Synthesis and Properties of New Molecule-Based Magnets Containing Mn(II), Cu(II) and Nitronyl Nitroxide Radical Cation

[摘要] As evidenced by the remarkable increase in publications on the topic, the field of molecule-based magnetism is flourishing. However, the critical temperature of the molecule-based magnets remains low and up to now there have not been compounds of this type used for any technological application. Four molecule-based magnets have been reported to remain magnetically ordered near room temperature.1-3 One of them decomposes at 350 K, and the other three demagnetize near 315 K. The crystal structures of these compounds have not all been determined.One of the outstanding features of molecule-based materials is that the magnetic properties may be transformed by quite small and subtle modifications in the molecular chemistry. Variations in the organic molecules may lead to modifications in the magnetism of the material. Sometimes the molecular fragments only serve as spacers and are not involved directly in the interaction between neighboring atoms.4 In other cases, the organic part, although not possessing magnetic moment or not being responsible for the transmission of the interaction is essential to the exchange process.5 In the last few years, numerous magnetic molecular units carrying spin and able to establish chemical connections with other magnetic units have been developed and investigated.6 Several magnetic studies have been performed on these polynuclear systems and theoretical models have been created to correlate the experimental results (susceptibility, magnetization) with the respective Hamiltonian.7 Comparing the theoretical models and the experimental data, it is possible to determine the exchange and/or anisotropy parameters of the system. The results confirm or contradict the type of interaction initially foreseen or desired (antiferromagnetic or ferromagnetic). From these studies, it is conceivable to elaborate new synthetic strategies to tailor materials with specific properties.The first examples of molecular materials possessing the properties of magnets appeared in 1986. Miller8 and co-workers, synthesized an organometallic compound with the formula [Fe(Me5Cp)2][TCNE] (Me5Cp = pentamethylcyclopentadienyl; TCNE = tetracyanoethylene) that presented a magnetic critical temperature Tc equal to 4.8 K. Simultaneously, Kahn9 and co-workers, described the synthesis of the coordination compound [MnCu(pbaOH)(H2O)3], (pbaOH = 2-hydroxy-1,3-propylenebis(oxamato)) with Tc = 4.6 K.In the beginning of the nineties, we initiated a research in the direction to increase the dimensionality of the compounds using a more 'organic' building block: [Cu(opba)]2- where opba stands for ortho-phenylenebis(oxamato) (see Figure 1a). We succeeded in designing two-dimensional magnets of formula [cat]2[M2 {Cu(opba)}3].S, where cat+ is a monovalent cation and S stands for solvent molecules. The first reported example10 of this series was the soft molecule-based magnet [Bu4N]2[Mn2{Cu(opba)} 3].6DMSO.H2O, with Tc = 15 K. When M = Co(II), a hard magnet11 is obtained (Hc = 3000 Oe), namely the [Bu4N]2[Co2{Cu(opba)} 3].3DMSO.3H2O with Tc of the order of 32 K.    In 1993, we obtained the compound [Me-Rad]2 [Mn2{Cu(opba)}3](DMSO) 2.2H2O,12 where Me-Rad+ is a nitronyl nitroxide radical cation (see Figure 1b). This compound, a magnet with Tc of 22 K, presents a rare interlocked structure. Moreover, by the time of the description of this compound, no molecular magnets were known with a resolved structure and Tc above of 15 K.A large number of reports have appeared in the literature on this [cat]2[M2{Cu(opba)}3 ].S system involving theoretical13,14 and different experimental studies,15-17 aiming the development of models to explain the observed magnetic behaviors. Also, numerous reports explored the potentialities of these bimetallic compounds18,19 through chemical variations. Although several groups have been working on this system, only recently the reproduction of the interlocked structure of the compound [Me-Rad]2[Mn2 {Cu(opba)}3](DMSO)2.2H 2O was achieved.20,21In this work we describe the synthesis of two new precursors (Pr-Rad)2[Cu(opba)].H2O (1) and (Bu-Rad)2 [Cu(opba)].2H2O (2), and two new molecule-based magnets, [Pr-Rad]2[Mn2{Cu(opba)}3 ].3.3DMSO.5H2O (3) and [Bu-Rad]2[Mn2{Cu(opba)}3 ].3DMSO.6H2O (4). With the present study we intended to verify the influence of changes in the radical cation in the interlocked structure, as well as the effect of these changes on the critical temperature. The results obtained through X-ray structural analysis, magnetic properties and elemental analysis and the critical temperature of the compounds indicate the interlocked structure to be the most likely for these new molecule-based magnets.  Experimental Syntheses: The radical 2-(pyridine-4-yl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide, abbreviated as Rad,22 the ligand opba, and the tetrabutylammonium salt of the copper(II) precursor (Bu4N)2[Cu(opba)] were prepared as already described.10 Pr-Rad+I: A mixture of 1.00 g (4.3 mmol) of Rad in 4.5 mL (7.85 g, 46 mmol) of 1-iodopropane and 1.0 mL of THF was stirred for 48 h, at 45 ÂºC. The green precipitate was collected by filtration, washed with THF and dried under vacuum. Yield: 1.42 g (82 %). Anal. Calc. for C15H23N3O2 I: C, 44.56; H, 5.75; N, 10.40. Found: C, 44.35; H, 5.77; N, 10.26%. Bu-Rad+I: A mixture of 1.0 g (4.3 mmol) of Rad in 5.0 mL (8.1 g, 44 mmol) of 1-iodobutane and 1.0 mL of THF was kept under stirring for 72 h, at 45 °C. The yellow-green precipitate was isolated by filtration, washed with THF and dried under vacuum. Yield: 1.12 g (62 %). Anal. Calc. for C16H25N3O2 I: C, 45.93; H, 6.04; N, 10.05. Found: C, 45.85; H, 5.68; N, 9.48%. (Pr-Rad)2[Cu(opba)].H2O (1): A solution of 0.69 g (0.86 mmol) of (Bu4N)2[Cu(opba)] in 15 mL of dichlorometane was added to a solution of 0.7 g (1.73 mmol) of PrRad+I- in 7.0 mL of chloroform. The mixture was heated at 40 °C and stirred for 90 min. The resulting brown polycrystalline precipitate was collected by filtration, washed with a 2:1 CH2Cl2/CHCl3 mixture, and dried in a dessicator under vacuum. Yield: 0.46 g (60 %). Anal. Calc. for C40H52N8O11 Cu: C, 54.32; H, 5.88; N, 12.67; Cu, 7.18. Found: C, 54.58; H, 5.92; N, 12.12; Cu, 6.94%. (Bu-Rad)2[Cu(opba)].2H2O (2): A solution of 0.53 g (0.67 mmol) of (Bu4N)2[Cu(opba)] in 2.0 mL of dichlorometane was added to a solution of 0.56 g (1.35 mmol) of BuRad+I- in 4.0 mL of chloroform under stirring at room temperature. The mixture was stirred for 30 min. The resulting brown polycrystalline precipitate was collected by filtration, washed with a 1:2 CH2Cl2/CHCl3 mixture, and dried in a dessicator under vacuum. Yield: 0.44 g (71 %). Anal. Calc. for C42H58N8O12 Cu: C, 54.21; H, 6.23; N, 12.04; Cu, 6.83. Found: C, 54.20; H, 5.95; N, 11.96; Cu, 6.43%. [Pr-Rad]2[Mn2{Cu(opba)} 3].3.3DMSO.5H2O (3): A solution of 0.010 g (0.056 mmol) of MnCl2.3H2O in 1.4 mL of DMSO was added to a solution of 0.21 g (0.23 mmol) of 1 in 6.4 mL of DMSO. The mixture was stirred at room temperature for 10 min. The color slowly turned green. The solution was then distributed into several shallow beakers that were then sealed and allowed to stand. The first crystals appeared within a few hours. They were collected after 5 days, washed with DMSO, and dried in a dessicator. Yield: 0.043 g (79%). Anal. Calc. for C67H88N12O30 S3.3Cu3Mn2: C, 41.07; H, 4.51; N, 8.63; S, 5.44; Cu, 9.79; Mn, 5.64. Found: C, 40.84; H, 4.28; N, 8.60; S, 5.40; Cu, 9.90; Mn, 5.38%. [Bu-Rad]2[Mn2{Cu(opba)} 3].3DMSO.6H2O (4): A solution of 0.0068 g (0.038 mmol) of MnCl2.3H2O in 1.3 mL of DMSO was added to a solution of 0.25 g (0.27 mmol) of 2 in 7.0 mL of DMSO. The mixture was stirred at room temperature for 10 min. The color slowly turned to green. The crystallization procedure was the same used for 3. The crystals were collected after 3 days, washed with DMSO, and dried in a dessicator. Yield: 0.034 g (91%). Anal. Calc. for C68H92N12O30 S3Cu3Mn2: C, 41.45; H, 4.67; N, 8.53; S, 4.89; Cu, 9.67; Mn, 5.58. Found: C, 41.41; H, 4.33; N, 8.66; S, 4.61; Cu, 9.90; Mn, 5.51%. Physical Techniques: Xray powder diffraction studies were performed in a Rigaku- Geigerflex equipment. Data were collected in the Bragg-Bretano mode with 2q varying from 4.0 to 40 and using a 0.020833° step; monochromatic CuKa radiation was used. Elemental analyses (C, H, N, Mn, Cu) were performed on a Perkin-Elmer 2400 apparatus. The magnetic properties were studied in the Quantum Design MPMS XL7 SQUID magnetometer, working in the dc mode between 2 and 300 K. The raw susceptibility data were corrected for the core diamagnetism estimated as: -397x10-6 and -433x10-6 emu mol-1 for the precursors 1 and 2, respectively and -790x10-6 and -766x10-6 emu mol-1 for the magnets 3 and 4, respectively.  Results and Discussion The synthesis of the magnets is very sensitive to the purity of the precursors and some other parameters. To form materials of stoichiometry Cu:Mn 3:2, the copper building block must be present in excess. If the Cu:Mn ratio in the solution is low, formation of MnCu(opba) chains23 occurs instead. The effect of reagents' concentration upon the reaction and on the formation of crystals is crucial. If the solution is over diluted no product is obtained, as Mn(II) is slightly sensitive and decomposes before or during the crystallization, producing an insoluble brown impurity. The use of water as a solvent favors formation of chains and the use of chloride is preferable to other salts.A variety of experiments were performed to find the best crystallization conditions. Beyond variations of concentration and Cu:Mn ratio, other parameters were considered such as the temperature, the type of crystallization flask (glass or polystyrene) and mixture of solvents. Regarding the stability of the magnets, it should be pointed out that, after removal from the mother liquor, they could be stored in a freezer for several months. PrecursorsThe magnetic susceptibility data for 1 and 2 are shown in Figure 2 in the form of the cMT versus T plot, cM being the molar magnetic susceptibility and T the temperature.    At room temperature cMT is equal to 1.05 emu K mol-1 for 1 and 1.21 emu K mol-1 for 2. These values are close to the theoretical value expected for 3 isolated spins S = 1/2 (two spins referring to the radicals and one to the copper) that can be calculated by the Curie law: Substituting the Bohr magneton (b) and Boltzmann constant (k) values into equation (1): and substituting the S value in equation (2) and considering g = 2: As T is lowered, cMT first increases, reaches a maximum at 10.6 K and 6.1 K with cMT = 1.56 emu K mol-1 and 1.59 emu K mol-1 for 1 and 2, respectively, and finally decreases rapidly. The profile of this curve indicates that in the two compounds some ferromagnetic interactions are operative. The decrease of cMT in the low-temperature region may be attributed to intermolecular interactions.The magnetic properties for the analogous precursor (Et-Rad)2[Cu(opba)].CH3CN.H 2O has already been described.20 Its cMT versus T curve shows the same outline as those of 1 and 2 and corresponds to a dimer ferromagnetically coupled and a spin S = 1/2 isolated. Gatteschi and co-workers have already shown that the orthogonality of a copper dx2-y2-type and a nitronyl nitroxide radical pp-type magnetic orbitals affords ferromagnetic coupling.24 This orthogonality can be obtained, for example, if the radical occupies an apical position of the Cu(II) ion, as in (Et-Rad)2[Cu(opba)].CH3CN.H 2O. The magnetic properties of this compound are very similar to those of the precursors 1 and 2, indicating that they have the same molecular structure. The structure of (Et-Rad)2[Cu(opba)].CH3CN.H 2O has been solved by a single crystal X-ray diffraction analysis and, as shown in Figure 3, consists of one [Cu(opba)]2-, two radical cations and molecules of CH3CN and H2O not coordinated. One of the radical cations is weakly linked to the copper while the other is relatively isolated.    X-ray powder diffraction were performed for 1 and 2 and the Rietveld method has been employed in structural analysis. For the sake of comparison, diffraction data were also obtained for (Et-Rad)2[Cu(opba)].CH3CN.H 2O and its crystal structure20 was used as a reference. The results obtained for the structure refinements for the three compounds are not of best quality. Nevertheless, as in the

[学科分类] 化学（综合）