Experimental and Theoretical Study of Halogen-Bonded Complexes of

DOI: 10.1021/cg100549u

Experimental and Theoretical Study of Halogen-Bonded Complexes of DMAP with Di- and Triiodofluorobenzenes. A Complex with a Very Short N 3 3 3 I Halogen Bond

2010, Vol. 10 3710–3720

Laila C. Roper, Carsten Pr€ asang, Valery N. Kozhevnikov, Adrian C. Whitwood, Peter B. Karadakov,* and Duncan W. Bruce* Department of Chemistry, University of York, Heslington, York YO10 5DD, U.K. Received April 26, 2010; Revised Manuscript Received July 1, 2010

ABSTRACT: X-ray single crystal structures are reported for 2:1 halogen-bonded complexes of 4-(N,N-dimethylamino)pyridine (DMAP) with 1,4- and 1,3-diiodotetrafluorobenzene and 1,3-diiodo-4,5,6-trifluorobenzene and for a 3:1 complex of DMAP with 1,3,5-triiodotrifluorobenzene. The complex between DMAP and 1,4-diiodotetrafluorobenzene shows the shortest halogen bond recorded for fluorinated iodoarenes. Model systems based on complexes between the same iodofluorobenzenes and ammonia are investigated by Hartree-Fock and DFT calculations to gain insights into the features of halogen bonding in di- and triiodo systems. The calculations reveal a weak charge-transfer component to the halogen bond and account for the lengthening of the C-I bond on complexation in terms of the C-I antibonding character observed within the localized molecular orbital describing the N 3 3 3 I bond. Introduction 1

2

Recognized since the mid 19th century, halogen bonding has recently become a more widely used noncovalent interaction for the preparation of supramolecular species. Understanding of halogen bonding developed through the 1970s to the 1990s,3 and now it is used routinely for the construction of new, supramolecular motifs.4 This experimental work has been accompanied by theoretical studies5 that have probed the nature of the Lewis acid/Lewis base interaction and have led to the concept of the σ-hole as a model for the partial positive charge that develops on the halide atoms that participate in halogen bonding.6 Information on the strength of halogen bonds is collected empirically from the literature, but more recent systematic studies have underpinned these observations in organic7,8 and inorganic4c,9 systems. This is arguably most simply and intuitively understood through the work on organic systems, where stronger halogen bonds accompany the presence of stronger Lewis bases7 or the presence of more electron-deficient iodine atoms acting as acceptors.8 Our interest in this interaction arose from our demonstration that two, non-liquid-crystalline components could be combined into a new, supramolecular entity using halogen bonding and that this new entity demonstrated liquid-crystal properties (1-5). We showed this first with alkoxystilbazoles in combination with iodopentafluorobenzene (1),10 and then, in collaboration with the Milan group, we demonstrated 2:1 complexes between stilbazoles and R,ω-diiodoperfluoroalkanes (2)11 and 1,4-diiodo- (3a) and dibromotetrafluorobenzenes (3b) (Figure 1).12 Examples of all of these were characterized crystallographically, and in each case, the halogen-bond length and the angle at the halogen were within the range normally found.2b However, an interesting observation was that, while halogen bonds formed between stilbazole and 1,4-dibromotetrafluorobenzene (3b), these complexes were not liquid crystalline, something we attributed to the appreciably weaker halogen bond between N and Br, which was not able to exist in the melt at modestly elevated temperatures. Then, in studying the thermal *E-mail: [email protected]. pubs.acs.org/crystal

Published on Web 07/19/2010

behavior of the 1,4-diiodootetrafluorobenzene complexes (3a), we found evidence for the rupture of some halogen bonding, consistent with the unusually large enthalpy observed for the transition between the SmA mesophase and the isotropic liquid. Furthermore, in studying stilbazole complexes of 1,3-diiodotetrafluorobenzene (4), we again saw evidence for halogen-bond rupture, and in this case, this interpretation of the thermal behavior was central to the explanation of the observed liquid crystal phase sequence, which involved the formation of a chiral nematic phase when none of the components was chiral.13 The above findings with the two isomers of diiodotetrafluorobenzene suggest a kinetic lability of certain halogen bonds, yet it is known that halogen bonds can be thermodynamically very strong, and this is particularly the case where halogen bonding involves a halide anion as the halogen bond acceptor.14 In these cases, there are two halogen bonds in the complex and so the question arises as to the possibility that complexation of the first Lewis base affects the electron density at the second iodine and so weakens the halogen bonding. This might be seen as analogous to the situation in aromatic dicarboxylic acids where two pKa values are observed, reflecting the electronic communication between the two carboxylate groups. Results Having used 4-(N,N-dimethylamino)pyridine (DMAP) successfully to undertake a systematic study of halogen bond strength,8 we therefore undertook analogous study of some DMAP complexes of di- and triiodo compounds to probe the question of whether complexation of the first Lewis base affects the electron density at the second (or third) iodine atoms. The study was undertaken crystallographically and is complemented by extensive calculations. The complexes studied are shown in Figure 2. Calculations. In order to investigate the binding of nitrogen bases to polyiodo acceptors and to probe the extent to which certain structural factors can affect the N 3 3 3 I halogen bond, a series of Hartree-Fock (HF) and density functional theory (DFT) calculations utilizing the B3LYP exchangecorrelation functional were carried out on the model systems r 2010 American Chemical Society

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Crystal Growth & Design, Vol. 10, No. 8, 2010

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Figure 1. Halogen-bonded liquid crystals.

Figure 2. Complexes under study.

M1-M9 shown in Figure 3, chosen to reflect complexes 6-9 (Figure 2), to be studied by X-ray methods. The high computational effort associated with these calculations necessitated the use of ammonia as Lewis base rather than DMAP itself. All HF and DFT calculations employed a composite basis set consisting of the standard aug-cc-pVDZ bases for H, C, N, and F, and the aug-cc-pVDZ-PP ECP basis for I.15 The geometries of all complexes were optimized

at the B3LYP/(H,C,N,F:aug-cc-pVDZ;I:aug-cc-pVDZ-PP) level of theory in two different ways, without and with inclusion of basis set superposition error (BSSE) corrections through the counterpoise (CP) method, using the standard gradient algorithms implemented in GAUSSIAN16 under the “Tight” convergence criteria. DFT is known to account reasonably well for the main features of the N 3 3 3 I interaction (see, for example, refs 17 and 18), which suggests that the

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Figure 3. Systems investigated by HF and DFT calculations. Italicized numbers represent HF Mulliken charges of the iodine atoms or ammonia molecules (i.e., the sum of the Mulliken charges on the nitrogen and hydrogens; this sum is zero for free ammonia) at the CP-corrected B3LYP geometries. Bond lengths are found under the bonds to which they correspond, with those in parentheses obtained using the CP correction; these latter values are regarded as the more accurate.

selected levels of theory should provide a sound basis for comparing the main trends exhibited by the halogen bonds in M1-M9. The more important results of the HF and DFT calculations are summarized in Figure 3, while the calculated energies can be found in Table 1. In all cases, the geometry optimizations produced structures of, or very close to, the maximum symmetry possible for each complex; as a consequence, the numbers shown in Figure 3 are only those describing symmetry-unique atoms or bonds. As shown by the N 3 3 3 I distances in Figure 3, the CP correction systematically increases the N 3 3 3 I distances by ca. 0.02 A˚; the BSSE has almost no effect on other bond lengths, including those of the I-C bonds involving the iodines participating in the halogen bonds. In the following discussion, we shall be making use of bond lengths coming from geometries optimized with the inclusion of the CP correction.

The calculations show that the engagement of an I in a N 3 3 3 I interaction is predicted to increase the corresponding I-C bond length, by between 0.016 A˚ (in M5) and 0.023 A˚ (in M1 and M7), in comparison to that in an isolated molecule. However, decreases in the I-C bond lengths found in complexes involving increasing numbers of halogen bonds (cf. M1 and M2, M3 and M4, M5 and M6, or M7, M8, and M9) amount to no more than 0.002 A˚, except for the case of M9, where the decreases in comparison to the cases of M7 and M8 are slightly more pronounced, at 0.007 A˚ and 0.005 A˚, respectively. The calculations match the experimental C-I bond lengths determined by single crystal X-ray methods for 1,4-diiodotetrafluorobenzene (2.075(1) A˚)19 and 1,3,5-triiodotrifluorobenzene (2.103(2) and 2.111(3) A˚;two different lengths are found).20 According to the results shown in Figure 3, the formation of additional halogen bonds should lead to an overall increase in the halogen bond lengths; for example, the association of a

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Figure 4. Mulliken atomic charges in M1 and M2 and the isolated precursor molecules (numbers in parentheses). Table 1. Total Energies and BSSE Energies for Complexes M1-M9 (see Figure 3), Corresponding to Geometries Optimized at the B3LYP/ (H,C,N,F:aug-cc-pVDZ;I:aug-cc-pVDZ-PP) and the CP-Corrected B3LYP/(H,C,N,F:aug-cc-pVDZ;I:aug-cc-pVDZ-PP) Levels of Theory complex

method

energy (a.u.)

M1

B3LYP CP-corrected B3LYP B3LYP CP-corrected B3LYP B3LYP CP-corrected B3LYP B3LYP CP-corrected B3LYP B3LYP CP-corrected B3LYP B3LYP CP-corrected B3LYP B3LYP CP-corrected B3LYP B3LYP CP-corrected B3LYP B3LYP CP-corrected B3LYP

-1276.209 212 -1276.208 454 -1332.788 062 -1332.786 513 -1276.209 179 -1276.208 441 -1332.787 825 -1332.786 231 -1176.965 885 -1176.965 146 -1233.543 323 -1233.541 874 -1472.159 525 -1472.158 74 -1528.737 910 -1528.736 385 -1585.315 326 -1585.313 086

M2 M3 M4 M5 M6 M7 M8 M9

BSSE energy (a.u.) 0.000 751

Table 2. Some Structural Data from the Solid-State Structures of Complexes 6-8

6 7aa 7ba

0.001 536 8 0.000 786 0.001 574 0.000 722

d(N 3 3 3 I)/A˚ 2.6672(17) 2.744(4) 2.780(4) 2.800(4) 2.783(4) 2.768(2) 2.892(2)

CÎN/ deg

INˆCipso/ degc

plane angleb/deg

d(C-I)/A˚

179.25 171.81 174.60 174.30 171.08 178.52 168.11

176.88 168.02 160.07 158.95 165.71 174.31 161.31

75.10 1.74 40.60 36.74 9.01 80.60 3.30

2.1212(18) 2.107(4) 2.105(4) 2.105(4) 2.100(4) 2.116(2) 2.100(2)

a 7a and 7b refer to the two crystallographically different complexes within the unit cell. b Torsion angle between the plane of the DMAP and the fluorinated ring. c Cipso refers to the 4-carbon of the pyridine ring of DMAP.

0.001 430 0.000 774 0.001 513 0.002 229

second NH3 with 1,4-diiodotetrafluorobenzene is predicted to add 0.028 A˚ to the N 3 3 3 I separation (cf. M1 and M2). This effect is more pronounced in the case of 1,3-diiodotetrafluorobenzene, with a difference of 0.035 A˚ between the optimized N 3 3 3 I bond lengths in M3 and M4. The addition of the second ammonia to 1,3,5-triiodotrifluorobenzene is expected to increase the N 3 3 3 I bond length by 0.032 A˚ (cf. M7 and M8) and that of the third ammonia by yet another 0.026 A˚ (cf. M8 and M9). The substitution of the F between the two iodines in M4 by a hydrogen (cf. M4 and M5) decreases the atomic charges on the iodines, which is accompanied by a substantial elongation of the optimized N 3 3 3 I bond, by 0.061 A˚. The findings on changes in C-I bond lengths and N 3 3 3 I distances broadly match those determined at a different level of theory by Lucassen et al. for pyridine complexed to 1,3,5-triiodotrifluorobenzene.21 Another aspect of the calculations relates the charges found at key atoms. Figure 3 shows that the positive charge on iodine is enhanced significantly on complexation (36% in M1) without greatly affecting the charge on the free iodine in 1:1 complexes. However, it is noticeable that an overall positive charge develops on the ammonia on complexation, which points to a chargetransfer component to the interaction, although, given the magnitude of the charge that develops (e.g., 0.0076 in M1), this is not a huge effect, suggesting that the electrostatic nature of the interaction dominates. The detail of this charge (re)distribution is shown in detail (Figure 4) for M1 and M2. It is interesting to observe that, in M1, the charge on the carbon attached to the complexed iodine effectively does not change, the charges on the ortho-carbons decrease by ca. 0.10, and the charge on the para-carbon increases by about 0.15, while, in M2, the charges

on the carbons attached to the complexed iodines increase by ca. 0.12, and those on all other carbons decrease by about 0.15-0.16 each. Despite the fact that the ammonia molecules are rather simplistic substitutes for the DMAP molecules in complexes 6-8 (see Figure 2), the calculations reproduce the qualitative trends in the experimental structural data included in Table 2 reasonably well. The increase in the N 3 3 3 I bond length in the sequence 6-7-8 is paralleled in the sequence M2-M4-M5, while the optimized C-I bond length in M2 is in particularly good agreement with the corresponding experimental value in 8. Preparation and Crystallography of Cocrystals. 2:1 Complexes. The first cocrystals prepared were 2:1 complexes of DMAP with 1,4-diiodotetrafluorobenzene (6), 1,3-diiodotetrafluorobenzene (7), and the related compound 1,3-diiodo4,5,6-trifluorobenzene (8) (Figure 2); single crystals of each complex formed readily on crystallization of 3:1 mixtures from THF, and key data are found in Table 2. Complex 6 crystallized in the P21/c space group and showed the expected 2:1 dimer, which was centrosymmetric about the centroid of the halogenated ring (Figure 5). The N 3 3 3 I separation was 2.6672(17) A˚, and the CÎN angle was 179.25°, while measuring the angle between N, I, and the ipso-carbon of the DMAP ring gave a slightly looser angle of 176.88°, which better reflects the disposition of the DMAP ring. The diiodobenzene ring makes an angle of 75.10° to the two DMAP molecules, which are (by symmetry-inversion center in the halogenated ring) coplanar. Interestingly, the C-I bond length was found to be 2.1212(18) A˚, which represents a statistically significant lengthening compared to 1,4-diiodotetrafluorobenzene itself; this is discussed below. Despite the fact that this is a 2: 1 complex, so far as we are aware, it carries the shortest N 3 3 3 I halogen bond known in neutral complexes of fluorinated iodoarenes. The solid-state packing is best described by considering segregated stacks of DMAP and the diiodobenzene (Figure 6). Thus, the diiodobenzenes propagate in a side-to-side fashion

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Figure 5. 2:1 complex between DMAP and 1,4-diiodotetrafluorobenzene.

Figure 6. Packing of complex 6.

Figure 7. Two different arrangements of stacked DMAP molecules in the solid-state structure of 6. The closer face-to-face motif is associated with the left-hand arrangement.

through an F 3 3 3 F contact, so that pairs of ortho fluorines from adjacent rings form a parallelogram where the intermolecular F 3 3 3 F contact is found as the short diagonal (2.797 A˚), while the separation along the long edge of the polygon is 2.937 A˚. The diiodobenzene rings propagate along the a-axis and are all strictly coplanar. The DMAP moieties stack along the a-axis in a head-to-tail fashion, but closer inspection shows there are two alternating arrangements (Figure 7). Thus, one pair is arranged at an interplanar separation of 3.391 A˚ and shows a slipped arrangement, while the other, at an interplanar separation of 3.603 A˚, shows two superimposed molecules in a head-totail arrangement with effective superposition of the ipso-carbons. The only other short contact (defined here and subsequently as being shorter than 0.1 A˚ less than the sum of the van der Waals’ radii of the two atoms) is between the iodine and one of the carbon atoms R to the ring nitrogen at 3.498 A˚. The distance to the other R-carbon is 3.622 A˚, showing a significant asymmetry in the halogen bond. Complex 7 crystallized in the P1 space group and also showed the expected 2:1 dimer, although this time there were two, independent dimers in the asymmetric unit and four

different N 3 3 3 I separations (Figure 8a). In essence, the two dimers are reasonably similar to one another, with each having one DMAP molecule essentially coplanar with the diiodobenzene and the other twisted out of plane. This latter DMAP also showed a larger INCipso angle at the pyridine nitrogen of the DMAP. A feature of all the halogen bonds is that they are slightly unsymmetric, in that there is a short contact to one R-carbon of the pyridine ring at between 3.44 and 3.50 A˚ (Figure 8b). In the crystal, the tetrafluorodiiodobenzene molecules are arranged in pairs in two independent slipped stacks (Figure 9; stacks A and B) that make an angle of 8.63° to one another between these stacks. In stack A, the fluoro arenes are separated by 3.383 A˚ and are arranged in a slipped, antiparallel fashion so that the closest iodine-iodine distance is 5.832 A˚; the distance between one of these rings and the next in the stack is 3.574 A˚, where an antiparallel relationship also exists, this time with an iodine-iodine separation of 4.215 A˚ (note that twice the van der Waals radius of iodine is 3.98 A˚). In stack B, the closest approach of the arene rings is 3.438 A˚, and the arrangement is very obviously antiparallel with an

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Figure 8. (a) The two independent dimers of complex 7; (b) the unsymmetric nature of the DMAP 3 3 3 iodine interaction in 7.

Figure 9. Stack A: (a) pair separated by 3.574 A˚; (b) pair separated by 3.383 A˚. Stack B: (c) pair separated by 3.438 A˚; (d) pair separated by 3.597 A˚. In all cases, views are perpendicular to the molecular planes of the pairs of molecules, which are, in each case, strictly parallel.

iodine-iodine separation of 5.834 A˚; the distance to the next arene is 3.597 A˚, where there is an iodine-iodine separation of 4.328 A˚. This separation of the diiodobenzenes causes a similar separation of DMAP molecules, and again there are two distinct stacks. In both cases, there is only a relatively uncorrelated, antiparallel arrangement of pairs of DMAP

molecules; the molecular planes of the closest pairs in a stack are not parallel, making an angle of 3.51° in one stack and 1.26° in the other, while alternate pairs of molecules are separated by either 7.136 or 7.068 A˚ (first stack) or by 6.892 or 6.958 A˚ in the other. The first stack is associated with stack A of the halogenated arenes, while the second stack is associated with stack B.

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The only remaining intermolecular contacts are H 3 3 3 F interactions, but the behavior in the two different halo arene stacks is different. Thus, in stack B, H 3 3 3 F interactions are found between meta-pyridine hydrogens and ring fluorines, while a third is between a ring fluorine and a methyl hydrogen of a DMAP moiety (Figure 10), whereas, in stack A, only the interaction with the methyl hydrogen is seen. Complex 8 also crystallized in the P1 space group, but here there is only one complex in the asymmetric unit; again, the complex is unsymmetric, with N 3 3 3 I separations of 2.768(2) and 2.892(2) A˚ (Figure 11). The longer halogen bond links two rings that are all but coplanar, making a mutual angle of 6.24°, while the shorter interaction links rings that are almost

Figure 10. Intermolecular hydrogen bonding in complex 7.

Roper et al.

perpendicular, with the angle between them being 85.04°. Interestingly, this longer halogen bond is unsymmetric with a short R-carbon 3 3 3 iodine separation of 3.474 A˚; the short halogen bond is very close to being totally symmetric with respect to the distances between iodine and the two pyridine R-C-H bonds. The packing of this complex is very different from that of the others, and the “repeat” unit is described as follows (Figure 12). Thus, at the center of the repeat, two DMAP moieties stack one upon another, rotated by 180° and with the pyridine nitrogen of one overlaying the amine nitrogen of the other, separated by 3.720 A˚. Expanding this forming stack from the center, a trifluorodiiodobenzene molecule is found facing each DMAP, which does not appear to sit in any particular disposition with respect to the DMAP, although the ipso-carbon of the DMAP sits beneath the centroid of the halogenated ring, separated by 3.482 A˚. The stacking sequence continues with two further DMAP moieties now almost at a right angle to the developing stack and adjacent to the fluorinated ring with an H 3 3 3 π interaction between a pyridine hydrogen of one DMAP and a fluorine-bearing carbon of the halogenated ring (2.831 A˚). The other DMAP has an identical interaction with the next halogenated ring in the stack. This repeating unit plus an extra C6HF3I2 ring is shown in Figure 12. The two orthogonal DMAP molecules then propagate in two directions: one that generates a stack of pairs of face-to-face DMAPs and the other that is end-to-end in nature, moving in and out of the plane of the page in Figure 12. However, it should be noted that all of this arrangement exists in the absence of any contacts shorter than 0.1 A˚ less than the sum of any pair of van der Waals radii. 3:1 Complex. Having found that rather strong halogen bonds were formed in 2:1 complexes, we were therefore keen to attempt the preparation of a 3:1 complex. This was indeed possible, and in our hands, it was possible to cocrystallize 1,3,5-CF3I3 and DMAP in a 3:1 ratio and isolate a simple 3:1 complex (9), which was found in the P21/m space group. Key distances and angles are found in Table 3, while two projections of the formula unit are shown in Figure 13. The complex contains two C6F3I3 3 3 3 DMAP interactions that are symmetry equivalent and a third that is distinct. Table 3. Significant Dimensions in the Crystal Structure of Complex 9

9

Figure 11. Molecular unit of complex 8.

Figure 12. Part of the packing arrangement in the crystal of complex 8.

d(N 3 3 3 I)/A˚ 2.765(3) 2.886(2)a

CÎN/ deg

INˆCipso/ deg

plane angleb/ deg

d(C-I)/A˚

176.15 168.58

176.88 160.41

2.53 13.04

2.111(3) 2.103(2)

a Symmetry equivalent interaction. b Torsion angle between plane of C6F3I3 and DMAP ring.

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The unique N 3 3 3 I separation is significantly shorter than the other and is associated with a much more linear halogen bond with a perfectly symmetric approach of the DMAP to the iodine. The other two are longer, with appreciable distortions from linearity and with an unsymmetric approach of the DMAP to the iodine, so that one R-C 3 3 3 I distance, at 3.468 A˚, is within the sum of the van der Waals’ radii and is significantly shorter than the distance to the other R-carbon (4.005 A˚). In the crystal structure of the parent trifluorotriiodobenzene,20 statistically there is a single C-I bond length (2.082(7), 2.069(7), and 2.070(6) A˚), and this is also the case in 9 (Table 3). The structure is organized in two main ways, although none of the intermolecular contacts now described is seen at 0.1 A˚ less than the sum of the van der Waals’ radii of the


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interacting species. Thus, there is a one-dimensional extension of the structure that propagates from the fluorine that bisects a pair of nonequivalent C-I bonds as it points toward two methyl hydrogens from a DMAP moiety (dF 3 3 3 H = 2.647 A˚; C-F 3 3 3 H angle = 167.76°) which in turn, through its nitrogen, binds another unit of C6F3I3, and so the chain continues. The two remaining fluorines of the C6F3I3 interact with single methyl hydrogens of different DMAP moieties, which in turn bind the 1-D chains together (Figure 14). In addition, there is an antiparallel, face-to-face arrangement of DMAP moieties with a separation of about 3.525 A˚ and with a small, lateral displacement. A second stacking arrangement has a DMAP arranged over a C6F3I3 moiety (the planes of the two rings make an angle of 2.53 A˚), with the amine nitrogen arranged parallel to a C-I bond and, hence, the pyridine nitrogen oriented along a C-F bond. This alternating motif propagates through the structure. Both stacking interactions are shown in Figure 15. There are no other short contacts. Discussion

Figure 13. Above (upper figure) and side (lower figure) views of the 3:1 complex, 9. In the upper figure, the black line is the mirror plane that bisects the C6F3I3 moiety.

Figure 14. Sheetlike structure of 9 held together by F 3 3 3 H contacts.

Comparison of the N 3 3 3 I separations shows that they are shortest in 6, and in fact, this separation is even shorter than that found in the 1:1 complex of DMAP with pentafluorobenzene (2.693(3) A˚);8 indeed, as noted above, we believe this to be the shortest N 3 3 3 I halogen bond of its type. This observation is perhaps at first sight surprising, as intuitively it might be predicted (as discussed earlier) that having two donors competing for two iodine atoms could weaken the overall interaction, something supported by the calculations where a 10% increase in d(N 3 3 3 I) on going from H3N 3 3 3 I-C6F4-I to H3N 3 3 3 IC6F4-I 3 3 3 NH3 is found. To be sure of this comparison between experiment and calculation, the ideal scenario would be to prepare and isolate a 1:1 complex between DMAP and 1,4diiodotetrafluorobenzene and evaluate the N 3 3 3 I bond length, but our experience shows that the isolation of such complexes is unlikely. For example, we have described previously the formation of 1:1 adducts between methoxystilbazole and 1,3diiodotetrafluorobenzene,13 and also 4-iodotetrafluorophenol,22 in which the “free” iodine forms a halogen bond to the alkoxy oxygen of a neighboring stilbazole in order to satisfy its

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electrophilic “needs” (Figure 16a and b). A related situation is found in the structure of the 1:1 adduct between methoxystilbazole and 4-iodotetrafluorophenol where the crystal is obtained from THF solution, as here the “free” iodine forms a halogen bond to a solvent oxygen (Figure 16c). Thus, it has not proved possible to prepare a 1:1 adduct in which there is a free iodine in an arrangement analogous to that of M1.

Figure 15. The two stacking motifs found in complex 8.

Roper et al.

The question then remains why such a short halogen bond is observed in complex 6. Table 2 shows that the angle at iodine makes this bond exceptionally linear, but plots of N 3 3 3 I bond lengths versus the halogen bond angle4 j do not show a significant correlation. The only other factor that may contribute is the packing efficiency in the solid state, which may to some extent be reflected by the density. Thus, the calculated density for 6 is higher (1.955 g cm-3) than that of 7 (1.917 g cm-3) or 8 (1.884 g cm-3), and while care must be taken in making such a comparison, especially as 6 crystallizes in a different space group from 7 and 8, it does suggest a consistent rationale. Complex 7 has four unique N 3 3 3 I halogen bonds, of which one is statistically shorter than the other three (although longer than that in 6), while, within the limits of the esds, the other three are the same length. There are no apparent solid-state effects that would appear to account for this. For 8, however, while one N 3 3 3 I separation is the same as the three in 7, the other is statistically much longer, at 2.892(2) A˚. In complex 9, there are two different N 3 3 3 I separations, which are very similar to those found in complex 8. In these cases, the halogen bonds are longer than that in complex 6, but the fact that in complex 7 four crystallographically independent N 3 3 3 I distances are observed makes it all but impossible to draw meaningful conclusions. A similar situation pertains in complex 8. Thus, while we have observed previously that predictions concerning halogen bond length and strength can be seen in single crystal structure determinations so that

Figure 16. Part of the crystal structure of (a) the adduct between 4-methoxystilbazole and 1,3-diiodotetrafluorobenzene and (b) 4-methoxystilbazole and 4-iodotetrafluorophenol showing in each case the formation of a I 3 3 3 O halogen bond from the otherwise “free” iodine; (c) shows the molecular structure of the adduct between 4-butyloxystilbazole and 4-iodotetrafluorophenol obtained from THF and shows the I 3 3 3 O halogen bond to the THF solvate.

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Figure 18. Schematic diagram to show the space-filling concept adopted by Triguero et al. and Metrangolo et al. in assembling 3:1 complexes of 1,3,5-trifluoro-2,4,6-triiodorobenzene. Figure 17. LMOs for M1 (upper) and M2 (lower). The orbitals are represented as isosurfaces at orbital values of (0.005 A˚-3/2 (assuming that distances are measured in A˚).

halogen bonding effects can dominate over intermolecular interactions,8 the data found here indicate clearly that this is not the case uniformly. Thus, the data we have do not allow the results of intuition and calculation to be borne out experimentally. However, an interesting observation comes from consideration of the covalent C-I bond lengths in the parent iodine compounds and their related complexes. This is possible for complexes 6 and 9, where, in the parent iodide compounds, C-I distances of 2.075(1) A˚ (6)19 and 2.069(7), 2.070(6), and 2.082(7) A˚ (9)20 are found. These distances are compared with those in the complexes, namely 2.1212(18) in 6 and 2.103(2) and 2.111(3) A˚ in 9. Thus, in the case of 6, there is an appreciable and statistically significant lengthening of the bond, while a similar effect is seen in 9 (assuming statistically significant lengthening is considered). Such observations are consistent with the predictions made by calculation and may be regarded as reliable, for while the length of the halogen bonds is clearly subject to crystal packing effects here, the C-I bond lengths require only the presence of halogen bonds to vary. Such an effect was rationalized by Moss and Goroff, who, as part of their investigations of 13C NMR parameters of iodoalkynes complexed with Lewis bases, performed calculations on the diiodoethyne/ammonia system and found mixing between the nonbonded pair of electrons on nitrogen and the C-I σ* orbital.23 In this study, we have chosen to examine this effect using localized molecular orbitals (LMOs), which show clearly that the LMO responsible for the halogen bond is antibonding over the C-I bond. This is true for both the 1:1 and 2:1 adducts of 1,4-diiodotetrafluorobenzene, M1 and M2 (Figure 17), and the two orbitals look almost the same, which is not unexpected, as LMOs corresponding to similar bonds usually look very similar. Our initial attempt to localize the HF/(H,C,N,F:aug-cc-pVDZ;I:aug-cc-pVDZ-PP) orbitals calculated at the CP-corrected B3LYP/(H,C,N,F:aug-ccpVDZ;I:aug-cc-pVDZ-PP) geometries of M1 and M2 using the Boys procedure24 implemented in GAUSSIAN failed. A second attempt to use the Boys procedure, this time within GAMESS-US,25 was also unsuccessful. However, the two other orbital localization procedures available within GAMESS-US, those of Edmiston-Ruedenberg26 and PipekMezey,27 converged without problems and produced very similar LMOs. The LMOs shown in Figure 17 were obtained using the Edmiston-Ruedenberg procedure.

In comparing theory and experiment, there is an interesting point relating to 9. Thus, previously21 it had been argued that a trihalide such as 1,3,5-trifluorodotriiodobenzene might not be able to form three halogen bonds based on the observation that only two halogen bonds were utilized in complexes with, for example, 4,40 -bipyridine, di(4-pyridyl)ethene, 1,4-bis((pyridin-4-yl)vinyl)benzene, and 1,3,5-tris((pyridin-4-yl)vinyl)benzene.21,28 DFT calculations at the PBE0/(apc1-aSDBDZ)// PBE0/(pc1-SDBDZ) level of theory of complexes between pyridine and 1,3,5-triiodotrifluorobenzene showed small elongations of the halogen bond on addition of successive pyridines, much as found here (ca. 1%/pyridine). However, the discussion also allowed for the possibility that the observed solid-state motifs that contained only two halogen bonds per 1,3,5-triifluorotriiodobenzene may have their origins in the fact that space would not be filled effectively by the 3:1 motif with such nitrogen bases. This idea was taken up by Triguero et al.29 and then by Metrangolo et al.,30 both of whom showed that if much stronger ionic halogen bonds were employed, then all three iodine atoms of 1,3,5-triifluorodotriiodobenzene could be halogen bonded, provided that the voids generated in the network were filled by a suitable cation, in their case either Et4Nþ or Et4Pþ. This is shown schematically in Figure 18. That 9 is isolated and characterized shows that very strong halogen bonds are not required, although it is likely that the ability to fill space is not an insignificant consideration. Conclusions Theory has shown that, in considering the complexation of Lewis bases to di- and triiodo Lewis acids, binding of a >1 Lewis base to the polyiodo compound lengthens the halogen bonds. These halogen bonds have a finite but small charge-transfer character, which classifies the halogen bond here as predominantly electrostatic in nature. Analysis of the shapes of the localized molecular orbitals responsible for the N 3 3 3 I halogen bonds shows that these exhibit significant antibonding character over the adjacent C-I bonds. This last observation is consistent with the observed increase in dC-I on complexation. However, experimentally the variation in halogen bond length in di- and triiodo Lewis acids is not found, and it seems clear that, in the systems examined here, solid-state packing effects dominate over small predicted variations in dN 3 3 3 I. This is best exemplified by the 2:1 complex between DMAP and 1,4-diiodotetrafluorobenzene, which shows the shortest N 3 3 3 I halogen bond known in neutral complexes of fluorinated iodoarenes. That the theoretically predicted reduction in halogen bond length is not an

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obstacle to the formation of multiple halogen bonds in polyiodo systems is further emphasized by the first definitive characterization of a 3:1 complex involving a neutral halogen bond between DMAP and 1,3,5-triiodotrifluorobenzene. Experimental Section DMAP and 1,4-diiodotetrafluorobenzene were used as received, while 1,3-diiodotetrafluorobenzene31 and 1,3,5-triiodotrifluorobenzene32 were prepared using literature methods. Solvents for crystallization were HPLC-grade and were used as received. X-ray Crystallography. Diffraction data were collected at 110 K, on a Bruker Smart Apex diffractometer with Mo KR radiation (λ = 0.71073 A˚) using a SMART CCD camera. Diffractometer control, data collection, and initial unit cell determination were performed using “SMART”.33 Frame integration and unit-cell refinement were carried out with “SAINTþ”.34 Absorption corrections were applied by SADABS.35 Structures were solved by direct methods using SHELXS-9736a and refined by full-matrix least-squares using SHELXL-97.36b All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were placed using a “riding model” and included in the refinement at calculated positions.

Acknowledgment. The authors thank the EPSRC for financial support to C.P. and V.N.K. Supporting Information Available: CIF files for complexes 6-9 plus the THF solvate of the butyloxystilbazole/1,3-diiodotetrafluorobenzene complex (CCDC deposition numbers 773127-773131). This material is available free of charge via the Internet at http:// pubs.acs.org.

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Experimental and Theoretical Study of Halogen-Bonded Complexes of

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