Kinetics and Thermodynamics of the Monomer-Dimer Equilibria of Dialkoxydibutylstannanes

Enthalpies and entropies of dimerization have been determined as functions of concentration for dibutyltin dipropoxide, dibutyltin dibutoxide, and dibutyltin diisopropoxide in toluene-d(8) and cyclohexane-d(12) solutions from the variation in Sn-119 NMR chemical shifts with temperature. The values of Delta H and Delta S obtained in toluene-d(8) for dibutyltin dibutoxide were -69.5 +/- 3.0 kJ mol(-1) and -197 +/- 10 J mol(-1) K-1, respectively, and for dibutyltin diisopropoxide -67.1 +/- 1.4 kJ mol(-1) and 244 9 J I K-1, respectively. Enthalpies and entropies of activation for this process for dibutyltin diisopropoxide have been derived by simulation of the temperature-dependent broadening of the Sn-119 NMR spectra. The free energy of activation for dimerization was 33 kJ mol(-1). The same thermodynamic parameters for a greater range of dibutyltin dialkoxides were derived theoretically by using MP2 single-point calculations on B3LYP-optimized geometries using the LANL2DZdp basis set with diffuse and polarization functions and its effective core potential for tin and 6-311G(2d,p)// 6-31G(d,p) for other atoms. Quite good agreement with the experimental results was achieved with this level of theory. The present study confirms that steric effects dictate the degree of dimerization for dibutyltin dialkoxides, with monomers becoming favored at room temperature when the alkoxide groups are changed from primary to secondary.