Preorganization of tetrathiamacrocyclic ligands: implications from computed and experimentally determined structures

The tetrathiamacrocycles trans-6,13-dihydroxy-1,4,8,11-tetrathiacyclotetradecane (L 1 ) and its benzo- (L 2 ), syn- and anti-dibenzo-substituted derivatives (L 3 and L 3′ , respectively) have been prepared by conventional methods, and a benzo-substituted-1,4,7,10-tetrathiacyclotridecane (L 4 ) was obtained by substitution of o-dichlorobenzene after activation with an organometallic fragment. The structures of L 1 –L 4 and of the four palladium(II) complexes [PdL 1 ]Cl 2 ·2H 2 O, [PdL 2 ][PF 6 ] 2 , [PdL 3′ ][PF 6 ]Cl and [PdL 4 ][PF 6 ] 2 were determined by X-ray crystallography. All the experimentally determined structures were accurately reproduced by molecular mechanics calculations, using a constant-parametrization scheme. Two parameters for the evaluation of the extent of preorganization of free macrocycle molecules, based on structural and strain-energy differences of the free macrocycles and their corresponding metal complexes, are introduced and discussed on the basis of the experimentally determined and computed structural data for L 1 –L 4 , their metal complexes and other similar ligand and metal complex systems. The implications for the design of new compounds leading to high stability and high metal-ion selectivity are outlined.