Dihalogen bonding

The reactivity of the initial halogen bonded complex has also received considerable attention. Husebye and coworkers have suggested a process whereby the dihalogen bond is cleaved to give a key cation intermediate and a halide anion (Eq. 4) [182]. This mechanism is consistent with that often proposed for nitrogen electron donor systems. 

Mez+)Me...H-C agostic bond (hydride bond) C-Hal+8... 8Hal-Y dihalogen bond (halogen bond)... 

Fig. 9.2 The fragment of the crystal structure of 4-iodobenzoic acid with C-I...I intermolecular interactions (dihalogen bonds -broken lines) black circles correspond to carbon atoms, grey to hydrogens, red to oxygens and violet to iodines, the positive and negative regions of the electrostatic potential for atoms being in contact are designated by + and —...

The mentioned here dual character of atoms of Groups V-VII, i.e. that o-holes are found in conjunction with regions of the negative electrostatic potential, results in the existence of interactions between symmetrically equivalent atoms. Thus the halogen-halogen (dihalogen) bonds [60] are observed as well as... 

The reactions discussed so far commonly involve breaking polarized bonds when mixed with a suitable reagent. As discovered in Chapter 10, Section 10.4.1, the dihalogens are polarizable in the presence of electron-rich or electron-poor species, and they also have relatively weak bonds. Many experiments confirm that dihalogen bonds (X-X) break in the presence of heat or light, via homolytic cleavage, to generate two radicals, X. Once the radical is formed, it can react with many different compounds. A radical is so reactive that it can even react with the C-H bond of an alkane. 

Although chlorine azide and bromine azide tend to give some dihalogen adducts, these reagents have been used in the presence of alcohol, ester and epoxy functions without interference.In the case of 17a-acetoxy-A -progesterone, a selective addition to the 6,7-double bond is obtained. ... 

Table 1 Calculated equilibrium bond distances [A] of the dihalogens as obtained with different methods applying the aug-cc-pVTZ basis set [35]...

Experimental information on the complexes between dihalogens and methylated amines is still comparatively scarce. Gas-phase investigations are available only for the complexes of trimethylamine with F2 [41 ] and with C1F [42], So far only a few theoretical investigations on XY- amine complexes have been presented [16,17,22,24,28,32,34,43,44]. On the basis of rotational spectroscopic analysis, the N(CH3)3 C1F complex was described as being dominated by a significant contribution of an ionic [(CH3)NC1]+- -F valence bond structure [41]. For the (CH3)3N F2 complex an even stronger tendency toward an ionic [(CH3)NF]+- F structure was reported [40]. 

Different types of halogen bonding as they occur in the interaction of small molecules have been reviewed. In all cases studied so far, the halogen bond turned out to have a number of characteristic properties reminiscent of the well-known hydrogen bond. The best investigated cases are those in which a dihalogen XY interacts with a Lewis base B. 

Keywords Lewis bases Dihalogens Halogen bond Angular geometry ... 

Efg Electric field gradient n-pair Non-bonding electron pair Tt-pair -bonding electron pair XY Generalised dihalogen molecules... 

This chapter is restricted to a discussion of halogen-bonded complexes B XY that involve a homo- or hetero-dihalogen molecule XY as the electron acceptor and one of a series of simple Lewis bases B, which are chosen for their simplicity and to provide a range of electron-donating abilities. Moreover, we shall restrict attention to the gas phase so that the experimental properties determined refer to the isolated complex. Comparisons with the results of electronic structure calculations are then appropriate. All of the experimental properties of isolated complexes B- XY considered here result from interpreting spectroscopic constants obtained by analysis of rotational spectra. 

The second conclusion concerns the difference Ar = rB...Hx(Z - X)-rB. -xy(Z X) between the Z to X distances in the two series B- HX and B- XY. Ar is positive and nearly constant for a given B and X, when XY is CI2, Br2, BrCl or ClF. Since the order of the internuclear distances is r(XY) > r(HX) for any given atom X, this result means the outer atom Y of the dihalogen molecule XY is always more distant from a given point in B for the complex B- XY than is the atom X from the same reference point in B for the complex B- HX. This second general result is relevant to the discussion of linear versus non-linear hydrogen and halogen bonds in Sect. 6. 

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