synthesis of several families with magnetic ordering temperatures as high as ∼125◦C.
Examples of soft and hard magnets with coercivities as high as 27 kOe have also been reported. Examples from our laboratory of organic-based magnets using the tetracya- noethylene radical anion, [TCNE]•−, are discussed. In addition, several molecule-based magnets based on Prussian Blue structured materials as well as dicyanamide are discussed.
Keywords. Magnetism; magnet; hysteresis; organic magnet.
PACS No. 75.50.-y
1. Introduction
Magnets are technologically important materials that are useful in every facet of our society. Control and improvement of magnetic properties in addition to their iden- tification of new phenomena, especially in combination with other useful properties, are important research and technological goals [1].
Commonly used magnets possess d- or f-orbital-based spin sites and have ex- tended network bonding in three dimensions (3-D). This 3-D bonding leads to their insolubility and energy-intensive metallurgical methods for their synthesis. The discovery of organic-based magnets [2] leads to the anticipation that improved sol- ubility and processing comparable to conventional polymers arises. Also, combining magnetic properties with other properties, e.g. magnetic ordering controllable light [3] or optical transparency [4], that are not currently available in commercial mag- nets may be important.
Organic, and more generally, molecule-based magnets have different structure types with respect to conventional magnets, and offer the opportunity for new phenomena, as well as combinations of properties not observed for commercial magnets. The first organic-based ferromagnet, i.e. ionic, noncovalent-bonded
‘zero-dimensional, organic-solvent soluble [FeCp∗2]•+[TCNE]•− (TCNE = tetra- cyanoethylene; Cp∗ = pentamethylcyclopentadienide) would not have been ex- pected to magnetically order due to the large through-space separations between spin sites [5,6]. Likewise, the soluble coordination polymer, [MnTPP]+[TCNE]•−
[H2TPP = tetraphenylporphyrin] [5a,7], would not have been expected to magnet- ically order due to its low dimensionality, as occurs.
The discovery of numerous bulk magnetism in organic and molecular materials now has worldwide interest [8] that has led to the development of new materials and new phenomena [5,6]. In particular, magnetic phenomena and its understanding in general has benefited from a renaissance in part due to contributions from organic- and molecule-based chemistry. Examples include the discovery of:
• Bulk ferro- and ferrimagnets based on organic/molecular components [5,9,10]
with critical temperatures exceeding room temperature [5,6,11].
• Prussian Blue structured, room temperature magnets [12–15].
• Clusters in high spin states with a large magnetic anisotropy and negative zero-field splitting can trap magnetic flux enabling a single molecule/ion to act as a magnet at low temperature (i.e. single molecule magnet) [16].
• Materials exhibiting large, negative magnetizations [17].
• Spin-crossover materials can have large hysteretic effects above room temper- ature [18].
• Photoinduced magnetism [3,19,20b].
• Electrochemical [17d,20] modulation of the magnetic behavior.
• Haldane conjecture [21a] and its realization [21b,c].
• Valence tautomers exhibiting spin crossover [22].
• Single chain magnets [23].
• Spin ladders [24].
• Giant [25] magnetoresistance effects.
• Colossal [26] magnetoresistance effects observed for 3-D network solids might be observed for molecule-based magnets.
The key component of any magnet is the electron spin. The stronger the spins couple the higher temperature magnetic ordering can occur. The coupling must not be compensated else antiferromagnetic ordering will result. While antiferro- magnetic coupling/ordering has less academic/technological interest with respect to ferromagnetic coupling/ordering, it consequently does not have extensive worldwide interest. Attainment of strong magnetic coupling to achieve ferri- or ferromagnetic ordering is far from trivial. Herein, in collaboration with Arthur J Epstein’s group (The Ohio State University), several classes of organic/molecule-based magnets have been synthesized and characterized, and are discussed [27].
2. TCNE-based magnets
In an effort to make a new organic-based metal decamethylferrocene, FeIICp∗2 was reacted with 7,7,8,8-tetracyano-p-quinodimethane (TCNQ) and two phases of [FeIIICp∗2]•+[TCNQ]•− were prepared [28]. One phase was characterized to be a metamagnet; i.e. it had an antiferromagnetic ground state, but above a critical field,Hcr, of 1.3 kOe, transition to a ferromagnetic-like state took place [28,29]. In addition, a third phase polymorph was discovered [30], and it ordered at 3.1 K as a ferromagnet [29,30]. The metamagnet and ferromagnet polymorphs possess 1-D
Figure 1. Structure of ferromagnetically coupled out-of-chain segments of 1-D chains of [FeCp∗2][TCNQ] (a) and [FeCp∗2][TCNE] (b) with alternating [FeCp∗2]•+ and [TCNQ]•−s or [TCNE]•−s [2b].
chains with alternatingS = 1/2[FeIIICp∗2]•+cations andS= 1/2[TCNQ]•− anions (figure 1a). This unexpected observation of magnetic ordering for the metamag- netic polymorph, led to an insight for stabilization of a ferromagnet [27]. That is, enhancement of the magnetic coupling (J) by increasing the spin density of the anion and using a smaller anion that additionally, as it is smaller, gets in closer proximity to the [FeIIICp∗2]•+cations enabling stronger coupling. TCNQ is approx- imately twice the size of TCNE, but with fewer atoms to delocalize the spin; the spin density of [TCNE]•−is twice that of [TCNQ]•−. Hence, replacement of TCNE tetracyanoethylene for TCNQ was targeted. Subsequently, [FeIIICp∗2]•+[TCNE]•−
was characterized to have the same structural motif as for [FeIIICp∗2]•+[TCNE]•−
(figure 1) and is a bulk ferromagnet with an ordering temperature,Tc, of 4.8 K, and a coercivity,Hcr, of 1 kOe at 2 K [2]. Table 1 summarizes the magnetic properties of ferromagnetic [FeIIICp∗2]•+[TCNE]•−.
After the discovery of [FeIIICp∗2]•+[TCNE]•− as the first organic-based magnet [2], many related magnetic materials were synthesized. In particular, [FeIIICp∗2]•+
[TCNE]•− (M = Cr, Mn, Fe) magnetically order with critical temperatures, Tc,
Table 1. Summary of properties of the molecular/organic bulk ferromagnet [FeIIICp∗2]•+[TCNE]•−.
Structure 1-D chains (figure 1b)
Solubility Conventional organic solvents
Critical/Curie temperature 4.8 K
Curie-Weissθconstant||(⊥) to 1-D chains +30 (+10) K
Spontaneous magnetization Yes forH= 0
Magnetic susceptibility||(⊥) to 1-D chains 0.00667 (0.00180) emu/mol (Obs. 290 K) Magnetic susceptibility||(⊥) to 1-D chains 0.00640 (0.00177) emu/mol (Calc. 290 K) Saturation magnetization||(⊥) to 1-D chains 16.3 (6) k emu Oe/mol
(calc. 16.7 k emu Oe/mol) Intrachain exchange interaction
||(⊥) to 1-D chains 27.4 K (19 cm−1) (8.1 K (5.6 cm−1))
Coercive field 1 kOe (2 K)
αcritical constant 0.09
βcritical constant ∼0.5
γcritical constant||(⊥) to 1-D chains 1.22 (1.19)
δcritical constant 4.4
Ferromagnetic ordering Yes – confirmed via neutron diffraction
57Fe M¨ossbauer Zeeman splitting 4.24 kG (4.2 K) forH= 0
that increase as M = Mn >Fe> Cr. Details of the magnetic properties of these materials are reviewed elsewhere [5c, 31–33].
V0(C6H6)2, bis(benzene)vanadium, is structurally quite similar to that of FeCp∗2, and its reaction with TCNE led to the formation of V(TCNE)x·yS (x∼2;y∼1/2), the first organic-based room temperature magnet (Tc ∼ 400 K) [11]. Later this magnet was also prepared via the reaction of TCNE and V0(CO)6[34] and use of the volatile V0(CO)6and TCNE precursors led to the development of a CVD (chemical vapor deposition) route to solvent-free thin films of the V[TCNE]y magnet [35].
X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) studies revealed that vanadium is present as VII and is coordinated by six nitrogens with V–N distances of 2.076 ˚A at 10 K (figure 2) [36].
V[TCNE]y is a disordered, amorphous substance with a low coercive field, Hcr, < 1 Oe at room temperature [37]. It is, however, a magnetic semiconduc- tor with a room temperature conductivity of∼10−4 S/cm, and magnetotransport studies indicate that electrons in valence and conducting bands are spin polarized, suggesting ‘spintronic applications [38]. M(TCNE)x (M = Mn [39], Fe [39,40] Co [39], Ni [39,41], Gd [42];x∼2;y∼1/2) and solid solutions VxM1−x(TCNE)x(M
= Fe [43], Co [44], Ni [41];x∼2;y∼1/2) magnets of composition have also been prepared.
In addition to magnets of V(TCNE)xcomposition, other electron acceptors have been reported to stabilize the ferrimagnetic ground state. These include solvated materials of V(TCNQ)2 (Tc = 52 K) [45], V(2,5-diethoxyTCNQ)2 (Tc = 106 K) [45], V(tetracyanopyrazene)2 (Tc = 200 K) [46], and V(dicyanoperfluorostilbene)2
(Tc = 205 K) [47] composition.
Manganoporphyrin-based magnets, e.g. [MnTPP][TCNE].solvent [H2TPP = tetraphenylporphyrin] are additional examples of TCNE-based magnets [5a,7]. This large family of ferrimagnets are all soluble 1-D coordination polymers possess- ing [TCNE]•− bridging between two Mn(III) ions (figure 3). These materials are
Figure 2. Local structure around each VII site V(TCNE)x deduced from XANES and EXAFS studies [36].
Figure 3. Segment of a typical 1-D· · ·[MnTPP]+[TCNE]•−· · ·coordination polymer showing [TCNE]•−trans-µ-N-σ-bonding uniformly to [MnTPP]+[7].
ferrimagnets resulting from the antiferromagnetic coupling of theS= 2 MnIIIwith S = 1/2 [TCNE]•− with critical temperatures, Tc, ranging up to 28 K [5a,48].
Furthermore, at low temperature they exhibit complex magnetic behaviors that include spin glass behavior, as is evident from the frequency-dependent (figure 4), and large coercivities of order 27 kOe at 2 K (figure 5) [49].
Figure 4. AC χ0(T) and χ00(T) at 10, 100 and 1000 Hz for [MnTBrPP]
[TCNE]·2PhMe [H2TBrPP = tetrakis(4-bromophenyl)porphyrin].
Figure 5. Temperature dependence of hysteresis loops observed for [MnT- BrPP][TCNE]·2PhMe [49].
Prussian Blue, FeIII4 [FeII(CN)6]3.xH2O, is the prototype for a large family of molecule-based magnets. The ideal Prussian Blue structure consists of → M ← C ≡ N → M0 ← N ≡ C → M ← [M = Fe(II); M0 = Fe(III) for Prussian Blue]
linkages identically along thea,b, and cunit cell axes (figure 6). The large family of magnetic materials results from substitution of the metal ions, as well as their
Figure 6. Idealized structure of Prussian Blue → M ← C ≡ N → M0
←C≡N →M ←linkages along thea,b, andcunit cell axes.
oxidation states. Thus, the number of spins per metal ion can be modulated leading to several magnetic behaviors. Ferromagnetic coupling leading to ferromagnetic or- dering may occur when spins on nearest neighbor metal ion sites reside in orthogonal orbitals. This is reported for CsNiII[CrIII(CN)6]·2H2O (Tc= 90 K) [50]. When the spin-containing orbitals are nonorthogonal the unpaired spins couple antiferromag- netically leading to a ferrimagnet. This is observed for CsMnII[CrIII(CN)6]·H2O (Tc = 90 K) [51]. Many magnets complying with this paradigm have been reported [12].
A decade ago Verdaguer’s group reported that the mixed-valent nonstoichiomet- ric VII/III[CrIII(CN)6]0.86·2.8H2O ordered as a ferrimagnet above room temperature at 315 K [13a]. Subsequently, K0.058VII/III[CrIII(CN)6]0.79·(SO4)0.058·xH2O that orders as a ferrimagnet at 372 K (99◦C) was reported [15]. Albeit more complex
Figure 7. Perpendicular view of a chain segment of M[N(CN)2]2 with each M being hexacoordinate and bridged byµ3-[N(CN)2]−.
than Verdaguer’s, it has greater air stability as samples exposured to oxygen for 128 h maintain aTc at 372 K. Simultaneously, Girolami [14b] reported a related stoichiometric magnet of KVII[CrIII(CN)6]· 2H2O composition (Tc = 376 K). Al- though formally containing only VII(S = 3/2), the magnetic ordering is attributed to the presence of VIII(S= 1) impurities.
The dicyanamide anion, –N≡C–N–C≡N–, is a five-atom conjugated spin-coupling bridge that can also provide a shorter three-atom –NCN– conjugated spin-coupling pathway through the central N. Metal complexes of M[N(CN)2]2(M = V, Cr, Mn, Fe, Co, Ni) composition have been reported to magnetically order [52,53]. Each M is six-coordinate and each [N(CN)2]− binds to three M’s, but cannot chelate, and thus forms a rutile-like structure (figure 7). MII[N(CN)2]2 magnetically order with Tc’s as high as 47 K for M = Cr. When M = Co and Ni, the materials order as ferromagnets, however, for M = V, Cr, Mn, Fe they order as canted antiferromagnets (or weak ferromagnets). CoII[N(CN)2]2 forms two polymorphs – α-(pink) andβ-(blue) that order at 8.7 K as a ferromagnet and at 8.9 K as a canted antiferromagnet, respectively. The coercive field forα-Co[N(CN)2]2 is 800 Oe and 7000 Oe for NiII[N(CN)2]2, which is unusual as Co-based magnets have higherTc’s as well as greater coercive fields with respect to Ni-based magnets due to single ion anisotropy.
A new molecule-based magnet using [CrIII(CN)6]3− and the cation of paddle wheel-structured ruthenium acetate, [Ru2(O2CMe)4]+, has recently been achieved [54]. Both ions are S = 3/2, but the latter has significant zero-field splitting (D/kB ∼ 90 K) [55]. Charge balancing requires three [Ru2(O2CMe)4]+’s per [Cr(CN)6]3−, and as the latter has six cyanides they are able to bond to six [Ru2(O2CMe)4]+’s, and each [Ru2(O2CMe)4]+ can bond to two [Cr(CN)6]3−. Hence a cubic charge compensated 3-D network structure is expected. Thus, [Ru2(O2CMe)4]3[Cr(CN)6] was anticipated to be cubic with a ∼ 13.3 ˚A, as well as magnetically order.
[Ru2(O2CMe)4]3[Cr(CN)6] was formed from the reaction of [Ru2(O2CMe)4]Cl and K3[Cr(CN)6] in water [54], and as anticipated it has a cubic 3-D extended structure. However, it possesses a second interpenetrating lattice (figure 8). Im- portantly, the temperature-dependentχT product decreases with decreasing tem- perature until∼100 K, but below 100 K, χT(T) increases dramatically indicative of magnet ordering, which is observed by other measurements at 33 K [54].
The hysteresis loop observed for [Ru2(O2CMe)4]3[Cr(CN)6] is unusual, (figure 9) [54]. The anomalous constricted nature of the hysteresis loop, although of unknown origin, is attributed to the presence of a second, interpenetrating lattice [57].
To minimize formation of the cubic lattice without a second interpenetrating lattice the methyl group of the acetate was substituted by the t-butyl group [58]. Shape and void volume modeling by the isosurface method [59] indi- cated that this cubic lattice was stable. But due to the larger t-butyl group [Ru2(O2CBut)4]3[Cr(CN)6] was anticipated to form a cubic, noninterpenetrating lattice. [Ru2(O2CBut)4]3[Cr(CN)6]·2H2O was prepared, but unexpectedly it did
Figure 8. Interpenetrating [Ru2(O2CMe)4]3[Cr(CN)6] cubic lattice; one lat- tice is black and the other is gray [56].
Figure 9. Hysteresis,M(H), for the [Ru2(O2CMe)4]3[Cr(CN)6] magnet with a second interpenetrating lattice [57].
Figure 10. Top view of layered (2-D) [Ru2(O2CBut)4]3[Cr(CN)6].
Mn(III) increased Tc from 4.8 to 8.8 K. Reaction V(C6H6)2 with TCNE led to the first room temperature organic-based magnet, which can be fabricated as thin films via CVD methodology. TCNE can also be reacted with Mn(II)porphyrins to form ferrimagnets of [Mn(III)porphyrin]+[TCNE]•− ·solvent composition. This family of magnets haveTc<28 K, exhibit spin glass behavior, and have enormous coercive fields (∼27 kOe) at 2 K. In our laboratory, molecule-based magnets based on dicyanamide of M[N(CN)2]2composition order as either ferromagnets or canted antiferromagnets depending on M, and haveTc ≤47 K. Using the [Ru2(O2CR)4]+ building block several new molecule-based magnets of [Ru2(O2CMe)4]3[Cr(CN)6] (Tc = 33 K) and [Ru2(O2CBut)4]3[Cr(CN)6] (Tc = 37.3 K) composition have been prepared. The former exhibits anomalous hysteretic behavior, with respect to the latter compound, and this is attributed to the presence of the second interpene- trating lattice. Thus, the study of molecule-based magnets continues to fascinate chemists worldwide.
Acknowledgment
The author gratefully acknowledges helpful assistance and discussions with A J Epstein (The Ohio State University), J L Dye (Michigan State University) and W W Shum (Utah). Continued support in part from the National Science Foundation Grant No. CHE 0110685, the US DOE (No. DE FG 03-93ER45504), and the AFOSR (No. F49620-03-1-0175) are also gratefully acknowledged.
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