The synthesis and structural characterisation of helical silver(I) complexes with short bis(pyridylimine) ligands

Three new disilver(I) double helicate complexes [Ag2L2] [ClO4](2) (1), [Ag2L2] [BF4](2) (2) [Ag2L2] [NO3](2) 3) and a polymeric monohelical species [Ag2L'(2)] [BF4](2) (4) have been generated via a self-assembly procedure to establish the effect of a side-chain on the subtle metal-ligand coding. The ligands, L = (C5H4N)C(CH3)-N-N-C(CH3) (C5H4N) and L' = (C5H4N) CH=N-N=CH(C5H4N), were both readily prepared imine-based systems, in which the two bis-bidentate chelating units are separated by a single bond. Complexes 1, 2 and 3 adopt a double helical architecture, in which each silver centre is bound to two pyridylimine units in a distorted tetrahedral coordination geometry. Complex 4 is formed as an infinite monohelical polymer in which each silver centre is coordinated in a distorted tetrahedral geometry by two bidentate binding units from different ligands and the ligands are in turn coordinated to two silver( I) centres. While the two pyridylimine units of the ligand L are able to twist both in its free form and in the double helicates 1, 2 and 3, ligand L' presents a planar conformation in both the free ligand and its monohelical complex 4. It is speculated that the arrangement of the two pyridylimine units around the N-N bond is essential to encode for a double helicate. Intra- and inter-molecular non-covalent interactions are found to stabilize the polymer 4 and provide opportunities for complexes 1, 2 and 3 to crystallize in different space groups.