Microenvironmental effects on equilibrium and photoredox chemistry of bromo complexes in reverse micelles

摘要

Kinetically labile bromo complexes are formed with main-group metal ions of s2> and s0> electron configurations both in homogeneous aqueous solutions and in water droplets inside reverse micelles. The local concentration of the Br-> ligand in the inner water pool can reach extremely high values if it is the counterion of the surfactant as in the case of cetyltrimethylammonium bromide (CTAB) dissolved in CH2Cl2 or CHCl3. Bromo complexes of the possibly highest coordination number are formed with Sb3+> (n=6), Bi3+> (n=6), and Pb2+> (n=4) in these systems at low water to surfactant molar ratio, W. Irradiation of these complexes results in ligand-to-metal charge-transfer (LMCT) reactions generating Br3-> as an end product. The quantum yields for the overall processes dramatically decrease upon increasing W in the CTAB/CHCl3 systems. This phenomenon can be attributed to a hydration effect enhancing the efficiency of the vibrational energy dissipation. In homogeneous aqueous solution of 0.1 M Br->, the predominant coordination number of bromomercurate(II) complexes is 4, while inside aerosol-OT reverse micelles (at the same local concentration of the ligand) it is only 3, owing to the dehydration of the reactants of the same charge in the complex equilibrium. Upon UV excitation, these complexes also undergo a LMCT reaction producing elemental mercury and Br3-> as end products.