1,3 Dichloro- and 1,3 dibromotetra-n-butyldistannoxane mixtures: ligand redistribution and fluxional dynamics in distannoxanes

The halogen redistribution reaction in the binary [nBu2SnCl]2O/[nBu2SnBr]2O system is examined by 119Sn- and 13C-NMR spectroscopy. Binary mixtures of [nBu2SnCl]2O and [nBu2SnBr]2O reach equilibrium rapidly at room temperature. The reactant dimers are found to be in equilibrium with all five possible mixed distannoxane dimers in the equimolar mixture. These mixed distannoxane dimers differ in the ratio of Cl and Br as well as the relative positioning of the halogens. The mechanism responsible for the rapid formation of the mixed Cl:Br distannoxane dimers is found to proceed via bimolecular collisions producing a four-centered transition state, which in turn undergoes a concerted exchange of the halogens. The equilibrium concentrations of the reactant and product dimers are well represented by a statistical distribution, indicating that Cl and Br exhibit equivalent donor abilities. At 298 K, the NMR spectral data are consistent with time-averaged structures arising from rapidly interconverting rigid ladder pairs. Lowering the temperature to 173 K failed to freeze out this fluxional process. A reversible configurational rearrangement is also observed in which rotation about the oxygenexocyclic tin bond results in the mutual exchange of halogens associated with the same exocyclic tin atom.