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Synthetic Metals 122 (2001) 535±542 HAT(CN)6: a new building block for molecule-based magnetic materials P.S. Szalay, J.R. GalaÂn-MascaroÂs, R. CleÂrac, K.R. Dunbar* Department of Chemistry, Texas A&M University, PO Box 30012, College Station, TX 77842-3012, USA Abstract The ability of reduced forms of HAT(CN)6 to be used as ligands to construct novel magnetic materials is reported and discussed herein. Binary compounds of ®rst row transition metals with discrete or extended polymeric structures are presented. All compounds exhibit characteristic paramagnetic behaviour except for a polymeric {Cu-HAT(CN)6} phase that behaves as a canted antiferromagnet below 6 K. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Organic radicals; Magnetic properties; N heterocyclic ligand; Coordination chemistry 1. Introduction Recent activity in molecule-based materials has led to important achievements in a number of technologically important areas. Novel molecular and extended networks constructed from organic and inorganic building blocks have been found to behave as molecule-based conductors and superconductors [1], optically active compounds [2] and magnets [3]. Furthermore, the versatility of molecule-based materials allows for the design of ``hybrid'' materials with the possibility of the co-existence of properties, or even synergy between them [4,5]. In particular, magnetism has been one of the ®elds that has undergone signi®cant development, with molecule-based magnetic materials that mimic and even improve on the properties of classic inorganic magnets (e.g. critical temperatures [6,7], coercive ®elds [8,9]) having been reported. A common feature of molecule-based magnets is the presence of polymeric networks of paramagnetic metals bridged by ligands capable of promoting magnetic interactions between the localized d-electrons. In addition to the different pathways for magnetic exchange through the ligand (s versus p), it is important to consider the size of the linker which is typically inversely related to the strength of the interaction. Consequently, most examples of molecular-based magnets are based on ligands consisting of only a few atoms such as CN [7,10±15], N(CN)2 [8,16±19], and C2O42 [9,20±23]. One strategy for inducing stronger magnetic interactions with longer linkers is to use organic * Corresponding author. Tel.: 1-979-845-5235; fax: 1-979-845-7177. E-mail address: [email protected] (K.R. Dunbar). radicals as ligands, due to the ability of unpaired electrons on the ligand to interact with those of the metals. Novel magnets have been obtained in this way with organic acceptors such as TCNE [6,24±28], and TCNQ [29,30]. In the vein of organoradical linkers, the hexanitrile derivative of 1,4,5,8,9,11-hexazatriphenylene, namely hexaazatriphenylenehexacarbonitrile (HAT(CN)6) (Scheme 1), offers interesting possibilities for preparing new magnetic materials with ®rst row transition metals. The cyclic voltammogram of HAT(CN)6 in acetonitrile, versus Ag/Ag, reveals two reversible one-electron reductions located at E1=2 0:064 and 0.450 V. Reduction by a third electron leads to decomposition as evidenced by an irreversible cathodic feature at 1.160 V (Fig. 1) [31].1 The accessibility of the ®rst two redox processes renders [HAT(CN)6]1 and [HAT(CN)6]2 2 viable candidates for isolation as paramagnetic ligands. In considering the two possible coordination modes of this ligand, the chelating bipyridine moieties in the central rings are expected to be favored over monodentate coordination at the terminal nitrile functionalities. With this situation, the coordination of up to three metals is possible, which would result in a triangular arrangement. Depending upon the metal/ligand ratio, one may anticipate different solid state architectures and molecular species to ensue. During the course of our studies, we noted the report of the compound {[Cu(dppe)]3[HAT(CN)6]} in which the [HAT(CN)6]1 ligand is chelated to three metal ions (see 1 Although four reversible reductions have been reported for HAT(CN)6 this result could not been reproduced with our purified samples. 2 Preliminary molecular orbital calculations indicate that the LUMO and LUMO 1 states are degenerate suggesting that the [HAT(CN)6]2 radical could be paramagnetic. 0379-6779/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 ( 0 1 ) 0 0 3 4 2 - 3 536 P.S. Szalay et al. / Synthetic Metals 122 (2001) 535±542 Scheme 1. footnote 2). This is the only compound of the [HAT(CN)6]1 radical that has been reported to date. In this case, however, the magnetic interaction of metal spins through the [HAT(CN)6]1 core could not be probed, since the Cu(I) centers are diamagnetic. In this paper, we report our initial studies on the deliberate reduction of HAT(CN)6 to [HAT(CN)6]1 and [HAT(CN)6]2 and the reactivity of these reduced species with transition metals. 2. Experimental 2.1. General procedures All solvents and chemicals were reagent grade and used without further puri®cation. All manipulations were carried out under an inert atmosphere using standard Schlenk-line techniques. The molecule HAT(CN)6 (1) was prepared according to the literature method and recrystallized from a methanol/acetonitrile solution [32]. Cyclic voltammetry (CV) experiments were carried out on a CH Instruments Electrochemical Work Station with a Ag/AgCl reference Fig. 1. Cyclic voltammogram of HAT(CN)6 in acetonitrile. electrode, a Pt disk as working electrode, and a Pt wire as an auxiliary electrode. Infrared spectra were recorded on solids suspended in Nujol on CsI plates using a Nicolet IR/42 FTIR spectrometer. Single crystal X-ray structural determination was performed on a Bruker SMART 1 K CCD platform diffractometer [33] with graphite monochromated Mo Ka Ê ). The frames were integrated in radiation (la 0:71069 A the Bruker SAINT software package [34], and the data were corrected for absorption using the SADABS program [35]. The structures were solved using the SHELXTL V.5.10 package [36]. Pertinent crystallographic data and re®nement parameters are provided in Table 1, and the corresponding CIF ®les are included as supplementary material. Electron spin resonance (ESR) spectra were recorded on a Bruker Electron Spin Resonance ESP300 E instrument equipped with an Oxford instrument Cryostat ESR900 in the range 4.2±300 K. Variable temperature magnetic susceptibility data were obtained in the range 2±300 K on polycrystalline samples using a Quantum Design, Model MPMS-XL. 2.2. Synthesis Co(HAT(CN)4O2)(H2O)44H2O (2), precipitates as an orange crystalline product from aerial evaporation of a solution containing Co(ClO4)26(H2O) (0.013 g, 0.034 mmol) dissolved in 2 ml of deionized water and HAT(CN)6 (0.008 g, 0.021 mmol) dissolved in 2 ml of acetonitrile. Ni(HAT(CN)4O2)(H2O)44H2O (3) is prepared under the same conditions by the reaction of Ni(ClO4)26(H2O) (0.013 g, 0.034 mmol) with HAT(CN)6 (0.008 g, 0.021 mmol). {(Cp2Fe)3[HAT(CN)6]}CH3CN (4), is prepared by mixing acetonitrile solutions of ferrocene (0.100 g, 0.528 mmol) and HAT(CN)6 (0.068 g, 0.176 mmol). Concentration of this solution either in vacuo or by evaporation in air results in the precipitation of dark green crystals. The compound displays a characteristic n(CBN) stretching mode at 2243 cm 1. [(Cp)2Co][HAT(CN)6] (5), is prepared by mixing acetonitrile solutions of HAT(CN)6 (0.304 g, 0.793 mmol) with cobaltocene (0.050 g, 0.264 mmol). The solution was concentrated and tetrahydrofuran was added to precipitate the green product which was collected by ®ltration, washed with tetrahydrofuran followed by diethyl ether, and dried in vacuo. The yield after drying is 75%. [(Cp)2Co][HAT(CN)6] displays a characteristic n(CBN) stretch at 2214 cm 1. The ESR spectrum of a solid sample at 4 K shows an isotropic signal with g 2:0084 and a line width of 7 G. Elemental analysis: Anal. Calcd. for C28N12H10Co1: C, 58.65; H, 1.76; N, 29.31. Found: C, 58.78; H, 1.92; N, 28.88. [(Cp)2Co]2[HAT(CN)6] (6) is prepared by combining acetonitrile solutions of HAT(CN)6 (0.075 g, 0.195 mmol) and cobaltocene (0.111 g, 0.585 mmol). The blue-green product, which precipitates from solution with concentration is collected by ®ltration, washed with tetrahydrofuran followed by diethyl ether, and dried in vacuo. The yield after drying is 85%. [(Cp)2Co]2[HAT(CN)6] displays a characteristic n(CBN) stretch at 2195 cm 1. Elemental analysis: Anal. P.S. Szalay et al. / Synthetic Metals 122 (2001) 535±542 537 Table 1 Crystallographic data and structural refinement parameters for HAT(CN)6 (1), {Co(HAT(CN)4O2)(H2O)4}4H2O (2), {Ni(HAT(CN)4O2)(H2O)4}4H2O (3) and {(Cp2Fe)3[HAT(CN)2]}CH3CN (4) C16H20N10O10Co Formula C18N12 Molecular weight 384.30 571.32 T (K) 110(2) 173(2) Ê) l (A 0.71069 0.71069 Space group 42/n n m P1 Ê) a (A 23.537(3) 8.571(2) Ê) b (A 23.537(3) 10.065(2) Ê) c (A 14.834(3) 13.571(3) a (8) 90.00 81.78(3) b (8) 90.00 77.43(3) g (8) 120.00 85.70(3) Ê 3) V (A 7178(2) 1129.8(4) Z 18 2 rcalc (g cm 3) 1.600 1.692 m (Mo Ka, cm 1) 1.11 8.43 F (0 0 0) 3456 586 Y Range 1.72±28.25 1.56±28.38 Reflections 1973 5236 I > ns(I) 2 2 Reflections/parameters 749/137 3199/398 Goodness-of-fit 0.834 1.023 R1a 0.0589 0.0541 R2 0.1449b 0.1252c P P a R1 h Fo Fc = Fo . i1=2 P P b R2 o Fo2 Fc2 2 = o Fo2 2 ; o 1= s2 Fo2 0:0916P2 where P Fo2 2Fc2 =3. C16H20N10O10Ni 571.09 173(2) 0.71069 P1 8.546(2) 10.072(2) 13.493(3) 82.07(3) 77.53(3) 85.49(3) 1121.7(4) 2 1.691 9.43 588 1.56±28.30 5253 2 2884/398 0.913 0.0556 0.1126d C50H33N13Fe3 983.44 173(2) 0.71069 P3 16.673(2) 16.673(2) 9.878(2) 90.00 90.00 120.00 2378.0(7) 2 1.373 9.52 1004 1.41±28.24 7587 2 6109/378 1.420 0.089 0.2335e o 1= s2 Fo2 0:077P2 . o 1= s2 Fo2 0:0642P2 . e o 1= s2 Fo2 0:100P2 . c d Calcd. for C38N12H20Co2: C, 59.86; H, 2.64; N, 22.04. Found: C, 58.95; H, 2.81; N, 22.49. M[HAT(CN)6]2 (M: Co (7) or Zn (8)) complexes are prepared by the reaction of an excess of the metal powder or chips with a vigorously stirred acetonitrile solution of HAT(CN)6 (0.200 g, 0.520 mmol). The resulting dark bluegreen solutions are ®ltered through Celite and reduced to dryness. The solid is washed with dichloromethane followed by diethyl ether and dried in vacuo. The yields after drying are 85% for both the Co and Zn compounds. The Co and Zn compounds display characteristic n(CBN) stretches at 2210 and 2212 cm 1, respectively. The results of ESR studies are presented in the results and discussion section of this manuscript. Elemental analysis: Anal. Calcd. for C36N24Co: C, 52.25; N, 40.62. Found: C, 51.32; N, 39.95. Anal. Calcd. for C36N24Zn: C, 51.84; N, 40.31. Found: C, 51.06; N, 39.73. 3. Results and discussion 3.1. Chemistry of neutral HAT(CN)6 The HAT(CN)6 molecule is a strongly electron-de®cient heterocycle. One consequence of this is that it avoids self-pcomplexation in its neutral form. Crystals obtained from a methanol/acetonitrile mixture reveal that neutral HAT(CN)6 molecules do not stack and, in fact, they are signi®cantly offset with respect to one another, forming staircase-like Ê , which chains (Fig. 2). The intermolecular spacing is 3.75 A Ê is larger than the expected van der Waals radii sum of 3.54 A [37]. The overall crystal packing is mainly dominated by perpendicular CN-p interactions which leads to a complicated, but highly symmetric, 3D hexagonal arrangement as nicely illustrated in Fig. 3. Although neutral HAT(CN)6 is expected to be a very weak ligand, reactions with aqueous ®rst row transition metal ions were nevertheless carried out. When a mixture of water/ acetonitrile was used for the reaction medium, the novel complex M(HAT(CN)4O2)(H2O)4 (M: Co, Ni) was isolated (Fig. 4). This result demonstrates the fact that HAT(CN)6 is hydrolytically unstable and undergoes cleavage of the two activated nitrile groups adjacent to the coordinated metal center. The result is a dianionic oxigen ligand. The coordination sphere of the metal center in these compounds is completed by water molecules. Ferrocene was initially employed as a reducing agent for HAT(CN)6, but proved to be an insuf®ciently strong reductant for this purpose. Thegreen crystalline product that was isolated from this reaction was shown by single-crystal X-ray crystallography to be the 3:1 phase {(Cp2Fe)3[HAT(CN)6]}CH3CN (Fig. 5). In support of the neutral assignment for the molecules in this compound is the n(CBN) stretch at 2243 cm 1 in the infrared spectrum. It appears that the ferrocene molecules act as templates in the formation of a layered structure. Within a 538 P.S. Szalay et al. / Synthetic Metals 122 (2001) 535±542 Fig. 2. Two different views of the HAT(CN)6 chain in 1. given layer, the ferrocene resides in the space adjacent to the chelating bipyridine moieties of HAT(CN)6. Adjacent layers are staggered such that the ferrocene in one layer lies underneath the cyano groups of the HAT(CN)6 in the layer above it. 3.2. Chemical reduction of HAT(CN)6 A more convenient and successful approach to chemically reducing HAT(CN)6 is to use cobaltocene. It is a suf®ciently Fig. 3. View of the 3D structure of HAT(CN)6 (1). P.S. Szalay et al. / Synthetic Metals 122 (2001) 535±542 539 dimerized in the solid state, perhaps due to the formation of integrated stacks with interactions between anions and cations. The reaction of neutral HAT(CN)6 with an excess of colbaltocene yields the salt [Cp2Co]2[HAT(CN)6]. The dianion is extremely sensitive to the presence of air or water, which renders its characterization dif®cult. The IR spectrum contains a characteristic n(CBN) mode at 2195 cm 1, which is lower in energy by 19 cm 1 relative to the monoradical [HAT(CN)6]1 and 48 cm 1 lower relative to neutral HAT(CN)6. 3.3. Reactions of HAT(CN)6 with metals Fig. 4. View of the M(HAT(CN)4O2)(H2O)4 complex present in compounds {Co(HAT(CN)4O2)(H2O)4}4H2O (2) and {Ni(HAT(CN)4O2)(H2O)4}4H2O (3). strong reducing agent to reduce HAT(CN)6 to both the monoradical [HAT(CN)6]1 and the doubly reduced [HAT(CN)6]2 forms. The compound [(Cp)2Co][HAT(CN)6] is isolated as a green powder from the reaction of cobaltocene with an excess of HAT(CN)6. Although single crystals have not yet been obtained, some characteristic features of the [HAT(CN)6]1 radical have been identi®ed. The IR spectrum contains a n(CBN) stretch at 2214 cm 1, which is shifted to lower energies from that of neutral HAT(CN)6. This is analogous to the shifts observed for other organic radicals with CBN groups [30,38±43]. The ESR spectrum of a solid sample of [(Cp)2Co][HAT(CN)6] at 4 K shows a typical isotropic resonance for an organic radical (S 1/2), with a g value of 2.0084(5) and with a narrow line width of 7 G. These data indicate that the [HAT(CN)6]1 radicals are not strongly Reactions of [(Cp)2Co][HAT(CN)6] with divalent ®rst row transition metal ions such as Mn, Fe, Co, and Ni in acetonitrile at room temperature yield dark green precipitates. The high degree of insolubility of these products in common solvents suggests the presence of extended structures. Assuming that the preferred [HAT(CN)6]1 binding mode is through the chelating N-sites and that the nitrile groups are suf®ciently bulky to force the tetrahedral versus octahedral geometry (Fig. 6), the expected stoichiometry for the binary metal-[HAT(CN)6] coordination polymers would be 3:2. The presence of the [HAT(CN)6]1 radical and the absence of [(Cp)2Co]2 was veri®ed by IR spectroscopy, but the low crystallinity of these samples precluded a structural study. Thus, it remains unknown if the compounds are discrete or polymeric, or if the solid state structure is controlled solely by metal±ligand interactions or also by ligand±ligand interactions. The magnetic measurements for these compounds revealed only paramagnetic behavior characteristic of the metal ions with no contributions from the ligand. This fact, taken together with the absence of signi®cant magnetic exchange between localized moments in the metal ions in all cases, hints at p±p dimerization of the ligand in the solid state. Such a situation would yield Fig. 5. View of the crystal structure of {(Cp2Fe)3[HAT(CN)6]}CH3CN (4) down the a (left) and c axis (right). 540 P.S. Szalay et al. / Synthetic Metals 122 (2001) 535±542 Fig. 6. Model for the M[HAT(CN)6]2 complex that emphasizes the steric role of the cyano substituents. diamagnetic dimers (S 0) similar to many other salts of organic radicals [44]. An alternative method for preparing binary transition metal compounds of [HAT(CN)6]1 is to directly react HAT(CN)6 with ®rst row transition metals such as Co and Zn. The resulting dark blue-green powders are soluble in polar organic solvents such as acetonitrile, which is a good indication that discrete molecules are being formed. Although X-ray quality single crystals have not been obtained, several spectroscopic characterization techniques have been employed that support their formulation as M[HAT(CN)6]2 (M: Co, Zn). The n(CBN) stretches of 2210 and 2212 cm 1 are consistent with the presence of [HAT(CN)6]1 . Both compounds are ESR silent in the solid state, but spectra obtained form acetonitrile/toluene frozen solutions at 4 K exhibit signals. The spectrum of the Zn compound consists of a single isotropic feature at g 2:0031(5) with DH 15 G. Hyper®ne coupling would not be expected due to the low natural abundance of 67 Zn (4%). The compound, Co[HAT(CN)6]2 however, with 59 Co (I 7/2, 100% abundant), exhibits an ESR signal with hyper®ne coupling to the Co nucleus at g 2:0051(5) with DH 480 G. Additionally, a broad signal was observed for the S 3/2 Co(II) at g 2:180. This g value is in good agreement with the one obtained from magnetic susceptibility studies of the compound, which revealed a high temperature moment of meff 4:2 B.M. and a g value of 2.17. These data indicate that tetrahedral Co(II) centers are magnetically isolated from each other. As con®rmed by ESR studies of the compound in the solid state, there is no contribution to the magnetization from [HAT(CN)6]1 presumably due to the intermolecular interactions in the solid state. The Zn compound is rendered diamagnetic in the solid state due to these stacking interactions. The reaction of anhydrous CuCl with HAT(CN)6 (3:2 ratio) in an acetonitrile/methanol mixture yields a shiny brown precipitate that is insoluble in all common solvents. This compound differs from the others in that the most intense n(CBN) stretch appears at 2132 cm 1, which is lower than the corresponding stretches for either the mono- or the dianion species. Also the elemental analysis indicates a high Cu content (Cu/HAT(CN)6 ratio >2:1), but no satisfactory stoichiometry could be calculated. The most interesting aspect of this product is the magnetic behavior. As Fig. 7 shows, wT continously decreases from room temperature, in accord with antiferromagnetic interactions between spin carriers. Since the stoichiometry for this compound is not known, the data are plotted per mol of Cu metal. The high temperature regime can be conveniently ®t to a Curie±Weiss law with C 0:37 emu/mol of Cu, which indicates that one unpaired electron is present per Cu atom. If we assume the Fig. 7. Temperature dependence of wT for the {Cu-[HAT(CN)6]} polymer (1000 G). Inset: temperature dependence of the magnetic susceptibility in the field-cooled and zero-field-cooled experiments (with an applied field of 15 G). P.S. Szalay et al. / Synthetic Metals 122 (2001) 535±542 541 transition metals with the pre-reduced [HAT(CN)6]1 anion. The structures of these salts are not known, but they show magnetic behavior indicative of a dimerization of the [HAT(CN)6]1 radicals in the solid state. Finally, and most interestingly, the reaction of HAT(CN)6 with CuCl yields a new molecule-based magnet. Although no structural information is currently available on this system, the fact that this compound shows spontaneous magnetization below 6 K with a coercive ®eld of 184 G at 2 K is an appealing result that con®rms our prediction that this ligand will produce interesting materials in the realm of molecule-based and molecular magnetism. Acknowledgements Fig. 8. Field dependence of the first magnetization of the {Cu[HAT(CN)6]} polymer measured at 2 K. Inset: hysteresis loop between 2000 and 2000 G. presence of Cu(II) centers (S 1/2) then this translates to no contribution from HAT(CN)6 radicals, most likely due to dimerization of these units in the solid state. Another possibility that cannot be discounted is the presence of diamagnetic Cu(I) centers with the [HAT(CN)6]1 radicals as spin carriers. The Weiss constant is large and negative (y 93:1 K), indicating that antiferromagnetic interactions are dominant between nearest neighbors. The most striking feature of the magnetic data appears at very low temperatures when wT reaches a minimum and then increases very rapidly, which is a signature of spontaneous magnetization. This hypothesis was con®rmed by performing ®eld-cooled and zero ®eld-cooled measurements (inset in Fig. 7), which reveal an obvious difference at 6 K. The ®eld dependence of the magnetization at 2 K (Fig. 8) does not saturate even at 7 T, which is indicative of some degree of canting between the individual spins. A hysteresis loop is observed for this compound (inset Fig. 8) with a coercive ®eld of 184 G at 2 K. 4. Conclusions Convenient outer-sphere chemical reduction routes of HAT(CN)6 to the radical [HAT(CN)6]1 species and the dianion [HAT(CN)6]2 have been described. Transition metals can also be used as reducing agents, but in these cases complexation of the metal with the reduced HAT(CN)6 species occurs. Discrete, soluble paramagnetic complexes are obtained by direct reaction of HAT(CN)6 with Co and Zn. We plan to use these molecules and related analogues as building blocks for extended structures. Insoluble polymeric binary salts have been obtained from reactions of divalent KRD gratefully acknowledges the National Science Foundation for support of this work (NSF CHE-9906583) and for funding the CCD X-ray instrument (NSF-9807975) and the SQUID magnetometer (NSF-9974899). JRGM thanks the Ministerio de Educacion y Cultura for a postdoctoral fellowship. References [1] J.M. Williams, J.R. Ferraro, R.J. Thorn, K.D. Carlson, U. Geiser, H.H. Wang, A.M. Kini, M.H. Whangbo, in: R.N. 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