Molecular Complexes of Halogens

Alternatively, as in Scheme B, BrCN forms only an outer sphere complex, with Fe(HjO) assuming the role of an acid catalyst. In both schemes the essential feature is a polarization of the YH-BrCN system in which YH acts as a donor and BrCN as an acceptor, not unlike the situation in molecular complexes of halogens with [Pg.384]

Figure 17.3 Structures of some molecular complexes of the halogens.

The complex between 1,10-phenanthroline and picric acid was stated to be a n-7i complex, not the expected n-n type.362 1,10-Phenanthroline also forms molecular complexes with phloroglucinol,363 halogenated p-nitrophenols 364 hexachlorocyclohexanes,365 and iodine.366,367 It forms an adduct with acetic anhydride.368 Electronic spectra of molecular complexes of 1,10-phenanthroline with porphyrins have been recorded.369... 

The most favorable conditions for obtaining molecular complexes of o-hydroxyazomethines of type 856 are created by use of strong Lewis acids, for example halides of metals of Groups III-V of the Periodic Table, and aprotic organic solvents with a low dielectric constant (hydrocarbons and their halogen-substituted derivatives). An example of such synthesis is the preparation of complexes of salicylideneimines with titanium and tin tetrachlorides (4.25) [101] ... 

The structure of ra-complexes has been examined by the X-ray crystallographic studies of solid samples and by the spectral measurement of heterocycle-halogen equilibria in solution. By the former approach the structure of the solid 1 2 pyridine-molecular iodine complex has been shown to consist principally of two pyridine molecules collinearly bonded to an iodine atom and of linear triiodide units. Moreover, in the 1 1 complex of dioxane and bromine, the heterocyclic oxygen-bromine-bromine linkages are also collinear.4 In the latter type of study, interest in the well-known phenomenon of brown iodine solutions has occasioned the measurement of the stability constants for many complexes of halogen and heterocycles.25, 29 Information concerning their structure in solution comes from a consideration of the relative size of such constants as a function of heterocycle structure. Thus, the fact that bromine complexes of both 8-bromo- and 8-methylquinolines possess stability constants (K = 1.1 and 4.8 liters/mole) much smaller than that of quinoline itself... 

Benzo[c]cinnoline forms molecular complexes with halogens in organic solvents thus attempts to brominate it with molecular bromine have been unsuccessful. Using a source of positive bromine, bromine/silver sulfate in sulfuric acid, Corbett and Holt found that reaction occurred at room temperature to give 27% of a monobromo and 4% of a dibromo derivative. These compounds were at first identified as 1-bromo- and l,4-dibromo-(or l,7-dibromo-)benzo[c]cinnolines, but the former was subsequently shown to be the 4 isomer. A reinvestigation of the reaction has shown that both 1- and 4-bromobenzo[c]cinnolines are primary products, formed in the ratio of ca. 2.3 1 at room temperature. The lower isomer ratio as compared with nitration in sulfuric acid probably reflects the greater steric demand of the attacking species. The dibromo compound formed is the 1,4-isomer. The formation of the octachloro derivative by chlorination of benzo[c]cinnoline in the presence of aluminum chloride has been mentioned,but no details are available. 

This method has proved applicable to the investigation of molecular complexes of metal halides with donor molecules [Kr 68, Ma 69, Ma 70a, Ma 73, To 69]. NQR examinations yield information on the effect of complex formation on the electronic structure of the halide, and hence indirectly on the locations of the halide ions and the donor molecules in the complex. NQR data have similarly thrown light on the structures of halogen complexes of organic amines [Bo 69, Br 72]. 

El Hage K, Piquemal JP, Hobaika Z, Maroun RG, Gresh N (2013) Could an anisotropic molecular mechanics/d5mamics potential account for sigma hole effects in the complexes of halogenated compounds J Comput Chem 34(13) 1125-1135. doi 10.1002/jcc.23242... 

Popov Al, Rygg RH (1957) Studies on the chemistry of halogens and of polyhalides. XI. Molecular complexes of pyridine, 2-picoline and 2,6-lutidine with iodine and iodine halides. J Am Chem Soc 79 4622-4625... 

It was shown by direct synthesis that the complex (C) was merely a molecular complex of two molecules of (Cl) and one of hydroquinone the lower molecular weight originally supposed was found to be due to dissociation. The compound (Cl) is unusually basic and forms adducts with acids such as phenols, FeCl3, and the halogen acids 132,133). 

The formation of halogen complexes with olefins does argue in favor of the molecular mechanism of halogenation because the complexes can participate in ion and radical reactions of transformation. It is difficult experimental problem to prove the molecular mechanism of these reactions because it should be based on the comprehensive kinetic study of the process and strong proofs that ions or radicals do not participate in the process. 

The 4,5-dihalogenothiazoles are obtained by cyclization-halogenation reactions as show in scheme 12 (3). 2-Acetamido-4,5-diiodothiazole has been obtained by Hurd and Wehrmeister (80). The triiodothiazole can be prepared by iodination by molecular iodine of the mercuric complex of 2-iodothiazole following the Travagli method (81). 

Kinetic studies have been carried out using the 1 1-complex iodobenzene dichloride as a source of molecular chlorine. In acetic acid solutions, the dissociation of this complex is slower than the rate of halogenation of reactive aromatics such as mesitylene or pentamethylbenzene, consequently the rate of chlorination of these is independent of the aromatic concentration. Thus at 25.2 °C first-order chlorination rate coefficients were obtained, being approximately 0.2 x 10-3 whilst the first-order dissociation rate coefficient was 0.16 xlO-3 from measurements at 25.2 and 45.6 °C the corresponding activation energies... 

First of all, the reaction pathways shown in Scheme 1 involve the formation of charge transfer complexes (CTC) between olefin and Br2- The formation of molecular complexes during olefin bromination had been hypothesized often (ref. 2), but until 1985, when we published a work on this subject (ref. 3), complexes of this type had been observed only in a very limited number of circumstances, all of which have in common a highly reduced reactivity of the olefm-halogen system, i.e. strongly deactivated olefins (ref. 4), or completely apolar solvents (ref. 5) or very low temperatures (ref 6). 

Platinum(II) isocyanide complexes of the general formulas [PtL4], [PtL3X], and PtL2X2 and substituted derivatives thereof (L = RNC, X = halogen, H, R, etc.), have been studied recently and will be the main subjects of discussion. It may be observed that much work on complexes of the first and third molecular stoichiometries had been reported previously. 

---