Noncovalent Interactions in 2-Mercapto-1-methylimidazole Complexes

Crystal Growth & Design , 2001, 1 (6), pp 501–507. DOI: 10.1021/ ... Citation data is made available by participants in Crossref's Cited-by Linking ...
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Noncovalent Interactions in 2-Mercapto-1-methylimidazole Complexes with Organic Iodides Julie I. Jay,‡ Clifford W. Padgett,† Rosa D. B. Walsh,† Timothy W. Hanks,§ and William T. Pennington*,†

CRYSTAL GROWTH & DESIGN 2001 VOL. 1, NO. 6 501-507

Department of Chemistry, Clemson University, Clemson, South Carolina 29634-0973, Department of Chemistry, Mesa State College, Grand Junction, Colorado 81501, and Department of Chemistry, Furman University, Greenville, South Carolina 29613 Received July 11, 2001;

Revised Manuscript Received September 7, 2001

W This paper contains enhanced objects available on the Internet at http://pubs.acs.org/crystal. ABSTRACT: 2-Mercapto-1-methylimidazole (mmim) forms complexes with organic iodides, 1,4-diiodotetrafluorobenzene (p-F4DIB), tetraiodoethylene (TIE), and 1,2-diiodotetrafluorobenzene (o-F4DIB), in which there are remarkably similar N-H‚‚‚S hydrogen bonding and S‚‚‚I halogen bonding patterns. Extended chains of alternating donor (mmim) and acceptor (p-F4DIB and TIE) molecules are formed through divalent I‚‚‚S‚‚‚I interactions. These chains are joined into double strands through N-H‚‚‚S interactions that link two mmim molecules into a dimer. In the third complex, N-H‚‚‚S bound dimers are linked into infinite chains through S‚‚‚I interactions involving a pair of o-F4DIB molecules. Thermal analysis indicates that decomposition in both of the F4DIB complexes proceeds through total loss of acceptor, while decomposition of the TIE complex involves a combination of acceptor loss and acceptor reaction. Melting in the o-F4DIB complexes appears to be preceded by a solid-state rearrangement or premelting phenomenon that is not observed in the other two complexes. Crystal data for mmim‚p-F4DIB: monoclinic space group, C2/c (no. 15), a ) 26.915(3) Å, b ) 7.9216(6) Å, c ) 14.2630(4) Å, β ) 105.618(1)°, V ) 2928.7(4) Å3, Z ) 8, Fcalc ) 2.34 g/cm3. Crystal data for mmim‚TIE: monoclinic space group, C2/c (no. 15), a ) 20.1789(12) Å, b ) 10.7824(10) Å, c ) 14.5072(2) Å, β ) 118.767(1)°, V ) 2766.9(3) Å3, Z ) 8, Fcalc ) 3.10 g/cm3. Crystal data for mmim‚ o-F4DIB: triclinic space group, P1 h (no. 2), a ) 7.4987(7) Å, b ) 8.1906(12) Å, c ) 11.6512(12) Å, R ) 83.245(4)°, β ) 84.695(3)°, γ ) 86.135(3)°, V ) 706.35(14) Å3, Z ) 2, Fcalc ) 2.43 g/cm3. Introduction Like hydrogen bonding, halogen bonding1 is strong, selective, and directional, making it ideal for geometrybased crystal design. In a recent comparison of these two interactions, halogen bonding was found to dominate over hydrogen bonding in driving the self-assembly of aromatic nitrogen donors and perfluoroiodide acceptors.2 Complexes based on this type of interaction have been used to overcome the low affinity of perfluorocarbons for hydrocarbons in solid and liquid phases,3 for the preparation of pseudo-polyhalide salts,4 for the formation of molecular metals and multicomponent molecular conductors,5,6 for the resolution of a racemic mixture of a bromoperfluorocarbon,7 and for the interconversion of polymorphic donors.8 Much of our work in this area has focused on complexes of aromatic nitrogen heterocycles with elemental iodine. The greater polarizability of I2 relative to the other halogens results in significantly stronger interactions.9 We have used this interaction to build extended chain structures such as pyrazine‚I210 and quinoxaline‚I2.11 With stronger donors, such as pyridine derivatives, the I2 is polarized to the extent that no extended interactions (i.e., with N‚‚‚I interactions at either end of the I2 molecule) are formed and only * To whom correspondence should be addressed. E-mail: billp@ clemson.edu. † Clemson University. ‡ Mesa State College. § Furman University.

molecular adducts result.8,11 With some strong donors, the polarization of the I2 molecule and the packing requirements of the donor result in the I2 molecule exhibiting amphoteric behavior, with one end acting as a Lewis acid to the nitrogen donor and the other end serving as Lewis base to a second I2 molecule that bridges two donor‚I2 moieties to form a neutral polyiodine species.12 To reduce polarization of the I2 molecule so that extended interactions can occur with strong donors, we have begun to use synthons of I2, i.e., organic iodides that resemble I2 with an organic spacer inserted between the acceptor sites. This has proven very successful, and we have prepared a wide variety of chain, layer, and network architectures based on organic iodide acceptors.13,14 Complexes of I2 and mixed halogens such as IBr with sulfur-based donors such as thioethers,15-18 thiones,19-23 and phosphorus sulfides24,25 are well-known. Many of these complexes exhibit amphoteric behavior similar to the nitrogen-based complexes discussed above. Complexes of sulfur donors with organic iodides are less wellknown, but have been reported. These include dithiane complexes with iodoform26 and diiodoacetylene,27 a complex of elemental sulfur (S8) with iodoform,28 a complex of iodo-ethylenedithiotetrathiafulvalene with bis(2-thioxo-1,3-dithiol-4,5-dithialoto)palladium(IV),29 and several iodoform solvates of metal isothiocyanates.30,31 2-Mercapto-1-methylimidazole (mmim), one of the most commonly employed drugs for the treatment of hyperthyroidism, functions as an iodine sponge,32 form-

10.1021/cg015538a CCC: $20.00 © 2001 American Chemical Society Published on Web 10/18/2001

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Table 1. Crystal Data mmim‚p-F4DIB formula Mw crystal system space group a/Å b/Å c/Å R (°) β (°) γ (°) V/Å3 Z Dc/g cm-3 µ/mm-1 transmission coefficient no. data measured no. data unique (Rmerge) no. obsd data (I > 2σ(I)) R1a wR2b

mmim‚TIE

C10H6N2F4SI2 516.03 monoclinic C2/c (no. 15) 26.915(3) 7.9216(6) 14.2630(4)

C6H6N2SI4 645.79 monoclinic C2/c (no. 15) 20.1789(12) 10.7824(10) 14.5072(2)

105.618(1)

118.767(1)

2928.7(4) 8 2.34 4.47 0.61/1.00 14024 2953 (0.024) 2448 0.0363 (0.0563) 0.0833 (0.0951)

2766.9(3) 8 3.10 9.13 0.44/1.00 13302 2834 (0.034) 2563 0.0492 (0.0553) 0.1245 (0.1302)

mmim‚o-F4DIB C10H6N2F4SI2 516.03 triclinic P1 h (no. 2) 7.4978(7) 8.1906(12) 11.6512(12) 83.245(4) 84.695(3) 86.135(3) 706.3(1) 2 2.43 4.63 0.61/1.00 6785 2837 (0.018) 2719 0.0332 (0.0359) 0.0820 (0.0844)

a R ) Σ||F | - |F ||/Σ|F | for observed data (I > 2σ(I); number in parentheses is for all data. b wR ) {Σ[w(F 2 - F 2)2]/Σ[w(F 2)2]}1/2 for 1 o c o 2 o c o observed data (I > 2σ(I); number in parentheses is for all data.

Table 2. Selected Distances (Å) and Angles (°)a mmim‚p-F4DIB

mmim‚TIE

mmim‚o-F4DIB

Distances C1-S1 C5-I1

1.704(5) 2.092(5)

C8-I2

2.090(5)

S1‚‚‚I1 S1‚‚‚I2b S1‚‚‚N1a

3.314(1) 3.342(1) 3.342(4)

C1-S1‚‚‚I1 C1-S1‚‚‚I2b C1-S1‚‚‚N1a I1‚‚‚S1‚‚‚I2b I1‚‚‚S1‚‚‚N1a I2b‚‚‚S1‚‚‚N1a C5-I1‚‚‚S1 C8-I2‚‚‚S1c

99.1(2) 92.9(2) 100.2(3) 156.4(1) 65.7(2) 92.3(2) 176.6(1) 168.4(1)

C1-S1 C5-I1 C5-I2 C6-I3 C6-I4 S1‚‚‚I1 S1‚‚‚I3b S1‚‚‚N1a I1‚‚‚I4d I2‚‚‚I4e

1.683(8) 2.103(9) 2.089(9) 2.096(8) 2.094(9) 3.302(2) 3.246(2) 3.306(7) 3.846(1) 4.041(1)

Angles C1-S1‚‚‚I1 C1-S1‚‚‚I3b C1-S1‚‚‚N1a I1‚‚‚S1‚‚‚I3b I1‚‚‚S1‚‚‚N1a I3b‚‚‚S1‚‚‚N1a C5-I1‚‚‚S1 C6-I3‚‚‚S1c C5-I1‚‚‚I4d C5-I2‚‚‚I4e C6-I4‚‚‚I1f C6-I4‚‚‚I2g

99.0(3) 97.0(3) 102.1(4) 155.6(1) 70.2(3) 88.7(3) 166.9(2) 171.4(2) 120.6(1) 153.5(1) 156.6(1) 116.7(1)

C1-S1 C5-I1

1.691(4) 2.112(4)

C6-I2

2.087(4)

S1‚‚‚I1 S1‚‚‚I2b S1‚‚‚N1a I1‚‚‚I2b

3.291(1) 3.843(1) 3.345(3) 4.056(1)

C1-S1‚‚‚I1 C1-S1‚‚‚I2b C1-S1‚‚‚N1a I1‚‚‚S1‚‚‚I2b I1‚‚‚S1‚‚‚N1a I2b‚‚‚S1‚‚‚N1a C5-I1‚‚‚S1 C8-I2‚‚‚S1b C5-I1‚‚‚I2b C8-I2‚‚‚I1b

113.7(2) 72.5(2) 104.7(2) 68.8(1) 126.0(2) 89.6(2) 171.5(1) 144.8(1) 125.6(1) 165.0(1)

a Atoms listed with a lower case letter were generated by the following symmetry operators: For mmim‚p-F DIB: (a) 0.5 - x, 0.5 - y, 4 -z; (b) 0.5 + x, 0.5 - y, 0.5 + z; (c) -0.5 + x, 0.5 - y, -0.5 + z. For mmim‚TIE: (a) -x, -y, 1 -z; (b) -0.5 + x, 0.5 + y, z; (c) 0.5 + x, -0.5 + y, z; (d) 0.5 - x, 0.5 + y, 1.5 - z; (e) x, -y, -0.5 + z; (f) 0.5 - x, -0.5 + y, 1.5 - z; (g) x, -y, 0.5 + z. For mmim‚o-F4DIB: (a) 1 - x, 1 y, 1 - z; (b) 1 - x, 1 - y, 2 - z.

ing a very stable donor‚acceptor complex with I2. However, the structure of this complex has not been determined. In this paper, we describe the preparation,

structural characterization, and thermal analysis of three complexes of this donor with organoiodide acceptors: 1,4-diiodotetrafluorobenzene (p-F4DIB), tetraiodoethylene (TIE), and 1,2-diiodotetrafluorobenzene (o-F4DIB). The crystal packing of all three complexes is dominated by very similar noncovalent interaction patterns consisting of N-H‚‚‚S hydrogen bonding and S‚‚‚I halogen bonding. Experimental Section Materials and Methods. 2-Mercapto-1-methylimidazole, 1,4-diiodotetrafluorobenzene, tetraiodoethylene, and 1,2-diiodotetrafluorobenzene were purchased from Aldrich Chemical Co. and were used as received. Solvents were obtained from commercial sources and were dried and/or purified by standard techniques and stored over activated sieves when necessary.

2-Mercapto-1-methylimidazole Complexes Carbon, hydrogen, and nitrogen analyses were performed by Atlantic Microlabs, Norcross, GA. Synthesis of mmim‚p-F4DIB. Mmim (0.0111 g; 0.0972 mmol) and p-F4DIB (0.0406 g; 0.101 mmol) were dissolved in methylene chloride (∼50 mL). Slow evaporation of solvent yielded 0.045 g of light yellow parallelepiped crystals of mmim‚ p-F4DIB (90% yield); mp ) 184-186 °C. Elemental analyses, calculated (observed): %C 23.27 (23.42), %H 1.17 (1.06), %N 5.43 (5.41). Synthesis of mmim‚TIE. Mmim (0.0150 g; 0.131 mmol) and TIE (0.0698 g; 0.131 mmol) were dissolved in methylene chloride (∼50 mL). Slow evaporation of solvent yielded 0.0731 g of light yellow parallelepiped crystals of mmim‚TIE (86% yield); mp ) 129-135 °C (decomposition). Elemental analyses, calculated (observed): %C 11.16 (11.66), %H 0.94 (0.87), %N 4.34 (4.38). Synthesis of mmim‚o-F4DIB. Mmim (0.00590 g; 0.0517 mmol) and o-F4DIB (0.0200 g; 0.0503 mmol) were dissolved in methylene chloride (∼50 mL). Slow evaporation of solvent yielded 0.0184 g of light yellow parallelepiped crystals of mmim‚p-F4DIB (71% yield); mp ) 110-112 °C. Elemental analyses, calculated (observed): %C 23.27 (24.19), %H 1.17 (1.33), %N 5.43 (5.97). X-ray Crystallographic Studies. X-ray powder diffraction analysis was used to verify sample purity and identity. Diffraction patterns obtained from bulk reaction products were compared to patterns calculated from single-crystal results using the program POWD10.13 Powder diffraction data were acquired on a Scintag XDS/2000 theta-theta diffractometer with Cu KR1 radiation (λ ) 1.54060 Å) and an intrinsic germanium solid-state detection system. Relevant crystallographic data for the single-crystal studies are given in Table 1. All measurements were made on a Rigaku AFC8S diffractometer with a Mercury CCD detector at room temperature (295 ( 1 K), with graphite-monochromated Mo KR radiation (λ ) 0.71073 Å). The data were collected to a maximum 2θ value of 52.9° in 0.5° oscillations (in ω) with two 10.0 s exposures (to identify detector anomalies). The data were corrected for Lorentz and polarization effects, and an absorption correction33 was applied to the data. Thermal Analysis. Thermal gravimentric analyses were performed on a Perkin-Elmer TGA 7 analyzer using the Pyris software package. Sample masses ranged from 8 to 17 mg. All calculations were performed on data represented as percent loss of starting mass. For onset calculations, the samples where heated at a constant rate of 5 °C/min from 50 to 400 °C (600 °C for mmim‚TIE) under nitrogen. Mass loss and onset calculations were performed by standard methods. Differential scanning calorimetry analyses were performed on a Mettler Toledo DSC 820e analyzer using the STARe software package. Samples sizes ranged from 4 to 10 mg and were heated from 30 to 160 °C at a constant rate of 5 °C/min. Data were represented as heat flow versus temperature. Onset and peak calculations were performed by standard methods.

Results and Discussion Selected distances and angles for the three complexes are given in Table 2. The crystal packing of all three complexes are dominated by noncovalent donor‚acceptor interactions involving both N-H‚‚‚S hydrogen bonding and S‚‚‚I halogen bonding interactions. In addition, I‚‚‚I interactions are present in mmim‚TIE. Most of these interactions are considerably shorter than accepted sums of van der Waals contact distances for the atoms involved (N: 1.55-1.61 Å; S: 1.79-1.80 Å; I: 1.98-2.00 Å)34,35 Crystal Structure of mmim‚p-F4DIB. The asymmetric unit contains one donor molecule and one acceptor molecule, each occupying a general position in the unit cell (Figure 1). Halogen bonding between the sulfur atom of the imidazole molecule and iodine atoms of

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Figure 1. Thermal ellipsoid plot (50% probability) of mmim‚ p-F4DIB (symmetry related atoms are generated by operations listed in Table 2.) W A 3D rotatable image in xyz format is available.

Figure 2. Double-stranded chain structure of mmim‚p-F4DIB W A 3D rotatable image in xyz format is available.

halogenated benzene molecules result in the formation of infinite chains. Adjacent donor‚acceptor pairs in the chain are related by an n-glide operation. Mmim molecules of two inversion-related chains form centrosymmetric dimers (centered at 0.25, 0.25, 0.0) through N-H‚‚‚S hydrogen bonding, linking the chains into double strands (Figure 2). The crystal packing is completed by stacking of double-stranded chains, with neighboring chains related by the C-centering. As is often observed in perfluorocarbon/hydrocarbon complexes, the donors and acceptors are segregated into separate regions (Figure 3). The S‚‚‚I distances observed in mmim‚p-F4DIB (3.314(1)-3.342(1) Å) are significantly longer than those to terminal I2 molecules (2.487 (3)-3.098(5) Å),21 but

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Figure 4. Thermal ellipsoid plot (50% probability) of mmim‚ TIE (symmetry related atoms are generated by operations listed in Table 2.) W A 3D rotatable image in xyz format is available.

Figure 3. Crystal packing of mmim‚p-F4DIB, viewed down the c-axis. W A 3D rotatable image in xyz format is available.

only slightly longer than those involving bridging I2 molecules (3.054(6)-3.197(6) Å).15 Comparison to the known complexes of dithiane with iodoform and diiodoacetylene is hampered by the low precision of these early investigations, but the contact distances (both involving bridging organoiodides) are essentially identical (3.27(1) Å for diiodoacetylene;27 3.32 Å for iodoform26) to those observed here. The N‚‚‚S interaction distance of 3.342(4) is similar to that observed in the parent compound (3.324(2)3.370(2) Å), which also forms hydrogen-bonded dimers,36 but while the planes of the two imidazole molecules of the dimer in the present compound are parallel (related by inversion symmetry) with a 1.42(1) Å vertical step between the planes, those of the parent compound are inclined at a dihedral angle of 76.7°. Crystal Structure of mmim‚TIE. The crystal structure of this complex (Figure 4) is very similar to that of mmim‚p-F4DIB. Donor and acceptor molecules are joined into chains through S‚‚‚I halogen bonding interactions and linked into double strands through formation of inversion-related mmim dimers involving N-H‚‚‚S hydrogen bonding. The S‚‚‚I (3.246(2)-

Figure 5. Double-stranded chain structure of mmim‚TIE W A 3D rotatable image in xyz format is available.

3.302(2) Å) and N‚‚‚S (3.306(7) Å) distances are comparable. Adjacent donor‚acceptor pairs in the chains are related by C-centering, the double strands are generated by an inversion center at the origin (Figure 5), and the planes of the two imidazole molecules of the dimer are parallel (1.06(1) Å step between planes). However, the packing of the chains in the TIE complex (Figure 6) is dominated by additional I‚‚‚I interactions, whereas the p-F4DIB complex is influenced by association into perfluoro- and hydrocarbon regions. I‚‚‚I interactions of 3.846(1) to 4.041(1) Å occurring between iodine atoms of chains related by 21 and c-glide operations are very similar to those observed in numerous other TIE complexes.13 Each of these interactions involves one iodine atom serving as a donor with a C-I‚‚‚I angle that approaches tetrahedral and another serving as acceptor with a C-I‚‚‚I angle that is nearly linear. Crystal Structure of mmim‚o-F4DIB. The nonlinearity of the acceptor sites in o-F4DIB prevents this complex from crystallizing with the same doublestranded chain structure seen in the p-F4DIB and TIE

2-Mercapto-1-methylimidazole Complexes

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Figure 6. Crystal packing of mmim‚TIE, viewed down the c-axis. W A 3D rotatable image in xyz format is available.

Figure 8. Infinite chains of mmim‚o-F4DIB W A 3D rotatable image in xyz format is available. Figure 7. Thermal ellipsoid plot (50% probability) of mmim‚ o-F4DIB (symmetry related atoms are generated by operations listed in Table 2.) W A 3D rotatable image in xyz format is available.

complexes; however, there are many similarities in the noncovalent interactions that occur. The complex crystallizes with one donor and one acceptor molecule in the asymmetric unit (Figure 7). As with the other complexes, a dimer of imidazole molecules is formed by N-H‚‚‚S hydrogen bonding (N‚‚‚S distance of 3.345(3) Å) across an inversion center (0.5, 0.5, 0.5); the imidazole planes are parallel, but in this case also nearly

coplanar with a vertical step of only 0.26(1) Å. A relatively strong S‚‚‚I halogen bonding interaction occurs between each sulfur and one of the iodine atoms of the acceptor (S1‚‚‚I1 3.291(1) Å). A weaker S‚‚‚I interaction (3.843(1) Å) between the sulfur and the iodine atoms of donor‚acceptor pairs related by an inversion center at (0.5, 0.5, 1.0) results in the formation of infinite chains of imidazole dimers doubly bridged by o-F4DIB molecules (Figure 8). As with the other perfluorinated complex, the packing of the chains is influenced by association into segregated perfluoro- and hydrocarbon regions (Figure 9).

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Figure 9. Crystal packing of mmim‚o-F4DIB, viewed down the a-axis. W A 3D rotatable image in xyz format is available. Table 3. Thermal Behavior of the Organoiodide Acceptors and Their Complexes

c

compound

melting pt (°C)

thermal onsetd (°C)

mass loss (%)

p-F4DIB TIE o-F4DIB mmim‚p-F4DIB

108-110a 146-148a 49-50a N/Ab

mmim‚TIE

129c

mmim‚o-F4DIB

99c

160 257 133 162 232 160 267 132 230

100 100 100 79 18 42 41 76 21

a Vendor-supplied data. b Melting endotherm not observed. Onset of melting by DSC. d Onset of mass loss by TGA.

Thermal Analysis. Heterocycle charge-transfer complexes with simple organoiodides typically decompose by fragmentation of the X‚‚‚I halogen bond. This event may be proceeded by melting of the complex, but sublimation of one component (usually the acceptor) from the solid has also been observed. Thermal analysis methods are effective tools for examining the stability of the crystal matrix and can also detect solid-state reactions between donor and acceptor at elevated temperatures. Table 3 shows the melting points, vaporization onset temperatures, and mass changes for the three acceptors and their complexes with mmim. Note that the pure acceptors are thermally stable and evaporate completely in a single event. The mmim‚o-F4DIB complex melts at 99 °C, then decomposes cleanly in two thermal events. The first, with an onset temperature of 132 °C, corresponds to a complete loss of the acceptor, while the second event, with an onset of 230 °C is the evaporation of the mmim

Jay et al.

donor. The melting transition and the evaporation of the o-F4DIB are observed as distinct events in the DSC. The decomposition of the mmim‚p-F4DIB complex is also straightforward. There is a mass loss at 162 °C that represents the volatilization of the acceptor. In the DSC, the melting endotherm cannot be clearly identified, but may lie under the leading edge of the vaporization endotherm. A second mass loss is observed with an onset temperature of 232 °C. As with the mmim‚o-F4DIB complex, this is the evaporation of the donor. The mmim‚TIE complex is quite different, and clearly undergoes a thermal reaction to give a new complex. The DSC shows that the complex melts with an onset of 129 °C. This is followed by a large exothermic event that occurs with an onset of 142 °C. While this chemical reaction is occurring, the complex undergoes another thermal event (onset 160 °C) in which 42% of the total mass is evolved. This temperature is not only significantly below the volatilization onset of pure TIE but also below the melting point. However, examination of the bright orange condensed volatiles from a control reaction run under an optical microscope showed that only TIE is evolved below 170 °C. Since TIE accounts for 82% of the sample mass, the remaining amorphous orange solid has a nominal mmim/TIE ratio of 2:1. Continued heating results in a second mass loss of 41% in a thermal event that has an onset temperature of 267 °C. This is not, however, simply the remaining TIE evolving since the mmim would also be expected to evolve at this temperature. The remaining solid only evolves mass slowly, retaining approximately 12% of the original mass at 600 °C. Visual inspection of the sample pan showed the material to be a black char of unknown composition. TIE has been observed to decompose to I2 and diiodoacetylene under certain conditions. It is likely that the thermal reaction is initiated by mmim-induced fragmentation of a C-I bond, giving an imidazole salt. Conclusion The results obtained in this study indicate that the interaction of sulfur donors with organoiodide acceptors is very similar to that with nitrogen donors. Unlike complexes with elemental iodine, in which the I2 is almost always terminal and is often amphoteric, organoiodides serve as bridging acceptors to sulfur donors. The similarity of the types of interactions that are found in mmim complexes with three relatively different acceptor molecules points to the potential of these components for crystal design. We are currently investigating mmim for polymorphic behavior. For this, or similar sulfur-based pharmaceutically important polymorphic materials, complex formation and subsequent decomposition could be very important for polymorph isolation, interconversion, or possibly for the preparation of new previously unobserved crystalline forms. Acknowledgment. Financial support of the National Science Foundation for J.I.J. (an NSF/REU participant at Clemson University; CHE-9987899) and for purchase of the CCD-based X-ray system used in this study (CHE-9808165), and of the NASA/Space Grant program provided by SC/EPSCoR is gratefully acknowledged.

2-Mercapto-1-methylimidazole Complexes Supporting Information Available: X-ray crystallographic information file (CIF) for all three complexes: mmim‚ p-F4DIB, mmim‚TIE, and mmim‚o-F4DIB. This material is available free of charge via the Internet at http://publs.acs.org.

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