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.