Low-Symmetry Self-Assembled Coordination Complexes with Exclusive Diastereoselectivity: Experimental and Computational Studies.

A pyridine/aniline appended unsymmetrical bidentate ligand -(4-(4-aminobenzyl)phenyl)nicotinamide, investigated in this work has two well-separated coordination sites. Combination of the ligand with -protected palladium(II) (i.e., PdL') and palladium(II) in separate reactions produced the corresponding PdL'L and extremely rare PdL type self-assembled binuclear complexes, respectively. Notably, both varieties of complexes are prepared from a common ligand system. Two diastereomers (i.e., (2,0) and (1,1)-forms) are possible for PdL'L type complex, whereas four diastereomers (i.e., (4,0), (3,1), (2,2), and (2,2)-forms) can be imagined for the PdL type complex. However, exclusive diastereoselectivity was observed, and the complexes formed belong to (1,1)-PdL'L and (2,2)-PdL forms. The diastereomers are predicted from NMR study in solution and DFT calculations in gas-phase and implicit-solvent media; however, single-crystal structures of both the complexes provided unambiguous support. The rare PdL type complex is studied in further detail. Parameters like counteranion, concentration, temperature, and stoichiometry of metal to ligand did not influence the diastereoselectivity in complex formation. DFT calculations show the (2,2) form to be the most stable, followed by the (3,1) isomer. The lowest conformational strain in the bound ligand strands in the (2,2)-arrangement along with optimal intermolecular interactions makes it the energetically most stable of all the isomers. Molecular dynamics (MD) simulations were carried out to visualize the self-assembly process toward the formation of PdL type complex and the free energy difference between the (2,2) and (3,1) isomers. Snapshots of MD simulation elucidate the step-by-step progress of complexation leading to the (2,2)-isomer.