Structure and conformational motion of seven-coordinate diorganotin(IV) complexes derived from salen and salan type ligands

Reaction of R₂SnCl₂ (R = Me, nBu, Ph) and the potassium salts of salenN₃H₃ (N,N′-bis(salicylidene)diethylenetriamine) and saleanN₃H₅ (N,N′-bis(o-hydroxybenzyl)diethylenetriamine) provided diorganotin(IV) complexes of the composition Me₂Sn(salenN₃H)]·solvate (solvate = 2.5H₂O, MeOH or DMSO), nBu₂Sn(salenN₃H)]·H₂O, Ph₂Sn(salenN₃H)]·2EtOH and Me₂Sn(saleanN₃H₃)]·2.5H₂O. In all compounds the tin atoms are seven-coordinate and have pentagonal-bipyramidal coordination environments, in which the organic substituents attached to the tin atoms occupy the axial positions. This occurs both in solution and the solid state; however, in solution the molecules are involved in conformational equilibria that require the presence of intermediates, in which the N → Sn bonds are dissociated. Although the saleanN₃H₃]²⁻ ligand is more flexible and basic, a very similar complexing behavior to that of salenN₃H]²⁻ has been found, and there is evidence that it is even a weaker ligand. Both ligands show the tendency to adopt a curved conformation within the complex, thus indicating that the dynamic process resembles the flapping of butterfly wings. However, the folding is reduced with increasing steric bulk of the organic substitutents attached to the tin atoms. The six-membered heterocyclic rings in the R₂Sn(salenN₃H)] derivatives have envelope conformation, while those in Me₂Sn(saleanN₃H₃)] have distorted boat-conformation. Thus, small changes in the hybridization and basicity of the nitrogen atoms cause significant differences of the stability and the dynamic behavior of the resulting molecules.