Halogen bonding in the crystal structures of 1 , 2-diiodo alkenes †

Halogen bonding is currently a widely discussed topic in the literature. In the last 15 years an increasing number of reviews (approximately 50) have been published on this subject. This type of intermolecular interaction was discovered about two centuries ago but was investigated in detail only in the last few years (for detailed mechanism and the theory of halogen bonding see ref. 2–6). Halogen bonds are relevant in the field of crystal engineering as well as in synthetic chemistry and material science. Many theoretical investigations have been published and halogen bonding has proved to be important in medicinal chemistry as well as in chemical biology. In 2013, the official IUPAC definition was released which states amongst others that typical halogen bonds R–X Y–Z are formed between a halogen bond donor (R–X, X = electrophilic halogen atom, e.g. I2, CH3Br, halonium ion) and a halogen bond acceptor Y (Y = lone pair possessing atom, a p-system or an anion). Additionally, the contact distance of X Y has to be shorter than the sum of the van der Waals radii, the R–X bond length should be elongated and the contact angle R–X Y should be near to 1801. Generally, iodine containing molecules form stronger halogen bonds than corresponding bromine, chlorine or fluorine containing derivatives. In most cases reported in the literature structures showing halogen bonding were obtained by forming co-crystals between two different compounds, one of which acts as the halogen bond donor and the other as the acceptor. A typical example is provided by the co-crystals of phenazine and 1,2,3,4-tetrafluoro-5,6-diiodo benzene or 4,40-bipyridine and tetraiodoethene. The other possible case – halogen bonds in crystals where the halogen bond donor and acceptor are included in the same molecule – is much less discussed. This includes structures of compounds containing e.g. iodine as halogen bond donor and nitrogen, oxygen, a halogen or a p-system as acceptor (I I, I O, I N, I C(p)). Diiodoolefines having additional halogen bond acceptor functionalities in the organic framework provide good examples for such structures. In these compounds halogen bonding should play a key role in determining the features of the crystal structure. In order to put light on the importance of halogen bonding as directing interaction in the formation of the structure in the solid state we have determined and investigated the crystal structures of a series of 1,2-diiodoolefines (Fig. 1). In this paper our aim is to investigate weak halogen bonds of iodine with the less strong halogen bond acceptors oxygen, iodine itself and aromatic, aliphatic or acetylenic p-systems. The systems we include in this study consist of the new crystal structures of nine 1,2-diiodoolefines determined in our laboratory and the crystal structures of several 1,2-diiodolefines from the literature that complement our data.


Introduction
Halogen bonding is currently a widely discussed topic in the literature. In the last 15 years an increasing number of reviews (approximately 50) have been published on this subject. This type of intermolecular interaction was discovered about two centuries ago 1 but was investigated in detail only in the last few years (for detailed mechanism and the theory of halogen bonding see ref. [2][3][4][5][6]. Halogen bonds are relevant in the field of crystal engineering 3 as well as in synthetic chemistry and material science. 4 Many theoretical investigations 5 have been published and halogen bonding has proved to be important in medicinal chemistry as well as in chemical biology. 6 In 2013, the official IUPAC definition was released which states amongst others that typical halogen bonds R-XÁ Á ÁY-Z are formed between a halogen bond donor (R-X, X = electrophilic halogen atom, e.g. I 2 , CH 3 Br, halonium ion) and a halogen bond acceptor Y (Y = lone pair possessing atom, a p-system or an anion). 7 Additionally, the contact distance of XÁ Á ÁY has to be shorter than the sum of the van der Waals radii, the R-X bond length should be elongated and the contact angle R-XÁ Á ÁY should be near to 1801. 7 Generally, iodine containing molecules form stronger halogen bonds than corresponding bromine, chlorine or fluorine containing derivatives. 3c In most cases reported in the literature structures showing halogen bonding were obtained by forming co-crystals between two different compounds, one of which acts as the halogen bond donor and the other as the acceptor. A typical example is provided by the co-crystals of phenazine and 1,2,3,4-tetrafluoro-5,6-diiodo benzene 8 or 4,4 0 -bipyridine and tetraiodoethene. 9 The other possible case -halogen bonds in crystals where the halogen bond donor and acceptor are included in the same molecule -is much less discussed. This includes structures of compounds containing e.g. iodine as halogen bond donor and nitrogen, oxygen, a halogen or a p-system as acceptor (IÁ Á ÁI, IÁ Á ÁO, IÁ Á ÁN, IÁ Á ÁC(p)).
Diiodoolefines having additional halogen bond acceptor functionalities in the organic framework provide good examples for such structures. In these compounds halogen bonding should play a key role in determining the features of the crystal structure. In order to put light on the importance of halogen bonding as directing interaction in the formation of the structure in the solid state we have determined and investigated the crystal structures of a series of 1,2-diiodoolefines ( Fig. 1). In this paper our aim is to investigate weak halogen bonds of iodine with the less strong halogen bond acceptors oxygen, iodine itself and aromatic, aliphatic or acetylenic p-systems. The systems we include in this study consist of the new crystal structures of nine 1,2-diiodoolefines determined in our laboratory and the crystal structures of several 1,2-diiodolefines from the literature that complement our data. Particular attention is directed towards how the molecules are arranged in the crystal with respect to the optimal space filling and at the same time optimal formation of weak interactions. This is of course dependent on the substituents and their acceptor abilities, which are connected to the double bond. We would like to find a kind of ''ranking'' of the interactions in terms of which interaction is responsible for the structural arrangement in these weak systems and which interactions are subordinate. What is stronger in this case: hydrogen bonds (HBs) or halogen bonds (XBs). From this systematic consideration we try to find general trends that could be suitable for crystal engineering of pure compounds/crystals concerning future applications.

Results and discussion
Synthesis 1,2-Diiodoolefines 1-8 ( Fig. 1) were all synthesized by 1,2-addition of I 2 to the corresponding acetylenes according to literature procedures. 10 The unsymmetrical diiodoolefine esters 5 and 6 are new and were prepared from the corresponding alkynes via the CuI catalyzed procedure published by Duan et al. (Fig. 2). 11 In all cases the trans isomers were obtained except for the acetal 7, where a mixture of the cis (7a) and trans (7b) derivatives was formed. From this mixture the two isomers could be separated by fractional crystallization and were structurally investigated via single crystal X-ray diffraction.

General remarks concerning the crystal structures
Single crystals, suitable for X-ray diffraction, of all compounds were obtained by recrystallization from EtOAc. Fig. 6 shows their molecular structures. A feature of most structures is a disorder of the diiodoalkene molecules over two positions. Only the structures of compounds 7a, 7b and 5 show no disorder. Here we discuss the higher occupied positions of the disordered atoms. For the diphenyldiiodide 1 and the alcohol derivative 4 the disorder concerns over 80% of the molecule. Hence the values of the atom distances and halogen bonds of these molecules have to be treated carefully in discussion and comparison with other data. Nevertheless, we include these data in the comparisons here, because the detected values of compounds 1 and 4 lie in the medium range of all here discussed compounds. Figures showing the disorder of all compounds are contained in the ESI. † Of the literature compounds from CSD there is only one disordered (RIDTOO 12 ). The structures of all literature compounds discussed here and some additional ones for the statistics with their names from CSD are depicted in Fig. 3 and 4.
The azide WIFVAJ 13 depicted in the bottom of Fig. 4 would fit perfectly in the topic of this comparison but the quality of the structural data is inadequate. That's why we exclude this data from this discussion. We also found the crystal structures of C 2 H 4 I 2 14 and C 2 H 2 I 2 14 in CSD which would also fit very well in the discussion here. Unfortunately the structures are from the year 1935 14 and thus the cif files contain not enough data for showing the structures with modern methods.

Crystal structures
The crystal structure of 8 contains the second shortest CQO bond (1.186(4) Å, Fig. 5) and the shortest C-I (trans) bond (2.109(4) Å) of our structures which is located under the shorter C-I bonds in this discussion (Fig. 8). The carboxyl group is strongly twisted out of the double bond plane (82.7(4)1 for OMe and À104.7(5)1 for CQO) compared to dimethyl fumarate, which is completely planar. 15   The crystal structure is governed by the shortest IÁ Á ÁO halogen bonds (3.005(3) Å) occurring in this work ( Fig. 9) with an almost linear C-IÁ Á ÁO angle (173.1(1)1) and a CQOÁ Á ÁI angle (117.4(2)1) that indicates an interaction between one of the lone pairs of the oxygen as halogen bond acceptor and the iodine as donor. This interaction is generated by both iodine atoms, and both carbonyl groups function as halogen bond acceptors, forcing the molecule to form four halogen bonds. Due to this, the molecules form layers parallel to the bc-plane that are stacked along the a-axis (Fig. 7).
Looking at the smaller, oxygen containing molecules from the literature (RIDTOO, 12 GIWTEM, 16 GIWTIQ, 16 NIMGOF 17 ), where no large substituent or phenyl ring is attached to the double bond, some common features and also differences in the crystal structures compared to 8 can be detected.
RIDTOO 12 is the only carboxylic acid containing molecule, that does not form IÁ Á ÁO halogen bonds (the less occupied disordered part seems to form an IÁ Á ÁO halogen bond but its arrangement fits perfectly in the network built up by the IÁ Á ÁI interactions). The structure is dominated by hydrogen bonds that cause the molecules to form dimers. These dimers are arranged in a way that a complex network is formed under assistance of IÁ Á ÁI halogen bonds that are the shortest here observed (3.801(1) Å) intermolecular IÁ Á ÁI atom distances (Fig. 10). Additionally, the molecular structure is almost planar (10.2(3)1) which most probably indicates that the twisting of all other carboxyl groups is due to halogen bonding.
In GIWTIQ 16 the hydrogen at the double bond is substituted by a methyl group. This steric increase of the substituent leads to a completely different crystal structure. The molecules form also dimers interconnected via hydrogen bonds. The carboxyl group is twisted out of the double bond plane by 81.2(9)1 and participates in IÁ Á ÁO halogen bonding (3.089(6) Å). The IÁ Á ÁO halogen bond ranges among the shorter ones of the here The trend goes to longer bonds compared to the literature value of 1.19 Å. 22 The reference of Allen et al. 21 (1.199 Å) is more appropriate. Fig. 6 Overview over the asymmetric units or molecular structures of our compounds 1-8 with the specific atom numbering. Symmetry codes: 1 [(a) (1 À x, 1 À y, 2 À z)]; 3 [(a) (1 À x, y, 1.5 À z)]; 8 [(a) (2 À x, Ày, 1 À z)]. DIAMOND representation, thermal ellipsoids are drawn at 50% probability level. detected distances (Fig. 9). Additionally, the molecules form also long IÁ Á ÁI halogen bonds (3.938(9) Å, Fig. 10). All intermolecular interactions lead to the formation of a complex network.
The bulkiness of the CH 2 OMe group included in the molecules of GIWTEM 16 instead of H or Me seems to be high enough to prevent the molecules from IÁ Á ÁI halogen bond formation. The molecules form chains via hydrogen bonds parallel to the bc-plane. Again the carboxyl group is twisted out of the double bond plane by 80.3(9)1 and the CQO bond forms IÁ Á ÁO halogen bonds (3.308(8) Å) which ranges amongst the longer here detected intermolecular IÁ Á ÁO distances (Fig. 9). The steric increase of the CH 2 OMe substituent leads to an elongation of the IÁ Á ÁO halogen bond compared to the value found in the crystal structure of GIWTOQ. 16 The molecules form also chains interconnected via the IÁ Á ÁO halogen bonds which results in the formation of layers.
If both substituents at the double bond of GIWTEM 16 are replaced by CH 2 OH groups as it is the case for NIMGOF 17 the molecules form hydrogen-and IÁ Á ÁO halogen bonds (3.429(1) Å) as already described in the original literature. 17 The molecules do not form dimers via hydrogen bonds and due to the high flexibility of the molecule there again is the possibility to form IÁ Á ÁI halogen bonds (3.875(3) Å) which range like the IÁ Á ÁO intermolecular distances amongst the longer ones (Fig. 10). All intermolecular interactions form a complex network.
Substitution of one CH 2 OH group by a phenyl ring leads to compound 4. As observed for the carboxyl groups in the previously discussed crystal structures, here the phenyl ring is twisted out of the double bond plane (À80.9(7)1). The hydroxyl groups make hydrogen bonds (Fig. 11) forming right twisted helical chains along the c-axis with four molecules forming a turn (Fig. 12).
The distance between the turns d(O1Á Á ÁO1 f ) is 7.532(3) Å. Starting from the asymmetric unit the molecules in the crystal also form a helix interconnected by I1Á Á ÁI2 e halogen bonds . DIAMOND representation, thermal ellipsoids are drawn at 50% probability level.  (3.927(3) Å) with I2 as the halogen bond donor and I1 as the corresponding acceptor. The turns of this second helix are three times as large as the helix formed by the hydrogen bonds (d(O1Á Á ÁO1 g ) = 22.597(2) Å) (Fig. 12). The molecules within the helix form chains along the c-axis via I1Á Á ÁC7 f contacts (3.563(1) Å). In the crystal structure there are also IÁ Á ÁO1 a (3.420(3) Å) contacts (Fig. 13). The I1Á Á ÁI2 e interaction is one of the longest, here observed IÁ Á ÁI halogen bonds (3.927(3) Å, Fig. 10). The IÁ Á ÁC(p) interaction is the atom distance concerning in the medium range (Fig. 14). Although the direction of the C-I bond and the orientation of the phenyl ring indicate almost no interaction. The IÁ Á ÁO halogen bond ranges among the longest here detected intermolecular IÁ Á ÁO interactions (Fig. 9). Consequently the main interactions that are responsible for this arrangement of the molecules in the crystal are the hydrogen bonds. However, despite the quite long IÁ Á ÁI distances, IÁ Á ÁI halogen bonds are also of importance in stabilizing the structure, as indicated by the corresponding bond angles (Tables in ESI †).
Replacing CH 2 OH by a methyl ester group as in the case of compound 5 the structural dominating interactions change completely. In the molecular structure of 5 the C1-I1 (2.118(1) Å) atom distance is a little elongated compared to the C2-I2 (2.109(1) Å) bond length, which compares well to the value of 8. The CQO atom distance (1.197(1) Å) of 5 is the longest CQO bond compared to our other structures (6, 8) but in the medium range of all structures (Fig. 5) and longer than the literature value. 21 In this crystal structure the phenyl ring is also twisted out of the double bond plane (91.0(5)1), same observations can be made for the carbonyl group (51.0(6)1), but this angle is smaller than expected and observed for the above discussed structures.
The intermolecular I2Á Á ÁC7(p) distance is the shortest of all substances in this comparison and highly directional (Fig. 14). The C2-I2 bond points directly towards the C7-C8 bond of the phenyl ring of the adjacent molecule. The two kinds of halogen bonds make the molecules form chains along the b-axis in the crystal structure of 5 (Fig. 15) and in fact there is no interaction between the chains other than van der Waals interactions.
Changing the ester from methyl to ethyl as in the case of 6 there again can be observed a different arrangement of the molecules in the crystal (Fig. 16).
The C-I bonds of 6 are much more similar than those of the methylester 5. The C1-I1 bond (2.116(5) Å) that is part of IÁ Á ÁC(p) interactions (3.530(3) Å) is the longer one. The phenyl substituent and the ester carbonyl group are twisted out of the double bond plane about 87.2(5)1 and 104.0(5)1, respectively. The latter value is comparable with the value that can be found in the crystal structure of the diester 8. The IÁ Á ÁC(p) halogen Fig. 11 Crystal structure of 4. One turn of the helix formed by the IÁ Á ÁI halogen bonds (I2 to I1 g ) includes three turns of the helix formed via hydrogen bonds (O1 to O1 j ; .6(2)1. Some (I)CQC(I)Ph residues are omitted for clarity. Symmetry codes: (a) (À0.25 + x, 1.25 À y, 0.75 + z), (c) (1.25 À x, 0.25 + y, 0.25 + z), (g) (x, y, 3 + z), (h) (x, y, 2 + z), (i) (1.5 À x, 1 À y, 1.5 + z), ( j) (x, y, 1 + z), (k) (1.5 À x, 1 À y, 2.5 + z). DIAMOND representation, thermal ellipsoids are drawn at 50% probability level. Fig. 12 Helix along the c-axis in the crystal structure of 4. All C-H hydrogen atoms are omitted for clarity. Symmetry codes: (a) (À0.25 + x, 1.25 À y, 0.75 + z), (b) (1.5 À x, 1 À y, À0.5 + z), (c) (1.25 À x, 0.25 + y, 0.25 + z). DIAMOND representation, thermal ellipsoids are drawn at 50% probability level. bond length lies in the medium range of all halogen bonds of this sort here detected (Fig. 14) and is responsible for the formation of chains that are dominating the crystal structure of 6. The steric increase caused by the insertion of the CH 2 group pushes the molecules away from each other and the intermolecular interactions become weaker. This results also in longer IÁ Á ÁO interactions (3.326(2) Å, 3.498(2) Å) compared to the values that can be detected in the crystal structure of 5. Compared to all values for the IÁ Á ÁO interactions in this comparison, the data of 6 belong to the longer ones (Fig. 9).
Changing the substituent from an ester to an ethyl acetal, there are two different isomers. For the (Z) isomer (7a) the two C-I distances are clearly different; the longer C2-I2 distance (2.115(3) Å) is associated with the phenyl bonded carbon atom and I2 does not form any halogen bonds. I1 with the shorter C1-I1 distance (2.091(3) Å) forms I1Á Á ÁO2 b halogen bonds (3.330(2) Å) that range between the longer IÁ Á ÁO halogen bonds here observed (Fig. 9). This observation is unusual, because for halogen bonding a slight elongation of the C-I bond (typically 2.1 Å) 22 would be expected. 2,7 The phenyl ring is clearly twisted out of the double bond plane (65.2(4)1) but not that much like in the structures discussed before. The molecules form zig-zag chains along the b-axis interconnected via the IÁ Á ÁO contacts. There are no further interactions between the chains, which are packed in the crystal to give an optimal space filling (Fig. 17).
Here we have the border line between steric hindrance and the ability to form halogen bonds, where also is the need for a little geometrical flexibility. Same observations can also be made in the crystal structure of MIQKUU, 23 the only acyclic cis compound from the literature. Here no halogen bonds can be observed and the molecule is quite rigid.
Keeping the phenyl substituent as a constant and changing the oxygen containing substituent against a H leads us to compound 3. In this crystal structure the C-I bond lengths are different and the phenyl ring is twisted out of the double bond plane (80.9(2)1). Here also IÁ Á ÁC(p) halogen bonds (3.573(1) Å) are the dominant intermolecular interactions which ranges under the longer ones in this discussion (Fig. 14). The molecules of 3 form chains along the a-axis head to tail connected via the IÁ Á ÁC(p) halogen bonds (Fig. 19). Here we have an example that shows, that a decrease in steric hindrance leads to a maximum of intermolecular interaction.
The largest change in crystal packing is caused by the substitution of the H to methyl. The asymmetric unit of compound 2 contains three independent molecules. Molecule 1 (contains I1) forms IÁ Á ÁC(p) halogen bonds (3.543(5) Å) whose lengths lie in the medium range of all here detected IÁ Á ÁC(p) interactions (Fig. 14). The phenyl ring of M1 is twisted out of the double bond plane about À79.0(1)1. These molecules form chains along the b-axis via the IÁ Á ÁC(p) interactions (Fig. 20).
In molecules 2 (containing I3) and 3 (containing I5) the phenyl rings are also twisted out of the double bond plane (À86.6(8)1 for M2 and 72.1(8)1 for M3). Each molecule forms a helical chain that can be converted into each other by symmetry. In both helices the alternating molecules M1 and M2 are interconnected via IÁ Á ÁI halogen bonds (I3Á Á ÁI6 a 3.884(6) Å, I4Á Á ÁI5 c 3.885(5) Å, Fig. 21) that range in the middle of all here detected IÁ Á ÁI interactions (Fig. 10). The helices itself are not connected with each other. Looking at the angles of the halogen bonds, there are two different tendencies. Two of the four angles are near to linear (I3Á Á ÁI6-C25 159.0(2)1, I5Á Á ÁI4-C16 173.2(2)1) and the other two are near to rectangular (I6Á Á ÁI3-C17 91.5(2)1, I4Á Á ÁI5-C26 97.7(2)1). These values indicate that I4 and I6 act as halogen bond donors and I3 and I5 act as halogen bond acceptors. Remarkable is that the corresponding C-I bonds of the donor atoms (I4/I6) are shorter compared to the C-I bond lengths of the acceptor atoms (I3/I5) because the opposite was expected, considering the XB definition of the IUPAC2. The helices are arranged parallel to the cell edges of the unit cell and the chains interconnected via IÁ Á ÁC(p) interactions are arranged between two helix strands (Fig. 22). The independent occurrence of IÁ Á ÁC and IÁ Á ÁI halogen bonds in the same crystal structure lets us conclude, that in this case the energy of both interactions is very similar. This left twisted helical chain arrangement is very similar to the crystal structure of 4.
interaction. This intermolecular distance is in the medium range of all interactions of this type here observed (Fig. 14). Also the overlap of the C1-I1 line with the adjacent phenyl substituent is warranted. The latter again is twisted out of the double bond plane (À80.5(8)1) as observed for the phenyl substituents of all other compounds discussed here. Via this intermolecular interaction the molecules form chains (Fig. 23).
In the crystal structure of EMUZIW 24 where one phenyl substituent is replaced by a bicyclic lactone there are two molecules in the asymmetric unit. Both substituents of both molecules are twisted largely out of the double bond plane although no IÁ Á ÁC(p) halogen bonds occur in this crystal structure due to the steric hindrance of the large substituents. One molecule of the asymmetric unit forms IÁ Á ÁI halogen bonds (3.835(4) Å) and chains interconnected via those. The other molecule forms also chains but those are interconnected by IÁ Á ÁO halogen bonds (3.347(5) Å).
In the crystal structure of ULEFUN 25 the molecule contains two terminal diiodo olefinic moieties and is generally quite flexible. The molecules are arranged in a way that leads to maximum halogen bond formation, four out of four iodine atoms form halogen bonds, including IÁ Á ÁI, IÁ Á ÁO and IÁ Á ÁC(p) interactions.
In the molecular structure of RUWRUX 26 there are two diiodo olefinic moieties nearby and twisted in a way that makes conjugation of the double bond unlikely. The molecule itself is quite flexible and is twisted in a way that a maximum halogen bond formation is possible. The asymmetric unit persists of two molecules with all in all eight iodine atoms. Seven of them form halogen bonds.

Discussion and conclusion
Looking at the similarities that can be found in the crystal structures of the compounds discussed above, a competition between the formation of hydrogen bonds and the formation of halogen bonds can be observed when both is possible. Also a competition between the different halogen bonding acceptors is present. All acids and alcohols form hydrogen bonds which always seem to be the dominant interaction and all others subordinate themselves. When halogen bonds occur in the crystal structure then the participating halogen bond acceptor like CO 2 R or Phenyl is twisted out of the double bond plane to ensure optimal contact conditions. This happens in a range that is clearly larger than single steric effects would induce. The reverse case is not always warranted. This observation lets us conclude that the energy of the halogen bond is higher than the energy the molecule wins via the conjugation.
Under exclusion of the very small molecule RIDTOO 12 where mostly hydrogen bonds and IÁ Á ÁI halogen bonds are relevant for the crystal structure formation, a general statement concerning the IÁ Á ÁO halogen bond formation can be made. Is an oxygen containing functional group included in the molecule that could act as halogen bond acceptor, then it functions as such. In the ester or acid groups occurring here, the included carbonyl group always forms the halogen bond. Those CQO functions seems to be the stronger halogen bond acceptor than the OH or OR groups that are also present. Exceptions are small molecules like RIDTOO 12 where stronger interactions like hydrogen bonds are possible. Also exceptions are rigid molecules with stericly demanding substituents in the cis configuration like 7a and MIQKUU, 23 where very weak or even no halogen bonds under participation of the oxygen occur.
As one can clearly see for the crystal structures that are built up by the molecules arranged in chains (7b, 1, 2, 3, 5) mostly IÁ Á ÁC(p) interactions are responsible for the chains. In the crystal structure of 5 the chains are built up by IÁ Á ÁO interactions supported by the IÁ Á ÁC(p) halogen bonds, resulting in the strongest IÁ Á ÁC(p) and the second strongest IÁ Á ÁO interactions.
When there are only iodine and a p-system present in a molecule to act as halogen bond acceptor as is the case for 1, 2, 3, C 2 I 2 and C 2 I 4 then, except for C 2 I 4 and parts of the crystal structure of 2, the heteroatomic IÁ Á ÁC(p) interaction is always preferred in the crystal structure instead of the homoatomic IÁ Á ÁI halogen bond. In this study no iodine interactions with olefinic double bonds could be observed. The main interactions occur between an iodine atom and the p-system of a phenyl ring. In the crystal structure of C 2 I 2 the halogen bond acceptor is the CRC Fig. 22 The helices form quad shaped subunits which form strands along the c-axis in the crystal structure of 2. The molecules that form IÁ Á ÁC(p) halogen bonds are between the strands of quad shaped units. DIAMOND representation, thermal ellipsoids are drawn at 50% probability level. (1 À x, 1 À y, 2 À z), (b) (Àx, 1 À y, 1 À z), (c) (À1 + x, y, À1 + z), (d) (1 + x, y, 1 + z), (e) (2 À x, 1 À y, 3 À z). DIAMOND representation, thermal ellipsoids are drawn at 50% probability level. triple bond. In the crystal structure of RETRIR 27 where a phenyl ring and a triple bond are included in the same molecule, the triple bond is the favored halogen bond acceptor.
For the IÁ Á ÁI halogen bond formation it is noticeable that they occur either in crystal structures consisting of smaller molecules like RIDTOO, 12 GIWTIQ, 16 NIMGOF, 17 2 and 4. The steric demand of the substituents of those molecules is either small or the substituents cannot act as halogen bond acceptor or both. That gives the iodine atoms the possibility to converge and interact. Or they occur in the crystal structures of larger flexible molecules with higher iodine content. There the molecules are able to twist themselves in a way of maximum halogen bond formation where also IÁ Á ÁI halogen bonds occur. Stating which intermolecular interaction is dominant in such molecules is hardly possible.
With this background the similarity (despite the opposite twisting: 2 left, 4 right) of the structural arrangement in the crystal structures of compounds 2 and 4 is remarkable, considering the diversity of interactions (OÁ Á ÁH, IÁ Á ÁO, IÁ Á ÁI, IÁ Á ÁC(p)) occurring in the crystal structure of 4. In the relevant part of the crystal structure of 2 there are solely IÁ Á ÁI halogen bonds. Nevertheless both crystal structures are dominated by helical chains with four molecules in a turn but the turns in the crystal structure of 4 are smaller because there the molecules are connected by hydrogen bonds. The corresponding helix built up by the IÁ Á ÁI halogen bonds is three times as large. In the crystal structure of 2 the asymmetric unit contains two independent molecules that form helices that are symmetry generated and also twisted into each other without connection. For the steric demand methyl and CH 2 OH seems to be equivalent in this case.
Discussing the question, whether the twisting of the phenyl ring at the CQC double bond is correlated due to the formation of halogen bonding to iodine the two isomers KAFBOJ 28 and KAFBID 28 can be considered (Fig. 24). In the case of KAFBOJ 28 clear halogen bonding of iodine to the p-system can be observed in the crystal. On the other hand, in the case of KAFBID 28 only hydrogen bonding and no halogen bonding is found. Looking at the torsion angles, regarding the phenyl substituents the twisting of the phenyl ring of KAFBOJ is with 74.4(7)1 much larger than the twisting in KAFBID (37.6(6)1). The larger twisting in the case of KAFBOJ is most probably caused by the effect of halogen bonding, which fits well to the observations made in the related diiodoolefins 1-7.

Materials and methods
All chemicals were commercially available and were used as received. NMR spectra were recorded with a JEOL EX 400 Eclipse instrument operating at 400.128 MHz ( 1 H) and 100.626 MHz ( 13 C).
Chemical shifts are referred to Me 4 Si ( 1 H, 13 C) as external standards. All spectra were measured, if not mentioned otherwise, at 25 1C. The assignment of the signals in the 1 H and 13 C NMR spectra is based on 2D ( 1 H, 1 H-COSY45, 1 H, 13 C-HMQC and 1 H, 13 C-HMBC) experiments. Mass spectrometric data were obtained with a JEOL Mstation JMS 700 spectrometer using the direct EI mode. The molecular structures in the crystalline state were determined by single crystal X-ray diffraction. For data collection an Xcalibur3 diffractometer equipped with a Spellman generator (voltage 50 kV, current 40 mA) and a Kappa CCD detector with an X-ray radiation wavelength of 0.71073 Å was used. The data collection was performed with the CrysAlis CCD software 29 and the data reduction with the CrysAlis RED software. 30 The structures were solved with SIR-92 or with SIR-2004, refined with SHELXL-97 and finally checked using PLATON. 31 The absorptions were corrected by SCALE3 ABSPACK multiscan method. 32 All relevant data and parameters of the X-ray measurements and refinements are given in Table 1. CCDC 1037179 (1), 1037183 (2), 1037178 (3), 1037181 (4), 1037184 (5), 1037182 (6), 988932 (7a), 988931 (7b) and 1037180 (8).