Chem. Mater. 1999, 11, 2205-2210
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Synthesis, Crystallography, and Magnetic Properties of 2-tert-Butylaminoxylbenzimidazole Jacqueline R. Ferrer,† Paul M. Lahti,*,† Clifford George,‡ Guillermo Antorrena,§ and Fernando Palacio§ Department of Chemistry, University of Massachusetts, Amherst, Massachusetts 01003, Laboratory for the Structure of Matter, Naval Research Laboratory, Washington, DC 20375, and Instituto de Ciencia de Materiales de Arago´ n, CSICsUniversidad de Zaragoza, 50009 Zaragoza, Spain Received March 11, 1999. Revised Manuscript Received May 17, 1999
The title nitroxide is stable at room temperature for months. ESR spectroscopy shows moderate spin delocalization from the nitroxide unit onto the benzimidazole unit. The radical crystallizes in the orthorhombic Pbca space group, forming columns that are joined by hydrogen bonds into a pseudo-2D motif. Magnetic susceptibility studies of a polycrystalline sample at low temperatures are best fit to a simple planar 2D Heisenberg model with S ) 1 /2 spin sites, having a Weiss constant of θ ) -4.24 K and an intraplane exchange interaction of J/k ) -1.60 K. Computational modeling of various close-contact dimeric interactions in the crystal structure suggest that the largest interaction in the system occurs between nearby nitroxide units related by a center of inversion and that this interaction is responsible for the overall antiferromagnetic interaction observed.
Introduction Much recent research effort has been focused on the synthesis and characterization of molecular magnetic materials. Investigations of new stable organic radicals and the organization of their spin moments through crystal engineering offer prospects for new types of magnetic materials. To achieve magnetic spin ordering in a bulk solid, one must first achieve appropriate crystal ordering. Qualitative models such as those of McConnell have attempted to delineate the types of three-dimensional ordering that lead to ferromagnetic behavior in the bulk solid state of an organic radical.1,2 One of the very promising strategies for controlling the solid-state packing behavior of organic molecules is through the use of hydrogen bonding. For example, Etter has classified hydrogen-bonded motifs in the solid state by graph set methodology.3 In addition, complementary hydrogen bonding of different molecules is a well-established theme in biological chemistry. The groups of Sugawara4,5 and Veciana6-9 have shown that †
University of Massachusetts. Naval Research Laboratory. CSICsUniversidad de Zaragoza. (1) McConnell, H. M. J. Chem. Phys. 1963, 39, 1910. (2) McConnell, H. M. Proc. R. A. Welch Found. Conf. 1967, 11, 1144. (3) Etter, M. C. Acc. Chem. Res. 1990, 23, 120-126. (4) Sugawara, T.; Matsushita, M. M.; Izuoka, A.; Wada, N.; Takeda, N.; Ishikawa, M. J. Chem. Soc., Chem. Commun. 1994, 1723. (5) Sugawara, T.; Izuoka, A. Mol. Cryst. Liq. Cryst. Sci. Technol., Sect. A 1997, 305, 41-54. (6) Cirujeda, J.; Ochando, L. E.; Amigo´, J. M.; Rovira, C.; Rius, J.; Veciana, J. Angew. Chem., Int. Ed. Engl. 1995, 34, 55. (7) Cirujeda, J.; Herna`ndez-Gasio´, E.; Panthou, F. L.-F.; Laugier, J.; Mas, M.; Molins, E.; Rovira, C.; Novoa, J. J.; Rey, P.; Veciana, J. Mol. Cryst. Liq. Cryst. 1995, 271, 1. (8) Cirujeda, J.; Herna`ndez-Gasio´, E.; Rovira, C.; Stanger, J.-L.; Turek, P.; Veciana, J. J. Mater. Chem 1995, 5, 243-252. (9) Novoa, J. J.; Deumal, M.; Kinoshita, M.; Hosokishi, Y.; Veciana, J.; Cirujeda, J. Mol. Cryst. Liq. Cryst. 1997, 305, 129-141. ‡ §
hydrogen-bonding hydroxyphenyl and dihydroxyphenyl groups may be attached to stable nitronylnitroxide radical moieties, yielding solid-state radicals with spinordering behaviors. Yoshioka and co-workers have attached the imidazole and benzimidazole hydrogenbonding groups to nitronylnitroxide spin-bearing centers to give stable solids with spin-ordering behavior.10,11 The prospects for using hydrogen bonding motifs in the area of molecular magnetism are thus very promising. In this paper, we describe the synthesis, X-ray crystallographic analysis, and magnetic characterization of 2-tert-butylaminoxylbenzimidazole, 1. This organic radical shows high stability under ambient conditions, presents two-dimensional (2D) ordering in the crystal phase, and exhibits antiferromagnetic (AFM) interactions between the radical sites at low temperatures, in a manner that is consistent with the crystalline order. Results Synthesis. The synthesis of radical 1 was accomplished by the route shown in Figure 1. Benzimidazole was protected by N-hydroxymethylation to give 2, which in turn was treated with n-butyllithium to give the corresponding carbanion in the 2-position due to the directing effect of the deprotonated protecting group. The carbanion was treated with 2-methyl-2-nitrosopropane12 and subjected to a protic quench to give hydroxylamine 3. Compound 3 oxidized quite easily in solution when exposed to air, and slowly turned reddish even in the solid state. We found it best to subject 3 quickly to (10) Yoshioka, N.; Irasawa, M.; Mochizuki, Y.; Kato, T.; Inoue, H.; Ohba, S. Chem. Lett. 1997, 251-252. (11) Yoshioka, N.; Irisawa, M.; Mochizuki, Y.; Aoki, T.; Inoue, H. Mol. Cryst. Liq. Cryst. 1997, 306, 403-408. (12) Stowell, J. C. J. Org. Chem. 1971, 36, 3055.
10.1021/cm990149d CCC: $18.00 © 1999 American Chemical Society Published on Web 07/07/1999
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Ferrer et al. Table 1. Hyperfine Coupling Constants for Radical 1 in Figure 2a
Figure 1. Synthesis of radical 1: (a) (CH2O)n, (b) n-BuLi, (c) Me3C-NdO, and (d) PbO2.
functionality
fitted hfc (gauss)
nitroxide N imidazole dN imidazole -N(H) benzimidazole 4,5,6,7 C-H
a(N) ) 10.16 G a(N) ) 2.13 G a(N) ) 0.79 G a(H) ) 0.86, 0.56, 0.37, 0.22 G
EPR-II computed hfc (gauss) 10.6 G 3.7 G 0.6 G
a Fitted hfc values obtained by WINSIM fit to experimental spectrum (ref 13). Correlation coefficient of fit was 0.986. Computed hfc values obtained using Gaussian 98 for model compound where the tert-butyl group in 1 is replaced by a methyl group.
Figure 3. FTIR spectra in the 2500-3500 cm-1 region for radical 1 at 12, 153, and 300 K as marked.
Figure 2. Experimental and simulated spectra for radical 1. Experimental spectra obtained at ν0 ) 9.8006 GHz in benzene, simulation carried out with WINSIM (ref 13).
suspension-phase oxidation with recently made lead dioxide to go directly to nitroxide 1, since the radical is more stable to air than precursor 3. The nitroxide itself recrystallizes from dichloromethane/hexane to give ruby-colored orthorhombic crystals that appear not to decompose over the course of several months of exposure to air. Spectroscopy. Nitroxide 1 was subjected to X-band electron-spin resonance (ESR) spectroscopy in benzene at room temperature. A typical spectrum is shown in Figure 2. In addition to the major hyperfine coupling constant (hfc) that is attributable to the nitroxide nitrogen, additional splitting is evident due to spin delocalization onto the benzimidazole ring. Line-shape fitting analysis of the spectrum was carried out using the WINSIM program of Duling13 to obtain the hfc data given in Table 1, and the simulated spectrum shown in Figure 2. The correlation coefficient of the fit was 0.986. The hfc were also estimated using the McConnell equation with spin densities estimated by computation (13) Duling, D. R. J. Magn. Res. 1994, B104, 105-110.
using the EPR-II method with the B3LYP14,15 hybrid density function wave function in Gaussian 98.16 Equation 1 was used where a(hfc) is the hfc in gauss, and Fi is the total atomic spin density computed on a nitrogen atom exhibiting hfc. A model compound was used in place of 1, with the tert-butyl group replaced by a methyl group. The predicted and experimental hfc are in good accord, as shown in Table 1.
a(hfc) ) (30 G) × Fi
(1)
The Fourier transform infrared (FTIR) spectrum of 1 showed the disappearance of the broad, strong -OH stretching peak that is characteristic of the precursor hydroxylamine 3. The N-H stretching remains at 3100 cm-1. Variable-temperature FTIR spectroscopic studies of 1 were carried out over 12-298 K. Figure 3 illustrates the changes observed in the spectrum in the 2500-3500 cm-1 region as a function of temperature. Crystallography. A ruby-colored 0.48 × 0.32 × 0.08 mm plate grown by slow evaporation from dichloromethane/hexane was selected for data collection, C11H14N3O, FW ) 204.25. Data were collected on a (14) Becke, A. D. Phys. Rev. A 1988, 38, 3098-3100. (15) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785789. (16) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Gill, P. M. W.; Johnson, B. G.; Robb, M. A.; Cheeseman, J. R.; Keith, T.; Petersson, G. A.; Montgomery, J. A.; Raghavachari, K.; Al-Laham, M. A.; Zakrzewski, V. G.; Ortiz, J. V.; Foresman, J. B.; Cioslowski, J.; Stefanov, B. B.; Nanayakkara, A.; Challacombe, M.; Peng, C. Y.; Ayala, P. Y.; Chen, W.; Wong, M. W.; Andres, J. L.; Replogle, E. S.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Binkley, J. S.; Defrees, D. J.; Baker, J.; Stewart, J. P.; Head-Gordon, M.; Gonzalez, C.; Pople, J. A. Gaussian; Gaussian Inc.: Pittsburgh, PA, 1998.
2-tert-Butylaminoxylbenzimidazole
Chem. Mater., Vol. 11, No. 8, 1999 2207 Table 2. Atomic Coordinates (×10-4) and Equivalent Isotropic Displacement Parameters (Å2 × 103) for Radical 1a
Figure 4. Schematic diagram of packing pattern for radical 1 and picture showing molecular structure orientation relative to unit cell axes.
computer-controlled diffractometer with an incident beam graphite monochromator (Bruker P4 with Cu KR radiation, λ ) 1.54178 Å, T ) 295 K). A least-squares refinement using 40 centered reflections within 11 < 2θ < 55° gave the orthorhombic Pbca cell, a ) 6.880(1), b ) 10.294(1), c ) 31.336(2) Å, with V ) 2219.2(2) Å, Z ) 8, dcalc ) 1.22 g/cm3. A total of 1505 reflections were measured in the θ/2θ mode to 2θmax ) 112°, of which there were 1448 independent reflections. Corrections were applied for Lorentz and polarization effects. A face indexed numerical absorption correction was applied, µ ) 0.66 mm-1; maximum and minimum transmission were 0.95 and 0.81, respectively. The structure was solved by direct methods and refined on F2 with a fullmatrix least-squares SHELTXTL97.17 The 140 parameters refined include the coordinates and anisotropic thermal parameters for all non-hydrogen atoms. Carbonbonded hydrogens used a riding model in which the coordinate shifts of the carbons were applied to the attached hydrogens, and C-H ) 0.96 Å, H angles idealized, and Uiso(H) ) (1.2 or 1.5)‚Ue(C). The final R values were R ) 0.041, and wR2 ) 0.095. The goodness of fit parameter was 1.02 and final difference Fourier excursions were 0.13 and -0.16 eÅ-3. Figure 4 shows a schematic of the crystal structure, as well as a simplified representation of the molecular order along the crystal axes. The figure shows the N-H‚‚‚N hydrogen bonds in the lattice as dashed lines; only the hydrogen atoms involved in this interaction are explicitly shown. The overall lattice is in effect a pair of bilayers. Each bilayer consists of stacks of nitroxide molecules that are hydrogen-bonded together to form planes of nitroxide radicals. Each stack within the sheet is canted perpendicular relative to its neighboring stack. Within each sheet, the nitroxide N-O bonds are all oriented syn to the N-H bond direction and all in the same direction, presumably due to the favorability of (17) Sheldrick, G. M. SHELTXTL97, Program for the Refinement of Crystal Structures; University of Go¨ttingen: Germany, 1997.
N(1) C(2) N(3) C(4) C(5) C(6) C(7) C(8) C(9) N(10) O(1) C(11) C(12) C(13) C(14) H(1A)
x
y
z
U(eq)
2910(3) 1876(3) 2302(3) 3712(3) 4699(4) 6031(4) 6410(4) 5458(4) 4105(3) 503(3) 500(3) -906(3) -1859(4) -2444(5) 217(4) 2690(30)
5385(2) 4383(2) 3242(2) 3534(2) 2720(2) 3267(3) 4599(3) 5425(2) 4868(2) 4630(2) 5785(2) 3639(2) 2923(3) 4368(3) 2718(2) 6220(20)
3646(1) 3807(1) 3638(1) 3336(1) 3056(1) 2786(1) 2789(1) 3064(1) 3335(1) 4123(1) 4277(1) 4293(1) 3923(1) 4543(1) 4582(1) 3727(7)
41(1) 36(1) 38(1) 36(1) 47(1) 55(1) 57(1) 50(1) 38(1) 41(1) 63(1) 42(1) 59(1) 76(1) 62(1) 49
a Space group Pbca (Z ) 8); a ) 6.880(1), b ) 10.294(1), c ) 31.336(2), R ) β ) γ ) 90°. H(1A) is the N-H hydrogen (other hydrogen atom coordinates are listed in Supporting Material, Table 5S).
local dipole alignment in this conformation. Within a bilayer, one sheet is related to the second by a typical benzenoid herringbone arrangement, such that the benzenoid C-H bonds of one sheet point into the π-cloud centroid of the other sheet. As a result, each bilayer has its nitroxide groups arrayed on the outside, and its benzimidazole aromatic cores on the inside. Each bilayer also has all of its N-O bonds aligned in the same direction. The lattice motif is completed by the presence of a center of symmetry relating one bilayer to another with its N-O aligned in the direction opposite to those in the symmetry-related bilayer. Table 2 gives relevant space group parameters and crystallographic coordinates. Full tables of atomic coordinates, bond distances, bond angles, anisotropic thermal parameters, and additional data collection and refinement parameters will be deposited with the Cambridge Crystallographic Database. Magnetic Studies. The sample of 1 used for magnetism studies was a polycrystalline powder with a weight of ∼55 mg. All experiments were performed using a Quantum Design SQUID magnetometer. The dc and ac magnetic susceptibilities (χ) were examined at zero external applied field over a temperature range of 1.850 K. In addition, dc magnetic susceptibility was carried out between 1.8 and 300 K at an applied field of 1 T. Magnetization versus applied magnetic field measurements (0-5 T) were carried out at 1.8 and 20 K. The magnetic susceptibility of the sample holder was measured and represents about 10% of the sample’s signal at room temperature and less than 1% below 25 K. For analysis of the susceptibility data, the Lande g factor and Weiss constant θ were taken as free parameters, and a fixed value of the spin quantum number S ) 1/2 was used, in accord with the radical nature of the spin bearing units. An additional term χd for the
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Figure 5. Magnetic susceptibility (dc) versus absolute temperature for radical 1.
Figure 6. Magnetization versus absolute temperature for radical 1 without diamagnetic correction, with theoretical diamagnetic correction, and with experimentally diamagnetic correction. The horizontal solid line represents the theoretical magnetization value for a radical (S ) 1/2) system.
temperature independent diamagnetic susceptibility contribution of the sample holder was also incorporated into the analysis model. Figure 5 shows a fit of the Curie-Weiss law over the 10-300 K temperature range against the experimental dc-susceptibility data. The values obtained for the fitted parameters were θ ) -4.24(4) K, g ) 2.029(3), and χd )2.81(1) × 10-4 emu/ Oe-mol. The χd term is of the same order of magnitude as the sum of the experimental diamagnetic contribution of the sample holder, plus the expected magnetism of the molecule (χd(theor)) based upon Pascal’s tables. Figure 6 shows the evolution of the effective moment µeff in Bohr magnetons (µB) with temperature. Three curves are shown: (a) the raw experimental data, (b) the data corrected by inclusion of the theoretical sample diamagnetism χd(theor), and (c) the data corrected by inclusion of the fitted diamagnetism χd(exp). A straight line indicates the value of µeff expected for an S ) 1/2 species, 1.73 µB. Discussion Solid-State Packing. Radical 1 stacks along the crystallographic a-axis, with hydrogen bonding between stacks along the b-axis. The stacking is assisted by the
Ferrer et al.
near-planarity of the radical in the solid statesthe torsional angle between the nitroxide N-O bond and the benzimidazole group is 171.6°. This planarization is apparently assisted by the dipolar interaction between the nitroxide N-O bond and the nearby benzimidazole N-H bond, which in all the radicals align in parallel to give favorable attraction between the partial negative charge of the oxygen atom and the partial positive charge of the hydrogen atom. The bilayer motif mentioned in the Results section brings the nitroxide groups to within 4.86 Å (O‚‚‚O distance) of one another at their closest approach across the inversion center from one bilayer to the next. The nitroxide-nitroxide distance along a stack is 6.88 Å, by translation along the a-axis. Between adjacent stacks of radicals, each N-H unit is hydrogen bonded to an imidazole nitrogen at a distance of 2.10 Å, resulting in a nitroxide-nitroxide distance of 5.84 Å along the b-axis. The NsH‚‚‚N angle is slightly bent from linearity at 154.1°. The combination of radical-radical π-stacking with hydrogen bonding between the stacks produces the overall pseudo-2D sheet motif found in the crystal structure. The Etter graph set classification of the hydrogen bonding in 1 is C(4).3 The major changes noted in the FT-IR spectrum with variable temperature were mainly in the 2500-3000 cm-1 region, where the C-H and hydrogen bonded N-H stretching bands are observed. As the temperature was decreased below room temperature, narrowing occurred in this overlapping envelope of peaks. This broad N-H stretch at 3100 cm-1 was particularly affected, becoming narrower and weaker until, at 12 K, only a shoulder spike at about 3120 cm-1 remained in that region of the spectrum. These are probably caused by tightening of the hydrogen-bonding order in the crystal lattice as thermal motion decreases, leading to an overall better defined N-H peak position. At the same time, the C-H stretching region also became narrower, with eventual resolution of multiple peaks as the apparent peak absorbance increased. The changes in the C-H stretching region are presumably associated with a slowing of the dynamic motion of the tert-butyl group on the nitroxide as temperature drops. Relationship between Solid-State Packing and Magnetic Behavior. The dc susceptibility of the sample was found to follow a Curie-Weiss law over 10300 K, with S ) 1/2, θ ) -4.24 K, and g ) 2.029(3). In the low-temperature range, the dc magnetic susceptibility χ vs temperature plot (Figure 5) shows a broad maximum at about 3 K. For several standard models of exchange coupling, we evaluated the parameter τmχm where τm ) kTmax/JS(S + 1) and χm ) χmaxJ/Ng2(µB)2. For these equations, S ) 1/2 for the radical species involved, k is the Boltzmann constant, µB is the Bohr magneton, g is the appropriate Lande factor, and J is the exchange constant. χmax is the maximum value of the susceptibility and Tmax is the temperature at that maximum (note that J is actually |J|). The following models were tested by nonlinear least-squares fitting to the data: linear chain (Bonner-Fisher model), simple planar, honeycomb, double layer, simple cube, and bodycentered cube.18 Among these models, the susceptibility behavior of 1 gives the best fit to a simple planar 2D Heisenberg
2-tert-Butylaminoxylbenzimidazole
Chem. Mater., Vol. 11, No. 8, 1999 2209
Table 3. Comparison of tmcm (see text definition) for Various Models tmcm
a
model
theoretical
linear chain simple planar honeycomb double layer simple cubic body-center cube
0.1256 0.1170 0.1093 0.1293 0.1135 0.1183
experimental 0.1175a
Value corrected for diamagnetism.
model with S ) 1/2 spin sites (Table 3). This is quite reasonable when compared to the crystal structure of 1, which exhibits a hydrogen-bonded 2D motif. If the planar 2D model is assumed with g ) 2 as appropriate for organic free radicals, we find an intraplane magnetic interaction of J ) -1.60 K. Although we have not yet observed the Neel magnetic phase transition temperature (TN) for 1 at temperatures down to 1.8 K, the data suggest that the transition is just below the experimental minimum for our apparatus. Further experiments would be desired to investigate the magnetic behavior of 1 at lower temperatures. Due to the relative symmetry of the crystal structure of 1, there are a limited number of radical-radical close contact interactions. These interactions should dominate intermolecular exchange in the solid state. We carried out computations on five different radical pairs shown in Figure 7, using the AM1-CI semiempirical molecular orbital method. This method has proven to be very useful for evaluating triplet-singlet gaps in open shell systems. The computations were simplified by replacing the tert-butyl groups with methyl groups, since the alkyl groups have minimal spin density. The computed exchange splittings are summarized in Table 4. Interestingly, all of the closest contact interactions are computed to be ferromagnetic except for one dominantly larger interaction, which is the nitroxide-nitroxide interaction through the inversion center. This is the closest approach between the nitroxide centers in the lattice, and is apparently close enough to give antiferromagnetic exchange interactions. While the computational method used is relatively simplistic, the results obtained are consistent with the observed behavior. It is important to consider all possible close contact interactions for radicals interacting in a crystal lattice, because it is not necessarily clear by inspection what interactions will be dominant. A variety of factors involving the overall overlap of sites with different spin densities must be considered. It has recently been demonstrated that simplified models of solid-state interaction that consider a single dominant interaction in a solid lattice are not well-correlated with the overall bulk exchange behavior.19,20 Statistical analysis of close contact distances in the crystal structures of organic magnetic crystals does not confirm simple McConnell (18) (a) Bonner, J. C.; Fisher, M. E. Phys. Rev. 1964, 135, A640. (b) Navarro, R. In Magnetic properties of layered transition metal compounds; de Jongh, L. L., Ed.; Kluwer Acad. Publ.: Norwell, MA, 1990; pp 105-190. (c) de Johng, L. J.; Miedema, A. R. Experiments on Simple Magnetic Model Systems; Taylor and Francis: London, 1974. (19) Deumal, M.; Novoa, J. J.; Bearpark, M. J.; Celani, P.; Olivucci, M.; Robb, M. A. J. Phys. Chem. A 1998, 102, 8404-8412. (20) Deumal, M.; Cirujeda, J.; Veciana, J.; Novoa, J. J. Adv. Mater. 1998, 10, 1461-1466.
Figure 7. Selected close contact arrangements in the crystal lattice for 1, which were used for computation of exchange on a pairwise basis (see Table 4). Arrows indicate positions of nitroxide oxygen atoms. Table 4. Estimates of Close-Contact Exchange Contributions in Crystalline 1
interaction type c-axis, “herringbone” interaction b-axis translation, interstack hydrogen bond b-axis 2-fold rotation a-axis translation, radical stacking c-axis inversion center
exchange interaction,a cal/mol
O‚‚‚O distance, Å
11.7 19.0
11.66 5.84
5.3 5.0 -62.0
7.84 6.88 4.86
a Carried out using program MOPAC93 version 1 from Fujitsu using the keyword set AM1 1SCF MECI OPEN(2,2) C.I.)(6,2) GEO-OK.
model pictures of argument. The bulk magnetic behavior of such crystals appears to be determined by the interplay of various different exchange interactions, and need not be determined by a single major interaction. Nevertheless, we feel that the analysis given above for 1 includes all of the important close contact interactions. Given that overall bulk magnetic behavior is often modeled as a geometric mean of multiple exchange interactions, it seems reasonable to assume similar qualitative behavior for 1. If this is so, the antiferromagnetic exchange through the inversion center should dominate the magnetic behavior of 1, as observed. Strategies for turning AFM to FM behavior in 1 should therefore focus on decreasing this interaction, or on
2210 Chem. Mater., Vol. 11, No. 8, 1999
changing the geometry of the interaction to make the interaction FM in nature. Conclusions Radical 1 is a new member of the family of hydrogenbonded stable organic radicals. Its high degree of solidstate order is a demonstration of the possibilities for crystallographic engineering of molecular magnetic materials. The solid-state ordering is consistent with the simple planar Heisenberg antiferromagnetic exchange interactions between the radicals. Such correlation of crystallographic structure-property relationships will be of crucial importance to achieve true predictability of electronic and magnetic properties. Simplified approaches typically used to correlate molecular packing with bulk molecular exchange have been statistically shown not to be strongly predictive of experimental behavior.19 As a result, more information is required for systems with well-defined packing and understandable magnetic behavior, to add to the structure-property database. Toward this end, we are presently exploring the effects of substituent replacements on 1 upon crystal packing and magnetic behavior. Experimental Section General Procedures. All reagents were obtained from commercial suppliers and used without further purification. Tetrahydrofuran (THF) was distilled from sodium/benzophenone immediately prior to use. Glassware was dried in an oven and assembled under a stream of argon. Melting points were taken in Pyrex capillaries on an Electrothermal apparatus and are uncorrected. 1H NMR spectra were recorded on a Bruker AC-200 spectrometer in deuterated solvents. Chemical shifts are reported in parts per million (ppm) downfield of internal standard tetramethylsilane on the delta scale (δ). Infrared spectra were obtained on a Midac M-9000 spectrometer interfaced to a Pentium PC with Grams32 software (Galactic Industries Corp.). Variable-temperature infrared spectrometeric studies were carried out using an APD Cryogenics Displex CS-202 K closed cycle circulating helium crystostat attached to vacuum line (
