (en molecules are omitted for clarity).


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Design and Magnetism of New Bimetallic Assemblies 2+

center at the N i of /rajw-[Ni(en)2] (7). A 1-D zigzag chain is formed by the alternate array of [Fe(CN)6] and c/j-[Ni(en)2] ions and two zigzag chains are combined by /raAw-[Ni(en)2] , providing a novel rope-ladder structure running along c axis (Figure 3). The [Fe(CN)6] " coordinates to three Ni(II) ions with its C N nitrogens in the meridional mode. The F e - N i ( l ) and Fe-Ni(2) distances are 5.145(2) and 4.993(2) A, respectively. In the crystal, the chains align along the diagonal line of the ab plane providing a pseudo 2-D sheet. The lattice water molecules are captured between the sheets. The nearest interchain Nil—Nil(-x-l. y+1, -z) and N i l - F e ( - x - l , -y+1, -z) separations in the sheet are 5.375 and 6.295 A, respectively. The nearest intersheet Nil-Fe(x-1, y, z), N i l - N i 2 ( x - 1 , y, z), and N i l - N i l ( x + 1 , y, z) separations are 6.494, 7.713, and 9.709 A, respectively. A disorder is seen in trans~[Ni(Qn)2] with respect to the en configuration in the lattice. 2+

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Magnetism. In our preliminary study magnetic measurements were made on polycrystalline sample obtained by the reaction with fra/w-[NiCl2(en)2] (7). The XMT value at room temperature was 4.95 c m K m o l (6.29 \i ) that increased with decreasing temperature, very abruptly below 20 K , up to a large maximum value of 86.1 c m K m o l at 14 K , and then decreased below this temperature (see Figure 4). The intrachain interaction between the adjacent Fe(III) and Ni(II) ions is evidently ferromagnetic. The abrupt increase in XMT around 20 K suggested the onset of longrange magnetic ordering and this was proved by our magnetization studies (7c = 18.6 K ) (7). In Figure 5 is given the field-dependence of the magnetization that shows an inflection at ca. 1 T and a tendency of saturation to M =7.6 Nji at 5 T. The saturation magnetization well corresponds to 8 N | i expected for ferromagnetically coupled N i ^ F e ^ . Based on the magnetization study this assembly is metamagnetic in nature. It is noticed that the crystalline sample with the rope-ladder structure differs in magnetic behavior from the polycrystalline sample described above. In this context we have to retract the previous conclusion based on the assumption that both the crystalline and polycrystalline samples have the same 1-D rope-ladder structure (7). The XMT VS. T curve of the crystalline sample shows a maximum at 14 K (see Figure 4) and the maximal value (10.22 c m K m o l ) corresponds to the value for S-r=4 of ferromagnetically coupled N i ^ F e ™ ^ The drop in XMT below 14 K suggests the operation of antiferromagnetic intersheet interaction. Non-zero magnetic moment exists in this compound that can be attributed to the residual z± contribution (12). Evidendy no long-range magnetic ordering occurs in the crystalline sample. It is likely that a disorder in the network structure occurs in the preparation of the polycrystalline sample by rapid precipitation. A candidate is the intercrossing of /ra/w-[Ni(en)2] between the rope-ladders, providing quasi 2-D (or 3-D) domains in the lattice. 3

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The bimetallic assemblies were obtained as black crystals by the 1:2 reaction of K [Fe(CN)6] and [Ni(pn) ]X (X=C1CV, B F " , PF ") in an aqueous solution (S). Analogous complexes with en ligand have been obtained which, however, are not included here because our spectroscopic studies have suggested a differing network structure for the en series. Two v(CN) modes are observed for this type at 2140 and 2110 c n r of which the former band, attributable to the bridging C N group, is more intense in this case. 3

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Crystal Structure of [Ni(pn) ]2[Fe(CN)6]C104-2H 0. The asymmetric unit consists of each one-half of [Fe(CN) ] - and CIO4- anions, one [Ni(pn) ] and water molecule (8). A l l the metal ions and the CI atom are at the special equivalent 2

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MT VS. T plots (per Fe^Nia) of [Ni(en)2]3[Fe(CN) ]2-2H2a

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H I T Figure 5. The field-dependence of magnetization of [Ni(en>2]3[Fe(CN)6]2• 2 H 0 obtained at 7 K . 2


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positions: Fe (0, 1/2, 0), N i ( l ) (1/2, 1/2, 0), Ni(2) (0, 0, 0), CI (1/2, 0, 0). The [Fe(CN)6] -ion coordinates to four adjacent [Ni(pn>2] cations through its four cyano nitrogens on a plane, providing a 2-D network of square-shaped structure (Figure 6). The mean Fe-C, C - N , and N-Ni bond distances are 1.949, 1.142, and 2.105 A , respectively, and the C(l)-N(l)-Ni(l) and C(2)-N(2)-Ni(2) bridge angles are 157.2(6) and 164.1(7) A, respectively. The Fe--Ni(l) and Fe~Ni(2) separations are 5.062(2) and 5.126(3) A, respectively. It is seen that /ra/w-[Ni(pn)2] align along a and b axes, giving a slight distortion of the quadrangle formed by four Fe ions to a rhombus. Perchlorate ion resides in each quadrangle and assumes two arrangements. In the bulk the 2-D sheets align along c axis with the intersheet Fe-Fe(0,1/2, 1) separation of 8.978(5) A. The nearest intersheet Fe~Ni(l) (x, y, z+1), Fe~Ni(2) (x, y+1, z+1) and Ni(l)-Ni(2) (x+1, y+1, z+1) separations are 9.852, 8.613 and 9.754 A, respectively. 3

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Magnetism. The three salts of this type are similar in magnetic behavior. Magnetic property of the perchlorate salt is described. The XMT VS. T curve is shown in Figure 7 and the XM VS. T plots in the low temperature region is given in the insert. The XMT VS. T curve has a maximum of 11.31 c m K m o l (9.51 u ) at 10 K that is very large for S-F=5/2 of ferromagnetically coupled N i ^ F e (4.38 c m K m o l and 5.91 u based on spin-only formalism). The XM VS. Tplots show around maximum at 10 K and a minimum at 6 K . Tne above results mean a ferromagnetic ordering occurs within the 2-D sheet and an antiferromagnetic interaction operates between the sheets. Near liquid helium temperature a non-zero magnetic susceptibility is observed which can be attributed to a residual & contribution (72). Magnetization studies have been made for mis assembly (Figure 8). The F C M curve obtained under a weak applied field of 3 G shows a peak at 10 K and a break near 8 K . Below 8 K a remnant magnetization is observed. The Z F C M curve shows two maxima at 8.0 and 9.9 K . The magnetic behavior in the temperature range of 8.0-9.9 K is typical of metamagnetic ordering and the peak at 8.0 K implies that the applied field is too weak to move domain walls below this temperature. Magnetic hysteresis studies at 4.2 K support the metamagnetic nature of this assembly (see Figure 9). A n abrupt increase in magnetization occurs around 3800 G , probably due to a spin flipping based on the intersheet antiferromagnetic interaction. The magnetization is not saturated at 20 T probably due to canted local spins. Such a canting of spin may arise from the magnetic ani sot ropy of the constituting metal ions. The remnant magnetization is 400 c m G mol and the coercive field is 290 G . The observed small remnant magnetization adds a support to the non-zero magnetism at zero field. Both the tetrafluoroborate and hexafluorophosphate salts showed similar metamagnetic ordering at 10.1 and 9.4 K , respectively. The remnant magnetization and the coercive field for the hexafluorophosphate salt are 260 c m G mol and 200 G , respectively. The field-dependence of the magnetization shows an inflection at ca. 4.5 k G and a gradual increase to M =3.9 Nu* at 80 kG. The maximum M value is smaller than 5 Nji expected for ferromagnetically coupled Ni^Fe™ suggesting that each local spin is canted and a very large magnetic field is necessary for overcoming this canting (Figure 10). 3

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From above discussion it is shown that [Fe(CN)6] *can adopt various bridging modes providing bimetallic assemblies of different network structures. The Ni/Fe ratio primarily relates to the dimensionality of the network structures. That is, type 1 of Ni/Fe=l has a 1-D zigzag chain structure, type 2 of NiA e=1.5 has a 1-D rope-ladder structure and type 3 of Ni/Fe=2 has a 2-D square sheet structure. A l l the three types show a ferromagnetic interaction operating within each chain or sheet through C N bridge. No magnetic ordering occurs in type 1 and type 2 of 1-D structure whereas a magnetic ordering occurs in type 3 of 2-D structure. The results demonstrate that 7


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Figure 8.

Design and Magnetism of New Bimetallic Assemblies 327

Temperature-dependences of magnetization (under 3 G) of

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The field-dependence of magnetization of [Ni(pn)2]2[Fe(CN)6]-

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3-D bimetallic network is important for developing ferromagnetic materials and such a 3-D network can be constructed based on, in general, [M(CN)6] and a planar M ' complex in M'/M=3 and using an appropriate counter ion. Studies in this line are in progress in our laboratory. n_

References 1. Kahn, O. Molecular Magnetism, VCH. Pub.: New York, 1993. 2. Kahn, O. In Organic and Inorganic Low-Dimensional Crystalline Materials, Delhaes, P., Drillon, B, Eds.; NATO ASI Ser.168; Plenum: New York, 1987; pp. 93. 3. Klenze, H.; Kanellalopulos, B.; Tragester, G.; Eysel, H. H. J. Chem. Phys. 1980, 72, 5819. 4. Gadet, V.; Mallah, T.; Castro, I.; Verdaguer, M . J. Am. Chem. Soc. 1992, 114, 9213. 5. Gadet, V.; Bujoli-Doeuff, M.; Force, L.; Verdaguer, M.; El Malkhi, K.; Deroy, A.; Besse, J. P.; Chappert,C.;Veillet, P.; Renard, J. P.; Beauvillain, P. In Molecular Magnetic Material; Gatteschi, D., et al., Eds.; NATO ASI Series 198; Kluwer: Dordrecht, 1990; p 281. 6. Mallah, S.; Thiebaut, S.; Verdaguer, M.; Veillet, P. Science 1993, 262, 1554. 7. Ohba, M.; Maruono, N.; Okawa, H.; Enoki, T.; Latour, J. M. J. Am. Chem. Soc. 1994, 116, 11566. 8. Ohba, M.; Okawa, H.; Ito, T.; Ohto, A. J. Chem. Soc., Chem. Commun. 1995, 1545. 9. Journaux, Y.; Kahn, O.; Zarembowitch, L.; Jaud, J. J. Am. Chem. Soc. 1983, 105, 7585. 10. Pei, Y.; Journaux, Y.; Kahn, O. Inorg. Chem. 1989, 28, 100. 11. Ohba, M.; Tamaki, H.; Matsumoto, N.; Okawa, H.; Kida, S. Chem. Lett. 1991, 1157. 12. Carlin, R. L. Magnetochemistry, Springer: Berlin, 1986. In Molecule-Based Magnetic Materials; Turnbull, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.

Molecule-Based Magnetic Materials - American Chemical Society

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