Monobromo complexes

The thermodynamic parameters (i.e.) the enthalpy and entropy values showed the formation of inner-sphere chloro complexes in the case of all the lanthanides. The enthalpies for the formation of monobromo complexes of lanthanides are also positive but smaller in magnitude than the corresponding chloro complexes. The complex formation enthalpies follow the sequence A//°(C1) > A//°(Br) > A//°(I) which is unusual for hard metal(III) ions. 

The enthalpies of formation of the monobromo complexes of these metal ions are also positive, but smaller than those of the chloro complexes. They decrease smoothly in the order La>Nd>Tb>Tm. It is thus found that the formation enthalpies change in the sequence A// (C1) > A/f°(Br) > A/f°(I), which would be unusual for hard metal(III) ions. This implies that, unlike the chloride ion, the bromide ion (and most certainly the iodide ion) forms outer-sphere complexes with the R(HI) and Y(III) ions in DMF. The log. 1, A/7j and AS° values of the monochloro complex of Y(III) in DMF are close to those of Ho(III) and Er(lll), as expected from their similar ionic radii. The logAli, AH° and A5) values for the monobromo complex of Y(III) are smaller than those of the monochloro complex, as in the case of lanthanide ions. Evidence for either an inner-or outer-sphere complex was obtained from Y-NMR spectra for Y(C104)3, YCI3, and YBr3 in DMF at room temperature. 

The reaction between Fe(IlI) and Sn(Il) in dilute perchloric acid in the presence of chloride ions is first-order in Fe(lll) concentration . The order is maintained when bromide or iodide is present. The kinetic data seem to point to a fourth-order dependence on chloride ion. A minimum of three Cl ions in the activated complex seems necessary for the reaction to proceed at a measurable rate. Bromide and iodide show third-order dependences. The reaction is retarded by Sn(II) (first-order dependence) due to removal of halide ions from solution by complex formation. Estimates are given for the formation constants of the monochloro and monobromo Sn(II) complexes. In terms of catalytic power 1 > Br > Cl and this is also the order of decreasing ease of oxidation of the halide ion by Fe(IlI). However, the state of complexing of Sn(ll)and Fe(III)is given by Cl > Br > I". Apparently, electrostatic effects are not effective in deciding the rate. For the case of chloride ions, the chief activated complex is likely to have the composition (FeSnC ). The kinetic data cannot resolve the way in which the Cl ions are distributed between Fe(IlI) and Sn(ll). 

The data for bromo complexes were obtained in aqueous methanolic solutions and outer-sphere bromo complexes with K = 1.3-1.9 were obtained for Pr, Nd, Sm, which are larger than the values of Ho (0.97) and Er (0.70). Chloro and bromo complex formation in dimethyl formamide studied by titration calorimetry [122] showed the evidence for MC12+, MCC. MCI3, and MCI4 species in solutions. In the case of bromide, monobromo and dibromo inner sphere complexes have been detected. The stepwise formation constants could not be determined for iodo complexes due to the small value of enthalpies of reaction. The stability constants data obtained in DMF are given in Table 4.8. 

The absorption bands in the infrared spectra of monobromo-substituted anilines in both the free molecules and the hydrogen bonded complexes with dimethylformamide (DMF), dimethyl sulfoxide (DMSO) and hexamethapol (HMPA) are studied134. The temperature... 

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. 

---