Halogen complexes reactions

RhX(CO)(PPh3)2 [X=halogen] complexes with hydrogen. The reaction for X=C1 is shown in Figure 11.3. 

These results have been rationalized74 by Heasley and coworkers by assuming that the primary function of the complexes is to limit the concentration of free halogen. In the reaction of free bromine where the reaction is second order in bromine, two or more molecules of halogen participate in the transition state while the halogen complexes with pyridine or amines impose a first-order mechanism by limiting the availability of free halogen (equation 40). 

Iridiuin(iv).—Group VII Donors. All reported studies of Ir concerned halogen complexes. Pure IrF4 has been prepared for the first time via reduction of IrFg on a hot filament of Ir or W. X-Ray powder pattern studies of this volatile, red-brown compound showed it to be different from IrFj or IrFs. A simple melt synthesis of K2lrFg has been developed involving the reaction ... 

Carbon-Halogen Bond Reactions Catalysed by Ni, Co and Pd Complexes... 

An attempt to directly convert hyellazole (245) to 6-chlorohyellazole (246) by reaction with N-chlorosuccinimide in the presence of a catalytic amount of hydrochloric acid led exclusively to 4-chlorohyellazole. On the other hand, bromination of 245 using NBS and a catalytic amount of hydrobromic acid gave only the expected 6-bromohyellazole (733). Alternatively, a direct one-pot transformation of the iron complex 725 to 6-bromohyellazole (733) was achieved by reaction with an excess of NBS and switching from oxidative cyclization conditions (basic reaction medium) to electrophilic substitution conditions (acidic reaction medium). Finally, a halogen exchange reaction with 4 equivalents of cuprous chloride in N,N-dimethylformamide (DMF) at reflux, transformed 6-bromohyellazole (733) into 6-chlorohyellazole (246) (602) (Scheme 5.73). 

Methylbenzene halogen complex of, 3 122 iodine monochloridecomplese, 3 109 Methylchlorosilanes hydrolysis, 42 149-150, 157 pyrolysis products of, 7 356-363 Methylcobalamin, 19 151, 152 Methyl-coenzyme M reductase, 32 323-325 EPR spectra, 32 323, 325 F43 and, 32 323-324 function, 32 324-325 Methyl-CoM reductase, 32 329 Methyl cyanide, osmium carbonyl complexes, reaction, 30 198-201 Methylcyclophosphazene salts, 21 70 synthesis, 21 109... 

Thus the complex fluorides of aluminum do not offer any advantages over other fluorinating agents in halogen-exchange reactions and so they have not found application in industry, although sodium hexafluoroaluminate is produced in large quantities. 

Another arylation method, in the case of nitrogen heterocycles, does not need a halogenated derivative but a heterocycle activated by triflic anhydride260,261 (reaction 22). Simple aryl halides usually do not react with phosphines and special methods therefore have to be used for their arylation. The most widely used is the complex salt method , in which an aryl halide is heated with a phosphine in the presence of a transition metal such as nickel (II)2e (reaction 23). The catalytic cycle probably takes place by means of a reduced nickel(I) complex, generated in situ from the starting nickel(II) salt this nickel(I) species could undergo an oxidative addition of the aryl halide to yield a transient nickel(III) adduct, which after the reductive elimination of the aryphosphonium affords the recovery of the first active-nickel(I) complex (reaction 24). 

An extensive review55 of the chemistry of halogen complexes containing carbon ligands, the hypervalent halogen compounds , has been published recently. This is written from an organic chemical viewpoint and includes discussion of the organic chemistry involved in reactions of these compounds. Only aspects directly related to coordination chemistry will be mentioned here. 

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