Thermodynamics of Coordination of Pyridine and Its Substituted Derivatives to Osmium Tetroxide

S.M. Nabavizadeh

Department of Chemistry, College of Sciences, Shiraz University, Shiraz, Iran, 71454

Complexation of osmium tetroxide (OsO₄) with pyridine and its substituted derivatives (i.e. 4-picoline, 3 -picoline, 4-tert-butylpyridine, methyl nicotinate, 3,4-dimethylpyridine, 3-chloropyridine, and 3-phenylpyridine) has been studied at different temperatures in benzene. Spectrophotometric measurements have been carried out to obtain the equilibrium constants and thermodynamic parameters. The equilibrium constants of the resulting 1:1 complexes are found to vary in the order 3,4-Me₂Py > 4-^tBuPy > 4-MePy > 3-MePy > Py > 3- PhPy > 3-C(O)OMePy ≈ 3-ClPy. All complexes are enthalpy stabilized whereas the entropy changes counteract the complexation. The results are discussed in terms of different basicities of the substituted pyridines.

Keywords: Osmium tetroxide, Pyridine derivatives, Thermodynamics, Spectrophotometry