Simple halogen bridges

According to CNDO/2 calculations for X, Y, Z = F, this anion possesses a nearly linear halogen bridge in addition to a decreased nucleophilicity and an energetic stability when compared with the simple anion SbY5Z 19). 

Figure 3.24 Formation of simple or double halogen bridges.

For complex formation, simple and well established techniques are brought into action. Splitting of halogen bridged dimers or replacement of weakly bonded ligands such as THF would be successful the presence of only weakly coordinating anions could support the transition from a linear (ylene) arrangement into a bent structure to form L M CL2 compounds. 

The iodo-bridged complexes 12 and 34 (formed by a simple halogen metathetic reaction of the corresponding chloro-bridged dimers with Nal in acetone, for example) both react with alkynes to afford heterocyclic products. 12 leads to a range of indoles 40 in a one-pot procedure, ... 

Second only to sulfur-based systems, nitrogen complexes are relatively well represented in the structural literature with 41 complexes reported. Of these, 25 are with I2 as the electron acceptor, 11 are with the interhalogen IC1, three are with Br2, and two are with IBr. As expected, in every case the halogen bond forms between the nitrogen and the softest halogen atom, i.e., iodine, in all of the complexes except those with dibromine. Most N I2 complexes, and all N Br2, N IBr, and N IC1 complexes are simple adducts, mode A. Exceptions for the diiodine complexes include bridging mode (B) observed for diazines, such as pyrazine [86], tetramethylpyrazine [86], phenazine, and quinoxaline [87], and for 9-chloroacridine [89] and the 1 1 complex of diiodine with hexamethylenetetramine [144] and amphoteric bridging mode (BA) observed for 2,2 -bipyridine [85], acridine [89], 9-chloroacridine [89], and 2,3,5,6-tetra-2/-pyridylpyrazine [91]. The occurrence of both B and BA complexes with 9-chloroacridine, and of B and A complexes and an... 

In method (ii), the group X (e.g., a halogen) may function as a three-electron donor when present as a bridging group, but as a one-electron donor when combined to one metal center. The simple act of bridgeopening creates a vacant coordination site on one metal atom. Behavior of this sort is invoked to explain the stereospecific incorporation of I3CO in the complex Os3(CO)10Cl2. 

The simple most systems displaying the structural motive described in Scheme 1 carry a halogen atom in /3-position to the radical center. An early controversy arising from stereochemical experiments deals with the equilibrium structure of these types of radicals.5,6 The stereochemical control observed in some of these reactions suggests that the halogen is either asymmetrically or symmetrically bridging the radical center, in particular if X = Br or I (Scheme 2). 

This simple approach in practice may require modification. In the first instance, the group 15 atom may still not be coordinatively saturated and, in particular with [RMXj] species, further electron density can be accepted into the Group 15 valence shell from the lone pairs on a halogen atom. If this occurred, the simplest product would be a dimer with two bridging halogen groups and such species are well known. 

The process probably involves coordination between the metal and the palladium via a bridging oxygen (boronic acids, silanols) or halogen (tin, zinc, magnesium), followed by internal transfer of the organic residue. The diagram shows a simple four-centre transition state, but more complex arrangements are possible. 

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